Review on Risk management techniques involved in curbing Arsenic Menace

About Authors:
Priya Pathak, Dr.H.H.Siddiqui*, Mr T.Mehmood
Faculty of pharmacy
Integral University,Lucknow

ABSTRACT
The article is a review on all the methods which have been evolved in recent times for curbing the menace caused by arsenic to our environment and humans. Arsenic poisoning is the most spread problem these days in India,as the main reasons of arsenic poisoning include the wastes being dumped by industries which in turn helps in increasing the amount of arsenic in groundwater. Mostly used method for removal of arsenic is oxidation of As(III) to As(V) and its subsequent removal through adsorption and/or precipitation.Generally alumina is used for adsorption purpose.Many methods have also been evolved for managing the increase of arsenic amount through wastes like Cement Solidification, Dolocrete Encapsulation Technology etc.which have been discussed later in the article. Many low cost technologies are also present like Arsenic removal plant fitted directly with hand pump, Co-precipitation-Sedimentation-Filtration under continuous flow system and use of Domestic filters.These techniques are used in case when large amount of water has to be treated as rest other technologies will become quite costly in this case.Although they cannot help in removing arsenic at large level but can be used in our daily life so that we could control arsenic pollution to some extent.

REFERENCE ID: PHARMATUTOR-ART-1160

Arsenic
Arsenic is a metal widely distributed in the earth’s crust .Arsenic is a member of group Va of the periodic table. Arsenic can exist in four valency states: –3, 0, +3 and +5. Arsenic appears in three allotropic forms: yellow, black and grey; the stable form is a silver-gray, brittle crystalline solid.

Levels of exposure to ARSENIC

• The World Health Organization has given a provisional guideline value of 10 µg/litre for arsenic in drinking-water as the practical quantification limit.
• The World Health Organization (WHO, 2000) has estimated that the unit risk for arsenic-induced lung cancer (risk estimate for lifetime exposure to a concentration of 1 µg/m³) is 1.5 × 10–3.
• Arsenic and arsenic compounds were evaluated by the International Agency for Research on Cancer (IARC, 1973).

There was sufficient evidence for carcinogenicity to humans and limited evidence for carcinogenicity to animals, and the overall evaluation was that arsenic and arsenic compounds are carcinogenic to humans.

Environmental levels and human exposure to ARSENIC

• Mean total arsenic concentrations in air from remote and rural areas range from 0.02 to 4 ng/m³.
• Mean total arsenic concentrations in urban areas range from 3 to about 200 ng/m³; much higher concentrations (> 1000 ng/m³) have been measured in the vicinity of industrial sources.
• Marine organisms normally contain arsenic residues ranging from < 1 to more than 100 mg/kg, predominantly as organic arsenic species such as arsenosugars (macroalgae) and arsenobetaine (invertebrates and fish).
• The daily intake of total arsenic from food and beverages is generally between 20 and 300 µg/day¹.

ARSENIC POISONING

It is a medical condition caused by increased levels of the element arsenic in the body.Arsenic poisoning interferes with the enzyme pyruvate dehydrogenase (PDH).This leads to cellular apoptosis episode.Poisoning with arsenic can raise lactate levels and lead to lactic acidosis.Inorganic Arsenic trioxide found in ground water particularly affects voltage-gated potassium channels , which can cause problems leading to death^2.

Arsenic determination techniques

A variety of methods are available for determination of arsenic:

1.Atomic absorption spectroscopy(AAS)and emission spectrometry(ES)

2.Microwave emission(ME)

3.DC helium method

4.Differential pulse polarography(DPP)

5.Hydride Generation Technique(HGT)

Hydride Generation Technique is the most sensitive technique whereas DPP is also well appreciated but is very laborious.

Usually for arsenic determination,arsine is first generated and then determined by AAS.It is difficult to measure small quantities of H₃As₄ by AAS.By AAS measuring different oxidation states of arsenic is also not possible.

The advantage with hydride generation technique is of distinguishing between arsenite,total inorganic arsenic and methylarsenic compounds³.

Polarographic Determination

Determination of AS(+3)by DPP:

Procedure:10ml of the sample at pH=0 is to be adjusted with Hydrochloric acid and is to be pippetted into cell and then 10 ml of supporting electrolyte HCL 1M and 2g of the resin are to be added.Nitrogen is to be bubbled through the solution.The polarographic determination by DPP technique is to be carried out under operational conditions.

The standard addition method is to be used for quantitative determination using Eppendrof pippets and standard arsenic solutions (20,50,100mg/l). Nitrogen is to be bubbled through the solutions for 10 mins,and the polarograms are to recorded.Both sample and each aliquot are to be measured three times,the mean value is to be recorded in calculation.The amount of arsenic added is to be plotted versus peak current and the arsenic concentration is to be obtained from this curve.A linear response is to be obtained for the range of concentration between 20 and 170mg/l⁶.

Management of Arsenic Poisoning

1.RISK MANAGEMENT OF ARSENIC IN CONTAMINATED ENVIRONMENTS

Risk management of contaminated sites includes source reduction, site remediation, and environmental protection. Selection of optimal risk management strategies requires consideration of core objectives such as technical practicability, feasibility, and cost effectiveness of the strategy and wider environmental, social, and economic impacts. Arriving at an optimal risk management solution for a specific contaminated site involves three main phases of the decision-making process. These include problem identification,development of problem solving alternatives (i.e., remediation technologies),and management of the site. The next section discusses the various remediation technologies considered suitable for managing As-contaminated soil and aquatic environments.

a. REMEDIATION OF ARSENIC-CONTAMINATED SOIL
Remediation of As-contaminated soil involves physical, chemical, and biological approaches that may achieve either the partial/complete removal of As from soil or the reduction of its bioavailability in order to minimize toxicity. A large variety of methods have been developed to remediate metal(loid)s-contaminated sites. These methods can also be applicable for the remediation of As-contaminated soils. The selection and adoption of these technologies depend on the extent and nature of As contamination,type of soil, characteristics of the contaminated site, cost of operation,availability of materials, and relevant regulations.

1. Physical Remediation
Major physical in situ treatment technologies to remediate metal(loid)-contaminated sites include capping, soil mixing, soil washing, and solidification.The simplest technique for reducing the toxic concentration of As in soils is mixing the contaminated soil with uncontaminated soil. This results in the dilution of As to acceptable levels. This can be achieved by importing clean soil and mixing it with As-contaminated soil or redistributing clean materials already available in the contaminated site. Another dilution technique,especially in cultivated soils, relies on deep ploughing, during which the vertical mixing of the contaminated surface soil with less contaminated subsoil reduces the surface contamination, thereby minimizing the potential for As uptake by plants and ingestion of As by grazing animals. However, in this method the total concentration of As in soil will remain the same.Soil washing or extraction has also been used widely for the remediation of metal(loid)-contaminated soils in Europe and this method may be applicable for As-contaminated soils to some extent evaluated an acid-washing process to extract the bulk of As(V) from a highly contaminated (2830 mg As kg^_1soil)Kuroboku soil (Andosol) so as to minimize the risk of As to human health and the environment. The contaminated soil was washed with different concentrations of hydrogen fluoride, phosphoric acid, sulfuric acid, hydrogen chloride, nitric acid, perchloric acid, hydrogen bromide, acetic acid, hydrogen peroxide, 3:1 hydrogen chloride–nitric acid, or 2:1 nitric acid–perchloric acid. Phosphoric acid proved to be most promising as an extractant,attaining 99.9% As extraction at 9.4% acid concentration. Sulfuric acid also attained a high percentage extraction. The acid-washed soil was further stabilized by the addition of lanthanum (La), cerium (Ce), and Fe(III) salts or their oxides/hydroxides, which form an insoluble complex with dissolved As. Both salts and oxides of La and Ce were effective in immobilizing As in the soil attaining less than 0.01 mg liter^_1As in the leachate.The success of soil washing largely depends on speciation of As present in the contaminated soils, as it is based on the desorption or dissolution of As from the soil inorganic and organic matrix during washing with acids and chelating agents. Although soil washing is suitable for off-site treatment of soil, it can also be used for on-site remediation using mobile equipment.

However, the high cost of chelating agents and choice of extractant may restrict their usage to only small-scale operations.Arsenic-contaminated soil may be bound into a solid mass by using materials such as cement, gypsum, or asphalt. However, there are issues associated with the long-term stability of the solidified material. Capping the contaminated sites with clean soil is used to isolate contaminated sites as it is less expensive than other remedial options.Such covers should obviously prevent upward migration of contaminants through the capillary movement of soil water. The depth of such cover or “cap” required for contaminated sites should be assessed carefully. Using a simulated experiment, demonstrated that when the water table is deeper than 2 m from the surface of cap, the upward migration of As through the cap is likely to be less than 0.5 m in 5 years.Where the water table is shallow enough to supply water to the surface (i.e.,1.5 to 2 m in most soils), dissolved As could take <10 years to reach the surface. They have also indicated that when the cap is of a different soil type than the underlying contaminated soil, a coarse-textured cap is very effective in reducing the capillary rise and therefore the cap should always be designed to include a coarser layer to break the capillary continuity.

2. Chemical Remediation
Remediation, based on chemical reactions, is becoming increasingly popular largely because of a high rate of success. A number of methods have been developed mainly involving adsorption, immobilization, precipitation,and comp lexation reactions .However, such methods are often expensive for the remediation of large areas. Two approaches are often used in the chemical remediation of metal(loid)-contaminated soils: (i) immobilization of metal(loid)s using inorganic and organic soil amendments in order to reduce their bioavailability and (ii)mobilization of metal(loid)s and their subsequent removal through plant uptake (phytoremediation) or soil washing. This section discusses the immobilization techniques used for the remediation of As-contaminated soil.Chemical immobilization is achieved mainly through adsorption/precipitation of As in contaminated sites through the addition of soil amendments.The mobilization of metal(loid)s in soils for plant uptake and leaching to groundwater can be minimized by reducing their bioavailability through chemical and biological immobilization. There has been interest in the immobilization of metal(loid)s using a range of inorganic compounds such as lime, P fertilizers (e.g., phosphate rocks) and alkaline waste materials, and organic compounds such as biosolids. Depending on the source, the application of P compounds can cause direct adsorption of As onto these materials, promote As complex formation, or induce desorption of As through competition. This method is considered more economical and less disruptive than the conventional remediation option of soil removal.Immobilization of As may be achieved by (i) changing the physical properties of the soil so that As is more tightly bound and therefore becomes less bioavailable; (ii) chemically immobilizing As either by sorption onto a mineral surface or by precipitation as a discrete insoluble compound; and/or (iii) mixing the contaminated soil with uncontaminated soil, thereby increasing the number of As-binding sites.

A number of organic and inorganic amendments are known to immobilize a range of metal(loid)s including As by chemical adsorption. These include ion-exchange resin, ferrous sulfate, silica gel, gypsum, clay minerals such as bentonite, kaolin, and zeolite, green sand, and liming materials.These materials are naturally occurring and nontoxic with a large specific surface area and a significant amount of surface charge. The use of naturally occurring clay minerals such as zeolite as adsorbents is a novel method for the remediation of metal(loid)-contaminated soils .The advantages of zeolite application are its high efficiency for retention of metal(loid)s in soils, low cost, and easy application⁹.

The effectiveness of soil additives in reducing contaminant mobility was assessed¹⁰. Their results indicated that the lowest amount of As was extracted when the soil was amended with beringite, steel shots, and their combination. Although the addition of hydroxyapatite decreased the mobility of metals such as Cd and Pb, it increased the mobility of As mainly due to H₂PO₄–AsO₄competition for the sorption sites. Therefore, the use of hydroxyapatite at multimetal(loid)-contaminated sites requires careful attention.Liming is increasingly being used as an important soil management practice in reducing the toxicity of certain metal(loid)s in soils. In addition to the traditional agricultural lime, a large number of studies have examined the potential value of other liming materials as immobilizing agents in reducing the bioavailability of a range of metal(loid)s in soils¹¹. However, the effect of liming soils on As mobility has been rather inconsistent. Lime addition to As-contaminated soil induces the formation of CaH(AsO₄)₂, thereby reducing the soluble As in the soil solution for plant uptake and leaching. However, the solubility product of this compound is greater than that for Fe and Al arsenates, which are readily formed in most soils. For this reason, liming is not practiced widely to overcome As toxicity in soils¹³, although liming has been reported to increase the immobilization of As ¹⁵ and to decrease the plant uptake of As¹⁶. The historical source of the contamination appears to be the ubiquitous use of As-based herbicides. Exposure pathway analyses showed that the highly mobile As posed a risk to both the groundwater and the residents living in the area¹⁷. The contaminated site was identified for industrial development with Australian industrial guidelines for As set at 500 mg kg_1 soil. Options for managing contaminated soil included in situ cleanup, excavation, and transport to landfill sites or application of risk-based land management strategy. Both in situ cleanup and excavation and transport to landfill were found to be prohibitively expensive and ranged from _$500,000 to $1,000,000.

A risk reduction strategy was adopted with the aim to reduce the mobility of As through chemical immobilization. Ferrous salt was used to generate in situ mineral phases to immobilize As. This reaction requires oxygen to be available to the soil and also generates considerable amounts of acid, which may be counterproductive to As immobilization in poorly buffered soils. The increased acidity could be neutralized by the amendment with lime. The redox conditions of the soil also influence the speciation of As, and an example of two possible redox couples is given later. Following initial detailed laboratory studies, amixture of Fe/Mn/gypsum was used as the stabilizing chemical¹⁸.

3. Biological Remediation

a. Bioremediation.Bioremediation of soils contaminated with organic compounds such as pesticides and hydrocarbons is widely accepted in which native or introduced microorganisms and/or biological materials, such as compost, animal manures, and plant residues, are used to detoxify or transform contaminants. There has been increasing interest in the application of this technology for the remediation of metal(loid)-contaminated soils, especially for those metal(loid)s that undergo biological transformation. Although it has several limitations, this technology holds continuing interest because of its cost effectiveness. The unique aspect in bioremediation is that it relies mainly on natural processes and does not necessarily require the addition of chemical amendments other than microbial cultures and biological wastes. Because As undergoes biological transformation in soil,appropriate microorganisms may be used for the remediation of As contaminated soils.Existing and developing in situ bioremediation technologies may be grouped into the following two broad categories.

i. Intrinsic bioremediation is where the essential materials required to sustain microbial activity exist in sufficient concentrations that naturally occurring microbial communities are able to degrade the target contaminants without the need for human intervention. This technique is better suited for remediation of soils with low levels of As over an extensive area.

ii. Engineered bioremediation relies on various approaches to accelerate in situ microbial degradation rates. This is accomplished by optimizing the environmental conditions by adding nutrients and/or an electron donor/acceptor, thus promoting the proliferation and activity of existing microbial consortia. It is favored for highly contaminated localized sites. Three approaches could be used in the bioremediation of As-contaminated soils: (i) As could be immobilized into microbial cells through biosorption (bioaccumulation), (ii) toxic As(III) could be oxidized to less toxic As(V), and (iii) As compounds could be removed from the soil by volatilization¹⁹.

i. Bioaccumulation: Microorganisms exhibit a strong ability to accumulate (bioaccumulation) As from a substrate containing very low concentrations of this element. Bioaccumulation is activated by two processes, namely biosorption of As by microbial biomass and its byproducts and physiological uptake of As by microorganisms through metabolically active and passive processes. Factors such as soil pH, moisture and aeration, temperature, concentration and speciation of As,soil amendments, and rhizosphere are known to influence the process of bioaccumulation of As in microbial cells.While a number of bacterial and fungal species have been known to bioaccumulate As, some algal species (Fucus gardneri and Chlorella vulgaris) are also known to accumulate As²⁰. This technique has often been used successfully to remove metal(loid) ions from the aquatic environment.

ii. Microbial redox reactions: Heterotrophic bacteria have been found to oxidize toxic As(III) in soils and sediments to less toxic As(V) and thus could play an important role in the remediation of contaminated environment²¹. Because As(V) is strongly adsorbed onto inorganic soil components, microbial oxidation could result in the immobilization of As. Strains of Bacillus and Pseudomonas spp²² and Alcaligenes faecalis ²³and Alcaligenes spp. ²⁴ were found capable of oxidizing As(III) to As(V).

A dissimilatory metal(loid) reduction has the potential to be a helpful mechanism for both intrinsic and engineered bioremediation of contaminated environments. Arsenic can be reduced to Aso, which is subsequently precipitated as a result of microbial sulfate reduction. Desulfototomaculum auripigmentum, which reduces both As(V) to As(III) and SO₂_ ₄to H₂S leads to As₂S₃precipitation ²⁵. Because arsenite is more soluble than As(V), the latter can be reduced to As(III) using bacteria in soil and subsequently leached.

iii. Methylation of As: A variety of microbes could transform inorganic As into its metallic hydride or methylated forms. Due to their low boiling point and/or high vapor pressure, these compounds are susceptible for volatilization and could easily be lost to the atmosphere²⁶. Methylation is considered a major biological transformation through which As is volatilized and lost. As discussed earlier, biomethylation of As in soils and aquatic systems is well documented,as it is important in controlling the mobilization and subsequent distribution of arsenicals in the environment²³.

Methanogenic bacteria, commonly present in sewage sludge, freshwater sediments, and composts, are capable of methylating inorganic As to volatile DMA. Arsenate, As(III), and MAA can serve as substrates in DMA formation.

Inorganic As methylation is coupled to the CH₄biosynthetic pathway and may be a widely occurring mechanism for As removal and detoxification ²³. In addition to bacteria, certain soil fungi also are able to volatilize As as methylarsine compounds, which are derived from inorganic and organic As species.Woolsondemonstrated the release of alkylarsines in a number of soils. Dimethylarsine and trimethylarsine are produced when soils were amended with inorganic and methylated arsenic herbicides. The organisms responsible for volatilization of As originate from diverse environments,suggesting that a number of species have the capacity to produce alkylarsines²³. In most cases, these organisms were tested in laboratory conditions; however,their performance should be assessed under field conditions in contaminated sites²⁷.

b. Phytoremediation. Phytoremediation is considered a subset of bioremediation that employs plants and their associated root-bound microbial community to remove, contain, degrade, or render environmental contaminants harmless. This terminology applies to all plant-influenced biological, chemical, and physical processes that aid in the remediation of contaminated medium²⁸. It involves soil–plant systems in which metal(loid)s-accumulating plants are grown in contaminated sites. It is considered an economically feasible and environmentally viable technology for remediating metal(loid)-contaminated systems. The effectiveness of this technology is,however, variable and highly site dependent.In phytoremediation, plants are exploited as a biopump that use the energy of the sun to remove water and contaminants from the soil to the above ground portion and return some of the products of photosynthesis back into the root zone in the form of root exudates involved in the (im)mobilization of contaminants. Transpiration is the driving force for phytoremediation.By removing water from the medium, plants help reduce erosion, runoff, and leaching, thereby limiting the movement of contaminants off-site. Some contaminants are taken up in the transpiration stream,where they may be metabolized, and may be eventually volatilized. By removing excess water from the soil profile, plant roots may also create an aerobic environment where metal(loid) mobility is reduced and biological activity is enhanced. Plants stimulate microbiological activity in the root zone by providing a carbon source from root exudates and decaying root materials.Phytoremediation technologies have been grouped into various categories that include phytostabilization, rhizofiltration, and phytoextraction ²⁸. In phytostabilization, transpiration and root growth are used to immobilize contaminants, including As by reducing leaching, controlling erosion, creating an aerobic environment in the root zone, and adding organic matter to the substrate that binds As. It involves the establishment of metal(loid)-tolerant vegetation on the contaminated site that is left in perpetuity. The stabilization of As in the root zone could be achieved through the addition of organic matter as well as soil amendments.

In rhizofiltration, the roots can be used to adsorb or absorb metal (loid)s, which are subsequently removed by harvesting the whole plant. In this case, metal(loid) tolerance and translocation of the metal(loid)s to aerial parts are largely irrelevant. In phytoextraction, plants can be grown on contaminated soil and the aerial parts [and the metal(loid)s they contain]harvested. In this case, plants need to be tolerant only if the soil metal(loid) content is very high, but they need to accumulate very high concentrations in their aerial parts. Phytoextraction involves repeated cropping of plants until the metal(loid) concentration in the soil has reached the acceptable

(targeted) level.Certain plants, termed “hyperaccumulators”²⁹,accumulate an inordinate concentration of metal(loid)s in their aboveground biomass. These plants may even accumulate metal(loid)s that are nonessential and often toxic to plants. The minimum concentration of As required for a plant to be classified as a hyperaccumulator of As was set at 1000 mg kg^_1(0.1%) on a dry weight basis³¹. The hyperaccumulation of metal(loid)s involves uptake of the soluble metal(loid) species by the root system, translocation to the aerial parts, and storage in a nontoxic form in the aerial portions. It suggested that this process necessarily requires tolerance to high concentrations of metal (loid)s.Using a combination of techniques, including X-ray absorption spectroscopy, the biological mechanisms involved in the accumulation of As in Indian mustard (Brassica juncea) was studied and the biochemical fate of As taken up by this plant was established. Arsenic was taken up by roots as oxyanions [As(V) and As(III)], possibly via the H₂PO^_4transport mechanism, and a small fraction was exported to the shoot via xylem. Once in the shoot, the As is stored as an As-III-tris-thiolate complex. The majority of the As remains in the roots as an As-III-tris-thiolate complex, which is indistinguishable from that found in the shoots and from As-III-trisglutathione.The thiolate donors are thus probably either glutathione or phytochelatins. Addition of the dithiol arsenic chelator dimercaptosuccinate to the hydroponic culture medium caused a fivefold increase in the As level in the leaves, although the total As accumulation was increased only marginally.This indicates that the addition of dimercaptosuccinate to As contaminated soils is likely to facilitate As bioaccumulation in plant shoots,a prerequisite for efficient phytoremediation strategy. The high cost of thiscompound, however, would be an economic concern unless the plants would be able to synthesize it.At present there are about 400 species of known terrestrial plants that hyperaccumulate one or more of several metal(loid)s. However, until recently no As-hyperaccumulating plants were reported. Ma et al.³¹discovered an As-hyperaccumulating plant, ladder brake (Pteris vittata L.), a terrestrial fern, which accumulates large amounts (23,000 mg kg^_1-dry weight basis) of As from soils. The unique property of As hyperaccumulation by the Chinese brake fern is of great significance in the phytoremediation of As-contaminated soils. Therefore, the potential of this fern for phytoremediation of As-contaminated soil was assessed by Tu et al. ³³in a glasshouse experiment using soils from an abandoned wood preservation site. Results have shown that the Chinese brake accumulated huge amounts of As from soil and that its As concentration increased with the growth period. The As concentration in the fronds was 6000 mgkg^_1dry mass after 8 weeks of transplanting and increased to 7230 mg kg^_1after 20 weeks. The As concentration increased as fronds aged, with old fronds accumulating as much as 13,800 mg As kg^_1. Another silver fern [Pityrogramma calomelanos (L.) Link] has also been reported to hyperaccumulate As up to 8350 mg kg^_1dry mass from soil containing 135 mg kg^_137. It occurs in tropical and subtropical regions of the world and is widely distributed in Thailand where it favors open, high rainfall areas. Some Arsenic uptake by plants is associated with the H₂PO^_4uptake mechanism,where presumably As(V) is taken up as a H₂PO^_4analogue. Therefore, there is a growing interest in using P fertilizer to enhance As uptake by plants. Tu and Ma³¹suggested that phosphate application may be an important strategy for the efficient use of Chinese brake (Pteris vittata L.) to phytoremediate As-contaminated soils. The addition of P fertilizer to As-contaminated soil was found to increase As solubility and mobility and thus increase plant uptake of soil As.

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B. REMOVAL OF ARSENIC FROM AQUATIC ENVIRONMENTS

As most cases of As toxicity in humans have resulted from the consumption of As-contaminated water, there have been intensive research efforts in developing technologies aimed at stripping As from water. A plethora of methods suitable for the removal of As from water at both household and community levels are currently available. These methods are primarily based on (i) removal of solid-phase As through coagulation, sedimentation, or filtration; (ii) removal of solution-phase As through ion exchange, osmosis, or electrodialysis; (iii) oxidation of As(III) to As(V) and its subsequent removal through adsorption and/or precipitation; (iv) biosorption using microorganisms; and (v) rhizofiltration using aquatic plants.

1. Physicochemial Methods

Filtration, adsorption, and chemical precipitation are the most common physicochemical methods used for stripping As from water. While the particulate As in water can be removed by simple filtration, the aqueous As can be removed through adsorption or precipitation followed by

filtration.

a. Filtration.Most of the domestic drinking water treatment systems for As removal involve filtration. For example, the “Pitcher filter” involving porous ceramics ³⁸ and sand filters  have been found to be effective in stripping As from water.It was noticed that the porous nanofiltration anion-exchange membrane removed about 90% of As(V) present in water at a concentration of 316 g liter^_1. Although this technology could achieve a high degree of As removal,it involves a high initial investment and high operation and maintenance costs.

b. Adsorption.A number of compounds, including activated alumina,Fe-coated sand, and ion-exchange resins are used to adsorb As. In most geologic environments, Fe₂O₃carries a positive surface charge that preferentially adsorbs As. Similarly, Al(OH)₃and silicate clays also adsorb large amounts of As.It wasinvestigated the removal of As from water using “brown gel,” which is a silica gel containing 6% of Fe(OH)₃, and observed that the maximum adsorption (17 g As kg^_1) of both As(III) and As(V) occurred at pH 6.It wasfound that synthetic Fe-floc [Fe(OH)₃], prepared by treating FeSO₄with NaOCl at pH 3.5–5.0, removed a large percentage of As from geothermal discharge water through coprecipitation.Similarly, Yuan examined the potential value of several Fe treated natural materials such as Fe-treated activated carbon, Fe-treated gel beads, and Fe oxide-coated sand in removing As from drinking water under both laboratory and field conditions. The Fe oxide-coated sand consistently achieved a high degree (>94%) of As(III) and As(V) removal. When the pH was increased from 5 to 9, As(V) adsorption decreased slightly, but As(III)adsorption remained relatively stable. Kimberlite tailings ³⁹ and iron-sulfide minerals such as pyrite and pyrrhotite were also found to be very effective adsorbents in stripping both As(III) and As(V) from water.Hlavay and Polyakdeveloped and tested novel adsorbents for As stripping. Porous support materials were granulated using Al₂O₃and/or TiO₂and then Fe(OH)₃was freshly precipitated onto the surface of these particles. The resulting Fe(OH)₃-impregnated porous adsorbent was dried at room temperature and packed into an ion-exchange column. These columns were found to remove >85% of As in water. The As(III) ions can primarily be adsorbed by chemical reaction on the surface of Fe(OH)₃. The neutral functional group of {kFeOH} reacts with H₂AsO^_3ions, and surface compounds of {kFeAsO₃H₂}, {kFeAsO₃H_},and {kFeAsO₂_} can beformed.Das et al.³⁹demonstrated the practical application of the adsorption technique in stripping As by developing a simple household device to remove As from groundwater used for drinking and cooking purposes. The system consists of a filter, tablet, and two earthen or plastic jars. The tablet contains Fe(III) salt, an oxidizing agent, and activated charcoal. The filter is made of mainly purified fly ash with binder. When the tablet is added to water (one tablet for every 20 liters), the As(III) ions are catalytically oxidized to As(V) ions in the presence of Fe(III), which are subsequently adsorbed onto activated charcoal and hydrous ferric oxide (Fe₂O₃–3H₂O). In addition to As(V), As(III) ions are also strongly adsorbed by Fe(III) oxides.The water is allowed to settle for about an hour and is then filtered. This stripping system has been installed in several locations in Bangladesh and West Bengal, and analytical resul ts have shown that generally 93–100% of the total As in water (with an initial concen tration of 149–463 g liter^{_ 1}) is removed.In year 2000 Khan etal evaluated the efficiency of a simple three-pitcher filter system consisting of ceramic filters (loca lly known as 3-kalshi) in stripping As from ground water. In the 3-kalshi assembly, the first kalshi (pot) contains Fe chips and coarse sand, the second contains wood charcoal and fine sand, and the third is the collector for the filtered water. Depending on the size of the filtering units , this system has been shown to be capable of reducing the As concentra tion in water from an init ial level of 1100g liter ^{_ 1}to be low the detection limit of 2 g liter ^{_ 1}with a corresponding decrease in dissolved Fe concentration (from 6000 to 200 g liter^{_ 1})⁴¹.

 c. Precipitation.Arsenate can be removed by precipitation/coprecipitation using Fe and Al compounds. Gulledge and O’Connor (1973)⁴⁴achieved a complete removal of As(V) from water using Fe₂(SO₄)₃at a pH range of 5 to 7.5. Hydrolyzing metal salts such as FeCl₃and alum [Al₂(SO₄)₃] have been shown to be effective in stripping As by coagulation. Hering was a scientist who achieved >90% removal of As(V) from water containing an initial concentration of 100 g As liter^_1. Shen⁴⁴removed As from drinking water by dosing with chlorine (Cl₂) and FeCl₃. Oxidation of As(III) to As(V) by Cl₂and the subsequent removal by precipitation were considered the mechanisms involved in this process.Treating drinking water with Fenton’s reagent (ferrous ammonium sulfate and H₂O₂) followed by passing through elemental Fe, As removal below the USEPA maximum permissible limit of 50 g liter^_1from an initial concentration of 2000 g liter^_1of As(III) was achieved.This method is simple and cost effective for use at community levels. Using a bench scale test, Mamtaz and Bache (2000)⁴⁵demonstrated that up to 88% of the As(III) in water could be removed by coprecipitation with naturally occurring Fe found in groundwater. One of the advantages in chemical precipitation method is that this can be used at both household and community levels. The materials are readily available and generally inexpensive.However, a problem of disposal of toxic sludge exists and it also requires trained operators.

2. Biological Methods

a. Phytoremediation using Aquatic Plants.Phytoremediation of Ascontaminated waters may be readily achieved by the use of aquatic plants because unlike soil, most of the As in water is available for plant uptake. In the case of soils, the plant must first solubilize the metal(loid)s in the rhizosphere and then should have the ability to transport it to the aerial tissue. The use of freshwater vascular plants for the removal of metal(loid)s from water has been long established. There are two approaches in using these plants for the remediation of polluted water: The first involves monospecific pond cultures of free-floating plants such as water hyacinth. The plants accumulate the metal(loid)s until a steady state of equilibrium is achieved. They are then harvested by removal from the pond. The second approach involves growing rooted emergent species in trickling bed filters. Rhizosphere microbes usually facilitate the removal of metal(loid)s in these systems. Rhizofiltration usually involves the hydroponic culture of plants in a stationary or moving aqueous environment wherein the plant roots absorb metal(loid)s from the water. Ideal plants for rhizofiltration should have extensive root systems and be able to remove metal(loid)s over an extended period.A field survey was done in which a number of terrestrial and aquatic plant samples were taken at several sites within the Taupo volcanic zone (TVZ) in New Zealand. The TVZ covers an area of 600,000 ha in the central North Island of New Zealand and the area is rich in geothermal activity. There have been previous reports of elevated As concentrations in some waterways and associated lands in the TVZ. The known sources of As pollution in the TVZ include (i) As arising from naturally occurring geothermal activity; (ii) geothermal bores that release As-rich water into the aquatic biosphere; (iii) runoff of As-based pesticides; (iv) As from timber treatment sites such as the pulp and paper mill at Kinleith; and (v) As added to lakes to control weeds (e.g., NaAsO₂added to Lake Rotorua). Data clearly display the difference of As accumulation between aquatic and terrestrial plants. Aquatic plants, , had As concentrations up to 4000 mg kg^_1on a dry matter basis. In contrast, terrestrial plants, showed much lower As concentrations. All the aquatic plants tested accumulated As at concentrations greater than 5 mg kg^_1on a dry matter basis, and none of the terrestrial plants tested had As concentrations surpassing 11 mg kg^_1. Most of the terrestrial plants tested were below the detection limit for As (0.5 mg kg^_1) even when growing in soil containing up to 89 mg As kg^_1. There is a diference in metal(loid) accumulation between aquatic and terrestrial plants  in terrestrial systems, for instance, the solubilization of As in the rhizosphere is necessary to allow the plant roots to take up and transport this element to the aerial parts of the plant. This is not the case when the plant grows in an aqueous medium, where the metal(loid) is already present  in a bioavailable form. Various reasons could be between aquatic and terrestrial plants. Aquatic plants, had As concentrations up to 4000 mg kg^_1on a dry matter basis. In contrast, terrestrial plants, showed much lower As concentrations. All the aquatic plants tested accumulated As at concentrations greater than 5 mg kg^_1on a dry matter basis, and none of the terrestrial plants tested had As concentrations surpassing 11 mg kg^_1. Most of the terrestrial plants tested were below the detection limit for As (0.5 mg kg^_1) even when growing in soil containing up to 89 mg As kg^_1.

b. Microbial Removal of Arsenic.Biosorption and biomethylation are the two important processes by which metal(loid)s, including As, are removed from water using microorganisms.The biosorptive process generally lacks specificity in metal(loid) binding and is sensitive to ambient environmental conditions, such as pH, solution composition, and the presence of chelators. Genetically engineered microorganisms (e.g., Escherichia coli) that express a metal(loid)-binding protein (i.e., metallothionein) and a metal(loid)-specific transport system have been found to be successful in their selectivity for accumulation of a specific metal (loid) in the presence of a high concentration of other metal(loid)s and chelating agents in solution ⁴⁶. These organisms also have potential application to remove specific metal(loid)s from contaminated soil and sediments.Biosorption is one of the promising technologies involved in removing As from water and wastewater. Several chemically modified sorbents have been examined for their efficiency in removing metalloids. Methylation is the most reliable biological process through which As can be removed from aquatic medium. Certain fungi, yeasts, and bacteria are known to methylate As to gaseous derivatives of arsine. Commercial application of biotransformation of metal(loid)s in relation to the remediation of metal(loid)-contaminated water was documented by Bender et al. (1995)⁵³. They examined the removal and transformation of metal(loid)s using microbial mats, which were constructed by combining cyanobacteria with a sediment inoculum from a contaminated site. When water containing high concentrations of metal(loid)s was passed through the microbial mat, there was a rapid removal of the metal(loid)s from the water. The mat was found to be tolerant of high concentrations of toxic metal(loid)s such as Cd, Pb, Cr,Se, and As (up to 350 mg liter^_1). Management of toxic metal(loid)s by the mat was attributed to the deposition of metal(loid) compounds outside the cell surfaces, as well as chemical modification of the aqueous environment surrounding the mat. Large quantities of metal(loid)-binding polysaccharides were produced by the cyanobacterial component of the mat. Photosynthetic oxygen production at the surface and heterotrophic consumption in the deeper regions resulted in steep gradients of redox condition in the mat.Additionally, sulfur-reducing bacteria colonized the lower strata, removing and utilizing the metal(loid) sulfide. Thus, depending on the biochemical characteristics of the microzone of the mat, the sequestered metal(loid)s could be oxidized, reduced, and precipitated as sulfides or oxides.

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C. MULTISCALAR-INTEGRATED RISK MANAGEMENT

A number of challenging issues need to be taken into consideration when devising strategies to manage As contamination of the environment. These include the following:

i. Complexity of As contamination—the severity and long-term persistence of As contamination are influenced by factors such as medium characteristics,site hydrogeology, land and water use, source term, chemical form and speciation, and target organism.

ii. Presence of multichemical species—As undergoes several biogeochemical transformation processes, resulting in the release of an array of chemical species that differ in their biogeochemical reactions, bioavailability, and biotoxicity.

iii. Extent and magnitude of As contamination of groundwater resource—for example, in Bangladesh, As in groundwater is derived from geological weathering of parent rock materials from the Indo-Gangetic alluvial plains spread over an area of millions of hectares

iv. Multipurpose end use of contaminated resources—water is used for drinking, cooking, and other household purposes and for irrigation;similarly, soil is used for agricultural production and recreational activities.

It is therefore important to formulate and/or devise integrated risk management strategies involving source avoidance, source reduction, and remediation.Source avoidance, which refers to avoiding the most contaminated source of the groundwater relative to certain geological strata, can be practiced to minimize the risk resulting from As contamination of soil and water resources. For example, in Bangladesh, shallow dug wells are increasingly becoming popular as an alternative to pump water from deeper strata.In some cases, the relatively contaminant-free strata are below 250-m deep zones. However, sanitation of these shallow wells is paramount to avoid gastroenteritis and other pathogenic-borne diseases. Another strategy is source reduction, which refers to removing or stopping the source of contamination.Source reduction can be achieved easily when the contamination source is of anthropogenic origin, such as those in landfills or similar point sources. As discussed earlier, in most regions, As contamination of groundwater is largely of geogenic origin, and source reduction may not be a feasible option to manage As contamination.

Remediation of contaminated soil and water resources requires both short-term and long-term solutions to the As problem. Therefore, the remediationstrategies should be aimed at multiscalar levels, i.e., household level to community and regional levels, representing the various levels of complexicity.Depending on the efficiency and cost effectiveness of the system, a combination of technologies may be required at certain levels.For example, at the least complex household level, remediation strategies involving only a simple filter (sorptive) system can be used to remove As (i.e., As stripping) from water used for drinking and cooking purposes, whereas at a more complex community level, more sophisticated precipitation technologies should be used to strip As from the community water supply so that cost can be shared and the system can be managed efficiently. More sophisticated stripping methods, which may require a series of a filtering–sorptive (precipitation) setup, are necessary in order to cope with the enormous volume of groundwater that needs to be treated before distribution to the community. Even at the community scale, the situation becomes even more complex when dealing with impacted soils, especially those geared for food production. In this case, land use is a very important factor to address. For example, in parks, applying soil amendments such as those high in Fe₂O₃may suffice to mitigate As risk. In contrast, technologies might be paired in a situation when the food chain might be compromised, as typified by rangeland, rice paddy, and so on. A viable approach in this circumstance is to apply phytoremediation during the initial period (1 to 2 years) to strip the “bioavailable” fraction, subsequently followed by soil amendments before committing to the intended land use. It is very important to observe that as the level of contamination becomes more complex, a monitoring scheme should be in place. Hence, a successful remediation scheme for an As-contaminated environment should aim for an integrated approach involving the possible combination of physical, chemical, and/or biological mechanisms.It is essential that the integration of remediation technologies should enhance efficiency, both technologically and economically, resulting in a reduction in the time required for achieving targeted levels of As. For example, phytoremediation is a promising new technology, which is relatively inexpensive and has been proven effective in the large–scale remediation of both soil and water resources. Further, it would also add “green” value (aesthetic) to the environment. Integrating physical, chemical, and/or bioremedial measures with phytoremediation could enhance a higher uptake of As by plants, can more effectively minimize biotoxicity through microbial and chemical immobilization, and can potentially eliminate As through the inducement of biomethylation and subsequent volatilization from the system⁵⁷.

2.Techniques for management of arsenic wastes

(a)CEMENT SOLIDIFICATION

Oretest Pty Ltd

Introduction
Oretest Pty Ltd in Western Australia utilise cement solidification for the treatment of arsenic waste. Cement solidification is a generic technology for the stabilisation and disposal of a range of inorganic wastes, especially those containing toxic metal such as mercury, cadmium, copper, nickel, lead and chromium. It can also be used to stabilise inorganic arsenic in the form of arsenite(As III) or arsenate (AsV).

The technology, because of its reliance on cement to provide a physical and chemical barrier to dissolution of the waste components, is not suitable for use with wastes containing organic components⁽⁵⁸⁾.

Technology Description
Free lime in the cement reacts with the arsenite(As III) in the waste to form an insoluble precipitate of calcium arsenite(III). This precipitate is highly insoluble above pH 10-11.The cement contains sufficient free lime to ensure the pH in the cement remains above the pH for dissolution, and hence retains the arsenic as an insoluble precipitate. The cement matrix also acts as a barrier to intrusion of leaching solutions, and extrusion of soluble species. When and if the pH drops, the rate of leaching of the arsenic is very slow, resembling diffusion through a clay barrier.

Performance
Oretest state that concentrated arsenic wastes are not large waste streams, and can therefore be treated onsite using portable equipment such as a cement mixer truck.The waste-cement-water mixture is poured into drums or bulk bags and allowed to set prior to disposal in a suitable landfill. Quality control (verification) is by TCLP on each batch of waste, with each batch being approximately 1 to 2 tonnes.

Safety and Environmental Risks
Oretest state that the treated material is similar to concrete, but with a lower compressive strength. Depending on requirements, the treated material can be set in drums, bulk bags or a tailings dam. There are no process emissions for suitable waste streams. However, the long term stability of the waste is unknown. When intrusion rates of leachants are low, and in low rainfall environments, leaching rates are estimated in geological time frames. The process has had a long history in the US. It is a low technology process using existing equipment, and all reactions occur at ambient temperature. It is therefore safe, reliable and easy to control.

The limitations of cement stabilisation technology are:

1.that the original volume of waste is increased (sometimes substantially) by the addition of the immobilisation media;

the large mass of waste that is generated which must be disposed to landfill;

the process is generally only applicable to inorganic waste streams, or residues from furnace processes; and

the long term stability of the waste is unknown⁽⁵⁹⁾.

Chemsal-Hudson

Introduction

A joint venture was developed by the Chemsal Group of companies and C. R. Hudson and Associates Pty Ltd which utilises a an immobilisation/stabilisation process for the treatment of concentrated arsenic wastes. During the research and development of this process over 50 other techniques and variations were examined, including the use of ferric iron salts to produce ferric arsenate. For the stoichiometric formation of ferric arsenate, large quantities of ferric salt are required giving rise to a 25-30 times bulking factor which is unacceptable according to Chemsal-Hudson. In addition, the Chemsal-Hudson research indicated better TCLP performance with the specialised calcium arsenate approach than the ferric arsenate approach.

Technology Description

The Chemsal-Hudson immobilisation/stabilisation process occurs in two stages. The first stage involves the pretreatment of arsenic based materials in a slurry reactor with the prime aim being to convert all arsenic present to the pentavalent state. This process is tailored to the specific type of material in question and is applicable to solids,liquids and sludges. The second stage converts the arsenic to calcium arsenate by the addition of lime (pH>12) and a blend of cementitious reagents that provide secondary binding of the calcium arsenate into the cementitious matrix ensuring low arsenic TCLP(<5 mg/L arsenic).

Performance

Chemsal-Hudson state that wastes containing up to 70% arsenic can be treated, and that their stabilised product readily complies with TCLP leachability requirements of 5 mg/l arsenic for disposal at a suitably licensed landfill.The materials handling systems are suitable for the processing of powders in sachets

and drums, soils, liquids, and sludges.

Safety and Environmental Risks

The Chemsal-Hudson facility is fully enclosed within a building with all waste handling and processing areas being negatively vented to a dual venturi/packed bed, wet scrubbing system. Detailed attention has been given to the safe depackaging, handling, and processing of arsenic based chemicals.

Limitations

The limitations of cement stabilisation technology are previously stated⁵⁸.

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Chemsal Remediation Pty Ltd

Introduction

Chemsal Remediation Pty Ltd is another joint venture company that combines the infrastructure and expertise of Chemsal with that of Pollution Solutions (Remediation)Pty Ltd of Tasmania. The joint venture is currently pilot testing proprietary technology that has to date demonstrated success in bioremediating an OC/arsenic matrix under benchtop conditions. Under an EPA R&D works approval, Chemsal Remediation is in the process of commissioning two five tonne batch reactors to demonstrate the performance of the technology.

Technology Description

The technology being deployed is bioremediation. Although this technique typically has a number of control and management problems in situ, by using a batch process the engineering of optimal conditions (eg nutrient, moisture, pH, oxygen, etc) for the bioremediation of OCPs and other organics can be more readily achieved and maintained.

The company offers a trial of the waste being considered for treatment in a laboratory pilot plant and can usually tell within about two to three weeks whether the waste is suitable for bioremediation by this technique. The exact details of this developmental technology is being kept secret at present.

Performance

It is intended that the product from the pilot plant will be streamed into the arsenic treatment plant for final processing to comply with TCLP leachability acceptance criteria at a licensed landfill. The bioremediation plant is expected to have the capability to treat arsenical compounds contaminated with:

organophosphates;

polycyclic aromatic hydrocarbons (PAHs);

PCBs;

aromatic hydrocarbons;

aliphatic hydrocarbons;

phenols;

cresols; and

halogenated solvents and compounds.

It is intended that the plant would be engineered with the same level of process control and safeguards as the existing arsenic treatment plant.

Limitations

The process is not yet commercialized.Historically, the bioremediation of persistent organochlorine type wastes has not been generally successful. For example, the biodegradation of OCPs such as DDT is known to be difficult. Achieving consistently effective biodegradation of wastes of varying composition and in the presence of arsenic, and potentially other cocontaminants such as copper, lead and chromium, is likely to be very difficult.However, initial results provided by the proponent appear promising.

Nationwide Industrial Service

Introduction

Nationwide Industrial Services (a division of Nationwide Oil Pty Ltd) specialised in the treatment of difficult to treat waste streams, including arsenicals, using cement fixation technology.The technology used is capable of handling concentrated arsenicals and arsenical mixtures, particularly sodium arsenite and copper chrome arsenate. Nationwide’s existing licence precludes the treatment of organochlorine pesticides (OCPs) or organophosphorus pesticides (OPPs), although such compounds as disodium

monomethyl arsonate (DSMA) can be treated.

Technology Description

The basic chemistry of the technology is to form an insoluble arsenic salt (normally a ferrosilicoarsenate or similar species) which is then physically bound within a mixture of flyash and cement. The flyash/cement system acts as both as an entrapment matrix and as a chemical (pH) buffer to prevent re-dissolution of the ferroarsenate.

Safety and Environmental Risks

Nationwide state that there are no process emissions

Limitations

Organic components in mixtures may preclude treatment, and OCPs and OPPs are excluded due to licence conditions. A cheap source of iron is required.The limitations of cement stabilisation technology are previously stated⁵⁹.

Cleanaway

Introduction

The waste streams treated by Cleanaway Technical Services (CTS) include:

1.plant residues;

2.copper arsenate residue from phosphate fertiliser manufacture;

3.arsenic pesticides (Coopers Sheepdip) recovery of arsenic.

The principal components of CCA are copper, chromium (hexavalent) and arsenic which are present in percent levels (1 to 10 %). The wastes are generally contaminated with gross solids such as wood, bricks and soils.

Cleanaway Technical Services in Newcastle NSW has been treating arsenic wastes since 1992. This included a variety of arsenic contaminated wastes and materials, with concentrations ranging from parts per million to 50% arsenic. The quantities treated range from small kilogram collections up to 300 tonnes of arsenic sludges and 160 cubic metres of contaminates solid process materials.The material treated ranged from laboratory chemical and farm chemical collections, to obsolete sheep dip, industrial wastes, contaminated materials from manufacturing plants, and arsenic reclamation industries.

Technology Description

The general chemistry used for treatment of CCA treatment plant residues involves reduction of the hexavalent chromium, followed by neutralisation and fixation with cement.Arsenic pesticides such as Coopers Sheepdip are treated for extraction and recovery of the arsenic. The general chemistry involves oxidation of the arsenic trioxide, filtration to remove the sulphur component, purification and concentration by a precipitation and extraction process. The end products are:

1.arsenic acid solution suitable for use in the manufacture of CCA formulations;

and

2.a chemically fixed sulphur cake containing some arsenic residues, which requires

disposal in a secure landfill.

The only preprocessing required is the decanting of the drums of waste, or bags of Coopers Sheepdip powders.

Coopers Sheepdip requires correct identification of the feed material, and the final arsenic extract must meet the specifications for preparation of CCA formulations.

Safety and Environmental Risks

Generally no emissions occur during the processing of the waste material. Any drums of dry material are wetted down prior to processing. Testing of the cured fixed product ensures compliance with the receiving landfill’s licence requirements. Testing is conducted by a NATA registered laboratory. Dust and process emissions associated with the treatment of Coopers Sheepdip would be controlled by suitable collection and scrubbing systems. The reliability of the process is dependent on the consistent nature of the feed stock, and Hazop studies would be used to engineer the risks to a minimum. The chemically fixed sulphur cake would require TCLP analysis to conform with landfill licence agreements.

Limitations

The limitations of cement stabilisation technology are previously stated.The treatment of arsenic pesticide (Coopers Sheepdip) is limited to when the formulation is in its commercial package. The process is also susceptible to contamination from other metals, particularly iron, and from organic compounds.The end use of the extracted arsenic is dependent on a market in the Treated Pine industry⁶⁰.

(b)DOLOCRETE ENCAPSULATION TECHNOLOGY

Introduction

Periclase Pty Ltd is an R & D company researching materials for the building and construction industry, and encapsulation technology for the safe disposal of toxic waste. The Dolocrete Encapsulation Technology has been developed by Periclase Pty Ltd over four years. The first trial was conducted in January 1996 on arsenic and toxic metal based waste. The technology is applicable to all waste streams with the only limitation being on current allowable practices and landfill limitations.

Technology Description

Dolocrete may be described as an alternative cementitious material capable of binding together a variety of aggregates, not normally associated with conventional concrete.The main component is “Doloment” which when mixed with a proprietary catalyst additive “Minic” to an aggregate mix, forms a “Dolomitic Cement”. The basic technology involves solidification of the matrix with microencapsulation of the waste material.Chemical bonds form within the natural magnesium based matrices. Encapsulation is effected by mixing the waste material with selected readily available chemicals. The technology differs from conventional cements where additives for concretes are liquid and are added during the mixing process. In Dolocrete technology, a pre-blended powder which include the catalyst (Minic) additive is required to be fully dissolved in the mixing water, prior to adding the mixing water to the agglomerate mix. Use of the powder needs careful supervision to ensure it is fully dissolved prior to use, however the mixing water does not have to be clean and sea water may be used.A pan-type mixer, having a positive paddle or screw action, is considered to provide the optimum mixing. Drum mixers, using a gravity mixing action, do not function well with the low water Dolocrete mixes, as cohesion of the mixes prevents material falling away from the drum sides during rotation. The resultant dolocrete slurry is then used as an integral part of a settable composition to effectively contain many waste materials in a manner that exceeds the requirements of less than 5 mg/L of the waste elements in the leachate.The treated material can be given structural integrity, with the strengths required controlled in the  treatment process. Tests are normally conducted at 14 days after treatment and again at 28 days, This is generally the longest time needed for solidification and bonding of the process.Where using cement technology is not always suitable for wastes containing organic material, the Dolocrete Technology encapsulates organic wastes without affecting the integrity of the hardened matrix.

Safety and Environmental Risks

The treated product is a hard concrete-like mass. Dolocrete states that there are no process emissions, and no by-products as a result of the process. All wastes are incorporated in the treated material, including, eg, contaminated overalls and gloves worn for protection. The reliability and safety of the process is governed purely by mixing formulae and good work practice. No inherent problems accompany the treatment material or the process.Verification of the treated material is by TCLP analysis.

Limitations

The current allowable disposal options are, by regulation, restricted to landfill. Only those organochlorine arsenical pesticide wastes with less than 50 mg/kg OCPs would be suitable for treatment and disposal to a suitable landfill. Future options (with acceptance of the technology) may include construction projects, eg, as road base material^58,59.

(c)GEOMELT

Introduction

The GeoMelt technology is a family of vitrification technologies developed around the in situ vitrification (ISV) process. The GeoMelt processes are commercially available mobile, thermal treatment processes that involves the electric melting of contaminated soils, sludges, or other earthen materials, wastes and debris for the purposes of permanently destroying, removing, and / or immobilising hazardous and radioactive contaminants. The GeoMelt-ISV process was developed by Battelle, Pacific Northwest National Laboratory, for the US Department of Energy. The ISV technology has been licensed to Geosafe Corporation and Geosafe has used ISV to successfully treat a number of Superfund sites in the US Geosafe has since developed the ISV process into a wider range of vitrification technologies. The GeoMelt processes are widely applicable to all soil types and all classes of contaminants including organics, heavy metals and radionuclides. The processes are available in Australia through Geosafe Australia Pty. Ltd. The GeoMelt-ISV process is being used at the Maralinga site in South Australia to treat burial pits containing soil and debris contaminated with plutonium and uranium as well as lead, barium, and beryllium.The GeoMelt process has been successfully used to treat concentrated wastes, including arsenic-bearing waste agricultural chemicals overseas. For this application,the drummed wastes were simply mixed with soil to facilitate the treatment process.

Technology Description

The GeoMelt process is a batch process that involves forming a pool of molten soil in a treatment zone between an array of four electrodes. The molten soil serves as the heating element of the process wherein electrical energy is converted to heat via joule heating as it passes through the molten soil. Melt temperatures typically range between 1,500 - 2,000°C. Continued application of energy results in the melt pool growing deeper and wider until the desired volume has been treated. When electrical power is shut off, the molten mass solidifies into a vitreous monolith with unequalled physical, chemical, and weathering properties compared to alternative solidification /stabilisation technologies. Individual melts up to 7 m deep and 15 m in diameter are formed during commercial operations. Large volumes of contaminated material requiring more than one batch melt are treated by making a series of adjacent melts resulting in the formation of one massive contiguous monolith. The process is operated on an around the clock basis and can achieve treatment rates of up to 150 tonnes per day. A stainless steel hood is employed, under a slight vacuum, to collect off-gases from the treatment zone. The off-gas treatment system can be designed to meet site specific requirements. The GeoMelt processes can be configured in several different ways to treat soils and wastes. The three primary treatment modes are as follows:

1.The GeoMelt-ISV process is the original GeoMelt process. ISV is used to treat contaminated soils and wastes in situ, where they are located, without the need for excavation and or other handling. This treatment method is referred to as the GeoMelt- in situ mode of treatment.

2.For many projects, contaminated soils, wastes and debris from a variety of on-site locations are collected and staged in a treatment area. This approach allows for more efficient treatment and provides the site owner the opportunity to consolidate many different waste streams. Liquid or concentrated drummed wastes can be mixed with soils and absorbents and treated in the configuration. This treatment configuration is referred to as the GeoMelt-staged in situ mode of treatment.

3.A third method of GeoMelt treatment involves the operation of a stationary batch treatment facility. This method involves an on-site treatment facility where wastes are brought to the treatment facility, staged in one or more reusable treatment cells, the wastes treated, the treated product removed from the cells, and the cells reused to treat additional wastes. The facility can be enclosed inside a building if desired. The treated product can be removed in solid form or can be discharged in its molten state. This treatment configuration is referred to as the GeoMeltstationary batch mode of treatment.GeoMelt processes result in a 25-50% volume reduction for most soils, and up to a 75% volume reduction for sludges and wastes that de-water and/or decompose during processing.

The volume reduction results in a subsidence volume above the vitreous monolith. In most GeoMelt-ISV or GeoMelt-staged in situ applications, the subsidence volume above the melts is filled with clean soil and the monoliths are left in the ground since they are no longer hazardous. Sites treated by the GeoMelt process are normally capable of future use without restriction associated with the vitreous monolith. However, if site requirements dictate that the monoliths be removed, the vitrified monoliths can be fractured and removed with conventional heavy equipment.

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Safety and Environmental Risks

The GeoMelt processes are relatively safe and represents a low risk to the environment as demonstrated by successful commercial operations in the US and in Japan. Factors that enhance the safety of the process are:

1.heavy metals and radionuclides are largely retained in the melt so the emissions of these species from the melt to the off-gas treatment system are minimal;

2.the treatment process is relatively slow; the melt grows at a rate of only a few cm per hour resulting in only a small fraction of the waste material being treated at any one time;

3.the off-gas treatment system is robust and has been demonstrated to be effective on a wide range of contaminant types;

4.organic and inorganic debris (eg plastic, wood, paper, steel drums, protective clothing and equipment) are destroyed or incorporated into the melt, therefore, separation and complete characterisation of the waste is not required;

5.for the in situ mode of treatment, the process does not require excavation and handling of contaminated soils so the risk to workers, the public, and the environment are minimised;

6.since the process treats wastes on-site, there is no requirement for, or risk from, the off-site transport of wastes;

7.no organic contaminants remain in the vitrified product;

8.??the vitrified product is extremely effective at immobilising heavy metals and radionuclides and the product far surpasses TCLP requirements;

9.the process equipment includes back-up safety systems and an alternate power supply in case of equipment or power failure.

Limitations

GeoMelt requires either soil, sand or some other similar earthen material to serve as the treatment media (melt). Concentrated liquids or other non-soil wastes can be mixed with soil for treatment. Treatment depths are limited to approximately 7 m from grade level for the in situ treatment mode. Other ISV configuration options exist for sites requiring greater treatment depths including restaging the materials.

Soil must have more than 1 % (w/w) combined alkali oxides. Most soils meet this criterion. An additive such as soda ash can be used if needed. For in situ applications, the water recharge rate must be less than 1 X 10-4 cm/s or dewatering or diversion techniques must be used. Fully water-saturated soils and wastes can be treated with the process if the recharge rate is controlled. Pretreatment is required for applications involving the in situ mode of treatment for sites containing high integrity sealed containers, such as drums. Alternatively, unacceptable materials such as sealed drums can be breached and the contents blended with soils when employing the staged in situ or stationary batch modes of treatment⁶⁰.

(d)BASE-CATALYSED DECHLORINATION

Introduction

Proponents of the Base-Catalysed Dechlorination (BCD) type of technologies may be able to handle OCP/arsenic type wastes, but may choose not to offer their services for the treatment of these wastes due to the small size of the projects involved.Consolidation of OCP/arsenic wastes into single, large projects may make these and other technologies more viable as the modification or configuration of equipment for larger projects will be more worthwhile. At present, this technology is being used to treat polychlorinated biphenyl (PCB) wastes in a liquid form.

Technology Description

The Base-Catalysed Dechlorination (or Base-Catalysed Decomposition) (BCD)process was developed to treat halogenated organic compounds, particularly PCBs.The process can involve direct dehalogenation or decomposition of waste materials,although it is essentially a liquids treatment process. More practically the process can be linked with a pretreatment step such as thermal desorption which yields a relatively small quantity of a condensed volatile phase for separate treatment.The BCD process involves the addition of an alkali or alkaline earth metal carbonate,bicarbonate or hydroxide. The proportions added range from 1 to about 20 percent by weight, the amount required being dependent on the concentration of the halogenated or non-halogenated organic contaminant contained in the medium.A hydrogen donor compound is employed, such as a paraffin oil solvent, to provide hydrogen for reaction, with the halogenated and non-halogenated contaminants, if these ions are not already present in the contaminated material. In order to activate these compounds to produce hydrogen ions a source of carbon must be added, either in solution or in suspension.The mixture is heated at a temperature and for a time sufficient to totally dehydrate the medium. After dehydration, the medium is further heated at a temperature between 200C and 400C for a time sufficient to effect reductive decomposition of the halogenated and non-halogenated organic contaminant compounds, typically 0.5 to 2hours. At this temperature the catalyst derived from the carbon source (egcarbohydrate) facilitates hydrogen transfer from donor compound to the organochlorine compound.

Generally, oxygen will not adversely affect the BCD process and therefore air does not need to be excluded. However, when applied to the decontamination of hydrocarbon fluids, either aliphatic or aromatic, air needs to be excluded in order to prevent ignition of the hydrocarbon at the elevated temperature of the BCD reaction. This is achieved by passing nitrogen gas through the reaction vessel. The treatment is usually carried out as a batch process with all steps completed within a single reactor.

OCPs which are contaminated with arsenic could potentially be treated with the BCD process. Trials have been done with ADI using BCD and were successful (Carlisle,1998). Although no arsenic was involved in these trials, it is believed that it would end up being extracted (via vacuum extraction at 150⁰?C) in an aqueous phase, or in the sludge, which would then require treatment, such as stabilisation^58,60.

(e)ELI ECO LOGIC

ELI Eco Logic operate the Eco Logic hydrogenation process in Kwinana, WA. Eco Logic is a hydrogenation process based on gas-phase thermo-chemical reaction of hydrogen with organic compounds. At 850?degree centigrade or higher, hydrogen  combines with organic compounds in a reaction known as reduction to form smaller,lighter hydrocarbons, primarily methane. For chlorinated compounds the reduction products include methane and hydrogen chloride. The reaction is enhanced by the presence of water, which acts as a reducing agent and a hydrogen source.Four waste preparation  and feed systems have been proposed to allow the treatmentof a variety of waste  materials including organic liquid waste streams, contaminated watery wastes,solid wastes such as soil or sediment, and gases, including product gas produced in the process. Product gas may contain products of incomplete destruction and these may be recycled through the system to ensure the product gas meets licensed emission limits.The mixture of gases and vaporised liquids are passed over electric heating elements situated around a central ceramic-coated steel tube of the reactor. Treated gases pass through a scrubber where water, heat, acid and carbon dioxide are removed. A caustic scrubbing agent is added to neutralise acids.The process uses hydrogen gas under pressure and care must be taken to operate the system to ensure that explosive air-hydrogen mixtures do not form.For most of the wastes treated, the product gas generated provides much of the process fuel needs. Chlorinated organics may be converted into fuel, and the chlorine is converted into a salt solution which will require disposal to a sewer (some arsenic may also be expected in the scrubber water). Desorbed solid waste can be disposed of to a landfill if other wastes constituents such as heavy metals are at acceptable levels.ELI state that they do not treat arsenical wastes (McEwan, 1998). The Eco Logic process is designed for OCP wastes, but is not able to accept OCPs contaminated with arsenic. The Eco Logic process is limited with respect to waste constituents such as arsenic, as the arsenic would volatilise and pass through the process,contaminating the scrubber water. Theoretically, if the arsenic concentrations were low enough initially in the waste, then the concentration in the resultant scrubber water would be low enough for discharge, either to sewer or for irrigation. However, to avoid any potential problems ELI advise that they will not accept OCP wastes that contain arsenic.

(f)XTALTITE

Introduction

Xtaltite (pronounced crystal-tight) is the trade name for a generic technology for incorporation of toxic waste elements into the crystal lattice structure of synthetic rocks (or polyphase ceramics). The process consolidates, immobilises and permanently isolates a wide range of toxic inorganic waste products. The technology has been developed to meet the demands of mining and industrial operations, such as smelter waste streams, which produce large volumes of toxic inorganic wastes, particularly arsenic trioxide estimated to be 10,000 tons per year.

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Technology Description

Xtaltite Corporation have developed a method of synthetic mineralisation specifically tailored to the stabilisation of heavy metal wastes, which is based on the ‘Synroc’ principle of synthetic mineral immobilisation of high level radioactive waste. Xtaltite have modified the chemistry of the process to further decrease the solubility of the synthetic mineral analogue.Characterisation of the waste stream is required, including a good understanding of the variability limits to the waste stream composition. Characterisation is primarily through powder X-ray diffraction in combination with microscopy to ensure the desired mineral assemblage has formed and the waste elements have entered the correct phases.

Although the stoichiometry of the chemistry must be precise to be able to produce a durable end product, the general formula is as follows;

Waste + quick lime + simple pre-cursors + oxygen + heat + cement = Xtaltite waste.

Several hours are required to develop the correct pre-cursors for Xtaltite production during firing at the hydrometallurgical stage. This ensures an insoluble Xtaltite product.The simple pre-cursors added to the mixture are proprietary formulations. The addition of alkali or acid,dependent on the waste stream, can act as a catalyst speeding up the hydrometallurgical stage. This may then need to be a batch process.

Major species are oxidised (to a higher valence state) to produce the final waste form phase. The reactions are carried out in two stages, hydrometallurgical and pyrometallurgical, to avoid sublimation of volatile species, particularly arsenolite in arsenic waste streams.The first stage of hydrometallurgy, where a precipitate is formed, ensures that all the elements are in suitable precursor configurations for a successful second stage.Pyrometallurgy is the second stage in which the precipitate is pressed into pellets and fired in air at temperatures of 1050 to 1200 C. This is performed in an oxidizing atmosphere to ensure the correct speciation. A binder, either a long-chain polymer or sodium silicate solution, may be added to the slurry enhance the mechanical stability of the unfired pellets.Xtaltite can be produced in the presence of iron and sulphur, which allows flexibility in pre-processing of any waste containing these elements.Xtaltite phase assemblages consist mainly of apatite-like minerals having the general formula A14A26(BO4)6X2, where A1 and A2 are larger cations such as Ca+ and Pb2+, B are smaller cations including As 5+ and P5+, and X are halogen or hydroxyl ions2.The key advantages of the Xtaltite process are:

1.volume effective, high waste loaded (more than 35 % w/w) ceramic;

2.??avoids the formation of secondary waste streams (by volatilisation);

3.??stability over geological time scales; and

4.cost effective one-off processing and disposal costs.

Safety and Environmental Risks

Risks associated with the transport and handling of Xtaltite are low as the product is in a pellet or a gravel form of high stability. Leaching tests can verify the stability of the final product, and the waste loading can be modified to achieve low leachability criteria.

Limitations

It is essential to understand the precise nature of elemental partitioning between the minerals, how such partitioning varies as a function of hydrometallurgical and pyrometallurgical processing, and the response to variations in waste stream composition.It is essential to ensure incompatible minor phases are avoided, as they may contribute to unacceptably high short term dissolution rates.The technology has the ability to:

1. remove both dissolved and suspended metals from the waste stream;

2. remove a variety of metal complexes;

3. work in the presence of high concentrations of background ions; and

4. remove anionic metals.

The absorptive filtration process removes inorganic contaminants, consisting mainly of metals, from aqueous waste streams with a wide range of contaminants concentrations and pH values⁶⁰.

NEW AND EMERGING TECHNOLOGIES

Introduction

A review of a recent publication by the USEPA’s Superfund Innovative Technology Evaluation (SITE) team indicates that a number of technologies are being developed that may have potential applications to arsenic wastes and OCP wastes containing arsenic.

(a)Pyrokiln Thermal Encapsulation

Svedala Industries Inc. have developed the Pyrokiln Thermal Encapsulation Process which is designed to improve conventional rotary kiln incineration of hazardous waste.The process introduces inorganic additives (fluxing agents) with the waste to promote incipient slagging or thermal encapsulation reactions near the kiln discharge. The thermal encapsulation is augmented using other additives in either the kiln or in the air pollution control baghouse to stabilise the metals in the fly ash. The process is designed to:

1. immobilise the metals remaining in the kiln ash;

2. produce an easily handled nodular form of ash; and

3. stabilise metals in the fly ash;

while avoiding the problems normally experienced with higher temperature “slagging kiln” operations.The basis of this process is thermal encapsulation. Thermal encapsulation traps metals in a controlled melting process operating in the temperature range between slagging and nonslagging modes, producing ash nodules that are 60 to 200 mm in diameter.

Wastes containing organic and metallic contaminants are incinerated in a rotary kiln.Metals with high melting points are trapped in the bottom ash from the kiln through the use of fluxing agents that promote agglomeration with controlled nodulisation.Metals with low melting and vaporising temperatures, such as arsenic, lead, and zinc,are expected to partially volatilise, partitioning between the bottom ash and the fly ash.Metals concentrated in the fly ash may be stabilised, if necessary, by adding reagents to the kiln and to the air pollution controls system to reduce leaching to below TCLP limits.

(b)Adsorptive Filtration

University of Washington has developed an adsorptive filtration process which removes inorganic contaminants (metals) from aqueous streams. An adsorbent ferrihydrite is applied to the surface of an inert substrate such as sand, which is then placed in one of three vertical columns. The contaminated waste stream is adjusted to a pH of 9 to 10 and passed through the column. The iron-coated sand grains in the column act simultaneously as a filter and adsorbent. When the column’s filtration capacity is reached (indicated by partial breakthrough or column blockage), the column is backwashed. When the adsorptive capacity of the column is reached (indicated by breakthrough of soluble metals), the metals are removed and concentrated for subsequent recovery with a pH-induced desorption process.The technology has the ability to:

1. remove both dissolved and suspended metals from the waste stream;

2. remove a variety of metal complexes;

3. work in the presence of high concentrations of background ions; and

4. remove anionic metals.

The absorptive filtration process removes inorganic contaminants, consisting mainly of metals, from aqueous waste streams with a wide range of contaminants concentrations and pH values.

(c)Vortec Corporation Oxidation and Vitrification Process

The Vortec Corporation has developed an oxidation and vitrification process for remediating soils, sediments, sludges, and mill tailings contaminated with organics,inorganics, and heavy metals. The process can oxidise and vitrify materials introduced as dry granulated materials or slurries.The basic elements of the process include:

1. a cyclone melting system;

2. a material handling, storage, and feeding subsystem;

3. a vitrified product separation and reservoir assembly;

4. a waste heat recovery air preheater;

5. and air pollution control subsystem; and

6. a vitrified product handling subsystem.

The cyclone melting system is the primary thermal processing system and consists of two major assemblies: a counter-rotating vortex (CRV) in-flight suspension preheater, and a cyclone melter. First slurried or dry contaminated soil is introduce into the CRV.

The CRV

1. uses the auxiliary fuel introduced directly into the CRV;

2. preheats the suspended waste materials along with any glass-forming additives mixed with oil; and

3. oxidises any organic constituents in the soil.

The average temperature of materials leaving the CRV combustion chamber is between 1,200 and 1,500 ⁰C, depending on the processed soils’ melting characteristics.The preheated solid materials leave the CRV and enter the cyclone melter, where they are dispersed to the chamber walls to form a molten glass product. The vitrified, molten glass product and the exhaust gases discharge from the cyclone melter and are separated.

The exhaust gases then enter an air preheater for waste heat recovery and are subsequently delivered to the air pollution control subsystem for particulate and acid gas removal. The molten glass product is delivered to a water quench assembly for subsequent disposal⁶⁰.

Features of the oxidation and vitrification process include:

1.uses various fuels, including gas, oil, coal, and waste;

2.handles waste quantities ranging from 5 tons per day to more than 400 tons per day;

3.recycles particulate residue collected in the air pollution control subsystem into thecyclone melting system. (These recycled materials are incorporated into the glass product, resulting in zero solid waste discharge); and

4.produces a vitrified product that is nontoxic according to USEPA TCLP standards.The product also immobilises heavy metals and has long term stability.

(d)Coordinate, Chemical Bonding, and Adsorption Process
Western Product Recovery Group Inc. developed the coordinate, chemical bonding,and adsorption (CCBA) process. The process converts heavy metals in soils,sediments, and sludges to nonleaching silicates. The process can also oxidizes organic in the waste stream and incorporate the ash into the ceramic pellet matrix. The solid residual consistency varies from a soil and sand density and size distribution to a controlled size distribution ceramic aggregate form. The residue can be placed back in its original location or used as a substitute for conventional aggregate. The process uses clays with specific cation exchange capacity as sites for physical and chemical bonding of heavy metals to the clay.The process is designed for continuous flow. The input sludge and soil stream are carefully ratioed with specific clays and then mixed in a high intensity mechanical mixer. The mixture is then densified and formed into green or unfired pellets of a desired size. The green pellets are then direct-fired in a rotary kiln for approximately 30minutes. The pellet temperature slowly rises to 1,100 degree Centrigrade creating the fired pellet’s ceramic nature. Organics on the pellet’s surface are oxidised,and organics inside the pellet are pyrolysed as the temperature rises. As the pellets  reach 1,100 ?C, the available silica sites in the clay chemically react with the heavy metals in the soil or sludge to form the final metal silicate product.

The process residue is an inert ceramic product, free of organics, with metal silicates providing the molecular bonding structure to preclude leaching. The kiln off-gas is processed in an afterburner and wet scrub system before it is released into the atmosphere. Excess scrubber solution is recycled to the front-end mixing process. The CCBA process has been demonstrated commercially on metal hydroxide sludges, and can also treat waste water sludges, sediments, and soils contaminated with most mixed organic and heavy metal wastes⁵⁹.

(e)Geo 2

Geo 2 are working with the NSW Department of Agriculture on a process to remove arsenic and DDT simultaneously from soil at former cattle dip sites in NSW. The technology uses a leaching agent to both destroy the organic components in the contaminated soil and remove arsenic, which ends up in the liquor. It is expected that arsenic would be removed from the liquor as a concentrated precipitate suitable for further treatment prior to disposal.As such, this process, used at such sites, can be seen as a generator of arsenical wastes, and such sources of waste should be included in any strategy designed to address waste management issues relating arsenical wastes and OCP/arsenical wastes⁵⁹.

3.Arsenic management in taxidermy
Use of arsenic as a preservative for the skin of birds etc is reffered as taxidermy.Spot tests such as the Weber’s test and the Macherey-Nagel paper are freely available and inexpensive methods that can identify arsenic in taxidermy collections and help to manage this contamination problem. These two spot tests were successfully calibrated against ICP-MS results on arsenic standard solutions and bird specimens. The spot test methods are sensitive enough to detect even background levels of arsenic. However, specimens that give negative results should be re-tested every two to three years⁶³.

Arsenic when present in museum collections requires an appropriate level of management, as does management of the information associated with the contaminated specimens. It is important for institutions to develop a protocol for handling arsenic contaminated objects that covers not just employees but also researchers and visitors⁶⁵. Specimens known, or suspected to contain arsenic should never be handled without appropriate protection. Nitrile gloves and a protective smock or apron, as well as a respirator, are necessary in dealing with these objects. These supplies should be disposed of in an appropriate way similar to other hazardous materials. Specimens testing positive for arsenic must have ‘‘Arsenic contaminated’’ clearly visible on their labels. This information must also be added to the museum paper and/or computer catalog. It should be noted that objects that tested negative might still contain arsenic. These objects should be inspected and tested every two to three years, as arsenic may

migrate from the interior of the specimen. Each test result, whether positive or negative, must be recorded in the specimen’s computer and/or paper catalog entry. These specimens should be stored separately whenever possible. Objects that are contaminated with arsenic should not be exhibited without appropriate conditions and/or decontamination to reduce the risk of exposure. A High-Efficiency Particulate Air (HEPA) vacuum could be used to absorb at least part of the arsenic powder on the specimen. This method may have restricted application in taxidermy because arsenic or arsenical soap was usually applied as a paste on the inner side of the specimen skin^67,68.

4.Low cost technologies

Arsenic removal plant fitted directly with hand pump.
Works based on the adsorption technique and function under continuous flow system.Arsenic removal plant can be attached with the hand-pump.The bed materials of such unit are activated carbon,silicious material and activated alumina.

Co-precipitation-Sedimentation-Filtration under continuous flow system.
It has been developed on the principle of co-precipitation of arsenic with the help of ferric chloride,multimedia filtration with sand and activated alumina which ensure 99% arsenic removal.

Domestic filters
All India Instituite of Hygiene and Public Health,Calcutta has developed this technique for arsenic removal in a domestic scale.Methodology is based on Coagulation-flocculation-sedimentation and filtration process.alum dose is necessary for arsenic removal.

School of Environment Studies(SOES),Jadavpur University,Calcutta had developed another type of domestic filter for removal of arsenic.Such filters are generally fitted with specially manufactured candles.Tablets of chemicals developed by SOES need to be added for removal of arsenic.Method for disposal of arsenic-adsorbed candles has been reported successful for arsenic removal^98,99.

Conclusion
All the methods described above are good in some or the other ways but the most commonly used method is oxidation of As(III) to As(V) and its subsequent removal through adsorption and/or precipitation.But these methods like ion exchange etc are used when small amount of water has to be treated for removal of arsenic.In case of large scale low cost technologies are used extensively because of their cheapness and easy to evaluate procedure.Various organizations are working these days on combining the low cost technologies with small scale techniques like adsorption,precipitation etc.

These low cost technologies are being practiced in our villages and areas which are prone to arsenic poisoning and satisfactory results have been obtained.

The arsenic being spread by wastes is also a very serious problem and this can be very nicely managed by techniques like Cement solidification and Dolocrete Encapsulation.Many new techniques are also emerging these days which have some benefit to the old ones,but these techniques are not very reliable and are not even easy to accomplish.They have not yet gain much popularity in India.

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