Batch-mixed iron treatment of high arsenic waters

Advances in metal(loid) oxyanion removal by zerovalent iron: Kinetics, pathways, and mechanisms

Metal(loid) oxyanions in groundwater, surface water, and wastewater can have harmful effects on human or ecological health due to their high toxicity, mobility, and lack of degradation. In recent years, the removal of metal(loid) oxyanions using zerovalent iron (ZVI) has been the subject of many studies, but the full scope of this literature has not been systematically reviewed. The main elements that form metal(loid) oxyanions under environmental conditions are Cr(VI), As(V and III), Sb(V and III), Tc(VII), Re(VII), Mo(VI), V(V), etc. The removal mechanisms of metal(loid) oxyanions by ZVI may involve redox reactions, adsorption, precipitation, and coprecipitation, usually with one of these mechanisms being the main reaction pathway and the other playing auxiliary roles. However, the removal mechanisms are coupled to the reactions involved in corrosion of Fe(0) and reaction conditions. The layer of iron oxyhydroxides that forms on ZVI during corrosion mediates the sequestration of metal(loid) oxyanions. This review summarizes most of the currently available data on mechanisms and performance (e.g., kinetics) of removal of the most widely studies metal(loid) oxyanion contaminants (Cr, As, Sb) by different types of ZVI typically used in wastewater treatment, as well as ZVI that has been sulfidated or combination with catalytic bimetals.

Role of Colocasia esculenta L. schott in arsenic removal by a pilot-scale constructed wetland filled with laterite soil

The role of plant Colocasia esculenta L. schott (C. esculenta) in arsenic removal was investigated in a pilot-scale constructed wetland (PCW), which was filled with laterite soil (19.90-28.25% iron by weight). This PCW consists of 2 sets of flow systems in parallel, with C. esculenta planted at a density of 20 plants/m² in one system and the other without any plants. The synthetic water containing arsenic concentration of 0.50 mg/l, with its pH controlled at 7.0 and influent flow at 1.5 m³/day. With C. esculenta, the arsenic in water decreased from 0.485 mg/l to 0.054 mg/l (89% removal), whereas, without C. esculenta, the arsenic decreased from 0.485 mg/l to 0.233 mg/l (52% removal). As for the fate of the influent arsenic, the C. esculenta was responsible for 65% of arsenic accumulation. Note that the arsenic was found mostly within the root zone depth (20-40 cm). It appears that such a high capacity of arsenic removal was enhanced both by the plants through rhizostabilization and by the iron-adsorbed process within the laterite soil bed. In addition, the arsenic removal was observed to increase along with the time from 30 to 90 days, and it reached to a maximum removal around 90 days, and then decreased after 122 days. Thus, the arsenic removal efficiency including mechanisms founded can then be applied in designing of constructed wetland for arsenic treatment from gold mine drainage with similar site/soil characteristic.

Versatility of iron-rich steel waste for the removal of high arsenic and sulfate concentrations in water

The aim of this work is to evaluate the application of a steel waste, basic oxygen furnace sludge (BOFS), rich in iron, to treat water contaminated with elevated arsenic and sulfate concentrations. In the first step, three doses (10, 60, and 80 g L^-1) of BOFS were tested to investigate the removal of As(III) and As(V) (67 mg L^-1) and sulfate (3700 mg L^-1) separately from an aqueous solution. In the second step, the efficacies of BOFS (10 g L^-1) and commercial ZVI (5 g L^-1) were compared to simultaneously remove arsenic and sulfate. The pH of the feed solution was adjusted to 2.5 and monitored during the experiment. The use of BOFS achieved arsenic removal up to 92% and sulfate removal of nearly 40% after 72 h of contact time. Use of BOFS also increased the solution pH to 12. Similar removal levels were achieved with both BOFS and ZVI. These results confirm the potential application of BOFS to remove high arsenic and sulfate concentrations from acidic solutions. The data obtained here should be used as a basis for further studies on the remediation of acid mine drainage with high concentrations of arsenic and sulfate using an abundant and low-cost steel waste.

Decontamination of spent iron-oxide coated sand from filters used in arsenic removal

Sand filters devised with iron-rich adsorbents are extensively promoted and deployed in the arsenic-prone south and south-east Asian countries (e.g., Bangladesh). The approach offers superior performance in removing arsenic while the spent sludge from the sand filters is an issue of concern due to the possibility of toxic releases after being discarded. In this work, a new technique is proposed for the treatment of spent iron-oxide coated sand (IOCS) from filters used in arsenic removal. Chelant-washing of the arsenic-loaded IOCS is combined with the solid phase extraction treatment to accomplish the objective. The unique point of the proposed process is the cost-effective scheme, which includes the option of recycling of the washing solvent beside the decontamination of the spent arsenic-rich sludge.

Technological options for the removal of arsenic with special reference to South East Asia

Arsenic contamination in ground water, used for drinking purpose, has been envisaged as a problem of global concern. However, arsenic contamination of ground water in parts of South East Asia is assuming greater proportions and posing a serious threat to the health of millions of people. A variety of treatment technologies based on oxidation, co-precipitation, adsorption, ion exchange and membrane process are available for the removal of arsenic from ground water. However, question remains regarding the efficiency and applicability/appropriateness of the technologies, particularly because of low influent arsenic concentration and differences in source water composition. Some of these methods are quite simple, but the disadvantage associated with them is that they produce large amounts of toxic sludge, which needs further treatment before disposal into the environment. Besides, the system must be economically viable and socially acceptable. In this paper an attempt has been made to review and update the recent advances made in the technological development in arsenic removal technologies to explore the potential of those advances to address the problem of arsenic contamination in South East Asia.

Background species effect on aqueous arsenic removal by nano zero-valent iron using fractional factorial design

This study describes the removal of arsenic species in groundwater by nano zero-valent iron process, including As(III) and As(V). Since the background species may inhibit or promote arsenic removal. The influence of several common ions such as phosphate (PO4(3-)), bicarbonate (HCO3-)), sulfate (SO4(2-)), calcium (Ca2+), chloride (Cl-), and humic acid (HA) were selected to evaluate their effects on arsenic removal. In particular, a 2(6-2) fractional factorial design (FFD) was employed to identify major or interacting factors, which affect arsenic removal in a significant way. As a result of FFD evaluation, PO4(3-) and HA play the role of inhibiting arsenic removal, while Ca2+ was observed to play the promoting one. As for HCO3- and Cl-, the former one inhibits As(III) removal, whereas the later one enhances its removal; on the other hand, As(V) removal was affected only slightly in the presence of HCO3- or Cl-. Hence, it was suggested that the arsenic removal by the nanoiron process can be improved through pretreatment of PO4(3-) and HA. In addition, for the groundwater with high hardness, the nanoiron process can be an advantageous option because of enhancing characteristics of Ca2+.

Removal of arsenate and arsenite from aqueous solution by waste cast iron

The removal of As(III) and As(V) from aqueous solution was investigated using waste cast iron, which is a byproduct of the iron casting process in foundries. Two types of waste cast iron were used in the experiment: grind precipitate dust (GPD) and cast iron shot (CIS). The X-ray diffraction analysis indicated the presence of Feo on GPD and CIS. Batch experiments were performed under different concentrations of As(III) and As(V) and at various initial pH levels. Results showed that waste cast iron was effective in the removal of arsenic. The adsorption isotherm study indicated that the Langmuir isotherm was better than the Freundlich isotherm at describing the experimental result. In the adsorption of both As(IH) and As(V), the adsorption capacity of GPD was greater than CIS, mainly due to the fact that GPD had higher surface area and weight percent of Fe than CIS. Results also indicated the removal of As(III) and As(V) by GPD and CIS was influenced by the initial solution pH, generally decreasing with increasing pH from 3.0 to 10.5. In addition, both GPD and CIS were more effective at the removal of As(III) than As(V) under given experimental conditions. This study demonstrates that waste cast iron has potential as a reactive material to treat wastewater and groundwater containing arsenic.

Performance of a zerovalent iron reactive barrier for the treatment of arsenic in groundwater: Part 2. Geochemical modeling and solid phase studies

Arsenic uptake processes were evaluated in a zerovalent iron reactive barrier installed at a lead smelting facility using geochemical modeling, solid-phase analysis, and X-ray absorption spectroscopy techniques. Aqueous speciation of arsenic is expected to play a key role in directing arsenic uptake processes. Geochemical modeling reveals contrasting pH-dependencies for As(III) and As(V) precipitation. At the moderately alkaline pH conditions typically encountered in zerovalent iron reactive barriers, As(III) is unlikely to precipitate as an oxide or a sulfide phase. Conversely, increasing pH is expected to drive precipitation of metal arsenates including ferrous arsenate. Bacterially mediated sulfate reduction plays an important role in field installations of granular iron. Neoformed iron sulfides provide surfaces for adsorption of oxyanion and thioarsenic species of As(III) and As(V) and are expected to provide enhanced arsenic removal capacity. X-ray absorption near edge structure (XANES) spectra indicate that arsenic is sequestered in the solid phase as both As(III) and As(V) in coordination environments with O and S. Arsenic removal in the PRB probably results from several pathways, including adsorption to iron oxide and iron sulfide surfaces, and possible precipitation of ferrous arsenate. Corrosion of granular iron appears to result in some As(III) oxidation to As(V) as the proportion of As(V) to As(III) in the solid phase is greater compared to influent groundwater. As(0) was not detected in the PRB materials. These results are broadly comparable to laboratory based studies of arsenic removal by zerovalent iron, but additional complexity is revealed in the field environment, which is largely due to the influence of subsurface microbiota.

Performance of a zerovalent iron reactive barrier for the treatment of arsenic in groundwater: Part 1. Hydrogeochemical studies

Developments and improvements of remedial technologies are needed to effectively manage arsenic contamination in groundwater at hazardous waste sites. In June 2005, a 9.1 m long, 14 m deep, and 1.8 to 2.4 m wide (in the direction of groundwater flow) pilot-scale permeable reactive barrier (PRB) was installed at a former lead smelting facility, located near Helena, Montana (USA). The reactive barrier was designed to treat groundwater contaminated with moderately high concentrations of both As(III) and As(V). The reactive barrier was installed over a 3-day period using bio-polymer slurry methods and modified excavating equipment for deep trenching. The reactive medium was composed entirely of granular iron which was selected based on long-term laboratory column experiments. A monitoring network of approximately 40 groundwater sampling points was installed in July 2005. Monitoring results indicate arsenic concentrations >25 mg L(-1) in wells located hydraulically upgradient of the PRB. Of 80 groundwater samples collected from the pilot-PRB, 11 samples exceeded 0.50 mg As L(-1); 62 samples had concentrations of arsenic at or below 0.50 mg L(-1); and, 24 samples were at or below the maximum contaminant level (MCL) for arsenic of 0.01 mg L(-1). After 2 years of operation, monitoring points located within 1 m of the downgradient edge of the PRB showed significant decreases in arsenic concentrations at depth intervals impacted by the emplaced zerovalent iron. This study indicates that zerovalent iron can be effectively used to treat groundwater contaminated with arsenic given appropriate groundwater geochemistry and hydrology. The study also further demonstrates the shortcomings of hanging-wall designs. Detailed subsurface characterization data that capture geochemical and hydrogeologic variability, including a flux-based analysis, are needed for successful applications of PRB technology for arsenic remediation.

Removal of As(V) and As(III) by reclaimed iron-oxide coated sands

This paper aims at the feasibility of arsenate and arsenite removal by reclaimed iron-oxide coated sands (IOCS). Batch experiments were performed to examine the adsorption isotherm and removal performance of arsenic systems by using the IOCS. The results show that the pH(zpc) of IOCS was about 7.0 +/- 0.4, favoring the adsorption of As(V) of anion form onto the IOCS surface. As the adsorbent dosage and initial arsenic concentration were fixed, both the As(V) and As(III) removals decrease with increasing initial solution pH. Under the same initial solution pH and adsorbent dosage, the removal efficiencies of total arsenic (As(V) and As(III)) were in the order as follows: As(V)>As(V)+As(III)>As(III). Moreover, adsorption isotherms of As(V) and As(III) fit the Langmuir model satisfactorily for the four different initial pH conditions as well as for the studied range of initial arsenic concentrations. It is concluded that the reclaimed IOCS can be considered as a feasible and economical adsorbent for arsenic removal.

Technology for remediation and disposal of arsenic

Groundwater contaminated with arsenic must be treated to meet stringent drinking water standards or guideline values. In recent years, several reliable, cost-effective, and sustainable treatment technologies have been developed, although improvements will continue to emerge as work continues. All treatment technologies work by concentrating arsenic at some stage of treatment. Large-scale use of arsenic removal systems generates arsenic-rich treatment wastes, and indiscriminate disposal of these sizable wastes may lead to environmental pollution. Safe disposal of arsenic-rich media is a growing environmental concern that needs to be addressed. For the developing world, arsenic-contaminated water requires some form of treatment to be sufficiently safe for consumption by local populations. Such treatment is particularly important where arsenic [particularly as As(III)] levels in raw water exceed 200 microg/L. At this level and above, >95% removal efficiency is required to produce water that meets international standards, an unlikely result in many locations. Alternative sources for securing safe water may also include rainwater harvesting, use of uncontaminated (filtered) surface waters, and water extraction from new deep tube wells and dug wells. There are disadvantages attendant to using these alternative water sources. For example, rainwater has few mineral salts and is subject to contamination from air pollution or by microbes, including pathogens. Similarly, surface waters, e.g., pond waters, or water from dug wells may require purification before use. Excessive pumping from deep tube wells may lower the water table sufficiently to allow entry of arsenic-contaminated waters from shallower horizons. Bioremediation and phytoremediation are more suitable to developing countries where sunlight is plentiful. In such countries, plant biodiversity is also great and may allow identification of plants suitable for bioremediation. In addition to removing arsenic from water, phytoremediation can also provide economic benefit to the people who apply the methods.

Raman spectroscopy and DFT calculations of As(III) complexation with a cysteine-rich biomaterial

Arsenite adsorption onto a protein-rich biomass and, more specifically, the chemical groups involved in the uptake were investigated using Raman spectroscopy and DFT calculations. The study was based on spectroscopic analyses of raw and arsenic-loaded biomass as well as standard samples of amino acids and arsenic salts. The predominant secondary structure of the protein was identified as the beta-sheet type, with some contribution from alpha-helix structures. The participation of sulphydryl groups from cystine/cysteine molecules during the adsorption of arsenite was demonstrated. Only the gauche-gauche-gauche (g-g-g) conformation type of the disulfide bonds was involved in arsenic complexation. The formation of a pyramidal trigonal As(HCys)(3) complex was modeled according to the density functional theory (DFT). The agreement of the DFT harmonic frequencies with the RAMAN spectra of the As(HCys)(3) complex demonstrated the relevant features of the cysteine-rich biomaterial regarding arsenic uptake as well as of the mechanism involved in the As(III)/biomass interaction at a molecular level. The results also illustrate that Raman spectroscopy can be successfully applied to investigate the mechanism of metal adsorption onto amorphous biomaterials.

Adsorption of arsenate on untreated dolomite powder

Raw dolomite powder was evaluated for its efficiency in adsorbing As(V) from water. An experimental setup comprised of a fluidized dolomite powder bed was used to assess the impact of various test variables on the efficiency of removal of As(V). Test influents including distilled water (DW), synthetic groundwater (SGW) and filtered sewage effluent (FSE) were employed to assess the effect of influent parameters on the adsorption process and the quality of the effluent generated. Dolomite exhibited good As(V) removal levels for distilled water (>92%) and synthetic ground water (>84%) influents at all initial As(V) concentrations tested (0.055-0.600 ppm). Breakthrough of dolomite bed occurred after 45 bed volumes for DW and 20 bed volumes for SGW influents with complete breakthrough taking place at more than 300 bed volumes. As(V) removal from FSE influents was relatively unsuccessful as compared to the DW and SGW influents. Partial removal in the order of 32% from filtered sewage effluent at initial concentration of 0.6 mg/L started at 75 bed volumes and gradually stopped at 165 bed volumes. Varying degrees of As(V) adsorption capacities were observed by the different test influents employed, which indicate that the adsorption of As(V) is adversely affected by competing species, mainly sulfates and phosphates present in the influent. The adsorptive behavior of dolomite was described by fitting data generated from the study into the Langmuir and Freundlich isotherm models. Both models described well the adsorption of dolomite. The average isotherm adsorptive capacity was determined at 5.02 mug/g. Regeneration of the dolomite bed can be achieved with the use of caustic soda solution at a pH of 10.5.

Arsenic removal from water/wastewater using adsorbents--A critical review

Arsenic's history in science, medicine and technology has been overshadowed by its notoriety as a poison in homicides. Arsenic is viewed as being synonymous with toxicity. Dangerous arsenic concentrations in natural waters is now a worldwide problem and often referred to as a 20th-21st century calamity. High arsenic concentrations have been reported recently from the USA, China, Chile, Bangladesh, Taiwan, Mexico, Argentina, Poland, Canada, Hungary, Japan and India. Among 21 countries in different parts of the world affected by groundwater arsenic contamination, the largest population at risk is in Bangladesh followed by West Bengal in India. Existing overviews of arsenic removal include technologies that have traditionally been used (oxidation, precipitation/coagulation/membrane separation) with far less attention paid to adsorption. No previous review is available where readers can get an overview of the sorption capacities of both available and developed sorbents used for arsenic remediation together with the traditional remediation methods. We have incorporated most of the valuable available literature on arsenic remediation by adsorption ( approximately 600 references). Existing purification methods for drinking water; wastewater; industrial effluents, and technological solutions for arsenic have been listed. Arsenic sorption by commercially available carbons and other low-cost adsorbents are surveyed and critically reviewed and their sorption efficiencies are compared. Arsenic adsorption behavior in presence of other impurities has been discussed. Some commercially available adsorbents are also surveyed. An extensive table summarizes the sorption capacities of various adsorbents. Some low-cost adsorbents are superior including treated slags, carbons developed from agricultural waste (char carbons and coconut husk carbons), biosorbents (immobilized biomass, orange juice residue), goethite and some commercial adsorbents, which include resins, gels, silica, treated silica tested for arsenic removal come out to be superior. Immobilized biomass adsorbents offered outstanding performances. Desorption of arsenic followed by regeneration of sorbents has been discussed. Strong acids and bases seem to be the best desorbing agents to produce arsenic concentrates. Arsenic concentrate treatment and disposal obtained is briefly addressed. This issue is very important but much less discussed.

Remediation strategies for historical mining and smelting sites

The environmental, social and economic problems associated with abandoned mine sites are serious and global. Environmental damage arising from polluted waters and dispersal of contaminated waste is a feature characteristic of many old mines in North America, Australia, Europe and elsewhere. Today, because of the efficiency of mining operations and legal requirements in many countries for prevention of environmental damage from mining operations, the release of metals to the environment from modern mining is low. However, many mineralized areas that were extensively worked in the 18th and 19th centuries and left abandoned after mining had ceased, have left a legacy of metal contaminated land. Unlike organic chemicals and plastics, metals cannot be degraded chemically or biologically into non-toxic and environmentally neutral constituents. Thus sites contaminated with toxic metals present a particular challenge for remediation. Soil remediation has been the subject of a significant amount of research work in the past decade; this has resulted in a number of remediation options currently available or being developed. Remediation strategies for metal/metalloid contaminated historical mining sites are reviewed and summarized in this article. It focuses on the current applications of in situ remediation with the use of soil amendments (adsorption and precipitation based methods are discussed) and phytoremediation (in situ plant based technology for environmental clean up and restoration). These are promising alternative technologies to traditional options of excavation and ex situ treatment, offering an advantage of being non-invasive and low cost. In particular, they have been shown to be effective in remediation of mining and smelting contaminated sites, although the long-term durability of these treatments cannot be predicted.

Oxidation and removal of arsenic (III) from aerated groundwater by filtration through sand and zero-valent iron

Removing arsenic from contaminated groundwater in Bangladesh is challenging due to high concentrations of As(III), phosphate and silicate. Application of zero-valent iron as a promising removal method was investigated in detail with synthetic groundwater containing 500 microg/L As(III), 2-3mg/L P, 20mg/L Si, 8.2mM HCO3-, 2.5mM Ca2+, 1.6mM Mg2+ and pH 7.0. In a series of experiments, 1L was repeatedly passed through a mixture of 1.5 g iron filings and 3-4 g quartz sand in a vertical glass column (10mm diameter), allowing the water to re-aerate between each filtration. At a flow rate of 1L/h, up to 8 mg/L dissolved Fe(II) was released. During the subsequent oxidation of Fe(II) by dissolved oxygen, As(III) was partially oxidized and As(V) sorbed on the forming hydrous ferric oxides (HFO). HFO was retained in the next filtration step and was removed by shaking of the sand-iron mixture with water. Rapid phosphate removal provided optimal conditions for the sorption of As(V). Four filtrations lead to almost complete As(III) oxidation and removal of As(tot) to below 50 microg/L. In a prototype treatment with a succession of four filters, each containing 1.5 g iron and 60 g sand, 36 L could be treated to below 50 microg/L in one continuous filtration, without an added oxidant.

Significance of iron(II,III) hydroxycarbonate green rust in arsenic remediation using zerovalent iron in laboratory column tests

We examined the corrosion products of zerovalent iron used in three column tests for removing arsenic from water under dynamic flow conditions. Each column test lasted 3-4 months using columns consisting of a 10.3-cm depth of 50:50 (w:w, Peerless iron:sand) in the middle and a 10.3cm depth of a sediment from Elizabeth City, NC, in both upper and lower portions of the 31-cm-long glass column (2.5 cm in diameter). The feeding solutions were 1 mg of As(V) L(-1) + 1 mg of As(III) L(-1) in 7 mM NaCl + 0.86 mM CaSO4 with or without added phosphate (0.5 or 1 mg of P L(-1)) and silicate (10 or 20 mg of Si L(-1)) at pH 6.5. Iron(II,III) hydroxycarbonate green rust (or simply, carbonate green rust) and magnetite were the major iron corrosion products identified with X-ray diffraction for the separated fractions (5 and 1 min sedimentation and residual). The presence of carbonate green rust was confirmed by scanning electron microscopy (hexagonal morphology) and FTIR-photoacoustic spectroscopy (interlayer carbonate stretching mode at 1352-1365 cm(-1)). X-ray photoelectron spectroscopy investigation revealed the presence of predominantly As(V) at the surface of corroded iron particles despite the fact that the feeding solution in contact with Peerless iron contained more As(III) than As(V) as a result of a preferential uptake of As(V) over As(III) by the Elizabeth City sediment. Extraction of separated corrosion products with 1.0 M HCI showed that from 86 to 96% of the total extractable As (6.9-14.6 g kg(-1)) was in the form of As(V) in agreement with the XPS results. Combined microscopic and macroscopic wet chemistry results suggest that sorbed As(III) was partially oxidized by the carbonate green rust at the early stage of iron corrosion. The column experiments suggest that either carbonate green rust is kinetically favored or is thermodynamically more stable than sulfate green rust in the studied Peerless iron corrosion systems.

Arsenic removal with iron(II) and iron(III) in waters with high silicate and phosphate concentrations

Arsenic removal by passive treatment, in which naturally present Fe(II) is oxidized by aeration and the forming iron(III) (hydr)oxides precipitate with adsorbed arsenic, is the simplest conceivable water treatment option. However, competing anions and low iron concentrations often require additional iron. Application of Fe(II) instead of the usually applied Fe(III) is shown to be advantageous, as oxidation of Fe(II) by dissolved oxygen causes partial oxidation of As(III) and iron(III) (hydr)oxides formed from Fe(II) have higher sorption capacities. In simulated groundwater (8.2 mM HCO3(-), 2.5 mM Ca2+, 1.6 mM Mg2+, 30 mg/L Si, 3 mg/L P, 500 ppb As(III), or As(V), pH 7.0 +/- 0.1), addition of Fe(II) clearly leads to better As removal than Fe(III). Multiple additions of Fe(II) further improved the removal of As(II). A competitive coprecipitation model that considers As(III) oxidation explains the observed results and allows the estimation of arsenic removal under different conditions. Lowering 500 microg/L As(III) to below 50 microg/L As(tot) in filtered water required > 80 mg/L Fe(III), 50-55 mg/L Fe(II) in one single addition, and 20-25 mg/L in multiple additions. With As(V), 10-12 mg/L Fe(II) and 15-18 mg/L Fe(III) was required. In the absence of Si and P, removal efficiencies for Fe(II) and Fe(III) were similar: 30-40 mg/L was required for As(II), and 2.0-2.5 mg/L was required for As(V). In a field study with 22 tubewells in Bangladesh, passive treatment efficiently removed phosphate, but iron contents were generally too low for efficient arsenic removal.

Removal of arsenic from synthetic acid mine drainage by electrochemical pH adjustment and coprecipitation with iron hydroxide

Acid mine drainage (AMD), which is caused by the biological oxidation of sulfidic materials, frequently contains arsenic in the form of arsenite, As(III), and/or arsenate, As(V), along with much higher concentrations of dissolved iron. The present work is directed toward the removal of arsenic from synthetic AMD by raising the pH of the solution by electrochemical reduction of H+ to elemental hydrogen and coprecipitation of arsenic with iron(III) hydroxide, following aeration of the catholyte. Electrolysis was carried out at constant current using two-compartment cells separated with a cation exchange membrane. Four different AMD model systems were studied: Fe(III)/As(V), Fe(III)/As(III), Fe(II)/As(V), and Fe(II)/As(III) with the initial concentrations for Fe(III) 260 mg/L, Fe(II) 300 mg/L, As(V), and As(III) 8 mg/L. Essentially quantitative removal of arsenic and iron was achieved in all four systems, and the results were independent of whether the pH was adjusted electrochemically or by the addition of NaOH. Current efficiencies were approximately 85% when the pH of the effluent was 4-7. Residual concentrations of arsenic were close to the drinking water standard proposed by the World Health Organization (10 microg/L), far below the mine waste effluent standard (500 microg/L).

Batch-mixed iron treatment of high arsenic waters

This paper develops batch-mixed treatment with zero-valent iron as a point-of-use technology, appropriate for arsenic removal from water stored within rural homes in Bangladesh and West Bengal, India, where arsenic poisoning has affected an estimated 20 million people. Batch tests with iron yielded the following results: (1) High arsenic removal (>93%) was achieved from highly arsenated waters (2,000 microg/L) over short contact times (0.5-3h) with iron filings added at doses ranging from 2500 to 625 mg/L; (2) Most rapid arsenic removal was observed in head-space free systems with sulphates present in solution, while phosphate buffers were observed to inhibit arsenic removal by iron; (3) The arsenic removed from water was found to be strongly bound to the elemental iron filings, such that the treated water could be decanted and iron could be reused at least 100 times; (4) Some iron dissolved into water over the contact period, at concentrations ranging from 100 to 300 microg/L, which are within safe drinking water limits. These results indicate that, with appropriate assessment of water chemistry in the affected region, treatment with metallic iron followed by simple decantation can be used as a practical, in-home, point-of-use technique for reducing human exposure to arsenic in drinking water.