H21C-1024 0800h
Redox Cycling and Arsenic Transport to Groundwater in Bangladesh
Arsenic is a contaminant in the groundwater of the Ganges delta. In Bangladesh alone, an estimated 57 million people may be drinking water with unsafe arsenic levels. The source of arsenic appears to be natural, solid-phase arsenic in the sediments, and various theories have been put forth regarding the modes of arsenic release to solution, ranging from oxidative or reductive degradation of arsenic-bearing solids to competitive ligand displacement by phosphate. Currently, reductive dissolution of Fe(III) (hydr)oxides and concomitant arsenic release is the most widely accepted explanation of the high arsenic concentrations in groundwater. However, much of the information about potential mechanisms of arsenic release has been gleaned solely from solution-phase data, and many puzzles linger concerning the distribution of arsenic. Only recently have studies been initiated that provide a comprehensive look at microbiology, hydrology, and chemistry of contaminated aquifers in Bangladesh. Using micro-X-ray fluorescence elemental mapping and micro-X-ray absorption near-edge structure spectroscopy, we have detected detrital arsenic-bearing sulfides in the aquifer sediments from our field site in Munshiganj, Bangladesh. The presence of detrital sulfides has been previously discounted, but their presence may, in fact, provide an important source of arsenic. Furthermore, their presence combined with a lack of ferric (hydr)oxides at depth is indicative of the reductive degradation of the latter phase. Rapid abiotic desorption of arsenic from sediments illustrates that a labile arsenic phase is easily transported through the aquifer sands. Addition of ferrihydrite, however, removes arsenic from solution and would minimize transport (a phenomenon not observed within the aquifers of Bangladesh). Based on our results, and in accordance with existing hydrological and biogeochemical data, reductive dissolution of ferric (hydr)oxides transpires in the surface and near-surface environments leaving sediments at well-depths of the Holocene aquifer devoid of such phases. Thus, redox cycling in sediments of the surface and near-surface liberates arsenic which is then transported relatively unimpeded to well-depths through the sandy aquifer.
H21C-1025 0800h
Arsenic Mobilization Influenced By Iron Reduction And Sulfidogenesis Under Dynamic Flow
Sulfidogenesis and iron reduction are ubiquitous processes that occur in a variety of anoxic subsurface and surface environments, which profoundly impact the cycling of arsenic. Of the iron (hydr)oxides, ferrihydrite possesses one of the highest capacities to retain arsenic, and is globally distributed within soils and sediments. Upon dissimilatory iron reduction, ferrihydrite may transform to lower surface area minerals, such as goethite and magnetite, which decreases arsenic retention, thus enhancing its transport. Here we examine how arsenic retained on ferrihydrite is mobilized under dynamic flow in the presence of {\it Sulfurosprillum barnesii} strain SES-3, a bacteria capable of reducing both As(V) and Fe(III). Ferrihydrite coated sands, loaded with 150 mg kg$^{-1}$ As(V), were inoculated with {\it S. barnesii}, packed into a column and reacted with a synthetic groundwater solution. Within several days after initiation of flow, the concentration of arsenic in the column effluent increased dramatically coincident with the mineralogical transformation of ferrihydrite and As(V) reduction to As(III). Following the initial pulse of arsenic, effluent concentration then declined to less than 10 $\mu$M. Thus, arsenic release into the aqueous phase is contingent upon the incongruent reduction of As(V) and Fe(III) as mediated by biological activity. Reaction of abiotically or biotically generated dissolved sulfide with iron (hydr)oxides may have a dramatic influence on the fate of arsenic within surface and subsurface environments. Accordingly, we examined the reaction of dissolved bisulfide and iron (hydr)oxide complexed with arsenic in both batch and column systems. Low ratios of sulfide to iron in batch reaction systems result in the formation of elemental sulfur and concomitant arsenic release from the iron (hydr)oxide surface. High sulfide to iron ratios, in contrast, appear to favor the formation of iron and arsenic sulfides. Our findings demonstrate that iron (hydr)oxides may quench reactions between sulfide and constituents sorbed to iron (hydr)oxide surfaces, forming elemental sulfur as opposed to sulfide-arsenic complexes. In addition, reductive transformation of iron (hydr)oxide by dissolved sulfide may release sorbed constituents. Hence, moderate to low concentrations of dissolved sulfide in association with iron (hydr)oxides may inhibit sequestration of important contaminants that are attenuated by Fe(III) and/or S(-II) bearing phases.
H21C-1026 0800h
Mathematical model for arsenic transformation and transport in the saturated zone
Toxic metalloids of abandoned mines, such as arsenic, emerge as a major contaminant. Arsenic occurs in natural waters as arsenite (As3+) or arsenate (As5+). Arsenite is more soluble and toxic than arsenate. Under oxidizing conditions, the predominant form of inorganic arsenic is arsenate, and arsenite is more prevalent under reducing conditions. It is known that microorganisms play a critical role in both the direct reduction and oxidation of the arsenic species. The biogeochemical redox processes and transport behavior need to be coupled in a reactive transport model to predict concentrations of the toxic inorganic arsenic in aqueous phase. A new reaction module describing the fate and transport of inorganic arsenic species are developed and incorporated into the RT3D (Reactive Transport in 3-Dimensions) code. The simulation results of numerical example demonstrate that the proposed model can describe the arsenate reduction to arsenite and its subsequent transport. To verify the model, laboratory column experiments are conducted. The arsenic transport results obtained from the model are compared with the experimental results. The reactive transport model of arsenic will be useful to predict the mobilization of arsenic, the evolution of its contaminant plumes, and the fate and transport of arsenic species in groundwater systems.
H21C-1027 0800h
Aquifer Vulnerability to Arsenic contamination in the Conterminous United States: Health Risks and Economic Implications
Arsenic is historically known be toxic to human health. Drinking water contaminated with unsafe levels of arsenic may cause cancer. The toxicity of arsenic is suggested by a MCLG of zero and a low MCL of 10 g/L, that has been a subject of constant scrutiny. The US Environmental Protection Agency (US EPA), based on the recommendations of the National Academy of Sciences revised the MCL from 1974 value of 50 g/L to 10 g/L. The decision was based on a national-level analysis of arsenic concentration data collected by the National Analysis of Water Quality Assessment (NAWQA). Another factor that makes arsenic in drinking water a major issue is the widespread occurrence and a variety of sources. Arsenic occurs naturally in mineral deposits and is also contributed through anthropogenic sources. A methodology using the ordinal logistic regression (LR) method is proposed to predict the probability of occurrence of arsenic in shallow ground waters of the conterminous United States (CONUS) subject to a set of influencing variables. The analysis considered the maximum contaminant level (MCL) options of 3, 5, 10, 20, and 50 g/L as threshold values to estimate the probabilities of arsenic occurrence in ranges defined by a given MCL and a detection limit of 1 g/L. The fit between the observed and predicted probability of occurrence was around 83% for all MCL options. The estimated probabilities were used to estimate the median background concentration of arsenic for different aquifer types in the CONUS. The shallow ground water of the western US is more vulnerable to arsenic contamination than the eastern US. Arizona, Utah, Nevada, and California in particular are hotspots for arsenic contamination. The model results were extended for estimating the health risks and costs posed by arsenic occurrence in the ground water of the United States. The risk assessment showed that counties in southern California, Arizona, Florida, Washington States and a few others scattered throughout the CONUS face a high risk from arsenic exposure through untreated ground water consumption. The risk analysis also showed the trade-offs in using different risk estimates as decision-making tools. A simple cost effectiveness analysis was performed to understand the household costs for MCL compliance in using arsenic-contaminated ground water. The results showed that the current MCL of 10 g/L is a good compromise based on existing treatment technologies
H21C-1028 0800h
Role of Sulfide, Selenate and Nitrate in Arsenite Oxidation in Mono Lake, CA
Arsenic is an important chemical constituent in Mono Lake where it occurs naturally at sufficient concentrations ($\sim$200 $\mu$M) to provide a source of energy for microbial metabolism (through oxidation or dissimilatory reduction). Arsenite and arsenic-thiol compounds are the dominant forms of arsenic present in seasonally anoxic regions of the water column when sulfide is present, but are rapidly converted to arsenate throughout the water column after holomixis. Experiments were conducted on surface water samples in April and August of 2004 to measure potential rates of aerobic As(III) oxidation to As(V) and to study the effect of co-oxidation of reduced S compounds. The presence of significant levels of arsenate ($\sim$20 M) in anoxic bottom waters suggests that anaerobic oxidation of arsenite may also occur in Mono Lake. Anaerobic arsenite oxidation experiments were conducted on samples collected from low oxygen (April) or anoxic (August) bottom waters. Selenate was tested as an electron acceptor for arsenite oxidation in April experiments, and separate experiments with nitrate and selenate were conducted in the August experiment. Surface water collected from Mono Lake in April and August of 2004 was enriched with arsenite or arsenite and sulfide and exposed to air for the duration of the experiment. Average rates of As(III) oxidation (arsenate production) were $\sim$25 $\mu$M/day in the arsenite treatments; 2 mM As(III) was converted to As(V) in 45 days. Rates increased 3-4-fold during the experiment, suggesting adaptation of the microbial community to the added arsenite. Oxidation was significantly faster in the arsenite + sulfide treatments ($\sim$200 $\mu$M/day). Similar results were observed in the August experiments, with much faster rates of arsenite oxidation in treatments with both arsenite and sulfide. No significant arsenate production was measured in killed controls, indicating that abiotic arsenite oxidation is slow. If similar rates occur in situ, all As(III) present in anoxic waters could easily be oxidized within a few days during turnover. Thus biological As(III) oxidation may be an important process over short time scales in Mono Lake. Anaerobic oxidation of arsenite (1 mM) using selenate (1 mM) as an electron acceptor occurred at a rate of $\sim$10 $\mu$M/day in April experiments. Rates were significantly faster in experiments conducted in August ($\sim$200 $\mu$M/day), possibly due to seasonal shifts in the microbial community or lake water chemistry. In both experiments, after 0.5-1 mM arsenate accumulated, arsenate reduction became the dominant arsenic metabolism, yielding a net decrease in arsenate concentrations. The oxidation of arsenite using selenate as the terminal electron acceptor represents a novel pathway for arsenite oxidation (and selenate reduction). Potential rates of anaerobic arsenite oxidation using nitrate (5 mM) as an electron acceptor were also measured in August samples. After a 5 day lag period, arsenite was oxidized at a rate of nearly 1 mM/day. Although the potential for anaerobic As(III) oxidation exists, this process is less significant than aerobic oxidation since nitrate and selenate are found at considerably lower concentrations in Mono Lake ($<$10 $\mu$M each). The results of these experiments show that 1) rapid biological oxidation of arsenite in Mono Lake can occur in both oxic and anoxic waters, 2) the observed potential rates of aerobic arsenite/thioarsenite oxidation are sufficient to oxidize all reduced arsenic species present within 1 week of turnover, and 3) at least two potential biological pathways of anaerobic arsenite oxidation exist in Mono Lake.
H21C-1029 0800h
Aqueous and Mineralogical Analysis of Arsenic in the Reduced, Circumneutral Groundwaters and Sediments of the Fraser River Delta, British Columbia
The aqueous and mineralogical geochemistry of the groundwaters and sediments of the Fraser River delta, BC is investigated. Groundwater and sediment samples were collected from two sites. The DND site is located in an upland area of Richmond BC, approximately 3 km upgradient from the north arm of the Fraser River. The Kidd2 site is located adjacent to the north arm of the Fraser River. A saline water wedge extends inland from the bed of the Fraser River to the subsurface beneath the Kidd2 site. Concentrations of dissolved arsenic (As) are relatively low at the DND site, while concentrations of approximately 30 ug/L dissolved As are present in the vicinity of the saline water wedge, at the Kidd2 site. The aqueous geochemical data for the Kidd2 site suggest that dissolved As is generated by the reduction of hydrous ferric oxide minerals. Sediment samples from both sites were analysed using a sequential extraction procedure (SEP) that was designed to target specific pools of solid-phase As. Results of the SEP analyses indicate that solid-phase As is present in the sediments at trace concentrations (<8 ppm). Processes such as sorption and the precipitation of arsenical sulphide minerals do not appear to be sufficient to mitigate the relatively elevated concentrations of dissolved As that are present at the Kidd2 site.
H21C-1030 0800h
Characterization of Mineral-Processing and Post-Depositional Controls on Arsenic-Bearing Gold Mine Tailings
Through 50 years of operations, arsenic-bearing tailings were produced at a gold mine in Ontario and deposited in a number of surface impoundments and ponds at the mine site. The tailings composition was dependent upon the mineral processing methods employed through time: roasting, cyanidation and sulphide flotation. Tailings and pond sediments were characterized by whole rock analysis, scanning electron microscopy, Rietveld refined powder X-ray diffraction, sequential extractions, x-ray absorption near edge structure spectroscopy (XANES) and x-ray absorption fine structure spectroscopy (XAFS). High aqueous arsenic concentrations were found to be associated with arsenic - bearing iron oxyhydroxides contained in historical roaster -derived tailings and tailings pond sediments. Under reducing conditions, a significant portion of the arsenic in these wastes is likely to be mobilized. Arsenic, primarily in the form of arsenopyrite, is found at relatively low concentrations in the freshly produced tailings, and is likely to remain stable if the tailings can be maintained under anaerobic, water-saturated conditions. Field and laboratory experiments suggest that arsenic may be immobilized as an undetermined sulphide phase under strongly reducing conditions.
H21C-1031 0800h
Cycling of Organoarsenic Compounds in Agricultural Watersheds
The use of the organoarsenical roxarsone, added to poultry feed to increase weight gain, results in elevated arsenic concentrations (10-50 mg/kg) in poultry litter. This litter is extensively applied to crop fields and pastures, both as a fertilizer and as a waste disposal technique, in agricultural regions. Using a combination of field sampling and laboratory experiments, we investigated the sources and sinks of arsenic within soils and natural waters in an agricultural watershed in the Shenandoah Valley of Virginia, USA, an area of intense poultry production. Surface, ground, and soil waters were collected in an instrumented field site to examine arsenic and other litter-derived species in different hydrologic compartments and different settings within the field site. We collected soil cores of the Frederick series, common in the Shenandoah Valley, from several areas experiencing different litter application histories in the valley to examine relationships between arsenic and physico-chemical properties of the soils. Last, we conducted a series of batch experiments to examine adsorption and biotransformation characteristics of roxarsone within Ap and Bt soil horizons of the Frederick soils. Results of these combined studies document a complex yet intriguing cycling of arsenic through the watershed, which will provide useful information for management of poultry litter in agricultural watersheds.
H21C-1032 0800h
Natural Arsenic in the Miocene Hawthorn Group, Florida: Wide Ranging Implications for ASR, Phosphate Mining, Private Well
In order to understand the mineralogical association and distribution of arsenic (As) in the Hawthorn Group we examined in detail the chemical and mineralogical composition of 370 samples that were collected from 16 cores in central Florida. In our study area the Hawthorn group consists primarily of a basal carbonate unit (the Arcadia Formation) and an upper siliciclastic unit (The Peace River Formation). The Peace River Formation contains appreciable amounts of phosphate and is currently being exploited for phosphate ore. Samples were taken for each Formation at intervals of 25ft. In addition to the interval samples we also took samples that contained visible pyrite crystals, iron oxides, green clays, phosphatic and organic material. These additional samples were collected because of their potential of high As concentrations. Arsenic concentrations were determined by hydride generation - atomic fluorescence spectrometry (HG-AFS) after digestion with aqua regia (3:1 HCl and HNO$_{3}$). The elements Fe, Na, Al, Si, Mg, Ca, S, P, and K were measured on the same solutions by inductively coupled plasma optical emission spectrometry (ICP-OES). The identification of discrete minerals was aided by scanning electron microscopy (SEM) and chemical compositions were obtained by electron-probe microanalyses (EMPA). Our study indicates that the average As concentrations significantly change from 9.0 ppm in the Peace River Formation to 3.0 ppm in the Tampa Member of the Arcadia Formation. As concentrations for all Hawthorn samples vary from 0.07 to 68.98 ppm ( $\mu$ = 5.6, $\sigma$ = 7.1). Our detailed mineralogical and geochemical study demonstrates that: (1) The As in the Hawthorn group varies from the formation to formation and is mostly concentrated in trace minerals, such as pyrite; (2) Concentrations of the As in pyrite crystals can vary drastically from a minimum of 0 ppm to a maximum of 8260 ppm; (3) Pyrite is an unevenly distributed throughout the Hawthorn Group; (4) Phosphate and organic material, clays, and iron oxides contain lower As concentrations contrasted to pyrite; (5) Pyrite occurs in framboidal and euhedral forms. Because phosphorous, arsenic and sulfur are chemically closely related, they often occur together in nature, thus posing a potential problem for the phosphate industry. There have been several occurrences of swine fatalities due to arsenic poisoning as a result of phosphate feed supplements. Information about the concentration, distribution and mineralogical association of naturally occurring As is important, because this is a first step to forecast its behavior during anthropogenic induced physico-chemical changes in the aquifer. Recently, aquifer storage and recovery (ASR) facilities in central Florida reported As concentrations in excess of 100 $\mu$g/L in recovered water. The ASR storage zone is the Suwannee Limestone, which directly underlies the Hawthorn sediments. It is crucial to the future of ASR in this area to understand the source and distribution of arsenic in the overlying Hawthorn Group and the cycling of arsenic in the Florida platform.
H21C-1033 0800h
Arsenic Levels in Tree Rings From a Contaminated Site in New Jersey, USA
Increment cores and stems were obtained from trees grown in arsenic (As) contaminated soil at the US EPA Vineland Superfund Site in New Jersey. The tree-ring widths were measured and crossdated to produce chronologies that were then compared with control chronologies created from trees sampled in nearby uncontaminated areas. Dendrochemical analysis by wet digestion and High Resolution Inductively Coupled Plasma Mass Spectrometry (HR ICP-MS) confirmed little As incorporation (0.05-1.0 ppm) in the xylem rings, and showed species-specific patterns in annual rings. Trees grown in As-contaminated soils had much higher xylem As than those from uncontaminated areas. Within individual trees, As levels were highest in the leaves, followed by the pith and bark, and are lowest in the xylem rings. These levels are several orders of magnitude lower than reported levels in roots and soils. Low growth periods in one pine sample, most notably in 1960's, appear to follow regional growth trends and correspond with higher As levels. One oak sample showed a unique fast growth period with normal As levels from 1955 to 1970, but a higher As period in the 1980's and early 1990's with no apparent growth anomaly. These periods may be related to either regional climatic, or local hydrologic and environmental changes induced by the operation of the Vineland Chemical Company and As disposal. The nutrient levels in xylem rings show little correlation with As. Our tree ring As results support the possibility of using As levels and profiles as a biomarker for local environmental contamination.
H21C-1034 0800h
Can Periodic Cicadas be used as a Biomonitor for Arsenical Pesticide Contamination?
Widespread use of arsenical pesticides on fruit crops, particularly apple orchards, during the first half of the 20th century is a significant source of arsenic to agricultural soil in the Mid-Atlantic region. Cumulative application rates may be as high as 37 Kg/hectare of arsenic in orchard areas. Brood X 17-year periodic cicadas (Magicicada spp.) emerged at densities up to 30,000 or more individuals per hectare in orchard and forest habitats during May-June, 2004, in Clarke and Frederick Counties, Virginia and in Berkeley and Jefferson Counties, West Virginia. These cicadas were sampled to evaluate the bioavailability of arsenic in orchard and non-orchard reference site soils. Potentially toxic elements, such as arsenic and other heavy metals bind to sulfhydryl groups, and thus may accumulate in keratin-rich tissues, such as cicada nymphal exuviae and adult exoskeletons. These cicadas feed on plant roots underground for 17 years before emerging to molt into their adult form. Adult cicadas have very limited dispersal, rarely traveling more than 50 m in a flight. As such, their body and exoskeleton keratin has potential value as a biomonitor for arsenic and other metals that is spatially referenced to local conditions for the duration of time the nymphs live in the soil. This study addresses the following research questions: (1) do the soils in and adjacent to orchard sites where arsenical pesticide was used contain elevated concentrations of arsenic and other metals relative to likely background conditions?; (2) can periodic cicadas be used as an easily sampled biomonitor measuring bioavailability of pesticide residues in soils?; and (3) do the concentration levels of arsenical pesticide residues in periodic cicadas emerging from contaminated orchard sites pose a dietary threat to birds and other wildlife that preferentially feed upon cicadas during emergence events?
H21C-1035 0800h
Arsenic Adsorption Onto Iron Oxides Minerals
The predominant form of arsenic in water is as an inorganic ion. Under different redox conditions arsenic in water is stable in the +5 and +3 oxidation states. Arsenic oxidation state governs its toxicity, chemical form and solubility in natural and disturbed environments. As (III) is found in anoxic environments such as ground water , it is toxic and the common species is the neutral form, H$_{3}$AsO$_{3}$. As (V) is found in aerobic conditions such as surface water, it is less toxic and the common species in water are: H$_{2}$AsO$_{4}$ $^{-}$ and HAsO$_{4}$ $^{- 2}$. The water pH determines the predominant arsenate or arsenite species, however, both forms of arsenic can be detected in natural water systems. Iron oxides minerals often form in natural waters and sediments at oxic-anoxic boundaries. Over time they undergo transformation to crystalline forms, such as goethite or hematite. Both As(V) and As(III) sorbs strongly to iron oxides, however the sorption behavior of arsenic is dependent on its oxidation state and the mineralogy of the iron oxides. Competition between arsenic and others ions, such fluoride, sulphate and phosphate also play a role. On the other hand, calcium may increase arsenic adsorption onto iron oxides. Electrokinetic studies and adsorption experiments were carried out in order to determine which conditions favour arsenic adsorption. Hematite, goethite and magnetite as iron based sorbents were used. Test were also conducted with a laterite soil rich in iron minerals. The focus of this study is to evaluate physical and chemical conditions which favour arsenic adsorption onto iron oxides minerals, the results contribute to an understanding of arsenic behaviour in natural and disturbed environments. Furthermore, results could contribute in developing an appropriate remediation technology for arsenic removal in water using iron oxides minerals.
H21C-1036 0800h
Determining the Influence of Groundwater Composition on the Performance of Arsenic Adsorption Columns Using Rapid Small-Scale Column Tests
The USEPA has established a more stringent drinking water standard for arsenic, reducing the maximum contaminant level (MCL) from 50 $\mu$g/L to 10 $\mu$g/L. This will affect many small communities in the US that lack the appropriate treatment infrastructure and funding to reduce arsenic to such levels. For such communities, adsorption systems are the preferred technology based on ease of operation and relatively lower costs. The performance of adsorption media for the removal of arsenic from drinking water is dependent on site-specific water quality. At certain concentrations, co-occurring solutes will compete effectively with arsenic for sorption sites, potentially reducing the sorption capacity of the media. Due to the site-specific nature of water quality and variations in media properties, pilot scale studies are typically carried out to ensure that a proposed treatment technique is cost effective before installation of a full-scale system. Sandia National Laboratories is currently developing an approach to utilize rapid small-scale columns in lieu of pilot columns to test innovative technologies that could significantly reduce the cost of treatment in small communities. Rapid small-scale column tests (RSSCTs) were developed to predict full-scale treatment of organic contaminants by adsorption onto granular activated carbon (GAC). This process greatly reduced the time and costs required to verify performance of GAC adsorption columns. In this study, the RSSCT methodology is used to predict the removal of inorganic arsenic using mixed metal oxyhydroxide adsorption media. The media are engineered and synthesized from materials that control arsenic behavior in natural and disturbed systems. We describe the underlying theory and application of RSSCTs for the performance evaluation of novel media in several groundwater compositions. Results of small-scale laboratory columns are being used to predict the performance of pilot-scale systems and ultimately to design full-scale systems. RSSCTs will be performed on a suite of water compositions representing the variety of water supplies in the United States that are affected by the new drinking water standard. Ultimately, this approach will be used to carry out inexpensive short-term pilot studies at a large number of sites where large-scale pilots are not economically feasible. Sandia National Laboratories is a multi-program laboratory operated by Sandia Corporation, a Lockheed Martin company, for the United States Department of Energy's National Nuclear Security Administration under Contract DE-AC04-94AL85000.
H21C-1037 0800h
Evaluation of a Chemical Fixation Technique for Remediation of Soils Contaminated with Arsenic Trioxide
The results of an experimental study designed to test a chemical fixation technique for remediation of arsenic-contaminated soils are reported. Soil samples were collected from two industrial sites where herbicide application contaminated the soil with arsenic trioxide. Weathering has redistributed and changed the speciation of the arsenic and caused contamination of soil water and groundwater. Remediation techniques requiring excavation of the affected soil are impractical due to the lack of site access for heavy equipment and the large area impacted. To address these concerns, an in situ treatment method was developed and tested in laboratory experiments. Column experiments were conducted to evaluate the effectiveness of the soil treatment method. Homogenized soil samples from each site were packed into duplicate 4"x18" Plexiglas columns and treated with ferrous sulfate solution using a flow rate of 2 ml/min until iron breakthrough was achieved. Another pair of packed columns was leached with DDI water to provide baseline data for the effect of the treatment solution matrix on arsenic mobility. All soil column effluents showed an initial spike of arsenic after onset of fluid flow; however, the soils undergoing treatment leached less than one half the amount of arsenic leached from the soil by DDI water. When the soil became saturated with the ferrous sulfate treatment solution, effluent solution arsenic concentrations fell below detectable levels. After breakthrough of the treatment solution was achieved, the treated soil columns were allowed to drain and cure for 7 days. One treated soil column and one untreated soil column were then continuously leached with EPA Method 1312 SPLP fluid to observe the effect of treatment on the mobility of arsenic. The second set of treated and untreated soil columns was also leached with SPLP fluid, but in a manner designed to simulate periodic rainfall events. Preliminary experimental results show that ferrous sulfate treatment dramatically decreases the amount of leachable arsenic, compared to untreated soil. After 30 hours of SPLP leaching, there was no detectable arsenic in the effluent of the treated soil column. In contrast, more than 5 mg/L arsenic was mobilized from the untreated soil in the same time period. These results indicate that in situ chemical fixation of arsenic-contaminated soils is an effective and low-cost treatment method for sites at which the use of heavy equipment is not feasible.
H21C-1038 0800h
New Sorbents for Removing Arsenic From Water
Elevated concentrations of arsenic in the drinking water used in many countries, including some of the poorest developing countries, and recognition that consuming this water can have serious consequences for human health, have led to increased investigations of ways to obtain safe water supplies. Finding new groundwater resources is a possible solution but this is a costly strategy that has no guarantee of success, particularly in areas where water is already a scarce commodity. The alternative is to treat water that is already available, but existing technologies are usually too expensive, too difficult to operate and maintain, or not completely effective when used in less developed countries or remote areas. There is therefore, an urgent need to find a simple and effective but inexpensive sorbent for arsenic that can be used to treat large volumes of water under less than ideal conditions. In this paper we present the results of field and laboratory trials that used a new, highly cost-effective, sorbent to remove arsenic from contaminated water. BauxsolT is the name given to the cocktail of minerals prepared by treating caustic bauxite refinery residues with Mg and Ca to produce a substance with a reaction pH of about 8.5, a high acid neutralizing capacity and an excellent ability to trap trace metals, metalloids and some other ionic species. The trapped ions are tightly bound by processes that include; precipitation of low solubility neoformational minerals, isomorphous substitution, solid-state diffusion, and adsorption; it is also an excellent flocculant. Although ordinary BauxsolT has an excellent ability to bind arsenate, and to a lesser extent arsenite, this ability can be further increased for particular water types by using activated BauxsolT or BauxsolT combined with small amounts of other reagents. Field trials conducted at the Gilt Edge Mine, South Dakota, showed that the addition of BauxsolT to highly sulfidic waste rock reduced the arsenic concentration in leachate water from 35 mg/L to less than 0.005 mg/L; the concentrations of trace metals were also lowered to environmentally acceptable levels and leachate acidity was neutralized. Arsenic concentrations in the leachate water have remained below 0.005 mg/L for the four years since the treatment was carried out and the concentrations of trace metals have remained well below regulatory limits. In another example, the use of BauxsolT blended with a small amount of jarosite successfully reduced the total arsenic concentration in an industrial processing water from 16.4 mg/L to less than 0.001 mg/L; the treatment also reduced the concentrations of Cd, Cr, Ni, Pb and Zn to environmentally acceptable values. In a final example, activated BauxsolT used in simple flow through columns reduced the arsenic concentration in potable water from about 2 mg/L to less than 0.001 mg/L. In all three trials the spent BauxsolT residue released almost no arsenic in a TCLP leaching test and easily met the criteria for classification as an inert solid so that there were no special requirements for the disposal of water treatment residues. In all three studies the BauxsolT-based products compared very favorably with other more costly sorbents that are available.