Mobile arsenic species in unpolluted and polluted soils

The fate and behaviour of total arsenic (As) and of As species in soils is of concern for the quality of drinking water. To estimate the relevance of organic As species and the mobility of different As species, we evaluated the vertical distribution of organic and inorganic As species in two uncontaminated and two contaminated upland soils. Dimethylarsinic acid (up to 6 ng As g(-1)), trimethylarsine oxide (up to 1.5 ng As g(-1)), 4 unidentified organic As species (up to 3 ng As g(-1)) and arsenobetaine (up to 15 ng As g(-1)), were detected in the forest soils. Arsenobetaine was the dominant organic As species in both unpolluted and polluted forest soils. No organic As species were detected in the contaminated grassland soil. The organic As species may account for up to 30% of the mobile fraction in the unpolluted forest floor, but never exceed 9% in the unpolluted mineral soil. Highest concentrations of organic As species were found in the forest floors. The concentrations of extractable arsenite were highest in the surface horizons of all soils and may represent up to 36% of total extractable As. The concentrations of extractable arsenate were also highest in the Oa layers in the forest soils and decreased steeply in the mineral soil. In conclusion, the investigated forest soils contain a number of organic As species. The organic As species in forest soils seem to result from throughfall and litterfall and are retained mostly in the forest floor. The relative high concentrations of extractable arsenite, one of the most toxic As species, and arsenate in the forest floor point to the risk of their transfer to surface water by superficial flow under heavy rain events.     

Metal contamination at a wood preservation site: characterisation and experimental studies on remediation

The aim of this investigation was to determine the occurrence of As, Cu, Cr and Zn in the soil at an abandoned wood preservation unit and to examine some possible extractants for the contaminants in the soil. The mean As content of the contaminated surface soils (0-10 cm) was 186 mg kg(-1), where as the mean concentrations of Cu, Cr and Zn in soils from the contaminated area were 26, 29 and 91 mg kg(-1), respectively. The elevated As content in the mineral soils is related to adsorption of inorganic As phases in the fine grained fractions, which are characterised by large surface area and high positive surface charge under the current acidic conditions. Cu and Cr were found to be rather mobile, which is reflected in their lower abundance in soils and significant accumulation in sediments in the drainage leaving the area. The fine fraction of the soil (<0.125 mm) has an average metal content increased by nearly 34% as compared to the <2-mm fraction conventionally used for the analysis and assessment of soil contamination. The <2-mm fraction constitutes approximately 65% of the total weight while the fine fraction (<0.125 mm) constitutes approximately 10%. These facts, taken together, are essential for the choice of remediation measures. Oxalate solutions have been tested as extractants for soil remediation. Dark acid oxalate extraction dissolves the amorphous Al- and Fe-oxides and hydroxides and mobilises the adsorbed inorganic As species. Oxalate also acts as a ligand for the cationic heavy metals, releasing them from exchangeable sites. With a three-step sequential leaching, up to 98-99% of the metals could be removed. At lower concentrations and higher pH, the leaching decreased to approximately 70%.     

Removal copper,chromium and arsenic from CCA-treated yellow pine by oleic acid

Removal of Cu, Cr and As metals from chromated copper arsenate (CCA) treated yellow pine wood samples with three different dimensions were investigated by extraction with oleic acid at four different pH levels. The concentrations of Cu, Cr and As were determined by XRF. The effects of pH, dimension and duration on remediation of CCA-treated wood samples were determined. Oleic acid was found to be very effective to remove copper, chromium and arsenic from CCA-treated wood samples especially at lower pH levels (pH=2.00 and 2.50). In addition, the best models estimate copper, chromium and arsenic leaching from CCA-treated wood samples by oleic acid remediation were determined by step-wise regression analysis.     

Partitioning of hydrophobic organic compounds within soil-water-surfactant systems

Understanding the partitioning of hydrophobic organic compounds (HOCs) within soil-water-surfactant systems is key to improving the use of surfactants for remediation. The overall objective of this study was to investigate the soil properties that influence the effectiveness of surfactants used to remediate soil contaminated with hydrophobic pesticides, as an example of a more general application for removing strongly sorbing HOCs from contaminated soils via in-situ enhanced sorption, or ex-situ soil washing. In this study, the partitioning of two commonly used pesticides, atrazine and diuron, within soil-water-surfactant systems was investigated. Five natural soils, one nonionic surfactant (Triton-100 (TX)) and one cationic surfactant (benzalkonium chloride (BC)) were used. The results showed that the cation exchange capacity (CEC) is the soil property that controls surfactant sorption onto the soils. Diuron showed much higher solubility enhancement than atrazine with the micelles of either surfactant. Within an ex-situ soil washing system, TX is more effective for soils with lower CEC than those with higher CEC. Within an in-situ enhanced sorption zone, BC works significantly better with more hydrophobic HOCs. The HOC sorption capacity of the sorbed surfactant (K(ss)) was a non-linear function of the amount of surfactant sorbed. For the cationic surfactant (BC), the maximal K(ss) occurred when around 40% of the total CEC sites in the various soils were occupied by sorbed surfactant. Below a sub-saturation sorption range (~20 g/kg), under the same amount of BC sorbed, a soil with lower CEC tends to have higher K(ss) than the one with higher CEC.     

Sorption materials for arsenic removal from water: a comparative study

Five different sorption materials were tested in parallel for the removal of arsenic from water: activated carbon (AC), zirconium-loaded activated carbon (Zr-AC), a sorption medium with the trade name 'Absorptionsmittel 3' (AM3), zero-valent iron (Fe(0)), and iron hydroxide granulates (GIH). Batch and column tests were carried out and the behavior of the two inorganic species (arsenite and arsenate) was investigated separately. The sorption kinetics of arsenate onto the materials followed the sequence Zr-AC > GIH = AM3 > Fe(0) > AC. A different sequence was obtained for arsenite (AC > Zr-AC = AM3 = GIH = Fe(0)). AC was found to enhance the oxidation reaction of arsenite in anaerobic batch experiments. The linear constants of the sorption isotherms were determined to be 377, 89 and 87 for Zr-AC, AM3 and GIH, respectively. The uptake capacities yielded from the batch experiment were about 7gl(-1) for Zr-Ac and 5gl(-1) for AM3. Column tests indicated that arsenite was completely removed. The best results were obtained with GIH, with the arsenate not eluting before 13100 pore volumes (inflow concentration 1 mg l(-1) As) which corresponds to a uptake capacity of 2.3 mg g(-1) or 3.7 g l(-1).     

Role of microbes in controlling the speciation of arsenic and production of arsines in contaminated soils

The influence of microbes on the speciation of arsenic and production of arsines in contaminated soils was investigated under laboratory conditions. Microbes were able to carry out reactions that resulted in changes in the speciation of arsenic in soil. The transformation of soil dominating species, arsenate [As(V)], under both aerobic and anaerobic conditions to arsenite [As(III)], monomethylarsonic acid [MMAA], dimethylarsinic acid [DMAA] and to volatile trimethylarsine [TMA] was, however, less than 0.5%, of which the production of TMA represented 0.02-0.3%. The volatilization process was also verified in the field, in the soil of a dumping area. The 'life-time' of arsines in air was, however, short and they were rapidly converted back to water soluble species, As(V) and trimethyl arsine oxide (TMAO).     

Arsenic speciation and mobilization in CCA-contaminated soils: influence of organic matter content

A laboratory incubation experiment was conducted to investigate the influence of organic matter content on arsenic speciation and mobilization in chromated copper arsenate (CCA)-contaminated soils. The study was performed with four synthetic CCA-contaminated soils, with a range of organic matter content (mixture of peat moss and poultry manure) varying between 0.5% and 15% (w/w), under unsaturated and aerobic conditions for 40 days. Changes in water-soluble arsenic speciation (As(V), As(III), MMAA, DMAA) were monitored over time in soil extracts by HPLC-ICP-MS and in the soil solid phase (As(III), As(V)) by a solvent extraction method. Irrespective of organic matter content, As(V) was the predominant soil bound and aqueous phase arsenic species. However, over 40 days, a high soil organic matter content (7.5% and 15%) was able to entail formation of soil bound As(III). Moreover, total water-soluble arsenic was positively correlated with dissolved organic carbon (r(2)=0.88). However, the organic matter content did not influence arsenic speciation in the soluble fraction; neither As(V) reduction nor arsenic biomethylation occurred within 40 days. An increase in dissolved organic carbon content promoted both As(V) and As(III) solubilization in soils. Also, over time, organic matter contents of 7.5% and 15% entailed the persistence of soluble As(V), likely due to the high content of dissolved organic compounds which prevented its sorption onto soil. Based on this data, the environmental risk of aerobic CCA-contaminated soils rich in organic matter may be due to an enhanced availability of soluble As(V) over time, rather than to the formation of the more toxic and more mobile As(III).     

Mobility of copper, chromium and arsenic from treated timber into grapevines

The use of chromated copper arsenate (CCA) treated timber posts as support structures in New Zealand vineyards has raised concerns regarding the release of heavy metal(loid)s from the treated timber into the environment. A laboratory experiment was set up to evaluate if post sealing by painting reduces the release rate of CCA metal(loid)s from timber posts. Three posts were painted (Gripset 38, Multi Purpose Bitumen Rubber) on the bottom part, submerged in freshwater, and the concentrations of copper (Cu), chromium (Cr) and arsenic (As) in the water were monitored over a period of 8 months. Three additional, unpainted posts were also used. The CCA contents in the water showed a clear difference between the painted and the unpainted posts, and painting the bottom of the posts reduced the release rate by 50-75%. To monitor the possible mobility of CCA from treated posts into grapevines further, an experiment with four-year-old grapevines planted into sixteen lysimeters was set up in a greenhouse. To half the lysimeters Cu, Cr and As were added to the soil surface at rates of 16, 20, and 12.5 mg/month from 15 November 2005 to 5 May 2006. The other lysimeters acted as a control. Soil solutions were collected at 50, 150 and 300 mm depths using suction cups after seven application of the CCA solution. The results showed that all the elements moved to a depth of 50 mm. Grape fruit, leaves and rachis were analysed for CCA metal(loid)s, but did not show any differences between the CCA-treated and control lysimeters. This indicated either that these metals were not taken up by grapevines or that their translocation from roots to the upper part of the vine was negligible. Further monitoring of CCA metal(loid)s in various parts of the grapevines, including roots, needs to be undertaken.     

Optimization of capacity and kinetics for a novel bio-based arsenic sorbent, TiO2-impregnated chitosan bead

The optimization of TiO₂-impregnated chitosan beads (TICB) as an arsenic adsorbent is investigated to maximize the capacity and kinetics of arsenic removal. It has been previously reported that TICB can 1) remove arsenite, 2) remove arsenate, and 3) oxidize arsenite to arsenate in the presence of UV light and oxygen. Herein, it is reported that adsorption capacity for TICB is controlled by solution pH and TiO₂ loading within the bead and enhanced with exposure to UV light. Solution pH is found to be a critical parameter, whereby arsenate is effectively removed below pH 7.25 and arsenite is effectively removed below pH 9.2. A model to predict TICB capacity, based on TiO₂ loading and solution pH, is presented for arsenite, arsenate, and total arsenic in the presence of UV light. The rate of removal is increased with reductions in bead size and with exposure to UV light. Phosphate is found to be a direct competitor with arsenate for adsorption sites on TICB, but other relevant common background groundwater ions do not compete with arsenate for adsorption sites. TICB can be regenerated with weak NaOH and maintain full adsorption capacity for at least three adsorption/desorption cycles.     

Extractability of metals and ecotoxicity of soils from two old wood impregnation sites in Finland

Four metal-contaminated soil samples were classified using physical methods, extracted by selective extraction procedures and analyzed for chemical concentrations. De-ionized water, 0.01 mol/l barium chloride, 1 mol/l ammonium acetate and concentrated nitric acid were used as extraction solutions. Ecotoxicity of water extracts and soil samples was analyzed in order to describe the bioavailability of the contaminants. Samples from old wood impregnation plants contained high amounts of As, Cu, Cr and Zn, which originated from chromated copper arsenate, ammoniacal copper-zinc arsenate, and ammoniacal copper quaternary compound. Total As concentrations of the heavily contaminated samples varied from 752 to 4340 mg/kg, Cu concentrations from 339 to 2330 mg/kg, Cr concentrations from 367 to 2,140 mg/kg and Zn concentrations from 79 to 966 mg/kg. The extractabilities of metals differed according to soil type, extractant and element. Cu and Zn were proposed to cause the highest toxicity in the water extracts of the soils. Ecotoxicity tests displayed rather high differences in sensitivity both for water extracts and for solid soil samples. Reproduction of Enchytraeus sp. was the most sensitive and seed germination of Lactuca sativa the least sensitive and the other tests were in decreasing order of sensitivity: Folsomia candida>reverse electron transport>MetPLATE>Toxichromotest>Allium cepa root growth>Lemna sp. growth. As a conclusion, polluted soils rich in sand retain heavy metals with less firm bindings, particularly in the case of Cu and Zn, than soils rich in clay, indicating that chemical methods for measuring the bioavailability of metals need to be optimized taking into account the soil type, acidity, redox state and the individual contaminants.     

Arsenic chemical species-dependent genotoxic potential in water extracts from two CCA-contaminated soils measured by DNA-repair deficient CHO-cells

Two soils with similar contamination levels from wood preservatives containing Chromium (Cr), Copper (Cu) and Arsenic (As) (CCA), were assessed for their general toxicity and genotoxicity. A set of water-based extraction methods, including pressurized liquid extraction (PLE), and batch leaching in milli-Q water and a weak CaCl₂-solution, was used to produce soil extracts containing available fractions of contaminants. In addition, to obtain indications of the contaminants' bioavailability and toxic potential the genotoxicity of the extracts was estimated by testing their ability to inhibit the growth of wildtype Chinese hamster ovary cells (CHO-cells) and three genetically modified pheno-types that are deficient in different DNA-repair mechanisms. Total extractable arsenic concentrations in the extracts were comparable between the sites. However, the genotoxic potential was clearly higher in soil R extracts. The differences in genotoxic responses were related to differences in inorganic arsenic speciation. The ratio of trivalent arsenic (As^III) to pentavalent arsenic (As^V) was higher in all soil extracts from soil R, regardless of the leaching method used. The results of the various combinations of soil extraction techniques and assays using the CHO-cell lines reflected important differences in arsenic speciation in the two soils and possible synergistic effects in CCA-related exposure. They also indicate that speciation and combinatory effects are factors that should be taken into account when assessing risks at former wood impregnation sites contaminated by CCA-agents.     

Arsenic retention and release in ombrotrophic peatlands

Organic matter can play an important role in the mobility and fate of As in the environment, but there is a lack of data on As biogeochemistry in ombrotrophic peatlands. The aim of this study was to investigate As retention and release in atmospherically contaminated ombrotrophic peat soils in the Peak District National Park (UK). Solid phase As concentrations in the peat soils exceed 25 mg kg^− 1. Solid phase As and Fe concentrations are closely correlated at sites where the peat is subjected to drying and oxic conditions. In a wetter zone of the bog, solid phase As and Fe distributions are decoupled, suggesting that As retention in these systems is not solely controlled by the presence of Fe oxides. Comparison of solid phase As and Pb distributions reveals that As has been subjected to post-depositional mobility in areas of water table fluctuation. Conversely, at permanently waterlogged locations As is immobile. Detailed stream water sampling reveals that As is released from the organic-rich uplands soils into the fluvial system. Dissolved As concentrations are highly variable, with values ranging from 0.20 to 7.28 μg l^− 1. Stream water As concentrations are elevated during late summer stormflow periods when there has been re-wetting of the peat after significant water table draw-down. Dissolved As is strongly correlated to dissolved organic carbon under stormflow and baseflow. The results of this study suggest that organic matter plays an important role in As dynamics in ombrotrophic peatlands, but further work is needed to identify the exact As binding and release mechanisms. Drying and re-wetting of ombrotrophic peat soils and associated changes in redox status has the potential to lead to increased As mobility. Further work is needed to provide information on how predicted climate change will influence As cycling at sites containing a legacy of atmospheric contamination.     

Anthropogenic contamination and risk assessment of heavy metals in stream sediments influenced by acid mine drainage from a northeast coalfield,India

The total concentrations and chemical partitioning of heavy metals in streambed sediments, collected around the Jaintia Hills coal deposit of northeast India, were studied using pollution indices and multivariate techniques to evaluate the risk and contamination levels from heavy metals and their possible origins. Results show that sediments close to mining sites have low pH (<4), high organic carbon, and contain significant amounts of Fe-oxyhydroxide phases (mainly, goethite and schwertmannite), which implies direct impact of coal mine drainage. The average concentrations of Fe, Cu, Co, Cd, Cr, and Zn exceeded the World average, and in some cases, Cd, Cu, Ni, and Cr concentrations exceeded the threshold effects level, which suggests they will be toxic to aquatic biota. Contamination factors (CF) show that the sediments are low to highly contaminated with Cd, Cu, Mn, Pb, Fe, and Zn and low to moderately contaminated with Co, Cr and Ni. The pollution load index (PLI), degree of contamination index (C _deg) and Nemerow integrated pollution index (NIPI) show that the sediments are moderately to highly contaminated, with the extent of pollution greatest nearest to the collieries. The potential ecological risk index (RI) shows low to considerable ecological risk from heavy metals in the sediments, with Cd having the high potential of risk, which also agrees with the risk assessment code (RAC). Multivariate statistical analysis suggests that the concentrations of the heavy metals in stream sediments are strongly influenced by Fe-oxyhydroxide phases and organic carbon derived from anthropogenic sources, mainly coal mining activities. Although a significant proportion of the Cd, Mn, and Ni in the sediments are partitioned into exchangeable and organic fractions, a sizable amount of metals are also found in the Fe–Mn fraction, suggesting that Fe-oxyhydroxides play a dominant role in controlling metal mobility in these stream sediments.     

Uptake, transport and transformation of arsenate in radishes (Raphanus sativus)

The localization and identification of arsenic compounds in terrestrial plants are important for the understanding of arsenic uptake, transformation and translocation within these organisms, and contributes to our understanding of arsenic cycling in the environment. High performance liquid chromatography inductively coupled plasma mass spectrometry (HPLC-ICP-MS), and X-ray absorption near-edge structure (XANES) analysis identified arsenite, arsenate and arsenic(III)-sulphur compounds in leaf, stem and root sections of Rhaphanus sativus (radish) plants grown in both arsenic contaminated mine waste, and arsenic amended liquid cultures. The total arsenic distribution was similar between the plants grown in mine waste and those grown hydroponically. Arsenate was the predominant form of arsenic available in the growth mediums, and after it was taken up by roots, X-ray absorption spectroscopy (XAS) imaging indicated that some of the arsenate was transported to the shoots via the xylem. Additionally, arsenate was reduced by the plant and arsenic(III)-sulphur compound(s) accounted for the majority of arsenic in the leaf and stem of living plants. In this study the application of synchrotron techniques permitted the identification of arsenic(III)-sulphur species which were "invisible" to conventional HPLC-ICP-MS analysis.     

Mineralogical and geochemical controls of arsenic speciation and mobility under different redox conditions in soil, sediment and water at the Mokrsko-West gold deposit, Czech Republic

Naturally contaminated soil, sediment and water at the Mokrsko-West gold deposit, Central Bohemia, have been studied in order to determine the processes that lead to release of As into water and to control its speciation under various redox conditions. In soils, As is bonded mainly to secondary arseniosiderite, pharmacosiderite and Fe oxyhydroxides and, rarely, to scorodite; in sediments, As is bonded mainly to Fe oxyhydroxides and rarely to arsenate minerals.The highest concentrations of dissolved As were found in groundwater (up to 1141 μg L^− 1), which mostly represented a redox transition zone where neither sulphide minerals nor Fe oxyhydroxide are stable. The main processes releasing dissolved As in this zone are attributed to the reductive dissolution of Fe oxyhydroxides and arsenate minerals, resulting in a substantial decrease in their amounts below the groundwater level. Some shallow subsurface environments with high organic matter contents were characterized by reducing conditions that indicated a relatively high amount of S^− 2,0 in the solid phase and a lower dissolved As concentration (70-80 μg L^− 1) in the pore water. These findings are attributed to the formation of Fe(II) sulphides with the sorbed As. Under oxidizing conditions, surface waters were undersaturated with respect to arsenate minerals and this promoted the dissolution of secondary arsenates and increased the As concentrations in the water to characteristic values from 300 to 450 μg L^− 1 in the stream and fishpond waters. The levels of dissolved As(III) often predominate over As(V) levels, both in groundwaters and in surface waters. The As(III)/As(V) ratio is closely related to the DOC concentration and this could support the assumption of a key role of microbial processes in transformations of aqueous As species as well as in the mobility of As.     

Comparative study of simultaneous removal of As, Cu, and Pb using different combinations of electrokinetics with bioleaching by Acidithiobacillus ferrooxidans

Different designs of electrokinetics were applied to simultaneously remove arsenic, copper, and lead from contaminated soils. Single electrokinetics (control) resulted in superior removal efficiencies for Cu (73.5%) and Pb (88.5%), though the removal of As (3.11%) was relatively little. Sequential bioelectrokinetics of bioleaching with Acidithiobacillus ferrooxidans and electrokinetics enhanced the removal of As (25%), while Pb exhibited a significant decrease in removal efficiency (10.6%), due to the formation of insoluble compounds. In order to improve the overall performance, integrated bioelectrokinetics was designed by inoculating A. ferrooxidans into the electrolyte after 5 or 15 days of electrokinetics. Lead (75.8%) and copper (72%) were effectively removed through electrokinetics, after which arsenic (35%) was more efficiently removed by bioleaching-enhanced electrokinetics. A pilot-scale experiment indicated that integrated bioelectrokinetics is an effective means of remediation of soils contaminated with multiple heavy metals and arsenic.     

Arsenic uptake and speciation in rice plants grown under greenhouse conditions with arsenic contaminated irrigation water

The accumulation of arsenic (As) by rice (Oryza sativa L.) is of great interest considering the dietary intake of rice is potentially a major As exposure pathway in countries where rice is irrigated with As contaminated groundwater. A small scale rice paddy experiment was conducted to evaluate the uptake of As by rice. Arsenic concentrations in rice tissue increased in the order grain相似文献   

Dislodgeable copper,chromium and arsenic from CCA-treated wood surfaces

Chromated copper arsenate (CCA) is commonly used to preserve wood, but its use poses risk of arsenic exposure. In order to evaluate the extent of exposure to As from physical contact with CCA-treated wood, dislodgeable As from treated wood surfaces (as well as Cu and Cr) was determined as a function of weathering time using dampened polyester wipe materials. Six sets of 2.5-m-long CCA-treated boards, three-four boards per set, were purchased from lumber yards and cut into 30- or 60-cm coupons. A total of 44 such coupons were placed outdoors and the dislodgeable CCA components from the surfaces of the wooden coupons were periodically determined over a 1- or 2-year period by a systematic wipe method followed by nitric acid extraction of the CCA components from the cloth. In all 316 samples, appreciable amounts of the three elements, Cu, Cr and As, were detected. The amounts of surface-dislodgeable As, the most potentially hazardous element and the one of major concern in this study, varied from 5 to 122 microg/100 cm(2) with an average value of 37+/-22 microg/100 cm(2). There was considerable variation in As dislodged among coupons, boards, sets and time. Test coupons that tended to release relatively higher (or lower amounts) over time initially, continued to do so over time. However, the amounts of arsenic dislodged over time did not follow a simple pattern. While the As dislodged tended to decrease with time during the first year, it approached the initial value or increased somewhat during the second year, presumably due to surface rejuvenation effects caused by erosion and weathering. When all the data were normalized to the initial values, no trend emerged, as indicated by the average normalized value of 1.0+/-0.4 for As dislodged over time. Apparently, on installations constructed with CCA-treated wood, arsenic may remain available for a number of years.     

Leaching, runoff and speciation of arsenic in a laboratory mesocosm

Leaching and runoff of arsenic (As) from the contaminated soil of an old wood impregnation plant, and fate in a recipient freshwater ecosystem, was studied in soil-water-sediment mesocosms in laboratory (0.9 m3; total water volume 200 l). During the 4-month experiment total leaching and discharge of As from regularly irrigated soil was approximately 40 mg, i.e. approximately 0.6% of total initial As content in the soil. Of the total As load, 7.5% remained in the water; 44% settled down to the shallow (water depth 5-30 cm) sediment zone; and 48.5% to the deeper (water depth 80 cm) sediment zone. The different arsenic species; arsenite [As(III)], arsenate [As(V)], monomethylarsonic acid (MMAA) and dimethylarsinic acid (DMAA), were analysed from irrigation and discharge water; mesocosm pool water; and sediment pore water using ion chromatography-inductively coupled plasma-mass spectrometry (IC-ICP-MS). The total amounts of arsenic in soil, water and sediment were determined by ICP-MS. Arsenic was leached out from the soil as As(V). In mesocosm water As(V) was the predominant dissolved species, but DMAA and particle bound species, were also detected. In shallow sediment, As(V) was the most abundant species together with some DMAA, whereas in deep sediment As(III) was the dominant species.     

Arsenic speciation and distribution in an arsenic hyperaccumulating plant

Arsenic-contaminated soil is one of the major arsenic sources for drinking water. Phytoremediation, an emerging, plant-based technology for the removal of toxic contaminants from soil and water, has been receiving renewed attention. Although a number of plants have been identified as hyperaccumulators for the phytoextraction of a variety of metals, and some have been used in field applications, no hyperaccumulator for arsenic had been previously reported until the recent discovery of Brake fern (Pteris vittata), which can hyperaccumulate arsenic from soils. This finding may open a door for phytoremediation of arsenic-contaminated soils. Speciation and distribution of arsenic in the plant can provide important information helpful to understanding the mechanisms for arsenic accumulation, translocation, and transformation. In this study, plant samples after 20 weeks of growth in an arsenic-contaminated soil were used for arsenic speciation and distribution study. A mixture of methanol/water (1:1) was used to extract arsenic compounds from the plant tissue. Recoveries of 85 to 100% were obtained for most parts of the plant (rhizomes, fiddle heads, young fronds and old fronds) except for roots, for which extraction efficiency was approximately 60%. The results of this study demonstrate the ability of Brake fern as an arsenic hyperaccumulator. It transfers arsenic rapidly from soil to aboveground biomass with only minimal arsenic concentration in the roots. The arsenic is found to be predominantly as inorganic species; and it was hypothesized that the plant uptakes arsenic as arsenate [As(V)I and arsenate was converted to arsenite [As(III)] within the plant. The mechanisms of arsenic uptake, translocation, and transformation by this plant are not known and are the objectives of our on-going research.