The toxicity and adverse health effects of arsenic are widely known. It is generally accepted that sorption/desorption reactions with oxy-hydroxide minerals (iron, manganese) control the fate and transport of inorganic arsenic in surface waters through adsorption and precipitation-dissolution processes. In terrestrial environments with limited reactive iron, recent data suggest organoarsenicals are potentially important components of the biogeochemical cycling of arsenic in near-surface environments. Elevated arsenic levels are common in South Texas from geogenic processes (weathering of As-containing rock units) and anthropogenic sources (a byproduct from decades of uranium mining). Sediments collected from South Texas show low reactive iron concentrations, undetectable in many areas, making oxy-hydroxide controls on arsenic unlikely. Studies have shown that eukaryotic algae isolated from arsenic-contaminated waters have increased tolerance to arsenate toxicity and the ability to uptake and biotransform arsenate. In this experiment, net uptake of arsenic over time by a freshwater cyanobacterium never previously exposed to arsenate was quantified as a function of increasing As concentrations and increasing N:P ratios. Toxic effects were not evident when comparing cyanobacterial growth, though extractions indicate accumulation of intracellular arsenic by the cyanobacterium. Increasing N:P ratios has minimal effect on net arsenate uptake over an 18 day period. However, cyanobacteria were shown to reduce arsenate at rates faster than the system can re-oxidize the arsenic suggesting gross arsenate uptake may be much higher. Widespread arsenate reduction by cyanobacterial blooms would increase arsenic mobility and potential toxicity and may be useful as a biomarker of arsenic exposure in oxic surface water environments.

Although the impacts of foreign species on aqueous transformations and arsenic adsorption by 2-line ferrihydrite (FH2) have been extensively studied, much less is known about the impact of transformation inhibitors on solid-state transformation of FH2 and arsenic adsorption. In this study, the influence of inhibitors (Si(IV), Mg(II), Al(III), Ti(IV), and Ci(citrate)), aging time, and heat treatment on FH2 transformation and arsenic adsorption was investigated. The FH2s were synthesized by mixing Fe(III) salts with an inhibitor at pH 7.5 and air drying for 2 d. With increases in Al/Fe molar ratio, FH2, poorly crystalline Al hydroxide, gibbsite, and bayerite were formed in the FH2-Al series, whereas FH2 was formed in the other FH2s. Heat treatment had a more considerable impact on the transformation, structure, and PZC of FH2 than aging at RT for 235 d. Upon heating the FH2s at 360 oC, most of the amorphous Fe oxide was transformed into hematite, whereas Si and Al had stronger retarding effects on transformation than the other inhibitors. Hematite and FH2 were identified with increases in Si/Fe molar ratio, whereas with increasing Al/Fe molar ratio, FH2 remained, gibbsite and bayerite were decomposed, and boehmite appeared as a trace component. However, the effect of Si was much more pronounced than that of Al in retarding the transformation. The adsorption density for As(III) on the FH2s (at pH 7) decreased in the order: FH2-Mg-2 > FH2 > FH2-Al-1, whereas As(V) followed the order: FH2-Al-1 > FH2-Mg-2 > FH2. Compared to aging at RT for 235 d, heat treatment at 360 oC resulted in significantly reduced arsenic adsorption. The heated FH2 showed a smaller adsorption capacity for arsenic compare to that of the other FH2s. In contrast, the heated FH2-Al and FH2-Si series showed much higher adsorption capacities for As(V) than any other FH2, whereas the heated FH2-Mg series exhibited the largest adsorption capacity of As(III) among the heated FH2s. It is concluded that the use of the FH2-Al, FH2-Mg, or FH2-Si series instead of pure FH2 as filter media in water treatment might achieve more efficient arsenic removal and enhance arsenic retention at waste-disposal sites.

In this study, total As (T-As) levels in superpermafrost groundwater at a site in the High Arctic doubled over the course of the summer thaw. This increase was not due to snow input, as levels in snow were negligible. This increase in T-As did not correspond with a decrease in Eh, nor a rise in soluble Fe(II). It did, however, correspond with a shift in As speciation from arsenate to arsenite suggestive of reducing conditions. In the absence of predominant reducing conditions, the highly alkaline nature of the melting snow and concomitant large input of HCO3- may have played an important role in the increase of As in groundwater during the summer thaw. <p>Laboratory studies found that dissolved As (D-As) release under anaerobic conditions depended on the organic matter content of soil, with organic soils releasing D-As under reducing conditions and mineral soils sequestering D-As. In temperate soils, the release of D-As from organic soils is greatly accelerated due to the activity of anaerobic microbes. In northern soils, the same phenomenon may occur, with greater microbiological activity in organic soils where there is more labile C and nutrients than in mineral soil. <p>The sequestration of As in mineral soil is postulated to have occurred due to preferential sorption of arsenite to ferrihydrite or possibly to green rust minerals present under anaerobic conditions. Supporting this, arsenite sequestration occurred to a greater extent compared to arsenate, which is in agreement with the relative affinities of these two species for ferrihydrite. <p>Evidence from this study suggests that the As cycle on Truelove Lowland is dominated by the desorption of As due to HCO3- input each year during the spring melt linked to the sorption of As to ferrihydrite or green rust present in underlying mineral soils. The sequential thawing of the soils active layer and large inputs of HCO3- are unique to northern environments. Thus, this delicate balance of two sorbing processes should be born in mind in northern development. Large inputs of soluble organic matter or nutrients could cause increased solubilization and mobility of D-As during the summer thaw when soils become flooded.

In this study, total As (T-As) levels in superpermafrost groundwater at a site in the High Arctic doubled over the course of the summer thaw. This increase was not due to snow input, as levels in snow were negligible. This increase in T-As did not correspond with a decrease in Eh, nor a rise in soluble Fe(II). It did, however, correspond with a shift in As speciation from arsenate to arsenite suggestive of reducing conditions. In the absence of predominant reducing conditions, the highly alkaline nature of the melting snow and concomitant large input of HCO3- may have played an important role in the increase of As in groundwater during the summer thaw. <p>Laboratory studies found that dissolved As (D-As) release under anaerobic conditions depended on the organic matter content of soil, with organic soils releasing D-As under reducing conditions and mineral soils sequestering D-As. In temperate soils, the release of D-As from organic soils is greatly accelerated due to the activity of anaerobic microbes. In northern soils, the same phenomenon may occur, with greater microbiological activity in organic soils where there is more labile C and nutrients than in mineral soil. <p>The sequestration of As in mineral soil is postulated to have occurred due to preferential sorption of arsenite to ferrihydrite or possibly to green rust minerals present under anaerobic conditions. Supporting this, arsenite sequestration occurred to a greater extent compared to arsenate, which is in agreement with the relative affinities of these two species for ferrihydrite. <p>Evidence from this study suggests that the As cycle on Truelove Lowland is dominated by the desorption of As due to HCO3- input each year during the spring melt linked to the sorption of As to ferrihydrite or green rust present in underlying mineral soils. The sequential thawing of the soils active layer and large inputs of HCO3- are unique to northern environments. Thus, this delicate balance of two sorbing processes should be born in mind in northern development. Large inputs of soluble organic matter or nutrients could cause increased solubilization and mobility of D-As during the summer thaw when soils become flooded.

Although the impacts of foreign species on aqueous transformations and arsenic adsorption by 2-line ferrihydrite (FH2) have been extensively studied, much less is known about the impact of transformation inhibitors on solid-state transformation of FH2 and arsenic adsorption. In this study, the influence of inhibitors (Si(IV), Mg(II), Al(III), Ti(IV), and Ci(citrate)), aging time, and heat treatment on FH2 transformation and arsenic adsorption was investigated. The FH2s were synthesized by mixing Fe(III) salts with an inhibitor at pH 7.5 and air drying for 2 d. With increases in Al/Fe molar ratio, FH2, poorly crystalline Al hydroxide, gibbsite, and bayerite were formed in the FH2-Al series, whereas FH2 was formed in the other FH2s. Heat treatment had a more considerable impact on the transformation, structure, and PZC of FH2 than aging at RT for 235 d. Upon heating the FH2s at 360 oC, most of the amorphous Fe oxide was transformed into hematite, whereas Si and Al had stronger retarding effects on transformation than the other inhibitors. Hematite and FH2 were identified with increases in Si/Fe molar ratio, whereas with increasing Al/Fe molar ratio, FH2 remained, gibbsite and bayerite were decomposed, and boehmite appeared as a trace component. However, the effect of Si was much more pronounced than that of Al in retarding the transformation. The adsorption density for As(III) on the FH2s (at pH 7) decreased in the order: FH2-Mg-2 > FH2 > FH2-Al-1, whereas As(V) followed the order: FH2-Al-1 > FH2-Mg-2 > FH2. Compared to aging at RT for 235 d, heat treatment at 360 oC resulted in significantly reduced arsenic adsorption. The heated FH2 showed a smaller adsorption capacity for arsenic compare to that of the other FH2s. In contrast, the heated FH2-Al and FH2-Si series showed much higher adsorption capacities for As(V) than any other FH2, whereas the heated FH2-Mg series exhibited the largest adsorption capacity of As(III) among the heated FH2s. It is concluded that the use of the FH2-Al, FH2-Mg, or FH2-Si series instead of pure FH2 as filter media in water treatment might achieve more efficient arsenic removal and enhance arsenic retention at waste-disposal sites.

Arsenic is not an essential element and can be toxic to both plants and animals in high concentration. There is a demonstrated association between soil arsenic (As) and the occurrence of straighthead (a physiological disorder in rice characterized by panicle sterility and yield loss); however, the relationship between grain-As accumulation and straighthead susceptibility in rice is not yet fully understood. The objective of the current study was to evaluate a set of diverse rice cultivars, including both indica and japonica subspecies, for total grain-As (TGAs) and As-species concentrations in 2004, 2005, and 2007, on a native (moderate As-concentration) paddy soil and an adjacent monosodium monomethylarsonate (MSMA) amended soil. Cultivars were evaluated under both continuously flooded and intermittently flooded (saturated) field conditions. The genotypic differences in the occurrence of straighthead, total grain-As (TGAs) and As-species concentrations, and their relationships with plant growth parameters, e.g., heading date, plant height, and yield were assessed. The cultivars exhibited a considerable range in both TGAs and grain-As species concentrations. In 2004 and 2005, twenty-one rice cultivars replicated on native soil under continuous flooding showed significant differences in TGAs and As-species concentrations by genotype and year. In 2005, heading was generally delayed in the rice cultivars, resulting in reduced yields that were likely associated with unusually high temperatures and prolonged exposure to stresses in the field, including prolonged flooding and associated soil-As induced stresses. Lower grain-As concentrations were generally associated with early maturing and high yielding genotypes, but with some exceptions. Total grain-As concentrations were not correlated to straighthead susceptibility suggesting that high As concentration in rice grain might not be a direct cause of the genotype-dependent panicle sterility associated with MSMA in soil. The rice cultivars grown on the MSMA-flooded treatment could be effectively differentiated for their relative straighthead susceptibility, with scores ranging from 1 to 8 for the most resistant to the most susceptible genotypes, respectively. In general, traits such as low grain-iAsIII concentration,early maturity, and high yield were correlated with straighthead resistance. In the MSMA-flooded treatment, very high grain-As accumulation resulted in elevated rice-grain dimethyl-AsV (DMAsV) concentration, whereas, the concentration of the more harmful inorganic-AsIII species was less affected. The TGAs and As-species concentrations were considerably higher in continuously--flooded soil than the intermittently-flooded soil. The variations in TGAs and grain-DMAsV concentrations were more highly influenced by water regime than by genotype, whereas, grain-iAIII concentrations were more highly genotype dependent. In the native soil with intermittent flooding, the concentrations of grain-DMAsV and the less desirable grain-iAIII concentrations were lowest. The study concluded that for attaining lower As accumulation in the rice grain both genotype selection and water management are potentially useful approaches.

Arsenic in drinking water is a problem in many developing nations such as Taiwan and Bangladesh. Currently, no oral binding agent exists for the mitigation of arsenic toxicity. The goals of this research were to 1) screen a variety of sorbents for their ability to sorb As from water and screen for potential nutrient interactions with vitamin A (VA) and riboflavin (RF) isotherms; 2) further describe the sorption of As to ferrihydrite using isothermal analysis and a simulated gastrointestinal model (GI), and by testing ferrihydrite’s ability to protect Hydra from As toxicity; 3) verify ferrihydrite’s safety and efficacy in a short term rodent model. Ferrihydrite was found to be the most effective sorbent for both As(III) and As(V). Exchanging SWy-2 with sulfur containing organic groups increased the sorption of both As(V) and As(III) compared to the parent clay, though the total As sorbed was much less than As sorption by ferrihydrite. Ferrihydrite and an industrially produced ferrihydrite (IPF) both sorbed As(V) and As(III) with high capacity. Both ferrihydrites also sorbed As(V) and As(III) at high capacity in the simulated GI model. Fe measured in the simulated GI tract was below tolerable daily limits for both ferrihydrite and IPF. Ferrihydrite at 0.25 percent w/w was found to protect Hydra up to 200 times the minimal effective concentration (MEC) for As(III) and over 2.5 times the MEC for As(V), while IPF at 0.25 percent w/w protected Hydra up to 200 times the MEC for As(III) and just over 2 times the MEC for As(V). IPF was apparently safe and well tolerated by the rats in our study over a period of 2 weeks. No statistically significant differences were seen in serum biochemistry, serum Fe, serum VA, or serum vitamin E between rats fed control diet versus those fed 0.5 percent w/w IPF. Ferrihydrite was found to reduce urinary As after a single gavage of 0.5 mL of 500 ppm As(III) or As(V). These results verify in vitro findings and suggest that ferrihydrite is apparently safe and effective as an enterosorbent for As.

The characterization of water treatment residuals containing arsenic was investigated in the first study. Arsenic desorption and leachability from the residuals were the focus of this study. Arsenic leaching from water treatment residuals was found to be underestimated by the toxicity characteristic leaching test (TCLP) due to the pH of the leachates being favorable for As(V) adsorption. Competitive desorption of arsenic with phosphate was significant because phosphate tends to compete with As(V) on the surface of the metal hydroxide for adsorption sites. However, arsenic desorption by the competition of sulfate and chloride was found to be negligible. The pH in the leachate was a critical variable in controlling arsenic stability in the residuals. The release of arsenic from the residuals was elevated at low and high pH due to the increase dissolution of the adsorbents such as Fe and Al hydroxides. In the second phase of the study, the stabilization techniques for arsenic contained residuals and were examined to develop methods to suitably stabilize arsenic to eliminate and/or minimize leaching. A decrease of arsenic leaching was achieved by the addition of lime to the residuals and believed to be due to the formation of less soluble and stable calcium-arsenic compounds. However, it is suggested that the ordinary Portland cement (OPC) should be added with the lime for the long term stabilization because lime can be slowly consumed when directly exposed to atmospheric CO2. The solidification and stabilization (S/S) technique with lime and OPC was shown to be successfully applied by the immobilization of a wide variety of arsenic tainted water treatment residuals.

The toxicity and adverse health effects of arsenic are widely known. It is generally accepted that sorption/desorption reactions with oxy-hydroxide minerals (iron, manganese) control the fate and transport of inorganic arsenic in surface waters through adsorption and precipitation-dissolution processes. In terrestrial environments with limited reactive iron, recent data suggest organoarsenicals are potentially important components of the biogeochemical cycling of arsenic in near-surface environments. Elevated arsenic levels are common in South Texas from geogenic processes (weathering of As-containing rock units) and anthropogenic sources (a byproduct from decades of uranium mining). Sediments collected from South Texas show low reactive iron concentrations, undetectable in many areas, making oxy-hydroxide controls on arsenic unlikely. Studies have shown that eukaryotic algae isolated from arsenic-contaminated waters have increased tolerance to arsenate toxicity and the ability to uptake and biotransform arsenate. In this experiment, net uptake of arsenic over time by a freshwater cyanobacterium never previously exposed to arsenate was quantified as a function of increasing As concentrations and increasing N:P ratios. Toxic effects were not evident when comparing cyanobacterial growth, though extractions indicate accumulation of intracellular arsenic by the cyanobacterium. Increasing N:P ratios has minimal effect on net arsenate uptake over an 18 day period. However, cyanobacteria were shown to reduce arsenate at rates faster than the system can re-oxidize the arsenic suggesting gross arsenate uptake may be much higher. Widespread arsenate reduction by cyanobacterial blooms would increase arsenic mobility and potential toxicity and may be useful as a biomarker of arsenic exposure in oxic surface water environments.

Extraction with sodium phosphate has been used as a method of accessing arsenic in soils. Arsenic extraction efficiency by phosphate from rice-paddy soils of Bangladesh usually has been low and highly variable between soils. The major objectives of this study were to examine the relationships between phosphate-extractable arsenic and soil iron-oxide composition and to investigate the experimental factors which might influence arsenic-extraction efficiency from rice-paddy soils of Bangladesh by phosphate. Statistical analysis of approximately 500 surface soils from Bangladesh indicated that phosphate-extractable arsenic was well correlated with total soil arsenic (r2 = 0.832) and oxalate-extractable arsenic (r2 = 0.825), though extraction efficiency varied widely (5 - 54 % of the total soil arsenic). The thanas with the lowest arsenic contents generally also had the soils with the lowest arsenic-extraction efficiencies. Quantity of phosphate-extractable arsenic was weakly correlated with the soil iron-oxide content, but extraction efficiency (i.e., the proportion of phosphate-extractable arsenic to total soil arsenic) was not correlated with any iron-oxide parameter. Arsenic extraction was strongly influenced by reaction variables such as sample grinding, phosphate concentration, principal counterion, reaction pH, and reaction time. The extraction efficiency was impacted by the influence of these individual factors on reaction kinetics and accessibility of arsenic adsorption sites for ligand exchange by phosphate. A portion of the arsenic was readily exchanged during the first few hours of extraction, followed by a much slower subsequent extraction. These results indicate that some of the arsenic is easily exchanged, but for a substantial portion of the arsenic, either the reaction kinetics is very slow or the sites are not accessible for reaction with phosphate. Extraction by phosphate is a useful procedure for the assessment of readily ligand-exchanged arsenic.