Earth Abides Arsenic Biotransformations.

In their comments, Bessinger et al. of Exponent, Inc. (1) consistently suggest that our approach overestimates leaching rates and by no means suggest that the leaching rates are underestimated. We believe that their underlying assumptions are questionable and that leaching rates may in fact increase if their assumptions are in error. Below are specific examples. Bessinger et al. (1) suggest that leaching rates were overestimated due to the absence of soil within the C&D lysimeters. That leaching rates would decrease with the inclusion of soil is debatable since there is considerable literature that supports an increase in arsenic mobility from soils containing organic and mineral components (2). Given this information it is possible that the inclusion of soil within the lysimeters could also increase leaching rates. Bessinger et al. state that the assumption of a constant leaching rate overestimates arsenic leaching. First, we should point out that anything other than a constant leaching rate would require speculation about the chemistry and infiltration rates occurring over time within the landfill. Furthermore, the C&D scenario showed no decline in leaching rate throughout the study (Figure 2 in ref 4 showed a relatively constant slope) and thus provided no basis upon which to render an assumption other than a constant leaching rate. Bessinger et al. imply that the non-weathered wood fraction in the lysimeters was too high resulting in greater arsenic leaching rates. The assumption that non-weathered wood leaches more than weathered wood is questionable, as weathered wood has been shown to release more arsenic, particularly after long periods of time (5). If weathered wood leaches more arsenic than non-weathered wood, we believe that Bessinger et al.’s primary assumption is in error and one can argue the opposite with respect to leaching rates. Comments made by Bessinger et al. regarding arsenic speciation are misleading. First, the Nico et al. (6, 7) papers cited focused on arsenic and chromium speciation in solid phases using XAS. Arsenic speciation information reported in Khan et al. (4) was for leachates, i.e., aqueous forms. It is inappropriate to validate results obtained from one technique for one matrix using data gained from other matrices. Additionally, Nico et al. (7) found free arsenate ions to be the dominant solution-phase arsenic form and state that “once in solution, arsenic no longer complexes with Cr or Fe”. Nico et al. (7) use our previous results (5) to support their findings. Second, Bessinger et al. incorrectly claims that we introduced uncertainty by using chemical methods incapable of distinguishing between As(V) species and that we reported only total As(V) concentrations. The technique we used, HPLC-ICP–MS, is currently the most widely used analytical method for determining arsenic species in aquatic environments and closer inspection of Figures 2, 3, and 4 in our paper (4) shows the results for all four arsenic species and the total. In their discussion of arsenic mobility below the landfill, Bessinger et al. contradicted their own arguments. In one case, they suggest an inconsistency between the speciation of arsenic from the lysimeters (primarily as As(V)) and those observed in groundwater wells (As(III)) near C&D landfills, which represents a failure to recognize that arsenic speciation can change within the subsurface. In another case, Bessinger et al. argue that arsenic would precipitate as orpiment (As2S3), although recent studies (8) show arsenic–sulfur complexes to play a key role in arsenic mobility and toxicity in sulfate-reducing environments. The observation that sulfate-reducing conditions may increase arsenic mobility thus refutes Bessinger et al.’s assumption that arsenic will be immobilized in the subsurface as a precipitant. In summary, the underlying assumptions made by Bessinger et al. are baseless and their argument that the lysimeter study overestimates arsenic leaching is unjustifiable. Bessinger et al. make inappropriate comparisons between Nico et al. and our article (4). Moreover, Bessinger et al. failed to recognize habitual changes in arsenic speciation and its effects on arsenic mobility in the subsurface, specifically under reducing conditions below landfill (3). Of particular concern is the lack of recognition by Bessinger et al. to the vast quantities of arsenic associated with the use and disposal of treated wood. As acknowledged in their comment and as observed in our data, about 10% of the landfills in Florida are currently experiencing elevated arsenic in their groundwater wells. These impacts are surprising given the time needed for arsenic to leach from the wood, travel through the aquifer, and ultimately be detected at groundwater-monitoring wells; a leaching process that can take tens to hundreds of years and which suggest greater arsenic impacts for the future. The large quantities of arsenic associated with CCA-treated wood and the potential for arsenic-containing leachate to migrate should be accounted for in the management of this material.

Aquatic arsenic: Toxicity, speciation, transformations, and remediation

Fate of arsenic during the interactions between Mn-substituted goethite and dissolved Fe(II)

Bioaccumulation, biotransformation and trophic transfer of arsenic in the aquatic food chain

Coupling of arsenic mobility to sulfur transformations during microbial sulfate reduction in the presence and absence of humic acid

Precipitation of arsenic-bearing solids as a secondary control on arsenic speciation in groundwater: Evidence from field study and geochemical analysis

Measurement of labile arsenic speciation in water and soil using diffusive gradients in thin films (DGT) and X-ray absorption near edge spectroscopy (XANES)

Transformation and mobilization of arsenic in the historic Cobalt mining camp, Ontario, Canada

Inorganic arsenic (iAs) has been classified as a type 1 carcinogen and has also been linked to several noncancerous health effects. Prior to 1995, the As(V) methylation pathway was generally considered to be a detoxification pathway, but cellular and animal studies involving MMA(III) (mono metyl arsonous acid) and DMA(III) (dimethyl arsinous acid) have indicated that their toxicities meet or exceed that of iAs, suggesting an activation process. In addition, thiolated arsenic metabolites were observed in urine after oral exposure of inorganic arsenic in some studies, for which the toxicological profile was not yet fully characterized in human cells. Studies have revealed that microorganisms from the gut environment are important contributors to arsenic speciation changes. This presystemic metabolism necessitates the development of protocols that enable the detection of not only inorganic arsenic species, but also pentavalent and trivalent methylated, thiolated arsenicals in a gastrointestinal environment. We aim to study the biotransformation of arsenic (As) using a Simulator of the Human Intestinal Microbial Ecosystem (SHIME). To be able to analyze the arsenicals resulting from biotransformation reactions occurring in this system, a method using liquid chromatography hyphenated to an inductively coupled plasma mass spectrometer (HPLC-ICP-MS) was developed. A Hamilton PRP-X100 anion exchange column was used. The method allowed separation, identification and quantification of As(III) (arsenite), As(V) (arsenate), DMA(V) (dimethylarsinicacid), MMA(V) (monomethylarsonicacid) and MMMTA (monomethylmonothioarsenate). Attempts to optimize the same method for also separating MMA(III) and DMA(III) did not succeed. These compounds could be successfully separated using a method based on the use of a Zorbax C₁₈ column. The properties of the column, buffer strength, pH and polar nature of mobile phase were monitored and changed to optimize the developed methods. Linearity, sensitivity, precision, accuracy and resolution of both methods were checked. The combination of the two methods allowed successful quantification of arsenic species in suspensions sampled in vitro from the SHIME reactor or in vivo from the human colon and feces.

Arsenic is a redox sensitive element which can exist in As(V), As(III), As(0), and As(-III) oxidation states under redox conditions in the natural environment. The mobility and toxicity of arsenic are determined by its oxidation states. Thermodynamic data in the literature are summarized and used to construct a pe-pH diagram for an arsenic-iron-sulfur system. The chemical species of arsenic in aqueous systems usually are not at equilibrium status because of slow rates of redox reactions. Microbially mediated reductions and oxidations of arsenic play an important role in the transformation of arsenic in sediments, soil, geothermal water, surface water, and water treatment sludge. Heterogeneous oxidation of As(III) species takes place in suspensions containing manganese dioxides, titanium dioxides, and clay minerals. The fate and transport of arsenic is closely related to the redox reactions of sulfur and iron. Oxidized arsenic species such as As(V) are typically adsorbed on iron oxides in soil and sediments under oxic conditions. At low redox potentials, reduced arsenic species such as As(III) are associated with sulfide and pyrite minerals. The occurrence of arsenic in groundwater is mainly attributed to reductive dissolution of iron oxides and oxidative dissolutions of arsenic-rich pyrite and sulfide minerals under moderate reducing conditions (i.e., arsenic mobilization zone).

Our studies have indicated that the relative concentration of Se or Hg to As in urine and blood positively correlates with percentage of inorganic arsenic (% Inorg-As) and percentage of monomethlyarsonic acid [% MMA (V)]. We also found a negative correlation with percentage of dimethylarsinic acid [% DMA (V)] and the ratio of % DMA (V) to % MMA (V). In another study, we found that a group of proteins were significantly over expressed and conversely other groups were under-expressed in tissues in Na-As (III) treated hamsters.
 Introduction.Inorganic arsenic (Inorg-As) in drinking water.One of the largest public health problems at present is the drinking of water containing levels of Inorg-As that are known to be carcinogenic. At least 200 million people globally are at risk of dying because of arsenic (As) in their drinking water1-3. The chronic ingestion of Inorg-As can results in skin cancer, bladder cancer, lung cancer, and cancer of other organs1-3. The maximum contamination level (MCL) of U.S. drinking water for arsenic is 10 ug/L. The arsenic related public health problem in the U.S. is not at present anywhere near that of India4, Bangladesh4, and other countries5.
 Metabolism and toxicity of Inorg-As and arsenic species.Inorg-As is metabolized in the body by alternating reduction of pentavalent arsenic to trivalent form by enzymes and addition of a methyl group from S-adenosylmethionine6, 7; it is excreted mainly in urine as DMA (V)8. Inorganic arsenate [Inorg-As (V)]is biotransformed to Inorg-As (III), MMA (V), MMA (III), DMA (V), and DMA (III)6(Fig. 1). Therefore, the study of the toxicology of Inorg-As (V) involves at least these six chemical forms of arsenic. Studies reported the presence of 3+ oxidation state arsenic biotransformants [MMA (III) and DMA (III)] in human urine9and in animal tissues10. The MMA (III) and DMA (III) are more toxic than other arsenicals11, 12. In particular MMA (III) is highly toxic11, 12. In increased % MMA in urine has been recognized in arsenic toxicity13. In addition, people with a small % MMA in urine show less retention of arsenic14. Thus, the higher prevalence of toxic effects with increased % MMA in urine could be attributed to the presence of toxic MMA (III) in the tissue. Previous studies also indicated that males are more susceptible to the As related skin effects than females13, 15. A study in the U.S population reported that females excreted a lower % Inorg-As as well as % MMA, and a higher % DMA than did males16.
 Abbreviation: SAM, S-adenosyl-L-methionine; SAHC, S-adenosyl-L-homocysteine.
 Differences in susceptibility to arsenic toxicity might be manifested by differences in arsenic metabolism among people. Several factors (for examples, genetic factors, sex, duration and dosage of exposure, nutritional and dietary factors, etc.) could be influence for biotransformation of Inorg-As,6, 17 and other unknown factors may also be involved.
 The interaction between As, Se, and Hg.The toxicity of one metal or metalloid can be dramatically modulated by the interaction with other toxic and essential elements18. Arsenic and Hg are toxic elements, and Se is required to maintain good health19. But Se is also toxic at high levels20. Recent reports point out the increased risk of squamous cell carcinoma and non-melanoma skin cancer in those treated with 200 ug/day of selenium (Nutritional Prevention of Cancer Trial in the United States)21. However, it is well known that As and Se as well as Se and Hg act as antagonists22. It was also reported that Inorg-As (III) influenced the interaction between selenite and methyl mercury23. A possible molecular link between As, Se, and Hg has been proposed by Korbas et al. (2008)24. The identifying complexes between the interaction of As and Se, Se and Hg as well as As, Se, and Hg in blood of rabbit are shown in Table 1.
 Influence of Se and Hg on the metabolism of Inorg-As.The studies have reported that Se supplementation decreased the As-induced toxicity25, 26. The concentrations of urinary Se expressed as ug/L were negatively correlated with urinary % Inorg-As and positively correlated with % DMA27. The study did not address the urinary creatinine adjustment27. Other researchers suggested that Se and Hg decreased As methylation28-31(Table 2). They also suggested that the synthesis of DMA from MMA might be more susceptible to inhibition by Se (IV)29 as well as by Hg (II)30,31 compared to the production of MMA from Inorg-As (III). The inhibitory effects of Se and Hg were concentration dependent28-31.
 The literature suggests that reduced methylation capacity with increased % MMA (V), decreased % DMA (V), or decreased ratios of % DMA to % MMA in urine is positively associated with various lesions32. Lesions include skin cancer and bladder cancer32. The results were obtained from inorganic arsenic exposed subjects32. Our concern involves the combination of low arsenic (As) and high selenium (Se) ingestion. This can inhibit methylation of arsenic to take it to a toxic level in the tissue.
 Dietary sources of Se and Hg.Global selenium (Se) source are vegetables in the diet. In the United States, meat and bread are the common source. Selenium deficiency in the US is rare. The US Food and Drug Administration (FDA) has found toxic levels of Se in dietary supplements, up to 200 times greater than the amount stated on the label33. The samples contained up to 40,800 ug Se per recommended serving.
 For the general population, the most important pathway of exposure to mercury (Hg) is ingestion of methyl mercury in foods. Fish (including tuna, a food commonly eaten by children), other seafood, and marine mammals contain the highest concentrations. The FDA has set a maximum permissible level of 1 ppm of methyl mercury in the seafood34. The people also exposed mercury via amalgams35.
 Proteomic study of Inorg-As (III) injury.Proteomics is a powerful tool developed to enhance the study of complex biological system36. This technique has been extensively employed to investigate the proteome response of cells to drugs and other diseases37, 38. A proteome analysis of the Na-As (III) response in cultured lung cells found in vitro oxidative stress-induced apoptosis39. However, to our knowledge, no in vivo proteomic study of Inorg-As (III) has yet been conducted to improve our understanding of the cellular proteome response to Inorg-As (III) except our preliminary study 40.
 Preliminary Studies: Results and DiscussionThe existing data (Fig. 1) from our laboratory and others show the complex nature of Inorg-As metabolism. For many years, the major way to study, arsenic (As) metabolism was to measure InorgAs (V), Inorg-As (III), MMA (V), and DMA (V) in urine of people chronically exposed to As in their drinking water. Our investigations demonstrated for the first time that MMA (III) and DMA (III) are found in human urine9. Also we have identified MMA (III) and DMA (III) in the tissues of mice and hamsters exposed to sodium arsenate [Na-As (V)]10, 41.
 Influence of Se as well as Hg on the As methyltransferase.We have reported that Se (IV) as well as mercuric chloride (HgCl2) inhibited As (III) methyltransferase and MMA (III) methyltransferase in rabbit liver cytosol. Mercuric chloride was found to be a more potent inhibitor of MMA (III) methyltransferase than As (III) methyltransferase30. These results suggested that Se and Hg decreased arsenic methylation. The inhibitory effects of Se and Hg were concentration dependent30.
 Influence of Se and Hg in urine and blood on the percentage of urinary As metabolites.Our human studies indicated that the ratios of the concentrations of Se or Hg to As in urine and blood were positively correlated with % Inorg-As and % MMA (V). But it negatively correlated with % DMA (V) and the ratios of % DMA (V) to % MMA (V) in urine of both males and females (unpublished data) (Table 3). These results confirmed that the inhibitory effects of Se as well as Hg for the methylation of Inorg-As in humans were concentration dependent. We also found that the concentrations of Se and Hg were negatively correlated with % Inorg-As and % MMA (V). Conversely it correlated positively with % DMA (V) and the ratios of % DMA (V) to % MMA (V) in urine of both sexes (unpublished data). These correlations were not statistically significant when urinary concentrations of Se and Hg were adjusted for urinary creatinine (Table 3). Interactions of As, Se, Hg and its relationship with methylation of arsenic are summarized in Figure 2.
 Sex difference distribution of arsenic species in urine.Our results indicate that females have more methylation capacity of arsenic as compared to males. In our human studies (n= 191) in Mexico, we found that females (n= 98) had lower % MMA (p<0.001) and higher % DMA (p=0.006) when compared to males (n= 93) (Fig. 3). The means ratio of % MMA (V) to % Inorg-As and % DMA (V) to %MMA (V) were also lower (p<0.05) and higher (p<0.001), respectively in females compared to males.
 The protein expression profiles in the tissues of hamsters exposed to Na-As (III).In our preliminary studies40, hamsters were exposed to Na-As (III) (173 pg/ml as As) in their drinking water for 6 days and control hamsters were given only the water used to make the solutions for the experimental animals. After DIGE (Two-dimensional differential in gel electrophoresis) and analysis by the DeCyder software, several protein spots were found to be over-expressed (red spot) and several were under expressed (green spot) as compared to control (Figs. 4a-c). Three proteins (one was over-expressed and two were under-expressed) of each tissue (liver and urinary bladder) were identified by LC-MS/MS (liquid chromatography-tandem mass spectrometry).DIGE in combination with LC-MS/MS is a powerful tool that may help cancer investigators to understand the molecular mechanisms of ca

Natural attenuation of arsenic in the Tinto Santa Rosa acid stream (Iberian Pyritic Belt, SW Spain): The role of iron precipitates

Removal of inorganic arsenic from As-spiked Lake Washington water by sorption onto activated alumina, iron-oxide-coated sand (IOCS), and an ion exchange resin was studied in laboratory systems. The sorption results could be rationalized only by invoking unanticipated changes in arsenic speciation between As(III) and As(V) in the feed reservoir. The changes in As speciation were subsequently confirmed in independent experiments, and the factors controlling the changes were investigated. Oxidation and reduction reactions sometimes occurred in the same system sequentially over a period of days to weeks, and in other systems both oxidation and reduction seemed to be proceeding simultaneously. Because As(III) has a much lower affinity for the adsorbents than does As(V), arsenic breakthrough was much more rapid when the influent arsenic was in the reduced form. In some cases, the combination of speciation changes in the influent and differential sorption of As(III) and As(V) generated breakthrough curves for total arsenic in which the breakthrough increased and then declined for a period before increasing again. Reduction of As(V) to As(III) was facilitated by filtration of the sample and by incubating the sample in the light, and it was impeded by the addition of PO 4 to solution. Neither filtration nor exposure to light had a discernible effect on the rate of As(III) oxidation. As(III) oxidation was facilitated by autoclaving the sample either before or after the As(III) was added. Although arsenic speciation changes are reproducible and explainable when investigating the behaviour of a given water sample under well controlled laboratory conditions, they are not easily extrapolated to other laboratory systems or field conditions, because they result from chemical and microbially mediated reactions that are strongly interrelated and evolve with time.

The aim of this research was to evaluate the total content of heavy metals in selected and frequently ‎used vegetables. Ten samples of the most common diet vegetables (chard, celery, garden cress, ‎spinach, Egyptian leek, wild mint, spring onion, arum, mallow, sunflower) were collected from the ‎local markets in Erbil, in the Kurdistan region of Iraq. The heavy metals were extracted from ‎vegetable samples by microwave-assisted acid digestion utilizing nitric acid/hydrogen peroxide, ‎followed by inductively coupled plasma-mass spectrometry (ICP-MS) to determine the ‎concentrations of As, Cd, Cr, Cu and Pb in the samples. The concentration ranges of heavy metals ‎found for the ten types of edible vegetables investigated are as follows (in µg g-1): As, 0.198-0.436; ‎Cd, <LOD-0.396; Cr, 1.653-11.915; Cu, 3.956-17.782 and Pb, 0.244-1.323. The method was ‎validated for total As, Cd, Cr, Cu and Pb extraction by analysing a certified reference material of ‎GBW10015-spinach. From health point of view estimated daily intake and carcinogenic risks of ‎heavy metals were evaluated. Estimated daily intake values of these heavy metals were found to be ‎lower than the maximum tolerable daily intake. This study also shows that the carcinogenic risk for ‎As, Cd, Cr and Pb exceeded the acceptable level. The results of this work recommend that consumers should be more cautious about levels of heavy ‎metals (especially As, Cd, Cr and Pb) in the analysed vegetable samples in order to make sure the ‎vegetable safety for consumers in Erbil city- the Kurdistan region of Iraq.

Arsenic speciation and transport associated with the release of spent geothermal fluids in Mutnovsky field (Kamchatka, Russia)