Absence Of Arsenate Research Articles

Neither glutamate nor alanine but arginine sensitizes BY-2 cells to arsenate.

Arsenic is toxic for plants. Our previous results showed that the application of proline enhanced the sensitivity of tobacco BY-2 cells to arsenate. In order to clarify the enhancement mechanism, we investigated the effects of other amino acids on the arsenate-stressed BY-2 cells. Glutamate at up to 10 m m did not affect the cell growth in the absence or presence of arsenate. Arginine at up to 10 m m did not affect the growth in the absence of arsenate but arginine at 10 m m enhanced the inhibition of the cell growth by arsenate. Alanine at up to 10 m m did not affect the cell growth under nonstressed condition but alanine at 10 m m significantly improved the cell growth under arsenate stress. These results suggest that alanine mitigates arsenate stress in BY-2 cells and that arginine like proline enhances the sensitivity of BY-2 cells to arsenate.

Comparative response of interfacial water structure to pH variations and arsenate adsorption on corundum (0 1 2) and (0 0 1) surfaces

The atomic-scale structure of the interface between metal oxide minerals and aqueous solutions regulates chemical reactions that are fundamental to many geological and environmental processes. Reactions occur at surface sites having multiple possible coordination states, each of which display distinct pH-dependent charging behavior that affects interactions with adsorbates and interfacial water. Our previous work has shown that corundum (0 0 1) surfaces induce weak spatial ordering of interfacial water that varies little between pH 5 and 9 but is substantially altered by the adsorption of arsenate. The (0 0 1) surface is dominated by doubly coordinated functional groups that are largely uncharged over the pH range examined, but other surfaces have functional groups in different coordination states with distinct charging behavior. Corundum (0 1 2) surfaces are known to induce stronger interfacial water ordering than (0 0 1) surfaces, but how this is affected by pH changes or ion adsorption has not been investigated. In this study, surface X-ray scattering measurements have been used to determine the response of interfacial water structure on corundum (0 1 2) surfaces to pH variations and the surface coverage of adsorbed arsenate. Comparison of interfacial water properties on (0 1 2) surfaces to those observed for (0 0 1) surfaces is then used to systematically investigate how surface site coordination state affects ion adsorption mechanisms, interfacial water structure, and the feedback between them. In the absence of arsenate, interfacial water displays little variation in its arrangement near the (0 1 2) surface between pH 5 and 9. This general invariance is also observed for the (0 0 1) surface, suggesting that over the pH range of most natural waters, surface site protonation-deprotonation appears inadequate to induce extensive restructuring of interfacial water. The adsorption of arsenate weakly perturbs interfacial water structure near the (0 1 2) surface, with both the spatial arrangement and ordering of the adsorbed water sites independent of arsenate surface coverage, in contrast to the substantial restructuring of interfacial water near the (0 0 1) surface. This suggest that on surfaces with initially weak water ordering [i.e., corundum (0 0 1) surfaces], the addition of a charged adsorbate species may induce water restructuring, but on surfaces with initially strong water ordering [i.e., corundum (0 1 2) surfaces], water structure is generally unperturbed. As arsenate forms coexisting inner- and outer-sphere complexes on both surfaces, adsorption mechanisms do not appear to control the resulting restructuring of interfacial water. Instead, the different surface functional groups present on the (0 1 2) and (0 0 1) surfaces, with their distinct charging behaviors, likely drive the response of interfacial water to arsenate adsorption. These observations suggest that on surfaces where water restructures upon ion adsorption, the dielectric constant, capacitances, and electrostatic activity corrections will all differ from the pristine surface. In contrast, on surfaces where water structure is unaltered by ion adsorption these parameters will be unperturbed. This should be considered during future refinements of surface complexation models to better predict adsorption behavior at environmental interfaces.

Response of interfacial water to arsenate adsorption on corundum (0 0 1) surfaces: Effects of pH and adsorbate surface coverage

The interaction of water with metal oxide mineral surfaces affects chemical reactions that are important to many natural processes. Previous studies demonstrate that the various functional groups on mineral surfaces control ion adsorption as well as the ordering of interfacial water, but the effect of water structure on the reactivity and dynamics of interfacial reactions has not been systematically investigated. In this study, surface X-ray scattering measurements on corundum (0 0 1) surfaces with and without arsenate over a range of pH conditions have been used to determine the response of interfacial water structure to arsenate adsorption. In the absence of arsenate, the structure of interfacial water near this surface varies little over the pH range of 5–9, suggesting that surface charging from protonation-deprotonation under the conditions studied is inadequate to induce extensive restructuring. In contrast, interfacial water undergoes substantial restructuring upon arsenate adsorption, indicating that charged surface complexes substantially perturb the arrangement and order of interfacial water on this surface. The overall interfacial water structure also varies proportionally with arsenate surface coverage, with adsorbed water layers moving closer to the surface and the extended layering of interfacial water showing reduced positional disorder as arsenate surface coverage increases. Systematic variations in interfacial water properties with increasing arsenate concentration at each pH value are consistent with the coexistence of two distinct water structures (in the absence and presence of adsorbed arsenate) that vary in proportion with adsorbate surface coverage. These observations demonstrate that the adsorption of arsenate alters the structure of interfacial water near corundum (0 0 1) surfaces, possibly through the modification of the charge state of surface sites or by providing new sites to which water may hydrogen bond. Such adsorbate-induced restructuring of interfacial water may contribute to the energetics of chemical reactions at mineral-water interfaces.

Performance and mechanism of simultaneous removal of chromium and arsenate by Fe(II) from contaminated groundwater

The feasibility and the mechanisms of simultaneous removal of chromium and arsenate by Fe(II) were investigated. In the absence of arsenate, chromium removal by Fe(II) increased to the maximum with increasing pH from ∼4 to ∼7 and then decreased with further increase in pH. Chromium removal by Fe(II) was controlled by the rate of Cr(VI) reduction by Fe(II) and the solubility of Fe 0.75Cr 0.25(OH) 3 at pH ≤ 7, but by the extent of Cr(VI) reduction under alkaline conditions. The presence of arsenate resulted in a decrease in chromium removal by Fe(II) under neutral and alkaline conditions as a result of the depression in the magnitude of Cr(VI) reduction by Fe(II) and sequestration of the Fe 0.75Cr 0.25(OH) 3 and FeOOH precipitation by HAsO 4 2 - . Arsenate removal by Fe(II) alone was trivial but was improved significantly at pH 4–9 due to the presence of 10–30 μmol L −1 chromate. It was the oxidative property of chromate that resulted in the oxidization of Fe(II) to Fe(III) concomitantly facilitating the removal of arsenate. Arsenate was removed by both adsorption and co-precipitation with Fe 0.75Cr 0.25(OH) 3 and FeOOH precipitates. EXAFS results revealed that arsenate mainly coordinated with Fe(III) rather than Cr(III) and arsenate formed bidentate-binuclear complexes with FeO(OH) octahydra as evidenced by an average Fe–As(V) bond distance of 3.25–3.26 Å.

Simultaneous removal of chromium and arsenate from contaminated groundwater by ferrous sulfate: Batch uptake behavior

Chromium and/or arsenate removal by Fe(II) as a function of pH, Fe(II) dosage and initial Cr(VI)/As(V) ratio were examined in batch tests. The presence of arsenate reduced the removal efficiency of chromium by Fe(II), while the presence of chromate significantly increased the removal efficiency of arsenate by Fe(II) at pH 6–8. In the absence of arsenate, chromium removal by Fe(II) increased to a maximum with increasing pH from 4 to 7 and then decreased with a further increase in pH. The increment in Fe(II) dosage resulted in an improvement in chromium removal and the improvement was more remarkable under alkaline conditions than that under acidic conditions. Chromium removal by Fe(II) was reduced to a larger extent under neutral and alkaline conditions than that under acidic conditions due to the presence of 10 μmol/L arsenate. The presence of 20 μmol/L arsenate slightly improved chromium removal by Fe(II) at pH 3.9–5.8, but had detrimental effects at pH 6.7–9.8. Arsenate removal was improved significantly at pH 4–9 due to the presence of 10 μmol/L chromate at Fe(II) dosages of 20–60 μmol/L. Elevating the chromate concentration from 10 to 20 μmol/L resulted in a further improvement in arsenate removal at pH 4.0–4.6 when Fe(II) was dosed at 30–60 μmol/L.

Mutational and gene expression analysis of mtrDEF, omcA and mtrCAB during arsenate and iron reduction in Shewanella sp. ANA‐3

Arsenate respiration and Fe(III) reduction are important processes that influence the fate and transport of arsenic in the environment. The goal of this study was to investigate the impact of arsenate on Fe(III) reduction using arsenate and Fe(III) reduction deficient mutants of Shewanella sp. strain ANA-3. Ferrihydrite reduction in the absence of arsenate was similar for an arsenate reduction mutant (arrA and arsC deletion strain of ANA-3) compared with wild-type ANA-3. However, the presence of arsenate adsorbed onto ferrihydrite impeded Fe(III) reduction for the arsenate reduction mutant but not in the wild-type. In an Fe(III) reduction mutant (mtrDEF, omcA, mtrCAB null mutant of ANA-3), arsenate was reduced similarly to wild-type ANA-3 indicating the Fe(III) reduction pathway is not required for ferrihydrite-associated arsenate reduction. Expression analysis of the mtr/omc gene cluster of ANA-3 showed that omcA and mtrCAB were expressed under soluble Fe(III), ferrihydrite and arsenate growth conditions and not in aerobically grown cells. Expression of arrA was greater with ferrihydrite pre-adsorbed with arsenate relative to ferrihydrite only. Lastly, arrA and mtrA were simultaneously induced in cells shifted to anaerobic conditions and exposed to soluble Fe(III) and arsenate. These observations suggest that, unlike Fe(III), arsenate can co-induce operons (arr and mtr) implicated in arsenic mobilization.

Influence of arsenate species on the formation of Fe(III) oxyhydroxides and Fe(II–III) hydroxychloride

Iron(III) oxyhydroxides were prepared by oxidation of aerated aqueous suspensions of Fe(II) hydroxide. The effects of arsenate species on their formation were studied by mixing FeCl 2·4H 2O, NaOH and Na 2HAsO 4 solutions. The intermediate and final products of the oxidation processes were characterised by X-ray diffraction, Infrared and Raman spectroscopy. Arsenate species were not reduced during the process but they influenced both oxidation stages, that is the formation of the intermediate Fe(II–III) compound and its subsequent oxidation into Fe(III) compounds. Arsenate species clearly inhibited the growth and hindered the crystallisation of GR(Cl −), the Fe(II–III) hydroxychloride that would have formed in the experimental conditions considered here. For the largest arsenate concentrations, the intermediate product was nanocrystalline and more likely consisted of clusters showing an ordering of atoms similar to that of GR(Cl −), isolated from each other by adsorbed arsenate species. The adsorption of As(V) prevented growth of these clusters into well-crystallised GR(Cl −). The arsenate species influenced similarly the second reaction stage by inhibiting the formation of well-ordered and crystallised Fe(III) compounds. Lepidocrocite, the final product in the absence of arsenate, was replaced by “6-line” ferrihydrite with increasing As(V) concentration, then “6-line” ferrihydrite was replaced by another poorly ordered compound, feroxyhite. These crystallised compounds were obtained together with an increasing part of nanocrystalline Fe(III) ox(yhydrox)ide(s).

Development of a novel biosensor for the detection of arsenic in drinking water

We sought to develop a whole-cell biosensor for the detection of arsenic in drinking water, a major problem in Bangladesh and West Bengal. In contrast to previously described systems, our biosensor would give a pH change as output, allowing simple detection with a pH electrode or pH indicator solution. We designed and modelled a system based on the arsenate-responsive promoter of the Escherichia coli arsenic detoxification system, using urease to increase pH in the absence of arsenate, and β-galactosidase (LacZ) to decrease pH in the presence of arsenate. The pH-reducing β-galactosidase part of the system was constructed and tested, and was found to give a clear response to arsenate concentrations as low as 5 ppb arsenic, well below the World Health Organisation (WHO) recommended limit of 10 ppb.

Coprecipitation of Arsenate with Metal Oxides: Nature, Mineralogy, and Reactivity of Aluminum Precipitates

Arsenic mobilization in soils is mainly controlled by sorption/desorption processes, but arsenic also may be coprecipitated with aluminum and/or iron in natural environments. Although coprecipitation of arsenic with aluminum and iron oxides is an effective treatment process for arsenic removal from drinking water, the nature and reactivity of aluminum- or iron-arsenic coprecipitates has received little attention. We studied the mineralogy, chemical composition, and surface properties of aluminum-arsenate coprecipitates, as well as the sorption of phosphate on and the loss of arsenate from these precipitates. Aluminum-arsenate coprecipitates were synthesized at pH 4.0, 7.0, or 10.0 and As/Al molar ratio (R) of 0, 0.01, or 0.1 and were aged 30 or 210 d at 50 degrees C. In the absence of arsenate, gibbsite (pH 4.0 or 7.0) and bayerite (pH 10.0) formed, whereas in the presence of arsenate, very poorly crystalline precipitates formed. Short-range ordered materials (mainly poorly crystalline boehmite) formed at pH 4.0 (R = 0.01 and 0.1), 7.0, and 10.0 (R= 0.1) and did not transform into Al(OH)3 polymorphs even after prolonged aging. The surface properties and chemical composition of the aluminum precipitates were affected by the initial pH, R, and aging. Chemical dissolution of the samples by 6 mol L(-1) HCl and 0.2 mol L(-1) oxalic acid/ oxalate solution indicated that arsenate was present mainly in the short-range ordered precipitates. The sorption of phosphate onto the precipitates was influenced by the nature of the samples and the amounts of arsenate present in the precipitates. Large amounts of phosphate partially replaced arsenate only from the samples formed at R = 0.1. The quantities of arsenate desorbed from these coprecipitates by phosphate increased with increasing phosphate concentration, reaction time, and precipitate age butwere always lessthan 30% of the amounts of arsenate present in the materials and were particularly low (<4%) from the sample prepared at pH 4.0. Arsenate appeared to be occluded within the network of short-range ordered materials and/or sorbed onto the external surfaces of the precipitates, but sorption on the external surfaces seemed to increase by increasing pH of sample preparation and aging. Furthermore, at pH 4.0 more than in neutral or alkaline systems the formation of aluminum arsenate precipitates seemed to be favored. Finally, we have observed that greater amounts of phosphate were sorbed on an aluminum-arsenate coprecipitate than on a preformed aluminum oxide equilibrated with arsenate under the same conditions (R = 0.1, pH 7.0). In contrast, the opposite occurred for arsenate desorption, which was attributed to the larger amounts of arsenate occluded in the coprecipitate.

High Intracellular Phosphorus Contents Exhibit a Correlation with Arsenate Resistance in Chlamydomonas Mutants

Pi in the medium relieved the toxicity of arsenate against cellular growth of Chlamydomonas reinhardtii. To investigate the relationship between intracellular P contents and arsenate resistance, we determined the intracellular P contents of arsenate-sensitive and arsenate-resistant mutants, which had been generated by random insertional mutagenesis. All 13 arsenate-resistant mutants showed higher P contents than the parent strain, while arsenate-sensitive mutants with high P contents were not found. In one of the arsenate-resistant mutants, AR3, the intracellular P content was about twice that in the wild type during growth in the absence of arsenate. Arsenate incorporation in AR3 was suppressed within 10 min after the addition of 1 mM arsenate, while Pi incorporation continued even after arsenate uptake ceased. Whereas the P content of the wild type decreased to half in the presence of 0.5 mM arsenate, almost the same degree (about 50%) of decrease was observed in AR3 cells grown in the presence of as much as 3 mM arsenate. AR3, in which PTB1, a homolog of a Pi transporter gene, had been disrupted, exhibited a higher activity of a high-affinity Pi transporter, suggesting that it may be due to a compensatory transport activity. These data suggest that the intracellular level of P is one of the important factors of arsenate resistance.

Soluble Reactive Phosphorus Measurements in Lake Water: Evidence for Molybdate‐enhanced Hydrolysis

AbstractMeasurements of ortho‐phosphate (PO4‐P) concentrations in natural waters obtained by molybdenum blue methodology are based on the assumption that molybdate (Mo) complexes only with PO4‐P (in the absence of arsenate and silicate interference) to form the reductant‐sensitive complex 12‐molybdophosphoric acid (12‐MPA). The hypothesis that Mo causes or accelerates PO4‐P release from bound sources or forms reductant‐sensitive complexes with organic compounds (organic‐PO4‐Mo) was tested by exposing filtered Lake Michigan water to acid, acid plus Mo simultaneously, and Mo, respectively, before measuring PO4‐P by the Chamberlain‐Shapiro extraction method. Not only does Mo accelerate hydrolysis in the presence of acid, but it also either causes hydrolysis or forms organic‐PO4‐Mo before samples are acidified. Although the relative amounts of 12‐MPA and organic‐PO4‐Mo formed in lake water cannot be assessed quantitatively, a major fraction of the blue color formed during routine analysis by this or by similar methods apparently is not the product of acid hydrolysis, but may also be due to reduction of organic‐PO4‐Mo. Future developments in molybdenum blue methodology aimed at minimizing hydrolysis must consider the sequence of reagent additions and Mo, as well as acid, contact times.

A Study of the Reaction of Glyceraldehyde with Glyceraldehyde 3-Phosphate Dehydrogenase

Abstract Kinetic data indicate that rabbit muscle glyceraldehyde 3-phosphate dehydrogenase possesses two types of diphosphopyridine nucleotide-binding sites. The actual chemical reaction (oxidation-reduction) occurs at tight sites. Binding of DPN to loose sites activates the reaction at the tight sites. These results are consistent with the concept that the enzyme is double headed, with oxidation-reduction heads (which contain the tight sites) and transfer heads (which contain the loose sites). Light scattering data are consistent with the concept that (in the absence of arsenate or phosphate) the enzyme can exist as a dimer (with an apparent weight average molecular weight of 2.8 x 105) which can dissociate upon dilution into monomers (with molecular weights of 1.4 x 105). Dimerization is inhibited by high concentrations of arsenate (0.05 m). This inhibition is overcome by adding DPN to the loose sites. The monomer is unreactive in the absence of arsenate and is markedly activated by adding DPN to the loose sites. The dimer is reactive in the absence of arsenate and is only slightly activated by adding DPN to the loose sites. A kinetic mechanism is proposed which agrees with these and most other observations. This mechanism is sequential with the addition of DPN and then the aldehyde to the oxidation-reduction head. This is followed by the addition of arsenate and then DPN to the transfer head. The mech anism of the reaction with glyceraldehyde and glyceraldehyde 3-phosphate seems to be the same.

FAILURE OF ARSENATE TO UNCOUPLE THE PHOSPHOTRANSACETYLASE SYSTEM IN CLOSTRIDIUM ACIDIURICI.

Sagers, Richard D. (Brigham Young University, Provo, Utah), Moshe Benziman, and Sigrid M. Klein. Failure of arsenate to uncouple the phosphotransacetylase system in Clostridium acidiurici. J. Bacteriol. 86:978-984. 1963.-The conversion of pyruvate to acetyl phosphate by extracts of Clostridium acidiurici required coenzyme A (CoA), an electron-carrier system (ferredoxin and nicotinamide adenine dinucleotide), and a divalent metal. Other cofactors may be involved but are not presently defined. The metal activates the system transferring acetyl units between CoA and phosphate. Acetyl CoA could be generated from pyruvate or from acetyl phosphate, but in both cases arsenate failed to uncouple the high-energy acyl compounds. The rate of acetyl transfer between acetyl phosphate and CoA was markedly decreased by 0.005 m arsenate, but the generation of acetyl CoA from pyruvate was essentially unaffected until the arsenate concentration exceeded 0.02 m. Close agreement was observed between the amount of pyruvate utilized and the amount of acetyl phosphate formed, both in the presence and absence of arsenate. The CoA-dependent exchange of P(32)O(4) ( identical with) with acetyl phosphate proceeded at a rate approximately one-eightieth of the rate of acetylation of CoA, indicating an equilibrium value for the phosphotransacetylase reaction similar to that observed for Clostridium kluyveri. The failure of arsenate to uncouple the C. acidiurici enzyme may indicate a high degree of specificity in relation to the acetyl unit acceptor, giving preference to phosphate over arsenate.

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