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PubMed2001

Solar oxidation and removal of arsenic at circumneutral pH in iron containing waters.

An estimated 30-50 million people in Bangladesh consume groundwater with arsenic contents far above accepted limits. A better understanding of arsenic redox kinetics and simple water treatment procedures are urgently needed. We have studied thermal and photochemical As(III) oxidation in the laboratory, on a time scale of hours, in water containing 500 micrograms/L As(III), 0.06-5 mg/L Fe(II,III), and 4-6 mM bicarbonate at pH 6.5-8.0. As(V) was measured colorimetrically, and As(III) and As(tot) were measured by As(III)/As(tot)-specific hydride-generation AAS. Dissolved oxygen and micromolar hydrogen peroxide did not oxidize As(III) on a time scale of hours. As(III) was partly oxidized in the dark by addition of Fe(II) to aerated water, presumably by reactive intermediates formed in the reduction of oxygen by Fe(II). In solutions containing 0.06-5 mg/L Fe(II,III), over 90% of As(III) could be oxidized photochemically within 2-3 h by illumination with 90 W/m2 UV-A light. Citrate, by forming Fe(III) citrate complexes that are photolyzed with high quantum yields, strongly accelerated As(III) oxidation. The photoproduct of citrate (3-oxoglutaric acid) induced rapid flocculation and precipitation of Fe(III). In laboratory tests, 80-90% of total arsenic was removed after addition of 50 microM citrate or 100-200 microL (4-8 drops) of lemon juice/L, illumination for 2-3 h, and precipitation. The same procedure was able to remove 45-78% of total arsenic in first field trials in Bangladesh.

Arsenic↗
PubMed2003

Immobilization mechanisms of arsenate in iron hydroxide sludge stabilized with cement.

Leaching tests, Fourier transform infrared spectroscopy (FTIR), extended X-ray absorption fine structure (EXAFS) spectroscopy, and thermodynamic modeling were performed to investigate arsenate [As(V)] immobilization mechanisms in iron hydroxide sludge stabilized with cement. The sludge from a groundwater remediation site in Tacoma, WA was mixed and immobilized with premixed cement to reach cement-to-sludge ratios of 2.5, 3.3, 5, 10, and 20 (wt premixed cement/wt dry sludge). The EXAFS analysis determined that As(V) formed bidentate mononuclear complexes on the iron hydroxide surface in the sludge. The adsorbed As(V) had a characteristic FTIR band at 830 cm(-1). Cement treatment converted the adsorbed As(V) to calcium arsenate precipitate with a FTIR peak at 860 cm(-1). The chemical forms of the As(V) were incorporated in an adsorption triple layer model (TLM) to describe the leaching behavior of As(V) in a pH range between 3 and 12. Cement treatment significantly reduced arsenic mobility because of the formation of the sparingly soluble calcium arsenate.

Arsenates↗
PubMed2004

TCLP underestimates leaching of arsenic from solid residuals under landfill conditions.

Recent revision of the arsenic in drinking water standard will cause many utilities to implement removal technologies. Most of the affected utilities are expected to use adsorption onto solid media for arsenic removal. The arsenic-bearing solid residuals (ABSR) from adsorption processes are to be disposed of in nonhazardous landfills. The Toxicity Characteristic Leaching Procedure (TCLP) tests whether a waste is hazardous or nonhazardous; most solid residuals pass the TCLP. However, the TCLP poorly simulates the alkaline pH, low redox potential, biological activity, long retention time, and organic composition of mature landfills. These same conditions are likely to favor mobilization of arsenic from metal oxide sorbents. This study quantifies leaching of arsenic from Activated Alumina (AA) and Granular Ferric Hydroxide (GFH), two sorbents expected to be widely used for arsenic removal. The sorbents were subjected to the TCLP, the Waste Extraction Test (WET), an actual landfill leachate, and two synthetic leachate solutions. Up to tenfold greater arsenic concentration is extracted by an actual landfill leachate than by the TCLP. Equilibrium leachate concentrations are not achieved within 18 h (the TCLP duration) and an N2 headspace and end-over-end tumbling increase the rate of arsenic mobilization. However, tests with actual landfill leachate indicate the WET may also underestimate arsenic mobilization in landfills.

Adsorption↗
PubMed2005

XAS study of iron and arsenic speciation during Fe(II) oxidation in the presence of As(III).

The speciation of As and Fe was studied during the oxidation of Fe(II)-As(III) solutions by combining XAS analysis at both the Fe and As K-edges. Fe(II) and As(III) were first hydrolyzed to pH 7 under anoxic conditions; the precipitate was then allowed to oxidize in ambient air for 33 h under vigorous stirring. EXAFS analysis at the As K-edge shows clear evidence of formation of inner-sphere complexes between As(III) and Fe(II), i.e., before any oxidation. Inner-sphere complexes were also observed when Fe became sufficiently oxidized, in the form of edge-sharing and double-corner linkages between AsIIIO3 pyramids and FeIIIO6 octahedra. XAS analyses at the Fe K-edge reveal that the presence of As(III) in the solution limits the polymerization of Fe(II) and the formation of green rust and inhibits the formation of goethite and lepidocrocite. Indeed, As(III) accelerates the Fe(II) oxidation kinetics and leads to the formation of nanosized Fe-As subunits of amorphous aggregates. These observations, rather than a presumed weaker affinity of As(III) for iron oxyhydroxides, might explain why As(III) is more difficult to remove than As(V) by aerating reducing groundwater.

Arsenic↗
PubMed2005

EXAFS analysis of arsenite adsorption onto two-line ferrihydrite, hematite, goethite, and lepidocrocite.

The modes of As(III) sorption onto two-line ferrihydrite (Fh), hematite (Hm), goethite (Gt), and lepidocrocite (Lp) have been investigated under anoxic condition using X-ray absorption spectroscopy (XAS). X-ray absorption near-edge structure spectroscopy (XANES) indicates that the absence of oxygen minimized As(III) oxidation due to Fenton reactions. Extended X-ray absorption fine structure spectroscopy (EXAFS) indicates thatAs(III)forms similar inner-sphere surface complexes on two-line ferrihydrite and hematite that differ from those formed on goethite and lepidocrocite. At high surface coverage, the dominant complex types on Fh and Hm are bidentate mononuclear edge-sharing (2E) and bidentate binuclear corner-sharing (2C), with As-Fe distances of 2.90 +/- 0.05 and 3.35 +/- 0.05 A, respectively. The same surface complexes are observed for ferrihydrite at low surface coverage. In contrast, As(III) forms dominantly bidentate binuclear corner-sharing (2C) sorption complexes on Gt and Lp [d(As-Fe) = 3.3-3.4 A], with a minor amount of monodentate mononuclear corner-sharing (1V) complexes [d(As-Fe) = 3.5-3.6 A]. Bidentate mononuclear edge-sharing (2E) complexes are virtually absent in Gt and Lp at the high surface coverages that were investigated in the present study. These results are compared with available literature data and discussed in terms of the reactivity of iron(III) (oxyhydr)oxide surface sites.

Adsorption↗
PubMed2002

A major human arsenic metabolite, dimethylarsinic acid, requires reduced glutathione to induce apoptosis.

Inorganic arsenicals are important environmental toxicants and carcinogens in humans. In mammals, including humans, inorganic arsenicals often undergo methylation, forming compounds such as dimethyarsinic acid (DMA). Recent evidence indicates DMA is a complete carcinogen in rodents while evidence for inorganic arsenicals as carcinogens in rodents remains equivocal. Thus, we studied the molecular mechanisms of in vitro cytolethality of DMA compared to that of the trivalent inorganic arsenical, sodium arsenite, using a rat liver epithelial cell line (TRL 1215). Arsenite was very cytotoxic in these cells (LC(50) = 35 microM after 48 h of exposure). With arsenite exposure, most dead cells showed histological and biochemical evidence of necrosis. Arsenite cytotoxicity increased markedly when cellular GSH was depleted with the glutathione synthase inhibitor, L-buthionine-[S,R]-sulfoximine (BSO). In contrast, DMA was nearly 3 orders of magnitude less cytotoxic (LC(50) = 1.5 mM) although evidence showed the predominating form of death was apoptosis. Surprisingly, GSH depletion actually decreased DMA-induced apoptosis. A glutathione scavenger, diethyl maleate (DEM), and a glutathione reductase inhibitor, carmustine, also prevented DMA-induced apoptosis. These data indicate that DMA requires intracellular GSH to induce apoptosis. Ethacrynic acid (EA), an inhibitor of glutathione S-transferase (GST) that catalyzes GSH-substrate conjugation, acivicin, an inhibitor of gamma-glutamyltranspeptidase (GGT) which catalyzes the initial breakdown of GSH-substrate conjugates, and aminooxyacetic acid (AOAA), an inhibitor of beta-lyase which catalyzes the final breakdown of GSH-substrate conjugates, all were effective in suppressing DMA-induced apoptosis. These findings indicate that DMA likely is conjugated in some form with GSH, and that it is this conjugate that induces apoptosis during subsequent metabolic reactions.

Animals↗
PubMed1968

The dependence on dicarboxylic acids and energy of citrate accumulation in depleted rat liver mitochondria.

The accumulation of some organic anions in the space inaccessible to sucrose of rat liver mitochondria was measured. In untreated mitochondria anions were apparently concentrated from 1mm applied concentration by between five- and 22-fold, depending on their charge. After depletion of endogenous reserves either with uncoupling agent or with oligomycin uptakes were decreased. The accumulation of citrate was restored by combinations of a dicarboxylic acid (malate, succinate, maleate or meso-tartrate) and energy. The energy could either be provided by oxidation of a suitable dicarboxylic acid or from ascorbate in the presence of tetramethylphenylenediamine, or from ATP. The restoration of citrate uptake is not necessarily accompanied by a gain of K(+), but a cation- and energy-linked citrate uptake can be induced with valinomycin. When citrate is added to mitochondria in the presence of malate the latter is competitively displaced. The anion accumulation could arise from an internal energy-linked positive potential.

Adenosine Triphosphate↗
PubMed1971

Differences between the conformation of arsanilazotyrosine 248 of carboxypeptidase A in the crystalline state and in solution.

Coupling of carboxypeptidase A crystals with diazotized arsanilic acid specifically labels tyrosine 248, an active-site residue of the enzyme. Many azophenols are yellow and their zinc complexes are red; the "yellow" absorption spectrum of zinc arsanilazocarboxypeptidase crystals is characteristic of the arsanilazotyrosyl group, not of the zinc complex. This is consistent with the interpretation of x-ray data on native crystals of carboxypeptidase A, indicating that tyrosine 248 and the zinc atom are too far apart to form a complex. However, the enzyme in solution is red, denoting the formation of a complex between zinc and arsanilazotyrosine 248. The most likely interpretation of the data is that the orientation of arsanilazotyrosine 248 in solution and in the crystal is different. If the unlabeled tyrosine 248 of native carboxypeptidase undergoes similar changes, these data may bear upon the low activity of the enzyme in the crystalline state and on the catalytic mechanism of the enzyme based on the crystal structure. The opportunities for analogous spectrochemical studies of other, similar systems are pointed out.

Arsenicals↗
PubMed1972

Similarities between the conformation of arsanilazotyrosine 248 of carboxypeptidase A in the crystalline state and in solution.

Modification of carboxypeptidase A(gamma) crystals (Anson) with diazotized arsanilic acid specifically labels tyrosine 248; at pH 8.2 the modified enzyme gives yellow crystals, but a red solution. It has been suggested that arsanilazotyrosine 248 forms a complex with the Zn cofactor accounting for the red color in solution, but that a complex is not formed in the crystal. However, the crystal structure of carboxypeptidase A(gamma) is unknown. We show here that crystals of carboxypeptidase A(alpha), whose crystal structure has been determined, are red both in solution and in the crystalline state (at pH 8.2) after modification with diazotized arsanilic acid. These new data are of importance in relating the structure in the crystalline state to the catalytic mechanisms, as based on the x-ray diffraction evidence. The activity of carboxypeptidase A in the crystal and in solution has a ratio of only 1/3 for the alpha form, in contrast to the ratio of 1/300 for the gamma form, with carbobenzoxyglycyl-L-phenylalanine as a substrate.A pH-jump experiment monitored by stopped-flow kinetics in a split-beam apparatus has revealed a single exponential rate when a solution of arsanilazotyrosine 248 carboxypeptidase A(alpha) at pH 6.7 (yellow) is increased to pH 8.5 (red). The rate constants obtained in this experiment are 6.1 sec(-1) at 3.0 mg/ml and 7.2 sec(-1) at 1.6 mg/ml concentration of enzyme.

Animals↗
PubMed1973

Conformations of arsanilazotyrosine-248 carboxypeptidase A alpha, beta, gamma, comparison of crystals and solution.

The spectra of the alpha, beta, and gamma forms of zinc monoarsanilazotyrosine-248 carboxypeptidase A are indistinguishable. At pH 8.2 their crystals are yellow, while their solutions are red, lambda(max) 510 nm. Absorption and circular dichroism-pH titrations of the modified zinc and apoenzymes demonstrate that the absorption band at 510 nm is due to a complex between arsanilazotyrosine-248 and the active-site zinc atom. Two pK(app) values, 7.7 and 9.5, characterize the formation and dissociation of this arsanilazotyrosine-248.Zn complex. On titrations of the apoenzyme, the absorption band at 510 nm is completely absent at all pH values. Instead, there is a single pK(app), 9.4, due to the ionization of the azophenol, lambda(max) 485 nm. Substitution of other metals for zinc results in analogous intramolecular coordination complexes with absorption maxima and circular dichroism extrema characteristic of the particular metal. Similar data and conclusions have been derived from studies of heterocyclic azophenol.metal complexes. The present studies demonstrate that the conformation of the crystals of all generally available alpha, beta, and gamma forms of the arsanilazoenzyme differs from that of their solutions. The spectra of the modified x-ray crystals, however, differ from those of all other carboxypeptidase forms and crystal habits studied. The internal consistency of their data, their interpretation, and the conclusions of Lipscomb and coworkers [Proc. Nat. Acad. Sci. USA (1972) 69, 2850-2854] are examined. Dissimilar chemical modification or conformation is thought to underlie these differences. The arsanilazotyrosine-248.zinc complex is a sensitive, dynamic probe of environmental conditions. Its response to changes in pH and physical state of the enzyme suggest different orientation of the arsanilazotyrosine-248 side chain in solution from that in the crystal. This finding calls for reexamination of the basis of the substrate-induced conformation change which has been thought to be critical to the mechanism, postulated on the basis of the x-ray structure analysis performed at pH 7.5.

Arsenicals↗
PubMed1975

Intramolecular arsanilazotyrosine-248-Zn complex of carboxypeptidase A: a monitor of catalytic events.

The intensely chromophoric intramolecular coordination complex formed between arsanilazotyrosine-248 and the active site zinc atom of azocarboxypeptidase A (Johansen, J. T. & Vallee, B. L. (1971) Proc. Nat. Acad. Sci. USA 68, 2532-2535) is a spectrokinetic probe of catalytic events. The interconversion of the azoTyr-248-Zn complex and its constituents is measured by stopped-flow pH and temperature-jump methods. The rate of interconversion, 64,000 sec-1, is orders of magnitude faster than that of the catalytic step itself (about 0.01-100 sec-1). Rapidly turned over peptide and ester substrates disrupt the azoTyr-248-Zn complex before hydrolysis occurs. As a consequence, formation of azoTyr-248, substrate binding, and catalysis can all be monitored while catalysis is actually in progress. The results of these dynamic studies specify a course of catalytic events, different from those postulated based on x-ray structure analysis. If azoTyr-248 is displaced, the direction is opposite to the inward movement postulated on the basis of x-ray studies and is not unique to induction by substrates, since rapid changes in pH also result in analogous spectral changes. AzoTyr-248 carboxypeptidase has all the features which are essential for mechanistic studies: (1) It is enzymatically active; (2) the spectra of the metal complex differ characteristically from those of its constituents; (3) it responds dynamically to environmental factors; and (4) the response time of the probe itself is much more rapid than is required for the measurement of the catalytic step. These combined kinetic and spectral properties of the metal complex render it a powerful spectrokinetic probe to visualize and discern microscopic details of the catalytic process.

Arsenicals↗