Arsenic dissolved sulfide

In natural waters, arsenic may exist as one or more dissolved species, whose chemistry would depend on the chemistry of the waters. Over time, arsenic species dissolved in water may (1) interact with biological organisms and possibly methylate or demethylate (Chapter 4), (2) undergo abiotic or biotic oxidation, reduction, or other reactions, (3) sorb onto solids, often through ion exchange, (4) precipitate, or (5) coprecipitate. This section discusses the dissolution of solid arsenic compounds in water, the chemistry of dissolved arsenic species in aqueous solutions, and how the chemistry of the dissolved species varies with water chemistry and, in particular, pH, redox conditions, and the presence of dissolved sulfides. Discussions also include introductions to sorption, ion exchange, precipitation, and coprecipitation, which have important applications with arsenic in natural environments (Chapters 3 and 6) and water treatment technologies (Chapter 7). 

Thiosulfuric acid is representative of a general class of acids, called thio acids or sulfo acids, in vhich one or more oxygen atoms of an oxygen add are replaced by sulfur atoms. For example, arsenic penta-sulfide dissolves in a sodium sulfide solution to form the thioarsenate ion, AsS -----, completely analogous to the arsenate ion, AsO ------- ... 

Under either reducing or oxidizing conditions, the solubilization of arsenic from sulfide phases can be subject to kinetic limitations. Mass transfer constraints, particularly in porous media, can result in localized saturation conditions near the surface of the solid. For oxidative dissolution, depletion of dissolved oxygen may limit dissolution kinetics. Microorganisms may also play a role in catalyzing such oxidative dissolution as has been demonstrated for pyrite oxidation (88) and thus dissolution rates may reflect the level of microbial activity (which may be subject, for example, to nutrient limitation). Thus, although equilibrium calculations indicate solubility constraints on dissolved arsenic concentrations, actual concentrations may be lower than the predicted equilibrium values due to slow dissolution kinetics or greater due to slow precipitation kinetics. 

EA Rochette, BC Bostick, GC Li, S Eendorf. Kinetics of arsenate reduction by dissolved sulfide. Environ Sci Technol 34 4714-4720, 2000. 

Newman et al. (56), and Rochette et al. (68) suggest that the reduction of arsenate by dissolved sulfide is very slow at circumneutral pH values. However, at pH values less than 5, the reduction rates of arsenate due to sulfide may be significant in natural systems, where half-lives as short as 21 hr have been reported (68) for this abiotic pathway (Table 3). Rochette et al. (68) also revealed the potential importance of intermediate As-O-S species in electron transfer reactions between sulfide and arsenate, such as H2 As OsS H2As 02S , and H2 As OS2. It is not known whether these chemical species may also serve as important redox active species for microbial metabolism. These authors have also compared the rates of As(V) reduction in the presence of sulfide versus those rates expected via dissimilatory reduction by an arsenate-respiring organism (strain SES-3) (54) and for those measured in lake sediments (69) at pH values less than 5, reduction rates due to dissolved sulfide can become more significant than reduction rates due to anaerobic respiration where As(V) is used as the terminal electron acceptor (Fig. 8). 

Table 3 Time Scales of Abiotic Reduction Rates of Arsenate to Arsenite in the Presence of Dissolved Sulfide, at pH Values Ranging from 4-7...

Thermodynamic calculations can be used to show the likelihood for precipitation of arsenic sulfides and formation of arsenic thiocomplexes in the anaerobic sludge environment of Facility K. Substituting average concentrations of As(lll) (4.5 X 10 M) and dissolved sulfide (2.3 X 10 " M) in sludge water into the relationships above gives the logarithm of the ion concentration product (loglCP) of 48.5 for Eq. (10) and 20.2 for Eq. (11). The ICP values... 

Arsenic(III) sulfide also dissolves in OH" and COs ", forming arsenites and thio-arsenites. The thioarsenites react with acids, forming AS2S3 ... 

Metals less noble than copper, such as iron, nickel, and lead, dissolve from the anode. The lead precipitates as lead sulfate in the slimes. Other impurities such as arsenic, antimony, and bismuth remain partiy as insoluble compounds in the slimes and partiy as soluble complexes in the electrolyte. Precious metals, such as gold and silver, remain as metals in the anode slimes. The bulk of the slimes consist of particles of copper falling from the anode, and insoluble sulfides, selenides, or teUurides. These slimes are processed further for the recovery of the various constituents. Metals less noble than copper do not deposit but accumulate in solution. This requires periodic purification of the electrolyte to remove nickel sulfate, arsenic, and other impurities. 

Reactions of the various sulfides of arsenic call for little further comment. AS2S3 bums when heated in air to give AS2O3 and SO2. Chlorine converts it to ASCI3 and S2CI2. It is insoluble in water but dissolves readily in aqueous alkali or alkali-metal sulfide solutions to give thioarsenites ... 

Precipitation is the most promising method for immobilizing dissolvable metals such as lead, cadmium, zinc, and iron.15 Some forms of arsenic, chromium, mercury, and some fatty acids can also be treated by precipitation.47 The common precipitating chemicals for metal cations are sulfide, phosphate, hydroxide, or carbonate. Among them, sulfide is the most promising, because sulfides have low solubility over a broad pH range. Precipitation is most applicable to sites with sand or coarse silt strata. 

The bomb contents are digested with concentrated hydrochloric acid, and material still undissolved is then digested with potassium hydroxide and hydrogen peroxide. A crude separation is made by a sulfide precipitation from the combined digestion solutions. The sulfides are dissolved in aqua regia, the solution is evaporated, and antimony in the residue is reduced to antimony (III) with hydroxylamine hydrochloride. The sample, in ammonium thiocyanate-hydrochloric acid medium, is loaded onto a Dowex 2 column (SCN" form). Arsenic and other impurities are eluted with aliquots of more dilute ammonium thiocyanate-hydrochloric acid solutions. Antimony is eluated with sulfuric acid and fixed in solution by addition of hydrochloric acid. The activity of the solution caused by the 0.56 MeV y-ray of 2.8-day 122Sb is counted. 

Direct fusion of the elements yields a number of arsenic sulfides, including AsuS3, AS4S4, As2S3, and As2S , the last two being obtained also by precipitation from arsenic (III) and arsenic (V) solutions, respectively. The disulfide dissolves in alkali sulfide solutions to form diio-arsenites ... 

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