Arsenic redox reactions

The course of the reaction has not been elucidated. Probably redox reactions involving cerium(IV) and arsenic(III) are catalyzed by iodide ions and organic iodine compounds with methylene blue acting as a redox indicator. 

Redox reactions may cause mobile toxic ions to become either immobile or less toxic. Hexavalent chromium is mobile and highly toxic. It can be reduced to be rendered less toxic in the form of trivalent chromium sulfide by the addition of ferrous sulfate. Similarly, pentavalent (V) or trivalent (III) arsenic, arsenate or arsenite are more toxic and soluble forms. Arsenite (III) can be oxidized to As(IV). Arsenate (V) can be transformed to highly insoluble FeAs04 by the addition of ferrous sulfate. 

Following consumption of dissolved O2, the thermodynamically favored electron acceptor is nitrate (N03-). Nitrate reduction can be coupled to anaerobic oxidation of metal sulfides (Appelo and Postma, 1999), which may include arsenic-rich phases. The release of sorbed arsenic may also be coupled to the reduction of Mn(IV) (oxy)(hydr)oxides, such as birnessite CS-MnCb) (Scott and Morgan, 1995). The electrostatic bond between the sorbed arsenic and the host mineral is dramatically weakened by an overall decrease of net positive charge so that surface-complexed arsenic could dissolve. However, arsenic liberated by these redox reactions may reprecipitate as a mixed As(III)-Mn(II) solid phase (Toumassat et al., 2002) or resorb as surface complexes by iron (oxy)(hydr)oxides (McArthur et al., 2004). The most widespread arsenic occurrence in natural waters probably results from reduction of iron (oxy)(hydr)oxides under anoxic conditions, which are commonly associated with rapid sediment accumulation and burial (Smedley and Kinniburgh, 2002). In anoxic alluvial aquifers, iron is commonly the dominant redox-sensitive solute with concentrations as high as 30 mg L-1 (Smedley and Kinniburgh, 2002). However, the reduction of As(V) to As(III) may lag behind Fe(III) reduction (Islam et al., 2004). 

Since rates of arsenic redox reactions are slow at room temperature (5), it is assumed that the oxidation state data represent adjustment of arsenic species to the electron activity of the solution at 300°C. A quantitative assessment of the Eh of the basalt-water system at 300°C requires high-temperature thermochemical data for aqueous arsenic species. Such data are not available and, therefore, approximations were used to calculate Eh at 300°C. 

Redox reactions in soils are affected by a number of parameters, including temperature, pH (see Chapter 7), and microbes. Microbes catalyze many redox reactions in soils and use a variety of compounds as electron acceptors or electron donors. For example, aerobic heterotrophic soil bacteria may metabolize readily available organic carbon using NO3, NOj, N20, Mn-oxides, Fe-oxides and compounds such as arsenate (As04 ) and selenate (Se04 ) as electron acceptors. Similarly, microbes may use reduced compounds or ions as electron donors, for example, NH4, Mn2+, Fe2+, arsenite (AsCXj), and selenite (SeO ). 

Iron(III)-arsenate compounds are stable under oxidizing conditions (Fig. 12.15). Assuming that redox conditions in the stratum become reductive, iron(III) converts to iron(II) and arsenate becomes arsenite (AsO3- or AsO ). As conditions reduce further, arsenic solubility is regulated by sulfides and pH (arsenic MCL is set at 0.05 mg L 1, arsenosulfides exhibit a solubility near 1 mg L 1). Arsenic redox reactions can be carried... 

Arsenic pentachloride was an elusive compound until it was finally obtained by the ultraviolet irradiation of an ASCI3/CI2 mixture at -105 °C. Above —50°C, a redox reaction occurs affording AsCb and molecular chlorine (see equation 14 above). The identity of AsCls was confirmed by elemental analysis and by comparison of its Raman spectrum with those of liquid PCI5 and SbCls. ... 

Arsenic can also be released indirectly as a result of other microbially induced redox reactions. For example, the dissimilatory iron-reducing bacterium Shewanella alga (strain BrY) reduces Fe(III) to Fe(II) in FeAs04-2H20,... 

Vagliasindi E. G. A. and Benjamin M. M. (2001) Redox reactions of arsenic in As-spiked lake water and their effects on As adsorption. J. Water Supply Res. Technology-Aqua 50, 173-186. 

The method is applicable for triorganoarsenic compounds bearing aryl or vinyl substituents (equations 76 , 77 and 78 ), while for the preparation of alkylarsenic halides this method has not been useful since only adduct formation and redox reactions have been reported between R3AS and AsXj (equations 79a and b) . Reaction of tris(trifluoromethyl) arsine with arsenic iodide, however, has been reported to afford a mixture of halides (equation 80) . In the reaction of unsymmetrically substituted compounds such as 4, cleavage of the aryl-arsenic bond is preferred to that of the alkyl-arsenic bond (equation 78 ). The cleavage of 4 by this method is in contrast to that of 3 in thermolysis (equation 75). 

Decomposes when heated above melting point, 536°F/280°C, producing toxic fumes of arsenic, lead. Lead arsenates may be subject to redox reactions. Both arsenic and lead are known human carcinogens. PLUMBOUS ACETATE (6080-56-4) Pb(CjH302)2 3H,0 Contact with acids forms acetic acid. Incompatible with oxidizers, bases, acetic acid alkalis, alkylene oxides, ammonia, amines, bromates, carbonates, citrates, chlorides, chloral hydrate cresols, epichlorohydrin, hydrozoic acid, isocyanates, methyl isocyanoacetate, phenols, phosphates, salicylic acid sodium salicylate, sodium peroxyborate, potassium bromate resorcinol, salicylic acid, strong oxidizers, sulfates, sulfites, tannin, tartrates, tinctures trinitrobenzoic acid, urea nitrate. On small fires, use dry chemical, Halon, or CO2 extinguishers. 

On the other hand, fluorination of alkyl- and aryldialkylaminochloro-phosphines, RPC1(NR2), with antimony (or arsenic) trifluoride under similar conditions to reaction (2) affords only the pentavalent fluoro-phosphorane RPFg(NR2) 288, 290) via the same type of redox reaction discussed previously in Section III,A for alkyl- and arylhalophosphines. 

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