Arsenic microbial pathways

Lewisite in soil may rapidly volatilize or may be converted to lewisite oxide due to moisture in the soil (Rosenblatt et al, 1975). The low water solubility suggests intermediate persistence in moist soil (Watson and Griffin, 1992). Both lewisite and lewisite oxide may be slowly oxidized to 2-chlorovinylarsonic acid (Rosenblatt et al, 1975). Possible pathways of microbial degradation in soil include epoxidation of the C=C bond and reductive deha-logenation and dehydrohalogenation (Morrill et al, 1985). Due to the epoxy bond and arsine group, toxic metabolites may result. Additionally, residual hydrolysis may result in arsenic compounds. Lewisite is not likely to bioaccumulate. However, the arsenic degradation products may bioaccumulate (Rosenblatt et al, 1975). 

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). 

The formation of arsines, a volatile form of arsenic, is known to occur via microbial activity (27). Soil microbes produce volatile arsenicals by a reductive methyl-ation pathway from inorganic and methylated forms of arsenic (29). Methylation in which a methyl ion is added to an arsenite ion is strictly a biological process... 

Volatilisation is the microbial biochemical process that methylates heavy metals. Several methyl-metal complexes have significant vapour pressures at room temperature. In this way metals methylated by the microbes just boil ofF. The best known metals to be treated in this fashion are mercury, selenium, tellurium, arsenic and tin. Much of the biomethylation pathway is unknown it appears to be very complex [26]. As a result of the complexity of the volatilisation process it has not been used in effluent treatment and is not discussed further herein. 

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