Arsenic redox reactions, rates

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. 

Since crushed basalt has been recommended as a major backfill component (1), experiments were completed to evaluate the rate of dissolved oxygen consumption and the redox conditions that develop in basalt-water systems under conditions similar to those expected in the near-field environment of a waste package. Two approaches to this problem were used in this study (l)the As(III)/As(V) redox couple as an indirect method of monitoring Eh and (2) the measurement of dissolved oxygen levels in solutions from hydrothermal experiments as a function of time. The first approach involves oxidation state determinations on trace levels of arsenic in solution (4-5) and provides an estimate of redox conditions over restricted intervals of time, depending on reaction rates and sensitivities of the analyses. The arsenic oxidation state approach also provides data at conditions that are more reducing than in solutions with detectable levels of dissolved oxygen. 

The usefulness of the As(m)-As(V) couple as a redox indicator in hydrothermal experiments is limited by the availability of thermochemical data and by the range of Eh dictated by analytical and reaction rate constraints. For low temperature (<150°C) short-term experiments, selenium oxidation state analysis may be more appropriate than arsenic, since the Se(VI)-Se(IV) reduction occurs at relatively high Eh (36). 

Anaerobic metabolism occnrs nnder conditions in which the diffusion rate is insufficient to meet the microbial demand, and alternative electron acceptors are needed. The type of anaerobic microbial reaction controls the redox potential (Eh), the denitrification process, reduction of Mu and SO , and the transformation of selenium and arsenate. Keeney (1983) emphasized that denitrification is the most significant anaerobic reaction occurring in the subsurface. Denitrification may be defined as the process in which N-oxides serve as terminal electron acceptors for respiratory electron transport (Firestone 1982), because nitrification and NOj" reduction to produce gaseous N-oxides. hi this case, a reduced electron-donating substrate enhances the formation of more N-oxides through numerous elechocarriers. Anaerobic conditions also lead to the transformation of organic toxic compounds (e.g., DDT) in many cases, these transformations are more rapid than under aerobic conditions. 

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). 

As in all liquid-redox processes, part of the sulfur is converted to thiosulfate, although the rate of formation is appreciably lower in the essentially neutral Thylox solution than in more alkaline solutions used in other processes. Hydrogen cyanide, which is absorbed in the absorber, reacts readily with the sulfur formed in the thionizer to yield sodium thiocyanate. Because of these side reactions, the active thioarsenate has to be replenished continuously by addition of arsenic oxide and sodium carbonate. 

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