Abiotic reduction

Figure 4.15 indicates the range of rates of O2 consumption in different soils. Oxygen is consumed in oxidation of inorganic reductants, such as Fe(II), as well as in oxidation of organic matter by microbes. Bouldin (1968) and Howeler and Bouldin (1971) compared measured rates of O2 movement into anaerobic soil cores with the predictions of various models, and obtained the best fits with a model allowing for both microbial respiration and abiotic oxidation of mobile and immobile reductants abiotic oxidation accounted for about half the O2 consumed. The kinetics of the abiotic reactions are complicated. They often depend on the adsorption of the reductant on solid surfaces as, for example, in... 

Sulfoxide Reduction Sulfoxide reduction is a two-electron-transfer reversible reaction resulting in thioethers. Organic sulfoxides are used mainly as agrochemicals, and their reduction (abiotic and microbially mediated) has been found in anaerobic soils, sediments, and groundwater (Larson and Weber 1994). 

In the presence of a strong reductant, abiotic reductive dechlorination of CPs is catalyzed by the reduced form of Vitamin B12 (Gantzer Wackett, 1991Smith Woods, 1994). These abiotic dechlorinations favor removal of m- and /(-chlorines, which differs substantially from reductive dechlorinations by anaerobic microbial consortia or by the CP-dehalogenating bacteria isolated to date. This indicates that abiotic dechlorinations of CPs are not central reactions in environmental or engineered anaerobic systems. 

There has been considerable interest in the abiotic dechlorination of chlorinated ethenes at contaminated sites. Reductive dehalogenation has therefore been examined using a range of reductants, many of them involving reduced complexes of porphyrins or corrins. 

A number of other abiotic reductions have been described ... 

Heijman CG, C Holliger, M A Glaus, RP Schwarzenbach, J Zeyer (1993) Abiotic reduction of 4-chloronitro-benzene to 4-chloroaniline in a dissimilatory iron-reducing enrichment culture. Appl Environ Microbiol 59 4350-4353. 

RDX and its partial reduction product hexahydro-l-nitroso-3,5-dinitro-l,3,5-triazine were degraded by K. pneumoniae to methylenedinitramine, and then to CHjO and methanol, while abiotic reactions produced NjO (Zhao et al. 2002). 

Transformation of DDT to DDD by reductive dechlorination has been demonstrated in a number of aquatic plants, although the reaction appears to be abiotic mediated by some component of the plants (Garrison et al. 2000). 

Humic acid and the corresponding fulvic acid are complex polymers whose structures are incompletely resolved. It is accepted that the structure of humic acid contains oxygenated structures, including quinones that can function as electron acceptors, while reduced humic acid may carry out reductions. These have been observed both in the presence of bacteria that provide the electron mediator and in the absence of bacteria in abiotic reactions, for example, reductive dehalogenation of hexachloroethane and tetrachloromethane by anthrahydroquininone-2,6-disulfonate (Curtis and Reinhard 1994). Reductions using sulfide as electron donor have been noted in Chapter 1. Some experimental aspects are worth noting ... 

Kessi J, KW Hanselmann (2004) Similarities between the abiotic reduction of selenite with glutathione and the dissimilatory reaction mediated by Rhodospirillum rubrum and Escherichia coli. J Biol Chem 279 50662-50669. 

Nijenhuis 1,1 Andert, K Beck, M Kastner, G Diekert, H-H Richow (2005) Stable isotope fractionation of tetrachloroethene during reductive dechlorination by Sulfospirillum multivorans and Desulfitobac-terium sp. strain PCE-S and abiotic reactions with cyanocobalamin. Appl Environ Microbiol 71 3413-3419. 

The principal abiotic processes affecting americium in water is the precipitation and complex formation. In natural waters, americium solubility is limited by the formation of hydroxyl-carbonate (AmOHC03) precipitates. Solubility is unaffected by redox condition. Increased solubility at higher temperatures may be relevant in the environment of radionuclide repositories. In environmental waters, americium occurs in the +3 oxidation state oxidation-reduction reactions are not significant (Toran 1994). 

Abiotic chemical transformation is the reduction of chemical concentrations by degrading the chemicals into other products. The most important chemical transformations are hydrolysis and oxidation/reduction reactions. 

Allard, B. and I. Arsenie. 1991. Abiotic reduction of mercury by humic substances in aquatic system — an important process for the mercury cycle. Water Air Soil Pollut. 56 457-464. 

A half-life of about 40 days was reported for hexachloroethane in an unconfined sand aquifer (Criddle et al. 1986). Laboratory studies with wastewater microflora cultures and aquifer material provided evidence for microbial reduction of hexachloroethane to tetrachloroethylene under aerobic conditions in this aquifer system (Criddle et al. 1986). In anaerobic groundwater, hexachloroethane reduction to pentachloroethane and tetrachloroethylene was found to occur only when the water was not poisoned with mercury chloride (Roberts et al. 1994). Pentachloroethane reduction to tetrachloroethylene occurred at a similar rate in both poisoned and unpoisoned water. From these results, Roberts et al. (1994) suggested that the reduction of hexachloroethane to tetrachloroethylene occurred via pentachloroethane. The first step, the production of pentachloroethane, was microbially mediated, while the production of tetrachloroethylene from pentachloroethane was an abiotic process. 

Roberts AL, Gschwend PM. 1994. Interaction of abiotic and microbial processes in hexachloroethane reduction in groundwater. Journal of Contaminant Hydrology 16 157-174. 

Schanke and Wackett [379] reported TeCA degradation by transition-metal coenzymes. cDCE (53%), tDCE (29%), VC (14%), ethylene (1%), and traces of 1,1,2-TCA were the products from this abiotic transformation with vitamin B12 and titanium(III) citrate. Both dechlorination and dichloroelimination had occurred the major route of degradation was reductive dihaloelimina-tion. 

The body of research on isotopic fractionation induced by sulfate and nitrate reduction provides insight into selenate, selenite and chromate reduction. For sulfate and nitrate oxyanions, reduction is generally microbially mediated, is irreversible, and involves a fairly large but variable isotopic fractionation. As described below, Se and Cr oxyanion reduction follows suit, though abiotic reactions may have a greater role in some transformations. 

Most of the reactions that involve significant fractionation of Se and Cr isotopes appear to be far from chemical or isotopic equihbrium at earth-surface temperatures. Redox disequilibrium is common among dissolved Se species. Dissolved Se(IV) and solid Se(0) are often observed in oxic waters despite their chemical instability (Tokunaga et al. 1991 Zhang and Moore 1996 Zawislanski and McGrath 1998). In one study of shallow groundwater, Se species were found to be out of equilibrium with other redox couples such as Fe(III)/Fe(II) (White and Dubrovsky 1994). The kinetics of abiotic Se(VI) reduction, like those of sulfate, are quite slow. In the laboratory, conversion of Se(VI) to Se(IV) requires one hour of heating to ca. 100°C in a 4 M HCl medium. 

More likely, there are ephemeral intermediate species with short residence times, and the reaction proceeds in several steps with several intermediates. In such a reaction pathway, changes in the relative rates of the reaction steps can result in changes in the fractionation. Furthermore, there may be multiple pathways by which a chemical transformation can occur. For example, transformation of Se(IV) to Se(0) could proceed via simple abiotic reaction, or via uptake of FlSeOj by a plant, reduction to Se(-ll) within the plant, incorporation into amino acids, death and decay of the plant, release of the Se(-II), and oxidation to Se". The overall transformation, from Se(lV) to Se(0), is the same, but because the two reaction pathways differ greatly, the overall isotopic fractionation may be greatly different. 

Startup effects. Startup effects must also be considered in the interpretation of laboratory experiments. For example, during sulfate reduction, the first small amormt of sulfur to pass through the chain of reaction steps would be subject to the kinetic isotope effects of all of the reaction steps. This is because it takes some time for the isotopic compositions of the pools of intermediates to become enriched in heavier isotopes as described above for the steady-state case. Accordingly, the first HjS produced would be more strongly enriched in the lighter isotopes than that produced after a steady state has been approached. This principle was modeled by Rashid and Krouse (1985) to interpret kinetic isotope effects occurring during abiotic reduction of Se(IV) to Se(0) (see below). Startup effects may be particularly relevant in laboratory experiments where Se or Cr concentrations are very small, as is the case in some of the studies reviewed below. 

Only one naturally relevant abiotic Se(VI) reduction process has been documented to date. Se(Vl) can be reduced to Se(TV) and ultimately to Se(0) by green rust , an Fe(II)- and Fe(lll)-bearing phase with sulfate occupying interlayer spaces (Myneni et al. 1997). Johnson and Bullen (2003) obtained an ese(vi)-se(iv) value of 7.4%o ( 0.2) for the Se(VI) reduction reaction. The result was not sensitive to changes in pH or solution composition within the ranges over which green rust is stable. 

In the earliest work, Krouse and Thode (1962) found the Se isotope fractionation factor Sse(iv)-se(o) to bc 10%o ( l%o) with hydroxylamine (NH2OH) as the reductant. Rees and Thode (1966) obtained a larger value, 12.8%o, for reduction by ascorbic acid. Webster (1972) later obtained 10%o for NHjOH reduction. Rashid and Krouse (1985) completed a more detailed study, and found that the fractionation factor varied with time over the course of the experiments. They explained the variations observed among the experiments in all four studies using a model in which reduction consists of two steps. With the rate constant of the second step two orders of magnitude smaller than the first, and kinetic isotope effects of 4.8%o and 13.2%o for the hrst and second steps, respectively, all the data (Table 3) were fit. Thus, kinetic isotope effects of apparently simple abiotic reactions can depend on reaction conditions. 

This range of isotopic fractionation (5.5%o to 9.1%o, excluding the early time points from Herbel et al. 2000) overlaps strongly with the range observed for abiotic reduction (10%o to 13%o Table 3). This suggests a fundamental difference between Se(lV) reduction and Se(Vl) reduction, in which microbial Se isotope fractionation is much smaller than that caused by abiotic reduction. 

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