Abiotic processes transfer

Figure 14.1 Schematic representation depicting the importance of electron transfer mediators as well as the concurrence of microbial and abiotic processes for reductive transformations of organic pollutants. Adapted from Schwarzen-bach et al. (1997).

So far, except for the iron(III)/iron(II) couple [reaction (6) in Table 14.2], we have considered reduction potentials of half reactions with an overall transfer of an even number of electrons (i.e., 2, 4, 6, etc.). However, in many abiotic multielectron redox processes, particularly if organic compounds are involved, the actual electron transfer frequently occurs by a sequence of one-electron transfer steps (Eberson, 1987). The resulting intermediates formed are often very reactive, and they are not stable under environmental conditions. In our benzoquinone example, BQ is first reduced to the corresponding semiquinone (SQ), which is then reduced to HQ ... 

Reduction of nitro aromatic compounds often appears to be a two-step process, in which a mediator is required for facile transfer of electrons from a bulk reductant to the contaminant. A well documented example is the coupling of organic matter oxidation by iron reducing bacteria to "abiotic" nitro reduction by biogenic Fe(II) that is adsorbed to mineral surfaces in a column containing aquifer material (36, 39, 76). 

The data in Figure 7.13 show reductive-dissolution kinetics of various Mn-oxide minerals as discussed above. These data obey pseudo first-order reaction kinetics and the various manganese-oxides exhibit different stability. Mechanistic interpretation of the pseudo first-order plots is difficult because reductive dissolution is a complex process. It involves many elementary reactions, including formation of a Mn-oxide-H202 complex, a surface electron-transfer process, and a dissolution process. Therefore, the fact that such reactions appear to obey pseudo first-order reaction kinetics reveals little about the mechanisms of the process. In nature, reductive dissolution of manganese is most likely catalyzed by microbes and may need a few minutes to hours to reach completion. The abiotic reductive-dissolution data presented in Figure 7.13 may have relative meaning with respect to nature, but this would need experimental verification. 

A brief discussion of some aspects of alcohol dehydrogenase will be used to illustrate the potential for catalysis. This system is chosen for illustration because it has been studied so extensively. Lessons drawn can be applied in a broader context. The 1,4-dihydropyridine (2a) is the reductant and this affords a nico-tinium ion (1) on transfer of hydride, as illustrated in equation (1). This process is mimicked in many abiotic systems by derivatives of (2 R = alkyl or benzyl), by Hantzsch esters (7), which are synthetically readily accessible, and 1,4-dihydro derivatives (8) of pyridine-3,5-dicarboxylic acid. A typical abiotic reaction is the reduction of the activated carbonyl group of an alkyl phenylglyoxylate (9), activated by a stoichiometric amount of the powerful electrophile Mg(CI04)2, by, for example, (2b equation 8). After acrimonious debate the consensus seems to be that such reactions involve a one-step mechanism (i.e. equation 5), unless the reaction partner strongly demands a radical intermediate, as in the reduction of iron(II) to iron(III). 

Along with the orbital diagram for it electron transfer (Figure 2), equation 1 implies an outer-sphere redox (second-order) mechanism. The reduction potential (E°) is calculated from the recently tabulated free-radical reduction potentials for a variety of half-reactions (16) Table I presents reduction potentials of interest. Thus, the oxidation of I by 02 is thermodynamically unfavorable by an abiotic, thermal, one-electron transfer process. Chloride, bromide, and bisulfide one-electron oxidations with 02 are also thermodynamically unfavorable with E° values of -2.57, -2.08, and -1.24 V, respectively. 

In the nonsterilized systems in which abiotic and biotic processes were not differentiated, Larson and Hufnal (1980) also reported that the Mn(IV) oxides are the most efficient, among the metal oxides, in oxidative polymerization of dissolved phenols. Ono et al. (1977) investigated the rate of radical formation on MnOi at high pH (pH 9) and proposed a mechanism that involves the removal of protons from hydroquinone before electron transfer. Stone and Morgan (1984a) reported that the reduction of Mn oxide by hydroquinone is a first-order reaction (with respect to oxide loading) and must occur on the oxide surface, i.e., phenols must form a surface complex prior to electron transfer. 

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