Linear Free Energy Relationships for the Biotic and Abiotic Reduction of Nitroaromatic Compounds

Nitroaromatic compounds (NACs) are ubiquitous environmental contaminants that are susceptible to biological and abiotic reduction. Prior works have found that for the abiotic reduction of NACs, the logarithm of the NACs’ rate constants correlate with one-electron reduction potential values of the NACs (<i>E</i>_H,NAC¹) according to linear free energy relationships (LFERs). Here, we extend the application of LFERs to the bioreduction of NACs and to the abiotic reduction of NACs by bioreduced (and pasteurized) iron-bearing clay minerals. A linear correlation (<i>R</i>² = 0.96) was found between the NACs’ bioreduction rate constants (<i>k</i>_obs) and <i>E</i>_H,NAC¹ values. The LFER slope of log <i>k</i>_obs versus <i>E</i>_H,NAC¹/(2.303<i>RT</i>/<i>F</i>) was close to one (0.97), which implied that the first electron transfer to the NAC was the rate-limiting step of bioreduction. LFERs were also established between NAC abiotic reduction rate constants by bioreduced iron-bearing clay minerals (montmorillonite SWy-2 and nontronite NAu-2). The second-order NAC reduction rate constants (<i>k</i>) by bioreduced SWy-2 and NAu-2 were well correlated to <i>E</i>_H,NAC¹ (<i>R</i>² = 0.97 for both minerals), consistent with bioreduction results. However, the LFER slopes of log <i>k</i> versus <i>E</i>_H,NAC¹/(2.303<i>RT</i>/<i>F</i>) were significantly less than one (0.48–0.50) for both minerals, indicating that the first electron transfer to the NAC was not the rate-limiting step of abiotic reduction. Finally, we demonstrate that the rate of 4-acetylnitrobenzene reduction by bioreduced SWy-2 and NAu-2 correlated to the reduction potential of the clay (<i>E</i>_H,clay, <i>R</i>² = 0.95 for both minerals), indicating that the clay reduction potential also influences its reactivity

Thermodynamic Controls on the Microbial Reduction of Iron-Bearing Nontronite and Uranium

Iron-bearing phyllosilicate minerals help establish the hydrogeological and geochemical conditions of redox transition zones because of their small size, limited hydraulic conductivity, and redox buffering capacity. The bioreduction of soluble U­(VI) to sparingly soluble U­(IV) can promote the reduction of clay-Fe­(III) through valence cycling. The reductive precipitation of U­(VI) to uraninite was previously reported to occur only after a substantial percentage of clay-Fe­(III) had been reduced. Using improved analytical techniques, we show that concomitant bioreduction of both U­(VI) and clay-Fe­(III) by <i>Shewanella putrefaciens</i> CN32 can occur. Soluble electron shuttles were previously shown to enhance both the rate and extent of clay-Fe­(III) bioreduction. Using extended incubation periods, we show that electron shuttles enhance only the rate of reduction (overcoming a kinetic limitation) and not the final extent of reduction (a thermodynamic limitation). The first 20% of clay-Fe­(III) in nontronite NAu-2 was relatively “easy” (i.e., rapid) to bioreduce; the next 15% of clay-Fe­(III) was “harder” (i.e., kinetically limited) to bioreduce, and the remaining 65% of clay-Fe­(III) was effectively biologically unreducible. In abiotic experiments with NAu-2 and biogenic uraninite, 16.4% of clay-Fe­(III) was reduced in the presence of excess uraninite. In abiotic experiments with NAu-2 and reduced anthraquinone 2,6-disulfonate (AH₂DS), 18.5–19.1% of clay-Fe­(III) was reduced in the presence of excess and variable concentrations of AH₂DS. A thermodynamic model based on published values of the nonstandard state reduction potentials at pH 7.0 (<i>E</i>′_H) showed that the abiotic reactions between NAu-2 and uraninite had reached an apparent equilibrium. This model also showed that the abiotic reactions between NAu-2 and AH₂DS had reached an apparent equilibrium. The final extent of clay-Fe­(III) reduction correlated well with the standard state reduction potential at pH 7.0 (<i>E</i>°′_H) of all of the reductants used in these experiments (AH₂DS, CN32, dithionite, and uraninite)

Iron(III)-Bearing Clay Minerals Enhance Bioreduction of Nitrobenzene by <i>Shewanella putrefaciens</i> CN32

Iron-bearing clay minerals are ubiquitous in the environment, and the clay–Fe­(II)/Fe­(III) redox couple plays important roles in abiotic reduction of several classes of environmental contaminants. We investigated the role of Fe-bearing clay minerals on the bioreduction of nitrobenzene. In experiments with <i>Shewanella putrefaciens</i> CN32 and excess electron donor, we found that the Fe-bearing clay minerals montmorillonite SWy-2 and nontronite NAu-2 enhanced nitrobenzene bioreduction. On short time scales (<50 h), nitrobenzene reduction was primarily biologically driven, but at later time points, nitrobenzene reduction by biologically formed structural Fe­(II) in the clay minerals became increasingly important. We found that chemically reduced (dithionite) iron-bearing clay minerals reduced nitrobenzene more rapidly than biologically reduced iron-bearing clay minerals despite the minerals having similar structural Fe­(II) concentrations. We also found that chemically reduced NAu-2 reduced nitrobenzene faster as compared to chemically reduced SWy-2. The different reactivity of SWy-2 versus NAu-2 toward nitrobenzene was caused by different forms of structural clay-Fe­(II) in the clay minerals and different reduction potentials (<i>E</i>_h) of the clay minerals. Because most contaminated aquifers become reduced via biological activity, the reactivity of biogenic clay–Fe­(II) toward reducible contaminants is particularly important