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Despite the broad consensus that humic substances (HS) can act as electron shuttles in bioreduction processes occurring in aquatic and terrestrial environments, no published studies have presented direct evidence that electron shuttling by HS plays a role in the mechanisms of Fe(III) bioreduction in soils. We conducted experiments in which two HS with very different chemical properties, a soil humic acid and a mixture of peat humic acid with aquatic organic matter, were added to an Ultisol and incubated under anoxic conditions to evaluate their effect on Fe(III) bioreduction by the indigenous microorganisms in the soil. Our experiments were designed to have the same initial reducing capacity available from the added HS but to have very different initial total carboxyl concentrations to examine the role of Fe(II) complexation by HS in the bioreduction process. The addition of HS significantly increased soluble Fe(II) production relative to that observed in control experiments without added HS. Moreover, after an initial period of incubation, soluble Fe(II) production was accelerated by the presence of a higher initial carboxyl content. These trends support the hypothesis that, at the initial stages of in Fe(III) bioreduction, electron shuttling dominates, whereas later Fe(II) complexation dominates. Our results apparently are the first to demonstrate directly that added HS can indeed enhance the bioreduction of Fe(III) minerals in soil by at least two different mechanisms.
The study of the interactions between dissolved Mo(VI) or W(VI) species and the surfaces of metal (hydr)oxides is relevant for two main areas: the optimization of the preparation of catalytic supports and the understanding of the environmental fate of these elements. For the latter, iron (hydr)oxides are the most important sinks for pollutants, and recently, their surface reactivity was the focus of many research works. In this work, we develop a joint approach, using in situ infrared spectroscopy and DFT simulations, to characterize the Mo(VI) and W(VI) species adsorbed on hematite (α-Fe 2 O 3 ). Surface sorbed polymers of tungstate and molybdate on hematite were identified for low pH and at high concentration of these elements, which is similar to the formation of polyoxometalates in solution phase. However, the surface speciation is different from the adsorption of polymolybdate or polytungstate already formed in solution and should be consistent with the growth of a surface polymer. For low concentrations/high pH conditions, the spectra are consistent with a monodentate surface complexation.
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