Reduction Pathway-Dependent Formation of Reactive Fe(II) Sites in Clay Minerals.

Structural Fe in clay minerals is an important, potentially renewable source of electron equivalents for contaminant reduction, yet our knowledge of how clay mineral Fe reduction pathways and Fe reduction extent affect clay mineral Fe(II) reactivity is limited. Here, we used a nitroaromatic compound (NAC) as a reactive probe molecule to assess the reactivity of chemically reduced (dithionite) and Fe(II)-reduced nontronite across a range of reduction extents. We observed biphasic transformation kinetics for all nontronite reduction extents of ≥5% Fe(II)/Fe(total) regardless of the reduction pathway, indicating that two Fe(II) sites of different reactivities form in nontronite at environmentally relevant reduction extents. At even lower reduction extents, Fe(II)-reduced nontronite completely reduced the NAC whereas dithionite-reduced nontronite could not. Our 57Fe Mössbauer spectroscopy, ultraviolet-visible spectroscopy, and kinetic modeling results suggest that the highly reactive Fe(II) entities likely comprise di/trioctahedral Fe(II) domains in the nontronite structure regardless of the reduction mechanism. However, the second Fe(II) species, of lower reactivity, varies and for Fe(II)-reacted NAu-1 likely comprises Fe(II) associated with an Fe-bearing precipitate formed during electron transfer from aqueous to nontronite Fe. Both our observation of biphasic reduction kinetics and the nonlinear relationship of rate constant and clay mineral reduction potential EH have major implications for contaminant fate and remediation.

Carbonate-Enhanced Transformation of Ferrihydrite to Hematite.

An elevated activity of (bi)carbonate in soils and sediments (pCO2, ∼2%) above current atmospheric CO2 (∼0.04%) could influence the iron cycling in mineral-water interfacial chemistry. However, the impact of (bi)carbonate on mineral transformation is unclear. Here, a model short range-ordered iron oxyhydroxide, two-line ferrihydrite, was used to evaluate the impact of (bi)carbonate on mineral transformation at near-neutral pH using experimental geochemistry, X-ray diffraction, X-ray absorption spectroscopy, transmission electron microscopy, and Fourier transform infrared spectroscopy. Results showed that (bi)carbonate promoted the transformation of ferrihydrite to hematite and retarded the goethite formation. As pCO2 increased from 408 to 20,000 ppmv at 40 °C, the transformation efficiency of ferrihydrite increased from 53 to 95%, and the formation of hematite increased from 13 to 76%. During the formation of hematite, a terminal ligand on a Fe(III)O6 octahedral monomer such as a hydroxyl or water was displaced to form Fe(III)O6 octahedral dimers and/or polymers. Because the Fe-O bond of ≡(Fe-O)2-CO is much weaker than that of ≡Fe-O-H, the -O2CO group can be more easily replaced by two terminal -OH groups; the dehydration/rearrangement between Fe(III)O6 octahedral monomers was enhanced under high pCO2. Results suggest that high carbonate activity is an important geochemical parameter controlling the occurrence of hematite in oxic environments and, in turn, iron cycling in the critical zone.

Biological redox cycling of iron in nontronite and its potential application in nitrate removal.

Biological redox cycling of structural Fe in phyllosilicates is an important but poorly understood process. The objective of this research was to study microbially mediated redox cycles of Fe in nontronite (NAu-2). During the reduction phase, structural Fe(III) in NAu-2 served as electron acceptor, lactate as electron donor, AQDS as electron shuttle, and dissimilatory Fe(III)-reducing bacterium Shewanella putrefaciens CN32 as mediator in bicarbonate- and PIPES-buffered media. During the oxidation phase, biogenic Fe(II) served as electron donor and nitrate as electron acceptor. Nitrate-dependent Fe(II)-oxidizing bacterium Pseudogulbenkiania sp. strain 2002 was added as mediator in the same media. For all three cycles, structural Fe in NAu-2 was able to reversibly undergo three redox cycles without significant dissolution. Fe(II) in bioreduced samples occurred in two distinct environments, at edges and in the interior of the NAu-2 structure. Nitrate reduction to nitrogen gas was coupled with oxidation of edge-Fe(II) and part of interior-Fe(II) under both buffer conditions, and its extent and rate did not change with Fe redox cycles. These results suggest that biological redox cycling of structural Fe in phyllosilicates is a reversible process and has important implications for biogeochemical cycles of carbon, nitrogen, and other nutrients in natural environments.

Impacts of detrital nano- and micro-scale particles (dNP) on contaminant dynamics in a coal mine AMD treatment system.

Pollutants in acid mine drainage (AMD) are usually sequestered in neoformed nano- and micro-scale particles (nNP) through precipitation, co-precipitation, and sorption. Subsequent biogeochemical processes may control nNP stability and thus long-term contaminant immobilization. Mineralogical, chemical, and microbiological data collected from sediments accumulated over a six-year period in a coal-mine AMD treatment system were used to identify the pathways of contaminant dynamics. We present evidence that detrital nano- and micron-scale particles (dNP), composed mostly of clay minerals originating from the partial weathering of coal-mine waste, mediated biogeochemical processes that catalyzed AMD contaminant (1) immobilization by facilitating heterogeneous nucleation and growth of nNP in oxic zones, and (2) remobilization by promoting phase transformation and reductive dissolution of nNP in anoxic zones. We found that dNP were relatively stable under acidic conditions and estimated a dNP content of ~0.1g/L in the influent AMD. In the AMD sediments, the initial nNP precipitates were schwertmannite and poorly crystalline goethite, which transformed to well-crystallized goethite, the primary nNP repository. Subsequent reductive dissolution of nNP resulted in the remobilization of up to 98% of S and 95% of Fe accompanied by the formation of a compact dNP layer. Effective treatment of pollutants could be enhanced by better understanding the complex, dynamic role dNP play in mediating biogeochemical processes and contaminant dynamics at coal-mine impacted sites.