Anoxic storage regenerates reactive Fe(II) in reduced nontronite with short-term oxidation

Reduction of U(VI) to U(IV) drastically reduces its solubility and has been proposed as a method for remediation of uranium contamination. However, much is still unknown about the kinetics, mechanisms, and products of U(VI) bioreduction in complex systems. In this study, U(VI) bioreduction experiments were conducted with Shewanella putrefaciens strain CN32 in the presence of clay minerals and two organic ligands: citrate and EDTA. In reactors with U and Fe(III)−clay minerals, the rate of U(VI) bioreduction was enhanced due to the presence of ligands, likely because soluble Fe 3+ − and Fe 2+ −ligand complexes served as electron shuttles. In the presence of citrate, bioreduced U(IV) formed a soluble U(IV)−citrate complex in experiments with either Fe-rich or Fe-poor clay mineral. In the presence of EDTA, U(IV) occurred as a soluble U(IV)−EDTA complex in Fe-poor montmorillonite experiments. However, U(IV) remained associated with the solid phase in Fe-rich nontronite experiments through the formation of a ternary U(IV)−EDTA−surface complex, as suggested by the EXAFS analysis. Our study indicates that organic ligands and Fe(III)-bearing clays can significantly affect the microbial reduction of U(VI) and the stability of the resulting U(IV) phase.

Section: Resultssupporting

confidence: 92%

Section: ■ Materials and Methodsmentioning

confidence: 92%

Combined Effects of Fe(III)-Bearing Clay Minerals and Organic Ligands on U(VI) Bioreduction and U(IV) Speciation

Zhang

Chen

Xia

et al. 2021

“…The mineral structural Fe(II) in farmland sediment was oxidized continuously during the course of 10 h (Figure c), which is different from the lakeshore sediment (Figure b). The oxidation pattern of mineral structural Fe(II) in farmland sediment was consistent with those of reduced clay minerals. − , That is, both mineral structural and surface-adsorbed Fe(II) were oxidized quickly in initial stage, and mineral structural Fe(II) was oxidized slowly in the later stage with regeneration of surface Fe(II) for oxygenation. , Together with the high abundance of clays (51%), it is reasonable to assign the mineral structural Fe(II) mostly to the silicates in farmland sediment. Contribution of structural Fe(II) in clays was supported by the production of more •OH upon oxygenation of farmland sediment after removing organic matter and Fe oxides (Figure S12c).…”

Section: Resultssupporting

confidence: 53%

Contaminant Degradation by •OH during Sediment Oxygenation: Dependence on Fe(II) Species

Xie

Yuan

Tong

et al. 2020

Self Cite

113

121

It has been documented that contaminants could be degraded by hydroxyl radicals (•OH) produced upon oxygenation of Fe(II)-bearing sediments. However, the dependence of contaminant degradation on sediment characteristics, particularly Fe(II) species, remains elusive. Here we assessed the impact of the abundance of Fe(II) species in sediments on contaminant degradation by •OH during oxygenation. Three natural sediments with different Fe(II) contents and species were oxygenated. During 10 h oxygenation of 200 g/L sediment suspension, 2 mg/L phenol was negligibly degraded for sandbeach sediment (Fe(II): 9.11 mg/g), but was degraded by 41% and 52% for lakeshore (Fe(II): 9.81 mg/g) and farmland (Fe(II): 19.05 mg/g) sediments, respectively. •OH produced from Fe(II) oxygenation was the key reactive oxidant. Sequential extractions, X-ray diffraction, Mössbauer, and X-ray absorption spectroscopy suggest that surface-adsorbed Fe(II) and mineral structural Fe(II) contributed predominantly to •OH production and phenol degradation. Control experiments with specific Fe(II) species and coordination structure analysis collectively suggest the likely rule that Fe(II) oxidation rate and its competition for •OH increase with the increase in electron-donating ability of the atoms (i.e., O) complexed to Fe(II), while the •OH yield decreases accordingly. The Fe(II) species with a moderate oxidation rate and •OH yield is most favorable for contaminant degradation.

“…In the case of no-ligand system, a direct contact between 2002 cells and rNAu-2 surface would preferentially oxidize structural Fe(II) around particle edges . Regeneration of edge Fe(II) through multistep electron hopping from interior Fe(II) to edge Fe(III) through Fe(III)–O/OH–Fe(II) entities is presumably driven by a reduction potential gradient, which is a slow process. , Thus, the extent of Fe(II) oxidation is limited. However, in the presence of ligands, the ligand-complexed Fe(III)/Fe(II) couple formed has a higher reduction potential relative to the structural Fe(III)/Fe(II) couple. , Given its small size, it is likely that complexed Fe(III) can access more structural Fe(II) sites, through both edges and basal planes of rNAu-2, thus increasing the extent of Fe(II) oxidation.…”

Section: Discussionmentioning

confidence: 99%

“…54 Regeneration of edge Fe(II) through multistep electron hopping from interior Fe(II) to edge Fe(III) through Fe(III)−O/OH−Fe(II) entities is presumably driven by a reduction potential gradient, which is a slow process. 35,70 Thus, the extent of Fe(II) oxidation is limited. However, in the presence of ligands, the ligand-complexed Fe(III)/Fe(II) couple formed has a higher reduction potential relative to the structural Fe(III)/Fe(II) couple.…”

Section: ■ Discussionmentioning

confidence: 99%

Promotion of Microbial Oxidation of Structural Fe(II) in Nontronite by Oxalate and NTA

Zhao

Jin

Sheng

et al. 2020

Iron redox cycling occurs extensively in soils and sediments. Previous research has focused on microbially mediated redox cycling of aqueous Fe. At circumneutral pH, most Fe occurs in solid phase, where Fe and organic ligands interact closely. However, the role of organic ligands in microbial oxidation of solid-phase Fe(II) is not well understood. Here, we incubated reduced nontronite NAu-2 (rNAu-2) with an iron-oxidizing bacterium and in the presence of oxalate and nitrilotriacetic acid. These ligands significantly enhanced the rate and extent of microbial oxidation of structural Fe(II) in rNAu-2. Aqueous and solid-phase analyses, coupled with biogeochemical modeling, revealed a pathway for ligand-enhanced bio-oxidation of solid-phase Fe(II): (1) dissolution of rNAu-2 to form aqueous Fe(II)−ligand complex;(2) bio-oxidation to Fe(III)−ligand complex; (3) rapid reduction of Fe(III)−ligand complex to Fe(II)−ligand complex by structural Fe(II) in rNAu-2. In this process, the Fe(II)−ligand and Fe(III)−ligand complexes effectively serve as electron shuttle to expand the bioavailable pool of solid-phase Fe(II). These results have important implications for a better understanding of the bioavailability and reactivity of solid-phase Fe pool in the environment.