Cr isotope fractionation during microbial reduction processes is commonly recognized as a promising tool in biogeochemistry and bioremediation. However, the mechanism of Cr isotope fractionation during microbial reduction is poorly understood. In this work, the relationship between the bacterial respiratory pathway and Cr isotope fractionation was investigated in Shewanella oneidensis MR-1. For comparison with the wild type, a mutant strain (ΔomcA/ΔmtrC) with a deficiency in extracellular Cr(VI) reduction was constructed by deleting the omcA and mtrC genes, which encode the terminal reductase during extracellular reduction. The magnitudes of Cr isotope fractionation (ε) for Cr(VI) reduction by the wild type and ΔomcA/ΔmtrC were −2.42 ± 0.68‰ and −2.70 ± 0.22‰, respectively. Surprisingly, the ε values were not significantly different between the two strains. This suggests that isotope fractionation is independent of the metal respiratory pathway during Cr(VI) reduction by S. oneidensis MR-1. Moreover, a three-step Cr isotope fractionation model that includes uptake, reduction, and efflux was proposed to exist during intracellular Cr(VI) reduction by S. oneidensis MR-1. The developed model provides a better understanding of Cr isotope fractionation during microbial reduction.

Converting CO2 into valuable chemicals and fuels is one of the most practical routes for reducing CO2 emissions while fossil fuels continue to dominate the energy sector. Noble-metal-free NiFe bimetal nanoparticles have shown good catalytic activity in CO2 conversion. Herein we theoretically evaluated the catalytic performance and possible mechanisms of NiFe-based nanoclusters for hydrogenating CO2 to form formic acid and CO through bicarbonate by using a periodic and self-consistent density functional theory (DFT) simulation. The theoretical results illustrated that NiFe nanoclusters could have good catalytic activity and selectivity for HCO3− reduction to formic acid and the possible pathway is that HCO3− preferred to react with adsorbed H atoms of H2 on NiFe alloy nanoclusters through the carbon atom site. Moreover, the NiFe alloy nanoclusters with the Fe atom exposed on the surface of the Ni cluster showed better performance with a lower energy barrier compared to that with Fe doped in the corner of the Ni cluster. However, the generation of CO from HCO3− reduction was shown to be neither thermodynamically nor kinetically favorable on NiFe alloy nanoclusters. Additionally, the simulation results also suggested that it was thermodynamically unfavorable for further hydrogenated reduction of formic acid to formaldehyde on NiFe alloy nanoclusters themselves as well as supported on graphene. In summary, a molecular-level insight of CO2 reduction to valuable products on NiFe nanoclusters is offered in this study, which may provide some useful information for guiding the design of NiFe-based catalytic materials for efficient CO2 conversion to useful fuels.

This study investigated the feasibility of nanoscale zero-valent iron (nZVI) for reductive dehalogenation of iodinated contrast medium diatrizoate (DTA). The impacts of various parameters, including nZVI dosage, DTA concentration, solution pH, aging time of nZVI, the presence of natural organic matter, and the type of competitive anions, on the dehalogenation of DTA as well as the formation of its reductive product 5-diacetamidobenzoate (DABA) using nZVI were evaluated. Furthermore, an aerobic biological post-treatment was conducted to study the biodegradability of reductive products of DTA dehalogenation using nZVI. The results showed that dosing with 0.5 g L−1 nZVI particles resulted in a rapid decrease in DTA concentration and a corresponding rise of the concentrations of DABA and I−. DTA dehalogenation in terms of its removal and DABA formation was enhanced with the increase in nZVI dosage but deteriorated when increasing solution pH. nZVI aging time had a negative impact on DTA dehalogenation. Natural organic matter at much low level could improve DTA dehalogenation, while had a negative influence at high concentrations. Contrary to sulfate, the presence of nitrate and phosphate strongly inhibited DTA removal using nZVI. The results also showed that the reductive product DABA could be degraded by aerobic biological post-treatment, suggesting DTA dehalogenation with nZVI may be a vital procedure for its biodegradability improvement and consequently complete removal.

•A high yield of formic acid was achieved for CO2 conversion with nano-Ni particles.•An exceptionally high selectivity was obtained for formic acid generation.•Nano-Ni catalyst displayed good stability to pH variation for CO2 reduction.•DFT elucidated attacking the C of HCO3− by the active H and hydroxyl was favourable.The hydrogenation of CO2 with gaseous hydrogen is currently believed to be the most commercially feasible synthetic method to resolve the serious worldwide greenhouse gas effects. However it is suffering from several disadvantages, such as expensive complex catalysts, and high energy consumption. In this study, a novel method for the hydrogenation of CO2 into formic acid with nano-Ni catalyst is developed. The effects of various parameters on the capability of the catalyst for CO2 conversion were investigated. Furthermore, the HCO3− reduction process on the catalyst was rationalized theoretically with density functional theory simulations. The prepared nano-Ni was demonstrated to be an available catalyst for CO2 conversion into formic acid by employing H2 as a hydrogen source at ambient temperature and almost constant pressure. Moreover, the nano-Ni catalyst displayed good tolerance with pH variations in CO2 reduction. The formation of formic acid from CO2 reduction on nano-Ni particles was enhanced with an increase NaHCO3 concentration and catalyst dosage. Additionally, theoretical analysis elucidated that the hydrogenation of CO2 into formic acid on nano-Ni catalyst was favorable over attacking the C of HCO3− by the active H and hydroxyl group.Download high-res image (158KB)Download full-size image

•nZVI exhibited insignificant inhibition on the activity of anaerobic granule sludge.•Fe fate and distribution after nZVI exposure to the AGS were investigated.•Changes of EPS contents and structure in the AGS were elucidated.•Variation of microbial community in the AGS after nZVI addition were explored.The study aimed at evaluating the influence of nano zero-valent iron (nZVI) on the activity of anaerobic granular sludge (AGS) from both macroscopic and microcosmic aspects using different methodologies. The tolerance response of AGS to nZVI was firstly investigated using short-term and long-term experiments, and also compared with anaerobic flocs. The Fe fate and distribution, the change of contents/structure of extracellular polymeric substances (EPS), and the variation of microbial community in the AGS after exposure to nZVI were further explored. Contrary to the anaerobic floc, insignificant inhibition of nZVI at dosage lower than 30 mmoL/L on the activity of AGS was observed. Additionally, the extra hydrogen gas released from the oxidation of nZVI was presumably suggested to stimulate the hydrogenotrophic methanogenesis process, resulting in 30% methane production enhancement when exposure to 30 mmoL/L nZVI. The microscopic analysis indicated that nZVI particles were mainly adsorbed on the surface of AGS in the form of iron oxides aggregation without entering into the interior of the granule, protecting most cells from contact damage. Moreover, surrounded EPS located outer surface of anaerobic granule could react with nZVI to accelerate the corrosion of nZVI and slow down H2 release from nZVI dissolution, thus further weakening the toxicity of nZVI to anaerobic microorganisms. The decrease in bacteria involved in glucose degradation and aceticlastic methanogens as well as the increase of hydrogenotrophic methanogens indicated a H2 mediated shift toward the hydrogenotrophic pathway enhancing the CH4 production.Download high-res image (208KB)Download full-size image