Extracellular electron transfer-dependent Cr(VI)/sulfate reduction mediated by iron sulfide nanoparticles

Synthesis mechanisms, property characterization, and environmental applications of biogenic FeS: A review

Iron sulfide (FeS) exhibits superior reactivity toward a wide range of contaminants, making it a promising candidate for environmental remediation in various media, including surface water, wastewater, soil, and groundwater. Driven by green and sustainable development principles, efficient, low-cost, and environmentally friendly biosynthesis has attracted considerable attention and has great environmental remediation potential. This review provides a comprehensive overview of the recent advances in biogenic FeS (bio-FeS), focusing on its synthesis mechanisms, performance characterization, and environmental applications. To the best of our knowledge, this is the first review exclusively dedicated to this emerging field. This review begins with an in-depth description of the four bio-FeS biosynthetic pathways, the primary actors of which are sulfate-reducing bacteria (SRB), iron-reducing bacteria (IRB), coupled SRB and IRB, and bio-extracts. Notably, SRB account for approximately half of the bio-FeS synthesis. Various characterization techniques, including scanning electron microscopy, X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, Fourier transform infrared spectroscopy, and Brunauer-Emmett-Teller analysis, have been discussed in depth to better understand the structure and properties of bio-FeS. In terms of morphological structure, the bio-FeS synthesized by SRB exhibited primarily flakes (similar to mackinawite), whereas the bio-FeS synthesized by IRB was chiefly spherical. Bio-FeS transportation, migration, and permeation properties were also explored. Furthermore, bio-FeS application in the removal of various contaminants, including arsenic, chromium, uranium, complex pollutants, rare earth elements, chlorinated hydrocarbons, and antibiotics, and their underlying mechanisms were discussed. Finally, the challenges and future research directions related to bio-FeS environmental applications were discussed. This review aims to provide valuable insights into bio-FeS synthesis and environmental applications, thereby supporting the development of innovative and sustainable technologies for environmental remediation.

Electroactive microorganism-assisted remediation of groundwater contamination: Advances and challenges

Groundwater contamination has become increasingly prominent, therefore, the development of efficient remediation technology is crucial for improving groundwater quality. Bioremediation is cost-effective and environmentally friendly, while coexisting pollutant stress can affect microbial processes, and the heterogeneous character of groundwater medium can induce bioavailability limitations and electron donor/acceptor imbalances. Electroactive microorganisms (EAMs) are advantageous in contaminated groundwater because of their unique bidirectional electron transfer mechanism, which allows them to use solid electrodes as electron donors/acceptors. However, the relatively low-conductivity groundwater environment is unfavorable for electron transfer, which becomes a bottleneck problem that limits the remediation efficiency of EAMs. Therefore, this study reviews the recent advances and challenges of EAMs applied in the groundwater environment with complex coexisting ions, heterogeneity, and low conductivity and proposes corresponding future directions.

Improved performance of Cr(vi)-reducing microbial fuel cells by nano-FeS hybridized biocathodes

Biocathode microbial fuel cells (MFCs) show promise for Cr(vi)-contaminated wastewater treatment. However, biocathode deactivation and passivation caused by highly toxic Cr(vi) and nonconductive Cr(iii) deposition limit the development of this technology. A nano-FeS hybridized electrode biofilm was fabricated by simultaneously feeding Fe and S sources into the MFC anode. This bioanode was then reversed as the biocathode to treat Cr(vi)-containing wastewater in a MFC. The MFC obtained the highest power density (40.75 ± 0.73 mW m^-2) and Cr(vi) removal rate (3.99 ± 0.08 mg L^-1 h^-1), which were 1.31 and 2.00 times those of the control, respectively. The MFC also maintained high stability for Cr(vi) removal in three consecutive cycles. These improvements were due to synergistic effects of nano-FeS with excellent properties and microorganisms in the biocathode. The mechanisms were: (1) the accelerated electron transfer mediated by nano-FeS 'electron bridges' strengthened bioelectrochemical reactions, firstly realizing deep reduction of Cr(vi) to Cr(0) and thus effectively alleviating cathode passivation; (2) nano-FeS as 'armor' layers improved cellular viability and extracellular polymeric substance secretion; (3) the biofilm selectively enriched a diversity of bifunctional bacteria for electrochemical activity and Cr(vi) removal. This study provides a new strategy to obtain electrode biofilms for sustainable treatment of heavy metal wastewater.

Elucidating interactive effects of sulfidated nanoscale zero-valent iron and ammonia on anaerobic digestion of food waste

In our previous study, anaerobic digestion of food waste could be effectively enhanced by adding sulfidated nanoscale zero-valent iron (S-nZVI) under high-strength ammonia concentrations. In this study, in order to further elucidate the specific interactive effects of S-nZVI and ammonia on anaerobic digestion of nitrogen-rich food waste, the methanogenic performance of anaerobic digestion systems respectively added with nanoscale zero-valent iron (nZVI) and S-nZVI were compared and monitored under different ammonia stress conditions. Both nZVI and S-nZVI could effectively stimulate the methanogenesis process among ammonia concentrations ranging from 0 to 3500 mg/L. However, the enhancing effects of S-nZVI and nZVI on anaerobic digestion of food waste were different, in which anaerobic digestion systems added with S-nZVI and nZVI performed best under 2500 mg/L of ammonia and 1500 mg/L of ammonia, respectively. Furthermore, the analysis of microbial communities suggested that ammonia stress enriched acetoclastic methanogens, while adding nZVI and S-nZVI into anaerobic digestions stimulated the process of hydrogenotrophic methanogenesis. Moreover, S-nZVI performed better in promoting the evolution of DIET-related microorganisms than nZVI, resulting in enhanced methane production under high ammonia-stressed conditions. This work provided fundamental knowledge about the interactive effects of S-nZVI and ammonia on the anaerobic digestion of food waste.

Detoxification mechanisms of electroactive microorganisms under toxicity stress: A review

Remediation of environmental toxic pollutants has attracted extensive attention in recent years. Microbial bioremediation has been an important technology for removing toxic pollutants. However, microbial activity is also susceptible to toxicity stress in the process of intracellular detoxification, which significantly reduces microbial activity. Electroactive microorganisms (EAMs) can detoxify toxic pollutants extracellularly to a certain extent, which is related to their unique extracellular electron transfer (EET) function. In this review, the extracellular and intracellular aspects of the EAMs’ detoxification mechanisms are explored separately. Additionally, various strategies for enhancing the effect of extracellular detoxification are discussed. Finally, future research directions are proposed based on the bottlenecks encountered in the current studies. This review can contribute to the development of toxic pollutants remediation technologies based on EAMs, and provide theoretical and technical support for future practical engineering applications.