Riboflavin as a non-quinone redox mediator for enhanced Cr(VI) removal by Shewanella putrefaciens

Cr(VI) removal during cotransport of nano-iron-particles combined with iron sulfides in groundwater: Effects of D. vulgaris and S. putrefaciens

Iron-based materials such as nanoscale zerovalent iron (nZVI) are effective candidates to in situ remediate hexachromium (Cr(VI))-contaminated groundwater. The anaerobic bacteria could influence the remediation efficiency of Cr(VI) during its cotransport with nZVI in porous media. To address this issue, the present study investigated the adsorption and reduction of Cr(VI) during its cotransport with green tea (GT) modified nZVI (nZVI@GT) and iron sulfides (FeS and FeS2) in the presence of D. vulgaris or S. putrefaciens in water-saturated sand columns. Experimental results showed that the nZVI@GT preferred to heteroaggregate with FeS2 rather than FeS, forming nZVI@GT-FeS2 heteroaggregates. Although the presence of D. vulgaris further induced nZVI@GT-FeS2 heteroaggregates to form larger clusters, it pronouncedly improved the dissolution of FeS and FeS2 for more Cr(VI) reduction associated with lower Cr(VI) flux through sand. In contrast, S. putrefaciens could promote the dispersion of the heteroaggregates of nZVI@GT-FeS2 and the homoaggregates of nZVI@GT or FeS by adsorption on the extracellular polymeric substances, leading to the improved transport of Fe-based materials for a much higher Cr(VI) immobilization in sand media. Overall, our study provides the essential perspectives into a chem-biological remediation technique through the synergistic removal of Cr(VI) by nZVI@GT and FeS in contaminated groundwater. ENVIRONMENTAL IMPLICATION: The green-synthesized nano-zero-valent iron particles (nZVI@GT) using plant extracts (or iron sulfides) have been used for in situ remediation of Cr(VI) contaminated groundwater. Nevertheless, the removal of Cr(VI) (including Cr(VI) adsorption and Cr(III) generation) could be influenced by the anaerobic bacteria governing the transport of engineered nanoparticles in groundwater. This study aims to reveal the inherent mechanisms of D. vulgaris and S. putrefaciens governing the cotransport of nZVI@GT combined with FeS (or FeS2) to further influence the Cr(VI) removal in simulated complex groundwater media. Our findings provides a chemical and biological synergistic remediation strategy for nZVI@GT application in Cr(VI)-contaminated groundwater.

Effect of soil organic matter-mediated electron transfer on heavy metal remediation: Current status and perspectives

Soil contamination by heavy metals poses major risks to human health and the environment. Given the current status of heavy metal pollution, many remediation techniques have been tested at laboratory and contaminated sites. The effects of soil organic matter-mediated electron transfer on heavy metal remediation have not been adequately studied, and the key mechanisms underlying this process have not yet been elucidated. In this review, microbial extracellular electron transfer pathways, organic matter electron transfer for heavy metal reduction, and the factors affecting these processes were discussed to enhance our understanding of heavy metal pollution. It was found that microbial extracellular electrons delivered by electron shuttles have the longest distance among the three electron transfer pathways, and the application of exogenous electron shuttles lays the foundation for efficient and persistent remediation of heavy metals. The organic matter-mediated electron transfer process, wherein organic matter acts as an electron shuttle, promotes the conversion of high valence state metal ions, such as Cr(VI), Hg(II), and U(VI), into less toxic and morphologically stable forms, which inhibits their mobility and bioavailability. Soil type, organic matter structural and content, heavy metal concentrations, and environmental factors (e.g., pH, redox potential, oxygen conditions, and temperature) all influence organic matter-mediated electron transfer processes and bioremediation of heavy metals. Organic matter can more effectively mediate electron transfer for heavy metal remediation under anaerobic conditions, as well as when the heavy metal content is low and the redox potential is suitable under fluvo-aquic/paddy soil conditions. Organic matter with high aromaticity, quinone groups, and phenol groups has a stronger electron transfer ability. This review provides new insights into the control and management of soil contamination and heavy metal remediation technologies.

Recent advances in enhanced technology of Cr(VI) bioreduction in aqueous condition: A review

Due to the extensive application of chromate in industry, chromium-contaminated water has emerged as a significant hidden danger that threatens human health and the safety of the ecological environment. The reduction of Cr(VI) to Cr(III) through microbial processes has become one of the most notable methods for remediating water polluted by chromium due to its economic efficiency and environmentally friendly nature. However, several issues persist in its practical application, such as low reduction rates, the need for additional nutrients, and challenges in solid-liquid separation. Therefore, there is a growing focus on seeking enhanced methods for Cr(VI) microbial reduction, which has become a key area of research. This review represents the initial effort to systematically classify and summarize the means of enhancing Cr(VI) microbial reduction. It categorizes the enhancement methods into two main approaches: microbial-based and multi-method combined enhancement, offering detailed explanations for their mechanisms. This research provides both inspiration and theoretical support for the practical implementation of the Cr(VI) microbial reduction method.

Preparation of a novel polypyrrole/dolomite composite adsorbent for efficient removal of Cr(VI) from aqueous solution

A novel adsorbent, deposited PPy on the DMI (PPy/DMI) composite, was successfully synthesized for Cr(VI) removal from aqueous solution. PPy/DMI composite was characterized by BET, SEM, TEM, XRD, and XPS. The SEM and TEM analyses revealed that DMI can greatly reduce the aggregation of PPy and significantly enhance its adsorption performance. The Cr(VI) removal was highly pH dependent. The high selectivity of PPy/DMI composite for Cr(VI) removal was found even in the presence of co-existing ions. The adsorption kinetic process followed the pseudo-second-order equation, demonstrating that the Cr(VI) adsorption behavior onto PPy/DMI is chemisorption. Furthermore, the intra-particle diffusion model implied that the adsorption was controlled by both liquid membrane diffusion and internal diffusion. The adsorption isotherm data fitted well with the Langmuir model with the maximum adsorption capacity (406.50 mg/g at 323 K) which was considerably higher than that of other PPy-based adsorbents. The Cr(VI) adsorption onto PPy/DMI composite was endothermic. The main mechanisms of Cr(VI) removal are involved in adsorption through electrostatic attractions, ion exchange, and in situ reduction. The results suggested that PPy/DMI composite could be a promising candidate for efficient Cr(VI) removal from aqueous solution.

Mechanism of Cr(VI) bioreduction by Clostridium sp. LQ25 under Fe(III) reducing conditions

The Cr(VI) bioreduction has attracted widespread attention in the field of Cr(VI) pollution remediation due to its environmental friendliness. Further in-depth research on the reduction mechanisms is necessary to enhance the efficiency of Cr(VI) bioreduction. However, the limited research on Cr(VI) bioreduction mechanisms remains a bottleneck for the practical application of Cr(VI) reduction. In this study, The Cr(VI) reduction of strain LQ25 was significantly improved when Fe(III) was used as an electron acceptor, which increased by 1.6-fold maximum within the set Cr(VI) concentration range. Based on this, the electron transfer process of Cr(VI) reduction was analyzed using strain LQ25. Based on genomic data, flavin proteins were found to interact closely with electron transfer-related proteins using protein-protein interaction (PPi) analysis. Transcriptome analysis revealed that flavin synthesis genes (ribE, ribBA, and ribH) and electron transfer flavoprotein genes (fixA, etfA, fixB, and etfB) were significantly upregulated when Fe(III) was used as the electron acceptor. These results indicate that the fermentative dissimilatory Fe(III)-reducing bacterial strain LQ25 mainly uses flavin as an electron shuttle for electron transfer, which differs from the common use of cytochrome c in respiratory bacteria. These findings on the mechanism of Cr(VI) bioreduction provide technical support for improving the efficiency of Cr(VI) reduction which promote the practical application of Cr(VI) bioreduction in the field of Cr(VI) pollution remediation.

Biostimulation effect of different amendments on Cr(VI) recovering microbial community

The present study used Cr(VI)-polluted microcosms amended with lactate or yeast extract, and nonamended microcosms as control, to investigate how a native bacterial community varied in response to the treatment and during the pollutant removal. Results suggested that providing electron donors resulted in a proliferation of a few bacterial species, with the consequent decrease in observed species richness and evenness, and was a driving force for the bacterial compositional shift. Lactate promoted, in the first instance, the enrichment of fermentative bacteria belonging to Chromobacteriaceae, including Paludibacterium, and Micrococcaceae as observed after 4 days. When the rate of Cr(VI) removal was maximum in microcosms amended with lactate, the most represented taxa were Pseudarcicella and Azospirillum. Using yeast extract as a carbon source and electron donor led instead to the significant enrichment of Shewanella, followed by Vogesella and Acinetobacter on the 4th day, corresponding to 90% of Cr(VI) removed from the system. After the complete Cr(VI) removal, achieved in 7 days in the presence of yeast extract, α-diversity was notably increased. The amendment-specific turnover of the enriched bacterial taxa resulted in a different kinetic of pollutant removal. In particular, yeast extract promoted the quickest Cr(VI) reduction, while lactate supported a slower, but also considerable, pollutant removal from water. Since it is reasonable to assume that a macroscopic effect, such as the observed Cr(VI) removal, involved the overrepresented taxa, deepening the knowledge of the native bacterial community and its changes were used to hypothesize the possible microbial pathways involved.

Goethite and riboflavin synergistically enhance Cr(VI) reduction by Shewanella oneidensis MR-1

Bioreduction of Cr(VI) is cost-effective and environmentally friendly, however, the slow bioreduction rate limits its application. In this study, the potential synergistic enhancement of Cr(VI) bioreduction by shewanella oneidensis MR-1 (S. oneidensis) with goethite and riboflavin (RF) was investigated. The results showed that the S. oneidensis reaction system reduce 29.2% of 20 mg/L Cr(VI) after 42 h reaction, while the S. oneidensis/goethite/RF reaction system increased the Cr(VI) reduction rate to 87.74%. RF as an efficient electron shuttle and Fe(II) from goethite bioreduction were identified as the crucial components in Cr(VI) reduction. XPS analysis showed that the final precipitates of Cr(VI) reduction were Cr(CH₃C(O)CHC(O)CH₃)₃ and Cr₂O₃ and adhered to the bacterial cell surface. In this process, the microbial surface functional groups such as hydroxyl and carboxyl groups participated in the adsorption and reduction of Cr(VI). Meanwhile, an increase in cytochrome c led to an increase in electron transfer system activity (ETSA), causing a significant enhancement in extracellular electron transfer efficiency. This study provides insight into the mechanism of Cr(VI) reduction in a complex environment where microorganisms, iron minerals and RF coexist, and the synergistic treatment method of Fe(III) minerals and RF has great potential application for Cr(VI) detoxification in aqueous environment.

Polycaprolactone-Modified Biochar Supported Nanoscale Zero-Valent Iron Coupling with Shewanella putrefaciens CN32 for 1,1,1-Trichloroethane Removal from Simulated Groundwater: Synthesis, Optimization, and Mechanism

1,1,1-Trichloroethane (1,1,1-TCA) is a typical organochloride solvent in groundwater that poses threats to human health and the environment due to its carcinogenesis and bioaccumulation. In this study, a novel composite with nanoscale zero-valent iron (nZVI) supported by polycaprolac-tone (PCL)-modified biochar (nZVI@PBC) was synthesized via solution intercalation and liquid-phase reduction to address the 1,1,1-TCA pollution problem in groundwater. The synergy effect and improvement mechanism of 1,1,1-TCA removal from simulated groundwater in the presence of nZVI@PBC coupling with Shewanella putrefaciens CN32 were investigated. The results were as follows: (1) The composite surface was rough and porous, and PCL and nZVI were loaded uniformly onto the biochar surface as micro-particles and nanoparticles, respectively; (2) the optimal mass ratio of PCL, biochar, and nZVI was 1:7:2, and the optimal composite dosage was 1.0% (w/v); (3) under the optimal conditions, nZVI@PBC + CN32 exhibited excellent removal performance for 1,1,1-TCA, with a removal rate of 82.98% within 360 h, while the maximum removal rate was only 41.44% in the nZVI + CN32 treatment; (4) the abundance of CN32 and the concentration of adsorbed Fe(II) in the nZVI@PBC + CN32 treatment were significantly higher than that in control treatments, while the total organic carbon (TOC) concentration first increased and then decreased during the culture process; (5) the major improvement mechanisms include the nZVI-mediated chemical reductive dechlorination and the CN32-mediated microbial dissimilatory iron reduction. In conclusion, the nZVI@PBC composite coupling with CN32 can be a potential technique to apply for 1,1,1-TCA removal in groundwater.

Moving towards the enhancement of extracellular electron transfer in electrogens

Electrogens are very common in nature and becoming a contemporary theme for research as they can be exploited for extracellular electron transfer. Extracellular electron transfer is the key mechanism behind bioelectricity generation and bioremediation of pollutants via microbes. Extracellular electron transfer mechanisms for electrogens other than Shewanella and Geobacter are less explored. An efficient extracellular electron transfer system is crucial for the sustainable future of bioelectrochemical systems. At present, the poor extracellular electron transfer efficiency remains a decisive factor in limiting the development of efficient bioelectrochemical systems. In this review article, the EET mechanisms in different electrogens (bacteria and yeast) have been focused. Apart from the well-known electron transfer mechanisms of Shewanella oneidensis and Geobacter metallireducens, a brief introduction of the EET pathway in Rhodopseudomonas palustris TIE-1, Sideroxydans lithotrophicus ES-1, Thermincola potens JR, Lysinibacillus varians GY32, Carboxydothermus ferrireducens, Enterococcus faecalis and Saccharomyces cerevisiae have been included. In addition to this, the article discusses the several approaches to anode modification and genetic engineering that may be used in order to increase the rate of extracellular electron transfer. In the side lines, this review includes the engagement of the electrogens for different applications followed by the future perspective of efficient extracellular electron transfer.

Bioreduction performance of Cr(VI) by microbial extracellular polymeric substances (EPS) and the overlooked role of tryptophan

Extracellular polymeric substances (EPS) have exhibited promising advantages in mitigating heavy metal contamination, e.g., single-valent silver (Ag(I)), trivalent gold (Au(III)), and hexavalent chromium (Cr(VI)). However, knowledge of the specific substrate in EPSs that supports Cr(VI) reduction has remained elusive. Here, we isolated a novel Cr(VI)-reducing strain with self-mediating properties in an aquatic environment with various pH values to investigate the mechanisms. After analysis by a batch assay coupled with X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), and scanning electron microscopy-energy dispersive spectrometry (SEM-EDS) spectroscopic techniques, it was found that Cr(VI) was reduced by the strain and soluble-EPS (S-EPS), and then, organo-trivalent chromium (organo-Cr(III)) was successfully formed. In addition, compared with other components of the strain, the strain and S-EPS completely removed Cr(VI), and the S-EPS exhibited a positive effect on Cr(VI) reduction with a strong monotonic correlation (R² = 0.999, p = 9.03 × 10^-5), indicating that the reduction is an EPS-dependent process. Specifically, the Cr(VI) reduction efficiency was enhanced to 48.85% and 99.4% after EPS and EPS plus tryptophan were added; their respective efficiencies were 3.94 and 8.02 times higher than that of the control assay in which the reductant was depleted. High-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) analysis showed that the tryptophan concentration concomitantly decreased by 61.54%. These findings highlighted the importance of S-EPS and tryptophan and improved our understanding of EPS for Cr(VI) reduction, which might provide a novel strategy for decontaminating targeted heavy metals in future applications.