Synergistic Effect of Soil Organic Matter and Nanoscale Zero-Valent Iron on Biodechlorination

A review of enhancement of chlorophenol bioremediation using synergistic effects between zero-valent iron and microorganisms

Chlorophenols (CPs) are a class of synthetic organic chemicals that are widely distributed in soil and groundwater, posing significant risks to human health and the environment due to persistence, acute toxicity, and potential carcinogenicity. Zero-valent iron (ZVI) has emerged as a promising remediation technique for CPs, but its efficacy is often hindered by surface passivation, non-target competition, and limited mobility in the subsurface. While CPs are inherently biodegradable, their high toxicity and the lack of functional enzymes in indigenous microbial systems restrict the effectiveness of bioremediation. Recently, a hybrid system integrating ZVI with microbial degradation draws increasingly research interests, paving out a new path for sustainable degradation of CPs. These systems leverage the synergistic interactions between ZVI and microorganisms to enhance CP biodegradation. This review provides a comprehensive analysis of the advancement. Key topics include the enhancement of electron transfer, alterations to microbial communities, mitigation of toxicity, and the interplay between other processes. Operation modes, ZVI dosage, and interactions with naturally occurring iron minerals, are also discussed in the context of applications in soil and groundwater remediation. Despite research efforts and successful implementations, critical knowledge gaps remain, particularly in regard to the characterization of microbial processes in natural systems, highlighting the need for future research.

Biochar-based composite microspheres embedded with zero-valent iron and soybean oil efficiently remove 1,1,1-trichloroethane and reshape microbial community in simulated groundwater

The increasing contamination of global groundwater by organic chlorine solvents poses a major threat to environmental and human health; however, there is a lack of structurally stable and effective materials for removing organic chlorine pollutants. In this study, biochar-based composite microspheres embedded with zero-valent iron (ZVI) and soybean oil were prepared and their effects on 1,1,1-trichloroethane (1,1,1-TCA) removal and the microbial community in simulated groundwater system were investigated. The composite microspheres achieved a remarkable 85.79% removal rate of 1,1,1-TCA after 360 h in groundwater, which was 1.63 times higher than that of ZVI + biochar microspheres (52.69%) and 1.33 times higher than that of soybean oil + biochar microspheres (64.50%). The composite microspheres also significantly reduced the oxidation-reduction potential to - 248.52 mV and maintained a neutral pH range of 6.8-7.2, thereby creating favorable conditions for long-term reductive dechlorination. The surface morphology of the composite was stable during degradation, reflecting its potential for long-term usage. The rich network structure of microspheres and the micropore structure of the biochar were conducive to the capturing of pollutants, safety of microorganisms, and slow release of organic carbon. 16S rDNA sequencing demonstrated that the composite significantly affected the diversity and stability of the microbial community, especially promoting the growth and interaction of dechlorinating and fermentative microorganisms in the groundwater and composite microspheres. The preliminary removal mechanisms included biochar-induced adsorption and ZVI-induced chemical reduction in the early stage and biochemical coupling of dechlorination in the middle and last stages. The biochar-based composite microspheres significantly enhanced the effectiveness and consistency of 1,1,1-TCA removal, potentially being applied to in situ enhanced reductive dechlorination of organochlorine solvents in site groundwater. Moreover, considering the abundant porous structure and easy availability of biochar, it can effectively promote the sustainability and cost-efficiency of the microspheres during application.

Biochars-inlaided nano zero-valent iron reactors: A tool for visualized analysis of soil-nanomaterials micro-interfacial interaction in soil remediation process

It is a great challenge to depict the evolution process of soil-nanomaterials micro-interfaces during soil remediation. A novel biochar loaded nano zero-valent iron (BC-nZVI) reactor with low density, high reactivity and suitable magnetism was prepared using the method we established. Fe0 nanoparticles (NPs) with the size <10 nm uniformly embedded in a layer of porous carbon networks, which attached firmly in the pores and outer surface of biochars. The BC-nZVI reactors efficiently degraded and mineralized pentachlorophenol (PCP) in soil in a wide pH range within 72 h at room temperature. BC-nZVI were easily collected from soil using sieving method or floating technology followed with magnetic separation. The evolution of interfacial properties of BC-nZVI was measured using XPS, Raman spectroscopy, XRD and SEM-EDS. Even supported by BCs, aggregation and oxidation of Fe NPs inevitably took place on the outer surface of BC-nZVI in PCP remediation process. For BC-nZVIFeSO4 prepared via traditional impregnation and carbothermal method, severe agglomeration and various aging products of Fe NPs were seen in the pores as well. Due to the protection of porous carbon networks, Fe NPs aggregation was greatly mitigated in the pores of BC-nZVI and the Fe2+/Fe3+ ratio on BC-nZVI surface was still as high as the fresh one. Meanwhile, about 10-20 % of carbon matrix was oxidized and turned into amorphous carbon. The Fe NPs corrosion products, broken carbon phase, and K+ of BC-nZVI reactors were released into soil and had little effect or even enhanced the soil bacterial community diversity and biomass. However, the BC-nZVI itself showed certain adverse effect to soil microbes.

Nano-micro materials regulated biocatalytic metabolism for efficient environmental remediation: Fine engineering the mass and electron transfer in multicellular environments

The escalating energy and environmental crises have spurred significant research interest into developing efficient biological remediation technologies for sustainable contaminant and resource conversion. Integrating engineered nano-micro materials (NMMs) with these biocatalytic processes offers a promising approach to improve the microbial performance for environmental remediation. Core to such material-enhanced hybrid biocatalysis systems (MHBSs) is the rational regulation of metabolic processes with the assistance of NMMs, where a fine engineered mass and electron transfer is beneficial for the improved biocatalytic activity. However, the specific mechanisms of those NMMs-enhanced microbial metabolisms are normally overlooked. Here, we review the recent progress in MHBSs, focusing primarily on the mass/electron transfer regulation strategies for an enhanced microbial behavior. Specifically, the NMMs-regulated mass and electron transfer in extracellular, interfacial, and intracellular environment are summarized, where the patterns of diverse microbiological response are discussed thoroughly. Notably, fine modifications of cell interfaces and intracellular compartments by NMMs could even endow the biohybrids with new metabolic functions beyond their natural capabilities. Further, we also emphasize the importance of matching the various metabolic demands of biosystems with the diverse properties of NMMs to achieve efficient environmental remediation through a coordinated regulation strategy. Finally, major challenges and opportunities for the future development and practical implementation of MHBSs for environment remediation practices are given, aiming to provide future system design guidelines for attaining desirable biological behaviors.

Biogenic amorphous FeOOH activated additional intracellular electron flow pathways for accelerating reductive dechlorination of tetrachloroethylene

Dissimilatory iron-reducing bacteria (DIRB) with extracellular electron transfer (EET) capabilities have shown significant potential for bioremediating halogenated hydrocarbon contaminated sites rich in iron and humic substances. However, the role and microbial molecular mechanisms of iron-humic acid (Fe-HA) complexes in the reductive dehalogenation process of DIRB remains inadequately elucidated. In this study, we developed a sustainable carbon cycling approach using Fe-HA complexes to modulate the electron flux from sawdust (SD), enabling almost complete reductive dechlorination by most DIRB (e.g., Shewanella oneidensis MR-1) that lack complex iron-sulfur molybdo enzymes. The SD-Fe-HA/MR-1 system achieved a 96.52% removal efficiency of tetrachloroethylene (PCE) at concentrations up to 250 μmol/L within 60 days. Material characterization revealed that DIRB facilitated the hydrolysis of macromolecular carbon sources by inducing the formation of amorphous ferrihydrite (FeOOH) in Fe-HA complexes. More importantly, the bioavailable FeOOH activated additional intracellular electron flow pathways, increasing the activity of potential dehalogenases. Transcriptome further highlight the innovative role of biogenic amorphous FeOOH in integrating intracellular redox metabolism with extracellular charge exchange to facilitate reductive dechlorination in DIRB. These findings provide novel insights into accelerating reductive dechlorination in-situ contaminated sites lacking obligate dehalogenating bacteria.

Enhancing pakchoi cabbage yield and quality but reducing human-disease risk of bacterial community from wastewater irrigation by combined nanoscale zerovalent iron and nitrification inhibitor

It was indispensable to seek effective and feasible measures to alleviate the adverse effects of wastewater irrigation. Nanoscale zerovalent iron (nZVI) and soil nitrogen management might enhance the vegetable yield and quality but mitigate the potential human-disease risks from wastewater irrigation. This study selected the nZVI and nitrification inhibitor as experimental objects. The planted pakchoi cabbage was irrigated with the tap water and wastewater and treated with nZVI and 3, 4-dimethylpyrazole phosphate (DMPP), respectively, the pakchoi cabbage yield and quality, soil enzyme activity and abiotic property, and human-disease risk of bacterial community were quantified. Compared with the control, the nZVI significantly enhanced the pakchoi cabbage yield by 51.5% but reduced the pakchoi cabbage nitrate content by 52.6% under wastewater irrigation condition. The nZVI alone had double-edged sword effects of increasing the pakchoi cabbage yield, reducing the pakchoi cabbage nitrate content and soil human-disease risk but inhibiting the system multifunctionality and soil bacterial community diversity and stability, under wastewater irrigation condition. The nZVI diminished human-disease risk via increasing the soil Firmicutes and Verrucomicrobiota ratios, and the extra DMPP could mitigate the negative effects of nZVI by increasing soil enzyme activity and stimulating soil Acidobacteria ratio. The combinations of nZVI and DMPP could not only enhance the pakchoi cabbage yield and quality but also reduce the human-disease risk of soil bacterial community from wastewater irrigation.

Multifunctional biomolecular corona-inspired nanoremediation of antibiotic residues

Facing complex and variable emerging antibiotic pollutants, the traditional development of functional materials is a "trial-and-error" process based on physicochemical principles, where laborious steps and long timescales make it difficult to accelerate technical breakthroughs. Notably, natural biomolecular coronas derived from highly tolerant organisms under significant contamination scenarios can be used in conjunction with nanotechnology to tackling emerging contaminants of concern. Here, super worms (Tubifex tubifex) with high pollutant tolerance were integrated with nano-zero valent iron (nZVI) to effectively reduce the content of 17 antibiotics in wastewater within 7 d. Inspired by the synergistic remediation, nZVI-augmented worms were constructed as biological nanocomposites. Neither nZVI (0.3 to 3 g/L) nor worms (104 to 105 per liter) alone efficiently degraded florfenicol (FF, as a representative antibiotic), while their composite removed 87% of FF (3 μmol/L). Under antibiotic exposure, biomolecules secreted by worms formed a corona on and modified the nZVI particle surface, enabling the nano-bio interface greater functionality, including responsiveness, enrichment, and reduction. Mechanistically, FF exposure activated glucose-alanine cycle pathways that synthesize organic acids and amines as major metabolites, which were assembled into vesicles and secreted, thereby interacting with nZVI in a biologically response design strategy. Lactic acid and urea formed hydrogen bonds with FF, enriched analyte presence at the heterogeneous interface. Succinic and lactic acids corroded the nZVI passivation layer and promoted electron transfer through surface conjugation. This unique strategy highlights biomolecular coronas as a complex resource to augment nano-enabled technologies and will provide shortcuts for rational manipulation of nanomaterial surfaces with coordinated multifunctionalities.

Environmental remediation approaches by nanoscale zero valent iron (nZVI) based on its reductivity: a review

The fast rise of organic and metallic pollution has brought significant risks to human health and the ecological environment. Consequently, the remediation of wastewater is in extremely urgent demand and has received increasing attention. Nanoscale zero valent iron (nZVI) possesses a high specific surface area and distinctive reactive interfaces, which offer plentiful active sites for the reduction, oxidation, and adsorption of contaminants. Given these abundant functionalities of nZVI, it has undergone significant and extensive studies on environmental remediation, linking to various mechanisms, such as reduction, oxidation, surface complexation, and coprecipitation, which have shown great promise for application in wastewater treatment. Among these functionalities of nZVI, reductivity is particularly important and widely adopted in dehalogenation, and reduction of nitrate, nitro compounds, and metal ions. The following review comprises a short survey of the most recent reports on the applications of nZVI based on its reductivity. It contains five sections, an introduction to the theme, chemical reduction applications, electrolysis-assisted reduction applications, bacterium-assisted reduction applications, and conclusions about the reported research with perspectives for future developments. Review and elaboration of the recent reductivity-dependent applications of nZVI may not only facilitate the development of more effective and sustainable nZVI materials and the protocols for comprehensive utilization of nZVI, but may also promote the exploration of innovative remediation approaches based on its reductivity.

Controllable chemical redox reactions to couple microbial degradation for organic contaminated sites remediation: A review

Global environmental concern over organic contaminated sites has been progressively conspicuous during the process of urbanization and industrial restructuring. While traditional physical or chemical remediation technologies may significantly destroy the soil structure and function, coupling moderate chemical degradation with microbial remediation becomes a potential way for the green, economic, and efficient remediation of contaminated sites. Hence, this work systematically elucidates why and how to couple chemical technology with microbial remediation, mainly focused on the controllable redox reactions of organic contaminants. The rational design of materials structure, selective generation of reactive oxygen species, and estimation of degradation pathway are described for chemical oxidation. Meanwhile, current progress on efficient and selective reductions of organic contaminants (i.e., dechlorination, defluorination, -NO2 reduction) is introduced. Combined with the microbial remediation of contaminated sites, several consideration factors of how to couple chemical and microbial remediation are proposed based on both fundamental and practical points of view. This review will advance the understanding and development of chemical-microbial coupled remediation for organic contaminated sites.

Synergistic binding mechanisms of co-contaminants in soil profiles: Influence of iron-bearing minerals and microbial communities

In contaminated soil sites, the coexistence of inorganic and organic contaminants poses a significant threat to both the surrounding ecosystem and public health. However, the migration characteristics of these co-contaminants within the soil and their interactions with key components, including Fe-bearing minerals, organic matter, and microorganisms, remain unclear. This study involved the collection of a 4.3-m-depth co-contaminated soil profile to investigate the vertical distribution patterns of co-contaminants (namely, arsenic, cadmium, and polychlorinated biphenyls (PCBs)) and their binding mechanisms with environmental factors. The results indicated a notable downward accumulation of inorganic contaminants with increasing soil depth, whereas PCBs were predominantly concentrated in the uppermost layer. Chemical extraction and synchrotron radiation analysis highlighted a positive correlation between the abundance of reactive iron (FeCBD) and both co-contaminants and microbial communities in the contaminated site. Furthermore, Mantel tests and structural equation modeling (SEM) demonstrated the direct impacts of FeCBD and microbial communities on co-contaminants within the soil profile. Overall, these results provided valuable insights into the migration and transformation characteristics of co-contaminants and their binding mechanisms mediated by minerals, organic matter, and microorganisms.

Pentachlorophenol affects doxycycline and tetracycline resistance genes in soil by altering microbial structure

Tetracycline antibiotics play a vital role in animal husbandry, primarily employed to uphold the health of livestock and poultry. Consequently, when manure is reintegrated into farmland, tetracycline antibiotics can persist in the soil. Simultaneously, to ensure optimal crop production, organochlorine pesticides (OCPs) are frequently applied to farmland. The coexistence of tetracycline antibiotics and OCPs in soil may lead to an increased risk of transmission of tetracycline resistance genes (TRGs). Nevertheless, the precise mechanism underlying the effects of OCPs on tetracycline antibiotics and TRGs remains elusive. In this study, we aimed to investigate the effects of OCPs on soil tetracycline antibiotics and TRGs using different concentrations of doxycycline (DOX) and pentachlorophenol (PCP). The findings indicate that PCP and DOX mutually impede their degradation in soil. Furthermore, our investigation identifies Sphingomonas and Bacillus as potential pivotal microorganisms influencing the reciprocal inhibition of PCP and DOX. Additionally, it is observed that the concurrent presence of PCP and DOX could impede each other's degradation by elevating soil conductivity. Furthermore, we observed that a high concentration of PCP (10.7 mg/kg) reduced the content of efflux pump tetA, ribosome protective protein tetM, tetQ, and passivating enzyme tetX. In contrast, a low PCP concentration (6.4 mg/kg) only reduced the content of ribosome protective protein tetQ. This suggests that PCP may reduce the relative abundance of TRGs by altering the soil microbial community structure and inhibiting the potential host bacteria of TRGs. These findings have significant implications in understanding the combined pollution of veterinary antibiotics and OCPs. By shedding light on the interactions between these compounds and their impact on microbial communities, this study provides a theoretical basis for developing strategies to manage and mitigate their environmental impact, and may give some information regarding the sustainable use of antibiotics and pesticides to ensure the long-term health and productivity of agricultural systems.

Fe/N co-doped magnetic porous hydrochar for chromium(VI) removal in water: Adsorption performance and mechanism investigation

Four kinds of Fe/N co-doped porous hydrochar were prepared by one/two-step N-doping schemes using microwave/traditional pyrolysis methods for removing Cr(VI) from aqueous phase. Heterocyclic-N was introduced through CO(NH2)2-based hydrothermal carbonization process, which could adjust the electronic structure of the hydrochar framework. Furthermore, Fe0 and Fe3O4 were embedded into hydrochar via carbothermal reduction reaction using FeCl3 as the precursor, which improved the reducibility and magnetism of the material. The modified hydrochar exhibited pH-dependency and rapid kinetic equilibrium, and the maximal adsorption amount of magnetic porous hydrochar obtained by microwave-assisted one-step N-doping (MP1HCMW) reached 274.34 mg/g. Meanwhile, the modified hydrochar had a high tolerance to multiple co-existing ions and the removal efficiency maintained above 73.91 % during five regeneration cycles. Additionally, MP1HCMW efficiently removed Cr(VI) via pore filling, electrostatic attraction, ion exchange, reduction, complexation, and precipitation. Summarily, Fe/N co-doped porous hydrochar was a feasible adsorbent with outstanding remediation potential for Cr(VI)-contaminated water.

Reciprocal Interactions of Abiotic and Biotic Dechlorination of Chloroethenes in Soil

Chloroethenes (CEs) as common organic pollutants in soil could be attenuated via abiotic and biotic dechlorination. Nonetheless, information on the key catalyzing matter and their reciprocal interactions remains scarce. In this study, FeS was identified as a major catalyzing matter in soil for the abiotic dechlorination of CEs, and acetylene could be employed as an indicator of the FeS-mediated abiotic CE-dechlorination. Organohalide-respiring bacteria (OHRB)-mediated dechlorination enhanced abiotic CEs-to-acetylene potential by providing dichloroethenes (DCEs) and trichloroethene (TCE) since chlorination extent determined CEs-to-acetylene potential with an order of trans-DCE > cis-DCE > TCE > tetrachloroethene/PCE. In contrast, FeS was shown to inhibit OHRB-mediated dechlorination, inhibition of which could be alleviated by the addition of soil humic substances. Moreover, sulfate-reducing bacteria and fermenting microorganisms affected FeS-mediated abiotic dechlorination by re-generation of FeS and providing short chain fatty acids, respectively. A new scenario was proposed to elucidate major abiotic and biotic processes and their reciprocal interactions in determining the fate of CEs in soil. Our results may guide the sustainable management of CE-contaminated sites by providing insights into interactions of the abiotic and biotic dechlorination in soil.

Fe-Based Nanomaterials and Plant Growth Promoting Rhizobacteria Synergistically Degrade Polychlorinated Biphenyls by Producing Extracellular Reactive Oxygen Species

Plant growth promoting rhizobacteria (PGPR) produce extracellular reactive oxygen species (ROS) to protect plants from external stresses. Fe-based nanomaterials can potentially interact with PGPR and synergistically degrade organic pollutants, yet they have received no study. Here, we studied how the interaction between a typical PGPR (Pseudomonas chlororaphis, JD37) and Fe-based nanomaterials facilitated the degradation of 2,4,4'-trichlorobiphenyl (PCB28), by comparing the zerovalent iron of 20 nm (nZVI20), 100 nm (nZVI100), and 5 μm; iron oxide nanomaterials (α-Fe2O3, γ-Fe2O3, and Fe3O4) of ca. 20 nm; and ferrous and ferric salts. Although all Fe materials (0.1 g L-1) alone could not degrade aqueous PCB28 (0.1 mg L-1) under dark or aerobic conditions, nZVI20, nZVI100, α-Fe2O3, and Fe2+ promoted PCB28 degradation by JD37, with the half-life of PCB28 shortened from 16.5 h by JD37 alone to 8.1 h with nZVI100 cotreatment. Mechanistically, the nanomaterials stimulated JD37 to secrete phenazine-1-carboxylic acid and accelerated the NADH/NAD+ conversion, promoting O2*- generation; JD37 increased Fe(II) dissolution from the nanomaterials, facilitating *OH generation; and the ROS gradually degraded PCB28 into benzoic acid through dihydroxy substitution, oxidation to quinone, and Michael addition. These findings provide a new strategy of nanoenabled biodegradation of organic pollutants by applying Fe-based nanomaterials and PGPR.

Functional materials contributing to the removal of chlorinated hydrocarbons from soil and groundwater: Classification and intrinsic chemical-biological removal mechanisms

Chlorinated hydrocarbons (CHs) are the main contaminants in soil and groundwater and have posed great challenge on the remediation of soil and ground water. Different remediation materials have been developed to deal with the environmental problems caused by CHs. Remediation materials can be classified into three main categories according to the corresponding technologies: adsorption materials, chemical reduction materials and bioaugmentation materials. In this paper, the classification and preparation of the three materials are briefly described in terms of synthesis and properties according to the different types. Then, a detailed review of the remediation mechanisms and applications of the different materials in soil and groundwater remediation is presented in relation to the various properties of the materials and the different challenges encountered in laboratory research or in the environmental application. The removal trends in different environments were found to be largely similar, which means that composite materials tend to be more effective in removing CHs in actual remediation. For instance, adsorbents were found to be effective when combined with other materials, due to the ability to take advantage of the respective strengths of both materials. The rapid removal of CHs while minimizing the impact of CHs on another material and the material itself on the environment. Finally, suggestions for the next research directions are given in conjunction with this paper.

In situ reductive dehalogenation of groundwater driven by innovative organic carbon source materials: Insights into the organohalide-respiratory electron transport chain

In situ bioremediation using organohalide-respiring bacteria (OHRB) is a prospective method for the removal of persistent halogenated organic pollutants from groundwater, as OHRB can utilize H₂ or organic compounds produced by carbon source materials as electron donors for cell growth through organohalide respiration. However, few previous studies have determined the suitability of different carbon source materials to the metabolic mechanism of reductive dehalogenation from the perspective of electron transfer. The focus of this critical review was to reveal the interactions and relationships between carbon source materials and functional microbes, in terms of the electron transfer mechanism. Furthermore, this review illustrates some innovative strategies that have used the physiological characteristics of OHRB to guide the optimization of carbon source materials, improving the abundance of indigenous dehalogenated bacteria and enhancing electron transfer efficiency. Finally, it is proposed that future research should combine multi-omics analysis with machine learning (ML) to guide the design of effective carbon source materials and optimize current dehalogenation bioremediation strategies to reduce the cost and footprint of practical groundwater bioremediation applications.

Earthworms enhance the bioremediation of tris(2-butoxyethyl) phosphate-contaminated soil by releasing degrading microbes

The escalating awareness of the environmental risks posed by organophosphorus flame retardants (OPFRs), e.g., tris(2-butoxyethyl) phosphate (TBOEP), necessitates the development of effective approaches to mitigate their adverse ecological effects. However, research on the remediation of OPFR-contaminated soil remains limited. In this study, a strategy is proposed to enhance the microbial remediation of TBOEP-contaminated soil through the introduction of exotic earthworms (Eisenia fetida). The presence of earthworms led to a substantial increase in the 28-d removal rates of TBOEP at concentrations of 0.05, 0.5, and 5 mg/kg, with improvements of 32.3 ± 2.0%, 33.2 ± 1.3%, and 33.0 ± 5.6% compared to rates in the absence of earthworms, respectively. The underlying mechanisms for this enhancement include the earthworm-mediated enrichment of TBOEP-degrading bacteria, particularly Rhodococcus, Flavobacterium, and Pseudomonas, and the transfer of Rhodococcus from the earthworm gut to the soil, resulting in an increased relative abundance within the soil. Concurrently, the earthworms stimulated soil peroxidase activity, facilitating the oxidative degradation of TBOEP. Furthermore, the rise in dissolved organic matter content following earthworm treatment fostered the growth of degrading bacteria in the soil. Rhodococcus emerged as a dominant contributor to soil TBOEP removal, consuming humic-like compounds in dissolved organic matter. This investigation underscores the significance of gut microbes and offers valuable insights for the application of earthworm-based remediation strategies in OPFR-contaminated soil.

Efficient degradation of hexabromocyclododecane using montmorillonite supported nano-zero-valent iron and Citrobacter sp. Y3

The coupling of modified nanoscale zero-valent iron (nZVI) with organohalide-degrading bacteria provides a promising solution for the remediation of hexabromocyclododecane (HBCD)-contaminated environments. However, the interactions between modified nZVI and dehalogenase bacteria are intricate, and the mechanisms of synergistic action and electron transfer are not clear, and requires further specific investigation. In this study, HBCD was used as a model pollutant, and stable isotope analysis revealed that organic montmorillonite (OMt)-supported nZVI coupled with the degrading bacterial strain Citrobacter sp. Y3 (nZVI/OMt-Y3) can use [¹³C]HBCD as the sole carbon source and degrade or even mineralise it into ¹³CO₂ with a maximum conversion rate of 100% within approximately 5 days. Analysis of the intermediates showed that the degradation of HBCD mainly involves three different pathways: dehydrobromination, hydroxylation, and debromination. The proteomics results showed that nZVI introduction promoted the transport of electrons and debromination. Combining the results from XPS, FTIR, and Raman spectroscopy with the analysis results of proteinomics and biodegradation products, we verified the process of electron transport and proposed a metabolic mechanism of HBCD degradation by the nZVI/OMt-Y3. Moreover, this study provides insightful avenues and models for the further remediation of HBCD and other similar pollutants in the environment.

Chemodiversity of soil organic matters determines biodegradation of polychlorinated biphenyls by a graphene oxide-assisted bacterial agent

A promising strategy for degrading persistent organic pollutants (POPs) in soil is amendment with nanomaterial-assisted functional bacteria. However, the influence of soil organic matter chemodiversity on the performance of nanomaterial-assisted bacterial agents remains unclear. Herein, different types of soil (Mollisol soil, MS; Ultisol soil, US; and Inceptisol soil, IS) were inoculated with a graphene oxide (GO)-assisted bacterial agent (Bradyrhizobium diazoefficiens USDA 110, B. diazoefficiens USDA 110) to investigate the association between soil organic matter chemodiversity and stimulation of polychlorinated biphenyl (PCB) degradation. Results indicated that the high-aromatic solid organic matter (SOM) inhibited PCB bioavailability, and lignin-dominant dissolved organic matter (DOM) with high biotransformation potential was a favored substrate for all PCB degraders, which led to no stimulation of PCB degradation in MS. Differently, high-aliphatic SOM in US and IS promoted PCB bioavailability. The high/low biotransformation potential of multiple DOM components (e.g., lignin, condensed hydrocarbon, unsaturated hydrocarbon, etc.) in US/IS further resulted to the enhanced PCB degradation by B. diazoefficiens USDA 110 (up to 30.34%) /all PCB degraders (up to 17.65%), respectively. Overall, the category and biotransformation potential of DOM components and the aromaticity of SOM collaboratively determine the stimulation of GO-assisted bacterial agent on PCB degradation.

Activation of chloride by oxygen vacancies-enriched TiO2 photoanode for efficient photoelectrochemical treatment of persistent organic pollutants and simultaneous H2 generation

Photoelectrochemical (PEC) activation of chloride ions (Cl^-) to degrade persistent organic pollutants (POPs) is a promising strategy for the treatment of industrial saline organic wastewater. However, the wide application of this technology is greatly restricted due to the general photoanode activation of Cl^- with poor capability, the propensity to produce toxic by-products chlorates, and the narrow pH range. Herein, oxygen vacancies-enriched titanium dioxide (O_v-TiO₂) photoanode is explored to strongly activate Cl^- to drive the deep mineralization of POPs wastewater in a wide pH range (2-12) with simultaneous production of H₂. More importantly, nearly no toxic by-product of chlorates was produced during such PEC-Cl system. The degradation efficiency of 4-CP and H₂ generation rate by O_v-TiO₂ were 99.9% within 60 min and 198.2 μmol h^-1 cm^-2, respectively, which are far superior to that on the TiO₂ (33.1% within 60 min, 27.5 μmol h^-1 cm^-2) working electrode. DFT calculation and capture experiments revealed that O_v-TiO₂ with abundant oxygen vacancies is conducive to the activation of Cl^- to produce more reactive chlorine species, evidenced by its high production of free chlorine (48.7 mg L^-1 vs 7.5 mg L^-1 of TiO₂). The as-designed PEC-Cl system in this work is expected to realize the purification of industrial saline organic wastewater coupling with green energy H₂ evolution.

Nanoscale Interactions of Humic Acid and Minerals Reveal Mechanisms of Carbon Protection in Soil

The concentrations of terrestrially sourced dissolved organic matter (DOM) have expanded throughout aquatic ecosystems in recent decades. Although sorption to minerals in soils is one major pathway to sequestrate soil organic matter, the mechanisms of organic matter-mineral interactions are not thoroughly understood. Here, we investigated the effect of calcium phosphate mineralization on humic acid (HA) fixation in simulated soil solutions, either with or without clay mineral montmorillonite (Mt). We found that Mt in solution promoted nucleation and crystallization of calcium phosphate (CaP) due to amorphous calcium phosphate clustering and coalescence on Mt surface, which contributed to the long-term persistence and accumulation of HA. Organic ligands with specific chemical groups on HA have higher binding energies to CaP-Mt than to CaP/Mt, according to dynamic force spectroscopy observations. Moreover, CaP-Mt formed in solution showed a great capacity for HA adsorption with a maximum adsorption quantity of 156.89 mg/g. Our findings directly support that Mt is crucial for DOM sequestration by facilitating CaP precipitation/transformation. This has an impact on how effectively we understand the long-term turnover of DOM and highlights knowledge gaps that might assist in resolving essential soil C sequestration issues.