Response of 2,4,6-trichlorophenol-reducing biocathode to burial depth in constructed wetland sediments

Review of cathodic electroactive bacteria: Species, properties, applications and electron transfer mechanisms

Cathodic electroactive bacteria (C-EAB) which are capable of accepting electrons from solid electrodes provide fresh avenues for pollutant removal, biosensor design, and electrosynthesis. This review systematically summarized the burgeoning applications of the C-EAB over the past decade, including 1) removal of nitrate, aromatic derivatives, and metal ions; 2) biosensing based on biocathode; 3) electrosynthesis of CH4, H2, organic carbon, NH3, and protein. In addition, the mechanisms of electron transfer by the C-EAB are also classified and summarized. Extracellular electron transfer and interspecies electron transfer have been introduced, and the electron transport mechanism of typical C-EAB, such as Shewanella oneidensis MR-1, has been combed in detail. By bringing to light this cutting-edge area of the C-EAB, this review aims to stimulate more interest and research on not only exploring great potential applications of these electron-accepting bacteria, but also developing steady and scalable processes harnessing biocathodes.

Complete 2,4,6-trichlorophenol degradation by anaerobic sludge acclimated with 4-chlorophenol: Synergetic effect of nZVI@BMPC and sodium lactate as an external nutrient

Ball-milled plastic char supported nano zero-valent iron (nZVI@BMPC) and their application combined with anaerobic sludge for microbial dechlorination of 2,4,6-trichlorophenol (2,4,6-TCP) were investigated. The XRD and FTIR analysis proved composition of zero valent states of iron, and the BET and SEM analysis showed that nZVI was uniformly distributed on the surface of BMPC. Successive addition of 1000 mg/L sodium lactate and nZVI@BMPC enhanced the acclamation of anaerobic sludge and resulted in the degradation of 4-CP within 80 days. The acclimated consortium with nZVI@BMPC completely degraded 2,4,6-TCP into CH4 and CO2, and the key dechlorination route was through 4-CP dechlorinaion and mineralization. The degradation rate of 2,4,6-TCP with nZVI@BMPC was 0.22/d, greater than that without nZVI@BMPC. The dechlorination efficiency was enhanced in the Fe2+/Fe3+ system controlled by nZVI@BMPC and iron-reducing bacteria. Metagenomic analysis result showed that the dominant de-chlorinators were Chloroflexi sp., Desulfovibrio, and Pseudomonas, which could directly degrade 2,4,6-TCP to 4-CP, especially, Chloroflexi bacterium could concurrently be used to mineralize 4-CP. The relative abundance of the functional genes cprA, acoA, acoB, and tfdB increased significantly in the presence of the nZVI@BMPC. This study provides a new strategy can be a good alternative for possible application in groundwater remediation.

Current advances of chlorinated organics degradation by bioelectrochemical systems: a review

Chlorinated organic compounds (COCs) are typical refractory organic compounds, having high biological toxicity. These compounds are a type of pervasive pollutants that can be present in polluted soil, air, and various types of waterways, such as groundwater, rivers, and lakes, posing a significant threat to the ecological environment and human health. Bioelectrochemical systems (BESs) are an effective strategy for the degradation of bio-refractory compounds. BESs improve the waste treatment efficiency through the application of weak electrical stimulation. This review discusses the processes of BESs configurations and degradation performances in different environmental media including wastewater, soil, waste gas and groundwater. In addition, the degradation mechanisms and performance-enhancing additives are summarized. The future challenges and perspectives on the development of BES for COCs removal are briefly discussed.

Unveiling the effect of different dissolved organic matter (DOM) on catalytic dechlorination of nFe/Ni particles: Corrosion and passivation effect

Dissolved organic matter (DOM), which is ubiquitously distributed in groundwater, has a crucial role in the fate and reactivity of iron materials. However, there is a lack of direct evidence on how different DOMs interact with nFe/Ni in promoting or inhibiting the dechlorination efficiency of chlorinated aromatic contaminants. By comparing humic acid (HA), fulvic acid (FA), and biochar-derived dissolved organic matter (BDOM) at different pyrolysis temperatures, we first demonstrated that the dechlorination effect of nFe/Ni on 2,4-dichlorophenol (2,4-DCP) depended on the nature of DOMs and their adsorption on nFe/Ni. HA showed an enhancing effect on the dechlorination of 2,4-DCP by nFe/Ni, while the inhibition effect of other DOMs resulted in the following dechlorination order: BDOM300 ≈FA>BDOM700 ≈BDOM500. The C2 component with higher aromaticity and molecular weight promoted the corrosion of nFe/Ni and the production of reactive hydrogen atoms (H*). The effects of different DOMs on nFe/Ni include that (1) HA accelerates the corrosion and H* production of nFe/Ni, (2) FA and BDOM300 enhance the corrosion but inhibit H* production, and (3) Both nFe/Ni corrosion and H* formation are suppressed by BDOM500/BDOM700. Therefore, this study will provide a reference for understanding the nature of DOM-nFe/Ni interaction and improving the catalytic activity of nFe/Ni when different DOMs coexist in practical applications.

Evaluating groundwater ecosystem dynamics in response to post in-situ remediation of mixed chlorinated volatile organic compounds (CVOCs): An insight into microbial community resilience, adaptability, and metabolic functionality for sustainable remediation and ecosystem restoration

The in-situ remediation of groundwater contaminated with mixed chlorinated volatile organic compounds (CVOCs) has become a significant global research interest. However, limited attention has been given in understanding the effects of these remediation efforts on the groundwater microbial communities, which are vital for maintaining ecosystem health through their involvement in biogeochemical cycles. Hence, this study aimed to provide valuable insights into the impacts of in-situ remediation methods on groundwater microbial communities and ecosystem functionality, employing high-throughput sequencing coupled with functional and physiological assays. The results showed that both bioremediation and chemical remediation methods adversely affected microbial diversity and abundance compared to non-polluted sites. Certain taxa such as Pseudomonas, Acinetobacter, and Vogesella were sensitive to these remediation methods, while Aquabacterium exhibited greater adaptability. Functional annotation unveiled the beneficial impact of bioremediation on the sulfur cycle and specific taxa such as Cellvibrio, Massilia, Algoriphagus, and Flavobacterium which showed a significant positive relationship with dark oxidation of sulfur compounds. In contrast, chemical remediation showed adverse impacts on the nitrogen cycle with a reduced abundance of nitrogen and nitrate respiration along with a reduced utilization of amines (nitrogen rich substrate). The findings of this study offer valuable insights into the potential impacts of in-situ remediation methods on groundwater microbial communities and ecosystem functionality, emphasizing the need for meticulous consideration to ensure the implementation of effective and sustainable remediation strategies that safeguard ecosystem health and function.

Advances and challenges in biocathode microbial electrolysis cells for chlorinated organic compounds degradation from electroactive perspectives

Microbial electrolysis cell (MEC) is a promising in-situ strategy for chlorinated organic compound (COC) pollution remediation due to its high efficiency, low energy input, and long-term potential. Reductive dechlorination as the most critical step in COC degradation which takes place primarily in the cathode chamber of MECs is a complex biochemical process driven by the behavior of electrons. However, no information is currently available on the internal mechanism of MEC in dechlorination from the perspective of the whole electron transfer procedure and its dependent electrode materials. This review addresses the underlying mechanism of MEC on the fundamental of the generation (electron donor), transmission (transfer pathway), utilization (functional microbiota) and reception (electron acceptor) of electrons in dechlorination. In addition, the vital role of varied cathode materials involved in the entire electron transfer procedure during COC dechlorination is emphasized. Subsequently, suggestions for future research, including model construction, cathode material modification, and expanding the applicability of MECs to removal gaseous COCs have been proposed. This paper enriches the mechanism of COC degradation by MEC, and thus provides the theoretical support for the scale-up bioreactors for efficient COC removal.

Effect of weak electrical stimulation on m-dichlorobenzene biodegradation in biotrickling filters: Insights from performance and microbial community analysis

The microbial electrolysis cell coupled with the biotrickling filters (MEC-BTF) was developed for enhancing the biodegradation of gaseous m-dichlorobenzene (m-DCB) through weak electrical stimulation. The maximum removal efficiency and elimination capacity in MEC-BTF were 1.48 and 1.65 times higher than those in open-circuit BTF (OC-BTF), respectively. Weak electrical stimulation had a positive impact on the characteristics of the biofilm. Additionally, microbial community analysis revealed that weak electrical stimulation increased the abundance of key functional genera (e.g., Rhodanobacter and Bacillus) and genes (e.g., catA/E and E1.3.1.32), thereby accelerating reductive dechlorination and ring-opening of m-DCB. Macrogenomic sequencing further revealed that electron transfer pathway in MEC-BTF might be mediated through extracellular electroactive mediators and cytochromes.

Response of microbial community to different media in start-up period of Annan constructed wetland in Beijing of China

Microbial community, as the decomposers of constructed wetland (CW), plays crucial role in biodegradation and biotransformation of pollutants, nutrient cycling and the maintenance of ecosystem balance. In this study, 9 water samples, 6 sediment samples, and 8 plant samples were collected in Annan CW, which has the functions of water treatment and wetland culture park. The characteristics of microbial community structure in different media were illustrated by using of high-throughput sequencing-based metagenomics approach and statistical analysis. Meanwhile, this study identified and classified human pathogens in CW to avoid potential risks to human health. The results showed that dominant bacteria phyla in CW include Proteobacteria, Bacteroides, Actinobacteria, Firmicutes and Verrucomicrobia. The distribution of microorganisms in three media is different, but not significant. And the pH and DO profoundly affected microbe abundance, followed by water temperature. The microbial diversity in sediments is the highest, which is similar with the detection of human pathogens in sediments. Moreover, compared with Calamus, Lythrum salicaria and Reed, Scirpus tabernaemontani has fewer pathogenic microorganisms. The distribution of microorganisms in the CW is complex, and a variety of human pathogens are detected, which is more prone to create potential risks to human health and should receive additional attention.

Impact of different zero valent iron-based particles on anaerobic microbial dechlorination of 2,4-dichlorophenol: Comparison of dechlorination performance and the underlying mechanism

The integration of iron-based materials and anaerobic microbial consortia has been extensively studied owing to its potential to enhance pollutant degradation. However, few studies have compared how different iron materials enhance the dechlorination of chlorophenols in coupled microbial systems. This study systematically compared the combined performances of microbial community (MC) and iron materials (Fe0/FeS2 +MC, S-nZVI+MC, n-ZVI+MC, and nFe/Ni+MC) for the dechlorination of 2,4-dichlorophenol (DCP) as one representative of chlorophenols. DCP dechlorination rate was significantly higher in Fe0/FeS2 +MC and S-nZVI+MC (1.92 and 1.67 times, with no significant difference between two groups) than in nZVI+MC and nFe/Ni+MC (1.29 and 1.25 times, with no significant difference between two groups). Fe0/FeS2 had better performance for the reductive dechlorination process as compared with other three iron-based materials via the consumption of any trace amount of oxygen in anoxic condition and accelerated electron transfer. On the other hand, nFe/Ni could induce different dechlorinating bacteria as compared to other iron materials. The enhanced microbial dechlorination was mainly due to some putative dechlorinating bacteria (Pseudomonas, Azotobacter, Propionibacterium), and due to improved electron transfer of sulfidated iron particles. Therefore, Fe0/FeS2 as a biocompatible as well as low-cost sulfidated material can be a good alternative for possible engineering applications in groundwater remediation.

Effects of different cathodic potentials on performance, microbial community structure and function for bioelectrochemical-stimulated dechlorination of 2,4,6-trichlorophenol in sediments

Bioelectrochemical systems with biocathodes constitute a promising means to enhance the biological dechlorination of 2,4,6-trichlorophenol (2,4,6-TCP) in constructed wetland (CW) sediments. However, the effect of different cathodic potentials on the structure and function of 2,4,6-TCP-reducing biocathode communities in CW sediments is largely unknown. Here, we evaluated the performance and microbial community structure of 2,4,6-TCP-reducing biocathode systems at different cathodic potentials (- 0.5, - 0.7, - 0.9, and - 1.1 V vs. saturated calomel electrode). The dechlorination efficiency of 2,4,6-TCP with the biocathode relatively increased by 16.02%-33.17% compared to that in the open circuit. The highest 2,4,6-TCP dechlorination efficiency (92.34 ± 0.86%) was observed at - 0.7 V in sediment, which may be due to the highest abundance of functional genera (e.g., Pseudomonas, Spirochaeta) at - 0.7 V. Metagenomic analysis provided new insights into the metabolic potential of microorganisms in CW sediments and suggested possible 2,4,6-TCP conversion pathways in sediments. 2,4,6-TCP was gradually dechlorinated to form 4-chlorophenol, followed by a ring-opening step via the activities of chlorophenol reductive dehalogenase and oxygenase (e.g., cprA, tfdB). Interestingly, micro-electrical stimulation enhanced the expression of chlorophenol reductive dehalogenase (cprA). Therefore, our findings at the molecular and gene expression levels provide insights into the effects of different cathodic potentials on the performance and community structure of 2,4,6-TCP-reducing biocathode systems in CW sediments.

Cometabolic biodegradation system employed subculturing photosynthetic bacteria: A new degradation pathway of 4-chlorophenol in hypersaline wastewater

4-chlorophenol (4-CP) as a toxic persistent pollutant is quite difficult treatment by using traditional biological processes. Herein, photosynthetic bacteria (PSB) driven cometabolic biodegradation system associated with exogeneous carbon sources (e.g., sodium acetate) has been demonstrated as an effective microbial technique. The biodegradation rate (r_i) can be at 0.041 d^-1 with degradation efficiency of 93% in 3094 lx. Through the study of subculturing PSB in absence of NaCl, it was found that 50% inoculation time can be saved but keeping a similar 4-CP biodegradation efficiency in scale-up salinity system. A new plausible biodegradation pathway for 4-CP in 4th G PSB cometabolic system is proposed based on the detected cyclohexanone generation followed by ring opening. It is probably ascribed to the increasement of Firmicutes and Bacteroidetes at phyla level classified based on microbial community. This study contributes to a new insight into cometabolic technology for chlorophenol treatment in industrial hypersaline wastewater.

Biomimetic synthesis technology for preparation of Fe3O4-encapsulated biochar using in highly efficient peroxodisulfate activation

The background value of iron in red soil is superior, primarily absorbed and homogeneously encapsulated in harvested biomass. However, this property on the high-value utilization of bionic iron-encapsulated biomass remains unknown. In this study, special biochar (Fe@BC) was obtained from this kind of biomass by one-step pyrolysis method, which was further used to activate peroxydisulfate (PDS) and degrade 2,4-dichlorophenol (2,4-DCP). The results showed that Fe₃O₄ was formed and homogeneously embedded in biochar at 500^oC. Comparing to catalysts prepared by impregnation pyrolysis (Fe/BC), Fe@BC exhibited excellent degradation performance (90.9%, k = 0.0037 min⁻¹) for 2,4-DCP. According to the free radicals quenching studies, hydroxyl radicals (·OH) and superoxide radicals (·O₂⁻) were the dominant reactive oxygen species (ROS) in Fe@BC/PDS system. Importantly, a PDS adsorption model was established, and the electron transport and PDS activation in the core-shell structure were demonstrated by DFT calculations. Therefore, this study could supply a high-performance catalyst and significant implications for high-value biomass utilization in red soil.