Stimulative mineralization of p-fluoronitrobenzene in biocathode microbial electrolysis cell with an oxygen-limited environment

Spatial distributions of microbial diversity in the contaminated deep groundwater: A case study of the Huaibei coalfield

The impact of coal mining activities on the structure of groundwater microbial communities in coal mining areas has gradually received academic attention. In this study, hydrochemical analysis and sequencing of the V4 region of the 16S rRNA gene were carried out using groundwater samples from the fourth aquifer in the loose layer of Cenozoic, the sandstone fissure aquifer in the coal measure strata of Permian, the Carboniferous Taiyuan Formation limestone aquifer, and the Ordovician limestone aquifer, at depths of 250 m, 600 m, 750 m, and 1000 m in monitoring wells. Results showed that the operational taxonomy units (OTUs) in the deep groundwater ecosystem were clustered distinguishably between the contaminated and the uncontaminated aquifers. The microbial community alpha-diversity of groundwater was significantly correlated with depth, and the richness of microbial community composition decreased with increasing depth. Proteobacteria (34.41%-97.41%), was found to be the dominant phylum, Gammaproteobacteria (10.05%-92.06%) was the dominant class and "Unassigned" (4.12%-64.72%) was dominant at the genus level. The number of endemic bacteria in the four aquifers was 1, 33, 99 and 11, respectively. It was also found that F^-, oxidation-reduction potential (ORP), and TOC were the main environmental variables affecting the groundwater all OTUs, abundant OTUs, and rare OTUs, respectively. These results indicate that the activity of rare OTU subcommunities increases gradually with increasing aquifer depth and that mining significantly enriched Thiovirga in deep groundwater. In addition, it was found that with the increase of depth, the effect of ORP on microbial community abundance decreased. This study deepens our understanding of the evolution characteristics of microbial communities in deep groundwater in coal mining areas. The unique characteristics of microbial communities characteristics of four aquifers with different depths provide a microbial perspective for understanding the characteristics of deep aquifers.

Biochar-based composites for remediation of polluted wastewater and soil environments: Challenges and prospects

Pharmaceuticals, heavy metals, pesticides, and dyes are the main environmental contaminants that have serious effects on both land and aquatic lives and necessitate the development of effective methods to mitigate these issues. Although some conventional methods are in use to tackle soil contamination, but biochar and biochar-based composites represent a reliable and sustainable means to deal with a spectrum of toxic organic and inorganic pollutants from contaminated environments. The capacity of biochars and derived constructs to remediate inorganic dyes, pesticides, insecticides, heavy metals, and pharmaceuticals from environmental matrices is attributed to their extensive surface area, surface functional groups, pore size distribution, and high sorption capability of these pollutants in water and soil environments. Application conditions, biochar feedstock, pyrolysis conditions and precursor materials are the factors that influence the capacity and functionality of biochar to adsorb pollutants from wastewater and soil. These factors, when improved, can benefit biochar in agrochemical and heavy metal remediation from various environments. However, the processes involved in biochar production and their influence in enhancing pollutant sequestration remain unclear. Therefore, this paper throws light on the current strategies, operational conditions, and sequestration performance of biochar and biochar-based composites for agrochemical and heavy metal in soil and water environments. The main challenges associated with biochar preparation and exploitation, toxicity evaluation, research directions and future prospects for biochar in environmental remediation are also outlined.

Enhanced bio-reductive degradation of fluoroglucocorticoids in the groundwater fluctuation zone by external electron donors: Performance, microbial community, and functional genes

This study evaluated the effectiveness of external electron donors on the bio-reductive degradation enhancement of fluoroglucocorticoids (FGCs) in the groundwater fluctuation zone during the wet season when reverse upward fluctuation of the groundwater table occurs and the dry season after the groundwater table declines. The results showed that the external electron donors, provided by the addition of nano zero-valent iron-modified biochar (nZVI@BC), inhibited the migration and enhanced the reductive defluorination of triamcinolone acetonide (TA), a representative FGC. The accumulation rate constant with temporal fluctuation depth and the attenuation rate constant with vertical fluctuation depth were -2.55 × 10^-3 and 4.20 × 10^-2, respectively, in the groundwater of the natural groundwater fluctuation zone (N-FZ). In contrast, the accumulation and attenuation rate constants were, respectively, 35.6% and 2.64 times higher in the groundwater fluctuation zone amended with nZVI@BC (nZVI@BC-FZ) as compared with those observed in the N-FZ. Furthermore, the decay rate constant of the TA residue in the dry season was 0.843 × 10^-2 μg/d in N-FZ and was 2.19 times higher in nZVI@BC-FZ. This enhancement effect, caused by the addition of external electrons, was positively correlated with the evolution of the microbial community and the expression of functional genes. The microbes evolved into functional genera with reductive dehalogenation (Xylophilus and Hydrogenophaga) and iron-oxidizing (Lysobacter, Pseudoxanthomonas, and Sphingomonas) abilities in the nZVI@BC-FZ system, which increased dehalogenation and iron oxide genes by a 4-5 order of magnitude. The utilization proportion of external electrons for TA metabolism was 50.04%, of which 30.82%, 10.26%, and 8.96% were utilized for defluorination, hydrogenation, and ring-opening, respectively. This study provides an effective method to reduce pollutant diffusion and enhance the bio-reductive degradation caused by groundwater table fluctuation.

Bioelectrochemical system for dehalogenation: A review

Halogenated organic compounds are persistent pollutants, whose persistent contamination and rapid spread seriously threaten human health and the safety of ecosystems. It is difficult to remove them completely by traditional physicochemical techniques. In-situ remediation utilizing bioelectrochemical technology represents a promising strategy for degradation of halogenated organic compounds, which can be achieved through potential modulation. In this review, we summarize the reactor configuration of microbial electrochemical dehalogenation systems and relevant organohalide-respiring bacteria. We also highlight the mechanisms of electrode potential regulation of microbial dehalogenation and the role of extracellular electron transfer in dehalogenation process, and further discuss the application of bioelectrochemical technology in bioremediation of halogenated organic compounds. Therefore, this review summarizes the status of research on microbial electrochemical dehalogenation systems from macroscopic to microscopic levels, providing theoretical support for the development of rapid and efficient in situ bioremediation technologies for halogenated organic compounds contaminated sites, as well as insights for the removal of refractory fluorides.

External potential regulated biocathode for enhanced removal of gaseous chlorobenzene in bioelectrchemical system

Microbial electrolysis cell (MEC) with a biocathode could provide extra reaction driving force for gaseous chlorobenzene (CB) removal. In this work, external potentials (-0.1 to -0.7 V vs. SHE) were applied to regulate the biocathodic activity. Results showed -0.3 V was the optimum potential, while the removal efficiency, dechlorination efficiency and Coulombic efficiency achieved 94%, 65%, and 89%, respectively. Electrochemical stimulation enriched dechlorination microorganisms (Achromobacter and Gordonia), and significantly improved CB mineralization efficiency, which was twice higher than that without additional potential at 300 mg m^-3 inlet concentration. Furthermore, electron transfer between biocathode and microorganisms was mainly through direct electron transfer (DET). A new integrated redox pathway for CB anaerobic degradation was proposed, in which CB was sequentially converted into 2-chlorophenol and 3-chlorocatechol, then dechlorinated to catechol, and finally mineralized into CO₂. Overall, this work provided an insight into gaseous CB bioelectrochemical degradation through the potential regulation.

Biochar for remediation of agrochemicals and synthetic organic dyes from environmental samples: A review

Application of agrochemicals in farming sector to control insects and pests; and use of synthetic organic dyes to color the products are increasing continuously due to the rapid growth of industries. During the application process many industries releases toxic agrochemicals and dyes in to the aquatic environment and on land without the proper treatment. Due to their toxicity the disposal of such chemicals is of utmost importance. Biochar offers the ability to remediate these substances from environmental matrices because of their high sorption ability of pollutants from water and soil. This review highlights the development and advancement of biochar-based treatment for abatement of agrochemicals and synthetic organic dyes, involving its technical aspects and the variables connected with removing these kinds of pollutants. Several optimization parameters like temperature, pH, chemical concentration, biochar properties, time, and co-existing ions have been elaborated. Literature survey shows that most of the researches on biochar application have been conducted in the batch mode. Hence there is an urgent need to apply this beneficial technique for the remediation of pollutants at the larger scale in the real water and soil samples. A comprehensive summary on sorption kinetics and adsorption isotherms with regards to pollutant removal is also presented. This review also covers the cost analysis of various techniques where biochar has been used as an adsorbent. Thus this review makes an easy roadmap for the further development in biochar and biochar based composites and expansion of these demanding areas of research in biochar and their applications.

A novel microbial electrolysis cell-A/O system treating cotton dyeing pretreatment wastewater: performance and microbial diversity analysis

The textile industry is developing rapidly in China. It generates large volumes of cotton dyeing pretreatment wastewater (CDPW). CDPW contains high concentrations of pollutants characterized by their strongly alkaline and recalcitrant nature for microbial degradation. This project aimed to evaluate the performance of a microbial electrolysis cell (MEC) coupled with anoxic/oxic (A/O) system (MEC-A/O) in treating CDPW, as well as analyze changes in microbial diversity. The results indicated that the effect of biological treatment in an electrolytic cell to treat CDPW was optimal at the voltage of 0.6V. The chemical oxygen demand (COD) removal efficiency under optimum conditions was 69.13%, higher than that of the A/O system alone (48.93%). Within a certain range, applied voltage was able to enhance microbial activity, increase the sludge concentration and enlarge the sludge particle size. At the same time, the applied voltage could effectively increase the abundance and the diversity of Bacteria and Archaea, as well as accelerate the degradation of pollutants.

Validation of effective roles of non-electroactive microbes on recalcitrant contaminant degradation in bioelectrochemical systems

Bioelectrochemical systems (BESs) have been widely investigated for recalcitrant waste treatment mainly because of their waste removal effectiveness. Electroactive microbes (EMs) have long been thought to contribute to the high effectiveness by interacting with electrodes via electron chains. However, this work demonstrated the dispensable role of EMs for enhanced recalcitrant contamination degradation in BESs. We revealed enhanced p-fluoronitrobenzene (p-FNB) degradation in a BES by observing a defluorination efficiency that was three times higher than that in biodegradation or electrochemical processes. Such an improvement was achieved by the collaborative roles of electrode biofilms and planktonic microbes, as their individual contributions to p-FNB degradation were found to be similarly stimulated by electricity. However, no bioelectrochemical activity was found in either the electrode biofilms or the planktonic microbes during stimulated p-FNB degradation; because no biocatalytically reductive or oxidative turnovers were observed on cyclic voltammetry curves. The non-involvement of EMs was further proven by the similar microbial community evolution for biofilms and planktonic microbes. In summary, we proposed a mechanism for indirect electrical stimulation of microbial metabolism by electrochemically generating the active mediator p-fluoroaniline (p-FA) and further degradation by a sequential combination of electrochemical p-FNB reduction and biological p-FA oxidation by non-EMs.

Optimization of a bioelectrochemical system for 2,4-dichloronitrobenzene transformation using response surface methodology

In the present study, a bioelectrochemical system (BES) was developed for 2,4-dichloronitrobenzene (DClNB) transformation. Response surface methodology (RSM) was applied to optimize the operational conditions, including the V/S ratio (volume of the BES/size of the electrode ratio), interval (D) (distance between the anode and cathode) and position (P) (proportion of the electrodes immerged in the sludge). The optimum conditions for the V/S ratio, interval and position were 40, 2.31 cm and 0.42. The pollutant removal rate and increase in Cl⁻ were 1.819 ± 0.037 mg L⁻¹ h⁻¹ and 11.894 ± 0.180 mg L⁻¹, which were close to the predicted values (1.908 mg L⁻¹ h⁻¹ and 12.485 mg L⁻¹). A continuous experiment indicated that the pollutant removal efficiency in the BES with 50% of the electrodes immerged in the sludge was 34.6% and 22.6% higher than that in the ones with 0 and 100% of the electrodes immerged in the sludge.

In the present study, a bioelectrochemical system (BES) was developed for 2,4-dichloronitrobenzene (DClNB) transformation.

Cathode potential regulation in a coupled bioelectrode-anaerobic sludge system for effective dechlorination of 2,4-dichloronitrobenzene

For enhanced dechlorination of 2,4-dichloronitrobenzene (2,4-DClNB), a coupled microbial electrosynthesis-upflow anaerobic sludge reactor (MES-UASB) was established, and the effect of cathode potential on the performance of combined process was investigated in this study. Results showed that a higher dechlorination efficiency of 78.5 ± 6.1% was achieved in the coupled MES-UASB at -660 mV, and the degradation rate of 4-chloroaniline (4-ClAn) reached 4.61 mg·L^-1·d^-1 within 120 h at -660 mV of cathode potential in batch experiments. The results of Illumina sequencing indicated that the biocathode operated at a lower potential favored the enrichment of dechlorination-related microbes such as Dehalobacter, Dehalococcoides and Anaeromyxobacter both in granular sludge and cathode biofilm. It could be speculated that a lower cathode potential is more feasible for the dechlorination of 2,4-DClNB due to the enrichment of dechlorination-related microbes as well as the production of electrons with higher energy for long-distance electron transfer (LDET).

Accelerated removal of high concentration p-chloronitrobenzene using bioelectrocatalysis process and its microbial communities analysis

p-Chloronitrobenzene (p-CNB) is a persistent refractory and toxic pollutant with a concentration up to 200 mg/L in industrial wastewater. Here, a super-fast removal rate was found at 0.2-0.8 V of external voltage over a p-CNB concentration of 40-120 mg/L when a bioelectrochemical technology is used comparing to the natural biodegradation and electrochemical methods. The reduction kinetics (k) was fitted well according to pseudo-first order model with respect to the different initial concentration, indicating a 1.12-fold decrease from 1.80 to 0.85 h^-1 within the experimental range. Meanwhile, the highest k was provided at 0.5 V with the characteristic of energy saving. It was revealed that the functional bacterial (Propionimicrobium, Desulfovibrio, Halanaerobium, Desulfobacterales) was selectively enriched under electro-stimulation, which possibly processed Cl-substituted nitro-aromatics reduction. The possible degradation pathway was also proposed. This work provides the beneficial choice on the rapid treatment of high-concentration p-CNB wastewater.

Microbial community and metabolic pathway succession driven by changed nutrient inputs in tailings: effects of different nutrients on tailing remediation

To solve the competition problem of acidophilic bacteria and sulfate-reducing bacteria in the practical application of mine tailing bioremediation, research into the mechanisms of using different nutrients to adjust the microbial community was conducted. Competition experiments involving acidophilic bacteria and sulfate-reducing bacteria were performed by supplementing the media with yeast extract, tryptone, lactate, and glucose. The physiochemical properties were determined, and the microbial community structure and biomass were investigated using MiSeq sequencing and qRT-PCR, respectively. Four nutrients had different remediation mechanisms and yielded different remediation effects. Yeast extract and tryptone (more than 1.6 g/L) promoted sulfate-reducing bacteria and inhibited acidophilic bacteria. Lactate inhibited both sulfate-reducing and acidophilic bacteria. Glucose promoted acidophilic bacteria more than sulfate-reducing bacteria. Yeast extract was the best choice for adjusting the microbial community and bioremediation, followed by tryptone. Lactate kept the physiochemical properties stable or made slight improvements; however, glucose was not suitable for mine tailing remediation. Different nutrients had significant effects on the abundance of the second enzyme of the sulfate-reducing pathway (p < 0.05), which is the rate-limiting step of sulfate-reducing pathways. Nutrients changed the remediation effects effectively by adjusting the microbial community and the abundance of the sulfate-reducing rate-limiting enzyme.

Biocatalysis mechanism for p-fluoronitrobenzene degradation in the thermophilic bioelectrocatalysis system: Sequential combination of reduction and oxidation

To verify the potentially synthetic anodic and cathodic biocatalysis mechanism in bioelectrocatalysis systems (BECSs), a single-chamber thermophilic bioelectrocatalysis system (R3) was operated under strictly anaerobic conditions using the biocathode donated dual-chamber (R1) and bioanode donated dual-chamber (R2) BECSs as controls. Direct bioelectrocatalytic oxidation was found to be infeasible while bioelectrocatalytic reduction was the dominant process for p-Fluoronitrobenzene (p-FNB) removal, with p-FNB removal of 0.188 mM d(-1) in R1 and 0.182 mM d(-1) in R3. Cyclic voltammetry experiments confirmed that defluorination in the BECSs was an oxidative metabolic process catalyzed by bioanodes following the reductive reaction, which explained the 0.034 mM d(-1) defluorination in R3, but negligible defluorination in controls. Taken together, these results revealed a sequentially combined reduction and oxidation mechanism in the thermophilic BECS for p-FNB removal. Moreover, the enrichment of Betaproteobacteria and uniquely selected Bacilli in R3 were probably functional populations for p-FNB degradation.

Effect of phosphoric acid as a catalyst on the hydrothermal pretreatment and acidogenic fermentation of food waste

The hydrothermal method was applied to food waste (FW) pretreatment with phosphoric acid as a catalyst. The content of soluble substances such as protein and carbohydrate in the FW increased after the hydrothermal pretreatment with phosphoric acid addition (⩽5%). The SCOD approached approximately 29.0g/L in 5% phosphoric acid group, which is almost 65% more than the original FW. The hydrothermal condition was 160°C for 10min, which means that at least 40% of energy and 60% of reaction time were saved to achieve the expected pretreatment effect. Subsequent fermentation tests showed that the optimal dosage of phosphoric acid was 3% with a VFA yield of 0.763g/gVSremoval, but the increase in salinity caused by phosphoric acid could adversely affect the acidogenesis. With an increase in the quantity of phosphoric acid, among the VFAs, the percentage of propionic acid decreased and that of butyric acid increased. The PCR-DGGE analysis indicated that the microbial diversity could decrease with excessive phosphoric acid, which resulted in a low VFA yield.

p-Nitrophenol degradation and microbial community structure in a biocathode bioelectrochemical system

The biocathode bioelectrochemical system (bioc-BES) was used forp-nitrophenol (PNP) degradation with sodium bicarbonate as the carbon source.

Electrical stimulation improves microbial salinity resistance and organofluorine removal in bioelectrochemical systems

Fed batch bioelectrochemical systems (BESs) based on electrical stimulation were used to treat p-fluoronitrobenzene (p-FNB) wastewater at high salinities. At a NaCl concentration of 40 g/liter, p-FNB was removed 100% in 96 h in the BES, whereas in the biotic control (BC) (absence of current), p-FNB removal was only 10%. By increasing NaCl concentrations from 0 g/liter to 40 g/liter, defluorination efficiency decreased around 40% in the BES, and in the BC it was completely ceased. p-FNB was mineralized by 30% in the BES and hardly in the BC. Microorganisms were able to store 3.8 and 0.7 times more K(+) and Na(+) intracellularly in the BES than in the BC. Following the same trend, the ratio of protein to soluble polysaccharide increased from 3.1 to 7.8 as the NaCl increased from 0 to 40 g/liter. Both trends raise speculation that an electrical stimulation drives microbial preference toward K(+) and protein accumulation to tolerate salinity. These findings are in accordance with an enrichment of halophilic organisms in the BES. Halobacterium dominated in the BES by 56.8% at a NaCl concentration of 40 g/liter, while its abundance was found as low as 17.5% in the BC. These findings propose a new method of electrical stimulation to improve microbial salinity resistance.

Cooperative role of electrical stimulation on microbial metabolism and selection of thermophilic communities for p-fluoronitrobenzene treatment

A novel thermophilic bioelectrochemical system (TBES) based on electrical stimulation was established for the enhanced treatment of p-fluoronitrobenzene (p-FNB) wastewater. p-FNB removal rate constant in the TBES was 78.6% higher than that of the mesophilic BES (MBES), the elevation of which owing to high-temperature overtook the rate improvement of 50.8% in the electrocatalytic system (ECS). Additionally, an overwhelming mineralization efficiency of 91.96% ± 5.70% was obtained in the TBES. The superiority of TBES was attributed to the integrated role of electrical stimulation and high-temperature. Electrical stimulation provided an alternative for the microbial growth independent energy requirements, compensating insufficient energy support from p-FNB metabolism under the high-temperature stress. Besides, electrical stimulation facilitated microbial community evolution to form specific thermophilic biocatalysis. The uniquely selected thermophilic microorganisms including Coprothermobacter sp. and other ones cooperated to enhance p-FNB mineralization.