Bioelectroventing: an electrochemical-assisted bioremediation strategy for cleaning-up atrazine-polluted soils

Stimulation of atrazine degradation by activated carbon and cathodic effect in soil microbial fuel cell

Soil microbial fuel cells (MFCs) have been increasingly studied in recent years and have attracted significant attention as an environmentally sustainable bioelectrochemical technology. However, the poor conductivity of the soil matrix and the neglect of the cathodic function have limited its application. In this study, quartz sand and activated carbon were subjected to investigation on their influence on atrazine degradation. Atrazine was introduced in different layers (cathode, upper layer) to explore the cathodic effect on atrazine removal. The results revealed that activated carbon could reduce the internal resistance (693 Ω) and generate the highest power density (25.51 mW/m²) of the soil MFCs, and thus increase the removal efficiency (97.92%) of atrazine. The dynamic degradation profiles of atrazine were different for different adding layers. The cathode electrode acted as an electron donor could increase the distance of the effective influence of the soil MFCs' cathode from the middle to the cathode layer. The cathode (region) and the region close to the cathode could degrade atrazine with the atrazine removal efficiencies ranging from 60.67% to 92.79%, and the degradation ability of the cathode was stronger than that of other layers. The degradation effect followed the order: cathode > upper > lower > middle). Geobacter, Desulfobulbus, and Desulfuromonas belonging to the δ-Proteobacteria class were identified as the dominant electroactive microorganisms in the anode layer, while their relative abundances are quite low in the upper and cathode layers. Pseudomonas is an atrazine-degrading bacterium, but its relative abundance was only 0.13-0.51%. Thus, bioelectrochemistry rather than microbial degradation was the primary driving force.

Passive electrobioremediation approaches for enhancing hydrocarbons biodegradation in contaminated soils

Electrobioremediation technologies hold considerable potential for the treatment of soils contaminated by petroleum hydrocarbons (PH), since they allow stimulating biodegradation processes with no need for subsurface chemicals injection and with little to no energy consumption. Here, a microbial electrochemical snorkel (MES) was applied for the treatment of a soil contaminated by hydrocarbons. The MES consists of direct coupling of a microbial anode with a cathode, being a single conductive, non-polarized material positioned suitably to create an electrochemical connection between the anoxic zone (the contaminated soil) and the oxic zone (the overlying oxygenated water). Soil was also supplemented with electrically conductive particles of biochar as a strategy to construct a conductive network with microbes in the soil matrix, thus extending the radius of influence of the snorkel. The results of a comprehensive suite of chemical, microbiological and ecotoxicological analyses evidenced that biochar addition, rather than the presence of a snorkel, was the determining factor in accelerating PH removal from contaminated soils, possibly accelerating syntrophic and/or cooperative metabolisms involved in the degradation of PH. The enhancement of biodegradation was mirrored by an increased abundance of anaerobic and aerobic microorganisms known to be involved in the degradation of PH and related functional genes. Plant ecotoxicity assays confirmed a reduction of soils toxicity in treatments receiving electrically conductive biochar.

Advances and prospects on the aquatic plant coupled with sediment microbial fuel cell system

Energy resource scarcity and sediment pollution perniciousness have become enormous challenges, to which research has been focused on energy recovery and recycle technologies to solve both above problems. The organic matter stored in anoxic sediments of freshwater ecosystem represents a tremendous potential energy source. The system of aquatic plant coupled with sediment microbial fuel cell (AP-SMFC) has attracted much attention as a more feasible, economical and eco-friendly way to remediate sediment and surface water and generate electricity. However, the research on AP-SMFC has only been carried out in the last decade, and relevant studies have not been well summarized. In this review, the advances and prospects on AP-SMFC were systematically introduced. Firstly, the annual publication counts and keywords co-occurrence cluster of AP-SMFC were identified and visualized by resorting to the CiteSpace software, and the result showed that the research on AP-SMFC increased significantly in the last decade on the whole and will continue to increase. The bibliometric results provided valuable references and information on potential research directions for future studies. And then, the research progress and reaction mechanism of AP-SMFC were systematically described. Thirdly, the performance of AP-SMFC, including nutrients removal, organic contaminants removal, and electricity generation, was systematically summarized. AP-SMFC can enhance the removal of pollutants and electricity generation compared with SMFC without AP, and is considered to be an ideal technology for pollutants removal and resource recovery. Finally, the current challenges and future perspectives were summarized and prospected. Therefore, the review could serve as a guide for the new entrants to the field and further development of AP-SMFC application.

An overview of in-situ remediation for nitrate in groundwater

Faced with the increasing nitrate pollution in groundwater, in-situ remediation has been widely studied and applied on field-scale as an efficient, economical and less disturbing remediation technology. In this review, we discussed various in-situ remediation for nitrate in groundwater and elaborate on biostimulation, phytoremediation, electrokinetic remediation, permeable reactive barrier and combined remediation. This review described principles of each in-situ remediation, application, the latest progress, problems and challenges on field-scale. Factors affecting the efficiency of in-situ remediation for nitrate in groundwater are also summarized. Finally, this review presented the prospect of in-situ remediation for nitrate pollution in groundwater. The objective of this review is to examine the state of knowledge on in-situ remediation for nitrate in groundwater and critically evaluate factors which affect the up-scaling of laboratory and bench-scale research to field-scale application. This helps to better understand the control mechanisms of various in-situ remediation for nitrate pollution in groundwater and the design options available for application to the field-scale.

A New Electrochemical Detection Technique for Organic Matter Content in Ecological Soils

The rapid detection of organic matter in soil is of great interest in agriculture, but the commonly used techniques require laboratory operation. Therefore, the development of a technique that allows rapid detection of soil organic matter in the field is of great interest. In this work, we propose an electrochemical-based approach for the detection of organic matter in soil particles. Since soil particles immobilized directly on the electrode surface can fall off during testing, we introduced graphene to coat the soil particles. The encapsulated soil particles can be stably immobilized on the electrode surface. We have investigated the electrochemical behavior of soil particles. The results show a correspondence between the electrochemical oxidation and reduction of soil particles and the organic matter content in them. We collected soil samples from three sites and constructed an electrochemical modeling, testing framework with stability based on multiple calibrations and random division of the prediction set. We used the equal interval partial least squares (EC-PLS) method for potential optimization to establish the equivalent model set. A joint model for the electrochemical analysis of organic matter in three locations of soil samples was developed for the commonality study.

New insights into the degradation of synthetic pollutants in contaminated environments

The environment is contaminated by synthetic contaminants owing to their extensive applications globally. Hence, the removal of synthetic pollutants (SPs) from the environment has received widespread attention. Different remediation technologies have been investigated for their abilities to eliminate SPs from the ecosystem; these include photocatalysis, sonochemical techniques, nanoremediation, and bioremediation. SPs, which can be organic or inorganic, can be degraded by microbial metabolism at contaminated sites. Owing to their diverse metabolisms, microbes can adapt to a wide variety of environments. Several microbial strains have been reported for their bioremediation potential concerning synthetic chemical compounds. The selection of potential strains for large-scale removal of organic pollutants is an important research priority. Additionally, novel microbial consortia have been found to be capable of efficient degradation owing to their combined and co-metabolic activities. Microbial engineering is one of the most prominent and promising techniques for providing new opportunities to develop proficient microorganisms for various biological processes; here, we have targeted the SP-degrading mechanisms of microorganisms. This review provides an in-depth discussion of microbial engineering techniques that are used to enhance the removal of both organic and inorganic pollutants from different contaminated environments and under different conditions. The degradation of these pollutants is investigated using abiotic and biotic approaches; interestingly, biotic approaches based on microbial methods are preferable owing to their high potential for pollutant removal and cost-effectiveness.

Sustainability in ElectroKinetic Remediation Processes: A Critical Analysis

In recent years, the development of suitable technologies for the remediation of environmental contaminations has attracted considerable attention. Among these, electrochemical approaches have gained prominence thanks to the many possible applications and their proven effectiveness. This is particularly evident in the case of inorganic/ionic contaminants, which are not subject to natural attenuation (biological degradation) and are difficult to treat adequately with conventional methods. The purpose of this contribution is to present a critical overview of electrokinetic remediation with particular attention on the sustainability of the various applications. The basis of technology will be briefly mentioned, together with the phenomena that occur in the soil and how that will allow its effectiveness. The main critical issues related to this approach will then be presented, highlighting the problems in terms of sustainability, and discussing some possible solutions to reduce the environmental impact and increase the cost-effectiveness and sustainability of this promising technology.

Microbial electrochemistry for bioremediation

Lack of suitable electron donors or acceptors is in many cases the key reason for pollutants to persist in the environment. Externally supplementation of electron donors or acceptors is often difficult to control and/or involves chemical additions with limited lifespan, residue formation or other adverse side effects. Microbial electrochemistry has evolved very fast in the past years - this field relates to the study of electrochemical interactions between microorganisms and solid-state electron donors or acceptors. Current can be supplied in such so-called bioelectrochemical systems (BESs) at low voltage to provide or extract electrons in a very precise manner. A plethora of metabolisms can be linked to electrical current now, from metals reductions to denitrification and dechlorination. In this perspective, we provide an overview of the emerging applications of BES and derived technologies towards the bioremediation field and outline how this approach can be game changing.

Combining electrokinetic transport and bioremediation for enhanced removal of crude oil from contaminated marine sediments: Results of a long-term, mesocosm-scale experiment

Marine sediments represent an important sink of harmful petroleum hydrocarbons after an accidental oil spill. Electrobioremediation techniques, which combine electrokinetic transport and biodegradation processes, represent an emerging technological platform for a sustainable remediation of contaminated sediments. Here, we describe the results of a long-term mesocosm-scale electrobioremediation experiment for the treatment of marine sediments contaminated by crude oil. A dimensionally stable anode and a stainless-steel mesh cathode were employed to drive seawater electrolysis at a fixed current density of 11 A/m². This approach allowed establishing conditions conducive to contaminants biodegradation, as confirmed by the enrichment of Alcanivorax borkumensis cells harboring the alkB-gene and other aerobic hydrocarbonoclastic bacteria. Oil chemistry analyses indicated that aromatic hydrocarbons were primarily removed from the sediment via electroosmosis and low molecular weight alkanes (nC₆ to nC₁₀) via biodegradation.

Electrodes boost microbial metabolism to mineralize antibiotics in manure

Livestock manures are potential sources of antibiotics in the environment. Sulfamethazine (SMZ), frequently used in veterinary medicine, can enter the environment by using manure as soil fertilizer due to its incomplete absorption in the animal gut and its unmetabolized excretion. The objective of this study was to evaluate the mineralization of ¹⁴C-labelled SMZ in manure under a new redox scenario provided by microbial electrochemical reactors, termed microbial electroremediating cells (MERC). These devices aim to overcome the electron acceptor limitation in bacterial oxidative metabolism by means of using electrodes to enhance the biodegradation of pollutants in the environment. Our results revealed that the total degradation of ¹⁴C-SMZ reached 43.5% in short term batch laboratory scale experiments under reducing conditions (-400 mV vs. Ag/AgCl). Actually, SMZ mineralization was enhanced up to 10-fold in the early stages (after 2 weeks) in comparison with an electrode-free natural attenuation assay. Moreover, mineralization showed a dependence on electrode potential, with negligible results for conditions set to +400 mV vs Ag/AgCl. The impact of merging electrodes and microorganisms for manure bioremediation suggests a promising future for this emerging technology to treat polluted livestock wastes and prevent soil and groundwater pollution.

Factors affecting the efficiency of a bioelectrochemical system: a review

The great potential of bioelectrochemical systems (BESs) in pollution control combined with energy recovery has attracted increasing attention.

Recent advances in microbial electrochemical system for soil bioremediation

Soil contamination poses a serious threat to ecosystem and human well-being. Compared to conventional physical and chemical treatment, the microbial electrochemical system (MES) offers a sustainable and environment-friendly solution for soil bioremediation. In principle, soil microbe degrades organic substrate and releases electron in anode region. The electron flows through electric circuit to the cathode and finally is accepted by oxygen or oxidized metals. With various inherent advantages, MES has been applied in petroleum hydrocarbon, chlorinated organics and heavy metals bioremediation in soils. This paper aims to review the recent advances of MES in soil bioremediation, including main mechanisms of contaminant removal with MES, configurations of soil MES and current development in bioremediation of soil contaminated by organic and inorganic pollutants. Moreover, challenges and future prospects of soil MES are discussed.

Opportunities for groundwater microbial electro-remediation

Groundwater pollution is a serious worldwide concern. Aromatic compounds, chlorinated hydrocarbons, metals and nutrients among others can be widely found in different aquifers all over the world. However, there is a lack of sustainable technologies able to treat these kinds of compounds. Microbial electro‐remediation, by the means of microbial electrochemical technologies (MET), can become a promising alternative in the near future. MET can be applied for groundwater treatment in situ or ex situ, as well as for monitoring the chemical state or the microbiological activity. This document reviews the current knowledge achieved on microbial electro‐remediation of groundwater and its applications.

Bioelectroventing: an electrochemical-assisted bioremediation strategy for cleaning-up atrazine-polluted soils

The absence of suitable terminal electron acceptors (TEA) in soil might limit the oxidative metabolism of environmental microbial populations. Bioelectroventing is a bioelectrochemical strategy that aims to enhance the biodegradation of a pollutant in the environment by overcoming the electron acceptor limitation and maximizing metabolic oxidation. Microbial electroremediating cells (MERCs) are devices that can perform such a bioelectroventing. We also report an overall profile of the ¹⁴ C-ATR metabolites and ¹⁴ C mass balance in response to the different treatments. The objective of this work was to use MERC principles, under different configurations, to stimulate soil bacteria to achieve the complete biodegradation of the herbicide ¹⁴ C-atrazine (ATR) to ¹⁴ CO₂ in soils. Our study concludes that using electrodes at a positive potential [+600 mV (versus Ag/AgCl)] ATR mineralization was enhanced by 20-fold when compared to natural attenuation in electrode-free controls. Furthermore, ecotoxicological analysis of the soil after the bioelectroventing treatment revealed an effective clean-up in < 20 days. The impact of electrodes on soil bioremediation suggests a promising future for this emerging environmental technology.