Enhanced simultaneous removal of nitrate and perchlorate from groundwater by bioelectrochemical systems (BESs) with cathodic potential regulation

Enhancement of nitrate reduction in microbial fuel cells by acclimating biocathode potential: Performance, microbial community, and mechanism

The enhancement of nitrate reduction in microbial fuel cells (MFCs) by acclimating biocathode potential was studied. An MFC system was started up, and measured by cyclic voltammetry to determine a suitable potential region for acclimating biocathode. The experimental results revealed that potential acclimation could efficiently improve denitrification performance by relieving the phenomenon of nitrite accumulation, and optimum performance was obtained at -0.4 V with a total nitrogen removal efficiency of 87.4 %. Subsequently, the characteristics of electron transfer behaviors were measured, suggesting that a positive correlation between nitrate reduction and the contribution of direct electron transfer emerged. Furthermore, a denitrification mechanism was proposed. The results indicated that potential acclimation was conducive to enhancing denitrifying enzyme activity and that the electron transport system activity could be increased by 5.8 times. This study provides insight into the electron transfer characteristics and denitrification mechanisms in MFCs for nitrate reduction at specific acclimatization potentials.

Ecotoxicity characterization assisted performance assessment of electro-bioremediation reactors for nitrate and arsenite elimination

The performance of combined reduction of nitrate (NO3 - ) to dinitrogen gas (N2 ) and oxidation of arsenite (As[III]) to arsenate (As[V]) by a bioelectrochemical system was assessed, supported by ecotoxicity characterization. For the comprehensive toxicity characterization of the untreated model groundwater and the treated reactor effluents, a problem-specific ecotoxicity test battery was established. The performance of the applied technology in terms of toxicity and target pollutant elimination was compared and analyzed. The highest toxicity attenuation was achieved under continuous flow mode with hydraulic retention time (HRT) = 7.5 h, with 95%, nitrate removal rate and complete oxidation of arsenite to arsenate. Daphnia magna proved to be the most sensitive test organism. The results of the D. magna lethality test supported the choice of the ideal operational conditions based on chemical data analysis. The outcomes of the study demonstrated that the applied technology was able to improve the groundwater quality in terms of both chemical and ecotoxicological characteristics. The importance of ecotoxicity evaluation was also highlighted, given that significant target contaminant elimination did not necessarily lower the environmental impact of the initial, untreated medium, in addition, anomalies might occur during the technology operational process which in some instances, could result in elevated toxicity levels.

Electroactive microorganism-assisted remediation of groundwater contamination: Advances and challenges

Groundwater contamination has become increasingly prominent, therefore, the development of efficient remediation technology is crucial for improving groundwater quality. Bioremediation is cost-effective and environmentally friendly, while coexisting pollutant stress can affect microbial processes, and the heterogeneous character of groundwater medium can induce bioavailability limitations and electron donor/acceptor imbalances. Electroactive microorganisms (EAMs) are advantageous in contaminated groundwater because of their unique bidirectional electron transfer mechanism, which allows them to use solid electrodes as electron donors/acceptors. However, the relatively low-conductivity groundwater environment is unfavorable for electron transfer, which becomes a bottleneck problem that limits the remediation efficiency of EAMs. Therefore, this study reviews the recent advances and challenges of EAMs applied in the groundwater environment with complex coexisting ions, heterogeneity, and low conductivity and proposes corresponding future directions.

Electrotrophic perchlorate reduction by a psychrotolerant acidophile isolated from an acid rock drainage in Antarctica

A new extremophilic isolate (USS-CCA7) was obtained from an acidic environment (pH ∼ 3.2) in Antarctica phylogenetically related to Acidithiobacillus ferrivorans; its electrotrophic capacities were evaluated in a three-electrode electrochemical cell. Cyclic voltammetry showed cathodic peaks of -428 mV, -536 mV, and -634 mV (vs. Ag/AgCl; pH = 1.7; 3 M KCl) for nitrate, oxygen, and perchlorate, respectively. The catalytic role of this microorganism was also observed by a decrease in the charge transfer resistance registered via electrochemical impedance spectroscopy. Five-day chronoamperometry of culture at pH = 1.7, USS-CCA7 showed a perchlorate removal rate of 19.106 ± 1.689 mgL^-1 day^-1 and a cathodic efficiency of 112 ± 5.2  %. Growth on electrodes was observed by epifluorescence and scanning electron microscopy. Interestingly, the results showed that toward higher pH, the cathodic peak of perchlorate is reduced in the voltammetric profiles. This study highlights the use of this psychrotolerant acidophile for the bioremediation of harsh perchlorate-pressured terrestrial under acidic conditions.

A. Jorand F, Barrière F, Etienne M. Challenges and Applications of Nitrate-Reducing Microbial Biocathodes

Bioelectrochemical systems which employ microbes as electrode catalysts to convert chemical energy into electrical energy (or conversely), have emerged in recent years for water sanitation and energy recovery. Microbial biocathodes, and especially those reducing nitrate are gaining more and more attention. The nitrate-reducing biocathodes can efficiently treat nitrate-polluted wastewater. However, they require specific conditions and they have not yet been applied on a large scale. In this review, the current knowledge on nitrate-reducing biocathodes will be summarized. The fundamentals of microbial biocathodes will be discussed, as well as the progress towards applications for nitrate reduction in the context of water treatment. Nitrate-reducing biocathodes will be compared with other nitrate-removal techniques and the challenges and opportunities of this approach will be identified.

Evaluation of electroactive denitrifiers at different potentials, temperatures and buffers based on microcalorimetry

Electroactive denitrifiers contribute to the nitrate removal in a bioelectrochemical system, but their metabolism and growth parameters remain vague. In this study, microcalorimetry as a suitable method was used to evaluate the metabolism and growth parameters of electroactive denitrifiers at different cathode potentials, temperatures and buffer solutions. The suitable cathode potential and temperature for electroactive denitrifiers were deemed as -100 mV and 30 °C, respectively. The suitable buffer was found to be phosphate buffer solution but can be replaced by bicarbonate buffer solution. When cultivated with bicarbonate buffer solution at -100 mV and 30 °C, electroactive denitrifiers achieved a specific nitrate removal rate of 2.20 ± 0.08 × 10^-10 mg NO₃^--N·(min·cell)^-1 and two growth rate constants (k₁ = 0.0051 ± 0.0004 min^-1, k₂ = 0.0030 ± 0.0004 min^-1), with gaseous nitrogen as the end product. The bioelectrochemical denitrification behaved as a two-step process, in which the nitrite reduction to gaseous nitrogen was the rate-limiting step.

Effect of hydraulic retention time on the electro-bioremediation of nitrate in saline groundwater

Bioelectrochemical systems (BES) have proven their capability to treat nitrate-contaminated saline groundwater and simultaneously recover value-added chemicals (such as disinfection products) within a circular economy-based approach. In this study, the effect of the hydraulic retention time (HRT) on nitrate and salinity removal, as well as on free chlorine production, was investigated in a 3-compartment BES working in galvanostatic mode with the perspective of process intensification and future scale-up. Reducing the HRT from 30.1 ± 2.3 to 2.4 ± 0.2 h led to a corresponding increase in nitrate removal rates (from 17 ± 1 up to 131 ± 1 mgNO₃^--N L^-1d^-1), although a progressive decrease in desalination efficiency (from 77 ± 13 to 12 ± 2 %) was observed. Nitrate concentration and salinity close to threshold limits indicated by the World Health Organization for drinking water, as well as significant chlorine production were achieved with an HRT of 4.9 ± 0.4 h. At such HRT, specific energy consumption was low (6.8·10^-2 ± 0.3·10^-2 kWh g^-1NO₃^--N_removed), considering that the supplied energy supports three processes simultaneously. A logarithmic equation correlated well with nitrate removal rates at the applied HRTs and may be used to predict BES behaviour with different HRTs. The bacterial community of the bio-cathode under galvanostatic mode was dominated by a few populations, including the genera Rhizobium, Bosea, Fontibacter and Gordonia. The results provide useful information for the scale-up of BES treating multi-contaminated groundwater.

Recent advances in the source identification and remediation techniques of nitrate contaminated groundwater: A review

Researchers have long been committed to identify nitrate sources in groundwater and to develop an advanced technique for its remediation because better apply remediation solution and management of water quality is highly dependent on the identification of the NO₃^- sources contamination in water. In this review, we systematically introduce nitrate source tracking tools used over the past ten years including dual isotope and multi isotope techniques, water chemistry profile, Bayesian mixing model, microbial tracers and land use/cover data. These techniques can be combined and exploited to track the source of NO₃^- as mineral or organic fertilizer, sewage, or atmospheric deposition. These available data have significant implications for making an appropriate measures and decisions by water managers. A continuous remediation strategy of groundwater was among the main management strategies that need to be applied in the contaminated area. Nitrate removal from groundwater can be accomplished using either separation or reduction based process. The application of these processes to nitrate removal is discussed in this review and some novel methods were presented for the first time. Moreover, the advantages and limitations of each approach are critically summarized and based on our own understanding of the subject some solutions to overcomes their drawbacks are recommended. Advanced techniques are capable to attain significantly higher nitrate and other co-contaminants removal from groundwater. However, the challenges of by-products generation and high energy consumption need to be addressed in implementing these technologies for groundwater remediation for potable use.

A critical review of existing mechanisms and strategies to enhance N2 selectivity in groundwater nitrate reduction

The pollution of nitrate (NO3-) in groundwater has become an environmental problem of general concern and requires immediate remediation because of adverse human and ecological impacts. NO3- removal from groundwater is conducted mainly by chemical, biological, and coupled methods, with the removal efficiency of NO3- considered the sole performance indicator. However, in addition to the harmless form of N2, the reduced NO3- could be transformed into other intermediates, such as nitrite (NO2-), nitrous oxide (N2O), and ammonia (NH4+), which may have direct or indirect negative impacts on the environment. Therefore, increasing N2 selectivity is a significant challenge in reducing NO3- in groundwater, which seriously impedes the large-scale implementation of available remediation technologies. In this work, we comprehensively overview the most recent advances in N2 selectivity regarding the understanding of emerging groundwater NO3- removal technologies. Mechanisms of by-product production and strategies to enhance the selective reduction of NO3- to N2 are discussed in detail. Furthermore, we proposed topics for further research and hope that the total environmental impacts of remediation schemes should be evaluated comprehensively by quantifying all potential intermediate products, and promising strategies should be further developed to enhance N2 selectivity, to improve the feasibility of related technologies in actual remediation.