Simultaneous arsenite oxidation and nitrate reduction at the electrodes of bioelectrochemical systems

Enhancing biocathode denitrification performance with nano-Fe3O4 under polarity period reversal

The presence of excessive concentrations of nitrate poses a threat to both the environment and human health, and the bioelectrochemical systems (BESs) are attractive green technologies for nitrate removal. However, the denitrification efficiency in the BESs is still limited by slow biofilm formation and nitrate removal. In this work, we demonstrate the efficacy of novel combination of magnetite nanoparticles (nano-Fe3O4) with the anode-cathode polarity period reversal (PPR-Fe3O4) for improving the performance of BESs. After only two-week cultivation, the highest cathodic current density (7.71 ± 1.01 A m-2) and NO3--N removal rate (8.19 ± 0.97 g m-2 d-1) reported to date were obtained in the PPR-Fe3O4 process (i.e., polarity period reversal with nano-Fe3O4 added) at applied working voltage of -0.2 and -0.5 V (vs Ag/AgCl) under bioanodic and biocathodic conditions, respectively. Compared with the polarity reversal once only process, the PPR process (i.e., polarity period reversal in the absence of nano-Fe3O4) enhanced bioelectroactivity through increasing biofilm biomass and altering microbial community structure. Nano-Fe3O4 could enhance extracellular electron transfer as a result of promoting the formation of extracellular polymers containing Fe3O4 and reducing charge transfer resistance of bioelectrodes. This work develops a novel biocathode denitrification strategy to achieve efficient nitrate removal after rapid cultivation.

Potential Self-Attenuation of Arsenic by Indigenous Microorganisms in the Nakdong River

The toxic element arsenic (As) has become the major focus of global research owing to its harmful effects on human health, resulting in the establishment of several guidelines to prevent As contamination. The widespread industrial use of As has led to its accumulation in the environment, increasing the necessity to develop effective remediation technologies. Among various treatments, such as chemical, physical, and biological treatments, used to remediate As-contaminated environments, biological methods are the most economical and eco-friendly. Microbial oxidation of arsenite (As(III)) to arsenate (As(V)) is a primary detoxification strategy for As remediation as it reduces As toxicity and alters its mobility in the environment. Here, we evaluated the self-detoxification potential of microcosms isolated from Nakdong River water by investigating the autotrophic and heterotrophic oxidation of As(III) to As(V). Experimental data revealed that As(III) was oxidized to As(V) during the autotrophic and heterotrophic growth of river water microcosms. However, the rate of oxidation was significantly higher under heterotrophic conditions because of the higher cell growth and density in an organic-matter-rich environment compared to that under autotrophic conditions without the addition of external organic matter. At an As(III) concentration > 5 mM, autotrophic As(III) oxidation remained incomplete, even after an extended incubation time. This inhibition can be attributed to the toxic effect of the high contaminant concentration on bacterial growth and the acidification of the growth medium with the oxidation of As(III) to As(V). Furthermore, we isolated representative pure cultures from both heterotrophic- and autotrophic-enriched cultures. The new isolates revealed new members of As(III)-oxidizing bacteria in the diversified bacterial community. This study highlights the natural process of As attenuation within river systems, showing that microcosms in river water can detoxify As under both organic-matter-rich and -deficient conditions. Additionally, we isolated the bacterial strains HTAs10 and ATAs5 from the microcosm which can be further investigated for potential use in As remediation systems. Our findings provide insights into the microbial ecology of As(III) oxidation in river ecosystems and provide a foundation for further investigations into the application of these bacteria for bioremediation.

Retention and recycling of granules in continuous flow-through system to accomplish denitrification and perchlorate reduction

This study employed a completely anoxic reactor and a gravity-settling design for continuously separating from flocculated biomass and hydraulically recycling granules back to the main reactor. The average chemical oxygen demand (COD) removal in the reactor was 98 %. Average nitrate (NO₃^--N) and perchlorate (ClO₄^-) removal efficiencies of 99 % and 74 ± 19 % were observed, respectively. Preferential utilization of NO₃^- over ClO₄^- led to COD limiting conditions, which resulted in ClO₄^- in the effluent. The average granule diameter in continuous flow-through bubble-column (CFB) anoxic granular sludge (AxGS) bioreactor was 6325 ± 2434 µm, and the average SVI₃₀/SVI₁ was > 90% throughout its operation. 16s rDNA amplicon sequencing revealed Proteobacteria (68.53% - 88.57%) and Dechloromonas (10.46% - 54.77%) to be the most abundant phylum and genus present in reactor sludge representing the denitrifying and ClO₄^- reducing microbial community. This work represents a pioneering development of CFB-AxGS bioreactor.

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.

Electrified bioreactors: the next power-up for biometallurgical wastewater treatment

Over the past decades, biological treatment of metallurgical wastewaters has become commonplace. Passive systems require intensive land use due to their slow treatment rates, do not recover embedded resources and are poorly controllable. Active systems however require the addition of chemicals, increasing operational costs and possibly negatively affecting safety and the environment. Electrification of biological systems can reduce the use of chemicals, operational costs, surface footprint and environmental impact when compared to passive and active technologies whilst increasing the recovery of resources and the extraction of products. Electrification of low rate applications has resulted in the development of bioelectrochemical systems (BES), but electrification of high rate systems has been lagging behind due to the limited mass transfer, electron transfer and biomass density in BES. We postulate that for high rate applications, the electrification of bioreactors, for example, through the use of electrolyzers, may herald a new generation of electrified biological systems (EBS). In this review, we evaluate the latest trends in the field of biometallurgical and microbial‐electrochemical wastewater treatment and discuss the advantages and challenges of these existing treatment technologies. We advocate for future research to focus on the development of electrified bioreactors, exploring the boundaries and limitations of these systems, and their validity upon treating industrial wastewaters.

Effects of anode/cathode electroactive microorganisms on arsenic removal with organic/inorganic carbon supplied

This study reports the effects of an external voltage (0 V, 0.4 V and 0.9 V) on soil arsenic (As) release and sequestration when amended with organic carbon (NaAc) and inorganic carbon (NaHCO₃), respectively, in a soil bioelectrochemistry system (BES). The results demonstrated that although an external voltage had no effect on the As removal capacity in an oligotrophic environment fueled with NaHCO_3, 93.6% of As(III) in the supernatant was removed at 0.9 V with an NaAc amendment. Interestingly, the content of As detected on the electrodes was higher than that removed from the supernatant, implying a continuous release of soil As under external voltages and rapid adsorption onto the electrodes, especially the cathode. In addition, the species of As on the cathode were similar to those in the supernatant (the As(III)/As(V) ratio was approximately 3:1), indicating that the removal capacity was independent of preoxidation. From the viewpoint of electroactive microorganisms (EABs), the relative abundances of the arrA gene and Geobacter genus were specifically enriched at the anode, thus signifying stimulation of the reduction and release of soil As in the anode region. By comparison, Bacillus was particularly abundant at the cathode, which could contribute to the oxidation and sequestration of As in the cathode region. Additionally, specific extracellular polymeric substances (EPSs) secreted by EABs could combine with As, which was followed by electrostatic attraction to the cathode under the effect of an electric field. Furthermore, the formation of secondary minerals and coprecipitation in the presence of iron (Fe) may have also contributed to As removal from solution. The insights from this study will enable us to further understand the biogeochemical cycle of soil As and to explore the feasibility of in situ As bioremediation techniques, combining the aspects of microbial and physicochemical processes in soil bioelectrochemical systems.

Electro-bioremediation of nitrate and arsenite polluted groundwater

The coexistence of different pollutants in groundwater is a common threat. Sustainable and resilient technologies are required for their treatment. The present study aims to evaluate microbial electrochemical technologies (METs) for treating groundwater contaminated with nitrate (NO₃^-) while containing arsenic (in form of arsenite (As(III)) as a co-contaminant. The treatment was based on the combination of nitrate reduction to dinitrogen gas and arsenite oxidation to arsenate (exhibiting less toxicity, solubility, and mobility), which can be removed more easily in further post-treatment. We operated a bioelectrochemical reactor at continuous-flow mode with synthetic contaminated groundwater (33 mg N-NO₃^- L^-1 and 5 mg As(III) L^-1) identifying the key operational conditions. Different hydraulic retention times (HRT) were evaluated, reaching a maximum nitrate reduction rate of 519 g N-NO₃^- m³_{Net Cathodic Compartment} d^-1 at HRT of 2.3 h with a cathodic coulombic efficiency of around 100 %. Simultaneously, arsenic oxidation was complete at all HRT tested down to 1.6 h reaching an oxidation rate of up to 90 g As(III) m^-3_{Net Reactor Volume} d ^-1. Electrochemical and microbiological characterization of single granules suggested that arsenite at 5 mg L^-1 did not have an inhibitory effect on a denitrifying biocathode mainly represented by Sideroxydans sp. Although the coexistence of abiotic and biotic arsenic oxidation pathways was shown to be likely, microbial arsenite oxidation linked to denitrification by Achromobacter sp. was the most probable pathway. This research paves the ground towards a real application for treating groundwater with widespread pollutants.

Arsenic adsorbent derived from the ferromanganese slag

Arsenic-contaminated groundwater has a severe negative impact on the health of living beings. Groundwater majorly contains arsenite (As(III)) as well as arsenate (As(V)). Among these two, the arsenite species are more carcinogenic, mobile, and lethal. Hence, it is more difficult to remove by conventional water treatment methods. Ferromanganese slag, waste generated from steel industries, has been utilized in this study for the development of arsenic adsorbent. A chemical treatment method is applied to the ferromanganese slag to prepare efficient arsenic adsorbent, and it is easy to scale up. An adsorbent with the capacity for simultaneous oxidation of As(III) and adsorption of total arsenic species can be efficient for arsenic decontamination. X-ray photoelectron spectroscopy and X-ray absorption near edge spectra techniques prove the As(III) oxidation capability of the developed material is about 70 ± 5% based on initial As(III) concentration. The adsorbent not only oxidizes the As(III) species but also adsorbs both the arsenic species. The Langmuir isotherm model estimates the maximum adsorption capacities at the equilibrium concentration of 10 μg/L are 1.010 ± 0.004 mg/g and 1.614 ± 0.006 mg/g for As(III) and As(V), respectively. The rate of adsorption of As(III) was higher compared to the As(V), which was confirmed by the pseudo-second-order kinetic model. Therefore, the treated water quality meets the World Health Organization and Indian drinking water standards.

Microbial Communities of Orange Tubercles in Accelerated Low-Water Corrosion

The rapid degradation of marine infrastructure at the low tide level due to accelerated low-water corrosion (ALWC) is a problem encountered worldwide. Despite this, there is limited understanding of the microbial communities involved in this process. We obtained samples of the orange-colored tubercles commonly associated with ALWC from two different types of steel sheet piling, located adjacent to each other but with different levels of localized corrosion, at a seaside harbor. The microbial communities from the outer and inner layers of the orange tubercles and from adjacent seawater were studied by pure culture isolation and metabarcoding of the 16S rRNA genes. A collection of 119 bacterial isolates was obtained from one orange tubercle sample, using a range of media in anaerobic and aerobic conditions. The metabarcoding results showed that sulfur and iron oxidizers were more abundant on the outer sections of the orange tubercles compared to the inner layers, where Deltaproteobacteria (which include many sulfate reducers) were more abundant. The microbial communities varied significantly between the inner and outer layers of the orange tubercles and also with the seawater but overall did not differ significantly between the two steel sheet types. Hence, we saw similar microbial communities in orange tubercles present, but different levels of localized corrosion, for two different types of colocated steel sheet piling. Metallurgical analysis found differences in composition, grain size, ferrite-pearlite ratio, and the extent of inclusions present between the two steel types investigated.IMPORTANCE The presence of orange tubercles on marine steel pilings is often used as an indication that accelerated low-water corrosion is taking place. We studied the microbial communities in attached orange tubercles on two closely located sheet pilings that were of different steel types. The attached orange tubercles were visually similar, but the extents of underlying corrosion on the different steel surfaces were substantially different. No clear difference was found between the microbial communities present on the two different types of sheet piling. However, there were clear differences in the microbial communities in the corrosion layers of tubercles, which were also different from the microbes present in adjacent seawater. The overall results suggest that the presence of orange tubercles, a single measurement of water quality, or the detection of certain general types of microbes (e.g., sulfate-reducing bacteria) should not be taken alone as definitive indications of accelerated corrosion.

In situ groundwater remediation with bioelectrochemical systems: A critical review and future perspectives

Groundwater contamination is an ever-growing environmental issue that has attracted much and undiminished attention for the past half century. Groundwater contamination may originate from both anthropogenic (e.g., hydrocarbons) and natural compounds (e.g., nitrate and arsenic); to tackle the removal of these contaminants, different technologies have been developed and implemented. Recently, bioelectrochemical systems (BES) have emerged as a potential treatment for groundwater contamination, with reported in situ applications that showed promising results. Nitrate and hydrocarbons (toluene, phenanthrene, benzene, BTEX and light PAHs) have been successfully removed, due to the interaction of microbial metabolism with poised electrodes, in addition to physical migration due to the electric field generated in a BES. The selection of proper BESs relies on several factors and problems, such as the complexity of groundwater and subsoil environment, scale-up issues, and energy requirements that need to be accounted for. Modeling efforts could help predict case scenarios and select a proper design and approach, while BES-based biosensing could help monitoring remediation processes. In this review, we critically analyze in situ BES applications for groundwater remediation, focusing in particular on different proposed setups, and we identify and discuss the existing research gaps in the field.

Progress Towards Bioelectrochemical Remediation of Hexavalent Chromium

Chromium is one of the most frequently used metal contaminants. Its hexavalent form Cr(VI), which is exploited in many industrial activities, is highly toxic, is water-soluble in the full pH range, and is a major threat to groundwater resources. Alongside traditional approaches to Cr(VI) treatment based on physical-chemical methods, technologies exploiting the ability of several microorganisms to reduce toxic and mobile Cr(VI) to the less toxic and stable Cr(III) form have been developed to improve the cost-effectiveness and sustainability of remediating hexavalent chromium-contaminated groundwater. Bioelectrochemical systems (BESs), principally investigated for wastewater treatment, may represent an innovative option for groundwater remediation. By using electrodes as virtually inexhaustible electron donors and acceptors to promote microbial oxidation-reduction reactions, in in situ remediation, BESs may offer the advantage of limited energy and chemicals requirements in comparison to other bioremediation technologies, which rely on external supplies of limiting inorganic nutrients and electron acceptors or donors to ensure proper conditions for microbial activity. Electron transfer is continuously promoted/controlled in terms of current or voltage application between the electrodes, close to which electrochemically active microorganisms are located. Therefore, this enhances the options of process real-time monitoring and control, which are often limited in in situ treatment schemes. This paper reviews research with BESs for treating chromium-contaminated wastewater, by focusing on the perspectives for Cr(VI) bioelectrochemical remediation and open research issues.

Bioelectrochemical Systems for Removal of Selected Metals and Perchlorate from Groundwater: A Review

Groundwater contamination is a major issue for human health, due to its largely diffused exploitation for water supply. Several pollutants have been detected in groundwater; amongst them arsenic, cadmium, chromium, vanadium, and perchlorate. Various technologies have been applied for groundwater remediation, involving physical, chemical, and biological processes. Bioelectrochemical systems (BES) have emerged over the last 15 years as an alternative to conventional treatments for a wide variety of wastewater, and have been proposed as a feasible option for groundwater remediation due to the nature of the technology: the presence of two different redox environments, the use of electrodes as virtually inexhaustible electron acceptor/donor (anode and cathode, respectively), and the possibility of microbial catalysis enhance their possibility to achieve complete remediation of contaminants, even in combination. Arsenic and organic matter can be oxidized at the bioanode, while vanadium, perchlorate, chromium, and cadmium can be reduced at the cathode, which can be biotic or abiotic. Additionally, BES has been shown to produce bioenergy while performing organic contaminants removal, lowering the overall energy balance. This review examines the application of BES for groundwater remediation of arsenic, cadmium, chromium, vanadium, and perchlorate, focusing also on the perspectives of the technology in the groundwater treatment field.

Bioelectroremediation of perchlorate and nitrate contaminated water: A review

Fresh water is a fundamental source for humans, hence the recent shrinkage in freshwater and increase in water pollution are imperative problems that vigorously affect the people and the environment worldwide. The breakneck industrialization contributes to the procreation of substantial abundance of wastewater and its treatment becomes highly indispensable. Perchlorate and nitrate containing wastewaters poses a serious threat to human health and environment. Conventional biological treatment methods are expensive and also not effective for treating wastewater effectively and incapable of in situ bioremediation. Bioelectrochemical systems are emerging as a new technology platform for a sustainable removal of such contaminants from wastewater streams. This article reviews the state of art of bioelectroremediation of contaminated waters with perchlorate and nitrate. Different aspects of this technology such as configuration and design, mode of operation and type of substrate are considered in detail.

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.