Complete bromate and nitrate reduction using hydrogen as the sole electron donor in a rotating biofilm-electrode reactor

Biological bromate reduction coupled with in situ gas fermentation in H2/CO2-based membrane biofilm reactor

Bromate, a carcinogenic contaminant generated in water disinfection, presents a pressing environmental concern. While biological bromate reduction is an effective remediation approach, its implementation often necessitates the addition of organics, incurring high operational costs. This study demonstrated the efficient biological bromate reduction using H2/CO2 mixture as the feedstock. A membrane biofilm reactor (MBfR) was used for the efficient delivery of gases. Long-term reactor operation showed a high-level bromate removal efficiency of above 95 %, yielding harmless bromide as the final product. Corresponding to the short hydraulic retention time of 0.25 d, a high bromate removal rate of 4 mg Br/L/d was achieved. During the long-term operation, in situ production of volatile fatty acids (VFAs) by gas fermentation was observed, which can be regulated by controlling the gas flow. Three sets of in situ batch tests and two groups of ex situ batch tests jointly unravelled the mechanisms underpinning the efficient bromate removal, showing that the microbial bromate reduction was primarily driven by the VFAs produced from in situ gas fermentation. Microbial community analysis showed an increased abundance of Bacteroidota group from 4.0 % to 18.5 %, which is capable of performing syngas fermentation, and the presence of heterotrophic denitrifiers (e.g., Thauera and Brachymonas), which are known to perform bromate reduction. Together these results for the first time demonstrated the feasibility of using H2/CO2 mixture for bromate removal coupled with in situ VFAs production. The findings can facilitate the development of cost-effective strategies for groundwater and drinking water remediation.

Synergistic effects boost electrocatalytic reduction of bromate on supported bimetallic Ru-Cu catalyst

Bromate is a commonly identified carcinogenic and genotoxic disinfection byproduct in water. In the present work, bimetallic Ru-Cu catalyst supported on carbon nanotube (RuCu/CNT) was prepared and the structural properties of the catalysts were characterized. The results show that the presence of Ru enhances the dispersion and reduction of Cu particles in the RuCu/CNT catalyst in comparison with the monometallic Cu catalyst supported on CNT (Cu/CNT). For electrocatalytic reaction on Cu/CNT, bromate is reduced on metallic Cu surface via a redox process. For Ru/CNT, highly active H* radicals are generated on metallic Ru surface via the Volmer process and are used for bromate reduction. As for the RuCu/CNT, bromate is reduced through two main pathways, including direct redox reaction on metallic Cu and indirect reduction by active H* radicals on Ru surface. Accordingly, RuCu/CNT exhibits the highest catalytic activity, ascribed to the synergistic effect between metallic Ru and Cu. Furthermore, the bimetallic catalyst displays much higher catalytic efficiency as compared with previously reported results. The pH, initial bromate concentration, in-situ electrochemical reduction of the electrodes and working potential have strong impacts on the removal efficiency of bromate on RuCu/CNT.

Removal of nitrate from agricultural runoff in biochar electrode based biofilm reactor: Performance and enhancement mechanisms

A biochar electrode based biofilm reactor was developed for advanced removal of nitrate from agricultural runoff. The corn-straw (Zea mays L.) biochar formed at 500 °C has an adsorption capacity of NO₃^--N up to 2.659 mg g^-1. After 45-day start-up phase, the removal efficiency of nitrate reached 93.4% when impressed current was 20 mA, hydraulic retention time was 12 h and chemical oxygen demand/total nitrogen (C/N) ratio was 0.56 without additional carbon source. In comparison, neither electrochemical reduction alone nor microbial denitrification alone could obtain the ideal nitrate removal efficiency. The results implied that bio-electrochemical reduction was the main way of nitrate removal in the biofilm electrode reactor (BER). The denitrification efficiency of 88.9% could still be obtained when C/N = 0. It is because biochar can significantly promote the utilization efficiency of cathode electrons by microorganisms. Thus, biochar is a promising electrode material, which provides a new idea for the optimization of BER.

Selective and Efficient Reduction of Nitrate to Gaseous Nitrogen from Drinking Water Source by UV/Oxalic Acid/Ferric Iron Systems: Effectiveness and Mechanisms

Nitrate (NO3−) reduction in water has been receiving increasing attention in water treatment due to its carcinogenic and endocrine-disrupting properties. This study employs a novel advanced reduction process, the UV/oxalic acid/ferric iron systems (UV/C2O42−/Fe3+ systems), in reducing NO3− due to its high reduction efficiency, excellent selectivity, and low treatment cost. The UV/C2O42−/Fe3+ process reduced NO3− with pseudo-first-order reaction rate constants of 0.0150 ± 0.0013 min−1, minimizing 91.4% of 60 mg/L NO3− and reaching 84.2% of selectivity for gaseous nitrogen after 180 min at pHini. 7.0 and 0.5 mg/L dissolved oxygen (DO). Carbon dioxide radical anion (CO2•−) played a predominant role in reducing NO3−. Gaseous nitrogen and NH4+, as well as CO2, were the main nitrogen- and carbon-containing products, respectively, and reduction pathways were proposed accordingly. A suitable level of oxalic acids (3 mM) and NO3− (60 mg/L) was recommended; increasing initial iron concentrations and UV intensity increased NO3− reduction. Instead, increasing the solution pH decreased the reduction, and 0.5–8.0 mg/L DO negligibly affected the process. Moreover, UV/C2O42−/Fe3+ systems were not retarded by 0.1–10 mM SO42− or Cl− or 0.1–1.0 mM HCO3− but were prohibited by 10 mM HCO3− and 30 mg-C/L humic acids. There was a lower reduction of NO3− in simulated groundwater (72.8%) than deionized water after 180 min at pHini. 7.0 and 0.5 mg/L DO, which meets the drinking water standard (<10 mg/L N-NO3−). Therefore, UV/C2O42−/Fe3+ systems are promising approaches to selectively and efficiently reduce NO3− in drinking water.

A comparative study on high-efficient reduction of bromate in neutral solution using zero-valent Al treated by different procedures

Bromate, a toxic by-product of bromide-containing drinking water after disinfecting with ozone, has attracted much attention in the past two decades. Traditional methods to activate zero-valent metals for reducing bromate are to eliminate their surface oxide layer by acid washing. In this work, for the first time, zero-valent Al (ZVAl) was surface treated by the following procedures including soaking, soaking and freeze-drying, soaking and heat-treating, and γ-Al₂O₃ covering Al particle surfaces (GCAP). It was found that all of above surface treated ZVAls have an obvious high efficiency for bromate reduction relative to pristine ZVAl. The bromate reduction rate is GCAP > soaking Al > freeze-drying Al > soaking and heat-treating Al > pristine Al, and using GCAP just 30 min is taken to completely reduce bromate to bromide in neutral solution. Mechanism analyses revealed that Al surface treating or covered by fine γ-Al₂O₃ phase can promote the hydration and breakage of Al surface passive oxide layer, resulting in a fast contact of inner Al with outside ions, leading to a high reduction rate of bromate in neutral solution. XPS analyses indicated that there are no bromate or bromide ions adsorbed on Al particle surfaces, implying that there is a high direct donating efficiency of electrons from inner Al to bromate ions in solution. Furthermore, GCAP has a good reusability and >90% bromate can be reduced even it was reused up to 4 cycles.

Fate and reduction of bromate formed in advanced water treatment ozonation systems: A critical review

Disinfection in water treatment and reclamation systems eliminates the potential health risks associated with waterborne pathogens, however it may produce disinfection by-products (DBPs) harmful to human health. Potentially carcinogenic bromate is a DBP formed during the ozonation of bromide-containing waters. To mitigate the problem of bromate formation, different physical/chemical or biological reduction methods of bromate have been investigated. Until now, adsorption-based physical method has proven to be more effective than chemical methods in potable water treatment. Though several studies on biological reduction methods have been carried out in a variety of bioreactor systems, such as in biologically active carbon filters and denitrifying bioreactors, the microbiological mechanisms or biochemical pathways of bromate minimization have not been clearly determined to date. Genetic analysis could provide a broader picture of microorganisms involved in bromate reduction which might show cometabolic or respiratory pathways, and affirm the synergy functions between different contributing groups. The hypothesis established from the diffusion coefficients of different electron donor and acceptors, illustrates that some microorganisms preferring bromate over oxygen contain specific enzymes which lower the activation energy required for bromate reduction. In addition, considering microbial bromate reduction as an effective treatment strategy; field scale investigations are required to observe quantitative correlations of various influencing parameters such as pH, ozone dose, additives or constituents such as ammonia, hydrogen peroxide, and/or chloramine, dissolved organic carbon levels, dissolved oxygen gradient within biofilm, and empty bed contact time on bromate removal or reduction.

Electro-assisted autohydrogenotrophic reduction of perchlorate and microbial community in a dual-chamber biofilm-electrode reactor

The electro-assisted autohydrogenotrophic reduction of perchlorate (ClO₄^-) was investigated in a dual-chamber biofilm-electrode reactor (BER), in which the microbial community was inoculated from natural sediments. To avoid the effect of extreme pH and direct electron transfer on perchlorate reduction, a novel cathode configuration was designed. The pH of the cathode compartment was successfully controlled in the range of 7.2-8.4 during whole experiment. The effective biological autohydrogenotrophic reduction of perchlorate was achieved using hydrogen generated in-situ on the electrode surface, and the removal rate of 10 mg L^-1 perchlorate reached 98.16% at HRT of 48 h. The highest perchlorate removal flux reached to 1498.420 mg m^-2·d^-1 with a 0.410 kW·h g-perchlorate^-1 energy consumption. The microbial community evolution in the BER was determined by high-throughput sequencing and the results indicated that the Firmicutes and Bacteroidetes were dominant at phylum level when perchlorate concentration was 10 mg L^-1 or lower. And the Proteobacteria became ascendant at the perchlorate concentration of 20 mg L^-1. The functional populations for perchlorate reduction were successfully enriched including Nitrosomonas (30%), Thermomonas (9%), Comamonas (8%) and Hydrogenophaga (3%). Meanwhile, the proportion of functional population in biofilm linked to perchlorate concentration. With the increase of influent perchlorate concentration, the perchlorate-reducing bacteria (PRB) were enriched successfully and became ascendant.

Bromate removal from water by acid activated and surfactant enriched Red Mud - the case of cooling water

Red Mud is a hazardous by-product of the Bayer process, used to produce alumina from bauxite, with ability to adsorb anions from water. Acid activation and enrichment with CetylTrimethylAmmonium Chloride (CTAC), a cationic surfactant, are employed to enable it to remove bromate initially from spiked double-distilled water. CTAC enrichment is found to substantially improve Red Mud's bromate removal ability in comparison to acid activation alone. Fourier Transformation Infrared Spectroscopy is used to evaluate the effectiveness of the enrichment process. Maximum CTAC loading is 0.037 g per g acid activated Red Mud (AARM). Adsorption is faster after CTAC enrichment. pH increase is found to adversely affect both AARM and acid activated CTAC enriched Red Mud's (CTAC-AARM) bromate removal capability, yet CTAC-AARM's ability proves more resistant to pH changes. Adsorption data fit best the Langmuir isotherm model for both adsorbers. The R² values for AARM and CTAC-AARM are 0.955 and 0.964 respectively. Maximum adsorbable bromate quantity is almost 2.5 times higher for CTAC-AARM in comparison to AARM. Finally, both Red Mud adsorbers are compared with respect to their ability to remove bromate from cooling water; an industrial matrix rich in competing ions. As cycles of concentration and pH appreciate, bromate adsorption is hindered regardless of the adsorber used. However, CTAC-AARM still performs better in removing bromate. It is proven that after suitable processing, Red Mud can re-enter the industrial cycle by playing a role in bromate removal from industrial waters.

Autotrophic induced heterotrophic bioreduction of bromate in use of elemental sulfur or zerovalent iron as electron donor

Bioreduction of bromate using elementary sulfur (S(0)) or zerovalent iron (Fe(0)) as electron donor was studied. After 60-day cultivation, the microbial consortium achieved high bromate conversion of 91.9 ± 2.1% and 90.0 ± 4.0% in the S(0) and Fe(0) amended tests. A coupling mechanism involving autotrophic oxidation of S(0) or Fe(0) to convert bicarbonate to volatile fatty acids followed by bromate reduction at the oxidation of the volatile fatty acids to CO₂ was proposed to interpret the noted reactor performances. The key functional strains including S(0) or Fe(0) oxidizing bacteria (Thiomonas and Ferrovibrio) and bromate reducing bacteria (Pseudoxanthomonas and Clostridium sp.) are identified. The studied system can provide an efficient BrO₃^- conversion way with no external organic carbon sources.

Synergistic adsorption and electrocatalytic reduction of bromate by Pd/N-doped loofah sponge-derived biochar electrode

In this work, a novel Pd/N-doped loofah sponge-derived biochar (Pd/NLSBC) material with three-dimensional (3D) network structure was prepared through the carbonization-impregnation method and applied as cathode for electrocatalytic bromate removal. The N-doped biochar not only increased the adsorption capacity of electrode, but also facilitated electron transfer, subsequently resulting in the high electrocatalytic activity for bromate removal. The results indicated higher bromate adsorption capacity of Pd/NLSBC electrode was favorable to the electrocatalytic bromate removal. The influences of significant operating factors including calcination temperature, initial solution pH, applied current intensity, and initial bromate concentration on electrocatalytic bromate removal were also optimized. Under the current intensity of 10 mA, Pd/NLSBC-800 exhibited the highest bromate removal efficiency (96.7 %) and the bromide conversion rate reached almost 100 % at the initial bromate concentration of 0.781 μmol L^-1. This process could be effectively performed over a wide range of pH (2.0-9.0) and be well fitted to the pseudo-first-order kinetic model under different conditions. The reaction mechanism study indicated that both direct electron transfer and indirect reduction by the active hydrogen atom (H*) contributed to the elctrocatalytic bromate removal. Meanwhile, Pd/NLSBC-800 electrode could maintain its high electrocatalytic activity for bromate removal after five cycles.

Phylogenetic characterization of bromate-reducing microbial community enriched anaerobically from activated sludge

Bromate is a weakly carcinogenic disinfection by-product generated from naturally existing bromide during ozonation. In order to uncover the phylogenetic diversity of bromate-reducing aquatic bacteria, the microbial mixed cultures were enriched anaerobically from various activated sludge samples with a high concentration of 3.6 mmol/L bromate which served as the electron acceptor in batch-fed reactors. Then the phylogenetic diversity was characterized by employing 16S rRNA gene-based clone libraries and high-throughput sequencing. Results showed that as highest as 48.65 mg/L [BrO₃^--Br] (0.61 mmol/L) could be reduced to Br^- within an operational cycle of 48 h, indicating the presence of bromate-reducing bacteria. The microbial analysis based on 16S rRNA gene clone libraries indicated that the dominant bacteria in the enriched consortium were affiliated to the genera of Aeromonas, which had been reported and implied its functional application to reduce bromate. In addition, some other subdivisions of bromate-reducing bacteria were enriched at different operational cycles. The abundance of the genus Rivibacter belonging to Comamonadaceae were increased after 10 cycles enrichment, which might represent another type of novel bromate bio-reducers. The study provided new insights for the phylogenetic diversity of the bromate-reducing microorganisms while further researches are required to verify the bromate bio-reduction mechanism.

Functional collaboration of biofilm-cathode electrode and microbial fuel cell for biodegradation of methyl orange and simultaneous bioelectricity generation

A distinctive process (BCE-MFC) was developed to explore the methyl orange (MO) degradation and simultaneous bioelectricity generation based on the functional collaboration of biofilm, electrolysis, constructed wetland, and microbial fuel cell. The biofilm-cathode electrode-microbial fuel cell (BCE-MFC) was capable of sustaining an excellent MO removal (100%) and bioelectricity production (0.63 V). BCE significantly enhanced MO biodegradability, thus resulting in a 56.3% improvement of COD removal in subsequent MFC. Bacillus was dominant in biofilm on cathode in BCE. In MFC, Proteobacteria phylum (64.84%) and Exiguobacterium genus (13.30%) were predominated in the anode region, probably basically responsible for electricity generation. Interestingly, relatively high content of Heliothrix sp. (9.94%) was found in the MFC designed here, which was likely to participate in electricity production as well. The proposed functional collaboration may be an effective strategy in refractory wastewater treatment and power production.

Microbial reduction of bromate: current status and prospects

Bromate is a disinfection byproduct (DBP) that forms during the ozonation of bromide-containing natural water, which may cause health risks to humans. In this review, we provide an overview of the mechanism of bromate formation, microbial communities and bioreactors that are responsible for bromate reduction. Bromate can be formed through two pathways of bromide oxidation by ozone or by ^·OH, and it can be removed by biological approaches. Members belonging to phyla of Spirochaetes, Proteobacteria, Firmicutes, Actinobacteria, Clostridium, Deinococcus-Thermus and Bacteroidetes have been identified as capable of reducing bromate to bromide. Multiple configurations of biofilm bioreactors have been employed to cultivate microbial communities to perform bromate removal. The rapid development of multiomics has and will continue to accelerate the elucidation of the mechanisms involved in bromate and other DBP conversions, as well as the interaction patterns among different bacterial subdivisions in the bioremoval of DBPs.

Indirect electrochemical reduction of nitrate in water using zero-valent titanium anode: Factors, kinetics, and mechanism

In this study, indirect electrochemical reduction with zero-valent titanium (ZVT) as anode successfully achieved the selective nitrate removal from simulated groundwater. The maximum nitrate removal efficiency and N₂ selectivity reached to 83.4% and 78.5% after 12 h, respectively. Experimental results demonstrated that the gaseous by-products (NO and N₂O) were negligible and the nitrate reduction process could be well depicted by pseudo-first-order kinetic model. Decreasing the pH value of electrolyte was favorable to electrical energy utilization efficiency and nitrate removal. The chloride ultimately showed inhibitory effects on electrochemical reduction of nitrate. During the electrochemical reaction, the ZVT lost electrons to generate the reducing agents (Ti³⁺ and Ti²⁺), which could afford electrons for nitrate reduction and form the solid by-products TiO_2.4Cl_0.2N_0.1. A 2-stage strategy, indirect electrochemical reduction + hypochlorite treatment (pre-reduction + post-oxidation), was developed to completely remove nitrate and the long-term performance of nitrate reduction was comprehensively evaluated. The effluent nitrate steadily kept at 8.8 mg N/L during 120 h continuous operation when the influent nitrate concentration was 25.9 mg N/L. Simultaneously, nitrite concentration was lower than 0.01 mg N/L, and ammonium and Ti ions were not detected in the effluent.

Bromate and Nitrate Bioreduction Coupled with Poly-β-hydroxybutyrate Production in a Methane-Based Membrane Biofilm Reactor

This work demonstrates bromate (BrO₃^-) reduction in a methane (CH₄)-based membrane biofilm reactor (MBfR), and it documents contrasting impacts of nitrate (NO₃^-) on BrO₃^- reduction, as well as formation of poly-β-hydroxybutyrate (PHB), an internal C- and electron-storage material. When the electron donor, CH₄, was in ample supply, NO₃^- enhanced BrO₃^- reduction by stimulating the growth of denitrifying bacteria ( Meiothermus, Comamonadaceae, and Anaerolineaceae) able to reduce BrO₃^- and NO₃^- simultaneously. This was supported by increases in denitrifying enzymes (e.g., nitrate reductase, nitrite reductase, nitrous-oxide reductase, and nitric-oxide reductase) through quantitative polymerase chain reaction (qPCR) analysis and metagenomic prediction of these functional genes. When the electron donor was in limited supply, NO₃^- was the preferred electron acceptor over BrO₃^- due to competition for the common electron donor; this was supported by the significant oxidation of stored PHB when NO₃^- was high enough to cause electron-donor limitation. Methanotrophs (e.g., Methylocystis, Methylomonas, and genera within Comamonadaceae) were implicated as the main PHB producers in the biofilms, and their ability to oxidize PHB mitigated the impacts of competition for CH₄.

Denitrifying microbial community with the ability to bromate reduction in a rotating biofilm-electrode reactor

In this study, the microbial community for bromate reduction in a rotating biofilm-electrode reactor (RBER) was investigated. Continuous experiment demonstrated that the bromate reduction by an auto-hydrogenotrophic microbial community was inhibited by high concentration nitrate (50mg/L). The bacterial diversity of RBER were examined through the analyse of 16S rRNA gene sequences of clone libraries. The results showed that the bromate-reducing bacteria were phylogenetically diverse at the phylum level, representing the Firmicutes, Proteobacteria, Bacteroidetes and Actinobacteria. The relative abundances of these microbial community represented 99.1% of all phylum in the biofilms when bromate served as the sole electron acceptor. Meanwhile, the Bacillus strains became the largest phylotype and represented about 37% of the total bacteria in the biofilm, indicating that the genus Bacillus played the key role in the auto-hydrogenotrophic process. Moreover, three new bacterial genera, Exiguobacterium, Arthrobacter and Chlorobium appeared with the respective relative abundance being about 7.37%, 1.81%, and 0.52%, which might be the bromate-specific reducing bacteria.

Effects of Electrical Stimulation on the Degradation of Azo Dye in Three-Dimensional Biofilm Electrode Reactors

Three-dimensional biofilm electrode reactors (3D-BERs) were constructed to degrade the azo dye Reactive Brilliant Red (RBR) X-3B. The 3D-BERs with different influent concentrations and external voltages were individually studied to investigate their influence on the removal of X-3B. Experimental results showed that 3D-BERs have good X-3B removal efficiency; even when the influent concentration was 800 mg/L, removal efficiency of 73.4% was still achieved. In addition, the X-3B removal efficiency stabilized shortly after the influent concentration increased. In 3D-BERs, the average X-3B removal efficiency increased from 52.8% to 85.4% when the external voltage rose from 0 to 2 V. We further identified the intermediate products via UV-Vis and gas chromatography-mass spectrometry (GC-MS) analyses, and discussed the potential mechanism of degradation. After the conjugate structure of X-3B was destroyed, all of the substances generated mainly consisted of lower-molecular-weight organics.