Energy-efficient treatment of refractory industrial effluent using flow-through electrochemical processes: Oxidation mechanisms and reduction of chlorinated byproducts.

While flow-through anodic oxidation (FTAO) technique has demonstrated high efficiency to treat various refractory waste streams, there is an increasing concern on the secondary hazard generation thereby. In this study, we developed an integrated system that couples FTAO and cathodic reduction processes (termed FTAO-CR) for sustainable treatment of chlorine-laden industrial wastewater. Among four common electrode materials (i.e., Ti4O7, ß-PbO2, RuO2, and SnO2-Sb), RuO2 flow-through anode exhibited the best pollutant removal performance and relatively low ClO3 and ClO4 yields. Because of the significant scavenging effect of Cl- in real wastewater treatment, the direct electron transfer process played a dominant role in contaminant degradation for both active and nonactive anodes though active species (i.e., active chlorine) were involved in the subsequent transformation of the organic matter. A continuous FTAO-CR system was then constructed for simultaneous COD removal and organic and inorganic chlorinated byproduct control. The quality of the treated effluent could meet the national discharge permit limit at low energy cost (∼4.52 kWh m3 or ∼0.035 kWh g1-COD). Results from our study pave the way for developing novel electrochemical platforms for the purification of refractory waste streams whilst minimizing the secondary pollution.

Tailored Metal-Organic Frameworks for Water Purification: Perfluorinated Fe-MOFs for Enhanced Heterogeneous Catalytic Ozonation.

An industrially viable catalyst for heterogeneous catalytic ozonation (HCO) in water purification requires the characteristics of good dispersion of active species on its surface, efficient electron transfer for ozone decay, and maximum active species utilization. While metal-organic frameworks (MOFs) represent an attractive platform for HCO, the metal nodes in the unmodified MOFs exhibit low catalytic activity. Herein, we present a perfluorinated Fe-MOF catalyst by substituting H atoms on the metalated ligands with F atoms (termed 4F-MIL-88B) to induce structure evolution. The Lewis acidity of 4F-MIL-88B was enhanced via the formation of Fe nodes, tailoring the electron distribution on the catalyst surface. As a result of catalyst modification, the rate constant for degradation of the target compounds examined increased by ∼700% compared with that observed for the unmodified catalyst. Experimental evidence and theoretical calculations showed that the modulated polarity and the enhanced electron transfer between the catalyst and ozone molecules contributed to the adsorption and transformation of O3 to •OH on the catalyst surface. Overall, the results of this study highlight the significance of tailoring the metalated ligands to develop highly efficient and stable MOF catalysts for HCO and provide an in-depth mechanistic understanding of their structure-function evolution, which is expected to facilitate the applications of nanomaterial-based processes in water purification.

New Insights into the Role of Humic Acid in Permanganate Oxidation of Diclofenac: A Novel Electron Transfer Mechanism.

Humic acid (HA) ubiquitously existing in aquatic environments has been reported to significantly impact permanganate (KMnO4) decontamination processes. However, the underlying mechanism of the KMnO4/HA system remained elusive. In this study, an enhancing effect of HA on the KMnO4 oxidation of diclofenac (DCF) was observed over a wide solution pH range of 5-9. Surprisingly, the mechanism of HA-induced enhancement varied with solution pH. Quenching and chemical probing experiments revealed that manganese intermediates (Mn(III)-HA and MnO2) were responsible for the enhancement under acidic conditions but not under neutral and alkaline conditions. By combining KMnO4 decomposition, galvanic oxidation process experiments, electrochemical tests, and FTIR and XPS analysis, it was interestingly found that HA could effectively mediate the electron transfer from DCF to KMnO4 in neutral and alkaline solutions, which was reported for the first time. The formation of an organic-catalyst complex (i.e., HA-DCF) with lower reduction potential than the parent DCF was proposed to be responsible for the accelerated electron transfer from DCF to KMnO4. This electron transfer likely occurred within the complex molecule formed through the interaction between HA-DCF and KMnO4 (i.e., HA-DCF-KMnO4). These results will help us gain a more comprehensive understanding of the role of HA in the KMnO4 oxidation processes.

Pulsed electric field drives chemical-free membrane stripping for high ammonia recovery from urine.

Recovering ammonia from waste streams (e.g., urine) is highly desirable to reduce natural gas-based NH3 production and nitrogen discharge into the water environment. Electrochemical membrane stripping is an attractive alternative because it can drive NH4+ transformation to NH3 via cathodic OH- production; however, the conventional configurations suffer from relatively low ammonia recovery (<80 %) and significant acid/material usage for ammonia adsorption. To this end, we develop a novel stack system that simply uses an oxygen evolution reaction to in-situ produce acid from water, enabling chemical-free ammonia recovery from synthetic urine. In batch mode, the percentage removal and recovery increased respectively from 74.5 % to 97.9 % and 81.8 % to 92.7 % when the electrode pairs increased from 2 to 4 in the stack system. To address the gas-sparging issue that deteriorated ammonia recovery in continuous operation, pulsed electric field (PEF) mode was applied, resulting in ∼100 % recovery under optimized conditions. At an ammonia removal rate of 35.1 g-N m-2 h-1 and electrical energy consumption of 28.9 kWh kg-N-1, our chemical-free system in PEF mode has achieved significantly higher ammonia recovery (>90 %) from synthetic urine. The total cost to recover 1 kg of NH3-N from real human urine was $15.9 in the proposed system. Results of this study demonstrate that this novel approach holds great promise for high ammonia recovery from waste streams, opening a new pathway toward sustainable nitrogen management.

Comparative study of Fe(II)/sulfite, Fe(II)/PDS and Fe(II)/PMS for p-arsanilic acid treatment: Efficient organic arsenic degradation and contrasting total arsenic removal.

As a widely used feed additives, p-arsanilic acid (p-AsA) frequently detected in the environment poses serious threats to aquatic ecology and water security due to its potential in releasing more toxic inorganic arsenic. In this work, the efficiency of Fe(II)/sulfite, Fe(II)/PDS and Fe(II)/PMS systems in p-AsA degradation and simultaneous arsenic removal was comparatively investigated for the first time. Efficient p-AsA abatement was achieved in theses Fe-based systems, while notable discrepancy in total arsenic removal was observed under identical acidic condition. By using chemical probing method, quenching experiments, isotopically labeled water experiments, p-AsA degradation was ascribed to the combined contribution of high-valent Fe(IV) and SO4•-in these Fe(II)-based system. In particular, the relative contribution of Fe(IV) and SO4•- in the Fe(II)/sulfite system was highly dependent on the molar ratio of [Fe(II)] and [sulfite]. Negligible arsenic removal was observed in the Fe(II)/sulfite and Fe(II)/PDS systems, while ∼80% arsenic was removed in the Fe(II)/PMS system under identical acidic condition. This interesting phenomenon was due to that ferric precipitation only occurred in the Fe(II)/PMS system. As(V) was further removed via adsorption onto the iron precipitate or the formation of ferric arsenate-sulfate compounds, which was confirmed by particle diameter measurements, fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. Through tuning solution pH, complete removal of total arsenic could achieve in all three systems. Among these three Fe-based technologies, the hybrid oxidation-coagulation Fe(II)/PMS system demonstrated potential superiority for arsenic immobilization by not requiring pH adjustment for coagulation and facilitating the in-situ generation of ferric arsenate-sulfate compounds with comparably low solubility levels like scorodite. These findings would deepen the understanding of these three Fe-based Fenton-like technologies for decontamination in water treatment.

Oxidation of amine-based pharmaceuticals with unactivated peroxymonosulfate: Kinetics, mechanisms, and elimination efficiency of NDMA formation.

Amine-based pharmaceuticals are a significant class of N-nitrosodimethylamine (NDMA) precursors. This study investigated the use of unactivated peroxymonosulfate (PMS) to control amine-based pharmaceuticals and their NDMA formation potential. Kinetic analysis and product identification revealed that sumatriptan and doxylamine primarily underwent reactions at their tertiary amine group, while ranitidine and nizatidine had both tertiary amine and thioether group as reaction sites. The NDMA formation from sumatriptan and doxylamine during post-chloramination was significantly reduced with the abatement of the parent contaminants, while the formation of NDMA remained high even if full abatement of ranitidine and nizatidine was achieved. Product formation kinetics and reference standard tests revealed the great contribution of transformation products to NDMA formation. Ranitidine could be oxidized to sulfoxide-type product ranitidine-SO and N-oxide type product ranitidine-NO. Ranitidine-SO exhibited a high NDMA yield comparable to that of ranitidine (>90%), while ranitidine-NO showed a low NDMA yield (2%). With further oxidation of ranitidine-SO at the tertiary amine group, NDMA formation was reduced by more than 90%. The underlying mechanism for the importance of the tertiary amine group in NDMA formation was demonstrated by quantum chemical calculation. These findings underscore the potential of PMS pre-oxidation on NDMA control.

Shifting Emphasis from Electro- to Catalytically Active Sites: Effects of Pore Size of Flow-Through Anodes on Water Purification.

A flow-through anode has demonstrated high efficiency for micropollutant abatement in water purification. In addition to developing novel electrode materials, a rational design of its porous structure is crucial to achieve high electrooxidation kinetics while sustaining a low cost for flow-through operation. However, our knowledge of the relationship between the pore structure and its performance is still incomplete. Therefore, we systematically explore the effect of pore size (with a median from 4.7 to 49.4 µm) on the flow-through anode efficiency. Results showed that when the pore size was <26.7 µm, the electrooxidation kinetics was insignificantly improved, but the permeability declined dramatically. Traditional empirical evidence from hydrodynamic modeling and electrochemical tests indicated that a flow-through anode with a smaller pore size (e.g., 4.7 µm) had a high mass transfer capability and large electroactive area. However, this did not further accelerate the micropollutant removal. Combining an overpotential distribution model and an imprinting method has revealed that the reactivity of a flow-through anode is related to the catalytically active volume/sites. The rapid overpotential decay as a function of depth in the anode would offset the merits arising from a small pore size. Herein, we demonstrate an optimal pore size distribution (∼20 µm) of typical flow-through anodes to maximize the process performance at a low energy cost, providing insights into the design of advanced flow-through anodes in water purification applications.

Efficient removal of As(III) from groundwaters through self-alkalization in an asymmetric flow-electrode electrochemical separation system.

It remains a great challenge to efficiently remove As(III) from groundwater using traditional technologies due to its stable electroneutral form. This study constructed an asymmetric flow-electrode electrochemical separation (AFES) system, which overcomes the drawback of H+ release from anodic carbon oxidation and achieves continuous self-alkalization function and highly efficient removal of As(III) from groundwater. At the applied voltage of 1.2 V and initial pH 7.5, the system could rapidly decrease the total As (T-As) concentration from 150.0 to 8.9 µg L-1 within 90 min, with an energy consumption of 0.04 kWh m-3. The self-alkalization was triggered by the generation of H2O2 from dissolved oxygen reduction and the adsorption of H+ on the cathode in the feed chamber, which significantly promoted the dissociation and oxidation of As(III), resulting in the removal of T-As predominantly in the form of As(V). The removal performance of T-As was slightly affected by the initial pH and coexisting ions in the feed chamber. The AFES system also exhibited considerable stability after 20 cycles of continuous experiments and superior performance in treating As-containing real groundwater. Moreover, the pH of the alkalized solution can be restored to the initial level by standing or aeration operation. This work offers a novel and efficient pathway for the detoxication of As(III)-contaminated groundwaters.

Site Engineering of Covalent Organic Frameworks for Regulating Peroxymonosulfate Activation to Generate Singlet Oxygen with 100 % Selectivity.

Singlet oxygen (1 O2 ) is an excellent reactive oxygen species (ROSs) for the selective conversion of organic matter, especially in advanced oxidation processes (AOPs). However, due to the huge dilemma in synthesizing single-site type catalysts, the control and regulation of 1 O2 generation in AOPs is still challenging and the underlying mechanism remains largely obscure. Here, taking advantage of the well-defined and flexibly tunable sites of covalent organic frameworks (COFs), we report the first achievement in precisely regulating ROSs generation in peroxymonosulfate (PMS)-based AOPs by site engineering of COFs. Remarkably, COFs with bipyridine units (BPY-COFs) facilitate PMS activation via a nonradical pathway with 100 % 1 O2 , whereas biphenyl-based COFs (BPD-COFs) with almost identical structures activate PMS to produce radicals (⋅OH and SO4 .- ). The BPY-COFs/PMS system delivers boosted performance for selective degradation of target pollutants from water, which is ca. 9.4 times that of its BPD-COFs counterpart, surpassing most reported PMS-based AOPs systems. Mechanism analysis indicated that highly electronegative pyridine-N atoms on BPY-COFs provide extra sites to adsorb the terminal H atoms of PMS, resulting in simultaneous adsorption of O and H atoms of PMS on one pyridine ring, which facilitates the cleavage of its S-O bond to generate 1 O2 .

Metal-Organic Framework with a Redox-Active Bridge Enables Electrochemically Highly Selective Removal of Arsenic from Water.

Selective removal of trace, highly toxic arsenic from water is vital to ensure an adequate and safe drinking water supply for over 230 million people around the globe affected by arsenic contamination. Here, we developed an Fe-based metal-organic framework (MOF) with a ferrocene (Fc) redox-active bridge (termed Fe-MIL-88B-Fc) for the highly selective removal of As(III) from water. At a cell voltage of 1.2 V, Fe-MIL-88B-Fc can selectively separate and oxidize As(III) into the less harmful As(V) state in the presence of a 100- to 1250-fold excess of competing electrolyte, with an uptake capacity of >110 mg-As g-1 adsorbent. The high affinity between the uncharged As(III) and the µ3-O trimer (-36.55 kcal mol-1) in Fe-MIL-88B-Fc and the electron transfer between As(III) and redox-active Fc+ synergistically govern the selective capture and conversion of arsenic. The Fe-based MOF demonstrates high selectivity and capacity to remediate arsenic-contaminated natural water at a low energy cost (0.025 kWh m-3). This study provides valuable guidance for the tailoring of effective and robust electrodes, which can lead to a wider application of electrochemical separation technologies.

Ferryl Ion in the Photo-Fenton Process at Acidic pH: Occurrence, Fate, and Implications.

Fenton processes produce reactive species that can oxidize organic compounds in natural and engineered systems. While it is well-documented that Fenton reactions produce hydroxyl radical (HO•) under acidic conditions, we demonstrated the generation of ferryl ion (FeIVO2+) in the UV/Fe(III) and UV/Fe(III)/H2O2 systems at pH 2.8 using methyl phenyl sulfoxide (PMSO) as the probe compound. Moreover, we clarified that FeIVO2+ is parallelly formed via the oxidation of Fe(III) by HO• and the O-O homolysis of [FeIII-OOH]2+ in the photo-Fenton process. The rate constant for the reaction between HO• and Fe3+ measured by laser flash photolysis was 4.41 × 107 M-1 s-1. The rate constant and quantum yield for thermal and photo O-O homolysis of [FeIII-OOH]2+ complex were 1.4 × 10-2 s-1 and 0.3, respectively, which were determined by fitting PMSO2 formation. While FeIVO2+ forms predominantly through the reaction between HO• and Fe3+ in the absence of H2O2, the relative contribution of [FeIII-OOH]2+ O-O homolysis to FeIVO2+ formation highly depends on the molar ratio of [H2O2]0/[Fe(III)]0, the level of HO• scavenging, and incident irradiance in the UV/Fe(III)/H2O2 system. Accordingly, an optimized kinetic model was developed by incorporating FeIVO2+-involved reactions into the conventional photo-Fenton model, which can accurately predict Fe(II) formation and contaminant decay in the UV/Fe(III) and UV/Fe(III)/H2O2 systems. Our study illuminated the underlying formation mechanism of reactive oxidative species in the photo-Fenton process and highlighted the role of FeIVO2+ evolution in modulating the iron cycle and pollutant abatement therein.

Heterogenous Iron Oxide Assemblages for Use in Catalytic Ozonation: Reactivity, Kinetics, and Reaction Mechanism.

Heterogeneous catalytic ozonation (HCO) has gained increasing attention as an effective process to remove refractory organic pollutants from industrial effluents. However, widespread application of HCO is still limited due to the typically low efficacy of catalysts used and matrix passivation effects. To this end, we prepared an Al2O3-supported Fe catalyst with high reactivity via a facile urea-based heterogeneous precipitation method. Due to the nonsintering nature of the preparation method, a heterogeneous catalytic layer comprised of γ-FeOOH and α-Fe2O3 is formed on the Al2O3 support (termed NS-Fe-Al2O3). On treatment of a real industrial effluent by HCO, the presence of NS-Fe-Al2O3 increased the removal of organics by ∼100% compared to that achieved with a control catalyst (i.e., α-Fe2O3/Al2O3 or γ-FeOOH/Al2O3) that was prepared by a conventional impregnation and calcination method. Furthermore, our results confirmed that the novel NS-Fe-Al2O3 catalyst demonstrated resistance to the inhibitory effect of high concentration of chloride and sulfate ions usually present in industrial effluent. A mathematical kinetic model was developed that adequately describes the mechanism of HCO process in the presence of NS-Fe-Al2O3. Overall, the results presented here provide valuable guidance for the synthesis of effective and robust catalysts that will facilitate the wider industrial application of HCO.

Microalgae production in human urine: Fundamentals, opportunities, and perspectives.

The biological treatment of source-separated human urine to produce biofuel, nutraceutical, and high-value chemicals is getting increasing attention. Especially, photoautotrophic microalgae can use human urine as media to achieve environmentally and economically viable large-scale cultivation. This review presents a comprehensive overview of the up-to-date advancements in microalgae cultivation employing urine in photobioreactors (PBRs). The standard matrices describing algal growth and nutrient removal/recovery have been summarized to provide a platform for fair comparison among different studies. Specific consideration has been given to the critical operating factors to understand how the PBRs should be maintained to achieve high efficiencies. Finally, we discuss the perspectives that emphasize the impacts of co-existing bacteria, contamination by human metabolites, and genetic engineering on the practical microalgal biomass production in urine.

Induced mazEF-mediated programmed cell death contributes to antibiofouling properties of quaternary ammonium compounds modified membranes.

Functionalized antibiofouling membranes have attracted increasing attention in water and wastewater treatment. Among them, contact-killing antibiofouling membranes deliver a long-lasting effect with no leaching or release, thus providing distinctive advantages. However, the antibiofouling mechanism especially in the vicinity of the membrane surface remains unclear. Herein, we demonstrate that mazEF-mediated programmed cell death (PCD) is critical for the antibiofouling behaviors of quaternary ammonium compounds modified membranes (QM). The viability of wild type Escherichia coli (WT E. coli) upon exposure to QM for 1 h was decreased dramatically (31.5 ± 1.4% of the control). In contrast, the bacterial activity of E. coli with the knockout of mazEF gene (KO E. coli) largely remained (85.8 ± 5.2%). Through addition of quorum sensing factor, i.e., extracellular death factor (EDF), the antibacterial activity was significantly enhanced in a dilute culture, indicating that the density-dependent bacterial communication played an important role in the mazEF-mediated PCD system in biofouling control. Long-term study further showed that QM exhibited a better antibiofouling performance to treat feedwater containing WT E. coli, especially when EDF was dosed. Results of this study suggested that the bacteria on the membrane surface subject to contact killing could modulate the population growth in the vicinity via quorum-sensing mazEF-mediated PCD, paving a way to develop efficient antibiofouling materials based on contact-killing scenarios.

Energy-efficient removal of trace antibiotics from low-conductivity water using a Ti4O7 reactive electrochemical ceramic membrane: Matrix effects and implications for byproduct formation.

The inevitably high energy consumption of traditional electrochemical processes to treat low-conductivity water has limited their wider application. Herein, we present an energy-efficient alternative, i.e., a Ti4O7 reactive electrochemical ceramic membrane (Ti4O7-REM) system with a superior mass transfer ability. For the removal of 10-200 µM norfloxacin (NOR) from low-conductivity (178-832 µS cm-1) water, the Ti4O7-REM system increased the kinetics rate constant by 4.3-34.0 times, thus decreasing the energy cost by 80.5-97.3% compared with a flow-by system. The rapid NOR removal was related to the enhanced direct electron transfer process in the Ti4O7-REM system, which allowed for higher resistance to HCO3- scavenging and a favorable reaction between NOR and the active sites. Meanwhile, this mechanism likely contributed to the less formation of inorganic chlorinated product, ClO3-, in the presence of Cl-. Although organic chlorinated byproducts were not detected during NOR degradation in the Ti4O7-REM system, Cl- influenced the speciation of the intermediates. A single-pass Ti4O7-REM system demonstrated 94-97% removal of trace antibiotics from real water samples in 30 s. The additional energy consumption (<0.02 kWh m-3) using a Ti4O7-REM system only contributed to 5.0-6.4% of the total in a typical tertiary wastewater treatment plant. Based on the above results, we can conclude that the convection-enhanced REM technique is viable for the purification of low-conductivity natural waters.

Natural lighting enhancing the algae proliferation and nitrogen removal in membrane-aerated bacterial-algal biofilm reactor.

Membrane-aerated bacterial-algal biofilm reactor (MABAR) is an emerging and novel technology in recent years, which has been attracting increasing attention due to its cost-effectiveness and superior removal performance of pollutants by versatile removal pathways in symbiotic bacterial-algal biofilm. However, the wider application of MABAR is hindered by the dilemma of insufficient algae biomass. In this study, an MABAR under natural sunlight was developed and operated for 160 d to access the feasibility of enhancing algae proliferation by natural lighting. Results showed that the MABAR with natural sunlight (nMABAR) demonstrated better performance of pollutants removal. High removal efficiencies of organic matter and NH4-N in nMABAR were 90 % and 92 %, respectively. In particular, the removal efficiency of TN in nMABAR, under less aeration, was up to 80 %, which was 15 % higher than the control reactor. The Chlorophyll-a content indicated that natural sunlight facilitated to algae growth in MABAR, and algae assimilation might be the dominant contributor to NH4-N removal. Moreover, there were microbial shifts in bacterial-algal biofilm in a response to the natural lighting, the nMABAR uniquely possessed a bacterial phylotype termed Thiocapsa, which could play an important role in bacterial nitrification. Algal phylotype Chlorophyceae significantly contributed to pollutants removal and synergistic relationship with bacteria. In addition, the superb performance of nMABAR under less aeration condition suggested that abundant algae were capable of supplying enough O2 for the system. These results provided insight into the natural lighting on algae-bacteria synergistic growth and cost-effective operation strategy for MABAR.

Indirect charging of carbon by aqueous redox mediators contributes to the enhanced desalination performance in flow-electrode CDI.

Reversible electrochemical separation based on flow electrodes (e.g., flow-electrode capacitive deionization (FCDI)) is promising to desalinate brackish water, a reliable alternative source of freshwater. The deployment of redox mediators (RMs) in FCDI offers an energy-efficient means to improve the process performance, but the nature of the RMs-mediated charge transfer remains poorly understand. We therefore systematically investigated commonly-used RMs including sodium anthraquinone-2-sulfonate (AQS), 4-hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO), hydroquinone (HQ) and ferricyanide ([Fe(CN)6]3-). Results showed that the desalination rate could be increased by over 260% with the addition of 10 mM [Fe(CN)6]3-. The lowest efficiency of AQS among the RMs should be ascribed to its reduction potential of -0.84 V (vs. Ag/AgCl) exceeding the potential (-0.48 V) of the negatively charged current collector at 1.2 V. While aqueous TEMPO and HQ could facilitate salt removal, their loss of efficiencies upon sorption onto the carbon surface indicated the insignificant pseudocapacitive contribution to ion migration. In-situ cyclic voltammetry measurements demonstrated the crucial role of the indirect charging of the flowable carbon materials to enhance the desalination performance in RMs-mediated FCDI. To sum up, results of this work pave a way to understand the RMs-mediated charge transfer and ion migration in FCDI, which would serve the purpose of design and optimization of the flow electrode systems for wider environmental applications.

Recent advances in electrocatalytic membrane for the removal of micropollutants from water and wastewater.

The increasing occurrence of micropollutants in water and wastewater threatens human health and ecological security. Electrocatalytic membrane (EM), a new hybrid water treatment platform that integrates membrane separation with electrochemical technologies, has attracted extensive attention in the removal of micropollutants from water and wastewater in the past decade. Here, we systematically review the recent advances of EM for micropollutant removal from water and wastewater. The mechanisms of the EM for micropollutant removal are first introduced. Afterwards, the related membrane materials and operating conditions of the EM are summarized and analyzed. Lastly, the challenges and future prospects of the EM in research and applications are also discussed, aiming at a more efficient removal of micropollutants from water and wastewater.

Biofouling suppresses effluent toxicity in an electrochemical filtration system for remediation of sulfanilic acid-contaminated water.

Electrochemical filtration system (EFS) has received broad interest due to its high efficiency for organic contaminants removal. However, the porous nature of electrodes and flow-through operation mode make it susceptible to potential fouling. In this work, we systematically investigated the impacts of biofouling on sulfanilic acid (SA) removal and effluent toxicity in an EFS. Results showed that the degradation efficiency of SA slightly deteriorated from 92.3% to 81.1% at 4.0 V due to the electrode fouling. Surprisingly, after the occurrence of fouling, the toxicity (in terms of luminescent bacteria inhibition) of the EFS effluent decreased from 72.3% to 40.2%, and cytotoxicity assay exhibited similar tendency. Scanning electron microscopy and confocal laser scanning microscopy analyses revealed that biofouling occurred on the porous cathode, and live microorganisms were the dominant contributors, which are expected to play an important role in toxicity suppression. The relative abundance of Flavobacterium genus, related to the degradation of p-nitrophenol (an aromatic intermediate product of SA), increased on the membrane cathode after fouling. The analysis of degradation pathway confirmed the synergetic effects of electrochemical oxidation and biodegradation in removal of SA and its intermediate products in a bio-fouled EFS, accounting for the decrease of the effluent toxicity. Results of our study, for the first time, highlight the critical role of biofouling in detoxication using EFS for the treatment of contaminated water.

Mechanistic insights into CO2 pressure regulating microbial competition in a hydrogen-based membrane biofilm reactor for denitrification.

CO2 is a proven pH regulator in hydrogen-based membrane biofilm reactor (H2-MBfR) but how its pressure regulates microbial competition in this system remains unclear. This work evaluates the CO2 pressure dependent system performance, CO2 allocation, microbial structure and activity of CO2 source H2-MBfR. The optimum system performance was reached at the CO2 pressure of 0.008 MPa, and this pressure enabled 0.18 g C/(m2·d) of dissolved inorganic carbon (DIC) allocated to denitrifying bacteria (DNB) for carbon source anabolism and denitrification-related proton compensation, while inducing a bulk liquid pH (pH 7.4) in favor of DNB activity by remaining 0.21 g C/(m2·d) of DIC as pH buffer. Increasing CO2 pressure from 0.008 to 0.016 MPa caused the markedly changed DNB composition, and the diminished DNB population was accompanied by the enrichment of sulfate-reducing bacteria (SRB). A high CO2 pressure of 0.016 MPa was estimated to induce the enhanced SRB activity and weakened DNB activity.