Understanding Rejection Mechanisms of Trace Organic Contaminants by Polyamide Membranes via Data-Knowledge Codriven Machine Learning.

Data-driven machine learning (ML) provides a promising approach to understanding and predicting the rejection of trace organic contaminants (TrOCs) by polyamide (PA). However, various confounding variables, coupled with data scarcity, restrict the direct application of data-driven ML. In this study, we developed a data-knowledge codriven ML model via domain-knowledge embedding and explored its application in comprehending TrOC rejection by PA membranes. Domain-knowledge embedding enhanced both the predictive performance and the interpretability of the ML model. The contribution of key mechanisms, including size exclusion, charge effect, hydrophobic interaction, etc., that dominate the rejections of the three TrOC categories (neutral hydrophilic, neutral hydrophobic, and charged TrOCs) was quantified. Log D and molecular charge emerge as key factors contributing to the discernible variations in the rejection among the three TrOC categories. Furthermore, we quantitatively compared the TrOC rejection mechanisms between nanofiltration (NF) and reverse osmosis (RO) PA membranes. The charge effect and hydrophobic interactions possessed higher weights for NF to reject TrOCs, while the size exclusion in RO played a more important role. This study demonstrated the effectiveness of the data-knowledge codriven ML method in understanding TrOC rejection by PA membranes, providing a methodology to formulate a strategy for targeted TrOC removal.

Trade-off between Endocrine-Disrupting Compound Removal and Water Permeance of the Polyamide Nanofiltration Membrane: Phenomenon and Molecular Insights.

The polyamide (PA) nanofiltration (NF) membrane has the potential to remove endocrine-disrupting compounds (EDCs) from water and wastewater to prevent risks to both the aquatic ecosystem and human health. However, our understanding of the EDC removal-water permeance trade-off by the PA NF membrane is still limited, although the salt selectivity-water permeance trade-off has been well illustrated. This constrains the precise design of a high-performance membrane for removing EDCs. In this study, we manipulated the PA nanostructures of NF membranes by altering piperazine (PIP) monomer concentrations during the interfacial polymerization (IP) process. The upper bound coefficient for EDC selectivity-water permeance was demonstrated to be more than two magnitudes lower than that for salt selectivity-water permeance. Such variations were derived from the different membrane-solute interactions, in which the water/EDC selectivity was determined by the combined effects of steric exclusion and the hydrophobic interaction, while the electrostatic interaction and steric exclusion played crucial roles in water/salt selectivity. We further highlighted the role of the pore number and residual groups during the transport of EDC molecules across the PA membrane via molecular dynamics (MD) simulations. Fewer pores decreased the transport channels, and the existence of residual groups might cause steric hindrance and dynamic disturbance to EDC transport inside the membrane. This study elucidated the trade-off phenomenon and mechanisms between EDC selectivity and water permeance, providing a theoretical reference for the precise design of PA NF membranes for effective removal of EDCs in water reuse.

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.

Enhancing Stability of Tannic Acid-FeIII Nanofiltration Membrane for Water Treatment: Intercoordination by Metal-Organic Framework.

Tannic acid (TA)-FeIII nanofiltration (NF) membrane has been demonstrated to possess more favorable removal of trace organic contaminants (TrOCs) over the conventional polyamide NF membrane. However, the drawback of acid instability severely hinders the practical application of TA-FeIII NF membrane in the treatment of (weak) acidic wastewater containing TrOCs (e.g., pharmaceutical wastewater, surface water, and drinking water). Herein, we introduced the MIL-101(Cr) nanoparticle, a kind of metal-organic framework (MOF), into the TA-FeIII selective layer to enhance the membrane acid stability. The acid-tolerance parameter of MIL-101(Cr)-stabilized TA-FeIII membrane (TA-FeIII-MOF membrane, 12,000 ppm/s-1) was two orders of magnitude larger than that of the TA-FeIII membrane (50 ppm/s-1), and the TA-FeIII-MOF membrane can withstand acid treatment at pH = 4 for more than 30 days. Meanwhile, the TA-FeIII-MOF membrane displayed increased water permeance from 9.5 to 12.7 L/(m2·h·bar) after the MOF addition, without compromising the selectivity. The enhanced acid stability for the TA-FeIII-MOF membrane was ascribed to an intercoordination mechanism, where FeIII centers (from TA-FeIII complex) coordinated with -COOH groups (from terephthalic acid of MOF) and CrIII centers (from MOF) coordinated with -OH groups (from TA of TA-FeIII complex), which was verified by the density functional theory calculation. This study highlights a new approach for the development of a TA-FeIII-based NF membrane with markedly enhanced acid stability, which is important for its real application in wastewater treatment and water reuse.

Green Solvent Cleaning Removes Irrecoverable Foulants from End-of-Life Membranes in Membrane Bioreactors: Efficacy and Mechanisms.

Removal of irrecoverable foulants, which cannot be removed by conventional chemical cleaning, from end-of-life (EOL) membranes remains a substantial challenge due to the strong interaction between the foulants and membrane matrix. Herein, we developed a green solvent cleaning strategy based on Hansen solubility parameters to achieve the removal of irrecoverable foulants from the EOL polyvinylidene fluoride (PVDF) membranes serving for 6 years in a large-scale membrane bioreactor (MBR). We selected methyl-5-(dimethylamino)-2-methyl-5-oxopentanoate (MDMO) as the green solvent due to its strong interaction with the PVDF material, which might enable the substitution of binding sites of irrecoverable foulants. After the MDMO cleaning, the water permeance of the EOL membrane recovered from 47.6 ± 4.7 to 390.9 ± 8.2 L m-2 h-1 bar-1 (with a flux recovery ratio of ∼100%), with its rejection ability and stability maintained. The main components of irrecoverable fouling were humic acid-like substances revealed by spectroscopic characterization. Molecular dynamic simulation further elucidated the cleaning mechanisms: the strong interaction of MDMO-PVDF enabled substitution of binding sites of irrecoverable foulants by MDMO, followed by desorption of the irrecoverable foulants from PVDF and diffusion of the irrecoverable foulants into the bulk phase of MDMO. Evaluation in a lab-scale MBR treating real municipal wastewater verified the reusability of green solvent cleaned-EOL membranes. This study provides a novel, effective, and green cleaning strategy to remove irrecoverable foulants and prolong the service life of membranes in MBRs, facilitating sustainable wastewater treatment using membrane-based processes.

Roles of Anion-Cation Coupling Transport and Dehydration-Induced Ion-Membrane Interaction in Precise Separation of Ions by Nanofiltration Membranes.

Nanofiltration (NF) membranes are playing increasingly crucial roles in addressing emerging environmental challenges by precise separation, yet understanding of the selective transport mechanism is still limited. In this work, the underlying mechanisms governing precise selectivity of the polyamide NF membrane were elucidated using a series of monovalent cations with minor hydrated radius difference. The observed selectivity of a single cation was neither correlated with the hydrated radius nor hydration energy, which could not be explained by the widely accepted NF model or ion dehydration theory. Herein, we employed an Arrhenius approach combined with Monte Carlo simulation to unravel that the transmembrane process of the cation would be dominated by its pairing anion, if the anion has a greater transmembrane energy barrier, due to the constraint of anion-cation coupling transport. Molecular dynamics simulations further revealed that the distinct hydration structure was the primary origin of the energy barrier difference of cations. The cation having a larger incompressible structure after partial dehydration through subnanopores would induce a more significant ion-membrane interaction and consequently a higher energy barrier. Moreover, to validate our proposed mechanisms, a membrane grafting modification toward enlarging the energy barrier difference of dominant ions achieved a 3-fold enhancement in ion separation efficiency. Our work provides insights into the precise separation of ionic species by NF membranes.

Self-Enhanced Decomplexation of Cu-Organic Complexes and Cu Recovery from Wastewaters Using an Electrochemical Membrane Filtration System.

Heavy metals in industrial wastewaters are typically present as stable metal-organic complexes with their cost-effective treatment remaining a significant challenge. Herein, a self-enhanced decomplexation scenario is developed using an electrochemical membrane filtration (EMF) system for efficient decomplexation and Cu recovery. Using Cu-EDTA as a model pollutant, the EMF system achieved 81.5% decomplexation of the Cu-EDTA complex and 72.4% recovery of Cu at a cell voltage of 3 V. The •OH produced at the anode first attacked Cu-EDTA to produce intermediate Cu-organic complexes that reacted catalytically with the H2O2 generated from the reduction of dissolved oxygen at the cathode to initiate chainlike self-enhanced decomplexation in the EMF system. The decomplexed Cu products were further reduced or precipitated at the cathodic membrane surface thereby achieving efficient Cu recovery. By scavenging H2O2 (excluding self-enhanced decomplexation), the rate of decomplexation decreased from 8.8 × 10-1 to 4.1 × 10-1 h-1, confirming the important role of self-enhanced decomplexation in this system. The energy efficiency of this system is 93.5 g kWh-1 for Cu-EDTA decomplexation and 15.0 g kWh-1 for Cu recovery, which is much higher than that reported in the previous literature (i.e., 7.5 g kWh-1 for decomplexation and 1.2 g kWh-1 for recovery). Our results highlight the potential of using EMF for the cost-effective treatment of industrial wastewaters containing heavy metals.

Dually Charged MOF-Based Thin-Film Nanocomposite Nanofiltration Membrane for Enhanced Removal of Charged Pharmaceutically Active Compounds.

Removal of pharmaceutically active compounds (PhACs) is of great importance in wastewater reclamation due to their potent negative impacts on human health. Typical polyamide nanofiltration (NF) membranes are negatively charged, which compromises their rejection rate of positively charged PhACs. Herein, we propose to rationally design a novel thin-film nanocomposite (TFN) NF membrane featuring a dually charged metal organic framework (MOF) to effectively remove both positively and negatively charged PhACs. Ethylenediamine (ED) was grafted to the coordinately unsaturated metal sites inside the MIL-101(Cr). The resulting ED-MIL-101(Cr) contained both strong positively charged amine groups inside its channels and negatively charged carboxyl groups at its surface. This dually charged nature of the MOF nanoparticles enabled the ED-MIL-101(Cr)-containing TFN membrane to achieve high rejection rates (mostly >90%) for both positively (terbutaline, atenolol, fluoxetine) and negatively charged PhACs (ketoprofen, diclofenac, bezafibrate). At the same time, the ED-MIL-101(Cr) TFN membrane had greatly improved water permeance (140% over the control membrane with MOF loading). Calculations based on density functional theory further confirmed the large energy barrier for the migration of both negatively and positively charged PhACs across the nanochannels of ED-MIL-101(Cr). This study highlights a promising potential of dually charged MOF-TFN membranes for efficient removal of trace organic contaminants in wastewater reclamation.

Hydrophilic Selective Nanochannels Created by Metal Organic Frameworks in Nanofiltration Membranes Enhance Rejection of Hydrophobic Endocrine-Disrupting Compounds.

Rejection of endocrine-disrupting compounds (EDCs) by thin-film composite (TFC) polyamide membranes remains a challenging issue in wastewater reclamation applications because of the unfavorable hydrophobic interaction between EDCs and membranes. Herein, we investigated the incorporation of hydrophilic metal organic frameworks (MOFs) into the polyamide layer to create water/EDC selective nanochannels for enhancing EDC rejection. Using MIL-101(Cr) MOF as a nanofiller, the water flux of the MOF0.20 TFC membrane (0.20 wt/v % MOF in n-hexane) was 2.3 times that of the control. The rejection rates against EDCs involving methylparaben, propylparaben, benzylparaben, and bisphenol A (BPA) by MOF0.20 were also significantly higher than the respective values of the control membrane, with the water/EDC selectivity (e.g., A/BBPA) of MOF0.20 approximately doubled compared to that of the control. Further single salt rejection and gold nanoparticle filtration tests confirmed that the hydrophilic nanochannels created by MOFs played a critical role in membrane transport, accounting for the significant enhancement of EDC rejection of the modified TFC membrane. This study demonstrates a promising membrane modification protocol using hydrophilic MOFs for achieving selective removal of EDCs and high-efficient wastewater reclamation using TFC membranes.

Shock resistance characteristic of a spiral symmetry stream anaerobic bio-reactor.

The shock resistance characteristic (SRC) of an anaerobic bioreactor characterizes the ability of the anaerobic community in the reactor to withstand violent change in the living environment. In comparison with an upflow anaerobic sludge blanket reactor (UASBR), the SRC of a spiral symmetry stream anaerobic bio-reactor (SSSAB) was systematically investigated in terms of removal efficiency, adsorption property, settling ability, flocculability and fluctuations in these parameters. A quantitative assessment method for SRC was also developed. The results indicated that the SSSAB showed better SRC than the UASBR. The average value (m value) of chemical oxygen demand removal rates of the SSSAB was 86.0%. The contact angle of granules in the SSSAB present gradient distribution, that is the m value of contact angle increasing from bottom (84.5°) to top (93.9°). The m value of the density at the upper and lower sections of the SSSAB were 1.0611 g·cm(-3) and 1.0423 g·cm(-3), respectively. The surface mean diameter of granules in the SSSAB increased from 1.164 to 1.292 mm during operation. The absolute m value of zeta potential of granular sludge at the upper and lower sections of the SSSAB were 40.4 mV and 44.9 mV, respectively. The weighted mean coefficient variance (C̅V̅) value indicated SSSAB was more stable than the UASBR.

Rheological behaviors of anaerobic granular sludge in a spiral symmetry stream anaerobic bioreactor.

The rheological behaviors of the anaerobic granular sludge (AGS) in a spiral symmetry stream anaerobic bioreactor (SSSAB) were investigated. Shear-thinning behavior, thixotropic behavior, concentration-viscosity behavior and temperature-viscosity behavior were evaluated based on the constitutive equation of the AGS. The results indicated that the Herschel-Bulkley model was able to adequately describe the constitutive relation of AGS in the SSSAB. The AGS also showed shear-thinning behavior as well as thixotropic behavior. The critical shear rate and network strength of the AGS were 61.8 s(-1) and 497.0 W m(-3), respectively. The relationship between the apparent viscosity and the sludge concentration was illustrated and explained by the Woodcock formula. The relationship between apparent viscosity of the AGS and temperature could be modeled using the Arrhenius equation. The AGS was significantly thermo-sensitive and its mean energy of activation was 14.640 kJ mol(-1). Notably, it was necessary to consider such behaviors in the hydrodynamic modeling of SSSAB in which shear condition, sludge concentration and temperature were in non-uniform distribution.

Anaerobic biodegradation enables zero liquid discharge of two-stage nanofiltration system for microelectronic wastewater treatment.

Nanofiltration (NF) is a promising technology in the treatment of microelectronic wastewater. However, the treatment of concentrate derived from NF system remains a substantial technical challenge, impeding the achievement of the zero liquid discharge (ZLD) goal in microelectronic wastewater industries. Herein, a ZLD system, coupling a two-stage NF technology with anaerobic biotechnology was proposed for the treatment of tetramethylammonium hydroxide (TMAH)-contained microelectronic wastewater. The two-stage NF system exhibited favorable efficacy in the removal of conductivity (96 %), total organic carbon (TOC, 90 %), and TMAH (96 %) from microelectronic wastewater. The membrane fouling of this system was dominated by organic fouling, with the second stage NF membrane experiencing a more serious fouling compared to the first stage membrane. The anaerobic biotechnology achieved a near-complete removal of TMAH and an 80 % reduction in TOC for the first stage NF concentrate. Methyloversatilis was the key genus involved in the anaerobic treatment of the microelectronic wastewater concentrate. Specific genes, including dmd-tmd, mtbA, mttB and mttC were identified as significant players in mediating the dehydrogenase and methyl transfer pathways during the process of TMAH biodegradation. This study highlights the potential of anaerobic biodegradation to achieve ZLD in the treatment of TMAH-contained microelectronic wastewater by NF system.

A review on artificial water channels incorporated polyamide membranes for water purification: Transport mechanisms and performance.

While thin-film composite (TFC) polyamide (PA) membranes are advanced for removing salts and trace organic contaminants (TrOCs) from water, TFC PA membranes encounter a water permeance-selectivity trade-off due to PA layer structural characteristics. Drawing inspiration from the excellent water permeance and solute rejection of natural biological channels, the development of analogous artificial water channels (AWCs) in TFC PA membranes (abbreviated as AWCM) promises to achieve superior mass transfer efficiency, enabling breaking the upper bound of water permeance and selectivity. Herein, we first discussed the types and structural characteristics of AWCs, followed by summarizing the methods for constructing AWCM. We discussed whether the AWCs acted as the primary mass transfer channels in AWCM and emphasized the important role of the AWCs in water transport and ion/TrOCs rejection. We thoroughly summarized the molecular-level mechanisms and structure-performance relationship of water molecules, ions, and TrOCs transport in the confined nanospace of AWCs, which laid the foundation for illustrating the enhanced water permeance and salt/TrOCs selectivity of AWCM. Finally, we discussed the challenges encountered in the field of AWCM and proposed future perspectives for practical applications. This review is expected to offer guidance for understanding the transport mechanisms of AWCM and developing next-generation membrane for effective water treatment.

Anaerobic hydrolysis of recalcitrant tetramethylammonium from semiconductor wastewater: Performance and mechanisms.

The treatment of tetramethylammonium hydroxide (TMAH)-bearing wastewater, generated in the electronic and semiconductor industries, raises significant concerns due to the neurotoxic, recalcitrant, and bio-inhibiting effects of TMAH. In this study, we proposed the use of an anaerobic hydrolysis bioreactor (AHBR) for TMAH removal, achieving a high removal efficiency of approximately 85%, which greatly surpassed the performance of widely-used advanced oxidation processes (AOPs). Density functional theory calculations indicated that the unexpectedly poor efficiency (5.8-8.0%) of selected AOPs can be attributed to the electrostatic repulsion between oxidants and the tightly bound electrons of TMAH. Metagenomic analyses of the AHBR revealed that Proteobacteria and Euryarchaeota played a dominant role in the transformation of TMAH through processes such as methyl transfer, methanogenesis, and acetyl-coenzyme A synthesis, utilizing methyl-tetrahydromethanopterin as a substrate. Moreover, several potential functional genes (e.g., mprF, basS, bcrB, sugE) related to TMAH resistance have been identified. Molecular docking studies between five selected proteins and tetramethylammonium further provided evidence supporting the roles of these potential functional genes. This study demonstrates the superiority of AHBR as a pretreatment technology compared to several widely-researched AOPs, paving the way for the proper design of treatment processes to abate TMAH in semiconductor wastewater.

Modification of ultrafiltration membrane with antibacterial agent intercalated layered nanosheets: Toward superior antibiofouling performance for water treatment.

Membrane fouling, especially biofouling induced by biofilm formation on membranes, can result in frequent cleaning or even replacement of membranes. Fabrication of membrane with excellent antibiofouling property is quite attractive due to its effectiveness and low-impact on the operation of membrane-based process. Herein, a cationic antibacterial agent, quaternary ammonium compound (QAC), was intercalated into the interlayer spaces of the MgAl layered double hydroxide (QAC/LDH) by self-assembly. The QAC/LDH composite was incorporated into polyethersulfone (PES) ultrafiltration (UF) membrane (PES-QLDH). The QAC/LDH enhanced the hydrophilicity, water flux, and resistance to organic fouling for the PES-QLDH membrane. The PES-QLDH membrane exhibited superior antibiofouling performance than the control PES membrane, with deposition of a thinner biofilm layer consisted of almost dead cells. The superior antibacterial activity inhibits the adhesion and growth of bacteria on the membrane surface, effectively retarding the formation of biofilms. Importantly, the synergistic effect of QAC and LDH in the PES-QLDH membrane resulted in a high biocidal activity based on both direct and indirect killing mechanisms. The PES-QLDH membrane maintained a stable and high antibacterial activity after several fouling-cleaning cycles. These results imply that the PES-QLDH membrane provides an effective and promising strategy for its long-term application in wastewater treatment.

Artificial intelligence-incorporated membrane fouling prediction for membrane-based processes in the past 20 years: A critical review.

Membrane fouling is one of major obstacles in the application of membrane technologies. Accurately predicting or simulating membrane fouling behaviours is of great significance to elucidate the fouling mechanisms and develop effective measures to control fouling. Although mechanistic/mathematical models have been widely used for predicting membrane fouling, they still suffer from low accuracy and poor sensitivity. To overcome the limitations of conventional mathematical models, artificial intelligence (AI)-based techniques have been proposed as powerful approaches to predict membrane filtration performance and fouling behaviour. This work aims to present a state-of-the-art review on the advances in AI algorithms (e.g., artificial neural networks, fuzzy logic, genetic programming, support vector machines and search algorithms) for prediction of membrane fouling. The working principles of different AI techniques and their applications for prediction of membrane fouling in different membrane-based processes are discussed in detail. Furthermore, comparisons of the inputs, outputs, and accuracy of different AI approaches for membrane fouling prediction have been conducted based on the literature database. Future research efforts are further highlighted for AI-based techniques aiming for a more accurate prediction of membrane fouling and the optimization of the operation in membrane-based processes.

Membrane fouling behaviors in a full-scale zero liquid discharge system for cold-rolling wastewater brine treatment: A comprehensive analysis on multiple membrane processes.

The challenge of water scarcity drives zero liquid discharge (ZLD) treatment to maximize reuse of industrial wastewater. Deciphering the characteristics and mechanisms of membrane fouling in the membrane-based ZLD system is crucial for the development of effective fouling control strategies. However, current studies only focused on the membrane fouling of single step, lacking in-depth understanding on the ZLD systems using multiple membrane processes. Herein, membrane fouling characteristics and mechanisms in a full-scale ZLD system for cold-rolling wastewater brine treatment were investigated via a comprehensive analysis on multiple nanofiltration (NF) and reverse osmosis (RO) membrane processes. The membrane fouling behaviors showed distinct characteristics along the wastewater flow direction in the ZLD system. Increasing amounts of foulants were deposited on the membrane surfaces with the sequence of the 1st pass RO, 1st stage NF, and 2nd stage NF processes. The organic fouling and silica scaling were more intensive in the 1st stage NF and 2nd stage NF for treating the brine of the 1st pass RO, as the foulants were rejected and concentrated by previous membrane processes. Severe inorganic fouling, containing amorphous SiO2, Al2O3, and Al2SiO5, occurred on the membrane surface of the 2nd pass RO membrane, due to the recirculated high-concentration silica, high water recovery, and concentration polarization. For the 3rd pass RO process, both the amounts of organic and inorganic foulants decreased dramatically, due to the low foulant concentration in its influent. This work provides a comprehensive understanding of membrane fouling in a membrane-based ZLD system, facilitating the development of membrane fouling control strategies for multiple membrane processes.

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.

Removal of p-toluenesulfonic acid from wastewater using a filtration-enhanced electro-Fenton reactor.

Rapid global industrialization accompanies the discharge of industrial wastewater. p-Toluenesulfonic acid (PTSA), a kind of aromatic sulfonate that belongs to the refractory organic pollutant, is one of the most widely used chemicals in pharmaceutical, dye, petrochemical and plastic industries. In this study, we developed a filtration-enhanced electro-Fenton (FEEF) reactor to remove PTSA from synthetic wastewater. A filtration-enhanced stainless-steel mesh (FESSM) was used as the cathode. Under the optimal operating conditions of applied voltage 2.5 V, pH = 3.0, addition of 0.2 mM Fe2+ and 1.0 mM H2O2 for 120 min, the removal efficiency of PTSA (initial concentration of 100 mg L-1) could reach 92.6%. Compared with the control anodic oxidation and conventional Fenton system, the FEEF system showed higher ˙OH yield and PTSA removal efficiency, with a lower effluent biological toxicity and operating cost. The enhanced mass transfer rate by the filtration in the FEEF system accelerated the regeneration of catalyst Fe2+ and further promoted the heterogeneous reactions. The Fe species on the surface of FESSM cathode possessed a gradient distribution, the inner layer was dominated by Fe and the outer layer was Fe3+. The degradation pathways of PTSA were proposed, including methyl hydroxylation, sulfonyl hydroxylation, ß-hydrogen hydroxylation, and ring-opening reaction. These results demonstrate that the novel FEEF system is a promising technology for the removal of refractory organic pollutants from industrial wastewater.