Fabrication of Tween-20 coated PVDF membranes for wastewater treatment: optimization of preparation parameters,removal and membrane fouling control performance

In the present study, polyoxyethylene (20) sorbitan monolaurate (Tween-20) was employed as a surface coating agent for hydrophilic modification of poly(vinylidene fluoride) microfiltration membranes. The optimized parameters for membrane preparation (i.e., coating temperature, coating concentration, coating time and drying time) were systematically investigated. Contact angle and transmembrane pressure were employed to evaluate the efficiency of the modified membranes, and the optimized parameters were proposed. The removal of chemical oxygen demand (COD) and suspended solids (SS), as well as fouling control performance, was further evaluated. The results showed that the optimized parameters were 40 °C, 4.5 mmol L⁻¹, 45 min and 45 min for coating temperature, coating concentration, coating time and drying time, respectively. Under these conditions, a hydration layer on the surface was formed, resulting in a more hydrophilic membrane surface. During domestic wastewater treatment in membrane bioreactor (MBR), the Tween-20 modified membrane exhibited better performance with rejection efficiencies of 94.56% and 97.53% for COD and SS, respectively. Tween-20 coating could mitigate the increase of transmembrane pressure and reduce the concentration of proteins accumulated on the membrane surface, which was effective for membrane fouling control. Simultaneously, the operation time of MBR was extended from 25 to 46 days. Furthermore, the stability of Tween-20 coated PVDF membrane was also verified. The results indicated that surface coating with Tween-20 is efficient and easy to be carried out, showing a great potential for application in MBR during wastewater treatment.

In the present study, polyoxyethylene (20) sorbitan monolaurate (Tween-20) was employed as a surface coating agent for hydrophilic modification of poly(vinylidene fluoride) microfiltration membranes.     

Computational fluid dynamics simulation as a tool for optimizing the hydrodynamic performance of membrane bioreactors

The hydrodynamic properties and shear stresses experienced by a membrane bioreactor (MBR) are directly related to its rate of membrane fouling. In this study, computational fluid dynamic models have been combined with cold model PIV experimental studies to optimize the performance properties of MBRs. The effects of membrane module height, number of aeration tubes and membrane spacing on liquid phase flow rates, gas holdup and shear stresses at the membrane surface have been investigated. It has been found that optimal MBRs experience the greatest shear forces on their surfaces at a distance of 250 mm from the aeration tube, around the 7 aeration tubes used to introduce gas and at the 40 mm spacings between the membrane sheets. Use of an aeration intensity of between 0.02 and 0.47 m³ min⁻¹ generated shear stresses that were 50–85% higher than the original MBR for the same aeration intensity, thus affording optimal membrane performance that minimizes membrane fouling.

The hydrodynamic properties and shear stresses experienced by a membrane bioreactor (MBR) are directly related to its rate of membrane fouling.     

New insights into the organic fouling mechanism of an in situ Ca2+ modified thin film composite forward osmosis membrane

In this study, the effect of organic substances on the fouling behavior of a thin film composite (TFC) membrane with in situ Ca²⁺ addition (TFC-Ca membrane) was evaluated. Bovine serum albumin (BSA), humic acid (HA) and sodium alginate (SA) were used as surrogate foulants for protein, natural organic substances and polysaccharides, respectively, thus enabling the analysis of foulant–membrane interaction in the membrane fouling process. Fouling experiments were carried out and the fouling mechanism was investigated by extended Derjaguin–Landau–Verwey–Overbeek (XDLVO) theory. SEM-EDX, ICP-OES and TOC analysis were applied to characterize the fouled TFC-Ca membrane. Results suggested that the interfacial free energies obtained from advanced contact angle measurements were correlated strongly with the rates of membrane fouling. In situ Ca²⁺ addition in the TFC membrane resulted in the decrease of the interfacial adhesion free energy (i.e., foulant–membrane interaction) and thus the mitigation of membrane fouling. The permeate flux of TFC-Ca FO membrane after organic fouling could be fully restored by simple physical cleaning. The antifouling mechanism of Ca²⁺ pre-binding carboxyl groups in the TFC-Ca FO membrane was demonstrated, which provides new insights into the development of antifouling TFC membranes in the future.

In this study, the effect of organic substances on the fouling behavior of a thin film composite (TFC) membrane with in situ Ca²⁺ addition (TFC-Ca membrane) was evaluated.     

Effect of applied voltage on membrane fouling in the amplifying anaerobic electrochemical membrane bioreactor for long-term operation

A novel and amplifying anaerobic electrochemical membrane bioreactor (AnEMBR, R2) was constructed and operated for a long time (204 days) with synthetic glucose solution having an average chemical oxygen demand (COD) of 315 mg L⁻¹, at different applied voltages and room temperatures. More than twice sodium bicarbonate was added for maintaining a pH of around 6.7 in the supernatant of the reactor R2, close to that of a control reactor called anaerobic membrane bioreactor (AnMBR, R1), after 138 days. And the transmembrane pressure (TMP) for the R2 system was only 0.534 bar at the end of operation and 0.615 bar for the R1 system. Although the electrostatic repulsion force contributed to pushing away the pollutants (proteins, polysaccharose and inorganic salt deposits, and so on), more microorganisms adsorbed and accumulated on the membrane surface after the whole operation, which might result in a rapid increase in membrane filtration resistance in the long-term operation. There were much more exoelectrogenic bacteria, mainly Betaproteobacteria, Deltaproteobacteria and Grammaproteobacteria, on the cathode and the dominant methanogen Methanothrix content on the cathode was three times higher than the AnMBR. The study provides an important theoretical foundation for the application of AnEMBR technology in the treatment of low organic strength wastewater.

A novel and amplifying anaerobic electrochemical membrane bioreactor was constructed and operated for a long time (204 days) with synthetic glucose solution having an average chemical oxygen demand (COD) of 315 mg L⁻¹, at different applied voltages and room temperatures.     

Pilot study on the effects of operating parameters on membrane fouling during ultrafiltration of alkali/surfactant/polymer flooding wastewater: optimization and modeling

Alkali/surfactant/polymer (ASP) flooding wastewater is commonly produced in enhanced oil extraction processes and needs to be properly treated prior to reuse due to the potential threat of formation damage. Ultrafiltration (UF) is an effective technique for treating ASP flooding wastewater to meet the requirements for reinjection water. Membrane fouling is the major challenge to UF application. In this study, the operating parameters were modified to research their effects on membrane fouling in a UF pilot study in Daqing, China. The effects of trans-membrane pressure (TMP), cross-flow velocity (CFV), concentration factor (CF) and temperature on membrane flux were systematically investigated, and optimal operating conditions were established by an orthogonal experiment. A temperature of 22 °C, TMP of 2.12 bar, CFV of 3.00 m s⁻¹ and CF of 5 were the most feasible operating conditions for the membrane types and raw water quality parameters in the study. The quality of the permeate met the water quality standards for injection to oilfield low-permeability layers. The results could provide a reference and guidance for practical operations. To learn more about the influences of the operating parameters, a model including external and internal pollution factors was developed based on the Hagen–Poiseuille equation and classical membrane fouling theory. The operating parameters had a more significant effect on external pollution than on internal pollution. The fouling on the membrane surface was much affected by TMP and CFV.

Ultrafiltration (UF) is an effective technique for treating ASP flooding wastewater to meet the requirements for reinjection water.     

Effects of a low-strength magnetic field on the characteristics of activated sludge for membrane fouling mitigation

This study aims to investigate the performance of a low-strength magnetic field in membrane bioreactors (MBRs) for membrane fouling mitigation and its effects on sludge characteristics and microbial community. The continuous operation of MBR with magnetic powder (MP-MBR) is denoted as the control-MBR, and a magnetic field is added to the MP-MBR to form the magnetic-MBR (M-MBR). The comparison between MP-MBR and M-MBR was conducted to treat synthetic wastewater. The results showed that the application of a low-strength magnetic field not only decreased the zeta potential and increased the particle size, but also improved the dehydrogenase activity and stimulated microbes to produce fewer SMPs, which markedly contributed to the improved filtration performance of MBRs. The variations in the microbial communities from the two MBRs at genus levels confirmed that the addition of a low-strength magnetic field significantly affected the microbial community and composition, further altered the microbial metabolites, and consequently affected the membrane fouling evolution.

This study aims to investigate the performance of a low-strength magnetic field in membrane bioreactors (MBRs) for membrane fouling mitigation and its effects on sludge characteristics and microbial community.     

Analysis of zwitterionic membrane fouling mechanism caused by HPAM in the presence of electrolytes

Membrane fouling has always been a tough issue that is urgent to solve. Electrolytes which are prevalent in wastewater have a major influence on membrane fouling. Therefore, it is of great significance to understand the role and fouling mechanism of electrolytes in the membrane fouling process. In this work, the zwitterionic membrane is used to process hydrolyzed poly(acrylamide) (HPAM) with the addition of electrolytes (CaCl₂, NaCl). Meanwhile, the effect of different electrolytes on the zwitterionic membrane fouling process by hydrolyzed poly(acrylamide) (HPAM) is systematically investigated. It was found that the flux recovery ratio (FRR) of the zwitterionic membrane is nearly 100% after treating HPAM with the addition of electrolytes. Therefore, molecular dynamics (MD) simulations were applied to illustrate the impact of electrolytes on the change of foulant structures and confirm the consequent effect of electrolytes on membrane fouling. According to the experiment and MD simulation results, it is found that the positive ion layer which exists between the HPAM and zwitterionic surface results in the excellent fouling resistance performance of the zwitterionic membrane. The zwitterionic membrane fouling mechanism is analyzed, which is helpful to the understanding of zwitterionic membrane fouling in high salinity wastewater.

The foulant HPAM and electrolyte CaCl₂ are used to investigate the fouling mechanism of zwitterionic membrane dealing with HPAM in saline wastewater.     

Theoretical study on molecular mechanism of aerobic oxidation of 5-hydroxymethylfurfural to 2,5-diformyfuran catalyzed by VO2+ with counterpart anion in N,N-dimethylacetamide solution

Vanadium-containing catalysts exhibit good catalytic activity toward the aerobic oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-diformyfuran (DFF). The aerobic oxidation mechanism of HMF to DFF catalyzed by VO₂⁺ with counterpart anion in N,N-dimethylacetamide (DMA) solution have been theoretically investigated. In DMA solution, the stable VO₂⁺-containing complex is the four-coordinated [V(O)₂(DMA)₂]⁺ species. For the gross reaction of 2HMF + O₂ → 2DFF + 2H₂O, there are three main reaction stages, i.e., the oxidation of the first HMF to DFF with the reduction of [V(O)₂(DMA)₂]⁺ to [V(OH)₂(DMA)]⁺, the aerobic oxidation of [V(OH)₂(DMA)]⁺ to the peroxide [V(O)₃(DMA)]⁺, and the oxidation of the second HMF to DFF with the reduction of [V(O)₃(DMA)]⁺ to [V(O)₂(DMA)₂]⁺. The rate-determining reaction step is associated with the C–H bond cleavage of –CH₂ group of the first HMF molecule. The peroxide [V(O)₃(DMA)]⁺ species exhibits better oxidative activity than the initial [V(O)₂(DMA)₂]⁺ species, which originates from its narrower HOMO–LUMO gap. The counteranion Cl⁻ exerts promotive effect on the aerobic oxidation of HMF to DFF catalyzed by [V(O)₂(DMA)₂]⁺ species.

The rate-determining reaction step is associated with the C–H bond cleavage of –CH₂ group of the first HMF molecule oxidized by [V(O)₂(DMA)₂]⁺ species, while counteranion Cl⁻ exhibits catalytically promotive effect.     

The preparation of a modified PVDF hollow fiber membrane by coating with multiwalled carbon nanotubes for high antifouling performance

In this study, an outer surface modified polyvinylidene fluoride (PVDF) hollow fiber membrane (HF-PVDF-CNT) was prepared by coating with dopamine (PD) and multiwalled carbon nanotubes (CNTs), to solve the problems of the instability of pure CNT mats fabricated by filter coating methods and membrane fouling in wastewater treatment. The modified membrane was assessed and characterized by various methods, including studies of its top surface and cross-sectional morphology, wettability, functional groups and electrical conductivity. The CNT material stability was evaluated during backwashing. The antifouling and filtering abilities of the unmodified and modified membranes were tested by monitoring the change in TMP and the rejection performance for different contaminants during filtration in bovine serum albumin solution (BSA), sodium alginate solution (SA) and humic acid solution (HA). Furthermore, HF-PVDF-CNT and electro-assisted HF-PVDF-CNT membranes were employed as the basic separation units in an anaerobic membrane bioreactor (AnMBR) system and an anaerobic electrochemical membrane bioreactor (AnEMBR) system, respectively. Characterization of the HF-PVDF-CNT membrane indicated that the CNT mats exhibited good stability, electrical conductivity and wettability. In filtration experiments using BSA, SA and HA solutions, the HF-PVDF-CNT membrane showed an obvious improvement compared with the HF-PVDF membrane in antifouling performance. During its application in the AnMBR and AnEMBR systems, the electro-assisted HF-PVDF-CNT membrane had greater effects than the HF-PVDF-CNT membrane on reducing fouling.

Backwashable CNT mats generated on the outer surface of a HF-PVDF membrane showed high antifouling performance.     

A hybrid electric field assisted vacuum membrane distillation method to mitigate membrane fouling

We proposed a novel method for vacuum membrane distillation (VMD) called Electric Field Assisted Vacuum Membrane Distillation (EVMD) that can be used to mitigate membrane fouling. A biaxial stretching polytetrafluoroethylene (PTFE) membrane was utilized as the base membrane, and multi-walled carbon nanotubes (MWCNTs) or a mixture of MWCNTs/graphene as a conductive substrate. During EVMD, the conductive PTFE membrane acted as the cathode while a stainless-steel wire mesh surrounding the conductive membrane acted as the anode. The effect of the per unit area loading mass (PUALM) of the conductive substrate on the membrane performance were investigated. Results revealed that for a PUALM of 10 g m⁻², the PTFE membrane not only exhibited excellent conductivity but also showed a high rate of gas flux. Doping graphene into the MWCNT conductive substrate led to the formation of nano-channels which served to improve the membrane distillation flux and the membrane hydrophobicity. The effects of the electric field strength as well as humic acid (HA) concentration on the antifouling performance during EVMD were also investigated. Results showed that during EVMD, the PTFE conductive membrane exhibited the best antifouling ability using an intermittent electric field with a field strength of 1.0 V cm⁻¹.

EVMD with a PTFE electro-catalytic membrane can effectively mitigate membrane fouling.     

Enhanced performance of porous forward osmosis (FO) membrane in the treatment of oily wastewater containing HPAM by the incorporation of palygorskite

Since the emergence of forward osmosis (FO), low energy requirements, low fouling propensity and high-water recovery have made it one of the most promising water purification technologies. However, there have been few reports focusing on the treatment of polymer flooding produced water (PFPW) using FO technology up to now. In the present work, porous FO membranes with/without palygorskite (Pal) nanoparticles were utilized as the separation membrane to evaluate the potential of a porous FO membrane in the treatment of oily wastewater containing HPAM and the effect of Pal nanoparticles on the FO performance was investigated. When the loading concentration of Pal in the membrane was 0.75 wt%, the water flux could reach 37.67 L m⁻² h⁻¹ by using 4 g L⁻¹ poly(sodium-p-styrenesulfonate) (PSS) as draw solution under a cross-flow rate of 18.5 cm s⁻¹, which was much higher than that for pure polysulfone (PS) membranes. Besides, the comparison between ultrafiltration (UF) and FO performance in treating HPAM solution indicated that FO possessed better antifouling capacity, since less decline and higher recovery of water flux were observed during the FO process. Furthermore, recycling the draw solution gave an almost unchanged water flux, which suggested the feasibility of draw solute regeneration in the FO process. This work broadens the application field of porous FO technology and may pave a new way in the treatment of PFPW.

Porous forward osmosis (FO) membranes with/without palygorskite (Pal) nanoparticles were utilized as the separation membrane to evaluate the potential of porous FO membrane in the treatment of oily wastewater containing HPAM.     

Nanostructure domains,voids, and low-frequency spectra in binary mixtures of N,N-dimethylacetamide and ionic liquids with varying cationic size

Classical molecular dynamics (MD) simulations were carried out on binary mixtures of N,N-dimethylacetamide (DMA) with hydroxide based ammonium ionic liquids (ILs), tetraethylammonium hydroxide (TEAH), tetrapropylammonium hydroxide (TPAH), tetrabutylammonium hydroxide (TBAH), at three different mole fractions of IL (X_IL). The solvation of DMA molecules by ions of ILs was studied by the combined distribution function (CDF). CDFs show that anions have strong correlations with the DMA due to the hydrogen bonding. Increasing the DMA disrupts the nanosegregated domains and causes changes in correlations of cation–DMA and anion–DMA. Also, increased translational motion of ions, as well as the fluidity of IL and a significant improvement in self-diffusion coefficients, are observed with the presence of more DMA. The structural microheterogeneity was investigated using the Voronoi tessellation method. Domain analysis confirms the formation of discreet domains by anions at all the mole fractions. The results also complement the experimental observations, which suggest that two types of aggregations are possible in given mixtures: below and above 0.5 X_IL. When the alkyl chain length on the cation increases, a notable decrease in ion translational motion was observed in the IL rich region. In the concentrated IL mixture, the self-diffusion coefficient of the cation is higher than that of the corresponding anion; further addition of IL (X_IL < 0.5) results in weaker interactions between DMA and anion when compared to DMA–cation. The mean collision time of each species is found to have an inverse relation with X_IL. The analysis of the vibrational density of states provides the low-frequency spectral feature of the mixtures.

Classical MD simulations were carried out on binary mixtures of DMA with hydroxide based ammonium ILs, TEAH, TPAH and TBAH, at three different mole fractions of IL.     

Performance and properties of coking nanofiltration concentrate treatment and membrane fouling mitigation by an Fe(ii)/persulfate-coagulation-ultrafiltration process

Coking nanofiltration (NF) concentrates, as typical wastewater with high salinity and refractory organics, have become one of the greatest challenges for “near-zero emission” processes. In our study, an advanced oxidation technology based on ferrous iron/persulfate (Fe(ii)/PS) and polyferric sulfate (PFS) coagulation coupled with ultrafiltration (UF) was used to treat NF concentrates and mitigate membrane fouling. Based on batch experiments, the optimal parameters of Fe(ii)/PS were obtained, during which we discovered that the slow reaction stage of total organic carbon (TOC) removal followed first-order degradation kinetics. Under the optimal reaction conditions, Fe(ii)/PS could efficiently mineralize 69% of organics in coking NF concentrates. In order to eliminate the iron floc generated in the Fe(ii)/PS step, a small amount of PFS (0.05 mM) was added to coagulate the iron floc, which could further improve the effluent quality so that the turbidity, iron content and TOC were significantly reduced by 79.18%, 98% and 21.79% respectively. Gas chromatography coupled with time-of-flight mass spectrometry (GC × GC-TOFMS) and fluorescence excitation-emission matrix spectrometry (EEM) were performed to characterize the removal of phenols, PAHs, quinolines and humic acids in NF concentrates which were responsible for UF membrane fouling. Moreover, scanning electronic microscopy (SEM) and atomic force microscopy (AFM) were conducted to study the surface of the UF membrane after treatment of NF concentrates. The result exhibited that the organic pollutants deposited on the UF membrane surface were reduced by Fe(ii)/PS-PFS pretreatment, and UF membrane flux was thus enhanced. Our results show the potential of the approach of applying Fe(ii)/PS-PFS-UF in NF concentrate treatment.

The removal effect of organics and the feasibility of membrane fouling mitigation with Fe(ii)/PS-PFS coupling technology.     

Fouling-resistant membranes for separation of oil-in-water emulsions

Separation by membrane technology of oily wastewater, especially emulsified oil/water mixtures, has become a topic of intensive study in recent years, and membrane fouling remains a challenge. In this work, porous polycarbonate membranes were coated with poly(diallyldimethylammonium chloride)/polystyrene sulfonate (PDDA/PSS) multilayers via the facile layer-by-layer deposition technique to improve their fouling resistance for separation of oil-in-water emulsions stabilized by ionic surfactants. The permeation flux for the virgin membrane was found to decrease by ∼90% in 10 cycles due to fouling. The membranes coated with PSS-capped PEMs were used to separate emulsions stabilized by sodium dodecyl sulfate, an anionic surfactant, whereas the ones modified with PDDA-capped PEMs were effective for separation of emulsions stabilized by cetyltrimethyl ammonium bromide, a cationic surfactant. Both retained up to 80% of their original permeation flux after 10 separation cycles. The purity of the filtrates was greater than 99.98%.

Membranes coated with a polyelectrolyte multilayer can resist fouling during the separation of emulsions stabilized by ionic surfactants.     

Fabrication of a hybrid ultrafiltration membrane based on MoS2 modified with dopamine and polyethyleneimine

The hydrophobicity of ultrafiltration membranes is the main cause of membrane fouling and reduced permeability, so it is necessary to improve the hydrophilicity and anti-fouling performance of ultrafiltration membrane materials. MoS₂ nanoparticles that were modified with polydopamine (PDA) and polyethyleneimine (PEI), named MoS₂-PDA-PEI, were added to fabricate a polyethersulfone ultrafiltration membrane (PES/MoS₂-PDA-PEI) for the first time. The effects of modified MoS₂ nanoparticles on membrane performance were clarified. The results indicated that the permeability, rejection, and anti-fouling capability of the hybrid PES/MoS₂-PDA-PEI membrane have been improved compared with the pristine PES membrane. When the content of MoS₂-PDA-PEI nanoparticles in the membrane is 0.5%, the pure water flux of the hybrid membrane reaches 364.03 L m⁻² h⁻¹, and the rejection rate of bovine serum albumin (BSA) and humic acid (HA) is 96.5% and 93.2% respectively. The flux recovery rate of HA reached 97.06%. As expected, the addition of MoS₂-PDA-PEI nanoparticles promotes the formation of the porous structure and improves the hydrophilicity of the membrane, thereby improving its antifouling performance.

The hydrophobicity of ultrafiltration membranes is the main cause of membrane fouling and reduced permeability, so it is necessary to improve the hydrophilicity and anti-fouling performance of ultrafiltration membrane materials.     

Fenton's reagent-enhanced supercritical water oxidation of wastewater released from 3-hydroxypyridine production

A study on Fenton''s reagent-enhanced supercritical water oxidation (SCFO) of wastewater released from 3-hydroxypyridine production was carried out in this paper. The effects of temperature, oxidant multiple, residence time, Fe²⁺ concentration, and pH on the degradation efficiency of wastewater were investigated. The Plackett–Burman test was designed to evaluate various factors, namely, temperature, oxidant multiple, and pH, which were found to significantly affect degradation efficiency. Response surface analysis was performed to optimize the parameter levels of the main influencing factors. The results indicated that the optimal conditions required for the oxidative degradation of wastewater in the SCFO systems were pH of 3, temperature of 473 °C, oxidant multiple of 7, Fe²⁺ concentration of 0.5 mg L⁻¹, and residence time of 262.6 s (flow rate: 1.5 mL min⁻¹). Under these conditions, the total organic carbon removal rate of the wastewater could reach 98.1%. The activation energy of the wastewater under SCFO conditions was 55.3 kJ mol⁻¹, and the pre-exponential factor A was 52.8 s⁻¹.

A study on Fenton''s reagent-enhanced supercritical water oxidation (SCFO) of wastewater released from 3-hydroxypyridine production was carried out in this paper.     

The alkali degradation of LDPE-based radiation-grafted anion-exchange membranes studied using different ex situ methods

Radiation-grafted anion-exchange membranes (RG-AEM) in alkaline membrane fuel cells (AEMFC) exhibit promising performances (low in situ resistances, high power outputs and reasonably high alkali stabilities). Much research is focused on developing AEMs with enhanced chemical stabilities in the OH⁻-forms at temperatures >60 °C. This study contributes towards this effort by providing a comparison of three different ex situ methods of screening alkali stabilities (where different laboratories conducted experiments on exactly the same batches of RG-AEM). Vinylbenzyl chloride monomer was radiation-grafted onto 25 μm thick low-density polyethylene (LDPE) precursor film in a single batch. This batch of grafted membrane was then split into three sub-batches, which were converted into RG-AEMs via amination with either: trimethylamine (TMA), N-methylpyrrolidine (MPY), or N-methylpiperidine (MPIP). Samples of each RG-AEM (l-AEM-TMA, l-AEM-MPY, and l-AEM-MPIP) were then distributed between the three collaborating institutes for evaluation using each institutes'' test protocols. Out of the three head-group chemistries, the l-AEM-TMA generally exhibits the best balance of conductivity and ex situ alkali degradation, especially in lower humidity environments. The l-AEM-TMA also exhibited interestingly high Cl⁻ ion conductivities (ca. 100 mS cm⁻¹) when heated to 80 °C in a relative humidity RH = 95% atmosphere, a measurement frequently overlooked in favour of determining conductivities of RG-AEMs submerged in water (conductivities of submerged RG-AEMs can be suppressed due to excessive water contents and swelling).

Three different ex situ alkali degradation protocols were compared on single batches of LDPE-based radiation-grafted anion-exchange membranes (containing trimethylammonium, N-methylpiperidinium, and N-methylpyrrolidinium headgroups).     

Polysaccharides derived from Enteromorpha prolifera for the removal of silver nanoparticle-humic acid contaminants by a coagulation-ultrafiltration process

Silver nanoparticles (AgNPs) pose serious health risks to humans as the adsorption between AgNPs and humic acid (HA) makes it difficult to remove them from surface water. To solve this problem, polysaccharides extracted from a marine alga, Enteromorpha prolifera (denoted as Ep), were used to eliminate the AgNP–HA composite contaminant via a coagulation-ultrafiltration (C-UF) process. The structure of Ep, AgNP–HA removal mechanism and membrane fouling were analyzed. The results indicated that the backbone of Ep was composed of (1 → 4)-linked l-rhamnopyranose, (1 → 4)-linked d-xylose and (1 → 4)-linked glucuronic acid. With the charge neutralization of PAC hydrolysates and the bridging-sweep role of Ep, AgNPs could be removed completely by the C-UF process. The coagulation performance and membrane flux were the highest when the PAC and Ep dosages were 2.0 mg L⁻¹ and 0.3 mg L⁻¹, respectively. In addition, when Ep was applied in the C-UF process, the flocs exhibited larger sizes, faster growth rates, better recovery ability and looser structures, which resulted in lower cake resistance and less pore blocking of the UF membrane. Consequently, the membrane flux could be improved by about 25–30% due to the addition of Ep.

Applying Ep as a coagulant aid in C-UF process could simultaneously remove AgNPs and reduce membrane fouling.     

Novel triangular silver nanoparticle modified membranes for enhanced antifouling performance

This study marks the first ever attempt at the successful fabrication of a novel reactive membrane to combat fouling through layer-by-layer (LBL) surface modification with polyelectrolyte (PE), followed by anisotropic triangular silver nanoparticles (TSNP). The morphology and the presence of TSNP on the membrane was confirmed by HR-TEM, FE-SEM and XPS. The charge density of the novel membrane (PE-TSNP) was increased 15.6 fold, as a result of the sharp-tip morphology of the TSNP forming tip-based “hot spots” on the membrane surface and high-atom-density active facets, which also enhanced the membrane hydrophilicity by 36%. Owing to these improved features, the novel membrane displayed remarkable antibacterial and anti-adhesion properties by achieving 100% bactericidal effect against high initial bacterial concentration (10⁷ CFU mL⁻¹). The membrane flux was improved by 31% while retaining a high flux recovery rate of 98.2% against biofouling. The membrane also mitigated organic and bio-organic fouling by maintaining high flux recovery rates of 96% and 95% respectively. As compared with a spherical silver nanoparticle modified membrane (PE-SSNP), the PE-TSNP membrane was 25.7% more hydrophilic and achieved 10% higher bacterial killing. Moreover, the novel membrane displayed 9.5%, 11.6%, and 14% higher flux recovery rates than that of the PE-SSNP membrane against biofouling, organic and bio-organic fouling respectively. Furthermore, the novel membrane retained a long-term biocidal capability of 93% even after 4 months of successive tests. ICP-MS revealed silver ion leaching of 4 μg L⁻¹ and the total silver loss of 14% from the PE-TSNP membrane after 14 days.

Enhanced antibacterial, anti-adhesion and fouling mitigation strategy imparted by shape dependent membrane modification via anisotropic triangular silver nanoparticles.     

Modification of fibrous membrane for organic and pathogenic contaminants removal: from design to application

In this study, a flexible multifunctional fibrous membrane for heterogeneous Fenton-like removal of organic and pathogenic contaminants from wastewater was developed by immobilizing zerovalent iron nanoparticles (Fe-NPs) on an amine/thiol grafted polyester membrane. Full characterization of the resulting polyester membranes allowed validation of successful grafting of amine/thiol (NH₂ or SH) functional groups and immobilization of Fe-NPs (50–150 nm). The Fenton-like functionality of iron immobilized fibrous membranes (PET–Fe, PET–Si–NH₂–Fe, PET–NH₂–Fe, and PET–SH–Fe) in the presence of hydrogen peroxide (H₂O₂) was comparatively studied in the removal of crystal violet dye (50 mg L⁻¹). The effect of pH, amount of iron and H₂O₂ concentration on dye removal was systematically investigated. The highest dye removal yield reached 98.87% in 22 min at a rate constant 0.1919 min⁻¹ (R² = 95.36) for PET–SH–Fe providing 78% toxicity reduction assessed by COD analysis. These membranes could be reused for up to seven repeated cycles. Kinetics and postulated mechanism of colour removal were proposed by examining the above results. In addition, the resultant membranes showed substantial antibacterial activity against pathogenic bacteria (Staphylococcus epidermidis, Escherichia coli) strains studied through disc diffusion-zone inhibitory and optical density analysis. These findings are of great importance because they provide a prospect of textile-based flexible catalysts in heterogeneous Fenton-like systems for environmental and green chemistry applications.

Multifunctional fibrous membrane for heterogeneous Fenton-like removal of organic and pathogenic contaminants from wastewater was developed by immobilizing zerovalent iron nanoparticles (Fe-NPs) on an amine/thiol grafted polyester membrane.