Surface Modification of Dense Membranes Using Radical Graft Polymerization Enhanced by Monomer Filtration

Surface graft polymerization is a promising way to modify membranes for improved performance. Redox-initiated graft polymerization of vinyl monomers is a facile and inexpensive method carried out at room temperature in aqueous media; however, its use is often limited by slow kinetics, low surface specificity, and excessive consumption of chemicals on undesired homopolymerization. It is shown that in the case of RO or NF membranes these drawbacks may be eliminated by utilizing the selectivity of the membranes toward monomers and carrying out the polymerization while applying pressure, i.e., under filtration conditions. Concentration polarization that ensues raises the concentration of reagents near the membrane surface and thereby drastically increases the rate of reaction and preferentially directs it towards surface grafting. Grafting experiments using 2-hydroxyethyl methacrylate and other monomers and characterization of modified membranes using permeability measurements, ATR-FTIR, AFM, XPS, and contact angle demonstrate that the required monomer concentrations can be drastically reduced, particularly when a small fraction of a cross-linker is added. As an additional benefit, this approach enables broadening the spectrum of utilizable monomers to sparingly soluble hydrophobic, charged, and macro-monomers, as was demonstrated using sparingly soluble ethyl methacrylate and 2-ethoxyethyl methacrylate and other monomers. Even though the kinetics of the process is substantially complicated by evolution and concentration polarization of oligomeric and polymeric species, especially in the presence of a cross-linker, it is well offset by the benefits of higher rate, specificity, and reduced monomer consumption

Bacterial Attachment to RO Membranes Surface-Modified by Concentration-Polarization-Enhanced Graft Polymerization

Concentration polarization-enhanced radical graft polymerization, a facile surface modification technique, was examined as an approach to reduce bacterial deposition onto RO membranes and thus contribute to mitigation of biofouling. For this purpose an RO membrane ESPA-1 was surface-grafted with a zwitterionic and negatively and positively charged monomers. The low monomer concentrations and low degrees of grafting employed in modifications moderately reduced flux (by 20–40%) and did not affect salt rejection, yet produced substantial changes in surface chemistry, charge and hydrophilicity. The propensity to bacterial attachment of original and modified membranes was assessed using bacterial deposition tests carried out in a parallel plate flow setup using a fluorescent strain of Pseudomonas fluorescens. Compared to unmodified ESPA-1 the deposition (mass transfer) coefficient was significantly increased for modification with the positively charged monomer. On the other hand, a substantial reduction in bacterial deposition rates was observed for membranes modified with zwitterionic monomer and, still more, with very hydrophilic negatively charged monomers. This trend is well explained by the effects of surface charge (as measured by ζ-potenital) and hydrophilicity (contact angle). It also well correlated with force distance measurements by AFM using surrogate spherical probes with a negative surface charge mimicking the bacterial surface. The positively charged surface showed a strong hysteresis with a large adhesion force, which was weaker for unmodified ESPA-1 and still weaker for zwitterionic surface, while negatively charged surface showed a long-range repulsion and negligible hysteresis. These results demonstrate the potential of using the proposed surface- modification approach for varying surface characteristics, charge and hydrophilicity, and thus minimizing bacterial deposition and potentially reducing propensity biofouling

Toward Improved Boron Removal in RO by Membrane Modification: Feasibility and Challenges

Membrane modification by concentration polarization (CP)-enhanced radical graft polymerization using a dilute aqueous solution of appropriate monomer was examined as a method for increasing rejection of boric acid by reverse osmosis (RO) membranes. On the basis of suggested physicochemical rationales a number of monomers were examined in order to determine those with the lowest affinity toward boric acid as compared to water. The improvement in the modified membrane performance was mainly attributed to sealing less selective areas (“defects”) inherently present in the original low pressure RO (LPRO) membranes. However, the effect clearly differed for different monomers. Among the examined monomers glycidyl methacrylate (GMA) exhibited the lowest affinity and the largest improvement in removal of boric acid along with a moderate loss of permeability and slightly improved NaCl rejection. Modification of LPRO membrane thus resulted in a membrane with a permeability in the brackish water RO (BWRO) range but with removal of boric acid and salt superior to those reported for most commercial BWRO membranes

Biofilm Formation and Biofouling Development on Different Ultrafiltration Membranes by Natural Anaerobes from an Anaerobic Membrane Bioreactor

Biofouling in anaerobic membrane bioreactors (AnMBRs) has not been studied widely. Moreover, the effect of membrane surface properties on biofilm formation beyond initial deposition is controversial. We investigated biofouling with polyvinyldifluoride, polyacrylonitrile, and zwitterion-modified polyethersulfone ultrafiltration membranes having different properties during 72 h filtration using natural anaerobes isolated from AnMBR and analyzed biofilm characteristics by physicochemical and molecular techniques. A decrease in membrane performance was positively correlated with biofilm formation on polyvinyldifluoride and polyacrylonitrile membranes, and as expected, physical cleaning effectively mitigated biofilm on hydrophilic and low-roughness membranes. Surprisingly, while the biofilm on the hydrophilic and low-surface roughness zwitterion-modified membrane was significantly impaired, the impact on transmembrane pressure was the highest. This was ascribed to the formation of a soft compressible thin biofilm with high hydraulic resistance, and internal clogging and pore blocking due to high pore-size distribution. Anaerobe community analysis demonstrated some selection between the bulk and biofilm anaerobes and differences in the relative abundance of the dominant anaerobes among the membranes. However, correlation analyses revealed that all membrane properties studied affected microbial communities’ composition, highlighting the system’s complexity. Overall, our findings indicate that the membrane properties can affect biofilm formation and the anaerobic microbial population but not necessarily alleviate biofouling

Fouling of Reverse Osmosis Membrane with Effluent Organic Matter: Componential Role of Hydrophobicity

Organic matter dissolved in tertiary effluents (effluent organic matter, EfOM) is the predominant organic membrane foulant in tertiary wastewater reverse osmosis (RO) desalination, ultimately causing biofouling. The interrelated effects of EfOM fractions of different hydrophobicity and polarity on membrane performance were studied by (i) examining each fraction’s overall effect on membrane permeability; (ii) analyzing the intrinsic hydraulic resistance induced by each fraction; (iii) studying their adsorption on the active layer of an RO membrane using a quartz crystal microbalance with dissipation monitoring (QCM-D); (iv) assessing their “dry” molecular mass when adsorbed on polyamide using localized surface plasmon resonance (LSPR) sensing; (v) analyzing their hydrodynamic radii by dynamic light scattering (DLS); and (vi) characterization using excitation–emission matrix (EEM) analysis and parallel-factor (PARAFAC) modeling. Hydrophobic and transphilic neutral fractions (containing ∼12.5% total organic carbon) have the greatest effect on membrane flux reduction and the highest hydraulic resistance and adhere most strongly to polyamide surfaces, resulting in the highest adsorbed “dry” mass. Therefore, in terms of their effect on RO permeate flux reduction, these fractions are the most detrimental in the EfOM mix. EEM analysis and associated PARAFAC modeling indicate that the main components causing this effect are mixtures of protein-like compounds, together with humic-like substances. Novel LSPR-based analysis elucidated the role of the fractions most detrimental to membrane permeability through measurement of dry mass surface concentration on a polyamide mimetic sensor. This study provides valuable insights into the roles of different EfOM fractions in RO membrane fouling and enhances our understanding of fouling during tertiary wastewater desalination

‘Should I stay or should I go?’ Bacterial attachment <i>vs</i> biofilm formation on surface-modified membranes

<div><p>A number of techniques are used for testing the anti-biofouling activity of surfaces, yet the correlation between different results is often questionable. In this report, the correlation between initial bacterial deposition (fast tests, reported previously) and biofilm growth (much slower tests) was analyzed on a pristine and a surface-modified reverse osmosis membrane ESPA-1. The membrane was modified with grafted hydrophilic polymers bearing negatively charged, positively charged and zwitter-ionic moieties. Using three different bacterial strains it was found that there was no general correlation between the initial bacterial deposition rates and biofilm growth on surfaces, the reasons being different for each modified surface. For the negatively charged surface the slowest deposition due to the charge repulsion was eventually succeeded by the largest biofilm growth, probably due to secretion of extracellular polymeric substances (EPS) that mediated a strong attachment. For the positively charged surface, short-term charge attraction by quaternary amine groups led to the fastest deposition, but could be eventually overridden by their antimicrobial activity, resulting in non-consistent results where in some cases a lower biofilm formation rate was observed. The results indicate that initial deposition rates have to be used and interpreted with great care, when used for assessing the anti-biofouling activity of surfaces. However, for a weakly interacting ‘low-fouling’ zwitter-ionic surface, the positive correlation between initial cell deposition and biofilm growth, especially under flow, suggests that for this type of coating initial deposition tests may be fairly indicative of anti-biofouling potential.</p></div

Photografting Graphene Oxide to Inert Membrane Materials to Impart Antibacterial Activity

Surface modification with bactericides is a promising approach to imparting membrane materials with biofouling resistance. However, chemical modification of membranes made from inert materials, such as polyvinylidene fluoride (PVDF) and polysulfone, is challenging because of the absence of reactive functional groups on these materials. In this study, we develop a facile procedure using benzophenone as an anchor to graft biocidal graphene oxide (GO) to chemically inactive membrane materials. GO nanosheets are first functionalized with benzophenone through an amide coupling reaction. Then, benzophenone-functionalized GO nanosheets are irreversibly grafted to the inert membrane surfaces via benzophenone-initiated cross-linking under ultraviolet irradiation. The binding of GO to the membrane surface is confirmed by scanning electron microscopy and Raman spectroscopy. When exposed to a model bacterium (Escherichia coli), GO-functionalized PVDF and polysulfone membranes exhibit strong antibacterial activity, reducing the number of viable cells by 90% and 75%, respectively, compared to the number with the pristine membranes. Notably, this bactericidal effect is imparted to the membranes without compromising membrane permeability or solute retention properties. Our results highlight the potential application of benzophenone chemistry in membrane surface modification as well as its promise in developing antimicrobial surfaces for a variety of environmental applications

Microporous Polyethersulfone Membranes Grafted with Zwitterionic Polymer Brushes Showing Microfiltration Permeance and Ultrafiltration Bacteriophage Removal

Virus removal from water using microfiltration (MF) membranes is of great interest but remains challenging owing to the membranes’ mean pore sizes typically being significantly larger than most viruses. We present microporous membranes grafted with polyzwitterionic brushes (N-dimethylammonium betaine) that combine bacteriophage removal in the range of ultrafiltration (UF) membranes with the permeance of MF membranes. Brush structures were grafted in two steps: free-radical polymerization followed by atom transfer radical polymerization (ATRP). Attenuated total reflection Fourier transform infrared (ATR–FTIR) and X-ray photoelectron (XPS) verified that grafting occurred at both sides of the membranes and that the grafting increased with increasing the zwitterion monomer concentration. The log reduction values (LRVs) of the pristine membrane increased from less than 0.5 LRV for T4 (∼100 nm) and NT1 (∼50 nm) bacteriophages to up to 4.5 LRV for the T4 and 3.1 LRV for the NT1 for the brush-grafted membranes with a permeance of about 1000 LMH/bar. The high permeance was attributed to a high-water fraction in the ultra-hydrophilic brush structure. The high measured LRVs of the brush-grafted membranes were attributed to enhanced bacteriophages exclusion from the membrane surface and entrapment of the ones that penetrated the pores due to the membranes’ smaller mean pore-size and cross-section porosity than those of the pristine membrane, as seen by scanning electron microscopy (SEM) and measured using liquid–liquid porometry. Micro X-ray fluorescence (μ-XRF) spectrometry and nanoscale secondary ion mass spectrometry showed that 100 nm Si-coated gold nanospheres accumulated on the surface of the pristine membrane but not on the brush-coated membrane and that the nanospheres that penetrated the membranes were entrapped in the brush-grafted membrane but passed the pristine one. These results corroborate the LRVs obtained during filtration experiments and support the inference that the increased removal was due to a combined exclusion mechanism and entrapment. Overall, these microporous brush-grafted membranes show potential for use in advanced water treatment

Advanced Tertiary Wastewater Treatment for Enhanced Effluent Quality by Electrically Conducting Membranes Operated at Altered Potential for Simultaneous Mitigation of Scaling and Biofouling

Electrically conducting membranes (ECMs) present a promising low-fouling technology for advanced tertiary wastewater treatment, particularly when employed as cathodes to resist biofouling. However, the elevated pH levels near the cathode lead to severe scaling. This study investigated alternating the applied potentials in carbon-nanotube-based ECM to simultaneously mitigate biofouling and scaling while producing high-quality effluent. The research demonstrates that alternating −2.5 V cathodic and 1.5 V (vs Ag/AgCl) anodic potentials at 5 s intervals significantly reduce scaling and biofouling. In a 48 h filtration experiment using real secondary effluents, there was a notable decrease in trans-membrane pressure by 110 mbar, contrasting with the 320 mbar increase observed under open-circuit potential (OCP) conditions. Surface analysis confirms the absence of scaling and biofilm on the ECM layer under alternating potential operation, in contrast to high biofilm formation on the ECM at OCP conditions. The low scaling was attributed to controlled pH levels and the limited biofilm formation to the measured reactive oxygen species, facilitating bacterial detachment. The short duration of low anodic potential under alternating potential may also decrease carbon nanotube (CNT) oxidation and extend ECM lifetime. ECM-based filtration produced higher-quality effluents than conventional chlorination-coagulation-sand-filtration tertiary treatments. These findings highlight the potential of ECM-based filtration for sustained and efficient advanced wastewater treatment, surpassing the current conventional tertiary treatment technologies