Abstract

The reverse osmosis (RO) filtration process, which uses semi-permeable membranes to achieve selective mass transport, has become the most versatile and efficient technique to produce fresh water from saline water and wastewater sources. A major challenge facing the widespread application of RO technology is membrane biofouling which is caused by the deposition and multiplication of microorganisms on the surface of the membranes required for the filtration process. Biofouling not only deteriorates membrane materials but also adds an energy burden to the system, increasing the operational cost of the RO technique. Modifying membrane surfaces with antibacterial materials is an effective technique to prevent the growth of biofilms while maintaining the original water purification capabilities of the membrane.
In this study, three district types of membrane-surface modifications were proposed as potential methods for mitigating biofouling on the RO membrane: (i) bacteria/biofilm-“defending” strategy: coating with zwitterionic and low-surface-energy polymers to prevent microorganism/protein deposition; (ii) bacteria/biofilm-“attacking” strategy: anchoring CuNPs to inhibit the propagation of microorganisms and (iii) combined bacteria/biofilm-“defending and attacking” strategy: grafting polymers and natural antibiofouling materials to not only prevent foulant deposition but potentially inhibit biofilm formation.
Hydrophilic compounds (polysulfonbetain, PSB) and low surface energy polymers (polydimethylsiloxane, PDMS) were grafted in combination to the membrane to control biofouling via the bacteria/biofilm-“defending” strategy. Results showed that surface hydrophilicity is critical for the deposition of bacteria and protein. Combining functionalization of different fouling-resistant materials (PDMS and pSB) achieved an enhanced antibiofouling performance.
CuNPs were proposed as a cost-efficient and quality-competitive material for fabricating antibiofouling membranes with bacteria-“attacking” functions. CuNPs modified membranes exhibited bacterial inactivation comparable to the widely used silver NPs. The multiple layers of CuNPs coating on surface mitigated the permeate flux decline caused by biofouling and the flux of the modified membrane was 20% higher than the control under the same experimental conditions.
The fouling resistant membrane with a combined bacteria/biofilm-“defending” and -“attacking” function was fabricated by grafting a fouling-resistant polymer (poly(sulfobetaine methacrylate), PSBMA) and a biofilm inhibiting amino acid (poly methacryloyl-L-Lysine, PLysMA) to the membrane via a polymer length controlling technique, electron transfer-atom transfer radical polymerization atom (ARGET-ATRP). When the defending moieties (PSBMA) were predominant on the exposed top of surface with the biofilm inhibiting material (PLysMA) underneath, the membrane significantly mitigated the flux decline caused by bacterial deposition and biofilm formation (50% higher than the control). Furthermore, complete flux recovery was observed on the former after two cycles of “fouling-cleaning”, while the control membrane only maintained 94% of initial flux under the same condition.
This study proposes cost-effective modifiers (CuNPs and PLysMA) for the fabrication of anti-biofouling membranes, enriches the knowledge around the application of GO in modification of anti-biofouling membranes and proposes controllable functionalization methods for the development of antibiofouling membranes. The strategies proposed in this study contribute to the investigation of novel anti-biofouling membranes and the development of facile membrane modification methods, which can further broaden the applicability of membrane processes in wastewater reclamation and increase the future fresh water supply.