Surface Modification of Polymeric Membranes with Thin Films and Silver Nanoparticles for Biofouling Mitigation

Membrane filtration is a highly efficient water treatment technique with substantial potential for helping overcome the global water crisis. However, biofouling, or the formation of biofilms on membranes, currently hinders the sustainable application of membrane filtration and is, in fact, widely considered to be one of the most challenging obstacles to overcome. Therefore, an effective membrane biofouling mitigation technique is urgently needed. The objective of this dissertation work was to investigate the influence of the surface modifications of membranes with polymeric thin films and silver nanoparticles (AgNPs) for biofouling mitigation. The surface-modified membrane’s anti-biofouling properties can be evaluated through quantitative assessments of the membrane’s bacterial anti-adhesive properties and antimicrobial properties. The first part of the dissertation effort focused on surface modifications with AgNPs and 2-bilayer PAH/PAA PEMs on a commercial polysulfone (PSU) microfiltration (MF) membrane. The membrane’s bacterial anti-adhesive properties were highly enhanced after PEM- and AgNP/PEM-modifications. Specifically, the deposition kinetics of Escherichia coli cells on the PEM- and AgNP/PEM-modified membranes were reduced and the removal efficiencies were significantly enhanced compared to those of the base membrane. Interaction force measurements demonstrated that the bacterial anti-adhesive properties exhibited by the membrane modified with a PEM film could be attributed to the highly swollen and hydrated PEMs that inhibit the direct contact or close approach of bacteria to the underlying membrane. AgNPs that were immobilized on the membrane surface imparted antimicrobial properties to the membrane and the degree of bacterial inactivation increased as a function of AgNP mass loading. In addition, the AgNP mass loading required for the inhibition of bacterial growth in our study was significantly lower than the AgNP loadings reported in other studies for membranes with AgNPs dispersed within the membrane matrix, hence implying that the distribution of AgNPs within the membrane plays an important role in controlling the membrane’s antimicrobial properties. The second part of this dissertation focused on surface modifications with PDA and AgNPs formed in situ on a laboratory-cast PSU MF membrane. AgNPs could be generated on the membrane surface through Ag+ ion reduction by the catechol groups in PDA by simply soaking the membrane in a AgNO3 solution. The AgNP mass loading was found to increase with increasing soaking time. The PDA film increased the surface hydrophilicity of the membrane and the PDA- and PDA/AgNP-modified membranes exhibited bacterial anti-adhesive properties. The AgNPs that were immobilized on the membrane through metal coordination imparted strong antimicrobial properties to the membrane. This technique for membrane surface modification paves a way to mitigate membrane biofouling by enhancing the membrane’s bacterial anti-adhesive and antimicrobial properties simultaneously and also provides a feasible method to replenish AgNPs on the membrane in situ in water treatment processes