Strategies for antibiofouling membranes
Thesis (PhD)--Stellenbosch University, 2015.
ENGLISH ABSTRACT: Membrane filtration is increasingly used for municipal and industrial wastewater treatment because it is an effective way to filter out bacteria and organic compounds. One of the largest problems is biofouling of the membranes, which may lead to blockage of the membrane with organic biofilms. Such blockages require expensive interruptions to the filtration process for periodic cleaning and cause a decrease in membrane lifespan. The ideal solution to the biofouling problem would be the production of novel polymeric membranes that do not biofoul. Theoretically, this can be accomplished in two ways: (1) provide membrane surfaces to which bacteria and organic material are incapable of adhering, and (2) introduce reactive compounds on the membrane surface that degrade the biofilm. In four data chapters written in the format of stand-alone publications, techniques were explored for the production of inherently anti-fouling membranes using the most current strategies of polymer synthesis and characterization. Specifically, Reversible Addition Fragmentation Chain Transfer (RAFT) polymerization is employed to graft an anti-adhesive layer of hydrophilic copolymer chains onto the surface of regenerated cellulose membranes. Then, the mechanism of control of the polymerization reaction of the hydrophilic copolymers was investigated in enable design of an optimized anti-adhesive surface layer. In the following two chapters, immobilization of different combinations of biomoleculedegrading enzymes on high surface area non-woven nanofibrous mats, produced via electrospinning, was investigated. The retention of their enzymatic activity was tested in order to first prove the principle that enzymes can retain activity once immobilized on a substrate. In the subsequent chapter two additional enzymes commonly used for biofilm remediation in clean-in-place (CIP) processes were immobilized on the nanofibers, on their own as well as in combination with each other. Surface modification of regenerated cellulose membranes to introduce zwitterionic hydrophilic copolymers revealed that grafting of copolymers of N-vinylpyrrolidone (NVP) and maleic anhydride (MAnh) could be achieved through an R-group approach of RAFTmediated polymerization. The MAnh contained in the polymer backbone or as end-groups of the polymer were converted to zwitterionic compounds. Upon exposure to bacteria, these membranes prevented adhesion of extracellular polymeric substances (EPS) and bacteria cells to the membrane surface. An investigation into the underlying kinetics of RAFTmediated polymerization showed that no control was achieved for 1:1 monomer ratios of NVP/MAnh for two types of RAFT agents. The lack of control was due to the acid-catalyzed dimerization of NVP occurring at a very large extent. Copolymerization was affected at even small amounts of MAnh but, once consumed, an initialization step for NVP was observed. This provides a means of incorporating short segments of the functional MAnh monomer in the copolymer whilst maintaining reasonable control over the polymerization using RAFT chain transfer agents. As an alternative to anti-cell adhesive surfaces, immobilization of enzymes on nanofibrous mats holds promise for the in situ degradation of organic biofilms. First, horseradish peroxidase and glucose oxidase were immobilized via nucleophilic addition of primary amines of the enzyme to reactive maleic anhydride residues in the copolymer backbone to prove the principle and illustrate that cascade reactions can be performed with catalytically active immobilized enzymes. Next, the combined immobilization of protease and α-amylase, two enzymes commonly used for biofilm remediation, enabled the degradation of protein and starch solutions. Co-immobilization led to an unexpected increase in activity of the αamylase, but at the same time a significant decrease in protease activity. The results obtained from the strategies explored in this thesis bode well for the future of manufacturing inherently anti-biofouling membranes and may very well lead to making membrane filtration economically more feasible to produce safe potable water, especially in the developing world.
AFRIKAANSE OPSOMMING: Geen opsomming