(386g) Characterization of Sulfonated Polysulfone Membranes Modified by Ion Beam Irradiation

For membranes to be competitive with water treatment conventional technologies, membrane processes need to operate with high rates of flux, have high degrees of selectivity and have high resistances to fouling. To accomplish these objectives together, modification of the membrane surface and/or microstructure may be necessary. One modification technique is ion beam irradiation. Ion beam irradiation is the bombardment of ions into the surface of the membrane. As the ions penetrate the membrane, they lose energy to the polymer [1]. This energy transfer results in bond breaking, bond formation and microstructure alterations, which cause changes in flux and selectivity as well as improve fouling resistance [2]. Previous studies by our research group have irradiated commercially-available water treatment membranes, and resulted in overall improvements [3-4]. The charge of the irradiated membrane was hypothesized to become more neutral based on observed decreases in selectivity of monovalent cations accompanied by no changes in flux. The selectivity of organics was improved by irradiation, which was evidenced by an increase in the rejection trend. Decreases in fouling accumulation/attachment were observed, while flux decline values due to irreversible fouling indicated no significant changes. Finally, biofouling analyses revealed that irradiation drastically decreased the ability of bacteria to adhere to the surface of the modified membrane. Based on the above results, ion beam irradiation was determined to improve the operation of modified membranes. However, our previous studies were proof of concept investigations and did characterize the modifications. The goal of our current research is to determine the effects of ion beam irradiation on surface morphology, microstructure and chemical structure of a modified commercial sulfonated polysulfone water treatment membrane. In order to determine the effects of ion beam irradiation on the chemical structure of the membrane, ATR/FTIR analysis was performed on the irradiated membrane and was compared with the ATR/FTIR analysis of virgin membrane. The only significance difference observed was with respect to the peak height at 1041 cm-1 wave number, associated with sulphonic ? benzene ring bonds (C-S), which was decreased by 19% after irradiation due to the breakage of some of the C-S bonds, with cross liking of the polymer occurring at these free sites. Changes occur only on the surface of the polymer, since ion beam irradiation was only used to modify the surface. We hypothesize that when sulphonic bonds were broken, after ion beam irradiation, a positive radical on the benzene ring was formed and H2SO4 was released. The free radical on the benzene ring was hypothesized to bind to a unbroken free sulphonic site to increase cross linking. These changes modified the surface morphology of the membrane and also decreased the negative charge of the membrane. We also focused on conducting bench-scale cross-flow filtration experiments to investigate the cake accumulation on the modified membrane. AFM analyses for the virgin and irradiated membranes, after approximately 14 hours of testing show that the roughness of the virgin membrane was approximately 195.39 nm, while that of irradiated membrane was approximately 3.49 nm. Thus, cake accumulation on the virgin membrane occurred faster than and was significantly greater than on the irradiated membrane. FTIR analysis was also performed on both the membranes after 14 hours of testing. The peak heights of virgin membrane were greater than those of the irradiated membrane, which also confirmed that fouling on the virgin membranes was greater than on the irradiated membrane. Thus, ion beam irradiation was observed to be an effective membrane modification technique.

References:

[1] E.H. Lee. Nuclear Instruments and Methods in Physics Research B 151 (1999) 29-41. [2] X. Xu and M.R. Coleman. Nuclear Instruments and methods in Physics Research b., 152 (1999b) 325-334. [3] K. Good, I.C. Escobar, X.L. Xu, M.R. Coleman, and M. Ponting. Desalination 146 (2002) 259-264. [4] S. King, I. Escobar, X. Xu. Environmental chemistry, 1 (2004)1-5.