Functionalization of polybenzimidizole membranes to impart negative charge and hydrophilicity

Polybenzimidizole (PBI) is a promising material for membrane separations due to its high mechanical and thermal stabilities. Two drawbacks that can hinder the use of PBI membranes in water applications are low hydrophilicity and neutral charge at neutral pH values. To investigate methods to increase hydrophilicity and charge, PBI membranes were cast in the form of flat sheets using the phase-inversion technique and were functionalized with groups designed to modify the membrane surface. Specifically, three functional agents (taurine, para-phenylene diamine, and ethylene diamine) were chosen for surface modification based on their potential to impart charges and hydrophilicity on the PBI membrane. The surface of the PBI membrane was activated using 4-(chloromethyl) benzoic acid (CMBA). Characterization of the membrane was done using Fourier transform infrared spectroscopy in attenuated reflectance mode (FTIR-ATR), contact angle measurement, ζ potential, and environmental scanning electron microscopy (ESEM). Modification resulted in increased hydrophilicity and negative surface charge coverage. Pure water permeability was found to decrease by 33% after surface activation and by ∼70% after chemical modification as compared to that of the unmodified PBI membrane. Monovalent salt rejection was investigated using sodium chloride feed at concentrations from 3.4 mM to 100 mM and at pH values of 7 and 10. At both pH values salt rejection decreased exponentially with increasing concentration, and modified membranes provided higher rejections than unmodified membranes.

Research highlights▶ Flat sheet asymmetric polybenzimidizole (PBI) membranes were successfully cast, and were functionalized with groups designed to modify the membrane surface. Specifically, three functional agents (taurine, para-phenylene diamine, and ethylene diamine) were chosen for surface modification based on their potential to impart charges and hydrophilicity on the PBI membrane. ▶ Modification led to an increase in hydrophilicity along with an overall increase in negative surface charge percentage surface activation and modification. ▶ Pure water permeability testing showed a nearly linear decrease in permeability as the amount of modification went up (i.e. surface activation to final modification). ▶ Sodium chloride rejection decreased exponentially as the feed concentration increased for all modifications as well as the unmodified membrane, with modified membranes always showing higher rejections than unmodified membranes. This trend was seen at feeds of pH 7 and pH 10. Promising salt rejections were observed upon modifying agent addition.