Tuning pore size and surface charge of poly(piperazinamide) nanofiltration membrane by enhanced chemical cleaning treatment

Abstract

Separation of divalent salts and small organics plays a paramount in biorefinery and food industries. Nanofiltration (NF) membranes featuring suitable pore aperture ranges enables achievement of this goal, yet it fails in attaining a high liquid permeance and selectivity due to the presence of a highly crosslinked polyamide (PA) layer. In this work, the separation efficiency of monosaccharides and divalent salts by NF can be significantly improved by using a facile post-treatment approach. Assisted by chemical cleaning using three types of commonly used agents (H2SO4, NaOH and NaClO), controlled hydrolysis of PA groups is adopted to modulate the crosslinking degree, pore size, and surface charge of PA layer. After systematically comparing the different post-treatment agents and optimizing the treatment conditions, 3000 ppm NaClO treatment for 48 h is selected as the best for enhancing the separation of xylose and Na2SO4. The un-crosslinked -NH groups that serve as sites chloritized by the NaClO in the post-treatment process trigger the hydrolysis of the PA chains, thus enabling the formation of a loose, strongly electronegative and narrow pore size distributed PA film. Such hydrolysis can also eliminate the “weak points” of the separation layer which are susceptible to pH and salts, and the membrane structure is strengthened and becomes more stable at different pH although the post-treated membranes have larger pores and more charges. This results in a remarkable decline in xylose rejection (from 40.0% to 18.0%) while maintaining the high retention of Na2SO4 (97.1%), leading to an improved separation factor from 2.7 to 5.4 with a 93.3% permeability improvement. The chlorinated NF membrane also maintains excellent separation performance in crossflow filtration process.