Structure adjustment for enhancing the water permeability and separation selectivity of the thin film composite nanofiltration membrane based on a dendritic hyperbranched polymer

Abstract

In this study, a dendritic amphiphilic hyperbranched polyester acyl chloride (HPE-COCl) with abundant terminal acyl chloride groups was proposed to incorporate into the polyamide skin layer via interfacial polymerization to adjust the structure and the separation performance of the thin film composite (TFC) nanofiltration (NF) membranes. The modified NF membranes based on HPE-COCl exhibit looser, more hydrophilic and negatively charged surface, thereby endowing TFC membranes with relatively higher water permeability and selectivity of monovalent/divalent salts, heavy metals/common monovalent salts, and dyes/common monovalent salts. Especially, the TFC-H2 membrane containing 0.07% w/v HPE-COCl significantly increases the water flux by 145% compared to the pristine membrane. Furthermore, the corresponding selectivity of Fe3+/Na+ increase about 5.09 times. The hypothesis on the basis of experimental results is that the HPE-COCl with alternating hydrophilic and hydrophobic units is prone to form a plurality of hydrophilic channels based on PEG units and hydrophobic microdomains based on benzoate units, which enhances the water permeation through capillary forces. Moreover, the strong negatively charged surface formed by high-density terminal carboxyl groups in HPE-COCl facilitates the passage of monovalent salts and retains the lager-sized heavy metal salts and dyes, thereby improving the selectivity of membranes. Thus, this work provides a new idea and method for the development of high performance nanofiltration membranes.