Effective inhibition of gypsum using an ion–ion selective nanofiltration membrane pretreatment process for seawater desalination

Abstract

Commercial reverse osmosis (RO) membranes experience severe inorganic scaling when treated with seawater. This results in a rapid flux decline that considerably compromises the energy efficiency of the desalination process. To address this issue fundamentally, we proposed an effective pretreatment process for seawater desalination using an ion–ion selective nanofiltration (NF) membrane. Such membranes were fabricated by a conventional interfacial polymerization method, solely using 2,2′-benzidinedisulfonic acid as the aqueous reactant, and trimesoyl chloride in the organic phase, respectively. Under the influence of size and Donnan exclusion, the custom-made membranes could effectively differentiate the ion transport behaviors of calcium and sulfate ions, demonstrating their ability to fractionate the ions that cause scaling. In brief, an ultralow rejection of Ca2+ (ca. 5.7%) was obtained by the most optimal membrane (BM-0.7) with a mean pore radius of 0.523 nm. Meanwhile, a large proportion of SO42- was retained at the concentrate side owing to the strong surface negativity of the membrane, and the ion composition varied significantly in the permeate stream of the BM-0.7 membrane. Thus, scaling induced by gypsum in the hybrid NF-RO desalination process was significantly alleviated both on the surface of the BM-0.7 membrane, owing to the lack of Ca2+ and on the surface of the RO membrane, owing to the insufficient amount of SO42-. In contrast, severe scaling was observed for the NF270 membrane (a commercial loose NF membrane procured from Dupont Company), owing to the high rejection of both ions, suggesting a simple fouling transfer from the RO membrane to NF one. Overall, this study provides a promising strategy of an NF pretreatment to modify the inherent water chemistry of seawater for inhibiting gypsum scaling in the seawater RO industry.