Nanofiltration membrane modification by UV grafting for salt rejection and fouling resistance improvement for brackish water desalination

Abstract

Two commercial NF membranes denoted as NF-1 and ASP30 were investigated in terms of applicability to desalinate ground water in Gaza where chloride-based salt concentrations are relatively high (1000mg/l) with 2200mg/l total dissolved solids (TDS). Initially, investigated membranes showed low chloride-based salt rejection as well as resistance to humic acid fouling which required certain improvement. Such improvement could be achieved by surface modification via UV-grafting. UV graft polymerisation is a common method applied to improve nanofiltration (NF) membrane properties due to: 1) lower operational costs 2) milder reaction conditions and 3) enhanced absorption selectivity of UV light without affecting the bulk polymer to increase salt rejection and humic acid fouling resistance was applied. The newly combined monomer of acrylic acid and ethylenediamine dihydrochloride grafted by UV initiated successfully increased NaCl rejection of the NF-1 membrane to 57–60%. Rejection of MgSO4 was increased from 89% to 94%, while for ASP30, NaCl and MgSO4 rejection increased from 33% and 32% to 57% and 71%, respectively. The best rejection for NF-1 was achieved at a low monomer concentration and moderate UV exposure, while ASP30 showed the best rejection at a high monomer concentration and low UV exposure. However, pure water flux was slightly reduced. Irreversible fouling caused by humic acid, common ground water foulant, was measured by the irreversible fouling factor (IRF), which was reduced after modification, indicating improved fouling resistance. These data show that this new combined monomer initiated by UV grafting can successfully improve NF membrane selectivity as well as fouling resistance.