Pressure-responsive nanofiltration for seawater desalination using sodium metasilicate sol-gel draw solution: Performance enhancement and silicate fouling mechanisms

The forward osmosis (FO) process for seawater desalination by sodium metasilicate (SMS) sol-gel draw solution faces challenges due to the high cost of cellulose triacetate (CTA), low water flux, and high costs. To address this issue, a cost-effective membrane of high water permeability and salt rejection is required. Pressure-responsive nanofiltration (PRNF) membranes present an opportunity for enhanced FO performance due to their superior water permeability and fouling resistance. This study evaluated the PRNF membranes against the CTA benchmark, measuring water flux, recovery, rejection rate, and reverse salt flux (RSF) across hydraulic pressures of 0 to 4 bar using DI water, NaCl, and seawater feeds with a 1 M SMS draw solution. PRNF outperformed CTA, demonstrating an increased water flux at 4 bar, reaching 40 LMH and 12.34 LMH in the PRNF and CTA for DI water feed, but decreased to 7.09 LMH and 4.46 LMH for the same membranes with SW feed. The rejection rate of Si and Na ions was comparable in both membranes, exceeding 99%. Membrane fouling was identified because of irreversible Mg/Ca silicate scaling. Then, the performance of PRNF was investigated at 4 bar with SW in five consecutive filtration cycles, and EDTA was added to the feed to prevent Ca/Mg silicate fouling. Using NaOH at pH 11 for membrane cleaning between cycles minimised Mg/Ca silicate fouling on the active layer while eliminating such fouling in the support layer. The investigation found that at pH 9, silica polymerization-initiated fouling while Mg crosslinking stabilized the gel, whereas at pH 11, fouling is triggered by Ca-induced silica scaling. Additional, cross-sectional SEM of the PRNF membrane did not detect severe fouling or pore blockages, while the EDX mapping indicated weakly localised depositions of inorganic salts. XRD analysis further showed minor crystalline deposits on the active layer, mainly associated with calcite (CaCO 3 ) and possible brucite (Mg(OH) 2 ), while the support layer had even weaker crystalline deposits. Overall, compared with the CTA membrane, the PRNF membrane exhibited better water flux and solute retention.