Efficient recovery of polyelectrolyte draw solutes in forward osmosis towards sustainable water treatment

Forward osmosis (FO) technology has long been constrained by the slow development of appropriate draw solutes (DS) and the relatively high cost associated with DS recovery. In this study, a series of polyelectrolytes, polyacrylic acid sodium salts (PAA-Na) with different molecular weights, were explored as DS for FO applications with a focus on the recovery using combined pH and microfiltration (MF). The FO system achieved a high water flux of 18.02 ± 0.51 LMH, low reverse salt flux (RSF) of 0.110 ± 0.004 gMH, and the J_S/J_W of 6.1 ± 0.3 mg L^{− 1} with 25 wt% PAA-Na (2000 Da) as the DS and DI water as the feed. The DS recovery efficiency by the combined pH + MF approach was 99.68% at pH of 4.35, and the operation cost was estimated at 0.037 $ m^{− 3}. Dynamic light scattering revealed that the hydrodynamic diameter of PAA increased with decreasing pH, resulting in PAA polymers precipitated as aggregates at the pH response point. The 25 wt% 2000 PAA-Na achieved the water flux of 11.56 ± 0.32 LMH from synthetic seawater and 17.19 ± 0.52 LMH from the treated wastewater. These results have demonstrated efficient and cost-effective recovery of PAA DS for FO-based applications.