Comparison of flux behavior and synthetic organic compound removal by forward osmosis and reverse osmosis membranes

Bench-scale forward osmosis (FO) and reverse osmosis (RO) experiments with both FO and RO membranes were used to investigate the systematic and mechanistic comparison of flux and removal behaviors of the relative hydrophilicities of several synthetic organic compounds (SOCs). The cellulose triacetate-based FO membrane exhibited relatively lower selectivity ratios based on the solution-diffusion model, indicating that the FO membrane has better separation properties than the polyamide-based RO membrane. And the reverse salt flux of FO and RO membranes was likely to be influenced by the combined selectivity effects of the active layer and internal concentration polarization (ICP) of the support layer. However, in active layer-facing-feed solution configuration in FO-mode, the RO membrane exhibited higher removal efficiency at the expense of severity of ICP and flux reduction. It was supported that this discrepancy behavior between retentions of SOCs and selectivity of salts primarily attributed to the ICP effect. Under higher cross-flow velocity operations in FO-mode, both the reduced external concentration polarization and retarded SOC diffusion from the reverse salt flux contributed to the improved SOC removal performance. The SOC removal percentage by the FO membrane with respect to molecular weight (MW) followed the order (MW, gmol^-1; removal, %): sulfamethoxazole (296.4; 90%)>carbamazepine (236.3; 83%)»atrazine (215.7; 49%)>4-chlorophenol (128.6; 39%)>phenol (94.1; 22%). For the FO membrane in RO-mode operation with SOCs of relatively small MW, breakthrough release was observed and was attributed to the FO membrane's porous, mesh fabric support backing layer. In addition, the batch adsorption and computational dynamics molecular modeling suggested that interaction affinity played a dominant role in the removal of SOCs and was generally correlated with their hydrophobicity. It was also demonstrated that the removal behavior of both FO and RO membranes with relative hydrophilicities of SOCs was mainly dominated by the steric hindrance mechanism during the FO process.