Evaluating Switchable Polarity Solvent Compatibility With Commercial TFC FO and RO Membranes

Switchable polarity solvents (SPS) are a promising candidate draw solution for forward osmosis (FO). The unique properties of the tertiary amine solvent cause it to be miscible with water as a bicarbonate salt in the presence of CO2, or immiscible as an organic liquid when CO2 is removed. Thus, these draw solutions are easily separable compared to thermolytic salt draw solutions and can generate high osmotic pressure with relatively high diffusivity compared to polymeric draw solutions. A common switchable solvent, dimethylcyclohexylamine (DMCHA), is a previously reported candidate FO draw solution which has demonstrated the capability to dewater sodium chloride solutions up to 5 mol/kg, but its basicity makes it incompatible with currently available cellulose triacetate (CTA) membranes. We investigate the stability of polyamide (PA) thin-film composite (TFC) reverse osmosis (RO) and FO membranes exposed to dimethylcyclohexylammonium hydrogen carbonate (DMCHAH HCO3) solution as models for long-term exposure to an SPS FO process and monitor the membrane performance metrics A (hydraulic permeability) and B (solute permeability) for sodium chloride and the solvent. We observe that PA selective layers appear to be unaffected by exposure to concentrated DMCHAH HCO3 on a month-long timescale, but screen various polymer support layers for degradation when exposed to DMCHAH HCO3. Finally we investigate the efficacy of RO polishing of SPS FO process effluent, which is necessarily saturated in DMCHA (~18 g/L), by evaluating solvent rejection in TFC RO membranes which have been exposed to saturated DMCHA solution.