Forward osmosis with direct contact membrane distillation using tetrabutylphosphonium p-toluenesulfonate as an effective and safe thermo-recyclable osmotic agent for seawater desalination

A phosphonium-based ionic liquid (IL) with lower critical solution temperature (LCST) property was assessed as a reusable draw solution (DS) for forward osmosis (FO). Tetrabutylphosphonium p-toluenesulfonate ([P4444]TsO) was successfully synthesized by neutralization reaction. Characterization results reveal its ability to generate sufficient osmotic pressure (14-68 bars for 0.5-2 M DS) to create a gradient across the FO membrane. Its thermal, physico-chemical and other colligative properties are favorable for its application as an osmotic agent. The LCST behavior of [P4444]TsO was found reversible and its phase separation from water can be done above its cloud point temperature T-c similar to 57 degrees C. In vitro cytotoxicity tests from LDH and MTT assay reveal that it can be safely used as DS at an effective concentration EC30 similar to 57 mg L-1 as its non-toxic level. Results from FO operations demonstrate that 2 M [P4444]TsO DS can effectively treat saline feed like seawater (0.6 M NaCl) with reasonable J(v) = 1.35 +/- 0.15 L m(-2)h(-1), low J(s) = 0.0038 +/- 0.00049 mol m(-2)h(-1), and considerably low specific solute flux (J(s)/J(v) - 0.0028 mol L-1). After FO, -98% of [P4444]TsO was precipitated by heating the DS at 60 degrees C and conveniently reused with consistent FO performance. Direct contact membrane distillation (DCMD) was found effective in removing the residual 2% [P4444]TsO in the DS supernatant to finally produce highquality effluent with concentrations way below the EC30 limit. Cost estimates for the entire process reveal the potential of FO combined with thermo-cyclic [P4444]TsO regeneration with DCMD for desalination application. (C) 2020 Elsevier Ltd. All rights reserved.

Automated summary

A phosphonium-based ionic liquid with lower critical solution temperature has been successfully synthesized and evaluated as a reusable draw solution for forward osmosis. It shows promising performance in treating saline feed, and can be conveniently regenerated and reused with consistent FO performance. The cost estimates reveal the potential of combining FO with thermo-cyclic regeneration for desalination applications.