Exploration of a cost-effective draw solution based on mixing surfactant and sodium chloride for forward osmosis desalination process

Abstract

A novel draw solution containing sodium chloride (NaCl) coupled with polyethylene glycol tert-octylphenyl ether surfactant (i.e., denoted as TX114) was used for the forward osmosis (FO) process to achieve high permeate flux and minimum specific reverse salt flux. Experimental results demonstrate that the mixture of 3M NaCl/0.6 mM TX114 draw solution greatly improved the water flux in the FO process (21.26 L/m2h) and reduced the specific salt leakage (Js/Jw=1.82 g/L) when feed solution was deionized water. Compared to the draw solution containing only NaCl, the mixture of NaCl/TX114 reduced the Js/Jw by 2.5 times because of the hydrophobic interaction between the FO membrane and the surfactant tails that constricted the membrane pore size and hence reduced the cross-membrane salt leakage. More importantly, the novel FO draw solution was efficiently recovered using a membrane distillation (MD) process, which could be energized by solar thermal energy or low-level waste heat. Indeed, the hybrid FO/MD process of a real seawater feed at the optimum conditions exhibited a stable water flux (6 L/m2h) and an excellent salt rejection (≈100%) for 25 h of operation, despite a thin cake layer of foulant was detected on the FO membrane surface. The results presented in this study indicate that the hybrid FO/MD process using the mixed NaCl/TX114 draw solute can serve as a membrane fouling-resistant and cost-effective seawater desalination platform to mitigate the increasing global fresh water demand.