Selective separation of chloride and sulfate by nanofiltration for high saline wastewater recycling

• Chloride and sulfate can be effectively separated by Desal-DL NF membrane.
• Desal-DL membrane presented high rejection to SO42− (>0.94).
• Special negative rejection to Cl− at high concentration of Na2SO4 was observed.
• Highly concentrated salt solution could be obtained through diafiltration process.

In order to effectively separate chloride (such as NaCl) and sulfate (such as Na2SO4) for high saline wastewater recycling, one commercial NF membrane (named Desal-DL) was employed in the permeation of the single and binary salt solutions of NaCl and Na2SO4 with a lab-scale cross-flow batch module, where the salt concentration ranged from 4 to 96 g L−1 and the operating pressures varied from 0.6 to 2.4 MPa as well as the temperature was kept under room temperature. The experimental results showed that the Desal-DL NF membrane had a low rejection to NaCl and a high rejection to Na2SO4 for single salt solutions. While for binary salt solutions, the membrane presented a bit higher rejection to SO42− and much lower rejection to Cl−, even special negative rejection to Cl− was observed when the concentration of Na2SO4 was high. This implies that NF is suitable to be used for the separation of Na2SO4 and NaCl from their binary solution, where Na2SO4 could be retained by the NF membrane and concentrated to high concentration while NaCl could pass through the membrane and might be diluted to low concentration with a diafiltration operation mode. Finally the selective separation of Na2SO4 and NaCl by NF diafiltration was simulated for the binary salt solution containing 23.4 g L−1 NaCl and 8.76 g L−1 Na2SO4. A highly concentrated solution of Na2SO4 (71.74 g L−1) and a relatively pure solution of NaCl (20.79 g L−1) were obtained, which favored the post-treatment of high saline wastewater for inorganic salts recycling.