Separation of ions with equivalent and similar molecular weights by nanofiltration: Sodium chloride and sodium acetate as an example

Wastewater containing equivalent and similar molecular weights (similar ions) generated from the production process have great recovery value. But effective separation is tough because of their similarities in chemical and physical properties. This work aims at comparing the potential for the separation of sodium acetate (NaAc) and sodium chloride (NaCl), which typically exists in pigment wastewater, through nanofiltration (NF) or reverse osmosis (RO) process. The effects of operational conditions such as pH, pressure, temperature, and feed concentration on the rejection of solutes for five commercial membranes were investigated. The DL membrane exhibited a higher separation factor than other NF and RO membranes. Under the optimum condition at 10 bar, pH = 10, and 25 °C, the DL membrane showed the best performance with the acetate rejection as high as 87.14% and the chloride rejection at only 0.98%. Other than common steric hindrance and Donnan exclusion, a synergic mechanism involving dehydration phenomenon and the apparent activation energy was proposed to explain the separation behavior of the similar ions. This work may provide new strategies to recycle valuable resources from wastewater or other complex mediums.