Tailored divalent-monovalent selectivity of nanofiltration and reverse osmosis membranes through controlled oxidation

The implementation of membrane technologies, especially nanofiltration (NF) and reverse osmosis (RO), has proven to be highly effective in water treatment applications where ion rejection is required. Extensive research has been made to demonstrate membranes with high water-solute selectivity. However, the rising need for sustainable and economically efficient processes has led to an increasing interest in solute-solute selectivity for targeted and precise separation applications such as resource recovery and extraction. Expanding beyond the traditional limitations of the permeability-selectivity trade-off contributes to the broader goal of selective separation. In this study, utilizing ultra-filtered treated wastewater, pristine and oxidized NF and RO membrane coupons were used to study the influence of oxidation through chlorination on solute-solute selectivity. The cation and anions monovalent/divalent separation factor initially increased with increasing the chlorine dose for both NF and RO membranes and then decreased when the chlorine dose increased beyond 8 K ppm-h. For monovalent-divalent ions/organics, the separation factor remained relatively consistent for both membranes. Interestingly, both membranes maintained high rejection rates for bulk organics and trace organics up to 10 K ppm-h chlorine dose. The results highlight the opportunities of controlled oxidation for tailoring the solute-solute selectivity.