Surface room-temperature treatment of polyamide-based reverse osmosis membrane by diazotization reagent: Perm-selectivity regulation and mechanism

The use of acidic nitrite as diazotization reagent has been adopted previously for performance improvement of finished polyamide-based reverse osmosis membrane through post-treatment under a low temperature. However, the mechanism of diazotization reagent treatment on the membrane intrinsic permeability coefficients for both solvent and solute is still not clear and low-temperature treatment is energy intensive. This work focused on the mechanism and the efficient regulation of perm-selectivity by surface treatment of polyamide-based reverse osmosis membrane with diazotization reagent under room temperature. Polyamide based membranes were treated with diazotization reagent under temperatures between 5.0 and 25.0 °C. Membrane physico-chemical property was characterized via instrumental analysis and membrane intrinsic permeation coefficient was evaluated via cross-flow permeation test. It was found that the diazotization reagent treatments under 5.0, 15.0 and 25.0 °C could effectively enhance membrane intrinsic permeability coefficient for water (A) by 14.1, 17.7 and 21.7%, lower membrane intrinsic permeability coefficient for NaCl (B) by 60.1, 50.0 and 40.3%, and improve membrane perm-selectivity (A/B) from 5.55 bar-1 to 15.76, 13.00 and 11.31 bar-1, respectively. Mechanism exploration revealed that both the diazotization reaction occurred with end amino groups and the N-nitrosylation reaction occurred with amide bonds had a great influence on the membrane performance. Diazotization reagent treatment under different pH demonstrated that the perm-selectivity of polyamide-based membrane could be finely regulated by controlling an optimized pH of diazotization reagent at 1.5. The performance regulation efficiency of diazotization reagent treatment was also found to be related with the initial performance of the treated membrane. Under the same condition, salt rejections of 96.5% and 98.0% could be improved up to 98.3% and 98.8%, respectively. This work not only explains the mechanism of diazotization reagent treatment on regulation of membrane perm-selectivity, but also provides a room-temperature modification method suitable for performance enhancement of newly produced membrane and performance restoration of used membrane.