Study of polyethyleneimine coating on membrane permselectivity and desalination performance during pilot-scale electrodialysis of reverse osmosis concentrate

A two-electrical stage and four-hydraulic stage pilot-scale electrodialysis system was used to investigate desalination performance and ion permselectivity of Ionics monovalent permselective membranes (CR671/AR112B) and normal grade membranes (CR67/AR204) (Suez Water Technologies & Solutions) to treat reverse osmosis (RO) concentrate in a brackish groundwater desalination plant. Monovalent permselective cation-exchange membrane CR671 was manufactured by surface modification of standard normal grade membrane CR67 with highly-branched polyethyleneimine coating. The covalently bonded polyethyleneimine layer on membrane surface was characterized by methylene blue dye test, Fourier transform infrared spectroscopy with attenuated total reflection (ATR-FTIR), and electrochemical impedance spectroscopy (EIS). The pilot-scale testing demonstrated the monovalent permselective membranes achieved the same desalting efficiency as the normal grade membranes under the same current density. The energy efficiency in terms of normalized salt removal decreased with the increasing current density (i.e., desalination level) but increased with feed water salt concentration. Polyethyleneimine coating on the CR671 membrane enhanced the selective transport of monovalent cations over divalent cations while having negligible impact on the membrane electrical resistance. The total stack resistance of electrodialysis was insensitive to concentration changes in the diluate and concentrate chambers during desalination of RO concentrate. The electrodialysis system achieved additional 55% of water recovery by treating RO concentrate, enhancing the overall water recovery from 82.5% of the primary RO to 92.1%. Scaling on the ion-exchange membranes and electrodes was effectively mitigated by addition of antiscalant and pH adjustment in concentrate and electrode rinsing streams. Electrodialysis was demonstrated a viable technology to modify product water quality and improve water recovery for desalination concentrate treatment.