Reverse Osmosis of Single and Mixed Electrolytes with Charged Membranes: Experiment and Analysis

Reverse osmosis experiments using single electrolytes, mixed electrolytes having common cations (sodium chloride/sodium sulfate and sodium hydroxide/sodium sulfate) and mixed electrolytes having common anions (sodium chloride/magnesium chloride and hydrochloric acid/magnesium chloride) were carried out with negatively charged membranes. Rejection occurred only due to the charge effect and was dependent upon volume flux, the ratio of the membrane charge density to feed concentration and the mole fraction in mixtures. Mono- and divalent anions in mixed electrolytes having common cations were found to be separated effectively. In particular, monovalent anion showed negative rejection. Mono- and divalent cations could not be separated as efficiently as mono- and divalent anions.Experimental results for single electrolyte solutions were well explained by applying the extended Nernst–Planck equation, and two transport parameters (effective charge density and structural parameters) were obtained. Rejection of ions in mixtures was well predicted by the extended Nernst–Planck equation with the transport parameters obtained from the single-electrolyte experiments.