Effect of operating parameters on the selectivity of nanofiltration phosphates transfer through a Nanomax-50 membrane

The present study deals with the retention by nanofiltration of synthetic solutions containing phosphate salts with a Nanomax-50 charged membrane. This work investigates the potential of the membrane for the selective retention of the phosphate anions H2PO4- and HPO42-. The first part of the study involves the characterization of the membrane used in filtration tests with aqueous solutions of neutral solutes (organic) and charged solutes (inorganic salts). In the second part, the effects of pressure, ionic strength and pH on the retention of phosphate anions are described. The results showed that the membrane is negatively charged at pH > 5 with a pore radius of about 0.45 nm and a hydraulic permeability around 24.6 10−12 m s−1 Pa−1. The values of the rejection rate of the phosphate anions are about 93% for HPO4- and 98% for HPO42-. The rejection rate of phosphate anions, particularly monovalent, depends on chemical parameters (feed concentrations, ionic strength and pH) and on transmembrane pressure. The experimental results were analyzed by the Spiegler-Kedem model and the transport parameters of phosphate anions: the reflection coefficient σ and solute permeability Ps were determined.