Desalination 168 (2004) 383–390 0011-9164/04/$– See front matter © 2004 Elsevier B.V. All rights reserved Presented at the EuroMed 2004 conference on Desalination Strategies in South Mediterranean Countries: Cooperation between Mediterranean Countries of Europe and the Southern Rim of the Mediterranean. Sponsored by the European Desalination Society and Office National de l’Eau Potable, Marrakech, Morocco, 30 May–2 June, 2004. *Corresponding author. Effect of salt type on mass transfer in reverse osmosis thin film composite membranes M. Khayet*, J.I. Mengual Department of Applied Physics I, Faculty of Physics, University Complutense of Madrid Av. Complutense s/n, 28040, Madrid, Spain Tel./Fax +34 (91) 3945191; email: khayetm@fis.ucm.es Received 30 January 2003; accepted 12 February 2004 Abstract Reverse osmosis separation of inorganic salts in aqueous solutions has been studied using polyamide thin film composite membrane, in spiral wound configuration. Various inorganic salts involving monovalent and divalent ions have been studied. The solute transport through the membrane and the mass transfer coefficient at the high pressure feed side of the spiral wound module were determined for each type of salt. The mass transfer coefficient of each inorganic salt was predicted from the experimental data for the reverse osmosis separation of sodium chloride as reference and the mass transfer empirical correlations. The values obtained were compared with the experimental ones. The free energy parameters for monovalent cations, monovalent anions and divalent cations have been calculated. These parameters were used to predict the reverse osmosis performance of inorganic salts in spiral wound reverse osmosis module. It was observed that the free energy values of the cations are negative while those of the anions are positive, suggesting that the surface of the membrane used is positively charged. Keywords: Reverse osmosis; Inorganic salts; Desalination; Thin film composite membranes; Spiral wound module 1. Introduction Aromatic polyamide membranes constitute one of the successful membranes developed for desalination, while spiral wound modules are widely used for commercial applications, ranging from reverse osmosis (RO) to ultrafiltration (UF). Therefore, the knowledge of the transport charac- teristics of such membranes and the predictability