Intermediate reverse osmosis ultrafiltration (RO UF) membranes for concentration and desalting of low molecular weight organic solutes

The performance of a new class of selective reverse osmosis membranes is described. Separation of water-soluble organic ions of molecular weight lower then a few hundred daltons from NaCl is demonstrated with an observed negative rejection of the latter. A negative salt rejection can be interpreted in terms of the Spiegler-Kedem analysis of reverse osmosis by introducing the Donnan exclusion correction into the driving force term responsible for salt transport. In practice, the negative salt rejection effect can be usefull exploited in industrial desalting-concentration processes such as in the manufacturing processes of dyestuffs and chemical intermediates. The increase in concentration of the retained charged organics, which occurs during the concentration step, causes the salt rejection value to become even more negative and, thus, very effective desalination can be achieved without excessive diafiltration. If diafiltration is still necessary, the required washing volume is reduced in a proportion to the negative values of salt rejection. As a result, the processing time and product losses are reduced, thereby favorably affecting the economics of the concentration-purification processes.