| The main objective of this dissertation was to investigate the demineralization of multicomponent aqueous solutions of inorganic electrolytes by two thin-film composite reverse osmosis membranes of the polyamide family: TFC{dollar}sp{lcub}rm TM{rcub}{dollar} (UOP Fluid Systems, San Diego, CA) and FT-30{dollar}sp{lcub}rm TM{rcub}{dollar} (FilmTec Corporation, Minneapolis, MN). The tests were performed with small portions of membrane sheets as well as spiral-wound elements.; The solutions tested were representative of many natural saline waters and wastewaters after typical RO pretreatments. Of particular interest were agricultural irrigation return waters such as those in the San Joaquin Valley, California. The solutions investigated consisted of mixtures of sodium, magnesium, chloride and sulfate ions together with trace amounts of nitrate, boric acid, selenate and biselenite. The feed solution characteristics and operating conditions utilized were: 25{dollar}spcirc{dollar}C; pH = 5.5; osmotic pressures in the range of 200 to 600 psi; and hydraulic pressures in the range of 400 to 1,000 psi.; Based on the results of this work it was evident that the permeation of the solutes through the membranes tested resulted from two distinct types of transport mechanisms: (1) the permeation of neutralized ion-pairs (i.e., cation-anion neutral complexes) formed at the upstream interface of the membrane; and (2) the ion-exchange-like permeation of counter-ions which move from radical site to radical site within the membranes.; While the rejection characteristics of the two membranes were comparable, each membrane proved to be superior for some types of trace components in the feed water. For example, the TFC{dollar}sp{lcub}rm TM{rcub}{dollar} membrane commonly gave a better rejection of the nitrate ion ({dollar}>{dollar}99 percent), and the FT-30{dollar}sp{lcub}rm TM{rcub}{dollar} membrane performed better with biselenite ({dollar}>{dollar}99.7 percent rejection) and boric acid ({dollar}>{dollar}82 percent rejection). Both membranes removed selenate to about the same degree ({dollar}>{dollar}99.94 percent rejection). A model is proposed which may be used to predict the removals of the electrolytes investigated over a wide range of solute composition. Finally, it was observed that the rejections obtained from the model were verified in spiral-wound elements containing the two types of commercial RO membranes. |
