The roles of pH and draw solute on forward osmosis process treating aqueous naphthenic acids

The effects of pH and draw solutes on the rejection of three naphthenic acid (NA) model compounds, namely cyclohexane carboxylic acid (CHA), 1-adamantaneacetic acid (AAA) and the refined Merichem mixture of NAs, by forward osmosis (FO) were studied. The rejection behavior of CHA and AAA were pH-depended (from pH 3 to 9) which further suggested that electrostatic repulsion was the dominant rejection mechanism. The rejection efficiency of Merichem NAs was maintained above 95%, which was not affected by the pH change (from 6 to 9). The water flux decline when Merichem NAs was used as feed solution was partially attributed to membrane surface fouling. Membrane surface contact angle changed with the properties of NAs in feed solution, which might suggest that the exposure of NA model compounds altered the membrane characteristics. Four inorganic salts-sodium chloride (NaCl), ammonia chloride (NH4Cl), sodium sulphate (Na2SO4), and calcium chloride (CaCl2)-were introduced as the draw salts and no significant difference was found between these draw solutes regarding the CHA rejection at pH = 9. The decreased reverse salt flux along with the water flux decline indicated that using CaCl2 as the draw solution caused membrane surface precipitation.