Sorption of the pharmaceuticals carbamazepine and naproxen to dissolved organic matter: Role of structural fractions

Pharmaceutical compounds and dissolved organic matter (DOM) are co-introduced into the environment by irrigation with reclaimed wastewater and/or application of biosolids. In this study, we evaluate the role and mechanism of interaction of the pharmaceuticals naproxen and carbamazepine with structural fractions of biosolids-derived DOM. Sorption interactions were estimated from dialysis-bag experiments at different pHs. Sorption of naproxen and carbamazepine by the hydrophobic acid fraction exhibited strong pH-dependence. With both pharmaceuticals, the highest sorption coefficients (K_DOC) were at pH 4. With the hydrophobic neutral fraction, pH affected only naproxen sorption (decreasing with increasing pH). Among the hydrophilic DOM fractions, the hydrophilic acid fraction exhibited the highest K_DOC value for carbamazepine, probably due to their bipolar character. In the hydrophilic acid fraction-naproxen system, significant anionic repulsion was observed with increasing pH. The hydrophilic base fraction contains positively charged functional groups. Therefore with increasing ionization of naproxen (with increasing pH), K_DOC to this fraction increased. The hydrophilic neutral fraction exhibited the lowest K_DOC with both studied pharmaceuticals. The K_DOC value of carbamazepine with the bulk DOM sample was higher than the calculated K_DOC value based on sorption by the individual isolated fractions. The opposite trend was observed with naproxen at pH 8: the calculated K_DOC value was higher than the value obtained for the bulk DOM. These results demonstrate that DOM fractions interact with each other and do not act as separate sorption domains.