Interactions of Pharmaceutical Compounds with Dissolved Organic Matter (DOM).

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 with six structural fractions of DOM.

Binding of naproxen and carbamazepine by the hydrophobic acid fraction (HoA) exhibited strong pH-dependence. With both pharmaceuticals, the highest binding coefficients (K_DOC) were at pH 4. With the hydrophobic neutral fraction (HoN), pH affected only naproxen binding (decreasing with increasing pH). Among the hydrophilic DOM fractions, the hydrophilic acid fraction (HiA) 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 (HiB) contains positively charged functional groups. Therefore with increasing ionization of naproxen (with increasing pH), K_DOC to this fraction increased. The hydrophilic neutral fraction (HiN) 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 binding 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. This study also demonstrates that the binding abilities of the DOM fractions can also significantly affect the mobility of pharmaceutical compounds in soils influenced by intensive irrigation with reclaimed wastewater or amended with biosolids.