Sorption of Phenanthrene by Humic Acid-Coated Nanosized TiO2 and ZnO

Phenanthrene sorption by nano-TiO2 and nano-ZnO particles was enhanced significantly by coated humic acids (HAS), implying that additional toxicity can be potentially given to these nano-oxides by adsorbed HOCs once released to the environment. Phenanthrene isotherms of adsorbed HA on nano-TiO2 and nano-ZnO were more nonlinear than that of their respective bulk HA. Both HA conformation changes and fractionation were observed upon HA adsorption on nano-TiO2 and nano-ZnO, which further affected phenanthrene sorption. Nano-TiO2 and nano-ZnO interacted with different functional groups of HA (i.e., phenolic OH with nano-TiO2, while COOH with nano-ZnO), leading to different conformations of adsorbed HA. Interaction of HA phenolic OH with nano-TiO2 increased the pi-polarity/polarizability of adsorbed HA and, consequently, its phenanthrene adsorption affinity and isotherm nonlinearity. Interactions of COOH groups on HA aromatic rings with nano-ZnO would also increase the pi-polarity/polarizability of adsorbed HA and its phenanthrene adsorption affinity, whereas interactions of COOH groups on HA aliphatic chains with nano-ZnO would make the adsorbed HA be in a more condensed state with lower partitioning affinity. Increase in adsorption and decrease in partitioning were responsible for the more nonlinear phenanthrene isotherms of adsorbed HA than bulk HA.