Sorption of nonpolar neutral organic compounds to low-surface-area metal (hydr)oxide- and humic acid- coated model aquifer sands

Abstract

The roles of mineral-bound humic acid (HA) and mineral surfaces in the sorption of six nonpolar neutral organic compounds with relatively high aqueous solubility, Sw , (1,2,4-trichlorobenzene, 1,4-dichlorobenzene, chlorobenzene, m-xylene, toluene, and benzene) to low-surface-area (i.e., ≤ 1.2 m2/g) metal (hydr)oxide- and HA-coated sands with low organic carbon fractions (i.e., 0.006% ≤ foc ≤ 0.044%) were investigated using well-characterized mineral surfaces [i.e., α-FeO(OH)- or Al2O3- coated sands], terrestrial HA, and solutions with relatively constant pH and ionic strength. Sorption isotherms of all six compounds to low-surface-area metal (hydr)oxide-and HA-coated sands were practically linear (i.e., 0.898 ≤ n ≤ 1.06), and resulted from a combination of sorption to both mineral-bound HA and mineral surfaces, with the dominance of either contribution depending on the properties of the sorbents (e.g., foc ) and organic compounds (e.g., Sw and Kow ). Compared to HA-associated high-surface-area, pure metal (hydr)oxides or clay minerals illustrating that loading levels of HA significantly impacted sorption affinity (i.e., Koc ) and linearity (i.e., n) for particularly hydrophobic compounds (i.e., phenanthrene, anthracene and pyrene) due to the changes in fractionation, and structural and chemical properties of mineral-bound HA, the subsequent changes of sorption affinity and linearity appeared to be insignificant for the sorption of organic compounds with relatively high Sw to low-surface-area metal (hydr)oxide- and HA-coated sands with low foc values. Thus, the predictive models for the sorption of organic compounds with relatively high Sw to low-surface-area metal (hydr)oxide- and HA-coated sands may not be remarkably improved by incorporating the complex changes of sorption affinity and linearity caused by the changes in fractionation, and structural and chemical properties of mineral-bound HA, although the mineral surfaces apparently caused physical and chemical changes of HA, and vice versa during adsorption onto mineral surfaces.