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

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.