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, S_w , (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% ≤ f_oc ≤ 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., f_oc ) and organic compounds (e.g., S_w and K_ow ). 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., K_oc ) 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 S_w to low-surface-area metal (hydr)oxide- and HA-coated sands with low f_oc values. Thus, the predictive models for the sorption of organic compounds with relatively high S_w 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.

Keywords:

• Fractionation
• metal (hydr)oxide- and HA-coated sands
• mineral-bound humic acid
• mineral surfaces
• nonpolar neutral organic compounds
• sorption

Acknowledgments

The authors wish to acknowledge financial support provided by both U.S. Environmental Protection Agency (EPA)'s Science to Achieve Results (STAR) Program (STAR R-82935501–0) and Korea Institute of Construction Technology (KICT)'s Core Technology Program (2011–0024-10–1) for this study. The opinions expressed in this paper are solely those of the author and are not necessarily consistent with the policies or opinions of the EPA and KICT. The authors also thank Dr. Charles D. Shackelford (Colorado State Univ., USA) for his helpful comments in preparation of this paper.