Association of humic acid with metal (hydr)oxide-coated sands at solid-water interfaces.

Mineral-bound humic acid (HA) can significantly modify the physicochemical properties of the mineral surfaces and vice versa, thereby influencing the fate and transport of organic pollutants in the subsurface. The effect of various mineral surfaces on the adsorption-desorption of dissolved bulk, terrestrial HA was evaluated using three model sorbents [uncoated, alpha-FeO(OH)-coated, and Al2O3-coated sands] at two equilibrium pH values. The results of SEM/EDS and XPS analyses revealed relatively uniform and stable metal (hydr)oxide coatings on quartz surface and the presence of the HA coating. Strong hysteresis effects were observed for both metal (hydr)oxide-coated sands whereas a weaker hysteresis effect was observed for uncoated sand, suggesting that the adsorption-desorption of HA to model sorbents is dependent on the affinity of chemical interactions between the HA and surface composition of model sorbents. Adsorption of HA molecules onto metal (hydr)oxide-coated sands can be attributed to ligand exchange for lower molecular weight (MW) HA fractions and hydrophobic interaction for higher MW HA fractions, illustrating that both kinetic and fractional adsorption-desorption of HA subcomponents are important considerations.