Sorption of two naphthoic acids to goethite surface under flow through conditions.

While the transport of low molecular weight organic acids was widely investigated, little is known about the mobility of the carboxylated aromatic compounds containing double rings in natural porous media. This study combines macroscopic (batch and column), microscopic (vibration spectroscopy), and surface complexation modeling to evaluate the mobility of two PAH degradation products: naphthoic acid (1-naphthoic acid (NA) and 1-hydroxy-2-naphthoic acid (HNA)), in porous media consisting of goethite-coated sand. The loss of ligands from aqueous solution was attributed to (1) a hydrogen-bonded surface complex present over the entire 3-10 pH range as well as protonated (2) surface and (3) bulk precipitates below pH 5. Mobility in column experiments was strongly affected by ligand functionality. Adsorption breakthrough predictions that make use of surface complexation parameters accurately predicted NA mobility. Those for HNA however predicted much less adsorption reactions than in the batch sorption experiments. Additional breakthrough experiments and test calculations confirmed that these differences were not related to sorption kinetics. HNA adsorption breakthrough data could only be predicted by lowering intrinsic complexation constant of the formation of hydrogen-bonded species, thereby suggesting modifications of the diffuse layer properties under flow conditions. These findings have strong implications in the assessment and prediction of contaminant transport and environmental remediation.