Sorption and Transport of Salicylate in a Porous Heterogeneous Medium of Silica Quartz and Goethite

Among transport studies of solutes in porous media, few works have combined microscopic speciation with macroscopic-scale investigations to describe the impact of antecedent sorbed silica on the transport of organic ligands in porous heterogeneous media. In this study, the sorption of salicylate (SA) to goethite-coated sand (GCS) was investigated under static and dynamic conditions by combining batch experiments and column tests with infrared spectroscopy. On the basis of infrared spectra, the salicylate adsorption was described by one type of iron site and a mononuclear bidentate surface complex. The intrinsic complexation constant deduced from batch modeling was successfully applied to estimate the sorbed amount under flow through conditions at various water velocities (0.038-0.768 cm/min). The shape of the breakthrough curve of SA was characterized by two fronts in both SA concentration and pH. This behavior could be likely explained by the mobilization of initially adsorbed silica from goethite surface upon SA sorption. The SA breakthrough can be interpreted as retention of SA on available surface sites up to their saturation and then on additional reactive sites, becoming free due to silicate desorption. This present work demonstrated the importance of sorbed silicate on Fe-oxides in the prediction of reactive transport of organic species on natural surfaces.