Cobinding of Pharmaceutical Compounds at Mineral Surfaces: Mechanistic Modeling of Binding and Cobinding of Nalidixic Acid and Niflumic Acid at Goethite Surfaces.

Although emerging contaminants rarely exist individually in environmental contaminated systems, only limited information on their adsorption mechanisms in multicomponent solutions is currently available. To address this shortcoming, this work examines for the first time the accuracy of a surface complexation model in predicting the cooperative adsorption of nalidixic acid (NA) and niflumic acid (NFA) at goethite (α-FeOOH) surfaces. Our model adequately predicts cobinding of an outer-sphere (OS) complex of NFA onto NA bound to goethite through metal-bonded (MB), hydrogen-bonded (HB), or OS complexes. More positive charge is introduced in the system via sodium interactions in order to describe the NFA adsorption at high NaCl concentrations in both single and binary systems. Our model confidently predicts multilayers of NA on goethite as well as NFA binding on goethite-bound NA over a large range of pH and salinity values as well as NA and NFA loadings. These findings have strong implications in the assessment and prediction of contaminant fate in multicomponent contaminated systems by invoking a nontraditional form of ligand-ligand interaction in this field of study.