Co-binding of pharmaceutical compounds at mineral surfaces: mechanistic modeling of binding and co-binding of nalidixic acid and niflumic acid at goethite surfaces

Abstract : Although emerging contaminants rarely exist individually in environmental contaminated systems, only limited information on their adsorption mechanisms in multi-components 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 (-FeOOH) surfaces. Our model adequately predicts co-binding 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, salinity as well as NA and NFA loadings. These findings have strong implications in the assessment and prediction of contaminant fate in multi-component contaminated systems by invoking a non-traditional form of ligand-ligand interaction in this field of study.