Molecular insights on the adsorption of some pharmaceutical residues from wastewater on kaolinite surfaces

Non-negligible concentrations of chemicals products are often contained in worldwide wastewaters. Although they can be eco-toxic, they are generally poorly removed in water treatment plants. Their accumulation in the environment has become a crucial problem since they present a serious risk to the flora and the fauna. An important fraction of these molecules are pharmaceuticals, including ibuprofen, paracetamol, aspirin, diclofenac -anti-inflammatory drugs, carbamazepine, and diazepam - anti epileptic medications. Their adsorption onto clay minerals and particularly on kaolinite offers an efficient and cheap perspective for wastewaters depollution. In this work, we investigated the adsorption mechanisms and the eventual competition of these molecules on kaolinite by means of density functional theory calculations. We show that these compounds adsorb more favorably than water on the kaolinite (001) basal aluminol-terminated surface mainly through hydrogen, 7C, and London interactions. The most strongly adsorbed molecule on kaolinite is paracetamol, with an adsorption energy of -159.4 kJ.mol(-1), while the less strongly adsorbed is diazepam, with an adsorption energy of -96.8 kJ. mol(-1). This latter is only adsorbed by means of dispersion interactions. Also, we show that the adsorption of these molecules on kaolinite presents no risk of generating toxic by-products and that, by heating the clay to 360 K, all these compounds can be desorbed, which demonstrates the regenerability of the material. Hence, kaolinite is an interesting and cheap candidate for pharmaceutical compounds removal from wastewaters. In addition, we unravel the mechanisms of adsorption of these molecules, offering a better understanding of the chemicals adsorption on a common clay mineral.