Influence of hydration/dehydration on adsorbed molecules: Case of phthalate on goethite

The behavior of adsorbed species on metal (hydr)oxides under water unsaturated conditions has been seldom studied despite its major implications in several domains such as geochemistry and heterogeneous catalysis. In this study, chemical forms of phthalate adsorbed on iron hydroxide (goethite) were investigated by infrared spectroscopy upon hydration and dehydration of goethite particle aggregates. Experimental results indicated important changes with relative humidity (RH) on chemical forms of phthalates adsorbed on goethite. With increasing RH from the dried state, bidentate phthalate surface complex is partially desorbed to form a protonated monodentate phthalate complex by using protons from water molecules. This protonated monodentate phthalate is still kept adsorbed on the goethite surface but becomes deprotonated at high RH values by solvating in liquid-like water covering on the goethite surface. DFT calculations made on a simplified system support these adsorption models. These results indicate that further studies are needed on the behavior of adsorbate in water unsaturated systems including hydration/dehydration processes. [Display omitted]