Adsorption of substituted nitrobenzenes and nitrophenols to mineral surfaces

Abstract

The adsorption of a large number of nitroaromatic compounds (NACs) to mineral surfaces, particularly to homoionic kaolinites, has been investigated. The results demonstrate that NACs may adsorb specifically and reversibly to the negatively charged siloxane surface of kaolinite. The strength of adsorption depends on the structure of the compound (i.e., type of substituent) and on the type of cation adsorbed to the siloxane surface. In the presence of strongly hydrated cations (e.g., Li⁺, Na⁺, Mg²⁺, Ca²⁺, Al³⁺), no significant specific adsorption of NACs is observed, while for more weakly hydrated cations (e.g., NH₄⁺, K⁺, Rb⁺, Cs⁺), the distribution coefficient, <I>K</I>_d, of a given NAC, increases with decreasing free energy of hydration of the cation. We propose that electron donor-acceptor (EDA) complexes between surface oxygens of the siloxane surface and a given NAC are responsible for the observed specific adsorption. Some simple model calculations indicate that such EDA complexes may have a significant impact on the transport and the fate of NACs (and, possibly, of other organic pollutants exhibiting electron acceptor properties) in the subsurface environment.