Chapter 11. Sorption III: Sorption Processes Involving Inorganic Surfaces

Summary

Adsorption to inorganic surfaces may be important in numerous environmental situations including air-soil exchanges of nonionic compounds, water-aluminosilicate exchanges of explosives like TNT, and water-mineral transfers of charged organic chemicals like ionic surfactants. In the case of air-surface exchanges, competition with water vapor is very important for polar surfaces. Hence multiparameter models of air-surface partition coefficients incorporate both solid-specific descriptors and the evolving chemical nature of those surfaces as they range from completely dry to completely water-wetted. For fully water-wet polar solids, organic compounds capable of specific interactions, such as nitroaromatic substances that participate in electron-donor:acceptor (EDA) interactions with siloxanes, must include these in their overall solid-water distribution coefficients. Also, because water-wet minerals exhibit surface charging, ionic organic sorbates may sorb via ion exchange. This near-surface association depends on the surface charge density, the other competing ions present in solution, and the hydrophobicity of the nonionic part of the organic sorbate's structure. Finally, specific surface bonding of certain organic ligands, especially ones that are bidentate, causes additional adsorption of such chemicals beyond ion exchange. Complexation data can be used with surface site density data to estimate the extent of formation of these surface-bound species.