Surface Reactions in Natural Aqueous Colloidal Systems

Abstract

Natural colloids are mixtures of inorganic and organic polymers resulting from weathering processes. These materials include layer-type aluminosilicates, hydrous oxides, and biodegraded organic matter. Regardless of their molecular structure, these polymeric constituents present reactive functional groups of two principal kinds at the colloid/aqueous solution interface: siloxane ditrigonal cavities and inorganic or organic hydroxy groups. The reactive proportion of each kind of functional group at a colloid surface depends on the extent of chemical weathering and the composition of the material cycling through the aqueous environment. – The principal mechanisms of reaction between surface functional groups and ions or molecules in aqueous solution, referred to generically as «adsorption», are (1) inner-sphere complexation, (2) outer-sphere complexation, and (3) association with a diffuse ion swarm. The role of these three mechanisms in diverse reactions, such as cation exchange, specific ion adsorption, and organic matter binding on colloids is discussed, as is the contribution these mechanisms make to surface charge development. The conceptual basis of surface charge variation for natural colloids is described in relation to the three adsorption mechanisms. – The formulation of the law of mass action for surface complexes on natural colloids involves stability constants analogous to those for aqueous complexes. The problem of determining the surface species activity coefficients in these stability constants is discussed along with recent activity coefficient models based on mean-field theory. Surface speciation models that incorporate both surface complexation and diffuse ion swarm association are reviewed and illustrated for adsorption phenomena in natural colloidal systems.