Chemisorption and Precipitation Reactions
Chemisorption reactions involving formation of mononuclear chemical bond between solutes in the solid solution and reactive surfaces in solid matrix are referred to as surface complexation reactions and are typically categorized by the relative binding strength of interaction between the adsorbate (species in solution) and the adsorbent (soil solid). One should note that the conceptual underpinning that guides our historical and contemporary understanding of chemisorption/surface complexation reactions is the notion that reactive surface sites can be viewed as a special type of ligand that coordinates to cations or exchanges with anions from solution (Schindler et al., 1976; Sigg and Stumm, 1981; Sposito, 1998, 1999). While our molecularlevel understanding of the physicochemical forces that govern the characteristics of these surface reactions has improved over the years, the underlying conceptual basis remains technically sound. ¤e driving force for molecules or ions in soil solution to form surface complexes with sites of adsorption in the soil solid matrix is governed by the physical and chemical properties of the adsorbate, sorption sites, and the soil solution (i.e., “solute,” “sorbent,” and “solvent” in the more general terminology in Stumm ). As an example, the probability that a cation will form an inner-sphere versus an outer-sphere surface complex will, in part, be governed by the relative free-energy change between the cation maintaining a water molecule versus a surface hydroxyl site within its immediate coordination sphere. Cations for which coordination with a water molecule is more energetically favorable will tend to favor outer-sphere surface complexes.