Elucidating Mechanisms of Competitive Sorption at the Mineral/Water Interface

This chapter provides insights into the current state of knowledge on the factors that influence the competitive sorption of different ions on soil components and soils. In soil environments, different cations, anions, and molecules are present together, and then mutual interactions occur among them for sorption on soil components (phyllosilicates, humic substances, variable charge minerals, carbonate, and microorganisms), organo-mineral complexes, and soils. The influence of competitive sorption on ion partitioning between two ions received great attention, but studies on the relative competition for sorption sites onto soil components or soils among various ions are rather scant. In the past decades, many researchers have carried out molecular-scale studies to couple macroscopic sorption investigations with direct characterization by spectroscopic techniques. A wide array of models has been also used to describe chemical species structures and distribution in solution, at the solid solution interfaces, and in the solid phase. This chapter first gives information on the factors that affect the sorption of cations and anions on phyllosilicates, organic matter, variable charge minerals (mainly Al- and Fe-oxides), and microorganisms, when added alone, and their capacity to form outer- or inner-sphere complexes on the surfaces of the soil components. Then, attention is given to the influence of the nature and concentration of inorganic and organic anions on the sorption of metal cations. The capacity of inorganic and organic ligands to enhance or inhibit cation sorption is discussed. Sorption increase is due to the formation of type A (metal ion bonded to both the sorbent and the ligand) or type B complexes (in which the ligand is bonded to the sorbent between the surface and the metal) on the surfaces of the sorbents or the formation of surfaces precipitates or by electrostatic interactions, whereas sorption decrease of a cation may occur when a ligand blocks common surface sites. The competition of two or more metal cations and the effect of an anion on their competition in sorption have also been discussed. The sorption of an anion inhibits the sorption of the competing anion(s) because it reduces the number of sites available on the sorbent (mainly variable charge minerals) and through changes in the electric potential. The competition in sorption between two or more ligands is affected by many factors such as pH, the nature of the sorbent, the affinity of each anion for the surfaces of the sorbent, the relative concentration of each anion, the surface coverage of the ligands, the sequence of addition of the competing anions, and the residence time. Particular attention has been given to the competitive sorption involving sulfate and phosphate as affected by other inorganic and organic ligands. Competitive sorption involving other ligands (e.g. carbonate, silicate, arsenate, arsenite, selenite, selenate, chromate, humic (HA) and fulvic acids (FAs), and selected low-molecular mass organic ligands (LMMOLs) has been also considered.