The sorption (retention) of metals including alkali (e.g., K), alkaline earth (e.g., Ca), transition (e.g., Cd and Ni), and oxyanions (e.g., phosphate, arsenate, and selenate) on soil mineral and organic constituents is one of the most important processes in controlling the fate, transport, and bioavailability of metals and oxyanions in soil and water environments. This review discusses various aspects of metal and oxyanion sorption on clay minerals and metal oxides and hydroxides, referred henceforth as metal-(oxyhydr) oxides, including: the role of surface functional groups; types of surface complexes and products; macroscopic and molecular scale assessments of sorption; surface precipitation; and the kinetics of sorption processes.
Adsorption, surface precipitation, and polymerization are all examples of sorption, a general term that is used when the retention mechanism at a surface is unknown. There are various metal sorption mechanisms that could occur at soil mineral surfaces involving both physical and chemical processes (Figure 1).
Adsorption can be defined as the accumulation of a substance or material at an interface between the solid surface and the bathing solution. Adsorption can include the removal of solute (a substance dissolved in a solvent) molecules from the solution, solvent (continuous phase of a solution, in which the solute is dissolved) from the solid surface, and attachment of the solute molecule to the surface. Adsorption does not include surface precipitation (the formation of a three dimensional phase product on a surface), or polymerization (formation of small multinuclear inorganic species such as dimers or trimers) processes It would be useful before proceeding any further to define a number of terms pertaining to retention (adsorption/sorption) of metal ions. The adsorbate is the material that accumulates at an interface, the solid surface on which the adsorbate accumulates is referred to as the adsorbent, and the ion in solution that has the potential of being adsorbed is the adsorptive. If the general term sorption is used, the material that accumulates at the surface, the solid surface, and the ion in solution that can be sorbed are referred to as sorbate, sorbent, and sorptive, respectively (1, 2).
The most important adsorbents/sorbents for metals and oxyanions in natural systems such as soils and sediments are clay minerals and metal-(oxyhydr) oxides (Table 1). These sorbents exhibit significant surface area and surface charge that play a pivotal role in ion sorption. The surface charge can be negative and invariant with pH in the case of constant charge clay minerals such as montmorillonite and vermiculite. The constant charge results from ionic substitution in the clay structure (in the past referred to as isomorphic substitution). The surface charge can also be variable becoming more negative with increased pH as surface functional groups (see discussion below) on clay minerals and metal-(oxyhydr) oxides deprotonate and more positive as pH decreases as surface functional groups protonate. Clay minerals such as kaolinite and Al- and Fe-oxides are considered variable charge minerals.
Adsorption is one of the most important chemical processes in soils. It determines the quantity of plant nutrients, metals, oxyanions, pesticides, and other organic chemicals that are retained on soil surfaces and therefore is one of the primary processes that affects transport of nutrients and contaminants in soils and sediments. Adsorption also affects the electrostatic properties, e.g., coagulation and settling, of suspended particles and colloids. Both physical and chemical forces are involved in adsorption of solutes from solution. Physical forces include van der Waals forces (e.g., partitioning) and electrostatic outer-sphere complexes (e.g., ion exchange). Chemical forces result from short-range interactions that include inner-sphere complexation that involves a ligand exchange mechanism, covalent bonding, and hydrogen bonding (1, 2).