SURFACE COMPLEXATION MODELING OF ARSENIC ADSORPTION TO MINERAL SURFACES

Arsenic is toxic to both plants and animals, including humans, and can accumulate in agricultural soils. Adsorption reactions on soil mineral surfaces potentially attenuate elevated inorganic soil solution As concentrations reducing contamination to groundwater. The two inorganic redox states of As, arsenite As(III) and arsenate As(V), differ both in their toxicity and adsorption behavior. Arsenate and arsenite adsorption behavior as a function of solution pH has been investigated on aluminum and iron oxides, clay minerals, and soils. Arsenic adsorption has been studied both in single ion systems and in the presence of a potentially competing anion species. Surface complexation models are chemical models that define specific chemical reactions, equilibrium constant expressions, mass balances, and charge balances. One of the advantages of these models over empirical adsorption isotherm equations is their ability to describe adsorption as a function of solution pH. The application of two of these models, the constant capacitance model and the triple layer model, to describe As adsorption behavior will be presented. Experimental and numerical methods of determining values for model adjustable parameters will be described. The ability of the models to predict binary competitive systems using adsorption constants determined in single ion systems will be evaluated and alternative system specific approaches to describe competitive systems presented. Simplifying assumptions and approximations are necessary to extend the model to As adsorption on natural materials, especially soils. The component additivity (CA) approach tries to predict adsorption of a complex mixture with surface complexation constants determined for the individual mineral phases present. In the generalized composite (GC) approach all mineral phases contributing to adsorption are represented by one general type of surface functional group. Examples of both modeling approaches to describing As adsorption by soils will be presented.