DIVISION S-2—SOIL CHEMISTRY Formulating the Charge-distribution Multisite Surface Complexation Model Using FITEQL

ality and crystallography of mineral surfaces involved, and the total concentration of the metal or oxyanion Sorptive interactions at the solid–water interface strongly influence present. However, most nutrient oxyanions and trace the bioavailability of many important nutrient oxyanions and trace metals occur at relatively low concentrations in natural contaminant metals in both natural and engineered settings. Recently, the charge-distribution multisite complexation (CD-MUSIC) model environments (micromolar or less) precluding precipitahas been developed to model ion adsorption behavior on variabletion. Therefore, adsorption processes ultimately control charge minerals. Although this model shows great promise, its use has the phase distribution and potential bioavailability of been limited by lack of incorporation into commonly used computer these ions (McBride, 1994). codes. In this study formulation of the CD-MUSIC model in the surBecause trace metals including Fe, Cu, Zn, and oxyface complexation modeling program FITEQL 4.0 is described, and anions such as Pi have central functions in many biodemonstrated using Cu2 and orthophosphate (Pi)-goethite adsorpchemical pathways, and occur at relatively low concention data. Mass-action and mass-balance expressions for Cu2 and Pi trations in pristine environments, organisms generally adsorption on goethite were developed using a combination of monotake up inorganic nutrient ions using transport systems dentate and bidentate surface species. Pauling’s rules were used to despecific for a particular chemical form. For example, termine the charge of surface adsorption sites and adsorption site density (nm 2) was calculated from crystallographic considerations. both microorganisms and plants generally take-up P and Electrostatic component coefficients in the mass-balance expressions Cu as free ions from solution—Pi (Frossard et al., 1995) were adjusted to reflect the actual charge of goethite adsorption sites, and Cu2 (Deighton and Goodman, 1995). However, Cu, thereby satisfying both the local charge balance for adsorbed species Zn, and other trace metals may become toxic to microand the global charge balance of the system as a whole. FITEQL 4.0 organisms, phytoplankton, and plants at concentrations was used to determine the best-fit equilibrium constants for the Cu2 as low as 10 9 M, with toxicity and free metal ion activity and Pi surface adsorption mass-action expressions, and the associated well correlated (Deighton and Goodman, 1995). Therespeciation of adsorbed ions. The speciation of adsorbed Cu2 ions was fore, determining the distribution and speciation of metdominated by a single monodentate surface species; whereas, two monoals and oxyanions between adsorbed and solution phases dentate and one bidentate surface species were required to adequately is essential for evaluating their potential bioavailability describe Pi adsorption. Formulating the CD-MUSIC model as outlined here provides a thermodynamically, electrostatically, and crystalloin the environment. graphically consistent approach for solving surface adsorption equilibMechanistic surface complexation models have been rium problems with FITEQL. widely used to estimate the distribution of oxyanions and to a lesser extent metals between sorbed and solution phases, as well as their ionic speciation within these S interactions at the solid–water interface phases. The underlying physics and chemistry of adsorpstrongly influence the bioavailability of many imtion phenomena are described in these models using portant nutrient oxyanions and trace contaminant metequilibrium adsorption reactions which place adsorbed als in both natural and engineered settings. Because of ions in specific electrostatic planes within an electrical the ubiquitous distribution and high specific surface area double layer (EDL) adjacent to the mineral surface. of colloidal minerals in the biosphere, mineral surfaces, The constant capacitance, diffuse double-layer (DDL), particularly metal (hydr)oxides, often act as reservoirs basic Stern, and triple layer models have been extenfor metals and oxyanions. Metals and oxyanions are sively applied to describe surface adsorption on mineral concentrated on mineral surfaces through precipitation surfaces (Westall and Hohl, 1980; Hayes et al., 1991; and both specific and nonspecific adsorption processes. Goldberg, 1992). These models typically require several The formation of surface microprecipitates as opposed user specified adjustable fitting parameters including to adsorbed species is controlled by a complex array of capacitance of the EDL and types of reactive surface geochemical factors including pH, ionic composition sites and their densities, and rely on an arbitrarily deand strength of embathing solution, chemical functionfined amphoteric description of ionizable surface functional groups. These shortcomings have recently been addressed through development of the CD-MUSIC Christopher J. Tadanier, Dep. of Geological Sciences, Virginia Polytechnic Institute and State Univ., Blacksburg, VA 24061; Matthew J. Abbreviations: SAI, specifically absorbed ion; CD, charge distribuEick, Dep. of Crop and Soil Environmental Sciences, Virginia Polytion; CD-Music, CD multisite complexation; DDL, diffuse-double technic Institute and State Univ., Blacksburg, VA 24061. Received layer; DL, diffuse layer; EDL, electrical double layer; NAI, nonspecifi14 May 2001 *Corresponding author ([email protected]). cally absorbed ion; Pi, orthophosphate; PZC, point-of-zero charge; TLM, triple layer model. Published in Soil Sci. Soc. Am. J. 66:1505–1517 (2002).