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DIVISION S-2—SOIL CHEMISTRY

Formulating the Charge-distribution Multisite Surface Complexation Model Using FITEQL

^a Dep. of Geological Sciences, Virginia Polytechnic Institute and State Univ., Blacksburg, VA 24061
^b Dep. of Crop and Soil Environmental Sciences, Virginia Polytechnic Institute and State Univ., Blacksburg, VA 24061

* Corresponding author (ctadanie{at}vt.edu)

Sorptive interactions at the solid–water interface strongly influence the bioavailability of many important nutrient oxyanions and trace contaminant metals in both natural and engineered settings. Recently, the charge-distribution multisite complexation (CD-MUSIC) model has been developed to model ion adsorption behavior on variable-charge minerals. Although this model shows great promise, its use has been limited by lack of incorporation into commonly used computer codes. In this study formulation of the CD-MUSIC model in the surface complexation modeling program FITEQL 4.0 is described, and demonstrated using Cu²⁺- and orthophosphate (P_i)-goethite adsorption data. Mass-action and mass-balance expressions for Cu²⁺ and P_i adsorption on goethite were developed using a combination of monodentate and bidentate surface species. Pauling's rules were used to determine the charge of surface adsorption sites and adsorption site density (nm^-2) was calculated from crystallographic considerations. Electrostatic component coefficients in the mass-balance expressions were adjusted to reflect the actual charge of goethite adsorption sites, thereby satisfying both the local charge balance for adsorbed species and the global charge balance of the system as a whole. FITEQL 4.0 was used to determine the best-fit equilibrium constants for the Cu²⁺ and P_i surface adsorption mass-action expressions, and the associated speciation of adsorbed ions. The speciation of adsorbed Cu²⁺ ions was dominated by a single monodentate surface species; whereas, two monodentate and one bidentate surface species were required to adequately describe P_i adsorption. Formulating the CD-MUSIC model as outlined here provides a thermodynamically, electrostatically, and crystallographically consistent approach for solving surface adsorption equilibrium problems with FITEQL.

Abbreviations: SAI, specifically absorbed ion • CD, charge distribution • CD-Music, CD multisite complexation • DDL, diffuse-double layer • DL, diffuse layer • EDL, electrical double layer • NAI, nonspecifically absorbed ion • P_i, orthophosphate • PZC, point-of-zero charge • TLM, triple layer model

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