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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

View larger version (45K): [in a new window]   Fig. 1. Electrical double layer defined by electrostatic planes in 3-plane surface complexation models. Charge separation due to adsorbed and electrolyte ions located in the electrostatic planes defines two molecular capacitors which are termed the inner- and outer-Helmholz layers. These layers are collectively known as the Stern Layer. A layer of diffuse electrolyte ions separates the Stern layer and bulk solution. Electrical potential decreases from some positive value at the mineral surface to zero in the bulk solution.

View larger version (54K): [in a new window]   Fig. 2. Goethite structure showing a double chain of FeO3(OH)3 octahedra that run parallel to the c axis. Singly, doubly, and triply coordinated surface oxygens are indicated.

View larger version (53K): [in a new window]   Fig. 3. Charge on singly, doubly, and triply coordinated surface oxygen functional groups in goethite using Pauling's rules. All Fe atoms are octahedrally coordinated and symmetrical distribution of charge is assumed.

View larger version (39K): [in a new window]   Fig. 4. Placement of ions in the electrostatic double layer of the CD-MUSIC model. Protonated surface oxygens lie in the 0-plane, ligands of specifically adsorbed ions are distributed between the 0- and 1-planes, and ion pairs are placed in the 2-plane.

View larger version (21K): [in a new window]   Fig. 5. Charge distribution (CD) f values for Cu2+ and Pi surface complexes with goethite assuming symmetrical distribution of central cation charge among surrounding ligands according to Pauling's rules. The CD f values represent the portion of central cation charge which is neutralized by surface ligands, and for symmetrical neutralization of cation charge are calculated as the fraction of total ligands situated in the surface 0–plane.

View larger version (15K): [in a new window]   Fig. 6. Copper2+ adsorption on goethite with 0.01 M NaNO3 background electrolyte. CuT = 0.785 mM. Goethite concentration 10 g L-1, specific surface area 50 m2 g-1. Open symbols represent experimentally determined Cu adsorption. Filled symbols with dotted lines represent adsorbed Cu species based on optimized CD-MUSIC fit to experimental data. Solid line represents cumulative total of adsorbed Cu species predicted by CD-MUSIC model.

View larger version (25K): [in a new window]   Fig. 7. Pi adsorption on goethite with 0.01 M NaClO4 background electrolyte. Pi,T = 40 µM. Goethite concentration 0.5 g L-1, specific surface area 66.5 m2 g-1. Open symbols represent experimentally determined Pi adsorption. Filled symbols with dotted lines represent adsorbed Pi species based on optimized CD-MUSIC fit to experimental data. Solid line represents cumulative total of adsorbed Pi species predicted by CD-MUSIC model.