Heavy metal adsorption on iron oxide and iron oxide-coated silica ...

127Goethite titration data were used to calibrate the TLM. Because FITEQL isunlikely to converge if the protolysis constants (logK a1 and logKa2) and electrolytebinding constants (logKcat and logKan) are simultaneously set as adjustable parameters(Hayes et al. 1991). The former was constrained based on the relationship that pHpzc isequal to (logKai+logKa2)/2. As a result, ApK a as well as the inner-layer capacitance (C1)were optimized outside of the FITEQL program. The outer-layer capacitance (C2) istraditionally set at 0.2 F/m 2 (Hayes et al. 1991). For the total site density (N a) of goethite,a value of —6 sites nm-2 was calculated (Tadanier and Eick, 2002; Peacock and Sherman,2004) based on a crystallographic consideration proposed by Hiemstra and van Riemsdijk(1996). This value was adopted for our work. Similar values for site density have beenused in other studies and proved reasonable in describing these systems (Hayes andLeckie, 1986; Miller and Sigg, 1992; Coughlin and Stone, 1995; Kooner et al. 1995;Venema et al. 1996; Robertson and Leckie, 1998; Liu and Huang, 2001). In summary,the TLM parameters optimized in FITEQL in this study were logK cat and logKan. Ionicstrength effects are accounted for in FITEQL using activity coefficients calculated withthe Davies equation (Davies, 1962).The TLM results for ApK a values varying from 1 to 5 (Table 9.1) show that,although the protolysis constants systematically decreased by 0.5 log units, the maximumchange in either logKcat or logKan is only approximately 0.14 log units. Therefore, theelectrolyte binding constants reflect little sensitivity to changes in ApK a. In addition, thesummation of the absolute values of logKcat and logKan is approximately equal to 2pHpzc•This relationship was used by Kosmulski (1996) to reduce the number of adjustableparameters. The optimum C1 value was found to be 2.0 F 111-2 for all ApKa values and