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

24Kanungo, 1994; Lee et al. 1996, 1998; Sauvé et al. 2000) and empirical Freundlichmodels (Dzombak and Morel, 1986; Mishra and Tiwary, 1995, 1998; Nakahara, 1996;Mishra et al. 1997; Gupta, 1998; Christophi and Axe, 2000, Vaishya and Gupta, 2004) todescribe equilibrium <strong>adsorption</strong>. However, a good fit using the Langmuir equation, orany other model without complementary molecular information, does not necessarilysupport the proposed mechanism (Schulthess and Sparks, 1991). For the Freundlichmodel, generally, if data can be fit with this isotherm, surface site heterogeneity is likely(Trapnell, 1955; Adamson, 1982). There are two limitations however in using theFreundlich equation (Schulthess and Sparks, 1991): first, it is only applicable to dilutesolutions, <strong>adsorption</strong> at high aqueous concentrations is generally overestimated and nomaximum <strong>adsorption</strong> is predicted; and, secondly, it does not explain the <strong>adsorption</strong>mechanism.The distribution coefficient, Kd, has been used extensively to describe thepartitioning of ions at the solid/liquid interface even though it is a function of pH,solution composition, solid/liquid ratio, and time (Lee et al. 1996; Jackson and Inch,1989; Kooner, 1993; Larsen and Postma, 1997). Misak et al. (1996) concluded that theerroneous logKd-pH plots in their study was caused by changes in the activity coefficientsof surface species even at sufficiently low surface coverages. Sauvé et al. (2000) studiedCd partitioning in 64 contaminated soil samples, where IQ varied over a range from 10 to100,000. Therefore, partitioning parameters achieved from one situation may not beapplied to another; <strong>adsorption</strong> modeling using a single parameter, Kd, has significantlimitations in the sense of <strong>adsorption</strong> prediction.