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

32this may be due to the fact that most models have been derived only from <strong>adsorption</strong>edges (Dyer et al. 2003) or that the surface reaction products may not be physicallymeaningful. To obtain a unique set of parameters describing the oxide/water interfaces,this study will use XAS derived surface complexes and calibrate models with datacovering a broad range of experimental conditions.The third hypothesis is that mechanistic SCM parameters for a single solute canbe applied to multi-solute systems. As discussed in the first hypothesis, <strong>adsorption</strong> sitesare highly related to surface functional groups and surface structures; <strong>metal</strong> ions will thenoccupy the same types of sites in the presence of other <strong>metal</strong> ions. The mechanisticmodeling parameters from single sorbate systems are thus expected to describe themultisorbate system. Although no direct XAS support was found, there are studiesshowing some success in modeling <strong>metal</strong> competition based on single sorbate parameters.Ali and Dzombak (1996) predicted sulfate and organic acids <strong>adsorption</strong> reasonably wellbased on single-sorbate data over a wide range of conditions. Palmqvist et al. (1999)showed single adsorbate modeling predicted <strong>adsorption</strong> competition for Cu and Zn butunderestimated Pb(II) <strong>adsorption</strong>. In this study, <strong>metal</strong> competition will be predictedusing SCMs from single sorbate systems. These models are based on XAS derivedmechanistic information and calibrated using data covering a significant range ofconditions.Finally, goethite is selected as the representative iron oxide sorbent in soils andsediments for this research. Goethite and hematite are thermodynamically the moststable Fe oxides under aerobic surface conditions and therefore, the most widespreadcrystalline Fe oxides in soils and sediments (Schwertmann and Cornell, 1991). However,