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

143Ca competition on HFO; however, the model failed at the lowest ionic strength (0.005)studied (Cowan et al. 1991). Results of this study indicate that, when coupled withspectroscopic data, parameters derived from single-adsorbate systems can be used topredict <strong>adsorption</strong> competition within reasonable uncertainty limits.9.5 SummaryA triple-layer surface complexation model has been used to simulate <strong>metal</strong> <strong>adsorption</strong>onto goethite surfaces. Constrained with spectroscopic information, a 2-pK, 2-site TLMsuccessfully predicted Ni and Zn <strong>adsorption</strong> in single-adsorbate systems. Curvature inthe constant-pH <strong>adsorption</strong> isotherms was accurately described using both high-affinityand low-affinity type sites. A unique set of parameters was found for each <strong>metal</strong> ion thatsuccessfully describes <strong>adsorption</strong> over a wide range of experimental conditions (six toseven orders of magnitude in adsorbate concentration, ionic strength ranging from 10 -3 to10-2, and an environmentally relevant pH range). Competition was also predicted withparameters calibrated using single adsorbate data. Results of this study suggest thataccurate chemical and physical representations of the complexes at the interfacial regionassist in developing mechanistically based predictive models. As a result, these uniquesets of parameters for Ni and Zn may inherently provide a more comprehensive depictionof surface interactions over a broad range of conditions.Although <strong>adsorption</strong> and competition on discrete oxide have been successfullydescribed using surface complexation models, to predict <strong>metal</strong> distribution and transportin soils and sediments these models needs to be tested using <strong>metal</strong> <strong>adsorption</strong> to iron