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

140data over a broad range of conditions when using a single set of model parameters.9.4 Ni and Zn CompetitionThe modeling results for single adsorbate systems (Table 9.2) indicate that the reactionconstants for Zn(II) <strong>adsorption</strong> are greater than that for Ni(II), suggesting Zn ions exhibita higher affinity than Ni ions for the goethite surface. This observation is in agreementwith other studies (Tiller et al. 1984; Crawford et al. 1993; Gupta, 1998; Dube et al.2001; Trivedi et al. 2001b; Jeon et al. 2003). Therefore, competition is expected whenboth ions are present and the site density is limited. Competition has been investigatedby many researchers. For example, Gadde and Laitinen (1974) concluded that Pb sorbedmore strongly than other ions (Zn, Cd, and Ti) on HFO and HMO surfaces; Balistrieriand Murray (1982) found that Mg(II) tends to suppress the <strong>adsorption</strong> of trace <strong>metal</strong>s(Cu, Pb, Zn, and Cd) on goethite; Christophi and Axe (2000) observed that goethite has agreater affinity for Pb than Cu; Trivedi et al. (2001b) found that Zn and Ni bind morestrongly than Ca to goethite; Bradl (2004) observed that Cd <strong>adsorption</strong> is stronglyinfluenced by the presence of competing cations, such as divalent Ca and Zn; andKanungo et al. (2004) found in a study on Co, Ni, Cu, and Zn sorption onto HMO that Niexperienced the greatest degree of displacement (Kanungo et al. 2004). However,competition between trace <strong>metal</strong>s in other studies may have been difficult to demonstrate,because site density may not have been limiting (Swallow et al. 1980; Palmqvist et al.1999).The competition experiments were performed at pH 6. Compared to singleadsorbate systems, Ni(II) <strong>adsorption</strong> decreased by approximately 23% due to presence of