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

36hours, goethite suspensions were centrifuged at 12000 RPM for 20 minutes using theSorvall® RC-28S centrifuge. The paste was re-suspended and centrifuged again toremove the background electrolyte (Nat and NO3- ions). The cleaning process wasrepeated until the conductivity of the supernatant after centrifugation reachedbackground. The goethite paste after cleaning was freeze-dried and stored for future use.Goethite was re-suspended and aged for 4 hours before use in <strong>adsorption</strong> studies toensure a stable particle size distribution (Trivedi, 2001).4.2 Pretreatment of SilicaAll silica samples were quartz, based on XRD analysis. The 0.2 mm (mean) silica was of99.995% (<strong>metal</strong>s basis) purity and purchased from Alfa Aesar (Alfa Aesar silica). The0.9 mm and 1.4 mm (mean) silica samples were provided courtesy of U.S. SilicaCompany (US silica). All silica samples were initially acid-washed (2 ml acid g -1 silica)with 10 -1 N HNO3 for 24 hours. After decanting the solution, the remaining silica wasrinsed repeatedly with DI water and then dried at 105 °C for 24 hours before storing inNalgene® containers for future use.4.3 Coating MethodsTwo methods were used for producing goethite-coated silica, a modified <strong>adsorption</strong>method based on Szecsody et al. (1994) and a modified precipitation method based onMeng and Letterman (1993a). To evaluate factors that influence the degree of coating,experiments were designed using a three-level fractional factorial method (Anderson andMclean, 1974) to reduce the number of studies and isolate effects. Factors investigated