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

41PZC include electrophoretic mobility, potentiometric titrations, streaming potential, andcation and anion <strong>adsorption</strong>.Potentiometric titrations were conducted with 1 g L-1 sorbent at three ionicstrengths (IS, 10 -3, 10-2, and 10-1 ) adjusted with NaNO3. The suspension was purged withhigh purity (99.999%) nitrogen gas to remove CO2. Titrations were conducted from pH 2to 10 for silica and coated silica and 4 to 10 for goethite with stepwise addition of 0.1 mltitrant (10 -1 and/or 10-2 N HNO3 or NaOH); pH values were recorded after anequilibration time of 2 minutes per step (Trivedi and Axe, 2001b).4.6 Adsorption StudiesIn all <strong>adsorption</strong> studies, bulk aqueous phase concentrations of <strong>metal</strong>s were used belowtheir solubility limit with respect to the thermodynamically stable form (Appendix B).Ionic strength, pH, and <strong>metal</strong> ion concentrations were adjusted with NaNO3, HNO3 andNaOH, and <strong>metal</strong> nitrate, respectively. Turbulence (Re 1.0x10 4) was maintained toeliminate resistance due to external mass transfer. All macroscopic <strong>adsorption</strong> studieswere conducted in Nalgene® containers with a contact time of 4 hours using a shaker (C-1 Platform Shaker, New Brunswick Scientific) at room temperature (-22 °C) and open tothe atmosphere.The amount of <strong>metal</strong> adsorbed was calculated from a mass balance by subtractingfinal aqueous concentration from initial concentration. Metal concentrations wereanalyzed with either Beckman Liquid Scintillation System (model LS6500) or graphitefurnace atomic absorption spectrophotometer (AA, Perkin Elmer 4110 ZL). When using