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

37included coating temperature (T), mean diameter of silica particle (D), Fe concentrationinitially applied (Feini), and aging time (t, for <strong>adsorption</strong> method only).In the modified <strong>adsorption</strong> method, discrete goethite was mixed with the silicasuspension at pH 7.5 and the factors varied according to the factorial design ofexperiment. The suspension was subsequently dried in the oven for 6 days, rinsed withDI water, and dried again for another 6 days. The drying temperature was varied from 35to 110 °C based on the design of an experiment.In the second method, goethite was synthesized under identical chemical andphysical conditions as discrete goethite except in the presence of silica. Ferric nitrate wasdissolved in a silica suspension, completely mixed, and purged with N2 gas. The systempH was raised to 12 by drop-wise addition of 10 N NaOH and aged for 4 hours.Subsequently, the suspension was aged for two weeks and rinsed with DI water. Thecoated silica was then dried for 3 days. In this method, temperature during aging rangedfrom 60 to 110 °C.To avoid inclusion of loosely attached oxide in characterization, abrasion studieswere performed on the coated silica at Re > 10 4 for 4 hours. The total Feconc. wasdetermined with a modified dithionite-citrate method based on Ross and Wang (1993)where the supernatant was then analyzed by atomic absorption spectrophotometry (AA)analysis (Clesceri et al. 1998). Amorphous Fe oxide was extracted with hydroxylaminehydrochloride-hydrochloric acid solution (0.25 M NH2OH•HC1, 0.25 M HC1) (Ross andWang, 1993). Crystalline Fe oxide is the difference between the total and amorphous Feconcentration.