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

CHAPTER 5CHARACTERIZATION OF IRON OXIDES AND SILICAIn this section, results are presented for characterizing iron oxide and silica. Goethite wasproduced in earlier work (Trivedi, et al. 2001a); HFO is synthesized following the samemethod as of Axe and Anderson (1995). Characterization analyses are conducted incorresponding studies and are summarized below.5.1 Characterization of Iron OxidesXRD analysis reveals that goethite is highly crystalline; the ESEM micrograph showsneedle shaped or acicular crystals of 0.5 to 1µm in length. In the particle size analysis,the goethite powder is re-suspended for 4 hours and the stabilized particles range from0.2 to 84 1.1m. The BET surface area of the goethite is 31 m 2 g-1 ; the pore size rangesfrom 2 to 220 nm and the porosity of goethite is 0.3%.Potentiometric titrations were performed to evaluate the PZC of goethite; theresult, 8.78 ± 0.97 (Figure 5.1), suggests slight drifting to a higher pH compared to thatfound earlier 7.8 ± 0.4 (Trivedi and Axe, 2000). Possible reasons for this difference maybe that potentiometric titrations were performed on different batches of goethite. Studies(Schwertmann et al. 1985; Villalobos et al. 2003) have shown that crystalline shape, sizeof particles, and the resulting reactivity of goethite depended on experimental conditionssuch as rate of addition and total base added, presence of sorbing ions, and aging time.Small variances between different batches may exists and influence the PZC. Previousstudies (Evans et al. 1979; Zeltner and Anderson, 1988) have shown that when thegoethite suspension was purged for 2 months, the PZC increased from 8.1 to 9.1 as45