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

65Extraction results after abrasion showed the degree of coating was in the range of0.59 to 21.36 mg Fe g -1 sand for <strong>adsorption</strong> and 0.67 to 8.5 mg Fe g -1 sand forprecipitation (Figure 6.1). These results are consistent with those observed in soils andsediments (Ryan and Gschwend, 1994; Coston et al. 1995; Wang et al. 1993; Pieper et al.1997). From the <strong>adsorption</strong> process, goethite constitutes greater than 95% (average96.8%) of the oxide, while it is less than 89% (average 62.9%) employing precipitation.This difference may be explained by silica solubility evaluated with MINEQL+ (version5.0) (Westall et al. 1976), where the dissolved Si(OH)4 ° concentration is about 3x10 -2 Mat pH 12 and 25 °C; these conditions represent those used for the aqueous coatingapplication (precipitation). Therefore, dissolved Si(OH)4 ° may be incorporated into theiron oxide structure during precipitation. The resulting less crystalline form of iron oxidemay be due to the presence of the substrate as well as silica dissolution, and has beenobserved by others (e.g., Anderson and Benjamin, 1985; Cornell et al. 1987) to inhibitcrystallization. In the <strong>adsorption</strong> method, dissolution is not relevant as goethite wasprepared before coating. Subsequently, in re-suspending the two minerals at pH 7.5 and25 °C, dissolved Si(OH)4° is approximately 104 M; however, the suspension is repeatedlyrinsed after aging.Given the observed oxide loadings, only silica was observed in the XRD patterns(Figure 6.2). Generally, the patterns of silica are similar to those of coated silica; thedifferences in the relative intensities may be attributed to the variability in preferredorientation of the silica sample. Nevertheless, powder residuals produced in theprecipitation process at 110°C were confirmed to be goethite (Figure 6.2, and enlarged inFigure 6.3 for detailed planes). As discussed earlier, Lo et al. (1997) found amorphous