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

5022% peak. The differences between the sample and the PDF may be attributed topreferred orientation. The width at the half height (WHH) (Figure 5.2) for the mostintense peak of US and Alfa Aesar silica are 0.14 and 0.12 '20, respectively.5.2.2 X-Ray Fluorescence (XRF)XRF was performed on silica samples to determine composition. Two original and eightacid-washed US silica samples were analyzed, while only one Alfa Aesar silica samplewas taken because the measured SiO2 content (100%) conformed to the reported99.995% purity. Alfa Aesar silica shows 100% purity (Table 5.3), while the original USsilica was 95.883% of SiO2 with some impurities of Al, Fe, and S. Retention of traceelements is reported to be related to their Al and Fe content (Meng and Letterman,1993a), and therefore, these Al and Fe impurities may influence <strong>adsorption</strong>. However,after acid-wash treatment of silica samples, the SiO2 content increased to 98.759% for allUS silica samples. Therefore, based on XRF analysis, Alfa Aesar silica may be the mostappropriate for <strong>adsorption</strong> studies. US silica requires pretreatment.5.2.3 Particle Size Analysis (PSA)Results of the particle size analysis for Alfa Aesar silica show that ionic strength and pHdo not influence the particle size distribution. Alfa Aesar silica has a mode of 0.22 mm(Figure 5.3), US silica has a mode of 0.99 mm (Figure 5.4), and again both ionic strengthand pH do not affect the particle size distribution. Considering that the silica particlesmay be applied in column studies for <strong>adsorption</strong>, Alfa Aesar silica, which has a muchsmaller particle size, may promote clogging when coated with iron oxide. Therefore,based on the particle size analysis, US silica is more suitable for <strong>adsorption</strong> studies.