Meso- and macroporous silica-based arsenic adsorbents: effect of pore size, nature of the active phase, and silicon release

Arsenic pollution in ground and drinking water is one of the major problems worldwide due to the natural abundance of arsenic by dissolution from ground sediments or mining activities as anthropogenic influence. To overcome this issue, iron oxides as low-cost and non-toxic materials have been widely studied as efficient adsorbents for arsenic removal, also when dispersed within porous silica supports. In this study, two head-to-head comparisons were developed to highlight insights on the As(V)-adsorptive ability of meso- and macrostructured silica-based adsorbents. First, the role of the textural properties of a meso-(SBA15) and macrostructured (MOSF) silica supports in affecting the structural-morphological features and the adsorption capacity of the active phase (Fe2O3) has been studied. Secondly, a comparison of the arsenic removal ability of inorganic (Fe2O3) and organic (amino groups) active phases was carried out on SBA15. Above all, since silica supports are commonly proposed for both environmental and biomedical applications as active phases carriers, we have investigated secondary silicon and iron pollution. The batch tests at different pHs revealed better performances for both Fe2O3-composites at pH 3. The values of qm of 7.9 mg/g (53 mg/gact) and 5.5 mg/g (37 mg/gact) were obtained for SBA15 and MOSF, respectively (gact stands for mass of the active phase). The results suggested that mesostructured materials are more suitable to disperse active phases as adsorbents for water treatment due to the obtainment of very small Fe2O3 NPs (about 5 nm). Besides studying the influence of the pore size of SBA15 and MOSF on the adsorption process, the impact of the functionalization was analyzed on SBA15 as the most promising sample for As(V)-removal. The amino-functionalized SBA15 adsorbent (3-Aminopropyltriethoxysilane, APTES) exhibited a qm of 12.4 mg/g and faster kinetics. Furthermore, issues associated with the release of iron and silicon during the sorption process causing secondary pollution were evaluated and critically discussed.