Different modes of adsorptions of arsenate on silica grafted with Fe3+-coordinated silanes

In order to optimize the adsorption of arsenate, we prepared silica based adsorbents by grafting Fe(3+)-coordinated N-(2-aminoethyl)-3-aminopropyltriethoxysilanes (aeaptes) in various Fe:aeaptes ratios (mixing ratios). Two kinds of mesoporous silica (MCM-41 and SBA-15) and fumed silica (Cab-O-sil M7D) were used as substrates. The adsorption of arsenate saturated at As/Fe=1, 2 and 3 on mesoporous silica based adsorbents from with Fe:aeaptes ratios of 1:1, 1:2 and 1:3, respectively. On the other hand, saturation occurred at As/Fe=1 on Cab-O-sil M7D-based adsorbents for all three Fe:aeaptes ratios. In addition to this dependence of the adsorption on the coordination conditions, two different modes were clearly distinguished in the adsorption isotherm for the MCM-41 based adsorbent prepared with a Fe:aeaptes ratio of 1:1. These two distinct adsorptions are likely to be due to the induction of well-defined coordination structures by this method. This result is significantly different from that found in a previous study where adsorbents were prepared by the grafting of aeaptes followed by the coordination of Fe(3+), where a single mode of adsorption was observed. The adsorptions were further analyzed using the Langmuir equation and by X-ray adsorption spectroscopy.