Adsorption of paraquat on mesoporous silica modified with titania: Effects of pH, ionic strength and temperature

The adsorption of the herbicide paraquat (PQ(2+).) on the binary system titania-silica has been studied in batch experiments by performing adsorption isotherms under different conditions of pH, supporting electrolyte concentration, and temperature. Adsorption kinetic on the studied material has also been carried out and discussed. PQ(2+) adsorption is very low on the bare silica surface but important on the composed TiO2-SiO2 adsorbent. In this last case, the adsorption increases by increasing pH and decreasing electrolyte concentration. There are no significant effects of temperature on the adsorption. The increase of the adsorption in TiO2-SiO2 seems to be related to an increase in acid sites of the supported titania and to the homogenously dispersion of the TiO2 nanoparticles over the silica support. The adsorption takes place by direct binding of Pe(2+) to TiO2 leading to the formation of surface species of the type SiO2-TiO2-PQ(2+). Electrostatic interactions and charge-transfer and outer-sphere complexes formations seem to play a key role in the adsorption mechanism. The analysis of thermodynamic parameters suggests that the adsorption on TiO2-SiO2 is endothermic and spontaneous in nature. (C) 2011 Elsevier Inc. All rights reserved.