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Selectivity of the Cd2+/Ca2+ exchange on modified rice hull silica

The rice hull ash is composed by 94% of SiO2, an agricultural waste that can be recovered and purified by a depolymerization reaction yielding an organo‐silicic gel. The purpose of this paper is to show that this silica can be used to fix Cd2+ from aqueous solution. The pH of hydrolysis of the organo‐silicic gel is the main factor modifying the distribution between the solid and the solution. The contact time between the Cd2+ solution and the solid was studied to optimize the sorption conditions. The equilibrium measurements were performed after 40 hours at room temperature. The competition with Ca2+ ions in the solutions was also studied in order to evaluate the selectivity of the Cd2+ fixation. It was found that the rice hull ash has a higher capacity to fix Cd2+ than the rice hull derivatives.