Adsorption study on natural clays as cement mineral additives: possibility of toxic metallic cations immobilization

Evironmetally safe mortars in which part of the cement binder was replaced by mineral additives (i.e. zeolite, bentonite and fly ash) were designed. Fly ash, as fine powdery byproduct of coal combustion, comprises heavy metals in its composition. Bentonite and zeolite are natural adsorbents with ability to immobilize certain toxic elements and prevent their migration from the mortar structure. In this study the ability of bentonite and zeolite to adsorb toxic cations present in fly ash leachate was investigated. Metallic cations were detected in quantities not higher than 52.6, 15.5, 52.4 and 22.7 mg/kg for Zn, Pb, Cu and Ni respectively. Adsorption kinetic was monitored using 0.1 mol/dm3 solutions of each of investigated cations (Zn2+, Pb2+, Cu2+ and Ni2+) as well as multicomponent solution of all these cations during predefined time intervals in the range from 10-1440 minutes. Adsorption isotherms were obtained in concentration range for each cation of single and multicomponent solution in the range from 0.10-0.30 mol/dm3. The obtained results of this investigation indicate that bentonite and zeolite are efficient adsorbents and, therefore, they can be applied in combination with fly ash in environmentally-safe construction material