Efficiency of volcanic ash-based porous geopolymers for the removal of Pb2+, Cd2+ and Hg2+ from aqueous solution

Abstract

The simultaneous removal of Pb2+, Cd2+ and Hg2+ from an aqueous solution using porous geopolymers from volcanic-ash GP8RHA (incorporating 24.2% of rice husk ash) and GP0 (without rice husk ash) serving as the reference, was performed. In batch experiments, the effects of pH, contact time, adsorbent dose, and metal ion concentration on the adsorption performance of the geopolymer products were evaluated at room temperature. From the obtained results, both geopolymer’s fixations towards metal adsorbates increased in the following order: Pb2+ ˃ Cd2+ ˃ Hg2+. The equilibrium time was reached after 30 min and 60 min for GP0 and GP8RHA, respectively, at pH 6, with 0.004 g of sorbent dose for a solution of an initial concentration of 10 ppm. Experimental data were correlated using adsorption isotherms and kinetic models. The pseudo-second order kinetic model and the Langmuir isotherm both fit the experimental results. Furthermore, physisorption mainly occurred on both adsorbent surfaces. When compared to GP0, GP8RHA sample promoted higher sorption capacities of metallic adsorbates, with the maximum adsorption capacities of 312.5 mg/g for Pb2+, 243.9 mg/g for Cd2+ and 232.6 mg/g for Hg2+. These findings suggest the potential use of porous geopolymers from volcanic ash in the treatment of metal-contaminated water.