Sorption properties of the activated carbon-zeolite composite prepared from coal fly ash for Ni²⁺, Cu²⁺, Cd²⁺ and Pb²⁺

Composite materials of activated carbon and zeolite have been prepared successfully by activating coal fly ash (CFA) by fusion with NaOH at 750 °C in N₂ followed by hydrothermal treatments under various conditions. Uptake experiments for Ni²⁺, Cu²⁺, Cd²⁺ and Pb²⁺ were performed with the materials thus obtained from CFA. Of the various composite materials, that were obtained by hydrothermal treatment with NaOH solution (ca. 4 M) at 80 °C (a composite of activated carbon and zeolite X/faujasite) proved to be the most suitable for the uptake of toxic metal ions. The relative selectivity of the present sorbents for the various ions was Pb²⁺ > Cu²⁺ > Cd²⁺ > Ni²⁺, with equilibrium uptake capacities of 2.65, 1.72, 1.44 and 1.20 mmol/g, respectively. The sorption isotherm was a good fit to the Langmuir isotherm and the sorption is thought to progress mainly by ion exchange with Na⁺. The overall reaction is pseudo-second order with rate constants of 0.14, 0.17, 0.21 and 0.20 L g/mmol min for the uptake of Pb²⁺, Cu²⁺, Cd²⁺ and Ni²⁺, respectively.