Sorption of Cu2+, Cd2+, and Pb2+ using modified zeolite from coal fly ash

The coal fly ash (CFA) was modified to zeolite X using fusion method with cationic exchange capacity (CEC) of about 140 meq/100 g. The zeolite was used as an effective sorbent for Cu2+, Cd2+, and Pb2+. The pseudo-second order kinetic model was appropriate for the description of the kinetic performance of the sorption. It required a longer time to reach equilibrium for higher initial metal concentration and lower sorbent dose but all reached equilibrium within 120 min. External mass transfer step seemed to take part as a rate limiting step for the sorption of Pb2+ at low initial concentration and high sorbent dose, on the contrary, the process was controlled more significantly by intraparticle diffusion step at high initial concentration and low sorbent dose. However, Cu2+ was found to be generally controlled by intraparticle diffusion step at all concentration range examined in this work. The sorption of Cd2+, on the other hand was controlled both by external mass transfer and intraparticle diffusion steps at all range of initial concentration. Langmuir and Dubinin–Radushkevich isotherms were applied to describe equilibrium data. The order of maximum sorption capacity in a unit of mol kg−1 was: Pb2+ (2.03) > Cu2+ (1.43) > Cd2+ (0.870). The sorption energy fell in the range of physic-sorption. Equilibrium sorption capacity and removal percentage were governed by both initial concentration and sorbent dose. A general mathematical model was developed for describing the sorption under the variations in initial metal concentration and zeolite doses. This model was proven to be reasonably accurate with additional sets of experiments.