Simultaneous removal of aqueous Zn2+, Cu2+, Cd2+, and Pb2+ by zeolites synthesized from low-calcium and high-calcium fly ash.

In this study, zeolites were synthesized from low-calcium (LCZ) and high-calcium (HCZ) fly ash, respectively. Subsequently, the zeolites were tested for their removal effectiveness for four aqueous cations, namely, Zn2+, Cu2+, Cd2+, and Pb2+, as a function of contact time, pH value, adsorbent dosage, and initial concentration of heavy metals. Both zeolites were characterized by X-ray diffraction, X-ray fluorescence spectrometry, scanning electron microscopy, specific surface area, and cation exchange capacity. The results show that HCZ mainly consists of an unnamed zeolite (Na6[AlSiO4]6·4H2O), whereas LCZ mainly consists of faujasite-type zeolite. The optimum sorption conditions were pH = 6.0; adsorbent dosage = 1.0 g·L-1; temperature = 25 °C; contact time = 100 min; and initial heavy metal concentration = 100 mg·L-1. The sorption kinetics of the four aqueous cations on both LCZ and HCZ followed the pseudo-second-order kinetic model, and the sorption isotherm data fitted well with the Langmuir isotherm model. For LCZ, the maximum adsorption capacities of Zn2+, Cu2+, Cd2+, and Pb2+ were 155.76, 197.86, 123.76, and 186.22 mg·g-1, respectively. For HCZ, the values were 154.08, 183.15, 118.91, and 191.94 mg·g-1, respectively. The zeolites were regenerated by NaCl solution (1 mol·L-1) and showed high removal efficiency. In conclusion, zeolites produced by fly ash are promising materials for removing Zn2+, Cu2+, Cd2+, and Pb2+ from wastewater.