Simultaneous removal of aqueous Zn, Cu, Cd, and Pb 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, Zn, Cu, Cd, and Pb, 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 (Na[AlSiO]·4HO), whereas LCZ mainly consists of faujasite-type zeolite. The optimum sorption conditions were pH = 6.0; adsorbent dosage = 1.0 g·L; temperature = 25 °C; contact time = 100 min; and initial heavy metal concentration = 100 mg·L. 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 Zn, Cu, Cd, and Pb were 155.76, 197.86, 123.76, and 186.22 mg·g, respectively. For HCZ, the values were 154.08, 183.15, 118.91, and 191.94 mg·g, respectively. The zeolites were regenerated by NaCl solution (1 mol·L) and showed high removal efficiency. In conclusion, zeolites produced by fly ash are promising materials for removing Zn, Cu, Cd, and Pb from wastewater.