Fly Ash Waste Conversion to Zeolite and Its Application for Cd2+ and Ni2+ Adsorption from Aqueous Solutions

Adsorption methods represent a common practice used for heavy metals removal from aqueous solutions. As adsorbent material, zeolites have an excellent adsorption capacity and present low environmental impact. The zeolite used for this study was synthesized from fly ash, a residue generated by thermal power plants, considering the need to reduce fly ash deposits and transform it into useful materials as part of the circular economy. Fly ash conversion to zeolite was performed via a modified hydrothermal method. To investigate the synthetic zeolite structure Scanning Electronic Microscopy, Fourier Transformed Infrared spectroscopy, X-ray diffraction and Raman spectroscopy were used. Chemical composition was revealed by X-ray Fluorescence and thermal behavior was investigated by thermogravimetric analysis. The influence of adsorbent dose and pH upon adsorption capacity of synthesized zeolite for Ni2+ and Cd2+ was investigated. The effect of contact time on the adsorption capacity of Cd2+ and Ni2+ was evaluated and it was found that the system reaches equilibrium after 30 min for Ni2+ and 60 min for Cd2+ due to the saturation of zeolite pores with metallic ions. It was found that the Langmuir model (Lineweaver-Burk equation) better describes the adsorption of both Ni2+ and Cd2+. The adsorption kinetics obeys the pseudo-second-order kinetic model (k2 = 0.0851 g/mg min for Ni2+ and k2 = 0.1780 g/mg min for Cd2+). The adsorption capacity of synthesized was found to be 13 mg/g for Ni2+ (initial Ni2+ solution concentration = 30 mg/L) and 17 mg/g for Cd2+ (initial Cd2+ solution concentration = 35 mg/L) at an adsorbent dose of 2 g/L and a contact time of 30 min. Reusability of the zeolite was also tested and it was found that it can be used for 5 cycles. The experimental study revealed that zeolite obtained from fly ash waste generated by a thermal plant represents a viable and sustainable alternative for removal Cd2+ and Ni2+ from aqueous solution.