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.

Fly ash-TiO₂ composites were obtained by sol–gel, and their catalytic performance was evaluated on removal of organic loading and expressed as Chemical Oxygen Demand (COD) from real wastewater samples. SEM, X-ray fluorescence, XRD, IR, Raman analysis and determination of PZC characterized the catalysts. The photocatalytic tests were carried out in a laboratory reactor under UV-VIS irradiation and influence of fly ash (FA) content, and irradiation time was investigated. The catalysts with FA content of 10%, 20% and 30% showed COD removal efficiencies higher than 50% after 5 h of irradiation. The COD degradation followed a pseudo-first order kinetic model. Obtained results proved that FA-TiO₂ composites could represent a suitable alternative for removal of organic loading from real wastewater.