Synthesis and Characterization of Zeolite A from Industrial Fly Ash as a Green, Cost-Effective Cd²⁺ and Pb²⁺ Adsorbent for Wastewater Applications

According to the large amount of fly ash waste generated from the use of lignite coal as the primary fuel for electricity generation in the Mae Moh district of Lampang province, Thailand, efforts have been made in waste management to reduce and repurpose this industrial byproduct. In this study, lignite coal fly ash was used to synthesize zeolite A adsorbents for the treatment of wastewater contaminated with heavy metals. Characterization of the synthesized zeolite using XRD, XRF, BET, and SEM methods confirmed that it is zeolite A, with a calculated Si/Al molar ratio of approximately 1.19, closely matching the theoretical ratio of zeolite A. This zeolite A exhibited a high crystalline phase and a mesoporous structure, having a specific surface area of 37.10 m²/g and a total pore volume of 0.06 cm³/g. The performance of this zeolite A was evaluated for the adsorption of Cd²⁺ and Pb²⁺ in prepared solutions. The removal efficiencies of zeolite A for Cd²⁺ and Pb²⁺ were 99.65% ± 0.1% and 93.90% ± 0.5%, with maximum adsorption capacities of 17.3 ± 0.6 and 8.8 ± 0.1 mg/g, respectively. Additionally, zeolite A demonstrated reusability for the adsorption of Cd²⁺ and Pb²⁺, maintaining a removal efficiency of 80.52% ± 0.1% for Cd²⁺ over five reuse cycles, and 96.83% ± 0.7% for Pb²⁺ over one reuse cycle. The adsorption of Cd²⁺ and Pb²⁺ by zeolite A followed the Langmuir isotherm model and pseudo-second-order kinetic model. Moreover, the adsorption of Cd²⁺ and Pb²⁺ by zeolite A was found to be a spontaneous, endothermic process, as evidenced by increasingly negative Gibbs free energy change (Δ<i>G</i>°) values with rising temperature. Density functional theory (DFT) calculations were also performed to investigate the binding of Cd²⁺ and Pb²⁺ ions to zeolite A, providing insight into why Cd²⁺ exhibits a slightly higher affinity than Pb²⁺. The results showed that Cd²⁺ ions have a marginally greater affinity for zeolite A compared to Pb²⁺ (-85.72 vs -85.39 kcal/mol), which aligns with experimental findings. This study offers an alternative approach for reducing industrial waste by repurposing it for valuable applications, contributing to sustainable waste management practices that align with the principles of the bio-circular-green economy.