Adsorption of Cd²⁺ ions onto zeolites synthesized from a mixture of coal fly ash and oil shale ash in aqueous media

Contaminated water, especially water containing high concentrations of toxic metal ions, is frequently discharged into the environment. Thus, developing advanced adsorbents with high adsorption capacities is essential for efficient pollutant removal. In this study, X- and A-type zeolites were synthesized using industrial waste materials, specifically oil shale ash and coal fly ash. Their physicochemical properties were characterized using XRD, SEM, and nitrogen adsorption-desorption analyses. The adsorption behaviors of Cd²⁺ ions onto the synthesized zeolites from aqueous solutions were thoroughly investigated, including kinetic and isotherm analyses. The results showed that the adsorption process adhered to pseudo-second-order kinetics and followed the Langmuir isotherm model for all zeolites. Maximum adsorption capacities were reached within 60 min, with a dosage of 1 g L^-1 and at pH 7. Among the zeolites, OSA75 demonstrated the highest Cd²⁺ adsorption capacity, reaching 236.41 mg g^-1, indicating that the incorporation of a small amount of coal fly ash significantly enhanced the adsorption performance. The dominant mechanism governing Cd²⁺ adsorption was identified as cation exchange.