Thermodynamic and kinetic study of heavy metal removal from aqueous solution using cerium-doped zinc oxide nanoparticles

The rapid rise in industrial activity has intensified heavy metal contamination of freshwater, with Pb 2+ , Hg 2+ , and Cd 2+ being among the most toxic and persistent pollutants. In this study, cerium-doped zinc oxide (Ce-doped ZnO) nanoparticles were synthesized via a simple precipitation route, structurally characterized by XRD, FE-SEM/EDX, BET, and DLS, and evaluated for their simultaneous adsorption of Pb 2+ , Hg 2+ , and Cd 2+ ions from aqueous media. XRD results confirmed the formation of hexagonal wurtzite ZnO with crystallite size decreasing from 36.1 nm for pure ZnO to 32.3 nm and 31.9 nm for 2 % and 4 % Ce-doped samples, respectively. BET analysis revealed a significant surface area enhancement (33.6 m 2 /g for 4 % Ce–ZnO), which contributed to improved adsorption performance. Batch adsorption experiments demonstrated that 4 % Ce–ZnO achieved maximum removal efficiencies of 94 % (Pb 2+ ), 87 % (Hg 2+ ), and 95 % (Cd 2+ ) under optimized conditions (pH 6.5, 0.025 g adsorbent, 30 mg/L metal ion concentration, and 220 min contact time). Kinetic modeling revealed that the Elovich model best described the adsorption process (R 2 > 0.93), suggesting chemisorption as the dominant mechanism, while Langmuir isotherm fitting indicated monolayer adsorption with maximum capacities (q max ) of 20.94, 17.54, and 19.98 mg/g for Pb 2+ , Hg 2+ , and Cd 2+ , respectively. Thermodynamic analysis confirmed the spontaneous (ΔG < 0) and exothermic (ΔH < 0) nature of adsorption, with a slight shift toward non-spontaneity for Hg and Cd at higher temperatures. Regeneration studies showed that Ce-doped ZnO retained >80 % efficiency after seven reuse cycles, highlighting its operational stability. Overall, Ce-doped ZnO nanoparticles represent an inexpensive, scalable, and reusable adsorbent with strong potential for industrial wastewater treatment and sustainable heavy metal remediation.