A novel chelating adsorbent, based on the functionalization of activated carbon (AC) derived from water hyacinth (WH) with melamine thiourea (MT) to form melamine thiourea-modified activated carbon (MT-MAC), is used for the effective removal of Hg²⁺, Pb²⁺, and Cd²⁺ from aqueous solution. Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and Brunauer–Emmett–Teller (BET) theory confirm the successful functionalization of AC with the melamine thiourea chelating ligand through the amidation reaction between the carboxyl groups of oxidized activated carbon (OAC) and the amino groups of melamine thiourea (MT) in the presence of dicyclohexylcarbodiimide (DCC) as a coupling agent. The prepared MT-MAC exhibited extensive potential for the adsorption of the toxic metal ions Hg²⁺, Pb²⁺, and Cd²⁺ from wastewater. The MT-MAC showed high capacities for the adsorption of Hg²⁺ (292.6 mg·g^–1), Pb²⁺ (237.4 mg·g^–1), and Cd²⁺ (97.9 mg·g^–1) from aqueous solution. Additionally, 100% removal efficiency of Hg²⁺ at pH 5.5 was observed at very low initial concentrations (25–1000 ppb).The experimental sorption data could be fitted well with the Langmuir isotherm model, suggesting a monolayer adsorption behavior. The kinetic data of the chemisorption mechanism realized by the melamine thiourea groups grafted onto the activated carbon surface have a perfect match with the pseudo-second-order (PSO) kinetic model. In a mixed solution of metal ions containing 50 ppm of each ion, MT-MAC showed a removal of 97.0% Hg²⁺, 68% Pb²⁺, 45.0% Cd²⁺, 17.0% Cu²⁺, 7.0% Ni²⁺, and 5.0% Zn²⁺. Consequently, MT-MAC has exceptional selectivity for Hg²⁺ ions from the mixed metal ion solutions. The MT-MAC adsorbent showed high stability even after three adsorption–desorption cycles. According to the results obtained, the use of the MT-MAC adsorbent for the adsorption of Pb²⁺, Hg²⁺, and Cd²⁺ metal ions from polluted water is promising.

In this study, stems and leaves of the papaya plant were employed to prepare a high-quality porous adsorbent via carbonization and chemical activation using phosphoric acid.