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Powerful adsorbents for heavy-metal removal from wastewater are attractive due to the growing effluent of industries. Developing hydrogels is a current research interest in heavy-metal adsorption from aqueous solutions. We prepared a novel melanin-based hydrogel from renewable chestnut shell pigment and acrylic acid by radical polymerization free from a traditional crosslinker. The synthesized material was characterized by Fourier transform infrared spectroscopy and scanning electron microscope. Its Cu(II)-adsorption performance from the water was evaluated by equilibrium isotherms, kinetics, and thermodynamics. The results indicate that: (1) The dry hydrogel showed a porous structure with a network of interconnected spindle-shaped bars, which makes it feasible to serve as an adsorbent; (2) The kinetic adsorption data followed both the pseudo-first-order and the pseudo-second-order models and both physical and chemical processes involved in the Cu(II) removal; (3) Cation exchanges with H⁺ from COOH and phenolic OH groups and with NH₄⁺ from –COONH₄ were likely the primary mechanisms of Cu(II) chemisorption adsorption onto the poly(AA/CSP) as forms of Cu²⁺ and CuOH⁺; (4) The equilibrium data were well fitted by the Langmuir isotherm with the maximum monolayer adsorption capacity of 200.3 mg/g; (5) The adsorption was a spontaneous and exothermic process co-driven by enthalpy and entropy.