Adsorption of Cd(II) from aqueous solutions by activated carbon prepared from grapevine

The sorption behavior of activated carbon (AC) derived from grapevine through phosphoric acid activation toward Cd(II) ions has been systematically investigated. Comprehensive characterization by low-temperature N₂ adsorption, FTIR spectroscopy, and Boehm titration revealed a high specific surface area (2285 m²/g) and developed mesoporosity (total pore volume 0.551 cm³/g). The influence of solution pH, sorbent dose, and initial Cd(II) concentration on adsorption efficiency was established. Batch experiments were carried out at 298 K over an initial Cd(II) concentration range of 30–850 mg/dm 3 ; the optimal operating conditions were established as pH = 6.45, sorbent dose 12.5 g/dm 3 , equilibrium contact time 90 min (with 95–98% of Cd(II) sorbed within the first 10–15 min). Sorption isotherms were found to be well described by Freundlich, Langmuir, Temkin, Dubinin–Radushkevich, Flory–Huggins, and Redlich–Peterson models, with the maximum sorption capacity reaching 35.97 mg/g (Langmuir, Flory–Huggins). The mean adsorption energy of 9.17 kJ/mol (Dubinin–Radushkevich) suggests an ion-exchange mechanism. Equilibrium analysis yielded the Gibbs free energy ΔG 0 = -13.76 kJ/mol confirms a spontaneous ion-exchange-controlled process. Sorption kinetics follow a pseudo-second-order model (k 2 = 7.29·10 −2 g·mg −1 ·min −1 , q e , calc = 8.25 mg/g), with mixed external/intraparticle diffusion control (D ext =1.63·10 −5 cm 2 /s, D̅ int = 9,89·10 −9 cm 2 /s,), confirming the dominance of intraparticle resistance. Regeneration tests using 1 M HCl as eluent over five consecutive adsorption–desorption cycles preserved ≈ 85% of the initial capacity, demonstrating satisfactory reusability. Quantum-chemical modeling confirmed that Cd(II) ions are preferentially adsorbed on carboxyl groups of the carbon matrix via ion exchange in the monohydrated form ([Cd(H₂O)]²⁺), with significant binding also observed at carbonyl centers. The potential contribution of π-complexation with conjugated carbon domains is also indicated. Overall, the results highlight the high efficiency and mechanistic features of grapevine-based AC as a promising, sustainable adsorbent for cadmium removal from aqueous media.