Untreated coffee residues (UCRs) were used to remove heavy metal ions from aqueous solutions. The effects of solution pH, heavy metal ions, and UCR concentrations on heavy metal ion removal by UCRs were determined. The removal percentage for heavy metal ions increased as pH and UCR doses increased; conversely, it decreased as the concentration of heavy metal ions increased. This study employed the pseudo firstand second-order kinetics models, the intraparticle diffusion model, and the Bangham model to simulate adsorption kinetics of Pb²⁺ and Zn²⁺ onto UCRs. Simulation results indicate that adsorption kinetics fit well with the pseudo second-order model. After reaction for 180 min at [adsorbate] = 20 mg/L, [UCRs] = 2 g/L, and pH 5, the adsorption percentage of Pb²⁺ and Zn²⁺ was 96% and 44%, respectively; moreover, the adsorption density of Pb²⁺ and Zn²⁺ onto UCRs was 9.7 and 4.4 mg/g (4.7×10⁻² and 6.8×10⁻² mmol/g), respectively. The adsorption of Pb²⁺ and Zn²⁺ onto UCRs is controlled mainly by surface diffusion.
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