This study aims to develop high-performance carbon adsorbents from plum pit shells and to evaluate the relationship between synthesis conditions, structural evolution, and Cu(II) adsorption performance in aqueous systems. The work addresses the valorization of agricultural waste into functional porous carbon materials suitable for environmental remediation applications. Plum pit shells were converted into carbon materials through controlled pyrolysis followed by KOH chemical activation. The influence of carbonization temperature, activation ratio, and thermal treatment conditions on material structure was systematically investigated. The resulting materials were characterized using SEM, XRD, FTIR, TGA, and nitrogen adsorption–desorption analysis, while adsorption behavior toward Cu(II) ions was evaluated through batch experiments under varying operational parameters, including pH, adsorbent dosage, contact time, and temperature. Equilibrium, kinetic, and thermodynamic models were applied to describe the adsorption process. The results showed that the optimized activated carbon achieved a specific surface area of up to 944.25 m²/g at a KOH-to-biochar ratio of 1:0.05. Cu(II) removal efficiency reached approximately 95% under optimal conditions (pH 5–6, 1.5–2.0 g/L dosage, 60–90 min contact time). Adsorption data were well described by the Langmuir model (R² > 0.99), indicating predominantly monolayer adsorption. Thermodynamic analysis confirmed that the process is spontaneous and exothermic, with negative Gibbs free energy values across the studied temperature range. The initial hypothesis regarding the formation of highly porous carbon structures and their strong affinity for Cu(II) ions was confirmed. A limitation of this study is that adsorption performance was evaluated using synthetic aqueous solutions rather than real wastewater systems, and regeneration cycles were not experimentally validated. Nevertheless, the materials demonstrate strong potential for practical water treatment applications. The practical value of this work lies in transforming low-cost agricultural waste into efficient adsorbents for heavy metal removal, contributing to sustainable water purification technologies. The originality of the study is in the systematic correlation of KOH activation parameters with pore development and adsorption performance for plum pit-derived carbon, an underexplored biomass precursor. The findings provide new insights into biomass-derived carbon design and support circular economy strategies for waste-to-material conversion.