This work focuses on determining the potential of activated carbon synthesised from coconut stem biomass (CSAC) as an effective adsorbent activated with hydrochloric acid (HCl) in adsorbing copper ions (Cu²⁺) from contaminated groundwater. Heavy metal pollution, particularly copper, poses serious environmental threats, and low-cost agricultural waste-derived adsorbents offer a sustainable solution for remediation. CSAC was synthesised via chemical activation with 0.1 M HCl and characterised using SEM, SEM-EDX, FTIR, and BET analysis. The surface area increased significantly from 82.465 to 230.096 m²/g post-activation. To evaluate the adsorption behaviour, batch experiments were performed under varying conditions of initial Cu²⁺ concentrations (25–150 mg/L), adsorbent dosage (2–3.5 g), pH (3–10), contact time (0–180 min), and temperature (298–338 K). Atomic absorption spectroscopy (AAS) was used to analyze the residual content of Cu²⁺ ions after the adsorption process. The kinetic analysis revealed that the adsorption process conformed to the pseudo-first-order model, with a high correlation coefficient (R² = 0.9782), indicating that physisorption primarily governed the mechanism. The Freundlich isotherm best describes the equilibrium data (R² = 0.9993), implying multilayer adsorption on a heterogeneous adsorbent surface. Furthermore, thermodynamic evaluations demonstrated that the process occurred spontaneously (ΔG < 0), was endothermic (ΔH = +22.19 kj/mol), and involved an increase in system disorder, as reflected by a positive entropy change (ΔS = +0.0711 kj/mol·K). Although conducted at a laboratory scale, the study shows significant potential for practical application in industrial wastewater treatment. The novelty of this research lies in utilizing HCl-activated coconut stem carbon as a cost-efficient and high-performance material for Cu(II) removal, addressing a gap in the use of underexplored agricultural waste materials. As future research, exploring multi-contaminant scenarios and assessing regeneration and cost-efficiency will be critical to confirming CSAC's viability in real-world industrial settings.