Sunflower stem pith-derived cellulose (SSPC) and nanocellulose (TOCN) were modified with polyethyleneimine (PEI) to enhance their adsorption of Cu²⁺ and Cr⁶⁺. PEI modification significantly enhanced the adsorption capacity for Cu²⁺, increasing it by 5.2 times compared to pure cellulose. Meanwhile, TOCN-PEI exhibited superior adsorption performance for Cr⁶⁺, attributed to its higher PEI graft density compared to SSPC. The adsorption kinetics were best described by the pseudo-second-order model, while the Langmuir isotherm model provided the best fit for equilibrium data, indicating monolayer adsorption. Thermodynamic analysis revealed that the adsorption process was exothermic (ΔH⁰ < 0) and less favorable at elevated temperatures. After five adsorption/desorption cycles, TOCN-PEI retained approximately 63.0 % of its initial adsorption efficiency, demonstrating reusability. Overall, TOCN-PEI is a promising adsorbent for Cu²⁺ and Cr⁶⁺ removal, offering exceptionally high Cu²⁺ adsorption capacity (217.3 mg/g), effective Cr⁶⁺ removal (196.7 mg/g), and good reusability.