Previous research has confirmed the presence of heavy metals in the Kerian River, Pulau Pinang, primarily due to escalating anthropogenic activities and uncontrolled industrial discharges. This environmental concern underscores the urgent need for efficient and sustainable remediation strategies. In response, the present study explores the use of a bio-based adsorbent derived from TEMPO-oxidized cellulose nanofiber (TEMPO-CNF), offering an eco-friendly and cost-effective alternative to conventional adsorbents. To enhance surface reactivity and structural integrity, the cellulose nanofibers were functionalized via TEMPO-mediated oxidation to introduce carboxylate (-COO) groups, followed by crosslinking with branched polyethyleneimine (bPEI) and citric acid (CA). The study systematically examined the effects of varying CA concentrations (5-30 % w/w relative to bPEI) on the adsorbent's physicochemical properties and heavy metal adsorption performance. Unlike previous approaches that employed either single-step oxidation or polymer grafting of cellulose-based materials, this work presents a synergistic dual-functionalization strategy-combining TEMPO oxidation with bPEI-CA crosslinking-to produce robust, highly reactive nanostructured sponges with improved adsorption capacity and mechanical stability. The optimal CA concentration was determined through evaluation of physical appearance and comprehensive characterization techniques, including Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, zeta potential analysis, Brunauer-Emmett-Teller (BET) surface area measurements, and inductively coupled plasma mass spectrometry (ICP-MS). The functionalized CNF sponge incorporating 20 % CA demonstrated superior Pb removal efficiency of 100 % from actual river water samples while maintaining excellent structural stability. These findings highlight the promise of TEMPO-CNF-based bio-adsorbents as sustainable and effective materials for real-world heavy metal remediation applications.