Water contamination with heavy metals, especially lead, is one of the major environmental challenges that has attracted the attention of many researchers for removal. In this study, nano-cellulose extracted from Chaetomorpha algae was immobilized on magnetic iron nanoparticles (Fe₃O₄) to prepare a bio‑magnetic adsorbent. The performance of this adsorbent in removing lead (II) ions from aqueous solutions was investigated under various conditions including pH, ion concentration, temperature, adsorbent dosage, and contact time. The maximum adsorption capacity was 2.5 mg of lead per gram of adsorbent, obtained at pH 5.2, an initial concentration of 7 mg/L, temperature of 45 °C, adsorbent dosage of 0.018 g/L, and a contact time of 120 minutes. Increasing the pH up to the optimum value enhanced the adsorption efficiency, while at higher pH values, formation of lead hydroxide precipitates caused a reduction in adsorption. Increasing the temperature up to 45 °C improved adsorption, but higher temperatures damaged the functional groups of the adsorbent. Theoretical studies using quantum chemical (DFT) calculations revealed that lead ions interact physically with the oxygen atoms of the cellulose rings, which enhances the stability of the adsorbent. Overall, the Fe₃O₄–nano-cellulose composite is an eco‑friendly and effective material for lead ion removal from water, and the combination of experimental and theoretical results paves the way for the development of new water‑purification adsorbents.