<p>The applications of ultrasonic energy have a broad interest in enhancing polymer properties, effectively enabling mini-emulsion formation of free radicals inside the polymer backbone. This method offers a rapid, energy-efficient approach, producing high-quality resulting polymer compared to conventional techniques. In this study, a novel cellulose-based biopolymer functionalized with 18-Crown-6 was synthesized via ultrasonic-assisted mini-emulsion free radical polymerization for the efficient removal of toxic lead (Pb²⁺) and cadmium (Cd²⁺) ions from aqueous solutions. The synthesized (CMC–co–18-Crown-6) biopolymer was thoroughly characterized using FTIR, UV–Vis, and SEM techniques to confirm structural integrity and successful functionalization, with an average pore diameter of 0.9 ± 0.2&#xa0;µm and an average fiber diameter of 1.2 ± 0.3&#xa0;µm. Batch adsorption experiments were conducted to investigate the influence of pH, contact time, and initial ion concentration on metal uptake. The maximum adsorption efficiency was observed at pH 5, with equilibrium reached at 120&#xa0;min. Kinetic studies revealed that the adsorption process followed a pseudo-second-order model, indicating chemisorption as the dominant mechanism. The polymer demonstrated higher affinity toward Pb²⁺ than Cd²⁺, attributed to the ionic size compatibility with the crown ether cavity. Regeneration studies showed over 90% adsorption efficiency retained after four cycles, indicating excellent reusability. These findings suggest that the 18-Crown-6-functionalized cellulose biopolymer is a promising, cost-effective, and environmentally friendly material for the targeted removal of hazardous heavy metals from contaminated water systems.</p> Graphical abstract <p></p>