Chromic acid-etched recycled low-density polyethylene functionalized with amidoxime groups for uranium extraction from seawater

Low-density polyethylene (LDPE) sheets derived from recycled food packaging bags were surface-modified with chromic acid etching and subsequently functionalized with amidoxime groups via gamma radiation-induced simultaneous co-grafting at 40 kGy of acrylonitrile (AN) and methacrylic acid (MAA) at 60:40 volume ratio for uranium extraction from seawater. Influence of etching time (0–30 min at 70 °C) on surface characteristics and functionalization efficiency was investigated. Surface analysis revealed that etching enhanced porosity and roughness, with an optimal duration of 5 min. This yielded the highest degree of grafting (117.92%) and amidoxime density (12.37 mol/kg). Structural characterization confirmed successful functionalization and improved thermal stability compared to pristine LDPE. The Langmuir and Freundlich models described uranium adsorption equally well. Field deployment in natural seawater (Chonburi province, Thailand) over 24 weeks achieved maximum adsorption capacity of 6.01 mg-U/g-adsorbent, representing a 102% improvement over non-etched samples. Reusability testing over eight adsorption–elution cycles yielded a cumulative uranium uptake of approximately 7 mg-U/g-adsorbent, which was 74% higher than the previously reported non-etched adsorbents, although capacity decreased by 48% after eight cycles. Competitive adsorption analysis in natural seawater identified uranium as a predominantly adsorbed element, followed by Zn, Fe, V, Ni, and Cu. These findings demonstrate that chromic acid pre-treatment of recycled LDPE is an effective and cost-efficient strategy for enhancing uranium recovery from seawater.