Harnessing waste PET bottles for sustainable Ca-MOF synthesis: a high-efficiency adsorbent for uranium and thorium†

The contamination of water sources by radioactive elements like uranium and thorium presents significant environmental and health challenges. Yet, emerging as a beacon of innovation, waste derived calcium metal–organic frameworks crafted from discarded plastic bottles and waste marble offer a sustainable solution. This novel approach not only tackles water purification challenges but also showcases the potential for waste recycling, symbolizing a fusion of environmental conservation and technological advancement. Multiple characterization techniques, including Fourier-transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FESEM), energy dispersive X-ray spectroscopy (EDX) mapping, and Brunauer–Emmett–Teller (BET) analysis, have verified the presence of abundant functional groups in the waste calcium MOF, revealing a microporous structure with a rod-like morphology. In batch experiments, the adsorbent's effectiveness was confirmed, achieving high removal efficiencies of 98.99% for U(VI) ions and 99.18% for Th(IV) ions at pH 5. Optimal performance was demonstrated using 6 mg and 3 mg of adsorbent per 10 mL from 50 mg L⁻¹ solutions of U(VI) and Th(IV), respectively, with removal times of 15 minutes for U(VI) and 10 minutes for Th(IV). The Box–Behnken design model was utilized to confirm the appropriateness of these parameters. Kinetic studies identified a pseudo-second-order model, while isotherm analysis confirmed compatibility with the Langmuir model, yielding maximum adsorption capacities of 829.18 mg g⁻¹ for U(VI) and 273.16 mg g⁻¹ for Th(IV). Additionally, the material demonstrated reusability for up to 5 consecutive cycles, highlighting its efficacy in adsorbing and regenerating U(VI)/Th(IV) from water.