Ultrafine cellulose nanofibers as efficient adsorbents for removal of UO₂²⁺ in water

Ultrafine cellulose nanofibers, 5-10 nm in diameter, were prepared from oxidation of wood pulp using the (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO)/NaBr/NaClO process followed by mechanical treatment. Carboxylate groups on the surface of these nanofibers provide negative charges, which are very effective to adsorb radioactive UO₂²⁺ in water, evidenced by static adsorption and high resolution transmission electron microscopy (TEM) measurements. The UO₂²⁺ adsorption capability of ultrafine cellulose nanofibers was about 167 mg/g, which is 2-3 times higher than those of typical adsorbents such as montmorillonite, ion imprinted polymer particles, modified silica particles/fibrous membranes, and hydrogels. The high UO ₂²⁺ adsorption capability can be attributed to the very high surface-to-volume ratio, high surface charge density, and hydrophilicity of ultrafine cellulose nanofibers, which can be used as effective media to remove radioactive metals from radio-nuclear wastewater.