Investigations of the pH-Dependent Adsorption of Uranyl Species on Phosphorylated Cellulose-Nanocrystal Ferrihydrite: Insights from Combined Raman Spectroscopy and Computational Analyses

Uranium contamination in groundwater is a major environmental concern due to its long half-life and chemical toxicity. We had previously developed the phosphorylated cellulose-nanocrystal–ferrihydrite (PCNCFH), a low-cost and sustainable nanomaterial, exhibiting a uranium adsorption capacity of 100 mg/g, and comprehensively characterized the adsorbent’s performance and stability. In this study, we specifically focus on elucidating how environmentally relevant uranyl species such as UO 2 (CO 3 ) 3 4–, UO 2 (CO 3 ) 2 2–, and (UO 2 ) 3 (OH) 5 + interact with PCNCFH across the typical groundwater pH range of 5–9. Raman spectroscopy confirmed the speciation of uranyl complexes in solution, and time-dependent Raman measurements enabled us to quantify the adsorption kinetics and equilibrium adsorption capacities ( q e ) of each species under controlled pH conditions. We find that UO 2 (CO 3 ) 3 4– exhibits significantly slower adsorption kinetics than the other complexes at all pH values. These results were also supported by IR spectroscopy and DFT modeling, revealing fundamental differences in interaction pathways that govern uranyl binding. Raman adsorption data were found to fit a pseudo-second-order model. The findings provide mechanistic insight essential for the rational design of species-specific and pH-optimized uranium remediation strategies for water utilities.