Synergistic role of uranyl ion in sorption of selenate oxyanion during Fe(II)-catalyzed transformation of ferrihydrite nanoparticles

Abstract Selenate oxyanions interact weakly with Ferrihydrite (Fh) and its transformation products, causing Se mobility and toxicity in the environment. Here, we investigated the sorption behaviour of selenate ion during Fe(II) aq. - catalyzed transformation of Fh under varying U and Fe(II) aq. concentration. Batch sorption study at pH ∼5.8 indicated increasing sorption of selenate ions on Fh with uranyl concentration (0.5–1.5 mM) and a fixed initial selenate concentration (1 mM), following Langmuir-type adsorption behaviour. Surface-sorbed selenate ions showed exponentially increasing retention behaviour on contact with varying [Fe(II) aq. ], suggesting surface co-precipitation with uranyl ions. The sorption behaviour is rationalized using the goethite (Gt) phase formation, as an index ( α Gt ), in the solids of the Fe(II) aq. -contacted U-Se-Fh sorption systems. This index, based on intensity corresponding to the (101) plane of Gt, inherently includes the effect of the crystallinity of Gt particles. In a six-hour kinetic study, selenate ion sorption parallels α Gt but a complex interplay of uranyl ions, selenate and Fe(II) aq. regulates α Gt by controlling nucleation-growth of Gt particles, which in turn defines sorption in the equilibrium state. These findings emphasize the role of co-contaminant in comprehensive understanding of the environmental behaviour of poorly sorbing contaminants.