Evidence for non-electrostatic interactions between a pyrophosphate-functionalized uranyl peroxide nanocluster and iron (hydr)oxide minerals

Abstract

The terminal oxygen atoms of the pyrophosphate groups in the uranyl peroxide nanocluster U₂₄Pp₁₂ ([(UO₂)₂₄(O₂)₂₄(P₂O₇)₁₂]⁴⁸⁻) are not fully satisfied by bond valence considerations and can become protonated. This functionality could allow for specific interactions with mineral surfaces, as opposed to the electrostatically-driven interactions observed between non-functionalized uranyl peroxide nanoclusters and mineral surfaces. The sorption of U₂₄Pp₁₂ to goethite and hematite was studied using batch sorption experiments as a function of U₂₄Pp₁₂ concentration, mineral concentration, and pH. A suite of spectroscopic techniques, scanning electron microscopy, and electrophoretic mobility measurements were used to examine the minerals before and after reaction with U₂₄Pp₁₂, leading to a proposed conceptual model for U₂₄Pp₁₂ interactions with goethite. The governing rate laws were determined and compared to those previously determined for a non-functionalized uranyl peroxide nanocluster. The rate of uranyl peroxide nanocluster sorption depends on the charge density and functionalized component of the uranyl peroxide cage. Electrophoretic mobility and attenuated total reflectance Fourier transform infrared spectroscopy analyses show that an inner-sphere complex forms between the U₂₄Pp₁₂ cluster and the goethite surface through the terminal pyrophosphate groups, leading to a proposed conceptual model in which U₂₄Pp₁₂ interacts with the triply-coordinated reactive sites on the (110) plane of goethite. These results demonstrate that the behavior of U₂₄Pp₁₂ at the iron (hydr)oxide-water interface is unique relative to interactions of the uranyl ion and non-functionalized uranyl peroxide nanoclusters.