Kinetic and thermodynamic studies on the interaction of europium(III) and phosphate with γ-Al2O3

Kinetics of europium sorption to four different aluminum (hydr)oxides: Corundum, γ-alumina, bayerite, and gibbsite

Kinetic studies can improve our understanding of the environmental mobility of radionuclides by providing insight into sorption mechanisms occurring at the mineral-water interface. While nearly identical equilibrium europium sorption edges are obtained for gibbsite (γ-Al(OH)₃), bayerite (α-Al(OH)₃), corundum (α-Al₂O₃), and γ-Al₂O₃, differences in surface acidity and crystal structure result in distinct reaction rates. Batch kinetic experiments were performed to investigate europium sorption to four aluminum (hydr)oxide minerals as a function of time (10-120 min), mineral concentration (0.5-50 m² L^-1), pH (5-7), and europium concentration (10^-8-10^-6 M). Linear and non-linear pseudo-first and pseudo-second order rate equations were used to fit the data. The overall reaction rates and reaction orders with respect to proton concentration, mineral concentration, and europium concentration were determined, leading to a governing rate law for each mineral system. Europium sorption is characterized by a two-step reaction with a fast initial sorption step followed by a slower sorption or aging process. The pseudo-second order equations resulted in better model fits of the data. Using the non-linear pseudo-second order rate constants, it was observed that the reaction orders for each mineral system were similar but resulted in different overall reaction rates following the trend bayerite ∼ γ-Al₂O₃ > corundum > gibbsite. However, the calculated reaction orders are similar to those previously reported for plutonium(IV/V) interactions with iron (oxyhydr)oxide minerals, suggesting that these binary systems share similar elementary reactions.

Kinetics of Uranyl Peroxide Nanocluster (U60) Sorption to Goethite

The unique properties of uranium-based nanomaterials may significantly impact our current understanding of the fate and transport of U(VI) in environmental systems. Sorption of the uranyl peroxide nanocluster [(UO₂)(O₂)(OH)]₆₀^60- (U₆₀) to goethite (α-FeOOH) was studied using batch sorption experiments as a function of U₆₀ concentration (0.5-2 g·L^-1), mineral concentration (100-500 m²·L^-1), and pH (8-10). The resulting rate law describing U₆₀ interactions with goethite at pH 9 was R = - k_rxn[U₆₀]^0.29±0.02[goethite]^1.2±0.1 where k_rxn = (6.7 ± 2.0) × 10^-4 (g·L^-1)^0.71±0.02(m²·L^-1)^-1.2±0.1(day^-1). The largest fraction of U₆₀ removed from solution was at pH 8, which is below the isoelectric point of the goethite used in this study. Site density calculations suggest that U₆₀ may exist on the goethite surface at a center-to-center distance of 5.4-6.5 nm, depending upon pH, which mirrors the center-to-center distance observed in the aqueous phase near the U₆₀ solubility limit. At pH 10, approximately 20% uranium was desorbed within 3 days. Analysis of the reacted mineral surface using X-ray photoelectron spectroscopy confirmed the presence of a single U(VI) species on the mineral surface, and electrospray ionization mass spectrometry revealed that U₆₀ remains intact during the sorption and desorption processes. These results demonstrate that the behavior of U₆₀ at the goethite-water interface is similar to that of discrete U(VI) but is governed by different sorption mechanisms and reaction kinetics, which has the potential to alter our current understanding of the fate and transport of uranium species in the environment.