Uranyl Adsorption and Speciation at the Fused Silica/Water Interface Studied by Resonantly Enhanced Second Harmonic Generation and the χ⁽³⁾ Method

Abstract

We report the first use of resonantly enhanced second harmonic generation (SHG) to study uranyl adsorption at a buried mineral oxide/water interface. Uranyl adsorption is studied in real-time, under flow conditions, and in the presence of environmentally relevant screening electrolyte concentrations. The in situ SHG spectrum of surface-bound uranyl reveals a well-defined resonance at 306 nm. By monitoring the SHG response at 306 nm, adsorption isotherms were collected for uranyl species at the fused silica/water interface at pH 7, and in the presence of aqueous carbonate. The measured adsorption free energies determined by the Langmuir isotherm are consistent with physisorption via hydrogen bonding. The speciation of the surface-active uranyl species at pH 7 was elucidated via a free energy versus interfacial potential analysis, which reveals that the uranyl adsorbates are either neutral or univalent cationic species. Complementary surface charge density data, obtained using the Eisenthal chi(3) technique, reveal that the charge on the ionic uranyl species adsorbing under the experimental conditions are positive. It is proposed that a mixture of neutral and univalent, cationic uranyl species is surface active at pH 7 and in the presence of carbonate ions. Insofar as the experimental conditions model those found in natural groundwater systems, the results of this work are valuable to the prediction and assessment of uranium pollution transport in groundwater and soils. Our thermodynamic results can also serve as important benchmarks for computer simulations of U(VI) transport in heterogeneous geochemical environments.

Evidence for Considerable Metal Cation Concentrations from Lithium Intercalation Compounds in the Nano–Bio Interface Gap

“…As shown previously, − second harmonic generation (SHG) χ (3) measurements can yield important information about binding thermodynamics and electrostatics. − SHG spectroscopy adsorption isotherms (Figure A) in which the SH signal intensity is monitored as a function of Co 2+ concentration in the presence of SLBs formed from DMPC and a 9:1 mixture of DMPC and DMPG at a constant salt concentration of 0.1 M allow us to explore the role of PG-lipids in promoting Co 2+ adsorption, to estimate the adsorption Gibbs free energy and the interfacial charge density, and to investigate reversibility, thereby demonstrating the wide utility of the χ (3) method for studying the nano–bio interface. Figure A shows the adsorption of Co 2+ to single-component lipid bilayers formed solely from DMPC results in little change in the SHG intensity as the Co 2+ concentration is increased.…”

Optical label-free and model-free probe of the surface potential of nanoscale and microscopic objects in aqueous solution

“…In practice, one who aims to extract the surface potential from a nonresonant SHS experiment applies the following 'Eisenthal-χ (3) method' [29]: The SH intensity scattered by particles in solutions is measured at a fixed scattering angle as a function of the ionic strength (c) of the solution in any (unspecified) polarization combination. This now widely applied procedure [20,23,27,29,30] is very similar to the method applied to planar interfaces [31][32][33][34][35][36] using the same expressions. However, by relying on a single equation to extract three parameters from an arbitrarily normalized data set, a unique solution for 0 cannot be obtained.…”

Qualitative and Quantitative Analyses of the Molecular-Level Interaction between Memantine and Model Cell Membranes

“…Besides, the third-order χ (3) term also contributes to the SFG signal of water due to the presence of positive charge at the interface. For a charged interface, the contribution of the third-order χ (3) to the second harmonic generation (SHG) signal has been scaled and developed into a methodology to investigate the interfacial adsorption, interaction, and reaction by the Eisenthal and Geiger groups. ,− Herein, there are two contributions (the second-order χ ( 2) and third-order χ (3) ) to the SFG signal intensity of the interfacial water. Without the addition of memantine, the SFG signals of interfacial water mainly come from the second-order χ (2) contribution.…”