U(VI) Adsorption and Speciation at the Acidic Silica/Water Interface Studied by Resonant and Nonresonant Second Harmonic Generation

Second harmonic generation (SHG) was used to study U(VI) interactions with a buried mineral oxide/water interface using pH 4 uranyl solutions that contained carbonate and environmentally relevant electrolyte concentrations. SHG resonance enhancement at 290 nm was observed at a NaCl concentration of 10 mM, whereas it was necessary to collect the adsorption isotherm using the nonresonant χ(3) technique at 1 mM NaCl concentration. Both methods resulted in the same (within one kT) adsorption free energy of −33 kJ/mol. This value is 10 kJ/mol less favorable than the free energy that was previously reported for the same system at pH 7, indicating that uranyl–surface interactions depend on pH. The charge density due to adsorbed uranyl species was also determined via the χ(3) technique to be +0.0031(5) C/m2, which is half the magnitude previously measured at pH 7. On the basis of the resonant and nonresonant SHG data, it is concluded that the surface-active uranyl species at pH 4 are neutral or univalent, cationic species, which, based on thermodynamic speciation calculations, can consist of the following aquated species present in bulk solution at pH 4: UO2(OH)2, UO2OH+, and UO2CO3. The results of this work are of value as benchmarks for geochemical transport models of uranium pollution resulting from acid mine drainage and nuclear waste storage.