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 ?⁽³⁾ technique at 1 mMNaCl 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 ?⁽³⁾ technique to be +0.0031(5) C/m ², 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 UO₂CO ₃. 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.