Interfaces play critical roles in chemical processes from nucleation, precipitation, dissolution, sorption to aggregation. For hydrated and hydroxylated metal oxides, hydroxides and oxyhydroxides, surface hydroxyls and adsorbed waters dictate the interfacial chemistry.  As the number of molecules at interfaces are small as compared to those in the bulk, results from common spectroscopy techniques such as IR, Raman, NMR and fluorescence are dominated by molecules in the bulk. Sum frequency generation – vibrational spectroscopy (SFG – VS) makes it possible to selectively probe the interfacial molecules due to its surface-/interface-specificity. In this work, we have applied SFG-VS in combination of Raman and FTIR spectroscopy to investigate the effect of metal ion (Cr, Eu) adsorption and γ-irradiation on the the surface hydroxyls and hydration waters on platelet nanoparticle gibbsite and boehmite, both are major constituents of high-level nuclear waste sludges, and contrasted with those in the bulk as revealed from the Raman spectral features.  Both metal ion adsorption and γ-irradiation lead to much more pronounced reduction of the spectral features of the surface hydroxyls relative to those in the bulk, indicating metal ion sorption on the surface and preferential radiolytic dehydrogenation of the hydroxyl groups. We also demonstrate that surface treatment by strong base, NaOH, results in lowered hydroxyl band intensity for gibbsite, consistent with possible deprotonation of surface hydroxyls in highly basic solutions.

Application of sum frequency generation – vibrational spectroscopy in complex chemical systems relevant to high-level tank waste

ACS Spring 2022

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