Metal Ion Adsorption and Speciation at the Mineral/Water Interface Studied by Second Harmonic Generation

The mobility of metal ions in soils is an area of concern for environmental scientists, biologists, geochemists, and pollution regulators. This mobility is dictated by interactions of metal species with the soil/water interface. Therefore, fundamental investigations into the adsorption thermodynamics, surface coverage, and speciation were performed for a series of environmentally relevant metal ions at a mineral/water interface. The silica/water interface was used as a model for naturally occurring soil/water interfaces. The fundamental, molecular-level results determined at this silica/water interface are of use in creating a predictive basis for understanding metal ion behavior in natural soil systems.;The nonlinear spectroscopy of second harmonic generation (SHG) was used for the adsorption studies. SHG is a surface-specific technique that allowed for real time, in situ monitoring of interfacial metal ion behavior under flow conditions, at environmentally relevant analyte and electrolyte concentrations. Initially, the Eisenthal chi(3) technique was used to quantify the thermodynamics of binding for a series of divalent metal ions at the silica/water interface and the carboxylic acid-functionalized silica/water interface. The measured adsorption free energies were observed to double when surface organic groups were present.;SHG methods were developed for determining the speciation of metal adsorbates at a mineral/water interface. For Sr(II) at the silica/water interface, a plot of measured free energies versus interfacial potential revealed that Sr(II) undergoes a reduction in its charge state, from +2 to +1 at screening electrolyte concentrations above 10 mM. This change in speciation is believed to result from the incorporation of an electrolyte anion into the Sr 2+ hydration sphere.;SHG was also utilized to study the interfacial adsorption of U(VI). With resonantly enhanced SHG, an in situ surface electronic spectrum was collected for U(VI) at the fused/silica water interface at pH 4 and 7. The measured adsorption thermodynamics indicated that uranyl adsorption is 10 kJ/mol less favorable at pH 4 than at pH 7. By combining a free energy verses potential analysis with resonantly enhanced SHG and chi(3) salt screening experiments, it was determined that a mixture of neutral and univalent, cationic uranyl species are surface active at both pH 4 and 7.