Nonlinear Optical Studies of Model Environmental InterfacesPublic Deposited
Surfaces and interfaces, which are ubiquitous in the environment, control the mobility,speciation, and ultimate fate of groundwater species. The presented work provides insight into the interfacial behavior of three environmental pollutants: hexavalent chromium, oxytetracycline, and morantel. Model surfaces are employed for these studies, including (1-102) α-Al<sub>2</sub>O<sub>3</sub> and a carboxylic acid-terminated adlayer on fused quartz, both of which are important environmental interfaces. To study interfacial phenomena, we exploit the power of the nonlinear optical technique second harmonic generation (SHG) because of its high sensitivity, surface selectivity, and ability to interrogate insulating materials. We demonstrate the power of SHG by obtaining orientation information for morantel adsorbed at the fused quartz-water interface, and the orientation differences for oxytetracycline adsorbed to several interfaces. We also improve the interpretation of physical parameters measured by SHG by determining the effects of the phase difference between the resonant and nonresonant contributions to the SHG signal. We begin applying the power of SHG to environmental interfaces by using the SHG χ<sup>(3)</sup> technique. We monitor the surface potential of a carboxylic acid-terminated siloxane adlayer, a model for the carboxylic acid groups of natural organic matter in soil, as a function of pH and observe two pKa values for the interface. The first of these pKa values, 5.6(2), is near typical values for bulk carboxylic acids, while the second, 9(1), is attributed to deprotonation occurring within a hydrogen-bonding network. From these measurements, we obtain thermodynamic state information for the surface, including the change in free energy density. Knowledge of the free energy density is important, since lowering this parameter is a major driving force for pollutant adsorption. Additionally, we study the adsorption of hexavalent chromium to α-Al<sub>2</sub>O<sub>3</sub> using resonantly enhanced SHG. In these experiments, we measure pH-dependent equilibrium adsorption constants. Modeling the experimental data allows us to decouple possible surface adsorption processes and provide equilibrium constants for each process. Our modeling suggests that chromate, CrO<sub>4</sub>-2, is the dominant adsorbed species. This work provides fundamental insight into the adsorption of hexavalent chromium to an Al<sub>2</sub>O<sub>3</sub>-water interface.