Interfaces and surfaces are ubiquitous in the environment. Heterogeneous binding events and reactions control the transport and speciation of dissolved pollutants in groundwater. Many heterogeneous processes occur at mineral oxide/water interfaces, and can be controlled either by the mineral oxide, or organic molecules bound to the surface. The prevalent natural organic molecules humic and fulvic acid contain carboxylic acid functional groups, which can chelate aqueous metal ions - a critical step in controlling the phase of metals in the environment. This work uses the nonlinear optical technique second harmonic generation (SHG) to study two common pollutants and organic adlayers at the interface between water and the most abundant mineral oxide, silica. SHG laser spectroscopy is well-suited for studying environmentally relevant interfaces. SHG allows for highly sensitive measurements under environmentally relevant temperatures and solute concentrations. By simultaneously employing complimentary techniques to probe the bulk, bulk concentration and surface coverage are correlated, allowing for the determination of kinetic, thermodynamic and structural information. In this work, SHG has been used to study the interaction of the agricultural antibiotic morantel with the silica/water interface. Isotherm measurements result in a ∆Gads of -42(3) kJ/mol at pH 7, consistent with a hydrogen-bonding interaction. Real-time tracking of morantel binding shows that the events are fully reversible, consistent with its high mobility in silica-rich soil environments. Additionally, SHG studies were carried out to characterize a carboxylic acid-functionalized silica/water interface between pH 2 and 12. Two acid-base equilibria are observed, and the high sensitivity of SHG to interfacial potential, surface charge density, and surface free energy density was demonstrated. The experimental methodology provides quantitative thermodynamic information necessary for understanding how solvated species interact with organic adlayers. SHG was used to follow the interaction of dissolved Mn(II) with carboxylic acid groups at a silica/water interface. Similarly to the morantel experiments, an isotherm was constructed showing a ∆Gads of -27.8(3) kJ/mol at pH 7 and fully reversible binding events. This work demonstrates the ability of SHG to probe pollutants that do not have an electronic transition in the UV-vis.

Last modified

• 08/16/2018

Creator

• Christopher Thomas Konek

DOI

• https://doi.org/10.21985/N2W71C

Subject

• Chemistry

Keyword

• morantel
• second harmonic generation
• manganese
• surfaces
• SiO2
• carboxylic acid

Date created

• 2007-08-09

Resource type

• Dissertation

Rights statement

• In Copyright