Attenuated Total Reflection-Fourier Transform Infrared and Atomic Force Microscopy-Infrared Spectroscopic Investigation of Suwannee River Fulvic Acid and Its Interactions with α-FeOOH

Suwannee River fulvic acid (SRFA) and its interaction with α-FeOOH have been investigated using two different spectroscopic techniques─attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy and atomic force microscopy-infrared (AFM-IR) spectroscopy. ATR-FTIR spectroscopy of SRFA thin films yields information on functional groups present within these films prepared from solutions of three different pH values (3, 6, and 8). This technique can also be used to probe the interaction of SRFA with α-FeOOH particle surfaces and the impact that pH has on these surface interactions. AFM-IR spectroscopy offers a different perspective as it probes both film morphology and spectral signals with nanoscale spatial resolution. Herein, we apply AFM-IR spectroscopy to investigate SRFA thin films and the interactions with α-FeOOH. Results from this study show that pH impacts the speciation of SRFA and its interaction with α-FeOOH. Furthermore, there are nanoscale and microscale heterogeneities in these thin films as shown in height images, point spectra, and spectral maps. Overall, these measurements using two different vibrational spectroscopic techniques provide insights into the heterogeneity of natural organic matter and its interactions with mineral surfaces.