Diffuse Reflectance and Transmission Fourier Transform Infrared (DRIFT) Spectroscopy of Humic and Fulvic Acids

Abstract

The infrared spectra of a humic and a fulvic acid from a peat were obtained using diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy and were compared with transmission Fourier transform infrared (FTIR) spectra using KBr pellets. The DRIFT spectra showed similar resolution to KBr pellet transmission spectra. Elimination of interference due to water bands is easy with DRIFT and possible changes in the spectra due to modification of H bonding and ionization of carboxylate groups in the KBr pellet is avoided. Comparison of the DRIFT spectra of humic substances with the DRIFT spectra of some reference compounds suggests that phenolic OH groups in fulvic acids are more H‐bonded than those in humic acids, but the COOH groups are similarly H‐bonded. Comparison of the C=O stretching band with those of the reference compounds suggests that either intramolecular H‐bonding between COOH and phenolic OH is not significant in humic substances or alkyl COOH groups contribute considerably in the 1720 cm⁻¹ band. The DRIFT spectra of Na and Cu(II) forms of humic acid powders show the capability of the method for metal‐ion binding studies. The possibility of using DRIFT spectroscopy for quantitative and semiquantitative evaluation is suggested by the similarity in the Kubelka‐Munk transformed DRIFT spectra and absorbance spectra using KBr pellets.