Spectroscopic characterization of pyrophosphate incorporation during extraction of peat humic acids

Extraction procedures represent a crucial point in the study of the molecular structure of humic substances (HS). Although alkaline extractants such as NaOH plus Na4P2O7 are usually employed for the extraction of the HS from soil and peat, little information concerning the interaction between extractants and humic acids is available. In this study, the results on the molecular structure of humic acids (HA) extracted and separated from a Sphagnum Irish peat sample with an alkaline pyrophosphate solution (0.1 M NaOH+0.1 M Na4P2O7) are reported. The HA were redissolved in 5 mM NaCl, fractionated in six cut-off fractions (nominal molecular weight: HA>300, 100-300, 50-100, 20-50, 10-20, and 5-10 kDa) using a tangential ultrafiltration system. All samples were dialyzed against distilled water and then freeze-dried. Fourier-transform infrared (FT-IR) and 31P nuclear magnetic resonance (NMR) spectroscopies were applied to characterize the incorporation of pyrophosphate (PP) in both the unfractionated HA and in the six HA fractions. Spectroscopic analyses showed that it was possible to assign a given pattern of bands to define the structure of phosphate: PP was present only in the unfractionated HA, whereas orthophosphate was present in both the high molecular weight and lower molecular weight fractions. The high concentration of phosphate found in the lower molecular weight HA fractions has been attributed to the presence of metals and the oxygenated groups of HA. The changes in the relative intensity of the bands attributed to oxygenated functional groups observed in the FT-IR spectra could be assigned to the interaction of phosphate groups with metal cations, which are involved in a coordination complex with the oxygenated groups.