SPECTROSCOPIC CHARACTERIZATION OF PYROPHOSPHATE INCORPORATION DURING EXTRACTION OF PEAT HUMIC ACIDS

Extraction procedures represent a crucial point in the study of the mo lecular structure of humic substances (HS). Although alkaline extracta nts such as NaOH plus Na4P2O7 are usually employed for the extraction of the HS from soil and peat, little information concerning the intera ction 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 pyrop hosphate solution (0.1 M NaOH + 0.1 M Na4P2O7) are reported. The HA we re redissolved in 5 mM NaCl, fractionated in six cut-off fractions (no minal molecular weight: HA >300, 100-300, 50-100, 20-50, 10-20, and 5- 10 kDa) using a tangential ultrafiltration system. All samples were di alyzed against distilled water and then freeze-dried. Fourier-transfor m infrared (FT-IR) and P-31 nuclear magnetic resonance (NMR) spectrosc opies were applied to characterize the incorporation of pyrophosphate (PP) in both the unfractionated HA and in the six HA fractions. Spectr oscopic 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 frac tions has been attributed to the presence of metals and the oxygenated groups of HA. The changes in the relative intensity of the bands attr ibuted to oxygenated functional groups observed in the FT-IR spectra c ould be assigned to the interaction of phosphate groups with metal cat ions, which are involved in a coordination complex with the oxygenated groups.