The Journal of Argentine Chemical Society
On-line version ISSN 1852-1428
PIDELLO, A et al. Characterization of electrochemical properties in fractions of different molecular size taken from natural soil organic matter of Argentine humid pampa. J. Argent. Chem. Soc. [online]. 2007, vol.95, n.1-2, pp. 10-19. ISSN 1852-1428.
Soil organic matter (SOM) is mainly formed by humic substances (HSs), which exhibit a highly complex structure. In order to establish causality relationships in redox interactions between HSs-microorganisms or HSs-soil abiotic factors, the characterization of HSs electroactive behavior needs to be researched in depth. To study the electrochemical characteristics of the molecular assemblies found in humic complexes, the components of HSs must be isolated and identified avoiding the use of drastic methods of separation and characterization. For these investigations a mild extractant, as ethylenediaminetetracetic acid (EDTA), is preferred, even it is less effective than alkali hydroxides in removing SOM. The objectives of this work were to fractionate the HSs compounds extracted with EDTA pH=7 with a molecular sieve (Sephadex G25) and to determine the significance and location in the electrochemical series of the electroactive pairs that make up the most important HSs fractions of these extracts. Eluates of the centrifuged Mollisol soil - EDTA pH=7 extracts were subjected to spectrophotometric scanning (220 and 415 nm) and the amount of electroactive compounds was determined in the fractions with higher absorbance (called f I and f II) through Differential Pulse Polarography (DPP) within a +300 to -900 mV range of potentials. Results showed that f I (excluded fraction which correspond to a molecular weight range assigned of humic acids) had a total concentration of electroactive compounds 10% lower than f II (that showed a molecular weight range assigned of fulvic acid). However, f I had a higher amount of electroactive compounds concentration at lower potentials (-200 to - 900 mV) and a lower concentration of electroactive compounds at higher potentials (-200 to +300 mV) than f II. The results also showed that the value of reducing status in f II was lower (about 23%) than f I. Our results raise the possibility that in front of biotic or abiotic redox effectors may exists electron exchanges between fractions of the HSs in accordance with the general concept that the energetically most favorable available electron acceptors are utilized first. The knowledge of the basic electrochemical nature of the HSs fractions of the studied Mollisol soil needs to be considered in future studies of the impact of many technological practices in our region, for instance, soil inoculation with microorganisms, soil fertilization practices or soil amendments with organic or inorganic compounds.
Keywords : Natural soils; Humic substances; Electrochemistry; Polarography.