Bonding and oxidation state of iron in humic complexes extracted from some Greek soils

Electron spin resonance and Mössbauer spectroscopy were used to obtain information on the nature of the bonding and of the oxidation state of iron in complexes of iron-humic substances, some prepared in the laboratory and others extracted from samples of Greek soils developed under different vegetative covers. In the synthetic FeHA complexes prepared at pH 2.8, iron was found to be both in trivalent and bivalent states. At pH 4.0, iron was in the trivalent state and only traces of Fe²⁺ were found.The ESR spectra of the natural HA's and FA's indicated that iron forms complexes with both of them. The Mössbauer spectra, however, did not provide verification for existence of Fe³⁺ organically bound to the humic acids. All the Mössbauer spectra of the natural humic acids showed minute quantities of iron in the ferrous state. The respective parameters indicated that the Fe²⁺ is directly bound to HA's. The ferrous iron probably resulted from the reduction of Fe³⁺ during the humification process. The Mössbauer spectrum of an FeFA complex prepared in the laboratory with a fulvic acid: metal ratio of (1:1) at pH 4.0 showed that iron occurred in both trivalent and bivalent states.Differences in the vegetative cover and in the taxonomic category of the soils studied did not have any effect on the Mössbauer parameters of the natural HA's, but differences were reflected in the ESR spectra of the FA's.