Iron extractibility from iron-humate complexes by a siderophore and a mixture of organic acids

Two well-characterized humic acids (HA), extracted from a volcanic soil and a leonhardite, were used to synthesize insoluble Fe complexes. The complexes were treated at different concentrations and contact times with a siderophore (deferoxamine mesylate) and a mixture of simple organic acids (citric, oxalic, tartaric, and ketoglutaric acids) naturally occurring in root exudates in order to evaluate the capacity of the extracting solutions to desorb Fe from the humic complexes. Results showed that iron can be extracted from the complexes according to the concentrations and pH of the extractants. The mixture of organic acids was generally a better extractant than the siderophore at the high concentration because of lower solution pH. Despite constant differences in solution pH, the extracting capacity of the two extractants was similar at lower concentrations. However, the extent of desorption varied with HA characteristics. Iron appeared to be more easily released from complexes with HA rich in aliphatic carbons than from those with HA rich in aromatic carbons. These findings were attributed to the stereochemical complexity of highly aromatic humic substances that prevents natural chelators to easily reach the Fe complexing sites. Key words: Humic substances, iron, siderophores, organic acids, plant chlorosis