PHENOLIC-ACID REDOX PROPERTIES - PH INFLUENCE ON IRON(III) REDUCTION BY CAFFEIC ACID

Phenolic acids are of great interest to soil chemists because their re dox properties affect the availability of micronutrients to plants. In order to provide information about the mechanisms that regulate the r eduction of Fe(III) at the soil-root interface, the redox activity of caffeic acid (CAD at different pH values in aqueous solution was inves tigated. The kinetics of the redox reaction was studied by using syste ms with Fe(III)/CAF molar ratios ranging from 1.1 to 10.9. At pH < 3, all systems showed the highest redox capacity: a maximum of nine elect rons for each molecule of CAF was released at a Pe(III)/CAF molar rati o of 10.9. At pH > 3, the redox capacity dramatically decreased and wa s very low above pH 4. At pH > 4, the reduction of Fe(III) was conside rably inhibited due to the formation of Fe(III)-CAF complexes as well as of Fe(OH)(3) precipitates, which are active in the adsorption of CA F. The amount of Fe(II) determined after 24 h of reaction suggests tha t not only the aquo ion, but also the hydrolized Fe(III)-soluble speci es, are active in the oxidation of the organic molecule. At about pH 4 , the ultraviolet/visible spectra revealed the presence of Fe(Im-CAF c omplexes, which gave rise to the formation of a black polymeric produc t. The infrared spectra of this product suggest that the CAF molecule interacts with the positively charged hydroxide through the carboxylat e groups. The reaction was not influenced by the presence of metal ion s such as Cu(II), Ni(II), Mn(II), Fe(II), and Zn(II). We have proposed a mechanism of the reduction and complexation process consistent with our results.