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Phenolic Acid Redox Properties: pH Influence on Iron(III) Reduction by Caffeic Acid

Dipartimento di Scienze Ambientali Agrarie e Biotecnologie Agroalimentari, Università di Sassari, Viale Italia no. 39, 1-07100 Sassari, Italy

Carlo Gessa

Istituto di Chimica Agraria, Università di Bologna, Viale Berti-Pichat, 10, I-40127 Bologna, Italy

Mauro Marchetti

Istituto per l'Applicazione delle Tecniche Chimiche Avanzate ai Problemi Agobiologici, C.N.R., Via Vienna 2, I-07100 Sassari, Italy

Marianna Usai

Istituto di Botanica Farmaceutica, Università de Sassari, via Muroni, 23/A, I-07100 Sassari, Italy

*Corresponding author.

ABSTRACT

Phenolic acids are of great interest to soil chemists because their redox properties affect the availability of micronutrients to plants. In order to provide information about the mechanisms that regulate the reduction of Fe(III) at the soil-root interface, the redox activity of caffeic acid (CAF) at different pH values in aqueous solution was investigated. The kinetics of the redox reaction was studied by using systems 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 electrons for each molecule of CAF was released at a Fe(III)/CAF molar ratio of 10.9. At pH > 3, the redox capacity dramatically decreased and was very low above pH 4. At pH > 4, the reduction of Fe(III) was considerably 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 CAF. The amount of Fe(II) determined after 24 h of reaction suggests that not only the aquo ion, but also the hydrolized Fe(III)-soluble species, are active in the oxidation of the organic molecule. At about pH 4, the ultraviolet/visible spectra revealed the presence of Fe(III)-CAF complexes, which gave rise to the formation of a black polymeric product. The infrared spectra of this product suggest that the CAF molecule interacts with the positively charged hydroxide through the carboxylate groups. The reaction was not influenced by the presence of metal ions 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.

Received for publication January 3, 1994.