Comparison of Processes for the Degradation of Trans-Cinnamic Acid: Anodic Oxidation, Electro-Fenton and Photoelectro-Fenton

INTRODUCTION

Olive mill wastes worsen environmental problems in Mediterranean countries. These wastewaters contain phenolic compounds used as fragrances and artificial flavours and other compounds such as trans-cinnamic acid.

The electrochemical techniques, especially the electrochemical advanced oxidation processes (EAOPs), are applied in many fields including the wastewater treatment. EAOPs are based on the in situ electrochemical generation of the hydroxyl radical (^●OH), which has so high oxidizing power that it is able to yield dehydrogenated or hydroxilated derivatives that can be oxidized up to their complete mineralization.

EXPERIMENTAL

In this work, the degradation of trans-cinnamic acid was studied by means of various EAOPs. Solutions of 100 mL of this compound in 0.05 M Na₂SO₄ of pH 3.0 were comparatively treated by anodic oxidation with electrogenerated H₂O₂ (AO-H₂O₂), electro-Fenton (EF) and photoelectro-Fenton (PEF) using a stirred tank reactor equipped with a 3 cm² boron-doped diamond (BDD) anode and a 3 cm² carbon-polytetrafluoroethylene air-diffusion cathode. In AO-H₂O₂, the acid and its intermediates were destroyed by ^●OH formed at the BDD surface, whereas in EF the mineralization rate was enhanced by ^●OH generated in the bulk from Fenton’s reaction between cathodically generated H₂O₂ and 0.5 mM Fe²⁺ added at the beginning of the experiment. In the case of PEF, the degradation process can be significantly enhanced under the photocatalytic action of UVA radiation provided by a 6 W artificial lamp. For each method, the influence of current density and trans-cinnamic acid content on TOC decay was examined. The results revealed that the PEF process was always the most efficient treatment, allowing an almost total mineralization of a 100 mg L^-1 TOC solution of trans-cinnamic acid after 180 min of electrolysis at 33.3 mA cm^-2. Generated short-chain aliphatic carboxylic acids as oxalic, acetic and fumaric in the above treatments were identified and quantified by ion-exclusion HPLC.

RESULTS

For all the EAOPs tested, TOC was gradually reduced at prolonged electrolysis. The oxidation ability increased in the sequence AO-H₂O₂ < EF < PEF. For each method, the mineralization was enhanced with increasing current density. PEF became the most potent method, yielding 98% mineralization after 180 min at 33.3 and 66.7 mA cm^-2, which showed that UVA light was a powerful tool.

TOC removal increased at a higher initial trans-cinnamic acid concentration, especially in the EF and PEF assays, thereby making the process much more efficient.

HPLC chromatograms of electrolyzed solutions exhibited well-defined peaks related to oxalic (t_r = 6.96 min), acetic (t_r = 15.1 min) and fumaric (t_r = 15.6 min) acids. The two latter acids are formed from the cleavage of the benzene ring of trans-cinnamic acid, being oxidized to oxalic acid that is directly and completely mineralized to CO₂in 240 min by PEF.

DISCUSSION

Trans-cinnamic acid solutions can be decontaminated at pH 3 and 25 °C using a BDD/air-diffusion cell. This is due to the oxidation with ^●OH formed from water oxidation at the BDD surface in AO-H₂O₂, the additional destruction with ^●OH produced in the bulk from Fenton’s reaction between generated H₂O₂ and added Fe²⁺in EF, and the synergistic action of photolysis of Fe(III)-carboxylate complexes under UVA irradiation in PEF.

The mineralization rate of trans-cinnamic acid increases in the order AO-H₂O₂ < EF < PEF under the same experimental conditions due to the larger production of ^●OH and the action of UVA light.

For each EAOP, higher mineralization rate was found at higher current density as a result of the higher generation of ^●OH at the BDD surface and in the bulk, but the mineralization current efficiency decreased because of the enhancement of parasite reaction of ^●OH.

An increase in trans-cinnamic acid concentration caused a higher TOC removal with superior current efficiency due to the minimization of parasite reactions of ^●OH by the presence of more organic matter.

The destruction of the parent molecule and its intermediates yielded final short-chain carboxylic acids such as fumaric, acetic and oxalic. The latter was the most persistent but its Fe(III) complexes could be rapidly photodecomposed in PEF.

The PEF process was the most potent EAOP giving rise to an almost total mineralization of solutions with 100 mg L^-1 TOC of trans-cinnamic acid after 180 min at 33.3 mA cm^-2.

The EAOPs tested demonstrated their feasibility to decontaminate water containing trans-cinnamic acid, opening the door to develop more effective technologies for the treatment of olive mill wastewaters.

ACKNOWLEDGEMENTS

The authors are grateful to MINECO (Spain) for financial support from project CTQ2013-48897-C2-1-R, co-financed with FEDER funds. The scholarship given to N.E Flores from SENECYT (Ecuador) is also acknowledged.