The decontamination of olive oil mill wastewater (OMWW) is of great interest in Mediterranean countries as the main producers of olive oil worldwide. The large variety of recalcitrant substances contained in OMWW and the difficulty to remove them or recycle the large volumes of wastewater encourage the development of new technologies for decontamination, like electrochemical advanced oxidation processes (EAOPs) that present high performance for recalcitrant compounds [1,2]. 4-Hydroxyphenylacetic acid (4-HPA) is a main component of this kind of wastewater [3]. This communication presents the effective removal of 4-HPA from an OMWW matrix by EAOPs including anodic oxidation with electrogenerated H₂O₂ (AO-H₂O₂), electro-Fenton (EF) and UVA photoelectro-Fenton (PEF).

EXPERIMENTAL

Analytical grade 4-HPA was purchased from Sigma. The OMWW was provided by a local company of olive oil extraction, which was characterized by analyzing its metal content, BOD, COD, TOC, conductance, TS, TDS and organic components before use. As a first step, the OMWW was filtered with a 45 µm cloth. As a second step, several mixtures with 100 mg L^-1 TOC of 4-HPA in OMWW were prepared. The OMWW/water ratio was varied from 20/80 to 100/0 v/v. Experiments were also carried out using 100 mg L^-1 TOC of 4-HPA in 50 mM of Na₂SO₄ to clarify the matrix effect. AO-H₂O₂, EF and PEF experiments were performed with 100 mL of each solution at pH 3.0 and 25 ºC using a stirred tank reactor with a 3 cm² boron-doped diamond (BDD) anode and a 3 cm² carbon-polytetrafluoroethylene air-diffusion cathode by applying 16.7 mA cm^-2 for 540 min. In the two latter methods, 0.50 mM Fe²⁺ was added as catalyst for Fenton’s reaction. PEF trials were run with a 6W UVA light.

RESULTS

The characteristics of the real OMWW were: 505.35 mg L^-1 of TOC, 1.51 mS of conductance, 1600 mg L^-1 of BOD, 5536 mg L^-1 of TS and 82 mgL^-1 of TDS. For each method, a gradual decay in TOC was found as the OMWW ratio in the sample increased. For example, a final TOC removal of 89% was achieved by AO-H₂O₂ in 50 mM Na₂SO₄, whereas it was reduced by 75% in a 60/40 OMWW/water mixture and 60% in pure OMWW. The best results were obtained by PEF, with final TOC decays of 98%, 88% and 87% in a the above media due to the combined action of ^●OH produced at the BDD surface from water oxidation, the ^●OH formed during Fenton’s reaction and the extra decomposition of Fe(III)-carboxylate complexes by UVA light. The kinetic assays also showed a drop in 4-HPA decay with raising OMWW content. Thus, total disappearance of the substrate was attained in 360 min by AO-H₂O₂ in 50 mM Na₂SO₄ and in 500 min for the other solutions. For the best process (i.e., PEF), these times diminished to 60 and 80 min, respectively.

CONCLUSIONS

4-HPA can be effectively degraded in an OMWW matrix using the EAOPs studied, especially PEF. This process was the most efficient one, reaching an almost total mineralization, to a larger extent in pure Na₂SO₄. The decay and mineralization of 4-HPA rose in the order AO-H₂O₂ < EF < PEF, regardless of the matrix used. The oxidation power of ^●OH formed from water oxidation at the BDD surface in AO-H₂O₂ was strongly enhanced by the additional ^●OH in the bulk from Fenton’s reaction, being the degradation improved under the potent action of UVA irradiation in PEF. For all the EAOPs studied, the presence of OMWW decelerated TOC removal and 4-HPA decay. This can be related to the parallel reactions of ^●OH with the organic components present in OMWW that act as oxidant scavengers.

ACKNOWLEDGEMENTS

We are grateful for economical support from project CTQ2016-78616-R (MINECO, FEDER, EU) and for the scholarship given to N.E Flores from SENESCYT (Ecuador).

BIBLIOGRAPHY

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[2] A. Thiam, E. Brillas, J.A. Garrido, R.M. Rodríguez, I. Sirés, Routes for the electrochemical degradation of the artificial food azo-colour Ponceau 4R by advanced oxidation processes, Appl. Catal. B: Environ. 180 (2016) 227.

[3] C. Belaid, M. Khadraoui, S. Mseddi, M. Kallel, B. Elleuch, J.F. Fauvarque, Electrochemical treatment of olive mill wastewater: Treatment extent and effluent phenolic compounds monitoring using some uncommon analytical tools, J. Environ. Sci. 25 (2013) 220.