UMTP and Department of Chemistry, University of Beira Interior, 6201-001 Covilhã, Portugal

doi: 10.4152/pea.201105343

Abstract

The influence of the hydrodynamics of the system on the extent of the electrochemical degradation of phenol, using a boron doped diamond (BDD) anode was investigated. Two different electrochemical cells were used: a batch stirred cell (A), with a volume of 200 mL and a BDD anode of 17.5 cm², and a batch with recirculation cell (B), with a BDD anode of 70 cm² and connected to a tank of 30 L. Assays were performed at different stirring speeds and applied current density of 300 A m^-2 , for cell A, and various flow rates, with an imposed current density of 100 A m^-2, for cell B. Chemical oxygen demand (COD) and total organic carbon (TOC) tests were performed to the samples collected throughout the assays, as well as UV-Vis spectrophotometric measurements. For cell A, after 2 h assay, COD removals between 84 and 94% and TOC removals ranging from 54 and 86 % were attained. For the assays run with cell B, during 10 h, COD and TOC removals varied from 27 to 51% and from 23 to 46%, respectively. The influence of the turbulence near the electrode's surface in the combustion efficiency was also analysed.

Keywords: phenol; boron doped diamond electrode; electrochemical oxidation; hydrodynamic conditions.

Introduction

Phenol is one of the pollutants whose degradation has been under intense study in the last decades. In fact, it is often chosen as model pollutant. G. Busca and collaborators presented an extensive review over the different ways to promote its degradation [1].

Electrochemical oxidation of effluents containing persistent organic pollutants is an alternative or tertiary treatment that has received much attention in the last years due to its interesting characteristics, since it is a clean process, can operate at low temperature and, in most cases, without adding any chemical and without sludge formation. Recently, the use of boron doped diamond (BDD) as anode material has proved to be very useful, since, besides its good mechanical and electrochemical stability, it has unique electrochemical characteristics, like large potential window for water decomposition [2], ability to produce hydroxyl radicals [3] poorly adsorbed on the electrode's surface [4] and that can perform the oxidation of the pollutants simultaneously in the bulk of the solution. Therefore, BDD anodes are an excellent material for the oxidation of all kinds of pollutants [5]. In particular, the electrooxidation of phenol and other phenolic compounds has already been carried out with BDD anodes, at bench scale, with good degradation rate [6,7]. However, to scale-up the process to an industrial level, a better understanding of the mass transfer during the electrochemical process is needed. The objective of this work is to study the influence of the hydrodynamic conditions on the mass transfer process during the anodic oxidation of phenol, using BDD as anode material, in two different electrochemical cells: a small batch stirred cell and a pilot cell with re-circulating flow.

Experimental

Two sets of electrochemical assays were run, using different experimental setups. The first one (A) consisted of an electrochemical cell, with a BDD anode of 17.5 cm² area, purchased from Adamant Technologies/CSEM, Switzerland, and the degradation was carried out in batch mode, for 2 h, at several stirring rates, using 200 mL of an aqueous 0.035 M Na₂SO₂ solution, containing 300 mg L^-1 of phenol (purchased from Aldrich). Current was imposed by a laboratory power supply GW, model GPS-3O3OD. Cell B was a DiaCel 196PVDF, with two monopol BDD electrodes (70 cm² area), from Adamant Technologies/CSEM, Switzerland, used with a Power supply DiaCell, PC-1500. These assays were performed in batch with recirculation mode, for 10 h, and a centrifugal pump, Wilo, Serie MHI, enabled the recirculation of the solution at the different flow rates tested. The composition of the solution was identical of that in cell A, but a volume of 30 L was used here and re-circulation flow rates of 200, 300 and 500 L h^-1 were tested.

The electrochemical assays were performed in galvanostatic mode, at imposed current densities of 300 and 100 A m^-2 for cells A and B, respectively. Phenol degradation tests were followed by UV-Visible spectrophotometry, with absorbance being measured from 200 to 600 nm, using a UNICAM -Helios-a UV/VIS spectrophotometer, by Chemical Oxygen Demand (COD) tests, using the closed reflux dichromate titrimetric method [8], and by Total Organic Carbon (TOC) determinations, performed in a Shimadzu TOC-VCPH apparatus.

Results and discussion

Fig. 1 presents the experimental COD removal as a function of the assays' duration, for the different experimental conditions tested with both electrochemical cells.