Electrooxidation of Diclofenac in Synthetic Pharmaceutical Wastewater Using an Electrochemical Reactor Equipped with a Boron Doped Diamond Electrode

Some studies have reported the abundance of drugs in groundwaters (1). Many of these compounds are not effectively removed by conventional wastewater treatment process. Some papers have reported the anodic oxidation treatment with BDD anode of wastewater contaminated with drugs (1-2). Massive quantities of hydroxyl radicals can be produced from water electrolysis at diamond surface, these radicals can oxidize the contaminants in the aqueous solution (3).

This paper presents a study the anodic oxidation of diclofenac in perchlorate medium at neutral pH in a FM01-LC reactor equipped with BDD anode. The inert perchlorate medium was studied to avoid byproducts of it during electrolysis; therefore, the diclofenac degradation can be mediated by the action of the hydroxyl radical produced on BDD surface.

Microelectrolysis tests were performed to determine the potential and current density limits, where diclofenac electrooxidation takes place. Electrooxidation of diclofenac were performed in a FM01-LC cell at different current density values of 10, 15 and 20 mA cm^-2 and at different mean linear flow velocities comprised between 14.6-58.4 cm s^-1.

 Electrooxidation evolution was estimated by COD analysis of samples taken at different times. Diclofenac concentration during electrochemical incineration was followed using UV-visible at 276 nm (2).

Figure 1 shows two typical sampled curves (j vs. E) in 0.5 M NaClO₄, pH 6.5, T = 298 K in absence of diclofenac without rotation speed (Fig. 1a) and at 300rpm of BDD RDE (Fig. 1b). The Tafel slope was inserted in Figure 1. From the analysis of this figure the anodic process corresponds to the oxidation of water. At potentials between 2.2-2.7 V vs. SHE the formation of hydroxyl radicals, takes place (3-4). While at E > 2.7 V vs. SHE the curve presents a considerable increase in the slope, which corresponds to the oxygen evolution reaction (3-4). Tafel curve analysis yield a slope of 0.810 V decade^-1. This value is higher than the reported by Michaud et al. (3) and Nava et al. (4) who report values of 0.23 and 0.25 V decade^-1, respectively. The difference of our Tafel slope can probably be associated with the manufacture of BDD, even when it was provided by the same supplier.

Figure 2 shows normalized COD vs. time curve at different values of j (10, 15 and 20 mA cm^-2) at constant u=29.2 cm s^-1. This figure revealed that hydroxyl radical formation, responsible to the degradation of diclofenac, is favored at j of 10 and 15 mA cm^-2; while at j of 20 mA cm^-2 the oxygen evolution reaction start to appear diminishing the degradation rate.

Results of COD depletion as a function of hydrodynamics (not shown herein) did not show dependence with fluid velocity owing to the diclofenac degradation involves a complex mechanism. The experimental set-up achieved 100% diclofenac mineralization with 78% current efficiency and energy consumption of 2.54 kWh m^-3 at j=15 mA cm^-2 and u=29.2 cm s^-1.

REFERENCES

1.  Sirés, I., Brillas, E.,  Environment International., 2012, 40, 212–229.

2. Zhao, X., Hou, Y., Liu, H., Qiang, Z., Qu, J. Electrochim. Acta., 2009, 54, 4172- 4179.

3. Michaud, P-A., Panizza, M., Ouattarra, L., Diaco, T., Foti, G., and Comninellis Ch., J. Appl. Electrochem., 2003, 33, 151-154.

4. Nava, J.L., Nuñez, F., González I., Electrochim. Acta., 2007, 52, 3229–3235.