Degradation of gestodene (GES)–17α-ethinylestradiol (EE2) mixture by electrochemical oxidation

In the last few years, a wide spectrum of molecules capable of affecting, to a greater or lesser extent, the endocrine system of several living organisms, including human beings, have been identified in the environment (Liu et al. 2011; Sahoo et al. 2012). Among these compounds can be included: steroid hormones such as gestodene (GES), 17α-ethinylestradiol (EE2), 17β-estradiol (E2), and drospirenone; β-blockers such as metoprolol; and pesticides such as 2,4-dichlorophenoxyacetic acid. Evidence has been gathered relating these types of molecules to the increase in the prevalence of cancers and malformations in human reproductive organs (Dickerson & Gore 2007), and to feminization, masculinization, and malformation of the reproductive system of lower organisms such as fish, birds, and reptiles (Brian et al. 2007). For this reason, they are generally identified as endocrine disrupting compounds (EDCs).

Due to the severe consequences related to the presence of EDCs, a great number of research groups have worked on the evaluation of technologies that may help reduce their presence in the environment. Among them are biological processes, combined biological–physicochemical processes, and the so-called advanced oxidation processes (AOPs). Fenton and photo-Fenton systems (Colombo et al. 2011), heterogenous photocatalysis (Czech & Rubinowska 2013), and several electrochemical processes, such as anodic oxidation and electro-Fenton system (Sirés et al. 2010), among others, belong to the AOPs group. Compared to conventional processes, their main advantage is that they achieve a profound transformation of the chemical structure of the contaminants, and do not only transfer them from one form or medium to another (Liu et al. 2009).

The mixture of GES and EE2, two steroid hormones, is frequently and intensively used as an oral contraceptive (Matějíček & Kubáň 2007). This explains its widespread presence in the environment (Velicu & Suri 2009), and the interest of various research groups to evaluate technologies that are appropriate for its degradation. Such is the case of Zhang et al. (2012), who reported the effectiveness of ozone in the degradation of the EE2 molecule, achieving a 94.7% removal of an initial concentration of 100 μg L⁻¹, using 9 mg L⁻¹ of ozone, pH 6, 10 mg L⁻¹ of natural organic matter and 20 mg L⁻¹ of suspended solids. On the other hand, Frontistis & Mantzavinos (2012) evaluated the sonodegradation of 110 μg L⁻¹ of EE2, observing a direct relationship between the rate of degradation of the molecule and the density of sonication power, while an inverse relationship with the system temperature was observed. It was also observed that organic matter delays the degradation process in an acidic medium (pH 3), while it accelerates it in a neutral medium (pH 7.8).

Frontistis et al. (2011) evaluated the anodic oxidation of 100 μg L⁻¹ of EE2, using a zirconium cathode and a boron-doped diamond (BDD) anode. The authors achieved the complete degradation of EE2 in 7 min using a current density of 2.1 mA cm⁻². The degradation of EE2, alone or mixed with E2, was studied by Guedes et al. (2008). These authors evaluated the ozonation and O₃/H₂O₂ processes. With the first process, they removed 99.7 and 100% of EE2 and E2, respectively, from a mixture having an initial concentration of 10 μg L⁻¹. These results were achieved when the reaction medium operated at pH 3 and 25 mg L⁻¹ of ozone. When the estrogens were treated separately, the results were similar, resulting in efficiencies greater than 99% for both compounds. In the O₃/H₂O₂ process, the addition of H₂O₂ did not improve estrogen removal. Currently, the study of the degradation of GES–EE2 mixture is insufficient. Only a few investigations have reported the degradation of the GES–EE2 mixture alone or in the presence of other hormones (Fu et al. 2007; Rokhina et al. 2012). Fu et al. (2007) evaluated the destruction of the mixture of eight molecules (10 μg L⁻¹ each one) using ultrasound (20 kHz). After 25 min of treatment, a degradation of about 80% of the total mixture, 82% of EE2, and 70% of GES was observed. The authors also evaluated the effect of the pH on the reaction medium (pH 3, 7, and 9) and observed that the greatest removal was achieved in acidic conditions. Moreover, the degradation kinetics of each molecule followed a pseudo-first order reaction profile.

On the other hand, Rokhina et al. (2012) evaluated the ozonation of 12 hormones, among them, EE2 and GES. The hormone mixture had an initial concentration of 100 μg L⁻¹of each one and degradation percentages above 99% were achieved with a 1 min treatment using an ozone dose of 5 mg L⁻¹. In view of this information, in the present article, data obtained from the degradation of the GES–EE2 mixture through anodic oxidation using BDD electrodes are shown. The tests were performed using the initial GES/EE2 concentration ratio present in pharmaceutical formulations of oral contraceptives. Parameters such as supporting electrolyte concentration, pH of the reaction medium, and the current supply to the system were controlled in order to determine the conditions in which the greater degradation percentage was achieved.

The analytical grade chemical substances, such as sodium sulfate, as well as the reagents used in the chemical oxygen demand (COD) determination were supplied by Sigma-Aldrich. The high-performance liquid chromatography (HPLC) grade solvents used in the chromatographic determinations were acquired from Sigma-Aldrich. The GES–EE2 working solutions were prepared using pharmaceutical formulations obtained from a local pharmacy store. In the preparation of each one of the working solutions, distilled water was used.

All the anodic oxidation tests of the GES–EE2 mixture were conducted in an undivided electrolytic cell with a reaction volume of 50 mL, using a Na₂SO₄ solution as supporting electrolyte. Considering the study reported by Frontistis et al. (2011), in all the cases, the initial concentration of the mixture was adjusted at 625 μg L⁻¹ of GES and 250 μg L⁻¹ of EE2, this mixture presenting an initial COD average concentration of 713 mg L⁻¹ and a standard deviation of 20.5 mg L⁻¹. As anode and cathode, parallel BDD electrodes (Adamant Technologies, Switzerland) having a working surface of 2.5 × 2.5 cm², separated by a distance of 2 cm, were used. The pH of the reaction medium was adjusted using 10% H₂SO₄. The current was supplied and controlled through a power supply unit (Extech Instruments Model 382270) operated under open potential. The reaction medium was kept at all times at room temperature (26 ± 2 °C), under constant agitation (850 rpm) and in complete darkness.

Witness tests were performed in order to ensure that the removal of COD present in the reaction medium could be attributed to the anodic oxidation and not to other uncontrolled factors, such as adsorption or evaporation. These tests were conducted under the same conditions in which the oxidation of the GES–EE2 mixture was evaluated (medium pH, supporting electrolyte concentration, agitation, darkness, and reaction time), the only difference being that in these tests the system did not receive electric current. During the reaction time of the witness tests, the reaction medium was monitored every 20 min and the residual COD of these samples was quantified.

The COD was determined using the closed reflex colorimetric method (APHA 2012). The organic matter was digested at 150 °C during 2 hours using potassium dichromate. The chromium ions produced were measured in a spectrophotometer UV/Vis (Hach DR-6000) at 605 nm.

The type and concentration of the supporting electrolyte, current density, electrode type and material, and pH of the reaction medium are among the main parameters controlling the efficiency of the anodic oxidation (De Amorim et al. 2013; Rabaaoui et al. 2013). A first set of tests was developed in order to determine, within a group of 27 different treatments, the conditions of pH, supporting electrolyte concentration, and current density supplied to the system, that would lead to the greatest degradation of the GES–EE2 mixture measured as a COD removal percentage (Equation (1)), as well as to evaluate the possible interaction between the factors, i.e., the dependence of the effect of one factor in the presence of the other factors. The slight variation observed in the residual COD concentration quantified in the witness tests did not reach a level of significance (α = 0.05, d.f. = 6, F = 0.556, p = 0.758), and thus the COD removal observed in the reaction medium during the degradation tests can be attributed to the anodic oxidation. The results obtained in this first set of tests show that the anodic oxidation is a promising alternative for reducing the concentration of the GES–EE2 mixture in aqueous medium.

In Figure 2, the direct relationship between pH and the removal of COD present in the reaction medium, mainly at low densities (16 to 32 mA cm⁻²), can be seen, while at 48 mA cm⁻² this relationship can only be observed when the concentration of the supporting electrolyte in the reaction medium was 0.05 M (8.70 mS cm⁻¹). The analysis of variance showed an interaction between the pH of the reaction medium and the supporting electrolyte (α = 0.05, d.f. = 4, F = 4.35, p = 0.004). This can be observed in Figure 2, where 0.05 M Na₂SO₄, independently of the current density applied to the system, permitted a greater effect of the pH on the COD percentage removal. In the same figure, it can be seen that, at a supporting electrolyte concentration of 0.02 M (3.58 mS cm⁻¹), independently of the current density, pH variation does not lead to a significant difference in the removal of the reaction medium. At 0.10 M Na₂SO₄ (16.06 mS cm⁻¹), low current densities show a significant pH effect on the COD removal efficiency, an effect that diminishes with the application of an increased current density to the system.

The anodic oxidation with BDD electrodes efficiently degraded both the pharmaceutical formulation mixture and the GES and EE2 active ingredients mixture. To degrade at least 93% of the active ingredient in GES and EE2, 5 and 10 min were required, respectively, while 15 min were necessary to achieve a 100% degradation of both molecules. Evidence was presented indicating that the studied factors (pH, supporting electrolyte concentration, and induced current) have a significant influence on the degradation of the GES–EE2 mixture, and follow a pseudo-first order kinetic profile. These results strengthen the arguments according to which anodic oxidation is a technology capable of degrading GES mixed with EE2 in an aqueous medium and can reduce the level of pollution caused by EDCs.