Electrochemical degradation of Ibuprofen on Ti/Pt/PbO₂ and Si/BDD electrodes

Abstract

The electrochemical oxidation of Ibuprofen (Ibu) was performed using a Ti/Pt/PbO₂ electrode as the anode, prepared according to literature, and a boron doped diamond (BDD) electrode, commercially available at Adamant Technologies. Tests were performed with model solutions of Ibu, with concentrations ranging from 0.22 to 1.75 mM for the Ti/Pt/PbO₂ electrode and 1.75 mM for the BDD electrode, using 0.035 M Na₂SO₄ as the electrolyte, in a batch cell, at different current densities (10, 20 and 30 mA cm⁻²). Absorbance measurements, Chemical Oxygen Demand (COD) and Total Organic Carbon (TOC) tests were conducted for all samples. The results have shown a very good degradation of Ibu, with COD removals between 60 and 95% and TOC removals varying from 48 to 92%, in 6 h experiments, with higher values obtained with the BDD electrode. General Current Efficiency and Mineralization Current Efficiency, determined for both electrodes, show a similar behaviour for 20 mA cm⁻² but a very different one at 30 mA cm⁻². The combustion efficiency was also determined for both anodes, and found to be slightly higher with BDD at lower current density and equal to 100% for both anodes at 30 mA cm⁻².

Introduction

Ibuprofen (Ibu) is a non-prescription, non-steroidal drug, used as an anti-inflammatory, analgesic and antipyretic in the human treatment of fever and pain. According to literature [1], several kilotons of this compound are produced worldwide each year, part of which is rejected to the effluents, excreted by patients in its original form or as metabolites from human biodegradation [2]. The occurrence of Ibu in water streams can also be due to an inadequate disposal of unused medication via solid waste and posterior landfill leachate. Despite environmental problems caused by the use of these types of drugs, their consumption is unlikely to be restricted since they are beneficial to humankind. In fact, their use is expected to grow [3]. In their review on the fate of pharmaceuticals during wastewater treatment processes, Jones et al. [4] present the idea that, in fact, drugs are reaching the aquatic system, although in very low concentrations, and they may present a potential hazard for human health, especially where no advanced wastewater treatments are used.

Ibuprofen is considered to be one of the most important pharmaceutical contaminants in sewage treatment plant (STP) influents [5]. Although 90% is lost during the passage of wastewater through a STP, Ibu and its first biotic products (hydroxy- and carboxy-ibuprofen) have been detected, among other pharmaceutical residues, in sewage waters and effluents and also in surface waters [1], [2], [3], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15].

Nebot et al. [16] conducted a quantification of human pharmaceuticals in water samples from the North of Scotland, by high performance liquid chromatography with tandem mass spectrometry detection, and found Ibu in wastewater effluents at a concentration of 405 ng L⁻¹. Identical studies were also performed in countries such as Germany, Spain, Switzerland, France, Italy, Sweden, Canada and Denmark, where the quantifications for Ibu in wastewater effluents varied from 60 to 3400 ng L⁻¹ [6], [9], [10], [11], [16], [17]. In a toxicity study, Cleuvers also found that the mixture of a few drugs could even augment the toxicity of each drug separately [18].

These facts have stimulated recent research for different processes for removing these types of compounds, namely through the use of biological [5], [19], [20], [21], [22], [23], photodegradation and solar photodegradation methods in the aquatic environment [11], [12], [24] and also advanced oxidation processes [25], [26], [27] that result in the formation of hydroxyl radicals, responsible for the oxidation process.

Another way to eliminate persistent pollutants is through the use of the electrochemical method. Recent studies have been conducted on the electrodegradation of pharmaceutical pollutants [28], [29], [30]. The application of electrochemical oxidation processes to eliminate organic matter does not require the use of chemicals, only a background electrolyte, already present in most effluents. Since electrochemical processes take place in a narrow film near the electrode’s surface, diffusion of the species from the bulk of the solution can be a limiting step, decreasing the current efficiency and increasing processing costs. This problem can be overcome if pollutants are present at higher concentrations. Since some countries are adopting membrane processes to be applied as tertiary treatments in order to remove some of the remaining persistent pollutants from the effluents of STPs, electrochemical processes can be used efficiently to degrade the concentrates formed during membrane operations. This solution is particularly viable if the appropriate material is chosen as the anode. In fact, several materials can be used as electrodes, namely electrocatalytic materials like PbO₂, SnO₂, Ti/Pt/PbO₂, Ti/SnO₂-Sb₂O₅ and boron doped diamond (BDD) [28], [31], [32], [33], [34], [35], [36], [37], [38]. For the latter, due to its particularly high overpotential for oxygen evolution, very high current efficiencies may be obtained and also a complete mineralization of the organic compounds. For these type of anodes, there are two possible controlling steps: (1) the rate of mass transfer from the bulk of the solution to the surface of the electrode (being the process controlled by diffusion) or (2) the rate electron transfer to the anode’s surface (giving origin to a current controlled process, also conventionally called “kinetic control”). In the former case, the variation of Chemical Oxygen Demand (COD) with time can be given by the following equation [37]:

$$\frac{\text{COD}}{\text{CO}{\text{D}}_0}=\exp \left(-\frac{A{k}_{\text{m}}}{V}t\right)$$

where COD is expressed in mol O₂ m⁻³, COD₀ is its initial value, t the time of treatment (s), A the electrode area (m²), V the volume of the treated solution (m³) and k_m the medium mass transfer coefficient (m s⁻¹). On the other hand, if the current density is low, the process can be current controlled, with the variation of COD with time given by [37]:

$$\text{COD}=\text{CO}{\text{D}}_0-\frac{jA}{nFV}t$$

where j is the applied current density, in A m⁻², and F is the Faraday constant. In order to determine the controlling step, the limiting current density, j_lim, has to be known, and it is given by [37]:

$$j_{\lim }=\left(\frac{1}{8}\right)F{k}_{\text{m}}\text{COD}$$

If j > j_lim, the electrochemical process is controlled by diffusion, whereas it is controlled by current when j < j_lim. In the latter case, only a higher current density would increase the rate of the reaction. On the other hand, an increase in concentration leads to an increase in the diffusion rate, according to Fick’s first law, leading to a higher degradation rate when the process is controlled by diffusion.

The present work reports on experimental results of the electrochemical oxidation of Ibu solutions at various experimental conditions, using two different materials as anodes, Ti/Pt/PbO₂ and BDD.