Double benefit of electrochemical techniques: Treatment and electroanalysis for remediation of water polluted with organic compounds

Abstract

Concern about the current pollution of water environments and the inefficacy of conventional water treatments for the elimination of refractory contaminants has placed electrochemistry in the spotlight. With the objective of demonstrating the diverse applications that electrochemical techniques can have in the area of water remediation, this study is focused on the use of three different methods: (i) electro-Fenton process with heterogeneous catalyst as the treatment for the degradation of the target compounds; (ii) cyclic voltammetry for the characterization of the electrochemical system, and (iii) differential pulse voltammetry for the monitoring of the evolution of the degradation process. Four organic compounds were selected as target pollutants: the ionic liquid 1,3-Bis(2,4,6-trimethylphenyl)imidazolinium chloride, Mesitol, Mesidine and 2,5-Xylidine. Results were corroborated and complemented with chromatographic and total organic carbon (TOC) measurements. After 420 min of heterogeneous electro-Fenton treatment, almost 80% of TOC abatement was achieved for the ionic liquid and more than 90% for Mesitol, Mesidine and 2,5-Xylidine. Cyclic voltammetry studies for Mesitol and Mesidine suggested the formation of a polymeric film which remains adsorbed on the electrode surface. Finally, it was possible to conclude that the coupling of differential pulse voltammetry with the heterogeneous electro-Fenton process provides useful information about the evolution of the degradation process of pollutants in just a couple of minutes.

Introduction

With the ever increasing amount and variety of pollutants detected in aquatic bodies, the concern and awareness about the necessity of a greater environmental protection has increased accordingly. This has resulted in the creation of more stringent environmental regulations [1,2]. Since conventional wastewater treatments have difficulties with degrading toxic and refractory organic pollutants, it is necessary to develop more efficient technologies that would allow meeting the new discharge regulations [3].

In this context, electrochemistry is a discipline that can play an important role in environmentally related applications [4,5]. Electrochemistry is defined as the study of the relationship between electrical signals and chemical systems that are incorporated into an electrochemical cell [6]. Exploiting this relationship between electricity and chemistry, electrochemical science and engineering can be applied to the detection, quantification and treatment of pollutants [7].

The use of electricity for water remediation was suggested for the first time in the UK in 1889. At that time, though, electrochemical technologies were not widely implemented due to the investment required and the cost of the electricity supply [1]. However, these techniques have evolved and nowadays can compete with other technologies in terms of costs, being even more efficient and compact. In fact, in some cases electrochemical processes may be of paramount importance considering the low efficiency of conventional treatments for the elimination of refractory contaminants [1,8]. It is in this scenario that the electrochemical advanced oxidation processes (EAOPs) are gaining increasing importance. EAOPs are based on the generation of highly reactive oxidative species, mainly the hydroxyl radical (·OH), which reacts and degrades the target pollutants [9]. The mechanism for the radical formation varies in the different EAOPs. In the case of electro-Fenton (EF) process, ·OH is generated at pH 3 through the reaction of ferrous iron with hydrogen peroxide (Eq. (1)), which are, respectively, electrochemically regenerated (Eq. (2)) and in-situ produced (Eq. (3)) at the cathode. In addition, using heterogeneous catalysis for the EF treatment presents several benefits, such as reducing the formation of iron hydroxide sludge and allowing an efficient recovery and reuse of the catalyst [2]. Heterogeneous EF (HEF) processes have demonstrated to be effective in the degradation of various recalcitrant pollutants such as dyes [10,11], pharmaceuticals [12], pesticides [13,14] or ionic liquids (ILs) [15,16]. However, frequently during the degradation process other toxic compounds may be generated, such as phenolic or aniline derivatives, which can be even more toxic than the parent substance [[17], [18], [19]]. Therefore, it is important to evaluate that those compounds are also eliminated during the treatment.

$${\text{Fe}}^{2+}+{\text{H}}_2{\text{O}}_2\to {\text{Fe}}^{3+}+·\text{OH}+{\text{OH}}^{-}$$

$${\text{Fe}}^{3+}+{\text{e}}^{-}\to {\text{Fe}}^{2+}$$

$${\text{O}}_2+2{\text{H}}^{+}+2{\text{e}}^{-}\to {\text{H}}_2{\text{O}}_2$$

Besides that, electrochemistry can be applied with analytical purposes. Electroanalysis are versatile techniques that provide high sensitivity, short analysis times, and a reduction in solvent, sample volume consumption and sample preparation time using inexpensive instrumentation compared to spectrophotometric or separation techniques. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) are among the most commonly used electrochemical techniques [20]. On the one hand, CV is useful for obtaining qualitative information about the properties and characteristics of the electrochemical process. Therefore, it is typically used for an initial electrochemical characterization of new systems. On the other hand, DPV is especially interesting for trace analysis, since it is one of the most sensitive voltammetric techniques. This sensitivity is obtained thanks to the double sampling of the current in each pulse period, just before the pulse and at the end of it, which allows the discrimination of the undesired capacity current from the required faradaic current by the subtraction of both currents sampled. Thus, it can be used to identify species based on the potential of the current peaks obtained during the analysis [20,21].

The objective of this study was to apply different electrochemical techniques to the assessment of the remediation of a water matrix: (i) HEF process for the degradation of the target compounds, (ii) CV for characterizing the electrochemical system and (iii) DPV for monitoring the degradation process evolution. For doing so, four organic compounds were investigated. Firstly, [IMes.HCl] was selected because ILs are considered as “contaminants on the horizon” to watch [22], and so their elimination should be addressed. Moreover, Mesitol and Mesidine were selected not only because they are toxic substances but also because they were identified as possible intermediates to be formed during the attack of the IL by hydroxyl radicals if the aromatic groups of the IL are separated from the main molecule by: (i) breaking the C–N bond between the aromatic and the imidazolinium rings (in which case Mesitol could be formed) or (ii) breaking the C–N/CN bonds of the imidazolinium ring (in which case Mesidine could be formed). Finally, 2,5-Xylidine was selected in order to investigate the effect that having less methyl substituents and in different positions (compared to Mesidine) would have on its electrochemical behavior.