Electro-oxidation of tetracycline in ethanol-water mixture using DSA-Cl₂ anode and stimulating/monitoring the formation of organic radicals

Highlights

• Tetracycline removal reaches 90% after 15 min electrolysis in ethanol-water.

• Ethanol-water medium favors hydroxyl, hydroxyethyl and ethoxy radical's formation.

• Ethanol is a convenient solvent because its low toxicity.

• The association of adsorption/desorption and electrolysis in ethanol is effective.

Abstract

Recent studies reported a new strategy of electro-oxidation of organic compounds using methanol as solvent. Considering its well-known toxicity, this work sought to evaluate the use of ethanol as an alternative solvent for pollutants degradation. Therefore, thorough analyses were performed in order to evaluate tetracycline (TC) electro-oxidation using DSA-Cl₂ anode in ethanol-H₂O solutions. The effects of solvent mixture, pH and current density on the degradation efficiency were evaluated. TC degradation in methanol-water and ethanol-water media resulted in very close removals of 95% and 90%, respectively, after 15 min of electrolysis at 10 mA cm⁻². In ethanol medium, the increase in current densities from 10 to 25 mA cm⁻² did not lead to significant changes in removal efficiency. The variation of the initial pH of the solution showed that the best removal efficiencies were obtained at neutral pH resulting in TC removals up to 90%, which is actually related to the molecular structure of TC. Through analysis using electron paramagnetic resonance (EPR), the formation of radicals such as hydroxyethyl (CH₃^●CHOH), hydroxyl (^●OH) and ethoxy (CH₃CH₂O^●) were detected, which effectively contributed toward the pollutant oxidation.

Introduction

Over the past few decades, there has been an increasingly growing interest in the detection of emerging pollutants (EPs) in environmental matrices due to the threats posed by the presence of these contaminants in the environment and the risks to human health caused by the exposure to these pollutants. A wide range of substances which are disposed of into the environment are considered EPs; these substances include pharmaceutical products, personal care products, pesticides, surfactants, among others. EPs can contaminate the soil, water and atmosphere, and have been found to pose serious threats to the ecosystem and to human health (Deblonde et al., 2011; Galindo-Miranda et al., 2019). In fact, most of these EPs are considered endocrine disruptors as they alter the endocrine functions of the body, causing a wide range of chronic diseases (Diamanti-Kandarakis et al., 2009). Considering that EPs are present in the environment in extremely low concentrations (of the order of μg L⁻¹), their removal process is often complex, and this has become a matter of grave concern to researchers and other stakeholders due to the aforementioned risks posed by these contaminants (Brillas, 2014; Montagner et al., 2017; Silva et al., 2016).

Pharmaceutical substances have become one of the main pollutants of water. Among these substances, antibiotics are the pollutants class seen to be of the greatest concern in the scientific community because of their widespread use by humans and animals for the treatment of infections and other illnesses (Santos et al., 2021). The human organism is incapable of fully metabolizing antibiotics; as such, the unmetabolized contents are eliminated through feces and urine, which are eventually disposed of into the environment via domestic sewage (Rivera-Utrilla et al., 2013; Parra et al., 2016; Fent et al., 2006). Tetracyclines are the second group of antibiotics that are most widely used in the world due to their low cost and broad spectrum of antibacterial activities; these antibiotics are commonly used for the treatment of various infections. Disturbingly though, when TC is ingested, the human body is only able to absorb 60% of the drug (Parra et al., 2016).

Because of the low concentrations of EPs found in aquatic environments, conventional wastewater treatments are unable to efficiently degrade these contaminants. In view of that, alternative advanced treatment techniques, such as electrochemical advanced oxidation processes (EAOP), have been developed with a view to addressing the inefficiencies of conventional treatment processes. EAOPs have been successfully employed for the removal of EPs from water and wastewater matrices (Malpass et al., 2012; Mello et al., 2021; Muñoz-Morales et al., 2019; Dionisio et al., 2020; Ding et al., 2020). In fact, several studies reported in the literature have demonstrated the high removal efficiency of EAOPs when applied for the treatment of different effluents (Monteil et al., 2019; Barrera-Díaz et al., 2014; Sirés et al., 2014); however, the high energy costs involved in the application of EAOPs have tended to limit their use in some cases. Recently, Muñoz-Morales et al. (2018) proposed an efficient alternative strategy to overcome this limitation; where the pollutants are initially concentrated through adsorption with activated carbon and then desorbed using organic solvent such as methanol, obtaining a concentrated solution of the target pollutant in low volume of methanol. This concentrated solution is subjected to electro-oxidation, where higher treatment efficiency is obtained with diminished energy consumption (Muñoz-Morales et al., 2019, 2020).

Since this is a novel strategy for the removal of micropollutants from wastewater, several studies still need to be carried out in order to fully understand the technique to effectively validate or evaluate its efficiency. More recently, Mello et al. (2021) studied the degradation of TC in methanol media where they compared the efficiencies of two commercial anodes: boron-doped diamond reactor (BDD) and dimensionally stable anode (DSA-Cl₂), as well as different supporting electrolytes: acid sulfuric and chloride anions. The degradation process was found to be more efficient when the DSA-Cl₂ electrode was applied in chloride medium. There are some research studies on the degradation of pollutants in methanol (Muñoz-Morales et al., 2018, 2019, 2020), nonetheless, methanol is a kind of toxic solvent, which is not favorable from an environmental and health perspective. As an alternative to this problem, this article proposes the use of ethanol in these processes. There are published articles that board the use of ethanol as a solvent for desorption of pollutants (Zhou et al., 2015; Reif et al., 2020; Larasati et al., 2020), however, there are no reports of the use of this solvent in pollutant degradation processes, being an innovative topic.

Bearing this in mind, the objective of this study was to analyze the degradation of tetracycline in ethanol/water. Under the technique proposed in this study, DSA-Cl₂ was used as anode and NaCl as electrolyte in the electrooxidation of TC. The study analyzed the behavior of ethanol during the degradation process while the formation of radical species was also investigated by electron paramagnetic resonance (EPR).