The generation of hydroxyl radicals and electro-oxidation of diclofenac on Pt-doped SnO₂–Sb electrodes

Abstract

Pt-doped SnO₂–Sb electrodes constitute promising candidates for the electrochemical abatement of refractory pollutants, but their efficacy to oxidize emerging pollutants remains uncertain. In this work, the electrochemical oxidation of diclofenac, pharmaceutical pollutant, on Pt-doped Ti/SnO₂–Sb electrodes has been studied by cyclic voltammetry and galvanostatic treatment in neutral medium. In parallel, the capability of these anodes to generate hydroxyl radicals (OHs) has been analyzed by in-situ UV spectroelectrochemical measurements. For comparison purposes, the responses of Ti/SnO₂–Sb and commercial Ti/Pt and BDD anodes were also evaluated. The voltammetric and electrolysis results show that the different Ti/SnO₂–Sb anodes can effectively oxidize and mineralize diclofenac, so their electrochemical activity lies in between that of Ti/Pt and BDD. The incorporation of small amounts of Pt (3–13 at.%) into the SnO₂–Sb coatings, despite hindering the OHs generation, enhances the kinetics and efficiency for diclofenac oxidation and mineralization. This better overall response is attributed to a synergy between diclofenac-Pt interaction and efficient OHs generation. Pt-doped Ti/SnO₂–Sb electrodes are then presented as a cheaper potential alternative to BDD for treating pharmaceutics pollutants in waters.

Introduction

Diclofenac, a nonsteroidal anti-inflammatory, analgesic, and antipyretic drug, is one of the most widely available pharmaceuticals worldwide [1]. Considering the continued release, biodegradation resistance and persistence [[2], [3], [4], [5], [6], [7]], and/or toxic effects [[8], [9], [10], [11]] of diclofenac, it is necessary to develop new methodologies for its elimination.

Various technologies have been proposed and reviewed for the removal of diclofenac [2,[12], [13], [14], [15], [16]]. Based on the generation of physisorbed hydroxyl radicals (OHs), the anodic oxidation of pollutants constitutes probably the simplest advanced oxidation process (AOP) [[17], [18], [19], [20]] and, thus, one of the most promising methods for the degradation of this drug [21]. This technology gathers several advantages to remove toxic biorefractory organic compounds [22]. However, its feasible application relies on the nature of the anode, which determines the efficiency and selectivity of the oxidation process, and the cost and durability [[17], [18], [19], [20],23].

To now, there are few studies on the anodic oxidation of diclofenac and most of them have been carried out with the Boron-Doped Diamond (BDD) anode [[24], [25], [26], [27]], demonstrating its capability to full mineralization and toxicity reduction. Despite its good performance, the practical utilization of the BDD is limited by its high cost and fragility (because it is generally supported as a thin film on silicon wafers). On the other hand, other electrodes, like Pt [25,28], PbO₂ [27] and carbon materials [24,28], were also investigated for diclofenac oxidation.

Exhibiting a high overpotential for the oxygen evolution reaction (OER), non-active SnO₂–Sb anodes constitute a promising cheaper and efficient alternative for the electro-oxidation of pollutants [17,29,30]. It was found that the incorporation of a small percentage of Pt (3–13 metal atomic percent (at.%)) remarkably increases their short service life [31,32] and their catalytic activity for phenol degradation [33,34] in different electrolytes. Hence, both fundamental and practical studies on the electrocatalytic properties and potential applicability of these anodes have aroused great interest.

It is generally assumed that the performance of a given anode towards the electrochemical abatement of organic pollutants is correlated with its capability to generate oxidizing OHs. Then, considering the observed catalytic effects of Pt [33,34], a promotion of OHs generation would be expected by incorporating Pt into the SnO₂–Sb anodes. Nevertheless, in a recent study [35] the electrocatalytic effect of Pt has been associated to a strong specific adsorption of phenol on Pt/PtOx sites. Despite this controversy, the influence of Pt doping on the capability of SnO₂–Sb anodes to produce OHs has been never investigated. Moreover, the catalytic activity of Pt-doped or undoped SnO₂–Sb anodes for the electro-oxidation of diclofenac has been never explored.

This work presents a study on the electrocatalytic activity of various Ti/SnO₂–Sb anodes towards the generation of OHs and the oxidation and mineralization of diclofenac, as one of the most important pharmaceuticals, in neutral medium. The effect of the Pt content on the SnO₂–Sb coating is investigated. For comparison purposes, the performance of Ti/Pt and BDD commercial anodes has been also analyzed. Considering that the anodic removal of diclofenac is a complex reaction, involving various concurrent electrochemical processes, the catalytic activity of the different anodes was studied by different techniques, such as cyclic voltammetry (CV), in-situ UV spectroelectrochemical measurements and galvanostatic electrolysis, and by using different fixed electrochemical conditions.

These conditions were selected with the intention of better discerning the electrocatalytic response of the electrodes, without optimization of the experimental conditions.