Photoelectrocatalytic degradation of chlortetracycline using Ti/TiO₂ nanostructured electrodes deposited by means of a Pulsed Laser Deposition process

Abstract

Ti/TiO₂ electrode was prepared by means of the Pulsed Laser Deposition method and used in a photoelectrocatalytic oxidation (PECO) process in order to degrade chlortetracycline (CTC). The deposited TiO₂ coatings were found to be of rutile structure. High treatment efficiency of CTC was achieved by the PECO process compared to the conventional electrochemical oxidation, direct photolysis and photocatalysis processes. Several factors such as current intensity, treatment time, UV lamp position and initial concentration of CTC were investigated. Using a 2⁴ factorial matrix, the best performance for CTC degradation (74.3% of removal) was obtained at a current intensity of 0.5 A during 120 min of treatment time and when the UV lamp was immersed in the solution in the presence of 25 mg L⁻¹ of CTC. The current intensity and treatment time were the main parameters influencing the degradation rate of CTC. Subsequently, a central composite design methodology has been investigated to determine the optimal experimental parameters for CTC degradation. The PECO process applied under optimal conditions (at current intensity of 0.39 A during 120 min with internal position of the UV lamp) is able to oxidize around 74.2 ± 0.57%, of CTC.

Introduction

In the past few years, there has been considerable interest to pharmaceuticals and personal care products (PPCPs) including antibiotics, hormones, anesthetics, etc. due to their discharges into the environment. These contaminants are soluble in surface water, groundwater, and even drinking water [1], [2], [3], [4]. Antibiotics belong to pharmaceutical compounds most often used as growth promoters of animals. The discharge of antibiotics into the environment is one of the major and global public health issues that need urgent action. Its annual usage has been globally estimated between 100,000 and 200,000 tones [5]. Chlortetracycline (CTC); oxytetracycline (OTC); and tetracycline (TC) are the most often used throughout the world [1], [6], [7], [8]. Antibiotics or their metabolites have been detected in surface water, ground water, sewage water, and drinking water at concentrations ranging from ng L⁻¹ to μg L⁻¹ [1], [9], [10]. These findings have raised concern regarding potential human health effects caused by low levels of antibiotics in drinking water, as well as the transfer and spread of antibiotic resistant genes among microorganisms [11]. Globally, a very low proportion (<1%) of these antibiotic compounds participates to the total DOC from pollutants in contaminated water, but their presence in water has to be taken into account owing to their potential toxicity for human. Thus, it is of great importance to develop efficient and cost-effective treatment technologies for removal of such compounds.

Conventional wastewater treatments such as traditional biological treatment methods are not always able to remove completely antibiotic compounds [1], [12]. It has been found by Ingerslev and Halling-Sorensen [13] that twelve sulfonamide antibiotics were not readily biodegradable in activated sludge. Likewise, Kummerer and Hartmann [14] investigated the effectiveness of the treatment of hospital and pharmaceutical wastewaters in several wastewater treatment plants in Germany. These works showed that many pharmaceutical products could not be oxidized during conventional biological treatments. However, other methods can be used to remove antibiotics such as adsorption, biosorption, ozonation, membrane techniques (reverse osmosis), advanced oxidation processes (AOPs), including O₃/H₂O₂, UV/O₃, UV/H₂O₂, H₂O₂/Fe²⁺, etc. [1], [15], [16], [17]. Among them, chemical oxidation processes have been successfully applied to degrade various pharmaceutical compounds present in contaminated water. UV and UV/H₂O₂ oxidation processes are used to degrade three antibiotics from tetracycline group (tetracycline; chlortetracycline; and oxytetracycline) [1]. In spite of the good oxidation of refractory organic pollutants, the complexity of these methods (advanced oxidation processes), high chemical consumption, and relatively high treatment costs constitute major barriers for large-scale applications [18].

In the same context, photocatalysis process combining UV and TiO₂ suspensions as catalyst was used by Reyes et al. [12] to degrade tetracycline as a model of antibiotic compounds. Degradation around 50% was recorded after 10 min of treatment using 0.5 g L⁻¹ of TiO₂ exposed to UV light. A renewed interest in photocatalysis has been spurred by the search for reliable, cost-effective water treatment processes. From this point of view, it can be interesting to develop photoelectrocatalytic oxidation (PECO) technique combining electrolytic and photocatalytic processes. This approach offers the possibility to delay the recombination of the electron–hole pairs (e⁻ _CB/h⁺ _VB) and the possibility to increase the lifetime of the latter. The applied external potential in PECO technique is the key factor because it accelerates the photocatalytic reaction.

The aim of the present study is to evaluate the efficiency of a PECO process using UV light and nanocrystalline TiO₂ photo-anodes for the efficient treatment of waters contaminated by CTC. To this end, an experimental design methodology [19] was put in place to investigate the influence of the principal experimental parameters (current intensity, treatment time, pollutant concentration, and UV lamp position) on the efficiency of the PECO process for CTC degradation. A second objective of this study was to use a statistical methodology for a rational analysis of the combination of operational factors that led to the best treatment process. Factorial design (FD) and central composite design (CCD) methodologies have been successively applied in order to point out the main and interaction effects of the factors and to optimize CTC degradation by the PECO process.