Reaction paths and efficiency of photocatalysis on TiO₂ and of H₂O₂ photolysis in the degradation of 2-chlorophenol

Abstract

The kinetics of 2-chlorophenol (2-CP) degradation and mineralization in the aqueous phase was investigated under irradiation at 254 nm, employing either photocatalysis in the presence of titanium dioxide, or hydrogen peroxide photolysis, to compare the efficiency of these photoinduced advanced oxidation techniques. Photocatalysis under 315–400 nm wavelength irradiation was also investigated. The concentration versus time profiles of the degradation intermediates catechol, chloro- and hydroxy-hydroquinone allowed the identification of the reaction paths prevailing under the different experimental conditions. Efficient CCl bond cleavage occurred as a consequence of direct light absorption by 2-CP, while hydroxyl radicals, photogenerated at the water–photocatalyst interface or during H₂O₂ photolysis, were the main oxidation agents, able to attack both 2-CP and its degradation intermediates. Highest degradation and mineralization efficiencies were achieved under H₂O₂ photolysis at 254 nm.

Introduction

Chlorophenols represent an important class of very common water pollutants. Because of their extensive use as fungicides, herbicides and wood preservatives [1], they can easily be found in soils and in aquatic environments. Other sources include the waste incineration or disinfection of sewage and industrial wastewater with chlorine, as well as discharges from paper mills, releasing them as by-products of chlorine-based bleaching [2]. Chlorophenols are persistent water pollutants under environmental conditions, due to the stability of the CCl bond, which is also responsible for their toxicity [3]. Most of them have been listed as toxic or priority pollutants by both the US Environmental Protection Agency and the European Commission [4], [5].

The increasing awareness of the possible environmental effects of chlorophenols has led to the demand for limiting their use and for the development of new methods of treating contaminated waters. Their degradation by conventional techniques is rather challenging, because of their stability and high solubility in water. Particular emphasis has been given in recent years to the use of advanced oxidation processes (AOPs) for water purification, some of which are based on either direct or sensitized photolysis; the degradation of chlorophenols by these means has been recently reviewed [6]. AOPs present the great advantage that they completely remove organic contaminants from the environment, not only from the aqueous phase, by transforming them into other organic compounds and finally into innocuous inorganic species. A correct application of AOPs requires the identification and possibly the monitoring of all intermediate species, which might be more toxic and/or persistent than the original contaminants.

In the present work detailed kinetic studies are reported on the aqueous phase degradation of 2-chlorophenol (2-CP), chosen as model chlorinated aromatic pollutant, employing two photoinduced AOPs, namely photocatalysis in the presence of titanium dioxide and hydrogen peroxide photolysis. 2-CP degradation under both photocatalysis [7], [8], [9], [10], [11], [12], [13] and H₂O₂ photolysis [14], [15], [16], [17], [18] has already been investigated by different research groups. Aim of the present study is a comparison between the water depollution efficiency of the two techniques employing the same irradiation source, also in relation to the rate of overall mineralization and to the intermediate species produced during 2-CP degradation. The concentration profiles of the aromatic intermediates have thus been monitored during the photodegradation treatments, aiming at identifying the reaction paths prevailing in each case and at ascertaining their effectiveness, also in relation to the evolution of the overall toxicity.