Sonocatalytic degradation of oxalic acid in the presence of oxygen and Pt/TiO₂

Highlights

• Oxidation of oxalic acid is enhanced under 360 kHz ultrasound, Pt/TiO₂ and oxygen.

• OH radicals formed by sonolysis contribute to catalytic activity enhancement.

• High frequency ultrasound leads to the highest oxalic acid degradation rate increase.

• 360 kHz ultrasonic irradiation drastically reduces the average catalyst aggregate size.

Abstract

In order to treat aqueous effluents containing organic pollutants, several techniques can be considered depending on the organic compound concentration. Sonochemistry appears to be a promising solution to answer water remediation issue. In fact, when submitted into a liquid, ultrasound can induce the nucleation, growth, and violent collapse of vapor/gas filled bubbles. However, despite the extreme local conditions observed during acoustic cavitation, using ultrasound alone is efficient only at low concentration in organic pollutants. In the present study, 0.05 M oxalic acid degradation kinetics were followed at 40 °C under various conditions, in presence or not of Pt/TiO₂ catalyst under silent conditions or ultrasound at 20 and 360 kHz. Experiments were achieved under controlled atmosphere and comparison between argon, Ar/O₂ (20 vol% O₂) and pure O₂ conditions was performed. Oxidation rate increase of oxalic acid was measured under Ar/O₂ atmosphere in presence of Pt/TiO₂ catalyst due to strong dispersion effect of both low and high ultrasonic frequency and formation of chemically active species by sonolysis. High frequency ultrasonic irradiation under Ar/O₂ atmosphere gives the highest kinetic increase compared to silent conditions with oxalic acid degradation rate around 13 μmol min⁻¹ at 40 °C with 2 g L⁻¹ of 3 wt% Pt on P25 TiO₂ catalyst.

Introduction

Water remediation and treatment of industrial wastewaters containing organic compounds are today's topics. Depending on the pollutant concentration several techniques can be considered for the treatment of aqueous effluents containing pesticides, carboxylic acids, chlorinated organic compounds, drugs or dyes [1], [2], [3]. For instance, advanced oxidation processes are one possible approach and involve the oxidation of the organic matter by hydroxyl radicals which are very reactive species [4], [5]. OH radicals can be in situ generated at near room-temperature by various routes involving ozone, hydrogen peroxide, homogeneous or heterogeneous catalyst with or without UV irradiation namely Fenton process, photocatalysis or ozone related systems [4], [6]. The main advantage of such processes is the ability of OH radicals to non-selectively react with organic compound until total mineralization. Nevertheless, this approach can be used only for low amount of aqueous pollutants within the range of μmol L⁻¹ to few mmol L⁻¹. For higher concentrations of organic compounds other techniques should be considered such as wet air oxidation or catalytic wet air oxidation [2], [3], [7]. Organic matter oxidation in that case is achieved in the presence of oxygen at temperatures from 130 to 320 °C and under very high pressures between 20 and 200 bars. These techniques are able to treat wastewaters with pollutant concentrations up to 0.1 mol L⁻¹; however, the drastic conditions needed can constitute a major drawback.

Sonochemistry is another promising approach that can be considered for water treatment [1], [6]. In fact, when submitted into a liquid, power ultrasound induces the nucleation, growth, and violent collapse of vapor/gas filled bubbles. This phenomenon known as acoustic cavitation produces a nonequilibrium plasma inside the cavitation bubbles which is responsible for in situ radical formation and possible organic degradation at the bubble–liquid interface [8]. Thus, the degradation of carboxylic acid such as oxalic or butyric acid was studied under high frequency ultrasonic irradiation in various conditions starting from few millimolar concentrations [9], [10]. However it appears that despite the extreme local conditions observed during acoustic cavitation and bubble collapse, ultrasound alone is efficient only at low concentration in organic pollutants [1], [11]. Coupling ultrasound with catalysts can be considered as one possibility to overcome these limitations and was actually employed for the degradation of various organic compounds such as phenol, detergents, dyes or drugs [1], [11], [12], [13], [14], [15], [16]. For instance, zero valent metals, such as copper or iron, were shown to enhance the degradation of phenol under ultrasonic irradiation [17]. In the same way, many efforts were focused on the understanding of the interaction between ultrasound and TiO₂ catalysts using low and high ultrasonic frequency irradiation [12], [13], [18], [19], [20], [21]. Under such conditions, oxidation of the organic matter is explained by the reaction with in situ generated OH radicals [13]. The use of noble metal based catalysts like Au/TiO₂ or platinum based under ultrasonic irradiation was also reported however the involved concentrations of pollutants still remain quite low and typically around 1 mM [22], [23].

In the present study, the coupling of ultrasound, 3 wt% of Pt on P25 TiO₂ catalysts and oxygen was considered and investigations on the degradation of 0.05 M oxalic acid were carried out. Oxalic acid was chosen since it is a quite refractory acid that can be formed during degradation of larger molecule and can be found in various industrial effluents [24]. Our main objective is to provide new insights on the sonocatalytic mechanism under near ambient conditions and the ability of such technique to treat organic compounds in higher concentration than considered in previously reported studies.