Preparation of PrFexCo1exO3/Mt catalyst and study on degradation of 2-hydroxybenzoic acid wastewater by catalytic wet peroxide oxidation

Abstract

In this study, the perovskite nanocomposite PrFexCo1exO3(Pr(S)) was successfully synthesized by the sol-gel method; PrFexCo1exO3/Al-pillared montmorillonite (Pr(S)/Mt) catalysts were prepared by impregnation (D) method and solid-melting (G) method, respectively, with Pr(S) as the active component and Al-pillared montmorillonite as the carrier. The catalysts were applied to treat the 2-hydroxybenzoic acid (2-HA)-simulated wastewater by catalytic wet peroxide oxidation (CWPO) technique, and the chemical oxygen demand (COD) removal rate and the 2-HA degradation rate were used as indicators to evaluate the catalytic performance. The results of the experiment indicated that the solid-melting method was more conducive to preparing the catalyst when the Co/Fe molar ratio of 7:3 and the optimal structural properties of the catalysts were achieved. The influence of operating parameters, including reaction temperature, catalyst dosage, H2O2 dosage, pH, and initial 2-HA concentration, were optimized for the degradation of 2-HA by CWPO. The results showed that 97.64% of 2-HA degradation and 75.23% of COD removal rate were achieved under more suitable experimental conditions. In addition, after the catalyst was used five times, the degradation rate of 2-HA could still reach 76.93%, which implied the high stability and reusability of the catalyst. The high catalytic activity of the catalyst was due to the doping of Co into PrFeO3, which could promote the generation of HO•, and the high stability could be attributed to the loading of Pr(S) onto Al-Mt, which reduced the leaching of reactive metals. The study of reaction mechanism and kinetics showed that the whole degradation process conformed to the pseudo-first-order kinetic equation, and the Langmuir-Hinshelwood method was applied to demonstrate that catalysis was dominant in the degradation process.

1. Introduction

In recent decades, the rapid development of the pharmaceutical industry has produced a large amount of pharmaceutical wastewater; they contain large amounts of antibiotics and pharmaceutical intermediates, which have the characteristics of complicated components, hard degradation, and strong toxicity [1]. If not treated properly, it will cause serious threats to human beings and the environment [2]. Due to the harm of pharmaceutical wastewater, the treatment of pharmaceutical wastewater has received strong attention from many countries.

At present, the common wastewater treatment technologies include adsorption, photocatalysis, microwave catalysis, ozone oxidation, etc. The adsorption method generally uses porous materials such as montmorillonite and kaolin as adsorbents, and the method mainly uses intermolecular forces between the adsorbent and the pollutant to adsorb the pollutant in the wastewater. The technology has low secondary environmental pollution and low equipment requirements [3], but the high cost of adsorbent regeneration has limited its further application. Nowadays, advanced oxidation processes (AOPs) are considered effective methods to treat pharmaceutical wastewater. In the process of AOPs, the generated hydroxyl radicals (HO•) mineralize pollutants in wastewater, which have strong oxidation activity and non-selectivity [4,5]. Photocatalysis is a kind of AOP that uses light to irradiate semiconductors to generate electrons and holes, which in turn mineralizes pollutants [6]. It has advantages such as high catalytic activity and environmental friendliness [7], but the use of visible light by semiconductors is limited and is not applicable to the degradation of actual wastewater. Microwave catalysis is also an effective technology for the degradation of pollutants. The thermal and excitation effects of microwaves produce large amounts of reactive oxygen species (ROSs) in the composite material, allowing for efficient and rapid degradation of pollutants [8,9], but the process is more energy intensive. Ozone has a strong oxidation ability and can produce a large amount of HO• in a short period to achieve complete degradation of pollutants, which is an environmentally friendly AOP, but the application cost is high and may also produce toxic by-products [10]. However, the catalytic wet peroxide oxidation (CWPO) technology with strong oxidation ability can overcome these disadvantages.