Effect of Zr⁺⁴ doping on characteristics and sonocatalytic activity of TiO₂/carbon nanotubes composite catalyst for degradation of chlorpyrifos

Highlights

• Carbon nanotube was oxidized with H₂O₂ in the presence of NH₄OH.

• Nanotitania/carbon nanotube and Zr⁺⁴ @nanotitania/carbon nanotube composites were prepared.

• All the prepared solid materials were characterized by different tools.

• Sonocatalytic degradation of chlorpyrifos was studied under different application conditions.

• Catalyst reusability was studied.

Abstract

Nanotitania/carbon nanotubes composites were prepared using the sol-gel method with different nanotitania: carbon nanotubes ratios. The composite sample was modified with zirconium cation (IV). The prepared composites were characterized with different techniques using thermogravimetric analysis (TGA), nitrogen adsorption, transmission electron microscopy (TEM), X-ray diffraction (XRD), energy dispersive X-ray (EDX), diffuse reflectance spectroscopy (DRS) and Fourier transform infrared spectroscopy (FTIR). Sonocatalytic degradation efficiency of the prepared sonocatalysts was tested for sonocatalytic degradation of chlorpyrifos at different application conditions such as the effect of chlorpyrifos concentration, temperature, catalyst dosage, ultrasonic power and the presence of radiation light. Zirconium modified nanotitania/carbon nanotubes composite (ZCT10) shows the maximum sonocatalytic activity (91.5% after 60 min) and obeyed pseudo-first order kinetic models. The optimum degradation efficiency was also confirmed at 2.0 g/L as catalyst dosage and increase with both of application temperature and ultrasonic power. The presence of radiation light can also enhance the activity of sonocatalysts. Catalyst reusability experiments showed that catalysts are well reusable for degradation of chlorpyrifos by the sonocatalytic process.

Introduction

Chlorpyrifos is one of the most widely used organophosphate insecticides [O, O-diethyl O-3, 5, 6-trichloro-2-pyridinyl phosphorothioate] due to its broad spectrum of activity [1]. It has been used in a variety of crops, including corn, soybeans, nuts, tree, alfalfa, citrus, wheat, peanuts and vegetables [2]. Degradation of chlorpyrifos in soil was found to be with half-lifetime about 10–120 days [3]. Hydrolysis of chlorpyrifos is accompanied by the formation of a compound known as 3, 5, 6-trichloro-2-pyridinol with adverse effects on the degradation of the parent compound and soil microbial activity [4]. Chlorpyrifos is responsible for the catalytic inhibition of acetylcholinesterase, which leads to in vivo accumulation of acetylcholine, resulting in the destruction of the nervous system and cell death [5]. Chlorpyrifos has been detected in air, rain, marine, streams, sediments, rivers, freshwater, fog and groundwater [6]. Degradation methods for organic pollutants such as oxidation are preferred because they can degrade organic pollutants rather than accumulating them on a certain medium creating another pollutant. Advanced oxidation processes such as heterogeneous Fenton catalysis, photocatalysis, and sonocatalysis are very important procedures in mineralization and decomposition of hazardous organic pollutants in water. Heterogeneous sonocatalysis is one of the most recent and promising advanced oxidation processes. Application of ultrasound irradiation to the organic aqueous solution is accompanied by the creation of acoustic cavitation, namely the formation, growth and implosive collapse of bubbles in solution. That bubbles collapse generates a localized hot spot with a temperature of about few thousand Kelvin and about one thousand atmospheric pressure [7] where nonvolatile organic pollutant molecules can hardly enter the bubbles for pyrolysis degradation. Also under the last conditions, decomposition of organic pollutants may be related to either direct pyrolysis and/or production of reactive radicals such as oxygen (^•O), hydrogen (^•H) and hydroxyl (^•OH). The following chain reaction equations (Eq. (1), (2), (3) explain the production of active species [8,9]:

H₂O+ Ultrasound radiation → ^•H+^•OH

O₂+ Ultrasound radiation →2^•O

^•O+H₂O → 2HO^•

Addition of semiconductive catalysts (sonocatalysts) with the ultrasound radiation to enhance the performance of sonochemical degradation of organics due to additional nucleation sites for development of cavity microbubbles and increasing the generation of new reactive radicals and raise the rate of pyrolytic degradation. ZnO [10], CdS [11] and TiO₂ [12] have been reported as efficient semiconductor sonocatalysts. Titanium dioxide is the most acceptable catalyst based on its chemical and biological inertness, non-toxicity and higher stability. The use of pure TiO₂ as a sonocatalyst is limited mainly by the difficult separation of TiO₂ particles from the solution after treatment and the recombination of the generated active holes and electrons, crystal growth and phase transformation. Many efforts have been made to avoid the last drawbacks in TiO₂ applications such as doping with another inorganic material such as CdS [11], SiO₂ [13], YAlO₃ [14], Fe₂O₃ [15] and CNTs [16]. Doping of TiO₂ sonocatalysts in suitable transition metals such as Fe⁺³ [17], La⁺³ [18], Cr⁺³ [19] and Cu⁺²/Zn⁺² [20] improved its catalytic activity by decreasing band gap and increasing charge carriers lifetime. To the best of our knowledge, there were no studies in pertinent literature about the application of the sonocatalytic process for chlorpyrifos removal on the surface of Zr⁺⁴@ nanotitania/carbon nanotube composites.

In this report, titanium dioxide was combined with different ratios with multiwalled carbon nanotube and doped in zirconium cation solution to improve its catalytic efficiency. The prepared catalysts were characterized by TGA, nitrogen adsorption, XRD, TEM, EDX, FTIR, and DRS techniques. Sonocatalytic degradation of chlorpyrifos was studied using the prepared catalysts at different application conditions.