Heterogeneous photocatalysis of Tordon 2,4-D herbicide using the phase mixture of TiO₂

Abstract

Titanium dioxide nanoparticles (nTiO₂) were used in heterogeneous photocatalysis for remediation of herbicide Tordon 2,4-D. These nTiO₂ was synthesized by the sol-gel method followed by thermal treatment at 450 °C for 2 h. The structural characterization by X-ray diffraction and Rietveld refinement confirm that the mixture of phases is 21.28 % of anatase (tetragonal) and 78.12 % of brookite (orthorhombic). All vibrational modes characteristic of anatase and brookite in the interval from 85 to 1100 cm⁻¹ were confirmed using the Raman spectroscopy. The composition of nTiO₂ was studied using the X-ray spectroscopy analysis (XPS), where TiO₂ and traces of Ti₂O₃ were confirmed. The nTiO₂ exhibit strong absorption in the region of 427 nm with direct optical bandgap (E_gap) calculated equal to 2.90 eV. For all catalytic tests with nTiO₂ as a catalyst there where high photocatalytic performance. Also, the total degradation for molecules of 2,4-D herbicide in aqueous solution was obtained at 45 °C for 40 min under UVc radiation. The toxicity tests with Allium cepa bulbs show that the solutions collected after the photocatalytic treatment with nTiO₂ nanoparticles was similar when compared with the negative control (distilled water) results. Moreover, the positive control (Tordon 2,4-D solution) and residue after the photolysis test exhibited high toxicity, inhibiting the growth and development of bulbs and roots.

Introduction

The pesticides are organic or organometallic compounds specifically classified by the exhibited action [[1], [2], [3], [4], [5]]. The agricultural activity, mainly in South America countries, has been reported by literature as of high risk due to the absence of strict inspection, poor investment in new technologies and the utilization of products potentially polluting, carcinogenic, mutagenic and of difficult degradation when exposed to natural conditions [6,7]. Among the potentially contaminating agrochemicals, are highlighted the herbicides, compounds used for weed control, widely used in the cultivation of sugarcane, soy, vegetables, and fructiferous [[8], [9], [10]].

The 2,4-Dichlorophenoxyacetic acid is a well-known component of herbicide formulations, commonly contained in the Tordon® 2,4-D herbicide that have been associated with numerous complications to human health [[11], [12], [13], [14]]. Once in contact with the environment, the Tordon 2,4-D can be leached by the rainfall action, reaching the surface and underground springs and contaminating air, soil, and biomass [15]. In the last decades, some studies were reporting the remediation of Tordon 2,4-D in the aqueous medium using activated carbon, electrochemical oxidation, biocatalysis, and heterogeneous photocatalysis [[16], [17], [18], [19]]. However, photocatalysis has advantages when compared to other processes, such as low cost, efficiency, and the reuse of the catalyst after several cycles [20].

Among the main semiconductors applied in the photocatalysis are highlighted the polymorphs of titanium dioxide, zinc oxide, silver chromate, tungsten trioxide, and zirconium oxide [[21], [22], [23], [24]]. Titanium dioxide (TiO₂) is a well-known semiconductor due to exhibit several advantages. For example, easy obtention using the conventional methods, low cost, strong absorption of UV radiation, and high performance in the oxidation of organic compounds in the aqueous medium [25].

The TiO₂ exhibit three polymorphs, the anatase phase ($I41/amd$) crystallizes in tetragonal structure, the brookite phase ($Pbca$) of orthorhombic structure and the rutile phase (${P4}_2/mmm$) that is more thermodynamically stable, have a tetragonal structure [26].

Boppellar et al. [27] report the obtention of pure TiO₂ polymorphs (anatase, brookite and rutile) and the phase mixture using the sol-gel method followed by hydrothermal method. These nanoparticles were applied in the photodegradation of rhodamine B (RhB) dye molecules in aqueous solution by heterogeneous photocatalysis.

In this paper, the synthesis, characterization and photocatalytic performance of TiO₂ nanoparticles (nTiO₂) obtained by the sol-gel method followed by thermal treatment at 450 °C for 2 h was reported. As also, the photodegradation of Tordon® herbicide under different conditions using the nTiO₂ as a catalyst.