Comparing the degradation of acetochlor to RhB using BiOBr under visible light: A significantly different rate-catalyst dose relationship

Highlights

• Increasing BiOBr significantly accelerate photodegradation of non-polar acetochlor.

• The rate-dose relationship of acetochlor/BiOBr and RhB/BiOBr is different.

• The adsorption capacity of acetochlor decreased as the BiOBr dose increased.

• More BiOBr will adsorb more polar substrates (like RhB).

Abstract

In this study, a weakly polar pesticide acetochlor in water was tried to degrade by BiOBr catalysts under visible light irradiation. Its degradation reaction rate increased exponentially with increasing catalyst dosage, which was significantly different from conventional photocatalytic patterns established for polar or water-soluble pollutants. Thus, the rate-dosage relationship of BiOBr/acetochlor was compared to that of BiOBr/Rhodamine B (RhB) (a typical strong polar dye pollutant in water) at different BiOBr dosages. The degradation rate of RhB increased with the BiOBr dosage and then plateaued due to more catalyst particles blocking the light into the inner solution, which is a typical behavior for heterogeneous photocatalysis of polar organic compounds in aqueous solution. While acetochlor behaved differently, its degradation rate increased exponentially with the BiOBr dosage, and no plateau was observed even up to 9.0 g/L BiOBr dosage. This special rate-dose relationship for weakly polar acetochlor was correlated with a unique adsorption property of BiOBr to acetochlor in water solution. There was a re-distribution effect that always exclude excessive acetochlor adsorbed on BiOBr surface regardless either substrates or BiOBr overdose. This finally resulted in every BiOBr particle having enough adsorption sites available for dissolved dioxygen to function thereby accelerate the degradation of weakly polar organic compounds in water.

Introduction

Acetochlor, which is a chloroacetanilide herbicide, is used for pre-emergence control of weeds for crops, such as beans and corn, and is one of the most widely used herbicides in the world. The aromatic ring and lack of strongly polar bonds (Fig. 1) prevent its binding to soil, so acetochlor is readily transferred from agricultural fields to water to cause environmental pollution. Natural way of the degradation of acetochlor residues in the land, and water is primarily direct photolysis under UV sunlight. The United States Environmental Protection Agency (USEPA) has classified acetochlor as a B-2 carcinogen [1]. Currently, advanced oxidation processes (AOP) for the degradation of acetochlor in water typically utilize ozone [2], [3], the Fenton reagent [4], γ-ray [5] and UV irradiation [6], [7], [8], in which acetochlor reacts stoichiometrically with homogeneous oxidants. These treatments can degrade acetochlor quickly, but it is costly to provide the necessary oxidant, such as O₃, H₂O₂ or high-energy radiation. Therefore, the development of novel heterogeneous photocatalytic systems that are activated by visible light and use the molecular oxygen in air as an oxidant is crucial. The TiO₂ photocatalytic reaction under UV light has been widely used to degrade many organic pollutants in water [9]. However, due to the polarity of its amide bond influenced by the CCl and COC bonds within acetochlor molecule, solid oxide photocatalysts, such as TiO₂, do not provide sufficient adsorption sites for the efficient degradation of acetochlor.

BiOBr responds to both UV and visible light irradiation [10] and has been used to degrade a wide variety of water-soluble organic contaminants including microcystin-LR [11], methyl orange [12], [13], [14], [15], rhodamine B (RhB) [16], [17], [18], phenol [16], ibuprofen [19], ciprofloxacin [20], tetrabromobisphenol A [21], benzotriazole [22] and guanine [23]. BiOBr has two separate valence bands (i.e., O 2p and Br 4p), and the conduction band is Bi 6p [9]. The excitation potential of the two valence bands is 2.6 eV (UV) and 2.25 eV (vis), respectively. The measured band gap of BiOBr is 2.45 eV. Therefore, BiOBr cannot directly oxidize water (E⁰ _OH/ = +2.7 V vs NHE) into hydroxyl radicals (OH). Instead, the OH free radicals are produced by a multi-step reaction on the conduction band, resulting in degradation of refractory organic compounds [24]. BiOBr has tetrahedral symmetry rather than the octahedral symmetry of the commonly used two-component oxide crystals of TiO₂, CdS and WO₃. The polarity of the BiO bond is also lower than that of the TiO bond. These characteristics affect the adsorption and degradation behavior as well as the potential for developing a more effective degradation system for weakly polar organic compounds, such as acetochlor.

In this investigation, the photocatalytic degradation of acetochlor on BiOBr under visible light irradiation (λ > 420 nm) was studied by monitoring total organic carbon (TOC) and chemical oxygen demand (COD) and the concentration of acetochlor using gas chromatography (GC). Our research focused on comparing the degradation of acetochlor with RhB, which is a typical water-soluble pollutant, under identical conditions. The optimal efficiency and relative mechanism for the degradation of the weakly polar acetochlor over BiOBr were determined by quantitatively analyzing the relationship between its degradation rate and catalyst dose. In addition, the dramatic difference in the adsorption vs degradation properties of the weakly polar acetochlor and strongly polar RhB (see Fig. 1) were also compared and confirmed through extending this BiOBr photocatalytic degradation to other weakly or strongly polar substrates in water.