A novel CuO/Fe₂O₃/ZnO composite for visible-light assisted photocatalytic oxidation of Bisphenol A: Kinetics, degradation pathways, and toxicity elimination

Highlights

• A new CuO/Fe₂O₃/ZnO catalyst was used for BPA degradation under visible light irradiation.

• The catalyst performed optimally in near-neutral conditions even under low loading.

• Adding H₂O₂ significantly enhanced degradation and mineralization of BPA.

• The main radical species responsible for BPA degradation were HO > h⁺ > O₂⁻ > ¹O₂.

• Effluent toxicity decreased significantly after treatment by CuO/Fe₂O₃/ZnO nano-particles.

Abstract

The present study presents a highly efficient, visible light-induced photocatalytic degradation of Bisphenol A (BPA) by a CuO/Fe₂O₃/ZnO composite, synthesized using a sol-gel combustion method. The catalyst was characterized by XRD, SEM, BET, EDX, DRS, TEM, and XPS techniques. The synthesized CuO/Fe₂O₃/ZnO catalyst was found to perform optimally at neutral pH and 0.04 g/L catalyst dosage, while H₂O₂ addition initiated further radical degradation pathways. HCO₃⁻ and NO₃⁻ in the water had a negative effect on BPA degradation, but SO₄²⁻, Cl⁻ and PO₄³⁻ demonstrated a significant increase in reaction kinetics, thus allowing a real-water application. Radical scavenger tests revealed that hydroxyl radicals and holes play the main role in BPA degradation and mineralization. Toxicity tests and probit analysis showed a significant decrease of LC₅₀ on Daphnia Magna neonates. Considering the exceptional characteristics and photocatalytic performance of CuO/Fe₂O₃/ZnO under visible light, its application could be an efficient alternative for the degradation of xenobiotic organic compounds from polluted waters.

Introduction

Bisphenol-A (BPA) is a well-known contaminant of emerging concern and one of the most extensively used organic chemicals in industry, such as plastics, food cans, sealant, and dental composites [1]. About 5.2 million tons were used worldwide in 2008, while due to its massive production and its recalcitrance towards the conventional treatment process, a large number of its derivatives are released to the environment [2]. BPA has a phenolic ring in its structure and can interact with estrogen receptors causing endocrine disorders in male and female species. Moreover, it plays an important role in diseases such as thyroid disorder, breast and prostate cancer, polycystic ovary syndrome (PCOS) and infertility [3], [4]. According to a cohort study in the U.S, BPA can be detected in up to 95.7% of adult urine samples [5], and due to adverse effects on human health and organisms it should be removed from effluents discharged to the environment [6].

To date, a series of conventional methods such as physical, chemical, and biological treatment processes have been used for BPA degradation [3]. Many degradation studies have been conducted using adsorption by adsorbents [1], coagulation [7], electrocoagulation [8], Fenton, and their derivative processes [9]. The biological processes were not efficient for biodegradation due to its inherent toxicity encountered in high concentrations [10], while physical processes can merely transfer BPA from aqueous solution to a solid adsorbent without any degradation [3]. The generation of highly reactive oxygen species (ROS), especially the powerful and non-selective hydroxyl radical in many different advanced oxidation processes (AOPs) techniques, led this process to be considered as an effective alternative to conventional methods and used for degrading recalcitrant chemical structures and bio-refractory compounds [11]. Besides, preventing the production of secondary waste streams is another advantage of these processes over the existing ones [12].

Several photocatalytic degradation processes were successfully used for the removal of recalcitrant and refractory compounds from water and wastewater due to their semiconductor properties [13], [14]. The most common catalysts used are based on TiO₂, ZnO, Bi₂O₃, BiOBr, AgBr, ZnO/CoMoO₄, ZnO/Ag/Ag₂WO₄, ZnO/NiWO₄/Ag₂CrO₄ and Ag₃PO₄ [15], [16], [17], [18]. However, due to the high tendency in photogenerated charges recombination, the vast majority of these catalysts present a low photodegradation activity under visible irradiation [19]. Among the various semiconductors, Zinc oxide (ZnO) has been used as a suitable photocatalyst in environmental remediation due to its non-toxic nature and relative stability [20], [21]. Nevertheless, ZnO has a wide bandgap (~3.2 eV) and hence has limited its application only in the exploitation of UV light spectrum [22]. Also, the wide bandgap caused the high recombination rate of the photo-induced charge carriers and leading to retardation of the degradation of pollutants [20].

Activation of this semiconductor catalyst in the visible region with metal and non-metal by forming composites can increase the amount of light-harvesting [23]. The combination of transition metals with ZnO can increase the absorption ability of light in the visible region and can inhibit the recombination of photogenerated carriers in the accumulation of electron and holes in different metal oxides [24]. It has been reported that oxygen vacancies modified ZnO behavior in photo-catalysis process, acting as a photo-excited electron trapping sites and thus decreasing or suppressing the electron-hole recombination [25]. The application of ZnO/CuO nanoparticle was investigated by Shirzadi et al. [24], and they reported this catalyst not only extends the visible light absorbance but also enhances the transfer of photogenerated electrons from the high conduction band to low one and efficient separation of electron-hole pairs. The presence of CuO along with ZnO can improve the photocatalytic activity and increase the spectral response has attracted significant interest, since the surface modification of the semiconductor oxides with metals such as copper (Cu) or iron (Fe), improved the photocatalytic activity against organic compounds through an interfacial charge transfer (IFCT) mechanism [26]. Hence, the synthesis and application of Cu/Fe/ZnO catalyst has advantages including enhancing visible light harvesting by semiconductor composite, effective exploitation of the photogenerated electrons, suppressing photo-excited electron recombination, increasing the oxidation reaction on deep valence inter-band holes and widening the photocatalytic sensitivity from UV to the visible light spectrum [26], [27].

In this work, we aim to (i) fabricate a CuO/Fe₂O₃/ZnO composite through sol-gel method and used for BPA degradation under visible light, (ii) determine the catalyst surface characteristics using scanning electron microscope (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), EDX (Energy-dispersive X-ray Spectroscopy), Brunauer-Emmett-Teller (BET) analysis, X-ray photoelectron spectroscopy (XPS) and differential reflection spectroscopy (DRS), (iii) clarify the effect of the involved parameters such as solution pH, catalyst dose, BPA concentration, and anionic strength on the BPA degradation and optimize each parameter, (iv) enhance the ability of treatment process in the presence of H₂O₂ as a potent electron acceptor, (v) understand the degradation kinetics in semiconductor or heterogeneous photo-catalysis modes, (vi) determine the BPA degradation mechanism using radical scavenger experiments, and (vii) evaluate the effluent toxicity changes in order to assess the risks of discharge of the treated streams.