Solar photocatalytic degradation of ibuprofen with a magnetic catalyst: Effects of parameters, efficiency in effluent, mechanism and toxicity evolution

Highlights

• OH.• and photo-hole contributed to photocatalytic degradation of ibuprofen

• Four commonly used oxidants showed different impacts on ibuprofen photodegradation.

• The process is efficient in treatment and mineralization of effluent and seawater.

• Three dominant pathways were proposed as mechanisms of ibuprofen degradation.

• A potential toxic product was detected and removed by the treatment process.

Abstract

The environmental-friendly photocatalytic process with a magnetic catalyst CoFe₂O₄/TiO₂ mediated by solar light for ibuprofen (IBP) degradation in pure water, wastewater effluent and artificial seawater was investigated systematically. The study aims to reveal the efficiency, the mechanism and toxicity evolution during IBP degradation. Hydroxyl radicals and photo-hole (h⁺) were found to contribute to the IBP decay. The presence of SO₄²⁻ showed no significant effect, while NO₃⁻ accelerated the photodegradation, and other anions including HCO₃⁻, Cl⁻, F⁻, and Br⁻ showed significant inhibition. The removal efficiency was significantly elevated with the addition of peroxymonosulfate (PMS) or persulfate (PS) ([Oxidant]₀:[IBP]₀ = 0.4–4), with reaction rate of 5.3–13.1 and 1.3–2.9 times as high as the control group, respectively. However, the reaction was slowed down with the introduction of H₂O₂. A mathematic model was employed to describe the effect of ferrate, high concentration or stepwise addition of ferrate was suggested to play a positive role in IBP photodegradation. Thirteen transformation products were identified and five of them were newly reported. The degradation pathways including hydroxylation, the benzene ring opening and the oxidation of carbon were proposed. IBP can be efficiently removed when spiked in wastewater and seawater despite the decreased degradation rate by 41% and 56%, respectively. Compared to the IBP removal, mineralization was relatively lower. The adverse effect of the parent compound IBP to the green algae Chlorella vulgaris was gradually eliminated with the decomposition of IBP. The transformation product C178a which possibly posed toxicity to rotifers Brachionus calyciflorus can also be efficiently removed, indicating that the photocatalysis process is effective in IBP removal, mineralization and toxicity elimination.

Introduction

Ibuprofen (IBP, a-methyl-4-[isobutyl] phenylacetic acid) is a non-steroidal, anti-inflammatory drug for the treatment of fever, muscle pain, arthritis, migraine and toothaches (Georgaki et al., 2014). It has been frequently detected in various water bodies due to its wide application and incomplete absorption by patients (a significant fraction in the parent form is excreted) (Quero-Pastor et al., 2014). In England and Wales, 0.025–0.475 μg/L IBP was detected from surface water and drinking water, respectively (Boxall et al., 2014). The trace level of pharmaceutical waste resulted in the imbalance of the aquatic ecosystem (Pietrini et al., 2015) and even posed threat to human health. For instance, IBP showed significant adverse effect on the growth of the algae Synechocystis sp. at the concentration of 0.0485 μM (Pomati et al., 2004). Besides, the reproduction and the survival of Oryzias latipes was inhibited when exposed to IBP solution of 0.485 μM and 0.00485 μM, respectively (Flippin et al., 2007; Han et al., 2010). Furthermore, ibuprofen shows an adverse impact on human kidney cells, liver cells, and gut microbiota species even over a very short-term exposure (W.E. Bennett Jr. et al., 2009).

Various technologies have been developed for the removal of IBP, including adsorption, biological degradation, and advanced oxidation processes (AOPs) (Dubey et al., 2010; Fu et al., 2019; Gu et al., 2019; Jia et al., 2020). Among the treatment processes, AOPs attract great attention because of the high efficiency and broad applicability in degradation of various contaminants. The treatment of IBP under different AOPs including photolysis (Kwon et al., 2018; Luo et al., 2018), homogeneous photocatalysis (Guo et al., 2018; Huang et al., 2018; Liu et al., 2018; Saeid et al., 2018) and heterogeneous photocatalysis (Gonçalves et al., 2020; Liang et al., 2020; Rather and Lo, 2020; Zhang et al., 2020) has been widely studied previously.

Despite the high efficiency, some toxic intermediates may be generated during the degradation of the parent compound (Chen et al., 2018; Ellepola et al., 2020; Grabarczyk et al., 2020). For example, during the photolysis process with UV-VIS, six transformation products formed during IBP degradation were determined and two of them were demonstrated to be toxic to fibroblasts and erythrocyte (Castell et al., 1987). Though IBP can be directly oxidized by ozone, with 99% removal efficiency in 20 min, the toxicity of the treated solution to Selenastrum capricornium was actually increased since some transformation products with higher toxicity were formed (Quero-Pastor et al., 2014). Therefore, it is necessary to monitor the generated intermediates and the toxicity of the treated solution during the degradation.

TiO₂ is one of the most used photocatalysts for treatment of micro-pollutants. Despite its high performance, the limited absorption of sunlight and the separation of TiO₂ from the treated water may result in its limitation for industrial scale applications. In our previous study, a catalyst was synthesized with a simple method and it was proven that the UV driven CoFe₂O₄/TiO₂ photocatalysis (UV/CoFe₂O₄/TiO₂) is efficient in sulfamethoxazole degradation. Sulfamethoxazole of 100 μM can be completely removed and 50% mineralization was reached in 5 h with matrix of pure water. And the low band gap energy of the catalyst (1.4 eV) (Gong et al., 2017) provides the potential of use of sunlight. Compared to the catalysts employed for antibiotics removal in previous studies (Iqbal et al., 2020, 2021; Kumari et al., 2020, 2021; Shah et al., 2020), the treatment process will be more economic due to the recyclable property of the catalyst. The separation of the catalyst from solutions is very easy and the photocatalytic activity of the recycled catalyst keeps intact even after five times of reuse (Gong and Chu, 2015). However, some transformation product generated during the treatment showed higher toxicity than the parent compound. To our best knowledge, the IBP removal by CoFe₂O₄/TiO₂ photocatalytic process stays practically unexplored. As a free and inexhaustible resource, the solar power of the sunlight has a good potential in development of new technology for contaminants removal. In this study, IBP degradation under simulated solar light (SSL) mediated CoFe₂O₄/TiO₂ process (SSL/CoFe₂O₄/TiO₂) was evaluated. Apart from the optimization of the reaction conditions, the effects of common anions and oxidants on IBP photodegradation were also investigated. The mechanism of IBP degradation was discussed by intermediates identification and radical determination. The toxicity evolution during the treatment process was assessed on algae and rotifers. In addition, treatment of WWTP effluent and artificial seawater spiked with IBP was determined.