Visible and UV photocatalysis of aqueous perfluorooctanoic acid by TiO₂ and peroxymonosulfate: Process kinetics and mechanistic insights

Highlights

• 100% PFOA was degraded within 9 h by TiO₂/PMS under visible light.

• 1.3:1 molar ratio of TiO₂/PMS was optimum for PFOA degradation.

• ${\text{SO}}_4^{•–}$ and photoinduced holes were the main active species.

• 100% PFOA in real wastewater was degraded within 2 h under UV light.

Abstract

The global occurrence and adverse environmental impacts of perfluorooctanoic acid (PFOA) have attracted wide attention. This study focused on the PFOA photodegradation by using photocatalyst TiO₂ with peroxymonosulfate (PMS) activation. Aqueous PFOA (50 mg L⁻¹) at the pH 3 was treated by TiO2/PMS under 300 W visible light (400–770 nm) or 32 W UV light (254 nm and 185 nm). The addition of PMS induced a significant degradation of PFOA under powerful visible light compared with sole TiO₂. Under visible light, 0.25 g L⁻¹ TiO₂ and 0.75 g L⁻¹ PMS in the solution with the initial pH 3 provided optimum condition which achieved 100% PFOA removal within 8 h. Under UV light irradiation at 254 nm and 185 nm wavelength, TiO₂/PMS presented excellent performance of almost 100% removal of PFOA within 1.5 h, attributed to the high UV absorbance by the photocatalyst. The intermediates analysis showed that PFOA was degraded from a long carbon chain PFOA to shorter chain intermediates in a stepwise manner. Furthermore, scavenger experiments indicated that ${\text{SO}}_4^{•–}$radicals from PMS and photogenerated holes from TiO₂ played an essential role in degrading PFOA. The presence of organic compounds in real wastewater reduced the degradation efficacy of PFOA by 18–35% in visible/TiO₂/PMS system. In general, TiO₂/PMS could be an ideal and effective photocatalysis system for the degradation of PFOA from wastewater using either visible or UV light source.

Introduction

Perfluoroalkyl substances (PFAS) have been extensively used in industry and consumer products since the 1950s (Janousek et al., 2019). Consequently, some PFAS, in particular perfluorooctanoic acid (PFOA), are widely detected in the aquatic environment to reach μg L⁻¹ and even up to mg L⁻¹ concentrations. For instance, Valsecchi et al. (2015) reported that the concentration of PFOA in the surface water of Bormida River, Italy ranged from 0.253 to 6.468 μg L⁻¹, with the mean value of 1.613 μg L⁻¹. In studying PFAS occurrence in surface water within a 10 km radius from a mega-fluorochemical industrial park, Liu et al. (2016) found that PFOA was at a severe contamination level with the concentration ranging from 0.0386 to 1707 μg L⁻¹. These levels constitute human health risks, which may lead to growth and reproduction toxicity, liver injury and even cancer from PFOA exposure (Bassler et al., 2019; Behr et al., 2018; Hurley et al., 2018). In February 2019, the United States Environmental Protection Agency (USEPA) established a multi-media, multi-program, national communication and research plan to address emerging environmental challenge from PFAS (https://www.epa.gov/pfas). Therefore, it is increasingly urgent to develop novel technologies with high efficacy and low cost for PFAS degradation.

Advanced oxidation processes (AOPs) such as ozonation, Fenton, ferrate and photocatalysis are widely used to degrade organic pollutants through free radicals, as reviewed by Sornalingam et al. (2016). AOPs based on hydroxyl radicals are effective for treating endocrine disrupting chemicals such as bisphenol A (Xiao et al., 2017), while sulfate radicals are effective in the degradation of pharmaceuticals in water (Gao et al., 2019). Of different AOPs, heterogeneous photocatalysis has drawn significant scientific attention to be applied for the treatment of organic pollutants including PFAS (Xu et al., 2017, 2018). Of different photocatalysts, titanium dioxide (TiO₂) has proven to be one of the most promising semiconductors for heterogeneous photocatalysis due to its wide band gap (3.14 eV), nontoxicity and long-term photostability (Yoo et al., 2018; Zhang et al., 2019). Hence, TiO₂ is widely used as co-catalyst synthesized with other materials during the photocatalytic process (Wang and Zhang, 2011). For example, Chen et al. (2012) investigated the accelerated TiO₂ (1000 mg L⁻¹) photodegradation of Acid Oange 7 (AO7) under visible light mediated by 614 mg L⁻¹ of peroxymonosulfate (PMS). PMS, derived from the Oxone (KHSO₅·0.5KHSO₄·0.5K₂SO₄), as an environmentally friendly oxidant, could produce sulfate radicals $\left({\text{SO}}_4^{•–}\right)$ in the solution and induce a remarkable synergistic effect in the combined TiO₂/PMS system (Feng et al., 2018; Jo et al., 2018). As a consequence, AO7 was fully degraded by TiO₂/PMS within 1.5 h, while only about 60% AO7 was removal by TiO₂ only (Chen et al., 2012). In another study, Khan et al. (2017) found that TiO₂/PMS (230/61 mg L⁻¹) could efficiently degrade lindane by visible light with 100% removal within 4 h. It has some obvious drawbacks such as the recombination of photo-generated charge carriers, which reduces the overall quantum efficiency (Cao et al., 2016; Pan et al., 2013).

For PFAS photodegradation, Park et al. (2018) synthesized graphene oxide/TiO₂ nanotubes array as catalysts irradiated by UV light, which achieved 83% PFOA degradation within 4 h. Wu et al. (2018) used ZnO-reduced graphene oxide combined with persulfate oxidation under UV light irradiation for PFOA degradation, and observed that almost 100% PFOA was removed within 4 h. However, such methods have some potential drawbacks in relation to real world applications. Firstly, these catalysts are commonly synthesized under certain conditions (i.e. high temperature and specific precursors), which unavoidably increase the cost and operation difficulty. Secondly, UV light was often the indispensable light source in the photocatalytic system to active the photocatalytic degradation of pollutants (Hao et al., 2019). At ground level, 44% of the sunlight energy is in the visible range, with only 3% in the ultraviolet range. Thus, it is difficult to utilize UV light from sunlight as photodegradation energy source, and the extra UV energy has to be provided for PFAS degradation, with UV-based treatment technologies.

The aim of this work was to investigate the feasibility of using TiO₂ with PMS for PFOA removal under visible light as a green technology. The objectives were to determine the kinetics and extent of PFOA photodegradation under visible light, the effects of catalyst dosage and initial solution pH on photodegradation, the reaction intermediates of PFOA photodegradation in the visible/TiO₂/PMS system, and the degradation pathway via the scavenger experiments. For comparison, the photodegradation was also conducted under UV light to evaluate the photocatalytic efficacy. In addition, PFOA photodegradation performance by TiO₂/PMS catalyst in real wastewater samples with a highly complex matrix was explored.