Short CommunicationDiscerning the inefficacy of hydroxyl radicals during perfluorooctanoic acid degradation
Graphical abstract
Introduction
Perfluoroalkyl substances (PFASs) are a class of over 3000 industrial chemicals that are contaminants of emerging concern due to their potential for bioaccumulation and persistence (Gómez et al., 2011; Saleh et al., 2019; Suja et al., 2009). PFOA is a widely researched PFAS that was historically used in leather, paper packaging, aqueous film forming foams (AFFFs) and as an additive to aviation fluid (Lopes da Silva et al., 2017). PFOA can also be formed in the environment through natural degradation of perfluorinated precursors such as 8:2 fluorotelomer alcohols (Ellis et al., 2004; Ju et al., 2008; Vestergren et al., 2008). However, PFOA is relatively recalcitrant due to its abundant high-strength C–F bonds (116 kcal/mol) (Lin et al., 2012; Liou et al., 2010).
Several advanced oxidation and advanced reduction processes have been shown to degrade PFOA, including UV-Fenton (Tang et al., 2012), UV-Fe(III) (Liu et al., 2013), sonochemical (Cheng et al., 2008; Lin et al., 2015; Vecitis et al., 2008), catalyzed H2O2 propagation (CHP) reactions (Mitchell et al., 2014), electrochemical (Gomez-Ruiz et al., 2017; Le et al., 2019; Schaefer et al., 2017; Urtiaga et al., 2015; Zhuo et al., 2011), UV, peroxone and heat-activated S2O82− (Eberle et al., 2017; Hori et al., 2005; Liu et al., 2012; Zhang et al., 2019), UV-SO32− (Song et al., 2013), and reduction with solvated electrons (Park et al., 2009) and nano-scale zerovalent iron (NZVI) (Arvaniti et al., 2015; Hori et al., 2006). Some studies indicate that OH may have an ancillary role in PFOA degradation. For example, tert-butyl alcohol (a OH scavenger) decreased the PFOA defluorination efficiency in a UV-Fenton treatment system (Tang et al., 2012). Similarly, we observed that other OH scavengers hinder PFOA defluorination in the presence of UV and Fe(III) (Liu et al., 2013). In these systems, OH was proposed to enhance PFOA degradation by either initiating PFOA decarboxylation (pathway 3, Figure S1) or by accelerating the conversion of intermediate perfluorinated organic radical to perfluorinated alcohol (pathway 4, Figure S1). Several other groups have also shown that OH may assist PFOA degradation (Chen et al., 2016, 2015; 2011; Gomez-Ruiz et al., 2018; Huang et al., 2016b; Lin et al., 2012; Song et al., 2012). However, other studies indicate that OH is ineffective in degrading PFOA (Hori et al., 2004; Szajdzinska-Pietek and Gebicki, 2000).
This paper seeks to clarify ambiguity in the literature about the role of OH in PFOA degradation. UV + H2O2 is a well-known advanced oxidation process (AOP) that relies primarily on OH generation (Khan et al., 2018; Xu et al., 2009). We gain insights into the role of OH in PFOA degradation by comparing UV photolytic treatment versus UV + H2O2, using nitrobenzene (NB) as OH probe to optimize H2O2 concentrations. Furthermore, we report electrical energy per order (EE/O) for our reactions to facilitate energy requirement comparisons with other studies.
Section snippets
Materials
Perfluorooctanoic acid (95% purity), perfluoroheptanoic acid (99% purity), perfluorohexanoic acid (≥97% purity), perfluoropentanoic acid (97% purity), perfluorobutanoic acid (98% purity), perfluoropropanoic acid (97% purity), nitrobenzene (≥99% purity) and fluoride standard (TraceCERT®, 1000 mg/L in water) were purchased from Millipore Sigma. Hydrogen peroxide 30% (w/v) was purchased from Fisher Scientific.
PFOA degradation using UV photolysis versus UV + H2O2 AOP
UVC irradiation experiments were carried out in a photoreactor that has been previously
Results and discussion
NB was used as a OH probe to optimize the H2O2 concentration and maximize OH production in our photoreactor (Li et al., 2017; Varanasi et al., 2018). A concentration of 5 g/L H2O2 resulted in the highest OH production, removing 94 ± 0.2% of 100 mg/L NB within 30 min under UV irradiation (Fig. 1). This corresponded to a steady state OH concentration of 4.56 × 10−13 ± 3.84 × 10−15 M (Table S1). H2O2 concentrations greater than 5 g/L were counterproductive to OH generation due to potential
Associated content
Supporting Information includes: (1) F− mass balance calculations and further details on EE/O calculations. (2) Figures of PFOA degradation mechanism in UV-Fenton and UV-Fe(III) treatment systems, -ln(NBt/NB0) versus time plots for NB degradation, light penetration experiments, and PFOA degradation by UV + H2O2 (300 g/L) over 3-day UV exposure. (3) Tables of OH concentrations at different H2O2 concentrations, and F− mass balance.
CRediT author statement
Hassan Javed: contributed intellectual input to this study, Conceptualization, Methodology, Investigation, Writing - original draft. Cong Lyu: contributed intellectual input to this study, Methodology, Investigation. Ruonan Sun: contributed intellectual input to this study, Investigation. Danning Zhang: contributed intellectual input to this study, Investigation, Visualization. Pedro J.J. Alvarez: contributed intellectual input to this study, Conceptualization, Supervision, Funding acquisition,
Acknowledgements
This research was supported by the NSF ERC on Nanotechnology-Enabled Water Treatment (EEC-1449500).
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