Impact of groundwater quality and associated byproduct formation during UV/hydrogen peroxide treatment of 1,4-dioxane
Graphical abstract
Introduction
1,4-Dioxane (C4H8O2) is a widely used solvent for a variety of industrial applications such as in the manufacture of chlorinated solvents (e.g., 1,1,1-trichloroethane), in products like adhesives, sealants, paint strippers, dyes, greases, varnishes, waxes, and in the manufacture of pharmaceuticals (Zenker et al., 2003). In addition to its intentional uses, 1,4-dioxane is also found as an impurity in consumer products such as deodorants, shampoos, and cosmetics, and as a byproduct during the manufacture of polyethylene terephthalate (PET) plastic (Mohr et al., 2010; Zenker et al., 2003). 1,4-Dioxane is classified as a probable human carcinogen based on evidence of carcinogenicity from animal studies showing increased incidences of nasal cavity, liver, and gallbladder tumors following oral exposure (ATSDR, 2012).
Wastewater discharge, unintended spills, and historical disposal practices have led to the widespread contamination of 1,4-dioxane in drinking water sources across the U.S. Analysis of the Unregulated Contaminant Monitoring Rule 3 (UCMR3) data from the U.S. EPA revealed that 1,4-dioxane was detectable at over 4000 sites across the U.S. (Adamson et al., 2017), and more than 600 sites were above the health-based drinking water reference level established by the U.S.EPA (0.35 μg L−1) representing one-in-a-million cancer risk (U.S.EPA, 2013). In New York State (NYS) alone, 516 sites were found above the detection limit (0.07 μg L−1) with 238 sites showing 1,4-dioxane concentrations higher than 0.35 μg L−1 as per UCMR3 data. Currently, there is no established federal maximum contaminant level (MCL) for 1,4-dioxane in drinking water, but several states have their own clean-up or notification levels ranging from 0.3 to 70 μg L−1. Recently, the NYS Department of Health has approved the nation’s first 1,4-dioxane standard at 1 μg L−1 (parts-per-billion, ppb) in drinking water (NYS Government, 2019).
1,4-Dioxane is highly persistent, soluble in water, resistant to (bio)degradation, non-volatile, and has a low sorption coefficient, making it extremely difficult to remove from water using conventional water treatment processes (e.g., by activated carbon, air stripping, etc.) (Zenker et al., 2003). Advanced oxidation processes (AOPs) are very effective at oxidizing 1,4-dioxane in drinking waters. Among various AOPs, ultra-violet (UV) based AOP including UV/H2O2 (Antoniou and Andersen, 2015; Coleman et al., 2007; Kim et al., 2006; Martijn et al., 2010; Maurino et al., 1997; Stefan and Bolton, 1998), UV/S2O82− (Antoniou and Andersen, 2015; Maurino et al., 1997; Zhao et al., 2014), UV/HOCl (Kishimoto and Nishimura, 2015; Zhang et al., 2019b), UV/NH2Cl (Patton et al., 2016; Zhang et al., 2019b), UV/Fe(II)/H2O2 (Chitra et al., 2012), UV–vis/ferrioxalate/H2O2 (Safarzadeh-Amiri et al., 1997), and the photoelectro-peroxone (PEP) process (Shen et al., 2017) have been extensively studied. A few studies have examined the generation of intermediates (e.g., aldehydes, organic acids, and the mono- and diformate esters of ethylene glycol) during UV/H2O2 treatment of 1,4-dioxane and have proposed possible degradation pathways (Maurino et al., 1997; Stefan and Bolton, 1998). Li et al. (2018) further investigated the cyto- and geno-toxicity of known intermediates from 1,4-dioxane degradation during UV-based AOP treatment (UV/H2O2, UV/S2O82−, and UV/NH2Cl) and suggested that formaldehyde and glycolaldehyde were the most cyto- and geno-toxic byproducts.
Although several studies have shown a good efficacy of decomposing 1,4-dioxane through UV-based AOPs (summarized in Table S1), scaling up or comparing these studies has been a challenge because of (i) varying experimental conditions, (ii) a lack of standardized reporting of system performance (e.g., electrical energy per order (EEO) and/or UV dose was often not reported), and (iii) a lack of testing performed under environmentally relevant conditions (e.g., high concentrations of 1,4-dioxane at parts per million levels were often tested). Although one study was performed at environmentally relevant levels of 1,4-dioxane (Swaim et al., 2008), the impact of water quality was not evaluated in the study. It is well known that some naturally occurring compounds in groundwater can scavenge radicals that may significantly impact the performance of AOPs (Crittenden et al., 2012), and thus, there is a need to evaluate source water quality impacts on the efficacy of AOP treatment of 1,4-dioxane.
In this study, a series of experiments at environmentally relevant conditions were performed to (i) comprehensively investigate the impact of pH, alkalinity, nitrate, natural organic matter (NOM), and iron on 1,4-dioxane removal, (ii) assess the formation of associated byproducts, and (iii) report variation in performance-related parameters (EEO and UV fluence-based rate constants) during bench-scale UV/H2O2 treatment. To the best of our knowledge, the impact of Fe (ferrous and ferric ions), alkalinity, and NOM on UV/H2O2 treatment of 1,4-dioxane has not been reported in literature. Organic acids (formic, acetic, and oxalic acids) and aldehydes (formaldehyde, acetaldehyde, and glyoxal) were monitored, which are known major byproducts formed during AOP treatment of 1,4-dioxane (Stefan and Bolton, 1998). These compounds are also common byproducts generated during AOP treatment of organic compounds (e.g., phenolic compounds) (Alnaizy and Akgerman, 2000; Scheck and Frimmel, 1995), and hence, additionally serve as representative byproducts to assess the relative levels generated from other organics (e.g., NOM) in groundwater. The results from experiments with synthetic source water were further compared with groundwater samples collected from sites contaminated with 1,4-dioxane.
Section snippets
Reagents and AOP system settings
All chemicals and solvents used in this study were of either certified ACS reagent grade or analytical grade with high purity and were purchased from Sigma-Aldrich or Fisher Scientific. Deionized water (DI water, Elix®) and ultra-pure Milli-Q® water (18.2 MΩ cm) were used throughout experiments.
A schematic diagram of the bench-scale AOP system is shown in Fig. 1. The UV reactor was equipped with a low-pressure mercury lamp (40 W, λ = 253.7 nm) and purchased from VIQUA (Ontario, Canada). The
1,4-Dioxane breakdown and byproducts formation during UV/H2O2 treatment
As expected, direct UV photolysis of 1,4-dioxane was not observed and H2O2 alone did not degrade 1,4-dioxane (Fig. 2a). In the presence of H2O2 (10 mg L−1) along with UV irradiation, 1,4-dioxane was rapidly decomposed by ∼90% in the first 3.5 min (UV fluence = 550 mJ cm−2) and became undetectable after 8 min (UV fluence = 1260 mJ cm−2) of operation (Fig. 2a). The calculated first-order rate constant was k = (1.13 ± 0.13) ✕10−2 s−1. Based on the UV fluence rate evaluated in our system, the
Conclusions
This study provides the first systematic evaluation of UV/H2O2 system performance to remove environmentally relevant levels of 1,4-dioxane by evaluating the impact of groundwater constituents most likely to alter the AOP process. Results demonstrated that nitrate and NOM are important groundwater parameters that can negatively impact the removal of 1,4-dioxane. Iron was shown to inhibit the degradation of 1,4-dioxane due to decreased UVT, although Fenton’s reaction-assisted removal (∼10%) of
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by a grant to the Center for Clean Water Technology (CCWT) from the NYS Environmental Facilities Corporation and NYS Department of Health (NYSDOH). The content is solely the responsibility of the authors and does not necessarily represent the official views of the sponsors. The authors would like to thank Drs. Scott Alderman, Lloyd Wilson, and Roger Sokol from the NYSDOH for their inputs. The authors would also like to thank Ms. Caitlin Asato from CCWT for help with
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