Sorption, Aerobic Biodegradation, and Oxidation Potential of PFOS

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Environmental Processes

Sorption, Aerobic Biodegradation and Oxidation Potential of PFOS Alternatives Chlorinated Polyfluoroalkyl Ether Sulfonic Acids hong chen, Youn Jeong Choi, and Linda S Lee Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b02913 • Publication Date (Web): 12 Aug 2018 Downloaded from http://pubs.acs.org on August 14, 2018

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Environmental Science & Technology

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Sorption, Aerobic Biodegradation and Oxidation Potential of PFOS Alternatives

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Chlorinated Polyfluoroalkyl Ether Sulfonic Acids

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Hong Chen,

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†, ‡

‡

Youn Jeong Choi and Linda S. Lee

*,‡

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Key Laboratory of Coastal Ecology and Environment of State Oceanic

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Administration, Department of Marine Chemistry, National Marine Environmental

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Monitoring Center, Linghe Street 42, Dalian 116023, China

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‡

Ecological Science and Engineering, Department of Agronomy, Purdue University, West Lafayette, IN 47907, USA

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*Corresponding author at: Department of Agronomy, Purdue University, West Lafayette, IN 47907, USA, Tel.: +1 765 494 8612; fax: +1 765 496 2926 E-mail address: [email protected] (L.S. Lee)

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Revision of es-2018-2913t for submission to Environmental Science & Technology July 27, 2018

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ACS Paragon Plus Environment


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Graphical Abstract

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ABSTRACT

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Global phase out of perfluorooctane sulfonic acid (PFOS) has led to increasing

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production of alternatives such as the chlorinated polyfluoroalkyl ether sulfonic acids

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(Cl-PFESAs) for which little is known on their environmental fate. In this study,

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sorption by soils, aerobic soil biodegradation, and oxidation potential of 6:2

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Cl-PFESA (9-chlorohexadecafluoro-3-oxanonane-1-sulfonate) and 8:2 Cl-PFESA

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(9-chlorooctadecafluoro-3-oxanonane-1-sulfonate) were evaluated. 6:2 Cl-PFESA

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sorption was quantified for aqueous and acetone/water solutions whereas 8:2 PFESA

29

was could only be accurately measured in acetone/water solutions. The log-linear

30

cosolvency model was applied and validated to estimate sorption of 8:2 Cl-PFESA.

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Only soil organic carbon (OC, 0.76 - 4.30%) was highly and positively correlated to

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sorption of the Cl-PFESAs (R2 > 0.96). The resulting log Koc values (OC-normalized

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sorption coefficients) are 4.01 ± 0.09 (n = 6) and 5.54 ± 0.05 (n = 4) L kg-1 for 6:2

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Cl-PFESA and 8:2 Cl-PFESA, respectively. Aerobic biodegradation in a loam soil at

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24 ± 0.5 °C showed negligible degradation of both Cl-PFESAs. Cl-PFESAs also

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remained unchanged in an unbuffered heat (50 °C)-activated 42 mM persulfate

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oxidation treatment. Therefore, Cl-PFESAs are equally recalcitrant as PFOS in

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addition to being more sorptive, thus with a higher bioaccumulation potential for a

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similar alkyl chain length.

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Keywords: F-53B; Cl-PFESAS; PFOS alternative; Soil; Persulfate Oxidation

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INTRODUCTION

Per- and polyfluoroalkyl substances (PFASs) are ubiquitous contaminants in the 1,2

44

environment and in wildlife

primarily from the direct emissions from

45

manufacturing facilities and use and disposal of PFAS-containing commercial

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products.3,4 In particular, perfluorooctane sulfonate (PFOS) has received the most

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attention because of the persistency, long-range transport potential, bioaccumulation

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properties and developmental, reproductive, and immunotoxic effects.5-8 The highest

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annual production of PFOS was estimated to be 4650 tons.3 In 2009, PFOS was added

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to the Annex B of the Stockholm Convention for Persistent Organic Pollutants.9 With

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the pressure to ban and limit the use of PFOS, various alternatives are being produced,

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such as 6:2 fluorotelomer sulfonate (6:2 FTSA).10 However, these compounds must be

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used at higher concentrations to sufficiently lower surface-tension, which is

54

paramount for covering the fuel to block oxygen access.10 Therefore, PFOS and its

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derivatives are still permitted in closed-loop systems of chrome plating in certain

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countries.11 China has been producing polyfluoroalkyl ether sulfonic acids (PFESAs)

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as a mist suppressant used in the chrome plating industry, under the trade name F-53B,

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since the 1970s12 with 20-30 tons produced annually.13 The major PFESA in F-53B is

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6:2 Cl-PFESA (9-chlorohexadecafluoro-3-oxanonane-1-sulfonate) with 8:2 Cl-PFESA

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(9-chlorooctadecafluoro-3-oxanonane-1-sulfonate) present as an impurity.8 Given

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their structure similarities (Table S1, graphical abstract) and similar physicochemical

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properties to PFOS, these PFESAs may be marketed as potential PFOS alternatives in

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commercial products, thus leading to their increased production and use.8,14 4


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Although PFESAs have been used for several decades, only recently have

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scientific studies on the emission, occurrence and behavior been initiated. Wang et

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al.14 were the first to evaluate the potential environmental toxicity of F-35B and the

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persistence and presence of 6:2 Cl-PFESA. They found high concentrations of 6:2

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Cl-PFESA in wastewater and surface water, which were not successfully removed by

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the wastewater treatment processess.14 Zebrafish assays showed F-53B to be

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moderately toxic14 and 6:2 Cl-PFESA was shown to interfere with the signaling of

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peroxisome proliferator-activated receptors much more than PFOS.15 Several studies

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investigated occurrence of Cl-PFESAs in surface water,14,16 drinking water, 17 sewage

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sludge,8 fish,13 frogs 18 and Greenland marine mammals.19 A few studies also included

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bioaccumulation and biomagnification of 6:2 Cl-PFESA in which it was found to be

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highly bioaccumulative in crucian carps,13 and biomagnify in the marine food chain 20

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as well as frogs at levels higher than PFOS.18 However, little to no information is

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available on sorption and biodegradation of Cl-PFESAs.

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Sorption and biodegradation are two important processes controlling the fate of

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organic compounds and their ecological risks in the environment. Based on available

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PFAS sorption data, sorption tends to be positively correlated to organic carbon (OC)

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content, but also appears to be influenced by clay content, pH and divalent

82

cations.21,22 Although Cl-PFESAs and PFOS have structural differences, they are

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similar with regards to components affecting sorption and model estimates of their

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octanol-water partition coefficients (Kow) for the acid forms;23 therefore, similar trends

85

in sorption may be expected, but data are lacking. The two key structural differences 5



86

between perfluoroalkyl sulfonates (e.g., PFOS) and Cl-PFESAs are the replacement

87

of one fluorine atom with chlorine and the insertion of an oxygen atom into the

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Cl-fluoroalkyl chain (Table S2). The latter may make Cl-PFESAs more readily

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degradable than PFOS, which has not been shown to biodegrade, although limited

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biodegradability was observed for perfluoroalkyl ether carboxylic acids (e.g.,

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ADONA and GenX).24 In a study mimicking a wastewater treatment scenario and

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using biological oxygen demand as a measure of biodegradability, Cl-PFESAs did not

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appear biodegradable.14 However, such information is limited since Cl-PFESAs

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concentrations and potential metabolites were not monitored. The present work

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focuses on quantifying sorption and aerobic biodegradation in surface soils of 6:2 and

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8:2 Cl-PFESAs. High sorption of 8:2 Cl-PFESA from water would require small soil

97

masses to large solution volumes leading to inaccuracy and substantial loss to

98

experimental vessels. Therefore, the log linear cosolvency model was applied and

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validated for estimating aqueous sorption coefficients using 6:2 Cl-PFESA for a

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subset of soils as was done previously for a series of fluorotelomer alcohols.25 In

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addition, the overall chemical oxidative potential of Cl-PFESAs was evaluated using

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heat-activated persulfate oxidation. Heat-activated persulfate forms free sulfate

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radicals (SO4-·), which has a similar oxidation strength as hydroxyl radicals (·OH), EO

104

= 2.6 eV and 2.7 eV, respectively; 26 however, SO4−· has been shown to be a more

105

effective oxidant for perfluoroalkyl acids and their precursors. For example, PFOA

106

was degraded by SO4−· but not by ·OH.27,28 Therefore, we chose SO4−· to investigate

107

the potential for oxidation of our target compounds. To our knowledge, this study is 6


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the first to report the sorption and biodegradation in soil as well as chemical oxidation

109

potential of novel PFESA analogues.

110 111

MATERIALS AND METHODS

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Chemicals. 6:2 Cl-PFESA, 8:2 Cl-PFESA and M8PFOS used as an internal standard

113

(isotopes were not available for the Cl-PFESAs), were purchased from Wellington

114

Laboratories. Acetonitrile (ACN) and methanol (MeOH) were obtained from

115

Sigma-Aldrich, with purity > 99%, HPLC grade. Inorganic chemicals including

116

sodium persulfate (Na2S2O8), calcium chloride (CaCl2), ammonium acetate and acetic

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acid were of reagent-grade purity. ENVI-Carb powder (120-400 mesh, 100 m2/g) was

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from Sigma-Aldrich.

119

Soils. Six surface soils varying in texture, pH, and OC content (Table S1) were used

120

in the sorption experiments. Two soils from West Lafayette, IN, U.S.A included one

121

sampled at the Purdue Student Farm (identified as PSF) immediately outside and area

122

used for organic farming and another sampled from the Agronomy Center for

123

Research and Education (identified as ARS) under a grassy area outside the

124

commercial crop production area. The other four soils were from China sampled from

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a rice paddy (CPS), under a grassy area (CGS), in a forested area (CFS) and along a

126

roadside (CRS). Soils were air-dried and homogenized (< 2 mm) before use except for

127

a portion of the ARS soils, which was used in the biodegradation studies. For the latter,

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the soil was moist sieved (< 2 mm) and stored at 4 °C until the start of the

129

biodegradation studies, which occurred within 3 months of sampling. 7



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Sorption isotherms. Sorption of the more soluble 6:2 Cl-PFESA was measured from

131

both aqueous solutions and acetone/water solutions with volume fraction acetone (fc)

132

of 0.1, 0.2, and 0.3 similar to what Liu et al.25,29 did for fluorotelomer alcohols. All

133

solutions contained 0.01 N CaCl2. For 8:2 Cl-PFESA, high sorption to both PP and

134

glass tubes (detailed in SI) led to sorption measurements from only acetone/water

135

solutions (fc = 0.2, 0.3, and 0.4). Initial solute concentrations of 2, 5 and 10 µg/L were

136

used in triplicates for each solution along with no chemical added soil controls in

137

duplicate and no soil added in duplicate for each concentration). Soil to solution ratios

138

ranged from 1:1 to 1:100 g:mL (detailed in Table S3) to target 20-80% sorption of the

139

chemical mass applied. For the two soils with the lowest OC% (CFS and CRS) where

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sorption was the lowest, 6:2 Cl-PFESA sorption experiments were only conducted in

141

aqueous solutions; sorption from acetone/water solutions required unrealistic soil

142

mass to solution ratios to achieve at least 20% sorption. Soils were weighed into

143

polypropylene (PP) tubes, chemical solutions added, and soil-solution slurries rotated

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end-over-end (40 rpm) at 24 ± 0.5 °C for 48 h which was deemed sufficient based on

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preliminary 5-d sorption tests in aqueous solutions for 6:2 Cl-PFESA. Sorption

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kinetics (approach to equilibrium) is faster in cosolvent-water solutions (log-linear

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increase in sorption rates with increasing fc).30 Sorbed concentrations were calculated

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by difference. Loss of Cl-PFESA mass by sorption to tubes or degradation for the

149

solute-solvent combinations was negligible as will be discussed.

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Using solver in Excel, sorption isotherms were fit to the Freundlich sorption

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model, Cs = Kf CwN , where Cw (mg L-1) and Cs, (mg kg-1) are the solution and sorbed 8


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phase concentrations at equilibrium, Kf is the Freundlich sorption coefficient (mg1-N

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LNkg-1) and N (unitless) is a measure of isotherm nonlinearity, as well as the linear

154

sorption model: Cs = Kd Cw

155

coefficient. The aqueous-phase sorption coefficients were also extrapolated (log

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Kd,w,exp) from acetone/water solutions using the log-linear cosolvency model as

157

illustrated for other compounds including fluorotelomer alcohols 25,29: log Kd,mix = log

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Kd,w,exp – ασc fc where σc and α are the cosolvency power and solvent-sorbet

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interaction terms, respectively.

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Degradation studies. Aerobic biodegradation studies were conducted using methods

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similar to those described by Liu et al.31 Briefly, soil (10 g air-dried weight) used

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within 3 months of field collection was added to sterile 125-mL amber glass serum

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bottles capped with butyl rubber aluminum crimp caps, wetted to approximately 75%

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field capacity using sterile water, and pre-incubated at 24 ± 0.5 °C in the dark for 7

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days. The field capacity for ARS soil was estimated to be 29.1% by A&L Labs

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(Lincoln, Nebraska), thus soil moisture content for the biodegradation study was

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~22%. After pre-incubation, concentrated 6:2 Cl-PFESA and 8:2 Cl-PFESA solutions

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(200 µg/mL) prepared in 1, 4-dioxane were added and mixed manually into each

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microcosm to give an initial concentration of 100 µg/kg soil. Sterile soil controls were

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prepared after the pre-incubation period by autoclaving three times at 103.4 KPa and

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121 °C for 2 h with one day in between the 1st and 2nd autoclave event and 2 days

172

between the 2nd and last autoclave event.32, 33 At each sampling date over a 105-d

173

period, samples were sacrificed for extraction in triplicate for the microbially active

or mix

where Kd,w

or mix

(L kg-1) is the linear sorption

9



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microcosms and in duplicate for autoclave-sterilized controls. Headspace O2 and CO2

175

levels were measured to confirm aerobic conditions and biological activity by

176

drawing 5-mL of headspace from microcosms using a needle syringe, and injecting on

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an Agilent 7890A gas chromatograph (GC) with a thermal conductivity detector

178

(TCD).

179

Sample extraction and extract clean-up. Soil microcosms in the aerobic

180

degradation study were extracted three times sequentially using 20 mL ACN followed

181

by two extractions with 20 mL of 90/10 v/v ACN/200 mM NaOH, which is based on

182

the method from previous report with slight modifications.8 After each solvent

183

addition, samples were sonicated at 45 °C for 60 min, centrifuged at 1400 rpm for 30

184

min (385 g), and supernatants transferred and combined into a single PP tube for each

185

microcosm. Clean-up of the combined extracts consisted of adding 1250 µL extract

186

into a 2-mL microcentrifuge tube with approximately 10 mg of ENVI-Carb powder

187

acidified with 75 µL of glacial acetic acid, vortexing, and centrifuging at 14,000 g for

188

20 min. The isotope mass-labeled PFOS (25 ng) used for an internal standard was

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added immediately prior to HPLC-MS/MS analysis. Recoveries for 6:2 Cl-PFESA

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and 8:2 Cl-PFESA in the soil extraction and clean-up step were 91.8 ± 1.6 % and 87.8

191

± 3.7 %, respectively (Table S2).

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Heat-activated persulfate oxidation studies. Persulfate (S2O82-) is a strong oxidant

193

that when heat-activated forms free sulfate radicals (SO4−·) that has been shown to

194

degrade several PFASs including perfluoroalkyl carboxylates and telomere-based

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perfluoroalkyl compounds.27,28,34 Persulfate oxidation of PFASs increases with 10


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increasing temperature,34, 35 increasing S2O82- concentration,34 and decreasing pH.34-36

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In unbuffered solutions, pH is acidic 28 and continues to decreases as S2O82- produces

198

SO4−·. For example, at 50 °C with 42 mM Na2S2O8 in unbuffered solutions, more than

199

50% of PFOA and 6:2 fluorotelomer sulfate degraded in less than 24 h.34 Therefore,

200

the latter conditions (42 mM Na2S2O8 , 50 °C, unbuffered solutions) were used to

201

explore the oxidation potential of 6:2 Cl-PFESA and 8:2 Cl-PFESA.34 Although

202

higher temperatures would increase degradation rates, we chose 50 °C so if

203

degradation occurred, we would be able to measure transformation intermediates.

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Individual Cl-PFESA reactions were done in 14-mL Pyrex centrifuge tubes with

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Teflon-lined screw-type caps with Na2S2O8 solutions prepared with 18 MΩ water and

206

stored at 4 °C prior to use. Cl-PFESAs prepared individually in methanol was

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aliquoted to tubes and the methanol evaporated prior to adding 6 mL of Na2S2O8

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solution resulting in an initial Cl-PFESA concentration of 100 µg/L. Reaction tubes

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were placed in a temperature-controlled water bath at 50 °C for a total of 50 h.

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No-oxidant controls were prepared similarly except using 6-mL 18 MΩ water. At each

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sampling time (0, 4, 9, 25, 50 h), duplicate reaction and control tubes were placed in

212

an ice bath and 6-mL methanol added. After coming to room temperature, a 1-mL

213

aliquot for each sample was transferred to an HPLC vial and the internal standard (25

214

ng) added prior to HPLC-MS/MS analysis. Measured Cl-PFESA concentrations in

215

controls at 0 h were designated as the initial Cl-PFESA concentration.

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HPLC-MS/MS analysis. Aqueous solutions and acetone/water solutions were diluted

217

1:1 with methanol before analysis. Quantification of 6:2 Cl-PFESA and 8:2 11



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Cl-PFESA was performed on a Shimadzu HPLC system with a gradient elution at 0.2

219

mL/min through a Kinetex C18 column (50 × 4.6 mm, 2.6 µm) coupled to an Applied

220

Biosystems Sciex API 3000 tandem mass spectrometer (MS/MS) in negative

221

electrospray ionization (ESI) mode. The mobile phase consisted of 0.15% acetic acid

222

in water (A) and 20 mM ammonium acetate in methanol (B) starting at 30% B,

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increased to 100% B in 6 min and maintained for 2 min, decreased to 30% B in 0.5

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min, and maintained at 30% B for 4.5 min. Injection volumes were 20 µL. M8PFOS

225

used as an internal standard was added to each vial immediately prior to HPLC

226

analysis. Concentrations were estimated using internal standard (M8PFOS) corrected

227

Cl-PFESA calibration curves prepared in 1:1 v:v MeOH:water.

228 229

RESULTS & DISCUSSION

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Sorption of 6:2 and 8:2 Cl-PFESA by soils. Sorption isotherms for 6:2 and 8:2

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Cl-PFESA are shown in Fig. 1 and 2 with linear model fits. Coefficients for linear and

232

Freundlich sorption model fits are summarized in Tables S4-S7. Freundlich N values

233

ranged from 0.80 ± 0.09 to 1.09 ± 0.15 with regression correlation coefficients (R2)

234

ranging from 0.971 to 1.000 (Tables S6 and S7). No consistent trend in nonlinearity

235

was apparent between compounds or with increasing cosolvent across soils being

236

negligible. This range in nonlinearity is similar to what has been observed for PFOS

237

sorption in aqueous solutions.21 All sorption isotherms were also reasonably

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well-fitted by the linear sorption model as well with R2 values ranging from 0.963 to

239

0.999 (Tables S4 and S5 and shown in Fig. 1 and 2). Therefore, the linear Kd values 12


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were used in the log-linear cosolvency model, which resulted in very good log-linear

241

correlations with R2 > 0.998 for both Cl-PFESAs on all four soils (Fig. 3).

242 243 244 245 246 247

Figure 1. Sorption isotherms for 6:2 Cl-PFESA from acetone/water solutions (fc = 0.0, 0.1, 0.2 and 0.3 for four soils (PSF, ARS, CPS and CGS) and from only aqueous solution (fc = 0.0) in two soils (CFS and CRS). % OC of soils follows: ARS > PSF > CGS > CPA > CFS > CRS (Table 1).

13



248 249 250 251 252

Figure 2. Sorption isotherms for 8:2 Cl-PFESA from acetone/water solutions (fc = 0.2, 0.3 and 0.4 in four soils (PSF, ARS, CPS and CGS). % OC of soils follows: ARS > PSF > CGS > CPA > CFS > CRS (Table 1).

253

For 6:2 Cl-PFESA, Kd,w (L kg-1) values measured directly from aqueous solutions

254

ranged between 77 and 435 L kg-1 (log Kd,w 2.07 to 2.64). The values extrapolated

255

from acetone/water solutions (log Kd,w,ext) resulted in a similar range 2.26 to 2.73, but

256

consistently higher than measured log Kd,w (Fig. 3A and Table 1) by 0.04 to 0.41 log

257

units. However, differences are less than 0.2 log units for 3 of the 4 soils, thus overall,

258

log Kd,w and log Kd,w,ext values agree reasonably well supporting the application of the

259

log-linear cosolvency model for estimating aqueous sorption coefficients for 8:2

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Cl-PFESA for which sorption from pure aqueous solutions could not be measured

261

reliably. The resulting log Kd,w,ext values for 8:2 Cl-PFESA ranged between 3.69 and

262

4.18, which are 1.52 ± 0.16 log units higher than observed for 6:2 Cl-PFESA across

263

soils. The difference between 8:2 and 6:2 PFESA translated to 0.76 ± 0.08 log units 14


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greater sorption per CF2 group, which is similar to the increase in sorption of 0.87 ±

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0.12 log units per CF2 group observed by Liu et al. 25 for a series of fluorotelomer

266

alcohols. This is also consistent with those for both the perfluorosulfonates and the

267

perfluorocarboxylates (0.5-1 log units),21,37,38 and that for the 6:2 and 8:2

268

fluorotelomer sulfonate (FtS) homologues (0.41 ± 0.059).39 Likewise, the difference

269

between the log Kow values predicted using EPA EPI Suite KOWWIN for 6:2 and 8:2

270

Cl-PFESA of 5.24 and 6.58, respectively (Table S2), is similar (0.67 log units per CF2

271

group).

272

273 274 275 276 277 278

Figure 3. Log-linear relationship between sorption coefficients in acetone/water solutions (Kd,mix) and volume fraction acetone (fc) measured from four soils for (A) 6:2 Cl-PFESA along with those measured directly from aqueous solutions (open symbols) and (B) 8:2 Cl-PFESA. The solid lines are the linear regression fits of log Kd, mix versus fc and the dotted lines are the regressions extrapolated to fc = 0 to estimate the aqueous sorption coefficient (log Kd, w, ext).

279

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280 281 282

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Table 1. Extrapolated (exp) log Kd,w,ext and associated cosolvency parameters (-ασc) for 6:2 Cl-PFESA and 8:2 Cl-PFESA determined by applying the log-linear cosolvency model to sorption measured from acetone/water solutions and the log Kd,w directly measured from aqueous solutions for 6:2 Cl-PFESA. soil

-ασ

log Kd,w,ext

R2

log Kd,w

log Koca

log Koc exta

-ασ

6:2 Cl-PFESA

283

log Kd,w,exta

R2

log Koc,exta

8:2 Cl-PFESA

PSF

7.42 (0.58)

2.68 (0.12)

0.994

2.64

ARS

7.33 (0.38)

2.73 (0.083)

0.997

2.54

4.01 ± 0.09

CPS

8.67 (0.38)

2.26 (0.081)

0.998

2.07

(n = 6)

CGS 8.13 (0.021) 2.63 (0.004)

0.999

2.22

11.4 (1.27)

3.94 (0.40)

0.988

4.24 ± 0.17

11.0 (0.14)

4.18 (0.04)

0.999

5.54 ± 0.05

(n = 4)

10.9(0.60)

3.69 (0.18)

0.997

(n = 4)

10.8 (0.57)

3.72 (0.18)

0.997

a

Log normal averages

16


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The cosolvency power (σc) of acetone for Cl-PFESAs. The values for ασc (slopes of

285

log Kd,mix versus a fc in Table 1) of acetone on 6:2 Cl-PFESA and 8:2 Cl-PFESA

286

sorption are 7.88 ± 0.53 and 11.0 ± 0.31 across the four soils, respectively. For

287

comparison, similar sorption studies with fluorotelomer alcohols (FTOHs) in acetone

288

water solutions resulted in ασc values of 7.15 ± 0.11 (n=5) and 11.4 ± 0.35 (n=4) for

289

8:2 and 10:2 FTOH, respectively.25,29 The α term (solvent-sorbent interaction) is

290

typically close to unity as observed for FTOHS (α = 1.02 ± 0.01 and 1.02 ± 0.02 for

291

8:2 FTOH and 10:2 FTOH, respectively); 25,29 therefore, ασc is good approximation of

292

the cosolvency power.

293

occur for both the PFESAs and the FTOHSs with ≈ 3.1 and 3.2 unit increase,

294

respectively. Increases in σc with increase molecular size (as reflected in increasing

295

log Kow) have been observed for other hydrophobic compounds.40 Therefore the

296

cosolvency power of acetone for 6:2 Cl-PFESA was higher than that for 8:2

297

Cl-PFESA, suggesting the increasing cosolvency power with increasing carbon-chain

298

length of the solute due to the increasing hydrophobicity.

299

Impact of soil properties on sorption for Cl-PFESAs. For both 6:2 and 8:2

300

Cl-PFESAs sorption was found to be strongly and positively correlated to OC fraction

301

(foc) (Fig. 4). Therefore, OC-normalized sorption coefficients (Koc = Kd/foc) were

302

calculated using Kd,w values for 6:2 Cl-PFESA and Kd,w,exp for 8:2 Cl-PFESA yielding

303

log Koc values of 4.01 ± 0.09 L kgoc-1 (n = 6, R2 = 0.964) and 5.54 ± 0.05 L kgoc-1 (n =

304

4, R2 = 0.968), respectively. This log Koc for 6:2 Cl-PFESA is at least one order of

305

magnitude higher than has been reported for PFOS (e.g., log Koc 2.57 ± 0.13 L kg-1for

Similar increases in ασc with the addition of two CF2 groups

17



306

PFOS determined on 4 sediments with 1.02 – 9.66% OC and pH 5.7 – 7.6).21

307

Although both PFOS and 6:2 Cl-PFESA have the same number of carbons, the

308

replacement of one F atom by the larger Cl atom and the addition of an oxygen atom

309

makes 6:2 Cl-PFESA have a larger molecular volume than PFOS (407.91 – 413.61 Å

310

compared to 381.77 Å), which contributes to the greater sorption.14,23 Therefore, 6:2

311

Cl-PFESA, a current PFOS alternative in the chrome plating industry, will be less

312

mobile, but more bioaccumulative than PFOS.13, 18

313 314 315

Figure 4. Relationship between soil-water distribution coefficients (Kd values) for 6:2 Cl-PFESA measured directly from aqueous solutions and 8:2 Cl-PFESA extrapolated from acetone/water solutions and the fraction of soil organic carbon.

316 317

Some previous reports have found statistically significant relationships between 30, 41, 42

318

sorption of PFASs and OC%

consistent with our findings while other studies

319

found OC to be insufficient as a strong predictor alone of sorption in some PFAS-soil

320

combinations.43-45, 39, 46 In a critical analysis of literature data, Li et al.22 showed that

321

sorption was significantly correlated with OC% and clay content for several PFASs

322

and in some cases statistically significant correlations with soil OC% and pH were 18


Page 18 of 28

Page 19 of 28


323

noted. Other than OC, we did not see any meaningful correlations between sorption of

324

the Cl- PFESAs and other soil properties including clay content, pH, and cation

325

exchange capacity (Table S8 and Fig. S1). The estimated pKa values for Cl-PFESAs

326

are very low (≤ 0.14)23 thus exist almost exclusively as organic anions in the

327

environment. For organic anions, electrostatic interaction would be primarily to

328

positively charged sites, which tend to be present in more acidic soils, or potentially

329

through cation bridging to cation exchange sites.47 Previous reports found that

330

contribution of electrostatic interactions to PFAS sorption increased when %OC
70% of ambient O2 levels for the first ten days. CO2 content increased

336

continuously from 0.6 % to 8.5% indicating active aerobic microbial reaction in the

337

system. Bottles were re-aerated weekly to ensure adequate O2 levels. The degradation

338

profiles for 6:2 Cl-PFESA and 8:2 Cl-PFESA in the ARS soil are shown in Fig. 5.

339

Mass recoveries slightly decreased over time except one data point for 8:2 Cl-PFESA

340

and averaged over time for 6:2 Cl-PFESA and 8:2 Cl-PFESA in autoclave-sterilized

341

soil are 87.3 ± 3.6% and 92.8 ± 8.2%, respectively, while the recoveries in live soil

342

are 89.9 ± 7.3% and 89.1 ± 7.2%, respectively. Comparing the mass recoveries

343

between live soil and auto-clave soil, it was concluded that no significant

344

biotransformation of 6:2 Cl-PFESA and 8:2 Cl-PFESA occurred during the 105-d 19



345

period in this soil.

346

347 348 349 350 351 352

Figure 5. Aerobic degradation of (A) 6:2 Cl-PFESA and (B) 8:2 Cl-PFESA in soil sampled immediately under a grassy area outside a commercial crop production area from the Agronomy Center for Research and Education (identified as soil ARS) in West Lafayette, IN, U.S.A in microbially active soils compared to autoclave-sterilized soil.

353 354

Persulfate oxidation for Cl-PFESAs. No heat-activated persulfate oxidation of 6:2

355

Cl-PFESA or 8:2 Cl-PFESA was observed with 42 mM persulfate at 50 °C over a

356

50-h period. 95.1 ± 3.5% versus 96.8 ± 3.3% 6:2 Cl-PFESA and 100.7 ± 3.1% versus

357

97.3 ± 6.4% 8:2 Cl-PFESA remained in the persulfate treated samples and controls,

358

respectively (Fig. 6). The pH of unbuffered 42 mM persulfate was ≈3.5 and decreased

359

during the reaction to a pH of ≈2 by 50 h. Similar results were observed for PFOS in

360

previous studies, whereas perfluoroalkyl carboxylates are easily oxidized by

361

heat-activated persulfate.34 The inability for persulfate radicals to attack both

362

Cl-PFESAs is an additional evidence that Cl-PFESAs will be persistent similar to

363

PFOS.

20


Page 20 of 28

Page 21 of 28


364 365 366

Figure 6. Heat-activated persulfate oxidation profiles of 6:2 Cl-PFESA (A) and 8:2 Cl-PFESA (B) at 50 °C in unbuffered solutions.

367 368

ENVIRONMENTAL IMPLICATIONS

369

Earlier research has shown that the estimated annual riverine discharge in China of

370

6:2 Cl-PFESA (0.2-6.9 t/y) was comparable to that of PFOS (1.4-11.0 t/y),16

371

indicating an emerging, significant amount of Cl-PFESAs are being used in

372

fluoropolymer manufacturing. With more stringent regulations on PFOS use, it is

373

reasonable to assume that the production and usage of Cl-PFESAs as a PFOS

374

alternative will continue to increase. The present study’s findings of more than an

375

order of magnitude higher sorption affinity of Cl-PFESAs than PFOS and strong

376

recalcitrance to biological and chemical degradation similar to PFOS together with

377

Cl-PFESA’s moderate toxicity and high bioaccumulation ability,13,14 Cl-PFESAs as an

378

alternative to PFOS should be treated cautiously.

379 380

ACKNOWLEDGEMENTS

381

This work was funded by the China Scholarship Council; the National Natural 21



382

Science Foundation of China, DuPont’s Center for Collaborative Research and

383

Education in Wilmington, DE; the Department of Agronomy at Purdue University;

384

and the USDA National Institute of Food and Agriculture Hatch Funds Accession No.

385

1006516.

386 387

SUPPORTING INFORMATION

388

The Supporting Information is available free of charge on the ACS Publications

389

website at DOI:

390 391

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Sorption, Aerobic Biodegradation, and Oxidation Potential of PFOS

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