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Partitioning Behavior of Bisphenol Alternatives BPS and BPAF Compared to BPA Youn Jeong Choi, and Linda S Lee Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b05902 • Publication Date (Web): 09 Mar 2017 Downloaded from http://pubs.acs.org on March 11, 2017

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Environmental Science & Technology

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Partitioning Behavior of Bisphenol Alternatives BPS and BPAF Compared to BPA

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Youn Jeong Choi1 and Linda S. Lee1,* 1

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Purdue University, Department of Agronomy, Ecological Science and Engineering Interdisciplinary Graduate Program, West Lafayette, IN 47907-2054

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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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Prepared for Environmental Science & Technology (Revision 1)

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Revised February 17, 2017

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ACS Paragon Plus Environment


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Graphical Abstract

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ABSTRACT With the pressure to ban or limit the use of bisphenol A (BPA), production of alternatives

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such as bisphenol AF (BPAF) and bisphenol S (BPS) are increasing, but little is known on their

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partitioning behavior for use in assessing distribution in the ecosystem. Octanol-water (DowpH)

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and soil-water partitioning were measured at several pH values for BPA, BPAF, and BPS.

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Sorption isotherms were constructed from measured aqueous and soil phase concentrations and

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were fit sufficiently well with a linear sorption model. pH-dependent distribution was observed

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in both octanol-water and soil-water systems particularly for BPS and BPAF, which have lower

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estimated pKa values than those for BPA. Accounting for soil organic carbon (OC) content and

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pH was sufficient to describe sorption reasonable well across the four soils (%OC 0.1 to 2.5, pH

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3.8-8.6); no other soil properties correlated well with bisphenol sorption. However, for a given

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soil especially for the two high clay low OC soils, BPS sorbed much more than expected relative

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to observed trends in DowpH and magnitude appeared correlated to % kaolinite; therefore, Ca2+-

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bridging of BPS to clay edge sites was assessed by comparing sorption from 0.01 N KCl and

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0.01 N CaCl2; however, no significant differences were observed.

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Keywords. Octanol-water partitioning, sorption, soils, pKa, pH-dependence, ionizables,

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hydrophobicity



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INTRODUCTION Bisphenol A (BPA) is a common environmental contaminant that has been linked to

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potential negative impacts on both human and ecosystem health. Early concerns led to extensive

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studies in BPA toxicology, chemistry, and environmental fate. BPA has been widely used as a

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monomer in the polycarbonate plastic and epoxy resin industries for several decades.1 In 2003,

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U.S. used 856,000 tons of BPA with 72% as polycarbonate plastic and 21% as epoxy resins.2

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Less than 5% of the BPA containing materials were used in the food container and printing paper

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industries.3 BPA can enter the body directly through drinking water and eating foods from BPA-

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containing products intake4 and through dermal contact with BPA-containing paper, most

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commonly thermal paper.5 The presence of BPA in human urine confirms BPA entry into the

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body.6 Furthermore, BPA can be released through discharges from wastewater treatment plants,

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thus entering the aquatic ecosystem.7 The major concern of BPA is its estrogenic activity8,9,

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which led to government and other organizations encouraging the development and use of

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alternatives.

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With pressure to ban and limit the use of BPA, various alternatives have been produced.

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Two BPA alternatives, bisphenol AF (BPAF) and bisphenol S (BPS) (Table 1), were selected for

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this study. Although there is an assumption that these alternatives are safer, their chemical

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structures are similar to BPA, thus so may be their endocrine disrupting potential.8–11 BPAF has

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been in use as a curing agent in the processing of fluorocarbon elastomers and fluorocarbon-

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based synthetic rubber, which is widely used to industry as O-rings, seals, and gaskets in

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processing plants due to their superior material characteristics, including thermal stability,

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chemical resistance and compression set resistance.12 The thermal paper industries selected BPS

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to replace BPA in their compounding epoxy resin due to its thermal stability.13 Unlike BPA, little


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is known about the environmental fate and transport of these alternatives. Analysis of BP

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analogues in sediment samples gathered from the US, Japan and Korea showed average

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concentrations of 117 ng/g for BPA (0-13,370 ng/g range), 0.05 ng/g for BPAF (0-4.13 ng/g

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range), 12.37 ng/g for BPS (0-1,970 ng/g range) and 210 ng/g for the total BP chemical group (0-

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25,300 ng/g range).14 BPA concentrations in sediments consistently increased until 2000 and

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then gradually decreased (data through 2012) due to the regulations against using BPA in plastic

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and resin manufacturing. There appeared to be a concomitant increase in the concentrations of

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the BPA alternatives in sediments from 2000 to 2012. These increases are likely due to the

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increase in BPS and BPAF concentrations in wastewater treatment discharges and land-applied

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biosolids. Application of municipal biosolids from wastewater treatment system is one major

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pathway of BPA release to the environment; 41% of biosolids are applied to agricultural soil as a

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fertilizer amendment 15. BPA concentration ranges reported in activated sludge include < 0.02 to

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39.8 mg/kg in Canada,16,17 < 0.01 to 236 mg/kg in Germany18,19 and 0.1 to 14.4 mg/kg in US.20

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BPA is a weak organic acid containing two ionizable hydroxyl groups with pKa values of

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9.6 and 10.2 (Table 1). Therefore, as pH increases, the anionic fraction of BPA increases

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resulting in subsequent decreases in sorption, which has been observed in soils (pH 4 to 10),

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activated carbon (pH 8 to 11), and zeolite (pH 9 to 11).21–25 BPAF and BPS are also weak acids,

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but more acidic than BPA with BPS estimated to have the lowest pKa values (estimated 7.4-8)

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(Table 1), thus pH-effects are expected to be greater for BPS in the environmentally relevant pH

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range than either BPA or BPAF.

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Research is limited on the partitioning and degradation in soil, sludge, and sediment and

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biosolids of BPA alternatives. Also experimental measurements of key chemical properties

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commonly used to predict environmental fate are lacking. Most literature on BPAF and BPS is



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focused on toxicological effects. The aim of this study was to examine the partitioning of BPA

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compared to BPAF and BPS in soil-water and octanol-water systems at different pH values.

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Sorption of BPA, BPAF and BPS was measured by independently quantifying aqueous and

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sorbed-solid phase concentrations on four soils varying in physical and chemical properties. pH-

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dependent octanol-water distribution coefficients (DowpH) were measured for three chemicals at

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several pH points with subsequent octanol-water distribution curves calculated over a wider pH

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range and correlated to predicted pH-dependent organic carbon (OC) normalized partition

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coefficients (DocpH).

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MATERIALS AND METHODS

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Chemicals and soils. BPA {4,4'-(propane-2,2-diyl)diphenol}, BPAF {4-[1,1,1,3,3,3-

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Hexafluoro-2-(4 hydroxyphenyl) propan-2-yl]phenol}, and BPS {4,4'-Sulfonyldiphenol} were

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obtained from Sigma Chemicals, St. Louis MO, USA and stored at room temperature.

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Deuterated BPA (d8-BPA) for use as an internal standard was purchased from Cambridge

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chemicals. Organic solvents including acetonitrile (ACN), methanol (MeOH), diethyl ether

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(DEE), 1-octanol were obtained as >99% purity, HPLC grade. Inorganic constituents including

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calcium chloride (CaCl2), sodium hydroxide (NaOH), potassium chloride (KCl), potassium

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hydroxide (KOH), and hydrochloric acid (HCl) for adjustment of ionic strength and pH were of

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reagent-grade purity. Stock solutions of target chemicals were prepared in pure methanol and

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stored at 4℃ individually. Aqueous chemical solutions were prepared by diluting the stock

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solutions into sterilized 5 mM CaCl2 or 10 mM KCl solution immediately before use. The

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volume fraction of methanol in all aqueous solutions used in the sorption studies was < 0.1%.


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Soils. Soils with distinctively different textures, pH and OC contents were selected

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(Table 2) including soil from the Purdue Student Farm (PSF-49), a forest soil (FRST-48), and

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two soils from the EPA soils database (EPA-09 and EPA-14). To prevent biological degradation,

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all soils were autoclave-sterilized using the method described by Wolf et al..26 Soil (air-dried)

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was added to sterile 35-mL glass tubes, adjusted to field capacity using sterile water and

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incubated for 72 h at ~22 ºC. After incubation, samples were autoclaved at 103.4 KPa and

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121 °C for 1 h, readjusted to field capacity, incubated again for 24 h, and autoclaved again for 1

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h. All glassware and deionized water were also sterilized by autoclaving.

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Adsorption isotherms. Each target chemical was dissolved in sterile 0.01 N CaCl2

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solution to achieve concentration of 10 50, 500, 500, and 1,000 g/L. Sterile solutions were

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added to sterilized centrifuge tubes containing autoclave-sterilized soil (Table S1 in SI). All

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concentrations, controls and blanks were performed in triplicate. Soils were pre-equilibrated for

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12 h in centrifuge tubes with sterile 0.01 N CaCl2 and capped with Teflon-lined screw caps

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before adding the target chemicals and rotating end-over-end (40 rpm) for 48 h as determined

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from the kinetic assessment. Single concentration (100 g/L) kinetic profiles for BPA, BPAF,

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and BPS conducted over a 5 or 7-d period indicate that a 48-h equilibration was sufficient for

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measuring equilibrium sorption isotherms (See SI for method and summarized data in Figure

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S2). The pH of the aqueous soil slurry was measured and then samples centrifuged at 1700 rpm

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for 45 min. Additional single-point isotherms were measured for BPS from 0.01 N KCl solutions

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at an initial concentration of 500 g/L to probe the role of the cation in the sorption process.

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Soil and aqueous phase extraction. After centrifugation, the supernatant was removed

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of which a 5-mL aliquot was extracted using 5 mL of DEE for BPA, BPAF and 9 ml for BPS.

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Preliminary tests for assessing if the addition of HCl or NaOH would improve extraction



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efficiencies were performed which yielded 0.5% and 101.3 recoveries, respectively, compared to

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86 % with no acid or base additions. Therefore, HCl (1 ml 0.01 M) was added to the aqueous

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aliquot lower pH before adding DEE.

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After extraction, 1.5-ml DEE was transferred to an HPLC vial, DEE was evaporated, and

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residuals were re-dissolved in 0.5-ml MeOH containing isotopically labelled BPA (d8-BPA)

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final concentration at 150 g/L. The soil plug was extracted twice with MeOH by rotating end-

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over-end at 35 rpm for ∼24 h at 22 ± 2 °C, and centrifuged at 1700 rpm for 45 min. Extracts

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were diluted 1:1 with MeOH containing the internal standard. Concentrations were determined

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using high HPLC/MS/MS. Since isotopically mass-labeled compound were not commercially

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available for BPS and BPA, matrix effects were determined for all three compounds by

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comparing signal responses for a known concentration of the target chemical spiked into

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methanol and blank matrix samples. The latter included methanol extracts of unamended soils

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and DEE extracts of the aqueous phases of the same samples. Concentrations determined for

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aqueous and solvent extracts from the sorption isotherms were then corrected accordingly using

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the quantified matrix factors (Table S5).

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pH-dependent octanol-water partition coefficients (DowpH). DowpH values were

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quantified using procedures modified from Karickhoff and Brown.27 Briefly, octanol and

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ultrapure water were pre-saturated with each other. Stocks of individual target solutes were

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dissolved individually in water-saturated octanol and diluted with water-saturated octanol to

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achieve a concentration in octanol of 100 mg/L. Solute concentrations were selected based on the

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estimated minimum and maximum DowpH values for each solute and the method limit of

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quantification (MLOQ) in both phases. The pH of octanol-saturated-water was adjusted to the

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targeted pH while maintaining a constant ionic strength of 50 mM with KCl, HCl and KOH.


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Octanol-water ratios varied from 250:0.1 to 1:100 to ensure that the concentrations in both

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phases reflected 20-60 % amount of the initial concentration and above MLOQs (Table. S4). For

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octanol/water ratios in which there was an insufficient volume of water to make pH

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measurements to validate target pH, octanol was saturated with pH and ionic strength adjusted

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water before use and stocks of individual target solutes were prepared in the octanol pre-

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saturated with pH adjusted water.

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Pre-saturated phases added to 35, 125, 250-ml amber glass bottle or 500-mL separatory

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funnels depending on the total water and octanol volumes needed while minimizing head space.

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Each pH-solute combination was done in triplicate, rotated for 24 hours followed by

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centrifugation of bottles at 1700 rpm for 20 min or for the separatory funnels, a static time long

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enough to allow for phase separation. The octanol phase was sampled, the remaining octanol

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phase was removed, aqueous phase aliquots were taken for HPLC-UV/FL analyses, and pH

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measured on the remaining aqueous phase. All aliquots were diluted in methanol prior to HPLC-

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UV/FL analyses and each analyzed in duplicate.

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Instrumental analysis. Bisphenol concentrations measured for most of the DowpH studies

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were determined using Shimadzu HPLC/UV-Fluorescence (FL) system with an Ascentis RP-

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Amide column (250 x 4.6 mm, 5um), 20 μL injection volume and an 80/20 v/v MeOH/water

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mobile phase at 0.5 mL/min. For BPA and PBAF detector wavelengths for UV were set at 280

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nm and FL at 230 nm (excitation, Ex)/310 nm (emission, Em). For BPS UV was set at 260 nm

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and FL at 260 nm (Ex), 310 nm (Em) for BPS. Retention times were 9.97, 12.75 and 7.42 min

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for BPA, BPAF, and BPS, respectively. For concentrations in the Kow studies that were below

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the detection limits of the HPLC-UV/FL systems and for all the sorption studies, a Shimadzu

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HPLC coupled to a Sciex API3000 mass spectrometer was used. Data were acquired using



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negative electrospray ionization in multiple reaction monitoring (MRM) mode. Chromatographic

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separation was performed on a Kinetex C18 column (100 × 2.0 mm, dp-5 μm) with an injection

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volume of 20 μL and an 80/20 v/v MeOH/0.15% acetic acid solution mobile phase at 0.3

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mL/min. The method limits of detection (MLOD) and the method limits of quantification

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(MLOQ) were defined as the minimum detectable amount of analytes from real samples in

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MRM mode with signal-to-noise ratios of 3:1 and 10:1. Additional compound-specific MS

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details are in the SI (Tables S2 and S3, Fig. S1).

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Modeling. Linear sorption isotherm models was fit to untransformed sorption data using

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solver in Excel. For predictive modeling of log DowpH versus pH, log-transformed data were fit

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using solver in Excel. To compare Kow between experimental results and predicted values, three

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freeware programs, EPISuite28, ACD/PhysChem Suite 29, and ChemAxon30 were selected, which

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are all provided in and linked to Chemspider (http://www.chemspider.com). All the programs

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allow property predictions by drawing the chemical structure of interest. KOWWIN within EPI

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Suite predicts Kow using the Atom/Fragment Contribution method combined with experimental

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data from its own data base. ACD/PhysChem Suite uses the ACD/Log P31 method is modified

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from CLOGP32 for which only a limited version of the program is freeware (pH dependency only

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estimated at two pre-defined values of pH 5.5 and 7.4). ChemAxon’s predictions for log P (i.e.,

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Kow) and log D (i.e., DowpH) are calculated by taking the average of values resulting from three

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different approaches: two QSAR (Quantitative Structure Activity Relationship) models, VG33

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and KLOP34, which accommodates electron delocalization, and the PHYSPROP database, which

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contains estimated and experimental values from various sources. ChemAxon also includes a

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method detailed by Csizmadia et al (1997) to account for electrolyte effects on partitioning.35

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RESULTS & DISCUSSION Experimental DowpH Values. Solute mass balance (sum of mass recovered in the octanol

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and water phases) was 100 ± 10 % in all pH-solute combinations with two exceptions: recovery

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was lower in one case (83.3% for BPAF, pH = 6.63) and higher (112.6 % for BPS, pH = 6.65) in

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the second case. The pH values targeted for DowpH measurements varied from ~4 to 12.7. For pH

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4 and 12.7, the initial and equilibrium aqueous pH values were similar and close to the targeted

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values. Meanwhile, pH of aqueous phase in other pH conditions between 4 and 12.7 was reduced

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(decreased by 0.08-1.37 unit) due to hydrogen ion released from dissociation of target

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compounds,

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Measured log DowpH values as a function of pH are summarized in Figure 1 along with a

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subset of measured or estimated values reported in the literature (further detailed in Table S6).

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For BPA, the log DowpH value measured at pH 4.18 where BPA is assumed to be 100% neutral

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(log KowH2A) is consistent with measured or estimated values reported in the literature (Fig. 1,

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Table S6) except for ChemAxon. All three approaches used by ChemAxon over predicts KowH2A

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by 0.39-0.89 log relative to other estimates. For BPAF and BPS, our measured values were

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compared with estimated values given that there were no previously measured values. For BPAF

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our measured log KowH2A values align well with the predictions from the USEPA EPISuite and

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ChemAxon, but not those from the ACD/PhysChem Suite, which yielded 1.33 log units lower

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values. We suspect that the electronegativity of the two perfluorocarbon groups is not adequately

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represented in the ACD/PhysChem Suite models as has been the case in predicting the behavior

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of perfluoroalkyl compounds. For BPS, predicted values from EPISuite and ACD/PhysChem are

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0.75 and 0.57 log units lower, respectively, than measured experimental results in this study

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(2.40 ± 0.02).



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As pH increases above > pKa,1 – 2, BPA becomes increasingly more ionized and log

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DowpH values decline by four log units between pH 4.18 and 12.64. Similar trends were observed

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with BPAF and BPS. Trends between the three BP analogues at a given pH follow: BPAF >

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BPA > BPS with for example, KowH2A for BPAF being one order of magnitude higher than for

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BPA and two orders of magnitude higher than for BPS. The high log KowH2A values for BPAF

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(4.64 ± 0.10) resulted in aqueous concentrations very low (0.34 g/L) even with higher starting

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concentrations of close to 200 mg/L and a 1:200 v:v octanol/water ratio.

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Predicting pH-dependent DowpH. The equilibrium solute distribution between octanol and water as a function of the pH (DowpH) can be described by Eq. 1, 𝑝𝐻

𝐻2𝐴 𝐻𝐴− 𝐴2− 𝐷𝑜𝑤 = 𝛼𝐻2𝐴 ∙ 𝐾𝑜𝑤 + 𝛼𝐻𝐴− 𝐾𝑜𝑤 + 𝛼𝐴2− 𝐾𝑜𝑤

Eq. 1

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where H2A, HA-, and A2- are the fraction of neutral (H2A), mono-anion (HA-), and di-anion

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(A2-), which sum to one and are defined in Eqs. 2, 3, and 4, respectively, and KowH2A, KowHA-, and

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KowA2- are the Kow values for the single species specified in the superscript. The fraction of three

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species can be described by the following 3 equations: 𝛼𝐻2𝐴 =

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𝛼𝐻𝐴− = 260

𝛼𝐴2− = 261 262

1 1+

10(𝑝𝐻−𝑝𝐾𝑎1)

+ 10(2𝑝𝐻−𝑝𝐾𝑎1−𝑝𝐾𝑎2)

Eq. 2

10(𝑝𝐻−𝑝𝐾𝑎1) 1 + 10(𝑝𝐻−𝑝𝐾𝑎1) + 10(2𝑝𝐻−𝑝𝐾𝑎1−𝑝𝐾𝑎2) 10(2𝑝𝐻−𝑝𝐾𝑎1−𝑝𝐾𝑎2 ) 1 + 10(𝑝𝐻−𝑝𝐾𝑎1) + 10(2𝑝𝐻−𝑝𝐾𝑎1−𝑝𝐾𝑎2 )

Eq. 3

Eq. 4

To fit Eq.1 to the measured pH-dependent data, log KowH2A and log KowA2- were fixed at

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the measured values while KowHA- was allowed to vary since it could not be determined

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independently. Values for pKa1 and pKa2 were fixed at experimentally obtained values for BPA


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(9.63 and 10.43 at 25ºC)21 and at ChemAxon estimates for BPS and BPAF since directly

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measured values were not available. Eq. 1 DowpH predictions for BPA, BPAF and BPS as a

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function of pH are shown as red solid lines in Figure 1, which fit well the measured pH effect on

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octanol/water distribution with resulting coefficients of determination (R2) for of BPA, BPAF,

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and BPS of 0.996, 0.997 and 0.984, respectively.

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For comparison, ChemAxon was used to predict DowpH versus pH at the ionic strength

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used in the current study (0.05 M) since it is the only freeware that accounts for ionic strength

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effects. No effect of ionic strength on partitioning of the neutral species in the ChemAxon

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predictions in agreement with the Setschenow equation from which enhanced partitioning well

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below 10% in the 0 to 0.1 M range is expected. At elevated pH values where BPA, BPAF, and

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BPS are essentially completely ionized (pH > 12, ~KowA2-), our values measured at an initial

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aqueous ionic strength of 0.05 M are more than one order of magnitude lower than the

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ChemAxon estimates. Although the Csizmadia et al (1997) method used by ChemAxon in

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predicting DowpH versus pH at the ionic strength is supposed to be suitable for 0.01 to 0.25 M

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ionic strengths, it appears to over predict ionic strength effects for the BPs.

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Unlike for the neutral species, increasing ionic strength is expected to substantially

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increase the octanol-water partitioning of organic anions by the formation and partitioning of

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ion-pairs as exemplified by Westall (1985)36 for chlorinated phenolates, Lee et al (1990)37 for

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pentachlorophenolate, and Johnson (1990)38 for methylated anilines. So for further comparison,

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ChemAxon was used to predict results for a lower ionic strength of 0.005 M, which resulted in

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more than an order of magnitude lower KowA2- and in closer agreement to those measured in the

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current study. The distribution of the ionized form is also influenced by the specific electrolyte

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composition (Lee et al., 1990), which affects the magnitude of ion pairing.



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Sorption isotherms. Mass recoveries for the isotherms across all soils are 102.5 ±

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12.6 % for BPA, 105.7± 12.2 % for BPAF, and 100.0± 20.3% for BPS (Table 3); therefore,

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extraction efficiencies were considered adequate and degradation negligible during the 48-h

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equilibration. Sorption isotherms constructed from measured solution (Cw, g/L) and sorbed

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phase (Cs, g/ kg) concentrations are summarized for solute sorption per soil type in Figure 2

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(linear and log-log sorption isotherms by soil type for each solute are shown in Figures S3 and

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S4). Linear and log-log isotherm fits are summarized in Table 3 and Table S7, respectively, for

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all three solutes and four soils. Linear Kd values ranged over 2 orders of magnitude for soils

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varying in OC (0.11 to 2.62%) and soil pH (4.3 to 8.6). Sorption data were fit reasonably well by

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the linear isotherm model with a goodness of correlation (R2) being >0.93 as summarized in

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Table 3. For BPS, the Freundlich N values, which represent the degree of nonlinearity are

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essentially unity. For BPA and BPAF, N values are within 10% of unity except for isotherms on

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the EPA09 soil, which is the most alkaline soil and has the lowest % organic carbon.

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The relative sorption trends between BPA, BPAF and BPS are similar for each soil with

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BPAF being sorbed the most and BPA being sorbed the least with the differences in sorption

303

magnitude following increasing %OC (FRST-48 ≥ PSF-49 > EPA-14 > EPA-09) (Fig. 2, Table

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3, Table S8). BPAF having the highest sorption was expected based on trends in solubility and

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Kow values. However, greater sorption of BPS compared to BPA was unexpected given that BPS

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has higher solubility and consistently lower Kow values than BPA at all pH values (Table 1, Fig.

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1). High sorption of BPS was most notable in the EPA09 and EPA14 soils with BPS and BPAF

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having very similar Kd values for the latter. The EPA soils have the lowest %OC (< 0.5%) and

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the highest % clay of the 4 soils investigated. Between the 2 EPA soils, EPA14 has the highest %

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clay (63.6% versus 17.4 %), and the highest kaolinite content (37% versus 0.7%) (Table S9),


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which is where potential for interaction with edge sites is the greatest. Therefore, we

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hypothesized that Ca-bridging anionic clay edges may be why BPS sorption is greater than

313

expected as has been observed with other organic anions.39 However, no significant differences

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were observed in the single-point Kd values for BPS in 0.01 N KCl compared to the isotherm

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generated with 0.01 N CaCl2 (open symbols in Fig. S5). The two EPA soils also had the highest

316

and lowest pH of the soils investigated with EPA14 being very acidic (pH=3.8). Therefore,

317

EPA14 has the greatest potential for the presence of positively charged edge sites with pKa value

318

reported for kaolinite as low as 5.0.49 EPA14 also has the highest amount of citrate-dithionite

319

extractable Fe (1.38 %), which may be from iron oxides where positive sites may also arise.

320

However, at this low soil pH value (3.8), all three bisphenols are essentially 100% neutral. This

321

suggests that other types of specific sorption mechanisms for BPS possibly involving the SO2

322

region of the molecule are significant.

323

The higher sorption than expected for BPS is also prompted further investigation of

324

ensuring that d8-BPA-corrected matrix effects did not skew BPS data and independently

325

quantifying specific compound by soil matrix effects for BPS and BPAF. However, applying the

326

matrix effects determined from the latter did not significantly change sorption coefficients; BPS

327

sorption remained higher than BPA sorption. Correlations between sorption and soil properties

328

other than % OC were evaluated including % clay, % silt, and even cation exchange capacity

329

although cation bridging appears insignificant (Fig. S5). Correlations are poor for all properties

330

except soil %OC (Figure S6 and Table S10). Therefore, other site-specific sorption mechanisms

331

yet to be determined must be significant for BPS.

332 333

pH-dependent Sorption. Although sorption increased with increasing %OC, R2 values for the linear regression of BPS sorption and %OC is noticeably poorer than observed for the



334

other compounds (Table S10), which is primarily due to the compounded effect of pH. Soil PSF-

335

49 has the highest %OC but also a high pH whereas the EPA-14 soil has a low %OC but also the

336

lowest pH. For BPS more than the other two bisphenols, soil pH values are much closer to within

337

2 pH units of the solute’s pKa (7.42, 8.0330). Even with BPA, pH effects can be observed when

338

comparing two soils with similar %OC but very different pH values (PSF-49 with 2.62% OC and

339

pH=7.8 versus FRST-48 with 2.38 %OC and pH=5.5), BPS exhibits a higher Kd in FRST-48

340

(15.34 L/kg) compared to PSF-49 (6.86 L/kg) relative to the difference in %OC between the two

341

soils. The reduced sorption of BPA has been observed at pH > pKa2 on activated carbon21,

342

montmorillonite40, graphene41, zeolite (Tsai et al., 2006b), and sediment42. For BPA, for all soils

343

except EPA09, soil pH < pKa1 – 2, thus BPA is essentially 100% neutral, and for EPA09, more

344

than 94% is as a neutral species. Therefore, log OC-normalized sorption coefficients (log Koc)

345

can be average across all soils. The resulting average log Koc for BPA of 2.57 ± 0.05 L/kgoc falls

346

in within the range reported in the literature (2.4-3.71, pH range 4.8- 8.1), 42–46 but is lower than

347

those estimated from various modeling suites (3.35-4.87, pH range 5.5-7.4). 28–30 For BPAF and

348

BPS using data just from the two soils where they are essentially neutral, average log KocHA

349

values of 3.50 ± 0.28 and 3.01 ± 0.20, respectively, were estimated. Although we noted there

350

must be additional sorption mechanisms impacting BPS given its unexpectedly higher sorption

351

than BPA, given the good correlations with %OC, assessing a Koc approach as a first estimate

352

seems reasonable.

353

For organic acids, both hydrophobic-type sorption to soil OC and pH-dependent

354

speciation must be considered in describing sorption over the environmentally relevant pH

355

range37. For the bisphenols with two ionizable groups, sorption distribution (Kd) can be described

356

by Eq. 5:


Page 16 of 28

Page 17 of 28

357


𝐾𝑑 = 𝛼𝐻2𝐴 ∙ 𝐾𝑑𝐻2𝐴 + 𝛼𝐻𝐴− 𝐾𝑑𝐻𝐴− + 𝛼𝐴2− 𝐾𝑑𝐴2−

Eq. 5

358

where KdH2A, KdHA-and KdA2- are the sorption distribution coefficients of the neutral (H2A) and

359

anionic (HA- and A2-) species. The described sorption coefficients normalized to the organic

360

carbon content of the soil can be expressed by

361

𝑝𝐻

𝐻2𝐴 𝐻𝐴− 𝐴2− 𝐾𝑑 = 𝐷𝑂𝐶 ∙ 𝑓𝑂𝐶 = 𝛼𝐻2𝐴 ∙ 𝐾𝑂𝐶 ∙ 𝑓𝑂𝐶 + (𝛼𝐻𝐴− 𝐾𝑂𝐶 + 𝛼𝐴2− 𝐾𝑂𝐶 ) ∙ 𝑓𝑂𝐶

Eq. 6

362

where KOCH2A, KOCHA-and KOCA2- are OC-normalized distribution coefficients of the neutral

363

(H2A) and anionic (HA- and A2-) species. OC-normalization of sorption affinity emphasizes

364

importance of organic carbon in soil to sorption of neutral organic chemical. Accordingly, if only

365

the neutral species is assumed to sorb appreciable and contribution of anionic species is

366

considered insignificant, Eq. 6 simplifies to:

367

𝑝𝐻

𝐻2𝐴 𝐷𝑜𝑐 = 𝛼𝐻2𝐴 ∙ 𝐾𝑖𝑜𝑐

Eq. 7

368

Validation of this assumption can be made by comparing predictions with measured observations

369

although the number of data points is limited. Eq. 7 predictions agree reasonably well (within a

370

factor of two or 0.3 log units) with measured values (Fig. 3) across the pH range of the four soils

371

(pH 3.8-8.6). One exception is BPS for the soil with the highest pH value and lowest %OC (pH

372

8.6 and 0.11 %OC) for which the measured value is 0.55 log units higher than predicted. This is

373

likely due to site-specific interactions, which appear to occur for all forms of BPS, but possibly

374

greatest for the ionized form of BPS, which dominates at pH 8.6.

375 376 377

ENVIRONMENTAL IMPLICATIONS The pressure to limit the use of BPA has led to increased production of BPA alternatives

378

such as BPAF and BPS, but with little known on their environmental behavior. This study

379

exemplified the pH-dependent partitioning of bisphenol alternatives with the greatest pH effect



380

observed for BPS, which has the lower pKa values. In general, OC and pH appeared sufficient to

381

normalize sorption across soils (0.11 to 2.62 %OC and pH 4.3-8.6) with partitioning dominated

382

by the neutral species. However, for BPS additional sorption mechanisms other than just simple

383

hydrophobic-type partitioning appears evident given that BPS consistently had the lowest DowpH

384

values and KowH2A, but not the lowest DocpH values. Much higher than expected sorption of BPS

385

was most evident on the 2 soils with low %OC and high clay content , which also had the highest

386

and lowest soil pH. Enhanced sorption mechanisms does not appear to include cation-bridging

387

mechanisms given that no difference was observed in sorption in the presence of a divalent

388

(Ca2+) versus monovalent (K+) environment and the lack of correlation to soil cation exchange

389

capacity. Future mechanistic sorption studies with BPS as well as persistence studies for both

390

BPA alternatives are needed to improve environmental fate predictions and risk management.

391 392

ACKNOWLEDGEMENTS

393

This work was funded in part by support provided by the Purdue Research Foundation and the Purdue

394

Agronomy Department.


Page 18 of 28

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395

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396 397 398

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(30) ChemAxon. JChem Suite-Marvin Suite.

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(39) Hyun, S.; Lee, L. S. Factors controlling sorption of prosulfuron by variable-charge soils and model sorbents. J. Environ. Qual. 2004, 33 (4), 1354–1361.

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(41) Xu, J.; Wang, L.; Zhu, Y. Decontamination of bisphenol A from aqueous solution by graphene adsorption. Langmuir 2012, 28, 8418–8425.

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(42) Zeng, G.; Zhang, C.; Huang, G.; Yu, J.; Wang, Q.; Li, J.; Xi, B.; Liu, H. Adsorption



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522 523


Table 1. Physical & chemical properties of bisphenol A, bisphenol AF and bisphenol S. Italicized values are predicted to differentiate from actual measured values. Name IUPAC Name

Bisphenol A (BPA) 4,4'-(propane-2,2diyl)diphenol

Bisphenol AF (BPAF) 4-[1,1,1,3,3,3-Hexafluoro2-(4 hydroxyphenyl) propan-2-yl]phenol

Bisphenol S (BPS) 4,4'-Sulfonyldiphenol

Structure

Mol. wt.

228.29

336.23

250.27

Log KocH2A

2.4-3.7142–46 3.3-4.8828,29

3.23-5.8828,29

2.20-3.8828,29

Log KowH2A

3.32 - 3.447,48 3.43 - 4.4028–30

2.82 - 4.7728–30

1.65 - 2.1428–30

pKa1, pKa2

9.63, 10.4321 9.78, 10.3930

9.13, 9.7430

7.42, 8.0330

Solubility28

146 mg/L

0.84 mg/L

1774 mg/L

524



525

526 527 528

Page 24 of 28

Table 2. Selected physicochemical properties of soils used in sorption study. Soil textured (%) Silt Clay

Soil

OCa (%)

soil pHb

CECc cmolc/kg

Sand

FRST-48

2.38

5.5

7.4

39.0

52.0

9.0

PSF-49

2.62

7.8

19.0

43.0

40.0

17.0

EPA-09e

0.11

8.6

12.4

7.1

75.6

17.4

EPA-14e

0.48

3.8

18.9

2.1

34.4

63.6

a

Percent organic carbon determined by loss on ignition (LOI) method; bpH of a 1:2 soil (g):water (mL) slurry; cCation exchange capacity determined by the ammonium acetate method. dParticle size analysis determined by hydrometer method. eAdditional information provided in Table S7.


Page 25 of 28

529


Table 3. Sorption coefficients from linear sorption isotherm model fits for BPA, BPAF and BPS on four soils

Soil

Linear (SD b)

R2 c

log Koc d

MR e (SD)

H2A f

log Kow g

8.49 (0.24) 8.53 (0.19) 0.61 (0.04) 1.47 (0.08)

0.989 0.993 0.932 0.952

2.55 2.51 2.74 2.49

101 (8) 106 (10) 101 (15) 102 (14)

1.000 0.985 0.922 1.000

3.431 3.425 3.396 3.431

141.77 (3.02) 116.50 (2.50) 1.43 (0.07) 7.92 (0.22)

0.994 0.994 0.961 0.987

3.78 3.65 3.11 3.22

109 (7) 111 (9) 96 (12) 104 (10)

1.000 0.955 0.781 1.000

4.647 4.627 4.54 4.648

Kd

a

BPA FRST-48 PSF-49 EPA-09 EPA-14 BPAF FRST-48 PSF-49 EPA-09 EPA-14 BPS

530 531 532 533

FRST-48 15.34 (0.37) 0.994 2.81 101 (14) 0.988 2.28 PSF-49 6.86 (0.48) 0.948 2.42 109 (15) 0.208 1.608 EPA-09 0.89 (0.03) 0.986 2.91 98 (12) 0.017 0.541 EPA-14 7.86 (0.28) 0.989 3.21 101 (12) 0.999 2.285 a linear distribution coefficient (L/kg): Cs= Kd Cw, where Cs (µg/kg) is the extractable sorbed concentration at solid and Cw (µg/L) is the equilibrium concentration of solute in solution; bstandard deviation; ccoefficient of determination; dlogarithm of organic carbon normalized partition coefficient (L/kgoc); eaverage % mass recovered ± standard deviation, n=15; ffraction of neutral species (H2A); g calculated log Kow at pH of each soil.



Page 26 of 28

534 6 4

2

log Dow pH

4 2

0 2

0

-2

0

-2

-2

(A) BPA 0

2

4

6

8 10 12 14

-4 0 2 4 6 8 10 12 14

pH

535 536 537 538 539 540 541

(C) BPS

(B) BPAF

pH

0

2

4

6

8 10 12 14

pH

log DpH OW

A I=0.005

B-Predicted

D-reference

Data point

A I=0.05

C-Predicted

E-refenrce

Figure 1. Measured log DowpH from this study, measured literature values for BPA (D47, E48 ), estimated values for BPAF and BPS at selected pH values (B29 and C28) and pH-dependent curves predicted using Eq. 1 (solid line) and [A] ChemAxon at two ionic strengths (dashed (I=0.005) and dotted lines (I=0.05)). log DowpH predictions used measured pKa1 and pKa2 from Bautista-Toledo21 for BPA and ChemAxon estimates for BPAF and BPS. [C] EPI Suite predictions for the neutral species is plotted at pH 4.

26 ACS Paragon Plus Environment

Page 27 of 28


50000

PSF

50000 40000

30000

30000

20000

20000

10000

10000

Cs (ug kg-1)

40000

0

BPA BPAF BPS

0 0

1000

FRST

200 400 600 800 1000

EPA09

0

8000

800

200 400 600 800 1000

EPA14

6000

600 4000 400 2000 200 0 0 0

200 400 600 800 1000

0

200 400 600 800 1000

Cw (ug L-1) 542 543 544

Figure 2. Sorption isotherms for (A) BPA, (B) BPAF and (C) BPS on four soils. Error bars represent standard deviation and lines are the linear isotherm model fits.

545



5

5

5

log DpH OC

BPA

BPAF

BPS

4

4

4

3

3

3

2

2

1

1

2

Page 28 of 28

log Doc log Koc-experiment

1

log Koc-reference

0

0 4

546 547 548 549 550 551

6

8

pH

10

0 4

6

8

10

pH

4

6

8

pH

Figure 3. Measured pH-dependent OC-normalized sorption coefficients (DocpH) in the pH range of 3 < pH < pKa,2+0.5 (solid symbols are from this study, open symbols are measured DocpH values in the literatures for individual soils) 42–46 compared to our DocpH estimates using predicted parameters from ChemAxon. The pH values in the points plotted are the pH values measured at sorption equilibrium.


Partitioning Behavior of Bisphenol Alternatives ... - ACS Publications

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