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Equilibrium relationship between SVOCs in PVC products and the air in contact with the product Clara M. A. Eichler, Yaoxing Wu, Jianping Cao, Shanshan Shi, and John C. Little Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b06253 • Publication Date (Web): 08 Feb 2018 Downloaded from http://pubs.acs.org on February 16, 2018

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Environmental Science & Technology

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Equilibrium relationship between SVOCs in PVC products and the

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air in contact with the product

3 4

Clara M. A. Eichler1, Yaoxing Wu2, Jianping Cao1, Shanshan Shi3, and John C. Little1*

5 6 7 8 9 10 11 12

1

Department of Civil and Environmental Engineering, Virginia Tech, Blacksburg, VA 24061, United States 2 Department of Environmental Engineering, Texas A&M University-Kingsville, Kingsville, TX 78363, United States 3 School of Architecture and Urban Planning, Nanjing University, Nanjing 210093, China *Corresponding author: [email protected]; Phone: (540) 231 0836

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1 ACS Paragon Plus Environment


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Abstract: Phthalates and phthalate alternatives are semi-volatile organic compounds (SVOCs)

16

present in many PVC products as plasticizers to enhance product performance. Knowledge of the

17

mass-transfer parameters, including the equilibrium concentration in the air in contact with the

18

product surface (y0), will greatly improve the ability to estimate the emission rate of SVOCs from

19

these products and to assess human exposure. The objective of this study was to measure y0 for

20

different PVC products and to evaluate its relationship with the material-phase concentrations

21

(C0). Also, C0 and y0 data from other sources were included, resulting in a substantially larger

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data set (Ntotal = 34, T = 25 °C) than found in previous studies. The results show that the

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material/gas equilibrium relationship does not follow Raoult’s Law and that therefore the

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assumption of an ideal solution is invalid. Instead, Henry’s Law applies and the Henry’s Law

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Constant for all target SVOCs consists of the respective pure liquid vapor pressure and an activity

26

coefficient γ, which accounts for the non-ideal nature of the solution. For individual SVOCs, a

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simple partitioning relationship exists, but Henry’s Law is more generally applicable and will be

28

of greater value in rapid exposure assessment procedures.

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TOC/Abstract Art

31 32 33



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Introduction

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Large numbers of mass-produced materials and products are present indoors. Many of these

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contain semi-volatile organic compounds (SVOCs), including plasticizers and flame retardants,

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which are an important class of indoor pollutants that are potentially associated with a range of

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adverse health effects including endocrine disruption, asthma, and allergies.1-3 The main routes

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of human exposure to SVOCs include inhalation of air and airborne particles, ingestion of dust

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and food, and dermal sorption from air or dermal contact with contaminated interior surfaces.4-5

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Understanding SVOC behavior in the indoor environment, and their associated human exposure

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pathways, requires identification of their sources, characterization of their physicochemical

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properties, and knowledge of their transport mechanisms. Models estimating human exposure to

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the SVOCs in building materials and consumer products rely on emission parameters which are

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often unavailable.6 Therefore, information is urgently needed which allows linking product

46

characteristics and emission parameters.

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For exposure assessment models, knowledge of the key parameter y0 (equilibrium concentration

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in the air directly adjacent to the product surface, µg/m3) will greatly improve our ability to

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predict the emission rate of SVOCs from consumer products.6-7

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measurements of y0 are either not available or too time-consuming to obtain, an estimate of y0

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based on existing product characteristics, typically the material-phase concentration C0 (µg/m3,

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sometimes also given in wt%) of the SVOC in the product, would be very useful.

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relationship between C0 and y0 as well as the influence of other parameters on this relationship

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has been the subject of several studies but remains poorly understood.7-10 Xu and Little11 showed

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in their study of SVOC emissions from polymeric materials that C0 can usually be assumed to be

56

constant for a long period of time due to very slow source depletion. They assumed a linear 4 ACS Paragon Plus Environment

In cases where accurate

The

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relationship between C0 and y0, concluding that y0 remains constant if C0 is constant and

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introducing K as the partition coefficient between C0 and y0.11 Ekelund et al.12 reported that di(2-

59

ethylhexyl) phthalate (DEHP) forms a thin liquid film on the product material surface, thus

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essentially acting as a pure compound. They concluded that y0 is equal to the vapor pressure of

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the pure compound p* and independent of C0, if C0 is high.12 Clausen et al.13 linked y0 to p*DEHP

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as a temperature-dependent variable, confirming the observation by Ekelund et al. Xu et al.14

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also used p*DEHP to approximate y0 in vinyl flooring. However, Little et al.6 gave examples of

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other SVOCs for which this approximation is not suitable, either because they do not form a pure

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liquid on the surface due to their low C0 values, or because the compound is in a solid state at

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room temperature. They stated further that a simple linear partitioning relationship between C0

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and y0 could only be assumed for C0 < 1%.6

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The use of Raoult’s Law (Eq. 6) to describe equilibrium might not apply to polymer solutions

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because of the large molecular size of polymers compared to SVOC molecules,6, 13 as discussed

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by Hawkes.15 Liang and Xu9 developed a new approach to measure y0 in polyvinyl chloride

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(PVC) products. They found a good linear relationship between C0 and y0 for C0 ≤ 13% for

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DEHP, and also described the slope of the line as a partition coefficient.9 Their results suggested

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a “critical level” of C0 which marks the transition from a linear relationship with y0 to another,

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unknown type of correlation.9 Liang and Xu acknowledged the influence of p* on y0, but

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rejected an approximation of y0 equal to p*. Based on a follow-up study on the influence of

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temperature on SVOC emissions, Liang and Xu16 observed that y0 was generally lower than

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p*DEHP and concluded that DEHP does not behave as a pure liquid in the products.16 Liu and

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Zhang10 investigated the ratio of y0 to p* in relation to C0 and suggested a polymer-solvent

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interaction parameter χ to explain the influence of C0 on y0/p*.


This parameter however


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decreases with decreasing C0.10 Most recently, Cao et al.8 established an empirical relationship

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between y0, C0, and p*.

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Summarizing, it is evident that both C0 and p* influence y0; however, the nature of the

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relationship remains unclear. The consensus appears to be that: 1) p* alone does not serve as an

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accurate predictor of y0, especially when C0 is low; 2) a linear relationship between C0 and y0

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might not be true for high C0 values; and 3) Raoult’s Law might not be applicable because the

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polymeric nature of PVC material deviates from ideal behavior.17 Additionally, temperature is an

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influence, as observed in several studies.8, 13, 16, 18 It should also be noted that almost all available

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studies focus on DEHP, which limits the generalization of the observed relationships for other

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compounds.

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The objective of this study was to measure y0 for different solid consumer products with known

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C0 values for a range of plasticizers, i.e., phthalates and phthalate alternatives, and to evaluate the

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relationship between C0 and y0 more broadly. Values of C0 and y0 from other studies were

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included in the evaluation, resulting in a data set with higher statistical significance (Ntotal = 34)

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than those used in previous studies. Since all y0 values used were measured at T = 25 °C, the

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effect of temperature was not included in this study. C0, which is usually given in µg/m3 or

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occasionally in wt% of PVC, was converted to mass fraction w and volume fraction ϕ, which

97

were then correlated to y0. In addition, p* can also be expressed in µg/m3, based on the Ideal Gas

98

Law.

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products and is crucial for more environmentally acceptable solutions with respect to product

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This work leads to an improved understanding of SVOC emissions from consumer

safety and exposure reduction.


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Materials and Methods

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Chemicals

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Details regarding the phthalates and phthalate alternatives used in the experiments of this study

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can be found in Table 1. D4-dibutyl phthalate-3,4,5,6 (d4-DiBP, 98 atom% D) used as internal

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standard (IS) and d4-di(2-ethylhexyl) phthalate-3,4,5,6 (d4-DEHP, 98 atom% D) used as

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surrogate for the determination of recovery rates were obtained from Sigma-Aldrich. Reagent-

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grade acetone and methanol used for cleaning and >99.9% dichloromethane (DCM) used as

108

solvent were obtained from Fisher Scientific, Waltham, MA.

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Table 1: Full names, abbreviations, CAS numbers, purity grades, molecular weights (MW), and

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supplier information of the phthalates and phthalate alternatives used in this study. Analyte

Abbreviation

CAS no.

Grade

Acetyltributylcitrate Di(2-ethylhexyl) adipate Di(2-ethylhexyl) phthalate Dioctyl terephthalate Diisodecyl phthalate Diisononyl phthalate 1,2-Cyclohexane dicarboxylic acid diisononyl ester

ATBC DEHA DEHP DEHT DIDP DINP DINCH

77-90-7 103-23-1 117-81-7 6422-86-2 24761-40-0 28553-12-0 166412-78-8

98% 99% ≥99.5% ≥99% ≥99% ≥99% ≥99%

MW (g/mol) 402.5 370.6 390.6 390.6 446.7 418.6 424.7

Supplier Sigma-Aldrich, St. Louis, MO

Just In Time Chemical Sales & Marketing, Inc., Linden, NJ

112 113

Source Materials

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Six children’s backpacks and four children’s toys were analyzed to obtain y0 of the plasticizers

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used in these products. Together, the products contained a total of four phthalates (DEHP,

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DEHT, DIDP, and DINP) and three phthalate alternatives (ATBC, DEHA, and DINCH) in

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different compositions and with varying material-phase concentrations C0. All products were 7 ACS Paragon Plus Environment


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obtained locally from a range of department stores. The toys were two plastic dinosaurs (DO-t1,

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about 17 cm, and DO-t2, about 22 cm), a plastic giraffe (TA-t1, about 20 cm), and a plastic turtle

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(TA-t2, about 13 cm). The backpacks (sample IDs: RO-b, ST-b, TA-b, TM-b, WA-b1, and WA-

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b2) and toys have been characterized regarding their plasticizer content by Xie et al.19 The

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backpacks were made of polyester with an inside PVC coating to ensure waterproofness, and

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PVC content ranged from 55.6 wt% to 74.6 wt%.19 The toys were made entirely of PVC.19

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Pieces of material were cut from the backpacks and used as SVOC source material, with the

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PVC-coated side facing upward. Because of the irregular shape of the toys, they were not

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directly suitable for use with the diffusive sampler, which requires a flat source material.

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Therefore, the toys were grated using a stainless steel grater and the gratings were then melted

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together to form a flat shape (Figure S1 in the Supplementary Information). For this purpose, the

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gratings were evenly distributed between two layers of aluminum foil and heated from both sides

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for several minutes with an iron (Hamilton Beach Durathon Model 19800) at a temperature of

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185 ± 15 °C.

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Diffusive Sampler

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The experimental method presented by Wu et al.20 to measure the surface/air partition coefficient

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Ks was modified for use in this study to measure y0. The design of the diffusive sampler is

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derived from the model described in the following section, with a schematic shown in Figure S2a.

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The cylindrical diffusion chamber consisted of a thin stainless steel washer and a disk made of an

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impervious material. The materials were either stainless steel (“stainless steel #2” in Wu et al.20)

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or steel. Both were purchased from SHARPE products, New Berlin, WI. The material surfaces

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and their respective Ks values for DEHP have been characterized by Wu et al.20. A circular

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stainless steel washer (22.225 mm ID, 34.925 mm OD, 0.254 mm thickness, Superior Washer & 8 ACS Paragon Plus Environment

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Gasket Corp., Rock Hill, SC) was placed between the SVOC source material and the impervious

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material disk. This way, one side of the disk was exposed to the SVOCs emitted from the source

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material. The thickness of the washer defines the diffusion distance between the emission source

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and the surface of the disk. Potential for contamination of the outer sides of the disk was

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minimized by covering it with a clean aluminum cap. A weight was placed on top of the

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chamber to hold the washer and the disk firmly against the flat source material surface. Air leaks

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in the diffusive sampler were assumed to be negligible. C0 has been found to be effectively

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constant (i.e., e.g., overall loss of DEHP from PVC flooring is only 0.002% after about one year's

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emission).11 With the temperature held constant, y0 can also be considered constant. The driving

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force for mass flux across the quiescent-air gap is the concentration in the air in equilibrium with

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the emission source and the concentration in the air adjacent to the sampling disk surface.

152

Because of the low chamber height, which is equal to the thickness of the washer, the

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accumulation of SVOCs on the chamber walls is assumed to be negligible.8, 14, 18 Using this

154

approach, a one-dimensional diffusion model was used to derive y0 from the experimental data.

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Before use, all parts of the diffusive sampler were thoroughly rinsed with methanol and acetone

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and blown dry with clean air. The chambers were placed in a temperature-controlled cabinet

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(Lunaire CEO-932) operated at 25 ± 0.5 °C. Experimental durations ranged from 7 to 26 days.

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Then, the disks were removed using pre-cleaned tweezers and immediately placed in 20 mL glass

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jars for extraction and analysis. Duplicate sampling was conducted for the backpacks at each

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time step. Because of the limited size of the re-shaped source material, only single sampling was

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conducted for the toys.



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Instrumentation and Quality Control

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The SVOCs accumulated on the surface of the disks were extracted using 10 mL of DCM. To

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monitor the recovery efficiency of solvent extraction, the solvent of each sample was spiked with

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10 µL of 20 µg/mL d4-DEHP solution prior to sample extraction. All extracts were subsequently

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concentrated to 500-1000 µL using a gentle stream of high purity nitrogen. A total of 10 µL of IS

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(250 µg/mL d4-DiBP) was added prior to analysis for volume correction and signal intensity

168

normalization.

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(GC/MS, Thermo Scientific Focus GC with Thermo Scientific DSQ II MS, Thermo Fisher

170

Scientific, Waltham, MA). Five-point calibration curves ranging from 100 to 2,000 ng/mL were

171

used for quantification of the target analytes. The R2 values of the calibration curves ranged from

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0.991 to 0.999.

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As a control for the quality of the toy preparation procedure, a piece of green vinyl flooring,

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which had served as SVOC source material in previous studies and is therefore well

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characterized9, 20-21, was grated and melted into shape. Diffusive samplers were placed on the re-

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shaped flooring material and sampled after 1, 2, 3, 9, 11, and 20 days. The measured y0 was

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compared to previous measurements of y0 of the original vinyl flooring, which showed good

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agreement.

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All glassware was heated to 400 °C for 2 h before each experiment to minimize contamination.

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Two procedural blanks using unexposed disks were included in each batch of samples to detect

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background contamination during the experimental process. The blanks were subject to the same

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extraction and analysis as were the actual samples. Of the seven targeted compounds, only

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DEHP, DEHA, DINCH, DINP, and ATBC were detected at low levels in some blanks of the

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backpack trials (≤ 100 ng/mL). Therefore, only the respective SVOC concentrations determined

All samples were analyzed using gas chromatography/mass spectrometry


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in actual samples were blank-corrected. 92% of the samples had solvent extraction recovery

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efficiencies within the range of 70 to 130% recommended by the U.S. Environmental Protection

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Agency for surrogate recovery.22 Recovery efficiencies of the other 8% of samples were between

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60% and 70%. This confirms the extraction efficiency and overall performance of the analytical

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method.

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Determination of y0

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Because the SVOC adsorption to the chamber walls is negligible, the SVOC diffusion model

192

developed by Wu et al.20 was simplified to include only the adsorption of SVOCs on the

193

impervious material surface (Figure S2b). In the sealed cylindrical chamber, SVOCs emitted

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from the source material at the bottom diffuse into the chamber air, which is quiescent and

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initially free of SVOCs. It is assumed that molecular diffusion of SVOCs is the only mechanism

196

of mass transport, following Fick’s law. As diffusion progresses, most of the SVOCs will adsorb

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to the surface of the impervious material at the top of the chamber. Based on these assumptions,

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a one-dimensional diffusion model was derived with the governing equation given as: ∂ , ∂ , =

∂ ∂

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(1)

200

where t is time (s), z is distance (m), Cg (µg/m3) is the gas-phase SVOC concentration, and D

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(m2/s) is the gas-phase diffusion coefficient. The boundary conditions are defined as:

Surface of emission source:

Impervious surface:

−

= =

∂ ∂ ∂ = = ∂ ∂ ∂

202


(2) (3)


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where L (m) is the chamber height, q (µg/m2) is the mass of SVOC adsorbed to the impervious

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surface material per unit surface area, and Ks (m) is the equilibrium surface/air partition

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coefficient of an SVOC which is characteristic for a specific surface/SVOC combination20. q was

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obtained as a function of time using the experimental setup described above.

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coefficients

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(http://archemcalc.com/sparc-web/calc#).

209

temperature and were estimated by solving the differential equation using the boundary

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conditions with q(t) as input (MATLAB R2012b, MathWorks, Inc.). R2 was used to determine

211

the best fitting model results.

212

Literature Data

213

Additional data for C0 and y0 were obtained from the literature, but estimated values for y0 based

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on p* were not included. The products, for which measured C0 and y0 data were available, were

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mainly flooring materials containing DEHP, DINP, diisobutyl phthalate (DiBP), and di-n-butyl

216

phthalate (DnBP) and crib mattress covers containing DINCH and DEHA.16, 23 In all studies, y0

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was measured at T = 25 °C, and all products were completely made of PVC.9,

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Furthermore, unpublished C0 and y0 data for three household products were included. These

219

products (plastic table covering, wall paper, and flooring material) are made of PVC and contain

220

DEHP in different weight percentages. Parameters were measured at 25 °C using the SPME-

221

method introduced by Cao et al.8 All additional data were treated in the same manner as the data

222

specifically acquired for this study.

223

Theoretical Relationship between C0 and y0

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Plasticizer in PVC is usually in its liquid state, while the large, irregularly shaped PVC molecules

225

are often at least partly aggregated or crystallized.24 Several theories exist to describe how

in

air

were

estimated

with

the

SPARC

Online

Diffusion Calculator

y0 and Ks are assumed to be constant at a given


16, 18, 21

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plasticizers behave in PVC.24-25 Chemically, the attraction between plasticizer and PVC is caused

227

by van der Waals forces and dipole-dipole interactions between the polarized C-Cl bond in the

228

PVC molecule chain and the polar parts of the plasticizer molecule.24 These interactions allow

229

solvation of the amorphous part of the PVC molecule by the plasticizer, but not of its crystalline

230

part.24 The plasticizer, however, does not interact with other components in a material, allowing

231

the plasticizer-PVC mixture to be described as a solution.26

232

Three general approaches to the relationship between C0 and y0 have been identified in the

233

literature: 1) y0 equals the pure compound vapor pressure p* and is independent of C0, at least for

234

C0 values larger than a critical value; 2) y0 is correlated to C0 by a partition coefficient; and 3) y0

235

is correlated to C0 by p*, when Raoult’s Law applies.

236

Generally, the relationship between liquid and gas phase at equilibrium is given as27 , = ∗ ∙ ∙ !

(4)

237

where y0,i is the gas-phase concentration of compound i in equilibrium with the solution, γi(l) is

238

the activity coefficient in solvent l, and xi is the mole fraction of the compound in the liquid

239

phase.

240

Both Henry’s Law and Raoult’s Law describe the partitioning of a compound between the liquid

241

phase and the gas phase.27 Henry’s Law can be written as27 , = ", ∙ !

(5)

242 243

where KH,i(l) is the Henry’s Law Constant of compound i in solvent l. Here, the solvent is PVC;

244

therefore the index l is omitted. For the applicability of Henry’s Law, the solution of compound i

245

does not have to be an ideal solution, i.e., the activity coefficient γi does not equal one, but is a 13 ACS Paragon Plus Environment


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constant. This assumption is often true if the compound in question is present in the solution at a

247

low fraction (i.e., it is a dilute solution). The slope of the line relating the fraction of the

248

compound in the material phase (usually the mole fraction x) to the fraction of the compound in

249

the gas phase is the Henry’s Law Constant (Figure 1a). For Raoult’s Law on the other hand,

250

ideal behavior is assumed,27 i.e., γi equals one, and the slope of the linear curve is equivalent to

251

the pure compound vapor pressure pi* (Figure 1a). This is usually applicable for large fractions

252

of the compound present in a solvent that is chemically similar. Raoult’s Law is given as27 = ∗ ∙ !

(6)

253 254

where pi is the partial pressure of compound i, which is equivalent to y0. The unit of pi depends

255

on the unit of pi*.

256

comparability.

Here, the concentration (µg/m3) is used instead of pressure units for

257 a)

b)

258

Figure 1: a) Relationship between y0 and x for ideal and non-ideal solutions; b) relationship

259

between y0 normalized by p* and x for ideal and non-ideal solutions.

260 14 ACS Paragon Plus Environment

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261


Equation 4 can be rearranged to , = ∙ ! ∗

(7)

262 263

Based on Equation7, the activity coefficient linearly correlates y0/p* and x and can therefore be

264

determined using linear regression. When the solutes in the polymeric solution are similar in

265

their interaction with the polymer, it can be hypothesized that γi is only weakly compound-

266

dependent within a given compound class, and can be reduced to γ (Figure 1b). In the case of an

267

ideal solution, the slope would then be one.

268

C0 given in wt% can be converted into the dimensionless mass fraction w (mass of plasticizer per

269

mass of PVC in a product) by Equation 8, while for C0 given in µg/m3, Equation 9 can be used.

#=

100#%

(8)

'()*

(9)

#=

270 271

ρPVC is the density of PVC (on average 1.5 g/cm3). The mole fraction x can be calculated based

272

on w.

273

characteristics of polymeric solutions including plasticizer-PVC solutions28-29, because of the

274

large molecular weight of polymers, which spans a wide range from about 20,000 g/mol to

275

80,000 g/mol or even 150,000 g/mol in some cases,30 and the significant size difference between

276

polymer and solute molecules. Instead, w in case of dilute solutions or the volume fraction ϕ is

However, x is often considered unrealistic and not appropriate to describe the



Page 16 of 26

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generally used instead of x in the context of polymeric solutions.26, 28, 31 For a PVC solution

278

containing n components, the volume fraction ϕ of compound i is29

+ =

,

,-.-/0

=

# /' # 1 − ∑435 #3 ∑435 3 + '3 '789

(10)

279 280

where V (m3) is volume and ρ (g/cm3) is density. In this study, both ϕ and w were used instead of

281

x when evaluating Henry’s Law and Raoult’s Law.

282

Results and Discussion

283

Modeling Results

284

The values for y0 of the grated and melted green vinyl flooring obtained by modeling were in

285

good agreement with previously measured values for the original material. For the re-shaped

286

material, y0 = 2.5 µg/m3 was obtained, while Liang and Xu9 reported 2.4 µg/m3. The consistency

287

of the measurements confirms the reliability of the preparation procedure. For data evaluation,

288

only the data for the original green vinyl flooring were used.

289

For the various products tested in this study, modeling of y0 yielded reasonable to very good

290

results with R2 values ranging from 0.44 to 0.97. Table 2 summarizes the modeling results and

291

the literature data, with additional information given in the Supplementary Information (Tables

292

S1 and S2). All modeling results can be found in Figures S3 and S4. The experimental data for

293

ATBC in the toy turtle were not suitable for use. With a C0 for ATBC in the turtle of 0.24% and

294

an ATBC background concentration of about 100 ng/m3, this is not surprising. Modeled y0

295

values ranged from 0.08 µg/m3 to 2.2 µg/m3. Additionally, the results for DEHT in sample TM-b

296

were excluded because the fitted Ks value did not match the expected range. All other Ks values 16 ACS Paragon Plus Environment

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were in agreement with our previous study20 and confirm the applicability of the diffusive

298

sampler.

299 300

Table 2: Data obtained in this study and from the literature for phthalates and phthalate

301

alternatives, including material-phase SVOC concentration C0, saturated vapor pressure p*,

302

volume fraction ϕ, and equilibrium concentration in the air directly adjacent to the product

303

surface y0. Absolute errors for C0, p*, and y0 are given if available. ϕ (-) 0.34

y0 (µg/m3)

25.5 ± 3.5

p* @ 25 °C (µg/m3)a 0.4 ± 0.05

0.56 ± 0.10

R2 (-) 0.75

DIDP

14.6 ± 3.7

0.1 ± 0.01

0.21

0.08 ± 0.14

0.95

DEHA DEHP DEHTb DINP DEHA DEHT DEHP DEHT DIDPb DINCH DINP DEHT

6.9 ± 0.1 0.1 ± 0.1 7.9 ± 4.3 4.2 ± 1.9 5.5 ± 2.5 5.4 ± 2.0 3.1 ± 2.3 6.9 ± 3.3

10.4 ± 0.03 2.5 ± 0.04 0.4 ± 0.05 0.4 ± 0.02 10.4 ± 0.03 0.4 ± 0.05 2.5 ± 0.04 0.4 ± 0.05

0.2 ±0.2 0.6 ± 0.6 16.9 ± 0.3

1.6 ± 0.01 0.4 ± 0.02 0.4 ± 0.05

0.11 2.20 ± 0.24 0.00 0.02 ± 0.005 0.11 0.80 ± 0.04 0.06 0.11 ± 0.02 0.01 1.30 ± 0.14 0.09 0.20 ± 0.11 0.04 0.25 ± 0.03 0.10 0.21 ± 0.04 < Detection limit 0.00 0.01 ± 0.003 0.01 0.06 ± 0.04 0.24 0.4 ± 0.08

0.87 0.44 0.97

DEHT

9.9 ± 0.5

0.4 ± 0.05

0.14

0.95

DEHT ATBC DEHT ATBCb

0.4 ± 0.05 13.2 ± 0.9 0.4 ± 0.05 13.2 ± 0.9

0.17 0.42 ± 0.06 0.02 0.95 ± 0.05 0.29 0.40 ± 0.07 Data not sufficient for modeling 0.32 2.5 ± 0.44

Product

Compound

C0 (wt%)

Backpack RO-b Backpack ST-b Backpack TA-b Backpack TM-b Backpack WA-b1 Backpack WA-b2

DEHT

Small dinosaur toy DO-t1 Big dinosaur toy DO-t2 Giraffe toy TA-t1

0.29 ± 0.09

Green vinyl flooring (re-shaped)b

DEHP

11.6 ± 0.4 1.7 ± 0.03 21.2 ± 1.1 0.24 ± 0.05 23.3 ± 3

Green vinyl flooring (original) Table cloth Wall paper Flooring

DEHP

23.3 ± 3

0.32

2.4

DEHP DEHP DEHP

16.2 2.8 13.4

0.23 0.04 0.19

1.8 0.19 1.7

PVC 1 PVC 2

DEHP DEHP

18.2 5.1

0.25 0.07

2.1 ± 0.4 0.8 ± 0.1

Turtle toy TA-t2

2.5 ± 0.04


Reference C0: Xie et al.19 y0: this study

0.94 0.94 0.57 0.68 0.56 0.84 0.77 0.73

0.84 0.74 0.82

0.83

Does not apply

C0: Liang and Xu9 y0: this study C0 and y0: Liang and Xu9 C0 and y0: this study using method described by Cao et al.8 C0 and y0: Cao et al.18


DiBP 4.3 1076 ± 49 DnBP 4.4 751 ± 23 Red vinyl flooring DEHP 6.1 2.5 ± 0.04 DiBP 3.8 1076 ± 49 DnBP 4.6 751 ± 23 Vinyl flooring 1 DEHP 13.0 ± 2 2.5 ± 0.04 Vinyl flooring 3 DINP 20.0 ± 3 0.4 ± 0.02 DEHP 0.1 ± 0.02 2.5 ± 0.04 Vinyl flooring 5 DnBP 9.0 ± 1 751 ± 23 DEHP 7.0 ± 1 2.5 ± 0.04 Crib matress cover 1 DINCH 11.0 ± 1 1.6 ± 0.01 Crib matress cover 2 DEHA 4.0 ± 1 10.4 ± 0.03 a Pure compound vapor pressures as measured in Wu et al.32 b Not included in the evaluation

0.06 0.06 0.09 0.05 0.06 0.18 0.28 0.00 0.12 0.10 0.16 0.06

68 ± 4.8 36 ± 2.5 0.90 50 25 2.30 0.42 0.02 25.90 1.49 0.70 1.05

Page 18 of 26

C0 and y0: Wu et al.21 C0 and y0: Liang and Xu9, 16

304 305

Evaluation of the Material/Gas Equilibrium Relationship

306

A total of Ntotal = 34 data points were included in the evaluation. Most data were available for

307

DEHP (N = 12) and DEHT (N = 7); N = 3 data points were available for DnBP, DEHA, and

308

DINP. DiBP and DINCH had two data points each and DIDP and ATBC had one data point

309

each. Most data for DEHP (83%) and all data for DiBP and DnBP came from the literature or

310

other sources.

311

Figure 2 shows the relationship between y0 and p* for all data points. Clearly the approximation

312

of y0 using p* as suggested by Ekelund et al.12 and Clausen et al.13 is not applicable, as y0 covers

313

almost the entire range between zero and p* for a single SVOC, e.g. DEHP. This observation,

314

however, is only valid for the presented data. One major difference between this study and the

315

work of Ekelund et al.12 is that in the latter, the temperature was much higher (100 °C). Because

316

the temperature has a great impact on y0,13,

317

purposes.

18

it is difficult to dismiss p* for approximation

318


Page 19 of 26


319 320

Figure 2: The SVOC gas-phase concentration adjacent to the source material y0 (µg/m3) as a

321

function of the pure compound vapor pressure p* (µg/m3), including data measured in this study

322

and additional data from literature and other measurements (Ntotal = 34, T = 25 °C).

323 324

All the data considered in this study can be organized according to Equation 7. y0/p* versus the

325

volume fraction ϕ is presented in Figure 3. The respective figure for the mass fraction w can be

326

found in Figure S5. In comparison to Figure 1b, if the data followed Raoult’s Law with the

327

assumption of an ideal solution, the slope (activity coefficient γ) of the regression curve (blue

328

line) would be close to one. Instead, the data clearly exhibits a linear trend with a slope greater

329

than one for both ϕ and w as independent variable and a standard error of the estimate of 5% in

330

both cases, which suggests a non-ideal equilibrium governed by Henry’s Law.

331

interesting to note that all target phthalates and phthalate alternatives can be summarized in this

332

manner. The difference between the results for ϕ and w is only minor, suggesting that both mass 19 ACS Paragon Plus Environment

It is very


333

and volume fractions could be used to represent the material phase. With γ in Equation 7

334

basically describing the deviation from non-ideal behavior based on the interaction between

335

solvent and solute, it is reasonable to expect that similar compounds have a similar γ in a solution.

336

Normalizing y0 with p* removes the compound-specific characteristics from the relationship,

337

leaving only solution-specific characteristics as the correlating factor. This expression shows that

338

the assumption of ideal behavior (Raoult’s Law) cannot be made for plasticizer-PVC solutions.

339

Instead, the pure compound vapor pressure p* and an activity coefficient γ have to be included to

340

correlate y0 and C0, with C0 expressed as mass fraction w or volume fraction ϕ.

341

Figure 3 also shows that some data points exceed y0/p* = 1. Theoretically, this should not be the

342

case because y0 should not be larger than the pure compound vapor pressure at a given

343

temperature. y0 and p* were both experimentally determined, and the uncertainties regarding

344

those measurements, given as errors bars in Figures 3 and S5, are most likely the reason for

345

overestimating y0 or underestimating p*, in this case for DEHT and DINP (Figure S6). However,

346

these values do not negate the general observation.

347

348 20 ACS Paragon Plus Environment

Page 20 of 26

Page 21 of 26


349

Figure 3: Relationship between y0 (µg/m3) normalized by p* (µg/m3) and the volume fraction ϕ,

350

including data measured in this study and additional data from literature and other measurements

351

(Ntotal = 34, T = 25 °C). Errors were calculated if data were available. The standard error of the

352

fitted slope is 0.18.

353 354

For single compounds, correlating either ϕ or w with y0 reveals that a linear trend can be

355

described as partitioning between gas and material phase (Figures S7 and S8). Linear regression

356

curves fitted to data available for DEHP and DEHT yields good results, with those based on ϕ

357

being slightly better. Figure 4 shows the regression curves of y0 for DEHP and DEHT using ϕ as

358

independent variable. The respective results for w can be found in Figure S9. The slope now

359

corresponds to a partition coefficient which could also be interpreted as a Henry’s Law Constant

360

(equal to a product of pi* and the activity coefficient γi). If sufficient data for one compound in a

361

similar matrix is available, this relationship can be used to approximate y0 based on a known C0

362

value.

363

364



Page 22 of 26

365

Figure 4: Relationship between the SVOC gas-phase concentration adjacent to the source

366

material y0 (µg/m3) and the volume fraction ϕ for DEHP (N = 12) and DEHT (N = 7) at T = 25

367

°C. Errors are given if available. The standard errors of the fitted slopes are 0.58 for DEHP and

368

0.13 for DEHT.

369 370

The results indicate that the equilibrium between gas and material phase of phthalates and

371

phthalate alternatives in PVC products can be described by Henry’s Law. The plasticizer-PVC

372

solution does not behave ideally, and the resulting Henry’s Law Constant consists of the pure

373

compound vapor pressure and an activity coefficient, which accounts for non-ideality. y0 is an

374

important parameter for the estimation of SVOC concentrations in the indoor air.

375

otherwise constant conditions, the SVOC concentration y in the air increases with increasing y0,

376

while at the same time the depletion period decreases.6 An approximation of y0 based on data for

377

C0 can be achieved if related data for the same compound is available, because y0 and ϕ (or w)

378

have been shown to correlate linearly. In this case, the slope represents a partition coefficient. If

379

insufficient y0 and C0 data is available for a specific compound, other similar compounds could

380

be used to establish the relationship between y0/p* and ϕ (or w). Based on the resulting activity

381

coefficient and p* for the specific compound, y0 can be estimated.

382

Only phthalates and phthalate alternatives in PVC products have been examined in this study.

383

The data set included all available data to date, and all values confirm the observations even

384

though the data comes from different sources. However, the effect of temperature was not

385

considered, and the results need to be confirmed for other SVOCs in other products. Also, fitting

386

Ks and y0 simultaneously may have introduced additional uncertainties into the accuracy of the


Under

Page 23 of 26


387

modelled y0 values. In general, the model can be further simplified, for example by assuming a

388

linear distribution of the gas-phase concentration inside the chamber.

389

It is possible that the heating process in the sample preparation step enhances the volatilization of

390

a compound, especially if this compound has a comparatively high vapor pressure. If it was also

391

only present at low concentrations, this could lead to the removal of some or most of the

392

compound from the material. ATBC in the turtle toy was only present with C0 = 0.24% and was

393

not detectable with the diffusive sampler. It is possible that the preparation step contributed to

394

this problem. Further development of the preparation step is therefore recommended. However,

395

at those low material-phase concentrations, the uncertainties are rather high anyway and all

396

results have to be handled carefully.

397 398

Associated Content

399

Supplementary Information available: Figure S1: Preparation of the toys for use with the

400

diffusive sampler. Figure S2: a) Schematic representation of the experimental setup; b) Schematic

401

representation of the adapted diffusive sampler and the diffusion model. Figure S3: Best-fit

402

modeling results for the backpacks. Figure S4: Best-fit modeling results for the toys. Table S1:

403

Additional data describing the samples investigated in this study. Table S2: Additional data

404

describing the data obtained from literature. Figure S5: Relationship between y0/p* and w. Figure

405

S6: a) y0/p* over ϕ and b) y0/p* over w for the different compounds. Figure S7: y0 as a function of

406

ϕ. Figure S8: y0 as a function of w. Figure S9: Relationship between y0 and w for DEHP and

407

DEHT.



408

Author Information

409

Corresponding Author

410

*Phone: (540) 231 0836; e-mail: [email protected] (J.C.L.).

411

ORCID

412

John C. Little: 0000-0003-2965-9557

413

Jianping Cao: 0000-0002-8633-388X

414

Notes

415

The authors declare no competing financial interest.

416

Acknowledgements

417

Support for this research was provided by the United States Environmental Protection Agency

418

(Grant Number RD-83560601-0).

419


Page 24 of 26

Page 25 of 26


420

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Equilibrium Relationship between SVOCs in PVC ... - ACS Publications

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