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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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Equilibrium relationship between SVOCs in PVC products and the
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air in contact with the product
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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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Abstract: Phthalates and phthalate alternatives are semi-volatile organic compounds (SVOCs)
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present in many PVC products as plasticizers to enhance product performance. Knowledge of the
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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
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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
24
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
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of greater value in rapid exposure assessment procedures.
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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
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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
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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-
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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*.
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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
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were then correlated to y0. In addition, p* can also be expressed in µg/m3, based on the Ideal Gas
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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
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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.
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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
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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
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normalization.
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(GC/MS, Thermo Scientific Focus GC with Thermo Scientific DSQ II MS, Thermo Fisher
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Scientific, Waltham, MA). Five-point calibration curves ranging from 100 to 2,000 ng/mL were
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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
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developed by Wu et al.20 was simplified to include only the adsorption of SVOCs on the
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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
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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:
−
= =
∂ ∂ ∂ = = ∂ ∂ ∂
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(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
208
(http://archemcalc.com/sparc-web/calc#).
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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
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the best fitting model results.
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Literature Data
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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
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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
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specifically acquired for this study.
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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
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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
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16, 18, 21
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plasticizers behave in PVC.24-25 Chemically, the attraction between plasticizer and PVC is caused
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by van der Waals forces and dipole-dipole interactions between the polarized C-Cl bond in the
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PVC molecule chain and the polar parts of the plasticizer molecule.24 These interactions allow
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solvation of the amorphous part of the PVC molecule by the plasticizer, but not of its crystalline
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part.24 The plasticizer, however, does not interact with other components in a material, allowing
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the plasticizer-PVC mixture to be described as a solution.26
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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.
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Equation 4 can be rearranged to , = ∙ ! ∗
(7)
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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
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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.
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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
17 ACS Paragon Plus Environment
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
Environmental Science & Technology
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
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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
Environmental Science & Technology
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
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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
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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
22 ACS Paragon Plus Environment
Under
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Environmental Science & Technology
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.
23 ACS Paragon Plus Environment
Environmental Science & Technology
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
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