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Jun 27, 2016 - At pH = 13, except for tributyl phosphate and tris(2-ethylhexyl) phosphate, 14 OP triesters were degraded with half-lives ranging from ...
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Organophosphate Flame Retardants and Plasticizers in Aqueous Solution: pH-Dependent Hydrolysis, Kinetics, and Pathways Guanyong Su,†,‡ Robert J. Letcher,*,†,‡ and Hongxia Yu∥ †

Ecotoxicology and Wildlife Health Division, Environment and Climate Change Canada, National Wildlife Research Centre, Carleton University, Ottawa, Ontario K1A 0H3, Canada ‡ Department of Chemistry, Carleton University, Ottawa, Ontario K1S 5B6, Canada ∥ State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, Nanjing 210023, China S Supporting Information *

ABSTRACT: Despite the growing ubiquity of organophosphate (OP) triesters as environmental contaminants, parameters affecting their aquatic chemical stabilities are currently unknown. The present study examined the pH-dependent (7, 9, 11, or 13) hydrolysis of 16 OP triesters in mixtures of 80 ng/mL for each OP triester over a period of 35 days at 20 °C. For the pH = 7, 9, and 11 solutions, 10 of the 16 OP triesters were stable and with no significant (p > 0.05) degradation. For the remaining 6 OP triesters, significant degradation occurred progressing from the pH = 7 to 11 solutions. At pH = 13, except for tributyl phosphate and tris(2-ethylhexyl) phosphate, 14 OP triesters were degraded with half-lives ranging from 0.0053 days (triphenyl phosphate) to 47 days (tripropyl phosphate). With increasingly basic pH the order of OP triester stability was group A (with alkyl moieties) > group B (chlorinated alkyl) > group C (aryl). Numerous OP diesters were identified depending on the pH level of the solution, whereas OP monoesters were not detectable. This is consistent with no significant (p > 0.05) depletion observed for 5 OP diesters in the same 4 solutions and over same 35 day period, suggesting OP diesters are end products of base-catalyzed hydrolysis of OP triesters. Our results demonstrated that pH-dependent hydrolysis of OP triesters does occur, and such instability would likely affect the fate of OP triesters in aqueous environments where the pH can be variable and basic.

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INTRODUCTION Organophosphate (OP) triesters are a large group of chemicals that have been used for decades as flame retardants (FRs) and plasticizers in various consumer products, such as plastics, textiles, wood, and many others materials.1−3 Specifically, the halogenated OP triesters, i.e., tris(1,3-dichloro-2-propyl) phosphate (TDCIPP), tris(2-chloroisopropyl) phosphate (TCIPP), tris(2-chloroethyl) phosphate (TCEP), 2,2-bis(chloromethyl)propane-1,3-diyltetrakis(2-chloroethyl) bisphosphate (V6), are mainly used as FRs to improve the resistance to fire by chemical or physical mechanisms.1 Nonhalogenated OP triesters are predominantly used as plasticizers and lubricants to regulate pore sizes.4 The exception is triphenyl phosphate (TPHP), which is also used in combination with halogenated FRs, i.e., halogenated bis(2-ethylhexyl) tetrabromophthalate (TBPH) and tetrabromobenzoate (TBB), and nonhalogenated mono-, di-, and tri-isopropylated triaryl phosphates (ITPs) in the FR formulation Firemaster 550 (FM550).4,5 Many OP triesters are additives and not chemically bonded to polymer products,5 and thus are easily released into the environment over the lifetime of these products. During the past few years, OP triesters have received more environmental attention due to an urgent demand for chemical substitutes/replacements for © 2016 American Chemical Society

the phased-out, commercial penta- and octa-BDE FR formulations.6 OP triesters can be released into the environment through urban wastewater and many of them can further distributed through the water cycle and can subsequently be released into various water bodies.1,7 Thus, there are increasing reports of OP triesters in various aqueous environments including river water,4 marine water,8 drinking water,9−11 and precipitation as well as in sewage treatment plant (STP) effluents/influents.9 The growing environmental ubiquity of OP triesters increases the urgency to continually address the knowledge gaps with respect to their environmental stabilities and fate in aqueous environments.1 Given the base molecular structures (i.e., ester bonds) of OP triesters, abiotic hydrolysis of the phosphate ester bond to a phosphoric acid is likely relevant with respect to their stability in aqueous environments. However, to our knowledge the abiotic stability with respect to relevant environmental parameters is limited to several OP Received: Revised: Accepted: Published: 8103

May 2, 2016 June 17, 2016 June 27, 2016 June 27, 2016 DOI: 10.1021/acs.est.6b02187 Environ. Sci. Technol. 2016, 50, 8103−8111

Article

Environmental Science & Technology

Figure 1. Chemical structures of 16 target organophosphate (OP) triesters. The OP triesters are subdivided into four groups based on their specific chemical structures. OP triesters of Group A contain nonhalogenated alkyl moieties; triesters of Group B contain chlorinated alkyl moieties; triesters of Group C contain aryl moieties; and triesters of Group D do not belong Groups A, B, or C.

2. MATERIAL AND METHODS 2.1. Standards and Reagents. The chemical structures of all 16 target compounds can be found in Figure 1. Triethyl phosphate (TEP), tripropyl phosphate (TPP), tributyl phosphate (TNBP), TCEP, tris(2-butoxyethyl) phosphate (TBOEP), TPHP, tris(2-ethylhexyl) phosphate (TEHP), tris(tribromoneopentyl) phosphate (TTBPP), and 2-ethylhexyl-diphenyl phosphate (EHDPP) were all purchased from Sigma-Aldrich (St. Louis, MO, U.S.A.). TCIPP was purchased from AK Scientific (Union City, CA, U.S.A.). TDCIPP and TMPP were purchased from TCI America (Portland, OR, U.S.A.), and the three isomers tris(4-bromo-3-methylphenyl) phosphate (T4B3MP), tris(3-bromo-4-methylphenyl) phosphate (T3B4MP) and tris(2-bromo-4-methylphenyl) phosphate (T2B4MP) were synthesized by GL Chemtech (Oakville, ON, Canada). The V6 was generously provided by Dr. Heather Stapleton (Duke University, NC, U.S.A.). Five isotopically enriched chemical standards were used for quantification of OP triesters. The d27-TNBP and d15-TEP internal standards were purchased from Cambridge Isotope Laboratories (Tewksbury, MA, U.S.A.), and the d15-TPHP internal standard was purchased from Wellington Laboratories (Guelph, ON, Canada). The d12-TCEP and d15-TDCIPP were

triesters, e.g. tris(methyphenyl) phosphate (TMPP), triethyl phosphate (TEP), TCIPP, TNBP, TEHP, and TPHP. Specifically, abiotic degradation was reportedly not expected for TCIPP in leachate from a sea-based solid waste disposal site.8 High stabilities were also reported for TNBP and TEHP in aqueous solutions under abiotic conditions.12 Aqueous stability information is also available for TPHP, TMPP, and TEP.13−16 For instance, TPHP was shown to be degraded in river water/sediment and pond sediment with half-lives ranging from 3 to 12 d, whereas degradation of TMPP was much slower with a half-life of 96 d.15,16 Regardless, precise experimental data of abiotic stabilities is lacking for most of the OP triesters, and the underlying pathways (i.e., to products of hydrolysis) are essentially unknown. In the present study, the aquatic stability of 16 environmentally relevant OP triesters was investigated as a function of time and under neutral and basic pH conditions. All possible pH-dependent products of hydrolysis (i.e., OP diester and monoester phosphoric acids) were also screened by use of high performance liquid chromatography-quadrupole-time-of-flight mass spectrometry (LC-Q-TOF-MS). To our knowledge, this study provides the first information on OP triester stability as a function of changing pH as well as elucidating the underlying mechanisms/pathways of aquatic degradation. 8104

DOI: 10.1021/acs.est.6b02187 Environ. Sci. Technol. 2016, 50, 8103−8111

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Environmental Science & Technology

Figure 2. Depletion of 16 OP triesters over a 35 day period in aqueous systems at nominal pH = 7, 9, 11, and 13 (see Table S1 for measured pH values), respectively. Using ultrahigh performance liquid chromatography-tandem quadrupole mass spectrometry with an atmospheric pressure chemical ionization source (UPLC-APCI(+)-MS/MS), the relative percent of the OP triester concentrations in solutions collected at day 0. The red bars represent the relative percent of the OP triester concentrations in solutions collected at day 35. Where there is a significant difference (p < 0.01) a “*” is marked beside the chemical names.

By use of a pH meter (Thermo Scientific, Beverly, MA, U.S.A.), accurate pH values (6.96 ± 0.08, 8.93 ± 0.01, 10.80 ± 0.02, and 12.96 ± 0.01) were determined for the four solutions systems at day 0 and 35 as listed in Table S1. 2.3. Experimental Design. Abiotic degradation experiments of OP triesters were conducted at a single concentration of 80 ng/mL. It has been shown that ester bond hydrolysis is independent of the initial concentration of target compounds.18 In the present hydrolysis study, an 80 ng/mL concentration for all individual OP triesters was also used. Although this 80 ng/ mL concentration is lower than the aqueous solubility limit of all 16 target OP triesters, we remained concerned that adsorption on glassware surfaces could still be possible for several of the more lipophilic triesters, i.e., EHDPP, TEHP, TMPP, T2B4MP, T3B4MP, T4B3MP, TBOEP, TNBP, and TTBPP. See details in the Text-S2. To minimize any possible “false-positive” hydrolysis degradation due to such potential adsorptive effects, two sequential experiments were designed and conducted. In the first experiment (Figure 2), concentrations of the investigated OP triesters were determined at two time points (0 and 35 d). In brief, 80 μL of the 16 OP triesters in methanol solution (20 μg/mL each) was added to each of 24, 15 mL precleaned glass tubes and blown to dryness under a gentle stream of nitrogen gas. A volume of 2 mL of the prewarmed reaction solutions were then added into the same tubes (6 replicates for each of four reaction solutions at nominal pH = 7, 9, 11, or 13). The bottles were vortexed well for 15 s, and for 3 of the 6 replicate samples from each reaction solution 2 mL of 2 M ammonium acetate buffer and 6 mL of fresh methanol were added to neutralize the pH and terminate any further pH-dependent hydrolysis of the OP triesters. It was emphasized that adding methanol is to wash down all the OP triesters that might be

prepared by and purchased from Dr. Vladimir Belov (Max Planck Institute for Biophysical Chemistry, Germany). The OP diester standards, di-n-butyl phosphate (DNBP) and diphenyl phosphate (DPHP), were purchased from Sigma-Aldrich. The other three OP diester standards, bis(1-chloro-2-propyl) phosphate (BCIPP), bis(2-butoxyethyl) phosphate (BBOEP), and bis(1,3-dichloro-2-propyl) phosphate (BDCIPP), were purchased from Dr. Vladimir Belov (Max Planck Institute, Germany). Purities of all OP standards were ≥97%. All reagents or solvents, including sodium azide (NaN3), ammonium acetate (NH4COOH) and UPLC-grade methanol were obtained from Sigma-Aldrich, with an exception of sodium chloride (NaCl) which was purchased from Merck KGaA (Darmstadt, Germany). 2.2. Aqueous Degradation. Since the hydroxide anion is a nucleophile for the ester bonds,17 base-catalyzed hydrolysis of the suite of OP triesters was investigated in both neutral (nominal pH = 7) and increasingly basic (nominal pH = 9, 11, and 13) solutions. Degradation experiments were conducted in four different aqueous solutions with volumes of 2 mL each. The compositions of the four aqueous solutions varied as a function of pH, and were detailed in Table S1 of the Supporting Information (SI). For all solutions, NaOH was used for the adjustment of pH level, and NaCl was added as a buffer to maintain the pH level and to be sure that all solutions had comparable ionic strengths. Adding NaN3 (final concentration: 0.02% (w/v)) was to avoid any possible microbial activities as a confounding factor to any observed degradation. The experiments are conducted and maintained at room temperature (20 ± 1 °C) and with shaking (90 rpm) in a water bath (Thermo Electron Corporation, Waltham, MA, U.S.A.). To avoid any potential effects from photolytic degradation, the water bath was covered to exclude light during the experiments. 8105

DOI: 10.1021/acs.est.6b02187 Environ. Sci. Technol. 2016, 50, 8103−8111

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Figure 3. Time-dependent hydrolytic degradation of OP triesters at nominal pH = 7, 9, 11, and 13 (see Table S1 for measured pH values) over a period of 4 days or 35 days (0 min, 15 min, 30 min, 1 h, 2 h, 4 h, 8 h, 1 d, 2 d, 4 d, 7 d, 14 d, 21 d, 28 d,and 35 d). For several OP triesters, the degradation curves were shown for a period of 4 days given their rapid degradation ( 0.01; see Figure 2) in pH = 7, 9, and 11 solutions over a period of 35 days (i.e., TCEP, TCIPP, TDCIPP, and V6); or due to the strong adsorption on glass walls (i.e., TMPP, T2B4MP, T3B4MP, T4B3MP, and EHDPP). Four OP treisters (TBOEP, TTBPP, TNBP, and TEHP) were not shown. Because a smooth degradation curve was not observed for TBOEP or TTBPP due to the severely strong adsorption on the glass wall, and no significant degradation was observed for TNBP or TEHP in all four pH solutions (t test, p > 0.01; in Figure 2). Using ultrahigh performance liquid chromatography-tandem quadrupole mass spectrometry with an atmospheric pressure chemical ionization source (UPLC-APCI(+)-MS/MS), and relative to time 0, the percent change in OP triester concentrations over time.

adsorbed on the glass surface. Then, a volume of 250 μL of this resulting solution and 20 μL of internal standards (100 ng/mL) were then combined in UPLC injection bottles, after which the collected samples were stored at −20 °C before UPLC-MS/MS analysis. In the next step, the other 3 of the 6 replicates of the prewarmed reaction solutions for each pH solution, were then placed into a water bath (20 ± 1 °C) for a period of 35 days, which were subsequently collected for UPLC-MS/MS analysis. In this experiment, 6 mL of methanol was directly added into the hydrolysis system, which can wash out and nullify any possible OP triester adsorption onto glassware surfaces. This allowed for the determination of the depletion rates of all 16 OP triesters over a period of 35 days at solution pH values of 7, 9, 11, and 13 with elimination of possible “false-positive” hydrolysis degradation caused by potential adsorptive effects. The second experiment (Figure 3) is a kinetics study, which was performed with respect to the pH-dependent degradation of several of the OP triesters, which were not susceptible to glass surface adsorption or could be corrected by control comparisons (pH = 7), i.e., TEP, TPP, TCEP, TCIPP, TDCIPP, V6, TPHP, TMPP, T 2 B 4 MP, T 3 B 4 MP, and T4B3MP. In brief, 80 μL of the OP triesters in methanol solution (20 μg/mL for each OP triester) was added into 15 mL glass tubes (n = 12), and blown to dryness under a gentle stream of nitrogen gas. A volume of 2 mL of the prewarmed reaction solutions were then added into tubes (3 replicates for each of four reaction systems of pH = 7, 9, 11, and 13). The tubes were vortexed well for 15 s, and immediately placed into

water bath for the degradation experiments. For each of the time points (0 min, 15 min, 30 min, 1 h, 2 h, 4 h, 8 h, 1 d, 2 d, 4 d, 7 d, 14 d, 21 d, 28 d, and 35 d), 50 μL of the reaction solution was collected from each of 12 tubes, pH neutralized with 50 μL of 2 M ammonium acetate buffer, diluted with 150 μL of methanol, spiked with 20 μL of internal standards (100 ng/mL) and stored at −20 °C until UPLC-MS/MS analysis. 2.4. Quantification of OP Triester/Diesters by UPLCMS/MS. The determination of OP triesters and diesters was performed on a Waters XEVO-TQ-S ultra high performance liquid chromatography-tandem quadrupole mass spectrometer in the positive atmospheric pressure chemical ionization mode (UPLC-APCI(+)-MS/MS) (Waters Limited, Milford, U.S.A.), and the detailed parameters are described in the SI. For more method details, please refer to our previous publications.19,20 2.5. Screening of Products of Hydrolysis by LC-Q-TOFMS. Screening of products of hydrolysis of OP triesters was carried out using an Agilent 1200 LC system, coupled with an Agilent 6250A quadrupole-time-of-flight mass spectrometer (LC-Q-TOF-MS; Agilent Technologies, Mississauga, ON, Canada). The LC system was equipped with an Luna 3u C18(2) 100A column (2.0 × 100 mm2, 3.0 μm particle size) (Phenomenex, Torrance, CA, U.S.A.). The mobile phase (A, water; B, methanol; both contain 2 mM ammonium acetate) flow rate was 0.5 mL/min, and the following gradient was employed: 5% B ramped to 70% B in 3 min (linear), and then ramped to 80% B in 12 min (linear), and ramped to 95% B in 3 min, and held for 7 min, followed by a change to 5% B and held 8106

DOI: 10.1021/acs.est.6b02187 Environ. Sci. Technol. 2016, 50, 8103−8111

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Table 1. Half-lives (t1/2, d) and Rate Constants (k, d−1) of 16 Organophosphate (OP) Triesters in Four Aqueous Solutions at Nominal pH = 7, 9, 11, and 13 (see Table S1 for Measured pH Values)a OP Triester Group TEP TPP TNBP TEHP Group TCEP TCIPP TDCIPP V6 Group TPHP TMPP T2B4MP T3B4MP T4B3MP Group TBOEP EHDPP TTBPP

pH = 7

pH = 9

pH = 11

pH = 13

NAb NA NA NA

NA NA NA NA

NA NA NA NA

t1/2 = 16 d; k = 0.0423 d−1; 1#c t1/2 = 33 d; k = 0.0213 d−1; 2# NA NA

NA NA NA NA

NA NA NA NA

NA NA NA NA

t1/2 = 0.083 d; k = 8.36 d−1; 3# t1/2 = 11 d; k = 0.0621 d−1; 4# t1/2 = 0.044 d; k = 15.6 d−1; 5# t1/2 = 0.11 d; k = 6.23 d−1; 6#

t1/2 = 112 d; k = 0.00618 d−1; 7# t1/2 = 94 d; Lineard t1/2 = 22 d; Linear t1/2 = 18 d; Linear t1/2 = 23 d; Linear

t1/2 = 77 d; k = 0.00906 d−1; 8# t1/2 = 110 d; Linear t1/2 = 21 d; Linear t1/2 = 18 d; Linear t1/2 = 21 d; Linear

t1/2 = 34 d; k = 0.0203 d−1; 9# t1/2 = 70 d; Linear t1/2 = 26 d; Linear t1/2 = 19 d; Linear t1/2 = 26 d; Linear

t1/2 = 0.0053 d; k = 130 d−1; 10# t1/2 = 0.027 d; k = 25.7 d−1; 11# t1/2 = 0.046 d; k = 15.2 d−1; 12# t1/2 = 0.066 d; k = 10.5 d−1; 13# t1/2 = 0.067 d; k = 10.4 d−1; 14#

NA t1/2 = 110 d; Linear t1/2 = 23 d; Linear

NA t1/2 = 130 d; Linear t1/2 = 22 d; Linear

NA t1/2 = 130 d; Linear t1/2 = 23 d; Linear

t1/2 = 21 d; Linear t1/2 = 0.11 d; k = 6.58 d−1; 15# t1/2 = 19 d; Linear

A

B

C

D

a Connected with Figure 4 by serial numbers 1−15. b“NA” means not available. No significant degradation is observed for this triester over a period of 35 days. ct1/2 and k was calculated based on an assumption that phosphate ester degradation reaction follow pseudo-first-order kinetics (No. 1− 15). dThese t1/2 values are estimated based on two time points of 0 and 35 d, because complete curves cannot be obtained due to adsorption on glass surfaces. We emphasize that future usage on these t1/2 values should be done with caution.

(t1/2, d), and t 1/2 = ln2/k. It should be noted that concentrations of some OP triesters (i.e., TMPP, T2B4MP, T3B4MP, T4B3MP, EHDPP) at pH = 13 have been corrected by controls (pH = 7), since glass adsorption was observed for these chemicals. For OP triesters where significant depletion was observed, but not 100% based on the first experiment (Figure 2) described in see section 2.3, and where adsorption occurred during the degradation experiment, their kinetic constants and t1/2 values were estimated based the two time points of 0 and 35 days and according to the following equation:

for 15 min for the next injection. The MS was operated using ESI source in the negative ion mode (ESI(−)), and its capillary voltage was 3.0 kV. Nitrogen was used as the drying and nebulizing gas and helium was used as the collision gas. Other parameters for MS were optimized as follows: gas temperature, 300 °C; drying gas, 10 L/min; nebulizer, 20 psi; fragmentor voltage, 250 V; skimmer voltage, 150 V. The Q-TOF instrument was tuned and calibrated with tuning calibration solution (G1969−85000, Agilent Technologies). The TOF-MS was operated at resolution (R) > 20000 at m/z 601.9790 and within 3 ppm mass error in mass range m/z 50−1700. For each run, TFA anion (m/z 112.9855) and HP-0921 (TFA adduct; m/z 1033.9881) were consistently introduced into the Q-TOF as reference masses. 2.6. Data Analysis. Data analysis was conducted in three steps. On the basis of the data generated from the first experiment (Figure 2) described in see section 2.3, a t-test analysis was performed on the concentration data of the OP triesters and for the time points of 0 and 35 d. This was done for each pH-value treatment to check whether there was any significant depletion of the OP triesters. If no significant difference was observed, then the half-lives (t1/2) were marked as not available (NA; see in Table 1), meaning that OP triester degradation did not occur for a specific aqueous solution over the period of 35 days. For OP triesters that were 100% depleted (i.e., TCEP, TDIPP, V6, TPHP, TMPP, T2B4MP, T3B4MP, T4B3MP, and EHDPP at pH = 13) in aqueous solutions at day 35, the kinetic rate constants (k, d−1) were calculated from the data generated from the second experiment (Figure 3) described in see section 2.3, and from the slope of the line plotting the ln of the fraction of the parent compound vs time. Here, pseudo-first-order kinetics (p < 0.0001 for all fitted linear curve; r2 > 0.93 for all curves with exceptions of TMPP (0.76) and EHDPP (0.74)) were selected to calculate the half-lives

t1/2 =

35C0 2(C0 − C35)

where C0 and C35 were concentrations of OP triesters at time points of 0 and 35 d, respectively. The degradation rate constants are not available for these chemicals (Table 1), and we emphasize that future usage on these t1/2 values should be done with caution.

3. RESULTS AND DISCUSSION 3.1. Stability and Degradation Kinetics. Sixteen target OP triesters in the present study were classified into four chemical subgroups, namely Groups A, B, C, and D (Figure 1). TEP, TPP, TNBP, and TEHP were classified into Group A as all three hydroxyl groups in phosphoric acid were substituted with alkyl groups (a fragment of the molecule with the general formula CnH2(n+1)). OPFRs of Group B were TCEP, TCIPP, TDCIPP, and V6 as all have chlorinated alkyl groups (with the general formula CnHxCly, x+y = 2(n+1)). TPHP, TMPP, T2B4MP, T4B3MP, and T3B4MP comprised Group C as all alkyl moieties are phenyl-based. Among these target OP triesters, TBOEP, EHDPP, and TTBPP did not fit structurally in Groups A, B, or C, and thus were designated as Group D (Figure 1). 8107

DOI: 10.1021/acs.est.6b02187 Environ. Sci. Technol. 2016, 50, 8103−8111

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Figure 4. Fitted linear curves for the time-dependent hydrolytic degradation of OP triesters in solutions at nominal pH = 7, 9, 11, and 13 (see Table S1 for measured pH values), and according to pseudo-first-order kinetics. Degradation constants (k, min−1, in Table 1) are calculated from the slope of the line by plotting the log of the fraction of the parent compound concentration versus time, and the half-lives (t1/2, in Table 1) are calculated as follows: t1/2 = ln 2/k. This is connected with Table 1 by serial numbers 1−15. For some OP triesters (i.e., TMPP, T2B4MP, T3B4MP, T4B3MP and EHDPP), adsorption of which might occur on glass surfaces (Text S2), their measured concentrations were corrected/normalized by those in pH = 7 solutions, and these chemicals were assumed to be stable in pH = 7 solutions over a short time (all less than 0.2 days).

No significant (p > 0.05, t-test) degradation was observed for all four OP triesters of Group A (TEP, TPP, TNBP, and TEHP) with nonhalogenated alkyl moieties and in solutions at pH levels of 7, 9, or 11 over a period of 35 days (Figures 2 and 3). In the solutions at pH = 13, significant (p < 0.01, t-test) degradation was observed for the two triesters with shorter alkyl chains (TEP, TPP), but the two OP triesters with longer alkyl chains exhibited great hydrolytic stability in the same solutions. The half-lives of TEP and TPP were estimated as 16 and 33 d, respectively, and the degradation rate constants of TEP and TPP were estimated as 0.0423 and 0.0213 d−1, respectively (Table 1 and Figure 3). These results probably suggest that the degradation rate constants decrease as a function of alkyl carbon chain length of OP triesters. Abiotic stability studies of TEP, TNBP, and TEHP have been previously conducted but under different conditions. 21 Specifically, the stability of TEP was studied in neutral (pH = 7) water systems at 101 °C, and its rate constant was reported to be 8.35 × 10−6 s−1 (t1/2 = 1.39 d).21 This is in contrast to the lack of degradation of TEP, we observed in a solution of pH = 7 at room temperature and over a period of 35 days. This result difference may suggest that any pH = 7 hydrolysis of TEP is likely accelerated by increased solution temperature.21 The high stabilities of TNBP and TEHP under the present basic solution conditions are consistent with a study that investigated the feasibility of an aqueous decontamination and an ex situ soil washing technique for the hydrolysis of OP triesters, suggesting that TNBP and TEHP were not hydrolyzed in solutions where sodium perborate (NaBO3, pH = 10.3) was added or under basic conditions (Na2CO3, pH = 10.7).12 OP triesters (TCEP, TCIPP, TDCIPP, and V6) of Group B (Figure 1) showed very similar degradation profiles compared to Group A OP triesters, but seemed to be degraded more rapidly under the same conditions. That is, there was no significant depletion in aqueous solutions of pH levels of 7, 9,

or 11 and over a period of 35 days. However, in solutions at pH = 13, the Group B OP triesters could be degraded extensively over a period of 35 days (Figures 2 and 3). It is worth noting that despite the one atom (chloride (Cl) or hydrogen (H)) difference in each moiety, the degradation rate of TCEP (t1/2 = 0.083 d) was much more rapid than that of TEP (t1/2 = 16 d) in solution at a pH = 13. Half-lives were estimated to be 0.083, 11, 0.044, and 0.11 d, and their degradation constant rates were estimated as 8.36, 0.0621, 15.6, and 6.23 d−1 for TCEP, TCIPP, TDCIPP, and V6, respectively (Table 1 and Figure 4). Previous hydrolytic stability data on these chloroalkyl OP triesters (i.e., TCEP, TCIPP and TDCIPP) have been predicted mainly from various mathematics models,22−24 not as a result of empirical laboratory experiments, and these modeling data are generally inconsistent with our present experimental results. For example, the predictive models and tools for assessing chemicals under the Toxic Substances Control Act (TSCA) in the United States suggested that TCEP may undergo abiotic degradation in the environment based on an estimated degradation half-life of 20 days at pH 5 to 9.22 However, from our present study, we found that TCEP showed great hydrolytic stability in solutions at pH 7 to 11, but was rapidly degraded at pH = 13. Another report suggested that TCIPP hydrolyzes very slowly in both alkaline and acidic aqueous environments.23 However, our experimental results suggested that TCIPP can be degraded quite rapidly in solution at pH = 13. Limited data for TDCIPP have suggested a resistance to pH-dependent hydrolysis in most environmental waters.24 Our data suggested that TDCIPP exhibits very similar degradation profiles compared to TCEP, TCIPP, and V6 with rapid hydrolysis in solutions at pH = 13. Group C OP triesters (Figure 1) exhibited totally different degradation profiles compared to OP triesters of Groups A or B. Specifically, the Group C OP triesters, TPHP, TMPP, T2B4MP, T3B4MP, and T4B3MP, contain aryl moieties and could be significantly degraded in all four solutions at pH 7, 9, 8108

DOI: 10.1021/acs.est.6b02187 Environ. Sci. Technol. 2016, 50, 8103−8111

Article

Environmental Science & Technology

ments.29 Compared to TPHP, in most cases TMPP has been reported at relatively low concentrations in aqueous samples,8 and the highest levels of TMPP were found in the ditch water samples collected around greenhouses in rural area of HigashiHiroshima.30 To our knowledge, no information is available for the concentrations of T2B4MP, T3B4MP, or T4B3MP isomers in environmental waters. Information on the concentrations of three Group D OP triesters in aqueous environments is currently lacking with the exception of TBOEP, where concentrations have been reported frequently and with levels being comparable to that of TNBP.9 These reported levels in aqueous environmental compartments are likely due to various factors, i.e., production volume, usage period, biotic transformation, as well as abiotic stability, which may suggest that environmental degradation is minimal for nonhalogenated OP triesters over a pH range of 7 to 11 in aqueous media. 3.2. Screening of Products of Hydrolysis of OP Triesters and the Underlying Mechanisms. On the basis of their molecular base structures, the most probable pathways of hydrolysis of OP triesters was hypothesized to be via the cleavage of ester bonds to form OP diester phosphoric acids with possible further degradation to OP monoester phosphoric acids. We investigated the products of hydrolysis of OP triesters, and all possible diester and monoester products were screened in degradation solutions collected at the time points of 0 d (as controls) and 35 d by use of LC-ESI(−)-ToF/MS or UPLC-TQ/MS. Numerous OP diesters, i.e., bis(2-chloroethyl) phosphate (BCEP; from TCEP or V6), bis(1-chloro-2-propyl) phosphate (BCIPP; from TCIPP), bis(1,3-dichloro-2-propyl) phosphate (BDCIPP; from TDCIPP), diphenyl phosphate (DPHP; from TPHP), bis(methylphenyl) phosphate (BMPP; from TMPP); bis(2-butoxyethyl) phosphate (BBOEP; from TBOEP), ethylhexyl monophenyl phosphate (EHMPP; from EHDPP) and three bis(bromo-3-methylphenyl) phosphate isomers (BBMP; from T2B4MP, T4B3MP or T3B4MP), were identified as products of the base-catalyzed hydrolysis of OP triesters. None of these diester degradation products were detectable in the control solutions. Furthermore, OP monoesters were not detectable in the same solutions of the hydrolyzed OP triesters over the period of 35 d. These results indicated that OP diesters are more stable than the parent OP triesters in same pH-variable aqueous solutions. This likely due to the phosphoric acid of the OP diester being deprotonated under basic conditions to its corresponding conjugate base. For example, several OP triesters (i.e., TCEP, TDCIPP, V6, TPHP) in solutions at pH = 13 were depleted in a very short time (less than 1 d), but their OP diester products were still detectable after 35 d. We subsequently conducted a mass balance study on selected OP triester-diester pairs, i.e., TPHP-DPHP, TBOEPBBOEP, TDCIPP-BDCIPP, and TCIPP-BCIPP, and the results demonstrated that the amounts of depleted OP triesters were very comparable to the amounts of OP diesters formed (Table S2). The abiotic stabilities of five OP diesters (DNBP, BCIPP, BDCIPP, DPHP, and BBOEP) were also investigated. No significant depletion (t-test, p > 0.01) was observed for all five of these OP diesters in aqueous solutions at all pH levels of 7, 9, 11, or 13 and over a period of 35 days (Figure S1). Although OP triesters in aqueous solutions were suspected to break down via cleavage of ester bonds, laboratory experiments have been limited to the study of only TPHP and TMPP.31 Specifically, Barnard et al. and Howard et al. found that alkaline hydrolysis of TPHP can result in a DPHP product,25,31 but further hydrolysis to monophenyl phosphate and (total

11, or 13 over 35 d. In aqueous solution at pH = 7, half-lives were estimated to be 112, 94, 22, 18, and 23 d for TPHP, TMPP, T2B4MP, T3B4MP, and T4B3MP (Table 1), and these values were very comparable to those obtained in the aqueous solutions at pH 9 and 11. However, when the pH was increased to 13, the degradation of Group C OP triesters increased. At pH = 13, the half-lives were estimated to be 0.0053, 0.027, 0.046, 0.066, and 0.067 d for TPHP, TMPP, T2B4MP, T3B4MP, and T4B3MP, and the constant rates were estimated at 130, 25.7, 15.2, 10.5, and 10.4 d−1, respectively (Table 1). The hydrolysis of TPHP was also investigated in a previous study, in which saturated solutions of the test substance were prepared by shaking an excess amount of the substance with distilled water or natural lake/river water, followed by filtration (11 μm) to remove most undissolved material and but pass through the microorganisms in natural water.25 Thus, this observed hydrolysis (19 or 3 d at pH 7 or 9 at 21 °C) of TPHP could be contributed to both abiotic and biotic activities, although the reported t1/2 was much lower than in our present study on TPHP. For TMPP, the half-life for TMPP was estimated to be 319 days at pH 7, 31.9 days at pH 8 and 3.19 days at pH 9.26 For OP triesters of Group D (Figure 1), our data suggested that TBOEP has hydrolytic stability similar to the OP triesters of Groups A and B, and was highly stable in solutions at pH 7, 9, or 11 (Figures 2 and 3). Increasing the pH to 13 resulted in a TBOEP half-life of 21 d (Table 1). In aqueous solutions at pH values ranging from 7 to 13, EHDPP hydrolysis was similar to Group C OP triesters, and the half-lives of EHDPP were 110, 130, 130, and 0.11 d at pH of 7, 9, 11 and 13, respectively. TTBPP hydrolysis was observed at all four pH levels with halflives of estimated to be 23, 22, 23, and 19 d in solutions of pH = 7, 9, 11, and 13, respectively (Figure 4 and Table 1). On the basis of the current literature, limited information is available for degradation of TBOEP, EHDPP or TTBPP. By use of predictive tools developed by USEPA, it is suggested that TBOEP may undergo environmental degradation based on estimated half-lives of 93−95 days at pH 9−5.22 However, in our present study, we did not observe significant depletion of TBOEP in aqueous solution at pH 7 to 11 over a period of 35 d. Using the same USEPA model, it was predicted that EHDPP would be hydrolyzed in the environment at a high alkaline pH of 9 based on an estimated half-life of 117 days,22 which is consistent with the results of our study.22 Since the phase-out of penta- and octa-BDE products, reports on the environmental occurrence of OP triesters are increasing, and most of the presently targeted 16 OP triesters have been detected in the aqueous environments. For example, all four Group A nonhalogenated OP triesters have been detected in environmental waters, and regardless of the monitored water types, the reported concentrations generally ranked as TNBP > TEP ≈ TPP > TEHP.9,27,28 Three OP triesters, TCEP, TCIPP and TDCIPP from Group B, have also been reported in various environmental water samples and at high concentrations.9 The greatest concentrations of chlorinated OP triesters were reported in raw water samples from a Japan Sea-based solid waste disposal site in which concentrations of TCEP, TCIPP and TDCIPP were 4.23−87.4, 11.3− 48.2, and 0.68−6.18 μg/L, respectively.8 Among the five OP triesters of Group C, TPHP has been detected in aqueous environments most frequently,8 and the greatest concentrations were reported in snow samples from an airport in northern Sweden, probably suggesting that the hydraulic liquid and oil used in aircraft were its sources to these outdoor environ8109

DOI: 10.1021/acs.est.6b02187 Environ. Sci. Technol. 2016, 50, 8103−8111

Article

Environmental Science & Technology Notes

dealkylated) phosphoric acid (H3PO4) was not observed. A similar hydrolysis pattern was also reported for TMPP, which was shown to be easily hydrolyzed in an alkaline medium to produce dicresyl phosphate and cresol, although TMPP was stable in neutral and acidic media at ambient temperatures.31 Strong linear correlative relationships were sometimes observed between log normalized OP triester degradation rate constants (k, d−1) and the leaving alcohol dissociation constants (pKa), i.e., plots between log k versus pKa of the leaving group.32−34 We also conducted a correlation analysis between OP triester log k (pH = 13 solutions) values and the leaving alcohol pKa values, and observed a negative (slope = −0.3010) and significant (p = 0.0087; r2 = 0.5535) linear curve (Figure S2 and Table S3). Thus, our results indicated that hydrolysis of OP triesters was partly driven by their leaving alcohol pKa values. 3.3. Environmental Implications. Given the ongoing reports on adverse effects, i.e., dermatitis,35 neurotoxicity,4,36 cardiotoxicity,37 genotoxicity, 38 as well as reproductive toxicity,39,40 of OP triester flame retardants and plasticizers, there is increasing scientific interest in the environmental fate of these chemicals. On the basis of numerous monitoring studies, most OP triesters currently under study, i.e., TEP, TPP, TNBP, TEHP, TCEP, TCIPP, TDCIPP, V6, TPHP, TMPP, TBOEP, and EHDPP, have been reported in river water, marine water, groundwater, tap water, sewage treatment plant (STP) effluents/influents or precipitation.9 The present study demonstrated that the hydrolytic stability to increasingly basic pH is in the order of Group A (with alkyl moieties) > Group B (OP triester with chlorinated alkyl moieties) > Group C (with aryl moieties) ∼ Group D (others). Thus, OP triesters with halogens and/or aryl groups are most unstable, and the most stable are purely alkyl-substituted (Group A) (even after a 35 day degradation period). These results indicate that, in the real environment, OP triesters, especially ones with aryl moieties, may exist more as OP diesters rather than triesters. In fact, the occurrence of OP diesters has been reported in human urine,20,41,42 which could be another source to the wastewater treatment plants and subsequently to aqueous environments. Current information on the toxicity of OP diesters is relatively rare. A recent study stated that OP diesters may have limited nuclear receptor activity compared to their parent triesters,43 but another study of comparison of toxicity between TPHP and DPHP found that DPHP altered more genes than TPHP.38 Future research in warranted on the environmental monitoring of OP diesters as well as triesters in aqueous environments, for example in wastewater treatment plant influents and effluents.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Environment Canada’s Chemicals Management Plan (CMP) (to R.J.L.) provided major funding for this project. Supplemental funding was from the Natural Science and Engineering Research Council (NSERC) of Canada (to R.J.L.).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.6b02187. Further details are given on the methods, assessment of adsorptive effects, OP diester hydrolysis studies, mass balance between OP triesters and diesters, and linear correlative relationships between log normalized k and pKa (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Tel.: 1-613-998-6696; fax: 1-613-998-0458; e-mail: robert. [email protected] (R.J.L.). 8110

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