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Mineral- and base-catalyzed hydrolysis of organophosphate flame retardants – A potential major fate-controlling sink in soil and aquatic environments Yida Fang, Erin Kim, and Timothy J. Strathmann Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b05911 • Publication Date (Web): 15 Jan 2018 Downloaded from http://pubs.acs.org on January 15, 2018

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Environmental Science & Technology

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Mineral- and base-catalyzed hydrolysis of

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organophosphate flame retardants – A potential

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major fate-controlling sink in soil and aquatic

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environments

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Yida Fang, Erin Kim, Timothy J. Strathmann*

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Department of Civil and Environmental Engineering, Colorado School of Mines, Golden,

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Colorado 80401, United States

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Table of Contents (TOC)/Abstract Art

ACS Paragon Plus Environment


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Abstract: The ubiquitous occurrence of organophosphate flame retardants (OPFRs) in aquatic and

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soil environments poses significant risks to human health and ecosystems. Here, we report on the

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hydrolysis of six OPFRs and three structural analogues in the absence and presence of metal

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(hydr)oxide minerals. Eight of the target compounds showed marked degradation in alkaline

14

solutions (pH 9-12) with half-lives ranging from 0.02 – 170 d. Kinetics follow a second-order rate

15

law with apparent rate constants for base-catalyzed hydrolysis (kB) ranging from 0.69 – 42,000 M-

16

1

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2 y, except for tris(2,2,2-trichloroethy) phosphate), rapid degradation is observed in the presence

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of metal (hydr)oxide minerals, with half-lives reduced to

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28 29 30


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31


Introduction

32

Organophosphate flame retardants (OPFRs; Table 1) are widely used as additives in residential

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and commercial products, including building materials, furniture, plastics, and electronic

34

equipment.1 Their production over the past decade has increased exponentially due to the phaseout

35

of brominated flame retardants. Because OPFRs are not chemically bonded to the materials they

36

are added to, they readily leach over time.2 Major pathways for entry into soil and aquatic

37

environments include rainfall/snow melting, laundry wastewater, stormwater runoff, and

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desorption from soil,3-6 and OPFRs have been detected in a number of aquatic systems, including

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wastewater treatment plants, surface water, sediments and groundwater.7-12 For example, the

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estimated discharge of tris(1-chloro-2-propyl) phosphate (TCPP) to U.S. rivers is 400 metric tons

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per year.4

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Despite benefits to public safety and property losses,7 release of OPFRs into the environment

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poses significant risks to human health and ecosystems. OPFRs have been detected in human milk,

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urine, serum, and adipose tissues,13-16 and potential health effects include cancer, neurotoxicity,

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endocrine disruption, and reproductive impacts.17-22 They have also been detected in various

46

aquatic organisms, raising additional concerns about their effects on impacted ecosystems.23-25

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Risks associated with OPFR release is compounded by their recalcitrance to many biotic and

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abiotic degradation processes.26,27 To date, only limited information is available regarding their

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natural attenuation.12 Reports of biodegradation have been limited to studies of aromatic OPFRs

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in activated sludge28 and tris(2-chloroethyl) phosphate (TCEP) in enriched cultures.29

51

Investigations of photochemical degradation of OPFRs in lake and river water revealed variable

52

persistence, with chloroalkyl phosphates being the most recalcitrant group.30,31 Their

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organophosphate ester core structure suggests that hydrolysis may be an important natural



54

attenuation mechanism.32 However, a recent report indicates that aqueous hydrolysis of OPFRs is

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extremely slow at pH conditions of natural waters, with appreciable hydrolysis only being

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observed under highly alkaline conditions (e.g., pH 13).33 Similarly, reports of OPFR reactions

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with other relevant nucleophiles (e.g., reduced sulfur species) have been documented, but

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experiments were limited to elevated temperatures and nucleophile concentrations (e.g., t1/2 = 6.3

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h at 50C and 10 mM thiophenolate, pH 9).34

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Past reports show that dissolved metal ions and mineral surfaces can catalyze the hydrolysis of

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some organophosphate pesticides (e.g., Mevinphos) and warfare agents (e.g., Soman).35,36

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Elevated concentration of selected transition metal ions, including FeII, CoII, NiII, were found to

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catalyze the hydrolysis of some organophosphate pesticides.37 Moreover, several mineral phases

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were found to catalyze the hydrolysis of organophosphates (e.g., p-nitrophenyl phosphate and

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paraoxon).38,39 While this suggests a potential sink for OPFRs, the nature of the side chains in

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OPFR structures differ somewhat from those found in other organophosphates. Thus, further

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studies are needed to evaluate the potential role of common soil minerals in catalyzing hydrolysis

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of OPFRs and assess the relative importance of these pathways to natural attenuation.

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This study examines for the first time the effects of common metal (hydr)oxide soil minerals on

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the hydrolytic transformation of OPFRs. Specifically, the aqueous degradation of nine OP triesters

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(six OPFRs and three structural analogs, Table 1) were simultaneously monitored in the absence

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and presence of six naturally occurring minerals [iron, aluminum, titanium and silicon

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(hydr)oxides] over a range of pH conditions. Application of sensitive liquid chromatography-

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tandem mass spectrometry (LC-MS/MS) methods enabled simultaneous measurement of the

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hydrolysis of all nine OPFRs in solution at concentrations low enough to prevent competition for

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surface catalytic sites. High resolution mass spectrometry (quadrupole time-of-flight; LC-qToF-


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MS) was also used to identify and compare OPFR transformation products in both homogenous

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and heterogeneous reactions. Results from kinetics experiments and product identification studies

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were used to confirm the controlling reaction pathways, reveal structure-reactivity trends, and

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identify mineral characteristics that contribute to the catalysis of OPFR degradation.

81 Table 1. OPFRs and Structural Analogues General Structure

Structures used in this study Cl O R

O

Tris(1,3-dichloro-2-propyl) phosphate (TDCPP) a

Tris(2,3-dibromopropyl) phosphate (TBPP)

Tris(2-chloroethyl) phosphate (TCEP)

Tris(1-chloro-2-propyl) phosphate (TCPP)

Tris(2-butoxyethyl) phosphate (TBEP)

Triethyl phosphate (TEP)

Tris(2,2,2-trichloroethyl) phosphate (TTCEP) b

Tris(2,3-dichloropropyl) phosphate (2,3-TDCPP)

Triisopropyl phosphate (TIPP)

a

Structures with bolded titles are organophosphate compounds used as flame retardants (OPFRs) or precursors for plasticizers and OPFRs. b Structures with italicized labels are structural analogues of OPFRs used here for structure-reactivity analyses.



82

Materials and Methods

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Reagents and Minerals. A full listing of chemical reagents and mineral solids is provided in

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Supporting Information (SI). The target compounds studied here are collectively referred to as

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OPFRs, but three structures not used as flame retardants or precursors were included to examine

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structure-reactivity trends.

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Batch Reaction Kinetics. A series of abiotic batch experiments were conducted to characterize

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the degradation of OPFRs. Detailed reaction conditions are provided in Table S1 of SI. Reactors

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were covered with aluminum foil and conducted under dark and anoxic conditions inside a

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thermostatted anaerobic glovebox (25C, 95% N2, 5% H2; Pd catalyst: Coy Laboratory; control

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experiments confirmed similar rates of OPFR degradation are observed in air-saturated and anoxic

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conditions). Individual reactors contained 30 mL water amended with 5 mM pH buffer and 10

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mM NaCl. For homogeneous reactions (pH 2-12), degradation of OPFRs was monitored for 64 d.

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For heterogeneous reactions (pH 6), sufficient quantity of each mineral was added to individual

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reactors to yield suspensions with 15 m2/L of mineral surface area, and degradation of the OPFRs

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was monitored for 16 d. Additional reactions were conducted with goethite over a wider pH range

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(6-9), and reactions were monitored for 64 d. Mineral suspensions were continuously mixed during

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reactions by magnetic stirrers to maximize the exposure of mineral surface to the surrounding

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solution. After equilibrating individual batch reactors overnight, reactions were initiated by

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introducing a mixture of the nine OPFRs. The initial concentration of each OPFR was 200 μg/L,

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corresponding to a total initial OPFR concentration (∑[OPFR]0) of ~4 μM (detailed procedure for

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preparing the OPFR mixture provided in the SI-1). This was much lower than the estimated

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mineral surface site concentration of all selected minerals. For example, α-FeOOH, which has the

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lowest surface site density (1.68 nm-2) among the six minerals, has an estimated surface site


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concentration of ~42 μM under conditions examined.40-42 Furthermore, tests conducted with

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individual OPFRs showed no difference in reaction rates when experiments were conducted using

107

the OPFR mixture. Aliquots of reaction solution were periodically collected for LC-MS/MS

108

analysis. Procedures for quenching reactions in the collected aliquots are described in SI.

109

Separate batch reactions were prepared with individual OPFRs to identify transformation

110

products. The conditions of these reactions (Table S2 in SI) were designed to allow for collection

111

of sample aliquots after the parent OPFR degraded through ~1, 2, and 3 half-lives. At these times,

112

aliquots were collected and processed as described above before analyzing by LC-qToF-MS.

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Analysis and Modeling. Details of LC-MS/MS and LC-qToF-MS methods are provided in SI.

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Acid dissociation constants (pKa) of alcohol structures corresponding to the OPFR leaving groups

115

were predicted using SPARC (Archem LLC). Pseudo first-order rate constants for individual batch

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reactions were determined by linear regression methods; second-order rate constants for base-

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catalyzed and neutral hydrolysis of individual OPFRs were determined by least-squares regression

118

analysis using Scientist for Windows (Micromath).

119 120

Result and Discussion

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Homogenous Aqueous Hydrolysis. Degradation of OPFRs was observed in both homogeneous

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and heterogeneous systems, with degradation kinetics following a pseudo-first-order rate law

123

(Figures S3-S4 in SI). Rate constants (kobs, d-1) are summarized in Table S1 in the SI. In the absence

124

of mineral surfaces, OPFR degradation was limited to alkaline pH conditions. Eight of the nine

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target compounds (TTCEP, TDCPP, TBPP, 2,3-TDCPP, TCEP, TCPP, TBEP, and TEP) showed

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significant degradation (defined as 20%) within the 64-d monitoring period; no apparent

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degradation of TIPP was observed within this period regardless of the pH conditions. Figure 1A



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shows that measured kobs values increased dramatically with increasing pH, exhibiting a linear

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relationship with changing OH- concentration. Apart from TTCEP, no significant degradation was

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observed at neutral or acidic pH conditions. It follows that OPFR degradation can be described by

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a second-order rate law, where kobs is proportional to [OH-]: 

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d [OPFR ]  k obs [OPFR] = k B [OH - ][OPFR] dt

(1)

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where kB is the apparent second-order rate constant for reaction with OH- (M-1 d-1). Such a rate law

134

is consistent with a base-catalyzed ester hydrolysis mechanism, wherein nucleophilic attack on the

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electrophilic phosphorus atom by OH- is followed by ester bond cleavage and release an alcohol

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leaving group.43 The reaction produces the corresponding phosphate diester and an alcohol as

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transformation products:

(2) 138 139

Solid lines in Figure 1A show the fit of Eq 1 to measured data, and the resulting kB values are

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provided in Table 2. It follows that hydrolysis of OPFRs is exceedingly slow at neutral and acidic

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conditions, and the reaction is only an important contributor to natural attenuation in highly

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alkaline waters. For the OPFR exhibiting the highest reactivity, TTCEP, appreciable rates of

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degradation are also observed at neutral and acidic pH conditions. Below pH 7, rates are

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independent of pH conditions, consistent with H2O reacting as the nucleophile. Thus, Eq 1 can be

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modified to consider both nucleophiles reacting in parallel:


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

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d [OPFR]  kobs [OPFR] = (kH2O [H 2 O]  k B [OH - ])[OPFR] dt

(3)

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Similar rate laws have been documented for carboxylic esters and carbamates.44,45 Comparing fit-

148

derived kB and kH2O values (Table 2), it is apparent that H2O is a much weaker nucleophile for

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TTCEP than OH-. This is consistent with the relative reactivity of different nucleophiles reported

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previously,43,46 as commonly described by the Swain-Scott equation:

k  log  Nu  =s  nNu,CH3Br  kH 2O 

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(4)

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where [kNu /kH2O] is the ratio of second-order rate constants for different nucleophiles in comparison

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to kH2O (representing reactivity of the background solvent), nNu,CH Br is the nucleophilicity of the

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attacking nucleophile determined empirically from reactions with CH3Br, and s is the nucleophilic

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sensitivity of the reaction of interest compared to the reference CH3Br reaction. The value of

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nNu,CH Br for OH- is 4.2, indicating that OH- is 104.2-fold more reactive with CH3Br than H2O (s =

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1 for reactions with CH3Br). Application of Eq 4 together with the reference nucleophilicity values

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for OH- (4.2) and H2O (1.0) provides an estimate of s = 1.92 for reaction with TTCEP. This

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suggests that nucleophile strength plays a greater role in reactions with TTCEP than the reference

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reaction with CH3Br.47 Previous studies on hydrolysis of organophosphate esters trimethyl

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phosphate (TMP) and TEP also reported greater sensitivity to nucleophile strength than CH3Br,

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exhibiting s values of 1.35 and 1.20, respectively.32,48 Since TMP and TEP have comparable s×

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nNu, CH Br values, we might assume other OPFRs that have similar structures with TTCEP will

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exhibit similar sensitivity towards OH- and H2O. Using this assumption, the kH2O value of TDCPP

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(the third most reactive target OPFR) can be estimated to be 110-6 M-1d-1, corresponding to a half-

3

3

3



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life of ~30 y at pH < 10, consistent with the fact that no TDCPP degradation was observed within

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the 64-d monitoring period under these conditions.

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To our knowledge, this is the first confirmation that degradation of commonly detected OPFRs

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follow a second-order rate law for base-catalyzed hydrolysis. Although a recent report by Su and

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coworkers33 included measurements of reaction half-lives of TDCPP and TCEP at pH 13, they

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were unable to measure rate constants at lower pH conditions (pH 7-11) to confirm a first-order

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dependence on [OH-]33. This is surprising in light of the present findings where the observed half-

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lives at pH 11 (~5.5 d for TDCPP, ~15 d for TCEP) are much shorter than the time period that the

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authors monitored. The source of the discrepancy is unclear, but findings from the current study

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are consistent with the expected pH-dependence for base-catalyzed hydrolysis reactions (i.e., 10-

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fold change in kobs value with each integer change in pH conditions). A full comparison between

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the half-lives of other OPFRs are provided in Figure S5 in SI. In the absence of minerals, the

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observed half-lives of most OPFRs ranged from 0.02 – 170 d between pH 9-12. However, under

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neutral pH conditions, most OPFRs appear to be rather inert. Table S3 shows that, apart from

180

TTCEP, the estimated half-lives of the target OPFRs range from 1 – 5,800 years for pH 6 - 8. Thus,

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in the absence of a suitable catalyst, OPFR hydrolysis in natural aquatic environment is not

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expected to be an important natural attenuation mechanism.


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183 Table 2. Second-order rate constants for base-catalyzed and uncatalyzed hydrolysis of OPFRs measured in homogeneous solutions. kB (M-1d-1) kH2O (M-1d-1) TTCEP 3.48 (±0.86) x 10-4 4.20 (±0.90 a)  104 TDCPP

1.42 (±0.27)  102

TBPP 2,3-TDCPP TCEP TCPP TBEP TEP

1.38 (±0.18)  103 1.27 (±0.19)  102 7.65 (±3.20)  101 2.46 (±0.60)  100 4.53 (±0.21)  100

TIPP

6.95 (±1.46)  10-1 c < 3.47  10-1

d

< 1.25  10-4 b < 1.25  10-4 < 1.25  10-4 < 1.25  10-4 < 1.25  10-4 < 1.25  10-4 < 1.25  10-4 < 1.25  10-4

a

Uncertainties represent one standard deviation of the least squares fit-derived values determined using Scientist for Windows (Micromath). b

Estimated by dividing the minimum observable kobs threshold value by H2O concentration (55.5 M). c Estimated based on kobs value measured at pH 12. d Estimated by dividing the minimum observable kobs threshold value by [OH-] at the highest pH examined (pH 12).



A

B

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Figure 1. (A) Influence of pH conditions on the kinetics of homogeneous aqueous hydrolysis of OPFRs. (B) Time courses for degradation of TDCPP in the presence of metal (hydr)oxide soil minerals at pH 6. Reaction conditions: [OPFR]0 = 200 g/L, 25C, 15 m2/L mineral solid surface area, 0.01 M NaCl. Solid lines in panel A represent model fits of Eqs 1 and 3 (resulting second-order rate constants listed in Table 2). The criterion threshold for the minimum value of kobs that could be accurately measured corresponds to a half-life of 200 d. Lines in panel B represent pseudo-first order model fits (resulting rate constants listed in Table S1 in SI). Error bars represent the triplicate-averaged standard error. Downward arrow in panel A indicates that the rate constant is an upper-bound estimate. ACS Paragon Plus Environment

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Mineral-Catalyzed Hydrolysis. Previous reports have shown that selected soil minerals can be

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potent catalysts of hydrolysis reactions,35,36,39 so a series of metal (hydr)oxide phases were

187

screened for their effects on OPFR degradation. Figure 1B shows a representative group of time

188

courses for TDCPP degradation observed at pH 6 in the absence and presence of metal (hydr)oxide

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minerals (other timecourses provided in Figure S5 in SI). Whereas the predicted half-lives (Table

190

3) of most OPFRs in homogeneous solution at pH 6 are estimated to exceed 10 y, half-lives are

191

reduced to 50 d). d Uncertainties represent triplicate-averaged standard errors.

192 193

Among the minerals screened, OPFRs degraded most rapidly in the presence of Fe(III)

194

(hydr)oxides (α-FeOOH, γ-FeOOH, and α-Fe2O3), whereas no apparent catalysis was observed in

195

the presence of 15 m2 L-1 of the aluminum and silicon oxides (γ-Al2O3 and SiO2). TiO2 catalyzed

196

degradation of TBPP and TBEP, but had no apparent effect on the other OPFRs. Within the three

197

Fe(III) (hydr)oxides, γ-FeOOH displayed greatest catalytic effect. The variation in reaction rate

198

between Fe(III) (hydr)oxides is possibly due to the different position of Fe(III) centers within each

199

oxide complexes,49 which can generate different catalytic effects to selected compounds.38 The

200

percentage of wetting surface on each Fe(III) (hydr)oxides is another reason that might affect their



201

ability to catalyzing hydrolytic reactions.50 As with homogeneous solutions, no TIPP degradation

202

was observed with any of the mineral surfaces, providing further indication that this particular

203

structure is very resistant to hydrolysis. More surprising was that no degradation of TCEP or TEP

204

was observed in mineral suspensions, despite being sensitive to base-catalyzed hydrolysis. In

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general, much less variation in rates of degradation are observed among the target OPFR structures

206

in suspensions of individual minerals (i.e., range of measured kobs values varies ~10-fold, Figure

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2B) than in the homogeneous base-catalyzed reactions (range of measured kobs values varies

208

~100,000-fold, Figure 1A). The compressed range of reactivities is attributed to decreased

209

importance of leaving group acidity in the rate-controlling step of the surface-catalyzed hydrolysis

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reactions. This is consistent with OPFR-surface coordination promoting ester bond cleavage,

211

possibly by stabilizing formation of the resulting alkoxide (-OR) group. The finding that mineral-

212

catalyzed hydrolysis is observed for TCPP but not TCEP, despite the latter exhibiting higher base-

213

catalyzed reactivity, suggests that surface-TCPP interactions have a more pronounced effect on

214

ester cleavage than surface-TCEP interactions. Hydrolysis of other classes of organophosphate

215

contaminants (e.g., pesticides, nerve agents) has also been reported to be associated with metal

216

oxide surfaces. For example, the presence of Al and Ti oxides increased the hydrolysis rate of

217

chlorpyrifos-methyl oxonate by ~10-fold at pH 7 while the presence of goethite caused a ~3-fold

218

increase.39 On the other hand, the presence of Fe(III) (hydr)oxide minerals reduced the half-life of

219

p-nitrophenyl phosphate, a phosphate monoester, by more than 30-fold, and Al and Ti oxides

220

caused similar catalytic effects as Fe(III) (hydr)oxides.38 In contrast, no mineral catalysis was

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observed for degradation of the pesticide demeton-S.51 These studies further support the

222

conclusion that hydrolysis-promoting interactions between OPFRs and mineral oxides are

223

compound and phase selective.


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Goethite (α-FeOOH) was further used as a representative Fe(III) (hydr)oxide mineral to examine

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the influence of solution conditions and heterogeneous reaction mechanism. Figure 2A shows the

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concentration change of TDCPP over the 64-d monitoring period in goethite suspensions

227

(comparable kinetic profiles for other OPFRs are provided in Figure S6). Most of the target OPFRs

228

showed accelerated degradation in the presence of goethite over the studied pH range (pH 6-8),

229

but the pH-dependent trends (Figure 2B) are muted in comparison to the effects of pH on

230

homogeneous solution. Furthermore, kobs values increase slightly when pH drops from pH 8 to 6,

231

opposite to the direction of the trend observed in alkaline homogenous solutions. The lack of strong

232

pH dependence is similar to earlier reports organophosphate hydrolysis catalyzed by metal

233

(hydr)oxides surfaces,38,39 and consistent with the pH-independent adsorption of nonionic

234

solutes.52,53 Like other nonionic organics, adsorption of the target OPFRs in the aqueous metal

235

(hydr)oxide suspensions is generally weak (i.e., extent of adsorption was too low to measure, being

236

less than the variability in the analytical measurements),52 but surface interactions are still critical

237

to promoting hydrolysis reactions. The lack of strong pH dependence also suggests that H2O

238

(rather than OH-) is the attacking nucleophile. The small variation in degradation rates observed

239

with changing pH may result from changes in weak hydrogen bonding interactions that occur upon

240

protonation/deprotonation of surface functional groups.

241

The range of OPFR half-lives were between 0.56 and 40 d, with TTCEP at pH 6 being the

242

shortest and TCPP at pH 8 being the longest in the goethite suspensions (Figure 2B). Their

243

corresponding kobs values were calculated and compared with the kobs values estimated in

244

homogeneous solutions at the same conditions (Figure 2C). The significant difference (i.e. 10 to

245

10,000,000-fold) between those kobs values shows goethite-catalyzed hydrolysis is much faster at

246

environmentally relevant conditions.



A

B

C

247

Figure 2. Influence of pH conditions on goethite-catalyzed hydrolysis of OPFRs. Detailed reaction conditions listed in Table S1; [OPFR]0 = 200 μg/L. (A) Observed time courses for dissipation of TDCPP, (B) Effect of pH on kobs values, (C) Differences between log(kobs) values measured in goethite suspensions versus homogeneous solutions at the same pH conditions. Error ACS Paragon Plus Environment bars represent triplicate-averaged standard errors.

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Transformation Products. Alkaline homogeneous batch reactors and goethite suspension (pH

249

6) batch reactors were prepared and amended with individual target OPFRs, and samples were

250

collected after ~1, 2, and 3 reaction half-lives to screen for transformation products (TPs) using

251

high resolution LC-qToF-MS. Potential TPs were analyzed from each qToF-MS signal and

252

compared with the unreacted samples as controls to avoid biased interpretation of the raw data,

253

despite hydrolytic TPs (i.e., OP diester, OP monoester) being the most plausible in this study.

254

Table 4 summarizes the TPs detected for each target OPFR. This analysis shows that all OPFR

255

transformation products detected were consistent with hydrolysis reactions, verifying that OH- and

256

goethite are acting principally to catalyze such transformations. Figure 3A-B show qToF-MS

257

spectra of two TPs identified during TDCPP reactions in goethite suspension (corresponding

258

MS/MS spectral fragmentation patterns are provided in SI). Figure 3C shows how MS spectral

259

peak areas associated with these two TPs grow over three consecutive half-lives of TDCPP

260

degradation, providing further confirmation that they are transformation products resulting from

261

degradation of the parent OPFR. qToF-MS analysis identified the corresponding OP diesters as

262

the major TP formed over the first three half-lives for most OPFRs in both homogeneous and

263

heterogeneous reactions, similar to the degradation products observed for other OP compounds.54

264

This indicates that the diesters are less reactive than their parent triesters. For TCPP, the

265

corresponding monoester was also detected in alkaline homogeneous solution, consistent with

266

further hydrolysis of the initial diester TP. Moreover, phosphate, the terminal hydrolysis

267

endproduct, was also detected in selected homogeneous alkaline reactions (TDCPP, 2,3 TDCPP,

268

and TCEP) and goethite-catalyzed reactions (TDCPP, 2,3 TDCPP, and TCPP), despite the strong

269

affinity between Fe(III) based minerals and phosphate.55,56 This is due to the very high sensitivity

270

of the analytical method used (LC-qToF-MS) in this study, which can measure many aqueous



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271

constituents to sub-ppt levels. Tests confirmed that sufficient residual aqueous phosphate remains

272

in solution after equilibration to be detected by LC-qToF-MS (e.g., 0.11 µM aqueous phosphate is

273

detected after equilibrating 0.46 µM phosphate with 1 g/L goethite for 2 d). The fact that the

274

corresponding monoesters were not detected under any of these reaction conditions indicates that

275

either the monoesters are unstable and rapidly hydrolyze to the phosphate terminal product or that

276

the phosphate ion results from an alternative reaction pathway for the parent OPFR or diester. OP

277

monoester and phosphate have been detected previously as hydrolysis products for other

278

organophosphate contaminants, but only at more extreme solution conditions (e.g., 75 0C, pH 4).57

279

To our knowledge, this is the first time OP monoester and phosphate have been confirmed as TPs

280

of organophosphate hydrolysis at environmentally relevant conditions.

281 Table 4. OPFR transformation products identified by LC-qToF-MS in alkaline homogeneous solutions and in goethite suspensions at neutral pH conditions.a Target OPFR TTCEP TDCPP TBPP 2,3-TDCPP TCEP TCPP TBEP TEP TIP

Diester TP Alkaline -FeOOH solution suspension             

Monoester TP Alkaline -FeOOH solution suspension



a

PO43- TP Alkaline -FeOOH solution suspension 



 

 

Detailed conditions for alkaline solution and goethite suspension reactions for each target OPFR are listed in Table S2 in SI.


Page 19 of 33


A

B

C

282

Figure 3. LC-ESI(+)-qToF-MS spectra for TPs identified during TDCPP reaction in goethite suspensions. Reaction conditions are provided in Table S2 in SI. (A) MS spectrum of the TDCPP diester observed at t = 3 day, (B) MS spectrum of phosphate observed at t = 6 day, (C) Timecourse for TDCPP degradation and the corresponding evolution of peak areas associated with the two TPs. Confirmation MS/MS spectra are provided in the SI. Error bars represent the triplicateaveraged standard errors.



Page 20 of 33

283

Influence of OPFR Structure. Results presented in Figures 1-2 demonstrate that both base-

284

and mineral-catalyzed degradation reaction rates are dependent upon OPFR structure. This is

285

especially the case for base-catalyzed reactions where kB values determined for the target OPFRs

286

range from 12), and the strong dependence on the estimated pKa values for base-catalyzed hydrolysis


Page 24 of 33

Page 25 of 33


368

reactions support mechanism 2. Clearly, formation of the unstable alkoxide intermediate during

369

ester bond cleavage is a major rate-determining factor, so direct surface interactions with this group

370

may be critical to promoting cleavage under non-alkaline conditions. Still, mechanism 1 cannot be

371

excluded, and additional experimental and theoretical investigations are needed to further

372

discriminate between the two pathways.

373 374

Environmental Significance

375

Growing detection of OPFRs in natural environments has increased concerns related to

376

potentially adverse human health and ecosystem effects, which are exacerbated by the limited

377

information on their attenuation mechanisms in natural systems. Given their ester-based structure,

378

hydrolysis was a logical candidate for controlling degradation of OPFRs in aquatic systems.

379

However, while hydrolysis of OPFRs was previously documented in highly alkaline solutions,

380

results presented here confirm that aqueous-phase hydrolysis is not expected to be a major sink for

381

these contaminants in aquatic systems. At the same time, results presented for the first time here

382

document the potential importance of common Fe(III) hydr(oxide) minerals as catalysts of OPFR

383

degradation via hydrolytic pathways. These phases are ubiquitous in many soils, so these reactions

384

may be a critical sink for OPFRs released into such systems (e.g., by stormwater percolation,

385

irrigation with treated wastewater). The fact that three different Fe(III) hydr(oxide) minerals all

386

exert significant catalytic effects suggests that such reactions will be widespread in Fe-rich soil

387

systems. Moreover, high surface area Si and Al oxide minerals failed to exert a catalytic effect,

388

indicating that OPFRs will be more persistent in soil and aquifer systems dominated by these

389

phases. Such findings need to be confirmed with whole soil samples of varying mineralogical

390

composition.



391

Findings from this study also indicate that OPFR diesters are likely to be a stable product of

392

OPFR hydrolysis processes, both in alkaline waters and Fe-rich soil systems. It follows that

393

concentrations of the diesters may exceed the parent OPFRs in many environments, yet very

394

limited information is available on the fate or toxicity of OP diesters in comparison to the parent

395

triesters, and their occurrence has been detected in human bodies, promoting environmental health

396

concerns.64,65 Based on these findings, it is recommended that OPFR occurrence studies add the

397

analysis of diesters to their protocols to provide a more comprehensive assessment of mass fluxes

398

in important environmental systems. This also leads to a recommendation for development of

399

authentic reference standards for diester TPs.

400

The observed structure reactivity trends, especially in alkaline solutions, can be potentially

401

applied to select less persistent OPFRs or design new OPFRs that will have lower persistence when

402

released into natural environments. The non-halogenated structures (TEP and TIPP), which

403

possess the least acidic leaving groups, are resistant to both alkaline and mineral-catalyzed

404

hydrolysis reactions. Thus, use of TEP might be discouraged based upon this finding. At the same

405

time, halogenated structures are often found to be less biodegradable than non-halogenated

406

analogues, but further study is needed to address the relative importance of biodegradation

407

processes and structure-reactivity trends before a definitive recommendation can be made.

408


Page 26 of 33

Page 27 of 33


409

Associated Content

410

Supporting Information. Experimental conditions and results of kinetics experiments, sample

411

quenching and storage protocols, description of analytical methods and characteristics of

412

transformation products and their MS/MS spectrums, figures illustrating degradation of OPFRs in

413

homogeneous and heterogeneous reactions, prediction and comparison of OPFRs half-life in the

414

absence of minerals, and a figure illustrating the relationship between kobs and leaving group

415

acidity in mineral suspensions are provided. This material is available free of charge via the

416

Internet at http://pubs.acs.org.

417 418

Author Information

419

Corresponding Author

420

*E-mail: [email protected]; phone: (303)-384-2226

421 422

Acknowledgments

423

Financial support was provided by the United States Department of Agriculture (Agriculture and

424

Food Research Initiative, Number 2014-67019-24024). Xin Xiao (Zhejiang University), Karl

425

Oetjen, and Christopher Higgins (Colorado School of Mines) provided assistance with mass

426

spectrometry analysis.

427 428 429 430 431



432

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