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Organophosphate Esters in Sediment of the Great Lakes Dandan Cao, Jiehong Guo, Yawei Wang, Zhuona Li, Kang Liang, Margaret B. Corcoran, Soheil Hosseini, Solidea M.C. Bonina, Karl J. Rockne, Neil C. Sturchio, John P. Giesy, Jing-fu Liu, An Li, and Guibin Jiang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b05484 • Publication Date (Web): 13 Jan 2017 Downloaded from http://pubs.acs.org on January 15, 2017

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Environmental Science & Technology

Organophosphate Esters in Sediment of the Great Lakes

Dandan Cao,† Jiehong Guo,‡ Yawei Wang,*,†,# Zhuona Li,‡ Kang Liang,† Margaret B. Corcoran,∥ Soheil Hosseini,§ Solidea M. C. Bonina,§ Karl J. Rockne,§ Neil C. Sturchio,∥ John P. Giesy,⊥ Jingfu Liu,† An Li,*,‡ Guibin Jiang†

†

State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for

Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China ‡

School of Public Health, University of Illinois at Chicago, Chicago, Illinois 60612, USA

§

Department of Civil and Materials Engineering, University of Illinois at Chicago, Chicago,

Illinois 60612, USA ∥

Department of Earth and Environmental Sciences, University of Illinois at Chicago, Chicago,

Illinois 60612, USA ⊥

Department of Veterinary Biomedical Sciences and Toxicology Centre, University of

Saskatchewan, Saskatoon SK S7N 0W0, Canada #

University of Chinese Academy of Sciences, Beijing 100049, China

*

Corresponding authors

Yawei Wang: Tel: +86 010 62849124; e-mail: [email protected]. An Li: Tel: 1-312-996-9597; e-mail: [email protected].

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ABSTRACT

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This is the first study on organophosphate esters (OPEs) flame retardants and plasticizers in the

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sediment of the Great Lakes. Concentrations of 14 OPEs were measured in three sediment cores

4

and 88 Ponar surface grabs collected from Lakes Ontario, Michigan, and Superior of North

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America. The sum of these OPEs (Σ14OPEs) in Ponar grabs averaged 2.2, 4.7, and 16.6 ng g-1 dw

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in Lakes Superior, Michigan, and Ontario, respectively. Multiple linear regression analyses

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demonstrated statistically significant associations between logarithm concentrations of Σ14OPEs

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as well as selected congeners in surface grab samples and sediment organic carbon content as

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well as a newly developed urban distance factor. Temporal trends observed in dated sediment

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cores from Lake Michigan demonstrated that the recent increase in depositional flux to sediment

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is dominated by chlorinated OPEs, particularly tris(2-chloroisopropyl) phosphate (TCPP), which

12

has a doubling time of about 20 years. Downward diffusion within sediment may have caused

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vertical fractionation of OPEs over time. Two relatively hydrophilic OPEs including TCPP had

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much higher concentrations in sediment than estimated based on equilibria between water and

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sediment organic carbon. Approximately a quarter (17 tonnes) of the estimated total OPE burden

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(63 tonnes) in Lake Michigan resides in sediment, which may act as a secondary source releasing

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OPEs to the water column for years to come.

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■ INTRODUCTION

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Phosphorus is one of the most effective elements for retarding fire.1 Organophosphate esters

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(OPEs) are widely used as flame retardants in plastics, textiles, electronic equipment, furniture,

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and building materials.2, 3 Non-chlorinated alkyl phosphates have also been used as plasticizers,

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hydraulic fluids, lacquer, anti-foam agents, glues and extractants for some metals.3 Use of OPEs

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began in the early 20th century, then increased rapidly during the 1940s and continued for later

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decades.1 With the phasing out of polybrominated diphenyl ethers (PBDEs) during the 2000s,

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production and use of OPEs as alternative flame retardants have increased significantly in recent

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years.4, 5

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Similar to other additive flame retardants, OPEs inevitably enter the environment during

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their production, use or disposal. It was estimated that approximately 80% of the annual

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consumption of OPE hydraulic fluids escaped to the environment through leakage from

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hydraulic systems in the 1960s.1 OPEs have been detected in various environmental

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compartments, including indoor air and dust, the atmosphere, wastewater and sludge, surface and

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drinking water, sediment, and biological samples including human blood, milk, and placenta.3, 6-8

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In the United States, there are at least four manufacturers of OPE flame retardants,9 although

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locations of their production facilities are not clear.

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In the Great Lakes region, concentration of OPEs in air was found to be ~100- to

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~1200-fold greater than those of brominated flame retardants.10, 11 Similarly, concentrations of

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OPEs in water collected from Lakes Erie, Michigan, and Huron were greater than concentrations

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of brominated flame retardants.12 OPEs were also found in fish, herring gulls and their eggs in

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the Great Lakes region.13-16 Sediments, which can serve as a sink or potential source of organic 3 ACS Paragon Plus Environment


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pollutants, have been found to be contaminated with OPEs in locations around the world.17-20

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However, to our knowledge, there were no published reports on OPEs in the sediments of the

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Great Lakes prior to this study. This represents a major gap in our knowledge of the

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environmental fate of OPEs in the largest freshwater system in the world.

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To date, relatively hydrophobic chemical pollutant groups have been investigated

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extensively in environmental monitoring efforts, especially those using sediment of natural

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waters. In comparison, the behavior of persistent organic pollutants with polar functional groups

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is less understood. Molecules of OPEs possess both polar and nonpolar moieties, thus are

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considered amphiphilic. The octanol-water partition coefficient (Kow) spans 10 orders of

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magnitude among the 14 OPE flame retardants and plasticizers listed in the Supporting

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Information, Table S1. Once in sediment, all of these OPEs are considered persistent with

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estimated half lives ranging from months to >4 years.21

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In this paper, we report measured concentrations of the 14 OPE flame retardants and

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plasticizers in sediment samples collected from Lakes Ontario, Michigan, and Superior. The

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acquired data set enabled an examination of the spatial distribution pattern and temporal trend as

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well as the influencing factors. The distribution between sediment particulate matter and pore

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water in the sediment was assessed, based on equilibrium partitioning theory; and the

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dependence of such phase distributions on the physicochemical property of the analytes was

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examined.

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■MATERIALS AND METHODS

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Collection and Characterization of Sediments. Sediments were collected from Lakes

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Michigan, Superior, and Ontario during 2010 to 2013 onboard the USEPA Research Vessel R/V 4 ACS Paragon Plus Environment

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Lake Guardian. Sampling locations are shown in Figure 1, and site information is provided in

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Table S2. A total of 88 Ponar grab samples were collected. Sediment cores were collected at

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three sites in southern (M009), central (M024), and northern (M047) Lake Michigan. Detailed

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sampling methods are provided in the Supporting Information.

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Water and solid contents, porosity, wet and dry bulk density, particle density, and

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sediment organic carbon (OC) were measured by use of standard methods described elsewhere.22

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Segments of cores were dated by use of profiles of 210Pb, 241Am, 226Ra, and 137Cs.23 Activities of

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these isotopes were quantified by use of gamma spectroscopy using well-type HPGe detectors.

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Mass sedimentation rate and focusing factor were derived from activities of

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previously.24

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Identification and Quantification of OPEs. Concentrations were measured for 14 OPEs,

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including trimethyl phosphate (TMP), triethyl phosphate (TEP), tri-n-propyl phosphate (TPrP),

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tri-iso-butyl phosphate (TiBP), tri-n-butyl phosphate (TnBP), tris(2-butoxyethyl) phosphate

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(TBEP), tris(2-ethylhexyl) phosphate (TEHP), triphenyl phosphate (TPhP), cresyl diphenyl

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phosphate (CDPP), tri-m-cresyl phosphate (TCrP), 2-ethylhexyl diphenyl phosphate (EHDPP),

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tris(2-chloroethyl)

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tris(1,3-dichloro-2-propyl) phosphate (TDCP). Chemical Structures and basic information on

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each of the 14 OPEs are summarized in Table S1.

phosphate

(TCEP),

tris(2-chloroisopropyl)

phosphate

210

Pb as detailed

(TCPP),

and

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Laboratory procedures included extraction by ultrasonication, clean-up using solid phase

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extraction cartridges, and analysis by liquid chromatography-tandem mass spectrometry

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(LC-MS/MS). Details of these methods are described in SI, and instrument operational



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parameters are summarized in Table S3. Typical extracted ion chromatograms of a sediment

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extract and an OPE standard mixture are given in Figure S1.

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Quality Control. A matrix blank was a sediment sample from about 50 cm depth of a core

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collected in 2011 from the Yellow Sea of China. Fourteen OPEs of 20 ng each were spiked into

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1.0 g of the matrix blank with triplicates. The recoveries of 12 OPEs ranged from 78% to 108%

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(Table S3). The recoveries of EHDPP and TEHP were 47% and 27%, respectively. The relative

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standard deviations (RSD) of measured OPE concentrations in the matrix spike ranged from

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3%-13% except for TEHP, which had an RSD of 38%. Results reported in this paper were not

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adjusted by the spiking recoveries.

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Three field blanks (Na2SO4, pre-purified by baking at 500 °C overnight) from Lakes Superior,

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Michigan, and Ontario, respectively, were processed in the same manner as collected sediment

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samples. TCPP, TiBP, TnBP, and TBEP were detected in the field blanks in the concentration

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range of N.Q. – 0.44 ng g-1 dw. On average, Σ14OPEs in the field blank comprised 1%, 10%, and

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23% of the concentration in Ponar grab sediment samples from Lakes Ontario, Michigan, and

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Superior, respectively. One procedural blank (consisting of only extracting solvent) was

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analyzed with every batch of samples, and TCPP, TiBP, TPhP and TnBP were commonly

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detected at lower levels (3 but Lake Michigan > Lake Superior (Figure 1, Table S4 and

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S5). This rank order is similar to those previously reported for PBDEs,25-27, 32 PCBs,25-27 and

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other emerging flame retardants.33,34 In Lake Superior, concentrations of Σ14OPEs were greater at

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southeastern sites (S001, S002, and S110) than at other sites (Table S4). Proximity of these sites

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to the cities of Sault Ste Marie and Marquette, MI, suggests inputs from local sources. In herring

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gull eggs collected in Agawa Rocks which is near sites S001 and S002, OPE concentrations

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appear to have greatly increased from 2004 to 2010.16 In Lake Michigan, higher concentrations

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(Σ14OPEs > 10 ng g-1 dw) and inventories (> 60 ng cm-2) were found in the southeast (M009,

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M011, and M061) and near Sleeping Bear Dunes (M093a). Sampling sites near Chicago and

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Milwaukee (M010, M020 and M030) are in non-depositional zone of Lake Michigan; sediments

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collected from these sites were sandy with less organic carbon ( 0.35 and p < 6×10-4, excluding TCPP and TiBP for which r > 0.24 and p < 0.03).

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The UDF2 values are included in Table S2 for all the sampling locations.

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In addition to potential source locations, the concentrations of OPEs exhibited increases

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with increasing sediment OC content. Studies on sorption mechanisms of OPEs on soil and

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sediment particles are currently scarce.36, 37 While organic matter of the sediments accommodates

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nonpolar organics predominantly by hydrophobic interaction, stronger specific interactions

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involving electron donor/acceptor or hydrogen bond could occur between numerous polar

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functional groups of the natural organic matter and the polar moieties of sorbates such as OPEs.

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With UDF2 and sediment OC content as independent variables, multiple linear regression

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models were developed and are presented in Figure 3 and Table S8. Sediment OC and impact of

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human activities are the main controlling factors influencing the distribution of OPEs in the

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sediment across the three lakes, together accounting for approximately 59% of the variance in

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concentration for Σ14OPEs.

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There is evidence that OPEs undergo long-range atmospheric transport.10, 38 The OPEs

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targeted in this work vary widely in their volatility, with vapor pressure ranging from 10-5

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to >100 Pa (Table S1). We found that the relative abundance of OPEs differs largely among the

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three lakes (Figure S2). In Lake Superior, the congeners with the greatest concentrations were

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relatively volatile TiBP and TnBP. In comparison, heavier congeners TBEP and TCrP had

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greater detection rates and concentrations in Lakes Michigan and Ontario. The mean ratio (TiBP

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+ TnBP) / (TBEP + TCrP) in Lake Superior was 10.7, which is much greater than those in Lakes

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Michigan (1.9) and Ontario (0.46). With higher vapor pressures, TiBP and TnBP are more likely



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to enter air and reach remote locations via atmospheric transport. In contrast, the heavier TBEP

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and TCrP have greater Koc thus are more likely to bind to sediments after local inputs.

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Temporal Trends and Downward Mobility. OPEs have already been used for >150 years;3

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however, information for production volume during the earlier years is not available. As shown

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in Figure 2, in deeper sediments dated before year 1900, concentrations of Σ14OPEs were low

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and relatively constant but still greater than the MDL. After 1900, concentrations and fluxes of

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Σ14OPEs began to increase and peaked in the 1970s to 1980s. A decrease was observed in

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sediments deposited in the 1990s; however, a rebound after 2000 was observed at all three sites

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although the number of data points is limited. At sites M047 and M024, the highest

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concentrations and fluxes of Σ14OPEs were in the uppermost segments of cores. At site M009,

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Σ14OPEs in surficial segment dated 2009 was 28 ng g-1 dw, which is almost double 16 ng g-1 dw

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observed in sediments deposited during the late 1990s, although still below the historical peak of

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41 ng g-1 dw in core segment deposited during the 1960s (Figure 2). Net depositional fluxes of

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OPEs at these sites were estimated using equation [S3]. The recent depositional fluxes were 0.83,

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0.19, and 0.25 ng m-2 yr-1 for Σ14OPEs at sites M009, M024, and M047, respectively. The

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observations are clear evidence of the re-emergence of OPEs as environment pollutants. The

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recent increasing trend is consistent with observations of OPEs in other environmental matrices,7,

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39

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consumer goods.4, 40, 41 The observed increases in the environment may also reflect the tripled

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worldwide production volume of OPEs - from approximately 100,000 tonnes per year in 1992 to

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341,000 tonnes per year in 2007.42

and can be attributed to large scale replacement of PBDEs with OPE flame retardants in


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Different time trends were observed among targeted OPEs. The above-mentioned

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increases in recent years appear to be dominated by the rapid increase of chlorinated congeners

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(Figure 2, lower panels), especially TCPP. Exponential increases were observed for TCPP at all

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three coring sites since the 1950s, with apparent doubling time t2 around 20 years (Figure 4). It

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has been reported that the consumption of TCPP has continued to increase since the mid-1960s.43

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After the phase-out of the more toxic TCEP and with increasing use of polyurethane foam,

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production of TCPP (a replacement of TCEP) has increased markedly from the 1980s.43 In 2000,

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worldwide production of TCPP was 36,000 metric tons; in recent years, TCPP represents ~80%

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of the chlorinated OPEs produced in Europe.44 TCPP was found to be the dominant (at 180 ng/g)

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among eight OPEs in the sediment of Liverpool Bay and the Rivers Mersey and Tees in UK.45 It

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was detected in air, water and bird eggs from the Great Lakes region.10-14, 16 These observations

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highlight the need to establish benchmarks for TCPP in various environmental media to prevent

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adverse biological effects. In contrast to TCPP, all alkyl and aryl OPEs with the exception of

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TBEP decreased expoentially at site M009 after 1960, with apparent half-lives (t0.5) ranging from

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15 to 40 years (Figure 4). The production of TCrP was predicted to decline ~5% each year

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during the period of 1978-1982.46 TBEP displayed no trend over the core depth. Both increases

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and decreases in the depositional fluxes were observed at M024 and M047 depending on

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individual OPEs, but most trends were statistically insignificant (p > 0.05).

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For compounds that are relatively water soluble, therefore mobile in aqueous phase, the

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chronological profile recorded in dated sediment cores may not be in good agreement with the

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actual deposition history. For example, extensive downward transport of herbicide atrazine (log

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Koc = 2.35) as well as some perfluorinated compounds in sediment cores of Lake Michigan was

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reported.23, 24 Assuming equilibrium conditions, the distribution between particle and pore water 13 ACS Paragon Plus Environment


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of sediment depends on hydrophobicity of the chemical (log Koc), the solid-to-water mass ratio

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(rsw), and the organic content of the particles (foc), but is independent of chemical concentration.

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Thus, the fraction of the compound in pore water of the sediment can be estimated using the

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equation below.47 Figure 5A illustrates the results of applying this equation to the sediment of

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this work.

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f iw =

1 1 + rsw ⋅ K oc ⋅ f oc

[eq. 1]

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The targeted OPEs vary widely in their affinity to organic matter, with log Koc ranging

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from 1.10 to 6.87 (Table S1). It is clear from Figure 5A, while OPEs with log Koc > ~5 are

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almost completely associated with sediment particles, a significant portion of OPEs with log Koc

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< ~3 can be present in pore water. This may have facilitated post-depositional transport to deeper

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sediment due to downward diffusion over time. We observed that, in all three cores, the summed

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fraction of less hydrophobic congeners (log Koc < 3.5) exhibits a rapid rise to near unity in deeper

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sediment (Figure 5B). Higher downward mobility of the more polar congeners relative to those

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more hydrophobic is likely to cause fractionation of a chemical mixture over the depth of

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sediments in a sufficiently long time period. The occurrence of natural fractionation of the OPEs

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in lake sediments over time is yet to be confirmed by future studies. Congener-specific

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information of the production and use history, especially that before 1900, is highly desired.

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Transformation after burial is another process that may affect the apparent time trend

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observed in sediment cores. Non-chlorinated alkyl and aryl OPEs, such as TiBP, TnBP, TCrP,

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TPhP and TBEP, tend to be more readily biodegradable than chlorinated OPEs such as TCPP,


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TCEP and TDCP.7, 42 However, the occurrence of degradation of OPEs cannot be confirmed

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based on the chronological profiles recorded in the three sediment cores of this work.

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Load Estimation and Phase Distribution between Water and Sediment.

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Concentration data are available for 11 OPEs dissolved in bulk water samples collected in spring

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2012 from Lake Michigan, at a central location (latitude 44.2, longitude -87.3) and a southern

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location (latitude 41.9, longitude –87.6).12 By multiplying the volume of water in Lake Michigan

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(4,920 km3) with the reported ΣOPEs concentration (9.3 ng/L)12, the total load of OPEs

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dissolved in the lake water is roughly 46 tonnes. Compared with the total load of 17 tonnes in the

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sediment (Table 1), it appears that about 27% of the OPEs in Lake Michigan resides in the

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sediment. This percentage is about 20% if the calculation is based on the same set of seven

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congeners (TCEP, TCPP, TDCP, TPhP, TnBP, TBEP, and TEHP) reported in water12 and

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sediment. The sum of 63 tonnes does not include the amounts in suspended or settling

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particulates and living biota, for which data are not available.

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TCPP, TnBP, and TBEP had >50% detection frequency in this work in the Ponar grab

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sediment samples collected in lower (latitude

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