Application of Dual Carbon–Bromine Isotope Analysis for Investigating

Feb 27, 2015 - Zuckerberg Institute for Water Research, Department of Environmental Hydrology and Microbiology, The Jacob Blaustein Institutes for Des...
0 downloads 0 Views 452KB Size
Subscriber access provided by UNIV LAVAL

Article

Application of dual carbon-bromine isotope analysis for investigating abiotic transformations of tribromoneopentyl alcohol (TBNPA) Faina Gelman, Anna Kozell, Yinon Yecheskel, Noa Balaban, Ishai Dror, Ludwik Halicz, and Zeev Ronen Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es504887d • Publication Date (Web): 27 Feb 2015 Downloaded from http://pubs.acs.org on March 4, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Environmental Science & Technology is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 20

Environmental Science & Technology

1

Application of dual carbon-bromine isotope analysis for investigating abiotic

2

transformations of tribromoneopentyl alcohol (TBNPA)

3 4

Anna Kozell1,2, Yinon Yecheskel3, Noa Balaban4, Ishai Dror3, Ludwik Halicz1, Zeev

5

Ronen4 and Faina Gelman1*

6

1

Geological Survey of Israel, 30 Malhei Israel st., Jerusalem 95501, Israel

7

2

Department of chemistry, The Hebrew University, Jerusalem 91904, Israel

8

3

Department of Earth and Planetary Sciences, Weizmann Institute of Science, Rehovot,

9

Israel.

10

4

11

Microbiology, The Jacob Blaustein Institutes for Desert Research, Ben-Gurion University

12

of the Negev, Sede Boqer Campus, 84990, Israel

Zuckerberg Institute for Water Research, Department of Environmental Hydrology and

13 14 15

Correspondence to FG: e-mail: [email protected]; tel: +972-2-5314208

16 17

Abstract

18

Many of polybrominated organic compounds, used as flame retardant additives, belong

19

to the group of persistent organic pollutants. Compound-specific isotope analysis is one

20

of the potential analytical tools for investigating their fate in the environment. However,

21

the isotope effects associated with transformations of brominated organic compounds are

22

still poorly explored. In the present study, we investigated carbon and bromine isotope

23

fractionation during degradation of tribromoneopentyl alcohol (TBNPA), one of the

24

widely used flame retardant additives, in three different chemical processes:

25

transformation in aqueous alkaline solution (pH 8); reductive dehalogenation by zero-

26

valent iron nanoparticles (nZVI) in anoxic conditions; and oxidative degradation by H2O2

27

in the presence of CuO nanoparticles (nCuO).Two-dimensional carbon-bromine isotope

28

plots (δ13C/Δ81Br) for each reaction gave different process-dependent isotope slopes

29

(Λ(C/Br)): 25.2 ± 2.5 for alkaline hydrolysis (pH 8); 3.8 ± 0.5 for debromination in the

30

presence of nZVI in anoxic conditions; and ∞ in the case of catalytic oxidation by H2O2

31

with nCuO. The obtained isotope effects for both elements were generally in agreement 1 ACS Paragon Plus Environment

Environmental Science & Technology

1

with the values expected for the suggested reaction mechanisms. The results of the

2

present study support further applications of dual carbon-bromine isotope analysis as a

3

tool for identification of reaction pathway during transformations of brominated organic

4

compounds in the environment.

5 6

2 ACS Paragon Plus Environment

Page 2 of 20

Page 3 of 20

1 2 3 4

Environmental Science & Technology

TOC/Abstract Art

TBNPA

3 ACS Paragon Plus Environment

Environmental Science & Technology

1

Introduction

2

Brominated organic compounds (BOCs) are produced in large quantities and applied in a

3

variety of consumer and industrial products [1]. Due to their wide use in everyday life,

4

they accumulate in the environment and living organisms [2-4]. Since many BOCs are

5

considered toxic for humans [5], evaluation of their degradation is of great importance for

6

environmental risk assessment and for effective treatment of the contamination.

7

During the last decades, compound-specific isotope analysis (CSIA) has been

8

demonstrated as an effective tool for examination of reaction mechanisms and deducing

9

compound degradation pathways in the environment [6]. This approach is based on the

10

fact that chemical bonds between the heavier isotopes are slightly stronger and, thus, are

11

broken slower than the bonds between the lighter isotopes. As a result, the remaining

12

(still non-reacted) fraction of the substrate becomes enriched by the heavier isotopes. The

13

main benefit of using the isotope approach is the possibility to track the fate of the

14

contaminants in complex environmental matrices without the need to identify and

15

quantify intermediates and products. The isotope enrichment factor (ε) may serve as a

16

parameter for a specific pathway. It can be calculated by a Rayleigh equation using the

17

relation between changes in isotopic composition and contaminant concentration [7].

18

Recently it has been demonstrated that multi-elemental isotope data are even more

19

informative and can be used for mechanism evaluation [7-16].

20

Although, multi-element isotopic effects associated with degradation of several types of

21

organic contaminants have been studied extensively, data for brominated organic

22

contaminants are still limited. Recently, Gas chromatograph – Multicollector-Inductively

23

Coupled Plasma Mass Spectrometer (GC-MC-ICPMS) has been introduced for bromine

24

isotope ratio analysis in organic compounds [17-19] and allowed determination of

25

bromine isotope enrichment factors in several enzymatic and chemical transformations

26

[16, 20-23].

27

When C-Br bond cleavage is the rate-limiting step of the transformation, normal carbon

28

and bromine apparent kinetic isotope effects (AKIE>1) (for AKIE calculation see eq.3

29

Experimental/Calculations) are anticipated; in the cases where other bonds cleavage (e.g.

30

C-C or C-H) are the rate-limiting step, no or insignificant bromine isotope effect is

31

expected. 4 ACS Paragon Plus Environment

Page 4 of 20

Page 5 of 20

Environmental Science & Technology

1

The magnitudes of the carbon and bromine kinetic isotope effects (KIEs) upon the

2

cleavage of C-Br bond estimated by using semi-classical Streitweiser limit model (see

3

Supporting Information) are 1.042 and 1.002, respectively. Streitweiser Limits for carbon

4

and chlorine KIEs during C-Cl bond breakage are 1.057 and 1.013, respectively [24].

5

Thus, for brominated compounds showing similar transformation pathways, the same or

6

slightly smaller carbon isotope effect is expected for brominated compounds, while Br-

7

KIE is likely to be about six times smaller than Cl-KIE.

8

The

9

tribromoneopentyl alcohol (TBNPA) (Scheme1), which is one of the most abundant

10

groundwater contaminants in the vicinity of the chemical industrial complex in the Negev

11

desert in Israel; concentrations up to 1 mg L-1 were detected in several monitoring wells

12

in that area [25]. Due to its relatively high water solubility (2 g L-1 at 25⁰C), TBNPA is

13

expected to migrate in the soil-aquifer zone with minimal retardation [26] . TBNPA is

14

considered toxic for humans and aquatic environment [27]. This compound belongs to

15

the group of brominated aliphatic compounds (e.g. dibromoneopentyl glycol,

16

tribromoneopentyl phosphate) which are widely used as additives in polymeric materials

17

and as reactive intermediates for production of high molecular weight brominated flame

18

retardants. The abundance of these compounds in environment, such as landfills and

19

other waste storage facilities is currently unknown.

20

considered as a representative of a broader group of brominated aliphatic compounds,

21

which often end-up as persistent environmental contaminants. To date the information

22

about the fate and degradation pathways for these compounds is scarce.

23

Several studies dealing with biotic and abiotic natural degradation of TBNPA in the

24

environment [25, 28] as well as its degradation by novel composite materials have been

25

reported in the literature during the last years [29, 30]. However, isotope effects

26

accompanying TBNPA decomposition in either biotic or abiotic reactions have not been

27

studied yet.

28

In the present study we investigate carbon and bromine isotope fractionation during

29

different scenarios of abiotic degradation of TBNPA, trying to find a correlation between

30

reaction pathways and observed isotope effects. The degradation reactions of TBNPA

31

investigated in the present study were: (1) transformation in alkaline solution (pH 8); (2)

present

study

focuses

on

a

specific

brominated

organic

compound,

To some extent TBNPA can be

5 ACS Paragon Plus Environment

Environmental Science & Technology

1

debromination by nZVI in anoxic conditions; and (3) oxidation by H2O2 in the presence

2

of nCuO catalyst. Mechanistic aspects of all these reactions have been studied previously

3

in laboratory experiments. While TBNPA transformation in slightly alkaline water is a

4

possible natural attenuation scenario [25], the two other processes represent engineered

5

systems for contamination treatment [29, 30]. Ezra et al. [25] reported that an

6

intramolecular nucleophilic substitution is the main pathway of TBNPA transformation in

7

alkaline solution (Scheme 1A). As C-Br bond cleavage is a rate-limiting step of the

8

reaction [25], both carbon and bromine isotope effects are expected during this process.

9

Reductive debromination mechanism has been proposed for TBNPA decomposition by

10

nZVI in anaerobic conditions [30]. It is usually accepted that reactions of alkyl halides

11

with ZVI via hydrogenolysis involve a single electron transfer (SET) leading to radical

12

formation as a first rate-limiting step [31]. The same scenario can be assumed for

13

TBNPA transformation by nZVI (Scheme 1B); in this case both carbon and bromine

14

isotope effects are expected. For oxidative decomposition of TBNPA by H2O2 catalyzed

15

by nCuO, it has been suggested [30] that reaction is based on the attack of TBNPA

16

molecule by reactive oxygen species (hydroxyl and superoxide radicals). Although the

17

reaction mechanism is still not fully understood, it can be hypothesized that TBNPA

18

decomposition occurs via proton abstraction from C-H bond, similarly to other processes

19

of oxidative decomposition of halogenated alkanes [32, 33] (Scheme 1C).

20

Revealing the isotope effects during the processes might provide additional information

21

and lead to a deeper understanding of the mechanistic aspects of all these reactions. We

22

expect that the data obtained will be in the future extrapolated to other brominated

23

organic compounds, transformed in the environment by similar pathways. This study

24

aims also to evaluate the potential of dual carbon-bromine isotope analysis for assessing

25

the degradation of TBNPA and analogous aliphatic brominated organic compounds in

26

environmental systems.

27 28 29 30 31 6 ACS Paragon Plus Environment

Page 6 of 20

Page 7 of 20

1

Environmental Science & Technology

Scheme 1. Suggested degradation pathways for TBNPA abiotic degradation

2 A. Alkaline solution (Intramolecular nucleophilic substitution) Br

Br

Br H2C

-Br-

CH2

C

C H2C

-H+

H2 C

H2C H2C

CH2 O-

Br

O C H2

Br

BBMO B. nZVI (Reductive dehalogenation (SET)) Br

Br

Br H2C

+ e-

CH2

C H2C

-

CH2

-Br

OH

Br

Br

.

H2C

CH2

+ H+

H2C

C H2C

CH3

C

CH2

H2C

OH

Br

Br

CH2 OH

TBNPA C. H2O2/CuO (Oxidative C-H cleavage)

+ OH. -H2O

Br

.C

H2C

Br

H

C H2C Br

3

CH2 OH

4 5 6

Experimental

7

Materials

8

TBNPA (> 98% pure) was obtained from TCI, (Australia). nCuO (29nm) were purchased

9

from Sigma-Aldrich. All used solvents and reagents were of analytical grade, triple

10

distilled water was used for the preparation of aqueous solutions.

11

Experimental setup

12

All the experiments were performed in duplicates.

13

Chemical transformation under alkaline conditions was performed with TBNPA solution

14

(100 mg L-1) in phosphate buffer (0.1M, pH 8). TBNPA solution in phosphate buffer with

15

pH 6 was used as a control. Forty ml VOC glass vial with 30 ml of TBNPA solution were 7 ACS Paragon Plus Environment

Environmental Science & Technology

1

held in the oven at 60 ˚C. Reaction in every single bottle was stopped by acidification to

2

pH 6 and organic components were immediately extracted into toluene (5ml), followed

3

by quantitative and isotope analyses. Extraction efficiency determined for the control

4

sample was > 95%. No changes in TBNPA concentrations have been detected in the

5

control experiments (pH 6) over a period of one month.

6

Catalytic transformation by nZVI was performed under argon blanket in a closed glass

7

bottle containing 250 ml of aqueous TBNPA solution (350 mg L-1) and 1 g of the

8

synthesized nZVI (wet weight). The nZVI was synthesized by reducing ferric ions (1 M

9

as FeCl3·6H2O) by dropwise adding NaBH4 (3 M) under an argon blanket, the formed

10

nZVI was then filtered out and washed with acetone followed by deionized water in an

11

anaerobic chamber. More details regarding the nZVI synthesis protocol are given in Dror

12

et al. [29] and Wang and Zhang [34]. The sample aliquots 3 ml were withdrawn at

13

different time intervals through a septum cap and filtered (0.22 µm) to remove

14

nanoparticles and extracted into 3 ml of toluene.

15

Catalytic degradation by nCuO was done in batch experiments as follows: 0.125g of

16

nCuO were suspended in 200 ml TBNPA solution (350 mg L-1 final concentration),

17

subsequently 50 ml hydrogen peroxide solution (30%) were added to initiate the catalytic

18

reaction. The solution was stirred at room temperature. High hydrogen peroxide

19

concentration was used to accomplish high degradation level of at least 90%. Sample

20

aliquots (3 ml) were withdrawn at different time intervals, filtered (0.22 µm) to remove

21

nanoparticles and extracted into 3 ml of toluene. No changes in TBNPA concentrations

22

have been detected in a control experiment conducted without nCuO particles.

23

Chemical and isotope analyses

24

Quantitative analysis of TBNPA and degradation products identification was done using

25

Gas Chromatograph – qMass Spectrometer (6890-5975 Agilent Technologies) equipped

26

with HP-5 capillary column (30 m, 0.25 mm, 0.25 µm). Helium was used as a gas carrier

27

at a flow rate of 1 mL min-1; the injector temperature was held at 250 °C. Oven was

28

heated from 60 °C to 250 °C with the rate 10°C min-1. The sample was injected in a split

29

(20:1) mode, liner i.d. 2mm. EI ionization (70eV) has been applied; analysis was

30

performed in a scan mode (m/z 35-350). TBNPA concentrations were calculated on the

31

basis of a calibration curve (R2>0.98) for TBNPA solution in toluene (concentration 8 ACS Paragon Plus Environment

Page 8 of 20

Page 9 of 20

Environmental Science & Technology

1

range from 50-1000 mg/L) prepared from the parent compound. Quantification of the

2

reaction products was not performed in the present study.

3

High Performance Liquid Chromatography (HPLC) equipped with a conductivity

4

detector (Alltech, model 650) and anion separation analytical column (Hamilton, PRP-

5

X100, 4.1, 150 mm) was used to measure bromine ions in the degraded samples. P-

6

hydroxybenzoic acid (4 mM) solution, adjusted to pH 8.5, was used as the mobile phase

7

at an isocratic flow of 3 mL min-1. Bromide concentrations were calculated on the basis

8

of a calibration curve (R2>0.99) developed from NaBr standard solutions treated in the

9

same manner as the samples.

10

Stable carbon isotope ratios were determined by GC-C-IRMS (Trace GC Ultra, Delta V

11

plus; Thermo Scientific). Oxidation oven (CuO/Ni/Pt) was held at 950 °C, reduction oven

12

at 650 °C. The same temperature program as used for GC-MS analysis was also applied

13

for the isotope analysis of the samples. δ13C values of the analytes were measured against

14

internal laboratory standard CO2 gas that was introduced at the beginning and at the end

15

of each run. Calibration of carbon isotope composition of CO2 gas was performed against

16

international standards USGS-40, urea #1,#2, #3 (Biogeochemical Laboratories Indiana

17

University) and reported in permil (‰) units relative to Vienna-Peedee Belemnite(V-

18

PDB).

19

Bromine isotope analysis was performed by GC-MC-ICPMS. The same instrumental

20

setup as described earlier [18] was employed. For each analysis 2 µl of the extract were

21

injected into a GC (HP 6890) interfaced to the MC-ICPMS (Nu Instruments). Strontium

22

external spike solution (SRM 987—US National Institute of Standards and Technology),

23

was continuously introduced into the system by an Aridus desolvation nebulizer for

24

correction of instrumental mass bias. Fine-tuning of the MC-ICPMS instrument was

25

performed according to the maximum signal of Sr. Signals of

26

83

27

MC-ICPMS system and details on the applied normalization of bromine isotope ratios

28

can be found in the Supporting Information. GC-MC-ICPMS provided the absolute

29

values of 81Br/79Br isotope ratio of the investigated compounds. In the present study the

30

ratios were expressed in permil units relatively to the bromine isotope ratio of the sample

31

at time zero according to eq.1:

86

Sr,

84

Sr,

81

Br,

79

Br, and

Kr were simultaneously collected by Faraday cups. Operating parameters for the GC-

9 ACS Paragon Plus Environment

Environmental Science & Technology

1

∆ , ‰

  



 ∗ 1000

(1)

2 3

Each of the samples was analyzed three times for carbon and bromine isotope

4

composition, respectively. Standard deviation (1SD) for average δ13C values were