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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
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Application of dual carbon-bromine isotope analysis for investigating abiotic
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transformations of tribromoneopentyl alcohol (TBNPA)
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Anna Kozell1,2, Yinon Yecheskel3, Noa Balaban4, Ishai Dror3, Ludwik Halicz1, Zeev
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Ronen4 and Faina Gelman1*
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1
Geological Survey of Israel, 30 Malhei Israel st., Jerusalem 95501, Israel
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2
Department of chemistry, The Hebrew University, Jerusalem 91904, Israel
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3
Department of Earth and Planetary Sciences, Weizmann Institute of Science, Rehovot,
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Israel.
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4
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Microbiology, The Jacob Blaustein Institutes for Desert Research, Ben-Gurion University
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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
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Abstract
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Many of polybrominated organic compounds, used as flame retardant additives, belong
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to the group of persistent organic pollutants. Compound-specific isotope analysis is one
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of the potential analytical tools for investigating their fate in the environment. However,
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the isotope effects associated with transformations of brominated organic compounds are
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still poorly explored. In the present study, we investigated carbon and bromine isotope
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fractionation during degradation of tribromoneopentyl alcohol (TBNPA), one of the
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widely used flame retardant additives, in three different chemical processes:
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transformation in aqueous alkaline solution (pH 8); reductive dehalogenation by zero-
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valent iron nanoparticles (nZVI) in anoxic conditions; and oxidative degradation by H2O2
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in the presence of CuO nanoparticles (nCuO).Two-dimensional carbon-bromine isotope
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plots (δ13C/Δ81Br) for each reaction gave different process-dependent isotope slopes
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(Λ(C/Br)): 25.2 ± 2.5 for alkaline hydrolysis (pH 8); 3.8 ± 0.5 for debromination in the
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presence of nZVI in anoxic conditions; and ∞ in the case of catalytic oxidation by H2O2
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with nCuO. The obtained isotope effects for both elements were generally in agreement 1 ACS Paragon Plus Environment
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with the values expected for the suggested reaction mechanisms. The results of the
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present study support further applications of dual carbon-bromine isotope analysis as a
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tool for identification of reaction pathway during transformations of brominated organic
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compounds in the environment.
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TOC/Abstract Art
TBNPA
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Introduction
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Brominated organic compounds (BOCs) are produced in large quantities and applied in a
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variety of consumer and industrial products [1]. Due to their wide use in everyday life,
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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.
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During the last decades, compound-specific isotope analysis (CSIA) has been
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demonstrated as an effective tool for examination of reaction mechanisms and deducing
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compound degradation pathways in the environment [6]. This approach is based on the
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fact that chemical bonds between the heavier isotopes are slightly stronger and, thus, are
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broken slower than the bonds between the lighter isotopes. As a result, the remaining
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(still non-reacted) fraction of the substrate becomes enriched by the heavier isotopes. The
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main benefit of using the isotope approach is the possibility to track the fate of the
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contaminants in complex environmental matrices without the need to identify and
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quantify intermediates and products. The isotope enrichment factor (ε) may serve as a
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parameter for a specific pathway. It can be calculated by a Rayleigh equation using the
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relation between changes in isotopic composition and contaminant concentration [7].
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Recently it has been demonstrated that multi-elemental isotope data are even more
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informative and can be used for mechanism evaluation [7-16].
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Although, multi-element isotopic effects associated with degradation of several types of
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organic contaminants have been studied extensively, data for brominated organic
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contaminants are still limited. Recently, Gas chromatograph – Multicollector-Inductively
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Coupled Plasma Mass Spectrometer (GC-MC-ICPMS) has been introduced for bromine
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isotope ratio analysis in organic compounds [17-19] and allowed determination of
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bromine isotope enrichment factors in several enzymatic and chemical transformations
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[16, 20-23].
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When C-Br bond cleavage is the rate-limiting step of the transformation, normal carbon
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and bromine apparent kinetic isotope effects (AKIE>1) (for AKIE calculation see eq.3
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Experimental/Calculations) are anticipated; in the cases where other bonds cleavage (e.g.
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C-C or C-H) are the rate-limiting step, no or insignificant bromine isotope effect is
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expected. 4 ACS Paragon Plus Environment
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The magnitudes of the carbon and bromine kinetic isotope effects (KIEs) upon the
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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
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and chlorine KIEs during C-Cl bond breakage are 1.057 and 1.013, respectively [24].
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Thus, for brominated compounds showing similar transformation pathways, the same or
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slightly smaller carbon isotope effect is expected for brominated compounds, while Br-
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KIE is likely to be about six times smaller than Cl-KIE.
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The
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tribromoneopentyl alcohol (TBNPA) (Scheme1), which is one of the most abundant
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groundwater contaminants in the vicinity of the chemical industrial complex in the Negev
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desert in Israel; concentrations up to 1 mg L-1 were detected in several monitoring wells
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in that area [25]. Due to its relatively high water solubility (2 g L-1 at 25⁰C), TBNPA is
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expected to migrate in the soil-aquifer zone with minimal retardation [26] . TBNPA is
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considered toxic for humans and aquatic environment [27]. This compound belongs to
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the group of brominated aliphatic compounds (e.g. dibromoneopentyl glycol,
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tribromoneopentyl phosphate) which are widely used as additives in polymeric materials
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and as reactive intermediates for production of high molecular weight brominated flame
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retardants. The abundance of these compounds in environment, such as landfills and
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other waste storage facilities is currently unknown.
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considered as a representative of a broader group of brominated aliphatic compounds,
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which often end-up as persistent environmental contaminants. To date the information
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about the fate and degradation pathways for these compounds is scarce.
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Several studies dealing with biotic and abiotic natural degradation of TBNPA in the
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environment [25, 28] as well as its degradation by novel composite materials have been
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reported in the literature during the last years [29, 30]. However, isotope effects
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accompanying TBNPA decomposition in either biotic or abiotic reactions have not been
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studied yet.
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In the present study we investigate carbon and bromine isotope fractionation during
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different scenarios of abiotic degradation of TBNPA, trying to find a correlation between
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reaction pathways and observed isotope effects. The degradation reactions of TBNPA
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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
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debromination by nZVI in anoxic conditions; and (3) oxidation by H2O2 in the presence
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of nCuO catalyst. Mechanistic aspects of all these reactions have been studied previously
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in laboratory experiments. While TBNPA transformation in slightly alkaline water is a
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possible natural attenuation scenario [25], the two other processes represent engineered
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systems for contamination treatment [29, 30]. Ezra et al. [25] reported that an
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intramolecular nucleophilic substitution is the main pathway of TBNPA transformation in
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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.
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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
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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
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TBNPA transformation by nZVI (Scheme 1B); in this case both carbon and bromine
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isotope effects are expected. For oxidative decomposition of TBNPA by H2O2 catalyzed
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by nCuO, it has been suggested [30] that reaction is based on the attack of TBNPA
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molecule by reactive oxygen species (hydroxyl and superoxide radicals). Although the
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reaction mechanism is still not fully understood, it can be hypothesized that TBNPA
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decomposition occurs via proton abstraction from C-H bond, similarly to other processes
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of oxidative decomposition of halogenated alkanes [32, 33] (Scheme 1C).
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Revealing the isotope effects during the processes might provide additional information
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and lead to a deeper understanding of the mechanistic aspects of all these reactions. We
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expect that the data obtained will be in the future extrapolated to other brominated
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organic compounds, transformed in the environment by similar pathways. This study
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aims also to evaluate the potential of dual carbon-bromine isotope analysis for assessing
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the degradation of TBNPA and analogous aliphatic brominated organic compounds in
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environmental systems.
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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
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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.
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Experimental setup
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All the experiments were performed in duplicates.
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Chemical transformation under alkaline conditions was performed with TBNPA solution
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(100 mg L-1) in phosphate buffer (0.1M, pH 8). TBNPA solution in phosphate buffer with
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pH 6 was used as a control. Forty ml VOC glass vial with 30 ml of TBNPA solution were 7 ACS Paragon Plus Environment
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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
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anaerobic chamber. More details regarding the nZVI synthesis protocol are given in Dror
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et al. [29] and Wang and Zhang [34]. The sample aliquots 3 ml were withdrawn at
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different time intervals through a septum cap and filtered (0.22 µm) to remove
14
nanoparticles and extracted into 3 ml of toluene.
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Catalytic degradation by nCuO was done in batch experiments as follows: 0.125g of
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nCuO were suspended in 200 ml TBNPA solution (350 mg L-1 final concentration),
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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
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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
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have been detected in a control experiment conducted without nCuO particles.
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Chemical and isotope analyses
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Quantitative analysis of TBNPA and degradation products identification was done using
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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
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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
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performed in a scan mode (m/z 35-350). TBNPA concentrations were calculated on the
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range from 50-1000 mg/L) prepared from the parent compound. Quantification of the
2
reaction products was not performed in the present study.
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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.
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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
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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
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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-
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∆ , ‰
∗ 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
