Science of the Total Environment 481 (2014) 260–265

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Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv

Stability of dioctyl sulfosuccinate (DOSS) towards hydrolysis and photodegradation under simulated solar conditions Sudha Rani Batchu, Cesar E. Ramirez, Piero R. Gardinali ⁎ Department of Chemistry and Biochemistry, Florida International University, Miami, FL, USA Southeast Environmental Research Center, Florida International University, Miami, FL, USA

H I G H L I G H T S • First study to report photolysis of DOSS • Photolysis of DOSS reported in environmentally relevant light sources in salt water • First study to report abiotic transformation products of DOSS

a r t i c l e

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Article history: Received 19 January 2014 Received in revised form 3 February 2014 Accepted 3 February 2014 Available online 4 March 2014 Keywords: Corexit components Dioctyl sulfosuccinate (DOSS) Deepwater Horizon Gulf oil spill Hydrolysis Photodegradation

a b s t r a c t Dioctyl sulfosuccinate (DOSS) is one of the main components of Corexit® EC9500A, a chemical dispersant formulation used at the surface and at depth during the response to the Deepwater Horizon incident. Despite being a high volume use chemical, data on its environmental stability are scarce. Hydrolysis and photodegradation of DOSS in both pure water and seawater were reported in the present study. DOSS photodegraded much faster under ultraviolet light source (254 nm, with half-life in hours) compared to relevant environmental light sources i.e., 350 nm and solar simulator (with half-lives in days). LC/MS–MS analysis of hydrolysis and photo-irradiated samples showed the presence of a common degradation product. MS/MS fragmentation of that product indicated a substitution of an octyl group by a hydroxyl group with a corresponding formula of C12H21O7S, which was confirmed by HRMS detection (Q-TOF, m/z 309.1017, +1.29 ppm). © 2014 Elsevier B.V. All rights reserved.

1. Introduction Corexit® EC9500A and Corexit® EC9527A are two chemical dispersants used in the remediation efforts of oil spills. Corexit® EC9500A was heavily used in the Deepwater Horizon (DWH) oil spill (USG, 2010) whereas Corexit® EC9527A was the main dispersant used in the Alaska North Slope oil spill remediation in 2006 (Fingas, 2008). Based on the published studies that are available, Corexit® formulations have low to moderate toxicity to most aquatic species (George-Ares and Clark, 2000; Hemmer et al., 2010; Judson et al., 2010). One of the surfactants used in both Corexit formulations is dioctyl sulfosuccinate sodium salt (DOSS, CAS number 577-11-7, Fig. 1). DOSS is a high production volume chemical in the U.S. (USEPA, 2013) and is also used as an emulsifying and wetting agent in formulations of laxatives, cosmetics, detergents,

⁎ Corresponding author at: 3000 NE 151st ST, FIU Biscayne Bay Campus, MSB-350, North Miami, FL 33181, USA. E-mail addresses: sbatc001@fiu.edu (S.R. Batchu), crami023@fiu.edu (C.E. Ramirez), gardinal@fiu.edu (P.R. Gardinali).

http://dx.doi.org/10.1016/j.scitotenv.2014.02.033 0048-9697/© 2014 Elsevier B.V. All rights reserved.

pesticides and other consumer products (NCBI, 2013). Corexit® EC9500A and Corexit® EC9527A have been shown to contain 21 ± 2% and 22 ± 5% of DOSS, respectively (Ramirez et al., 2013). Besides the DOSS introduced to the Gulf of Mexico (GOM) as point source during the response to the DWH incident, GOM waters also receive DOSS through diffuse non-point sources such as riverine and estuarine discharges (Hayworth and Clement, 2012). Studies of DOSS biodegradation in seawater seem contradictory. Garcia et al. (2009) have shown that DOSS is susceptible to biodegradation in both aerobic and anaerobic conditions in fresh waters. A more recent study by Campo et al. (2013) found that DOSS was biodegraded by 98% in 8 and ~ 38 days in cultures from surface and deep Gulf of Mexico waters (at 25 °C and 5 °C, respectively). However, Kujawinski et al. (2011) reported that DOSS was detected 300 km from the well 64 days after the Corexit application has ceased at concentrations reaching up to 2.1 μg/L, and the article results suggest that this compound is not biodegradable and any decrease in the concentration could be attributed only to dilution effects. In light of these contradictory findings, it is important to monitor DOSS and its degradation products in the environment as the released dispersed oil moves and transforms. Stability of

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undergo photolysis under sunlight (Jasper and Sedlak, 2013; Zeng and Arnold, 2013). However, the abundance of natural sunlight depends on many factors such as latitude (Li et al., 2002), depth of the photic zone and overcast conditions. For this reason, the results of the experiments conducted with natural sunlight may pose a challenge in comparing them. To overcome that, experiments were conducted with a SunTest, which is a surrogate of natural sunlight. The SunTest XLS produces a continuum of wavelengths from 300 nm to 800 nm by using a xenon lamp. The wavelength distribution and the intensity of the xenon lamp are very similar to those of natural sunlight (Diepens and Gijsman, 2007).

Fig. 1. Structure of DOSS.

DOSS towards photolysis and hydrolysis has not been studied before and is the main objective of this work. Photostability of DOSS was studied using UV light (254 and 350 nm) and in simulated solar radiation. The photolysis was studied in natural seawater and in pure water simultaneously to assess the importance of indirect versus direct photolysis processes. 2. Materials and methods 2.1. Chemicals Neat DOSS sodium salt was purchased from Acros Organics (Geel, Belgium). Certified DOSS and DOSS-13C4 standards were purchased from Cambridge Isotopes Laboratories (Andover, Massachusetts, USA). Stock and working solutions of standards were prepared in acetonitrile. Optima LC/MS grade formic acid, acetonitrile and water were purchased from Fisher Scientific (Fairlawn, New Jersey, USA). Sodium hydroxide and sulfuric acid (certified A.C.S. grade) and ammonium hydroxide (trace metal grade) were also purchased from Fisher Scientific. Artificial seawater was prepared using the commercially available Instant Ocean® sea salt to 3.5% w/v. 2.2. Salt water Salt water was taken from the shore at Bill Baggs State Park at Key Biscayne, Florida, USA, using a solvent-rinsed amber glass bottle. Salt water characteristics are shown in Table 1. Salt waters (SW) were filtered (0.2-μm) and stored in the dark at b4 °C until the experimental solutions were prepared, typically within a month. 2.3. Light sources employed in the study Photodegradation experiments were conducted using Rayonet UV photochemical reactors (Southern New England Ultraviolet Co., Branford, CT) and a SunTest XLS Tabletop Xenon Exposure System (ATLAS Material Testing Technology LLC, Chicago, Illinois, USA). The photochemical reactor was operated with 16-mercury vapor lamps (UV 254 nm) or black light phosphor bulbs (UV 350 nm). UV 254 nm radiation is included in the present study to identify the photolysis products. UV 350 nm is comparable to the range of the UVA region (315 nm– 400 nm) of sunlight and hence commonly used to predict the photodegradation of organic compounds in the environment (Lam et al., 2003; Radjenovic et al., 2009; Sturini et al., 2009). In the environment, the environmental fate of organic compounds depends on its ability to

2.4. Sample irradiation Standard solutions of DOSS (100 μg/L) were prepared in reverse osmosis deionized water (RODW) and SW. The photolysis experiments were performed once with internal replicates. Three 30-mL quartz tubes (Southern New England Ultraviolet Co., Branford, CT) were used for experiments in the photochemical reactor. The first tube was filled with 30 mL of RODW (blank) and the second and third were filled with solutions of DOSS in RODW, irradiating one of them while the other one was wrapped with aluminum foil (dark control). The same procedure was repeated using SW. All the tubes were placed on a merry-go-round to ensure uniform irradiation in the photochemical reactor chamber. For SunTest experiments, DOSS solutions were placed in three UV transparent Nasco WHIRL-PAK 2 oz bags and then the bags were floated in a water bath with circulating water during exposure to keep the solution at a constant temperature (25 °C). At regular intervals 1000 μL of the test solution was collected into a 2-mL LC amber vial containing 484 μL of acetonitrile, fortified with DOSS-13C4 (15.8 μL, 1.9 mg/L) and subsequently analyzed by LC–MS/MS. 2.5. Hydrolysis experiments The test solution of standard DOSS (80 μL, 1.5 mg/L) in RODW was transferred to two vials each containing 5 mL of 0.01 M sulfuric acid and 0.01 M sodium hydroxide, respectively. Control solutions (no acid or base added) were prepared by spiking standard DOSS (80 μL, 1.5 mg/L) into 5 mL of RODW. The same procedure was repeated with artificial seawater. After 24 h reaction time, the pH was neutralized with formic acid or ammonium hydroxide. Then, 1000 μL of test solution was placed in a 2 mL amber vial containing 484 μL of acetonitrile, fortified with DOSS-13C4 (15.8 μL, 1.9 mg/L) and subsequently analyzed by LC–MS/MS. 2.6. Liquid chromatography Liquid chromatography analysis was performed according to a previously published methodology by the same authors (Ramirez et al., 2013). HPLC separation was performed using an Accela quaternary pump equipped with a HTC-PAL autosampler system (Thermo Scientific, USA). Liquid chromatography was carried out using a Hypersil Gold

Table 1 Characteristics of salt water used in the experiment. Parameter

Salt water

pH Dissolved organic carbon (mg-C/L) Electrical conductivity (μS/cm) Salinity (ppt)

7.9 1.37 88,000 36

Fig. 2. Structure of degradation product DP1. Fragments a and b, masses monitored for the neutral loss experiments.

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Fig. 3. ESI-parent ion scan chromatograms of control DOSS and base hydrolysis sample (left) after 24 h of reaction. MS/MS spectra of DP1 obtained in triple quadrupole mass spectrometer (right).

aQ column C18 (50 mm × 2.1 mm, 3 μm) equipped with a Hypersil Gold aQ pre-column (10 mm × 4.6 mm × 3 μm) (Thermo Scientific, USA). Injection volume was 20 μL. Separation was performed in 10 min with a flow rate of 0.325 mL/min, using a binary gradient mobile phase consisting of 0.1% formic acid in water and 0.1% formic acid, 0.9% water, and 99% acetonitrile. 2.7. Mass spectrometry Analytes were ionized by electrospray ionization operating in the negative mode. A TSQ Quantum Access QqQ Mass Spectrometer (Thermo Scientific, USA) was operated in selective reaction monitoring (SRM) mode. MS operating conditions and SRM transitions were also similar to the previous methodology, except for the different scan modes (full scan MS and two MS/MS scan modes (neutral loss and parent ion scans)) employed for structure determination of products. Using full scan mode (m/z: 50–450), only one product was identified by subtracting spectra of the control sample from the corresponding acid or base treated sample. The products identified in QqQ were confirmed using a high resolution mass spectrometer (HRMS), quadrupole time of flight mass spectrometer (Q-TOF-MS). 3. Results and discussion 3.1. Hydrolysis experiments In the parent ion scan, MS was programmed to scan for precursor ions producing a product ion with m/z 81 (fragment a, Fig. 2) whereas in neutral loss experiments, MS scanned for the precursor ion that decomposes into a product ion with the loss of a neutral molecule i.e., 130 amu (fragment b, Fig. 2). Both DOSS and its hydrolysis product presented signals in both neutral loss [M-130]− and parent ion scans (m/z = 81) indicating that the hydrolysis product preserved the

sulfonate moiety and at least one intact octyl group. The chromatograms of the parent ion scan and the MS spectrum are shown in Fig. 3. With this information, along with its observed mass to charge ratio (m/z 309.1), the hydrolysis product was identified as the product of a des-octylation reaction via substitution with a hydroxyl group (degradation product 1, Fig. 4). To verify the proposed structure, HRMS was performed on a 6530 QTOF Mass Spectrometer (Agilent Technologies, Santa Clara, USA) equipped with an electrospray ionization (ESI) source operating in the negative ionization mode, using the following parameters: gas temperature: 325 °C; drying gas: 5 L/min; nebulizer: 30 psi; sheath gas temperature: 375 °C; sheath gas flow: 12 L/min; Vcap: 3.5 kV, fragmentor: 146 V; and skimmer: 46 V. Accurate mass spectra recorded at a resolution of 5000 in the mass range 50–500 showed an ion with accurate mass of m/z = 309.1017 (Fig. 4), which deviates from the proposed structure (C12H21O7S, m/z = 309.1013) by + 1.29 ppm. A search in the ChemSpider database using the exact mass yielded only two possible structures (diisobutylsulfosuccinate and dibutylsulfosuccinate). None of these compounds would produce the [M-130]− signal in the neutral loss scan, therefore the accurate mass confirms the proposed structure. The proposed reaction mechanism for the base hydrolysis of DOSS is shown in Fig. 4. The mechanism likely implies a nucleophilic attack by the OH− ion ester group leading to the formation of 1-carboxy-3-(2ethylhexyloxy)-3-oxopropane-1-sulfonate (DP1) and 2-ethylhexan-1olate. The same degradation product 1 (DP1) was also observed in acid hydrolysis but at a much lower concentration and it might be because the abundance of DP1 is low or it quickly decomposed further in acidic medium. However, the absence of any other products might support poor hydrolysis of DOSS in acidic medium. Ester hydrolysis in the presence of strong base leading to the formation of carboxylic acid is very well studied; however, very few studies documented alkaline hydrolysis of DOSS (Cross, 1998; Mukherjee et al., 1994; NLM, 2013). The same hydrolysis product was also reported in biodegradation experiments of

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DOSS in laboratory microcosms by Campo et al. (2013), although no further confirmation of this product was performed during that study.

3.2. Photodegradation studies The photodegradation of DOSS was followed by monitoring the loss of the molecular ion as a function of irradiation time. The plots of Ln (Ct/C0) versus time (Ct is the concentration of DOSS at a given time and C0 is initial concentration) is linear, suggesting that the photolysis of DOSS is a first order reaction and therefore rate constant can be obtained by the slope of the curve. Degradation at the most energetic wavelength (UV 254 nm) was rather fast with a half-life of 0.6 h for RODW and 3.4 h for SW. The 6-fold reduction of DOSS photolysis in salt water (Fig. 5) suggests a predominant indirect photolysis process that could be attributed to a photogenerated species such as the hydroxyl radical [OH•]. Previous studies found a positive correlation between [OH•] and the photolysis rate of anionic surfactants (Daneshvar et al., 2002; Horvath and Huszank, 2003; Leu et al., 1998; Lin et al., 1999). Under a different set of experimental conditions (pH 10.5, DOSS = 300 mg/L, H2O2 = 300 mM) DOSS had a half-life of 0.14 h using UV 254 nm (Olmez-Hanci et al., 2011) and the faster photodegradation rate could have been caused by the higher concentration of hydroxyl radicals generated by H2O2. Additionally, chloride in salt water used in the present study may have acted as scavengers of hydroxyl ions, which could also contribute to the lower photodegradation rate observed (Liao et al., 2001). As expected, degradation was much slower with half-lives of 17, 14 days at 350 nm and 5, 7 days in SunTest for RODW and SW, respectively (Fig. 5). The kinetic parameters of DOSS in various light sources are shown in Table 2. The half-life values in SunTest were based on continuous irradiation, and photolysis rates may be significantly lowered by depth and meteorological conditions in the natural environment.

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There are no reported values for DOSS photodegradation in the literature yet. Kujawinski et al. (2011) reported that DOSS was persistent in gulf waters up to 64 days after its application has been ceased. Studies by Campo et al. (2013) with microbial cultures isolated from the surface and deep sea of the Gulf of Mexico showed significant biodegradation of DOSS (98%) in 8 days at surface conditions and 38 days at depth. Our surface simulated solar irradiation results (in SW) are consistent but rates were slower than the biodegradation results showing half-lives in the order of 6.54 to 13.8 days. Because the photo- and biodegradation rates in deep ocean waters will likely be lower, DOSS would be expected to be more stable than what is shown in Table 2, in deep waters. In environmentally relevant light sources (350 nm and under solar simulation) there is no significant enhancement or attenuation of DOSS photolysis rate in SW compared to RODW, implying that the effect of matrix components (dissolved organic matter and chloride ion) is minimal in these light sources. The irradiation samples obtained at UV 254 nm (RODW) were run in HRMS which confirmed the formation of the same DP1 observed in the hydrolysis experiments.

4. Conclusion Photodegradation studies at 350 nm and under simulated solar radiation showed that photolysis could be an important factor in controlling the environmental fate of DOSS in surface waters. Estimated half-lives for DOSS (in SW) were 13.8 and 6.54 days under 350 nm and simulated solar radiation, respectively. The effect of the matrix composition was also found to be minimal under these light sources, which suggest that photodegradation of DOSS is a molecule-specific process. Although kinetic data showed rather fast degradation rates under simulated solar conditions in the lab, rates under natural conditions could be attenuated

Fig. 4. Mass spectra of DOSS and its hydrolysis product DP1 obtained in Q-TOF high resolution mass spectrometer and the proposed mechanism for the formation of DP1 (m/z 309.1014) from the parent molecule.

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multiple sources may complicate interpretations in nearshore environments. The fate of DOSS requires more study, and no data is available for DP1. Therefore, further research is necessary. Conflict of interest The authors declare that there is no conflict of interest. Acknowledgments This work was supported by BP Exploration and Production Inc. and the BP Gulf Coast Restoration Organization. The Environmental Analysis Research Laboratory acknowledges the support from the Thermo Scientific Corporation in the development of this work. This is SERC contribution 656. References

Fig. 5. First order curves of DOSS at UV 254 nm (a), 350 nm (b) and SunTest (c). Note that the unit of the x-axis in the insert of figure 5a is hours.

by a number of environmental factors. However, the existence of multiple routes for degradation, combined with dilution and dispersion processes could make the detection of DOSS an analytical challenge. The main phototransformation product (DP1, m/z 309.1017, + 1.29 ppm), obtained due to the loss of an octyl side chain, was also observed under acidic and alkaline hydrolysis conditions. These results suggest that DP1 may be a good marker in assessing the fate of DOSS in salt water for extended periods of time. However, the presence of

Table 2 Kinetic data of DOSS under different light sources. Light source

Matrix

k (h−1)

r2

t1/2 (d)

UV 254 nm UV 254 nm UV 350 nm UV 350 nm SunTest SunTest

RODW SW RODW SW RODW SW

1.1 0.22 0.0017 0.0021 0.0059 0.0044

0.995 0.995 0.995 0.994 0.974 0.981

0.0252 0.143 17.0 13.8 4.83 6.54

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