A simple method for determination of corexit 9527® in natural waters

Marine Environmental Research 31 (1991) 69-78

~

~

~.

A S i m p l e M e t h o d for D e t e r m i n a t i o n o f C o r e x i t 9 5 2 7 ® in Natural Waters

Genine M. Scelfo & Ronald S. Tjeerdema* Aquatic Toxicology Program, Institute of Marine Sciences, University of California, Santa Cruz, California 95064, USA (Received 20 March 1990; revised version received and accepted 19 November 1990)

ABSTRACT A methodJor determination of Corexit 9527~ in natural waters was developed to meet the demand for effective monitoring of anionic surfactant-based oil spill dispersants. Incorporating ion-pair formation with bis( ethylenediamine ) copper(H), extraction of the complex into methylisobutylketone, and flame atomic absorption spectroscopy, the method is suitable for a concentration range o.['2 to lOOmg/liter, with precision as low as 5% relative standard deviation for samples in the mid- to high-range. Only a small sample volume is required (10 ml ), allowing rapid analysis of multiple samples and providing sensitivity in the range most requirecl for monitoring during the first few hours after application, when toxic impacts are most probable. Sensitivity may be substantially increased for trace analysis by increasing sample volume. Overall, the method is simple, rapid, sensitive within the range required for monitoring, requires a small sample volume, and is suitable for both marine and fresh waters.

INTRODUCTION

While chemical dispersants have been integral to oil spill response for at least a quarter-century, the acute effects of both dispersants and dispersed oil on marine life have only recently become a consideration in dispersant use (Crain, 1984; NRC, 1989). Until now, research has focused on the effects of * To whom correspondence should be addressed. 69 Marine Environ. Res. 0141-1136/91/$03-50 @ 1991 Elsevier Science Publishers Ltd, England. Printed in Great Britain

70

Genine M. Scelfo, Ronald S. Tjeerdema

dispersed oil, rather than those from dispersants alone. However, neither aerial- nor surface-spray systems are precise, producing significant overspray (Crain, 1984), and dispersants from oil droplets easily partition into the water column (NRC, 1989). Under present use guidelines, dispersant concentrations in the upper 10 cm of water after application may range from 20 to 70mg/liter (M. M. Singer, personal communication), exceeding concentrations toxic to the sensitive life stages of many marine organisms (NRC, 1989). Thus, measurement of dispersant concentrations in natural waters is necessary to both monitor their application rates and estimate toxic effects. While dispersed oil may be extracted and measured by spectrophotometry, no satisfactory analytical methods exist for measuring low levels of chemical dispersants in either the presence or absence of oil (Hazel et al., 1970; Nagy & Penrose, 1982; Haynes et al., 1984). Simple, rapid, and sensitive methods for quantitation of low dispersant concentrations in both marine and fresh waters are needed for effective monitoring during the period immediately after application, when toxic effects are most probable. The active components in most dispersant products are surfactants, which effectively reduce oil-water interfacial tension by micelle formation to produce an oil-water dispersion. The dispersive properties of surfactants arise from their bipolar molecular structure; they generally consist of a polar, hydrophilic 'head' and a non-polar, hydrophobic 'tail', and the charge of the hydrophilic portion determines classification as either nonionic, anionic, cationic or amphoteric. Dispersants are complex mixtures of surfactants and solvents, and while their exact compositions are considered proprietary by manufacturers, their chemical characteristics are known. In general, the present lack of suitable analytical methods is due to the inherent difficulty of analyzing unknown and complex mixtures that require functional group, rather than simple species, analysis (Nagy & Penrose, 1982). Corexit 9527 ~, currently the preferred agent licensed for use in California, is a mixture of both nonionic (48%) and anionic (35%) surfactants in an aqueous solvent containing ethylene glycol monobutyl ether (17%); the surfactants include ethoxylated sorbitan mono- and trioleates (nonionic) and sodium dioctyl sulfosuccinate (SDS; anionic; Canevari, 1971). The most direct approach to measure Corexit 9527 ~ would be to analyze for either the anionic or nonionic surfactants (simultaneous analysis would be inefficient); however, there are advantages to using the anionic component. First, it is a single species (SDS) rather than a variable and complex mixture of oligomers, and it comprises over one-third of the total dispersant formula. Second, methods for anionic surfactant analysis are generally less tedious and more quantitative than those for nonionic agents; those for nonionic surfactants involve polyoxyethylene reactions, and reactivity is effected by

Determination o f Corexit 9 5 2 T ~ in natural waters

71

the number of oxyethylene units, which varies (Cross, 1987). Also, reactions are not well defined or stoichiometric, and are only semi-quantitative (Crisp, 1987; Cross, 1987). The nonionic surfactant component of Corexit 952?" is a complex mixture of ethoxylated sorbitan esters which differ in the length of both alkyl and polyoxyethylene chains. Variability exists between both manufacturer lots and in the environment due to degradation. As toxic effects are most serious during the immediate post-application period, when dispersant concentrations are in the ppm range, a suitable method for monitoring must be rapid, simple, sensitive within this range, require small sample volumes, and be applicable to both marine and fresh waters. Some of the above criteria are met by the method of Crisp et al. (1975). They used bis(ethylenediamine) copper(II), to form an ion-pair with anionic surfactants, and flame atomic absorption spectroscopy (AAS) for analysis. While suitable for both marine and fresh waters, the method involves both a solvent extraction and an acid back-extraction. Gagnon (1978) modified the method and improved sensitivity by using a graphite furnace; however, both require samples of at least 100 ml and are designed for analysis of single surfactants, not mixtures. Our method, similar to that of Crisp et al. (1975), also involves ion-pair formation of anionic surfactants with bis(ethylenediamine) copper(ll). An excess of bis(ethylenediamine) copper(II) complex (to drive ion-pair formation) is added to a water sample containing Corexit 9527 '~. The neutral species (copper)x-(ethylenediamine)~. (surfactant): is extracted into methylisobutylketone (MIBK) and its concentration is determined from copper analysis by flame AAS without an acid back-extraction. Crisp et al. (1975) determined the molar ratio of sodium dioctyl sulfate to bis(ethylenediamine) copper(II) to be 2:1. However, since the exact formula of Corexit 9527 ~ is unknown, both the exact complexing and molar ratio of surfactant to bis(ethylenediamine) copper(IIi cannot be determined. While the copper(II) ion will mainly complex with sodium dioctyl sulfosuccinate, other components may also be complexed or interfere with SDS-copper complexation. While it will not determine the molar concentration of the complexed component, our method can be accurately used to determine the concentration of Corexit 9527'~ in water samples with a calibration curve.

MATERIALS AND METHODS Instrumentation

A Perkin-Elmer (Norwalk, CT) Model 2380 atomic absorption spectrophotometer equipped with a hollow cathode copper lamp and an oxidizing

72

Genine M. Scelfo, Ronald S. Tjeerdema

air-acetylene flame was used. The lamp current was 8 mA, the wavelength for copper determination was 324.7 nm, and the spectral band pass was 0"7 nm.

Chemicals Copper sulfate pentahydrate, a m m o n i u m sulfate, HC1, and mono- and dibasic sodium phosphate were purchased from Mallinckrodt (St. Louis, MO), while ethylenediamine and MIBK were obtained from Fisher Scientific and MCB Chemicals, Inc. (Houston, TX), respectively (all were reagent grade). The bis(ethylenediamine) copper(II) reagent was prepared by dissolving 62"3 g copper sulfate pentahydrate, 49.6g a m m o n i u m sulfate, and 45.1 g (50ml) ethylenediamine in distilled water; final volume was adjusted to 1 liter. A 100 mg/1 stock solution of Corexit 9527 ~ in either filtered sea water or purified fresh water was prepared daily; serial dilution was avoided due to cumulative error from glass partitioning. Prior to use, sea water was passed through a 1/~m glass fiber filter, and fresh water was passed through a MilliQ'~ water purification system (Millipore, Bedford, MA).

Analysis Water samples (10 ml) were placed into conical glass centrifuge tubes with teflon-lined screw caps; glassware was conditioned by rinsing with an aliquot of sample prior to analysis. Sample pH was adjusted to 7.0 by adding 100#1 0.1y HC1 to sea water samples or 1 ml 1M phosphate buffer (pH 7) to fresh water samples, and samples were mixed by vortexing for 10 s. Bis(ethylenediamine) copper(II) (500~1) was then added, and the samples were vortexed for 30 s, then shaken for 5 min using a Wrist-Action ~-~shaker (Burrell Corp., Pittsburgh, PA). The neutral copper-ethylenediaminesurfactant complex was extracted with the addition of 1 ml MIBK, followed by vortexing for 30s. The ratio of sample-chelating a g e n t - M I B K was maintained at 20:1:1, and samples were centrifuged at 2000 rpm for 3 min for phase separation. Aliquots (100 ktl) of the organic extract were aspirated into the flame using a teflon micro-sampling cup, and copper absorbance readings were averaged over 6 s. Since the molar ratio is unknown, Corexit 9527 f~ concentrations were determined from calibration curves of Corexit 9527 "~ concentration versus copper absorbance. Spectrophotometer performance was optimized daily for each set of analyses; both blanks and standards were measured with every set of u n k n o w n samples, and their concentrations were calculated from a calibration curve that was derived daily.

Determination o f Corexit 9527 ® in natural waters

73

30

20

< 10 L9

0

~

,

,

10

20

-

,

-

30

,

•

,

40

•

50

Corexit 9527 Concentration (rag/L) Fig. 1. A c a l i b r a t i o n curve o f c o p p e r ( l l ) a b s o r b a n c e versus C o r e x i t 9527 ~ c o n c e n t r a t i o n in purified fresh w a t e r (r 2 = 0-992). Vertical b a r s r e p r e s e n t SD, a n d n was 7 o r 8.

RESULTS

Calibration The calibration curves were linear for both purified fresh ( r 2 = 0-992) and sea water (r 2 = 0"981) in the 10-50 mg/1 concentration range (Figs 1 and 2). The concentration of Corexit 952T* (Xmg/1) may be calculated from a measured absorbance (Y) with the appropriate linear regression equation obtained from the calibration curves: fresh water, Y = 0 . 0 0 6 X - 0.047; sea water, Y = 0 . 0 0 3 X - 0.030. There is a reproducible curvature between 0 and 10 rag/1 for both fresh and sea water; most likely an artifact of the flame AAS method, it was also reported by Crisp et al. (1975), but not by G a g n o n (1978), who used flameless AAS. Accurate concentration determinations in this range can be readily obtained through transformation of the data using natural logarithms (fresh water r 2 = 0.995; sea water r 2 = 0"998). The method is also suitable for water concentrations of up to 100 mg/1 following establishment of an appropriate standard curve. 15 ¸

{ 10 5 G

~

. 10

Corexit

Fig. 2.

, 20 9527

,

,

,

30

40

Concentration

.

,

.

50

(mg/L)

A c a l i b r a t i o n curve o f c o p p e r ( I I ) a b s o r b a n c e versus C o r e x i t 9527*: c o n c e n t r a t i o n in filtered sea w a t e r ( r 2 = 0"981). Vertical b a r s r e p r e s e n t SD, a n d n was 7 o r 8.

74

Genine M . Scelfo R o n a l d S. 77jeerdema

TABLE l Precision of the Method in Purified Fresh Water C o r e x i t 9527" (mg/qiter)

N

5 l0 20 30 40 50

8 7 8 8 8 8

Copper(l[I absorbance . . . . . . . . M e a n ( x lO 2) S D ( × IO 2) R S D (%)"

0"51 1"72 7-17 I 1"56 18"34 25"26

0"07 0"23 0'90 0'68 0'94 1'34

13 13 13 6 5 5

" RSD, relative standard deviation.

Precision and detection limits Precision was assessed by repeatedly analyzing standard solutions c o n t a i n i n g 5, 10, 20, 30, 40 a n d 50mg/1 C o r e x i t 9527"' (Tables 1 a n d 2); it r a n g e d f r o m 2 4 % relative s t a n d a r d d e v i a t i o n at low c o n c e n t r a t i o n s to 5 % at high c o n c e n t r a t i o n s . T h e s t a n d a r d d e v i a t i o n o f a set o f 12 b l a n k a b s o r b a n c e r e a d i n g s was 4 x 10 4 for fresh w a t e r a n d 2 x 10 4 in sea water, a n d the limit o f d e t e c t i o n (the c o n c e n t r a t i o n w h i c h gives an a b s o r b a n c e e q u a l to twice this s t a n d a r d d e v i a t i o n ) was 1"7 mg/1 f o r fresh w a t e r a n d 2"3 rag/1 for sea w a t e r (as C o r e x i t 9527 '~).

M e t h o d recovery R e c o v e r y is c o m m o n l y m e a s u r e d to e v a l u a t e an a n a l y t i c a l m e t h o d . H o w e v e r , w h e n using c o m p l e x a t i o n , the m o l a r r a t i o o f a n a l y t e to

TABLE 2 Precision of the Method in Filtered Sea Water C o r e x i t 9527" (mg/liter)

5 10 20 30 40 50

N

Copper(ll) absorbance

..................... M e a n ( × lO 2) S D [ × IO 2) 7 8 7 8 8 8

0.17 0.61 2.57 5.76 8.61 13.17

" RSD, relative standard deviation.

0.02 0.15 0.19 0.69 0-66 0.78

R S D (%)"

11 24 7 12 8 6

Determination o f Corexit 9527 r~ in natural waters

75

complexing agent must be known. Although the anionic component of Corexit 9527-" complexes with bis(ethylenediamine) copper(II), the molar ratio ofcomplexation is unknown. Therefore, recovery cannot be accurately determined. Previous methods have utilized singular anionic detergents which complex with bis(ethylenediamine) copper(II) at known ratios (Crisp et al., 1975; Gagnon, 1978). However, as long as the analytical response is both linear and reproducible, the method is suitable for environmental monitoring. Interferences

Statistical comparison of the purified fresh and sea water standard regression lines showed a significant difference in their slopes (P<0"01), indicating possible interference from the increased ionic strength of sea water (Zar, 1974). These results differ with those of Crisp et al. (1975), who demonstrated that formation of the coppermthylenediamine surfactant complex was not seriously affected by the inorganic ions in sea water. Natural organic substances (including kelp-based surfactants) may also potentially interfere with the method. However, they are corrected for by analyzing matrix blanks and subtracting them from the sample data.

DISCUSSION The method commonly utilized for determination of anionic surfactants in natural waters involves extraction as an ion-pair with methylene blue (a cationic dye) into organic solvent, followed by spectrophotometric analysis (Longwell & Maniece, 1955; APHA, 1985). However, it requires 1 liter of sample, is time consuming (involving multiple extractions), and for marine samples requires an additional sublation step for both preconcentration and to remove interfering anions (Kozarac et al., 1975). Yamamoto & Motomizu (1987) developed a solvent extraction-based spectrophotometric method using ethyl violet as the cationic dye reagent. Although requiring a single extraction, it also requires both a large sample volume and a wash to eliminate chloride interference from sea water. Fluorescent dyes have also been investigated as ion-association reagents for anionic surfactants. Two methods described by Rubio-Barroso et al. (1988a, b) use spectrophotometric analysis of the ion-pair; however, while both require only a single extraction and a small sample volume, neither is applicable to complex mixtures or sea water samples. Cationic organometallic complexes present a better alternative to organic dye systems (Waters & Taylor, 1977). In general, methods using

76

Genine M. Scelfo, Ronald S. Tjeerdema

metal complexes are faster, simpler, and more economical (requiring smaller sample and extraction solvent volumes). Taylor & Fryer (1969) pioneered use of metal complexation to determine the concentration of anionic surfactants in wastewater with the ferroin cation and colorimetry; the presence of metal ion in the complex facilitates use of AAS, as demonstrated by Le Bihan & Courtot-Coupez (1970). They used flame AAS to indirectly determine the concentration of nionic surfactants by measuring the copper absorbance of a 1,10-phenanthroline-surfactant ion-pair. While suitable for sea water, the method requires 1 liter of sample and three separate extractions. However, by alternatively using a flameless atomization method (graphite furnace AAS), they increased sensitivity and reduced sample volume by a factor of 100; they also eliminated one extraction by blank correcting with the data from two extracts (Le Bihan & Courtot-Coupez, 1970). Gallego et al. (1986) used flow-injection analysis and AAS with continuous liquid-liquid extraction of the 1,10-phenanthroline-surfactant ion-pair. While the method is simple, automated, and requires a small sample volume, it is not applicable to either complex mixtures or sea water samples. Crisp et al. (1975) were the first to use bis(ethylenediamine) copper(ll) for determination of anionic surfactants. In their method, detergent anions are extracted into chloroform following complexation with the bis(ethylenediamine) copper(II) cation. The chloroform extract is backextracted with dilute HNO3, and flame AAS is used to determine copper in the aqueous phase. Using a 150ml sample, the detection limit is 30~g/1 for both fresh and sea water. However, the necessary acid back-extraction and large sample volume reduce both the speed and efficiency required for analyzing multiple samples. While Gagnon (1978) lowered the detection limit and eliminated the need for an acid back-extraction by using flameless AAS, the method still requires large sample volumes and is not applicable to complex mixtures. The acid back-extraction is eliminated by using MIBK as the extraction solvent; MIBK can be aspirated directly with flame AAS, and its elimination also removes the need to transfer samples to additional glassware. Glasspartitioning, resulting in sample loss, is a significant problem when using surfactants. It is minimized by using one set of glassware, which is preconditioned with sample prior to use, and centrifugation for phase separation. The small sample volume allows rapid analysis of multiple samples and provides sensitivity in the range required for monitoring Corexit 9527", during the first few hours after application, when toxic impacts are most probable. Finally, sensitivity may be increased for trace analysis by either increasing sample volume or by using flameless AAS. Our method represents a significant advance in the analysis of surfactant-based

Determination of Corexit 9527~ in natural waters

77

oil spill dispersants in natural waters. It is simple, rapid, sensitive within the range required for monitoring, requires a small sample volume, and is suitable for both marine and fresh waters.

A C K N O W L E D G E M ENTS We thank Kenneth W. Bruland, Robert P. Franks, Michael Martin (California Department of Fish and Game), and Marshall Sylvan for assistance. Support was provided by the U C S C - C D F G Cooperative Toxicology Research Program (Chapter 1429), and the Coastal Toxicology Program, U C Toxic Substances Research and Teaching Program.

REFERENCES American Public Health Association (1985). Standard MethodsJor the Examination of Water and Wastewater, 16th edn. Washington, DC, pp. 577-89. Canevari, G.P. (1971). Oil Slick Dispersants and Methods. US Patent Office No. 3,793,218. Patented 1974, Washington, DC. Crain, O. L. (1984). A multifaceted approach to applying dispersants. In Oil Spill Chemical Dispersants--Research, Experience, and Recommendations, ed. T. Allen. ASTM Technical Publication 840, Philadelphia, PA, pp. 428-35. Crisp, P. T. (1987). Trace analysis of nonionic surfactants. In Nonionic Surfactants .... Chemical Analysis, ed. J. Cross. Marcel Dekker, Inc., New York, pp. 77 116. Crisp, P. T., Eckert, J. M. & Gibson, N. A. (1975). The determination of anionic detergents with the bis(ethylenediamine) copper(II) ion. Anal. Chim. Acta, 78, 391-6. Cross, J. (1987). Introduction to nonionic surfactants. In Nonionic Surfactants--Chemical Analysis, ed. J. Cross. Marcel Dekker, Inc., New York, pp. 3-28. Gagnon, M. J. (1978). Note on a rapid and sensitive method for the determination of anionic detergents in natural waters at the ppb level. Wat. Res., 13, 53-6. Gallego, M., Silva, M. & Valcarcel, M. (1986). Indirect atomic absorption determination of anionic surfactants in wastewaters by flow-injection continuous liquid-liquid extraction. Anal. Chem., 58, 2265-9. Haynes, D. L., Kelly, D.G., Smith, J. H. & Fernandez, E. L. (1984). Developing Methods for Analyzing Oil Dispersants in Seawater. National Technical Information Service PB 84-238328, US Department of Commerce, Springfield, VA. Hazel, C. R., Kopperdohl, F., Morgan, N. & Thomsen, W. (1970). Evaluating Oil Spill Cleanup Agents--Development of Testing Procedures and Criteria. California State Water Resources Control Board, Publication No. 43, Sacramento, CA. Kozarac, Z., Cosovic, B. & Branica, M. (1975). Spectrophotometric determination of anionic surfactants in seawater. Mar. Sci. Comm., 1, 147-63. Le Bihan, A. & Courtot-Coupez, J. (1970). Dosage de traces de detergents anioiques et cationiques dans l'eau de m e r e t les eaux douces par spectrophotometric l'absorption atomique. Bull. Soc. Chim. France, 1,406-11.

78

Genine M. Scelfo, Ronald S. 7~/eerdema

Longwell, J. & Maniece, W. D. (1955). Determination of anionic detergents in sewage, sewage effluents, and river waters. Analyst, 80, 167 71. Nagy, E. & Penrose, W. R. (1982). Analysis of oil dispersants. In Oil and Dispersants h7 Canadian Seas Research Appraisal and Recommendations, ed. J. B. Sprague, J. H. Vandermuelen & P. G. Wells. Economic and Technical Review Report EPS 3-EC-82-2, Ottawa, Canada, pp. 140 3. National Research Council (1989). Using Oil Dispersants on the Sea. National Academy Press, Washington, DC. Rubio-Barroso, S., Gomez-Rodriguez, M. & Polo-Diez, L.M. (1988a). Fluorometric determination of anionic surfactants by extraction as the rhodamine-B ion pair. Microchem. J., 37, 93 98. Rubio-Barroso, S., Gomez-Rodriguez, M. & Polo-Diez, E.M. (1988b). Fluorometric determination of anionic surfactants by extraction as safranine-Y ion pairs. Anal. Chim. Acta, 206, 351-5. Taylor, C. G. & Fryer, B. (1969). Tile determination of anionic detergents with iron(I1) chelates-application to sewage and sewage effluents. ,4nalvst, 94. 1106 16. Waters, J. & Taylor, C.G. (1977). The colorimetric estimation of anionic surfactants, In Anionic Sm;lactants Chemical Analysis, ed. J. Cross. Marcel Dekker, Inc., New York, pp. 193-220. Yamamoto, K. & Motomizu, S. (1987). Solvent extraction spectrophotometric determination of anionic surfaclants in seawater. AnaO'st, 112, 1405 8. Zar, J. H. (1974). Biostatistical Analysis. Prentice-Hall, Inc., Englewood CliflA, N J, USA.