In order to develop models that can predict the environmental behavior and ee ff cts of chemicals, reliable experimental data are needed. However, for anionic surfactants the number of ecotoxicity studies is still limited. The present study therefore aimed to determine the aquatic ecotoxicity of three classes of anionic surfactants. To this purpose we subjected daphnids (Daphnia − − − magna) for 48 h to alkyl carboxylates (C CO ), alkyl sulfonates (C SO ), and alkyl sulfates (C SO ) with different carbon x 2 x 3 x 4 chain lengths (x). However, all surfactants with x > 11 showed less than 50% immobility at water solubility. Hence, EC − −1 − −1 − −1 values for only few surfactants could be gathered: C CO (16 mg L), C CO (0.8 mg L ) and C SO (13.5 mg L ). 9 2 11 2 11 4 Data from these compounds showed an increase in ecotoxicity with a factor 4.5 per addition of a hydrocarbon unit to the alkyl chain, and a factor 20 when replacing the sulfate head group by a carboxylate head group. Unfortunately, we could not test carboxylates with a broader variety of chain lengths because solubility limited the range of chain length that can be tested. Keywords Aquatic ecotoxicity · Daphnia · Solubility · Anionic surfactants · Alkyl sulfonates · Alkyl sulfates · Alkyl carboxylates Numerous new organic chemicals are produced yearly for 2000; Roberts and Costello 2003; Boeije et al. 2006; Hodges application in industry and consumer products (CEFIC et al. 2006; Qi et al. 2011), the data is still too limited to 2014). For environmental risk assessment of new and exist- compare the effect between different surfactant groups (i.e., ing chemicals, an understanding of their environmental surfactants with different head group structures), certainly behavior and effects is required, but for anionic surfactants for anionic surfactants. In this study, we therefore focused the number of ecotoxicity studies is still limited. For the on generating aquatic ecotoxicity data for anionic surfactants development of predictive models such as quantitative struc- from three different surfactant groups. ture–activity relationships (QSARs) for surfactants, more Anionic surfactants are high production volume chemi- experimental data for these group of compounds are there- cals which are present in many consumer products and con- fore needed. Although some toxicity tests on surfactants sequently also in the environment (Sanderson et al. 2006; have been performed thus far (Schüürmann 1990; Roberts CEFIC 2014). Their amphiphilic and electrostatic properties 1991; Versteeg et al. 1997; Wong et al. 1997; Dyer et al. make them very efficient compounds for the detergent indus- try. At the same time, these properties result in a very die ff r - ent environmental behavior compared to e.g. neutral organic * J. Hammer compounds (Jones et al. 2003; Guo and Gaiki 2005). Unlike email@example.com for common neutral organic pollutants, their accumulation Institute for Biodiversity and Ecosystem Dynamics and potential effects can therefore not always be correlated (IBED), University of Amsterdam, P.O. Box 94248, with predicted octanol–water partition constants (log K ) ow 1090 GE Amsterdam, The Netherlands (Tolls and Sijm 1995). Institute for Risk Assessment Sciences, Toxicology Division, The ecotoxicity of organic compounds (quantified by the Utrecht University, P.O. Box 80177, 3508 TD Utrecht, concentration causing a 50% effect; EC value) is usually The Netherlands determined in standardized Daphnia magna acute ecotoxic- Ecofide, Singel 105, 1381 AT Weesp, The Netherlands ity tests according to OECD guideline 202 (OECD 2004). KWR Watercycle Research Institute, P.O. Box 1072, For some surfactants within a specific surfactant group 3430 BB Nieuwegein, The Netherlands Vol.:(0123456789) 1 3 100 Bulletin of Environmental Contamination and Toxicology (2018) 101:99–104 −1 (i.e., homologues sharing the same head group), toxicity is OECD guideline 202, containing 266 mg L CaCL ·2H O, 2 2 −1 observed to increase with increasing alkyl chain length due and 112 mg L MgSO ·7H O. Concentrations of KCl and 4 2 −1 to increased hydrophobicity (Roberts 2000; Roberts et al. NaHCO were 5 and 65 mg L respectively (OECD 2004). 2013; Barmentlo et al. 2015). At the same time, hydropho- The test media was buffered to pH 7 ± 0.3 with NaOH −1 bicity affects the bioavailability of surfactants by decreas- (66 mg L ) and 3-(N-morpholino)propanesulfonic acid −1 ing the solubility, but also by increasing sorption to other (MOPS; 1.046 g L ). phases (Pittinger et al. 1989). Bioavailability of anionic The D. magna were exposed to the selected compounds in surfactants is also influenced by the electrostatic character - 48 h immobility tests (OECD 2004). Per experiment five test istics of the head group, which can result in ion-pairing with concentrations, a solvent control (0.25% methanol without 2+ 2+ divalent inorganic cations (e.g., Ca or Mg ) (Rodriguez the test compound) and a control were tested with four rep- et al. 2001; Yan et al. 2010). The standard medium in the licates per treatment. Each replicate consisted of a glass tube D. magna toxicity test (OECD 2004) contains a relatively filled with 20 mL of test solution, spiked with 50 µL (0.25% high total ionic strength that includes divalent cations and of total volume) methanol containing the test compound. solubility problems can therefore be expected for some sur- The tubes were randomly distributed in a climate controlled factants. The determination of EC values for (ionic) com- fume hood (20 ± 1°C), with a light–dark regime of 16:8 h. pounds with a low solubility using OECD guideline 202 The experiment was started by introducing five neonates can therefore be challenging. However, since experimental (younger than 24 h) into each tube. After 48 h, the number of data for anionic surfactants is still much needed, the aim of animals not responding to stimulation was scored. Hardness, the present study was to employ the standardized D. magna oxygen concentration, temperature and pH were measured ecotoxicity test to determine the aquatic ecotoxicity of three at the start and the end of the experiments and were within classes of anionic surfactants: alkyl carboxylates, alkyl sul- the range prescribed by OECD guideline 202 (OECD 2004). fonates, and alkyl sulfates. The concentration of the test compounds was analyzed by extracting a 200 µL water sample from each replicate at the start and the end of the experiment, an injection standard Materials and Methods was added and the sample was subsequently diluted with 750 µL of methanol and stored in a freezer (− 18°C) until All test compounds had a typical surfactant structure con- chemical analysis. taining a hydrophobic alkyl chain and a hydrophilic ion- All anionic surfactants were detected with a triple quad- ized head group. Sodium salts of linear alkyl sulfates rupole mass spectrometer (MDS SCIEX API 3000 MS/MS (C SO ; with alkyl chain lengths C , C, C and C ) System from Applied Biosystems, Bleiswijk, The Nether- x 4 11 13 15 16 and linear alkyl sulfonates (C SO; C, C, C and C ) lands) with a Turbo Ion spray source operated at 400°C. A x 3 11 13 14 15 were obtained from Research Plus (South Plainfield, NJ). solvent delay switch (Da Vinci, Rotterdam, The Netherlands) Sodium salts of linear alkyl carboxylates (C CO; C , was used to prevent introduction of inorganic constituents x 2 9 C, C, C , and C ) were obtained from Sigma-Aldrich, from water samples into the MS. Chromatograms were inte- 11 13 14 15 (Zwijndrecht, The Netherlands). All organic compounds grated with Analyst 1.4.2 (Applied Biosystems). Concentra- had purities higher than 98%. Ammonium acetate was pur- tion–response relationships and the corresponding 48 h EC chased from Sigma-Aldrich. Methanol was obtained from values were calculated according to Haanstra et al. (1985) by Biosolve (Valkenswaard, The Netherlands). Ultrapure water fitting a logistic curve (Eq. 1) to the percentage of mobility was obtained from a Millipore water purification system (100% − immobilization) versus the surfactant concentration (resistivity > 18 MΩ/cm, Merck Chemicals, Amsterdam, in the water phase. The Netherlands). y(x)= The daphnid D. magna Straus was selected as test organ- (1) 1 + e (log x − log a) 10 10 ism to determine the aquatic ecotoxicity of surfactants. Juvenile daphnids (clone 4) aged < 24 h were obtained from where y(x) is the mobility at concentration x (in %), a is the −1 adults between 2 and 3 weeks old. Continuous cultures EC value (in mg L ), b is the slope of the curve, c is y(0) were maintained in Elendt M4 medium and fed with the which equals the average mobility of the control and x is the −1 alga Chlorella vulgaris. At regular intervals (about every surfactant concentration in water (in mg L ). Data analyses 3 months), acute toxicity tests were performed with the were performed with SPSS software (IBM Corp 2013) and reference toxicant K Cr O to check whether the sensitiv- Graphpad Prism Version 7.0 (GraphPad Software 2017). 2 2 7 ity of the daphnids culture was within the limits (EC , −1 24 h = 0.6–2.1 mg L ) as set by the guideline (OECD 2004). The medium used in the toxicity experiments consisted of the standard OECD medium that was prepared according to 1 3 Bulletin of Environmental Contamination and Toxicology (2018) 101:99–104 101 Results and Discussion A total of 14 surfactants with varying alkyl chain lengths from three surfactant groups (alkyl sulfates, alkyl sul- fonates, and alkyl carboxylates) were tested. Due to their hydrophobicity and electrostatic charge, anionic sur- factants with long alkyl chains often poorly dissolve in water containing inorganic cations. We therefore decided to first test the effect of saturated water solutions at maxi- mum aqueous solubility (S ) on the daphnids. To this end we stirred an excess of compound for 48 h in standard OECD medium under the standard conditions of the D. magna toxicity tests (OECD 2004). For the compounds − − that caused more than 50% immobility of the daphnids Fig. 1 Effect of head group on ecotoxicity of C SO and C CO 11 4 11 2 at S , a concentration range was tested in order to obtain to Daphnia magna after 48 h of exposure. Both dose–response curves were calculated according to Haanstra et al. (1985). The EC con- concentration–response relationships and to derive EC centrations are plotted with their 95% confidence intervals as solid values. black symbols (the 95% confidence interval of C SO is too small 11 4 We were unable to dissolve alkyl sulfonates (C SO ) x 3 to be seen) in the OECD medium at sufficiently high concentrations to cause any effect. This may have been a result of the pres- most studies focused on C SO . Persoone et al. (1989) ence of (divalent) cations in the aqueous phase. Cations 12 4 −1 − reported an EC value of 9.6 mg L for C SO in a D. are known to affect the hydration of anionic surfactants 50 12 4 magna 24 h toxicity test and Dyer et al. (1996) found an and often lowers their critical micelles concentration −1 2+ EC value of 5.5 mg L in a 48 h Ceriodaphnia dubia (CMC) (Yan et al. 2010). Divalent cations such as Ca 2+ toxicity test (comparable sensitivity to D. magna (Ver- and Mg can furthermore form ion pairs containing two steeg et al. 1997)). Both values are in line with our data surfactant monomers and one divalent cation, or form for C SO , as toxicity generally increases from 24 to bridges between monomers and charged sites on sorbents 11 4 −1 48 h exposure and an EC value of 5.5 mg L is close (Haftka et al. 2015). For the alkyl carboxylates (C CO ) x 2 to the expected EC concentration increase when a and the alkyl sulfates (C SO ), compounds with an alkyl x 4 hydrocarbon (–CH –) unit is added to the alkyl chain of chain longer than C were badly soluble in the OECD C SO (see next paragraph). The dose–response curve of medium and showed less than 50% immobility at S . 11 4 − −1 C CO provided an EC concentration of 0.80 mg L Hence, no further ecotoxicity tests were performed for 11 2 50 −1 (95% CI 0.7–0.9 mg L ) (Fig. 1). Toxicity data for D. these compounds. magna are scarce for C CO , a 36x higher EC50 value Because of the solubility problems of the tested com- 11 2 −1 (EC = 29 mg L ) was reported by Lundahl and Cabri- pounds in the OECD medium, EC values for only denc (1978) in a 24 h ecotoxicity test, and an EC value few anionic surfactants could be generated: C CO , 9 2 −1 − − of 1.3 mg L was reported by the European Chemical C CO and C SO . Because one pair of these sur- 11 2 11 4 Agency (2014). While, we were unable to acquire the exact factants contains equal alkyl chain lengths and differ - − − experimental details of the toxicity test of Lundahl and ent surfactant head groups (C CO and C SO ), and 11 2 11 4 − − Cabridenc (1978), their analysis was performed using the another pair ( C CO and C CO ) differs in chain length 9 2 11 2 Methylene Blue Active Substance (MBAS) essay which with equal head group, we had two single opportunities is meanwhile retracted as a standard method by ASTM. to evaluate the effect of head group structure and alkyl Comparing the dose–response curves and EC val- chain length on the toxicity of the anionic surfactants. − − ues for C SO and C CO shows that the head group However, note that these interpretations are based on only 11 4 11 2 − − has a significant effect on ecotoxicity (Fig. 1). The alkyl a single pair of surfactants. For C CO, C SO and 9 2 11 4 chains of both compounds are of the same length and the C CO analyzed concentrations were respectively ± 10%, 11 2 effect of hydrophobicity is subsequently similar (Ham- ± 10% and ± 30% lower compared to nominal concentra- mer et al. 2017). Therefore, the difference in EC values tions. During the 48 h D. magna toxicity experiments 100% is likely a result from the different molecular properties of control survival was recorded. From the dose–response − − − −1 the surfactant head groups (SO vs. CO ). Besides the curve of C SO an EC value of 13.5 mg L was 4 2 11 4 50 −1 head group structure, the most notable distinction between derived (95% CI 13.2–13.8 mg L ) (Fig. 1). We were the properties of these two surfactant groups is the differ - unable to find any EC values of C SO in literature as 50 11 4 ence in pK [4.8 for C CO (Haynes 2015), and − 3.6 for a x 2 1 3 102 Bulletin of Environmental Contamination and Toxicology (2018) 101:99–104 C SO (COSMOlogic 2015)]. The pK value is partly a report of the European Chemical Agency (2013). The results x 4 a result of the charge distribution over a molecule and shows from Lundahl and Cabridenc are questionable (see previous what fraction of the compound is in the ionic form at cer- paragraph) and both studies lack experimental details about tain pH. While these compounds are in the OECD medium medium composition and only mention the duration of the both for > 99% present in their ionic (de-protonated) form, tests. Toxicity between C and C carboxylate differed with 11 9 the difference charge distribution between both molecules a factor of ∼ 23 compared (Fig. 2), which is a factor of ∼ 4.5 still affects their behavior in the aqueous phase and their per hydrocarbon unit added to the alkyl chain. This is some- interaction with other phases. For example, alkyl carboxy- what higher than the increments found for other surfactant lates are much better hydrated than alkyl sulfates (Vlachy groups in previous studies [between 2.4 and 3.4 (Lundahl et al. 2009), which also affects their electrostatic interac- and Cabridenc 1978; Maki and Bishop 1979; Hodges et al. tion with sorbents (Rabin and Stillian 1994). Furthermore, 2006)]. An increase in the alkyl chain length increases the the difference in charge distribution may affect the uptake hydrophobicity of the compound and thus increases the of the anionic surfactants in cell membranes due to their sorption to the membrane lipid (Könnecker et al. 2011). At zwitterionic properties (Scherer and Seelig 1989). Badly longer alkyl chain lengths (> C ) the toxicity is expected to hydrated compounds are usually more affected by local further increase, but this effect is not detectible using the D. charges and have more difficulty to partition into membranes magna toxicity test due the low solubility of the compounds than well hydrated compounds (Jing et al. 2009; Roberts in the OECD medium. The factor ~ 4.5 increase in toxicity et al. 2013). The C CO surfactant may therefore parti- with addition of a carbon atom to the alkyl chain is based 11 2 tion more effectively into cell membranes of the daphnids on only two chemicals. This data set is limited and could be compared to C SO which explains why alkyl carboxylates regarded as a shortcoming of the study. Unfortunately, we 11 4 were approximately 20 times more toxic compared to their could not test more compounds because of the solubility sulfated counterparts. problems (limits) of the longer chain carboxylates in the The effect of the alkyl chain length on surfactant tox- calcium rich test medium of the Daphnia test. Another test icity was studied by comparing the EC values of organism that requires another medium composition (less − − C CO and C CO . The dose–response curve for C CO calcium) could avoid this shortcoming. 9 2 11 2 9 2 −1 showed an EC concentration of 16.0 mg L (95% CI The main reason why ecotoxicity could not be detected −1 14.8–17.3 mg L ), see Fig. 2. Just like for the previously for most of the test compounds probably lies in the presence discussed surfactants, literature data on the toxicity of of cations in the aqueous solution of the D. magna tests, C CO to D. magna is inconsistent and details about the which can affect the solubility and bioavailability of ani- 9 2 experimental setup were difficult to obtain. We were able to onic surfactants. In an attempt to generate more ecotoxicity find two EC concentrations from literature: first, again a data, we decided to change the composition of the origi- −1 very high EC concentration of 135 mg L from a 24 h D. nal OECD medium and study the effect of divalent cation − − magna toxicity test by Lundahl and Cabridenc (1978). Sec- concentration on the ecotoxicity of C CO and C CO . 9 2 11 2 −1 ond, a reported EC value of 16 mg L , which is equal to To this end, four different media were prepared with dif- 2+ 2+ our experimentally derived EC value and originates from a ferent concentrations of Ca and Mg , while maintaining 2+ 2+ original Ca :Mg ratio (Naddy et al. 2002). A concen- 2+ −1 tration of Ca of 10 mg L was selected as the lowest concentration, because lower concentrations will affect with D. magna survival (Hessen et al. 2000). The highest con- 2+ −1 centration of Ca tested was 80 mg L , conform with the original OECD guideline 202. The resulting EC concentra- tions varied slightly, but did not differ significantly between medium compositions. Hence, the medium with the lowest ionic strength may already contain enough cations to cause precipitation of anionic surfactants. The D. magna toxicity test is a well-accepted and stand- ardized toxicity test which has generated ecologically rel- evant toxicity data for many organic compounds. However, the medium proposed in the OECD guideline for D. magna Fig. 2 Effect of alkyl chain length on ecotoxicity of C CO and 11 2 is of high ionic strength and this can result in solubility prob- C CO on Daphnia magna after 48 h of exposure. Both dose– 9 2 lems for compounds that are already barely soluble in water response curves were calculated according to Haanstra et al. (1985). and for compounds that maintain an electrostatic charge The EC concentrations are plotted with their 95% confidence inter - (Waaijers et al. 2013). The D. magna toxicity test appeared vals as solid black symbols 1 3 Bulletin of Environmental Contamination and Toxicology (2018) 101:99–104 103 ether sulfates. Environ Toxicol Chem 19:608–616. h t t p s : / / d o i . unable to produce reliable results for most of the surfactants org/10.1002/etc.56201 90312 tested in this study. For hazard assessment purposes of ani- European Chemical Agency (2013) Regulation (EU) No. 528/2012 onic surfactants, alternative approaches should therefore concerning decanoic acid be investigated that either exclude the influence of divalent European Chemical Agency (2014) Regulation (EU) No. 528/2012 concerning lauric acid cations present in the test medium or endpoints should be GraphPad S (2017) GraphPad Prism version 7.00 for Windows selected that are affected at concentrations below the aque- Guo Y, Gaiki S (2005) Retention behavior of small polar compounds ous solubility of the surfactants. Furthermore, because on polar stationary phases in hydrophilic interaction chromatog- anionic surfactants are known to have an affinity for soil raphy. 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Environ Sci Technol 49:11053–11061. https ://doi. the D. magna tests, we were able to determine the effect of org/10.1021/acs.est.5b029 01 surfactant alkyl chain length and head group composition Hammer J, Haftka JJ-H, Scherpenisse P et al (2017) Fragment-based on the aquatic ecotoxicity of a select group of anionic sur- approach to calculate hydrophobicity of anionic and nonionic surfactants derived from chromatographic retention on a C18 factants. However, these interpretations were based on only stationary phase. Environ Toxicol Chem 36:329–336. https :// a single pair of surfactants. doi.org/10.1002/etc.3564 Haynes WM (2015) CRC handbook of chemistry and physics, Acknowledgements The present study was supported by the Dutch 96th edn. CRC Press, Florida Technology Foundation STW, which is part of The Netherlands Hessen DO, Alstad NEW, Skardal L (2000) Calcium limitation Organization for Scientific Research and is partly funded by the Min- in Daphnia magna. J Plankton Res 22:553–568. https ://doi. istry of Economic Affairs (Stichting voor de Technische Wetenschap- org/10.1093/plank t/22.3.553 pen). Additional funding was received from Deltares (Utrecht, The Hodges G, Roberts DW, Marshall SJ, Dearden JC (2006) The Netherlands) and Environmental Risk Assessment and Management aquatic toxicity of anionic surfactants to Daphnia magna—a (ERASM), which is a partnership of European detergent and surfactant comparative QSAR study of linear alkylbenzene sulphonates products. We would like to thank BSc students Tessa de Bruin, Remy and ester sulphonates. Chemosphere 63:1443–1450. https://doi. Mulders, and Linde de Herder from the University of Amsterdam for org/10.1016/j.chemo spher e.2005.10.001 their support with the experimental work and Rineke Keijzers from IBM Corp (2013) IBM SPSS statistics for Windows version 22.0.0. Ecofide for supplying the daphnids. 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