Environ. Sci. Technol. 2009, 43, 5712–5718
Modeling Nonlinear Sorption of Alcohol Ethoxylates to Sediment: The Influence of Molecular Structure and Sediment Properties STEVEN T. J. DROGE,* LEIRE YARZA-IRUSTA, AND JOOP L. M. HERMENS Institute for Risk Assessment Sciences (IRAS), Utrecht University, Yalelaan 2, 3584 CM Utrecht, The Netherlands
Received February 12, 2009. Revised manuscript received May 6, 2009. Accepted May 18, 2009.
The nonlinear sorption of individual alcohol ethoxylate (AE) homologues was studied as a function of the chemical structure of AE and properties of six marine sediments and three clay minerals. All sorption data for both sediments and clays are well described by a dual-mode model, combining a Langmuir and linear sorption term. The nonlinear isotherms of a single homologue on different substrates almost overlap when sorbed concentrations are expressed per specific surface area. Below and above the Langmuir maximum capacity, isotherms approach linearity. Accordingly, it is demonstrated for nine individual AE that the two linear sorption coefficients for the clay mineral illite are predictive within a factor of two for a North Sea sediment. The linear sorption term at high concentrations is likely related to bilayer formation on the mineral surfaces, for both clays and sediments. Adsorption and bilayer formation to mineral surfaces dominate the sorption behavior of most AE homologues to the tested marine sediments. The two fitted sorption coefficients correlate well with the polar and nonpolar chain lengths of the AE. The enhanced nonlinearity of isotherms for AE with longer ethoxylate chains is explained by both an increasing adsorption coefficient and a decreasing bilayer formation affinity with additional ethoxylate units.
Introduction Alcohol ethoxylates (AEs) are nonionic surfactants of which worldwide more than one million metric tons are consumed annually (1). Small fractions will be discharged into the environment via treated and untreated effluents (2, 3). Trace levels of AEs have been observed in coastal and freshwater sediments in the vicinity and downstream of discharge locations (4-6). AEs are complex technical mixtures of homologues, varying in alkyl chain length and with wide distributions of the number of ethoxylate (ethylene oxide) units. Toxicity tests with a single AE showed that the survival of amphipods depends on the freely dissolved concentrations in the sediment system (7). The sorption processes of AEs in sediments should thus be well understood in order to interpret the sediment-sorbed concentrations from monitoring studies, to define risk levels for AE in the sediment * Corresponding author e-mail:
[email protected]. Present address: Department of Analytical Environmental Chemistry, Helmholtz Centre for Environmental Research - UFZ, Permoserstrasse 15, 04318 Leipzig, Germany. 5712
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compartment and to understand the mobility of AE in the environment. Current risk assessment procedures for AE consider hydrophobic interactions with the organic matter fraction in the sediment as the dominating sorption process and apply constant sorption coefficients for each individual AE to estimate aqueous concentrations (6, 8, 9). AE indeed sorb to organic rich phases such as sewage sludge (McAvoy and Kerr 2001, in ref 9) and sediment with high organic carbon contents (10). Sorption of AE also strongly depends on the alkyl chain length, suggesting a linear-hydrophobic based process (9). Several studies with natural sediment, however, have demonstrated nonlinear sorption isotherms for individual AEs. Sediment-water distribution coefficients (Kd) for AEs in these studies were better correlated to the clay fraction than to the organic matter content (11-14). AEs are well-known to adsorb to clay minerals and this process has been extensively studied and reviewed (15, 16). Strong adsorption of AEs to clay minerals is possible due to the extensive polar interactions of the repetitive ether oxygens in the ethoxylate chains with hydrophilic surfaces. At concentrations where hydrophilic surfaces become saturated, AEs form bilayer aggregates due to sorbate-sorbate interactions between the alkyl chains of dissolved monomers and adsorbed surfactants (17-19). Alkylethoxylates are also known to intercalate the surface area between sheets of expandable clay minerals such as montmorillonite (20, 21). This internal surface area can be more than ten times the external surface area. Therefore, it has been suggested that a high fraction of swelling clays in sediments strongly contributes to the overall sorption of AEs in sediment (13, 14). Sorption of alkyl ethoxylates to natural sediments is suggested to be a combination of sorption to both organic matter and mineral surfaces (9, 22), and this should somehow be accounted for in risk assessment procedures. Most sorption studies with AE or alkylphenol ethoxylates (APEOs) only focused on the concentration range close to the critical micelle concentration (CMC) (13). This is partly due to the limited ability of applied analytical techniques to study freely dissolved concentrations readily below the point where adsorption sites become saturated, e.g. via surface tension measurements (18, 23), microcalorimetry (24, 25), and ellipsometry (26). The analysis of the freely dissolved concentration by the solid-phase microextraction (SPME) method combined with LC-MS detection strongly facilitates sorption studies with fine clays, even at low concentrations, especially since there is no need to physically separate the aqueous phase from the sorbent (27). In a previous study (28), we showed that the dual-mode sorption model (DMM) in eq 1, which combines a single Langmuir adsorption component and a linear term, successfully describes the nonlinear sorption data for three individual AE homologues to a marine sediment Cs )
C S,max · b · C aq + KII · Caq 1 + b · Caq
(1)
where CS,max is the maximum adsorption capacity of an adsorption phase in the sediment, b (KL in ref 28) is a parameter related to the adsorption affinity, and KII (Kp in ref 28) is the sorption coefficient of the linear sorption component in the sediment. The DMM (eq 1) distinguishes two separate sorption phases in sediment, and fitted DMM isotherms provide two sorption coefficients: the Langmuir sorption coefficient (KI), defined by the product CS,max · b in 10.1021/es900452p CCC: $40.75
2009 American Chemical Society
Published on Web 06/19/2009
eq 1, and the linear sorption coefficient (KII). It was hypothesized that the nonlinear sorption term represents adsorption to clay minerals and that the linear term represents absorption into organic matter (28). The aim of this work was to further examine the relation between the DMM parameters and properties of both AEs and marine sediments and thereby better understand the various sorption mechanisms of AEs in marine sediments. The main focus of this paper is to elucidate the role of clay minerals in the sorptive behavior of individual AE compounds. The first objective of this study was therefore to study the sorption of AE to kaolinite, bentonite and illite. These clays are representative for the type of minerals that dominate the clay fraction in most marine sediments (29). Their surface properties differ widely, e.g. kaolinite has both siloxane and aluminum hydroxide surfaces, bentonite and illite both only siliceous surfaces. Due to the relatively low surface charge, bentonite is an expanding clay mineral, creating external and interlayer surfaces. illite is a nonexpanding mineral and was chosen as a model mineral surface to study the sorption behavior of a set of individual AEs. The second objective was to examine the contribution of sorption to clay minerals to the overall sorption in marine sediments. Sorption tests were performed on various marine sediments to study the relation between sorption coefficients and sediment properties. Furthermore, the sorption behavior for one marine sediment was studied with the same set of individual AE structures that were used for illite, allowing for a comprehensive comparison between these two sorbents. As a third objective, we aimed to provide a quantitative relation between sorption parameters and the alkyl chain length and number of ethoxylate (EO) units, for both the marine sediment and illite, thereby offering a starting point for the improved risk assessment of AEs.
Experimental Methods Chemicals, Solvents, SPME Fibers. Tested AEs were polyethylene glycol alkyl ethers with linear alkyl chains and of the general formula [CH3(CH2)xO(CH2CH2O)yH]. Annotated as Cx+1EOy, C12EO3, C12EO5, C12EO6, C12EO8, C14EO6, and C14EO8 (all >98% TLC) were from Fluka (Buchs, Switzerland), and C14EO11 and C14EO14 were synthesized by J. Tolls and H. C. Kwint in 1996 (>97% GC, HPLC,1H NMR). Artificial seawater (GP2) was prepared according to standard EPA procedures (30). Except for KBr (Sigma-Aldrich, Zwijndrecht, The Netherlands) the seawater salts were from Merck (Darmstadt, Germany). To prevent biodegradation of AE during the tests, both 100 mg/L sodium azide (NaN3) (Merck) and 0.4 mL of a 37% formaldehyde solution (methanol free, Sigma-Aldrich) were added to all test vials. HPLC-quality methanol (Labscan, Dublin, Ireland) and highly pure deionized water (Millipore Waters, Amsterdam, The Netherlands) were used. The SPME fiber, with a 108 µm diameter glass core and a 34.5 µm polyacrylate (PA) coating (volume 15.4 µL/m), was custommade by Polymicro Technologies (Phoenix, AZ) and cut to pieces of 50 mm. Sorbents. Kaolinite and bentonite were obtained from Keramikos (Haarlem, The Netherlands), and illite was purchased as fine powdered green clay from Argiletz (France). Kaolinite and bentonite were hydrated for one day in GP2, after which the supernatant (30 min at 1670 g) was carefully decanted and the wet pellet homogenized. The test vials with the added clay had to be sonicated for at least 30 s to achieve well dispersed suspensions. Dry illite readily dispersed and was therefore chosen for studying the sorption behavior of the various AEs. Three marine sediments were sampled in the Dutch North Sea using a box corer, at locations ‘Oyster grounds’ (OG), ‘Frisian front’ (FF), and just north of the Frisian front (NFF), all three considered sedimentation areas. Three intertidal sediments were sampled by hand. These sediments
comprised of those from an intertidal mudflat (Oesterput, OP) in the Dutch Eastern Scheldt estuary, on a mudflat near the city of San Fernando (SF) in the bay of Cadı´z (Spain), and at La Antilla (LA), close to Huelva (Spain). Particles >2 mm were removed, and all sediments were stored wet at 4 °C. To avoid batch variability, subsamples from below the aerobic upper layer of the stored samples were used as wet sorbent in all sorption tests. The dry weight of the wet substrates was determined on triplicate subsamples by drying at 105 °C overnight. The fraction of organic carbon (foc) in the sorbents was determined using a NA 1500 NCS elemental analyzer (Fisons, Milan, Italy) after treatment with 1 M hydrochloric acid. Grain size distribution was measured using a Malvern Mastersizer-S (Malvern Ltd., Malvern, UK). Specific surface area of the sorbents was measured by N2 sorption at 77K (BET-method) and a standardized ethylene glycol monoethyl ether (EGME) method (31), which was based on the retention of a monolayer of EGME on P2O5 dried sorbents after immersion in EGME and excess removal by vacuum suction over CaCl2. Table 1 presents foc and EGME available area only; further details are presented in Table S-2 in the Supporting Information (SI). Sorption Experiments and Extractions. Sorption tests were performed in similar batch tests as described earlier (28). Aiming to cover the concentration range between detection limit and CMC, one or more additional sets of experiments followed initial range-finding pilots for most of the presented isotherms. Disposable SPME fibers were used in all tests to determine freely dissolved concentrations. The SPME method cannot be used to study sorption above the CMC in combination with a mass balance, since it only relates to the freely dissolved concentration (27). Highest tested concentrations were always at least within a factor of two below the estimated CMC in seawater, which were estimated to be a factor of two lower than measurements at low salinity (27). Detection limits depend on the fiber-water partition coefficients (Kfw) and quantitation limits of AE in the methanol SPME extracts (∼1 ng/mL). Previously published fiber-water partition coefficients (Kfw) were used (27), except for C12EO3, C12EO5, C12EO6, and C14EO14, for which Kfw values were determined with duplicate fibers at four different concentrations. For this purpose, the aqueous phase was also analyzed using C18-SPE columns, as described in ref 27. Table S-1 (SI) shows the results of new reported Kfw values. The clear dependency of the Log Kfw values on both alkyl chain lengths (+0.52 log units per CH2 unit) and the number of ethoxylate units (-0.26 log units per EO unit) is presented in the SI. C12EO8 was the model alcohol ethoxylate for which sorption isotherms on the three clay minerals and the six sediments were obtained. For the other AE compounds, sorption isotherms were obtained only for illite and the NFF sediment. Duplicate fiber measurements were used for all tests, except for sorption tests with C12EO3 and C14EO6, where one 40 mm fiber was used per test vial. Disturbance of the equilibrated systems due to uptake in the fibers was thereby minimized. The fiber duplicates were highly comparable, with relative standard deviations of the overall difference between duplicates
