Anal. Chem. 1996, 68, 921-929
Determination of the Quaternary Ammonium Surfactant Ditallowdimethylammonium in Digested Sludges and Marine Sediments by Supercritical Fluid Extraction and Liquid Chromatography with Postcolumn Ion-Pair Formation Pilar Ferna´ndez,† Alfredo C. Alder,* Marc J.-F. Suter, and Walter Giger
Swiss Federal Institute for Environmental Science and Technology (EAWAG), CH-8600 Du¨ bendorf, Switzerland
To study the phasing-out of the quaternary ammonium surfactant ditallowdimethylammonium cation (DTDMAC), concentrations of the cation in anaerobically stabilized sewage sludges were determined before and after its replacement by better degradable compounds. DTDMAC was quantitatively extracted from digested sludges using 380 atm of supercritical CO2 modified with 30% methanol at 100 °C. Determination of DTDMAC was performed by normal-phase HPLC with postcolumn ion-pair formation and extraction with no sample cleanup. Mean concentrations of DTDMAC in sludges from five different municipal sewage treatment plants in Switzerland decreased from 3.67 g/kg (in 1991) to 0.96, 0.47, and 0.21 g/kg of dry sludge in 1992, 1993, and 1994, respectively. The precision of the method in digested sludge for 0.1-6.0 g/kg of dry matter, as indicated by the relative standard deviation, was typically 7%. The influence of the sample matrix was studied by performing supercritical fluid extraction (SFE) in coastal marine sediments. While SFE and a conventional liquid extraction method gave equal DTDMAC concentrations in sludges, the extraction of marine sediment samples yielded 30-40% higher DTDMAC values for SFE compared to those obtained by liquid extraction. The 94% drop in DTDMAC concentrations in digested sludges is due to the replacement of this substance and is a clear result of the producers’ voluntary ban on its use in Europe. Quaternary ammonium surfactants show a high affinity for negatively charged surfaces, making them suitable for industrial applications and as components of consumer products.1,2 The major uses of cationic surfactants are as fabric softeners and antistatic agents in laundry detergents. Important industrial applications are related to manufacturing of organomodified clays and biocides.1,2 The worldwide consumption of cationic surfactants was estimated to exceed 250 000 tons in 1986.2 In 1987, consumption of cationic surfactants in laundry detergents was approximately 80 000 tons in the United States and 72 000 tons in Europe.1 † Current address: Environmental Chemistry Department, C.I.D.-C.S.I.C., Jorge Girona Salgado, 18-26, 08034 Barcelona, Catalonia, Spain. (1) Boethling, R. S.; Lynch, D. G. In The Handbook of Environmental Chemistry; de Oude, N. T., Ed.; Springer-Verlag: Berlin, 1992; Vol. 3, p 144. (2) Boethling, R. S. In Cationic Surfactants; Cross, J., Singer, E. J., Eds.; Marcel Dekker, Inc.: New York, 1994; Vol. 53, p 95.
0003-2700/96/0368-0921$12.00/0
© 1996 American Chemical Society
During the textile washing process, a fabric softener is applied in the rinse cycle, where the active ingredient adsorbs onto fabrics and makes textiles feel soft. During the next washing process, quaternary ammonium surfactants are transferred to the aqueous phase by the excess of anionic surfactants and discharged to the aquatic environment through wastewaters, mainly as ion-pair complexes.3,4 The complexes are hydrophobic but largely ionic in character.5 Cationic surfactants adsorb strongly to soils, sediments, and suspended particles because of hydrophobic and electrostatic interactions with negatively charged surfaces. The most widely used active ingredient in fabric softeners has been ditallowdimethylammonium chloride (DTDMAC).2 DTDMAC, typically containing homologues with alkyl chain lengths of C16 and C18, have a rather low water solubility, leading to a moderately high partition coefficient (log Kow ) 2.69).6 Owing to its physicochemical properties and its nonbiodegradability in anoxic environments, DTDMAC is significantly enriched in anaerobically stabilized sludges and anoxic sediments. Therefore, DTDMAC has been replaced in Europe since 1991 by new quaternary ammonium compounds that contain ester functions in the long hydrophobic chains and are expected to have an improved degradibility in the environment.7-9 The use of DTDMAC in fabric softeners in other regions has, to our knowledge, not changed. In Switzerland, the detergent industry started a voluntary phase-out of DTDMAC in the second part of 1991, and thus DTDMAC no longer is employed as an ingredient in fabric softeners. The consumption of DTDMAC, estimated by the Swiss detergent industry, decreased steadily from approximately 1140 tons in 1989 to 100 tons in 1994.10 (3) Matthijs, E.; Gerike, P.; Klotz, H.; Kooijman, J. G. A.; Karber, H. G.; Waters, J. Removal and Mass Balance of the Cationic Fabric Softener Di(hydrogenated)tallow Dimethyl Ammonium Chloride in Activated Sludge Sewage Treatment Plants; European Association of Surfactants Manufacturers (AIS/CESIO): Brussels, Belgium, 1992. (4) Gerike, P.; Klotz, H.; Kooijman, J. G. A.; Matthijs, E.; Waters, J. Water Res. 1994, 1, 147. (5) Scowen, R. V.; Leja, J. Can. J. Chem. 1967, 45, 2821. (6) Woltering, D. M.; Larson, R. J.; Hopping, W. D.; Jamieson, R. A.; de Oude, N. T. Tenside, Surfactants, Deterg. 1987, 24, 286. (7) Giolando, S. T.; Rapaport, R. A.; Larson, R. J.; Federle, T. W.; Stalmans, M.; Masscheleyn, P. Chemosphere 1995, 30, 1067. (8) Puchta, R.; Krings, P.; Sandku ¨ hler, P. Tenside, Surfactants, Deterg. 1993, 30, 186. (9) Waters, J.; Kleiser, H. H.; How, M. J.; Barratt, M. D.; Birch, R. R.; Fletcher, R. J.; Haigh, S. D.; Hales, S. G.; Marshall, S. J.; Pestell, T. C. Tenside, Surfactants, Deterg. 1991, 28, 460.
Analytical Chemistry, Vol. 68, No. 5, March 1, 1996 921
Mass balance studies in 1986 and 1987 at three municipal treatment plants in Germany reported removal of DTDMAC between 95 and 98%, of which only 36-43% was due to primary degradation.3 In the primary settling tanks of three sewage treatment plants, the removal of DTDMAC due to adsorption onto suspended solids varied from 23 to 51%. The reported DTDMAC concentrations in digested sludges ranged from 3.8 to 5.1 g/kg of dry sludge.3 Enrichment of DTDMAC in sewage sludge has also been observed in another study in Germany, where concentrations of 8.9-9.2 g/kg were determined.11 Few data have been published on cationic surfactants in aquatic sediments. Levels of
