Sources, transport and reactivity of anionic and non-ionic surfactants in several aquatic ecosystems in SW Spain: A comparative study
Introduction
Surfactants comprise a broad group of chemical compounds synthesized to exhibit tensioactive properties that make them useful for employment as a key ingredient of household and industrial detergents, but also in personal care products and pesticide formulations, among other applications. These compounds can be classified, according to their charge, into several types, where those of the anionic and non-ionic classes show the highest volumes of production. According to the data reported by CESIO (Comité Européen des Agents de Surface et de leurs Intermediaries Organiques), 998 ktons of anionics and 1231 ktons of non-ionics were manufactured during the year 2000 in the EU, these together account for about 90% of the total production of synthetic surfactants. Linear alkylbenzene sulfonates (LAS) are the most representative anionic surfactants (434 ktons), closely followed by alkyl ethoxysulfates (AES) and the non-ethoxylated alkyl sulfates (404 ktons both together). Alcohol polyethoxylates (AEOs) are the main non-ionic surfactants produced in Europe (747 ktons). The recent restrictions in the use of alkylphenol polyethoxylates (APEOs) in household detergents mainly due to the estrogenic properties shown by their metabolites (Jobling et al., 1996) have reduced their production to 116 ktons.
Once used, surfactants are discharged via wastewater treatment plants (WWTPs) into aquatic environments. Thus, a considerable number of studies have reported the presence of LAS in many parts of the world, in both water (Bester et al., 2001, Eichhorn et al., 2002, Ding et al., 1999, González-Mazo et al., 1997, González-Mazo et al., 1998, León et al., 2002, Marcomini et al., 2000, Takada and Ogura, 1992, Terzic and Ahel, 1994, Trehy et al., 1996, Ying, 2006) and sediment phases (Bester et al., 2001, Folke et al., 2003, González-Mazo et al., 1998, León et al., 2002, Marcomini et al., 2000, Trehy et al., 1996), with values typically ranging from less than 50 to more than 1000 μg L−1 and from less than 0.1 to several tens of mg kg−1 respectively, depending on the distance from urban wastewater discharge sources and type of wastewater treatment. Several papers also consider LAS intermediates, the sulfophenylcarboxylic acids (SPCs), which appear mostly in the water column at concentrations up to 100 μg L−1 due to their low hydrophobicity (Ding et al., 1999, Eichhorn et al., 2002, González-Mazo et al., 1997, León et al., 2002, Marcomini et al., 2000, Trehy et al., 1996). With respect to the non-ionics, the potential estrogenicity of the metabolites of NPEOs (nonylphenol polyethoxylates, which are the major class of APEOs) has resulted in these intermediates being studied even more than the parent compound, which shows levels from 0.01 to 50 mg kg−1 in sediments (Bester et al., 2001, Jonkers et al., 2003, Jonkers et al., 2005, Marcomini et al., 2000, Maruyama et al., 2000, Naylor et al., 1992). Thus, the presence of nonylphenol (NP) and NPEOs with one and two ethoxylated units (EO) -which are hydrophobic degradation intermediates- has been widely detected in sediments from rivers (Ahel et al., 1994, Isobe and Takada, 2004, Jonkers et al., 2003, Lee Ferguson et al., 2001, Naylor et al., 1992), lakes (Bennett and Metcalfe, 1998) and the sea bed (Bester et al., 2001, Lee Ferguson et al., 2001, Marcomini et al., 2000), ranging from 0.1 to 72 mg kg−1. Concentrations of up to 100 μg L−1 have been found in surface waters (Ahel et al., 1994, Ahel et al., 2000, Isobe and Takada, 2004, Jonkers et al., 2003, Jonkers et al., 2005) for the more polar nonylphenol ethoxycarboxylates (NPECs). On the other hand, data concerning the presence of aliphatic surfactants such AES and AEOs are rather limited. One of the main reasons for this could be that conventional high performance liquid chromatography coupled to ultraviolet or fluorescence detectors (HPLC-UV-FLD) and gas chromatography coupled to mass spectrometry (GC-MS) cannot be used, as these compounds lack volatilization capability and fluorescence. However, by developing specific protocols, some authors have found concentrations between 0.1 and 2 μg L−1 for AES (Neubecker, 1985, Pojana et al., 2004, Popenoe et al., 1994) and AEOs (Dunphy et al., 2001, Eadsforth et al., 2006, Fendinger et al., 1995) in surface waters. The presence of AES in sediment, showing concentration values ranging from 0.02 to 0.40 mg kg−1, has recently been reported for the first time by our group in a previous study (Lara-Martín et al., 2005) and confirmed later by Sanderson et al. (2006) in other areas. The few papers dealing with the occurrence of AEOs in sediments (Lara-Martín et al., 2006a, Petrovic et al., 2002) have shown levels ranging from 0.04 to 9.9 mg kg−1, although these data may underestimate the true values because in these studies the analysis was conducted on a limited set of homologs and ethoxymers.
With respect to the environmental behavior of surfactants, sorption and degradation have been described as two of the main processes affecting these compounds in aquatic ecosystems (Ying, 2006). Thus, a non-conservative behavior has been described for LAS (Eichhorn et al., 2002, León et al., 2002, Takada and Ogura, 1992, Terzic and Ahel, 1994) and NPEOs (Ahel et al., 1994, Isobe and Takada, 2004, Jonkers et al., 2003, Lee Ferguson et al., 2001, Maruyama et al., 2000) in rivers and estuaries. The percentage of LAS sorption onto particulate matter is commonly below 20% although it appears to be higher at higher salinities (González-Mazo et al., 1998), while a relatively fast degradation into SPCs (most of them found in the dissolved form) has been confirmed in Spanish (León et al., 2002) and Brazilian rivers (Eichhorn et al., 2002). NPEOs have shown a higher affinity for suspended solids (more than 25% is adsorbed according to Jonkers et al., 2005) while their polar metabolites (NPECs) are only present in the aqueous phase (Isobe and Takada, 2004, Jonkers et al., 2003). In-situ degradation by progressive shortening of the ethoxylated chain has been also detected (Maruyama et al., 2000) and the highly hydrophobic compounds generated by this process are often preserved in sediments (Bennett and Metcalfe, 1998). It is especially remarkable that this kind of study has never been carried out for AES and AEOs in spite of their production volumes being comparable to those for aromatic surfactants (LAS and NPEOs). Given this situation our main object in the present work is to perform a comparative study for the first time concerning the distribution and environmental behavior of these four main surfactants in three different Spanish aquatic ecosystems by: a) determining their occurrence in water, suspended solids and sediments; b) identifying their distribution and sources in each area; and c) characterizing their reactivity attending to their sorption capacities onto the particulate phase, changes in their homolog/ethoxymer patterns and the generation of carboxylated metabolites in the water column due to degradation processes.
Section snippets
Sampling areas
Sediment and water samples were collected using an inflated boat, at several sampling stations located in three different aquatic ecosystems in the region of Cadiz, in the southwest of Spain (Fig. 1). Zone A is a tidal marine channel (named the Sancti Petri channel) which is located in the south of the Bay of Cadiz, a salt marsh environment. This channel connects the inner part of the bay with the Atlantic Ocean and is 18 km in length. It has a reduced depth (between 3 and 6 m) and the tidal
Identification of contamination sources
Concentrations of target compounds in sediments from the selected areas spanned a wide range of values, from less (<0.1 mg kg−1) to more (several tens of mg kg−1) contaminated points. In general terms, similar values for LAS and NPEOs have been detected previously in other aquatic environments in Europe (Bester et al., 2001, Folke et al., 2003, González-Mazo et al., 1998, León et al., 2002, Marcomini et al., 2000, Trehy et al., 1996) and America (Ahel et al., 1994, Jonkers et al., 2003, Lee
Conclusions
The present study provides several new insights into the distribution and environmental fate of surfactants in different aquatic systems, especially in the case of AES and AEOs, where there are almost no data. Surfactants are ubiquitous in sediments from both freshwater and coastal marine environments, acting as a sink for most hydrophobic organic compounds. Higher values have been detected next to urban wastewater discharge points and industrial/naval activities. However, significant decreases
Acknowledgements
We express our gratitude to I. Gude and O. Mansilla for their help during the samplings and to E. de Miguel (SCCYT UCA) for his technical support with the LC-MS system. We also thank Petroquímica Española S.A. and KAO Corp. for supplying us with the surfactants standards, and F. Ventura and J.A. Field for donating the carboxylated intermediates. This study was carried out under the CICYT R&D Project REN2001-2980-C02-01.
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