Surfactants (also called surface active agents or wetting agents) are organic chemicals that reduce surface tension in water and other liquids. The most familiar use for surfactants are soaps, laundry detergents, dishwashing liquids and shampoos. Other important uses are in the many industrial applications for surfactants in lubricants, emulsion polymerisation, textile processing, mining flocculates, petroleum recovery, wastewater treatment and many other products and processes. Surfactants are also used as dispersants after oil spills.

There are hundreds of compounds that can be used as surfactants and are usually classified by their ionic behaviour in solutions: anionic, cationic, non-ionic or amphoteric (zwiterionic). Each surfactant class has its own specific properties.

There are many sources of surfactants that are discharged into natural waters. Industrial sources include textile, surfactants and detergent formulation. Surfactants are also used in laundries and households and are therefore found in discharges from sewage treatment works. They also have agricultural applications in pesticides, dilutants and dispersants (McNeely et al 1979).

Surfactants are compounds composed of both hydrophilic and hydrophobic or lipophobic groups. In view of their dual hydrophilic and hydrophobic nature, surfactants tend to concentrate at the interfaces of aqueous mixtures; the hydrophilic part of the surfactant orients itself towards the aqueous phase and the hydrophobic parts orients itself away from the aqueous phase into the second phase.

The hydrophobic part of a surfactant molecule is generally derived from a hydrocarbon containing 8 to 20 carbon atoms (e.g. fatty acids, paraffins, olefins, alkylbenzenes). The hydrophilic portion may either ionise in aqueous solutions (cationic, anionic) or remain un-ionise (non-ionic). Surfactants and surfactant mixtures may also be amphoteric or zwitterionic (CCME 1992).

The table below gives some examples of major commercial and industrial surfactants.

Nonylphenol and its ethoxylates (NPEs) are one of the types of surfactants causing concern. The primary source of nonylphenolic compounds in the aquatic environment is due to the incomplete degradation of NPE (nonylphenol ethoxylate) surfactants during sewage treatment, and therefore it is unlikely to be present in the aquatic environment in the absence of other NPE degradation by-products (such as nonylphenol mono- and diethoylates (NP1EO and NP2EO) and nonylphenoxy carboxylic acids (NPEC)).

Concentrations of nonylphenol in surface waters vary widely but locally high concentrations (sometimes in excess of 100 µg l^-1) have been reported, especially in areas receiving industrial and sewage discharges. Higher concentrations (several mg kg^-1) have been detected in sediments, although much of this is unlikely to be bioavailable (Whitehouse et al 1998a).

In view of their hydrophilic nature, surfactants tend to be water soluble to some degree. Depending on the specific chemicals, solubility varies from very soluble (e.g. some anionic surfactants) to insoluble (e.g. some cationic surfactants) (Lewis and Wee 1983) .

Anionic surfactant are not appreciably sorbed by inorganic solids. On the other hand, cationic surfactants are strongly sorbed by solids, particularly clays. Significant sorption of anionic and non-ionic surfactants has been observed in activated sludge and organic river sediments. Depending on the nature of their hydrophobic moieties, non-ionic surfactants may be sorbed onto surfaces. Some surfactants have been found to alter the sorption to surfaces of coexisting chemical species, such as metals (CCME 1992).

In general, surfactants in modern day use are considered to be biodegradable under conditions of efficient sewage treatment, The rates of degradation depend partially on the chemical structure. Surfactants containing linear hydrophobes are generally more biodegraded than those containing branched hydrophobes. Nonylphenol and some of its ethoxylates are not readily degraded during sewage treatment (CCME 1992).

Because of the hundreds of compounds that can be used as surfactants and because the toxicity (and potential to be present in sediment) and bioaccumulation potential will vary according to the type of surfactant, an assessment is not possible here.

An exhaustive literature review on the toxicity of surfactants to marine organisms has not been carried out for the purposes of this profile. The information provided in this section is taken from existing review documents for one surfactant (nonylphenol) as an example (Whitehouse et al 1998a). The most sensitive groups of organisms have been identified.

Whitehouse et al (1998a and 1998b) reviewed data on the saltwater toxicity of nonylphenol and octylphenol. By way of an example, their conclusions for nonylphenol are presented below.

The authors reported that, in acute studies with saltwater species, the mysid shrimp Mysidopsis bahia was the most sensitive species, where a 96 hour LC50 of 43 µg l^-1 was reported. Corresponding 96 hour LC50s values for fish were higher, ranging from 135 µg l^-1 for fathead minnow Pimephales promelas to 3,000 µg l^-1 for cod Gadus morhua. Nonylphenol was generally toxic to algae at concentrations greater than 500 µg l^-1 although the lowest 96 hour EC50 (for growth) was 27 µg l^-1 in the marine diatom, Skeletonema costatum.

Some toxicity data for sediment-dwelling organisms were also presented (although it relates to freshwater organisms). Whitehouse et al (1998) found nonylphenol dosed into sediment would not be readily bioavailable. Much higher levels of nonylphenol were required in sediment than in water to cause adverse effects to the sensitive midge larvae Chironomus tentans .

Whitehouse et al (1998a) also investigated data on the endocrine disrupting effects of nonylphenol. While no data were available for saltwater organisms, nonylphenol concentrations of 20 µg l^-1 and greater were found to cause effects related to oestrogenicity.

With regard to bioaccumulation, Whitehouse et al (1998a) found bioaccumulation factors for aquatic organisms to be around 300. However, much higher values were found for algae (but this may have been due to adsorption) and when radio-labelled nonylphenol was used.

Potential effects include:

• toxicity of nonylphenol and octylphenol to algae, invertebrates and fish at concentrations above the respective EQSs of 1 microg l^-1 (annual average) and 2.5 microg l^-1 (maximum allowable concentration) for nonylphenol and 1 microg l^-1 (annual average) and 2.5 microg l^-1 (maximum allowable concentration) for octylphenol in the water column;
• accumulation of nonylphenol in sediments though bioavailability is considered to be low;
• nonylphenol has been found to have endocrine disrupting effects in freshwater organisms at concentrations of 20 microg l^-1.

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