User Guide to Thresholds and Classification - Environmental ...

338User Guide for Thresholds and Classificationsproposed that a cut-off limit of 700 for the molecular weight could be applied. However, this cut-off has beensubject to criticism and an alternative cut-off of 1000 has been proposed in relation to exclusion ofconsideration of substances with possible indirect aquatic effects (CSTEE, 1999). In general,bioconcentration of possible metabolites or environmental degradation products of large molecules shouldbe considered. Data on bioconcentration of molecules with a high molecular weight should therefore becarefully evaluated and only be used if such data are considered to be fully valid in respect to both the parentcompound and its possible metabolites and environmental degradation products.Surface-active agentsSurfactants consist of a lipophilic (most often an alkyl chain) and a hydrophilic part (the polar headgroup).According to the charge of the headgroup, surfactants are subdivided into classes of anionic, cationic, nonionic,or amphoteric surfactants. Due to the variety of different headgroups, surfactants are a structurallydiverse class of compounds, which is defined by surface activity rather than by chemical structure. Thebioaccumulation potential of surfactants should thus be considered in relation to the different subclasses(anionic, cationic, non-ionic, or amphoteric) instead of to the group as a whole. Surface-active substancesmay form emulsions, in which the bioavailability is difficult to ascertain. Micelle formation can result in achange of the bioavailable fraction even when the solutions are apparently formed, thus giving problems ininterpretation of the bioaccumulation potential.Experimentally derived bioconcentration factorsMeasured BCF values on surfactants show that the BCF may increase with increasing alkyl chain length andbe dependant of the site of attachment of the head group, and other structural features.Octanol-water-partition coefficientThe octanol-water partition coefficient for surfactants can not be determined using the shakeflask or slowstirring method because of the formation of emulsions. In addition, the surfactant molecules will exist in thewater phase almost exclusively as ions, whereas they will have to pair with a counter-ion in order to bedissolved in octanol. Therefore, experimental determination of KOW does not characterise the partition ofionic surfactants (Tolls, 1998). On the other hand, it has been shown that the bioconcentration of anionic andnon-ionic surfactants increases with increasing lipophilicity (Tolls, 1998). Tolls (1998) showed that for somesurfactants, an estimated log KOW value using LOGKOW could represent the bioaccumulation potential;however, for other surfactants some ‗correction‘ to the estimated log KOW value using the method ofRoberts (1989) was required. These results illustrate that the quality of the relationship between log KOWestimates and bioconcentration depends on the class and specific type of surfactants involved. Therefore,the classification of the bioconcentration potential based on log KOW values should be used with caution.Conflicting data and lack of dataConflicting biconcentration factor dataIn situations where multiple BCF data are available for the same substance, the possibility of conflictingresults might arise. In general, conflicting results for a substance, which has been tested several times withJanuary 2012 EPA0109