LINEAR ALKYLBENZENE SULFONATE (LAS) - UNEP Chemicals

OECD SIDS LINEAR ALKYLBENZENE SULFONATE (LAS)
Remarks: A significant number of additional literature articles report data on the
environmental fate of LAS. An additional bibliography of literature citations
for LAS can be found in an Appendix to this dossier.
(b)
Type of Measurement: Background [X]; At contaminated site [ ]; Other [ ]
Medium: soil
Remarks: Where surfactant and hydrophobic organic compounds (HOCs) co-exist in
soil-water systems there are a number of possible interactions which can
occur simultaneously: 1) distribution of surfactant between monomeric,
hemimicellar and miscellar forms, 2) competition for hydrophobic
adsorption sites between the surfactant and HOC and 3) partitioning of
HOC among soil hydrophobic adsorption sites, surfactant micelles and
hemimicelles. The interaction of HOCs with surfactant monomers is
usually very weak and insignificant. At concentrations where micelles and
hemimicelles are present interactions can take place. Sorbed HOCs can be
solubilised by free micelles, resulting in mobilisation. HOCs in solution
are in equilibria between sorption onto hydrophobic adsorptive sites on the
soil, partitioning into hemimicelles – both resulting in immobilisation, and
partitioning into free micelles. Whether the HOCs are previously sorbed
onto soil or are in solution, partitioning into micelles, hence mobilisation, is
favoured by increasing surfactant concentration. A model has been put
forward describing the effect of non-ionic surfactant on the distribution of
HOC in a soil-water system. In simple terms the model illustrates that
sorbed surfactant molecules tend to increase HOC sorption onto soil by
increasing its fractional organic carbon content, and free surfactant tends to
decrease sorption by increasing the apparent aqueous solubility of the
HOC.
The biodegradation of HOCs in soil can be enhanced by surfactants due to
enhanced solubility in the presence of micelles. In some cases however,
biodegradation appears to be inhibited by micelles forming a barrier to the
degrading organism. Any such inhibition is unlikely to be prolonged due to
the biodegradable nature of most modern surfactants. In fact, it is very
unlikely that micelles would be present in sludge-amended soils due to the
low concentration of surfactants.
Although there is evidence that surfactants can effect the fate and behaviour
of HOCs in soil, the potential for detergent ingredients to cause significant
effects is limited due to the relatively low concentrations found compared
with critical micelle concentrations (CMCs). In addition, the effective
CMC in environments such as soil and sediments is generally much higher
than in clean water systems. Typical soil concentrations of LAS, the most
heavily used surfactant in domestic detergents, are significantly lower than
those required to produce micelles in pore water. Therefore, it is unlikely
that surfactants present in domestic detergents will contribute significantly
to the mobilisation of HOCs in sludge-amended soil.
Reference: Haigh, S.D. 1996. A review of the interaction of surfactants with organic
contaminants in soil. The Science of the Total Environment 185:161-170.
UNEP PUBLICATIONS 174