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Environmental fate & pathways

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Description of key information

In the key study (Temmink and Klapwijk 2004), the adsorption-desorption of LAS in activated sludge was determined in batch experiments. The Kp for commercial LAS was 2,500 L/kg, corresponding to a log Kp of 3.4.Sorption equilibrium was achieved rapidly, within 5 -10 minutes. Desorption was less pronounced, but still reached rapid equilibration. A sludge-water partition coefficient Kp of 3,210 L/kg volatile suspended solids is also reported for C12LAS (log Kp = 3.5) based on the work of Feijtel et al. (1999). Those researchers found that only 2-8% of commercial LAS was present as dissolved C12LAS, with the remaining 92-98% adsorbed to the sludge. Despite this high degree of sorption, more than 99% of the LAS load was removed by biodegradation, showing that the adsorbed fraction as well as the soluble fraction of LAS is readily available for biodegradation.

In a supporting study, the kinetics of the absorption-desorption of LAS as well as equilibrium isotherms were determined in batch studies for commercial LAS as well as LAS homologues C10, C11, C12and C13. A multiple pseudo-first order kinetic model provided the best fit to the kinetic data, indicating the presence of two adsorption-desorption processes. Equilibrium adsorption and desorption data demonstrate acceptable fit (R2>0.99) to a model consisting of a Langmuir plus a quadratic term, which provided an integrated description of the experimental data over a wide range of LAS concentrations (5-1800 mg/L tested). At low concentrations, the Langmuir term explained the adsorption of LAS on soil sites which were highly selective of the n-alkyl groups and covered a very small fraction of the soil surface area, whereas the quadratic term described adsorption on the much larger part of the soil surface and on LAS retained at moderate to high concentrations.

Mackay et al. (1996) conducted five-stage Level III fugacity modelling for LAS that included evaluative, regional and local-scale models. The level I and II models each resulted in LAS partitioning in air, water, soil and sediment at percentages of 0, 26, 56 and 18%, respectively. The overall residence time of LAS is predicted to be 100 hours with removal primarily by biodegradation in water (76%) and partitioning to sediment (13%). Impacts of LAS are predicted to be restricted to local receiving waters and their sediments and biota. In the Level III Fugacity Model, when discharges are directly to water, the residence time is predicted to be 33 hours and more than 99% remains in the water. However, in shallower receiving water more partitioning to sediments might be expected. When discharge is to soil, the residence time is predicted to be 28 days because of the slower biodegradation rate (compared to water) and little transfer to other media. Using the ChemCAN 4 model and assuming 90% LAS discharge to soil and 10% to water, LAS partitioning in air, water, soil and sediment is predicted to be 0, 0.64, 99.35 and 0.004%.

Level III fugacity modelling was also conducted by ECETOC (1993; reliability not assigned due to uncertainty regarding the input parameters) to predict LAS concentrations in air, biota, sediment, arable soil, suspended solids and water. LAS concentrations were predicted to be highest in soil and suspended solids.

Additional information

Computer modelling predicts that, if released to the environment, over half of LAS would partition to soil, and the remainder would be found in water and sediment. If released to water, it is predicted to be removed by biodegradation in under two days; when released to soil, removal is predicted to take 28 days. Similar investigations using laboratory studies and computer modeling indicate that when commercial LAS is mixed with activated sludge, almost all of the LAS (92 -98%, Kp=2,500 L/kg, log Kp= 3.4) would sorb to the sludge in under 10 minutes, and that while most of the added LAS sorbed to the sludge, it was available for biodegradation, which removed more than 99%. LAS is not anticipated to be found in air to any significant amount due to the low vapor pressure.