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Environmental fate & pathways

Adsorption / desorption

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Reference
Endpoint:
adsorption / desorption, other
Remarks:
other: partition coefficient
Type of information:
experimental study
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: Peer-reviewed research publication.
Qualifier:
no guideline followed
Principles of method if other than guideline:
A sorption-isotherm and the kinetics of adsorption and desorption of LAS to activated sludge were determined in batch experiments. Three different biodegradation tests were also carried out (an OECD 301F ready biodegradation test; a batch activated sludge [BAS] test; and a "by-pass" test developed to mimic condition of the pilot scale activated sludge plant).
GLP compliance:
no
Type of method:
batch equilibrium method
Media:
sewage sludge
Radiolabelling:
no
Test temperature:
not reported
Analytical monitoring:
yes
Details on sampling:
At least every other day 24-h samples of 2.5L influent and 2.5L effluent were collected in PE bottles and total (sum of adsorbed and dissolved) LAS concentrations were determined using an HPLC method adapted from Feijtel et al. 1995. At least once a day 200 mL grab samples were taken from the aeration tank and return sludge and transferred into PE centrifuge tubes for determination of dissolved and absorbed LAS. The sludge samples were immediately centrifuged for 15 min at 3500 rpm. The supernatant was transferred into PE bottles and preserved by 3% formalin and stored for a maximum of 10 days at 4°C until further analysis. Representative aliquots of pre-settled influent, final effluent, or supernatant of the centrifuged sludge samples were passed over 6 mL preconditioned C18 SPE columns at a rate not exceeding 10 mL/min. The SPE columns were washed with 2 mL methanol/water and eluted with 5 mL of methanol. The eluate was then passed through strong anion exchange (SAX) columns, washed, eluted and subsequently evaporated to dryness under a gentle flow of nitrogen gas. The dry residue was dissolved in 2-5 mL of HPLC mobile phase.
Details on matrix:
Monitoring data were collected in a pilot-scale municipal activated sludge treatment plant. The plant consisted of a completely mixed aeration tank (490L) and a secondary settler (280L). The plant was operated at C12 LAS influent concentrations between 2 and 12 mg/L and at sludge retention times of 10 and 27 days.
Details on test conditions:
A sorption-isotherm and the kinetics of adsorption and desorption of LAS to activated sludge were determined in batch experiments. Three different biodegradation tests were also carried out (an OECD 301F ready biodegradation test; a batch activated sludge [BAS] test; and a "by-pass" test developed to mimic condition of the pilot scale activated sludge plant). Only the sorption results are presented here.
Phase system:
other: log Kp
Type:
other: log Kp
Value:
3.5 dimensionless
Remarks on result:
other: C12 LAS
Phase system:
other: log Kp
Type:
other: log Kp
Value:
3.4 dimensionless
Remarks on result:
other: C11.6 LAS
Adsorption and desorption constants:
Kp (C12 LAS): 3210 L/kg (log Kp = 3.5)
Kp (commercial C11.6 LAS mixture): 2,500 L/kg (log Kp = 3.4)
Details on results (Batch equilibrium method):
Sorption equilibrium was achieved rapidly, within 5-10 minutes. Desorption was less pronounced, but still reached rapid equilibration. The sludge-water partition coefficient Kp of 3210 L/kg volatile suspended solids is reported. Applying the same QSAR for the commercial C11.6 LAS mixture results in a value of log Kp = 3.4 (i.e., Kp = 2500 L/kg), consistent with Feijtel et al. 1999 (see section 3.3.1(b)). In the other experiments conducted in this study, only 2-8% was present as dissolved C12 LAS, 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.
Conclusions:
The log Kp for commercial LAS was 3.4.
Executive summary:

The adsorption-desorption of LAS in activated sludge was determined in batch experiments. The Kp for commercial LAS was 2,500 L/kg, with a log Kp of 3.4.

Description of key information

The adsorption-desorption of LAS in activated sludge was determined in batch experiments. The Kp for commercial LAS was 2,500 L/kg, with a log Kp of 3.4.

Key value for chemical safety assessment

Other adsorption coefficients

Type:
log Kp (solids-water in activated sewage sludge)
Value in L/kg:
2 500

Additional information

The adsorption-desorption of LAS in activated sludge was determined in batch experiments. The Kpfor commercial LAS was2,500 L/kg, corresponding to a log Kpof 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 Kpof 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 another study, the kinetics of LAS adsorption and desorption to soil were investigated. Concentrations of 5 -1800 mg/L LAS and exposures times of up to 61 hrs were studied. The adsorption/desorption rates were found to depend on the carbon chain length of the homologue, and the adsorption/desorption behaviour was found to follow a combined Langmuir/quadratic model.