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Environmental fate & pathways

Adsorption / desorption

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Endpoint:
adsorption / desorption, other
Remarks:
Further information on adsorption/desorption
Type of information:
experimental study
Adequacy of study:
key study
Study period:
Not reported
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
study well documented, meets generally accepted scientific principles, acceptable for assessment
Qualifier:
no guideline followed
Principles of method if other than guideline:
Laboratory experiments and field measurements were conducted to determine the extent and reversibility of the sorption of organotins and their major degradation products to lake sediments under typical natural conditions and to check whether the same sorption mechanism(s), as found for humic acids in earlier work, govern the sorption of organotins to sediment organic matter. The postdepositional fate of these chemicals in sediments was also investigated. Sorption-desorption batch experiments were therefore conducted using contaminated freshwater harbour sediments and two certified organotin containing marine sediments. To distinguish between the organotins originally present in these sediments and the freshly sorbed organotins, sorption experiments were carried out using perdeuterated standard compounds. The sediment-water distribution ratios determined in the laboratory were compared with in situ distribution ratios calculated from solid and pore water concentration depth profiles measured in two dated sediment cores from two different harbours.
GLP compliance:
not specified
Remarks:
GLP status is not required for environmental fate studies.
Radiolabelling:
yes
Test temperature:
22 ± 2 °C
Analytical monitoring:
yes
Matrix no.:
#4
Matrix type:
other: Sediment Core from Harbour Wӓdenswil (W2)
% Clay:
11
% Silt:
81
% Sand:
8
% Org. carbon:
0.001
pH:
7
Matrix no.:
#5
Matrix type:
other: Sediment Core from Harbour Enge
% Clay:
14
% Silt:
84
% Sand:
2
% Org. carbon:
0.001
pH:
7.2
Details on matrix:
FIELD MEASUREMENTS
> Sampling, Sample Storage, and Organotin Analysis
Harbour waters and sediment cores were sampled from several harbours of Lake Zurich and Lake Constance.
Activity records from local authorities ensured that harbours in which sediments were sampled had not been dredged for at least 20 years. Harbour water was sampled in polycarbonate flasks and analysed within 24 h. Sediments, including pore water, were collected with a gravity corer using either PVC or Plexiglas liners of 6 and 12 cm inner diameter, respectively. The sediment cores in PVC liners were transported to the laboratory, and on the same day they were photographed, extruded, and sliced at 1, 2, or 20 cm intervals. Although PVC is known to contain organotins such as stabilisers, no organotin contamination was observed in the sediments, which was attributed to the short contact time of <4 h. The samples were transferred to polycarbonate bottles and freeze-dried. The sediment cores in Plexiglas liners (E2 and W2 in Figure 4) which were used for a more detailed study of the in-situ partitioning and transport of organotins were extruded on-site and sliced at 1, 2, or 4 cm intervals. The samples were then transferred to 250 mL polycarbonate centrifuge bottles and transported to the laboratory, where they were centrifuged on the same day at 20 000g for 30 min. The sediment pore water was decantedand filtered. The solid sediment samples were freeze-dried and kept at -25 °C until analysis.

> Determination of Particulate Organic Carbon (POC), Grain Size Distribution, and Water Content of the Sediments
Total (organic and inorganic) carbon content of the freeze-dried sediments as well as hydrogen and nitrogen were determined using a CHNS-Analyser. Inorganic carbon was determined by acid digestion of the sediment samples and subsequent titration of the released carbon dioxide using a CO2-Coulometer 5011. The POC content was calculated as the difference between total carbon and inorganic carbon content. The coarse fraction of the sediments (>150 µm) was sieved and weighed. The size distribution of the smaller particles was determined with a Mastersizer X laser particle sizer. The water content was calculated as the weight difference between the wet and the freeze-dried sediments.

> Determination of Dissolved Organic Carbon (DOC) in Sediment Pore Water
The dissolved organic carbon (DOC) of the filtered pore water (0.45 µm) was determined using a Shimadzu 500 TOC analyser calibrated with phthalic acid solutions. Prior to analysis, the samples were acidified with one drop of 32% HCl.
Details on test conditions:
LAKE WATER
- Source: lake Zurich
- Pre-treatment of lake water: the water was filtered through 0.45 µm polycarbonate filters and purged with nitrogen for 4 h. The water was then buffered to pH 7.3 with 1 mM MOPS buffer.

TEST SYSTEM
- Type, size and further details on reaction vessel: For each of the four parallel experiments, 10 L of the buffered water were filled in 12.5-L polycarbonate containers, stirred at 22 ± 2 °C using a polycarbonate coated magnetic stir bar, and purged again with nitrogen for 1 h.


LABORATORY EXPERIMENTS
> Determination of Sediment-Water Distribution Coefficients (Kdobs) and Equilibrium Kinetics of OT and Perdeuterated OT Compounds
Kdobs = Cs/Cw
where
Cs: concentration in sediment
Cw: concentration in aqueous phase of equilibrium

Kdobs was determined with organotin contaminated sediments and lake water using a three batch approach. Each batch series included a duplicate determination of Kdobs and two blanks containing only organotin and organotin-dn spiked water.
For each batch series, three sequential batch experiments were performed in duplicate. In the first batch, 10 L of the filtered and buffered lake water was spiked with 5000 ng of perdeuterated organotin compound (C0 = 500 ng/L), and 5.0 g of dried lake sediment W2-18 (freeze-dried sediment from 18 to 20 cm depth of sediment core W2 collected in harbour Wӓdenswil, lake Zurich) or of certified reference sediment PACS-1/PACS-2 were suspended in the polycarbonate containers. The containers were closed airtight, and the headspace was continuously purged with nitrogen throughout the experiment. After 24 and 48 h, the sediment was allowed to settle, and 500 mL water samples were removed from the container. After a contact time of 72 h, the sediment was filtered. The water samples and 1/3 of the filter-wet sediment were analysed for organotin concentration.
For the second and the third batch, the remaining 2/3 and 1/2 of the wet sediments from the first and second batch, respectively, were suspended in newfractions of 10 L of pretreated lake water. To be able to observe desorption of the freshly sorbed perdeuterated organotins, these compounds were not spiked to the second batch but again to the third batch at a concentration of 100 ng/L. Water samples and sediment were collected and analysed as described for the first batch.
Two blanks that were carried out simultaneously with identical organotin concentrations, but without sediment, were used to quantify organotin losses to the polycarbonate container surfaces and during filtration.
Sample No.:
#1
Type:
log Kd
Value:
3.89 L/kg
pH:
7.35
Temp.:
22 °C
Matrix:
Sediment layer from core W2, 18-20 cm depth
% Org. carbon:
58
Remarks on result:
other: laboratory experiment
Sample No.:
#1
Type:
other: log Doc r
Value:
5.12 L/kg
pH:
7.35
Temp.:
22 °C
Matrix:
Sediment layer from core W2, 18-20 cm depth
% Org. carbon:
58
Remarks on result:
other: laboratory experiment
Remarks:
result normalised for organic carbon content
Sample No.:
#2
Type:
log Kd
Value:
3.68 L/kg
pH:
7.35
Temp.:
22 °C
Matrix:
PACS-1
% Org. carbon:
37
Remarks on result:
other: laboratory experiment
Sample No.:
#2
Type:
other: log Doc r
Value:
5.11 L/kg
pH:
7.35
Temp.:
22 °C
Matrix:
PACS-1
% Org. carbon:
37
Remarks on result:
other: laboratory experiment
Remarks:
result normalised for organic carbon content
Sample No.:
#3
Type:
log Kd
Value:
3.15 L/kg
pH:
7.35
Temp.:
22 °C
Matrix:
PACS-2
% Org. carbon:
32
Remarks on result:
other: laboratory experiment
Key result
Sample No.:
#3
Type:
other: log Doc r
Value:
4.65 L/kg
pH:
7.35
Temp.:
22 °C
Matrix:
PACS-2
% Org. carbon:
32
Remarks on result:
other: laboratory experiment
Remarks:
result normalised for organic carbon content
Sample No.:
#4
Type:
log Kd
Value:
3.26 L/kg
pH:
7
Matrix:
Harbour Wӓdenswil
% Org. carbon:
58
Remarks on result:
other: in situ determination
Sample No.:
#4
Type:
other: log Doc r
Value:
4.61 L/kg
pH:
7
Matrix:
Harbour Wӓdenswil
% Org. carbon:
58
Remarks on result:
other: in situ determination
Remarks:
result normalised for organic carbon content
Sample No.:
#5
Type:
log Kd
Value:
3.58 L/kg
pH:
7.2
Matrix:
Harbour Enge
Remarks on result:
other: in situ determination
Sample No.:
#5
Type:
other: lod Doc r
Value:
4.92 L/kg
pH:
7.2
Matrix:
Harbour Enge
Remarks on result:
other: in situ determination
Remarks:
result normalised for organic carbon content
Adsorption and desorption constants:
> Laboratory sorption experiments
- Sorption of the organotins to the sediments tested was fast and reversible.
- At ambient pH values, the sorption of triorganotins (and probably also of the di- and monoorganotin compounds) to sediments is dominated by complex formations of the positively charged organotin-species with oxygen ligands present in the particulate organic matter. The fast kinetics and reversibility of sorption can be rationalised by considering that in aqueous solution, the polar particulate organic matter ligands are more easily accessible as compared to the more hydrophobic domains.

> Field data
- Kd obs did not depend on the water/sediment ratio
- sorption equilibrium was established in both cores (i.e., no aging phenomena were observable)

Since very similar Doc values were determined in both the field and laboratory, the sorption mechanisms can be expected to be the same (i.e. readily reversible complexation of OTs by negatively charged POM-ligands).
Validity criteria fulfilled:
not applicable
Conclusions:
Findings from the study indicate that sorption of organotin compounds to sediments is a fast and reversible process involving primarily particulate organic matter constituents as sorbents. In general, organotin compounds will readily desorb and, therefore, be more readily bioavailable. However, organotins present in the sediment can be expected to be bioavailable to benthic organisms.
Executive summary:

Laboratory experiments and field measurements were conducted to determine the extent and reversibility of the sorption of organotins and their major degradation products to lake sediments under typical natural conditions and to check whether the same sorption mechanism(s), as found for humic acids in earlier work, govern the sorption of organotins to sediment organic matter. The postdepositional fate of these chemicals in sediments was also investigated. Sorption-desorption batch experiments were therefore conducted using contaminated freshwater harbour sediments and two certified organotin containing marine sediments (PACS-1 and PACS-2). To distinguish between the organotins originally present in these sediments and the freshly sorbed organotins, sorption experiments were carried out using perdeuterated standard compounds. The sediment-water distribution ratios determined in the laboratory were compared with in situ distribution ratios calculated from solid and pore water concentration depth profiles measured in two dated sediment cores from two different harbours.

Findings from the study indicate that sorption of organotin compounds to sediments is a fast and reversible process involving primarily particulate organic matter constituents as sorbents. In general, organotin compounds will readily desorb and, therefore, be more readily bioavailable. The log Kd values that were determined for MBTCl3 ranged from 3.15 to 4.92 L/kg. Experimental results normalised for carbon content in the sediment indicated that log Doc values were in the range of 5.12 to 4.65 which are in the same range as the in situ measured values of 4.61 to 4.92 L/Kg.

Endpoint:
adsorption / desorption: screening
Data waiving:
study scientifically not necessary / other information available
Justification for data waiving:
the study does not need to be conducted because the substance has a low octanol water partition coefficient and the adsorption potential of this substance is related to this parameter
Reason / purpose:
other:

Description of key information

There is a single relevant and reliable study available to address the endpoint. The experimentally derived Log Doc r value of 4.65 was selected for chemical safety assessment as it is considered the most reliable (due to being obtained in controlled experimental conditions) and results in a worst-case scenario with regards to potential exposure to the substance in the modelling conducted in the CSR.

Key value for chemical safety assessment

Koc at 20 °C:
44 668.36

Other adsorption coefficients

Type:
other: log Doc r (Distribution coefficient sediment/water normalised for carbon content)
Value in L/kg:
4.65
at the temperature of:
22 °C

Other adsorption coefficients

Type:
log Kp (solids-water in sediment)
Value in L/kg:
3.15
at the temperature of:
22 °C

Additional information

Until very recently, the substance was considered to exhibit properties, including rapid hydrolysis, such that waivers were submitted to cover this endpoint as certain tests were not considered technically feasible.  As part of the ongoing CoRAP evaluation additional investigations have been, and are currently being, conducted and studies are still ongoing.  Current evidence suggests that the substance may in fact behave differently than originally thought and as such the results presented in Berg et al. (2001) are considered to accurately reflect the behaviour of the substance with regards to its adsorption/desorption.