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Adsorption / desorption

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Description of key information

Teflubenzuron was very quickly and strongly adsorbed to 4 soils (K’ 159.4 to 936.4, KOC 21139 to 32556), and desorbed very little. Teflubenzuron is classified as being of low mobility to immobile. The arithmetric mean of the KOC is 26062.

Key value for chemical safety assessment

Koc at 20 °C:
26 062

Additional information

One valid study on the adsorption/desorption of teflufbenzuron is available and described as key study.


 


Celamerck (1986): Soil Adsorption/Desorption Study with CME 134. DocID according to Draft Assessment Report prepared in compliance with Regulation (EC) No 1107/2009: TZ-620-005; crossreference to Draft Assessment Report according to Regulation (EC) No 1107/2009: CA 7.1.2/01


 


The adsorption of 14C-CME 134 on soil was investigated with four different types of soil (sand, sandy loam, silt loam, clay loam) using a solution of the compound in 0.01 M CaCl2 (9.46 µg/L) and a soil-to-solution ratio of 1:5.


 


After a six hours treatment period, the concentrations in the supernatant solutions of the adsorption test samples with sand, sandy loam, silt loam and clay loam were measured and the quantities adsorbed were calculated under consideration of the no-soil control sample. It was found that 96.9 % (sand), 98.8 % (sandy loam), 99.1 % (silt loam) and 99.4 % (clay loam) of the amount of CME 134, still dissolved in the control sample, were adsorbed after the six hours treatment period. These values show that CME 134 is very strongly adsorbed on all soils tested.


 


Desorption tests which were performed subsequently with the treated soil samples revealed that 6.1 %, 3.7 %, 1.2 % and 1.3 % of the radioactivity adsorbed could be desorbed again from sand, sandy loam, silt loam and clay loam, respectively, during a twofold desorption period of 24 hours each. It is very unlikely that these trace amounts still consist of the unchanged parent compound only, since the results of the soil metabolism studies under aerobic and anaerobic conditions indicate a distinct degradation after a corresponding period of time.


 


Further investigations to determine adsorption isotherms were not possible in the case of CME 134. As no-soil control samples showed a continuous decrease of parent compound concentration, even without soil, it is not possible to achieve equilibrium conditions. This, however, is necessary to determine adsorption isotherms.


 


In consideration of all results and facts, it can safely be concluded that CME 134 itself shows practically no tendency to migrate once it is applied on soil. This is confirmed by the very low solubility of the compound in water (20 µg/L) and by the high adsorption rate on all soil types tested (≥ 97 %).


 


 


A second study of adsorption/desorption was performed with the major degradation product 3,5-dichloro-2,4-difluoro-phenylurea.


 


This study is summarized in the following for sake of completeness but has however not been documented in a separate IUCLID Endpoint Study Record.


 


Batelle (2003h): Adsorption/Desorption Characteristics of 3,5-Dichloro-2,4-Difluoro[14C]phenylurea (CL902374) to Soil. DocID according to Draft Assessment Report prepared in compliance with Regulation (EC) No 1107/2009: 2003/1010659; crossreference to Draft Assessment Report according to Regulation (EC) No 1107/2009: CA 7.1.2/02


 


The adsorption characteristics of 3,5-dichloro-2,4-difluorophenylurea were determined for three test soils and 0.01 M CaCI2 solution. The soils used were Speyer 2.2 (loamy sand), Speyer 3A (sandy silty loam) and Speyer 5M (silty sand).


Values for adsorption coefficient coefficient KD ranged from 13.3 to 29.6. Values for the adsorption coefficient based on organic carbon content ranged from 898 to 1720 in the soils tested. The adsorption process of 3,5-dichloro-2.4-difluoro[U-14C]phenylurea was fast and reached maximum levels after 1 hour in all three soils. In adsorption isotherms tests values of Freundlich adsorption coefficients (KadsF) ranged from 21.7 to 27.5.


Desorption kinetics and desorption isotherms were investigated to determine the desorption characteristics of 3,5-dichtoro-2.4-difluoro[U-14C]phenylurea from soil following an initial adsorption phase. The desorption process of 3,5-dichloro-2.4-difluoro[U-14C]phenylurea reached levels of 14 to 18 % in all three soils after 1 hour. The values of the Freundlich desorption coefficients (KdesF) ranged from 1.7 to 2.9.


 


 


Conclusion


 


In the study (Celamerck, 1986) the adsorption/desorption behaviour of teflubenzuron was investigated in four soils. The notifier concluded that teflubenzuron strongly adsorbed to soil with Koc values in a range from 21139 to 32556 mL/g. However, the RMS considers that these values are subject to uncertainty. High adherence to glass was reported in the study (approximately 16%). Equilibration was not achieved due to the rapid depletion of teflubenzuron in water, even when tested in the absence of soil as a control. OECD 106 recommends direct measurement from soil in these circumstances. However, the results were still based on measuring remaining concentrations in water, with no overall mass balance. Therefore the RMS considers that the results from the study may include some inaccuracies, but is satisfied that teflubenzuron is strongly sorbed to soil and that any disprecancies in the values will not affect the risk assessment.


 


In the second study (Batelle, 2003) to the adsorption/desorption behaviour of 3,5-dichloro-2,4-difluorophenylurea (the single soil metabolite > 10%AR) was investigated in 3 soils. The Koc values ranged from 898 to 1720 mL/g. High adherence to glass (9.9% AR) was noted in this study. However, equilibration was achieved and following supernatant and soil extractions a parental mass balance across soil and supernatant of 90% AR was seen therefore accounting for all radioactivity. Analysis was based on supernatant and soil extraction as recommended in OECD 106. Therefore the RMS considered the values to be reliable.