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Environmental fate & pathways

Adsorption / desorption

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Description of key information

The available information suggests that Stilbene Fluorescent Whitening Agents have not an univocal adsorption/desorption behaviour describable by a range of values: they are expected to be more or less mobile depending on the soil type and conditions.

Key value for chemical safety assessment

Koc at 20 °C:
10 000

Additional information

Adsorption/desorption screening with HPLC cannot technically be performed on the members of the Stilbene Fluorescent Whitening Agents category.

 

Three studies are available for the assessment of the adsorption potential of the substances belonging to the Stilbene Fluorescent Whitening Agents category: they were performed on CAS 16090-02-1 (morpholino derivative, disulphonate), CAS 16470-24-9 (dihydroxyethylamino tetrasulphonated salt) and on CAS 68971-49-3 (dihydroxyethylamino, esasulphonated).

Results of the test performed on CAS 68971-49-3
6 Na+ Eurosoil 5 Eurosoil 3 LUFA 2.3 Eurosoil 2 Eurosoil 4
Water solubility: 278 g/l at 20 °C pH 9
Dry Weight [%] 97.7 96.9 94.6 96.5 97.3
pH (Aqua demin.)  3.59 6.98 6.9 7.04 7.09
Organic Carbon [%] 5.96 3.01 0.64 3.72 1.31
Clay (<0.002 mm) [%]  6 17 10 22.6 20.3
Silt (0.002-0.063 mm) [%]  12.7 36.8 31 64.1 75.7
Sand (0.063-2 mm) [%]  71.6 46.4 59.1 13.4 4.1
Cation Exchange Capacity [mval/100g] 24.1 16.6 5.2 28.9 17.3
Koc [ml/g] 72.1 110 423 761 874
log Koc 1.8 2.04 2.6 2.8 2.9
Mobility High High medium low low

 Results of the test performed on CAS 16470-24-9  Results of the test performed on CAS 16090-02-1
4 Na+ LUFA 2.3
sandy loam
LUFA 2.1
sand
LUFA 2.2
loamy sand
2 Na+ LUFA 2.2
loamy sand
LUFA 2.1
sand
LUFA 2.3
sandy loam
Water solubility: 650 g/l at 20 °C pH 9; 406 g/l at 20 °C pH 7.9 Water solubility: 1.9 g/l at 20 °C pH 10.5
pH 6.6 5.9 5.8 pH 5.8 5.9 6.6
max. water cap. (g/100 g TM) 35.3 26.1 44 max. water cap. (g/100 g TM) 44 26.1 35.3
Organic Carbon [%] 1.34 0.7 2.29 Total N [%] 0.17 0.06 0.11
<0.02 mm [%]  19.6 7.9 10.1 Organic Carbon [%] 2.29 0.7 1.34
Clay (<0.002 mm) [%]  8.3 3.5 5.1 <0.02 mm [%]  10.1 7.9 19.6
Silt (0.002-0.063 mm) [%]  3.9 1.9 2.1 Clay (<0.002 mm) [%]  5.1 3.5 8.3
Sand (0.063-2 mm) [%]  88 94.6 92.9 Silt (0.002-0.063 mm) [%]  2.1 1.9 3.9
Cation Exchange Capacity [mval/100g] 9.5 4.9 9.7 Sand (0.063-2 mm) [%]  92.9 94.6 88
Total-N [%] 0.11 0.06 0.17 Cation Exchange Capacity [mval/100g] 9.7 4.9 9.5
Koc [dm3/g] 2.84 4.21 10 Koc [dm3/g] 0.86 1.04 2.24
Koc [ml/g] 2840 4210 10000 Koc [ml/g] 860 1040 2240
logKoc 3.4 3.6 4 logKoc 2.9 3 3.3
Mobility slight slight immobile Mobility low low slight
% of asdorbed test item not desorbed  >96.1  >95.4  >96.2 % of asdorbed test item not desorbed  >73.9  67.2  >83.8
>96.2 >94.9 >96.1 75.6 >72.0 >84.0

As all the substances belonging to the category, the test items are very soluble in water and they are characterized by a negative log kow. The experimental Koc values obtained describe different adsorption potentials: in the cases of CAS 16090-02-1 and CAS 16470-24-9 the substances can be slightly mobile to immobile. In the case of CAS 68971-49-3 the substances resulted to be high, medium and low mobile on the basis of the different soils used.

 

The distribution of a chemical between soil and aqueous phases is a complex process depending on a number of different factors: the chemical nature of the substance, the characteristics of the soil and climatic factors such as rainfall, temperature, sunlight and wind. The increasing (or decreasing) of the mobility seems to be not clearly influenced by a particular condition assayed in the studies.

Analysing the tables data, the pH is the parameter that mainly undergoes to changes related to Koc values, nevertheless its effective impact can be considered not linear. Furthermore, the pH values do not help to trace a common behaviour: in the case CAS 68971-49-3 and CAS 16090-02-1 lower pHs correspond to higher mobility, while in the case of CAS 16470-24-9 the trend resulted inverted. In the case of CAS 68971-49-3 the clay and sand soil contents seems to be related to the Koc. In all cases, the organic carbon content and the soil cation exchange capacity seem not directly influence the chemicals’ mobility.

 

The numerous phenomena and mechanisms involved in the process of adsorption of a chemical by soil cannot be completely defined by a simplified laboratory model: the tests performed give an indication about adsorption as process of the binding of a chemical to surfaces of soils but they do not distinguish between different adsorption processes (physical and chemical adsorption) and such processes as surface catalysed degradation, bulk adsorption or chemical reaction.

No calculation of the mass balance as well as no desorption kinetic was possible during the test conducted on CAS 68971-49-3 because an accurate quantization method has not been found during the test. On the contrary, in the two studies performed with CAS 16090-021 and CAS 16470-24-9 the percentages of substance not desorbed were in the range of 67 -84 % and 94 – 96 %, respectively. The low percentage of desorption recorded during the tests can be considered as an indication that the substances under investigation are able to interact with the soil substrates chemically; however, it cannot be established the strength of this interaction.

 

In conclusion, the available information suggests that Stilbene Fluorescent Whitening Agents have not an univocal adsorption/desorption behaviour describable by a range of values: they are expected to be more or less mobile depending on the soil type and conditions.

 

In order to limit the hazard for soil compartment, the risk assessment for the substances belonging to the category has been performed using the highest experimental Koc value. In this way, the Predicted Environmental Concentrations in soil are maximized (for further details, see the Category Justification Report).

[LogKoc: 4.0]