222 OP

Adsorption / desorption

Description of key information

The Adsorption/Desorption behavior of the test item could not be determined, as the test item is an inorganic substance that is nearly insoluble in water at the test conditions necessary for performing the study according to one of the guidelines OECD 121, OECD 106 and ISO 18479.

Additional information

1.    Statement

1.1 Selection of the appropriate method for the adsorption/desorption screening

1.1.1 OECD 121:

The estimation of the Adsorption coefficient on Soil and on Sewage Sludge using High Performance Liquid Chromatography (HPLC) does not apply because the test item is an inorganic substance that is not UV active and therefore cannot be detected by commonly available HPLC with UV-detection. Furthermore, since the method according to OECD 121 is validated and mostly applied for typical organic substances (e.g. aromatic amines), it may not be applicable to inorganic substances such as the test item.

Therefore, it was considered applying the OECD 106 guideline for screening of the adsorption /desorption.

1.1.2 OECD 106:

With the test design as described in the OECD guideline 106, two different types of matrices can be used:

1.1.2.1 Estimation of the adsorption/desorption behavior of a chemical on different soil types

One precondition for applicability of this test is the water solubility of the test item. In case the water solubility is quite low, difficulties may arise due to the fact that the concentration in the aqueous phase cannot be analyzed with sufficient accuracy.

Another precondition for applicability of the test is the availability of an analytical method with sufficient accuracy.

Since the test item is an inorganic substance containing phosphorus and aluminium, detection of both the aluminium as well as the phosphorus content may be disturbed by the high background level of both elements phosphorus and aluminium that occur in significant amount in all soil types proposed by the guideline for testing. Therefore the soil matrices may not be suitable for performing the test since no analytical method is available that could determine either aluminium or phosphorus coming from the test item with sufficient accuracy.

 

1.1.2.2 Performance of the adsorption/desorption screening using activated sludge according to ISO 18479

As an alternative to the soil matrices, the same adsorption/desorption screening method can be applied but using activated sewage sludge as matrix instead. Although sewage sludge also contains a high content of phosphorus, it may be technically possible to detect the aluminium content by the available cuvette test.

1.2 Preliminary test on the solubility of the test item

Prior to the testing of the Adsorption/Desorption behaviour of the test item solubility testing was performed with buffer solutions at pH 4, 7 and 9.

Analysis of the test item were performed by spectoscopic determination of aluminium after reaction with Eriochromacyamin R, employing a readily available cuvette test.

The test item was incubated at room temperature for approx. 16 h (overnight). No aluminium could be detected at the pH levels 4 and 7 since the test item dissolved in an amount less than 0.02 mg/L which is the detection limit of the analytical method. At pH 9, a small amount of the test item dissolved at room temperature. Since the test item is an inorganic salt, it is supposed to dissociate in buffered solution, however, the solvation seems to be depending on the pH value. Analysis by the cuvette test showed an aluminium content of 1.8 mg/L at pH 9 which corresponds to a transformation of approx. 8 % of the applied test item. Based on the chemical properties of dissolved aluminium salts in water, as reported in literature (e.g. Holleman-Wiberg, „Inorganic Chemistry“, First English edition, Academic Press, de Gruyter, 2001), it is assumed that the hydrolysis at pH 9 could be interpreted as formation of the aqua-hydroxo-aluminate complex ion. 

 

As the test item itself is an inorganic substance that is not soluble at the test conditions according to the respective guidelines, neither the principles of the OECD guideline 121 nor the principles of the OECD guideline 106 (or ISO 18479) could be applied for the test item, thus no further testing could be performed.

 

 

The Adsorption/Desorption behavior of the test item could not be determined, as the test item is an inorganic substance that is nearly insoluble in water at the test conditions necessary for performing the study according to one of the guidelines OECD 121, OECD 106 and ISO 18479.