Silicic acid, aluminum magnesium sodium salt

Adsorption / desorption

Link to relevant study record(s)

Reference

Endpoint:

adsorption / desorption: screening

Type of information:

experimental study

Adequacy of study:

key study

Study period:

November 2016

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

guideline study

Qualifier:

according to guideline

Guideline:

OECD Guideline 106 (Adsorption - Desorption Using a Batch Equilibrium Method)

Qualifier:

according to guideline

Guideline:

EU Method C.18 (Adsorption / Desorption Using a Batch Equilibrium Method)

GLP compliance:

yes (incl. QA statement)

Type of method:

batch equilibrium method

Media:

soil

Radiolabelling:

no

Test temperature:

The test was performed at 22.5 ± 0.5 °C.

Analytical monitoring:

yes

Details on sampling:

In the preliminary test, one soil/solution ratio with the highest amount of soil was tested only: 1 : 1 (30 g soil and 30 mL test solution).
For equilibration, the air-dried soil samples (30 g each) were shaken with 27 mL of a 0.01 M CaCl2-solution for 2 days. After this time, 3 mL of filtrated test item stock solution (nominal concentration 50 g/L, see chapter 4.1.3) was added to 27 mL preparations to result in a test item solution with nominal concentration 5 g/L.To the blank samples 3 mL of a 0.01 M
CaCl2-solution was added. Six replicates of mixtures with the test item solution per soil were prepared (two replicates per sampling time: 0 h, 24 h and 48 h). Six blanks per soil and one replicate of CaCl2 solution (without test item) was prepared in order to determine the background signal.

Details on test conditions:

The mixtures were shaken for 48 h at 22.5 °C. After 1 minute shaking two replicates with the test item and two blanks per soil were sampled as an initial value
(0 h). The initial concentration of silicon in the stock solution was determined with 8.30 mg/L Si.

Key result

Type:

log Koc

Remarks on result:

other: According the test results the determination of the adsorption of Silicic acid Aluminium Magnesium Sodium Salt on soils is not possible.

Details on results (Batch equilibrium method):

In the blank solutions on day 0, the concentration of silicon was higher than the silicon concentration in the test item solutions after subtraction of the blank value. Furthermore, the measured silicon concentration in the test solution with soil Lufa 2.2 has changed from 8.30 mg/L to 5.34 mg/L Si after one minute shaking of the solution with the soil. The concentration of silicon in Eurosoil 5 after one minute shaking lay in the similar range as the start value in the stock solution (8.48 mg/L Si).
During the test the measured silicon concentration in the blank solutions with soil Lufa 2.2 and Eurosoil 5 increased incessantly from 11.06 to 17.02 mg/L Si and from 8.50 to 21.08 mg/L Si, respectively, despite the 48 h equilibration phase before start of the test. After 48 h test duration, the concentration of silicon in Eurosoil 5 test item solution remained unchanged in comparison to the start vale (0 h). In contrast, the concentration of silicon in Lufa 2.2 test item solution was 59 % lower than the initial value, although the value in the blank solution increased incessantly. This is probably due to the precipitation of silicon in Lufa 2.2 test item solution during the test instead of adsorption, as different Si ions were probably present in the test mixture (obtained from the soil and from the test item). Furthermore, the properties of dissolved silicon are strongly dependent on the pH value. The measured pH values in Eurosoil 5 and in Lufa 2.2 test item solutions were 3.1 and 6.8, respectively.

Table: Mean Concentrations on Day 0

Soil	Sample	Mean Value Si	Amount Si – blank	Mean Value Si	RSD
		mg/L	mg/L	mg/L	%
Eurosoil 5	Blank 1 and 2	8.50	-	-	0.5
	Flask 1	17.18	8.68	8.48	3.2
	Flask 2	16.79	8.29
Lufa 2.2	Blank 1 and 2	11.06	-	-	3.8
	Flask 1	16.14	5.08	5.34	6.9
	Flask 2	16.67	5.60

Table: Results

Soil	Sample	Mean Value Si on day 0	Mean Value Si after 24 h	Mean Value Si after 48 h	Difference after 48 h
		mg/L	mg/L	mg/L	%
Eurosoil 5	Blank	8.50	15.71	21.08	+147.9
	Test Solution*	8.48	8.09	8.52	+0.5
Lufa 2.2	Blank	11.06	14.59	17.02	+.53.9
	Test Solution*	5.34	2.87	2.18	-59.1

*the concentration of Si after subtraction of the blank value

Conclusions:

The determination of the adsorption of Silicic acid Aluminium Magnesium Sodium Salt on soils is not possible.

Executive summary:

Testing of the adsorption of Silicic acid, aluminium magnesium sodium salt on soils according OECD 106 failed, in a preliminary study to determine the concentration of silicon in the blank value in comparison to the test item solution the the concentration of silicon was higher than the silicon concentration in the test item solution after subtraction of the blank value.

Test solution (test item) and soil contain beside same cations (natrium, magnesium, aluminium) also silicon on both sides. Depending on parameters like reaction time, pH value etc. various equilibrium concentrations of silicon in soil and test solution occured including transfer of silicon from soil to test solution. Therefore an analytical determination of adsorption is technically not possible.

Description of key information

Testing of the adsorption of Silicic acid, aluminium magnesium sodium salt on soils according OECD 106 failed:

Test solution (test item) and soil contain beside same cations (natrium, magnesium, aluminium) also silicon on both sides. Depending on parameters like reaction time, pH value etc. various equilibrium concentrations of silicon in soil and test solution occured including transfer of silicon from soil to test solution. Therefore an analytical determination of adsorption is technically not possible.