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The low solubility of silicon may be the reason that the number of studies can be found regarding the aquatic toxicity of Si/silica is limited. No studies on acute or chronic aquatic effect of elemental silicon have been located. More often it is the impurities of commercial substances which may raise some concern over Si-based industrial products rather than the Si-element itself. For sparingly soluble substances such as metals and metalloids, environmental hazard assessments can be performed using data on particle dissolution and metal release by using the OECD T/Dp (transformation/dissolution) protocol (OECD 2001). The principle of the T/D protocol is to determine the concentration of metal elements that are released from sparingly water soluble substances. These data are then compared with data that are already available for each of the elements to determine if the concentration released is above the concentration expected to cause an effect in the environment. Information on the different elements and their potential effects to the environment is discussed in the following section.

 

Information regarding solubility of silicon:

Silicon powder dissolves relatively sparingly in distilled water, but dissolves to much higher extent in natural waters (e.g. in brackish and sea water). Technically aquatic reliable and valid tests for silicon (applying dissolved material) can be carried out.

 

The solubility of silicon for particles sized less than 0.05 mm of Si HG, Si LG and PCS after 168 hours of exposure at 100 mg/l load were tested and results, as dissolved %, are as follows:

Silicon HG 12 % ± 0.7 % (at pH 7.2)

Silicon LG 3.2 % ± 0.2 % (at pH 7.2)

Silicon PCS 32 % ± 1 % (at pH 7.2)

Normally, the dissolved fraction in ecotoxicity tests refers to the fraction that passes through a filter of 0.45 μm. It should be noted, however, that this definition may not necessarily refer to the metals/elements in solution. In the range of 0.01- 0.45 μm colloid inert particles remain suspended.

 

Soluble amorphous silica or sparingly soluble forms of silica have been studied on aquatic species, fish, daphnia and algae in few studies. These results can be used partly as a read across/Weight of Evidence approach (WoE) material in support of the silicon hazard assessment.

Performing ecotoxicity tests on sparingly water soluble substances, and in particular metals and metalloids, can be problematic. Therefore, the Transformation/Dissolution (T/D) protocol is recommended as a first tier for assessing testing requirements (OECD 2001). The T/D protocol has been performed on Silicon (low grade) for a period of 7 and 28 days representing potential acute and chronic effects respectively. The T/D protocol is a relevant approach particularly for sparingly water soluble metals. The 7 day test is only representative of acute toxicity, and for substances which are produced/imported at high tonnage, chronic data may be required which can be obtained from the full 28 day transformation/ dissolution protocol.

The T/D test protocols were performed (7 and 28 days) according to the methods presented in section 4.1.1.1 (Lillicrap et al, 2011). In addition, the results from the T/Dp tests are also presented in section 4.1.1.1 and are summarised below in Table A10.:

 

Table A10. 7 day transformation/dissolution data.

Identification

conc (mg/L)

pH

Cu(μg/L)

Pb (μg/L)

Si (mg/L)

Zn (μg/L)

DWC (7day)

-

6

3.69

0.034

<0.02

1.28

-

8.5

1.13

0.02

0.26

1.18

Si low grade (7 day)

1

6

2.26

0.029

<0.02

1.18

1

8.5

0.67

0.01

0.27

1.07

10

6

3.92

0.027

<0.02

1.81

10

8.5

0.78

0.006

0.26

1.44

100

6

3.03

0.030

<0.02

2.16

100

8.5

0.69

0.008

0.24

0.95

DWC (28 day)

5.5

2.37

0.0310

<0.02

1.72

8.5

0.88

<0.005

0.26

1.40

Si low grade (28 day)

5.5

6.89

0.028

<0.02

1.59

8.5

0.76

<0.005

0.28

0.81

Comparing the data from the 7 and 28 day T/Dp tests with the limits recommended in the EU Risk Assessment Reports (RAR) as shown in Lillicrap et al. (2010), the data indicate that the impurities present in the silicon (low grade) should not cause an effect to aquatic organisms and hence these substances should require no further testing and silicon does not require environmental hazard classification.