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Due to its chemical nature sodium aluminate is not stable under environmental conditions (pH-values between 4 and 9) (for details see section environmental fate and pathways). The main decomposition products of sodium aluminate in water are 1) sodium hydroxide, 2) aluminium hydroxide and its dehydrate form, aluminium oxide.

 

1) Sodium hydroxide (NaOH)

 

In water the decomposition product sodium hydroxide dissociates into the sodium ion (Na+) and hydroxyl ion (OH-) both having a wide natural occurrence. The environmental fate of sodium hydroxide was addressed in a detailed risk assessment reported by the European Union (2007).

Since sodium aluminate is used as flocculant in sewage treatment plants and as a coagulant for drinking water, sodium hydroxide emission applies solely to water.

Due to the high water solubility and the low vapor pressure of sodium hydroxide, water is the relevant compartment for sodium hydroxide.

 

If emitted to waste water that is treated in a biological STP or WWTP, virtually the total amount of sodium hydroxide will end up in the effluent of the STP.An addition of NaOH to surface water may increase the pH, depending on the buffer capacity of the water. In general the buffer capacity preventing shifts in acidity or alkalinity in natural waters is regulated by the equilibrium between carbon dioxide (CO2), the bicarbonate ion (HCO3-) and the carbonate ion (CO32-). If the pH is < 6, un-ionised CO2 is the predominant species. At pH values of 6 - 10 the bicarbonate ion (HCO3-) is the predominant species and at pH values > 10 the carbonate ion (CO32-) is the predominant species. In the majority of natural waters the pH values are between 6 and 9, thus the bicarbonate concentration is most important for the buffer capacity. Besides this natural occurring buffer capacity, NaOH from production to STPs/WWTPs and receiving waters are well controlled in the EU. Taking into account the existing EU Directives for pH control for surface water and many Member States additional national regulations to control the pH of waste waters and surface waters it is concluded that STPs and surface waters are sufficiently protected with regard to pH changes. The European Union (2007) risk assessment report comes to the conclusion that it cannot be completely ruled out that there are (some) sites with NaOH discharges to the aquatic environment, resulting in significant pH changes and effects on biological STPs/WWTPs or receiving surface waters. However, the available data clearly indicate that neutralisation of NaOH containing waste waters and effluents is common practice, either from a legal point of view (legislation for surface waters) or from a practical point of view (protection of the functioning of biological STPs/WWTPs). Regarding surface water, the enforcement of the (EU) legislation is an important issue for the validity of this conclusion. Thus, for reasons explained, NaOH is not further considered here.

 2) Aluminium hydroxide and aluminium componds

 

The relevant component with regard to hazard profile of sodium aluminate in the aquatic compartment is aluminium and various aluminum forms present in the environment. The complex environmental chemistry of aluminium is addressed in several reports and public available sources to date (e.g. Sposito 1995, WHO 1997, EURAS 2007).

 

As decomposition product of sodium aluminate in the aquatic environment, aluminium hydroxide itself is highly insoluble. A study on the transformation / dissolution of the environmentally relevant “lead transformation product” aluminium hydroxide tested in accordance with the OECD transformation / dissolution protocol (CIMM 2007) demonstrated that the amount of aluminum released by the samples and by the blanks were undistinguishable both at pH 6 and pH 8. 

 

Table: Summary of the short term (7 days) and long term (28 days) transformation/dissolution data obtained for Al(OH)3

Loading

pH

Endpoint

Al, µg/L

St. Des.

CV, %

Blanks

6

7 days

2

0.00

0

1 mg/L

6

7 days

3

1.86

72

1 mg/L

6

28 days

4

2.07

55

100 mg/L

6

7 days

2

1.67

79

Blanks

8

7 days

3

0.00

0

1 mg/L

8

7 days

3

0.49

15

1 mg/L

8

28 days

3

0.00

0

100 mg/L

8

7 days

4

1.39

40

Nevertheless, various aluminium species and metal complexes, as well as organic complexes, including dissolved and particulate forms existdepending on certain environmental conditionssuch as pH, alkalinity, temperature, dissolved organic carbon, dissolved inorganic carbon and anion concentration. Under normal environmental condition, only a limited amount of the total aluminium contribution to the aquatic environment will be available in form of dissolved aluminium.At pH < 5.5, the free ion (Al3+) becomes the prevalent form, along with the inorganic monomeric complexes [AlF, Al(OH)x and Al(SO4)]. The increased availability at this pH is considered to be reflected in higher toxicity. At pH 6.0–7.5, solubility declines due to the presence of insoluble Al(OH)3. At higher pH (pH > 8.0), the more soluble Al(OH)4-species predominates, which again increases availability. Therefore, in order to address the toxicity of aluminium, test results are presented as total and dissolved aluminium if stated in the report. Considering the insolubility of aluminium hydroxide, the toxic effects derived from dissolved aluminium represent the worst-case scenario for aluminum released into aquatic compartment though sodium aluminate application.

 

In the available studies on different aluminium compounds a broad range of different test conditions are reported concerning concentration range, pH, hardness and DOC etc.. The results of these studies allow a ‘more realistic’ view on the complicated environmental chemistry of aluminates in natural water. To reflect the complex situation, thus, read-across to studies conducted with various analogue substances was performed and used for a weight of evidence approach to consider the environmental toxicology of aluminium.

 

The results of these studies demonstrate the complexity and variability of toxic effects of aluminium on aquatic organisms, but do not indicate a clear hazard profile. Depending on test conditions, test organisms or aluminium compounds tested, the results varied extremely. Basically, toxic effects decrease with increasing of DOC of the test media. However, since the acute toxicity to fish and the chronic effects on invertebrates decrease significantly, if the test solutions were filtered. The formulation of aluminium organic complexes may be the reason for the reduced toxicity. Influence of the hardness of test media was only species specific. No significant effects on fish and algae were observed, but on invertebrates. The variability of toxicity is significantly affected by the pH value of the test medium. Moreover, adsorption of aluminium species on fish gills and cuticle of invertebrates, both in soluble and insoluble forms, seem also to be a significant mode of toxic action.

At pH-values up to 6, acute toxicity ranged from 6 µg/L (Ceriodaphnia dubia and algae) to 224 µg/L (fish) for dissolved Al and 7 µg/L (algae) to 10500 µg/L (Ceriodaphnia dubia) for total aluminium, excluding the result received under 0 mg/L DOC, because this is not considered to be a standard test condition. Invertebrates, followed by algae appear to be more sensitive than fish. In the single available chronic fish test (60-d) a NOEC of 124 µg/L based on the length is reported.

At pH ranges of 6 to 8 (6.3 – 7.8), only data for total Al are available.Acute toxicity was determined from 600 µg/L (algae) to 14000 µg/L (fish). Algae were the most sensitive species. Regarding the chronic toxicity, fish appear to be the most sensitive (NOEC, 60-d, 57 µg/L, based on the number of hatched).

At pH-values above 8, acute and chronic toxicity is within the same range for all three species., LC50/EC50-values in the range of 150 µg/L (algae) – 790 µg/L (fish) were determined for dissolved Al and 317 µg/L (algae) – 7000 µg/L (fish) for total Al. Algae seem to be even more sensitive than daphnids. However, there are fewer data available for invertebrates. Regards the chronic toxicity, the lowest NOEC value of 10 µg/L was determined for Ceriodaphnia dubia exposed to unfiltered medium for 7 days. The low endpoint may result from adsorption of aluminium species to the cuticle of test animals. Otherwise, the NOECs were comparable in regard to the acute toxicity, ranging from 400 µg/L (algae and Ceriodaphnia dubia) to 14 mg/L (fish).

Resulting from the reported studies algae are most sensitive to aluminium compounds, followed by invertebrates, with acute effect concentrations reported as low as 20 μg/L (ErC50) of dissolved aluminium. In algae, aluminium is considered to interfere with intracellular phosphorus metabolism or glucose metabolism early in the glycolytic pathway.

 

Although aluminium hydroxide, decomposition product of sodium aluminate, showed no toxicity up to its solubility. However, in acidic environment, different aluminium species could be released. Therefore , the toxic effects of dissolved aluminium in aquatic system should be taken in consideration. As far as data available, toxicity data for aluminium applied for the PNEC derivation are based on the concentrations of dissolved aluminium at pH 8 and at pH 6, respectively. Nevertheless, PNECs derived from dissolved aluminium represent the worst-case scenario for aluminum released into aquatic compartment, which clearly overestimate the toxicity effect of sodium aluminate itself.

EURAS (2007) Development of a high quality aquatic ecotoxicity database for Al metal, Al oxide and Al hydroxide. European Union (2007) European Union Risk Assessment Report. Sodium hydroxide. CAS 1310-73-2. EINECS No. 215-185-5. Targeted Risk Assessment. Sposito (ed) (1995) The environmental chemistry of aluminium. Crc Pr Inc, 480 pp, 2nd edition. WHO (1997) Aluminium. Environmental Health Criteria 194.