Hexacalcium hexaoxotris[sulphato(2-)]dialuminate(12-)

Toxicity to soil microorganisms

Description of key information

Ettringite is not expected to have detrimental effects on soil microorganisms.  See discussion in this section.

Additional information

Due to its chemical nature Ettringit is not stable under natural environmental conditions. Due to its chemical nature Ettringite is not stable under natural environmental conditions. The main degradation products are calcium sulfate (dihydrate) with limited solubility resulting in free calcium and sulfate ions and insoluble aluminium hydroxides and insoluble aluminium oxides (at neutral pH range).

The relevant compound to consider with regard to terrestrial toxicity of Ettringite is aluminium.

For calcium sulfate, calcium and sulfate ions:

Calcium sulfate, calcium and sulfate ions are ubiquitous in the environment. Calcium is an important constituent of most soils and the minerals found in soil are mostly compounds of calcium with other substances. Furthermore, calcium sulfate, as Gypsum, is used as an inorganic fertiliser to improve soil quality. Important applications include:

•for the reclamation of sodic soils through ion exchange (calcium replacing sodium)

•to reduce run-off water and its resulting erosion in dry agricultural areas as an ameliorant for acidic subsoils and soils in forestry

•to improve Ca- and S-nutritional elements in agriculture (rape and cereals)

•Gypsum is also useful as an additive for soils with high levels of sodium

Sulfur-reducing bacteria comprise several groups of bacteria that use sulfate as an oxidizing agent. Calcium sulfate is not toxic to these forms of bacteria and since it is used to enhance the quality of soil, it is expected that calcium sulfate would not be toxic to soil microorganisms.

Given the extensive and continued use of calcium sulfate as a fertiliser and for chemical treatments of soils and its natural occurrence in the environment, it is considered that calcium sulfate would not have a detrimental effect on soil microorganisms.

For aluminium

Aluminium is the most abundant metallic element in the Earth's crust. Based on its ubiquitous occurrence the present natural background concentration far outweighs anthropogenic contributions of aluminium to the terrestrial environment. As detailed in the endpoint summary on terrestrial toxicity in general further toxicity testing on terrestrial organisms is considered unjustified and waiving based on exposure consideration is applied.

However, for reasons of completeness existing data on the terrestrial toxicity of aluminium are provided in addition and summarised here.

Illmer et al. (1995) studied the toxicity of aluminium to soil microorganisms, using an undefined microbial biomass, by following biomass, microbial respiration, CM-cellulase activity and N-mineralisation. Microbial biomass decreased with increasing available aluminum concentration, independent of pH (2.9 to 3.5). From about 3 mg C biomass/g DM/h at 0 µmol/g DM Al, the microbial biomass decreased to approximately 1 mg C biomass/g DM/h at 87 µmol/g DM Al. Microbial respiration showed a slight, non-significant tendency to decrease with increasing available Al concentration. CM-Cellulase activity (representative of the C-Cycle) did not significantly correspond to an increased available Al concentration. The N-mineralization (representative of the N-Cycle) was negatively influenced by an increased available Al concentration. Kinraide & Sweeney (2003) investigated Rhizobium leguminosarum bv. trifolii strains streaked on agar plates after addition of AlCl3. Colonies were streaked with and without 48h incubation at 25 °C and colonies counted 72 hours after the respective streaking. The ratio between number of colonies streaked after 48 hours incubation and the number of colonies streaked without incubation is referred to as growth in terms of fold increase. Inhibition of cell growth (increase in cell number) by AlCl3 in solutions was observed at pH 4.4. Little or no cell increases occcured at 2 µM AlCl3 with die-off of the inoculum at higher concentrations. The toxicity of 1 µM AlCl3 increased with increasing pH (range 4.0 to 6.0).