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Environmental fate & pathways


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When Ettringite is diluted by water, it will dissociate forming Calcium-, Aluminium- and Sulfate ions. Aluminium ions hydrolyse readily and precipitation of Al(OH)3 and release of free sulfate and calcium ions will occur. Hydrolysis of aluminium ions has two possible “directions” towards a neutral pH, i.e. base hydrolysis and acid hydrolysis. Both acid and base hydrolysis of aluminum results in precipitation of aluminium hydroxide, rapidly.  Ettringite decomposes to aluminum hydroxide when added to water and can therefore not be considered as water soluble.

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Additional information

Zhao et al. (2009). investigated the effect of pH in the range of 4.0 to 6.4 on the aluminium chloride hydrolysis at low concentration level. As coagulant aluminium chloride was diluted with deionised water. At pH 4.0, mono- and dimeric aluminum species [Al(OH)2(H2O)2-3]+and [Al2O2(OH)(H2O)0-5]+ were detected as main products. With increasing pH, hydrolysis and polymerization increased. At pH 5.0, aluminum species mainly aggregated and assemblied to median polymeric species (Al6-Al10 species) and these to large polymeric species (Al11- Al21). At pH 5.8, metastable median and large polymers decomposed into small aluminum species and disaggregated into dimeric species. With pH 6.4, the majority of aluminum formed to Al(OH)3 amorphous flocs. In alkaline solution, depends on the concentration of aluminates, the soluble aluminate ions, Al(OH)4 ‾ and Al2O(OH)62 - ions are considered to be the totally dominating species (Szabo at el. 1978).

Aluminium hydroxide, Al(OH)3, is amphoteric. It is soluble in both strong acids and strong bases. Nevertheless,the transformation/dissolution study according to OECD 29 showed that at somewhat neutral pH (pH 6 and pH 8), the solubility of aluminium hydroxide is minimal. In all tested conditions, the amounts of aluminium released, both at high loading (100 mg/L) and low loading (1 mg/L), by the samples and the blanks were undistinguishable (CIMM, 2007).