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Environmental fate & pathways

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The forms of aluminium present in the environment depend on many factors, especially pH, alkalinity, temperature, dissolved organic carbon, dissolved inorganic carbon and anion concentration. Aluminium, as a metal, is insoluble in water. In contrast, aluminium chloride is highly soluble in water and dissociating to aluminium hydroxide and hydrochloric acid. Depending on the buffer capacity of an aquatic system AlCl3 may change the pH towards acidic. The pH of an aquatic system is also important for any further reactions and the distribution of AlCl3. At pH < 5.5 Al3+ becomes the prevalent form. At higher pH >8.0 the more soluble Al(OH)4- species predominate. As already mentioned, under circum neutral conditions AlCl3 will dissociate to Al(OH)3 and HCl. However, in natural waters, the chemistry of aluminium becomes more complicated due to the presence of alternative ligands like naturally occurring organic substances (including humic and fulvic acids) and inorganic minerals, which compete for complexation with the hydroxide ion. Thus, under relevant environmental pH conditions Al3+ will react to aluminium hydroxide which is forming complexes, precipitating to gibbsite, or Al3+ adsorb immediately to organic as well as to inorganic particles.

In conclusion, at pH values between 5.5 and 8.0 AlCl3 will dissociate in water into Cl- and Al3+ ions. The latter may adsorb immediately to organic particles and precipitates, forms inorganic alumino-silicate minerals (clay and silt) or reacts to insoluble Al(OH)3.  

 

References:

- Environment Agency (2007). Science Report Proposed EQS for aluminium, Feb 2007

- Environment Canada (2000). State of science report for Aluminium chloride, Aluminium nitrate and Aluminium sulphate, Canadian Environmental Protection Act, Dec 2000