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Under REACH (ECHA 2008, Chapter R.7B – Endpoint Specific Guidance), the term ‘Hydrolysis’ refers to the “Decomposition or degradation of a chemical by reaction with water”, and this is a function of pH (i. e., abiotic degradation). Aluminium persists in the environment irrespective of whatever chemical species form as a result of hydrolysis, although it may form insoluble aluminium hydroxides that precipitate out of solution. Characterization of aluminium in environmental media is typically based on total aluminium concentrations inclusive of all specific chemical forms or species. Since hydrolysis changes the chemical form but does not decompose aluminium and since characterization of total aluminium considers all chemical forms, the concept of degradation of aluminium by hydrolysis needs to be considered differently from that of organic substances during the consideration of its environmental fate.

Aluminum is a strongly hydrolysing metal but is relatively insoluble in the neutral pH range (6.0–8.0) which is the most commonly observed range of environmental pH for aquifers.

Nevertheless the stability of aluminium salts in the environment need to be considered in a different context from that used to determine fate for organic substances: As a metal, speciation, the partitioning of aluminium among different physical and chemical forms, occurs.

Both speciation and solubility of aluminum are affected by a several environmental parameters, notably pH, but also water temperature, dissolved organic carbon (DOC) content, to some extent hardness and the presence and concentrations of numerous ligands.

Metals in solution may be present as dissolved complexes, as “free” or aquo ions, in association with particles, as colloids or as solids in the process of precipitating low solubility species. Colloidal particles (0.001 to 1μm) are implicated in the transport of metals in river ecosystems, and accumulation of metals in sediment and biofilm and the transfer to biota. The reactivity of aluminum, as well as geochemical behaviour, bioavailability and toxicity, depend upon its speciation.

In solution inorganic ligands such as sulphate (SO42-), fluoride (F-), phosphate (PO43-), bicarbonate (HCO3-) and hydroxide (OH-), inter alia can form strong complexes with aluminum. While organic ligands can be formed with oxalic, humic and fulvic acids. The proportions and type of complexes that are formed in solution are determined by the relative concentrations of the inorganic and organic ligands.

Interactions with pH and DOC are well reported and primordial to the fate and behaviour of aluminum. DOC will complex with aluminium in water, forming aluminium-organic complexes and reducing concentrations of monomeric forms of aluminum. 1 mg DOC/L at pH 4.5, complexes approximately 0.025 mg Al/L with complexing capacity increasing with pH. Environment Canada (2010) modelled fractions of dissolved organic aluminum for various rivers in Canada using the MINEQL+ and WHAM; According to their calculations the importance of complexation with dissolved organic material (DOM) decreased over the pH range 7.0 to 8.5, but this may be related to the reduction of the Al3 +and AlOH 2 +species at these pHs which can associate with DOM.

The hydrolytic products of mononuclear aluminum combine to form polynuclear species in solution in aquifers. Aluminum starts to polymerize when the pH of an acidic solution is above 4.5: 2Al(OH)(H2O)52+Al2(OH)2(H2O)84++ 2H2O.

Polymerization leads to larger structures, and ultimately to the formation of the Al13polycation.

Refs . Environment Canada Health Canada, (2010) Priority substance list assessment report. follow-up to the state of science report, 2000: Aluminium chloride, aluminium nitrate aluminium sulphate