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Environmental fate & pathways

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Aluminum zirconium chloride hydroxide is an inorganic substance which will rapidly dissociate into aluminum, zirconium, chloride and hydroxide ions upon dissolution in the environment. The chemistry of the relevant metals aluminium and zirconium in aqueous solutions is complex. Depending on concentrations, pH and other ions being present, Al and Zr do not exist as simple metal cations, but instead form various species, most relevantly hydroxides and hydroxide-oxides.

Zirconium is one of the 20 most abundant elements in the earth's crust (at approximately 0.03%). Zirconium displays under most environmental conditions, a very low mobility, mainly due to the low solubility of the hydroxide Zr(OH)4. This limits the concentration of dissolved Zr in most natural solutions (fresh water, seawater as well as soil and sediment porewater) to <0.05 μg/L. Depending on the pH of the environmental medium, different zirconium species exist in solution, including Zr4+, and various hydroxides. At pH 7, a Zr(OH)2(CO3)22-complex may form, but the complex is unstable and Zr(OH)4forms with decreasing pH. The hydro-bicarbonate (Zr(OH)4-HCO3-H2O) complex may be the most significant Zr complex in natural water (http://www.gtk.fi/publ/foregsatlas, accessed on 12.03.2013).

Results of the solubility testing (OECD Series on Testing and Assessment No. 29) seem to confirm the lack of zirconium solubility at environmentally relevant levels. At a loading of 1 mg aluminum zirconium chloride hydroxide /L (corresponding to 0.19 µg Al and 0.10 mg Zr) of water or environmental OECD test medium at pH 6 and pH 8, respective dissolved zirconium concentrations were below the detection limit of ca. 0.3 µg/L at all time-points while dissolved Al levels ranging from 0.01 – 0.12 mg/L were measured.

Thus, regarding the environmental fate and toxicity of aluminum zirconium chloride hydroxide (if any), it can be assumed that it will not be driven by zirconium. Therefore, full read-across to other aluminum substances considering a typical aluminum content of ca. 19.4% is justified.

Abiotic degradation

The endpoints "Phototransformation of an element in water, soil or air" are not relevant for substances that are assessed using a read-across approach on an elemental basis, i.e., based on the exposure and effects of aluminium, expressed as elemental Al.

 

The term“Hydrolysis” refers to the“Decomposition or degradation of a chemical by reaction with water,” and this as a function of pH (i.e., abiotic degradation). The need for testing may be waived if “The substance is highly insoluble in water”, or if“The substance is readily biodegradable”. The latter property of a substance assumes a rapid mineralization of the substance and therefore hydrolysis tests will provide little information.

 

As mentioned previously, in the case of the current substance the chemical safety assessment is based on elemental metal concentration, i.e., the assessment of aluminium is conducted regardless of the pH- dependent speciation in the environment. Hence, as the assessment is based on the element concentration (i.e., Al), physicochemical processes like decomposition and degradation by reaction with water are not relevant. This elemental-based assessment (pooling together of all speciation forms) can be considered as a worst-case assumption for the chemical assessment.

 

In general, (abiotic) degradation is an irrelevant process for inorganic substances, including aluminum zirconium chloride, that are assessed on an elemental basis.

Biotic degradation

For inorganic substance like aluminium and zirconium salts for which the chemical assessment is based on the elemental concentration (i. e., pooling all inorganic speciation forms together), biotic degradation is an irrelevant process, regardless of the environmental compartment that is under consideration: biotic processes may alter the speciation form of an element, but it will not eliminate the element from the aquatic compartment by degradation or transformation. This elemental-based assessment (pooling all speciation forms together) can be considered as a worst-case assumption for the chemical assessment.

References:

Driscoll CT, Postek KM., 1996, The chemistry of aluminium in surface waters. In: Sposito G., editor, The environmental chenistry of aluminium. 2nd edition. Boca Raton (FL): CRC Press. p: 363-418.

Exley C. 2003. A biogeochemical cycle for aluminium. Journal of Inorganic Biochemistry 97:17.