Dialuminium zinc tetraoxide

Endpoint summary

Additional information

Due to its low solubility, Zinc aluminium oxide will mainly remain undissolved under neutral environmental conditions and will deposit in soil or sediment. A minor fraction may dissolve into the aqueous phase under acidic or alkaline conditions. In this case each metal will undergo speciation in function of the environmental conditions. The speciation and adsorption of metals in the environment depends on a number of parameters, such as pH, redox potential, DOC and the presence of anions or complexing agents.

Experimental data on the adsorption/desorption behaviour of Zinc aluminium oxide are not available, but the fate of the individual metals in the environment has been thoroughly investigated.

Zinc

The data available on the fate of zinc in the environment has been comprehensively reviewed in the EU Risk Assessment Report for zinc metal (ECB, 2008). The central information from this report is given here and further details can be found in the original report.

Zinc in fresh water or seawater can occur in both suspended and dissolved forms and is partitioned over a number of chemical species. Zinc in freshwater can be divided in several classes, as for instance hydrated zinc ions, zinc ions complexed by organic ligands (humic and fulvic acids), zinc oxy ions and zinc adsorbed to solid matter. The distribution over free zinc and zinc complexes has been found to be roughly 30% and 70%, respectively, in European surface waters (Cleven et al., 1993; Jansen et al., 1998).

Adsorption to suspended matter and bed sediment is also an important factor for the behaviour of zinc in aquatic systems. Several phosphates (WHO, 1996), hydroxides, clay minerals and organic matter are important for the adsorption of zinc in aerobic waters (Cleven et al., 1993). The efficiency of these materials in removing zinc from the solution varies according to their concentrations, pH, redox potential (Eh), salinity, nature and concentrations of complexing ligands and the concentration of zinc. The metal carriers in sediments are clay minerals, silicates (quartz, feldspars), calcium carbonate and organic matter. The accumulation of zinc in the sediments from surface water increases with decreasing size of the sediment particles (Cleven et al., 1993).

Precipitation of soluble zinc compounds appears to be significant only under reducing conditions in waters with high zinc concentrations, particularly when the pH is higher than 8 (Cleven et al., 1993). Low pH is important to maintain zinc in solution, generally as the free ion, although the control of maintaining zinc in solution is related to the dissolved organic matter. Under anaerobic conditions and in the presence of sulphide ions, precipitation of zinc sulphide limits the mobility of zinc. Under aerobic conditions, desorption of zinc from sediments occurs at increasing salinity due to the displacement of adsorbed zinc ions by alkali and alkaline earth cations, which are abundant in brackish and saline waters (WHO, 1996). An increase in the dissolved and suspended fractions of zinc in estuarine water was reported in the mixing zone between fresh and brackish water, mainly due to increased residence time in estuaries (WHO, 1996).

The speciation of zinc in the aquatic compartment is of high complexity and depends highly on abiotic factors, such as pH, (dissolved) organic matter content, redox potential, etc. It is assumed that speciation is very relevant for the migration of zinc through sediment, for the distribution of zinc among its truly dissolved and non-dissolved forms.

Aluminium

A number of chemical factors can alter the speciation of aluminium, thereby affecting the extent of adsorption and desorption of aluminium on suspended particles, as a result aluminium speciation is complex and changes significantly with changes in pH.   In the absence of organic matter, Al3+is the predominant aluminium species at low pH (less than 5.5). As pH increases above 5.5, aluminium-hydroxide complexes formed by hydrolysis become increasingly important and dominate aqueous aluminium speciation (Figure 4.2.1-1, attached in Chapter 5.4.1; provided by the Aluminium REACH Consortium). The presence of a moderate amount of organic matter in soft water (2 mg/L as dissolved organic carbon or DOC is used here) results in organically complexed aluminium being the dominant aluminium form when the pH is between 4 and 7. Above pH 7, anionic aluminium hydroxide predominates, although organically complexed aluminium remains the second most important form of dissolved aluminium. 

 Aluminium speciation can also include the formation of insoluble polymeric aluminium-hydroxide species.  Polymeric aluminium hydroxides tend to exist as amorphous colloids and solid phases. The kinetics of this transformation to polymeric species, including aqueous colloids and amorphous precipitates, depends on many factors but typically occurs over a time scale of minutes to hours. Subsequent formation of more crystalline solid phases may take additional time, as much as a few days. As a result of these relatively slow transformations from dissolved to crystalline forms of aluminium, there is a considerable range of solubilities that have been reported for aluminium hydroxide solid phases (Lindsay and Walthall, 1996).

 As a result of this dynamic chemistry, the amount of aluminium associated with suspended particles is dependent on the chemical conditions. Factors that are known to affect aluminium speciation, such as pH and DOC, are also known to affect adsorption and desorption from particle surfaces. To illustrate this further, the amount of aluminium associated with suspended particles was estimated by chemical simulation that included aqueous aluminium speciation (inorganic and organic), aluminium solubility, and complexation by NOM. For these simulations a NOM concentration of 4 mg/L (2 mg/L as DOC) and a total suspended solids (TSS) concentration of 1 mg/L were chosen to represent a reasonable lower bound for the range of values of these substances that would be expected in the environment. Suspended particles were assumed to be composed primarily of silica (80%) with a small amount of clay (10%) and particulate organic matter (10%). Aluminium concentrations were set to the maximum allowable by solubility with amorphous gibbsite at a temperature of 20⁰C. Under these conditions, the amount of aluminium bound to particles as a result of surface complexation (i.e. adsorption) was pH dependent, but was typically less than 8% of the total aluminium at pH 6, and was further reduced to below 1% at pH values above 7 (Figure 4.2.1.-2A). This distribution was similar in both soft and hard waters. The corresponding Log Kd values for this distribution are shown in Figure 4.2.1.-2B, with values between 3 and 5.  Very similar results were obtained with higher DOC concentrations of 4 mg/L.

 

References:

Cleven, R.M.F.J., Janus, J.A., Annema, J.A and Slooff, W. (Eds.). 1993. Integrated Criteria Document Zinc. RIVM report 710401028, National Institute of Public Health and the Environment, Bilthoven, The Netherlands. (Originally published in 1992, as RIVM-Report 710401019: “Basisdocument Zink”, In Dutch).

European Chemicals Bureau (ECB), Risk assessment Zinc metal CAS-No.: 7440-66-6, EINECS-No.: 231-175-3, Final report, May 2008, EUR 24587 EN - 2010

Jansen, R.A.G., H.P. van Leeuwen, R.F.M.J. Cleven and M.A.G.T. van den Hoop. 1998. Speciation and lability of zinc(II) in river waters. Environ. Sci. Technol. 32, 3882-3886.

Lindsay W. L. and Walthall P. M. (1996). The solubility of aluminium in soils. In The environmental chemistry of aluminium. (G. Sposito, ed.), pp. 333–361. USA: Lewis Publishers, Boca Raton

WHO. 1996. Environmental Health Criteria for Zinc, Draft, summary, evaluation, conclusions and recommendations of the IPCS task group. Final report publiehed in 2001 (Environmental Health Criteria Series 221: Zinc, International Programme on Chemical Safety, World Health Organoization, Geneva