Dialuminium zinc tetraoxide

Adsorption / desorption

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 of metals in the environment depends on several parameters, such as pH, redox potential and the presence of anions.

Experimental data on the adsorption/desorption behaviour ofZinc aluminium oxideare 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.

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).

Solids-water partition coefficients (log Kpsusp) in suspended matter derived from measurements over the period 1983-1993 range from 3.43 to 5.04 (Popp and Laquer, 1980; Li et al., 1984; Stortelder et al., 1989; Venema, 1994; Koelmans and Radovanovic, 1997; Yland and Smedes, 1996; Yland, 1996).

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; 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:

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

Li, Y et al. 1984. Desorption and coagulation of trace elements during estuarine mixing. Geochimica et Cosmochimica Acta, 48: 1879-1884

Koelmans, AA and H Radovanovic. 1997. Modelling trace metal distribution in surface waters, Model formulations and calibration. Wageningen Agriculture University.

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

Popp, C.J. and Laquer, F. 1980.Trace metal transport and partitioning in the suspended sediments of the Rio Grande and tributaries in central New Mexico: Chemosphere, v. 9, pp. 89-98.

Stortelder et al. 1989. Perspectives for water organisms (part 1 and 2). DBW/RIZA Nota No. 89.016a+b, Lelystad, NL.

Venema. 1994. Gehalte in zwevend stof, meten of berekenen? RIZA werkdocument 94064x, RIZA, Lelystad, NL.

Yland, E. 1996. Partitiecoëfficiënten tussen sediment en water voor metalen berekend uit mariene monitoringsgegevens en analyse van het aandeel van de achtergrondwaarde op de zwevend stof/water coëfficiënt. RIKZ/OS96.141 x (in Dutch).

Yland, E and Smedes F. 1996. Partitiecoëfficiënten tussen zwevend stof en water voor metalen berekend uit mariene monitoringsgegevens.RIKZ/OS96.117 x (in Dutch).