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log Koc is not an appropriate endpoint for determination of adsorption of aluminium salts

Key value for chemical safety assessment

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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). 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).

Due to its 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 20C. 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.