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Environmental fate & pathways

Adsorption / desorption

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Description of key information

Study scientifically unjustified because it is to be expected from the physico-chemical properties of the substance that the absorption 
potential (log Koc very low) is very low and the log Kow of the inorganic substance is < 3 (REACH regulation EU 1907/2006 Annex IX column 2).
In the publication of Kim et el. (1997) the property and structure of aqueous KAI(SO4)2* 12H20 solutions were investigated by the measurements of
diffusion coefficients and concentration gradients developed in vertical columns of supersaturated solutions. The
diffusion coefficients decreased linearly with increasing concentration up to the saturation point and decreased more
rapidly with increasing concentration in the supersaturated region. This drastic decrease of diffusion coefficient at
supersaturated concentrations was similar to that observed in other supersaturated aqueous solutions which was
attributed to the formation of solute clusters in metastable solutions. A cluster diffusion model employing the solution
viscosity and thermodynamic data can provide a good correlation between predicted and experimental diffusion
coefficients in undersaturated solutions, but fails to provide a reasonable prediction of the much lower values of diffusion
coefficients in supersaturated solutions. The average size of the diffusing entities estimated from the diffusion coefficient
data showed a steady growth with increasing concentration in the undersaturated region and a faster growth with
increasing concentration in the supersaturated region. The average cluster sizes in supersaturated solutions were
estimated from the data of concentration gradients in vertical columns and solution thermodynamics using the concept
of the number and weight average molecular weights. The estimated cluster size increased with increasing degree of
supersaturation as well as solution "age", and showed good agreement with those estimated from the diffusion coefficient

Key value for chemical safety assessment

Additional information

With the study results of Adsorption/desorption.002 (Kim et al., 1997) there were found indirect evidences of ionic association and cluster formation in concentrated and supersaturated solutions of KAI(SO4)2* 12H20 from the measurements of diffusion coefficients and concentration gradients in vertical columns. Strong ionic association was suspected even in the solutions of low concentration, and the sizes of the clusters increased with increasing concentration. The clusters in the supersaturated region increased much more drastically with increasing concentration and extended time. The theoretical prediction of the diffusivity and the analysis of column concentration gradients in the supersaturation range require consideration of structural details of the solution, as well as the thermodynamic solution properties.

The diffusion is one of the important main ways for transport and distribution of chemical substances:

In general, there are four main ways by which small molecules cross biological lipid membranes:

1. Passive diffusion. Diffusion occurs through the lipid membrane.

2. Filtration. Diffusion occurs through aqueous pores.

3. Special transport. Transport is aided by a carrier molecule, which act as a “ferryboat.”

4. Endocytosis. Transport takes the form of pinocytosis for liquids and phagocytosis for solids.

The first and third routes are important in relation to pharmacokinetic mechanisms. The aqueous pores are too small in diameter for diffusion of most drugs and toxicant, although important for movement of water and small polar molecules (e.g., urea). Pinocytosis is important for some macromolecules (e.g., insulin crossing the bloodbrain barrier).

In the publication from Kim et al. Fig. 1 shows the diffusion coefficient measured as a function of solution concentration at 25 °C.

Diffusion coefficient declined slowly but continuously with concentration in the undersaturated region and declined more rapidly in the supersaturated region.

Table 1 and 2 of the publication shows the influence of concentration and time to density, molecular weight, number of molecules

and diameter of clusters. Fig. 2 shows this influence in graphic form.

Diffusion of the heavily hydrated ionic aggregates in KAl(SO4)2*12H20 solutions can be considered as a motion of clusters.

With increasing of concentration of the substance the cluster increases and diffusion decreases.

This could be the reason why aluminium potassium bis (sulphate) is less toxic for human and environment cell membranes than aluminium sulfate because the clusters of aluminium potassium bis (sulphate) diffuse slower through the cell membranes than aluminium sulfate.