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Environmental fate & pathways

Adsorption / desorption

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Soil adsorption/ desorption studies with magnesium carbonate are not possible due to analytical difficulties, the solubility characteristics of magnesium carbonate in water and its inorganic nature.

Irrespective of the method selected to monitor the test material, analytical difficulties could arise from the addition of magnesium carbonate to soil. A significant contribution of dissolved magnesium would be expected in all samples due to the presence of magnesium salts as a naturally occurring mineral fraction in soils. Therefore, it would not be possible to analytically distinguish between contributions to the analysed solution magnesium concentration originating from the test material and that originating from the soils themselves.

Magnesium carbonate presents only limited solubility in water and solubility characteristics are known to be sensitive to both solution pH and also the presence/partial pressure of carbon dioxide (which dissolves to give carbonate or bicarbonate ions). Therefore, changes with respect to aqueous phase pH and dissolved carbon dioxide concentrations on exposure to soils may lead to shifts in the relative solubility of the test material and potential precipitation and sedimentation on centrifugation of the samples; a process which can not be separated analytically from any true adsorption onto the soil phases.

The organic carbon content of the soils is not anticipated to play a significant role in the mobility of simple inorganic salts such as magnesium carbonate and therefore the actual test endpoint, that of an organic carbon normalised adsorption coefficient (Koc) is probably not actually valid/ relevant for this type of substance. For the magnesium content at least, pH, water potential and carbon dioxide partial pressure amongst others will be controlling factors.

Marcussen et al (2008) have managed to measure a distribution coefficient for magnesium in sediment. Distribution coefficients (Kd) are expressed as the ratio between elemental concentrations on the sediment material in mg/kg and the concentration in the pore water in mg/L. The observed distribution coefficient value for magnesium ranged from 234 to 1820.