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Environmental fate & pathways

Adsorption / desorption

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Description of key information

According to REACH Annex VIII, section 9.3.1, column 2 "a study does not need to be conducted if  based on the physicochemical properties the substance can be expected to have a low potential for adsorption  .e.g, the substance has a low octanol water partition coefficient, or the substance and its relevant degradation products decompose rapidly".  Ammonium thiocyanate as a representative member of the category is readily biodegradable according to the results of an OECD 301D test.  The thiocyanates in this category all have estimated log Kow values less than zero, indicating a low potential for adsorption.    However, SCN- can adsorb to soils with a large anion exchange capacity (Fe, Al and Mn oxides, clay minerals), especially at high temperatures.  The surfaces of most soil particles have low anion exchange capacity, thus this mechanism would be of minor relevance.

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Additional information

Two studies have been reviewed for adsorption/desorption potential of thiocyanates. The first study (Misra and Misra, 1991) is based on adsorption tests only, with no tests on desorption potential. This study indicates a potential for adsorption for sodium and ammonium thiocyanates. This test is considered not reliable as there are serious doubts about whether equilibrium was reached in the test, and also on the identity of the anion (CNS- vs SCN-). Due to sodium and potassium cations being normally present in the environment, and also for ionizing substances, it is the cations that are normally adsorbed to the humic acids in soils. The second study (Brown and Morra, 1993) shows the importance of organic carbon in providing an energy source for microorganisms that can degrade thiocyanates in soils, especially at temperatures <= 30oC . At temperatures above 40oC, sorption to soils with a large anion exchange capacity may serve to immobilize SCN- in soils. However due to the fact that most soils surfaces have low anion exchange capacity, this mechanism of adsorption is considered less relevant.