Leaching tests mechanism

Leaching of chemicals from complex materials or matrices is a complicated phenomenon in which many factors may influence the release of the specific organic compounds and inorganic ions. Important factors include major element chemistry, pH, redox, complexation, liquid to solid ratio, contact time, and biological activity. To describe fully the leaching of SWMs/COMs under field conditions, a battery of leaching tests was specifically designed to simulate various physical and chemical release mechanisms. 

One of the more important factors affecting the isolation of radioactive waste is the rate of release of the radioactivity from the solid waste form to the environment. The most probable mechanism for release and transport of radioactivity from a solid waste form is by leaching of radioactive isotopes with groundwater. The objective of leach-testing various waste forms is to evaluate the rate at which specific hazardous radionuclides migrate from waste if and when the waste form comes in contact with groundwater. In this paper, measurement of leach rates of radioactive waste by a method which incorporates neutron activation is described. 

Two leach test procedures were used a modified IAEA dynamic leach test and a static leach test. In the former test the leachant was gently agitated on a mechanical shaker and was changed at selected intervals. A typical sampling schedule was three times daily for the first week, twice per day for week 2, once per day for weeks 3 through 5, and once per week through week 9. 

W. E. Shively, The Chemistry and Binding Mechanisms Involved with Leaching Tests of Heavy Metals Solidified and Stabilized with Portland Cement, M. S. Thesis, Univ. of New Hampshire, Durham, NH (1984). 

For the purposes of describing leaching tests, the description of the leaching mechanisms hence the description of the types of tests are simplified into those which are relevant to granular materials and those which are suitable for monolithic materials. 

Rankers RH, Hohberg I (1991) Leaching tests for concrete containing fly ash - evaluation and mechanism. In Goumans JJJM, van der Sloot HA, Aalbers ThG, Maastricht RT (eds) Waste materials in construction. Elsevier, Amsterdam, pp 275-282... 

However, given that other, supposedly stable and leach-resistant catalysts that we had previously studied seemed to operate solely by a leaching mechanism, with no supported Pd apparently active for the reaction (vide supra) and that other authors had found precatalysts of this type to operate by solely a leaching mechaiusm in Heck conversions of aryl iodides (19), we decided to investigate this system further using the solid poison testing method. 

The scope of this work was not intended to include study and analysis of leaching mechanisms beyond this point, but the data do show that the mechanism is dependent on time, sampling frequency, and type of element. Also, over the long testing period, two different mechanisms account for the release of material. More work is needed to increase our understanding of these high-level waste glass-solution interactions. 

Ray, I., and Thiele, H., Long Term Leaching of Silicate Systems Testing Procedure, Actinides Behavior and Mechanism, Karlsruhe Report KFK-2456 (1977). 

A9.3.5.9.2 Polymers are typieally not available in aquatic systems. Dispersible polymers and other high molecular mass materials can perturb the test system and interfere with uptake of oxygen, and give rise to mechanical or secondary effects. These factors need to be taken into account when considering data from these substances. Many polymers behave like complex substances, however, having a significant low molecular mass fraction which can leach from the bulk pol)mier. This is considered further below. 

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