Trace element extractions, kinetics

The use of chemical modelling to predict the formation of secondary phases and the mobility of trace elements in the CCB disposal environment requires detailed knowledge of the primary and secondary phases present in CCBs, thermodynamic and kinetic data for these phases, and the incorporation of possible adsorp-tion/desorption reactions into the model. As noted above, secondary minerals are typically difficult to identify due to their low abundance in weathered CCB materials. In many cases, appropriate thermochemical, adsorption/desorp-tion and kinetic data are lacking to quantitatively describe the processes that potentially affect the leaching behaviour of CCBs. This is particularly tme for the trace elements. Laboratory leaching studies vary in the experimental conditions used (e.g., the type and concentration of the extractant solution, the L/S ratio, and other parameters such as temperature and duration/ intensity of agitation), and therefore may not adequately simulate the weathering environment (Rai et al. 1988 Eary et al. 1990 Spears Lee, 2004). 

Microdialysis to monitor the chemistry of extracellular space in human tissue is a challenging technique that has been applied in the neurosciences to measure neurochemical composition in vivo.104 Microdialysis coupled online with in-tube solid phase extraction (SPE) and ICP-MS has been developed by Sun et al.105 for the in vivo determination of several trace metals in order to study the transfer kinetics of trace elements in brain extracellular fluid. 

As measurements in these protocols are made in equilibrium conditions, only thermodynamic information is obtained. However, kinetic extraction-desorption studies are a more correct approximation to the distribution of species in natural media (Aulitiia and Pickering, 1988 Bermond et al., 1998 Ortiz-Viana et al., 1999 Fangueiro et al., 2002 Gismera et al., 2004). The desorption rate constants of the trace element in sediments and soils can be related to its mobility and toxicity. 

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