Kinetic extraction-desorption studies

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. 

Laboratory experiments, transport modeling, field data, and engineering cost analysis provide complementary information to be used in an assessment of the viability of an MNA approach for a site. Information from kinetic sorption/ desorption experiments, selective extraction experiments, reactive transport modeling, and historical case analyses of plumes at several UMTRA sites can be used to establish a framework for evaluation of MNA for uranium contamination (Brady et al, 1998, 2002 Bryan and Siegel, 1998 Jove-Colon et al, 2001). The results of a recent project conducted at the Hanford 100-N site provided information for evaluation of MNA for a °Sr plume that has reached the Columbia River (Kelley et al, 2002). The study included strontium sorption-desorption studies, strontium transport and hydrologic modeling of the near-river system, and evaluation of the comparative costs and predicted effectiveness of alternative remediation strategies. 

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). 

Time is another variable for this methodology, where a 24—48-h extraction time is generally typical for many batch extractions (e.g. Anderson and Christensen, 1988 Yin et al, 2002) but it may also vary and extend into several weeks according to the aim of the study (Jopony and Young, 1994). Longer equilibration times are normally selected to try to take into account the slow kinetics of the organic-metal desorption. 

The method of soil suspensions extracts is based on metal desorption/dissolution processes, which primarily depend on the physico-chemical characteristics of the metals, selected soil properties and environmental conditions. Metal adsorption/ desorption and solubility studies are important in the characterization of metal mobility and availability in soils. Metals are, in fact, present within the soil system in different pools and can follow either adsorption and precipitation reactions or desorption and dissolution reactions (Selim and Sparks, 2001). The main factors affecting the relationship between the soluble/mobile and immobile metal pools are soil pH, redox potential, adsorption and exchange capacity, the ionic strength of soil pore water, competing ions and kinetic effects (e.g. contact time) (Evans, 1989 Impelhtteri et al., 2001 McBride, 1994 Sparks, 1995). 

This method is already relatively old and has been used for the past several years in organic chemistry and biochemistry. High kinetic energy (several keV) primary ions, e.g., Ar, bombard a surface on which the sample has been deposited. Under these conditions, ions are extracted from the surface and can be analyzed, Benninghoven and co-workers [102] presented a number of examples carbohydrates, alkaloids, amino acids (and derivatives) and peptides. As with the other methods, both positive and negative ionization modes are possible (Fig. 14). More recently, the same author [103] demonstrated the possibility of studying non-volatile nucleic acids and compared the results obtained with the other desorption methods. Sensitivity limits are on the order of ng. 

Chai et al. (46) developed a novel automated gas chromatographic technique to study slow kinetic processes. The technique uses multiple headspace extraction and can be applied to reactions involving volatile formation or adsorp-tion/desorption phenomena. 

Generally, TPD can be described as the measurement of the rate of desorption of adsorbed molecules, as a function of temperature. Therefore, this method can be useful in the extraction of very important information. It can be used in the identification and characterization of sites active in adsorption and catalytic reactions, in the study of adsorption states, binding energies, surface concentration and desorption kinetics. To summarize, this method is nowadays very important and often applied for the characterization of materials used as catalysts. In this domain, two main areas of applications are the characterisation of acid/base properties of solid materials, what is essential for understanding their reactivity and the determination of kinetic and thermodynamic parameters of desorption processes or decomposition reactions. 

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