Trace elements desorption

Concentrations of trace elements in soil solution may be controlled by the solubility of certain solid phases via dissolution/(co-)precipitation or by other physicochemical and biological processes such as adsorption-desorption, complexation, and redox reactions. 

Kingery W.L, Oppenheimer S.F., Elan F.X., Selim H.M. Adsorption/desorption hysteresis of trace metals with soil components A Dynamical systems approach. Proceedings of the Fifth International Conference on the Biogeochemistry of Trace Elements, Vienna, Austria. 1999. 

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). 

Interest in trace element speciation studies in natural waters has increased considerably during the last decade. It has become apparent that data on total concentrations of any element rather than on individual well defined chemical entities, are often inadequate to identify transport mechanisms, ultimate fate and toxicity of particular elements to organisms. A study of the different trace metal species and their relative distribution will assist in understanding the chemical processes that take place in the highly reactive estuarine zone and in the open sea. These processes include the rate at which chemical processes take place, the participation in geochemical processes (precipitation/dissolution, adsorption/desorption). 

Like colloidal material, surfaces have complex-ing sites for trace elements and the same formalism as that described for colloids can be used. Understanding the partitioning of metals and more generally trace elements between water and solids is crucial for fundamental smdies on transport, bioavailability, and fate of trace elements in river systems. For example, the spatial and temporal trends of metal or radionuclide partitioning between dissolved and suspended sohds is a major issue for understanding and predicting the pathways of pollutants in the environment. As a consequence, an impressive literature focuses on experimental studies of trace-element adsorption/desorption on synthetic surfaces (mostly hydrous oxides). However, field-based studies aimed at assessing the importance... 

Li Y.-H., Burkhardt L., and Teraoka H. (1984) Desorption and coagulation of trace elements during estuarine mixing. Geochim. Cosmochim. Acta 1879-1884. 

For many of the more abundant elements, such as Al, Fe, and Mn, precipitation of mineral forms is common and may greatly influence or even control their solubility. For most trace elements, direct precipitation from solution through homogeneous nucleation appears to be less likely than adsorption-desorption, by virtue of the low concentration of these metals and metalloids in soil solutions in well-aerated dryland soils. When soils become heavily polluted, metal solubility may reach a level to satisfy the solubility product to cause precipitation. Precipitation may also occur in the immediate vicinity of the phosphate fertilizer zone, where the concentration of heavy metals and metalloids present as impurities may be sufficiently high. Precipitation of trace metals as sulfides may have a significant role in metal transformation in reduced environments where the solution sulfide concentration is sufficiently high to satisfy the solubility product constants of metal sulfides (Robert and Berthelin, 1986). 

FACTORS AFFECTING THE SORPTION-DESORPTION OF TRACE ELEMENTS IN SOIL ENVIRONMENTS... 

The aim of this chapter is to provide the current state of knowledge on the factors that affect the mobility of trace elements in soil environments. Special attention is given to the influence of inorganic and organic ligands, including nutrients and root exudates, on the sorption—desorption processes of trace elements in cationic and anionic forms on/from soil components and soils. 

Sorption of trace elements onto soil components is greatly affected by pH, ionic factors, nature of the sorbents, redox reactions, and so on, but the sorption of elements in cationic form differs greatly from that of elements in anionic form. The presence of organic and inorganic ligands (including nutrients) in soil environments has a very important role in the sorption-desorption processes of trace elements. 

Ainsworth et al. (1994) observed that oxide aging did not cause hysteresis of trace element cation sorption-desorption. Aging the hydrous ferric oxide with trace elements cations resulted in hysteresis with Cd and Cu, but little hysteresis was observed with Pb. The extent of reversibility with aging for Co, Cd, and Pb was inversely proportional to the ionic radius of the ions (i.e., Co < Cd < Pb). The authors attibuted the hysteresis to Co and Cd incorporation into a recrystallizing solid (probably goethite) via isomorphic substitution, not to micropore diffusion. 

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