ENVIRONMENTAL SIGNIFICANCE OF ADSORPTION—DESORPTION OF TRACE ELEMENTS IN SOILS — A REVIEWK. JEEVAN RAO
Absorption-desorption reactions play an important role in controlling the concentrations of metal contaminants in the soil solution. The accumulation of trace elements in carbonates can be a significant immobilization process in some soils. The process of Se accumulation in carbonates may be used to reduce trace elements availability, and, therefore, toxicity in natural systems. Both NI and Mn are found to form surface precipitate structures on calcite that are different from thermodynamically favoured corresponding mineral phases. The extent of desorption of Cd from soils is dependent on the affinity with which it is adsorbed. A long linear relation between the Kd of Cd in soils and the amount of Cd desorbed was established, which is environment. Evidence exists for Cd2., Pb2, incorporation into amorphous Fe and Al oxides by coprecipitation, although there is some question regarding the structural location of these metals. Manganese oxides play a vital role in redox and adsorption processes in soilsThe strong binding of Zn and Pb by soil phyllomagnates accounts for their relatively low mobility and bioavailability measured in crops and provides a molecular-level explanation to the low hazard imparted to these trace metals despite their relatively high concentration. The pl-I had little effect on As (V) sorption over the pH range from 2 to 8.5. Increasing ionic strength increases As (V) sorption on the soils. In soils Cr reacts with both organic and inorganic colloids. The various studies on Cr (III) desorption conformed to Langmuir-type equation. Although desorption related contribution to soil solution is low, the unadsorbed Cr (III), due to low level of sorption coupled with longer duration required to attain equilibrium, may not only be mobile in soils leading to groundwater contamination but also exerts adverse effects on soil organisms and plants. Addition of Cr (III) above the sorption capacity of soils may aggravate the situation. The environmental significance of adsorption-desorption of various other trace elements in soils were reviewed.
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