The Chemical Dynamics of Cadmium in the Soil Environment

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Abstract Cadmium (Cd) is known to be toxic to plants and animals and can be transported to both ground and surface water through specific physical mechanisms. The chemical dynamics of Cd and other similar (heavy) metals in the soil environment is controlled by its interactions with both solid and aqueous phases of soil. The concentration of Cd and other metals in soil solution is highly influenced and regulated by processes such as adsorption-desorption, ion exchange, precipitation-dissolution, and soil and solution phase composition. Such processes, in turn, are function of soil properties such as pH, charge density and distribution, thickness of diffuse double layer and the activity of Cd present in solution. This report intends to provide a comprehensive and critical review to the effect of such properties on Cd transport. 1. Introduction Historically, the discover of Cadmium (Cd) goes back to the eighteenth century (1817) by the German scantiest Friedrich Stromeyer who found it as an impurity in zinc carbonate. In the modern world, Cd is used in many industrial, urban, and agricultural applications ([1], [2]) and is often found together at sites contaminated with heavy metals. As Cd is a toxic metal to humans [3] and animals alike, the environmental and soil contamination with Cd is becoming of great concern in the last few decades. Therefore, it is important for chemists, environmental scientists and engineers to understand the chemistry of Cd interactions in soils to understand its bioavailability. Millions of tonnes of hazardous waste containing CD are generated in the world on yearly basis. Because of the inefficient waste handling techniques and hazardous waste leakage in the past, thousands of sites, es... ... middle of paper ... ...in terms of two basic mechanisms: specific adsorption, which is characterized by more selective and less reversible reactions including chemisorbed inner-sphere complexes, and nonspecific adsorption (or ion exchange), which involves rather weak and less selective outer-sphere complexes [14]. It is established that specific adsorption brings about strong and irreversible binding of heavy metal ions with organic matter and variable charge minerals while nonspecific adsorption is an electrostatic phenomenon in which cations from the pore water are exchanged for cations near the surface. Cation exchange is a type of outer-sphere complexation with only weak covalent bonding between metals and charged soil surfaces. The process is naturally reversible and occurs rather quickly as it is typical for reactions which are diffusion-controlled and of electrostatic nature [14].

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