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Leaching Process

Leaching is the process by which constituents of a solid material are released into a contacting water phase. Although some species may be more of an environmental concern than others, the leaching process is indiscriminant such that all constituents (e.g., major or minor matrix components as well as inorganic, organic and radionuclide contaminants) are released under a common set of chemical phenomena which may include mineral dissolution, desorption and complexation, and mass transport processes. In turn, these phenomena are affected by certain factors that can alter the rate or extent of leaching. Among these factors are:
  • internal chemical and physical reactions
  • external stresses from the surrounding environment
  • physical degradation of the solid matrix due to erosion or cracking, and
  • loss of matrix constituents due to the leaching process itself.
Leaching Factors
Physical and Chemical Factors Influencing Leaching

The process of leaching includes the partitioning of contaminants between a solid and liquid phase (e.g., assuming local equilibrium) coupled with the mass transport of aqueous or dissolved constituents. Mass transport is the summation of diffusion, hindered diffusion, tortuosity effects, and effective surface area effects through the pore structure of the material to the environment. Important chemical factors, those that influence the liquid-solid partitioning (LSP) of a constituent, include solution pH, redox, the presence of dissolved organic matter, and biological activity. Physical factors, such as relative hydraulic conductivity, porosity and fill geometry, play an important role in determining the rate at which constituents transport through a solid into a passing liquid phase.

The process itself is universal, as any material exposed to contact with water will leach components from its surface or its interior depending on the porosity of the material considered.