Assessment of Physical Leaching Processe

The physical processes controlling the desorption of some elements (B, Cd, Co, Mn, Ni, and Sr) from soils in a continuous leaching system representing the human stomach are investigated here by fitting experimental leaching data to a thermodynamical desorption profile model. The kinetic data reveals the release of elements from soil with artificial gastric solution at various flow rates (42–122 mL min-1) to support the work, followed by analysis of the release processes using individually coupled plasma mass spectrometry (ICP-MS). The leaching profiles of the various elements were fitted to a mathematical model incorporating two transport processes (liquid film diffusion and apparent solid phase diffusion) to determine the effective external mass transfer coefficient (β) and the apparent intraparticle solid diffusion coefficient (D). A system of partial differential equations was solved numerically with finite difference discretization of the computational domain allowing the rate limiting physical desorption processes for each element to be determined. The (thermodynamic) driving force of the leaching process is defined by the distribution coefficient (Kd) between soil and leachant. Although the Kd values investigated are very similar (ca. 6–15 L kg-1) for the elements studied with the exception of (B ca. 2 L kg-1), the leaching profiles are very different due to diffusion-limited processes. This results in quantifiable parameters for the liability of elements in soil upon ingestion which may be implemented in future risk assessment protocols.