Mass Transfer Principles in Column Percolation Tests: Initial Conditions and Tailing in Heterogeneous Materials
Initial conditions (pre-equilibrium or after the first flooding of the column), mass transfer mechanisms and sample composition (heterogeneity) have a strong impact on leaching of less and strongly sorbing compounds in column percolation tests. Mechanistic models as used in this study provide the ne...
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| Online Access: | https://www.mdpi.com/1996-1944/14/16/4708 |
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| author | Binlong Liu Michael Finkel Peter Grathwohl |
| author_facet | Binlong Liu Michael Finkel Peter Grathwohl |
| author_sort | Binlong Liu |
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| description | Initial conditions (pre-equilibrium or after the first flooding of the column), mass transfer mechanisms and sample composition (heterogeneity) have a strong impact on leaching of less and strongly sorbing compounds in column percolation tests. Mechanistic models as used in this study provide the necessary insight to understand the complexity of column leaching tests especially when heterogeneous samples are concerned. By means of numerical experiments, we illustrate the initial concentration distribution inside the column after the first flooding and how this impacts leaching concentrations. Steep concentration gradients close to the outlet of the column have to be expected for small distribution coefficients (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>K</mi><mi>d</mi></msub><mo><</mo><mn>1</mn></mrow></semantics></math></inline-formula> L kg<sup>−1</sup>) and longitudinal dispersion leads to smaller initial concentrations than expected under equilibrium conditions. In order to elucidate the impact of different mass transfer mechanisms, film diffusion across an external aqueous boundary layer (first order kinetics, FD) and intraparticle pore diffusion (IPD) are considered. The results show that IPD results in slow desorption kinetics due to retarded transport within the tortuous intragranular pores. Non-linear sorption has not much of an effect if compared to <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>K</mi><mi>d</mi></msub></mrow></semantics></math></inline-formula> values calculated for the appropriate concentration range (e.g., the initial equilibrium concentration). Sample heterogeneity in terms of grain size and different fractions of sorptive particles in the sample have a strong impact on leaching curves. A small fraction (<1%) of strongly sorbing particles (high <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>K</mi><mi>d</mi></msub></mrow></semantics></math></inline-formula>) carrying the contaminant may lead to very slow desorption rates (because of less surface area)—especially if mass release is limited by IPD—and thus non-equilibrium. In contrast, mixtures of less sorbing fine material (“labile” contamination with low <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>K</mi><mi>d</mi></msub></mrow></semantics></math></inline-formula>), with a small fraction of coarse particles carrying the contaminant leads to leaching close to or at equilibrium showing a step-wise concentration decline in the column effluent. |
| first_indexed | 2024-03-10T08:38:25Z |
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| language | English |
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| spelling | doaj.art-bb2269053d68442d921127e77d756b6d2023-11-22T08:31:41ZengMDPI AGMaterials1996-19442021-08-011416470810.3390/ma14164708Mass Transfer Principles in Column Percolation Tests: Initial Conditions and Tailing in Heterogeneous MaterialsBinlong Liu0Michael Finkel1Peter Grathwohl2Center for Applied Geoscience, University of Tübingen, Schnarrenbergstraße 94-96, 72076 Tübingen, GermanyCenter for Applied Geoscience, University of Tübingen, Schnarrenbergstraße 94-96, 72076 Tübingen, GermanyCenter for Applied Geoscience, University of Tübingen, Schnarrenbergstraße 94-96, 72076 Tübingen, GermanyInitial conditions (pre-equilibrium or after the first flooding of the column), mass transfer mechanisms and sample composition (heterogeneity) have a strong impact on leaching of less and strongly sorbing compounds in column percolation tests. Mechanistic models as used in this study provide the necessary insight to understand the complexity of column leaching tests especially when heterogeneous samples are concerned. By means of numerical experiments, we illustrate the initial concentration distribution inside the column after the first flooding and how this impacts leaching concentrations. Steep concentration gradients close to the outlet of the column have to be expected for small distribution coefficients (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>K</mi><mi>d</mi></msub><mo><</mo><mn>1</mn></mrow></semantics></math></inline-formula> L kg<sup>−1</sup>) and longitudinal dispersion leads to smaller initial concentrations than expected under equilibrium conditions. In order to elucidate the impact of different mass transfer mechanisms, film diffusion across an external aqueous boundary layer (first order kinetics, FD) and intraparticle pore diffusion (IPD) are considered. The results show that IPD results in slow desorption kinetics due to retarded transport within the tortuous intragranular pores. Non-linear sorption has not much of an effect if compared to <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>K</mi><mi>d</mi></msub></mrow></semantics></math></inline-formula> values calculated for the appropriate concentration range (e.g., the initial equilibrium concentration). Sample heterogeneity in terms of grain size and different fractions of sorptive particles in the sample have a strong impact on leaching curves. A small fraction (<1%) of strongly sorbing particles (high <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>K</mi><mi>d</mi></msub></mrow></semantics></math></inline-formula>) carrying the contaminant may lead to very slow desorption rates (because of less surface area)—especially if mass release is limited by IPD—and thus non-equilibrium. In contrast, mixtures of less sorbing fine material (“labile” contamination with low <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>K</mi><mi>d</mi></msub></mrow></semantics></math></inline-formula>), with a small fraction of coarse particles carrying the contaminant leads to leaching close to or at equilibrium showing a step-wise concentration decline in the column effluent.https://www.mdpi.com/1996-1944/14/16/4708leaching testequilibrium conditionnon-equilibrium conditionmodellingsorption kineticsnon-linear sorption |
| spellingShingle | Binlong Liu Michael Finkel Peter Grathwohl Mass Transfer Principles in Column Percolation Tests: Initial Conditions and Tailing in Heterogeneous Materials Materials leaching test equilibrium condition non-equilibrium condition modelling sorption kinetics non-linear sorption |
| title | Mass Transfer Principles in Column Percolation Tests: Initial Conditions and Tailing in Heterogeneous Materials |
| title_full | Mass Transfer Principles in Column Percolation Tests: Initial Conditions and Tailing in Heterogeneous Materials |
| title_fullStr | Mass Transfer Principles in Column Percolation Tests: Initial Conditions and Tailing in Heterogeneous Materials |
| title_full_unstemmed | Mass Transfer Principles in Column Percolation Tests: Initial Conditions and Tailing in Heterogeneous Materials |
| title_short | Mass Transfer Principles in Column Percolation Tests: Initial Conditions and Tailing in Heterogeneous Materials |
| title_sort | mass transfer principles in column percolation tests initial conditions and tailing in heterogeneous materials |
| topic | leaching test equilibrium condition non-equilibrium condition modelling sorption kinetics non-linear sorption |
| url | https://www.mdpi.com/1996-1944/14/16/4708 |
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