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Mobile Organic Sorbent Affected Contaminant Transport in Soil

Numerical Case Studies for Enhanced and Reduced Mobility

Lehrstuhl für Bodenkunde, Department für Ökologie, Wissenschaftszentrum Weihenstephan, Technische Universität München, D-85350 Freising-Weihenstephan, Germany

View larger version (41K): [in a new window]   Fig. 1. Size class spectra of particles and aggregates of different origin in the soil environment. The shaded area represents the region of mobile materials. The dotted line designates the upper size limit of the operationally defined dissolved fraction. Note the small and medium-sized colloidal particles are part of this fraction, too.

View larger version (52K): [in a new window]   Fig. 2. Interaction pathways considered by the numerical case studies. The interactions can be of linear or nonlinear type, spontaneous or kinetically controlled. (A) Conceptual model for the transport of mobile organic sorbents (MOS) through soil. (B) Conceptual model for MOS enhanced contaminant mobility (Scenario A). (C) Conceptual model for reduced mobility due to cosorption (Scenario C). (D) Conceptual model for reduced mobility due to cumulative MOS-sorption (Scenario D). Note that for Scenario C the organic fraction of the bulk solid phase is a function of time. The solid phase is composed of different organic and inorganic sorbents. Effects of the mobile sorbents on solution phase properties like surface tension can be considered as well.

View larger version (25K): [in a new window]   Fig. 3. Breakthrough of mobile organic sorbents (MOS) with different affinity to soil. The interaction parameters are chosen such that the corresponding breakthrough curves represent different MOS or MOS fractions. filled circles, DKD = 0.18, MOS hydrophilic, low content of sorbing minerals; triangles, DKD = 1.8, MOS hydrophilic, higher content of sorbing minerals; squares, DKD = 22.4, MOS intermediate, low content of sorbing minerals; diamonds, DKD = 112, MOS intermediate, higher content of sorbing minerals; inverted triangles, DKD = 224, MOS hydrophobic, low content of sorbing minerals; circles, DKD = 673, MOS hydrophobic, high content of sorbing minerals.

View larger version (36K): [in a new window]   Fig. 4. Breakthrough of (a,b) a dummy heavy metal and (c,d) a dummy hydrophobic contaminant in the presence of increasing concentrations of mobile organic sorbents (MOS) for Scenario A, enhanced mobility. Given are the breakthrough curves for the free CHf and the MOS-bound solute CHb. For this scenario, no sorption of the MOS to the immobile solid phase is allowed (DKD = 0). The symbols represent the following MOS concentrations (kg m–3): circles, 0; triangles, 0.005; squares, 0.01; diamonds, 0.05; inverted triangles, 0.1.

View larger version (35K): [in a new window]   Fig. 5. Breakthrough of (a,b) a heavy metal and (c,d) a hydrophobic contaminant in the presence of increasing concentrations of mobile organic sorbents (MOS) for Scenario B, reduced mobility due to cosorption. Given are the breakthrough curves for the free CHf and the MOS-bound solute CHb. For this scenario, sorption of the MOS to the immobile solid phase is allowed (DKD = 22). As MOS model substance, the hydrophilic MOS fraction (KD = 0.015 m3 kg–1) was chosen which sorbs nonspecifically to the bulk solid phase (D = 1470 kg m–3). The dashed line (a,b) gives the breakthrough of this MOS fraction for both solutes. The symbols represent the following MOS concentrations (kg m–3): circles, 0; triangles, 0.005; squares, 0.01; diamonds, 0.05; inverted triangles, 0.1.

View larger version (30K): [in a new window]   Fig. 6. Breakthrough of (a,b) a heavy metal and (c,d) a hydrophobic contaminant in the presence of increasing concentrations of mobile organic sorbents MOS) for Scenario B. Given are the breakthrough curves for the free CHf and the MOS-bound solute CHb. As MOS model substance, the hydrophobic MOS fraction (KD = 1.870 m3 kg–1) was chosen which sorbs specifically to the bulk solid phase (D = 120 kg m–3). The dashed line (a,b) gives the breakthrough of this MOS fraction for both solutes. The symbols represent the following MOS concentrations (kg m–3): circles, 0; triangles, 0.005; squares, 0.01; diamonds, 0.05; inverted triangles, 0.1.

View larger version (25K): [in a new window]   Fig. 7. Breakthrough of HOC in the presence of mobile organic sorbents (MOS) according to Scenario B, cosorption. Given is the limit situation for the evolution of a shoulder in the breakthrough curve.

View larger version (18K): [in a new window]   Fig. 8. Breakthrough of a hydrophobic contaminant in the presence of increasing concentrations of mobile organic sorbents (MOS) for Scenario B. The sorption of the solute was assumed nonlinear following a Freundlich-type sorption isotherm with p = 1.15. Given are the breakthrough curves for the free CHf and the MOS-bound solute CHb. As MOS model substance, the hydrophobic MOS fraction (KD = 1.870 m3 kg–1) was chosen which sorbs specifically to the bulk solid phase (D = 120 kg m–3). The solid line indicates the breakthrough of this MOS fraction. The symbols represent the following MOS concentrations (kg m–3): circles, 0; triangles, 0.001; filled squares, 0.005; diamonds, 0.01; diamonds, 0.05; open squares, 0.1.

View larger version (18K): [in a new window]   Fig. 9. Depths profiles of the fraction of the bulk solid phase which provides sorption sites for the hydrophobic organic contaminants. (a) Depth profiles for increasing mobile organic sorbents (MOS) concentrations as calculated after 280 pore volumes have been exchanged. (b) Depth profiles for 0.050 kg MOS m–3 for different moments. Initially, the amount of organic matter was set to 0.03 10–3 kg m–3.

View larger version (19K): [in a new window]   Fig. 10. Breakthrough of a hydrophobic organic contaminant in the presence of increasing concentrations of mobile organic sorbents (MOS) for Scenario C, the cumulative sorption scenario. Given are two situations. (a) Linear hydrophobic organic compounds interaction of the hydrophobic organic compounds with the solid phase organic matter and intermediate MOS (KD = 1.870 m3 kg–1) with a high amount of MOS-specific sorption sites.