Ion Exchange and Diffusive Mass Transfer During Miscible Displacement Through an Aggregated Oxisol1
- P. Nkedi-Kizza,
- P. S. C. Rao,
- R. E. Jessup and
- J. M. Davidson2
Effluent breakthrough curves (BTCs) for four tracers (3H2O, 14CH3OH, 36Cl, and 45Ca) were measured over a wide range of porewater velocities (3 to 164 cm/hour) using water-saturated columns of Ione soil, an aggregated Oxisol. Increasing asymmetry of the measured BTCs for all tracers with increasing pore-water velocity was attributed to decreasing residence time for solute transfer by diffusion between inter- and intra-aggregate regions. The measured BTCs for 36Cl and 45Ca illustrated the role played by the pH-dependent soil surface charge in determining the retention and transport of ionic species.
Measured BTCs were compared with those calculated using a simulation model where (i) convective-dispersive solute flow was limited to the inter-aggregate regions, (ii) solute transfer into and out of equivalent spherical aggregates was described by Fick's second law of diffusion, and (iii) isotopic exchange was considered to be an instantaneous reversible process and was described with a linear exchange isotherm. For the slightly sorbed solutes (3H2O and 14CH3OH), all model parameters were independently estimated. Owing to the strong pH-dependence of isotopic exchange and the inability to exactly duplicate in batch experiments the chemical environment (pH and ionic strength) of the soil columns, exchange coefficients (KD) for 36Cl and 45Ca were estimated by curve-fitting the model to BTCs measured at the low pore-water velocity (3 cm/hour). These KD values and all other independently estimated parameters were then used to simulate 36Cl and 45Ca BTCs for the high pore-water velocity (120 cm/hour). Good agreement was found between measured and calculated BTCs for all tracers. These data and simulations demonstrated the coupling between physical processes (convective-dispersive flow, diffusive mass transfer) and chemical processes (ion exchange, pH-dependent charge) in determining the shapes and positions of BTCs obtained from water-saturated aggregated soils with a variable surface charge.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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