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Ion Exchange and Diffusive Mass Transfer During Miscible Displacement Through an Aggregated Oxisol
Author(s) -
NkediKizza P.,
Rao P. S. C.,
Jessup R. E.,
Davidson J. M.
Publication year - 1982
Publication title -
soil science society of america journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.836
H-Index - 168
eISSN - 1435-0661
pISSN - 0361-5995
DOI - 10.2136/sssaj1982.03615995004600030006x
Subject(s) - chemistry , diffusion , mass transfer , analytical chemistry (journal) , ion exchange , oxisol , ion , soil water , mineralogy , thermodynamics , environmental chemistry , soil science , chromatography , geology , organic chemistry , physics
Effluent breakthrough curves (BTCs) for four tracers ( 3 H 2 O, 14 CH 3 OH, 36 Cl, and 45 Ca) 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 36 Cl and 45 Ca 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 ( 3 H 2 O and 14 CH 3 OH), 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 ( K D ) for 36 Cl and 45 Ca were estimated by curve‐fitting the model to BTCs measured at the low pore‐water velocity (3 cm/hour). These K D values and all other independently estimated parameters were then used to simulate 36 Cl and 45 Ca 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.