Prediction of Cation Transport in Soils Using Cation Exchange Reactions

Abstract

Mathematical models that describe the transport of multiple ions in the soil profile are discussed. The transport models are of the convective- dispersive type and it is assumed that ion exchange is the retention reaction which governs the distribution of cations between solution and adsorbed phases. A commonly used model assumes that equilibrium reaction between solution and exchanger phases for any two cations is not influenced by other species present in solution. The necessary inputs are the cation exchange capacity and exchange selectivity coefficients for each cation pair. This classical approach has been modified by incorporating selectivity coefficients that vary with the fractional coverage on the exchanger phase. This approach more accurately represents the cation exchange reactions and has yielded improved description of Ca, Mg, and Na breakthroughs in effluent from soil columns. Nonequilibrium ion exchange behavior has been incorporated into the classical transport model using two different approaches. The first approach uses the two-region concept (mobile and immobile water) where it is postulated that physical nonequilibrium governs the ion transfer between mobile and immobile regions. The second nonequilibrium approach is based on the assumption that cation exchange reactions exhibit kinetic behavior. Depending on the type of clay and ions present in the soil, two types of cation exchange sites have been characterized; one which exhibits equilibrium and the other which shows kinetic behavior.KeywordsSoil ColumnCation TransportSelectivity CoefficientMiscible DisplacementCation Exchange ReactionThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.