Anion transport involving competitive adsorption during transient water flow in an andisol

Andisols containing allophane and other poorly crystalline materials, carry variable surface charge, and adsorb otherwise inert anions such as Cl- and NO3-. The objective of this study was to predict composition-dependent, retarded transport of anions during unsteady water flow in an Andisol. One-dimensional absorption experiments in Kannondai subsoil (Hydric Hapludand) columns were conducted with CaCl2, Ca(NO3)(2), and mixed CaCl2-Ca(NO3)(2) solutions at different concentrations (0.00462-0.115 mol L(-1)). Because of nonlinear, competitive adsorption of Cl- and NO3- by the soil, the observed retardation of anion penetration depended on the total concentration and composition of the invading solution. Nitrate ion had a smaller affinity to the adsorption sites than Cl- (the selectivity coefficient K-V = 0.51) and showed a greater penetration in the presence of Cl-. The adsorption of Cl- and NO3- much exceeded the concurrent SO42- desorption from the soil. This suggests that SO42- was so strongly adsorbed that exchange reaction with the invading anions proceeded only to a limited extent. Thus, the adsorption of the monovalent anions was largely due to the increase in the anion exchange capacity of the soil in response to an increase in the ionic strength of the bulk solution. An approximate theory was developed for anion transport involving competitive adsorption in which total monovalent anion adsorption, obeying a Langmuir-type equation, is allocated via K-V to the adsorption of Cl- and NO3-. Employing the adsorption parameters determined from separate steady state teaching experiments, the theory successfully described the measured anion content profiles.