Determining competitive nitrate and chloride adsorption in an andisol by the unsaturated transient flow method

Abstract

Adsorption of monovalent electrolyte anions during transport process in Andisols is largely due to the increase in the total anion adsorption rather than through anion exchange with strongly adsorbed native S04 2−. The unsaturated transient flow method has been developed for determining adsorption isotherms for weakly reactive ions without causing excessive desorption of strongly adsorbed native ions. The objective of this study was to extend the method to determine NO3 − adsorption isotherms in an Andisol in the absence or presence of Cl competing for the adsorption sites Kannondai subsoil (Hydric Hapludand), premixed with a Ca(NO3)2 or CaCl2-Ca(NO3)2 solution at different concentrations, was packed into sectionable columns, and one-dimensional water absorption experiments were conducted. Anion adsorption by soil, Q n, and the liquid-phase concentration, C n, prior to the water imbibition were obtained from the plots of the anion content vs. water content in the region beyond the ‘plane of separation,’ where the antecedent solution was accumulated. The values of Qn and C n found in a series of column experiments were then used to construct the isotherms describing the solution composition-dependent adsorption of NO3 − and Cl−. e fitted to Langmuir-type equations, the maximum adsorption for NO3 − (Qmax= 27,1-29.0 mmolc kg−1) was consistently smaller than that for Cl− (Qmax = 455 46.1 mmolc kg−1), while the empirical constant K (= 0.0238 - 0.0274 m3 molc −1) was insensitive to the anion species. The anion content profiles predicted by the inferred adsorption isotherms closely agreed with the measured content profiles.