DefinePK

Abstract

Anion and cations from applied agrochemicals move differently during their bulk solution flow through sandy and clayey soils. Saturated inert and reactive porous media were generated from solid glass beads and sepiolite clay minerals to mimic sandy and clayey soils. At various flow rates, solutions of potassium bromide (KBr) of various concentrations were separately displaced by deionised water through above media. Concentration of the Br– and K+ were determined with ion-specific electrodes and bulk solution by electrical conductivity (EC). Non-linear curve fitting technique, CXTFIT 2.0 was used to parameterize the breakthrough curves (BTCs) of each displacement for the anion, the cation and the bulk solution, separately. Shapes of BTCs and parameters including retardation factor (R) and coefficient of hydrodynamic dispersion (D) were used to interpret the results. For inert media, (a) the matching BTCs of the anion, cation and bulk solution were observed, (b) the values of R did not differ significantly from unity, and (c) the values of D were identical and changed with pore-water velocity, (v). For reactive media, the BTCs of anion appeared before and those of cations appeared after those of bulk solution. The values of R were less than unity for the anion, greater than unity for the cation and close to unity for the bulk solution. However, the values of D were unaffected by nature of solutes or solution. Results of this study show that separation of ions observed during transport through reactive material means that the composition as well as the concentration of a solution change continuously during flow through reactive soils i.e. clay or clay loam. These results have implications for exchange of cations and exclusion of anions from the soils of varying net negative charges and cation exchange capacity.