APPLYING BATCH-DETERMINED RETENTION PROPERTIES TO PREDICT SULFATE TRANSPORT THROUGH SOIL COLUMNS

Abstract

Equilibrium properties of sulfate retention by soil under different experimental conditions were determined and evaluated with regard to their impact on subsequent transport simulations. The sulfate retention characteristics of a silty clay soil (Typic Hapludult) were examined in sulfate solutions at 3 initial pHs, and with or without phosphate in solution. Langmuir and Freundlich adsorption isotherms were fit to the data with low standard errors. An increase in pH from values ranging between 4.28 and 4.55 to values ranging between 5.96 and 6.47 decreased the estimated maximum retention from 4.7 to 3.4 mmol/kg soil. The addition of a phosphate solution further decreased estimated maximum sulfate retention from 3.4 to 2.5 mmol/kg. Independently determined transport parameters combined with batch-determined retention properties accurately predicted sulfate transport through a small laboratory column when column effluent pH and batch experiment pH were similar. Retention properties were not transferable from laboratory tube to small column when batch and column pH differed or when solutions contained phosphate during batch experiments but not during the column experiment. Relative concentration of the simulated breakthrough curves for a 0.1 m M sulfate solution equalled 0.5 after 8.5 and 34 pore volumes for sulfate retention characteristics determined at pH 5.96–6.47 and 4.28–4.55, respectively. The results indicate that reliable predictions of transport from batch-determined retention properties are possible, but only if batch conditions closely match conditions for specific transport situations. © Williams & Wilkins 1991. All Rights Reserved.