Adsorption of chloride, phosphate and phthalate on fractions obtained froma Chilean andisol.

The adsorption of chloride, phthalate, and phosphate on an unfertilized sam ple of Osorno soil is studied. The clay fraction (< 2 mu m, FF) and the cla y fraction without organic matter (S/MO) were obtained from this Andisol. T he experimental adsorption isotherms on both fractions were described throu gh Langmuir's models (uniform and two-surface models). No temperature effects were observed on adsorption isotherms, in the range 4 degrees C to 25 degrees C, for the three anions. Adsorption of phosphate increases as equilibrium time increases, due to its strong interaction with the surface. The same effect was observed for FF sample at high phthalate concentration. No time effect on chloride adsorption was observed. Adsorption of the three anions is higher on FF sample, showing the high cap acity of soil organic matter to bind them, which is not suitably described by the surface charge parameter here presented. It is possible to describe the chloride adsorption through the uniform surf ace Langmuir model, but obtained parameters do not have physical meanings. Phthalate and phosphate adsorption are described by the uniform surface and two-surface models respectively. Under similar experimental conditions the Langmuir maximum adsorption capac ity has the sequence phosphate > phthalate > chloride.