KINETICS OF MOLYBDATE AND PHOSPHATE SORPTION BY SOME CHILEAN ANDISOLS

Abstract

The kinetics for the sorption of molybdate and phosphate by four Chilean Andisols have been determined. About 55% of the molybdate and 61% of the phosphate was sorbed in the first 0.5 h, after which sorption slowly increased, reaching 90% for molybdate and 97% for phosphate after 72 h. At the same time, OH- ions were released into the external solution, raising its pH by 0.85 units for molybdate and by 0.65 units in the case of phosphate. These observations indicated that both anions were sorpbed by a ligand exchange mechanims. Among the five kinetic models examined (Table, 2), the Elovich equation gave the best fit of the experimental data (R2 = 0.93 to 0.97, standard error = 0.35 to 0.94). The sorption rate constant (α) for both anions was related to the organic matter (OM) content of the soils, especially the content of Al- and Fe-humus complexes. The α values for molybdate were 2.24×1015 mmol kg-1 h-1 for the Vilcún soil (15% OM), 2.49 x1012 mmol kg-1 h-1 for the Pemehue soil (16% OM), 8.76×1010 mmol kg-1 h-1 for the Osorno soil (20% OM), and 3.11×107 mmol kg-1 h-1 for the Piedras Negras soil (24% OM). The corresponding values for phosphate were 3.89×107, 5.21×1010, 3.11×1012 and 1.08×1016 mmol kg-1 h-1. The desorption rate constant (β) for the four soils (in the above order) ranged from 0.47 to 0.28 for molybdate, and 0.22 to 0.39 mmol kg-1 h-1 for phosphate. The results suggest that the mineralogical composition and organic matter content of the Andisols control the kinetics for the sorption of both molybdate and phosphate. Molybdate appeared to have a high affinity for Fe- and Al-oxides, while phosphate was largely sorbed to Fe-and Al-humus complexes.