INTERACTIONS OF HUMIC SUBSTANCES WITH ALLOPHANIC COMPOUNDS

Organic matter from two Chilean Andisols, Vilcun and Osorno soils, wer e extracted and fractionated into humic (HA) and fulvic acids (FA). Hu mic acid were chemically characterized and their molecular weight (MW) distribution was obtained by gel permeation chromatography. The HA fr om the Vilcun soil had a lower MW than the HA from the Osorno soil and consequently had a higher acidity. Humic acid were used to coat synth etic allophanic materials which carried ferrihydrite coatings. The iso electric point (IEP) of the synthetic allophanic materials was reduced from 8.6 to near 3.0 and surface area was reduced from 450 m(2)/g to 120 m(2)/g depending on the HA level. The surface pKa values showed th at the HA buffer capacity generally reduced the surface acidity of all ophahic materials, but the HA from the Vilcun soil at a higher HA leve l which slightly increased the surface acidity due to an observed lowe r condensation degree. Phosphate (P) adsorption capacity was reduced t o a greater degree in the HA from the Osomo soil model than in the HA Vilcun one, indicating a positive correlation with surface acidity. Th e differential isosteric heat of P adsorption (Delta H) showed similar energy sites at lower P adsorbed levels in the Osorno and Vilcun soil models, but that with over 40% surface coverage, the Vilcun soil mode l had higher energetic reaction than the Osorno one.