Interactions of humic substances with allophanic compounds

Abstract

Organic matter from two Chilean Andisols, Vilcun and Osorno soils, were extracted and fractionated into humic (HA) and fulvic acids (FA). Humic acid were chemically characterized and their molecular weight (MW) distribution was obtained by gel permeation chromatography. The HA from 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 synthetic allophanic materials which carried ferrihydrite coatings. The isoelectric point (IEP) of the synthetic allophanic materials was reduced from 8.6 to near 3.0 and surface area was reduced from 450 m²/g to 120 m²/g depending on the HA level. The surface pKa values showed that the HA buffer capacity generally reduced the surface acidity of allophahic materials, but the HA from the Vilcun soil at a higher HA level which slightly increased the surface acidity due to an observed lower condensation degree. Phosphate (P) adsorption capacity was reduced to a greater degree in the HA from the Osomo soil model than in the HA Vilcun one, indicating a positive correlation with surface acidity. The differential isosteric heat of P adsorption (?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 model had higher energetic reaction than the Osorno one.