Phosphate adsorption in allophanic soils

The kinetics and heats of phosphate adsorption were measured on the <2 μm Na‐saturated fractions of three allophanc‐rich soils from Japan. Between 50 and 2250 μmol P g−1 as sodium phosphate were added to the soil fractions at pH 5 and pH 7, and at initial concentrations of 5 and 25 × 10−4m to avoid aluminium phosphate precipitation.

An initial ‘instantaneous’ adsorption associated with exposed sites and, simultaneously, two inverse exponential rates of adsorption on internal and freshly forming external sites were observed. These rates are attributed to changes in the microstructure of allophane and to the desorption of organic matter held on allophanic surfaces. This interpretation is strongly supported by corresponding changes in the heats of adsorption with time. Calorimetry clearly indicates that when very large amounts of phosphate are added, new and very reactive surfaces are progressively exposed.

More phosphate was adsorbed when the soil was acid and when the soil contained less organic matter.

RESP-08355