Effects of duration of anaerobiosis and reoxidation on phosphate sorption characteristics of an acid soil

Abstract

Suspensions of a silt loam were incubated under reducing conditions for 1, 6, 9, 18, 27 and 45 days at a pH of 6.5 or 6.6. Additional suspensions of the same soil which had been previously reduced at varying pH levels were reoxidized. Phosphate sorption properties of the soil were determined after the various incubation periods using normal sorption isotherm techniques. There was a large increase in the phosphate sorption capacity after only 1 day of reduction. An apparent decrease in sorption between 1 and 18 days was caused by a consistent decrease in bonding energy. Increasing iron concentrations in solution suggested that these effects were due to increasing dissolution of iron hydrous oxides and exposure of equivalent but less energetic adsorption sites which were previously inaccessible. The largest adsorption occurred at 27 days due, apparently, to the precipitation of amorphous and more reactive ferroso-ferric hydroxides. By 45 days the sorption capacity had decreased and was only slightly greater than that of the untreated soil. Between 18 and 27 days of anaerobiosis were required to cause a large increase in labile and solution phosphate. After reoxidaton there was a large decrease in phosphate sorption capacity at pH 6.6 and 8.0 but little change at pH 5.0. There was a similar decrease in bonding energy at the higher pH levels, but a large increase at pH 5.0. A fourfold increase in solution phosphate concentration occurred at pH 8, while relatively small changes occurred-at the other pH levels. These differences were attributed to changes in the solubilities and surface properties of iron compounds associated with the previous reduced conditions and pH levels. The results suggest that reoxidation increases phosphate sorption only if the pH or Eh during reduction is low enough to maintain a high iron concentration in solution.