The reactions of phosphorus in thermally treated soils

Abstract

The reactions of phosphorus in thermally treated soils UUSITALO Risto, HARTIKAINEN Helinä Thermal treatment is a common practice to reclaim soils contaminated with organic pollutants. We studied whether the treated soil can be used as a growth medium or be mixed with cultivated soils as an alternative to storing in heaps. This paper describes the thermally induced changes in soil P reserves and in the ability of soil to retain added P. Two surface soil samples (sandy loam and silty clay) were treated at 600°C and 1000 °C. The changes in particle size distribution, organic C, surface area, oxalate-extractable Al (Alox) and Fe (Feox) and Chang-Jackson inorganic P fractions were measured. The desorption-sorption isotherms were used to describe the sorption properties. Heating decreased the amount of clay size particles (in the clay soil from 46% to 11% at 600° and to 9% at 1000°C) and reduced the surface area by 90 %. At 600°C, Alox markedly increased in the clay soil due to the breakdown of clay mineral lattices, but was not affected in the coarse textured soil. At 1000°C Alox fell to a very low level as a result of partial fusing of the soil particles. On the other hand, Feox decreased regularly with increasing temperature. The release of P from SOM increased the fractionable inorganic P at 600°C. The changes in NH4F-P and NaOH-P assumed to represent P bound by Alox and Feox, respectively, followed the alterations in their reaction components. The changes in H2SO4-P at 1000°C showed that in the clay soil P was to a marked extent trapped within the aggregates formed during heating and inside the highly crystalline oxides of Al and Fe. In the sandy loam soil most of the P sorbed by oxides was only transferred to the acid-soluble form. At 1000°C both soils lost their ability to retain added P. Keywords: soil remediation, phosphorus fractions, phosphorus sorption A.