Influence of steam sterilisation on soil chemical characteristics, trace metals and clay mineralogy

Abstract

Soil sterilisation with steam under pressure and additives, such as CaO or KOH that react exothermically in water, is an alternative technology to fumigants. The influence of soil sterilisation (system ‘Bioflash’, Ecoflash SC 600) on the chemical behaviour of organic matter, heavy metals (Cd, Cu, Zn and Pb) and clay mineralogy was investigated on three typical soil materials (Ap horizons) with different carbonate contents. These materials were treated in the laboratory in wooden containers. Soil heating produced temperatures ranging between 60–80 °C for at least 20 min. The treatment generally seemed to increase the pH values and may lead in the long-term to a certain alkalisation with high dosages of CaO or KOH. The effects of the CaO treatment on the K and Ca speciation in the soil were rather small, whereas those of the KOH treatment were more pronounced for K. The addition of KOH favoured the fixation of K in the interlayers of smectite. The XRD measurements were in agreement with the chemical speciation of K. The K chemistry was affected by the treatment by higher contents of the fixed fraction. Organic matter partially decreased after the soil heating treatment. The behaviour of the heavy metals was also affected. In some cases, the EDTA-extractable fractions of Cu, Zn or Pb as well as the soluble fractions of Cu and Zn were distinctly changed after the treatment. The soil-heating treatment probably affected the type and amount of organic ligands and also the adsorbing surfaces that control the solubility of the heavy metals. The heavy metal solubility, however, still remained low after the treatment. The effects of the soil sterilisation on the chemical and mineralogical properties did not seem to be dramatic. It is, however, difficult to predict possible changes if the treatment is repeated over years to decades. We presume that there might be quite substantial effects (especially on the behaviour of organic matter under certain circumstances) in the long term if, as seen at some sites, changes (i.e. decrease) are already measurable after one treatment. Mulching or the addition of organic manure would be possible measures to counter a destabilisation of the soil (i.e. increase in soil erosion susceptibility) and a continuous disintegration of the organic matter pool.