Soil sterilisation methods for use in OECD 106: How effective are they?

Under many circumstances chemical risk assessments for pharmaceuticals and other substances are required to differentiate between 'loss' of a chemical from the aqueous phase as a result of abiotic (sorption or precipitation reactions) or biotic (biodegradation) processes. To distinguish only abiotic processes, it is necessary to work under sterile conditions. Reported methods include poisoning the soil with sodium azide, irradiation and autoclaving. However, a key aspect of any testing is the representativeness of the matrix and so any sterilisation procedure needs to ensure that the integrity of the sample is maintained, in particular particle size distribution, pH and organic carbon partitioning potential. A number of controlled laboratory experiments were performed on 3 different types of soil. Results indicated that none of the methods successfully sterilised the soils and some physico-chemical changes in soils were identified post-treatment. Autoclaving destroyed the soil structure, therefore potentially affecting its sorption behaviour and sodium azide changed the pH of the loam soil solution by 0.53 pH units. Gamma irradiation exhibited least disruption to the tested soils physico-chemical properties. It was therefore concluded that gamma irradiation was the best available method for sterilising soils in preparation for sorption-desorption experiments; however care needs to be taken with this method to ensure that microbial activity is absent, or quantified if present. The changes to soils after sterilisation varied depending on the individual soil properties, indicating that soils should be studied on a case-by-case basis.