Properties of an aged phenanthrene-contaminated soil and its response to bioremediation processes

Purpose Persistent organic compounds exhibit declining extractability and bioavailability to microorganisms and other soil organisms with increasing contact time or ageing. Among the possible mechanisms are the association of organic compounds with natural organic matter. Ageing can reduce the negative effects that an organic pollutant may have on the biological, biochemical properties and phytotoxicity of the contaminated soil. The aim of the present work was to evaluate, under laboratory conditions, the response of an agricultural soil contaminated with phenanthrene (Phe) and subjected to 2-year ageing and the effects and potential remediation capabilities of compost and an effective Phe-degrading bacterial culture.Materials and methods A fresh agricultural soil was contaminated with Phe, supplemented with compost at two different rates and subjected to 2-year ageing under laboratory conditions. Phe was detected by HPLC analysis. The main biochemical and chemical properties of soil amended or not with compost were measured before and after the ageing process by using standard techniques. Germination tests were performed with Cucumis sativus L. and Lepidium sativum L. Results and discussion Significantly reduced amounts of Phe were extracted after 650 days and a greater amount of Phe accumulated in the soil humic fractions and preferentially in the humin–mineral fraction. A decrease of the extractable Phe from the 650-day aged samples occurred in the presence of the Phe-degrading bacterial culture but only after further 21–50 days of incubation. A similar decrease was measured also in the non-inoculated samples. The higher values of arylsulphatase and phosphatase measured after 650 days of incubation seem to indicate that the indigenous bacteria of the soil adapted to and survived the presence of the aged Phe, also because of its possible reduced toxicity. A little residual phytotoxicity was still measured indicating that the low amount of available Phe was high enough to reduce the germination of the two plant species. Conclusions The results support that complex phenomena occur in a soil when a contaminant is present and persist for long time in it. The higher amounts of Phe extracted by the humic fractions indicate that ageing of Phe actually occurred and was favoured by the presence of the high level of organic matter in the soil. Tested parameters showed different response to the presence of aged compound. Therefore, the choice of the biochemical parameter to use as indicator of soil quality might be crucial for obtaining correct and easily interpretable results.