An appropriate site characterization performed in a timely and cost-efficient manner is an essential step towards the sustainable remediation of contaminated sites. In the case of soil bioremediation, reliable methods for monitoring and assessing soil microflora and its activity, and microcosms testing site-specific biodegradation of pollutants, are a prerequisite for successful technology selection and planning.

The aims of this research were: (1) to evaluate several biological soil-testing methods for characterizing the biodegradation processes in soil; (2) to test the usefulness of a chemical method (cyclodextrin extraction) for simulating the biologically available contaminant fraction; and (3) to compare the chemical and biological test methods with each other, in order to find the most appropriate model for testing the biodegradation potential and its enhancement in contaminated soil.

Laboratory-scale microcosm experiments on soils spiked with diesel and transformer oil were performed and evaluated, using chemical analyses and biological methods indicating different endpoints of microbial degradation in soil: contaminant decrease; concentration of degrading microbial cells; and aerobic respiration rate by measuring the dehydrogenase enzyme playing a role in respiration or CO2 as an end-product. As a part of the biological methodology, the following biotests were applied: (1) measurement of contaminant-degrading cell concentration using the dilution method, and statistical evaluation by the most probable number method; (2) soil dehydrogenase enzyme activity test; (3) soil respiration test in a ‘closed bottle’; and (4) soil respiration test in a self-designed aerated system (dynamic soil reactors). The chemical analytical methods included: (1) organic solvent extraction followed by gravimetry; (2) organic solvent extraction followed by gas chromatography; and (3) extraction by aqueous cyclodextrin solutions followed by gas chromatography.

The results of the chemical and biological tests show a better agreement for diesel oil, than for transformer oil, which is chemically less homogeneous and less biodegradable. The most coherent results were obtained by the aerobic respiration test and by chemical analyses after hydroxypropyl-beta-cyclodextrin extraction (HEH). The correlation between these two and all other tests proved that HEH extraction of the soil followed by analyses may yield a chemical method which is able to properly simulate real biodegradation. In the case of both diesel- and transformer-oil, very good correlation was obtained between the results of cyclodextrin extractions and the results of soil respiration in aerated reactors, which is the system that best models the real environment. Our results suggest that cyclodextrin extraction is suitable for the prediction of hydrocarbon degradation in soil.

Key words: bioavailability, biodegradation,  biological soil testing methods, cyclodextrin, microcosm, soil bioremediation, testing biodegradation 

DOI 10.2462/09670513.956