Biological degradation of selected hydrocarbons in an old PAH/creosote contaminated soil from a gas work site

An old PAH/creosote contaminated soil (total similar to 300 mu g PAH/g soil ) from a former gas work site in Stockholm, Sweden, has been treated at 20 degrees C with the addition of various nutrients and inoculated with bacter ia (isolated from the soil) to enhance the degradation of selected hydrocar bons. Microcosm studies showed that the soil consisted of two contaminant f ractions: one available, easily degraded fraction and a strongly sorbed, re calcitrant one. The bioavailable fraction, monitored by headspace solid pha se microextraction, contained aromatics with up to three rings, and these w ere degraded within 20 days down to non-detectable levels (ng PAH/g soil) b y both the indigenous bacteria and the externally inoculated samples. The n utrient additives were: a minimal medium (Bushnell-Haas), nitrate, nitrite, potting soil (Anglamark, Sweden), sterile water and aeration with Bushnell -Haas medium. After 30 days treatment most of the sorbed fractions were sti ll present in the soil. Stirring or mechanical mixing of the soil slurries had the greatest effect on degradation, indicating that the substances were too strongly sorbed for the microorganisms. When stirring the choice of nu trient seemed less important. For the non-stirred samples the addition of n itrate with the bacterial inoculum showed the best degradation, compared to the other non-stirred samples. At the end of the experiments, accumulation s of metabolites/degradation products, such as 9H-fluorenone, 4-hydroxy-9H- fluorenone, 9,10-phenanthrenedione and 4H-cyclopenta[def]phenanthrenedione were detected. The metabolite 4-hydroxy-9H-fluorenone increased by several orders of magnitude during the biological treatments. Microbial activity in the soil was measured by oxygen consumption and carbon dioxide production.