Using biodegradation kinetics to measure availability of aged phenanthreneto bacteria inoculated into soil

The rate of biodegradation of pollutants in soil can be limited by the poll utant's availability to microorganisms. We have developed a bioassay for th e availability of phenanthrene to bacteria: that degrade phenanthrene in so il. The assay uses a soil in which phenanthrene is degraded very slowly. Th e rate of phenanthrene mineralization in this soil may be increased substan tially through bioaugmentation with a bacterial inoculum. By delaying inocu lation, it is possible to manipulate the time phenanthrene is present in so il before accelerated biodegradation begins. A phenanthrene concentration m uch lower than the affinity constant of the inoculum is added; thus, biodeg radation kinetics approach first order. Because the phenanthrene first-orde r rate constant for the inoculum is the same regardless of the phenanthrene residence time in soil, the change in phenanthrene availability to the ino culum can be measured over time, The availability of phenanthrene to bacter ia declined in a biphasic double exponential pattern with time. The initial rapid, decline in availability resembled the change in amount of phenanthr ene extracted from soil with hexane-water. However, after phenanthrene had been present in the soil longer than 300 h, the fraction extracted with hex ane-water declined faster than the substrate available to the bacterial ino culum, suggesting that the bacteria are able to access a pool of phenanthre ne unavailable to hexane.