MASS-TRANSFER LIMITATION OF BIOTRANSFORMATION - QUANTIFYING BIOAVAILABILITY

Biotransformation is controlled by the biochemical activity of microor ganisms and the mass transfer of a chemical to the microorganisms. A g eneric mathematical concept for bioavailability is presented taking ba th factors into account. The combined effect of mass transfer of a sub stance to the cell and the intrinsic activity of the cell using the su bstance as primary substrate, is quantified in a bioavailability numbe r (Bn). The concept can easily be extended to secondary substrates. Th e approach has been applied to explain the observed kinetics of the bi otransformation of organic compounds in soil slurries and in percolati on columns. The model allowed us to predict threshold concentrations b elow which no biotransformation is possible. Depending on the environm ental system and the chemical involved, predicted threshold concentrat ions span a range of 11 orders of magnitude from nanograms to grams pe r liter and match with published experimental data. Mass transfer-and not the intrinsic microbial activity-is in most cases the critical fac tor in bioremediation.