DIFFUSION AS A RATE CONTROLLING STEP IN HEAVY-OIL BIODESULFURIZATION PROCESSES

Microbial degradation of hydrocarbons depends on various chemical and physical factors (viz, temperature, electron accepters, nutrients, pH, substrate characteristics) and on the presence of degrading microorga nisms. Even if all these factors are optimized, oil biodegradation can still be retarded by diminished availability of the compound. Setti e t al. (1992) have shown that n-alkanes affect the biodegradation of ar omatic sulfur compounds by an aerobic Pseudomonas sp. This communicati on illustrates further investigations into the diffusion effect that c ontrols the biodegradation of aromatic sulfur compounds. Polycyclic ar omatic hydrocarbons (PAHs) can be converted in the dissolved state onl y. This implies that mass-transfer from the solid phase to the aqueous phase might be rate-limiting. Dibenzothiophene (DBT) biodegradation i s a good example of this model. The presence of a fatty acid (hexadeca noic acid, aC16) during the fermentation affects DBT degradation. We p ropose a model in which aC16 surrounds the DBT molecule forming a mice lle which favors a co-metabolic process between the fatty acid and the aromatic sulfur compound. Similar behavior is seen when DBT is dissol ved in n-dodecane(C12) and n-hexadecane (C16). There are two important parameters that affect DBT degradation in this system: the surface ar ea of the substrate and the DBT concentration in C12. DBT degradation is limited by the diffusion of the sulfur compound from the organic ph ase to the adsorbed microorganism on the hydrocarbon. Diffusion increa ses with the DBT concentration, and the DBT degradation rate is signif icantly higher than that reported for DBT alone. In this case, a carri er effect of the n-alkane in the DBT degradation is shown. A barrier e ffect is suggested when the DBT concentration is below the limit for w hich the degradation rate is lowest. Our investigations show that the presence of a co-substrate, such as fatty acids or n-alkanes, affects the bioavailability of the aromatic sulfur compound in aerobic conditi ons.