Some effects of clay particles on the microbial degradation of naphthalene

Mineralization and bioluminescence assays were used to examine the effects of clay particles on the microbial degradation of naphthalene in aqueous suspensions. An abiotic study of the ability of six different clays to absorb naphthalene led to the selection of a low surface-area kaolinite and a high surface-area montmorillonite to be used in biological studies. Both of the selected clays demonstrated negligible sorptive capacities for naphthalene. In 12-hour batch mineralization assays using the Pseudomonas isolate 5R, the rate and extent of naphthalene mineralization ,ms observed to be retarded in suspensions containing 44% kaolinite when compared with suspensions containing 17% kaolinite or 17% montmorillonite. In a series of bioluminescence assays using the Pseudomonas reporter strain HK44, a characteristic light response was identified in which increasing the concentration of clay in the suspension from 0 to 17% had the effect of decreasing the magnitude and the rate of light production. The effect of the clay was determined to be due to factors other than interference with the light detection method. The inhibitory effect of the clay was found to be similar in both mineralization and bioluminescence assays, although after an initial exposure period, light output appeared to be independent of metabolic rate. Also, reproducibility of the results from bioluminescence experiments was highly dependent on the cell growth procedure.

Negative findings were made concerning possible rate­ limitations due to adsorption, boundary layer mass transfer, and intraparticle diffusion. The inhibitory effect was not observed when direct clay-microbe contact was eliminated by using cells immobilized in alginate beads. On the basis of these observations as well as certain assumptions concerning the transport of naphthalene into the cell, a conceptual model has been proposed in which attachment of the cells to clay particles results in blockage of diffusion sites on the cell membrane and a decrease in the rate of naphthalene uptake. The implications of these findings regarding operation of slurry-based bioreactors, and future use of bioluminescent reporter technology in treatability studies are discussed.