Bioavailability of phenanthrene intercalated into an alkylammonium–montmorillonite clay

We have investigated the availability of phenanthrene, intercalated into a montmorillonite clay saturated with tetradecyltrimethylammonium (TDTMA) cations, as a carbon and energy source to two bacterial species. When phenanthrene was provided in the ‘free’ form (i.e. in the absence of clay), the number of viable bacterial cells (Burkholderia sp.) increased from 2.7×106 at the beginning to 1.5×108 after 5 days of incubation. The doubling time during the exponential growth phase was 26 h. At the end of the incubation period (11 days) 54.3±9.8% of added phenanthrene was degraded by Burkholderia sp., and 90.4±4.3% by Sphingomonas sp. By contrast, bacterial growth and phenanthrene degradation were negligibly small to non-existent when phenanthrene was intercalated into TDTMA–montmorillonite whilst no depletion of phenanthrene was detected in the abiotic culture flasks. Similarly, scanning electron microscopy of the filtered culture containing ‘free’ phenanthrene showed areas of cell proliferation whereas very few cells were visible when phenanthrene was provided in the form of a TDTMA–clay intercalate. The biostability of clay-intercalated phenanthrene may be ascribed to immobilization of the substrate, and its physical inaccessibility to degrading bacteria and their enzymes.