Effect of metal ions and metal nanoparticles encapsulated in porous silica on biphenyl biodegradation by Rhodococcus erythropolis T902.1

Biodegradation of biphenyl was carried out by Rhodococcus erythropolis T902.1 in presence of nanometer-sized metallic (Co, Pd, Ag and Cu) nanoparticles (NPS) synthesized by the sol–gel process. In order to prevent their agglomeration, the metallic NPs (1–2 nm diameter) were anchored inside microporous silica crystallites and named Co/SiO2, Pd/SiO2, Ag/SiO2 and Cu/SiO2 samples, respectively. They were added at low concentrations of 10−6, 10−5 and 10−4 M of metal in the culture medium, and their impact was compared with that of the simple metal ions added as cobalt, palladium, silver or copper salts. The cultures containing Pd/SiO2 or Co/SiO2 samples at 10−4 M of metal achieved a 50 % higher biphenyl degradation yield after 18 days of incubation and improved R. erythropolis T902.1 growth compared with those without (positive control) or with silica particles only. The highest biodegradation performance, i.e., 107 ± 3 ppm/day, which was about 85 % higher than in control conditions without NPs, was recorded in 250-mL baffled flasks stirred at 150 rpm with Co/SiO2 sample at 10−4 M Co. Furthermore, the stimulating effect of NPs on biphenyl biodegradation seems to also depend on the thermal treatment conditions applied to NPs since the experimental results indicated that, after calcination, the cobalt oxide NPs at a concentration of 10−4 M were more effective than the reduced cobalt NPs with a degradation yield of 81 ± 1 and 77 ± 2 %, respectively, after 18 days. On the other hand, the results showed that the addition of 10−4 M of Cu2+ or Ag+ ions or the addition of Cu/SiO2 or Ag/SiO2 samples at 10−4 M of metal have an inhibitory effect on biphenyl biodegradation. However, Cu2+ and Ag+ ions were more toxic to the R. erythropolis T902.1 bacteria than the respective Cu or Ag NPS anchored inside silica particles. Moreover, this work showed that in these conditions, the activity of catechol 1,2-dioxygenase (a critical enzyme in aromatic biodegradation pathway) was severely inhibited, whereas the presence of 10−4 M of Co2+ ions or Co/SiO2 sample stimulated the enzyme activity compared to the conditions without NPs.