Isolation, identification and subsequent application of Gram-positive bacterial strains for the bioremediation of...

Benzophenone (BZP) is a persistent environmental pollutant that can induce serious negative effects on the human health and aquatic life. Because of its hydrophobic nature and resistance to natural degradation, it tends to bioaccumulate in the aquatic ecosystem. The present study was aimed at developing an efficient bioremediation system for the degradation of BZP in wastewater. A total of 150 bacterial strains were isolated from wastewater collected from the twin cities of Rawalpindi and Islamabad, Pakistan. Approximately 86% of the isolates were identified in samples of municipal wastewater, and 14% were isolated in the effluent of soap industries. The potential of the isolated strains was assessed for the degradation of BZP. Based on the initial screening, only 3.3% of the isolates were able to produce biomass at different concentrations (10-1000 mg L^-1) after 48 h of incubation. Following the first screening, the two most effective bacterial isolates, Bacillus cereus (DK2) and Bacillus pumilus (S4), identified through their cell growth and morphological, physiological, and phylogenetic characteristics, were selected for further investigation. In the first 72 h, DK2 and S4 degraded more than 60% of BZP. DK2 showed 67.5% degradation, while S4 showed 69.5% degradation. The consortium (DK2 and S4) showed 86% degradation after 72 hours of incubation in liquid MSM under aerobic conditions. Degradation behaviour was accompanied by growth (CFU mL^-1) after 120 hours. The BZP degradation reached 100% ± 2.35% after 96 h under the optimum environmental conditions, which included a pH of 7, an incubation temperature of 30 °C, 1.0 g L^-1 of ammonium nitrate as the nitrogen source, and yeast extract (2.0 g L^-1) as an additional carbon source. Gas chromatography-mass spectrometer (GC-MS) analysis exhibited a multistage degradation pathway that included benzophenone ring cleavage and producing compounds like phenyl cyclohexyl ketone, benzophenone dimethyl ketal, 2-cyclohexen-1-one, hydroxylation, demethylation, benzcarbothioc acid, and heptacosane. These compounds are less harmful substances than parent compound. The degradation data was best-fitted with the pseudo-first-order (PFO1) kinetic model, as evident from the rate constant of 0.8 for S4, and pseudo-second-order model (PSO2) for DK2, as indicated by the rate constant of 0.9. The present work provides insights into the biological transformation of BZP and highlights that Bacillus cereus and Bacillus pumilus, both individually and in a mixed consortium, are promising strains for the bioremediation of wastewater contaminated with BZP.