EVALUATION OF COMETABOLIC DEGRADATION OF TRICHLOROETHYLENE BY BURKHOLDERIA CEPACIA G4 ENCAPSULATED IN POLYETHYLENE GLYCOL POLYMER

Abstract

Trichloroethylene (TCE) can be mineralized through cometabolism by microorganisms expressing mono or dioxygenase if provided with a primary growth substrate. However, it is hard to maintain high cell density and survival of such precious biocatalyst at a contaminant plume in the groundwater environment. One solution to this problem is to provide a proper shelter to microorganisms by encapsulation, which offers protection from predation, toxicity, and advection. This study investigated the effectiveness of an encapsulation material, polyethylene glycol (PEG) polymer for cometabolic removal of TCE by Burkholderia cepacia G4, using toluene as the primary growth substrate. B. cepacia G4 was encapsulated in cylindrical PEG pellets with dimensions of Ø4mm×4mm. Two ranges of TCE i.e. low range (0.1, 0.5 and 1 mg/L) and high range (5, 10 and 20 mg/L) were investigated with two concentrations of toluene i.e. 10 and 50 mg/L. Temperature and pH were maintained at 28°C and 7.2, respectively. All experiments were carried out in batch mode. Results confirmed that PEG encapsulated B. cepacia G4 can degrade TCE and toluene simultaneously with negligible lag time. High removal efficiencies for TCE were observed at 50 mg/L of toluene while it was relatively low at 10mg/L. A 100% removal was observed in the low range with toluene 50 mg/L while, the efficiencies varied from 50 to 90% in same range with toluene 10mg/L. For low range in suspended cultures with toluene 50mg/L, a 100% removal efficiency was observed at 0.1 mg/L of TCE while, it was about 80% for 0.5 and 1 mg/L of TCE, respectively. For high range of TCE, removal efficiencies were 44 to 72% with 50 mg/L of toluene while the removal for same range with 10 mg/L of toluene was 23 to 44%. Suspended cultures were strongly suppressed at high range of TCE due to toxicity. Suspended cells removed 23-55% of TCE with 50mg/L of toluene. There was almost no removal of TCE at the high range with 10 mg/L toluene due to high maintenance energy demands. The values of removal efficiency for this case ranged from 1 to 22%. Relatively better removal for high range with 50 mg/L of toluene shows that at high substrate availability microorganisms could maintain the cometabolic activity. The suspended cultures were strongly suppressed at a higher concentration of both toluene and TCE degradation rates decreased as the TCE concentration was increased. On the other hand in encapsulated cultures, there was no significant drop in toluene degradation rates even at high range of TCE, showing that encapsulation served the purpose well. Transformation yield ranged as 0.0099-0.51 mg-TCE/mg-toluene for 10mg/L of toluene while it ranged as 0.0031-0.15 mg-TCE/mg-toluene for 50mg/L of toluene available. In suspended cultures, this range was 0.012-0.098 mg-TCE/mg-toluene at 10mg/L of toluene to 0.0024-0.086 mg-TCE/mg-toluene at 50mg/L of toluene available, respectively