STUDIES ON BIODEGRADATION OF CHLOROPHENOLS BY RHODOCOCCUS

Abstract

The present study was carried out with the objective of isolating a potential Rhodococcal strain for the efficient degradation of wide range of chlorophenols and phenolics. Various kinetic parameters for the biological degradation of mono and dichlorophenols were evaluated by the isolated Rhodococcus sp. A mixed substrate study for the degradation of chlorophenol was carried out with phenol and cresols in order to simulate actual phenolic wastewater and the degradation pattern of the substrates observed. A coculture study was planned to study the microbial interaction of the isolated Rhodococcus strain with a fast growing bacterial population to predict the survival viability of the strain in the natural system. A Pseudomonas sp. being widely prevalent in natural conditions was selected for the coculture study. Biotreatment studies of synthetic wastewater containing chlorophenols were carried out by immobilizing the isolated Rhodococcus sp on a gravel medium in a trickling filter reactor. A promising strain of Rhodococcus sp. Ml was isolated from garden soil with the capability of degrading a wide range of phenolics (phenol, o-cresol, m-cresol and pcresol) and chlorophenols (2-chlorophenol, 4-chlorophenol, 2,4-dichlorophenol and 2,4,6-trichlorophenol). The degradation efficiency of the strain was improved by subjecting it to induction using different aromatic compounds like toluene, benzoic acid, phenol and catechol. Benzoic acid induced culture of Rhodococcus sp. Ml was observed to degrade chlorophenols in least time period with negligible lag. Optimization study for temperature and pH was carried out using benzoate induced culture of Rhodococcus sp. Ml. Optimum temperature and pH for the growth CiJ and degradation of chlorophenol by Rhodococcus sp. Ml was in the range of 32 to 35°C and 7.0 to 8.0 respectively. Rhodococcus sp. Ml showed the ability to grow up to pH 11.5 and is thus suitable for use in the treatment of alkaline waste. Benzoate induced culture of Rhodococcus sp. Ml acclimatized to respective chlorophenols was used for degradation study of chlorophenols. Complete degradation of 2-chlorophenol, 4-chlorophenol, 2,4-dichlorophenol and 2,4,6-tnchlorophenol was observed up to a concentration of 300, 100, 50 and 10 mgl"1 respectively. Evaluation of kinetic parameters showed inhibitory kinetics as there was decrease in specific growth rate, growth yield and substrate uptake rate for 2-chlorophenol, 4-chlorophenol and 2,4-dichlorophenol with increase in substrate concentrations. Mixed substrate study of 2-chlorophenol by Rhodococcus sp. Ml in mixture with phenol and p-cresol showed simultaneous but preferential pattern of degradation of the substrates. Phenol and p-cresol was observed to be the preferred substrate to be utilized preferentially with respect to chlorophenol. Simultaneous degradation of chlorophenol could possibly be due to cometabolism resulting into transformation. Decrease in degradation rate of the substrates in mixed substrate form with respect to pure substrate degradation indicated competitive inhibition pattern of substrate utilization. Coculture study of Rhodococcus sp. Ml with Pseudomonas fluoresceins PI showed competitive type of interaction between the populations for the degradation of 2-chlorophenol and phenol. Pseudomonas fluorescens PI evolved as the dominant species during growth on 2-chlorophenol. However, the case was reversed during growth on phenol where Rhodococcus sp. Ml was observed to be the dominant

population. Growth on p-cresol showed neutral type of interaction between both the populations. Biological treatment by a fixed film system using a trickling filter reactor was carried out for the treatment of 2-chlorophenol and 4-chlorophenol. Organic and hydraulic loading was optimized for the effective removal of the chlorophenols from synthetic wastewater. The trickling filter reactor was then operated at optimal organic and hydraulic loading for the treatment of 2-chlorophenol and 4-chlorophenol.