The kinetics of biodegradation of trans-4-methyl-1-cyclohexane carboxylic acid

Abstract

This thesis presents the study of biodegradation factors of a candidate naphthenic acid compound, the trans isomer of 4-methyl-1-cyclohexane carboxylic acid (trans-4MCHCA). Low molecular weight components of naphthenic acids such as trans-4MCHCA are known to be toxic in aquatic environments and there is a need to better understand the factors controlling the kinetics of their biodegradation. In this study, a relatively low molecular weight naphthenic acid compound and a microbial culture developed in our laboratory (primarily Alcaligenes paradoxus and Pseudomonas aeruginosa) were used to study the biodegradation of this candidate naphthenic acid. The purpose of the research was to evaluate the kinetic parameters and model the biodegradation of this compound in three bioreactor systems: batch reactors, a continuously stirred tank reactor and immobilized cell reactors. In batch reactors, the maximum specific growth rate (0.52±0.04 dˉ¹) of the consortium at 23°C and neutral pH was not highly variable over various initial substrate concentrations (50 to 750 mg•lˉ¹). Batch experiments indicated that biodegradation can be achieved at low temperatures; however, the biodegradation rate at 4°C was only 22% of that at room temperature (23°C). Biodegradation at various pH values indicated a maximum specific growth rate of 1.69±0.40 dˉ¹ and yield (0.41±0.06 mg•mgˉ¹) at a pH of 10. Study of the candidate substrate using a continuously stirred tank reactor and the microbial culture developed in the batch experimentations revealed that the kinetics of the candidate naphthenic acid are best described by the Monod expression with a maximum specific growth rate of 1.74±0.004 dˉ¹ and a half saturation constant of 363±17 mg•Lˉ¹. The continuously stirred tank reactor achieved a maximum reaction rate of 230 mg•(l•d)ˉ¹ at a residence time of 1.6 d (39 h). Two high porosity immobilized cell reactors operating continuously over three months were found to consume trans-4MCHCA at a rate almost two orders of magnitude higher than a continuously stirred tank reactor. The immobilized cell systems attained a maximum reaction rate of 22,000 mg•(l•d)ˉ¹ at a residence time of 16 minutes. High porosity immobilized cell reactors were shown to effectively remove a single naphthenic acid substrate in continuously fed operation to dilution rates of 90 dˉ¹. A plug flow model best represented the degradation in the immobilized cell systems and was demonstrated to be a useful tool for studying the effects of parameter variation and prediction of reactor performance. This work highlights the potential of augmented bioremediation systems for the degradation of naphthenic acids.