Abstract

Mass transfer and biodegradation of naphthalene and methyl and dimethyl substituted naphthalenes were studied in a novel bead mill bioreactor and in a conventional roller bioreactor. For all tested PAH compounds, mass transfer rates in the bead mill bioreactor were significantly faster than those in the roller bioreactor. The volumetric mass transfer coefficient for naphthalene, 2-methyl naphthalene and 1,5- dimethyl naphthalene, determined in the bead mill bioreactor, were as high as 0.71, 0.12, 0.31 h-1, respectively. These values indicate that the mass transfer rate was dependent on the structure and physical properties of the PAH compound. The candidate bacterium used in this study (Pseudomonas putida ATCC 17484) was able to utilize both naphthalene and 2- methyl naphthalene but was incapable of efficient degradation of 1,5-dimethyl naphthalene. The biodegradation rate of naphthalene and 2-methyl naphthalene in the bead mill bioreactor were 57.6 and 11.4 mg/L-h, respectively. These rates were significantly faster than those achieved in the conventional roller bioreactor (10.5, 2.7 mg/L-h for naphthalene and 2- methyl naphthalene, respectively). The presence of naphthalene in the bioreactor resulted in a 37% increase in biodegradation rate of 2-methyl naphthalene during co metabolism experiments.

Introduction

Contamination of surface and ground waters with organic pollutants is a major environmental concern in North America. Many of these pollutants are composed of a variety of polycyclic aromatic hydrocarbons (PAHs) which are known to be carcinogenic, mutagenic and toxic [1, 2]. In Western Canada, PAHs and recalcitrant naphthenic acids co-occur with a diverse range of hydrocarbons present in oil sand deposits. In situ bioremediation has appeared as a promising approach for cleanup of contaminated sites, but it is a very slow process and cannot be controlled effectively, leading to uncertainty about the outcomes of implementing such a clean-up process [1,3]. Utilization of microbial cells with the ability to decompose PAHs in a controlled environment (ex-situ bioremediation) has improved both biodegradation rate and efficiency of the treatment process. One of the main barriers in successful ex-situ bioremediation of PAHs and recalcitrant naphthenic acids is the hydrophobicity and partial solubility of these compounds in aqueous phases, resulting in appreciable mass transfer limitations within the bioreactor. Stirred tank bioreactors offer an advantage with respect to mass transfer from the solid phase to the liquid phase but inherent to the design of stirred tank bioreactors, particles will always cling to the agitation system, baffles and walls. This together with lifting and separation of particles from the liquid phase by sparged air, required for the metabolism of the microbial cells, prevent the efficient biodegradation of PAHs. These problems could be overcome in a conventional roller bioreactor but even in this system mass transfer limitation is still a barrier in efficient biodegradation of PAHs [4–5]. To address this challenge we have developed a novel Bead Mill Bioreactor (BMB), in which inert particles (glass beads) are used for improving mass transfer from the solid and gas phases into the aqueous phase.

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