Comparison of phenanthrene biodegradation by free and immobilized cell systems: formation of hydroxylated compounds

Abstract

One of the foremost environmental issues having a key role in the feasibility study of polycyclic aromatic hydrocarbons (PAHs) biodegradation is the concern of the toxicity of the formed intermediate metabolites. In this study, biodegradability of phenanthrene (PHE) at initial concentrations of 100-500ppm and its hydroxylated intermediate metabolites (IMs) in aqueous phase were investigated using free cells (FC) and immobilized cells (IC) in polyvinyl alcohol (PVA) cryogel beads. Results showed that both FC and IC systems were capable of complete PHE biodegradation at initial concentrations lower than 250ppm after 7days, though IC system showed a higher PHE removal rate. The maximum IM concentrations observed at initial PHE concentrations of 100 and 250ppm were 20 and 49ppm for FC system, whereas 7.4 and 19ppm were obtained for IC system, respectively, and IMs were finally removed after 7days. Similarly, at 500ppm, IC system resulted in higher removal of PHE compared to FC system. However, during the 7-day period for FC system, IMs concentration rose up to 59ppm, while for IC system, IMs concentration reaches a maximum at day 5 and thereafter it follows a negative rate. It was also shown that resorcinol as an indicator of hydroxylated aromatic metabolites at concentrations of 0-100ppm can well be biodegraded by free and immobilized cell systems. No prohibition on PHE biodegradation could hence occur due to IMs formation. Additionally, stability of IC system was examined in repeated-batch cultures, showing the effective removal of PHE up to nine reuse cycles.