Degradation of naphthalene in aqueous solution using a microbial symbiotic system founded by degrading and ureolytic bacteria

Abstract

Although single bacteria have been applied to the Polycyclic Aromatic Hydrocarbons (PAHs) remediation, its efficacy is severely restricted by long degradation periods and low efficacy. A microbial symbiotic system founded by two or more bacterial strains may be an alternative to traditional remediation approaches. Its construction is, however, hampered by antagonistic interactions and remains challenging. The present work proposed a microbial symbiotic system consisting of the naphthalene degrading bacteria and the non-PAHs degrading bacteria and improved their interspecies interactions by using sequential inoculation. The non-PAHs degrading bacteria were inoculated after the inoculation of the naphthalene degrading bacteria. The sequential inoculation not only promoted the non-PAHs degrading bacteria to use the metabolites of the naphthalene degrading bacteria as an energy source but developed a resistance of the two bacterial strains to naphthalene. Vaterite and aragonite were identified following urea hydrolysis by the non-PAHs degrading bacteria. The faster precipitation rate in naphthalene degradation by the symbiotic system elevated the proportion of vaterite, allowing more naphthalene and its metabolites to be wrapped in or attached to minerals with the bacteria through the physisorption (van der Waals force) and chemisorption (Ca-π interaction with aromatic rings) and promoting the formation of aggregates. The formation of aggregates further reduced the mobility of naphthalene. Results indicate that 40% of naphthalene in the non-inoculated sterile control group was quickly released into the atmosphere, causing serious public concerns regarding health safety. According to the thermogravimetry-gas chromatography mass spectrometry (TG-GC/MS) analysis, no trace of naphthalene was found in the samples, indicating that the degrading bacteria fully degraded naphthalene after its adsorption. As a result, the degradation efficiency of 100% was attained using the symbiotic system even at 200 mg/L naphthalene. The findings underscore the relative merits of the symbiotic system applied to the remediation of naphthalene in an aqueous solution.