| Summary: | 博士 === 國立交通大學 === 環境工程系所 === 97 === Polycyclic aromatic hydrocarbons (PAHs) are widely distributed over the environment. Due to their toxicity, recalcitrance, carcinogenicity and mutagenicity, the removal/degradation of PAHs is one of the most important research issues. Among the PAHs, three-ring PAHs i.e., fluorene (FLU) and phenanthrene (PHE), are the most frequent and abundant pollutants found in the contaminated sites. FLU and PHE degradations are reported under sulfate-reducing conditions or by sulfate-reducing bacteria (SRB). However, the metabolites produced, metabolic pathway(s) and the optimal biodegradation conditions under sulfate-reducing environment are not well understood. In order to obtain and understand the FLU and PHE metabolisms such as degradation kinetics, metabolic mechanisms, optimal operating conditions and the interactions between metabolites and parent compound, a series of aqueous batch experiments were conducted using a sulfate reducing bacterial enrichment culture (87 ± 6%).
In the biodegradability study, batch experiments were conducted with FLU (5 mg/L), PHE (5 mg/L) and a mixture of the two (5 mg/L each). After 21 d of incubation, 88% of FLU and 65% of PHE were degraded by SRB. However, a decrease in biodegradation efficiency was observed in the presence of both FLU and PHE. Throughout the study, sulfate reduction was coupled with biomass growth and PAH biodegradation indicating that SRB were the major group of microorganisms responsible for the degradation of FLU and PHE. Using the GC-MSD analysis, phenol was identified as the metabolite of FLU and 2-methyl-5-hydroxybenzaldehyde, 1-propenyl-benzene, p-cresol and phenol were identified as the metabolites of PHE. These metabolites infer that hydration, hydrolysis, dehydrogenation and decarboxylation are the mechanisms responsible for the biodegradation of FLU and PHE. Based on the observations, novel metabolic pathways of FLU and PHE by the enriched SRB were proposed.
In the optimization study, batch biodegradation experiments were designed using the rotatable central composite design with five levels. The designed concentrations were 2-50 mg/L for PHE, 480-3360 mg/L for sulfate, and 5-50 mg/L for initial biomass. Experimental results indicated that the biomass concentration was the most significant factor, followed by the sulfate and PHE concentrations. In the present study, the desirability functions methodology (DFM) was applied to find out the maximum specific PHE removal rate (Rs). A maximum Rs of 9.0 mg/g VSS-d was estimated when the initial PHE, sulfate and biomass concentrations were 18.5, 841 and 50 mg/L, respectively. The Rs observed in the optimization experiments was higher than the values reported in the previous studies demonstrating the superiority of the enriched SRB culture for the biodegradation of PHE. Subsequently, a set of confirmation experiments were performed under the optimal PHE, sulfate and biomass concentrations i.e., 18.5, 841 and 50 mg/L, respectively; the results matched well with the Rs estimated using DFM. In addition, the results of adsorption experiments exhibited that the adsorption of PHE on biomass was proportional to the initial concentration of PHE. When PHE was added in the biotic system, large quantity of PHE was adsorbed in the biomass instantly and the sorbed PHE was further degraded by the SRB.
Finally, in the metabolite biodegradation study, the ability of sulfate-reducing enrichment culture for the utilization of phenol and p-cresol (5 and 10 mg/L) as carbon and energy sources was examined. In another experiment, the phenol and p-cresol were incubated simultaneously in the enrichment culture with PHE (5 mg/L each) to study the substrate interactions during the biodegradation process and to assess the potential effect of presence and accumulation of phenol and p-cresol on PHE degradation. Experimental results demonstrated that p-cresol was rapidly degraded without a lag phase. About 88% and 65% p-cresol degradations were reached within 21 d for the experiment at 5 mg/L and 10 mg/L of p-cresol, respectively. However, no significant degradation of phenol was recorded. The outcomes of PHE biodegradation experiments in the presence of phenol and p-cresol show that the degradation of phenol or p-cresol is higher than their degradations in the single compound system i.e., system added with only phenol or p-cresol. Nevertheless, the presence of both phenol and p-cresol in the system slightly inhibited the degradation of PHE.
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