Removal and biodegradation of polycyclic aromatic hydrocarbons by a microalgal species, selenastrum capricornutum
Student thesis: Doctoral Thesis
Polycyclic aromatic hydrocarbons (PAHs) are a class of bioaccumulative, toxic, carcinogenic and/or mutagenic organic compounds and must be removed from contaminated wastewater before discharge into our environment. Microorganisms are widely investigated to remove PAHs but most related studies were focused on bacteria and fungi. The present study therefore aims to explore the potential and mechanisms of a freshwater green microalga, Selenastrum capricornutum, to remove PAHs at their dissolved concentrations, namely phenanthrene (PHE, 1 mg l-1), fluoranthene (FLA, 0.25 mg l-1) and pyrene (PYR, 0.1 mg l-1), under free and alginate immobilized conditions. Free live S. capricornutum could remove the three PAHs through rapid adsorption, followed by absorption and bioaccumulation. PAHs were then gradually degraded inside the cells. The removal of PAHs by S. capricornutum increased with the initial cell density applied. At the optimal initial cell density, 1 x 107 cells ml-1, 88% of PHE were removed in 96 hours while FLA and PYR were completely removed in 48 and 24 hours, respectively. In 96 hours, 70% of PHE were degraded while FLA and PYR were completely degraded in 72 and 48 hours, respectively. Dead microalgal cells could only adsorb and absorb PAHs (60 to 80%) but not degrade PAHs. Among the three PAHs, PHE was the most difficult PAH to be removed by S. capricornutum, followed by FLA, and PYR was the easiest one. PHE also had the lowest degradation rate while FLA was the highest, suggesting that 4-ring FLA and PYR were degraded more easily than 3-ring PHE by S. capricornutum. A total of twelve PAH metabolites were found during degradation by S. capricornutum. Three of them were identified as 2-, 3- and 9-hydroxyphenanthrenes. Other possible metabolites were proposed as 1-hydroxyphenanthrene, dihydrodiols of PAHs, di-hydroxylated PAHs and tri-hydroxylated PAHs. Mono- and di-hydroxylated PAH metabolites were detected at the same time, indicating that PAHs might be metabolized through both mono- and di-oxygenase pathways. The quantities of PAH metabolites decreased with time, reflecting that PAH metabolites were further broken down and detoxified. After immobilization in alginate beads, S. capricornutum could still remove and degrade PAHs similar to that of free cells, indicating the feasibility of using immobilized microalgae to bioremedy PAH contamination. At the optimal cell density of immobilized S. capricornutum (2.5 x 106 cells bead-1), 92% of PHE and all FLA were removed in 168 hours while PYR was completely removed in 24 hours. Not only cells, some PAHs were adsorbed onto alginate matrix. Retaining of PAHs in alginate reduced the bioavailability of PAHs to the microalgal cells leading to lower PAH degradation rate than that of free cells. In 168-hour treatment, 73% degradation of PHE was achieved by immobilized microalgal beads while FLA and PYR were completely degraded. By increasing the number of beads applied to the PAH-contaminated medium, the removal and degradation of PAHs by immobilized S. capricornutum could be enhanced. The present study demonstrated that S. capricornutum, under both free and alginate immobilized conditions, could remove and degrade a mixture of PAHs simultaneously and effectively. The mechanisms were rapid initial adsorption, absorption and bioaccumulation then gradually degraded inside the cells. Although alginate matrix helped to accumulate PAHs and immobilized algae had comparable PAHs removal as the free cells, the degradation rates were lower in immobilized microalgal beads as adsorption onto alginate matrix would reduce the bioavailability of PAHs.
- Selenastrum capricornutum, Organic compounds removal, Polycyclic aromatic hydrocarbons, Environmental aspects, Purification, Sewage