以硫酸還原菌處理土壤中多環芳香族碳氫化合物之研究

Abstract

多環芳香族碳氫化合物 (PAHs) 具毒性、致癌性及致畸胎性，是目前世界已知的致癌化學物最龐大的一群，且被美國環保署列為優先評估之項目。PAHs 大部分為人為產生，主要是因碳氫化合物不完全燃燒所致。其為穩定的疏水性化學物質，非常容易吸附在有機物含量高的土壤顆粒中，且難以被去除。針對土壤中PAHs的去除方法，好氧處理已廣泛被研究，但在實際污染的區域只有表層土壤是好氧，其餘有99%皆為厭氧狀態。

現今利用厭氧生物處理土壤中多環芳香烴 (PAHs) 的研究逐漸增加，根據研究顯示在硫酸還原的條件下 PAHs 能有效地被降解，但仍缺乏以硫酸還原菌 (Sulfate-reducing bacteria, SRB) 為優勢菌種處理 PAHs 的研究，且鮮少進行影響因子與最適操作條件的探討。故本研究嘗試以硫酸還原菌為優勢菌種處理受 PAHs 污染之土壤，以期能提升PAHs的降解效率。

研究中首先討論 PAHs 微波萃取的最適化條件。討論的因子有四種，分別為萃取溫度、萃取時間、萃取溶劑的種類及其添加量。研究結果顯示萃取土壤中 PAHs 的最佳萃取溶劑為丙酮/二氯甲烷 (1:1, v/v)。且在96oC 萃取17分鐘的條件下，添加36 mL 的萃取溶劑可使 PAHs 有效地從土壤中萃取出來，其回收率可達 95% 以上。研究中並利用中央合成實驗設計 (Central composite design, CCD) 搭配反應曲面方法 (Response surface methodology, RSM)，進行實驗之規劃與結果之分析，探討 pH 值、微生物濃度及共基質濃度 (乙醇) 對 PAHs 降解之影響，找出其最適化操作條件。實驗結果顯示在微酸性的情況下，其PAHs降解的效果較佳，且微生物含量越高其去除效率也越好，添加多量的額外基質並無顯著地增加PAHs的降解。在乙醇濃度低於35 mg/L、pH 介於4~6.5且微生物濃度達到 66.5 mg/L 以上時，PAHs 降解率可達到 90% 以上。此外，研究中也探討不同微生物濃度與不同 PAHs 濃度對於 PAHs 的降解速率常數 (K值) 之影響並與其他研究進行比較。研究結果發現，大部份 PAHs 之零階與一階動力常數值較其它相關研究 (非硫酸還原菌為優勢菌種) 高，顯示利用 SRB 為優勢菌種處理土壤中的PAHs 之降解成效佳，符合預期的假設。

Polycyclic aromatic hydrocarbons (PAHs) are known to be the large group of carcinogens in the world, and potentially have toxic and mutagenic danger. The principle anthropogenic source of PAHs is by incomplete combustion of hydrocarbon. The USEPA has identified 16 PAHs as priority pollutants. The hydrophobic compounds like PAHs, have the strong tendency to sorb onto the soil. As for the biodegradation of PAHs, aerobic treatment has been well studied. However, the oxic zone of polluted area only exist in the surficial (1-10 mm) and the 90-99% of pollutants are under anoxic conditions. Biodegradation of PAHs in anaerobic conditions has been studied extensively. PAHs are reported to be effectively degraded under sulfate-reducing condition. But, only very limited studies have investigated the biodegradation of PAHs by SRB as a priority microorganisms. In addition, the influence of factors and the optimization of biodegradation conditions were seldom discussed. The objective of this study was to improve PAHs degradation efficiency in soil by using sulfate-reducing bacteria as the priority microorganism. As a precursor in the investigation, the optimization of microwave-assisted extraction of PAHs was studied. Temperature, time, solvent type and solvent volume are the main factors considered in this study. The result shows that the mixture of Acetone/Dichloromethane (1:1, v/v) is the best extraction solvent. Recovery of PAHs was over 95% when extracted under 96oC in 17 minutes by adding 36 mL of solvent. In this study, central composite design (CCD) and response surface methodology (RSM) were used for the experimental design and data analysis. The impact of pH, biomass concentration and co-substrate (ethanol) concentration on PAHs degradation were investigated and the conditions were optimized. The result shows that the PAHs degradation is better under acidic environment with high biomass concentration. The addition of co-substrate has not increased the PAHs biodegradation efficiency. The degradation rate was over 90% when the ethanol concentration was less than 35 mg/L, pH was between 4 to 6.5 and the biomass concentration was more than 66.5 mg/L. From the observations, the influence of biomass and PAHs concentration on PAHs degradation rate was calculated (in terms of rate coefficient K) and compared with other researchers. The calculated zero-order and first-order kinetic coefficient are higher than reported in the literature. This shows that the PAHs degradation rate could be elevated by sulfate-reducing bacteria as priority microorganism.