| Summary: | 碩士 === 國立成功大學 === 環境工程學系碩博士班 === 93 === Surfactin and rhamnolipid are two types of biosurfactants with different chemical structures. In this study, we found that both of them owned relatively low critical micelle concentrations (CMC), which were 35 and 52 mg/L for surfactin and rhamnolipid, respectively. Also, both of the two biosurfactants can achieve significantly high emulsification index (E24) which resulted in the emulsification of diesel oils. The addition of 120 mg/L surfactin achieved E24 of 58%, which resulted in the emulsification of 58% diesel oil. However, only 48 mg/L rhamnolipid was needed to achieve the E24 (62%), and 62% of diesel oil was emulsified. Based on the two important characters, relatively CMC and high E24, this study focused on 1) the addition concentrations of the two biosurfactants, 2) ammonia concentrations, and 3) pH values in the environment, in order to investigate the effect of biosurfactants addition on diesel oil biodegradation.
In the study of adding different concentrations of biosurfactant, when adding different concentrations of surfactin to the diesel contaminated soil solution, the optimal microbial growth rate was attained with 40 mg/L of surfactin. The specific growth rate was increased from 0.041/hr to 0.055/hr, compared with the non-addition group. When adding 40 mg/L of surfactin, the degradation rate was raised from 20.1 mg/L/hr, with the non-addition group, to 86.7 mg/L/hr, which were about four-time improvement in the degradation rates. In spite of this, addition of high concentration of surfactin was found to cause an inhibition of the microbial activity. The specific growth rate dropped to 0.009/hr when 400 mg/L of surfactin was added. When adding different concentrations of rhamnolipid, the microbial growth curve indicated that an optimal bioactivity can be achieved with 80 or 160 mg/L of rhamnolipid addition. Compared with the non-addition group, the specific growth rate was increased from 0.021/hr to 0.047/hr, and the diesel degradation rate was raised from 20.5 to 137.3 mg TPH-d/L/hr. An approximately seven-fold enhancement of the diesel degradation was achieved.
In the study of pH investigation, with no addition of the biosurfactant, the optimal diesel degradation was 63.7 mg TPH-d/L/hr when pH value was 7.2. Extreme high (8.4) or low (5.2) pH values resulted in relatively low diesel degradation rate, 40.1 and 19.7 mg TPH-d/L/hr, respectively. With the addition of surfactin, at the pH value of 8.4, both of the specific growth rate and the diesel degradation rate reached the optimal. At the pH value of 5.2, the specific growth rate dropped from 0.051/hr (pH 8.4) to 0.013/hr; and the diesel degradation also decreased from 91.7 (pH 8.4) to 7.2 TPH-d mg/L/hr. Similarly, with the addition of rhamnolipid, the highest diesel degradation rate was attained when the pH values were around 6.3-8.4. At pH 5.2, the diesel degradation rate dropped from 85.7 (pH 7.4) to 36.2 TPH-d mg/L/hr。
In the study of ammonia investigation, with no addition of the biosurfactant, the optimal growth rate and diesel degradation rate were found with the ammonia concentrations of 200-300 mg/L. When adding 40 mg/L of surfactin, the specific microbial growth rate could achieve 0.057/hr with the ammonia concentration f 200 mg/L. Yet, the specific growth rate decreased to 0.035/hr when ammonia concentration was only 50 mg/L. When adding 50 mg/L of rhamnolipid, about equivalent diesel degradation (52.1-65.4 TPH-d mg/L/hr) was reached with the ammonia concentration of 100-300 mg/L. However, when 450 mg/L of ammonia concentration was provided, diesel degradation became significantly low (9.2 mg TPH-d/L/hr).
In the study of adding biosurfactants to diesel-contaminated soil, it was observed that, with the addition of either surfactin or rhamnolipid, the remained diesel was apparently lower than that in the control set after 68 days operation. With the addition of surfactin (40 mg/kg), the remained diesel in the soil was 1655 TPH-d mg/kg. With the addition of rhamnolipid (50 mg/L), the remained diesel was only lower than 250 mg TPH-d/kg. At the same time, the remained diesel in the control set was around 4678 TPH-d mg/kg which was originally 6981 TPH-d mg/kg.
In the study of molecular biomonitoring with DGGE, it concluded that with the addition of rhamnolipid, the microbial community was more diverse than that in the control set and in the surfactin addition. Also, in the result of Marker comparison, it indicated the existence of diesel-degrading bacteria in both the experimental and the control sets.
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