| Microbial remediation technique has become one of the major tools for the remediation of persistent organic pollutants (POPs) contaminated soils due to its advantages of lower cost and less secondary pollution. Due to the strong hydrophobic interactions between soils and POPs such as polycyclic aromatic hydrocarbons (PAHs) and organochlorine pesticide (OCPs), a large portion of POPs is difficult to release from soil surface into aqueous phase and become available to microbial. Furthermore, as interactions between POPs and soils depends on type of soils and POPs, bioavailability of POPs in different soils is also varied, which results in difficulties in evaluating the ecological risk of POPs and feasibility of the bioremediation. Therefore, effectively evaluating bioavailability of POPs in different soils and taking measures to stimulate POPs biodegradation are hotpots in the research of risk assessment and soil bioremediation.According to the literature, the amount of hydroxypropyl-p-cyclodextrin (HPCD) extractable fraction for PAHs less than4-rings could reflect the strength of interaction between PAHs and soils, and further predict bioavailability of PAHs in soils. However, a bias was observed when using HPCD extraction method to predict PAHs bioavailability in soils of different types. As organic matter is the main component of soil that controls bioavailability of POPs, its composition and difference in sorption capacity for different contaminants might have an impact on the ability of HPCD extraction method in predicting POPs bioavailability, which requires further research. Meanwhile, as cyclodextrin could enhance the dissolution of poorly-soluble POPs, it shows potential to promote biodegradation of POPs. Less study was reported, which examined effect of cyclodextrin on biodegradation of different POPs. In this study, on the one hand, organic matter fractions from different soils/sediments were isolated to investigate how their different behaviors affect bioavailability of POPs and accuracy of HPCD extraction method in predicting POPs bioavailability. POPs of different type or mixture was tested to examine the feasibility of HPCD extraction method in predict their bioavailability in soils. On the other hand, interaction between cyclodextrin and POPs was examined to investigate the mechanism of cyclodextrin in promoting biodegradation. The study would provide theoretical support for predicting the risk of POPs in soils, evaluating the feasibility for bioremediation of different POPs contaminated soils and promoting the efficiency of soil bioremediation. The main research content and results were shown as follows:(1) Different organic matter fractions including humin and humic acid, isolated from seven soils/sediments and artificially contaminated with phenanthrene, were used to determine the main fraction of organic matter that limits the bioavailability of phenanthrene and the key factor that affects the accuracy of HPCD extraction method in predicting bioavailability of phenanthrene. The results showed, compared with humic acid and the bulk soil/sediment, humin exhibited a stronger interaction with phenanthrene, and phenanthrene sorbed by humin was more difficult to be extracted by HPCD and biodegraded. For humin and the bulk soil/sediment, the amount of HPCD-extractable phenanthrene and biodegradable phenanthrene was linearly correlated, which exhibits an approximate1:1relationship. This result demonstrates HPCD extraction method could predict biodegradation of phenanthrene in both humin and the source soils/sediments. But for the humic acid fraction, the biodegradation rates of phenanthrene in all seven humic acid samples were above90%. The extent of phenanthrene biodegradation was significantly higher than the extent of HPCD extraction. That suggests a portion of sorbed phenanthrene in humic acid could be directly degraded by bacterial and the existence of excess humic acid might affect the accuracy of HPCD extraction in predicting bioavailability of phenanthrene.(2) In order to determine the reliability of HPCD extraction method in predicting bioavailability of different POPs, relationships between HPCD extractability and biodegradability of PAHs or DDTs in soils were examined. In soils polluted with a mixture of five PAHs, the amount of HPCD-extractable and biodegradable fraction exhibits an approximate1:1relationship for FLU, PHE, FLA and PYR. But for ANTH, the amount of HPCD-extractable fraction was significantly higher than the amount of biodegradable fraction. In DDTs polluted soils, the amount of HPCD-extractable and biodegradable fraction exhibits an approximate1:1relationship for p,p’-DDT and o,p’-DDT. However, for p,p’-DDD and p,p’-DDE, the amount of HPCD-extractable fraction was significantly higher than amount of biodegradable fraction. This result demonstrated the accuracy of HPCD extraction method in estimating bioavailability of different POPs was also affected by the differences in properties of POPs.(3) Molecular docking and experiments were combined to examine extractability of P-cyclodextrin ((3-CD) and HPCD for phenanthrene from two model non-aqueous phase liquids, hexadecane and diethylhexyl phthalate. Effect of cyclodextrin on biodegradation of phenanthrene in the non-aqueous phase liquids was further investigated. The results demonstrated cyclodextrin could promote partitioning of phenanthrene into the aqueous phase, thus improve the biodegradation rate of phenanthrene. The effect of HPCD on enhancing biodegradation was better than β-CD, which was attributed to the higher association constant of HPCD for phenanthrene. The biodegradation rate was positively correlated with amount of cyclodextrin-extractable phenanthrene, which demonstrated that the enhancement of cyclodextrin on phenanthrene biodegradation was mainly attributed to the increased partitioning of phenanthrene by cyclodextrin.(4) Effect of HPCD on biodegradation of PAHs and DDTs was examined both in aqueous solvent and in soils. For a mixture of five PAHs in aqueous solvent, in the presence of HPCD, the time needed for complete biodegradation of FLU, PHE, FLA and PYR was reduced from7days to5days. And the extent of biodegradation of ANTH could also be increased from63%to75%after7-day biodegradation test. For soils aged with PAHs for20days, in the presence of HPCD, bioavailability was significantly increased only for ANTH (p <0.05). For soils aged for100days, bioavailability of ANTH, FLA and PYR was significantly increased by HPCD (p<0.05). For FLU and PHE, the bioavailability was not significantly affected with the presence of HPCD in soils aged for20days and100days. It suggested effect of HPCD on bioavailability of PAHs was influenced by the hydrophobicity of PAHs and aging time of soils. For DDTs, the presence of HPCD decreased their biodegradation rate to the extent of30%-50%. Biodegradation rate of DDTs in soils was also significantly decreased with the presence of HPCD (p<0.05). The results indicated biodegradation of DDTs was inhibited by HPCD, and the inhibition could be explained by the high association constant between HPCD and DDTs, causing DDTs combined with HPCD difficult to be degraded by bacterial. |
PDF Full Text Request