The Dissipation Process Of Pentachlorophenol In Millimeter Rhizosphere

Pentachlorophenol (PCP, C6Cl5OH) is a kind of ionizable hydrophobic organic contaminants (HIOCs). The highest concentrations of PCP are usually found in soil and aquatic sediments. The investigation on the environmental behaviors of PCP in soil and the key controlling factors involved will be effective and helpful to achieve the phytoremediation of PCP-contaminated soils.This thesis aimed to evaluate the biodegradation of PCP in millimeter rhizosphere and the corresponding microbiological and biochemical response, and develop the alternative phytoremediation technique for PCP contamination. The results for this research can be summarized as follows:1. A glasshouse experiment was conducted to investigate the rhizosphere effects on the biodegradation of PCP and on the dynamic response of soil microbial community structure, using a specially designed rhizosbox where ryegrass seedlings were grown in soil spiked with PCP at concentrations of 20mg PCP-kg~-1 and 50mg PCP-kg~-1 soil. Changes of PCP concentrations with increasing distance from the root compartment of the rhizosbox were then determined. The most dissipation of PCP in planted soil was at the 2mm and 3mm from the root zone where total PCP decreased to 0.69 mg PCP·kg~-1 and 1.69 mg PCP ·kg~-1, respectively with the spiked concentration of 20 mg PCP·kg~-1 and 50 mg PCP·kg~-1. Meanwhile, good negative correlations with the total PLFAs were found in both low and high PCP spiked treatments (p<0.05). The degradation gradient followed the order: near-rhizosphere > root compartment > far-rhizosphere soil zones for both concentrations where ryegrass {Lolium perenne L.) were grown. In contrast, there were no differences in both PCP-contaminated soils with distance in the unplanted soils.2. The phosphatase activities and urease activities increases in both soils companied by the enhanced dissipation of PCP, which were higher in the near-rhizosphere than that in the far-rhizosphere soils. The results suggest that the effect of root exudate is essential in the dissipation of xenobiotics such as PCP in soil.3. Phospholipid fatty acid (PLFA) profiles, consisted of quantitativedetermination of each PLFA component and the shift in PLFA patterns, can provide comprehensive information for detecting the microbial community. Several fatty acids are known to be "signatures" of subsets of microbial community. Therefore, the specific PLFA profiles can be used to characterize different groups of bacteria. Furthermore, a phospholipid pattern change in the related samples could be indications of a shift in microbial community. In this research, an investigation was conducted using the PLFA profiles to follow shifts in microbial community structure and to illustrate the mechanism of the PCP dissipation in the rhizosphere of ryegrass. We found that the gram-negtive bacteria were relatively gathering in the 2mm and 3mm near rhizosphere, respectively in both 20 mg PCPkg"1 and 50 mg PCP-kg"1 spiked concentration, where presented the highest dissipation and lowest residues. Thus some conclusions were made that the gram-negtive bacteria may be the mainly microbial factors involved in the accelerated process of PCP dissipation in the rhizosphere.