The Interface Behavior Of Pentachlorophenol In Soil-Water System And Its Dissipation Process In Millimeter Rhizosphere

Pentachlorophenol (TCP, C₆Cl₅OH) is an ionizable hydrophobic organic contaminant (HIOC). As a pesticide, herbicide, and antiseptic, it was once used worldwide, and has been designated as a priority pollutant and a probable human carcinogen. In the 1970s PCP was popularly used in China in fighting against snail fever and as a herbicide. Due to slow biodegradation, PCP may present a toxicity risk in contaminated soils for prolonged periods of time. Hence it is not surprising that it still causes environmental problems at many locations. The highest concentrations of PCP are usually found in soil and aquatic sediments. Understanding the behavior of PCP and its environmental risk requires an assessment of the processes influencing its fate, transport, bioactivity and persistence in soils. Investigation of the environmental behavior of PCP in soil (e.g. sorption/desorption, residue and dissipation, and the controlling key factors involved are thus necessary and effective.This dissertation, which is, in part, supported by the National Nature Science Foundation for Distinguished Young Scholars of China (No. 40425007) and the Teaching and Research Award Program for Outstanding Young Teachers in Higher Education Institutions of China, aimed to understand the interface behavior of PCP in the soil-water system, determining the potential contributions of soil physicochemical properties and its organic/inorganic components to PCP retention, evaluating the dissipation of PCP in the rhizosphere and the corresponding microbiological and biochemical responses, and developing alternative rhizo-remediation techniques to decrease PCP contamination. The information derived from this work will contribute to a better understanding of the soil-plant-microbe interactions and their impacts on the environmental behavior of organic contaminants in the soil-water system. The main experiments and conclusions are as follows:(1) Sorption of PCP by pure minerals and humic acids were measured to obtain additional perspective on the potential contributions of both clay minerals and soil organic matter (SOM) to contaminant retention in soils. Four types of common soil minerals and two kinds of humic acids (HAs) were tested. The sorption affinity for PCP was in the order: HAs >> K-montmorillonite>>Ca-montmorillonite > goethite > kaolinite. Such differences in sorption capacity were attributed to the crucial control of HAs. Clay minerals also had a contribution, especially K-montmorillonite, which played an important, if not dominant, role in the controlling process of PCP sorption. Modulating the cation type and composition onclay mineral surfaces through cation exchange processes provides an environmentaly-safe protocol to manipulate the mobility and availability of organic contaminants, which could have applications in environmental remediation. By removing 80 % (on average) of the organic carbon from the soils with H2O2, the sorption decreased by an average of 50 %. The sorption reversibility was also greatly increased. Considering the uncharged mineral fractions in soil before and after HzQrtreated, the main variation in sorption behavior of the soil might thus be related to the removed organic carbon and the reduced pH. This strongly indicated the interactions between SOM and clay minerals on PCP sorption as a function of pH.(2) Information about the sorption mechanism of PCP was gained by constructing highly detailed isotherms over the widest possible concentration range. Using the dual-mode model (DMM), sorption isotherms of PCP were accurately predicted, the sorption capacities were determined, and the mechanisms of adsorption (hole-filling) and partitioning were elucidated. Sorption-desorption hysteresis in the present study was strongly indicated in ten soils with different properties. Such hysteresis might result from the pore deformation and entrapment of PCP molecules within organic matter. Total nitrogen in soils also has its contribution in these processes. Correlations obtained from stepwise regression analyses confirmed that pH, soil organic carbon and organic carbon fractions, as well as particle size distribution are the main factors responsible for the sorption and desorption hysteresis processes. These factors interacted to influence the fate of PCP sorption-desorption in soil. The empirical models developed in this study accurately predict PCP sorption and desorption hysteresis phenomena in the soils that were investigated.(3 ) Dissipation of PCP in soil was investigated and the chemical relationships with soil properties were studied. The results indicate that the dissipation of extractable PCP residues can be described using first-order kinetics equations, with a half-life (T1/2) ranging from 6.5 to 173.3 d. The sharply different patterns of PCP dissipation in different soils were closely related to soil properties. Correlations obtained from stepwise regression equations were significant (PO.01) between soil parameters and extractable PCP residues (R2=0.974**) as well as T1/2 values (R2= 0.882**). The dissipation dynamics of PCP in soil was most accurately predicted by using pH together with organic carbon content (OC) and soil particle size distribution.(4) A glasshouse experiment was conducted using a specially designed rhizobox where ryegrass seedlings were grown for 53 days in a soil spiked with PCP at concentrations of 8.7 ±0.5 and 18+0.5 mg kg"1 soil to investigate rhizosphere effects on the dissipation of PCP. The soil in the rhizobox was divided into six separate compartments at various distances fromthe root surface. Changes in PCP concentrations with increasing distance from the root compartment of the rhizobox were then assessed. The largest and most rapid loss of PCP in planted soil was at 3 mm from the root zone where total PCP decreased to 0.20 and 0.65 mg kg"1 respectively with the two PCP treatments. The dissipation gradient followed the order: near-rhizosphere > root compartment > far-rhizosphere soil zones for both PCP concentrations where ryegrass was grown. In contrast, there was no difference in PCP concentration with distance in the unplanted soil. The increases in both soil microbial biomass carbon and the activities of soil urease and phosphatase were related to the enhanced dissipation of PCP, which was higher in the near-rhizosphere than far-rhizosphere soil. The results suggest that the effect of root proximity is important in the dissipation of xenobiotics such as PCP in soil.(5) Further investigations were conducted using phospholipid fatty acids (PLFAs) profiles to follow the millimeter spatial response of the soil microbial community with the purpose to illustrate the mechanism of dissipation gradients of PCP in the rhizosphere of ryegrass (Lolium perenne L). The response of PLFAs profiles was related to the gradient influence of root exudates and PCP dissipation, showing a clear shift from hydroxyl to saturated taxa, then to fungi, then to gram-negative bacteria and gram-positive bacteria, then to actinomycetes, and then to arbuscular mycorrhizal fungi. This kind of development in microbial community finally resulted in the accumulation of arbuscular mycorrhizal fungi, actinomycetes and bacteria in the 3 mm rhizosphere layer, where the spiked PCP also exhibited both the highest dissipation rate and lowest residue concentration. Based on the stepwise regression analysis, the five fatty acids of 16:lto5,16:0, il7:0, al7:0 and 10Mel8:0 were confirmed to be the most important microbial factors controlling the rhizosphere specificity of PCP dissipation, strongly indicating the microbial synergistic controlling mechanisms of bacteria, arbuscular mycorrhiza and actinomycete that are involved in the accelerated process of PCP dissipation in the rhizosphere.(6) The remediation effect and mechanisms of root exudates on rhizo-remediation in PCP polluted soil were studied in a simulated rhizo environment. The residual dissipation of PCP in soil differed with addition of different rates of root exudates. There were significant correlations between the dissipation of PCP and the soil biochemical indices of microbial biomass carbon (0?), nitrogen (Nmic), carbon nitrogen ratio (Cmic/Nmic), microbial quotient and enzyme activities in soil. At the lowest rate (13.38 TOC mg kg"1), the PCP residual was smallest and remediation was most effective;at this time, the soil microbial biomass carbon, nitrogen, microbial quotient, dehydrogenase activities showed the largest responses. Themicrobial biomass nitrogen, microbial quotient and dehydrogenase activities appear to be sensitive biological indices indicating good soil environmental quality in rhizo-remediated soil, previously polluted with PCP.