Partition Of Polycyclic Aromatic Hydrocarbons Between Soil, Water And Root

Many organic pollutants entering into soil cause serious problems to ecological risks and human health. Remediation of the contaminated soils is of worldwide concerns nowadays. Understanding the partition of polycyclic aromatic hydrocarbons (PAHs) is paramount of determinations of remediation strategies for soil contamination.In this dissertation, the sorption of phenanthrene as a representative of PAHs by soils was investigated. The influence and its mechanisms of DOM and heavy metals on phenanthrene sorption by soils were evaluated. The uptake and sorption of PAHs by plant roots and their correlations to root compositions were elucidated. Main original results are shown as follows.(1) The sorption of phenanthrene by seven soils differing in soil organic carbon contents(f_oc) could be well described by the Linear-type model. The simulated distribution constants (K_d) were observed to positively correlate to f_oc.(2) The influence of soil inherent dissolved organic matter (DOM) and exotic DOM on phenanthrene sorption by six soils differing in soil f_oc was investigated. The experimented exotic DOM was extracted from straw wastes. The K_d values for phenanthrene sorption by deionized water-eluted soils were found to be 3.13%～21.5% larger than those by the corresponding control soils. This indicates that the presence of soil inherent DOM impedes the sorption of phenanthrene by soils. Moreover, the sorption enhancement was in positive correlation with f_oc. As to the exotic DOM, its influence on phenanthrene sorption was related to its added concentrations in soil-water system. Over the range of tested exotic DOM concentrations (0～106 mg DOC/L), the K_d values for phenanthrene sorption by soil increased first and decreased thereafter with the increase of added exotic DOM concentrations. The presence of the added exotic DOM in lower concentrations clearly promoted phenanthrene sorption by soil. By contrast, higher added DOM concentrations (≥52 mg DOC/L) obviously impeded the sorption of phenanthrene in this tree-phase system including soil, water and DOM. The impacts of exotic and inherent DOM on phenanthrene sorption were suggested to primarily relate to the association of phenanthrene with DOM in solution and the 'cumulative sorption' between soil solids and aqueous solution.(3) Phenanthrene sorption by soils that have been contaminated by Pb, Zn and Cu was investigated using a batch experimental method. Phenanthrene sorption isotherms by the soil samples with or without heavy metals were well described by a linear sorption model with the R² values greater than 0.96. The distribution coefficients (K_d) and the soil organic carbon-normalized distribution coefficients (K_oc) by heavy metal-contaminated soils were significantly higher than those by the control soil. The soil samples contaminated with higher contents of heavy metals manifested the larger sorption coefficients (K_d and K_oc). The concentrations of dissolved organic carbon (DOC) in equilibrium solution for phenanthrene sorption were observed lower than those by control soil. Our calculations indicate that the less competition for phenanthrene sorption from the decreased DOC contributed little to the enhanced phenanthrene sorption by heavy metal-contaminated soils. But the calculated sorption coefficients of phenanthrene by the DOC sorbed on soil surfaces (K_Ph/soc) were about 2-3 orders of magnitude larger than the sorption coefficient by the inherent soil organic matters, which we attribute to be the predominant mechanism responsible for the enhanced sorption by the soil samples contaminated with heavy metals.(4) The sorption and uptake of phenanthrene by roots differing in lipid, in all cases, could also be well described by the Linear-type model. The simulated distribution constants (K_d') for root uptake were observed to positively correlate to root lipid contents. In addition, the distribution constants for phenanthrene sorption were higher than those of uptake. This could be the reasons that phenanthrene be taken up by roots and subsequent translocated into shoots through the transpiration streams. In addition, the decrease of phenanthrene contents in roots was also due to the plant growth dilute and degradation of PAHs in plants.