The Sorbed Forms Of Phenanthrene In Soils And Its Bioavailability

Due to the lipophicity and persistance of hydrophobic organic contaminants (HOCs), it may pose great risk to ecosystem and human health, and cause much attention. Through sorption, deposition processes, HOCs could be accumulated in soil/sediment-kind natural geosorbents. Therefore, soil/sediment-kind natural geosorbents could be the sink of HOCs. Sorption/desorption is the key process controlling the transport and transformation of HOCs in soil/sediment. HOCs interact with soil/sediment organic matter (SOM) by different interactions, and sorbed on different sites exist in different sorbed forms in soil/sediment. HOCs in different sorbed forms has different desorption thermodynamic and kinetic properties. Desorption process plays an important role in controlling the bioavailability and eco-risk of the sorbed HOCs. Hence, investigating the sorption/desorption processes of HOCs in soil/sediment-kind natural geosorbent and desorption behaviors of the HOCs in different sorbed forms may facilitate the understanding of bioavailability and the assessment of the eco-risk posed by sorbed HOCs in soil/sediment.Phenanthrene was sleceted as the model HOCs and its sorption and desorption behavior on several natural geosorbents were investigated. According to the differences in desorption characteristics, the sorbed phenanthrene in natural geosorbents was operationally seperated into three fractions. Based on the operational fraction, a triple-compartment was proposed and applied into the study of the bioavailability of sorbed phenanthrene in soils. The main conclusions drawn in this dissertation are as following:(1) Both concentration-and time-dependence of the phenanthrene sorption and desorption processes were observed. The affinity of the sorbent for the sorbate decreases progressively when the specific interaction sites become filled with increasing sorbate concentration, leading to the nonlinearity of the sorption isotherms and the increase of the desorption percentages. The shape of the sorption isotherms changed with the contact time, which indicated that besides the diffusion of contaminants molecules in inter-and inner SOM particles, the interactions between phenanthrene molecules and SOM also play important roles in the sorption of phenanthrene to natural geosorbents. The sorption rates of phenanthrene varied with the characteristics of sorption domains:the rate of sorption in the partitioning domain is faster than that in the adsorption domains; this may be explained by the accessibility governed by the SOM conformation. Moreover, the entrance of foreigner molecules may lead to the transformation SOM conformation from rigid domain to rubbery domain, this phenomenon was termed as "swelling" or "conditioning effects". The higher the concentration of foreigner molecules, the more obvious this phenomenon is.(2) The heterogeneity of the sorption sites energy distribution varied with the composition and conformation of SOM. The site energy distribution in partitioning sorption domains is more homogenous than that in adsorption domains. The sorption sites with different sorption energy show varying accessibilities to phenanthrene moleculars, sorption sites with higher sorption energy has lower accessibility than those with lower sorption energy. This may be ascribed to the sorption sites with higher sorption energy, such as adsorption domains, are internal to the SOM matrix may be accessed more slowly due to the steric hindrance of the outer domains and unevenly distributed with respect to access by sorbing molecules. The phenanthrene molecules sorbed on lower energy sorption sites have higher potential to be desorbed, resulting into the site energy distribution curve shift towards the higher energy direction.(3) A fraction of phenanthrene was sorbed irreversibly and cannot be desorbed in the desorption process, and the isotherm of desorbed compartment is not linear, indicating that the reversible sorption compartment can not be ascribed to the partitioning solely, some specific interactions also contribute to the reversible sorption compartment. Therefore, a triple-compartment desorption model (TCDM) was proposed to fit the desorption isotherms. According to the desorption isotherm’s characteristics, the sorbed phenanthrene could be fractionated into linear sorption compartment, reversible Langmuir-type sorption compartment and irreversible compartment in this triple-compartment model. A new approach was proposed to calculate the partitioning coefficient Kp for the linear sorption compartment from the concentration of contaminants in solid phase and liquid phase in the sorption and first desorption cycle. As indicated by the value of Kp calculated by this method that the organic carbon content corresponding the linear partitioning sorption is different, hence, calculating KP from total organic carbon content is questionable. The residual solid concentration after the termination of desorption experiment was considered as the irreversible sorption compartment. The TCDM proposed fits the desorption isotherms in this study very well, and it may facilitate the understanding of the thermodynamic properties of desorption process.(4) According to the desorption rates, the sorbed phenanthrene in natural geosorbents was separated into fast desorption, slow desorption and irreversible sorption compartment, total sorption could be described as the sum of these three compartments. Based on desorption rates, a triple compartment sorption model (TCSM) was proposed. The contribution of each compartment to the total sorption varied with the equilibrium concentration:at low concentrations the sorption dominated by irreversible sorption compartment and slow desorption compartment, while the fast desorption compartment contributed more at high concentrations. The sorption isotherm of the fast desorption compartment fitted by linear sorption model very well, and the slow desorption and irreversible sorption compartment exhibit Langmuir type sorption behavior. The desorption rates decreased with the increasing affinity between phenanthrene molecules and SOM, and the comparison TCSM with TCDM also suggested that there is an underlying relationship between desorption thermodynamic properties and kinetic properties. The fractionation of sorbed phenanthrene in soils by desorption rates is more operational, it may provide a powerful tool to study the bioavailability and eco-risk of HOCs in soil/sediment.(5) A semi-permeable membrane based micro-extraction (SPM-ME) technique was proposed to determine concentration of the free dissolved HOCs in soil pore-water. This method can accurately determine the free dissolved HOCs in soil pore-water avoiding the overestimation of desorption potential of sorbed HOCs by partitioning theory. Also this method is suitable to determine the free dissolved HOCs in low concentration system, which could not be determine by other method. The SPM-ME is a nondestructive, nondepletive and in-situ method for the determining of free dissolved HOCs in soil pore-water.(6) The ratio of Cworm/Csoil increased with the increasing spiked concentration, this may be due to that the sorption sites with high energy becomed filled and more phenanthrene molecules were sorbed on low energy sites leading to the increasing potential of desorption and bioavailability. The determined Cworm is higher than the predicted value from Cpore-water by equilibrium partitioning theory, indicating that earthworm did not just uptake phenanthrene from pore-water, it also can accumulate phenanthrene through digestion of soil particles. The contribution of each biouptake route to the total accumulation varied with spiked phenanthrene concentrations, the higher the spiked phenanthrene concentration, the more uptake through porewater. BCFpore_water derived from normalizing Cworm with Cporewater decreased with increasing concentration of phenanthrene in porewater, indicating that digestion contributed more to the total accumulation at low concentration. Earthworm can not only utilize the linear sorption compartment in soil, it can also uptake the nonlinear sorption compartment. Aging decreased the bioavailability of sorbed phenanthrene in soils, this may be explained by the change of the distribution of phenanthrene in different sorbed forms. Accrodding to triple compartment model, more phenanthrene was distributed in irreversible sorption compartment, leading to the decreasing bioavailability.