Sorption And Transport Of Hydrophobic Organic Compounds In Soil-Water System Studied By Liquid Chromatographic Method

The transport, transfer and bioavailability of hydrophobic organic compounds (HOCs) in soil-water systems are greatly affected by their sorption process, and soil/water distribution coefficient (Kd) is a measurement of HOC sorption potential, thus it is very important to obtain the Kd of HOCs. Kd can be obtained by direct measurement (e.g., batch equilibrium method) or indirect prediction (e.g., linear free energy relationships). Both approaches have their respective advantages, and are used simultaneously in practice. In this study we evaluated high perfbrmance liquid chromatographic (HPLC) method to obtain Kd from both approaches.Firstly, we developed a novel method for Kd measurement based on ideal nonlinear chromatographic theory, in which nonlinear sorption isotherm of HOCs can be obtained by a simple integration from the breakthrough curves (BTCs) measured by soil column liquid chromatography (SCLC). A significant advantage of this method over batch equilibrium method is that it can measure the sorption isotherm in low concentration range, whereas it is difficult for batch method. Another advantage of this method is that not only it can be used to measure sorption isotherm, but also to study the HOC transport. The feasibility and validity of this method used for measurement of sorption isotherms and transport studies were evaluated by comparing it with both the batch equilibrium and traditional column methods, and identical results were achieved.Secondly, the use of HPLC and SCLC method for Kd prediction was evaluated using a wide range of chemicals and soils. A multi-column HPLC method was developed for Kd prediction using the retention parameters measured on HPLC columns with different selectivities as molecular descriptors of multiple regression equations. The multi-column HPLC method was proved to be applicable to a widerange of chemical types, and it overcomes the limitation of the traditional HPLC method, which is only applicable for congeneric chemicals or chemicals with similar properties.Because Kd varies as a function of soil properties, it is not a part of the intrinsic properties of a specific chemical, which is a site-specific value that may vary with different soil types. To develop a rapid Kd prediction method for specific types of soils, we evaluated the SCLC method among a wide range of soils with significantly different properties. The correlations between the retention factors measured on soils with water or methanol-water mixtures as mobile phases and Kd measured by batch equilibrium experiments were investigated for five different types of soils, and the results show that the SCLC is suitable and accurate for Kd prediction.Finally, for understanding the sorption and transport of HOCs in soil-water systems, linear solvation energy relationships (LSERs) were used to quantitatively characterize the molecular interactions between HOCs and soils based on the correlations between solute molecular descriptors and retention factors measured by SCLC. The results show that the solute size (Vx) and the HBA basicity are the two dominant factors that governing HOC retention on soils, and the HBD acidity and the excess polarizabilities (R) are the two minor solute descriptors. But the dipolarity/polarizability almost has no contribution to HOC retention on soils. Also the molecular interactions involved in adsorption of HOCs on soil mineral-black carbon complex were characterized by LSER, and we for the first time found the HOC adsorption is favored by the planarity of solutes, and the contribution of the planarity was quantitatively estimated In addition the influence of soil solution on HOC sorption on soil was also studied using LSER by modifying the mobile phase of SCLC with soil solution, and results indicate that the soil solution can influence HOC sorption by cringing the properties of soil surface or aqueous phase.