Thermodynamic and kinetic models for the extraction of essential oil from savory and polycyclic aromatic hydrocarbons from soil with hot (subcritical) water and supercritical CO 2

Abstract

Mechanisms that control the extraction rates of essential oil from savory ( Satureja hortensis) and polycyclic aromatic hydrocarbons (PAHs) from historically-contaminated soil with hot water and supercritical carbon dioxide were studied. The extraction curves at different solvent flow-rates were used to determine whether the extractions were limited primarily by the near equilibrium partitioning of the analyte between the matrix and solvent (i.e. partitioning thermodynamics, or the “elution” step) or by the rate of analyte desorption from the matrix (i.e. kinetics, or the “initial desorption” step). Two simple models were applied to describe the extraction profiles obtained with hot water and with supercritical CO 2: (1) a model based solely on the thermodynamic distribution coefficient K D, which assumes that analyte desorption from the matrix is rapid compared to elution, and (2) a two-site kinetic model which assumes that the extraction rate is limited by the analyte desorption rate from the matrix, and is not limited by the thermodynamic ( K D) partitioning that occurs during elution. For hot water extraction, the thermodynamic elution of analytes from the matrix was the prevailing mechanism as evidenced by the fact that extraction rates increased proportionally with the hot water flow-rate. This was also confirmed by the fact that simple removal calculations based on a single K D (for each essential oil compound) gave good fits to experimental data for flow-rates from 0.25 to 4 ml/min. In contrast, supercritical CO 2 extraction showed only minimal dependence on flow-rate, and the simple K D model could only describe the initial 20–50% of the extraction. However, a simple two-site kinetic model gave a good fit for all CO 2 flow-rates tested. The results of these investigations demonstrated that very simple models can be used to determine and describe extractions which are limited primarily by partitioning thermodynamics, or primarily by desorption kinetics. Furthermore, these results show that the time required for the recovery of essential oil from savory with hot water can be minimized by increasing flow-rate, with little change in the total volume of water required. In contrast, raising the flow-rate of supercritical CO 2 has little effect on the mass of essential oils recovered per unit of time, indicating that optimal recovery of these compounds with supercritical CO 2 (amount recovered for the lowest amount of CO 2) requires longer extraction times rather than faster flow-rates.