Studies of hydrophobic organic pollutant interactions with cyclodextrin: implication for groundwater remediation

Henry’s Law constants of several common, subsurface hydrophobic organic pollutants (HOPs) including trichloroethylene (TCE), perchloroethylene (PCE) and benzene, toluene, ethylbenzene and o-xylene (BTEX), were measured over a range of temperatures (35, 45, 55 and 65°C) and cyclodextrin (CD) concentrations (0, 10, 20, 50 and 100 g L-1). In aqueous CD solutions, Henry’s constant values decrease by a power law relationship with increasing CD concentration due to an apparent solubility enhancement caused by HOP partitioning to the hydrophobic cavity of CD molecules. The temperature dependence of air-water partitioning under the influence of CD was well described by the van’t Hoff equation for all HOPs tested. A three-phase equilibrium model was used to interpret air-water-CD partitioning data. Our results show that HOP CD-water partition coefficients decrease linearly with increasing temperature. Adsorption isotherms of TCE to the granular activated carbon (GAC), Norit C GRAN, were determined for aqueous solutions containing a range of CD concentrations (0, 20, 50, 100 g L-1). The adsorption coefficient and constant of apparent and rescaled Freundlich isotherms was determined by the nonlinear regression of versus total aqueous concentration and free water phase concentration,respectively. Our results showed that in aqueous CD solutions, the adsorption to GAC decreased with increasing CD concentration due to the interaction of TCE with CD in the aqueous phase. A linear relationship was derived with the linear coefficient of 0.999. Breakthrough curves of single solute, TCE were obtained over a range of flow velocities (1-10 ml∙min-1) and HPCD concentrations (0, 20, and 50 g∙L-1). Relationships between the pore velocities and two-site non-equilibrium parameters obtained from CXTFIT program were illustrated for this specific adsorption. Comparison of original breakthrough curve under different CD concentrations to the solubility-enhancement-rescaled ones showed (1) all 3 curves exhibited similar scaled times at which they reach 50% of the input concentration and (2) cyclodextrin increased the time needed for TCE to reach equilibration between GAC and liquid phases. TCE at higher flow rate had insufficient time to interact with GAC such that non-equilibrium processes were more significant at higher flow rate