Cyclodextrin-enhanced electrokinetic removal of phenanthrene from a model clay soil

Removal of hydrophobic organic contaminants (HOCs) from saturated low-permeability subsurface environments using a solubility-enhanced electrokinetic remediation process is demonstrated for a model system. Phenanthrene, hydroxypropyl-beta-cyclodextrin (HPCD), and kaolinite were selected as a representative HOC, HOC solubility-enhancing agent, and model clay soil, respectively. Electrokinetic (EK) column experiments were conducted under various operating conditions, and the results were interpreted in terms of the EK properties and expected phenanthrene solubilization of the test systems. No significant effects of HPCD on the EK properties of kaolinite were observed. Initial pore solution pH values dictated the initial electroosmotic flow (EOF) and charge flow rates through the test samples. However, with increasing EK operating times, low pH values (i.e., near or below the point of zero charge of kaolinite) dominated over most of the column length in unbuffered systems, thereby decreasing the EOF and charge flow rates with time. To minimize these adverse effects, pH control of the anode reservoir with a Nar CO3 buffer was used to keep EOF and charge flow rates high. EK experiments using HPCD solutions showed greater phenanthrene removal from the kaolinite samples, and the removal efficiency depended on the HPCD concentration used. Longer EK operating times without pH control were generally not beneficial for removing phenanthrene because of the low EOF rates obtained after 3 days. The best overall phenanthrene removal was obtained by flushing the anode reservoir with a high HPCD concentration prepared in the Na2CO3 buffer solution. The results obtained from this preliminary study show that an EK process combined with HPCD flushing and pH buffering may be a good remediation alternative for removing HOCs from low-permeability subsurface environments.