Mechanism for reverse electroosmotic flow and its impact on electrokinetic remediation of lead-contaminated kaolin

The direction of electroosmotic flow in clay is normal from the anode to the cathode, and the opposite direction is rarely observed. However, electroosmotic flow from the cathode to the anode was observed in kaolin acidified to pH 4 by acetic acid during an electroosmosis experiment. It had an impact on the electrokinetic remediation occurring with lead-contaminated kaolin. The experimental results indicated that reverse electroosmotic flow from the cathode to the anode was caused by a decrease in the absolute value of the soil zeta potential due to the compressed double electric layer and the hydrophilic carboxyl groups in acetate ions. The reverse electroosmotic flow was stronger than normal electroosmotic flow from anode to cathode. The reverse electroosmotic flow had an impact on migration of lead ions in the lead-contaminated kaolin during electrokinetic remediation experiments. The experimental results indicated that the rate for removal of Pb was increased by 10.1% due to the reverse electroosmotic flow. A micro-mechanism model for lead ion migration was built according to the functions of electric field and electroosmotic flows on lead ions, and it explained well the mechanism for the impact of reverse electroosmosis on lead ion removal. The micro-mechanism model indicated that when the direction of the stronger electroosmotic flow is the same as the migration direction of Pb, the rate for removal of Pb is improved, and vice versa.