pH Variation and Its Effect on Metal Concentration during Electrokinetics

Heavy metal contaminated soils pose significant threat to the environment and human health. Electrokinetic remediation is an emerging remedial technology that removes the contaminants from silty or clayey soils by applying an electric potential across contaminated and saturated soils using a pair of electrodes. The anode attracts negatively charged contaminants, and the cathode attracts positively charged contaminants, such as heavy metal ions. When the cationic contaminants reach the cathode, they can be pumped out with the contaminated water and disposed or treated safely. However, the electrokinetics is complicated by the metal precipitation before the metal ions can reach the cathode. This is caused by electrolysis that occurs at both electrodes. At the anode, water is oxidized, releasing oxygen and creating H⁺ that results in an acid front moving towards the cathode. At the cathode, hydrogen is evolved and a base front of hydroxyl ions (OH^–) is generated and moves to the anode. The metal ions migrating toward the cathode tend to react with OH^– when they reach the base front to form metal hydroxides, which precipitate from the pore fluid as solid form.