Effect of voltage gradient on integrated electrochemical remediation of contaminant mixtures

Abstract

This paper evaluates the effect of voltage gradient on the efficiency of integrated electrochemical remediation (IECR) of low permeability soils contaminated with both heavy metals and polycyclic aromatic hydrocarbons (PAHs). The IECR remediation process aims to oxidize organic contaminants within the soil by the electro-osmotic delivery of hydrogen peroxide (H2O2) and a Fenton-like oxidation process, as well as simultaneously removing heavy metals by electro-osmotic advection and electromigration. In Fenton-like oxidation, the native soil iron is utilized as a catalyst to decompose H2O2 to generate free hydroxyl radicals that oxidize PAHs into relatively benign products such as carbon dioxide, water and oxygen. A series of bench-scale experiments was performed on kaolin (a low permeability soil) spiked with nickel (a representative heavy metal) and phenanthrene (a representative PAH) each at a concentration of 500 mg per kg of dry soil under two voltage gradients, 1 and 2 VDC/cm. The H2O2 solution in two different concentrations at 5% and 10% was introduced at the anode, and each experiment was conducted for a total duration of four weeks. The results showed that increasing the voltage gradient from 1 VDC/cm to 2 VDC/cm did not increase the electro-osmotic delivery of H2O2 significantly. Phenanthrene removal from the soil was negligible in all the experiments; however, 28% and 34% of the phenanthrene were oxidized within the soil in the 5% and 10% H2O2 experiments, respectively, under 1 VDC/cm. The phenanthrene oxidation increased to about 32% and 42% under 5% and 10% H2O2 concentrations, respectively, under 2 VDC/cm. Nickel migrated towards the cathode and then precipitated close to the cathode, due to high pH conditions in all the experiments. Nickel migration was slightly higher in the case of 2 VDC/cm than in the case of 1 VDC/cm, due to greater migration of the acidic pH front towards the cathode under 2 VDC/cm. Overall, the results showed that an increase in the voltage gradient from 1 VDC/cm to 2 VDC/cm improved overall remedial performance slightly, and alternative strategies to increase H2O2 delivery and prevent precipitation of metals near the cathode are required for achieving significantly higher remedial efficiencies.