Iodide-Enhanced Electrokinetic Remediation of Mercury-Contaminated Soils

Abstract

This study investigates using an iodide-enhanced solution at the cathode during electrokinetic treatment to optimize the removal of mercury from soils. The experimental program consisted of testing two types of clayey soils, kaolin, and glacial till, that were initially spiked with 500 mg/kg of Hg͑II͒. Experiments were conducted on each soil type at two voltage gradients ͑1.0 or 1.5 VDC/cm͒ to evaluate the effect of the voltage gradient when employing a 0.1 M KI solution. Additional experiments were performed on each soil type to assess the effect of using a higher iodide concentration ͑0.5 M KI͒ when using a 1.5 VDC/cm voltage gradient. The tests conducted on the kaolin soil showed that when the 0.1 M KI concentration was employed with the 1.0 VDC/cm voltage gradient, approximately 97% of the mercury was removed, leaving a residual concentration of 16 mg/kg in the soil after treatment. The tests conducted on glacial till indicated that it was beneficial to use the higher ͑0.5 M KI͒ iodide concentration and the higher ͑1.5 VDC/cm͒ voltage gradient to enhance mercury removal, because, under these conditions, a maximum of 77% of the mercury was removed from the glacial till, leaving a residual concentration of 116 mg/kg in soil after electrokinetic treatment. Compared to kaolin, the lower mercury removal from the glacial till soil is attributed to the more complicated soil composition, such as the presence of carbonates and organic matter, which caused Hg͑II͒ to adsorb to the soil and/or exist as an immobile chemical species.

Particle morphology and mineral structure of heavy metal-contaminated kaolin soil before and after electrokinetic remediation

“…Electrokinetic remediation of heavy metals in soils has been investigated at the University of Illinois at Chicago [2][3][4][5][6]. These studies demonstrated that in low buffering soils such as kaolin with water as the electrolyte solution, targeted metal cations such as nickel and cadmium migrate toward the cathode and are immobilized before they reach the cathode.…”

In situ remediation technologies for mercury-contaminated soil

“…As Hg also forms soluble species according to the following reactions (Cox et al 1996;Reddy et al 2003b):…”

Electrokinetic extraction of lead from kaolinites: II. Experimental investigation

“…As the transformation of most heavy metals is pH dependent, keeping the soil at low pH appears to be a plausible solution. Techniques being investigated include simultaneous injection of enhancement agents, such as purging solutions (synthetic or natural), chelating agents, and complexing agents including sulfuric acid, citric acid (Eykholt and Daniel 1994;Gu et al 2009b), EDTA (Yeung et al 1996), iodine/iodide lixiviant (Cox et al 1996;Reddy et al 2003), humic acid, sodium acetate solution, nitric acid, to transform the contaminants to their mobile phases so that they can be effectively removed; acetic acid depolarization of the cathode reaction, and use of ion-selective membrane to veil hydroxide ions back-transport at the cathode (Rødsand et al 1995); and conditioning of electrodes and reservoir solutions to specific pH to eliminate the adverse impacts of electrode reactions (Lee and Yang 2000). More research on these physicochemical soil-contaminant interactions is certainly needed to develop better enhancement techniques for electrokinetic extraction (Yeung 2009b).…”