Electromigration of cadmium in contaminated soils driven by single and multiple primary cells
Introduction
Electrokinetic (EK) remediation is a very effective technique to remove toxic metals from contaminated soils. A comprehensive review of EK soil remediation has been provided in several previous studies [1], [2], [3], [4], [5]. Briefly, with the application of electric field to the polluted site, pollutants will migrate towards anode or cathode by electromigration, electro-osmosis and electrophoresis [1], [2], [3]. Much interest has been focused on the electromigration of heavy metals in soils [6], [7], [8], [9], [10], [11], [12], [13]. It has been well established that the removal mechanism of heavy metals by EK technique involved two steps, the desorption of heavy metals from soils to pore solution and the subsequent movement by electromigration, secondarily by electro-osmosis and diffusion [1], [2], [3]. It is impossible to move the pollutants adsorbed on soils by electromigration.
Generally, about 1 V/cm potential gradient was used in EK remediation in both experimental and field test [3], [4], [5], [14], [15]. Actually, the potential may attribute to the removal efficiencies of heavy metals from two aspects. On one hand, electrolysis of water on anode produces much H+, which forms an acidic front towards cathode by electromigration. The acid front may largely contributes to the desorption of heavy metal from contaminated soils into soil pore solution [1], [2], [3]. On the other hand, such a high potential may accelerate the electromigration rate of the positively charged heavy metals in the soil pore solution. Thus, relatively shorter remediation duration was achieved by EK process. High electric energy consumption is one of the most important factors restricting the application of EK technique [15], [16], particularly in developing countries such as China. Therefore, it is very meaningful to develop new methods to reduce the energy consumption. Supposed that the contribution of potential to the desorption was subtracted or the heavy metals have been dissolved in soil pore solutions, much lower potential gradient can be used to drive electromigration. This supposition is reasonable in some specific cases, such as acidic soils or controlled acid conditions artificially so that heavy metals have been desorbed from soils.
In the present study, we tentatively used a primary cell, instead of the traditional dc electric power, to drive the electromigration of heavy metals in soils. The schematic diagram of the technique is shown in Fig. 1. Iron (Fe) and carbon (C) are used as anode and cathode, respectively. When Fe and C are connected with wire, a primary cell is formed due to the difference of electrode potentials. Electrode reaction on C is the evolution of H2 (reaction (1)) and on Fe is the corrosion of iron (reaction (2)), so the electrode potential on C is higher than that on Fe. Electric current is from C to Fe in the wire and from Fe to C in the soil, as shown in Fig. 1. Cations migrate from Fe to C and anions migrate contrarily:C: 2H+ + 2e− → H2, φθ(H+, H2) = 0 VFe: Fe − 2e− → Fe2+, φθ(Fe2+, Fe) = −0.440 V
This study investigated the electromigration of cadmium, a representative heavy metal pollutant, in soils driven by the primary cell. The objectives are (1) to explore the feasibility of using Fe/C primary cell to drive the electromigration of cadmium in soils; (2) to research the effect of acid in Fe and C compartment on the electrokinetic properties and the electromigration of cadmium; (3) to investigate electromigration of cadmium by the series of several primary cells so as to increase the remediation area.
Section snippets
Chemicals and materials
CdCl2·2.5H2O (Tingxin Chemical Reagent Factory, China, >99.0%) was used as the source of cadmium. Scrap iron was provided from the mechanical factory of Huazhong University of Science and Technology, China. It was first washed in boiling NaOH (1%) solution for 20 min to remove oil and then in H2SO4 (1%) to remove rust. Finally, it was washed with tap water and deionized water to be neutral. Crustose carbon was purchased from Kexin Chemical Co. Ltd., China. It was washed by tap water and
Effect of aqueous solution on electric potential
The standard potential between C and Fe is 0.440 V [21]. However, the potential was affected by the concentration of H+ and Fe2+, which can be described by Nernst equation [21]:E = φ(H+, H2) − φ(Fe2+, Fe)E = φθ(H+, H2) − φθ(Fe2+, Fe) + RT/2F ln[α(H+)2α(Fe)/α(H2)α(Fe2+)]E = 0.440 − RT/2F ln[α(Fe2+)p(H2)] − 0.059pHwhere R is the Avogadro number, T the standard temperature, F the Coulomb constant, and α(H+), α(Fe), α(H2) and α(Fe2+) are the activities of the corresponding species in the brackets. Generally, the
Conclusions
The present study investigated the feasibility of using Fe/C primary cell to drive the electromigration of cadmium in soils. The effect of acid in Fe and C compartment on the electrokinetic properties and the electromigration of cadmium were studied. Furthermore, the electromigration of cadmium by the series of several primary cells was also researched. The main conclusions were drawn as follows:
- (1)
In the primary cell system, the addition of acid to C compartment increased the electric potential,
Acknowledgement
This work was supported by the key project of Natural Science Foundation of Hubei Province (no. 2006ABD005).
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