Electrokinetic remediation of Zn and Ni-contaminated soil

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Abstract

The feasibility of catholyte conditioning with the acidic solution and pre-treatment of soil with acidic solution was investigated with the electrokinetic remediation of Zn and Ni contaminated field soil. The extraction of Zn and Ni from soil increased with the decrease in pH of the extracting solution and nitric acid was very effective to extract Zn and Ni from the soil. Conventional electrokinetic treatment and acetate buffer circulation method were not effective to remove Zn and Ni from the soil. Pre-treatment of the soil with acidic solution enhanced the desorption of Zn and Ni and catholyte conditioning with this solution was effective in maintaining the overall soil pH within the electrokinetic cell. The catholyte conditioning and pre-treatment method enhanced the removal of Zn and Ni up to 41% and 40% after operation for 4 weeks. More than 96% of Zn and Ni removed by electrokinetic remediation were due to the electromigration. Catholyte conditioning and the pre-treatment method is effective in enhancing metal removal in electrokinetic remediation.

Introduction

Recently, heavy metal contamination has caused serious environmental and human health problems in abandoned mine and industrial sites. Soil washing and solidification/stabilization have been used to remediate soil contaminated with heavy metals in Korea [1], [2], [3]. Even though soil washing is an effective technology to remove heavy metals from sandy or silt soil, it is not effective for fine-grained soil [1]. Even though stabilization and solidification is the most common choice for treating metal-contaminated soil, these techniques are not applicable to zinc and nickel-contaminated soil because aqua-regia extraction is the Korean standard test method for nickel and zinc. Even though stabilization and solidification method is applied to treat zinc and nickel-contaminated soil, the total mass of the metals is not changed, and most of zinc and nickel are extracted from the treated-soil by the aqua-regia method. Currently, a separation technique needs to be applied to remediate zinc and nickel-contaminated soil in Korea.

Electrokinetic (EK) remediation is one of promising separation technologies and the most effective technique for fine-grained and clayey soil [4], [5]. It is effective to remove organic compounds, heavy metals and radionuclides from soil, mine tailings, sludge and sediment [2], [5], [6]. In electrokinetic remediation, the removal mechanisms are electromigration, electroosmosis and electrophoresis. In metal removal using the electrokinetic process, a hydrogen ion is produced at the anode due to the hydrolysis reaction of water. The hydrogen ion is transported toward the cathode by an electric field and is exchanged with cationic metals such as zinc and nickel onto soil surface. The desorbed metal ions are moved toward the cathode by electromigration. Generally, an acidic solution is preferred to extract or desorb cationic metals from soil, which means higher removal efficiency [2]. The control of soil pH using various methods is a common choice to enhance the removal efficiency of pollutants in the electrokinetic process [2], [3], [7], [8], [9], [10], [11]. However, change in the soil pH influences the zeta potential of the soil surface and the direction of electro-osmotic flow is highly dependent on the zeta potential or surface charge of the soil [2], [3]. More negative zeta potential of the soil surface enhanced the more electro-osmotic flow. If the direction of electroosmotic flow is toward the cathode, then the removal of cationic metal might be enhanced, while the removal might decrease in the case of the opposite direction.

This study investigated the feasibility of conditioning the catholyte with an acidic solution and the pre-treatment of soil with an acid on the electrokinetic remediation of Zn and Ni contaminated soil.

Section snippets

Soil sample

The soil used in this study was contaminated by soot in power plant stack, which contains a high concentration of Ni and Zn, in a power plant. Initial concentrations of Ni and Zn were 1324 and 1632 mg/kg, respectively. The soil was sampled in the field and sieved using mesh 10 and soil of <2 mm was used in electrokinetic experiments. The soil was silty loam and the organic content was 10.3%. The initial pH and water content of the soil were 6.8% and 20%.

Experimental apparatus

Fig. 1 shows a diagram of EK experimental

Basic extraction experiment

Fig. 2 shows the extraction efficiency of Zn and Ni by nitric acid and equilibrium pH after washing. The extraction efficiency of Zn and Ni from soil increased sharply as the pH of acidic washing solution decreased from 4 to 1. As the initial pH of the extracting solution decreased from 2 to 1, the extraction efficiency increased dramatically from 0.2% to 39.7% for Zn and from 0.7% to 16.8% for Ni. Except pH 1.0, the extraction of nickel and zinc from soil was negligible and the equilibrium pH

Conclusions

In this study, the feasibility of catholyte conditioning and pre-treatment of soil with an acidic solution for an electrokinetic experiment was investigated in a laboratory study. Zn and Ni were not extracted by the nitric acid solution of less than 0.1 M. Conventional electrokinetic and acetate buffer circulation was not effective to remove Zn and Ni from the contaminated soil. Catholyte conditioning played a major role in maintaining the soil pH as acidic, which enhanced the overall removal of

Acknowledgement

This research was supported a grant from Kumoh National Institute of Technology.

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