Electrokinetic remediation of Zn and Ni-contaminated soil
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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