# Enhanced Electrokinetic Remediation for the Removal of Heavy Metals from Contaminated Soils
## 1 Introduction  
Soil is affected by various anthropogenic activities, including mining, waste dump, deforestation, urbanization, intensive agriculture practices and change in land use [1]. As a result, soils have been contaminated with a variety of organic and inorganic contaminants [2]. The metal-contamination in the soil is of special concern because metals show special toxicity for living organisms affecting the development of microorganisms and plants, enter the food change and extend the toxicity to animals and humans [3]. Contaminating metals cannot be degraded, and their removal from the soil is usually difficult, time-consuming and costly [4]. Many studies published in literature dealt with the removal of heavy metals from soil. However, the efforts in developing and testing various innovative technologies did not still result in a reliable and feasible technology to be generally applied in the remediation of metal-contaminated soils [5, 6]. Recently, research has focused on the development of high-sensitivity sensors, usually based on electrochemical technology [7, 8]. These sensors can be used for the detection of heavy metal ions in various media, allowing a new way to monitoring metal-contamination in hard-to-reach locations and remote environments. Electrokinetic remediation has been proposed as an effective technology for the remediation of metal-contaminated soils [9]. Electrokinetic technology uses a low-intensity DC electric field to mobilize and transport the metals out of the soil. The benefits of electrokinetics rely on its capacity to be applied in situ with minimum disturbing of the surface, the possibility of treating low permeability soils and the low-cost in energy. However, electrokinetics is affected by the geochemical soil conditions that may limit the solubilization of metals. In order to increase the solubility of the metal in the soil interstitial fluid, various methods have been proposed [10]. The acidification of soil with the acid front electrogenerated in the anode is the most common option [11]. This option is usually combined with the depolarization of the electro-reduction of water on the cathode tosuppress the alkaline front [12]. The controlled addition of mineral acid in the cathode allows for the neutralization of the basic front and favors the acidification of the soil with the subsequent solubilization of metals (for example, Co\({}^{2+}\), Ni\({}^{2+}\), Cd\({}^{2+}\),... ). Alternatively, alkaline metals (for example, chromate) can be extracted from the soil matrix favoring the development of the basic front on the cathode and suppressing the acid front on the anode by the addition of alkali (e.g., NaOH). The modification of the soil pH with the acid or alkali front from cathode or anode is an effective method for the solubilization of metals in many cases, but it is important to consider the geochemistry of soils and contaminating metals. As an example, [PERSON] et al. [13] used the controlled addition of ammonia to solubilize copper and arsenic. Ammonia forms a stable complex with copper [Cu(NH\({}_{3}\))\({}_{4}\)]\({}^{2}\) in alkaline conditions that can be transported towards the cathode by electromigration. Conversely, various authors [12; 14] used sulfuric acid to neutralize the basic environment of the cathode to mobilize the metals due to the acidification of the acid front from the anode. This method was very effective for metals, such as Cd\({}^{2+}\), Co\({}^{2+}\) and Zn\({}^{2+}\), but Pb was not removed because it forms an insoluble salt with sulfate [14]. These results proved that the chemistry of facilitating agents with the metal-soil system is of crucial importance for a successful remediation. Furthermore, the acidification or alkalinization of soil may induce unacceptable changes in soil properties. The soil will require additional treatment to restore the natural properties of soil at the end of the remediation process [9].