Washing enhanced electrokinetic remediation for removal cadmium from real contaminated soil

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Abstract

The main objective of this study is to evaluate the combination of electrokinetic remediation and soil washing technology in order to remove cadmium from contaminated soil. This paper presents the results of an experimental research undertaken to evaluate different washing and purging solutions to enhance the removal of cadmium from a real contaminated soil during electrokinetic remediation. Two different experimental modules were applied in the laboratory. Soil was saturated with tap water, while acetic and hydrochloric acids, as well as ethylenediaminetetraacetic acid (EDTA) were used as purging solutions in the first module. Results show that there was a decrease of cadmium concentration near anode, but a significant increase in the middle of the cell, due to the increasing pH. Citric, nitric and acetic acids were used for soil washing and purging solutions in the second module. In this case, an 85% reduction of cadmium concentration was achieved. Therefore, results indicate that soil pH and washing solutions are the most important factors in governing the dissolution and/or desorption of Cd in a soil system under electrical fields.

Introduction

The release of heavy metals in biologically available forms, as a result of human activity, may damage or alter both natural and man made ecosystems [1]. The chemical form (speciation) of heavy metals in soil solution is greatly dependent on the metal element concerned, pH and presence of other ions, etc. [2]. Cadmium is a non-essential heavy metal pollutant of the environment resulting from various agricultural, mining and industrial activities and also from the exhaust gases of automobiles [3]. It has been considered as an extremely significant pollutant due to its high toxicity and greater solubility in water which determines its wide distributions in aquatic ecosystems [4]. Cadmium has been suspected causing symptoms of hypertension, angiopathy, kidney and bone function decay [5].

Great efforts have been made to find ways to remove contaminated species from soil. In search of alternative techniques, there has been an increasing interest for in situ treatment, without excavation of the soil. The electrokinetic process is a great promise for remediation of polluted soils, as it has high removal efficiency and time effectiveness in low permeability soils [6]. Electrokinetic remediation can be used to treat soils contaminated with inorganic species [7], [8], [9], [10], [11], [12], [13], organic compounds [14], [15], [16] and radionuclides [17], [18].

The main mechanisms of contaminants movement in the electrical field involved in electrokinetic technology are electromigration of ionic species and electroosmosis. Electromigration can be defined as the migration of ionic species present in the soil void fluid. Cations move towards the cathode, while anions move towards the anode. In some cases, electromigration probably contributes significantly to the removal of contaminants, especially at high concentrations of ionic contaminants and/or high hydraulic permeability of soil [19]. Electroosmosis in a pore occurs due to the drag interaction between the bulk of the liquid in the pore and a thin layer of charged fluid next to the pore wall that, like a single ion, is moved under the action of the electric field in a direction parallel to it. This phenomenon produces a rapid flow of water in low permeability soils and probably contributes significantly to the decontamination process in clay soils [13], [20]. The removal of contaminants would have the advantage of these two concurrent movements of electromigration and electroosmosis. In the soil two other transport mechanisms, advection and diffusion, exist. Hence, when electrical current is applied to the soil, all four transport mechanisms have to be considered [21].

An important advantage of this electrochemical technique is the high degree of control of flow direction that can be achieved because the material move along electric field lines that are defined by the electrode placement [22].

When low dc current is applied to a porous medium, the electric current leads to electrolysis reactions at the electrodes, which generate an acidic medium at the anode and an alkaline medium at the cathode.2H2O  4e  O2  + 4H+ (anode)2H2O + 2e  H2  + 2OH (cathode)The H+ generated at the anode moves through the soil towards the cathode by ion migration, pore fluid flow, pore fluid advection and diffusion. On the other hand, the reduction reaction at the cathode zone dissociates water to form H2 and OH during electrolytic dissociation. Consequently, the pH value near the cathode increases. The H+ and OH ions generated by the electrolytic dissociation move across the pore fluid within soil particles towards either the anode or the cathode [19], [23]. Both soil pH and electrolysis reactions at the electrodes play a critical role in the electrokinetic process.

Acar et al. [24] demonstrated that the movement of this acid front together with migration and advection of the cations and anions under electrical gradients constitutes the mechanisms of removal contaminants from soils. The factors influencing the acid/base profile across the porous medium would significantly affect the flow, the flow efficiency and the extent of ion migration and removal in electrokinetic soil processing. The movement of the acid front would cause desorption of cations from the soil surfaces and facilitate their release into the pore fluid. This reaction, associated with the concurrent electroosmotic flow, reinforces metal removal from the soils. As a result, metals are deposited at the cathode and anions at the anode [25].

In low buffering soils, the pH of the soil decreases to 2–3 near the anode and increases to 8–12 near the cathode due to the electrolysis reaction at the electrodes. When heavy metals enter into basic conditions, they adsorb to soil particles or precipitate as hydroxides, oxyhydroxides, etc. and in acidic conditions, those ions desorb, solubilize and migrate [18]. In order to remove the heavy metals from soils, different liquids, other than water, can be used near the electrodes. The cathode reaction should be depolarized to avoid the generation of hydroxides and their transport in the soil [25]. The selected liquids, also known as purging solutions, should induce favorable pH conditions in the soil, and/or interact with the heavy metals, so that the heavy metals are removed from the soil [26].

Recently, researchers have tried to develop soil washing techniques in which soil-bound contaminants are transfered to the liquid phase by desorption and solubilization. Several washing solutions have been investigated, such as water, acids, bases, chelating agents, alcohols and other additives [27], [28]. In practice, acid washing and chelator soil washing are the two most prevalent removal methods [29], [30]. The action of metal and washing solution (ligand) may be expressed by the following equation [31]:Me+LMeL,k=[MeL][Me][L]where Me represents a metal cation, L represents a ligand anion and k is the formation constant.

Ethylenediaminetetraacetic acid (EDTA) is the most commonly used chelate because of its strong chelating ability for different heavy metals [27], [30], [32]. This chelating agent removes trace metals with less impact on soil properties than decontamination systems, using acids as flushing agents and being slowly degradable by microorganisms [33]. The ability to extract the metals without inducing a strong acidification of medium is a very desirable characteristic. The problems with EDTA are that it may complexes strongly with a variety of metals in soils including alkaline earth cations such as Al, Ca, Fe and Mn, may bind to the soil solid phase and no longer be available for the removal of contaminants [34]. It is also relatively expensive and given the tonnes of soils that need remediation, this often leads to an excessively costly remediation [32].

Citric acid forms mononuclear, binuclear or polynuclear and bi-, tri- and multidentate complexes, depending on the type of metallic ion. For example, metals, such as Fe and Ni, form bidentate, mononuclear complexes with two carboxyl acid groups of the citric acid molecule. Copper, Cd and Pb form tridentate, mononuclear complexes with citric acid involving two carboxyl acid groups and the hydroxyl group [35]. Because citric acid is relatively inexpensive, rather easy to handle, and has a comparatively low affinity for alkaline earth metals (Ca, K and Mg), it is a suitable candidate for soil washing [36].

A number of studies have also been conducted to determine the metal extraction efficiency of strong mineral acids, including HNO3 and HCl [37]. These acids show a significant potential to extract metal ions from the soil. However, their use is associated with a number of disturbing physical, chemical and biological properties [36]. When HCl was used the final soil pH was 1, raising the concern an increase contaminant mobility, a decrease soil productivity and adverse changes in the soil's chemical and physical structure due to mineral dissolution [38]. Another concern using HCl is its possible electrolysis and chlorine gas formation when it reaches the anode compartment [18].

Decontamination can be accomplished through in situ soil washing in which a soil solution is applied to the unexcavated contaminated zone by flooding or sprinkling it in order to extract pollutants from the soil. The migration of contaminants into the ground water must be prevented by using proper control measures specific to each location. The effectiveness of in situ washing is limited by the permeability of the soil in its undisturbed state. Soils with permeability of less than 10−4 cm/s are considered unsuitable for in situ washing, in which cases excavation of the contaminated soil followed by on-site clean-up by washing can provide a viable alternative [39].

The objective of this work was to examine the effectiveness of electrokinetic removal of cadmium using different washing and purging solutions. Two sets of experiments were conducted where soil was saturated by water and different acids were used as purging solutions. A third set of experiment where soil was washed by acids was also conducted. The distribution of cadmium in the soil during the experimental time is also examined. The optimum conditions of the above experiments will be used for in situ application of electrokinetic process.

Section snippets

Description of soil

Real soil used was obtained from an abandoned military area, since it was polluted with numerous heavy metals. The composition of the soil used in the experiments is shown in Table 1.

Experimental set-up

Two different electrokinetic cells were used in the experiments. The first instrument (Fig. 1a) consisted of a cylinder, two electrodes compartments, two tubes, two electrolyte solutions reservoirs and a power supply (Statron, 0–300 V, 0–1.2 A). The contaminated soil was placed into a plexiglass cylinder 50 cm in

Soil saturated by water

Fig. 2 shows the pH, electroosmotic flow and electroosmotic velocity variations for tests I that were performed under water saturation of soil, whereas acid was used in the anode and cathode reservoirs. Due to the electrolysis reactions, H+ is produced at the anode, causing pH values around 2.5. The OH produced at the cathode results in a pH-increase to around 11. The cumulative electroosmotic flow was calculated by measuring the changes of volume in the electrode reservoir. The electroosmotic

Conclusions

Based on the experiments conducted in this study, the following conclusions can be drawn:

  • (1)

    The main results for tests I are identical. There is no significant removal of Cd from the soil (<24%) during the electrokinetic process. After 25 days of treatment, there is a high decrease of Cd concentration in the area near the anode and a significant increase in the middle of the cell.

  • (2)

    There is a continuously removal of Cd towards the cathode area during the experimental time, where it accumulates as

Acknowledgements

The financial support of the Marie-Curie Fellowship is gratefully acknowledged. We also thank Prof. Dr. W. Calmano and Dr. H. Stichnothe of the Technical University of Hamburg–Harburg (Germany) for their experimental assistance.

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