ELECTRODIALYTIC REMEDIATION OF SOIL

POLLUTED WITH HEAVY METALS

Key Parameters for Optimization of the Process

H. K. HANSEN, L. M. OTTOSEN, L. HANSEN, B. K. KLIEM, A. VILLUMSEN and G. BECH-NIELSEN*

Department of Geology and Geotechnical Engineering, The Technical University of Denmark, Lyngby, Denmark*Department of Chemistry, The Technical University of Denmark, Lyngby, Denmark

In this paper, the importance of some parameters for the ef® ciency of electrodialytic soilremediation are evaluated. The parameters investigated are pH, the limiting current densityand the addition of desorbing agents to the soil. These three parameters are found to be of

the greatest importance. Results show that electrodialytic soil remediation can be optimized byunderstanding and adjusting these parameters. For scaling up the remediation method, theseparameters are of crucial importance.

Keywords: electrodialytic soil remediation; heavy metals; pH; limiting current density;complexing agents

INTRODUCTION

Electrodialytic remediation is a new method of removingheavy metals from polluted soil. The method uses a DCcurrent as cleaning agent, and together with a combinationof ion exchange membranes, the process is optimized onthe removal of heavy metals, and products coming from theelectrode reactions are prohibited from entering the soil(Hansen et al.1 , Ottosen et al.2 ). The principle of the methodis given in Figure 1. After remediation, the heavy metals endup in compartments II (cations) and IV (anions). Mem-branes 2 and 3 are in contact with the soil on one side ofthe membrane and with an aqueous solution on the otherside. Membranes 1 and 4 are used to control the electrodereactions better and to avoid certain species coming fromthe polluted soil participating in the electrode reactions(if the soil was a marine soil Cl± could be oxidized to Cl2 atthe anode). The remediation process has by now been ableto remove Cu, Cr, Pb, Zn, and Hg from polluted soil2 ,(Hansen et al.3 ).

Several parameters are important for the process; boththe desorption of the heavy metals from the soil, the heavymetal speciation and the ef® ciency and economy of theprocess. These parameters are among others (Hansenet al.4 ): pH, redox-potential, temperature, current density,electroosmosis, and water content of the soil duringremediation. Some soil characteristics are important foref® ciency, especially the clay and carbonate content, theparticle size distribution and the content of organic matter.Addition of desorbing agents to the soil can be animportant tool to improve the ef® ciency of the remediationmethod.

In this paper, some key parameters for the remediationprocess are discussed in order to be able to predict andoptimize scale up from laboratory scale to full scalevia bench scale equipment. These key parameters arepH, the limiting current density and the addition of

desorbing/complexing agents to the soil in order to havethe heavy metals in ionic form in the soil solution.

pH

The pH of the soil liquid has a crucial importance forthe ef® ciency of electrodialytic soil remediation. Boththe speciation of the heavy metals and the desorption/adsorption equilibrium are highly pH dependent. Heavymetals like Cu, Zn and Pb are known (Pourbaix5 ) to becationic species at low pH and anionic, due to complexingwith OH± at higher pH. For Cr and As, the redox levelcan furthermore change between high and low pH as wellas the charge of the species.

A remediation experiment was carried out on a Danishsoil polluted with Cu (2300 mg/kg DM), Pb (830 mg/kgDM) and Zn (2400 mg/kg DM)3 . Furthermore, the soilcontained 13000mg/kg DM of Ca. The contaminated soilwas mixed with distilled water, and put into compartmentIII (see Figure 1). The electrodialytic remediation cellwas cylindrical with a surface area of 50.0 cm2 , and thecontaminated soil compartment was 15 cm long. Theelectrodes were platinized titanium rods with a diameterof 3 mm and a length of 3 cm. The remediation experimentwas run for 54 days at a current density of 0.2 mA/cm2 .After the experiment the soil was cut into 10 slices andheavy metal concentrations and pH were measured ineach slice. Figure 2 shows the normalized Cu, Pb , Zn andCa concentration in each slice together with pH.

From the pro® les on the ® gure it can be noticed that Cais removed ® rst followed by Zn and ® nally Cu and Pb.This can be explained by the pH curve, because Ca isdesorbed at neutral or weak acidic pH (= 6), Zn at weakacidic pH (= 5±6), and Cu and Pb at pH lower than 4.5(Alloway6). When desorbed, the metal cations move inthe electric ® eld towards the cathode. Closer to the cathode,

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the pH is higher, and the metals precipitate. Thereforethe shapes of the curves for Zn, Cu and Pb are as shown.For Ca, the pH will never be so high that Ca reprecipi-tates, and no accumulation will occur. For remediationef® ciency, the mobilization of Ca decreases the ef® ciencyregarding the heavy metal. The optimal situation consistsin maintaining Ca immobile as CaCO3 , as shown later inthis paper.

It is important to measure pH during electrodialyticremediation in order to discover areas of the soil where aprecipitation of released heavy metals could happen.

LIMITING CURRENT DENSITY

The electrodialytic soil remediation process is obviouslydependent on the electric current density. The higher thecurrent density the faster the ions are moving in order tocarry the current in the soil. If the current density is toohigh, the remediation is hindered by different processesoccurring in the soil and at the membrane surfaces. Dueto the large amount of clay particles, organic material, andiron and manganese oxides in the soil, the soil will havenet negatively charged surfaces. This means that the soilcan be regarded as a cation exchanger. Therefore most ofthe current through the soil will be carried by cations. Inelectrodialytic soil remediation (as shown in Figure 1) thesoil will be adjacent to an anion exchange membrane at

the anode side of the process. This means that a cationexchanger (the soil) is placed together with an anionexchanger (the membrane). In this interface between thetwo exchangers, the ions are removed in the direction ofthe electrodes according to their charges. Therefore theinterface will be depleted of ions rapidly. This can also befound from the expression for the limiting current density,Ilim:

Ilim

zFDC

d

1

tm t s

1

where z is the charge, F is Faraday’ s number, D is thediffusion coef® cient of the counter-ion in the ® lm layeradhering to the membrane, C is the bulk solution con-centration, d is the ® lm layer thickness, while tm and ts

are the transport numbers of the counter-ion transportedin the membrane and solution, respectively. In the caseof soil adjacent to the membrane, the solution is the soilliquid.

The low concentration of anions in the soil (C low), thetortuosity of the soil pores (D low) and the poor mixingin the soil (d high) all result in a low limiting currentdensity for the anion exchange membrane adjacent tothe soil. When the current exceeds this critical value, thetransport of anions in the soil to the anion exchangemembrane surface becomes insuf® cient to carry theapplied current, and water molecules are dissociatedinto H+ and OH± ions at the soil/membrane interface inorder to supplement the current. The OH- ions will passthrough the anion exchange membrane to compartmentIV (see Figure 1), whereas the H+ ions will move throughthe soil volume.

The limiting current at the cation exchange membraneshould be higher than at the anion exchange mem-brane due to a higher concentration of free cations. Thiscan also be deduced from equation (1) because the term(tm ± ts) will be smaller for cations than anions due to thehigher transport number for cations than for anions inthe soil. If the current used in the remediation cell islower than the limiting current for the cation exchangemembrane, but higher than the limiting current for theanion exchange membrane, no OH±-front will be devel-oped, whereas a certain amount of water dissociation atthe anion exchange membrane will result in a H+ -frontmoving through the soil volume, and this can be valuablefor desorption of cationic heavy metals from the soilparticles.

The following experiments verify these considerations.Four remediation experiments with different currentdensities on a soil polluted with 1300 mg/kg DM copperwere carried out. Before remediation the contaminatedsoil was mixed with distilled water to reach a water contentof 18%, and put into compartment III (see Figure 1). Theelectrodialytic remediation cell was cylindrical with asurface area of 12.5 cm2 , and the contaminated soilcompartment was 10 cm long. The 4 remediation experi-ments were run for 25 days each with current densities of0.12, 0.24, 0.4 and 0.75mA/cm2 , respectively. In allexperiments the electrolyte in compartments I, II, IV andV was 0.01 M NaNO3 adjusted to about pH 2 with HNO3 .After the experiments, the soil was cut in 6 slices of equalthickness. pH and copper content was measured in each

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Figure 1. The principle of electrodialytic soil remediation.

Figure 2. The relative distribution of Ca, Zn, Cu, Pb and pH in the soilafter remediation

slice, and in Figure 3 and Figure 4 normalized Cu-contentand pH, respectively, are given as a function of the distancefrom the cation exchange membrane 2 for the fourexperiments.

It is clearly seen that copper has moved towards thecathode, and that a higher current density means a fasterremediation of Cu. The copper content in the slices closestto the anode can be regarded as clean (under the limitingvalues given by the Danish EPA of 200mg/kg DM). ThepH has been lowered at the anode side of the soil in allexperiments, which indicates that the limiting currentdensity for the soil/anion exchange membrane interfacehas been exceeded even at the lowest current density. Theaccumulation of Cu in the direction of the cathode due topH differences is seen as in Figure 2. For the highest currentdensity the concentration of Cu is triple the originalconcentration in the slice closest to the cathode. Thisindicates that Cu precipitates in this case in larger amountsthan at the 3 other current densities. The pH has in factincreased in comparison to the original pH of the soil, andthis must be due to OH± production in the cathode side ofthe soil. Therefore, the limiting current for the soil/cation

exchange membrane has been exceeded for this usedcurrent density of 0.75 mA/cm2 , and water has beendissociated in order to carry the current from the soil tothe cation exchange membrane surface. It was further seenduring these experiments that no Cu arrived to compart-ment II in the experiment with 0.75mA/cm2 . The powerconsumption per cleaned soil volume for the 4 experimentswere (beginning with the lowest current density) 0.14Wh/cm3 , 0.25 Wh/cm3 , 1.58Wh/cm3 and 3.72 Wh/cm3 . Thisindicates that for the experiment with the highest currentdensity, the voltage drop across the soil was very high dueto the Cu precipitation zone. For this soil and under theconditions used the optimal current density would be inthe order of 0.4 mA/cm2 to allow a compromise betweenremediation time and power consumption.

ADDING COMPLEXING AGENTS

The ef® ciency of electrodialytic remediation of soil couldbe affected by some soil and pollution characteristics.This could be when the heavy metals either are sorbedstrongly to the soil particles or precipitated as nearlyinsoluble salts, when the speciation of the heavy metals isinexpedient for electrodialytic remediation, or when thereis a high amount of harmless ions in the soil as in the casewhen a large amount of CaCO3 is present. In these casesit could be necessary to add desorbing or complexingagents to the soil in order to desorb the heavy metals, andto extract selectively heavy metals from the soil particlesinto the soil solution.

Two laboratory remediation experiments to show thenecessity to add complexing agents were carried out withhighly Cu-polluted Danish soil (20 g/kg dry soil) rich incarbonate (11%) (Ottosen et al.7 ). In one experiment thesoil was pretreated with acid (1 M HCl) in order to eliminatethe buffering capacity of the soil. The electrolyte solutionswere 0.01 M NaNO3 adjusted to about pH 2 with HNO3 .In the second experiment the soil was pretreated with2.5% NH3 and the electrolyte solutions were 2.5% NH3

media. The total current passed through the soil was inboth cases 0.15 equivalents.The soil was cut into slices, andthe Cu-content in each slice was measured. Figure 5 shows

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Figure 3. The relative Cu concentration in the soil after remediation usingdifferent current densities.

Figure 4. pH in the soil after remediation using different current densities.Figure 5. Normalized Cu pro® les in the soil at the end of the tworemediation experiments7.

the normalized Cu-content as a function of the distancefrom cation exchange membrane 2.

From Figure 5 is seen that Cu only has been removedfrom the soil slice closest to the anode in the case whenHCl was added to the soil. Thus the remediation occursvery slowly. Here the current is mainly carried by Ca2 + andH+ ions similar to Figure 2. Similar accumulating effectdue to pH difference through the soil volume is seen tohappen. When ammonia is added to the soil, Cu andammonia will form [Cu(NH3 )4 )]

2 + complexes. Thesecomplexes are charged and will thus move in the appliedelectric ® eld. When NH3 is added to the soil, pH willincrease and carbonate will not be dissolved. Furthermore,the remediation is clearly improved with NH3 comparedto acid addition. No accumulation of Cu is seen, becauseno pH jump is created. When dissolved or desorbed fromthe soil particles Cu is transported completely out of thesoil. The reduction in Cu content in the half of the soilvolume closest to the anode is around 93%. If furthercurrent was passed through the soil more Cu would havebeen removed.

Figure 6 shows the electrical resistance over the reme-diation cell for the two cases as a function of remediationtime7 . It is seen that in the case of ammonia additionthe resistance is much lower than with acid addition. Thisis due to higher amount of mobile ions throughout thewhole soil volume. So ammonia addition will both give amore ef® cient Cu removal with electrodialytic remediationtogether with a lower power consumption.

DISCUSSIONÐ SCALING UP

The effects of the three operating parameters on theperformance of a electrodialytic soil remediation processenable the conditions for large scale remediation to beestimated. Of course the practical adjustment of some ofthe parameters could be dif® cult for large scale processes.Especially, the addition of complexing agent should bedone in the most practical way. An addition of complexingagent could be done by pouring the liquid over the soiltogether with a pumping of liquid through the bottom ofthe soil volume. Electroosmotic addition of liquid fromthe anode side of the remediation process would be analternative solution.

The pH is dif® cult to adjust during remediation in a

homogenous way, so pH should be followed carefully.Changes in pH with time could be monitored and pH couldbe optimized for the remediation by addition of acid orbase depending on the heavy metal involved.

The electric current density for large scale electro-dialytic soil remediation should be chosen from smallerlaboratory experiments carried out with the same soil. It isexpected that the limiting current would be the same insmall and large scale remediation. Most convenientlythe electric current density should be kept below thelimiting current density. On the other hand, too lowelectric current densities would result in large remediationtimes. If pH is measured continuously in the soil closestto the cation exchange membrane, indication of exceedingthe limiting current for the membrane/soil interface couldbe observed.

The distance between the electrode units in large scaleremediation should also be considered. A large distancewould correspond to fewer electrode units per volume ofsoil, but also to longer remediation times. If the currentdensity is kept below the limiting current density for thesoil/cation exchange membrane interface, the electricalresistance over the soil volume would be equally distri-buted. If the limiting current density is exceeded, the mainfraction of the ohmic resistance would be due to the soilslice closest to the cathode.

CONCLUSIONS

When scaling up electrodialytic soil remediation toactual process scale the three key parameter discussed inthis paper must be considered.

The pH is of crucial importance for the desorption ofheavy metals from the soil. Furthermore, the pH affectsthe mobility and the speciation of heavy metals.

The current density must be kept below the limitingcurrent density for the soil cation exchange membraneinterface to avoid production of hydroxide ions in the soilby water splitting. The hydroxide ions could stop themigration of heavy metals by precipitation and this part ofthe soil would have a high electrical resistance.

Addition of desorbing/complexing agents to the soil isnecessary in situations where

(1) the heavy metals are strongly sorbed to the soil,(2) a speci® c soil composition complicates the reme-diation, and(3) the speciation of the heavy metals is inappropiatefor electrodialytic soil remediation.

REFERENCES

1. Hansen, H. K., Ottosen, L. M., Laursen, S. and Villumsen, A., 1997,Electrochemical analysis of ion-exchange membranes with respect toa possible use in electrodialytic decontamination of soil polluted withheavy metals, Sep Sci Technol, 32(15): 2425±2444.

2. Ottosen, L. M., Hansen, H. K., Laursen, S. and Villumsen, A., 1997,Electrokinetic remediation of soil polluted with copper from woodpreservation industry, Environ Sci Technol, 31: 1711±1715.

3. Hansen, H. K., Ottosen, L. M., Kliem, B. K. and Villumsen, A., 1997,Electrodialytic remediation of soils polluted with Cu, Cr, Hg, Pb andZn, J Chem Technol Biotechnol, 70: 67±73.

4. Hansen, H. K., Ottosen, L. M., Hansen, L., Kliem, B. K., Villumsen, A.and Bech-Nielsen, G., 1998, Electrodialytic soil remediation. Reviewof important parameters, Submitted.

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Figure 6. Changes in resistance during the remediation experiments7.

5. Pourbaix, M., 1967, Atlas of electrochemical equilibria in aqueoussolutions, Advances in Chemistry Series 67, (American ChemicalSociety, Washington, DC).

6. Alloway, B. J., 1995, Heavy Metals in Soils, (Blackie Academic &Professional, Chapman & Hall, London, UK).

7. Ottosen, L. M., Hansen, H. K., Hansen, L., Kliem, B. K., Bech-Nielsen,G., Pettersen, B. and Villumsen, A., 1998, Electrodialytic soilremediationÐ Improved conditions and acceleration of the processby addition of desorbing agents to the soil, Contaminated Soil ’ 98,Proc Sixth Int FZK/TNO Conf on Contaminated Soil, 17± 21 May 1998,Edinburgh, UK, (Thomas Telford, London, UK) pp. 471±478.

ADDRESS

Correspondence concerning this paper should be addressed toDr H. K. Hansen, Department of Geology and Geotechnical Engineering,The Technical University of Denmark, Building 204, DK-2800 Lyngby,Denmark.

This paper was ® rst presented at the 5th European Symposium onElectrochemical Engineering, held at Exeter, UK, 24± 26 March 1999. Theproceedings of the conference are published in the IChemE SymposiumSeries, No 145.

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