SEPARATIONS
Decontamination of Soils by Membrane Processes: Characterization
of Membranes under Working Conditions
R. de Lara,†J. Rodrı´guez-Postigo,†F. Garcı´a-Herruzo,‡ J. M. Rodrı´guez-Maroto,‡and J. Benavente*,†
Departamento de Fı´sica Aplicada I and Departamento de Ingenierı´a Quı´mica, Facultad de Ciencias, Universidad de Ma´laga, 29071 Ma´laga, Spain
An electrodialytic soil remediation process was applied for the removal of cadmium (1000 mg/kg of dry soil) from spiked kaolin. The method consists of the application of a dc electric current to the soil placed between ion-exchanger membranes, which allows for the separation of the soil from electrode solutions. Characterization of cationic and anionic membranes was done by measuring the membrane potential and electrical resistance under operating conditions. Experimental results and modeling indicate that both membranes work close to ideal conditions (t+≈1), although a small fraction of the H+ions (tH+membrane≈0.13tH+solution) is allowed to circulate through the anionic membrane and less than 1% of OH-ions pass through the cationic one. A model for ion transport involved in the process across both ion-exchanger membranes used in soil remediation was also developed, and good agreement was obtained when compared with the experimental results. Cadmium analysis of different sections of the soil column showed a high removal from the zones closer to the anode and a significant accumulation in the vicinity of the cathode.
Introduction
Electrodialytic decontamination of soils polluted by heavy metals is a technique under development,1-6 and it might be considered as an alternative to other cleaning procedures in the case of low-permeability soils. The method consists of the use of a dc current in com-bination with ion-exchange membranes, in a way simi-lar to that operating in conventional electrodialysis processes.7,8 The polluted soil is placed between one cation-exchange membrane and an anion-exchange membrane, and the current drives heavy metals (cations and anions) from the soil to the external solutions. To avoid the presence of certain undesirable species from the soil that could affect the electrodes, another pair of ion-exchange membranes can eventually be used to isolate the electrode compartments. To date, several different kinds of metals such as Cu, Cd, Cr, Pb, Hg, and Zn have been removed from polluted soil by the electrodialytic technique.9-16
Chemical and electrical conditions during soil reme-diation can be rather different from those usually existing in a typical membrane separation process, and frequently, manufacturer’s specifications do not provide complete and useful information about some important parameters of the process. For this reason, character-ization of the membranes under working conditions must be performed. Specifically, characterization of the membranes in contact with solutions similar to those
existing during operation in soil remediation is carried out. Among other key parameters, it is necessary to consider the transport of heavy metals through the membranes, the combination of both electrical potential and concentration gradients, the electrical resistance of the membranes, and the effect of pH on both the chem-ical nature of membranes and the different transport parameters.
In this work, the electrodialytic decontamination of a carbonated soil polluted with Cd2+ions is considered.
For that reason, cationic and anionic exchange branes have been characterized measuring the mem-brane potential and the electrical resistance with the membranes in contact with aqueous CdCl2solutions. As a result of the electrode reactions, H+and OH-ions can be produced, which can move through the soil (acidic and basic fronts), thereby affecting the deposition/ desorption of charged species. Therefore, the effect of salt concentration and pH on membrane parameters (mainly ion transport numbers and electrical resistance) must also be considered. A model for the transport of ions (Cd2+, H+, Cl-, and OH-) across both ion-exchange
membranes under working conditions in soil remedia-tion (simultaneous applicaremedia-tion of electrical and concen-tration gradients) is also presented.
Experimental Section
Characterization of Membranes.Two commercial ion-exchange membranes, a cation exchanger CR67-HMR-402 (C-Ex) and an anion exchanger AR204-SZRA-412 (A-Ex), supplied by Ionics Iberica were studied. The membrane characteristic parameters given by the sup-plier are summarized in Table 1.
* To whom correspondence should be addressed. Fax: +34 952132000. E-mail: [email protected].
†Departamento de Fı´sica Aplicada I. ‡Departamento de Ingenierı´a Quı´mica.
400 Ind. Eng. Chem. Res.2005,44,400 407
10.1021/ie040202u CCC: $30.25 © 2005 American Chemical Society Published on Web 12/23/2004
Electrochemical measurements were carried out with the membrane samples in contact with aqueous CdCl2 solutions at different concentrations and a temperature of 25.0(0.5 °C. To verify the reliability of the results, some measurements were also made with NaCl solu-tions, and the corresponding values were compared with those indicated in Table 1.
Membrane potential and electrical resistance mea-surements were carried out in a test cell similar to that described elsewhere.17 The membranes were tightly clamped between two glass half-cells with a free area of 0.64 cm2 by using silicone rubber rings. The whole system was introduced into a thermostatic bath. To detect possible concentration polarization at the mem-brane surfaces,18a magnetic stirrer was placed at the bottom of each half-cell, and its stirring rate (ω) was externally controlled; measurements were carried out at two stirring rates: ω)0 andω)525 rpm.
The electromotive force (∆E) between the two sides of the membranes caused by a concentration gradient was measured by using two reversible Ag/AgCl elec-trodes connected to a digital voltmeter (Yokohama 7552, 1-GΩinput resistance). Measurements were carried out keeping the concentration in the anode compartment,
C1, constant (C1)0.01 M) and gradually changing the concentration in the cathode compartment,C2, fromC2 )10-3M toC
2)0.1 M.19
The electrical resistance of the membrane system was also determined using an ac bridge (Wayne Kerr, PCA 6425) for 40 different frequencies ranging between 100 Hz and 300 kHz. Measurements were carried out at two given concentrations, 0.01 and 0.05 M, with the mem-branes placed in the membrane holder (Rem) and without the membrane (solution resistance, Re). The effect of pH was considered by measuring the electrical resistance of both membranes at a given concentration (0.01 M NaCl) but different pH’s: 2.3, 3.5, 6.0, 7.5, and 9.1.
Transport of H+ Ions across the Anion-Ex-changer Membrane. The transport of H+ ion was measured with a cell similar to that indicated above, but in this case, the membrane area and volume were 1.54 cm2 and 20 cm3, respectively, for each half-cell. HNO3and NaNO3solutions of the same molar concen-trations were placed at each side of the membrane, and the variation of pH in the solution of NaNO3 was determined as a function of time. Measurements were
carried out at five different concentrations of HNO3: 5 × 10-2, 3.5 × 10-2, 2.5 × 10-2, 1× 10-2, and 5 × 10-3 M.
Transport of Cd2+, H+, OH-, and Cl-Ions under Concentration and Electrical Potential Gradients. Graphite electrodes connected to the dc source (ISO-Tech IPS601A) with a maximum voltage of 60 V were used in the study of ion transport (Cd2+, H+, OH-, and
Cl-) under dynamic conditions. The initial concentra-tions of CdCl2 solutions on the two sides of the mem-brane were 1 and 1000 ppm, and the current density was 10 mA/cm2. The solution concentrations were determined at different time instances until 170 min had elapsed. Measurements were carried out with C-Ex and A-Ex membranes for both opposite external condi-tions, meaning that the electrical potential and the concentration gradient were acting either with or against one another. No decrease in the membrane efficiency was observed during the experiments, even after several uses.
Soil and Electrodecontamination Procedure. The soil used for these experiments was a mixture of 95% commercial kaolin (100%) and 5% sodium bicar-bonate. Kaolin was used because it has a low perme-ability, a negligible organic content, and a low cation-exchange capacity. Sodium bicarbonate, which acts a buffer, was added to simulate a natural basic soil, which is quite difficult to clean. The particle size distributions together with other properties such as hydraulic con-ductivity are listed in Table 2. The electroosmotic flow was found to be almost negligible because of the presence of the membranes. Thus, no effort was made to characterize the soilζpotential.
A schematic diagram of the cell used in this study is shown in Figure 1. The system consists of a glass cell
with two electrode compartments, each of 0.64 cm2 cross-sectional area (previously described), and a tube of the same cross-sectional area and 12-cm length that contains the polluted soil. A solution of NaCl (1 M) was used as the electrolyte in both electrode compartments. The initially neutral pH of the solution changes along the process because the electrolysis of water occurs at the electrodes according to the following reactions
After the anode and cathode reservoirs had been filled with the indicated solution, a dc constant voltage of 60
Table 1. Characteristics of the Membranes
membrane property C-Ex A-Ex
specific weight (mg/cm2) 13,7 13,7
thickness (µm) 560-580 500 water content of
wet resin only (%)
46 46
capacity (mequiv/g of dry resin)
2.10 (minimum) 2.40 (minimum)
average transport number (0.01-1.0 M NaCl)
tNa+)0.94(0.05 tCl-)0.96(0.04
Table 2. Properties of the Soil Tested
particle size distribution (%)
clay (e0.002 mm) 80
silt (0.02-0.002 mm) 16
sand (2-0.02 mm) 4
density (g/cm3) 2.36
water content (%, w/w, dry basis) 35 hydraulic conductivity (cm‚s-1) 1.3×10-8
organic matter (%) exempt cation-exchange capacity (mequiv/100 g) 1.5-2.0
pH 4.7(0.2
Figure 1. Schematic representation of the cell for laboratory-scale decontamination.
At the anode: 2H2OfO2(g)+4H
++
4e
-At the cathode: 4H2O+4e
-f2H2(g)+4OH
-V was applied. This potential gradient (5 -V/cm) was used to obtain an experimental current density in the range of the most typical operating conditions used by other authors20-22(0.1-1 mA/cm2). A multimeter was used to measure the voltage and current flow through the soil during the experiments. The membranes were placed between the electrode compartments and the contaminated soil.
Sample Handling and Analysis.The experiments were carried out using kaolinite as a model of a low-permeability material. For all experiments, the com-mercial kaolin was spiked for 24 h with a solution of CdCl2‚2.5H2O (99.7%), to reach a final concentration of cadmium in the soil of 1000 mg of Cd2+/kg of dry soil.
Then, the water phase was removed by heating at 80 °C for 48 h. The solid concentration of aliquots of spiked kaolin was analyzed by acid digestion followed by atomic absorption spectrometry (AAS). Then, the soil was thoroughly mixed with the desired amount of deionized water to obtain a moisture of 35% (w/w, dry basis) in order to operate close to saturation. Afterward, the mixture was placed into the cell, and once the soil was fully packed, the cell assembly was completed. Finally, the cathode and anode compartments were filled with the electrolytic solution.
During the test operation, the electrical current and the pH were measured periodically ,and samples of the compartments were obtained to measure the metal concentration by AAS (Perkin-Elmer 3100).
At the end of the experiments, the soil was extruded from the cell and divided into 12 slices to study the spatial distribution of the residual metal concentration, as well as that of the pH value. For the chemical analysis of the soil, the Cd2+
was extracted using nitric acid (65%) under reflux conditions for at least 24 h to complete the extraction. After extraction, the samples were centrifuged (Heraeus Sepatech, model Labofuge AE) at 5000 rpm for 5 min. The supernatant was analyzed using atomic absorption spectrometry. The soil pH was measured using a ratio 1 M KCl/soil of 2.5 (w/ w) with a Crison model Basic 20 pH meter.
Results and Discussion
Characterization of the Membranes. Measured electrical potential differences at both sides of the studied ion-exchange membranes for CdCl2 and NaCl solutions at two stirring rates are shown in Figure 2. For comparison, values of the electrical potential for ideal cation-exchange (t+ ) 1) and anion-exchange (t-)1) membranes are also included in Figure 2 (solid and dashed lines, respectively). As can be observed, the experimental points differ slightly from the theoretical lines, and only small deviations from ideal behavior were found at low concentrations. Cation and anion transport numbers in the membranes were determined from the experimental values using the equation23
where R and F are the gas and Faraday constants, respectively;Tis the thermodynamic temperature of the system;νiis the stoichiometric coefficient of ioni(i) +or-);ν)ν++ν-; andaiis the activity coefficient of
NaCl in solutioni, withi)1 (anode compartment) and 2 (cathode compartment) . t+ is the cation transport
number, which represents the fraction of the total current carried by the cations (t++t-)1).
Values for ion transport numbers in the two ion-exchange membranes are reported in Table 3. As can be observed, very good agreement between the values obtained with NaCl solution and those reported in Table 1 exists. These results indicate that the A-Ex and C-Ex membranes can be considered practically ideal for both solutions in the range of concentrations studied, in agreement with results reported in the literature for charged membranes.23,24
The effect of the solution pH on the transport number was also considered, and Figure 3 shows the variation of the measured electrical potential difference (∆E) with pH for the C-Ex membrane at two given concentration ratios (C1/C2 ) 0.5 and C1/C2 )2). An average value for∆Ein the whole range of pH (between 4 and 10) can be determined, and the following transport numbers were obtained
∆E) - ν
νi RT
Ft+ln
(
a1a2
)
(1)Figure 2. Membrane potentials of NaCl and CdCl2solutions at
two stirring rates. Solid line: C-Ex,2,ω)0 rpm;4,ω)525
rpm. Dashed line: A-Ex,9,ω)0 rpm;0,ω)525 rpm.
Table 3. Average Transport Numbersaof Ion-Exchange
Membranes
t+(CdCl2) t+(NaCl)
membrane
ω)
0 rpm
ω)
525 rpm
ω)
0 rpm
ω)
525 rpm
C-Ex 1.00(0.06 1.05(0.09 0.91(0.06 0.93(0.04 A-Ex 0.08(0.06 0.09(0.06 0.09(0.06 0.09(0.06
aErrors are standard deviations of at least three experiments
using the same piece of membrane.
C1/C2)0.5: t+)0.94(0.09 forω)0 rpm
Figure 4 shows the electrical resistance of the mem-brane system (Rem) versus frequency for both mem-branes in contact with CdCl2and NaCl solutions. Clear differences in Rem values depending on both the salt concentration and the electrolyte can be observed. The values also depend on the type of membrane, with higher values being obtained for the anion-exchange membrane. The effect of solution pH onRemwas also considered, and only small differences in the electrical resistance were obtained, which are attributable to the modification of the solution conductivity. Moreover, differences between the membrane electrical resistances and the solution resistances were in the range of 4-6%, so the extra energy consumption due to the presence of the membrane in the cell used in electrodialytic soil remediation can be neglected.
Transport of H+Ions across the Anion-Exchange Membrane. The movement of protons through the anion-exchange membrane in absence of electrical cur-rent was verified by means of a set of five experiments at the experimental conditions indicated in the Soil and Electrodecontamination Procedure paragraph.
When the current is zero, the H+ concentration gradient between the two compartments produces the
movement of H+through the membrane. This is rep-resented by the equation
where Kis the transport coefficient (cm‚s-1),V is the solution volume of the half-compartment (cm3), and S is the membrane surface area in contact with the solutions (cm2). [H+]
Noand [H+]Naare the concentrations of protons (mol‚cm-3) measured in the solutions of HNO
3 and NaNO3, respectively.
The transport coefficients obtained by fitting these experiments were used to simulate the evolution of the pH in the NaNO3solution. As can be seen in Figure 5, the agreement between the experimental and modeled pH values is good. For the five tests performed, the adjustment was similar to the one shown in Figure 5. This fact indicates the transport of H+ through the anionic membrane. Nevertheless, there is a significant retention of H+on this membrane, whereas almost no OH- circulates through the cationic one. Therefore, movement of protons across the soil should be expected. Figure 6 shows the change in the value of the effective transport coefficient with changing HNO3concentration. Figure 3. Membrane potential versus pH for C-Ex membrane.
Positive values: C1/C2)0.5. Negative values: C1/c2)2.0,ω)
525 rpm;2,ω)0 rpm.
C1/C2)2: t+)0.87(0.06 forω)0 rpm
t+)0.86(0.07 forω)525 rpm
Figure 4. Electrical resistance versus frequency for both mem-branes with two concentrations of NaCl and CdCl2.
Figure 5. Experimental and model results for pH in the NaNO3
solution with 5×10-2M HNO 3.
Figure 6. Relation of transport coefficients to HNO3
concentra-tion.
Vd[H
+
]Na
dt )KS([H
+
]No-[H+]Na)
∴ d[H
+
]Na
([H+]No-[H+]Na)
)KS
As can be seen in this figure, the relation between the two parameters is essentially linear, and as observed, the five experiments show a reduction of the transport as the initial acid concentration decreases.
Model for the Transport of Cd2+, Cl-, H+, and OH-Ions through Ion-Exchange Membranes un-der Dynamic Conditions.A one-dimensional model was developed for simulating the transport of Cd2+, Cl-,
H+, and OH-ions through cation- and anion-exchange membranes. It was assumed that the ions move from aqueous solution when a dc current is applied to the system. Then, the evolution of the chemical species in the anolyte and the catholyte is considered by (a) a mass balance for each ion, (b) transport numbers for each ionic species through the ion-exchange membranes, (c) chemical equilibrium.
Moreover, it was also assumed that the only factor to be considered is the electrokinetic migration across the system and the fact that there are neither complexes nor chemical species different from those previously mentioned. With these assumptions, the equations describing the model are as follows:
(i) The mass balance for the ith ion in volume Vjis
given by
whereVjis the volume of solution in electrode compart-mentj(anodic or cathodic, cm3),C
ijis the concentration
of theith ion in thejth compartment (mol‚cm-3),tiis the transport number of theith chemical species,Iis the current passing through the system (A), zi is the charge of the ith ion, F is the Faraday number (C‚equiv-1), andGijis the rate of production of theith ion by electrochemical (oxidation/reduction) reactions (mol‚s-1).
Table 4 provides a summary of the equations for each ion involved in the transport.
(ii) The transport numbers for cationic and anionic species through the membranes were calculated as
whereui+andui-are the electrochemical mobilities of
the ith cation and anion, respectively; [Ciz+] an and [Ciz-]
catare the concentrations of the cations inside the anode cell and the anions inside the cathode cell, respectively; fi is the ratio between the product uiCi
inside the membrane and in the solution; and ∑ represents the sum of the electrical transport of all ions through the membrane.
(iii) Moreover, it is necessary to consider the chemical equilibrium for both cadmium hydroxide precipitation and water ionization
Therefore, the model for simulating the chemical species present inside the two cells consists of eight differential equations. These equations were integrated forward in time by a fourth-order Runge-Kutta algo-rithm. The results of the model were compared with the experimental results with the object of determining the value of the effective mobility through the membranes. To solve these equations, the calculations were done with the kinetic processes, electromigration, and elec-trochemical reactions, which are considered instanta-neous, separately from the chemical equilibria, in a similar fashion as was done elsewhere.25In this way, for each increment of time, we calculate first the change of the concentrations of the species by kinetic processes and then the chemical equilibria (without changes in the time).
Figure 7 shows the experimental results and the model predictions for C-Ex membrane for different concentrations of Cd2+
in the electrode compartments. As can be seen, the model compares adequately with the experimental results and predicts the significant decrease of Cd2+in the more concentrated electrolyte, which, in the first case (Figure 7a), is principally due to its reduction in the cathode compartment, as shown by the following reaction
This reaction prevents the reduction of water and the corresponding generation of hydroxyl ions in the cathode cell. This effect, together with H+transport across the membrane, permits acidic pH to be reached simulta-neously in both electrode compartments.
On the other hand (Figure 7b), for a higher concen-tration of cadmium in the anode compartment, a decrease of this value was observed as a result of cadmium transport across the membrane. However, the cadmium concentration in the catholyte does not in-crease because subsequent precipitation in this cell occurs because of the basic pH value resulting for the reduction of water.
Table 4. Model Equations for the Ions Involved on the Process
Vd[Ci]cat
dt V
d[Ci]an dt i
[Cd2+]
- 1
zCd2+F
[
(
[Cd2+]cat KS+[Cd2+]cat
)
-t
Cd2+
]
-ItCd2+
zCd2+F (4)
[H+] ItH+
zH+F
I(1-tH+)
zH+F (5)
[OH-] I
zOH-F
[
1-
(
[Cd2+
]cat KS+[Cd2+]cat
)
-t
OH-
]
ItOH
-zOH-F
(6)
[Cl-] - ItCl
-zCl-F
ItCl
-zCl-F
(7)
VjdCij
dt ) tiI ziF
+G
ij (3)
Transport numbers for cations
[ti+]m)
fi+zi+ui+[Ci z+
]an
∑
i)1 2fi+zi+ui+[Ci z+
]an+
∑
i)1 2
fi-zi-ui-[Ci z
-]cat (8)
Transport numbers for the anions
[ti-]m)
fi-zi-ui-[Ci z
-]cat
∑
i)1 2fi+zi+ui+[Ci z+
]an+
∑
i)1 2
fi-zi-ui-[Ci z
-]cat (9)
Cd2++2 OH-aCd(OH)2(S) KS)[Cd
2+
][OH-]2)2×10-14 (10)
H2OaH++OH- KW)[H+][OH-])10-14 (11)
In Figure 8 is depicted a case using a higher cadmium concentration in the catholyte and A-Ex membrane. The same reduction in cadmium concentration is observed, initially accompanied by an acidic pH in the cathode cell because of H+ transport across A-Ex membrane. Despite this, the Cd2+
ion reduction suffers a competi-tion for water reduccompeti-tion particularly when the cadmium concentration decreases below 10 mg/L. At this point, a significant increase in the hydroxyl concentration occurs, and the pH in the cathode reservoir quickly increases (Figure 8). Recall that, although the protons are transported through both types of membranes, the
transport rate for protons across the anionic-exchange membrane is lower than that across the cationic-exchange membrane.
The values of ratio fi giving the best fit to the experimental results are included in Table 5 along with the electrochemical mobilities in free solution. These results confirm the essentially ideal behavior of both membranes and the fact that a significant number of protons can cross through the anionic membrane, in agreement with the results reported above.
Soil Electrodecontamination Results.The previ-ous results were applied to the study of the electrodia-lytic decontamination of a column of soil spiked with cadmium (1000 mg/kg). A decrease of the electrical current density from 5 to 0.5 mA/cm2occurs during the process close to the end of the experiment. This decrease can be attributed to the increase of the electrical resistance in the system associated with the decrease in the concentration of free ions in solution in some sections of the soil. The strong reduction in the elec-troosmotic flux when the ion-exchange membranes are present in the cell should also be pointed out, but a value of 0.15 L/m2‚h for this flux was obtained, under similar external conditions, during decontamination without membranes. Therefore, the movement of water in the soil is restricted by the ion-exchange membranes so that electroosmosis is of minor importance during the electrodialytic remediation process. This fact indicates that the use of membranes not only limits the amount of applied current wasted in carrying undesirable ions between the electrode compartments, but also clearly reduces the electroosmosis, one of the mechanisms for ion transport.
Figure 9 shows the evolution of the pH and cumula-tive mass of cadmium collected in the cathode reservoir. As can be expected, after 1 h of operation, the pH’s in the cathodic and anodic solutions reach values of 11 and 3.5, respectively. As observed in this figure, the cad-mium mass in the cathode increases linearly along the process, indicating that the cadmium is continuously removed from the soil.
After the electrokinetic experiment was stopped, the pH values and the residual concentrations of cadmium in the different portions of soil were measured. pH
Figure 7. Experimental results and model prediction for C-Ex. (a) Catholyte, 1000 mg/L Cd2+; anolyte, 1 mg/L Cd2+. b) Anolyte, 1000 mg/L Cd2+; catholyte, 1 mg/L Cd2+.
Figure 8. Experimental results and model prediction for A-Ex. Catholyte, 1000 mg/L Cd2+; anolyte, 1 mg/L Cd2+.
Figure 9. Cadmium collected in cathodic compartment and evolution of pH.0, pHcat;4, pHan;2, Cdan;9, Cdcat
Table 5. Values of Ratiofiand Electrochemical
Mobilities for the Ions Transported
H+ OH- Cd2+ Cl -cation-exchange membrane,fi 1 <0.01 1 0.05
anion-exchange membrane,fi 0.13 1 0.04 1
distributions and initial and final cadmium concentra-tion profiles are depicted in Figure 10.
Very acidic pH values in the soil close to the anode were obtained (pH≈2), but the pH increased along the column to a neutral pH value at a relative distance from the anode of about 75%. This result is in agreement with the transport of protons through the anion-exchange membrane, while the hydroxyl ions cannot cross the cation-exchange membrane or do so in a very small amount.
The cadmium analysis of the 12 slices obtained from the soil column shows that those closer to the anode are essentially free of contamination, which indicates the success of electrodialytic operation. However, there is an appreciable accumulation of cadmium in the proxim-ity of the cathode, because of precipitation due to the pH value (∼6.5) as a consequence of the bicarbonates remaining in this region of the column. All of these observations clearly show the progress of the decon-tamination and acidic fronts, as well as the necessity of a longer decontamination time to complete elimina-tion of the metal.
Conclusions
The use of ion-exchange membranes in soil electrore-mediation avoids the migration of undesirable species toward the soil while the ions are transported from the soil into the electrode compartments and only a very limited amount of the applied current is wasted in carrying undesirable ions. Although the anionic mem-brane has a positive charge, a small fraction of the transport of the protons from the anionic compartment into the soil is allowed. No other cations are observed to move through this membrane. The transport number for protons through the anionic membrane was calcu-lated as 13% of the transport number for a free solution. Meanwhile, the cationic membrane acts essentially as an ideal rectifier for all hydroxyl anions present, avoid-ing the eventual presence of a basic front in the soil. Therefore, the pH inside the column evolves from pH )6.5 toward acidic values (pH)2), with an acid front that migrates from the anode to the cathode. As a consequence, if the remediation is not performed for enough time, Cd accumulates close to the cathode, where the pH remains at 6.5.
Extra energy consumption due to the presence of membranes in electrodialytic soil remediation can be
neglected because of their high relative conductivity under operating conditions. The membranes restrict the movement of water in the soil, diminishing the elec-troosmotic contribution to the total transport of con-taminants. The removal of cadmium by electrodialytic remediation progresses adequately. After the experi-mental time of 10 h, 90% Cd removal was achieved on the half of the column closer to the anode, whereas the overall removal was only 17%, as a consequence of accumulation due to precipitation induced by the pres-ence of bicarbonate. Therefore, longer times would be necessary to obtain a complete cleaning of this type of soil.
Acknowledgment
We thank Ionic Iberica S.A. for generously supplying the membranes used in this work. The authors are also grateful to the reviewers for their useful comments.
Nomenclature
ai)activity coefficient of NaCl solution
Cij)concentration of theith ion in thejth compartment
(mol‚cm-3)
[Ciz+]an )concentration of the cations inside the anode
(mol‚cm-3)
[Ciz-]cat)concentration of the anions inside the cathode
(mol‚cm-3)
∆E)electrical potential difference (V) F)Faraday constant (C‚equiv-1)
fi+)ratio of the productuiCiinside the cationic membrane
to that in the solution
fi-)ratio of the productuiCiinside the anionic membrane
to that in the solution
Gij)rate of production of theith ion by electrochemical
reactions (mol‚s-1)
[H+]No ) concentration of protons in HNO3 solutions
(mol‚cm-3)
[H+]Na ) concentration of protons in NaNO3 solutions
(mol‚cm-3)
I)current passing through the system (A) K)transport coefficient (cm‚s-1)
R)gas constant (J‚mol-1‚K-1)
Rem )electrical resistance of the system with the
mem-branes (Ω)
Re)electrical resistance of the solution (Ω)
S)membrane surface area (cm2)
ti)transport number of theith chemical species
t+)cation transport number
t-)anion transport number
ui+)electrochemical mobility of theith cation (cm2‚V-1‚s-1)
ui-)electrochemical mobility of theith anion (cm2‚V-1‚s-1)
V)solution volume of a half-compartment (cm3)
Vj)volume of solution in electrode compartmentj(anodic
or cathodic) (cm3)
zi+)charge of theith cation (equiv‚mol-1)
zi-)charge of theith anion (equiv‚mol-1)
νi)stoichiometric coefficient of ion (i) +or-) ω)stirring rate (rpm)
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Figure 10. Cadmium and pH distributions in the soil for electrodialytic experiment.
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Received for review July 22, 2004 Revised manuscript received October 22, 2004 Accepted November 3, 2004 IE040202U
