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Ind. Eng. Chem. Res. 2005, 44, 400-407
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 combination with ion-exchange membranes, in a way similar 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 remediation 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, characterization of the membranes under working conditions must be performed. Specifically, characterization of the membranes in contact with solutions similar to those * 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.
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 chemical 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 membranes have been characterized measuring the membrane potential and the electrical resistance with the membranes in contact with aqueous CdCl2 solutions. 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 remediation (simultaneous application of electrical and concentration gradients) is also presented. Experimental Section Characterization of Membranes. Two commercial ion-exchange membranes, a cation exchanger CR67HMR-402 (C-Ex) and an anion exchanger AR204-SZRA412 (A-Ex), supplied by Ionics Iberica were studied. The membrane characteristic parameters given by the supplier are summarized in Table 1.
10.1021/ie040202u CCC: $30.25 © 2005 American Chemical Society Published on Web 12/23/2004
Ind. Eng. Chem. Res., Vol. 44, No. 2, 2005 401 Table 1. Characteristics of the Membranes membrane property specific weight (mg/cm2) thickness (µm) water content of wet resin only (%) capacity (mequiv/g of dry resin) average transport number (0.01-1.0 M NaCl)
C-Ex
A-Ex
13,7 560-580 46
13,7 500 46
2.10 (minimum)
2.40 (minimum)
tNa+ ) 0.94 ( 0.05 tCl- ) 0.96 ( 0.04
Table 2. Properties of the Soil Tested particle size distribution (%) clay (e0.002 mm) silt (0.02-0.002 mm) sand (2-0.02 mm) density (g/cm3) water content (%, w/w, dry basis) hydraulic conductivity (cm‚s-1) organic matter (%) cation-exchange capacity (mequiv/100 g) pH
80 16 4 2.36 35 1.3 × 10-8 exempt 1.5-2.0 4.7 ( 0.2
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 solutions, and the corresponding values were compared with those indicated in Table 1. Membrane potential and electrical resistance measurements 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 membrane surfaces,18 a 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 and ω ) 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 electrodes connected to a digital voltmeter (Yokohama 7552, 1-GΩ input 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, from C2 ) 10-3 M to C2 ) 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 membranes 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-Exchanger 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. HNO3 and NaNO3 solutions of the same molar concentrations 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 (ISOTech 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 concentrations of CdCl2 solutions on the two sides of the membrane 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 conditions, 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 bicarbonate. Kaolin was used because it has a low permeability, a negligible organic content, and a low cationexchange 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 conductivity 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 soil ζ potential. A schematic diagram of the cell used in this study is shown in Figure 1. The system consists of a glass cell
Figure 1. Schematic representation of the cell for laboratoryscale decontamination.
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
At the anode:
2H2O f O2(g)+ 4H+ + 4e-
At the cathode: 4H2O+ 4e- f 2H2(g)+ 4OHAfter the anode and cathode reservoirs had been filled with the indicated solution, a dc constant voltage of 60
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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 lowpermeability material. For all experiments, the commercial 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
∆E ) -
()
a1 ν RT t+ ln νi F a2
(1)
where R and F are the gas and Faraday constants, respectively; T is the thermodynamic temperature of the system; νi is the stoichiometric coefficient of ion i (i ) + or -); ν ) ν+ + ν-; and ai is the activity coefficient of NaCl in solution i, with i ) 1 (anode compartment) and 2 (cathode compartment) . t+ is the cation transport
Figure 2. Membrane potentials of NaCl and CdCl2 solutions 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 Numbersa of Ion-Exchange Membranes t+(CdCl2) membrane C-Ex A-Ex
ω) 0 rpm
ω) 525 rpm
t+ (NaCl) ω) 0 rpm
ω) 525 rpm
1.00 ( 0.06 1.05 ( 0.09 0.91 ( 0.06 0.93 ( 0.04 0.08 ( 0.06 0.09 ( 0.06 0.09 ( 0.06 0.09 ( 0.06
a Errors are standard deviations of at least three experiments using the same piece of membrane.
number, which represents the fraction of the total current carried by the cations (t+ + t- ) 1). Values for ion transport numbers in the two ionexchange 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 ∆E in the whole range of pH (between 4 and 10) can be determined, and the following transport numbers were obtained
C1/C2 ) 0.5: t+ ) 0.94 ( 0.09 for ω ) 0 rpm t+ ) 0.91 ( 0.10 forω ) 525 rpm
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C1/C2 ) 2: t+ ) 0.87 ( 0.06 for ω ) 0 rpm t+ ) 0.86 ( 0.07 for ω ) 525 rpm Figure 4 shows the electrical resistance of the membrane system (Rem) versus frequency for both membranes in contact with CdCl2 and 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 on Rem was 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 current 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
Figure 5. Experimental and model results for pH in the NaNO3 solution with 5 × 10-2 M HNO3.
Figure 6. Relation of transport coefficients to HNO3 concentration.
movement of H+ through the membrane. This is represented by the equation
V
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.
Figure 4. Electrical resistance versus frequency for both membranes with two concentrations of NaCl and CdCl2.
d[H+]Na ) KS([H+]No - [H+]Na) dt ∴
d[H+]Na +
+
([H ]No - [H ]Na)
)
KS dt V
(2)
where K is 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+]No and [H+]Na are the concentrations of protons (mol‚cm-3) measured in the solutions of HNO3 and NaNO3, respectively. The transport coefficients obtained by fitting these experiments were used to simulate the evolution of the pH in the NaNO3 solution. 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 HNO3 concentration.
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Table 4. Model Equations for the Ions Involved on the Process V
i [Cd2+]
[H+]
V
d[Ci]an
dt
[(
[ti-]m )
dt
) ]
ItCd2+ zCd2+F
(4)
ItH+
I(1 - tH+)
(5)
zH+F
zH+F
-
[OH-]
1
I
-
[Cd2+]cat
zCd2+F K + [Cd2+] S cat
zOH-F [Cl-]
d[Ci]cat
[ ( 1-
[Cd2+]cat 2+
KS + [Cd ]cat
- tCd2+
) ] - tOH-
ItClzCl-F
ItOH-
(6)
zOH-F ItCl-
(7)
zCl-F
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 under 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 Vj is given by
Vj
t iI dCij ) + Gij dt ziF
(3)
where Vj is the volume of solution in electrode compartment j (anodic or cathodic, cm3), Cij is the concentration of the ith ion in the jth compartment (mol‚cm-3), ti is the transport number of the ith chemical species, I is the current passing through the system (A), zi is the charge of the ith ion, F is the Faraday number (C‚equiv-1), and Gij is the rate of production of the ith 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
Transport numbers for cations [ti+]m )
fi+zi+ui+[Ciz+]an 2
2
fi+zi+ui+[Ciz+]an + ∑fi-zi-ui-[Ciz-]cat ∑ i)1 i)1
Transport numbers for the anions
(8)
fi-zi-ui-[Ciz-]cat 2
2
(9)
fi+zi+ui+[Ciz+]an + ∑fi-zi-ui-[Ciz-]cat ∑ i)1 i)1 where ui+ and ui- are the electrochemical mobilities of the ith cation and anion, respectively; [Ciz+]an and [Ciz-]cat are 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
Cd2+ + 2 OH- a Cd(OH)2(S) KS ) [Cd2+][OH-]2 ) 2 × 10-14 (10) H2O a H+ + OH-
KW ) [H+][OH-] ) 10-14
(11)
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 algorithm. 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 electrochemical reactions, which are considered instantaneous, separately from the chemical equilibria, in a similar fashion as was done elsewhere.25 In 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
Cd2+ + 2e- f Cd0 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 simultaneously in both electrode compartments. On the other hand (Figure 7b), for a higher concentration 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 increase because subsequent precipitation in this cell occurs because of the basic pH value resulting for the reduction of water.
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Figure 9. Cadmium collected in cathodic compartment and evolution of pH. 0, pHcat; 4, pHan; 2, Cdan; 9, Cdcat Table 5. Values of Ratio fi and Electrochemical Mobilities for the Ions Transported H+
OH-
Cd2+
Cl-
cation-exchange membrane, fi 1
