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Published online 11 January 2008
Published in Soil Sci Soc Am J 72:56-62 (2008)
DOI: 10.2136/sssaj2007.0131
© 2008 Soil Science Society of America
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SOIL CHEMISTRY

Wien Effect Determination of Adsorption Energies between Heavy Metal Ions and Soil Particles

Yu-Jun Wanga, Cheng-Bao Lia, Wei Wangb, Dong-Mei Zhoua, Ren-Kou Xua and Shmulik P. Friedmanc,*

a Institute of Soil Science, Chinese Academy of Sciences, P.O. Box 821, Nanjing 210008, China
b College of Resource and Environ. Science, Nanjing Agricultural University, Nanjing11 210095, China
c Institute of Soil, Water, and Environ. Sciences, Agricultural Research Organization, the Volcani Center, Bet Dagan 50250, Israel

* Corresponding author (vwsfried{at}agri.gov.il).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Gibbs mean free adsorption energies between cations and charged soil particles are a measure of physicochemical interactions between ions and soil particles. The distribution of Gibbs free adsorption energies could not be determined experimentally before the development of Wien effect measurements in dilute soil suspensions. In the present study, energy relationships between heavy metal ions and particles of yellow-brown and black soils (an Alfisol and a Mollisol) were inferred from Wien effect measurements in dilute suspensions, in deionized water, of homoionic soil particles (<2 µm) of the two soils saturated with ions of five heavy metals. The results show that the mean Gibbs free binding energies of the heavy metal ions with yellow-brown and black soil particles diminish in the order Pb2+ > Zn2+ ≥ Cu2+ > Cd2+ > Cr3+, where the range of binding energies for yellow-brown soil (5.39–8.54 kJ mol–1) is less than that for black soil (8.39–9.88 kJ mol–1). The electrical field-dependent mean Gibbs free adsorption energies of these heavy metal ions for yellow-brown and black soils descend in the order Cu2+ > Cd2+ > Pb2+ > Zn2+ > Cr3+ and Cu2+ > Zn2+ > Pb2+ > Cd2+ > Cr3+, respectively. The mean Gibbs free adsorption energies of Cu2+, Zn2+, Cd2+, Pb2+, and Cr3+ at a field strength of 150 kV cm–1, for example, are in the range of 0.5 to 2.1 kJ mol–1 for the two soils.

Abbreviations: EC, electrical conductivity


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The interactions of heavy metal cations with soil particles have been studied extensively and reported in the literature of soil and environmental sciences (Bergseth and Stuanes, 1976; Cavallaro and McBride, 1978; Farrah and Pickering, 1978; Tyler, 1978; Echeverria et al., 1998; Gomes et al., 2001; Jain and Sharma, 2002; Wang et al., 2003; Polcaro et al., 2003; Bradl, 2 004; Diatta et al., 2004). Most evaluations of these cation–soil particle interactions have relied on inference from adsorption isotherms (Forbes et al., 1976; Bradl, 2004; Cavallaro and McBride, 1978; Echeverria et al., 1998; Jain and Sharma, 2002; Wang et al., 2003; Pohlmeier and Lustfeld, 2004; da Fonseca et al., 2005; Gupta and Bhattacharyya, 2005), whose determination is laborious and time consuming. Soil chemists usually determine the Gibbs free adsorption energy, {Delta}G, from measurements of the equilibrium constant (distribution coefficient), Kc = Cads/Cl, the ratio between the adsorbed (Cads) and solution (Cl) concentrations, {Delta}G = –RTln(Kc) (da Fonseca et al., 2005; Lin and Lin, 2005), or from the intercept ({Delta}S/R, where S is entropy) and slope (–{Delta}H/RT, where H is enthalpy) of a Van't Hoff curve, ln(Kc) = f(1/T), obtained from Kc measurements at several temperatures, {Delta}G = {Delta}H – T{Delta}S (Jain and Sharma, 2002; Gupta and Bhattacharyya, 2005). It is also common to quantify the interactions between various ions and particles of clay minerals or soils in terms of the affinity parameter of a best-fit Langmuir isotherm, whether the assumptions behind the originally derived Langmuir model are applicable or not (Olsen and Watanabe, 1957; John, 1972; Holford and Mattingly, 1975; Saeed, 1977; Ajwa and Tabatabai, 1997; Mandal and Hazra, 1997; Undabeytia et al., 2002). Recently, Critter and Airoldi (2003) experimentally determined the ion-exchange equilibrium on the interface between cationic latosol soils and aqueous solutions, and calculated {Delta}G by linearization of the Langmuir equation. The negative {Delta}G values that they determined represented spontaneous ion exchange with the hydrogenated soils, which indicated high affinities of these soils for Ca and Pb. To summarize, the investigation of energy relationships between cations and soil particles in the second half of the 20th century was based on indirect deduction rather than direct measurement. Since there was no practical and simple method to determine the binding or adsorption energy of cations to soil particles, this attribute was not studied or reported extensively.

Recently, a new method for evaluating the interactions of ions with charged colloidal particles, based on measurements of the Wien effect in suspensions, has been developed (Li and Friedman, 2003; Li et al., 2002, 2003, 2005). In a previous study (Li et al., 2005), a new method to determine the binding (or bonding) energy and the adsorption energies of cations with soil particles, in terms of mean {Delta}G was presented; it is based on measurement of the Wien effect in soil suspensions. The Wien effect method is superior to the above-mentioned methods for determining adsorption energies of counter ions in general and heavy metal ions in particular on soil particles as it directly measures adsorption and not exchange, and is less laborious. In the present study, we used the new method to evaluate the mean Gibbs free binding and adsorption energies of five heavy metal ions (Cu2+, Zn2+, Cd2+, Pb2+, and Cr3+) on yellow-brown and black soil particles of <2 µm in diameter.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Soil Samples
The tested yellow-brown soil (an Alfisol from Nanjing, Jiangsu) and a black soil (a Mollisol from Haerbin, Heilongjiang), which were expected to carry only negative charge, were collected from a depth of about 1 m. The dominant clay minerals of the yellow-brown soil are hydromuscovite and vermiculite. The principle clay mineral of the black soil is hydromuscovite; it also contains some saponite and chlorite and some organic matter in its clay fraction. The basic properties of the two soils are listed in Table 1 .


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  		Table 1. Properties of the tested soils, including clay, organic matter, and Fe2O3 content, cation exchange capacity (CEC), and pH in water.

 
The clay fraction of <2 µm in diameter was separated by sedimentation, dried, and ground. The positive and negative charge densities of the clay fractions of the two soils at different pH values, determined according to the method of Schofield (1949), are presented in Fig. 1. The soil particles carried no significant positive charge, as required for the proposed method, and their negative charge densities were pH dependent in the relevant pH range of 4.5 to 6.0 that characterized our suspensions (Table 2 ).


Figure 1
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  		Fig. 1. Negative and positive charge densities of the clay fractions (<2 µm) of the soils as a function of pH.

 

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  		Table 2. The electrical conductivity under weak electrical fields (EC0) and pH of the various tested soil suspensions of solids concentration (cp) of 10 g L–1, cation exchange capacity at prescribed pH (CEC), equivalent ionic conductivity ({lambda}), fraction of ionized ions under weak electrical fields (f0), Gibbs mean free binding energy ({Delta}Gbi) as determined by EC0 measurements; total concentrations of heavy metal ions in suspensions (Ci), the activity of heavy metal ions in the supernatant ({alpha}i,), the active fraction of heavy metal ions in suspensions (f0i) and the Gibbs mean free binding energy ({Delta}Gbia) as determined from ion activity measurement.

 
Preparations of Homoionic Soil Samples and Suspensions
The clay fractions of the two soils were saturated with the five different heavy metal cations by three sequential equilibrations with 1 mol L–1 solutions of the chlorides of these heavy metals. The clay samples were then centrifuged (104 · g, 5–25 min) and the concentrated Cl– supernatants were discarded. Deionized water was added to the precipitate, with thorough stirring, and the new suspensions were centrifuged again. When the supernatant was found to be turbid, a mixture of deionized water with ethanol (H2O/ethanol 1:1–1:1.5) was added.

The supernatants were discarded and their electrical conductivity (EC) and Cl– concentrations were measured. This procedure was repeated up to 11 times, until the supernatant contained no detectable Cl– ions. The Cl––free clay fraction was then dried and ground.

Suspensions were prepared by adding deionized water to soil samples in 50-mL plastic bottles to a solids concentration of 10 g L–1. The plastic bottles were sealed and shaken for 30 min, and the suspensions were dispersed ultrasonically for 45 min. The homoionic suspensions were allowed to stand for about 7 to 10 d, to achieve sufficient equilibration of ion reactions in the suspensions, before measurements in strong electrical fields. The electrical conductivities under weak electrical fields (EC0) and the pH of the suspensions of the two soils saturated with the various heavy metal ions are presented in Table 2.

Wien Effect Measurement Procedure
The weak-field EC of the suspensions was determined with a regular conductivity bridge to ensure that it was well within the measurement range of 500 {Omega} to 20 k{Omega} of the short high-voltage pulse (SHP)-2 apparatus. The SHP-2 is similar to the SHP-1 apparatus, described in detail in Li and Friedman (2003), except for its extended resistance range of 500 {Omega} to 20 k{Omega} and its higher maximal voltage drop of up to 30 kV. The test sample of each suspension was poured into the electrode cell, which was connected to the apparatus via regular Cu wires and crocodile clips, and the resistance of the variable resistor was set to about the expected resistance of the test sample. An initial, relatively low voltage of about 1.5 kV was selected, and the spark gap button was pressed to initiate a short pulse. The needle of the galvanometer jumped to the right or left, depending on the direction of the current in the comparison circuit, which, in turn, depended on whether the variable resistance was higher or lower than that of the electrode cell. The variable resistor was adjusted and the procedure was repeated, usually for three to five pulses, until a minimal needle jump was achieved, indicating that the resistances of the variable resistor and the test sample were equal. The resistance of the variable resistor was then determined with a regular meter. It was necessary to wait for a few seconds between successive pulses to allow full charging of the high-voltage capacitor and to allow the suspension in the electrode cell to cool down to its initial temperature of 25°C. The suspension was stirred with a thin Plexiglas rod between pulses to ensure homogeneity. After the resistance had been determined for a given applied voltage, the voltage was raised to the next required level and the above procedure was repeated. The measurements were terminated when the voltage had been raised to a level at which sparking (dielectric breakdown) through the suspension occurred. Then the measurements were repeated in the reverse order using the same suspensions, proceeding from high to low voltages, to eliminate the effects of possible long-term heating and other irreversible phenomena. The data presented in Fig. 2 are the means of these two sets of measurements; the standard errors were usually smaller than the symbols on the graphs. All the reported measurements were made at a constant temperature of 25°C.


Figure 2
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  		Fig. 2. Dependence on field strength of electrical conductivity of suspensions (10 g kg–1) of (A) yellow-brown and (B) black soil particles saturated with various heavy metal ions in deionized water.

 
Evaluation of Binding and Adsorption Energies
The derivation of the equations used for computing the binding (or bonding) and adsorption energies between cations and charged soil particles from the Wien effect measurements in soil suspensions was discussed in full in Li et al. (2005), and is briefly summarized here.

Binding Energy Evaluation
It was assumed that the metallic cations and the unidentified anions made similar contributions to the overall suspension EC, i.e., that the ionized cations accounted for half of the suspension EC at weak fields (EC0). If all of the cations on the surfaces of the soil particles at saturation could be ionized, the corresponding EC (ECu, S m–1) would be ECu=2CECcp{lambda} where CEC is the cation exchange capacity (molc kg–1), cp is the particle concentration of the suspension (kg L–1), and {lambda} the equivalent conductivity (S m–1 molc–1 L) of the heavy metal cation. Thus, the degree of dissociation or the active fraction of cations can be evaluated by

Formula 1[1]
Marshall's formula for calculating the mean free binding energy of a cation i in terms of the Gibbs free energy is (Marshall and Barber, 1949; Marshall, 1950)

Formula 2[2]
where R is the universal gas constant (8.315 J mol–1 K–1), T is the thermodynamic temperature, {alpha}i represents the activity of the cation, ci the total concentration of the cation in a soil suspension, and f0 the active fraction.

If it is assumed that there is an analogy between Marshall's active fraction and the fraction of ionized cations that contribute to the suspension EC, Eq. [1] can be substituted into Eq. [2] to yield an approximate expression for evaluating the mean Gibbs free binding energy of the heavy metal cations to the soil particles from the measurements of the weak field electrical conductivity, EC0:

Formula 3[3]

Adsorption Energy Evaluation
We described above a method for evaluating the mean Gibbs free binding energy, which is an average binding energy of the total heavy metal cation population. Below we provide the equations for evaluating the spectrum of adsorption energies from the Wien effect measurements. By analogy to Eq. [2], the change in free chemical energy of a cation in a soil suspension, when the suspension system changes from State 1 to State 2 at constant temperature and pressure, is given by {Delta}G = RT ln({alpha}2/{alpha}1), where {alpha}2 and {alpha}1 are the activities of the suspension's component, i.e., the heavy metal cations under a weak and a strong electrical field. Therefore, repeating our use of the approximate analogy between the electrophoretic mobility and the activity of the ionized cations, the mean Gibbs free adsorption energy of the cations can be represented as

Formula 4[4]
where EC and EC0 represent the electrical conductivity of the suspension under the strong and weak electrical fields, respectively. This expression enables us to evaluate the mean adsorption energy of the population of cations released from the soil particles as the electrical field was increased from zero to E, and application of the expression to a series of measurements of the Wien effect, EC(E), provides a spectrum of the cation adsorption energies. The increase in the mobility of the cations in the suspension stems from the work done by the applied electrical field in overcoming the binding forces between the cations and the soil particles. The positive adsorption energies calculated with Eq. [4] reflect the work done by the applied electrical field. For the sake of clarity, all binding or adsorption energies have been assigned positive signs here, which is why there is no minus sign in the above equations.


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Mean Gibbs Free Binding Energy
The parameters required for calculating the mean Gibbs free binding energy ({Delta}Gbi) from the EC0 measurements by means of Eq. [3] and mean free binding energies are presented in Table 2. The {Delta}Gbi values, calculated from the EC0 measurements, diminished in the order Pb2+ > Zn2+ ≥ Cu2+ > Cd2+ > Cr3+ for both soils. This sequence of the divalent cations is similar to previously reported adsorption sequences (Gomes et al., 2001; Diatta et al., 2004), but it was somewhat unexpected that the mean free binding energy of the trivalent Cr3+ was smaller than those of all the divalent heavy metal ions. This may be because the high degree of hydrolysis of Cr3+ in water, enhanced by the process of leaching the homoionic particles with distilled water, resulted in its transformation into divalent Cr(OH)2+, together with monovalent H+ cations. The hydrolysis constant (pK) of Cr3+ (3.9) is much smaller than those of Pb2+ (7.2), Zn2+ (8.2), Cu2+ (6.5), and Cd2+ (9.7) (Wen and Sho, 1977). It is also notable that the {Delta}Gbi values of the yellow-brown soil were smaller than those of the black soil, probably because of the higher organic matter content in the black soil (Table 1). The measured {Delta}Gbi values of Cd2+ with yellow-brown and black soil particles (6.78 and 9.07 kJ mol–1, respectively) were in reasonable agreement with those found by Li et al. (2005) (7.1 and 8.2 kJ mol–1, respectively). The bottom part of Table 2 presents the mean Gibbs free binding energies, {Delta}Gbia, evaluated according to Marshall's method (Eq. [2]) from measurements of the activity of the heavy metal cations in the supernatant, as determined by means of atomic absorption. We estimate that the {Delta}Gbia evaluation is less accurate because of the difficulties in obtaining a transparent supernatant by means of centrifuging. Nevertheless, the values of the mean Gibbs free binding energies yielded by the two methods are in reasonable agreement, except, perhaps, for Cd2+ with each of the two soils, for which the values of {Delta}Gbi, obtained from EC0 measurements, were the smallest among those of the various cations, and the values of {Delta}Gbia, obtained from activity measurements, were higher.

Electrical Conductivity–Field Strength Relationships
The effects of field strength on the electrical conductivities of the suspensions, in deionized water, of yellow-brown and black soil particles saturated with various heavy metal ions are shown in Fig. 2A and 2B, respectively. With weak electrical fields (~15 kV cm–1), the EC values of the suspensions ranged from 0.006 to 0.01 mS cm–1, and were closely related to the nature of the saturating heavy metal ions; they diminished in the order Zn > Cr ≥ Pb ≥ Cu = Cd for both soils. Negative Wien effects [EC(E) smaller than EC0] appeared at applied fields in the range of ~15 to 50 kV cm–1, and the field strength at the minimum, Em, of the concave segment of the EC vs. E curve was about 25 kV cm–1 for the suspensions containing Cu2+, Zn2+, Cd2+ and Pb2+. For the suspensions containing Cr3+, however, negative Wien effects appeared at E values ranging from 15 to ~100 kV cm–1, and Em was about 50 kV cm–1.

It has been suggested previously that the negative second Wien effect arises from a double-layer polarization that causes a temporary readsorption of ions on the soil particle dipoles (within the finite pulse duration of 10–6–10–5 s), which leads, in turn, to a decrease in the measured transient electrical conductivity (Schelly and Astumian, 1984). The particle polarization mechanism is probably controlled by surface conductance, which means that the induced dipole is in the direction of the applied external field and that its magnitude is determined by the ratio between the surface and bulk-solution conductivities ({sigma}s/r{sigma}l, in which {sigma}s is the surface conductivity, {sigma}l is the bulk solution conductivity, and r is the particle radius; Dukhin, 1993). As the applied external field increases, two things happen: (i) particle polarization (proportional to E) occurs, with the consequently increased readsorption of bulk ions; and (ii) stripping of ions from the adsorbed particles increases. Up to a certain field strength, Em, polarization and readsorption dominate, and the measured transient EC decreases toward a local minimum ECm (Em), but as E increases further, ion stripping begins to dominate. The competition between these two opposing processes determines the upwardly concave shape of the EC vs. E curve around the local minimum, ECm (Em). We previously found (Jiang et al., 2006), for example, that a suspension of electrodialyzed clay fractions of yellow-brown soil in dilute Cd(NO3)2 solution exhibited a significant negative Wien effect with an ECm (Em) at about 100 kV cm–1. In contrast to this, the suspension of homoionic clay particles of yellow-brown soil saturated with Cd2+ exhibited only a slight negative Wien effect at weak fields, with Em at about 25 kV cm–1. The repeatable minor negative Wien effect with homoionic particles suspended in deionized water and the major negative Wien effect in suspensions of electrodialyzed soil particles seem to be real phenomena, whose complex mechanisms are worth further investigation.

Beyond the concave segments of the EC vs. E curves, the EC increased more steeply for the suspensions containing Cu2+, Zn2+, Cd2+, and Pb2+ than for the two suspensions containing Cr3+. At field strengths >100 kV cm–1, the EC of the suspensions were in the order Zn > Cu > Cd ≥ Pb >> Cr.

Mean Gibbs Free Adsorption Energy
The mean Gibbs free adsorption energies of all heavy metal ions released at a given applied field, as evaluated by means of Eq. [4], are presented in Fig. 3. In addition to the plotted mean Gibbs free adsorption energies, {Delta}Gad(E) of the various heavy metal ions on the yellow-brown and black soil particles (Fig. 3A and 3B, respectively), sample evaluation for three field strengths, {Delta}Gad (100, 150, and 200 kV cm–1), are listed in Table 3 . The {Delta}Gad values for the various heavy metal ions clearly diminish in the order Cu2+ > Cd2+ > Pb2+ > Zn2+ > Cr3+ for the yellow-brown soil, and Cu2+ > Zn2+ > Pb2+ > Cd2+ > Cr3+ for the black soil, with the positions of Cd2+ and Zn2+ reversed in the two sequences. This difference may stem from the higher content of particulate organic matter and associated dissolved organic molecules in the black soil than in the yellow-brown soil (13.6 and 5.4 g kg–1, respectively). It was reported, for example, that the amount of Cd adsorbed onto kaolinite, bentonite, and clay fractions of soils increased in the presence of low (0–2 mmol L–1) concentrations of low-molecular-weight organic (oxalic, citric, and acetic) acids (Zhang et al., 2004; Liao and Xie, 2004; Liao, 2006). The {Delta}Gad values of Cu2+, Pb2+, Cd2+ and Cr3+ for yellow-brown soil particles were larger than or equal to those for black soil particles, whereas the {Delta}Gad values of Zn2+ were lower for the yellow-brown soil. This is contrary to the general tendency for the binding energies, {Delta}Gbi, of all heavy metal ions to black soil particles to be larger than those to yellow-brown soil particles. The evaluated maximum Gibbs free adsorption energies of up to 3.5 kJ mol–1 (Fig. 3) indicate that the applied electrical fields are not strong enough to strip off the heavy metal cations bound tightly in the Stern layer. From an environmental or agricultural point of view, the loosely bound cations are more readily exchangeable in the soil and thus the characterization of their adsorption energies is of greatest importance.


Figure 3
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  		Fig. 3. Mean Gibbs free adsorption energy as a function of field strength for suspensions, in deionized water, of (A) yellow-brown soil and (B) black soil particles saturated with various heavy metal ions (particle concentration of 10 g kg–1).

 

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  		Table 3. Fraction of released (active or conducting) cations, f, and mean Gibbs free adsorption energies ({Delta}Gads) of various heavy metal ions adsorbed on the surfaces of yellow-brown soil and black soil particles at several field strengths (evaluated by means of Eq. [4] and the measured EC vs. E curves [Fig. 2]).

 
Interestingly, the dielectric strengths of the suspensions are different for the same type of metal between the two different soils, especially for Zn2+. For most of the metal ions, the breakdown points in the black soil are higher than those in the yellow-brown soil. This is not true for the Zn2+ suspensions, however, for which the dielectric breakdown occurred at about 220 kVcm–1 in the yellow-brown soil and at about 180 kVcm–1 in the black soil (Fig. 2). These trends in the dielectric strengths are opposite to the mean Gibbs free adsorption energies (Table 3).


   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Energy relationships between heavy metal ions and particles of yellow-brown and black soils were inferred from measurements of electrical conductivity under weak field conditions and from Wien effect measurements in dilute suspensions of homoionic soil particles, assuming an approximate analogy between the electrophoretic mobility and the activity of the ionized cations. The results show that the mean Gibbs free binding energies of the heavy metal ions with yellow-brown soil and black soil particles diminish in the order Pb2+ > Zn2+ ≥ Cu2+ > Cd2+ > Cr3+, where the range of binding energies for the yellow-brown soil (5.39–8.54 kJ mol–1) is less than that for the black soil (8.39–9.88 kJ mol–1). The electrical-field-dependent mean Gibbs free adsorption energies of these heavy metal ions for the yellow-brown and black soils descend in the order Cu2+ > Cd2+ > Pb2+ > Zn2+ > Cr3+ and Cu2+ > Zn2+ > Pb2+ > Cd2+ > Cr3+, respectively. The Wien effect method is superior to other methods for determining (the spectrum of) adsorption energies of counter ions in general and heavy metal ions in particular on soil particles, as it directly measures adsorption and not exchange and is less laborious. Therefore, it should be used for obtaining a data set of counter ion–soil particle or heavy metal–soil particle interactions that can serve for predicting competitive adsorption, mobility, and bioavailability of (heavy) metal ions of environmental or agricultural interest on various soils.


   ACKNOWLEDGMENTS
 
We acknowledge the support of the National Science Foundation of China under Project no. 40401030. We are also grateful to Mikhail Borisover for fruitful discussions.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
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Received for publication April 9, 2007.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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Y.-J. Wang, C.-B. Li, W. Wang, J. Jiang, D.-M. Zhou, R. Xu, and S. P. Friedman
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