- Research Article
- Open Access
Wien effect of Cd/Zn on soil clay fraction and their interaction
© The Author(s) 2018
- Received: 23 November 2017
- Accepted: 5 February 2018
- Published: 13 February 2018
The coexistence of Cd2+ and Zn2+ ions in nature has a significant influence on their environmental behaviors in soils and bioavailability for plants. While many studies have been done on the mutual toxicity of Cd2+ and Zn2+, few studies can be found in the literature focused on the interaction of Cd2+ and Zn2+ on soil clay fractions especially in terms of energy relationship.
The binding energies of Cd2+ on boggy soil (Histosols) particles and Zn2+ on yellow brown soil (Haplic Luvisols) particles were the highest, while those of Cd2+ and Zn2+ on paddy soil (Inceptisols) particles were the lowest. These results indicated that Cd2+ and Zn2+ have a strong capacity to adsorb in the solid phase at the soil–water interface of boggy soil and yellow brown soil, respectively. However, both Cd2+ and Zn2+ adsorbed on paddy soil particles easily release into the solution of the soil suspension. Unlike the binding energy, the higher adsorption energies of ions in boggy and yellow brown soils showed a weak binding force of ions in boggy soil and yellow brown soil. A 1:1 ratio of Cd2+ to Zn2+ promotes the mutual inhibition of their retentions. Cd2+ and Zn2+ have high mobility and bioavailability in paddy soil and yellow drab soil (Ustalfs), whereas they have high potential mobility and bioavailability in boggy soil and yellow brown soil.
In the combined system, Zn2+ had preferential adsorption than Cd2+ on soil clay fractions. Boggy soil and yellow brown soil have a low environmental risk with lower mobility and bioavailability of Cd2+ and Zn2+ while paddy soil and yellow drab soil present a substantial environmental risk. In the combined system, Cd2+ and Zn2+ restrain each other, resulting in the weaker binding force between ions and soil particles at a 1:1 ratio of Cd2+–Zn2+.
- Wien effect
- Combined pollution
- Soil particles
In China, about 13% of soils included in the national assessment of soil contamination hold excessive amounts of inorganic pollutants . Most of these contaminants are metals and metalloids that can be found in fertilizers  or in several classes of hazardous residues that are improperly disposed of in the soils . Such elements can pose a threat to human health because of their accumulation throughout the food chain [4–7].
Both cooperative and antagonistic toxic effects are reported for soil pollutants such as those for Cd2+ and Zn2+. Turner  found that Cd2+ might increase Zn2+ uptake in some plants as a result of root damage in the higher Cd2+. Wu et al.  suggested that Cd2+ enhances Zn2+ accumulation in chloroplast (FII), whereas Zn2+ addition decreases the concentration of Cd2+ in root trophoplast (FII). Van Gestel and Hensbergen  observed that Cd2+ and Zn2+ had antagonistic and synergistic effects on the growth and reproduction of the Collembola, respectively.
The adsorption process at the solid–water interface strongly affects both mobility and bioavailability of pollutants in soil. Despite this, most research efforts in this field using empirical sorption isotherms or surface complexation models have focused on the behavior of single adsorbate [11–14]. Even the direct assessment of binding strengths between adsorbates and soil particles does not account for the competitive effects that take place in multi-adsorbate environments such as those found in soils [15–19].
Sorption studies carried out in multi-reactive media can better characterize the behavior of pollutants in soils. The use of soil samples and multi-metal solutions allowed Gomes et al.  to assess not only the competition effects on the metal distribution coefficients but also the effects of intrinsic cation properties such as the trend to hydrolysis and the softness parameter of Misono on the preferential metal adsorption. Using a competitive two-metal adsorption isotherm Ming et al.  observed that the co-adsorption of Zn2+ enhanced the Cd2+ mobility in soils.
The measurement of the Wien effect in colloidal suspensions is a straightforward and rapid method established by Li et al.  for determining binding energies associated with ion adsorption. This approach is based on the so-called Wien effect and colloidal electrolyte theory as proposed by Marshall and Krinbill . The Wien effect is a deviation from Ohm’s law characterized by the enhancement of the electrical conductivity (EC) of a suspension in response to the application of an increasing electrical field (E) . We have been successfully measuring this effect in suspensions of whole soils and soil minerals during the last decade [18, 25].
The Wien effect curves (EC vs. E) allows for quantifying the mean free ion binding and adsorption energies and the intensity of ion stripping from soil particles [15, 18, 22, 26, 27]. However, so far, this procedure was only applied to single metal systems. Therefore, in this paper we used the Wien effect to evaluate the co-adsorption impacts on the binding energies of Cd2+ and Zn2+ retained on soil clay fractions.
Properties of the studied soil samples
Chinese soil type
US soil type
EC (μs cm−1)
CEC (cmol kg−1)
DOC (mg L−1)
FeDCB (g kg−1)
Yellow drab soil
Yellow brown soil
Mineralogical composition of the clay fractions of the studied soils
Yellow brown soil
Yellow drab soil
The preparation of soil clay fractions saturated with Cd2+ and Zn2+ and suspensions
The soil clay fractions were suspended in solutions containing 0.4 mol L−1 of positive charges from both Cd2+ and Zn2+. The solutions were prepared through the dissolution of amounts of reagent grade Cd(NO3) 2 · 4H2O and Zn(NO3) 2 · 6H2O sufficient for obtaining Cd2+:Zn2+ molar ratios of 3:1 [0.15 mol L−1 Cd(NO3)2 and 0.05 mol L−1 Zn(NO3)2], 1:1 Cd:Zn [0.10 mol L−1 Cd(NO3)2 and 0.10 mol L−1 Zn(NO3)2], and 1:3 Cd:Zn [0.05 mol L−1 Cd(NO3)2 and 0.15 mol L−1 Zn(NO3)2]. The surfaces of the clay fraction particles were saturated with Cd2+ and Zn2+ through three successive batches that included agitation, centrifugation, supernatant disposal and resuspension in the initial solution. Thereafter, the non-adsorbed ions were removed from particle surfaces through successive batches with deionized water that were repeated until the EC stabilization of the supernatants in a minimal value. The saturated particles were then oven-dried, ground, sieved and analyzed for their total amounts of Cd and Zn in an atomic absorption spectrophotometer Hitachi 2000 after microwave-assisted acid dissolution . Aqueous suspensions (10 g L−1) of the saturated particles were prepared in 50-mL centrifuge tubes, agitated for 30 min and then sonicated for 45 min. After 10 additional 1-h agitations intercalated with 23 h of standing, the equilibrated suspensions were used in the measurements of the Wien effect for assessing binding energies of Cd and Zn cations adsorbed to the soil clay fractions.
Wien effect measurements
The electrical conductivity under strong electrical fields was measured with the SHP-2 (short high-voltage pulse) apparatus. Both the SHP-2 apparatus features and the measuring procedure were detailed elsewhere [15, 19, 22, 27, 33]. Before measuring the Wien effect with the apparatus, the weak-field EC of the sample was determined with a regular conductivity meter (DDS-310, Shanghai Kangyi instrument Co.). The measurements were conducted at 25 °C to ensure that the suspension resistance was within the range from 200 Ω to 20 kΩ. The strong-field ECs measurements were carried at 25 °C by applying a voltage drop that increased from 1.0 kV up to the sparking occurrence (dielectric breakdown). The electrode spacing was kept constant at 1 mm. The first set of measurements of the Wien effect was carried out from low to high field strengths whereas the second one was performed in reverse order to eliminate possible effects of long-term heating and other irreversible phenomena. In consideration of the lower energy contributed by the applied electrical field, the outer-sphere complexes were examined by the Wien effect method. After the measurements of the Wien effect the suspensions were centrifuged and the supernatants were filtered and analyzed for Cd and Zn. All measurements were performed in duplicate.
Binding energies of adsorbed Cd2+ and Zn2+
For the sake of clarity, the minus sign of Eq. (3) was omitted and to the binding/adsorption energies will be assigned positive signs. The binding energy reflects the distribution proportion of metal ions on the interface of solid–solution. In Eq. (3), the numerator represents the EC expected from the contribution of all metal ions in the suspension under ideal condition. The denominator represents the EC contributed by the dissolved metal ions. The larger the binding energy is, the smaller the denominator is, that is, there are less dissolved metal ions in suspensions.
Adsorption energies of adsorbed Cd2+ and Zn2+
Given the non-selectivity of the electrical conductivity measurement, in a mixed system only the total adsorption energy of cations can be assessed, similar to the adsorption energy of single ions.
Equation (4) enables one to evaluate the mean free adsorption energy of all cations stripped off from the soil clay fraction particles as the electrical field increased from zero to E. The application of Eq. (4) to a series of measurements of the Wien effect, EC(E), can provide a spectrum of the cation adsorption energies, ∆Gads(E). If the metal strongly adsorbs to the soil clay fraction particles, fewer ions will be dissolved at a weak E and the EC enhancement at the high E will be substantial, i.e. the adsorption energy will be high.
Cd2+ and Zn2+ on the soil clay fraction particles and in the equilibrium solutions
The contents of Cd2+ and Zn2+ in soil colloid particles and corresponding supernatant, the binding energies of Cd2+ and Zn2+ on different soil colloid particles at various ratio of Cd2+ and Zn2+, related parameters to calculate the binding energy
EC0 (mS cm−1)
Metal in colloid particles (mmol kg−1)
ECiu (mS cm−1)
Metal in supernatant (mmol L−1)
ECi0 (mS cm−1)
△Gbi (kJ mol−1)
Yellow brown soil
Yellow drab soil
Yellow brown soil
Yellow drab soil
Yellow brown soil
Yellow drab soil
Electrical conductivity of suspensions as a function of field strength
The weak-field electrical conductivity (EC0) is proportional to the concentration of dissolved ions found in solution. Most of EC0 values of the suspensions containing clay-sized particles extracted from the paddy soil (Inceptisols) and yellow drab soils (Ustalfs) were the highest measured ones (Table 3). This finding could be due to the fact that soils had substantial amounts of dissolved organic matter which, in turn, carries electrical charges and forms soluble complexes with metals that are liable to ionize .
Mean Gibbs free binding energy
The binding energies of Cd2+ and Zn2+ in the mono-metal systems were higher than those in the competitive systems. Shaheen et al. [35, 36] observed lesser retentions of Cd2+ and Zn2+ in the competitive systems. Furthermore, the Cd2+ binding energy was lower in the competitive system while that of Zn2+ had no significant reduction compared to that observed in the single system. Antoniadis and Tsadilas  also obtained similar results, mainly because the binding strength between Cd2+ and soil particles was smaller and prone to be affected by competitive ions.
Boggy soil and yellow brown soil showed higher affinities for Cd2+ and Zn2+, respectively, while paddy soil has lower affinity for both cations. In other words, boggy soil and yellow brown soil attracted Cd2+ and Zn2+, respectively. On the other hand, both Cd2+ and Zn2+ tend to escape from the particles of paddy soil into soil solution.
Mean Gibbs free adsorption energy
Mean free adsorption energies of ions adsorbed in soil colloid particles with four soil types at field strength of 100, 150, 200, 250 kV cm−1
Ratio of Cd2+/Zn2+
Yellow brown soil
Yellow drab soil
Soil heavy metal contamination is not only single-metal ion contamination but also a binary pollution. In this study, the suspension Wien effect was used to investigate the interactions between two species of heavy metal ions and four types of soil clay fraction particles in the Cd2+–Zn2+ binary systems at various ratios of Cd2+ to Zn2+. The binding energy and adsorption energy spectra of Cd2+ and Zn2+ for the soil clay fraction particles were calculated based on the EC-E curves. Compared to the binding energies of Cd2+ and Zn2+ in the mono-metal system, more Zn2+ was electrostatically adsorbed on the clay fraction particles of the four studied soils than Cd2+. The binding energy results in the competitive system suggested that Cd2+ is inclined to combine with the clay fraction particles of boggy soil and Zn2+ tend to combine with clay fraction particles of yellow brown soil. For paddy soil, Zn2+ and Cd2+ are prone to partition into soil solution, resulting in the increase of the mobility and bioavailability of metal ions. Although soil clay fraction particles have a strong attraction to certain ions, the binding force between ions and soil particles may be weak. The adsorption energies of ions in boggy soil and yellow brown soil were higher, indicating weaker electrically adsorbing strength between ions and soil particles. Consequently, mobility and bioavailability of Cd2+ and Zn2+ are poor in boggy soil and yellow brown soil, respectively, while the potential mobility and bioavailability of Cd2+ and Zn2+ are high with the external force provided by the applied electrical field. In the combined system there was antagonism between Cd2+ and Zn2+ resulting in a weaker binding force between ions and soil particles at a 1:1 ratio of Cd2+–Zn2+. Furthermore, the affinity of ions to soil particles decreased with the increase of the proportion of ions (Cd2+ or Zn2+) in the Cd2+–Zn2+ binary samples.
YJW planned the experiments together with CBL and DMZ. TTF did the vast majority of the lab-work and was the main author of the manuscript and compiled all the data. All six authors contributed significantly to the writing of the manuscript. TTF was in charge of the Wien effect data collection and analysis with YJW, JG and MEA. YJW started this project with CBL and DMZ several years ago and it was their initial experiments and results that laid the foundation for the data presented within this manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
Availability of data and materials
The datasets supporting the conclusions of this article are included within the article.
Consent for publication
The manuscript is approved by all authors for publication.
Ethics approval and consent to participate
This work was supported by the National Natural Science Foundation of China (Project No. 41422105) to YJW and the Natural Science Foundation of Jiangsu Province (Project No. BK20130050) to YJW and the Knowledge Innovation Program of the Chinese Academy of Sciences (ISSASIP1612) to YJW.
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