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Environmental Science and Pollution Research

, Volume 24, Issue 30, pp 23598–23606 | Cite as

Streaming potential method for characterizing interaction of electrical double layers between rice roots and Fe/Al oxide-coated quartz in situ

  • Zhao-dong Liu
  • Hai-cui Wang
  • Jiu-yu Li
  • Ren-kou Xu
Research Article

Abstract

The interaction between rice roots and Fe/Al oxide-coated quartz was investigated through zeta potential measurements and column leaching experiments in present study. The zeta potentials of rice roots, Fe/Al oxide-coated quartz, and the binary systems containing rice roots and Fe/Al oxide-coated quartz were measured by a specially constructed streaming potential apparatus. The interactions between rice roots and Fe/Al oxide-coated quartz particles were evaluated/deduced based on the differences of zeta potentials between the binary systems and the single system of rice roots. The zeta potentials of the binary systems moved in positive directions compared with that of rice roots, suggesting that there were overlapping of diffuse layers of electric double layers on positively charged Fe/Al oxide-coated quartz and negatively charged rice roots and neutralization of positive charge on Fe/Al oxide-coated quartz with negative charge on rice roots. The greater amount of positive charges on Al oxide led to the stronger interaction of Al oxide-coated quartz with rice roots and the more shift of zeta potential compared with Fe oxide. The overlapping of diffuse layers on Fe/Al oxide-coated quartz and rice roots was confirmed by column leaching experiments. The greater overlapping of diffuse layers on Al oxide and rice roots led to more simultaneous adsorptions of K+ and NO3 and greater reduction in leachate electric conductivity when the column containing Al oxide-coated quartz and rice roots was leached with KNO3 solution, compared with the columns containing rice roots and Fe oxide-coated quartz or quartz. When the KNO3 solution was replaced with deionized water to flush the columns, more K+ and NO3 were desorbed from the binary system containing Al oxide-coated quartz and rice roots than from other two binary systems, suggesting that the stronger electrostatic interaction between Al oxide and rice roots promoted the desorption of K+ and NO3 from the binary system and enhanced overlapping of diffuse layers on these oppositely charged surfaces compared with other two binary systems. In conclusion, the overlapping of diffuse layers occurred between positively charged Fe/Al oxides and rice roots, which led to neutralization of opposite charge and affected adsorption and desorption of ions onto and from the charged surfaces of Fe/Al oxides and rice roots.

Keywords

Electric double layers Fe oxide Al oxide Rice roots Surface charge Ion adsorption 

Introduction

Large areas of variable charge soils are distributed in the tropical and subtropical regions in the south of China (Yu 1997). The variable-charge soils contain two types of soil colloids, that is, negatively charged phyllosilicates and positively charged Fe and Al oxides. Qafoku et al. (2004) discovered that the coexistence of negatively charged phyllosilicates and positively charged Fe/Al oxides was mainly responsible for the simultaneous adsorption of cations and anions of an electrolyte without net release of other ions into soil solution by variable-charge soils, called “salt adsorption.” This was due to overlapping of electrical double layers around Fe/Al oxides and silicate minerals (Qafoku and Sumner 2002; Li et al. 2009). Other studies also indicated that there was overlapping of electrical double layers between oppositely charged particles (Hou et al. 2007a; Li and Xu 2008; Li et al. 2009), which influenced surface charge properties and adsorption of various cations on soils and phyllosilicates (Wang et al. 2011, 2013b; Xu et al. 2011).

Plant roots with abundant surface functional groups exhibit ion exchange properties (Meychik and Yermakov 1999). In soil solution, plant root surfaces are negatively charged due to dissociation and association of surface functional groups (–COOH, –OH, –NH2, and –H2PO4) from cell walls and cell membranes (Kinraide et al. 1992; Meychik et al. 2005; Wu and Hendershot 2008). Therefore, electric double layers also form at the interface between plant roots and soil solution. Plant roots can be regarded as special charged surfaces. Similarly, interaction of electric double layers between plant roots and positively charged particles in soils may occur, which can affect the surface charge properties of plant roots and root/soil interface. However, this hypothesis requires testing.

The electrochemical characteristics of plasma membranes of plant roots have been investigated extensively to examine their charge properties (Kinraide et al. 1998). The surface potential of plasma membranes of plant roots is usually calculated with specific models (Gouy–Chapman–Stern model) (Kinraide 2003; Wang et al. 2013a). A web-accessible computer program was developed for calculating the potentials and ion activities on cell-membrane surfaces of plant roots (Kopittke et al. 2014). The electrochemical characteristics of plant root surfaces were also studied through measuring zeta potential of cell walls of plant roots using a micro-electrophoresis method, and thus surface potential of plant roots was characterized indirectly (O'Shea et al. 1990; Zheng et al. 2004). These studies were useful for understanding the surface charge properties of plant roots. However, the calculation of surface potential of plasma membranes is complicated and several assumptions were made. Before measurement of zeta potential of plant root cell walls, complicated pretreatments are required to separate cell walls from plant roots (Zheng et al. 2004). These pretreatment procedures may destroy the integrity of plant roots. Moreover, the electrochemical characteristics of plant root surfaces are not entirely represented by those of the plasma membrane and cell walls. Recently, our research group developed a streaming potential apparatus for zeta potential measurements of plant roots in situ (Li et al. 2015). Relative to the plasma membrane or cell wall, it will be of practical significance to investigate the interaction of electrical double layers between plant roots and Fe/Al oxides in situ.

In this study, Fe or Al oxide-coated quartz was used as a substitute for variable charge soils to investigate the interaction of electrical double layers between Fe/Al oxides and rice roots through leaching experiments and to measure zeta potential of mixtures of rice roots with Fe/Al oxide-coated quartz. The main objective was to evaluate the feasible of streaming potential method to characterize the interactions between plant roots and Fe/Al oxide-coated quartz.

Material and methods

Preparation of Fe and Al oxide-coated quartz

Quartz was obtained from a commercial supplier (Sinopharm Chemical Reagent Co. Ltd., Shanghai, China) and mechanically dry-sieved to get a narrow size fraction using 20–60 mesh (0.25–0.83 mm). The quartz was cleaned thoroughly before use: ultrasonicated in 0.01 M NaOH for 0.5 h and then rinsed with deionized water (this step was repeated three times), then ultrasonicated for an additional 0.5 h in 0.01 M HCl three times before a final thorough rinsing with deionized water. The quartz was then air-dried.

Fe oxide-coated quartz was prepared using the following procedures: quartz was mixed with a solution of 0.3 M Fe(NO3)3 and titrated gradually with 1.0 M NaOH to pH 7.0. The suspension was left to stand at 25 °C for 48 h and then the Fe oxide-coated quartz was washed with deionized water until electrical conductivity reached a constant value. The washed Fe oxide-coated quartz was air-dried.

Al oxide-coated quartz was prepared using the following procedures: quartz was mixed with a solution of 0.5 M AlCl3, and titrated gradually with 1.0 M NaOH to pH 7.0. The suspension was left to stand at 25 °C for 48 h, and then the Al oxide-coated quartz was washed with deionized water until electrical conductivity reached a constant value. The washed Al oxide-coated quartz was air-dried.

Rice roots

Seeds of rice (Oryza sativa L.) were surface sterilized with 30% H2O2 for 15 min, followed by thorough washing with deionized water, soaked in deionized water for 4 h, and germinated in a plastic container on gauze at 25 °C in darkness. When the seedlings had grown to about 2 cm tall, they were moved into a controlled environment growth chamber with day/night temperatures of 27/20 °C, day length of 14 h, light intensity of 375 μmol photon m−2 s−1, and relative humidity of 70% and grown in a modified nutrient solution suggested by International Rice Research Institute (Fan et al. 2007). The composition of nutrient solution was as follows: 0.75 (NH4)2SO4, 1.5 NaNO3, 0.32 NaH2PO4·2H2O, 0.5 K2SO4, 1.7 MgSO4·7H2O, and 1.0 CaCl2 (mM); and 9.1 MnCl2·4H2O, 0.16 CuSO4·5H2O, 0.15 ZnSO4·7H2O, 0.07 (NH4)6Mo7O24·4H2O, 18 H3BO3, and 40 FeSO4·7H2O-EDTA (μM). The nutrition solution had pH 5.5 and was renewed every 3 days. After 15 days, uniform seedlings were selected and transplanted to PVC pots (10 cm diameter and 14.5 cm high, 12 plants per pot). After 25 days, the plant root was excised and washed with deionized water. Then, some roots were used to directly measure the streaming potential, and the rest were air-dried and preserved for the follow-up experiments (dry root was preserved at the room temperature in darkness).

Leaching experiments with dilute KNO3 solution

Rice roots and quartz with or without Fe/Al hydroxides were dry-packed into a glass chromatographic column (10 cm × 2.6 cm, Shanghai, China) with 80-μm nylon net on both ends. Prior to the experiments, the packed column was connected to a pump (BT00-300M, Baoding, China) and flushed upward with deionized water operating at a rate of 0.5 mL min−1 for at least 12 h until the column effluent was clear and electrical conductivity reached a constant value. Pore volume (PV) measured by weighing method was 8.95 mL.

The experiment was conducted in the following two steps: (1) 0.1 mM KNO3 solution was pumped into leaching column from its bottom at a constant velocity of 0.5 mL min−1 for about 6.70 PVs. (2) About 5.03 PVs of deionized water were pumped into the column. The leachates were collected and their electric conductivity (EC) and concentrations of K+ and NO3 in the leachate were measured regularly. K+ was determined by flame photometry. The concentration of NO3 was determined using UV spectrophotometry (Pansu and Gautheyrou 2006).

Measurement of streaming potential

The Fe/Al oxide-coated quartz, rice roots, and their mixture were loaded into the measuring cell (length of 3 cm and inner diameter of 1.4 cm) of the streaming potential apparatus (Li et al. 2015). A multi-meter was used to measure streaming potential (ΔE) through a pair of non-polarizable Ag/AgCl electrodes. A conductivity meter was used to measure electrical resistance of the measuring cell with a pair of Pt electrodes. All electrodes were fixed on ends of the measuring cell. The liquid pressure difference at ends of the measuring cell was measured by a liquid manometer. Electrolyte solution was pumped into the measuring cell. The streaming potential varied with the hydraulic gradient (ΔP) obtained by adjusting a valve, and then the ratio of ΔE to ΔP was obtained. The zeta potential (ζ) was calculated using the Helmholtz–Smoluchowski equation (Childress and Elimelech 1996), in the following form:
$$ \zeta =\frac{\varDelta E}{\varDelta P}\frac{\mu }{{\varepsilon \varepsilon}_0}\kappa $$
where ΔEP is the slope of the streaming potential versus applied pressure curve, μ is the dynamic viscosity of the solution, ε is the permittivity of the test solution, ε 0 is the permittivity of free space, and κ is the solution conductivity.

The streaming potential (ΔE) was changed with the differences of liquid pressure (ΔP) between two ends of measuring cell. While the ΔEP and zeta potential did not change with ΔP for a given charged material, zeta potential cannot be measured directly. Therefore, the streaming potential was measured and then the zeta potential was calculated from ΔEP for the investigation of the interactions between rice roots and Fe/Al oxide-coated quartz in the present study.

ATR-FTIR analysis

ATR-FTIR analysis of rice roots was carried out using a FTIR spectrometer Nicolet iS10 (Thermo-Scientific) with a diamond crystal in the spectral range of wave number 650–4000 cm−1, with a resolution of 4 cm−1 and a scan number of 64, using the ATR technique.

Statistic analysis

Data processing and statistical analyses were carried out using SPSS ver. 20.0 for Windows (Chicago, IL, USA). One-way analysis of variance (ANOVA) was used for all experiments to ascertain whether the differences between treatments and between two cultivars were significant at P < 0.05 (Pearson’s correlation).

Results and discussion

Zeta potentials of fresh and dry roots

The absolute value of zeta potential of dry and fresh rice roots increased with increasing solution pH (Fig. 1). The main reason was that dissociation of the acidic functional groups on the root surface increased with the increasing pH, resulting in more negative charge on the root surface. Compared with fresh roots, the absolute value of zeta potential increased for air-dried roots. Zeta potential of rice roots increased by 74.9, 18.9 and 1.1% for pHs 3.76, 6.21, and 8.55, respectively, after being air-dried. When roots were air-dried, the root volume reduced and the surface charge density increased. Although plant roots can absorb water and expand during streaming potential measurement, they are not completely restored to their original shape. However, the difference in zeta potential between fresh and dry roots was < 3 mV at various pH values. Therefore, the effect of air-drying on the zeta potential of roots could be neglected.
Fig. 1

Comparison of zeta potential between fresh roots and dry roots. Zeta potential was measured with the streaming potential method using NaCl as the supporting electrolyte at electric conductivity of 80 μS cm−1. The bars are the standard errors of the means (n = 6). Different letters show significant differences among treatments (P < 0.05; LSD’s test)

The zeta potential of rice root surfaces measured using the streaming potential apparatus was significantly correlated with the potential at the outer surface of root–cell plasma membranes and the zeta potential of cell walls of rice roots (Li et al. 2015). This provides a novel method for investigating the interaction between charged plant roots and soil particles. Protonation and deprotonation of functional groups (–COOH, –OH, –NH2, and –H2PO4) result in charge on the plant root surfaces (Meychik and Yermakov 1999). The ATR-FTIR spectra were similar for fresh and air-dried roots (Fig. 2). Therefore, the air-drying process did not change the surface functional groups on rice roots. As it is difficult to save the fresh roots and conduct the follow-up experiments, dry roots were used for studying the charge characteristics of rice roots and interaction between roots and charged particles in the present study.
Fig. 2

ATR-FTIR spectra of fresh and dried roots of rice

Effect of pH and electrolyte concentration on zeta potential

The change of solution properties altered the charge properties of charged surfaces, thus affected their zeta potentials. Effects of pH and electrolyte concentration on zeta potential of rice roots and Fe/Al oxide-coated quartz are shown in Figs. 3 and 4. The zeta potential of rice roots was negative in the range of pH from 3.5 to 8.0, and the isoelectric point (IEP) was not observed in the zeta potential–pH curve. Therefore, rice roots carried a net negative charge in this pH range. Absolute value of zeta potential of rice roots increased with the increasing pH (Fig. 3), indicating that the amount of surface negative charge increased. The zeta potential of Al oxide-coated quartz was positive in the pH range studied. Therefore, Al oxide-coated quartz carried a net positive charge. Zeta potential of Al oxide-coated quartz decreased with increasing pH (Fig. 3), showing that the amount of surface positive charge was reduced. A similar change trend of zeta potential with pH was observed for Fe oxide-coated quartz (Fig. 3). IEP was not observed in the zeta potential–pH curve for Al oxide-coated quartz. However, IEP was observed at pH 5.5 in the zeta potential–pH curve for Fe oxide-coated quartz, suggesting that net surface charge of Fe oxide-coated quartz changed from positive to negative for solution pH > 5.5. The amount of positive charge was much greater on Al oxide- than Fe oxide-coated quartz at the same pH, consistent with zeta potential of Fe/Al oxides measured by electrophoresis (Liu et al. 2013, 2015).
Fig. 3

Effect of pH on zeta potential of rice roots and Fe/Al oxide-coated quartz. Zeta potential was measured with the streaming potential method using NaCl as the supporting electrolyte at electric conductivity of 80 μS cm−1. The bars are the standard errors of the means (n = 6)

Fig. 4

Effect of electrolyte concentration on zeta potential of rice roots and Fe/Al oxide-coated quartz at pH 6.21. The bars are the standard errors of the means (n = 6)

The absolute values of zeta potentials of Fe/Al oxide-coated quartz and rice roots decreased with increasing electrolyte concentration (Fig. 4), consistent with theoretical predictions (Yu 1997). Increasing electrolyte concentration reduced the thickness of the diffuse layers of electric double layers on Fe/Al oxides and rice roots and thus decreased the absolute value of zeta potential. The zeta potential was greater for Al oxide- than Fe oxide-coated quartz at the same ionic strength, demonstrating that the positive charge was also greater on Al oxide- than on Fe oxide-coated quartz.

Comparison of zeta potential between the rice roots and the binary system containing Fe/Al oxide-coated quartz and rice roots

When the Fe/Al oxide-coated quartz particles and rice roots were in contact with electrolyte solutions, two types of electric double layers were formed on their surfaces. In the binary system containing Fe/Al oxide-coated quartz and rice roots, the overlapping of the diffuse layers of the two types of electric double layers may occur between oppositely charged adjacent surfaces, which would influence surface chemical properties at the Fe/Al oxide particles/roots interface and change zeta potentials of the binary systems containing Fe/Al oxide-coated quartz particles and rice roots (Fig. 5). The overlapping interactions of the diffuse layers on positively charged Fe/Al oxides and negatively charged rice roots were similar with those on Fe/Al oxides and negatively charged kaolinite and quartz reported in our previous studies (Hou et al. 2007a; Li and Xu 2008; Li et al. 2016). The differences of zeta potentials between the binary systems containing oppositely charged particles and the single system of kaolinite were used to express the interaction extent between oppositely charged particles in our previous studies (Hou et al. 2007a; Li and Xu 2008). Similarly, the differences of zeta potentials between the binary systems containing Fe/Al oxide-coated quartz particles and rice roots and the single system of rice roots were used to express the interaction extent between Fe/Al oxides and rice roots in this study.
Fig. 5

Zeta potential of rice roots and binary systems containing Fe/Al oxide-coated quartz and rice roots. Zeta potential was measured with the streaming potential method using NaCl as the supporting electrolyte at electric conductivity of 80 μS cm−1. Mass ratio of Fe/Al oxide-coated quartz to rice roots was 70:3. The bars are the standard errors of the means (n = 6)

The zeta potentials of the binary systems containing Fe/Al oxide-coated quartz particles and rice roots were higher than those of rice roots alone (Fig. 5), suggesting that the overlapping of the diffuse layers on positively charged Fe/Al oxide-coated quartz particles and negatively charged rice roots reduced the effective negative charge on the roots, similar to the interactions of Fe/Al oxides with kaolinite (Hou et al. 2007a; Li and Xu 2008). The amount of positive charge was greater on Al oxide- than on Fe oxide-coated quartz particles at the same pH (Fig. 3), which led to a stronger interaction of rice roots with Al oxide- than with Fe oxide-coated quartz particles, and a greater shift of the zeta potential to positive values compared with Fe oxide-coated quartz particles (Fig. 5).

Confirmation of the overlapping of diffuse layers by leaching experiments

The overlapping of diffuse layers on positively charged Fe/Al oxides and negatively charged rice roots was confirmed by the results of leaching experiments (Fig. 6). The EC of the leachates from the columns contained Fe/Al oxide-coated quartz and rice roots was lower than that from the column contained quartz and rice roots. The EC of the leachate from the column that contained Al oxide-coated quartz was much lower than the leachates from the columns that contained quartz and Fe oxide-coated quartz (Fig. 6). The changing trends of NO3 concentration in the leachates were similar with EC. The concentration of NO3 in leachate of the column containing Al oxide-coated quartz was much lower than the leachates of the columns that contained quartz and Fe oxide-coated quartz, suggesting that more NO3 was adsorbed in Al oxide-containing system compared with Fe oxide-containing system and quartz with rice roots. The concentration of K+ in the first five PV for all three columns was similar and very low. After the fifth PV, K+ concentration in the leachate of the column containing Al oxide-coated quartz was much lower than the leachates of the columns containing quartz and Fe oxide-coated quartz (Fig. 6). These results suggested that more K+ and NO3 were adsorbed simultaneously in the column containing Al oxide-coated quartz than those in the columns containing Fe oxide-coated quartz and quartz, which was also responsible for the lower EC in the leachates from the column containing Al oxide-coated quartz than the other two columns.
Fig. 6

Changes of electric conductivity and concentrations of K+ and NO3 in the leachates of the column packed with rice roots and quartz with or without Fe/Al oxides leached with 0.1 mM KNO3. The bars are the standard errors of the means (n = 3)

K+ and NO3 were all electrostatically adsorbed by negatively and positively charged surfaces, respectively. During the adsorption, K+ and NO3 enter into diffuse layers of the electrical double layers on negatively and positively charged surfaces and act as counter ions. According to the principles of surface chemistry, ions were adsorbed by charged surfaces electrostatically through ion exchange reactions (Qafoku and Sumner 2002; Li et al. 2009). When K+ and NO3 were adsorbed by rice roots and Fe/Al oxide-coated quartz in the binary systems in the leaching experiments, the equivalent cation and anion should be released into leachates and EC values of the leachates from different binary systems should be similar. However, the EC values of leachates from the columns containing Fe/Al oxide-coated quartz were lower than the EC of leachate from the column containing quartz (Fig. 6), indicating that more K+ and NO3 were adsorbed, but less cation and anion were released in the binary systems containing Fe/Al oxide-coated quartz than that containing quartz. This is due to overlapping of diffuse layers on oppositely charged surfaces. The overlapping of diffuse layers led to neutralization of positive charge on Fe/Al oxides with negative charge on rice roots partially (Fig. 5). When K+ and NO3 entered into the diffuse layers on rice roots and Fe/Al oxides to balance surface charges on these charged surfaces, the overlapping of diffuse layers on oppositely charged surfaced decreased and simultaneous adsorption of K+ and NO3 occurred in the binary system containing oppositely charged surfaces (Qafoku and Sumner 2002; Li et al. 2009). The extent of diffuse layer overlapping on oppositely charged surfaces depended on the amounts of surface charges on these surfaces (Hou et al. 2007b). The greater amount of positive charge on Al oxide-coated quartz led to the larger extent of diffuse layer overlapping between Al oxide-coated quartz and rice roots compared with Fe oxide-coated quartz, which was the reason for more adsorption of K+ and NO3 simultaneously and a greater reduction in the leachate EC in the binary system containing Al oxide-coated quartz and rice roots than that containing Fe oxide-coated quartz and rice roots.

Overlapping of the diffuse layers on oppositely charged surfaces and its effect on neutralizing opposite charge are completely reversible with change of ionic strength (Qafoku and Sumner 2002; Li et al. 2009; Li and Xu 2013). After leaching experiments with KNO3, the leaching solution was replaced with deionized water to flush the column, and an opposite trend was observed for both EC and concentrations of K+ and NO3 in the leachates (Fig. 7), compared with the data shown in Fig. 6. The leachate EC and concentrations of K+ and NO3 in the leachate from the column containing rice roots and Al oxide-coated quartz were greater than in the leachates from the columns containing rice roots and Fe oxide-coated quartz or quartz. These results suggested that the stronger electrostatic attraction between oppositely charged surfaces of Al oxide and rice roots enhanced desorption of K+ and NO3 in the binary systems containing Al oxide-coated quartz and rice roots, compared with the systems containing rice roots and Fe oxide-coated quartz or quartz, and subsequently, the overlapping of diffuse layers on oppositely charged surfaces of Al oxide and rice roots occurred again. Therefore, the data presented in Fig. 7 further confirmed the overlapping of diffuse layers on oppositely charged Al oxide and rice roots.
Fig. 7

Changes of electric conductivity and concentrations of K+ and NO3 in the leachates of the column packed with rice roots and quartz with or without Fe/Al oxides leached with deionized water. The bars are the standard errors of the means (n = 3)

The results from column leaching experiments in Figs. 6 and 7 also suggested that the overlapping of diffuse layers between rice roots and Fe/Al oxides affected adsorption and desorption of ions in the binary system containing Fe/Al oxides and rice roots. The addition of electrolyte solution into the binary systems led to simultaneous adsorption of cations and anions due to the overlapping of diffuse layers on oppositely charged surfaces. However, when the binary systems with ions adsorbed were leached with deionized water, the overlapping of diffuse layers on oppositely charged surfaces enhanced the desorption of cations and anions from charged surfaces due to the decrease in effective surface charge and electrostatic attraction to ions.

Environmental implications

Fe/Al oxide particles are important active components in soils, especially in variable charge soils in tropical and subtropical regions, and mainly exist as coatings on soil phyllosilicate mineral particles (Jefferson et al. 1975; Hendershot and Lavkulich 1983; Qafoku et al. 2000). Therefore, the interaction of electric double layers between the variable charge soils and plant roots can occur under field conditions when plants are growing. The overlapping interaction of diffuse layers between Fe/Al oxide particles and plant roots decreased the effective charge at the interface of Fe/Al oxide particles/plant roots and consequently affect the adsorption, absorption, and transport of nutrients, metals, heavy metals, and organic pollutants by plant roots (Wang et al. 2014). Soil contamination with heavy metals due to industrial contamination occurred in tropical and subtropical regions of India, China, South America, and Africa. The accumulations of these toxic metals in plant tissues, stems, fruits, and leaves have adverse effects on humans, animals, and other plant live. The overlapping of diffuse layers on plant roots with those on soil Fe/Al oxides decreased the effective negative charge on plant roots and thus the adsorption affinity of the roots to heavy metal cations and subsequently reduced the transport of these metals from the roots to stems, as well as leaves and fruits of the plants. This was beneficial to reduce the environmental risks of heavy metals in soils to humans and animals.

In summary, zeta potential reflects the charge status of the charged surface when it contacts with electrolyte solution. The streaming potential method can characterize the zeta potentials of Fe/Al oxide-coated quartz particles, rice roots, and the interaction between them as influenced by solution conditions. Root surface potential of plants plays a significant role in ion adsorption (Nagata and Melchers 1978; Kinraide et al. 1992, Kinraide 2003, 2006; Wang et al. 2011, 2014) and cell–cell interactions (Nagata and Melchers 1978). With the help of the streaming potential method, the overlapping of diffuse layers on rice roots with those on Fe/Al oxides was confirmed in the present study. This is the first time that the overlapping of diffuse layers on plant roots with those on Fe/Al oxides has been reported to our knowledge. The overlapping of diffuse layers on plant roots with those on Fe/Al oxides will decrease the effective negative charge on plant roots and thus the adsorption affinity of the roots to heavy metal cations and subsequently may inhibit the absorption and transport of heavy metals from roots to stems, as well as leaves and fruits of the plants.

Conclusions

The streaming potential method can be used to measure zeta potential of rice roots and its mixture with Fe/Al oxide-coated quartz and study the interactions of Fe/Al oxides with plant roots. The overlapping of diffuse layers occurred between positively charged Fe/Al oxides and negatively charged rice roots, which led to neutralization of positive charge on Fe/Al oxides with negative charge on rice roots and affected adsorption and desorption of ions onto and from the charged surfaces of the oxides and rice roots. The greater amount of positive charge on Al oxide led to the stronger interaction of Al oxide-coated quartz with rice roots and the greater effect of the interaction on adsorption and desorption of ions on Al oxide and rice roots. The results obtained in this study will provide useful references for understanding the interaction between plant roots and Fe/Al oxides in variable charge soils and its effect on adsorption and uptake of ions by plant roots.

Notes

Funding information

This study was supported by the National Natural Science Foundation of China (41230855).

References

  1. Childress AE, Elimelech M (1996) Effect of solution chemistry on the surface charge of polymeric reverse osmosis and nanofiltration membranes. J Membr Sci 119:253–268CrossRefGoogle Scholar
  2. Fan XR, Jia LJ, Li YL, Smith SJ, Miller AJ, Shen QR (2007) Comparing nitrate storage and remobilization in two rice cultivars that differ in their nitrogen use efficiency. J Exp Bot 58:1729–1740CrossRefGoogle Scholar
  3. Hendershot WH, Lavkulich LM (1983) Effect of sesquioxide coatings on surface charge of standard mineral and soil samples. Soil Sci Soc Am J 47:1252–1260CrossRefGoogle Scholar
  4. Hou T, Xu RK, Zhao AZ (2007a) Interaction between electric double layers of kaolinite and Fe/Al oxides in suspensions. Colloid Surf A 297:91–94CrossRefGoogle Scholar
  5. Hou T, Xu RK, Tiwari D, Zhao AZ (2007b) Interaction between electrical double layers of soil colloids and Fe/Al oxides in suspensions. J Colloid Interface Sci 310:670–674CrossRefGoogle Scholar
  6. Jefferson D, Tricker M, Winterbottom A (1975) Electron-microscopic and Mössbauer spectroscopic studies of iron-stained kaolinite minerals. Clay Clay Miner 23:355–360CrossRefGoogle Scholar
  7. Kinraide TB (2003) The controlling influence of cell-surface electrical potential on the uptake and toxicity of selenate (SeO4 2−). Physiol Plant 117:64–71CrossRefGoogle Scholar
  8. Kinraide TB (2006) Plasma membrane surface potential (ψpm) as a determinant of ion bioavailability—a critical analysis of new and published toxicological studies and a simplified method for the computation of plant ψpm. Environ Toxicol Chem 25:3188–3198CrossRefGoogle Scholar
  9. Kinraide TB, Ryan PR, Kochian LV (1992) Interactive effects of Al3+, H+, and other cations on root elongation considered in terms of cell-surface electrical potential. Plant Physiol 99:1461–1468CrossRefGoogle Scholar
  10. Kinraide TB, Yermiyahu U, Rytwo G (1998) Computation of surface electrical potentials of plant cell membranes-correspondence to published zeta potentials from diverse plant sources. Plant Physiol 118:505–512CrossRefGoogle Scholar
  11. Kopittke PM, Wang P, Menzies NW, Naidu R, Kinraide TB (2014) A web-accessible computer program for calculating electrical potentials and ion activities at cell-membrane surfaces. Plant Soil 375:35–46CrossRefGoogle Scholar
  12. Li SZ, Xu RK (2008) Electrical double layers’ interaction between oppositely charged particles as related to surface charge density and ionic strength. Colloid Surf A 326:157–161CrossRefGoogle Scholar
  13. Li JY, Xu RK (2013) Inhibition of acidification of kaolinite and an Alfisol by aluminum oxides through electrical double-layer interaction and coating. Eur J Soil Sci 64:110–120CrossRefGoogle Scholar
  14. Li SZ, Xu RK, Li JY (2009) Interaction of electrical double layers between oppositely charge particles in variable-charge soils as related to source to salt adsorption. Soil Sci 174:27–34CrossRefGoogle Scholar
  15. Li ZY, Liu Y, Zheng YY, Xu RK (2015) Zeta potential at the root surfaces of rice characterized by streaming potential measurements. Plant Soil 386:237–250CrossRefGoogle Scholar
  16. Li ZY, Xu RK, Li JY, Hong ZN (2016) Effect of clay colloids on the zeta potential of Fe/Al oxide-coated quartz: a streaming potential study. J Soils Sediments 16:2676–2686CrossRefGoogle Scholar
  17. Liu ZD, Li JY, Jiang J, Hong ZN, Xu RK (2013) Adhesion of Escherichia coli to nano-Fe/Al oxides and its effect on the surface chemical properties of Fe/Al oxides. Colloid Surf B 110:289–295CrossRefGoogle Scholar
  18. Liu ZD, Wang HC, Li JY, Hong ZN, Xu RK (2015) Adhesion of Escherichia coli and Bacillus subtilis to amorphous Fe and Al hydroxides and their effects on the surface charges of the hydroxides. J Soils Sediments 15:2293–2303CrossRefGoogle Scholar
  19. Meychik NR, Yermakov IP (1999) A new approach to the investigation on the tonogenic groups of root cell walls. Plant Soil 217:257–264CrossRefGoogle Scholar
  20. Meychik NR, Nikolaeva JI, Yermakov IP (2005) Ion exchange properties of the root cell walls isolated from the halophyte plants (Suaeda altissima L.) grown under conditions of different salinity. Plant Soil 277:163–174CrossRefGoogle Scholar
  21. Nagata T, Melchers G (1978) Surface charge of protoplasts and their significance in cell-cell interaction. Planta 142:235–238CrossRefGoogle Scholar
  22. O'Shea P, Walters J, Ridge I, Wainright M, Trinci APJ (1990) Zeta potential measurements of cell wall preparations from Regnellidium diphyllum and Nymphoides peltata. Plant Cell Environ 13:447–454CrossRefGoogle Scholar
  23. Pansu M, Gautheyrou J (2006) Handbook of soil analysis-mineralogical, organic and inorganic methods. Springer-Verlag, BerlinCrossRefGoogle Scholar
  24. Qafoku NP, Sumner ME (2002) Adsorption and desorption of indifferent ions in variable charge subsoils-the possible effect of particle interactions on the counter-ion charge density. Soil Sci Soc Am J 66:1231–1239CrossRefGoogle Scholar
  25. Qafoku NP, Sumner ME, West LT (2000) Mineralogy and chemistry of some variable charge subsoils. Commun Soil Sci Plant Anal 31:1051–1070CrossRefGoogle Scholar
  26. Qafoku NP, Evan R, Noble A, Baert G (2004) Variable charge soils: their mineralogy, chemistry and management. Adv Agron 84:159–215CrossRefGoogle Scholar
  27. Wang YP, Xu RK, Li JY (2011) Effect of Fe/Al oxides on desorption of Cd2+ from soils and minerals as related to diffuse layer overlapping. Soil Res 49:231–237CrossRefGoogle Scholar
  28. Wang YM, Kinraide TB, Wang P, Zhou DM, Hao XZ (2013a) Modeling rhizotoxicity and uptake of Zn and Co singly and in binary mixture in wheat in terms of the cell membrane surface electrical potential. Environ Sci Technol 47:2831–2838CrossRefGoogle Scholar
  29. Wang YP, Xu RK, Li JY (2013b) Effect of Fe/Al hydroxides on desorption of K+ and NH4 + from two soils and kaolinite. Pedosphere 23:81–87CrossRefGoogle Scholar
  30. Wang YM, Kinraide TB, Wang P, Hao XZ, Zhou DM (2014) Surface electrical potentials of root cell plasma membranes: implications for ion interactions, rhizotoxicity, and uptake. Int J Mol Sci 15:22661–22677CrossRefGoogle Scholar
  31. Wu YH, Hendershot WH (2008) Cation exchange capacity and proton binding properties of pea (Pisum sativum L.) roots. Water Air Soil Pollut 200:353–369CrossRefGoogle Scholar
  32. Xu RK, Xiao SC, Jiang J, Wang YP (2011) Effects of amorphous Al(OH)3 on the desorption of Ca2+, Mg2+, and Na+ from soils and minerals as related to diffuse layer overlapping. J Chem Eng Data 56:2536–2542CrossRefGoogle Scholar
  33. Yu TR (1997) Chemistry of variable charge soils. Oxford University Press, New YorkGoogle Scholar
  34. Zheng SJ, Lin XY, Yang JL, Liu Q, Tang CX (2004) The kinetics of aluminum adsorption and desorption by root cell walls of an aluminum resistant wheat (Triticum aestivum L.) cultivar. Plant Soil 261:85–90CrossRefGoogle Scholar

Copyright information

© Springer-Verlag GmbH Germany 2017

Authors and Affiliations

  • Zhao-dong Liu
    • 1
    • 2
  • Hai-cui Wang
    • 1
    • 2
  • Jiu-yu Li
    • 1
  • Ren-kou Xu
    • 1
  1. 1.State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil ScienceChinese Academy of SciencesNanjingChina
  2. 2.University of Chinese Academy of SciencesBeijingChina

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