Yue-nan Li, Yan-sheng Gu, Man-zhou Li, Guang-jie Huo, Xi-ping Wang, Zhi-jie Xu, Jie Yue, Dan Du,Man-ge Geng

1 State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan 430074, China.

2 Natural Resources Monitoring Institute of Henan Province, Zhengzhou 450016, China.

Abstract: The plant samples of Bidens pilosa were collected from a coal gangue vacant site and its surrounding area, located in central China, to study the remediation effect of the plant species on heavy metal (HM) contamination in both natural and electrokinetic (EK) conditions. The analytical results showed that the effect of phytoextraction and bioconcentration on the heavy metals in the sample of the EK group is more significant than those in the other control group. Compared with the results of natural condition, under the EK condition the concentrations of cadmium (Cd), lead (Pb), copper (Cu) and zinc (Zn) in the stems and leaves of the Bidens pilosa increased to 0.40 mg/kg, 4.23 mg/kg, 7.27 mg/kg, 830.24 mg/kg, respectively,with their increments of 292%, 1 731%, 141%, 2 076%. For root samples, the Cd, Pb, Cu and Zn concentrations increased to 0.52 mg/kg, 4.36 mg/kg, 10.87 mg/kg, and 98.12 mg/kg and the increase rates were 1 034%, 140%, 29%, and 181%, respectively. The phytoextraction efficiency of the Bidens pilosa was significantly higher than that of control group. The removal efficiency of Cd, Pb, Cu and Zn in soil increased to 26%, 72%, 27%, and 79% with the EK applied. In addition, the mechanism of HM migration,extraction and enrichment in Bidens pilosa under the EK condition was discussed.

Keywords: Electrokinetic; Heavy metal; Bidens pilosa; Coal gangue area; Phytoextraction Received: 30 Dec 2019/ Accepted: 10 Dec 2020

Introduction

Phytoremediation has recently become one of the most popular research topics in the field of environmental science because of its costeffectiveness, environmental-friendliness and insitu remediation capacity (Zeng et al. 2019; Zhu et al. 2019; Jacob et al. 2018). However, there are certain limitations of this approach to dealing with contamination problem, which mainly include the dependence on contaminant bioavailability and long duration and depth of the bio-treatment(Cameselle et al. 2013; Couto et al. 2015).Electrokinetic (EK) assisted phytoremediation may enhance phytoaccumulation by promoting contaminant bioavailability (solubilization and selective mobilization enhancement of metal/metalloid desorption-transport). It combines the advantages of both phytoremediation and EK technologies that drive contaminants to migrate from soil to plants(Zhang et al. 2016; Hassan et al. 2018).

Many small scale pot experiments were conducted to validate the effectiveness of EK assisted phytoremediation. Cang et al. (2011)suggested that 2 V/cm of voltage gradient was optimum for Indian mustard (Brassica juncea)phytoremediation. Bi et al. (2011) stated that alternating current (AC) changed not only the migration direction and distribution of heavy metals (HM), but also enhanced the bioenergy of plants and improved the plant accumulation efficiency of HMs. Chen et al. (2007) conducted pot experiments and the results showed that the amount of copper (Cu) absorbed by ryegrass increased by 311 mg/kg with an electric field of 1 V/cm applied. Their pot experiments also showed that the direct current (DC) electric field promoted the growth of Sedum alfredii. Under DC field, the cadmium (Cd) absorption of the above-ground(AG) part of Sedum alfredii was significantly increased. When the voltage was 1.0 V/cm, the duration of EK assistance was 6 hours per day, and the electric field direction was exchanged once every two days. The Cd uptake of Sedum alfredii was the highest, reaching 90.6 μg/kg (Xu, 2015).Yao et al. (2015) found that the HM concentration in the AG part of Sedum alfredii was significantly increased under EK condition for 20 days and 0.5 V/cm. The phytoextraction efficiency dropped significantly after 40 days of continuous EK application.

Zhou et al. (2005) used citric acid to make a complexation with trivalent chromium (Cr3+) in the soil. Pazos et al. (2006), Xi et al. (2009) and Lu(2009) conducted a polarity exchange test on EK,in which the H+was produced by the anode and the OH-generated by the cathode to ensure the neutralization during the test, preventing the formation of alkaline and acid area. Zheng et al.(2007) proposed a moving electrode technology which reduced the pH value of the soil near the electrode, and increased the free state of HMs and their removal rate. However, the aforementioned researches subject to secondary pollution or excessive soil acidification, and these adverse influences would undermine the eco-friendliness advantage of phytoremediation.

In brief, EK assisted phytoremediation technologies mentioned above have the advantages of insitu, green, no secondary pollution, simple operation, low operation, low energy consumption, and low cost, from which HM pollutants can be recycled and reused (Cameselle et al. 2013; Zhou et al. 2011; Siyar et al. 2020). However, such technology is still in the small scale experimental stage, and requires further improvement. Moreover,the application of the EK assisted phytoremediation by using Bidens pilosa has not been reported.Therefore, in this study a coal gangue vacant site with its surrounding areas was selected as the research area. Based on the investigation of HM pollution and the screening of the autochthonous phytoextractor (Li et al. 2017; 2018), Bidens pilosa was selected to carry out the in-situ EK assisted phytoremediation experiments to compare the characteristics of the phytoextraction and bioconcentration between the target HMs under the EK condition and natural conditions. Subsequently, the results could provide the scientific foundation and large scale implement guidance for using EK assisted phytoremediation with Bidens pilosa in coal mining areas.

1 Materials and methods

1.1 Research site and HM pollution

The in-situ EK assisted phytoremediation site is located at a coal gangue vacant area in Henan,central China (Fig. 1). The soils in the surrounding area of the mine have been polluted by Cd, Pb, Cu,Zn and Cr (Li et al. 2018), which are the most common toxic heavy metals (Agwu et al. 2018).There are 46.2% of the total samples with potential ecological risk index higher than 300, which caused strong or extremely strong ecological risk(Li et al. 2018). The main constituents of coal gangue are SiO2, Al2O3and Fe2O3. The gangue pH is 7.6-8.4 and the soil pH is 7.1-8.2.

Fig. 1 Map showing the research site

1.2 Experiment design

Two experiment plots were designed in order to compare the results between the natural (control)group and the EK group. In each plot, Bidens pilosa was planted on February 28th, 2013, and harvested on October 25th, 2013. There were in total 72 plant samples and 24 soil samples collected from the experimental plots.

As shown in Fig. 2, the EK system mainly included DC power supply and aluminum foil electrode. The DC power supply is RXN-603D (0-60 V, 0-3 A adjustable). The aluminum foil paper was cut into strips of 15×12 cm and buried vertically into the contaminated soil of the experimental plots. The aluminum foil paper was connected to the DC power source to serve as electrodes, with a voltage of 30 V and an electrical current of 30 mA. In order to alleviate the adverse effects of excessive acidification and alkalization of the soil in the anode and cathode regions during the experiment, the DC power was cut off for 12 hours (8:00 pm to 8:00 am of the next day) every day as well as in the case of rainfall. In addition,the experimental plots were watered on every Friday, with the intension of the site soil dilution to avoid excessive concentration of H+and OH-ions.

1.3 Sample preparation and HM determination

The detailed sampling scheme is shown in Table 1.The plant samples were washed three times using distilled water and then oven-dried. Subsamples were ground and mixed to form a representative sample, sieved through 100 mesh sieve, and digested with an H2O2-HClO4-HF-HNO3method in PTFE jars on an electric heating plate. Two Chinese national standard samples (GBW07427,GBW10048) and one blank sample were used as references to control the quality of the HM determination process. The total concentrations of cadmium (Cd), lead (Pb), copper (Cu), chromium(Cr) and zinc (Zn) in the plants and soil samples were measured by inductively coupled plasmaatomic emission spectroscopy (ICP-AES, IRIS Intrepid II XSP, Thermo Scientific, Waltham, MA,USA). The recovery rates of the standard samples were within 90%-110%.

Fig. 2 In-situ EK assisted phytoremediation system setup

Table 1 Detailed sampling scheme for EK and control groups

1.4 Parameters for analysis

Two parameters that widely used in phytoremediation experimental tests are Bio Concentration Factor (BCF) and Translocation Factor (TF)(Tangahu et al. 2011; Badr et al. 2012; Van der Ent et al. 2013). The BCF refers to the ratio of heavy metal concentration in plants to that in soil, while the TF means the ratio of the concentration of the same HM in the AG and root (Sun et al. 2009;Pratas et al. 2013; Paulo et al. 2014). They can be calculated using the following equations:

Fig. 3 Heavy metal concentrations in the AG (blue bars) and root (red bars) of the Bidens pilosa in the control group

In addition, for assessing the removal efficiency of soil HMs in the remediation site, the soil HM removal rate is used, which is presented in Equation (3).

Where:Reis the soil HM removal efficiency(%),Rbis HM concentration of the soil before remediation (mg/kg) andRathe HM concentration of the soil after remediation (mg/kg).

2 Results and discussion

2.1 Phytoextraction in control group

The analytical results showed that, under natural condition, the Cd concentration was 0.102 mg/kg in the AG part of the Bidens pilosa, and that of the root was 0.046 mg/kg, with a TF value of 2.217.The Cd BCF of the AG part and root were 0.16 and 0.07, respectively. The concentration of Pb was 0.236 mg/kg in the AG part and 1.815 mg/kg in the plant root, with a associated TF of 0.130. The Pb BCF of the AG and root were 0.005 and 0.038,respectively. For Cu, the concentrations of the AG and root were 3.010 mg/kg and 8.395 mg/kg. This resulted in the Cu TF lower than 1, which was 0.359. The Cu BCF of AG part was 0.10, which was lower than that of the root (0.28). The Zn concentration of the AG and the root were 38.15 mg/kg and 34.90 mg/kg, with the TF of 0.577 (1.09). The Cr (Zn) BCFs in the AG and of the root were 0.14 (0.41) and 0.24 (0.37), respectively, which is shown in Fig. 3. These results show that the Bidens pilosa has a great potential of HM phytoextraction, which consists with the earlier report (Li et al. 2017). However, the relative low BCFs suggests that there are certain limitation factors, such as low proportion of the exchangeable HM ions in total HM concentration (Ma et al.2015; Bian et al. 2019).

2.2 Phytoextraction in EK assisted group

Under EK condition, the Cd concentration was 0.40 mg/kg in the AG and 0.52 mg/kg in the root.The AG and root BCFs were 0.82 and 1.07,respectively, with a TF value of 0.76. The concentration of Pb was 4.32 mg/kg in the AG and 4.368 mg/kg in the root of the Bidens pilosa. The AG and root BCFs were both 0.13; and its TF was 0.99. The Cu concentration in the AG part was 7.27 mg/kg and 10.87 mg/kg in the root. The AG and root BCFs were 0.27 and 0.41, respectively;and its TF was 0.669. The Cr concentrations were 5.31 mg/kg in plant AG part and 13.74 mg/kg in the root, respectively, and the AG and root BCFs were 0.09 and 0.24, with a TF of only 0.387. The concentration of Zn was 830.25 mg/kg in the AG and 98.13 mg/kg in the root. The AG and root BCFs were 10.20 and 1.21 with a TF of 8.46 (Fig. 4).

Fig. 4 Heavy metal concentrations in the AG (blue bars) and root (red bars) of Bidens pilosa under EK condition

2.3 Comparison between control and EK groups

Compared with the control group, there is considerable improvement in terms of the HM concentrations for Cd, Zn, Cu and Mn in the EK conditions. For example, the Zn concentration in the plant AG part is 38.15 mg/kg in the control group, but it is 830.24 mg/kg in the EK group,which indicates an improvement rate of 2 076.26%.Similarly in the EK condition, the Pb concentration increases to 4.32 mg/kg, with an improvement rate of 1 731.78%; the Cd improvement rate is 292.16%and that of Cu is only 141.69%. It is also noted that the concentration of Cr decreases from 9.15 mg/kg in the control group to 5.31 mg/kg in the EK group, with a reduction rate of 42.02%. Under the EK condition, the bioconcentration effects of the four elements in the root have been significantly improved except for Cr. Among them, the largest increase is Cd, which increases from 0.04 mg/kg to 0.52 mg/kg, and the increase rate is 1 034.78%.Followed by Zn, the concentration increases from 34.90 mg/kg to 98.12 mg/kg in the control group,the improvement rate is 181.16%. The other two elements have an increase rate of about 30%-141%.Cr decreases from 15.880 mg/kg to 13.73 mg/kg,and its decreasing rate is 13.49% (Table 2).

Table 2 Comparison of the HM concentrations in Bidens pilosa under natural and EK conditions

As for the BCF, under the EK condition, the BCFs of Cd, Zn, Cu and Mn have been significantly improved. The highest increment is Pb, which increases from 0.5% to 13%. The BCF of Zn increases from 40% to 1 020%. The BCF of Cd increases from 16% to 81.76%. The Cu increase rate is 172%. Cr decreases from 13% to 9%, with a decease rate of 32% (Table 3). The TFs of Zn, Pb and Cu all increase to different degrees in the EK conditions. The increase rates of Zn and Pb are 674% and 661%, respectively, while the TFs of Cd and Cr reduce by 65% and 32% (Table 4).

Table 3 Comparison of the BCF in Bidens pilosa under natural and EK conditions

Table 4 Comparison of TF in Bidens pilosa under natural and EK conditions

The HM removal efficiencies in soil were calculated and listed in Table 5, which indicates that under EK condition the removal efficiency of Cd, Zn, Cu and Mn in the soil is significant. The highest HM removal efficiency can be found in Zn,with a removal rate increasing from 12% to 79%;and that of Cd increases from 5% to 26%.However, the removal efficiency of Cr drops from 19% to 13%, with a decline rate of 32%. This suggests that the EK assistance strongly promotes the removal of Zn, Cd, Cu and Pb in the soil by Bidens pilosa, but that does not work well with Cr in the soil.

Electrical drive helps HM ions and metal compounds migrate and then be absorbed by plants(Zhao et al. 2012; Liu et al. 2015). Due to the electronic current, the H+of the soil solution in the anode region increases, which changes the pHvalue and redox conditions of the soil (Cang et al.2011; Reddy and Cameselle, 2009; Reddy and Chandhuri, 2009; Siyar et al. 2020). Moderate acidic environment improves the solubility and bioavailability of HMs (Jin et al. 2019). A large number of these ions migrates to the cathode,increasing the concentration of ions in the soil to the root of the Bidens pilosa in the cathode region,which creates a favorable condition of phytoextraction of these metal ions. The moderate acidic environment caused by weak current in the rhizosphere may convert some HMs into a bioavailable state (Hodko et al. 2000). At the same time, the weak current effect may stimulate the growth of biomass (Ahemad, 2019), forming cell structural conditions favorable for the selective absorption and transport of HM ions (Ahemad,2019; Manoj et al. 2020). The application of EK could reduce the transformation of Cr (VI) into Cr(III), which is less bioavailable (Kalčíková et al.2016). This may explain both the decline of Cr phytoextraction and its removal efficiency.

Table 5 Comparison of the soil HM removal efficiency by Bidens pilosa under natural and EK conditions

Aboughalma et al. (2008) stated that a continuous DC electrical field tended to induce pH changes in the soil due to the acid and basic fronts generated by the electrolysis of water. To reduce this adverse effect, this study introduced the periodic supply of power and irrigation which could help dilute and neutralize the excessive accumulation of H+and OH-in the anode and cathode regions, avoiding the formation of peracid or alkali environment. Furthermore, irrigation may also wash the surface of the inert membrane attachment on the electrode and the anode oxygen bubble and the cathode hydrogen bubble, which assists in increasing the energy consumption efficiency. Compared to the control group, the EK assistance not only promotes the growth of Bidens pilosa, but also increases the amount of HMs extracted by the Bidens pilosa.

Due to the limitation of research, the mechanism of EK-induced physicochemical processes affecting the migration of HMs is yet not fully understood. The migration mechanism of soluble HMs (ionic state, exchangeable state) to plants, the root exudates and microbial interactions, the transport mechanism of proteins on the root cell membrane, the mechanism of detoxification, and the mechanism of induction complexation strengthening phytoremediation need further research.

3 Conclusions

The in-situ remediation experiments and comparative study have shown that the HM concentration of Bidens pilosa in the EK group is significantly higher than that in the control group.The plant AG part concentrations of Cd, Pb, Cu and Zn increase from 0.10 mg/kg, 0.23 mg/kg,3.01 mg/kg and 38.15 mg/kg in natural condition to 0.40 mg/kg, 4.32 mg/kg, 7.27 mg/kg and 830.24 mg/kg in EK condition, respectively.Accordingly, the concentrations of Cd, Pb, Cu and Zn in the root increase from 0.04 mg/kg,1.81 mg/kg, 8.39 mg/kg and 34.90 mg/kg in the control group to 0.52 mg/Kg. 4.36 mg/kg,10.87 mg/kg and 98.12 mg/kg in the EK group.Under the EK condition, the phytoextraction efficiency of Bidens pilosa is significantly improved compared with that of natural conditions.The improvement rate of HM removal from the soil for Cd, Pb, Cu and Zn are 420.00%, 22.04%,68.75%, and 558.33%, respectively. Overall, the EK assisted phytoremediation by Bidens pilosa shows potential in a large scale field practice.

Acknowledgements

This study was financially supported by Henan Land and Resources Department (2011-622-38).