Impact of electrokinetic-assisted phytoremediation of heavy metal contaminated soil on its physicochemical properties, enzymatic and microbial activities
Introduction
Phytoremediation is a cost-effective and environment-friendly remediation technique for clean-up contaminated soils. The main advantages of phytoremediation include low cost of implementation and practicality [1], [2]. However, there are also some limitations, such as the need for long remediation periods, and its dependency on the solubility or availability of metals in the rhizosphere. In recent years, a new remediation technology has been developed by combining electrokinetic (EK) with phytoremediation to decontaminate metal-polluted soil.
O’Connor et al. [3] applied a direct-current (DC) electric field together with phytoremediation to restore a Cu- and Cd-contaminated soil. Uptake of Cu and Cd by ryegrass (Lolium perenne L.) increased near the cathode. Similarly, Lim et al. [4] examined the effect of applied voltage, treatment time, and use of chelators on the uptake of lead (Pb) from a polluted soil using Indian mustard. The electric field (together with 0.5 mmol EDTA kg−1 soil) improved Pb uptake by Indian mustard up to 2–4 times relative to the control (without DC electric field). Zhou et al. [5] found that the shoot tissue Cu concentration of ryegrass in treatments which combined EDTA and a vertical DC electric field was 1.46 times of that in those treatments without a DC electric field. Aboughalma et al. [6] studied the influences of AC (alternating current) and DC electric field on potato (Solanum tuberosum var. Kuras) uptake of metals. The results showed that the Zn and Cu uptake in the potato roots under DC treatments was higher than those in the control and AC treatments. Therefore, electrokinetic-assisted phytoremediation technology has a demonstrated-potential to restore contaminated soil.
However, a key requirement for any new remediation methods is to demonstrate that they are not only effective for clean-up but also have little negative effects on soil physicochemical properties and soil health [7], [8], [9]. To our knowledge, there are only very limited reports regarding the effect of electrokinetic-assisted phytoremediation on soil properties.
The possible effects during such combined remediation may result from flowing factors: the DC electric field, plant, and their interaction. Firstly, the electrode reactions results in soil acidification near the anode and alkalization near the cathode, and hence the soil pH and EC change following DC treatments [10], [11]. Chen et al. [12] found that the available soil nitrogen, phosphorus and potassium contents were 1.44, 4.31 and 2.25 times of the initial levels, while soil enzymatic activities were observed to change under DC electric field treatments. Lear et al. [13], [14] and Cang et al. [15] studied the changes of microbial population and soil enzymatic activities after EK treatments. The results showed that the accumulation of pollutants and acidity near the anode were the main reasons for the adverse changes in soil microbial population. However, these studies did not examine the change in available nutrients and microbial properties in different soil sections (i.e. near the anode and near the cathode) and the underlying factors and mechanisms. Secondly, plant growth itself can impact upon the soil properties. Generally, plant growth would improve soil physicochemical properties and increase soil microbial activities [16], [17], but the DC electric field influences plant growth and the soil metal fractionation. Currently, the combined impact of DC electric field and plant on soil physicochemical properties and microbial activities are unclear.
Therefore, the aim of this study is to investigate the change of electric current, available soil nutrients, and microbial activities after electrokinetic-assisted phytoremediation, and to disclose the effect mechanism of DC electric field and plant together on soil properties. These results will be used to guide the practical use of this technology.
Section snippets
Soil
A multiple heavy metal polluted soil (Stragnic Anthrosols) was sampled from a uncultivated land near a smelter in Fuyang City, Zhejiang Province, China, from a depth of 0–20 cm. Soil samples were air-dried and sieved through a nylon sieve (Ø 0.85 mm). Its cation exchange capacity (CEC) was 21.3 cmol (+) kg−1. The soil pH was 7.96 and soil electrical conductivity (EC, 1:2.5 soil to water) was 0.28 mS cm−1. The contents of soil organic carbon (SOC), clay content, and CaCO3 were 20.3 g kg−1, 16.1%, and
Electric current and plant biomass of mustard shoot and root
Fig. 2 shows that the electric currents were 35–47, 6–8, 15–26 and 35–55 mA for the treatments CK-2 V, Y-1 V, Y-2 V, and Y-4 V, respectively. Among the different treatments, the currents in Y-4 V treatment were higher than those in Y-1 V and Y-2 V treatments. For the same voltage treatments (2 V cm−1), the currents in CK-2 V without plant were higher than those in Y-2 V with plant. Generally, the electrical currents are positively correlated with the soil water contents during EK process [24], [25]
Conclusions
In view of the results obtained in the present study, it can be concluded that EK-assisted phytoremediation influences the soil physicochemical properties, enzymatic and microbial activities in different soil regions (anode, middle and cathode regions), especially in those treatments where higher voltages were applied. The operational conditions, especially the electric voltages and currents, were very important for these changes in soil properties. The DC electric field affected soil pH, EC,
Acknowledgements
This work was supported by the National Natural Science Foundation of China (41125007, 20807044) and the International Research Staff Exchange Scheme - ELECTROACROSS (269289). A special thank to Kopittke Peter M. (the University of Queensland, Australia) for his review of the language in this manuscript is due.
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