Electrokinetic remediation of Cd-contaminated soil using low voltage gradients coupled with array adsorption zone and polarity exchange
Graphical Abstract
Introduction
Heavy metal contamination in soil is a global challenge that restricts economic development and threatens human health (Gorospe, 2012, Ji et al., 2017, Toth et al., 2016, Kumar et al., 2018). Various remediation techniques have been proposed to address soil contaminated by heavy metals, such as bioremediation (Guo et al., 2020, Zheng et al., 2020a, Jiang et al., 2019), electrokinetic remediation (EKR) (Ma et al., 2010, Rezaee et al., 2018a, Xu et al., 2020) and immobilization techniques (Zheng et al., 2020b, Basta and Mcgowen, 2004, Qi et al., 2020). Bioremediation is greatly limited by environmental factors and time-inefficient. Immobilization techniques focus on reduction of bioavailability of heavy metals, so immobilized heavy metals in the soil remain risky in the presence of environmental disturbances.
Over the past few decades, EKR technology is an emerging technique, which is considered as one of the most potential separation technologies for contaminant removal from soil (Wen et al., 2020). In a typical EKR system, heavy metals migrate through electroosmosis, electromigration, and electrophoresis and accumulate on/near the electrodes (Virkutyte et al., 2002, Rezaee and Asadollahfardi, 2018b). However, the removal efficiency of heavy metals using EKR technology alone is rather limited. It is one of the obstacles that the migration of heavy metal was hindered by precipitation of metal hydroxides due to the generation of OH- at the cathode (Probstein and Hicks, 1993). Many methods have been proposed to improve the EKR efficiency, including conditioning catholyte pH (Zhou et al., 2004), ion exchange membranes (Ottosen et al., 2003), approaching anodes (Shen et al., 2007), polarity exchange (Pazos et al., 2006), combination with adsorption (Baskaran et al., 2020, Hussain et al., 2013, Ma et al., 2010) or permeable reactive barrier (He et al., 2020). These single or combined enhanced processes commonly apply 1 V·cm−1 or greater voltage gradient to obtain greater efficiency (Wen et al., 2020). Strong electric field leads to side effects including noticeable pH change, higher soil temperature and faster drop in moisture content, which in turn inhibits the increase in removal efficiencies (Zhou et al., 2020). These side effects will not only have a negative impact on the soil, but also greatly increase energy consumption. Therefore, an enhanced method with high heavy metal removal efficiency at low voltage gradients should be developed. The study of Cai et al. (2021) indicated that there is a high potential for EKR to remove Cd from agricultural soils with low voltage and low energy consumption. However, the studies on EKR using low voltage gradients are rather limited.
A hybrid electrokinetic–adsorption cell was operated at the voltage gradient of 0.2 V·cm−1 for 35 days and 36% of Cd was removed (Mu'Azu et al., 2016a). When the voltage gradient of 0.5 V·cm−1 was applied to the electrodynamic geosynthetics EKR, the Cd content was reduced by 26% (Tang et al., 2017). All these results suggest that the removal efficiency of heavy metals was not efficient enough when low voltage gradients were applied, which was due to the weak migration of heavy metals. During the remediation process, it is difficult for all heavy metals to accumulate near the cathode, in the cathode electrolyte or in the treatment zones. Moreover, conventional adsorption zone was usually a rectangular zone filled with adsorbent and installed near the cathode (Mu'Azu et al., 2016a, Baskaran et al., 2020). There have been no enhancement methods to improve the removal efficiency of heavy metals by capturing heavy metals in the array adsorption zones composed of staggered adsorption columns at low voltage gradients. Zhao et al. (2022) coupled 0.2 V·cm−1 voltage gradient with granular activated carbon to remediate Cd-contaminated soil, and the removal efficiency achieved 61.05% after 28 days. The result suggested that it is feasible to achieve Cd removal from soil by short-distance migration under the action of a low-voltage electric field. Therefore, it may be possible to try to improve the removal efficiency of heavy metals by using the array adsorption zone. Various reports have shown that the polarity exchange prevents or re-dissolutes precipitates of heavy metals in the soil near the cathode (Lu et al., 2012, Zhou et al., 2017), but the reciprocating motion caused by polarity exchange and its effects were neglected. It is estimated that the reciprocating motion of heavy metals is a side effect that reduces the removal efficiency in conventional EKR, and the negative effect is weakened after arranging the array adsorption zones.
Herein, an enhanced EKR method that uses low voltage gradient (0.2 V·cm−1) coupled with array adsorption zone and polarity exchange was proposed, which was used for the remediation of Cd-contaminated soil. The objectives of this study were (1) to confirm the reciprocating motion of Cd2+ due to polarity exchange and its effects on electrokinetic remediation coupled with polarity exchange (PEKR) and PEKR coupled with array adsorption zone (PEKR-AAZ), (2) to identify the removal efficiency of Cd by PEKR-AAZ, (3) to further evaluate the soil remediation effect via pot cultivation of wheat. Overall, this study aimed to provide a potential efficient EKR technology for remediating Cd-contaminated soils by applying a low voltage gradient.
Section snippets
Chemicals and materials
All chemicals were analytical grade and purchased from Tianjin Comio Chemical Reagent Co., Ltd. (Tianjin, China). Soil samples were collected from 0 to 20 cm surface layer of a farmland in Tianjin city, China (E 117°2′, N 39°13′). The soil samples were air-dried and ground, and passed through a 2 mm sieve. The physical and chemical properties of the soil samples are shown in Table 1. The tested soil was classified as alkaline clay. The collected soil was contaminated with Cd(NO3)2 solution and
Effect of polarity exchange on soil pH
Fig. 2a shows the pH profile of EKR after 10 d and 30 d treatment. Soil pH decreased to 4 at the soil section close to anode (P1), and increased to 10 at the soil section close to cathode (P6). The change in soil pH is due to the electrolysis of water producing H+ at the anode and OH- at the cathode. As a result, the soil region near the anode became acidic and the region near cathode became alkaline. Moreover, the production of OH- leads to the production of hydroxide precipitation of Cd,
Conclusion
The feasibility of PEKR-AAZ technology using low voltage gradient (0.2 V·cm−1) to remediate Cd-contaminated soil was investigated and Cd could be efficiently removed from contaminated soil. The reciprocating motion of Cd due to polarity exchange is a side effect in conventional PEKR, but is overcome in PEKR-AAZ and may even facilitate the removal of Cd. The pot experiments on wheat demonstrate that the bioavailability of heavy metals can be significantly reduced by PEKR-AAZ remediation. The
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The study was supported by the China National Key Research and Development (R&D) Program (No. 2019YFC1904102).
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