Influence of polarity exchange frequency on electrokinetic remediation of Cr-contaminated soil using DC and solar energy
Introduction
Soil is fundamental to human survival and development. Soil pollution caused by industrialization, urbanization, agricultural intensification and economic development was s serious threat to human health and environmental safety (Kim et al., 2012; Lee and Kim, 2010; Ma et al., 2018). Chromium is a toxic element that negatively affects animals and plants metabolic activities, Cr(VI) is oxidizability and high permeability to skin, and teratogenic and carcinogenic effects (Kaviyarasi et al., 2021; Shiv et al., 2021). Currently, soil heavy metal pollution was becoming more and more serious. which showed that the total exceeding standard rate was 16.1 %. Among these results, 19.4 % of the arable land sites exceeded the environmental quality standard (Zhou et al., 2018).
Electrokinetic remediation (EKR) was a relatively cheap, effective and environmentally friendly technology, it could effectively remediate heavy metals contaminated soil, even the soil with low permeability (Vocciante et al., 2017; Wang et al., 2019). In typical EKR applications, electrodes were inserted on both sides of the soil, and low direct current was applied, so pollutants could reach the anode or cathode region through electromigration, electroosmosis and electrophoresis (Kim et al., 2015; Masi et al., 2017; Rezaee et al., 2019). The above process was accompanied by electrolysis reactions and geochemical reactions. The oxidation reaction occurs in the anode compartment (2H2O→4H++4e−+O2) to generate hydrogen ions (H+), and the reduction reaction take place in the cathode compartment (2H2O+2e−→H2+2OH−) to generate hydroxide ions (OH−) (Hassan et al., 2015; López-Vizcaíno et al., 2017; Villen-Guzman et al., 2015). H + and OH– during EKR process divide the whole soil area into acidic and alkaline areas. Chemical precipitation and adsorption of heavy metals usually occurred under basic pH conditions, which could block the pore of soil and simultaneously result in the generation of focusing phenomenon (Li et al., 2020), thus preventing the migration of heavy metals and reducing the efficiency of remediation. Through polarity exchange, the decrease of pollutant removal efficiency caused by "focusing effect" and soil acidification can be effectively reduced, which reverses the polarity of the electric field and neutralizing the H+ and OH− generated (Cai et al., 2015; Lu et al., 2012; Zhou et al., 2018).
Polarity exchange electrokinetic remediation can effectively reduce the accumulation phenomenon in the cathode region of soil, enhance the current, and improve the removal efficiency of Cr in the soil, especially the removal rate of Cr(III). Zhou et al. (2018) used traditional electrokinetic remediation and polarity exchange electrokinetic remediation to test Cr-contaminated soil. Electrokinetic parameters such as current value, electroosmosis, soil pH value, and removal efficiency of three valence states of Cr were studied. The removal efficiency of Cr in different valence states was improved by using polarity exchange electrokinetic remediation instead of traditional electrokinetic remediation technology, and the removal rates of total Cr was 43.65 %, Cr(VI) was 91.88 % and Cr(III) was 19.32 %.
The problem of polarity exchange electrokinetic remediation was the reduced remediation efficiency due to the repeated migration of pollutants. Lu et al. (2012) found that the removal of chromium 88 % and cadmium 94 % under the polarity exchange electrokinetic every 48 h, and the concentration of pollutants in the middle part significantly higher than that at both ends. This meant that the technology could reduce the "focusing effect" of pollutants, but it brought about the problem of reciprocating migration of pollutants.
Many studies had been conducted on the EKR of heavy metals, but only paid little attention to Cr-contaminated soil and polarity exchange electrokinetic remediation. In addition, the conversion between Cr(VI) and Cr(III) during the polarity exchange electrokinetic remediation process has rarely been discussed, and few studies have been conducted on the evolution of current with the concentration of Cr in electrolytes and the factors affecting pH during the process. In this study, the feasibility of using direct current and solar exchange electrodes for remediation of chromium-contaminated soil was investigated in comparison. The results would help to understand the effect of different valence chromium forms on the removal of chromium in soil.
Section snippets
Experimental soil
Chromium-free soil was from Huaqiao University, Fujian Province, China, with a depth of 20 cm. The grain size analysis showed that the texture was loam. The samples were dried at room temperature without rocks, weeds and plant roots, and then crushed to remove large particles through a 10-mesh (2 mm) sieve.
The contaminated soil was prepared by adding K2Cr2O7 solution to the soil, and the content of chromium in the prepared soil reached 1200 mg/kg. After the soil was stabilized for a period of
Changes in electric voltage
Electric voltage is one of the key factors in the EKR of contaminated soil (Rojo et al., 2012). In general, the higher the voltage gradient is, the better the remediation efficiency. The variation trends in electrical voltage during EKR were given in Fig. 2. Fig. 2 shows that the changes in electrical voltage in Tests 1, 2, and 3 were different from those in Tests 4 and 5. In Tests 1, 2 and 3, the electrical voltage remained constant because of the constant DC power supply, so there was overlap
Conclusions
- (1)
Electric voltage was the most important factor affecting the removal efficiency of electrokinetic remediation. When the voltage was constant and stable, the removal efficiency was better. The Cr removal efficiency of DC power was higher than that of solar cells, mainly because solar cells could not provide stable voltage.
- (2)
The results showed that the polarity exchange frequency, electrifying time, voltage and current had influence on the pH of electrolyte and soil, the direction and quantity of
Declaration of Competing Interest
We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled.
Acknowledgements
This work was financially supported by Fujian province Science and Technology project Foundation (2017I01010015), Xiamen Technology project Foundation (3502Z20173050, 3502Z20173052), Henan Technology project Foundation (192102310040), Subsidized Project for Postgraduates’ Innovative Fund in Scientific Research of Huaqiao University (17013087054).
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These authors contributed equally to this work.