Influence of polarity exchange frequency on electrokinetic remediation of Cr-contaminated soil using DC and solar energy

Abstract

Electrokinetic remediation (EKR) is a remediation method that can effectively remove heavy metals from soil. Traditional EKR uses an external power source to consume electrical energy, in the present study, through the experiment of different electrode exchange frequency to remediate Cr contaminated soil, the effect of polarity exchange electric remediation and the influencing factors of Cr(VI) to Cr(III) conversion are discussed. Five experiments were conducted using DC power supply with conventional EKR (T1), DC power supply with polarity exchange EKR (T2, T3) and solar energy with EKR (T4, T5). The results showed that the T1 experiment had the maximum removal rate of total Cr (42.03 %) and Cr(VI) (85.50 %) but had an adverse effect on soil pH. T2, which exchanged polarity every 24 h, and T3, which exchanged polarity after 84 h, had smaller adverse effects on soil pH, but the removal efficiency was reduced due to repeated migration. The reduction of Cr(VI) to Cr(III) occurred during the EKR processes, the content of Cr(III) increased greatly. Furthermore, few differences were observed between DC power and solar power on the EKR experiments. Therefore, the combination of electrokinetics and solar energy is a feasible method for Cr-contaminated soil.

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.