Liang Chen
School of Civil Engineering, Tianjin University, Tianjin 300072, China
E-mail: liangchen@tju.edu.cn
Abstract: This study explored the application of a Vertical Electrokinetic system (V-EK) with multilayer electrodes in shallow soil to form an “Electric Sieve” to mitigate and prevent the soil salinization caused by salts rising from shallow groundwater in the coastal areas. In the model V-EK system, the electric resistances of soil column, reversely corresponding to salinity, at the applied voltages 4, 10 and 20 V were 266, 487 and 1272 Ω, respectively. Meanwhile, lower electrical conductivity (EC, between 67-230 μs/cm) were observed in the soil within 50 cm below the surface at the voltages of 10 V and 20 V, which was much lower than the minimum value (581 μs/cm) of the control with no current applied. For the control column without EK treatment (0 V), soil in the surface layer had the highest EC value at 1721 μS/cm due to the salts rising from the bottom, and the EC values of soil beneath the surface were in the range of 581-1127 μS/cm. Compared to control column, the level of ions in the surface soil significantly declined after V-EK treatment, especially for the column with voltage at 10 V and 20V. When voltage was at 20 V, Na+ was detected at a range of 0.06-0.08mg/g in the surface soil, a >99% reduction when compared to the controls. Similar efficacy was observed for chloride (Cl-), in the V-EK column with the voltages at 10 V and 20V.
Keywords: Chloride, Electrodes; Electrokinetics; Salinity, Costal area
Introduction
Soil salinity is one of the main reasons for soil degradation that has drawn wide attention under the background of global change, brings about an environmental issue of worldwide significance (Bessaim et al., 2019; Mao et al., 2016; Li 2018; Wang et al., 2018; Hamdan et al., 2014). At the world level, saline soil extends over all the continents including Africa, Asia, Australasia, and America (Yan et al., 2013; Litalien & Zeeb, 2020). General statistics estimates that 7% of the earth’s continental surface area (Rengasamy, 2006; Cho et al. 2010), and at least 20% of arable soil and half of the uncultivated soil in the world are affected by varying levels of salts (Klouche et al., 2020; Nouri et al., 2017; Long et al., 2016; Scudiero et al., 2016; Cho et al. 2010). Coastal area is one of the main areas affected by soil salinity under the comprehensive action of human and natural factors, which is formed by the marine and terrestrial sedimentation and directly associated with seawater intrusion (He et al. 2015; Li et al., 2014). Besides, 70% of the world population concentrated in coastal areas (Fan et al., 2018; Hamdan et al., 2014), and this make a huge and continuously increasing demand for landscape re-construction in urban and peri-urban. However, landscaping is enormously restricted in coastal soils, due to the high salinity, and low vegetation cover lead to a fragile ecosystem for coastal saline land.
In coastal characterize by low elevation and flat terrain, global warming makes evaporation more intense as well as sustained sea level rise, which aggravates salinization (Xie et al., 2019; Yu et al., 2014; Xu et al., 2020). Meanwhile, evaporation causes the continue rise of capillary water from saline phreatic, which is one of the main reasons for increasing salinity in coastal area (Lou et al. 2018; Chi et al., 2019). To mitigate and prevent the soil salinization, many conventional remediation methods had been investigated and applied, including soil substitution, soil drainage, irrigation, chemical amendments and salt-accumulating plant (Yang et al., 2018; Kim et al. 2010; Choi et al. 2012; Jo et al. 2015; Li et al. 2016). However, most have failed mainly due to freshwater shortage (Fei et al. 2018; Liu et al. 2018), higher cost (Chen et al. 2015), longer reclamation time (Sun et al. 2017; Jo et al. 2012) and not environmentally sustainable (Chen et al., 2019). Additionally, fine-grained soil in coastal areas render remediation work more difficult. To find the effective and economic salt remediation for this kind soil with low permeability porous media is a challenge, especially in coastal area where re-salinization is easily happened because of the continuous supplement of salts from the shallow saline phreatic water and sea (Qadir et al. 2000; Chi et al., 2019; Fan et al., 2018; Yu et al., 2014; Walter et al., 2018).
As an innovative, sustainable and inexpensive technology, Electrokinetic (EK) has considerable potential for the remediation of saline soils with low permeability. Cho et al. (2010) utilized EK to achieve a removal efficiency of 99 and 95% for nitrate and chloride ions, respectively; and Kim et al. (2010) reached a removal efficiency of 100% for sodium and 58% for chloride ions from tidelands. Similar result obtained by Lee et al. (2012) that Na+ and Cl- were easily removed from the soil (>97%) by electromigration During 1 or 2 weeks and the ions such as chloride, sodium, and nitrate were reduced to over 90% after in situ EK process applied to two ridges in a greenhouse for growing chrysanthemum (Choi et al., 2012). By using EK, Kim et al. (2013) observed that the removed cations from saline soils (EC = 13.7 dS/m) including Ca2+, Mg2+, K+, and Na+ were 19.5%, 34.4%, 58.9%, and 89.6%, respectively; and the removed anions including Cl-, NO3-, and SO42- were 47.9%, 91.5%, and 67.6%, respectively. Klouche et al. (2020) achieved removal rates of 83% and 58% for sodium and calcium ions, respectively after 15 days of EK treatment. Generally, the EK removal of including Na+, NO3-, and Cl-removal were quite high compared to the EK removal of Ca2+, Mg2+, and SO42- (Abou Shady, 2016; Choi et al., 2012; Kim et al., 2012; Kim et al., 2013). Besides, the percentage reduction of EC was found to be 87% (64 days), 73-83% (60 days), and 64-90% (1 or 2 weeks), as noticed by various EK cases (Lee et al., 2011; Choi et al., 2012; Lee et al., 2012).
For EK remediation, two key factors including the electrode materials and the distance between electrodes are always significantly considered due to the corrosion resistance of electrode materials and the removal efficiency and limitations of EK. Copper, iron, titanium, stainless steel, graphite, platinum and some inert metal coating materials are commonly used electrode materials that accepted by the cost effective or the corrosion resistance (Table S1). Generally, corrosion reactions occur at the anode together with electrolysis reaction. Materials such as Cu and Fe could be oxidized and dissolved that results in the introduction of corrosive products into the soil. Thus, these materials may not be suitable for long-term applications (Jia et al., 2005). Meanwhile, the spacing between electrodes correlated with the process efficiency and the cost, the voltage gradient of 1 V/cm is usually used if both removal rate and energy are considered (Cho et al., 2012).
However, the electrode configurations should be seriously considered during the application of EK. Hamdan et al (2014) discussed electrode configurations influence the active area of the electric field, suggested that 2D electrode configuration has a smaller inactive electric field area than a 1D electrode configuration. Zhang et al (2010) applied both horizontal and vertical electric fields and found it significantly effective when prevent Cr (VI) from migrating downward. In coastal areas, shallow groundwater makes salt rise to surface by capillary water continually, render most remediation methods unsustainable. Moreover, the low permeability of soil makes salt difficult move downwards when using irrigation related techniques.
Therefore, Vertical Electrokinetic system (V-EK) with multilayer electrodes was considered as an “Electric Sieve” were proposed in this work to mitigate and prevent the surface soil salinization caused by salts rising from shallow groundwater in coastal area, and the influences of electric action on salt transport in aquifer - vadose zone system were explored. Different to the traditional perspective of saline soil restoration that used the horizontal electrical field configurations to transfer contaminants to both side of electrodes in combination with the saturated water conditions or irrigation, we focus on the mitigation and inhibition and of the salt rising from shallow groundwater by the vertical EK system with multilayer electrodes, which could be considered as an “Electric Sieve”.
Materials and Methods