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