IJERD – International Journal of Environmental and Rural Development (2023) 14-2

Research article
erdChanges in Soil Physical Properties Owing to Soil Reduction
Treated with Electrokinetic Treatment
TAKAHIKO NAKAMURA
Faculty of Regional Environment Science, Tokyo University of Agriculture, Tokyo, Japan

NARONG TOUCH*
Faculty of Regional Environment Science, Tokyo University of Agriculture, Tokyo, Japan
Email: nt207118@nodai.ac.jp
Received 17 September 2023

Accepted 26 December 2023(*Corresponding Author)

Abstract Increasing soil water content due to soil reduction is generally confirmed in rice
paddy soils after flood irrigation or in littoral sediments with high organic matter content.
This could be caused by soil aggregates during the reduction process through biological andelectrostatic phenomena. In the literature, changes in liquid and plastic limits owing to soil
reduction treated with electrokinetic treatment (ET) have been reported; however, there wasno report relating to changes in other soil physical properties, such as grain size distribution(GSD) and water-holding capacity (WHC). Thus, this study aimed to examine changes in soil
physical properties caused by soil reduction treated with ET. Changes in GSD, hydraulicconductivity, and WHC were examined in laboratory experiments to understand soil
aggregates due to soil reduction. During ET application, a decrease in electrical conductivity
was observed, indicating the cohesion of ions (soil aggregate). This resulted in increases in
the percentage of particles ranging from 0.075-0.212 mm, hydraulic conductivity, and WHC.
However, particle dispersion occurred when the electrical current was high (10 mA), resulting
in a significant decrease in hydraulic conductivity and WHC. Therefore, it can be said thatsoil aggregates can develop electrostatically. Thus, ET can be used for developing soil
aggregate.
Keywords soil reduction, electrokinetic treatment, soil aggregate, grain size distribution,
hydraulic conductivity, water-holding capacity

INTRODUCTION
In rice paddy fields, oxygen diffusion from air into the soil is suppressed after flood irrigation on thesoil surface, causing soil reduction. Touch et al. (2021) experimentally examined soil redox potential(ORP) after making a water layer on paddy soils. They observed that soil reduction occurred in 45
days. Generally, the water content of paddy soil increases temporally during soil reduction. Ozaki etal. (2013) observed an increase in water content from 30% to 50% after flooding irrigation in winter.
Moreover, an increase in water content due to soil reduction occurs in littoral sediments. Fukui et al.
(2012) reported that the water content of sediments with high organic matter content is higher thanthat of sediments with low organic matter content. Hattori et al. (2018) reported an increase in watercontent with an increase in total organic carbon. Therefore, it can be said that organic matter
influences the water content of sediments. However, the mechanisms behind the increasing watercontent owing to soil reduction or organic matter remain unclear.
It is believed that an increase in water content owing to soil reduction occurs through soil
aggregate by biochemical and electrostatic mechanisms. Generally, the released substances during
the decomposition of organic matter can act as binders in soil aggregate. With the proceeding oforganic matter decomposition, and fungal hyphae development, water-stable aggregates increased
and then decreased (Limura and Egawa, 1956). Many researchers have since focused on thecorrelation between soil organic matter and soil aggregate (Okolo et al., 2020; Mustafa et al., 2020).

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Soil aggregate may also occur through electrostatic bonding. Liaki et al. (2010) investigatedphysicochemical effects on clay after ET application. They observed changes in water content owing
to ET application and pointed out that ET application varied the zeta potential of clay particles,
causing changes in share strength, liquid limit, and plastic limit of clayey soils. However, the effectsof ET on other soil physical properties remain unknown.
OBJECTIVE
This study aims to examine changes in soil physical properties owing to soil reduction treated with
ET. This can be done by inducing reduction reactions in upland soils with a constant flow of electricalcurrent using a potentiostat (current fixation). Specifically, changes in grain size distribution (GSD),
hydraulic conductivity, and water-holding capacity (WHC) are examined. In addition, changes in ion
concentration present in soil pore water after the generation of electrical current are determined.
METHODOLOGY
Experimental Procedures and Operations
Figure 1 shows the experimental device, comprising a cylindrical bottle with an inner diameter anda height of 120 mm and 150 mm, respectively. First, the upland soil collected from the farmland wasplaced in the bottle until it reached a height of 20 mm from the bottom, and an electrode (anode) was
placed on the soil layer. 30 mm of the soil layer was then placed on the electrode (Fig. 1a). Finally,
tap water was poured over the soil layer. The bottle was then placed in a container (360 mm in width,
510 mm in length, and 300 mm in height) filled with tap water (Fig. 1b). In the container, an electrode(cathode) was submerged close to the water surface.
The electrode material was carbon cloth (News Company, PL200-E), which was heated at
500°C for 1 h before using it, as Nagatsu et al. (2014) suggested. The heated carbon cloth with awidth of 10 cm and height of 10 cm was separated into fibers to form a brush-type electrode (Fig.
1c) used in the experiments.
Potentiostat

(b)

RE

WE

CE

120 mm

100 mmTap water

Upland soil

Electrons

300 mm(a)

50 mm

Cathode20 mm

Tap water
510 mmCarbon fiber electrode(c)Fig. 1 Experimental devices and operations
To generate an electrical current, the anode, cathode, and reference electrode (Toyo Corp., WRE-7A) were connected to a potentiostat (Hokuto, HA-151B) using the circuit shown in Fig. 1b. A
potentiostat was used to maintain a constant electrical current of 4, 6, 8, and 10 mA. The currentⒸ ISERD

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generation lasted for 7 days. As the treatment duration was less than 7 days, the effects of organic
matter decomposition on soil aggregate are thought to be less significant.
Analyses
Soil sampling was conducted at the end of the experiments. Surface soil of 1 cm was removed, andthe residue was centrifuged at 6000 rpm for 5 min (As One, CN-2060) to extract pore water. The
electrical conductivity (EC) and calcium ion (Ca2+) of extracted pore water were measured using an
EC meter (Horiba, D-74) and a Ca2+ meter (Horiba, Ca-11), respectively.
Grain size distribution (GSD), hydraulic conductivity, and water holding capacity (WHC) were
measured to examine changes in soil physical properties owing to soil reduction. GSD, hydraulicconductivity, and WHC were measured using the wet sieving method, the falling head permeability
test, and the centrifugation method, respectively. In the falling head permeability test, the soil sample
was placed in a 100 cm3-sampler can with a soil surface area of approximately 20 cm 2 and a soilheight of 0.4-1.2 cm. In the centrifugation method, the soil sample was centrifuged at 500 rpm (pF =2) and 3900 rpm (pF = 3.8).
RESULTS AND DISCUSSION
Changes in Ion Conditions in Soil Pore Water Owing to Soil Reduction
Figure 2 shows changes in EC and Ca2+ concentration in soil pore water at different electrical currents.
EC is an index that indicates the total ion concentration. Generally, changes in ion concentration bycohesion or dispersion cause a variation in EC. For example, from Fig. 2a, EC decreased from 120
mS/m to 28 mS/m with an increase in electrical current from 0 mA to 6 mA and increased from 28
mS/m to 68 mS/m with an increase in electrical current from 6 mA to 10 mA.

(b) Ca2+ concentration(a) Electrical conductivity (EC)

Fig. 2 Electrical conductivity and Ca2+ concentration at different electrical currents
In other words, decreases in ion concentration occurred when the electrical current increased
until 6 mA, and the release of ions occurred when the current was higher than 6 mA. This can be
confirmed in Fig. 2b, wherein Ca2+ concentration decreased from 150 mg/L to 31 mg/L when theelectrical current increased from 0 mA to 6 mA. However, when the electrical current was higher
than 6 mA, Ca2+ concentration increased from 31 mg/L to 145 mg/L. These results suggest that ion
cohesion occurs in soils with an increase in electrical current from 0 mA to 6 mA. Interestingly, ion
dispersion occurs in soils with an increase in electrical current from 6 mA to 10 mA. Furthermore, it
was observed that ion cohesion or dispersion strongly correlates with electrical current, and thecorrelation coefficient was higher than 0.95 (Fig. 2). It is thought that this cohesion or dispersion
induces soil aggregates, resulting in changes in soil physical properties.
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Based on the results of the sieve analysis, the residual rates on the sieve range of 0.075-0.212 mm,
0.212-0.850 mm, and 0.850-2 mm were determined, and Fig. 3 shows their relationship with theelectrical current. In Fig. 3, the correlation coefficients were 0.22 and 0.41 for the particle size rangeof 0.85-2 mm and 0.212-0.850 mm, respectively. This suggests that soil reduction has less effect onparticles larger than 0.212 mm.
Interestingly, the residual rate of the particle size range of 0.075-0.212 mm increased with anincrease in electrical current, with a correlation coefficient of 0.61. Thus, it can be said that soil
reduction causes the cohesion of soil particles less than 0.075 mm, contributing to the increase inresidual rate. Liaki et al. (2010) and Wang et al. (2022) reported that generating electrical current in
the soil causes changes in the zeta potential of soil particles, leading to cohesion or dispersion of theparticles responding to an increase or decrease in zeta potential.
35

R = 0.22

Residual rate (%)

30
25

R = 0.41

0.85<D<2mm

200.212<D<0.85mm

15

0.075<D<0.212mm

10

R = 0.61

5
0
0

2

4
6
8
Electrical current (mA)

10

Fig. 3 Particle cohesion due to soil reduction
Changes in Hydraulic Conductivity Owing to Soil Reduction
Figure 4 shows the hydraulic conductivity at different electrical currents and its relationship with EC.
The hydraulic conductivity increased from 0.29 to 0.45 mm/day when the current increased from 0
mA to 8 mA (Fig. 4a). Based on Fig. 2, the cohesion of particles occurred in the electrical current
range of 0-6 mA. This cohesion induced soil aggregates, leading to an increase in hydraulicconductivity.

(a) Hydraulic conductivity(b) Correlation of hydraulic conductivity with EC

Fig. 4 Hydraulic conductivity at different electrical currents and its relationship with EC
Furthermore, the dispersion of particles occurred in the electrical current range of 6-10 mA.
However, there was no decrease in hydraulic conductivity when the current was 8 mA, while there
was a decrease when the current was 10 mA (Fig. 4a). This indicates that cohesion and dispersion ofparticles influence the hydraulic conductivity of soils, and a high rate of dispersion causes largeⒸ ISERD

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decreases in hydraulic conductivity. It was also observed that hydraulic conductivity had a strong
correlation (R = −0.84) with EC, and a larger hydraulic conductivity with a lower EC (cohesion state).
Changes in Water-Holding Capacity Owing to Soil Reduction
Generally, soil aggregates influence not only hydraulic conductivity but also the WHC of soils.
Figure 5 shows the water content at pF 2 and 3.8. A similar trend was observed with the variation in
hydraulic conductivity. The water content at pF 2 increased with an increase in electrical current
from 0 mA to 8 mA, and a strong correlation (R = 0.83) was observed (Fig. 5a). An increase in thewater content at pF 3.8 was also observed (Fig. 5b); however, the correlation was less significant (R= 0.54). In addition, a large decrease in water content was observed when the current was 10 mA(Fig. 5a). From Fig. 5a, it can also be said that the cohesion of soil particles induces soil aggregates,
which increases WHC; however, high rates of particle dispersion decrease WHC.
Commonly, there are three types of water in soil: adhesion (the water on the surface of soil
particles), cohesion (the water attached to adhesion water), and gravitational (the water that flows
with gravitational force). The water content at pF larger than 1.8 refers to the amounts of adhesion
and cohesion water. From these results, because a strong correlation between the electrical current
and the water content at pF 2 was observed, it is thought that only cohesion water is influenced byelectrical current generation (soil reduction).
33

Water content at pF = 3.8 (%)

Water content at pF = 2 (%)

53

52
51

R = 0.83
50

49

32

31

R = 0.54

30
29
28

48

0

2

4
6
Electrical current (mA)810(a) Water content at pF 2

0

2

4
6
Electrical current (mA)810(b) Water content at pF 3.8

Fig. 5 Hydraulic conductivity at different electrical currents and its relationship with EC
CONCLUSIONS
Laboratory experiments were conducted to examine the electrostatic effects of soil reduction on thephysical properties of soil. Specifically, we examined changes in GSD, hydraulic conductivity, and
WHC after introducing an electrical current into the soil (i.e., soil reduction caused by ET). Whenthe electrical current was increased from 0 mA to 10 mA, EC and Ca2+ concentrations decreased
when the current was less than 6 mA and increased when the current was higher than 6 mA. Thissuggests that the cohesion of particles occurs in soils due to soil reduction; however, a high current(10 mA) can lead to the dispersion of soil particles. From GSD, the residual rate of the particle size
range of 0.075-0.212 mm increased when the current increased, indicating that soil reduction causes
soil aggregates. Furthermore, the hydraulic conductivity had a strong correlation (R = −0.84) with
EC, indicating that cohesion or dispersion influences the hydraulic conductivity of soils. This study
also confirmed that hydraulic conductivity and WHC increased when the cohesion of soil particles
occurred. Therefore, it can be concluded that soil aggregates can form electrostatically during soil
reduction. In other words, ET can be used to develop soil aggregates and change the physicalproperties of soils.
ACKNOWLEDGMENTSⒸ ISERD

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The authors gratefully acknowledge partial funding from the Tokyo University of Agriculture:
FY2021 Grant-in-Aid for Sustainable Agriculture Research Projects. The authors would like to thankthe students of the Rural Environmental Engineering Laboratory, Tokyo University of Agriculture,
for their efforts in collecting data.
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