IJERD – International Journal of Environmental and Rural Development (2023) 14-1

Research article
erd

Reuse of Unavailable Phosphate in Soils Using Solar CellPowered Electrokinetic Treatment
NARONG TOUCH*
Faculty of Regional Environment Science, Tokyo University of Agriculture, Tokyo, Japan
Email: nt207118@nodai.ac.jp

TAKAHIKO NAKAMURA
Faculty of Regional Environment Science, Tokyo University of Agriculture, Tokyo, Japan
Received 18 December 2022

Accepted 10 May 2023(*Corresponding author)

Abstract Electrokinetic treatment (ET) is widely used for soil remediation, specifically forremoving heavy metals from soil. Till date, we have focused on the nutrients released in soils
using a solar cell-powered ET, which contributes to increasing the productivity of organicfarming. Previously, when a 1.5 V solar cell was used in ET, the release of ammonium ionsand the growth of microorganisms were observed. However, no release of phosphate ions wasobserved. Thus, it is presumed that a 1.5 V solar cell is insufficient to release phosphate insoils. Therefore, this study aims to examine the effects of solar cell voltage on the release ofphosphate in soils. In laboratory experiments, an anode (oxidation reaction) and a cathode(reduction reaction) were installed in andosol mixed with cow manure compost, and a 3 or 6
V solar cell was used to produce an electric current between the two electrodes. Sixty-two days
after the current generation, the chemical properties of the soil were measured. The results
suggest that soil redox conditions can be controlled by changing the solar cell voltage. Fromthe pH distributions, a higher voltage of solar cells was used and a large change in pH wasobtained. Furthermore, a decrease in soil redox potential was obtained by increasing the solarcell voltage. From the phosphate distributions, a 6 V solar cell is required to release phosphatein soils. It is recommended that applying ET powered by a 6 V solar cell (potential gradient:0.12 V/cm) facilitates nutrient release in soils, which is the most important factor in organic
farming.
Keywords solar cell, voltage, electrokinetic treatment, soil reduction, phosphate, organic
farming

INTRODUCTION
According to the Ministry of Agriculture, Forestry, and Fisheries of Japan, organic farming is astrategy to achieve sustainable agriculture. The government has proposed many countermeasures tofacilitate organic farming (MAFF, 2019). Hence, productivity-enhancing technologies andimplementation systems are required.
In organic farming, because nutrients are supplied by the decomposition of organic matter,facilitating this decomposition is crucial in providing higher productivity. As the decomposition oforganic matter depends on the Soil Chemical Environment (SCE), controlling SCE is imperative inenhancing the productivity of organic farming. According to Yan and Hou (2018), understandingSCE is useful for ensuring healthy soil that can sustain crop production. Therefore, to provide higher
productivity in organic farming, a technology that can control the SCE to activate bacterial
metabolism for enhancing the decomposition of organic matter is required.
Electrokinetic Treatment (ET) is a technology for generating a low-intensity electrical field
between two electrodes buried in soils by applying a direct current or constant voltage (an external
power supply). ET has been widely used for a few decades to remove contaminants, such as dyes
and heavy metals (Hanay et al., 2009, Almomani and Baranova, 2013). Kim et al. (2010) reportedchanges in soil pH and electrical conductivity owing to ET application in soil. Additionally, ET canⒸ ISERD32

IJERD – International Journal of Environmental and Rural Development (2023) 14-1

be used to control SCE. Previously, a battery was used to apply a direct current or a constant voltage,
but now a solar system is also used as an external power supply of ET (Liu et al., 2020). However, alarge potential gradient, for example, 3-5 V/cm, is commonly applied to soils, which causes large
variations in the SCE. This, in turn, influences soil biology, limiting the decomposition of the organic
matter.
We thought that by decreasing the potential gradient (i.e., the solar cell voltage), the effects on
soil biology could be reduced. As such, in our previous study (Touch and Nakamura, 2022), a 1.5 V
solar cell (potential gradient: 0.03 V/cm) was used in ET. Following the application, the release ofammonium ions and the growth of microorganisms were observed, which would also be useful fororganic farming. However, no release of phosphate was observed. Thus, a 1.5 V solar cell wasinsufficient to release phosphate in soils. As increasing the voltage of the solar cell facilitates
reactions on the electrode surface in soils, using a higher voltage of the solar cell may cause therelease of phosphate.
OBJECTIVE
This study aims to examine the effects of solar cell voltage on the release of phosphate in soils.
Specifically, we examined changes in electrode potential by increasing the solar cell voltage, the soil
redox conditions (pH and redox potential), and nutrient release following the application of solar
cell-powered ET. This was conducted by applying ET to andosol mixed with cow manure compost
with a generated electric current for two months.
METHODOLOGY
Experimental Procedures and Operations
Commercial products, i.e., dried cow manure compost (fully matured, nitrogen 1.09%, phosphate2.29%, potassium 2.28%) and andosol (volcanic derived soils with high contents of organic matterand aluminum compounds), were used in laboratory experiments. First, the compost was mixed withandosol at a volume ratio of 47% (generally, 40-60% was used). The mixture was then used in theexperiments, which were conducted under three conditions without treatment (Control), treated witha 3 V solar cell (SC-3V), and treated with a 6 V solar cell (SC-6V). For Control, an electrode wasinstalled in the mixture to measure the soil potential continuously (Fig. 1a).

⊕Voltage logger⊝ (for potential measurement)(a) Control(c) Electrode: carbon fibers

2 cm Tap water (prevention of soil drying by evaporation)

14 cm

Reference electrodeCow manure compost-mixed andosol
Voltage logger(b) SC-3V or SC-6V⊝
Solar cell (3 or 6 V)
1ΩRemoval of 1 cm-surface soil before soil sampling⊕⊝⊕ Voltage logger⊝

3 cm

14 cm(d) Soil sampling
Cathode (10 cm ×10 cm)

Anode (10 cm ×10 cm × 2)

49 cm
Soil sampling at intervals of 5 cm
1 cm

56 cm
Plastic container: 14 cm × 56 cm × 26 cm

Fig. 1 Experimental devices and operationsⒸ ISERD

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For SC-3V or SC-6V, two electrodes were installed in the soil layer (Fig. 1b). To generate anelectrical current, one electrode was connected to the positive terminal, and the other was connectedto the negative terminal of a solar cell (3 V or 6 V) using the circuit shown in Fig. 1b. Different
voltages of the solar cells were obtained by connecting a 1.5 V solar cell (Tamiya, 1.5 V-500 mA) inseries. An external resistance of 1 Ω was loaded between the anode and solar cell. The electrodematerial 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 a width of 10 cm and heightof 10 cm was separated into fibers to form a brush-type electrode. The electrode was placed in aplastic net (1-mm mesh) and installed in the soil layer (Fig. 1c). The anode surface area was 0.02 m2(10 cm × 10 cm × 2), while the cathode surface area was 0.01 m2 (10 cm × 10 cm). A smaller cathodesurface area compared with the anode surface was made to obtain a higher reduction at the cathode.
After installing both electrodes, the container was filled with tap water. Consequently, a 2 cm
water layer was formed on the soil surface, which can prevent the soil from drying owing toevaporation. The voltage at both terminals of the external resistance was measured every 15 minusing a voltage logger (T&D Corp., MCR-4V) to calculate the circuit current according to Ohm’slaw. The current density was obtained by dividing the current by the surface area of the anode, i.e.,
0.02 m2. The electrode and a reference electrode (Toyo Corp., W-RE-7A) placed in the soil layer
were connected to the voltage logger using the circuit shown in Figs. 1a and 1b to measure theelectrode potential.
Analyses
Two months after the experiments started, soil sampling was conducted. The surface soil of 1 cm
was removed, and pH and redox potential (ORP) were measured using a pH/ORP meter (Horiba, D50) at intervals of 5 cm from the anode. Soil samples were collected (Fig. 1d) after pH and ORP
measurements. Furthermore, each soil sample was centrifuged at 6000 rpm for 5 min (As One, CN2060) to extract pore water. Finally, each ion concentration in pore water (phosphate, PO43-;ammonium, NH4+; potassium, K+) was measured. The ion concentration was measured using
PACKTEST (Kyoritsu Corp., WAK), except for K+ concentration which was measured using a K+
meter (Horiba, K-11).
RESULTS AND DISCUSSION
Variations of Electrode Potential with Different Voltages of Solar Cells

Electrode potential (V)Figure 2 depicts temporal changes in the electrode potential during the experiments. For SC-3V andSC-6V, the electrode potential refers to the potential of the cathode in which a reduction reaction
occurs. For Control, the electrode potential decreased temporally, and mostly stabilized at -0.48 V
on Day 62 after the experiment started. The low potential was observed because cow manure compostwas used, and it took approximately two months for its reduction. On Days 50-60, the peak electrodepotential ranged from -0.70 V (night-time) to -0.94 V (daytime) for SC-3V and -0.90 (night-time) to-1.14 V (daytime) for SC-6V.
0.5

Control

SC-3VSC-6V0-0.5-1-1.5Connection failure-20510152025303540
Operation time (day)45505560Fig. 2 Comparison of electrode potential with and without using the solar cell
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IJERD – International Journal of Environmental and Rural Development (2023) 14-1

1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0SC-3V

Current density (A/m2)

Current density (A/m2)

Compared with the Control, more reduction was obtained when a high-voltage solar cell wasused, and there was a large decrease in the electrode potential when the solar cell voltage wasincreased from 3 V to 6 V. This is because the reactions on the electrode are more active owing toan increase in electrical current when the voltage of the solar cell increases. As shown in Fig. 3, thecurrent density was approximately 0.06-1.30 A/m2 for SC-6V and 0.01-0.61 A/m2 for SC-3V. The
system current can vary using different solar cell voltages, that is, by changing the solar cell voltage,soil reduction can be controlled.

SC-6V

0.14
0.12

SC-3V

SC-6V

0.1
0.08
0.06
0.04
0.02
00

5 10 15 20 25 30 35 40 45 50 55 60 65
Operation time (day)253035 40 45 50 55
Operation time (day)60

65

Fig. 3 Comparison of current densities generated by SC-3V and SC-6V
Changes in Redox Conditions at the End of the Experiment
Figure 4 shows changes in pH and ORP due to the current generation at the end of the experiment.
In Fig. 4, 0 and 49 cm refer to the soil samples at the anode (oxidation) and cathode (reduction),respectively (SC-3V and SC-6V). In Fig. 4a, the pH at the anode was 6.21 for Control, which
decreased to 6.18 for SC-3V and 3.56 for SC-6V. These decreases were caused by oxidation reactions
at the anode. However, the pH at the cathode was 6.44 for Control, which increased to 6.72 for SC3V and 7.11 for SC-6V. These increases were caused by reduction reactions at the cathode. From thepH distributions, a distance of 25 cm from the electrode were influenced by the current generation
using a 3 V or 6 V solar cell.

Fig. 4 Changes in pH and ORP due to current generation at the end of the experiment
Decreased soil ORP was observed on using a 3 V or 6 V solar cell compared to Control, except
at the anode of SC-6V. The soil ORP ranged from -150 to -112 mV for Control and -190 to -150 mVfor SC-3V and SC-6V, respectively (Fig. 4b) which disagrees with our previous study (Touch andNakamura, 2022) that reported an increase in ORP when using a 1.5 V solar cell. By increasing thesolar cell voltage, a large decrease in soil ORP can be obtained.
Nutrient Release Owing to Current GenerationⒸ ISERD

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The distributions of ammonium (NH4+), phosphate (PO43-), and potassium (K+) are shown in Fig. 5.
The release of NH4+ was observed, which parallels our previous study (Touch and Nakamura, 2022).
A 1.8- to 2.3-fold increase in NH4+ concentration was observed within 5−20 cm of the anode (Fig.
5a). However, no significant difference was observed between SC-3V and SC-6V at 10−30 cm of
the anode, suggesting that a 3 V solar cell is enough to release NH4+ in soils. The increase in NH4+
concentration was low near the cathode compared with that near the anode. This is because of thereduction reactions near the cathode, which induce denitrification. In Particular, the NH4+
concentrations in SC-6V were lower than those of Control within 5 cm from the cathode.
No differences in PO43- concentration were observed between Control and SC-3V (Fig. 5b),suggesting no release of PO43- despite using a 3 V solar cell. Interestingly, a 2- to 7-fold increase inPO43- concentration was observed within 20−45 cm of the anode for SC-6V. Within 15 cm of theanode, no increase in PO43- concentration was observed. This was because of the release of calcium
ions at this distance (Fig. 5c) following the decrease in pH (Fig. 4a), leading to the fixation of PO43by the calcium ions. An increase in K+ concentration was also observed. Compared with the Control,
maximum 1.3-fold (SC-3V) and 1.6-fold (SC-6V) increases in K+ concentration were observed
within 5-20 cm of the anode (Fig. 5d). However, a smaller increase in K+ concentration was observed
near the cathode (20-45 cm of the anode), except for the cathode. A decrease in sediment pH induced
the dissociation of metal compounds, causing the release of metal ions (e.g., Ca2+, K+), which can fix
PO43- in littoral sediments (Touch et al., 2017; Touch et al., 2018).
In summary, applying ET powered by a 6 V solar cell can release nutrient compounds in soils,
such as NH4+, PO43-, and K+. These releases are caused by the enhancement of organic matterdecomposition in soils and the decrease in soil pH following ET application.

Fig. 5 Comparison of nutrient concentration distributionsⒸ ISERD

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IJERD – International Journal of Environmental and Rural Development (2023) 14-1

CONCLUSIONSLaboratory experiments were conducted to examine the effects of solar cell voltage on the release ofphosphate in soils. Specifically, we examined changes in the electrode potential by increasing thesolar cell voltage, the soil redox conditions (pH and redox potential), and nutrient release after
applying solar cell-powered ET to andosol mixed with cow manure compost. The system current
increased as the solar cell voltage increased, which effectively induced reactions on the electrode.
By changing the solar cell voltage, electrode potential, and soil redox conditions were controlled.
From the pH distributions, 25 cm from each electrode was influenced by the application of ET. Alarger influence was obtained with a higher voltage of the solar cell. From the ORP distributions, alarge decrease in soil ORP was obtained by increasing the solar cell voltage. Hence, soil redox
conditions can be varied using solar cells of different voltages. From the PO43- distributions, a 6 V
solar cell is required to release PO43- in soil. It was also highlighted that decreasing the pH near theanode decreased the PO43- concentration. Finally, applying ET powered by a 6 V solar cell (potentialgradient: 0.12 V/cm) facilitates nutrient release in soils.
ACKNOWLEDGMENTS
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.
REFERENCES
Almomani, F. and Baranova, E.A. 2013. Kinetic study of electro-Fenton oxidation of azo dyes on boron-doped
diamond electrode. Environmental Technology, 34 (11), 1473-1479, Retrieved from DOI https://doi.org/10.1080/09593330.2012.758644
Hanay, O., Hasar, H., Koçer, N.N. and Ozdemir, O. 2009. Removal of Pb from sewage sludge byelectrokinetics, Effect of pH and washing solution type. Environmental Technology, 30 (11), 1177-1185,Retrieved from DOI https://doi.org/10.1080/09593330903144058
Kim, K.J., Cho, J.M., Baek, K., Yang, J.S. and Ko, S.H. 2010. Electrokinetic removal of chloride and sodium
from tidelands. Journal of Applied Electrochemistry, 40 (6), 1139-1144, Retrieved from DOI https://doi.
org/10.1007/s10800-010-0082-1
Liu, R., Wang, H., Liu, Z. and Tao, C. 2020. Electrokinetic remediation with solar powered for electrolytic
manganese residue and researching on migration of ammonia nitrogen and manganese. Journal of Water
Process Engineering, 38, 101655, Retrieved from DOI https://doi.org/10.1016/j.jwpe.2020.101655
Ministry of Agriculture, Forestry and Fisheries of Japan (MAFF). 2019. Current situation and policy on organic
agriculture in Japan, Retrieved from URL https://www.maff.go.jp/e/policies/env/sustainagri/
attach/pdf/organicagri-1.pdf
Nagatsu, Y., Tachiuchi, K., Touch, N. and Hibino, T. 2014. Factors for improving the performance of sedimentmicrobial fuel cell. Journal of Japan Society of Civil Engineers Ser B2 (Coastal Engineering), 70 (2), 10661070, Retrieved from DOI https://doi.org/10.2208/kaigan.70.I_1066 (in Japanese)
Touch, N. and Nakamura, T. 2022. Benefits of applying microbial fuel cell technology in organic farming forsustainable agriculture. International Journal of Environmental and Rural Development, 13 (2), 49-54.
Touch, N., Takata, H. Yamaji, S. and Hibino, T. 2018. Changing organic matter characteristics of littoral
sediment by solar cell-combined sediment microbial fuel cell. International Journal of Environmental
Science and Development, 9 (3), 62-66, Retrieved from DOI 10.18178/ijesd.2018.9.3.1074
Touch, N., Yamaji, S., Takata, H. and Hibino, T. 2017. Solar cell-combined sediment microbial fuel cell forpreserving sediment and water environments. International Journal of Environmental Protection, 7 (1), 3745, Retrieved from URL http://paper.academicpub.org/Paper?id=17530
Yan, B. and Hou, Y. 2018. Soil chemical properties at different toposequence and fertilizer under continuous
rice production, A review. IOP Conference Series: Earth and Environmental Science, 170 (3), 032107,
Retrieved from URL https://iopscience.iop.org/article/10.1088/1755-1315/170/3/032107/pdfⒸ ISERD37