IJERD – International Journal of Environmental and Rural Development (2022) 13-1

Research article
erd

Evaluating Ion Exchange Capacity of Molten Slag for
Hydroponic System
ANTONIO PEREZ FUENTES
Graduate School of Agro-Environmental Science, Tokyo University of Agriculture, Tokyo, JapanSARVESH MASKEY
Faculty of Regional Environment Science, Tokyo University of Agriculture, Tokyo, Japan

HIROMU OKAZAWA*
Faculty of Regional Environment Science, Tokyo University of Agriculture, Tokyo, Japan
Email: h1okazaw@nodai.ac.jp

YURI YAMAZAKI
Faculty of Agriculture, Tottori University, Tottori, JapanTOMONORI FUJIKAWA
Faculty of Regional Environment Science, Tokyo University of Agriculture, Tokyo, JapanTOSHIMITSU ASAI
Faculty of Regional Environment Science, Tokyo University of Agriculture, Tokyo, Japan
Received 1 February 2022

Accepted 25 April 2022(*Corresponding Author)

Abstract Hydroponic farming promotes high-efficiency water, fertilizers, and highproductivity under a controlled environment. However, the use of this system needs
consistent application of fertilizers, increasing the cost of operations. Molten Slag (MS), a
waste from the Municipal Solid Waste Incineration has essential nutrients that can be usedas fertilizer. The objective of this study was to clarify the releasing process of nutrients bymolten slag to be applied as a substrate in hydroponic system. For this, anion and cation
contents of three varieties of Molten Slag (MS1, MS2, MS3) were determined in thelaboratory by a shaking method. The nutrient content, P2O5, K2O, MgO and CaO were in
MS1, MS3 and MS2. The quantification ion released was performed at 1, 6, 12 and 24
hours. It was observed that MS3 had performed the best condition in the releasing processof K+, Mg+, SO42- and Cl-. Although MS1 had higher nutrient content, MS3 released higher
contents of those minerals among the samples. According to the result of this study, it was
confirmed that MS3 has a high potential for hydroponic farming.
Keywords molten slag, hydroponic system, ion exchange capacity, releasing nutrientsINTRODUCTION
Around 200 billion tons of fertilizers are produced annually for agricultural purposes (FAOSTAT,
2021). The use of these fertilizers and water supply tends to increase to provide food safety for 9
million people toward 2050. The benefit of fertilizers in agricultural production is remarkable since
it improves yields and keeps fertile soil. On the other hand, the constant use of fertilizers not only
increase the costs but hazard the ecological balance with releasing of greenhouse gases andeutrophication to water bodies (Sedlacek et al., 2020).
Molten Slag (MS) is a by-product of the Municipal Solid Waste Incineration -MSWI- process(Fig. 1). The process to generate molten slag starts with the addition of wastes inside a furnace, thiswaste can be paper, plastic, glass, steel, aluminum, food waste/raw garbage, waste oil, among
others. After burning most of the organic wastes, the residues are separated as ferrous materials(aluminum, steel, incombustibles), vitreous material and molten slag. The gas generated in thoseⒸ ISERD

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furnaces goes through a chamber for purification and generation of energy, and the fly ash is alsocollected for future processing. The composition of molten slag varies according to the diverse
wastes in every municipality and the type of furnace in which they are processed (Czop andŁaz´niewska-Piekarczyk, 2020). Molten slag shares similarities to the slag generated by the ironand steelmaking process, and they have been tested for correct use as a cement aggregate, roadbuilding and for soil correction (Dubey et al., 2019; Zeng et al., 2020; Devnita et al., 2021). Altland,
et al. (2015) described the possibility of up taking micro-nutrients from these types of slag and thespontaneous vegetation in landfills where molten slag is storage (Gomes et al., 2016) opens to
discussion if the contents of Ca, K, Mg, and Na, from molten slag can be a good source of nutrientsfor plants. Molten slag collected from MSWI has not been used in agriculture and recent studies(Kobayashi et al., 2004; Sekito et al., 2014) found that there was not leaching of heavy metals inseveral samples analyzed. As the generation of wastes increases around the world, this study
reveals the possibility to release beneficial ions to assess the growth of crops with this material.

Fig. 1 Municipal solid waste incineration process
OBJECTIVE
The objective of this study was to quantify and record the releasing process of nutrients released
and retained by three types of molten slag from the Kanto region in Japan and estimate theavailability of those nutrients for crop production under hydroponic conditions.
METHODOLOGY
We collected three types of molten slag from Kofu (MS1), Kamiina (MS2) and Nishi-Akikawa(MS3), the dried content of CaO, MgO, K2O and P2O5 was determined in principle, then theanalysis of releasing those minerals was carried out. First, we prepared 10 g of every sample inside
a bottle with 50 mL of Ultra-Pure Water and two replicates. The samples were shaken at 180 rpm
for 1, 6, 12 and 24 hours. Electrical conductivity (EC), pH and anion/cation contents were
measured. Anion and cation contents in the water samples, which were filtered by membrane filter
with a diameter of 0.45 μm, were measured by a spectrometric method using Ion analyzer (IA-300,
HORIBA). Only seven ions were involved in the discussion, considering plant growth, and theconcentration is given in mg/L. ANOVA and T-test were applied to find any significant difference
among the samples.

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RESULTS AND DISCUSSION
Chemical Properties of Molten Slag
MS1

28

24

Concentration (mg/L)

Concentration (mg/L)

20

20

16

16

12

12

20

16

12

8

8

8

4

4

4

0

0

0
1

6

Na+
Na+

12

1

24

Elapsed time (h)
K+
Mg+
K+
Mg+

MS3

28

24

24

Concentration (mg/L)

MS2

28

6

Na+
Na+

Ca+
Ca+

12

Elapsed time (h)
K+
Mg+
K+
Mg+

1

24

Ca+
Ca+

6

Na+
Na+

12

Elapsed time (h)
K+
Mg+
K+
Mg+

24

Ca+
Ca+

Fig. 2 Amount of cation released by every type of slag
along 1, 6, 12 and 24 hours of shaking treatment
Table 1 Chemical properties of three varieties of molten slag
Per Wind Dried Material
pH
Units

EC

CEC

Per Dried Material
NH4-N

Inorg. N

3

Bulk density

P2 O 5

K2O

MgO

CaO

Zn

Cu

Fe

mS/cm

meq/100g

kg/m

mg/100g

mg/100g

mg/100g

mg/100g

mg/100g

mg/100g

mg/kg

mg/kg

mg/kg

MS1

7.17

0.04

ND

1.58

0.6

0.6

21

2.6

8.6

142

48

208

34.7

MS2

7.78

0.04

ND

1.59

0.9

0.9

12

1.6

6.2

42

34

106

12.9

MS3

7.84

0.06

0.1

1.51

0.6

0.6

7

2.2

5.3

103

102

182

40.7

Obtained by dry method in Katakura Co Op Agri LabsND: Not Detected

MS1 has higher content of P2O5, K2O, MgO, CaO, and Cu as shown in Table 1. On the other hand,MS2 has the lowest nutrient content in all aspects, except for P2O5 and MgO, including the highest
density.
Releasing of Cations by Molten Slag
During the test, pH values did not raise levels higher than nine, but this value is still consideredhigh for crop development. It was expected that MS1 would have the largest EC due to thesignificant amounts of CaO, MgO and K2O, however, after 24 hours none of those ions tend toincrease in this material. The leachate of calcium explains the peak values observed at 6 hours ofshaking into the water, producing carbonation's reaction (Huntzinger et al., 2009).
K+ plays a significant role in many synthetic, physiological and biochemical processes inplants. This element is present in the soil in low concentrations, from 0.2% to 3.3% of the total soil
mass (Barker and Pilbeam 2015). Although a low value of K+ was found in this releasing process ofmolten slag, the concentrations in the dried material are acceptable. The range goes from 0.0011 to
0.0028 mg/g in MS2 and from 0.0037 to 0.0057 mg/g in MS3. At least 16%, 21% and 31% werereleased from the MS1, MS2 and MS3, respectively. MS3 is the most suitable supplying more K +into the water.
Even though MS1 has more significant amounts of MgO than MS3, it was observed that thepercentage of Mg+ released by MS1 is lower compared to MS3, producing almost the double
amount of Mg+ released in the same period. In comparison, MS2 has around 0.062 mg/g andreleased 0.0068 mg/g. It is only about 10% of MgO.
For the efficient process to obtain better gas in the incineration process, calcium oxide is added
to clean the impurities (Zheng et al., 2018), this component remains in the slag and becomes themost abundant next to the silicates. MS1 was the material with the highest content in manyⒸ ISERD

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compounds, having values of Ca2+ superior to 1.4 mg/g; moreover, after 24 hours in the shaking
process, it was determined that the values of the Ca2+ released are still low compared to 26%
released from the MS2. Calcium is not an essential nutrient for plants, but it is a requirement. An
excessive amount of Ca2+ can result in plants toxicity. Plants can allocate around 100 mg/g per dry
matter (Römheld, 2012; White and Brown, 2010). When the delivery of Ca2+ to the soil exceeds theplant’s uptake capacity, there is a risk of accumulation of Ca2+ at the surface of the roots and mostof the time to the precipitation as CaCO3, CaSO4, or Ca oxalate in rhizocylinders around the roots
induced by P deficiency (Newman and Römheld, 1998; Marschner and Rengel, 2012). Ca2+
released by MS3 ranges between 7.5 and 13%, meanwhile in MS1 and MS2, it goes from 4.6 to
7.7% and from 13.5 until 26%, respectively.
The primary function of Na+ seems to be to maintain osmolality in the plant tissues of the
halophyte seepweed under NaCl conditions (Mori et al., 2010). However, Na+ is not considered anutrient for most plant species. Subbarao et al. (1999) showed that it could be a substitute forpotassium in Beta sp. to a large extent and, it has been accepted as a micronutrient in some C4
plants that require Na+ for cotransport. Although Na+ values were not obtained in the dried content
analysis of the samples, it was observed that there was a leaching of sodium along with the shaking
test, there was not a variation among the shaking period, but there is a significant difference among
the three samples. Assuming that the contents of sodium in MS2 are also lower than the following
two samples as occur for the minerals, this is the reason why this sample had the lowest trend, as
observed in Fig. 2. High Na/Ca ratios may lead to a deficiency of Ca2+ in Sodic or saline soils dueto the excess of Na+ in the soils (White, 2015) that’s why it is important to consider that ratios of
Na/Ca are also balanced. The ratio of Na/Ca of Molten slag goes from 0.03 to 0.05 in all thesamples with contents from 0.020 mg/g in MS1, 0.011 mg/g in MS2 and 0.022 mg/g in MS3.
MS1

10

8

4

Concentration (mg/L)

Concentration (mg/L)

6

6

4

2

2

1

6

12

24

Elapsed time (h)ClCl-

PO4PO43-

6

4

2

0

0

MS3

10

8

8

Concentration (mg/L)

MS2

10

0
1

6

12

24

1

6

SO4SO42-

ClCl-

PO4PO43-

SO4SO42-12

24

Elapsed time (h)Elapsed time (h)ClCl-

PO43PO4-

SO4SO42-

Fig. 3 Anion released by every type of slag along 1, 6, 12 and 24 hours
Anion Concentration
From the three samples analyzed in this study, only MS2 showed a small fraction of PO42- released;
it is possible to say that 0.3 mg of PO42- are available in the water to be taken by the plants fromthis material. The range of PO42- in MS2 goes from 0.0008 mg/g to 0.0018 and finally to 0.0012 mg
along the treatment. Only MS2 could perform this condition from the three samples since MS1 andMS3 didn’t show great changes during the same period.
SO42- is an important nutrient that acts as a major structural component of proteins, also
participates in several processes like photosynthesis, N2 fixation, chlorophyll production, andcatalytic activities of many enzymes (Mills and Jones, 1996; Hawkesford, 2010). An unbalanced
application of N fertilizers with low or lack of sulfur can indicate the plant’s sulfur deficiency. In
principle, the samples of molten slag released around 0.013 mg/g of SO42- at the first hour, but after
24 hours, it was only MS3 which increased to 0.019 mg/g. Since the molten slag can’t provide
nitrogen such as NH4+ or NO3-, the sufficiency of sulfates in agricultural systems will be directly
related to the content from an external nitrogen source. Although molten slag can provide few SO42to crops systems, this quantity is not enough to accomplish the plant requirements.
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Cl- concentrations up to 500 mg/L have been supplied in sweet onion and demonstrated thatthe requirement of chlorides is only exceeded by N, P and K (Randle, 2005). Furthermore, theconcentration observed in the releasing process from the three types of molten slag doesn’t exceed
more than 3 mg/L by 200 g of MS. Therefore, this amount of Cl- found doesn’t represent a hazard
for crop growth but enriches the micronutrients uptake by plants in the water content.
It was observed that MS1 is the material with the highest content of nutrients per dried soil.
This condition does not guarantee that the releasing amounts of nutrients from molten slag isrelated to the mineral content in every type. Although MS2 does not have great amounts ofnutrients, it was observed that EC increased due to high amounts of Ca2+ released.
CONCLUSION
In this study, three varieties of molten slag under releasing treatment of nutrients for crop system
proved a remarkable difference among the quantities of nutrients released by every type of slag
analyzed. MS3 had performed the highest amounts of nutrients released by g of sample, findingthat the percentage of K+ available for plants is higher in MS3 than MS1 and MS2. Even when thecontent of Ca2+ is lower in MS2, the final concentration in the water was higher than the following
two samples, which can increase pH levels due to the chemical reactions of Ca 2+ when it contacts
water. It was indeed MS3 that released more Mg+ ions into the water and SO42- that can make adifference in plant nutrition. Certainly, PO42- is one of the major important anions for cropdevelopment yet low concentrations were found in this experiment: therefore, deep analysis and adifferent methodology should be considered to set the availability of this nutrient in crop systems.
It is expected that MS3 will perform better plant growth as it promises essential nutrients for plantdevelopment. Since the melting furnace is feed by same type of raw material, a comprehensive
study will be focused on the differences of every of system combined with analysis switching
sampling periods to confirm the releasing process.
ACKNOWLEDGEMENTS
The authors are grateful to KOBELCO ECO-SOLUTION Co., Ltd. for providing us the materials
for this study. This research was supported by Tokyo University of Agriculture.
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