Applicability of Alkaline Waste and By-products as Low Cost Alternative Neutralizers for Acidic Soils

Abstract

Acidic soils can induce several negative impacts, especially in agricultural fields. To address these problems, lime is often applied to increase the pH value of acidic soils. Calcium carbonate is the most common and conventional agricultural lime; however, it is a natural and scarce resource. To promote a recycling-based society, alternative neutralizers with lower costs that use alkaline waste and by-products are essential. Therefore, we investigated the effectiveness and applicability of three types of autoclaved lightweight aerated concrete, recycled concrete, steel slag as basic oxygen furnace slag, and fly ash (mainly particles less than 0.106 mm and 0.106–2 mm in size), as alternative neutralizers for three representative acidic soils through laboratory neutralization experiments. The neutralization performance was evaluated by measuring the additive weight percentage of each neutralizer required to convert each acidic soil to neutral soil (pH 7). For neutralizers with two particle sizes, the finer fraction clearly showed lower additive weight percentages indicating higher neutralization performance. Among the six tested alkaline waste and by-products, the steel slag exhibited the highest neutralization performance. In particular, finer fraction steel slag exhibited a high neutralization performance, similar to that of the conventionally used calcium carbonate. This result suggests that fine steel slag (particle size < 0.106 mm) is the most promising and suitable alternative neutralizer.

1. Introduction

Acidic soils are classified by the Food and Agriculture Organization (FAO) of the United Nations as having pH value less than 5.5. These soils are globally distributed; however, they predominantly occur in both the humid northern temperate zone and humid tropics, which are mainly covered by coniferous forests, and savannah and tropical rain forests, respectively. Approximately 30% of the global ice-free land area is covered by acidic soils.¹⁾

Soil acidification occurs naturally, particularly in areas where the mean annual precipitation exceeds the mean annual evapotranspiration. Acidification is often accelerated by several natural and anthropogenic processes, such as acid deposition (e.g., acid rain and snow) and soil amendments for intensive agriculture (e.g., fertilizers and manure). There are several potential negative effects of soil acidification, especially in agricultural fields. For instance, high acidity causes the dissolution of aluminum (Al), manganese (Mn), and iron (Fe) as toxic chemical components and deficiency of nutrients, including calcium (Ca), magnesium (Mg), and phosphorous (P), which seriously affect plant growth and threaten crop production.^2,3,4,5,6,7)

Acid sulfate soils are also distributed worldwide. These soils often contain sulfide minerals, such as pyrite (FeS₂), which are stable under anaerobic conditions. Once sulfate soils are oxidized, for example, by anthropogenic processes such as civil engineering works and mining activities, sulfide minerals immediately react with air and water, resulting in the production of sulfuric acid. This acid reacts with the surrounding geology (soils, sediments, and rocks), often causing the dissolution of harmful heavy metals and trace elements, including arsenic (As) and lead (Pb), which are natural sources of soil and groundwater contamination. In civil and environmental engineering, acid sulfate soils have a high risk of degrading infrastructure that consists of concrete and iron.^8,9,10,11)

To prevent negative impacts of soil acidification on plant growth in relation to crop production, lime is often added to increase soil pH. The application of lime could also be a suitable solution for the problems associated with acid sulfate soils. Among the conventional neutralizers, such as calcium carbonate, calcium oxide, and calcium hydroxide, calcium carbonate is the most common utilized agricultural lime; however, these neutralizers are naturally occurring and scarce. Recently, several studies have evaluated the effectiveness and applicability of alternative lower cost neutralizers that use alkaline waste and by-products, especially for promoting a recycling-based society. For example, Roy and Joy (2011) assessed the dose-based effects of fly ash on the chemical and microbial properties of laterite cropland soil through short-term laboratory and field studies.¹²⁾ The pH values in this soil increased clearly with increasing dose (5%, 10%, 20%, and 40% w/w) and time (0, 15, and 60 days). After the experiments, the initial pH value of 6.34 ± 0.04 (n = 5) had increased to 7.70 ± 0.03 (n = 5). Tozsin et al. (2014) evaluated the effectiveness of marble quarry waste (MQW) and marble cutting waste (MCW) on the neutralization of soil acidity using a laboratory incubation test.¹³⁾ The results showed that the initial soil pH of 4.71 increased to 6.36 and 6.84, after using MQW and MCW, respectively. Several previous studies have utilized alkaline slag, an industrial by-product derived from the ammonia-alkali production of sodium carbonate.^14,15,16) These studies suggest that slag is highly effective in correcting soil acidity.

Focusing on acid sulfate soils, Katoh et al. (2018) used steel slag (basic oxygen furnace slag) as an amendment for the reuse of excavated sedimentary rocks (mudstones) containing natural pyrite and cadmium (Cd), a harmful heavy metal.¹⁰⁾ The sedimentary rocks with more than 0.5%wt steel slag remained within a neutral to alkaline pH range, and the Cd released was less than 0.1 mg/kg, even after 90 days. Several laboratory tests have suggested that steel slag is appropriate for reusing excavated sedimentary rocks containing pyrite.

Acid mine drainage (AMD), which is generally produced by the oxidation of sulfide minerals, (e.g., pyrite), has high acidity and high concentrations of harmful heavy metals and trace elements. This process can lead to the production of acid sulfate soils. Studies on AMD-impacted soils remediated using drinking water treatment residuals (WTRs) have been conducted by RoyChowdhury et al. (2018; 2019).^17,18) RoyChowdhury et al. (2019) used both Al-based and Ca-based WTRs to treat AMD-impacted soil with high concentrations of Fe, nickel (Ni), zinc (Zn), Pb, and As through a greenhouse column and scaled-up simulated field experiments.¹⁸⁾ The field study was carried out using 5% w/w WTR applied to a soil where vetiver, which prevents soil erosion, was cultivated. The results showed that soil pH increased significantly from 2.69 to 7.20, effectively decreasing the concentrations of harmful chemical components in the leachate. The low leachability of harmful chemical components from the soil was likely due to adsorption onto the WTR surface. In addition to these important findings, other studies have utilized different kinds of waste and by-products to effectively adsorb harmful heavy metals and trace elements.^{19,20,21,22,23,24)} Thus, there is great potential to apply waste and by-products, especially with alkaline properties, for neutralization of acidic soils and immobilization of harmful heavy metals and trace elements in acid soil leachates. To effectively promote a recycling-based society, it is essential to understand the effectiveness and applicability of promising lower cost alternative neutralizers, obtained from waste and by-products. In particular, there are limited studies using a wide variety of waste and by-products for the neutralization of acidic soils.

The objectives of this study were: (i) to characterize the basic chemical properties of six selected alkaline waste and by-products, using two particle sizes expected as alternative neutralizers and three conventional neutralizers; (ii) to investigate the effect of neutralizer quantity on pH and electrical conductivity (EC) in three representative acidic soils through laboratory neutralization experiments; (iii) to identify the additive weight percentage of each neutralizer to neutralize three acidic soils (pH 7); and (iv) to compare the neutralization performance between alternative and conventional neutralizers to evaluate the effectiveness and applicability of selected waste and by-products.

2. Materials and Methods

2.1. Tested Geological Media and Neutralizers

Three acidic soils, hereafter referred to as acidic geological media, volcanic ash soil (VAS), marine sediment (MS), and laterite (L), were selected for neutralization under laboratory conditions. VAS is a typical soil type in Japan, with the sample in this study collected from the Tokyo area. MS was sampled from Nagano Prefecture, Japan. This sediment was deposited during the Neogene period and may contain sulfide minerals, which potentially release strongly acidic water. L is widely distributed, especially in tropical regions, with this sample collected from outside Japan. These three acidic geological media were manually crushed, sieved using a 2 mm mesh sieve, and air-dried for subsequent experimentation.

Autoclaved lightweight aerated concrete (AAC), recycled concrete (RC), steel slag as basic oxygen furnace slag (SS), and fly ash (FA) were selected as promising alternative neutralizers for acidic geological media. Three types of AAC, domestic AAC (AAC-D), AAC from outside Japan (AAC-OJ), and AAC slightly containing FA from outside Japan (AAC-FA-OJ) were utilized in this study. Three conventional neutralizers, calcium carbonate (CC), calcium oxide (CO), and calcium hydroxide (CH), were selected as control and reference materials. Samples of these neutralizers were used from both industrial product and chemical reagent sources. For the three types of AAC, RC, and SS, two particle size fractions of less than 0.106 mm and 0.106–2 mm were prepared as fine and coarse sized fractions, respectively. All other alternative and conventional neutralizers (FA, CC, CO, and CH) were prepared to the fine sized fraction (< 0.106 mm).

The pH and EC of the geological media and neutralizers were determined with triplicate samples using solid (tested material) to liquid (pure water) ratios of 1:2.5 and 1:5, respectively. For finer fractions of alternative and conventional neutralizers, further characterization of the chemical composition was performed using a wavelength dispersive X-ray spectrometer (Axios, Malvern Panalytical Ltd., United Kingdom).

2.2. Laboratory Neutralization Experiment

Neutralization experiments used a conventional method to evaluate the effectiveness and applicability of alternative neutralizers for acidic geological media by comparing them with conventional neutralizers as control and reference materials. Firstly, 5 g of each air-dried acidic geological medium, 0 to 1200 mg of each neutralizer, and 12.5 mL of pure water were mixed in a 100 mL polypropylene bottle. Each bottle was cured at 20°C and relative humidity of 60% for 24 h. After the curing time, each bottle was shaken at 180 rpm for 5 h, and excess CO₂ produced during the shaking process was removed by aeration using an air compressor at a rate of 2 L/min. The pH and EC of the supernatant were measured immediately using precisely calibrated portable meters. Each experiment with triplicate samples was continued until neutralization (pH 7) was completed.

3. Results and Discussion

3.1. Chemical Properties of Geological Media and Neutralizers

Table 1 presents the average pH and EC (μS/cm) values with coefficient of variation (%) for all the geological media and neutralizers tested in this study. The chemical compositions (wt%) of both alternative and conventional neutralizers for finer-sized fractions (< 0.106 mm) are reported in Table 2. The basic chemical properties of conventional neutralizers used in industrial products and chemical reagents are listed in Tables 1 and 2. No substantial differences were observed between both types of conventional neutralizers.

Table 1. Averaged pH and electrical conductivity (μS/cm) values for acidic geological media and neutralizers. Values in parentheses illustrate coefficient of variation (%).

Geological medium	pH	Electrical conductivity (μS/cm)
VAS	5.54 (1.0)	160 (1.3)
MS	4.75 (0.3)	55 (3.1)
L	4.91 (0.6)	22 (2.6)

Neutralizer	Less than 0.106 mm (finer fraction)	0.106–2 mm (coarser fraction)	Less than 0.106 mm (finer fraction)	0.106–2 mm (coarser fraction)
AAC-D	9.87 (0.4)	9.79 (0.5)	1933 (6.0)	1627 (0.4)
AAC-OJ	9.28 (0.6)	9.00 (0.5)	1020 (3.5)	900 (2.9)
AAC-FA-OJ	8.72 (0.9)	8.70 (0.7)	1237 (8.3)	890 (8.1)
RC	10.85 (0.2)	10.93 (0.8)	750 (1.3)	567 (2.0)
SS	12.49 (0.0)	12.48 (0.1)	7067 (0.8)	7200 (0.0)
FA	10.62 (0.1)	–	1257 (0.6)	–
CC (industrial product)	9.03 (0.5)	–	53 (0.0)	–
CO (industrial product)	13.02 (0.2)	–	7667 (3.3)	–
CH (industrial product)	12.69 (0.3)	–	8167 (1.9)	–
CC (chemical reagent)	8.93 (0.3)	–	51 (2.0)	–
CO (chemical reagent)	13.02 (0.1)	–	7700 (1.3)	–
CH (chemical reagent)	12.75 (0.2)	–	8067 (1.4)	–

VAS, volcanic ash soil; MS, marine sediment; L, laterite; AAC-D, domestic autoclaved lightweight aerated concrete (AAC); AAC-OJ, AAC from outside Japan; AAC-FA-OJ, AAC slightly containing FA from outside Japan; RC, recycled concrete; SS, steel slag as basic oxygen furnace slag; FA, fly ash; CC, calcium carbonate; CO, calcium oxide; CH, calcium hydroxide.

Table 2. Chemical compositions (wt%) of neutralizers.

Neutralizer	Chemical composition (wt%)
	CaO	SiO2	Al2O3	Fe2O3	MgO	Na2O	K2O	SO3	Other
AAC-D	36.74	53.85	3.31	1.94	0.62	0.16	0.36	2.42	0.60
AAC-OJ	32.82	57.59	3.30	1.85	1.23	0.08	0.86	1.85	0.42
AAC-FA-OJ	27.48	41.34	17.31	6.20	1.61	0.27	3.16	1.37	1.26
RC	35.44	40.13	11.05	6.17	1.81	1.02	1.07	1.69	1.62
SS	53.74	15.50	3.53	21.41	–	0.03	0.01	0.83	4.95
FA	3.96	54.63	29.81	4.88	1.65	0.82	0.91	0.50	2.84
CC (industrial product)	98.46	0.24	0.15	0.15	0.85	–	0.01	0.02	0.12
CO (industrial product)	98.11	0.29	0.15	0.13	1.03	–	0.01	0.09	0.19
CH (industrial product)	98.29	0.23	0.12	0.15	0.92	–	0.02	0.10	0.17
CC (chemical reagent)	99.89	–	–	–	–	–	–	–	0.11
CO (chemical reagent)	99.24	0.04	0.02	0.02	0.46	–	–	0.01	0.21
CH (chemical reagent)	99.22	0.04	0.01	0.03	0.50	–	–	0.02	0.18

AAC-D, domestic autoclaved lightweight aerated concrete (AAC); AAC-OJ, AAC from outside Japan; AAC-FA-OJ, AAC slightly containing FA from outside Japan; RC, recycled concrete; SS, steel slag as basic oxygen furnace slag; FA, fly ash; CC, calcium carbonate; CO, calcium oxide; CH, calcium hydroxide.

All three geological media showed pH values from 4.75–5.54. According to the soil pH classification by the United States Department of Agriculture (USDA) Natural Resources Conservation Service,²⁵⁾ both MS and L are classified as very strongly acidic soils (i.e., pH 4.5–5.0). In contrast, VAS is slightly weaker and belongs to the strongly acidic soil classification (i.e., pH 5.1–5.5). Tested waste and by-products as alternative neutralizers generally indicated strongly alkaline pH values compared to CC (approximately pH 9), which is the most common and conventional agricultural lime. In particular, SS was very strongly alkaline at a pH of 12.49 (Table 1). These alkaline properties may have originated from the high CaO content (53.74%, Table 2). FA showed relatively strong alkalinity at pH 10.62; however, it mainly consisted of SiO₂ and Al₂O₃, with the CaO content being much lower (3.96%).

3.2. Variations of pH and EC Values in Laboratory Neutralization Experiments

Figure 1 shows the variations in pH values for VAS, MS, and L by adding AAC-D, AAC-OJ, AAC-FA-OJ, RC, SS, FA, and CC (industrial product) with finer-sized fractions. Across all three geological media, the pH values clearly increased with increasing amounts of each neutralizer. A markedly higher additive amount was required to complete the neutralization in VAS compared to MS and L, which supports previous evidence stating that VAS has a higher pH buffering capacity.²⁶⁾ Both MS and L appeared to undergo stronger weathering processes, as suggested by their surface conditions and significantly lower EC values (Table 1), probably resulting in a lower amount required for neutralization.

Fig. 1.

pH variations in (a) VAS, (b) MS, and (c) L by adding AAC-D, AAC-OJ, AAC-FA-OJ, RC, SS, FA, and an industrial product of CC with finer fraction sizes. Error bars express standard deviation (1σ). Abbreviations: VAS, volcanic ash soil; MS, marine sediment; L, laterite; AAC-D, domestic autoclaved lightweight aerated concrete (AAC); AAC-OJ, AAC from outside Japan; AAC-FA-OJ, AAC slightly containing FA from outside Japan; RC, recycled concrete; SS, steel slag as basic oxygen furnace slag; FA, fly ash; CC, calcium carbonate. (Online version in color.)

Variations in the EC (μS/cm) values for VAS, MS, and L with the addition of AAC-D, AAC-OJ, AAC-FA-OJ, RC, SS, FA, and CC (industrial product) with finer-sized fractions are illustrated in Fig. 2. As observed in the pH variations (Fig. 1), the EC values also increased with increasing amounts of each neutralizer. In the agricultural field, the EC values of soils have been utilized as an important indicator for evaluating soil salinity, with values less than 2000 μS/cm classified as non-saline soils.²⁷⁾ The EC values of the VAS after the addition of each neutralizer were clearly higher than those of the MS and L, because VAS originally had a higher EC value of 160 μS/cm (Table 1). As such, a larger amount of each neutralizer was required for neutralization, which is likely due to the higher pH buffering capacity in comparison to MS and L. However, the EC values of all three geological media after neutralization satisfied the 2000 μS/cm classification of non-saline soils. This laboratory neutralization experiment suggested that substantial salinization does not occur with the addition of each neutralizer to the three selected acidic geological media.

Fig. 2.

Electrical conductivity (μS/cm) variations in (a) VAS, (b) MS, and (c) L by adding AAC-D, AAC-OJ, AAC-FA-OJ, RC, SS, FA, and an industrial product of CC with finer fraction sizes. Error bars express standard deviation (1σ). Abbreviations: VAS, volcanic ash soil; MS, marine sediment; L, laterite; AAC-D, domestic autoclaved lightweight aerated concrete (AAC); AAC-OJ, AAC from outside Japan; AAC-FA-OJ, AAC slightly containing FA from outside Japan; RC, recycled concrete; SS, steel slag as basic oxygen furnace slag; FA, fly ash; CC, calcium carbonate. (Online version in color.)

3.3. Effectiveness and Applicability of Alkaline Waste and By-products as Alternative Neutralizers

The additive weight percentages (%) of all tested neutralizers are shown in Fig. 3. For AAC-D, AAC-OJ, AAC-FA-OJ, RC, and SS, which have two particle sizes, the finer fraction clearly showed lower additive weight percentage requirement (i.e., higher neutralization performance) compared to the coarser fraction. For both types of conventional neutralizers, the results were similar to each other.

Fig. 3.

Additive weight percentages (%) of neutralizers required to neutralize acidic geological media to pH 7. Abbreviations: VAS, volcanic ash soil; MS, marine sediment; L, laterite; AAC-D, domestic autoclaved lightweight aerated concrete (AAC); AAC-OJ, AAC from outside Japan; AAC-FA-OJ, AAC slightly containing FA from outside Japan; RC, recycled concrete; SS, steel slag as basic oxygen furnace slag; FA, fly ash; CC, calcium carbonate; CO, calcium oxide, CH, calcium hydroxide. (Online version in color.)

Among the conventional neutralizers, the highest neutralization performance (i.e., the lowest additive weight percentages required) was observed for CO and CH; however, CC also exhibited a similar neutralization performance. As CC is the most common and conventional agricultural lime, it was used as the control and reference material for the following section. The additive weight percentages of CC required for acidic geological media neutralization were 1.6–2.0%, 0.3%, and 0.3% for VAS, MS, and L, respectively (Fig. 3). Among the tested alternative neutralizers, SS showed the highest neutralization performance. Especially, the additive weight percentages of SS at finer particle size fractions were 1.8%, 0.5%, and 0.4% for VAS, MS, and L, respectively. These high neutralization performances are similar to those of CC, the most common agricultural lime. Considering these results, the SS of the finer fraction is the most promising and suitable alternative neutralizer for the three acidic geological media. In contrast, FA showed the lowest neutralization performance, although the pH was relatively strongly alkaline (pH = 10.62) (Table 1).

According to previous studies, steel slag, including basic oxygen furnace slag, generally consists of Ca silicate (Ca₂SiO₄, larnite), tri-Ca silicate (Ca₃SiO₅, rankinite), and calcium oxide (CaO).²⁸⁾ X-ray diffraction analyses (MiniFlex, Rigaku Corporation, Japan) detected these main calcium minerals in SS used in this study. Among the alternative neutralizers, SS had a markedly higher CaO content, thus showing the highest neutralization performance (Table 2 and Fig. 3), whereas FA had the lowest CaO content and as such the lowest neutralization performance. The relationships between the CaO contents and additive weight percentages for all tested neutralizers, including conventional neutralizers, were evaluated in Fig. 4. The additive weight percentage decreased (i.e., the neutralization performance increased) with increasing CaO content. These results suggest that an indication of the neutralizer performance in acidic geological media can be quickly predicted using the CaO content.

Fig. 4.

Relationship between neutralizer CaO content (%) and additive weight percentage (%) for neutralization to pH 7. Abbreviations: VAS, volcanic ash soil; MS, marine sediment; L, laterite. (Online version in color.)

However, in the practical utilization of waste and by-products as alternative neutralizers, harmful heavy metals and trace elements are sometimes present at higher concentrations.^28,29,30,31) Therefore, detailed leaching and other appropriate stability tests for harmful heavy metals and trace elements are required prior to their application to acidic geological media.

4. Conclusions

In this study, we have investigated the effectiveness and applicability of a series of conventional and alternative neutralizers to neutralize the pH of acidic geological media. These materials were separated into three types of AAC (AAC-D, AAC-OJ, and AAC-FA-OJ), RC, SS, and FA, with all but FA having two particle sizes expected as alternative neutralizers. The effect of the additive amount of each neutralizer on the pH and EC in each acidic geological medium was also experimentally investigated.

In all three geological media, the pH and EC values clearly increased with increasing amounts of neutralizer. A markedly higher amount was required for neutralization of VAS, likely due to VAS having a higher pH buffering capacity compared to MS and L, which were suggested to undergo stronger weathering processes. For AAC-D, AAC-OJ, AAC-FA-OJ, RC, and SS, which were prepared as both finer and coarser particle fractions, the finer fraction clearly showed lower additive weight percentages required (i.e., higher neutralization performance). Among the six alkaline waste and by-products tested as alternative neutralizers, SS showed the highest neutralization performance. Specifically, the additive weight percentages of SS with the finer fractions were 1.8%, 0.5%, and 0.4% for VAS, MS, and L, respectively. Oppositely, the additive weight percentages of CC for an industrial product and a chemical reagent for neutralization were 1.6–2.0%, 0.3%, and 0.3% for VAS, MS, and L, respectively. The finer fraction of SS represented similar neutralization performances to CC, which is the most common and conventionally used agricultural lime, suggesting that it is the most promising and suitable alternative neutralizer.

Harmful heavy metals and trace elements are sometimes present in higher concentrations in waste and by-products. Therefore, detailed leaching and other appropriate stability tests for these harmful chemical components are essential before practical application of waste and by-products as alternative neutralizers for acidic geological media.

Acknowledgments

This study was supported by the ISIJ (The Iron and Steel Institute of Japan) Research Promotion Grant, the Collaborative Research Project (2019-15, 2020-12, 2021-13, and 2022-2) of the Research Institute for Natural Hazards and Disaster Recovery, Niigata University, Japan, a Research Grant from the Maeda Engineering Foundation, Japan, and the Science and Technology Research Partnership for Sustainable Development (SATREPS) of the Japan Science and Technology Agency (JST) and Japan International Cooperation Agency (JICA) (JPMJSA1701).

References

• 1)   H. R.  von Uexküll and  E.  Mutert: Plant Soil, 171 (1995), 1. https://doi.org/10.1007/BF00009558
• 2)   W. D.  Bowman,  C. C.  Cleveland,  Ĺ.  Halada,  J.  Hreško and  J. S.  Baron: Nat. Geosci., 1 (2008), 767. https://doi.org/10.1038/ngeo339
• 3)   K.  Fujii,  S.  Funakawa and  T.  Kosaki: Pedologist, 55 (2012), 415. https://doi.org/10.18920/pedologist.55.3_415
• 4)   D.  Tian and  S.  Niu: Environ. Res. Lett., 10 (2015), 024019. https://doi.org/10.1088/1748-9326/10/2/024019
• 5)   K. W. T.  Goulding: Soil Use Manag., 32 (2016), 390. https://doi.org/10.1111/sum.12270
• 6)   Y.  Shavrukov and  Y.  Hirai: J. Exp. Bot., 67 (2016), 15. https://doi.org/10.1093/jxb/erv437
• 7)   E. W.  Slessarev,  Y.  Lin,  N. L.  Bingham,  J. E.  Johnson,  Y.  Dai,  J. P.  Schimel and  O. A.  Chadwick: Nature, 540 (2016), 567. https://doi.org/10.1038/nature20139
• 8)   A.  Akcil and  S.  Koldas: J. Clean. Prod., 14 (2006), 1139. https://doi.org/10.1016/j.jclepro.2004.09.006
• 9)   G.  Du Laing,  J.  Rinklebe,  B.  Vandecasteele,  E.  Meers and  F. M. G.  Tack: Sci. Total Environ., 407 (2009), 3972. https://doi.org/10.1016/j.scitotenv.2008.07.025
• 10)   M.  Katoh,  S.  Moriguchi,  N.  Takagi,  Y.  Akashi and  T.  Sato: J. Soils Sediments, 18 (2018), 1194. https://doi.org/10.1007/s11368-017-1892-0
• 11)   T.  Saito and  N.  Watanabe: Int. J. GEOMATE, 19 (2020), 36. https://doi.org/10.21660/2020.71.5782
• 12)   G.  Roy and  V. C.  Joy: Ecotoxicol. Environ. Saf., 74 (2011), 769. https://doi.org/10.1016/j.ecoenv.2010.10.041
• 13)   G.  Tozsin,  A. I.  Arol,  T.  Oztas and  E.  Kalkan: J. Environ. Manag., 133 (2014), 374. https://doi.org/10.1016/j.jenvman.2013.12.022
• 14)   J.-Y.  Li,  N.  Wang,  R.-K.  Xu and  D.  Tiwari: Pedosphere, 20 (2010), 645. https://doi.org/10.1016/S1002-0160(10)60054-9
• 15)   M. M.  Masud,  J.-Y.  Li and  R.-k.  Xu: J. Soils Sediments, 15 (2015), 365. https://doi.org/10.1007/s11368-014-0993-2
• 16)   R.-y.  Shi,  J.-y.  Li,  N.  Ni,  K.  Mehmood,  R.-k.  Xu and  W.  Qian: J. Soils Sediments, 17 (2017), 2116. https://doi.org/10.1007/s11368-017-1673-9
• 17)  A. RoyChowdhury, D. Sarkar and R. Datta: Remediation, 28 (2018), 75. https://doi.org/10.1002/rem.21562
• 18)  A. RoyChowdhury, D. Sarkar and R. Datta: Environ. Sci. Pollut. Res., 26 (2019), 144414. https://doi.org/10.1007/s11356-019-04785-z
• 19)   G. N.  Paranavithana,  K.  Kawamoto,  Y.  Inoue,  T.  Saito,  M.  Vithanage,  C. S.  Kalpage and  G. B. B.  Herath: Environ. Earth Sci., 75 (2016), 484. https://doi.org/10.1007/s12665-015-5167-z
• 20)   G. M. P.  Kumara,  T.  Saito,  S.  Asamoto and  K.  Kawamoto: Int. J. GEOMATE, 14 (2018), 44. https://doi.org/10.21660/2018.42.7148
• 21)   U.  Kumarasinghe,  Y.  Sakamoto,  T.  Saito,  M.  Nagamori,  C. S.  Kalpage,  G. B.  Herath,  M. I. M.  Mowjood and  K.  Kawamoto: Int. J. GEOMATE, 15 (2018), 225. https://doi.org/10.21660/2018.51.7179
• 22)   U.  Kumarasinghe,  K.  Kawamoto,  T.  Saito,  Y.  Sakamoto and  M. I. M.  Mowjood: Process Saf. Environ. Prot., 120 (2018), 118. https://doi.org/10.1016/j.psep.2018.09.003
• 23)   G. M. P.  Kumara,  K.  Kawamoto,  T.  Saito,  S.  Hamamoto and  S.  Asamoto: J. Environ. Eng., 145 (2019), 04019078. https://doi.org/10.1061/%28ASCE%29EE.1943-7870.0001597
• 24)   A. B. P.  Bandara,  G. M. P.  Kumara,  A.  Matsuno,  T.  Saito,  T. T. V.  Nga and  K.  Kawamoto: Int. J. GEOMATE, 19 (2020), 22. https://doi.org/10.21660/2020.74.9176
• 25)  The United States Department of Agriculture (USDA) Natural Resources Conservation Service: Soil Quality Indicators: pH, Soil Quality Information Sheet, https://www.fs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb5293776.pdf, (accessed 2022-12-27).
• 26)   J.  Espinosa: Better Crops Plant Food, 80 (1996), 28.
• 27)  The United States Department of Agriculture (USDA) Natural Resources Conservation Service: Soil Electrical Conductivity, Soil Quality Indicators, https://www.nrcs.usda.gov/sites/default/files/2022-10/Soil%20Electrical%20Conductivity.pdf, (accessed 2022-12-27).
• 28)   N. M.  Piatak,  M. B.  Parsons and  R. R.  Seal II: Appl. Geochem., 57 (2015), 236. https://doi.org/10.1016/j.apgeochem.2014.04.009
• 29)   D. K.  Gupta,  U. N.  Rai,  R. D.  Tripathi and  M.  Inouhe: J. Plant Res., 115 (2002), 401. https://doi.org/10.1007/s10265-002-0057-3
• 30)   C. J.  Engelsen,  H. A.  van der Sloot,  G.  Wibetoe,  H.  Justnes,  W.  Lund and  E.  Stoltenberg-Hansson: Cem. Concr. Res., 40 (2010), 1639. https://doi.org/10.1016/j.cemconres.2010.08.001
• 31)   T.  Sano,  S.  Miura,  H.  Furusawa,  S.  Kaneko,  T.  Yoshida,  T.  Nomura and  S.  Ohara: J. Wood Sci., 59 (2013), 307. https://doi.org/10.1007/s10086-013-1337-3