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Journal of Resources and Ecology >

2023 , Vol. 14 >Issue 4: 767 - 774

DOI: https://doi.org/10.5814/j.issn.1674-764x.2023.04.009

Evaluation and Improvement of Mine Site Quality

Variation of Nutrients and Salinity for Applying Fly Ash and Wood Vinegar in Coal Gangue Substrate

  • HAN Xiang , 1, 2 ,
  • ZHANG Chaoying 1, 2 ,
  • GENG Yuqing , 1, 2, * ,
  • CHEN Lin 1, 2 ,
  • HAN Xiuna 1, 2
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  • 1. School of Forestry, Beijing Forestry University, Beijing 100083, China
  • 2. The Key Laboratory of Silviculture and Conservation of Ministry of Education, Beijing Forestry University, Beijing 100083, China
*GENG Yuqing, E-mail:

HAN Xiang, E-mail:

Received date: 2022-08-24

  Accepted date: 2023-01-30

  Online published: 2023-07-14

Supported by

Key Research and Development Program of China(2017YFC0504404)

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Abstract

Coal gangue and fly ash are major industrial solid wastes containing some nutrients associated with organic or mineral matter. Vegetation restoration depends on high-quality soil in mining sites. Exploring the effects of wood vinegar (WV) irrigation and fly ash addition on the variation in chemical properties of the coal gangue substrate can provide a theoretical basis and data support for the reconstruction of mine soils and resource utilization of mine solid wastes. The indoor soil incubation experiment was conducted by adding fly ash at rates of 0, 10%, 20% and 50% to a coal gangue substrate mixed with coal gangue and raw soil. We analyzed the nutrients and salinity of the mixture of coal gangue substrate and fly ash (CGSFA) after irrigating distilled water (DW) and WV. The results showed that the addition of fly ash decreased the pH of the CGSFA mixture under DW irrigation, and WV irrigation increased the pH of the CGSFA mixture compared with DW irrigation. The addition of fly ash could increase the contents of dissolved organic carbon (DOC), available phosphorus (AP) and available potassium (AK) and decrease the contents of total organic carbon (TOC) and total nitrogen (TN) of the coal gangue substrate. Compared with DW irrigation, WV irrigation increased the contents of TOC, DOC, active organic carbon (AOC) and AP of the CGSFA mixture by 19.3%, 931.1%, 228.1% and 15.6% and decreased the contents of TN and AK by 10.6% and 35.1%, respectively. In addition, the addition of fly ash increased the contents of K+, Ca2+ and SO42- and decreased the Na+ content in the coal gangue substrate. The irrigation of WV increased the contents of Mg2+, HCO3-, Cl- and SO42- of the CGSFA mixture and reduced the contents of K+ and Na+. Overall, fly ash addition and WV irrigation can improve the nutrients and salinity of the mixture of the coal gangue substrate. Considering the accumulation of HCO3-, Cl- and SO42-, it is recommended to choose 10% to 20% fly ash addition to coal gangue substrate and irrigation with WV for the reconstruction of mine soils.

Key words： coal gangue; fly ash; wood vinegar; nutrient

Cite this article

HAN Xiang , ZHANG Chaoying , GENG Yuqing , CHEN Lin , HAN Xiuna . Variation of Nutrients and Salinity for Applying Fly Ash and Wood Vinegar in Coal Gangue Substrate[J]. Journal of Resources and Ecology, 2023 , 14(4) : 767 -774 . DOI: 10.5814/j.issn.1674-764x.2023.04.009

1 Introduction

Coal is the fundamental energy of China. With the rapid economic development, the focus of coal mining has gradually shifted to the northwest, where the fragile ecological environment is restrained by scarce water (Yang and Xu, 2021). Large-scale coal mining practices have gathered many coal gangue hills, occupying lots of lands and causing environmental problems (Dong et al., 2021). Restoration of native vegetation on coal gangue hills by covering topsoil has become the main pattern to promote socioeconomic development (Liu et al., 2018). Due to the shortage of fertile soil or topsoil, the raw soil with poor structure and nutrients, i.e., sediment in the river rapids or soil parent material, is often used as soil for planting. The raw soil is expensive for transport, and the ecosystem environment in the other region can be destructed subsequently. Moreover, coal gangue contains high organic matter and other mineral components. The combination of raw soil and coal gangue materials was considered essential for vegetation reconstruction (Li and Wang, 2019). Huang et al. (2013) used crushed coal gangue as the main component of the growing medium for ecological restoration of gangue heap. Ke et al. (2021) reported the effect of coal gangue used as the raw material, including the particle size and content, on ecological substrates for vegetation restoration properties. The coal gangue is dominated by mesoporous, with tiny specific surface area, low water holding capacity and few available nutrients (Jablonska et al., 2017; Ashfaq et al., 2020).
Fly ash is a fine particle characterized by a large specific surface area, low bulk density and alkaline reaction. It can be used as an inorganic amelioration agent for agricultural soil because it contains essential macro and micro-nutrients for plant growth (Feng et al., 2005; Jala and Goyal, 2006; Yao et al., 2014). The addition of fly ash can improve the water‐holding capacity and permeability of coal gangue (Wang et al., 2017; Zheng et al., 2022). Wang and Cai (2008) revealed that adding substrate amendments such as fly ash to coal gangue piles can motivate plant growth. The coal gangue and fly ash have been explored in a reasonable proportion as a material for soil reconstruction in mining areas (Wang et al., 2021). However, problematic characteristics, such as high pH and soluble salt contents, can negatively affect plant biomass, especially at higher application rates (Haynes, 2009; Yu et al., 2019). Little is known about the variation of the nutrient element of the coal gangue mixed with different addition rates of fly ash in reconstructed mine soil.
Coal gangue and fly ash are major industrial solid wastes in China that contain nutrients associated with organic or mineral matter. In the natural environment, weathering processes can destroy the particle structure and causes the release of nutrients (Zygadlo and Wozniak, 2010). Whereas the low availability of some nutrient elements and a slow degradation rate lead to an infertile state with deficient useful nutrients and poor structures in the initial phases of reconstructed mine soils (Yin et al., 2016). The rapid release of nutrients from soil materials is a critical process for enhancing soil fertility (Zhu et al., 2008). Chemical weathering is the major way for soil parent materials to release nutrients. Acid leaching experiments have revealed that coal gangue subjected to acidic immersion can be changed regarding mineral composition, particle release, surface microstructure and pore size distribution (Qiang et al., 2014; Qi et al., 2018). In addition, the fly ash modified by sulfuric acid can mobilize acid-soluble metal ions and increase the specific surface area (Xu et al., 2010). Wood vinegar (WV), also known as pyroligneous acid, is a liquid material with condensed and high oxygenated organic acids obtained during the slow pyrolysis of woody biomasses (Jindo et al., 2022). It is a promising additive for soil treatment due to its antioxidant activity, low cost and readily collection (Benzon and Lee, 2017; Sun et al., 2020). Previous studies have demonstrated that acids can release nutrient elements, whereas no research has reported the effect of WV on mine solid wastes. We hypothesize that WV irrigation can change the chemical properties of the coal gangue and fly ash mixture compared with distilled water (DW) irrigation. Hence, the study aims to investigate the variation of nutrients and salinity 1) in the substrate of coal gangue with raw soil under the condition of different addition rates of fly ash and 2) in the mixture of coal gangue substrate and fly ash under the condition of WV and DW irrigation. The research results can provide a theoretical basis and data support for the utilization of mine solid wastes and the reconstruction of mine soils.

2 Materials and methods

2.1 Experimental material

The coal gangue was collected from the gangue dump in Yangchangwan coal mine of Shenhua Ningxia coal industry, Ningxia Hui Autonomous Region. The raw soil was sampled from the natural soil near the gangue dump. For the consistency of the main properties of materials, fly ash was obtained from the West Laifeng Power Plant in Wuhai City, Inner Mongolia Autonomous Region. The materials were prepared by air-drying, then pulverized and sieved through a 2 mm sieve after collecting, and kept in clean polythene boxes for further analysis. The WV was obtained from Huaixin park, Fengtai District, Beijing, after clarifying the by-products of poplar wood carbonization at 550 °C under slow pyrolysis. The pH of the WV was 3.40. The chemical properties of the test materials are shown in Table 1.
Table1 Main chemical properties of test materials
Material pH EC
(ds m-1)		 TOC
(g kg-1)		 TN
(g kg-1)		 AP
(mg kg-1)		 AK
(mg kg-1)
Coal gangue 6.73 0.21 113.89 2.30 4.83 15
Raw soil 9.26 0.13 2.55 0.01 4.33 20
Fly ash 8.72 1.99 3.11 0.03 9.56 105

Note: TOC: Total organic carbon; TN: Total nitrogen; AP: Available phosphorus; AK: Available potassium.

2.2 Experiment design

The simulation experiments of pot incubation were conducted indoors from July to November 2020. The plastic pot is 13 cm in height and 8 cm in diameter, with six small holes of 1 cm in diameter at the bottom for fluid drainage and ventilation. Before filling the pot, the coal gangue and raw soil were mixed into the coal gangue substrate according to the mass ratio of 1:1 in the previous research (Dong et al., 2020). Then, 0 (CK), 10% (F1), 20% (F2) and 50% (F5) of fly ash were added and mixed fully. Finally, the mixtures of the coal gangue substrate and fly ash (CGSFA) were loaded into the pot with a shovel. Thus, four different CGSFA mixtures were prepared. To keep similar compaction in all pots, the polyoxymethylene cylinder was put down five times in the vertical direction at the same height. In the experiment, eight treatments were designed, including four CGSFA mixtures irrigated with DW and four CGSFA mixtures irrigated with WV, and each treatment was repeated three times (Table 2). The CGSFA mixture was irrigated until there was liquid leakage from the bottom. Then, 250 ml of DW or WV was irrigated on the 20th and 50th days. After incubation for three months, the CGSFA mixture was fully mixed and sampled to determine the chemical properties.
Table 2 Experimental design for addition rates of fly ash and the two irrigation methods
Addition rate (%) Irrigation method
DW WV
0 DWCK WVCK
10 DWF1 WVF1
20 DWF2 WVF2
50 DWF5 WVF5

Note: DWCK means treatment under distilled water irrigation without fly ash. DWF1, DWF2, DWF5 means treatment under distilled water irrigation with 10%, 20%, 50% of fly ash addition. WVCK means treatment under wood vinegar irrigation without fly ash. WVF1, WVF2, WVF5 means treatment under wood vinegar irrigation with 10%, 20%, 50% of fly ash addition. The same below.

2.3 Analyse methods

The pH was determined by an acidity meter with a soil- water ratio of 1: 5. Total organic carbon (TOC) was determined by the dichromate oxidation/dilution heat method, and total nitrogen (TN) was determined by the Kjeldahl method. Dissolved organic carbon (DOC) was extracted and determined using the Multi N/C 3100 Analyzer, and active organic carbon (AOC) was determined using potassium permanganate (Weil et al., 2003). Available phosphorus (AP) was extracted with sodium bicarbonate and then determined by molybdenum antimony anti-spectrophotometry. Available potassium (AK) was extracted with ammonium acetate and then analyzed by flame photometry directly (Lu, 2000).
The soil water-soluble salts were leached at a soil/water ratio of 1:5. Additionally, K+ and Na+ were determined by flame photometry; Ca2+ and Mg2+ were determined by EDTA complex titration; CO32− and HCO3− were determined by double-indicator titration; Cl− was determined by AgNO3 titration; SO42− was determined by indirect titration with EDTA.

2.4 Data analysis

One-way analysis of variance (ANOVA) was used to analyze the differences in pH, nutrients and salinity of the CGSFA mixture with different addition rates of fly ash. The significance of differences was compared using the Duncan method under the DW and WV irrigation treatments. Two- way ANOVA was used to analyze the effects of different irrigation methods and fly ash addition as well as their interactions on pH, nutrients and salt variation of the CGSFA mixture. The significance of the difference was tested by the Duncan method. These analyses were performed using SPSS (version 21.0; IBM, Armonk, NY, USA).

3 Results

3.1 Variation of pH value

The pH of the CGSFA mixture shows different trends with the fly ash addition under the two irrigation methods (Fig. 1). Compared with the DWCK, the CGSFA mixture added with fly ash shows an obviously decreased pH under DW irrigation and no difference under WV irrigation.
Fig. 1 The pH of the CGSFA mixture under the two irrigation methods Note: Lowercase letters indicate significant differences under DW irrigation a>b>c (P<0.05), and capital letters indicate significant differences under WV irrigation.
Two-way ANOVA shows that the pH of the CGSFA mixture is significantly influenced by the irrigation method and fly ash addition (Table 3). The pH of the CGSFA mixture irrigated with WV is significantly higher than that irrigated with DW, indicating that WV irrigation can increase the pH of the CGSFA mixture. The addition of fly ash significantly decreases the pH of the CGSFA mixture. Moreover, their interaction on the pH variation of the CGSFA mixture is significant.
Table 3 Two-way ANOVA for analysis of the pH of the CGSFA mixture under the two irrigation methods
Parameter Irrigation method (I) Addition rate (A) Interaction (I×A)
F I(Sig.) P F A(Sig.) P F P
pH 69.153 B, A <0.001 35.899 a,b,b,b <0.001 37.086 <0.001

Note: I(Sig.) indicated significant differences between the DW and WV irrigation, respectively. A>B (P<0.05). A(Sig.) indicated significant differences among the different rates of fly ash addition (CK, F1, F2 and F5), respectively. a>b>c>d (P<0.05). The same as Table 5 and Table 7.

3.2 Nutrient variation of the CGSFA mixture

The nutrient content of the CGSFA mixture under the two irrigation methods differs from that of fly ash addition (Table 4). The contents of TOC, DOC and AOC change under the two treatments. The TOC content decreases significantly with the increasing rate of fly ash addition. The DOC content increases significantly under WV irrigation but shows no changes under DW irrigation with the addition of fly ash. The AOC content of DWCK is significantly higher than that of fly ash addition under DW irrigation, and the AOC content of WVF5 and WVF2 is higher than that of WVCK under WV irrigation. This phenomenon implies that the changing trend of the AOC content is opposite under the two irrigation methods. The TN content decreases, and the content of AP and AK increases significantly with fly ash addition.
Table 4 Nutrient content of the CGSFA mixture under the two irrigation methods
Treatment TOC (g kg-1) DOC (g kg-1) AOC (g kg-1) TN (g kg-1) AP (mg kg-1) AK (mg kg-1)
DWCK 66.66±3.84a 0.36±0.01a 3.31±0.15a 1.26±0.01a 3.78±0.16d 26.67±1.67c
DWF1 60.73±1.05a 0.41±0.15a 2.97±0.35b 1.18±0.01b 6.07±0.11c 23.33±1.67c
DWF2 56.60±3.23b 0.42±0.05a 2.93±0.13b 1.10±0.01c 7.91±0.31b 38.33±1.67b
DWF5 45.41±2.05c 0.45±0.03a 2.29±0.34b 0.76±0.00d 10.65±0.22a 68.33±1.67a
WVCK 73.08±2.02a 3.35±0.02c 8.36±1.09b 1.14±0.02a 5.99±0.22d 20.00±0.00b
WVF1 71.01±0.85a 2.55±0.18d 7.88±0.12b 1.05±0.01b 6.83±0.21c 20.00±0.00b
WVF2 72.06±2.12a 4.56±0.31b 9.09±0.42b 0.93±0.01c 7.65±0.30b 28.33±1.67a
WVF5 57.65±2.75b 7.44±0.42a 12.37±0.36a 0.73±0.00d 12.38±0.06a 33.33±1.67a

Note: Data in the table are expressed as mean ± standard error. Different letters in the same column indicate significant differences between different treatments (P<0.05). The same as Table 6.

Both irrigation methods and fly ash addition at different rates can significantly affect the nutrient content of the CGSFA mixture, and the difference lies in degrees (Table 5). Specifically, the effect of irrigation methods is more obvious on TOC, DOC, and AOC but weaker on TN and AP comared with that of fly ash addition. It is noteworthy that WV irrigation can increase the TOC, DOC, AOC and AP conents of the CGSFA mixture by 19.3%, 931.1%, 228.1% and 15.6% and decrease the contents of TN and AK by 10.6% and 35.1% compared with DW irrigation. With the addition of fly ash, AP and AK contents increase, the TN content sharply decreases, and the DOC content shows no changes.
Table 5 Two-way ANOVA for analysis of the nutrient content of the CGSFA mixture under the two irrigation methods
Nutrient Irrigation method (I) Addition rate (A) Interaction (I×A)
F I(Sig.) P F A(Sig.) P F P
TOC 41.615 B,A <0.001 21.273 a,ab,b,c <0.001 1.208 0.339
DOC 35.945 B,A <0.001 0.828 a,a,a,a 0.498 2.415 0.104
AOC 383.101 B,A <0.001 6.074 a,b,b,b 0.006 13.050 <0.001
TN 208.071 A,B <0.001 626.757 a,b,c,d <0.001 14.360 <0.001
AP 52.339 B,A <0.001 341.472 d,c,b,a, <0.001 12.826 <0.001
AK 181.500 A,B <0.001 172.167 c,c,b,a <0.001 49.944 <0.001

3.3 Changes in the salinity of the CGSFA mixture

The salinity of the CGSFA mixture under the two irrigation methods is shown in Table 6. It is notable that the Na+ content decreases significantly, and K+, Ca2+, Mg2+, HCO3-, Cl-, and SO42- contents increase significantly for the CGSFA mixture with the addition of fly ash under DW irrigation. The addition of fly ash increases the Mg2+ content, while no significant difference is found between the treatments with the addition rate of fly ash. The addition of fly ash decreases the content of Na+ and increases the contents of K+, Ca2+, and total anions under WV irrigation. In contrast, the variation of salt ions is slight in the fly ash addition treatment under WV irrigation. No obvious difference is found in the contents of K+, Na+, Ca2+, HCO3- and Cl- between WVCK and WVF1 as well as the contents of K +, Mg2+ and Cl- between WVF2 and WVF5.
Table 6 Salt ion content of the CGSFA mixture under the two irrigation methods
Treatment K+ (mg kg-1) Na+ (mg kg-1) Ca2+ (g kg-1) Mg2+ (mg kg-1) HCO3- (mg kg-1) Cl-( mg kg-1) SO42- (g kg-1)
DWCK 5.00±0.00c 238.33±1.67a 0.16±0.00d 24.40±12.20b 35.79±2.15c 68.63±9.47c 2.21±0.24d
DWF1 15.00±0.00b 188.33±6.67b 1.09±0.07c 276.53±40.67a 41.48±1.41bc 130.17±9.47bc 5.12±0.70c
DWF2 15.00±0.00b 175.00±0.00c 2.63±0.07b 215.53±20.33a 45.55±0.81b 160.93±6.26b 7.92±0.14b
DWF5 20.00±0.00a 93.33±4.41d 3.09±0.13a 276.53±40.67a 55.31±2.15a 461.50±22.06a 9.68±0.49a
WVCK 5.00±0.00b 190.00±8.66a 0.93±0.13c 223.67±53.80b 129.32±6.14c 385.77±54.59b 4.08±0.63d
WVF1 5.00±0.00b 173.33±11.67a 1.20±0.12c 325.33±40.67a 218.79±33.76bc 587.88±40.77ab 7.20±0.24c
WVF2 10.00±0.00a 135.00±5.77b 2.07±0.41b 284.67±40.67ab 284.67±5.87b 730.35±40.17a 8.80±0.80b
WVF5 11.67±1.67a 46.67±6.01c 3.33±0.18a 528.67±107.59a 508.33±100.37a 892.23±139.39a 10.88±0.37a
The two-way ANOVA shows that both the irrigation methods and fly ash addition can significantly influence the salinity characteristics of the CGSFA mixture (Table 7). The F value indicates that irrigation methods are the dominant factors affecting the salt ions of the CGSFA mixture except for Na+, Ca2+ and SO42-. Compared with DW irrigation, WV irrigation increases the contents of Mg2+, HCO3-, Cl- and SO42- of the CGSFA mixture by 71.8%, 540.6%, 216.1% and 27.1% and decreases the contents of K+ and Na+ by 42.2% and 21.6%. In addition, the addition of fly ash increases the contents of K+, Ca2+ and SO42-, decreases the Na+ content, and increases the contents of Mg2+, HCO3- and Cl- only at a high rate. Their interaction significantly affects the contents of K+, HCO3- and Ca2+ of the CGSFA mixture.
Table 7 Two-way ANOVA for analysis of the salt ion content of the CGSFA mixture under the two irrigation methods
Salt ion Irrigation method (I) Addition rate (A) Interaction (I×A)
F I(Sig.) P F A(Sig.) P F P
K+ 196.000 A,B <0.001 120.000 d,c,b,a <0.001 28.000 <0.001
Na+ 64.800 A,B <0.001 174.997 a,b,c,d <0.001 2.741 0.077
Ca2+ 1.238 A,A 0.282 90.386 d,c,b,a <0.001 4.749 0.015
Mg2+ 14.973 B,A 0.001 9.914 c,ab,b,a 0.001 1.811 0.186
HCO3- 82.081 B,A <0.001 10.225 b,b,b,a 0.001 8.351 0.001
Cl- 119.381 B,A <0.001 21.688 c,b,b,a <0.001 1.626 0.223
SO42- 17.420 B,A 0.001 73.219 d,c,b,a <0.001 0.658 0.590

4 Discussion

4.1 The effect of fly ash addition on nutrient and salinity of the coal gangue substrate

Fly ash is rich in alkali metal elements, such as sodium, calcium and magnesium, showing a high pH. A study has suggested that applying fly ash to soil increases the pH of the soil (Wang et al., 2021). However, another study has shown that fly ash application does not shift the pH of the mixed substrate due to its low buffering capacity (Isa et al., 2012). Whereas, in our study, the pH after adding fly ash is significantly lower than that of the DWCK under DW irrigation (Fig. 1). The reason is related to the lower pH of fly ash than raw soil (Table 1), and the incubation process can neutralize the alkalinity of fly ash due to fly ash weathering (Ogut et al., 2009). In addition, a large number of inorganic ashes such as SiO2, Al2O3 and Fe2O3 contained in fly ash undergo redox reactions with air and water, further leading to the acidification of the coal gangue substrate (Chen et al., 2007).
Soil carbon, nitrogen and phosphorus are important for providing essential nutrients for plant growth and sustaining soil fertility. Nutrients in coal gangue can be released to meet plant growth needs in the weathering process (Cai et al., 2015). The content of dissolved nutrients is correlated with the mineral, chemical composition and lithology (Zhao et al., 2020). The carbon or nitrogen content of fly ash is negligible due to their oxidation, whereas phosphorus and potassium are kept during coal combustion (Yao et al., 2014). Our results demonstrate that TOC and TN contents decrease, and AP and AK contents of the coal gangue substrate increase with the increasing addition rate of fly ash, which is partly supported by Doongar et al. (2013). They reported that the availability of nitrogen and potassium declined significantly, whereas the availability of phosphorus increased when the addition of fly ash was at higher levels. This result is obviously due to the differences in the composition of coal gangue and fly ash in different regions. AOC is a sensitive indicator measured by the lower KMnO4 concentration. In this study, the AOC content after adding fly ash is lower than that of the DWCK under DW irrigation. Moreover, no significant decrease is found in the fly ash addition treatment, indicating that AOC in fly ash is smaller than the coal gangue substrate.
The content of principal oxides in fly ash from China is usually in descending order: SiO2 > Al2O3 > Fe2O3 > CaO > K2O > MgO >SO3 >Na2O (Liu et al., 2021). The variation depends on the type of coal used, combustion conditions, collector setup, etc. (Yao et al., 2014). Fly ash is an important source of extracts (Mupambwa et al., 2015), while soluble salts characteristics needs to be paid attention to for the reconstructed mine soil. Our results reveal that the content of Na+ is the highest, followed by Ca2+ among the cation, and SO42- dominates in anions in the coal gangue substrate, which is not consistent with the result reported by Zhang et al. (2018), who found that cations were dominated by K+ and Na+ and anions were dominated by HCO3-. The difference is related to the chemical properties of the coal gangue substrate. The contents of Ca2+, Mg2+ and K+ of the CGSFA mixture increase significantly with the increase in fly ash addition, which is related to the affluence of fly ash oxides (Liu et al., 2021). The decrease of Na+ is due to the substitution of Ca2+ and Mg2+ and the leaching process (Huang et al., 2021). In the anions of the CGSFA mixture, SO42- dominates, and its content increases significantly with the addition of fly ash. A higher SO42- content results from the sulfate minerals contained in coal and coal gangue. The average sulfur content in raw coal is high and is called high sulfur coal in Inner Mongolia (Dai, 2000; Ma, 2020). In addition, sulfur minerals in coal can be transformed into SO2 and absorbed by alkaline substances with coal combustion (Tan and Li, 2008). The contents of HCO3- and Cl- of the CGSFA mixture increase tardily with the addition of fly ash at a high rate, which indicates that fly ash contains a small amount of HCO3- and Cl- (Pan et al., 2020; Yan et al., 2022).

4.2 The effect of irrigating wood vinegar on nutrient and salinity of the CGSFA mixture

WV is an acidic organic compound with over 50% of acetic acid, and WV irrigation can increase the concentration of H+ and acidify, thus lowering its pH (Grewal et al., 2018). However, WV irrigation did not reduce the pH of the CGSFA mixture in our study. The previous research demonstrated that the HCO3- content can directly affect soil alkalinity and increase the pH of the soil (Liu et al., 2019). Carbonate is one of the typical minerals in raw coal (Shao et al., 2019). Additionally, WV activates the alkaline material in coal gangue and fly ash, promotes Ca2+ and Mg2+ dissolution, and increases the HCO3- content (Qiang et al., 2014). This may be the reason for the increase in pH of the CGSFA mixture under WV irrigation.
The content of organic carbons, such as TOC, DOC and AOC, in the CGSFA mixture under the WV irrigation treatment is higher than that under DW irrigation treatment, which is related to the organic carbon in WV. In addition, WV or DW irrigation strongly affects organic carbon. The decrease in the TN content of the CGSFA mixture under the WV irrigation treatment is related to the slight amount of nitrogen in fly ash and the strong effect of fly ash addition (Li and Wang, 2014). WV has a metal-complexing ability with iron and aluminum oxides, thus reducing the adsorption/fixation of phosphate (Benzon and Lee, 2017). The availability of P depends on the characteristics of coal gangue and fly ash and the contents of Ca, Fe and Al in the CGSFA mixture. The AP content under the WV irrigation treatment is higher than that under DW irrigation, which is related to the transformation of inorganic phosphorus in fly ash due to the organic acid and the complexation of WV with metal oxides. The WV activates the mobilization of acid-soluble metal ions and promotes the dissolution of Ca2+ and Mg2+ (Xu et al., 2010; Qiang et al., 2014), causing the contents of Ca2+ and Mg2+ under WV irrigation to be higher than those under DW irrigation. In contrast, leaching leads to a decrease in K+ and Na+ due to high mobility.
For the composition of salt anions, Cl- has higher mobility and is more harmful than other anions. The Cl- content under WV irrigation is significantly higher than that under DW irrigation, which is supported by Peng et al. (2018), who found that increasing acidity could promote the release of Cl-. Although Cl- is one of the essential trace elements for plants, high content can increase salt damage and further limit plant growth (Qian et al., 2022). Moreover, WV irrigation increases the Cl- content of the CGSFA mixture, and subsequent accumulation of Cl- in mine soil can limit chlorine-resistant vegetation. In this study, WV irrigation increases the SO42- content significantly, which may be because the hydration of SO3 in fly ash is promoted by WV irrigation.
Soil salinity levels directly limit the uptake of water and nutrients and plant growth. However, the effects of salt ions on soil fertility and plant are different. For the cations in the salt composition, Ca2+, Mg2+ and K+ are useful nutrients, and higher Na+ levels increase pH and negatively impact plant growth (Yu and Yang, 1982). The cation variation of the CGSFA mixture suggests that WV irrigation is useful for the reconstruction of mine soils. The CGSFA mixture is dominated by SO42-, which accounts for 67.17% to 80.59% of the total salt mass, and belongs to the sulfate-type saline-alkali soil. Sulfur is an essential nutrient for plant growth, and plants need to absorb sulfate from the soil. Fly ash addition and WV irrigation can change the salt ion composition, with a decreased Na+ content and a significantly increased SO42- content. Since high salt may be harmful to plants, it is recommended to choose 10% to 20% fly ash as well as sulfur-tolerant and salt-tolerant plants for the CGSFA mixture combined with WV irrigation. Moreover, our experiment was conducted indoors, and field trials based on reconstruction with the CGSFA mixture are needed in the future.

5 Conclusions

The reconstruction of mine soils with coal gangue and fly ash is an ideal way to utilize mine solid wastes. Indoor experiments were conducted, including adding fly ash to the coal gangue substrate and irrigating WV. We demonstrated that the addition of fly ash decreased the pH of the CGSFA mixture under DW irrigation, and WV irrigation increased the pH of the CGSFA mixture compared with DW irrigation. Two-way ANOVA indicated that the addition of fly ash could increase the contents of DOC, AP, AK, K+, Ca2+ and SO42- and decrease the contents of TOC, TN and Na+ in the coal gangue substrate. Compared with DW irrigation, WV irrigation increased the contents of TOC, DOC, AOC, AP, Mg2+, HCO3-, Cl- and SO42- of the CGSFA mixture and reduced the contents of K+ and Na+. It is recommended to choose 10% to 20% fly ash addition to coal gangue substrate and irrigation with WV for the reconstruction of mine soils.

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