Physicochemical Properties and Planting Performance of Artificial Soil Developed from Multiple Coal-Based Solid Waste Materials
1. Introduction
The total annual coal production in China is about 4 billion tons, resulting in a huge amount of CSWM generated in the process of coal development and utilization [1]. These materials include coal gangue (CG), fly ash (FA), desulfurization gypsum (DG), gasification slag, and bottom ash [2]. The issues associated with the disposal of these CSWM remain severe; the majority of the remaining CSWM is directly deposited and landfilled, resulting in serious environmental contamination [3,4]. Further improving the resource utilization efficiency of CSWM has become an urgent need for the development of a low-carbon society. Simultaneously, it is noteworthy that the middle and upper reaches of the Yellow River Basin serve as the primary coal reservoir [5,6], contributing to around 41.7% of China’s overall coal production. However, the region mostly experiences a semi-arid and arid climate, leading to significant issues in land desertification and ecological degradation. The current ecological restoration of mines is encountering significant problems related to limited soil supplies. Therefore, the purpose of this study was to develop AS using CG, FA, and DG, so that it can be applied in mine ecological restoration. The high-value utilization of coal-based solid wastes has been studied not only on a laboratory scale, but also in pilot tests. The test results reached the laboratory level [7,8].
Currently, there is a wide range of research being conducted on the disposal and use of CSWM. The predominant research and application fields of CG are in engineering fillers [9,10]. These fillers are typically employed in engineering applications such as mine backfill, subgrade fill, and foundation pit backfill. In addition, numerous researchers have conducted investigations into the viability of using CG to make construction products, including cement [11,12], ceramics, sintered bricks, and lightweight aggregates for concrete [13,14]. Additionally, numerous studies have demonstrated that the mineral composition of CG is similar to that of soil and that it also contains a significant amount of nutrients and trace elements [9]. Moreover, the organic matter content in CG is usually 2–10 times that of soil, which is very favorable for plant growth [15,16]. So, some scholars have conducted research on the production of soil amendments and fertilizers derived from CG [16]. FA is the primary solid waste generated by coal-fired power plants. It possesses favorable volcanic ash properties, making it predominantly utilized as an additive in cement and concrete production [17]. Numerous studies have demonstrated that FA serves as an effective soil conditioner, leading to notable enhancements in the physical structure, chemical properties, and microbial environment of soil. FA can also provide the macronutrients and micronutrients required for plant growth [18]. It was found that the application of modified FA had a positive effect on the remediation of heavy metals in sodium-containing soil. This application could improve soil pH value, enhance soil water retention capacity and promote the formation of soil aggregates [19]. DG is extensively utilized in the field of agriculture, proving its efficacy in enhancing the physical and chemical characteristics of soil. It has been observed to effectively regulate the loss of soil nutrients and replenish trace elements in the soil, thereby improving crop yields [20,21,22]. Moreover, DG has demonstrated significant potential in ameliorating saline-sodic soils, resulting in enhanced seedling emergence, reduced pH levels, and increased water retention capacity of the soil [22]. This study is also in line with the concept of the circular economy, as proposed by domestic and foreign scholars, with the goal of transforming waste into high-value materials and achieving resource sustainability [23,24].
In general, CG, FA, and DG exhibit favorable plant compatibility and can serve as effective soil amendments to enhance plant development. Consequently, these materials possess considerable potential for utilization in ecological contexts. However, previous studies mostly introduced these materials into the soil as external additives with the aim of enhancing soil characteristics. However, this approach does not adequately address the issue of insufficient soil resources for ecological restoration in mining areas. Indeed, it has been observed that CSWM, such as CG and FA [25,26,27], possess abundant mineral constituents such as aluminates, silicates, and oxides of iron and aluminum. These mineral components are close relatives to the natural soil. Hence, it is inferred that CSWM have the potential to be used as ecological soil, but there are few relevant studies. If these materials are suitable for direct use as planting soil in mining ecological restoration, it would enhance the efficiency of CSWM consumption and significantly conserve planting soil resources. In this study, three types of coal-based solid waste materials, CG, FA, and GG, were selected and mixed to prepare AS. The physicochemical properties of AS, including bulk density, specific gravity, porosity, field capacity, ammonium nitrogen (AN), effective phosphorus (AP), quick-acting potassium (AK), pH, and electrical conductivity (EC), were tested considering different raw material ratios. At the same time, Chinese Leymus chinensis was selected for planting experiment on AS to study the growth characteristics of this plant, including stem height, root length and biomass.
Coal gangue, fly ash and desulfurization gypsum are by-products of coal mines and coal-fired power plants and are often considered waste. By using these wastes for the preparation of artificial soil, the comprehensive utilization of resources is realized, the dependence on natural resources is reduced, and the problem of waste disposal is solved. It can reduce the exploitation of natural soil and help protect the natural environment. Some coal mine wastes have different physical and chemical properties, and by mixing and blending, the texture of artificial soil can be adjusted to make it more suitable for plant growth. This customized artificial soil can meet the specific needs of different plants for soil and improve soil quality. Finally, the direction of this study was determined based on the lack of natural vegetation soil in the study area.
4. Discussion
CG is a prevalent CSWM that generally comprises soil-like constituents, including silicates, oxides, and carbonates [51,52], in addition to substantial quantities of organic matter, N, P, and K. CG is an acidic mineral, and as the amount of CH increases, the EC and pH values decrease. The particle size of coal gangue is relatively large, so the field capacity and porosity of the AS exhibit a decline as the CG content increases. FA has a large specific surface area; approximately 980 m2/kg of FA was utilized in this study. Therefore, through field water capacity experiments and soil property measurements, it could be confirmed that the porosity and water retention capacity of AS increased. Moreover, FA is rich in nutrients and can supply the AS with nutrients. The alkaline FA chosen for this study was capable of controlling the acidic environment of the AS. The addition of DG and FA altered the structure of the AS, increased its permeability and water retention capacity, and transformed its texture from suboptimal to optimal. Based on the comparison graphs of plant height and growth, it can be observed that as FA and DG content increased and CG gradually decreased, plant growth initially improved and then declined. Therefore, regulating FA and DG content within the range of 20%–30% can effectively stimulate plant growth in the AS prepared by CSWM.
From the perspective of resource utilization, artificial soil solves the problem of large accumulations of coal-based solid waste in coal mine areas, and the raw material is accumulated waste, avoiding pollution to the environment and groundwater and other natural resources. In addition, water resources are scarce in the study area, and normal soil ecological slope protection will increase the cost. Compared with normal soil, artificial soil is easy to obtain, lower in cost, has better nutrient content and water retention performance, will reduce the risk of pests and diseases, and improve the health of the vegetation growth environment. The raw material of the artificial soil is prepared from coal-based solid waste material, which not only solves the problem of waste accumulation but also beautifies the environment. This idea is conducive to environmentally friendly and sustainable development and the goal of ecological restoration of mines. Coal-based solid waste materials support plant growth conditions and can provide sufficient nutrients, water retention and ventilation. The preparation process for artificial soil is simple, easy to manage, and can be easily realized in practical implementations. The concept of preparing artificial soil conforms to the reality of local shortages of water and soil. The use of artificial soil for slope ecological restoration is beneficial to society and the environment and can reduce soil erosion and improve water quality.
5. Conclusions
(1) The physical properties of artificial soil prepared with coal gangue, fly ash and desulfurization gypsum are different. The porosity increases with the decrease in coal gangue content. The porosity of C100 is 0.48 and that of C40F20D40 is 3.1. The bulk density decreases with the decrease in coal gangue content. The water retention capacity increased with the increase in fly ash and desulfurization gypsum content. From C100 to C40F40D20, the field water retention capacity increased from 0.148 to 0.368, and the saturated water content increased from 0.223 to 0.46, respectively.
(2) With different proportions of the three coal-based solid waste materials, the chemical properties of artificial soil also differed. Under the condition of no vegetation growth, the pH of C100 was 3.56 at a minimum, and that of C40F40D20 was 12.45 at a maximum. However, under the influence of plant growth for 30 days, the pH of each experimental group was close to neutral. This result is attributed to two factors: plant respiration and microbial metabolism. The additions of CG, FA, and DG can all alter the physical structure and chemical properties of AS, but their modification mechanisms are different. FA facilitates the generation of an alkaline environment and the release of ammonium nitrogen (AN). DG contributes sulphate ionic compounds.
(3) The preparation of artificial soil (AS) for plant growth can be achieved through the composite formulation of three commonly used materials: coal gangue (CG), fly ash (FA), and desulfurization gypsum (DG). Specifically, the incorporation of FA and DG at a ratio of 20%–30% enhances the AS texture and water retention capacity, promoting plant growth. Based on the growing situation of plants and the physicochemical characteristics of the AS, the following order can be established to indicate the advantages and disadvantages of each ratio: C80 > C60 > C100 > C40.
