Enhancing Restoration of Arid Mining Area Using Lignite-Based Superabsorbent Gel

TL;DR

A new superabsorbent gel made from lignite helps restore dry mining areas by retaining water and improving plant growth.

Contribution

A lignite-based superabsorbent gel is developed for ecological restoration in arid mining areas with high water retention and reusability.

Findings

The gel achieved 522 g·g−1 water absorption in distilled water and 65.5 g·g−1 in salt solution.

The gel retained over 81.8% of its water absorption capacity after five reuse cycles.

Field trials showed improved vegetation coverage in mining areas using the gel.

Abstract

This research designed a high-performance superabsorbent gel aligned on the integration of lignite humic residue (LHR) with a polymeric organic network in order to address ecological restoration challenges in the arid mining area in Xinjiang. This water-retaining agent was synthesized by employing solution polymerization techniques using acrylic acid (AA) and acrylamide (AM) as monomers, lignite hydrothermal residue (LHR) as a functional additive, and ammonium persulphate (APS) as the initiator. The resulting lignite hydrothermal residue–polyacrylic gel composite material was obtained by using N,N′-methylene-bisacrylamide (MBA) as the primary crosslinking agent. The water absorption capacity and mechanical strength of the acrylic gel were further enhanced by specifically incorporating low-cost, safe, and non-toxic lignite humic residue (LHR). The performance test indicated that this gel achieved a maximum water absorption of 522 g·g−1 in distilled water and 65.5 g·g−1 in 0.9% sodium chloride solution. Its reusability and water absorption capacity remained above 81.8% even after five cycles of natural dehydration and reabsorption. The method for synthesizing this superabsorbent gel effectively constructs a soil water retention network structure, improving the soil microenvironment, and enhancing plant salt tolerance. The field trial results showed that the application of this LHR-AA-AM superabsorbent gel considerably improved vegetation coverage in mining areas. Hence, this study provides an efficient and economical superabsorbent material for ecological restoration of saline–alkali land in arid regions without soil replacement, demonstrating promising application prospects.

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Full text

1. Introduction

The mining area in Xinjiang is situated within China’s arid region, characterized by harsh climatic conditions. The area experiences prolonged and exceptionally severe winters, with strong winds throughout the year, resulting in low precipitation. In parallel, the mining area’s soil exhibits high alkalinity and extremely low organic matter content, rendering the ecosystem inherently fragile [1]. Mining operations have further exacerbated soil degradation and salinization, presenting formidable challenges for regional vegetation restoration. The mining area’s remote location further increases the prohibitively high planting costs associated with using imported soil for ecological restoration. Consequently, there is an urgent need to develop an efficient and viable ecological restoration technique that does not require soil replacement. The current strategies for improving saline–alkali soils primarily include three remedial approaches: (1) physical remediation (such as drip irrigation [2] and soil replacement [3]), (2) biological remediation (such as microbial remediation [4,5] and salt-tolerant crops [6,7]), (3) chemical remediation (such as gypsum [8], biochar [9,10], and hydrogels [11,12,13]). However, physical and biological methods are often constrained in practical application by limitations such as high costs, extended timelines, and environmental dependency. Consequently, economical and efficient chemical amelioration techniques have garnered significant attention in recent years, emerging as a key research direction within this field.

Superabsorbent polymers play a vital role in advancing sustainable agriculture and forestry through their exceptional water absorption and nutrient retention capabilities [14,15,16]. In mine ecological restoration, superabsorbent gels demonstrate significant practical value by enhancing soil structure, improving water and nutrient retention capacity, and reducing soil erosion, thereby substantially promoting vegetation recovery and growth [17,18,19]. In response to environmental protection requirements, research and applications are progressively shifting toward natural biodegradable polymeric materials (such as chitosan, cellulose, and alginates) [12,20,21]. However, these materials commonly exhibit insufficient water absorption, poor salt tolerance, relatively high costs, and inadequate recyclability [22], necessitating further refinement. In contrast, inorganic superabsorbent polymers exhibit distinct advantages in mechanical strength and salt tolerance, offering complementary properties to organic polymers [23,24,25]. Through rational composite design, it is possible to combine the strengths of both materials, yielding superabsorbent polymers with superior performance and broader applicability. This approach addresses the demand for efficient, sustainable water-retaining materials in the agriculture, forestry, and ecological restoration sectors.

Grafting inorganic non-metallic oxides is a common strategy for enhancing the mechanical strength of hydrogels. Although nano-TiO_2_ particles are widely utilized due to their excellent chemical stability and surface hydrophilicity, their high cost has constrained large-scale application [26]. Consequently, recent research has shifted focus to utilizing low-cost natural inorganic minerals and industrial/agricultural waste materials as grafting agents. These are incorporated into superabsorbent polymers as an effective and economical reinforcement method [27,28]. Brown coal constitutes approximately 16% of China’s total coal resources and serves as a vital feedstock for humic acid (HA) extraction. As a natural, economical, and safe plant nutrient carrier and synergist [29,30], humic acid can be efficiently extracted for achieving high-value utilization of brown coal [31]. However, the existing extraction processes are constrained by limited humic acid extraction efficiency and poor conversion rates, generating substantial humic-acid-rich residues. These residues not only occupy land but also represent significant resource wastage. As a result, advancing the resource utilization of brown coal residues holds considerably important [32]. In this study, we present a straightforward method for developing brown coal residue–acrylic acid (AA)–acrylamide (AM) superabsorbent gel (LHR-AA-AM) for ecological restoration in arid coal mining areas.

2. Results and Analysis

2.1. Optimal Material Ratios and Reaction System Optimization

This experiment determined the optimal reaction conditions for the superabsorbent gel through a combination of single-factor and orthogonal designs. Factors influencing the swelling rate of the lignite slag–acrylic acid–acrylamide composite water-retaining agent were investigated, including temperature, reaction time, degree of neutralization, and ammonium persulphate (APS) dosage.

Orthogonal experiments were conducted to determine the optimal synthesis conditions using gel water absorption in both distilled water and 0.9% NaCl solution as the evaluation criteria. The experimental design involved screening key variables, including raw material ratios, crosslinking agent (N,N′-methylene-bisacrylamide (MBA)) dosage, and brown coal slag (LHR) content. The specific reaction conditions are listed in Table 1. Analysis of variance indicated that the MBA dosage was not a statistically significant factor. Based on the combined water absorption performance in distilled water and 0.9% NaCl solution, Experiment 2 was selected as the optimal reaction condition.

Subsequently, single-factor experiments were conducted to investigate the effects of reaction temperature, reaction time, APS dosage, and LHR dosage on the superabsorbent gel. R_water_ represents the range of each factor (in distilled water). R_saltwater_ represents the range of each factor (in saltwater). K_water1–4_ denotes the average of all experimental results for different factors at different levels (in distilled water). K_saltwater1–4_ denotes the average of all experimental results for different factors at different levels (in saltwater).

Under the synthesis conditions (acrylic acid neutralization degree was 70%, mass ratio (g/g) w(AA):w(LHR):w(AM):w(MBA):w(APS) = 100:10:100:0.4:0.8, as shown in Figure 1a), the gel’s water absorption capacity exhibited a temperature-dependent optimum value, peaking at 70 °C (Figure 1a). At temperatures below 70 °C, reduced initiator activity led to insufficient radical generation and sluggish polymerization, resulting in weak crosslinking, increased soluble components, and poor mechanical strength. The optimum temperature of 70 °C balanced radical generation with reaction kinetics, forming an effective three-dimensional network structure. However, temperatures exceeding 70 °C resulted in three adverse effects: (1) accelerated initiator decomposition, yielding low-molecular-weight gels with shorter main chains; (2) decreased crosslink density; (3) accelerated exothermic polymerization, potentially triggering thermal runaway and ultimately producing viscous, low-strength products with diminished water absorption capacity.

Under the synthesis conditions (acrylic acid neutralization degree of 70%, mass ratio (g/g) w(AA):w(LHR):w(AM):w(MBA):w(APS) = 100:10:100:0.4:0.8, as shown in Figure 1b, the swelling properties exhibited a significant correlation with reaction time. When the reaction time was insufficient, polymerization was incomplete, and inadequate crosslinking prevented the formation of a fully developed three-dimensional crosslinked network structure. This resulted in an increased proportion of soluble components, thereby reducing the water absorption capacity. At the optimal reaction time of 3 h, both polymerization and crosslinking reactions proceeded fully and uniformly. However, with further increases in reaction time, the likelihood of side reactions and excessive crosslinking increased. Over-crosslinking significantly enhanced the rigidity of the gel network, which not only restricted the elastic deformation of gel chains but also substantially reduced the available hydration space within the network structure. Consequently, the water absorption capacity of the gel declined.

Under controlled synthesis conditions (acrylic acid neutralization degree of 70%, reaction temperature 70 °C, mass ratio (g/g) w(AA):w(LHR):w(AM):w(MBA) = 100:10:100:0.4), the gel’s water absorption capacity exhibited significant dependence on the amount of APS initiator, showing an initial increase, followed by a decrease (Figure 1c). This optimal behavior stems from competing radical-mediated mechanisms: (1) at low APS concentrations (<0.6%), insufficient radical generation leads to incomplete network formation and increased soluble components; (2) conversely, excessive APS (>0.6%) generates excessive radicals, accelerating termination reactions, reducing gel molecular weight, and causing over-crosslinking, thereby constraining the expansion of the three-dimensional network structure. Equilibrium was attained at an APS dosage of 0.6%, where the radical concentration was optimal for forming an effective porous network structure, thereby maximizing water absorption capacity.

Under controlled reaction conditions (reaction temperature of 70 °C, mass ratio (g/g) w(AA):w(LHR):w(AM):w(MBA):w(APS) = 100:10:100:0.4:0.6), the influence of the neutralization degree on the water absorption capacity of superabsorbent gels was systematically investigated. As illustrated in Figure 1d, water absorption exhibits a pronounced peak at a neutralization degree of 70%, characterized by an initial increase, followed by a gradual decline in water absorption rate with further increases in neutralization degree. This phenomenon arises from two competing mechanisms: at lower neutralization degrees (<70%), the high reactivity of acrylic acid monomers facilitates copolymerization and homopolymerization reactions that are difficult to control. Concurrently, insufficient ionizable groups result in low osmotic pressure, thereby diminishing water absorption capacity. Conversely, at excessively high neutralization levels (>70%), sodium acrylate (CH_2_=CHCOONa) exhibits lower reactivity than acrylic acid (CH_2_=CHCOOH). This slows grafting reaction rates and reduces crosslinking density, ultimately increasing soluble components and diminishing the gel’s water absorption capacity.

2.2. Characterization of Relevant Properties

2.2.1. Water Retention Performance of Superabsorbent Gels

Figure 2 indicates that the water retention capacity of samples after water absorption saturation exhibited temperature-dependent variations. Specifically, when exposed to different thermal conditions, the material’s water retention performance demonstrated significant differences, indicating its thermoresponsive characteristics.

The experimental results showed that within 6 h, the water retention capacity plummeted from 46.5% at 30 °C to merely 14.3% at 60 °C. This thermal response indicated two important characteristics: (1) the gel’s crosslinked network maintained excellent structural stability under low-temperature conditions; (2) even at elevated temperatures, the gel retained measurable water adsorption capacity.

Comparative analysis revealed that the evaporation rate of water at 60 °C was significantly accelerated compared to 30 °C. This was primarily attributed to two mechanisms: (i) enhanced molecular thermal motion promoting the evaporation of free water; (ii) disruption of the hydrogen bond network typically stabilizing the water molecules within the gel matrix.

2.2.2. Swelling Rate of Superabsorbent Gel in Water

Figure 3 demonstrates that during the initial hydration phase, the gel exhibited rapid water absorption capacity and a pronounced swelling rate. Its absorption kinetics display marked time dependency: the swelling rate is initially swift but gradually diminished over time. This superabsorbent gel demonstrated a significant advantage in water absorption ratio compared to commercial superabsorbent gels, though it slightly trails commercial products in maximum water absorption rate.

2.2.3. Repeated Water Absorption Performance of Superabsorbent Gels

Figure 4 demonstrates that, after five consecutive absorption–desorption cycles, the water absorption ratios of both superabsorbent gels decreased compared to their initial performance. Compared to commercial superabsorbent gels, the LHR-AA-AM superabsorbent gel exhibited relatively better water absorption performance after five repeated water absorption cycles. This behavior is attributed to the damage sustained by the gel’s internal crosslinked structure during repeated wetting and drying cycles. This damage weakened the elasticity of the gel chain network, impairing its ability to retain and absorb water as effectively as in its initial state. Concurrently, a small proportion of urea and humic acid within the gel dissolved in water during this process, further reducing the gel’s water absorption capacity.

2.2.4. Water Absorption Properties in Different Salt Solutions

Regarding the selection of metal ions, sodium chloride (NaCl) constitutes the primary component in the salinization of soils within arid mining areas and is frequently employed to simulate saline–alkaline environments. Calcium chloride (CaCl_2_) is the hardness ion commonly present in soils, and ferric chloride (FeCl_3_) was selected to investigate the unique effects of multivalent ions (as iron-rich effluents are common in acidic mine drainage). Consequently, these three ions were selected for testing.

Figure 5 demonstrates that the water absorption capacity of the gel exhibited significant concentration-dependent behavior with different salt solutions. The experimental results indicate that both the equilibrium swelling degree and water absorption capacity decreased markedly with increasing salt concentration. This trend was most pronounced in NaCl solutions, where the water absorption rate plunges sharply from 178 g/g to 38.6 g/g as the NaCl concentration rises from 0.01 mol/L to 0.15 mol/L. The mechanisms underlying swelling inhibition primarily involve (1) a reduction in the osmotic pressure gradient across the gel network; (2) the formation of complexes between multivalent metal ions and carboxylate groups, leading to network contraction and subsequent decrease in liquid retention capacity.

Furthermore, the water absorption capacity of the gel exhibited a negative correlation with the valence state of metal ions. At equivalent concentrations, water absorption followed the order: NaCl > CaCl_2_ > FeCl_3_. This phenomenon arises because multivalent cations (such as Ca^2+^ and Fe^3+^) possess a stronger charge-shielding effect, enabling them to more effectively neutralize ionized groups within the gel and thereby inhibit its swelling capacity.

This study demonstrates that the water absorption behavior of gels in diverse ionic solution environments is governed by multiple interrelated factors, including solution concentration, the gel’s chemical structure, and metal ion characteristics.

2.2.5. Fourier-Transform Infrared Spectroscopy Analysis

The gel structure was characterized using Fourier-transform infrared spectroscopy (FTIR) (Figure 6). The spectrum exhibited absorption peaks corresponding to key functional groups: significant changes were observed in the O–H stretching vibration peaks within the range of 3707–3663 cm^−1^, manifested as peak broadening and a shift toward lower wavenumbers, strongly indicating the formation of new hydrogen bonds or graft copolymerization reactions between functional groups on the LHR (e.g., hydroxyl and carboxyl groups) and the AA–AM gel chains. The characteristic peaks at 2566 and 2552 cm^−1^ correspond to S–H bond stretching vibrations in LHR, while the peak at 2162 cm^−1^ corresponds to C≡N stretching vibrations in LHR. The peaks at 1499 and 1468 cm^−1^ are attributed to aromatic C=C skeletal vibrations; those at 1380 and 1324 cm^−1^ correspond to amide C–N stretching vibrations; the peaks at 1113 and 1149 cm^−1^ are assigned to C–O–C stretching vibrations in AA–AM; the peaks at 872 and 848 cm^−1^ correspond to aromatic C–H out-of-plane bending vibrations; and the peaks at 587 and 575 cm^−1^ are ascribed to vibrations of silicon–aluminum oxides, likely Si–O stretching vibrations. The above results indicate that LHR and AA–AM are tightly integrated through strong, multiple hydrogen-bond interactions and possible secondary aromatic-ring interactions, forming a novel hybrid network structure with strong interactions [31].

2.2.6. Thermogravimetric Analysis

The TG and DTG curves of the gel (Figure 7) reveal its thermal degradation process unfolds in five distinct stages [17].

(i)The weight loss in the initial stage (below 192 °C) was approximately 13.5%. This stage was attributed to the loss of adsorbed moisture from the air and bound water in the superabsorbent resin.(ii)In the subsequent second stage (192–331 °C), the sample weight loss of 6.9% was caused by the decomposition of the small molecules in the hydrogel.(iii)During the third stage (331–414 °C), the sample weight loss of 16.9% was attributed to the breakdown of the three-dimensional network structure due to the decomposition of polymer side chains and main chains.(iv)In the fourth stage (414–498 °C), the sample weight loss of 23.7% resulted from the decomposition of the crosslinked network and aromatic structures, along with the breakdown of aromatic ring structures in any humic acid components present.(v)In the final stage (498–800 °C), the sample weight loss of 9.3% involved the slow oxidation of residual carbon and the decomposition of inorganic constituents. At elevated temperatures, the remaining carbon skeleton underwent further oxidation, ultimately forming stable char residue.

2.2.7. XRD Analysis

In Figure 8, the lignite humic residue (LHR) spectrum exhibits pronounced diffraction peaks at 2θ = 21.82° and 2θ = 26.75°, corresponding to zeolite and silica [33,34]. In the gel spectrum, these peaks transformed into weaker, lower-intensity peaks. This indicates that the uniform distribution of lignite residue within the gel disrupts the crystalline structure, resulting in the formation of an amorphous gel. Based on the above analysis, the XRD test results confirmed the successful preparation of LHR-AA-AM.

2.2.8. Elastic Modulus Analysis

The strain test in Figure 9 demonstrates that, throughout the process, G′ > G″ (240 Pa > 108 Pa), indicating that the material exhibits a predominantly elastic response and displays the characteristics of a solid gel. The obtained gel shows elastic gel properties, featuring a highly stable network structure with exceptional structural integrity.

2.2.9. SEM Morphological Analysis

Through SEM morphological analysis of the LHR-AA-AM gel, Figure 10a–c reveal the fine characteristics of the gel’s microstructure under three different magnifications of 500×, 1000×, and 5000×. The structure exhibits a structure resembling ‘lamellar stacking + interwoven pores’, with pores between the lamellae further refining and crosslinking points becoming more distinct. This refined structure indicates the uniform crosslinking of gel chains during polymerization. This ensures consistent swelling across all regions during water absorption, preventing structural damage from localized over-swelling. It also provides the necessary space for long-term water storage.

2.2.10. Analysis of Root Formation in Pothos Using Superabsorbent Gels

Take two glass bottles of the same size and specifications, and add 500 mL of distilled water to each bottle. Divide the bottles into two groups: one group without any gel, serving as the blank control group, and the other group where 1 g of LHR-AA-AM gel particles is added to each bottle. Evenly disperse and plant pothos efficient extraction in each glass bottle. The experiment lasts 30 days, during which the rooting of the pothos is observed.

This study demonstrated that the application of superabsorbent gels exerted a significant positive effect on the rooting process of pothos cuttings. The experimental results (Figure 11) indicated that application of the LHR-AA-AM gel containing residual humic acid significantly enhanced the rooting process of the pothos cuttings. Compared to the control group, the experimental cuttings exhibited accelerated rooting onset and higher rooting rates. Furthermore, the root systems of the experimental pothos demonstrated superior morphological characteristics including increased root number, greater root length, well-developed fibrous roots, and healthy coloration. The establishment of a robust root system further facilitated above-ground growth, as evidenced by the deeper green foliage and more rapid shoot development, whereas the leaves of the control group gradually turned yellow and withered. This outcome was primarily attributed to the auxin-like activity of humic acid: its ability to improve the rhizosphere microenvironment and its stimulatory influence on plant metabolic process. Consequently, the residual HA in lignite humic residue acts as an effective promoter of root growth in plants.

2.2.11. Soil Improvement Status

Field planting: First, till and level the soil of the planting plot, and determine the application rate of superabsorbent hydrogel particles (water-retaining agent) based on soil conditions. Spread the water-retaining agent at the bottom of the sowing furrow or planting hole, then cover it with a 3–5 cm layer of soil. The recommended application rate for superabsorbent hydrogel particles is 1.0–2.0 kg per mu. After applying the water-retaining agent, proceed with sowing or transplanting the plants, preferably selecting drought-tolerant and salt-tolerant varieties. Upon completion, water regularly and maintain moist soil during the germination period.

By optimizing the dosage of superabsorbent gels and biofertilizers, the planting of Calligonum (1), Suaeda salsa (2), Corethrodendron scoparium (3), and Ammopiptanthus mongolicus (4) in the arid mining area achieved an overall survival rate exceeding 80% (as shown in Supplementary Figures S2 and S3). This demonstrates that the water-retaining agent played a crucial role during the plant rooting and germination stages. Annual irrigation water consumption per mu was reduced to just 70 m^3^, substantially conserving irrigation water and demonstrating the efficacy of the water retention agent under mining area’s arid, wind-blown sand climate. Table 2 presents a systematic comparative analysis of the physicochemical properties and fertility indicators between native soil and cultivated soil. The results reveal that the original habitat soil exhibits extremely low organic matter content, strong alkalinity, and elevated salinity (total salt content of approximately 20 g kg^−1^), characteristic of typical saline–alkali soil. Following the cultivation of salt-tolerant and drought-resistant plants, significant improvements were observed in soil physicochemical parameters such as fertility indicators: organic matter, nitrate nitrogen, ammonium nitrogen, available phosphorus, and available potassium all increased. Concurrently, the soil pH decreased and total salinity was substantially reduced.

The results indicate that compared to the native habitat soil, the cultivated soil exhibited significant improvements in both soil pH and chemical fertility. Specifically, organic matter and essential nutrients including nitrogen, phosphorus, and potassium markedly increased, while soil pH shifted toward neutrality, thereby creating a more favorable soil environment for plant growth. These findings demonstrate that through measures such as the application of superabsorbent gels and the implementation of appropriate irrigation practices, barren habitat soil can be successfully transformed into fertile cultivation soil.

3. Conclusions

This study successfully synthesized a novel, low-cost network superabsorbent gel based on lignite humic residue (LHR), which also functions as a slow-release fertilizer. The synthesis process was optimized using a combination of single-factor and orthogonal experiments: an LHR dosage of 0.5 g, an acrylic acid neutralization rate of 70%, a reaction temperature of 70 °C, and an acrylamide (AM) dosage of 5 g. Under these conditions, the resulting gel exhibited a water absorption rate of 521.2 g·g^−1^ for deionized water and 65.5 g·g^−1^ for physiological saline solution, demonstrating excellent water retention capacity and salt tolerance. Field trials concluded in the arid, sandstorm-prone climate of Santang Lake demonstrated that this superabsorbent gel significantly improved plant survival rates, lowered soil pH, and enhanced soil organic matter content, proving its practical value in the ecological restoration of mining areas. This research not only provides a viable technical solution for ecological remediation in mining areas but also offers novel insights for developing low-cost, eco-friendly superabsorbent gels for application in ecological restoration within harsh environments. Subsequent work will focus on large-scale technological deployment alongside long-term monitoring and assessment of ecological effects on soil.

4. Materials and Methods

4.1. Experimental Materials

Acrylic acid (AA, Shanghai Aladdin Bio-Chemical Technology Co., Ltd., Shanghai, China), acrylamide (AM, Shanghai Aladdin Bio-Chemical Technology Co., Ltd., Shanghai, China), N,N-methylene-bisacrylamide (MBA, Shanghai Aladdin Bio-Chemical Technology Co., Ltd., Shanghai, China), ammonium persulfate (APS, Tianjin Zhiyuan Chemical Reagent Co., Ltd., Tianjin, China), sodium hydroxide (NaOH, Tianjin Xinbote Chemical Co., Ltd., Tianjin, China), sodium chloride (NaCl, Tianjin Zhiyuan Chemical Reagent Co., Ltd., Tianjin, China), commercial water-retaining agents (acrylate polymers), and lignite humic residue (LHR) were procured. (The main components are humus and silicon-aluminum oxides [35].)

4.2. Experimental Method

An appropriate quantity of lignite residue (LHR) was weighed, dispersed in water, and sonicated for 20 min prior to use. Subsequently, 5 g of acrylic acid (AA) was added to a 250 mL three-neck flask. The degree of neutralization of the system was adjusted to 70% by slowly adding a 40% NaOH solution dropwise. Thereafter, acrylamide (AM), the crosslinking agent N,N′-methylene-bisacrylamide (MBA), and the brown coal residue suspension were added successively.

The reaction temperature was then raised to 70 °C, after which ammonium persulfate (APS) was slowly introduced dropwise as the initiator. The polymerization reaction was allowed to proceed for 3 h at constant temperature under continuous magnetic stirring to ensure homogeneous mixing. Throughout the polymerization process, nitrogen gas was continuously purged into the system to provide an inert atmosphere.

After completion of the reaction, the product was washed three times with distilled water to remove unreacted monomers and soluble impurities. The washed product was placed in a glass Petri dish and dried in an oven at 80 °C until it was fully set and no longer tacky to the touch. The dried material was then cut into small pieces and ground using a pulverizer.

A specific quantity of the dried product was weighed for the determination of the water absorption ratio.

Following the above-described procedure, a series of LHR-AA-AM superabsorbent gels was synthesized by varying reaction temperature, reaction time, degree of neutralization, APS dosage, AA:AM ratio, LHR dosage, and MBA dosage.

4.3. Orthogonal Experiment

Factors: The experimental variables ere Factor A, Factor B, and Factor C—namely, raw material ratios, crosslinking agent (MBA) dosage, and brown coal slag LHR.

Levels: Different values were taken by each factor. Each factor had four distinct levels: Level 1, Level 2, Level 3, and Level 4.

Data Analysis Method:

Upon completion of the 16 experiments, 16 results were obtained, and range analysis was subsequently performed as follows:

Calculate the average of indicators for each factor at different levels. For instance, calculate the sum of all experimental results for Factor A at Level 1:

$$[eqn]$$

Similarly, compute K_AWater2_, K_Awater3_, K_Awater4._

Since both absorption capacity in distilled water and saltwater needed to be evaluated simultaneously, the corresponding sums (e.g., K_saltwater1–4_) were also calculated for Levels 1–4.

The same calculations were performed for Factors B and C.

The range (R) for each factor was calculated.

$$[eqn]$$

For example,

$$[eqn]$$

Determine the order of factor significance and identify the optimal level combination.

Order of significance: A factor with a larger range (R) indicates that changes in its levels have a greater impact on the experimental results, making it a primary factor. Factors are ranked in order of importance based on their R values.

Optimal level combination: For each factor, compare the averages across its levels. The level with the most favorable value (e.g., highest if maximizing absorption) is identified as the optimal level for that factor. Combining the optimal levels of all factors yields the theoretically optimal condition.

4.4. Swelling Performance

Approximately 0.1 g of the superabsorbent gel sample was accurately weighed and recorded as mass m. The sample was placed into a tea bag, which was then sealed and immersed in the test liquid (0.9% sodium chloride solution or distilled water). The gel was allowed to absorb the liquid until it reached full swelling equilibrium.

The swollen tea bag was then removed from the liquid and suspended vertically until no free liquid was observed to drip. The total mass of the swollen tea bag was subsequently measured and recorded as m_1_. Separately, the mass of an empty tea bag after immersion under identical conditions was measured and recorded as m_2_ to account for the liquid absorbed by the tea bag itself.

$$[eqn]$$

where m is the mass of the dry gel, m_1_ is the mass of the tea bag, with gel after equilibrium swelling, and m_2_ is the mass of an empty blank experimental tea bag after immersion. R is the water absorption rate.

4.5. Repeated Water Absorption Performance

A specified quantity of the gel was used to evaluate its repeated water absorption capacity. Following the water absorption test described in Section 4.3, gel samples that had completed one absorption cycle were dried in an oven. The dried samples were then subjected to a subsequent water absorption test. This absorption–drying cycle was repeated multiple times to assess the repeated water absorption performance of the gel.

4.6. Water Absorption Rate

An appropriate quantity of the sample was placed in a tea bag and immersed in deionized water. At predetermined time intervals (1, 3, 5, 10, 20, 30, 60, 120, 180, and 300 min), the tea bag was removed, surface moisture was gently blotted with filter paper, and the sample was weighed. The water absorption rate of the gel at each time point was calculated using Formula (1). By calculating the water absorption rates at different time periods, the water absorption kinetics curve of the gel was analyzed.

4.7. Salt Tolerance Performance

Sodium chloride (NaCl), calcium chloride (CaCl_2_), and ferric chloride (FeCl_3_) salt solutions were prepared at concentrations of 0.01, 0.03, 0.05, 0.1, and 0.15 mol/L. The superabsorbent gel was immersed in these different concentrations of the solutions, removed after complete swelling, and weighed, and the water absorption rate was calculated using Formula (1).

4.8. Gel Characterization

Scanning electron microscopy analysis: Gel was swollen to 2–3 times its original volume, rapidly frozen with liquid nitrogen, and freeze-dried for 24 h. Microstructural morphology was subsequently observed and documented using a field-emission scanning electron microscope (Model JSM-7001F, JEOL Ltd., Akishima, Tokyo, Japan).

Fourier-transform infrared spectroscopy: gel powder was mixed with potassium bromide, pressed into pellets, and scanned using a VERTEX 70 RAMI spectrometer (Bruker Technologies GmbH, Saarbrücken, Germany) for spectral recording.

X-ray diffraction analysis: X-ray diffraction patterns were recorded using a D8 advance diffractometer (Bruker Technologies GmbH, Saarbrücken, Germany) over a scanning range of 2θ = 5–80°, at a scanning rate of 5°/min.

Thermogravimetric analysis: Thermogravimetric analysis was conducted under a nitrogen atmosphere using a Hitachi STA7300 analyzer (Hitachi Ltd., Tokyo, Japan). The temperature was raised from room temperature to 800 °C to record the thermal weight loss behavior.

Rheological behavior testing: The rheological behavior of the hydrogel was measured at room temperature using a rheometer (Anton Paar, MCR 302, Graz, Austria). A strain of 500% was applied to the gel, and the test was conducted for 20 min.

4.9. Analysis of Physicochemical Soil Properties and Fertility Assessment

Organic matter was measured with potassium dichromate capacity—external heating method.

Nitrate nitrogen and ammonium nitrogen were extracted with 0.01 M calcium chloride, determined by a continuous flow analyzer (Bran+Luebbe AA3, Hamburg, Germany).

Available phosphorus: sodium bicarbonate extraction—molybdenum antimony anti-colorimetric method was determined by an Agilent CARY60 UV spectrophotometer (Agilent Technologies, Santa Clara, CA, USA).

Available potassium: ammonium acetate extraction—atomic absorption spectrometry was determined by Thermo Fisher S-series atomic absorption spectrophotometer (Thermo Fisher Scientific, Waltham, MA, USA).

pH was determined by Mettler Toledo FiveEasy Plus pH meter (Mettler Toledo, Columbus, OH, USA).

Determination of water-soluble salts: total salts were measured by the dry residue method; K^+^ and Na^+^ were measured by atomic absorption spectrometry; other ions were measured by chemical titration.