Characteristics of Chinese Weathered Coal from Six Geographical Locations and Effects on Urease Activity Inhibition

Abstract

Weathered coal is known to have potential inhibitory effects on urease activity, thus reducing the loss of nitrogen from fertilizer such as ammonia. This means that it can be used as a urea enhancer to promote urea utilization efficiency. However, the variation in its composition and structure has impeded the optimal utilization of this resource. In this study, we collected Chinese weathered coal from six representative geographical locations and analyzed its elemental and substance composition, as well as determined its chemical structure via Fourier transform infrared spectroscopy and investigated its effects on urease (soybean meal) activity. The results showed evident variation in the composition and structure among the different weathered coal samples, especially in the pH values, humic acid and ash content, and aromaticity. All six weathered coal samples significantly inhibited urease activity, and the inhibitory effect was enhanced with the elevated proportion of weathered coal introduced to urea. When the additive proportion of weathered coal increased, the weathered coal, characterized as having a higher humic acid content and a more aliphatic structure, showed a more rapid increase in the urease activity inhibition rate, while there was only a slight effect when the weathered coal had a low humic acid content and high atomicity. Therefore, the former type of weathered coal was more sensitive to the additive proportion. Furthermore, there was no consistent rule when the same proportion of weathered coal from different geographic locations was blended into urea, which might be attributable to other unexplored factors.

1. Introduction

Weathered coal is a metamorphic coal and is characterized by a low calorific value, poor stability, and a high ash yield and moisture content, making it a poor choice for the various established industries that use coal as a raw material [1]. In addition, low demand in the industry leads to its accumulation, resulting in a waste of storage space and representing a likely source of environmental pollution [2]. Consequently, exploring ways to utilize weathered coal is an urgent economic and social challenge. Due to the huge amount of opencast coal mines, the weathered coal reserves in China are estimated to be more than 10 billion tons [3], higher than the amount in other countries. In China, weathered coal has an uneven distribution, and the reserves are mainly distributed in Inner Mongolia as well as in Shanxi and Xinjiang in the north regions and in Jiangxi and Yunnan in the south regions. Therefore, the samples collected from these geographical regions in China represent the most common characteristics observed in weathered coal.

In general, weathered coal is characterized as having a higher humic acid content than other materials, and this offers a higher cation exchange capacity, a more porous structure, and more active functional groups [1]. These features confer the capacity of weathered coal to exchange, chelate, or absorb nutrients in the soil and in applied fertilizers and to affect the activity of soil enzymes to enhance their availability for plant growth [1]. However, weathered coal from different geographical locations shows large variations in the above characteristics due to differences in the composition of their parent coals and the subsequent environmental and climatic transformations in different geographical locations [1,4]. Humic acid content has been used as an important and basic index to evaluate different weathered coals. For example, the humic acid content of weathered coal from Lingshi exceeded that from Huozhou by 57.1% [5]. Besides humic acid content, differences also exist in the elemental composition and pH value of weathered coal from different mines [6]. These differences determine the effectiveness of weathered coal as an agent to enhance maize germination and nutrient availability [5,6]. However, previous studies rarely paid attention to the structure of weathered coal even though it is closely related to the function of weathered coal.

Urea is the dominant nitrogen fertilizer in agriculture around the world. However, after being applied to soil, 10–19% of the nitrogen will be lost through ammonia volatilization, resulting in low fertilizer utilization efficiency and in economic losses and serious environmental pollution [7,8,9]. The inhibition of urease enzyme activity is a common method that is widely adopted to suppress ammonia volatilization and to further increase the urea utilization rate [9]. Weathered coal has been used to suppress NH₃ volatilization from urea-fertilized soil by inhibiting urease enzyme activity [10,11]. Because of its abundance of weathered coal, China has a long-established history of investigating the effects of weathered coal on urea utilization. For example, Sun et al. [12] reported that the combination of weathered coal and urea slowed down the release of nitrogen and that the amount of N loss could be lowered by 45.4% when compared to the application of urea alone. Gao et al. [13] blended weathered coal and urea with a ratio of 3:10 and found that during the 35-day incubation period, the inhibitory rate of urease activity by weathered coal increased with time. On the 35th day, urease activity with the addition of weathered coal demonstrated the biggest difference when single urea was applied as the treatment, and the inhibitory rate of urease activity reached its highest value: 27.3%. Sun et al. [5] added the same amount of weathered coal and urea into soil and investigated the dynamics of urease activity during a 10-day incubation period. They observed a lower inhibitory rate of 5.7–14%, with the highest inhibitory rate being observed on the 6th day of the incubation period. The inhibitory effect of weathered coal on urease activity has also been shown in maize cultivation experiments by Lu et al. [14]. These authors showed that the maximum inhibitory rate occurred at the jointing stage of maize growth and decreased after that. However, other studies reported the opposite effect: Kołodziej et al. [15] and Sugier et al. [16] reported that the addition of weathered coal to reclaimed soil actually increased urease activity. Thus, most previous research has concentrated on the effects of the geographical origin of weathered coal on urease activity. There have been few comparative studies identifying this effect by weathered coals from different geographical locations, although other studies have shown different structural and compositional features associated with coal performance [1,4]. Lu et al. [14] found that the weathered coal from Xinjiang produced a higher inhibitory rate than coal from Shanxi, even though the weathered coal from Xinjiang had a lower dissociative humic acid content than the coal from Shanxi. Our lack of knowledge on the effects of the chemical properties of weathered coal on urease activity impedes the optimal utilization of weathered coal. There is also a lack of investigation analyzing the relationship between the chemical properties of weathered coal and urease activity, which also impedes the optimal utilization of weathered coal.

In order to better utilize weathered coal resources, experiments investigating the effects of weathered coal with a different chemical structure on urease activity and clarifying the relationship between the generated effects and the characteristics of weathered coal need to be conducted. Due to its wide existence and easy extractability, bean urease was selected to simulate the soil urease response to variable urea solutions and to evaluate the effectiveness of urease inhibitors. In this paper, we collected weathered coal from six different and representative geographical locations in China to investigate (a) their compositional and structural characteristics and (b) their effects on bean urease activity to understand the relationship between the characteristics of weathered coal and the inhibition of urease activity. The knowledge derived from this study should provide a reference for the efficient utilization of nitrogen fertilizer through the use of weathered coal resources.

2. Materials and Methods

2.1. Collection and Processing of Weathered Coal

Six weathered coal samples were collected from depths of 0–10 cm from the working faces of different typical geographical coal mines in China, and their geographical locations and the corresponding identification codes used in this paper are shown in Figure S1 and

Table 1. Proximate and ultimate analysis results of weathered coal samples from different geographical locations.

Sample	Geographical Location	Proximate Analysis 1 (%)		Ultimate Analysis 1
		Moisture	Ash	Content (%)				Atomic Ratio
				C	H	N	O 2	H/C	O/C	N/C
TL	Tongliao, Inner Mongolia	13.0	34.2	52.5	2.60	1.06	43.8	0.59	0.63	0.02
WH	Wuhai, Inner Mongolia	15.8	18.6	66.4	2.94	1.15	29.5	0.53	0.33	0.01
LF	Linfen, Shanxi	23.0	31.3	52.2	2.22	1.09	44.5	0.51	0.64	0.02
CJ	Changji, Xinjiang	28.3	12.4	57.7	2.16	1.00	39.2	0.45	0.51	0.01
PX	Pingxiang, Jiangxi	18.4	40.6	45.7	3.57	0.57	50.1	0.94	0.82	0.01
ZT	Zhaotong, Yunnan	10.0	19.1	54.5	4.61	1.06	39.9	1.02	0.55	0.02
Variation coefficient 3 (%)		33.9	38.3	11.1	52.4	16.1	16.4	55.0	25.2	33.3

Values in the table are means of triplicate determinations. ¹ Calculated on an air-drying base and dry ash-free base. ² Calculated by subtracting the sum of percentage of C, H, and N. ³ Indicates the differences among weathered coal samples from different geographical locations.

. Seven to ten sub-samples were collected from each location to obtain a mixture sample, and the conspicuous waste rock and other sediment were discarded. All of the samples were air-dried to constant weight, pulverized, and crushed to pass successively through a 100 mesh sieve (passing particles <149 μm) and a 200 mesh sieve (passing particles <74 μm). Subsequently, the standard proximate analysis for coal particles (at particle size level <149 μm), including moisture and ash analysis, was carried out. Specifically, the moisture content of the weathered coal was determined using the air-drying method by ovening roughly 1 g of weathered coal samples at 105 °C to a constant weight according to International Standard ISO 589:2008 (Hard coal—Determination of total moisture), while the ash analysis was conducted using the slow ashing method, in which the sample was placed into a muffle furnace for ignition to a constant weight, as described in International Standard ISO 1171:2010 (Solid mineral fuels—Determination of ash). The elemental composition of the samples (particle size of <74 μm) was determined using an elemental analyzer (Elementar Analysensysteme GmbH, Vario MAX CN Carlo Erba NA1500) with acetanilide as the standard regent. The ultimate analysis was calculated using the elemental composition data after subtracting the ash and moisture. The oxygen content was calculated by subtracting the sum of percentage of C, H, and N. To ensure accuracy, the above analysis was carried out in triplicate, and the mean values are presented in the tables. In addition, the variation coefficient for each parameter between samples from different geographical locations was calculated as follows:

$$\mathrm{Variation} \mathrm{coefficient}=\frac{\sqrt{\frac{{{\displaystyle \sum }}_{i=1}^n{\left(x_i-\overline{x}\right)}_i^2}{n}} }{\overline{x}}$$

(1)

where x_i is the measured parameter from each weathered coal sample; $\overline{x}$ is the mean value of all weathered coal samples; n equals 6.

2.2. Conventional Chemical Analysis of Weathered Coal

The humic acid content of the weathered coal was performed according to International Standard ISO 5073:2013 (Brown coals and lignites—Determination of humic acid) by extracting humic acid with alkaline sodium pyrophosphate solution and then evaluating the content using the potassium dichromate volumetric method. The pH value and exchange capacity of weathered coal samples (at particle size level <149 μm) were determined by potentiometry according to the methods proposed by the Institute of Coal Chemistry, Chinese Academy of Science [17].

The E₄/E₆ ratios, total acidity, and the composition of the acidic functional groups were determined with weathered coal samples at a particle size level <149 μm, as per Zhang et al. [18].

To ensure accuracy, the above analysis was carried out in triplicate, and the mean value is presented in the tables. The variation coefficient for each parameter between the samples from different geographical locations was calculated according to Formula (1).

2.3. FTIR Spectroscopy of Weathered Coal

2.3.1. FTIR Measurements

FTIR spectra were surveyed to analyze the type and relative content of the major functional groups in weathered coal semiquantitatively [19]. FTIR measurements were carried out with an FTIR 650 spectrometer (Tianjin Gangdong Sci. & Tech. Development Co., Ltd., Tianjin, China) using the traditional KBr pellet method. Each spectrum was obtained via a collection of 32 scans in the scan range of 4000–400 cm⁻¹ and at a resolution of 4 cm⁻¹.

2.3.2. FTIR Spectral Processing

Following the literature [20,21,22], the obtained spectra were divided into four absorbance bands for the semi-quantitative analysis of functional groups in coal, namely hydroxyl structures (3600–3000 cm⁻¹), aliphatic structures (3000–2800 cm⁻¹), oxygen-containing structures (1800–1000 cm⁻¹), and aromatic structures (900–700 cm⁻¹). Within each of the absorbance bands, the raw spectral data were handled as: the baseline correction by connecting the left and right points of the interval with a straight line; the curve-fitting analysis with the Gauss combination function, which is known to be an optimal approach for the deconvolution of FTIR spectra [23]; the integral of the peak areas using a least-squares interactive procedure in Origin 9.0 software. Thus, the independent characteristic peaks, including the peak height, band shape, and width were separated from the overlapping curve. Details about the fitting methods and goodness of fit criteria can be found elsewhere [20,23]. Detailed FTIR band assignments for various functional groups have been summarized in Table S1 according to Painter et al. [24], Wang and Griffiths [25], Li and Zhu [26], Wang et al. [21], Li et al. [27], among others.

2.3.3. Calculation of FTIR Parameters

The FTIR parameters in Table S2 were calculated using the data derived from the integrated areas for each peak derived from the semi-quantitative FTIR analysis to evaluate the chemical characteristics of the weathered coal samples [21,23,26,28,29,30,31,32,33]. In addition, the apparent aromaticity (f_a) of the coal samples was used to characterize the aromatic carbon fraction. According to the report by He et al. [33], given that only aromatic (C_ar) and aliphatic (C_al) carbons are included in all the types of carbon atoms, f_a can be calculated using the following equations:

$$\frac{{\mathrm{H}}_{\mathrm{al}}}{\mathrm{H}}=\frac{{\mathrm{H}}_{\mathrm{al}}}{{\mathrm{H}}_{\mathrm{al}}+{\mathrm{H}}_{\mathrm{ar}}}=\frac{{\mathrm{A}}_{3000-2800}}{{\mathrm{A}}_{3000-2800}+{\mathrm{A}}_{900-700}}$$

(2)

$$\frac{{\mathrm{C}}_{\mathrm{al}}}{\mathrm{C}}=\left(\frac{{\mathrm{H}}_{\mathrm{al}}}{\mathrm{H}}\times \frac{\mathrm{H}}{\mathrm{C}}\right)/\frac{{\mathrm{H}}_{\mathrm{al}}}{{\mathrm{C}}_{\mathrm{al}}}$$

(3)

$$f_{\mathrm{a}}=1-\frac{{\mathrm{C}}_{\mathrm{al}}}{\mathrm{C}}$$

(4)

where H_al/H is the ratio between the concentrations of the aliphatic (H_al) and the total hydrogen atoms (H), as determined from the integrated absorbance band areas at 3000–2800 cm⁻¹ (A_3000–2800) for H_al and 900–700 cm⁻¹ (A_900–700) for aromatic hydrogen (H_ar), respectively; C_al/C represents the aliphatic carbon fraction; H/C is the ratio of hydrogen atoms to carbon atoms calculated according to the ultimate analysis; H_al/Cal is approximately 1.8 for coals [23,34] and is the proportion of hydrogen and carbon in the aliphatic groups.

2.4. Preparation of Materials for Assay of Urease Activity

2.4.1. Preparation of Urea Containing Weathered Coal

To ensure a nitrogen content higher than 45% (minimum outlined in the Chinese standard for commercial urea), we set the proportions of weathered coal blended into urea as 1%, 2%, and 3%. Briefly, urea containing weathered coal samples were prepared by physically blending 0.2000, 0.4000, and 0.6000 g of weathered coal (particle size of <149 μm) into 19.8000, 19.6000, and 19.4000 g of urea (analytical reagent, purchased from Xilong Scientific Co., Ltd., Shantou, China), respectively. Then, the mixture was ground through a 100 mesh sieve (particle size of <149 μm) to obtain a urea containing weathered coal product containing 1%, 2%, and 3% of weathered coal by weight, respectively. Urea without weathered coal was processed in the same way to be used as a control sample (UN) for comparative purposes.

2.4.2. Preparation of Soybean Meal Urease

Soybean (a commercially available product from Langfang Sancan Grain Processing Co., Ltd., Langfang, China) was oven-dried at 80 °C for eight hours to obtain a constant weight and was ground through a 200 mesh sieve (particle size of <74 μm) to obtain urease. It was stored at −20 °C until use.

2.4.3. Assay of Urease Activity

The urease activity was measured by referring to GB/T 8622-2006 (Determination of urease activity in soya bean products for feeds), and triplicate measurements were performed for each sample. A 3.0000 g amount of the urea or the urea containing weathered coal was dissolved in 100 mL of phosphate buffer (pH = 7.0 ± 0.1, mixture of 50 mL 0.05 M Na₂HPO₄ and 50 mL 0.05 M KH₂PO₄) to obtain urea or urea containing weathered coal solutions. A 0.0200 g amount of urease was added to 10 mL urea or urea containing weathered coal solution and cultivated for 30 min ± 10 s at 30 ± 0.5 °C. Then, 10 mL of 0.1 M HCl solution was added to terminate cultivation. The solution was shaken up and cooled down to below 20 °C. Finally, titration was conducted with an automatic potential titration (ZDJ-5, LeiCi, Shanghai) by recording the volume of 0.1 M NaOH solution necessary to reduce the pH value of the solution to 4.7. Meanwhile, cross-referencing was conducted by adding 10 mL 0.1 HCl before cultivation instead of when cultivation was complete. One unit (U) of urease activity is defined as the weight of the released amino nitrogen per minute during the urea hydrolysis of 1 g of urease at 30 ± 0.5 °C and pH 7.0. The inhibition rate of urease activity was calculated using the following formula:

$$\mathrm{Inhibition} \mathrm{rate} (\%)=\frac{A_{UC}-A_{UW}}{A_{UC}}\times 100$$

(5)

where A_UC and A_UW are the urease activity of the controlled urea and urea containing weathered coal, respectively.

3. Results and Discussion

3.1. Proximate and Ultimate Analysis

The results of the proximate and ultimate analyses for the weathered coal samples are presented in Table 1. It is clear that the samples showed a higher variation coefficient in the moisture, ash, and hydrogen contents than the other investigated parameters. Among all of the samples, CJ had the highest moisture content and the lowest ash content. At 40.6%, the ash content of PX was the highest in all of the samples, reflecting that more ash was involved in coal during the weathered coal formation process in Pingxiang, Jiangxi. The results of the ultimate analysis showed that carbon and oxygen were the main compositional elements in all of the weathered coals, and their contents accounted for more than 95% of the total weight. Apart from the index of the proximate analysis, the elemental variation could also be used to evaluate the compositional differences among the samples. Compared to other samples, PX had a lower carbon and nitrogen content and a higher hydrogen and oxygen content. However, WH exhibited a reverse trend. Atomic ratios can be used to provide information concerning the structural characteristics of humic material [35]. In this study, ZT occupied a higher H/C atomic ratio than other samples, indicating a lower aromaticity and more aliphatic structure for ZT [36]. The O/C atomic ratio of PX exceeded that of WH by 1.48 times, a signal that there are more oxygen-containing functional groups in PX [37].

3.2. Conventional Chemical Analysis

The results of the conventional chemical analysis showed that the determined index was widely spaced across the habitats (

Table 2. Conventional chemical analysis of weathered coal samples from different geographic locations.

Sample	pH Value	E4/E6	ΔlogK	Humic Acid (%)	Exchange Capacity (mmol/g)	Acidic Functional Groups (mmol/g)
						Total Acidic Groups	Carboxylic Groups	Phenolic Hydroxyl Groups 1
TL	4.92	3.48	0.53	39.6	2.46	4.48	1.13	3.35
WH	5.16	3.48	0.57	46.6	2.79	3.90	1.01	2.89
LF	5.16	3.68	0.57	33.8	2.48	4.10	0.48	3.62
CJ	5.11	3.60	0.56	45.3	2.87	4.63	1.18	3.45
PX	4.73	3.47	0.57	27.6	2.47	3.37	0.70	2.67
ZT	4.24	4.64	0.68	36.8	2.90	4.39	0.63	3.76
Variation coefficient 2 (%)	6.70	11.2	8.09	17.1	7.31	10.2	31.0	11.8

Values in the table are means of triplicate determinations. ¹ Calculated according to the differences in the total acidic groups and the carboxylic groups. ² Indicates the differences among weathered coal samples from different geographical locations.

). The humic acid content and carboxylic groups had a higher variation coefficient than other indexes listed in Table 2. CJ had a similar value for humic acid content as WH, and their values significantly exceeded those of the others. This is consistent with the trend observed in the carbon content in Table 1. The variation coefficient of the carboxylic group content among different samples was 31.0%, more than triple that in the total acidic groups and phenolic hydroxyl groups. The carboxylic group content in TL, WH, and CJ was higher than one mmol/g, but that in LF, PX, and ZT was less than one mmol/g. E₄/E₆ (the absorbance ratio of weathered coal solution at 465 and 665 nm) and the ΔlogK value (the logarithm of the absorbance ratio of weathered coal solution at 400 and 600 nm) were used to assess the aromaticity of the materials containing organic carbon [38,39]. Among all of the selected samples, ZT exhibited the highest E₄/E₆ and ΔlogK values, indicating lower aromaticity and a more aliphatic structure [38,39]. This is in agreement with the results of the higher H/C atomic ratio of ZT in Table 1. Exchange capacity is an indicator that predicts a material’s nutrient-absorbing capacity. In this study, ZT exhibited a higher exchange capacity, a signal that ZT could absorb more nutrients than the other samples. The variation in the pH values among the observed samples was smaller than other indicators. ZT showed the minimum value, which was attributed to its high acidic functional groups and exchange capacity. However, PX had the lowest humic acid content as well as the lowest exchange capacity and total acidic groups. This can be attributed to its high ash content.

3.3. FTIR Analysis of Weathered Coal

3.3.1. Global FTIR Characteristics of Weathered Coal

The FTIR spectra were collected to assess the structural characteristics of all of the samples (Figure 1), and Table S1 shows their band assignments. Despite different geographical locations, all of the samples showed a similar characteristic absorbance band. This is an indication that samples have similar surface structures and chemical functional groups [40]. A small but distinct peak at around 2925 cm⁻¹ attributed to the aliphatic structure was seen clearly in the ZT spectra but not at all in the spectra of the other samples. This implies that sample ZT has a higher aliphatic structure [30]. The vibration intensity at 1800–1500 cm⁻¹ in PX was not as strong as that in other samples, indicating a lower content of oxygen-containing functional groups [30]. This is consistent with the results in Table 2, which show that sample PX had less total acidic groups. The spectra of sample CJ showed the lowest vibration intensity at around 1100–1000 cm⁻¹, while PX had the highest value, which implies that sample CJ has the lowest ash content but that PX has the most [24]. This also corresponds with the results of the proximate analysis in Table 1.

3.3.2. Curve-Fitting of the FTIR Spectra

The complexity of the structure and substance usually leads to overlapping in the absorption bands, preventing the accurate analysis of the functional groups in coal [23], limiting the structural information obtained from the raw spectra. In order to determine finer structural details, the raw spectra were divided into four characteristic absorbance bands, and each band was subjected to curve-fitting around the spectral peaks [20,21,33]. It was found that the curve-fitting process for all of the samples in the present study had R² > 0.99 and the regular residual within ±5%, as presented in Figures S2–S7. There were 16–19 peaks for oxygen-containing structures, while other structures only contained 4–6 peaks. The peak assignment is listed in Table S1. Considering the fact that a small fraction of water was probably still present or was absorbed from the moist air into coal pellets after drying, the band related to the hydroxyl structures was not analyzed in detail to avoid spurious structural assignment to the original coal structure [23]. The vibrational peaks (3000–2800 cm⁻¹), belonging to the aliphatic structures in the samples that could not be directly distinguished from the raw spectra in Figure 1, are clearly able to be observed after curve-fitting, as shown in Figures S2–S7. In addition, the stronger absorption at the asymmetric stretching vibrations of CH₂ (2922 cm⁻¹) than at the symmetric stretching vibrations of CH₂ (2854 cm⁻¹) in all samples confirmed the presence of long aliphatic CH₂ chains [41].

3.3.3. Structural Parameters from the FTIR Spectra

The above analysis in the FTIR spectra only provided a qualitative description of the structural information. Calculating the quantitative structural parameters by integrating the peak area will result in a more intuitive message [21,23,30]. Although many indexes derived from FTIR spectra have been used to evaluate the chemical characteristics of coal [42,43], the weak absorbance peak area is usually limited, and its signal is usually affected by the environment. Therefore, in this study, only relatively intense and stable absorbance peaks or area ratios were calculated, and the curve-fitting parameters, including the area ratios of various bands, are listed in Table S2, and the results are presented in

Table 3. Structural parameters determined from FTIR spectra from different geographical locations. Variation coefficient indicates the differences among weathered coal samples from different geographical locations.

Sample	fa1	AR1 2	AR2 2	Ahy/Aet2	ACH2/ACH32	AC=O/AC=C2	A Factor 2	C Factor 2
TL	0.97	13.0	8.75	4.75	5.29	0.71	0.58	0.42
WH	0.90	1.93	1.85	2.16	9.36	0.58	0.63	0.37
LF	0.98	12.8	16.3	11.5	8.05	0.40	0.72	0.28
CJ	0.97	10.7	6.21	1.66	4.43	0.33	0.75	0.25
PX	0.95	3.80	8.74	3.11	3.17	0.47	0.68	0.32
ZT	0.49	0.67	0.12	2.17	3.12	0.66	0.60	0.40
Variation coefficient 3 (%)	21.6	78.9	82.6	88.2	46.5	28.3	10.6	20.6

Values in the table are means of triplicate determinations. ¹ Apparent aromaticity of coal samples. ² Indicated and calculated according to Table S2. ³ Indicates the differences among weathered coal samples from different geographical locations.

.

The structural parameters in Table 3 show that these selected indexes had different variation coefficients with a wide range of 10.6–88.2%. AR1 and AR2 exhibited greater variation coefficients, indicating that aromaticity varied among different weathered coal samples. Sample LF had an AR1 value of 12.8 and an AR2 value of 16.3, both of which are greater than those observed in other samples, signaling a relatively higher abundance of aromatic carbon in sample LF [31]. This reconfirms the results that sample LF also has relatively lower H/C (Table 1). Similar to above, sample TL also had aromatic rings with more condensation. Therefore, samples TL and LF had an abundant aromatic structure. Moreover, the lower values of 0.67 for AR1 and 0.12 for AR2 were investigated in sample ZT, which were attributed to low aromaticity with a higher abundance of aliphatic groups [31]. In addition, sample ZT showed apparent aromaticity (f_a), achieving a value of 0.49, lower than the rest of the samples by around 50%. This is in accordance with the phenomenon in which only sample ZT showed obvious vibration in the aliphatic region of the global FTIR spectra. Taking the high H/C and E₄/E₆ value in sample ZT into consideration, sample ZT had a more aliphatic structure and lower aromaticity. Moreover, with the exception of sample ZT, the apparent aromaticity of other samples showed a narrow variation within the range of 0.90–0.98. The calculation of f_a was based on the hypothesis that all carbon presents with an aromatic or aliphatic structure in coal [27,33]. However, weathered coal is characterized with a higher degree of oxidation, and abundant carbon can be attributed to oxygen-containing groups [1]. Therefore, it is not feasible that f_a represents the aromaticity of weathered coal.

Another structural parameter with a high coefficient of variation was A_hy/A_et, whose maximum was 11.5 in sample LF, exceeding the values of the rest of the samples by more than two times. This characteristic indicates that there was more moisture retained in sample LF [32], consistent with the results for moisture content in Table 1. However, the higher moisture content of sample CJ could not be predicted in this parameter, which can be ascribed to its high content of humic acid but little ash. The greater values of A_CH2/A_CH3 in samples WH and LF suggest that they had longer alkyl chains bound to aromatic rings than the weathered coal collected from the other coal mines [31]. However, the ratio of A_CH2/A_CH3 in sample ZT was 3.12, a third of that in sample WH, implying a relatively compact structure with less space between aromatic clusters and indicating that the aliphatic side chains become more branched [31,44].

The last three parameters, A_C=O/A_C=C, A factor and C factor, were calculated with the same base and have been widely used for describing the maturation level of coal [45]. In addition, the value of A factor ranged from 0.58 to 0.75, and CJ, LF, and PX showed greater values, emphasizing strong hydrocarbon-generating potentials of source rocks, which is also in good agreement with its high hydrogen content and high volatile matter [21,31]. Samples TL and CJ showed a C factor with a greater value, reflecting their higher coal maturation level and slighter coalification [33]. These results are consistent with sample ZT having lower aromaticity and the condensation of the aromatic domains.

3.4. Cluster Analysis of Weathered Coal

To classify the weathered coal samples from different geographical locations, clustering analysis for individuals was conducted using the single linkage method (in Figure 2). The results showed that ZT was apparently different from the other investigated samples. This is attributed to its higher aliphatic structure investigated above. Subsequently, the high ash content led to sample PX separating from the other weathered coal. The other four samples, which were collected from the northern region of China, showed a closer distance than those from ZT and PX and were classified into two sub-clusters. The joined sub-cluster of TL and LF was the reflection of a similar structure in ash and humic acid content, and they also showed relatively higher aromaticity in the value of f_a, AR1, and AR2. Samples WH and CJ exhibited higher pH values as well as higher carbon and humic acid contents, resulting in the formation of their joined sub-cluster.

3.5. Inhibition of Weathered Coal on Urease Activity

All of the investigated values of the inhibition rate of urease activity after the addition of weathered coal in Figure 3 were above zero, verifying that the addition of weathered coal inhibits urease activity [10,13,15,16]. This indicates that in this study, weathered coal had the potential to be used as an urease inhibitor to delay urea hydrolysis and reduce ammonia volatilization. Moreover, significant differences were also determined for the urease inhibition rate among different additive proportions of weathered coal (Table S3), and almost a positive correlation was found (Figure 3). A similar finding was also reported by Saha et al. [46], who found that the more the brown coal blended into urea, the lower the urease activity in soil, resulting in slower urea-N hydrolysis and NH₄⁺-N release. Hence, for this study, when the additive proportion of weathered coal was 3%, urea containing weathered coal showed high potential in reducing ammonia volatilization loss. However, the above result was inconsistent with the results obtained when weathered coal was used as a soil conditioner [47,48]. Li et al. [48] suggested that urease activity is slightly inhibited when there is a limited amount of weathered coal input and is significantly elevated when the application rate of Chinese weathered coal increased. This might be attributed to the different methods of application and cultivation: in Li’s study, there was only a single application of weathered coal into reclaimed soil, while a mixture of urea and weathered coal was applied in the solution used for cultivation in the present study. During solution cultivation, the easily lowered pH value of the solution and increased humic acid content originating from the addition of weathered coal was potentially attributed to the inhibition of urease activity [49,50]. However, the inherent mechanism of lowering urease activity is still not clear and should be explored further. Moreover, the weathered coal from the different geographical locations exhibited dissimilar increased rates during urease activity inhibition along with the elevated additive proportion (Figure 3). Sample ZT had a more rapid increase rate than the weathered coal collected from other geographical locations, indicating that the effectiveness on urease inhibition observed in sample ZT was more concentration dependent than that in other samples. However, there was a small increase in sample PX in terms of urease activity inhibition among different additive proportions of weathered coal, suggesting that the weathered coal that was incorporated into urea had a stable effect on urease hydrolysis when the incorporated proportion varied. The rest of the samples showed a similar slope, with values between those obtained for ZT and PX. This trend is consistent with the cluster analysis results in Figure 2, indicating that the parameters of weathered coal influenced their increased rate of urease inhibition and thus affected urea hydrolysis and ammonia volatilization. The FTIR structural parameters of sample ZT showed lower AR1 and AR2 values than other samples (Table 3), indicating a higher aliphatic structure or a smaller molecular weight. Furthermore, weathered coal with more aliphatic structure usually had a higher exchange capacity for urease binding [51] and a lower pH value, resulting in deep urease activity inhibition [52]. These contributed to sample ZT having the highest increase rate of urease activity inhibition in response to the elevated additive proportion. When urease activity inhibition is too strong, it will lead to an insufficient nitrogen supply during the earlier stages of crop growth, and thus, caution regarding the additive proportion of weathered coal should be taken when blending the ZT sample into urea to achieve optimal nitrogen release. The smaller increase in the inhibition effect observed in the PX sample might be the result of its high ash but low humic acid content. In general, the ash in weathered coal consisted of silicic oxide and metallic oxide [1], and neither of them affected urease activity. Additionally, humic acid could be bound with urease to inhibit urease activity, and its inhibitory effect could be enhanced due to the elevated additive proportion of humic acid [53,54,55]. Therefore, the low humic acid content contributed to the PX sample having a smaller increase in urease activity inhibition. This provided a reference for the utilization of weathered coal resources as a urease inhibitor and indicates that more precise additive proportions should be used for weathered coal with higher humic content and aliphatic structure, while that with discrepant characteristics could be applied with rough additive proportions.

When the same aliquot of weathered coal from different geographical locations was introduced, the inhibition rate of urease activity varied. However, WH and TL always showed less urease activity inhibition than other samples, while the stronger inhibition was observed in the urea containing LF weathered coal. This attributed the inhibition rate of urease activity by TL and WH to a significantly lower value than that achieved by other samples, while that by LF was significantly higher than the others (Table S3). As shown in Figure 2, TL and WH had no similar characteristics, with the exception of both being collected from coal mines in Inner Mongolia; thus, the compositional and structural characteristics were not enough to explain the differences in the inhibition rate of weathered coal. In this study, weathered coal was used without any purification or treatment, resulting in various impurities and microorganisms still being present in the samples, and this is partially responsible for the similar urease activity inhibition by the weathered coal samples from TL and WH. Conversely, the irregular changes in the pH of the solution, even though it only represents a limited effect, might also contribute to this complex phenomenon. However, the above information was not certain until a further exploration was conducted to verify the inherent mechanism.

Above all, the weathered coal samples that were selected for use in this study showed significant urease activity inhibition when it was introduced into urea, and the inhibition rate increased with an additive proportion of weathered coal. The higher increase in the inhibition rate occurred for the ZT sample, which was characterized as having a higher humic acid content and lower aromaticity, indicating that the concentration played an important role on its effectiveness of urease activity inhibition. For each additive proportion, the weathered coal collected from Linfen, Shanxi, always showed a stronger urease inhibition, while TL and WH had a weaker urease inhibition, indicating that LF seemed to be more suitable for use as a urease inhibitor and that it is not wise to directly apply TL and WH without modification. However, the results from the solution cultivation experiment were limited, and the precise effects and optimal conditions of weathered coal should be verified using soil cultivation and agricultural production.

4. Conclusions

Weathered coal collected from different geographic locations had disparate material and elemental compositions and exhibited various structural features, especially in humic acid and ash content, in their structural composition and aromaticity. All six weathered coal samples showed that the capacity to inhibit urease activity increased when the proportion of weathered coal introduced to urea increased. Sample ZT had a higher humic acid content and more aliphatic structure. Sample ZT also showed a more rapid increase in urease activity inhibition as the additive proportion increased, and thus, a more accurate addition in proportion should be considered for weathered coal with the above characteristics. The weathered coal collected from Linfen, Shanxi, always showed stronger urease inhibition, indicating that LF seemed to be more suitable when used as the urease inhibitor. However, due to the complexity of weathered coal, there is no consistent rule between urease activity inhibition and the characteristics of weathered coal when the same proportion of weathered coal from different geographic locations is blended into urea.