Technosol Development Based on Residual Fraction of Coal Tailings Processing, Agro-Industrial Waste, and Paper Industry Waste

Abstract

In order to obtain solutions to the coal mining region demand in southern Brazil for high-performance and low-cost technosols within a concept of mineral circularity and sustainable development of the coal sector, this study aims to evaluate the functional feasibility of the combined use of a residual fraction of coal tailing, waste from the rice and poultry agro-industry, as well as the cellulose industry, as raw materials for technosol development. Characterizations (XRF, LOI, ICP-OES, pH, XRD, and sulfur forms) were performed to adjust the waste proportions used in the constructed soil, as defined based on the clay content of the coal tailing, the organic carbon content of the poultry litter, and technical recommendations for the use of synthetic gypsum in agricultural soils. Based on the characterizations of the residues, a greenhouse experiment was designed, consisting of four technosol formulations (T1–T4). Initially, an ecotoxicity test was conducted with the treatments in contact with Allium cepa L. (onion) to determine the degree of technosol toxicity. Field tests were then carried out, which were replicated three times in a completely randomized block design, with the germination and growth of Lactuca sativa L. (crisp lettuce) as a bioindicator. The fertility of the treatments was analyzed eight weeks after the establishment of the experiment and compared to standard values for agricultural soils. The characterization performed with the individual wastes demonstrated the potential for application in technosols according to current regulations. The ecotoxicity tests showed that the soil was not toxic to the plant in any of the treatments studied. The field experiment demonstrated no difference in germination between the treatments. However, the development of lettuce seedlings occurred only in technosol formulations T2 and T3, highlighting the potential of the studied residues for the construction of technosols.

1. Introduction

Brazilian coal deposits have a total of 32 billion tons and are concentrated in the southern region of the country [1]. In the state of Santa Catarina, they are associated with the lithologies of the Rio Bonito formation. This geological formation, consisting of successive sedimentary cycles of sandstone, siltstone, and shale packages, is divided into the Barro Branco Layer, Bonito Layer, and Irapuá Layer, with mining predominantly underground and concentrated in the first two layers [2]. Due to the reduced thickness of the layers and the significant presence of iron disulfide nodules (FeS₂), commonly called pyrite, the raw coal ore (run-of-mine, ROM) needs to undergo physical beneficiation processes to meet the standards of the power generation industry [2]. Therefore, it is estimated that 50% of all coal extracted (ROM) is discarded as tailing [3]. In the Santa Catarina coal region alone, around 300 million tons of tailing have been accumulated since the beginning of the 20th century, and it is estimated that 300 thousand tons of tailing are generated every month [3,4].

Residual fractions from various beneficiation processes have the potential to recover sulfur (S), offering an alternative to reduce Brazil’s dependency on imports, as noted in the Official Gazette of the Union No. 115 [5]. The country consumes large amounts of sulfur, mainly as sulfuric acid, which is used in the chemical industry, steelmaking, and agriculture, with about 55% of it being utilized for producing agricultural fertilizers [6].

The environmental issues from sulfur-rich coal mining tailing, such as acid mine drainage (AMD) and heavy metal contamination, along with the need to restore degraded mining areas, are driving research into the potential for repurposing ROM coal beneficiation waste in Brazil’s transformation industry [5].

In recent years, circular processes have become strategies to move away from linear production processes, focusing on controlling the treatment flow and final disposal of generated waste [7]. One such strategy is the development of technosols from the combined use of various urban, industrial, and agro-industrial wastes, as well as sources of S from coal mining waste, as fertilizing materials for the recovery of degraded areas [8,9,10]. The application of organic amendments in degraded areas is one of the quickest practices for recovering biological activity and soil fertility [11,12]. However, this practice can modify the composition of the indigenous microbiota, depending on the characteristics of the organic amendment, requiring compositional studies of the residues used [13]. Ensuring sustainability in the use of technosols is essential for achieving long-term ecological balance and soil health restoration [14,15,16].

The combination of these materials with plant-supporting substrates resulting from anthropogenic activity [8,17,18] can allow the combined recovery of the physical–chemical and biological characteristics of the degraded soil [19,20]. The combined use of various agricultural, agro-industrial, and urban wastes can enhance soil by improving particle size, pH, nutrient content, organic matter, and water retention. This process, as outlined in the 2008 Technical Instruction of Wastes [21], is key to developing technosols derived from waste materials. Among the materials used as inputs for improving soil nutritional conditions, poultry litter stands out, a byproduct of chicken production widely used in agriculture as organic fertilizer, with high functional potential for providing organic matter and nutrients to soils [22].

Another material is rice husk, used raw or as ash from burning for the cogeneration of energy, which has great potential for aeration, drainage, and air exchange at the root base and is a lightweight and porous material [23], as well as a source of organic carbon, nitrogen, potassium, phosphorus, calcium, and magnesium [24]. Lime mud, a byproduct of the pulp and paper industry, also fits into this range of materials for its potential as an alkaline conditioning agent based on calcium [25]. These and other wastes are functionally present in the production processes of some industries, and their combined use in technosols represents an important opportunity for the transition to a Circular Economy [9]. A functional technosol requires a substrate mixture containing balanced amounts of carbon, nitrogen, phosphorus, and mineral elements that support plant growth [10]. Its capacity to sustain life depends on the availability of macro- and micronutrients as well as protection against toxic elements [26].

To address the demand of the southern region of Brazil for high-performance, low-cost technosols within a concept of mineral circularity and sustainability, this study aims to evaluate the functional feasibility of the combined use of a residual fraction of coal tailings, wastes from the rice agro-industry and poultry production, and wastes from the pulp and paper industry in the development of this environmentally sustainable product.

2. Materials and Methods

2.1. Raw Materials

The residual fraction of coal tailing (RFC) for the development of technosol application was collected from the beneficiation of ROM-stage coal extracted from an open-pit mining operation located in the Santa Catarina Coal Basin (SCCB) and originating from the Barro Branco coal layer. The Barro Branco coal layer is considered the most important in the SCCB and can be generally divided into three sub-layers with the following lithological variations: Banco, Quadração, and Forro [1].

The mining company, which supplied the residual fraction, implements a beneficiation process consisting of several stages, including crushing and separating coal from other waste materials using an air table, a cyclone, and a fine sedimentation basin, where the material fraction of interest is collected. It is important to note that if this residual material is not valorized after processing, it is disposed of in a controlled landfill. This disposal is carried out in compliance with current legislation [27], which primarily regulates the concentration of toxic contaminants based on leaching tests and pH measurements.

The other materials used were poultry litter (PL), rice husk (RH), and lime mud (LM). These materials were collected from companies located in municipalities near the SCCB (Figure 1).

The poultry litter was obtained in the municipality of Timbé do Sul, SC, from the production of chickens for slaughter, with the birds remaining on the litter for 5–6 cycles (batches) of approximately 28 days each, resulting in an average organic carbon concentration of 28%. It is noteworthy that the average production of poultry litter is 2.2 kg per chicken [28], and considering that poultry farming has grown exponentially in recent years, with Brazil’s chicken production in 2023 being approximately 56 billion heads [29], it is estimated that the generation of this waste exceeds 123 Tg per year. Poultry litter is recommended for application in agricultural soils as a supplier of organic carbon, nitrogen, and macronutrients (P, K, Ca, and Mg) [24].

The rice husk was collected from a rice processing industry in the municipality of Meleiro, SC. It is estimated that about 2.7 million tons of rice husk are produced annually throughout Brazil [30], with the southern region of Santa Catarina producing approximately 1.25 million tons per year, representing 67% of the state’s production and 11% of the national production [31]. The husk comprises 22% of the gross weight of a rice grain [32], resulting in the production of 275 thousand tons per year in the southern region of Santa Catarina. This waste is mostly used as material for burning (cogeneration of energy for drying the cereal) or sent to controlled landfills, adding no value to the waste.

Similarly, lime mud, a byproduct of the pulp and paper industry, was collected in the municipality of Lages, SC, where the largest paper producer and exporter in the country is located [33]. It is estimated that about 0.5 tons of lime mud are generated per ton of pulp produced [34]. According to NBR 10004 [35], lime mud is classified as Class IIA waste (non-inert) due to the presence of sodium, chloride, and sulfate, leached or solubilized outside acceptable concentration limits [36]. Currently, only 8.9% of the total solid waste generated in the paper industry is reused as raw material by other industrial sectors, while another 9.2% is sent to landfills [34]. Among all the waste generated in paper production, lime mud accounts for 5% of production, corresponding to 700 thousand tons per year [37].

2.2. Waste Characterization

The different wastes were chemically and mineralogically characterized. The RFC was chemically characterized by X-ray fluorescence (XRF), loss on ignition (LOI), pH, sulfur forms, and inductively coupled plasma–optical emission spectrometry (ICP-OES) and mineralogically by X-ray diffraction (XRD).

For the chemical characterization of the RFC by XRF, an energy-dispersive X-ray fluorescence spectrometer, model EDX 7000 (Shimadzu, Tokyo, Japan), was used, and the loss on ignition was performed at 950 °C (combined water, organic, and inorganic matter), according to ASTM D7348 [38]. The determination of the chemical composition of the other wastes used (rice husk, poultry litter, and lime mud) is important so that they can be used as materials/fertilizers in soil construction without causing negative environmental impacts. The quantification of the extractable (exchangeable) content of some chemical elements of interest (micro and macronutrients and trace elements) was carried out through a leaching test [39], as described above for the RFC.

To determine the pH, a pH meter model AK95 (AKSO, Taipei City, Taiwan) was used. For the analysis, 4.0 g of the RFC (as generated without drying) was mixed with 100 mL of deionized water. The mixture was stirred on a magnetic stirrer for 15 min, followed by a 5 min rest. The pH reading of the RFC was taken at two moments: immediately after collection/preparation and after 12 months of storage in a closed container.

The sulfur forms in the RFC were obtained by total sulfur analysis, performed via instrumental analysis, and sulfate and pyrite sulfur. This determination was made by elemental analysis by combustion, using equipment model CHN 628 SERIES SULFUR (LECO, Saint Joseph, MI, USA), where 0.20 g of the material was placed in the equipment and burned up to a temperature of 1350 °C. Pyrite sulfur was determined using a 2 g sample of the material, to which hydrochloric acid was added. The material obtained from the acid washes was used to determine this type of sulfur using a potassium dichromate titrant. Sulfate sulfur was obtained from the filtrate of the hydrochloric acid test. Since total sulfur is the sum of organic sulfur, pyrite sulfur, and sulfate sulfur, organic sulfur was obtained by difference.

The extractable content of chemical elements in the RFC extract was analyzed by ICP-OES. A 100 g aliquot was mixed with 500 mL of deionized water and subjected to leaching by constant agitation for 7 days. The 500 mL aliquot of the leached extract was subjected to inductively-coupled plasma–optical emission spectrometry (ICP-OES) testing. The equipment used for the tests was the Agilent 720 ICP-OES (Agilent, Santa Clara, CA, USA).

The mineralogical characterization of the RFC by XRD was conducted using an X-ray diffractometer, model LabX XRD 6100 (Shimadzu, Japan) with Cu radiation, tube voltage of 40 kV, and current of 30 mA, reading between 4° and 70° (2θ) and speed of 0.02°/s.

2.3. Technosols Formulations

After characterizing the waste materials, four different mixtures were defined for the technosol formulation, varying the quantities of the used waste materials. The four formulations were named: Technosol 1 Negative Control (T1, RFC); Technosol 2 (T2, RFC + PL + RH); Technosol 3 (T3, RFC + PL + RH + LM); and Technosol 4 Positive Control (T4, vermiculite + PL).

The compositions and quantities of each component are presented in

Table 1. Technosol formulations (T1–T4), mass (g), and mass percentage (%) of each technosol component.

Technosols Formulations	Composition	Amount (g)	Amount (%)
T1	RFC	554	100.00
T2	RFC	464.17	83.79
	PL	34.43	6.21
	RH	55.40	10.00
T3	RFC	463.40	83.65
	PL	34.43	6.21
	RH	55.40	10.00
	LM	0.77	0.14
T4	Vermiculite	277	50.00
	PL	277	50.00

and were defined according to the maximum and minimum levels of nutrient input [24] and in accordance with the legislation of the state of Santa Catarina [40]. The quantity of each material was calculated to obtain 554 g of technosol (equivalent to the mass of soil in each plant cultivation container). Due to the ultrafine characteristics of the RFC (<13 μm), RH was added at 10% w/w to improve the physical structure and water percolation of the technosol. The organic carbon (C-org) present in the PL was used to calculate the necessary amount to achieve an organic matter content in the soil (OMS) of 3% (OMS = C-org × 1.724), which is the standard in most soils of the SCCB. Rational analysis [41] was applied for the calculation of the clay percentage in the RFC, which served as a parameter for the dosage (D) of LM (LMD) in T3. The LMD was calculated by multiplying the clay percentage of the RFC (56%) by the factor 50 (LMD = 2800 kg ha⁻¹), resulting in 1.55 g kg⁻¹ clay.

2.4. Ecotoxicity Tests with Allium cepa L. as a Bioindicator

The technosol formulations were subjected to toxicity analysis through an ecotoxicity test with the plant Allium cepa L. (onion) to evaluate the effects and toxicity of the technosol components on living organisms. Initially, solubilized extracts were produced for each formulation T1–T4, according to the USEPA–SW 846 standard [42]. The test was conducted with the extracts of the materials in amounts proportional to those used for the composition of the constructed soils, using the solubilized extract of the RFC as a negative control and mineral H₂O as a positive control. All existing roots of the onion bulbs were cut, the dry cataphylls were removed, and the bulbs (n = 6 per formulation) were exposed to 50 mL of the solubilized extract of each formulation in Falcon tubes. The onions were placed in the Falcon tubes in contact with the sample of the solubilized extract at room temperature for 7 days and protected from light. After 7 days, the number of roots that grew, the length of the longest root, and the total root biomass were evaluated for each formulation.

The ecotoxicity test data for A. cepa. are presented as the mean ± SD. The variables were checked for normality distribution using the Shapiro–Wilk test and tested for significant differences between treatments regarding the biomarkers: number of roots, maximum root length (cm), and total root biomass (g) using the Kruskal–Wallis test, followed by Dunnett’s post-hoc test. The software PAST 4.03 was used for all statistical toxicity tests compared to the treatments, and statistical significance was set at a p-value of 5%.

2.5. Germination and Growth Tests of Lactuca sativa L.

After performing the ecotoxicity tests, the compositions of the technosols were evaluated in a greenhouse using Lactuca sativa L. (crisp lettuce) as a bioindicator plant. Lettuce was selected for its ease and rapid cultivation and its broad application in research on developing materials from waste, thus allowing comparison with other bibliographies. For the technosol experiment, 10 lettuce seeds per sample unit (554 g in each pot) were used and replicated three times for each formulation. The experiment lasted 8 weeks, as recommended in the Liming and Fertilization Manual for the States of Rio Grande do Sul and Santa Catarina, as the necessary period for plant germination and development [24]. The performance of the formulation was evaluated by the germination percentage and survival of the lettuces over the eight-week period.

2.6. Chemical Fertility Analyses of Technosols

After eight weeks of cultivating L. sativa, the formulations were subjected to chemical fertility analyses according to the standard soil fertility analysis parameters [43], having their values of extractable aluminum, extractable calcium, SMP index, extractable magnesium, organic matter, pH in water, extractable potassium, extractable sodium, extractable phosphorus, potential acidity, base sum, cation exchange capacity (CEC), base saturation, and clay content quantified.

The performance evaluation of the formulation was conducted by comparing the chemical fertility parameters with reference values established by Sociedade Brasileira de Ciência do Solo (SBCS, Brazilian Society of Soil Science,

Table 2. Soil parameters used for indication of liming and NPK fertilization in agricultural crops, in accordance with SBCS/NRS (2016) [24].

Parameters	Range
	Very Low	Low	Medium	High	Very High
pH (H2O)	<5	5.1–5.4	5.5–6	>6
Pexchangeable (mg dm−3)	0–7	7–14	14–21	21–42	>42
K (mg dm−3)	10–30		40–90	90–180	>180
OM (g kg−1)	≤25		26–50	>50
Ca (mmolc dm−3)		≤20	21–40	>40
Mg (mmolc dm−3)		≤5	6–10	>10
CEC (mmolc dm−3)		≤750	760–1500	1510–3000	>3000
V (%)	<45	45–64	65–80	>80
Parameter	Textural Class
	I	II	III	IV
Clay (g kg−1)	>600	401–600	201–400	≤200

) [24].

3. Results and Discussion

3.1. Waste Characterization

The results of the X-ray fluorescence (XRF) of the RFC of ROM coal are presented in

Table 3. X-ray fluorescence (XRF) and loss on ignition of the RFC with the percentages of the main oxides.

Sample	Oxides (%)											LOI
	SiO2	Al2O3	SO3	Fe2O3	CaO	K2O	TiO2	MgO	MnO	P2O5	ZrO2
RFC	47.87	22.03	1.19	3.20	0.97	2.25	1.37	0.72	0.01	ND	0.06	20.20

LOI: Loss on ignition; ND: not detected.

. The major oxides are silica (SiO₂), alumina (Al₂O₃), sulfur (SO₃), iron (Fe₂O₃), and potassium (K₂O).

The obtained results evidenced a high content of SiO₂ and Al₂O₃, 47.87% and 22.03%, respectively, presenting an Al₂O₃/SiO₂ ratio < 0.7. Based on this ratio, three classes are defined. The first class consists of the highest percentages of SiO₂, where the Al₂O₃/SiO₂ ratio is <0.3, with quartz and feldspar as the main components, hence characterized by low clay content and a granulometric range with larger particle sizes. The second class, with an Al₂O₃/SiO₂ ratio between 0.3 and 0.5, is primarily composed of kaolinite and illite, with a small amount of quartz, feldspar, and calcite. The third class (Al₂O₃/SiO₂ > 0.5) is predominantly composed of fine-diameter kaolinite (tendency to be clayey). According to the classification, the analyzed fraction falls into the third class (Al₂O₃/SiO₂ > 0.5), having high plasticity [44,45].

The fraction presented low contents of SO₃, Fe₂O₃, CaO, K₂O, TiO₂, and MgO. The percentage of loss on ignition evidences the presence of organic material in the RFC, with a volatile content of 20.2%. The pyritic fraction found in SCCB coal originates from the association of marine and estuarine sediments, rich in sulfates (SO₄²⁻), with the organic matter that formed the coal during the deposition process [1]. The SCCB deposits follow a pattern of chemical contents linked to the place of origin, where the sulfur contents mostly vary from 3.4 to 7.7% by mass, most of which occur in the form of pyritic sulfur [1]. The sulfur content of the RFC was considered low; however, it remained within the permitted values in accordance with the reference legislation [24].

The iron content present in the waste should be low, equivalent to 1.5% of the sample, as high iron values, both in fertilizers and soil, become toxic to plants [46]. The RFC presented values slightly above the ideal for application (2.24% Fe) [24] but still considered a low iron content compared to other literature [47], as it is a waste from coal extraction.

Many precipitates have their solubilities altered due to the hydrogen ion potential as a consequence of the alteration in ion concentration through reactions with H₃O⁺ or HO⁻ in the medium [48]. To maintain the low toxicity of micronutrients present in the soil, it is ideal for the pH to remain in a range of 6.0 to 7.0, being stabilized close to neutral. Considering the pH and the chemical composition of the residual fraction, it is observed that there are no elements considered toxic or that hinder the growth or development. The residual fraction had a near-neutral pH from collection and storage, equivalent to 6.8 and 7.5, respectively.

The analysis of sulfur content (

Table 4. Forms and percentage of sulfur present in the sample of the RFC.

Sample	Sulfur Forms and Content (%)
	Pyritic	Sulfate	Organic	Total
RFC	60.0	10.0	30.0	100.0

), quantifying all forms of sulfur present in the sample, that is, pyritic, sulfate, and organic sulfur, was performed according to ASTM D2492 [49].

The results showed a low percentage content of total sulfur in the RFC sample, with a value of 0.476% S (result obtained from SO₃ presented in Table 3), being in the form of pyritic, sulfate, and organic sulfur, with 60%, 10%, and 30%, respectively. The predominant sulfur content in coal residues is pyritic [47], as well as the sulfur present in the residual fraction detected by XRF, as described in Table 3. Pyritic sulfur, in contact with O₂ and H₂O, forms ferrous sulfate (FeSO₄) and sulfuric acid (H₂SO₄), producing acid mine drainage (AMD), which is one of the main environmental problems of coal extraction [6]. Thus, the selected sample showed potential for the suggested application, mainly due to its low pyritic sulfur content.

RFCs are composed of minerals found in sedimentary rocks, typically rich in silicon and aluminum oxides and often hydrated, including quartz, kaolinite, illite, feldspar, potassium, mica, carbonates, and sulfides [45]. The results of the XRD analysis (Figure 2) allowed the detection of the presence of three mineralogical phases: quartz (SiO₂, reference code: 01-089-8935), kaolinite ((Al₄(OH)₈(Si₄O₁₀), reference code: 01-074-1784), illite ((KH₃O)(AlMgFe)₂(SiAl)₄O₁₀[(OH)₂H₂O], reference code: 01-070-3754) and pirite ((FeS2), reference code: 03-065-3321).

In the RFC, pyrite is the phase related to sulfates and iron sulfides. This phase was found in small quantities in the XRD analysis and in the chemical characterization based on the SO₃ content from XRF and forms of sulfur. The high silicon dioxide (SiO₂) content is related to the presence of silica minerals (mainly quartz sand) and the clay mineral kaolinite [50]. Compounds containing Si, including quartz (SiO₂), can be used in agricultural crops as a substitute for limestone [51]; however, its reactivity varies depending on particle size, the dose used, the type and soil, and the contact time between the material and the soil [20]. The contact time between the components of the technosols in this study and the field experimentation was not sufficient to release the reactivity of Si, necessitating the addition of an alkalizing material (lime mud) to control the pH.

In RFC, the pH level is directly related to the pyrite content of the samples. One factor that accelerates pyrite oxidation is the exposure of wastes to the soil surface, as the process becomes aerobic [52], speeding up the AMD formation and consequently acidifying the sample [6]. Therefore, the higher the pyrite content in the RFC, the greater the tendency for residue oxidation over time and, thus, the reduction in pH.

The primary formation factor influencing the concentration of elements in the soil solution is the source material [40]. Table 5 corresponds to the ICP-OES analysis, which quantifies the heavy metal content of the materials used in the technosol formulations: poultry litter, rice husk, lime mud, and the selected residual fraction of coal tailing. All parameters analyzed in the leaching test were below the limits established by NBR 10005/2004 [39].

The RFC presented macroelement concentrations within the range of values presented by the remaining components of the technosol, with the exception of K, with the lowest value (0.5 mg dm⁻³). In the RFC and RH (Table 5), the concentrations of the macroelements Ca, Mg, K, and Na were below global average values, according to Allen (1989) [53], similar to what was observed for K, Mg, and Na in the LM. Conversely, the PL exhibited macroelement concentrations within the expected range, except for Mg and Ca. Therefore, PL was considered the primary source of macro and micronutrients in the technosols formulated, with the exception of Ca, whose main source was LM with 501.1 mg dm⁻³. The PL used in the study showed values of approximately 37.95 mg dm⁻³ and 61.4 mg dm⁻³ of total phosphorus (P) and potassium (K), respectively, which are considered high when compared to other studies [54,55]. The content of these elements in the PL varies according to the batches of animals raised on the same litter, as birds raised for meat production release approximately 70% of the P and 80% of the K ingested in their diet [56]. Thus, an increase in the number of batches cultivated on the same litter promotes a proportional increase in nutrients in the PL [22,55], especially P and K, which are not lost to the environment in gaseous forms [55]. In addition to these two important macronutrients, the PL was the major source of the macronutrients. Magnesium (22.24 mg dm⁻³) and sodium (34.45 mg dm⁻³), as well as the micronutrients copper (1.13 mg dm⁻³), total iron (0.41 mg dm⁻³), manganese (0.375 mg dm⁻³), and zinc (0.175 mg dm⁻³) (Table 5), reinforcing its status in contributing to the performance of the tested technosols. The micronutrient Fe in the technosols also contributed to RFC, with this component of the technosols being the only one to present Al in its composition.

Table 5. Chemical element content (mg dm⁻³) of the technosol components obtained through ICP-OES analysis.

Elements	Waste (mg dm−3)				Reference Values NBR 10004/2004 (mg dm−3)	SWM (mg dm−3)
	RFC	PL	RH	LM
Aluminum (Al)	1.3	<0.1	<0.1	<0.1	NA	100–2000
Barium (Ba)	0.04	0.01	<0.10	<0.10	70	500 *
Boron (B)	<0.1	0.2	<0.1	<0.1	NA	0.2–5
Calcium (Ca)	14.1	16.1	0.75	501.1	NA	100–2000
Copper (Cu)	<0.01	1.13	<0.009	<0.009	NA	0.1–3
Iron (Fe)	0.12	0.41	<0.10	<0.10	NA	50–1000
Magnesium (Mg)	4.455	22.245	0.49	<0.10	NA	40–500
Manganese (Mn)	<0.01	0.375	<0.10	<0.10	NA	5–500
Molybdenum (Mo)	<0.01	0.03	<0.01	<0.01	NA	0.01–0.2
Phosphorus (P)	<0.02	37.955	2.185	<0.05	NA	NA
Potassium (K)	0.5	61.4	10.7	1.1	NA	50–500
Silver (Ag)	<0.01	<0.01	<0.01	0.115	5	NA
Selenium (Se)	<0.01	<0.01	<0.01	0.03	1	10 *
Sodium (Na)	3.35	34.45	2.3	3.8	NA	20–200
Vanadium (V)	<0.01	0.01	<0.01	<0.01	NA	100 *
Zinc (Zn)	<0.01	0.175	<0.10	<0.10	NA	1–40 *

NA—Not available; SWM = soil world means according to Allen [53] and Brooks [57], when indicated by *.

Table 5. Chemical element content (mg dm⁻³) of the technosol components obtained through ICP-OES analysis.

Elements	Waste (mg dm−3)				Reference Values NBR 10004/2004 (mg dm−3)	SWM (mg dm−3)
	RFC	PL	RH	LM
Aluminum (Al)	1.3	<0.1	<0.1	<0.1	NA	100–2000
Barium (Ba)	0.04	0.01	<0.10	<0.10	70	500 *
Boron (B)	<0.1	0.2	<0.1	<0.1	NA	0.2–5
Calcium (Ca)	14.1	16.1	0.75	501.1	NA	100–2000
Copper (Cu)	<0.01	1.13	<0.009	<0.009	NA	0.1–3
Iron (Fe)	0.12	0.41	<0.10	<0.10	NA	50–1000
Magnesium (Mg)	4.455	22.245	0.49	<0.10	NA	40–500
Manganese (Mn)	<0.01	0.375	<0.10	<0.10	NA	5–500
Molybdenum (Mo)	<0.01	0.03	<0.01	<0.01	NA	0.01–0.2
Phosphorus (P)	<0.02	37.955	2.185	<0.05	NA	NA
Potassium (K)	0.5	61.4	10.7	1.1	NA	50–500
Silver (Ag)	<0.01	<0.01	<0.01	0.115	5	NA
Selenium (Se)	<0.01	<0.01	<0.01	0.03	1	10 *
Sodium (Na)	3.35	34.45	2.3	3.8	NA	20–200
Vanadium (V)	<0.01	0.01	<0.01	<0.01	NA	100 *
Zinc (Zn)	<0.01	0.175	<0.10	<0.10	NA	1–40 *

NA—Not available; SWM = soil world means according to Allen [53] and Brooks [57], when indicated by *.

RH, whether carbonized or not, is a non-reactive substrate, as confirmed by the low levels of the elements (Table 5). Being a non-reactive product and mainly having a physical function in soil structuring, its nutritional levels can be considered null or low [10].

Gaskin et al. [25] indicated that LM has the potential to be used as an agricultural liming material due to its capacity to neutralize soil acidity due to the presence of calcium and magnesium in its composition. As shown in Table 5, LM presented a high calcium content, 501.1 mg dm⁻³, standing out as the main source of this element for the technosols. Studies by He et al. [58] and Gaskin et al. [25] demonstrated that LM has concentrations of heavy metals comparable to or lower than agricultural lime [59] and was thus considered a potential residue for lime substitution. Table 5 shows low contents for sodium, with 3.8 mg dm⁻³, and also for the heavy metals silver and selenium, with 0.115 mg dm⁻³ and 0.03 mg dm⁻³, respectively. Heavy metals, in the concentrations presented, are below the reference values set by NBR 10004/2004 [35] and are not considered soil contaminants [40].

The bioavailability of Cr, Cu, Ni, Pb, and Zn in all soil components was also analyzed according to the state legislation of Santa Catarina [40], given the environmental concerns related to the use of multiple waste sources in the composition of technosols. All of these elements in all components of the technosols (Table 5) are below the recommended level for agricultural application [24].

In the soil, chemical elements (micro and macronutrients) often accumulate in the upper layer (0–20 cm), making them more accessible to plant roots and microorganisms and, therefore, available for absorption [60]. The availability or contamination by trace elements is related to the accumulation and transport processes of these elements with the clay fraction, which is responsible for the interactions of solid and liquid interfaces [61]. The main toxic element in tropical soils is aluminum (Al), which, under acidic soil conditions, is free in the solution and can thus be absorbed by plants, causing damage to the root system [62]. Acid mine drainage (AMD) treatment plants in Brazil mainly use sodium hydroxide or lime to neutralize the wastewater from coal extraction and beneficiation [63]. The metallic precipitates are removed from the water in settling ponds [64], where the RFC used in this study was collected. The reagents used for AMD treatment are related to the alteration in Al and Fe values in RFC, though not significantly.

The poultry litter (K, P, Na, Mg, Cu, Fe, Mn, and Zn) and lime sludge were the main sources of macro and micronutrients for the tested technosol formulations, according to Kabata-Pendias and Mukherjee [65] in their classification of the function of elements in soil fertility.

3.2. Ecotoxicity of Technosols

The observed values for the biomarkers: number of roots, length of the longest root (largest root), and total root biomass of A. cepa in the solubilized extract of each tecnosol formulation are presented in Figure 3a–c, respectively.

The number of roots that grew and the length of the longest roots of A. cepa exposed to the leachates from both formulations T2 and T3 showed no significant differences between the formulations and also did not show differences when compared to the negative control (T1, RFC) and the positive controls T4 (vermiculite + PL) and mineral water. In relation to root biomass (Figure 3c), there was a significant difference between treatments T4 and T1 (p < 0.009), and between T4 and T2 (p < 0.007), with T1 and T2 demonstrating mean root biomass values higher than T4 (0.33 ± 0.08 g), evidencing that there was some nutritional variation between treatments, related to root mass.

Regarding the analyzed variables, the results obtained in the onions exposed to the leachates were either similar (T1 = 64.92 ± 8.17, T2 = 68.73 ± 2.14, and T3 = 60.3 ± 8.15 cm) to those exposed to the control (H₂O = 70.48 ± 11.4 cm); (T1 = 31.17 ± 6.37, T2 = 35.17 ± 8.26, T3 = 31.5 ± 9.29 roots) to those exposed to the control (H₂O = 28.67 ± 6.38 roots); or better than (T1 = 0.54 ± 0.14, T2 = 0.55 ± 0.16 g) to those exposed to the control (H₂O = 0.45 ± 0.12 g). These findings signal the absence of toxic effects in the tested formulations and indicate clearly the potential use of the RFC used in this experiment as a raw material for the construction of technosols, mainly for the restoration of areas degraded by coal mining. The valorization of RFC as raw material would contribute to reducing the volume of coal mining wastes to be sent to controlled deposits and provide an alternative valorization option for the byproducts PL and RH, thus promoting the transition to a Circular Economy in mining, a fact already demonstrated in other studies [10,45,66] carried out with residual fractions of ROM coal beneficiation, as developed in SCCB.

3.3. Germination and Growth of Lactuca sativa L.

The interaction between the components of the technosol can be observed through the growth of plants. Figure 4 illustrates the compositions and the initial growth of the plants used as bioindicators.

The results showed no difference between treatments concerning germination; however, the growth of the bioindicator plant differed between all formulations. Formulation T3 showed the longest survival time and the largest size of the lettuce seedlings, continuing to develop for six weeks. This treatment also showed the most appropriate development of the foliar system, thus having the highest potential result. Formulation T2 also presented an initial foliar system, although it was less developed visually compared to formulation T3.

Formulation T1, or negative control, showed initial lettuce development, but its growth ceased in the early weeks, and the soil became excessively compacted. The RFC used had a high percentage of clay minerals (kaolinite and illite), which restricted the transport and development of the root system, as observed in the literature [67]. Additionally, the structure of the RFC as soil proved inadequate for the development of lettuce plants, most likely due to the low amount of space between the particles, driven by the material’s density.

3.4. Fertility of the Technosols

The results of the fertility tests of the technosols are presented in

Table 6. Fertility analysis of technosol formulations (T1–T4).

Variables	Technosols Formulations
	T1	T2	T3	T4
Extractable Aluminum (mmolc dm−3)	<0.01	<0.01	<0.01	<0.01
Extractable Calcium (mmolc dm−3)	3.61	2.83	3.00	1.30
Index in SMP	8.07	7.82	7.82	5.70
Extractable Magnesium (mmolc dm−3)	7.83	8.99	8.81	5.47
OM (g kg−1)	18.20	24.80	21.10	26.00
pH (H2O)	8.10	7.70	7.80	7.14
Extractable Potassium (mg dm−3)	178.50	1814.40	1845.20	911.10
Extractable Sodium (mg dm−3)	60.10	444.50	459.40	22.40
Extractable Phosphorus (mg dm−3)	<0.10	243.50	272.50	272.27
Potential Acidity (mmolc dm−3)	0.40	0.54	0.54	5.79
Sum of Bases (mmolc dm−3)	12.16	18.39	18.54	9.20
CEC (mmolc dm−3)	12.60	18.90	19.10	15.40
V (%)	96.77	97.15	97.17	59.93
Clay content (g kg−1)	8.00	20.00	24.00	6.00

, and the reference values for soils in the states of Santa Catarina and Rio Grande do Sul are in Table 2.

The T2 and T3 formulations (Table 6) showed low clay content (<200 g kg⁻¹), low CEC values (<750 mmol_c dm⁻³), very high exchangeable P (>42 mg dm⁻³), high exchangeable K (>90–180 mg dm⁻³), high pH (>6), very low OM content (≤25 g kg⁻¹), low Ca levels (≤20 mmol_c dm⁻³) and medium Mg levels (6–10 mmol_c dm⁻³) when compared with the soil parameters (Table 2).

The activity and mobility of chemical elements (micro and macronutrients) in the solution depend on various properties, such as soil pH, redox potential, cation exchange capacity (CEC), competition for adsorption sites, anionic binding, ionic strength, soil solution composition, and the physicochemical characteristics of the material from which the soil originated [68]. Mobility, in turn, is influenced by specific surface area, soil texture and density, amount of organic matter, mineralogical composition, and the amount and type of metals present [69].

Base saturation is the proportion of the cation exchange capacity occupied by bases. Soils with a base saturation of less than 50% have charges occupied by components of acidity (H or Al) and need correction [47], while soils with more than 70% indicate no need for liming. All four formulations showed base saturation above 50%, indicating they are not considered acidic soils. This data can be reinforced by the analysis of extractable aluminum, showing insignificant values and potential acidity.

The soil pH value is fundamental for controlling chemical, physical, and biological aspects, and depending on the soil pH level, it can lead to a balance of macro and micronutrients [65]. Indeed, both macro and micronutrients are highly available to plants according to SBCS [24], showing that the proposed mixtures in all formulations are positive for plant growth in terms of pH. In all formulations conducted, especially in formulations T2 and T3, macro and micronutrients are available in sufficient quantities for plants, indicating that the proposed mixtures are satisfactory for plant growth in terms of pH and chemical composition.

The arrangement of particles defines soil porosity, which in turn affects soil aeration and the movement of soil solution within it. A well-structured soil has a high level of organization and a lower tendency to compact, although achieving this requires time. In contrast, soils with massive structures have few or rare macropores, making water and air movement and root growth practically nonexistent [70]. The technosol produced in formulation T1 has a poorly developed structure, resulting from compacted soils with little macropore organization. Therefore, the absence of lettuce growth can be related to this characteristic.

Technosol formulation (T3) shows low clay concentration and low cation exchange capacity (CEC). Low CEC values are often associated with low clay content and/or low soil organic matter (SOM). It should be noted that the rational analysis performed to determine the clay content in RFC was based on the results of XRD and particle size analysis and considers three groups of materials present in the clay fraction: silicate clays (kaolinite, smectite, illite, and vermiculite); iron and aluminum oxides (goethite, hematite, and gibbsite); and amorphous materials (humic substances and organic materials) [71,72]. Conversely, in soil fertility analysis, three types of minerals are considered: total clay (finest soil fraction < 2 µm, regardless of specific mineral type); silicate clays (montmorillonite, illite, and kaolinite) with different CEC and water retention characteristics; and non-silicate minerals (iron oxides and aluminum hydroxides), which can influence soil fertility and structure differently compared to aluminum silicates [41]. Differences in parameters evaluated between rational and soil fertility analysis explain, for example, the differences obtained in RFC clay content of 56% obtained by rational analysis and 8% in fertility analysis (Table 6).

It is also worth noting that the RFC, the largest mass and volume component of the soil, was artificially ground instead of naturally decomposed and degraded to reach a smaller particle size, which explains the lack of correlation between particle size and cation exchange capacity (CEC). Combined with artificial grinding, kaolinite group clay minerals exhibit low cation exchange capacity (CEC) in the order of 1 to 10 mmol_c dm⁻³ [73], which can be improved by adding organic matter since organic matter associated with mineral sources improves soil CEC [46]. In the CEC data in Table 6, it is possible to analyze the increase in this variable in T2 and T3 compared to T1, possibly related to the addition of organic matter to the soil.

The chemical element contents in RFC are below the reference values of the technical standard NBR 10004 [35], highlighting the possibility of using it as a component of technosol. Similar results were obtained by Amaral Filho et al. (2020) [74], Weiler et al. (2020) [75], and Zocche et al. (2023) [10] in experiments with technosols based on coal mining waste.

Comparing the data obtained in formulation T3, as a reference for higher application potential, with the studies mentioned above, this technosol showed the highest macronutrient (K, P, and Na) levels. These levels may be related to the addition of poultry litter to the technosol, while the other studies considered here opted for different organic matter sources, such as malt waste [74] and urban waste [75]. According to SBCS [24], the proportion of total N in mineral and exchangeable form is, on average, 25% in chicken litter, 15% in laying hen litter, 30% in sewage sludge, 25% in liquid cattle manure, 60% in liquid pig manure, and 5% in composted pig manure and litter, which can vary according to the fertilizer batch. These values compared to poultry litter indicate that the K, P, and Na values observed in T3 originated from this input.

As observed in the results obtained in the present study, except for the values of P, K, and Mg, which can be considered adequate, as well as the available Al content and potential acidity, which were low, the other soil fertility parameters are far below those needed for maintaining plant productivity. However, considering that no toxicity was evidenced in T3 and T2 concerning L. sativa germination and survival, it can be inferred that both technosols suggest the potential for the restoration of degraded areas, obviously requiring continuous management practices of constructed soils to achieve desired plant productivity levels.

After the proper characterization and confirmation of the aggregate potential of residues and byproducts for the construction of technosol, the technical and economic management of these materials is initiated [75]. Robinson et al. [76] estimated the cost of reconstructing 0.30 m of topsoil per hectare at GBP 61,053 (USD 83,036, based on an exchange rate of 1.36 for converting GBP to USD in 2022). On the other hand, the costs for reconstructing a 0.30 m layer of topsoil per hectare in coal-mined degraded areas in the SCCB range from up to USD 31,000 to USD 88,582 [10]. These costs include project design, handling of spoil, transportation of mineral components (clay materials and topsoil), earthworks, fertilizers (NPK), acquisition and transport of organic additives, seeds and seedlings, and labor [77].

Besides the costs, some parameters of the newly constructed soils must be adjusted, such as the CEC values, which were lower than those of typical agricultural soils of the region. This condition should be improved with long-term use of technosols, and new formulations and proportions of materials and adjustments should be tested in order to correct this soil condition.

4. Conclusions

Based on the chemical and physical characterizations, the residual fraction of ROM coal was confirmed as having potential for application in the technosol formulations, according to the Santa Catarina state legislation. However, for the development of this technosol to become a reality, it is important that the separation of residual fractions from coal beneficiation occurs in distinct stages and that the sample of the residual fraction with the highest potential is obtained selectively. The disposal of these materials in landfills as a single residue by mining companies makes waste valorization unfeasible.

In the ecotoxicity assay conducted with A. cepa, it was possible to determine the level of toxicity of the technosols produced on a laboratory scale. Root growth ranged from 60.3 to 68.73 mm, indicating that there were no significant differences between the treatments. Treatment 2 exhibited the best average results considering the three biomarkers assessed.

Under the field experimental conditions, the growth of lettuce in the technosols did not show significant differences either; however, Treatment 3 demonstrated the longest survival time and the largest size of lettuce seedlings, remaining in development for six weeks.

Based on the conducted tests, the residues or byproduct components of technosols presented amounts of macro and micronutrients necessary to meet the nutritional needs of the tested plants, also indicating that no toxicity was observed, although the soil structures were not adequate, as they showed a high degree of compaction, indicating the need to improve the proportion of structuring material. In this case, the percentage of rice husk, or even the addition of other residues, such as rice husk ash, to improve structure and drainage capacity promoted better soil development time. When dealing with technosols for the restoration of degraded areas, the management of the newly constructed soil should include liming procedures and the addition of micro and macronutrients as needed, not only to allow germination but also to support plant development and productivity.

Considering sulfur as a strategic chemical compound for the country, the potential use of residual fractions of ROM coal becomes even more relevant to reduce dependence on imports and contribute to reducing the impacts of new mining activities and environmental liabilities resulting from the coal industry. These indications of potential are crucial for the transition to a Circular Economy. Certainly, other possibilities are hidden within the different residual fractions and should, therefore, be revealed through valorization studies to be used as secondary mineral sources.