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Article

Valorization of Waste Mineral Wool and Low-Rank Peat in the Fertilizer Industry in the Context of a Resource-Efficient Circular Economy

by
Marta Huculak-Mączka
*,
Dominik Nieweś
,
Kinga Marecka
and
Magdalena Braun-Giwerska
Department of Engineering and Technology of Chemical Processes, Faculty of Chemistry, Wroclaw University of Science and Technology, 50-373 Wroclaw, Poland
*
Author to whom correspondence should be addressed.
Sustainability 2025, 17(15), 7083; https://doi.org/10.3390/su17157083
Submission received: 17 June 2025 / Revised: 25 July 2025 / Accepted: 31 July 2025 / Published: 5 August 2025
(This article belongs to the Topic Circular Economy in Interdisciplinary Perspective: Valorization of Raw Materials, Sustainable Products, and Pro-Ecological Industrial Developments)
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Abstract

This study aims to evaluate eco-innovative solutions in the fertilizer industry that allow for waste valorization in the context of a resource-efficient circular economy. A comprehensive reuse strategy was developed for low-rank peat and post-cultivation horticultural mineral wool, involving the extraction of valuable humic substances from peat and residual nutrients from used mineral wool, followed by the use of both post-extraction residues to produce organic–mineral substrates. The resulting products/semifinished products were characterized in terms of their composition and properties, which met the requirements necessary to obtain the admission of this type of product to the market in accordance with the Regulation of the Minister for Agriculture and Rural Development of 18 June 2008 on the implementation of certain provisions of the Act on fertilizers and fertilization (Journal of Laws No 119, item 765). Elemental analysis, FTIR spectroscopy, and solid-state CP-MAS 13C NMR spectroscopy suggest that post-extraction peat has a relatively condensed structure with a high C content (47.4%) and a reduced O/C atomic ratio and is rich in alkyl-like matter (63.2%) but devoid of some functional groups in favor of extracted fulvic acids. Therefore, it remains a valuable organic biowaste, which, in combination with post-extraction waste mineral wool in a ratio of 60:40 and possibly the addition of mineral nutrients, allows us to obtain a completely new substrate with a bulk density of 264 g/m3, a salinity of 7.8 g/dm3 and a pH of 5.3, with an appropriate content of heavy metals and with no impurities, meeting the requirements of this type of product. A liquid fertilizer based on an extract containing previously recovered nutrients also meets the criteria in terms of quality and content of impurities and can potentially be used as a fertilizing product suitable for agricultural crops. This study demonstrates a feasible pathway for transforming specific waste streams into valuable agricultural inputs, contributing to environmental protection and sustainable production. The production of a new liquid fertilizer using nutrients recovered from post-cultivation mineral wool and the preparation of an organic–mineral substrate using post-extraction solid residue is a rational strategy for recycling hard-to-biodegrade end-of-life products.

1. Introduction

The growing global population, climate variability, and ongoing soil degradation are major challenges driving the need for innovative and sustainable agricultural solutions to achieve food security [1,2,3]. In response to these challenges, the European Commission has promoted the concept of a circular economy (CE), emphasizing the recovery and reuse of materials across sectors [4,5]. This includes utilizing byproducts from one industry as raw materials in another, particularly in agriculture, where resource efficiency is crucial.
Agricultural systems generate large volumes of waste, and intensification trends due to urbanization and globalization have led to reduced arable land [6]. To address this, sustainable fertilization practices aim to reduce the consumption of mineral fertilizers, improve nutrient efficiency, and incorporate organic and secondary raw materials into fertilizer formulations [7,8,9]. These priorities align with Regulation (EU) 2019/1009, which encourages the increased use of organic and waste-based components in fertilizing products [10,11,12,13]. These changes take into account the need to increase the content of organic matter in soil and act in accordance with the principles of sustainable agriculture and biocircular management.
These issues are closely related to the use of natural and waste organic materials, particularly carbon-bearing ones, in the technologies for producing useful humic products [14,15]. Given the rising demand for humic substances across industries and the growing market value of these products [14,15,16,17,18], their importance in the global economy is increasing. Among the available carbon-based raw materials, low-rank resources such as peat are especially promising due to their abundance and alignment with EU policies promoting efficient, non-energy use of nonrenewable resources [19,20,21,22]. Humic and fulvic acids (fractions of humic substances) can be obtained by conventional extraction or extraction assisted by physical processes. Both fractions can be used directly or after prior treatment in agriculture, chemistry, medicine, etc. Regardless of the method of obtaining humic substances, the process results in a specific type of post-extraction waste, which is still a valuable organic raw material and can be used in other processes. This aligns with the goals of a circular economy by promoting waste valorization and reducing environmental impact.
Changes in cultivation techniques are another solution to the challenges of shrinking arable land, urbanization, water scarcity, and climate change. Soilless culture systems in an almost completely controlled environment are a relatively modern cultivation technology that works in closed-loop systems with a recirculating water/nutrient solution [23,24].
Rock wool is widely used in hydroponics due to its stable pH, excellent air–water retention, high porosity, and sterile, homogeneous structure [25,26]. In North America and Europe, most greenhouse-grown vegetables such as tomatoes, cucumbers, and peppers are cultivated on mineral substrates [25,27,28]. However, these soilless systems generate large amounts of difficult-to-recycle mineral wool waste (up to 52 m3 per hectare), which contains residual nutrients, pesticides, and pathogens [29,30].
After one growing season, the substrate still holds valuable components, making landfilling an unsustainable solution unless it is properly treated. Reports in the literature indicate that spent mineral wool from cultivation can be reused as a component of horticultural substrates or applied as a soil improver when combined with organic materials, such as, for example, biochar [31]. Nonetheless, landfilling remains a common practice in many countries, and illegal dumping, especially in Central and Eastern Europe, causes environmental issues such as water eutrophication and airborne dust pollution. Rising costs of fertilizer raw materials and the need for responsible resource use underscore the potential for nutrient recovery from used mineral wool in fertilizer production.
As indicated above, peat and horticultural mineral wool are two widely used materials in agriculture and horticulture. However, both pose environmental challenges at the end of their lifecycle. Post-cultivation mineral wool is difficult to biodegrade and is often improperly disposed of. Similarly, peat, as a nonrenewable resource, should be managed efficiently—including when it becomes a residue after the extraction of valuable humic substances—and reused as a secondary raw material in line with the principles of minimizing material losses and circular economy practices.
The use of biowaste and organic materials in soil amendments and fertilizers is well known, as is the extraction of humic substances from carbon-rich materials. However, despite the rapid development of humic substance extraction technologies, little attention is paid to the post-extraction residue, which remains a material still rich in organic matter.
After one growing season, mineral wool substrate still retains valuable nutrients. Recovering nutrients from used mineral wool offers an innovative, eco-friendly way to produce high-quality liquid fertilizers. Unlike other waste-based fertilizers, mineral wool has not been used for this purpose before. This method expands product options and reduces raw material costs compared to conventional fertilizer production. However, this process also results in unmanaged waste in the form of post-extraction mineral wool.
The valorization of these post-extraction materials (peat and mineral wool) is thus both an environmental necessity and an opportunity for circular resource use. The novelty of this study lies in this integrated reuse strategy, transforming both solid and liquid outputs into marketable fertilizer products.
To fill this research gap, the present study proposes a comprehensive valorization pathway for two post-extraction waste streams, based on the following objectives:
-
Extraction of humic substances from peat and demonstrating that post-extraction peat still holds value as a secondary raw material;
-
Recovery and reuse of nutrients from hard-to-recycle post-cultivation mineral wool in the form of a liquid fertilizer product or intermediate;
-
Reuse of post-extraction peat and post-extraction, post-cultivation mineral wool in the production of a new organic–mineral horticultural substrate;
-
Initial attestation tests of liquid fertilizers and substrates necessary to place these products on the market.

2. Materials and Methods

2.1. Materials

Peat and mineral wool were selected due to their widespread use in agriculture and their potential for sustainable valorization after use. The reuse of both post-extraction residues—peat remaining after humic substance extraction and mineral wool following nutrient recovery—offers valuable opportunities for developing fertilization strategies aligned with circular economy principles.
To obtain humic substances, peat from Żuławy Wiślane, Poland, was used. Before the extraction, the peat was dried at 62 ± 2 °C until a constant mass was obtained, ensuring the removal of excess moisture without decomposing organic compounds, and then passed through 2.0 mm mesh sieves.
Waste mineral wool from hydroponic cucumber and tomato crops, collected as mat residues from farms in Kalisz County, Wielkopolska (Kalisz, Poland), was used. Before nutrient extraction, the samples were dried at 30 °C for 24 h to enable effective grinding and sieving without fully removing moisture, which remained beneficial for the subsequent extraction process. The partially dried samples were then ground and sieved through a 2.0 mm mesh.

2.2. Extraction of Humic and Fulvic Acids from Peat

The extraction process of humic substances from peat was based on a procedure recommended by the International Humic Substances Society (IHSS), which was in accordance with the International Standard ISO 19822:2018 [32]. This document establishes a method for the determination of humic acids (HAs) and acidic hydrophobic fulvic acids (FAs), which is applicable to dry and liquid materials used as ingredients in commercial fertilizers, soil amendments, and geological deposits. As a result, we obtained high-purity HAs and FAs with optimal extraction efficiency. During the process, a post-extraction residue is formed as a byproduct after the direct extraction of humic substances from peat. According to the circular economy, post-extraction peat as waste from one production process will be used as an organic secondary raw material in the production of organic–mineral horticultural substrates.
Extraction of humic substances was started by removing polyvalent cations such as Ca2+ from peat using hydrochloric acid. The raw material was acidified to pH 1–2 using 1 M HCl, and then the ratio of 1 g of dry sample/10 mL of 0.1 M HCl solution was obtained. The sample was shaken on a laboratory shaker for 1 h and then separated by decantation. The residue was neutralized with 1 M NaOH, and 0.1 M NaOH was added to acquire a ratio of sample to extract of 1:10. Extraction was performed for 4 h at room temperature under mechanical mixing, and the suspension formed during extraction was allowed to settle overnight. The supernatant was separated by centrifugation and acidified to pH 1 with 6 M HCl. The suspension was left to settle for 16 h. After this time, the gel of humic acids was separated from the supernatant containing fulvic acids by filtration.
Fulvic acids were separated from the fraction containing dissolved fulvic acids and other organic compounds using XAD-8 resin. The adsorbed hydrophobic fulvic acids were removed from the resin using 0.1 M NaOH. Afterwards, the solution was concentrated in a rotary evaporator and dried in a laboratory dryer at 62 ± 2 °C until a constant mass was obtained. The samples prepared in this way were subjected to further tests.

2.3. Characterization of Humic Acids, Fulvic Acids, and Peat Before and After Extraction

Peat is a well-known component of substrates with very good physicochemical properties. The post-extraction peat material as an organic component should have similar features. To characterize the peat waste, which is also a secondary raw material in the presented solution, the input and post-extraction peat, humic acids, and fulvic acids were subjected to standard analyses used in the analysis of humic substances. The possibility of their use is related to their structure and specific properties. To characterize the humic substance fraction and the peat before and after extraction, elemental analysis, FTIR spectroscopy, and solid-state CP-MAS 13C NMR spectroscopy were used.
The elemental composition (C, H, N) of the samples was determined using a Vario EL CUBE elemental analyzer (Elementar, Hanau, Germany) under standard operating conditions. Calibration was conducted with sulfanilamide as a reference material, and sample sizes were approximately 7 mg. Each measurement was performed in triplicate. Oxygen was calculated by difference on an ash-free basis.
Fourier transform infrared (FTIR) spectroscopy was conducted using a VERTEX70 spectrometer (Bruker, Bremen, Germany) with a spectral range from 400 to 4000 cm−1 and a resolution of 2 cm−1 under a nitrogen atmosphere. Samples were prepared as KBr pellets. Operating temperature was ambient.
Solid-state 13C Cross Polarization Magic Angle Spinning Nuclear Magnetic Resonance (CP-MAS 13C NMR) spectroscopy was carried out using a Bruker Avance III 300 MHz spectrometer (Bruker, Germany) with a 4 mm MAS probe. Spectra were acquired at a spinning rate of 10 kHz in the range of −300 to 300 ppm.

2.4. Procedure of Nutrient Extraction from Waste Mineral Wool to Produce Liquid Fertilizer

For the recovery of fertilizer substances from waste mineral wool, a two-stage liquid-solid extraction system was used. In the proposed concept, recovered nutrients were extracted using 0.1 M EDTA. The first stage of extraction was carried out for 30 min by heating the slurry to 50 °C. The second extraction step was also carried out for 30 min without a heating process. Extraction was performed with a mass ratio of solid to liquid phase of φ = 1:10.
The concept was based on the use of the obtained extract containing residual mineral wool nutrients and standard fertilizer components to produce liquid fertilizer with microelements. It was assumed that 75% of the input would be the extract obtained from waste wool with the use of 0.1 M EDTA solution.
The first ingredient that has a significant impact on the composition of the product was potassium dihydrogenphosphate, which is the result of neutralization of phosphoric acid with potassium hydroxide. The neutralization is carried out in the presence of water until the pH is in the range 5.5–7.0 (preferably 6.0–6.5) at a temperature not exceeding 70 °C, preferably in the range 40–50 °C. Due to the energy release during the process, it is necessary to use a water jacket reactor to remove the heat. The obtained thermal energy is preferably used for heating in the subsequent stages of fertilizer production—dissolving the solid components and introducing ammonium nitrate and urea.
The phase of dissolving the solid components should take place at a temperature of approx. 40 °C. Citric acid is introduced before the addition of manganese and iron sulfate to reduce the pH if the pH is higher than 6. The last step is adding ammonium nitrate and urea. It is important to keep the temperature constant and above 40 °C. This importance of stability is due to the strong absorption of heat from the environment by these components.
The final pH should be suitable for use in horticulture—between 5 and 6. The final value can be adjusted by adding citric acid (reducing the pH) and triethanolamine (increasing the pH).
The proposed solution was based on the use of material waste, i.e., post-cultivation horticultural mineral wool, in the production of fertilizing products and on the use of the post-extraction residue (after extracting valuable materials from waste horticultural mineral wool) as a component in the production of a new organic–mineral horticultural substrate.

2.5. The Reuse of Post-Extration Peat and Mineral Wool in the Production of a New Organic–Mineral Horticultural Substrate

The proposed method of recycling two important materials currently used in agriculture, low-energy peat and post-cultivation mineral wool, generates post-extraction waste. One of the proposals was the use of post-extraction mineral wool containing residual amounts of macro- and microelements together with post-extraction carbon-bearing material, which is a residue from the process of obtaining valuable humic and fulvic fractions, to produce innovative substrates containing nutrients. In such a substrate, the ratio and choice of individual components play an important role in determining the final parameters of the product.
Post-extraction materials may undergo various changes, including, for example, alterations in chemical composition, a reduction in soluble organic compounds, or modifications in physical structure. However, they can still retain beneficial properties, such as porosity, water retention capacity, and other important characteristics. These features are essential, as they enhance nutrient bioavailability, support microbial activity, and improve overall substrate performance in reuse applications.
In accordance with the proposed solution, post-extraction mineral wool as a secondary raw material should be free of biological contamination and, if necessary, sterilized to eliminate microorganisms. The disinfection step may be omitted if the extractant used in the previous processes had bactericidal and fungicidal properties. Sterilization can be performed, among others, by irradiating the raw material with UV lamps, heating it in microwave dryers, or bathing it in selected chemical solutions.
Mineral wool after mineral extraction should be properly stored and possibly additionally sterilized if it was not processed into a garden substrate immediately after the end of filtration. The post-extraction rock wool is then mixed with the post-extraction peat and optionally mixed with standard NPK fertilizer with microelements. It is possible to prepare specialized substrates for specific crops by selecting individual fertilizer components and combining them to enrich the substrate with nutrients. The proposed universal horticultural substrate is based on post-extraction peat (60 wt%) and post-extraction mineral wool (40 wt%) with the addition of standard NPK fertilizer with microelements.
If the product requires a higher pH than the one obtained, the mixture can be made alkaline. This was especially important due to the acidic pH of the peat, and the pH of the produced substrate can be adjusted by adding dolomite or previously introduced fertilizer components. From a technical point of view, one production line should be served by two independent dedusting lines. The first one should dedust the unsterilized waste zone, and the second one after sterilization. Waste from the second system should be classified and recycled to the production process. As indicated in the diagram (Figure 1), red lines represent dedusting, blue lines represent stream recirculation, purple lines indicate waste, and black lines represent the main process.
The gray fields in the diagram indicate an optional step in the process depending on the desired characteristics of the substrate. The yellow fields represent intermediate products and products that are being proposed as a resource-efficient circular economy approach to valorizing waste mineral wool and peat in the fertilizer industry.

2.6. Initial Attestation Tests of Liquid Fertilizer and Substrate Necessary to Place Products on the Market

The results of physical, physicochemical, chemical, and biological tests were required to obtain a permit to place the product on the market. New fertilizers or agents supporting the cultivation of plants should be of appropriate composition and quality and meet requirements in terms of the content of impurities.
For this purpose, in the proposed liquid fertilizer and the substrate, the content of the following nutrients was determined: nitrogen (N), phosphorus (P), potassium (K), magnesium (Mg), calcium (Ca), sodium (Na), copper (Cu), zinc (Zn), manganese (Mn), iron (Fe), boron (B), and molybdenum (Mo). We also determined the content of heavy metals (Cr, Cd, Ni, Pb, Hg) and biological pollutants: the presence of live eggs of intestinal parasites (Ascaris sp., Trichuris sp., Toxocara sp.) and bacteria of the genus Salmonella sp. In the case of the organic–mineral substrate’s composition, pH, dry matter content, organic matter content, bulk density, and salinity were also determined. To ensure the reliability of the analytical results, all sample handling procedures adhered to standard protocols for contamination control and sample integrity. Quality control steps included the use of blanks, reference standards, and replicate analyses for all tests. Methods were validated according to relevant ISO and EU guidelines, ensuring the reproducibility and accuracy of results across batches.
Physical, physicochemical, chemical, and biological tests of the fertilizer and agent supporting the cultivation of plants were carried out in accordance with the applicable methodology issued by authorized or accredited bodies (Article 4(4)(1) of the Act of 10 July 2007 on fertilizers and fertilization (Journal of Laws of 2021, Item 76) and Section 2 and Section 4 of the Regulation of the Minister for Agriculture and Rural Development of 18 June 2008 regarding the implementation of certain provisions of the Act on fertilizers and fertilization (Journal of Laws No 119, item 765).
The contents of P, Na, Ca, Mg, B, Cu, Fe, Mn, Mo, and Zn, as well as of Cr, Cd, Ni, and Pb, were determined by inductively coupled plasma optical emission spectrophotometry (ICP-OES). The content of N was determined using Kjeldahl’s method by the reduction of nitrates to ammoniacal nitrogen with chromium powder, mineralization with concentrated sulfuric acid, distillation, and titration of ammonia. The content of K was determined by flame atomic emission spectrometry (FAES). Mercury was determined using AAS with the mercury vapor amalgamation technique. pH was measured by a potentiometric method. Moisture, dry matter, and organic matter contents were determined gravimetrically by measuring the weight loss of the sample. The detection of Salmonella sp. was performed in accordance with cultivation methods on artificial substrates complemented by biochemical tests. The detection of live eggs of the intestinal parasites Ascaris sp., Trichuris sp., and Toxocara sp. was carried out by isolation and incubation techniques and confirmed by microscopic observations. The physical properties of the substrate, i.e., bulk (volumetric) density and salinity, were determined in accordance with European standards PN-EN 13040:2009 [33] and PN-EN 13038:2011 [34], respectively.
This study was conducted under controlled laboratory conditions. As such, it did not address variability in raw material composition, economic feasibility, long-term stability, or field validation under real agricultural conditions. Furthermore, no life cycle assessment (LCA) was performed to evaluate the environmental impact. These limitations underscore the importance of further applied research to assess the scalability and practical implementation of the proposed solution.

3. Results

3.1. Physicochemical Characterization of Humic Acids, Fulvic Acids, and Peat Before and After Extraction

Both peat and rock wool are widely used in growing media. Peat, due to its favorable properties and alignment with sustainable strategies for nonrenewable resource use and the non-energy valorization of carbonaceous waste, is increasingly used as an important industrial raw material for humic substance production. Regardless of the method of obtaining humic substances, the process results in a specific type of post-extraction waste, which is still a valuable organic raw material in other processes. Figure 2 shows the first stage, in which humic and fulvic acids are extracted from peat, generating post-extraction peat as a by-product to be used as a component of a new organic–mineral substrate. The nature of such waste depends on the physical and chemical parameters of the process and the degree of extraction of humic substances. To assess the quality and structural changes resulting from the extraction, the raw material (peat), products (humic and fulvic acids—HAs, FAs) and waste (post-extraction peat—post-peat) were subjected to elemental analysis and analyzed by FTIR and solid-state CP-MAS 13C NMR spectroscopy.
Elemental analysis and the C/H, C/N, and C/O ratios in the humic and fulvic acids extracted from peat and in raw material before and after extraction are reported in Table 1.
The elementary composition of organic substances and the ratio of individual atoms H/C, O/C, and C/N determine the dynamics of their transformations and allow for the prediction of their structural formulas. It was assumed that the H/C atomic ratio is an indicator of aromatization and condensation. The O/C atomic ratio provides information regarding the content of oxygen functional groups; additionally, the C/N ratio is a measure of the degree of maturity and durability [35,36]. The type of humic and fulvic acids can be determined from the ratios of H/C and O/C atoms and the Van Krevelen diagram [37,38].
To characterize the tested organic materials before and after the extraction of humic substances, Fourier transform infrared absorption spectroscopy was also used. The obtained spectra allow for the identification of the present functional groups and the configuration of bonds, but their interpretation due to the presence of many absorption bands was not easy and unambiguous. Despite the complex and heterogeneous nature of humic compounds, which is reflected in wide and overlapping peaks, FTIR spectra for all tested materials (Figure 3) have bands characteristic of this kind of compound.
Comparing the spectra for the tested samples, four main ranges of the spectrum, characteristic of functional groups, can be distinguished [39]: hydroxyl functional groups (3700–3000 cm−1), aliphatic functional groups (3000–2800 cm−1), oxygen functional groups (1800–1000 cm−1), and aromatic functional groups (900–700 cm−1).
The results obtained from FTIR spectroscopy and other basic analyses agree with those obtained using solid-state CP-MAS 13C NMR spectroscopy, which indicate the relative content of different types of carbon atoms. Analyzing the 13C NMR spectrum, four ranges were considered: 0–45 ppm, related to the presence of alkyl carbon component atoms (carbons in chains and rings, methyl groups, and primary aliphatic amines, among others)—C-alkyl; 45–110 ppm (aliphatic carbons bonded to single or double oxygen or nitrogen atoms in carbohydrate structure)—O-alkyl C; 110–160 ppm, the range covering carbon atoms occurring in aromatic connections (C-H and phenolic)—C-aromatic; and 160–210 ppm, indicating carbon atoms in carboxylic structures (carboxylic acids, ketones, aldehydes, ester, and amide carbons)—C-carboxylic [39,40,41,42]. Solid-state CP-MAS 13C NMR spectra of HAs, FAs, and peat before and after their extraction are shown in Figure 4, and the results from the integration of the major regions of the chemical shift regions are listed in Table 2.

3.2. Physicochemical and Biological Characteristics of the New Organic–Mineral Substrate and Liquid Fertilizer

For an organic fertilizer, organic–mineral fertilizer, mineral fertilizer not marked “EC fertilising products” (in accordance with Regulation (EU) 2019/1009 of the European Parliament and of the Council of 5 June 2019), or agent supporting the cultivation of plants to be placed on the market, authorization by the agriculture minister is required; alternatively, authorization can be obtained in another EU country if the fertilizer meets the criteria for quality and content of impurities. Manufacturers or importers should submit full documentation concerning the quality of the product, test results for relevant samples, and opinions regarding its safety to obtain permits. If the quality requirements are not met or if it is shown that the product poses a health risk to humans or animals, the permit may be withdrawn. In Poland, the authority issuing the permit to place fertilizing products (other than EC) on the market is the Ministry of Agriculture and Rural Development based on the criteria set out in the Regulation of the Minister for Agriculture and Rural Development of 18 June 2008 on the implementation of certain provisions of the Act on fertilizers and fertilization (Journal of Laws No 119, item 765).
The first product to be proposed was a liquid fertilizer based on an extract containing nutrients recovered from waste mineral wool by using 0.1 M EDTA solution. Figure 5 shows the second stage of the framework, in which nutrients are extracted from post-cultivation mineral wool and used in the production of liquid fertilizer. Table 3 shows the physicochemical and biological characteristics of the waste mineral wool-based liquid fertilizer.
Mineral wool after the process of extracting nutrients becomes a post-extraction residue. The second most valuable waste was peat after the extraction of humic and fulvic acids from it. In accordance with the current challenges of the closed-loop strategy, both post-extraction residues were reused as a secondary raw material in a new product, an organic–mineral substrate.
The second product to be proposed was a universal horticultural substrate composed of post-extraction peat (60 wt%) and post-cultivation mineral wool (40 wt%), supplemented with standard NPK fertilizer and microelements. Figure 6 shows the third stage of the framework, in which this organic–mineral substrate was produced using post-extraction waste materials. A tested growing medium qualifies as an agent supporting the cultivation of plants, and fertilizers are subject to the same requirements and procedure for placing the product on the market. Table 4 shows the physicochemical and biological characteristics of the organic–mineral substrate.

4. Discussion

4.1. Qualitative Assessment of Humic Acids, Fulvic Acids, and Peat Before and After Extraction

Humic acids commonly have much higher carbon concentrations and, consequently, slightly lower H/C atomic ratios than fulvic acids, which is typically associated with a higher aromatic content [43]. The higher carbon content, the relatively high hydrogen content, and the lower oxygen content in humic acids prove that they are a hydrophobic fraction with a smaller number of aliphatic groups. Additionally, the correspondingly changing values of the H/C and O/C atomic ratios confirm this relationship. A H/C ratio of 1.0 and a low O/C value indicate a decrease in the polarity of the aliphatic groups [44,45]. The increase in hydrophobicity causes a decrease in solubility, acidity, and oxygen content. The elemental analysis data show that FAs have lower carbon content and higher oxygen content than other samples. Then, the O/C ratio indicates an increase in oxygenated functional groups such as carboxylic groups, which have acidic characteristics and therefore have a higher acidity and are hydrophilic and well soluble over the pH range. The high C/N ratio for FAs, due to the low N content and high O/C ratio, is another characteristic of the increase in aliphatic structures.
By analyzing the elementary composition of peat before and after extraction, it can be concluded that a significant part of the organic substance remains in the waste. In the context of peat raw material and its residues (post-peat), the H/C ratio provides information about the stability, maturity, and degree of condensation of the organic matter contained in it [45]. The higher oxygen content in raw peat suggests that humus fractions with valuable functional groups have been successfully extracted. As the C content increases and the hydrogen content often decreases, the H/C ratio decreases. This phenomenon results from the condensation of rings of aromatic compounds included in the organic substance in the extraction residue. This result proves the high degree of polymerization and condensation of the organic substance in the waste.
The stability of the organic matter derived from peat was demonstrated by the contents of carbon and nitrogen. The C/N ratio in an organic substance determines the dynamics of its changes in the environment [46]. Humification, a measure of the extent to which fresh organic matter has been transformed into recalcitrant humic substances [47], is one way to characterize the degree of peat decomposition. A low C:N ratio is usually equivalent to a high humification degree [48]. Organic substances richer in carbon and poor in nitrogen, as is the case with peat, are a more sustainable source of energy and matter for microorganisms. It can be assumed that preparations based on peat and post-extraction residues will undergo slow mineralization.
Several characteristic bands are clearly marked in the FTIR spectra for all tested samples. The broadest and largest band was located at approximately 3400–3000 cm−1 and is characteristic of the stretching of O-H bonds of hydroxyl groups that are capable of forming hydrogen bonds, mainly carboxylic acids, alcohols, and phenols, as well as the stretching of N-H bonds from amides and amines [40,46,49]. The next sharp and well-defined absorption bands centered at 2940 cm−1 and 2840 cm−1 are attributed to the symmetrical stretching vibration of aliphatic C-H bonds of methyl -CH3 and methylene -CH2 groups, respectively [50], and indicate a high share of aliphatic structures. The presence of oxygenated functional groups favors the interaction of humic substances with organic and inorganic compounds, especially metallic ions, which favors their availability for plants [51]. From 1720 to 1600 cm−1, the presence of a series of symmetrical stretching of C=O and C=C double bonds of ketones, aldehydes, esters, and quinones varies depending on the type of sample. Peat before and after extraction has the characteristic of a wide band. In the humic acid spectrum, two visible separate peaks at approximately 1620 and 1715 cm−1 are observed. The first band indicates the presence of C=C bonds in aromatic linkages (in the benzene ring), and C=O bonds in quinones, aldehydes, and ketones cannot be excluded [37,52,53]. The second clearly marked absorption is characteristic of the presence of C=O carbonyl groups in peptides, aldehydes, and ketones, C=O double bonds, and C=O stretch bonding of carbonyl groups in ketones and quinones [37,49,50]. This result indicates that the material contains more oxygen-containing active groups, such as benzene ring structures, alcohols, or phenols. The occurrence of C=O bonds confirms the moderate and higher maturity of humic acids in comparison to fulvic acids [37]. Additionally, in contrast to the other spectra, the FTIR spectrum shows a wide absorption band centered at 1430 cm−1, which indicates the presence of C-H bonds of methyl and methylene groups [46,54]. Only for fulvic acids can the presence of an intense and well-defined band with an absorption center at 1150 cm−1 also be observed. This band absorption can arise from Si-O vibration in silica [53] but is dominant only in the spectra of fulvic acids, whereas it is absent in peat and humic acids. Therefore, it is more likely that this peak confirms the presence of carboxylic groups bound to metal cations [46]. In agricultural practice, it proves the ability of fulvic acids to maintain soil fertility by supplying the plants with minerals necessary for development and growth. The main difference in the spectra of FAs and the others is reflected in the fingerprint area of less than 1000 cm−1, where many of the vibrations are deformed. Fulvic acids showed absorption in the range of 900–700 cm−1 due to aromatic structures with isolated aromatic hydrogens (880 cm−1), N-O bonds with nitrogen esters, N-H vibrations, and non-NH2 vibrations (695 and 634 cm−1) [37,41,53].
There was no major difference in the FTIR spectra of peat before and after extraction. Moreover, the spectra of peat samples are more similar to the spectra of humic acids, which proves their similarity in structure and that part of the organic matter remains in the raw material. In the context of using post-extraction peat as an organic component of the horticultural substrate, this method is advantageous; additionally, the post-extraction residue is characterized by a loosened structure, which may result in greater availability and the possibility of interacting with other elements of the system. These spectra indicate that the peat after extraction contains both aromatic and aliphatic structures, as well as hydroxyl, carboxyl, and oxygen-containing functional groups.
The Fourier transform infrared absorption spectra of HAs and FAs obtained from peat confirm, however, that HAs have greater structural complexity and a relatively higher molecular weight than FAs. This was evidenced by the differing presence of several absorption bands visible in these spectra and the different intensities of the peaks.
In the C aliphatic range of NMR spectra (0–110 ppm) in all tested materials, the dominant peak at 30 ppm arises from alkyl carbon components such as methyl, methylene, and methine carbons. However, in the O-substituted alkyl C region, two well-resolved peaks at 55 and 75 ppm and a smaller one at 105 ppm are observed only in peat and post-peat samples. These peaks are attributed to methoxy groups associated with lignin, carbon rings, and anomeric carbons in polysaccharides [40,55,56,57]. This result was confirmed by the percentage of total aliphatic C in the range of 0–110 ppm, which for peat and post-extraction peat is above 60% and suggests a relatively high amount of aliphatic components in their structures. However, the peat and post-peat NMR spectra also show well-defined peaks in the aromatic region centered at 130 ppm and 151 ppm. These are assigned to unsaturated carbons or aryl-C with protonated aromatic C and carbon bonded to phenolic OH and indicate the content of a more condensed structure. In the carboxylic range of spectra (60–110 ppm), only FAs show a stronger absorption peak in the chemical shift range of 160–185 ppm, which corresponds to carboxyl carbon, amide, and ester functionalities. This indicates that the molecular structure of FAs contains many carboxylic groups with relative contents of 25.9% of carbon in the carboxylic structure. The C contents of the aromatic structure for FAs were not very different, although they were higher in HAs, at 23.5 and 26.7%, respectively. The data are consistent with the results of the elemental and infrared analysis and suggest that post-peat has a relatively condensed structure with a high C content (43.8%) and a reduced O/C atomic ratio but is still rich in alkyl-like matter (63.2%) but devoid of some functional groups in favor of the extracted fulvic acids.

4.2. Qualitative Assessment of the New Organic–Mineral Substrate and Liquid Fertilizer

The disposal of post-cultivation mineral wool slabs as one of the popular substrates used in soilless culture is very difficult due to their nonbiodegradability. However, waste mineral wool contains residues of important nutrients, which can be recovered and reused as a component for new products. According to the proposal of waste mineral wool valorization, preprocessing of this horticultural waste includes extraction of nutrients and their use in the production of liquid fertilizer. The innovativeness of this solution is based on the idea of using waste wool as a raw material to obtain useful products that have the potential to replace classic fertilizers by reducing the consumption of nonrenewable mineral raw materials for their production. The demonstration of the possibility of recovery of essential nutrients contained in waste mineral wool may allow for obtaining cost-effective liquid fertilizer.
The tested liquid fertilizer meets the quality requirements for fertilizer in terms of the content of NPK nutrients. It also contains a significant amount of magnesium and manganese. The concentration of heavy metals in this sample was lower than the acceptable concentration. There was also no presence of microbiological contamination. Based on the negative test for Ascaris sp., Trichuris sp., and Toxocara sp. eggs, pathogenic bacteria of the genus Salmonella sp., and heavy metals in the sample, the proposed fertilizer can potentially be used for commercial purposes. The product is suitable for use in agricultural crops, both for fertilization before sowing or planting plants and for feeding during the growing season.
During the last two decades, there has been a significant development in the market and research on substrates. Various research efforts have been conducted to find ways to transform agricultural, industrial, and municipal wastes into materials that can be used in growing media. Standardizing and characterizing the qualitative characteristics of the substrate, as well as monitoring and regulating the conditions under which plants grow, are of particular importance. The quality of the substrate is one of the most important factors influencing the proper growth and development of plants. The physical properties of the substrate are an important criterion for its quality. The precise characterization of physical features is becoming increasingly important and is influenced by economic considerations and high technological progress. This process creates the possibility of mechanization and automation of large-scale production; therefore, it is important to repeat and standardize all factors that determine the cultivation and growth of plants. A variety of analyses were carried out to evaluate the substrate, including pH, EC, bulk density, organic matter, and nutrient content, as well as biological and heavy metal contamination.
The tested substrate has a pH-H2O of 5.3 and a salinity (EC) expressed with reference to NaCl at the level of 7.8 g/dm3. The optimum pH for most cultivated plants is 5.6–6.5, although values between 5.0–5.5 and 6.5–7.0 may not result in challenges for most crops [58]. As the proposed substrate is relatively acidic, it can be used for plants that are tolerant of lower pH conditions. Other than the pH value of growing media, salinity is a limiting factor for crop growth. Salinity is a measure of the total concentration of dissolved salts by electrical conductivity (EC). Managing nutrient supply and salt concentration is very important, especially in closed soilless systems [58]. The acceptable salinity of the substrates is approx. 6 g/dm3. The salinity of the proposed substrate should be assessed as relatively high, which also indicates the high content of nutrients. The total nitrogen content was 0.32% N, phosphorus was 0.75% P, potassium was 0.25% K, magnesium was 1.04% Mg, and calcium was 3.24% Ca. The product also contains micronutrients. The sodium content is 0.38% Na, which in relation to other nutrients is high. The product contains 37.6% organic matter in dry matter. For proper growth and development, different plant species have different preferences regarding nutrient ratios in the medium. It is important to maintain the ratio between the metallic macronutrients in the root zone, since excessive Ca:K or Mg:K concentrations can result in the accumulation of these ions [59].
The proportions of individual minerals in the tested product do not seem optimal from the point of view of the nutritional needs of crops, with an excess of magnesium, calcium, and phosphorus in relation to nitrogen and potassium. The chemical composition of the product requires modification to adapt to the nutritional requirements of crops when used as a growing medium. An additional potential application of the product, without modifying its chemical composition, can be its utilization as a soil conditioner to increase the content of organic matter and nutrients in arable land.
Using waste materials to produce new fertilizing products invites the risk of introducing various types of pollutants into crops and the environment [60]. The content of heavy metals in the tested sample was lower than that allowed in an agent supporting the cultivation of plants in accordance with the Regulation of the Minister for Agriculture and Rural Development of 18 June 2008 on the implementation of certain provisions of the Act on fertilizers and fertilization (Journal of Laws No 119, item 765). Neither the substrate nor its components contained any microbiological contaminants. The sample was free of Ascaris sp., Trichuris sp., and Toxocara sp. eggs and pathogenic bacteria of the genus Salmonella sp. The quality of this product should not raise any objections, but its conscious composition is an important supplement to health and safety.

4.3. Practical Implications and Limitation of the Study

The results of this study confirm that the proposed valorization pathway holds practical potential for sustainable agriculture. By extracting humic substances from peat and recovering nutrients from post-cultivation mineral wool, it was possible to create value-added products from materials that would otherwise become waste. The resulting liquid extract meets nutritional standards comparable to those of commercial fertilizers, while the remaining solid mixture forms a structurally stable, porous, and biocompatible substrate with favorable pH and salinity properties.
This dual valorization approach not only minimizes the volume of poorly biodegradable waste produced in soilless cultivation systems but also promotes the efficient use of peat as a source of humic substances and the reuse of post-extraction residues as components for new growing media. As such, it provides a promising solution for recycling nutrient-rich waste streams, including biowaste, thereby reducing landfill dependency and generating additional economic value.
Moreover, avoiding landfilling contributes to environmental protection by conserving natural resources, lowering greenhouse gas emissions, and reducing the need for raw material extraction. A particularly serious issue is the illegal disposal of waste mineral wool in forest areas, wastelands, and natural ecosystems, which leads to environmental degradation. The integration of these recovered materials into horticultural applications thus supports circular resource use, enhances environmental sustainability, and may result in significant cost savings for agricultural producers.
This study was limited to laboratory-scale evaluations. Economic feasibility, variability in raw material quality, long-term stability, and field validation under real agricultural conditions were not addressed. Additionally, no life cycle assessment (LCA) was performed to quantify environmental impact. These limitations highlight the need for further applied research.

5. Conclusions

The study demonstrated that both peat and its post-extraction residue, as well as the humic and fulvic acids derived from it, exhibit diverse structural and chemical properties that determine their potential applications in sustainable agriculture. Elemental analysis, FTIR, and 13C NMR spectroscopy revealed that humic acids were richer in carbon and more hydrophobic due to a higher degree of aromaticity and condensation, while fulvic acids contain more oxygen-containing functional groups, making them more acidic and highly soluble. These properties indicate the complementary roles of HAs and FAs in improving soil properties and nutrient availability.
Importantly, the analysis confirmed that post-extraction peat retains a significant amount of organic matter, including functional groups similar to those in raw peat. This suggests its potential for reuse as a component of organic–mineral substrates, contributing to efficient resource use and waste reduction. Spectroscopic data further confirmed that the extraction residue remained rich in aliphatic and aromatic structures, which additionally supports its value as a horticultural material.
The study confirmed that the extract obtained from mineral wool after cultivation contained nutrients at concentrations suitable for use as liquid fertilizers. Its composition was comparable to that of commercial products. Furthermore, the organic–mineral substrate formed by combining post-extraction peat with mineral wool after cultivation and nutrient extraction resulted in a structurally stable, porous, and biocompatible product. Its properties, including pH, salinity, and nutrient retention capacity, indicate strong potential as a sustainable alternative to conventional horticultural growing media.
Comprehensive characterization supports the validity of the proposed strategy for valorizing low-energy peat and waste mineral wool through the extraction of valuable substances and the reuse of residues in the production of substrate and fertilizer. This approach aligns with circular economy principles and sustainable agriculture by reducing environmental impact, increasing the share of waste materials in new processes, and offering new opportunities for resource recovery in agricultural practice.

Author Contributions

Conceptualization, M.H.-M. and D.N.; methodology, M.H.-M., D.N., K.M. and M.B.-G.; formal analysis: K.M. and M.B.-G.; writing—original draft preparation, M.H.-M. and D.N.; writing—review and editing, M.H.-M. and M.B.-G.; supervision, M.H.-M. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Ministry of Science and Higher Education of Poland within the framework of a science subsidy for 2025 which was realized in the Department of Engineering and Technology of Chemical Processes, Wroclaw University of Science and Technology (no. 8211104160-K24W03D05).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Proposal for valorization of waste mineral wool and low-rank peat for circular economy.
Figure 1. Proposal for valorization of waste mineral wool and low-rank peat for circular economy.
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Figure 2. Process of humic and fulvic acid extraction from peat, resulting in products (HAs, FAs) and post-extraction peat.
Figure 2. Process of humic and fulvic acid extraction from peat, resulting in products (HAs, FAs) and post-extraction peat.
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Figure 3. FTIR spectra of humic and fulvic acids and peat before and after extraction.
Figure 3. FTIR spectra of humic and fulvic acids and peat before and after extraction.
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Figure 4. 13C NMR spectra of humic and fulvic acids and peat before and after extraction.
Figure 4. 13C NMR spectra of humic and fulvic acids and peat before and after extraction.
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Figure 5. Process of nutrient extraction from post-cultivation mineral wool, resulting in a liquid fertilizer product and post-extraction post-cultivation mineral wool.
Figure 5. Process of nutrient extraction from post-cultivation mineral wool, resulting in a liquid fertilizer product and post-extraction post-cultivation mineral wool.
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Figure 6. Process of producing an organic–mineral horticultural substrate from post-extraction peat and post-cultivation mineral wool.
Figure 6. Process of producing an organic–mineral horticultural substrate from post-extraction peat and post-cultivation mineral wool.
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Table 1. Elemental composition of humic and fulvic acids and peat before and after extraction.
Table 1. Elemental composition of humic and fulvic acids and peat before and after extraction.
SampleElemental Composition (%)Atomic Ratio
NCHOH/CO/CC/N
HAs3.150.64.741.70.90.816.4
3.050.05.641.41.10.816.6
3.151.35.340.31.00.816.5
Average3.150.65.241.11.00.816.5
FAs0.919.910.968.35.52.521.2
1.120.28.470.34.23.517.9
1.119.18.571.34.53.716.9
Average1.119.79.370.04.73.218.7
Peat2.941.16.449.61.61.214.1
2.642.75.649.11.31.116.4
2.738.05.653.71.51.414.0
Average2.740.65.950.81.41.314.8
Post-Peat3.346.16.344.31.41.014.0
3.548.56.241.81.30.913.7
3.547.86.442.31.30.913.7
Average3.447.46.342.81.30.913.8
Table 2. Integrated areas of the main spectral bands of solid-state CP–MAS 13C NMR spectra of humic and fulvic acids and peat before and after extraction.
Table 2. Integrated areas of the main spectral bands of solid-state CP–MAS 13C NMR spectra of humic and fulvic acids and peat before and after extraction.
SampleDistribution of Carbon Chemical Shift (ppm), %
0–4545–110110–160160–210
Alkyl CCarbohydrate CAromatic CCarboxylic C
HAs25.329.426.718.6
FAs22.128.523.525.9
Peat28.333.723.614.3
Post-Peat25.337.924.412.4
Table 3. Physicochemical and biological characteristics of the waste mineral wool-based liquid fertilizer.
Table 3. Physicochemical and biological characteristics of the waste mineral wool-based liquid fertilizer.
Parameter, UnitDetermined
in the Sample		Minimum Requirements for Mineral Fertilizers According to Journal of Laws No 119, Item 765 and of 2008
N, wt%2.5 ± 0.31.0
P2O5, wt%3.3 ± 0.51.0
K2O, wt%6.65 ± 0.391.0
Mg, wt%1.3 ± 0.2-
Ca, wt%0.01 ± 0.01-
Na, wt%0.33 ± 0.05-
Cu, mg/kg105 ± 13-
Zn, mg/kg193 ± 24-
Mn, mg/kg1049 ± 126-
Fe, mg/kg2.5 ± 0.3-
B, mg/kg96.2 ± 11.6-
Mo, mg/kg12.8 ± 1.3-
Cr, mg/kg<5.0100
Cd, mg/kg<0.55
Ni, mg/kg6.8 ± 0.960
Pb, mg/kg<5.0140
Hg, mg/kg<0.012
Presence of pathogens and eggs of intestinal parasites
Ascaris sp.,
number of eggs/1 kg of dry matter		Not determined
Trichuris sp.,
number of eggs/1 kg of dry matter		Not determined
Toxocara sp.,
number of eggs/1 kg of dry matter		Not determined
Salmonella sp.,
per 100 g of sediment		Not determined
Table 4. Physicochemical and biological characteristics of the organic–mineral substrate.
Table 4. Physicochemical and biological characteristics of the organic–mineral substrate.
Parameter, UnitDetermined
in the Sample		Minimum Requirements for Mineral Fertilizers According to Journal of Laws No 119, Item 765 and of 2008
N, wt%0.32 ± 0.05-
P, wt%0.75 ± 0.12-
K, wt%0.25 ± 0.03-
Mg, wt%1.04 ± 0.17-
Ca, wt%3.24 ± 0.50-
Na, wt%0.38 ± 0.06-
Cu, mg/kg32.9 ± 3.98-
Zn, mg/kg64.3 ± 7.88-
Mn, mg/kg545 ± 65.6-
Fe, mg/kg1007 ± 121-
B, mg/kg11.5 ± 1.37-
Mo, mg/kg9.25 ± 1.08-
Cr, mg/kg22.9 ± 2.8100
Cd, mg/kg<0.55
Ni, mg/kg26.6 ± 3.260
Pb, mg/kg20.0 ± 2.4140
Hg, mg/kg0.07 ± 0.012
Moisture, %6.0 ± 0.1-
Organic matter, %37.6 ± 2.3-
pH-H2O5.3 ± 0.2-
EC, g NaCl/dm37.8 ± 0.6-
Bulk density, g/m3246 ± 50-
Presence of pathogens and eggs of intestinal parasites
Ascaris sp.,
number of eggs/1 kg of dry matter		Not determined
Trichuris sp.,
number of eggs/1 kg of dry matter		Not determined
Toxocara sp.,
number of eggs/1 kg of dry matter		Not determined
Salmonella sp.,
per 100 g of sediment		Not determined
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Huculak-Mączka, M.; Nieweś, D.; Marecka, K.; Braun-Giwerska, M. Valorization of Waste Mineral Wool and Low-Rank Peat in the Fertilizer Industry in the Context of a Resource-Efficient Circular Economy. Sustainability 2025, 17, 7083. https://doi.org/10.3390/su17157083

AMA Style

Huculak-Mączka M, Nieweś D, Marecka K, Braun-Giwerska M. Valorization of Waste Mineral Wool and Low-Rank Peat in the Fertilizer Industry in the Context of a Resource-Efficient Circular Economy. Sustainability. 2025; 17(15):7083. https://doi.org/10.3390/su17157083

Chicago/Turabian Style

Huculak-Mączka, Marta, Dominik Nieweś, Kinga Marecka, and Magdalena Braun-Giwerska. 2025. "Valorization of Waste Mineral Wool and Low-Rank Peat in the Fertilizer Industry in the Context of a Resource-Efficient Circular Economy" Sustainability 17, no. 15: 7083. https://doi.org/10.3390/su17157083

APA Style

Huculak-Mączka, M., Nieweś, D., Marecka, K., & Braun-Giwerska, M. (2025). Valorization of Waste Mineral Wool and Low-Rank Peat in the Fertilizer Industry in the Context of a Resource-Efficient Circular Economy. Sustainability, 17(15), 7083. https://doi.org/10.3390/su17157083

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