Introduction

Biomass waste is the largest group of residues from various sectors such as municipal waste, sewage sludge, agricultural waste, or food industry waste. The available techniques for valorizing biomass should convert it into valuable products or recover energy from it, in line with a closed-loop economy. Many technologies can be applied to valorize waste biomass, including thermal (Khademi et al. 2023; Naveen et al. 2023), chemical (Zhang et al. 2023; Baniamerian et al. 2023), or biological techniques (Sharma et al. 2023; Fernández-Domínguez et al. 2023).

Hydrothermal carbonization (HTC) is a promising thermochemical waste treatment technique. Its great advantage is the ability to treat materials with higher moisture content, which is not possible with other thermal processes, such as pyrolysis or incineration. Subcritical water is the reaction environment in the HTC process, which allows the decomposition of biopolymers and organic compounds under conditions of elevated temperature (150–260 °C) and elevated autogenous steam pressure (2–10 MPa). The HTC temperature range given in the literature is relatively wide, with the low range starting between 150 and 180 °C (Funke and Ziegler 2010; Ghanim et al. 2016; Olszewski et al. 2019; Shen 2020; Kulbeik et al. 2021) and the high range usually being 260 °C and 280 °C (McGaughy and Reza 2018; Shen 2020; Niinipuu et al. 2020; Xiong et al. 2021; Hansen et al. 2022). Many works recommend that the optimum HTC temperature be close to 200 °C for BSG (Jackowski et al. 2020) as well as for many other types of biomass (Aragón-Briceño et al. 2020, 2021a, b; Sobek et al. 2022; Mlonka-Mędrala et al. 2022). Such a temperature has been reported for commercial-scale HTC installations (Sobek et al. 2022). The HTC processes occurring are complex, with hydrolysis, dehydration, decarboxylation, polymerization, and carbonization steps distinguished, leading to the formation of hydrochar rich in polyaromatic and phenolic structures (Shen 2020). Controlling the process conditions provides the possibility of obtaining hydrochar with different properties. The conversion of lignocellulosic biomass is strongly dependent on its chemical structure and the intensity of the HTC process (temperature, residence time). The decomposition of hemicellulose occurs at the lowest temperature (180 °C), cellulose at 200 °C and lignin requires more energy due to its aromatic structure and begins to decompose above 260 °C (Zhang et al. 2021). Hydrochar can have several applications, such as biofuels (Zhang et al. 2021), catalysts (Liu et al. 2021a), and adsorbents (Feng et al. 2019; Babeker and Chen 2021) or soil amendment (Islam et al. 2021a).

Their soil-conditioning properties include high carbon content (Fregolente et al. 2021), biological activity, regulation of soil pH (de Jager and Giani 2021), and release of nutrients, including nitrogen and phosphorus (Wang et al. 2020). Nutrient release studies have shown that hydrochars activate the short-term movement of nutrients in the soil, which is more rapid compared to pyrolysis-derived biochar with a more stable release profile (Jafari Tarf et al. 2022). The hydrochar will likely be suitable for short- to medium-term cultivation; however, long-term studies are needed to confirm this. Hydrochars, due to their porous structure, can also serve as a delivery agents for plant growth-promoting bacteria, providing them with a safe environment when introduced into the soil (Thunshirn et al. 2021).

The conditions under which hydrochar is produced strongly affect its composition and thus its effect on plants (Islam et al. 2021b). It has been noted that the presence of hydrochar could have various effects on plant germination. The favorable impact of hydrochar on germination is attributed to its properties that alter soil parameters (water retention and improved nutrient availability). At the same time, the unfavorable germination-inhibiting effect is associated with the phytotoxic properties of the hydrochar itself (presence of residual organic compounds). An in-depth meta-analysis of this topic has shown that the addition of hydrochar in high doses significantly reduces the germination or the weight of the aboveground part (Luutu et al. 2022). Research is therefore needed to determine a safe dose of hydrochar that can be used in plant trials (Puccini et al. 2018).

The brewing industry generates huge amounts of solid waste that must be managed and converted into value-added products. Brewer's spent grain (BSG) is the most abundant waste produced in this industry, with an estimated 0.2 kg of this waste generated for every liter of beer, representing nearly 40 Mt of waste for the global beer market (Dessì et al. 2022). The material is rich in cellulose, hemicellulose, and lignin, which gives it potential as a feedstock for HTC (Maqhuzu et al. 2021). The hydrochars formed by the hydrothermal treatment process have a developed surface rich in numerous functional groups that can bind nutrient ions (Si et al. 2023). Such properties can be exploited to produce innovative micronutrient-enriched hydrochars that can support plant growth by providing the desired components. Those formulations will benefit plants both due to their introduction into the soil as amendments, and as delivery agents for nutrients that can be released slowly from the materials. In this work, we proposed enriching hydrochars with micronutrients by the traditional immersion sorption method and the spray method, which is more favorable from the point of view of process implementation (shorter process time, less energy required for drying).

Implementing micronutrient-enriched hydrochars for fertilizer purposes should include two groups of experiments, verifying the feasibility of carbon application and its possible toxicity and the effect of prepared formulations on plant growth and development in long-term tests. Our research hypothesis assumed that micronutrient-enriched hydrochars would act as nutrient delivery agents for plants to support their growth. To prove this hypothesis, we designed a series of experiments to (1) select a safe dose of hydrochars use in plant tests (phototoxicity tests for various doses of hydrochars produced in different process temperatures of 140–200 °C), (2) produce micronutrient-enriched fertilizer hydrochars (Cu, Zn and Mn) and study their physicochemical properties, (3) compare two enrichment methods (immersion and spraying methods), and (4) verify the utility of the enriched hydrochars in plant tests (pot trials).

Materials and methods

Hydrochar preparation

The BSG was obtained from a brewery located at the Wroclaw University of Science and Technology (Poland). BSG was stored in a cooler at a temperature of 5 °C prior to the HTC experiment. A diagram of the experimental setup (Figure S1) shows the autoclave rig. The autoclave was filled with 3.8 L of wet BSG, with a moisture content of 70%, determined with a Radwag moisture analyzer (MA.X2.A model) at 105 °C. HTC experiments were carried out at temperatures of 140 °C, 170 °C, and 200 °C. After being heated up to the selected temperature, the biomass was kept in the heated reactor for 30 min. Subsequently, the heating was switched off and the rig was left to cool overnight.

Preliminary usability analysis

Germination tests are a simple and quick experiment to assess the phytotoxicity of fertilizer materials. To evaluate the utility of hydrochars as delivery agents for fertilizer micronutrients and a potential source of nitrogen, unenriched materials (HTC140.0, HTC170.0, and HTC200.0) were tested in cucumber (Cornichon de Paris, Legutko). Seeds were placed on Petri dishes filled with sterile filter paper sawdust and stratified (temperature: 4 °C, time: 2 days). Plant tests were conducted for 10 days at 25 °C under equal light conditions (2400 lx, with a 16-h day, and 8-h night cycle). The test materials were applied on the third day of the experiment at five application rates calculated on the basis of the nitrogen requirements (140 kg N/ha): 0, 1%, 1%, 20%, 100%, and 150%. On the last day of the experiment, the biometric parameters (root ball and aboveground plant parts) were evaluated. Roots were analyzed using an Epson scanner (model PerfectionV850 Pro, Poland), while chlorophyll content was analyzed using an OPTI-SCIENCES chlorophyll meter (model CCM-300, USA).

Hydrochar enrichment

Immersion sorption

Enrichment was carried out on materials from the HTC process performed at three temperatures (HTC140.0, HTC170.0, and HTC200.0). For this purpose, a solution of micronutrients was prepared by dissolving sulfate salts (CuSO4, ZnSO4, and MnSO4) in demineralized water. The total concentration of micronutrients was 15,000 mg/L, where there were 5000 mg/L Cu(II), 5000 mg/L Mn(II), and 5000 mg/L Zn(II)). The pre-milled material (3 g) was placed in a dish containing 200 ml of tricathion solution (pH 5). The process was carried out with continuous stirring for 24 h at room temperature. After this time, the material was drained and air-dried (HTC140.IS, HTC170.IS, and HTC200.IS).

Spray sorption

To perform hydrochar enrichment using the spray method, a solution of micronutrients was prepared from sulfate salts containing 5000 mg/L Cu(II), 5000 mg/L Mn(II), and 5000 mg/L Zn(II). On the Petri dish, a thin layer of material (5 g) was placed and sprayed with 37.5 ml of the previously prepared solution. The application of the elements was carried out continuously, with the material stirred during the procedure. The enriched hydrochar was dried in two variants: (1) in air for 30 days (HTC140.SS, HTC170.SS, and HTC200.SS) and (2) in air for 30 days and at 110 °C for 24 h (HTC140.SS1, HTC170.SS1, and HTC200.SS1). The material was stored in sealed containers.

Materials characterization

Thermogravimetric analysis (TGA)

Thermogravimetric analysis (TGA) for HTC140.0, HTC170.0, and HTC200.0 materials was performed on a 5E-MAC6710 thermogravimetric analyzer (CKIC, China) equipped with an analytical balance (± 0.1 mg) (Mettler-Toledo, USA). Each sample (0.5 g) was placed in an open ceramic crucible in the apparatus and heated from room temperature to the temperature of 900 °C with a heating rate of 2 °C/min, under an inert atmosphere of pure nitrogen (99.99%) with a flow at a level of 5 l/min. At the final analysis temperatures, the samples were stabilized for 10 min. Each sample was analyzed in triplicate to observe possible discrepancies in the results. The results were consistent; hence, the deviations were not presented in the charts.

Infrared spectroscopy (FT-IR)

Infrared spectroscopy (FT-IR) was performed using IRAffinity-1S Shimadzu equipped with Specac ATR (transmittance mode, 32 scans, and 400–4000 cm−1 wavelength range).

Textural analysis

Textural properties of the hydrochars were determined by nitrogen adsorption (77 K) with the QUANTACHROME AUTOSORB 1-C apparatus. Before analysis, the samples were degassed at 423 K for 24 h under high vacuum. The specific surface area of tested materials was calculated using the Brunauer–Emmett–Teller (BET) method. The volume of micropores was calculated using the t-plot method (de Boer). The total pore volume was obtained from the amount of adsorbed nitrogen vapor at a relative pressure close to unity. It was assumed that the pores were then filled with liquid adsorbate. The average pore diameter was obtained from the total pore volume and BET surface area, assuming a cylindrical pore geometry. The cumulative pore volume was calculated using the NLDFT method (Horttanainen et al. 2017).

Extraction test

To verify the leachability and bioavailability of elements present in spray-enriched hydrochar (HTC140.SS1, HTC170.SS1, HTC200.SS1), extraction was performed in water (EN 15958:2011) and in neutral ammonium citrate solution (EN 15957:2011) (Izydorczyk et al. 2020).

Water extraction

1 g of spray-enriched hydrochar was weighed and transferred to an Erlenmeyer flask containing 110 ml of ultrapure water. The flask was shaken at 180 rpm at room temperature for 30 min. The sample was then filtered and subjected to multi-element analysis. The experiment was carried out analogously for the three materials.

Neutral ammonium citrate extraction

1 g of material was weighed and transferred to an Erlenmeyer flask containing 100 ml of neutral ammonium citrate at 65 °C. The solution was shaken on an orbital shaker (180 rpm) at a constant temperature of 65°C for 60 min. The filtered solution was diluted to 250 mL and subjected to multi-element analysis. Experiments were performed for individual materials. Leachability and bioavailability were calculated based on Eq. 1:

$$\mathrm{BL}=\frac{{C}_{E}\bullet {V}_{E}}{{C}_{S}\bullet {m}_{S}}\bullet 100$$
(1)

where BL—bioavailability/leachability (%); CE—concentration of elements in the extract (mg/L); VE—volume of the extract (L); Cs—the content of elements in the spray-enriched hydrochar (mg/kg); and mS—the mass of the extracted spray-enriched hydrochar (kg).

Nutrient release

To examine the release of nutrients from the enriched materials (HTC140.SS1, HTC170SS1, and HTC200SS1), a 16 × 20 XK glass column was used. The test material (2 g) was transferred to a column, and then a diluent simulating soil condition (NaNO3, 1% m/m) was applied using a peristaltic pump with a flow rate of 2 mL/min. Samples were taken every 30 min at 3 h, and then after 24, 48, 72, 96, and 120 h. The samples were subjected to multi-element analysis. The experiment was carried out analogously for three materials.

Pot tests

Plant studies in germination tests showed phytotoxic effects at doses greater than 20% (with respect to plant nitrogen requirements). Pot tests were carried out on hydrochars subjected to 30-day air aging and dried at 110 °C for 72 h, enriched with micronutrients (Cu(II), Mn(II), and Zn(II)). To perform pot tests, multiplexes (5 × 4) were used. According to preliminary studies, tests were carried out on cucumber seeds (Cornichon de Paris, Legutko). The tests were carried out on a commercial all-purpose soil with pH = 5.5–6.5. The experiment was carried out for 30 days under constant environmental conditions presented in Sect. "Preliminary usability analysis." The groups were fertilized with prepared enriched hydrochar (HTC140.SS1, HTC170SS.1, and HTC200SS.1) at four doses (with respect to the nitrogen requirements (140 kg N/ha): 1%, 20%, 100%, and 150%). The control groups were the unfertilized group and the group fertilized with preparations from mineral salts with a composition analogous to HTC140.SS1, HTC170SS.1, and HTC200SS.1. On the last day of the experiment, plant growth parameters were evaluated according to the methodology presented in Sect. "Preliminary usability analysis." The dried and mineralized plants were subjected to multi-element analysis.

Analytical methods

Sample preparation

Hydrochars and dried plants (vase tests) were digested in a microwave digestion system (Start D, Milestone, Italy). According to the following scheme, microwave digestion was assisted by the application of ultrapure concentrated acid-aided microwave digestion, according to the following scheme: (1) hydrochar samples (0.5 g) were digested with a two-acid mixture of 7.5 ml HCl and 2.5 ml HNO3; (2) plant biomass samples from the pot test, with a mass of approximately 0.5 g, were digested with 5 ml of HNO3 (Merck, Germany). After digestion, the solutions were filtered and diluted 100 times.

Elemental composition analysis: ICP-OES

The multi-elemental composition was assessed by ICP-OES (inductively coupled plasma—optical emission spectrometry) on a Vista spectrometer (Varian, Australia). The analysis was performed separately for hydrochar, plant materials, and different extracts with full compensation of the effects of the matrix, taking into account the principles of measurement traceability. The nitrogen and carbon content were determined by thermoconductometry using a CN Vario MACRO Cube elemental analyzer (ELEMENTAR Analysensysteme GmbH, Germany).

Statistical analysis

The t analysis of normality of distribution, homogeneity of variance, and statistical differences was performed. The normal distribution was evaluated using the Shapiro–Wilk test (p > 0.05). In the case of a normal distribution, the Mann–Whitney test (p < 0.05) was used to evaluate statistical differences. For results with a normal distribution, homogeneity of variance analysis was performed using the Brown–Forsythe test (p > 0.05). The normality of distribution was then analyzed using the t-test (p < 0.05) or Welch's test (p < 0.05) for homogeneous and heterogeneous variance, respectively. Statistical analyses were performed using Statistica 13.1 software (Tibco, USA).

Results and discussion

Preliminary usability analysis

The use of the HTC process to convert biomass represents a sustainable approach to obtaining new alternative sources of nutrients for plants. Hydrochars are a valuable source of nitrogen (Toufiq Reza et al. 2016), phosphorus, potassium (Xiong et al. 2021), organic carbon, or fertilizer micronutrients (Puccini et al. 2018). To verify the usefulness of the materials from BSG, germination tests were performed on three hydrochars (HTC140.0. HTC170.0, and HTC200.0). The application was carried out at four doses (with respect to nitrogen requirements): 0.1%, 1%, 20%, and 100%.

In all cases, application of a dose above 20% of the nitrogen requirement (> ok. 0.6 g/ pan) caused a phytotoxic effect (Table 1). Growth arrest was observed on day 5 of the tests. The inhibition of germination in the case of hydrochar application was mainly attributed to the presence of polyphenols and volatile fatty acids (Puccini et al. 2018). Other work has also observed that, regardless of the raw material used, fresh hydrochars can have an inhibitory effect on plant growth (Karatas et al. 2022). This can be reduced by washing the material in demineralized water (Bargmann et al. 2013), or subjecting the material to an aging process (Farru et al. 2022). However, promising results were obtained for biometric parameters for groups fertilized with a dose of 0.1–20%, where a statistically significant increase in stem length was observed compared to the non-fertilized group. The best results were observed for the groups with an applied dose of 20%. For the growth parameters of the root ball, it was reported that the length and volume of the root decreased as dose increased, but the data were not statistically significant compared to the unfertilized group. The highest mass of fresh sprouts was obtained for the groups fertilized with a 0.1% hydrochar dose. The 25% increase in biomass compared to the unfertilized group was recorded for the HTC140.0 group.

Table 1 The effect of non-enriched hydrochar dosage on cucumber growth parameters—germination tests
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The results show great potential for the use of hydrochars as a source of nitrogen and micronutrients delivery agents, but it would be necessary to minimize the phytotoxic effect at higher doses. In this study, an attempt was made to eliminate the negative effect in a short-term aging process and successive drying at 110 °C for 72 h. The effectiveness of removing the chemical compounds responsible for the inhibition effect was verified by thermogravimetric analysis and pot tests.

Hydrochar enrichment: results discussion

Due to the sorptive properties of hydrochar, or treated hydrochar (Liu et al. 2020), the materials have been applied as sorbents used in wastewater treatment, flue gas treatment (Deng et al. 2020; Liu et al. 2021), or antibiotic removal (Cheng et al. 2021). Many researchers have attempted to increase the adsorption capacity of hydrochars by modification of the surface or composite formation (Tran et al. 2017; Zhang et al. 2019). Among others, an increase in the sorption capacity of volatile organic compounds has been achieved by treatment with phosphoric acid(V) and activation with potassium hydroxide (Zhang et al. 2019). The high content of nitrogen and other macro- and micronutrients makes this type of material a valuable fertilizer material. There are several reports on research into the agricultural use of hydrochars (Catenacci et al. 2022), but none of the works presents the possibility of using their sorption properties to enrich them with the desired fertilizer elements.

Basic forms of hydrochars are characterized by high total nitrogen contents of 3.18%, 3.52%, and 3.70% for HTC140.0, HTC170.0, and HTC200.0, respectively (Table 2). As expected, as the carbonization temperature increases, the carbon content of the materials increases (HTC140.0: 45.53%, HTC170.0: 53.54%, and HTC200.0: 57.39%). The presence of other macronutrients in the form of oxides ranges from 0.78 to 0.18%. The content of micronutrients (Cu, Mn, Zn, and Fe) does not exceed 200 mg/kg in the raw material.

Table 2 The content of macro- and microelements in materials before and after enrichment in Cu(II), Mn(II), and Zn(II) ions
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This paper proposes two methods for enriching hydrochars with micronutrients (Cu, Mn, and Zn): immersion sorption and the author's spray sorption method. The conventional method of conducting the process in a ternary system leads to enrichment of the material mainly in Cu(II) ions. The sorption of both Zn(II) and Mn(II) ions occurs with much lower efficiency (several times lower compared to Cu(II)), which is due to the interaction between the ions present in solution (Zherebtsov et al. 2017). Furthermore, the use of the immersion method resulted in the loss of macronutrients such as phosphorus, potassium, calcium, and magnesium, which were leached into the solution containing micronutrients during sorption. The release of K+, Ca2+, and Mg2+ ions appears to be directly related to one of the mechanisms of the sorption process, involving ion exchange (Chen et al. 2023). In the case of K+ ions, their intense release may be related to the metal charge in the material, where 2 ions of the element are removed to allow the sorption of one ion of the micronutrient (Carreira et al. 2023). The even distribution of elements in the material, thus avoiding interactions between ions, and no loss of macronutrients can be achieved by enriching the material with the spray method. Hydrochars enriched by this method are characterized by Cu, Mn, and Zn contents in the desired ratio of 1:1:1 (HTC140.SS, HTC170.SS, and HTC200.SS). The advantage of the method is that the proportion of the required elements can be adjusted, and thus no waste is generated in the form of a post-sorption solution. Drying the materials concentrated the components by evaporating excess water (HTC140.SS1, HTC170.SS1, and HTC200.SS1). Further studies focused on spray-enriched materials because of the simplicity of the potential commercialization of the process and its waste-free nature.

Material characterization

Thermogravimetric analysis (TGA)

Thermogravimetric (TG) curves of hydrochars (HTC) obtained at 140–200 °C HTC temperature range are presented in Fig. 1. For clarity, the TG results of the hydrochars after 30 days of aging dried at 110 °C (24 h), enriched hydrochars, and enriched hydrochars after 30 days of aging dried at 110 °C (24 h) are presented in Supplementary, Figure S2 (A-C).

Fig. 1
figure 1

TG course of the spent grain hydrochars (HTC) obtained in the temperature range of 140–200° C

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In the case of biomass materials, the TG results are typical of three stages. Stage I, up to a temperature of 140–150 °C, is mainly to dehydration and light organic compounds. Stage II (the temperature range strongly depends on the biomass type, most often up to temperatures 550–600 °C) can be called active pyrolysis, where compounds forming the structure of the raw material are thermally transformed into gaseous products at a different rate, which depends on the chemical structure of the raw materials. Stage III, called passive pyrolysis, is connected to the formation of solid residues (ash).

The BSG are typical of ca. 10% fat, 25% protein, and 55–60% carbohydrate (Naibaho and Korzeniowska 2021; Iadecola et al. 2022). Low temperature of HTC (140–200 °C) probably caused thermal transformation of lipids and partially proteins, as the activation energy of the mentioned is much lower compared to carbohydrates (34.02, 53.01, and 187.43 kJ mol−1, respectively (Chen et al. 2018)(Chen et al. 2018)(Chen et al. 2018).

About 5% mass loss was observed at temperatures up to 150 °C. In this case, not only dehydration but also lipids and protein thermal degradation can be observed. Both lipids and proteins are to a small degree of thermal degradation below 150 °C (Kebelmann et al. 2013). Above a temperature of 150 °C, the intensity of decomposition increases. In stage II, all components of the raw material are decomposed. As TG results, elemental analysis indicates that a narrow range of HTC temperature affected the thermal stability of the products but to a minor extent. Ca. A 10% increase in carbon content (Table 2) can be observed in Stage II of the TG course (Fig. 1). Carbon structures with a shorter chain undergo more rapid thermal degradation in the case of HTC200.0 compared to HTC products obtained in lower process temperatures. The completion of the thermal degradation process was observed in a similar temperature range (580–600 °C) in the case of all the analyzed raw materials. Solid TG products were measured in the range of 2.53–2.66 wt%. As it can be observed in the case of HTC products processing, drying and impregnation processes (0.5% of the total element content at the impregnation stage) do not influence strongly the TG course (Figure S2). The drying of HTC products for obvious reasons did not affect the solid residue content. Impregnation caused an increase in solid residue up to the value range of 3.19–3.24 wt% (the subsidized micronutrients are not transformed into volatile products, they remain a solid residue, which can be observed in Figure S3).

Infrared spectroscopy (FT-IR)

FT-IR-ATR spectra of HTC products were collected in the wavelength range of 400–4000 cm−1 to identify their chemical functional groups, including carboxyl, hydroxyl, amino, and amide groups—main components of carbohydrates, lipids, and proteins. Results are presented in Figure S4. In the range of ca. 3280 cm−1 (peak maxima), H stretching vibration of -OH groups, primary amines, and N–H stretching, it can be observed that an increase in the HTC temperature flattened the peak. Taking into account the temperature of the process and the activation energy of individual chemical bonds, the proportion of -OH groups is likely to be reduced. The peak located around 3010 cm−1 is associated with unsaturated fatty acids from lipids (C = CH- vibration). The interesting thing is that the peak appears only in case of the sample obtained at the HTC temperature of 140 °C. The spectra region in the range of 3100–2800 cm−1 is characteristic of C-H stretching vibration of lipids and C-H aliphatic chains. Peaks appear in the case of all studied materials, which confirms the TGA results, that the materials did not undergo drastic conversion at the applied HTC parameters. In case of the sample obtained at the temperature of 140 °C, characteristic peak at 1743 cm−1 can be related to ester or carboxylic linkages in lignin and hemicellulose. The HTC obtained at 200 °C does not show the indicated peak (probably the temperature was high enough for partial decomposition of lignin/hemicellulose). In the range of 1750–1650 cm−1, carbonyl groups (C = O) stretching was observed. In the range of 1120–980 cm−1, carbohydrate (C–O–C) of polysaccharides can be observed (Shapaval et al. 2014; Silbir and Goksungur 2019; Iadecola et al. 2022).

Textural analysis

According to the IUPAC classification, the nitrogen adsorption isotherms depicted in Figure S5 are type II isotherms with a little H3-type hysteresis loop. The observed shape of isotherms is characteristic of non-porous or macroporous materials; simultaneously, the hysteresis loop is evidence of capillary condensation, which occurs during multilayer adsorption on samples consisting of aggregates of plate-like particles, which can make slit-like pores.

Table 3 gives the textural characteristics of the brewers’ spent grain hydrochars (HTC) obtained in a 140–200 °C HTC temperature range. As one can see, all the tested samples have low specific surfaces in the range of 1.5–4.8 m2/g, low total pore volume below 0.04 cm3/g, and similar pore diameter (about 28–29 nm) obtained by the non-local density theory method (NLDFT). The influence of the HTC temperature on the textural properties is small and visible, mainly in the average pore diameter, which changes from 42.5 to 32.8 nm with the temperature increase. This observation corresponds to the course of the curves of cumulative pore volumes presented in Figure S6. The hydrochar obtained at 200° C shows a more significant increase in pore volume in the range of 3–10 nm than other samples, demonstrating the greater proportion of narrow pores in the HTC-200.0 case.

Table 3 Textural properties of hydrochars obtained by HTC at 140–200 °C
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Extraction test

The bioavailability of micronutrients was evaluated by extraction tests. Studies clearly show that micronutrients introduced by the spray method are available to plants. Table 4 shows the results obtained for extraction in water and neutral ammonium citrate.

Table 4 Extraction tests in water and neutral ammonium citrate-enriched hydrochars
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The micronutrient balance in the soil has a major impact on crop yields. It is estimated that the problem of micronutrient deficiency affects 5–35% of the world’s soils, which is associated with the occurrence of abiotic stress in plants (Dhaliwal et al. 2022; Gui et al. 2022). Higher values of bioavailability coefficients were obtained with ammonium citrate. This is a positive effect. The use of ammonium citrate is a kind of simulation of soil conditions. Therefore, it can be concluded that micronutrients introduced into the soil in this form will be available to plants, while limiting their leaching during heavy rainfall, as confirmed by the lower bioavailability coefficients obtained for samples subjected to water extraction (Izydorczyk et al. 2020). Supplementation of micronutrients in bioavailable forms for plants has a positive effect in mitigating the effects of climate change and consequently on the quantity and quality of crops (Samoraj et al. 2022). The application of fertilizers containing low-available nutrients for plants causes the need to fertilize the crop several times in one growing season (Skrzypczak et al. 2022a). Increased application leads to excessive accumulation of elements in the soil and, consequently, causes environmental concerns (Huang et al. 2023).

Nutrient release

In recent years, interest has grown significantly in the use of nutrient-enriched biochars as a sustainable and innovative method to provide both macronutrients (Fachini et al. 2022) and micronutrients (Skrzypczak et al. 2022a, b). In studies, enriched biochar produced by pyrolysis in the presence of macronutrients has been proven to exhibit slow release (Lustosa Filho et al. 2020). The same conclusions were also reached in the case of biochar enrichment by sorption in micronutrients (Skrzypczak et al. 2022a, b). Valorization of hydrochars formed by HTC processes is not a fully developed direction. Many studies focus on material application, but few propose solutions for additional valorization of processed waste. Li et al. (2021) research proposed enriching hydrochar in phosphorus by conducting the HTC process in concentrated phosphoric acid(V). An enrichment of 20% m/m was achieved. The study did not evaluate the release of the macronutrient and only focused on the effective stabilization of Pb in soils. The release time of elements plays an extremely important role in the process of delivering nutrients to plants (Labus et al. 2021).

In this study, the release of micronutrients from spray-enriched hydrochars was evaluated. Release was carried out through a constant supply of medium in the form of sodium nitrate (1% m/m), which has properties that simulate soil conditions (Fig. 2). The analysis shows that the highest release occurs in the first hour of the process for HTC140SS.1, HTC170SS.1, and HTC200SS.1. The transition of ions into solution from HTC140.SS1 in the first hour is gradual until equilibrium is reached. In the case of HTC170.SS1 and HTC200.SS1, the tech process occurs much faster and a flattening of the curve is observed in the first few minutes. This may be due to the lack of binding to active sites on the surface of the material (Skrzypczak et al. 2022a, b). The rapid release effect is also influenced by the high ionic content of the tested materials, which causes electrostatic cation-cation repulsion (Bajpai and Sharma 2004). After this time, the release decreases significantly. It was observed that as the HTC temperature increased, the materials released the sorbed components faster. The release of components from HTC200.SS1 was almost double that of HTC140.SS1. In each case, the highest release rates were recorded for Zn(II) and Cu(II). In the case of Mn(II), the amount of released ions is significantly lower for all three analyzed variants.

Fig. 2
figure 2

Release of micronutrients with HTC140.SS1 (A), HTC170.SS1 (B), and HTC200.SS1 (C)

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The summarized results indicate that an increase in the production temperature of the material significantly increases the mobility of nutrients into solution. The most favorable properties as a micronutrient transporter in this case are shown by the material produced at 140 °C, which is characterized by a gradual release of components from the first minutes of the release time.

Pot tests

Research on plants is an extremely important part of confirming the effectiveness of fertilizer formulations. Pot tests are applications conducted in the soil, preceding field tests. The literature shows that hydrochars not only promote the improvement of soil conditions, but also have a significant impact on minimizing the effects of fertilizer on the environment (Ro et al. 2016). In soil studies, hydrochar has been shown to prevent nutrient leaching from sandy soils (Ro et al. 2016), minimize nitrogen oxide emissions (Zhou et al. 2018), or promote increased water retention in sandy soils (Abel et al. 2013). In vitro studies indicate inconclusive effects of untreated hydrochar on plant growth (Thuille et al. 2015; Xiong et al. 2021).

Growth inhibition was observed in the HTC170.SS1 and HTC200.SS1 groups with an applied dose of 150% (Table 5). Application of the same dose to HTC140.SS1 did not cause phytotoxic effects. The highest fresh plant mass was obtained for the group fertilized with 1% material produced at 170 °C (52.0 g). The HTC170.SS1 group had the highest root ball parameters: approximately twice the root length, three times the root area, and five times the volume compared to the mineral-fertilized group (M170). The highest chlorophyll content (347 ± 47 mg/m2) and the longest stem length (9.11 ± 3.22) were recorded for the HTC170.SS1 fertilized group (150%). For this group, a correlation was observed that as the dose of material increased, the length of the stem increased, while the parameters of the root ball decreased. The same dependence was not found for the groups fertilized with materials disposed at 140 °C and 200 °C.

Table 5 Effect of enriched hydrochar dosage on plants growth parameters—pot test
Full size table

The content of selected macronutrients and micronutrients in the dry matter is shown in Table 6. Analysis indicates that the nitrogen content of plants increases with increasing doses of hydrochar. A particular increase can be observed in the group fertilized with a 150% dose of HTC170SS.1, where the nitrogen content increased by approximately 100% compared to the group fertilized with a 1% dose and compared to the group with applied mineral fertilizer (100% dose) (M170). The smallest increase was recorded for the groups fertilized with HTC200SS1. According to reports, the lower the C:N ratio, the faster nitrogen is released into the soil (Brust 2019). In most cases, the C:N ratio for hydrochar is between 20 and 30 (Ebrahimi et al. 2022). The higher dose of hydrochar also affected the bioavailability of the phosphorus present in the soil. There are many reports on the use of hydrochar as a valuable source of both nitrogen and phosphorus (Aragón-Briceño et al. 2021a). Ebrahimi et al. (2022) demonstrated in their work that hydrochar formed in the co-carbonization of sludge and biomass at 180 °C leads to improved availability of phosphorus and soil nitrate. As expected, with increasing dosages of formulations, in most cases there was an increase in Cu, Mn, Zn, and Fe derived directly from the raw material immediately prior to enrichment. The best effects of micronutrient enrichment were observed for the group fertilized with a 100% dose of HTC170.SS1.

Table 6 Contents of macro- and micronutrients in dry mass of sprouts—pot test
Full size table

Conclusions

The study confirmed that the temperature of the HTC process using BSG affects the physicochemical properties of the materials produced. It was observed that as the treatment temperature increases (140 °C, 170 °C, 200 °C), the materials are characterized by an increased carbon and nitrogen content and a larger specific surface area, as well as a decrease in pore diameter. Preliminary usability tests in plants of unenriched materials have shown phytotoxic effects at doses above 20% of nitrogen requirements (140 kg/ha). A novel approach to enrichment of hydrochars by spray sorption allowed optimal proportions of micronutrients from the Cu(II), Mn(II), and Zn(II) groups. In vitro tests confirmed the high bioavailability of nutrients and their low leachability in water. In studies of the kinetics of nutrient release in a solution that simulates soil conditions, it was observed that the higher the processing temperature of brewery waste, the faster the transfer of micronutrients to the nutrient solution. The inhibition effect of plant growth was eliminated by subjecting the materials to 30-day aging tests and sequential drying at 105 °C for 72 h. The soil application of enriched and pretreated hydrochars from brewery waste did not show phytotoxic effects on cucumber growth at optimal doses. In addition, multicomponent analysis showed that the application of hydrochars improves the bioavailability of selected macronutrients. Additional enrichment of the material with micronutrients (Cu(II), Mn(II), and and Zn(II)) resulted in significantly higher contents of these elements in plants compared to groups with applied mineral fertilizers. The study shows that the use of hydrochars from brewery waste has great potential as a nitrogen source and micronutrient carrier for the agrochemical sector.