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Article

Biochar Derived from Sewage Sludge: The Impact of Pyrolysis Temperature on Chemical Properties and Agronomic Potential

by
Justyna Kujawska
1,*,
Edyta Wojtaś
1 and
Barbara Charmas
2
1
Faculty of Environmental Engineering, Lublin University of Technology, 20-618 Lublin, Poland
2
Faculty of Chemistry, Maria Curie-Skłodowska University, 20-031 Lublin, Poland
*
Author to whom correspondence should be addressed.
Sustainability 2024, 16(18), 8225; https://doi.org/10.3390/su16188225
Submission received: 19 August 2024 / Revised: 3 September 2024 / Accepted: 19 September 2024 / Published: 21 September 2024
(This article belongs to the Special Issue Recycling Biomass for Agriculture and Bioenergy Production)
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Abstract

:
The rising volume of sewage sludge from urbanization poses substantial environmental and public health concerns, underscoring the urgency for the implementation of effective waste management strategies. The objective of this study was to evaluate the influence of pyrolysis temperature on the chemical composition and agronomic potential of biochar derived from sewage sludge. The pyrolysis process was conducted at temperatures ranging from 400 °C to 800 °C, and the resulting biochar was analyzed for pH, electrical conductivity, metal content, and carbon fractions. Additionally, phytotoxicity tests were conducted to assess the impact of the biochar on plant germination. The findings indicated that elevated pyrolysis temperatures resulted in an elevated alkalinity, electrical conductivity, and concentration of alkali metals in the biochar. Conversely, these processes resulted in a reduction in total organic carbon content and an increase in heavy metal content, which may limit the potential for biochar to be used in agricultural applications. The phytotoxicity tests indicated that the biochar produced at lower temperatures (400 °C) exhibited positive effects on plant growth when administered at doses of 5 and 10 t·ha−1. Conversely, the biochar produced at higher temperatures (800 °C) demonstrated significant toxicity. The findings indicate that the pyrolysis temperature is a critical factor in determining the suitability of biochar for agricultural applications. The production of biochar at lower temperatures may offer agronomic benefits, whereas the use of higher temperatures increases stability but is associated with the risk of higher heavy metal concentrations.

1. Introduction

Sewage sludge, a by-product of wastewater treatment, contains a complex mixture of organic matter, nutrients, heavy metals, and various contaminants, including pathogenic microorganisms and antibiotics [1,2]. The rising volume of sewage sludge, driven by urbanization and population growth, is intensifying these challenges, necessitating the implementation of effective management strategies to mitigate environmental and health risks [3,4]. One of the principal issues associated with sewage sludge is the presence of antibiotic-resistant microorganisms and bacteria, which present a considerable health risk if not adequately managed. The presence of these pathogens has the potential to facilitate the transmission of disease and contribute to the exacerbation of the antibiotic resistance crisis, particularly when sludge is utilized as a soil amendment [5]. Furthermore, sewage sludge contains a range of heavy metals and other toxic elements that have the potential to contaminate soil and water resources, thereby adding to the challenges associated with the safe disposal and reuse of this material [6,7]. Despite these risks, sewage sludge can be a source of nutrients and organic matter that can increase soil fertility and support agricultural productivity [8,9]. Pyrolysis offers a promising technology for the conversion of sludge into valuable products, including bio-oil and biochar, which can be employed for energy generation and other applications [10]. Key factors affecting the efficiency of pyrolysis, such as temperature, residence time, particle size, and heating rate, can be optimized using advanced modeling techniques such as artificial neural networks [11]. In addition, the analysis of pyrolysis products reveals their potential as a source of energy and as a raw material for the production of high-value chemicals, which is part of the concept of a circular economy and sustainable development. Research into the optimization of pyrolysis conditions and the detailed characterization of the products obtained is an important step towards effective waste management and environmental protection [12]. The properties of biochar, including its adsorption ability, carbon stability, and capacity to enhance soil quality, directly contribute to the realization of individual sustainable development goals (SDGs) set out in the sustainable development goals (SDGs) framework. The adsorptive ability of biochar is of paramount importance for the realization of SDG 6 (clean water), while coal stability is of significant consequence for SDG 13 (activities in the field of climate change). Furthermore, its properties as a soil additive support SDG 2 (zero hunger) and SDG 15 (life on land). The utilization of biomass waste for the generation of biochar is aligned with SDG 12 (responsible consumption) and SDG 7 (clean and reliable energy) through the promotion of resource efficiency and the generation of clean energy. Consequently, biochar emerges as a pivotal technology that facilitates sustainable development through environmental protection, climate change mitigation, and the enhancement of agricultural productivity [13]. The pyrolysis process, which involves the thermal decomposition of organic material in anaerobic conditions, allows sewage sludge to be transformed into a stable form of carbon, namely biochar. This can serve as a long-term carbon store in the soil, thus contributing to carbon sequestration and reducing greenhouse gas emissions [10]. An increase in pyrolysis temperature results in the intensification of dehydration, decarboxylation, and decarbonylation reactions in the biomass, which in turn leads to a reduction in the H/C (hydrogen/carbon) and O/C (oxygen/carbon) ratios in the biochar. These ratios are of great importance in determining the chemical composition and reactivity of the biochar [14]. A reduction in these ratios is frequently sought after as it indicates a greater degree of charring and stability in the final product, thereby rendering the biochar more suitable for a variety of applications, including soil improvement and carbon sequestration. By modifying the H/C and O/C ratios, it is possible to modify the properties of the biochar to suit specific applications, including soil improvement, water treatment, and carbon sequestration. The ability to vary pyrolysis conditions and utilize a range of feedstocks enables the precise tuning of these parameters [15]. The research conducted by Cayuela et al. (2015) demonstrated that biochar with an H/C ratio of less than 0.3 is an extremely effective method for reducing N2O emissions [16] In a study conducted by Wang et al. (2015), it was demonstrated that biochar with a high aromatic carbon content and a low O/C and H/C ratio is more conducive to soil carbon sequestration [17]. Furthermore, the pyrolysis temperature has a marked effect on the properties of biochar, influencing its physical, chemical, and functional characteristics [18]. A number of studies have examined these effects, underscoring the significance of temperature in the development of biochar for specific applications. An increase in pyrolysis temperature results in an expansion of the specific surface area and micropore volume of the biochar. To illustrate, the biochar derived from biosolids exhibited an increase in specific surface area from 48.25 m2·g−1 to 65.74 m2·g−1 and micropore volume from 0.0313 cm3·g−1 to 0.0369 cm3·g−1 with an elevation in temperature from 700 °C to 900 °C [19]. Similarly, the biochar from waste biomass showed increased BET surface area and zeta potential with increasing temperatures from 400 °C to 800 °C, indicating enhanced surface modification and thermal stability [20]. Furthermore, the chemical properties of biochar are also contingent upon temperature. It has been observed that elevated pyrolysis temperatures tend to result in a reduction in yield and nitrogen content, while concurrently increasing the carbon content and aromaticity. This phenomenon can be attributed to the volatility of nitrogen compounds and the intensification of the charring process, which increases carbon retention and reduces water-soluble carbon content [21]. Furthermore, the temperature has an impact on the acid–base properties of biochar. At elevated temperatures, there is a reduction in the number of acid functional groups and an increase in the number of base functional groups, which affects the adsorption capacity [22]. However, it should be noted that the optimal temperature for biochar production may vary depending on the intended use. For instance, biochar produced at lower temperatures, such as 400 °C, has been demonstrated to be a more suitable microbial carrier, promoting plant growth and nitrogen fixation in agricultural applications [23]. A pyrolysis temperature exceeding 400 °C effectively inactivates pathogens and antibiotic-resistant bacteria, thereby reducing their presence in the resulting biochar [24]. A significant challenge lies in the necessity of precisely aligning the pyrolysis temperature with the intended biochar application. This may entail the monitoring of biochar properties to ensure the attainment of the desired product characteristics.
The interest in biochar, a material with a wide range of potential applications, is growing in various fields, including environmental protection, agriculture, and industry. This is due to the unique sorption, antibacterial, and agronomic properties of biochar. The effective sorption of heavy metals, including chromium and cadmium, by biochar can be attributed to the presence of numerous functional groups of oxygen and minerals, which enhance its adsorption capacity [25]. Furthermore, the modification of biochar with nanoparticles of silver and copper results in the development of notable antibacterial and antioxidant properties, in addition to high efficiency in the removal of impurities, such as antibiotics [26].
The use of biochar as a soil additive is becoming increasingly important, as it offers a range of promising agronomic and environmental benefits. To ensure the safe incorporation of biochar into soil, it is essential to select an appropriate dose and to monitor soil health and plant growth, as well as to assess ecotoxicity. The ecotoxicity assessment of biochar is dependent on a number of factors, including the raw material used, the pyrolysis conditions, and the application rate. This is evidenced by numerous studies. The results of ongoing studies indicate that biochar can have both beneficial and harmful effects on the environment. For example, the Cantou study, which evaluated biochar produced from disparate raw materials, determined that 28% of the samples were toxic to organisms. This toxicity was attributed to such factors as elevated salinity and volatile organic compounds present in the biochar produced at lower pyrolysis temperatures (350 °C) [27]. This emphasizes the necessity of taking into account the particular attributes of biochar, including its elemental composition and pyrolysis temperature, when evaluating its ecotoxicity.
Despite the extensive research conducted on the production of biochar and its environmental impact, there remain significant research gaps that require further analysis. The variety of raw materials employed in the production of biochar, including sewage sludge, and the variability of production methods, render the generalization of test results a challenging endeavor. It is therefore evident that further research is required in order to take into account the specific raw materials and production parameters in question, with a view to developing precise guidelines for the production of biochar that is safe for the environment. On this basis, it is possible to indicate the research gap related to the determination of the optimal conditions for the production of biochar, which will allow for the balancing of agronomic benefits in order to minimize the risk of heavy metal pollution. In response to this need, it is necessary to evaluate the impact of varying sewage sludge pyrolysis temperatures on the physicochemical characteristics of biochar, including its chemical composition, heavy metal content, and phytotoxicity indices. The objective of this study was to ascertain the optimal conditions to produce biochar, with a view to maximizing its benefits as a soil amendment while minimizing the risk associated with the presence of harmful substances. This analysis is vital for the formulation of recommendations for the safe and effective utilization of biochar in agricultural contexts, as well as for the comprehension of the long-term impact of its deployment on the natural environment.

2. Materials and Methods

The sewage sludge was obtained from the Hajdów wastewater treatment plant in Lublin. The sludge was subjected to a drying process at 105 °C in a laboratory dryer (Binder Classic Line E Series Dryer, Binder GmbH, Tuttlingen, Germany) for a period of 24 h. The pyrolysis process was conducted in an inert atmosphere (N2, 100 cm3·min−1) by gradually increasing the system temperature to 400 °C, 500 °C, 600 °C, and 800 °C. An increase in temperature of 10 °C per minute was employed for all experimental samples. The resulting biochar was synthesized at the Faculty of Chemistry, Maria Curie-Skłodowska University in Lublin. After pyrolysis, the biochar produced was thoroughly mixed to ensure homogeneity before samples were taken for analysis. Random sampling was used to select portions of biochar for various chemical analyses. The biochar samples were ground and sieved to a mesh size of 2 mm. The use of a 2 mm sieve to filter the biochar has proven an effective method for removing larger impurities that can occur in biochar. Furthermore, the application of biochar with smaller, more uniform particles has been shown to enhance the ease of use. The resulting biochar types were designated BSS400, BSS500, BSS600, and BSS800, which correspond to the pyrolysis temperatures employed for the sewage sludge.
The pH and electrical conductivity (EC) were determined with a soil:water ratio of 1:2.5 using an Orion Versa Star multifunctional device (Thermo Fisher Scientific, Waltham, MA, USA). The carbon (C), organic carbon (TOC), inorganic carbon (TIC), residual organic carbon (ROC), hydrogen (H), and moisture contents of the biochar samples were analyzed using an RC 62 LECO device (Leco Corporation, Saint Joseph, MI, USA). The nitrogen content was estimated by the Kjeldahl method on a Kjeldahl TM 8200 Foss Tecator system (Foss, Hoganas, Sweden). The determination of metals in biochar was conducted using an Agilent 8900 ICP MS Triple Quad mass spectrophotometer (Agilent Technologies, Santa Clara, CA, USA), following the extraction of the samples through digestion with concentrated HCl/HNO3, in accordance with the methodology outlined in USEPA 1997, Method 3051a [28]. All biochar analyses were performed in triplicate for each sample. This repetition allowed for identifying and checking for any inconsistencies or anomalies in the data.
A phytotoxicity test was conducted on a series of soil mixtures, comprising the following types of chernozem with biochar at the following doses: The concentrations of biochar in the soil mixtures were 5 t·ha−1, 10 t·ha−1, 20 t·ha−1, and 40 t·ha−1. The composition of the mixtures and their nomenclature are presented in Table 1.
The toxicity of the mixtures was evaluated using the commercially available Phytotoxkit test kit from Tigret [29]. The test assesses the germination capacity of cress seeds exposed to soil mixtures for a period of three days. The control soil was prepared using unmodified soil. A total of 100 mL of soil was added to the test plate, along with the biochar. The soil under examination was irrigated with distilled water. The water saturation was calculated in accordance with the instructions provided by the test manufacturer. The test plates were incubated at 25 °C for a period of three days. The germination index values were calculated in accordance with the following formula:
GI = E · F,
where:
  • GI—germination index, %;
  • E—ratio of the number of germinated seeds in the tested substrate and the number of germinated seeds in the control;
  • F—ratio of the average length of roots of plants obtained in the tested substrate to the average length of roots in the control [30]

Statistical Analysis

In order to evaluate the discrepancies between the groups and the interconnections between the acquired data, two distinct statistical analyses were conducted. Tukey’s test and correlation analysis were employed for this purpose. Prior to conducting the Tukey test, an analysis of variance (ANOVA) was employed to ascertain whether there were notable discrepancies between the groups. When the results of the ANOVA demonstrated statistical significance (p < 0.05), the subsequent analysis was conducted using the Tukey test, with the objective of identifying specific pairs of groups that exhibited differential characteristics with respect to the variables under examination. The groups were designated with letters, whereby identical letters signify that the groups are homogeneous and exhibit no statistically significant differences [31].
A correlation analysis was employed to evaluate the interrelationship between the pyrolysis temperature and the remaining data. All statistical analyses were conducted using the Statistica package, version 13 (TIBCO Software Inc., Palo Alto, CA, USA). The level of statistical significance was set at p < 0.05 for all analyses.

3. Results and Discussion

Table 2 presents the results of the tests of pH, electrical conductivity (EC), and alkali metal content in biochar. The analysis of these parameters permits an understanding of the impact of the pyrolysis process, including temperature, on the final properties of biochar. This enables the biochar production process to be tailored to specific requirements and potential risks, such as phytotoxicity caused by excess salt, to be mitigated.
The pH of biochar increases with pyrolysis temperature. The tested biochar was characterized by an alkaline pH (pH = 8.1–10.96). The biochar produced at the lowest temperature, 400 °C, showed the lowest pH value, which was significantly different from those of other types of biochar. The increase in pH with increasing pyrolysis temperature is observed for both sewage sludge biochar and biochar produced from other raw materials [32]. Studies show that high pyrolysis temperatures lead to the formation of more alkaline biochar, resulting from the decarboxylation of organic acids and an increase in the concentration of carbonates in the biochar [33]. In addition, the observed increase in pH can be attributed to the increase in inorganic components resulting from the separation of alkali metals from the organic matrix as a result of elevated temperatures [34].
The application of elevated pyrolysis temperatures results in the generation of a biochar with a more alkaline character. As the pyrolysis temperature rises, the organic biomass constituents, including cellulose, hemicellulose, and lignin, undergo complete distribution. This distribution results in the release of volatile compounds (e.g., CO2, CO, CH4, and other organic gases) and leaves a constant residue comprising predominantly coal. The distribution of organic acids and the loss of acidic functional groups (such as carboxylic and phenolic groups) at higher temperatures result in a reduction in the overall acidity of biochar. This is corroborated by Lan et al. studies, who observed a reduction in acid functional groups and an increase in functional groups of bases in the biochar produced from banana peel with an increase in temperature from 100 °C to 500 °C [22].
Additionally, elevated temperatures facilitate the concentration of inorganic minerals (e.g., calcium, magnesium, potassium) in biochar as the organic matter is evaporated. These minerals frequently manifest as oxides, carbonates, or silicates, which are characteristically alkaline. The relative amount of these alkaline compounds in biochar increases with pyrolysis temperature, thereby contributing to its alkalinity. Murtaza et al. (2024) observed an increase in pH and calcium carbonate, potassium, and phosphorus concentration in biochar with an increase in temperature, as well as an increase the alkalinity of the biochar when studying biochar derived from various raw materials, including eggplant and bark Acacia nilotica [35].
Furthermore, an increase in pyrolysis temperature affects the surface and porosity of the biochar, which can consequently impact its alkalinity. In the Kim et al. study, it was demonstrated that the biochar derived from biosolids exhibited an enhancement in the appropriate surface area and volume of micropores as the temperature increased from 700 °C to 900 °C. This resulted in a reinforced interaction with acidic components within the soil, thereby elevating the alkalinity of the biochar [19].
At elevated temperatures during pyrolysis, the biochar undergoes a transformation, becoming increasingly aromatic and graphitic in its structure. An increase in aromaticity results in a reduction in the number of functional groups that could contribute to acidity, which in turn leads to an increase in the pH of biochar [20]. Electrical conductivity values also increase with increasing temperature. The electrical conductivity of biochar pyrolyzed at temperatures above 500 °C increased ten-fold compared to biochar obtained at 400 °C (EC = 64 µS cm−1) and was statistically significantly higher than the EC value for BS400 biochar. Such results indicate an increase in soluble salt content with increasing pyrolysis temperature. However, a review of the literature shows that the electrical conductivity of sewage sludge biochar varies. Yuan et al. and Huang et al. showed that the EC values in sewage sludge biochar decreased with the increasing pyrolysis temperature of the sludge [36,37]. On the other hand, in the studies by Pendio et al. and Gasco et al., similar to the studies presented here, the value of the electrical conductivity increases [38,39]. Differences in electrical conductivity values can result from differences in sludge composition, temperature, and pyrolysis time. These differences affect the salt content and solubility, resulting in variable EC results in the studies. High-salinity biochar should be applied to soil under controlled conditions to reduce the risk of phytotoxicity [40].
Biochar is characterized by high alkali metal contents, and the dominant alkali metal is calcium. The concentration of sodium, magnesium, potassium, and calcium in biochar increases statistically significantly with pyrolysis temperature. The concentration of these metals in the biochar produced at 800 °C is more than twice as high as in the biochar produced at 400 °C. The high content of alkali metals in biochar may cause problems in the choice of biochar management method. The high content of K, Ca, and Mg in biochar is beneficial when biochar is used as a soil amendment. These elements are essential nutrients for plants and can improve soil fertility by increasing mineral availability. Calcium in biochar can act as a de-acidifier, neutralizing soil acidity and improving soil structure [41]. As shown by Zhao et al., sewage-sludge-derived biochar contains higher concentrations of alkali metals than biochar of plant and animal origin. The high concentrations of alkali metals are due to the composition of sewage sludge, which is rich in both organic matter and inorganic metals [42]. The study analyzed the effect of pyrolysis temperature on the content of heavy metals in biochar obtained from sewage sludge. The analysis of heavy metals in biochar allows for the assessment of environmental hazards, especially in the context of introducing biochar into soil, agriculture, and land reclamation. During the pyrolysis of sewage sludge, heavy metals may be concentrated or redistributed, affecting their mobility and availability in the environment [43]. The results of the heavy metal content are presented in Figure 1 and Figure 2, which show that the tested biochar has different concentrations of heavy metals. It was observed that increasing the pyrolysis temperature leads to an enrichment of the biochar in most of the heavy metals, except for cadmium and lead. The results obtained were compared with the specific classes defined by the European Biochar Certificate (EBC) guidelines for the use of biochar. These classes have specific thresholds and ecological requirements adapted to the intended use in both organic (EBC-AgroOrganic) and conventional (EBC-Agro) agriculture [44]. In addition, they were compared with the heavy metal concentrations in biochar defined by the International Biochar Initiative (IBI). The IBI guidelines were developed to ensure the safe use of biochar in agriculture and soil remediation [45].
In the analyzed biochar types, the concentrations of cadmium and lead showed statistically significant differences between all types of biochar. The concentrations of cadmium in the biochar produced at temperatures of 500 °C and 600 °C (0.23 mg kg−1, 0.34 mg kg−1) were ten times lower than in the biochar obtained during pyrolysis at temperatures of 400 °C and 800 °C (3.17 mg kg−1, 4.2 mg kg−1). The results indicated that the cadmium concentration in BS400 and BS800 biochar exceeded the EBC-Agro limit of 0.7 mg kg−1, whereas for BS500 and BS600 biochar, the cadmium concentration remained within both this limit and the higher EBC-Agro limit of 1.5 mg kg−1.
The highest lead concentration (54.25 mg kg−1) was found in the biochar obtained at the lowest pyrolysis temperature of 400 °C, and this value exceeded the lead concentration limit specified for EBC-AgroOrganic. The remaining biochar types tested were within the lead concentration limits specified for both EBC-AgroOrganic and EBC-Agro. According to the European Biochar Certificates (EBC) guidelines, the maximum allowable lead concentrations are 45 mg kg−1 for EBC-AgroOrganic and 150 mg kg−1 for EBC-Agro.
The concentrations of chromium, copper, nickel, and zinc in the biochar tested increased as the pyrolysis temperature of the sewage sludge increased. The concentrations of these metals in the biochar produced at pyrolysis temperatures of 400 °C and 500 °C showed statistically significant differences compared to the biochar obtained at temperatures of 600 °C and 800 °C. On the other hand, the differences in metal concentrations between the biochar produced at 600 °C and 800 °C were not statistically significant. All the biochar types tested significantly exceeded the allowable limits for copper (70 mg kg−1 for EBC-Agroorganic and 100 mg kg−1 for EBC-Agro), nickel (25 mg kg−1 for EBC-Agroorganic and 50 mg kg−1 for EBC-Agro), and zinc (200 mg kg−1 for EBC-Agroorganic and 400 mg kg−1 for EBC-Agro) specified in the European Biochar Certificate guidelines. For chromium, samples BS400 and BS600 exceeded the limits set for the EBC-Agro category (70 mg kg−1 for EBC-Agroorganic and 90 mg kg−1 for EBC-Agro).
The biochar types tested did not meet the requirements of the heavy metal concentration limits specified by the European Biochar Certificate, but were within the ranges of heavy metal concentrations in biochar specified by the International Biochar Initiative. The ranges of heavy metal concentrations specified by the International Biochar Initiative (IBI) are up to 200 mg kg−1 for lead, up to 1.4 mg kg−1 for cadmium, up to 120 mg kg−1 for chromium, up to 600 mg kg−1 for copper, up to 60 mg kg−1 for nickel, up to 450 mg kg−1 for zinc, and up to 1 mg kg−1 for mercury [45]. Similarly, when the concentrations of heavy metals were compared with the permissible contents of these metals in sewage sludge in the countries of the European Union [46], all the tested biochar samples contained significantly lower amounts of heavy metals than the limits that may occur in sewage sludge used in agriculture and land reclamation for agricultural purposes. According to the Regulation of the Minister of Climate and Environment on the use of municipal sewage sludge, these values cannot exceed: for cadmium, 20 mg kg−1; for copper, 1000 mg kg−1; for nickel, 300 mg kg−1; for lead, 750 mg kg−1; for zinc, 2500 mg kg−1; and for chromium, 500 mg kg−1 [46].
When compared with the results of other studies, the results obtained are consistent with the general trends (Table 3). In the study by Chun et al., the metal content in the biochar from sewage sludge increased with pyrolysis temperature up to 500 °C; above this temperature, the metal concentration remained constant. The exception was cadmium, the concentration of which was about 100 mg kg−1, and the lowest content was found at the highest pyrolysis temperature. In the conducted study, the metal concentrations were different: zinc and chromium levels were nearly half as much, copper levels were reduced to one-fourth, and cadmium and lead concentrations were found to be ten times greater [47]. In contrast, studies by Lu et al. (2015) showed that the concentration of heavy metals in biochar from sewage sludge increased slightly with pyrolysis temperature, except for Cd, as in the presented studies. The differences in concentration between biochar types were not as significant as in the presented studies; for example, the concentration of chromium in the biochar produced at 400 °C was 100 mg kg−1, while in the biochar produced at 700 °C, the concentration was 103 mg kg−1 [48]. Gasco et al. studied the properties of three types of biochar produced from different raw materials: wood chips, paper sludge mixed with wheat hulls, and sewage sludge. An analysis of the heavy metal content of these biochar types showed that the highest concentrations of heavy metals were found in the biochar produced from sewage sludge. The concentration of lead in sewage sludge biochar was 138 mg kg−1 compared to 17.94 mg kg−1 in the biochar from paper sludge mixed with wheat husks and only 2.26 mg kg−1 in the biochar from wood chips [39]. These comparative studies are important for increasing the knowledge about biochar and developing safe methods for its management.
Differences in heavy metal concentrations in the biochar from sewage sludge are mainly due to the chemical properties of this sludge. The variable nature of the raw materials and the different conditions under which the experiments were conducted confirm the instability of biochar properties and indicate the need for monitoring studies.
Metals such as Cu, Pb, Cr, Ni, and Zn can occur in various chemical forms such as sulfides, carbonates, and hydroxides, which are characterized by low vapor pressure even at elevated temperatures. This means that even at temperatures of up to 750 °C, these compounds do not evaporate to any significant extent, limiting the release of metals in gaseous form to the atmosphere. In addition, the chemical stability of sulfides, carbonates, and hydroxides means that they do not readily break down into simpler components, further reducing the risk of metal emissions to the environment [33]. At higher pyrolysis temperatures, heavy metals can be converted into less mobile chemical forms, reducing their toxicity and the risk of leaching into groundwater. As a result, biochar can be used more safely in agriculture, minimizing the risk of soil and water contamination. Studies by Lu et al. confirm that the biochar produced from sewage sludge by pyrolysis poses minimal risk to soil and groundwater when used as a soil amendment [48]. The risk of soil and groundwater contamination associated with biochar was found to be much lower than the risk associated with the direct application of sewage sludge. In addition, a meta-analysis by Nkoh et al. found a reduction in the bioavailability of heavy metals due to the adsorption capacity of biochar [50].
The pyrolysis temperature is of significant importance with regard to the stabilization of heavy metals in biochar. The heavy metals present in sediments occur mainly in the form of silicates, phosphates, oxides, carbonates, sulfides, hydroxides, and organic complexes [51]. Consequently, the characteristics of heavy metal migration are closely related to their boiling point temperatures, which are a function of the types of compounds that arise in sediments and reaction conditions [52]. At the boiling point of the metal or one of its accompanying compounds, the reaction temperature reaches a point at which the metal is volatilized, resulting in a reduction in its concentration in biochar.
Prior research has demonstrated that the temperature, apparatus, and pyrolysis techniques exert a considerable influence on the attributes of biochar enrichment with heavy metals, the dispersion of chemical traits, the toxicity of leaching, and the bioavailability of coal [53]. The studies conducted by Liu et al. have demonstrated that metals, such as Cu, Cr, and Ni, are prone to migration during the pyrolysis process. This is likely attributable to their robust thermal stability, which renders these metals and their compounds resistant to decomposition or volatility, thereby enabling their retention in the bio-oil [54]. The findings of Devi and Saroha, and Shao et al. also corroborate that as the pyrolysis temperature rises, Cu, Cr, and Ni, which in sediments occur in bioavailability, combine with organic matter and become more stable [55,56].
Zhang et al. studied the lead content in sewage sludge after pyrolysis and found that the lead concentration in sewage sludge after pyrolysis at 450 °C was over 90% lower than in the sludge before pyrolysis. However, at 550 °C, the lead concentration increased again [53]. Wang and Tomita suggest that the volatilization of Pb is caused by the reaction of its oxide with carbon at high temperatures, leading to the reduction of lead to a gas and its subsequent volatilization [57].
In studies on biomass pyrolysis, Stals et al. found that a significant amount of cadmium (Cd) migrated from biochar to the liquid phase at temperatures between 350 °C and 550 °C. This transformation was associated with the decomposition of CdCO₃ at around 400 °C, leading to the formation of CdO and CO2. CdO was then reduced by carbon at around 650 °C, causing the volatilization of cadmium in gaseous form [58]. However, Liu et al. (2018) demonstrated that the Cd content in biochar remained high at 950 °C [54].
These differences may reflect significant variations in the composition of the sludge used in different experiments.
The results of the total organic carbon (TOC), total inorganic carbon (TIC), residual organic carbon (ROC), and dissolved organic carbon (DOC) contents are presented in Table 4. An analysis of the changes in carbon content as a function of pyrolysis temperature helps to optimize the biochar production process in order to obtain a material with the desired properties that can be used, for example, in agriculture, soil remediation, or carbon sequestration.
The total organic carbon content decreases statistically significantly with increasing pyrolysis temperature. The biochar produced at 800 °C has half the TOC of the biochar produced at 400 °C. The TOC content of biochar is crucial for assessing its stability, nutrient content, and overall effectiveness in various applications. The studies by Tag et al. also show a decrease in TOC in sludge-derived biochar with increasing temperature. However, both total organic carbon and total carbon values are lower compared to biochar from plant biomass, where the TOC content for the biochar from grape waste is 72.3%, 68.1% from orange pomace, and 56.6% from poultry manure [59]. In Paiva’s control studies, it was shown that increasing the pyrolysis temperature of materials such as pine bark, eucalyptus sawdust, bamboo, bagasse, coffee husks, and olive cake caused an increase in the TOC content of biochar. On the other hand, when chicken manure was used as a raw material, as in the studies, the TOC content increased with the pyrolysis temperature [21]. This may have occurred because the emissions from sewage sludge and animal waste are unstable and quickly released with an increase in pyrolysis temperature, even before supplying the biochar with recalcitrant carbon compounds [60].
Biochar contains low concentrations of inorganic carbon, with no statistically significant differences between the samples tested. Despite the intense release of volatile organic carbon during pyrolysis at high temperatures, this process does not affect the inorganic carbon content as it mainly affects organic carbon.
Dissolved organic carbon refers to any type of dissolved organic carbon that can originate from different sources, both natural and anthropogenic. In the biochar studied, DOC values decreased with increasing pyrolysis temperature. The highest value (65.03%) was obtained at 400 °C, which was ten times higher than in other biochar types. The lowest DOC values, which were not statistically significant, were recorded for the biochar produced at temperatures of 600 °C and 800 °C. The studies conducted by Mancinelli et al., Albuquerque et al., Zhang et al., and Lin et al. have documented a decrease in dissolved organic carbon (DOC) concentrations with increasing pyrolysis temperature [61,62,63,64]. In a study conducted by Smith et al. (2016), a 100 °C increase in pyrolysis temperature resulted in a 41–44% reduction in DOC content for lignin-derived biochar, particularly in the 300–500 °C temperature range [65]. During pyrolysis, DOC can be broken down into simpler molecules or incorporated into stable forms of carbon, such as those present in biochar. These processes can occur even at relatively low temperatures, showing that DOC is susceptible to transformation even in the early stages of pyrolysis [60,64].
Residual organic carbon (ROC) is defined as the portion of total organic carbon (TOC) formed at temperatures between 450 °C and 600 °C. It is characterized by a high degree of stability and resistance to degradation. Typically, ROC is found in chemical bonds within stable compounds. The residual organic carbon content of the biochar exhibited a statistically significant correlation with the pyrolysis temperature of the sludge, demonstrating an increase in this parameter with rising temperature. The highest residual organic carbon (ROC) value was recorded for the biochar produced at 800 °C (13.67%), which was ten times higher than the ROC value in biochar produced at 400 °C. Residual organic carbon is more stable and less susceptible to degradation, which indicates that it can remain in the soil for an extended period, potentially lasting for hundreds to thousands of years. This makes it crucial for long-term carbon sequestration, as it facilitates the retention of carbon in ecosystems over extended periods, thereby reducing its emission to the atmosphere in the form of CO2 [66]. The high residual organic carbon values observed in sewage sludge biochar indicate that the biochar contains a substantial quantity of carbon compounds that are resistant to degradation. This may indicate a high potential for the long-term carbon sequestration of biochar, which is beneficial in the context of reducing carbon dioxide emissions to the atmosphere. Furthermore, the presence of residual organic carbon may also influence the chemical and physical stability of biochar, rendering it a more durable material for utilization in soil enhancement and long-term carbon sequestration.
The impact of pyrolysis temperature on the composition of biochar, specifically the concentrations of carbon (C), hydrogen (H), and oxygen (O), was investigated. The findings are presented in Table 5. The elemental analysis of the biochar subjected to varying pyrolysis temperatures revealed a notable decline in the concentrations of carbon (C), hydrogen (H), and oxygen (O) with increasing temperature. During pyrolysis, a greater loss of oxygen was observed in comparison to hydrogen; however, these differences were not statistically significant. As a consequence of the accelerated loss of oxygen, biochar is distinguished by a low H/C ratio [67]. The atomic ratios of O/C and H/C can be employed to assess the durability of biochar and its impact on the environment [12]. The tested biochar exhibited low H/C ratios, ranging from 0.04 to 0.07. In the studies conducted by Ippolito et al. [67] the H/C ratio in the biochar derived from manure and sewage did not undergo any alteration with the alteration of pyrolysis temperature. A lower H/C ratio is indicative of a higher degree of aromaticity and greater structural stability of the biochar [68]. The findings of Cayuela et al. (2015) indicate that the biochar with an H/C ratio of less than 0.3 has the potential to significantly reduce nitrous oxide emissions, making it an effective tool in climate change mitigation strategies [16].
The O/C ratio in the biochar subjected to testing decreased in conjunction with the rise in temperature at which the sewage sludge underwent pyrolysis. The reduction in O/C ratios indicates that at elevated temperatures, greater quantities of aromatic carbon structures are formed, which results in these biochar types exhibiting enhanced chemical stability and reduced susceptibility to biological degradation [69]. The O/C ratios of the BSS600 and BS800 biochar types were found to be less than 0.2, which, according to the criteria proposed by Spokas, indicates that the half-life of these biochar types is greater than 1000 years. Conversely, the O/C ratios in the BSS400 and BSS500 biochar fell within the range of 0.2 to 0.6. In this case, the half-life of the biochar is estimated to be between 100 and 1000 years, in accordance with the criteria proposed by Spokas et al. [70]. As demonstrated by Spokas et al. (2010), the O/C ratios in the biochar derived from forestry sources (0.58–0.68) and agricultural biomass (0.55–0.73) are markedly higher than those observed in the biochar produced from natural and fossil fuels (0.01–0.38) [70]. In accordance with the standards for biochar set forth by the International Biochar Initiative, BSS500, BSS600, and BSS800 biochar types may be employed for the purpose of carbon sequestration within the environment, thereby limiting the emission of carbon to the atmosphere. Additionally, the H/C ratio value must not exceed 0.6, as this is one of the parameters utilized to distinguish the biochar that is approved for use in agricultural settings [45].
A correlation analysis was performed between pyrolysis temperature and the tested properties of biochar to identify the relationship between these parameters. Correlation analysis allows for the determination of how the change in pyrolysis temperature affects various properties of biochar. This allows for the determination of the temperatures that are conducive to obtaining biochar with the desired properties, including those that are non-toxic and suitable for practical applications. The results of this analysis are presented in Table 6.
A strong positive correlation (r = 0.9) was identified between the pyrolysis temperature and the content of K, Na, Cu, Zn, Ni, Ca, Mg, ROC, Cr, H, and Na, as well as pH and conductivity. This indicates that as the temperature increases, the values of these parameters tend to decrease. The N, O, H/C, and O/C ratios exhibited a strong negative correlation, indicating a notable decline in these values with elevated pyrolysis temperatures. The negative correlations observed for elements such as carbon (C) and oxygen (O) indicate that elevated pyrolysis temperatures may result in enhanced carbonization and the loss of volatile compounds. De Morais investigated the relationship between pyrolysis temperature and biochar properties, with a particular focus on nitrogen content and electrical conductivity. Similarly, as observed in the aforementioned studies, there is a negative correlation between temperature and total nitrogen content [71].

Phytotoxicity Analysis

A phytotoxicity analysis was conducted to assess the effect of biochar on the germination index (GI) of cress. Figure 3 presents the results of the phytotoxicity test of soil (chernozem) mixtures with biochar added at different rates (5 t·ha−1, 10 t·ha−1, 20 t·ha−1, and 40 t·ha−1). The aim of the study was to understand which conditions associated with the use of biochar are most favorable for plant growth and which may limit their development.
The highest germination index (GI) values were observed for the mixture with biochar pyrolyzed at 400 °C, at a dose of 5 t·ha−1 (101.32%), which suggests that under these conditions, biochar provides the optimal conditions for plant growth. The lowest germination indices (GI) were observed in mixtures with biochar pyrolyzed at 800 °C, particularly at doses of 10t and above. This suggests that germination is limited under these conditions. The statistical analysis demonstrated that the GI values in mixtures with biochar pyrolyzed at temperatures of 400 °C and 800 °C exhibited a statistically significant difference. In comparison to the toxicity criteria outlined by Emino and Warman (2004), it can be concluded that the mixture comprising biochar pyrolyzed at 400 °C at a dose of 40 t·ha−1, that with biochar pyrolyzed at 600 °C at a dose of 40 t·ha−1, and mixtures with biochar pyrolyzed at 800 °C at each applied dose exhibits high toxicity (GI < 50%). The mixtures with biochar pyrolyzed at 400 °C and 500 °C at a dose of 5 t·ha−1 exhibited a GI value greater than 100, indicating their potential use as a phytonutrient. The mixture with biochar pyrolyzed at 500 °C at a dose of 10t reached a GI value of 93.71%, which suggests no phytotoxicity, as it falls within the range of GI > 80%. The remaining mixtures exhibited a GI range of 50–80%, indicating a moderate degree of phytotoxicity [72]. The results of the conducted studies demonstrate that the dose of biochar and the pyrolysis temperature have a significant impact on plant germination. Gondek’s literature review indicates that plant growth is influenced by multiple factors, including the dose of biochar, the pyrolysis temperature, the type of biochar, the type of raw material from which the biochar is produced, and the method of conducting the pyrolysis process [73]. Mierzwa et al. (2016) conducted a study to assess the ecotoxicity of the biochar obtained from poultry waste. The results of their studies indicated that soil with a lower addition of biochar (2.5 t·ha−1) exhibited greater toxicity than soil with a higher dose (5 t·ha−1) [74]. The studies conducted by Gasco et al., Mierzwa et al., and Błaszczyk et al. on the ecotoxicity of various types of biochar yielded inconclusive results regarding the impact of biochar on plants. The data collected indicated both positive and negative effects [5,39,75].
The increasing prevalence of biochar as a soil amendment necessitates the implementation of monitoring studies, complemented by a more robust program of ecotoxicological assessments in soil matrices, particularly in field settings. Furthermore, there is a pressing need for research into the mechanisms of biochar action on organisms. This will facilitate the safe and effective utilization of biochar in diverse settings, thereby minimizing the potential for adverse environmental impacts while optimizing the advantages associated with its deployment.

4. Conclusions

Testing shows that the pyrolysis temperature significantly affects the properties and agronomic potential of biochar derived from sewage sludge. As the pyrolysis temperature is increased, the alkalinity of the resulting biochar is observed to increase. Furthermore, the electrical conductivity (EC) and concentrations of alkali metals, including sodium, magnesium, potassium, and calcium, also increase with rising temperature. This indicates that the biochar produced at higher temperatures possesses a higher salinity.
Furthermore, the pyrolysis temperature influences the concentration of heavy metals in biochar. An increase in temperature results in an elevated concentration of most heavy metals, which may impede the safe utilization of biochar in agriculture, particularly if they exceed the permissible standards. Nevertheless, the concentrations of heavy metals in the tested biochar remain below the limits specified by IBI, indicating the potential for their use, provided that appropriate monitoring and risk assessment are conducted.
The production of biochar at elevated temperatures results in a reduction in the H/C and O/C atomic ratios, which suggests enhanced stability and the potential for long-term soil carbon sequestration.
The phytotoxicity results indicate that biochar produced at lower temperatures, such as 400 °C, may enhance plant growth, whereas biochar produced at higher temperatures (600–800 °C), particularly at rates exceeding 10 t ha−1, may exhibit phytotoxic properties. The optimal biochar rate is contingent upon the pyrolysis temperature and has the potential to significantly influence plant germination rates.
The results of the correlation analysis confirm the existence of strong correlations between the pyrolysis temperature and the chemical parameters of the biochar, thereby emphasizing the necessity of precisely tailoring the pyrolysis process to the intended use of the biochar.
The study demonstrated that pyrolysis temperature is a pivotal factor in determining the chemical properties of sewage-sludge-derived biochar and its agronomic potential. The optimization of the pyrolysis process has the potential to result in the production of biochar with desirable properties, both in terms of enhancing soil quality and carbon sequestration, while concurrently reducing the risk of contaminant content.
It is recommended that further studies be conducted to assess the ecotoxicity of biochar, particularly in field conditions, in order to ensure its safe and effective use in agriculture.

Author Contributions

Conceptualization, J.K. and E.W.; methodology, E.W. and B.C.; software, J.K.; validation, J.K.; formal analysis, J.K.; investigation, J.K., E.W. and B.C.; resources, J.K.; data curation, J.K. and E.W.; writing—original draft preparation, J.K. and E.W.; writing—review and editing, J.K. and E.W.; visualization, J.K.; supervision, B.C.; project administration, J.K.; funding acquisition, J.K. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.

Acknowledgments

This research was funded in whole by National Science Centre, Poland, Miniatura 7 [DEC-2023/07/X/ST10/00655].

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Concentration of cadmium, chromium, nickel, and lead in biochar produced at pyrolysis temperatures of 400 °C, 500 °C, 600 °C, and 800 °C. The letter indicators at the average content values indicate the statistically homogeneous groups (Tukey Homogeneous Groups). The presence of the indicator designates the lack of a statistically significant difference between them.
Figure 1. Concentration of cadmium, chromium, nickel, and lead in biochar produced at pyrolysis temperatures of 400 °C, 500 °C, 600 °C, and 800 °C. The letter indicators at the average content values indicate the statistically homogeneous groups (Tukey Homogeneous Groups). The presence of the indicator designates the lack of a statistically significant difference between them.
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Figure 2. Concentration of zinc and copper in the biochar types produced at pyrolysis temperatures of 400 °C, 500 °C, 600 °C, and 800 °C. The letter indicators at the average content values indicate the statistically homogeneous groups (Tukey Homogeneous Groups). The presence of the indicator designates the lack of a statistically significant difference between them.
Figure 2. Concentration of zinc and copper in the biochar types produced at pyrolysis temperatures of 400 °C, 500 °C, 600 °C, and 800 °C. The letter indicators at the average content values indicate the statistically homogeneous groups (Tukey Homogeneous Groups). The presence of the indicator designates the lack of a statistically significant difference between them.
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Figure 3. The germination index for the tested mixtures with biochar. The letter indicators at the average content values indicate the statistically homogeneous groups (Tukey Homogeneous Groups). The presence of the indicator designates the lack of a statistically significant difference between them.
Figure 3. The germination index for the tested mixtures with biochar. The letter indicators at the average content values indicate the statistically homogeneous groups (Tukey Homogeneous Groups). The presence of the indicator designates the lack of a statistically significant difference between them.
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Table 1. The composition of the mixtures employed in the ecotoxicological test.
Table 1. The composition of the mixtures employed in the ecotoxicological test.
NameSewage Sludge Pyrolysis TemperatureDose
PBSS400_5400 °C5 t·ha−1
PBSS400_10 10 t·ha−1
PBSS400_20 20 t·ha−1
PBSS400_40 40 t·ha−1
PBSS500_5500 °C5 t·ha−1
PBSS500_10 10 t·ha−1
PBSS500_20 20 t·ha−1
PBSS500_40 40 t·ha−1
PBSS600_5600 °C5 t·ha−1
PBSS600_10 10 t·ha−1
PBSS600_20 20 t·ha−1
PBSS600_40 40 t·ha−1
PBSS800_5800 °C5 t·ha−1
PBSS800_10 10 t·ha−1
PBSS800_20 20 t·ha−1
PBSS800_40 40 t·ha−1
Table 2. Physicochemical properties of biochar.
Table 2. Physicochemical properties of biochar.
ParametersDescriptionUnitsBSS400BSS500BSS600BSS800
pH 8.1 a10.96 b10.61 b10.43 b
EC µS·cm−164 a570 b657 c600 c
NaMeanmg·kg−11041.87 a864.17 b1671.22 c2073.14 d
SD 0.332.972.461.59
MgMeanmg·kg−15192.49 a4373.4 b8235.40 c10,850.50 d
SD 0.561.280.981.97
KMeanmg·kg−13134.80 a2464.29 b4325.49 c6096.55 d
SD 0.210.602.981.60
CaMeanmg·kg−152,571.49 a43,165.29 b77,753.90 c107,067 d
SD 1.750.660.791.52
The letter indicators at the average content values indicate the statistically homogeneous groups (Tukey Homogeneous Groups). The presence of the indicator designates the lack of a statistically significant difference between them.
Table 3. Comparison of heavy metal content in biochar derived from sewage sludge by different researchers.
Table 3. Comparison of heavy metal content in biochar derived from sewage sludge by different researchers.
Sewage Sludge Pyrolysis TemperatureContent of Heavy Metals
[mg·kg−1]		References
400 °CCd = 4.1; Cr = 3.9; Cu = 3.9; Ni = 5.2; Pb = 43.2; Zn = 52.7[37]
500 °CCd = 4.0; Cr = 3.8; Cu = 2.1; Ni = 2.1; Pb = 36.6; Zn = 3.3
600 °CCd = 4.1; Cr = 3.8; Cu = 3.9; Ni = 2.4; Pb = 38.5; Zn = 10.1
700 °CCd = 4.1; Cr = 3.8; Cu = 2.9; Ni = 2.6; Pb = 19.9; Zn = 6.4
350 °CCd = 1.53; Cr = 110.59; Cu = 134.67; Ni = 37.38; Pb = 62.39; Zn = 2060[49]
550 °CCd = 2.01; Cr = 142.9; Cu = 169.93; Ni = 49.31; Pb = 78.54; Zn = 2603
750 °CCd = 0.4; Cr = 150.93; Cu = 178.91; Ni = 50.18; Pb = 81.65; Zn = 2763
400 °CCd = 225; Cr = 118; Cu = 213; Ni = 95.4; Pb = 4900; Zn = 986[33]
500 °CCd = 235; Cr = 106; Cu = 215; Ni = 97.7; Pb = 5120; Zn = 1040
600 °CCd = 229; Cr = 106; Cu = 209; Ni = 101; Pb = 5250; Zn = 1090
700 °CCd = 123; Cr = 103; Cu = 227; Ni = 103; Pb = 5250; Zn = 1090
400 °CCd = 3.17; Cr = 43.77; Cu = 452.28; Ni = 72.34; Pb = 54.25; Zn = 1039.31
500 °CCd = 0.23; Cr = 109.13; Cu = 572.15; Ni = 87.16; Pb = 11.24; Zn = 1713.39[our study]
600 °CCd = 0.34; Cr = 134.13; Cu = 800.4; Ni = 129.66; Pb = 13.73; Zn = 2000.61
800 °CCd = 4.2; Cr = 174.08; Cu = 1252.11; Ni = 181.17; Pb = 36.3; Zn = 3330.7
Table 4. Total organic carbon (TOC), total inorganic carbon (TIC), residual organic carbon (ROC), and dissolved organic carbon (DOC) in the studied biochar.
Table 4. Total organic carbon (TOC), total inorganic carbon (TIC), residual organic carbon (ROC), and dissolved organic carbon (DOC) in the studied biochar.
Type of BiocharTOCTICROCDOC
%
BS40032.33 a0.18 a1.99 a65.03 a
BS50023.26 b0.09 b2.67 b5.53 b
BS60011.54 c0.07 b8.19 c2.67 c
BS80017.89 d0.09 b13.67 d2.41 c
The letter indicators at the average content values indicate the statistically homogeneous groups (Tukey Homogeneous Groups). The presence of the indicator designates the lack of a statistically significant difference between them.
Table 5. The elemental composition of the biochar.
Table 5. The elemental composition of the biochar.
Type of BiocharElemental CompositionAtomic Ratio
CHNOH/CO/CC/N
%
BS40034.51 a1.31 a4.21 a23.07 a0.040.677.67
BS50026.03 b1.44 b2.87 b8.7 b0.060.338.10
BS60025.28 b0.81 c1.01 c3.42 c0.070.1311.42
BS80026.17 b0.76 d1.36 c2.3 d0.040.0913.15
The letter indicators at the average content values indicate the statistically homogeneous groups (Tukey Homogeneous Groups). The presence of the indicator designates the lack of a statistically significant difference between them.
Table 6. The Pearson r correlation coefficient values between pyrolysis temperature and the chemical parameters of biochar.
Table 6. The Pearson r correlation coefficient values between pyrolysis temperature and the chemical parameters of biochar.
r r
pH0.558TOC−0.685
EC0.692TIC−0.644
Na0.910ROC0.978
Mg0.982DOC−0.709
K0.997C−0.659
Ca0.991H0.956
Cd0.371N−0.823
Cr0.952O−0.790
Cu0.994H/C−0.810
Ni0.992O/C−0.814
Pb-0.175
Zn0.993
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Kujawska, J.; Wojtaś, E.; Charmas, B. Biochar Derived from Sewage Sludge: The Impact of Pyrolysis Temperature on Chemical Properties and Agronomic Potential. Sustainability 2024, 16, 8225. https://doi.org/10.3390/su16188225

AMA Style

Kujawska J, Wojtaś E, Charmas B. Biochar Derived from Sewage Sludge: The Impact of Pyrolysis Temperature on Chemical Properties and Agronomic Potential. Sustainability. 2024; 16(18):8225. https://doi.org/10.3390/su16188225

Chicago/Turabian Style

Kujawska, Justyna, Edyta Wojtaś, and Barbara Charmas. 2024. "Biochar Derived from Sewage Sludge: The Impact of Pyrolysis Temperature on Chemical Properties and Agronomic Potential" Sustainability 16, no. 18: 8225. https://doi.org/10.3390/su16188225

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