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Article

Influence of Technical Parameters of the Pyrolysis Process on the Surface Area, Porosity, and Hydrophobicity of Biochar from Sunflower Husk Pellet

by
Katarzyna Wystalska
,
Anna Kwarciak-Kozłowska
* and
Renata Włodarczyk
Faculty of Infrastructure and Environment, Czestochowa University of Technology, Dabrowskiego 69, 42-201 Czestochowa, Poland
*
Author to whom correspondence should be addressed.
Sustainability 2023, 15(1), 394; https://doi.org/10.3390/su15010394
Submission received: 25 November 2022 / Revised: 16 December 2022 / Accepted: 22 December 2022 / Published: 26 December 2022
(This article belongs to the Section Environmental Sustainability and Applications)
Browse Figures
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Abstract

:
Biochar is a product that has been of interest to many researchers in recent years. The use and positive effect of biochar depend on its properties, which in turn result primarily from the type of substrate used for production and the technical parameters of the pyrolysis process used. From the point of view of sustainable development, agricultural raw materials, such as sunflower husks, are good materials for biochar synthesis. The research aimed to determine the effect of changing the technical parameters of the pyrolysis process (i.e., temperature, heating rate, and residence time) on the properties of biochar obtained from sunflower husk pellets. The pellets were heated to 480 °C, 530 °C, and 580 °C. The applied heating rate for 480 °C was 4.00 and 7.38 °C·min−1, for 530 °C it was 4.42 and 8.15 °C·min−1 and for 580 °C it was 4.83 and 8.92 °C·min−1. Determining these properties is important due to the use of biochar, e.g., in the processes of sorption of pollutants from the water and soil environment. The technical parameters of the pyrolysis process used allowed us to obtain hydrophilic materials with porosity in the range of 10.11% to 15.43% and a specific surface area of 0.93 m2·g−1 to 2.91 m2·g−1. The hydrophilic nature of biochar makes it possible to use them in the processes of removing inorganic pollutants and polar organic pollutants. The presence of macropores in biochar may contribute to the improvement of water management in the soil and affect the assimilation of microelements by plants. The low content of heavy metals in biochar does not pose a threat to the environment.

1. Introduction

The assumptions of sustainable development include, among others: proper management of natural resources, sustainable consumption, production, and care for technological development following nature [1,2]. These assumptions fit in very well with activities aimed at waste management and the production of usable materials from them. The use of agricultural waste as biomass for the synthesis of biofuels and biochar is the right approach to achieve SDGs (sustainable development goals) [3]. Companies producing sunflower oil retain large amounts of sunflower husks, which are most often processed into pellets and then used as fuels. Thermal conversion of biomass allows not only the use of this product as low-emission fuel [4] but also as biochar—a material used in environmental protection, agriculture, or specific industries [5,6], often called a key component of a sustainable economy. From the point of view of sustainable development, agricultural resources such as wheat straw, bagasse, rice husks, coffee husks, or sunflower husks are good materials for biochar synthesis [3,7,8] Biochar was included for the first time as a promising negative emission technology (NET) in the IPCC special report published in 2018. Biochar recovery from agricultural biomass has numerous advantages, including biomass decay prevention, fossil energy offsets, and reduced emissions of harmful greenhouse gases while simultaneously improving physicochemical and biological properties, soil quality and bioenergy [3,9,10,11].
In addition, lignocellulosic biomass is a good substrate for obtaining biochar with a well-developed surface [12]. Heat treatment improves the porous structure of biochar by removing the volatiles from the starting plant material [13]. Biochar is a fine-grained char with a high content of organic carbon and low susceptibility to degradation, obtained in the process of pyrolysis of biomass and biodegradable waste [5,14]. The use of slow pyrolysis (carried out in more than a few minutes) for the production of biochar [15] results in a higher yield of biochar, and the process itself is treated as a more environmentally friendly and sustainable technology [16]. According to [17], biochar produced during slow pyrolysis improves the available water content in both fine-grained and coarse-grained soils.
As a substrate for the production of biochar, waste biomass and biomass dedicated for this purpose can be used, while the use of the former seems to be more appropriate due to the need to manage a specific type of waste
Recently, biochar has been at the center of interest of many scientists, due to the specific properties that enable it to be used in many different environmental processes related to, among others, the removal of pollutants from water, soil, or gases [16,18,19,20]. The properties of the biochar produced are determined by the type of raw material used [21] and the parameters of the pyrolysis process, i.e., temperature [22,23,24,25,26], heating rate [27,28] and reaction time [29] or additionally applied chemical modification [29] or heat treatment [30]. According to [29], the temperature of pyrolysis affects the physical and chemical properties of biochar, such as surface area, functional groups, pore structure, and elemental composition. In terms of sorption mechanisms, the importance of pore-filling mechanisms and hydrophobic interactions increases with the increase in pyrolysis temperature [29]. The selection of appropriate parameters and the processed material allows for the production of a product with the desired properties [31,32]. It was assumed that the optimal temperature range used for the production of biochar is 500–800 °C [29], however, the higher the pyrolysis temperature, the higher the energy consumption of the process. According to [33], the best pyrolysis temperature is considered to be temperatures in the range of 400–550 °C, and the best raw material for the production of biochar comes from the category which includes, among others, straw, husks, and seeds.
The analysis of the results of work carried out by other researchers showed the possibility of using biochar from sunflower husks, among others: to improve soil properties in terms of water retention [34], as a material increasing the sustainable production of soybeans [35], to remove copper ions [36] and pharmaceuticals [37] from aqueous solutions.
The effectiveness of the use of biochar in sorption processes is the result of their physicochemical properties. As indicated by [16,21,29], when removing organic pollutants on biocarbon sorbents, it is important for phenomena such as the formation of hydrogen bonds, pore filling, electrostatic interactions, cation/p/π–π interactions, Lewis acid–base interactions, and hydrophobic interactions. The presence of -OH and -COOH on the surface of biochar makes it more hydrophilic (attracting water/wettable), which promotes hydrophobic interactions with organic pollutants with a high degree of hydrophobicity [21]. Increasing the pyrolysis temperature above 550 °C reduces the presence of oxygen functional groups on the surface of biochar, which determines its sorption potential [33].
The specific surface area and associated porosity are among the main features of biochar that determine the efficiency of pollutant removal [12,38]. The greater the specific surface area and the developed porous structure, the better the potential sorption effects of various pollutants. Highly porous materials provide the appropriate surface area, pollutant adsorption, and transport routes, and the volume and surface area of micropores affect the adsorption capacity of specific pollutants [29].
An important feature of biochar is the level of hydrophobicity, which has a large impact on the adsorption of pollutants from the water phase [30,39]. Biochar with higher hydrophobicity (produced at higher temperatures) can sorb organic pollutants (mainly due to higher specific surface area and microporosity) [16,23], while biochar produced at lower temperatures may have better pollutant removal capabilities for inorganic pollutants [19] and polar organic pollutants, through functional groups containing oxygen, precipitation and electrostatic attraction [16]. The adsorption of organic pollutants on biochar depends on the solubility of pollutants; if they have hydrophobic functional groups in their structure, they can be attached to biochar due to hydrophobic interactions [40].
The hydrophobicity of biochar can also be important, e.g., in terms of creating waterproof hydrophobic soil covers with the addition of biochar. As demonstrated by Zhang et al. in 2022 [41], modification of biochar with special agents allowed for a significant increase in their hydrophobicity, which had a positive impact on the effectiveness of the created soil covers.
Therefore, the aim of the conducted research was to determine changes in the specific surface area, porosity, and the degree of wettability of biochar obtained from sunflower shell pellets, caused by a change in the technical parameters of the pyrolysis process, i.e., temperature, heating rate, and residence time. These features, in terms of the use of biochar in pollutant sorption processes, affect the efficiency of these processes.

2. Materials and Methods

2.1. Substrates for the Production of Biochar

As a raw material for the production of biochar, sunflower husk pellets were used (Figure 1).

2.2. Applied Parameters of the Pyrolysis Process

The pellets were converted in the PRW-S100 × 780/11 furnace (Czylok, Jastrzębie Zdrój) in a nitrogen atmosphere (5 dm3/min). The pellets were placed in the furnace chamber and heated to 480 °C, 530 °C, and 580 °C. The applied heating rate for 480 °C was 4.00 and 7.38 °C·min−1, for 530 °C it was 4.42 and 8.15 °C·min−1, and for 580 °C it was 4.83 and 8.92 °C·min−1. The residence times of the sample at the set temperature were 10 and 60 min [42]. The samples were left in the furnace chamber until they reached ambient temperature.

2.3. Determination of Selected Heavy Metals

Prepared portions of biochar (5 g) were poured into a bottle with 50 mL of distilled water. The bottles with the suspension were shaken on a laboratory shaker (150 rpm) for 24 h. The suspension was then filtered through a hard filter. Selected heavy metals were determined by ICP-OES Thermo Elemental IRIS INTREPID II XSP DUO, in 3 replications.

2.4. Determination of Porosity and Specific Surface Area

The analysis of porosity and specific surface area was carried out using a PoroMaster 33 mercury porosimeter equipped with Quantachrome Instruments for Windows software. Tests using a mercury porosimeter were carried out three times for each of the tested sorbent samples. Tests using a mercury porosimeter allow us to obtain the following information: pore size distribution, porosity, pore size and shape, and particle size distribution of the tested material.

2.5. Hydrophilicity Tests

In assessing the wettability of the tested biochars, a method based on the observation of a drop applied to the material was used. The static method was used: a drop (3 μL of water) was applied to a previously polished (water paper, fineness 2500) and degreased surface, and the contact angle was assessed. Θ—contained between the surface of the solid and the tangent exposed from the contact point of the solid phase, liquid or gaseous defined for the liquid phase. By convention, solids that have contact angles Θ < 90° are considered wettable; these materials have high surface energy (when the wetting liquid is water, these materials are called hydrophilic). On the other hand, those materials with a contact angle Θ > 90° are considered non-wettable (hydrophobic or lyophobic = low surface energy). Wettability tests were performed 3–5 times for a given sample, and the results were averaged. The contact angle was determined using a MicroCapture microcamera.

3. Results and Discussion

3.1. Specific Surface Area and Porosity

The analysis of the specific surface area of selected products obtained at each of the given temperatures showed that the value of the specific surface area of biochar (Figure 1) from sunflower hulls was small, in the range of 0.93–2.91 m2·g−1, and did not show any clear dependence on the temperature used (Table 1).
The average value of the specific surface area (calculated for each group of applied temperatures) was the highest for the temperature of 530 °C (2.46 m2·g−1), then for the temperature of 580 °C (1.62 m2·g−1). This could be due to the partial removal of volatile organic compounds from the processed biomass at the two temperatures used. A clear dependence of the effect of temperature on increasing the specific surface area was shown by other researchers [43], who determined the specific surface area of biochar obtained from various substrates (soybean stover and peanut shell) at 300 °C, at the level of 5.61 m2·g−1 and 3.14 m2·g−1 and higher, respectively, and 420.3 m2·g−1 and 448.2 m2·g−1 for biochar obtained at 700 °C. In turn, Liu et al. [44] obtained biochar from pine wood at a temperature of 700 °C, with a specific surface area of 29 m2·g−1, while the product obtained at a temperature of 300 °C had a specific surface area with a lower value of 21 m2·g−1. A clear dependence of the specific surface area on the pyrolysis temperature was also shown by Song et al. [45], who obtained a change in the specific surface area of biochar from about 3 m2·g−1 to about 6 m2·g−1, increasing the temperature from 300 °C to 600 °C. The results obtained by the above researchers [43,44,45] confirm the dependence of the specific surface area on the temperature of the pyrolysis process. However, in our case, the temperature gradient used was 50 °C and was probably too small to translate into a clear effect of changing the specific surface area. However, Giudicianni et al. [46] marked clear differences in the values of the specific surface area (approx. 50–270 m2·g−1) of biochar obtained from different types of biomass at the same temperature, equal to 873K. The biochar obtained in our research were characterized by a low specific surface area in the entire range of used temperatures. The value of the specific surface area probably results from the nature of the transformed substrate (sunflower husks), which has been subjected to mechanical processing (pellet production), consisting in pressing with the use of a certain pressure. This treatment could have affected the value of the determined specific surface area of the biochar. Similar results were obtained by Kufka et al. [47], who produced biochar at a temperature of 400 °C from pine-spruce pellets, characterized by a specific surface area of 0.9 m2·g−1, and showed good results when using it for the sorption of non-ferrous metals. An important factor influencing the specific surface area is also the residence time of the sample at a given temperature. As shown in [48], the surface areas of biochar increased when the residence time was in the range of 30–90 min, while they decreased when the residence time was in the range of 90–150 min. In our research, the minimum stop time was 10 min and the maximum stop time was 60 min. Such values were adopted for economic reasons. According to [16], with an increase in the residence time from 10 min to 60 min, the specific surface area increases, and then from 60 min to 100 min, there is a slight decrease. In our case, a clear relationship between the increase in the specific surface area and the increase in the residence time of the sample from 10 to 60 min was not observed. Based on porosimetry tests, the value of open porosity and density of the biochar was also estimated. They were, respectively: open porosity 10.11–15.43%, density 0.76–1.67 g·cm−3. The results showed that the heating rate and residence time can influence the porosity of the biochar. However, no clear trends were observed. Researchers [35] presented the characteristics of biochar (pyrolysis at 450–550 °C) from sunflower husks, and recorded a total pore surface of 19.01 m2/g and a total porosity of 75.92%. The total pore volume of sunflower husk biochar in the study by Saleh et al. [36] was 12 mm3·g−1, while the BET-specific surface area was 3.85 m2·g−1, for biochar produced at 450 °C. In turn, in the study by [33], sunflower husk granules were pyrolized (temp. 650 °C), and the total porosity of the biochar and the total specific pore volume of 15.5% were obtained, respectively.
The porosity value, despite the relatively high pyrolysis temperature, was at a moderate level. This confirms the assumption that the pelletization of sunflower hulls affects the porosity of the biochar produced. As a result of pyrolysis, the authors [32] observed an increase in pore diameters above 0.4 μm. Figure 2 presents SEM photos of the microstructure of the biochar produced at different magnifications (from 350× to 10,000×).
For a comparison of the surface morphology of the fabricated objects, SEM photos of the surface morphology of biochar from walnut shells were included. The aim was to show how the type of substrate used affects the nature and properties of the biochar. Comparing both biochars, it is found that the biochar made from walnut has a less developed microporous structure. In the case of biochar from sunflower husk pellets, the difference in surface morphology is associated with a higher sorption potential. In the case of biochar from sunflower husk pellets, voids are visible, probably macropores. A characteristic honeycomb-like arrangement is also visible, which probably reflects the carbon skeleton of the biological capillary structure of the lignocellulosic raw material [31]. Pore diameters are in the range of approx. 934 nm–1.89 μm. In studies [34], the average pore radius was 0.24 µm, however, these researchers used non-pelletized sunflower husks for the production of biochar, which could have resulted in a better porous structure of the biochar. According to [29], the rate of heating is an important factor affecting the surface area and porosity of biochar, as it affects the heat and mass transfer inside the particles. However, excessive heating rates can have adverse effects such as the melting of biochar particles—resulting in a smooth surface [29]. Similarly, a too-long residence time at high temperatures may have a negative effect on the porous structure [12]. Therefore, in our research, low reaction temperatures and low heating rates were used, within the following ranges:
  • for the heating rate of 65 min—7.4–8.9 °C·min−1,
  • for the heating rate of 120 min—4.0–4.8 °C·min−1.
Such selected temperature and heating rates (slow pyrolysis) result in good biochar yield and moderate porosity. Too high a rate of heating can cause a reduction in pore volume, as confirmed by research results [35]. The presence of macro- and mesopores in biochar determines the size of the internal surface and is important in adsorption processes. Macropores act as transport routes allowing access to smaller pores (Figure 3a) (classification according to IUPAC, www.iupac.org, accessed on 1 September 2022). The parameters of the sunflower husk pellet pyrolysis process: the heating rate and the time in which the sample was stopped, have a significant impact on the biochar porosity. Stopping the sample at the temperature of 480 °C allows for partial removal of volatile compounds, through which the material obtains high open porosity (Figure 3b), and the degree of surface development is increased.
The grain size (Table 2) of the tested biochar was in the range of 0.29–1.99 mm [42]. Higher temperatures and shorter heating and residence time result in finer biochar particles. Slower pyrolysis results in coarser biochar particles.
As indicated in the literature, the adsorption capacity of the material increases with the decrease in the size of biochar particles. Therefore, when using biochar as an adsorbent for hydrophobic pollutants, it is recommended to use particles with a diameter of several micrometers [29]. In the case of pollutant adsorption using biochar with hydrophilic properties, the size of biochar particles does not affect the process. Fragmentation of biochar from sunflower husk pellets to grain sizes of 0.4 to 1.6 mm on average will affect the adsorption effect in a narrow range for selected types of pollutants.

3.2. Hydrophilicity

The analysis of the wettability of biochar showed the dependence of the level of hydrophilicity on the temperature of the pyrolysis process (Table 3). The increase in the duration of the pyrolysis process and the residence of the material at a given temperature translates into an increase in the temperature obtained during the process and affects the nature of the biochar. As the process temperature increases, the contact angle of the obtained product decreases, i.e., the material is more hydrophilic (Figure 4, Figure 5 and Figure 6). As indicated [18], biochar prepared at low temperatures has more surface polar functional groups and is better suited to remove inorganic pollutants. As demonstrated in studies [33], increasing the pyrolysis temperature to over 550 °C increases the hydrophobicity of the biochar structure due to the loss of oxygen. In the case of the material that was subjected to the longest exposure, among the tested samples, in the pyrolysis process, i.e., 480 °C /120/60, the drop was absorbed immediately (Figure 5).

3.3. Content of Selected Heavy Metals

In terms of the potential use of biochar as sorbents of pollutants from the aquatic environment or as a soil improver, an analysis of the content of heavy metals (Table 4) in the biochar was carried out. The possible release of metals from biochar could worsen the environment.
The conducted tests showed low concentrations of heavy metals in the produced biochar (Zn, Cu, Pb). However, the presence of Cd and Cr at a measurable level was found only in three samples. Analyzing the influence of the pyrolysis temperature on the presence of metals in the biochar, no clear influence of this factor was observed. Cu, Zn and Pb are metals with low boiling points, but definitely higher than those used in our research. In only a few cases concerning the content of Zn, the highest of the applied temperatures resulted in an increase in the concentration of Zn in the respective biochar. This was probably due to the loss of the remaining biochar components at 580 °C. As shown in [48], the content of heavy metals in biochar is affected by the time the sample is kept at a given temperature. These researchers showed an increase in metal content with increasing residence time ranging from 30 to 90 min. Probably due to small differences in the adopted temperatures in our experiment, (gradient 50 °C) the effect of the pyrolysis temperature and the sample residence time on the content of metals in biochar was not visible.

3.4. Content of Selected Biochar Components and pH

The total carbon content in the tested biochar, produced with different pyrolysis parameters, was quite high and ranged from 70.53% to 81.96% [42]. The C content in biochar produced from plant biomass is usually higher than in biochar of other origins because the higher lignin content favors carbonization [26]. In biochar produced at a heating rate in the range of 7.38–8.92 °C·min−1 and a heating time of 60 min, a higher content of total carbon was observed than at a heating time of 10 min [42]. The heating rate in the range of 4.0–4.83 °C·min−1 and the applied heating time of 60 min resulted in a higher total carbon content than in biochar produced at the same heating rate but with a shorter heating time of 10 min. The content of total nitrogen in the tested biochar ranged from 1.21 to 1.42% and decreased with increasing pyrolysis temperature [42]. A similar phenomenon was observed by other researchers [23]. The rate and time of heating had no significant effect on the nitrogen content. As described in the publication [40], the pH of the produced biochar was in the range of 9.37–10.32—the higher pH of the biochar produced at higher temperatures was caused by the increase in the content of alkaline cations. Studies of the phosphorus content in aqueous extracts made of biochar showed that the content was in the range of 3–120 mg·kg−1 and clearly increased in the case of extracts made of biochar from increasingly higher pyrolysis temperatures [42]. This was probably due to the higher content of this component in biochar produced at higher temperatures (which is confirmed by the results of other studies [22], in forms given for dissolution. An inverse relationship was observed for Ca and Mg. Higher pyrolysis temperature resulted in a lower concentration of these components in water extracts. Higher concentrations of alkaline elements are usually noted in biochar originating from higher temperatures [49,50], their lower concentration in extracts is probably due to the presence of this component in forms less susceptible to leaching. Potassium was not determined in aqueous extracts. Sodium concentrations were observed in the range of 8–15 mg·kg−1. The concentration of Fe ranged from 1–6 mg·kg−1.

4. Conclusions

Transformation of waste biomass in the pyrolysis process is a technology for the sustainable production of materials that can be potential adsorbents of pollutants from the water environment and soil. The technical parameters of the pyrolysis process (temperature, heating rate, and sample residence time) allowed us to obtain hydrophilic materials with porosity in the range of 10.11% and 15.43% and a specific surface area of 0.93 m2·g−1 and 2.91 m2·g−1. The low value of the specific surface area may be the result of sunflower shell pelletization before the pyrolysis process. Increasing the value of the specific surface area and the volume of pores requires the use of a physical or chemical modification of the biochar. The hydrophilic nature of biochars makes it possible to use them in the processes of removing inorganic pollutants and polar organic pollutants. The hydrophilic character of biochar produced in the tested temperature range may positively affect the structure of the soil. The presence of macropores in biochar contribute to the improvement of water management in the soil and affect the assimilation of microelements by plants. The low content of heavy metals in biochar does not pose a threat to the environment. The presence of microelements can positively affect the growth and yield of plants. The technical parameters of pyrolysis used are positive in the energy aspect, and the relatively low pyrolysis temperature and low heating rate can positively affect the chemical nature of the biocarbon surface (the presence of specific functional groups).

Author Contributions

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

Funding

This research was funded by the statute subvention of Czestochowa University of Technology, Faculty of Infrastructure and Environment.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data sharing not applicable.

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the man script, or in the decision to publish the results.

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Figure 1. (a) Pellets from sunflower husks. (b) Biochar made from pellets.
Figure 1. (a) Pellets from sunflower husks. (b) Biochar made from pellets.
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Figure 2. Surface morphology of biochar from sunflower husk pellets (sample produced with parameters 480/120/60) and biochar from walnut shells, made using a scanning electron microscope (magnification from 350× to 10,000×).
Figure 2. Surface morphology of biochar from sunflower husk pellets (sample produced with parameters 480/120/60) and biochar from walnut shells, made using a scanning electron microscope (magnification from 350× to 10,000×).
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Figure 3. Dependence of the change in the unit pore volume (a) and the specific surface area (b) of the produced biochar with the applied parameters of the pyrolysis process to the size of the pores.
Figure 3. Dependence of the change in the unit pore volume (a) and the specific surface area (b) of the produced biochar with the applied parameters of the pyrolysis process to the size of the pores.
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Figure 4. Image of the distribution of a drop of water recorded during the analysis of the degree of wettability of the surface of products obtained with the following parameters: (a) 480 °C/65/10, and (b) 530 °C/65/10.
Figure 4. Image of the distribution of a drop of water recorded during the analysis of the degree of wettability of the surface of products obtained with the following parameters: (a) 480 °C/65/10, and (b) 530 °C/65/10.
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Figure 5. Changes in the wetting angle over time in the product obtained with the parameters 480 °C/120/60.
Figure 5. Changes in the wetting angle over time in the product obtained with the parameters 480 °C/120/60.
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Figure 6. Wettability tests of biochar samples obtained with the following parameters: (a) 480 °C/65/10, (b) 530 °C/65/10, (c) 480 °C/120/10.
Figure 6. Wettability tests of biochar samples obtained with the following parameters: (a) 480 °C/65/10, (b) 530 °C/65/10, (c) 480 °C/120/10.
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Table
Table 1. Open porosity, surface area and density of obtained biochar (a [42]).
Table 1. Open porosity, surface area and density of obtained biochar (a [42]).
Pyrolysis Temperature
[°C]
Heating Rate
[°C·min−1]/Residence Time
[min]		Open
Porosity
[%]		Specific Surface [m2·g−1]Density
[g·cm−3]
480 a7.38/1015.431.220.76
4.00/1010.431.330.92
7.38/6013.231.761.67
4.00/6013.561.531.22
5308.15/1015.252.560.81
4.42/1014.792.911.42
8.15/6014.152.831.23
4.42/6010.131.531.22
5808.92/1010.451.991.14
4.83/1012.231.481.12
8.92/6010.672.071.17
4.83/6010.110.931.15
Table
Table 2. Grain size for biochar obtained at different temperatures.
Table 2. Grain size for biochar obtained at different temperatures.
The Temperature of the
Pyrolysis Process
[°C]		Heating Rate
[°C·min−1]/Residence Time
[min]		Grain Size [mm]
480 7.38/101.596 ± 0.269
530 8.15/101.113 ± 0.169
580 8.92/100.800 ± 0.245
480 4.00/100.940 ± 0.366
530 4.42/100.753 ± 0.272
580 4.83/101.370 ± 0.584
480 4.00/600.733 ± 0.117
530 4.42/601.503 ± 0.409
580 4.83/600.907 ±0.234
480 7.38/600.850 ±0.265
530 8.15/600.420 ±0.125
580 8.92/601.026 ± 0.332
Table
Table 3. Contact angle for biochar obtained at different temperatures.
Table 3. Contact angle for biochar obtained at different temperatures.
The Temperature of the
Pyrolysis
Process
[°C]		Heating Rate
[°C·min−1]/Residence Time
[min]		Contact Angle [o]
4807.38/10
8.15/10
8.92/10		51°
53032°
580The drop is absorbed immediately.
4804.00/10
4.42/10
4.83/10		63°
530The drop is absorbed immediately.
580The drop is absorbed immediately.
4804.00/60


4.42/60
4.83/60		Wettability changes over time.
After 1s—53°, after 2s—46°, after 3s—46°, after 4s—52°, after 8s—0°.
530The drop is absorbed immediately.
580The drop is absorbed immediately.
4807.38/60
8.15/60
8.92/60		49°
530The drop is absorbed immediately.
580The drop is absorbed immediately.
Table
Table 4. Content of selected heavy metals in biochar.
Table 4. Content of selected heavy metals in biochar.
The Temperature of the
Pyrolysis Process [°C]		Heating Rate [°C·min−1]/Residence Time
[min]		CdCrCuPbZn
mg kg−1
480 7.38/10-29.46 ± 0.5832.83 ± 0.8945.62 ± 12.7756.10 ± 0.09
530 8.15/10--24.57 ± 0.9544.08 ± 9.5435.48 ± 0.27
580 8.92/10-8.30 ± 0.8227.40 ± 0.3045.29 ± 2.6654.50 ± 0.34
480 4.00/10--27.43 ± 0.3148.77 ± 9.8648.01 ± 0.57
530 4.42/10--33.18 ± 1.9143.95 ± 6.7457.10 ± 0.50
580 4.83/103.51 ± 0.43-50.40 ± 0.6748.30 ± 3.11123.00 ± 1.84
480 4.00/60--29.74 ± 0.1549.44 ± 4.0356.50 ± 0.21
530 4.42/60--30.86 ± 2.2046.99 ± 8.1148.86 ± 5.30
580 4.83/60--31.43 ± 2.0043.94 ± 4.1076.10 ± 2.06
480 7.38/60--33.53 ± 1.2552.80 ± 5.5667.80 ± 1.63
530 8.15/60--27.12 ± 3.1451.60 ± 10.4047.46 ± 0.29
580 8.92/60--34.41 ± 2.1578.70 ± 3.99115.00 ± 1.94
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Wystalska, K.; Kwarciak-Kozłowska, A.; Włodarczyk, R. Influence of Technical Parameters of the Pyrolysis Process on the Surface Area, Porosity, and Hydrophobicity of Biochar from Sunflower Husk Pellet. Sustainability 2023, 15, 394. https://doi.org/10.3390/su15010394

AMA Style

Wystalska K, Kwarciak-Kozłowska A, Włodarczyk R. Influence of Technical Parameters of the Pyrolysis Process on the Surface Area, Porosity, and Hydrophobicity of Biochar from Sunflower Husk Pellet. Sustainability. 2023; 15(1):394. https://doi.org/10.3390/su15010394

Chicago/Turabian Style

Wystalska, Katarzyna, Anna Kwarciak-Kozłowska, and Renata Włodarczyk. 2023. "Influence of Technical Parameters of the Pyrolysis Process on the Surface Area, Porosity, and Hydrophobicity of Biochar from Sunflower Husk Pellet" Sustainability 15, no. 1: 394. https://doi.org/10.3390/su15010394

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