1 Introduction

In the last dozen or so years the research on biochar production was intensified. The growing interest in this material results from its numerous advantages: this is the material rich in carbon, with a large degree of porosity, large specific surface area with a stable structure, a large content of surface functional groups and a great cation exchange capacity [1,2,3]. In addition, biochar has a wide range of applications: agriculture—soil improvement [4],environmental protection—removal of harmful gases, i.e., CO2, H2S [5], NH3 [6, 7],contaminants from the aqueous solutions—pharmaceuticals [8], dyes ([9], pesticides [10], in soil remediation from inorganic and organic pollutants [11],in energy recovery technologies—renewable fuel, catalysts [12].

Biochar can be obtained in the pyrolysis process in the inert atmosphere from virtually any type of biomass, which makes it a competitive material, much cheaper compared to the commercial materials [1, 6, 13, 14]. Besides the carbon adsorbent, there are obtained also other products such as: oil and synthesis gas. The percentage of the obtained products depends on the heating rate, temperature and pressure used during the pyrolysis [13, 15]. The precursors used to obtain biochar are e.g. wood biomass: sawdust [16, 17], cones [18,19,20], food or agricultural waste: nut shells [21, 22], cereal straw [1], cotton stalks [23], corn cobs [24, 25], rice husks [26], banana peels [27], coffee spent grains [28],natural fertilizers: chicken manure [29] and waste paper [30].

The adsorption properties of biochar could be improved due to development of its porosity, e.g. as a result of physical or chemical activation. Physical activation includes two stages: carbonization of the carbon precursor in the inert gas atmosphere and activation at a high temperature with the use of an oxidizing gas—carbon dioxide, steam, air or a mixture of CO2 and steam [31, 32]). The chemical activation can take place in two ways. The first method of activation includes the carbon precursor impregnation with a chemical activator, followed by heating at an elevated temperature (carbonization/activation) [33]. The other one includes two stages. After the initial carbonization of the carbon precursor (300 to 600 °C), the biochar is impregnated with a chemical agent and then activated (700 to 1200 °C) [33].

The chemical activation is more agvantageous than the physical one due to its greater efficiency, larger surface areas and the more developed porous structure of the obtained materials [34]. The use of the activation process with KOH as an activating agent leads to the formation of a well-developed porous structure on the biochar surface with a predominance of micropores as well as the appearance of active functional groups [35, 36]. It was proved that the content of micropores on the surface of active carbon increases slightly with the increasing extent of impregnation [37]. This is due to a more intensive reaction between the KOH and carbon surface which results in a greater release of CO2 and CO gases as well as the micropores formation [38]. Zhu et al. [39] obtained a biochar with a large specific surface area (782–2435 m2/g) and developed microporosity (0.301–0.625 cm3/g) using the Paulownia sawdust as a carbon precursor. The raw sawdust was preheated in the oven (250 °C, 2 h, N2), then the material was impregnated with KOH (80 °C, 12 h) and the activation process took place. There were used the following conditions: impregnation KOH/C weight ratio 2:1, 4:1 and 6:1; activation temperatures 600, 700, 800 and 900 °C; activation time 0.5, 1 and 2 h. In turn, Yang et al. [40] obtained structured porous biocarbons in the two-stage process involving hydrothermal treatment and carbonization of the Cedar wood sawdust. The dried sawdust (105 °C, 24 h) was poured with the 5 wt% KOH solution and subjected to the hydrothermal treatment (the bottle hermetically closed with a lid, the temperature 120 °C, for: 30 min, 2 h and 8 h). After the hydrothermal treatment, the suspension was separated by filtration and dried at 105 °C until the stable weight. The obtained product was carbonized (800 °C, 5 °C/min, 1.5 h) in the argon atmosphere. The activated biochars were characterized by the following parameters: specific surface area (875–908 m2/g), volume of micropores (0.406–0.420 cm3/g). Quan et al. [41] obtained a biochar from the pine sawdust. The pyrolysis was performed at 700 °C (temperature increase 10 °C/min) for 2 h. Then the activation was made with KOH. After the pyrolysis the biochar was mixed with KOH at the weight ratio of 4:1, placed in the boats in the tube furnace, and the activation was performed at the temperatures of 700, 800, 900 °C for 2 h in the nitrogen atmosphere. The obtained materials were characterized by the large specific surface area SBET = 1729 − 2331 m2/g and the volume of micropores Vmi = 0.671 − 0.795 cm3/g. Liu et al. [42] used the coconut shell as the biochar precursor. The crude material was impregnated with the KOH solution followed by activation at 700 °C. The obtained biochar had the developed microporous structure (Vmi = 0.315 cm3/g) and the large specific surface area (860 m2/g). Marques et al. [43] obtained the apple tree small branches biochar impregnated with the KOH solution and then activated at 800 ℃ (1–4 h). The finally obtained biochars were characterized by the large specific surface (2114–2472 m2/g) and the developed microporosity (91–98%). Oginni et al. [33] compared the biochars obtained from two types of biomass: Kanlow Switchgrass (KS) and Public Miscanthus (PM) as a result of one and two-stage activation. The raw biomass was heated at 500 °C. Then the starting biomass and the biochar after the pyrolysis were impregnated with KOH at the weight ratio of 1:1. The activation of the materials obtained by these methods was performed at 900 °C for 0.5 h. The specific surface area values ​​were higher for the materials obtained as a result of raw biomass impregnation (one-step activation). The SBET values ​​differed depending on the precursor and the method of preparation (KS: 599–1272 m2/g and PM: 957–1597 m2/g). Applying various carbon precursors and two different methods (carbonization-activation and impregnation-activation) at two different KOH/carbon ratios Elmouwahidi et al. [44] obtained materials with a developed porosity. To sum up, the porous structure and properties of the ultimately obtained biochars depend on both the nature of the starting material and to a large extent on the parameters of the activation process [45].

Toxic gases that pollute the environment pose a serious threat to human health [46]. One of these gases is ammonia. Right after dinitrogen oxide (N2O), this gas is one of the most common compounds in the atmosphere [46, 47]. Ammonia is a poisonous gas which is also a valuable chemical raw material used in industry, among others, for the production of varnishes, plastics, rubbers, disinfectants, battery-killing agents, fertilizers and in cooling installations as a cooling gas [48, 49]. When the content of ammonia in the atmosphere exceeds 35 ppm, it has a negative effect on human and animal health [50]. Therefore, for the sake of the safety of the natural environment and people, it is very important to eliminate this gas from the surrounding atmosphere.

One of the most frequently used methods of removing ammonia polluting the air, taking into account the low cost, simplicity of implementation and efficiency in a wide range of concentrations, are adsorption processes [51]. The adsorbents that work best during this process should have a well-developed surface and porosity, which will ensure great ammonia removal capacity [49].

The aim of the paper was to obtain a biochar with developed porosity from the low-cost KOH-activated sawdust and to investigate the influence of the type of carbon precursor on the physicochemical and sorption properties against the gas phase impurities represented by ammonia. To sum up, it was crucial for us to obtain effective adsorbents with a great adsorption capacity for removal of ammonia and possibly in the future of other toxic gases from the air, e.g. nitrogen oxides NOx, sulfur oxides: SO2 and SO3 as well as carbon oxides: CO and CO2.

2 Experimental

2.1 Chemicals

The ultra-high purity (UHP) nitrogen 5.0 (99.999%) for preparation of biochars was purchased from Air Liquide (5.0), Poland. Potassium hydroxide for activation of the porous structure of biochars was purchased from POCh, Poland.

The gases, UHP helium 5.0 (99.999%) for cleaning (stabilization) of biochar and temperature-programmed desorption of ammonia (TPD), and UHP 10% ammonia in helium for the pulse chemisorption were purchased from Air Liquide, Poland.

2.2 Materials

The sawdust used as the carbon precursors was obtained from the treatment of the following deciduous trees: oak, hornbeam, cherry and apple. The raw sawdust was ground in the laboratory mill (MF 10, IKA, Germany) to collect the grain size fraction smaller than 2 mm. Then the materials were placed in the quartz boats and heated in the tube furnace (MRT-4, Czylok, Poland) to 800 °C at the heating rate of 2 °C/min with the additional isothermal stages at 180 °C (2 h), 400 °C (2 h) and the final temperature 800 °C (3 h). The process proceeded in the nitrogen atmosphere at the gas flow rate of 20 dm3/h. The obtained biochars were designated C–O (Carbon–Oak), C–H (Carbon–Hornbeam), C–A (Carbon–Apple) and C–C (Carbon–Cherry).

For developing develop the porous structure of the obtained biochars, the activation process was conducted. The obtained materials were mixed with KOH at the weight ratio of 1:2 and 1:4 (biochar:KOH), then placed in the quartz boats and annealed (5 °C/min) in the tube furnace to 600 °C at the nitrogen flow rate of 20 dm3/h. The materials were soaked at 600 °C for 2 h. The obtained materials were designated C–O1-2, C–H1-2, C–A1-2, C–C1-2 and C–O1-4, C–H1-4, C–A1-4, C–C1-4.

2.3 Characterization

The low-temperature nitrogen adsorption/desorption isotherms were measured at 77 K using the ASAP 2020 volumetric analyzer (Micromeritics Inc., Norcross, GA, USA) in the Structural Research Laboratory at Jan Kochanowski University in Kielce. Just before the measurements, all samples were degassed at 200 °C for at least 2 h.

Scanning electron microscopy images were obtained using the scanning electron microscope (SEM Zeiss Ultra Plus, EDS Bruker Quantax 400, Germany). During the measurements the voltage of 2 kV was applied. The energy-dispersive X-ray spectroscopy (SEM/EDX, acceleration: 15 kV) was used for the quantitative measurements.

The Perkin–Elmer Spectrum 400 FT-IR/FT-NIR spectrometer (Perkin–Elmer, Waltham, MA, USA) with the endurance single bounce diamond, attenuated total reflection (ATR) cell was used for spectra registration. The spectra 4000–650 cm−1 were recorded. All materials were dried and ground before the measurements.

The Boehm’s titration method [52, 53] was used for determination of the functional acidic and basic oxygen surface groups. The 0.2 g mass weights of the biochars were dispersed in the sodium bicarbonate, sodium carbonate, sodium hydroxide and sodium ethoxide solutions (all at the concentration of 0.1 mol/dm3) for determination of the functional acidic groups. However, hydrochloric acid (0.05 mol/dm3) was used for determination of the total basic groups. After shaking for 48 h at room temperature the suspensions were filtrated and 10 cm3 of the filtrate was titrated with 0.1 mol/dm3 HCl for determination of the acidic groups and with 0.05 mol/dm3 NaOH for the total basic group determination. The titration endpoint was determined using the methyl orange indicator.

The biochars pHpzc (point of zero charge) was analyzed by means of the method used in [54, 55]. At the beginning the solutions of the pH between 3 and 12 in the 0.01 mol/dm3 NaCl solution were prepared by adding 0.1 or 1 mol/dm3 of HCl and 0.1 or 1 mol/dm3 of NaOH. The biochar samples (0.15 g) were added to the solutions (50 cm3) with an appropriate pH value. Next, they were shaken in the incubator (Orbital Shaker-Incubator ES-20, Grant-bio) for 180 min at 298 K. Then the final pH was measured. After this time, the relationships between the final and the initial values of pH were determined. The pHpzc indicates the intersection point of the experimental curves and pHinitial = pHfinal line [54, 55]. The pH-meter inoLab pH 730, WTW was used for the pH value measurement.

The thermal behavior and composition of the volatile and fixed carbon in the biochars were determined using the Derivatograph C (Paulik, Paulik & Erdey, MOM, Budapest). The tested materials (about 20 mg) were placed in the small corundum crucible. Al2O3 was used as the reference material. The analyses were made in air (O2) or inert (N2) atmosphere in the temperature range from 20 to 1200 °C (heating rate 10 °C/min). The TG, DTG and DTA curves were registered.

The content of volatile carbon, i.e., less humified organic matter (%VC) was determined from the TGA data in the N2 atmosphere in the temperature range 150–900 °C (TG%900,N2), assuming that the physically bound water (moisture) is desorbed up to 150 °C. The ash percentage (%A) is the residue after the complete thermal degradation of the material in the O2 atmosphere at 1200 °C (TG% 1200,O2). The content of fixed carbon, more humified organic matter (%FC) was determined as the difference of TG% 1200,O2 and TG% 900,N2. The share of the thermostable biochar fraction (Cthermo, poorly thermodegradable, ash-free, in relation to the dry mass) was also determined as the content of stable substances (%FC) in relation to the sum of volatile (%VC) and fixed substances (% FC) [56] This parameter is considered reliable for assessing the level of organic matter stability in pyrocarbons, compost materials or other organic wastes [57].

The pulse chemisorption and temperature-programmed desorption (TPD) measurements of ammonia were made using the automatic AutoChem II 2920 analyzer (Micromeritics, Norcross, GA, USA). Before testing all biochars were degassed at 200 °C for 2 h (ASAP 2020, Micromeritics, Norcross, GA, USA). The first stage of the measurements was the stabilization of the material (50 mg of biochar in the quartz reactor) in the helium atmosphere at the temperature of 250 °C (20 °C/min for 40 min).

The chemisorption studies were carried out at the three temperatures: 0 °C, 10 °C and 20 °C. The standard gas (10% NH3 in helium) was dosed from the loop of known volume. The TCD detector registered the next doses. Dosing was performed until the surface of the analyzed material was saturated. The total amount of gaseous ammonia adsorbed on the biochar surface was calculated. In the next stage the experiments of thermo-programmed desorption were performed. The temperature increased from the measurement temperature, at which the chemisorption process was initially conducted to 250 °C (with the temperature increase rate of 10 °C/min).

2.4 Calculations

The porous structure standard parameters: specific surface area, pore volume as well as pore size distribution were determined based on the experimental nitrogen adsorption isotherms. Determination of the specific surface area (SBET) was made in the range of the relative pressure from 0.05 to 0.20 taking into account the nitrogen molecule surface area equal to 0.162 nm2 [58]. Determination of the total pore volume (Vt) was made from one point of the adsorption isotherm at the relative pressure p/p0 = 0.99 [59]. To calculate the pore size distribution functions (PSDs), the non-local density functional (NLDFT) method was applied for the carbon slit-shaped pores which characterizes the surface energetical heterogeneity and geometrical corrugation [60, 61]. The calculations were made applying the numerical program SAIEUS (Micromeritics). The maxima of the PSD curves were used to determine the micropore widths, wmi. Microporosity, expressed in % was calculated as the ratio of the micropore volume Vmi to the total pore volume Vt.

3 Results and discussion

The main factors influencing the structural and surface properties of carbon materials obtained from the industrial waste are: (1) the nature of the carbon precursor, its composition and structure; (2) activation method, type and amount of activating substance. In the presented research the source of the organic matter was the sawdust of four different kinds of trees: oak, hornbeam, apple and cherry. Despite the fact that they are different species of trees, their structure was not significantly diversified. Figure 1a and b presents the experimental nitrogen adsorption–desorption isotherms for the studied biochars. All isotherms are Type I according to the IUPAC classification [62]. The large adsorption values ​​at low relative pressures indicate a strongly developed structure of micropores in the tested materials. The almost parallel course of the isotherms to the axis of relative pressures in the range of medium and high pressures (p/p0 ~ 0.2–0.9) is confirmed by a very small share of mesopores.

Fig. 1
figure 1

Nitrogen adsorption–desorption isotherms for the biochars obtained based on the carbon precursors: oak and hornbeam sawdust (a) as well as apple tree and cherry tree sawdust (b)

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The lowest situated isotherms determined for the non-activated C–O and C–H biochars (Fig. 1a) as well as C–A and C–C (Fig. 1b) indicate their least developed surface and porosity. The use of KOH as an activating agent resulted in intensive development of the surface and structure of the pores, especially in the area of micropores. The influence of the amount of the activating agent is also clearly visible on the isotherms (Fig. 1). The isotherms determined for the materials with the maximum KOH content (1:4) are the highest situated, indicating the most effective development of the surface and volume of micropores. These observations are confirmed by the values of the structural parameters in Table 1.

Table 1 Structural parameters of the biochars
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The specific surface area (SBET) values ​​for the unactivated biochars ranged from 333 m2/g for the C–O material to 387 m2/g for the C–C material while the values ​​of the total pore volume (Vt) ranged from 0.16 cm3/g for C–O to 0.21 cm3/g for C–C. The activation with KOH has brought very good results. The activation by mixing carbon with KOH at the ratio 1:2 resulted in the over twofold increase in the specific surface area and Vt value. For all materials obtained in this way, the surface values ​​ranged from 714 m2/g for the C–O1-2 material to 788 m2/g for the C–C1-2 material, while the Vt values ​​ranged from 0.33 cm3/g for C–O1-2 to 0.40 cm3/g for C–A1-2. On the other hand, the activation by mixing carbon with KOH at the ratio 1:4 improved these parameters. The values ​​increased and ranged from 718 m2/g for the C–H1-4 material to 912 m2/g for the C–A1-4 material. The Vt values ​​ranged from 0.33 cm3/g for the C–H1-4 material to 0.43 cm3/g for the C–A1-4 material. As the aim of these studies was to obtain a biochar with well-developed microporosity, the volumes of micropores (Vmi) should be dominant. Our goal has been achieved. The micropore volumes determined for the materials with the most developed porosity, i.e., C–H1-4 and C–A1-4, are 0.32 cm3/g and 0.40 cm3/g, respectively. A large proportion of the micropores: from 85 to 97% was observed for all activated biochars (Table 1). It was proved that the biochar/KOH mass ratio plays an important role in the development of the porous structure. The development of the porous structure takes place as a result of the intercalation of metal (potassium) into the carbon structure and it is the larger the greater the impregnation factor is, owing to which there is formed a larger number of pores. The general reaction is as follows:

$${\text{4 KOH}} + {\text{C}} \to {\text{K}}_{{2}} {\text{CO}}_{{3}} + {\text{K}}_{{2}} {\text{O}} + {\text{2 H}}_{{2}} ,$$
(1)

however, after [63] it runs multi -stage according to the reactions:

$${\text{2 KOH}} \to {\text{K}}_{{2}} {\text{O}} + {\text{H}}_{{2}} {\text{O}};$$
(2)
$${\text{C}} + {\text{H}}_{{2}} {\text{O}} \to {\text{H}}_{{2}} + {\text{CO}};$$
(3)
$${\text{CO}} + {\text{H}}_{{2}} {\text{O}} \to {\text{H}}_{{2}} + {\text{CO}}_{{2}} ;$$
(4)
$${\text{K}}_{{2}} {\text{O}} + {\text{CO}}_{{2}} \to {\text{K}}_{{2}} {\text{CO}}_{{3}} .$$
(5)

Moreover, if the temperature or contact time is too high or the optimal KOH content is exceeded, the activation yield decreases, pore sizes get larger and the micropores share is reduced. Then too intensive reaction of KOH with carbon takes place, causing significant losses of carbon matter. Moreover, as a result of the decomposition of an excessive amount of KOH in the system, the water is formed (reaction 2) [64, 65]. At the high temperature of the process, the water causes the excessive carbon gasification [45].

The selection of the carbon precursor plays also an important role in the development of the porous structure. The sawdust has a tubular structure [45, 66]. This type of structure, existing in the raw sawdust, allows them to adsorb effectively the reagent thus enabling the biochar activation [45].

The pore size distribution functions (PSDs) for the biochars were calculated using the DFT method. They are presented in Fig. 2a and b.

Fig. 2
figure 2

Pore size distributions (PSDs) calculated by the DFT method for the biochars obtained based on the carbon precursors: oak and hornbeam sawdust (a) as well as apple tree and cherry tree sawdust (b)

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The maximum can be seen on all PSD curves, indicating the presence of pores of the sizes < 2 nm, which corresponds to the width of micropores. As follows from Table 1 the micropore sizes range from 0.55 to 0.65 nm in the case of the non-activated biochars while in the case of the materials after the KOH activation process the values vary from 0.60 to 0.64 nm.

In order to analyze the morphology of the tested biochars before and after the activation with KOH, SEM imaging was performed (Figs. 3, 4).

Fig. 3
figure 3

SEM images for the biochars: C–O (a), C–O1-2 (b), C–O1-4 (c) and C–H (d), C–H1-2 (e), C–H1-4 (f)

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Fig. 4
figure 4

SEM images for the biochars: C–A (a), C–A1-2 (b), C–A1-4 (c) and C–C (d), C–C1-2 (e), C–C1-4 (f)

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As follows from Figs. 3 and 4 depending on the carbon precursor the analyzed carbon materials are characterized by different porous structures. The unactivated biochars obtained based on the oak (C–O, Fig. 3a) and horn (C–H, Fig. 3d) have poorly developed porous structures. The carbons C–A (Fig. 4a) and C–C (Fig. 4d) were characterized by slightly better developed porosity in the case of the non-activated materials which was confirmed by the SEM images. It can be clearly visible that the activation with KOH caused the development of porosity of the tested biochars (Figs. 3b, c, e, f, 4b, c, e, f). In all cases after the KOH activation there was observed a layered structure which can indicate the order of the tested biochars. To sum up, it can be stated that the KOH activation affects the microporous structure of the obtained carbons including the size and volume of micropores but it does not affect mesoporosity (Table 1).

The EDS (X-ray energy dispersion spectroscopy) studies were carried out simultaneously with the SEM analysis. This technique makes the qualitative and quantitative elemental analyses of the biochars possible [67]. Table 2 presents the results of EDS chemical composition microa

Table 2 The EDS analysis for the biochars
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nalysis for the non-activated and KOH activated biochars.

Analyzing the obtained EDS results it can be observed that the basic chemical components of all tested biochars are carbon and oxygen and, to a much smaller extent, potassium. Only in the case of the biochar obtained from the sawdust of fruit trees, i.e., apple and cherry trees, the presence of calcium ions was additionally found. The presence of carbon and oxygen results from the presence of cellulose, hemicellulose and lignin in the wood biomass as well as in the obtained biocarbons [68, 69]. The amount of carbon in the non-activated materials ranges from 85.50 to 90.86% w/w. After the biochar activation the values ​​are slightly smaller and range from 72.02 to 84.03%. The oxygen content is from 8.74 to 12.06% and from 13.94 to 22.19% for the inactivated and activated biochars, respectively. This element is bound in the form of surface oxygen groups, and the increase in its content along with that in the degree of impregnation indicates an increase in the number of these groups on the surface of the tested materials. The presence of other elements, such as potassium and calcium, is largely influenced by the chemical composition of the biomass, i.e., raw sawdust.

The presence of the surface functional groups was confirmed by the ATR-FTIR analysis (Figs. 5, 6).

Fig. 5
figure 5

FTIR spectra for the biochars obtained from the hornbeam and oak sawdust

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Fig. 6
figure 6

FTIR spectra for the biochars obtained from the apple tree and cherry tree sawdust

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The low-intensity bands in the range of 3755–3576 cm−1 for all analyzed materials correspond to the asymmetric and symmetric stretching vibrations of the surface O–H groups [20, 70,71,72]). In turn, the 2174 cm−1 band is related to the presence of carbon monoxide (CO) in the tested materials [70]. The vibrations associated with the presence of double bonds between carbon atoms or those of carbon and oxygen are responsible for the intense band at 1740 cm−1 [73, 74]. The band at 1570 cm−1 is characteristic of the C=C bond [75]. On the other hand, the bands in the range of 1500–1100 cm−1 (exactly at 1366 cm−1, 1232 cm−1, 1100 cm−1) indicate the presence of carbonyl groups (C=O) [76]. Their presence on the biochar surface is consistent with the results of the Boehm titration (Table 3). In turn, the 1000 cm−1 and 1005 cm−1 bands are characteristic of the C–OH stretching vibrations [76]. The band in the range from 900 to 750 cm−1 indicates the C–H bending vibrations [72]. It should be highlighted that activation changes the chemical structure of the biocarbons surface. The band 1570 cm−1 increases with the increasing KOH ratio, while the bands at 1740, 1366 and 1232 cm−1 increase at the 1–2 ratio but decrease at the 1–4 KOH ratio.

The Boehm’s titration method [52, 53] was used to characterize the chemical nature of biochars surface. The individual acid groups undergo neutralization in the following order: carboxylic − neutralized when influenced by NaHCO3; carboxyl + lactone − affected by Na2CO3; carboxylic + lactone + phenolic—neutralized by NaOH; carboxylic + lactone + phenolic + carbonyl − neutralized when influenced by C2H5ONa. Summing up, the content of basic functionalities was determined by titration of unreacted HCl that was previously used to neutralize the basic groups using the NaOH solution. The surface functional groups have a significant impact on the adsorption process, ensuring selectivity in the removal of specific pollutants. The functional groups affect the surface charge of the adsorbent and thus its adsorption capacity [77].

The content of the functional groups determined on the surface of the studied KOH activated biochars are presented in Table 3.

Table 3 The biochars functional surface groups determined by the Boehm method and the pHpzc values
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The obtained results indicate the predominance of acidic and basic groups on the surface of the tested biochars. Note that an increase in the KOH ratio results in an increase mainly in the carboxyl and carbonyl groups and to a much smaller extent in the phenolic ones, which is consistent with the pHpzc data. As can be seen, the number of functional groups depends on the precursor from which the biochars were obtained and on the activator content (KOH).

The point of zero charge (pHpzc) determines the value of pH at which the net surface charge on the adsorbent equals zero [54, 78]. The value of pHpzc determined for the tested materials from the graphs (Figs. 7, 8) is approx. 6.6–6.9 (Table 3) indicating that in the solution of pH > pHpzc the biochar surface has a negative charge while for pH < pHpzc the surface has a positive charge [54, 78]. The pHpzc values are consistent with the results obtained by the Boehm's method (Table 3).

Fig. 7
figure 7

Point of zero charge (pHpzc) for the biochars obtained from the oak and hornbeam sawdust

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Fig. 8
figure 8

Point of zero charge (pHpzc) for the biochars obtained from the apple tree and cherry tree sawdust

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The thermal analysis was applied for estimation of the thermal stability of the materials. Figures 9 and 10 show the TG%, DTG and DTA curves for the biochars obtained from various organic precursors.

Fig. 9
figure 9

TG%, DTG and DTA curves for the biochars based on the oak (a) and hornbeam (b) sawdust

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Fig. 10
figure 10

TG%, DTG and DTA curves for the biochars obtained based on the apple tree (a) and cherry tree (b) sawdust

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As follows from the analysis of the mass loss curves (TG%) the starting biochars are thermally stable only up to the temperature of ~ 300–400 °C and the complete thermal degradation of the carbon material is over in the temperature range of 800–1000 °C, depending on the organic material precursor. The KOH impregnation causes reduction in the thermal stability, the process of thermal degradation begins and ends at the temperatures lower than it is observed for the unmodified biochars. This is related to better development of porosity and surface functionalities due to the use of the activator. Moreover, activation with the use of KOH resulted in the carbon content decrease and significant oxygen content increase, which occur mainly in the form of surface functional groups in activated materials. These functionalities are a subject to desorption and thermal degradation much more easily than the ordered carbon structures, therefore the thermal stability of activated biochars is smaller. The analysis of the TG% = f (T) curves (Figs. 9, 10) shows that the type of organic precursor and the action of the activating agent affect the thermal stability.

On the basis of the TGA analysis performed in the atmosphere of O2 and N2, a proximate assessment of the type of carbon matter in the tested biocarbons was made. The results of research on the ash content, volatile and fixed carbon as well as the share of fixed carbon matter in the biocarbons are presented in Table 4.

Table 4 The proximate analysis and thermostability indices of biocarbons
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According to data analysis in Table 4 the activation with the use of KOH changes the nature of the structure of the carbon material effectively. As the activation degree increased, the volatile matter (% VC) contents increased significantly, while the fixed carbon contents (% FC) decreased slightly in the biocarbons obtained from different biomass sources (Table 4). At the same time it was found that the share of the thermostable carbon matter (% Cthermo) in the starting biocarbons is very similar (~ 90%, Table 4) whereas the activation with an increasing amount of KOH causes a slight decrease in the share of the thermally stable fraction. This confirms that pyrolysis at the temperature of 800 °C is an effective method of obtaining stable biochars while the additional modification changes the materials structure effectively reducing their thermal stability.

The results of pulse chemisorption and temperature-programmed desorption (TPD) of ammonia for the studied biochars are presented in Table 5. When analyzing the data of the pulse chemisorption of ammonia there was observed the following dependence: for all tested biochars, ammonia adsorption increases with the increasing temperature. The adsorption capacity of the obtained biochars depends on several factors: the carbon precursor, the degree of development of the porous structure and the size of the specific surface area. The developed microporous structure, and thus the large specific surface area (Table 1) play a key role in the process of ammonia adsorption on the tested adsorbents. The largest adsorption capacity was demonstrated for the C-A1-4 material: 3.05 mmol/g (temperature 20 °C), the remaining biochars were characterized by a slightly smaller value of this parameter. The smallest adsorption capacity value was obtained for the material C–H1-2: 2.24 mmol/g. The results of EDS, FTIR Boehm’s titration and pHpzc clearly show that the KOH activation causes oxidation of the surface and formation of oxygen functional groups in which those of strongly acidic character dominate. This has a positive effect on the ammonia chemisorption.

Table 5 The NH3 adsorption capacity and TPD for the tested activated biochars
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Based on results of the ammonia temperature programmed desorption TPD (Table 5) it was found that a very small amount of ammonia was desorbed although quite a large amount of this gas was adsorbed. The small NH3 desorption is related to the dominant presence of acidic groups (which proves that it is chemisorbed on the acidic groups). However, such an effect is beneficial because the tested carbon materials can trap the toxic gas ammonia.

Table 6 summarizes the maximum sorption capacities of the tested biochars and other carbon materials in order to compare their adsorption capacity from the gas phase with respect to NH3 based on the literature data. It was shown that the tested biochars are characterized by good adsorption properties.

Table 6 The comparison of the maximal adsorption capacity in relation to NH3 for the tested biochars with that of other carbon adsorbents
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4 Conclusions

The paper proves that sawdust is a cheap and good carbon precursor. Moreover, the activation with KOH enables preparation of adsorbents with very good structural parameters, large specific surface area and developed microporosity. After the activation process, the adsorbents were characterized by an ordered porous structure. The type of organic precursor and the action of the activator affect the thermal stability of the obtained biochar. The predominance of acidic and basic groups was observed on the surface of all tested biochars. It was proved that the obtained carbon adsorbents remove the harmful gas, which is ammonia, from the gas phase effectively. Moreover, very little of this gas is desorbed, so the resulting biochars can be promising as far as the trap of NH3 and possibly of other toxic gases is concerned.