In this work, Norway spruce (Picea abies (Karst) L.) bark was employed as a precursor to prepare activated carbon using zinc chloride (ZnCl2) as a chemical activator. The purpose of this study was to determine optimal activated carbon (AC) preparation variables by the response surface methodology using a Box–Behnken design (BBD) to obtain AC with high specific surface area (SBET), mesopore surface area (SMESO), and micropore surface area (SMICR). Variables and levels used in the design were pyrolysis temperature (700, 800, and 900 °C), holding time (1, 2, and 3 h), and bark/ZnCl2 impregnation ratio (1, 1.5, and 2). The optimal conditions for achieving the highest SBET were as follows: a pyrolysis temperature of 700 °C, a holding time of 1 h, and a spruce bark/ZnCl2 ratio of 1.5, which yielded an SBET value of 1374 m2 g−1. For maximised mesopore area, the optimal condition was at a pyrolysis temperature of 700 °C, a holding time of 2 h, and a bark/ZnCl2 ratio of 2, which yielded a SMESO area of 1311 m2 g−1, where mesopores (SMESO%) comprised 97.4% of total SBET. Correspondingly, for micropore formation, the highest micropore area was found at a pyrolysis temperature of 800 °C, a holding time of 3 h, and a bark/ZnCl2 ratio of 2, corresponding to 1117 m2 g−1, with 94.3% of the total SBET consisting of micropores (SMICRO%). The bark/ZnCl2 ratio and pyrolysis temperature had the strongest impact on the SBET, while the interaction between temperature and bark/ZnCl2 ratio was the most significant factor for SMESO. For the SMICRO, holding time was the most important factor. In general, the spruce bark AC showed predominantly mesoporous structures. All activated carbons had high carbon and low ash contents. Chemical characterisation indicated that the ACs presented disordered carbon structures with oxygen functional groups on the ACs’ surfaces. Well-developed porosity and a large surface area combined with favourable chemical composition render the activated carbons from Norway spruce bark with interesting physicochemical properties. The ACs were successfully tested to adsorb sodium diclofenac from aqueous solutions showing to be attractive products to use as adsorbents to tackle polluted waters.
Graphical abstract
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Hinweise
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1 Introduction
Activated carbon is a porous carbon material that has been subjected to reaction with gases and/or chemicals (e.g. ZnCl2) before, during, or after pyrolysis to obtain beneficial physicochemical and adsorptive properties [1]. Activated carbon (AC) can be produced with specific properties to meet the needs for low-cost and high-performance materials for more sustainable technologies [2‐4]; it is one of the most common materials used as adsorbent [5‐8] and catalyst and electrode material in environmental, chemical, and energy storage applications [9, 10].
To produce efficient ACs, a carbon source precursor (e.g. any biomass) is carbonised at temperatures above 500 °C in an inert atmosphere, in a process called pyrolysis [1, 8‐10]. After pyrolysis, the fixed carbon is modified through chemical and/or physical activation [1, 8, 9]. Generally, pyrolysis and activation aim to generate AC with large specific surface area (SBET), pore volume, micropore area (SMICRO), and mesopore area (SMESO), and beneficial surface functionality—such as hydrophobicity and a large number of functional groups [6, 7, 11]. These properties depend on the manufacturing pyrolysis process [9, 10, 12]. The AC characteristics are severely influenced by several factors, such as (i) biomass precursor properties (chemical and structural), (ii) pyrolysis method (conventional, microwave-assisted, and/or hydrothermal), (iii) pyrolysis conditions (temperature, heating rate, and holding time), (iv) activation method (chemical and/or physical), and (v) activation conditions (carbon precursor/activator ratio, holding time, etc.) [8‐12].
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However, it is not trivial to elucidate how and to which extent these factors affect the AC production process and resulting characteristics [11, 13]. Using the design of experiments (DoE) methodology enables identifying significant factors for specific processes and how they affect the resulting product properties [11, 13, 14]. DoE also provides resource efficiency by creating experimental spaces that maximise the output of relevant data from a minimum number of experiments for building empirical models that correlate responses with the experimental factors [14, 15].
DoE has been applied successfully to optimise the experimental conditions for AC preparation from precursors such as sewage sludge [11], coconut shell [16], bamboo [17], seed pods [18], jatropha hull [19], polycarbonate [20], and Turkish lignite [21]. Two factors were analysed when Karacan et al. [21] employed DoE to produce highly porous AC from lignite: chemical impregnation ratio (ranging from 0 to 4) and activation temperature (ranging from 500 to 900 °C). The authors found that the optimum condition to prepare AC with the highest SBET was at a temperature of 800 °C with a chemical impregnation ratio of 2.05. Ayyalusamy and Mishra [22] targeted the optimal conditions for the preparation of ACs from polyethylene terephthalate by evaluating the effect of three main factors (activation temperature, holding time, and chemical impregnation ratio) on two responses (high surface area and yield). Through DoE analysis, impregnation ratio and activation time were found to have the most significant effects on SBET values of the ACs. The optimised experimental response values were 537 m2 g−1 and 12.57% for SBET and AC yield, respectively, from optimal preparation conditions of 37.63% ratio of chemical activator, 600 °C, and 30 min.
In this work, Norway spruce (Picea abies (Karst.) L.) bark was used as a biomass precursor to produce activated carbons with highly developed porosities. Norway spruce covers large areas of Europe, accounting for more than 30 million ha [23], and has substantial economic importance for the wood market in Scandinavia and Europe. Yearly, only in Sweden, 90 Mm3 standing volumes are harvested for industrial utilisation. Around 10–15% of this volume consists of bark that falls out as a low-value side-product at the sawmill and pulp industries. Spruce bark is rich in phenolic compounds, such as condensed tannins [24], and its main components comprise cellulose, hemicellulose, and lignin—making it very suitable as a precursor for AC [25].
In this work, Norway spruce bark ACs were produced by chemical activation using ZnCl2 as activator reagent. The literature reports that ZnCl2 acts as a superior activating agent compared to the others, producing AC with much higher developed porosity [8, 18]. As a result, ZnCl2 provides ACs with high surface area and mass yield values [8]. In addition, ZnCl2 is well known for developing AC mesoporosity which is very suitable for solid–liquid separation [18]. The activation with ZnCl2 works as a standard method to make mesoporous AC. In addition, the ZnCl2 has the advantage of using lower activation temperatures [8], making the AC preparation process cheaper when compared to other chemical activators, e.g. KOH and H3PO4. Combining all these factors can be extremely attractive to industry interests, making the process more economically feasible.
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One potential drawback of using ZnCl2 is its potential toxicity if it is not managed correctly in the activation process; however, the acid leaching procedure that the pyrolysed materials are subjected using 6 M HCl at 80 °C under reflux eliminates practically all ZnCl2 compounds from the carbon matrix [5‐8, 18]. This procedure eliminates the toxicity of the AC prepared with ZnCl2 [5‐8, 18].
To the best of our knowledge, only two studies with spruce bark as the main precursor for the preparation of AC have previously been reported [12, 26]. Besides, no optimisation study for the production of ACs from spruce bark using DoE has been reported yet. Furthermore, the effects of pyrolysis temperature, holding time, and ZnCl2/bark ratio on specific surface area (SBET), mesopore area (SMESO), and micropore area (SMICRO) were investigated and optimised using DoE. In addition, the resulting ACs were fully characterised by elemental analysis, X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, hydrophobicity index, and surface morphologies (SEM). To evaluate a possible suitable and efficient application for the prepared ACs, they were tested to adsorb sodium diclofenac (DFC) from aqueous solutions.
2 Materials and methods
2.1 Raw materials
The spruce bark was delivered from a pulp and paper mill and prepared at the Biomass Technology Centre (BTC), Swedish University of Agricultural Sciences, Umeå, Sweden. The wet bark was dried in a custom-made plane drier at 40 °C, shredded with a screen size of 15 mm (Lindner Micromat 2000, Lindner-Recyclingtech GmbH, Spittal an der Drau, Austria), hammer-milled with a screen size of 4 mm (Bühler DFZK 1, Bühler Group, Uzwil, Switzerland), representatively sampled according to ISO 18,135:2017, and cutting-milled with a screen size of 200 µm using a Fritsch Pulverisette 14 mill equipment. The zinc chloride, ZnCl2, was acquired from Sigma-Aldrich, and tap water was used throughout the AC preparation.
2.2 AC preparation
The ACs were prepared through a one-step pyrolysis activation process according to a procedure previously reported in [5‐8, 11]. A total of 15.0 g of spruce bark was blended with the ZnCl2, and about 30.0 mL of water was added during mixing to form a homogeneous paste [5‐8, 11]. The paste was dried at 105 °C for 24 h, was then put in a metallic crucible, and was heated with a fixed heating rate of 10 °C min−1 under nitrogen flow of 600 mL min−1 in a conventional high-temperature oven. After the set temperature was reached, the sample was treated for a specific holding time. After treatment, the reactor was cooled under an N2 flow until the temperature reached 200 °C before the sample was taken out. To remove residual ZnCl2, samples were boiled for 2 h at 75 °C with a 6.0 M HCl solution under reflux conditions and washed with deionised water until the washing fluid obtained a stable pH.
2.3 Experimental design
The spruce bark pyrolysis and activation were performed according to a Box–Behnken design (BBD). BBD is a factorial combination of a minimum of three factors with incomplete block designs. In each block, one factor is held at the central point (0), while the others vary according to four different combination values for the upper (+ 1) and lower (− 1) limits [27, 28]. Our experimental design comprised 15 experiments which included 12 factorial points and three centre points for the factors pyrolysis temperature (°C), pyrolysis holding time (h), and ZnCl2/bark dry matter mass ratio ( −), as listed in Table 1. Based on the literature and preliminary experiments, these three factors and their ranges were chosen [11, 13‐17]. The running order of the experiments was randomised to minimise the effects of the uncontrolled factors.
Table 1
Experimental Box–Behnken design (BBD) matrix
Sample name
Temperature
Holding time
ZnCl2/bark mass ratio
Temperature (°C)
Holding time (h)
ZnCl2/bark mass ratio ( −)
Coded factor levels
Uncoded factor levels
AC1
1
− 1
0
900
1
1.5
AC2
1
0
1
900
2
2
AC3
0
0
0
800
2
1.5
AC4
− 1
1
0
700
3
1.5
AC5
0
0
0
800
2
1.5
AC6
− 1
0
1
700
2
2
AC7
0
0
0
800
2
1.5
AC8
0
1
1
800
3
2
AC9
0
1
− 1
800
3
1
AC10
1
0
− 1
900
2
1
AC11
− 1
0
− 1
700
2
1
AC12
0
− 1
1
800
1
2
AC13
0
− 1
− 1
800
1
1
AC14
− 1
− 1
0
700
1
1.5
AC15
1
1
0
900
3
1.5
The studied responses were mass yield (%); BET surface area, SBET (m2 g−1); mesopore surface area, SMESO (m2 g−1); micropore surface area, SMICRO (m2 g−1); and pore volume (cm2 g−1).
Minitab software (version 20, Minitab Inc., USA) was used for the BBD analysis to elucidate factor influences on the responses (SBET, SMESO, and SMICRO) and generate factor values for optimal responses.
2.4 AC characterisation
The mass yield (%) was calculated from the dry matter quota after and before activation.
Analyses of surface areas (SBET, SMESO, and SMICRO) and pore volume were carried out by N2 sorption/desorption analysis (Tri Star 3000 apparatus, Micrometrics Instrument Corp.). Before the analysis, samples were degassed at 180 °C for 3 h in an N2 atmosphere. The surface areas, pore size distribution, and pore volume were calculated by multipoint nitrogen gas sorptiometry according to the Brunauer–Emmett–Teller (BET) principle. BET and t-plot equations were used to calculate the SBET, SMESO, and SMICRO.
The elemental analysis was performed using an elemental analyser (EA-IsoLink, Thermo Fisher Scientific). Shortly, 0.05 g oven-dried samples were used to determine total carbon (C), nitrogen (N), oxygen (O), and hydrogen (H) contents. The ash mass fraction was determined by subtracting the C, N, O, and H mass fractions from the total mass of the sample.
XPS spectra were collected using a Kratos Axis Ultra DLD electron spectrometer using a monochromated Al Kα source operated at 150 W. An analyser pass energy of 160 eV for acquiring survey spectra and a pass energy of 20 eV for individual photoelectron lines were used. The samples were gently hand-pressed using a clean Ni spatula into the powder sample holder. Because activated carbon is conductive, no charge neutralisation system was used. The binding energy (BE) scale was calibrated following the ASTM E2108 and ISO 15,472 standards. Processing of the spectra was accomplished with the Kratos software.
Raman spectra were collected using a Bruker Bravo spectrometer (Bruker, Ettlingen, Germany) attached to a docking measuring station. Shortly, 0.5 g of AC samples were manually ground using an agate mortar and pestle, placed in 2.5-mL glass vials, and scanned in the 300–3200 cm−1 spectral range at 4 cm−1 resolution for 256 scans. Min–Max normalisation over the 1000–2000 cm−1 region and smoothing (9 points) was done using the built-in functions of the OPUS software (version 7, Bruker Optik GmbH, Ettlingen, Germany). No baseline correction was needed.
The H2O vapour adsorption isotherms were determined by dynamic vapour sorption (DVS Advantage, Surface Measurement Systems). Shortly, 0.02 g was used to get the isotherms. Equilibrium moisture contents (EMCs) were collected by recording the sample weight at each 1% RH in the 0–10% range, each 2.5% RH in the 10–20% range, each 5% in the 20–50% range, and each 10% in the 50–95% range during adsorption, and at each 20% during desorption.
The hydrophobicity/hydrophilicity index (HI) was calculated according to a method previously reported in the literature [22]: 0.3 g of each AC was placed into 5-mL beakers and inserted into plugged 1.5-L E-flasks with saturated atmosphere solvent vapour (water or n-heptane) using 80 mL of each solvent. The beakers were placed in the centre of the E-flasks to avoid contact with the flask walls. After 24 h, the beakers were removed and weighed. The weight gained was used to calculate the maximum H2O vapour adsorption.
2.5 Diclofenac (DCF) batch adsorption tests
The DCF solution used for the adsorption tests was at an initial concentration of 2000 mg L−1 and a pH of 6.0. Aliquots of 20.00 mL of DFC were added to 50.0-mL Falcon flat tubes containing 30 mg of each AC. The Falcon tubes containing DCF and ACs were agitated in a shaker model TE-240 for 4 h. Afterwards, to separate the DFC and ACs, the flasks were centrifuged. After adsorption, the residual solution of DCF was quantified using a UV–Visible spectrophotometer (Shimadzu 1800) at a maximum wavelength of 285 nm. The amount of DCF adsorbed by the ACs was calculated using Eq. (1):
$$q=\frac{\left({C}_{o}-{C}_{f}\right)}{m}.V$$
(1)
3 Results and discussion
3.1 Yield and textural characteristics of the ACs
Table 2 shows SBET, SMESO, SMICRO, SMESO%, SMICRO%, pore volume, and mass yield of the prepared ACs. The mass yield of the produced ACs ranged from 30.3 to 41%. The lowest yield was obtained for AC15 (900 °C, 3 h, and ZnCl2/bark ratio 1.5), and the highest for AC6 (700 °C, 2 h, and ZnCl2/bark ratio 2). It has previously been reported that a higher pyrolysis temperature and longer holding time reduce the mass yield [29, 30] due to more volatile compounds exiting the biomass during the pyrolysis.
Table 2
Textural properties and mass yield of the ACs
Sample name
Experimental conditions (°C:h:ratio)
SBET (m2 g−1)
SMESO (m2 g−1)
SMICRO (m2 g−1)
SMESO% (%)
SMICRO% (%)
Pore volume (cm3 g−1)
Mass yield (%)
AC1
900:1:1.5
1019
837
182
82.1
17.9
0.57
37.5
AC2
900:2:2
1212
1132
80
93.4
6.6
0.78
35.1
AC3
800:2:1.5
1181
935
246
79.2
20.8
0.68
37.7
AC4
700:3:1.5
1316
1152
164
87.5
12.5
0.80
39.0
AC5
800:2:1.5
1344
1008
226
75.0
25.0
0.69
40.3
AC6
700:2:2
1346
1311
35
97.4
2.6
0.87
41.0
AC7
800:2:1.5
1298
1122
222
86.4
13.6
0.76
39.0
AC8
800:3:2
1185
68
1117
5.7
94.3
0.74
40.3
AC9
800:3:1
1067
526
541
49.3
50.7
0.42
30.3
AC10
900:2:1
1018
562
456
55.2
44.8
0.56
40.7
AC11
700:2:1
1196
564
632
47.2
52.8
0.63
38.3
AC12
800:1:2
1294
1244
49
96.1
3.9
0.83
38.5
AC13
800:1:1
739
385
354
52.1
47.9
0.56
38.6
AC14
700:1:1.5
1374
1124
251
81.8
18.2
0.80
31.5
AC15
900:3:1.5
1076
207
869
19.2
80.8
0.63
30.1
For instance, Bergna et al. [29] produced ACs from birch and spruce wood chips at two different activation holding times: 2 and 4 h, with reported mass yield values of 10.4% and 5.5% for birch and 15.2% and 11.5% for spruce, respectively. Sulaiman et al. [30] pyrolysed cassava stem to produce AC with yields varying from 3.8 to 25.7%. In another work [31], ACs were prepared by using CO2 and MgCl2 as activation agents. The yield of the AC prepared with CO2 was 6.54%, while MgCl2 activation resulted in a yield of 18.1%. The abovementioned studies presented a substantially lower yield compared to the ACs prepared in this work. Generally, ZnCl2 activation gives higher yields than activation with other chemical reagents [29, 30]. Impregnation with ZnCl2 results in degradation of the cellulosic material, and carbonisation produces dehydration that results in charring and aromatisation of the carbon skeleton, thereby avoiding huge losses [6, 8].
All produced ACs, regardless of experimental conditions, exhibited high SBET values (Table 2). The highest SBET value of 1374 m2 g−1 was obtained through pyrolysis at 700 °C for 1 h and a ZnCl2/bark ratio of 1.5 (AC14). This AC preparation condition corresponded to the lowest temperature and holding time and the mid-level for the ZnCl2/bark ratio in the experimental design. Also, ACs with SBET above 1300 m2 g−1 were generated at three other experimental conditions (see Table 2), indicating that spruce bark is a promising and feasible precursor for producing ACs with high specific surface areas and well-developed porosities using different conditions.
As indicated by SMESO and SMESO% values, the spruce bark ACs showed mesoporous structures; for instance, the percentage of mesopores (with relation to SBET values) of AC6, AC12, and AC2 was 97.4%, 96.1%, and 93.4%, respectively. The presence of mesopores is highly desirable in ACs for adsorption and energy storage applications because they ensure wetting and liquid transport throughout the bulk of the AC [2, 3, 9, 10]. In contrast, AC8 (prepared at 800 °C, 3 h, ZnCl2/bark ratio of 1) and AC15 (prepared at 900 °C, 3 h, ZnCl2/bark ratio of 1.5) exhibited highly microporous structures: 94.3% and 80.8% of the total SBET, respectively.
Comparing the obtained results in Table 2 with international literature, Dos Reis et al. [11] produced ACs from sewage sludge and reported SBET values up to 679 m2 g−1. Danish et al. [32] used Acacia mangium wood to make AC and reported an SBET of 1767 m2 g−1, with 95% of the total surface area as mesopores. In another work, Li et al. [20] prepared ACs from Chinese chestnut burs; the highest SBET was 1254.5 m2 g−1 with a microporosity ratio of 87.2%.
Bouchelta et al. [33] produced ACs from biomass of Algerian date pits with high-developed microporosity. The AC with the highest SBET (1467 m2 g−1) was obtained at 700 °C and 4 h of holding time. They concluded that increasing the holding time favoured the development of microporosity and led to high SBET values. Mistar et al. [34] synthesised ACs from yellow bamboo (Bambusa vulgaris “Striata”), and all prepared ACs presented predominantly microporous structures. The AC’s microporosity increased significantly with pyrolysis temperature and chemical activator ratio.
Galiatsatou et al. [35] used olive pulp and peach stones as precursors for AC preparation and concluded that extended holding times favoured mesopore development. Hu et al. [36] reported that high chemical ratios, long holding times, and high temperatures increase the mesoporosity in ACs. They suggested that a high ZnCl2/biomass ratio creates new mesopores by widening the micropores.
The SBET values of the AC samples in this study increased as follows: AC13 < AC10 < AC1 < AC9 < AC15 < AC3 < AC8 < AC11 < AC2 < AC12 < AC7 < AC4 < AC5 < AC6 < AC14, while the mesoporosity increased from AC8 < AC15 < AC11 < AC9 < AC10 < AC7 < AC5 < AC3 < AC14 < AC1 < AC7 < AC4 < AC2 < AC12 < AC6. By these results, it is difficult to establish correlations between SBET values and single pyrolysis parameters, but by applying the DoE data analysis methodology, it is possible to clarify the relations between experimental factors and SBET and micro-mesoporosity formation. These results are presented in the following section.
3.2 Response surface plots, influence of the factors, and statistical analysis
3.2.1 Response surface plots
The response–contour surface plot is a valuable and visual tool for understanding how selected factors affect the studied responses by providing a “map” of the fitted model’s response values at all possible settings within the range of the experimental design. However, it is important to remember that the visuals represent model data, not experimental values, to avoid overinterpretation of results and underestimating model errors.
Figure 1A–I show trends for the factors’ effects and their interactions on the specified responses for the ACs. In addition, optimum conditions can be found within the region with the highest response values. ACs with the highest SBET are obtained at shorter activation holding times, low pyrolysis temperatures, and high ZnCl2/bark ratios (see Fig. 1A–C). In more detail, ACs with the highest SBET values are obtained at holding times no longer than 2.5 h, pyrolysis temperatures no higher than 800 °C, and a ZnCl2/bark ratio up to 2.0.
Fig. 1
Contour plots for effects of pyrolysis temperature, holding time, and ZnCl2/bark ratio on SBET (A–C), SMESO (D–F), and SMICRO (G–I)
×
The highest mesopore surface area (SMESO) is obtained at pyrolysis temperatures up to 800 °C, holding times up to 2 h, and ZnCl2/bark ratios around 1.5–2 (see Fig. 1D–F). High micropore surface areas (SMICRO) require longer activation holding times (close to 3 h) and a pyrolysis temperature up to 800 °C. Here, the effect of the ZnCl2/bark ratio is more challenging to interpret due to possible interaction effects (see Fig. 1G–I).
Surface contour plots show the behaviour of the studied factors on the SBET, SMESO, and SMICRO responses; however, elucidating factor and interaction importance need additional analysis. For this purpose, Pareto charts and normal plots are used and analysed in the below section.
3.2.2 Pareto charts and normal plots
From experimental results, the effects of the factors and their interactions on the SBET, SMESO, and SMICRO can be statistically evaluated by use of a Pareto chart; it aims to graphically demonstrate the significance and the relationship of the studied factors on the related responses [11, 13, 15]. The bars visualise the factors and their interactions [11]. The dotted line is associated with the p-value [11]; the factors or interactions that exceed this line have a statistically significant effect on the studied response [11]. Therefore, for the SBET response, the Pareto chart (Fig. 2A) shows that the ZnCl2/bark ratio (C), followed by pyrolysis temperature (A), is statistically significant (α = 0.05) factors while holding time (B) in the studied range does not influence the SBET values significantly. The square for the factors ZnCl2/bark ratio (CC) and holding time (BB) and the interaction between holding time and ZnCl2/bark ratio (BC) have significant but weak effects on the SBET. For the SMESO, the interaction between holding time and ZnCl2/bark ratio (BC) is the most critical factor (Fig. 2B), while for SMICRO, holding time (B) is the most influential. All other factors with t-test values that do not cross the reference line are statistically insignificant (α = 0.05) [11, 13].
Fig. 2
Pareto charts of the standardised effects of pyrolysis temperature, holding time, and the ratio of ZnCl2 on A SBET, B SMESO, and C SMICRO for the AC preparation (at a minimum statistically significant level of 95% confidence)
×
Morali U et al. [36] optimised the preparation of AC from sunflower seed extracted meal using DoE. They concluded that the ZnCl2/biomass ratio was one of the most influential factors for the SBET values of the ACs. In another work, dos Reis et al. [11] used DoE to optimise the AC textural properties and similarly found that the ratio of ZnCl2 played a crucial role for the SBET values. In carbon activation, ZnCl2 is intercalated into the carbon matrix to produce pores at temperatures above its melting point, which act as templates to create and develop porosity [35, 36]. The melted ZnCl2 expands the existing micropores into mesopores leading to a higher SBET [35, 36]. Moreover, in this activation process, reactions between the carbon atoms in the precursor material and ZnCl2 in extended carbon interlayers contribute to creating pores [9, 35]. In addition, ZnCl2 chemical activation increases the carbon content by forming an aromatic graphitic structure, leading to a higher SBET.
The Pareto chart helps to identify the factors and interactions most decisive for a selected response [11]. However, it does not show if the effect is positive or negative; a normal plot provides such information (see Fig. 3). In the normal plot, the significant factors are labelled with red symbols; those that negatively affect the response are situated far to the left of the fit line and vice versa [11]. Consequently, high temperature decreases the SBET (Fig. 3A), and a long holding time increases the micropore area (Fig. 3C). A high ZnCl2/bark ratio increases the SBET values (Fig. 3A), and interactions between holding time and the ZnCl2/bark ratio increase the mesopore area. The remaining factors and their interactions have no statistically significant effects (α = 0.05) on the responses under the studied conditions.
Fig. 3
Normal plots of the standardised effects of pyrolysis temperature, holding time, and the ratio of ZnCl2 on A SBET, B SMESO, and C SMICRO for the AC preparation (at a minimum statistically significant level of 95% confidence). Negative and positive effects are separated at the 50% level
×
Our results agree with previous literature, showing that the concentration of ZnCl2 during activation substantially affects the SBET values of the ACs [5‐8, 37, 38]. This result is reasonable because when the ZnCl2/precursor ratio increases, more metallic ions are complexed with the functional groups present in the biomass during the mixing step (between biomass and ZnCl2) [39, 40]. After pyrolysis, the inorganic compounds occupy a considerable volume of the carbonaceous matrix. When these inorganics are leached out during the HCl washing step, the resulting AC materials display well-developed pore network structures and high SBET [39, 40].
In addition, the negative effect of the temperature on the SBET is also reported in the literature [11, 41]. The authors concluded that increased pyrolysis temperatures induce shrinkage of the carbon structure, resulting in a reduction in SBET and pore volume. This finding is also reported by several researchers, which showed a negative effect of pyrolysis temperature on the SBET [3, 8, 21, 25].
3.3 AC preparation: comparison with literature
Any precursor rich in carbon can be converted to activated carbon with tailored properties targeting different applications such as adsorbents, catalysts, and anodes for energy storage devices. However, to produce an efficient AC, careful considerations of pyrolysis conditions must be made since the conditions play an essential role in the final AC’s characteristics. Moreover, robust optimisation studies are required to make the production process sustainable and environmentally friendly.
DoE has been used to optimise AC preparation in several previous studies (see Table 3). Comparing the results obtained in this work with international literature gives a more accurate evaluation of whether the DoE was applied and appropriately conducted and how effective it can be to optimise the preparation of porous AC. For an efficient comparison, the optimisation of the production of ACs, using several different DoE, is displayed in Table 3.
Table 3
Factors and outcomes from DoE-assisted studies on AC production
Precursor
DoE method
Activation reagent
Studied factors and levels
Studied responses
Main factor
Main outcomes
Ref
Sewage sludge
23 full factorial design
11 runs
ZnCl2
Temperature: 500, 650, 800 °C
Holding time: 15, 37.5, 60 min
Ratio in ZnCl2/precursor: 0.5:1, 1:1, 1.5:1
SBET
The most significant factors were pyrolysis temperature and interaction between pyrolysis temperature, holding time, and ratio of ZnCl2:sludge
The optimum condition was at pyrolysis temperature of 500 °C, holding time of 15 min, and a ratio of ZnCl2:sludge of 0.5
Table 3 shows that the pyrolysis temperature, holding time, and chemical/precursor ratio are the most studied factors, and SBET is the most evaluated response. A large variety of carbon precursors have been treated under different preparation conditions. Table 4 shows a considerable variation in response values (especially for SBET) with biomass precursors and preparation conditions.
Table 4
Elemental composition of the spruce bark ACs
Sample name
C (%)
N (%)
H (%)
O (%)
Ash (%)
C/N
C/O
AC1
93.7
0.51
0.79
2.11
2.9
183
44.4
AC2
93.4
0.52
1.20
2.38
2.5
179
39.2
AC3
94.5
0.46
0.98
2.12
1.9
205
44.6
AC4
93.4
0.43
0.95
2.27
2.9
217
41.1
AC5
94.5
0.44
0.88
2.18
2.0
214
43.3
AC6
93.6
0.42
1.10
2.13
2.7
222
43.9
AC7
93.3
0.43
1.12
1.94
3.2
222
48.1
AC8
93.3
0.43
0.99
1.87
3.4
217
49.9
AC9
94.0
0.50
0.98
2.02
2.5
188
46.5
AC10
93.2
0.64
1.18
2.65
2.3
145
35.2
AC11
93.9
0.50
1.11
2.38
2.1
187
39.5
AC12
93.8
0.49
0.99
2.20
2.5
191
42.6
AC13
94.8
0.50
1.13
2.47
1.1
189
38.4
AC14
93.3
0.43
1.02
2.77
2.5
217
33.7
AC15
94.5
0.42
1.06
1.65
2.4
225
57.3
Abioye et al. [42] employed a DoE Box–Behnken design (with 15 runs in total) to optimise oil palm shell ACs’ textural properties. Three factors were evaluated: pyrolysis temperature: 800, 850, 900 °C; holding time: 20, 30, 40 min; and CO2 flow rate: 200, 300, 400 cm3 min−1. Two responses were analysed: SBET and micropore volume, SMICRO. The authors found that the holding time had the most profound effects on SBET and SMICRO values [42]. Also, the SBET values ranged from 291 to 574 m2 g−1. The optimum condition was identified as 900 °C, 40 min, and CO2 flow rate: 400 cm3 min−1.
dos Reis et al. [11] used DoE (a full factorial design with 12 runs in total) to optimise the production of ACs from sewage sludge through conventional and microwave heating methods. The authors evaluated three factors with SBET as the response. The factors were pyrolysis temperature or microwave power, holding time, and chemical activator proportion. Factor conditions were temperature (500, 650, 800 °C), microwave power (700, 840, 980 W), holding time (15, 37.5, and 60 min for conventional pyrolysis and 8, 10, and 12 min for the microwave methods), and the ZnCl2:sludge ratio (0.5, 1.0, and 1.5). The optimum factor combination for producing AC with the highest surface area was found at 500 °C, 15 min, and a ZnCl2:sludge ratio of 0.5 for conventional pyrolysis, while it was 980 W, 12 min, and a ZnCl2:sludge ratio of 0.5 for microwave pyrolysis. The optimal conditions generated ACs with SBET values of 679 m2 g−1 and 501 m2 g−1 for conventional and microwave pyrolysis, respectively. The most critical factors for conventional pyrolysis were temperature and the interaction between pyrolysis temperature, holding time, and chemical activator. The interaction between radiation power, holding time, and the chemical activator ratio was the most influential for microwave pyrolysis.
Comparing our results with the literature data (Table 3) reveals that ACs with very high SBET and well-developed mesoporosity can be produced from Norway spruce bark under optimal pyrolysis conditions. The literature on AC production covers a plethora of precursors and experimental matrices of pyrolysis conditions and activation methods. It is not always possible to compare the results from different studies, but it is safe to conclude that the optimum methodology varies with the precursor properties. Also, further systematic studies are needed to increase the knowledge on how suitable combinations of biomass precursors, pyrolysis conditions, and chemical activation procedures to obtain ACs with textural and functional properties for specific applications.
3.4 Chemical characterisation and morphology of the activated carbons
3.4.1 Elementary analysis
Table 4 shows the results for the elemental composition of the prepared ACs. For comparison, a typical commercial AC has 88% of C, 0.5% of H, 0.5% of N, and 3–4% ash [47]. All ACs prepared from spruce bark through ZnCl2 activation had a C content above 93% (Table 5), which can be considered a high value compared with the literature. For example, Correa et al. [47] employed a range of different biomasses, based on alpha-cellulose, xylan, kraft lignin, to produce ACs, and their C content varied from 76.9 to 87.8%. In another work [48], coconut shell was used to prepare ACs, and the highest C content was 82.66%. High carbon content reflects good AC characteristics due to the carbon’s physicochemical, electrical, and thermal properties. Also, an elevated C content is often a sign of low ash content, and ashes in AC are often responsible for reducing SBET and surface functionalities. Surface functionality is vital for many AC applications, such as adsorption of pollutants in waters and as electrode materials in energy storage systems.
Table 5
XPS spectral deconvolution (at%) of AC1, AC9, AC13, and AC14
O1s
Samples
C1s
O1s
C/O
C = O
C–OH, C–O–C
π-π* excitation
AC1
94.8
5.0
19.0
1.46
2.80
0.73
AC9
94.7
4.9
19.2
1.24
3.02
0.67
AC13
94.7
4.4
21.4
1.51
2.21
0.70
AC14
94.5
5.2
18.2
2.10
2.40
0.70
The prepared ACs had relatively high oxygen content. This feature may reflect a high hydrophilicity index, a good quality for AC applications in solid/liquid systems [47]. In addition, the ACs had high C/O ratios, which indicates the formation of hydroxyl groups on the AC surfaces (this was analysed by XPS analysis and discussed in the following section). Also, high C/O ratios indicate that the spruce bark AC structures are similar to near-perfect graphene layers. Sample AC15 had the highest (57.3) and AC14 the lowest (33.7) C/O values.
3.4.2 X-ray photoelectron spectroscopy (XPS)
The elemental composition and chemical state of the ACs were analysed with XPS. It was employed to understand AC surface chemistry and assess the effects of the ZnCl2 treatment and the relationship between spruce bark composition and pyrolysis conditions. Four AC samples (AC1, AC9, AC13, and AC14) were studied (Fig. 4). The samples were selected based on their SBET values (from lowest (AC13) to highest (AC14) and two in between (AC1 and AC9).
Fig. 4
XPS C1s and O1s spectra of AC1, AC9, AC13, and AC14
×
As can be seen, Fig. 4 shows spectra related to C1s and O1s, carbon and oxygen bonds, respectively [14, 49, 50]. The deconvolution of the C1s spectrum indicates that all samples contain graphitic/aromatic C = C or hydrocarbon C–C (~ 284.3 eV), C-O in phenolic hydroxyl or ether groups (~ 286.5 eV), carbonyl C-O (~ 287.5 eV), and ester O = C–O (~ 289.1 eV), graphitic carbon being the major component [49].
O1s spectra were deconvoluted to three chemical oxygen states with binding energies at around 530.8–531.2 eV, 532.6–533.1 eV, and 534.8–536.3 eV. These binding energies could correspond to oxygen singly bonded to carbon in aromatic rings, in phenols and ethers (533.2–533.8 eV), or to oxygen double-bonded with carbon in carbonyl and quinone-like structures (530.8–531.2 eV) [51], confirming the presence of some functional groups on the ACs’ surfaces.
The quantitative information from XPS is shown in Table 5. The surface C content of all four ACs did not vary significantly (from 94.5 to 94.8%). In contrast, the O content showed a larger variation (from 4.4 to 5.2%), and the values match the values presented by elementary analysis (see Table 4). The highest O1s content (5.2%) was found in the AC14 sample, suggesting that this sample had more functionalities when compared to the others, especially for functional groups related to C = O, C–OH, and C–O–C (see Table 5). The AC14 sample had the highest SBET and amount of functional groups, suggesting high efficiency as an adsorbent for water decontamination and electrode material for energy storage systems [2, 9, 18, 37].
3.4.3 Raman spectroscopy
Raman spectroscopy analysis was performed to evaluate the degree of graphitisation of the prepared ACs. Using Raman, it was possible to determine the ratio of the ID/IG bands (see Fig. 5). The lower ID/IG value indicates that ACs exhibit more perfect and orderly graphite structures with a high graphitisation degree [52‐54], while a higher ID/IG peak intensity ratio reveals more structural defects in carbon materials. Moreover, ID/IG values also serve to identify the size of the sp2 domain, related to graphene structure, in the AC structure [53, 55]. Figure 5 shows that AC6 (700 °C, 2 h, 2) had the highest graphitisation degree, followed by AC14 (700 °C, 1 h, 1.5)—presenting ID/IG values of 0.85 and 0.88, respectively. These values indicate that both samples (AC6 and AC14) had highly disordered structures. Interestingly, these samples also presented SBET values among the highest in the experimental design: 1343 m2 g−1 (AC6) and 1374 m2 g−1 (AC14). On the other hand, AC1, AC2, AC10, and AC12 presented ID/IG values higher than 1.00. AC1, AC2, and AC10 samples were pyrolysed at 900 °C while AC12 at 800 °C.
Fig. 5
Ratio of ID/IG bands of the ACs
×
3.4.4 Hydrophobic-hydrophilic analysis (HI)
The surface characteristics of the prepared ACs were evaluated by solvent vapours of different polarities using n-heptane (less polar) and water (more polar) [18, 56, 57]. The hydrophobic-hydrophilic ratio is an important parameter to estimate the tendency of the material to adsorb compounds that are organic or water-based [56]. Figure 6 displays the calculated mass ratio of n-heptane to water uptake by all ACs. It is observed that all the AC surfaces are predominantly hydrophilic (see Fig. 6) since HI values are lower than 1.0. Hydrophilic groups on AC surfaces are related to H-bonding and oxygen groups (already shown in Fig. 4) known to be present on biomass AC surfaces [18].
Fig. 6
Ratio of n-heptane and water adsorption values
×
However, AC2, AC8, and AC12 presented HI values above 1.0, which means they have a hydrophobic behaviour. Interestingly, these three samples were made with a ZnCl2:bark ratio of 2 and at higher temperatures (800 and 900 °C); it seems that a high ZnCl2 concentration and high temperature lead to more hydrophobic surfaces. Leite et al. [18] prepared AC from avocado seed biomass and stated that an increase in pyrolysis temperature increased the HI values of the ACs. However, the hydrophobic/hydrophilic behaviour of ACs is not only influenced by the experimental pyrolysis condition and activation method. It also depends on the number of functional groups and types present on the AC’s surface, the AC’s aromatisation rate, and the chemical nature of the biomass precursor [35, 49, 57].
3.4.5 Surface morphology (SEM analysis)
SEM was employed to examine the effects of pyrolysis and activation conditions on the surface morphology characteristics of the AC samples. All SEM micrographs (Fig. 7) show rugosity, an irregular structure, holes, and cavities to a lesser or greater extent that agrees with the SBET and pore structure analyses. The SEM pictures reveal differences that indicate that the pyrolysis conditions and the ratio of ZnCl2 did influence the surface characteristics of the ACs.
Fig. 7
SEM images of AC samples AC1, AC2, AC4, AC6, AC7, AC8, AC9, AC13, and AC14. All at 5 K of magnification
×
Macropores are present in all samples. Macropores are very important if the AC is used as an adsorbent to remove water pollutants or electrodes in energy storage systems. They serve as vectors for the solution passage through the pores until it attains the micropores and mesopores. Depending on the preparation conditions, the AC external surfaces exhibited macropores of different sizes and shapes, reflecting their SBET, SMICRO, and SMESO values.
The ACs were found to be very porous regardless of the preparation condition. During the pyrolysis, it seems that the cavities and holes on the AC’s surfaces resulted from the ZnCl2 state transformation—its low melting point at 290 °C and the boiling point at 732 °C are fused into the biomass matrix, thereby creating a denser structure and well-developed pore network [5, 6, 11, 18].
3.4.6 Water vapour sorption
Water vapour sorption isotherms for the AC1, AC4, AC9, AC13, AC14, and AC15 are shown in Fig. 8. According to the IUPAC classification [58], all isotherms are very close to type V, characterised by little water uptake at low relative pressures and the presence of a hysteresis loop over a big part of the pressure range. Although the isotherms exhibited similar types and shapes, the hysteresis forms differ, indicating that the pyrolysis conditions influence the AC characteristics as previously discussed in the manuscript.
Fig. 8
Water vapour adsorption isotherms at 30 °C
×
The H2O isotherms were not influenced by the AC’s textural properties, such as SBET, because the AC with the highest SBET presented one of the lowest H2O uptake values. Hence, the AC water vapour sorption may, contrary to the N2 adsorption, be very dependent on the surface chemistry, such as the presence of functional groups on the AC surface.
Water adsorption isotherms for most porous carbons exhibit a hysteresis loop. The size and shape of the loop varied between ACs with different pore sizes and pore structures. From the isotherms displayed in Fig. 8, it is possible to see that AC8—the most microporous sample (94.6%)—exhibited the longest and widest hysteresis. Gallego-Gomez reported that the completion of the micropore filling delays the onset of multilayer adsorption, leading to hysteresis at high relative pressure [59].
3.4.7 DFC adsorption
In this work, Norway spruce bark activated carbons were tested to remove DCF from aqueous solutions to evaluate their suitability for the adsorption process (see Fig. 9). The ACs displayed very high adsorption capacity values (272–417.4 mg g−1). Their physicochemical properties can explain the high efficiency of the AC in removing DFC. All ACs presented very high SBET and well-developed pore structure in the range of mesoporosity with few samples mainly composed of micropores. Both micro- and mesoporous ACs are highly efficient to adsorb DFC molecules with a small molecule size (1.015 nm) [62]. The DCF molecule can easily be accommodated in pores bigger than its size, which is the case of micropores up to 2.0 nm and mesopores up to 50.0 nm.
Fig. 9
DFC adsorption capacity on spruce bark activated carbons
×
The results exhibited in Fig. 9 indicate that the prepared ACs could be successfully employed to remove DFC and organic emerging compounds from aqueous solutions.
Comparing our results with the literature data, the adsorption capacity (q) of the best performing AC (AC14) is comparable and even higher to the adsorption capacities of many different adsorbents reported in the literature (see Table 6). Compared to ACs produced in other works, the high diclofenac adsorption capacity efficiency can be attributed to the small particle size and high developed porosity and the functional groups present on the surface of the ACs.
Table 6
Comparison of the adsorption capacities for diclofenac using different adsorbents
Table 6 shows that Norway spruce bark AC (AC14) had the second-highest adsorption capacity (417.4 mg g−1) among all presented adsorbents. The highest q value (444.44 mg g−1) was reached by polyethyleneimine-functionalised sodium alginate/cellulose nanocrystal/polyvinyl alcohol core–shell microspheres (PVA/SA/CNC)@PEI). Although PVA/SA/CNC)@PEI presented a higher q value than AC14, its production cost is incredibly much higher when compared to that of AC14. Consequently, if the production cost is added to the desirable properties, ZnCl2-activated Norway spruce AC could be classified as an excellent adsorbent to remove DFC from aqueous solutions.
4 Conclusion
Norway spruce bark can be a very suitable feedstock for the production of activated carbons with well-developed porosity. Response surface methodology based on Box–Behnken design was confirmed to be effective in optimising activated carbons’ preparation. Three responses were studied: SBET, mesoporosity (SMESO), and microporosity (SMICRO). The maximum values for the responses were obtained at the following conditions: SBET: 700 °C, 1 h, and 1.5:1 (sample AC14), SMESO: 700 °C, 2 h, and 2:1, and SMICRO: 800 °C, 3 h, and 2:1 ZnCl2:bark ratio. The highest SBET, SMESO, and SMICRO were 1374, 1311, and 1117 m2 g−1, respectively. The ZnCl2 ratio and pyrolysis temperature were the most critical factors for the SBET, while the interaction between temperature and ZnCl2 ratio was the most significant factor for SMESO. For the SMICRO, holding time was the most important factor.
The spruce bark AC showed predominant mesoporous structures and relatively high yield compared to the literature. Based on ID/IG values, the AC samples with the highest SBET values presented much more disordered structures. The water vapour results suggested that the H2O adsorption was not controlled by the textural properties of AC but rather by their chemical properties and the availability of functional sites. The DFC adsorption data showed that the AC displayed very high adsorption capacity values (272–417.4 mg g−1), and the sample with the highest SBET (AC14) also exhibited the highest q value (417.4 mg g−1) for DFC removal.
The large surface area, interesting chemical structure, and high DFC adsorption capacity of the ACs in this study show a great potential of Norway spruce bark residues as a precursor material for the production of AC with good adsorption properties.
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