**1. Introduction**

Biomass is a renewable and widespread resource that, if utilized sustainably, can help to reduce the emission of carbon dioxide that directly affects global warming [1]. Agricultural and forestry residues and by-products from biobased industries can be used as feedstock for energy production and material applications to replace fossil fuel sources [2,3].

Norway spruce (*Picea abies* (L.) Karst.) is one of the most common and economically valuable trees for the European forest industry as it is widely distributed from central to boreal and eastern Europe [4]. Together with pine and birch, spruce is the most common tree species in Sweden; these three combined comprise more than 90% of standing volume [4]. The Swedish annual forest harvest amounts to approximately 90 Mm<sup>3</sup> standing volume [5], and they are economically very important for sawmill and paper and pulp industries. However, in the production of sawn timber, pulp, and paper, only the stem wood is used— the remaining components can be considered industrial by-products. Around 10–15% of the feedstock volume delivered to the forest industries consists of bark currently mainly utilized as fuel and other low-value applications [5,6]. Consequently, research to employ bark as a precursor for value-added and eco-friendly material products is motivated.

Using biomass to produce biobased carbon materials (BBC) such as biochar (BC), activated carbon (AC), carbon composite materials (CCM) is an application with grea<sup>t</sup> potential. It reduces fossil carbon use and can provide new types of functionalities [6–14]. BBC is the oldest, most common, and efficient material for removing pollutants from aqueous media [7–13]. Besides its chemical stability, surface functionalities, high porosity, and specific surface area are essential characteristics for efficient application in the adsorption process [8–16]. However, high-purity activated carbons are expensive; therefore, the use of other biobased carbon materials can be explored as adsorbents for the removal of pollutants and micropollutants [8,10–16].

Adsorption is seen as one of the most suitable treatment methods for tackling pollutants from contaminated water and wastewaters due to its simple operating conditions, high efficiency, and low-cost employment. To design a very efficient adsorption process, the BBC must be prepared to achieve suitable properties.

The BBC properties are highly dependent on pyrolysis conditions and activation methods [14,17]. For instance, chemical activation can create carbon materials with ultrahigh BET surface area (SBET) and porosities because of extensive micro and mesoporosity development. Each pore structure has a specific role in the adsorption process, e.g., the micropore structure contributes significantly to the SBET values and the adsorption of smallsized contaminants (e.g., metallic species and small organic molecules) [13,17]. Mesopores are essential as vectors to the surface areas within the carbon material particle, and their respective quantities are primarily dependent on the pyrolysis conditions and activation method. The mesopore structure is vital for larger-molecule adsorption, which is the case for dyes and colored effluents.

It is estimated that over 10,000 different dyes and pigments are used in the food, leather, cosmetics, and textile industries, e.g., only the textile industry consumes up to 200,000 tons of dyes yearly, thereby generating large amounts of colored effluents [18]. These colored effluents are, if not adequately treated, discharged into the environment, where they are potentially harmful to the aquatic systems and ecosystem integrity. Besides, many dyes are reported as mutagenic and carcinogenic [19]. Therefore, these effluents must be treated before their discharge into the environment, and the adsorption process using biomass-activated carbon is one the most suitable treatment process [12–16].

The purpose of this study was to investigate the potential of spruce bark residues as a precursor to producing efficient carbon-based materials by pyrolysis, using KOH (KOH-BBC) and ZnCl2 (ZnCl2-BBC) as chemical activators. The effect of the chemical reagents on the BBC characteristics such as morphology, specific surface area and porosity, surface chemistry, BBC composition, hydrophobicity index, carbon yield, and adsorption of two dyes and different synthetic effluents were evaluated.

#### **2. Materials and Methods**

#### *2.1. Preparation of BBCs*

Norway spruce (*Picea abies* Karst.) bark was delivered from a pulp and paper mill in northeast Sweden and prepared at the Biomass Technology Centre (BTC), Swedish University of Agricultural Sciences, Umeå, Sweden. The wet bark was dried in a custommade plane drier at 40 ◦C, shredded with a screen size of 15 mm (Lindner Micromat 2000, Lindner-Recyclingtech Gmbh), hammer-milled with a screen size of 4 mm (Bühler DFZK 1, Bühlergroup), representatively sampled according to ISO 18135:2017, and

cutting-milled with a screen size of 200 μm using a Fritsch Pulverisette 14 mill (FRITSCH GmbH, Germany).

The pyrolysis was done in a single pyrolysis-step preparation according to a previously reported procedure [20–24]. First, 15.0 g of the spruce bark was mixed in a weight ratio of 1:1 with each chemical activation agen<sup>t</sup> (KOH and ZnCl2). During the mixing, about 40.0 mL of water was added to form homogeneous pastes. These two pastes were dried in an oven at 105 ◦C for 24 h. The dried pastes were placed in a metallic crucible and treated thermally in a conventional high-temperature oven under a nitrogen flow of 600 mL min−1. They were heated from 20 to 800 ◦C at a rate of 10 ◦C min−<sup>1</sup> and held at 800 ◦C for 60 min. The oven was turned off to cool down the pyrolyzed samples while the nitrogen flow was kept, and when the temperature dropped to 200 ◦C, the nitrogen flow was shut off. The pyrolyzed materials were milled with a screen size of 200 μm and completely leached out by conventional leaching using 0.1 M HCl, under reflux, for 2 h for KOH-BBC and using 1.0 M HCl, under reflux, for 2 h for ZnCl2-BBC [13,15–17]. The BBC preparation procedure is summarized in Figure 1.

**Figure 1.** The BBC preparation procedure.

#### *2.2. BBC Characterization*

## 2.2.1. Textural Properties

The adsorbent's textural properties, especially for biobased carbon materials, are crucial for evaluating their applications and potential application efficiencies. Common BBCs are almost always heterogeneous, having an unknown range of pore sizes and a range of pore shapes, blocked and network pores [12,14,15].

The samples' surface morphology was observed with scanning electron microscopy (SEM) (55-VP, Supra, Zeiss), using an acceleration voltage of 20 kV and magnification ranging from 100 to 20,000.

N2 adsorption/desorption isotherm analysis (Tristar 3000 apparatus, Micrometrics Instrument Corp., Norcross, GA, USA) was performed to quantify the porosity (by DFT method) and surface area (BET method). Before the analysis, the sample was degassed at 180 ◦C for 3 h in an N2 atmosphere. The specific surface area was calculated in the relative pressure interval of 0.05–0.3 using the Brunauer–Emmett–Teller (BET) method [16,22]. Mesopore size and distribution were calculated by the Barrett–Joyner–Halenda (BJH) method from desorption curves while the micropore area values were calculated by the

t-plot method [16,20,22]. The percentage of the mesopore and micropore areas were calculated based on the SBET values [22].

#### 2.2.2. Elemental Analysis, Yield (%), Raman Spectroscopy, and Zeta Potential

The elemental analysis was carried out to evaluate the volatiles and fixed carbon contents and quantify the elemental composition of the BBCs, respectively. The analysis was made using a CHN Perkin Elmer M CHNS/O Analyzer, model 2400.

The yield (%) was calculated from the dry matter quota of the biomass precursor after and before activation.

Raman spectroscopy is widely applied to characterize BBCs. It is applied to obtain structural information on the bulk of carbon materials. Raman spectra were recorded on a Renishaw inVia Raman spectrometer (Renishaw, Kingswood, UK) at 633 nm HeNe laser.

Zeta-potential was performed to obtain the charge (whether positive or negative) of the BBCs. It was determined at pH 7 using a potential analyzer (Zetasizer Nano ZS90, Malvern Panalytical, Malvern, UK).

#### 2.2.3. Water Vapor Sorption and Hydrophobicity/Hydrophilicity

The BBCs' water vapor sorption isotherms and the hydrophobicity/hydrophilicity index (HI) are used to determine the properties of the BBC/water interface and the water molecules' ability to attach to the BBC surface, which both may influence dye adsorption.

The BBCs' H2O vapor adsorption isotherms were determined by dynamic vapor sorption (DVS Advantage, Surface Measurement Systems) at 25 ◦C, where RH was varied from 0 to 95% and back in 5% steps. The hydrophobicity/hydrophilicity index (HI) was performed according to a method previously reported in the literature [23]: 0.3 g of each BBC was placed into 5 mL beakers and inserted into plugged 1.5 L E-flasks with saturated atmosphere solvent vapor (water or n-heptane) using 80 mL of each solvent. The beakers were placed in the center 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 vapor adsorption.

#### *2.3. Dye Adsorption Analysis*

#### 2.3.1. Batch Adsorption Studies

Aliquots of 20.00 mL of 30.00–1000.0 mg L−<sup>1</sup> of RB-4 and RO-16 were added to 50.0 mL Falcon flat tubes containing 30 mg (dosage of 1.5 g <sup>L</sup>−1) of each BBC [20,24,25]. The Falcon tubes containing RB-4 or RO-16 and BBCs were agitated in a shaker model TE-240 between 0.1–12 h to obtain the kinetics data. Afterward, to separate the dyes and BBCs, the flasks were centrifuged. After adsorption, the residual solutions of RB-4 and RO-16 were quantified using a UV-Visible spectrophotometer (Shimadzu 1800) at a maximum wavelength of 595 and 494 nm, respectively.

The amount of RO-16 adsorbed by the BBCs and the percentage of removal were calculated using Equations (1) and (2), respectively [20,24,25]:

$$q = \frac{\left(\mathbb{C}\_0 - \mathbb{C}\_f\right)}{m} \cdot V \tag{1}$$

$$\% \, Bernoulli = 100 \cdot \frac{\left(\text{C}\_0 - \text{C}\_f\right)}{\text{C}\_0} \tag{2}$$

where *q* is the amount of selected dye uptake by the BBCs (mg g<sup>−</sup>1); *C*0 is the initial dye concentration in contact with BBCs (mg <sup>L</sup>−1), *Cf* is the final concentration (mg <sup>L</sup>−1) after adsorption, *V* is the volume of dye solutions (L) in contact with the BBCs, and *m* is the BBC mass (g).

#### 2.3.2. Adsorption Kinetics and Equilibrium Analysis

Adsorption kinetics provides information on the adsorption rate, the adsorbent's performance, and the mass transfer mechanisms [20,23–25]. Knowing the adsorption kinetics is crucial for designing efficient adsorption systems.

The RO-16 adsorption kinetics of the KOH-BBC and ZnCl2-BBC samples were evaluated at two initial concentrations: 500 and 700 mg L−1. The suitability of different models for predicting the adsorption kinetics was assessed by analyzing R2adj and SD values.Pseudo-first-order (PFO) model, pseudo-second-order (PSO) model, and general order models were used to evaluate the kinetic adsorption process [23–25].

The mathematical representations of pseudo-first-order, pseudo-second-order, and general order are shown in Equations (3)–(5), respectively.

$$q\_t = q\_{t^\circ} \left[ 1 - \exp\left(-k\_1 \cdot t\right) \right] \tag{3}$$

$$q\_t = \frac{k\_2 \cdot q\_c^2 \cdot t}{1 + q\_c \cdot k\_2 \cdot t} \tag{4}$$

$$q\_t = q\_\varepsilon - \frac{q\_\varepsilon}{\left[k\_N \cdot (q\_\varepsilon)^{n-1} \cdot t \cdot (n-1) + 1\right]^{1/(n-1)}}\tag{5}$$

Equilibrium isotherms are used to determine the adsorption affinity and dye removal mechanisms of the adsorption systems [8,23–25]. Each adsorption system (individual adsorbent material and adsorbate) has a unique isotherm, and the quantity of adsorbed adsorbate on an adsorbent depends on both the BBC's and the solution's properties. Therefore, equilibrium studies are mandatory to evaluate and establish adsorbent efficiency.

The equilibrium process was analyzed by Langmuir, Freundlich, and Liu's models. The fit quality was assessed through statistical indicators such as *R*2, *<sup>R</sup>*<sup>2</sup>*adj*, and *SD*. See further details about these indicators in references [13,14,20,23].

Langmuir, Freundlich, and Liu's models are shown in Equations (6)–(8), respectively.

$$\eta\_{\varepsilon} = \frac{Q\_{\text{max}} \, \_{\text{L}} \mathbf{C}\_{\text{L}} \, \_{\text{c}}}{1 + K\_{\text{L}} \, \_{\text{c}} \mathbf{C}\_{\text{c}}} \tag{6}$$

$$q\_{\varepsilon} = K\_F \mathcal{C}\_{\varepsilon}^{1/nF} \tag{7}$$

$$q\_{\epsilon} = \frac{Q\_{\max} \cdot (K\_{\S} \cdot \mathcal{C}\_{\epsilon})^{nL}}{1 + (K\_{\S} \cdot \mathcal{C}\_{\epsilon})^{nL}} \tag{8}$$

Detailed information about all these equations can be found in the literature [10,23,24].

#### 2.3.3. Preparation of the Dyeing Synthetic Effluents

De-ionized water was used for the preparation of all solutions used in the dye adsorption experiments. RB-4 (C23H14N6Cl2O8S2) and RO-16 (C20H17N3O10S3Na2) were obtained from Sigma Aldrich, Sweden. The stock solution was prepared by dissolving the dye in distilled water to 2.00 g L−1. Working solutions were obtained by diluting the dye stock solution to the required concentrations without adjusting the pH.

Synthetic effluents with different compositions (see Table 1) were prepared to test the BBCs' applicability for treating real effluents.

#### 2.3.4. Analytical Control of the Measurements and Statistical Evaluation of Nonlinear Models

The adsorption equations were fitted using the nonlinear approach obtained by the Simplex method and the successive interactions of the Levenberg–Marquardt algorithm [10,15–17,21]. This fitting was acquired by the nonlinear fitting facilities of the Microcal Origin 2020 software, and they were used to fit the kinetic and equilibrium data. The determination coefficient (*R*2), adjusted determination coefficient (*R*<sup>2</sup>*adj*), and the

standard deviation of the residues (SD) were employed to analyze the suitability of the models [10,15–17,21–25].


**Table 1.** Effluent compositions and concentrations.

Residual standard deviation measures the difference between the theoretical and experimental amounts of dyes removed from solutions. The *R*2, *<sup>R</sup>*<sup>2</sup>*adj*, and *SD* are given in Equations (9)–(11), respectively [21–25].

$$R^2 = \left(\frac{\sum\_{i}^{n} \left(q\_{i, \text{exp}} - \overline{q}\_{i, \text{exp}}\right)^2 - \sum\_{i}^{n} \left(q\_{i, \text{exp}} - q\_{i, \text{model}}\right)^2}{\sum\_{i}^{n} \left(q\_{i, \text{exp}} - \overline{q}\_{i, \text{exp}}\right)^2}\right) \tag{9}$$

$$R\_{adj}^2 = 1 - \left(1 - R^2\right) \left(\frac{n-1}{n-p-1}\right) \tag{10}$$

$$SD = \sqrt{\left(\frac{1}{n-p}\right) \cdot \sum\_{i}^{n} (q\_{i,\,\exp} - q\_{i,\,\text{model}})^2} \tag{11}$$

where *qi*,*model* represents the individual theoretical *q* value predicted by the model. *qi,exp* represents each experimental *q* value. *qexp* is the average of the experimental *q* values. *n* and *p* represent the number of experiments and model parameters, respectively.

#### **3. Results and Discussion**

*3.1. BBC Characteristics*

3.1.1. Textural Properties and Porosity

The SEM images show remarkable differences between both microstructures (see Figure 2). The ZnCl2-BBC has a dense structure, with more elongated cavities and holes of different sizes and shapes (Figure 2A,C) that should have been formed during the leaching step with 6.0 mol L−<sup>1</sup> HCl. Additionally, it is observed that ZnCl2-BBC presents a rough surface.

KOH-BBC (Figure 2B,D) presents ordered macropore structure and holes with lower diameter in its surface, which should be attributed to the lower concentration of HCl (1.0 mol <sup>L</sup>−1) used in the leaching step. Both BBC presents irregular particle size and rough surface.

The ZnCl2-BBC was prepared by catalyzed dehydration and elimination of carbonyl and hydroxyl groups during the heat treatment [26,27], and that the ZnCl2 (due to its low melting point at 290 ◦C and the boiling point at 732 ◦C) is fused into the biomass matrix, thereby creating a denser structure and a microporous network [27,28]. On the other hand, KOH activation provokes the breakage of C–O–C and C–C bonds, creating pores and well-developed porosity [26]. Additonally, an uneven distribution of KOH in the bark matrix can promote hyperactivation during the pyrolysis process, resulting in pore wall demolition and widening of the micropores into mesopores [26]. These structural transformations are beneficial for the BBC's physical adsorption of RB-4 and RO-16 and effluents treatment.

**Figure 2.** SEM images of BBCs: (**A**) ZnCl2-BBC at 1.2 K of magnification, (**B**) KOH-BBC at 1.2 K of magnification, (**C**) ZnCl2-BBC at 5 K of magnification, (**D**) KOH-BBC at 5 K of magnification.

The N2 isotherms for ZnCl2-BBC and KOH-BBC (Figure 3) can be ascribed to a type I isotherm. A type I isotherm (also mentioned as Langmuir isotherm) is typical for microporous materials (with pore diameter <2 nm) [16]. Higher amounts of N2 are adsorbed at low relative pressures for microporous materials, and when it is close to 1, the curve may reach a limiting value or rise if larger pores are present [16].

Although both BBCs exhibited a Type I isotherm, the adsorbed N2 volumes differed significantly (Figure 3). The KOH-treated BBCs had an almost 30% higher SBET (1067 m<sup>2</sup> g<sup>−</sup>1) than the ZnCl2-BBC (754 m<sup>2</sup> g<sup>−</sup>1) (Table 2). The external surface area and the micropore and mesopore volumes agree with these results. Hence, it can be concluded that KOH-activation produced a BBC with better textural properties and better adsorption performance than ZnCl2-activation. The percentage of mesopores in KOH-BBC (49.29%) is higher when compared with the ZnCl2-BBC sample (43.51%), while the share of micropores is higher in ZnCl2-BBC (56.49%) when compared to KOH-BBC (50.71%) (See Table 2). However, both micro and mesoporous materials are highly efficient to adsorb organic molecules with small sizes and, therefore, suitable for adsorption of RB-4 (size of 1.59 nm) and RO-16 (size of 1.68 nm).

**Figure 3.** N2 isotherm curves for ZnCl2-BBC and KOH-BBC (**A**) and pore distribution curves (**B**).



The pore size distributions derived from the BJH plots of both BBC samples are displayed in Figure 3B. The chemical activation seemed to affect the pore structure of the BBC samples. BBC-KOH showed a much higher distribution of the pores in the range of large micropores or small mesopores, 1.72–2.24 nm (see the line with squares). According to the BJH plots, both samples possess large quantities of micropores and homogeneous and small mesopores. The creation of large micropores and small mesopores enhanced the BBC-KOH sample, which is in good agreemen<sup>t</sup> with the porosity data.

Literature data reveals significant variance in SBET values depending on the type of biomass and preparation conditions (Table 3). For instance, Sipola et al. [8] prepared activated carbon from scots pine (*Pinus sylvestrus*) and spruce (*Picea* spp.) barks for wastewater purification and found specific surface areas ranging from 200 to 600 m<sup>2</sup> g<sup>−</sup>1. In another work [9], the spruce bark porous materials were produced and employed in methylene blue dye adsorption. The materials presented SBET ranging from 351 to 1275 m<sup>2</sup> g<sup>−</sup><sup>1</sup> and were successfully employed in the dye removal from aqueous solutions. In addition, a specifically high SBET (2330 m<sup>2</sup> g<sup>−</sup>1) was achieved with rice plant residue as a biomass precursor. However, in that case, a highly complex preparation procedure was required: First, pre-carbonization at 500 ◦C for one hour followed by NaOH washing; Secondly, BBC was mixed with KOH at a ratio of 1:4 (biomass: KOH) and pyrolyzed at 800 ◦C for 30 min and then, followed by HCl washing to remove the inorganic compounds. Consequently, due to the cumbersome procedure, the high SBET comes with a high cost. The SBET values of the ZnCl2− and KOH-activated Norway spruce bark BBCs are comparable with BBCs from several other biomass precursors, but in this case, the manufacturing method is simple, and the feedstock material highly available and cheap.

**Table 3.** Comparison of KOH-BBC and ZnCl2-BBC preparation methods and SBET for a variety of biomass precursors.



**Table 3.** *Cont.*

#### 3.1.2. Elemental Analysis, Carbon Yield, Raman Spectroscopy, Zeta-Potential, and FTIR

The carbon content of the spruce bark ZnCl2- and KOH-activated BBCs was 94.8% and 91.6%, respectively (see Table 4). These values are very high compared to literature; Correa et al. [35] produced several BBCs from different biomasses, and the carbon content varied from 76.9 to 87.8%, while Duan et al. [36] obtained 82.66% of carbon content in BBC made from coconut shells. High carbon content can reflect good adsorption efficiency because hydrophobic interactions of the aromatics of BBC can interact with organic molecules. In addition, high carbon content means less ash content, and ashes in the BBC reduce SBET and functional groups, which hinder the adsorption process. Concerning the oxygen content, KOH-BBC presented higher content when compared to ZnCl2-BBC; this can positively influence the carbons' hydrophilicity index and the water/dye adsorption behavior [35].

$$HI = \frac{\frac{amount\ of\ water\ vapor\ (mg)}{mass\ of\ BBC\ (g)}}{\frac{amount\ of\ h - hcp\tan\ vapor\ (mg)}{mass}of\ BBC\ (g)}\tag{12}$$

The *BBC* yield from pyrolysis and KOH activation was approximately one-third of the ZnCl2 treatment (Table 4). This result indicates a strong reaction between bark and KOH during the pyrolysis process. Via breakage of C–O–C and C–C bonds, KOH can play a catalytic role in the material's volatilization, leading to a low carbon yield [37]. Impregnation with ZnCl2 results in degradation of the cellulosic material that, combined with the dehydration during carbonization, leads to charring and aromatization of the carbon skeleton. These pyrolytic conditions inhibit the formation of tar and reduce mass loss [38]. As a result, BBC production by ZnCl2 activation generally provides higher yields than when using other chemical reagents [38].


**Table 4.** Properties and elementary analysis of activated carbons.

The Raman spectra of the ZnCl2- and KOH-BBCs are shown in Figure 4. The D and G bands indicate the degree of defective structure and the activated carbons' graphitization, respectively [28,30]. These bands' position, area, and intensity can also show differences in the structural characteristics [34,39]. Both samples' D- and G-bands are located at around 1340 and 1593 cm<sup>−</sup>1, corresponding to the defect/disorder-induced structures in the BBCs' graphite layers and the vibration of sp2-bonded carbon atoms in a twodimensional hexagonal lattice, respectively [28,34]. The relative strength intensity (ID/IG) represents the degree of defect in the BBCs—higher values indicate more defects [30]. The obtained ID/IG values were 1.1 and 0.99 for ZnCl2-BBC and KOH-BBC, respectively, i.e., the graphitization level in the KOH-BBC was slightly higher than in the ZnCl2-BBC [28,30].

**Figure 4.** Raman spectra of BBC samples.

The Zeta-potential of both BBCs were negative, with a slightly higher value for the KOH-BBC (see Table 4). The negative charging comes from COO–, –COH–, and –OH– functionalities that can positively affect the adsorption process [40].

FTIR was employed to identify the presence of the functional groups on BBCs samples. The FTIR spectra of the BBC samples are presented in Figure 5. It is possible to identify that the different chemical treatments affected the chemical functionalities on the BBCs. In KOH-BBC, the presence of peaks in between 4000–3600 cm<sup>−</sup><sup>1</sup> represents the O–H stretching vibration in carboxyl and phenol groups [10–12,15]. The sample treated with

KOH also exhibited a sharper and broader transmittance band at 3410–3535 cm<sup>−</sup><sup>1</sup> when compared with the ZnCl2-treated sample, which is assigned to the O–H stretching mode of hydroxyl groups and adsorbed water [11,12,15]. The peaks at 2948 cm<sup>−</sup><sup>1</sup> (asymmetric) and 2875 cm<sup>−</sup><sup>1</sup> (symmetric) are related to the CH– stretching and appeared only in the sample treated with ZnCl2. A new peak at 2373 cm<sup>−</sup><sup>1</sup> is observed only in KOH-BBC, which is assigned to hydrogen-bonded OH. The peaks at around 1542–1574 cm–1 are assigned to the asymmetric stretching of O=C of carboxylates. The band at 1138–1160 cm<sup>−</sup><sup>1</sup> are related to CO– of alcohols, and at around 963–1009 cm<sup>−</sup><sup>1</sup> to the OCC—-a stretch of an ester is identified [10–12,15]. These functional groups on BBCs surfaces are often related to a good adsorption efficiency process [10–12,15].

**Figure 5.** FTIR spectra of BBC samples (**A**) ZnCl2-BBC and (**B**) KOH-BBC.

3.1.3. Water Vapor Adsorption Isotherms, Hydrophilicity Index (HI)

Water vapor adsorption isotherms for both BBCs are shown in Figure 6. According to the IUPAC classification [41], both isotherms are very close to type V, characterized by low levels of water uptake at low relative pressures and the presence of a hysteresis loop over the majority of the pressure range. Adsorption of water vapor was higher for KOH-BBC than for ZnCl2-BBC (see Table 4), indicating a more hydrophilic surface for KOH-BBC than ZnCl2-BBC.

**Figure 6.** Water sorption isotherms for KOH-BBC and ZnCl2-BBC samples at 25 ◦C.

The N2 and H2O isotherms differ both in type and shape. Although there is a nonexisting correlation between these two techniques, it is worth pointing out that N2 adsorption generated type I isotherms, while H2O adsorption yields isotherms of type V. Different on isotherm curves may be because the process is complex and does not depend only on the porosity. The adsorption of water vapor on biomass materials is known to be dependent on surface chemistry. BBC materials have plenty of surface functional groups, which initiate predominant water adsorption through the hydrogen bonding between a water molecule and surface functional groups.

#### *3.2. Dye Adsorption Analysis*
