*3.1. Thermogravimetric Analysis (TGA) of Rice Husk (RH)*

As mentioned above, a pre-pyrolysis test of the RH sample was carried out by a thermal analyzer under an inert atmosphere at a fixed heating rate of 10 ◦C/min. The test conditions were very similar to the pyrolysis experiments for producing RH biochar. Figure 1 presents the TGA/TGA curves for the RH sample, showing that the significant degradation temperature was less than 400 ◦C. As shown, there were two apparent peaks and one shoulder as the pyrolysis temperature increased from room temperature to 400 ◦C. In the first peak, mass loss should occur in the form of water vapor (moisture) between 60 and 200 ◦C. For a lignocellulosic biomass, hemicellulose and a smaller amount of cellulose may be the most labile polymeric components as compared to lignin [1]. This implies that the TGA shoulder appeared at a temperature of around 300 ◦C, which is lower than that of lignin. During the stage of pyrolysis, the complex reactions (e.g., cracking, condensation) involve depolymerization and scission, thus causing a continual mass loss in the form of vapors such as moisture, CO<sup>2</sup> and volatile organics [44]. The final peak around 400 ◦C can be attributed to the thermal degradation of most cellulose and a smaller amount of lignin. In order to produce porous biochar from RH, the pyrolysis experiments thus started from 400 ◦C, where more lignin was pyrolyzed to form the products of char and tar. At higher temperatures (400–1000 ◦C), the mass loss can be attributed to the thermal degradation of most lignin and inorganic minerals (e.g., carbonates, chlorides, oxides) [26].

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**Figure 1.** Thermogravimetric analysis (TGA) curve (blue color) and derivative thermogravimetry (DTG) curve (red dash line) of rice husk (RH) at a heating rate of 10 °C/min. **Figure 1.** Thermogravimetric analysis (TGA) curve (blue color) and derivative thermogravimetry (DTG) curve (red dash line) of rice husk (RH) at a heating rate of 10 ◦C/min. (DTG) curve (red dash line) of rice husk (RH) at a heating rate of 10 °C/min.

0.20

**Derivative Weight (%/min)**

#### *3.2. Yields and Pore Properties of Resulting Biochar 3.2. Yields and Pore Properties of Resulting Biochar*  3.2.1. Yields of Resulting Biochar

#### *3.2. Yields and Pore Properties of Resulting Biochar*  3.2.1. Yields of Resulting Biochar

110

**Yield (%)**

3.2.1. Yields of Resulting Biochar The biochar yield was calculated by the ratio of biochar mass to RH mass loaded into each pyrolysis experiment (around 5 g). As the pyrolysis temperature increased, more condensable products were formed as water and organic components, but there remained less carbonized solids as biochar due to the complex degradation reactions in progress. Figure 2 illustrates the variation in the yield of the resulting RH-based biochar as a function of temperature (400–900 °C) for a holding time of 30 min, indicating a decreasing trend. Although not shown here, the yields of the resulting RH-based biochar The biochar yield was calculated by the ratio of biochar mass to RH mass loaded into each pyrolysis experiment (around 5 g). As the pyrolysis temperature increased, more condensable products were formed as water and organic components, but there remained less carbonized solids as biochar due to the complex degradation reactions in progress. Figure 2 illustrates the variation in the yield of the resulting RH-based biochar as a function of temperature (400–900 ◦C) for a holding time of 30 min, indicating a decreasing trend. Although not shown here, the yields of the resulting RH-based biochar prepared at 900 ◦C for four holding times (0, 30, 60 and 90 min) also indicated this trend. The biochar yield was calculated by the ratio of biochar mass to RH mass loaded into each pyrolysis experiment (around 5 g). As the pyrolysis temperature increased, more condensable products were formed as water and organic components, but there remained less carbonized solids as biochar due to the complex degradation reactions in progress. Figure 2 illustrates the variation in the yield of the resulting RH-based biochar as a function of temperature (400–900 °C) for a holding time of 30 min, indicating a decreasing trend. Although not shown here, the yields of the resulting RH-based biochar prepared at 900 °C for four holding times (0, 30, 60 and 90 min) also indicated this trend.

**Temperature (oC) Figure 2.** Variation in the yield of resulting RH-based biochar as a function of temperature. **Figure 2.** Variation in the yield of resulting RH-based biochar as a function of temperature.

**Figure 2.** Variation in the yield of resulting RH-based biochar as a function of temperature. 3.2.2. Pore Properties of Resulting Biochar

3.2.2. Pore Properties of Resulting Biochar 3.2.2. Pore Properties of Resulting Biochar The potential applications of biochar for water retention, sorption of contaminants and as a habitat for microorganisms are strongly dependent on its pore properties, which include specific surface area, density, porosity and morphology [45]. In the present study, all pore properties of the biochar samples were based on N<sup>2</sup> gas adsorption–desorption at −196 ◦C and helium gas displacement. Table 1 lists the pore properties of the resulting

biochar (BRH series), which included the BET surface area (SBET), total pore volume (Vt), true density (ρs), particle density (ρp) and porosity (εp). Although particle density can be determined by a mercury (Hg) pycnometer due to the high surface tension of Hg and its inability to enter any pore of the porous sample [41], it was calculated by the values of total pore volume and true density in the present study [42].

$$
\rho\_\mathbf{p} = \rho\_\mathbf{s} / (\mathbf{V}\_\mathbf{t} \times \rho\_\mathbf{s} + 1) \tag{1}
$$

**Table 1.** Pore properties of resulting RH-based biochar.


<sup>a</sup> Sample notation indicated the resulting biochar (BRH-temperature-time) produced at the temperature of 400–900 ◦C for a holding time of 0–90 min using 5 g dried rice husk (RH).<sup>b</sup> BET surface area (SBET) was based on relative pressure range of 0.05–0.30.<sup>c</sup> Total pore volume (Vt) was obtained at relative pressure of around 0.99.<sup>d</sup> True density (ρs) was measured by the helium displacement method. <sup>e</sup> Particle density (ρp) was calculated from the total pore volume (Vt) and true density (ρs).<sup>f</sup> Particle porosity (εp) was calculated from the particle density (ρp) and true density (ρs).

Furthermore, the porosity (εp) can be obtained by subtracting the ratio of particle density to true density from 1 [43]:

$$
\varepsilon\_{\rm P} = 1 - \left(\wp\_{\rm P}/\wp\_{\rm s}\right) \tag{2}
$$

As listed in Table 1, there were obvious variations in the pore properties of the biochar as a function of pyrolysis temperature (400–900 ◦C) and holding time (0–90 min). Similar to numerous studies [45,46], there was a positive correlation between the BET surface area (or porosity) and pyrolysis temperature. When the pyrolysis temperature increased from 400 to 900 ◦C, there was greater formation of pyrogenic amorphous biochar [27], thus causing a greater pore space or more pores with the pyrolysis temperature as charring intensity increased. Noticeably, the pore properties of the biochar continuously increased as the pyrolysis temperature increased up to 900 ◦C (for a fixed holding time of 30 min), or as the holding time was extended up to 90 min (at a fixed pyrolysis temperature of 900 ◦C). This implies that the structural breakdown of the resulting biochar produced at higher temperatures, probably due to sintering or fusion [45], was not observed in this work [47]. In the present study, the process parameter of pyrolysis temperature had a more significant effect on the pore properties of biochar as compared with the holding time (or reaction residence time). This is because the extent of physical changes (e.g., mass loss) for the biochar was highly dependent on the temperature during pyrolysis processing (seen in Figure 1). From the viewpoint of efficient energy use, the pyrolysis conditions at 500 ◦C for 30 min may be suitable for the production of RH-based biochar with a high BET surface area of 211 m2/g when compared with the resulting biochar (BET surface area of 279 m2/g) prepared at 900 ◦C for 90 min.

The BET surface area of the resulting biochar was also linked to its N<sup>2</sup> adsorption– desorption isotherms, which can be further transformed into its pore size distribution based on the BJH method [41]. Figure 3 depicts the N<sup>2</sup> adsorption–desorption isotherms and pore

growth space.

size distributions of the resulting biochar prepared at various pyrolysis temperatures for a holding time of 30 min. Furthermore, Figure 4 shows the similar characteristics of the resulting biochars prepared at 900 ◦C for different holding times. As shown in Figures 3 and 4, the corresponding N<sup>2</sup> adsorption–desorption isotherms and pore size distributions of the resulting biochars were consistent with those in Table 1. Furthermore, the biochar pyrolyzed at higher temperatures showed more pronounced pore development, including micropores and mesopores. Therefore, these biochars possess the so-called Type I and Type-IV shapes (isotherms) [41]. This mesoporous feature will be beneficial in providing water retention, adsorption capacity and microbial growth space. characteristics of the resulting biochars prepared at 900 °C for different holding times. As shown in Figures 3 and 4, the corresponding N2 adsorption–desorption isotherms and pore size distributions of the resulting biochars were consistent with those in Table 1. Furthermore, the biochar pyrolyzed at higher temperatures showed more pronounced pore development, including micropores and mesopores. Therefore, these biochars possess the so-called Type I and Type-IV shapes (isotherms) [41]. This mesoporous feature will be beneficial in providing water retention, adsorption capacity and microbial

tion of RH-based biochar with a high BET surface area of 211 m2/g when compared with

therms and pore size distributions of the resulting biochar prepared at various pyrolysis temperatures for a holding time of 30 min. Furthermore, Figure 4 shows the similar

The BET surface area of the resulting biochar was also linked to its N2 adsorption–desorption isotherms, which can be further transformed into its pore size distribu-

the resulting biochar (BET surface area of 279 m2/g) prepared at 900 °C for 90 min.

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**Figure 3.** N2 adsorption–desorption isotherms (upper) and pore size distributions (lower) of resulting biochar prepared at various temperatures for a fixed holding time of 30 min. **Figure 3.** N<sup>2</sup> adsorption–desorption isotherms (**upper**) and pore size distributions (**lower**) of resulting biochar prepared at various temperatures for a fixed holding time of 30 min.

**Figure 4.** N2 adsorption–desorption isotherms (upper) and pore size distributions (lower) of resulting biochar prepared at 900 °C for various holding times. **Figure 4.** N<sup>2</sup> adsorption–desorption isotherms (**upper**) and pore size distributions (**lower**) of resulting biochar prepared at 900 ◦C for various holding times.

In order to observe the porous texture of the resulting biochar, SEM analyses at 15 kV were performed on its gold-coated surface with different magnifications (i.e., 200 and 1000). As shown in the SEM images (Figure 5), the representative biochar (i.e., BRH-900-30) has fine pores on the surface. These pores will be more formed from gas vesicles at higher pyrolysis temperatures, resulting in better pore properties such as specific surface area and porosity (Table 1). In addition, the resulting biochar also exhibits a rigid, irregular and rough surface due to its rigid silica composition and the rigorous carbonization at a higher temperature (i.e., 900 °C) for a longer holding time (i.e., 30 min). In this regard, the RH-based biochar may be a good medium for possible applications in In order to observe the porous texture of the resulting biochar, SEM analyses at 15 kV were performed on its gold-coated surface with different magnifications (i.e., 200 and 1000). As shown in the SEM images (Figure 5), the representative biochar (i.e., BRH-900-30) has fine pores on the surface. These pores will be more formed from gas vesicles at higher pyrolysis temperatures, resulting in better pore properties such as specific surface area and porosity (Table 1). In addition, the resulting biochar also exhibits a rigid, irregular and rough surface due to its rigid silica composition and the rigorous carbonization at a higher temperature (i.e., 900 ◦C) for a longer holding time (i.e., 30 min). In this regard, the RH-based biochar may be a good medium for possible applications in water retention and wastewater treatment in the soil environment due to its highly porous structure.

water retention and wastewater treatment in the soil environment due to its highly po-

rous structure.

**Figure 5.** SEM images (×200, left; ×1000, right) of the resulting biochar (BRH-900-30). **Figure 5.** SEM images (×200, **left**; ×1000, **right**) of the resulting biochar (BRH-900-30). **Figure 5.** SEM images (×200, left; ×1000, right) of the resulting biochar (BRH-900-30).

#### *3.3. Chemical Characterization of Resulting Biochar 3.3. Chemical Characterization of Resulting Biochar 3.3. Chemical Characterization of Resulting Biochar*

As described in Section 2.4, the EDS system, which is commonly used alongside SEM, was used to enable semi-quantitative elemental analysis on the surface of the resulting biochar. Figure 6 shows an EDS spectrum from the resulting biochar (BRH-900-30), observing the presence of carbon (C), oxygen (O) and silicon (Si). As a preliminary quantification, the corresponding contributions to the elemental composition are 18.15, 4.66 and 40.19 wt%, respectively. The presence of C and O in the RH-based biochar should arise from its lignocellulosic precursor (i.e., RH). The most significant peak is assigned to the presence of Si due to the high content of silica (SiO2) in the RH. The rich presence of silica in the RH-based biochar can be further identified by X-ray diffraction (XRD) [48] or X-ray photoelectron spectroscopy (XPS) analysis [49]. As described in Section 2.4, the EDS system, which is commonly used alongside SEM, was used to enable semi-quantitative elemental analysis on the surface of the resulting biochar. Figure 6 shows an EDS spectrum from the resulting biochar (BRH-900-30), observing the presence of carbon (C), oxygen (O) and silicon (Si). As a preliminary quantification, the corresponding contributions to the elemental composition are 18.15, 4.66 and 40.19 wt%, respectively. The presence of C and O in the RH-based biochar should arise from its lignocellulosic precursor (i.e., RH). The most significant peak is assigned to the presence of Si due to the high content of silica (SiO2) in the RH. The rich presence of silica in the RH-based biochar can be further identified by X-ray diffraction (XRD) [48] or X-ray photoelectron spectroscopy (XPS) analysis [49]. As described in Section 2.4, the EDS system, which is commonly used alongside SEM, was used to enable semi-quantitative elemental analysis on the surface of the resulting biochar. Figure 6 shows an EDS spectrum from the resulting biochar (BRH-900-30), observing the presence of carbon (C), oxygen (O) and silicon (Si). As a preliminary quantification, the corresponding contributions to the elemental composition are 18.15, 4.66 and 40.19 wt%, respectively. The presence of C and O in the RH-based biochar should arise from its lignocellulosic precursor (i.e., RH). The most significant peak is assigned to the presence of Si due to the high content of silica (SiO2) in the RH. The rich presence of silica in the RH-based biochar can be further identified by X-ray diffraction (XRD) [48] or X-ray photoelectron spectroscopy (XPS) analysis [49].

**Figure 6.** EDS spectrum of the resulting biochar (BRH-900-30). **Figure 6.** EDS spectrum of the resulting biochar (BRH-900-30). **Figure 6.** EDS spectrum of the resulting biochar (BRH-900-30).

In general, the chemical characteristics of biochar mainly comprise aromatic C and inorganic minerals, which are dependent on the starting feedstock and pyrolysis conditions applied. The presence of functional groups on the surface of biochar plays a vital role in the soil and water environments. For example, the addition of biochar to soil has been proven to enhance the sorption capacities for heavy metal ions by the electrostatic attraction [45], which can be attributed to the high contents of oxygen-containing functional groups on the surface due to the negatively charged surface. The FTIR spectrum of the resulting biochar (BRH-900-30) shown in Figure 7 has the following features [23,50,51]. The peak at 3460 cm−1 can be assigned to the presence of adsorbed water and hydrogen-bonded biochar O-H groups. The band between 1500 and 1700 cm−1 could be attributed to C=O stretching or C=C groups in aromatic rings. The sharp peak around 1385 cm−1 could be assigned to the symmetric deformation band of –CH3 groups. The In general, the chemical characteristics of biochar mainly comprise aromatic C and inorganic minerals, which are dependent on the starting feedstock and pyrolysis conditions applied. The presence of functional groups on the surface of biochar plays a vital role in the soil and water environments. For example, the addition of biochar to soil has been proven to enhance the sorption capacities for heavy metal ions by the electrostatic attraction [45], which can be attributed to the high contents of oxygen-containing functional groups on the surface due to the negatively charged surface. The FTIR spectrum of the resulting biochar (BRH-900-30) shown in Figure 7 has the following features [23,50,51]. The peak at 3460 cm−1 can be assigned to the presence of adsorbed water and hydrogen-bonded biochar O-H groups. The band between 1500 and 1700 cm−1 could be attributed to C=O stretching or C=C groups in aromatic rings. The sharp peak around 1385 cm−1 could be assigned to the symmetric deformation band of –CH3 groups. The In general, the chemical characteristics of biochar mainly comprise aromatic C and inorganic minerals, which are dependent on the starting feedstock and pyrolysis conditions applied. The presence of functional groups on the surface of biochar plays a vital role in the soil and water environments. For example, the addition of biochar to soil has been proven to enhance the sorption capacities for heavy metal ions by the electrostatic attraction [45], which can be attributed to the high contents of oxygen-containing functional groups on the surface due to the negatively charged surface. The FTIR spectrum of the resulting biochar (BRH-900-30) shown in Figure 7 has the following features [23,50,51]. The peak at 3460 cm−<sup>1</sup> can be assigned to the presence of adsorbed water and hydrogen-bonded biochar O-H groups. The band between 1500 and 1700 cm−<sup>1</sup> could be attributed to C=O stretching or C=C groups in aromatic rings. The sharp peak around 1385 cm−<sup>1</sup> could be assigned to the symmetric deformation band of –CH<sup>3</sup> groups. The peak at around

1115 cm−<sup>1</sup> could be the stretching vibration of C-O and the vibration of silica bonds. Si-O framework bands at around 1115 cm−<sup>1</sup> and below 800 cm−<sup>1</sup> were evident [17]. These FTIR results were in accordance with those in the EDS analysis (Figure 6), enhancing the slight hydrophilicity on the surface of the RH-based biochar. These functional groups will impact the soil pH and interaction with ionic contaminants in soil, causing higher cation exchange capacity (CEC) and adsorption capacities for cations (e.g., metal ions). silica bonds. Si-O framework bands at around 1115 cm−1 and below 800 cm−1 were evident [17]. These FTIR results were in accordance with those in the EDS analysis (Figure 6), enhancing the slight hydrophilicity on the surface of the RH-based biochar. These functional groups will impact the soil pH and interaction with ionic contaminants in soil, causing higher cation exchange capacity (CEC) and adsorption capacities for cations (e.g., metal ions).

peak at around 1115 cm−1 could be the stretching vibration of C-O and the vibration of

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**Figure 7.** FTIR spectrum of the resulting biochar (BRH-900-30). **Figure 7.** FTIR spectrum of the resulting biochar (BRH-900-30).

#### **4. Conclusions 4. Conclusions**

In this work, a series of biochars were prepared from RH at various pyrolysis conditions, which were designed by the matrix of temperature (400–900 °C) and holding times (i.e., 0–90 min) based on the results of a pre-pyrolysis test by TGA. The pore properties (i.e., BET surface area and porosity) of the resulting RH-based biochar were clearly positively linked to the studied ranges of pyrolysis temperature and holding time. The maximal pore properties with the BET surface area of around 280 m2/g could be obtained from the conditions at 900 °C for a holding time of 90 min. The porosity of the optimal biochar was as high as 0.316. In addition, the resulting biochar showed characteristic of microporous and mesoporous structures based on the N2 adsorption–desorption isotherms and pore size distributions. The results of the EDS and FTIR analyses also supported the slight hydrophilicity on the surface of the RH-based biochar due to the richness in oxygen/silica-containing functional groups. Based on the physicochemical properties determined, the RH-based biochar could be an excellent material for possible applications in water conservation, wastewater treatment and soil amendment. In this work, a series of biochars were prepared from RH at various pyrolysis conditions, which were designed by the matrix of temperature (400–900 ◦C) and holding times (i.e., 0–90 min) based on the results of a pre-pyrolysis test by TGA. The pore properties (i.e., BET surface area and porosity) of the resulting RH-based biochar were clearly positively linked to the studied ranges of pyrolysis temperature and holding time. The maximal pore properties with the BET surface area of around 280 m2/g could be obtained from the conditions at 900 ◦C for a holding time of 90 min. The porosity of the optimal biochar was as high as 0.316. In addition, the resulting biochar showed characteristic of microporous and mesoporous structures based on the N<sup>2</sup> adsorption–desorption isotherms and pore size distributions. The results of the EDS and FTIR analyses also supported the slight hydrophilicity on the surface of the RH-based biochar due to the richness in oxygen/silica-containing functional groups. Based on the physicochemical properties determined, the RH-based biochar could be an excellent material for possible applications in water conservation, wastewater treatment and soil amendment.

**Author Contributions:** Conceptualization, W.-T.T.; methodology, Y.-Q.L. and H.-J.H.; validation, Y.-Q.L. and H.-J.H.; data curation, Y.-Q.L.; formal analysis, Y.-Q.L.; resources, Y.-Q.L.; writing—original draft preparation, W.-T.T.; writing—review and editing, W.-T.T.; supervision, W.-T.T. All authors have read and agreed to the published version of the manuscript. **Author Contributions:** Conceptualization, W.-T.T.; methodology, Y.-Q.L. and H.-J.H.; validation, Y.- Q.L. and H.-J.H.; data curation, Y.-Q.L.; formal analysis, Y.-Q.L.; resources, Y.-Q.L.; writing—original draft preparation, W.-T.T.; writing—review and editing, W.-T.T.; supervision, W.-T.T. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding. **Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable. **Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable. **Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors express sincere thanks to the Instrument Center of National Pingtung University of Science and Technology for the assistance in the scanning electron microscope (SEM)/energy-dispersive X-ray spectroscopy (EDS) analysis. Additionally, the authors are thank Ming-Shou Tang, Chin-Hsien Chang and Tsung-Hsien Kuo (undergraduate students at the Department of Environmental Engineering and Science) for conducting the pyrolysis experiments.

**Conflicts of Interest:** The authors declare no conflict of interest.
