1

Characterizations of Activated Carbons from Golden Shower (Cassia fistula) upon 1

Different Chemical Activation Methods with Potassium Carbonate 2

3

Hai Nguyen Trana,b, Sheng-Jie Youa,* and Huan-Ping Chaoa,* 4

aDepartment of Environmental Engineering, Chung Yuan Christian University, Chungli 32023, Taiwan 5

bDepartment of Civil Engineering, Chung Yuan Christian University, Chungli 32023, Taiwan 6

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*Corresponding authors: Tel: +886 3 2654911 and +886 3 2654914, E-mails: sjyou@cycu.edu.tw (S-J. You) and hpchao@cycu.edu.tw (H-P. Chao)

mailto:sjyou@cycu.edu.twmailto:hpchao@cycu.edu.tw

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Abstract 9

Activated carbons (ACs) were synthesized from golden shower (GS) through chemical 10

activation with K2CO3. Two synthesis processes were used: one-stage and two-stage processes. 11

In the one-stage process, GS that was impregnated with K2CO3 was directly pyrolyzed 12

(GSAC), and the two-stage process consisted of (1) pyrolytic or hydrolytic carbonization to 13

produce biochar (GSB) or hydrochar (GSH) and (2) subsequent chemical activation was 14

defined as GSBAC and GSHAC, respectively. The characteristics of the ACs and their 15

precursors, including thermal stability and textural, physicochemical, structural, and crystal 16

properties, were thoroughly investigated. Results showed that the characteristics of ACs 17

strongly depend on the method used for their synthesis. The Brunauer–Emmett–Teller surface 18

area followed the order GSAC > GSHAC > GSBAC > GSB > GSH > GS. The existence of 19

acidic groups was determined through Fourier transform infrared spectroscopy and Boehm 20

titration. The excellent adsorptive capacities of the ACs were confirmed from the iodine 21

number (1568–2695 mg/g) and methylene number (143–233 mg/g). 22

Keywords: Activated carbon, chemical activation, hydrochar, biochar, golden shower 23

3

1. Introduction 24

Activated carbons (ACs) with high specific surface areas (SBET) play a vital role in 25

adsorbing organic compounds in the water treatment process. According to an industry market 26

research report [1], world demand for AC is estimated to increase 8.1 percent per year to 2.1 27

million metric tons in 2018. Nevertheless, the high cost of commercial ACs prevents their 28

large-scale use. Lignocellulose materials have been considered as abundant renewable 29

precursors that can be used for manufacturing ACs at lower cost. Typically, ACs can be 30

synthesized through two well-known processes: physical and chemical activation. The 31

chemical activation process can be conducted in one or two stages. In the one-stage process, 32

which is the most common method, raw materials are directly mixed with certain activating 33

reagents and the mixture is pyrolyzed. By contrast, the two-stage process comprises (1) a 34

precarbonization process (i.e., pyrolysis or hydrolysis) and (2) chemical activation. Pyrolysis, 35

a dry carbonization process, is conducted at high temperatures (400–1200 °C) in an inert 36

atmosphere (i.e., N2 or Ar atmosphere), under vacuum conditions, or under oxygen-limited 37

conditions for producing biochar. In hydrolysis, a wet carbonization process, the raw materials 38

are dispersed in an autoclave containing a given solution (i.e., H2O, or H3PO4). Subsequently, 39

the autoclave temperature is controlled (150–350 °C) for 2–24 h at a specific pressure for 40

producing hydrochar [2]. 41

Many previous investigations have shown that ACs prepared through the two-stage chemical 42

activation process have a larger surface area and higher adsorption capacity than those 43

prepared using the one-stage chemical activation process. For example, Basta et al. [3] 44

prepared ACs from rice straw-by using the one-stage and two-stage processes. Their results 45

4

showed that two-stage KOH activation through prepyrolysis carbonization was more efficient 46

in producing ACs with high SBET and high adsorption capacity for methylene blue (MB). A 47

similar result was reported by other investigators [4, 5]. For hydrothermal carbonization, 48

Fernandez et al. [6] reported that SBET of orange-peel-derived ACs produced from hydrochar 49

in water was twice that of orange-peel-derived ACs in H3PO4 acid solution. ACs with 50

extremely high SBET, synthesized from commercial organic precursors (i.e., furfural, glucose 51

starch, cellulose, eucalyptus sawdust, and saccharides), have been precarbonized through a 52

hydrothermal process [7, 8]. However, very few studies have compared the characteristics of 53

ACs prepared from the three chemical activation methods. 54

Notably, activating reagents have a strong effect on AC characteristics. ACs activated by 55

K2CO3 show the highest SBET, nearly 2000 m2/g, compared with those activated by Na2CO3, 56

KOH, NaOH, ZnCl2, and H3PO4 [9]. In another comparison study on the efficiency of 57

chemical agents (i.e., K2CO3 and KOH) in the synthesis of ACs [10], ACs activated by K2CO3 58

and KOH through the optimal carbonization process exhibited similar yields, SBET, and 59

micropore volumes. Furthermore, environmentally friendly chemical agents play a major role 60

in industrial and environmental applications. In particular, K2CO3 is not a deleterious 61

chemical, and it is frequently used in food additives. Therefore, K2CO3 was used as an 62

activating agent in the current study. 63

Cassia fistula (commonly known as golden shower) is a very popular ornamental plant in 64

tropical and subtropical regions, and it can be used as a lignocellulose precursor for 65

synthesizing ACs because it is relatively abundant, a renewable resource, and costs less. Hanif 66

et al. [11] investigated the biosorption of Ni(II) in industrial wastewater by various GS 67

5

biomasses. The results demonstrated that GS was an excellent biosorbent for Ni(II) removal 68

from industrial effluents. Later, the biosorption of chromium(III) and chromium(VI) in 69

aqueous solutions by untreated and pretreated GS biomass was studied by Abbas et al. [12]. 70

Although GS has been used as a biosorbent for removing heavy metal ions, its characteristics 71

have not been reported. 72

In this study, golden shower (GS) was used as a lignocellulose precursor for synthesizing ACs 73

through various chemical activation methods by using K2CO3. The characteristics of the ACs, 74

comprising thermal, physicochemical, textural, morphological, and crystal properties, surface 75

functional groups, and adsorption capacities for iodine and MB, were thoroughly investigated. 76

In this paper, the characteristics of the ACs’ precursors are also discussed. 77

2. Materials and Methods 78

2.1. Preparation of feedstock, biochar, hydrochar, and their ACs 79

Pods of GS were collected from Taipei in Taiwan, and the seeds were removed. The materials 80

shown in Figure S1 were washed with tap water at least thrice and then with deionized 81

distilled (DD) water to remove any adhering dirt or impurity. They were then placed in an 82

oven at 80 °C for 24 h. The dried GS was ground and sieved to obtain particles in the size 83

range from 0.106 to 0.250 mm. 84

Approximately 50 g of the powdered GS was taken in a porcelain crucible covered with a lid. 85

The crucible was placed in a muffle furnace (Deng Yng DF 40, Taiwan) in a limited-oxygen 86

atmosphere at 800 °C for 4 h for obtaining biochar (GSB). Hydrochar (GSH) was prepared 87

6

through a hydrothermal carbonization process. Approximately 30 g of the powdered GS was 88

mixed with 120 mL of DD water in a 200 mL Teflon-lined autoclave at 190 °C for 24h. 89

ACs were synthesized through chemical activation processes with K2CO3. The samples were 90

immersed in K2CO3 solution with a weight ratio of 1:1 (K2CO3/precursors), and this was 91

followed by pyrolysis in limited-oxygen conditions at 800 °C for 4 h (Figure 1). Chemically 92

activated GS, GSB, and GSH are referred to by the abbreviations GSAC, GSBAC, and 93

GSHAC, respectively, and GSAC, GSBAC, GSHAC, and GSB were porous carbon materials. 94

After carbonization, the samples were thoroughly washed with 0.1 M HCl for dissolving ash 95

and inorganic salts. Finally, DD water was used to wash the samples until the pH of the 96

filtrates reached a constant value. The samples were then dried and sieved, and they were 97

stored in brown bottles until use. 98

Figure 1 99

2.2. Characteristics of feedstock, biochar, hydrochar, and their ACs 100

2.2.1. Thermal characteristics 101

The thermal stability of ACs and their precursors was measured by thermo-gravimetric 102

analysis (TGA; DuPont TA Q50, USA) under air atmosphere. The experiment was carried out 103

from room temperature to 900 oC at a heating rate of 10 oC/min. 104

2.2.2. Physicochemical characterization 105

7

Proximate analysis was performed by following an international standard procedure (ASTM). 106

The bulk (apparent) density and hardness (abrasion) number were obtained from the literature 107

[13]. 108

2.2.3. Textural properties 109

N2 adsorption–desorption isotherms were measured using a Micromeritics ASAP 2020 110

sorptometer at 77 K. The Brunauer–Emmett–Teller (BET) method was employed to compute 111

the specific surface areas (SBET). Furthermore, the micropore surface area (Smicro) and 112

micropore volume (Vmicro) were determined using De Boer’s t-plot (statistical thickness) 113

method. The external (nonmicropore) surface area (Sexternal) was calculated as the difference 114

between SBET and Smicro. The total pore volume (Vtotal) was estimated in terms of the amount of 115

adsorption at a relative pressure (P/P0) of 0.98 by using the Horvath–Kawazoe method. Pore 116

size distributions were measured using Brunauer’s MP method, and the nonmicropore volume 117

(Vnon-micro) was computed by subtracting the micropore volume from the total pore volume. 118

2.2.4. Morphological and crystal properties 119

The morphology of the adsorbents was obtained using scanning electron microscopy (SEM; 120

Hitachi S-4800, Japan). To identify possible crystalline structures, adsorbents were analyzed 121

using X-ray diffraction (XRD; PANalytical PW3040/60 X’Pert Pro) with Cu Kα radiation (λ = 122

0.154 nm). The scan angle 2θ covered the range 5° < 2θ < 60° and the scan speed was 3°/min. 123

2.2.5. Surface chemistry 124

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The functional groups present on the adsorbent surface were detected using Fourier transform 125

infrared spectroscopy (FTIR; FT/IR-6600 JASCO); the adsorbent particles were pelleted using 126

KBr. The spectra were scanned from 4000 to 650 cm−1. The pH value of the adsorbents at the 127

point of zero charge (pHPZC) was determined using the solid addition method, which is 128

analogous to the drift method [14]. 129

The acidic groups and basic sites on the adsorbent surfaces were determined through Boehm 130

titration by following the standardization protocol proposed by Goertzen et al. [15]. The total 131

numbers of acidic (i.e., carboxylic, phenolic, and lactonic, as shown in Figure S2) and 132

carboxylic groups on the adsorbents were computed as the numbers of moles neutralized by 133

NaOH and NaHCO3, respectively. The number of phenolic groups was assumed to be the 134

difference between the number of moles neutralized by NaOH and Na2CO3, and the number of 135

lactonic groups was assumed as the difference between the moles neutralized by Na2CO3 and 136

NaHCO3. Furthermore, the total number of basic sites of adsorbents was calculated as the 137

number of moles neutralized by HCl. The numbers of moles of adsorbent surface 138

functionalities (nASF, mmol/g) neutralized by NaOH (pKa = 15.74), Na2CO3 (pKa = 10.25), 139

NaHCO3 (pKa = 6.37), and HCl were determined from the following equations: 140

a

NatNatHClHCla

CNaASF mV

CVCVCVV

n

2

.2 OHOH00O32

(1) 141

a

NatNatHClHClaNaASF mV

CVCVCVVn OHOH00OH/NaHCO

.3

(2) 142

a

NatNataHClASF mV

CVCVVn OHOH00

. (3) 143

9

where Vo (in millimeter) is the volume of NaOH/Na2CO3/NaHCO3/HCl solution added 144

initially; Co (in molarity) is the concentration of NaOH/Na2CO3/NaHCO3/HCl when Vo is 145

extracted; VHCl (in milliliter) and CHCl (in molarity) are the volume and concentration of HCl 146

solution added to aliquots taken from Vo, respectively; Va (in milliliter) is the volume of 147

aliquot taken from Vo; m (in gram) is the mass of adsorbents; and Vt(NaOH) (in milliliter) and 148

Ct(NaOH) (in molarity) are the concentration and volume of the titrant in the back titration, 149

respectively. 150

2.2.6. Adsorption properties 151

The iodine number (ASTM D4607-14) is the amount of iodine adsorbed (milligram) by 1.0 g 152

of carbon. Three weighed carbon doses were transferred into three Erlenmeyer flasks, and to 153

each flask, 10 mL of 5% (by weight) HCl solution was added. The mixture was boiled for 1.0 154

min to remove any trace of sulfur and then cooled to room temperature. Subsequently, 50 mL 155

of 0.1 N iodine solution was added to each flask. The flasks were immediately stoppered, 156

shaken vigorously for 30 s, and immediately filtrated. A 25-mL aliquot of each filtrate was 157

directly titrated with a standardized 0.1 N sodium thiosulfate solution (Na2S2O3.5H2O) until 158

the solution became pale yellow. The endpoint was determined by adding 2 mL of starch until 159

the solution turned colorless. The titration experiment was conducted in duplicate, and mean 160

values were considered. It was ensured that filtrate normalities (C) determined from eq. (4) 161

were not within the range of 0.08 to 0.04 N. The amount of iodine adsorbed per gram of 162

carbon (X/M; milligram per gram) was calculated using eq. (5). The iodine number is the 163

value of X/M at the residual iodine concentration (C) of 0.02 N. 164

10

FSNC .1 (4) 165

MSBDFA

MX ..

(5) 166

where N1 is the normality of sodium thiosulfate (in equivalent per liter); F is the amount of 167

filtrate (in milliliter); and S is the amount of sodium thiosulfate (in milliliter). Furthermore, we 168

have A = 0.1 × 12693.0; B = N1 × 126.93; and DF = (I + H)/F, where DF denotes the dilution 169

factor, I represents the amount of iodine (in milliliter), and H is the amount of 5% HCl used 170

(in milliliter). 171

MB number was defined as the amount (in milligram) of MB adsorbed by 1.0 g of carbon, and 172

Barton’s method [16] was used for determining it. Approximately 0.1 g of the carbon samples 173

was added to Erlenmeyer flasks containing 25 mL at a MB concentration of 1000 mg/L. The 174

flasks were covered with a parafilm and shaken using an orbital shaking incubator (S300R-175

Firstek) with 150 rpm at 25°C for 24h. The MB concentration was determined using 176

a Genesys 10 UV-Vis spectrophotometer (Thermo Scientific, USA) at maximum wavelength 177

of 665 nm. The amount of MB uptake at equilibrium, qe (mg/g), was calculated using the mass 178

balance equation: 179

( )o ee

C C Vqm

(6)

where C0 is the initial MB concentration (in milligram per liter), Ce is the equilibrium 180

concentration of MB (in milligram per liter), m (in gram) is the mass of the adsorbent, and V 181

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(in liter) is the volume of the solution. All the chemicals used were of analytical reagent grade, 182

and DD water was used in all the experiments. 183

3. Results and Discussion 184

3.1. Thermal characteristics 185

The pyrolytic characteristics of raw GS were determined using the thermogravimetric analyzer, 186

and they are presented in Figure 2a. Four overlapping peaks for the maximum weight loss are 187

visible in the thermogravimetric curve. The weight loss temperature (Tmax) at 56.2°C was 188

assigned to the vaporization of moisture (≈4.11%), and the thermal decomposition peaks at 189

Tmax of 307, 332, and 451°C were attributed to the thermal degradation of hemicellulose 190

(19.37%), cellulose (38.95%), and lignin (37.57%) in GS, respectively. Yang et al. [17] noted 191

that the pyrolysis of pure hemicellulose and cellulose occurred rapidly and that the weight loss 192

of pure hemicellulose occurred at 220–315 °C (Tmax = 268 °C) whereas that of pure cellulose 193

occurred at 315–400 °C (Tmax = 355 °C). It is noteworthy that the weight of the residue 194

remains virtually constant from 500–900 °C, suggesting that complete carbonization of GS 195

requires a minimum temperature of 500 °C. 196

For the GSH material, the disappearance of the peak corresponding to the decomposition of 197

hemicellulose is evident in Figure 2c, indicating that the hydrothermal process at 190 °C can 198

completely decompose the hemicellulose component of raw GS. However, cellulose and 199

lignin in GS might not be decomposed during the hydrothermal process, and this possibility is 200

supported by a previous report [17]. 201

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The onset and endset temperatures of the adsobents are extrapolated on the basis of the 202

intersection of two tangents to the TG curves. As expected, the onset temperatures, which 203

denote the temperatures at which weight loss begins, show an appreciable shift. Furthermore, 204

the endset temperatures of the adsorbents (Figure 2) also show a considerable change and 205

follow the order of GSBAC > GSAC ≈ GSHAC > GSB > GSH > GS. Therefore, it can be 206

concluded that the ACs are thermally more stable than their precursors. 207

Figure 2 208

3.2. Physicochemical characterization 209

Table 1 shows that the porous carbon materials had a relatively low percentage of moisture, 210

total ash, and volatility. These analysis results are in accord with the TGA readings in Figure 2. 211

Generally, a low percentage of ash and moisture indicate high quality of AC, and the low 212

volatile content reflects high potential for industrial applications. Notably, the hardness 213

numbers of the (a measure of ACs’ resistance to attrition) are strongly dependent on those of 214

the precursors of the ACs. The hardness numbers follow the order of GSBAC > GSHAC > 215

GSAC for ACs and GSB > GSH > GS for their precursors (Table 1). The results indicate that 216

precursors with higher hardness numbers can be used to obtain ACs with higher hardness. 217

In Table 1, the elements containing C and O are the primary composites. The O/C molar ratio 218

has been used as a reference for surface hydrophilicity because it is indicative of polar-group 219

content, with the polar groups most likely to have been derived from carbohydrates. Moreover, 220

the H/C molar ratio has been used as an indicator of the degree of aromatization and 221

carbonization [18]. The H/C molar ratio of GSAC (0.10), GSBAC (0.03), and GSHAC (0.06) 222

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decreased rapidly compared with their precursors—GS (1.37), GSB (0.19), and GSH (0.62), 223

respectively, implying that the ACs had higher aromaticity and were thermally more stable. 224

The analogous results of TGA analysis also indicated that the ACs showed higher thermal 225

stability than their precursors. The relatively higher H/C molar ratio of GSH indicates that 226

original organic plant residues, such as cellulose and lignin (Figure 2), remained in hydrochar. 227

The porous carbon samples (i.e., GSAC, GSBAC, GSHAC, and GSB) had similar O/C molar 228

ratios, suggesting that they shared similar affinity for water. 229

The GSBAC yield (84.16%) was greater than the GSHAC (63.77) and GSAC yields (57.67). 230

This might result from the complete degradation of hemicellulose, cellulose, and lignin in GS 231

during the previous pyrolysis process for producing GSB (Figure 2). The greater GSHAC 232

yield compared with the GSAC yield was attributed to the hemicellulose in GS that was 233

decomposed by the hydrothermal process to produce GSH (Figure 2). 234

Table 1 235

3.3. Textural properties 236

The adsorption/desorption isotherms of the ACs and their precursors are presented in Figure 237

S4. Clearly, the adsorption isotherms of all the ACs and biochar sample belong to Type I of 238

the International Union of Pure and Applied Chemistry (IUPAC) classification, and the Type I 239

isotherm is a typical characteristic of micropores with a small external surface area [19]. 240

Furthermore, a wide knee (hysteresis loop) is present in the adsorption/desorption isotherms of 241

the porous carbon materials. Hysteresis appearing in the multilayer range of physical 242

adsorption isotherms is usually related to the adsorbent with micropore or mesopore structures. 243

14

According to the IUPAC nomenclature, porous carbon materials exhibit the H4-type 244

hysteresis loop, which is associated with narrow slit-like pores. 245

Table 2 lists the textural parameters of the ACs and their precursors. Both the raw material 246

(GS) and hydrothermal treated material (GSH) had low SBET and pore volume. Nevertheless, 247

after the chemical activation process, the SBET values of the ACs increased sharply in 248

comparison with those of their precursors, with the order being GSAC (8.14–1413 m2/g) > 249

GSHAC (24.8–1238 m2/g) > GSBAC (597.8–811.9 m2/g). The micropore volume accounted 250

for a major part of the total pore volume, and its share in the total pore volume decreased in 251

the order: GSAC > GSHAC > GSAC > GSB. A higher percentage of micropore volume might 252

result in higher adsorption capacity of small molecules. Surprisingly, the GSB produced by 253

slow pyrolysis at 800 °C also exhibited favorable textural characteristics, indicating its 254

potential for use as an adsorbent. 255

Table 2 256

From a practical and economical perspective, because the yield is as important as the surface 257

area, the product SBET × total yield can be considered as a performance criterion for any 258

porous carbon preparation process [20]. Although the total GSAC yield was the lowest (see 259

Table 1), the product of SBET × total yield was the highest (814.8 m2/g) for this material; the 260

product values were 789.75 m2/g, 683.24 m2/g, and 137.37 m2/g for GSHAC, GSBAC, and 261

GSB, respectively. Clearly, GSAC is economically beneficial. 262

3.4. Morphological and crystal properties 263

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Figure 3 shows the morphology of the synthesized ACs and their precursors. The raw GS and 264

hydrothermal material had low porosities and fairly rough surfaces. These observations 265

matched with their low SBET and total pore volume (Table 2). The microspheres in GSH, 266

which are visible in Figure 2, were formed from the high cellulose fraction in GS during 267

hydrothermal carbonization [7]. However, the high lignin content (Figure 2) might prohibit 268

the formation of a pathway from the high number of carbon spheres in the GSH sample. 269

Furthermore, the poorly developed pores of GSH might result from the weak decomposition 270

of cellulose and lignin during the hydrothermal process. A marked change can be observed in 271

the morphology of GSB when the lignocellulose material is pyrolyzed at a high temperature. 272

The surface morphology of GSB shows well-developed pores and fairly high surface area 273

(SBET ≈ 598 m2/g), and therefore, GSB can be considered as a green adsorbent for use in 274

wastewater treatment processes. 275

Notably, the synthesized AC samples were characterized by irregular and heterogeneous 276

surface morphology with sponge-like structures. The formation of well-developed pores of 277

various sizes and shapes in the ACs during pyrolysis resulted from the evaporation of K2CO3 278

[21]. Okada et al. [22] highlighted the high SBET of ACs prepared from waste newspaper 279

through K2CO3 activation, which may intercalate and expand the interlayers of adjacent 280

hexagonal network planes. The planes consist of C atoms, and pore formation is enhanced 281

because of poorly developed hexagonal planes, which are similar to poorly developed planes 282

in graphite. Another reason for enhanced pore formation might be the diffusion of K2CO3 283

molecules into the rudimentary pores created during pyrolysis and the consequent acceleration 284

of the K2CO3–C reaction [23]. Pore development in the char during pyrolysis plays a crucial 285

16

role in improving SBET and the total pore volume. Therefore, it can be inferred that the surface 286

morphology of the ACs and biochar strongly depended on their preparation method. 287

Figure 3 288

XRD patterns of the crystallinities of the six adsorbents are presented in Figure 4. The XRD 289

patterns of raw GS and GSH show high crystallinity, with two sharp peaks at 2θ values of 290

approximately 15° and 22°. These peaks are characteristic of crystalline cellulosic materials 291

[24]. The other adsorbents show weak crystallinity. Cellulose in the GS and GSH with a 292

parallel alignment and a crystalline structure resulted from hydrogen bond interactions and 293

van der Waal’s forces between adjacent molecules [25]. Hemicellulose and lignin are 294

amorphous in nature [24]. The XRD spectra of GSB and AC samples showed the absence of 295

the observed peaks for the adsorbents, suggesting that (1) the structure of GSB and ACs is 296

predominantly amorphous and (2) cellulose and lignin in GS and GSH are completely 297

decomposed at the pyrolysis temperature of 800 °C. 298

Figure 4 299

3.5. Surface chemistry 300

Qualitative information on functional groups present on the adsorbent surfaces and their 301

spectroscopic assignments is provided in Figure 5. The spectrum of GS is typical of 302

lignocellulose materials [7, 26]. The bands at 3700–3000 cm−1 are assigned to (O–H) 303

stretching vibrations in hydroxyl groups in hemicellulose, cellulose, and lignin [27]. 304

Furthermore, the bands observed in the range of 3000–2800 cm−1 are related to asymmetric 305

17

and symmetric C–H stretching vibrations of the methyl (–CH3–) and methylene (–CH2=) 306

groups, which are expected to be present in hemicellulose, cellulose, and lignin. The TGA 307

results of GSH (see Figure 2c) confirm that hemicellulose in GS was decomposed during the 308

hydrothermal process. Consequently, the peak at 2924 cm−1 in the GSH spectrum reflects 309

aliphatic C–H stretching in the methyl and methylene groups in cellulose and lignin. The 310

intense bands observed for aliphatic CHn nearly disappeared in the spectra of porous carbon 311

materials, indicating the complete loss of labile aliphatic compounds, and a marked decrease 312

in the nonpolar group content in the carbonization process [28]. The presence of a carbon-313

carbon triple bond (C≡C) in disubstituted alkynes can be inferred from the bands in the region 314

of 2450–2150 cm−1. The C≡C vibrations in alkyne groups of biochar and the three ACs were 315

more intense than those in raw GS, because of the release of volatile matter [26]. 316

The characteristics of carboxylic and lactonic groups (C=O) are evident from the bands at 317

1800–1650 cm−1. The decrease in the most insensitive peaks at 1800–1650 cm−1 for the 318

porous carbon materials indicated that C=O groups were decomposed at the high 319

carbonization temperature, and thermal decomposition can be evaluated using [29]. This 320

inference matches with the results of Boehm titration. The amount of carboxylic and lactonic 321

groups present on the porous carbon materials decreased sharply by approximately 93%–97% 322

and 78%–92% compared with raw GS, respectively (Table 3). 323

Similarly, the bands around the region from 1650 to 1480 cm−1 can be attributed to the C=C 324

double bonds in aromatic rings. The presence of aromatic benzene rings can also be 325

recognized from the bands in the 970–730 cm−1 region, which belong to the aromatic C–H 326

18

out-of-plane bending mode. Finally, the observed band between approximately 1290 and 970 327

cm−1 corresponds to stretching C−O groups [26]. 328

Figure 5 329

The electrical state of the adsorbent surfaces in solutions was characterized by the point of 330

zero charge (PZC). The pH value at which the net (external and internal) surface charge of an 331

adsorbent is zero is defined as pHPZC [30]. Figure S3 and Table 3 show a plot and pHPZC 332

values of adsorbent samples, obtained using the pH drift method. Obviously, the pHPZC values 333

of the ACs were strongly dependent on the chemical activation method used (pHPZC of GSAC > 334

GSBAC > GSHAC). 335

Table 3 336

Table 3 presents quantitative information on the acidic and basic groups on the adsorbent 337

surfaces; the information was obtained through Boehm titration. Generally, any adsorbent 338

typically coexists with both acidic and basic properties in solutions. However, the dominant 339

concentration of the total acidic groups or total basic groups is strongly dependent on the 340

pHPZC values. When pHPZC < 7.0, the total amount of acidic groups should dominate; 341

otherwise, the total amount of basic groups should dominate. Clearly, the changes in the 342

pHPZC values are in accordance with the change in the total acidic groups and total basic 343

groups. 344

3.6. Adsorption properties 345

19

The adsorption properties of the ACs and biochar were evaluated on the basis of the iodine 346

number and MB number. Iodine molecules (≈0.27 nm) can be adsorbed into micropores (pore 347

width > 1 nm) of porous materials [31], whereas MB molecules with a minimal molecular 348

cross section of approximately 0.8 nm require the minimal pore size distribution to be 1.33 nm 349

[32]. 350

The iodine number of commercial AC is typically 900 mg/g, with the iodine values being 351

greater than 1000 mg/g for high grades of AC. The adsorption capacities of the materials for 352

iodine are listed in Table 4, and a linear correlation between the iodine number and micropore 353

parameters (i.e., volume and surface area) is presented in Figure S5. As expected, the ACs 354

show a high iodine number in the following order: GSHAC (2695 mg/g) > GSAC (2604 355

mg/g) > GSBAC (1568 mg/g). These iodine values are relatively higher than those of 356

commercial ACs. Several previous investigations have reported that ACs produced from 357

lignocellulose and asphalt materials exhibited a high iodine number, exceeding 2000 mg/g. 358

Nowicki et al. [33] synthesized ACs from walnut shells through chemical activation with 359

KOH, and they claimed that the iodine numbers of ACs were 2160 mg/g at a pyrolysis 360

temperature of 700 °C, and approximately 2000 mg/g at 800 °C. Similar high iodine numbers 361

(2208 mg/g) have been noted in other studies too [34]. Furthermore, Kandah et al. [35] 362

reported that asphalt-derived AC showed an iodine number that was approximately thrice that 363

of commercial AC. 364

The adsorption capacities of MB with a concentration of 1000 mg/L on biochar and ACs are 365

displayed in Table 4. The adsorption capacity (in milligram per gram) and removal efficiency 366

(in percentage) of MB at 24 h decreased in the order of GSBAC (233.6 mg/g; 94.7%) > 367

20

GSAC (210.2 mg/g; 85.5%) > GSHAC (143.1 mg/g; 57.8%) > GSB (32.6 mg/g; 13.5%). 368

Graham (1955) investigated the nominal limiting pore diameter for adsorption of MB by the 369

five commercial ACs and concluded that the average nominal limiting pore diameter for MB 370

adsorption was approximately 1.3 nm. The adsorption amounts of MB on GSB and ACs are 371

related to the adsorption capacities of organic compounds with high molecular weight on the 372

adsorbents. Because organic compounds with high molecular weight cannot enter micropores, 373

the adsorption capacities of porous carbon materials for MB are not proportional to their SBET. 374

Both SBET and the pore size are the factors that determine the highest adsorption capacity. 375

Table 4 376

Table S1 presents a comparison of the textural properties, total yield, and SBET × total yield of 377

the ACs synthesized in this study with those of ACs prepared in previous investigations. 378

Clearly, the yield of AC synthesized under oxygen-limited conditions was considerably higher 379

than that of AC prepared in an inert nitrogen atmosphere. The values of the specific surface 380

area and total pore volume of both GSAC and GSHAC were in accordance with the values 381

presented in the literature; however, the values for GSBAC did not agree with literature values. 382

This might be attributed to GSBAC being produced with a lower impregnation ratio of the 383

activating reagent/ precursor. 384

3. Conclusions 385

The characteristics of ACs synthesized from GS indicated the SBET values followed the order 386

of GSAC > GSHAC > GSBAC > GSB > GSH > GS. Micropore volumes of the ACs were 83% 387

for GSAC, 73% for GSHAC, and 68% for GSBAC. Typical acidic functional groups present 388

21

on the surface of ACs were accurately identified through FTIR and Boehm titration. Excellent 389

adsorption properties of the ACs were confirmed from the high iodine numbers and the MB 390

index. On the basis of favorable characteristics, GS can be considered an excellent precursor 391

for synthesizing ACs through chemical activation. 392

Acknowledgements 393

This current work was financially supported by Chung Yuan Christian University (CYCU) in 394

Taiwan. First author would like to thank CYCU for the Distinguished International Graduate 395

Students (DIGS) scholarship to pursue his doctoral studies. 396

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4. Oh, G.H. and C.R. Park, Preparation and characteristics of rice-straw-based porous 407

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5. Kennedy, L.J., J.J. Vijaya, and G. Sekaran, Effect of two-stage process on the 409

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6. Fernandez, M.E., et al., Development and characterization of activated hydrochars 413

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7. Sevilla, M., A.B. Fuertes, and R. Mokaya, High density hydrogen storage in 416

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8. Huang, F.-C., et al., Preparation of activated carbon using micro-nano carbon spheres 419

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Potassium Carbonate And Potassium Hydroxide. Applied Surface Science, 2014. 293: 425

p. 138-142. 426

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494

495

27

FIGURE CAPTIONS 496

Figure 1. Schematic illustration of the preparation procedure for GS, GSB, GSH, and their 497

activated carbons (GSAC, GSBAC, and GSHAC) 498

Figure 2. Thermo-gravimetric analysis of the ACs and their precursors 499

Figure 3. Scanning electron microscopy (SEM) of the ACs (d-f) and their precursors (a-c) 500

Figure 4. The X-ray diffraction spectra of the ACs and their precursors 501

Figure 5. FTIR spectra of activated carbons and their precursors 502

503

504

505

TABLE CAPTIONS 506

Table 1. Physical and chemical characteristics of the ACs and their precursors 507

Table 2. Textural parameters of the ACs and their precursors 508

Table 3. Surface chemistry of the ACs and their precursors 509

Table 4. Iodine number and Methylene number of ACs and biochar 510

511

28

512

Figure 1. Schematic illustration of the preparation procedure for GS, GSB, GSH, and their 513

activated carbons (GSAC, GSBAC, and GSHAC) 514

515

29

516

517

518

Figure 2. Thermo-gravimetric analysis of the ACs and their precursors 519

30

520

521

522

523

524

525

526

527

528

529

530

531

532

533

534

Figure 3. Scanning electron microscopy (SEM) of the ACs (d-f) and their precursors (a-c) 535

(a) GS (d) GSAC

(b) GSB (e) GSBAC

(c) GSH (f) GSHAC

31

536

537

538

539

540

541

542

543

544

Figure 4. The X-ray diffraction spectra of the ACs and their precursors 545

546

547

32

548

Figure 5. FTIR spectra of the ACs and their precursors549

33

Table 1. Physical and chemical characteristics of ACs and their precursors 550

GS GSB GSH GSAC GSBAC GSHAC Ultimate analysis C (wt %) 53.39 77.29 60.64 71.04 76.31 75.61 H (wt %) 6.14 1.26 3.15 0.59 0.19 0.35 N (wt %) 0.84 0.56 1.05 0.60 0.65 1.20 O (wt %)a 39.63 20.89 35.16 27.77 22.85 22.84 H/Cb 1.37 0.19 0.62 0.10 0.03 0.06 O/Cb 0.56 0.20 0.44 0.29 0.22 0.23 Proximate analysis Moisture (wt %) 1.80±0.28 2.23±0.29 3.12±0.18 4.86±0.25 3.13±0.72 5.30±0.79 Volatile (wt %) 76.23±0.53 14.33±3.87 63.11±0.85 13.10±2.33 14.24±3.81 15.43±2.02 Total ash (wt %) 1.17±0.30 0.89±0.08 0.45±0.10 0.93±0.04 0.60±0.15 0.85±0.68 Fixed carbon (wt %) 20.93±0.78 84.73±3.77 33.31±0.95 81.11±2.31 82.03±2.98 78.41±2.53 Physicochemical properties Total yield (%) - 22.97±3.14 - 57.67±3.20 84.16±1.46 63.77±1.38 Hardness (%) 49.07±2.00 83.05±2.32 77.78±1.70 58.01±3.18 80.89±0.63 76.04±4.26 Bulk density (g/cm3) 0.66±0.022 0.40±0.007 0.32±0.001 0.11±0.002 0.37±0.020 0.17±0.002 pH (1:20) before washed 5.43±0.33 10.58±0.03 4.83±0.02 11.45±0.05 12.65±0.05 11.37±0.23

pH (1:20) after washed - 8.46±0.27 4.20±0.17 8.15±0.20 9.71±0.03 8.81±0.33 aCalculated by difference; batomic ratio; mean values ± standard deviation. 551

34

Table 2. Textural parameters of ACs and their precursors 552

553 GS GSB GSH GSAC GSBAC GSHAC

BET surface area (m2/g) 8.14 598 24.80 1,413 812 1,238

Langmuir surface area (m2/g) 17.93 772 37.74 1,846 1,030 1,685

External surface area (m2/g) - 160 - 101 128 109

Micropore surface area (m2/g) - 438 - 1,302 684 1,130

Total pore volume (cm3/g) 0.01 0.30 0.08 0.66 0.38 0.68

Micropore volume (cm3/g) - 0.16 - 0.55 0.26 0.50

Non-micropore volume (cm3/g) - 0.14 - 0.11 0.12 0.18

Micropore/total pore volume (%) - 53.97 - 83.22 68.60 73.10

Average pore diameter (nm) - 3.08 - 2.92 3.73 4.44

Median pore width (nm) - 0.63 - 0.65 0.65 0.67

Total yield x SBET (m2/g) - 137.34 - 814.84 683.24 789.75 554

35

Table 3. Surface chemistry of ACs and their precursors 555

556

GS GSB GSH GSAC GSBAC GSHAC

Total acidic groups (mmol/g) 8.74±0.21 0.72±0.03 2.12±0.01 0.77±0.07 0.55±0.10 1.20±0.05

+ Phenolic 0.37±0.38 0.05±0.04 0.53±0.08 0.19±0.09 0.22±0.16 0.39±0.15

+ Lactonic 1.75±0.21 0.25±0.07 0.83±0.05 0.34±0.13 0.13±0.09 0.37±0.08

+ Carboxylic 6.61±0.14 0.41±0.04 0.76±0.04 0.25±0.02 0.20±0.03 0.43±0.03

Total basic sites (mmol/g) 0.12±0.03 0.33±0.08 0.10±0.05 1.56±0.02 0.60±0.06 0.99±1.31

Total groups (mmol/g) 8.86 1.05 2.21 2.34 1.15 2.18

Density of aciditya (C/m2) - 0.12 - 0.05 0.07 0.09

Density of total groupsb (C/m2) - 0.17 - 0.16 0.14 0.17

pHPZC 3.75±0.35 6.65±0.21 4.30±0.14 8.55±0.07 7.75±0.07 5.25±0.21

aDensity of acidity = (total acidic groups in mmol/g x Faraday's constant of 96.5 in C/mmol)/SBET in m2/g; bDensity of total groups 557

calculated similarly to density of acidity; mean values ± standard deviation. 558

559

36

Table 4. Iodine number and Methylene number of ACs and biochar 560

561

GSB GSAC GSBAC GSHAC

Iodine number (mg/g) 794±6.16 2604±2.26 1568±4.54 2695±0.88

Methylene number (mg/g)

+ Contact time of 5 min 4.15±1.46 177.81±0.67 164.05±0.63 21.37±1.25

+ Contact time of 24 h 32.62±2.89 210.21±0.92 233.59±1.20 143.14±1.98

Pore volume (> 1.3 nm; cm3/g)a 0.072 0.176 0.103 0.160

aCalculated from incremental BJH (Barret–Joyner–Halenda) pore volumes larger than the pore with of 1.3 nm (cm3/g); mean values ± 562

standard deviation.563

37

Supplementary information for: 564

565

Characterizations of Activated Carbons from Golden Shower (Cassia 566

fistula) upon Different Chemical Activation Methods with Potassium 567

Carbonate 568

Figure captions 569

Figure S1. The pods bar of Cassia fistula (Golden Shower) (a), and their particle sizes of 0.106-570

0.25 mm (b) 571

Figure S2. Acidic oxygen functionalities on surfaces of the adsorbents 572

Figure S3. Point of zero charge of ACs and their precursors 573

Figure S4. Nitrogen adsorption/desorption isotherms (a) of GS, GSB, GSH, GSAC, GSBAC, and 574

GSHAC; and pore size distributions (b) of GSB, GSAC, GSBAC, and GSHAC 575

Figure S5. The correlation between micropore volume, micropore sufface area, and iodine number 576

577

Table caption 578

Table S1. Comparison on textural properties, total yield, and SBET x total yield of ACs in this study 579

with other ACs published in literatures 580

581

38

582

583

584

585

586

587

588

589

590

591

592

Figure S1. The pods bar of Cassia fistula (golden shower) (a), and their particle sizes of 0.106-593

0.25 mm (b) 594

595

(a)

(b)

39

596

597

Figure S2. Acidic oxygen functionalities on surfaces of the adsorbents 598

599

40

600

Figure S3. Point of zero charge of ACs and their precursors 601

(Digitals in parenthesis indicate values of pHpzc ± standard deviation) 602

603

41

604

605

Figure S4. Nitrogen adsorption/desorption isotherms (a) of GS, GSB, GSH, GSAC, GSBAC, and 606

GSHAC; and pore size distributions (b) of GSB, GSAC, GSBAC, and GSHAC607

42

608

609

Figure S5. The correlation between micropore volume, micropore sufface area, and iodine number610

43

Table S1. Comparison on textural properties, total yield, and SBET x total yield of ACs in this study with other ACs published in literatures

Activated carbon SBET (m2/g) Pyrolysis

conditions Yield (%)

Micro-pore volume/total pore volume

Activating reagents

Yield x

SBET References

ACs produced from the one-stage chemical activation process (GSAC)

Kraft lignin of spruce wood 2000

800 oC, 1 h, N2 flow

40.4 0.85/0.95 K2CO3 (1:1) 808 Hayashi et al.(200)

Chickpea husk 1778 800 oC, 1 h,

N2 flow 22.5 0.75/0.82 K2CO3 (1:1) 400 Hayashi et al. (2002)

Golden shower 1413 800 oC, 4 h,

O2 absence 57.7 0.55/0.66 K2CO3 (1:1) 815 This study

Soybean oil cake 1352 800 oC, 1 h,

N2 flow 11.56 0.40/0.68 K2CO3 (1:1) 156 Tay et al. (2009)

Palm shell 1170 800 oC, 2 h,

N2 flow 18.9 0.57/no mentioned

K2CO3 (1:1) 221 Adinata et al. (2007)

Grape seed 1166 800 oC, 2 h,

N2 flow 24.6 0.46/0.48 K2CO3 (1:1) 287 Okman et al. (2014)

Grape seed 1090 800 oC, 1 h,

N2 flow 20.1 0.41/0.47 KOH (1:1) 219 Okman et al. (2014)

Soybean oil cake 618 800 oC, 1 h,

N2 flow 4.11 0.143/0.29 KOH (1:1) 25 Tay et al. (2009)

Commercial rice straw pulp 430

800 oC, 1 h, N2 flow 8.5 0.16/0.51

KOH (4:1) 37 Basta et al. (2009)

Raw rice straw 420 800 oC, 1 h,

N2 flow 17.5 0.15/0.31 KOH (4:1) 74 Basta et al. (2009)

44

Activated carbon SBET (m2/g) Pyrolysis

conditions Yield (%)

Micro-pore volume/total pore volume

Activating reagents

Yield x

SBET References

ACs produced from the two-stage chemical activation process via pre-pyrolysis carbonization (GSBAC)

Rice straw 2200 800 oC, 1 h,

N2 flow - 0.54/ 1.15 KOH (4:1) - Oh and Park (2002)

Raw rice straw 1554 800 oC, 1 h,

N2 flow 13.5 0.59/0.93 KOH (4:1) 210 Basta et al. (2009)

Commercial rice straw pulp 1393

800 oC, 1 h, N2 flow

18 0.25/0.76 KOH (4:1) 251 Basta et al. (2009)

Rice straw 1010 800 oC, 1 h,

N2 flow - 0.34/ 0.64 KOH (1:1) - Oh and Park (2002)

Golden shower 812 800 oC, 4 h,

O2 absence 84.2 0.26/0.38 K2CO3 (1:1) 683 This study

Rice husk 379 800 oC,

N2 flow 39.2 0.39/0.65 H3PO4

(85% by weight)

149 Kennedy et al. (2004)

45

Activated carbon SBET

(m2/g)

Pyrolysis

conditions

Yield

(%)

Micro-pore volume/total pore volume

Activating

Reagentsa

Yield

x

SBET

References

ACs produced from the two-stage chemical activation process via pre-hydrolysis carbonization (GSHAC)

Eucalyptus sawdust 2252 700 oC, 1 h,

N2 flow 36 0.91/1.03

KOH

(4:1) 810 Sevilla et al. (2011)

Commercial starch 2194 700 oC, 1 h,

N2 flow 37 0.92/1.01

KOH

(4:1) 812 Sevilla et al. (2011)

Commercial furfural 2179 700 oC, 1 h,

N2 flow 34 0.91/1.03

KOH

(4:1) 741 Sevilla et al. (2011)

Commercial glucose 2121 700 oC, 1 h,

N2 flow 43 0.91/1.00

KOH

(4:1) 912 Sevilla et al. (2011)

Commercial cellulose 2047 800 oC, 1 h,

N2 flow 33 0.74/0.98

KOH

(4:1) 676 Sevilla et al. (2011)

Commercial glucose 1612 800 oC, 3 h,

N2 flow - 0.35/ 0.86

KOH

(3:1) - Huang et al. (2014)

46

Commercial Sucrose 1491 800 oC, 3 h,

N2 flow - 0.73/0.79

KOH

(5:1) - Huang et al. (2014)

Commercial Xylose 1386 800 oC, 3 h,

N2 flow - 0.25/0.83

KOH

(5:1) - Huang et al. (2014)

Golden shower 1238 800 oC, 4 h,

O2 absence 63.8 0.50/0.68

K2CO3

(1:1) 790 This study

Commercial tapioca flour 986.2

800 oC, 1 h,

N2 flow 39.01 0.46/0.57

KOH

(3:1) 385 Pari et al. (2014)

Orange peel 618 600 oC, 1 h,

N2 flow 23.5 0.29/0.39

H3PO4

(4:1) 145 Fernandez et al. (2015)

aThe ratio in parenthesis indicates the impregnation weight ratio of activating reagent to precursor

47

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26

27

28

29

30