Role of Inherent Inorganic Constituents in SO2 Sorption Ability of

Subscriber access provided by The Chinese University of Hong Kong

Article

The role of inherent inorganic constituents in SO2 sorption ability of biochars derived from three biomass wastes Xiaoyun Xu, Da Xuan Huang, Ling Zhao, Yue Kan, and Xinde Cao Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b03077 • Publication Date (Web): 28 Oct 2016 Downloaded from http://pubs.acs.org on October 29, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Environmental Science & Technology is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 29

Environmental Science & Technology

1

The role of inherent inorganic constituents in SO2 sorption ability of biochars

2

derived from three biomass wastes

3 4

Xiaoyun Xu, Daxuan Huang, Ling Zhao, Yue Kan, Xinde Cao*

5

School of Environmental Science and Engineering, Shanghai Jiao Tong University,

6

Shanghai 200240, China

7 8

*

Corresponding author. Tel: +86 21 3420 2841. E-mail: [email protected].

9

1

ACS Paragon Plus Environment


10 11

ABSTRACT Biochar is rich in both organic carbon and inorganic components. Extensive

12

work has attributed the high sorption ability of biochar to the pore structure and

13

surface chemical property related to its organic carbon fraction. In this study, three

14

biochars derived from dairy manure (DM-biochar), sewage sludge (SS-biochar), and

15

rice husk (RH-biochar), respectively were evaluated for their SO2 sorption behavior

16

and the underlying mechanisms, especially the role of inherent inorganic constituents.

17

The sorption capacities of SO2 by the three biochars were 8.87-15.9 mg g-1. With the

18

moisture content increasing from 0% to 50%, the sorption capacities increased by up

19

to about 3 times, mainly due to the formation of alkaline water membrane on the

20

biochar surface which could promote the sorption and transformation of acidic SO2.

21

DM- and SS-biochar containing larger mineral constituents showed higher sorption

22

capacity for SO2 than RH-biochar containing less mineral components. CaCO3 and

23

Ca3(PO4)2 in DM-biochar induced sorbed SO2 transformation into K2CaSO4.H2O and

24

CaSO4.2H2O, while the sorbed SO2 was converted to Fe2(SO4)3.H2SO4.2H2O,

25

CaSO4.2H2O, and Ca3(SO3)2SO4.12H2O in SS-biochar. For RH-biochar, K3H(SO4)2

26

might exist in the exhausted samples. Overall, the chemical transformation of SO2

27

induced by biochar inherent mineral components occupied 44.6%-85.5% of the total

28

SO2 sorption. The results obtained from this study demonstrated that biochar as a

29

unique carbonaceous material could distinctly be a promising sorbent for acidic SO2

30

removal in which the inorganic components played an important role in the SO2

31

sorption and transformation.

32 33

2


Page 2 of 29

Page 3 of 29


34

Graphic Abstract

35 36 37

3



38

INTRODUCTION

39

Sulfur dioxide is a great threat to atmospheric environment and human health [1]. In

40

2014, China standards applicable to new large boilers set the limit value of SO2

41

emission as low as 300 mg m-3 for solid fuels, 25% lower than the previous one [2].

42

The increasingly strict rules and regulations urgently need to develop the techniques

43

for reducing emission of SO2.

44

Among the various desulfurization methods used for SO2 removal, sorption of

45

SO2 on porous carbon materials shows several advantages such as high operation

46

flexibility and low maintenance cost [3-5]. Microporous structure and surface

47

chemical property of carbon materials have been proved to be mainly responsible for

48

the effective SO2 removal, especially in the presence of O2 and H2O [6-8]. SO2 can

49

catalytically be oxidized to SO3 in an SO2-O2 atmosphere and hydrated to H2SO4 in an

50

SO2-O2-H2O atmosphere [7]. As a result, a lot of studies have focused on

51

improvement of pore structure or modification of surface chemical property to

52

enhance SO2 removal performance. For example, various activations with strong

53

acid/base or ZnCl2 were applied to improve pore structure and surface basicity was

54

enhanced via incorporation of basic nitrogen functionalities [9] or introduction of

55

alkaline substance, such as KOH, CaO, and CaCO3 [10]. Since SO2 is an acidic gas,

56

the high alkalinity of carbon materials would favor its sorption. However, the

57

introduction of additives by pretreatment might cause several drawbacks, including

58

blocking the pores in carbon materials, possible weak binding of the additives to the

59

carbon [11]; and the relatively high production cost resulted from the multistep

60

activation procedures [12].

61

Biochar, a carbon-rich byproduct of biomass pyrolysis, is quite similar to

62

activated carbon with respect to high surface area and rich surface functional groups

4


Page 4 of 29

Page 5 of 29


63

[13]. Lots of work has attributed the high sorption ability of biochar to the pore

64

structure and surface chemical property related to its organic carbon fraction [14, 15].

65

However, in addition to the richness of organic carbon, biochar also contains a fair

66

amount of inorganic constituents such as alkali metals (K, Na) and alkali earth metals

67

(Ca, Mg) [16]. Furthermore, biochar is naturally alkaline with the most of pH values

68

being over 8 [17]. Our recent work showed that the high alkalinity and rich minerals

69

of biochar played an important role in the effective removal and transformation of

70

H2S [18]. Therefore, we assumed that these distinct properties of biochar might also

71

favor sorption and catalytic oxidation of acidic SO2. It is hypothesized that minerals in

72

the biochar could further react with SO2 dissolved in the alkaline water membrane on

73

the biochar surface to form various sulfite or sulfate which would in turn enhance SO2

74

sorption.

75

In this paper, we aimed to investigate the sorption behavior of SO2 by biochars

76

as well as the underlying mechanisms, especially the role of minerals in the

77

transformation of SO2. Specifically, (1) three representative biochars with different

78

mineral content and species were prepared from dairy manure, sewage sludge, and

79

rice husk, respectively; (2) the effect of O2 and H2O on the catalytic oxidation process

80

of SO2 by biochars and the resulted S forms were determined; and (3) the contribution

81

of minerals in the removal of SO2 and the transformation of SO2 during sorption were

82

elucidated.

83 84

MATERIALS AND METHODS

85

Preparation and characterization of biochar. Sewage sludge was collected

86

from a wastewater treatment plant located in Southern Shanghai city, China. Dairy

87

manure and rice husk were sampled from a farm in a suburban of Shanghai city,

5



88

China. The composition of the biomass was shown in the supporting information.

89

After being dried to a constant weight at 105 °C, these waste solids were ground to

90

less than 2 mm and then subjected to a typical slow pyrolysis process for biochar

91

production as described previously [18]. Briefly, the ground biomass was put in in a

92

stainless steel reactor in a Muffle Furnace and heated under N2 condition at a rate of

93

10 ºC min-1. When the temperature reached 500 ºC, the heating was kept for 4 h and

94

the resulting solid residue was called as biochar. The biochars derived from dairy

95

manure, rice husk, and sewage sludge were referred to as DM-biochar, RH-biochar

96

and SS-biochar, respectively. The resulted biochars were ground and passed through a

97

0.5-mm sieve for later characterization and SO2 sorption.

98 99

The concentrations of C, H, and N in biochars were determined using the CHNS/O Analyzer (Perkin Elmer, 2400 II). Mineral elements (Ca, Mg, K, etc.) were

100

analyzed using ICP-AES (ICAP6000 Radial, Thermo) after biochar was digested with

101

the USEPA Method 3050B [19]. To determine the ash content without decomposing

102

the inherent minerals such as carbonates and silicate, biochars were heated at 600°C

103

in a muffle oven under air atmosphere for 2 h. The pH of the biochar was measured in

104

solution with de-ionized water at a ratio of 1:20 after 24 h equilibrium using the

105

pH/Ion 510 Bench Meter. The surface area and pore volume was detected by N2

106

adsorption isotherms at 77 K using a Surface Area and Porosimetry Analyzer

107

(ASAP2060, Micromeritics Inc., USA). The mineral phases of biochar were

108

characterized by X-ray diffraction (D/max-2200/PC, Japan Rigaku Corporation)

109

operating at 35 kV and 20 mA, and data was collected over the 2θ range from 10 to 50

110

using Cu Kα radiation with a scan speed of 2o per minute. All the characterization was

111

made in three replicates.

112

SO2 sorption. Dynamic tests were carried out at room temperature to evaluate 6


Page 6 of 29

Page 7 of 29


113

the capacity of biochars for SO2 removal. Biochar samples were packed into a glass

114

column (25 cm length, 5 cm diameter). The filled length of the sample was 2.5 cm

115

with the mass of DM-, SS-, and RH-biochar being about 4.0±0.2, 8.0±0.4, and

116

3.8±0.2 g, respectively due to the difference of their densities. A mixture of 0.5%

117

(5000 ppm) SO2 and ambient air was then passed through the column of adsorbent at

118

1.5 L min-1. The outlet concentration of SO2 was monitored using flue gas meter

119

(KM900, KANE, UK). The test was stopped until the outlet concentration equaled the

120

inlet concentration. The sorption capacities of each sorbent in terms of mg SO2 per g

121

of biochar were calculated by integration of the area above the sorption curves, and

122

from the SO2 concentration in the inlet gas, flow rate, sorption time, and mass of

123

adsorbent. The detailed calculations were shown in the supporting information. For

124

each sample, the experiments were carried out with at least three replicates. The

125

difference of determined capacities agreed within 4%.

126

To evaluate the effect of moisture content on the sorption capacity of SO2 by

127

biochars, moist air with 100% relative humidity at room temperature was circulated

128

through the biochar bed at a flow rate of 2 L min-1 until the moisture content of the

129

biochars reached 10, 20, 30, and 50 wt%. Then the humid biochars were used for the

130

dynamic tests of SO2 sorption run in the air. To determine the effect of O2 during the

131

sorption process of SO2 by biochars, the breakthrough capacity experiments were run

132

in N2 at 50 wt% moisture content and compared with air at the same moisture.

133

Transformation of SO2 during the sorption. After the breakthrough capacity

134

experiments were completed, the original and exhausted biochars were freeze-dried

135

for the further characterization analysis. The pH value and the water soluble SO42-

136

concentration in the biochars were determined. The possible less soluble sulfate

137

minerals in the exhausted biochars were characterized for by XRD analysis. To

7



138

further evaluate the chemical nature of S-containing species, the differential

139

thermogravimetric (DTG) experiments were carried out in a TGA instruments

140

(Mettler Toledo, Switzerland). The peak maximum temperature and range of the DTG

141

curve provide information on the nature of the sorbed S species while the size of the

142

peak is related to their amount. The heating rate was 10 oC min-1 and the final

143

temperature was 900 oC. The analyses were performed under a constant flow of 50

144

mL min-1 of N2.

145

Contribution of mineral components of biochar to SO2 sorption. In order to

146

determine the role of mineral components of biochar in SO2 sorption, inorganic

147

fraction was separated from biochar according to the method described in a previous

148

study [20]. Briefly, about 0.5 g biochar was put in an open crucible and heated at

149

500°C for 6h in the atmosphere in the Muffle Furnace to eliminate the carbon fraction.

150

The SO2 sorption capacities of biochar minerals were also determined through

151

dynamic tests as done in the section “SO2 sorption” and the SO2 transformation

152

mechanism was conducted as done in the section “Transformation of SO2 during the

153

sorption”. In order to quantify the contribution of biochar minerals, a calculation was

154

conducted by normalizing SO2 sorption by biochar minerals into that by the whole

155

biochar.

156 157 158

RESULTS AND DISCUSSION Characteristics of biochars. The selected physico-chemical properties of the

159

DM-, SS-, and RH-biochars are shown in Table 1. The RH-biochar had a higher

160

specific surface area and pore volume than the other two biochars, which was perhaps

161

due to the relative higher proportion of hemicelluloses, cellulose, and lignin fractions

162

in the rice husk (Table S1). All three biochars were alkaline, and the DM-biochar had

8


Page 8 of 29

Page 9 of 29


163

the highest pH (Table 1). The difference in pH among these three biochars may be due

164

to the different content of alkali metals (K, Na, et al), alkali earth metals (Ca and Mg)

165

and carbonates (Table 1), which were the main alkaline substances in the biochar [17].

166

Although SS-biochar had the highest content of Ca+Mg+K (7.74%), compared to the

167

other two biochars, its pH was the lowest due to the fact that SS-biochar was rich in

168

Fe (2.21%), which could hydrolyze to produce H+ in the solution. These basic sites of

169

biochars could facilitate sorption of acidic SO2 [21].

170

All three biochars were rich in C, but the C content varied greatly among the

171

three types of biochars, following the order RH-biochar > DM-biochar > SS-biochar

172

(Table 1), which is consistent with the previous work that biochars derived from plant

173

residues normally had the highest C, followed by biochars produced from animal

174

waste and municipal waste [22]. Rice husk is rich in hemicelluloses, celluloses and

175

lignin while sludge and dairy manure are rich in minerals (ash) (Table S1), so

176

RH-biochar contained the highest C and SS- and DM-biochar had a much higher

177

nutrient and mineral content (Table 1). High content of mineral elements in

178

SS-biochar allowed the existence of calcite (CaCO3), maghemite (γ-Fe2O3), and some

179

clay minerals such as anorthite (CaAl2Si2O8) and muscovite (KAl2Si3AlO10(OH, F)2)

180

(Fig.S1), agreeing with the reports by Qian and Jiang [23]. Thus, the SS-biochar had

181

the highest ash content (61.4%) than the other two biochars (55.3%, 33.2% for DM-

182

and RH-biochar, respectively). Noted that the ash content of 33.2% for RH-biochar

183

was somewhat overestimated due to the incomplete ashing resulted from the possible

184

mutual protection between carbon and silicon rich in the RH-biochar as reported by

185

Xiao et al [24]. Both SS- and DM-biochars had a high Ca content (2.17% for

186

DM-biochar, 6.57% for SS-biochar) that was present as CaCO3 (Fig. S1). High Mg in

187

the DM-biochar allowed its association with high Ca and P (Table 1) and was present

9



188

as the mineral whitlockite (Ca, Mg)3(PO4)2 (Fig. S1). Compared to SS-biochar and

189

DM-biochar, no obvious crystal peaks were identified in RH-biochar due to the much

190

less mineral species and content (Table 1). Besides, the rich noncrystalline C in

191

RH-biochar might cause the high background intensity and thus weaken the relative

192

peak intensity of other minerals [25]. Different physico-chemical properties of three

193

biochars such as surface area, alkalinity, especially mineral species and content may

194

lead to different performance in SO2 sorption as well as the underlying mechanism

195

[26].

196

Sorption of SO2 by biochars and effects of H2O and O2. The SO2 sorption

197

curves of three biochars in the presence of air with different moisture content are

198

shown in Fig. 1 and the values of the sorption capacities are given in Table S2. For all

199

three biochars, the sorption of SO2 without any moisture (0%) was very fast, with

200

breakthrough time of only about 30 seconds, and thereafter the shape of the sorption

201

curve was almost a vertical line until saturation. It means a limited sorption of SO2 by

202

biochars under no moisture condition with sorption capacities of three biochars being

203

8.9-15.9 mg g-1, following the decreasing trend of DM-biochar > SS-biochar >

204

RH-biochar (Table S2). However, the sorption was greatly improved and increased as

205

the moisture increased from 0% to 50% except for SS-biochar (Fig. 1). The SO2

206

sorption by SS-biochar increased with the moisture increasing from 0% to 20% and

207

then decreased hereafter (Fig. 1), this is probably because too much H2O tend to

208

induce aggregation of the clay minerals rich in SS-biochar [1] which occupied or

209

blocked the active sites for SO2 sorption. The sorption capacities of SS-biochar at 20%

210

and 50% moistures were 38.4 mg g-1 and 34.5 mg g-1, increased by about 3 times and

211

2.6 times, respectively, compared to that without moisture (Table S2). With the

212

moisture content in DM-biochar increased from 0% to 50%, the breakthrough and

10


Page 10 of 29

Page 11 of 29


213

saturation time continuously increased from 32 and 260 seconds to 100 and 650

214

seconds, respectively. Correspondingly, the sorption capacity increased from 15.9 mg

215

g-1 to 64.3 mg g-1, elevated by about 3 times (Table S2). Similarly, the sorption

216

capacity of RH-biochar increased from 8.9 mg g-1 at 0% moisture to 13.3 mg g-1 at 50%

217

moisture, elevated by about 50% (Table S2). It has been reported that the presence of

218

moisture was a necessity to obtain a superior performance of SO2 sorption by carbon

219

materials since H2O could change the chemistry of the system and the mechanism of

220

adsorption [6]. All three biochars used in this study had pH values of above 8.90

221

(Table 1), the presence of H2O could favor the formation of alkaline water membrane

222

on the biochar surface which would facilitate the removal of acidic SO2 [27].

223

Generally, the sorption process is governed by surface chemistry and pore

224

texture of the sorbent [4]. It has been reported that surface area, pore volumes, and the

225

pore size distribution of carbon materials played a dominant role in the SO2 sorption

226

[28, 29]. Table 1 shows that the surface areas and pore volume of three biochars are

227

RH-biochar > SS-biochar > DM-biochar, however, the contrary trend of the SO2

228

sorption capacity was observed as DM-biochar > SS-biochar > RH-biochar (Table S2).

229

The discrepancy suggested that the SO2 sorption by the biochar was not controlled by

230

its surface area. The sorption trend seems to agree with that of the mineral contents

231

(Table 1 and S2). Our previous studies indicated that heavy metals or H2S uptake by

232

biochars was attributed to a combined process of surface adsorption and precipitation

233

with biochar minerals [30, 31]. Therefore, we inferred that the inherent mineral

234

components in the biochar might also be involved in the SO2 sorption, which would

235

be demonstrated in next sections.

236

After SO2 sorption, surface area and pore volume of the biochars with no

237

moisture (0%) greatly decreased, compared to the original samples (Table S3). It is

11



238

probably due to the SO2 molecules anchored on the pore surface as described by Sun

239

et al [1]. Increasing the moisture content to 50% increased surface area and pore

240

volume of the exhausted DM- and SS-biochar which was even larger than the original

241

samples (Table S3). This is probably because high moisture enhanced the

242

transformation of biochar inherent minerals such as calcite to amorphous sulfate

243

minerals in the SO2-sorbed biochar which tend to have relative higher surface area

244

and pore volume [32]. The above results again demonstrated that porous structure and

245

chemical properties of sorbents could have a coupling influence on SO2 removal in

246

the presence of O2 and H2O due to the catalytic oxidation and hydration of SO2 to

247

H2SO4 [4, 33].

248

Figure S2 compares the SO2 sorption under N2 and air condition at 50% moisture.

249

The corresponding sorption capacities are presented in Table S2. Although there were

250

changes in the breakthrough and saturation time for biochars with different carrier

251

gases, all three biochars showed reduced sorption capacities for SO2 in the N2 carrier

252

gas with the decrease ranging from 8.2% to 30%, compared to air as carrier gas

253

(Table S2). The results demonstrated that the presence of O2 could enhance the

254

adsorption process though the effect was not as much as the moisture content

255

(49.9%-3040%). Previous work also showed that presence of O2 did not dramatically

256

affect the adsorption process of SO2 by KMnO4 modified activated carbon fibers or

257

calcium carbonate but SO2 breakthrough capacity was greatly increased when H2O

258

was present [27, 32].

259

Transformation of SO2 during its sorption by DM-biochar. After SO2 sorbed,

260

DM-biochar showed obvious decline of pH with the values decreased from 10.2 in the

261

original biochar to 7.07 as the moisture increased to 50% (Fig. 2). The decrease of pH

262

further demonstrated that acidic SO2 was sorbed and dissolved in the alkaline water 12


Page 12 of 29

Page 13 of 29


263

membrane on the biochar surface. XRD patterns of DM-biochar after SO2 sorption

264

(Fig. 3) show stronger peak intensity of sulfate with higher moisture content. The

265

exhausted DM-biochar at 0% and 10% moistures did not show obvious peaks of new

266

minerals compared to the original one. At 20% moisture, the exhausted DM-biochar

267

started to show the new peaks of K2CaSO4.H2O, and the strongest peak intensity of

268

CaSO4.2H2O was observed at 50% moisture (Fig. 3), proving that high moisture

269

enhanced the formation of insoluble sulfate. Ca in CaSO4.2H2O and K2CaSO4.H2O

270

might come from the transformation of Ca3(PO4)2 to Ca(H2PO4)2.H2O and the

271

dissolution of CaCO3, which was evidenced by XRD analysis in which the peak of

272

Ca3(PO4)2 and CaCO3 in the original sample decreased or disappeared after SO2

273

sorption while the new peak of Ca(H2PO4)2.H2O appeared (Fig. 3). The series of

274

transformation of SO2 to calcium sulfate minerals resulted from the catalytic

275

oxidation reaction of SO2 on carbon materials in the presence of O2 and H2O [34]. The

276

change in water soluble SO42- further supported this transformation. As the moisture

277

increased, more SO2 was sorbed and oxidized into SO42-, but the amount was not

278

enough for formation of calcium sulfate minerals, so water soluble SO42- increased.

279

When the moistures increased over 30%, insoluble calcium sulfate minerals were

280

largely formed, resulting in the decrease of water soluble SO42- (Fig. 2).

281

The DTG curve for DM-biochar before SO2 sorption revealed two peaks at

282

25-200 oC and 600-800oC, which referred to the loss of physically adsorbed water and

283

the decomposition of minerals such as calcium carbonate or phosphate, respectively

284

(Fig. 4). After SO2 sorption on DM-biochar without moisture (0% moisture), the DTG

285

pattern didn’t show any obvious change, compared with the original biochar, which

286

means that a limited and weak physical sorption of SO2 may happen on the biochar

287

surface. For the exhausted DM-biochar at 50% moisture, two strong peaks appeared at

13



288

25-200 oC, indicating more SO2 was weakly sorbed on the surface of biochar in the

289

presence of H2O. The new peak between 150 and 200 oC appeared in the exhausted

290

DM-biochar at 50% moisture was probably attributed to the crystal water in

291

CaSO4.2H2O, Ca(H2PO4)2.H2O, and K2CaSO4.H2O formed during the SO2 sorption

292

and could dehydrate at the temperature between 100 and 200 oC [35, 36] (Fig. 3).

293

Another new peak between 200 and 400 oC in the exhausted DM-biochar at 50%

294

moisture was normally attributed to the loss of H2SO4 [26, 37]. Since H2SO4 was

295

unlikely existed as the system was still close to neutral or even basic after SO2

296

sorption (Fig. 2), the new peak was probably resulted from the decomposition of

297

Ca(H2PO4)2 which started to decompose at 200 oC [36]. Moreover, the new peak

298

between 700 and 850 oC was probably attributed to the decomposition of dehydrated

299

calcium sulfate minerals such as CaSO4.2H2O and K2Ca(SO4)2.H2O. The results of

300

DTG analysis showed the formation of the new calcium sulfate minerals after SO2

301

sorption, which is consistent with the XRD analysis (Fig. 3). Besides, the exhausted

302

biochar at 50% moisture reveal the reduced peak intensity between 600 and 700 oC

303

which is related to calcium carbonate or phosphate existing in the original

304

DM-biochar (Fig. 4). This indicated occurrence of the reaction of CaCO3 and

305

Ca3(PO4)2 in the original biochar with sorbed SO2, in accordance with the

306

disappearance of the peaks for CaCO3 and Ca3(PO4)2 after SO2 sorption in XRD

307

analysis (Fig. 3). Based on the discussion above, the transformation of SO2 during the

308

sorption by DM-biochar was probably as follows:

309

SO2(gas) ↔ SO2(ads)

310

2SO2(ads) + 2H2O + O2→ 2SO42- + 4H+

311

4H++ Ca3(PO4)2 + H2O→ Ca(H2PO4)2·H2O + 2Ca2+

312

2H++ CaCO3→ Ca2+ + CO2↑ + H2O 14


Page 14 of 29

Page 15 of 29


313

SO42- + Ca2+ + 2H2O → CaSO4·2H2O↓

314

SO42- + CaSO4·2H2O + 2K+→K2Ca(SO4)2·H2O↓ + H2O

315

Transformation of SO2 during its sorption by SS-biochar. The pH values in

316

the exhausted SS-biochar reduced from 8.40 in the original biochar to 6.65 as the

317

moisture increased to 50% (Fig. 2). It again demonstrated that acidic SO2 was sorbed

318

and dissolved in the alkaline water membrane on the biochar surface. XRD analysis of

319

the exhausted SS-biochar with no moisture (0%) did not show obvious new peaks

320

compared to the original sample (Fig. 3). The exhausted SS-biochar at 10% moisture

321

started to show the new peaks of Fe2(SO4)3·H2SO4.2H2O and CaSO4·2H2O, and the

322

intensities of those peaks became stronger with the increasing moisture content. The

323

exhausted SS-biochar at 50% moisture even showed the apparent new peak of

324

Ca3(SO3)2SO4·12H2O (Fig. 3). Baltrusaitis et al [32] studied the effect of water on the

325

reactions of SO2 with calcium carbonate and found that water catalyzed the formation

326

of CaSO3 to a larger extent than CaSO4, indicating that high moisture content was

327

favorable for the formation of sulfite. Besides, the pores in SS-biochar at 50%

328

moisture might be blocked by the “water cluster” [38] or the aggregation of the clay

329

minerals rich in SS-biochar, which hindered the transport of O2 and therefore affected

330

the complete oxidation of S(IV) to S(VI). The change in water soluble SO42- (Fig. 2)

331

further supported the transformation of SO2.

332

Like DM-biochar, the DTG curve for SS-biochar before SO2 sorption also

333

revealed two peaks at 25-200 oC and 600-750oC (Fig. 4), linking to the removal of

334

physically adsorbed water and the decomposition of minerals, respectively. The DTG

335

pattern of exhausted SS-biochar without moisture didn’t show any obvious change,

336

compared to the original sample. The DTG pattern of the exhausted SS-biochar at 50%

337

moisture showed several new peaks at about 100, 140, 600-650,700-900 oC, which

15



338

correspond to the loss of the weakly adsorbed SO2 on the surface, the decomposition

339

of crystal water, sulfite minerals, and sulfate minerals, respectively (Fig. 4), in

340

accordance with the appearance of Ca3(SO3)2SO4·12H2O, CaSO4·2H2O and

341

Fe2(SO4)3·H2SO4·2H2O in XRD patterns of SS-biochar after SO2 sorption at 50%

342

moisture. The exhausted biochar also showed the decrease of the peak between 600

343

and 750 oC related to calcium carbonate in the original SS-biochar, indicating the

344

reaction of calcium carbonate in the original biochar with sorbed SO2, which was

345

consistent with the decrease of the peak CaCO3 in XRD pattern (Fig. 3). Based on the

346

discussion above, the transformation of SO2 during the sorption by SS-biochar was

347

probably as follows:

348


349

SO2(ads) + H2O ↔ H2SO3*↔2H+ + SO32-

350

2SO2(ads) + 2H2O + O2→ 2SO42- + 4H+

351

2H+ + CaCO3→ Ca2+ + CO2↑ + H2O

352

2SO32- + 3Ca2+ + SO42- + 12H2O → Ca3(SO3)2SO4·12H2O↓

353

SO42- + Ca2+ + 2H2O →CaSO4·2H2O↓

354

4SO42- + Fe2O3 + 8H+→Fe2(SO4)3·H2SO4·2H2O +·H2O

355

Transformation of SO2 during its sorption by RH-biochar. Sorption and

356

dissolution of acidic SO2 in the alkaline water membrane on the RH-biochar surface

357

reduced pH from 9.5 in the original biochar to 5.58 at 50% moisture (Fig. 2).

358

Dissolved SO2 was further oxidized into sulfate in the presence of O2 and high

359

moisture, resulting in increase in the water soluble SO42- (Fig. 2). When the moisture

360

increased to over 20%, less soluble calcium sulfate minerals would be formed, as a

361

result, the water soluble SO42- decreased (Fig. 2). However, the production of less

362

soluble sulfate might be limited and not enough to be detected in the XRD and DTG

16


Page 16 of 29

Page 17 of 29


363

analysis (Fig. 3 and 4). XRD analysis showed the formation of K3H(SO4)2 in the

364

exhausted samples with the moisture increasing from 20% to 50%. Presence of H+ in

365

the K3H(SO4)2 is probably because the pH of biochar after SO2 sorption was reduced

366

to within the range of acidity (4.53-5.61) (Fig. 2). The DTG pattern for RH-biochar

367

before SO2 sorption revealed two peaks at 50-200 oC and 600-800 oC which were

368

linked to the removal of physically adsorbed water and the decomposition of calcium

369

carbonate, respectively (Fig. 4). No obvious change in DTG patterns for all exhausted

370

RH-biochars occurred compared with the original biochar (Fig. 4), indicating that SO2

371

was physically sorbed by RH-biochar that was readily desorbed by purging during the

372

DTG analysis. On the other hand, K3H(SO4)2 formed during SO2 sorption could not be

373

decomposed at the studied temperature range in the DTG analysis. Based on the

374

discussion above, the transformation of SO2 during the sorption by RH-biochar was

375

probably as follows:

376


377

2SO2(ads) + 2H2O + O2→ 2SO42- + 4H+

378

3K+ + 2SO42-+ H+→ K3H(SO4)2

379

In short, the catalytic oxidation of SO2 by all three biochars started from the

380

adsorption of SO2 to the biochar surface, and then the dissociation in the water film

381

and the gradual oxidation to SO42-, followed by reaction with the biochar mineral

382

components to form sulfate products. Overall, interaction of mineral with SO2 played

383

a great role during sorption by all three biochars.

384

Contribution of inherent mineral components in biochars to the sorption. In

385

order to further determine the role of mineral components in biochar, inorganic

386

fraction (minerals) was separated from biochar by removing organic carbon and

387

subjected to the SO2 sorption. The breakthrough and saturation time of SO2 sorption 17



388

by DM-biochar minerals increased to 340 seconds and 1100 seconds, with increase by

389

240% and 89.7%, respectively, compared to original DM-biochar (Table 2). For SO2

390

sorption by SS-biochar minerals, the breakthrough and saturation time increased by

391

10.0% and 38.9%, respectively, compared to the original SS-biochar. RH-biochar

392

minerals also showed much longer breakthrough and saturation time with increase by

393

100% and 47.8%, respectively, compared to the original RH-biochar (Table 2). All

394

three biochar minerals showed much higher sorption capacities for SO2, with increase

395

by 36.6% -123%, compared to the corresponding biochar. If the SO2 sorption per

396

gram mineral was normalized into that per gram biochar, the contribution of minerals

397

to SO2 sorption by biochars was 44.6%-85.5% (Table 2), further demonstrating that

398

minerals in biochar may play an important role in the sorption of SO2. Compared to

399

DM-biochar and SS-biochar, the RH-biochar contained less mineral content and less

400

mineral contribution to the SO2 sorption (Table 1 and 2). Therefore, the RH-biochar

401

had smaller capacity for SO2 sorption (Table S2).

402

The XRD patterns for all three minerals after SO2 sorption showed almost the

403

same new peaks of sulfate as the corresponding SO2-sorbed biochars but the peak

404

intensity was much higher due to the concentrated minerals, compared to those in

405

their biochars (Fig. 3 and Fig. S3). However, there still existed some differences in the

406

sulfate species between biochars and corresponding minerals after SO2 sorption. For

407

instance, DM- and RH-biochar minerals after SO2 sorption did not show the peaks of

408

CaSO4.2H2O and K3H(SO4)2, respectively which were observed in the corresponding

409

exhausted biochars (Fig. S3 and Fig. 3). Element K and Ca enriched in the DM- and

410

RH-biochar minerals resulted in the sulfate existed in the form of K2Ca(SO4)2.H2O

411

instead of CaSO4.2H2O and K3H(SO4)2, respectively. The SS-biochar mineral after

412

SO2 sorption did not show the peak of Ca3(SO3)2SO4.12H2O as in the exhausted 18


Page 18 of 29

Page 19 of 29


413

biochar (Fig. 3 and Fig. S3), indicating that minerals might enhance the oxidation of

414

SO2 from S(IV) to S(VI). The XRD results of biochar minerals after SO2 sorption

415

further proved that biochar minerals contributed to the sorption of SO2 via the

416

chemical reactions.

417

Environmental implication. Results from this study indicated that biochars

418

derived from dairy manure, sludge, and rice husk could be promising sorbents for SO2

419

capture. Presence of moisture and O2 could facilitate the sorption or reaction of SO2

420

with biochar. Mineral components in the biochar could react with the sorbed SO2 to

421

form various sulfate minerals, enhancing removal of SO2 by biochars. Richness of

422

mineral components in the biochar is mainly responsible for high retention of SO2 via

423

formation of stable sulfate minerals.

424

Besides, a large amount of organic wastes such as animal manure and crop

425

residues are produced annually around the world. Turning these waste products into

426

carbonaceous materials like biochar that can sorb contaminants have environmental

427

implications for improving waste management and protecting the environment.

428

Moreover, biochar was initially developed for dealing with the challenge of climate

429

change caused by the greenhouse effect. The exhausted biochar after desulfurization

430

could still be buried in the soil to fulfill its role in the carbon sequestration and

431

gypsum as the main product of desulfurization could improve crop yields and soil

432

quality [39].

433

SUPPORTING INFORMAITON

434

Composition of the biomass and Calculation of the sorption capacity; Composition of

435

biomass used for biochar production (Table S1); Breakthrough time, saturation time,

436

and capacity of SO2 sorption by DM-, SS- and RH-biochars at different moisture

437

contents and carrier gases (Table S2); Pore parameters of biochar before and after SO2 19



438

sorption at different moisture contents (Table S3); XRD patterns of DM-, SS- and

439

RH-biochars (Figure S1); Breakthrough curves of SO2 sorption by DM-, SS- and

440

RH-biochars in the air and N2 carrier gases (Figure S2); XRD patterns of biochar

441

minerals before and after SO2 sorption at 50% moisture (Figure S3). This material is

442

available free of charge via the Internet at http://pubs.acs.org.

443

ACKNOWLEDGEMENTS

444

This work was supported in part by National Natural Science Foundation of China

445

(No. 21537002, 21428702).

20


Page 20 of 29

Page 21 of 29


446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489

References 1. Sun, F.; Gao, J.; Liu, X.; Tang, X.; Wu, S. A systematic investigation of SO2 removal dynamics by coal-based activated cokes: The synergic enhancement effect of hierarchical pore configuration and gas components. Applied Surface Science 2015, 357, 1895-1901. 2. Ministry of Environmental Protection of the People,s Republic of China, Emission standard of air pollutants for boiler (GB 13271-2014); http://kjs.mep.gov.cn/hjbhbz/bzwb/dqhjbh/dqgdwrywrwpfbz/201405/W020140530580815383678. pdf. 3. Sethupathi, S.; Bashir, M. J.; Akbar, Z. A.; Mohamed, A. R. Biomass-based palm shell activated carbon and palm shell carbon molecular sieve as gas separation adsorbents. Waste Management & Research : the Journal of the International Solid Wastes and Public Cleansing Association, ISWA 2015, 33, 303-12. 4. Karatepe, N.; Orbak, Đ.; Yavuz, R.; Özyuğuran, A. Sulfur dioxide adsorption by activated carbons having different textural and chemical properties. Fuel 2008, 87, 3207-3215. 5. Boualem, T.; Debab, A.; Martinez de Yuso, A.; Izquierdo, M. T. Activated carbons obtained from sewage sludge by chemical activation: gas-phase environmental applications. Journal of Environmental Management 2014, 140, 145-151. 6. Bagreev, A.; Bashkova, S.; Bandosz, T. J. Adsorption of SO2 on activated carbons: the effect of nitrogen functionality and pore sizes. Langmuir 2002, 18, 1257-1264. 7. Raymundo-Piñero, E.; Cazorla-Amorós, D.; Salinas-Martinez de Lecea, C.; Linares-Solano, A. Factors controling the SO2 removal by porous carbons: relevance of the SO2 oxidation step. Carbon 2000, 38, 335-344. 8. Moreno-Castilla, C.; Carrasco-Marin, F.; Utrera-Hidalgo, E.; Rivera-Utrilla, J. Activated carbons as adsorbents of sulfur dioxide in flowing air. Effect of their pore texture and surface basicity. Langmuir 1993, 9, 1378-1383. 9. Raymundo-Pinero, E.; Cazorla-Amoros, D.; Linares-Solano, A. The role of different nitrogen functional groups on the removal of SO2 from flue gases by N-doped activated carbon powders and fibres. Carbon 2003, 41, 1925-1932. 10. Macias-Perez, M. C.; Bueno-Lopez, A.; Lillo-Rodenas, M. A.; Sahnas-Martinez de Lecea, C.; Linares-Solano, A. SO2 retention on CaO/activated carbon sorbents. Part I: Importance of calcium loading and dispersion. Fuel 2007, 86, 677-683. 11. Fan, L.; Chen, J.; Guo, J.; Jiang, X.; Jiang, W. Influence of manganese, iron and pyrolusite blending on the physiochemical properties and desulfurization activities of activated carbons from walnut shell. Journal of Analytical and Applied Pyrolysis 2013, 104, 353-360. 12. Nieto-Márquez, A.; Atanes, E.; Fernández, A.; López-Villa, S.; Cambra, A.; Fernández-Martínez, F. Effective adsorption of sulfur dioxide by activated carbons containing calcium carbonate from deinking paper sludge. Industrial & Engineering Chemistry Research 2014, 53, 15620-15628. 13. Ahmad, M.; Rajapaksha, A. U.; Lim, J. E.; Zhang, M.; Bolan, N.; Mohan, D.; Vithanage, M.; Lee, S. S.; Ok, Y. S. Biochar as a sorbent for contaminant management in soil and water: a review. Chemosphere 2014, 99, 19-33. 14. Jiang, T. Y.; Jiang, J.; Xu, R. K.; Li, Z. Adsorption of Pb(II) on variable charge soils amended with rice-straw derived biochar. Chemosphere 2012, 89, 249-256. 15. Tong, X.; Li, J.; Yuan, J.; Xu, R. Adsorption of Cu(II) by biochars generated from three crop 21



490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533

straws. Chemical Engineering Journal 2011, 172, 828-834. 16. Zhao, L.; Cao, X.; Wang, Q.; Yang, F.; Xu, S. Mineral constituents profile of biochar derived from diversified waste biomasses: implications for agricultural applications. Journal of Environmental Quality 2013, 42, 545-52. 17. Yuan, J. H.; Xu, R. K.; Zhang, H. The forms of alkalis in the biochar produced from crop residues at different temperatures. Bioresource Technology 2011, 102, 3488-3497. 18. Xu, X.; Cao, X.; Zhao, L.; Sun, T. Comparison of sewage sludge- and pig manure-derived biochars for hydrogen sulfide removal. Chemosphere 2014, 111, 296-303. 19. U. S. E. P. A. Test methods for evaluating solid waste, Laboratory Manual Physical/Chemical Methods. U.S. Gov. Print Office, Washington, DC. 1986. 20. Kuhlbusch, T. Method for determining black carbon in residues of vegetation fires. Environmental Science & Technology 1995, 29, 2695-2702. 21. Muniz, J.; Herrero, J. E.; Fuertes, A. B. Treatments to enhance the SO2 capture by activated carbon fibres. Applied Catalysis B-Environmental 1998, 18, 171-179. 22. Zhao, L., Zheng, W., Cao, X.D. Distribution and evolution of organic matter phases during biochar formation and their importance in carbon loss and pore structure. Chemical Engineering Journal 2014, 250, 240-247. 23. Qian, T. T.; Jiang, H. Migration of phosphorus in sewage sludge during different thermal treatment processes. ACS Sustainable Chemistry & Engineering 2014, 2, 1411-1419. 24. Xiao, X.; Chen, B.; Zhu, L. Transformation, morphology, and dissolution of silicon and carbon in rice straw-derived biochars under different pyrolytic temperatures. Environmental Science & Technology 2014, 48, 3411-3419. 25. Guerrero, M.; Ruiz, M. P.; Millera, Á.; Alzueta, M. U.; Bilbao, R. Characterization of biomass chars formed under different devolatilization conditions: Differences between rice husk and eucalyptus. Energy Fuels 2008, 22, 1275-1284. 26. Bashkova, S.; Bagreev, A.; Locke, D. C.; Bandosz, T. J. Adsorption of SO2 on sewage sludge-derived materials. Environmental Science & Technology 2001, 35, 3263-3269. 27. Liu, W.; Adanur, S. Desulfurization properties of modified activated carbon fibers and activated carbon fiber paper. Journal of Industrial Textiles 2013, 44, 513-525. 28. Lua, A. C.; Guo, J. Adsorption of sulfur dioxide on activated carbon from oil-palm waste. Journal of Environmental Engineering 2001, 127, 895-901. 29. Raymundo-Pinero, E.; Cazorla-Amoros, D.; De Lecea, C. S. M.; Linares-Solano, A. Factors controling the SO2 removal by porous carbons: relevance of the SO2 oxidation step. Carbon 2000, 38, 335-344. 30. Xu, X.; Cao, X.; Zhao, L.; Zhou, H.; Luo, Q. Interaction of organic and inorganic fractions of biochar with Pb(II) ion: further elucidation of mechanisms for Pb(II) removal by biochar. Rsc Advances 2014, 4, 44930-44937. 31. Cao, X.; Ma, L.; Gao, B.; Harris, W. Dairy-manure derived biochar effectively sorbs lead and atrazine. Environmental Science & Technology 2009, 43, 3285-3291. 32. Baltrusaitis, J.; Usher, C. R.; Grassian, V. H. Reactions of sulfur dioxide on calcium carbonate single crystal and particle surfaces at the adsorbed water carbonate interface. Physical Chemistry Chemical Physics 2007, 9, 3011-3024. 33. Lee, Y. W.; Park, J. W.; Choung, J. H.; Choi, D. K. Adsorption characteristics of SO2 on activated carbon prepared from coconut shell with potassium hydroxide activation. Environmental 22


Page 22 of 29

Page 23 of 29


534 535 536 537 538 539 540 541 542 543 544 545 546 547 548

Science & Technology 2002, 36, 1086-1092. 34. Lizzio, A. A.; DeBarr, J. A. Mechanism of SO2 removal by carbon. Energy & Fuels 1997, 11, 284-291. 35. Borrachero, M.; Payá, J.; Bonilla, M.; Monzó, J. The use of thermogravimetric analysis technique for the characterization of construction materials. Journal of Thermal Analysis and Calorimetry 2008, 91, 503-509. 36. Ioiţescu, A.; Vlase, G.; Vlase, T.; Doca, N. Kinetics of decomposition of different acid calcium phosphates. Journal of Thermal Analysis and Calorimetry 2007, 88, 121-125. 37. Arcibar-Orozco, J. A.; Rangel-Mendez, J. R.; Bandosz, T. J. Reactive adsorption of SO2 on activated carbons with deposited iron nanoparticles. Journal of Hazardous Materials 2013, 246-247, 300-309. 38. Do, D.; Do, H. A model for water adsorption in activated carbon. Carbon 2000, 38, 767-773. 39. Watts, D. B.; Dick, W. A. Sustainable uses of FGD gypsum in agricultural systems: introduction. Journal of Environmental Quality 2014, 43, 246-52.

549 550

23



Page 24 of 29

Table 1. The selected physico-chemical properties of the DM-, SS- and RH-biochars Specific surface area Biochar

a

Mineral elements (%)

Pore volume

(m2 g-1)

(cm3 g-1)

DM-biochar

4.83a

0.014

SS-biochar

10.1

RH-biochar

41.5

C

H

N

S

Ash

pH Ca

Mg

Fe

Mn

K

P

(%)

(%)

(%)

(%)

(%)

10.2

2.17

0.76

0.60

0.04

2.32

4.84

50.0

2.04

2.17

0.58

55.3

0.022

8.90

6.57

0.64

2.21

0.05

0.53

1.70

28.7

1.49

3.28

1.23

61.4

0.041

9.78

0.96

0.23

0.13

0.03

2.90

0.81

51.2

1.93

0.55

0.06

33.2

The means of three replicates (n=3)

24


Page 25 of 29


Table 2. Contribution of minerals to the SO2 sorption by biochars in the presence of air at 50% moisture content Biochar

Mineral Fraction

Breakthrough time

Saturation time

Sorption capacities

SO2 capacity of minerals

Contribution of mineral in

(%)

(s)

(s)

(mg g-1)

normalized into biochar

SO2 sorption by biochar

Biochar

Minerals

Biochar

Minerals

Biochar

Minerals

(mg g-1)

(%)

DM-biochar

39.9a

100

340

580

1100

43.8

83.7

33.4

76.3

SS-biochar

62.6

50

55

450

580

27.0

36.9

23.1

85.5

RH-biochar

20.0

70

140

230

340

13.3

29.8

5.95

44.6

a

The means of three replicates (n=3)

25



DM-biochar

1.0

C/Co

0.8 0.6 0.4

0% 10% 20% 30% 50%

0.2 0.0

0

100

200

300

400

500

600

700

Time (s)

SS-biochar

1.0 0.8

C/Co

0.6 0.4

0% 10% 20% 30% 50%

0.2 0.0

0

100

200

300

400

500

600

700

Time (s)

1.0

RH-biochar

C/Co

0.8 0.6 0.4

0% 10% 20% 30% 50%

0.2 0.0

0

50

100

150

200

250

300

350

400

Time (s)

Figure 1. Breakthrough curves of SO2 sorption by DM-, SS- and RH-biochars at different moisture contents.

26


Page 26 of 29

Page 27 of 29


60 10 50 -1

7

40 30

2-

pH

8

SO4 (mg g )

9

20

6

10

5 DM-biochar

0

4 --

Original 0%

10%

20%

30%

50%

Moisture content

40

10 9

-1

20

7

2-

8 pH

SO4 (mg g )

30

6 10

5

SS-biochar

4

0

--

Original 0%

10%

20%

30%

50%

Moisture content

6 10 5

2-

pH

3

7

-1

4

8

SO4 (mg g )

9

2

6

1

5 RH-biochar

4

0 --

Original 0%

10%

20%

30%

50%

Moisture content

Figure 2. Changes in pH and soluble SO42- in DM-, SS- and RH-biochars after SO2 sorption at the moisture contents from 0% to 50%, compared to the original biochars. The pH and SO42- values are the means of three replicates (n=3).

27



Q

DM-biochar

Gy

Page 28 of 29

Q

K

C,Gy K,W K,Sy Gy Gy W

Ch Ch

50% 30% 20% 10% 0%

Cp W

Original

10

15

20

25

30

35

40

2ϴ (°)

Q

SS-biochar

Ct F,Gy

Q,Gy I,M So F

Ct

F

M

F,A Gy F Gy Gy I,M

50% 30% 20% 10%

A

G

10

15

20

25

0%

C

Original

30

35

40

2ϴ (°)

RH-biochar

Q Ps 50% 30% 20% 10% 0% Original

10

15

20

25

30

35

40

2ϴ (°)

Figure 3. XRD patterns of DM-, SS- and RH-biochars before and after SO2 sorption at the moisture contents from 0% to 50%. A, Anorthite; C, CaCO3; Ct, Ca3(SO3)2SO4.12H2O; Cp, Ca3(PO4)2; Ch, Ca(H2PO4)2.H2O; G, γ-Fe2O3; Gy, CaSO4.2H2O; F, Fe2(SO4)3.H2SO4.2H2O; I, Illite; M, Muscovite; K, K2Ca(SO4)2.H2O; Ps, K3H(SO4)2; Q, Quartz; So, Silicon Oxide; Sy, KCl; W, (Ca,Mg)3(PO4)2.

28


Page 29 of 29


0.12 Original Air, 0% Air, 50%

DM-biochar Weight derivative (%/oC)

0.10 0.08 0.06 0.04 0.02 0.00 100

200

300

400

500

600

700

800

900

Temperature (oC)

0.10

Weight derivative (%/oC)

SS-biochar

Original Air, 0% Air, 50%

0.08

0.06

0.04

0.02

0.00 100

200

300

400

500

600

700

800

900

Temperature (oC)

0.08 Original Air, 0% Air, 50%

Weight derivative (%/oC)

RH-biochar 0.06

0.04

0.02

0.00 100

200

300

400

500

600

700

800

900

Temperature (oC)

Figure 4. The DTG curves for DM-, SS- and RH-biochars before and after SO2 sorption at 0% and 50% moistures.

29


Role of Inherent Inorganic Constituents in SO2 Sorption Ability of

Recommend Documents