Influences of Temperature and Metal on Subcritical Hydrothermal

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Release and Catalytic Effect of Heavy Metal during Subcritical Hydrothermal Liquefaction of Hyperaccumulator: Implications for the Recycling of Hazardous Hyperaccumulators Feng Qian, Xiangdong Zhu, Yuchen Liu, Quan Shi, Longhua Wu, Shicheng Zhang, Jianmin Chen, and Zhiyong Jason Ren Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b03756 • Publication Date (Web): 27 Jan 2018 Downloaded from http://pubs.acs.org on January 30, 2018

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Environmental Science & Technology

ACS Paragon Plus Environment


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1

Influences of Temperature and Metal on Subcritical Hydrothermal

2

Liquefaction of Hyperaccumulator: Implications for the Recycling

3

of Hazardous Hyperaccumulators

4

Feng Qian,† Xiangdong Zhu,†,‡,* Yuchen Liu,† Quan Shi,§ Longhua Wu,# Shicheng Zhang,†, ¢, *

5

Jianmin Chen,†, ¢ Zhiyong Jason Ren‡

6

†

7

Environmental Science and Engineering, Fudan University, Shanghai 200433, China

8

‡

9

CO 80309, United States

Shanghai Key Laboratory of Atmospheric Particle Pollution and Prevention (LAP3), Department of

Department of Civil, Environmental, and Architectural Engineering, University of Colorado Boulder, Boulder,

10

§

State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China

11

#

Key Laboratory of Soil Environment and Pollution Remediation, Institute of Soil Science, Chinese Academy

12

of Sciences, Nanjing 210008, China

13

¢

Shanghai Institute of Pollution Control and Ecological Security, Shanghai 200092, China

14 15

*

Corresponding

author

Tel/fax:

+86-21-65642297;

16

(Xiangdong Zhu), [email protected] (Shicheng Zhang)

17 18


E-mail:

[email protected]

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19


ABSTRACT

20

Waste Sedum plumbizincicola (S. plumbizincicola), a zinc (Zn) hyperaccumulator during

21

phytoremediation, was recycled via a subcritical hydrothermal liquefaction (HTL) reaction into

22

multiple streams of products, including hydrochar, bio-oil, and carboxylic acids. Results show

23

approximately 90% of Zn was released from the S. plumbizincicola biomass during HTL at an

24

optimized temperature of 220 °C, and the release risk was mitigated via HTL reaction for

25

hydrochar production. The low-Zn hydrochar (~ 200 mg/kg compared to original plant of

26

1558mg/kg) was further upgraded into porous carbon (PC) with high porosity (930 m2/g) and

27

excellent capability of carbon dioxide (CO2) capture (3 mmol/g). The porosity, micropore

28

structure, and graphitization degree of PCs were manipulated by the thermal recalcitrance of

29

hydrochar. More importantly, results showed that the released Zn2+ could effectively promote the

30

production of acetic acid via the oxidation of furfural (FF) and 5-(hydroxymethyl)-furfural

31

(HMF). Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) with

32

negative electrospray ionization analysis confirmed the deoxygenation and depolymerization

33

reactions and the production of long chain fatty acids during HTL reaction of S. plumbizincicola.

34

This work provides a new path for the recycling of waste hyperaccumulator biomass into

35

value-added products.

36



37

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INTRODUCTION

38

Phytoremediation can be an effective approach to remove heavy metals from contaminated

39

soil with high efficiency and biocompatibility.1-3 For example, Sedum plumbizincicola (S.

40

plumbizincicola) is a heavy metal hyperaccumulator that is capable of absorbing zinc (Zn) from

41

soil in extremely high concentration (~10,000 mg/g).4-6 However, one remaining major barrier

42

for the application of phytoremediation is the disposal of large quantities of contaminated plant

43

biomass. If not treated appropriately, the accumulated metals may return to the environment and

44

cause secondary contamination.

45

Hydrothermal liquefaction (HTL) has recently emerged as an environmentally friendly

46

technology to recycle waste biomass and organic materials to produce bio-oils and chemicals,7-9

47

and its capability of separating metals from hyperaccumulators have been demonstrated very

48

effective.10,

49

landfilling,11 HTL showed superior performance in separating heavy metals without further

50

contamination. Moreover, it can fully take advantage of the hyperaccumulator biomass resources

51

by recycling metals, chemicals, and hydrochar.

11

Compare to other technologies such as incineration,12 pyrolysis,13,

14

and

52

It has been recently reported that the accumulated Zn in S. plumbizincicola could be

53

transferred to a liquid phase after HTL at supercritical temperatures (~370 °C).10 In that case,

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metals in liquid phase can be effectively recycled via base and electrochemical precipitation or

55

coagulation-flocculation.15 Meanwhile, a series of small molecule chemicals such as acids,

56

furans and ketones can be obtained by membrane filtration and column chromatography.

57

However, to obtain supercritical condition of high temperature (> 300 °C) and pressure (> 10


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MPa), the process requires high energy consumption.16, 17 This drawback can be circumvented by

59

using subcritical HTL with much lower energy demand.18 However, little is known with regard to

60

heavy metal release and transformation mechanisms. Meanwhile, the potential risks of heavy

61

metals in hydrochar also need to be evaluated.

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Recently, metal ions such as nickle (Ni2+), lead (Pb2+), and zinc (Zn2+) were reported to have

63

catalytic effects on the transformation of bio-oil components.19-22 For example: (1) Metal ions

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can increase bio-oil yield due to the promotion of a hydrogenation reaction.19 Moreover, they can

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change the boiling point distribution of bio-oils via the formation of nitrogen and oxygen

66

containing compounds.19 (2) Metal ions can improve the hydrolysis of cellulose to glucose and

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the decomposition of glucose to promote the production of organic acids.20 For example, Wang et

68

al. found that Pb2+ catalyzed the selective cleavage of C3-C4 bond of fructose to produce lactic

69

acid, which obtained a yield of 70% at 190 °C.21 (3) Metal ions, including Zn2+, can catalyze the

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oxidative degradation of 5-(hydroxymethyl)-furfural (HMF) and furfural (FF) to acetic acid.22

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Therefore, it is important to investigate the effect of released metal ions on the bio-oil products

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of a hyperaccumulator HTL reaction.

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Bio-oil compositions are extremely complicated with molecular weights ranging from 30 to

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2000 Da.23 Although gas chromatography-mass spectrometry (GC-MS) is widely used to

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examine bio-oil products derived from biomass HTL reaction,24, 25 the molecular composition of

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bio-oils is not yet fully understood. The Fourier transform ion cyclotron resonance mass

77

spectrometry (FT-ICR MS) coupled with negative electrospray ionization (ESI) was recently

78

employed to reveal the compositions of bio-oils at the molecular level.26, 27 This new technology



79

offer a great opportunity to illustrate the molecular mechanism of released metals on the

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transformation of bio-oil during an HTL reaction.

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In this study, we used S. plumbizincicola as a model Zn hyperaccumulator to investigate the

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feasibility of recycling heavy metal accumulated biomass into small molecule chemicals and

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hydrochar using subcritical HTL reaction. The effects of HTL temperature on Zn release and

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chemical compositions of bio-oil were investigated. In addition, the release risks of Zn in

85

hydrochar were evaluated using the modified European Community Bureau of Reference (BCR)

86

sequential extraction method and Toxicity Characteristic Leaching Procedure (TCLP). To

87

increase the hydrochar porosity for utilization, hydrochar with a low Zn content (~ 200 mg/kg)

88

was further activated to a porous carbon (PC) for carbon dioxide (CO2) capture. The effect of

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HTL temperature on the porosity of as-prepared PCs was also considered. More importantly, the

90

catalytic effects of released Zn2+ on HTL products were studied, and ESI FT-ICR MS analysis

91

was used to provide a comprehensive understanding of the chemical composition of bio-oil

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products. Mechanisms for production of acetic acid by Zn2+ catalysis were also proposed.

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EXPERIMENTAL SECTION

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Chemicals and Materials

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S. plumbizincicola was collected from a Zn contaminated site in Zhejiang, China. After the S.

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plumbizincicola was washed to remove impurities, it was dried at 80 °C and crushed into

97

particles with granularity less than 0.25 mm for the HTL reaction. The total concentration of Zn

98

in the S. plumbizincicola was 1558 mg/kg. Concentration of Mg, Ca was 1247 and 39975 mg/kg,


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and concentration of other major metal contaminants including Cu, Pb and Cd was 59.5, 5.2 and

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2.9 mg/kg, respectively. Zinc chloride and extraction reagents (acetic acid, hydroxylammonium

101

chloride, hydrogen peroxide, and ammonium acetate) used for the BCR sequential extraction

102

method, were of analytical grade and purchased from Aladdin Reagent Corporation (Shanghai).

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Standard

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2-methoxy-phenol, 4-amino-phenol, hydroquinone, cyclopentanone, 2-cyclopentenone and

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3-methyl-2-cyclopentenone, FF, 5-methyl-furfural, and HMF, were purchased from the same

106

company.

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S. plumbizincicola biomass HTL Reaction, Separation and Extraction

GC

grade

compounds

for

the

GC-MS

quantification,

including

phenol,

108

A mixture of 15 grams of S. plumbizincicola and 150 mL of diluted hydrogen chloride (HCl)

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solution was loaded into a 250 mL autoclave for subcritical HTL. The autoclave was sealed and

110

heated to the desired temperature (190-310 °C) for 2 h with a stirring speed of 180 rpm. The

111

temperature was changed from 190 to 310 °C to discern the effects of reaction temperatures on

112

the metal distribution and bio-oil products. The Zn2+ amount (0-0.5 g) was adjusted at 220 °C to

113

examine the catalytic effect of Zn2+ on the S. plumbizincicola HTL reaction. After HTL reaction,

114

the autoclave was cooled to room temperature. The solid and liquid phases were then collected

115

and separated by filtration. The liquid phase was then extracted with an equal volume of ethyl

116

acetate with 1 h shaking at 150 rpm. The ethyl acetate soluble solution was evaporated in a rotary

117

evaporator at 77 °C to obtain the bio-oils. The liquid phase (the phase before extraction of

118

bio-oil), bio-oils and solid phase samples obtained from HTL are denoted as LP-X-Y and BO-X-Y,



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and HC-X-Y, respectively, where X represents the HTL temperature (190-310 °C), and Y is the

120

Zn2+ content (0-0.5 g).

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Characterizations of Liquid Products

122

The functional groups of the bio-oils were analyzed by a Fourier transform infrared

123

spectroscopy (FTIR, Nexus470) over a wavenumber range of 4000 to 400 cm−1. Bio-oils were

124

analyzed using a GC-MS system (Thermo, FOCUS DSQ) with a HP-5MS column (30 mm ×

125

0.25 mm × 0.25 µm). 1 µL of ethyl acetate solution of bio-oils was injected into the GC injector

126

at a temperature of 280 °C. The GC oven temperature was programmed at a rate of 20 °C/min

127

from 60 °C (held for 2 min) to 300 °C (held for 5 min). The carrier gas was helium with a flow

128

rate of 1 mL/min. Main compounds, as determined by their relatively peak areas, were

129

quantitatively analyzed. The yields were calculated based on initial S. plumbizincicola mass

130

(Equation 1). Yields ( x ) =

131

132

mass of x (g) × 100 % mass of biomass (g)

(1)

where x is the compound produced in the HTL reaction of S. plumbizincicola.

133

Acids in the liquid phase were analyzed by a GC (Shimadzu 2010) with a tabilwax-DA

134

column (30 m × 0.32 mm × 0.25 µm). The GC oven temperature was programmed at a rate of

135

15 °C/min from 80 °C (held for 3 min) to 210 °C (held for 2 min). The injected temperature was

136

set at 250 °C, and the injected volume was 1 µL. The carrier gas was nitrogen with a flow rate of

137

40 mL/min.

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The liquid phase was further analyzed by an ESI FT-ICR MS (Bruker Apex ultra) with a 9.4 T


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superconducting magnet. The liquid phase sample was dissolved in methanol to produce a 0.2

140

mg/mL solution. The as-prepared solutions were injected into the electrospray source at an

141

injection rate of 180 µL/h. Conditions for negative ion formation were typically 4.0 kV spray

142

shield voltage, 4.5 kV capillary column introduced voltage, and -320 V capillary column end

143

voltage. Time domain data sets were co-added with 128 acquisitions to improve the

144

signal-to-noise ratio. FT-ICR mass spectra were internally calibrated with deuterium-labeled

145

stearic acid. The m/z values that were greater than six times of the standard deviation of the

146

baseline noise, falling between m/z 100 - 800 with relative abundance, were exported to a

147

spreadsheet.28

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Synthesis of Magnetic Carbon Composites

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Hydrochar with a low Zn content (~200 mg/kg) was further modified to PCs to increase its

150

porosity. Typically, 1.5 g of hydrochar and 2.5 mmol of iron (III) chloride (FeCl3) as the catalyst

151

were mixed with 10 mL of deionized water. The mixtures were then shaken for 12 h and dried in

152

air at 80 °C for 4 h. Samples were activated at a heating rate of 10 °C/min to the desired

153

temperature (600-800 °C) and held for 1 h. The CO2 gas flow was controlled at 0.2 L/min.

154

The as-prepared PCs are denoted as PC-X-Y, where X represents the HTL temperature

155

(190-310 °C) and Y is the additional Zn2+ content. The activation mechanism of CO2 in porosity

156

improvement catalyzed by iron salt can be found in our previous literature.29

157

Characterizations of Hydrochar and PCs

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A modified BCR sequential extraction was used to examine the changes of heavy metal forms



159

within the hydrochar. TCLP method was used to investigate the release risk of heavy metals in

160

hydrochar. Details about BCR sequential extraction procedures and TCLP were described in Text

161

S1 and S2. Hydrochar was also characterized with thermogravimetry (TG) and derivative

162

thermogravimetry (DTG) to examine its thermal stability. Heating conditions were from room

163

temperature to 800 °C under CO2 gas at a heating rate of 20 °C/min. The elemental compositions

164

(C, H, N) of hydrochar were identified with an elemental analyzer (Vario EL III). The ash

165

contents of hydrochar were calculated by heating samples at 600 °C for 2 h in air. The porosity

166

of the PCs was analyzed with N2 adsorption-desorption isotherms at -196 °C and CO2 adsorption

167

isotherms at 0 °C by using a Quantachrome FL33426. The narrow micropore size (< 1.2 nm)

168

distribution was examined using the non-local density functional theory model of CO2 adsorption.

169

The crystal structures of the PC samples were examined using a powder X-ray diffraction (XRD)

170

equipped with Cu Ka radiation (40 kV, 40 mA) between 2θ ranges of 10-90°. Raman spectra

171

were analyzed using a LabRam-1B spectrometer equipped with a 514 nm He-Ne laser.

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RESULTS AND DISCUSSION

173

Effects of HTL Temperature on Zn Release and Hydrochar Activation

174

Figure 1a shows that diluted HCl (0.24 mol/L) increased the release of Zn from the S.

175

plumbizincicola plant into the liquid phase and left small amount of Zn in hydrochar (solid

176

product of S. plumbizincicola HTL). The release of primary plant metals, such as Mg and Ca,

177

during the HTL reaction exhibited similar tendency. Further, it was found that HTL temperature

178

obviously affected the Zn distribution between hydrochar and the liquid phase (Figure 1b). At an


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HTL temperature of 220 - 280 °C, hydrochar retained low concentration of Zn (~10%), partly

180

because the acidity of liquid phase at that temperature promoted the release of Zn. This was

181

evidenced by an analysis of acids in the liquid phase at different temperatures (Table S1). Among

182

the temperatures tested, the highest yield of acetic acid was also at 220 °C (the temperature of

183

lowest concentration of Zn in hydrochar) and no other acids were detected. In that case, Zn in S.

184

plumbizincicola could be largely released via HTL process. Similarly, Mg and Ca were most

185

easily released from S. plumbizincicola at an HTL temperature of 220 °C (Figure S1).

186

The modified BCR sequential extraction was used to assess the metal risk of hydrochar for its

187

safe utilization.30 Figure 1c shows that the acid exchangeable and easily reducible fraction

188

percent of Zn in hydrochar decreased along with HTL temperature increase. Accordingly, the

189

oxidizable and residual fraction of Zn in hydrochar was increased with an increase in HTL

190

temperature. This can be explained that interactions between organic compositions of hydrochar

191

with Zn were promoted with elevated HTL temperatures, producing a more stable fraction.31 Mg

192

and Ca both yielded similar metal fraction results (Figure S2).

193

Toxicity Characteristic Leaching Procedure (TCLP) is often used to determine whether the

194

treated waste is hazardous of metal leaching risk by the United States Environmental Protection

195

Agency. TCLP-extractable Pb concentration of biomass and hydrochar derived from 190 -

196

310 °C is 0.25 and 0.08, 0.13, 0.21, 0.18 and 0.30 mg/L, respectively. And TCLP-extractable Cd

197

concentration is 0.24 and 0.23, 0.21, 0.11, 0.29 and 0.09 mg/L, respectively. As a result, the

198

release risks of Pb and Cd in biomass and hydrochar were very low because the

199

TCLP-extractable Pb and Cd concentration is lower than the critical level in hazardous waste (5



200

mg/L for Pb and 0.5 mg/L for Cd).

201

Compared to the critical level of Zn in hazardous waste (25 mg/L), biomass had great Zn

202

leaching risk, while produced hydrochar is relatively safe for Zn leaching risk (Figure 1d).

203

Further, all HTL derived hydrochar had much lower Zn TCLP leaching concentration than

204

biomass (agreed well with the decreased acid exchangeable fraction and easily reducible fraction

205

of hydrochar to biomass), indicating that HTL is an effective way to reduce the metal leaching

206

risk of hyperaccumulator biomass. It can be explained that Zn tend to transform into a more

207

stable fraction through complexation reactions.34

208

Hydrochar with a low Zn content (~200 mg/kg) and low release risk of Zn made it feasible to

209

fabricate PCs from hydrochar for reuse. The basic physicochemical properties of hydrochar are

210

shown in Table S2. An element analysis of hydrochar indicates that its aromaticity increased with

211

an increase in HTL temperature. Therefore, different properties of hydrochar influenced CO2

212

activation reaction (C + CO2 → 2CO) and the resulted product porosity (Figure 2a). The

213

activation temperature for optimal porosity is similar for most hydrochar (~770 °C), however, it

214

was greatly reduced in the HC-190-0 sample. TG and DTG results confirmed that more unstable

215

components of hydrochar were lost at low temperatures (< 400 °C) for the HC-190-0 sample

216

(Figure 2b, c). Therefore, a lower temperature (650 °C) was required for the activation of the

217

HC-190-0 sample. In addition, the optimal porosity from the activation of HC-190-0 sample (426

218

m2/g) was also lower than that of the other four samples (860 - 930 m2/g), partly because the

219

residual hydrochar components were inert to a CO2 reaction at the examined activation

220

temperature. As a result, the properties of PCs were strictly controlled by the thermal


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221


recalcitrance of their hydrochar precursors.35

222

The CO2 adsorption, XRD, and Raman analysis of PC samples with peak porosity from the

223

surface area curves of different hydrochar are compared in Figure 2. As shown in Figure 2d, PCs

224

showed four peaks at around 0.47, 0.52, 0.58 and 0.82 nm of the narrow micropore size

225

distribution. This contributes to PC samples with good CO2 adsorption capacity (1.5 - 3 mmol/g),

226

indicating that hydrochar is a promising precursor for fabrication of CO2 adsorption material. It

227

should be pointed out that higher HTL temperature based hydrochar (high thermal recalcitrance)

228

produced a PC with a more developed micropore structure, resulting in stronger CO2 adsorption

229

H2O C capacity. XRD results show that Fe3O4 (JCPDS 65-3107, Fe 3+  → Fe 2 O 3  → Fe 3O 4 ) was

230

the main crystal for different HTL temperature hydrochar-derived PCs (Figure S3). Higher HTL

231

temperature based hydrochar produced a smaller crystal size of Fe3O4 (concluded from the

232

Scherrer’s equation). Raman spectra indicated that PCs derived from higher HTL temperature

233

hydrochar had a higher graphitization degree (decreased ID/IG value from 3.35 of PC-190-0 to

234

2.84 of PC-310-0), partly due to the requirement of a higher activation temperature (> 220 °C)

235

(Figure S4). Therefore, the micropore structure and graphitization degree of PCs and the crystal

236

sizes of iron oxides were controlled by thermal recalcitrance of their hydrochar precursors.35

237

Effects of HTL Temperature on Bio-oil Properties

238

Element analysis indicates that the contents of C and O in bio-oil were greatly affected by the

239

HTL temperature (Table S3). Acids, phenolic compounds, ketones and furans were identified as

240

primary components of the bio-oils by GC-MS analysis (Figure S5 and Table S4). These



241

compounds are likely originated from dehydration, decarboxylation, deoxygenation and

242

aromatization reactions during the S. plumbizincicola HTL process.36

243

As shown in Figure 3a, furans (mainly FF and HMF) dramatically decreased with increasing

244

HTL temperature, whereas acetic acid shows the opposite trend. The acetic acid production can

245

be explained by a two-step reaction process involving hydrogenolysis of carbohydrates and a

246

subsequent oxidation reaction. The first step converts carbohydrates to FF and HMF.

247

Carbohydrates are primarily derived from hydrolysis of cellulose, as evidenced by the

248

disappearance of C=O vibration in carbohydrates at 1668 cm-1 in FTIR spectra when the HTL

249

temperature was above 190 °C (Figure S6).37 The second step further converts the produced FF

250

and HMF to acetic acid by oxidation.38 It also should be pointed out that small amount of ketones

251

and phenols were produced at HTL temperatures of 280 °C and 310 °C (Figure 3a), implying that

252

the decomposition of cellulose and hydrolysis of ether bonds from lignin can be increased at

253

higher temperature (> 250 °C).39

254

To further distinguish the chemical compositions, FT-ICR MS was used to analyze the

255

compositions of the liquid phase.27, 40 Samples of LP-220-0 and LP-280-0 were selected for

256

FT-ICR MS analysis, due to their different chemical compositions of liquid phase derived from

257

different reaction stages in the previous analysis (Figure 3a). As shown in Figure S7, more than

258

3500 peaks were detected in LP-220-0, while that number decreased to about 2000 peaks in

259

LP-280-0. This should be due to the degradation reaction at high HTL temperatures of 280°C.

260

Furthermore, the main mass distribution of liquid phases changed from m/z 350 - 400 of

261

LP-220-0 to m/z 300 - 350 of LP-280-0 (Figure S8). It indicated that large molecules may


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262

undergo depolymerization, dehydration and deoxygenation reactions to form lower molecular

263

weight compounds (m/z 300 - 350) at higher HTL temperatures of 280 °C.41 This finding agrees

264

well with the generation pathways of acidic, phenolic and furan based compounds in GC-MS

265

analysis. Detailed molecular formulas for the compounds detected by FT-ICR MS in LP-220-0

266

and LP-280-0 are shown in Tables S5 and S6, respectively. More compounds with relatively

267

small molecules were observed, further verifying that HTL temperature affects the HTL reaction

268

of S. plumbizincicola.

269

Mass scale-expanded segments (m/z 283) showed the mass difference of 36.4 mDa for the

270

replacement of O with CH4 (Figure S9). C18H19O3- in LP-280-0 was derived from the compounds

271

with higher oxygen numbers (> 3), indicating that a deoxygenation reaction occurred at a higher

272

HTL temperature of 280 °C. Specifically, the double bond equivalents (DBE) of C16H11O5-,

273

C17H15O4- and C18H19O3- gradually decreased from 11 to 9, indicating the deoxygenation process

274

of the carbonyl group (C=O) of compounds. It also should be noted that the content of stearic

275

acid (C18H35O2- with DBE value of one) dramatically decreased at an HTL temperature of 280 °C,

276

indicating the possible decomposition of long chain acids at higher HTL temperatures of 280 °C.

277

The results also agreed with the increased acid production at 220 °C in bio-oils analyzed by

278

GC-MS (Table S4).

279

Figure 4 shows the plots of carbon number versus DBE and the Van Krevelen diagram for

280

LP-220-0 and LP-280-0. The DBE of compounds derived from a high HTL temperature (280 °C)

281

exhibited higher relative abundance in the low carbon number (C10 - C17), indicating the progress

282

of a depolymerization reaction. C18H33O2- with a DBE value of two (labeled as Compound 1 in



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283

Figure 4a) was likely octadecenic acid.26 The main compound of the LP-280-0 sample was

284

possibly hexadecanoic acid (C16H31O2- with DBE value of one, labeled as Compound 2 in Figure

285

4b).42 These results indicate the possible transformation of octadecenic acid into hexadecanoic

286

acid, further suggesting a depolymerization reaction at higher HTL temperatures of 280 °C.

287

The relative abundance of Ox, N1Ox, and N2Ox class species were normalized and summarized

288

in Figure S10. Compounds derived at a low HTL temperature (220 °C) were mainly comprised

289

O1-O13 species, with O8 as the most abundant O class (Figure S10a). However, compounds

290

derived at a high HTL temperature (280 °C) demonstrated narrower Ox species of O2-O9 with O6

291

and O7 as the most abundant O classes. These results verify the hypothesis that deoxygenation

292

reaction occurred during the HTL process. Similar results of N1Ox and N2Ox class species in

293

LP-220-0 and LP-280-0 samples further confirmed the progress of the deoxygenation reaction at

294

increased HTL temperatures (Figure S10b, c).

295

To further characterize the aromatic compounds and the degree of aromatization in the liquid

296

phase during HTL, a modified aromaticity index (AImod) was introduced and calculated based on

297

elemental compositions of compounds (Equation 2).

298

AI mod =

1 + C − 0.5O − S − 0.5H C − 0.5O − S − N − P

(2)

299

where C, O, S, H, N, and P refer to the carbon, oxygen, sulfur, hydrogen, nitrogen and

300

phosphorous numbers, respectively. Compounds were considered aliphatic when AImod ≤ 0.5;

301

aromatic when 0.5< AImod < 0.65; and condensed aromatic structures when AImod ≥ 0.65.43 In that

302

case, Figure S11 shows the classifications of compounds in the liquid phase. Notably, a high

303

HTL temperature (280 °C) slightly increased the relative ratio of aromatic and condensed


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304


aromatic compounds, indicating the progress of aromatization reaction during HTL.

305

A Van Krevelen diagram indicates plots of elemental H/C ratios against O/C ratios, which

306

facilitates the identification of reaction types during HTL.44 Figures 4c and 4d show the Van

307

Krevelen diagram divided into four regions based on the different H/C and O/C ratios described

308

elsewhere.44 The size of the spot in the diagram represents the relative abundance of each mass

309

peak.45 The abundance of compounds with structures similar to cellulose and hemicellulose was

310

larger at an HTL temperature of 220 °C, indicating the decomposition of cellulose and

311

hemicellulose at this temperature. This was further supported by the production of acids analyzed

312

by GC-MS (Figure 3a and Table S4). More lignin-derived compounds were present at an HTL

313

temperature of 280 °C, suggesting increased lignin depolymerization at higher HTL temperatures

314

of 280 °C.46

315

In summary, the FT-ICR MS analysis results support the findings that deoxygenation,

316

aromatization and depolymerization reactions occurred during the HTL reaction of S.

317

plumbizincicola.

318

Effect of Released Zn2+ on Bio-oil Products Properties

319

To simulate the role of released Zn2+ on the properties of the bio-oil products, different Zn2+

320

amounts were added during the HTL reaction of S. plumbizincicola at 220 °C. Figure S12 shows

321

that the yield of bio-oil gradually increased as Zn2+ increased, due to promotion of hydrogenation

322

by the catalysis of Zn2+.47

323

The components of bio-oils were further studied to learn more about the Zn2+ catalytic effect.



324

Figure 3b indicates that Zn2+ can catalyze the oxidation of FF and HMF into acetic acid.22 The

325

concentrations of phenols and ketones experienced less change, implying that the

326

transformations of cellulose to ketones and lignin to phenols were independent of Zn2+ content

327

(Table S7).

328

As a result, reaction pathways for acetic acid production by Zn2+ catalysis were proposed in

329

Figure S13. Cellulose can be hydrolyzed to glucose by the coordination of Zn2+ with the

330

hydroxyl group of cellulose.48 Then, glucose may undergo two pathways to produce acetic acid:

331

the first is through isomerization from glucose into fructose via the Lobry de Bruyn-Alberda-van

332

Ekenstein transformation catalyzed by Zn2+, followed by HMF production through a dehydration

333

reaction.48 Acetic acid can then be formed by the oxidation of HMF.20 The second step to produce

334

acetic acid is through a ring-opening reaction, followed by a dehydration process to produce FF,

335

which can then be oxidized to produce acetic acid catalyzed by Zn2+.

336

Samples of LP-220-0, LP-220-0.05, and LP-220-0.25 were selected for the analysis of FT-ICR

337

MS due to their large differences in the production of acetic acid and furans (Figure S14). As

338

shown in Figures 5a and S16, molecular weight of compounds in the liquid phase decreased by

339

~50 when 0.25 g of Zn2+ was added, partly due to catalytic effect of Zn2+ on the hydrolysis of

340

cellulose and the decomposition of carbohydrate.48 A Van Krevelen diagram revealed a slight

341

decrease in the abundance of compounds with structures similar to cellulose (Figure S15). DBE

342

values of compounds in liquid phase further indicated that compounds with lower carbon number

343

(C5 - C10) were produced (Figure S16). These results well verified the catalytic effect of Zn2+ on

344

the hydrolysis of cellulose to produce compounds with low molecular weights. Compounds with


Page 18 of 34

Page 19 of 34


345

DBE values of one and two O in their molecules are likely fatty acids, such as C14H27O2-,

346

C15H29O2-, C17H33O2- and C18H35O2- (Table S8). The relative abundance of these fatty acids

347

decreased as Zn2+ content increased, indicating a catalytic effect of Zn2+ in producing long chain

348

acids.

349

Mass scale-expanded segments of m/z 255 were shown in Figure 6. The C16H31O2- with DBE

350

of 1 was probably palmitic acid.49 The relative abundance of C16H31O2- increased as Zn2+

351

increased, which supports the conclusion that there is a higher production of acids at high Zn2+

352

content. Besides, other compounds with DBEs greater than four (the DBE of benzene is four)

353

were likely aromatic compounds. The contents of these compounds increased as Zn2+ content

354

increased, indicating that Zn2+ may also catalyze the decomposition of lignin to produce aromatic

355

compounds. A Van Krevelen diagram also showed a slight decrease in the compounds with

356

structures similar to lignin (Figure S15). The relative abundance of Ox, N1Ox and N2Ox class

357

species was normalized and summarized in Figures 5b and S19. Notably, a greater number of

358

compounds with one lower oxygen were produced after Zn2+ was added (Figure 5b), providing

359

evidence that a deoxygenation reaction catalyzed by Zn2+ occurred during the HTL process.

360

In summary, the FT-ICR MS analysis results indicate that Zn2+ has catalytic effects on the

361

deoxygenation reaction and the production of compounds with low molecular weight and long

362

chain acids during the HTL reaction of S. plumbizincicola.

363

Environmental Implications

364

This work demonstrates that subcritical HTL can be an alternative option to recycling



Page 20 of 34

365

hyperaccumulator biomass into carbon materials and a series of chemicals (bio-oil, carboxylic

366

acids, ketones and furans) and therefore potentially address the disposal problem of

367

hyperaccumulators associated with phytoremediation.

368

More than 90% of the Zn in S. plumbizincicola can be released into aqueous solutions at

369

220 °C. The released Zn2+ can effectively catalyze the reaction and increase the yield of bio-oil

370

and acetic acid. In the medium and long terms, there is potentially a great demand for acetic acid,

371

because acetic acid is extensively used as an important raw material for organic synthesis and as

372

an industrial solvent.20 Zn and organic acids in aqueous solution can be collected via reported

373

techniques such as chemical precipitations and membrane filtration.15

374

value-added products and energy makes subcritical HTL more beneficial than traditional

375

pyrolysis or supercritical HTL process, also further improving economic viability of

376

phytoremediation.13 The resultant hydrochar with low Zn content (~ 200 mg/kg) can further be

377

activated to produce PC with excellent porosity (930 m2/g), a promising adsorbent for CO2

378

capture.

379

ACKNOWLEDGEMENTS

16, 17

The recovery of

380

This research was supported by the National Natural Science Foundation of China (No.

381

21407027, 21577025), the National Key Technology Support Program (No. 2015BAD15B06),

382

and the International Postdoctoral Exchange Fellowship Program of China supported by Fudan

383

University.

384

Supporting Information


Page 21 of 34


385

Procedures of BCR sequential extraction and TCLP are shown in Text S1 and S2. Acetic acid

386

concentration is presented in Table S1. Basic physicochemical properties of hydrochar and

387

bio-oils are shown in Tables S2 and S3. Compounds of bio-oils detected by GC-MS are shown in

388

Tables S4 and S7. Molecular formulas of compounds in the liquid phase are listed in Tables S5,

389

S6 and S8.

390

Distribution of metals is shown in Figures S1 to S2. XRD and Raman spectra of PCs are

391

presented in Figures S3 and S4. Total ion chromatogram and FTIR spectra of bio-oils are shown

392

in Figures S5 and S6. FT-ICR analyses are presented in Figures S7 to S11 and S14 to S17. The

393

effect of Zn2+ content on the yields of bio-oils is shown in Figure S12. Proposed mechanisms are

394

presented in Figure S13.

395

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



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Figure captions: Figure 1. (a) Distribution of metals (Zn, Mg, Ca) in hydrochar and liquid phase with and without hydrochloric acid (HCl, 0.24 mol/L) addition via HTL of S. plumbizincicola at 250 °C, (b) distribution of Zn between hydrochar and liquid phase through HTL conversion of S. plumbizincicola at various temperatures (190 - 310 °C), (c) percentage of different Zn fraction in S. plumbizincicola and hydrochar at various temperatures (190 - 310 °C) using modified BCR method (F1: acid exchangeable fraction; F2: easily reducible fraction; F3: oxidizable fraction; F4: residual fraction), (d) TCLP concentrations of biomass and hydrochar produced at various temperatures (190 - 310 °C). Experiments in (a), (b) and (d) were performed in duplicate. Figure 2. (a) BET surface area variations of hydrochar-derived porous carbon (PC) at different activation temperature, (b) TG thermograms of hydrochar samples produced under different HTL temperature, (c) DTG thermograms of hydrochar samples produced under different HTL temperature, (d) narrow micropore size distribution of PC samples with peak porosity from the surface area curves of different hydrochar and (e) adsorption isotherms of CO2 (at 0 °C) of PC samples with peak porosity from the surface area curves of different hydrochar (inset image of d). Figure 3. (a) Yields of main compounds (acetic acid, furans, ketones and phenols) in bio-oils produced under different temperature HTL process (typical compounds in bio-oils were quantified based on standard compounds: furans contain furfural, 5-methyl-furfural and 5-(hydroxymethyl)-furfural; ketones include cyclopentanone, 2-cyclopentenone and



3-methyl-2-cyclopentenone; phenols contain phenol, 2-methoxy-phenol, 4-amino-phenol and hydroquinone), (b) Effects of Zn2+ content on the yields of main components in bio-oils based on original biomass mass at 220 °C hydrothermal temperature. Analysis of yields of main components were performed in duplicate. Figure 4. (a) Plots of carbon number versus DBE for LP-220-0, (b) plots of carbon number versus DBE for LP-280-0, (c) Van Krevelen diagram of compounds in LP-220-0 and (d) Van Krevelen diagram of compounds in LP-280-0 (plot sizes represent relative abundance of compounds). Figure 5. (a) Mass distribution of LP-220-0, LP-220-0.05 and LP-220-0.25 by negative-ion ESI FT-ICR MS, (b) relative abundance of Ox class species of LP-220-0, LP-220-0.05 and LP-220-0.25. Figure 6. Mass scale-expanded segments (m/z 255) of FT-ICR MS for LP-220-0, LP-220-0.05 and LP-220-0.25 samples (numbers in the brackets are the DBE values of relevant compounds).


Page 28 of 34

Page 29 of 34


a

Mg

b

Liquid Ca

80 60 40

60

HCl water

HCl water

60 40

20

F1

190

60

biomass HC-190-0 HC-220-0 HC-250-0 HC-280-0 HC-310-0

40

0

80

0

HCl water

Zn concentration (mg/L)

Percentage of different fraction (%)

80 c

Liquid

20

20 0

Hydrochar

100 Distribution (%)

Distributions (%)

100

Hydrochar Zn

F2

F3

F4

220 250 280 Temperature (oC)

310

d

45

30

15

0 biomass 190

Figure 1


280 220 250 o Temperature ( C)

310


a HC-190-0 HC-220-0 HC-250-0 HC-280-0 HC-310-0

800 600

HC-280-0

770 C

HC-250-0

60

HC-220-0

HC-190-0

40

200 0

20

650

700 750 o Temperature ( C)

HC-250-0 HC-310-0

0.0

200

800

600

800

Temperature ( C) c

dV(D) (cm3/nm/g)

1.5

HC-220-0

400 o

HC-280-0

-0.4

1.0

CO 2 adsorbed (mmol/g)

600

Deriv. Weight (%/oC)

HC-310-0

80

400

-0.2

b

100

o

Weight (%)

BET surface area (m2/g)

1000

Page 30 of 34

d

e

3

PC-190-0 PC-220-0 PC-250-0 PC-280-0 PC-310-0

2 1 0 0.0 0.2 0.4 0.6 0.8 1.0 Pressure (bar)

0.5

HC-190-0 0.0

-0.6 200

400 600 o Temperature ( C)

800

0.4

0.6

0.8

1.0

Pore diameter (nm)

Figure 2


1.2

Page 31 of 34


5 a

3 acetic acid

b acetic acid

3

furans ketones

0.2 0.1

phenols

0.0

Yields (%)

Yields (%)

2 1

4 2 1.2 0.8 0.4

furans

0.02

ketones

0.01

phenols

0.00 190

220

250

280

0.0

310

o

0.1

0.2

0.3

Zn2+ (g)

Temperatures ( C)

Figure 3


0.4

0.5


20

20

b LP-280-0

a LP-220-0

Depolymerization

15 DBE

DBE

15

Page 32 of 34

10

5

10

5

Compound 1

Compound 2

0

0 5

10

15

20

25

30

5

10

15

Carbon number Lipids

c LP-220-0

Cellulose/hemicellulose

1.5 Lignins

2.0

1.0

0.5 0.0

25

30

Carbon number

H/C

H/C

2.0

20

Lipids

Cellulose/hemicellulose

d LP-280-0

1.5 Lignins

1.0

Condensed aromatics

0.2

0.4

0.6

0.8

0.5 0.0

O/C

Condensed aromatics

0.2

0.4

O/C Figure 4


0.6

0.8

Page 33 of 34


6 b Relative abundance (%)

LP-220-0.25 LP-220-0.05

20

LP-220-0 10

LP-220-0 4

2 LP-220-0.05 LP-220-0.25 0 O

1 O

2 O 3 O 4 O 5 O 6 O 7 O 8 O 9 O 10 O 11 O 12 O 13

0 060

-5

50

55

50

0 50 0

45 0-

0

0 40

045 40

0

35 0-

035

00

30

-3

50 25 0

-2 20 0

-2

00

0 15 0

Relative abundance (%)

30 a

Ox class species

m/z distribution 0 20 015

Figure 5



C16H15O3255.10462(9)

3

C16H31O2255.23301(1)

-

2

Intensity *10

8

1

8

10

0

C14H11N2O3 255.06631(10) C11H11O7255.05110(6)

Page 34 of 34

LP-220-0

C12H15O6255.08748(5) C13H19O5255.12387(4)

LP-220-0.05

3 2

x

1 0

LP-220-0.25

Intens.

4 3 2 1 0

255.00

255.05

255.10

255.15

255.20

m/z Figure 6

33 ACS Paragon Plus Environment

255.25

255.30

Influences of Temperature and Metal on Subcritical Hydrothermal

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