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Pyrolytic Temperature Dependent and Ash Catalyzed Formation of Sludge Char with Ultra-High Adsorption to 1-Naphthol Xiao Qing Liu, Hong-Sheng Ding, Yuan-Ying Wang, Wu-Jun Liu, and Hong Jiang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b04536 • Publication Date (Web): 05 Feb 2016 Downloaded from http://pubs.acs.org on February 6, 2016

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Environmental Science & Technology

Pyrolytic Temperature Dependent and Ash Catalyzed Formation of Sludge Char with Ultra-High Adsorption to 1-Naphthol

Xiao-Qing Liu, Hong-Sheng Ding, Yuan-Ying Wang, Wu-Jun Liu, Hong Jiang*

CAS Key Laboratory of Urban Pollutant Conversion, Department of Chemistry, University of Science and Technology of China, Hefei 230026, China

* Corresponding author: Dr. Hong Jiang Fax: 86-551-63607482; E-mail: [email protected]

ACS Paragon Plus Environment


TOC Art

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ABSTRACT

3

Massively produced sewage sludge brings a serious problem to environment.

4

Pyrolysis is a promising and bifunctional technology to dispose the sewage sludge and

5

recover energy, in which a large amount of pyrolytic sludge char is also produced. In

6

this study, we proposed a value-added utilization of sludge char. We prepared an

7

adsorbent with ultra-high capacity for hydrophobic organic pollutant (1-naphthol) by

8

pyrolysis of sludge and removal of the ash moiety from the sludge char. The

9

adsorptive behavior of the adsorbent is strongly dependent on the pyrolytic

10

temperature of sludge, and the maximum adsorption capacity of 666 mg g–1 was

11

achieved at 800 °C, which is comparable to deliberately modified graphene. Further

12

exploration indicated that the robust adsorption to 1-naphthol is attributed to the

13

catalytic effect of ash in sludge which facilitated the formation of more orderly

14

graphitic structures and aromaticity at high pyrolytic temperatures.

15



16

INTRODUCTION

17

The activated sludge method is a globally employed technology for sewage treatment

18

because of its low cost and easy operation.1 However, the disposal of large amounts of

19

excess sludge from wastewater treatment plants is a serious problem. Landfilling of

20

sludge, which was previously used extensively for sludge disposal, is banned by many

21

countries because of the decrease in available land area and possible secondary

22

pollution from leachate.2, 3 Thus, pyrolysis has emerged as a promising method for

23

sludge disposal and energy recovery.4-6

24

Pyrolysis of sludge produces minimal poisonous gases, such as dioxin, because

25

sludge is processed in anoxic atmosphere and at a relatively low temperature

26

(400–600 °C) during pyrolysis.2 The main products of sludge pyrolysis are bio-oil and

27

pyrolytic sludge char. The former can be used as liquid fuel,7 whereas the latter

28

mainly contains ash, C, and oxygen-containing functional groups and can thus be used

29

as soil conditioner to increase soil fertility and remediate soil pollution.8-11

30

Surface water, groundwater, and soil contamination by synthetic aromatic

31

compounds is one of the most intractable environmental problems that pose a threat to

32

both human beings and wildlife. 1-naphthol, which is extensively used as an industrial

33

material, is of special concern because of its acute toxicity and negative effects on the

34

environment. The treatment of high concentrations of aromatic wastewater and the

35

remediation of environmental contamination have attracted considerable attention.

36

Biological methods are usually inefficient because of the poor bioavailability of

37

1-naphthol. Chemical and physical treatment processes, such as ozonation, Fenton

38

oxidation, and membrane technology, have been developed for the decomposition or

39

separation of 1-naphthol. However, these processes are costly. The adsorption of

40

1-naphthol from a contaminated environment with commercial adsorbents, such as


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activated carbon and carbon nanotubes, is an alternative option.12, 13

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Removal of 1-naphthol by pyrolytic sludge char is a cost-effective and

43

difunctional approach when compared with adsorption by commercial adsorbents.On

44

one hand, the pyrolytic sludge char obtained from sludge pyrolysis is cheap and

45

readily available, and must be disposed. On the other hand, pyrolytic char is an

46

efficient soil conditioner and adsorbent for organic pollutants. The adsorption

47

behavior of biochars derived from biomass wastes at different temperatures has been

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well reported,14-16 whereas only a few reports focused on the adsorptive behavior of

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sludge char.10,

50

significant differences in composition and chemical properties. For instance,

51

functional groups on the surface of char may be affected by the high nitrogen and

52

phosphorus contents of sludge (35-85%).5, 17-19 The high mineral content in sludge

53

char can participate in the adsorption of organic compounds, just like mesoporous

54

organosilica.20,

55

interactions with organic contents.22

11

21

The adsorption properties of sludge chars vary because of the

Also, the ash can influence the pyrolysis process, through

56

Currently, the adsorption capacities of most carbonaceous adsorbents including

57

carbon nanatubes to 1-naphthol are lower than 100 mg g–1 except those of graphene

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based materials (Table S1). It is particularly promising if an adsorbent with high

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adsorption capacity and low cost can be obtained from sludge char. Zou et al. deashed

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and activated the sludge char, and obtained purified sludge char with largest surface

61

area of 346 m2 g–1 at 600 oC and adsorption capacity of 96 mg g–1 for phenol.10 Gu et

62

al. copyrolyzed sludge and reed straw to increase the adsorption capacity of sludge

63

char.11 The adsorption capacity of sludge char are thus comparable to that of

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commercial activated carbon by these efforts. Nevertheless, it is still low and needed

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to be enhanced.



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Herein, we prepared an adsorbent with high adsorption capacities(666 mg g–1)

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towards 1-naphthol by pyrolysis of the sewage sludge. The main objective of this

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study is try to reveal the underpinned relation between pyrolytic temperature as well

69

as sludge components and adsorption performance of obtained material, which is

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particularly important to the practical application of sludge pyrolysis technology. To

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this end, we 1) determined the adsorption capacities of sludge chars of original sludge,

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deashed sludge, and deashed char obtained at different temperatures (300-800 oC); 2)

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investigated the change of carbon structure of different chars by Raman, XPS, and

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nitrogen adsorption-desorption isotherms; and 3) analyzed the effect of ash by SEM

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and energy dispersive spectroscopy. On the basis of the gathered data, we inferred that

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the adsorption performance of sludge char is attributed to the temperature-dependent

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properties of chars and ash catalytic interaction.

78 79

EXPERIMENTAL SECTION

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Materials. All reagents used in this work were of analytical grade. Sludge was

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gathered from a sewage treatment plant of Hefei, southeast of China. The sludge was

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first air dried outside for several days, and then heated at 80 °C (for retaining as much

83

as organic compounds) in the oven for 20 h to remove the moisture. Finally, the dried

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sludge was sieved after smashing through a 120 mesh sieve and stored in a dryer for

85

further use.

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Characterization Methods of Sludge and Sludge Chars. Thermogravimetric

87

analysis (TGA) of sewage sludge was conducted using a TGA instrument (SDT Q600,

88

TA, USA) under a nitrogen atmosphere, followed by heating from room temperature

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to 900 °C at a rate of 10 °C min–1. Derivative thermogravimetric (DTG) curves were

90

analyzed on the basis of derivative weight loss. Elemental (C, H, N) analyses were


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conducted using an elemental analyzer (Elementar Analysensysteme GmbH,

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Germany). Oxygen content was determined by a mass balance. H/C and (O +N)/C

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atomic ratios were calculated to evaluate the aromaticity and polarity of the pyrolytic

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sludge char, respectively. The samples were firstly degassed at 300 °C using a vacuum

95

for 300 minutes. Then the specific surface areas (SSAs) were measured, with N2

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adsorption determined by a Tristar II 3020M surface area analyzer. Fourier transform

97

infrared (FTIR) spectroscopy was recorded in the 4000–400 cm–1 region by a Nicolet

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FTIR Spectrophotometer (model 6700). X-ray photoelectron spectroscopy (XPS)

99

analysis was performed on an ESCALAB 250 xi system equipped with an Al Kα

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X-ray source operating in CAE mode. Raman experiments were conducted with a

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LABRAM-HR spectrometer using the green line of an argon laser (λ=514.5 nm) as

102

the excitation source. The morphologies of the samples were observed by scanning

103

electron microscopy (SEM, Sirion 200, FEI electron optics Co., USA). SEM-EDS

104

images of the samples were obtained by energy dispersive spectroscopy (EDS, INCA

105

energy, UK). The ash composition of sewage sludge is determined by X-ray

106

fluorescence spectrometry (XRF, XRF-1800, SHIMADZU Co., Japan).

107

Preparation of Sludge Char. TGA of the sludge was conducted to determine

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the suitable pyrolytic temperature (Fig. S1) before preparing the sludge chars. Apart

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from a small peak that appeared at 60 °C owing to the volatilization of water,23

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sludge decomposition mainly occurred at 200 to 500 °C. Two strong peaks appeared

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at 276 and 326 °C, and the corresponding pyrolysis rates were 0.22% and 0.21% °C–1,

112

respectively. The two strong peaks were caused by the decomposition of aliphatic

113

acids and saccharides.24 The shoulder between 400 and 500 °C was caused by protein

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decomposition.3, 24 The pyrolysis of the sludge was almost completed with 43.6%

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weight loss when the temperature reached 500 °C. When the temperature continued to



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rise, a small quantity of residual organic matter in the sludge further decomposed and

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aromatized, such that the weight loss only increased 4.0% from 500 to 800 °C. The

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decomposition of inorganic carbonate caused a small peak at 620 °C.25 According to

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the TGA curve, we selected 300, 400, 500, and 800 °C to pyrolyze the sludge and

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prepare sludge chars (SCs), and the samples were named SC300, SC400, SC500, and

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SC800, respectively. 300 and 400 °C corresponded to the maximum weight loss

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temperature range, 500 °C was the temperature near the end point of sludge pyrolysis,

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and 800 °C was the final temperature corresponding to sludge char carbonization.

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The pyrolytic process was conducted in a horizontal furnace equipped with a

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quartz tube. First, 15 g of preprocessed sludge was put into the quartz tube, which was

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then placed in the horizontal furnace. Second, Ar of 99.99% purity was introduced to

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the channel at 100 mL min–1 for at least 1 h to keep the air out. The furnace was then

128

heated to the target temperature at a heating rate of 5 °C min–1. Then it is holding at

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the target temperature for 4 hours. After cooling down, the quartz tube was taken out,

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and sludge char was obtained. To determine the ash content of char, a certain amount

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of sludge char was placed in a crucible and burned in a muffle furnace at 600 °C for 4

132

h. The obtained ash was also used to measure the capacity for 1-naphthol.

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Sorption Experiments. A 50 mL solution containing 100 mg L–1 of 1-naphthol,

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200 mg L–1 of NaN3 (to inhibit the degradation by incidental bacteria), and 0.25 g of

135

SCs were mixed in each 100 mL conical flask during the dynamic adsorption

136

experiment. The flasks were then placed on a rotating shaker and agitated at 180 rpm

137

and 25 °C. At different times, 0.5 mL of the solution was taken and passed through a

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0.45 µm filter membrane to measure the concentration of residual solution after

139

adsorption.

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The adsorption capacities of chars and ash were determined through another


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batch of experiments. The dynamic experiment results indicated that more than 80%

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of 1-naphthol was sorbed onto SCs in 4 d. Therefore, the adsorption time in this

143

experiment was 4 d. Other conditions were the same as those of the dynamic

144

experiment, and all sorption experiments were run in duplicate.

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The

1-naphthol

concentration

in

the

solution

was

determined

by

146

high-performance liquid chromatography equipped with an UV detector using

147

acetonitrile−water (70:30, v/v) as the mobile phase at a flow rate of 1 mL min–1. The

148

UV wavelength used for 1-naphthol detection was 292 nm. The amount of 1-naphthol

149

sorbed by the sorbent was calculated by the difference between the concentration of

150

control solution and the solution with adsorbents.

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RESULTS AND DISCUSSION

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Characterization of Sludge and Chars. The sludge feedstock contained 47.2%

154

of volatiles, 4.9% of fixed carbon, and 2.5% of moisture. The detailed composition of

155

the sludge and chars obtained at different temperatures is listed in Table 1. The

156

decrease in H/C (the ratio of H to C) and (O+N)/C (the ratio of O and N to C) with

157

increasing temperature also indicates the gradual volatilization of organic compounds

158

from sludge.

159

The SSA of sludge chars increases with increasing temperature, reaches the

160

maximum for SC400, and then decreases (Table 1 and Fig.S2). As the temperature

161

increased from 300 to 800 oC, the decomposition of organic fraction in the sludge was

162

significantly enhanced, with the final carbon content of the char decreasing from

163

22.3% to 14.8%. A more condensed aromatic structure forms and some mineral pores

164

collapse as temperature further increases.26, 27 These conditions result in a decrease in

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the SSA of SC500 and SC800, and similar results are also found in other relevant



166

work, in which the wood or crop were pyrolyzed at different temperatures to obtain

167

biochar with various surface areas.

168

observed when the pyrolysis temperature was increased to 400 °C. The SSAs SC300

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was 4.58 m2 g–1. The value was significantly smaller than those of SC400, SC500, and

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SC800 (Table 1), possibly because of the significant increase in the releases of

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volatiles in this temperature range, which is in good agreement with the TGA results.

28, 29

Notably, a significant increase in SSA was

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Ultra-High Adsorption Capacity of Adsorbent. The adsorption capacity of

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sludge chars is affected by numerous factors, including SSA, chemical structure, and

174

composition. Given the SSAs of sludge chars are significantly less than those of chars

175

with similar 1-naphthol adsorption capacity,30 other factors must serve more

176

important functions. The TGA curves reveal that sludge chars contain a large amount

177

of ash (Fig. S1). And the composition analysis shows that the ash content of sludge

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chars increased from 50% to 82% with increasing temperature (Table 1). The high

179

content of ash may impact the adsorption properties of SCs. To study the ash

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influence on the adsorption performance of SCs, ash and the organic moiety of SCs

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were separated, and used to adsorb 1-naphthol, respectively.

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The ash in sludge char is removed with 1 M HCl and 10% (v/v) HF treatment

183

(deashed chars are denoted as DA300, DA400, DA500, and DA800).31 Meanwhile,

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SC300, SC400, SC500, and SC800 were incinerated to obtain ash samples (ASH300,

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ASH400, ASH500, and ASH800). The adsorption behaviors of 1-naphthol by

186

different adsorbents are shown in Fig. 1. Pseudo-second-order kinetics is a commonly

187

used dynamic models describing external diffusion of adsorbate, internal diffusion

188

adsorption, and diffusion inside particle. The relationship between adsorption capacity

189

and time is coincided with the pseudo-second-order kinetics model (Fig. 1b and d,

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R2=0.989-0.997 of SCs, 0.994-0.999 of DAs). The equation of pseudo-second-order


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192

kinetics model is described in Eq. 1. t 1 1 = + t 2 qt k2 qe qe

(1)

193

where qt, qe [mg g–1] are the concentrations at t and equilibrium, respectively; Ce [mg

194

L–1] is the equilibrium solution concentration; k2 is the adsorption constant.

195

The maximum adsorption capacity of SCs (Table S2) is approximately 67 mg g–1,

196

which is comparable with that of common adsorbents reported (Table S1). The

197

adsorption capacities of DAs (Fig. 1c) are significantly higher than those of SCs.

198

Figure 1c and Table S2 show that the adsorption capacity of DA significantly

199

increases with preparation temperature. As shown in Fig. S3, the maximum adsorption

200

capacity of DA800 calculated from the Langmuir model is 666.67 mg g−1. It is

201

wonderful that the adsorption performance of low-cost DA800 exceeds carbon

202

nanotubes, and is comparable to the modified graphene (Table S1). The ash

203

adsorption results showed that the 1-naphthol adsorption capacity of all ash samples

204

are at 8 to 10 mg g–1 (Table S2), which is significantly less than the overall

205

adsorption capacity of SCs.

206

The Dominant Factors Influencing the Adsorption Performance of DAs.

207

Surface Structure and Compositions. The surface structure and compositions of DAs

208

and DCs were analyzed by XPS (Fig. 2, Fig. S4 and Table S3). The energy spectrum

209

of C1s comprises five kinds of peaks. They are C–C/C=C(284–285eV),

210

C–O(alcoholic hydroxyl group, phenolic hydroxyl group or ether, 285.4–286.0eV),

211

C=O(carbonyl or quinonyl, 287eV), C–N(pyrrolic-N, protein–N, amine–N, and

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pyridinic–N, 285.4eV),32-34 and O=C–O(carboxyl or ester group, 288.2–288.7eV).35, 36

213

The percent of each functional group is listed in Table S3. C=O can be obtained in

214

DA300 and DA400; as temperature further increases, C–O remains as the only

215

oxygenous group in DA500 and DA800. The percentage of C–N group decreases



216

slightly with temperature. The content of C–C/C=C in 300, 400, 500 has small change,

217

while it is obviously lower than that in DA800. These results show that the

218

O-containing groups of DAs decrease with temperature, which indicates the increase

219

in carbonization degree with temperature. The percent of each functional group is

220

listed in Table S3, and the content of C–C/C=C，C–N，C–O，C=O in DC300, 400, 500

221

and 800 differs slightly. These results indicate that temperature has little effect on the

222

surface functional groups of DCs.

223

Surface Functional Groups. The functional groups on the surface of SCs and

224

DAs and DCs were analyzed by FTIR (Fig. 3 and Fig. S5, respectively). In Fig. 3a,

225

the peak at 3426 cm−1 is assigned to the stretching vibration of –OH. As the

226

temperature rises, the peak intensity is gradually weakened. The bands at 2925, 2850,

227

1450, and 1378 cm−1 represent the stretching vibration of –CH2– and disappear as the

228

temperature rises above 400 °C. The ester C=O at 1720 cm−1 diminishes at 400 °C.

229

The aromatic C=C at 1630 cm−1 slightly diminishes under 500 °C and nearly

230

disappeared at 800 °C. The N–O band at 1539 cm−1 disappeared above 300 °C. The

231

peaks at 790 and 465 cm−1 are respectively assigned to the stretching vibrations

232

Si–O–Si and O–Si–O groups and are widespread in all SCs.

233

Figure 3b is the FTIR spectra of DAs. Similar to Fig. 4a, the weak peak at 3381

234

cm−1 is attributed to the stretching vibration of –OH, and disappears at 800°C. The

235

bands at 2918, 2845, 1454, and 1387 cm−1 is assigned to the stretching vibration of

236

–CH2– and disappear as the temperature rises above 400 °C. The ester C=O at 1711

237

cm−1 of DAs diminished at 500 °C not 400 °C of SCs. The C=O of SC400 maybe

238

mostly covered by the ash of chars. The aromatic C=C at 1616 cm−1 slightly

239

diminishes as temperature increases and is much weaker at 800 °C. The change of

240

FTIR spectra indicates O-containing groups on both chars and deashed chars decrease


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with the temperature, which coincides with the XPS survey. Compared with SCs, the

242

intensity of Si–O–Si and O–Si–O groups in DAs is much weaker, indicating most ash

243

in SCs is removed.

244

Notably, the surface area and functional groups are two of the main factors

245

influencing the adsorption performance of the char materials.37 Chemical activation

246

with alkali may significantly increase the surface area of the char,38 whereas most of

247

the surface functional groups are also eliminated under the high temperature alkaline

248

conditions.39 Thus, given the adsorption capacity of the sludge char is closely related

249

to the surface functional groups, additional activation is not needed for 1-naphthol

250

adsorption and just increase the preparation cost.

251

Aromatization Degree. Raman spectra were obtained to analyze the

252

microstructure of chars. Figure 4a shows that graphite and amorphous carbon peaks

253

appear in SC300. As temperature rises, the two peaks in SC400, SC500, and SC800

254

become clear. These results indicate that the carbonization degree of sludge increases

255

with increasing temperature. Figure 4b shows the Raman spectra of DAs. Different

256

from SCs, graphite and amorphous peaks of DA300 was clear, indicating the presence

257

of inorganic matter in sludge affected the evolution of the average char

258

microstructure.40 D and G bands found at 1350 and 1580 cm−1 are evidence of

259

amorphous and graphite carbons, respectively. The curve fitting of band combinations

260

for chars (Fig. S6) was applied with line shapes Lorentz (for band D1, D2, D4) and

261

Gaussian (for band D3). G, D1, D2, D4 bands are at about 1580, 1350, 1620, and

262

1200 cm−1 respectively, and D3 band is at about 1500 cm−1.41 From the fitting results

263

in Table S4, ID1,2,3,4/IG of SCs and DAs decreases approximately with the increase of

264

treatment temperature, IG/Iall increases with treatment temperature rising, except a

265

similar value between 400 and 500 °C chars, suggesting the aromaticity of chars



13

266

increases with the increasing temperature. Solid state

C NMR spectroscopic

267

technique also demonstrated the increase of aromatic cluster size of wood chars with

268

temperature rising.42 Chars derived from high temperature with an aromatic structure

269

can form a π-π bond with 1-naphthol.43 However, it is strange that the ID1,2,3,4/IG and

270

IG/Iall of chars (DCs) obtained from deashed sludge show a

271

the temperature (Fig.S7 and Table S4).

irregularly change with

272

Pyrolytic Temperature Dependent Ash Catalysis. XPS, FTIR, and Raman

273

analyses show that the O-containing functional groups of chars decrease with the

274

increasing pyrolytic temperature, whereas the aromatization degree increases. A

275

positive correlation was previously reported between organic-carbon-normalized

276

sorption coefficients (Koc) and aromatic C contents of chars derived from cellulose

277

and chitin. This correlation was caused by the π-π interaction of phenanthrene and

278

naphthalene adsorption on chars.44, 45

279

Although the ash in sludge has a low adsorption capacity, such as inorganic salts

280

and silicon, may affect the properties of obtained chars in the sludge pyrolysis. We

281

carried out an experiment to verify the effect of ash. The sludge were treated with 1 M

282

HCl and 10% (v/v) HF to remove the ash,31 and then the deashed sludge were dried

283

and pyrolyzed at 300, 400, 500, and 800 oC. The obtained chars are denoted as DC300,

284

DC400, DC500, and DC800. The adsorption of 1-naphthal by these chars was tested

285

and the results were shown in Fig. S8. Strangely, the pyrolytic temperature has limited

286

effect on the adsorption performance of DCs, and the maximum adsorption capacity

287

of DCs is lower than 300 mg g−1. It can be explained that the organic compounds in

288

sewage sludge are easy to be thermochemically decomposed and leave the carbon

289

skeleton which is not further changed with the increasing temperature.46 The

290

adsorption capacities of DA300 and DA400 are lower than those of DC300 and


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DC400, respectively, whereas DA500 is higher than DC500. With the increasing

of

292

pyrlytic temperature, the adsorption capacity of DA800 is far higher than that of

293

DC800. It has been reported that ash has catalytic effect on biomass pyrolysis,47 and

294

the pyrolytic temperature and time also affect the catalytic performance of ash in

295

sewage sludge. At high temperatures, new mineral phases with high catalytic activity

296

are formed.48, 49 We compared the surface morphology of original sludge and different

297

chars by SEM (Fig. S9). SEM images show that the DA800 is obvious poriferous

298

compared to DC800, the SSA of DA800 and DC800 is 200 and 80 m2 g−1,

299

respectively, which suggests ash play an important role in the formation of

300

high-capacity char of 1-naphthal.

301

The composition of ash is determined by XRF. As shown in Table S5, the main

302

contents of ash are SiO2, Al2O3, P2O5, Fe2O3. The SEM-EDS line spectra of

303

SC300-800 and DA800 are shown in Fig. 5 and the detailed lines of C, O, Si, and Al

304

are displayed in Fig. S10. The main elements of SCs are C, O, Si, Al, P, Mg, N, Fe, Ca,

305

and Zn, which is in agreement with the results of XRF. The line spectra of C and O of

306

SC300 (Fig. 5a, Fig. S10a1-a4) are similar, indicating the oxygen are mainly

307

connected with carbon at 300 oC. As temperature increased, the spectra of Si and Al of

308

SC400 (Fig. 5b and Fig.S10b1-b4) becomes alike, while the spectra of C and O are

309

different, which suggests that the Si and Al combined together and O separated from

310

C. Similar spectra of O, Si, and Al are observed in SC500 (Fig. 5c and Fig. S10c1-c4),

311

indicating that aluminosilicates probably formed at 500 oC, which is consistent with

312

the literature.[48] The peaks of C, O, Si, and Al are all alike in the spectra of SC800

313

(Fig. 5d and Fig. S10d1-d4), illustrating closely interactions among these elements.

314

These results suggests that at high temperatures, new ash phases formed and may also

315

react with C and O, thus affecting carbon morphology or structure.22 Compared with



316

SCs, there is much more carbon of DA800 (Fig. 5e and Fig. S10e1-e4), and the ash

317

content decreases a lot, indicating most of ash was removed after washing.

318

On the basis of aforementioned discussion, we proposed a formation pathway of

319

high-capacity adsorbent to 1-naphthol which is a pyrolytic temperature dependent

320

ash-catalyzed process (Fig. 6). At low temperature (i.e., 300 oC), the biomass in

321

sewage sludge are seldom decomposed, and the adsorption capacity of char to

322

1-naphthol is low; when the temperature reaches 400 oC, the biomass in sludge is

323

decomposed, and new phases of carbon formed which increases the adsorption of

324

1-naphthol, while the ash may decelerated pyrolysis process as minerals hinder heat

325

diffusion and volatiles release.50 As the temperature exceeds 500 oC, most volatiles in

326

sludge are decomposed and the remained char with aromatic carbon exhibits a high

327

adsorption to 1-naphthol, while ash exert a catalytic function on the formation of the

328

char with greater aromatization degree.

329

Environmental Implication. The yield of DA800 is 19.8 wt% (Table S6),

330

suggesting that the adsorbent can be massively produced. Meanwhile, DA800 has

331

good adsorption performance in solutions of pH from 5 to 9, and little effect is

332

observed at three different temperatures (15, 25, and 35 oC) (Fig.S11), indicating it

333

has the potential application under environmentally relevant conditions. A concern

334

should be addressed is that the potential threat of heavy metals. We conducted a heavy

335

metal ion release experiment under the same condition as those of the adsorption

336

experiment according to Toxicity Characteristic Leaching Procedure (TCLP). The

337

sludge chars of SC300–800 studied in this work were mixed with the TCLP leachant

338

containing 0.064 M NaOH and 0.10 M acetic acid (the pH of the leachant is 4.93), the

339

mass ratio of char to leachant was 1:20. The mixed solution was stirred at 30 rpm and


Page 16 of 30

Page 17 of 30


340

25 oC for 18 h. Then the leachant was filtered to measure Pb, Cd, Cu, and Zn contents.

341

The result was shown in Table S6. No metal ions except Zn was detected. And the

342

maximum concentration of Zn was less than 2.0mg L−1, the limit of Zn release

343

according to national standard of China (GB8978-1996). The leaching experiment

344

results indicate that the chars obtained from pyrolysis of sewage sludge in some cities

345

is suitable for environmental reuse. In other cases, the leaching test is necessary

346

before the reuse of sewage sludge chars.

347

ASSOCIATED CONTENT

348

Supporting Information. Tables S1 to S7 and Figs.S1 to S11 are provided in SI.

349

These materials are available via the Internet at http://pubs.acs.org.

350

AUTHOR INFORMATION

351

Corresponding Author

352

*Fax: +86-551-3607482; e-mail: [email protected].

353

Notes

354

The authors declare no competing financial interest.

355

ACHNOWLEDGEMENTS

356

The authors gratefully acknowledge financial support from National Key

357

Technology R&D Program of the Ministry of Science and Technology

358

(2012BAJ08B00) and National 863 Program (2012AA063608-01).

359 360



361

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Page 22 of 30

Page 23 of 30


507 Sample

Table 1 Elemental Composition and SSA of Sludge and Char Yield

Ash

C

H

O

N

H/C

O/C

(O+N)/C

SSA

Sample

(%)

(%)

(%)

(%)

(%)

(%)

Sludge

100

45.4

26.1

4.5

19.3

4.7

2.06

0.55

SC300

70.0

63.3

22.3

2.5

8.5

3.5

1.32

0.29

0.42

4.58

DA300

1.77

SC400

60.7

71.8

17.3

1.6

6.9

2.5

1.08

0.30

0.42

28.14

DA400

10.45

SC500

59.7

73.4

16.0

1.4

7.2

2.1

1.01

0.34

0.44

13.58

SC800

54.7

82.0

14.8

0.8

2.1

0.3

0.67

0.10

0.12

19.01

(m2 g−1)

SSA (m2 g−1)

0.71

508 509 510 511


DA500 DA800

98.01 200.70


512

Figure Captions

513

Figure 1 a) Adsorption kinetics of 1-naphthol on SCs obtained at different

514

temperature

515

Pseudo-second-order kinetics simulation of 1-naphthol adsorption on SCs, c)

516

Adsorption kinetics of 1-naphthol on DAs (initial 1-naphthol concentration 200 mg

517

L–1, sorbent 0.3 g L–1), d) Pseudo-second-order kinetics simulation of 1-naphthol

518

adsorption on DAs.

519

Figure 2 XPS analysis of DAs

520

Figure 3 FTIR analysis of SCs a) and DAs b).

521

Figure 4 Raman spectroscopy of SCs a) and DAs b).

522

Figure 5 SEM-EDS line spectra of SC300 (a), SC400 (b), SC500 (c), SC800 (d),

523

DA800 (e).

524

Figure 6 Schematic of pyrolytic temperature-dependant and ash catalytic formation of

525

high-capacity of adsorbent.

(initial 1-naphthol concentration 100 mg L–1, sorbent 0.5 g L–1), b)

526 527 528 529 530 531 532 533 534 535 536


Page 24 of 30

Page 25 of 30


a

b

c

d

537 538

Figure 1

539



540

541 542

Figure 2

543


Page 26 of 30

Page 27 of 30


544 545

a

b

546 547

Figure 3

548



549 550

a

b

551 552

Figure 4

553


Page 28 of 30

Page 29 of 30


554

a

b

c

d

e

555 556

Figure 5

557



558

559 560

Figure 6

561 562


Page 30 of 30

Pyrolytic Temperature Dependent and Ash Catalyzed Formation of

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