Accepted Manuscript

Influence of pyrolysis temperature on characteristics and heavy metal adsorptiveperformance of biochar derived from municipal sewage sludge

Tan Chen, Yaxin Zhang, Hongtao Wang, Wenjing Lu, Zeyu Zhou, YuanchengZhang, Lulu Ren

PII: S0960-8524(14)00553-7DOI: http://dx.doi.org/10.1016/j.biortech.2014.04.048Reference: BITE 13344

To appear in: Bioresource Technology

Received Date: 18 March 2014Revised Date: 13 April 2014Accepted Date: 15 April 2014

Please cite this article as: Chen, T., Zhang, Y., Wang, H., Lu, W., Zhou, Z., Zhang, Y., Ren, L., Influence of pyrolysistemperature on characteristics and heavy metal adsorptive performance of biochar derived from municipal sewagesludge, Bioresource Technology (2014), doi: http://dx.doi.org/10.1016/j.biortech.2014.04.048

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customerswe are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, andreview of the resulting proof before it is published in its final form. Please note that during the production processerrors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

1

Influence of pyrolysis temperature on characteristics and heavy metal adsorptive

performance of biochar derived from municipal sewage sludge

CHEN Tana, ZHANG Yaxinb, WANG Hongtaoa∗, LU Wenjinga, ZHOU Zeyua,

ZHANG Yuanchenga, REN Lulu

a

a School of Environment, Tsinghua University, Beijing 100084, China

b College of Environmental Science & Engineering, Hunan University, Changsha

410082, China

Abstract: To investigate systematically the influence of pyrolysis temperature on

properties and heavy metal adsorption potential of municipal sludge biochar,

biophysical dried sludge was pyrolyzed under temperature varying from 500 °C to

900 °C. The biochar yield decreased with the increase in pyrolysis temperature, while

the ash content retained mostly, thus transforming the biochars into alkaline. The

structure became porous as the temperature increased, and the concentrations of surface

functional group elements remained low. Despite the comparatively high content of

heavy metal in the biochar, the leaching toxicity of biochars was no more than 20% of

the Chinese standard. In the batch experiments of cadmium(II) adsorption, the removal

capacity of biochars improved under higher temperature, especially at 800 °C and

900 °C even one order of magnitude higher than that of the commercial activated

carbon. For both energy recovery and heavy metal removal, the optimal pyrolysis

∗ Corresponding author. Address: School of Environment, Tsinghua University, Beijing

100084, China. Tel.: +86 10 62773438. E-mail addresses: htwang@tsinghua.edu.cn.

2

temperature is 900 °C.

Keywords: biochar, municipal sewage sludge, fast pyrolysis, temperature, heavy metal,

adsorption.

1. Introduction

The disposal and utilization of municipal sewage sludge are a growing conern and have

been identified as “a future waste problem”. There are some conventional disposal

methods of sewage sludge, including sanitary landfill with necessary

immobilized/stabilized treatment, combustion, utilization as raw construction material,

anaerobic digestion to recycle methane and hydrogen, and application as fertilizer in

agronomy. However, these options have gradually been prohibited because of

stringent regulations, thus developing an economically and environmentally

acceptable treatment for municipal sludge is a critical social issue. Pyrolysis is an

alternative technology that is cost-effective and clean. It can simultaneously recycle

high-value fuel gas (e.g. hydrogen), and reduce solid waste. The other advantages of

pyrolysis include concentrating on heavy meals (Zhai et al., 2012), and reducing the

releasing of organic micropollutants and pathogens (Yuan et al., 2013).

In pyrolysis, sewage sludge is proved as good feedstock material owing to the

compositions of hydrocarbons and inorganic materials (Nipattummakul et al., 2010;

Pedroza et al., 2014). During pyrolysis process, with the sweep flow of inactive gas,

the sewage sludge is heated to a high temperature without oxygen. In the gas, liquid

and solid phases, various products are generated, namely, syngas, tar or bio-oil, and

3

biochar. For energy production which is the main goal of the process, the pyrolysis

conditions are controlled to produce the maximum amounts of syngas and tar. Han et

al. (2012) report that the syngas produced by fast pyrolysis of biophysical dried

sludge is rich in hydrogen, with a maximum H2 of more than 40 vol.% and an H2 yield

ratio of 0.0181 g/g dried sludge at 900 °C.

Biochar, the solid by-product of pyrolysis, is a form of carbon black containing

carbon materials ranging from elemental or graphitic to a small amount of

polyaromatic carbon (Chun et al., 2004). Many studies report that biochar may restore

degraded soil, increase crop yield, fix carbon dioxide and adsorb contaminants.

Although the specific area and micropore volumn of biochars are much smaller than

those of commercial activated carbon (AC), the adsorption capability of biochars with

respect to both organic pollutants and heavy metals is similar to or even better than

that of AC, with low cost due to no need of activation (Chun et al., 2004). The biochar

derived from sewage sludge is a carbon-mineral adsorbent with abundant mineral

oxides (Singh et al., 2010), and the adsorption mechanisms vary depending on the

biochar properties and the adsorption conditions (Cao et al., 2009; Harvey et al., 2011;

Lu et al., 2012).

The physical and chemical properties of biochar depend on both the characteristics of

the feedstock source and the pyrolysis conditions. Among pyrolysis conditions,

temperature plays the key role because it does not only affect production distribution

but also influences the nature of biochar (Kim et al., 2012; Mendez et al., 2013; Yuan

et al., 2013). When pyrolysis temperature is higher, less biochar is generated and the

4

microstructure develops more effectively. If the temperature is too high, the loss of

carbon and other functional group elements on the surface is excessive. The chemical

composition, pH, surface charge and thermal stability of biochar, as well as the heavy

metal fate in the biochar body, are also functions of pyrolysis temperature.

In using biochar as a potential material for environment remediation, the factors to

consider include adsorption performance and environmental friendliness (especially

with respect to the behavior of heavy metal). These factors are determined by biochar

characteristics. Thus, defining the relationship between biochar properties and

pyrolysis temperature, which is the decisive pyrolysis condition, is essential. This

study systematically investigates the influence of pyrolysis temperature on the

properties of biochar derived from municipal sewage sludge and its potential to

adsorb heavy metal. Biophysical dried sewage sludge is pyrolyzed in a horizontal

quartz reactor under pyrolysis temperature ranging from 500 to 900 °C. The yield

ratio, elemental distributions, specific surface area, thermal stability, Fourier

transform infrared (FTIR) spectra of the resultant biochars are analyzed. To examine

the environmental safety and adsorptive performance of these biochars, the leaching

toxicity and batch equilibrium experiments are also closely conducted.

2. Material and Methods

Throughout this study, analytical reagent (AR) grade chemicals and deionized water

were used. All of the labware was soaked in dilute nitric acid at least overnight,

thoroughly flushed with tap water, and washed three times with deionized water. The

commercial AC was purchased from Beijing Modern Eastern Fine Chemical in China,

5

and ground through a 40 mesh sieve (0.45 mm).

2.1. Biochar preparation

The biophysical drying and fast pyrolysis process were described by Han et al. (2012).

Biophysical drying combines with the heat produced in the aerobic fermentation of

microbes and the convection effect generated by forced aeration, which is a more

energy-efficient method of sludge drying than conventional methods. The product of

biophysical drying has a fine-particle morphology and a non-compact structure, which

effectively transfers heat during pyrolysis.

The biophysical drying reactor consisted of a 159 L adiabatic cylindrical vessel

(stainless steel, 1000 mm high, an internal diameter of 450 mm) with leachate

drainage, aeration and monitoring systems. Municipal sewage sludge with an initial

moisture content of 82.1% was sampled from the Xiaojiahe Municipal Sewage

Treatment Plant in Beijing, China, and mixed with pine bark, a structure material, as

the mass ratio of 2:1 (sludge : bark). The moisture content of the sewage sludge

reduced to approximately 25% after seven days of drying. Subsequently, the sludge

was sieved apart from the pine bark, and air dried to constant weight with a moisture

content of 2.33±0.09% and an ash content of 48.02±0.46% (dry basis).

Fast pyrolysis was conducted in a horizontal fixed-bed reactor (an internal diameter of

50 mm, 1200 mm long, and a quartz tubular in the electric furnace). 3 g dried sludge

was placed in a porcelain boat on the non-heating end of the quartz tubular. The

pyrolysis temperature was selected from 500 °C to 900 °C, which is the common

temperature range of fast pyrolysis. The set temperature reached after about 30 min of

6

heating with carrier gas (N2) sweeping at 0.3 L·min-1

, and the porcelain boat was fed

into the heating zone with an N2 flow rate of 0.03 L·min-1. After 20 min, the pyrolysis

process was stopped, and then the porcelain boat was removed from the heating zone

and cooled with N2 sweeping at 0.3 L·min-1

for 30min. The solid yield was the

biochar, which was ground through a 40 mesh sieve (0.45 mm). Prior to additional

experiments, no pretreatment was conducted. The obtained biochars were abbreviated

as BC500, BC600, BC700, BC800 and BC900 respectively, according to the pyrolysis

temperature.

2.2. Biochar characterization

2.2.1 General nature

The element analysis of C, H, O and N was performed by an EA3000 elemental

analyzer (EuroVector, Italy). Biochar samples were calcinated to constant weight

during about 120 min at 650 °C, and ash content was calculated as the mass residual

percentage of the samples. The mixing ratio of biochar sample and deionized water

was 1:10 (w./v.), and the pH value of the sludge was recorded as the pH of the biochar.

The point of zero charge of biochar pH (pHPZC) was determined according to

Mahmood et al. (2011). The cation exchange capacity (CEC) of the biochars were

tested using cobalt hexamine ion exchange method as proposed by Hu et al. (2000).

2.2.2 FTIR spectra

FTIR spectra were investigated in the 4000~400 cm−1

region under a 4 cm−1

resolution using a Spectrum GX spectrometer (Perkin Elmer, USA). The baseline of

the raw data was adjusted and then the modified data was normalized, both by

7

OMNIC 8.0.342 software (Thermo Scientific, USA).

2.2.3 Thermal stability

The thermogravimetric (TG) and derivative thermogravimetric (DTG) analyses were

conducted using a TGA/DSC 1 STARe system (Mettler Toledo, Switzerland), at a

heating rate 10 °C/min from 25 °C to 1000 °C, and under the static air atmosphere of

N2 (flow rate of 20 mL/min).

2.2.4 Micro morphology and surface characters

Approximately 10 mg biochar was dispersed in 20 mL deionized water by sonication

for 30 min and picked up onto a carbon-coated copper grid. After the sample was air

dried, the micro morphology was observed by an S-5500 scanning electron

microscope (Hitachi, Japan). The surface area, pore size, pore volumn and fractal

dimension were determined by the N2 adsorption-desorption isotherm at 77 K using

an Autosorb-1-C gas sorption system (Quantachrome, USA). Prior to measurement,

the samples were outgassed at 300 °C for 4 h.

2.2.5 Heavy metal content and leaching toxicity

The heavy metal content in the biochars was determined by microwave digestion with

chloronitric acid and the inductively coupled plasma optical emission spectroscopy

(ICP-OES) method using an IRIS Intrepid II XSP Spectrometer (ThermoFisher, USA).

The extraction procedure for leaching toxicity was using sulphuric acid and nitric acid

method based on the Chinese standard HJ/T 299-2007. The identification standard of

extraction toxicity for hazardous wastes of China (GB 5085.3-2007) was applied.

2.3. Heavy metal adsorption

8

A stock solution of Cd2+

was prepared by dissolving Cd(NO3)2·4H2O in deionized

water at a Cd2+ concentration of 2000 mg/L. Cd2+-bearing solutions were prepared by

diluting the stock solution to specific concentrations ranging from 10 to 200 mg/L.

Before adsorption, the pH level of Cd2+

-bearing solutions was not adjusted, and was

measured as initial pH.

50 mg of either biochar or AC was placed into a 40 mL glass bottle. Subsequently, 25

mL Cd2+ solution was added, and this solution was intensively mixed using a vortex

maker. After stirring by a thermostatic box for 4 h at 25±1 °C, the suspension was

filtered with 0.45 µm polysulfone filter membrane. The residual Cd2+

and release Ca2+

concentrations were determined by ICP-OES (IRIS Intrepid II XSP Spectrometer,

ThermoFisher, USA). All adsorption experiments were run in triplicate, and the blank

solution was measured for comparison.

The removal percentage and removal capacity of Cd2+ were calculated as follows:

( )0 0/ 100%

eR C C C= − ×

(1)

0( ) /e

Q C C V m= − (2)

where, R is the removal percentage of Cd2+

(%); C0 and Ce are the initial and

equilibrium concentrations of Cd2+ (mg/L); Q is the removal capacity of Cd2+ at

equilibrium (mg/g); V is the volumn of the solution (mL) and m is the weight of either

biochars or of AC (mg).

3. Results and Discussion

3.1. General natures of the biochars

The properties of the biochars, including yield rate, ash content, elemental

9

composition, pH, pHPZC and CEC, are summarized in Table 1. As the pyrolysis

temperature rises from 500 to 900 °C, the percentage of biochar yield reduces by ten

percents from 63.10% to 53.31%. A similar observation is reported by Mendez et al.

(2013) and Yuan et al. (2013). The loss of non-ash content (fixed carbon and volatile

matters) in the sludge is great, as confirmed by the variations in the amount of C, N, O

and H elements. However, the ash content almost completely remains, which indicats

that: (1) very little heavy metal runs off into the gas-phase syngas and the liquid-phase

tar, and heavy metals almost entirely maintains in the solid-phase biochar; (2) the

carbonaceous materials transform into hydrocarbon compounds as gas and aromatic

hydrocarbons as tar.

During pyrolysis, the loss of volatile matters takes away a lot of surface functional

group elements (H, O and N). The contents of remaining H, O and N are respectively

0.70%, 10.45% and 1.54% through pyrolysis at 500 ℃, while decline rapidly to

0.11%, 2.44% and 0.53% at 900 °C. Meanwhile, since the element C exists as both

fixed carbon and volatile matters, the retaining C only decreases by less than 2% from

500 °C to 900 °C. As a result, the atomic ratio reduces, amorphous carbon increases,

surface functional groups decrease and microstructure developes, which complies

with the solution of the FTIR spectra in section 3.2. The ratio of molar H/C as a

carbonization degree parameter is always lower than 0.1, thus suggesting the biochars

with strong carbonization and high aromaticity can resist decomposition (Yuan et al.,

2013). The molar O/C ratio of biochars is higher than that of AC (Chun et al., 2004)),

which indicates the biochars with more polar-groups have higher hydrophilicity than

10

some categories of commercial AC. These changes in H/C and O/C also illustrate that

dehydrogenative polymerization and dehydrative polycondensation occur during

pyrolysis, with significant loss of oxygen and aliphatic hydrogen (De Filippis et al.,

2013). The molar N/C ratio, with the same tendency of H/C and O/C, decreases with

pyrolysis temperature increasing, suggesting the surface functional groups of the

biochars reducing.

Almost all metal oxides and minerals as ash content maintain in the biochars, thus,

attributed to these alkaline substances, the pH of the biochars induce alkalinity. The

pH values of BC500, BC600, BC700, BC800 and BC900 gradually increase at 8.81,

9.54, 11.11, 12.18 and 12.15 respectively, keeping stable at the strong basic pH level

of 12. Other researchers also reported that the pH level of the biochars derived by

pyrolysis increased (Mendez et al., 2013), as a result of the release of alkali salts from

the pyrolytic structure (Chen et al., 2011). The presence of metal oxides and minerals

also make the pHPZC of the biochars higher than 8, which indicates that the surface of

biochars is positive charged when the solution is acid, neutral or even weak alkaline,

and will exclude the metal cations. The surface complexion may be not the main

mechanism of the heavy metal adsorption. The well-buffered neutral to alkaline

properties of the biochar strongly immobilize the metals within Kistler et al. (1987).

In alkaline environment, heavy metal ions transform into precipitation of very low

solubility, thus the high pH of the biochars ensures the safety of heavy metal leaching.

The abundant alkali metals (Na, K) and alkaline-earth metals (Ca, Mg) in the ash are

released, and the vacant sites can be replaced by other cations, thus the CEC of the

11

biochars are large with the maximum 247.5 cmol·kg-1

of BC900. BC600 has the CEC

of 30.81 cmol·kg-1, which is consistent with a similar feedstock under the same

pyrolysis temperature (Mendez et al., 2013). Another reason for the increased basicity

may be that the organic nitrogen present as amine functionalities transforms into

pyridine-like compounds, and the amount of acidic surface functional groups decrease

as the oxygen percentage losses when the temperature increases, both of which can

result in surface basicity enhancing (Bagreev et al., 2001; De Filippis et al., 2013).

3.2. FTIR

Changes in surface functional groups are also reflected by the FTIR spectra. The

FTIR spectra of the biochars and AC are presented in Supplementary material (Fig.

S-1), and the relevant peaks attributed to special functional groups and compounds are

summarized in Table 2. The peaks at wavenumbers of 3420, 1420, 1035 and 780 cm-1

show the surface carbon structure of the biochars. This structure consists of chain

hydrocarbons and functional groups such as hydroxyl and aromatic rings. The

aromatic structure will provide π-electron, which is reported to have potential to bond

heavy metal cations strongly (Harvey et al., 2011). A higher pyrolysis temperature

heightens the peaks at 3420 cm-1 and 1420 cm-1, and increases the distribution rate of

the -OH and -CH2- structure, due to the decomposition of chain hydrocarbons and the

reduction of oxygen functional groups. Meanwhile, the peaks at 780 cm-1

weaken,

thus indicating that the dehydrogenation reaction intensifies under the high

temperature. The strong peaks of 1035 cm-1

demonstrate that during pyrolysis, various

forms of oxygen in the substrate sludge transform primarily into directly banding with

12

the adjacent carbon element, thereby integrating into carbon chain in the form of

carbon-oxygen single bond.

Compared with AC, the peaks of biochars are fewer and weaker, and this means that

the surface functional groups distribute skimpier, and the abundance of the surface

functional group species is poorer, lacking of keton, aldehyde, lacton and carboxyl

(carbon-oxygen double bond functional groups).

3.3. Thermal stability

The TG curves (Supplementary material, Fig. S-2(A)) illustrates that the order of

thermal stability is BC500 ＜ BC600 ＜ BC700 ＜ BC800 ＜ BC900. BC800

and BC900 are more stable than AC in the temperature range from 25 to 1000 °C,

however BC500, BC600 and BC700 are more stable than AC only at low temperature,

but less stable at high temperature. The DTG curves (Supplementary material, Fig.

S-2(B)) shows the mass loss rate at the special temperature. AC losses weight mainly

at approximately 100 °C, the majority of which is moisture and volatile. In contrast,

the biochars typically lose mass from 600 °C to 700 °C, because the carbonate in ash

content (Mendez et al., 2013) and the aromatic structure (Wu et al., 2011) decompose.

Additionally, within the entire temperature range, the mass losses in BC800 and

BC900 are less than 20 % and 10 %, respectively, which also confirms their strong

thermal stability.

3.4. Surface characters and micro morphology

The scanning electron microscopy (SEM) photos of BC900 (Supplementary material,

Fig. S-3) reveal the well-developed micro pore structures. As displayed in Table 3, the

13

surface area, pore volumn, pore size and fractal dimension of the biochars increase

gradually, as the pyrolysis temperature promotes. The data on surface area and pore

volumn of the biochars is in agreement with previous studies using similar feedstocks

(Bagreev et al., 2001; Lu et al., 1995; Mendez et al., 2013; Rio et al., 2005; Yuan et al.,

2013). The specific area of the biochars is considerably less than that of AC (727 m2/g,

as determined in this study), because it is not activated. However, the adsorption

activity of the biochars is better than AC, in combination with the surface functional

group distribution, the results suggesting the main mechanism for heavy metal

adsorption on the biochars is not surface complextion. The higher fractal dimension

indicates the improved porous structure. Figure 1 showes that the N2 adsorption

amount increases with the increasing of the pyrolysis temperature, except at 600°C.

The distribution curves of pore size (Figure 1, inset) suggest the pore distributions of

the biochars are all similar with the peak horizontal position (3.80 nm approximately),

whereas the pore channels of high-temperature biochars are more uniform. Although

the N2 adsorption amount of the biochars is significantly less than AC, the pore size

distribution is basically the same. According to IUPAC classification (Sing et al.,

1985), the N2 adsorption-desorption isotherms of all the biochars correspond to a

similar Type IV isotherm and Type H2 hysteresis loop behavior, which indicates that

(1) the pores of the biochars are mainly mesopores (pores of widths between 2 nm to

50 nm), which is consistent with both the measured value of the pore size in this study

and that obtained in a previous study (Rio et al., 2005); (2) capillary condensation will

occur in the pores; and (3) the pores are with narrow necks and wide bodies, often

14

refered to as “ink bottle pores”.

At 600 °C, an exception to the law on changes in surface area and pore volumn versus

pyrolysis temperature is observed. In the temperature range of 550~650 °C, the sludge

melts and converts into intermediate thermoplastic phase, which is softened. In

addition to the evolution of secondary volatiles, gases bubbles out and the pore

enlargement phenomenon forms (Lu et al., 1995). Thus, BC600 with relatively large

pores (compared with BC500 and BC700) has the lowest surface area and pore

volumn.

3.5. Heavy metal content and leaching toxicity

As provided in Table 4, the heavy metal content of the biochars is significantly higher

than that of AC. In the biochars, P, Al, Ca, Fe, K and Mg are at the high level of

1%~4%, whereas Cr, Cu, Mn and Na range from 0.01% to 0.1%, and Cd and Pb are

trace in the biochars. This suggests the content of hazardous metals is low, and the

precipitation generated by alkaline substances (particularly Ca-compounds) and

phosphate will lower the leaching toxicity to below the safe level (Hu et al., 2013).

The results of the extraction procedure are presented in Table 5. All the concentrations

of extraction solutions are extremely low, and some even cannot be detected. The

measured value does not reach a fifth of the limit value, thus, the leaching toxicity of

the biochars is within the secure level. The phenomenon that heavy metal is of high

content in biochar but little by leached, is also reported previously (He et al., 2010;

Kistler et al., 1987). This finding may be ascribed not only to the immobilized

precipitation generated as mentioned above (see section 3.1), but also to the

15

vitrification formed during the high temperature pyrolysis, embedding heavy metals

into solid solutions (Park & Song, 1998).

3.6. Adsorption of Cd2+

Biochars remove a greater percentage of Cd2+

and possess a higher removal capacity

than AC does (Figure 2(A) and (B)). The removal percentage of Cd2+

by AC decreases

sharply from 28.86% at an initial Cd2+

concentration of 9.50 mg/L to less than 5%

when the Cd2+ concentration increases. By contrast, all of the biochars display higher

removal percentage, and BC800 and BC900, in particular, almost removes 100% Cd2+

of the initial Cd2+

concentration range from 0 to 50 mg/L. The removal capacity of AC

does not exceed 2.5 mg/g, whereas, the removal capacity of the biochars is promoting

with the pyrolysis temperature rising. Although the removal capacity of BC900 is

slightly lower, the adsorption activities of BC800 and BC900 are similar, with the

capacity of almost more than 15 mg/g, which is ten times higher than that of AC at the

same initial Cd2+

concentration (see Figure 2(A) inset). There are also some reports on

cadmium adsorption by biochars derived from different raw materials, such as the

maximum sorption capacity of about 25 mg/g for straw (Remenarova et al., 2012),

6.22 mg/g for household biowaste (Qin et al., 2012) and 26.32 mg/g for corn straw

(Liu et al., 2012). Compared with these results, the biochars derived from sewage

sludge in this study performs good heavy metal adsorptive efficiency. Figure 3 shows

the pH influence on Cd2+

species. When the solution pH is higher than 8, the amount

of soluble Cd reduces sharply, however, when the pH value reaches 10, soluble Cd2+

ions disappear almost completely in the solution. Although the alkali (earth) metals in

16

the minerals and metal oxides can buffer pH, the pH values after equilibrium are still

neutral or acid (see Figure 2(C)), except for BC700, BC800 and BC900 with only low

initial Cd2+ concentrations. The released Ca2+ concentrations increase with the initial

Cd2+

concentration rising (see Figure 2(D)), which suggests the other mechanism of

Cd2+

may be cation exchange, Ca2+

releasing from the mineral matrix and the site

replacing by Cd2+

forming new bond.

4. Conclusions

Biophysical dried sludge was fast pyrolyzed at temperatures from 500 °C to 900 °C.

With the temperature rising, the yield of biochars decreased, ash content and

microstructure development of biochars promoted. However, the volatile matters lost

more, and the surface functional groups remained seldom. The biochars were of good

thermal stability, and the leaching toxicity of the biochars remained within a safe level.

The adsorption of Cd2+ by biochar is significantly higher than by AC, and the main

mechanism may be surface precipitation and ion-exchange. 900 °C is the optimal

pyrolysis temperature to both recover energy and adsorb heavy metal.

Acknowlegements

The authors gratefully acknowledge the financial support of the Fund from Natural

Science Foundation of China (No. 41371472) and the Major Science and Technology

Program for Water Pollution Control and Treatment of the Ministry of Environmental

Protection of China (No. 2011ZX07317-001). The authors also thank Dr. Rong HAN

for her valuable comments on the experiment design.

17

Figure Capions

Figure 1 Nitrogen adsorption-desorption isotherms and pore size distribution curves

(insert) of the biochars and commercial AC.

Figure 2 Cd2+

adsorption performance of the biochars and commercial AC.

Equilibrium conditions: 4 h, 25±1 °C. (A) Cd2+

removal percentage versus Cd2+

initial

concentration; inset of (A) Cd2+ removal capacity at a Cd2+ initial concentration of

47.44 mg/L versus categories of absorbents; (B) Cd2+ removal capacity versus Cd2+

initial concentration; (C) pH of the solution versus Cd2+

initial concentration; (D) Ca2+

release concentration versus Cd2+

initial concentration.

Figure 3 Speciation of Cd(Ⅱ) in aqueous as a function of pH, simulating via Visual

MINTEQ ver. 3.0. pH ranges from 1 to 13 pH unit; and the total Cd concentration is

0.001 mol/L (112.41 mg/L).

18

2 4 6 8 10 12 14 16 18 20

0.000

0.005

0.010

0.015

0.020

0.025

0.030

0.035

0.0 0.2 0.4 0.6 0.8 1.0

0

20

40

60

200

250

300

dV

/dW

(cm

3 g

-1 n

m-1

)

Pore Diameter (nm)

AC

BC500

BC600

BC700

BC800

BC900

Adso

rpti

on

Volu

me(

cm3 g

-1, S

TP

)

Relative Pressure (P/P0)

Figure 1 Nitrogen adsorption-desorption isotherms and pore size distribution curves

(insert) of the biochars and commercial AC.

19

0 50 100 150 200

0

20

40

60

80

100

0 50 100 150 200

0

5

10

15

20

25

0 50 100 150 200

2

4

6

8

10

12

0 50 100 150 200

0

10

20

30

40

50

60

Cd

2+ r

emo

val

per

cen

tage

(%)

Cd2+

initial concentration (mg/L)

(B)

Cd

2+ r

emo

val

cap

acit

y (

mg

-Cd/g

-ab

sorb

ent)

Cd2+

initial concentration (mg/L)

(C)

(A)

AC

BC500

BC600

BC700

BC800

BC900

blank

initial pH

pH

(p

H u

int)

Cd2+

initial concentration (mg/L)

(D)

Ca2

+ r

elea

se c

once

ntr

atio

n (

mg

/L)

Cd2+

initial concentration (mg/L)

AC BC500 BC600 BC700 BC800 BC9000

4

8

12

16

20

24

Cd

2+ r

emo

val

cap

acit

y (

mg

-Cd/g

-abso

rben

t)

Cd2+

initial concentration

= 47.44 mg/L

20

Figure 2 Cd2+

adsorption performance of the biochars and commercial AC. Equilibrium conditions: 4 h, 25±1 °C. (A) Cd2+

removal percentage

versus Cd2+

initial concentration; inset of (A) Cd2+

removal capacity at a Cd2+

initial concentration of 47.44 mg/L versus categories of

absorbents; (B) Cd2+

removal capacity versus Cd2+

initial concentration; (C) pH of the solution versus Cd2+

initial concentration; (D) Ca2+

release concentration versus Cd2+ initial concentration.

21

0 2 4 6 8 10 12 14

0.0000

0.0002

0.0004

0.0006

0.0008

0.0010

concen

trat

ion (

mo

l/L

)

pH (pH uint)

Cd2+

Cd(OH)2(s)

Total Disolved Cd

Figure 3 Speciation of Cd(II) in aqueous as a function of pH, simulating via Visual

MINTEQ ver. 3.0. pH ranges from 1 to 13 pH unit; and the total Cd concentration is

0.001 mol/L (112.41 mg/L).

22

Table 1 Main properties of the biochars pyrolyzed under temperature of 500~900 °C.

BC500 BC600 BC700 BC800 BC900

Yield percentage/wt.% 63.10±0.50 60.25±1.54 58.66±0.74 54.71±1.34 53.31±0.48

Ash content percentage/wt.% 74.21±0.55 77.90±1.40 81.53±0.50 83.93±0.07 88.07±0.56

Ash remaining ratioa/% 99.83 100.06 101.97 97.90 100.09

Volatile matter and fixed carbon

loss ratiob/%

67.95 73.77 78.66 82.68 87.47

Elemental

analysis/wt.%

C 17.46±0.64 18.40±2.43 16.92±0.55 16.20±2.29 15.92±2.74

H 0.70±0.13 0.34±0.09 0.21±0.06 0.03±0.05 0.11±0.11

O 10.449±0.613 7.353±0.371 6.860±0.111 3.641±0.104 2.439±0.575

N 1.54±0.06 1.38±0.14 0.95±0.07 0.50±0.07 0.53±0.07

C+H+O+N 30.15 27.47 24.94 20.37 19.00

Atomic ratio

H/C 0.088 0.057 0.042 0.037 0.079

O/C 0.449 0.300 0.304 0.169 0.115

N/C 0.075 0.064 0.048 0.026 0.029

pH (S/L=1:10) 8.81 9.54 11.11 12.18 12.15

pHPZC 8.58 9.04 9.91 10.18 10.17

CEC/cmol·kg-1

76.75±6.53 30.81±2.67 50.34±2.73 126.62±9.16 247.51±7.49

a Ash remaining ratio = Ash content percentage of biochar × Yield percentage / Ash content percentage of sludge × 100%;

23

b Volatile matter and fixed carbon loss ratio = 1 - (1 - Ash content percentage of biochar) × Yield percentage / (1 - Ash content percentage of

sludge) × 100%.

25

Table 2 Analysis of the peaks of FTIR spectra.

Peak

position（ cm-1）

Functional group References

~3420 OH stretching vibration (Droussi et al., 2009; Iqbal et al.,

2009)

2883 C–H stretching vibration (Pan et al., 2009)

1564 C=C stretching vibration

-CONH-

C=O stretching vibration of

keton, aldehyde, lacton and

carboxyl

(Cao et al., 2009; Chia et al., 2012;

Droussi et al., 2009)

(Zhang et al., 2011)

(Julian Moreno-Barbosa et al., 2013)

~1420 C-H stretching vibration of

CH2 and CH3

(Droussi et al., 2009; Zhang et al.,

2011)

~1070 -OH (Inyang et al., 2011)

~1035 C-O stretching vibration (Chia et al., 2012; Droussi et al.,

2009; Zhang et al., 2011)

~780 aromatic ring C-H (Zhang et al., 2011)

26

Table 3 Microstructure properties of the biochars.

BC500 BC600 BC700 BC800 BC900

BET surface area/m2·g 25.424 20.268 32.167 48.499 67.603

Pore size/nm a

3.743 3.758 3.745 3.771 3.840

Pore volumn/cm3·g

b 0.05608 0.05268 0.06842 0.08989 0.09855

Fractal dimension (NK model) c

2.6373 2.6415 2.6752 2.7372 2.8666

Fractal dimension (FHH model) c

2.6989 2.6674 2.7019 2.7560 2.7920

a Barrett, Joyner and Halenda (BJH) model, desorption data;

b P/P0=0.99;

c Adsorption data.

27

Table 4 Element content in the biochars and in the commercial AC.

Elements/mg·kg-1 BC500 BC600 BC700 BC800 BC900 AC

Al 28417.35 28332.37 32848.06 34454.77 35501.94 8531.10

Ca 59286.90 62719.38 64373.64 65839.62 69560.08 10928.86

Cd 3.37 3.70 nd nd nd 0.07

Cr 100.34 100.94 114.58 105.81 112.02 9.35

Cu 202.44 208.48 242.30 201.49 183.71 3.18

Fe 31123.70 33606.39 35326.67 35769.17 37202.34 11320.53

K 8518.52 8498.59 9938.32 9289.42 8684.18 1161.18

Mg 14736.07 15457.60 16369.21 16573.98 17523.36 2926.83

Mn 749.27 760.43 833.45 799.40 836.45 196.82

Na 1173.02 1724.09 1383.51 1930.98 3417.89 5350.27

P 18185.58 18760.23 20350.03 19348.41 20238.79 276.36

Pb 51.52 nd nd nd 5.81 nd

nd = not detected.

28

Table 5 Heavy metal concentration in the solution of the extraction procedure of biochars. Mixed solution of sulphuric acid and nitric acid with

pH = 3.20±0.05; S/L=1:10 (w./v.).

Heavy

metals/mg·L-1 BC500 BC600 BC700 BC800 BC900

Chinese limit

value

Ba nd nd 1.014 1.187 2.797 100

Cd 0.025 0.027 0.011 0.033 0.007 1

Total Cr nd 0.031 nd nd nd 15

Cu 0.162 0.026 0.067 0.132 0.190 100

Ni 0.045 nd nd 0.020 nd 5

Pb 0.516 nd 0.885 0.732 0.926 5

Zn 0.032 0.008 0.518 0.042 0.002 100

nd = not detected.

30

References

1. Bagreev, A., Bandosz, T.J., Locke, D.C. 2001. Pore structure and surface chemistry of adsorbents

obtained by pyrolysis of sewage sludge-derived fertilizer. Carbon, 39(13), 1971-1979.

2. Cao, X., Ma, L., Gao, B., Harris, W. 2009. Dairy-Manure Derived Biochar Effectively Sorbs Lead and

Atrazine. Environmental Science & Technology, 43(9), 3285-3291.

3. Chen, X., Chen, G., Chen, L., Chen, Y., Lehmann, J., McBride, M.B., Hay, A.G. 2011. Adsorption of

copper and zinc by biochars produced from pyrolysis of hardwood and corn straw in aqueous solution.

Bioresource Technology, 102(19), 8877-8884.

4. Chia, C.H., Gong, B., Joseph, S.D., Marjo, C.E., Munroe, P., Rich, A.M. 2012. Imaging of

mineral-enriched biochar by FTIR, Raman and SEM-EDX. Vibrational Spectroscopy, 62, 248-257.

5. Chun, Y., Sheng, G.Y., Chiou, C.T., Xing, B.S. 2004. Compositions and sorptive properties of crop

residue-derived chars. Environmental Science & Technology, 38(17), 4649-4655.

6. De Filippis, P., Palma, L.D., Petrucci, E., Scarsella, M., Verdone, N. 2013. Production and

Characterization of Adsorbent Materials from Sewage Sludge by Pyrolysis. CHEMICAL

ENGINEERING TRANSACTIONS, 32, 205-210.

7. Droussi, Z., D’orazio, V., Provenzano, M.R., Hafidi, M., Ouatmane, A. 2009. Study of the

biodegradation and transformation of olive-mill residues during composting using FTIR spectroscopy

and differential scanning calorimetry. Journal of Hazardous Materials, 164(2–3), 1281-1285.

8. Han, R., Liu, J., Zhang, Y., Fan, X., Lu, W., Wang, H. 2012. Dewatering and granulation of sewage

sludge by biophysical drying and thermo-degradation performance of prepared sludge particles during

succedent fast pyrolysis. Bioresource Technology, 107, 429-436.

9. Harvey, O.R., Herbert, B.E., Rhue, R.D., Kuo, L.-J. 2011. Metal Interactions at the Biochar-Water Interface: Energetics and Structure-Sorption Relationships Elucidated by Flow Adsorption

Microcalorimetry. Environmental Science & Technology, 45(13), 5550-5556.

10. He, Y.D., Zhai, Y.B., Li, C.T., Yang, F., Chen, L., Fan, X.P., Peng, W.F., Fu, Z.M. 2010. The fate of Cu,

Zn, Pb and Cd during the pyrolysis of sewage sludge at different temperatures. Environmental

Technology, 31(5), 567-574.

11. Hu, H.Y., Liu, H., Shen, W.Q., Luo, G.Q., Li, A.J., Lu, Z.L., Yao, H. 2013. Comparison of CaO's

effect on the fate of heavy metals during thermal treatment of two typical types of MSWI fly ashes in

China. Chemosphere, 93(4), 590-596.

12. Hu, X., Lv, G., Yang, Y. 2000. Determination of cation-exchange capacity in clay [Co(NH3)6]3+

exchange method. Chinese Journal of Analytical Chemistry, 28(11), 1402-1405. (in Chinese)

13. Inyang, M., Gao, B., Ding, W., Pullammanappallil, P., Zimmerman, A.R., Cao, X. 2011. Enhanced

Lead Sorption by Biochar Derived from Anaerobically Digested Sugarcane Bagasse. Separation Science

and Technology, 46(12), 1950-1956.

14. Iqbal, M., Saeed, A., Zafar, S.I. 2009. FTIR spectrophotometry, kinetics and adsorption isotherms

modeling, ion exchange, and EDX analysis for understanding the mechanism of Cd2+ and Pb2+ removal

by mango peel waste. Journal of Hazardous Materials, 164(1), 161-171.

15. Julian Moreno-Barbosa, J., Lopez-Velandia, C., del Pilar Maldonado, A., Giraldo, L., Carlos

Moreno-Pirajan, J. 2013. Removal of lead(II) and zinc(II) ions from aqueous solutions by adsorption

onto activated carbon synthesized from watermelon shell and walnut shell. Adsorption-Journal of the

International Adsorption Society, 19(2-4), 675-685.

16. Kim, K.H., Kim, J.Y., Cho, T.S., Choi, J.W. 2012. Influence of pyrolysis temperature on

physicochemical properties of biochar obtained from the fast pyrolysis of pitch pine (Pinus rigida).

Bioresource Technology, 118, 158-162.

17. Kistler, R.C., Widmer, F., Brunner, P.H. 1987. Behavior of chromium, nickel, copper, zinc, cadmium,

mercury, and lead during the pyrolysis of sewage sludge. Environmental Science & Technology, 21(7),

704-708.

18. Liu, Y., Qin, H., Li, L., Pan, G., Zhang, X., Zheng, J., Han, X., Yu, X. 2012. Adsorption of Cd~(2+)

and Pb~(2+) in aqueous solution by biochars produced from the pyrolysis of different crop feedstock.

31

Ecology and Environmental Sciences, 21(1), 146-152. (in Chinese)

19. Lu, G.Q., Low, J.C.F., Liu, C.Y., Lua, A.C. 1995. Surface area development of sewage sludge during

pyrolysis. Fuel, 74(3), 344-348.

20. Lu, H., Zhang, W., Yang, Y., Huang, X., Wang, S., Qiu, R. 2012. Relative distribution of Pb2+

sorption mechanisms by sludge-derived biochar. Water Research, 46(3), 854-862.

21. Mahmood, T., Saddique, M.T., Naeem, A., Westerhoff, P., Mustafa, S., Alum, A. 2011. Comparison of Different Methods for the Point of Zero Charge Determination of NiO. Industrial & Engineering

Chemistry Research, 50(17), 10017-10023.

22. Mendez, A., Tarquis, A.M., Saa-Requejo, A., Guerrero, F., Gasco, G. 2013. Influence of pyrolysis

temperature on composted sewage sludge biochar priming effect in a loamy soil. Chemosphere, 93(4),

668-676.

23. Nipattummakul, N., Ahmed, I., Kerdsuwan, S., Gupta, A.K. 2010. High temperature steam

gasification of wastewater sludge. Applied Energy, 87(12), 3729-3734.

24. Pan, J., Li, G., Chen, Z., Chen, X., Zhu, W., Xu, K. 2009. Alternative block polyurethanes based on

poly(3-hydroxybutyrate-co-4-hydroxybutyrate) and poly(ethylene glycol). Biomaterials, 30(16),

2975-84.

25. Park, J.K., Song, M.J. 1998. Feasibility study on vitrification of low-and intermediate-level

radioactive waste from pressurized water reactors. Waste Management, 18(3), 157-167.

26. Pedroza, M.M., Sousa, J.F., Vieira, G.E.G., Bezerra, M.B.D. 2014. Characterization of the products

from the pyrolysis of sewage sludge in 1 kg/h rotating cylinder reactor. Journal of Analytical and Applied

Pyrolysis, 105(0), 108-115.

27. Qin, H.-z., Liu, Y.-y., Li, L.-q., Pan, G.-x., Zhang, X.-h., Zheng, J.-w. 2012. Adsorption of Cadmium

in Solution by Biochar From Household Biowaste. Journal of Ecology and Rural Environment, 28(2),

181-186. (in Chinese)

28. Remenarova, L., Pipiska, M., Hornik, M., Rozloznik, M., Augustin, J., Soja, G. 2012. CADMIUM

AND ZINC SORPTION FROM SINGLE AND BINARY SOLUTIONS BY STRAW BIOCHAR: THE ROLE

OF FUNCTIONAL GROUPS. Ict Press, Prague.

29. Rio, S., Faur-Brasquet, C., Le Coq, L., Le Cloirec, P. 2005. Structure characterization and adsorption

properties of pyrolyzed sewage sludge. Environmental Science & Technology, 39(11), 4249-4257.

30. Sing, K.S.W., Everett, D.H., Haul, R.A.W., Moscou, L., Pierotti, R.A., Rouquerol, J., Siemieniewska,

T. 1985. REPORTING PHYSISORPTION DATA FOR GAS SOLID SYSTEMS WITH SPECIAL

REFERENCE TO THE DETERMINATION OF SURFACE-AREA AND POROSITY

(RECOMMENDATIONS 1984). Pure and Applied Chemistry, 57(4), 603-619.

31. Singh, B., Singh, B.P., Cowie, A.L. 2010. Characterisation and evaluation of biochars for their

application as a soil amendment. Australian Journal of Soil Research, 48(6-7), 516-525.

32. Wu, H., Zhao, Y., Long, Y., Zhu, Y., Wang, H., Lu, W. 2011. Evaluation of the biological stability of

waste during landfill stabilization by thermogravimetric analysis and Fourier transform infrared

spectroscopy. Bioresource Technology, 102(20), 9403-9408.

33. Yuan, H., Lu, T., Zhao, D., Huang, H., Noriyuki, K., Chen, Y. 2013. Influence of temperature on

product distribution and biochar properties by municipal sludge pyrolysis. Journal of Material Cycles

and Waste Management, 15(3), 357-361.

34. Zhai, Y., Peng, W., Zeng, G., Fu, Z., Lan, Y., Chen, H., Wang, C., Fan, X. 2012. Pyrolysis

characteristics and kinetics of sewage sludge for different sizes and heating rates. Journal of Thermal

Analysis and Calorimetry, 107(3), 1015-1022.

35. Zhang, B., Xiong, S., Xiao, B., Yu, D., Jia, X. 2011. Mechanism of wet sewage sludge pyrolysis in a

tubular furnace. International Journal of Hydrogen Energy, 36(1), 355-63.

32

Highlights

� Municipal sewage sludge was pyrolyzed at various temperature.

� Pyrolysis temperature influences the properties of biochar strongly.

� Biochars perform better than commercial activated carbon on heavy

metal adsorption.

� The purpose of both energy recovery and heavy metal removal can

achieve at 900 °C.