Accepted Manuscript Title: A comparison of biochars from lignin, cellulose and wood as the sorbent to an aromatic pollutant Author: Jianfa Li Yimin Li Yunlu Wu Mengying Zheng PII: DOI: Reference:

S0304-3894(14)00689-X http://dx.doi.org/doi:10.1016/j.jhazmat.2014.08.033 HAZMAT 16207

To appear in:

Journal of Hazardous Materials

Received date: Revised date: Accepted date:

29-4-2014 29-7-2014 11-8-2014

Please cite this article as: J. Li, Y. Li, M. Zheng, A comparison of biochars from lignin, cellulose and wood as the sorbent to an aromatic pollutant, Journal of Hazardous Materials (2014), http://dx.doi.org/10.1016/j.jhazmat.2014.08.033 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Highlights Wood biochar’s composition is close to cellulose biochar obtained at same HTT.

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Wood biochar’s properties were not dominated by lignin despite its recalcitrance. Lignin biochar had a sorption capacity comparable to cellulose and wood biochars.

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Lignin biochar is superior to other biochars on efficient biomass utilization.

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High partition contribution favors desorption of nitrobenzene from biochars.

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A comparison of biochars from lignin, cellulose and wood as the sorbent to an aromatic pollutant

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Jianfa Li*, Yimin Li, Yunlu Wu, Mengying Zheng

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College of Chemistry and Chemical Engineering, Shaoxing University, Shaoxing,

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Zhejiang 312000, P.R. China

* Corresponding author: College of Chemistry and Chemical Engineering, Shaoxing

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+86 575 8834 1524; fax: +86 575 8834 1521

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University, Huancheng West Road 508, Shaoxing, Zhejiang 312000, P.R. China. Tel:

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E-mails: [email protected] (Jianfa Li); [email protected] (Yimin Li)

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ABSTRACT Biochars’ performance as the sorbent to pollutants is dependent on their compositions

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and surface characteristics, which are then related to the feedstock used for biochar preparation. The objective of this work is to probe the feedstock’s influence on

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biochar’s sorption property through a comparative study on biochars from lignin,

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cellulose and wood prepared at 400 ºC and 600 ºC, respectively. Elemental and spectral analyses demonstrated that the wood biochar had a composition and carbonization

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degree close to the cellulose biochar but much different from the lignin biochar

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prepared at the same temperature, suggesting that lignin is not dominant to properties of plant-derived biochars. The lignin biochar showed a sorption capacity comparable to

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both cellulose and wood biochars as the sorbent to nitrobenzene, with a higher partition

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contribution to the total sorption due to the lower carbonization of lignin. In general, the

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lignin biochar is a good candidate of sorbent to aromatic pollutants, and is advantageous over the other two species with its efficient carbon utilization. Keywords: Biochar; Lignin; Sorption; Nitrobenzene; Desorption

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1. Introduction Biochars have received growing attentions in recent years, because of their great

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potential in carbon sequestration and soil improvement [1]. Generally, biochars are produced by pyrolysis of biomass at a relatively low to moderate temperature (200~700

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°C) and in an oxygen-limited or inert atmosphere. Due to the particular properties of

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these carbon-rich products, biochars are good sorbents for many environmental pollutants such as heavy metals [2-4], aromatics [5, 6], pesticides [7, 8] and

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pharmaceuticals [9]. According to previous research results, the sorption capacity of

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biochars is closely related to their carbonization degree and surface properties, such as specific surface area (SSA) and functional groups, which are then dependent on the

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pyrolysis process (particularly heat treatment temperature (HTT)) and feedstock [10, 11].

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For example, the biochars obtained at relatively high HTT (>500°C) showed high

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degree of carbonization, as indicated by low H/C atomic ratios and high specific surface areas [12]. In addition, the elemental compositions and surface characteristics of biochars also rely on the types of feedstock used for pyrolysis [13]. In spite of the variety of biomass types, most of them contain carbohydrate biopolymers (e.g. cellulose and lignin) as the major carbonizable components. Therefore, investigation on the thermal decomposition behavior of these biopolymers is useful for predicting the compositions and properties of plant-derived biochars, as well as their sorption behaviors to pollutants. Previous research has discovered that cellulose is readily decomposed at a relatively low temperature (0.4) comparable to both cellulose and wood biochars. When biochars from same feedstock obtained at different temperatures were

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compared, it can be seen that the two lignin biochars (LG600 and LG400) showed

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different sorption behavior. The higher sorption of nitrobenzene by LG600 biochar was

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observed at lower aqueous concentrations (Ce/Cs0.1). However, the biochars prepared at higher temperature (600 ºC) (CL600 and PW600) from both cellulose and wood feedstocks showed higher sorption than those obtained at 400 ºC (CL400 and PW400). To elucidate the different sorption behaviors of nitrobenzene by various biochars, the sorption isotherms were analyzed by Freundlich equation which is expressed as the below,

log Qe = log K F + N sorp log Ce

(eq. 1)

Where Qe is the amount sorbed per unit weight of sorbent at equilibrium (mg/kg), Ce is

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the equilibrium concentration (mg/L). The regressed parameters are listed in Table 3. All the sorption isotherms fit the Freundlich equation (eq. 1) well with correlation

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coefficient R2>0.98. Nsorp values close to 1 for the isotherms of cellulose and wood are indicative of a partitioning-dominant mechanism of nitrobenzene sorption by the two

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feedstocks, which is in agreement with previous reports [17]. The alkali lignin is soluble

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in water within 24 h, so its sorption to nitrobenzene was not measured. Fitting the sorption data by charred samples with Freundlich equation (eq. 1) gives Nsorp values

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ranging from 0.193 to 0.342, and logKF values from 4.29 to 4.83, both are comparable

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to the published values of nitrobenzene sorption by pine-needle derived biochars [5]. However, these Freundlich parameters are not adequate for exploring the various

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sorption mechanisms of biochars from different types of biomass feedstock. Particularly,

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the obvious differences on chemical compositions (e.g. H/C and O/C ratios) and surface

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characteristics (e.g. SSA and porous structures) make the comparison complicated. (Fig. 4 and Table 3 here)

For this reason, the mixed adsorption and partition mechanism proposed by Chen et

al. [5] was used here to investigate the different sorption behaviors of these biochars. Namely, the total sorption (QT=Qe) of nitrobenzene by biochars can be defined as the sum of the amounts contributed by adsorption (QA) and by partition (Qp), respectively. Considering the saturation of surface adsorption and progressive increase of partition contribution at relatively high equilibrium concentration in aqueous phase, the equation (2) is then transformed into a linear relationship between Qe and Ce as shown in

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Page 16 of 35

equation (3). (eq. 2)

QT = Qe = Q Amax + K PCe

(eq. 3)

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QT = Q A + QP

where QAmax is the saturated adsorption capacity of a biochar, and KP the partition

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coefficient. According to the isotherm shape shown in Figs.4(a-c), the linear increase of

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Qe with Ce was observed at high solution concentrations, so the linear regression was

conducted at the concentrations of Ce/Cs>0.1. The regression results are shown in Table

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3 with correlated coefficients R2>0.90. Then the relative contribution of partition (CP, %)

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to the total sorption at each equilibrium concentration can be calculated by equation (4), and change of CP (%) with Ce (mg/L) for various biochars is shown in Fig. 5. CP = ( K PCe / Qe ) × 100%

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(Fig. 5 here)

(eq. 4)

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As can be seen, the highest contribution of partition to the total sorption was observed on LG400 biochar, corresponding to the lowest degree of carbonization as concluded by elemental analysis and surface characterization. The recalcitrance of lignin makes it only partially carbonized, so the LG400 biochar contained relatively high amount of NOMs, which acted as the partition phase for absorption of nitrobenzene [5]. In contrast, the least contribution of partition was obtained on CL600 biochar with the highest degree of carbonization and lowest content of partition phase, implying a sorption mechanism dominated by surface adsorption on this biochar. The sorption of nitrobenzene by other biochars can be attributed to a mechanism of surface

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adsorption combined with partition, and the relative contribution of partition increased with the equilibrium concentration according to Fig. 5. Considering the C-containing

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fraction of various biochars acted as the partition phase, the C-normalized partition coefficient KOC was calculated by dividing KP by C-content of biochars [22]. When KOC

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values for biochars obtained at the same temperature were compared, the contribution of

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partition to the total sorption decreased in the order: LG400 > PW400 > CL400, and LG600 > PW600 > CL600. But KOC value for the low temperature (400 ºC) biochar is

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apparently higher than that for high temperature (600 ºC) biochar from the same

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feedstock, suggesting higher contribution of partition to the total sorption for the low temperature biochars [5]. The ranks are consistent with the decreasing order of O/C

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atom ratios (as indicative of polarity) of these biochars.

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The SSA-normalized surface adsorption ( QA,SSAmax = Q Amax / ( SSAi MWnitrobenzene ) [30]

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was calculated for evaluating the contribution of surface adsorption to the total sorption. The calculated QA,SSAmax values for three biochars prepared at high HTT (600 ºC) are comparable to the theoretical maximum mono-layer adsorption capacity of nitrobenzene (2.677 μmol/m2) [5]. Among these three biochars, the relatively lower QA,SSAmax value was obtained for the PW600 biochar, which may be related to its developed micro-pores. And the somewhat higher QA,SSAmax value for the CL600 biochar is corresponding to its developed meso-pores, in which nitrobenzene molecules may overlap easily on the adsorption surface. However, for the three biochars obtained at relatively low temperature (400 ºC), the calculated QA,SSAmax values are much higher than the

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theoretical value. The reason may be attributed to the stronger specific interactions between nitrobenzene and surface polar groups of low temperature biochars [5, 23],

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when the higher polarity of these biochars than those high temperature (600 ºC) biochars was considered. Besides, the unusually high QA,SSAmax values should be a result

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of underestimated N2-BET SSA for these low-temperature biochars, in which the

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condensed NOMs may block the entry of N2 into porous structure in the N2-adsorption measurement at 77 K [29, 31].

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Desorption hysteresis was observed in the sorption-desorption cycle of nitrobenzene

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by each biochar (Figs. 4(a-c)), and such kind of partial irreversibility for sorption by other biochars has also been reported previously [32-34]. If artificial causes were

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eliminated, desorption hysteresis is generally linked to a pore filling mechanism [31,

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35]. Braida et al. [36] observed the swelling of charchoal by sorption of benzene, and

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proposed that desorption hysteresis was related to pore deformation, which resulted in the entrapment of some benzene molecules in micropores. This explanation is well applicable for the fraction of sorbate adsorbed on the biochar surface, but may not for the fraction partitioning to NOMs. To evaluate the contribution of partition to the desorption hysteresis, the hysteresis indices ( HI = N desorption / N sorption ) was calculated for each sorption-desorption cycle, and results are shown in Table 4. As can be seen, the highest HI, also implying the most readily desorption, was obtained for LG400 biochar, corresponding to a highest KOC value as indicative of partition contribution in sorption. In contrast, the lowest HI was got for desorption from the CL600 biochar, and

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corresponding to the lowest KOC value for the sorption. Interestingly, a positive relationship of HI with KOC was obtained as shown in Fig. 6. The results suggest that the

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fraction of nitrobenzene absorbed in the partition phase is more liable to be desorbed than that adsorbed in the biochar surface. In the view of partition contribution to the

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sorption (KOC) and desorption (HI), the sorption property of PW400 biochar to

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nitrobenzene is close to the CL400 biochar, which may be related to their similar compositions. And if the three biochars obtained at a relatively higher temperature (600

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ºC) were compared, we can see that the sorption/desorption property of wood biochar

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(PW600) is between that of lignin biochar (LG600) and cellulose biochar (CL600), implying the increasing influence of lignin as an biomass component on the sorption

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4. Conclusions

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(Table 4 and Fig. 6 here)

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property of biochar prepared at higher HTT.

The biochars from lignin, cellulose and wood are different in elemental

compositions, surface characteristics and sorption properties. Generally, the wood biochar has a yield and composition close to the cellulose biochar, while the lignin biochar showed its difference with much higher yield and O/C atomic ratio. The wood biochar contained more developed microporous structure than biochars from lignin and cellulose. When nitrobenzene was used as the sorbate, the lignin biochar showed sorption capacity comparable to both cellulose and wood biochars. However, due to the lignin’s recalcitrance, the partition contributed more to the sorption and desorption by

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the lignin biochar. The results herein are useful for predicting the biochars’ properties from feedstock of so much variety, and also suggest a potential way for efficient

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utilization of lignin as the industrial byproduct.

Acknowledgment

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This work was supported by the National Natural Science Foundation of China

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(41271475) and Research Project for Public Welfare of Shaoxing City, China (2012B70084).

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Appendix A. Supplementary data

Supplementary data associated with this article can be found, in the online version,

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at xxxxxx.

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[34] S.M. Martin, R.S. Kookana, L.Van Zwieten, E. Krull, Marked changes in herbicide sorption–desorption upon ageing of biochars in soil, J. Hazard. Mater. 231–232

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Figure captions Fig. 1 IR spectra of biochars and feedstocks

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Fig. 2 Pore size distribution of biochars Fig. 3 SEM images of biochars

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Fig. 4 Sorption and desorption isotherms of nitrobenzene by biochars ((a) lignin

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biochars, (b) cellulose and cellulose biochars, and (c) wood and wood biochars) Fig. 5 Change of partition contribution (CP) with equilibrium concentration (Ce)

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Fig. 6 Relationship of hysteresis index (HI) with partition coefficient (KOC)

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Table 1 Yields and elemental compositions of biochars and feedstocks Yield

& biochar

(%)

C-yielda (%)

pHb

Elemental compositions C(%) H(%)

O(%)

C+H+O(%)

H/Cc

O/Cc

1.35

0.530

1.83

0.868

1.63

0.716

84.5

0.675

0.260

95.6

0.542

0.120

100

100

9.01

47.1

5.31

33.3

85.8

Cellulose

100

100

6.60

42.1

6.43

48.7

97.2

Wood

100

100

5.62

46.6

6.33

44.5

97.4

60.2

3.39

20.9

65.2

83.3

CL400

23.7

44.6

6.72

79.3

3.59

12.7

PW400

29.0

47.9

6.71

77.0

3.45

14.6

95.0

0.538

0.142

LG600

57.5

81.9

67.1

1.19

14.3

82.6

0.213

0.160

CL600

18.3

40.3

7.81

92.7

1.86

3.14

97.7

0.241

0.025

PW600

20.8

40.1

7.39

89.8

1.81

4.44

96.0

0.242

0.037

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10.4

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LG400

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10.5

cr

Lignin

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Biomass

Calculated by C − yield (%) = C(%)biochar ×Yieldbiochar / C(%) feedstock .

b

Measured at 24 h after hydration of 0.1 g biochar sample in 10 mL pure water.

c

Atomic ratios.

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a

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Table 2 Porous structure of biochars MPA,

ESA,

TPV,

MPV,

m2/g

m2/g

m2/g

cm3/g

cm3/g

0.00158

0

18.0

0.0275

0.00222

10.5

0.022

0.0119

1.04

CL400

21.9

PW400

33.3

/

1.24

3.88 22.8

APD, nm 89.1

ip t

LG400

SSA,

5.61

4.86

cr

Biochar

259

200

59.3

0.138

0.102

2.68

CL600

349

280

68.9

0.193

0.143

3.41

PW600

368

321

47.5

0.190

0.164

2.33

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LG600

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SSA: specific surface area; MPA: micropore area; ESA: external surface area; TPV:

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d

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total pore volume; MPV: micropore volume; and APD: average pore diameter.

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i cr us

Linear in high concentration (Ce/Cs>0.1)

Freundlich R2

KP

KOC

QAmax

(mL/g)

(mL/g)

(mg/g)

1.35±0.05

0.917±0.019

Wood

1.54±0.04

0.987±0.018

0.995

LG400

4.35±0.02

0.342±0.009

0.988

CL400

4.51±0.01

0.283±0.004

0.997

PW400

4.29±0.02

0.322±0.009

0.987

LG600

4.69±0.01

0.199±0.005

0.991

83.8±9.2

CL600

4.83±0.01

0.193±0.007

0.983

40.3±4.6

0.199±0.004

0.995

74.3±4.3

PW600

ce

4.71±0.01

0.994

ed

Cellulose

pt

Nsorp

Ac

logKF

M

Sample

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Table 3 Regression parameters of sorption isotherms by biochars and feedstocks

QSSAmax

R2

μmol/m2

13.7±0.3

32.5

-0.40±0.23

0.996

/

34.2±1.3

73.4

-1.13±0.82

0.986

/

92.2±5.4

0.987

720

0.926

52.1

0.972

20.9

120±7.7

0.905

3.77

43.5

199±3.1

0.916

4.63

82.8

138±3.2

0.976

3.04

181±8.0 88.1±9.4 111±7.2

300 111 145 125

141±6.7 85.8±5.5

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Table 4 Regression parameters of desorption isotherms by biochars Freundlich / Desorption Ndesorp

R2

HI

LG400

5.16±0.010

0.0892±0.0044

0.979

0.261

CL400

5.25±0.007

0.0351±0.0029

0.940

0.124

PW400

5.16±0.006

0.0455±0.0025

0.973

0.164

LG600

5.24±0.005

0.0273±0.0022

0.945

0.137

CL600

5.34±0.005

0.0128±0.0023

0.893

0.0663

PW600

5.27±0.005

0.0215±0.0020

0.920

us

cr

logKF

ip t

Sample

Ac ce p

te

d

M

an

0.108

29

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Ac ce p

te

d

M

an

us

cr

ip t

Figure 1

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Ac

ce

pt

ed

M

an

us

cr

i

Figure 2

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Ac

ce

pt

ed

M

an

us

cr

i

Figure 3

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Ac ce p

te

d

M

an

us

cr

ip t

Figure 4

Page 33 of 35

Ac

ce

pt

ed

M

an

us

cr

i

Figure 5

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Ac

ce

pt

ed

M

an

us

cr

i

Figure 6

Page 35 of 35