全 文 ：Chinese Journal of Chemical Engineering, 16(3) 401—406 (2008)
Peanut Shell Activated Carbon: Characterization, Surface Modification
and Adsorption of Pb2+ from Aqueous Solution
XU Tao (徐涛) and LIU Xiaoqin (刘晓勤)*
State Key Laboratory of Materials-oriented Chemical Engineering, Nanjing University of Technology, Nanjing
210009, China
Abstract Metal ion contamination of drinking water and waste water, especially with heavy metal ion such as
lead, is a serious and ongoing problem. In this work, activated carbon prepared from peanut shell (PAC) was used
for the removal of Pb2+ from aqueous solution. The impacts of the Pb2+ adsorption capacities of the acid-modified
carbons oxidized with HNO3 were also investigated. The surface functional groups of PAC were confirmed by Fou-
rier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), Boehm titration. The textural
properties (surface area, total pore volume) were evaluated from the nitrogen adsorption isotherm at 77 K. The ex-
perimental results presented indicated that the adsorption data fitted better with the Langmuir adsorption model. A
comparative study with a commercial granular activated carbon (GAC) showed that PAC was 10.3 times more effi-
cient compared to GAC based on Langmuir maximum adsorption capacity. Further analysis results by the Langmuir
equation showed that HNO3 [20% (by mass)] modified PAC has larger adsorption capacity of Pb2+ from aqueous
solution (as much as 35.5 mg·g－1). The adsorption capacity enhancement ascribed to pore widening, increased
cation-exchange capacity by oxygen groups, and the promoted hydrophilicity of the carbon surface.
Keywords activated carbon, phosphoric acid, adsorption, wastewater treatment, surface modification
1 INTRODUCTION
In recent years, lead ions have been introduced
into natural water from a variety sources such as acid
battery manufacturing, metal plating and finishing,
tetraethyl lead manufacturing, mining, ammunition
and the ceramic glass industries [1]. Lead poisoning in
human causes severe damage not only to the kidney
and liver, but also to nervous and reproductive sys-
tems. Therefore, there are tremendous efforts on re-
ducing the concentration of heavy metals in effluent
wastewaters in view of environmental protection as
well as to meet stringent permissible discharge levels.
The problems connected with lead ions pollution
are abated by processes such as chemical precipitation,
ion exchange, liquid membrane extraction, electrode
deposition, activated carbon adsorption and biological
processes [2]. Among these methods, carbon adsorp-
tion is the most attractive one because of its efficiency,
economical feasibility and the ease for the treatment
of wastewater containing lead ions.
China ranks first in peanut production, which
represents a potential of 4500 thousand tons of peanut
shells produced each year [3] and this material has
shown to be an excellent source of high quality and
low cost activated carbon. Recently, it has been found
that activated carbon obtained from peanut shell
treated with phosphoric acid at moderate temperatures
(400 and 500°C) has high specific surface area and
pore volume, as well as exhibiting a large adsorption
capacity towards heavy metal ions such as Cu2+, Zn2+,
Pb2+ and Ni2+ [4-7]. Wilson et al. [8] have shown that
activated carbon obtained from peanut shell has metal
ion adsorption efficiencies greater than two commer-
cial carbons. However, the previous studies don’t give
the theoretical explanations to the influence of surface
functional groups of the peanut shell activated carbon
on metal ion adsorption capacities, and there is a lack
of information on the surface modified H3PO4-activated
peanut shell activated carbon and its performance in
removing lead ions from aqueous solutions.
The primary objective of this study is to discuss
the lead ion adsorption capacity of H3PO4-activated
peanut shell carbon and acid surface modified peanut
shell activated carbons. The effects of important fac-
tors such as the metal ion concentration of the solution
and the surface functional groups of the adsorbents on
the equilibrium capacities are discussed.
2 MATERIALS AND METHODS
2.1 Adsorbate
A stock solution of Pb2+ (100 mg·g－1) was pre-
pared by dissolving Pb(NO3)2 in deionized water and
acidified with 10 ml of concentrated HNO3 (pH＝1)
to prevent hydrolysis.
The stock Pb2+ solution was diluted using deion-
ized water to different lead ion concentration from 2
mg·g－1 to 80 mg·g－1 for adsorption experiments.
2.2 Adsorbent
Peanut shells were obtained from Shandong
province of China and they were milled to between
0.85-2.00 mm (10 and 20 mesh). The milled peanut
shells were mixed with phosphoric acid solution (58%
by mass) (Shanghai Chemical Agent Inc., China) at a
ratio of 1︰1 by mass (shells: H3PO4 solution). The
mixture was stirred well for 2 h and then conveyed to
a stainless steel tube with a diameter of 5 cm and a
length of 80 cm which was placed in a tube heating fur-
nace (Shanghai Experimental Furnace Inc., Shanghai).

Received 2007-09-10, accepted 2008-04-21.
* To whom correspondence should be addressed. E-mail: liuxq@njut.edu.cn
Chin. J. Chem. Eng., Vol. 16, No. 3, June 2008 402
The dry sample was heated at a heating ramp of
10°C·min－1 in a nitrogen atmosphere and was held at
550°C for 2 h. After natural cooling to room tempera-
ture the peanut shell carbon samples were crushed to
less than 1 mm size; and all the carbon samples were
weighted and washed with 0.1 mol·L－1 HCl to remove
surface ash. Then the carbons were washed with de-
ionized water to remove the hydrochloric acid and
dried in a vacuum drying oven at 150°C overnight.
The obtained carbons were placed in a desiccator, and
allowed to cool for 24 h. The yield of peanut shell
activated carbon in this experiment is about 40% (by
mass). The experimental apparatus for preparation of
peanut shell activated carbon was shown in Fig. 1.

Figure 1 The experimental apparatus for preparation of
activated carbon
1—temperature controller; 2—tube heating furnace; 3—stainless
steel tube; 4 — thermoelectric couple; 5 — vacuum pump;
6—nitrogen cylinder
The peanut shell activated carbon was then
treated with HNO3 at different volume concentrations
(5%, 20% and 50%). For the treatment with HNO3,
the PAC was mixed with different acid concentrations
and the suspension (1 g of PAC per 10 ml of nitric
solution) was stirred at 90°C until brown gas, which
was considered as NO2, stopped to evolve (about 2 h).
After stirring, the mixture was cooled and then
washed thoroughly with distilled water until all ni-
trates were eliminated. Four carbon samples including
raw peanut shell carbon and HNO3-modified peanut
shell carbon are described in Table 1.
Table 1 Description of test peanut shell activated carbons
Sample Conditions of preparation and modification
PAC impregnation with 58% (by mass) H3PO4 in acid/precursor ratio of 1 and heat treated for 2 h at 550°C
PAC-N1 PAC treated with 5% HNO3
PAC-N2 PAC treated with 20% HNO3
PAC-N3 PAC treated with 50% HNO3
It is known that granular carbons have been
proven to be highly effective adsorbents for the re-
moval of heavy metal ions from aqueous solution. For
the sake of comparison, a commercial activated car-
bon for water purification obtained from Wuhan Lin-
feng Inc., Hubei is also used.
2.3 Determination of porous characteristics
This was performed by the adsorption of N2 at 77 K
using Micrometrics Surface Area Analyzer (ASAP2010,
Micromeritics Inc., USA). Before measuring the ad-
sorption of N2, the sample was subjected to degassing
for 2 h at a final pressure of 133.32×10－4 Pa. The total
pore volume (Vp) was estimated from the volume of
N2 (as liquid) held at a relative pressure (P/P0) of 0.95.
2.4 Surface chemistry characterization
FTIR spectra
The untreated peanut shell activated carbon and
HNO3 treated activated carbons (PAC-N1, PAC-N2
and PAC-N3) were examined using Fourier transform
infrared (FTIR) spectroscopy. The spectra of the sam-
ples were recorded between 4000 and 400 cm－1 using
a AVATAR-360 FTIR spectrometer (Thermo Nicolet
Inc., USA).
XPS
Sample PAC was inserted into the EscaLab 210
(V.G. Scientific Ltd.) photoelectron spectrometer using
a nonmonochromatized AlKα radiation (1486.6 eV),
the source operated at 15 kV and 34 mA. The
high-resolution scans were performed over the
280-294, 527-540, 128-138eV ranges (C1s, O1s, P2p
spectra, respectively) for samples.
2.5 Adsorption of lead ion
For a given set of analysis, 300 mg of peanut
shell activated carbon or commercial GAC was placed
in some 80-ml beakers, and 30 ml of a lead ion solu-
tion was added to each. Then the beakers were placed
in a thermostatic swing bed and agitated at a constant
temperature of 25°C. The equilibrated solution was
subjected to ICP-AES (Optima 2000 DV, Perkin
Elmer, USA) analysis in order to determine the con-
centration of residual lead ion. The difference between
the initial and final lead concentration was used to
calculate the amount of lead ion adsorbed.
3 RRESULTS AND DISCUSSION
3.1 Pore structure and surface functional groups
The results of the structural investigation of sam-
ples with the following carbon surface modification
are listed in Table 2. The decrease in the BET surface
area and the widening of the micropore structure
brought about by strong oxidants may be attributed to
the destruction of the pore walls. It was found that the
physical morphology of the PAC is also affected by
the strength of the oxidizing agent. When nitric acid
oxidation is carried out using solution concentrations
Table 2 Selected structural properties of
the carbon samples
Carbon SBET/m2·g－1 Vtotal/cm3·g－1
PAC 1019 0.754
PAC-N1 910 0.693
PAC-N2 867 0.621
PAC-N3 559 0.377
GAC 1057 0.639
Chin. J. Chem. Eng., Vol. 16, No. 3, June 2008 403
higher than 20%, the texture is greatly changed. Se-
vere oxidation at extreme conditions practically de-
stroys the porous structure of the original PAC due to
the erosion of the pore walls. Hence HNO3 oxidation
not only alters the chemical properties of the PAC
surface but also its texture.
3.2 Surface chemistry
3.2.1 FTIR spectra
Infrared spectroscopy provides information on
the chemical structure of the adsorbent material. Fig. 2
shows FTIR spectra of the synthetic peanut shell car-
bon (PAC) and the acid modified peanut shell carbons
(PAC-N1, PAC-N2 and PAC-N3). The most charac-
teristic change is observed with the range of 1700-1730
cm－1. The band centered at 1720 cm－1 is ascribed to
the stretching vibrations of carboxyl groups on the
edges of layer planes or to conjugated carbonyl groups
(C O in carboxylic acid and lactones groups). The
spectra of oxidized carbons PAC-N1, PAC-N2 and
PAC-N3 show a considerable amount of carboxyl
groups in comparison to the non-treated carbon sam-
ple (PAC). The relative intensity of this band increases
with increasing HNO3 concentration up to 20% and
then it decreases.
The broad band between 1300 and 900 cm－1 in
PAC, PAC-N1 and PAC-N2 samples has a maximum
at 1070-1080 cm－1. The absorption in this region is
the characteristic for phosphorus and phosphor carbo-
naceous compounds. The peak at 1070-1080 cm－1
may be ascribed to ionized linkage P+ O－ in acid
phosphate esters and to symmetrical vibration in a
chain of P O P (polyphosphate). The shoulder at
1180-1220 cm－1 may be assigned to the stretching
mode of hydrogen-bonded P O, to O C stretching
vibrations in P O C (aromatic) linkage and to
P OOH [9]. The spectra (Fig. 2) therefore suggest the
formation of P-containing carbonaceous structures
like acid phosphates and polyphosphates in synthetic
phosphoric acid-activated peanut shell carbons.
3.2.2 XP spectra
For the sake of a better understanding of the sur-
face chemical characteristics of the H3PO4-acticated
peanut shell carbon, XPS Peak 4.1 is employed to de-
convolve the high resolution C1s, O1s and P2p peaks, as
shown in Fig. 3. The high resolution C1s, O1s and P2p

Figure 2 FTIR spectra of raw peanut shell carbon (PAC) and HNO3-oxidized peanut shell carbon samples
(PAC-N1, PAC-N2 and PAC-N3)

Figure 3 The C1s, O1s, P2p XPS spectra of untreated PAC
Chin. J. Chem. Eng., Vol. 16, No. 3, June 2008 404
spectra reveal the presence of several peaks for each
elemental composition as well as the range of separate
peak positions corresponding to the different binding
energies (BE) and their relative peak area are shown in
Table 3.
The deconvolution of the P2p spectra yielded two
main peaks [10]: peak1 (134.0-134.6 eV), metaphos-
phate (M[PO3]n, n＝2-8); peak2 (133.6-134.7 eV),
(PhO)3PO compounds (Ph presents the phenyl group,
C6H5). In either case, the P atom is bonded to four O
atoms by one double bond and three single bonds.
When P is bonded to one C atom and three O atoms
[e.g., in CH3OP(OH)2], the P2p peak moves to ca.
133.6 eV. As can be seen in Table 3, the relative peak
area of peak 2 is larger than that of peak 1 (9.91 %
versus 6.87 %), so the above assignment suggests the
P atom in our peanut shell activated carbon (PAC) is
mainly present on the carbon surface by bonding to O
atoms. This result fitted well to the result of the PAC
FTIR analysis.
3.3 Pb2+ equilibrium adsorption isotherms
Langmuir equation and Freundlich equation [11]
were applied for adsorption equilibrium for both PAC
and GAC. Adsorption isotherm data for Pb2+ adsorp-
tion were plotted and presented in Fig. 4 (a). Equilib-
rium data obtained for the two adsorbents were fitted
to the Langmuir and Freundlich isotherms. The fol-
lowing expressions of a straight line were used, found
by means of mathematical transformation of isotherms.
For Langmuir isotherm:
e e
e 0 0
1C C
Q Q b Q
= + (1)
where Ce is the equilibrium concentration (mg·L
－1),
Qe is the amount adsorbed at equilibrium (mg·g
－1), Q0
(mg·g－1) and b (mg－1) are Langmuir constants related
to maximum adsorption capacity and energy of ad-
sorption, respectively.
For Freundlich isotherm:

(a)

(b)
Figure 4 Adsorption isotherm of Pb2+ on untreated PAC
and GAC (a); HNO3 treated PAC-N1, PAC-N2 and PAC-N3 (b)
(pH＝2.5; agitation time, 24 h; PAC concentration, 10 g·L－1;
initial Pb2+ concentration (C0) : 2 mg·L
－1, 4 mg·L－1, 8 mg·L－1,
16 mg·L－1, 32 mg·L－1 and 64 mg·L－1)
—— Langmuir; - - - - Freundlich
e f e
1ln ln lnQ K C
n
= + (2)
where Ce is the equilibrium Pb2+ concentration (mg·L
－1),
and Kf and n are the Freundlich constants incorporat-
ing all the factors effecting adsorption capacity, an
indication of favorability of Pb2+ adsorption onto ad-
sorbent. The values of the Langmuir and Freundlich
Table 3 Fitted C1s, O1s and P2p peak parameters deduced from XPS spectra for PAC carbon sample
PAC
Peak Binding energy/eV Possible assignment
Relative peak area/% Full-width at half-maximum/eV
C1s 284.5±0.1 C* H, non-functionalized sp2 C 96.86 2.703
(81.91)① 286.1±0.1 C* OH, phenolic, alcohol or ether groups 0.85 1.597
288.7±0.1 C* O, ester groups 0.28 1.034
289.5±0.3 COOH, carboxyl acidic groups 1.99 1.384
O1s 530.6±0.2 C* O, carbonyl groups 60.5 2.854
(15.58)① 532.3±0.3 C* OH, ester groups 14.2 1.582
533.5±0.2 C* O C*, ether oxygen 22.1 2.181
534.3±0.2 COOH, carboxyl acidic groups 3.37 1.034
P2p 132.9±0.4 Na2HPO4 standard peak 84.1 3.072
(2.51)① 134.7±0.3 (PhO3)PO (Ph＝ C6H5) 9.91 0.513
134.2±0.4 M[PO3]n (n＝2-8) (metaphosphate) 6.87 2.796
① Atomic percentage of carbon, oxygen, phosphor from XPS data.


Chin. J. Chem. Eng., Vol. 16, No. 3, June 2008 405
isotherm constants at different Pb2+ concentration for
Pb2+ removal were calculated from the slope and in-
tercept of the plots of Ce/Qe versus Ce and lg Qe versus
lg Ce, respectively, and the results are shown in Table 4.
Table 4 Langmuir and Freundlich isotherm constants for
PAC, modified PACs and GAC
Langmuir constants Freundlich constants
Adsorbents
Q0/mg·g－1 b/mg－1 R2 Kf 1/n R2
GAC 2.337 0.3196 0.9978 0.8588 0.2510 0.9090
PAC 24.02 0.3684 0.9984 9.540 0.2632 0.9299
PAC-N1 25.21 0.3435 0.9992 9.482 0.2664 0.9609
PAC-N2 35.46 0.3927 0.9992 13.36 0.2632 0.9376
PAC-N3 10.93 0.3032 0.9997

3.824 0.2773 0.9074

As presented in Table 4, a high value of coefficient
of correlation, R2 for both PAC and GAC (0.9984 and
0.9978, respectively) indicates good agreement be-
tween experimental and predicted data using the
Langmuir equation. Q0 and b were determined from
the Langmuir plots and listed in Table 4. It is noted
that the ratio of Q0 values of PAC and GAC works out
to be 10.3.
Figure 4 (b) presents the adsorption isotherms for
the uptake of Pb2+ on HNO3-modified peanut shell
activated carbons PAC-N1, PAC-N2 and PAC-N3. The
correlation coefficients for the linear Langmuir re-
gression fits are much larger than that for the
Freundlich plot (also presented in Table 4). So the
Langmuir model could describe well the adsorption
isotherms for the uptake of Pb2+ from aqueous solu-
tion throughout the whole range of concentrations.
Liquid-phase oxidation with HNO3 (20%, by volume)
greatly enhances the adsorption capacity of PAC (up
to 35.46 mg·g－ 1), since the HNO3-treated carbon
showed intensely oxidized carbon surfaces and thus
possessed more adsorption sites available for the se-
questration of lead ions from aqueous solution. The
existence of surface oxygen functionalities was de-
tected in this work, these acidic groups lead to a very
low point of zero charge [12] values and greatly in-
crease the surface charge, which makes Pb2+ more
accessible to the inner pores.
However, the lowest removal capacity is
achieved by the strong nitric acid solution (50%, by
volume) oxidized peanut shell activated carbon
(PAC-N3) (low to 10.93 mg·g－1). This result may be
attributed to the loss of the PAC surface area and the
total pore volume. As presented in Table 2, the strong
nitric acid solution (50%, by volume) treatment re-
duces the surface area of PAC by 45.2 %, and the pore
volume (Vtotal) reduces from 0.754 cm3·g
－1 to 0.377
cm3·g－1. Some reports have found that treatment with
HNO3 can substantially affect the physical morphol-
ogy of the activated carbons [13, 14]. In this work,
drastic conditions used in the oxidation step lead to
the collapse of some pore walls, so the very low sur-
face area and pore volume lead to the small adsorption
capacity of PAC-N3.
In accordance with the results of spectroscopic and
adsorption studies, the mechanism of adsorption of
Pb2+ may be explained based on the Bronsted-Lowry
acid-base mode l [15] and ion exchange model [16].
For H3PO4-activated peanut shell carbon (PAC) and
HNO3-oxidized peanut shell carbon (PAC-N1, PAC-N2
and PAC-N3), some carbon-oxygen complexes ( COOH,
C* OH and C* O C*) and phosphor-oxygen com-
plex (POx) are usually present, which render the car-
bon surface slightly polar. And these surface oxy-
gen-contained complexes hydrolytic water molecules
may ion exchange with Pb2+. The reactions mecha-
nism may be shown as follows:
COOH + Pb2+ + H2O→ COOPb+ + H3O+ (3)
C* OH+Pb2++H2O→C* OPb+ + H3O+ (4)
( COOH)2 + Pb2+ + 2H2O→ ( COO)2Pb + 2H3O+
(5)
C* O C* + 2H2O→ 22 2C OH + + 2OH－
2( 22 2C OH + ) + Pb2+→ (C2O)2Pb2+ + 4H+ (6)
POx + xH2O→ P(OH)x+ + x(OH)－
2P(OH)x+ + Pb2+→ 2(POPb)x+ + 2xH+ (7)
The reactions proposed above present differences
in adsorption capability and indicate a variety in
mechanism of interaction between carbon surface and
ionic metal spices present in aqueous solution.
4 CONCLUSIONS
Results on adsorption of lead on peanut shell ac-
tivated carbon showed that this material is an effective
adsorbent for the removal of Pb2+ from aqueous solu-
tion. It could be employed for the economic treatment
of wastewater containing the heavy metal studied, as
this adsorbent was derived from an agricultural waste
by-product and had a high adsorption capacity.
A comparative study with a commercial activated
carbon showed that H3PO4-activated peanut shell car-
bon is 10.3 times more efficient compared to GAC
based on the Langmuir adsorption capacity. The XPS
and FTIR studies indicated that some surface functional
groups such as carbonyl, carboxyl and phosphor- oxy-
gen complexes were detected and the HNO3 modified
peanut shell activated carbon generated a significantly
large number of these functional groups, which sup-
ports a large adsorption capacity of Pb2+ from the
aqueous solution.
The chemical characters of PAC surface domi-
nated the Pb2+ adsorption process and the mechanism
of Pb2+ removal could be explained on the base of ion
exchange model and Bronsted-Lowry acid-base model.
Removal of Pb2+ from aqueous solution was consid-
erably enhanced by chemical HNO3 oxidation, which
may be related to pores widening, increased
cation-exchange capacity by oxygen groups, and the
promoted hydrophilicity of the carbon surface.
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