Journal of Cleaner Production 238 (2019) 117904

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Journal of Cleaner Production

journal homepage: www.elsevier.com/locate/jclepro

Combined physical and chemical activation of sludge-based adsorbent

enhances Cr(Ⅵ) removal from wastewater
Liguo Zhang a, **, Jingshi Pan a, Lei Liu b, Kang Song c, Qilin Wang d, *
a
School of Environment, South China Normal University, Guangdong Provincial Engineering Technology Research Center for Wastewater Management and
Treatment, Guangzhou, 510006, China
b
School of Environment and Civil Engineering, DongGuan University of Technology, DongGuan, 523808, China
c
State Key Laboratory of Freshwater Ecology and Biotechnology, Institute of Hydrobiology, Chinese Academy of Sciences, Wuhan, 430072, China
d
Centre for Technology in Water and Wastewater, School of Civil and Environmental Engineering, University of Technology Sydney, Ultimo, NSW, 2007,
Australia

a r t i c l e i n f o a b s t r a c t

Article history: To enhance the adsorption properties of sludge-based adsorbents (SBAs), the physical activation and the
Received 30 April 2019 combined physical and chemical activation were examined comparatively. The surface composition and
Received in revised form structure of modified SBAs were characterized using BET surface area, XRD and FTIR, the adsorption
19 July 2019
behavior of modified SBAs was investigated for Cr(Ⅵ) removal from wastewater. For CSU-NaOH, the
Accepted 2 August 2019
Available online 3 August 2019
optimum adsorption time and pH were 0.5 h and 2.5, respectively. The maximum adsorption capacity by
CSU-NaOH was 15.3 mg g
1 for Cr(VI) removal, which was over 2 times of those by carbonized sludge and
Handling editor: Prof. Jiri Jaromir Kleme carbonized sludge with urea addition at 25  C. The adsorption kinetics could be fitted with the pseudo
second-order model for three adsorbents, the adsorption isotherm could be fitted with Langmuir model.
Keywords: The thermodynamic analysis indicated that the adsorption process of three adsorbents for Cr(Ⅵ) removal
Sludge-based adsorbents was spontaneous and endothermic. The reusability tests showed that the removal rate of Cr(VI) was kept
Adsorption over 95% by desorbed CSU-NaOH sample until the fifth cycle. The enhancement of adsorption properties
Chromium can be attributed to both the increase of BET surface area and the improvement of surface functional
Urea
groups of SBAs. The advantages of higher adsorption capacity and adsorption rate of CSU-NaOH suggest
Activation
that CSU-NaOH is an effective adsorbent for Cr(Ⅵ) removal.
© 2019 Elsevier Ltd. All rights reserved.

1. Introduction heavy metals removal from wastewater, however, the traditional

commercial carbons have limited performance for metal removal.
The excessive discharge of heavy metals into environment have Therefore, it is important to develop alternative low-cost adsor-
resulted in the pollution of waters and soil. And heavy metals are bents (Xu et al., 2015).
usually non-biodegradable and do harm to living tissues (Li et al., The production of municipal sewage sludge increases yearly
2011). As one of typical heavy metals, Chromium (Cr) accumu- with the increasing wastewater treatment rate worldwide (Zhou
lates in the food chain and poses threat to human physiology, et al., 2014; Wei et al., 2018). As a type of byproduct during
therefore, it has to be removed from the effluent (Altas et al., wastewater treatment, sewage sludge should be treated and
2011；; Lin et al., 2018). disposed properly to avoid secondary pollution (Samolada and
The main removal methods for heavy metals include adsorption, Zabaniotou, 2014; Wang et al., 2017, 2019). Therefore, it is vital to
electroflotation, reverse osmosis, membrane separation, ion- find out a secure, reasonable and high-efficiency sludge utilization
exchange, solvent extraction, chemical precipitation, etc. way (Hii et al., 2014). The carbon-bearing sewage sludge is a
(Agrafioti et al., 2014). Adsorption is an effective technology for promising material for adsorbent preparation, and lots of re-
searchers focus on developing sludge-based adsorbents (Xu et al.,
2015; Isabela et al., 2018).
* Corresponding author. Generally, porosity and surface area of adsorbents are two key
** Corresponding author. concerns for the production of sludge-based adsorbents, and the
E-mail addresses: zhanglg@scnu.edu.cn (L. Zhang), Qilin.Wang@uts.edu.au studies showed that the surface area of carbonized sludge is usually
(Q. Wang).

https://doi.org/10.1016/j.jclepro.2019.117904
0959-6526/© 2019 Elsevier Ltd. All rights reserved.
2 L. Zhang et al. / Journal of Cleaner Production 238 (2019) 117904

maximized at high temperature (Bagreev et al., 2001a; Devi and 2.2. Analytical methods
Saroha, 2017). A great deal of research was carried out on phys-
ical activation or chemical activation. Physical activation proceeds The BET surface area was measured with a specific surface area
by the progressive burn-off of carbon fraction (Jeon et al., 2018). analyzer (ASAP2020) using the adsorption isotherms of gas
Physical activation of sludge by CO2 and steam was also examined adsorption (N2, 77 K). X-ray diffraction (XRD) was carried out with
(Jindarom et al., 2007). an X-ray diffractometer (Bruker D8 Adanvance, Germany) with Cu-
Chemical activation can improve the surface chemistry prop- Ka irradiation (40 kV, 40 mA). Surface functional groups of sludge-
erties of sludge-based adsorbents. The effectiveness of the principal based adsorbent were measured with the Fourier transform
activation reagents, including KOH, ZnCl2 and H2SO4, was investi- infrared spectrometer (Nicolet 6700, USA) in the range of
gated (Zhai et al., 2004; Ros et al., 2006; Yu and Zhong, 2006). The 400e4000 cm
1. The elemental analysis of raw sludge sample was
surface area of 297 m2 g
1 was obtained at 950  C and hold time for determined using a Thermoquest NA2100 elemental analyzer giv-
1 h (Bagreev et al., 2001b). The surface area of 382 m2 g
1 was ing the mass percentage of carbon, oxygen, phosphorus, chlorine,
achieved with KOH activated sludge (Wang et al., 2008). The surface and sulfur. The metal contents were measured using the induc-
area of 289 m2 g
1 was attained using H3PO4 as a reagent for acti- tively coupled plasma emission spectrometry (ICP-OES). The con-
vating sewage sludge (Zhang et al., 2005). tent of each element in the sludge was determined by the energy
In order to elevate the porosity and surface area of carbonized dispersive spectroscopy (EDS). The concentration of Cr (Ⅵ) was
sludge, Chandrashekhar et al. (2019) investigated the method of measured using the Diphenylcarbonylhydrazine spectrophotom-
mixing sludge with other low-ash materials. Tao et al. (2015) re- etry. The leaching tests of heavy metals were conducted according
ported the adsorption capacity of 51.3 mg g
1 for Pb2þ removal by a to HJ557-2010.
sludge-based adsorbent after HNO3 treatment. The facile one-pot
synthesis was employed to prepare sewage sludge biochar coated
by carboxymethyl chitosan for improving Pb(II) and Hg(II) removal, 2.3. Boehm titration
and the results indicated that the eNH2, eOH, eCOOH functional
groups played a key role in Pb(II) and Hg(II) removal (Ifthikar et al., The Boehm titration tests were conducted in 250 mL flasks at
2018). It was reported that the adsorption capacity of ferric- 25  C. The 0.1 g sludge samples were added into 50 mL HCl
activated sludge-based adsorbents was higher than non-activated (0.05 mg L
1), NaOH (0.05 mg L
1), NaCO3 (0.05 mg L
1) and
one for tetracycline removal (Yang et al., 2016). However, the acti- NaHCO3 (0.05 mg L
1), respectively. The mixtures were agitated on
vation conditions are relatively complicated when considering the a shaker at 150 rpm for 24 h and filtered. The excessive HCl was
pre-treatment and post-treatment for getting a better BET surface added into the latter three mixtures. Back titration was conducted
area. In addition, even though the post-treatment by HCl or NaOH by NaOH using phenolphthalein as an indicator.
was usually used to enhance BET surface area of sludge-based ad-
sorbents, the mechanism of inorganic fraction's contribution was
not known (Smith et al., 2009). And the chemical activation by 2.4. Adsorption tests
ZnCl2 will give rise to a risk of introducing heavy metal into the
prepared sludge-based adsorbents. The effect of initial pH of 1.5e8.0 on Cr (Ⅵ) adsorption was
Therefore, the aim of present work is to examine the synergistic conducted in 150 mL flasks at 25  C. The 0.2 g CSU or CSU-NaOH
effect of a combined activation method of physical activation fol- was added into 50 mL of Cr (Ⅵ) solution (15 mg L
1). This mixture
lowed by NaOH activation for sludge-based adsorbents preparation was agitated on a shaker at 150 rpm until the equilibrium was
with pentaerythritol addition, and further to investigate its reached. The solution pH was adjusted by 0.1 mol L
1 NaOH and
adsorption properties for Cr(Ⅵ) removal from wastewater. 0.1 mol L
1 HCl solution. The supernatant was taken for the anal-
ysis. The removal rate and adsorption capacity were calculated by
the change of its concentration. The initial Cr(Ⅵ) concentrations
were adjusted from 10 to 80 mg L
1.
2. Materials and methods Adsorption kinetics was studied in 250 mL flasks at 25  C. The
0.4 g CSU and 0.4 g CSU-NaOH were added into 100 mL Cr (Ⅵ) so-
2.1. Preparation of sludge-based adsorbents lution (15 mg L
1), respectively. The mixtures were agitated on a
shaker at 150 rpm. The solution pH was adjusted by 0.1 mol L
1
The raw sludge was collected from the secondary sedimentation NaOH and 0.1 mol L
1 HCl solution at 2.5 ± 0.1. The Cr(Ⅵ) concen-
tank of a local wastewater treatment plant in Guangzhou. The trations in supernatant were measured from 1 to 360 min.
sludge was dried in the oven at 105  C for 24 h to make its weight The models commonly used to describe adsorption kinetics
constant. The sample sludge was grounded into fine powder and include the pseudo first-order model and pseudo second-order
separated into particle size (<0.074 mm) by a sieve of 200 mesh model and intra-particle diffusion model. The pseudo first-order
(Yang et al., 2016). Then the sample was reserved in a dryer. model and pseudo second-order model equations are given as
The pretreated sludge was mixed with urea and pentaerythritol
at a mass ratio of 1:1:1. The sludge was carbonized in a tubular lgðqe
qt Þ ¼ lgqe
k1 , t (1)
furnace (N2 as purge gas with a flow rate of 300 mL min
1). The
heating rate was 3  C$min
1. The mixture was heated to 850  C, t 1 t
¼ þ (2)
and held at the final temperature for 0.5 h. The above sample was qt k2 q2e qe
called Carbonized sludge with urea addition (CSU), and corre-
spondingly the sample without urea addition was called carbonized where k1 (min
1) is the rate constant of pseudo first-order
sludge (CS). The 2 g of CSU sample was further impregnated in adsorption, and k2 (g$mg
1$min
1) is the rate constant of pseudo
10 mL of 3 mol L
1 NaOH for 24 h. Then the sample was washed and second-order adsorption. The qe (mg$g
1) and qt (mg$g
1) are the
filtered until the filtrate was neutral. The absorbent was dried and amount of adsorbed concentrations at equilibrium and at time t,
grounded into fine powder for future use. The above sample was respectively.
called CSU modified by NaOH (CSUeNaOH). The intraparticle diffusion equation is given as
 L. Zhang et al. / Journal of Cleaner Production 238 (2019) 117904 3

3. Results and discussion

qt ¼ ks t 1=2 þ C (3)
3.1. Characterization of sludge-based adsorbents
where qt (mg$g
1) is the adsorption amount of the sludge adsor-
bent to Cr(VI) at t time. The T (min) is the adsorption time. The The elemental analysis of raw sludge was shown in Table 1. The
ks(mg$g
1 min
1/2) is the intra powder diffusion model adsorption surface area and pore structure of raw sludge, CS, CSU and CSU-
rate constant. The C is the intercept. NaOH were shown in Table 2. The BET surface areas of raw sludge
The Langmuir and Freundlich equations are commonly used for and CSU were 5.9 m2 g
1 and 109.4 m2 g
1, respectively. The addi-
describing the adsorption behavior of adsorbents, and the equa- tion of urea and pentaerythritol during carbonization increased
tions are expressed as about 100% surface area of CSU sample. The surface area increased
more than 20 times after modification, the pore volume increased,
qm KL Ce and the pore diameter decreased. It indicated that micropores and
qe ¼ (4)
ð1 þ KL Ce Þ mesopores were mainly formed in the CSU sample during
carbonization. However, further NaOH modification decreased the
1=n surface area of CSU-NaOH sample.
qe ¼ KF C e (5) The XRD graphs of CS, CSU and CSU-NaOH were shown in Fig. 1.
In the XRD graph of CS, the main peak appeared at 2q ¼ 26.9 . In the
where qm (mg$g
1) and KL (L$mg
1) are Langmuir constants related
XRD graph of CSU, the obvious peak appeared at 2q ¼ 21 and 26.9 .
to adsorption capacity and energy of adsorption, respectively. KF
In addition, the spectrum of CSU-NaOH showed several peaks at
((mg$g
1)(L$mg
1)1/n) and n are Freundlich constants related to the
21, 26.9 and 40 , indicating the presence of silica, iron oxide and
adsorption capacity and intensity, respectively.
calcium oxide (Bhattacharya et al., 2008). The sharp peak indicated
The thermodynamic parameters (DG, DH, and DS) were also
that quartz was the main phase of CS, CSU and CSU-NaOH samples.
used to evaluate the adsorption thermodynamics at 20, 30 and
The XRD results revealed that the diffraction strength of CSU and
40  C, respectively. Gibbs free energy DG was the standard of
CSU-NaOH samples both decreased.
spontaneous process, which is described in the following formula:

DG ¼
RT lnKL (6) 3.2. FT-IR and Boehm titration

where DG (kJ$mol
1) is the Gibbs free energy change. T (K) is the As shown in Fig. 2, the FT-IR spectra showed that the absorption
absolute temperature. The R (8.314 kJ mol
1$K
1) is the constant, peak of CSU-NaOH sample was weakened, indicating the decom-
and KL (L$mg
1) is the Langmuir constant. position of organic matter in sludge-based adsorbents during
Similarly, the entropy change and enthalpy change of the system activation treatment (Iqbal et al., 2009). The NeH band was
was obtained by the following equation: observed at 468 cm
1. The IR spectra between 781 and 808 cm
1
could be attributed to the stretching vibrations of Metal-OH. The
lnKL ¼ DS=R
DH=RT (7) stronger absorption peaks at 1032-1056 cm
1 and 570 cm
1, which
can be explained in terms of phosphate groups, PeO and OePeO
where DH (kJ$mol
1) is the reaction enthalpy. S (kJ$mol
1$K
1) is
stretching (Velghe et al., 2012). The absorption peak at 1032-
the entropy of the reaction. The R (8.314 kJ mol
1$K
1) is the con-
1035 cm
1, also associated with SieOeSi groups, indicating the
stant, and KL (L$mg
1) is the Langmuir constant.
existence of silicate functional groups (Pan et al., 2011). The band at
1508-1604 cm
1 was the absorption peaks of C]O, C]C and N]O,
which was attributed to the addition of urea during sludge-based
2.5. Reusability test and leaching test adsorbents preparation. The band at 3300-3500 cm
1 represents
the stretching vibration of the hydroxyl group, indicating the ex-
The desorption tests were conducted in 150 mL flasks at 25  C. istence of moisture in sludge (Gu et al., 2014). The improvement of
0.5 g saturated sludge-based adsorbents samples were added into adsorption performance of CSU-NaOH can be attributed to the
50 mL NaOH (2 mol L
1), H2SO4 (2 mol L
1) and HNO3 (2 mol L
1), occurrence of carbonyl group caused by NaOH activation.
respectively. The CSU-NaOH sample was impregnated for 12 h to The contents of functional groups on all samples were deter-
desorb Cr (Ⅵ). The Cr (Ⅵ) concentration and total chromium con- mined using Boehm titration method, and the results were given in
tent in the filtrate were measured. The control experiment was Table 3. Acidic groups were the dominate groups in all the samples.
carried out with deionized water. The sludge-based adsorbents The types and total amount of functional groups of CSU and CSU-
samples were washed with deionized water until the filtrate was NaOH changed after activation treatment. The acidic groups of
neutral. Then the adsorbents were dried in the oven at 105  C and CSU-NaOH were slightly decreased, including carboxyl group,
then stored for next-cycle use. The above desorbed CSU-NaOH phenol hydroxyl group. However, the alkaline groups of CSU-NaOH
sample was then added into 15 mg L
1 Cr (Ⅵ) solution to
examine the reusability of sludge-based adsorbents. The duration
Table 1
time of adsorption tests was kept for 4 h of each cycle. The Cr(Ⅵ) Elemental analysis of raw sludge sample.
concentration in filtrate was measured to calculate the Cr(Ⅵ)
Item (wt%) Measurements Item (wt%) Measurements
removal rate.
Leaching test was carried out at a solid/liquid ratio of 1:10, i.e. 1 C 43.36 Ca 0.71
O 45.63 Al 2.69
g of adsorbent was added into 10 mL deionized water, and the so-
P 1.68 Mg 0.42
lution was vibrated at 110 rpm for 8h, and then the sample was Cl 0.02 Na 0.05
settled down for 16 h, the filtration of supernatants was conducted S 0.4 K 0.49
with a syringe filter (0.45 mm pore size). The metal contents were Ti 0.08 Zn 0.04
measured using the inductively coupled plasma emission Si 3.56 Mn 0.03
Fe 0.84 Ni 0.02
spectrometry.
4 L. Zhang et al. / Journal of Cleaner Production 238 (2019) 117904

Table 2
BET surface areas and pore properties of sludge-based adsorbents.

Sample BET surface area(m2$g
1) Pore volume(mL$g
1) Pore size(nm)

Dried sludge 5.9 0.025 16.704

CS 51.9 0.102 7.833
CSU 109.4 0.065 2.373
CSU-NaOH 97.9 0.053 2.152

Fig. 1. XRD graphs of carbonized sludge and activated SBAs.

Fig. 3. Effect of pH on Cr(Ⅵ) removal.

There exist following equations:

H2 CrO4 4 HCrO

4 þH
þ
(8)

HCrO
2

4 4 CrO4 þ H
þ
(9)

2HCrO
2

4 4 Cr2 O7 þ H2 O (10)
Therefore, in this study, better adsorption capacity at pH of 2e3
may be attributed to the large number of Hþ ions present at these
pH values. The surface of sludge adsorbent was positive potential
due to protonation, and chromate ions were absorbed through
electrostatic interaction at low pH. The surface of sludge adsorbent
Fig. 2. FT-IR spectra of carbonized sludge and activated SBAs. was negative potential due to proton ionization at high pH. The
electrostatic repulsion was not conducive to the adsorption of
Table 3
Contents of all kinds of surface functional groups (mmol$g
1).

Sample Base groups Acidic groups Carboxyl Lactone groups Phenol

CS 0.05 1.45 0.8 0.05 0.6

CSU 0 1.95 1 0.05 0.9
CSU-NaOH 0.25 1.75 1 0.125 0.625

were slightly increased.

3.3. Effects of pH and adsorption time

As shown in Fig. 3, within the range of pH 1.0e2.5, the Cr (Ⅵ)

removal for the CSU and CSU-NaOH was 97% and 99%, respectively.
However, the Cr(Ⅵ) removal rate significantly decreased with
increasing pH when the pH reached over 3.0, which indicated that
pH was the key factor for removing Cr(Ⅵ). The main chemical


speciation of Cr(Ⅵ) were Cr2O2
7 (pH < 1), HCrO4 (pH 2e6) and
CrO2

4 in neutral and alkaline conditions (pH > 6) (Altas et al., 2011). Fig. 4. Effect of adsorption time on Cr(Ⅵ) removal.
 L. Zhang et al. / Journal of Cleaner Production 238 (2019) 117904 5

chromate ions. Therefore, the optimum pH was 2.5 in this study. model. Linear regression coefficients of CSU and CSU-NaOH by
Based on Fig. 4, the Cr (Ⅵ) removal of CS and CSU samples was Langmuir model were both over 0.97, which indicated that single
45% and 80% at 2 h, respectively. However, the Cr (Ⅵ) removal rate molecule adsorption mechanism was predominant for Cr(VI)
of CSU-NaOH reached up to 99% at 0.5 h. The adsorption rate of adsorption by CSU and CSU-NaOH.
CSU-NaOH for Cr (Ⅵ) increased significantly, therefore, the The maximum Cr(VI) adsorption capacity was 15.3 mg g
1 by
adsorption equilibrium time mostly depended on the properties of CSU-NaOH, which was over 2 times of those by CS and CSU at 25  C
sludge-based adsorbents. and initial Cr(VI) concentration of 15 mg L
1. Altas et al. (2011) re-
ported that the monolayer adsorption capacity of the adsorbent
3.4. Adsorption kinetics and thermodynamics was 8.67 mg g
1 at 20  C when the initial Cr(VI) concentration was
100 mg L
1. The chemical activated sludge-derived adsorbent with
The comparisons of kinetic constants were shown in Table 4 and ZnCl2 was used to remove heavy metals, the adsorption capacity of
Table 5, respectively. The linearized results of kinetic data were 15.4 mg g
1 was achieved for Cr(III) removal at pH 3.0, however, the
shown in Fig. 5. initial Cr(III) concentration was not specified (Rozada et al., 2008).
Based on Table 4, the pseudo second-order kinetic model was fit Compared with traditional physical activation, the carboniza-
for the explanation of Cr(Ⅵ) adsorption process due to high value of tion with urea addition is thus much simpler, and does not require
correlation coefficient. These suggested that pseudo second-order intensive energy or pressure input (Fitzmorris et al., 2006).
kinetics was the rate control step for Cr(Ⅵ) adsorption onto Compared to chemical activation with ZnCl2, combined activation
sludge-based adsorbents, which indicated that sludge adsorbent has the advantage of without heavy metals introduced into
for Cr(Ⅵ) removal was a chemical process. adsorbent (Bagreev et al., 2001b). However, the direct quantitative
The adsorption capacity of CSU for Cr(Ⅵ) increased with the comparison of all these adsorbents is difficult without back-to-back
extension of adsorption time. At the initial stage of adsorption, the experiments since many factors (e.g. temperature, pH and initial
adsorption rate increased dramatically from 0 to 30 min because of pollutant concentration, among others) could influence the results
more surface active sites on the sludge adsorbents and high mass (Smith et al., 2009).
transfer rate in solution. The increase of Cr(Ⅵ) removal rate was not The thermodynamic parameters of three sludge-based adsor-
obvious after 4 h, which indicated that adsorption of Cr(Ⅵ) ach- bents at different temperatures were shown in Table 7. The DG
ieved adsorption equilibrium. However, the Cr(Ⅵ) adsorption by values at different temperatures were all negative, which suggested
CSU-NaOH reached adsorption equilibrium at 30 min. At adsorp- that the adsorption of Cr(VI) for three sludge-based adsorbents
tion equilibrium state, the removal rates of Cr(Ⅵ) by CSU and CSU- were spontaneously. The DG values decreased when the tempera-
NaOH were 89.04% and 99.99%, respectively. ture increased, which suggested that the spontaneous degree also
The linearized results of intraparticle diffusion model were increased with the increase of temperature. Based on the formula,
given in Fig. 5(c). The intraparticle diffusion data were summarized the DH and DS values were both positive, which indicated that the
in Table 5. At the first stage of intraparticle diffusion model, the CS, adsorption process was an endothermic reaction, and the increase
CSU and CSU-NaOH all fitted the data well, linear regression co- of temperature faciliated the reaction rate.
efficients of intraparticle diffusion model were all over 0.91. How-
ever, the adsorption data of CSU-NaOH were not suitable for the 3.6. Mechanisms of combined activation of sludge-based
second stage of intraparticle diffusion model, linear regression adsorbents
coefficients of intraparticle diffusion model were only 0.70.
Meanwhile, the particle diffusion curve did not go through the Due to the inherent properties of containing carbon and inor-
origin. It suggested that the particle diffusion process was divided ganic oxide, such as SiO2, Fe2O3 and CaO, municipal sewage sludge
into three stages of external surface biosorption, gradual bio- has the advantage of producing adsorbent (Aktas and Cecen, 2007).
sorption and final equilibrium stage. The gradual biosorption stage However, the direct carbonization usually gives rise to low surface
was the rate-controlled step. The liquid membrane diffusion and area and adsorption capacity. Therefore, pentaerythritol was added
surface adsorption process were able to affect the adsorption rate. to enhance carbon content of prepared adsorbent, and the addition
of urea aimed to improve the surface area and porosity of adsorbent
3.5. Adsorption isotherms and thermodynamic analysis through the gas release during the carbonization of sludge at high
temperatures.
Isothermal adsorption was mainly used to determine the The commonly used nucleating agents during carbonization
interaction between adsorbent and adsorbate and the adsorption were SiO2, Fe2O3 and calcium silicate, the composition was similar
mechanism (Gulnaz et al., 2006). The Langmuir and Freundlich to those inorganic oxide in sewage sludge. As shown in Table 1, the
asdorption isotherms were obtained at 25  C. The linearized sludge contained the elements which usually contained in nucle-
Langmuir and Freundlich equations were shown in Fig. 6. Table 6 ating agents. For example, the stretching bonds of SieO, SieOeSi
showed the Langmuir and Freundlich isotherm constants. Both and SieOeC were observed in the FT-IR spectra (Fig. 2), which was
the Langmuir model and Freundlich model fitted the adsorption derived from large amounts of Si in the sludge. The nucleating
data well within the concentration ranges studied. However, the agents facilitated the release of ammonia. Urea was used as the
adsorption data of CSU-NaOH were more suitable for Langmuir foaming agent containing nitrogen, which could release ammonia

Table 4
Kinetic constants for Cr(VI) adsorption on three sludge-based adsorbents.

Sample Pseudo first-order kinetics Pseudo second-order kinetics

qe(mg$g
1) k1(min
1) R2 qe(mg$g
1) k2(g$mg
1$min
1) R2

CS 0.9766 0.0078 0.9348 1.7535 0.0487 0.9977

CSU 1.3772 0.0043 0.9184 3.3613 0.0327 0.9975
CSU-NaOH 2.1958 0.0091 0.8589 13.351 0.0374 0.9997
6 L. Zhang et al. / Journal of Cleaner Production 238 (2019) 117904

Table 5
Kinetic constants of intraparticle diffusion model for Cr(VI) adsorption on three sludge-based adsorbents.

Sample First stage Second stage

ks(mg$g
1$min
1/2) C R2 ks(mg$g
1$min
1/2) C R2

CS 0.1135 0.4779 0.9399 0.0059 1.5865 0.8693

CSU 0.1573 1.5165 0.9823 0.0457 2.4951 0.7014
CSU-NaOH 0.2987 10.458 0.9113 0.0556 12.442 0.7913

Fig. 6. Adsorption isotherms of Cr(Ⅵ) on three sludge-based adsorbents

(a) Langmuir isotherm
(b) Freundlich isotherm.

during the carbonization of sewage sludge at high temperature, and

simultaneously generated complex organic matter. The pentaery-
thritol was used as the carbon source to enhance carbon content in
sludge-based adsorbents, which was favorable for improving BET
surface area of adsorbents (Tao et al., 2015).
The homogeneous system was formed by mixing dried sludge,
urea and pentaerythritol in the weight ratio of 1:1:1. Through the
growing and merging of foam, the carbonized porous structure was
finally fixed. Compared with dried sludge and carbonized sludge,
the BET surface area of CSU sample increased by 20 times and 2
times, respectively (Table 2). The surface area decreased after alkali
treatment, however, the adsorption performance of CSU-NaOH was
better than CSU, it can be inferred that the adsorption performance
Fig. 5. Adsorption kinetics of Cr(Ⅵ) on three SBAs (a) Pseudo first-order kinetics (b) of CSU-NaOH was not only relate to its surface area, but also related
Pseudo second-order kinetics (c) Intraparticle diffusion model. to the change of functional groups on the surface of sludge-based
adsorbents. Based on the FT-IR spectra, the NeH and N]O bonds
 L. Zhang et al. / Journal of Cleaner Production 238 (2019) 117904 7

Table 6
Adsorption isotherm constants for Cr(Ⅵ) removal by three sludge-based adsorbents.

Sample Langmuir Freundlich

KL (L$mg
1) qm (mg$g
1) R2 KF (mg$g
1)(L$mg
1)1/n n R2

CS 0.0586 7.2310 0.8432 1.3100 2.8450 0.8777

CSU 0.2708 7.6570 0.9708 3.4610 5.3850 0.9645
CSU-NaOH 1.5058 15.267 0.9953 7.7518 3.8491 0.8235

Table 7
Thermodynamic parameters for Cr(VI) removal.

T(K) DG0(kJ$mol
1) DH0(kJ$mol
1) DS0(kJ$mol
1$K
1)

CS 293
9.94
303
10.25 4.432 0.048
313
10.93
CSU 293
13.24
303
14.11 26.55 0.135
313
16.00
CSU-NaOH 293
17.14
303
18.43 11.19 0.097
313
19.08

Fig. 8. Reusability of CSU-NaOH for Cr(Ⅵ) removal.

were observed, which were able to react with the urea added
during carbonization. The appearance of C]O, C]C, and carbonyl Cu, Zn, Pb, Cr, Cd and Ni in the sample of carbonized sludge, Cu, Zn
stretching bonds were the derived from pentaerythritol and NaOH. and Ni had relatively higher concentrations. For CSU-NaOH sample,
The mechanisms of sludge-based adsorbents preparation were there existed only lower contents of Cu and Cr. The concentrations
described in Fig. 7. of heavy metals were all lower than Chinese Standard of Hazardous
Waste Identification (GB5085.3e2007).
3.7. Reusability test, leaching test and cost analysis Table 9 shows the cost analysis of combined activation of
sludge-based adsorbents. Based on the preparation method, urea,
To examine the reusability of adsorbed CSU-NaOH, its desorp- pentaerythritol and NaOH are the main chemicals used for the
tion tests were conducted with deionized water, H2SO4, HNO3 and activation of sludge-based adsorbents. The total chemical cost of
NaOH. As shown in Fig. 8, the adsorbent desorbed by NaOH had the CSU-NaOH preparation was 17.4 $/kg. The improvement of
highest reusability among all desorption reagents, and the Cr(VI) adsorption capacity by combined activation was over than 110%
removal rate was kept over 95% by desorbed CSU-NaOH sample compared with carbonized sludge (Table 6).
until the fifth cycle. However, the desorption reagents of H2SO4 and
HNO3 had limited desorption performance, which were nearly all 4. Conclusion
lower than 60% of removal rate for Cr(VI) in each cycle. As a sig-
nificant factor for application, the high reusability indicated that The feasibility of enhancing the adsorption properties of sludge-
CSU-NaOH would be an effective adsorbent for Cr(Ⅵ) adsorption based adsorbents was investigated through combined activation
(Devi and Saroha, 2017). method for Cr(Ⅵ) removal. The following conclusions can be drawn
Table 8 shows the leached concentrations of heavy metals with from this study:
distilled water. The results showed that there existed six metals of The combined physical activation and chemical activation

Fig. 7. Mechanisms of combined activation of sludge-based adsorbents.

8 L. Zhang et al. / Journal of Cleaner Production 238 (2019) 117904

Table 8
Concentrations of heavy metals leached from the carbonized sludge and activated adsorbents (mg/L).

Sample Cu Zn Pb Cr Cd Ni As Hg

CS 4.87 0.92 0.002 0.05 0.004 2.114 e e

CSU 0.019 e e 0.001 0.003 0.007 e e
CSU-NaOH 0.003 e e 0.015 e e e e
Maximum allowable valuea 100 100 5 15 1 5 5 0.1

“d“: below detection limit.

a
Based on Chinese Standard of Hazardous Waste Identification (GB5085.3e2007).

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This research was supported by the grants of National Natural Prod. 196, 626e634.
Science Foundation of China (No. 41372050) and National Sleep Pan, Z., Tian, J., Xu, G., Li, J., Li, G., 2011. Characteristics of adsorbents made from
biological, chemical and hybrid sludges and their effect on organics removal in
Foundation of Guangdong Province (No. 2016A030313432). Dr Qilin
wastewater treatment. Water Res. 45, 819e827.
Wang acknowledges Australian Research Council for funding sup- Ros, A., Lillo-Rodenas, M.A., Fuente, E., Montes-Moran, M.A., Martin, M.J., Linares-
port through Discovery Early Career Researcher Award Solano, A., 2006. High surface area materials prepared from sewage sludge-
(DE160100667) and Discovery Project (DP170102812). based precursors. Chemosphere 65 (1), 132e140.
Rozada, F., Otero, M., Moran, A., Garcia, A.I., 2008. Adsorption of heavy metals onto
sewage sludge-derived materials. Bioresour. Technol. 99, 6332e6338.
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