Anal. Method Environ. Chem. J. 3 (4) (2020) 30-39

Research Article, Issue 4

Analytical Methods in Environmental Chemistry Journal

Journal home page: www.amecj.com/ir

AMECJ

Adsorption methodology: Synthesis of Nano-

structured nitrogen-doped porous carbon adsorbents for

perchloroethylene vapor adsorption

Mohammad Ghasemi Kahangi

a,b

, Alimorad Rashidi

and Mohammad Samipoorgiri

Chemical Engineering Department, Islamic Azad University, North Tehran Branch, Tehran, Iran

Carbon and Nanotechnology Research Center, Research Institute of Petroleum Industry, Tehran, Iran

ABSTRACT

The perchloroethylene (PCE, tetrachloroethylene), as a representative

of chlorinated ethylenes and volatile organic compounds (VOCs), can

be easily transported and remain in the atmosphere due to its volatility

and stability properties. As a result, there is a crucial need to reduce

this pollution to the extent permitted by international standards.

The concentration of PCE determined with Gas chromatography–

mass spectrometry analyzer (GC-MS). Activated nanocarbons

(ACs) doped with nitrogen functional groups were prepared using

the walnut shell as a precursor to evaluate their adsorption capacity

for PCE vapors. Several techniques, including scanning electron

microscopy (SEM), N

adsorption-desorption, and the Fourier

transforms infrared spectrometry (FTIR), were applied to characterize

the physical-chemical properties of the ACs. It is found that the

PCE adsorption considerably increased on the nitrogen-doped ACs

(KNCWS) due to their structural and surface charge properties. By

conducting kinetic study, the pseudo-rst-order model matched well

with experimental data that could indicate reversible adsorption of

the PCE on heteroatom doped ACs. The sips model agreed well with

the equilibrium adsorption of PCE on the nitrogen-doped ACs, and

the maximum adsorption capacities for PCE reached 166, 285, and 95

mg/g for KNCWS-11, KNCWS-21, and KNCWS-31, respectively.

Also, the concentration of PCE were online measured based on

nitrogen-doped ACs as solid-phase extraction (SPE) by the GC-MS

as analytical procedure. Therefore, the nitrogen-doped ACs was good

choices for the removal of PCE vapors.

Keywords:

Perchloroethylene,

Adsorption procedure,

Gas chromatography mass spectrometry,

Porous Nano carbon,

Nitrogen-doped adsorbent.

ARTICLE INFO:

Received 12 Aug 2020

Revised form 5 Oct 2020

Accepted 12 Nov 2020

Available online 30 Dec 2020

* Corresponding Author: A.M. Rashidi

E-mail addresses: rashidiam@ripi.ir

https://doi.org/10.24200/amecj.v3.i04.125

------------------------

1. Introduction

Atmospheric air, as an important topic of the

environment that ensures the lives on Earth, not

only contains natural and vital compounds, but also

a set of undesirable and unnatural materials [1].

Since the existence of volatile organic compounds

(VOCs) in the atmosphere can cause severe health

problems. They have attracted a lot of attention

and many efforts have been made to removal

them in recent years [2]. In addition, numerous

VOCs contribute to the degradation of the

stratospheric ozone layer, which is part of climate

change. VOCs are also considered a precursor to

the secondary formation of PM2.5 particulate

Perchloroethylene vapor adsorption by N-doped porous carbon Mohammad Ghasemi Kahangi et al

matter [3]. According to various sources, volatile

organic compounds in atmospheric air are divided

into several main groups of alkanes, alkenes,

alkynes, aromatics, volatile organic compounds

of chlorine, and volatile organic compounds of

sulfur. Chlorine volatile organic compounds are

much more environmentally toxic than other

VOCs. Specically, Cl-VOCs are primarily

known as stable, biogenic, and biodegradable

compounds in the environment. Most of them are

toxic, and carcinogenic. Therefore, today we need

to research effective techniques for managing

currents contaminated with Cl-VOCs that are

economically viable [4]. Perchloroethylene (PCE,

tetrachloroethylene) as a representative of chlorine

ethylenes is a synthetic chlorine hydrocarbon known

for its exceptional solubility and low inammability.

PCE is a colorless and sweet-smelling liquid with

highly solubility in water that is widely utilized

as a solvent in industrial processes like metal

degreasing, drying, and drug, pesticides, adhesives,

and antifreeze production [5]. Due to the prevalence

of leakage and its inappropriate disposal, large

amounts of PCE enter the environment in industrial

sites [6]. PCE with a vapor pressure of 18.5 mm

Hg at 25 °C is predicted to exist only as vapor in

the ambient atmosphere [7]. According to OSHA,

PCE levels above 100 ppm can cause neurological

effects that can damage to the central nervous

system in humans [8]. Many methods have been

reported to control environmental pollution, such

as adsorption, advanced oxidation, electrochemical

and photocatalytic method. Because of low cost,

simplicity in design, and operation, the adsorption

method is widely used among the various methods

to remove the Cl-VOCs. Cost-effective, efcient,

and sustainable methods for removing VOCs are

still a challenge and need more attention in this

regard. Zeng et al. (2015) found that activated

carbon aganite-nanocomposite (β-FeOOH-AC)

has outstanding capacity for PCE adsorption. It is

shown that PCE removal with an adsorbent dose

of 8 g/L and an initial concentration of 100 mg/L

can reach 97.83%. Adsorption kinetics indicates

that the quasi-second-order model ts well with the

experimental data. The Langmuir and Freundlich

isotherm models were reasonably tted to describe

the PCE adsorption behavior on β-FeOOH-

AC from water. The thermodynamic results of

adsorption show that the PCE adsorption process

on β-FeOOH-AC is a spontaneous, endothermic,

monolayer, and multilayer adsorption process

joint with a physical process that occurs through

an ion-exchange surface adsorption mechanism

[9]. In the adsorption process, the price of the

adsorbent is one of the most important factors in

the economics of the process, so most researchers

try to build and select a cheap adsorbent in this

process [10]. Lignocellulose wastes are a good

option for cheap adsorbents, as the existing wastes

enter efcient cycles. Activated carbon used to

prevent normal landlling in landlls is recyclable

and economically competitive with other methods.

In addition, the solid phase extraction as analytical

techniques, such as the needle trap extraction,

the tube extraction, the sorption trap, solid-phase

microextraction (SPME), the thin-lm extraction,

dynamic SPE, and stir bar SPE were used for

determination PCE and VOCs in air by GC-MS.

This method has the dual advantage of reducing

waste production and greenhouse gas emissions.

Accordingly, In this research, by using the walnut

shell as a precursor, a series of urea modied

carbons were activated by KOH and the effect of in

situ N-doping on the textural properties as well as

PCE adsorption performance were investigated. In

addition the proposed procedure based on nitrogen-

doped ACs coupled to GC-MS analyzer. Moreover,

Different analysis (BET, FE-SEM, and FT-IR) were

employed to characterize the as-prepared ACs.

2. Experimental

2.1. Apparatus and Reagents

The gas chromatography (GC) equipped with loop

injector was used for etrachloroethylene analysis

in air (Agilent GC, 7890A, GC-MS, Netherland).

The detector of triple quadrupole (MS) have used

in Agilent GC. Due to injection process, the slide

of plunger carrier down and tighten the plunger

thumb screw until nger-tight. The sampling

Anal. Method Environ. Chem. J. 3 (4) (2020) 30-39

valves introduce a sample into the carrier gas

stream and valves were used to inject a sample gas

in gas streams. Iranian walnut shell was used as the

precursor of nanocarbons. Moreover, KOH, urea,

and HCl with analytical grade were purchased from

Merck. The etrachloroethylene (perchloroethylene,

PCE) was purchased from Sigma Aldrich

([HOCH

N(CH

COOH)]; CAS Number: 93-

62-9). Acetone and ethanol prepared from Sigma,

Germany.

2.2. Preparation of N-doped nanocarbons

Walnut shell was respectively crushed, washed,

and dried at 100 °C for 24 hours. The precursor

was powdered and sieved into 150 µm. The

carbonization of powdered WS was carried out

under nitrogen gas stream by a heating rate of 5 °C/

min to 600 °C for about 1 hour (CWS). Then urea–

as a source of nitrogen–was mixed with different

ratios of carbonized samples (1:1, 2:1, and 3:1)

and heated at 400 °C for 1 hour through a heating

rate of 3.33 °C/min (NCWS). Next, the nitrogen-

doped carbon was mixed with KOH with the ratio

of 1:4 and activated under N

using a heating rate

of 7.5

C per minute up to 900 °C for 1 hour. The

as-prepared samples were washed with 1 M HCl

and hot distilled water until neutral pH and nally

were dried in the vacuum oven at 80 °C for 5h. The

as-prepared samples were denoted as KNCWS-xy

in which xy represents the ratio of urea to CWS.

2.3. Characterization

To investigate the specic surface area, pore

diameter, and total pore volume, adsorption/

desorption isotherm of nitrogen was performed

using a Micromeritics ASAP 2020 Plus device.

Each sample was degassed at vacuum at 250 °C

and nitrogen uptake was performed at 196 °C. To

provide the morphology of as-prepared ACs, a

TESCAN MIRA3 eld emission scanning electron

microscope (FE-SEM) was used. Reectance

fourier transform infrared (FT-IR) spectra were

collected from the absorbance intensities of

functional groups of as-prepared ACs by Thermo

Nicolet AVATAR 360 within the 4000˗400 cm

-1

wave number range.

2.4. Procedure

Figure 1 shows a schematic of a PCE adsorption

laboratory system operating according to the law of

thermodynamic equilibrium between the liquid and

gas phases. This device is used to absorb PCE in

the range of 0-1000 ppm, 25 °C, and atmospheric

pressure.

Continuous N

-PCE ow is produced by the

ow of N

through a liquid PCE tank to obtain a

certain concentration of PCE in the gas stream by

changing the ratio of concentrated and dilute N

ow. Concentrated N

ows out of the bubble tank

and ows continuously onto the activated carbon

sample located in the adsorbent bed. Contaminated

passed through the adsorbent and the process

Fig. 1. Schematic of the PCE adsorption laboratory system

Perchloroethylene vapor adsorption by N-doped porous carbon Mohammad Ghasemi Kahangi et al

continued until the adsorbent saturated with PCE

and the gas ow at the inlet and outlet of the column

was analyzed online with GC-MS analyzer. Using

the refractive index, the time required for saturation

can be achieved [11].

3. Results and Discussion

3.1. FE-SEM

SEM micrographs of the carbonized walnut

shell (Fig. 2a) has a smooth, pore-free surface in

which, after nitrogen doping (Fig. 2b), a number

of macropores are seen on the sample. Figure 2c

shows a spongy structure containing uniform,

very ne cavities of micro and nano size, showing

that severe morphological changes have occurred

during the activation process and that the active

nano carbons bear no resemblance to the carbonized

sample. To better understand the textural properties

of activated nano carbons, the surface was analyzed

by physical adsorption of N

3.2. Physical Properties

adsorption/desorption isotherms were measured

to evaluate the textural properties of the samples.

The adsorption isotherms of Figure 3a are of type

I according to the IUPAC classication, and its

desorption is of type H4. According to research,

H4 type hysteresis rings are related to the narrow

slit of the specimen [12]. The isotherms have a soft

curvature in the relative pressure between 0 and

0.4 owing to the presence of cavities larger than

the average adsorbed diameter. On the other hand,

Fig. 2. FE-SEM images of a) CWS, b) NCWS-21, c) KNCWS-21

Anal. Method Environ. Chem. J. 3 (4) (2020) 30-39

more curvature of the isotherm knee at low relative

pressure and a slight increase in N

adsorbed

with increasing pressure indicate expansion

(enlargement) of the micropore structure. The

hysteresis ring exhibited at a relative pressure of

about 0.5 indicates the denite presence of meso-

cavities in the samples (P/P

> 0.4) [13]. As can

be seen from the wider circle of KNCWS-21

and KNCWS-11 hysteresis, the participation

of mesopores is greater. This is in line with our

ndings, which show a maximum average pore

width of 2.48 nm for KNCWS-21 and 2.41 nm for

KNCWS-33 [14]. Based on the distribution shown

in Figure 3b, the cavities mainly contain sizes

less than 10 nm, which indicates the presence of

both micropores and mesopores. With increasing

the urea to carbon ratio from 1:1 to 2:1, the peak

intensity in the pore distribution curves of the

samples increased and then decreased to a ratio of

3:1. The degradation of the structural parameters of

the KNCWS-31 sample compared to KNCWS-21

and occurred due to the destruction of the walls

between the cavities or the blockage of the cavities

with excessive nitrogen [15] (Table 1).

Fig. 3. N

adsorption/desorption isotherms(a) and BJH pore size distribution of KNCWSs(b).

(a)

(b)

Perchloroethylene vapor adsorption by N-doped porous carbon Mohammad Ghasemi Kahangi et al

3.3. FT-IR

The diagrams in Figure 4 are the results of the FT-IR

tests (a) CWS, (b) NCWS-21, and (c) KNCWS-21.

The samples show almost the same spectrum,

however, some of the weak/strong functional

groups have disappeared. Nitrogen doping as well

as different activation conditions have resulted in

changes in the carbon sample spectrum. 3426 ~ 3409

-1

bandwidth is seen for all samples, indicating

tensile vibration of N-H groups or tensile vibration

of O-H hydroxyl groups of phenol, alcohol, and

carboxylic acid [16]. The broadband in the range of

2922 ~ 12851 cm

-1

is related to the asymmetric tensile

vibration of CH

, which can be attributed to the -CH-

bond on the carbon surface. -CH- bonds may belong

to alkyl groups such as methyl, methylene groups,

or aldehyde groups [17]. The bandwidth at 1612 ~

11376 cm

-1

is due to amides, pyridine, and C=N,

indicates nitrogen functional groups at temperatures

higher than crude carbon [18]. The peak range of

1030 ~ 1099 cm

-1

is also attributed to the C-N tensile

vibration [19]. Therefore, analysis of FT-IR spectra

conrms the presence of N-containing groups in the

synthesized samples.

3.4. Equilibrium adsorption

Figure 5 shows the performance of both chemical

adsorption processes (nitrogen doping) and

physical adsorption (increasing surface area and

pore volume). Due to the trend of PCE adsorption

isotherms, intensication of carbon activation has

improved the adsorption process. Also, increasing

the adsorption of PCE by adding nitrogen plays an

important role for functional groups, because the

Fig. 4. FT-IR spectra of the as-synthesized KNCWs

Table 1. Textural properties of the KNCWSs

Specimens S

BET

D V

tot

mic

⁄ V

tot

/g) (nm) (cm

/g) (%)

KNCWS-11

2461 2.41 1.48 8.78

KNCWS-21

3225 2.48 1.99 10.05

KNCWS-31

2319 2.03 1.17 20.5

Anal. Method Environ. Chem. J. 3 (4) (2020) 30-39

Fig. 5. Isotherm of PCE adsorption on KNCWs

nature of PCE is electron-accepting, and nitrogen

groups are electron-giving and cause chemical

adsorption. It should be noted that the high share of

mesopores also facilitates the transfer of PCE mass

and increases adsorption. According to the results

of PCE adsorption, it can be acknowledged that the

reason for the signicant decrease in the adsorption

rate of the KNCWS-31 sample is the decrease in

its physical characteristics, which occurred due

to the destruction of the walls between cavities or

blockage of cavities with excessive nitrogen.

According to Table 2, the modulus of adsorption

equilibrium data for the adsorption of PCE on

doped activated nano carbons was consistent with

the Langmuir, Freundlich, and Sips models [20].

However, the Sips model showed higher values

of R

than the Langmuir and Freundlich models

for all samples studied. In this experiment, the

b-constant of the Sips model was approximately

zero, indicating that the Sips model was reduced to

the Freundlich model. This means that multilayer

adsorption occurs with a non-uniform distribution

of heat and tensile adsorption across the surface of

doped activated nano table 2.

3.5. Kinetic adsorption

In order to evaluate the speed of the adsorption

process and to determine the process speed control

stage, kinetic modeling is performed. Kinetic

data were evaluated using pseudo-rst-order [21]

and pseudo-second-order [22] models to provide

a suitable model for the kinetic behavior of the

studied gravity (Fig. 6).

The adsorption kinetics and correlation coefcients

of PCE on doped and non-doped activated nano

carbons are shown in Table 3. Both pseudo-rst-

order and pseudo-second-order equations can

predict the adsorption process under experimental

conditions. Given the calculated values of q

and R

the pseudo-rst-order equation better describes the

adsorption of PCE. The values of R

in the pseudo-

rst-order equation have highe r values than the

pseudo-second-order equation. The results show

that the adsorption of PCE in doped activated nano

carbons belongs to the pseudo-rst-order equation,

which indicates the reversible adsorption between

PCE and the adsorbent table 3.

4. Conclusion

In this study, a series of nitrogen-doped activated

nano carbons were synthesized using walnut shell

and evaluated to adsorb perchloroethylene (PCE)

vapor. In practice, the vapor adsorption capacity

of PCE using nitrogen-doped activated carbon

Perchloroethylene vapor adsorption by N-doped porous carbon Mohammad Ghasemi Kahangi et al

Fig. 6. Kinetics of PCE adsorption on KNCWs

Table 2. The parameters of Langmuir, Freundlich, and Sips isotherms

of PCE adsorption on doped activated nano carbons

Isotherms Adsorbents

KNCWS-11 KNCWS-21 KNCWS-31

Langmuir

348.3383 703.6625 168.2004

b 0.00065812 0.00069629 0.0010431

R-squared 0.99962 0.99824 0.99847

Freundlich

50.3955 179.2698 23.3665

b 0.0042402 0.0020152 0.009701

n 1.4203 1.4426 1.7413

R-squared 0.99839 0.99586 0.99721

Sips

232.0382 399.4676 100.7033

b 0.0013378 0.0018034 0.0022754

n 0.78099 0.64366 0.48246

R-squared 1.00000 0.99989 0.99979

Table 3. Pseudo-rst and second-order kinetics parameters of PCE

adsorption on doped activated nano carbons

Isotherms

KNCWS-11

Adsorbents

KNCWS-21 KNCWS-31

pseudo-rst-order

140.582 295.911 84.529

0.0309 0.031 0.095

R-squared

0.990 0.982 0.981

pseudo-second-order

176.946 382.646 0.001

0.0001 7.603 0.001

R-squared

0.970 0.960 0.995

Anal. Method Environ. Chem. J. 3 (4) (2020) 30-39

varies considerably due to their structural and

surface roperties. Nitrogen plays an important

role in the adsorption of PCE. Samples of

KNCWS-11, KNCWS-21, and KNCWS-31 at

initial concentrations of 1000 ppm have adsorption

rates of 166, 285, and 95 mg g

-1

, respectively.

Therefore, the use of nitrogen-doped activated

nano carbons as a green adsorbent provides a cost-

effective means of combating biomass waste and

can partially reduce the climate change caused by

PCE vapor. Contaminated PCE/ N

at the inlet and

outlet of the column was analyzed online with GC-

MS analyzer.

5. Acknowledgments

The authors wish to thank the Chemical Engineering

Department, Islamic Azad University, North Tehran

Branch, Tehran, Iran

6. References

[1] M. Słomińska, P. Konieczka, J. Namieśnik,

The fate of BTEX compounds in ambient air,

Crit. Rev. Environ. Sci. Technol., 44 (2014)

455–472.

[2] L. Yu, L. Wang, W. Xu, L. Chen, M. Fu, J.

Wu, D. Ye, Adsorption of VOCs on reduced

graphene oxide, J. Environ. Sci., 67 (2018)

171–178.

[3] S. Jafari, F. Ghorbani-Shahna, A. Bahrami,

H. Kazemian, Adsorptive removal of toluene

and carbon tetrachloride from gas phase

using zeolitic imidazolate framework-8:

Effects of synthesis method, particle size, and

pretreatment of the adsorbent, Microporous

Mesoporous Mater., 268 (2018) 58–68.

[4] C. Dai, Y. Zhou, H. Peng, S. Huang, P. Qin,

J. Zhang, Y. Yang, L. Luo, X. Zhang, Current

progress in remediation of chlorinated

volatile organic compounds: A review, J. Ind.

Eng. Chem., 62 (2018) 106–119.

[5] R.E. Doherty, A history of the production and

use of carbon tetrachloride, tetrachloroethylene,

trichloroethylene and 1,1,1-trichloroethane in

the United States: Part 1--historical background;

carbon tetrachloride and tetrachloroethylene,

Environ. Forensics., 1 (2000) 69–81.

[6] B. Huang, C. Lei, C. Wei, G. Zeng, Chlorinated

volatile organic compounds (Cl-VOCs)

in environment sources, potential human

health impacts, and current remediation

technologies, Environ. Int., 71 (2014) 118–

138.

[7] C. Barton, Tetrachloroethylene, in: P.

Wexler (Ed.), Encycl. Toxicol. (Third Ed.,

Third Edition, Academic Press, Oxford, pp.

498–502, 2014. https://doi.org/https://doi.

org/10.1016/B978-0-12-386454-3.00436-X.

[8] J.D. Tucker, K.J. Sorensen, A.M. Ruder,

L.T. McKernan, C.L. Forrester, M.A. Butler,

Cytogenetic analysis of an exposed-referent

study: perchloroethylene-exposed dry

cleaners compared to unexposed laundry

workers, Environ. Heal., 10 (2011) 16.

[9] Y. Zeng, Z. Zeng, T. Ju, F. Zhang,

Adsorption performance and mechanism of

perchloroethylene on a novel nano composite

β-FeOOH-AC, Micropor. Mesopor. Mater.,

210 (2015) 60–68.

[10] Z. Rouzitalab, D.M. Maklavany, S.

Jafarinejad, A. Rashidi, Lignocellulose-based

adsorbents: A spotlight review of the effective

parameters on carbon dioxide capture process,

Chemosphere, 246 (2020). https://doi.

org/10.1016/j.chemosphere.2019.125756.

[11] E. Jangodaz, E. Alaie, A.A. Safekordi, S.

Tasharro, Adsorption of ethylbenzene

from air on metal–organic frameworks

MIL-101(Cr) and MIL-53(Fe) at room

temperature, J. Inorg. Organomet. Polym.

Mater., 28 (2018) 2090–2099.

[12] M. Thommes, K. Kaneko, A. V Neimark, J.P.

Olivier, F. Rodriguez-Reinoso, J. Rouquerol,

K.S.W. Sing, Physisorption of gases, with

special reference to the evaluation of surface

area and pore size distribution (IUPAC

Technical Report), Pure Appl. Chem., 87

(2015) 1051–1069.

[13] M.S. Shafeeyan, W.M.A.W. Daud, A.

Houshmand, A. Arami-Niya, Ammonia

modication of activated carbon to enhance

Perchloroethylene vapor adsorption by N-doped porous carbon Mohammad Ghasemi Kahangi et al

carbon dioxide adsorption: Effect of pre-

oxidation, Appl. Surf. Sci., 257 (2011) 3936–

3942.

[14] Z. Rouzitalab, D. Mohammady Maklavany,

A. Rashidi, S. Jafarinejad, Synthesis of

N-doped nanoporous carbon from walnut

shell for enhancing CO2 adsorption capacity

and separation, J. Environ. Chem. Eng., 6

(2018) 6653–6663.

[15] J. Chen, J. Yang, G. Hu, X. Hu, Z. Li, S. Shen,

M. Radosz, M. Fan, Enhanced CO2 capture

capacity of nitrogen-doped biomass-derived

porous carbons, ACS Sustain. Chem. Eng., 4

(2016) 1439–1445.

[16] A. Heidari, H. Younesi, A. Rashidi, A.A.

Ghoreyshi, Evaluation of CO2 adsorption

with eucalyptus wood based activated carbon

modied by ammonia solution through heat

treatment, Chem. Eng., J. 254 (2014) 503–

513.

[17] A. Kongnoo, P. Intharapat, P. Worathanakul,

C. Phalakornkule, Diethanolamine

impregnated palm shell activated carbon for

CO2 adsorption at elevated temperatures, J.

Environ. Chem. Eng., 4 (2016) 73–81.

[18] Z. Xiong, Z. Shihong, Y. Haiping, S. Tao, C.

Yingquan, C. Hanping, Inuence of NH3/

CO2 modication on the characteristic of

biochar and the CO2 capture, Bioenergy

Res., 6 (2013) 1147–1153.

[19] Z. Geng, Q. Xiao, H. Lv, B. Li, H. Wu, Y. Lu,

C. Zhang, One-step synthesis of microporous

carbon monoliths derived from biomass

with high nitrogen doping content for highly

selective CO2 capture, Sci. Rep., 6 (2016)

4–11.

[20] N.S. Nasri, U.D. Hamza, S.N. Ismail, M.M.

Ahmed, R. Mohsin, Assessment of porous

carbons derived from sustainable palm solid

waste for carbon dioxide capture, J. Clean.

Prod., 71 (2014) 148–157.

[21] N. Álvarez-Gutiérrez, M. V. Gil, F. Rubiera,

C. Pevida, Kinetics of CO2 adsorption on

cherry stone-based carbons in CO2/CH4

separations, Chem. Eng. J., 307 (2017) 249–

257.

[22] Y.S. Ho, G. McKay, Pseudo-second order

model for sorption processes, Process

Biochem., 34 (1999) 451–465.