Nghiên cứu sự hấp phụ của 2,4 d và 2,4,5 t trong môi trường nước bằng vật liệu ống nano cacbon (CNTs) tt tiếng anh

MINISTRY OF EDUCATION AND TRAINING

MINISTRY OF NATIONAL DEFENSE

ACADEMY OF MILITARY SCIENCE AND TECHNOLOGY
--------------------------

HOANG KIM HUE

STUDY OF THE ADSORPTION OF 2,4-D AND 2,4,5-T
FROM AQUEOUS SOLUTION
ONTO CARBON NANOTUBES (CNTs)

Specialization: Theoretical and Physical chemistry
Code: 9 44 01 19

SUMMARY OF PhD THESIS IN CHEMISTRY

HA NOI - 2019


The dissertation completed at:
Academy of Military Science and Technology

Academic supervisors:
1. Dr Lam Vinh Anh
2. Dr To Van Thiep

Reviewer 1: Prof.Dr Ta Ngoc Don
Hanoi University of Science and Technology
Reviewer 2: Assoc.Prof.Dr Vu Anh Tuan

Vietnam Academy of Science and Technology
Reviewer 3: Assoc.Prof.Dr Nguyễn Manh Tuong
Academy of Military Science and Technology

The dissertation will be defended in front of Doctor Examining Committee
held at Academy of Military Science and Technology at … … …, … … …
…, 2019.

The thesis can be found at:
- The Library of Academy of Military Science and Technology
- Vietnam National Library


THE SCIENTIFIC PUBLICATIONS
1. Hoang Kim Hue, Lam Vinh Anh, To Van Thiep, Pham Trung Kien
(2016), Some initial results of the purification of carbon nanotubes (CNTs),
synthesized by the chemical vapor deposition (CVD) method, Journal of
catalysis and adsorption, 5(3), pp. 52 - 62.
2. Hoang Kim Hue, Lam Vinh Anh, To Van Thiep, Phung Thi Lan (2017),
Study of the purification of carbon nanotube materials to be applied in the
adsorption of 2,4-D herbicide in the aqueous solution, Journal of catalysis
and adsorption, 6(2), pp. 100 - 106.
3. Hoang Kim Hue, Lâm Vinh Anh, To Van Thiep, Nguyen Hoang Dung
(2017), Study of the activation of carbon nanotube materials to be applied
in the adsorption of 2,4-dichlorophenoxyacetic acid, Journal of
ScienceReseach and Military Technology, 52, pp. 186 - 193.
4. Hoang Kim Hue, Lam Vinh Anh, To Van Thiep (2018), Study of the
adsorption of 2,4-dichlorophenoxyacetic acid from the aqueous solution
onto carbon nanotubes, Vietnam Journal of Chemistry, 56(2), pp. 191 - 197.
5. Hoang Kim Hue, Lam Vinh Anh, Dinh Bao Trong (2018), Study of the

adsorption of 2,4-dichlorophenoxyacetic acid from the aqueous solution
onto activated carbon, Vietnam Journal of Chemistry, 56(2), pp. 208 - 214.
6. Hoang Kim Hue, Lam Vinh Anh, Le Minh Cam (2018), Comparative
Study of 2,4-Dichlorophenoxyacetic Acid Adsorption onto Alkali-activated
Carbon Nanotubes and Activated Carbon, Eleventh internation conference
on the Remediation of chlorinated and Recalcitrant compounds, California,
US, pp. 32.
7. Hoang Kim Hue, Lam Vinh Anh (2018), Study of the adsorption of 2,4,5T herbicide in the aqueous phase on activated carbon nanotubes, Journal of
catalysis and adsorption, 7(1), pp. 78 - 85.



1
INTRODUCTION
Due to Vietnam war’s consequences, Bien Hoa, Phu Cat and Da Nang air
bases were severely polluted by 2,4-dichlorophenoxyacetic acid (2,4-D),
2,4,5-trichlorophenoxyacetic acid (2,4,5-T) herbicides. The estimated
volume of contaminated soils and sediments is approximately 700000 m3.
To date, a volume of nearly 90000 m3 at Da Nang air base has been treated
by In-pile thermal desorption with the support of Vietnamese and American
governments. And a volume of 225000 m3 of contaminated soils and
sediments has been treated by the isolation landfilling at Phu Cat, Bien Hoa
and Da Nang air bases. A huge volume of remaining contaminated soils,
sediments and water at Bien Hoa air base requires a suitable treatment
technology. Nevertheless, all existing treatment technologies in Vietnam
produce the solution of herbicides as a by-product, then a further treatment
by adsorbents is required.
Currently, there is a fast development in the study and application of
adsorbents in the environmental treatment. Scientists continue to invent new
materials with better adsorption capability. In some recent decades, carbon

nanotubes (CNTs) have drawn attention in study.
CNTs have uniformly porous structure, porous force, hydrophobicity and
ability to form π - π interaction with molecules of 2,4-D, 2,4,5-T and Dioxin.
In addition, CNTs also possesses thermal stability, then, they could be
recycled. Therefore, CNTs are forecasted to be a promising adsorption
material for the treatment of the solution of 2,4-D, 2,4,5-T and Dioxin.
The dissertation title: “study of the adsorption of 2,4-D and 2,4,5-T from
the aqueous solution onto carbon nanotubes (CNTs)”.
The objective of the dissertation:
To set up a purification and activation procedure from raw CNTs,
synthesized in Vietnam to produce purified and activated CNTs to be used
in the adsorption of 2,4-D and 2,4,5-T herbicides.
To study the adsorption characteristics of 2,4-D and 2,4,5-T herbicides on
purified CNTs and activated CNTs in the aqueous solution.
Main content of the dissertation:
Developing a purification procedure from raw CNTs in Vietnam.
Selecting activation condition for purified CNTs.
Studying factors, affecting the adsorption capability of 2,4-D on purified
CNTs and activated CNTs.
Studying and setting up the adsorption isotherm, kinetics and
thermodynamic parameters and activation energy of the adsorption of


2
2,4-D on purified CNTs and activated CNTs.
Studying and setting up the adsorption isotherm, kinetics and
thermodynamic parameters and activation energy of the adsorption of
2,4,5-T on activated CNTs.
Academic and practical contributions of the dissertation:
Studied and developed the purification procedure from raw CNTs in

Vietnam and the activation methods from purified CNTs to increase purity,
specific surface area, porous volume and 2,4-D adsorption capability of
CNTs.
Studied and defined adsorption characteristics of 2,4-D and 2,4,5-T on
purified CNTs and activated CNTs.
Structure of the dissertation:
Introduction; Chapter 1: Overview; Chapter 2: Study objects and
methods; Chapter 3: Results and discussions; Conclusion.
CHAPTER 1. OVERVIEW
1.1 Introduction of 2,4-D and 2,4,5-T herbicides
1.1.1 Use history and toxicity
2,4-D and 2,4,5-T were used in agriculture since 40th decade of the
previous century. Their compounds were studied and mixed into herbicides
to spray in the south from 1961 to 1971 during the Vietnam war. Currently,
2,4-D and 2,4,5-T have been prohibited in many countries due to its severe
toxicity to eyes, nerve system, endocrine, immune system and its possibility
to cause blood cancer. Especially, 2,4,5-Trichlorophenol molecules as a
by-product of the decomposition of 2,4,5-T could combine together to form
Dioxin if existing long time in the environment.
1.1.2 Structure, chemical and physical properties of 2,4-D and 2,4,5-T
- 2,4-D and 2,4,5-T are among acids with phenoxy group, and there is
conjugated π electron system of benzene ring in their molecules.
- pKa,2,4-D = 2.73; pKa,2,4,5-T = 2.88.
- logKOW,2,4-D = 2.81; logKOW,2,4,5-T = 4.00.
- In aqueous environment, 2,4-D and 2,4,5-T dissolve into acidic radicals.
1.1.3 Source and current situation of 2,4-D and 2,4,5-T pollution in Vietnam
- Herbicides used in the agriculture.
- Herbicides used by US army during the Vietnam war.
1.1.4 Some treatment methods for the source of herbicides, used in Vietnam
war

1.1.5 Current situation of research on the adsorption of 2,4-D and 2,4,5-T
onto carbon materials in the aqueous environment


3
1.2 Carbon nanotube materials and their adsorption characteristics to
organic compounds
1.2.1 Overview of carbon nanotubes
1.2.2 Structure of CNTs
1.2.2.1 Crystalline structure
Carbon nanotubes (CNTs) are made by rolling up of sheet of graphene
into a hollow cylinders with two ends covered by fullurene hemispheres.
The diameter of the carbon nanotube is about a few nm, with a length of
some μm to several centimeters. The covalent carbon atoms together make
up the tightened six-sided rings. Each carbon atom has four electrons in the
outer layer that make up the three bonds σ that have sp2 and π orbital hybrids.
However, according to number of walls in the tube, carbon nanotues are
divided into single walled nanotubes (SWCNTs) and multi walled
nanotubes (MWCNTs).
Due to the interaction force π-π, carbon nanotues tend to aggregate into
bundles, the distance between tubes in a bundle is approximately 0.34 nm.
In addition, carbon nanotubes could have hetero-elemental defects,
expanding or narrowing the six-sided rings.
1.2.2.2 Pore structure
The surface area of the carbon nanotubes depends on the tube diameter,
the number of walls, open ends or closed ends, the aggregation into bundles
or seperation, functional groups on the surface of tubes or metal impurities
in the tube core.
The tube core of SWCNTs has a diameter of less than 2 nm while, the
diameter of the tube core of MWCNTs is normally from 2 to 15 nm.

1.2.3 Surface chemistry of CNTs
1.2.4 Adsorption characteristics of organic compounds on CNTs
1.2.4.1 Non-uniform adsorption
1.2.4.2 Multi-mechanisms of interaction
Hydrophobic interaction, electrostatic interaction, π-π interaction,
hydrogen bonding interaction.
1.2.4.3 Factors affecting the adsorption
- Affected by the properties of CNTs: surface area, capillary volume, tube
diameter, oxygen functional groups, aggregation into bundles.
- Affected by organic compounds: molecular geometry, functional
groups.
- Affected by environmental conditions: temperature, pH, ionic force.
1.2.5 Method of preparing CNTs
1.2.5.1 Method of synthesis


4
Carbon nanotubes can be synthesized by arc, laser and chemical vapor
deposition (CVD) methods. Among them, the CVD method is widely used
because of the high synthesis yield, but CNTs synthesized by this method
contain many impurities such as amorphous carbon, aromatic organic
compounds and metals.
1.2.5.2 Purification methods
- Chemical methods using techniques such as: oxidation in the gas phase,
oxidation in liquid phase, electro-oxidation and treatment with HCl.
- Physical methods using techniques such as filtering, centrifugation and
tempering.
- Integration method: combining physical and chemical methods.
1.2.5.3 Activation method
Chemical methods is more effective in increasing the surface area and in

narrowing capillary size distribution of CNTs than physical methods.
1.3 Theory of adsorption applied in the dissertation
1.3.1 Concept and classification of adsorption
1.3.2 Equation of adsorption isotherm
The forms of the Langmuir (1.12) and Freundlich (1.15) can be expressed
respectively as:
1
KL Ce
(1.12)
(1.15)
qe = qm
qe = KF Cne
1 + KL Ce
Whereas qmax is the maximum adsorption capacity (mg/g), and K L is the
Langmuir adsorption equilibrium constant (L/mg), which is related to the
free energy of adsorption. KF (mg1-n.g-1.Ln) represents the adsorption
capacity when adsorbate equilibrium concentration equals to 1, and n
represents the degree of adsorption dependence at equilibrium
concentration. KT is the equilibrium binding constant, BT is related to heat
of adsorption, T is the absolute temperature (K).
Whereas, Ce (mg/L) is the concentration of the solute at the equilibrium,
qe: the the adsorbed amount at the equilibrium (mg/g), K L: Langmuir
constant, KF (l/g) and: Freundlich constants.
1.3.3 Equation of adsorption kinetics
- Two equations of pseudo first and second order apparent kinetics can be
expressed as (1.17) and (1.18), respectively:
t
1
1
=

+ t (1.18) v0 = k2 q2e (1.19)
ln(qe - qt ) = lnqt - k1 t (1.17)
q t v0 q e
Whereas, k1 (minutes-1) and k2 (g/mg.minutes): the rate constant of first
and second pseudo apparent kinetics, qt: the adsorbed amount at the moment
t (mg/g), v0: initial adsorption rate.


5
- Weber - Morris diffusion kinetics model is expressed as:
(1.21)
qt = kd t1⁄2 + L
Where kd (mg/g min1/2) is the intra-particle diffusion rate constant
(mg/g.minutes0.5).
If L is equal to 0: the intra-particle diffusion controls the adsorption rate.
If L is different to 0: both film diffusion and intra-particle diffusion affect
the adsorption.
1.3.4 Thermodynamic and kinetic conditions
CHARPTER 2. STUDY OBJECTS AND METHODS
2.1 Study objects
- 2,4-D and 2,4,5-T herbicides in the aqueous environment.
- CNTs synthesized in Vietnam (CNT-TH).
2.2 Chemicals and equipment
2.2.1 Hóa chất
Reference substances of 2,4-D and 2,4,5-T have the purity of 99.9 %.
Toluene, ACN, HCl, HF, HNO3, CH3COOH, KOH, NaOH and CaCl2 have
analytical purity grade, N2 is made in Vietnam with the purity of 99.999 %,
CNT-TH is synthesized in Vietnam by CVD method with tube diameter of
10 to 30 nm and SBET: 170 - 200 m2/g. CNT-TQ is MWCNTs of China with
tube diameter of 10 to 20 nm and tube length of 5 - 15 µm and the purity of

over 97 %, Shirasagi activated carbon - Z1 (AC) is made in Japan, used in
the study of the integration technology.
2.2.2 Equipment
Agilent HP-1100: high performance liquid chromatography (HPLC),
Agilent GC-6890: gas chromatograph and Agilent MS 5975: signal detector,
AAS - 300 - USA: atomic absorption spectrometer, Hanna HI 2211: pH
meter, Mettler Toledo AB204: analytical Balance, Soxhlet extractor,
Ultrasound machine, Shaking machine with temperature control, Vacuum
dryer; SRJX Tube Furnace - 2,5 - 13.
2.3 Study methods
2.3.1 Set up the purification procedure for CNT-TH
The purification process is based on techniques that include: Soxhlet
extraction, oxidation in liquid phase with HNO3, oxidation in air, treatment
with HCl and HF, tempered at 900 ˚C in N2 gas.
The efficiency (HTC) of the purification process and Fe removal efficiency
(HFe) are calculated according to equations (2.1) and (2.2) as follows:
mFe,s
ms
(
)
H
%
=
∙100 (2.2)
(
)
HTC % =
∙100
(2.1)
Fe

mFe,t
mt


6
Whereas, mt: the sample weight (g) before the purification, ms: the sample
weight after the purification (g), mFe,t: the amount of Fe in the sample before
the purification, mFe,s: the amount of Fe in the sample after the purificaiton.
2.3.2 Investigation of activation conditions for CNT-TC
The mixture of CNT-TC and KOH at the ratio of a/1 was mechanically
ground, then heated at a temperature of x ˚C in N2 gas atmosphere with flow
rate of z mL/min. Rinsed the product with HCl and distilled water to neutral
medium, dried and stored in a desiccator.
2.3.3 Investigation of the adsorption process
2.2.3.1 Preparation of the adsorption solution
2.3.3.2 Adsorption conditions
The adsorption process was studied by batch method. The investigated
concentrations of was in the range from 52.2 to 205.7 mg/L with 2,4-D and
from 53.0 to 200.0 mg/L with 2,4,5-T.
- Adsorption isotherm was studied at: volume of solution (V): 50 mL, 2,4D adsorbent amount (m2,4-D): 50 mg, 2,4,5-T adsorbent amount (m2,4,5-T): 25
mg, pH = 6, temperature: 30 ˚C, shaking speed: 150 rpm, sampling time: 24
hours.
- Adsorption kinetics was studied at: V: 100 mL, m2,4-D: 100 mg, m2,4,5-T:
50 mg, pH = 6, temperature: 30 ˚C, shaking speed: 150 rpm, sampling time:
1, 2, 5, 8, 10, 12, 15, 20, 30, 40, 60, 90 and 120 minutes.
- Condition investigation: temperature: 10, 20, 30 and 40 ˚C, pH: 3 - 9,
ionic force (concentration of CaCl2) at 0, 0.005, 0.01, 0.1, 0.5 and 1 mol/L.
Sample volume, filtered by ultrafiltration membrane: 0,5 mL.
2.3.3.3 Determination of the adsorption capacity of the material
The adsorption amount at the time (qt,mg/L) and at the equilibrium

(qe,mg/L) are calculated as follows:
C0 - Ct
C0 - Ce
(2.3)
(2.4)
qt =
∙V
qe =
∙V
m
m
Adsorption efficiency (HHp):
C0 - Ce
HHP =
∙100
(2.5)
C0
Whereas, C0, Ct, Ce (mg/L) are concentrations of 2,4-D or 2,4,5-T in the
solutions at the initial time, moment t and at the equilibrium, respectively.
2.2.3.4 Set up adsorption isotherm
Following the linear regression of experimental data.
2.2.3.5 Set up adsorption kinetics
Following the linear regression of experimental data.
2.2.3.6 Evaluation of the fitting of the isothermal model


7
The fitting level of the model was evaluated based on the correlation
coefficient of linear equation (R2) value and average relative error (ARE),
calculated by the following formula:

p
|qe,tti - qe,tni |
100
(2.6)
ARE=
∑
p
qe,tni
i-1

Whereas, p is number of data points, qe,tni: experimental adsorption
amount (mg/g) at the point i, qe,tti: calculated adsorption amount at the point
i (mg/g).
2.3.4 Determination of 2,4-D and 2,4,5-T concentrations on HPLC
The concentration of 2,4-D and 2,4,5-T in solution was analyzed on HPLC,
according to the following parameters: mobile phase of ACN: H2O: Acetic
acid at the ratio of 50: 49: 1 (V: V: V), flow rate: 1 ml / min, injected sample
volume: 20 μl, wavelength: λ = 280 nm, column temperature: 30˚C.
2.3.6 Analysis methods of material composition and structure
Characteristics of structure and composition of materials were detected by
modern methods such as TEM, XRD, EDX, IR, Raman, TEM, SEM, TGA
/DTA, N2 adsorption-desorption isotherm, acid-base titration, analysis of
impurities in CNTs by GC-MS method, analysis of Fe in CNTs by AAS
method.
CHAPTER 3. RESULTS AND DISCUSSIONS
3.1 Study on the purification of carbon nanotubes
3.1.1 Structural characteristics and impurities of CNT-TH
3.1.1.1 Structural characteristics
Lin (Cps)

400

CNT-TH

300
CNT-TQ

200
100
0
20

40

60

80

2-Theta-Scale

Figure 3.1: XRD diagram of
Figure 3.2:TEM image
CNT-TH and CNT-TQ
of CNT-TH
XRD diffraction spectra of CNT-TH and CNT-TQ were similar (Figure
3.1), with the characteristic intensity of the graphite reflectance planes. TEM
image also indicated that CNT-TH is in tubular form with closed ends, outer
diameter of the tube from 10 to 30 nm (Figure 3.2), proving that CNT-TH
is MWCNTs. CNT-THs have their specific surface area, volume and
capillary diameter of 170 m2/ g, 0.897 cm3/g and 21.133 nm, respectively.

8
3.1.1.2 Impurity components and purity of CNT-TH

Figure 3.5: TGA/DTA diagram of CNT-TH Figure 3.6: EDX diagram
in air environment
of CNT-TH
Results of GC-MS, TEM, TGA/DTA analysis (Figure 3.5) and EDX
analysis (Figure 3.6) showed that CNT-TH has organic impurities,
amorphous carbon, Al, Fe and Si. The purity of CNT-TH was determined to
be about 75.43 %.
3.1.2 Set up the purification procedure for CNT-TH
3.1.2.1 Removal of organic impurities
Organic impurities can be removed by Soxhlet extraction of CNT-TH
with toluene, extraction time is 6, 12, 24 and 48 hours. Results of GC-MS
extraction showed that the suitable extraction time was 24 hours.
3.1.2.2 Removal of metal impurities
* Selection of techniques to remove metals
Al can be removed with HNO3 or HCl, Si can be
removed by HF. The most difficult is the removal
of Fe, locked in the core CNT-TH. Therefore, it is
necessary to select a technique to remove Fe.
(a)
Results showed that the KL2 obtained during
CNT-TH treatment with HNO3 had the lowest Fe
content of 0.06 %, but the HHP(2,4-D) of KL2 was
lower than that of CNT-TH. Therefore HNO3 is not
used to purify CNT-TH. If only oxidized in air or
(b)

treated with HCl, HHP(2,4-D) and HFe inconsiderably
increased. When CNT-TH was oxidized in air, then Figure 3.9: SEM image
treated with HCl (HK4), HFe increased to 73.56 % of KL2 (a) and KL4 (b)
and HHP(2,4-D) increased from 64.39 to 69.56 %. Besides, carbon nanotubes
of the sample KL4 was not cut into short sections like in the sample KL2
(Figure 3.9). In addition, if the above procedure was repeated twice, HFe
increased to 94.42 % and HHP(2,4-D) to 80.16 %. Thus, the chosen techniques
for the removal of Fe, Al and Si are oxidation in air and treatment with HCl,
HF. Oxidation of CNT-TH in the air also removed the amorphous cabon.


9
Therefore, it is necessary to investigate appropriate oxidation conditions in
the air.
* Selection of oxidation conditions in air
The oxidation conditions in the air were investigated including
temperatures at 360, 400, 420, 440 and 460 ˚C, time of oxidation in air at 1,
2, 3, 4 and 5 hours fort he first time. The oxidation times were 20, 40, 60
and 90 minutes fort the second time. Selection criteria based on the
evaluation of HFe, HHP(2,4-D) and HTC included the temperature: 440 ˚C, first
oxidation time : 4 hours, second oxidation time: 40 minutes.
3.1.2.3 Removal of oxygen functional groups
The oxidation in the air, however, occurs "more smoothly" than HNO 3
oxidation, but also forms oxygen-based groups on the surface of CNTs.
This reduces the purity of carbon nanotubes and the adsorption capacity of
2,4-D can be reduced, if the content of the oxidizing group on the CNTs is
between 3.84 and 22.8 %. Therefore, CNTs need to be tempered at 900 ˚C
in N2 gas. In this study, the duration of tempering process was 0, 0.5, 1, 2, 3
and 4 hours. Results showed that the appropriate tempering time was 1 hour.
Conclusion:

Based on the above results, the
suitable purification process for
CNT-TH was shown in Figure 3.14.
The application of the purification
process for CNT-TH samples was able
to remove 94.42 % Fe and the
adsorption capacity of 2,4-D increased
from 64.3 to 83.24 %. Purified carbon
nanotube samples were symbolized as Figure 3.14: Purification procedure
for CNT-TH
CNT-TC.
3.1.3 Physico-chemical characteristics and purity of CNT-TC
3.1.3.1 Physico-chemical properties
XRD and TEM results showed that the
purification process did not destroy the
crystalline structure of MWCNTs. CNT-TC
have long filament shape with hollow core
and open ends. Black particles in the core and
on the walls were considerably reduced. The
BET specific surface area increased from 170
to 267 m2/g and the capillary volume Figure 3.16: TEM image of
CNT-TC
increased from 0.897 to 1.426 cm3/g.


10

(a)

100

95
90
85

(b)

1700

SBET (m2/g)

Hiệu suất hấp phụ
(%)

3.1.3.2 Purity of CNT-TC
The TGA results showed that CNT-TC lost
about 99.91 % by weight due to the burning
of CNTs at 500 - 700 °C. The residue
obtained after 900 °C is about 0.09 %, being
consistent with the amout of Fe of 0.08 %,
determined by AAS. In addition, there were
almost no signals of Al, Si and Fe in EDX
diagram of CNT-TC. There were only signals Hình 3.20: Giản đồ EDX
của CNT-TC
for 97.70 % of C and 2.30 % of O.
Therefore, the purity of CNT-TC is about 97.61 % and CNT-TC has the
higher quality grade than CNT-TQ.
3.2 Study of material chemistry
3.2.1 Activation conditions of CNT-TC
1200
700

200

80
CNT-TC CNT-HK CNT-HNa

CNT-TC CNT-HK CNT-HNa

AC

AC

100
98
96
94
92
90

600

(a)
SBET (m2/g)

Hiệu suất hấp phụ
(%)

Figure 3.21: Adsorption capacity of 2,4-D (a) and surface area (b)
of CNT-TC, CNT-HK, CNT-HNa and AC (C0 = 52.2 mg/L)
(b)

500

400
300
200

1

2

3

4

5

6

7

8

1

2

3

4

5

6

7

8

Ratio of KOH/CNT-TC

Ratio of KOH/CNT-TC

Figure 3.22: Effect of KOH ratio, used to activate CNT-TC on the
adsorption of 2,4-D (a) and the surface (b) of HKi (C0 = 52.2 mg/L)
98
96
94
92

99

Adsorption
efficiency (%)

Adsorption
efficiency (%)

Adsorption
efficiency (%)

100

98
97

500 600 700 800 900 1000

Temperature (˚C)

98
96
94

96

90

100

0 0.5 1 1.5 2 2.5 3

Time (hours)

0

500

1000

1500

Gas blowing rate (mL/min)

Figure 3.23
Figure 3.24
Figure 3.25
Figure 3.23, 3.24, 3.25: Effect of temperature, time and N2 blowing rate on
the adsorption of 2,4-D on HKi (C0 = 52.2 mg/L)


11
Appropriate conditions to activate CNT-TC included: KOH as activating
agent, ratio of KOH/CNT-TC: 5/1, temperature: 800 ˚C, activation time: 1
hour and N2 blowing rate: 500 mL/min.
3.2.2 Physico chemical characteristics of CNT-HKi

0.12

Absorbance(Abs)

dV/dD (cm3/g.nm)

Figure 3.27: TEM image of CNT-HK5
0.09
0.06
0.03
0

0.05
0.04

0.03

CNT-TC
CNT-HK5

0.02
0.01

0
2

3

4

5

D (nm)

CNT-TC
CNT-HK5

6

3500

CNT-HK3
CNT-HK7

Wavenumber

Relative intensity

Figure 3.29: Capillary size
distribution of CNT-TC, CNT-HKi
1.2
1
0.8
0.6
0.4
0.2
0
1000

2500

1500

500

(cm-1)

Figure 3.30: IR Spectra of
CNT-TC and CNT-HK5
CNT-TC
CNT-HK5

1200

1400

1600

1800

Frequency (cm-1)

Figure 3.32: Raman spectra of CNT-TC and CNT-HK5
Table 3.8: pHPZC of CNT-TC and CNT-HKi
Material
CNT-TC
CNT-HK3 CNT-HK5 CNT-HK7
pHPZC
8.45
7.40
7.10
6.80
According to the XRD results, the activation did not destroy the
crystalline structure of MWCNTs, but increased the defect (Figure 3.27).
The specific surface area was significantly increased from 267 to 540 m2/g
and the volume of the capillary in the diameter range from 3.2 to 4.2 nm was
increased to 2.5 times (Figure 3.29). Activated CNTs did not have any
different functional groups from CNT-TC (Figure 3.30), but the pHPZC value
decreased with the increase in the KOH/CNT-TC ratio (Table 3.8). In
addition, Raman analysis showed that the density per unit area of sp2 carbon
decreased after activation (Figure 3.32).


12

Adsorption
efficiency (%)

Adsorption
efficiency (%)

qe(mg/g)

qe(mg/g)

qe (mg/g)

3.3 Study of adsorption kinetics of 2,4-D and 2,4,5-T on carbon
nanotubes
3.3.1 Some factors affecting the adsorption capacity of 2,4-D on CNT-TC
and CNT-HKi
CNT-TC
CNT-HK3
3.3.1.1 Effect of initial concentration of 2,4-D
CNT-HK5
CNT-HK7
3.3.1.2 Effect of temperature
150
The adsorption capacity of 2,4-D of 120
90
CNT-TC and CNT-HKi decreased with the
60
increase of temperature (Figure 3.34). This is a
30
50 75 100 125 150 175 200

sign of exothermic adsorption and physical
Cₒ (mg/l)
nature.
Figure 3.33: Effect of initial
3.3.1.3 Effect of pH
concentration of 2,4-D
The adsorption capacity of 2,4-D on
48
(a)
CNT-TC and CNT-HKi decreased as the pH of
46
the solution approached pHPZC, then negligibly
44
decreased if the pH of the solution exceeds the
42
40
this pH value (Figure 3.35). This could be
0
10
20
30
40
50
explained by the electrostatic interactions.
T (˚C)
Indeed, at pKa, 2,4-D= 2.73 < pHdd < pHPZC,
52
(b)
2,4-D existed as anion form in the solution
while the surface of adsorbents was positively 51.5

51
charged, therefore, there was electrostatic
interaction between them. The electrostatic 50.5
0
10
20
30
40
50
attraction force decreased as the pH
T (˚C)
approached pHPZC and the electrostatic
repulsion force occured at pHdd > pHPZC as the Figure 3.34: Effect of
temperature
surface of the material is negatively charged.
100
(a)
Thus, the adsorption capability decreased as
95
90
the electrostatic attraction decreased and the
85
negligibly changed when the repulsion force
80
between them appeared.
75
2 3 4 5 6 7 8 9 10
3.3.1.4 Effect of ionic force
pH
The 2,4-D adsorption capacity of CNT-TC

100
(b)
99
and CNT-HKi increased as the concentration
98
of CaCl2 in the solution increased. Because the
97
presence of salts in the solution induced
96
95
pressing on the diffusion layer on the material,
2 3 4 5 6 7 8 9 10
then favored the electrostatic attraction and
pH
thus favored the adsorption.
Figure 3.35: Effect of pH


13
3.3.2 Investigation of adsorption isotherm models of 2,4-D on CNT-TC and
CNT-HKi
3.3.2.1 Set up Langmuir adsorption isotherm model
Langmuir isotherm parameters were found to describe the adsorption
equilibrium of 2,4-D on CNT-TC/CNT-HKi and displayed in the table 3.9.
Table 3.9: Langmuir isotherm parameters of the adsorption of 2,4-D on
CNT-TC and CNT-HKi
T
qm
qmdt
KL

ARE
Material
RL
R2
2
(˚C) (mg/g) (µg/m ) (L/mg)
(%)
10 84.03 313.56 0.1475 0.1149 0.9980 5.87
20 84.03 313.56 0.0982 0.1631 0.9955 6.69
CNT-TC
30 83.33 310.95 0.0837 0.1861 0.9944 4.52
40 79.37 296.14 0.0754 0.2024 0.9947 4.49
10 140.85 304.86 0.5000 0.0369 0.9979 12.18
20 138.89 300.63 0.3258 0.0555 0.9964 14.92
CNT-HK3
30 135.14 292.50 0.2426 0.0731 0.9948 9.56
40 128.87 281.10 0.1778 0.0972 0.9944 11.33
10 156.25 289.35 0.4476 0.0410 0.9968 13.85
20 151.52 280.58 0.3568 0.0509 0.9964 13.87
CNT-HK5
30 147.06 272.33 0.2528 0.0704 0.9944 12.83
40 142.86 264.55 0.1813 0.0955 0.9929 11.25
10 158.73 287.56 0.4286 0.0427 0.9940 16.49
20 153.85 278.71 0.3283 0.0551 0.9942 16.10
CNT-HK7
30 149.25 270.39 0.2659 0.0671 0.9937 13.80
40 147.06 266.41 0.1915 0.0908 0.9908 12.95
In table 3.9, the dependence of Ce/qe on Ce was displayed as linear lines
with the correlation coefficient R2 larger than 0.9908. Thus, parameters of
Langmuir models were highly reliable. On the other hand, the value R L is

between 0 and 1, indicating that the adsorption is favorable in the
investigated concentrion range. However, the value of ARE is quite large
with lowest value from 4.49 to 16.49 %. Therefore, Langmuir model could
be used to describe the adsorption isotherm equilibrium of 2,4-D on
CNT-TC and CNT-HKi.
The value of qm (mg/g) characterized the saturated monolayer per unit
weight of the material showing that the material with high specific surface
area and high capillary size distribution in the range of 3.2 - 4.2 nm high
will have higher 2,4-D adsorption capacity.
However, comparing the adsorption amount per unit surface area of the
material (qmdt, µg/m2) showed the opposite side. Thus, activation reduced


14
the adsorption capacity per unit area of the surface of the material. On the
other hand, according to the Raman results, activation reduced the number
of sp2 hybrid carbon per unit area of the surface, while sp2 hybrid carbon
atom CNTs has π orbital that can interact π-π with benzene ring in the
2,4-D molecule. Thus, it can be assumed that the decrease in π-π interaction
reduced the adsorption capacity per unit surface area of the material after
activation.
3.3.2.2 Freundlich isotherm model
Table 3.10: Freundlich isotherm parameters of the adsorption
of 2,4-D on CNT-TC and CNT-HKi
Material

T (˚C) KF (L/g)

n

1/n

R2

ARE (%)
10
35.823 5.7703 0.1733 0.9826
1.80
20
30.229 4.9628 0.2015 0.9748
2.71
CNT-TC
30
27.259 4.5935 0.2177 0.9900
1.33
40
24.179 4.3197 0.2315 0.9736
2.66
10
65.543 5.0582 0.1977 0.9563
6.16
20
59.033 4.8591 0.2058 0.9891
3.09
CNT-HK3
30
52.342 4.5208 0.2212 0.9818
3.17
40
45.966 4.2845 0.2334 0.9934

1.83
10
66.853 4.5086 0.2218 0.9766
4.74
20
61.529 4.3821 0.2282 0.9803
3.06
CNT-HK5
30
53.678 4.0733 0.2455 0.9861
3.42
40
46.829 3.8565 0.2593 0.9907
2.39
10
67.518 4.5025 0.2221 0.9818
4.66
20
61.886 4.3802 0.2283 0.9922
2.75
CNT-HK7
30
55.857 4.1425 0.2414 0.9894
3.01
40
49.043 3.9063 0.2560 0.9926
2.32
Freundlich isotherm parameters of the adsorption of 2,4-D on CNT-TC
and CNT-HKi in the table 3.10 indicated that the relationship between lnCe
and lnqe is linear with the correlation coefficient R2 larger than 0.9563. The

value of ARE is from 1.33 to 6.16 %, smaller than Langmuir model.
Therefore, Freundlich model is more fitting than Langmuir model.
The fitting of Freundlich model indicated that the surface of the material
is not uniform. As mentioned in above discussions, the adsorption of 2,4-D
on CNT-TC and CNT-HKi was dominated by the π-π interaction and
electrostatic force.


15
3.3.3 Determination of thermodynamic parameters of the adsorption of
2,4-D on CNT-TC and CNT-HKi
Thermodynamic parameters of the adsorption of 2,4-D on CNT-TC and
CNT-HKi were determined and presented in the table 3.13.
Table 3.13: Thermodynamic parameters of the adsorption
of 2,4-D on CNT-TC and CNT-HKi
ΔG0T (kJ/mol)
ΔH0298
ΔS0298
Material
283 K 293 K 303 K 313 K (kJ/mol) (J/mol.K)
CNT-TC
-13.387 -11.007 -11.508 -9.541 -49.144 -127.250
CNT-HK3 -16.749 -16.148 -14.804 -13.862 -45.069 -99.593
CNT-HK5 -15.802 -15.411 -14.223 -13.203 -41.184 -89.010
CNT-HK7 -15.887 -15.445 -14.654 -13.609 -37.382 -75.447
Table 3.13 indicated that the adsorption enthalpy change (∆H0298 ),
entropy change (ΔS0298 ), Gibb free engery change (∆G0T ) at 283, 293, 303
and 313 K were all negative. Therefore, the adsorption was spontaneous,
exothermic and the order of the system was increased. The adsorption heat
was in the range from 20 to 80 kJ/mol, characterizing the electrostatic

interaction and physical adsorption. With results from the investigation of
pH effect in the item 3.3.1.3, it was concluded that the electrostatic
interaction had significant impact on the adsorption capacity. On the other
hand, experiments were done at pH of 6, smaller pHPZC of CNT-TC and
CNT-HKi, then the electrostatic interaction was the attaction force. It is
notable that the 2,4-D adsorption heat on the study materials decreased with
the reduction of the pHPZC value. It was possible that the narrow gap between
pHPZC value of the adsorbents and pH value of the solution led to the
decrease in the electrostatic attraction force, reducing the exothermic heat
of the system.
3.3.4 Study of the adsorption of 2,4,5-T on CNT-HK5 and its comparison
with 2,4-D
3.3.4.1 Comparison of hydropobicity of 2,4-D and 2,4,5-T
Bipolar momentums of the bond between C-OCH2COOH and C-Cl were
named as: µ1 and µ2.
μ2,4-D = √μ21 + μ22 - μ1 μ2

(3.15)

μ2,4,5-T = √μ21 + μ22 - 2μ1 μ2 (3.16)

From the expressions (3.15) and (3.16), it was observed that: μ2,4,5-T <
𝜇2,4−𝐷 indicated that 2,4,5-T was less disolved in water than 2,4-D.


16
Therefore, 2,4,5-T was more hydrophobic than 2,4-D. This result was also
consistent with the value KOW, 2,4,5-T larger than KOW, 2,4-D.
3.3.4.2 Set up the adsorption isotherm of 2,4,5-T on CNT-HK5
Isothermal parameters of Langmuir and Freundlich models established to

decribe the adsorption of 2,4,5-T on CNT-HK5 were presented on the table
3.14.
Table 3.14: Isothermal parameters of Langmuir and Freundlich models
for the adsorption of 2,4,5-T on CNT-HK5
Parameters of isothermal models
T (˚C)
Langmuir
qm (mg/g) KL (L/mg)
RL
R2
ARE (%)
10
200.000
0.4505
0.0402 - 0.0110 0.9977
14.15
20
196.078
0.4080
0.0442 - 0.0121 0.9982
12.95
30
196.078
0.3091
0.0575 - 0.0159 0.9967
13.41
40
192.307
0.2905
0.0610 - 0.0169 0.9980

10.49
Freundlich
KF (L/g)
n
1/n
R2
ARE (%)
10
117.378
8.4962
0.1177
0.9752
3.08
20
113.137
8.2169
0.1217
0.9828
2.50
30
107.254
7.7640
0.1288
0.9886
1.91
40
101.291
7.2993
0.1370
0.9905

1.83
3.3.4.3 Determination of thermodynamic parameters of the adsorption of
2,4,5-T on CNT-HK5
Table 3.15: Thermodynamic parameters of the adsorption
of 2,4,5-T on CNT-HK5
T (˚C) ΔG0T (kJ/mol) ΔH0298 (kJ/mol) ΔS0298 (J/mol.K)
283
- 25.225
293
- 25.137
- 43.685
- 64.443
303
- 24.211
313
- 23.350
3.3.4.4 Comparison of the adsorption capacity of 2,4,5-T and 2,4-D on
CNT-HK5
Two typical parameters of the adsorption capacity of the system is qm and
KF. These values of the adsorption of 2,4-D and 2,4,5-T on CNT-HK5 at 10,
20, 30 and 40 ̊C were displayed in the table 3.16.


17
Table 3.16: Comparison of qm and KF values of the adsorption
of 2,4-D and 2,4,5-T on CNT-HK5 at different temperatures
qm (mg/g)
KF (L/g)
Temperature˚C
2,4-D

2,4,5-T
2,4-D
2,4,5-T
10
156.250
200.000
66.853
117.378
20
151.515
196.078
61.529
113.137
30
147.059
196.078
53.678
107.254
40
142.857
192.307
46.829
101.291
Table 3.16 indicated that at all investigated temperatures, the maximum
adsoprtion capacity of 2,4,5-T on CNT-HK5 was always higher that than
value of 2,4-D on CNT-HK5 and KF value of 2,4,5-T on CNT-HK5 was
always higher than that value of 2,4-D on CNT-HK5. On the other hand, the
adsorption heat of 2,4,5-T on CNT-HK5 was also higher than that value of
2,4-D on CNT-HK5
While, 2,4-D and 2,4,5-T have similar molecular structure, the

hydrophobicity of 2,4,5-T is higher than that of 2,4-D. Therefore, it could
be concluded that the hydrophobic interaction force played a role in the
adsorpion of 2,4-D and 2,4,5-T on CNT-HK5.
3.3.5 Summarization of study results of adsorption thermodynamics
The results of the study can be summarized as follows:
1. The adsorption of 2,4-D and 2,4,5-T on CNT-TC and CNT-HKi
showed signs of physical adsorption, occurring on the heterogeneous
surface of the material in terms of energy. The adsorption equilibrium was
best described in the Freundlich model.
2. There were several types of adsorption forces that were active
simultaneously in the adsorption of 2,4-D and 2,4,5-T on CNT-TC and
CNT-HKi such as electrostatic forces, hydrophobic interaction forces and
π-π interaction forces, where electrostatic forces had the primary influence
on the adsorption.
3. The adsorption of 2,4-D and 2,4,5-T on CNT-TC and CNT-HKi was
spontaneous, exothermic and order-increased. The heat generated in the
adsorption was from 37.382 to 49.144 kJ/mol.
3.4 Kinetics study of the adsorption of 2,4-D and 2,4,5-T on CNTs
3.4.1 Effect of contact time on the adsorption capacity of 2,4-D on CNT-TC
and CNT-HKi
In all cases of the adsorption of 2,4-D on CNT-TC and CNT-HKi,
increasing the contact time from 1 to 5 minutes led to the significant increase
in the adsorption capacity at all different concentrations, then, the increase
was slow and the equilibrium was reached.


18
3.4.2 Establishing the adsorption kinetics of 2,4-D on CNT-TC and
CNT-HKi 3.4.2.1 Động học khuếch tán Weber - Morris
45

35

2

4

6

8

10

y = 0.6087x + 48.114
R² = 0.8998

46
43

y = 6.2948x + 35.779
R² = 0.8608
0

12

2

4

46

43
40

qt (mg/g)

qt (mg/g)

49

8

10

12

52

(c)
y = 0.0594x + 50.666
R² = 0.9239
y = 0.6363x + 48.191
GĐ1
R² = 0.9214
GĐ2
y = 5.6994x + 37.081
GĐ3
R² = 0.9151

6

t1/2(min1/2)

t1/2(min1/2)
52

GĐ1
GĐ2
GĐ3

40

30
0

(b)

y = 0.0814x + 50.396
R² = 0.9114

49

qt (mg/g)

y = 0.0954x + 43.07
R² = 0.7858
y = 1.6004x + 36.591
GĐ1
R² = 0.9303
GĐ2
GĐ3

y = 6.5579x + 25.396
R² = 0.9391

40

qt (mg/g)

52

(a)

y = 0.0609x + 50.685
R² = 0.8621

49

y = 0.3875x + 49.316
R² = 0.8494

46

GĐ1
GĐ2
GĐ3

y = 3.49x + 42.545
R² = 0.8645

43

(d)

40
0

2

4

6

t1/2(min1/2)

8

10

12

0

2

4

6

8

10

12

t1/2(min1/2)

Figure 3.48: Weber - Morris diffusion kinetics of the adsorption
of 2,4-D on CNT-TC (a), CNT-HK3 (b), CNT-HK5 (c) and CNT-HK7 (d)
Based on Weber - Morris model, the adsorption of 2,4-D on CNT-TC
could be separated into three steps: 1-5 minutes, 5-20 minutes and 20-120
minutes. The value of Li is different from 0, indicating both film diffusion
and intra-particle diffusion governed the adsorption rate.
3.4.2.2 Pseudo first order adsorption kinetics
The correlation value R2 of linear lines, displaying the relationship
between t and ln(qe-qt) was low, therefore, the pseudo fist order adsorption
model was not fit to decribe the whole adsorption process of 2,4-D on
CNT-TC and CNT-HKi.
3.4.2.3 Pseudo second order adsorption kinetics
The correlation value R2 of linear lines, displaying the relationship
between t/qt and t, and parameters of pseudo second order kinetics model of
the adsorption of 2,4-D on CNT-TC and CNT-HKi were displayed in the
table 3.18.
In the table 3.18, the correlation value R2 in all cases are greater than
0.9996. The adsorption capacity at the equilibrium, calculated from the
pseudo second order model (qe,tt) was similar to that empirical value (qe,tn).
Therefore, the adsorption of 2,4-D on CNT-TC and CNT-HKi was well
decribed by the pseudo second order kinetics model.
In the table 3.18, the initial concentration of 2,4-D had different impact
on the initial adsorption rate (v₀), depending on the adsorption material.

19
Table 3.18: Parameters of the pseudo second order kinetics equation
of the adsorption of 2,4-D on CNT-TC and CNT-HKi
Parameters
Con.
qe,tn
qe,tt
k2
v0
R2
(mg/L)
(mg/g)
(mg/g) (g/mg.min) (mg/g.min)
CNT-TC
C01
43.973
44.248
0.0460
90.090
1
C02
53.331
53.476
0.1093
312.500
1
C03
60.512
60.976
0.0708

262.158
1
C04
71.266
71.429
0.0332
169.492
0.9999
C05
72.885
72.993
0.0196
104.167
0.9998
C06
81.237
80.645
0.0116
75.758
0.9996
CNT-HK3
C01
51.240
51.282
0.0809
212.766
1
C02
70.849
70.922

0.0497
250.000
1
C03
91.123
90.909
0.0484
400.000
1
C04
104.602
105.263
0.0475
526.316
1
C05
112.284
112.360
0.0720
909.091
1
C06
128.285
128.205
0.0608
1000.00
1
CNT-HK5
C01
51.279

51.282
0.0951
250.000
1
C02
72.517
72.464
0.0501
263.158
1
C03
93.400
93.458
0.0477
416.667
1
C04
112.342
113.636
0.0430
555.556
1
C05
118.186
117.647
0.0723
1000.00
1
C06
141.327

140.845
0.0630
1250.00
1
CNT-HK7
C01
52.292
51.282
0.1311
344.828
1
C02
72.596
72.464
0.0732
384.615
1
C03
94.845
95.238
0.0735
666.667
1
C04
115.747
116.279
0.0561
769.231
1
C05

121.309
121.951
0.0672
1000.00
1
C06
148.170
149.254
0.0561
1250.00
1
With CNT-TC, v0 increased from 90.090 to 312.500 (mg/g.min) as the
initial concentration of 2,4-D increased from 52.2 to 75.833 (mg/L).
However, if C0 was continued to increase to 205.672 (mg/L), then v0
decreased to 75.758 (mg/g.min). Because, when the initial concentration of
2,4-D was low, the number of adsorption sites on the surface of CNT-TC
was sufficient for the adsorption. However, as the number of adsorption sites


20
was unchanged, continuing to increase the initial concentration of 2,4-D led
to the adsorption competition between 2,4-D molecules on adsorption sites,
then, reduced v0. With CNT-HK3, CNT-HK5 and CNT-HK7, v0 increased
from 212.766 to 1000.00 (mg/g.min), from 250.000 to 1250.00 (mg/g.min)
and from 344.828 to 1250.00 (mg/g.min), respectively as the initial
concentration of 2,4-D increased in the investigated range. Therefore, v0 of
CNT-HKi was much higher than that value of CNT-TC. It could be
explained by the fact that the capillary volume in the diameter range of
3.2-4.2 nm of CNT-HKi was 2.5 times higher than that value of CNT-TC.
In addition, the aggregation of bundles of CNT-HKi created more capillaries

with open ends at middle of tubes, then, the capillary force of CNT-HKi was
higher than CNT-TC. This was also the reason why the value of v0 of
CNT-HKi was higher than CNT-TC.
3.4.3 Effect of temperature and activation energy on the adsorption of
2,4-D on CNT-TC and CNT-HKi
Table 3.19: Pseudo second order kinetics parameters of the adsorption
of 2,4-D on CNT-TC and CNT-HKi at different temperatures
(C03 = 102.026 mg/L)
T
qe,tn
qe,tt
k2
v0
Material
R2
(˚C) (mg/g) (mg/g) (g/mg.phút) (mg/g.phút)
10 65.489 65.790
0.0340
147.059
1
20 64.802 64.935
0.0516
217.391
0.9996
CNT-TC
30 60.512 60.976
0.0708
262.158
1
40 56.761 57.471

0.1044
344.828
0.9999
10 94.641 95.238
0.0230
208.333
1
20 92.882 93.458
0.0358
312.500
1
CNT-HK3
30 91.123 90.909
0.0484
400.000
1
40 88.399 88.496
0.0751
588.235
1
10 97.192 98.039
0.0196
188.679
1
20 95.934 96.154
0.0309
285.714
1
CNT-HK5
30 93.400 93.458

0.0477
416.667
1
40 91.598 91.743
0.0743
625.000
1
10 97.835 98.039
0.0274
263.158
1
20 96.613 97.087
0.0442
416.670
1
CNT-HK7
30 94.845 95.238
0.0735
666.667
1
40 92.243 92.593
0.1060
909.091
1
Parameters of the pseudo second order kinetics of the adsorption of 2,4-D
on CNT-TC and CNT-HKi at the initial concentration C03 of 102.026
mg/L at 10, 20, 30 and 40 ˚C were presented in the table 3.19.


21

From the table 3.19, the value k2 increased with an increase of
temperature. If the dependence of k2 on temperature was assumed to follow
the Arrhenius equation, then the apparent activation energy of the adsorption
(Ehp) was calculated and presented in the table 3.20.
Table 3.20: Activation energy of the adsorption
of 2,4-D on CNT-TC and CNT-HKi
Material
CNT-TC
CNT-HK3 CNT-HK5 CNT-HK7
Ehp (kcal/mol)
6.370
6.784
7.785
8.052
In the table 3.20, Ehp of the adsorption systems was always smaller than
8.052 kcal/mol, consistent with published studies, stated that the activation
energy of the physical adsorption was less than 8 kcal/mol or 40 kj/mol.
Therefore, it could be assumed that the adsorption of 2,4-D on CNT-TC and
CNT-HKi had physical nature and this was consistent with the results of
adsorption thermodynamics.
3.4.4 Kinetics study of the adsorption of 2,4,5-T on CNT-HK5 and its comparison
with 2,4-D
3.4.4.1 Kinetics study of the adsorption of 2,4,5-T on CNT-HK5
The adsorption of 2,4,5-T on CNT-HK5 could be considered as three
stages when describing by Weber - Morris and the pseudo second order
kinetics model was also fit to describe the whole adsorption process like the
adsorption of 2,4-D on CNT-HK5.
3.4.4.2 Effect of the initial concentration on the adsorption kinetics
Parameters of apparent adsorption kinetics of the adsorption of 2,4,5-T on
CNT-HK5 at different initial concentration of 2,4,5-T were displayed in the

table 3.22.
Table 3.22: Parameters of apparent adsorption kinetics
at different initial concentration of 2,4,5-T
Parameters
Con.
qe,tn
qe,tt
k2
v0
R2
(mg/L)
(mg/g)
(mg/g) (g/mg.min) (mg/g.phút)
C01
104.095
104.167
0.0174
188.679
1
C02
142.202
142.857
0.0094
192.308
1
C03
164.869
163.934
0.0133
357.143

1
C04
177.122
178.571
0.0112
357.143
0.9999
C05
178.817
178.571
0.0224
714.286
0.9999
C06
195.454
196.078
0.0217
833.333
0.9999