i
MODIFIED RICE HUSK AND ACTIVATED CARBON FILTERS FOR THE
REMOVAL OF ORGANICS AND HEAVY METALS IN WATER
ABDURRAHMAN GARBA
A thesis submitted in
fulfilment of the requirement for the award of degree of
Doctor of Philosophy in Science
Faculty of Science, Technology and Human Development
Universiti Tun Hussein Onn Malaysia
FEBRUARY, 2017
iii
DEDICATION
To Allah (S.W.T), then to my parents and all members of my extended family
iv
ACKNOWLEDGEMENTS
In the name of Allah, the most beneficent, the most merciful. All praises be unto Him,
the lord of the worlds for making it possible for me to come this far regarding the
pursuit of PhD in Science. I would like to appreciate and thanks my main supervisor
Dr. Hatijah Binti Basri for the support, guidance, advice and constructive criticism
during the entire duration of this research. I also wish to extend my special gratitude
to my co-supervisor Associate Professor Dr. Noor Shawal Nasri for his immense
contribution towards making this research successful.
I wish to appreciate the financial support provided by the office for research,
innovation, commercialization and consultancy management (ORICC) of University
Tun Hussein Onn Malaysia for the financial support under GIPS Vot number 1253 for
experimental and other aspects of this research. I also wish to thank the centre for
graduate studies of UTHM for the assistance they rendered during conferences and
other academic issues.
Special thanks to my parents, especially my mother for the constant prayers,
care, love and encouragement. I appreciate the love, support, understanding and
patience from my beloved wife Aisha Sani Muhammad and my children Abubakar,
Hafsat and Abdullah. Special thanks to Mr. Ishak Ayub and his colleagues at food
technology laboratory for the help and understanding they exhibited during the course
of this work. Also special thanks to my brothers Bello, Abdullah and the entire
extended family for their support. Special thanks to my friends at UTM – MPRC, UTM
community and all others too numerous to mention.
Lastly, I acknowledge the support of my employer, Federal Ministry of
Education, Abuja, Nigeria, for the study leave granted to me. Special thanks to all my
colleagues at FGGC Bakori for their support and friendliness
v
ABSTRACT
Discharge of untreated industrial effluents containing heavy metals and organics is
hazardous to the environment because of their toxicity and persistent nature. At the
same time, agricultural waste poses disposal challenges, which can be converted into
value added products like adsorbents that could serve as tools for contaminants
abatement. Previous findings proved that, adsorption was a sustainable, economical
and lucrative separation technique for the removal of such contaminants. This thesis
presents the fabrication of a filter for the removal of organics and heavy metals in
water which was prepared from treated rice husk and modified activated carbon (AC).
The analysis of AC via Brunauer-Emmett-Teller (BET) surface area and scanning
electron microscopy evidenced porosity of 707 m2/g as surface and a pore volume of
0.31 cm3/g. The elemental and thermogravimetric analysis proved that AC contain
48.7% carbon, while the Fourier transform infrared spectroscopy shows that the
surface contains functional groups such as O-H, C=C, C-O, C-O-C and C-H. The
experimental results were fitted with fixed-bed adsorption models to understand the
adsorbate-adsorbent relationship. Fixed-bed adsorption studies show that, the highest
adsorption capacity of 248.2 mg/g and 234.12 mg/g for BPA and phenol respectively
was obtained at 250 mg/L concentration and 9 mL/min flow rate. The results also
revealed 73 % and 87 % as the highest removal capacity for heavy metal Pb and Cd
respectively at 20 mg/L concentration and 9 mL/min flow rate. For sustainability,
regeneration of the spent AC was carried out in a microwave which showed 75% yield
after five cycles, while the rice husk was eluted with 0.1M hydrogen chloride and
37.8% efficiency was achieved after three successive cycles. The UV lamp
incorporated in the filter shows total inactivation of E. coli after 7 minutes.
vi
ABSTRAK
Pelepasan efluen industri tidak terawat yang mengandungi logam berat dan bahan
organic adalah berbahaya kepada alam sekitar ekoran daripada ketoksikan bahan. Oleh
itu, penyingkiran bahan daripada air minuman adalah sangat penting bagi melindungi
kesihatan dan alam sekitar. Dalam masa yang sama, penyingkiran sisa pertanian juga
merupakan cabaran, yang boleh ditukarkan menjadi produk berharga seperti penjerap
yang boleh mengurangkan kuantiti bahan cemar ini. Kajian terdahulu menunjukkan
teknik penjerapan adalah mesra alam, lestari, ekonomi dan boleh menjadi teknik
pemisahan yang berharga bagi penyingkiran bahan cemar tersebut. Tesis ini
membentangkan tentang fabrikasi penapis bagi tujuan penyingkiran bahan cemar
organic dan logam berat yang disediakan daripada sekam padi terawat dan karbon
teraktif terubahsuai (AC). Analisis luas permukaan dengan menggunakan teknik
Brunauer-Emmett-Teller (BET) dan mikroskopi imbasan electron (SEM)
menunjukkan keliangan AC adalah 707 m2/g dengan isipadu liang 0.31 cm3/g. Analisis
unsur dan termogravimetri menunjukkan AC mengandungi 48.7% karbon, manakala
spektroskopi infra-merah jelmaan fourier menunjukkan permukaan mengandungi
kumpulan-kumpulan berfungsi O-H, C=C, C-O, C-O-C dan C-H. Keputusan
eksperimen telah dipadankan dengan model penjerapan fixed-bed untuk memahami
hubungi elemen dijerap dan penjerap. Kajian penjerapan fixed-bed menunjukkan
kapasiti penjerapan tertinggi pada 248.2 mg/g dan 234.12 mg/g bagi BPA dan fenol
masing-masing pada kepekatan larutan 250 mg/L dan kadar alir 9 mL/min. Keputusan
juga menunjukkan kapasiti penyingkiran 73 % and 87 % bagi logam berat Pb dan Cd
pada kepekatan 20 mg/L dan kadar alir 9 mL/min. Untuk kelestarian, penjanaan
semula bagi AC terpakai yang dijalankan menggunakan ketuhar gelombang mikro
menunjukkan 75% hasil setelah lima kitaran. Sekam padi yang dilalukan dengan
larutan hidrogen klorida 0.1M menunjukkan kecekapan 37.8% yang berjaya dicapai
setelah tiga kitaran. Lampu UV yang masukkan ke dalam penapis menunjukkan
penyahaktifan semua E. coli setelah tujuh minit.
vii
CONTENTS
DECLARATION ii
DEDICATION iii
ACKNOWLEDGEMENTS iv
ABSTRACT v
ABSTRAK vi
CONTENTS vii
LIST OF TABLES xiii
LIST OF FIGURES xv
LIST OF SYMBOLS xix
LIST OF ABREVIATIONS xxi
LIST OF APPENDICES xxiii
CHAPTER 1 INTRODUCTION 1
1.1 Research Background 1
1.2 Problem Statement 4
1.3 Research Objectives 5
1.4 Research Scope 6
1.5 Research Significance 7
1.6 Structure of the Thesis 8
CHAPTER 2 LITERATURE REVIEW 10
2.1 Activated Carbon 10
2.2 Preparation of Activated Carbon 12
2.2.1 Raw Materials for Carbon Production 13
2.2.2 Carbonization Process 14
viii
2.2.3 Activation Process 16
2.3 Types of Activated Carbon 21
2.4 Microwaves 23
2.4.1 Microwave Heating 23
2.4.2 Microwave Heating for Activated Carbon
Regeneration 25
2.5 Activated Carbon Modification 26
2.5.1 Acid Treatment 27
2.5.2 Basic Treatment 28
2.5.3 Heat Treatment 29
2.5.4 Microwave Treatment 29
2.5.5 Biological Treatment 30
2.5.6 Miscellaneous Modification Methods 31
2.6 Activated carbon Properties 31
2.6.1 pH 31
2.6.2 Surface Area 31
2.6.3 Ash Content 32
2.6.4 Pore size distribution 33
2.6.5 Chemical properties 33
2.7 Uses and applications of Activated carbon 34
2.8 Cost of activated carbon production 34
2.9 Adsorption Isotherms 37
2.9.1 IUPAC Classification of isotherms 38
2.10 Fixed–bed adsorption models 42
2.10.1 Clerk model 43
2.10.2 Thomas model 43
2.10.3 Adams–Bohart model 44
2.10.4 Bed depth service time (BDST) model 44
2.10.5 Yoon–Nelson model 45
2.10.6 Wang model 46
2.10.7 Wolborska model 46
2.10.8 Dose – response model 47
2.10.9 Modified dose–response model 47
2.11 Error Analysis 47
ix
2.11.1 The sum of the squares of the error (ERRSQ)
48
2.11.2 Hybrid fractional error function (HYBRID) 48
2.11.3 Average relative error (ARE) 48
2.11.4 Sum of squares (SS) 49
2.11.5 Sum of absolute errors (EABS) 49
2.11.6 Marquardt’s percent standard deviation
(MPSD) 49
2.11.7 Nonlinear chi–square test (x2) 50
2.11.8 Sum of absolute error (SAE) 50
2.11.9 Root mean square error (RMSE) 50
2.11.10 Mean square error (MSE) 51
2.12 Rice Husk 51
2.12.1 Properties of Rice Husk 52
2.12.2 Rice Husk Pre-treatment 52
2.12.3 Uses and Applications of Rice Husk 54
2.13 UV Disinfection 55
2.13.1 Effectiveness of UVC Disinfection 57
2.13.2 Mechanism of UV Disinfection 59
2.13.3 Applications of UV radiation 61
CHAPTER 3 METHODOLOGY 62
3.1 Introduction 62
3.2 Activated Carbon (AC) Preparation 64
3.2.2 Carbonization Process 67
3.2.3 Activation Process 67
3.2.4 Physical Activation 69
3.2.5 Chemical Activation 70
3.2.6 Characterization of Activated Carbon 71
3.2.7 Field–Emission Scanning Electron
Microscopy (FESEM) 71
3.2.8 Nitrogen Adsorption 72
x
3.2.9 Fourier Transform Infrared Spectroscopy
(FTIR) 72
3.2.10 pH 72
3.2.11 Proximate Analysis 73
3.2.12 Ultimate Analysis 73
3.2.13 Thermogravimetric/Derivative
Thermogravimetric Analysis (TG/DTG) 73
3.3 Ammonia Treatment 74
3.4 Fabrication of the filter 74
3.4.2 Removal efficiency of Activated Carbon 77
3.4.3 Fixed-Bed Adsorption Procedure 77
3.4.4 Fixed-Bed Breakthrough Curves Modeling 79
3.4.5 Error Analysis 79
3.4.6 Regeneration Procedure 80
3.5 Rice Husk Preparation 82
3.5.1 Materials 82
3.5.2 Characterization of Rice Husk 84
3.5.3 Treatment with Sodium Hydroxide 85
3.5.4 Measurement of Pb2+ and Cd2+
Concentrations in water 85
3.5.5 Removal efficiency of Rice Husk 85
3.5.6 Fixed-bed Adsorption Procedure 85
3.5.7 Breakthrough Curves modeling 86
3.5.8 Error Analysis 87
3.5.9 Regeneration Procedure 87
3.6 Sand Preparation 88
3.6.1 Materials 88
3.7 Preparation of E. coli 88
3.7.1 Materials 88
3.7.2 Methods 89
3.7.3 Gram Staining 89
3.7.4 Preparation of inoculum 90
3.7.5 Preparation of 0.5 McFarland standard 90
xi
3.7.6 Use of McFarland’s standard in macro
dilution procedure 91
3.7.7 Preparation of Serial dilutions of E. coli 92
3.7.8 Measurement of E. coli in water 93
3.7.9 Disinfection of E. coli by the UV lamp 93
CHAPTER 4 RESULT AND DISCUSSIONS 94
4.1 Introduction 94
4.2 Preparation of Activated carbon 95
4.2.1 Proximate and Ultimate analysis of the
precursor 95
4.2.2 Effect of carbonization time and temperature
on the carbon yield 96
4.3 Characterization of Activated Carbon 99
4.3.1 Fourier Transform Infrared Spectroscopy 99
4.3.2 Acid/base properties of activated carbon 100
4.3.3 Field Emission Scanning Electron
Microscopy 100
4.3.4 Thermogravimetric analysis 102
4.3.5 Zeta Potential Analysis 104
4.3.6 Surface Area and Pore Volume Analysis 104
4.4 Regeneration and Adsorption/Desorption cycles of
AC 105
4.5 Thermodynamic studies for Bisphenol A (BPA) and
Phenol Adsorption 107
4.5.1 Effect of temperature on BPA and Phenol
Uptake 107
4.5.2 Determination of free energy change (∆G),
enthalpy change (∆H) and entropy change
(∆S) 109
4.5.3 Estimation of activation energy 110
4.5.4 Mechanism of Phenol Adsorption 111
4.6 Fixed-bed Adsorption Studies 112
xii
4.6.1 Bisphenol A Uptake 112
4.6.2 Phenol Uptake 118
4.6.3 Phenol Breakthrough Curve Modeling 121
4.7 Comparison between synthesized and the
commercially prepared carbon 124
4.8 Preparation of Rice Husk 126
4.8.1 Characterization of Rice Husk 126
4.8.2 Lead Uptake 129
4.8.3 Lead Breakthrough Curve Modeling 134
4.8.4 Cadmium Uptake 135
4.8.5 Cadmium Breakthrough Curve Modeling 139
4.8.6 Comparison between Pd and Cd uptake by
the rice husk 141
4.8.7 Regeneration cycles of Rice Husk 145
4.9 Disinfection of E.coli using UV lamp 145
4.10 Cost of the filter production 146
CHAPTER 5 CONCLUSION AND RECOMMENDATIONS 147
5.1 Conclusion 147
5.2 Recommendations 148
REFERENCES 150
APPENDIX 180
VITA 192
xiii
LIST OF TABLES
2.1 Commercially Prepared AC with their Surface Area 32
2.2 Production cost of char in some parts of the world 36
2.3 Summary of some studies indicating dose response data
for water disinfection using UV-LEDs 58
4.1 Proximate and ultimate analysis of raw palm oil shell use
in this study 96
4.2 Proximate and ultimate analysis of this study compared
with other studies 96
4.3 Effect of carbonization time on weight loss 98
4.4 Weight loss of oil palm shell and yield during
carbonization 98
4.5 Acid–Base properties of the precursor, char and
activated carbon 100
4.6 Thermodynamic parameters of BPA and phenol
adsorption onto modified activated carbon 109
4.7 The Adams–Bohart, Thomas and Yoon–Nelson model
constants f or the BPA adsorption by modified activated
carbon packed column 116
4.8 The Adams–Bohart, Thomas and Yoon–Nelson model
constants for the phenol adsorption by modified
activated carbon column. 122
4.9 Comparison of maximum adsorption capacity of some
AC’s for BPA and phenol 125
4.10 The Adams–Bohart, Thomas and Yoon–Nelson model
constants for the adsorption of lead by treated rice husk
packed column 133
xiv
4.11 The Adams–Bohart, Thomas and Yoon–Nelson model
constants for cadmium adsorption by treated rice husk
packed column. 139
4.12 Regeneration efficiency at different cycles 145
4.13 Cost of producing 1 unit of the filter 146
xv
LIST OF FIGURES
2.1 Internal structure of activated carbon adsorbent 12
2.2 Oil palm shell processing field 14
2.3 Carbonization reaction scheme of a carbonaceous
material 15
2.4 Flowsheet for the production of activated carbon 16
2.5 The GAC Contactor 21
2.6 Types of activated carbon (A) PAC, (B) GAC and (C)
EAC 22
2.7 The electromagnetic spectrum 23
2.8 The magnetic and electric fields of microwaves 24
2.9 Microwaves interactions with different materials 25
2.10 Thermal gradient for conventional (A) and microwave
heating (B) 26
2.11 Type I isotherm plot for nitrogen adsorption 39
2.12 Type II isotherm plot for nitrogen adsorption 39
2.13 Type III isotherm plot for nitrogen adsorption 40
2.14 Type IV isotherm plot of nitrogen adsorption 40
2.15 Type V isotherm plot of nitrogen adsorption 41
2.16 Type VI isotherm plot of nitrogen adsorption 41
2.17 Uses of Rice husk 55
2.18 The types of UV radiations in the ultraviolet region 57
2.19 UVC Radiation disrupts DNA 60
2.20 Applications of UV radiations 61
3.1 Flow chart for the overall Methodology process 63
3.2 Oil Palm Shell sun dried after washing 65
3.3 Oven drying of Oil Palm Shell 65
3.4 The grinding machine 66
xvi
3.5 Sieves and shaker for size separation 66
3.6 Granulated oil palm shell after sieving 66
3.7 Oil palm shell char after carbonization process 67
3.8 Schematic diagram for carbonization and activation
process 69
3.9 The pyrolysis set–up 70
3.10 Materials for impregnation 71
3.11 Magnetic stirrer for mixing the impregnated char 71
3.12 Schematic diagram of the fabricated filter 75
3.13 The PG Instruments UV–Vis Spectrometer 76
3.14 The Fixed–bed Adsorption set up 79
3.15 Modified microwave oven with glass reactor 81
3.16 Schematic diagram for microwave regeneration set–up 81
3.17 Rice Husk sun dried after washing 83
3.18 The drying of rice husk in hot air oven 83
3.19 The Milling Machine 83
3.20 The sieves and shaker 84
3.21 The Ground and Sieved Rice Husk 84
3.22 The Wickerham’s card 91
3.23 Serial dilution and plating of the bacteria 92
4.1 The FTIR spectra of the activated carbon samples 99
4.2 FESEM images of (A) RPS, (B) CPS and (C) PCAC 101
4.3 TG/DTG curves for RPS (A), CPS (B), and PCAC (C) 103
4.4 Zeta potential for the modified activated carbon 104
4.5 Type I isotherm plot from adsorption and desorption of
nitrogen by PCAC 105
4.6 Desorption efficiency and Yield of PCAC-AM after five
cycles 106
4.7 Effects of temperature on BPA and phenol removal at
concentration of 50 mg/L, pH 6.5, 150 rpm and
adsorbent dose of 0.1g. 108
4.8 Linear plot of lnKc against 1/T at initial concentration of
50 mg/L, pH 6.5, steering speed of 150 rpm and
adsorbent dose of 0.1g 111
xvii
4.9 Effect of initial BPA concentration at constant bed
height of 3 cm and flow rate of 6 mL/min 113
4.10 Effect of flow rate on adsorption of BPA at bed height
of 3 cm and 50 mg/L concentration 114
4.11 Effect of adsorbent bed height at constant initial BPA
concentration of 50 mg/L and flow rate of 6 mL/min. 115
4.12 Effect of initial phenol concentration at 3 cm bed height
and flow rate of 6 mL/min 119
4.13 Effect of flow rate at bed height of 3 cm and 50 mg/L
initial phenol concentration. 120
4.14 Effect of bed heights at constant phenol concentration of
50 mg/L and flow rate of 6 mL/min 121
4.15 FTIR spectra of the rice husk 126
4.16 SEM image of raw rice husk 127
4.17 EDS of Raw Rice Husk 128
4.18 SEM image of the rice husk after treatment 128
4.19 EDS of Rice Husk treated with NaOH 128
4.20 XRD result for the raw and treated rice husk 129
4.21 Effect of initial lead concentration at constant bed height
of 2.8 cm and flow rate of 3 mL/min 130
4.22 Effect of lead solution flow rate at constant bed height
of 2.8 cm and 5 mg/L concentration. 131
4.23 Effect of bed heights at constant lead concentration of 5
mg/L and 3 mL/min flow rate. 132
4.24 Effect of initial cadmium concentration at constant bed
height of 2.8 cm and flow rate of 3 mL/min 136
4.25 Effect of cadmium solution flow rate at constant bed
height of 2.8 cm and 5 mg/L concentration 137
4.26 Effect of bed heights at constant cadmium concentration
of 5 mg/L and 3 mL/min flow rate 138
4.27 Effect of initial Pb or Cd concentration at constant bed
height of 2.8 cm and flow rate of 3 mL/min 142
4.28 Effect of Pb and Cd solution flow rate at constant bed
height of 2.8 cm and 5 mg/L concentration 143
xviii
4.29 Effect of bed heights at Pb and Cd concentrations of
5mg/L and 20 mg/L and constant flow rate of 3 mL/min
144
4.30 Percentage disinfection of E. coli with time 146
xix
LIST OF SYMBOLS
°C Degrees Celsius
µm Micrometre
A Absorbance
Å Angstrom
BaCl Barium Chloride
Ce Concentration in equilibrium
CO Carbon monoxide
Co Initial Concentration
CO2 Carbon dioxide
g Gram
GHz Giga Hertz
h hour
H2 Hydrogen
H2O Water
H2SO4 Hydrogen sulphide
H3PO4 Phosphoric acid
HCl Hydrogen Chloride
K Degrees Kelvin
K2CO3 Potassium Carbonate
KOH Potassium Hydroxide
L Litre
lbs Pound-mass
m Activated Carbon Dosage/ Mass
mg Milligram
Mins Minutes
mm Millimetre
n Number Factors
xx
N Number of Experiment
N2 Nitrogen Gas
NaCl Sodium Chloride
NaOH Sodium Hydroxide
NH3 Ammonia
Nm Nanometre
nm Nanometre
q Amount adsorbed
V Volume
ZnCl2 Zinc Chloride
λmax Maximum Wavelength
xxi
LIST OF ABREVIATIONS
AAS Atomic Absorption Spectroscopy
AC Activated Carbon
BAC Bead Activated Carbon
BDST Bed Depth Service Time
BET Brunauer-Emmett-Teller
BJH Barret-Joyner-Helenda
BPA Bisphenol A
CFU Colony Forming Unit
cm Centimetre
CPS Carbonized Palm Shell
DBP Disinfection Bye-product
DFT Density Functional Theory
DNA Deoxyribonucleic acid
EA Elemental Analysis
EAC Extruded Activated Carbon
EBCT Empty Bed Contact Time
EDS Energy Dispersive X-ray Spectroscopy
EMB Eosin Methylene Blue
FCC Federal Communications Commission
FESEM Field Emission Scanning Electron Microscopy
FTIR Fourier Transform Infrared Red Spectroscopy
GAC Granular Activated Carbon
GHG Green House Gases
IUPAC International Union of Pure and Applied Chemistry
MW Microwave
PAC Powdered Activated Carbon
PCAC Furnace Prepared-Potassium Carbonate Activated Carbon
xxii
PCAC-AM Ammonia treated-Potassium Carbonate Activated Carbon
RH-NaOH Sodium Hydroxide treated Rice Husk
RPS Raw Palm Shells
SEM Scanning Electron Microscopy
TGA Thermogravimetric Analysis
UV Ultraviolet radiation
UV-LED Ultraviolet – Light-emitting diode
VOC Volatile Organic Compounds
XRD X-ray Diffraction
xxiii
LIST OF APPENDICES
APPENDIX TITLE PAGE
A List of publications and conferences attended 179
B Instruments used for the characterization of the
adsorbents 183
C UV – Visible calibration data and curves 185
D Structure of E. coli and differences between gram
positive and gram negative bacterium 187
E Pictorial presentation of the fabricated filter 189
F Pictorial presentation of the UV lamp and its
specifications 190
1
CHAPTER 1
INTRODUCTION
1.1 Research Background
Water is colourless, odourless, tasteless and transparent liquid that without it, life is
impossible. It is the only substance on earth that naturally exist in all the states of
matter, and is also interchanging between this various forms. In comparison with other
common liquids, water is unique because it has high absorption and retention capacity
for heat due to hydrogen bonds that exist between its molecules. It is a valuable
resource of outmost importance to both plants and animals for their survival. It is also
a good liquid solvent because it can dissolve a great variety of compounds ranging
from simple salts to complex organic compounds. Water is a natural resource that is
essential to human survival. Therefore, there is need for its improvement and
preservation continuously on daily basis. Although, about three-quarter of the earth
was covered by water, not all of it is available for human consumption. This is because,
only 3 % of the water is fresh, while the rest are usually found in oceans and seas,
which are saline in nature (Gleick & Ajami, 2014).
Water is of paramount importance to human beings in terms of both domestic
and industrial use, its preservation from contamination at point and non-point sources
is necessary (Garba, et al., 2016). The rapid growth in the area of human population
and industries worldwide has increased the problems in the environment, specifically
in the quality of water resources. Recently, the quality of water resources is declining
on daily basis as a result of human activities such as indiscriminate disposal of
pollutants into the water bodies through industrialization, civilization, environmental
2
changes and agricultural practices (Ali et al., 2012). Heavy metals pollution is one of
the many causes of water pollution that brings about environmental degradation and
deterioration which affects both humans and the aquatic system (Das et al., 2008).
Heavy metals is a group name usually used for metals and metalloids that were
associated with contamination and are toxic to both plants and animals. Commonly,
they are the metallic and metalloid chemical elements that have a relatively high
density values between 63.5 to 200.6 and a specific gravity greater than 5 (Fu & Wang,
2011; Zhao et al., 2016). They are non-biodegradable and toxic even at low
concentrations. Thus, they could be harmful to most organisms at certain level of
exposure and absorption. Common elements considered as heavy metals include Cu,
Cd, Cr, Co, Pb, Hg, Se, Ni, Zn and As. Some researchers incorporate Ag, Be, Ba, Mn,
Mg, Fe, Li, Tl, Ti, V, Sr and Sb as heavy metals (Fu & Wang, 2011; Park et al., 2016;
Zhou & Haynes, 2010). It is a well-known fact that the anthropogenic sources of heavy
metals into our water resources are power plants, extractive and metal ore processing,
metallurgical, chemical, electronic, nuclear, agricultural practices and waste disposal.
This pollution presents a serious problem that requires immediate attention and
subsequent removal from water resources (Witek-Krowiak, 2013). Heavy metals are
highly toxic as they were prioritized as the primary inorganic pollutants for surface
water and groundwater as well as the environment due to their mobility in the aquatic
ecosystem which affects both aquatic life and the other higher life forms. Water is the
medium for and source of entry of this poisonous elements into the human body in
which their intake beyond the permissible limits cause toxicity to the humans (Khan
et al., 2008). Several methods were being used for heavy metals removal from water
such as coagulation, precipitation, ion exchange, photocatalytic degradation,
electrochemical deposition, flotation, membrane filtration and adsorption (Ahmed &
Ahmaruzzaman, 2016; Barakat, 2011; Baruah et al., 2015; Duru et al., 2016; Goyal &
Masram, 2015; Lofrano et al., 2016; Patil et al., 2016; Vunain et al., 2016).
In addition, organic pollutants is another group of pollutants that contaminate
water bodies. They are carcinogenic, mutagenic, teratogenic, and have high
bioaccumulation in nature if they are not removed from the water (Kümmerer et al.,
2016). Therefore, organic pollutants were persistent in the environment because of
such characteristics. Several types of organic pollutants were discovered in both
surface and ground water bodies. These organic pollutants could be sourced from
hydrocarbons, plasticizers, pesticides, oils, detergents, pharmaceuticals, greases,
3
biphenyls and phenols (Ali et al., 2012; Barceló, 2005; Tran et al., 2015). The
pollutants have a general characteristics such as presence of a cyclic rings which are
either of aromatic or aliphatic nature, absence of polar functional groups and
substitution by a variable number of halogens in which chlorine was the preferred
halogen substituted (Ali et al., 2012). Phenols and phenolic derivatives are other class
of organic pollutants that were considered as priority pollutants because they are
detrimental to humans even at low concentrations. Phenols and its derivatives can
contaminate drinking water even at a low concentrations of 0.005 mg/L as it gives the
water a significant bad taste and bad odour thereby making the water unfit for use
(Aksu, 2005; Gupta & Balomajumder, 2015). Phenols and some of its derivatives were
released from manufacturing industries such as pharmaceutical, plastic, paper,
petroleum and petrochemicals, pesticides, and other manufacturers (Park et al., 2013).
Literature reports several methods for organic compounds removal in aqueous media.
The basic principles of their removal methods depends on their properties such as
physical, chemical and biological. Some of the methods applied for the removal of
phenols and its derivatives include reverse osmosis, chemical oxidation,
electrochemical oxidation, ion exchange, solvent extraction, photocatalytic
degradation, electro–dialysis, chemical coagulation and adsorption (Abussaud et al.,
2015; Al-Rashdi et al., 2011; Ali et al., 2012; Bazrafshan et al., 2016; Cheng et al.,
2016; Garba, et al., 2016; Singh & Balomajumder, 2016a, 2016b).
Furthermore, presence of infectious diseases that were caused by pathogenic
bacteria such as E. coli were the most common health risk associated with drinking
water. The waterborne pathogens were mostly introduced into the water bodies
through human and animal faeces. In some instances, the bacteria can also exist
naturally as autochthonous microorganism in the water. Cattles were considered as the
main repository of E. coli where by the infected animals can approximately excrete
between 102 to 105 CFU/g of faeces. Also, the introduction of fresh manure to the crops
as fertilizer or for amendment of soil can basically enhance the spread of the
pathogenic bacteria to the farmland and the environment at large. This is of great
concern because, the bacteria may remain active from several days up to more than a
year (Ongeng et al., 2011; Williams et al., 2007). The E. coli can easily be transported
from the manure to the surface or groundwater by stormwater run-off, leaching,
precipitation etc (Bradford et al., 2013; Mohanty et al., 2014; Zhang et al., 2014). The
removal methods for pathogenic bacteria in water and wastewater is a complex process
4
that is significantly affected by some factors such as retention time, seasonal
variations, vegetation and the composition of the water. The most frequently used and
well established removal methods for pathogenic bacteria include starvation to death
or predation, filtration, sedimentation and adsorption (Suzuki et al., 2014; Varga &
Szigeti, 2016; Wu et al., 2016).
From all of the above listed methods for the removal of heavy metals, organics
and bacteria from water, adsorption technology was the most commonly applied
alternative. This is because, adsorption is cheap, green, sustainable, and can be
regenerated. Fundamentally, adsorption is the accumulation of a substance on the
surface or in the interface of an adsorbent. In the case of heavy metals, organics and
bacterial removal, the process takes place between the surface and interface of a solid
adsorbent and a contaminated water. The liquid contaminant is usually regarded as the
adsorbate, while the adsorbing material is known as the adsorbent (Çeçen & Aktas,
2011).
1.2 Problem Statement
Presence of water pollutants such as heavy metals, organics and bacteria poses a
serious problem that requires immediate attention. This is because, their ingestion and
accumulation is dangerous to human health and the environment. For instance, heavy
metals such as lead, cadmium, chromium, arsenic, mercury and others are serious
common environmental pollutants that are generated from human activities. Heavy
metals are toxic, can be incorporated into the food chain, and can exist in the
environment as dissolved or free ions that could be absorbed into the soil, sediment or
may bioaccumulate into the biota. Acute intoxication of these metals may trigger
damage to the central nervous system, lungs, liver, kidney, cardiovascular and
gastrointestinal system, bones and endocrine glands (Lakherwal, 2014; Liang et al.,
2014).
Also, organic compounds such as phenols and its derivatives are water
pollutants that were listed in the priority hazardous substances. These demonstrate
their serious toxicities to human health and the ecology (Bazrafshan et al., 2012; Busca
et al., 2008). The ingestion of water that is contaminated with phenol and its
derivatives into the human body may result in protein degeneration and also damage
5
to the central nervous system, eye, heart, peripheral nerve, skin, kidney, liver, and
pancreas (Asghar et al., 2014).
Furthermore, presence of pathogenic bacteria like E. coli in water is a public
health hazard. These is because the microbes may lead to serious illnesses such as
gastrointestinal, kidney failure, and urinary tract infections which is a significant cause
of death in many parts of the world (Alshehri et al., 2014).
Adsorption using agricultural waste materials has been found to be
undoubtedly one of the most popular and most widely used technology in water
treatment, but most of filters that utilizes adsorption technology were still a little
expensive. This is due to the fact that, commercially prepared activated carbon was
incorporated into the filter.
It is on this note that this research focuses on fabrication of an efficient water
filter that utilizes cheap activated carbon prepared from agricultural waste materials so
as to convert the waste materials into value added products which helps to provide
solution to agricultural waste disposal problem and at the same time using the products
to remove contaminants in the environment. Although, fabrication of water filter is a
highly researched area, but more has be done in the area so as to get filters that are less
expensive, green, sustainable and have value for money. The novelty of this work is
in preparation of a cheap water filter using cheap precursors that utilizes simple and
efficient methods which does not require the usage of high energy or consumption of
large amount of chemicals. Also, the filter was in modular form using cheap plastic
containers as casing so as to give room for easy assembling and unlimited expansion.
In addition, germicidal ultraviolet lamp was incorporated into the filter to ensure the
total elimination of bacteria in the produced water.
1.3 Research Objectives
Based on the existing problem statement, the objectives of the research are
summarized as follows;
i. To develop efficient adsorbents from low cost agricultural waste as
adsorption media and to modify the adsorbents so as to improve their
surfaces for higher adsorption of the contaminants.
6
ii. To characterize the adsorbents in terms of textural, thermal and chemical
properties in order to understand their suitability and as well as their
adsorption characteristics under fixed–bed adsorption mode.
iii. To evaluate the adsorption capacities of the regenerated adsorbents using
microwave heating for activated carbon and elution with 0.1 M HCl for the
rice husk.
iv. To evaluate the bacterial disinfection abilities of the incorporated UV lamp
for their total elimination in the filter media.
1.4 Research Scope
To achieve the above mentioned objectives, the following scopes were drawn;
i. Adsorbents were prepared from low-cost agricultural waste materials. This
include activated carbon prepared from palm oil shell by carbonization at
800 C and activation at the same temperature. Modification of the carbon
was carried out using 6.6 M ammonia solution to improve its hydrophobic
surface. The rice husk was prepared by treatment with 2 % NaOH in order
to remove lignin and improve its adsorption capacity for heavy metals.
ii. The characterization of the adsorbents covers: surface chemistry (FTIR,
and elemental composition), thermogravimetric analysis (TGA, fixed
carbon, ash and volatile matter), surface morphology and texture (BET
surface area SEM and XRD).
iii. Adsorptive capacities of the prepared adsorbents were investigated in a
fixed – bed mode using synthetic solutions prepared from standard
solutions of heavy metals for lead and cadmium analysis, phenol and
bisphenol A for organic compounds anlysis and E. Coli solution for
bacterial analysis.
7
iv. Reusability of the adsorbents were determine after five successive
adsorption/desorption cycles for activated carbon and three cycles for the
rice husk.
v. Inactivation properties of the UV lamp was investigated and the results
were expressed as percentage removal.
1.5 Research Significance
The main contribution of this study is fabrication of an inexpensive water filter from
agricultural waste materials such as oil palm shell for the removal of organics, heavy
metals and pathogenic bacteria from water. The fabricated filter demonstrated
favourable adsorption capacity for the listed contaminants and the adsorbents in the
filter media remain durable after reasonable successive regeneration cycles.
In Malaysia there is large amount of palm shells produced from oil palm
processing industries. The use of this cheap and abundant precursor materials in
production of adsorbents for subsequent use in the removal of heavy metals, organics
and microbes from water will go a long way in reduction of environmental pollution
which helps in environmental sustainability and in turn curb the potential health risks
associated with this contaminants in water.
In addition, converting such agricultural waste into value added products such
as adsorbents would serve as a way to mitigate the disposal challenges posed by this
waste materials to the environment. Also, the industries involved in conversion of such
waste materials would serve as an indirect way of revenue generation and
simultaneously for job creation.
Furthermore, application of this cheap water filter would help in reduction of
water shortages. This is because the filter could be used in recycling of grey water
from (ablution water, laboratories, kitchens, etc). The filter could also help in
harvesting rain water for areas that incorporate rain water as part of its source of
domestic water.
8
1.6 Structure of the Thesis
The thesis comprises of five chapters and each chapter discusses specific areas of the
research as presented below:
Chapter 1: Introduction
This chapter provides introduction to the research, background of the study, the
problem statement, objectives of the research, scope of the research and as well how
the thesis was organized.
Chapter 2: Literature Review
This chapter focuses on review of the relevant studies conducted in this area. The
synthesis of the adsorbents and methods of modifications for a specific contaminant
uptake from the water. The use of microwave heating for activated carbon
regeneration.
Chapter 3: Methodology
This chapter discusses raw materials selection, experimental approaches, equipment
employed for the research and general procedures followed in carrying out this
research. These include the carbonization and activation processes, characterization
and modification of the adsorbents, formulation of the synthetic heavy metals, organics
and E. coli solutions in water and the fixed-bed adsorption process.
Chapter 4: Results and Discussion
This chapter presents the results of using oil palm shell for the production of AC under
conventional heating method and as well the treatment of rice husk for heavy metals
uptake. The chapter also discusses the characteristics of the prepared adsorbents and
their applications for contaminants. The fixed–bed experimental results are also
discussed and their correlations with the adsorption models and were respectively
verified. Regeneration results of the spent adsorbents after five successive adsorption–
regeneration cycles were also discussed.
9
Chapter 5: Conclusion and Recommendations
This chapter discusses the important inferences arrived at based on the results obtained
from the research and it also present the recommendations for future studies in the
area.
10
CHAPTER 2
LITERATURE REVIEW
2.1 Activated Carbon
Activated carbon (AC) is a microporous material that is predominantly amorphous
solid with a highly developed internal surface area and pore volume. These unique
properties are responsible for its high adsorption capacity which were exploited in
many different liquid and gas phase applications (Adinata et al., 2007; Arami-Niya et
al., 2010). Activated carbon has a slight positive charge on it which make it even more
attractive to contaminants. When a liquid or gas passes over the positively charged
surface of the carbon, the negatively charged ions from the contaminants were
attracted to the surface of the carbon matrix (Alslaibi et al., 2013). The ease of
regeneration of carbon gives it added advantage of making it economical and this
significantly reduces the cost of purification process.
AC can be synthesized through carbonization followed by subsequent
activation of the carbonaceous materials that contains lots of organic but few inorganic
constituents like coal, wood, corn cobs, fruit shells and other agricultural wastes in an
inert atmosphere (Mercier et al., 2014; Okman et al., 2014). AC is available in a
various forms ranging from granular (GAC), powdered and extruded (EAC) activated
carbon depending on their method of production and the prospective application.
Activated carbons are widely used in various applications for gas or liquid
phase aqueous solutions such as portable water and treatment, industrial purification,
air purification, chemical recovery operations, organic and inorganic contaminants
removal from industries, toxins remediation by respirators, odour removal, natural and
11
hydrogen gas storage, catalyst support, pharmaceuticals and electrode materials in
capacitors and batteries (Sayğılı et al., 2015; Tseng, 2006; wang, et al., 2014). Various
studies were published regarding the utilization of ACs as adsorbents mostly for
environmental remediation. This processes were achieved due to well-developed
porous nature of the carbons, large surface area and sufficient surface functionalities
of the carbons. This physicochemical parameters (porous structure, large surface area
and surface chemistry) can be enhanced by the type of raw material and the activation
process employed (Inagaki, 2009).
Amongst the wide range of precursors used for activated carbon production
(fruit shell, fruit stones, corn cobs, wood, coal bagasse, pulp etc.), oil palm shell is one
of the most employed for water and wastewater uses because of its huge availability
and effectiveness. Surface area of ACs ranges between 500 to 3000 m2/g depending
on the composition of the precursor and the activation process used (Bandosz, 2006).
Among the AC properties, pore size distribution has been shown to play an
instrumental role in its adsorption process as transportation of adsorbate from the
aqueous medium to the internal structures or pores of the adsorbent is mainly through
the mesopores and macropores. These distributions determines the adsorption capacity
of the adsorbent (Bandosz, 2006). Hierarchically porous ACs comprised of
interconnected pores such as micropores which has pore size between 0 – 2 nm (20 Å)
and it determines the surface area of carbon as it constitutes about 95% of the carbon
surface area, mesopores which are also known as transitional pores were responsible
for adsorbates movement from the surface of AC to the internal pores with internal
pores between 2 – 50 nm (20 - 500 Å) and macropores with pores above 50 nm (500
Å) respectively as presented in Figure 2.1 (Bhandari et al., 2015; Yamada et al., 2007).
12
Figure 2.1 Internal structure of activated carbon adsorbent (Bendosz, 2006)
2.2 Preparation of Activated Carbon
There are basically two processes involved in activated carbon production which are
physical or chemical treatment. Each process involves two steps namely; carbonization
and activation which were responsible for design of the shapes and sizes of the
activated carbons (Al-Swaidan & Ahmad, 2011; Danish et al., 2011; Jun et al., 2010;
Yahya et al., 2015). In physical treatment process, the precursor material is first
carbonized, followed by activation step using either steam or carbon dioxide as
activant. On the contrary, in chemical treatment process, the precursor material is
impregnated with activating reagent, then heated under inert atmosphere to drive away
the volatile components (Ahmad et al., 2013; Arami-Niya et al., 2010). The chemical
reagents used in chemical treatment process could dissolve cellulosic components of
the precursor material which initiates formation of cross-links (Örkün et al., 2012).
Chemical activation has been shown to have more advantages over physical activation.
This advantages include; chemical activation generally requires lower carbonization
temperatures, produces carbons with higher yield, produce carbons with higher surface
area, involves one step, and allows for tailoring the porosities of the produced carbons.
However, there are some disadvantages associated with chemical activation such as
thorough washing of the carbon produced is required so as to remove the impurities
blended with the activating reagent and also the activating reagent could be corrosive
(Hirunpraditkoon et al., 2011; Sing, 2014).
Macropores
Mesopores
Micropores
13
2.2.1 Raw Materials for Carbon Production
Any carbon-rich or lignocellulosic solid material can be a potential candidate for
activated carbon production. However, the most crucial factors taken into
consideration for the selection of an appropriate raw material are that the material must
be non-hazardous, easily available, inexpensive with high carbon and low inorganic
contents, highly resistant to abrasion, thermally stable, highly rich in volatile contents
as their evolution during pyrolysis results into development of a porous char and must
have small pore diameter which translates into higher exposed surface area and
consequently high adsorption capacity (Ali et al., 2012; Kyzas & Deliyanni, 2015).
Common raw materials used include agricultural waste (such as oil palm shell, coconut
shell, coal, rice husk, corn cobs, waste tires, pulp mill residue, resins etc.) (Ioannidou
& Zabaniotou, 2007).
Malaysia is the second largest palm oil producer and exporter with an annual
estimation of about 73.74 million tons of biomass as residue. About 6 – 7 % of such
biomass waste were left over as palm shells (as seen in Figure 2.2). Oil palm shell is
one of the most common raw materials used for activated carbon production in
Malaysia which could be due to its enormous availability and its demonstration of
other good properties such as high fixed-carbon content, low ash and low inorganic
materials. High carbon content in form of volatiles (usually above 50 %), helps to
develop basic pore, while presence of inorganic materials influence the adsorption
capacities of the carbon and high ash content make the carbon to be more hydrophilic
(Mushtaq et al., 2015; Ohimain & Izah, 2017; Rafatullah et al., 2012; Yahya et al.,
2015). It is also reported that oil palm shell based activated carbon have predominantly
microporous pore structure and this accounts for its high available internal surface
area that is found to be ideal for adsorption of small organic pollutant molecules
(Ioannidou & Zabaniotou, 2007; Zhang et al., 2006).
14
Figure 2.2 Oil palm shell processing field
2.2.2 Carbonization Process
Carbonization is a critical stage for activated carbon production because it is during
this process that the micropores of the carbon starts to form. Carbonization can be
defined as the thermal degradation of a carbonaceous material in the absence of oxygen
which turn the material into different products that consist of solid (the char as
residue), liquid (high molecular weights that condense after cooling) and gas in form
of light molecular weights (Bandosz, 2006; Çeçen & Aktas, 2011). There are
essentially two stages in a carbonization process namely; primary and secondary
carbonization (Fernández et al., 2011).
The primary carbonization stage involved evolution of volatiles through
thermal decomposition which is also known as devolatilization of the material. At this
stage, the reactions that takes place were mainly dehydration, dehydrogenation,
decarbonilation and decarboxylation (Bandosz, 2006). On the other hand, the
secondary stage involved decomposition reaction that takes place within the solid
matrix and as well further reaction between the released volatiles and the residue as
presented in Figure 2.3.
15
Figure 2.3 Carbonization reaction scheme of a carbonaceous material (Bandosz,
2006)
At the secondary carbonization stage, the large molecular compounds or the
char breaks further into smaller gas molecules such as carbon monoxide, carbon
dioxide and hydrogen due to collision with gaseous materials from primary
carbonization. The factors that play an important role in the above process include
carbonization temperature, heating rate and residence time. Carbonization is also
regarded as slow pyrolysis because it takes place at relatively lower heating rates at
temperatures below 400 °C for a very long residence time that can take several hours
or even days (Bandosz, 2006).
However, one of the primary products of pyrolysis that is in solid form (char)
is a carbonaceous residue that mainly consist of elemental carbon that originates from
the thermal decomposition of precursor material after the completion of the pyrolysis
process. In other words, it could be regarded as the residue of an unconverted biomass
material used in pyrolysis process. The char plays an important role in the pyrolysis as
it has tendencies to catalyse or sometimes participate in secondary reactions. The
applications of the char varies considerably according to its characteristics. It is widely
applied as feedstock for production of activated carbon, nanofilaments, the gasification
process to obtain hydrogen rich gas, producing high surface area catalyst used in
electrochemical capacitors and also used in industries as solid fuel for boilers
(Fernández et al., 2011). In this study, the char produced is mainly used for activated
carbon synthesis.
16
2.2.3 Activation Process
Activation can be achieved thermally by heating the solid product (char) obtained from
carbonization stage. Although, the char has started developing rudimentary pores as a
result of carbonization process, it is unsuitable to be used as adsorbent until the porous
structure is fully developed through activation. The activation process of char could
be achieved by either physical or chemical means. In either case, heating of the char is
required even though the temperature range is higher in physical activation due to
absence of chemical activant. However, efficiency of activated carbon depends on
modifications of its properties to increase its affinity towards certain contaminants
(Çeçen & Aktas, 2011; Lu et al., 2012).
The functional groups at the surface of activated carbon were derived from the
following sources namely; the precursor, thermal treatment, chemical treatment and
activation methods (Li et al., 2011). Furthermore, activation process can be one-step
or two steps. When carbonization and activation process are carried out
simultaneously, then it is regarded as one-step activation. On the contrary, if the two
processes were carried out one after the other, that is carbonization followed by
activation process, then it is called two-step activation process as presented in Figure
2.4 (Bandosz, 2006; Nasri, et al., 2014).
Figure 2.4 Flowsheet for the production of activated carbon (Bandosz, 2006)
Raw
Material
Crushing,sieving
Mill
Briquetting
Preoxidation
Wash Carbonization Activation
Pre-treatments ActivationCarbonizationPrecursor
17
2.2.3.1 Physical Activation
For physical activation process, the raw material is first of all washed, dried and milled.
It is then carbonized under oxygen deficient environment by flow of nitrogen to a
temperature between 600 to 700 °C. The carbonized sample is then activated under
flow of CO2 or steam to a temperature of about 600 to 900 at 2 – 20 °C/min. The
product is then washed with distilled water until the washing’s pH is close to 7, dried
in oven and then stored in desiccator for use (Demiral et al., 2011).
The physical or thermal treatment method involved two stages; thermal
decomposition otherwise known as carbonization of the precursor material followed
by activation of the char. This process is basically referred to as dry oxidation which
involved the reaction between the precursor and gaseous steam, gases and steam or
mixture of CO2 and air at temperature above 700 °C. A greater uniformity of the pores
are usually achieved by activation with CO2 compared to other physical activants. This
process involved the removal of moisture and non-carbon portions of the material to
produce a carbon skeleton with latent pore structure. The carbonization process
converts the precursor to char with high fixed carbon content for activation purpose.
Carbonization temperature is usually achieved at temperature range between 400 – 850
°C under nitrogen flow so as to create inert atmosphere within the system. On the other
hand, activation temperature ranges from 600 – 900 °C which allow the conversion of
the carbonized material to activated carbon (Yahya et al., 2015).
The activation process is usually carried out in the presence of activating gases
such as carbon dioxide, steam or air, or combinations of any of the relevant gases
mentioned above. The oxidizing gases reactions with the biomass give rise to the pore
creation and development. During this process, if the carbon atoms were removed from
inside of the rudimentary pores formed during carbonization, the opened micropores
become enlarged and the closed micropores opened up. However, if the atoms were
removed from the surface of the material, no new pores will be formed but it assist in
the reduction of the particle size. In physical activation, gases were utilized as
activating agents and does not produce waste water thereby making the process to be
an environmental friendly technology. In brief, the activation process increases the
pore volume and surface area of the precursor greatly through the removal of volatile
18
components and also from carbon burn-off (Ioannidou & Zabaniotou, 2007). The
overall chemical reactions can be presented as follows;
H2O + CX CO + H2 + CX-1 at (800 – 900 °C)
CO2 + CX 2CO + CX-1 at (800 – 900 °C)
O2 + CX 2CO + CX-2 at (800 – 900 °C)
O2 + CX CO2 + CX-1 at (< 600 °C)
However, the physical activation process for production of microporous activated
carbon have some inherent drawbacks which include longer time of production and
consumption of high energy. Another disadvantage of this method is that large amount
of internal carbon mass is removed to obtain a well-developed internal structure which
may translate to lower yield of product from the precursor material. The carbonization
and activation procedure of the precursor materials are usually carried out in direct-
fired rotatory kilns or furnaces. The materials used for constructing kilns and furnaces
are usually steel and other refractories designed to withstand high temperatures during
the processes. Recently, Microwave have also been employed for the activation of char
with immense results of short heating period, simplicity of operation and cost
effectiveness (Deng et al., 2010; Foo & Hameed, 2011; Liu et al., 2010). After the
activation procedure, the reactor was cooled down to about 50 °C under nitrogen gas
flow and the resultant AC produced is then washed with distilled water until washing’s
pH is close to 7.
2.2.3.2 Chemical Activation
Chemical activation could be a single or two step process for the production of
activated carbon. For a single step, the carbonization and activation of the precursor
material takes place at the same time. Which means, the raw material is first
impregnated with the chemical activant, followed by simultaneous carbonization and
activation process. While for the two steps, the carbonization and activation are carried
out consecutively in which the precursor is first carbonized in either furnace or
microwave oven, the product (char) is then impregnated with the chemical activant
and the activation is later carried out. In the chemical activation process, the precursor
19
is mixed with a solid chemical activant which is an acidic, basic or a salt containing
alkali or alkali earth metals. The activating agents dehydrates the material and as such
control its thermal decomposition by preventing the formation of the pyrolytic residue
and thus improving the carbon yield. The temperatures applied during chemical
activation process are usually lower than those used during carbonization process.
Therefore, internal pore structure development is better achieved in the case of
chemical activation process. Other advantages of chemical activation include;
simplicity of use, lower activation temperatures, no need of previous carbonization of
the precursor and development of good porous structure. Despite the above mentioned
advantages, the chemical activation process has a disadvantage of post-activation
washing of the carbon to remove residual reactants and inorganic materials (ash) which
makes the process time and energy consuming, tedious and expensive (Abioye & Ani,
2015; Viswanathan et al., 2009).
Chemical activation is a process in which the raw material is impregnated by a
chemical compound such as H2SO4, H3PO4, KOH, K2CO3, ZnCl2 or any other
inorganic chemical compound and the impregnated material is pyrolysed to degrade
and dehydrate the organic materials content and then it is washed to remove the
activating agents. The dehydration of the lignocellulosic material results into charring,
aromatization and creation of the porous structure in the carbon skeleton (Kushwaha
et al., 2012; Nor et al., 2013).
In chemical activation process, the raw material is first washed, dried and
crushed into desired particle sizes before impregnation with the chemical activant. The
activant concentration is adjusted to obtain a required mass ratio of the activant to the
raw material, the mixture is then stirred until homogeneity, and it is then carbonized
and activated in an inert atmosphere at temperatures between 400 – 800 °C. The
product is then washed to remove residual activant. Washing is carried out using 0.1
M solution of HCl and followed by distilled water till the pH of the washings is
constant and close to 7. The washed product is then dried and stored in desiccator. It
is important to note that, adjusting operation conditions and parameters such as
carbonization or activation temperature, duration of heating, activant-precursor ratio
and the choice of activant allows for determination of the optimum operating
conditions for the production of carbon with properties for a specific application (Foo
& Hameed, 2012b). Studies revealed that, chemical activation process involved less
soaking time and lower temperatures compared with physical activation process
20
thereby reducing energy cost and consequently production cost, though chemical
activation process requires treatment of residual waters from the washings. However,
studies reports that adsorption capacity increases continuously with an increase of the
activant-precursor ratio until a point where further increase has no effect on the
activated carbon’s properties.
In short, the chemical activant play the following roles during chemical activation:
Bond initiation and bond formation with the weakly connected bridges that
translate to dehydration and elimination reactions that release volatiles to
create a stronger cross-linked structure.
Tar and other liquids (such as acetic acid and methanol) minimization and
reduction of particles or volume shrinkages during thermal treatment resulting
into an adsorbent with high yield and high porosity
Modification of the adsorbent’s surface functionalities by forming surface
complexes or functional groups (Viswanathan et al., 2009).
Similarly, a large number of carbonaceous materials often regarded as agricultural
waste such as oil palm shell (Nasri et al., 2015), coconut shell (Nasri, et al., 2014), rice
husk (Kalderis et al., 2008), cocoa shell (Ahmad et al., 2013), corn cobs and fruit
waste (Ali et al., 2012) have all been converted successfully into activated carbon
through physical or chemical activation process. Activated carbons with high surface
area and a wide open structure have been synthesized through chemical activation of
carbonaceous raw materials with chemicals such as potassium carbonate. In addition,
it is important to note that the properties of the synthesized product depends on the
starting raw material, carbonization conditions, and suitable activation process and
post treatment methods employed.
Among the chemical activants employed, K2CO3 is very effective for activated
carbon production. The reaction between carbon skeleton and K2CO3 in an inert
atmosphere leads to reduction of the salt and oxidation of the carbon to generate
porosity. However, chemical activants like KOH and NaOH were hazardous,
expensive and corrosive, while others like ZnCl2 were unfriendly to the environment
and also poses waste disposal challenge (Adinata et al., 2007). Therefore, K2CO3 is
employed in this study for purification of water and wastewater.
21
2.3 Types of Activated Carbon
Activated carbons are generally categorized and manufactured as either powdered,
granular (GAC), extruded or pelletized (EAC), bead activated carbon (BAC) and
polymer coated activated carbon. The most commonly used among the types were
GAC and PAC. The PAC comprised of ground carbon that have very fine particle size.
It is usually added to water treatment either in dry form or as slurry. The PAC‘s
adsorption capacity for the removal of tastes and odours depends on factors like
adequate mixing, contact time, adsorbent dosage and concentration of the
contaminants that brings about the taste/odour (Bandosz, 2006).
GAC has relatively larger particle size in comparison to PAC and this also
made it to have a smaller external surface. PAC has disadvantages such as high
operation cost when use continuously, cannot be regenerated, produces large amount
of sludge and infiltrate through filters causing dirt in water (Bandosz, 2006). Also,
GAC can remove organics as well as taste and odour causing compounds. The GAC
is used in a filter medium as a layer or as separate filter known as GAC contactors.
The GAC contactors are similar to filters as they are all designed to provide adequate
contact time between the contaminants and the filter medium. The contact time can be
determined by calculating the empty bed contact time (EBCT) which is also known as
detention time by dividing the volume of the filter with flow rate. The calculation is
coined as EBCT because the volume taken up by the GAC is not taken into
consideration. However, GAC has advantage of regeneration after use compared to
PAC (Bandosz, 2006). Below is a schematic diagram of a GAC contactor (Figure 2.5).
Figure 2.5 The GAC Contactor
22
In addition, most forms of activated carbon are non-polar in nature, which
made them to have great affinity for non-polar compounds. Therefore, they are
effectively employed in the removal of organic contaminants including phenolics
(Bandosz, 2006). The different types of AC are presented in Figure 2.6.
Figure 2.6 Types of activated carbon (A) PAC, (B) GAC and (C) EAC (Bandosz,
2006)
23
2.4 Microwaves
Microwaves are between infrared radiation and radio waves in electromagnetic
spectrum region. Microwaves have electromagnetic waves with wavelength in the
range 0.001 to 1 m, which were equivalent to frequencies between 300 to 0.3 GHz.
This range of electromagnetic spectrum were used for many applications such as radar,
telecommunication, heating and various industrial processes (Fernández et al., 2011;
Ranji, 2014). Figure 2.7 shows the electromagnetic spectrum
Figure 2.7 The electromagnetic spectrum (Lam et al., 2016)
2.4.1 Microwave Heating
Two frequencies were selected for heating purposes by the Federal Communications
Commission (FCC) for microwave heating applications. The frequencies reserved
were 0.915 and 2.45 GHz. Microwave heating is different from the conventional
heating because energy is rather transferred to the material being heated. The material
to be heated must have dielectric properties in which the heating is achieved when the
dipoles in the dielectric material were exposed to alternating electromagnetic field that
will make them to approximately realign themselves 2.5 billion times per second for a
microwave operating at frequency of 2.45 GHz. This movement translate into rotation
of dipoles and energy is released in form of heat from internal resistance to the rotation
(Fernandez et al., 2014).
24
Materials are characterized based on their interactions with the microwaves.
Microwaves consist of two components that are perpendicular to one another, namely
electric and magnetic fields as shown in Figure 2.8. Different materials exhibit
different dielectric properties and hence different interactions with the microwaves.
Based on this factor, the materials are categorized into conductors, transparent,
absorbers and mixed absorbers. The ability of any material to get heated by a
microwave radiation depends on its loss factor where the high absorbing materials are
regarded as having high loss factor, while those with low absorbance were categorized
as low factor. Conductors such as metals cannot be heated with microwaves because
as they interact with microwaves, their surfaces reflect off the radiation. On the other
hand, transparent materials such as glass allow the radiation to pass it. The mixed
absorbers such as composite materials display mixed characteristics of either
absorption, reflection or transparency (Figure 2.9). Despite some materials were
categorized as having low loss factor or poor microwave absorbers, sometimes do
absorb and become heated in the process when heated in heated above their critical
temperatures due to temperature dependent nature of microwave heating (Appleton et
al., 2005). The only materials that perfectly absorbs microwave radiation and become
heated during the process are those that have good dielectric properties.
Figure 2.8 The magnetic and electric fields of microwaves (Lam et al., 2016)
150
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