ABSTRACT
Rice husk, a potential source of activated carbon, is
locally available in Nigeria but underutilized thereby constituting solid waste
menace in the environment. Incidentally, large volume of wastewater containing
high concentrations of toxic contaminants such as phenol and other contaminants
are continuously being generated as a result of increased industrial
activities. Agricultural waste materials including rice husk, when suitably
modified could serve as activated carbon for efficient removal of wastewater
contaminants. In this study, rice husk activated carbon was produced by
carbonization followed by activation with phosphoric acid. Batch adsorption
experiment was conducted on effect of process variables (carbonization
temperature, initial phenol concentration, adsorbent dosage, contact time,
solution pH and temperature) on the adsorption of phenol onto rice husk activated
carbon. Equilibrium isotherm, kinetics and thermodynamics of the adsorption
process were studied. Response surface methodology was then employed for
modeling and optimization of the process variables. Under the optimum condition
for the production of rice husk activated carbon developed, characterization of
the rice husk activated carbon was carried out and column adsorption experiment
was conducted in two modes: Rapid Small Scale Column Test (RSSCT) and Large
Scale Column Test (LSCT) using process wastewater collected from Kaduna
refinery and Samaru stream water. RSSCT was utilized to study the effect of
process variables: flow rate and bed depth and the experimental data obtained
were fitted into column models (BDST, Thomas and Yoon-Nelson), while the LSCT
was employed to obtain the peak adsorption performance. For the batch
adsorption, process variables were observed to have significant effect on the
performance of the adsorption process. Adsorption capacity and removal
efficiency of 2.4 mg/g and 96 % were attained and equilibrium was found to
occur between 10-20 minutes.
Adsorption capacity and removal efficiency were also observed to be maximum at
pH=4 and better favoured at lower temperatures. The adsorption process was
found to be inconsistent with the assumption of Langmuir monolayer but
conformed to Freudlinch assumption of multilayer and physical adsorption. The
data followed pseudo-second order kinetic model and intra-particle diffusion
was not the only rate-limiting step. The adsorption process was also found to
be exothermic. The model equations developed have correlation coefficient (R2)
values of 0.9979 and 0.9981 for removal efficiency and adsorption capacity
respectively. Optimization revealed that the optimum temperature for the
production of rice husk activated carbon is 441.46 oC. The
models were experimentally validated under optimum condition. Phosphoric acid
modification of rice husk was also observed to enhance the surface area from
12.47 to 102.4 m2/g and micropores from 2.4 to 1.82 nm. This was
confirmed from the SEM micrographs while FTIR analysis revealed the existence
of oxy- and phosphorous-oxy-containing functional groups. RSSCT shows that
performance of the column adsorption was significantly affected by bed depth
and flow rate. The column experimental data fitted well to Thomas and
Yoon-Nelson models. LSCT shows that the column experiment attained relatively
high adsorption capacity of 55.106 mg/g at exhaustion point of 0.9; this
implies the peak adsorption capacity of the rice husk activated carbon.
CHAPTER ONE
INTRODUCTION
1.1 Preamble
Biomass waste especially agricultural wastes have, in recent
times, been receiving increased attention for many reasons; among which are
depletion of global fossil fuel reserves, economic and environmental benefit,
availability and renewability (Deng et al., 2010; Igboro, 2010; Mohamed et al.,
2010; Liu and Zhang, 2011; Wang et al., 2011a; Lim et al., 2012; Chen et al.,
2013). These agricultural wastes constitute a great source of emerging
bio-products (bio-fuel, bio-chemicals and bio-materials) and their efficient
utilization will provide a viable platform for effective solid waste management
(Kalderis et al., 2008a; Singh et al., 2008; Purnomo et al., 2011; Boumaza et
al., 2012; Sahin and Saka, 2013). Adsorbents that are used in removal of
contaminants from water and wastewater include activated alumina, zeolites and
activated carbon (Ahmaruzzaman, 2008; Lin and Juang, 2009; Soto et al., 2011;
Ali et al., 2012; Han et al., 2013). Nowadays, activated carbons
(bio-materials) prepared from agricultural wastes are receiving increasing
attention due to their relatively low cost, availability and ease in processing
(Krishnani et al., 2008; Sahu et al., 2009b; Chand et al., 2009; Njoku and
Hameed, 2011).
Water is an essential necessity for the well being of all living
things. It is unique in its physiological roles to all living resources and
hence indispensable to all life forms. Therefore, among the various forms of
pollution, water pollution is of greatest concern (Bhatnagar and Minocha,
2006). It is a known fact that increasing global industrialization has led to
the continuous generation of large amounts of wastewater containing toxic
pollutants. The presence of these toxic contaminants is associated with adverse
effect and has therefore led to increased strict regulation of water pollution hence; making the treatment of wastewater generated from
industrial activities a necessity before being discharged into the environment
(Sahu et al., 2009b; Calero et al., 2013; Moussavi et al., 2013).
Consequently, pollution control and management have evolved
many technologies for the treatment of wastewater (Qadeer and Akhtar, 2005;
Bhatnagar and Minocha, 2006). These technologies and methodologies which differ
in their performance and effectiveness include coagulation, filtration, ion
exchange, sedimentation, solvent extraction, adsorption, electrodialysis,
chemical oxidation, disinfection, chemical precipitation and membrane
separation (El-Naas et al., 2010; Liu et al., 2010). Among the various
available technologies for water pollution control, adsorption process is
considered relatively more promising because of its convenience, ease in
operation and simplicity of design (Han et al., 2008; Kadhim and Al-Seroury,
2012; Song et al., 2013). Various kind of contaminants can be removed from
polluted water by adsorption process due to the availability of diverse form of
adsorbents especially activated carbon which makes it have a broader
applicability in water pollution control (Kalderis et al., 2008b; Fierro et al.,
2008; Liu et al., 2010; Aidan, 2012).
1.2 Rice and Rice
Husk
Rice is the third most important cereal crop grown around the
world with an annual output of more than 650 million tons (Song et al., 2013).
Nigeria has an annual local production of 3-4 million tons of paddy rice
(Cadoni and Angelucci, 2013) and about 20-25 % of this is rice husk. Rice husk
which is a layer protecting rice grain (Nhapi et al., 2011) is the major
by-product obtained from rice processing (Zhang et al., 2011a). It consists of
cellulose (32.24 %), hemicelluloses (21.34 %), lignin (21.44 %), water (8.11
%), extractives (1.82 %) and mineral ash (15.05 %) as well as high percentage
of silica in its mineral ash, which is 94.5-96.34 % (Ngah and
Hanafiah, 2008; Demibras, 2009; Masoud et al., 2012). Rice husk also has an
average particle diameter of 4-5mm and bulk density of 96-160 kg/m3 (Bansal et
al., 2009). In recent times, there have been deliberate attempts to utilize
large quantities of rice husks from rice mills for useful purposes. These
include the use as animal feed, bedding materials, soil conditioner,
fertilizer, solid fuel for steam generation, bio-fuel, a source of organic and
inorganic chemicals, porous carbon, catalyst, abrasives components, refractory
and insulating materials, paper and board manufacturing, among others (Patel,
2005; Srivastava et al., 2008; Lin et al., 2013). Rice husk is insoluble in
water, has good chemical stability, high mechanical strength and possesses a
granular structure, making it a potential adsorbent material (Ngah and Hanafiah,
2008; Chakraborty et al., 2011). It constitutes approximately 20-25 % of the
total grain weight depending on the variety and it is abundantly and locally
available at almost no-cost but underutilized thereby constituting solid waste
in the environment (Masoud et al., 2012). It is very difficult to decompose and
traditionally, it is being disposed on land resulting in aesthetic pollution,
eutrophication and perturbation in aquatic life (Foo and Hameed, 2009; Li et al.,
2011a; Zhu et al., 2012). Application of heat treatment on rice husk can lead
to different type of residue depending on oxygen supply (Foo and Hameed, 2009).
When rice husk is burnt in air, a residue, white rice husk ash consisting of
pure silica (95 %) is obtained. But heating rice husk in limited supply or
absence of oxygen (pyrolysis) produces a black rice husk ash containing varying
amount of carbon and silica. This residue when appropriately modified can be of
useful purposes such as in water and wastewater treatment. The suitability of
rice husk for removal of wastewater contaminants largely depends on its
modification processes (Ye et al., 2010).
Over 2000 wastewater chemical contaminants have been
identified among which over 600 are of organic origin (Agarry and Aremu,
2012a). Phenolic compounds are among the most common organic pollutants and are
present in effluents from coal processing, plastic, disinfectant, pesticides,
pharmaceutical, petroleum and steel industries (Kennedy et al., 2007; Beker et
al., 2010). Phenol, an aromatic compound with the following properties,
molecular formula: C6H5OH, molecular weight: 94.11g/mol, molecular size:
0.42-0.72 nm, cross sectional area: 0.414 nm2, melting point: 40.9 oC, boiling
point: 181.75 oC, water solubility: 93 g/l, has always been chosen as a model
pollutant in the field of environmental research (Busca et al., 2008; Liu et al.,
2010; Liu et al., 2011; Song et al., 2013). Phenol is an important toxic
compound listed as a priority pollutant by environmental protection agencies
because of its high toxicity and possible accumulation in the environment
(Suresh et al., 2011; Moreno-Virgen et al., 2012). Phenol is the 11th of the
126 chemical priority pollutants by United State Environmental Protection
Agency (USEPA) with odour threshold of 0.04 ppm (Omar, 2012). The concentration
of phenol in wastewater varies from 0.1-6,800 mg/l and the permissible limit of
phenol concentration in wastewater by Federal Environmental Protection Agency
(FEPA) is 0.5 mg/l (Otekunefor and Obiukwu, 2005) while concentration in
portable water by World Health Organization (WHO) is 0.001 mg/l (Bada, 2007;
Kamble et al., 2008). Long exposure to low level of phenol in water can cause
severe health hazard which include liver damage, diarrhea, dark urine and mouth
ulcer. Phenol is a strong eye and respiratory irritant (Belgin et al., 2008).
Bad taste and odor is an indication of the presence of phenol in water (Kermani
et al., 2006). As a result of toxic and inhibitory characteristics, phenols are
very difficult or impossible to remove by biological treatment processes, hence
studies are being conducted with the
aim of decreasing their concentrations to allowable limits or
converting them to less toxic and hazardous forms that may be released easily
into the environment (Ahmaruzzaman 2008; Blanco-Martinez et al., 2009; Soto et
al., 2011; Girish and Ramachandra, 2012). In view of these characteristics, it
was chosen for focus in this study.
1.4 Statement of
the Problem
High concentration of chemical contaminants in industrial
wastewater can cause hazard when discharged into the environment (Abdelkareem,
2013). These hazards include aesthetics, health disorder, damage to aquatic
life and environment (Girods et al., 2009; Nabais et al., 2009; Lazo-Cannata et
al., 2011). Phenols are among the most common water pollutants that can cause
hazards including health hazard which may lead to death. Amongst hydrocarbons
present in refinery wastewater, phenol is one of the main dissolved components
and it is also one of the most difficult hydrocarbons to degrade biologically
(Benyahia, 2004; Kamble et al., 2008). Industries such as Kaduna Refinery
continuously generate large volume of wastewater that contains high concentration
of phenol (Alhamed, 2009; Nadavala et al., 2009) thereby making wastewater
treatment an integral part of their activities. Ultimately, this results in
increased cost of production and has led to the continuous search for simple,
cleaner and cost-effective treatment approach.
Most of the treatment methods for wastewaters are not without
their drawbacks such as high capital and operational cost, regeneration cost
and residual disposal (Bansal et al., 2009; Lakshmi et al., 2009; Chowdury et
al., 2011). Treatment cost for these methods ranges from 10-450 US Dollars per
cubic meter of treated water except for adsorption which is in the range of
5-200 US Dollars (Ali et al., 2012). Consequently, adsorption
has been identified as one of the most efficient techniques
for the removal of wastewater contaminants because of the potentials of
low-cost adsorbents especially activated carbon from agricultural wastes (Singh
et al., 2008; Krishnani et al., 2008; Naiya et al., 2009). More so that the increasing
demands for food production will lead to additional generation of agricultural
wastes such as rice husk which increase additional challenges in solid waste
disposal. For example, in an attempt to become self-sufficient in rice
production, Nigeria has been on the path of boosting its rice production
(Cadoni and Angelucci, 2013) which will invariably lead to increased generation
of rice husk (as solid waste) in the environment.
As a result of the above, there is ongoing research into
modification of these agricultural wastes with the aim of improving their
efficiencies and effectiveness in adsorption process. Rice husk is an
agricultural waste material that is cheaply available as a by-product of rice
processing but requires pretreatment (modification) for better performance as
an adsorbent. These modification processes may be very complex, sometimes
comprising of several stages of production, elevated temperatures for pyrolysis
and gas activation as well as being time-consuming (Kalderis et al., 2008a; Liu
et al., 2011). Influence of thermal and chemical pretreatment of rice husk for
phenol adsorption from wastewater has been studied but little is known of the
combined techniques in batch and column studies especially direct pyrolysis
preceding phosphoric acid activation. Therefore, this research is on the
production of rice husk activated carbon by direct pyrolysis preceding
phosphoric acid modification for adsorption of phenol and other wastewater
contaminants.
The outcome of this research will be of immense economic
benefit to the rice processing centers that generate rice husk by adding value
to their by-product. It will also be of benefit to industries that generate
phenol-containing wastewater like petroleum refinery through the availability
of simple and cost-effective treatment approach. It will also be of benefit by
adding value to the by-product (bio-char) of pyrolysis of rice husk for the
production of bio-fuel and/or chemicals. It will also be of environmental
benefit to the government and public.
1.6 Aim and
Objectives
The aim of this research is to produce combined thermal and
chemical pretreated rice husk activated carbon and apply it in the adsorption
of phenol and water treatment.
The obtectives of this research are:
i. To investigate the influence of combined thermal and chemical
pretreatment of rice husk on phenol adsorption.
ii. To investigate the effect of adsorbent dosage, initial phenol
concentration, contact time, pretreatment temperature, pH and temperature on
batch adsorption of phenol using the thermal and chemically treated rice husk
activated carbon.
iii. To study the equilibrium isotherm, kinetics and
thermodynamics of the adsorption process.
iv. To develop model equations for the process variables and
determine the optimum operating conditions (using Response Surface
Methodology).
v. To characterize the rice husk activated carbon under optimum
condition.
vi. To investigate the effect of flow rate and bed depth on
column adsorption of phenol using the thermal and chemically treated rice husk
activated carbon as well as to analyze the column experimental data using
column models.
1.7 Limitation of
the Study
This study is specifically
limited to the use of thermal and chemically treated rice husk activated carbon
in the adsorption of phenol from simulated and refinery wastewater and
treatment of general contaminants from communal raw water. While the use may have
prospects in application to no-phenolic wastes in industrial effluent, these
were not specifically covered in this study due to obvious constraint of
finance, equipment and time. Another limitation encountered in this study was
that the heating rate of the furnaces available could not be preset thereby
making it difficult to study the effect of heating rate. However, heating rate
was estimated by taking note of the temperature attained in 1 hour and dividing
it by 60 minutes.
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