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.

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).

1.3       Phenol
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.

1.5       Justification of the Study
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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