1.1 BACKGROUND OF STUDY
Activated carbon, also widely known as activated charcoal or activated coal is a form of carbon which has been processed to make it extremely porous and thus to have a very large surface area available for adsorption or chemical reactions (Mattson et al., 1971). The word active is sometimes used in place of activated. It is characterized by high degree of micro porosity. A gram of activated carbon can have a surface area in excess of 500 m2 Sufficient activation for useful applications may come solely from the high surface area, though further chemical
treatment generally enhances the adsorbing properties of the material. Activated carbon is most commonly derived from charcoal. Waste biomass is getting increasing attention all over the world for activated carbon development as it is renewable, widely available, cheap and environmentally friendly resource. The common method of development is thermochemical (Kumar et al., 2005). The main concern is the removal of chemical component by adsorption from the liquid or gas phase (Bansal et al. 1988). Today, activated carbon has been produced from various biomass such as corncob, rice husk, cherry stones, coconut shells, palm shells, to mention but a few. Preparation of activated carbon with ultra-high specific surface area from biomass
such as lignin, corncob, cornstalk, dates, etc., has attracted much attention. Among these carbon sources, corncob is a good precursor for preparing carbon with ultrahigh specific surface area (Li, 2007). The carbons prepared from corncob have been used in wastewater treatment such as removal of organic pollutants (Sun et al., 2006).
However, a comprehensive study of activating corncob with different activation strategies to prepare carbon with ultra-high specific surface area and pore volumes, and their subsequent performance in water purification as the impurity adsorption has not to our knowledge been reported. Therefore, in this study we report the synthesis of ultra-high surface area carbon materials using two preparation strategies namely, chemical activation procedure using a chemical activator such as ammonium sulphate ((NH4)2SO4) and microwave-synthesized activation procedure. We also report the adsorption capacity of those carbons for water purification. To prepare activated carbon, conventional heating method is usually adopted, in which the heat is produced by electrical furnace. However, in some cases, the thermal process may take several hours, even up to a week to reach the desired level of activation (Yuen et al., 2009). Another problem related to the furnace is that the surface heating does not ensure a uniform temperature for different shapes and sizes of samples. This generates a thermal gradient from the hot surface to the kernel of the sample particle, blocks the effective diffusion of gaseous products to its surroundings and finally results in activated carbon quality decrease (Peng et al., 2008). Furthermore, there is a considerable risk of overheating or even thermal runaway (exothermic process) of portion of sample, leading to the complete combustion of the carbon (Williams et al., 2008).
Recently, microwave has been widely used in preparation and regeneration of activated carbon. The main difference between microwave devices and conventional heating systems is heating pattern. In microwave device, the energy is directly supplied to the carbon bed. The conversion of microwave energy is not by conduction or convection as in conventional heating, but by dipole rotation and ionic conduction inside the particles (Jones, 2002). Therefore, the treatment time can be significantly reduced through microwave heating.
1.2 STATEMENT OF PROBLEM
In recent years, increasing awareness of environmental impact of organic and inorganic compounds has prompted the purification of waste water prior to discharge into natural waters. A number of conventional treatment technologies have been considered for treatment of waste water contaminated with organic substance. Among them, the adsorption process has been found to be the most effective method while activated carbon is regarded as the most effective material for controlling this organic load. Common active carbons available are usually developed by thermo chemical means using activating agents and heating ovens, thus producing activated carbons which take a longer time with limited pore structures. With the advent of microwave technology, a better and efficient activated carbon can be produced within a short period and a cheaper cost.
1.3 OBJECTIVE OF THE RESEARCH
The aim of this research project is to determine and compare the performance of chemically and microwave synthesized activated carbon from corn cob.
1.4 SIGNIFICANCE OF THE RESEARCH
When this research project is successfully completed, it will provide the following
i. Corn cobs are abundant in Nigeria.
ii. Encourage the establishment of industries that will use Agricultural waste materials to produce activated carbon.
iii. It will create job opportunities, thereby reducing unemployment in the country.
iv. It will attract foreign exchange for Nigeria as activated carbon has
very wide industrial applications.
1.5 SCOPE OF RESEARCH
This research work focuses on the following:
i. Preparation of activated carbon from corn cob by thermal and microwave means
ii. Comparative study of the adsorption capacities of chemically and microwave synthesized activated carbon.
The term adsorption refers to the accumulation of a substance at the interface between two phases such as solid and liquid or solid and gas. The substance that accumulates at the interface is called ‘adsorbate’ and the solid on which adsorption occurs is ‘adsorbent’. Although certain phenomenon associated with adsorption were known in ancient times, the first quantitative studies were reported by C.W. Scheele in 1773 (Mantell, 1951) on the uptake of gases by charcoal and clays. This was followed by Lowitz’ observations who used charcoal for decolorization of
tartaric acid solutions. Larvitz in 1792 and Kehl in 1793 observed similar phenomenon with vegetable and animal charcoals, respectively. However, the term ‘adsorption’ was proposed by Bois-Reymond but introduced into the literature by Kayser (Abrowski, 2001). Ever since then, the adsorption process has been widely used for the removal of solutes from solutions and gases from air atmosphere. At the surface of the solids, there are unbalanced forces of attraction which are responsible for adsorption. In cases where the adsorption is due to weak van der Waals forces, it is called physical adsorption. On the other hand, there may be a chemical bonding between adsorbent and adsorbate molecule and such type of adsorption is referred as chemisorption.
2.2 ACTIVATED CARBON DEVELOPMENT
Activated carbon is nothing but carbon produced from carbonaceous source materials like corn cob, nutshells, peat, wood, coir, lignite, coal and petroleum pitch. It can be produced by any one of the following described processes:
2.2.1 PHYSICAL REACTIVATION
By this process precursor is developed into activated carbons using gases. This is generally done by using one or a combination of the following processes:
Carbonization: Material having appreciable carbon content is pyrolyzed at temperature ranging between 600–900 °C, in the absence of oxygen (usually in inert atmosphere with gases like argon or nitrogen) using a furnace.
Activation/Oxidation: in this process raw material or carbonized material is exposed to oxidizing atmospheres (carbon monoxide, oxygen, or steam) at temperatures above 250°C, usually in the temperature range of 600–1200°C.
2.2.2 CHEMICAL ACTIVATION
Before carbonization, the raw material can be impregnated with certain chemicals. The chemical needs to be typically an acid, strong base, or a salt (ammonium sulphate, phosphoric acid, potassium hydroxide, sodium hydroxide, zinc chloride, respectively). The role of the activating agent is to improve the pore size of the activated carbon in order to improve its adsorption capacity. After impregnation, the raw material needs to be carbonized at lower temperatures (450–900 °C). It is believed that the carbonation / activation step proceeds simultaneously with the
chemical activation. Chemical activation is preferred over physical activation owing to the lower temperatures and shorter time needed for activating material.
2.2.3 STEAM ACTIVATION
The use of steam for activation can be applied to virtually all raw materials. Varieties of methods have been developed but all of this shares the same principle of initial carbonization at 500°C to 600°C followed by activation with steam at 800°C to 1100°C. Since the overall (converting carbon to carbon dioxide) is exothermic, it is possible to utilize this energy and have a self-sustaining process.
Initial, gasification of the carbonize material with steam occurs and is shown in the following reaction known as water-gas reaction.
C + H2O CO + H2 (- 31 Kcal)
This reaction maintains temperature by partial burning of the CO and H2
CO + 1
/2O2 CO (+ 67 Kcal)
H2O + 1
/2O2 H2O (+ 58 Kcal)
C + O2 CO2 (+ 94 Kcal)
2.3 PROPERTIES OF ACTIVATED CARBON
The properties of activated carbon can be discussed under physical and chemical
2.3.1 PHYSICAL PROPERTIES
The most important physical property of activated carbon is the surface area of the activated carbon. For specific applications, the surface area available for adsorption depends on the molecular size of the adsorption and the pore diameter of the activated carbon. Generally, liquid-phase carbons are characterized as having a majority of pores of gas phase adsorbents are 3mm in diameter and smaller. They require larger pores due to the essence of rapid diffusion of the liquid.
The density of activated carbon, together with its specific adsorptive capacity for a given substance can be used to determine grades of activated carbon required for an existing system.
The mechanical strength and the resistance of the particles are important where pressure drop and carbon losses are concern.
2.3.2 CHEMICAL PROPERTIES
1. IODINE NUMBER: Iodine number is defined as the milligrams of iodine adsorbed by one gram of carbon when the iodine concentration in the residual filtrate is 0.02 normal. Iodine number is the most fundamental parameter used to characterize activated carbon performance. It is a measure of activity level (higher number indicates higher degree of activation), often
reported in mg/g (typical range 500–1200 mg/g). It is equivalent to surface area of carbon between 900m²/g and 1100m²/g. It is the standard measure for
liquid phase applications.
2. MOLASSES NUMBER: Some carbons are more adept at adsorbing large molecules. Molasses number or molasses efficiency is a measure of the mesopore content of the activated carbon by adsorption of molasses from solution. A high molasses number indicates a high adsorption of big molecules (range 95–600). Molasses efficiency is reported as a percentage (range 40%–185%). The European molasses number (range 525–110) is inversely related to the North American molasses number.
3. TANNIN ADSORPTION: Tannins are a mixture of large and medium size molecules. Carbons with a combination of macropores and mesopores adsorb tannins. The ability of a carbon to adsorb tannins is reported in parts per million concentrations (range 200 ppm–362 ppm).
4. DECHLORINATION: Some carbons are evaluated based on the dechlorination half-life length, which measures the chlorine-removal efficiency of activated carbon. The dechlorination half-value length is the depth of carbon required to reduce the chlorine level of a flowing stream from 5 ppm to 3.5 ppm.
5. APPARENT DENSITY: Higher density provides greater volume activity and normally indicates better-quality activated carbon.
6. HARDNESS/ABRASION NUMBER: It is a measure of the activated carbon’s resistance to attrition. It is an important indicator of activated carbon to maintain its physical integrity and withstand frictional forces imposed by backwashing, etc. There are large differences in the hardness of activated carbons, depending on the raw material and activity level.
7. ASH CONTENT: Ash reduces the overall activity of activated carbon and it reduces the efficiency of reactivation. The metal oxides (Fe2O3) can leach out of activated carbon resulting in discoloration. Acid/water soluble ash content is more significant than total ash content.
8. PARTICLE SIZE DISTRIBUTION: The finer the particle size of an activated carbon, the better the access to the surface area and the faster the rate of adsorption kinetics. In vapour phase systems this needs to be considered against pressure drop, which will affect energy cost.
2.4 STRUCTURE OF ACTIVATED CARBON
A proper glance at the molecular and crystalline structure of carbon helps to understand the structure of carbon. However, activated carbon is a micro porous inert carbon with a large internal surface and this surface, organic molecules from liquids or gases can adsorb. Adsorption is the natural phenomenon in which molecules from the gas or liquid phase are attached to the surface to the solid. Carbon material are activated by series of processes which include removal of all water (dehydration), conversion of organic matter to elemental carbon, driving off the non-carbon portion (carbonization), burning off tars and enlargement of pores (activation).
The basic structural unit of activated carbon is closely approximated by the structure of pure graphite. The graphite crystal is composed of layers of fused hexagons held by weak Van-der-waal forces. Activated carbon is a disorganized form of graphite due to impurities and the method of preparation. The structure developed is a function of carbonization and activation temperature. In terms of pores structure, the adsorbent pores can divided into three basic classes:
- Macro pores
- Transitional or meso pores
- Micro pores
The micro pores are developed primarily during carbon activation and result in the large surface area for adsorption to occur. Activated carbons contain:
- Bulk atoms that are natural
- Surface atoms that are real
- Corner atoms that are very reactive and even react with metals.
2.5 APPLICATIONS OF ACTIVATED CARBON
The uses of activated carbon products are diverse as they are used in virtually every aspect of life. They are important and hence cannot be overemphasized.
Some of these applications include:
a. ANALYTICAL CHEMISTRY: Activated carbon, in 50% w/w combination with celite , is used as stationary phase in low-pressure chromatographic separation of carbohydrates (mono-, di-trisaccharides) using ethanol solutions (5–50%) as mobile phase in analytical or preparative protocols.
b. ENVIRONMENTAL APPLICATIONS: Activated carbon is usually used in water filtration systems. Carbon adsorption has numerous applications in removing pollutants from air or water streams both in the field and in industrial processes such as: Spill cleanup, Groundwater remediation, Drinking water filtration, Air purification and Volatile organic compounds capture from painting, dry cleaning, gasoline dispensing operations & other processes. Activated carbon is also used for the measurement of radon concentration in air.
c. MEDICAL APPLICATIONS: Activated carbon is used to treat poisonings and overdoses following oral ingestion. It is thought to bind the poison and prevent its absorption by the gastrointestinal tract. In cases of suspected poisoning, medical personnel administer activated carbon on the scene or at a hospital’s emergency department. Dosing is usually 1 gram/kg of body
mass (for adolescents or adults, give 50–100 g), usually given only once, but depending on the drug taken, it may be given more than once.
d. FUEL STORAGE: Research is being done in testing various activated carbons’ ability to store natural gas and hydrogen gas. The porous material acts like a sponge for different types of gases. The gas is attracted to the carbon material via Van der Waals forces. Some carbons have been able to achieve bonding energies of 5–10 kJ per mol. The gas may then be desorbed
when subjected to higher temperatures and either combusted to do work or in the case of hydrogen gas extracted for use in a hydrogen fuel cell. Gas storage in activated carbons is an appealing gas storage method because the gas can be stored in a low pressure, low mass, low volume environment that would be much more feasible than bulky on board compression tanks in vehicles.
e. GAS PURIFICATION: Filters with activated carbon are usually used in compressed air and gas purification to remove oil vapors, odors, and other hydrocarbons from the air. Activated carbon filters are used to retain radioactive gases from a nuclear boiling water reactor turbine condenser. The air vacuumed from the condenser contains traces of radioactive gases.
The large charcoal beds adsorb these gases and retain them while they rapidly decay to non-radioactive solid species. The solids are trapped in the charcoal particles, while the filtered air passes through.
f. CHEMICAL PURIFICATION: Activated carbon is commonly used to purify solutions containing un-wanted colored impurities such as during a recrystallization procedure in Organic Chemistry.
g. Distilled alcoholic beverage purification: Activated carbon filters can be used to filter vodka and whiskey of organic impurities which can affect color, taste, and odor. Passing an organically impure vodka through an activated carbon filter at the proper flow rate will result in vodka with an identical alcohol content and significantly increased organic purity, as judged by odor and taste.
h. MERCURY SCRUBBING: Activated carbon, often impregnated with iodine or sulfur, is widely used to trap mercury emissions from coal-fired power stations, medical incinerators, and from natural gas at the wellhead.