CHAPTER TWO LITERATURE REVIEW The surge of industrial activities has intensified more environmental problems as seen for example in the deterioration of several ecosystems due to the accumulation of dangerous pollutants

CHAPTER TWO
LITERATURE REVIEW
The surge of industrial activities has intensified more environmental problems as seen for example in the deterioration of several ecosystems due to the accumulation of dangerous pollutants. Apart from the environmental damage, human health is likely to be affected as the presence of toxic wastes beyond a certain limit brings serious hazards to living organisms (Febrianto et al., 2009). Phenol and substituted phenols are one of the important categories of aquatic pollutants, which are considered as toxic, hazardous and priority pollutants (Bhatnagar & Minocha, 2009). The main sources of phenol which are released into the aquatic environment are the wastewater from industries such as coke ovens in steel plants, petroleum refineries, resin, petrochemical and fertilizer, pharmaceutical, chemical and dye industries (Ahmaruzzaman & Sharma, 2005).
Several treatment methods have been applied to remove phenolic compounds from aqueous solutions, such as biological treatment using live and dead organisms, catalytic wet oxidation and adsorption technology using activated carbons prepared from various precursors. Other methods include air stripping, incineration, ion exchange and solvent extraction. For instance, petrochemical and chemical industries are concentrated in the South Durban area in South Africa where there is extreme contamination of ground and surface water and members of the community have consistently complained of high levels of cancer (Butler & Hallowes, 2002).
Adsorption is gaining interest as one of the most effective processes for treatment of industrial effluent containing toxic materials. The occurrence of non-biodegradable wastes in streams and lakes threatens the use of water resources and various treatment methods have been used for the removal of these wastes. Among these methods, adsorption using commercial activated carbon has proven to be efficient, however it is highly expensive. Hence in recent years there has been a continuous search for locally available and cheaper adsorbent.

2.1 Adsorption Process
In adsorption process, two substances are involved. One is the solid or the liquid on which adsorption occurs and it is called adsorbent. The second is the adsorbate, which is the gas or liquid or the solute from a solution which gets adsorbed on the surface.
Adsorbent: This is the substance on whose surface the adsorption occurs.
Adsorbate: This is the substance whose molecules get adsorbed on the surface of the adsorbent (i.e. solid or liquid).
Adsorption is different from absorption. In absorption, the molecules of a substance are uniformly distributed in the bulk of the other, whereas in adsorption, molecules of one substance are present in higher concentration on the surface of the other substance.
…… (2.1)
Adsorption is influenced by the nature of solution in which the contaminant is dispersed, molecular size and polarity of the contaminant and the type of adsorbent. Hence, it is important to be able to relate the amount of contaminant adsorbed from the wastewater stream to the amount of adsorbent needed to reduce the contaminant to acceptable levels (Rowe & Abdel-Magid, 1995). The presentation of the amount of solute adsorbed per unit weight of the adsorbent as a function of the equilibrium concentration in bulk solution at constant temperature is termed the adsorption isotherm. Adsorption isotherm models can be regarded as benchmark for evaluating the characteristic performance of an adsorbent.
2.1.1 Types of Adsorption
Adsorption can be classified into two types based on the nature of forces that exist between adsorbate molecules and adsorbent:
1. Physical Adsorption (Physisorption): If the force of attraction existing between adsorbate and adsorbent are Vander Waal’s forces, the adsorption is called physical adsorption. It is also known as Vander Waal’s adsorption. In physical adsorption the force of attraction between the adsorbate and adsorbent are very weak, therefore this type of adsorption can be easily reversed by heating or by decreasing the pressure.
2. Chemical Adsorption (Chemisorption): If the force of attraction existing between adsorbate and adsorbent are almost same strength as chemical bonds, the adsorption is called chemical adsorption. It is also known as Langmuir adsorption. In Chemisorption the force of attraction is very strong, therefore adsorption cannot be easily reversed.
Physisorption Chemisorption
Low heat of adsorption (20-40 kJ mol-1) High heat of adsorption (40-400 kJ mol-1)
Force of attraction are Van der Waal’s forces Forces of attraction are chemical bond forces
It usually takes place at low temperature and decreases with increasing temperature It takes place at high temperature
It is reversible It is irreversible
It is related to the ease of liquefaction of the gas The extent of adsorption is generally not related to liquefaction of the gas
It is not very specific It is highly specific
It forms multi-molecular layers It forms monomolecular layers
It does not involve any activation energy It involves activation energy
Fig. 2.1: Comparison between Physisorption and Chemisorption
(Source: Literature Survey)
Factors Affecting Adsorption:
The extent of adsorption depends upon the following factors:
Nature of adsorbate and adsorbent.
The surface area of adsorbent.
Activation of adsorbent.
Experimental conditions. E.g., temperature, pressure, etc.
2.1.2 Adsorption Isotherm Models
Analysis of the isotherm data is important to develop an equation which accurately represents the results and which could be used for design purposes and to optimize an operating procedure. Langmuir and Freundlich models are the most common theoretical equilibrium isotherms applied in solid/liquid systems (Ho, 2004; Basha et al., 2008), and the models are extensively used due to their Simplicity and ease of interpretation. Likewise, linear regression has been frequently used to evaluate the model parameters (Basha et al., 2008). However, equilibrium isotherms such as the Temkin, two site Langmuir, Langmuir-Freundlich (Sips isotherm), Redlich-Peterson, Toth, and Dubinin-Radushkevitch can also be used to model experimental data (Onyango et al., 2004).
2.1.2.1 Langmuir Adsorption Isotherm
The Langmuir isotherm also called the ideal localized monolayer model was developed to represent chemisorption (Wang et al., 2009). Langmuir (1918) theoretically examined the adsorption of gases on solid surfaces, and considered sorption as a chemical phenomenon. The Langmuir equation relates the coverage of molecules on a solid surface to concentration of a medium above the solid surface at a fixed temperature. This isotherm is based on the assumption that; adsorption is limited to mono-layer coverage, all surface sites are alike and can only accommodate one adsorbed molecule, the ability of a molecule to be adsorbed on a given site is independent of its neighbouring sites occupancy, adsorption is reversible and the adsorbed molecule cannot migrate across the surface or interact with neighbouring molecules (Febrianto et al., 2009; Sarkar ; Acharya, 2006). By applying these assumptions and the kinetic principle (rate of adsorption and desorption from the surface is equal), the Langmuir equation can be written in the following hyperbolic form:
q_e=q_max (K_L C_e)/(1 + K_L C_e ) …… (2.2)
this equation is often written in different linear forms (Febrianto et al., 2009):
1/q_e = (1/(K_L q_max )) 1/C_e + 1/q_max ….… (2.3)
C_e/q_e = 1/q_max C_e+ 1/(K_L q_max ) …… (2.4)
where qe is the adsorption capacity at equilibrium (mg/g), qmax is the theoretical maximum adsorption capacity of the adsorbent (mg/g) and, as such, can be thought of as the best criterion for comparing adsorptions (Ho et al., 1995), KL is the Langmuir affinity constant (l/mg) and Ce is the supernatant equilibrium concentration of the system (mg/l). However, it should be realized that the Langmuir isotherm offers no insights into aspects of adsorption mechanism (Liu ; Liu, 2008).
2.1.2.2 Freundlich Adsorption Isotherm
Initially, the Freundlich isotherm was of an empirical nature which was later interpreted as sorption to heterogeneous surfaces or surfaces supporting sites of varied affinities. It is assumed that the stronger binding sites are occupied first and that as the degree of site occupation increases, the binding strength decreases. (Davis et al., 2003). Adsorption of organic and inorganic compounds on a wide variety of adsorbents can be described by Freundlich isotherm (Febrianto et al., 2009). According to this model the adsorbed mass per mass of adsorbent can be expressed by a power law function of the solute concentration as (Freundlich, 1906):
q_e= K_F ?C_e?^(1?n) …… (2.5)
where KF is the Freundlich constant related with adsorption capacity (mg/g), n is the heterogeneity coefficient (dimensionless). The linear expression of Freundlich equation is written in logarithmic form as follows:
log??q_e ?= log??K_F+ (1?n)? log??C_e ? …… (2.6)
The plot of log qe versus log Ce has a slope with the value of 1/n and an intercept magnitude of log KF. On average, a favourable adsorption tends to have Freundlich constant n between 1 and 10. Larger value of n (smaller value of 1/n) implies stronger interaction between the adsorbent and the adsorbate while 1/n equal to 1 indicates linear adsorption leading to identical adsorption energies for all sites. Generally, linear adsorption occurs at very low solute concentrations and low loading of the adsorbent (Site, 2001).
2.1.3 Adsorption Kinetic Models
Adsorption equilibria studies are important in determining the efficiency of adsorption. Added spite of this, it is also necessary to identify the adsorption mechanism type in a given system. With the purpose of investigating the mechanism of adsorption and its potential rate-controlling steps that include mass transport and chemical reaction processes, kinetic models have been exploited to test the experimental data. In addition, information on the kinetics of metal/organic compound uptake is required to select the optimum condition for full-scale batch adsorbate removal processes. Adsorption kinetics is expressed as the solute removal rate that controls the residence time of the adsorbate in the solid–solution interface.
Generally, several steps are involved during the sorption process by porous sorbent particles: (i) Bulk diffusion; (ii) External mass transfer (boundary layer or film diffusion) between the external surface of the sorbent particle and the surrounding fluid phase; (iii) Intra-particle transport within the particle; and (iv) Reaction kinetics at phase boundaries.
In practice, kinetic studies were carried out in batch reactions using various initial adsorbate concentrations. Adsorption kinetic models have been proposed to clarify the mechanism of sorption from aqueous solution on to an adsorbent. Several adsorption kinetic models have been established to understand the adsorption kinetics and rate-limiting step. These include Lagergren’s pseudo-first and second-order rate model, Weber and Morris sorption kinetic model, Natarajan and Khalaf first-order reversible reaction model, etc.
2.1.3.1 Lagergren’s Model
Lagergren’s kinetics equation has been most widely used for the adsorption of an adsorbate from an aqueous solution. Vast majority of the adsolutes in the adsorption systems from the articles studied were aqueous phase pollutants such as metal ions, dyestuffs, and contaminating organic compounds. At large, the adsorbents were activated carbon (Onganer & Temur, 1998; Kadirvelu & Namasivayam, 2000; Dai, 1994), materials of biological organic compounds (Yamuna & Namasivayam, 1993; Kandah, 2001), agricultural by-products such as banana pith (Namasivayam & Kanchana, 1992), palm-fruit bunch (Nassar, 1997), corn pith (Namasivayam et al., 2001), cow dung (Das et al., 2000), sago (Quek et al., 1998), coconut husk (Manju et al., 1998), and orange peel (Namasivayam et al., 1996) and inorganic adsorbents such as fly ash (Viraraghavan & Ramakrishna, 1999; Panday et al., 1985), polyacrylamide grafted hydrous tin(iv)oxide gel (Shubha et al., 2001), Fe(III)/Cr(III) hydroxide (Namasivayam et al., 1994), chrome sludge (Lee et al., 1996), magnetite (Ortiz et al., 2001), kaolinite (Atun & Sismanoglu, 1996), and bituminous shale (Tütem et al., 1998).
Lagergren’s original paper expressed the pseudo-first order rate equation for the liquid-solid adsorption system in 1898 and was summarised as follows:
ax/dt=k(X -x) ………………………(a)
X and x (mg g-1) are the adsorption capacities at equilibrium and at time t, respectively.
k (min-1) is the rate constant of pseudo-first order adsorption.
Equation (a) was integrated with boundary conditions t = 0 to t = t and
x = 0 to x = x:
ln?(X/(X – x))=kt …………………………(b)
and
x=X(1-e^(-kt) ) ……..…………………(c)
equation (b) may be rearranged to the linear form:
log??(X-x)= log?X-k/2.303 t? ………….(d)
The most popular form used is:
log??(q_e-q_t )= log??q_e-k_1/2.303 t? ? …… (2.7)
qe and qt (mg g-1) are the adsorption capacities at equilibrium and at time t respectively. k1 (min-1) is the rate constant of pseudo-first order adsorption.
Consequently, the sorption data was also studied by second order kinetics
dq/dt=k_2 ?(q_e-q_t)?^2 …………………………. (i)
where k_2 is the rate constant of pseudo- second order adsorption.
After integration,
1/(q_e-q_t )=1/q_e +k_2 t …………………………….. (ii)
This can be written in the linear form on further simplification
t/q_t =1/(k_2 ?q_e?^2 )+t/q_e …… (2.8)
The applicability of this equation can be studied by a plot of t/qt vs. t.

2.1.3.2 Intra Particle Diffusion
The most commonly used technique for identifying the mechanism involved in the adsorption process is by fitting the experimental data in an intra-particle diffusion plot. Previous studies by various researchers showed that the plot of Qt versus t0.5 represents multi linearity, which characterizes two or more steps involved in the adsorption process. According to Weber and Morris, an intra particle diffusion co-efficient Kp is defined by the equation:
K_p= Q_t?t^0.5 or q_t= K_p t^(1?2)+C …… (2.9)
Thus the Kp (mg/g min 0.5) value can be obtained from the slope of the plot of Qt (mg/g) versus t0.5 and C is the intercept.
2.2 Types of Adsorbent
2.2.1 Commercial Adsorbents
2.2.1.1 Zeolites
Zeolites are aluminosilicate minerals containing exchangeable alkaline and alkaline earth metal cations (normally Na, K, Ca and Mg) as well as water in their structural framework. The physical structure is porous, enclosing interconnected cavities in which the metal ions and water molecules are contained. Zeolites have high ion exchange and size selective adsorption capacities as well as thermal and mechanical stabilities (Wang et al., 2009). Also, zeolites can be either synthetic (Hui et al., 2006) or natural (Rubio, 2006). They have been used as water softeners (Ali ; El-Bishtawi, 1997), chemical sieves and adsorbents (Hui et al., 2005) for a long time. However, zeolites become unstable at high pH (Basu, et al., 2006) and for this reason; chemicals are added to adjust the pH, which makes this process expensive. The process of regenerating zeolite packed beds dumps salt water into the environment. Furthermore, the use of zeolites does not reduce the level of most organic compounds (Johnson, 2005).
2.2.1.2 Silica gel
Silica gel is a non-toxic, inert and efficient support and is generated by decreasing the pH value of the alkali silicate solution to less than ten. The solubility of silica is then reduced to form the gel and as the silica begins to gel, cells in silica are trapped in a porous gel, which is a three-dimensional SiO2 network (Chaiko et al., 1998). Porous silica gel is an inorganic synthetic polymeric matrix often used to entrap cells and its use for entrapment is called the sol-gel technique (Weller, 2000). Reactive sites of silica gel exist in large numbers, and therefore, the number of immobilized organic molecules is high, which results in good sorption capacity for metal ions (Rangsayatorn et al., 2004; Chaiko et al., 1998).
2.2.1.3 Activated alumina
Activated alumina is a filter media made by treating aluminium ore so that it becomes porous and highly adsorptive. It can also be described as a granulated form of aluminium oxide. Activated alumina removes a variety of contaminants that often co-exist with fluoride such as excessive arsenic and selenium (Farooqi et al., 2007).
The medium requires periodic cleaning with an appropriate regenerant such as alum or acid in order to remain effective. Activated alumina has been used as an effective adsorbent especially for point of use applications (Ghorai ; Pant, 2005; Bouguerra et al., 2007). The main disadvantage of activated alumina is that the adsorption efficiency is highest only at low pH and contaminants like arsenites must be pre-oxidized to arsenates before adsorption. In addition, the use of other treatment methods would be necessary to reduce levels of other contaminants of health concern (Johnson, 2005).
2.2.1.4 Activated carbon
The most widely used adsorbent for industrial applications is activated carbon (Ho, 2004). In the 1940’s, activated carbon was introduced for the first time as the water industry’s main standard adsorbent for the reclamation of municipal and industrial wastewater to a potable water quality (Huang, et al., 2009). It has been found as a versatile adsorbent due to its high capacity of adsorption because of small particle sizes and active free valences. The structure consists of a distorted three dimensional array of aromatic sheets and strips of primary hexagonal graphic crystallites (Stoeckli, 1990). This structure creates angular pores between the sheets of molecular dimensions which give rise to many of the useful adsorption properties of activated carbon (Stoeckli, 1990; Innes et al., 1989). In spite of this, due to its high cost of production, activated carbon could not be used as the adsorbent for large scale water treatment. Moreover, the regeneration of activated carbon is difficult due to the use of costly chemicals, high temperatures, and hence, its regeneration is not easily possible on a commercial scale. Commercial activated carbon, which has high surface area and adsorption capacity, is a potential adsorbent for removing heavy metals and dissolved organic compounds from wastewater. However, preparing activated carbon is relatively complicated and involves carbonization and activation stages.
According to the IUPAC definitions the pore sizes of activated carbon can roughly be classified as micropores ( 50 nm) (Stoeckli et al., 2002). The macropores act as transport pathways, through which the adsorptive molecules travel to the mesopores, from where they finally enter the micropores. Thus, macro- and mesopores can generally be regarded as the highways into the carbon particle, and are crucial for kinetics. The micropores usually constitute the largest proportion of the internal surface of the activated carbon and contribute most to the total pore volume (Rodriguez-Reinoso ; Linares-Solano, 1989).
Activated carbon has both chemical and physical effects on the substance where it is used as a treatment agent. Activity can be separated into adsorption, mechanical filtration, ion exchange and surface oxidation. Adsorption is the most studied of these properties in activated carbon (Cheremisinoff ; Morresi, 1978). Heavy metal removal by adsorption using commercial activated carbon has been widely used. However, high costs of activated carbon and 10-15% loss during regeneration makes its use prohibitive in the developing countries like South Africa (Vimal et al., 2006). Commercial activated carbon also requires complexing agents to improve its removal performance for heavy metals. Therefore this situation no longer makes it attractive to be widely used in small-scale industries because of cost inefficiency (Sandhya ; Kurniawan, 2003). This has led to a search for cheaper carbonaceous substitutes. In order to overcome the problems associated with the activated carbon, low cost adsorbents derived from agricultural waste is proposed in the present work.
2.2.2 Low Cost Adsorbents
In a developing country like South Africa, materials which are locally available in large quantities such as agricultural wastes and industrial by-products can be utilized as low cost adsorbents. Conversion of these materials into adsorbents for wastewater treatment would help to reduce the cost of waste disposal and provide an alternative to commercial activated carbon (Kurniawan et al., 2006). The adsorption of toxic waste from industrial wastewater using agricultural waste and industrial by-products has been massively investigated (Basu et al., 2006; Wan ; Hanafiah, 2007; Srivastava et al., 2006). Several reviews can be referred to that discuss low-cost adsorbents application for industrial wastewater treatment (Kurniawan et al., 2006; Babel and Kurniawan, 2003; Crini, 2005; Pollard et al., 1992).

Fig. 2.2: Possible classification of low-cost adsorbents
(Source: Literature Survey Compiled by Grassi et al., 2012)

2.2.2.1 Agricultural Wastes
Production of activated carbon from agricultural wastes serves a double purpose by converting unwanted, surplus wastes to useful, valuable material and provides an efficient adsorbent material for the removal of pollutants from wastewater. In recent years, more attentions have been gained by the biomaterials which are by-products or the wastes of large-scale industrial processes and agricultural waste materials. A range of adsorbents such as orange peel, grass, leaf, wheat shells, heartwood, rice husk, saw dust of various plants, bark of the trees, groundnut shells, coconut shells, black gram husk, hazelnut shells, walnut shells, cotton seed hulls, waste tea leaves, Cassia fistula leaves, maize corn cob, jatropa deoiled cakes, apple, banana, soybean hulls, grapes stalks, water hyacinth, sugar beet pulp, sunflower stalks, coffee beans, arjun nuts, and sugarcane bagasse have been reported to be used to remove or recover heavy metals and dissolved organic compounds from aqueous solutions.
Karnitz et al. (2007) reported the use of chemically modified sugarcane bagasse to adsorb heavy metal ions and Mukherjee et al. (2007) studied the adsorption of phenol using an adsorbent derived from sugarcane bagasse as well. This shows that agricultural wastes are versatile; they can be used for sorption of both inorganic and organic wastes.
Effective use of biomass wastes has become one of the promising fields of the treatment of heavy metals due to the low cost as well as their environmentally friendly nature (Shao et al., 2011). Wong et al. (2003) investigated this agricultural wastes were extensively used for the removal of heavy metals due to their abundance in nature. Besides that, it has been used for adsorbing metal ions due to the characteristic functional groups (Tarley et al., 2004).
Agricultural waste materials being economic and eco-friendly due to their unique chemical composition, availability in abundance, renewable, low in cost and more efficient are seem to be viable option for heavy metal remediation. These promising agricultural waste materials are used in the removal of metal ions either in their natural form or after some physical or chemical modification (Sud et al., 2008). But, many studies have shown that the adsorption capacity of these adsorbents may be increased by their treatment with chemical reagents (Tarley et al., 2004). In general, raw lignocellulosic adsorbents were modified by various methods to increase their sorption capacities because metal ion binding by lignocellulosic adsorbents is believed to take place through chemical functional groups such as carboxyl, amino, or phenolics. More recently, great effort has been contributed to develop new adsorbents and improve existing adsorbents. Many investigators have studied the feasibility of using low-cost agro-based waste materials (Demirbas, 2008).
2.2.2.2 Industrial By-Products
Many industrial wastes are high in carbon content and offer significant potential for conversion into carbonaceous chars which may then be further activated to yield porous adsorbents. Like agricultural waste, industrial by-products such as fly ash, used tyres, waste iron, metallic iron, hydrous titanium oxide, and blast furnace slag are inexpensive and abundantly available (Kurniawan et al., 2006). These materials can be chemically modified to enhance their removal performance. However, unlike those from agricultural waste, adsorbents from this source can be obtained from industrial processing only. In South Africa, several such wastes currently pose a variety of disposal problems due to bulk volume, auto reactivity or physical nature like oily wastes and scrap tyres. Thus, the controlled pyrolysis of these wastes combined with the reuse of porous products contributes to a minimisation of handling difficulties (Pollard et al., 1992). Some of these industrial by-products combine good adsorption capacities and buffering effect, which assure almost complete removal of heavy metal ions without preliminary correction of the initial pH being necessary.
Fly ash, an industrial solid waste of thermal power plants is one of the cheapest adsorbents having excellent removal capabilities for different wastes. South Africa produces approximately 28 million tons of coal fly ash per annum (Reynolds et al., 2002). Only 5% of the fly ash is used as a construction material while the rest is stored in ash damps, which in turn have to be rehabilitated increasing the cost of ash handling (Woolard et al., 2000). Sen and De (1987) carried out a study on the adsorption of mercury using coal fly ash and it was reported that the maximum adsorption capacity of 2.82 mg Hg2+/g took place at a pH range of 3.5 – 4.5 and that adsorption followed the Freundlich model. In another work, a comparative adsorption study was carried out by Jain et al. (2001) using carbon slurry waste obtained from a fertilizer plant and blast furnace sludge, dust, and slag from steel plant wastes as adsorbents for the removal of dyes. It was found that carbonaceous adsorbent prepared from the fertilizer plant waste exhibited a good potential for the removal of dyes as compared to the other three adsorbents prepared.
2.3 Mechanism of Adsorption
Sud et al. (2008) reported that the removal of metal ions from aqueous streams using agricultural materials is based upon metal adsorption. The process of adsorption involves a solid phase (sorbent) and a liquid phase (solvent) containing a dissolved species to be sorbed. Due to high affinity of the sorbent for the metal ion species, the latter is attracted and bound by rather complex process affected by several mechanism involving chemisorptions, complexation, adsorption on surface and pores, ion exchange, micro precipitation, heavy metal hydroxide condensation onto the biosurface, and surface adsorption, chelation, adsorption by physical forces, entrapment in inter and intrafibrillar capillaries and spaces of the structural polysaccharides network as a result of the concentration gradient and diffusion through cell wall and membrane (Sud et al., 2008).
In order to understand how metals bind to the biomass, it is essential to identify the functional groups responsible for metal binding. Most of the functional groups involved in the binding process are found in cell walls. Plant cell walls are generally considered as structures built by cellulose molecules, organized in microfibrils and surrounded by hemicellulosic materials (xylans, mannans, glucomannans, galactans, arabogalactans), lignin and pectin along with small amounts of protein (Dewayanto, 2010).
2.3.1 Various Adsorbents Used for Adsorption of Phenol and Its Derivatives
In recent years literature surveys show that a large number of alternative adsorbents have been studied to replace activated carbon. The review presents the summary of the removal of phenol and its derivatives by using following adsorbents by investigators in research works. Also the comparison of adsorption capacities for various phenolic compounds on adsorbents was shown in the Figure 2.3.

Fig. 2.3: Comparison of adsorption capacities for phenolic compounds on various adsorbents
(Source: Literature Survey Compiled by Bazrafshan et al., 2016)
Zarei et al., (2013), studied the efficiency of Moringa peregrina tree shell ash for the removal of phenol from aqueous solutions; the examination was carried out in a batch system. According to the results of this study, it was found that the Moringa peregrina tree shell ash is not only a low-cost adsorbent but also has a high performance in the removal of phenol from aqueous solutions (Zarei et al., 2013). In another research, the adsorption potential of pistachio-nut shell ash in a batch system was studied by Bazrafshan et al. (2012b) for the removal of phenol from aqueous solutions. The possibility of using rice husk and rice husk ash for removal of phenol from aqueous solution was investigated by Mahvi et al. (2004). Activated carbon prepared from rubber seed coat (RSCC), an agricultural waste by-product has been used for the adsorption of phenol from aqueous solution by Rengaraj et al. (2002b). Rao and Viraraghavan, (2002), have investigated the use of nonviable pretreated cells of Aspergillus niger to remove phenol from an aqueous solution. Five types of non-viable pretreated A. niger biomass powders were used as a biosorbent to remove phenol present in an aqueous solution at a concentration of 1000 g l-1. Sulfuric acid pretreated A. niger biomass was found effective in the removal of phenol present in an aqueous solution at a concentration of 1000 g l-1 (Rao ; Viraraghavan, 2002). Findings of Tor et al. (2006) on the application of neutralized red mud for removal of phenol from aqueous solution showed that the neutralized red mud was an effective adsorbent for the removal of phenol from aqueous solutions. Higher phenol removal by neutralized red mud was possible provided that the initial phenol concentration was low in the solution (Tor et al., 2006). The potential of tendu (Diospyros melanoxylon) leaf refuse from local industry, which itself is a solid waste disposal menace and its chemically carbonized product to adsorb phenol was investigated by Nagda et al. (2007). Activated carbon derived from avocado kernels (AAC) was evaluated for its ability to remove phenol by Rodrigues et al. (2011).
Adsorption of phenol on natural clay for phenol removal from aqueous solutions have investigated by Djebbar et al. (2012). The phenol removal potential of clay, a low cost and abundantly available material has been investigated by Nayak and Singh (2007). Activated carbon derived from rattan sawdust (ACR) was evaluated by Hamid and Rahman (2008) for its ability to remove phenol from an aqueous solution in a batch process. Abdelwahab and Amin (2013) have analyzed the removal of phenol from aqueous solution by adsorption onto Luffa cylindrical fibers (LC). Adsorption study for phenol removal from aqueous solution on activated palm seed coat carbon (PSCC) was carried out by Rengaraj et al. (2002a). A comparative study with a commercial activated carbon showed that PSCC is two times more effective than commercial activated carbon (CAC) (Rengaraj et al., 2002a). The vegetable sponge of cylindrical loofa, a natural product which rows in the north of Algeria, was used by Cherifi et al. (2009). Abdelkreem (2013) explored the possibility of using olive mill waste to remove phenol from aqueous effluents. The experimental studies on removal of phenol from waste water in a fluidized bed column using coconut shell activated carbon as an adsorbent have been reported by Kulkarni et al. (2013). Arris et al. (2012) showed that cereal by-product, an abundant natural material, can be used effectively and efficiently for the removal of phenol from wastewater. Rushdi et al. (2011) showed that Jordanian zeolite tuff can be used as a low cost adsorbent for the removal of phenol from water.
Another investigation of the use of three carbonaceous materials, activated carbon (AC), bagasse ash (BA) and wood charcoal (WC), as adsorbents was studied by Mukherjee et al. (2007). Srivastava et al. (2006) research deals with the adsorption of phenol on carbon rich bagasse fly ash (BFA) and activated carbon-commercial grade (ACC) and laboratory grade (ACL). The study showed that the bagasse fly ash (BFA) is an effective adsorbent for the removal of phenol from aqueous solution (Srivastava et al., 2006). Karatay and Donmez (2014) have carried out the research on an economical phenol bio-removal method using Aspergillus versicolor an agricultural wastes as a carbon source. Viraraghavan and Alfaro (1998) examined the effectiveness of less expensive adsorbents such as peat, fly ash and bentonite in removing phenol from wastewater by adsorption. Batch adsorption research by Kilic et al. (2011) for the removal of phenol from aqueous solution have been carried out by using activated carbon obtained from tobacco residue by chemical activation using K2CO3 and KOH as activation agents. A natural bentonite modified with a cationic surfactant, cetyl trimethyl ammonium bromide (CTAB), was used as an adsorbent for removal of phenol from aqueous solutions by Senturk et al. (2009). Application of a chemically modified green macro alga as a biosorbent for phenol removal has carried out by Aravindhan et al. (2009). The potential of bentonite for phenol adsorption from aqueous solutions was investigated by Banat et al. (2000). The removal of phenol (Ph) and 2-chlorophenol (2-CPh) from aqueous solution by native and heat inactivated fungus Funalia trogii pellets investigated by Bayramoglu et al. (2009). Batch adsorption experiments were conducted by Bahdod et al. (2009) to investigate the removal of phenol from wastewater by addition of three apatites {porous hydroxyapatite (PHAp), crystalline hydroxyl- (HAp) and fluoroapatite (FAp)}. The adsorption of phenol from aqueous solutions was investigated using a carbonized beet pulp in the inert nitrogen atmosphere by Dursun et al. (2005). Results in comparative studies on adsorptive removal of phenol by three agro-based carbons, which have investigated by Srihari and Das (2008) showed that the black gram husk (BGH) is an effective adsorbent for the removal of phenol from aqueous solution when compared with green gram husk (GGH) and rice husk (RH).
Activated carbons prepared from tamarind nutshell, an agricultural waste by-product, have been examined by Goud et al. (2005). Another Experiment have been conducted by Kermani et al. (2006) to examine the adsorption of phenol from aqueous solutions by rice husk ash and granular activated carbon (GAC). Phenol removal from aqueous system by jute stick has studied by Mustafa et al. (2008). In Siboni et al. (2013) research activated red mud containing iron and calcium as major components was applied to treat synthetic wastewater in a batch reactor. In another research, the adsorption of phenol from wastewater was investigated using sawdust as adsorbent by Dakhil (2013). Moyo et al. (2012) investigated the possibility of Saccharomyces cerevisiae as an alternative adsorbent for phenol removal from aqueous solution. Adsorption of phenol from aqueous solution was investigated using sodium zeolite as an adsorbent by Saravanakumar and kumar (2013). The application of Colocasia esculenta as an alternative adsorbent for the removal of phenol from aqueous solution was investigated by Obi and Woke (2014). The potential of employing wheat husk for phenol adsorption from aqueous solution was studied by Jagwani and Joshi (2014). The potential of activated carbon prepared from Typha orientalis Presl to remove phenol from aqueous solutions was studied by Feng et al. (2015). Coconut shell has been converted to activated carbon through chemical activation with KOH by Hu and Srinivasan (1999). The properties of the carbon produced were dependent on the impregnation ratio and the activation temperature. The removal capabilities were found to be 206, 267 and 257 mg/g for phenol, 4-chlorophenol and 4- nitrophenol respectively. It was found that the adsorption increased with increase in agitation time and initial concentration while acidic pH was favourable for the adsorption of TCP. The maximum adsorption capacity was 716.10 mg/g.
Coconut husk was used to remove 2,4,6-trichlorophenol under optimized conditions by Hameed et al (2008). The effect of activation temperature, activation time and KOH to char impregnation ratio were studied. The adsorption capacity was found to be 191.73 mg/g. Namasivayam and Kavitha (2006) utilized coir pith carbon has an adsorbent for understanding the mechanism of the phenol removal. It was found that the adsorption capacities of 48.31, 19.12 and 3.66 mg/g were obtained for phenol, 2,4-dichlorophenol and p-chlorophenol. The FTIR studies shows that the participation of the specific functional groups in adsorption interaction, while SEM studies visualized the formation of the adsorbed white layer on the phenol surface. The applicability of shells, seed coat, stone and kernels of various agricultural products as adsorbents for the removal of toxic pollutants from water has been investigated. The feasibility of activated carbon from almond shell, hazelnut shell, walnut shell and apricot stone for the removal of phenol has been investigated by Aygun et al. (2003), and found that the adsorption capacity of 70.4, 100, 145 and 126 mg/g was obtained for almond shell, walnut shell, hazel nut shell and apricot stone respectively. It was found that the impregnating agents and activating agents had influence on phenol removal. The effectiveness of the almond shell carbon for the treatment of pentachlorophenol from water was performed by Santos et al. (2008), and a saturation adsorption capacity of 9.6 mg/g was obtained under continuous flow experiments. The nature of sorption on almond shells carbon was understood by focusing on the structural and chemical characterization of the carbons.
2.3.2 Properties of Agricultural Adsorbent
Agricultural materials particularly those containing lignin and cellulose as the main constituents shows potential adsorption capacity for metals and organic compounds. Other components are hemicellulose, extractives, lipids, proteins, simple sugars, starches, water, hydrocarbons, ash and many more compounds that contain a variety of functional groups present in the binding process. (Dewayanto, 2010).
The functional groups present in biomass molecules are acetamido groups, carbonyl, phenolic, structural polysaccharides, amido, amino, sulphydryl carboxyl groups alcohols and esters. These groups have the affinity for metal complexation. The presence of various functional groups and their complexation with heavy metals during adsorption process has been reported by different research workers using spectroscopic techniques that facilitate metal complexation which helps for the sequestering of heavy metals (Sud et al., 2008).
Agricultural waste usually has high moisture content that required removal through physical treatments which include natural drying under the direct sunlight, room drying, and oven drying at certain temperature. Dried materials are normally ground to obtain the specific granular size and can directly be applied as an adsorbent or transformed into carbonaceous adsorbent by pyrolysis (Dewayanto, 2010).
Chemical treatment of agricultural wastes can extract soluble organic compounds and enhance chelating efficiency using different kinds of modifying agents such as base solutions (sodium hydroxide, calcium hydroxide, sodium carbonate), mineral and organic acid solutions (hydrochloric acid, nitric acid, sulfuric acid, tartaric acid, citric acid), organic compounds (ethylenediamine, formaldehyde, methanol), oxidizing agent (hydrogen peroxide), and dye (Reactive Orange 13). Chemically modified adsorbents can provide better performance for removing soluble organic compounds, eliminating coloration of the aqueous solutions and increasing efficiency of metal adsorption (Dewayanto, 2010).
2.4 Chlorophenols
2.4.1 Sources and Usage of Chlorophenols
Most of the commercially important chlorophenols are obtained by direct chlorination of phenol using chlorine gas or for the higher chlorinated phenols, the chlorination of lower chlorinated phenols at high temperatures (WHO, 1989). In the technical product, there are impurities of other chlorophenol isomers or chlorophenols with more or less chlorine. The heavy chlorophenols are mainly contaminated by polychlorophenoxyphenols, chlorodibenzoparadioxins and chlorodibenzofurans. Emissions are mainly due to the manufacture, storage, transportation and application of chlorophenols. Because the higher chlorinated phenols are produced at higher temperature, the contamination of the higher chlorinated phenols is greater than that of the lower chlorinated phenols (WHO, 1989). However, due to their broad-spectrum antimicrobial properties, chlorophenols have been used as preservative agents for wood, paints, vegetable fibres and leather and as disinfectants. In addition, they are used as herbicides, fungicides and insecticides and as intermediates in the production of pharmaceuticals and dyes, (WHO, 1989).
2.4.2 Effects of Chlorophenols
Chlorophenols can be absorbed through the lungs, the gastro-intestinal tract and the skin, 80% of it can be excreted via the kidneys without undergoing any transformation. The toxicity of chlorophenols depends upon the degree of chlorination, the position of the chlorine atoms and the purity of the sample. Chlorophenols have an irritating effect on the eyes and on the respiratory tract. Toxic doses of chlorophenols cause convulsions, shortness of breath, coma and finally death. After repeated administration, toxic doses may result in damage to the inner organs (primarily liver) and the bone marrow. Pentachlorophenol has a toxic effect on embryos in animal experiments (lethal at higher concentrations). Technical PCP may possibly be carcinogenic not least due to contamination. Mutagenic potential cannot be excluded, (WHO, 1989).
In the aquatic environment, chlorophenols may be dissolved in free or complexed form or adsorbed on suspended matter. Removal is mainly by way of biodegradation which is rapid when adapted microorganisms are already present. However, PCP is biodegraded much more difficultly than other chlorophenols. Chlorophenols are also removed from water by photodecomposition and volatilisation. Finally, adsorption of chlorophenols on suspended matter plays a role in the amount of chlorophenols in water: light chlorophenols are hardly fixed whereas PCP is fixed very strongly, (WHO, 1989).

2.5 Almond Nut Shells
The tropical Almond Terminalia catappa (Indian almond) belongs to the family Combrataceae is a fruit of a large spreading tree distributed throughout the tropics and coastal environment (Species profiles, 2006). The fruit is a sessile, laterally compressed, ovoid to ovate and smooth skinned drupe. The oil containing seeds are encased in a tough fibrous husk with a fleshy pericarp. This corky fibrous endocarp (nut) of the fruit and shells are waste materials and can be collected on community basis for reuse. Almond nut shells are abundant, inexpensive and readily available lignocellulosic material.

Plate 1: Almond (Terminalia catappa) nut shells
2.6 Instrumentation of FT-IR Spectroscopy
Fourier Transform infrared spectroscopy (FTIR) originates from Fourier transform (a mathematical process) which is required to convert the raw data into the actual spectrum. It is a technique which is used to obtain an infrared spectrum of absorption or emission of a solid, liquid or gas. It takes advantage of asymmetric molecular stretching, vibration and rotation of chemical bonds as they are exposed to designated wavelengths of light, and transform the signal from the time domain to its representation in the frequency domain (Wikipedia.org).

Fig. 2.4: Fourier Transform Spectrometer
(Image Source: Wikipedia Commons)
The normal instrumental process is as follows:
1. The Source: Infrared energy is emitted from a glowing black-body source. This beam passes through an aperture which controls the amount of energy presented to the sample (and, ultimately, to the detector).
2. The Interferometer: The beam enters the interferometer where the “spectral encoding” takes place. The resulting interferogram signal then exits the interferometer.
3. The Sample: The beam enters the sample compartment where it is transmitted through or reflected off of the surface of the sample, depending on the type of analysis being accomplished. This is where specific frequencies of energy, which are uniquely characteristic of the sample, are absorbed.
4. The Detector: The beam finally passes to the detector for final measurement. The detectors used are specially designed to measure the special interferogram signal.
5. The Computer: The measured signal is digitized and sent to the computer where the Fourier transformation takes place. The final infrared spectrum is then presented to the user for interpretation and any further manipulation.
Because there needs to be a relative scale for the absorption intensity, a background spectrum must also be measured. This is normally a measurement with no sample in the beam. This can be compared to the measurement with the sample in the beam to determine the “percent transmittance.”
This technique results in a spectrum which has all of the instrumental characteristics removed. Thus, all spectral features which are present are strictly due to the sample. A single background measurement can be used for many sample measurements because this spectrum is characteristic of the instrument itself.
The light passes through a beamsplitter, which sends the light in two directions at right angles. One beam goes to a stationary mirror then back to the beamsplitter. The other goes to a moving mirror. The motion of the mirror makes the total path length variable versus that taken by the stationary-mirror beam. When the two meet up again at the beamsplitter, they recombine, but the difference in path lengths creates constructive and destructive interference ‘an interferogram’.
The recombined beam passes through the sample. The sample absorbs all the different wavelengths characteristic of its spectrum, and this subtracts specific wavelengths from the interferogram. The detector now reports variation in energy versus time for all wavelengths simultaneously. A laser beam is superimposed to provide a reference for the instrument operation (Wikipedia.org).
A mathematical function called a Fourier transform allows to convert an intensity-vs.-time spectrum into an intensity-vs.-frequency spectrum.
The Fourier transform:

A(r) and X(k) are the frequency domain and time domain points, respectively, for a spectrum of N points.
2.7 Instrumentation of UV-Visible Spectroscopy
Ultraviolet–visible spectroscopy or ultraviolet-visible spectrophotometry (UV-Vis or UV/Vis) refers to absorption spectroscopy or reflectance spectroscopy in the ultraviolet-visible spectral region. This means it uses light in the visible and adjacent (near-UV and near-infrared (NIR)) ranges. UV/Vis spectroscopy is routinely used in analytical chemistry for the quantitative determination of different analytes, such as transition metal ions, highly conjugated organic compounds, and biological macromolecules. Spectroscopic analysis is commonly carried out in solutions but solids and gases may also be studied.
The functioning of this instrument is relatively straightforward. A beam of light from a visible and/or UV light source (coloured red) is separated into its component wavelengths by a prism or diffraction grating. Each monochromatic (single wavelength) beam in turn is split into two equal intensity beams by a half-mirrored device. One beam, the sample beam (coloured magenta), passes through a small transparent container (cuvette) containing a solution of the compound being studied in a transparent solvent. The other beam, the reference (coloured blue), passes through an identical cuvette containing only the solvent (Michigan State Univ.edu).

Fig. 2.5: Schematic for a UV-Vis spectrophotometer
(Image Source: Wikipedia Commons)
The intensities of these light beams are then measured by electronic detectors and compared. The intensity of the reference beam, which should have suffered little or no light absorption, is defined as I0. The intensity of the sample beam is defined as I. Over a short period of time, the spectrometer automatically scans all the component wavelengths in the manner described. The ultraviolet (UV) region scanned is normally from 200 to 400 nm, and the visible portion is from 400 to 800 nm (Michigan State Univ.edu). If the sample compound does not absorb light of a given wavelength, I = I0. However, if the sample compound absorbs light then I is less than I0, and this difference may be plotted on a graph versus wavelength, as shown on the right. Absorption may be presented as transmittance (T = I/I0) or absorbance (A = log I0/I). If no absorption has occurred, T = 1.0 and A = 0. Most spectrometers display absorbance on the vertical axis, and the commonly observed range is from 0 (100 % transmittance) to 2 (1 % transmittance). The wavelength of maximum absorbance is a characteristic value, designated as ?max . Different compounds may have very different absorption maxima and absorbances. Intensely absorbing compounds must be examined in dilute solution, so that significant light energy is received by the detector, and this requires the use of completely transparent (non-absorbing) solvents. The most commonly used solvents are water, ethanol, hexane and cyclohexane. Solvents having double or triple bonds, or heavy atoms (e.g. S, Br ; I) are generally avoided. Because the absorbance of a sample will be proportional to its molar concentration in the sample cuvette, a corrected absorption value known as the molar absorptivity is used when comparing the spectra of different compounds. This is defined as:
Molar Absorptivity, ? = A/cl
(where, A = absorbance, c = sample concentration in moles/liter ; l = length of light path through the cuvette in cm).