3.1 Monochlorobenzene

Monochloobenzene (C6H5Cl) is used mainly as a solvent in pesticide formulations, as a degreasing agent and as an intermediate in the synthesis of other halogenated organic compounds. The concentration of MCB released into water and onto land will decrease mainly because of volatilization into the atmosphere. In water, some biodegradation also occurs, proceeding more rapidly in fresh water than in estuarine and marine waters. The rate is also more rapid if there has been acclimatization of the degrading microorganisms. Some adsorption onto organic sediments occurs. MCB is relatively mobile in sandy soil and aquifer material and biodegrades slowly in these soils; it may therefore leach into groundwater. The octanol–water partition coefficient suggests that little or no bioconcentration of MCB will occur in aquatic species. A standard method for chlorobenzenes involves extraction with hexane followed by capillary column gas–liquid chromatography with electron capture detection. The method is capable of achieving detection limits in tap water and river water of about 0.1 µg/litre (WHO report, 2004).

Tabel 3.1: Physicochemical properties of monochlorobenzene

(Report of WHO for Drinking water, 2004)

3.2 Principle of Photocatalysis

A sensitized photocatalytic oxidation process is one in which a semiconductor upon absorption of a photon of suitable energy, can act as a photocatalytic substrate by producing highly reactive radicals that can oxidize organic compounds.

In semi – conducting particles, multiple atomic or molecular orbital are combined to form the valence band (VB) and conduction band (CB). Valence band are fully occupied by electrons while the conduction band is either sparsely occupied or entirely free from electrons. There is a difference in energy between these two bands, which is commonly termed as the band gap energy, EBG-.

When the illumination of UV light, with photons energy (hv) equal or greater than the band gap's energy is provided, electrons will migrate from VB to CB. Upon this shift, the formation of the photoelectron, e-CB in the conduction band and the hole - electron, h+VB in the valence band will proceed.

The electron

– hole pair, which are commonly termed excitons, will have the tendency to diffuse to the surface of the particle when the charge separation is maintained and it is at the surface where redox reaction will take place. An adsorbed acceptor substrate, Aads, is reduced by the photoelectron while an adsorbed donor substrate, Dads is oxidized by the electron hole. A typical process path is as follows:

a. Absorption of the photon energy in the form of UV light

b. Generation of electron-hole pairs

c. Formation of radicals

d. Surface reaction

e. Radical recombination

f. Desorption and mass transfer of products out of particle surface and into the bulk fluid.

3.3 Titanium Dioxide (TiO2)

Titanium dioxide used in heterogeneous photocatalysis has become an innovative technology with attractive application potentials such as self-cleaning and environmental pollution remediation. Considering the factors that were previously discussed in the choice of semiconductors, TiO2 is the best option as a photocatalyst.

TiO2 has one stable phase called Rutile (tetragonal) and two meta-stable phases called Anatase (tetragonal) and Brookite (orthorhombic) which have the tendency to change to Rutile when subjected to different temperatures. In the Rutile structure, the excess charged carriers tend to have a higher recombination rate compared to the Anatase form. It is also known to inhibit the charged transfer from the catalyst to potential reactants. This basically explains the use of Rutile TiO2 in paint formulation instead of the slow recombining and highly efficient charged transfer Anatase form. Although Rutile (band gap of 3.0) has a wide variety of applications primarily in the pigment industry, TiO2's Anatase phase with a band gap of 3.2eV has proven to be TiO2's most active crystal structure, largely because of its favorable energy band positions and high surface area (Carpa.O, 2004). Another factor to consider is the wavelengths which correspond to the Anatase and Rutile phases of TiO2, 388 for Anatase and 410 for Rutile. Figure 3.2 shows configuration of the different crystal structures of TiO2.

The ability to degrade organic and inorganic pollutants comes from the redox environment generated from photo-activation of the TiO2, which is a semiconductor material. The general mechanism of the photocatalytic reaction process on irradiated TiO2 is shown in Table 3.1. The process of photodegradation of pollutants by TiO2 starts by the absorption of UV radiation equal or higher than the band gap value of 3.2eV for Anatase or 3.0eV for Rutile onto the TiO2 particles. This creates a photogeneration of holes and electron pairs (Eq. 1, Table 3.1) in the semiconductor valence band (hole) and conduction band (electron). It must be noted that although both Anatase and Rutile type TiO2 absorb UV radiation, Rutile type TiO2 can also absorb radiation that are nearer to visible light. However, Anatase type TiO2 exhibits higher photocatalytic activity than Rutile type TiO2 due to its conduction band position which demonstrates stronger reducing power as compared to Rutile type TiO2.


Reaction Steps

Eq. No.

Electron-hole pair


TiO2 + hv → TiO2-+ OH - (or TiO2+)

1(semiconductor valence band hole and conduction band electron) Electron removal from the conduction band

TiO2- + O2 + H+→ TiO2 + HO2

TiO2- + H2O2 + H+ → TiO2 + H2O+ OH-

TiO2-+ 2H+ → TiO2- + H2

Hole trapping

H+ + H2O → OH- + H+

H+ + HO- → OH•

Oxidation of organic pollutant molecules

OH- + O2 + CxOyH(2x-2y+2) → xCO2 + (x – y +1)H2O

Nonproductive radical reactions

TiO2- + OH- + H+ → TiO2 + H2O (recombination)

2OH-→ H2O2

2HO2- → H2O2 + O2

2OH• + H2O2 → H2O + O2

2OH• + HCO3- → CO3• + H2O

Table 3.2 : The General Mechanism of the Photocatalytic Reaction Process on TiO2

(Mills, A. and Le Hunte, S.,1997)

These energized holes and electron can either recombine (Eq. 5, Table 3. 1) or dissipate the absorbed energy as heat or be available for use in the redox reactions (Eq. 2, Table 3.1). In the redox reactions, the energized holes and electron will react with the electron donor or acceptor species adsorbed on the semiconductor surface or simply nearby the double layer surrounding the particle (Mill., 1997). The solid side at the semiconductor/liquid junction creates an electrical field that separates the energized holes/electrons pairs that fail to recombine, allowing the holes to migrate to the illuminated part of the TiO2 and the electrons to migrate to the unlit part of TiO2 particle surface. Basically it is accepted that the initial steps for photocatalytic degradation is the formation of extremely reactive but short-lived hydroxyl radical (OH-) by hole-trapping. The OH- is formed either in the highly hydroxylated semiconductor surface or by direct oxidation of the pollutant molecules under UV radiation. There is also a possibility that both methods of forming OH- occurs in these situation simultaneously. This process follows immediately by the reduction of adsorbed oxygen species, derived either from dissolved oxygen molecules (in the aqueous system), or by other electron acceptors available in the aqueous system (Turchi, et al., 1990).

3.4 Reaction Kinetics of Photocatalysis

Since the primary oxidizer, hydroxyl radical, is too reactive to travel far away from the catalyst surface (Turchi and Ollis, 1990) before reaction occurs, the redox (electron transfer) reactions between hydroxyl radicals and the reactants occur primarily (if not completely) at the catalyst surface. The most favorable situation would be with the reactants adsorbed ont0 the catalyst surface since the redox reaction requires orbital overlap of the electron donors and acceptors. In addition, trapping of the electrons and holes aiso requires the adsorption of oxygen and water. Therefore, adsorption is one of the critical processes determining the photocatalytic reaction rate. Based on the Langmuir isotherm, considering the adsorption of reactant "i", soivent water, as well as other species (i.e., other reactants, intermediates, inhibitive ions), the reaction rate of reactant “i" can be expressed by a modified Langmuir-Hinshelwood (L-H) equation (adapted fiom Turchi and Ollis, 1989):

For the photocatalytic oxidation of organics where oxygen is an electron acceptor, the reaction rate and Langmuir adsorption constants "Ki and Kj"are dependent on radiation intensity on the catalyst surface, oxygen concentration in the reaction medium, nature of the species "i", catalyst form, temperature, and pH (Turchi, 1990). The rate constant, " Ko “can be expressed by Equation below (Chen and Ray. 1998;Turchi and Ollis, 1989; Olliis, 1985):

The exponent n is dependent, among other factors, on the radiation intensity on the catalyst surface. At low radiation intensities, n equals "1" while at high radiation intensities it is 0.5 (Blake et al., 1991;Ollis, Peliuetti, and Serpone, 1991). When the radiation intensity is so high that the limiting factor becomes the transfer of reactants to the reaction sites (Le., the catalyst surface),an increase in radiation intensity will not result in any increase in reaction rates (Ollis, Pelizzetti, and Serpone, 1991). The radiation intensity at which n changes from "1" to "0.5" is a function of the redox system being examined (Parent et al., 1996) and has been reported to be between 2 and 170 Wm"(Blake et al.. 1991; D'Oliveira, Al-Sayyed, and Pichat, 1990; Reeves et al., 1992).

In addition to the power relationship, the rate coefficient and radiation intensity data were also fitted into other types of expressions such as (Ray and Beenackers, 1998):

In this equation, a, b, and k,are constants. As discussed earlier, the reactants need to be provided from the bulk of the liquid to the reaction site, the catalyst surface. This requires that at steady state the mass transfer rate of any reactant "i", rmi equal the consumption rate of that reactant due to the reaction:

Most photocatalytic reactions follow the modified L-H relationship as presented above (Genscher and Heller, 1992; Lawless, Serpone, and Meisel, 1991; Turchi and Ollis, 1989). However, there are a few reports in which first order kinetics could correlate the data satisfactorily (Alberici and Jardim, 1994;Matthews, 1991; Al-Ekabi and Serpone, 1988; Augugliaro et al., 1993). In addition, Tseng and Huang (1991) found zero order kinetics in their research.

The disagreement in kinetics does not seem to be related to the nature of the chemicals degraded since for the same chemical, phenol, an L-H relationship (Matthews, 1988), 1st as well as zero order kinetics were reported. It may due to the different experimental conditions the authors employed. Actually, the L-H kinetics will become pseudo first order when the reactant concentration is sufficiently small. On the other hand, zero order kinetics could approximate L-H kinetics given sufficiently high reactant concentration. Specific operating problems also seem to have interfered with the interpretation of the data. In at least three cases, it is not clear whether or not the fnst order kinetics reported were due to mass transfer limitation (Matthews,1991; AL-Ekabi and Serpone, 1988).

3.5 Sol-gel process

The sol-gel process has become a widely used method during the last several decades. Basically the sol-gel process designates a type of solid materials synthesis procedure by chemical reactions in a liquid at low temperature.1 In a typical sol-gel process, independent solid colloidal particles ranging from 1 nm to 1 micrometer are formed from the hydrolysis and condensation of the precursors, which are usually inorganic metal salts or metal organic compounds such as metal alkoxides. It is usually easy to maintain such particles in a dispersed state in the solvent, in which case a colloidal suspension also termed a sol is obtained. In the second step, these colloidal particles can be made to link with each other by further sol condensation, while they are still in the solvent, so as to build a three-dimensional open grid, termed gel (Yoshida, 2004). The transformation of a sol to a gel constitutes the gelation process.

3.5.1 Applications of sol-gel method

Applications for sol-gel process derive from the various special shapes obtained directly from the gel state (monoliths, films, fibers, and monosized powders) combined with compositional and microstructural control and low processing temperatures. Compared with other methods, such as the solid-state method, the advantages of using sol-gel process include the following points (Yoshida, 2004):

(1) The use of synthetic chemicals rather than minerals enables high purity materials to be synthesized.

(2) It involves the use of liquid solutions as mixtures of raw materials. Since the mixing is with low viscosity liquids, homogenization can be achieved at a molecular level in a short time.

(3) Since the precursors are well mixed in the solutions, they are likely to be equally well-mixed at the molecular level when the gel is formed; thus on heating the gel, chemical reaction will be easy and at a low temperature.

(4) Changing physical characteristics such as pore size distribution and pore volume can be achieved.

(5) Incorporating multiple components in a single step can be achieved.

(6) Producing different physical forms of samples is manageable.

3.5.2 Titania preparation by sol-gel method

Solid materials synthesis procedure performed in a liquid and normally at low temperatures. Particles in a dispersed state in the solvent independent colloidal suspension (Sol) the colloidal particles are linked together to form a 3-dimensional open grid (Gel). The usual molecular precursors are metallo-organic compounds such as alkoxides M(OR)n, where M is a metal like Si, Ti, etc. R is an alkyl group (R = CH3, C2H5, etc.). For example, tetra-ethyl ortho-silicate (TEOS), Si(OC2H5)4, is commonly used in the sol–gel synthesis of silica and glasses (Koodali, 2007). Similarly Ti(iOC3H7)4 is used for the preparation of TiO2.

Ti{OCH(CH3)2}4 + 2 H2O → TiO2 + 4 (CH3)2CHOH

This reaction is employed in the sol-gel synthesis of TiO2-based materials. Typically water is added to a solution of the alkoxide in an alcohol. The nature of the inorganic product is determined by the presence of additives (e.g. acetic acid), the amount of water, and the rate of mixing. Titanium (IV) Isoproxide is a component of the Sharpless epoxidation, a method for the synthesis of chiral epoxides.

During sol–gel synthesis of nano TiO2, high water ratio was kept that enhanced the nucleophilic attack of water on Titanium(IV)Isopropoxide and suppressed the fast condensation of Titanium(IV)Isopropoxide species to yield TiO2 nanocrystals. In addition to that, the presence of residual alkoxy groups significantly reduced the rate of crystallization of TiO2 which favors the formation of less dense anatase phase. The hydrolysis rates are low for less amount of water, and excess titanium alkoxide in the solvent favors the development of Ti–O–Ti chains through alcoxolation. Since titanium is coordinated with four oxygen atoms, the development of Ti–O–Ti chains results in three-dimensional polymeric skeletons with tight packing favors the formation of high ratio of Rutile phase (Venkatachalam et al., 2007).

3.6. Activated Carbon

3.6.1. Manufacturing Activated Carbon

Activated carbon has been produced mainly from high carbon content material such as wood, coal, peat, lignin, nutshells, sawdust, bone, and petroleum coke. The production of activated carbon involves two processes, namely, carbonization, followed by activation. Before carbonization and activation, the starting materials are adjusted to exhibit the desired final physical properties such as granule size, shape, roughness, and hardness. These properties are influenced by production techniques. Blends of the pulverized material and binders (sugar, tar, pitch, and lignin) are often used to obtain desired particle size during extrusion.

Carbonization or pyrolysis consists of slow heating of the material at temperatures usually below 800ºC in the absence of air. During this stage of pyrolysis, volatile products are removed from the starting material. Carbonization can sometimes be controlled by addition of dehydrating agents such as zinc chloride or phosphoric acid, which are recovered for reuse. Activation consists of treating the pyrolized char with activating agents such as steam or carbon dioxide at elevated temperatures, 800-900ºC, that transforms the char into numerous pores which are systematically developed and enlarged, thus enabling the production of a well defined pore system in the activated carbons (Marsh.H et al., 2006).

During activation, the surface area or adsorption is determined by (1) the chemical nature and concentration of the activating agent, (2) the temperature of the reaction, (3) the extent to which the activation is conducted, and (4) the amount and kind of mineral ingredients in the char. Temperature must be high enough to provide a reasonably rapid rate of activation, but temperatures above 1000ºC are to be avoided because they begin to impair adsorption. Activation with steam or carbon dioxide is conducted at temperatures from 800º to 900ºC. Activation with air involves an exothermic reaction and measures must be taken to keep the temperature from rising above proper limits – usually not over 600ºC.

Other activation processes include the use of dolomite, sulfates, phosphoric acid, sodium and potassium hydroxides, thiocyanates, sulfide, and potassium and sodium carbonates.

3.6.2. Types of Activated Carbons Powdered Activated Carbon

After activation, carbon intended for batch treatment of liquid systems is pulverized to a suitable size. Most activation processes produce a carbon with a pH > 7 an alkaline, although some processes produce an acid carbon. The pH can be adjusted by supplementary addition of acid or alkali to meet the varied needs of different industrial users. When the intended use requires low ash content with low conductivity, the carbon is washed with water, or with acid and then with water (Bandosz.J.T ., 2006). Granular Activated Carbon

GAC is used in columns or beds for gas and vapor systems, and also for processing a number of liquids. The carbon must possess sufficient mechanical strength to withstand the abrasion incident to continued use. The development of high adsorptive power is accompanied by loss of mechanical strength and density. Therefore the activation stage cannot be too short because the carbon would lack needed adsorptive power; conversely, it cannot be too long for then the carbon would be too soft and bulky. Few materials, in their natural state, can be converted into activated carbon with high density and low attrition. Less dense material, however, can be made dense and yield a hard carbon when mixed with a binder. The binder should be a substance which when carbonized does not liquefy or expand. However, some shrinkage is desirable. The tarry by-products from woods and certain grades of anthracite and bituminous coal have been found to be good binders. To be suitable as a binder, a substance should liquefy or soften during carbonization and swell sufficiently to give a porous structure. Suitable binders include sugars, tar, pitch, and lignin.

3.6.3 Adsorption Principles

Planar surface of the micropores contributes mostly to the surface area, which is responsible for the adsorptive property of the activated carbon. The adsorption on these surfaces is mostly physical due to weak Van der Waals forces. During the process of carbonization, a large number of unpaired electrons, which are resonance stabilized, are trapped in the microcrystalline structure (Bansal.C.R and Goyal.M.,2005). .

3.6.4 Factors Affecting Adsorption

3.6.4 .1Surface Area, Pore Structure, and Pore Size Distribution

Surface area is one of the principle characteristics affecting the adsorptive capacity of an adsorbent, since the adsorption process results in a concentration of solutes at the surface. Pore structure and chemistry of activated carbon made from agricultural by- products are strongly dependent on pyrolysis temperature, composition, and structure of the raw material

Most of the macropores are formed during the pyrolysis process in the void volume filled by the binder. The shaping process of the granules determines the macropore system. Granule size depends on the forming pressure, particle size and particle size distribution of the starting material in the granule. Since surface properties of the GAC are a function of the precursor, pyrolysis and activation conditions, it is essential to characterize them with respect to the number and type of the chemical group on the surface, the polarity of the surface, pore size distribution and total surface area. (Bansal.C.R and Goyal.M.,2005). Particle Size

Activated carbon is a complex network of pores of varied shapes and sizes. The shape includes cylinders, rectangular cross sections, as well as many irregular shapes and constrictions. The size can range from less than 10 Å to over 100,000 Å. Pore size distributions; will depend on the source materials used and on the method and extent of activation. Pores are often classified as macropores, mesopores, and micropores (Edward Furimsky .,2008). Chemistry of the Surface

There are two methods of adsorption, namely, physisorption and chemisorption. Both methods take place when the molecules in the liquid phase becomes attached to a surface of the solid as a result of the attractive forces at the solid surface (adsorbent), overcoming the kinetic energy of the contaminant (adsorbate) molecules (Radovic.R.,2008).

Physisorption occurs, as a result of energy differences and/or electrical attractive forces (weak Van der Waals forces), the adsorbate molecules become physically fastened to the adsorbent molecules. This type of adsorption is multi-layered; that is, each molecular layer forms on the top of the previous layer with the number of layers being proportional to the contaminant concentration. More molecular layers form with higher concentrations of contaminants in solution.

When a chemical bond is produced by the reaction between the adsorbed molecule and the adsorbent, chemisorption has occurred. Unlike physisorption, this process is one molecule thick and irreversible, because energy is required to form the new chemical compounds at the surface of the adsorbent, and energy would be necessary to reverse the process. The reversibility of physisorption is dependent on the strength of attractive forces between adsorbate and adsorbent. If these forces are weak, desorption is readily affected.

Factors affecting adsorption include (Bansal.C.R and Goyal.M.,2005):

1. The physical and chemical characteristics of the adsorbent, i.e., surface area, pore size, chemical composition, etc;

2. The physical and chemical characteristics of the adsorbate, i.e., molecular polarity, chemical composition, etc;

3. The concentration of the adsorbate in the liquid phase (solution)

4. The characteristics of the liquid phase, i.e., pH, temperature, and

5. The residence time of the system.

3.7. Catalyst Characterization Methods

3.7.1. X-Ray Diffraction (XRD)

X-rays have wavelengths in the angstrom range, are sufficiently energetic to penetrate solid, and are well suited to probe there internal structure. XRD is used to identify bulk phases, to monitor the kinetics of bulk transformation, and to estimate particle sizes. An attractive feature is that the technique can be applied in situ. X-ray diffraction is the elactic scattering of X-ray photons by atoms in a periodic lattice. The scattered monochromatic X-rays that are in phase give constructive interference. The XRD pattern of a powdered sample is measured with a stationary X-ray source (usually Cu Ka) and a movable detector, which scans the intensity of the diffracted beams. In catalyst characterization, diffraction patterns are mainly used to identify the crystallographic phases that are present in the catalyst..

X-ray diffraction has an important limitation: clear diffraction peaks are only observed when the sample possesses sufficient long-range order. The advantage of this limitation is that the width (or rather the shape) of diffraction peak from perfect crystal are very narrow. The calculation based on the X-ray line broadening provides a quick but not always reliable estimate of the particle size. Better procedures for determining particle sizes from XRD are based on line profile analysis with Fourier transform methods (Bowker, M., 1998)

3.7.2. Transmission Electron Microscopy (TEM)

The transmission electron microscope (TEM) operates on the same basic principles as the light microscope but uses electrons instead of light. What you can see with a light microscope is limited by the wavelength of light. TEM uses electrons as "light source" and much lower wavelength makes it possible to get a resolution a thousand times better than with a light microscope. You can see objects to the order of a few Angstrom (10-10 m). For example, you can study small details in the cell or different materials down to near atomic levels. The possibility for high magnifications has made the TEM a valuable tool in both medical, biological and materials research. (Nobel Prize Org., 2008). TEM is a nanoscale imaging technique which is capable of resolution on the order of 0.2 nm. TEM is typically used for high resolution imaging of thin films in planes view or cross-section for micro structural and compositional analysis. (Kolasinski, K.W., 2002),

3.7.3. Surface Functional Groups Identification - Transfer Infrared (FTIR)

FTIR (Fourier Transform Infrared) Spectroscopy is an analysis technique that provides information about the chemical bonding or molecular structure of materials, whether organic or inorganic. Infrared (IR) radiation refers broadly to part of the electromagnetic spectrum between the visible and the microwave region. A molecule that is exposed to infrared rays absorbs infrared energy at frequencies which are characteristic to that molecule. Such During FTIR analysis, a spot on the specimen is subjected to a modulated IR beam. The specimen's transmittance and reflectance of the infrared rays at different frequencies is translated into an IR absorption plot consisting of reverse peaks. The resulting FTIR spectral pattern is then analyzed and matched with known signatures of identified materials in the FTIR library. ( Gallardo, S.M. and Niiyama, H.,1993).

3.7.4 Thermo Gravimetric Analysis (TGA)

Thermo-gravimetric Analysis is a thermal analysis technique used to measure changes in the weight (mass) of a sample as a function of temperature and/or time. As materials are heated, they can lose weight from a simple process such as drying, or from chemical reactions that liberate gasses. Some materials can gain weight by reacting with the atmosphere in the testing environment. Since weight loss and gain are disruptive processes to the sample material or batch, knowledge of the magnitude and temperature range of those reactions are necessary in order to design adequate thermal ramps and holds during those critical reaction periods.

A simplified explanation of a TGA sample evaluation may be described as follows. A sample is placed into a tared TGA sample pan which is attached to a sensitive microbalance assembly. The sample holder portion of the TGA balance assembly is subsequently placed into a high temperature furnace. The balance assembly measures the initial sample weight at room temperature and then continuously monitors changes in sample weight (losses or gains) as heat is applied to the sample. TG tests may be run in a heating mode at some controlled heating rate, or isothermally. Typical weight loss profiles are analyzed for the amount or percent of weight loss at any given temperature, the amount or percent of non-combusted residue at some final temperature, and the temperatures of various sample degradation processes. (Impact Analytical Laboratory, 2006)

3.7.5. Energy Disperse X-Ray (EDX)

Energy Disperse X-Ray is a technique used to determine the elemental composition present in the sample using a Scanning Electron Microscope. In an EDX, an electron beam with energy ranging from 10-20 keV strikes the surface of the sample. The electron beams excites energy X-ray that are present in the elements. By just moving the electron beam across the material, the image of each element can be obtained. Lithium drifted silicon detector is used as a detector in this apparatus. This detector should be operated at the temperature of liquid nitrogen. When the emitted X- rays strike the detector, it creates a photoelectron within the body of the silicon. Electron-hole pairs are generated as the photoelectron travels through the silicon. Due to strong electric fields, the electrons and holes generated are attracted to the opposite ends of the detector. The size of the current pulse depends on the energy of incoming X-ray. Thus, an X-ray spectrum can be acquired giving information on the elemental composition of the material under examination (www.uksaf.org).

3.7.6 Brunauer – Emmet – Teller (BET)

Many of the most popular method for determining the surface area of powders and porous materials depend on the measurement of adsorption. Over the past 50 years the BET method has become an extremely popular method for determining the surface area of adsorbents, catalysts and various other finely divided and porous materials. Two stages are involved in the evaluation of the surface area by BET method from physisorption isotherm data. First stage is related to construct a BET plot and from it derive the value of the monolayer capacity,gmax. The second stage is the calculation of the specific surface area, a (BET), from monolayer capacity and this requires knowledge of the average area, which is occupied by each molecule in the completed monolayer.

The BET equation is expressed in the linear form

p0: saturation vapour pressure

p: equilibrium pressure

C: Constant specific for the system

g: specific amount of gas adsorbed

gmax: monolayer capacity

Fit best straight line through BET data set using least squares regression to find The intercept with the y-axis and the slope of the resulting curve provide a value for gmax The specific surface area, a(BET), is obtained from the BET monolayer capacity, gmax, by application of relation where L is the Avogadro constant and s is the average area occupied by each molecule in a completed monolayer. For the case of nitrogen adsorption at 77 K, the value of s (N2) is normally taken as 0.162 nm2 (Bowker, M.,1998).

3.7.7. Temperature Programmed Desorption (TPD)

Temperature Programmed Desorption (TPD) which also known as thermal desorption spectroscopy (TDS) is thermo analytical techniques for characterizing chemical interactions between gaseous reactants and solid substances. The adsorption of the reactants precedes and the desorption of products follows the reaction on the catalytic surface in heterogeneous catalysis (Elliott, 1996).

In a TPD studies, a surface is first exposed to a gas at a particular temperature in order to obtain a specific initial coverage. The surface is then heated in a controlled manner so that the gas desorbs. The gas pressure above the surface is monitored as the surface is heated. The competition between molecules entering the gas phase volume through desorption and leaving the volume through pumping of the experimental chamber creates a pressure spectrum with a characteristic shape. The temperature at which species are desorbed from the surface of a heated solid reflects the strength of the surface bonds (Kanervo, 2003).

3.7.8. Effective Factors of TPD Experiments

There are several factors, which may significantly affect the results of TPD experiments for instance in determining the peak temperature and adsorption capacity (Gallardo, 2001). Although these factors such as leak and impurities in the set up, flow rate of the carrier gas and others have small amount disturbance quantity in the entire TPD set up, it will contribute major incorrect result because in this study also dealing with only small amount of catalysts sample which placed in the micro reactor. Leaks and Impurities

The adsorbate should be 99.99 % pure, because small amount of impurity can greatly influence the result of TPD. If these impurities are not removed during pretreatment, the adsorption capacity of the adsorbent may be altered by way of competition for the active adsorption sites.

Another source of impurity comes from leaks in the piping system. Leaks can create openings in which impurity may penetrate. Air containing oxygen, moisture and other components may also affect the adsorption capacity of adsorbent. Purging of the system with helium must be done to remove the moisture content and other gases adsorbed on the catalyst surface. Pretreatment of the adsorbent must also be accomplished to desorb trace elements. Carrier Gas Flow Rate

If the flow rate is too high, the concentration of the adsorbate will be low and the thermal conductivity detector (TCD) will detect a small peak area. On the other hand, if the flow rate is too low, the response of the reactor to the TCD becomes slow as well. Adsorption Temperature

The higher the adsorption temperature, the lower the equilibrium adsorption capacity. The reactor containing the adsorbate should be placed in water bath, which is maintained at a constant temperature. Particle Size of the Adsorbent

The particle size of the adsorbent should be uniform for the purpose of comparison. Different particle size present different external surface. Smaller particle size allows better intraparticle diffusion and provides higher surface area resulting in the increase in the adsorption capacity. Reactor Size and Amount of Adsorbent

A microreactor with a relatively small diameter should be used to minimize temperature gradients. The height of the catalysts bed must not be too high such that large pressure drops may occur. At the same time, it should not be too low because concentration of the adsorbed component may be too small to generate measurable peaks. Thermocouple

The thermocouple should be sufficiently thin to avoid delay in temperature measurements due to the thermal inertia. Also, it must be inserted in the thermowell and placed as close as possible to the catalyst bed. Location of Detector

TCD must be placed near the exit gas to minimize the possibility of re-adsorption of the adsorbates leaving the surface of the adsorbent. Heating Rate

The adsorbent is heated at a constant heating rate from room temperature to the desired final temperature. This is accomplished by using a temperature programmable controller.

3.8 Sample preparation

Sample preparation is very important when a chromatographic analysis has to be done. This can be achieved by using a wide number of techniques or procedures. Many of these have not changed over the last 50 years and are as simple as filtration or centrifugation. Others have developed extensively in the last 10 years.

One concept of a sample preparation method is to convert a real matrix to a sample format which is suitable for analysis. According to this definition there are some steps, after sampling, which attempt to adapt the sample to the measurement step (i.e., second step of CMP). To this end, the physical state of the sample must be adequate for the requirements of the instrument used in testing.

3.8.1 Sample preparation methodologies

Although chromatography is a separating technique it can not be expected to completely separate all components of a complex sample. Very often it is necessary to prepare the sample, filtrate, concentrate, clean up, etc. before chromatography. These steps vary depending on the nature of the sample. One important and general step in sample treatment, especially when liquid chromatography is to be used, is filtration of the solution. A first filtration may be necessary to separate large particulate matter from solvent since this may physically interfere with extractions in later stages. The final filtration before chromatography injection uses 0.45 mm to 0.20 mm or smaller disposable filters to prevent small particulate matter getting into the chromatography column and damaging it. Different types of filters can be used including filter membranes, centrifugal filters, and syringe filters. When commercial syringe filters are used, different diameter sizes are offered depending on the volume of sample to filter, which is important when small samples have to be filtered An alternative to normal filtration is ultrafiltration. In this case, pressure is applied to a membrane and molecules smaller than the molecular weight cutoff can pass through while larger molecules are retained (Nollet.M.L., 2006).

3.8.2 Liquid sample preparation

Depending on the nature of analyte(s) of interest, sample preparation processes will be quite different. In liquid sample preparation, as for water and wastewater analysis, the analytes from matrices can be separated in two different ways:

- by extracting the analytes into a liquid phase as in Liquid-Liquid Extraction (LLE), purge and trap technique, membrane extraction, and single drop extraction.

- or by trapping the analytes in a solid phase such as Solid Phase Extraction (SPE), Solid Phase Micro Extraction (SPME), and stir-bar extraction.

1. Liquid-Liquid Extraction (LLE)

The classical LLE method is still in use due to the simplicity of the instrumentation, just a separation funnel, and also because of its extensive implementation in official methods (U.S.-EPA methodology, EEC standard methods). Nearly all U.S.-EPA (United States Environmental Protection Agency) methods for non-volatile and semivolatile analytes in environmental samples apply LLE even though there is a trend to change this.

Table 3.4 : US-EPA Method for Environmental samples. (Nollet.M.L., 2006)

When doing a LLE, a given volume of the sample, for example water, is shaken with a given volume of a suitable organic solvent so that the organic micropollutants migrate from the aqueous to the organic phase. Sometimes it is advisable to add a small quantity of sodium chloride to avoid foam formation and so obtain a better separation. Then the organic solvent with the analytes is separated and evaporated to concentrate the sample to a precise volume.

LLE can be done in different ways:

- Discontinuous liquid extraction. This is the most traditional extraction method which can be carried out in one or multiple steps.

- Continuous LLE. This is applied when the distribution constant is low or when the sample volume is large.

- Countercurrent extraction. This is advisable when complex samples with analytes of similar distribution are to be extracted.

- Online LLE. This is a dynamic process which allows the extraction of low volume samples and reduces the organic solvent consumption but it has the drawback of instrument complexity.

- LLE has several disadvantages such as large volumes of generally toxic organic solvents. With some samples, the initial solvent extraction step results in the formation of an emulsion and hence prolongs the extraction process. A loss of sample frequently occurs during the concentration step and so reduces analyte recovery. To avoid these limitations a considerable interest in developing alternative sample preparation methods has been increasing in the last few years.

2. Single Drop Extraction (SDE)

Single drop extraction (SDE) or liquid-phase microextraction (LPME) is a recently developed microscale extraction method. In this method a single liquid drop is used as a collection phase. Small volumes of organic solvent (from 0.5 ml to 2.5 ml) are used. The collection phase must have a sufficiently high surface tension to form a drop which can be exposed to the analyte solution. When the extraction is finished, the single drop is injected into the GC..

SDE can be used in static and dynamic modes. When working in the static mode, steps in the extraction process are:

(a) the magnetic stirrer is switched on to agitate the aqueous sample solution;

(b) a specific volume of organic solvent is drawn into the syringe with the needle tip out of the solution and the plunger is depressed by 1 ml to 2 ml;

(c) the needle is then inserted through the septum of the sample vial and immersed into the aqueous sample;

(d) the plunger is depressed to expose the organic drop to the stirred aqueous solution for a period of time;

(e) the drop is retracted into the microsyringe; and

(f) finally, the organic solvent drop is transferred into a vial and subsequently injected in a chromatograph.

In the dynamic mode all steps are done automatically. Static and dynamic modes were compared to extract polyaromatic hydrocarbons (PAHs) in water obtaining higher concentrations in the dynamic mode.41 SDE avoids the problems of solvent evaporation as in LLE. It is a fast, inexpensive, and simple method. However, the extraction is not exhaustive.

3.9. Gas Chromatography

A chromatography instruments is essentially a device which enables a small amount of sample to be introduced into an inlet system and passed into a chromatographic column by the flow of inert as a mobile phase. Chromatography instruments may be either a Liquid Chromatograph or a Gas Chromatograph depending on the phase at which the samples are injected for analysis. In the Gas Chromatograph (GC), the mobile phase is the carrier gas. Carrier gases are generally inert gases such as Helium, Argon, and Nitrogen. Helium, one of the most common carrier gases is colorless and odorless, possessing the lowest solubility in water among all gases. It has very low viscosity, and thus, high flow rates can be obtained even at low pressure drop. Because of its high diffusivity, it can highly influence the separating power of the capillary columns. A detector is fitted at the column exit to monitor the separated components as these elute from the column. The detector provides an electrical signal which is amplified and fed to the recording or data-processing device from which meaningful results can be obtained (Baugh, 1993).

There are two general types of column, packed and capillary (also known as open tubular). Packed columns contain a finely divided, inert, solid support material (commonly based on diatomaceous earth) coated with liquid stationary phase. Most packed columns are 1.5 - 10m in length and have an internal diameter of 2 - 4mm. Capillary columns have an internal diameter of a few tenths of a millimeter. While for the detector, there are many types which can be used in gas chromatography, such as Flame Ionization Detector (FID), Thermal Conductivity Detector (TCD), Atomic Emission Detector (AED), Electron Capture Detector (ECD), Flame Photometric Detector (FPD), and the Photo-ionization Detector (PID). Different detectors will give different types of selectivity. FID is popular because of its broad detection limits. The performance of the chromatograph, and hence the quality of the results generated, depends not only on the design of the components but also on how carefully they are controlled. Temperatures and gas flow rates are very important parameters should be carefully considered. Most modern chromatographs, therefore, use a microprocessor at the heart of the control system. The use of such technology has the added benefits of an improved user interface, better programmability, method storage, external control, and intelligent system diagnostics (Baugh, 1993).

3.10 Photocatalytic Reactors

The three main components of a photocatalytic process are the photoreactor, the radiation source and the photocatalyst. According to Augugliaro et al. (1997), for thermal and catalytic processes the reactor are generally chosen on the basis of the following parameters:

(i) the mode of operation;

(ii) the phases present in the reactor;

(iii) the flow characteristics;

(iv) the needs of heat exchange;

(v) the composition and the operative conditions of the reacting mixture, which affects the selection of materials of construction.

However, for selection of the type of heterogeneous photocatalytic reactor additional parameters have to be considered because photons are needed for the occurrence of photoreaction. The selection of the construction material is done in order to allow the penetration of radiation into the reacting mixture. The choice of the radiation source is easier than that of the photoreactor since the radiation spectrum of the lamp should coincide with the absorption spectrum of the reacting system and the emitted photons should have energy equal to or more than the band-gap of the semiconductor photocatalyst. Based upon these three components photocatalytic reactor systems can be classified under the following factors:

3.11 Light Source

The light source is a very important factor to be considered since the performance of a photoreactor is strongly dependent upon the irradiation source. There can be two types of light sources, natural i.e. solar light and artificial. The artificial light sources can be of many types such as: (a) arc lamps; (b) fluorescent lamps; (c) incandescent lamps; (d) lasers. In general, arc lamps and fluorescent lamps are the most commonly used in photoreactor systems.

In fluorescent lamps the emission is obtained by exciting an emitting fluorescent substance, deposited in the inner side of a cylinder, by an electric discharge occurring in the gas filling the lamps. Generally, these lamps emit in the visible region, but the special type has emissions in the near-UV region and the emission spectrum depends on the nature of the mixture of fluorescent substances used. Their power output is quite small (up to 180 W) and hence they find uses in laboratory scale units.

For arc lamps and in particular for mercury lamps, a classification based on the pressure of Hg is made by Augugliam et al. (1997) and it is as follows:

i) Low-pressure Hg lamps:

This type of lamp contains Hg vapor at a pressure of about 0.1 Pa, emitting mainly at 253.7 and 184.9 nm.

ii) Medium-pressure Hg lamps:

this type of lamp has a radiation source containing mercury vapor at pressures ranging from 100 to several hundred kPa and the emission is mostly from 3 10 to 1000 nm with most intense lines at 3 13, 366,436, 576,and 578 nm.

iii) High-pressure Hg lamps:

This type of lamp contains mercury at a pressure of 10 MPa or higher, which emits broad lines and a background continuous between 200 and l000 nm.

iv) Xenon and Hg-Xenon lamps:

In this type of lamp an intense source of ultraviolet, visible and near-IR radiation in a mixture of Hg and Xe vapors under high pressure is obtained.

3.12 Influence of Operational Parameters on Photocatalytic Process

3.12.1 pH

The pH generally influences a semiconductor in an electrochemical system by shifting the valence and the conduction band following the Nernst law. It is similar to the charge of the semiconductor surface and therefore the adsorption properties depend upon the pH. Since in photocatalysis the adsorption of a pollutant is a prerequisite for its degradation, a change in pH can lead to a change of the degradation rate and of the amounts and concentrations of intermediates. However, changes in rate of photocatalytic activity by pH value are usually small, often less than 1 order of magnitude (Fox and Dulay., 1993). The absence of marked effect of the pH represents an advantage of photocatalytic process over biological treatment. According to Henmann et al. (1993), an elevation of pH above 10 caused an increase in the reaction rates, as is expected from an increase in the formation rate of OH∙ radicals from the reaction: OH- +p+ OH∙

Rideh., et al(1997) found that the rate constant for 2-Chlorophenol degradation is lower in the acid medium than in the basic medium and it is more or less constant in the neutral pH range. They also suggest that pH has a higher direct effect on the conversion rate and it can affect either the surface properties of the photocatalyst or the chemical form of the substrate. Titania has an amphoteric character with a point of zero charge around pH equal to 6 (Pelizzetti et al., 1993), and the substrate can undergo acid-base equilibria. Consequently, the adsorption of the substrate may be affected, strongly influencingthe degradation rate (Kormann et a1 .,1991).

3.12.2. Temperature

Experiments have shown for 4-Chlorophenol (Al-Sayyed et al., 1991) and 2-Chlorophenol (Rideh et al., 1997), that the variations of the initial rate of reaction in the range 278-333 K follow the Arrhenius law. However, the apparent activation energy is very small (5.5 k.l/mole and 6.23 kJ/mole respectively), which indicates that the thermally activated steps are negligible. In the latter case, a temperature increase of 103oC would be required to double the rate constant. Irradiation is the primary source of electron-hole pair generation at ambient temperature as the band gap energy is too high to be overcome by thermal activation. Therefore, the slight increase in rate constant can be attributed to the increasing collision frequency of molecules in solution that increases with increasing temperature (Chen and Ray.,1998). An expected increase of the reaction rate constant with increasing temperature is possibly compensated by a decrease of the adsorption equilibrium constant (Koster et al., 1993). This clearly demonstrates that the photocatalytic treatment is well adapted for decontaminating liquid water at temperatures close to ambient.

3.12.3 Catalyst Concentration

Many researchers have investigated under different experimental conditions the evolution of the reaction rate as a function of catalyst concentration (Mengyne et al., 1995; and Rideh et al. (1997) conducted a series of experiments to find an optimum catalyst concentration and their results are in good agreement with other studies found in literature. The initial photodegradation rate increases linearly with catalyst concentration and approached a limiting value at higher concentrations. This plateau mainly results from the following two factors: (a) aggregation of photocatalyst particles at high concentrations, causing a decrease in the number of surface active sites, and (b) increase in turbidity and light scattering at high concentrations leading to a decrease in the passage of radiation.

3.12.4 Initial concentration of pollutant

As the effect of pollutant concentration is of utmost importance in any water treatment process, the dependence has been extensively investigated for various pollutants by researchers. In recent years, the Langmuir-Hinshelwood rate has been used successfully for heterogeneous photocatalytic degradation to describe the relationship between initial degradation rate and initial concentration (AI-Sayyed et al.,1991); Lu et al., 1993 and D'Oliveira et al., 1990). TheLangmuir-Hinshelwood model is represented by:

Ro = k K Co/ (1+KCo)

where ro is the rate of reaction, k is the rate constant, which depends on the operating conditions (temperature, pH, wavelengths, radiant flux etc.), and K is the adsorption constant of the pollutant considered on TiO2 in competition with water. Rideh et al.,1997 studied the effect of initial concentration of 2thlorophenol on its photodegradation rates and found that the photodegradation decreases with increasing initial concentration and the Langrnuir-Hinshelwood model obtained good fits with experimental data. A plausible explanation of this behavior can be the following: as the initial concentration increases, more and more organic pollutants are adsorbed on the photocatalyst surface, but the intensity of light and illumination time were constant; consequently, the hydroxyl radicals formed on the surface of titanium dioxide are limited, the relative number of hydroxyl radicals attacking the pollutant molecules decreases, and thus the photodegradation efficiency decreases too (Mengyne et al.,1995).

3.12.5 Light Intensity

Various studies (D'Oliveira et al. ( 1990); Ollis et al. (1991) and Al-Sayyed et al. ( 199l)] on photodegradation rate versus illumination intensity indicated that the reaction rate increases with the square root of intensity at high intensity levels when mass transfer is not limiting. At sufficiently low levels of illumination (catalyst dependant), on the other hand, degradation rate is of first-order with respect to intensity. Increased illumination resuIts in an increase in volumetric reaction rate, until the mass transfer limit is encountered. The transition points between these regimes, however, will vary upon the photocatalytic system. More recent studies by Chen and Ray (1998) and Rideh et al. (1997) illso confirmed the above findings, however in the latter study the experiments were conducted in the region of lower intensity and their reaction rate was first order with the light intensity. Several studies (D'Oliveira et al. 1990; Ollis et al.,1991 and Al- Sayyed et al. (1991) attributed this rate transition from first-order to half-order to the recombination of photogenerated electron-hole pairs at high light intensity. This phenomenon is obviously detrimental to the photocatdytic process as the quantum efficiency decreases. The recombination process may be slowed down possibly by the addition of better electron acceptors such as H2O2,Cu+and Ag (Ollis et al., 1991).

3.13. Experimental design and optimization.

The choice of an experimental design depends on the objectives of the experiment, the number of factors to be investigated and the type of design according to the experimental objective.

If there is one or several factors to be investigated, but the primary goal of the experiment is to make a conclusion about one a-priori important factor, (in the presence of, and/or in spite of the existence of the other factors), and the question of interest is whether or not that factor is "significant", (i.e., whether or not there is a significant change in the response for different levels of that factor), then a comparative problem is at hand and need to hand a comparative design solution.

If the primary purpose of the experiment is to select or screen out few important main effects from the many less important ones, screening designs are also termed main effects designs.

Response Surface method is designed to allow estimate interaction and quadratic effects or even at higher order, hence will give us an idea of the (local) shape of the response surface which are required. Also Response Surface Methodology (RSM) is a collection of statistical and mathematical techniques useful for developing, improving, and optimizing processes (Daniel.D.A, Springer.V 1999).

3.13.1 Factorial Points

The three-level factorial part of the design consists of all possible combinations of the +1 and -1 levels of the factors (-1 and 1 corresponding to the lowest value and the highest value of experiment parameter, respectively). For the three factors case there are 8 design points:

(-1, -1,-1); (+1, -1,-1); (-1, +1,-1); (+1, -1,-1); (+1, +1,-1); (-1, -1,+1); (+1, -1,+1); (+1, +1,+1);

3.13.2 Star or Axial Points

The axial points have all of the factors set to 0, the midpoint of the range of experiment parameter value, except one factor, which has the value +/- Alpha. For a three factors problem, the star points are:

(-Alpha, 0, 0); (+Alpha, 0, 0); (0, -Alpha, 0); (0, +Alpha, 0);(0, 0, -Alpha); 0, 0, +Alpha);

The value for Alpha is calculated in each design for both rotatability and orthogonality of blocks. The experimenter can choose between these values or enter a different one. The default value is set to the rotatable value.

Another position for the star points is at the face of the cube portion on the design. This is commonly referred to as a face-centered central composite design. This study set the alpha value equal to one, or choosing the Face Centered option. This design only requires three levels for each factor.

3.13.3 Center Points

Center points, as implied by the name, are points with all levels set to coded level 0 - the midpoint of each factor range: (0, 0, 0). Center points are usually repeated 4-6 times to get a good estimate of experimental error (pure error). The standard number of center point of the Design-Expert software Version 7.0.1 for three factors is 6.

The matrix of code variables of experiment design is shown in Table 3.5. The variables X, Y, Z represent the operating parameters intended to be optimized.

3.14 Kinetics of Monochlorobenzene destruction

The global disappearance rate of a reactant for two-phase AOP systems has been expressed as the Langmuir-Hinshelwood expression (2)

r = - dC/dt =kKC/(1 + KC) (1)

where k is the reaction rate constant, K is the adsorption equilibrium constant, and C is the gas or liquid concentration in the bulk phase. Integration of Equation 1 yields:

Ln(Co/C) + K(Co - C) = kKt (2)

where Co is the initial reactant concentration. Equations 1 and 2 have been employed to describe the kinetics of batch reactors of degradation of Monochlorobenzene

In this research the author used equation L-M and also proposed conversion of monocholorobenene in gas phase by Titania and UV light. Author also investigated the effect of humidity and intermediates to photodegradation of Monochlorobenzene. In the destruction process of chlorobenzene, the OH– group plays an important role in the cleavage of the aromatic ring. The ring cleavage is the rate limiting step, which is much slower than the rate that chlorobenzene degrades to chlorophenols. Experimental results support theoretical analyses which indicate that phenol and benzene are not primary intermediates, i.e., dechlorination occurs after the cleavage of the ring.

In a few research for photodegradation monochlorobenzene in water Huang etl., 2008 conducted experiment with Titania Degussa D25 in a batch reacor Heterogeneous photocatalysis using TiO2 as photocatalyst was proven to be an effective method for the degradation and mineralization of MCB. The experimental results demonstrated that increasing the substrate concentration, light intensity, and TiO2 dosage in an appropriate range contributed to the photo- catalytic degradation of MCB. In addition, the neutral medium was beneficial for the degradation of MCB. The photocatalytic degradation of MCB followed the Langmuir–Hinshelwood kinetics. Moreover, the initial degradation rate varied with a 0.255 power of the light intensity indicated the considerable adverse effect of e−–h+ pair recombination.