Chapter - III THEORETICAL CONCEPTS 3.1 Advanced Oxidation Processes AOPs are promising methods for the remediation of wastewaters containing recalcitrant organic compounds such as pesticides, surfactants, coloring matters, pharmaceuticals, phenolic wastes and endocrine disrupting chemicals. AOPs involve two stages of oxidation discussed above: 1) the formation of strong oxidants (e.g., hydroxyl radicals) and 2) the reaction of these oxidants with organic contaminants in water. However, the term advanced oxidation processes refer specifically to processes in which oxidation of organic contaminants occurs primarily through reactions with hydroxyl radicals (63). 3.2 Hydroxyl radicals The main mechanism of AOPs function is the generation of highly reactive free radicals. Hydroxyl radicals (HO ) are effective in destroying organic chemicals because they are reactive electrophiles (electron preferring) that react rapidly and nonselectively with nearly all electron-rich organic compounds. They have an oxidation potential of 2.33 V and exhibit faster rates of oxidation reactions comparing to conventional oxidants such as H2O2 or KMnO4 (64). Once generated, the hydroxyl radicals can attack organic chemicals by radical addition, hydrogen abstraction and electron transfer (65). In the following reactions, R is used to describe the reacting organic compound. R + HO ROH (1) R + HO R + H2O (2) Rn + HO Rn-1 + OH- (3) 49
These radicals are very reactive, attack most organic molecules, and are not highly selective (66, 67). Table 3.1 shows oxidation potentials of various oxidants. S.No Oxidant Oxidation potential (v) 1 Fluorine 3.03 2 Hydroxyl radical 2.80 3 Atomic Oxygen 2.42 4 Ozone 2.07 5 Hydrogen peroxide 1.78 6 Perhydroxyl radical 1.70 7 Permanganate 1.68 8 Hypobromous acid 1.59 9 Chlorine dioxide 1.57 10 Hypochlorous acid 1.57 Table 3.1 Oxidation potentials of various oxidants 3.3 Classification of AOPs A great number of methods are classified under the broad definition of AOPs. Most of them use a combination of strong oxidizing agents (e.g H2O2, O3) with catalysts (e.g. transition metal ions) and irradiation (e.g. ultraviolet, visible). Among different available AOPs producing hydroxyl radicals, titanium dioxide/uv light process, hydrogen peroxide/uv light process and Fenton s reactions seem to be some of the most popular technologies for wastewater treatment as shown by the large amount of data available in the literature. Fig 3.1 represents the wavelength under which various photodegradation methodologies take place 50
Figure 3.1 Wavelength Range of Photochemical Degradation These AOPs are classified either as homogeneous or heterogeneous. Homogeneous processes are further subdivided into processes that use energy and processes that do not use energy (Figure 3.2). 51
Figure 3.2 Classification of Advanced Oxidation Processes 3.4 AOP Technologies used in the present study The following sections describe a wide range of advanced oxidation systems that are currently studied in this thesis for their possible use in phenolic wastewater treatment. Ultraviolet Irradiation (UV)-Photolysis Hydrogen Peroxide/ Ultraviolet Irradiation (H2O2/UV) Fenton s reaction Photo-Fenton oxidation TiO2-catalyzed UV Oxidation (UV/TiO2) 52
3.5 Homogenous advanced oxidation processes Homogeneous AOPs using UV radiation are generally employed for the degradation of compounds that absorb UV radiation within the corresponding range of the spectrum. The compounds that absorb UV light at lower wavelengths are suitable for such type of photo-degradation. 3.5.1 UV Photolysis Direct photolysis involves the interaction of light with molecules in addition to water to bring about their dissociation into fragments, with the following mechanistic pathway: R + hν Intermediates (4) Intermediates + hν CO2 + H2O + R - (5) This process appears to be less effective than other processes where radiation is combined with hydrogen peroxide or ozone, or where homogeneous, heterogeneous catalysis or photocatalysis are employed. The most common sources of UV light are continuous wave low pressure mercury vapor lamps (LP-UV), continuous wave medium pressure mercury vapor lamps (MP-UV), and pulsed-uv (P-UV) xenon arc lamps. Both LP-UV and MP-UV mercury vapor lamps produce a series of line outputs, whereas the xenon arc lamp produces continuous output spectra. 3.5.2 Hydrogen Peroxide and Ultraviolet Radiation (H2O2/UV) This advanced oxidation process involves the formation of hydroxyl radicals generated by the photolysis of H2O2 and the corresponding propagation reactions. The photolysis of hydrogen peroxide occurs when UV radiation (hν) is applied, as shown in the following reaction: H2O2 + hν 2HO (6) 53
The molar absorptivity of hydrogen peroxide at 253.7 nm is low, about 20M 1 cm 1 and HO radicals are formed per incident photon absorbed. This technique requires a relatively high dose of H2O2 and/or a much longer UV-exposure time the rate of photolysis of hydrogen peroxide has been found to be ph dependent and increases when more alkaline conditions are used, because, at 253.7 nm, peroxide anions HO 2 may be formed, which display a higher molar absorptivity than hydrogen peroxide, 240M 1 cm 1 (63), by the following reaction: HO 2 + hν HO + O (7) The absorptivity of hydrogen peroxide may be increased by using UV lamps with output at lower wavelengths. The main reactions of the system are the following ones: Initial reactions kp H2O2 + hν 2HO (8) k p H2O2 H2O + ½O2 (9) Propagation HO + H2O2 HO 2 + H2O (10) HO 2 + H2O2 HO + H2O + O2 (11) HO 2 + HO 2 HO + HO + O2 (12) Termination HO + HO 2 H2O + O2 (13) HO + HO H2O2 + O2 (14) And radicals recombine, as shown below: 2OH H2O2 (15) 54
When the water to be treated has a higher absorbance, it may compete for the radiation with the hydrogen peroxide. This is one of the drawbacks of this method. An H2O2/UV system can totally mineralize any organic compound, reducing it to CO2 and water. However, generally, in real-life scenarios, such a drastic process is not necessary. The toxicity of oxidation products is not a problem since they are easily degraded (68). Hydrogen peroxide can be added either as a single slug dose or at multiple points in the system. The optimum dose of H2O2 should be determined for each water source based on bench and pilot-scale testing. 3.5.2.1 Advantages H2O2 is highly soluble and can be added to the source water at high concentrations H2O2/UV processes can generate larger amounts of hydroxyl radicals than O3/UV processes for equal amounts of energy used to add the oxidants to the source water. 3.5.2.2 Disadvantages The presence of residual hydrogen peroxide in the treated effluent will promote biological re-growth in the distribution system. The method is expensive due to the cost of necessary devices and the energy requirements. 3.5.3 Fenton and Photo Fenton s Oxidation 3.5.3.1 Fenton s Oxidation The Fenton reagent, a mixture of hydrogen peroxide and iron (II) salt, is discovered by Henry J.H. Fenton. He described the oxidation power of hydrogen 55
peroxide on certain organic molecules in which OH radicals are produced from hydrogen peroxide under the addition of Fe(II) as a catalyst. This system is considered as the most promising treatment among AOPs for remediation of highly contaminated waters (69). The main reactions of the system are as follows: Initial reactions H2O2 H2O + ½O2 (16) H2O2 + Fe 2+ Fe 3+ + HO + HO (17) H2O2 + Fe 3+ Fe(OOH) 2+ + H + Fe 2+ + HO 2 + H + (18) HO + Fe 3+ Fe(OH) 2+ Fe 2+ + HO (19) Propagation HO + H2O2 HO 2 + H2O (20) HO 2 + H2O2 HO + H2O + O2 (21) HO 2 + HO 2 HO + HO + O2 (22) Termination Fe 2+ + HO Fe 3+ + HO (23) HO 2 + Fe 3+ Fe 2+ + H + + O 2- (24) HO + HO 2 H2O + O2 (25) HO + HO H2O2 + O2 (26) Because of its simplicity, the Fenton reaction is the process most often applied when it is necessary to remove recalcitrant compounds. However, the major drawback of the Fenton reaction is the production of iron sludge waste, which paved the way for the development of the photo-fenton process, that uses UV or solar light for the reduction of Fe(III) oxalate back to Fe(II) oxalate resulting 56
in a drastic reduction of the sludge waste. Even though Fenton process is able to destroy the toxic organic compounds in a relatively short time, it cannot lead to complete mineralization of organic compounds. The extent of mineralization was reported to be around 40 60% [70], depending on the amount of reagents employed. 3.5.3.2 Photo Fenton oxidation The degradation velocity of organic pollutants is significantly enhanced when UV visible light at wavelengths greater than 300 nm is added to the reaction. In this process, the photolysis of iron (III) complexes allows regeneration of iron (II), which can further react with more hydrogen peroxide. The reactions involved in the process are: Fe 2+ + H2O2 Fe 3+ + HO + HO (27) hν Fe 3+ + H2O2 Fe 2+ + HO + H + (28) hν H2O2 2HO (29) This process reduces the formation of the sludge waste that is produced in the original Fenton process. However, it is necessary to exhaustively control the ph of the medium similar to dark Fenton. Generally, the ph range should be between 2.6 and 3 for the best performance of the system. 3.5.3.3 Factors affecting Fenton and Photo fenton Many constituents of the water as well as the conditions in which the process is carried out can substantially affect reaction rates and stoichiometry, due to effects in the solution chemistry. 57
3.5.3.3.1 ph The performance of this reactive system is a function of ph and it significantly influences the decomposition of organic compounds. Literature review showed Fenton and photo Fenton system s performance to be efficient at an optimum ph 3. At high ph values (>4), the decomposition rate decreases due to the precipitation of Fe(OH), while for ph values below the optimal, the rate decreases due to the lower light absorption coefficient associated with the iron complexes formed (71). At ph 3, approximately half of the Fe(III) is present as Fe +3 and half as Fe(OH) +2 ion which is the photo-active species. Below this ph the concentration of Fe (OH) +2 declines and at higher ph, Fe(III) precipitates as oxyhydroxides. 3.5.3.3.2 Effect of hydrogen peroxide concentration [H2O2] The concentration of hydrogen peroxide has an important influence in the degradation of organic compounds. The reaction rate tends to increase with increasing hydrogen peroxide concentration (69). Because of the series of intermediates that take place in the process, sufficient hydrogen peroxide must be added in order to push the reaction beyond that point. This fact is frequently seen when pre-treating a complex organic wastewater for toxicity reduction. As the H2O2 dose is increased, a steady reduction in COD may occur with little or no change in toxicity until a threshold is attained, whereupon further addition of hydrogen peroxide results in a rapid decrease in wastewater toxicity (72). Addition of excess H2O2 exceeding the optimum limit do not improve the degradation efficiency, which might be attributed to auto decomposition of H2O2 to oxygen and water, and the recombination of OH radicals. Moreover higher concentrations of hydrogen peroxide act as free-radical scavenger itself, thereby decreasing the 58
concentration of hydroxyl radicals and reducing compound elimination efficiency (73). 3.5.3.3.3 Effect of catalytic ferrous ion concentration [Fe +2 ] The feature of an optimal dose range for iron catalyst is characteristic of Fenton s reagent, although the definition of the range varies between contaminated waters. Typical ranges are 1 part iron per 5 25 parts of hydrogen peroxide (w/w) (74). Addition of iron salt above the optimal dosage does not affect the degradation, even when the concentration is doubled. This is due to the fact that, at a Fe (II) concentration higher than the optimum, the rate of hydroxyl radicals originated from the decomposition of H2O2 is so high that much of hydroxyl radicals are consumed by the side reactions before they are utilized for the removal of the pollutant [75]. Moreover, it would result in brown turbidity that hinder the absorption of the UV light required for photolysis and also would cause the recombination of OH radicals (72). 3.5.3.4 Advantages Fenton process requires very little energy compared to Photofenton a n d other oxidation technologies that utilize O3 or UV. Fenton & Photo Fenton processes produce no vapor emissions and, therefore, require no off-gas treatment or air permits. The process is carried out at room temperature and pressures. Since reactions take place in homogeneous phase, there are no mass transfer limitations. Iron is a highly abundant and non-toxic element. The photo-fenton process can use photons within a broader wavelength range as compared to heterogeneous photocatalysis (76). 59
3.5.3.5 Disadvantages An iron extraction system is needed to remove residual iron from the treated water, which may increase the costs for the system. A very low ph (<4) environment is necessary to keep the iron in solution (77). Therefore, ph adjustment before and after treatment will be required. The requisite acid and base injections will increase the operational & maintenance costs. The photo-fenton process using artificial light involves an additional energy consumption to run the lamps. 3.6 Heterogenous advanced oxidation processes Heterogeneous advanced oxidation processes generally use catalysts to carry out the degradation of compounds. The term heterogeneous refers to the fact that the contaminants are present in the aqueous phase, while the catalyst is in the solid phase. The latter accelerates the chemical process because of the existence of electron-hole pairs. The photogenerated holes and electrons give rise to oxidation and reduction processes, respectively. In an aqueous solution, the water molecules adsorbed to the catalyst, are oxidized giving rise to OH radicals. As the process is usually carried out in aerobic conditions, the species to be reduced is oxygen, generating the superoxide radical. OH radicals subsequently oxidize organic pollutants adsorbed onto the surface of the catalyst. 3.6.1 Photocatalysts 60
A good photocatalyst should be photoactive, able to utilize visible and /or near UV light, biologically and chemically inert, photostable, inexpensive and nontoxic. In order for a semiconductor to be photochemically active as a sensitizer the redox potential of the photogenerated valence band hole must be sufficiently positive to generate OH radicals, which can subsequently oxidize the organic pollutant. The redox potential of the photo generated conductance band electron must be sufficiently negative to be able to reduce absorbed oxygen to superoxide. Si, TiO2, ZnO, WO3, CdS, ZnS, SrTiO3, SnO2, WSe2, Fe2O3, etc can be used as photocatalysts (78). Titanium dioxide (TiO2) in particular has been demonstrated to be an excellent catalyst for photooxidation of a variety of organic and inorganic compounds (79). 3.6.2 Titanium dioxide (TiO2) Titanium is the ninth most abundant element on the earth s crust. In its most stable form as an oxide, it is found in three possible crystalline forms: brookite, rutile and anatase. Only rutile and anatase are relevant from a photocatalytic point of view, the latter having the highest photocatalytic activity (80). TiO2 is a semiconductor with band gap energy of around 3.2 ev. Advantages of TiO2 are its chemical inertness, photo stability, high reaction rates and relatively low price. Among the different TiO2 forms available, Degussa P-25 is one of the most efficient forms with a particle size of around 25 nm. 3.6.2.1 Process fundamentals TiO2 produces pairs of electrons and holes by absorbing Ultraviolet (UV) radiation from sunlight or illuminated light source. The electron of the valence band of titanium dioxide becomes excited when illuminated by light. The excess 61
energy of this excited electron promotes the electron to the conduction band of titanium dioxide therefore creating the negative electron (e - ) and positive hole (h + ) pair. This stage is referred as the semiconductor s photo excitation state. The energy difference between the valence band and the conduction band is known as the Band Gap. Wavelength of the light necessary for photo excitation is 388nm. The positive-hole of titanium dioxide breaks apart the water molecule to form hydrogen gas and hydroxyl radical. The negative electron reacts with oxygen molecule to form super oxide anion. This cycle continues when light is available. The schematic representation of TiO2 Photocatalysis is presented in Fig 3.3. Figure 3.3 Schematic of TiO2 Photocatalysis Figure 3.3 Schematic representation of TiO2 Photocatalysis 3.6.2.2 Optimum Operating Parameters 3.6.2.2.1 Amount of the catalyst Catalyst concentration is to be used only till an optimum value (81), as using excess catalyst reduces the amount of photo-energy being transferred in the medium due to opacity offered by the catalyst particles. It should also be noted 62
that the optimum value is dependent on the type and concentration of the pollutant, as well as the rate of generation of free radicals. 3.6.2.2.2 Initial concentration of the reactant For highly concentrated effluents, absolutely no destruction may be observed and dilution is essential in this case. Beltran et al. (1993a) (82) have shown that the destruction of atrazine is 70% for initial concentration of 10 ppm whereas for 100 ppm, only 45% of the initial atrazine is degraded. Langmuir Hinshelwood type of models (83) can be used to relate the observed rates with the initial concentration of the pollutant, but the model parameters will be strongly dependent on the composition of the effluent as well as other reactor operating conditions. 3.6.2.2.3 Effect of ph ph has a complex effect on the rates of photocatalytic oxidation and the observed effect is generally dependent on the type of the pollutant as well as the zero point charge (zpc) of the semiconductor used in the oxidation process, i.e. more specifically on the electrostatic interaction between the catalyst surface and the pollutant. The adsorption of the pollutant and hence the rates of degradation will be maximum near the zpc of the catalyst (84). For some of the pollutants, which are weakly acidic, rate of photocatalytic oxidation increases at lower ph due to an increase in the extent of adsorption under acidic conditions (85). Some of the pollutants, which undergo hydrolysis under alkaline conditions or undergo decomposition over a certain ph range, may show an increase in the rate of photocatalytic oxidation with an increase in the ph (86). Fox and Duley (1993) (87) and Matthews and McEvoy (1992b) (88) have reported that ph had marginal effect on the extent of degradation over the range of ph studied in their work. Since the 63
effect of ph cannot be generalized, it is recommended that laboratory scale studies are required for establishing the optimum conditions for the operating ph. 64