applications of heterogeneous photocatalysis for industrial wastewater ...

Environmental Engineering and Management Journal, September 2002, Vol.1, No.2, 187-196 http://omicron.ch.tuiasi.ro/EEMJ/

“Gh. Asachi” Technical University of Iasi, Romania _________________________________________________________________________

APPLICATIONS OF HETEROGENEOUS PHOTOCATALYSIS FOR INDUSTRIAL WASTEWATER TREATMENT Anca Florentina Caliman1*, Carmen Teodosiu2, Ion Balasanian1 Faculty of Industrial Chemistry, Bd. D. Mangeron 71A, Iasi 6600 1 Department of Inorganic Technology 2 Department of Environmental Engineering ________________________________________________________________________

Abstract Refractory organic compounds (priority pollutants) are difficult to remove from industrial wastewaters by conventional methods and, especially in the case of biological treatment, these compounds may slow down or even stop the microorganisms activity. Heterogeneous photocatalysis may be considered a viable alternative for the removal of refractory organics due to several important advantages such as: complete mineralization or formation of more readily biodegradable intermediates when complex organic compounds are treated, no need of auxiliary chemicals, no residual formation, easily operation and maintenance of the equipment. This paper presents a literature survey of the research conducted in the field of heterogeneous photocatalysis, providing information on the possibilities and efficiencies encountered in the application of this process for industrial wastewater treatment for the removal of different types of refractory organic compounds. The basic fundamental principles are described, as well as the influence of the main parameters governing the heterogeneous photocatalytic process such as: wavelength, mass of catalysts, type and initial concentration of refractory organic contaminants, type of charge-trapping species, initial pH, temperature, light intensity. The possibilities to oxidise the organic compound from different industrial wastewaters are also reviewed in order to identify the active catalysts and the operating conditions, but also to investigate the correlations between all factors influencing the photocatalytic process.

Keywords: Heterogeneous photocatalysis, wastewater treatment, industrial wastewaters, refractory organic compounds

1.

Introduction

The removal of refractory pollutants from industrial wastewaters is presently of great concern since most of these compounds are toxic, mutagenic and/or

* Author to whom all correspondence should be addressed: e-mail: [email protected], tel/fax: +40-0232-237594

Caliman et al./Environmental Engineering and Management Journal 1 (2002), 2, 187-196

carcinogenic, even at low concentrations causing serious problems for the environment and human health. The ineffectiveness of conventional methods in removal of refractory organic pollutants led to the necessity to develop other efficient wastewater treatment processes. The advanced oxidation processes (AOPs) are considered as attractive alternatives, since they transform hazardous pollutants into compound with a more reduced impact on the environment. Among the various types of AOPs, the heterogeneous photocatalysis has been the subject of many researches due to several important advantages such as: • organic compound can be oxidised to CO2, H2O and/or to more readily biodegradable intermediates with removal efficiencies up to 95-99 %; • no auxiliary chemical agents are required; • no secondary pollution is caused, since no wastes are produced; • excepting the photoreactor, very simple and cheap equipment is needed; • the installations are easily operated and maintained. Although heterogeneous photocatalysis has proved to be an efficient method for the destruction of priority organic pollutants from wastewaters, its utilisation depends mainly by the high cost of energy necessary to generate UV light from electric lamps. Considering the need to replace traditional sources of energy with renewable ones in the context of sustainable development, alternative sources of energy for the photocatalytic processes were proposed, i.e. the use of solar photons instead of those from artificial sources to initiate photochemical reactions. In tropical countries, sunlight is an important source of energy that is not presently used to its full potential, often due to price and availability of land, while in countries with a temperate climate, solar irradiation is less effective. UV irradiation is more expensive but has as main advantage the 24 h/day availability, which means an average of 5 times higher than of the solar systems. In these cases, the economic assessment of the process is very important in order to evaluate the extent to which, the use of solar photons is cheaper than their generation by electricity. This paper presents a literature survey of the research conducted in the field of heterogeneous photocatalysis, providing information on the possibilities to apply the heterogeneous photocatalysis for certain types of industrial wastewaters. There are discussed the process conditions such as to obtain the maximum efficiencies, from both technological and economical points of view. 2. Principles of heterogeneous photocatalysis Heterogeneous photocatalysis is an AOP in which illuminated semiconductors catalyse the oxidation of the organic pollutants through photogeneration of electrons in the conduction band and positive holes in the valence band. The electrons react with the molecular oxygen adsorbed on the catalysts surface reducing it to O-2•, while the holes oxidise either the organic molecules or the OH- and the H2O molecules to HO•. The generated radicals are strong oxidants, which react further with the organic pollutants, completely 188

Applications of heterogeneous photocatalysis for industrial wastewater treatment

mineralising most of them to CO2, H2O and mineral acids. This mechanism is schematically depicted in Fig. 1. conduction band e-

O-2• O2

Eg

HO•

hν h+

H2O

valence band Fig.1. The catalysts/solution interface under illumination

Most of the experiments regarding the removal of refractory organics from aqueous media were done using TiO2, ZnO, WO3, SnO2, CeO2, SrTiO3, NiOSrTiO3, CdS, ZrO2, MoO3 as photocatalysts. Among these, TiO2 and ZnO have proved high photocatalytic activity. Apart of physical and chemical properties of the catalysts, such as solubility in water, corrosion and inactivation, achievement of high quantum efficiencies (Ф) is an important aspect that must be taken into account when a catalyst is selected for the photocatalytic process, since a major part of the operational cost of such a process is due to the electrical energy necessary for UV photon emission. The majority of the applications of heterogeneous photocatalysis uses TiO2, because it is chemically stable under many conditions, is non-toxic, relatively inexpensive and easily available, has a suitable bandgap energy, a high photoreactivity and efficiency in oxidising a variety of organic compounds. In order to improve the photocatalytic efficiency and to decrease the amount of radiation required for initiating the process, TiO2 was modified through ion doping or deposits of metal particles: Pt-TiO2, NiO-SrTiO2 (Suri et al., 1993), Ag-TiO2 (Sökmen and Özkan, 2002), Au+3-TiO2 (Li and Li, 2002) or Ti/Fe mixed oxide colloids with different iron content (Bahnemann et al., 1993). The efficiency of the process when immobilised photocatalysts are used is lower than that obtained in presence of dispersed titania particles. Many experiments were done with immobilised TiO2 on a variety of supporting materials: glass beads, fiber glass, silicon, quartz, activated carbon and zeolites (Langford et al., 1999).

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3. Factors influencing the heterogeneous photocatalysis process 3.1.Wavelength Titanium dioxide (anatase) has an energy bandgap of 3.2 eV (Ollis et al., 1991; Zhang et al.,1994) and can be activated by UV illumination with a wavelength of 280-387.5 nm. Artificial UV light can be generated by xenon or mercury lamps, but oxygen is rapidly consumed in the system and a chemical oxidant has to be added in order to allow the oxidation to proceed (Mathews, 1993). Another option that may be considered is vacuum-UV radiation (VUV), having λ < 200 nm. The vacuum-ultraviolet processes receive more attention after excimer light sources became available. Excimers are unstable molecular complexes formed through silent electrical discharge, which release at decomposition, their binding energy in the form of UV radiation. The photolysis of water below 185 nm produces hydroxyl radicals and hydrogen atoms with high efficiency, since water absorbs the VUV radiation with high cross section and quantum yield. The most efficient excimer source for VUV photolysis of water is the Xe excimer lamp, with a wavelength of 172 nm (Laszlo and Dombi, 1998; Laszlo and Dombi, 1999). Considering the fact that the heterogeneous photocatalysis use for wastewater treatment depends first on energy costs, many research subjects were focused on the possibilities to use solar light as source of energy even if the TiO2 absorbs only 3% from the solar spectrum. The photodegradation of the protocatechuic acid (3,4-dehydoxybenzoic acid), a refractory polyphenolic compound, typically found in olive processing and wine distillery wastewaters, realised under solar illumination on a sunny day (9,5 mW/cm2), and on a cloudy day (2,1 mW/cm2), as well as under artificial irradiation, shows that the time required for 50 % degradation of the organic compound was 2 times faster on the sunny day, and 2.5 times slower, on the cloudy day, both results being compared with the case when blacklight fluorescent tubes were used. These results are presented in Fig.2 (Poulios et al., 1999). The photocatalytic degradation of the dyes from wastewaters resulted from leather industry, by solar oxidation processes, revealed that the quantum efficiency of ZnO powder is significantly higher than that of TiO2, as a result of the fact that ZnO absorbs a larger fraction of the solar spectrum, being thus, more suitable for dyes degradation in the sunlight (Sakthivel et al., 1999). 3.2. Concentration, type and support of the photocatalyst As for any catalytic process, the initial rate of reaction increases directly proportional with the catalyst loading towards a limit value that corresponds to complete absorption of the photons, over which the rate becomes independent of the mass of catalyst. This limit depends on the geometry and the operational conditions of the photoreactor (Herrmann, 1999). 190


Fig.2. Photocatalytic degradation of 50 mg/L protocatechuic acid under artificial and natural illumination in the presence of 1g/L TiO2 nano-powder P-25

When suspensions of photocatalyst are used, its dosage depends on the initial solute concentration and could be explained in terms of availability of active sites on catalyst surface and the penetration of photoactivating light into the suspension. The availability of active sites increases with catalysts loading, but the light penetration and hence the photoactivated volume of the suspension is diminished. At low solute concentrations, when active sites are in excess, these opposing effects are balanced and change of the catalysts loading lead to minor differences on the rates of degradation. At high solute concentrations, the effect of active sites is more important than that of the diminished photoactivated volume. Thus, a significantly greater degradation efficiency is achieved at increased catalyst loading, which also results in an increased number of absorbed photons (Shankar et al., 2001). Nepollian et al. (1999) have studied the photocatalytic degradation of a textile dye in the presence of sunlight using TiO2 and ZnO, concluding that ZnO was more efficient in the removal of colour, while TiO2 revealed a better photodegradation efficiency, as presented in Table 1. In the first case, the phenomenon can be explained by the fact that the size of the ZnO particles is smaller than that of TiO2, facilitating high light absorption over ZnO and much scattering over TiO2. In the second case, the loss of ZnO photocatalytic activity is explained as a result of its photocorrosivity. TiO2/Pt revealed a better efficiency than TiO2 in photodegradation of phenols and chlorophenols (Macoveanu et al., 1997). When immobilised photocatalysts are used on walls of tubes of several millimetres diameter, mass transfer influence may appear (Mathews, 1987; Ollis et al.,1989). Generally, the particle sizes for 100% anatase TiO2 (laser powders) are ranged between 5-20 nm, while for the anatase (70%) and rutile (30%) mixture 191


TiO2 (Degussa P-25 powder), these are ranged between 20-30 nm. For the immobilised photocatalysts, the layer thickness is of 0.2-0.6 µm. Table 1. The effect of initial concentration of the photocatalyst on the photodegradation efficiency. Amount of ZnO/TiO2 (mg/100 mL)

Initial concentration of the dye (mg/L)

200

300

400

500

Photodegradation efficiency (η%) ZnO TiO2 ZnO TiO2

ZnO

TiO2

ZnO

TiO2

100

63

86

52

84

43

67

30

52

200

89

98

86

98

75

83

54

67

300

92

98

90

98

87

96

80

90

400

98

98

94

98

90

98

86

98

500

98

98

98

98

98

98

98

98

Solar irradiation time: 8 h

3.3. Concentration and type of organic pollutant The photocatalytic degradation of the organic compounds in water showed a correlation between the reaction rates and the concentration of reactant. Generally, the photocatalytic degradation of various reactants obeys a pseudofirst order kinetics described by eq. 1, as for example, 4-nitrophenol (San et al., 2002), protocatechuic acid (Poulios et. al, 1991), or a simple LangmuirHinshelwood kinetics, given by eq. 2, such as: 4- nitrotoluene (Vohra and Tanaka, 2002), phenol (Ilisz and Dombi, 1998), r0 = k · c

r0 = k 1

(1)

K.c 1 + K.c

(2)

in which: r0- degradation rate, c- solute concentration, k- pseudo-first rate reaction constant, k1- reaction rate constant showing the tendency of the compound to be converted when adsorbed, K- apparent adsorption constant. The two parameters k1 and K in the Langmuir- Hinshelwood expression are usually interpreted as reflecting the limiting rate at high concentrations of solute, when essentially all the available sites capable of adsorbing the solute are filled. The adsorption constant becomes progressively more important as the solute concentration is decreased (Mathews, 1991a). Thus, a solute recognised as difficult to oxidise at high concentrations may be oxidised comparatively rapidly at low concentrations, because it appears to be strongly adsorbed. In any case, these constants describe the 192


rate of degradation for particular experimental conditions, covering a range of solute concentrations. Generally, the kinetics follows a Langmuir-Hinshelwood mechanism. For dilute solutions (c < 10-3 M ), K· c 5x 10-3 M), K· c >>1 and the reaction rate is maximum. In other cases, such as the photocatalytic degradation and mineralization of chlorobenzoic acids, a kinetics of zero order was found, even at low concentrations, probably due to a strong chemisorption on titania with the saturation of the hydroxylic adsorption sites (Herrmann, 1999). The following classes of refractory organics usually found in wastewaters are destroyed using heterogeneous photocatalytic process: hydrocarbons, halogenated aliphatics or aromatics, phenols, polycyclic aromatics, polyaromatic hydrocarbons (naphtalene, phenantrene), textile dyes (reactive and acid dyes), pesticides and antibiotics. The effect of initial concentration of 4-nitrophenol on the photocatalytic degradation rate was investigated over the concentration range of (3.5-9.5) x10-5 mol/L. The results, presented in Table 2, show that the photodegradation rate increases with the initial concentration of the organic compound increasing, until a limiting value (7.6 x 10-5 mol/L) above which it begins to decrease (San et al., 2002). Table 2. Effect of initial concentration of 4-nitrophenol on the photodegradation rate C0 (10-5 mol/L)

K (10-3 /min.)

r0

3,5

12,69 ± 0,53

0,9930

5,2

7,78 ± 0,30

0,9940

6,8

6,72 ± 0,53

0,9941

7,6

4,27 ± 0,16

0,9943

9,3

3,12 ± 0, 13

0,9931

Degradation on TiO2 of different nitrophenols depends on the positions of the nitro substituents, while the increase of the number of substituents leads to a decrease of the degradation rate (Macoveanu et al., 1997). Studying the photochemical degradation of different chlorophenols in the presence of TiO2, Pandiyan et al. (2002) concluded that, depending upon the degree of dechlorination, the decomposition rate decreases in the order: 4-chlorophenol >2,4-dichlorophenol >2,4,6-trichlorophenol 3.4. Type of charge-trapping species The most common electron acceptor in photocatalytic processes is molecular oxygen, which prevents the electron-hole recombination by reacting with the excess electrons from the conduction band.

193


Literature reports indicate an enhancement of the photocatalytic rate upon addition of different compounds, as electron acceptors, in order to inhibit efficiently the recombination process: 9 H2O2 (Hofstadler and Bauer, 1994); 9 K2S2O8 (Shankar et al., 2001); 9 KBr (Al-Ekabi et al., 1993); 9 AgNO3 (Ilizs et al., 1999; Ilisz and Dombi, 1999) 9 FeCl3 (Sakthivel et al., 2000); 9 potassium peroxymonosulphate (oxone) (Al-Ekabi et al., 1993). 9 Fenton’s reagent H2O2 (Malato et al., 2002). For industrial wastewaters, where the mineralization of the organic pollutants is of great interest, the use of H2O2 or oxone may be justified. 3.5. Temperature The optimum temperature for heterogeneous photocatalytic process is generally ranged between 20-80°C. At low temperatures the adsorption of the final reaction products is favoured, desorption of which tends to be the rate-limiting step. When temperature increases above 80°C, the exothermic adsorption of reactants is disfavoured and this tends to become the rate-limiting step (Blanco and Malato, 2000). 3.6. Light intensity The rate of degradation of organic compounds in aqueous solution is dependent on the light intensity. At illumination levels appreciably above 1 sun equivalent, the reaction rate increases with the square root of intensity. At weaker levels of illumination, the reaction rate is of first order in intensity (Ollis et al., 1991). Because absorption of photons is first order in intensity, it results that at low intensities, the quantum efficiency is a constant, and at higher intensity levels, it decreases proportional to I-0.5, indicating a decrease in efficiency for intense lamps or concentrated solar sources. Investigating the photocatalytic degradation of 4-nitrotoluene in aqueous suspensions, on Pt/TiO2 and TiO2, Vohra and Tanaka (2002) found that light intensity could be a factor in determining Pt loading effect on the degradation rate, while Pt-TiO2 is effective only when used at a high intensity light source. 3.7. pH The optimum pH value, at which the efficiency of the degradation of the organic compounds in water is maximum, depends on the initial composition and characteristics of the wastewater. A detailed analysis of the best pH conditions should consider also the intermediate compounds that are formed. Nishida and Ohgaki /1994/ showed that complete investigations concerning the following aspects: - the electric and ionic profiles of the catalyst and the surface; 194


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