Volatile organic compounds


1.1 Background of the Study

Environmental pollution and destruction on a global scale urgently calls for totally new environmentally friendly, inexpensive, clean chemical technology and processes. The civilian, commercial and defense sectors of most advanced industrialized and developing nations are faced with tremendous environmental problems related to the remediation of hazardous wastes, contaminated ground water and the control of toxic air contaminants. Natural fresh water is one of the most valuable resources in our planet. Nowadays quality of natural water is strongly affected by industry wastewaters including many toxic contaminants from human activities. Volatile organic compounds (VOCs) are hazardous substances and are known to be present in wastewater.

Printing industry also generates many VOCs and exhausts them to environment. In printing processes, many VOCs are included in the ingredients of various printing inks that are known to have carcinogenic, mutagenic and teratogenic effects. Different printing processes use different kinds of printing inks. The typical VOCs in the Printing industry are Toluene, Methyl Ethyl Ketone (MEK), Glycol Ethers, Xylene (mixed isomers), Tetrachloroethylene, Methyl Isobutyl Ketone, Methanol, 1,1,1-Trichloroethane, Dichloromethane, Ethylene Glycol and Chlorobenzene (www.epa.gov/osw/inforesources/pubs/infocus/printing.pdf.,VOCs report from Australian Printing Association.,1999). In the developed countries as United State, Canada, Australia governments and organizations consider printing environment strictly. There are many rules and regulations to minimize the amount of VOCs emitted to the environment in printing industry. However, in the developing countries like the Philippines and Vietnam surveys from printing industry show the harmful impact of this industry to environment and are summarized in Chapter 2. The most concerned problem is that wastewater from printing plants is rarely treated before flowing to the drainage system. Therefore, there is a great demand to develop environmental-friendly and cost effective methods for VOCs destruction applied in printing industry.

Methods for wastewater treatment can be divided into three main methods such as physical, biological and chemical processes. Each technology has disadvantages and limitations in practical applications. Assessment of these environment processing technologies will be also presented in Chapter 2.

Photocatalytic or photoactivated reactions are applicable to a wide range of valuable industrial processes such as air and water remediation, organic synthesis, photo destruction of toxic compounds, and purification of drinking water (Carp. O et al., 2004). The anatase form of TiO2 has been the most extensively employed in photocatalytic reactions because of its high activity and chemical stability. (Hoffmann et al., 1995; Mills, A. and S. LeHunte, 1997).

As a durable photocatalyst with good photocatalytic properties, titanium dioxide (titania) has been extensively studied for its preparation and application in the photocatalytic process for last twenty years [Hoffmann et al., 1995]. One of the major advantages of titania based photocatalytic oxidation technology is that the degradation of organic pollutants in air and water can be achieved at ambient temperature and pressure. In addition to its relatively high activity and chemical stability, titania is non toxic and inexpensive. However, the relatively low quantum efficiencies and the requirement of near ultraviolet light energy for activation are currently the two major limitations for its large-scale application (Mills, A. and S. LeHunte, 1997).

The activities of titanium dioxide as a photocatalyst depend on its physical and chemical properties and characteristic, such as its morphology, crystal structure (anatase or rutile), primary particle size and surface area of the catalyst. All above properties and characteristics can vary significantly with the catalyst synthesis methods. Several researchers (Ohtani, B.et al.,1997) found out that crystal phase and crystallinity are the key factors with respect to the photoactivity of titania, where photoactivity increases with crystallinity. Anatase phase has higher photocatalytic activity than the rutile phase does in most of the studies (Hoffmann et al., 1995). The reason for the lower activity could be that the electron-hole recombination in rutile phase is faster than in the anatase phase.

Titania, as a catalyst, is normally used in the form of powders or slurry. Generally speaking, powders and slurry have relatively high surface area and high reactivity, plus they are relatively easy to produce. As we know, an increase in surface area could increase the apparent photocatalytic activity of a catalyst. Either increasing the porous structure or decreasing the particle size can usually increases in surface area. Therefore, nanosized and porous structured catalyst have been a long term study goal for researchers in this area.

However, as the particle size of the catalyst decreases, the density of the electrons and holes on the surface could also increase and then the chance of the electron-hole recombination at the surface of the particles increases. Therefore, decreasing the particle size increases the available surface for reaction, but also increases the rate of electron-hole recombination at the surface (Carp. O et al., 2004). Those two effects compete each other and result in an optimum particle size.

1.2 Statement of the Problem

Photocatalysis is still in the developmental stage for VOCs removal from waste water. In the past two decades, many investigations have been conducted in photocatalytic oxidation of VOCs and several types of photocatalyst systems have been developed (Blake, 2001). Photocatalysis has some following limitations as: incomplete mineralization of some aromatics and halothanes, generation of relatively stable intermediate(s) (such as phosgene from trichloroethylene), which are more toxic than the initial substrate, not useful in breaking down large volume of spoilage, but capable of destroying accumulations, deactivation due to by-products.

There are two following directions needed to develop photocatalyst as Titania. Researchers optimize the synthesis of catalyst, in order to obtain catalysts with a defined crystal structure, smaller particle sizes ( Nano Titania ), and the ability to use various support materials (Titania composites). Another direction conducted is design and development a new form photocatalys by doping Titania with other metals, with high selectivity, which can operate effectively under visible and/or solar irradiation (Gianluca et al, 2004).

In the first direction, studies show the reactivity of Titania improved by loading on some supports. The supporting material should be transparent or at least allow some UV radiation to pass through it and be chemically inert or non-reactive to the pollutant molecules, its intermediates and the surrounding aqueous system. The supporting material should sufficiently bond via either physically or chemically to the TiO2 without reducing the Titania's reactivity. The supporting material should have a high surface area and a strong adsorption affinity towards the pollutants (organic or inorganic compounds) to be degraded. This criteria reduce or eliminate the intermediates produced during the photocatalytic degradation while further increasing mass transfer rates and processes for an efficient photodegradation. The supporting material should allow for fast and easy photocatalyst recovery and re-use with or without regeneration (Gianluca et al, 2004).

Based on these criteria activated carbon is one ideal material use as support for Titania. Many researches were conducted to synthesized Nano Titania-Activated carbon composites (Gianluca et al, 2004). These nano composites were subjected to test their performance with representative pollutans in wastewater and results were positively reported. There are a few studies of VOCs in wastewater due to their volatility and solubility in water. Chlorobenzene is used as ink solvent in printing industry. Its properties are describled in Chapter 3. VOCs treatment in printing wastewater by photocatalysis technology is still a new research topic and potential to be applied in printing industry. In this research, the performance of photocatalysis will be also enhanced by coupling with Activated carbon to make Nano photocatalysis composite.

1.3 Objectives of the Study

The aim of this research is to destroy Clorobenzene in simulated wastewater of printing industry by nano Titania (nanoTiO2-AC) photocatalysts prepared by sol-gel method under UV irradiation.

The following are the specific objectives of the study:

(1) To synthesize a composite catalyst ( Titania with Activated Carbon) by the sol-gel method for photocatalytic degradation of Chlorobenzene in water.

(2) To characterize the catalysts synthesized by BET, TEM, XRD, TGA, and FTIR methods.

(3) To determine adsorptive properties of synthesized Titania-AC composite with Chlorobenzene by TPD method.

(4) To compare the photocatalytic activity of the catalysts synthesized and commercial Titania as Degussa 25 with Chlorobenzene in water.

(5) To optimize operating parameters in photodegradation process such as: initial concentration of Chlorobenzene, Titania loading, pH value, UV intensity.

(6) To study kinetics of photocatalytic reaction of Chlorobenzene

1.4 Scope and Limitations

Chlorobenzene is popular VOCs in printing wastewater as well as one toxic in Priority toxic list of EPA and will be the object in this reseach. Through the collaboration organized by the AUN/SEED-Net, the research for photocatalysis of VOCs is intended to include three main phases. Phase one includes preparation and characterization of Nano photocatalysist composite. The photocatalyst was synthesized using the sol-gel method with Titanium (IV) Isopropoxide as the precursor material. Phase two consists of optimization the operating parameters in photocatalyic degradation process. Finally phase three will focus on kinetics study and mechanism to photocatalytic degradation to chlorobenzene. Data gathered will be valuable information for assumed process of VOCs destruction could be applied in printing industry in Philippines, Vietnam and Thailand.

The research is limited to only one typical VOC and it is Chlorobenzene. Also activated carbon from Burapha university will be used to synthesize Nano Titania-AC composite. This research continued laboratory-scale stage but will not include pilot plan investigation and commercialization.

1.5 Significance of the Study

Pollution in wastewater from printing industry is considered to be major environmental problem that needs to be treated. The effluent containing contaminants as VOCs in printing industries has various compositions depending on what kind of ink used. Release of these effluents without treatment can contaminate the soil and groundwater.

Advanced oxidation processes (AOPs) are proved to be potential methods for treatment industry wastewaters and thus these methods is applied globally. Among these methods, Photocatalysis using UV-TiO2 system is one of the most efficient for substantial treatment of these wastewaters.

In many environment Acts, many regulations for industry wastewater are approved. VOCs and their concentrations is limited therefore treatment of the wastewater streams containing VOCs by TiO2-UV system would be a suitable method for the printing industry to meet environment demands. With advantages of photocatalysis, printing industry would control its pollutions with affordable expenses.

The research will bring benefits for environment scientists and printing enterprises because it is one of initial research of printing environment. The next research could be extended to fulfill limited aspects of this research.

If the study is found to be effective, this research will contribute to the development of to remove the water pollutants as hazardous volatile organic compounds. Otherwise, the study can be used as good basis for successive research steps as reactor design or kinetics modeling. In addition, data acquired from all experimentations will be use for further research studies in pilot plant testing section.

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