Chemical Degradation Methods for Wastes and Pollutants

Environmental and Industrial Applications

Matthew A. Tarr

• Publisher: CRC Press
• Year: 2003
• Print ISBN:
  9780824743079
• eBook ISBN:
  9781135527327
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Environmental Science

Chemical Degradation Methods for Wastes and Pollutants focuses on established and emerging chemical procedures for the management of pollutants in industrial wastewater and the environment. This reference offers an in-depth explanation of the degradation process, mechanisms, and control factors affecting each method, as well as issues crucial to th

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Ozone-UV Radiation-Hydrogen Peroxide Oxidation Technologies

Fernando J.Beltrán
Universidad de Extremadura, Badajoz, Spain

I. INTRODUCTION

Processes involving the use of ozone, UV radiation, and hydrogen peroxide, characterized by the generation of short-lived chemical species of high oxidation power, mainly the hydroxyl radical, are classified as advanced oxidation technologies (AOTs). Possibly, the term may be attributed to Glaze et al. [¹], who pointed out that hydroxyl radical oxidation is the common feature of these processes. The importance of these processes is due to the high reactivity and redox potential of this free radical that reacts nonselectively with organic matter present in water. In practical cases, these processes present a high degree of flexibility because they can be used individually or in combination depending on the problem to be solved. For instance, for phenols or substances with high UV molar absorption coefficients, ozone or UV radiation can be used alone, respectively, without the need of any additional reagent, such as hydrogen peroxide. Another advantage of these AOTs is that they may be applied under mild experimental conditions (atmospheric ambient pressure and room temperature).

The need for the application of these AOTs is based on different social, industrial, environmental, and even academic reasons. The increasing awareness of society for the quality of drinking water has led to the establishment of maximum contaminant levels of priority pollutants in drinking water [1,2]. The preparation of ultrapure water is needed for some industrial activities such as those derived from the pharmaceutical and electronic processes. Also, the release of wastewater into natural environmental reservoirs is another concern; recycling of wastewater is already in progress in countries where the lack of water is a national problem [4]. Finally, academic interest exists because the study of these AOTs allows testing the application of some physical and chemical laws and engineering theories (mass, energy, and/or radiation conservation equations, kinetic modeling, absorption theories, etc.) to the environmental problems of water treatment.

Because of the aforementioned reasons, the number of research works and applications based on these AOTs in the treatment of water has increased considerably during the past 20 years. Numerous publications that refer to different aspects of these processes have so far been published in journals such as Ozone Science and Engineering, Water Research, Ozone News, IUVA News, and the Journal of Advanced Oxidation Technologies. In addition, several books on the subject are available, such as that edited by Langlais et al. [5] on applications and engineering aspects of ozone in water treatment and that of Doré [6] on the chemistry of oxidants. Reviews are also abundant, including those of Camel and Vermont [7] on ozone involving oxidation processes, Reynolds et al. [8] and Chiron et al. [9] on the oxidation of pesticides, Legrini et al. [10] on photochemical processes, Yue [11] on kinetic modeling of photooxidation reactors, and Scott and Ollis [12] on the integration of chemical and biological oxidation processes for wastewater treatment.

In this chapter, AOTs based on ozone, UV radiation, and hydrogen peroxide are presented with special emphasis on their fundamental and application aspects. Related literature of research studies and applications, especially those appearing in the last decade, are also listed, and specific examples of laboratory and scale-up studies are described in separate sections.

II. BACKGROUND AND FUNDAMENTALS OF O₃/UV/H₂O₂ PROCESSES

O₃/UV/H₂O₂ processes are characterized by the application of a chemical oxidant (ozone and/or hydrogen peroxide) and/or UV radiation. Individual description of properties and reactivities of these oxidation technologies is necessary to understand their synergism when used in combination for the treatment of specific water pollutants or wastewaters. However, because combined processes (O3/H2O2, UV/H2O2, or O3/UV) are usually recommended in real situations, a general description of the processes and fundamentals of the individual and integrated O3/UV/H2O2 technologies is also presented in the following sections.

A. General Description

Ozone-or UV-radiation-based technologies (O₃/UV/H₂O₂) are chemical oxidation processes applied to water treatment for the degradation of individual pollutants or the reduction of the organic load (chemical oxygen demand, COD) and improved biodegradability of wastewaters. In addition, ozone and UV radiation alone can be used for disinfection purposes; in fact, this was their first application in water treatment [13,14]. In addition, these AOTs, particularly ozonation, can be used to enhance the efficiency of other processes such as Fe–Mn removal [15,16], flocculation-coagulation-sedimentation [17,18], biological oxidation [¹²], or biological degradation of organic canbon in granular activated carbon [19-21].

O₃/UV/H₂O₂ AOTs are suitable for the treatment of water containing organic pollutants in concentrations not higher than some tens of milligrams per liter. However, these technologies can also be used to treat concentrated solutions. In addition to concentration, factors such as molecular structure of pollutant, aqueous organic matrix, pH, etc. are variables that affect the efficiency and applicability of O₃/UV/H₂O₂ AOTs for practical application. For wastewater treatment, O3/UV/H2O2 AOTs are used in combination with biological oxidation processes because of the enhancement achieved on the biological oxygen demand (BOD). In fact, another feature of O₃/UV/ H₂O₂ AOTs is that they steadily transform high molecular weight substances into more oxygenated lower molecular weight substances, which involves an increase of BOD [22,23]. Examples of studies on wastewater treatment that give a general view of the application of O₃/UV/H₂O₂ AOTs are those of Rice and Browning [24] and, more recently, by Rice [25] on the use of ozonation, or Zhou and Smith [26], Rivera et al. [27], and Kos and Perkowski [28] for combined oxidation involving UV radiation.

O3/UV/H2O2 AOTs, together with other processes treated in different chapters (such as Fenton oxidation), can be named ambient (temperature and pressure), advanced oxidation technologies, in contrast with other AOTs such as hydrothermal oxidation processes that require pressures and temperatures above 1 MPa and 150°C, respectively, and which are more suitable for the treatment of concentrated wastewaters. It is evident that appropriate ranges of concentrations for the different oxidation technologies cannot be exactly established but some recommended values have been reported [29]. Fig. 1 shows some possible recommended ranges of concentrations for these types of AOTs.

O₃/UV/H₂O₂ AOTs generally involve two oxidation/photolysis routes to remove foreign matter present in water. Thus, ozone, hydrogen peroxide, and/or UV radiation can react individually or photolyze directly the organic in water. However, when used in combination, they can degrade pollutants by oxidation through hydroxyl free radicals generated in situ. Hydroxyl radicals have the largest standard redox potential except for fluorine (see Table 1).

Figure 1 Oxidation process advisable according to COD of water. (WAO, wet air oxidation. SCWAO, supercritical wet air oxidation).

In addition, they react very rapidly with almost all types of organic substances through reactions whose rate constants vary from 107 to 1010 M-1 s-1 [30]. Table 2 gives a list of rate constant values of these reactions.

Because of the high and similar values of the rate constants, it is said that these free radicals react nonselectively with the organic matter present in water, although, as deduced from the above range of values, there are compounds that react with them almost three orders of magnitude faster than others. Among the most common water pollutants, phenols and some pesticides are substances that react rapidly with hydroxyl radicals, whereas some organochlorine compounds are less reactive.

Another feature of these AOTs is that they are destructive types of water pollution removal processes because they eliminate compounds rather than transfer them to another medium. Thus, carbon adsorption or stripping transfers pollutants from one phase (water) to another phase such as a solid phase (carbon) or a gas phase (air). In the latter case, purification of air is required so that an additional step (i.e., carbon adsorption) is also needed, which implies higher processing costs.

Table 1 Standard Redox Potential of Some Oxidant Species

Table 2 Rate Constants of the Reaction Between the Hydroxyl Radical and Organic Compounds in Water

At first sight, however, the main drawback of O₃/UV/H₂O₂ AOTs is the high processing cost, mainly because both ozone and UV radiation require a continuous feed of energy for process maintenance, as well as high capital costs for ozone generators and photoreactors. However, the development of improved ozonators and UV lamp technologies has made these processes more amenable in pra...

Table of contents

• COVER PAGE
• TITLE PAGE
• COPYRIGHT PAGE
• ENVIRONMENTAL SCIENCE AND POLLUTION CONTROL SERIES
• PREFACE
• CONTRIBUTORS
• 1: OZONE-UV RADIATION-HYDROGEN PEROXIDE OXIDATION TECHNOLOGIES
• 2: PHOTOCATALYTIC DEGRADATION OF POLLUTANTS IN WATER AND AIR: BASIC CONCEPTS AND APPLICATIONS
• 3: SUPERCRITICAL WATER OXIDATION TECHNOLOGY
• 4: FENTON AND MODIFIED FENTON METHODS FOR POLLUTANT DEGRADATION
• 5: SONOCHEMICAL DEGRADATION OF POLLUTANTS
• 6: ELECTROCHEMICAL METHODS FOR DEGRADATION OF ORGANIC POLLUTANTS IN AQUEOUS MEDIA
• 7: THE ELECTRON BEAM PROCESS FOR THE RADIOLYTIC DEGRADATION OF POLLUTANTS
• 8: SOLVATED ELECTRON REDUCTIONS: A VERSATILE ALTERNATIVE FOR WASTE REMEDIATION
• 9: PERMEABLE REACTIVE BARRIERS OF IRON AND OTHER ZERO-VALENT METALS
• 10: ENZYMATIC TREATMENT OF WATERS AND WASTES