Advanced Oxidation Processes for Water and Wastewater Treatment
370
Advanced Oxidation Processes for Water and Wastewater Treatment
370Hardcover
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Product Details
| ISBN-13: | 9781843390176 |
|---|---|
| Publisher: | IWA Publishing |
| Publication date: | 03/01/2004 |
| Pages: | 370 |
| Product dimensions: | 6.12(w) x 9.25(h) x 0.75(d) |
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CHAPTER 1
Introduction
Simon A. Parsons and Mike Williams
1.1 INTRODUCTION
Advances in chemical water and wastewater treatment have led to a range of processes termed advanced oxidation processes (AOPs) being developed. The processes have shown great potential in treating pollutants of low or high concentrations and have found applications as diverse as groundwater treatment, municipal wastewater sludge destruction and volatile organic compounds (VOCs) treatment. AOPs have been developed and studied over the past 30 years and the scientific literature surrounding their development and application is extensive (Figures 1.1 and 1.2). Suty et al. (2003) reviewed the literature from 1975 until 2000 and showed the diversity of AOPs reported in the literature and how the number of papers had significantly increased decade after decade. This trend was seen for all AOPs and those papers referring directly to water treatment.
This book aims to give the reader an overview of the range of AOPs available and cover both the underpinning science behind the processes and where its current or future applications may lie. In this chapter we will look at a background to oxidation and where opportunities for AOPs exist.
1.1.1 Oxidation
The ultimate aim of the oxidation of pollutants in water is to 'mineralise', that is to convert the constituents of an organic pollutant into simple, relatively harmless and inorganic molecules:
• carbon to carbon dioxide,
• hydrogen to water,
• phosphorous to phosphates or phosphoric acids,
• sulphur to sulphates,
• nitrogen to nitrates,
• halogens to halogen acids.
Oxidation can be generalised as follows:
• Inorganic compounds. The removal of electrons to produce a higher oxidation state Fe2+ going to Fe3+.
• Organic compounds. The combination of carbonaceous material with oxygen to produce a more heavily oxygenated compound; for instance, in wine, ethanol to acetic acid. 'Forced' oxidation of hydrocarbons for water purification purposes should, ultimately, form carbon dioxide and water.
Each oxidation is accompanied by a corresponding reduction so that the charge balance is preserved. The driving force for oxidation is the stability of the ultimate products. Heats of formation (in kJ mol-1) are:
Carbon dioxide (g) -393
In contrast, the heats of formation (in kJ mol-1) of organic molecules are typically:
Ethylamine (C2H7N) -74
So conversion of ethylamine to carbon dioxide (2 per molecule), water (3.5 per molecule) and nitrate (1 per molecule) is thermodynamically favourable, because nearly 2000 kJ mol-1 of energy is released.
Strong oxidising agents are formed from the electronegative elements of the top right-hand corner of the periodic table (F, Cl, Br, O, S). The relative strength of oxidants is shown in Table 1.1. Here conventional oxidants such as chlorine, permanganate and ozone are compared to the hydroxyl radical. It is this radical that gives many of the AOPs covered in this book their power.
1.1.2 Free radicals
Many strong oxidants are 'free radicals', of which the hydroxyl radical HO• is the most powerful oxidising species after fluorine. The aim of most AOPs is to produce the hydroxyl radical in water. For example, the hydroxyl radical is able to oxidise a wide range of organic compounds significantly (109) faster than ozone (Table 1.2). A free radical is not an ionic species but is formed from an equal cleavage of a two-electron bond.
HO:OH -> HO• + •OH photolysis
Each HO• is uncharged and two HO• will recombine to form HOOH (also uncharged). The symbol • indicates the radical centre and represents a single unpaired electron.
Once a free radical reaction has been initiated by photolysis, ozone, hydrogen peroxide, heat, etc. then a series of simple reactions will ensue. The complexity of the chemistry of such systems is due to the very large number of reactions that are possible. With such complicated mechanisms it is very difficult to predict all of the products of an oxidation.
The rate of oxidation will depend on radical concentration, oxygen concentration and pollutant concentration. Many factors can affect the radical concentration, such as pH, temperature, the presence of ions, the type of pollutant, as well as the presence of scavengers such as the bicarbonate ion, which is an effective radical trap.
1.2 AOPS FOR WATER AND WASTEWATER TREATMENT
Developments in chemical treatment have made a range of AOPs suitable for water and wastewater applications. Table 1.3 lists those AOPs that have been developed and whilst the list is not comprehensive it does highlights the range and variety of processes developed or under development that could have applications in water and wastewater treatment. A number of these processes are commercially available and in some instances widely used. Ultraviolet (UV) for instance has over 3000 applications in Europe as a disinfection processes and is used in the US for treating groundwater pollutants such as methyl tertiary-butyl ether (MTBE) and N-nitrosodimethylamine (NDMA). Process such as the chemical combinations of H2O2, O3 and UV, Fenton's reagent, wet air oxidation, supercritical water oxidation and electron beam have all been used at full scale. Other processes such as photocatalysis and ultrasound have been evaluated at pilot scale but many of the process are being developed at the laboratory bench. While the list includes individual processes much research has been undertaken using a combination of the processes, such as UV, ultrasound and ozone together, which offer significant kinetic and performance benefits over each of the processes alone.
The literature on applications is too varied to cover all topics so here those range of applications covered at the recent AOP3 conference have been reviewed (Vogelpohl, 2003). Firstly generic areas have been identified and it can been seen that AOPs have found applications in all areas of water and wastewater treatment (Table 1.4). AOPs are available as an alternative processes option for removing pollutants from liquid, solid and gaseous streams. Examples of specific contaminants and wastes treated by AOPs covered at the conference are shown in Table 1.5. While some of the applications are more viable than others the overall conclusion from the literature is that as processes develop and the need for greater, faster removal of pollutants is required then AOPs will find their place in water and wastewater treatment flowsheets.
CHAPTER 2
UV photolysis: background
Mihaela I. Stefan
2.1 INTRODUCTION
Over the past two decades, photochemical technologies have become more attractive than the conventional treatment technologies, such as air stripping, adsorption on the activated carbon and biodegradation, for pollutant removal from the contaminated environment. Photochemical technologies are simple and clean, cost-effective in many applications, and, often, give the end-user the dual benefit of both environmental contaminant treatment and disinfection. Ultraviolet (UV)-driven advanced oxidation processes (AOPs) are primarily based on the generation of powerful oxidising species, such as the hydroxyl radical (•OH), through the direct photolysis of hydrogen peroxide (H2O2), or, through photo-induced processes as in the photo-Fenton type reactions or photocatalysis. In UV direct photolysis, the contaminant to be destroyed must absorb the incident radiation and undergo degradation starting from its excited state. The typically low concentrations of pollutants in the contaminated water and, in general, the low efficiency of photodissociation resulting from the light absorption event, limit the industrial applications of the UV photolysis process as compared to the hydroxyl radical-driven technologies, where the light absorption by the target pollutant is not absolutely required. However, there are many cases when the target pollutants are strong absorbers of the UV radiation, and therefore, their UV photolysis may become a significant component during the treatment by any UV-driven AOP. Sometimes, a contaminant can be treated by both direct photolysis and •OH radical-induced processes. In such cases, the optimisation of the treatment process is driven by economic considerations.
There is growing interest in the application of UV light to environmental contaminant treatment, interest particularly driven by the overall public concern about the potential carcinogenic and toxic effects of contaminants in water, and by the progressively more restrictive requirements regarding the admissible levels of such contaminants in aquatic environment imposed by the regulatory agencies. As a result of this increased interest, numerous research studies have been undertaken on the UV light-induced degradation of various classes of organic and inorganic compounds of environmental concern, and a rich published literature has become available in this domain.
In this chapter, we shall briefly discuss the principles of UV direct photolysis, as well as the characteristics of the UV lamps used in UV light-based water remediation, and some process efficiency parameters. In the chapter, we have followed IUPAC recommendations, 1996 (http://www.unibas.ch/epa/) with regard to terms and definitions used in photochemistry.
2.2 FUNDAMENTALS OF UV PHOTOLYSIS
2.2.1 Electromagnetic radiation and light absorption by chemical species
In 1666, Isaac Newton was first to observe the diffraction of a white light beam when passing through a prism, opening the investigation of the properties of the light components. Almost 150 years later (1800–1801), Sir William Herschel and J.W. Ritter discovered that there was radiant energy beyond the two ends of the visible light, which were identified as infrared and UV regions, and in 1804, Thomas Young showed that the invisible chemically active radiations beyond the violet end of spectrum were subject to the laws of interference. Such an observation, along with further extended investigations, indicated that the visible, UV and infrared chemical effects were all properties of the same electromagnetic radiation, differing from each other only in frequency.
2.2.1.1 Electromagnetic radiation and bond dissociation energies
Light has both wave and particle properties (Calvert and Pitts, 1966; Parker, 1968). Light is a form of electromagnetic radiation, which explains its wave properties, such as reflection, refraction, diffraction, interference and polarisation. The relationship between the wavelength λ (m, usually given in nm in photochemical application) and the frequency v (s-1) of radiation is
λ = c/v (2.1)
where c = 3.0 x 108ms-1 is the velocity of light in a vacuum. The particle properties of light involve the absorption/emission of light, and can explain the photoelectric effect and photochemical reactions, based on Planck's quantum theory of radiation. Light is absorbed or emitted in discrete units of energy E, called quanta or photons (h), which are related to the frequency of radiation through Equation (2.2):
E = hv = hc/λ = hcv (2.2)
where h = 6.6256 x 10-34 J s is a proportionality constant (Planck's constant), and v – is wave number (m-1, usually given in cm-1). Equation (2.2) is known as Planck's Law of Radiation and expresses the wave/particle duality of light, that is, light comes in discrete packets of energy called photons or quanta, which have a frequency, and thus, wavelength, associated with them. One mole (6.02 x 1023) of photons (quanta) is called an einstein, therefore, the energy of 1 einstein of wavelength λ (nm) is
E = 6.02 x 1023 hc/λ = 1.1966 x 105/λ kJ einstein-1 (2.3)
UV radiation is usually defined as the electromagnetic radiation of wavelength between 4 and 400 nm (Koller, 1965) [according to other sources, between 10 and 400 nm (CRC Handbook, 1991)], the spectral domain that covers the gap between the X-ray and visible regions. Figure 2.1 shows the electromagnetic spectrum from 100 to 1000 nm.
The UV spectral range of interest for the UV photolysis applications in water is the UVC [200–280 nm (Phillips, 1983)], where both the pollutants and the water constituents (dissolved organic and inorganic compounds) absorb the radiation. Some authors consider the UVB within 280–320 nm range (Sonnemann, 1992), others define it from 290 to 320 nm (Moore, 1996). The vacuum UV (VUV) region within the 100–200 nm range is of a particular interest for AOPs, since these radiations are absorbed by water, according to reaction (2.4), thereby generating highly reactive species that further induce oxidative degradation of the dissolved organic pollutants (Adam and Oppenländer, 1986; Jakob et al., 1993; Oppenländer, 2002; Oppenländer and Gliese, 2000).
H2O + hv(λ(<190nm) -> H• + •OH (2.4)
Organic compounds also absorb in the VUV spectral range, but when dissolved in aqueous solution, water is the main absorber, as it is present at a concentration of about 55.5 M, which is typically a million times or more the concentration of any contaminant present. VUV photolysis has commercial applications, particularly in the total organic carbon removal in the ultrapure water.
It is interesting to examine the energies carried by UV radiations, and to compare them with the energies required for chemical bond dissociation. Table 2.1 lists some examples of single bond dissociation energy ΔE° (kJ mol-1) and the corresponding 'threshold' wavelength λD (nm), defined as the maximum wavelength for which the photon energy matches the bond energy required to result in a homolytic bond cleavage.
Theoretically, any radiation of λ < λD, therefore, carrying energies [MATHEMATICAL EXPRESSION OMITTED] can break the corresponding chemical bond, as a result of light absorption. However, whether or not such a process occurs is determined by two factors: the probability of the light absorption event, which depends on the optical properties of the compound that are quantified in its absorption spectrum, and the probability that the excited state reached through the light absorption process proceeds to a chemical reaction, which is expressed as the quantum yield.
2.2.1.2 Interaction of light with molecules in solution: absorption, electronic spectra and water background impact
The first law of photochemistry (Grotthus–Draper law) states that only the light that is absorbed by a molecule can be effective in producing a photochemical change in that molecule (Calvert and Pitts, 1966). The quantitative aspects of light absorption in the solution by a particular constituent are expressed by the Beer–Lambert law, which states that the fraction of light absorbed by the system does not depend on the incident spectral radiant power Poλ (Wm-1, commonly used unit Wnm-1), and the amount of light absorbed is proportional to the number of the constituent molecules absorbing the radiation. The law expression is given in Equation (2.5):
[MATHEMATICAL EXPRESSION OMITTED] (2.5)
where Pλ and Poλ are the transmitted and the incident spectral radiant power, respectively, C(M) is the concentration of the constituent of interest in the irradiated solution, l(m, common unit cm) is the irradiation pathlength, and αλ and ελ are the decadic attenuation coefficient of the medium (if water, commonly referred as the 'water background') (m-1, commonly used unit cm-1) and the decadic molar absorption coefficient (m2 mol-1, commonly used unit M-1cm-1) of the constituent in that medium, respectively. The sum αλ + ελC is called absorption coefficient a (m-1, frequently cm-1) of medium, accounting for both constituent and water background absorption per unit of irradiation pathlength.
(Continues…)
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Table of Contents
Preface, ix,
List of contributors, x,
1. Introduction Simon A. Parsons and Mike Williams, 1,
2. UV photolysis: background Mihaela I. Stefan, 7,
3. UV light-based applications Mihaela I. Stefan and Christian T. Williamson, 49,
4. UV/H2O2 processes Tuula A. Tuhkanen, 86,
5. Fenton processes S. Wadley and T.D. Waite, 111,
6. Semiconductor photocatalysis Andrew Mills and Soo-Keun Lee, 137,
7. Photoelectrocatalysis processes T.A. Egerton and P.A. Christensen, 167,
8. Ultrasound processes T.J. Mason and C. Pétrier, 185,
9. Radiation processes W.J. Cooper, P. Gehringer, A.K. Pikaev, C.N. Kurucz and B.J. Mincher, 209,
10. Wet air oxidation processes L. Patria, C. Maugans, C. Ellis, M. Belkhodja, D. Cretenot, F. Luck and B. Copa, 247,
11. AOPs for VOCs and odour treatment K. Van Craeynest, H. Van Langenhove and R.M. Stuetz, 275,
12. Advanced oxidation of textile industry dyes I. Arslan-Alaton, 302,
13. Water treatment applications Simon A. Parsons and Anthony Byrne, 329,
Index, 347,