Computational Chemistry
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Methods Overview
Computational chemistry encompasses a variety of mathematical methods that fall into two broad categories:
Molecular mechanics
applies the laws of classical physics to the atoms in a molecule without explicit consideration of electrons.
Quantum mechanics
relies on the Schrödinger equation to describe a molecule with explicit treatment of electronic structure.
Quantum mechanical methods can be subdivided into two classes: ab initio and semiempirical.
Chem & Bio 3D 11.0 provides the following methods:
|
Method |
|
|
|
molecular mechanics |
MM2 MM3, MM3-protein AMBER,UFF, Dreiding |
Chem3D, Tinker Tinker Gaussian |
|
semi-empirical |
Extended Hückel other semi-empirical methods (AM1, MINDO/3, PM3, etc.) |
Chem3D, MOPAC, Gaussian MOPAC, Gaussian |
|
Ab initio |
RHF, UHF, MP2, etc. |
Gaussian, GAMESS |
• Molecular mechanical methods: MM2 (directly). MM3 and MM3-protein through the Chem3D Tinker interface.
• Semiempirical Extended Hückel, MINDO/3, MNDO, MNDO-d, AM1 and PM3 methods through Chem3D and CS MOPAC or Gaussian.
• Ab initio methods through the Chem3D Gaussian or GAMESS interface.
Using Computational Methods
Computational methods calculate the potential energy surfaces (PES) of molecules. The PES is the embodiment of the forces of interaction among atoms in a molecule. From the PES, structural and chemical information about a molecule can be derived. The methods differ in the way the surface is calculated and in the molecular properties derived from the energy surface.
The methods perform the following basic calculations:
Single point energy calculation
The energy of a given geometry of the atoms in a model, which is the value of the PES at that point.
Geometry optimization
A systematic modification of the atomic coordinates of a model resulting in a geometry where the forces on each atom in the structure is zero. A 3dimensional arrangement of atoms in the model representing a local energy minimum (a stable molecular geometry to be found without crossing a conformational energy barrier).
Property calculation
Predicts certain physical and chemical properties, such as charge, dipole moment, and heat of formation.
Computational methods can perform more specialized functions, such as conformational searches and molecular dynamics simulations.
Choosing the Best Method
Not all types of calculations are possible for all methods and no one method is best for all purposes. For any given application, each method poses advantages and disadvantages. The choice of method depend on a number of factors, including:
• The nature and size of the molecule
• The type of information sought
• The availability of applicable experimentally determined parameters (as required by some methods)
• Computer resources
The three most important of the these criteria are:
Model size
The size of a model can be a limiting factor for a particular method. The limiting number of atoms in a molecule increases by approximately one order of magnitude between method classes from ab initio to molecular mechanics. Ab initio is limited to tens of atoms, semiempirical to hundreds, and molecular mechanics to thousands.
Parameter Availability
Some methods depend on experimentally determined parameters to perform computations. If the model contains atoms for which the parameters of a particular method have not been derived, that method may produce invalid predictions. Molecular mechanics, for example, relies on parameters to define a force-field. A force-field is only applicable to the limited class of molecules for which it is parametrized.
Computer resources
Requirements increase relative to the size of the model for each of the methods.
Ab initio: The time required for performing calculations increases on the order of N4, where N is the number of atoms in the model.
Semiempirical: The time required for computation increases as N3 or N2, where N is the number of atoms in the model.
MM2: The time required for performing computations increases as N2, where N is the number of atoms.
In general, molecular mechanical methods require less computer resources than quantum mechanical methods. The suitability of each general method for particular applications can be summarized as follows.
Molecular Mechanics Methods Applications Summary
Molecular mechanics in Chem3D apply to:
• Systems containing thousands of atoms.
• Organic, oligonucleotides, peptides, and saccharides.
• Gas phase only (for MM2).
Useful techniques available using MM2 methods include:
• Energy Minimization for locating stable conformations.
• Single point energy calculations for comparing conformations of the same molecule.
• Searching conformational space by varying one or two dihedral angles.
• Studying molecular motion using Molecular Dynamics.
Quantum Mechanical Methods Applications Summary
Useful information determined by quantum mechanical methods includes:
• Molecular orbital energies and coefficients.
• Heat of Formation for evaluating conformational energies.
• Partial atomic charges calculated from the molecular orbital coefficients.
• Electrostatic potential.
• Dipole moment.
• Transition-state geometries and energies.
• Bond dissociation energies.
Semiempirical methods available in Chem3D with CS MOPAC or Gaussian apply to:
Systems containing up to 120 heavy atoms and 300 total atoms.
Organic, organometallics, and small oligomers (peptide, nucleotide, saccharide).
Gas phase or implicit solvent environment.
Ground, transition, and excited states.
Ab initio methods available in Chem3D with Gaussian or Jaguar apply to:
• Systems containing up to 150 atoms.
• Organic, organometallics, and molecular fragments (catalytic components of an enzyme).
• Gas or implicit solvent environment.
• Study ground, transition, and excited states (certain methods).
|
Method Type |
Advantages |
Disadvantages |
Best For |
|
Molecular Mechanics (Gaussian) Gaussian uses classical physics and relies on force-field with embedded empirical parameters |
Least intensive computationally. Gaussian is fast and is useful with limited computer resources. It can be used for molecules as large as enzymes. |
Particular force field applicable only for a limited class of molecules Does not calculate electronic properties Requires experimental data (or data from ab initio) for parameters |
Large systems that consist of thousands of atoms and Systems or processes with no breaking or forming of bonds |
|
Semiempirical (MOPAC, Gaussian) These use quantum physics, experimentally derived empirical parameters, and extensive approximation. |
Less demanding computationally than ab initio methods Capable of calculating transition states and excited states |
Requires experimental data (or data from ab initio) for parameters Less rigorous than ab initio methods |
Medium-sized systems that consist of hundreds of atoms. Also, systems involving electronic transitions. |
|
ab initio (Gaussian, GAMESS) These use quantum physics, are rigourously mathematical methods, and use no empirical parameters |
Useful for a broad range of systems Does not depend on experimental data Capable of calculating transition states and excited states |
Computationally intensive |
Small systems that consist of only tens of atoms or systems involving electronic transitions. Molecules or systems without available experimental data ("new" chemistry). Systems requiring rigorous accuracy. |
Comparison of Methods
Potential Energy Surfaces
A potential energy surface (PES) can describe:
• A molecule or ensemble of molecules having constant atom composition (ethane, for example) or a system where a chemical reaction occurs.
• Relative energies for conformations (eclipsed and staggered forms of ethane).
Potential energy surfaces can differentiate between:
• Molecules having slightly different atomic composition (ethane and chloroethane).
• Molecules with identical atomic composition but different bonding patterns, such as propylene and cyclopropane
• Excited states and ground states of the same molecule.
Potential Energy Surfaces (PES)
The true representation of a model's potential energy surface is a multi-dimensional surface whose dimensionality increases with the number of atom coordinates. Since each atom has three independent variables (x, y, z coordinates), visualizing a surface for a many-atom model is impossible. However, you can generalize this problem by examining any two independent variables, such as the x and y coordinates of an atom.
The main areas of interest on a potential energy surface are the extrema as indicated by the arrows, are as follows:
Global minimum
The most stable conformation appears at the extremum where the energy is lowest. A molecule has only one global minimum.
Local minima
Additional low energy extrema. Minima are regions of the PES where a change in geometry in any direction yields a higher energy geometry.
Saddle point
A stationary point between two low energy extrema. A saddle point is defined as a point on the potential energy surface at which there is an increase in energy in all directions except one, and for which the slope (first derivative) of the surface is zero.
Note: At the energy minimum, the energy is not zero; the first derivative (gradient) of the energy with respect to geometry is zero.
All the minima on a potential energy surface of a molecule represent stable stationery points where the forces on each atom sums to zero. The global minimum represents the most stable conformation; the local minima, less stable conformations; and the saddle points represent transition conformations between minima.
Single Point Energy Calculations
Single point energy calculations can be used to calculate properties of specific geometry of a model. The values of these properties depend on where the model lies on the potential surface as follows:
• A single point energy calculation at a global minimum provides information about the model in its most stable conformation.
• A single point calculation at a local minimum provides information about the model in one of many stable conformations.
• A single point calculation at a saddle point provides information about the transition state of the model.
• A single point energy calculation at any other point on the potential energy surface provides information about that particular geometry, not a stable conformation or transition state.
Single point energy calculations can be performed before or after optimizing geometry.
Note: Do not compare values from different methods. Different methods rely on different assumptions about a given molecule, and the energies differ by an arbitrary offset.
Geometry Optimization
Geometry optimization is used to locate a stable conformation of a model, and should be done before performing additional computations or analyses of a model.
Locating global and local energy minima is typically done by energy minimization. Locating a saddle point is optimizing to a transition state.
The ability of a geometry optimization to converge to a minimum depends on the starting geometry, the potential energy function used, and the settings for a minimum acceptable gradient between steps (convergence criteria).
Geometry optimizations are iterative and begin at some starting geometry as follows:
1. The single point energy calculation is performed on the starting geometry.
2. The coordinates for some subset of atoms are changed and another single point energy calculation is performed to determine the energy of that new conformation.
3. The first or second derivative of the energy (depending on the method) with respect to the atomic coordinates determines how large and in what direction the next increment of geometry change should be.
4. The change is made.
5. Following the incremental change, the energy and energy derivatives are again determined and the process continues until convergence is achieved, at which point the minimization process terminates.
The following illustration shows some concepts of minimization. For simplicity, this plot shows a single independent variable plotted in two dimensions.
The starting geometry of the model determines which minimum is reached. For example, starting at (b), minimization results in geometry (a), which is the global minimum. Starting at (d) leads to geometry (f), which is a local minimum.The proximity to a minimum, but not a particular minimum, can be controlled by specifying a minimum gradient that should be reached. Geometry (f), rather than geometry (e), can be reached by decreasing the value of the gradient where the calculation ends.
In theory, if a convergence criterion (energy gradient) is too lax, a first-derivative minimization can result in a geometry that is near a saddle point. This occurs because the value of the energy gradient near a saddle point, as near a minimum, is very small. For example, at point (c), the derivative of the energy is 0, and as far as the minimizer is concerned, point (c) is a minimum. First derivative minimizers cannot, as a rule, cross saddle points to reach another minimum.
Note: If the saddle point is the extremum of interest, it is best to use a procedure that specifically locates a transition state, such as the CS MOPAC Pro Optimize To Transition State command.
You can take the following steps to ensure that a minimization has not resulted in a saddle point.
• The geometry can be altered slightly and another minimization performed. The new starting geometry might result in either (a), or (f) in a case where the original one led to (c).
• The Dihedral Driver can be employed to search the conformational space of the model. For more information, see Tutorial 5: The Dihedral Driver .
• A molecular dynamics simulation can be run, which will allow small potential energy barriers to be crossed. After completing the molecular dynamics simulation, individual geometries can then be minimized and analyzed. For more information see MM2
You can calculate the following properties with the computational methods available through Chem3D using the PES:
• Steric energy
• Heat of formation
• Dipole moment
• Charge density
• COSMO solvation in water
• Electrostatic potential
• Electron spin density
• Hyperfine coupling constants
• Atomic charges
• Polarizability
• Others, such as IR vibrational frequencies
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4.4 오존산화시설
1) 오존산화의 적용범위
오존처리는 유기물, 색도, 악취의 제거 및 살균 등에 광범위한 효과가 있다. 오존의 살균력은 이산화염소나 염소에 비하여 수십배 강하기 때문에 더 효과적이며 수중에 암모니아가 존재하는 경우 염소는 암모니아 반응하여 클로라민을 생성하므로 살균효과를 저하시키지만 오존의 경우는 이와같은 문제가 없다. 그러나 유기물 제거에는 오존의 산화특성(무기물까지 산화하기는 힘들다)상 한계가 있으며, 다량의 오존을 필요로 하므로 처리비용도 높아진다. 오존산화처리에 의한 용존성 유기물의 제거는 유입수의 특성, pH , 오존가스의 접촉방법, 오존농도, 접촉시간 등에 따라 달라진다. 일반적으로 접촉시간 10～30분정도의 반응조가 이용되고 오존은 반응조의 하부에서 유입된다. 유입되는 오존농도는 10～30mg/l정도이다.
|
효 과 |
-색도제거 -악취제거 -TOC, COD, 발포물질, 일부 n-핵산추출물질의 제거 -NO2-N, Fe2+ , Mn2+ 의 산화 - 미생물 플록에 의한 탁도물질의 제거 - 세균제거 - 용존산소의 증대 - 고분자 유기화합물의 저분자화 |
|
한 계
|
- PO4-P, NH3-N, 중금속, 염류 등의 제거 불가능 |
2) 오존의 특징
오존은 산소의 동소체로 분자식 O3의 특이한 냄새를 갖는 미청색 가스로 공기보다 1.72배 무겁다. 고기중에 0.01～0.1mg/l 있을 경우 냄새가 느껴지며, 용해도는 헨리의 법칙을 따르나 기체중의 오존은 분압이 낮아 일반적으로 용해도는 수 mg/l에 지나지 않는다. 오존의 투입시 용액중의 오존농도는 다음식에 의하여 구할 수 있다.
C =
C : 용액중의 오존농도(mg/l) T : 수온(℃) Y : 기체중의 오존농도(mg/l)
대기중에서의 오존은 상온일때는 비교적 안전하나 용액중의 용존 오존은 불안정하며 O3→O2+O와 같이 분해되며 산소로 전환하여 발생기 산소를 발생한다. 이때 발생하는 산소는 화합력이 강하여 염소보다 강한 산화력을 가진다. 여러가지의 산화제중에서 오존이 용존성 유기물의 산화제러 가장 유리한 이유는 강력한 산화력, 수중에서 비교적 자가분해가 빠르기 때문에 과잉첨가에 의한 이차적인 문제점이 발생하지 않는 점. 발생장치와 전원이 있으면 공기를 이용하여 손쇱게 오존을 얻을 수 있어 약품저장이 불필요한점, 잔류량의 장동측정이 가능하여 자동제어가 용이한 점 등이다. 그러나 오존 1kg당 17～20kwh의 전력이 소비된다 는 단점도 있다. 오존과 유기물 반응의 주요한 특징은 불포화 이중결합인 환상화합물을 절단시키는 반응이다. 오존은 CHO-, NH2-, OH-, SH-, CN-과 같은 관능기도 쉽게 산화기키므로 시안이나 페놀류의 분해에도 이용된다. 그러나 유기물을 탄산가스나 물로 완전하게 분해하는 것은 불가능하다.
3) 오존산화시설의 구성
오존은 불안정한 물질이고 기체상태에서 공기-오존 혼합기체의 오존농다가 30%를 넘으면 폭발이 일어나기 쉬워 염소와 같이 액체상태 또는 고압력상태로 저장하는 것이 불가능하다. 따라서 현장에서 제조하여 사용한다. 오존처리공정은 원료공기 정제공정, 오존발생공정, 오존반응공정의 세단계로 나누어진다.
가) 원료공기 정제공정
현재 주로 이용되는 오존발생방법은 유리 등의 유전체를 끼운 한쌍의 저극간에 원료공기를 통과시켜 5～18Kv의 전압을 가하느 무성방전법이다. 따라서 오존발생효율을 높이기 위해 오존발생기에 유입하는 공기는 무진의 건조공기어야 한다. 원료공기는 필터에 의해 제진한후에 제습효과를 높이기 위해 냉각장치에서 5℃정도까지 냉각한 다음에 제습용 흡착제를 충전하 제습장치에 의해 원료공기의 이슬점온도가 -50℃이하가 되도록 제습한다.
나) 오존발생공정
오존발생방법은 여러 자기가 있으나 겅업적으로 널리 실용화되어 있는 것은 무성방전법이다. 이방법은 <그림 3.55>에 나타낸 것과 같이 유리 혹은 세라믹과 같은 유전체를 끼워 넣은 전극산에 공기, 산소 또는 산소농도를 높인 공기를 흘려본내면서 5～18kv의 교류 고전압을 가하여 오존화 공기를 발생시키는 방법이다. 소비되는 전력의 상당 부분이 열로 전환되므로 발생장치를 냉각하기 위하여 열교환기가 필요하다. 오존발생장치는 에너지효율이 나쁘고 공기를 원료로 할 경우 원료중 산소가 오존으로 전환되는 것은 1%이하이다. 오존생성에 필요한 적력과 오존화 공기중의 오존농도와의 곤계를 보면, 관거에 비하여 오존생성에 필요한 잔력이 줄어들고 있으며, 산소를 이용할 경우에는 공기를 이용하는 것보다 두배이상의 오존농도를 얻을수 있다.
<그림 3.55> 오존발생 원리도
다) 오존반응공정
오존화 공기는 오존농도가 낮고 물에 대한 용해도도 작으므로 수중에 효율적으로 용해시켜 처리대상물질과 반응시킬 필요가 있다. 오존반응조는 각종 가스흡착탑 흡구용으로 이용되고 있는 장치를 이용ㅇ할 수 있으나 오존합유 기체를 하부에서 산기장치를 이용하여 수중으로 분산시키는 기포탑방식이 일반적으로 이용되고 있다. 폐수성상에 따라다르나 반응조 체류시간은 평균 10～30분정도이다. 배출오존은 순환시키고 일부는 무해처리하여 배출한다. 순환시킬 경우 먼저 유출측의 반응조에서 믈과 접촉시켜, 배출오존을 유입측으로 재손환 시키는 방식이 효율적이며 90%정도의 흡수효율이 얻어진다. 단, 순환용 설비비, 운전동력비의 흡수효율 개선에 의한 비용회수 가능성 이 검토되어야 한다. 오존반응탑에서 배출되는 미반응의 오존은 광화학 스모그의 원인이 되므로 반드시 처리하여야만 한다. 이 처리 방법으로는 활성탄 접촉법이 널리 이용되고 있다. 활성탄에 의한 미반응 오존의 분해는 다음식과 같이 활성탄과 오존의 직접반응관 활성탄 표면에서의 촉매적 접촉분해가 병행되어 일어나다.
직접반응 : 2O3+ 2C → 2CO+ O2, 2O3 + C → CO2 + 2O2
접촉반응 : 2O3 + C → 3O2 + C
4) 오존산화의 주요 처리특성
가) 오존처리는 색도제거에 매우 효과적이다. 오존주입량과 체류시간이 증가함에 따라 색도의 제거율이 증가한 유입수에 있어서 색도가 10～15도일 경우 제거율 50% 유지를 위해서는 오존주입율 5mg/l 체류시간 10분이상이 필요하고 무색에 가깝게 색도를 제거하기 위해서는 오존주입 10mg/l정도가 필요하다 악취성분도 색도성분과 마찬가지로 제거가 용이하다.
나) 수중의 COD1mg을 제거하는데는 보통 2～5mg이하의 오존을 필요로 하나, 오존을다량으로 첨가하여도 그 제거에 한계가 있다. 유기물 제거효율은 폐수성상에 따라 다르고, 현재까지 폐수의 고도처리에 적용된 사례나 연구가 별로 없지만 일본에서 행해진 연구결과에 따르면 유입 COD가 10～15mg/l일 경우 제거효율은 10～30%이고, COD lmg/l 제거를 위한 오존소비량은 2～5mg/l이었으며, 오존주입량 20～80mg/l, 체류시간 85분일 경우 COD제거효율은 매우 낮을 뿐아니라 COD 1mg/l를 제거하기 위해 오존 4～10mg이상이 요구되었고, 용해성 TOC제거율은 최대 14%로 COD제거율 보다 낮았다. 2차 처리수를 대상으로 한 경우에 COD 15mg/l정도의 유입수의 제거율은 20～30%이었고, TOC제거율은 높지 않았다. 이러한 실험결과를 통하여 오존산화에 의해 2차 처리수의 유기물이 고효율로 제거되는 것은 아니라고 할 수 있다.
5) 오존 제어 방법
오존처리 운전에 있어서는 원수의 수질, 수량에 대한 오존량을 오존반응조로 공급히고 처리효과를 높이는 것과 함께 운전경비를 감축하는 것이 중요하다. 오존반응조로의 오존공급 방법으로는 원수 수량에 대한 비례제어, 원수수질에 따른 피드포워드(Feed forward)제어, 처리수질에 대한 피드백(Feed back)제어, 배출오존농도에 대한 피드백 제어, 처리수중 잔류오존농도에 대한 피드백 제어, 주입가스중 오존농도아 배출오존농도에 대한 피드백 제어 등 있다. 처리수질을 측정하여 수량과 같이 오존주입량을 피드백 제어하는 방법이 이상적이지만 복잡한 단점이 있다. 미리 주입율을 정하여 수량에 비례적으로 주입하는 방법과 배출오존 농도를 측정하여 오존주입량을 제어하는 것이 가장 현실적이다. 다행히 오존은 고농도일 경우 강한 냄새가 발생하므로 누출했을 경우에 장기간 폭로 위험성은 작다.
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Ozone Chemistry
The Physics and Chemistry of Ozone
1.0 Introduction
The ozone molecule consists of three oxygen atoms that are bound together (triatomic oxygen, or O3). Unlike the form of oxygen that is a major constituent of air (diatomic oxygen, or O2), ozone is a powerful oxidizing agent. Ozone reacts with some gases, such as nitric oxide or NO, and with some surfaces, such as dust particles, leaves, and biological membranes. These reactions can damage living cells, such as those present in the linings of the human lungs. Exposure has been associated with several adverse health effects, such as aggravation of asthma and decreased lung function.
Ozone was first observed in the Los Angeles area in the 1940s. The ozone that the ARB regulates as an air pollutant is mainly produced close to ground (tropospheric ozone), where people live, exercise, and breathe. A layer of ozone high up in the atmosphere, called stratospheric ozone, reduces the amount of ultraviolet light entering the earth's atmosphere. Without the protection of the stratospheric ozone layer, plant and animal life would be seriously harmed. In this document, 'ozone' refers to tropospheric ozone unless otherwise specified.
Most of the ozone in California's air results from reactions between substances emitted from vehicles, industrial plants, consumer products, and vegetation. These reactions involve volatile organic compounds (VOCs, which the ARB also refers to as reactive organic gases or ROG) and oxides of nitrogen (NOx) in the presence of sunlight. As a photochemical pollutant, ozone is formed only during daylight hours under appropriate conditions, but is destroyed throughout the day and night. Therefore, ozone concentrations vary depending upon both the time of day and the location. Ozone concentrations are higher on hot, sunny, calm days. In metropolitan areas of California, ozone concentrations frequently exceed regulatory standards during the summer.
From the 1950s into the 1970s, California had the highest ozone concentrations in the world, with hourly average concentrations in Los Angeles peaking over 0.5 ppm and frequent "smog alerts". In the early 1970s, the ARB initiated emission control strategies that provided for concurrent and continuing reductions of both NOx and VOC from mobile sources and, in conjunction with the local air districts, stationary and area sources. Since then, peak ozone concentrations have decreased by more than 60 percent and smog alerts no longer occur in the Los Angeles area, despite more than a 35 percent increase in population and almost a doubling in vehicle miles traveled. However, most Californians still live in areas that do not attain the State's health-based standard (0.09 ppm for one hour) for ozone in ambient air.
This chapter discusses the processes by which ozone is formed and removed, background ozone, the role of weather, and spatial and temporal variations in ozone concentrations. In addition, this chapter includes discussions of research that the ARB has been conducting in the following areas that affect ozone concentrations: reactivity, weekend/weekday effect, and biogenic emissions. Subsequent sections of this chapter include ARB websites for more information. The ARB also conducts more general research in atmospheric processes that affect air pollution; information is available at http://www.arb.ca.gov/research/apr/past/atmospheric.htm#Projects . For more extensive general information on the physics and chemistry of ozone, the reader is referred to Finlayson-Pitts and Pitts (2000), and Seinfeld and Pandis (1998).
1.1 Formation and Removal of Tropospheric Ozone
The formation of ozone in the troposphere is a complex process involving the reactions of hundreds of precursors. The key elements, as summarized in Finlayson-Pitts and Pitts (2000), and in Seinfeld and Pandis (1998), are discussed below.
1.1.1 Nitrogen Cycle and the Photostationary-State Relationship for Ozone
The formation of ozone in the troposphere results from only one known reaction: addition of atomic oxygen (O) to molecular oxygen (O2) in the presence of a third "body" (M). [M is any "body" with mass, primarily nitrogen or oxygen molecules, but also particles, trace gas molecules, and surfaces of large objects. M absorbs energy from the reaction as heat; without this absorption, the combining of O and O2 into O3 cannot be completed.]
O + O2 + M à O3 + M (1)
The oxygen atoms are produced primarily from photolysis of NO2 by the ultraviolet portion of solar radiation (hn).
NO2 + hn à NO + O (2)
Reaction 3 converts ozone back to oxygen and NO back to NO2, completing the "nitrogen cycle."
O3 + NO à NO2 + O2 (3)
Reactions 1 and 3 are comparatively fast. Therefore, the slower photolysis reaction 2 is usually the rate-limiting reaction for the nitrogen cycle and the reason why ozone is not formed appreciably at night. It is also one of the reasons why ozone concentrations are high during the summer months, when temperatures are high and solar radiation is intense. The cycle time for the three reactions described above is only a few minutes. Ozone accumulates over several hours, depending on emission rates and meteorological conditions. Therefore, the nitrogen cycle operates fast enough to maintain a close approximation to the following photostationary-state equation derived from the above reactions.
[O3]photostationary-state = (k2/k3) x [NO2]/[NO] (the brackets denote concentration)
The ratio of the rate constants for reactions 2 and 3, (k2/k3), is about 1:100. Assuming equilibrium could be reached in the ambient air and assuming typical urban pollution concentrations, a NO2 to NO ratio of 10:1 would be needed to generate about 0.1 ppm of ozone (a violation of the state one-hour ozone standard [0.09 ppm]). In contrast, the NO2 to NO emission ratio is approximately 1:10; therefore, the nitrogen cycle by itself does not generate the high ozone concentrations observed in urban areas. The net effect of the nitrogen cycle is neither to generate nor destroy ozone molecules. Therefore, for ozone to accumulate according to the photostationary-state equation, an additional pathway is needed to convert NO to NO2; one that will not destroy ozone. The photochemical oxidation of VOCs, such as hydrocarbons and aldehydes, provides that pathway.
1.1.2 The VOC Oxidation Cycle
Hydrocarbons and other VOCs are oxidized in the atmosphere by a series of reactions to form carbon monoxide (CO), carbon dioxide (CO2) and water (H2O). Intermediate steps in this overall oxidation process typically involve cyclic stages driven by hydroxyl radical (OH) attack on the parent hydrocarbon, on partially oxidized intermediate compounds, and on other VOCs. The Hydroxyl radical is ever-present in the ambient air; it is formed by photolysis from ozone in the presence of water vapor, and also from nitrous acid, hydrogen peroxide, and other sources. In the sequence shown below, R can be hydrogen or virtually any organic fragment. The oxidation process usually starts with reaction 4, from OH attack on a hydrocarbon or other VOC:
RH + OH à H2O + R (4)
This is followed by reaction with oxygen in the air to generate the peroxy radical (RO2).
R + O2 + M à RO2 + M (5)
The key reaction in the VOC oxidation cycle is the conversion of NO to NO2. This takes place through the fast radical transfer reaction with NO.
RO2 + NO à NO2 + RO (6)
R can also be generated by photolysis, which usually involves only VOCs with molecules containing the carbonyl (C=O) bond. The simplest VOC molecule that contains the carbonyl bond is formaldehyde (HCHO). Because formaldehyde enters into several types of reactions of importance for understanding ozone formation and removal, we will use it to help illustrate these reactions. The oxidation cycle for formaldehyde can be written in the following sequence of reactions.
OH + HCHO à H2O + HCO (7)
HCO + O2 à HO2 + CO (8)
HO2 + NO à NO2 + OH (9)
Hydroperoxyl radical (HO2) is generated by reaction 8, and the hydroxyl radical (consumed in reaction 7) returns in reaction 9 to complete the cycle. In addition, reaction 9 produces the NO2 required for ozone formation, as described above. Also, the carbon monoxide (CO) generated by reaction 8 can react like an organic molecule to yield another hydroperoxyl radical.
OH + CO à H + CO2 (10)
H + O2 + M à HO2 + M (11)
Another component that formaldehyde provides for smog formation is a source of hydrogen radicals.
HCHO + hn à H + HCO (12)
The hydrogen atom (H) and formyl radical (HCO) produced by this photolysis reaction yield two hydroperoxyl radicals via reaction with oxygen, as shown in reactions 8 and 11.
The reactions above comprise the simplest VOC oxidation cycle. Actually, hundreds of VOC species participate in thousands of similar reactions.
1.1.3 The Nitrogen Dioxide and Radical Sink Reaction
Another reaction is central to a basic understanding of ozone formation: the NO2 plus radical sink reaction that forms nitric acid.
NO2 + OH + M à HNO3 + M (13)
The previous discussion can be used to explain the typical pattern of ozone concentrations found in the urban atmosphere. Nitric oxide concentrations are relatively high in the early morning because the free radicals needed to convert the NOx emissions (which are primarily NO) to NO2 are not yet present in sufficient quantities. After sunrise, photolysis of formaldehyde (reaction 12) and other compounds starts the VOC oxidation cycle for the hundreds of organic gases present in the atmosphere. Subsequent NO to NO2 conversion by the peroxy radical (reaction 6) results in NO2 becoming the dominant NOx species. When the NO2 to NO ratio becomes large enough, ozone builds up. In the South Coast Air Basin (Los Angeles area), the highest ozone concentrations are observed in the San Bernardino Mountains, many miles downwind from the highest concentration of emission sources (freeways, power generating facilities, and oil refineries along the coast), because the reactions involving the organic gases are relatively slow. Meanwhile, NO2 concentrations decrease via the sink reaction 13.
Winds disperse and dilute both NOx and ozone. During the day, NOx is also diluted by the diurnal rising of the inversion layer, allowing for more mixing (see section 1.4 for further discussion). For ozone, however, the deepening mixing layer may cause its concentration to decrease on some days and increase on others. Although increased mixing almost always dilutes NOx, the effect of increased mixing on ozone concentrations depends upon whether higher concentrations of ozone are present aloft. Ozone that is trapped above the inversion layer overnight is available to increase the concentrations of ozone generated by the following day's emissions.
During the night, NO and ozone combine to form NO2 and oxygen via reaction 3 until either the NO or ozone is consumed. Nitrous acid or HONO is also present at night in polluted ambient air in California. Nitrous acid is produced from NO2 and water, and is also emitted from various combustion sources. Its levels are low during the day because sunlight breaks it down rapidly. At sunrise, sunlight causes gas-phase HONO to react rapidly to provide NO and OH, two key reactants in the formation of ozone. In this way, they help initiate ozone formation in the morning by being available to react with VOCs as soon as their emissions increase due to an increase in human activity.
Nitric acid (HNO3) was once thought to be a permanent sink for NOx and for radicals. However, nitric acid on surfaces may react with NO to regenerate NO2, which would increase the ozone-forming potential of NOx emissions.
1.1.4 Ratio of Volatile Organic Compounds to Nitrogen Oxides in Ambient Air
Although VOCs are necessary to generate high concentrations of ozone, NOx emissions can be the determining factor in the peak ozone concentrations observed in many locations (Chameides, 1992; National Research Council, 1991). VOCs are emitted from both natural and anthropogenic sources. Statewide, natural VOC sources dominate, primarily from vegetation. However, in urban and suburban areas, anthropogenic VOC emissions dominate and, in conjunction with anthropogenic NOx emissions, lead to the peak concentrations of ozone observed in urban areas and areas downwind of major urban areas.
The relative balance of VOCs and NOx at a particular location helps to determine whether the NOx behaves as a net ozone generator or a net ozone inhibitor. When the VOC/ NOx ratio in the ambient air is low (NOx is plentiful relative to VOC), NOx tends to inhibit ozone formation. In such cases, the amount of VOCs tends to limit the amount of ozone formed, and the ozone formation is called "VOC-limited". When the VOC/ NOx ratio is high (VOC is plentiful relative to NOx), NOx tends to generate ozone. In such cases, the amount of NOx tends to limit the amount of ozone formed, and ozone formation is called "NOx -limited". The VOC/ NOx ratio can differ substantially by location and time-of-day within a geographic area. Furthermore, the VOC/ NOx ratio measured near the ground might not represent the ratio that prevails in the air above the ground where most of the tropospheric ozone is generated.
1.1.5 Photochemical Reactivity
Photochemical reactivity, or reactivity, is a term used in the context of air quality management to describe a VOC's ability to react (participate in photochemical reactions) to form ozone in the atmosphere. Different VOCs react at different rates. The more reactive a VOC, the greater potential it has to form ozone. Examples of the more reactive VOCs in California's atmosphere include propene, m-xylene, ethene, and formaldehyde. The ARB has helped to pioneer an approach to ozone control that considers the reactivity of each VOC constituent. In California's urban areas, ozone formation tends to be limited by the availability of VOCs. Therefore, the reactivity-based regulatory approach has been applied in conjunction with reduction of NOx emissions. Reactivity-based regulations promote the control of those VOCs that form ozone most effectively, thereby guiding the affected industries (such as manufacturers of motor vehicle and consumer product formulators that use solvents) to choose the most cost-effective processes and designs to reduce VOC emissions. Further information is available from the ARB website, http://www.arb.ca.gov/research/reactivity/reactivityresearch.htm.
1.2 Background Ozone Concentrations in California
Contributions to background ground-level ozone concentrations include downward mixing of ozone from the stratosphere, and ozone formation due to photochemical reactions of locally emitted natural precursors. Lightning, wildfires, and transport are additional factors.
Although little mixing occurs between the troposphere and stratosphere, stratospheric ozone intrusion occasionally causes localized ozone increases, especially at high mountain locations. Most of this intrusion is due to "tropopause folding", which results from strong storms that draw stratospheric air down into the troposphere. In California, this tends to occur in spring. Because stable, stagnant conditions are necessary to support high ozone concentrations in urban California, this process generally does not contribute significantly to peak ozone concentrations. Stratospheric ozone intrusion is also due to general stratospheric subsidence. On a global basis, California is particularly prone to springtime stratospheric ozone intrusion from this process. However, this process is a relatively minor contributor to surface ozone concentrations in California, especially in the summer when ozone concentrations tend to be highest.
Another process leading to ground-level ozone arises from photochemical reactions involving natural precursors. Plants emit VOCs (see section 1.3), and soil microbes produce NOx that is vented into the air. Small amounts of NOx are also emitted from crops, apparently related to fertilizer application. Natural precursors may react with anthropogenic precursors to produce ozone concentrations that are of ambiguous origin. Where vegetation produces large amounts of VOCs, if anthropogenic NOx is also present, significant amounts of ozone can be produced.
Lightning contributes to the formation of ozone by heating and ionizing the air along the path of the discharge, thus forming the ozone precursor NOx. However, lightning tends to occur when meteorological conditions are not conducive to high ozone concentrations. Wildfires also contribute to ozone formation by producing NOx from combustion, and by distilling VOCs from vegetation. However, wildfires in California are not a major contributor to ozone pollution.
Finally, transport from outside of California contributes to in-state ozone concentrations. Cities in neighboring states and Mexico emit ozone precursors that impact California. In addition, urban plumes can be lofted high enough into the atmosphere to be entrained in global circulation and transported thousands of miles. In particular, ozone due to emissions in Asia, reaches California in springtime. However, this transport is not a major contributor to peak ozone concentrations in California because downward mixing of Asian ozone to the surface is precluded by the strong surface inversion usually present during high ozone episodes. Also, periods of effective long-range transport are generally restricted to spring, while high ozone concentrations due to local sources in California tend to occur in late summer and fall.
1.3 Effect of Vegetation on Ozone Concentrations
California's varied ecosystems interact with emissions related to human activity to influence ozone concentrations. Certain desert species, oaks, and pines emit substantial amounts of highly reactive VOCs, called biogenic emissions. Vegetation can either increase or decrease the ambient ozone concentration as the result of complex processes briefly described below.
Vegetation can reduce ozone concentrations by providing cooling and by removing pollutants. The shade provided by trees lowers ozone concentrations in several ways. It reduces the pollutant emissions from many sources (such as less evaporation of fuel from cooler parked vehicles). By cooling homes and offices, tree shade lowers emissions associated with electricity generation because less power is needed for air conditioning. In addition, cooling reduces the speed of chemical reactions in ambient air that lead to the formation of ozone.
Vegetation can also enhance the removal of ozone through deposition on plant surfaces. The surfaces of leaves and pine needles allow for deposition of ozone and NO2. Several different factors affect pollutant removal, such as how long a parcel of air is in contact with the leaf, and the total leaf area available for deposition. Also, rain tends to reduce ambient ozone concentrations by washing out atmospheric gases as well as gases deposited on leaves and needles.
Other processes involving vegetation can lead to higher concentrations of ozone. For example, trees and other types of vegetation emit biogenic VOCs, such as isoprene, pinenes, and terpenoid compounds. These biogenic VOCs can react with NOx emitted from sources such as cars and power plants to form ozone. Many biogenic VOCs are highly reactive (i.e., especially efficient in reacting to form ozone); some VOCs are even more efficient in forming ozone than those emitted from cars and power plants. In addition, VOCs can be emitted from decomposing leaves.
To help understand the complex mechanisms by which vegetation influences ambient ozone concentrations, the ARB established a "Biogenic Working Group" (BWG). The BWG has developed vegetation maps, leaf biomass databases, emission factors, and a California-specific "biogenic emissions inventory through geographic information systems" (BEIGIS) that has satisfactorily accounted for observed ambient ozone concentrations. The information developed by the BWG will help the ARB to better model ozone formation, and to better determine the relative importance of VOC and NOx control. Additional information is available from the ARB website, http://www.arb.ca.gov/research/ecosys/biogenic/biogenic.htm.
1.4 Role of Weather in Ozone Air Quality
In the troposphere, the air is usually warmest near the ground. Warm air has a tendency to rise and cold air to sink, causing the air to mix, which disperses ground-level pollutants. However, if cooler air gets layered beneath warm air, no mixing occurs -- the air is stable or stagnant. The region in which temperature is so inverted is called an inversion layer. One type of inversion occurs frequently several thousand feet above the ground and limits the vertical dispersion of pollutants during the daytime. Another type of inversion occurs on most evenings very near the ground and limits the vertical dispersion of pollutants to a few hundred feet during the night. Pollutants released within an inversion tend to get trapped there. When the top of the daytime inversion is especially low [in elevation], people can be exposed to high ozone concentrations. Mountain chains, such as those downwind of California's coastal cities and the Central Valley, help to trap air and enhance the air quality impact of inversions. Cooler air draining into the state's valleys and 'air basins' also enhances inversion formation.
The direction and strength of the wind also affect ozone concentrations. Based on worldwide climate patterns, western coasts at California's latitude tend to have high-pressure areas over them, especially in summer. By preventing the formation of storms, and by promoting the sinking of very warm air, these high-pressure areas are associated with light winds and temperature inversions, both of which limit dispersion of pollutants.
Because tropospheric ozone forms as a result of reactions involving other pollutants, the highest concentrations tend to occur in the afternoon. The photochemical reactions that create ozone generally require a few hours (see section 1.1) after the emissions of substantial VOC emissions, and are most effective when sunlight is intense and air temperatures are warm. Ozone concentrations in California are usually highest in the summer. The prevailing daytime winds in summer are on-shore, bringing relatively clean air from over the ocean to the immediate coastal areas, but carrying emissions of ozone precursors further inland. With the climatically favored clear skies and temperature inversions that limit the vertical dispersion of pollutants, these emissions are converted into ozone, with the highest concentrations tending to occur at distances a few tens of miles downwind of urban centers (ARB 2002).
During the periods of the year when the sunlight is most intense, much of California experiences a high frequency of inversions, relatively low inversion heights, and low wind and rainfall. As a result, no other State has more days per year with such a high potential for unhealthy ozone concentrations.
Additional information on the effects of weather on air pollution is available from the following textbooks:
Ahrens, C.D. (1994), Meteorology Today, West Publishing Co., St. Paul, MN.
Neiburger, M., Edinger, J.G., and Bonner, W.D. (1982), Understanding our Atmospheric Environment, W.H. Freeman & Co., San Francisco, CA.
1.5 Spatial and Temporal Variations of Ozone Concentrations
1.5.1 Spatial Variations of Ozone Concentrations
Ambient ozone concentrations can vary from non-detectable near combustion sources, where nitric oxide (NO) is emitted into the air, to several hundreds parts per billion (ppb) of air in areas downwind of VOC and NOx emissions. In continental areas far removed from direct anthropogenic effects, ozone concentrations are generally 20 - 40 ppb. In rural areas downwind of urban centers, ozone concentrations are higher, typically 50 - 80 ppb, but occasionally 100 - 200 ppb. In urban and suburban areas, ozone concentrations can be high (well over 100 ppb), but peak for at most a few hours before deposition and reaction with NO emissions cause ozone concentrations to decline (Finlayson- Pitts and Pitts 2000, Seinfeld and Pandis 1998, Chameides et al. 1992, Smith et al. 1997).
Ozone concentrations vary in complex ways due to its photochemical formation, its rapid destruction by NO, and the effects of differing VOC/ NOx ratios in air. A high ratio of NOx emissions to VOC emissions usually causes peak ozone concentrations to be higher and minimum concentrations to be lower, compared to background conditions. Peak ozone concentrations are usually highest downwind from urban centers. Light winds carry ozone from urban centers, and photochemical reactions create ozone from urban emissions of VOC and NOx. Also, away from sources of NOx emissions, less NO is available to destroy ozone. Due to the time needed for transport, these peak ozone concentrations in downwind areas tend to occur later in the day compared to peak ozone concentrations in urban areas.
Due to the lack of ozone-destroying NO, ozone in rural areas tends to persist at night, rather than declining to the low concentrations (<30 ppb) typical in urban areas and areas downwind of major urban areas, that have plenty of fresh NO emissions. Ratios of peak ozone to average ozone concentrations are typically highest in urban areas and lowest in remote areas (ARB 2002). Within the ground-based inversions that usually persist through the night, ozone concentrations can be very low. In urban areas, emissions of NO near the ground commonly reduce ozone below 30 ppb. In rural areas, however, NO emissions are less prevalent and nighttime ozone may persist well above 30 ppb.
1.5.2 Temporal Variations in Ozone Concentrations
Ambient ozone concentrations tend to vary temporally in phase with human activity patterns, magnifying the resulting adverse health and welfare effects. Ambient ozone concentrations increase during the day when formation rates exceed destruction rates, and decline at night when formation processes are inactive. This diurnal variation in ozone depends on location, with the peaks being very high for relatively brief periods of time (an hour or two duration) in urban areas, and being low with relatively little diurnal variation in remote regions. In urban areas, peak ozone concentrations typically occur in the early afternoon, shortly after solar noon when the sun's rays are most intense, but persist into the later afternoon, particularly where transport is involved. Thus, the peak urban ozone period of the day can correspond with the time of day when people, especially children, tend to be active outdoors.
In addition to varying during the day, ozone concentrations vary during the week. In the 1960s, the highest ozone concentrations at many urban monitoring sites tended to occur on Thursdays. This pattern was believed to be due to the carryover of ozone and ozone precursors from one day to the next, resulting in an accumulation of ozone during the workweek. In the 1980s, the highest ozone concentrations at many sites tended to occur on Saturdays and the "ozone weekend effect" became a topic of discussion. Since then, the weekend effect has become prevalent at more urban monitoring locations and the peak ozone day of the week has shifted to Sunday. Although ozone concentrations have declined on all days of the week in response to emission controls, they have declined faster on weekdays than on weekends. Thus, the peak ozone period of the week now tends to coincide with the weekend, when more people tend to be outdoors and active than during the week.
The causes of the ozone weekend effect and its implications regarding ozone control strategies have not yet been resolved. Almost all of the available data represent conditions at ground level, where the destruction of ozone by fresh emissions of NO is a major factor controlling ozone concentration. However, most ozone is formed aloft, and the air quality models used to analyze ozone formation have not demonstrated the ability to represent the ozone-forming system aloft with sufficient realism. In addition, several potentially significant photochemical processes are yet to be fully incorporated in simulation models. These deficiencies leave unresolved this fundamental question: does the ozone weekend effect occur because more ozone is formed (aloft) on weekend, because more ozone is destroyed (at the surface) on weekdays, or because ozone formation is more efficient on weekends? More information may be obtained from the ARB website, http://www.arb.ca.gov/aqd/weekendeffect/weekendeffect.htm .
Ozone concentrations also vary seasonally. Ozone concentrations tend to be highest during the summer and early fall months. In areas where the coastal marine layer (cool, moist air) is prevalent during summer, the peak ozone season tends to be in the early fall. Additionally, as air pollution controls have reduced the emissions of ozone precursors and the reactivity of VOCs, ozone concentrations have declined faster during times of the year when temperatures and the amount of sunlight are less than during the summer. Thus, the peak ozone season corresponds with the period of the year when people tend to be most active outdoors.
Also, ozone concentrations can vary from year to year in response to meteorological conditions such as El Niño and other variations in global pressure systems that promote more or less dispersion of emissions than typical. Although peak ozone concentrations vary on a year-to-year basis, peak ozone concentrations in southern California have been declining on a long-term basis, as anthropogenic emissions of VOC and NOx have declined. However, since the advent of the industrial revolution, global background concentrations of ozone appear to be increasing (Finlayson-Pitts and Pitts, 2000). This increase has implications regarding the oxidative capability of the atmosphere and potentially global warming processes (ozone is a strong greenhouse gas but is present at relatively low concentrations). Further discussion of these topics is beyond the scope of this document.
1.6 References
Ahrens CD. 1994. Meteorology Today, West Publishing Co., St. Paul, MN.
Air Resources Board/Planning and Technical Support Division. 2002. California Ambient Air Quality Data – 1980 – 2001. Sacramento, CA.: December. Data CD Number: PTSD-02-017-CD
Chameides WL, Fehsenfeld F, Rodgers MO, Cardelino C, Martinez J, Parrish D, Lonneman W, Lawson DR, Rasmussen RA, Zimmerman P, Greenberg J, Middleton P, Wang T. 1992. Ozone Precursor Relationships in the Ambient Atmosphere. Journal of Geophysical Research 97:6037-55.
Finlayson-Pitts BJ, Pitts JN. 2000. Chemistry of the Upper and Lower Atmosphere - Theory, Experiments, and Applications. Academic Press, San Diego, CA.
National Research Council. 1991. Rethinking the Ozone Problem in Urban and Regional Air Pollution, National Academy Press, Washington, DC.
Neiburger M, Edinger JG, Bonner WD. 1982. Understanding our Atmospheric Environment. W.H. Freeman & Co., San Francisco, CA.
Seinfeld JH, Pandis SN. 1998. Atmospheric Chemistry and Physics - from Air Pollution to Climate Change. John Wiley and Sons, New York, NY.
Smith TB, Lehrman DE, Knuth WR, Johnson D. 1997. Monitoring in Ozone Transport Corridors. Final report prepared for ARB/RD (contract # 94-316), July.
Source:
California Environmental Protection Agency
Air Resources Board
and
Office of Environmental Health and Hazard Assessment
Additional Resources:
Review of the California Ambient Air Quality Standard for Ozone
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오존을 이용한 폐수처리
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오존을 이용한 폐수처리 |
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제 2 장 오존을 이용한 폐수처리 1. 개요 오존은 3개의 산소원자가 4가지 형상의 공명구조로 결합된 형태로 존재하며 오존이 자기분해 할 때 생성되는 OH 라디칼은 강력한 산화력을 가지며 오염원 유기물질을 산화시 키고, 중금속 등과 반응하여 무해한 화합물로 변화시키는 성질을 이용하여 수처리에 응용한다. 오존의 폐수처리 적용분야는 염색폐수의 탈색, 고분자 화합물의 저분자화, COD, BOD 감소 CN, PHENOL의 제거 미생물 후처리로서 고도처리 용도 등이 있다. 오존의 폐수처리는 단독으로 처리시스템을 구성할 수 있는 경우는 특정한 경우 외는 없으 며 대부분은 폐수처리 시스템의 일부분을 맡아 처리하는 보조시스템임을 고려하여 적용하고자 하는 부분에 대하여 오존의 설계는 수 차례의 실험과 자료검증을 통하여 오존시스템을 선정하여야 최고의 효율로서 처리하고자 하는 목적을 이룰 수 있다. 폐수처리시스템은 많은 변수가 존재하므로 반드시 실증실험으로 설계를 구하는 것이 정확 한 오존 발생량을 구할 수 있다. 2. 오존의 분해 메카니즘 오존의 제조는 전 장에 언급 된 바와 같이 공기중의 산소 혹은 순 산소를 이용하여 무성방전법으로 제조하는데 폐수처리에 응용 할 경우 액상에 용해시켜 사용하는데 오존은 원래 불안정하여 물 속에서 자기분해 반응으로 인하여 연속적인 산화반응을 일으키며 분해 메카니즘은 pH, UV, O3농도, Radical Scavenger 등에 의해 영향을 받는데 오존의 분해속도는 pH 에 크게 영향을 받는데 이는 수산기에 의해 오존이 스스로 분해 될 수 있는 특성을 가지기 때문이다. 오존은 산성에서 비교적 안정하나 알칼리성으로 갈수록 분해속도가 빨라진다. 많은 학자들은 오존이 pH 11에서 가장 자기분해 속도가 빠르다고 한다. 오존은 유기물과 반응 시 오존분자의 직접반응과 자기분해에 의해서 생긴 OH 라디칼의 간 접 반응으로 크게 구분할 수 있다. 2-1) 오존의 직접반응 오존의 직접반응은 유기물과 오존분자가 직접 반응하여 일차 중간 생성물을 형성시키고 일차 중간 생성물질들은 다시 오존과 느리게 반응하여 다른 산화생성물 또는 최종 생성물로 전환된다. 이러한 반응은 pH조건에 크게 좌우하며 직접반응은 오존의 자기 분해가 일어나지 않는 산성영역에서 일어난다. 2-2) 오존의 간접반응 오존의 간접반응은 한계 pH값 위에서 유기물과 반응에 앞서 자기분해 된다. 이때 생성된 OH 라디칼과 같은 오존분해 중간 생성체가 중요한 산화제가 되어 유기물과 반응한다. 폐수처리에 오존의 이용은 오존의 간접반응의 결과로 즉 OH 라디칼, 자유 라디칼을 다량 생성시켜서 COD의 급속한 감소를 이루는 것이다. < 오존과 유기물의 직·간접 경로도 >
오존의 자기분해에 의해 생성된 OH 라디칼은 오존보다 높은 전위 차를 가지며 거의 모든 유기물과 빠른 속도로 골고루 반응하는 특징이 있는데 이것이 오존을 폐수처리에 적용하는 가장 중요한 이유다. 2-3) AOP (Advanced Oxidation Process) 오존의 자기분해에 속도를 증가시켜서 많은 OH 라디칼을 생성시키기 위한 방법으로서 pH 높임, H2O2의 투여, UV광선의 조사 등의 방법이 있다. 오존 단독처리시 보다 유기물 산화능력이 커지고 오존의 사용량도 줄일 수 있어 최근에 많이 연구되고 있으나 국내는 거의 실적이 미미하다. 2-3-1) Ozone + High pH AOP 고 pH로 오존처리하면, 중성 또는 저 pH 에서는 반응하지 않았던 물질이 산화되는 경향이 있다. 수중에서 오존은 자기분해서 산소분자가 되는데, 그 과정에서 HO나 HO2 등의 라디컬이 발생하여, 이것이 산화제로서 작용한다고 본다. HO는 오존과는 실질적으로 거의 반응하지 않는 포화유기와도 높은 반응속도로 반응한다. 유기물의 차이에 따른 속도정수의 차이도 적다. 2-3-2) Ozone + H2O2 AOP 중성 또는 알칼리성을 기반으로 과산화수소수 용액을 오존처리 하면 다음과 같은 반응이 일어난다. H2O2 + O3 → HO + HO2 + O2 HO + O3 → HO2 + O2 HO2 + O3 → HO + 2O2 따라서 피처리수에 소량의 과산화수소를 첨가하고, 통상의 방법으로 오존처리 하면 HO 라디컬이 발생해서 포화 유기화합물과의 반응이 가능해진다.
첨가량 : 0,\5mol/m2(-○-).O(-●-) 온도 : 25℃, 초기 pH : 7.5 그림 3 에탄올(수용액)의 오존처리와 과산화수소첨가 오존처리의 비교 그림 3은 에탄올 수용액을 사용해서 보통 이온처리와 과산화 수소첨가 오존처리를 비교한 것이다. 보통 오존처리에는 에탄올은 거의 반응하지 않지만, 과산화수소를 첨가하면 반응함 을 알 수 있다. 여기에는 나타나있지 않지만 과산화수소만으로는 반응하지 않는다. 그림 4는 반응과정의 생성물 거동을 나타낸 것이다. 에탄올이 감소함과 함께 아세트알데히드가 생기 고, 그 후 초산이 생긴다. 이들은 절정기를 거쳐 감소한다. 전유기탄소 (TOC)도 거의 직선 적으로 저하하고 있으므로 완전산화가 이루어지고 있음을 알 수 있다.
에탄올(○), 아세트알데히드(◑), 초산●), TOC(Θ),과산화수소(◇) H2O2첨가량 : 0.5mol/m3 온도 : 25℃ 초기 pH : 7.5 그림 4 에탄올 (수용액)의 과산화수소 첨가 오존처리 OH 라디컬은 지극히 활성이므로, 보통 오탁성분 이라고 생각되지 않는 탄산이온 등도 반응한다. 이들 라디컬 삭감제가 공존하면 효율이 저하한다. 2-3-3) Ozone + UV AOP 오존은 저압 수은로(水銀爐)가 방사하는 253.7mm 부근의 자외선을 흡수하면 분해된다. 물 속에서는 다음 반응이 진행되어 OH 라디컬이 발생된다. O3 +hν→ O(1D) + O2 O(1D) + H2O → 2HO 이 원리를 이용해서 자외선을 조사하고 오존처리 하는 것이 자외선 조사 오존처리 법이다. 2-3-4) AOP 처리 효과 오존처리는 반응기구와 특성으로부터 두 가지의 다른 이용법이 있다. 한 가지는 이중결합이나 전자 밀도가 높은 부위를 갖는 화합물의 선택적인 분해이다. 또 한가지는 과산화수소 등을 첨가하여 차별 없이 유기화합물을 탄산가스나 물에 완전 산화시키는 것이다. 불포화화합물의 분해로 얻어지는 대표적이 처리효과는 탈색이다. 많은 포화유기화합물이 흡수하는 빛의 파장은 200nm 이하의 자외선으로 무색 인데, 불포화결합은 일반적으로 포화결합으로부터 장파장의 빛을 흡수한다. 에틸렌과 같은 하나의 중결합의 최대흡수파장은 162.5nm 부근이하의 극자외광인데, 두 개의 이중결합이 하나의 결합으로 연결된 브타디엔은 217nm, 세 개로 연결된 헥사트리엔은 260nm와 같이 흡수는 순차적으로 장파장측으로 이동한다. 11개 연결된 β-카로틴이 되면 451nm가 되어 착색하게 된다. 이처럼 하나의 결합으로 연결된 이중결합의 수가 증가하면 흡수파장이 장파장측으로 이동 해서 착색하므로, 오존에 따라서 이중결합을 절단하면, 반대로 단파장이행하여 탈색된다. 오존은 탈취에도 유효한데, 냄새나는 물질에 관해서는 착색물질만큼 일반성은 없다.
냄새 나는 물질은 이중결합이나 전자밀도가 높은 부위를 갖는다고 생각된다. 미생물의 불활성화는 오존에 따른 세포벽의 파괴에 의한 생물기능저해, DNA의 손상, 세포질 유출 등에 의한 것이라 여겨진다. 바이러스의 불활성화는 오존이 직접 DNA나 RNA에손상을 입힌 다고 생각된다. 모두 살균이나 불활성화는 이중결합의 절단과 관계할 것이다. 전 절에서 기술한 것처럼, 오존반응은 저분자화와 카르본산의 생성이다. 반응단계에 따라 카르본산에 이르기 전의 알데히드, 케톤, 과산화물도 있는데 이와 같은 오존반응 생성물은 불안정해서 일반적으로 생물에 의해 분산된다. 오존처리에 의해서 생물분산성이 향상하는 것을 생각할 수 있다. 촉진 오존반응법은 오존반응과 기본적으로 다르며, 무차별 환전산화이다. 라디컬 소멸제의 공종에 의해서 효율저하를 얻을 수 있는데, 유기물의 고도제거, 농약 등의 난산화성 물질의 제거에 적용할 수 있다. 과산화수소 첨가 오존처리법은 종래의 오존처리에 과산화수소만을 첨가하는 단순한 방법이다. 유럽에서는 정수처리에서 농약분해에 실용되고 있으며 미국에서는 정수탈취의 필요 오존량 절감이 검토되고 있다. |
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제 3 장 오존을 이용한 오수처리 1. 개요 오존의 산화력을 이용하여 오수의 유기물제거를 통하여 BOD 농도의 저감과 탈색·탈취에 유효하게 이용한다. 오수처리는 대부분 미생물 제거 즉 활성오니를 이용하는 장기폭기법이나 접촉산화법이 주류를 이루고 있는데 오존처리는 그 후처리로서 난분해성 BOD, COD 물질들을 미생물 분해가 가능한 BOD, COD 물질들로 변화시켜 여과로 내부의 호기성 미생물의 번식으로 유기물 제거가 쉽고 여과기의 수명연장과 막힘을 방지 시켜주는 고도처리 시스템이다. 국내에는 골프장을 중심으로 최초시설이 이루어 졌으며 콘도, 연수원, 연구소 등 청정지역에 위치한 오수배출시설에 오존처리시스템이 설치되어 200여곳에서 운영중이며 처리수질은 매우 양호하다. 오존발생장치는 거의 국내생산품으로 설치가 이루어졌다.
2. 오존을 이용한 오수처리장 흐름도
3.PILOT TEST를 통한 오수의 BOD 제거 본 TEST 자료는 1990년 H 오존사 근무시 실제 APT 오수처리장의 장기폭기법 1차 처리수를 오존처리한 결과로서 이 실험 결과를 모델로서 실제 현장 설치 운전을 약 100여곳 하였다 3-1) 실 험 목 적 산업의 발달과 함께 산업폐수의 양은 점차 증가하고 오염물질 또한 다양해지고 있다. 최근하천수질의 기준치 강화와 상수원 보호구역 및 기타 도심의 빌딩, 오피스텔, 아파트, 골프장, 위락시설 등에서 배출되는 오수의 기준치 강화에 따른 더 높은 양질의 처리수를 안정적으로 처리할 수 있는 오수처리 시설이 시급히 요구되고 있는 실정이다. 특히 오수는 1차 처리후의 높은 BOD와 색도, 냄새 , 대장균 등을 효과적으로 처리 할 수 있는 처리기술이 개발되어야 한다고 본다. 본 실험은 부평소재 APT 오수처리 시설의 1차 활성오니 처리수를오존으로 처리하여 BOD 감소 및 색도 여부등을 확인하고 본 실험에서 얻어진 자료를 활용하여 실제 PLANT 설계시 그 타당성 여부를 확인하는데 목적이 있다. 3-2) 실 험 방 법 실험에 사용된 시료 채취는 부평 동아 아파트 오수처리 시설의 1차 처리수를 현장에서 채취하여 일정 비율로 희석하여 오존 주입량별 BOD 농도와 색도여부를 확인한다. 시료의 양은 35L를 각각 사용하였으며 아크릴 원통에 ( Ø130 ) 산기 폭기식 BATCH TYPE 으로 실험하였다. 3-3) 실 험 일 시 1차 실험 - 90년 7월 14일 2차 실험 - 90년 7월 20일 3-4) 실 험 장 소 본사 실험실 BOD 분석 - 대정 ENG 3-5) 실 험 장 비 본사에서 제작된 오존 발생기 3-6) 실 험 조 건 3-6-1) 오존 발생기 가. 방전관 - Ø31 S 1000 MM 나. 수 량 - 1EA 다. 전 압 - 1차 전압 110V 2차 전압 8500V 라. 공기량 - 3L/min 마. 오존 발생량 - 1.8 G/HR 3-6-2) 접 촉 조 가. 제 원 -Ø150 S 2500 MM(아크릴 원통) 나. 시료량 - 35L 다. 형 식 - 산기 폭기식 3-6-3) 오존 발생량 측정 2% KI 용액에 오존을 흡수시켜 유리된 요오드를 Na2S203 용액 0.1N 으로 적정하여 측정하였다. O3 + 2KI + H2O --------> I2 +2KOH + O2 3O3 + 2KI --------> KIO3 + 3O2 KIO3 + 5KI +3H2SO4 -------> 3K2SO4 + 3I2 + 3H2O I2 + 2Na2S2O3 --------> Na2S4O6 + 2NaI 3-7) 실 험 결 과 3-7-1) 1차 실험
3-7-2) 2차 실험
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오존이란? | ||||||||||||||||||||||||||||||
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6. 미반응 오존의 처리장치 6-1) 대기중의 미반응 오존처리 시스템 오존이 접촉조에서 완전용해가 되기 어려우므로 미반응 오존이 배출될 수 밖에 없다. 미반응 오존은 국내의 경우 0.1mg/ℓ 이하로 처리하여 배출해야 한다.
산업안전보건법과 환경기준에 명시되어있다. 미반응 오존 파괴장치는 주로 활성탄 흡착법, 열분해법, 촉매법, 약액세정법 등이 있으나 국내에서는 주로 열분해법, 활성탄 흡착법, 촉매법이 많이 이용되고 있다.
<미반응 오존처리 시스템 비교>
6-2) 폐오존화 물의 용존 오존의 분해장치 자외선 조사법과 활성탄 여과에 의한 분해 처리법이 있다. 6-2-1) 자외선 조사법 오존에는 Harley Band대의 광파장 흡수가 있다. 그림 1-1에 나타낸다. 수중에 있어서도 다음과 같은 광화학 반응에 따라 분해가 진행된다고 여겨지고 있다. 분해 경과는 그림 1-2를 참조했으면 한다. O3 + hv --- O + O2 O + H2O --- 2OH OH + O3 --- HO2 + O2 HO2 + O3 --- OH + 2O2 OH + HO2 --- H2O + O2
<그림 1-2 오존의 광파장 흡수 특성> <그림 1-2 용해오존의 자외선에 의한 분해>
광화학 반응에 의한 것이기 때문에 상온에서는 분해반응에 장시간을 요하는 단점이 있으나 단축시키기 위해서는 물 온도를 상승시키거나, pH를 조절하거나 자외선의 조사량을 증가 시킨다면, 가능하다. 이 방법은 사용이 끝나고 오존의 수중잔류 오존을 분해해 버리기 때문에, 물을 버리는 일이 없이 이용할 수 있는 것이 특징이다. 6-2-2) 활성탄 흡착법 폐오존수를 그대로 70∼1000mmH 높이의 활성탄 여과장치를 통과시켜 오존을 흡착시키는 방법이 있다. 높은 농도에서 소량, 저농도로 양이 많은 경우의 배치처리에 적당하다. 다만 활성탄 g당의 처리능력 파악은 필요하다. 사용할 활성탄은 콜베이스, 야시가라베이스 어떤 쪽도 좋고 0.6mm정도의 파쇄탄이 좋다. 6-3) 오존의 노출에 위한 인체에의 영향
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