The Nobel Prize in Chemistry 1995

F. Sherwood Rowland

decomposition of ozone

When ozone is produced it will decay rapidly, because ozone is an instable compound with a relatively short half-life. The half-life of ozone in water is a lot shorter than in air (see table 1). Ozone decays in water under drinking water conditions (pH: 6-8,5), partly in reactive OH-radicals. Therefor, the assessment of an ozone process always involves the reactions of two species: ozone and OH-radicals. When these OH-radicals are in the dominant particles in the solution, it is called an advanced oxidation process (AOP). The decay of ozone in OH-radicals in natural waters is characterized by a fast initial decrease of ozone, followed by a second phase in which ozone decreases by first order kinetics [15]. Dependent on the quality of the water, the half-life of ozone is in the range of seconds to hours. Factors influencing the decomposition of ozone in water are temperature, pH, environment and concentrations of dissolved matter and UV light. Here, the main influence factors for ozone decomposition will be discussed.

Influence factors

1. Temperature

Temperature has an important influence on the half-life of ozone. Table 1 shows the half-life of ozone in air and water. In water the half-life of ozone is much shorter than in air, in other words ozone decomposes faster in water [1]. The solubility of ozone decreases at higher temperatures and is less stable. On the other hand, the reaction speed increases with a factor 2 or 3 per 10 °C [5,6]. Principally, ozone dissolved in water cannot be applied when temperatures are above 40 °C, because at this temperature the half-life of ozone is very short.

2. pH

As mentioned above, ozone decomposes partly in OH-radicals. When the pH value increases, the formation of OH-radicals increases. In a solution with a high pH value, there are more hydroxide ions present, see formulas below. These hydroxide ions act as an initiator for the decay of ozone:

1 O3 + OH- ? HO2- + O2 2 O3 + HO2- ? •OH + O2 •- + O2

The radicals that are produced during reaction 2 can introduce other reactions with ozone, causing more OH-radicals to be formed.

In addition the pH influences acid/base equilibriums of some compounds and also the reaction speed of ozone. This applies also to the reaction with scavenger CO32-, which is also pH dependant (Pka HCO32-/CO32- = 10,3).

Figure 1 shows that the decay of ozone in a basic environment is much faster than in an acid environment.

Figure 1: effect of the pH on the decay of ozone (T = 15 °C)

3. Dissolved solids concentration

Dissolved ozone can react with a variety of matter, such as organic compounds, viruses, bacteria, etc. As a result, ozone decomposes to other matter; see figure 2. This figure illustrates that the half-life of ozone in distilled water is much shorter, compared to tap-water.

Figure 2: Ozone decomposition in different types of water at 20 °C. 1 = double-distilled water; 2 = distilled water; 3 = tap water; 4 = groundwater of low hardness; 5 = filtered water from Lake Zurich (Switzerland); 6 = filtered water from the Bodensee (Switzerland)

Ozone decomposes in water in OH-radicals. Dependent on the nature of the dissolved matter, these can accelerate (chain-reaction) or slow down the decay of ozone. Substances that accelerate this reaction are called promoters. Inhibitors are substances that slow down the reaction.

When water is ozonized, one often uses the term 'scavenging capacity'. Scavengers are entities that react with OH-radicals and slow down the chain-reaction. The scavenging capacity can be defined as follows [16]:

kOH-DOC[DOC] + kOH-HCO3-[HCO3-] + kOH-CO32-[CO32-]

4. Carbonate and bicarbonate

Scavengers slow down the chain-reaction. This is because after the reaction of scavengers with OH-radicals, the reaction products do not react with ozone any further. Carbonate is a scavenger with a strong effect. The addition of carbonate (CO32-) can increase the half-life of ozone [5,6]. The effect on the reaction speed is highest at low concentrations. Above 2 mmol-1 for ozonisation and 3 mmol l-1 for advanced oxidation process (AOP), the decrease in the reaction speed is negligible [6].

When a solution mainly undergoes indirect reactions (with OH-radicals), for instance in a solution with a high pH value or an AOP-process, the presence of scavengers is undesired. The scavengers react very fast with OH-radicals and lower the oxidation capacity. For this kind of processes a low scavenging capacity is required.

Carbonate (CO32-) ions are a much stronger scavengers than bicarbonate (HCO32-) ions (reaction speed CO32-: k = 4,2 * 108 M-1s-1 and reaction speed HCO3-: k = 1.5 * 107 M-1s-1). That is why in an ozone process under drinking water conditions, the bicarbonate concentration is less important [6]. Figure 3 illustrates the relation of the carbonate ratio, bicarbonate ratio and pH.

Natural Organic Material

Natural organic material (NOM) exists in every kind of natural water and is often measured as dissolved organic carbon (DOC). NOM reduces the quality of the water with regard to color and odor. Ozone can be used in water treatment, for the reduction of the concentration of NOM. The concentration of NOM in natural waters can vary from 0,2 – 10 mg l-1 [6]. The influence of NOM on ozone is twofold. Dependent on the type of NOM, it can be oxidized directly by NOM. This is the case for compounds which easily react with ozone, such as double bonds, activated aromatic compounds, deprotonated amines and sulphide [15]. On the other hand, OH-radicals can react with NOM (indirect reaction) and act as a promoter or as a scavenger. In natural waters, it is difficult to determine the stability of ozone as a result of the indefinite effect of NOM. That means it is not possible to estimate the fraction that accelerates or slows down the reaction

I was born on June 28, 1927, the second of three sons, in the small central Ohio town of Delaware, the home of Ohio Wesleyan University. My father and mother had moved there the previous year when he took the position of Professor of Mathematics and Chairman of the Department at Ohio Wesleyan. All of my elementary and high school education was received in the Delaware public schools from an excellent set of teachers. The Delaware school system then believed in accelerated promotion, so that I entered first grade at age 5 and skipped the fourth grade entirely, with the result that I entered high school at 12 and graduated a few weeks before my sixteenth birthday. The college preparatory curriculum was strong on Latin, English, History, Science and Mathematics. The academic side of high school was easy for me, and I enjoyed it. In several summers of my early teens, the high school science teacher entrusted to me during his two week vacations the operation of the local volunteer weather station, an auxiliary part of the U.S. weather service-maximum and minimum temperatures and total precipitation. This was my first exposure to systematic experimentation and data collection.

Our home was filled with books, and all of us were avid readers. My reading at that time ran toward naval history, which was complemented with realistic scale-models and simulated naval battles using an elaborate mathematical system for rating each warship and the effects of combat on them. During my sophomore year in high school, my math teacher, who also coached tennis and basketball, encouraged me to take up tennis - which led me onto the varsity tennis team for my junior and senior years, and into a full decade of intense athletic competition. As a senior, I played on the varsity basketball team.

After graduation from high school in 1943, almost all of my male classmates immediately entered the military services. However, because I was still well under the compulsory draft age of 18, I enrolled at Ohio Wesleyan and attended the university year-round for the next two years. During these war years, only 30 or 40 civilian males were on campus, plus about 200 naval officer trainees and 1,000 women. With so few men available, I played on the University basketball and baseball teams, and wrote much of the sports page for the University newspaper.

My accelerated academic schedule made me eligible for my final year of university in June, 1945, as I approached my 18th birthday. However, with the fighting in the Pacific and the continuing military draft, I enlisted in a Navy program to train radar operators. The Pacific war ended while I was still in basic training near Chicago, and I served the next year in several midwestern Naval Separation Centers, as the 10,000,000 Americans who had preceded me into the military were returned to civilian life. A major amount of this Navy time was devoted to competitive athletics for the Navy base teams, and I emerged after 14 months as a non commissioned officer with a rating of Specialist (Athletics) 3rd class. My first real opportunity to see the rest of the United States came when I was transferred to San Pedro, California for discharge from the Navy.

I then hitchhiked 2000 miles back to Ohio, traveling through Yosemite and Yellowstone Park on the way.

This year away from the academic life convinced me that at age 19, there was little reason for me to seek a quick finish to my undergraduate education. I therefore arranged my schedule to take two more years rather than one to graduate, and continued to play basketball on the university team. My coursework at Ohio Wesleyan emphasized science within a liberal arts curriculum, with more or less equal amounts of chemistry, physics and mathematics, and majors in all three fields. As had been the case in high school, I really enjoyed the academic side of university life.

I do not honestly remember when the decision that I would go to graduate school was made. My father had studied for his Ph.D., and all of us took it for granted that I would, too. Furthermore, both my parents had firm convictions that the University of Chicago, which each had attended, was not just the best choice for graduate work, but the only choice. So I applied to the Department of Chemistry at the University of Chicago for Fall 1948, and was duly admitted. All service veterans were entitled to a certain number of months (27 in my case) of paid university education, and I had not used any of these credits during my undergraduate years at Ohio Wesleyan because faculty children did not pay tuition, and I lived at home. I therefore didn't apply for any of the teaching assistantships or academic fellowships, and was quite surprised after arriving in Chicago to find that many of my fellow students were being paid by the University to attend graduate school. In subsequent years, I was supported by an Atomic Energy Commission (A.E.C.) national fellowship.

At that time, the Chemistry Department of the University of Chicago had a policy of immediately assigning each new graduate student to a temporary faculty adviser prior to the choice of an individual research topic. My randomly assigned mentor was Willard F. Libby, who had just finished developing the Carbon-14 Dating technique for which he received the 1960 Nobel Prize. Bill Libby (although I never called him anything but "Professor Libby" until I was more than 40 years old) was a charismatic, brusque (on first meeting, "I see you made all A's in undergraduate school. We're here to find out if you are any damn good!") dynamo, with a very wide range of fertile ideas for scientific research. I settled automatically and happily into his research group, and became a radiochemist working on the chemistry of radioactive atoms. Almost everything I learned about how to be a research scientist came from listening to and observing Bill Libby.

The first nuclear reactor had been built by Enrico Fermi in 1942 under the football stands at the University of Chicago, and the post-war university had managed to capture many of the leading scientists from the Manhattan Project into the Physics and Chemistry departments. My impression at the time (and now in retrospect 45 years later) was that this was an unbelievably exciting time in the physical sciences at the University of Chicago. My physical chemistry course was taught by Harold Urey for two quarters and in the third quarter by Edward Teller; inorganic chemistry was given by Henry Taube; radiochemistry by Libby. I also attended courses on Nuclear Physics given by Maria Goeppert Mayer and by Fermi. (The chemistry student grapevine said, "Go to any lecture that Fermi gives on any subject"). Urey and Fermi already had been awarded Nobel Prizes, and Libby, Mayer and Taube were to receive theirs in the future.

My thesis concerned the chemical state of cyclotron-produced radioactive bromine atoms. The nuclear process not only creates a radioactive atom, but breaks it loose from all of its chemical bonds. These highly energetic atoms exist only in very, very low concentration, but can subsequently be traced by their eventual radioactive decay. Bill Libby gave his graduate students an unusual amount of leeway in how they chose to use their time, and was a superb research superviser - supporting, encouraging, but never letting one forget that intensive critical thought, together with unrelenting hard work on experiments, underlay all progress in our research.

My interest in competitive athletics also continued unabated in graduate school. Because of the atypical structure of its undergraduate college system, the University of Chicago, unlike almost all other American universities, permitted graduate students to compete in intercollegiate athletics. During my first graduate year, I played both basketball and baseball for the University teams. I continued to play baseball for the University during the spring for two more years, and spent both of those summers playing semi-professional baseball for a Canadian team in Oshawa, Ontario. Each winter I also played for several basketball teams around the city of Chicago.

Without a doubt, however, the major extracurricular event of those four years at the University of Chicago was meeting and then marrying on June 7, 1952, Joan Lundberg, also a graduate of the University. We have now shared more than 43 years of married life - and shared is really the descriptive word. I finished my Ph. D. thesis in August of 1952, and we went off to Princeton University in September of that year for my new position of Instructor in the Chemistry Department. Our daughter Ingrid was born in Princeton in the summer of 1953, and our son Jeffrey in Huntington, Long Island, in the summer of 1955.

In each of the years from 1953-55, I spent the summer in the Chemistry Department of the Brookhaven National Laboratory. An early experiment there of putting a powdered mixture of the sugar glucose and lithium carbonate into the neutron flux of the Brookhaven nuclear reactor resulted in a one-step synthesis of radioactive tritium-labeled glucose, an article in Science, and a new sub-field of tritium "hot atom" chemistry. The A.E.C. also expressed considerable interest in this tracer chemistry, and offered support for continuation of the research.

In 1956, I moved to an Assistant Professorship at the University of Kansas, which had just completed a new chemistry building including special facilities for radiochemistry. Contract support from the A.E.C. was already approved, and in place when I arrived that summer. Several excellent graduate students interested in radiochemistry joined my research group that summer, and were shortly joined by others and by a series of postdoctoral research associates, including many from Europe and Japan. This research group was very productive for the next eight years, chiefly investigating the chemical reactions of energetic tritium atoms and I moved through the ranks to a full Professorship. Both Ingrid and Jeff grew up knowing the members of the group - meeting everyone at our regular home seminars, and from an early age occasionally visiting the laboratory. During these Kansas years, too, the everyday routine was that the entire family came home for lunch. Later on in California, Ingrid and Jeff each worked regularly (but unpaid) drafting slide and journal illustrations for the chemistry department, and thereby continuing to know the members of my research group.

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