Photochemistry of natural water systems Scientists at a recent workshop sponsored by NATO assessed the progress in this new area of research , Photolysis PAW
Classical photochemistry, which began in the mid-19th century, involved studies of the reactions of chemicals in a solvent exposed to a band of radiation. After irradiation, the changes in the chemicals were observed. In recent years, these studies have been extended to chemicals in natural waters—both marine and freshwaters. Marine and freshwaters are a veritable broth of chemicals, a complex mixture of dissolved organic and inorganic chemicals, transition metal ions, and biological species. Exposure of this chemical broth to sunlight causes the formation of reactive species that lead to further reaction among the broth's ingredients. The sun is the light source and the energy supply for photochemical reactions in water systems. The major portion of the solar energy that penetrates the atmosphere is absorbed by the oceans, which occupy 70% of the Earth's surface. Much of this energy is used for photosynthesis and heating the ocean's surface. The remainder is absorbed and can initiate photochemical reactions that could play a significant role in the chemistry and biology of the oceans. As sunlight penetrates a column of freshwater or coastal marine water, the great bulk of the solar radiation is absorbed by naturally dissolved or particulate substances. Research conducted during the past few years indicates that a small but significant portion of the radiation absorbed by the dissolved natural substances results in the formation of reactive species— electronically excited molecules—that are capable of participating in a variety of chemical reactions. Basically, all photochemical reactions must satisfy a fundamental requirement; only light that is absorbed by a system can induce a chemical reaction. Although 80-90% of the visible and ultraviolet radiation incident upon 568A
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As"r
Dust
PAW1, PAS
Photolysis
Suspended particles
Mixing
Sedimentation
Bottom sediment
Schematic diagram of important processes in environmental photochemistry in water and air
Environ. Sci. Technol., Vol. 17. No. 12, 1983
the ocean is potentially available to initiate chemical reactions, it does not follow that all of this light initiates photoreactions. Most of it is used in photophysical processes, for example energy transfer, fluorescence, and phosphorescence. Under certain conditions an excited species does not react chemically but serves as a donor: Through energy transfer it excites another molecule, the acceptor, which becomes the reactive species. The efficiency of either the photochemical or photophysical process is a function of both the properties of the reaction environment and the character of the species in an excited state. The fundamental quantity that is used to describe the photoefficiency of any photo process is the quantum yield, which can be generally defined as the number of events occurring divided by the number of photons absorbed. Scientists working in this exciting
new area of photochemistry assembled recently at a workshop that was sponsored by the Marine Sciences Panel of the NATO Scientific Affairs Program and held at the Woods Hole Océanographie Institution on Cape Cod. The workshop brought together 35 scientists, many of whom had not previously met. Some are performing laboratory studies; others are working in the real world of freshwater and marine chemistry. The field of photochemistry has grown considerably in the past three years. (A critical review of the field will be published in ES& Τ early next year.) Photochemistry plays a significant role in the marine environment. But an understanding of marine photochem ical processes and their effects on the organic fraction is still only rudimen tary. An examination of the light ab sorbers in seawater reveals that many potential reactions are possible in the
0013-936X/83/0916-0568A$01.50/0
© 1983 American Chemical Society
OH
Dtrect photolysis PA
Direct photolysis Mixing
Pw
Indirect photo processes (photosensitized and free radicals)
Symbols: P w = chemical dissolved in water; P E = chemical sorbed o n sediments or microbiota; P A = chemical in its vapor state; P A S = chemical sorbed on atmospheric particulates; P AW = chemical in cloud droplets.
marine environment. Many more reactions and significant processes, as yet not understood, are occurring. In a review article in 1981, Rod Zika closed by saying, "The experimental elucidation of these processes is complicated by the simultaneous involvement of a maze of reactions and conditions which are controlling them." In classical photochemistry, reactions that are very slow are usually considered unimportant, and hence of little interest. But in the marine environment, very slow reactions can be significant if they are the only operating mechanism for a particular process or if they compete favorably with other abiotic or biotic mechanisms contributing to the same process. The absorption of sea water in the region between 290 and 700 nm is a gradual curve with maxima in the red and in the ultraviolet. For most processes in spectroscopy and photochemistry,
water is considered to be transparent to near ultraviolet and visible light. Therefore, in the marine environment, only those compounds that absorb at wavelengths longer than 290 nm are candidates for primary photochemical processes. This is because sunlight does not go below 290 nm, and in the oceans the "broth" is dilute enough that water does absorb some of this light. The broth In the atmosphere, reactive species include ozone, hydrogen peroxide, CH3O2H· radical, peroxyl radical (HO2·), formaldehyde, and hydroxyl radical (HO·). Several of these species are also involved in photochemical transformations in natural water systems. However, it is impossible to quantify the fluxes (concentrations of species per surface area) of these species in water systems at this time.
Humic substances are found dissolved in all waters, but they are not always the same chemically. In every case they are mixtures of complex organic chemicals. Photochemistry may be implicated in the formation of marine humic substances. Whereas marine humic substances are remarkably homogeneous, terrestrially derived humics are more diverse. Yet a third class of humics is found in freshwater. By definition, humic acids are acid-insoluble, base-extractable materials; fulvic acids are soluble in acids and bases. In terms of chromophores and functional groups, one recent theory is that the marine humics are unsaturated fatty acid polyelectrolyte mixtures. The fulvics, on the other hand, appear to be phenol carboxylates and aromatic dicarboxylates containing polyelectrolyte mixtures. In aquatic environments, transition metal ions are present in very low concentrations. Some of these metal ions are essential nutrients; others are highly toxic to aquatic organisms. Although some 20 transition metal ions are found in natural waters, perhaps a half dozen appear to be involved in photochemical processes. They are Co (II and III), Cu(II), Fe(III), Mn(II), Hg(II), and Cr(III). Organic free radicals were discovered some 60 years ago. Many kinds of organic free radicals have been studied in the gas phase and in organic solvents, but relatively few studies have been carried out in water itself. Although only a few percent of studies are in water, hundreds of organic compounds have been studied superficially in water. Scientists now recognize that organic free radicals are ubiquitous in the environment and in biological systems. But an understanding of the kinds of radical species that occur in natural waters is still very limited. Free radicals, nevertheless, are formed on photolysis of natural waters. The source of free radicals, as well as oxidizing species, appears to be the dissolved humic materials in natural water systems. In ocean waters, where humic materials are in fairly low concentrations, photolysis of the inorganic species, nitrite, also yields the radicals N O · and H O · , in an interesting analogue to their formation and role in the atmosphere. Of particular interest to environmental chemists are oxy free radicals that could be formed from peroxides, excited humic acids, and hydrogen peroxide. The chemistry of these radicals is well understood, and absolute rate constants are available for their Environ. Sci. Technol., Vol. 17, No. 12, 1983
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reactions with a wide range of organic compounds in organic solvents or in the gas phase. However, interactions of oxy radicals with inorganic species, particularly those found in aquatic systems, are now largely unknown. Radicals can be detected and char acterized by ESR spectroscopy and spin traps, but using this latter tech nique in seawater presents serious drawbacks. The technique, flash spectroscopy, promises to provide useful kinetic information for envi ronmental samples, but a disadvantage is that it does not allow specific iden tification of chemical intermediates. Although many photochemical re actions produce free radicals in natural waters, they are not the only source of radicals. Two nonphotochemical sources may be important in many environments; these are biological and thermal chemical processes. Docu mentation of these processes is still very sketchy. The evidence was re viewed by Oliver Zafiriou in a review published this year.
Roger Adams: Scientist and Statesman by D. Stanley Tarbell and Ann Tracy Tarbell
Examines the life of Roger Adams — a man unparalleled in his con tribution to the development of organic chemistry. A comprehensive biographical sketch of Roger Adams, whose work was important in the development of American chemistry and chemical education. Adams's early years, edu cation, and career achievements in academia, industry, research, and government are described. His con tributions to Illinois chemistry in par ticular and the education of chemists are expounded. CONTENTS Introduction · Early Years and College · Germany and Harvard, 1912-16· Illinois, 1916-26 · Academic Progress · Service and Research to 1942 · Government Service, 1940-48 «Illinois and Research, 1943-67 · Broader Horizons · Career Achievements of Roger Adams's Ph.D.s. 1918-58 · Career Achievements of Roger Adams's Postdoctorates, 1936-59 240 pages (1981) Clothbound US & Canada $13.95 Export $16.95 LC 81-17625 ISBN 0-8412-0598-1 240 pages (1981) Paper U S & Canada $9.95 Export $11.95 LC 81-17625 ISBN 0-8412-0711-9 Order from: American Chemical Society Distribution Office —16 1155 Sixteenth St., N.W. Washington, D.C. 20036 or CALL TOLL FREE 800-424-6747 and use your credit card.
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Oxidizing species Produced as intermediates or final products when waters are irradiated by sunlight, oxidizing species were first discovered in water systems in 1966; hydrogen peroxide was found by sci entists in near-shore waters of the Gulf of Mexico, off the Texas coast. In the past three years, there have been many measurements of hydrogen peroxide concentrations in a variety of natural and polluted aquatic systems. Hydro gen peroxide is found in nearly all waters. Its presence is well established in most natural waters; one of its im portant reactions is to produce the strongly oxidizing hydroxyl radical (HO·). Hydrogen peroxide is formed in natural water samples on irradiation, but disappears fairly rapidly in reac tions in the dark. The reactions that destroy hydrogen peroxide can only be speculated on at this time. Scientists agree that its formation is tied closely to the production of intermediary hydroperoxyl radical and its anion (.O2H/O2-). Hydroperoxides and peroxy radi cals, two other oxidizing species, have also been shown to exist in natural waters, although by indirect methods. They cannot be measured by direct methods. Although there are only a few references to these species in the chemical literature, scientists have been active in proposing and testing hypotheses on the mechanism for the formation and destruction of these I oxidizing species.
Environ. Sci. Technol., V o l . 17, No. 12, 1983
The formation of singlet oxygen has been shown to occur; this species is involved in the phototransformation of dissolved organic materials in the aquatic environment. One reason why singlet oxygen is such an important intermediate in various photochemical and photophysical processes is its rel atively long lifetime in solution (3 μβ). This long lifetime makes possible the oxidation of various substrates such as polyunsaturated fatty acids, sterols, and amino acids. Scientists point out that oxidation by the singlet oxygen species is greatly reduced in the open sea, but that it cannot be neglected on a large time scale. Nevertheless, singlet oxygen is important as an intermediate in the phototransformation of some pollutants and other materials in nat ural water systems. Ozone, another species, is also im portant; its flux has been approxi mated. Globally, approximately 15 mmol/m 2 of ozone impinge on the surface of the ocean. As much as 98% of the available ozone can be ac counted for by its reaction with iodide (I~) in ocean waters. This means that ocean water is a sink for atmospheric ozone. This significant observation awaits future confirmatory studies. The majority of inorganic anions in natural water systems do not undergo reactions in direct sunlight; however, there are exceptions. For example, nitrite ion leads to the production of the hydroxyl radical. The main sig nificance of the nitrite anion in seawater may be that it produces this re active oxidizing species, not that it re moves nitrate from surface waters. Looking ahead The interactions of photoproduced species in natural water systems are beginning to be unraveled by scientists working in this area. In the future, scientists need to develop more sensi tive techniques for detecting and characterizing radical species in these systems. —Stanton Miller Additional Reading Zafiriou, Oliver G. In "Chemical Oceanogra phy"; Riley J. P.; Chester, R., Eds.; Academic Press: United Kingdom, 1983; Chapter 48, "Natural Water Photochemistry," pp. 339-76. Zepp, Richard G. In "The Role of Solar Ultra violet Radiation in Marine Ecosystems"; Calkins, John, Ed.; Plenum Press, 1982; "Photochemical Transformations Induced by Solar Ultraviolet in Marine Systems," pp. 293-427. Zika, Rod G. In "Marine Organic Chemistry"; Elsevier Oceanography Series, 31; Elsevier Scientific Publishing Company: New York, N.Y., 1981; Chapter 10, "Marine Organic Photochemistry," pp. 299-326.
