1

Variation in cosmic and terrestrial radiation in the continental US. ( m r e d y )

Exposure in mremly

0 <50 60-70 80-90 1M120

50-60 70-80

will be 1 in 500 or 200 in 100 000. The risk from uranium mines under the proposals is therefore a much higher health risk than that now existing from elemental phosphorus plants and nearly two orders of magnitude higher than that proposed for elemental phosphorus plants. It should be noted, however, that the average lifetime of a mine is 5-15 y. so an individual would probably not be subject to this risk for more than 15 y.

The widely divergent potential health risks under these proposed regulations raise the questions of what level of health risk is so low that it should not be reduced by any regula- tion or what level is so high that addi- tional means besides the regulation should be taken to protect the general population. In other words, is there a level of health risk so small that we should call it trivial, or is there a risk so high that an educational campaign

should be initiated to inform the ex- posed population of this risk? Some would argue that no risk is too small if it can be reduced still further. But since it is obviously not possible to regulate every radioactive atom or carcinogenic molecule emitted, establishing a gen- erally acceptable minimum health risk above which regulation should be considered is perhaps an important area of research and discussion for those pollutants for which no threshold can be established.

One very common argument in these discussions is to compare the risks from uranium mine emissions, for example, to the risks from riding in an automobile at 60 mph or from taking one ride in an airplane. Here, volun- tary risks are being compared to what are usually considered involuntary risks. The people living near a facility emitting radionuclides may not be free to move for economic or other reasons. Furthermore, they may be totally un- aware of any risk from such a facility. Such comparisons of voluntary vs. in- voluntary risks may not serve to ad- vance rational discussion because two essentially different kinds of risks are involved.

In establishing a general, maximum acceptable health risk, the population exposed might also be taken into con- sideration. A lifetime health risk of 1 in 500 for 30 000 people might be of much more concern than it is for 30. For example, at present, there are currently only about 30 individuals living so near active uranium mill tailings that they might be subject to the maximum risk. Anotherconsider- ation might be how the risk and the population are expected to change over time. Some risks might last for thou- sands of years with little change, such as those from uranium mill tailings. Others might be expected to last only 100 or fewer years, such as those from the vents from a particular uranium mine.

Health risks resulting from regula- tions developed within EPA's Office of Radiation Protection vary widely. If the risks to the general population from low levels of ionizing radiation or from other pollutants are compared across federal agencies, they are found to vary even more. This situation ap- pears inconsistent with EPA Admin- istrator Ruckelshaus's June 1983 speech before the NAS, in which he urged that "our science analysis be vigorous and the quality of our data be high" and that "we. . . make uniform the way in which we manage risk across the Federal regulatory agen- cies." -Bette Hileman

Environ. Sci. Technol.. VoI. 17. No. 12, 1983 507A

Photochemistry of natural

Scientists at a recent workshop sponsored by NATO assessed the

progress in this new area of research

Classical photochemistry, which began in the mid-19th century, in- volved studies of the reactions of chemicals in a solvent exposed to a band of radiation. After irradiation, the changes in the chemicals were ob- served.

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 re- actions in water systems. The major portion of the solar energy that pene- trates.the atmosphere is absorbed by the oceans, which occupy 70% of the Schematic diagram of important processes in environmental photochemistry Earth's surface. Much of this energy in water and air is used for photosynthesis and heating the ocean's surface. The remainder is the Ocean is potentially available to new area of photochemistry assembled absorbed and can initiate photo- initiate chemical reactions, it does not recently at a workshop that was spon- chemical reactions that could play a follow that all of this light initiates sored by the Marine Sciences Panel of significant role in the chemistry and photoreactions. Most of it is used in the NATO Scientific Affairs Program biology of the oceans. photophysical processes, for example and held at the Woods Hole Oceano-

As sunlight penetrates a column of energy transfer, fluorescence, and graphic Institution on Cape Cod. The freshwater or coastal marine water, the phosphorescence. Under certain con- workshop brought together 35 scien- great bulk of the solar radiation is ab- ditions an excited species does not tists, many of whom had not previously sorbed by naturally dissolved or par- react chemically but serves as a donor: met. Some are performing laboratory ticulate substances. Research con- Through energy transfer it excites studies; others are working in the real ducted during the past few years indi- another molecule, the acceptor, which world of freshwater and marine cates that a small but significant par- becomes the reactive species. chemistry. The field of photochemistry tion of the radiation absorbed by the The efficiency of either the photo- has grown considerably in the past dissolved natural substances results in chemical or photophysical process is a three years. (A critical review of the the formation of reactive species- function of both the properties of the field will be published in ES&Tearly electronically excited molecules-that reaction environment and the charac- next year.) are capable of participating in a variety ter of the species in an excited state. Photochemistry plays a significant of chemical reactions. The fundamental quantity that is used role in the marine environment. But an

Basically, all photochemical reac- to describe the photoefficiency of any understanding of marine photochem- tions must satisfy a fundamental re- photo process is the quantum yield, ical processes and their effects on the quirement; only light that is absorbed which can be generally defined as the organic fraction is still only rudimen- by a system can induce a chemical re- number of events occurring divided by tary. An examination of the light ab- action. Although 80-9Wo of the visible the number of photons absorbed. sorbers in seawater reveals that many and ultraviolet radiation incident upon Scientists working in this exciting potential reactions are possible in the

5661 Environ. Sci. Technol.. VoI. 17. No. 12, 1983 0013-936X/83/0916-0568/\$01.50/0 @ 1983 American Chemical Society

, p, o n

Humic substances are found dis- solved in all waters, but they are not always the same chemically. In every case they are mixtures of complex or- ganic chemicals. Photochemistry may be implicated in the formation of ma- rine humic substances. Whereas ma- rine humic substances are remarkably homogeneous, terrestrially derived humics are more diverse. Yet a third class of humics is found in fresh- water.

By definition, humic acids are acid-insoluble, base-extractable ma- terials; 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 contain- ing 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, per- haps a half dozen appear to be involved in photochemical processes. They are

I,mIRC1 pnoro p Co (I1 and 111). Cu(Il), Fe(III), Mn(II), Hg(II), and Cr(II1).

Organic free radicals were discov- ered some 60 years ago. Many kinds of organic free radicals have been studied in the gas phase and in organic sol- vents, but relatively few studies have been carried out in water itself. Al- though only a few percent of studies are in water, hundreds of organic compounds have been studied super- ficially in water. Scientists now rec- ognize that organic free radicals are ubiquitous in the environment and in biological systems. But an under- standing 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 con- centrations, photolysis of the inorganic species, nitrite, also yields the radicals NO* and HO-, in an interesting ana- logue to their formation and role in the atmosphere.

Of particular interest to environ- mental chemists are oxy free radicals that could be formed from peroxides, excited humic acids, and hydrogen peroxide. The chemistry of these rad- icals is well understood, and absolute rate constants are available for their

~

(photosensitized and

Syrnbob Pw = Chemical di88Oiw.3 in watee Ps = chemksl sort& on sediments 01 micmbiOIa; PA = chemic# in its vapor S1a10: PAS = Chemical sort& on atmo8pheriC partioulater: Pew = chemical in cloud (ImpIet~

marine environment. Many more re- actions and significant processes, as yet not understood, are occurring. In a review article in 1981, Rod Zika closed by saying, “The experimental eluci- dation of these processes is compli- cated by the simultaneous involvement of a maze of reactions and conditions which are controlling them.”

In classical photochemistry, reac- tions that are very slow are usually considered unimportant, and hence of little interest. But in the marine envi- ronment, very slow reactions can be significant if they are the only operat- ing mechanism for a particular process or if they compete favorably with other abiotic or biotic mechanisms contrib- uting to the same process. The ab- sorption of seawater in the region be- tween 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.

me broth

In the atmosphere, reactive species include ozone, hydrogen peroxide, CH302H. radical, peroxyl radical ( H o p ) , formaldehyde, and hydroxyl radical (Hoe). Several of these species are also involved in photochemical transformations in natural water sys- tems. However, it is impossible to quantify the fluxes (concentrations of species per surface area) of these species in water systems at this time.

Environ. Scl. Techml.. Vol. 17. No. 12, 1983 5091

Roger Adams: Scientist and Statesman Sy D. Stanley Tarbell snd 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, 191 2-16 Illinois, 191b26 Academic Progress Service and Research to 1942 Government Service, 1940-48 lllinoisand Research, 1943-67 Broader Horizons Career Achievements of Roger Adams‘s Ph D s. 1918-58 Career Achievements of Roger Adams s Postdoctorates, 193659

240 pages (1 981) Clothbound US & Canada $13.95 Export $16.95

240 pages (1981) Paper US & Canada $9.95

LC 81-17625 ISBN 0-841 2-0598-1

Export $1 1.95 LC 81 -1 7625 ISBN 0-841 2-071 1-9

Order from: American Chemical Society Distrlbution Office - 16 1155 Slxteenth St., N.W. Washington, D.C. 20036 or CALL TOLL FREE 800-424-6747 and use your credit card.

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.

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 natura1 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 a t this time. Scientists agree that its formation is tied closely to the production of intermediary hy- droperoxyl radical and its anion (*02H/02-).

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 oxidizing species.

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 ps). 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/m2 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 sea- water 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. I n “Chemical Oceanogra-

phy”; Riley J. P.; Chester, R., Eds.; Academic Press: United Kingdom, 1983; Chapter 48, “Natural Water Photochemistry,” pp.

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.

Zika, Rod G. I n “Marine Organic Chemistry”; Elsevier Oceanography Series, 3 1 ; Elsevier Scientific Publishing Company: New York, N . Y . , 1981; Chapter 10, “Marine Organic Photochemistry,” pp. 299-326.

339-16.

293-427.

570A Environ. Sci. Technol., Vol. 17, No. 12, 1983