Photochemistry of Environmental Aquatic Systems

288 Pages·1987·4.93 MB·English

by  Rod G. Zika and William J. Cooper (Eds.)

ACS SYMPOSIUM SERIES 327
Photochemistry
of Environmental
Aquatic Systems
Rod G Zika EDITOR
University of
William J. Cooper, EDITOR
Florida International University
Developed from a symposium sponsored by
the Divisions of Geochemistry and Environmental Chemistry
at the 189th Meeting
of the American Chemical Society,
Miami Beach, Florida,
April 28-May 3, 1985
American Chemical Society, Washington, DC 1987
In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
Library of Congress Cataloging-in-Publication Data
Photochemistry of environmental aquatic systems.
(ACS symposium series, ISSN 0097-6156; 327)
"Developed from a symposium sponsored by the
Divisions of Geochemistry and Environmental
Chemistry at the 189th Meeting of the American
Chemical Society, Miami Beach, Florida, April 28-
May 3,1985."
Bibliography: p.
Includes index.
1. Environmental chemistry—Congresses
2. Photochemistry—Congresses
Congresses. 4. Aquatic ecology—Congresses.
I. Cooper, William J. II. Zika, Rodney G., 1940-
III. American Chemical Society. Division of
Geochemistry. IV. American Chemical Society.
Division of Environmental Chemistry. V. American
Chemical Society. Meeting (189th: 1985: Miami
Beach, Fia.) VI. Series.
TD193.P48 1987 628.Γ61 86-26489
ISBN0-8412-1008-X
Copyright© 1987
American Chemical Society
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PRINTED IN THE UNITED STATES OF AMERICA
In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
ACS Symposium Series
M. Joan Comstock, Series Editor
Advisory Board
Harvey W. Blanch
University of California—Berkeley USDA, Agricultural Research Service
Alan Elzerman W. H. Norton
Clemson University J. T. Baker Chemical Company
John W. Finley James C. Randall
Nabisco Brands, Inc. Exxon Chemical Company
Marye Anne Fox W. D. Shults
The University of Texas—Austin Oak Ridge National Laboratory
Martin L. Gorbaty Geoffrey K. Smith
Exxon Research and Engineering Co. Rohm & Haas Co.
Roland F. Hirsch Charles S.Tuesday
U.S. Department of Energy General Motors Research Laboratory
Rudolph J. Marcus Douglas B. Walters
Consultant, Computers & National Institute of
Chemistry Research Environmental Health
Vincent D. McGinniss C. Grant Willson
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In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
Foreword
The ACS SYMPOSIUM SERIES was founded in 1974 to provide a
medium for publishing symposia quickly in book form. The
format of the Series parallels that of the continuing ADVANCES
IN CHEMISTRY SERIES except that in order to save time the
papers are not typese
by the authors in camera-ready form. Papers are reviewed under
the supervision of the Editors with the assistance of the Series
Advisory Board and are selected to maintain the integrity of the
symposia; however, verbatim reproductions of previously pub
lished papers are not accepted. Both reviews and reports of
research are acceptable, because symposia may embrace both
types of presentation.
In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
Preface
PHOTOCHEMISTRY IS NOT A NEW FIELD, and numerous books exist that
cover the basic areas of photochemistry. The study of photochemistry in
aqueous solutions, and in particular its application to environmental
processes, is relatively new.
The field of aquatic photochemistry encompasses a wide diversity of
areas within environmental science. Natural waters receiving solar radiation
are active photochemica
secondary processes are occurring
with both living and nonliving particulate matter. Naturally occurring humic
substances are relatively efficient initiators of photochemical reactions.
Many xenobiotic chemicals in natural waters undergo either direct or
indirect photochemical transformations.
Within the areas of water and waste water, photochemistry may be the
result of open contact chambers or it may be used as a treatment process
to control organic compounds or for disinfection. Within atmospheric
studies, numerous photochemically induced reactions occur in cloud
droplets.
Studies that are presently being conducted in the field range from
purely phenomenological descriptions of photoproducts in the environment
to time-resolved laser spectroscopic studies of primary photoprocesses. This
book is a first attempt to compile chapters dealing with this complex and
rapidly evolving field of environmental science.
ROD G. ZIKA
Rosenstiel School of Marine and Atmospheric Sciences
Division of Marine and Atmospheric Chemistry
University of Miami
Miami, FL 33149
WILLIAM J. COOPER
Drinking Water Research Center
Florida International University
Tamiami Campus
Miami, FL 33199
August 23, 1986
vii
In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
Chapter 1
Introduction and Overview
William J. Cooper1 and Frank L. Herr2
1Drinking Water Research Center, Florida International University, Miami, FL 33199
2Office of Naval Research, Code 422CB, Arlington, VA 22217
Sunlight that arrives at the surface of the earth contains
substantial amounts of energy When surface waters which contain
either natural or anthropogeni
sunlight, light initiate
Sunlight induced photochemical reactions in surface waters may
broadly be defined as environmental aquatic photochemistry. Within
aquatic photochemistry, it is possible to envision reactions
involving either inorganic and/or organic molecules. These
chemicals could be either natural or anthropogenic and may
participate in either homogeneous or heterogeneous reactions. Very
often in these environments, a complex array of primary and
secondary photoprocesses are occurring simultaneously.
Surface waters are diverse in nature. They might be near
shore or inland wetland environments or mid-oceanic oligotrophic
water. Until recently, sunlight induced photochemistry was not
recognized as an important pathway for the transformation of
natural and anthropogenic chemicals in surface waters. It is now
well established that photochemically mediated processes are
important in most, if not all, areas of aqueous phase environmental
chemistry. Both direct, primary, and indirect photoprocesses have
been documented in natural waters.
The fact that all of these possibilities exist is exciting.
There are a seemingly endless number of combinations and
permutations to study; however, caution should be used. The
potential factors affecting any one study are so complex that
extreme care must be taken when interpreting data obtained from
natural systems. On the other hand, extrapolating data obtained in
laboratory studies to natural environments generally requires the
use of many assumptions.
Numerous reviews have been published that detail various
aspects of aquatic photochemistry (1-11). This book brings
together a group of papers representing a number of topics, i n
order to provide the reader with an appreciation of the complexity
of the field and, at the same time, a glimpse of various areas
0097-6156/87/0327-0001$06.00/0
© 1987 American Chemical Society
In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
2 PHOTOCHEMISTRY OF ENVIRONMENTAL AQUATIC SYSTEMS
within the relatively young field of environmental aquatic
photochemisty. This book is not an exhaustive compilation of the
field of environmental aquatic photochemistry.
Direct Photoreactions
A wide variety of substances with active chromophores at
wavelengths found in the surface solar spectrum occur in natural
waters. Some of these substances undergo direct photolysis, that
is a chemical change that results as a direct consequence of the
absorption of photons by the substance. Conceptually, direct
photoreactions are the simplest and usually the easiest type of
process to study in natural waters. Since the reaction proceeds
rapidly to products from the primary excited state manifold, the
physical characteristics of the reactant's environment usually have
only small effects on the reaction. Such reactions can often be
studied in pure and/o
reactant·
Comparatively few natural molecules f i t into this category
(5), and most of these exhibit only weak absorbances at the high
energy threshold of the surface solar spectrum. Most naturally
occurring compounds are therefore quite transparent to incident
solar radiation and reactions of these compounds which proceed via
direct photolysis are the exception rather than the rule. Most
examples of direct photochemical reactions are found among the
numerous studies done on xenobiotic substances (1,6), where the
environmental rate is often derived from laboratory measurements
which are carried out in organic solvents because of the limited
solubility of the compounds. If the electronic absorption spectra
and the quantum yield for the compound are determined in water or
in an organic solvent system which gives a good approximation to
water, then the calculation of an environmental rate is a
relatively simple matter. For more complex molecules, the reaction
quantum yield is generally wavelength independent (12) and the
direct photolysis rate constant can be computed for a specific
location and time from the electronic absorption spectra, the
quantum yield, and the solar spectral irradiance.
Indirect Photoreactions
The high light transparencies of most compounds in natural water to
solar radiation dictate that direct photo-reactions of these
compounds are either not possible or represent only a minor
reaction pathway. Indirect photochemical reactions for such
compounds can only occur either through reaction with reactive
molecules in ground or excited states that are themselves products
of primary photochemistry, or through photosensitized reactions in
which the excited state species of some chromophore transfers an
electron or energy to the compound. The importance of both of
these indirect reaction pathways in natural water systems is now
recognized. A substantial amount of evidence exists in the
literature implicating indirect reaction mechanisms in the
photochemistry of both xenobiotic and natural compounds (4,5,6,10).
The complexity of natural water systems and the host of
In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
1. COOPER AND HERR Introduction and Overview 3
simultaneous photo-reactions which are occurring frequently make a
clear mechanistic interpretation of the data difficult.
Secondary photo-oxidants. There are three recognized primary
sources of secondary oxidants in natural aquatic environments. The
first of these arises from in situ primary photo-reactions which
often produce free radicals and other reactive products. The range
of reactivity for free radicals varies from the very strong
oxidizing species like OH to weakly oxidizing and more selective
species like CO^ , NO, and 0 · Subsequent reactions of these
2
radicals can lead to other reactive products. For example, OH in
seawater and other halide containing_waters will rapidly convert to
dihalide anion radicals like Br^ and I (5, 13). Hydrogen
2
abstraction reactions from organic compounds by such inorganic
radicals can produce radicals which in turn might react with oxygen
to form organo-peroxy radicals and possibly peroxides. Reactive
non-radical products ca
example is believed t
which in turn is generated primarily from dissolved humic
substances (HS) (14-16). The compexity of the reaction sequence is
shown in the following equations:
*
HS + hr > HS li]
HS* + 02 > HS+' + V [2]
20 "' + 2H+ > [3J
2 H2°2 + °2
HS+* +0 > HS0 #+ [4]
2 2
HS0 '+ + RH > HS0 H + R* + [5]
2 2
H 0 > 20H' [6]
2 2
HS0 H > HSO* + OH* 17]
2
OH' + X" > HOX"" [8]
HOX"' + X" > OH" + V [9]
(where X is a halide)
Although this set of reactions does not indicate a l l of the
possible steps involved, i t is interesting to note that the initial
primary photoprocesses resulted in 10 secondary products. In
addition, the two peroxide molecules (i.e. H 0 and HSO^H) are new
2 2
chromophores and function as reservoirs for further radical
production.
A second source for secondary oxidants comes from thermal
reactions when reactive photo-products such as H 0 react with
2 2
other constituents of the water. Reactions of peroxides with
various transition metals, for instance, will produce 0 or OH
2
radicals and often the metal ion is converted to a new reactive
oxidation state. This sort of process has been demonstrated for
In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
4 PHOTOCHEMISTRY OF ENVIRONMENTAL AQUATIC SYSTEMS
the Cu(II)/Cu(I) couple in seawater (17). Reactions such as these
may be far removed from the initial photochemical process; however,
they are a consequence of it and may have the important function of
acting as a redox buffer in environmental water systems.
The third source of secondary photo-oxidants arises from
fluxes of atmospheric oxidants through the surface of natural water
bodies. Although these products do not originate from aquatic
photo-reactions, they nevertheless augment the in situ reaction
sources of seconday reactive products and must be taken into
account, especially when attempting to quantify reaction processes
in the steep gradient region near the air-water interface.
Thompson and Zafiriou (18) have examined the chemical impact on
natural waters and have calculated air-water fluxes for many of the
diverse atmospheric oxidants.
Photosensitized Processes. One of the most studied indirect
photoreactions is th
energy transfer from th
chromophores in environmental waters. This process has been
observed in both seawater (19) and freshwater (20,21). The
evidence suggests that singlet oxygen is a common product in
natural water systems, but its importance relative to other
secondary photo-products in affecting the chemistry is uncertain.
Its limited significance is a function of its selective reactivity,
low rate constants and its rapid relaxation to the ground state in
aqueous media.
Photosensitization via energy transfer in dilute solutions of
sensitizer and receptor is in general a low efficiency process.
This will probably hold true for most environmental aquatic
systems. Much higher efficiencies might, however, be obtained in
heterogeneous reaction environments such as micelles, particles, or
other interfaces where sensitizer and reactant are concentrated.
Photosensitization by electron transfer, as shown in equation
2 above, has also been observed (22,23). The occurrence of 0^
(14,16) in natural waters is good evidence that these processes are
occurring, at least for oxygen reduction. With electron acceptors
other than oxygen, this process is probably similar to energy
transfer in that there is a low probability for other electron
acceptors to be involved except in heterogeneous environments.
Heterogeneous Reactions
Surface mediated processes are also an important consideration in
natural water photochemistry. In aqueous media, two different
surface/interfaces may occur that result in heterogeneous
reactions. The two interfaces considered here are liquid-solid and
liquid-liquid. Surface processes in geochemistry and aquatic
enviroments have been covered in more detail in two recent books
and the reader is referred to these volumes for more details
(24,25).
Natural water systems often contain particulate matter. The
particulate matter may be either living or non-living. Algae are
the predominant component of the living particulate matter. The
In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
1. COOPER AND HERR Introduction and Overview 5
non-living particulate matter is primarily inorganic in
composition, but may have organic matter associated with i t .
Particulate-liquid interfaces are known to participate in thermally
and/or photochemically mediated reactions.
Surface microlayers are implicated in many chemical processes.
They are exposed to the full solar spectrum of light arriving at
the surface, and are often important in photochemically mediated
reactions. The microlayer, associated with most natural waters, is
considerably different in chemical composition from the underlying
water column (26). Hence, the photochemically mediated reactions
that take place in this layer may differ substantially from those
in the water column. The differences in the reactions may be one
of kinetics (rates of the reaction), or maybe mechanistic in nature
and the reaction(s) proceed(s) via different pathways resulting in
different reaction products.
One extreme case of the liquid-liquid interface is the oil
spill (slick) problem.
recently been reviewed
processes may and do occur simultaneously, and that the chemistry
associated with studies of this type are extremely complex ·
Natural microlayers form on most water surfaces. There are
numerous difficulties encountered when studying them. One of the
most perplexing problems centers around the collection of natural
samples of microlayers. Another difficulty is that processes
occurring in the microlayer are often not well characterized (26).
Thus, innovative approaches are required to study processes similar
to those in natural waters.
In the case of particulate matter, different types of
reactions may be involved. Reactions that occur on the surfaces of
the non-living particulate matter may involve direct
photochemically mediated reactions in surface complexes. Another
possibility is surface semiconductor redox reactions. In the case
of the living algal particulate matter, a third process would be
photosensitized reaction on the surface of algae.
The particulate surface heterogeneous reactions may result in
primary and/or secondary processes. Most of the examples of algal
associated processes suggest secondary reactions resulting from
exudates in the aqueous phase.
Quantifying Environmental Photoprocesses
As in many areas of environmental science, one of the most
difficult aspects of environmental photochemistry is extrapolating
laboratory based experiments to the natural environment. One tool
that is becoming used more frequently is that of mathematical
models to predict the distribution of photoproducts in the
environment (12). Modeling aquatic photoprocesses is complex, for
in order to describe in detail the observed products, i t is
necessary to understand quantum yields throughout the solar
spectrum, formation rates, in many cases decomposition rates (the
photoproducts are rarely conservative), absorbance characteristics
of the aquatic system, and physical mixing of the water masses.
In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1987.