Oliver C. Zafirinu Woods Hole Oceanographic Institution Woods Hole, Mass. 02543 Jacques Joussot-Dubien Universiti de Bordeaux 1 33405 Talence, France Richard G . Zepp Environmental Protection Agency Athens, Ga. 30613
Rod G . Zika University of Miami Rosenstiel School of Marine and Atmospheric Science Miam', Fla. 33149 Photochemical reactions may affect the photic zone, the surface sunlit layer of freshwaters, oceans, and estuarine regimes where fresh and saltwaters begin to interact. Hydrosphere photochemistry influencesthe world's oceans (about 70% of the Earth's surface), and the illuminated portion of the hydrosphere is especially active and diverse. 'Ransport, material exchanges, and biology (including aquatic primary pro3 S A Environ. Sci.Technol..MI. 18,No. 12. 1884
ductivity) are more intense in this area Sept. 12-16, 1983 (I). This review, than they are in deeper waters. drawn largely from the experience and In addition to being naturally signifi- reports of those present at that meeting, cant, this zone of surface waters is used surveys the area for the nonspecialist as the receptacle for many liquid, solid, and presents results, problems, and opand airborne wastes. At the same time, portunities for research. More detailed however, we view these surface layers references and discussion are available as essential for recreation, aesthetics, in recent reviews (2-4) and meeting abtransportation, food, and sources of stracts (5). freshwater. Thus it is important to unFor brevity, we cite primarily recent derstand the processes for the cycling references not mentioned in other reof materials in the photic zone. views. A much more extensive bibliogUntil the past decade, the role of pho- raphy, including references from other tochemical reactions as a component of reviews and from the NATO meeting photic zone processes was virtually ig- abstracts, is available from any of the nored, but now a growing number of authors (6). studies show numerous reactions and effects. New reports appear often, so Chemical and environmental scope the tally of a dozen elements (Mn, Fe, There are a number of photoreactions Cu, Hg, TI,C, N, 0, S, CI, Br, and I) that occur in the environment: Hydrcinvolved in these processes is likely to gen peroxide forms in freshwater and require frequent revision. seawater; the free radical NO reaches To explore progress and exchange detectable steady-state levels in the ideas in the field of freshwater and ma- equatorial Pacific; a photochemical (dirine photochemistry, an international mnethane) rearrangement product is group of scientists met under NATO found in shallow-water encrusting corsponsorship in Woods Hole, Mass., als (7);inorganic phosphate quantita0013936X184/0916-0358A$01.5010
0 1984 American Chemical Society
tively forms an iron-organic-phosphate complex in freshwater bog lakes; photolysis is the major sink for pentachlorophenol (PCP) in experimental ponds (8);a lampreycide photolyzes in a tributary of the Great Lakes (9); streams receiving the effluent from TNT manufacture rapidly redden in sunlight; the insecticide disulfoton p h e tooxidizes in rice paddies to sulfoxide; and spilled oil rapidly forms surfactants and toxic, water-soluble products. Table l summarizes important aspects of these photoprocesses and others, covering many of the elements affected by sunlight-induced photochemistry. Because many of these studies are continuing, the last two columns of Table l represent current thinking, not definitive conclusions, on probable mechanisms and likely effects. Figure 1 shows the rapidity of some of these processes and the good match between models and measurements in favorable cases. The examples in Table I contain noteworthy similarities between freshwater and saltwater systems. These include the widespread involvement of unidentified organic chromophores in processes involving either net redox reactions, energy transfer, or both, the almost universal importance of dissolved oxygen as an acceptor of energy or electrons and as a participant in secondary reactions, the common occurrence of secondary free radical reactions of organic and inorganic species, and the much higher incidence of direct photolysis of pollutants than of natural molecules of low molecular weight and known structure. Although these observations show factors common to all natural water systems, there are many important differences between freshwater and saltwater environments: Illuminated zones of freshwaters are generally much shallower and more variable than those of oceanic central gyres and pristine lakes having up to 1 % of surface light at 100-I50 m. Dissolved organic compounds and suspended particulates, living and nonliving, are variable in both environments but usually are one to four orders of magnitude lower in seawater. Seawater pH is 8 & 0.5, whereas freshwater pH is 7 & 3. Seawater's ionic strength is 0.7; freshwater's is 0.001-3. The anionic composition of seawater is constant, dominated by chloride and sulfate; freshwaters vary, but are usually dominated by bicarbonates. Thus, major differences between freshwater and saltwater are found. For example, OH radical reacts with the
abundant bromide ion in seawater, but this ion is nearly absent in freshwater. The naNre and concentrations of organic chromophores differ significantly, and in the ocean chloride ion is a better nucleophile than water. However, the prevalence of reports concerning direct photoreactions in fresh vs. saltwater is an artifact of the paucity of studies of anthropogenic compounds in seawater as compared with freshwater. Many synthetic compounds, such as polynuclear and heteroaromatic compounds, can absorb sunlight. But in nature, the simpler ions and molecules in freshwater and seawater are transparent to sunlight, and the dominant chromo phores are complex organic geopolymers, biopigments, and suspended minerals. Their structures often are unknown.
Underwater light and photolysis Solar radiation is fundamental to all the photochemical and photobiological processes of natural waters. The amount, spectral quality, and spatiotemporal distribution of sunlight are key variables at the surface and within the water column. The light field is best understood-and is often best quantified-by means of models. Recently, relevant measurements and models of the solar radiation field in the air and under water have undergone considerable development for use in atmospheric chemistry studies, for evaluating photobiological effects of solar UV, and for evaluating satellite imagery such as Coastal Zone Color Scanner results. The downwelling spectral irradiance at depth z, E(z,X), depends on three factors: atmosphere-transmitted irradi-
ance, which is incident on the water surface; transmission through the airwater interface; and optical properties of water and dissolved and suspended materials, which determine the spectral attenuation properties of the water column. Reliable estimates of surface insolation are available as a function of wavelength, solar elevation, and atmospheric optical properties, such as haze and ozone column (10-12), and can be made when the sky is clear or when it is cloudy. These estimates are extremely useful for longer time scales. However, field measurements often last anywhere from a few minutes to a few days, during which cloudiness frequently has an important effect that deviates from the long-term average. Loss of light in crossing the air-water interface can usually be neglected; it is only a few percent for solar elevations above 20 degrees. The difficult parameter to evaluate is the wavelength-dependent underwater light field, which also depends on the highly variable absorption properties of water. The diffuse attenuation cwfficient, KAX), which characterizes light penetration and mathematically defines absorption in a scattering medium, can be accounted for by the empirical model of Baker and Smith (13). This model agrees with marine observations to within *IO% between 300 nm and 700 nm. It accounts for KAX) as a sum of contributions from water, chlorophyll, dissolved organic material, and mineral matter (Figure 2). Models such as this one will prove useful for coastal and open ocean problems, although spatiotemporal variations in the amount and kinds of mineral matter and biopigEnviron. Sci. Technol.,Vol.18. NO.12, 1984 359A
I
meats may limit their usefulness in estuaries and highly colored waters. The optical factors discussed above can now be combined with the physicochemical properties' absorption coefficient ( 6 ) and qUanNm yield (*,which may be dependent on medium) to give the in situ photolysis rate in the water column for compound P a t depth z, due to downward irradiance E in a narrow band centered at wavelength h:
wavelength to give an overall photolysis rate at some depth in the water column:
2.3JA2 (A)e(A)E(z. A) iii' [Pl,dA A,
Rates computed at shallow depths using Quation 2 agree closely with experimental results (Figure 1) (14). A second integration over depth (not shown) gives the rate per unit area, a quantity that is dimensionally commensurate with other flux estimates, such as atmcspheric input and biological rate. It is important to minimize confusion Here & is the average cosine, which c o m t s for radiation arriving at the de- by using definitions consistently (15. tector in planes not normal to it (13). 16). The type (diffuse or beam attenuaThe equation can be integrated over tion), path length units, and exponential
360A Environ. Sci. Technal..Voi. 18. No. 12,1984
base of absorption coeffiients should be. stated. Quantum yield calculations should specify use of total light absorbed vs. light absorbed by a specific component, importance and treatment (if any) of scattering, and loss of substrate vs. product appearance. Measuring the underwater light field is an obvious alternative to modeling it. Appropriate instrumentation and analytical methods have been described (17, le), but the equipment is sophisticated and expensive, and the measurements are difficult. Most investigators rely on simpler measurements of total or broadband (such as UV) in-air irradiance to normalize for the effects of cloudiness. Water column properties, such as concentration of chlorophyll, organic carbon, and suspended material
(or simply an absorption spectrum in waters with high chromophore content and low scattering), can be used to approximate KAX) during an experiment (Figure 2). Environmental photochemists also use light fields in the laboratory. Systems such as solar simulators (xenon arc lamps with appropriate filters) can approximate surface sunlight at most wavelengths, but not easily in the 2% 305-nm range. Alternatively, on-theroof experiments use sunlight, usually either with actinometer solutions or radiometers to measure doses. Excessive heating, freezing, light focusing hy container walls, dirty walls, and inhomogeneous radiation fields may yield artifacts. Thin layers of borosilicate glass often transmit well down to 305 nm,but they must be checked piece by piece. Below 305 nm, quartz or Vycor glassware is needed. Since the underwater light spectrum varies markedly with depth, broadband studies can accurately measure only the near-surface effects for optically thin samples or vertically integrated effects and absorption coefficients as a function of wavelength. Unfortunately, despite the advent of lasers and interference Fdters, it is often difficult to obtain useful intensities in monochromatic beanis at a l l wavelengths of interest. This limitation can be severe in environmentid studies in which low rates are important, trace analyses require large samples, or parallel irradiation of many samples is required. Numerous investigators have used radiation below 290 nm (usually mercury arcs) for convenience, to obtain higher rates, or in initial studies. 7his practice cannot be recommended, even for survey work, because the results bear no easily verifiable relation to those that would be obtained at longer wavelengths.
-
FIGURE 1
Comparison of measured and calculated half-lives for direct photolysis 37
-
Methyl parathion.
-
. -
-
-
0 Dibenzothiophene
Dibenzo[c, glcarbazole 0 Benz[a]anthracene
OBenzcquinoiine
'
1
2
Hours
Measured half-life
particles, and many of their direct reactions prohably occur in the particulate phase. The simple inorganic components of natural waters also are generally transparent to sunlight. 111 the domain of particle-free water without dissolved organic chromophores, water absorbs nearly all the light-a situation that is approached at times in oceanic central gyres. Water itself photodissociates inefficiently to H+ and OH- ions (@, so this is essentially a nc-effect pathway. D i t photolysis The rarity of direct photoreactions of Known chromophores. Direct pho- inorganic chromophores is underscored tolysis reactions involve light-absorb- by the fact that nitrite and nitrate are the ing entities called chromophores, only well-studied cases. Nitrogen oxwhich undergo chemical change as a ides and OH radicals are photoproducts direct consequence of absorbing pho- (Tables 1, 2) (2, 4). Nitrate is quite untons. The primary products may partic- reactive, while at the sea surface nitrite ipate in further, secondary reactions. shows a net loss of about 10% per day. The direct photolysis of known dis- A few additional weak chromophores solved molecules with known chromo- with some known photochemistry are phores is conceptually the simplest type iodate, uranyl ion, hydrogen peroxide, of photochemical reaction. However, and ferrous ions (19); their environcomparatively few natural molecules of mental photochemistry is little studied. known structure fit this case; a few ex'Ransition metal ions and their inoramples are carbonyl compounds, ganic complexes have photochemically methyl iodide, riboflavin, tryptophane, inactive weak absorption bands in the thiamine, and vitamin BIZ. Chloro- UV-visible region. The photoreactive phylls, carotenoids, and polyunsatu- charge transfer bands of simple metal rated fatty acids are hydrophobic sub complexes of natural inorganic ligstances that sorb readily onto aquatic ands-hydroxide, carbonate, chloride,
Days
and sulfate-are usually at wavelengths below 290 nm, where there is no sunlight. Organic and inorganic metal complexes that do react often follow one of two pathways (Table I) (2): charge transfer (ligand-to-nletal and metal-to-ligand) or ligand exchange (e.g., photoaquation). As noted previously, many petrcchemical and other xenobiotic compounds absorb sunlight effectively and react with significant quantum yields (Table 1). Some are hydrophilic enough to react in the aqueous phase, including metal-organic complexes such as FeNTA and Fe-EDTA, polar aromatic and heteroaromatic compounds, and synthetic materials such as phenols, iodides, aldehydes, ketones, nitro compounds, and azo compounds. It is not possible to derive much useful information about direct photolysis from theory, although the literature may indicate the most probable products and mechanisms. For example, organic iodides, peroxides, and disulfides often form radicals by homolysis. Phenols may photoionize to form phenoxy radicals and hydrated electrons. Double bonds isomerize, and aldehydes and ketones react by the well-known Norrish type 1 and type 2 pathways. UnfortuEnviron.Sci.Technol..Vol.18, No. 12. 1984 361A
LILC L
?activityof some radicals and short-lived oxidants with natural components Radicals and triplet slates Na(ura1 Wale, Components
BrHCOj C0,Z02
1-
OH
-
< 6 % 104 8 x 105 2 x 103
3x
104
-
Os-
ROO
RO
-
-
-
0-200 (?) ?
?
?
DOM HOOH (CH3,S Mecad Maior Products
2x104
?
53
!,s~ ~ ~ ~ i i t m~ !i /r tihwc,VAT0 Advanced Research Insriruw. ore (clockwise from u p p r 1c.h): Hod Zika. Riduird Zepp. Jucques Joussor-Duhien, and Oliver C. Z~!firiou. Environ. S~i.Technol.,Vol.18, No. 12. 1984 371A
