Photoc
waters
ryof
Many compounds and environments are affected by sunlight-induced photochemistry
Oliver C. Zafiriou Woods Hole Oceanographie Institution Woods Hole, Mass. 02543 Jacques Joussot-Dubien Université 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 Miami, 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 influences the world's oceans (about 70% of the Earth's surface), and the illuminated portion of the hydrosphere is especially active and diverse. Transport, material exchanges, and biology (including aquatic primary pro358A
Environ. Sci. Technol., Vol. 18, No. 12, 1984
ductivity) are more intense in this area than they are in deeper waters. In addition to being naturally significant, this zone of surface waters is used as the receptacle for many liquid, solid, and airborne wastes. At the same time, however, we view these surface layers as essential for recreation, aesthetics, transportation, food, and sources of freshwater. Thus it is important to understand the processes for the cycling of materials in the photic zone. Until the past decade, the role of photochemical reactions as a component of photic zone processes was virtually ignored, but now a growing number of studies show numerous reactions and effects. New reports appear often, so the tally of a dozen elements (Mn, Fe, Cu, Hg, Tl, C, N, O, S, CI, Br, and I) involved in these processes is likely to require frequent revision. To explore progress and exchange ideas in the field of freshwater and marine photochemistry, an international group of scientists met under NATO sponsorship in Woods Hole, Mass.,
Sept. 12-16, 1983 (1). This review, drawn largely from the experience and reports of those present at that meeting, surveys the area for the nonspecialist and presents results, problems, and opportunities for research. More detailed references and discussion are available in recent reviews (2-4) and meeting abstracts (5). For brevity, we cite primarily recent references not mentioned in other reviews. A much more extensive bibliography, including references from other reviews and from the NATO meeting abstracts, is available from any of the authors (6). Chemical and environmental scope There are a number of photoreactions that occur in the environment: Hydrogen peroxide forms in freshwater and seawater; the free radical NO reaches detectable steady-state levels in the equatorial Pacific; a photochemical (di7T-methane) rearrangement product is found in shallow-water encrusting corals (7); inorganic phosphate quantita-
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tively forms an iron-organic-phos phate 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 photooxidizes in rice paddies to sulfoxide; and spilled oil rapidly forms surfactants and toxic, water-soluble products. Ta ble 1 summarizes important aspects of these photoprocesses and others, cover ing many of the elements affected by sunlight-induced photochemistry. Because many of these studies are continuing, the last two columns of Ta ble 1 represent current thinking, not de finitive conclusions, on probable mech anisms 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 1 contain noteworthy similarities between fresh water and saltwater systems. These in clude • the widespread involvement of un identified organic chromophores in processes involving either net redox re actions, energy transfer, or both, • the almost universal importance of dissolved oxygen as an acceptor of en ergy or electrons and as a participant in secondary reactions, • the common occurrence of sec ondary free radical reactions of organic and inorganic species, and • the much higher incidence of di rect photolysis of pollutants than of nat ural molecules of low molecular weight and known structure. Although these observations show factors common to all natural water systems, there are many important dif ferences between freshwater and salt water 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 100-150 m. • Dissolved organic compounds and suspended particulates, living and non living, are variable in both environ ments 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 seawa ter 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
Estuarine area. This satellite view of the mouth of the Chesapeake Bay, where fresh and saltwaters begin to interact, shows an area of photochemical activity in the environment. abundant bromide ion in seawater, but this ion is nearly absent in freshwater. The nature and concentrations of or ganic chromophores differ signifi cantly, and in the ocean chloride ion is a better nucleophile than water. However, the prevalence of reports concerning direct photoreactions in fresh vs. salt water 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 com pounds, can absorb sunlight. But in na ture, 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 un known. 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 quanti fied—by means of models. Recently, relevant measurements and models of the solar radiation field in the air and under water have undergone considera ble 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 ζ, £(ζ,λ), 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 atmo spheric 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, dur ing 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 coeffi cient, Κ/(λ), 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 ±10% between 300 nm and 700 nm. It accounts for K/{\) as a sum of contributions from water, chloro phyll, 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
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TABLE 1
A diversity of natural water photoprocesses Environment
Substrates
Products
Probable mechanisms
Marine and freshwaters
Natural organic chromophores and pigments, C. See also Figure 6
C· + H 0 2 o r C · + AH-
H atom transfer to 0 2 or A
C+ + ·02-
Electron transfer to 0 2 Energy transfer to 0 2 •0 2 ~ disproportionation; other? Direct photolysis
c + o2 HOOH
Marine
N02-
•NO + -OH
Br-sEA, CO3 2 -, RH
•Br 2 -,
R-
ROO·
0 2 addition
CH3I
C H 3 + I-
Direct photolysis
C 0 3 - , R·
2+
Changes Ν speciation; induces NO air-sea flux Changes radical selectivity; affects water treatment Oxidizes organic radicals Changes h speciation; air-sea exchanges Postulated source of bioavailable Mn Altered cycling-toxicity of Cu
Mnaq
Cu(ll)
Cu(l)CI
H 2 0 2 / 0 2 " reduction of Cu(ll); charge transfer to metal
Fe(lll)-organic complex
Fe(ll) + C 0 2
? Consumes 0 2
Fe(lll)-organic-P04 complex
Fe(il) + P 0 4
Unknown
Surface films
|- (sea), organics
HOI, oxidized organics
0 3 diffusion
Change I - air-sea exchange; surface film properties
Polluted waters Oil spills
R H , A r H , R2S
R = 0 , R C 0 2 - , ArOH R 2 SO
Ar(CH 2 )„CH 3
Phenylalkanones, phenylalkanols Azo, azoxy dyes
Free radicals; direct photolysis; singlet oxygen Anthraquinonesensitized Unknown
Changes spreading emulsification and toxicity of oil Model study
Cu(ll)-organic complex
Effluent (explosives manufacture)
TNT, nitroaromatics
Herbicides
2,4-D
Pesticides
Disulfoton
Preservatives
Pentachlorophenol
Domestic waste
Fe(lll)-NTA
ments may limit their usefulness in es tuaries and highly colored waters. The optical factors discussed above can now be combined with the physicochemical properties' absorption coeffi cient (e) and quantum yield (Φ, which may be dependent on medium) to give the in situ photolysis rate in the water column for compound Ρ at depth z, due to downward irradiance £ in a narrow band centered at wavelength λ:
Jà[py\
=
V dt Λ, λ 2.3Φ(λ)β(λ)Ε(ζ,λ)ίΡ]ζ
(1)
Here ûd is the average cosine, which corrects for radiation arriving at the detector in planes not normal to it (13). The equation can be integrated over Environ. Sci. Technol., Vol. 18, No. 12, 1984
Unknown
Numerous
M n 0 2 (colloidal)
Freshwaters
360A
• OH radical redox and abstraction
Likely effects
Oxidation, reduction, hydrolysis products Disulfoton sulfoxide
Direct photolysis
Phenols, quinones, acids, C 0 2 , C l Fe(ll) + amine + C 0 2 + CH20
Initiated by direct photolysis of PCP Charge transfer to metal
Singlet oxygen
wavelength to give an overall photolysis rate at some depth in the water column:
-m -
2.3 (λ2 Φ (K)e(\)E(z, λ) Πι1 [P]zd\ λ]
Rates computed at shallow depths using Equation 2 agree closely with experi mental results (Figure 1) (14). A sec ond integration over depth (not shown) gives the rate per unit area, a quantity that is dimensionally commensurate with other flux estimates, such as atmo spheric input and biological rate. It is important to minimize confusion by using definitions consistently (15, 16). The type (diffuse or beam attenua tion), path length units, and exponential
Oxidation of organics; reduction of 0 2 ; dissolution of colloidal Fe; bioavailability of Ρ
Red dyes are aesthetic, possible biotoxicity problem Complex Product more soluble and toxic in some tests Complex Degrades NTA; induces Fe(ll) autooxidation
base of absorption coefficients should be stated. Quantum yield calculations should specify use of total light ab sorbed vs. light absorbed by a specific component, importance and treatment (if any) of scattering, and loss of sub strate vs. product appearance. Measuring the underwater light field is an obvious alternative to modeling it. Appropriate instrumentation and ana lytical methods have been described (17, 18), but the equipment is sophisti cated and expensive, and the measure ments are difficult. Most investigators rely on simpler measurements of total or broadband (such as UV) in-air irra diance 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 Kj{X) 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 290305-nm range. Alternatively, on-theroof experiments use sunlight, usually either with actinometer solutions or radiometers to measure doses. Excessive heating, freezing, light focusing by 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 filters, it is often difficult to obtain useful intensities in monochromatic beams at all wavelengths of interest. This limitation can be severe in environmental 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. This 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. Direct photolysis Known chromophores. Direct photolysis reactions involve light-absorbing entities called chromophores, which undergo chemical change as a direct consequence of absorbing photons. The primary products may participate in further, secondary reactions. The direct photolysis of known dissolved molecules with known chromophores is conceptually the simplest type of photochemical reaction. However, comparatively few natural molecules of known structure fit this case; a few examples are carbonyl compounds, methyl iodide, riboflavin, tryptophane, thiamine, and vitamin B 1 2 . Chlorophylls, carotenoids, and polyunsaturated fatty acids are hydrophobic substances that sorb readily onto aquatic
FIGURE 1
C o m p a r i s o n of m e a s u r e d a n d c a l c u l a t e d half-lives for direct photolysis -//1
τ
ι ι
ι»? ι
ι \l
h-r
Methyl p a r a t h i o n ·
37 -
35
ir T
ία Q
23
Quinoline (
-
î/Pyr/1
8 ΰ
F
6 -
• Dibenzothiophene
Benzo[a]pyrene
ο
- Dibenzo[c, gr]carbazole • Benz[a]anthracene
3
' '
6
21 23 Days
35
37
Measured half-life Source: Reference 14
particles, and many of their direct reactions probably occur in the particulate phase. The simple inorganic components of natural waters also are generally transparent to sunlight. In 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 (4), so this is essentially a no-effect pathway. The rarity of direct photoreactions of inorganic chromophores is underscored by the fact that nitrite and nitrate are the only well-studied cases. Nitrogen oxides and OH radicals are photoproducts (Tables 1, 2) (2, 4). Nitrate is quite unreactive, while at the sea surface nitrite shows a net loss of about 10% per day. A few additional weak chromophores with some known photochemistry are iodate, uranyl ion, hydrogen peroxide, and ferrous ions (79); their environmental photochemistry is little studied. Transition metal ions and their inorganic complexes have photochemically inactive weak absorption bands in the UV-visible region. The photoreactive charge transfer bands of simple metal complexes of natural inorganic ligands—hydroxide, carbonate, chloride,
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 1) (2): • charge transfer (ligand-to-metal and metal-to-ligand) or • ligand exchange (e.g., photoaquation). As noted previously, many petrochemical 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
TABLE 2
Reactivity of some radicals and short-lived oxidants with natural components Oxidants
Radicals and triplet states Natural Water Components
OH
ROO
RO
NO
3
H20
ciBr
HCCv CO32o2 IN02DOM HOOH
(CH3)2S Mered Major Products Total Rate (per s e c o n d )
HOOH/ ROOH
UPC —
