Photolysis In The Environment

Two types of photolysis processes occur in aquatic systems: direct photolysis and indirect photolysis (Fig. 3.2.5). In direct photolysis, the target contaminant (in this case, the pharmaceutical compound) absorbs a solar photon. In an indirect photolysis mechanism, the target does not need to or is unable to absorb light because another chromophore in the system such as dissolved organic matter acts as a sensitizing species.

Direct photolysis of pharmaceuticals is initiated by photon absorption, a fact that is well known to pharmacists who dispense medication in amber bottles and advise patients to stay out of the sun when taking

Photolysis Reaction Environment
Fig. 3.2.5. Direct and indirect photolysis in aquatic systems.

Wavelength (nm) Wavelength (nm)

Fig. 3.2.6. Spectral overlap of sunlight emission and nitrofurantoin absorbance and ranitidine absorbance (mE is equal to millieinsteins). Nitrofurantoin and ranitidine absorption indicated by a dashed line and sunlight emission is indicated by a solid line. Spectra are based on the data from Refs. [14,15]. Sunlight emission was calculated using SMARTS [67] for Minneapolis, MN in July at noon.

Wavelength (nm) Wavelength (nm)

Fig. 3.2.6. Spectral overlap of sunlight emission and nitrofurantoin absorbance and ranitidine absorbance (mE is equal to millieinsteins). Nitrofurantoin and ranitidine absorption indicated by a dashed line and sunlight emission is indicated by a solid line. Spectra are based on the data from Refs. [14,15]. Sunlight emission was calculated using SMARTS [67] for Minneapolis, MN in July at noon.

certain medications. The phototoxicity of some drugs has led to an active area of research on this topic that complements the study of the environmental photochemistry of pharmaceuticals (see Section 3.2.3).

The rate of photon absorption is determined by the action spectrum, or the product of the compound's absorbance spectrum and the solar spectrum. Examples of the overlap between light absorbance and the solar spectrum for nitrofurantoin and ranitidine are shown in Fig. 3.2.6. Note that both of these compounds contain similar chro-mophores, a furan and a nitro group, and that nitrofurantoin, which has these groups in conjugation, has larger overlap with the solar spectrum due to the longer wavelength absorption feature.

The excited state of a molecule is short-lived and may undergo various physical or chemical relaxation processes. Physical relaxation processes, such as vibrational energy loss, energy transfer to another species, or emission of a photon lead to the regeneration of the parent compound. Only those processes that lead to chemical changes in the parent compound lead to a decrease in the concentration of the species being photolyzed. Such transformations may include fragmentation, isomerization/intramolecular rearrangement, H-abstraction, dime-rization/polymerization, and electron transfer. The fraction of chemical transformation events per photon absorbed is defined as the quantum yield (F) for that process. This value may range from 0 to 1, but values between 0.0001 and 0.1 are common for compounds that are photodegraded on reasonable time scales (half-lives of minutes to days). The first-order rate constant for the transformation is given by where kobs, the rate constant for direct photolysis, is equal to the product of the quantum yield, the molar absorptivity of the compound (e), and the solar irradiance (I), integrated over all wavelengths (1).

The photolysis rate is a function of both the rate of light absorption (i.e., the action spectrum) and the quantum yield. Note that a compound with a large spectral overlap with sunlight and a small quantum yield may be more persistent than a compound with a small spectral overlap and a large quantum yield. In the example shown in Fig. 3.2.6, ranitidine has a smaller rate of absorption than nitrofurantoin, yet both compounds degrade at similar rates due to the fact that ranitidine has a higher quantum yield [14,15]. Another example is the anti-inflammatory medication diclofenac, which absorbs very little light with wavelengths greater than 300 nm (leading to a small spectral overlap) compared with the structurally similar mefenamic acid (which absorbs wavelengths up to 400 nm), both shown in Fig. 3.2.7. The quantum yield for diclofenac is ^0.1 [16-18] while that for mefenamic acid is 0.00015 [19]. While mefenamic acid absorbs 100 times more sunlight, it is 1000 times less efficient at being transformed, resulting in a direct photolysis half-life for mefenamic acid that is ^10 times greater than that of diclofenac.

280 320 360 400 440 480 520 560 600 Wave length (nm)

Wavelength (nm)

Fig. 3.2.7. Spectral overlap of sunlight emission and diclofenac absorbance and mefenamic acid absorbance (mE is equal to millieinsteins). Diclofenac and mefenamic acid absorption indicated by a dashed line and sunlight emission is indicated by a solid line. Spectra are based on the data from Ref. [19]. Sunlight emission was calculated using SMARTS [67] for Minneapolis, MN in July at noon.

280 320 360 400 440 480 520 560 600 Wave length (nm)

Wavelength (nm)

Fig. 3.2.7. Spectral overlap of sunlight emission and diclofenac absorbance and mefenamic acid absorbance (mE is equal to millieinsteins). Diclofenac and mefenamic acid absorption indicated by a dashed line and sunlight emission is indicated by a solid line. Spectra are based on the data from Ref. [19]. Sunlight emission was calculated using SMARTS [67] for Minneapolis, MN in July at noon.

All compounds, whether or not they absorb solar photons, are potentially subject to indirect photolysis. In an indirect photochemical mechanism, a sensitizer absorbs light and subsequently reacts directly with the substrate or produces a reactive intermediate that reacts with the substrate. The principal light-absorbing species in indirect photolysis is the dissolved organic matter (DOM) present in natural waters. Photoexcitation of DOM leads to the production of a variety of photo-chemically produced reactive intermediates (PPRIs) including the reactive oxygen species hydroxyl radicals (HO*), singlet oxygen (1O2), peroxy radicals (ROO*), and superoxide (O2*) as shown in Fig. 3.2.5. Other PPRIs, such as triplet (excited) DOM and hydrated electrons, are also produced and can react with pharmaceutical pollutants. It should also be mentioned that while DOM is the main sensitizing species in natural waters, other light-absorbing species may also generate PPRIs, such as nitrate and nitrite that produce hydroxyl radicals in sunlight.

Reaction rates with PPRIs are dictated by the product of their steady-state concentration and their bimolecular reaction rate constant. The specificity of the PPRI (i.e., the chemical functional groups it will react with) varies widely. For example, hydroxyl radical is a non-specific oxidant that reacts with most organic compounds at diffusion controlled rates, either by hydrogen atom abstraction from sp3 hybridized C-H bonds or addition to C-C double bonds. Steady-state concentrations of hydroxyl radical in sunlit waters range from 10~18 to 10~15mol/L [20], leading to a disappearance rate of approximately

10~5-10~8s_1 (half-life of ~1-1000 days) for even the most unreactive organic pollutants. At the other end of the selectivity spectrum is singlet oxygen, which only reacts with specific functional groups such as electron rich olefins, phenolates, and sulfides. The fact that it is present in higher steady-state concentrations (10~12-10~14mol/L, [20]) leads to reaction with singlet oxygen being the dominant photochemical loss mechanism for substrates containing these high-reactivity functional groups.

A good example is cimetidine, a photostable compound in pure water under sunlight irradiation that is rapidly degraded photochemically in DOM-containing waters. The PPRI responsible for its degradation is singlet oxygen, which reacts rapidly with cimetidine's imidazole ring [15]. Other PPRIs do not appear to be important due to the fact that the total reaction rate constant for cimetidine loss matches the calculated reaction rate constant based on the steady-state concentration of singlet oxygen and the bimolecular reaction rate constant.

Overall, the total rate constant for loss via photolysis will be the sum of the first-order direct photolysis rate constant and the second-order rate constants for reactions with PPRIs multiplied by the steady-state concentration of the appropriate PPRI species as shown in

In this expression, the first term represents the rate of direct photolysis (Eq. 3.2.1). The second term represents the sum of all indirect photolysis pathways, which are each the product of the second-order rate constant for reaction of the species of interest with a PPRI, ki,j, and the concentration of the respective PPRI. Depending on the relative importance of these two terms, compounds may react solely via direct photolysis, solely via indirect photolysis, or by a combination of the two. Thus, a continuum of photolysis reactivity is expected, as shown in Fig. 3.2.8 for several compounds for which the environmental photolysis has been studied. The exact ratio of direct to indirect processes will be a function of the quantum yield, the light-absorbing properties of the compound, the magnitude of the rate constants for the reactions with the PPRIs, and the PPRI steady-state concentrations. The rate of light absorption by the compound and the PPRI steady-state concentration will be a function of time of day (light intensity) as well as environmental conditions.

Fig. 3.2.8. Continuum of photoreactivity with respect to direct and indirect processes. The contribution of direct photolysis decreases from left to right. The PPRI listed is the species that contributes most to indirect photolysis for the particular compound. Photolysis rates/mechanisms are compiled from Refs. [15,17,19,31,34,36,38].

Fig. 3.2.8. Continuum of photoreactivity with respect to direct and indirect processes. The contribution of direct photolysis decreases from left to right. The PPRI listed is the species that contributes most to indirect photolysis for the particular compound. Photolysis rates/mechanisms are compiled from Refs. [15,17,19,31,34,36,38].

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  • cameron
    When does photolysis occur?
    3 years ago

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