Basic Processes

In natural systems, the majority of chromophoric substances are comprised of humic substances (HS). These are the brownish materials, which mainly derive from plant debris, are leached into freshwater systems, and, ultimately, into the oceans. Wherever they interact with light, a series of chemical reactions occur. They absorb both ultraviolet (UV) and visible light (VIS) in the wavelength range (290) 300-600 nm. These chromophores are activated many times a day. One calculation says that, for instance, on a sunny day in Lake Greifensee (Switzerland), each chromophore in the lake's epilimnion is activated 270 times, that is, ten times or more per hour. The lightabsorption capacity is, in most cases, linked to the presence of »-electrons and delocalized ^-electron systems which are available from heteroatoms, aromatic rings, or conjugated double bonds. These are the so-called 'chromophores'. By energy absorption, the outermost electron orbitals gain energy, and electrons are elevated from their lowest energy state (ground state, S0) to a higher energy state (the excited state, denoted Si or S*). Molecules in excited states are more reactive than their ground states (S0).

In order to understand the principles of photolysis, an energy-level diagram (Figure 1) can be used to illustrate basic processes after light absorption in chromophores. In organic molecules, two types of excited states following absorption of UV/VIS-light (symbolized as hv) are known: short-lived singlet (S1) and long-lived triplet states (Ti). The latter cannot be populated directly. Immediately after light absorption, excited singlet states (S1) are formed, which are usually short-lived (nanoseconds, 10~10s). Excited singlet states can deactivate radiationless by internal conversion (IC), by a radiant process called fluorescence (F), or can contribute to the formation of triplet states (T1) via intersystem crossing (ISC). Furthermore, electron transfer to suitable acceptors (R) may occur or even the ejection of free dissolved electrons (eaqu). As a result, free radicals of chromophores are formed which undergo further reactions.

Because of their comparably long lifetimes (micro- to milliseconds, ^10~6-10~3 s), triplet states account for the majority of photochemical reactions. In addition to the radiation-less (IC) and radiant deactivation to the ground state, which is called phosphorescence (P), reactions like electron and proton transfer may occur. These reactions mainly depend on the redox potentials of the involved redox pairs. When electrons are transferred to molecular oxygen, the super oxide anion ("O;T) is formed. In addition to electron transfer reactions, energy transfer from T1 to molecular oxygen is an important pathway, which is

Energy transfer, AE = 94 kJ mol-1

Energy transfer, AE = 94 kJ mol-1

Electron transfer

Figure 1 Energy-level diagram to illustrate processes after light absorption (hv) in a chromophore: S0, ground state; S-i, first-excited singlet state; T-, first-excited triplet state. Relaxation processes of S^ Franck Condon relaxation (FC), fluorescence (F); internal conversion (IC) (black), and intersystem crossing (ISC) to T1 which deactivates by phosphorescence (P), and internal conversion (IC) (red). Photochemistry may occur both from S! and T- via electron transfer to suitable acceptors R and energy transfer to molecular oxygen resulting in the formation of singlet oxygen (1O2).

Electron transfer

Figure 1 Energy-level diagram to illustrate processes after light absorption (hv) in a chromophore: S0, ground state; S-i, first-excited singlet state; T-, first-excited triplet state. Relaxation processes of S^ Franck Condon relaxation (FC), fluorescence (F); internal conversion (IC) (black), and intersystem crossing (ISC) to T1 which deactivates by phosphorescence (P), and internal conversion (IC) (red). Photochemistry may occur both from S! and T- via electron transfer to suitable acceptors R and energy transfer to molecular oxygen resulting in the formation of singlet oxygen (1O2).

followed by the formation of singlet oxygen (1O2) that is considered as one of the most reactive species in nature.

'Direct photochemical' reactions are immediate chemical changes to the chromophore such as isomerization, bond cleavage, or degradation of larger into smaller molecules as a consequence of electron transfer reactions. In the presence of oxygen, photochemical decarboxylation and formation of CO2 are observed inHS, which are usually enhanced by the presence of iron in HS complexes.

The different reaction products in Figure 2 are summarized as 'reactive oxygen species' (ROS). Super oxide anions, for example, may lead to the formation of a whole cascade of other ROS (Figure 2). The individual ROS have very different half-lives, from only a few microseconds, as for 1O2, to well over 1 h as for H2O2. Depending on production rates and lifetimes, average steady-state concentrations for ROS from 10-1 to 10~2 M are found in natural waters (Table 1).

Production and particularly gross ecological effects are summarized in Figure 3. The light-induced formation of ROS is called sensitization, and the photoexcited molecule itself the sensitizer. Although the sensitizer molecule returns without modification to the ground state, the photogenerated reactive species can attack any suitable target in their neighborhood, including the sensitizer itself. Photolysis by ROS or other photosensitizers (TiO2) is referred to as 'indirect photolysis'. In fact, ROS account for the majority of photodegradation reactions observed with HS. Any photosensitized reaction involves the transfer of energy, hydrogen atoms, protons, or electrons. The importance of oxygen in the photooxidation of natural organic matter is evident from

Table 1 Examples for production pathways and steady-state concentrations of ROS in natural waters

Reactive

Steady-state

oxygen

concentration

species (ROS)

(M)

Production

1O2

2 x 10-15-10-12

Energy transfer

• OH

1.5 x 10-18-10-16

Photolysis of HS

• OH

4 x 10~17-

Photolysis of NO3 from

2 x 10~16

H2O2 (photo-Fenton-

-10~3

reaction)

•OH

eaqu

10-17

Charge transfer

• O2

10-11-10-1°

Charge transfer via eaqu~

H2O2

10~2-3 x 10~7

From disproportion

of'O2"

oxygen consumption studies. Oxygen plays a pivotal role as the initial scavenger of radicals that are produced during irradiation of water. This leads to the generation of alkoxy and peroxy radicals that decay to stable oxygenated species.

In the following section, the environmental relevant formation pathways of OH-radicals will be discussed in more detail. Besides natural chromophoric organic compounds, inorganic chromophores play a major role in photochemistry. This applies for nitrate-rich freshwaters and for first-year sea ice with its algal flora, which may account for as much as 25% of the primary production in ice-covered Antarctic waters.

Enzymatic decay

Figure 2 Summary of ecochemical sources and removal pathways of ROS (red) in natural waters including singlet oxygen (1O2), superoxide anion ('O2), hydrogen peroxide (H2O2), and hydroxyl radicals ('OH). Mn+ and Me(n "1) denote metals in the n+ or n(-1)+ oxidation state, NOx- the nitrate or nitrite anion, and '?' unknown pathways. 'humics' = (dissolved) chromophoric organic carbon. These processes are put into a more ecological framework in Figure 3. Modified from Kieber DJ, Peake BM, Scully NM (2003) Reactive oxygen species in aquatic ecosystems. In: Helbling EW and Zagarese H (eds.) UV Effects in Aquatic Organisms and Ecosystems, pp. 251-288. Cambridge: Royal Society of Chemistry- Reproduced by permission of The Royal Society of Chemistry (RSC) on behalf of the European Society for Photobiology.

\\ 1

Aquatic

NO3-

i 1

Reactive oxygen species: 1O2, 'OH, H2O2

Reactive oxygen species: 1O2, 'OH, H2O2

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