92
E. Pelizzetti . P. Calza
lular malfunctions. It is unclear, however, whether their loss by active or passive processes is common for either healthy or senescent cells. A variety of thermal chemical
processes may produce radicals. It has been suggested that thermal oxidation of
phenols by Mn02 may generate humic materials (Shin do and Huang 1982).
Radical fate: The principal fate of many of the more reactive radicals is their transformation to a new radical; generally the daughter radical will be less reactive and more
selective than its parent. This type of transformation is quite common and occurs during the reactions of radicals with the major constituents of fresh and natural waters,
because these components are always light-element even-electron structures.
Harvey (1983) has noted that marine humic materials may be primarily lipid-derived
autoxidation/condensation products, formed in part from the action of thermally and
photochemically generated radicals as well as other reactants with biogenic polyunsaturated triglycerides.
Finally, to a limited extent, less reactive radicals may simply accumulate in certain
water systems until their steady state levels are significant. The fate of radicals in the
environment may therefore be summarized as follows: transformation, redox stabilization, radical formation and possibly simple accumulation.
The oxidants participate in important non-radical reactions and also frequently
generate new radicals, thus serving as radical reservoirs. The efforts to understand the
role of photochemically produced free radicals in natural waters are complicated by
several factors. Free radicals react readily with one another, with transition metals, and
more slowly with many organic compounds.
The ·OH radical: the ·OH radical is the most reactive photochemically produced free
radical in the atmosphere, while its role in an aqueous system is less clearly understood.
Flash photolysis studies have demonstrated that it is formed in sea water. The possible
sources of the ·OH radicals in sea water are N0 2 , NO;, H20 2, Fe 2 + and dissolved organic
matter. The significant known sources and sinks, togeilier with typical daytime steadystate concentration of hydroxyl radical in atmospheric and surface waters are reported
in Table 3.1.
Peroxy radicals and superoxide radical anion. In addition to the ·OH radical formation rate, Hoigne (1990) has demonstrated the formation and estimated the concentrations in some natural waters of other major radical types, as organic peroxy radicals
and superoxide radical anion. They have suggested that the fastest ROO· secondary reactions would give half-lives of a few days at the surface for reactive compound classes,
such as phenols, aromatics, amines and hydroquinones.
The major fonts of both radicals, together with their fates, are reported in Table 3.2.
3.3.2.3
Heterogeneous Photochemistry
All natural waters contain non-living organic particles, mineral particles and living organisms, as well as colloidal materials. In addition to iliese core materials, binding to particles
by ions and solutes (especially hydrophobic organic compounds) also exposes the colloidal materials to ilie different and often enhanced reactivity of ilie particulate phase.
E. Pelizzetti . P. Calza
lular malfunctions. It is unclear, however, whether their loss by active or passive processes is common for either healthy or senescent cells. A variety of thermal chemical
processes may produce radicals. It has been suggested that thermal oxidation of
phenols by Mn02 may generate humic materials (Shin do and Huang 1982).
Radical fate: The principal fate of many of the more reactive radicals is their transformation to a new radical; generally the daughter radical will be less reactive and more
selective than its parent. This type of transformation is quite common and occurs during the reactions of radicals with the major constituents of fresh and natural waters,
because these components are always light-element even-electron structures.
Harvey (1983) has noted that marine humic materials may be primarily lipid-derived
autoxidation/condensation products, formed in part from the action of thermally and
photochemically generated radicals as well as other reactants with biogenic polyunsaturated triglycerides.
Finally, to a limited extent, less reactive radicals may simply accumulate in certain
water systems until their steady state levels are significant. The fate of radicals in the
environment may therefore be summarized as follows: transformation, redox stabilization, radical formation and possibly simple accumulation.
The oxidants participate in important non-radical reactions and also frequently
generate new radicals, thus serving as radical reservoirs. The efforts to understand the
role of photochemically produced free radicals in natural waters are complicated by
several factors. Free radicals react readily with one another, with transition metals, and
more slowly with many organic compounds.
The ·OH radical: the ·OH radical is the most reactive photochemically produced free
radical in the atmosphere, while its role in an aqueous system is less clearly understood.
Flash photolysis studies have demonstrated that it is formed in sea water. The possible
sources of the ·OH radicals in sea water are N0 2 , NO;, H20 2, Fe 2 + and dissolved organic
matter. The significant known sources and sinks, togeilier with typical daytime steadystate concentration of hydroxyl radical in atmospheric and surface waters are reported
in Table 3.1.
Peroxy radicals and superoxide radical anion. In addition to the ·OH radical formation rate, Hoigne (1990) has demonstrated the formation and estimated the concentrations in some natural waters of other major radical types, as organic peroxy radicals
and superoxide radical anion. They have suggested that the fastest ROO· secondary reactions would give half-lives of a few days at the surface for reactive compound classes,
such as phenols, aromatics, amines and hydroquinones.
The major fonts of both radicals, together with their fates, are reported in Table 3.2.
3.3.2.3
Heterogeneous Photochemistry
All natural waters contain non-living organic particles, mineral particles and living organisms, as well as colloidal materials. In addition to iliese core materials, binding to particles
by ions and solutes (especially hydrophobic organic compounds) also exposes the colloidal materials to ilie different and often enhanced reactivity of ilie particulate phase.