or heterogeneous Fenton techniques especially with the use of
semiconductors or resins (146,150– 168 ).
Both conventional and improved photo-Fenton methods play a
significant role in the degradation of various organic compounds,
differing in their structure (aliphatic or aromatic), such as
hydrocarbons ( 169 ), alcohols and their derivatives ( 170 ), organic
acids: salicylic (OHC 6 H 4 COOH) ( 171 ), oxalic (COOH) 2 ( 157 ),
citric (OH)C 3 H 4 (COOH) 3 ( 172 ), maleic (CHCOOH) 2 ( 158 ), formic
(HCOOH) ( 173 – 175 ), phenol (C 6 H 5 OH) and its derivatives
(159,163,176,177), poly(ethylene glycol) (H(OCH 2 CH 2 )nOH)
( 178 ), and many others organic pollutants (2,179).
The photo-Fenton processes are explored as photochemical pre-
treatment of nonbiodegradable and ubiquitous environmental
pollutants and/or extremely toxic compounds in wastewaters, such
as persistent organic dyes under visible light irradiation
(151,154,180,181) and under UV irradiation (139,182), azo dye fac-
tory wastewaters (140,162,183– 185 ), herbicides ( 186 – 188 ),
pesticides (152,153,189,190), insecticides ( 191 ), pharmaceuticals
and wastewaters from medical laboratories ( 192 – 197 ), surfactants
( 198 ), industrial effluents with persistent toxic pollutants ( 199 ),
industrial solvents and wastewaters (167,200), chlorinated
solvents ( 201 ), and municipal wastewater ( 202 ). The photo-Fenton
process was proposed to improve the biodegradability of especially
biorecalcitrant wastewater, coming from textile industry, and the
method was also suggested for water disinfection ( 203 – 205 ).
A.2. Photoreduction in other iron systems
A similar oxidative photodegradation of organic substances in
the presence of iron(III) complexes without any added H 2 O 2
was explored over a long time(2,16,17,95,206,207). The studies
proved that generation of LL(Eq. 26) or OHradicals (Eq. 25)
in the photoreduction of [FeIII(LL) 3 ] or [FeIII(OH)L 5 ], respec-
tively, leads simultaneously to direct or indirect oxidation of
organic ligands (LL) or other substances (via OHradicals). The
Fe(II) re-oxidation by atmospheric molecular oxygen is strongly
pH dependent and in general too slow ( 147 ) to be used for sewage
treatment, although it works perfectly under natural environ-
mental conditions.
The Fe(II)/Fe(III) cycling and consequent radical generation
was studied repeatedly and effects of various parameters were
considered. DOM, oxygenation, and solar irradiance were found
to be the most important factors controlling the nature and con-
centrations of Fe(II) and Fe(III) complexes in water basins.
There is a large number of possible thermal and photochemical
318 ZOFIA STASICKA