Decontamination of water by combined solar advanced oxidation processes and biotreatment - Solar Energy Sciences and Engineering Applications - page 377

Figure 12.2.1 Normal solar irradiance (I) on the Earth's surface (ASTM E891-87, air mass 1.5), main
light absorbing gases and light absorption of Fe 3 + species.

If organic substances (quenchers, scavengers, or in the case of wastewater treatment,
pollutants) are present in the system Fe 2 + /Fe 3 + /H 2 O 2 , they react in many ways with
the hydroxyl radicals generated. The organic radicals generated continue reacting, pro-
longing the chain reaction and thereby contributing to reducing the consumption of
oxidants in wastewater treatment by Fenton and photo-Fenton. In aromatic pollutants,
the ring system is usually hydroxylated before it is broken up during oxidation, typi-
cally into intermediate degradation products containing quinone and hydroquinone
structures. In any case, sooner or later, ring opening reactions further mineralize
the molecule. One important drawback of the Fenton method, especially for total
mineralisation of organic pollutants, is that carboxylic intermediates cannot be fur-
ther degraded. Carboxylic and dicarboxylic acids (L: Mono- and Dicarboxylic acids)
are known to form stable iron complexes, which inhibit the reaction with peroxide
(Kavitha and Palanivelu, 2004). Hence, the catalytic iron cycle reaches a standstill
before total mineralisation is accomplished (Equation 12.2.4).

H 2 O 2 , dark
−→

Fe 3 + + [FeL n ] x +

→

nL no further reaction (12.2.4)

The primary step in the solar photoreduction of dissolved ferric iron is a ligand-to-
metal charge-transfer reaction in which intermediate complexes dissociate as shown
in the reaction in Equation 12.2.5. The ligand may be any Lewis base able to form
a complex with ferric iron (OH − ,H 2 O, HO 2 ,Cl − , R-COO − , R-OH, R-NH 2 etc.).
Depending on the reacting ligand, the product may be a hydroxyl radical such as in
Equation 12.2.6 or other radical derived from the ligand. The direct oxidation of an
organic ligand is possible as well, as shown for carboxylic acids in Equation 12.2.7.

[Fe 3 + L] [Fe 3 + L] ∗ −→ Fe 2 + + L • + h ν −→ (12.2.5)

[Fe(OH)] 2 + + Fe 2 + + OH • h ν −→ (12.2.6)

R)] 2 + + Fe 2 + +

R •

[Fe(OOC − h ν → CO 2 + (12.2.7)