Page 184 - Industrial Wastewater Treatment, Recycling and Reuse

158 Industrial Wastewater Treatment, Recycling, and Reuse

3.3 FENTON CHEMISTRY

The Fenton process involves the application of iron salts and H 2 O 2 to pro-
duce hydroxyl radicals. Ferrous ion is oxidized by H 2 O 2 to ferric ion, a
hydroxyl radical, and a hydroxyl anion. Ferric ion is then reduced back (typ-
ically in the presence of irradiations) to ferrous ion, a peroxide radical, and a
proton by the same H 2 O 2 . The Fenton reaction generally occurs in an acidic
medium between pH 2 and 4 and involves the following possible steps
(Masomboon et al., 2009; Rodriguez et al., 2003).
2+ • 3+
Fe +H 2 O 2 ! OH + OH +Fe (3.10)
+
Fe 3+ +H 2 O 2 ! Fe 2+ +H + HOO • (3.11)
3+ • 2+ +
Fe + HOO ! Fe +H +O 2 (3.12)
•
Fe 2+ + OH ! Fe 3+ +OH (3.13)
• •
OH + H 2 O 2 ! H 2 O + HOO (3.14)
•

Fe 2+ + HOO ! HOO +Fe 3+ (3.15)
• •
OH + OH ! H 2 O 2 (3.16)
•
OH + organics ! products + CO 2 +H 2 O (3.17)
s , while the rate of reac-
The rate of reaction (3.10) is around 63 M 1 1
s
tion (3.11) is only 0.01–0.02 M 1 1 (Kang et al., 2002; Martinez et al.,
2003). This indicates that ferrous ions are consumed more rapidly
than they are produced. The hydroxyl radicals will degrade organic com-
3+
pounds through reaction (3.17), and H 2 O 2 can also react with Fe via
reaction (3.11).
Many researchers have studied Fenton chemistry for the oxidation of
different organic pollutants, including aromatic and phenolic compounds,
pesticides, herbicides, and organic dyes (Bigda, 1996; Karci et al., 2012; Kusic
et al., 2006; Ma et al., 2005; Martinez et al., 2003; Sun et al., 2007; Xue et al.,
2009). In the Fenton-reagent-driven oxidation of organic pollutants, the
important parameters that need to be considered to get the optimized results
include ratio of H 2 O 2 to ferrous ion concentration, operating pH, and con-
centration of pollutant. Although successful on a laboratory scale, this process
finds lesser application on an industrial scale because of its ineffectiveness in
reducing certain refractory pollutants such as acetic acid, acetone, carbon tet-
rachloride, methylene chloride, n-paraffins, maleic acid, malonic acid, oxalic
acid, and trichloro-ethane, and due to the high cost of chemical reagents used