11.2.7 Zerovalent Iron
Zerovalent iron (ZVI) powder is an environmental friendly reducing agent which has
been widely studied for environmental remediation in recent years. ZVI is readily
oxidized to Fe
2+ ion in the presence of oxidants. In the aqueous systems, this
phenomenon leads to the dissolution of solid which is the primary cause of metal
corrosion. ZVI was used to degrade the azo dye in the aqueous medium by Devi et al.
[26]. The influence of various reaction parameters like effect of iron dosage,
concentration of H 2 O 2 /ammonium per sulfate (APS), initial dye concentration, and
effect of pH and the influence of radical scavenger are studied, and optimum
conditions are reported. The degradation rate decreased at higher iron dosages and
also at higher oxidant concentrations due to the surface precipitation which deactivates the iron surface. The rate constant for the processes Fe
0 /UV and Fe
0 /APS/UV
is twice compared to their respective Fe
0 /dark and Fe
0 /APS/dark processes. The rate
constant for Fe
0 /H 2 O 2 /UV process is four times higher than Fe
0 /H 2 O 2 /dark process.
The increase in the efficiency of Fe
0 /UV process is attributed to the cleavage of
stable iron complexes which produces Fe
2+ ions that participates in cyclic Fenton
mechanism for the generation of hydroxyl radicals. The increase in the efficiency of
Fe
0 /APS/UV or H 2 O 2 compared to dark process is due to continuous generation of
hydroxyl radicals and also due to the frequent photo reduction of Fe
3+ to Fe
2 .
Though H 2 O 2 is a better oxidant than APS in all respects, it is more susceptible to
deactivation by HO
• scavengers. The decrease in the rate constant in the presence of
HO
• scavenger is more for H 2 O 2 than APS. Iron powder retains its recycling
efficiency better in the presence of H 2 O 2 than APS. The decrease in the degradation
rate in the presence of APS as an oxidant is due to the fact that generation of free
radicals on iron surface is slower compared to H 2 O 2 . Also, the excess acidity
provided by APS retards the degradation rate as excess H
+ ions acts as hydroxyl
radical scavenger. The degradation of methyl orange (MO) using Fe
0 as an aciddriven process shows higher efficiency at pH 3. The efficiency of various processes
for the decolorization of MO dye is of the following order: Fe
0 /H 2 O 2 /UV > Fe
0 /
H 2 O 2 /dark > Fe
0 /APS/UV > Fe
0 /UV > Fe
0 /APS/dark > H 2 O 2 /UV % Fe
0 /
dark > APS/UV. Dye resisted to degradation in the presence of oxidizing agent in
dark. The degradation process was followed by UV–vis and GC–MS spectroscopic
techniques. The complete degradation of the dye was achieved in the presence of
oxidizing agent when the system was amended with iron powder under UV light
illumination. The concentration of Fe
2+ leached at the end of the optimized degradation experiment is found to be 2.78 Â 10
À3 M. With optimization, the degradation
using Fe
0 can be effective way to treat azo dyes in aqueous solution.
11.2 Heterogeneous Photo-Fenton Reaction
269
Zerovalent iron (ZVI) powder is an environmental friendly reducing agent which has
been widely studied for environmental remediation in recent years. ZVI is readily
oxidized to Fe
2+ ion in the presence of oxidants. In the aqueous systems, this
phenomenon leads to the dissolution of solid which is the primary cause of metal
corrosion. ZVI was used to degrade the azo dye in the aqueous medium by Devi et al.
[26]. The influence of various reaction parameters like effect of iron dosage,
concentration of H 2 O 2 /ammonium per sulfate (APS), initial dye concentration, and
effect of pH and the influence of radical scavenger are studied, and optimum
conditions are reported. The degradation rate decreased at higher iron dosages and
also at higher oxidant concentrations due to the surface precipitation which deactivates the iron surface. The rate constant for the processes Fe
0 /UV and Fe
0 /APS/UV
is twice compared to their respective Fe
0 /dark and Fe
0 /APS/dark processes. The rate
constant for Fe
0 /H 2 O 2 /UV process is four times higher than Fe
0 /H 2 O 2 /dark process.
The increase in the efficiency of Fe
0 /UV process is attributed to the cleavage of
stable iron complexes which produces Fe
2+ ions that participates in cyclic Fenton
mechanism for the generation of hydroxyl radicals. The increase in the efficiency of
Fe
0 /APS/UV or H 2 O 2 compared to dark process is due to continuous generation of
hydroxyl radicals and also due to the frequent photo reduction of Fe
3+ to Fe
2 .
Though H 2 O 2 is a better oxidant than APS in all respects, it is more susceptible to
deactivation by HO
• scavengers. The decrease in the rate constant in the presence of
HO
• scavenger is more for H 2 O 2 than APS. Iron powder retains its recycling
efficiency better in the presence of H 2 O 2 than APS. The decrease in the degradation
rate in the presence of APS as an oxidant is due to the fact that generation of free
radicals on iron surface is slower compared to H 2 O 2 . Also, the excess acidity
provided by APS retards the degradation rate as excess H
+ ions acts as hydroxyl
radical scavenger. The degradation of methyl orange (MO) using Fe
0 as an aciddriven process shows higher efficiency at pH 3. The efficiency of various processes
for the decolorization of MO dye is of the following order: Fe
0 /H 2 O 2 /UV > Fe
0 /
H 2 O 2 /dark > Fe
0 /APS/UV > Fe
0 /UV > Fe
0 /APS/dark > H 2 O 2 /UV % Fe
0 /
dark > APS/UV. Dye resisted to degradation in the presence of oxidizing agent in
dark. The degradation process was followed by UV–vis and GC–MS spectroscopic
techniques. The complete degradation of the dye was achieved in the presence of
oxidizing agent when the system was amended with iron powder under UV light
illumination. The concentration of Fe
2+ leached at the end of the optimized degradation experiment is found to be 2.78 Â 10
À3 M. With optimization, the degradation
using Fe
0 can be effective way to treat azo dyes in aqueous solution.
11.2 Heterogeneous Photo-Fenton Reaction
269