Page 186 - Industrial Wastewater Treatment, Recycling and Reuse

160 Industrial Wastewater Treatment, Recycling, and Reuse

3.3.2 Optimum Operating Conditions
3.3.2.1 Operating pH
The pH of the system has been observed to significantly affect the degrada-
tion of pollutants and the optimum pH has been observed to be about 3 in
the majority of cases (Tang and Huang, 1996; Venkatadri and Peters, 1993;
Zhonga et al., 2009); thus it is recommended as the operating pH. At lower
pH (pH<2.5), the formation of (Fe(II) (H 2 O)) 2+ occurs, which reacts more
slowly with H 2 O 2 and, therefore, produces lesser quantum of hydroxyl rad-
icals, thereby reducing the degradation efficiency. In addition, the scaveng-
ing of the hydroxyl radicals by hydrogen ions becomes important at a very
3+
low pH (Tang and Huang, 1996). The reaction of Fe with H 2 O 2 is also
inhibited. At an operating pH of >4, the decomposition rate decreases
because of the reduction in the free iron species in the solution, probably
due to the formation of Fe (II) complexes with the buffer inhibiting the for-
mation of free radicals. It can also be due to the precipitation of ferric oxy-
hydroxides (Lin and Lo, 1997), which inhibit the regeneration of ferrous
•
ions. Also, the oxidation potential of HO radicals is known to reduce with
an increase in the pH.

3.3.2.2 Number of Ferrous Ions
Usually, the rate of degradation increases with an increase in the concentra-
tion of ferrous ions, although the extent of increase is sometimes observed to
be marginal above a certain concentration as reported by Lin et al. (1999)
and Kang and Hwang (2000). Also, an enormous increase in ferrous ions will
lead to an increase in the unutilized quantity of iron salts, which will con-
tribute to an increase in the TDS content of the effluent stream; this is not
permitted (Masomboon et al., 2009). Thus, laboratory-scale studies are
required to establish the optimum loading of ferrous ions under similar con-
ditions unless data is available in the open literature.

3.3.2.3 Concentration of H 2 O 2
The concentration of H 2 O 2 plays a more crucial role in deciding the overall
efficacy of the degradation process. Usually, it has been observed that the
percentage degradation of the pollutant increases with an increase in the dos-
age of H 2 O 2 until it reaches an optimum value (Masomboon et al., 2009;
Zhonga et al., 2009). However, care should be taken when selecting the
operating oxidant dosage. The residual H 2 O 2 contributes to COD, and
hence excess H 2 O 2 is not recommended as it would add to excess COD
•
and also due to its OH scavenging effect at higher concentrations. Also,