Page 192 - Industrial Wastewater Treatment, Recycling and Reuse

166 Industrial Wastewater Treatment, Recycling, and Reuse

3.4.2.6 Effect of Ionic Species
The presence of ionic species may affect the degradation process via the
effect on the process of adsorption of the contaminants on catalyst and its
subsequent reaction with hydroxyl radical ions, and/or absorption of UV
light. This is a very important point that needs to be considered because
real-life industrial effluents will contain different types of salts at different
concentrations. Generally, these salts are in ionized or dissociated forms.
Many examples have been given in the literature (e.g., Kormann et al.,
1991; Wei et al., 1990; Yawalkar et al., 2001) regarding the effects of various

anions and cations. In general, it can be said that CO 3 , HCO 3 (they act as

radical scavengers and also affect the adsorption process), and Cl (affects the
adsorption step strongly and also partly absorbs UV light) ions have strong
detrimental effects on the degradation process, whereas other anions such as
sulfate, phosphate, and nitrate affect the degradation efficiency only margin-

ally. Yawalkar et al. (2001) have studied the effect of SO 4 ,CO 3 ,Cl , and

HCO 3 ions on the overall degradation rates and reported that the detri-

mental effects are observed in the order SO 4 < CO 3 < Cl < HCO 3 .

For cations the results are generally contradictory, more on the negative
side except for some reports of marginal enhancement due to the presence
3+ 2+
of Fe /Cu ions at very low concentrations (Wei et al., 1990). More work
is required in this area before the generalized effect of cations on the efficacy
of photocatalytic degradation can be well established.
3.5 HYBRID METHODS
The different AOPs discussed above are capable of degrading organic pol-
lutants to a certain extent depending on the pollutant to be treated. All have
to overcome some drawbacks for their efficient operation. The efficacy of
the process depends strongly on the rate of free radical generation along with
the extent of radical contact with the contaminant molecules. Thus, efficient
design should aim at maximizing both these quantities. The similarity
between the mechanism of destruction in the case of different advanced oxi-
dation techniques and some of the common optimum operating conditions
point toward the synergism between these methods and the fact that com-
binations of these AOPs should give better results as compared to individual
techniques. Moreover, some of the drawbacks of the individual techniques
can be overcome by some features of other techniques. For example, the
efficiency of photocatalytic oxidation is severely hampered by two main
factors, mass transfer limitations and fouling of the solid catalyst; however,