Environmental Engineering Reference
In-Depth Information
Many studies in literature suggest that the amount of catalyst plays a
major role in the degradation of dyes in photocatalytic systems. It is recom-
mended to determine the optimum loading for an efficient removal of dye.
Generally the rate of photocatalytic degradation increases with an
increase in the amount of catalyst. By increasing photocatalyst concentra-
tion the exposed surface area increases, which absorbs more numbers of
photons, and as a result the rate of photocatalytic degradation of the dyes
increases [156].
However, after a certain amount of photocatalyst concentration the
treatment efficiency may keep constant or decrease. This can be explained
in terms of availability of active sites on photocatalyst surface and the light
penetration of photoactivating light. Because by using the photocatalyst
concentration more than necessary, the light penetration through the solu-
tion becomes difficult, and hence, the photoactivated volume of the suspen-
sion is reduced. In such a condition, part of the catalyst surface probably
becomes unavailable for photon absorption and dye adsorption [149,157].
Initial Dye Concentration
The studies in literature reveal that increasing dye concentration will
reduce photocatalytic degradation [143,158,159]. This can be attributed to
the quantity of intermediates increased with the increasing dye concentra-
tion. At higher dye concentration, a significant amount of UV light might
be absorbed by dye molecules rather than the catalyst, reducing the photo-
degradation efficiency [160,161].
Sacco et al. concluded in their study that at constant reaction times, the
increase of initial dye concentration will lead to a decrease in photocata-
lytic activity because the increase in the color intensity of the solution will
reduce the light penetration into the aqueous medium, meaning that the
path length of photons inside the solution will decrease [157].
Another point is that the active site of the catalyst surface area is fixed.
If the concentration of dye increases, since a limited number of dye mol-
ecules can attach at the active site of the catalyst, the rate of degradation
decreases. Competitions between dye molecules to attach to active site also
affect the rate of degradation [162].
pH
The interpretation of pH effects on the efficiency of dye photocatalytic oxi-
dation is a difficult and interesting task because of its several roles.
Previous studies indicate that pH may affect photocatalytic oxidation
efficiency in a number of ways [143]. For example, the charge of the dye
molecules with ionizable functional groups and the surface of the cata-
lyst that can be used are both pH dependent. Acid-base properties of the
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