Nanotechnostructured Catalysts TiO Nanoparticles for Water Purification - Nanotechnologies for Water Environment Applications - page 70

Extreme temperature condition is generally expected to have adverse influence
on the reaction rate. Herrmann (1999) has reasoned that, at very low temperature (below
zero), desorption of the final products becomes a rate limiting step. On the other end, if
temperature is above 80 o C, the exothermic adsorption of substrate becomes disfavored
and rate limiting steps, due to the exothermic nature of this process. Fortunately, the
common operating temperature of most water purification processes is in the range of
optimum photocatalytic reaction. There is no need to artificially adjust the temperature,
unless extreme temperature condition is met.

3.4.7 Presence of Interference Species

The occurrence of interference ions is rather common in industrial wastewater
(Konstantinou and Albanis, 2004). It is common to encounter a wastewater effluent
containing a mixed composition of pollutants and other species, such as inorganic ions,
organic solvents, chelating agents, organic matter and humic substances. The presence
of the interference species may pose both plausible and prohibitive influence on the
photocatalytic reactions via surface perturbation, charge carriers inhibition, and ROS
species reactions. Thus, it is very important to consider the presence of interference
agents on the applications of photocatalytic processes for the purification of water or
wastewater.

3.4.7.1 Anion Effects

In general, the presence of commonly occurring inorganic anions, including Cl - ,
NO 3 - , NH 4 - , CO 3 - , HCO 3 - , PO 4 3- and SO 4 2- , exhibits restraining effects on photocatalytic
reaction rates (Bhatkhande et al., 2002; Konstantinou and Albanis, 2004). This
observation can be explained by the reaction of electron holes and OH• with anions (Eqs.
3.27 and 3.28) (Konstantinou and Albanis, 2004), which can be viewed as electron holes
and OH• scavengers (Fox and Dulay, 1993). Some anions, such as Cl - , act as an UV
light absorber as well. The adsorption of these ions onto TiO 2 surface also may prevent
the reaction (Konstantinou and Albanis, 2004). This competition of adsorption not only
depletes the surface active sites, but also forms a highly polar layer near the particle
surface, blocking the diffusion of organics to the active sites (Fox and Dulay, 1993).

h tr + + Cl - Cl• (Eq. 3.27)

OH• + Cl - Cl• + OH - (Eq. 3.28)

Even though inorganic radicals, such as Cl•, also have oxidizing activity, they
are not as active as electron holes and OH•. Hence, the presence of these anions
normally retards the reaction rate. Yawalkar et al. (2001) compared the influence of
anions on the degradation of phenol, and found the order of interference to be: SO 4 2- <
CO 3 2- < Cl - < HCO 3 - .