The primary oxidizing agents are the surface bound hydroxyl
radicals having extremely short life span (10−9 s), the fact
which prevents them from diffusing to a long distance. Therefore,
only microorganisms that adhered to the photocatalyst
surface can efficiently react with the hydroxyl radicals. This
may lead to oxidation and disruption of the cytoplasmic membrane.
The direct measurement of the concentration of hydroxyl
radicals in water under specific experimental conditions is not
an easy task, because their concentration in the steady-state is
low (10−9 M), due to their high reactivity toward water. The
indirect measurements of their concentration can be realized
by using a probe compound, e.g. para-chlorobenzoic acid (p-
CBA). The concentration of p-CBA decreases in the solution
due to its reaction with the hydroxyl radicals. This is related to
the hydroxyl radicals overall production by the photocatalyst.
Some substances, like tert-butanol and methanol, can be used as
scavengers for a specific kind of hydroxyl radicals, making a useful
tool for assessing the capacity of the designed photocatalytic
coating.
The addition of the scavengers (methanol and tert-butanol) to
the test solution, along with the test microorganism, enables the
determination of the extent of the survival of the microorganisms
in the presence of a certain type of hydroxyl radicals. Namely, the
results of some experiments showed that one type of scavengers
react with a certain kind of hydroxyl radicals, leaving the other
free to react with Escherichia coli as the test organism.
The objective of the present work was to study the photocatalytic
and self-cleaning properties of the clay roofing tiles, as a
porous ceramic substrate covered with a mesoporous TiO2 coating.
For this purpose, photochemical experiments were carried
out using p-CBA as a probe compound and two different kinds of
scavengers (methanol and tert-butanol), to test the existence of
hydroxyl radicals on the surface coating. Besides, microbiological
tests of inactivation of the bacteria Pseudomonas aeruginosa,
also carried out in the presence/absence of the same scavengers,
were employed to study the nature of the hydroxyl radicals
involved.