Photocatalytic Processes for the Removal of Dye - Green Chemistry for Dyes Removal from Wastewater - page 131

strong and less selective, resulting in complete mineralization, iii) the pro-
cess does not consume any expensive oxidizing chemical, and iv) the pho-
tocatalysts are less expensive, non-hazardous, stable and reusable [1,30,31].
UV-light-assisted dye degradation is a very popular technique, but recent
studies have also found the use of natural sunlight for dye degradation as a
promising and sustainable route. Direct solar radiation was used as a light
source (between 9 am and 4 pm) with different ranges of solar light intensi-
ties at different locations throughout the year [32-37]. Both non-concen-
trating and concentrating solar reactors were utilized for the degradation of
different dyes under several experimental conditions [32-37].
There are five main parameters that affect photo-oxidation of dye
molecules such as: i) photocatalyst loading, ii) initial dye concentration,
iii) solution pH, iv) light intensity, and v) electron scavenger. These will be
discussed in the next sections.

4.6.1 Effect of Photocatalyst Loading

Mehrotra et al . [30, 38] categorized photocatalytic reaction into two distinct
regions depending on the catalyst concentration. At a low catalyst concentra-
tion (<0.05 g L -1 ), the overall rate is entirely controlled by reaction kinetics
and is known as kinetic regime. However, at a high catalyst concentration
(0.05-2 g L -1 ), the reaction rate depends on several factors such as: i) exter-
nal mass transfer, ii) catalyst agglomeration, iii) light shielding via catalyst,
among others. In the kinetic regime, the initial rate is directly proportional
to the catalyst concentration. That means, by using twice the photocatalyst
mass the conversion also doubles, and the reaction rate remains unchanged.
As mentioned by Konstantinou and Albanis [39], the dye degradation rate
increases in the catalyst concentration range of 400-500 mg/L, whereas
reduced degradation rate is observed above 2000 mg/L. In the photocata-
lytic process, there is an optimum concentration level of the photocatalyst
beyond which the reaction becomes independent of the photocatalyst mass.
At optimum catalyst loading, the complete photocatalyst surfaces would be
illuminated with the incident light. Once the catalyst concentration exceeds
the optimum level, a fraction of the photocatalyst surface would become
unavailable for light absorption and dye adsorption, thus shielding the effect
of the particle on the light by excess particles [39,6].

4.6.2 Effect of Initial Dye Concentration

Initial concentration of the pollutant would affect the degradation
rate, and thus the effect of initial concentration is always studied.
Photocatalytic degradation kinetics of different dyes over TiO 2 followed