Aims:
The aim of this work was to investigate the impact of light modulation parameters on the degradation of
terephtalic acid, an organic model pollutant, within a heterogeneous photocatalytic system under visible light. For this
purpose, a fixed bed photocatalytic reactor, irradiated by white-light LEDs matrix controlled by a system for light dimming,
was used. The bed consisted of a nitrogen-doped titania photocatalyst deposited on polystyrene pellets.
Background:
Wastewater containing TPA can be treated into conventional aerobic biological units. However, the
mineralization of TPA is slow and inefficient and its presence influences negatively the biodegradation efficiency because
this pollutant inhibits microbial growth. Nowadays innovative technologies named advanced oxidation processes (AOPs),
such as heterogeneous photocatalysis with UV and visible light, ozonation, Fenton oxidation have gained popularity for
effective organic destruction of TPA from wastewater. The heterogeneous photocatalytic oxidation process of the TPA
under visible light is the most advantageous process in terms of both fixed and operating costs.
Objective:
In this work the successful application of light modulation techniques to degradation of TPA using a
photocatalytic system with supported visible active photocatalysts (N-doped TiO2) immobilized on polystyrene pellets was
reported. In particular, sinusoidal lighting has been used analyzing the influence of the period of oscillation and the
amplitude of the light modulation on the reaction kinetics, in such a way as to minimize the times and energy costs for the
process.
Methods:
To evaluate the influence of light modulation on the efficiency of the TPA removal, a discontinuous system
composed by a Recirculating Photocatalytic Fixed Bed Reactor (RPFBR) irradiated by a matrix of white light LEDs was
used. The flat geometry of photoreactor guarantees the efficient excitation of photocatalyst. An amount of 250 mL of
aqueous solution with initial TPA concentration of 12.5 ppm was applied in the photocatalytic tests lasting 180 min of
irradiation fixed or sinusoidal modulated.
Results:
The results show that the variation of the oscillation period of the sinusoidal modulation has a relevant influence
on the photodegradation of TPA and a maximum value of the apparent kinetic constant, 0.0045 min-1 is found when the
period of oscillation is 0.24 s. The sinusoidal modulation with optimal amplitude is that with current between 50-200 mA,
that shows the highest value of the apparent kinetic constant, equal to 0.0046 min-1. The optimal sinusoidal modulation, as
a consequence is with current between 50-200 mA and period of 0.24 s. From the data collected from the tests, it is possible
to evaluate the energy cost necessary to obtain the reduction of 90% of the terephthalic acid in 1 m3 of polluted water for
each modulation (E E/O ), and compare these values with other tests for TPA degradation reported in the literature. The
internal comparison and with the three systems of literature showed the optima sinusoidal modulation of LEDs matrix
permits a strong reduction the electrical energy consumption.
Conclusion:
Photocatalytic tests have demonstrated the improvement of the process energy efficiency using the light
modulation technique . A further confirmation of the advantage of light modulation was obtained by comparing the energy
costs required for the abatement of 90% of the terephthalic acid in 1m 3 of the photocatalytic system. Finally, a mathematical
model for photocatalytic degradation of terephthalic acid within the recirculating fixed bed photocatalytic reactor, irradiated
by white-light LEDs was developed.
