Development of photocatalyst for effective utilization of solar energy for wastewater treatment by Leena V. Bora

Guided by: Dr. R. K. Mewada With Synopsis and CD 13EXTPHDE113

ABSTRACT:
Due to rapid industrialisation, large amount of wastewater is generated. The
textile industries which uses dyes and other dyes manufacturing industries, in
particular, discharge highly coloured dyes that can adversely affect plants, marine life
and humans. Treatment of this wastewater prior to its discharge into natural water
bodies is essential since it could be carcinogenic and toxic. To combat the issue,
governments have laid down discharge norms that need to be followed while treating
the wastewater. While conventional treatment methods are energy and cost intensive,
and generally do not completely degrade the pollutant within optimal operating
parameters, advanced oxidation processes (AOP) can mineralise the organic chemicals
to harmless water and carbon dioxide. Photocatalysis is one such AOP which thus
treats the effluent with the help of solar energy. In the vicinity of city of Ahmedabad
and in the state of Gujarat many dyes and textile industries are present. Current work
is an attempt to propose better option to conventional techniques for degradation of
such dyes from wastewater.
Most of the development in this area has been focussed on TiO2 as a
photocatalyst. However, owing to its characteristic bandgap, it is able to utilise only
the ultra-violet region, which is only ~4% of the solar spectrum. Ag2CO3, on the other
hand, has a relatively narrow bandgap and can work in visible region (~47% of the
solar spectrum). However, narrow bandgap often results in rapid recombination of the
photogenerated electrons and holes. In order to arrest recombination, Ag2CO3 can be
coupled with other photocatalysts having appropriate band positions and together they
can exhibit better results.
The main objective of this work is to evaluate the treatability of a model dye
sample in an efficient and effective manner. In the present study, a series of hybrid
Ag2CO3/SiC and Ag2CO3/ZnO nanostructures have been successfully synthesised
through a simple precipitation route. The photocatalytic performance was evaluated
by the degradation of MB since it is non-toxic and hence suitable for educational labwork.
All experiments were performed under natural solar irradiation. It was observed
that formation of a heterojunction improves the photoactivity of the hybrid
photocatalysts by inducing a charge transfer between Ag2CO3 and SiC/ZnO. The
photocatalysts were characterized by XRD, SEM, TEM, TGA-DTG, BET and UVVis/
NIR. Among all the hybrid combinations prepared, the best photocatalytic results and formal quantum efficiency (FQE) under natural sunlight were obtained with
AGSC-12 and AGZN-22 composites. It is observed that under experimental
conditions, AGSC-12 was able to completely decolourise the dye in 90 min with an
FQE of 0.154%. Similarly, AGZN-22, under the experimental conditions could
decolourise MB completely in just 20 min. The FQE of AGZN-22 was found to be
0.884%. The performance of both these photocatalysts was observed to be much better
than that of the conventionally popular TiO2.
Factors viz., photocatalyst dosage, solution pH, solar intensity, substrate and
its initial concentration and speed of agitation were found to influence phototreatment.
Corresponding optimum parametric values have been found and reported
in terms of FQE. The performance of the photocatalysts have also been investigated
on real industrial effluent and rate expression developed in terms of TOC. Also, a CPC
solar photoreactor (CPC-P) has been designed and fabricated and investigated for its
performance. Its performance was found to be better than that of the laboratory reactor.
While the lab reactor could decolourise MB by 40% with AGSC-12, the discoloration
was 60% in the CPC-P. With AGZN-22, the discolorations were 70% and 80% in the
lab reactor and CPC-P, respectively.
All the work has been carried out in direct natural solar light and would be
useful in real-time solar photocatalytic design of reactors and other solar applications
for environmental remediation.