(12b) Cavitation and Immobilised Photo-Catalysis for Effluent Treatment: A Comparative Study of Individual and Combined Operations

Cavitation
and Immobilised Photo-catalysis for Effluent Treatment:
A
Comparative Study of Individual and Combined Operations

Sebastien De-Nasri,
Sanjay Nagarajan, Peter K. J. Robertson and Vivek. V. Ranade*

Multiphase Flows, Reactors &
Intensification Group (mRIg)

School of Chemistry and Chemical
Engineering

Queen’s University Belfast

Belfast BT9 5AG

Email: V.Ranade@qub.ac.uk
Background

Unsustainable strain on freshwater sources globally could be
reduced if industries reuse their wastewater after treatment. Wastewater
treatment (WWT) plants, which remove pollutants from wastewater, commonly use conventional
physico-chemical processes to treat wastewater. These conventional processes
are energy intensive, expensive or use environmentally unfavourable chemicals
for treatment and hence an alternative has to be sought ^1,2.

Advanced oxidation processes (AOPs) are a form of advanced treatment
which are gaining attention as a potential cost-effective method of treating
wastewater. AOPs are characterised by their in situ generation of hydroxyl
radicals to mineralise organic pollutants. Various AOPs include Fenton,
photo-Fenton, photocatalysis, cavitation, ozonation and electrochemical
treatment. In Fenton and ozonation based processes, chemicals are added to induce
hydroxyl radical production, whilst electrochemical utilises electricity to
treat wastewater by inducing oxidation processes with electrodes. Cavitation
and photocatalysis are both processes which have been extensively studied for
water treatment, and recent studies show great potential with large scale
operation ^1,3,4.

Cavitation is the formation, growth and subsequent collapse of
microbubbles in a liquid, and is of interest for WWT due to its ability to
completely mineralise organic pollutants without the need of complex catalysts
or extreme operating conditions. The two commonly studied types of cavitation
are acoustic cavitation (AC) and hydrodynamic cavitation (HC). HC relies on
cavity generation through induced pressure variations in a flowing liquid using
a cavitating device whereas AC relies on the propagation of ultrasound in a
liquid medium for cavity formation. HC can be easily scaled up for large scale
operation due to its continuous nature and low energy requirement when compared
to AC.

Photocatalysis is another emerging AOP which is a light driven
chemical reaction capable of producing reactive radical species through
irradiation of a semi-conductor. Key characteristics of photocatalysis are its
ambient operating conditions, low operating costs, and complete mineralisation
of pollutants without the formation of undesired intermediates, as such, this process
has feasible applications in wastewater treatment ^4–7. Commonly,
photocatalysts are suspended in solution to obtain an improved irradiated area
to volume ratio to perform photocatalysis. However, the need to separate these
nanoparticles is a major limiting step for scale-up. Conversely,
immobilised photocatalyst (ImPC) systems that do not require a separation
process could be used. Poor mass transfer in ImPC and poor diffusivity of OH
radicals in bulk⁸ are however issues
faced by ImPC. These limitations can be partially overcome by operating it in
combination with other AOPs, or as a continuous or recirculating batch systems
to enhance mass transfer.

The extent of degradation of pollutants is dependent on the
quantity of hydroxyl radicals produced, which is increased significantly when
combinations of AOPs are combined ⁹. HC is an effective
AOP which is operated in continuous flow, this characteristic is ideal for
coupling with ImPC process. By combining HC with ImPC, mass transfer
limitations can be negated through the continuous flow, whilst the degradation
efficiency of both processes is increased through increased production of
hydroxyl radicals. Fig. 1 depicts such a coupled process.

Figure 1 - schematic diagram of
cavitation coupled with an immobilised photocatalyst system
Scope of present work

Both HC and AC produces in situ OH radicals as a result of cavity
collapse. Although methods of cavity generation are different, the chemistry of
the reactions upon bubble collapse is equivalent in both cases. Hence, for the ease
of experiments, this paper will focus on AC coupled with ImPC (sonophotocatalysis)
which will then be used for translating it to a HC-ImPC system. Sonophotocatalysis
has been extensively studied in the past for degrading a range of pollutants and
proves to be an effective hybrid technique. However, the only combinations
reported have used suspended photocatalysts with AC ¹⁰. Due to the issues
surrounding the scale-up of a suspended photocatalyst system, ImPC coupled with
AC/HC is suggested in this work. By effectively degrading a model pollutant
through ImPC and cavitation, an effective hybrid system can be employed which
has neither mass transfer limitations nor a necessary separation process. The
results will provide a sound basis for design of such hybrid systems for
effluent treatment.
Experimental methods

Acetone
was used as the model pollutant for degradation experiments. The photocatalyst
used was TiO₂ P25. The ImPC/cavitation system is described Fig. 1.,
the pollutant will be initially passed through a cavitating device (Sonics VCX
500 Ultrasonic processor) before being treated with photocatalysis. Given the
nature of this system, treatment will consist of multiple passes occurring
within a certain time. Individual processes (AC and
ImPC) and their optimal operating parameters will be determined (amplitude,
catalyst coating) through initial experiments. Sonophotocatalysis experiments will
then be carried out and optimised. By comparing degradation data for individual
and hybrid processes, the extent of synergy of the two processes can be
quantitively compared and assessed against other hybrid systems in literature.
Results and discussion

Processes
were compared on a basis of acetone degradation. Acetone was selected as a
model pollutant as it is a common industrial solvent, it also possesses a
simple organic structure and has a well-documented reaction scheme with
hydroxyl radicals, reducing the likelihood of unknown intermediates forming.
Furthermore, sonophotocatalysis has been established as a process which can
degrade a range of pollutants from a variety of industries, which does not need
to be further established by investigating complex pollutants.

Initial
experiments with AC have been carried out using 20kHz Sonics VCX 500 Ultrasonic
processor, where the amplitude has been optimised (90%) for acetone solution (1000ppm)
during 15 mins of sonication. Analysis was performed using a Cary 300 Scan,
UV–vis Spectrophotometer at 263 nm, with a scan rate of 400 nm/min. Detailed results
including optimised conditions for individual ImPC catalyst coating, and hybrid
process will be presented at AIChE2018.  
Summary and outlook

This
work focuses on comparing the performance of a batch recirculating system
hosting an ultrasound/hydrodynamic cavitation reactor and an immobilised
photoreactor in series. The performance of this sonophotocatalytic system is
compared to the individual processes based on the degradation of a model
pollutant - acetone. Additionally, the effect of operating parameters of
cavitation reactors and weight of photocatalyst coating on the degradation
performance will be tested and optimised. This comparison will allow the extent
of synergy between the systems to be determined and would form the basis for
developing a mathematical model to understand sonophotocatalytic degradation
performance for a broader range of effluents. This approach and work presented
is useful for future development of immobilised photocatalytic – hydrodynamic
cavitation hybrid systems.

Keywords: AOP,
Sonophotocatalysis, process intensification, immobilised photocatalyst

 
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