Topics in Current Chemistry (2020) 378:29
1 3
1 Introduction
The presence of recalcitrant pollutants in wastewater streams has shown an increasing trend in recent years, in particular due to the advent of newer chemicals required
by human beings, such as novel pharmaceutical drugs for tackling various ailments. A
similar trend has been seen for pesticides due to the aim to achieve higher production
from agricultural fields. These newly developed chemical compounds are often biorefractory in nature and have a longer half-life than many of their predecessors and represent a challenge to the efficacy of the conventional wastewater treatment methods.
Consequently, focus has been on developing newer oxidation schemes based on, for
example, advanced oxidation processes [1, 2].
Among the different advanced oxidation processes proposed in recent years, photocatalytic oxidation based on both ultraviolet (UV) and solar irradiations is considered to
be one of the more promising approaches. One target of such treatment is the destruction
of complex pollutants, either completely or by converting the pollutant into smaller, easily digestible and non-toxic compounds [3, 4]. The two most important factors contributing to the efficacy of a photocatalytic oxidation system are the catalyst and the operation
of the photocatalytic reactor, including the design [5]. The most common problems associated with the photocatalyst are limited activity of the catalyst, uneven particle size distribution, lack of control over the catalyst morphology and possible deactivation during
the operation. The actual operation of photocatalytic oxidation is limited by mass transfer resistances based on the heterogeneous nature of the catalyst, oxidants and effluent.
In this article, I highlight the important aspects of tackling these commonly observed
disadvantages in photocatalytic oxidation operations using ultrasound.
The passage of ultrasound through a liquid medium causes the formation of cavities
(either gas filled or vaporous depending on the medium). The generated cavities grow
under the driving pressure field, controlled by the amplitude and power of the ultrasound, and ultimately collapse, violently releasing significant energy [6, 7]. The phenomenon is described as cavitation, and the net effects are the generation of local hot
spots, liquid circulation currents and turbulence as well as production of free radicals
(both of the chemical compounds present and the oxidants). The effects are classified
as being either physical or chemical, and these can be tailored based on the operating
conditions of the ultrasound system. The physical effects, such as turbulence and micromixing, are expected to be beneficial in terms of improving catalyst synthesis and eliminating mass transfer resistances in the actual photocatalytic oxidation operation [8]. The
chemical effects in terms of radical production can be synergistic for photocatalytic oxidation [9]. An in-depth analysis of the application of ultrasound for both catalyst synthesis and the actual operation of photocatalytic oxidation is presented in subsequent sections of this article.
2 Mechanistic Understanding of Cavitation and Its Effects
Cavitation generated by the passage of ultrasound through a liquid has been
described as acoustic cavitation. When the ultrasound waves are introduced into the
liquid, the rarefaction and compression cycles drive different stages of cavitation.
72
Reprinted from the journal
1 3
1 Introduction
The presence of recalcitrant pollutants in wastewater streams has shown an increasing trend in recent years, in particular due to the advent of newer chemicals required
by human beings, such as novel pharmaceutical drugs for tackling various ailments. A
similar trend has been seen for pesticides due to the aim to achieve higher production
from agricultural fields. These newly developed chemical compounds are often biorefractory in nature and have a longer half-life than many of their predecessors and represent a challenge to the efficacy of the conventional wastewater treatment methods.
Consequently, focus has been on developing newer oxidation schemes based on, for
example, advanced oxidation processes [1, 2].
Among the different advanced oxidation processes proposed in recent years, photocatalytic oxidation based on both ultraviolet (UV) and solar irradiations is considered to
be one of the more promising approaches. One target of such treatment is the destruction
of complex pollutants, either completely or by converting the pollutant into smaller, easily digestible and non-toxic compounds [3, 4]. The two most important factors contributing to the efficacy of a photocatalytic oxidation system are the catalyst and the operation
of the photocatalytic reactor, including the design [5]. The most common problems associated with the photocatalyst are limited activity of the catalyst, uneven particle size distribution, lack of control over the catalyst morphology and possible deactivation during
the operation. The actual operation of photocatalytic oxidation is limited by mass transfer resistances based on the heterogeneous nature of the catalyst, oxidants and effluent.
In this article, I highlight the important aspects of tackling these commonly observed
disadvantages in photocatalytic oxidation operations using ultrasound.
The passage of ultrasound through a liquid medium causes the formation of cavities
(either gas filled or vaporous depending on the medium). The generated cavities grow
under the driving pressure field, controlled by the amplitude and power of the ultrasound, and ultimately collapse, violently releasing significant energy [6, 7]. The phenomenon is described as cavitation, and the net effects are the generation of local hot
spots, liquid circulation currents and turbulence as well as production of free radicals
(both of the chemical compounds present and the oxidants). The effects are classified
as being either physical or chemical, and these can be tailored based on the operating
conditions of the ultrasound system. The physical effects, such as turbulence and micromixing, are expected to be beneficial in terms of improving catalyst synthesis and eliminating mass transfer resistances in the actual photocatalytic oxidation operation [8]. The
chemical effects in terms of radical production can be synergistic for photocatalytic oxidation [9]. An in-depth analysis of the application of ultrasound for both catalyst synthesis and the actual operation of photocatalytic oxidation is presented in subsequent sections of this article.
2 Mechanistic Understanding of Cavitation and Its Effects
Cavitation generated by the passage of ultrasound through a liquid has been
described as acoustic cavitation. When the ultrasound waves are introduced into the
liquid, the rarefaction and compression cycles drive different stages of cavitation.
72
Reprinted from the journal