Solar photocatalysis for environmental remediation

174 Pages·2019·8.486 MB·English

by  Chakrabarti, Sampa

SAMPA CHAKRABARTI
The Energy and Resources Institute
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To
my mentor
Prof. Binay Kanti Dutta

Preface
From the very childhood, the brightness or tejas of the sun fascinated
me. At that age, I wondered if, one fine ‘morning’, that red ball
did not appear over the horizon, there would be no more ‘morning’
in our life. Afterwards, I found that Rig Veda, the oldest of Hindu
scriptures, considered the Sun God or Aditya the most powerful.
Later, when I visited Rome and Egypt, I found that the Sun God
was highly regarded in ancient Roman and Egyptian civilizations.
With the advent of civilization, energy crisis deepened and the
search for an alternative source of energy began. Although solar
energy had been used from ancient times for various applications,
innovations in solar energy technology began in 1767 when Swiss
scientist Horace-Bénédict de Saussure built the first solar collector.
Meanwhile, the definition of civilization started changing during
the Industrial Revolution. Industrial Revolution changed people’s
way of life and drastically increased the demand for energy. The use
of fossil fuels to meet this huge energy demand led to the pollution
of the environment. Other developmental activities also resulted
in environmental pollution. Consequently, the conventional energy
resources began to deplete. At the same time, pollution began to
result in the deterioration of the environment quality. Awareness
concerning the environment began in the 1980s. There are several
remedial solutions for environmental problems with the use of
advanced technologies; however, most of them require huge energy.
Therefore, this is a vicious cycle and to break the cycle, renewable
energy should be used for environmental remediation. In tropical
viii Preface
countries like India, abundantly available solar energy is the most
suitable source of renewable energy.
This book aims to highlight the possible use of solar energy for
environmental remediation or pollution control. Energy is needed for
the treatment of solid, liquid, or gaseous wastes. Advanced oxidation
techniques, nowadays used for water and wastewater treatment,
require energy for exciting electrons to generate reacting species. In
an effluent treatment plant, energy is required for operating pumps
and other equipment. The use of solar energy for these purposes
does not cause environmental pollution, thus improving the quality
of environment. Researchers all over the world are working on this
area. In most cases, the research is still confined to laboratories. The
studies on solar energy for environmental applications are somehow
inadequate. With respect to experimental conditions, a tremendous
diversity in the way of reporting experiments was encountered.
While artificial light is used, a description of light sources (e.g., the
type of lamp, manufacturer, model number, intensity or power, and
spectral data) is crucial. In addition, the distance from the artificial
light source and the material of construction of the photoreactor
also play significant roles for standardizing the protocol of photo-
assisted experiments.
The book discusses the use of solar heat and light for
environmental remediation. Sun is an endless source of energy.
Heat and light are two forms of solar energy. Both heat and light of
the sun can be utilized for environmental remediation. Ultraviolet
radiation of sunlight kills the disease-causing microbes or pathogens
in water and also facilitates photocatalytic detoxification. The
different aspects of controlling the pollution caused by solid, liquid,
and gas wastes with the help of solar energy, as well as basics of
photocatalysis, are dealt with in Chapter 1. Flat collectors are the
simplest in construction and use, but depending on the intensity of
solar radiation in different geographical locations, it is sometimes
necessary to use concentrating collectors. Chapter 2 describes
various types of solar energy collectors generally used in reactors
for the control of solid, liquid, and gaseous pollutants.
During the early years of industrialization, industrial wastewater
used to be discharged into the river without any treatment in almost
all countries of the world. Nowadays wastewater treatment has
become mandatory, not only by the law but also by the absolute
necessity of clean and clear water for the survival of mankind.
Preface ix
Exposure to solar energy decreases harmful pathogens in drinking
water; the SODIS Protocol is accepted by World Health Organization
(WHO). The biodegradable pollutants in wastewater can be taken
care of by the activated sludge and other bacteriological/microbial
processes. The non-biodegradable organic pollutants from various
industries can be treated efficiently using advanced oxidation
processes (AOP), such as photo-Fenton and photocatalytic reactions,
the excitation energy being provided by the sunlight. The advantage
is twofold—utilization of solar energy and savings of fossil fuels.
Water and wastewater treatment using sunlight is described in
Chapter 3.
With rapid industrialization, a significant amount of solid waste
is also generated as a result of anthropogenic activities. Municipal
solid wastes contain plastics among other biodegradable wastes.
Sludges are generated from various industrial effluent treatment
processes. Polymeric waste materials are non-biodegradable and
release toxic gases when they are incinerated. Many waterbodies in
India are strewn with plastic bags that have severely degraded the
water quality. If plastic carry bags could be produced from a plastic
film that could be naturally degraded in presence of water, air and
sunlight, the problem of plastic pollution would not be so severe.
Solar energy can be used for the photocatalytic degradation of plastic
wastes and is described in Chapter 4. The treatment of solid wastes
other than plastic is not covered in the book. In fact, the remedial
measure suggested is a preventive one in which photodegradable
plastics are synthesized before use. Chapter 5 deals with the control
of air pollution with the help of solar energy. Ambient air may
be contaminated by different volatile organic compounds, besides
particulate solids, SO , NO , and greenhouse gases. Using solar
x x
energy assisted advanced oxidation techniques, volatile organic
compounds can be removed from air. NO and SO can also be
x x
removed by sunlight-enhanced photocatalytic oxidation. Building
materials, windowpanes, wallpapers, and tiles are impregnated with
photocatalysts and with the help of sunlight, polluted air can be
treated. Hydrogen is considered to be the future energy source for
mankind. Fuel cell research is advancing. It will be a clean energy
with inexhaustible supply. To break the energy–environment nexus,
fuel cell is going to play an important role. Photocatalytic water
splitting using solar energy will definitely be a milestone in the
fuel cell research if it can be implemented properly. Solar splitting