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ELECTROCHEMISTRY FOR A CLEANER ENVIRONMENT
National Centre for Catalysis Research
M. HELEN
Research Scholar
Environment and Pollution
Pollution is the action of environmental
contamination with man-made waste
Possibilities offered by Electrochemistry

With a rapidly growing world population and an increasing number of reports on
detrimental effects on the environment, its protection has become a major issue

The strategies for environmental protection in industry generally include
processes for waste treatment as well as development of new processes or
products which have no or less harmful effects on the environment

Electrochemistry has important roles to play in both types of strategies

Electrochemical processes can be used for recovery or treatment of effluents
from industrial or municipal plants. Industrial electrochemistry has undergone a
development towards cleaner processes and more environmentally friendly
products

Electrochemical sensors are effective and inexpensive devices for environmental
monitoring of an increasing range of toxic substances

A big and important class of environmental problems can be found in the energy
and transportation sectors

Electrochemistry offers unique ways to generate pure electric power at high
efficiency in fuel cells or to store it in batteries
Electrochemistry, with its unique ability to oxidize or reduce compounds at a
well-controlled electrode potential and by just adding (at the anode) or
withdrawing (at the cathode) electrons, offers many interesting possibilities
in environmental engineering
Reaction
Eo
Reaction
Eo
F2 + 2e- ---> 2F-
+2.87
Fe3+ + 3e- ---> Fe
-0.04
Co3+ + e- ---> Co2+
+1.80
Sn2+ + 2e- ---> Sn
-0.14
Cl2 + 2e- ---> 2Cl-
+1.36
Ni2+ + 2e- ---> Ni
-0.25
O2 + 4H+ + 4e- ---> 2H2O
+1.23
Co2+ + 2e- ---> Co
-0.29
NO3- + 4H+ + 3e- ---> NO + 2H2O
+0.96
Fe2+ + 2e- ---> Fe
-0.41
Ag+ + e- ---> Ag
+0.80
Zn2+ + 2e- ---> Zn
-0.76
Fe3+ + e- ---> Fe2+
+0.77
2H2O + 2e- ---> H2(g) + 2OH-
-0.83
I2 + 2e- ---> 2I-
+0.54
V2+ + 2e- ---> V
-1.18
Cu+ + e- ---> Cu
+0.52
Mn2+ + 2e- ---> Mn
-1.18
Cu2+ + 2e- ---> Cu
+0.34
Al3+ + 3e- ---> Al
-1.66
Cu2+ + e- ---> Cu+
+0.15
Na+ + e- = Na
-2.71
2H+ + 2e- ---> H2
0.00
Li+ + e- = Li
-3.04
Strong reducing agents
Strong oxidizing agents
Standard Reduction Potentials (in Volts), 25oC
Electrochemical processes for waste treatment
Anodic processes


To oxidize organic pollutants to harmless products
To remove toxic compounds from flue gases
Cathodic processes

To remove heavy metal ions from waste water solutions

In both types of electrode processes, the operating conditions must be carefully
controlled in order to avoid side reactions.

In aqueous solutions, which are most often used, the side reactions are mainly
oxygen evolution at the anode and hydrogen evolution at the cathode.

These side reactions lower the current efficiency thereby increasing the
operating costs, and may disturb the process because of vigorous gas evolution or
pH changes at the electrodes.

Not only can the two electrodes of the electrochemical cell be used in purification
processes, but the ion-selective membranes that are often placed between the
electrodes to have a selective transfer of only anions or cations can also.

New electrodialytic processes using such membranes have been developed, which
can solve a variety of environmental problems.
Cyanide Poisoning
Electroplating
- Zinc, copper, cadmium, silver, gold, brass and
nickel are commonly plated using cyanide solutions
Cyanide solutions - intrinsic cleaning ability
- effective in keeping metals in solution during the plating
process
- to obtain very finely grained metal deposits
Cyanide
- harms the brain and heart, and may cause coma and death
High concentration of cyanide (> 1 M)
Electrochemical process
Anode (graphite or stainless steel) : cyanide is oxidized
CN- + 20H-
CNO- + H20 + 2e-
Cathode: metal deposition with hydrogen evolution as a side
reaction.
Temperature
Current density
- 50-90 ºC
- 500 A m-2
Low concentration in outlet stream
Chemical method
CN-
CNO- using hypochlorite (NaClO) as an oxidizing agent
Chlorine in Drinking Water
Effective disinfectant - destroy many of the bacteria in your
drinking water
Chlorinated hydrocarbons - chlorine reacts with decomposed
plant and animal materials
- solvents and disinfectants, bleaching in the pulp and
paper industry (chlorinated phenols)
Chlorine - irritant to the eyes, skin, the upper respiratory tract, and the lungs
Dechlorination
 Combustion
 membrane separation, adsorption on activated carbon and stripping,
chemical oxidation with air, ozone or other oxidants
 Chemical reduction techniques, such as catalytic dehalogenation with
hydrogen or other reducing agents
 Biological techniques using special microorganisms or enzymes
 Electrochemical dechlorination - either anodically or cathodically
p-chlorophenol and pentachlorophenol - anodically on lead dioxide
First step
chlorine is substituted by hydroxyl radicals formed from water
Subsequent steps
further oxidation yields quinone, which decomposes into maleic
acid, oxalic acid (primarily) and carbon dioxide.
Oxygen and significant amounts of ozone were formed as byproducts at the anode
Risk
chloride ions formed may be oxidized to hypochlorite, which can
then form chlorinated organic compounds
Cathodic Electrochemical Dechlorination
RCl + H+ + 2e-
RH + C1-
Electrode material - thin graphite/carbon fibres high specific surface area
and high overpotential for the competing hydrogen
evolution reaction
Removal of heavy metal ions
Cu, Hg, Zn, Cr and Cd
Waste waters containing heavy metal ions - generated in metallurgical and
electroplating industries and in the manufacture of printed circuit boards.
Conventional purification - uses hydroxide precipitation gives voluminous
metal hydroxide sludge that has to be disposed of
Complexed metal ions in alkaline solutions-hydroxide precipitation is not a
viable method
Cathodic removal of heavy metal ions
attractive alternative process
Metal can be recovered in its pure metallic form
Anode
- three-dimensional electrode or
just a planar electrode for e.g.
oxygen evolution
Cathode - graphite particles, expanded metal,
metal wool & graphite fibres
 Three-dimensional electrodes -offer both a
high specific surface area and high mass
transport rate conditions.
 Metal concentration can be reduced from
100 to 0.1 ppm at a residence time of a few
minutes
 Operational costs are favourable compared with classical waste water
treatment systems
 the space required by the process is low
 deposited metal in the cathode may be recovered as a concentrated solution
by chemical dissolution
Electrodialytic processes
Aqueous streams containing e.g. NaCl and
Na2S04 - chemical processing operations




Flue gas scrubbing
Metal pickling
Fermentation
Rayon manufacture
Splitting of sodium sulfate solutions into sodium hydroxide and sulfuric acid solutions
Applications

Large scale brackish and seawater desalination and salt production.

Small and medium scale drinking water production (e.g., towns & villages,
construction & military camps, hotels & hospitals)

Water reuse (e.g., industrial laundry wastewater, produced water from oil/gas
production, metals industry fluids)
Electrochemical remediation of soils
Restoration of contaminated soils
Anode -oxygen evolution
H20
1/2 02 + 2H+ + 2e-
Cathode- hydrogen evolution
H20 + 2e-
H2 + 20H-
Ions will move
–
due to migration, diffusion and convection
Heavy metal ions
-
move to the cathode
Organic compounds
-
by means of the electroosmotic flow
Electrochemical gas purification
I step : Absorption of the gaseous species in a liquid
II step: Electrochemical conversion of them to less
harmful products
 Reduction of chlorine to chloride
 Oxidation of nitrous oxides to nitric acid
 Sulfur dioxide to sulfuric acid
The reduction or oxidation - directly at the electrode or indirectly via a redox mediator
Chemically
Bromine as a mediator to oxidize SO2
SO2 + Br2 + 2H20
Electrochemically
Bromine is regenerated
2 HBr
H2S
H2 + 1/2 S2
Br2 + H2
H2S04 + 2HBr
Electrochemical power sources for cleaner electrical energy

Thermal combustion of fossil fuels in power plants and vehicles is
a major environmental problem in modern society

The immediate damage of air pollution has been estimated to cost
about three times more than the fossil fuels themselves

The most important gaseous impurities in the flue gas from
electricity generation plants are C02, NO, SO2 and dust particles

C02 is a major contributor to the greenhouse effect and NO,
contributes to the acidification of water and soil, eutrophication,
and the formation of smog
Global warming and climate change
Battery
 Road traffic alone generates more than 50 % of the total emissions
of nitrogen oxides, carbon monoxide and hydrocarbons.
 The only vehicles that are likely to meet “zero emission vehicles”
demands are electric vehicles.
 In order to meet these new regulations ‘The Big Three’, General
Motors, Ford and Chrysler in the USA decided in January 1991 to
form a consortium, The United States Advanced Battery Consortium
(US ABC), for cooperation towards improved power sources for
electric vehicles.
US ABC
Pb/PbO2
Specific energy/
W h kg-1
Peak specific power/
W kg-1
Discharge
cycles
80-100
150-200
600
200
400
1000
35-40
150-300
100-1000
lithium-metal sulfide, lithium-polymer,
lithium-ion batteries, metal hydride-nickel
oxide, zinc-air and zinc-nickel oxide
Representation of a lead acid battery
Charging of the battery
Discharging of the battery
Anode: Sponge metallic lead
Cathode: Lead dioxide (PbO2)
Electrolyte: aqueous sulfuric acid
Applications: Motive power in cars, trucks,
standby/backup systems
Battery vs Capacitors

Batteries - used in all-electric vehicles or in hybrid vehicles that use
also combustion engines for propulsion.

The batteries are then mainly used for acceleration and for citydriving,
while the combustion engine gives a reasonable range.

In this application, a high peak power density of the battery is a major
requirement, while the energy density, which determines the all-electric
range, is less important compared to all-electric vehicles.

An interesting alternative, or complement, to batteries is
elctrochemical capacitors (also called ultra capacitors or
supercapacitors), which can give peak power densities greater than 1
kW kg-l while the energy density is only 2-10 % of that stored in a
battery)
Applications




Regenerative backing in hybrid vehicles,
cold engine starting,
backup power systems and
digital electronic devices
Electrochemical capacitor

An electrochemical capacitor stores the electrical energy
electrostatically by charging of the electrochemical double
layer at the electrode/electrolyte interface

In some systems intermediates are adsorbed on the
electrode surface or intercalated into the electrode
material, which gives an additional so called pseudocapacitance that may be 10 to 100 times higher than the
double layer capacitance

In a finely porous electrode with a high specific surface
area, fairly high amounts of electrical energy may be
stored per unit volume or mass

Research and development has been going on since the early 1990s to develop
ultracapacitors using various types of carbon, doped conducting polymers, and metal
oxides as electrode materials

The electrolyte may be aqueous, organic or a solid polymer

Ultracapacitors with aqueous electrolyte can store 1.5 W h kg-1 and deliver 1 kW kg-1,
while the best values reported for devices using an organic electrolyte are 5-7 W h
kg-l and 2 kW kg-1
Fuel Cells
Direct Energy Conversion vs Indirect Technology
Thermal Energy
ICE
Mechanical Energy
Fuel Cell
Chemical Energy
Electrical Energy
Applications
Submarine with fuel cell propulsion - German Navy
Electrochemical sensors
 Monitoring of toxic compounds - Electrochemical sensors are convenient and
effective devices
 An electric signal can be related directly to the concentration of the compound
being measured
 Sensing pollutants - as potentiometric, amperometric or voltammetric sensors
Potentiometric Sensors
 These sensors measure the electrical potential of an electrode
when no current is flowing. The signal is measured as the potential
difference (voltage) between the working electrode and the
reference electrode
 The working electrode's potential must depend on the
concentration of the analyte in the gas or solution phase
 Ion-selective electrodes can be used to determine for example pH,
fluoride and cyanide concentrations in water
 The concentration of toxic gases such as sulfur and nitrogen oxides
can also be determined with potentiometric sensors
Amperometric sensors
 These sensors measure current at a fixed
potential
 The current is then proportional to the
concentration of the measured species
 The Clark electrode for measuring oxygen
concentration is the classic example
 Its general principle also works for toxic
gases like CO, NO, NO*, SO2 and H2S
 A sensor can be made selective by a
suitable choice of electrode potential and
electrode material
 An array of such selective sensors can be
built into one device for monitoring flue
gases and other gas streams containing
several toxic components
Photoelectrochemical methods
Photoanode
2h+ + H2O
2H+
+
½ O2
Cathode
2e- + 2H+
H2
 Recent advances in photoelectrochemistry have led to new, interesting possibilities,
both for treatment of pollutants and for conversion of solar energy from light to
electricity
 In the first case, suspensions of semiconductor particles can be used to harness the
light with production of electrons and holes in the solid, which can destroy pollutants
by means of reduction and oxidation, respectively
 In this way, air or water containing organic, inorganic or microbiological pollutants can
be effectively treated
 Photoelectrochemical cells for electricity production offer a sustainable way to
generate electricity, e.g. for charging batteries in electric vehicles
 With semiconductor electrodes using dye sensitized nanocrystalline Ti02 films an
efficiency of 12 % has been reported
 Compared to conventional photovoltaic cells, this type of photoelectro-chemical cell is
less expensive, since it uses inexpensive raw materials, is easily fabricated
In brief

A variety of selected electrochemical processes and devices for
environmental protection have been presented

Some, but not all of them, have also been tested at pilot scale and some have
reached commercialization

In some cases, it is only a matter of time, further development work (and
investment) being required

In other cases, chemical or biological processes are preferred because they
are competitive and do not require expertise in electrochemistry and
electrochemical engineering

It may be expected that the number of electrochemical processes for
treatment or prevention of pollution will increase in the future due to their
specific advantages in a number of applications

A major beneficial impact of electrochemistry on the environment would be
the future introduction of fuel cell or battery driven vehicles
Thank you
Delhi to observe Earth Hour every month
The Delhi government has proposed to hold an Earth Hour on the last working day of every
month, urging residents to switch off non essential lights to save power on the lines of the
recently held global campaign.
India is a signatory to the United Nations Educational, Scientific and
Cultural Organization's (UNESCO) World Heritage Convention adopted
in 1972. The main goal of the World Heritage Convention is to identify
and protect monuments of great cultural and natural heritage throughout
the world. In signing the Convention, a country pledges to conserve the
World Heritage sites located in its own territory and protect its national
heritage. The application for a site to be accepted as the World must
come from the country itself. The application process includes
submission of a plan detailing how the site is managed and the
measures assuring its continued protection. In some cases, UNESCO
identifies conditions to a country before accepting a site as a world
heritage monument.
For example, at the time Delphi was nominated by Greece, a plan was in
the works to build an aluminum plant nearby. The Greek government
was asked to find an alternative location for the plant, did so, and Delphi
was accepted onto the World Heritage List. In other cases, such as the
Giza Pyramids, UNESCO asks the country for remediation of potential
threats. In 1995, the Pyramids were threatened by a highway project
near Cairo which would have seriously damaged the monument.
Negotiations with the Egyptian government resulted in a number of
alternative solutions which replaced the disputed project. Ultimately, the
treaty is not binding by the force of an ultra-national body but rather left
to the discretion of the country. The Agra area currently has three world
heritage sites: the Taj Mahal, Agra Fort and Fatehpur Sikri.
The Issue: Environmental pollution spurred by industry and automobiles has long been
observed to be progressively destroying the Taj Mahal's white marble surface. Petitions
of Indian environmentalists have led to a series of court challenges in the Indian Supreme
Court and lower courts. The conflict has often pitted business and labor interests against
environmentalists and preservationists as well as India's need to protect its cultural
heritage versus its need to provide jobs for its citizens.
2. Description: Mark Twain once remarked the world is divided between two types of
people: those who have seen the Taj Mahal and those who have not. The Taj is one of the
most recognizable landmarks in the world and the image most associated with India. The
Mughal emperor Shah Jahan erected the Taj Mahal at Agra as a mausoleum in memory of
his beloved wife, Arjumarid Bano Begum; (popularly known as Mumtaz Mahal "favored of
the court"), who died in A.D. 1630. Begun in 1632 AD, it took 20,000 men working every
day over 22 years to complete. It is heralded by many as the greatest work of Mughal
architecture.
India has experienced exponential industrial growth in recent years. Increasingly, people
have left villages for urban centers in order to try and find work. The result of this
industrialization has often been overcrowded cities and dense pollution. Agra is no
exception. It has been identified as a "pollution intensive zone" by the World Health
Organization (WHO). It is estimated that the area around the Taj contains five times the
amount of suspended particles (such as sulfur dioxide) that the Taj Mahal could handle
without sustaining everlasting damage. India has been involved in a "greening" campaign
particularly in regards to its national monuments.
More recently, India has begun to try and attract more tourists: this has created a
dilemma how to market its best Tourist attraction without causing significant damage to
it in the process.