National Centre for Catalysis Research ELECTROCHEMISTRY FOR A CLEANER ENVIRONMENT M. HELEN Research Scholar.

National Centre for Catalysis Research

ELECTROCHEMISTRY FOR A CLEANER ENVIRONMENT

M. HELENResearch Scholar

Pollution is the action of environmental contamination with

man-made waste

Environment and Pollution

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

Possibilities offered by Electrochemistry

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

F2 + 2e- ---> 2F- +2.87

Co3+ + e- ---> Co2+ +1.80

Cl2 + 2e- ---> 2Cl- +1.36

O2 + 4H+ + 4e- ---> 2H2O +1.23

NO3- + 4H+ + 3e- ---> NO + 2H2O +0.96

Ag+ + e- ---> Ag +0.80

Fe3+ + e- ---> Fe2+ +0.77

I2 + 2e- ---> 2I- +0.54

Cu+ + e- ---> Cu +0.52

Cu2+ + 2e- ---> Cu +0.34

Cu2+ + e- ---> Cu+ +0.15

2H+ + 2e- ---> H2 0.00

Fe3+ + 3e- ---> Fe -0.04

Sn2+ + 2e- ---> Sn -0.14

Ni2+ + 2e- ---> Ni -0.25

Co2+ + 2e- ---> Co -0.29

Fe2+ + 2e- ---> Fe -0.41

Zn2+ + 2e- ---> Zn -0.76

2H2O + 2e- ---> H2(g) + 2OH- -0.83

V2+ + 2e- ---> V -1.18

Mn2+ + 2e- ---> Mn -1.18

Al3+ + 3e- ---> Al -1.66

Reaction Eo

Na+ + e- = Na -2.71

Li+ + e- = Li -3.04

Standard Reduction Potentials (in Volts), 25oCS

tron

g ox

idiz

ing

agen

ts

Strong reducing

agents

Electrochemical processes for waste treatment

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.

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

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

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 - 50-90 ºCCurrent density - 500 A m-2

High concentration of cyanide (> 1 M)

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

Chlorine - irritant to the eyes, skin, the upper respiratory tract, and the lungs

- solvents and disinfectants, bleaching in the pulp and paper industry (chlorinated phenols)

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

Dechlorination

First step

chlorine is substituted by hydroxyl radicals formed from water

Subsequent steps

further oxidation yields quinone, which decomposes into maleicacid, oxalic acid (primarily) and carbon dioxide.

Oxygen and significant amounts of ozone were formed as by-products at the anode

Riskchloride ions formed may be oxidized to hypochlorite, which can then form chlorinated organic compounds

p-chlorophenol and pentachlorophenol - anodically on lead dioxide

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

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 processMetal can be recovered in its pure metallic form

Removal of heavy metal ions

Cu, Hg, Zn, Cr and Cd

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

Splitting of sodium sulfate solutions into sodium hydroxide and sulfuric acid solutions

Aqueous streams containing e.g. NaCl and Na2S04 - chemical processing operations

Flue gas scrubbing Metal pickling Fermentation Rayon manufacture

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)

Applications

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

I step : Absorption of the gaseous species in a liquid

II step: Electrochemical conversion of them to less harmful products

Chemically

Bromine as a mediator to oxidize SO2

SO2 + Br2 + 2H20 H2S04 + 2HBr

Electrochemically

Bromine is regenerated

2 HBr Br2 + H2

H2S H2 + 1/2 S2

Electrochemical gas purification

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

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

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.

Specific energy/ W h kg-1

Peak specific power/W kg-1

Discharge cycles

US ABC 80-100 150-200 600

200 400 1000

Pb/PbO2 35-40 150-300 100-1000lithium-metal sulfide, lithium-polymer, lithium-ion batteries, metal hydride-nickel oxide, zinc-air and zinc-nickel oxide

Battery

Representation of a lead acid battery

Charging of the battery Discharging of the battery

Anode: Sponge metallic leadCathode: Lead dioxide (PbO2)Electrolyte: aqueous sulfuric acidApplications: Motive power in cars, trucks,

standby/backup systems

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)

Battery vs Capacitors

Regenerative backing in hybrid vehicles,

cold engine starting, backup power systems and digital electronic devices

Applications

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 pseudo-capacitance 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

Electrochemical capacitor

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

Fuel Cell

ICEThermal Energy Mechanical Energy

Chemical Energy Electrical Energy

Applications

Submarine with fuel cell propulsion - German Navy

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

Electrochemical 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

Potentiometric 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

Amperometric sensors

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

Photoelectrochemical methodsPhotoanode

2h+ + H2O 2H+ + ½ O2

Cathode

2e- + 2H+ H2

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

In brief

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.