INTRODUCTION:

Potable waters are varied widely in composition and corrosivity. Some are extremely aggressive, while others cause negligible attack, unfortunately the latter are a rather small minority. Surface supplies generally approach saturation with respect to dissolved oxygen. As a result, they are usually quite corrosive unless it is naturally inhibition by laying down a protective film or deposit by water. Generally deep well supplies are devoid of dissolved oxygen. As a result, practically noncorrosive, but, unfortunately, absorptive of oxygen is difficult to prevent particularly when subject to treatment like clarification, settling etc. A number of inhibitors available for treatment of potable water is drastically limited by portability considerations. Moreover, levels of the permissible inhibitors are considerably restricted by portability requirements. Economics considerations frequently lead to even more drastic limitation of treatment levels. Development of green corrosion inhibition formulations like Amino (tri methylene phosphonic Acid) [ATMP] Zn²⁺and citrate for potable water system has been the focus of attention for the past one decade. This paper reviews the current trends in green chemistry which are certain to play a vital role in sustainability of safe potable water.

Access to water supply as of 2002:¹

In 2002, 1.1 billion people lacked access to improved water sources, which represented 17% of global population. Moreover, the world’s population is expected to increase every year by 74.8 million people. The goal of achieving ‘safe potable water for all’ is a more difficult than Education for all’ in developing countries like India According to WHO statistics, “one-sixth of humanity currently lacks access to any form of improved water supply within 1k.m. of their homes”².

Root causes for the problem:

The root causes for the problem are industrial expansions using water intensive technologies and water polluting technologies, population explosive and disposal of industrial and sewage waste waters without appropriate treatment. WHO pointed out that 86% of urban waste water is discharged from Latin America and Carribean and also 65%of waste water in Asia discharged untreatably into rivers, lakes and oceans². For example, in India, 1:1 million liters of raw sewage is dumped into the Ganges River every minute².

According to WHO observations, 1g of faces (in untreated water) may contain 10 million viruses, 1 million bacteria, 1000parasite assists and 100 worm eggs². Due to drinking of unsafe water, 1.8 million people die every year from diarrohoeal diseases including (cholera). Most of these 1.8 million 90% are children under age 5, mostly in developing countries 88% of diarrohoeal disease is attributed to unsafe water supply. Every year, there are 1.5 millions of clinical hepatitis¹.

Industries like tannaries, chloralkali industry, battery industry dye industry drugs and pharmaceutical industry, electroplating industry and others are the sources of pollution of ground water sources. Fertilizers and pesticides used in agriculture are also of the great concern in the recent times in the countries like India.

Apart from the above, without any intervention of humanity, in many regions across the globe, several ground water sources contain higher levels of fluorides and arsenic naturally due to leaching of minerals. Between 28 and 35 million people consume drinking water with arsenic levels for higher than permissible limit in Bangladesh¹. The ground water contaminated with arsenic has been found in countries like Argentina, Bangladesh, Chile, China, Mexico, Thailand, Myanmar and USA¹. 26 million people in china and almost an equal number from India suffer from dental fluorosis. Over 1 million people suffer from skeletal fluorosis¹.

Specifications of safe potable drinking water:

As per Indian standard IS10500: 1991 first revision, the specifications for safe drinking water (3) is given in the table.

Drinking Water Specification as per Indian Standards (3)

S. No	Substance or Characteristic	Requirement (Desirable Limit)	Undesirable Effect Outside the Desirable Limit
Essential Characteristics:
1	Turbidity, NTU, Max	5	Above 5, consumer acceptance decreases
2	pH value	6.5 to 8.5	Beyond this range the water will affect the mucous membrane and/or supply system
3	Total Hardness (as CaCO3) mg/l, Max	300	Encrustation in water supply structure and adverse effects on domestic use
4	Iron (as Fe) mg/l, Max	0.3	Beyond this limit taste/appearance are affected, has adverse effect on domestic use and water supply structures, and promotes iron bacteria
5	Chlorides (as CI) mg/l, Max	250	Beyond this limit, taste, corrosion and palatability are affected
6	Residual, free Chlorine, mg/l, Max	0.2	Beyond this limit, taste, corrosion and palatability are affected

Desirable Characteristics:
7	Dissolved Solids mg/l, Max	500	Beyond this palatability decreases and may cause gastrointestinal irritation
8	Calcium (as Ca) mg/l, Max	75	Encrustation in water supply structure and adverse effects on domestic use
9	Copper (as Cu) mg/, Max	0.05	astringent taste, discoloration and corrosion of pipes, fitting and utensils will be caused beyond this
10	Manganese (as Mn) mg/l, Max	0.1	Beyond this limit taste/appearance are affected
11	Sulphate (as SO₄) mg/l, Max	200	Beyond this causes gastro-intestinal irritation when magnesium or sodium are present
12	Nitrate (as NO₃) mg/l, Max	45	Beyond this methaemoglobinemia takes place
13	Fluoride (as F) mg/l, Max	1.0	Fluoride may be kept low as possible. High fluoride may cause fluorosis
14	Phenolic compounds (as C₆H₅OH) mg/I, Max	0.001	Beyond this, it may cause objectionable taste and odour
15	Mercury (as Hg) mg/I, Max	0.001	Beyond this, the water becomes toxic
16	Cadmium (as Cd), mg/I, Max	0.01	Beyond this, the water becomes toxic
17	Selenium (as Se), mg/I, Max	0.01	Beyond this, the water becomes toxic
18	Arsenic (as As), mg/I, Max	0.05	Beyond this, the water becomes toxic
19	Cyanide (as CN), mg/I, Max	0.05	Beyond this limit, the water becomes toxic
20	Lead (as Pb), mg/I, Max	0.05	Beyond this limit, the water becomes toxic
21	Zinc (as Zn), mg/I, Max	5	Beyond this limit, it can cause astringent taste and an opalescence in water
22	Anionic detergents (as MBAS), mg/I, Max	0.2	Beyond this limit it can cause a light froth in water
23	Chromium (as (Cr₆⁺) mg/I, Max	0.05	may be carcinogenic above this limit
24	Polynuclear aromatic hydrocarbons (as PAH) g/I, Max	-	May be carcinogenic
25	Mineral oil, mg/I, Max	0.01	Beyond this limit undesirable taste and odour after chlorination take place
26	Pesticides mg/I, Max	Absent	Toxic
27	Radioactive materials: a) Alpha emitters Bq/I, Max b) Beta emitters pci/I, Max	- -
28	Alkalinity mg/I, Max	200	Beyond this limit taste becomes unpleasant
29	Aluminium (as AI), mg/I, Max	0.03	Cumulative effect is reported to cause dementia
30	Boron, mg/I, Max	1	-

Besides the above, WHO guidelines give the permissible limits for the organic compounds, herbicides, pesticides/insecticides are shown in table.

Permissible limits for organic compounds, herbicides, pesticides/insecticides as per WHO guidelines⁴

S. No	Name of the Compound	Permissible limit
Organic Compounds
1	Benzene	0.01mg/L
2	Carbon tetrachloride	0.004mg/L
3	1,2, dichlorobenzene	1mg/L
4	Formaldehyde	0.9mg/L
5	Hexachlorobenzene	1 ug/L
6	Styrene	0.02mg/L
7	Tetrachloroethene	0.04mg/L
8	Trichloroacetic acid	0.2mg/L
9	1,2,4, trichlorobenzene	0.02mg/L
10	Chloroform	0.2mg/L
11	Vinyl chloride	0.005mg/L
12	1,4 dichlorobenzene	0.3 mg/L
Herbicides
1	Atrazine	0.002mg/L
2	Bentazone	10.3mg/L
3	Cyanazine	0.0006mg/L
4	Metolachlor	0.01mg/L
5	Simazine	0.002mg/L
Pesticides/Insecticides
1	Carbofuran	0.007mg/L
2	Heptachlor	0.03ug/L
3	Methyl parathion	9ug/L

Sustainability of Safe Potable Water: Toward Green Chemistry and Green Technologies:

The synthetic chemical industry now produces billions of tons per year of upto 70,000 commercial substances⁵. Several synthetic chemicals are distributed now globally in the environment, food web, and human tissues posing great risk to humans and wild life. The environmental policies, which give the guidelines for quality of waste waters discharged from industry, are not successful at preventing large scale chemical pollution of water sources over a period of time. The approach has been to shift chemicals from one environmental medium to another, from one place to another, from one chemical form to another.

While continuing the effluent treatment technologies to convert harmful species into a harmless one, simultaneous development of ecologically sound technologies (green technologies) should be given special emphasis and thrust and the environmental policies should act as driving force for the practical implementation of green technologies and only green technologies. It is relevant to quote Joe Thornton⁶ in this context. “The current approach to environmental regulation is inherently adequate to address the scope and complexity of this problem. A fundamental shift in paradigm is required to protect human health and ecological systems from chemically induced damage. The concept of green chemistry represents a critical part of this shift, but it is not sufficient per se. Only if green chemistry is conceived as part of a new policy based on precautionary, democratically guided implementation of sustainable production technologies, will contribute significantly to establishing an ecologically compatible product base”.

Green Chemistry:

‘Green Chemistry is the design of chemical products and processes that reduce or eliminate the use and generation of hazardous substances, whereas environmental chemistry is the chemistry of natural environmental of pollutant chemicals in nature’⁸.

‘Green chemistry is an overarching philosophy of chemistry rather than a discipline of it. It applies to organic chemistry, inorganic chemistry, biochemistry, analytical chemistry, even physical chemistry.

In 2005, Ryoji Noyori identified three developments in green chemistry viz/, use of supercritical carbon dioxide as green solvent, aqueous hydrogen peroxide for clean oxidations and the use of hydrogen in asymmetric synthesis gather examples of green chemistry are supercritical water oxidation, on water reactions and solid state synthesis.

Supramolecular chemistry is a new area in which research is currently going on. Synthesis of new chemicals is done in solid state without need for any solvents. A typical example is that cycloaddition of trans 12 bis (4-pyridyl) ethylene is directed by resorcinol in the solid state in the presence of UV light to give 100% yield⁶.

Cycloaddition of trans-1,2-bis (4-pyridyl) ethylene

Paul Anastas of Environmental Protection Agency of USA and John C Warner developed 12 principles of green chemistry⁸ which are mentioned below.

Principles of Green Chemistry:

1. Prevent waste: Design chemical syntheses to prevent waste, leaving no waste to treat or clean up.

2. Design safer chemicals and products: Design chemical products fully effective, yet have little or no toxicity.

3. Design less hazardous chemical syntheses: Design syntheses to use and generate substances with little or no toxicity to humans and the environment.

4. Use renewable feedstock: Use raw materials and feedstock that are renewable rather than depleting. Renewable feedstock are often made from agricultural products or are the wastes of other processes; depleting feedstock are made from fossil fuels (petroleum, natural gas, or coal) are mined.

5. Use catalysts, not stoichiometric reagents: Minimize waste by using catalytic reactions. Catalysts are used in small amounts and can carry out a single reaction many times. They are preferable to stoichiometric reagents, which are used in excess and work only once.

6. Avoid chemical derivatives: Avoid using blocking or protecting groups or any temporary modifications if possible. Derivatives use additional reagents and generate waste.

7. Maximize atom economy: Design syntheses so that the final product contains the maximum proportion of the starting materials. There should be few, if any, wasted atoms.

8. Use safer solvents and reaction conditions: Avoid using solvents, separation agents, or other auxiliary chemicals. If these chemicals are necessary, use innocuous chemicals. If a solvent is necessary, water is usually the best medium.

9. Increase energy efficiency: Run chemical reactions at ambient temperature and pressure whenever possible.

10. Design chemicals and products to degrade after use: Design chemical products to break down to innocuous substances after use so that they do not accumulate in the environment.

11. Analyze in real time to prevent pollution: Include in – process real – time monitoring and control during syntheses to minimize or eliminate the formation of byproducts.

12. Minimize the potential for accidents: Design chemicals and their forms (solid, liquid, or gas) to minimize the potential for chemical accidents including explosions, fires, and releases to the environment.

Green Technologies: A conceptual understanding:

Any technology involves the use of process to develop a new product, which is ultimately brought into civilization. A green technology is ecologically sound. It does not generate hazardous substances. It does not pollute the environment. For example, a hydrogen-oxygen fuel cell technology provides us energy and the ultimate product of the overall electrochemical reaction is water. Therefore, it is an example of green technology in the field of energy.

Green technologies for water treatment:

It is well known that chromates and nitrites are excellent inhibitors for corrosion prevention/control of carbon steel, used as material of construction in heat exchangers. These chemicals are added in small concentrations of a few mg/L to protect carbon steel from corrosion and increase the life of heat exchangers. But, because of the toxicity of hexavalent chromium present in chromates and also of the toxicity of nitrite at very low concentrations, use of these chemicals as corrosion inhibitors in cooling water systems has been banned all over the world. Development of green corrosion inhibition formulations for protecting carbon steel, copper and other metals and alloys has been the focus of attention for the past one decade. results in this field of research are quite encouraging and are already put in practice in industry. Some examples of environmental friendly corrosion inhibitor formulations are worth mentioning. Synergistic mixtures containing hydroxy sthylidene phosphonate (HEDP) and Zn²⁺,⁹ HEDP, Zn²⁺ and ascorbate¹⁰, Nitrilotris methylene phosphonate, Zn²⁺ and ascorbate¹¹. ATMP and Zn²⁺⁽¹²⁾ are proved to be an excellent green inhibited for prevention of corrosion of carbon steel in cooling water system. In the temary systems, the concentrations of both Zn²⁺ and phosphonate are further reduced, while improving the efficiency with ascorbate or citrate, which are environmentally friendly. All the thermal power stations under the control of National Thermal Power Corporation have already switched over to the use of HEDP-Zn²⁺ combinations as inhibitors for cooling water systems. The current trend in the field of research on corrosion inhibitors for different industrial applications is essentially with a fcous on “green inhibitors”.

Other examples of green technologies

Are use of membrane cells in chloralkali industry replacing the mercury cells, using chromium (III) salt technology in leather tanning industry replacing the use of hexavalent chromium salt, use of hydrogen oxygen fuel cells in the energy sector, use of bio fuels, hydrogen as energy source and technologies based on solar energy in the energy sector, use of vegetable dyes and other non toxic or relatively less toxic dyes in dyes industry.

Solid state chemical synthesis and Zero waste technology are some of the priorities for the emerging green technologies. Recovery of heavy metals from industrial effluents, conversion of a toxic substance into a non toxic one before the discharge of industrial effluent will convert some of the existent technologies into green technologies. Even sewage treatment technologies to convert into non polluting waste water before discharge is the need of the hour. In order to meet the demand for safe drinking water by the growing population, development of green technologies for desalination f water will be a priority in the twenty first century.

CONCLUSION:

For the past two centuries several technologies have made a significant impact on human civilization. The significant impact has a positive dimension and also a negative dimension. The negative dimension is pollution of the environment which includes all the water resources. Water infrastructure is lifeline for community health and prosperity. As already listed in agenda of United Nations sustainability of safe potable water is the most important new millennium goal for the survival of human kind. There is no doubt that green technologies will offer one of the solutions for sustainability of safe potable water. There is a need for the legislation by all the governments to give a thrust on green technologies without looking much into the economic part of it. There is a need for bringing greater awareness and for sensitizing the scientists, technologists, entrepreneurs and the consumers towards promotion of green technologies. There is a need for greater synergism in the efforts of all those concerned to bring out the results of greater relevance to protect the drinking water. Principles of green chemistry and of green technologies must play an integral part of curriculum in chemistry, chemical engineering and technology, energy, metallurgy and materials engineering and all other allied disciplines of science and technology. Twenty first century is going to witness the emergence of green technologies, which have a vital role in sustainability of safe potable water.

REFERENCES:

1. Water, Sanitation and hygiene links to health, facts and figures updated November 2004. www. who.in/water_sanitation_health/publications/facts 2004/index.htm.

2. Chemical and Engineering News, 82, No.29, pp.25-30.

3. Drinking water specification, Indian standard IS 10500:1991 First Revision

4. Guidelines for Drinking water quality, WHO, third edition, incorporating first addendum, volume-1, Recommendations, 2003.

5. Environmental outlook for the chemicals industry, Organization for Economic Cooperation and Development, Paris, 2000.

6. Joe Thornton, Pure Applied Chemistry, 73, 2001, pp 1231-36.

7. Green Chemistry, Wikipedia, ww.wikeppedia.org

8. Ryoji Noyori, Chemical Communications, 14, 2005, pp 1807-11.

9. Anastas, P., and Warner, J.C., Green Chemistry: Theory and Practice, Oxford University Press, New York, 1998, p 30.

10. S. Rajendran, B.V.Appa Rao and N.P. Palanisamy, Anti-Corrosion Methods and Materials, 47, 2000, pp.83-87.

11. B.V.Appa Rao and S.Srinivasa Rao, Bulletin of Electrochemistry, 21, 2005, pp 139-144.

12. C.Thangavelu, P.Patric Raymond, S.Rajendran and M.Sundaravadivelu proc- 15^th National Congress on corrosion control, Chennai, 2010.

Received on 02.02.2011 Modified on 03.03.2011

Asian J. Research Chem. 4(7): July, 2011; Page 1033-1037