engineering chemistry dr. payal...

Engineering Chemistry Dr. Payal Joshi

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Environmental Aspects of Chemistry Green Chemistry Introduction: ‘Green Chemistry’ is the universally accepted term to describe the movement towards more environmentally acceptable chemical processes and products. It encompasses education, research, and commercial application across the entire supply chain for chemicals. The term ‘Green Chemistry’ was coined in 1991 by P.A.Anastas in a special program launched by US EPA (US Environmental Protection Agency). Definition of Green Chemistry: ‘The design of chemical products and processes that are environmentally benign and reduce negative impact to human health and environment.’ Green chemistry incorporates a new approach to the synthesis, processing and application of chemical substances in such a manner as to reduce threats to health and the environment. This new approach is also known as: Environmentally benign chemistry or Clean chemistry or Benign-by-design chemistry. Green chemistry is sustainable chemistry. There are several important respects in which green chemistry is sustainable: • Economic: Green chemistry normally costs less in strictly economic terms (to say nothing of environmental costs). • Materials: By efficiently using materials, maximum recycling, and minimum use of raw materials, green chemistry is sustainable with respect to materials. • Waste: By reducing as far as possible, or even totally eliminating their production, green chemistry is sustainable with respect to wastes. Green chemistry is commonly presented as a set of 12 principles proposed by Anastas and Warner. The principles comprise instructions for professional chemists to implement new chemical compounds, new syntheses and new technological processes. The first principle describes the basic idea of green chemistry — protecting the environment from pollution. The remaining principles are focused on such issues as atom economy, toxicity, solvent and other media using consumption of energy, application of raw materials from renewable sources and degradation of chemical products to simple, nontoxic substances that are friendly for the environment.

12 Principles of Green Chemistry

1. Prevention It is better to prevent waste than to treat or clean up waste after it has been created. Explanation: Hundreds of tonnes of hazardous waste are released in air, water and land by industry every hour of the day. Chemical industry is the biggest source of such waste. Today, stringent legislation has increased testing methods to classify many chemicals as hazardous and toxic. For a chemical industry, waste management is costly and time consuming. Hence, it’s the old saying, ’prevention is better than cure.’ Legislation increasingly forces industry and users of chemicals to change – both through substitution of hazardous substances in their processes or products and through the reduction in the volume and hazards of their waste.

i) Green Chemistry: Principles of green chemistry with examples, (Numerical Problems based on Atom economy).

ii) E-Waste: Definition, classification and management of e-waste.


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2. Atom Economy Synthetic methods should be designed to maximize the incorporation of all materials used in the process into the final product. Explanation: In most of the organic reactions, along with the desired products, by- products are also formed which have no utility. Any by-product of a chemical reaction for which there is no profitable use is a ‘waste’. Green chemistry requires that new processes should be devised such that maximum starting material is present in the product. *This point is explained in the latter part of the chapter. 3. Less Hazardous Chemical Synthesis Wherever practicable, synthetic methods should be designed to use and generate substances that possess little or no toxicity to human health and the environment. Explanation: Starting material should be selected such that it should be least toxic. Compounds like benzene, pyridine, beta-naphthyl amines are carcinogenic which can result in intermediate, final products and by-products of toxic nature. Hence, synthesis starting with them should be avoided. Alternative route of synthesis allows Green Chemistry approach of ‘benign by design’ to be applied at the design stage, help assure the sustainability of new products. The ‘ideal synthesis’ can be shown as below,

Features of an ‘Ideal synthesis’

4. Designing Safer Chemicals Chemical products should be designed to effect their desired function while minimizing toxicity. Explanation: After the drug is synthesized, clinical trials are carried out. If it is found toxic or having serious side effects, it is immediately discarded. Drug molecules are then modified keeping the basic chemical structure same so as to maintain its desired function, whilst reducing its toxicity, eg, Thalidomide: This drug was used for treating morning sickness in pregnant women. It was found that women who were administered thalidomide drug had babies with teratogenic effects. 5. Safer Solvents and Auxiliaries The use of auxiliary substances (e.g. solvents, separation agents, etc.) should be made unnecessary wherever possible and innocuous when used.


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Explanation: Solvents end up as more waste than that generated during the reaction. Toxic solvents used to dissolve reactants should be avoided wherever possible. Ether, acetone, benzene are highly inflammable as they cause fire hazards. Solvents like, CCl4, CHCl3, benzene and pyridine carry health risks. Using safer solvents like water, supercritical CO2 etc., wherever possible is the recommended greener approach. eg., Green solvents like supercritical fluids such as, SF-carbon dioxide have efficiently replaced toxic chemical reagents. 6. Design for Energy Efficiency Energy requirements of chemical processes should be recognized for their environmental and economic impacts and should be minimized. If possible, synthetic methods should be conducted at ambient temperature and pressure. Explanation: Catalysts accelerate the rate of reaction and hence they can be carried out at lower temperatures and pressures. This results in reduction of the energy requirements. Use of fossil fuels needs to be avoided as they cause pollution. Petroleum fuels results in air pollution. Use of microwaves and ultrasound requires lesser energy for the overall chemical processes. This is due to the fact that such reactions involve proper and uniform heat transfer and minimal wastage of energy. eg., Microwaves and sonication are newer energy-efficient preparation techniques that are used to synthesize various products of industrial importance. 7. Use of Renewable Feedstocks A raw material or feedstock should be renewable rather than depleting whenever technically and economically practicable. Explanation: Feedstocks are the main ingredients that go into the production of chemical products. Green chemistry tries, when possible, to utilize benign, renewable feedstocks as raw materials. From the point view of green chemistry, combustion of fuels obtained from renewable feedstocks is more preferable than combustion of fossil fuels from depleting finite sources. For example, many vehicles around the world are fueled with diesel oil, and the production of biodiesel oil is a promising possibility. As the name indicates, biodiesel oil is produced from cultivated plants oil, e.g. from soya beans. eg., Synthesis of Adipic Acid

Traditional/Conventional Route


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Alternative Feedstock route/Greener Pathway

Large amounts of adipic acid [HOOC(CH2)4COOH] are used each year for the production of nylon, polyurethanes, lubricants and plasticizers. In step 1 of conventional method, benzene — a compound with convinced carcinogenic properties — is a standard substrate for the production of this acid. In step 2, the oxidation of cyclohexane with air may lead to an uncontrolled reaction. It has the risk of explosion. Not all of the cobalt catalysts can be recovered. This may lead to the disposal of a heavy metal to the environment. In step 3, nitrous oxide (N2O) gas is produced as a by-product which is a greenhouse gas. In green pathway, starting material, glucose, is harmless. Glucose can be converted into adipic acid by E. coli that is used to catalyze two steps of the reaction. This reduces the use of chemical reagents with significant toxicity. 8. Reduce Derivatives Unnecessary derivatization (use of blocking groups, protection/deprotection, temporary modification of physical/chemical processes) should be minimized or avoided if possible, because such steps require additional reagents and can generate waste. *Note: No explanation required for principle 8 9. Catalysis Catalytic reagents (as selective as possible) are superior to stoichiometric reagents. Explanation: Catalysis is truly a well-established technology. In petroleum refineries, catalysts are fundamental to the success of many processes. Catalysts are required in very small amounts. Moreover, they can be recovered as they are not consumed, making them reusable. Stoichiometric reagents are required in large amounts resulting in unwanted by products, thereby reducing the atom economy. Catalytic reactions are faster and require lesser energy than regular synthesis without catalyst. 10. Design for Degradation Chemical products should be designed so that at the end of their function they break down into innocuous degradation products and do not persist in the environment. Explanation: Materials that are produced must be benign (harmless) or readily biodegradable (easily digestible by the microorganisms to produce simpler and safer products). If they are not easily degraded, they accumulate resulting in environmental issues. eg.,Hydrogen peroxide is used as a sterilant to degrade biomedical syringes, plastics, etc.


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11. Real-time analysis for Pollution Prevention Analytical methodologies need to be further developed to allow for real-time, in-process monitoring and control prior to the formation of hazardous substances. Explanation: Processes should be continuously monitored and all the products formed should be analyzed. If any toxic material is detected then the conditions should be modified to prevent its formation. eg., Ethylene glycol formation needs constant check, since at higher temperatures; a toxic substance such as dioxin is produced. 12. Inherently Safer Chemistry for Accident Prevention Substances and the form of a substance used in a chemical process should be chosen to minimize the potential for chemical accidents, including releases, explosions, and fires. Explanation: A major concern with respect to flammable, reactive, and explosive substances is their widespread industrial use. Actually, such materials are relatively safe inside of manufacturing plants and properly secured storage areas. The greater threat comes from their transport. This is illustrated by very frequent transportation accidents involving rail cars, trucks, barges, and pipelines that result in explosions, fires, and release of corrosive materials. Failure of protective measures can result in a bad accident or serious harm to worker health. In manufacture of explosives such as, TNT (trinitroglycerol), extreme precautions should be taken. Example: Bhopal Gas tragedy Atom Economy: Yield is universally accepted metric for measuring efficiency of a chemical synthesis. It provides a simple and understandable way to measure success of a synthetic route. Green Chemistry teaches us that yield is not enough. It fails to allow for reagents that have been consumed, solvents and catalysts that will not be fully recovered, and, most importantly, the often laborious and invariably resource- and energy-consuming separation stages such as, water quenches, solvent separations, and distillations. Efficiency of a reaction is most commonly measured in terms of percentage yield of the reaction. Theoretical yield is initially calculated based on the amount of reagent used in the reaction. Percentage yield is the actual yield calculated after the synthesis. Consider the following reaction,

(1g) = 1/1 x 92/78 x1.0 = 1.18 g. If the actual yield of the reaction was found to be 1.16g then, Percentage Yield = 1.16/ 1.18 x 100 = 98.3 % As per green chemistry, a higher yield is not sufficient. We are looking at actual mass of reactants incorporated in products. Atom economy is the measure of how efficiently the atoms of the reactants in any reaction are incorporated into the desired product.


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% AE = FW of C7H8/FW of C6H6+ CH3Cl = 92/78+50.5 x 100 = 71.8% Atom economy shows the actual number of atoms incorporated into products. Around 20% is waste that needs treatment. b.

% AE = [C4H2O3/C6H6 + 4.5 x (16x2)] x 100 % AE = [98/78+ 144] x 100 = 44.1% c. CH3-CH=CH2 + Cl2 Æ Cl-CH2-CH=CH2 + HCl

Thus, % AE = [C3H5Cl/C3H6 + Cl2] x 100 = [76.5/42+71] x 100 = 67.7% d. C2H6 + Br2 Æ C2H5Br + HBr (At.wt of Br = 80) % AE = [FW of main products/FW of all reactants]x100 = [C2H5Br]/[C2H6] + [Br] = (109/ 30 + 160) x 100 = 109/190 x 100 = 57.3% e. C4H8 + 3O2 → C4H2O3 + 3H2O Maleic anhydride is the main product. %AE = [C4H2O3]/[ C4H8+3O2] = (98/56 + 96) x 100 = 64.47%

+ 4.5 O2

V2O5O

O

O

+ 2CO2 + 2H2O


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E-Waste Management Introduction: Electronic waste, e-waste is a category of surplus, obsolete, broken, or discarded end-of-the-lifespan electrical or electronic devices. It includes all secondary computers, entertainment device electronics, mobile phones, and other items such as television sets and refrigerators, whether sold, donated, or discarded by their original owners destined for reuse, resale, recycling, or disposal. The processing of electronic waste in developing countries causes serious health and pollution problems due to the fact that electronic equipment contains some very serious contaminants such as lead, beryllium, cadmium, and brominated flame retardants. Even in developed countries recycling and disposal of e-waste involves significant risk. Most of the electronic gadgets are ‘designed for the dump.’ Today, electronic equipments after end-of-the life are harder to upgrade, easy to break and impractical to repair. E-waste has resulted in global toxic emergency. Example: Silicon Valley is the most poisoned communities in the US. IBM report stated that their workers making Si chips had 40% chances of miscarriages and serious forms of cancer. Electronic equipments/ products which have become obsolete due to: a. Latest advancement in existing technologies. Like mobiles phones replaced pagers within a

year or two. b. Changes in design, style and status. For example, advanced versions of cell phones are

regularly & rapidly replacing existing handsets. c. Nearing the end of their useful life.

Classification/ Sources of e-waste: E-waste encompasses ever growing range of obsolete electronic devices such as: a. Household Appliances: Washing machines, Dryers, Refrigerators, Air-conditioners,

Vacuum cleaners, Coffee Machines, Irons, Toasters, etc. b. Office, Information & Communication Equipment: PCs, Laptops, Mobiles, Telephones,

Fax Machines, Copiers, Printers etc. c. Entertainment & Consumer Electronics: Televisions, VCR/DVD/CD players, Hi-Fi sets,

Radio, DVDs, CDs, floppies, tapes, etc. d. Lighting Equipment: Fluorescent tubes, sodium lamps etc. (Except: Bulbs, Halogen Bulbs). e. Electric and Electronic Tools: Drills, Electric saws, Sewing Machines, Lawn Mowers etc. f. Toys, Leisure, Sports and Recreational Equipment: Electric train sets, coin slot machines,

treadmills etc.

Table.1 enlists the various hazardous substances found in e-waste and their health effects.

Substances in e-waste

Source Hazard

Arsenic LEDs Chronic exposure can lead to skin diseases, lung cancer

Barium Cathode ray tubes (CRTs) Short-term exposure can lead to brain swelling, muscle weakness, heart damage

Cadmium Ni-Cd batteries, printer inks Lung cancer, kidney damage, bone disorder (osteoporosis)


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Lead CRT screens, batteries, printer boards

Appetite loss, constipation, fatigue, persistant headaches

Lithium Lithium-ion batteries Mercury Alkaline batteries Brain and liver damage, if

ingested or inhaled Chromium VI CDs, data tapes DNA damage, permanent eye

injury Tetrabromo bisphenol A (TBBA)

Fire retardants in plastics Severe hormonal disorder

PVC Cable insulations Skin disorders Americium (radioactive)

Medical devices, smoke detectors Poisoning, acute respiratory distress leading to death

Dioxins Fire retardants in plastics Malformations of the foetus, decreased reproduction and growth rates and cause impairment of the immune system

CFCs Refrigerants, Cooling unit, Insulation foams

Skin cancer, DNA damage

Beryllium Power supply boxes containing silicon controlled rectifiers and x-ray lenses

Classified human carcinogen-Chronic Beryllium disease affecting the lungs

Selenium Photoelectric cells, photocopiers, fax machines

Selenosis-neurological abnormalities

Hazardous E-Waste Disposal Techniques: Incineration, open fire burning and landfills are employed as processing techniques at large as well as small scale. But these techniques pose problem of environmental pollution in surrounding area. 1. Incineration: Incineration is the process of destroying waste through burning. Incineration is associated with a major risk of generating and dispersing contaminants and toxic substances in environment because of the variety of substances found in e-waste. This is especially true for incineration without prior treatment eg., flue gas purification, Cu, present in PCBs and cables, acts as catalyst for the generation of extremely toxic polybrominated dioxins (PBDDs) and furans (PBDFs) when brominated flame-retardants are incinerated at low temperature (600-800°C). PVC, found in significant amounts in e-waste, is highly corrosive when burnt and also induces the formation of dioxins. Incineration also leads to the loss valuable of trace elements which could have been recovered if they had been sorted and processed separately. 2. Open Burning: Open fires burn at relatively low temperatures release many more pollutants than in a controlled incineration. Inhalation of open fire emissions can trigger asthma attacks, respiratory infections, and cause other problems such as coughing, wheezing, chest pain, and eye irritation. Chronic exposure may lead to diseases such as emphysema and cancer. Burning PVC releases hydrogen chloride (HCl) which on inhalation mixes with water in the lungs to form hydrochloric acid. This acid can lead to corrosion of the lung tissues, and several respiratory complications. Often open fires burn with a lack of oxygen, forming carbon monoxide, which


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poisons the blood when inhaled and extended exposure can be fatal. The residual particulate matter in the form of ash is prone to fly around in the vicinity and can also be dangerous when inhaled. 3. Land-filling: One of the most widely used methods of waste disposal. When e-waste is dumped and land-filled, it results in leaching of toxic metals in the soli resulting in soil and ground water pollution. eg, Mercury leaches, when circuit breakers are destroyed. Lead leach from broken lead-containing glass, cathode ray tubes from TVs and monitors. Besides leaching, vaporization is also of concern in landfills. For example, volatile compounds such as, mercury can be released to atmosphere due to vaporization. Landfills are prone to uncontrolled fires which can release toxic fumes. Management of E-Waste (Eco-friendly method): It is estimated that 75% of electronic items are stored due to uncertainty of how to manage it. These electronic junks lie unattended in houses, offices, warehouses etc. and normally mixed with household wastes, which are finally disposed at landfills. This necessitates implementable management measures and state-of-the-art recycling techniques as follows viz; 1. Detoxication: a. Electronic waste processing usually first involves dismantling the equipment into various

parts (metal frames, power supplies, circuit boards, plastics). b. In this process critical components are removed from the e-waste in order to avoid dilution

and / or contamination of these materials with toxic substances during the downstream processes.

c. Critical components include, e.g., Lead glass from CRT screens, CFC gases from refrigerators, light bulbs and batteries.

Advantages of this process are the human's ability to recognize and save working and repairable parts, including chips, transistors, RAM, etc. Disadvantage of the process is that the labor is cheapest in countries with the lowest health and safety standards. 2. Shredding: Mechanical processing is the next step in e-waste treatment. a. It is normally an industrial large scale operation with sophisticated mechanical separator,

with screening and granulating machines to separate constituent metal and plastic fractions, which are sold to smelters or plastics recyclers.

b. It is done to obtain concentrates of recyclable materials and also to further separate hazardous materials.

c. Typical components of a mechanical processing plant are: i) Crushing units; ii) Shredders. d. Magnets and eddy currents are employed to separate glass, plastic, and ferrous and

nonferrous metals, which can then be further separated at a smelter. e. Hazardous smoke and gases are captured, contained, and treated to mitigate environmental

threat. 3. Refining: Refining of resources in e-waste is possible with technologies to get back raw material with minimal environmental impact. Most of the fractions are refined or conditioned in order to be sold as secondary raw materials or to be disposed of in a final disposal site.


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eg, Leaded glass from CRTs is reused in car batteries, ammunition, and lead wheel weights, or sold to foundries as a fluxing agent in processing raw lead ore. Copper, gold, palladium, silver, and tin are valuable metals sold to smelters for recycling. An ideal electronic waste recycling plant combines dismantling for component recovery with increased cost-effective processing of bulk electronic waste. Conclusion: A growing trend in electronic waste management is reuse. Reuse is preferable to recycling because it extends the lifespan of a device. Devices still need eventual recycling, but by allowing others to purchase used electronics, recycling can be postponed and value can be gained from device use. Making electronic company manufacturers of dealing with e-waste is called ‘extended producer responsibility’ or ‘product take back.’ There is a need for turning the company strategy from ‘designed for the dump’ to ‘designed to last.’

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