Environmental Chemistry (Sustainable Chemistry)

Green Chemistry

Green chemistry, also known as sustainable chemistry, is a philosophy of chemical product and process design that aims to reduce or eliminate the use and generation of hazardous substances. It is based on a set of guiding principles that promote environmentally benign chemical practices.

Introduction:

The primary goal of green chemistry is to develop chemical processes and products that are:

• Environmentally friendly: Minimizing pollution and waste.
• Economically viable: Cost-effective and efficient.
• Safer: Reducing risks to human health and the environment.

The Twelve Principles of Green Chemistry:

1. Prevention: It is better to prevent waste than to treat or clean up waste after it has been created.
2. Atom Economy: Synthetic methods should be designed to maximize the incorporation of all materials used in the process into the final product.
3. Less Hazardous Chemical Syntheses: Wherever practicable, synthetic methods should be designed to use and generate substances that possess little or no toxicity to human health and the environment.
4. Designing Safer Chemicals: Chemical products should be designed to effect their desired function while minimizing their toxicity.
5. Safer Solvents and Auxiliaries: The use of auxiliary substances (e.g., solvents, separation agents) should be made unnecessary wherever possible and innocuous when used.
6. Design for Energy Efficiency: Energy requirements for 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.
7. Use of Renewable Feedstocks: A raw material or feedstock should be renewable rather than depleting whenever technically and economically practicable.
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.
9. Catalysis: Catalytic reagents (as selective as possible) are superior to stoichiometric reagents.
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.
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.
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.

Green Chemistry In Day-To-Day Life:

Green chemistry principles are increasingly being applied in everyday products and processes:

• Biodegradable Plastics: Developing plastics from renewable resources (like starch, PLA) that break down in the environment, reducing plastic waste.
• Water-Based Paints: Replacing solvent-based paints with water-based paints significantly reduces the emission of volatile organic compounds (VOCs), which are harmful air pollutants.
• Environmentally Friendly Cleaning Products: Using biodegradable soaps and detergents, and avoiding harsh chemicals.
• Safer Solvents: Replacing toxic solvents like benzene and carbon tetrachloride with safer alternatives like supercritical $CO_2$, ethanol, or water in various industrial and laboratory processes.
• Energy Efficiency: Designing processes that require less energy, such as using catalysts that operate at lower temperatures and pressures.
• Renewable Energy Sources: Using solar, wind, and hydroelectric power reduces reliance on fossil fuels, thereby lowering greenhouse gas emissions.
• Green Catalysts: Developing more efficient and selective catalysts reduces waste and energy consumption.
• Sustainable Agriculture: Promoting organic farming practices that minimize the use of synthetic pesticides and fertilizers.

Prevention Of Corrosion (from Metals And Non-metals)

Prevention Of Corrosion:

Corrosion is the degradation of metals due to electrochemical reactions with their environment. Preventing corrosion is crucial for maintaining the structural integrity and lifespan of metallic materials.

• Barrier Protection: Coating the metal surface with a protective layer that physically separates it from the corrosive environment.
  • Painting: Applying layers of paint or varnish creates a barrier. The paint may also contain anti-corrosive pigments.
  • Oiling/Greasing: Applying oil or grease to metal surfaces, commonly used for machinery parts, prevents contact with moisture and oxygen.
  • Electroplating: Depositing a thin layer of a more corrosion-resistant metal (e.g., chromium, nickel, tin) onto the surface of the metal to be protected using electrolysis. Tin plating on iron is used for food cans because tin is less reactive than iron and forms a protective oxide layer.
  • Galvanization: Coating iron or steel with a layer of zinc. Zinc is more electropositive than iron, so it corrodes preferentially, protecting the iron (sacrificial protection). Even if the zinc layer is scratched, it continues to protect the iron electrochemically.
  • Anodizing: Primarily used for aluminium. It involves making the metal part the anode in an electrolytic cell, where a thick, protective oxide layer (e.g., $Al_2O_3$) is formed on the surface. This layer can be dyed for decorative purposes.
• Sacrificial Protection: This method involves connecting the metal to be protected with a more reactive (more electropositive) metal. The more reactive metal acts as the anode and corrodes sacrificially, protecting the less reactive metal (which acts as the cathode).
• Galvanization of Iron: As mentioned above, zinc corrodes instead of iron.
• Magnesium or Zinc Anodes: Used to protect underground pipelines, ship hulls, and offshore structures. These are periodically replaced as they corrode.
• Alloying: Forming alloys with metals that are inherently more resistant to corrosion.
• Stainless Steel: An alloy of iron, chromium (typically >10.5%), nickel, and sometimes molybdenum. Chromium forms a thin, transparent, and tough passive oxide layer ($Cr_2O_3$) on the surface, which protects the underlying metal.
• Brass and Bronze: Alloys of copper, which are generally more corrosion-resistant than pure copper in certain environments.
• Cathodic Protection: This is a technique used to prevent corrosion of a metal surface by making it the cathode of an electrochemical cell. This can be achieved in two ways:
  • Using a More Reactive Metal (Sacrificial Anode): As described above.
  • Using an Impressed Current: Connecting the metal structure to be protected to the negative terminal of a DC power supply, while the positive terminal is connected to an inert anode (e.g., graphite, platinum) buried in the electrolyte (e.g., soil, seawater). This forces electrons onto the metal surface, preventing oxidation.