How Fertiliser From Farms Can Turn a Lake Green and Lifeless

Pesticides sprayed on fields, fertiliser washed off by rain and untreated sewage all end up in rivers, lakes, groundwater and soil. Some pollutants cause disease directly, while others quietly strip water of oxygen or build up along food chains. Green chemistry tries to stop pollution before it starts.

This lesson covers the major water and soil pollutants and their sources, and the principles of green chemistry.

Water is polluted by pathogens, organic waste, plant nutrients, heavy metals and industrial chemicals from sewage, farms and factories, while soil is polluted mainly by pesticides, excess fertilisers and industrial waste.

Water pollutants and their sources:

- Pathogens — bacteria and other microbes from sewage and animal waste, causing diseases such as cholera and typhoid
- Organic waste — sewage and waste from food-processing and paper mills; bacteria decomposing it use up dissolved oxygen
- Plant nutrients — nitrates and phosphates from fertilisers and detergents
- Heavy metals — lead, mercury and cadmium from industry and mining, which are toxic and accumulate in living things
- Industrial chemicals — acids, oils, dyes and polychlorinated biphenyls from factories

Biochemical oxygen demand. BOD is the amount of oxygen bacteria need to break down the organic matter in a given volume of water. Clean water has a low BOD, while heavily polluted water has a high BOD.

Eutrophication:

- Excess nitrates and phosphates make algae grow rapidly into a thick green layer on the surface
- The layer blocks light, and when the algae die, bacteria decomposing them use up the dissolved oxygen
- Fish and other aquatic animals die for lack of oxygen

Chemicals in drinking water. Excess fluoride mottles teeth and damages bones, excess nitrate can cause blue baby syndrome, and lead harms the kidneys, liver and nervous system.

Soil pollutants:

- Pesticides — insecticides such as DDT, herbicides and fungicides; many are non-biodegradable and concentrate up food chains, a process called biomagnification
- Fertilisers — excess use alters soil chemistry and washes into water bodies
- Industrial waste — fly ash, heavy metals and plastics dumped on land

An everyday example. Lakes in several Indian cities turn bright green in summer as fertiliser runoff and sewage feed the algae, and some even froth with detergent foam.

The substance. A biodegradable pollutant can still be harmful — sewage breaks down naturally, but its decomposition consumes the oxygen that fish need.

Green chemistry is the design of chemical products and processes that reduce or eliminate hazardous substances, so that pollution is prevented at its source instead of being cleaned up afterwards.

Key principles:

- Prevention — it is better to avoid producing waste than to treat it later
- Atom economy — design reactions so that as many reactant atoms as possible end up in the product
- Safer chemicals and solvents — use less toxic substances, and replace harmful organic solvents with water, supercritical carbon dioxide or no solvent at all
- Energy efficiency — run reactions at room temperature and pressure where possible
- Renewable feedstocks — use raw materials from plants rather than petroleum
- Catalysts — use selective catalysts instead of large amounts of reagents
- Degradable products — design chemicals that break down harmlessly after use

Worked example — atom economy. Atom economy compares the mass of the desired product with the total mass of the reactants:

$$\text{Atom economy}=\frac{\text{molar mass of desired product}}{\text{total molar mass of reactants}}\times 100$$

In the addition ${\mathrm{C}}_2{\mathrm{H}}_4+{\mathrm{H}}_2\to {\mathrm{C}}_2{\mathrm{H}}_6$, every atom ends up in ethane:

$$\text{Atom economy}=\frac{30}{28+2}\times 100=100\text{ per cent}$$

In the substitution $\mathrm{C}{\mathrm{H}}_4+\mathrm{C}{\mathrm{l}}_2\to \mathrm{C}{\mathrm{H}}_3\mathrm{Cl}+\mathrm{HCl}$, hydrogen chloride is a by-product:

$$\text{Atom economy}=\frac{50.5}{16+71}\times 100=58\text{ per cent}$$

Green chemistry in practice:

- Dry cleaning with liquid carbon dioxide instead of tetrachloroethene, a harmful chlorinated solvent
- Bleaching paper with hydrogen peroxide and a catalyst instead of chlorine
- Making ethanal from ethene by catalytic oxidation in water, giving a high yield with little waste

An everyday example. Farmers who switch to neem-based pesticides follow the green chemistry idea of using safer, degradable chemicals from a renewable plant source.

The substance. Green chemistry is about design, not clean-up — a treatment plant that removes a pollutant helps, but a process that never makes the pollutant is greener.