Using the Earth's resources and obtaining potable water

Chemistry · Using resources. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Humans use the Earth's resources for warmth, shelter, food, transport and materials. Some resources are finite and will run out. Sustainable development means meeting the needs of people today without stopping future generations from meeting theirs.

Potable water is water that is safe to drink. It is not pure: it still contains low levels of dissolved salts, but harmful microbes have been removed. Fresh water is filtered and sterilised. Sea water must be desalinated, which needs a lot of energy.

Waste water must be treated before it goes back into rivers. Higher Tier students also learn two newer ways to get copper from low-grade ores: phytomining and bioleaching.

Rules to remember

Fresh water to potable water: choose a suitable source, pass it through filter beds to remove solids, then sterilise it with chlorine, ozone or ultraviolet light.
Desalination: distillation, or reverse osmosis using membranes. Both need large amounts of energy, so they are expensive.
Sewage treatment: screening and grit removal, then sedimentation to give sewage sludge and effluent.
Sludge is digested by bacteria without air (anaerobic); effluent is treated by bacteria with air (aerobic).
Phytomining: plants take up metal compounds, are harvested and burned, and the ash contains the metal compounds.
Bioleaching: bacteria produce a solution called a leachate that contains metal compounds.
Copper is then obtained from these compounds by displacement with scrap iron or by electrolysis.

Common mistake

Students write that potable water is pure water. Pure water contains only water molecules; potable water still has dissolved substances but is safe to drink.

Worked example

A waste heap contains rock with too little copper to be worth mining in the usual way. A company treats it with bacteria and later adds scrap iron. Explain how copper is obtained.

The method is bioleaching: the bacteria produce a leachate, a solution that contains copper compounds.
Iron is more reactive than copper, so scrap iron displaces copper from the solution.
Scrap iron is cheap and would otherwise be waste, so this is a low-cost way to collect the copper.

Answer: Bacteria make a solution of copper compounds (leachate), and scrap iron displaces the copper metal from it.

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