Global challenges Study Guide

Traditional methods for extracting metals from their ores, such as smelting, are energy-intensive and can cause significant environmental pollution, releasing sulfur dioxide and greenhouse gases. To mitigate these issues, biological methods like bioleaching and phytoextraction have been developed. These methods offer less environmentally damaging alternatives, particularly for low-grade ores where conventional techniques are not economically viable.

Bioleaching involves using bacteria to convert insoluble metal compounds in ore into soluble compounds. These soluble compounds can then be extracted, for example, by electrolysis or displacement. The process begins by crushing the ore and mixing it with water and specific bacteria. The bacteria oxidise the metal sulfides, releasing the metal ions into solution. For instance, *Thiobacillus ferrooxidans* bacteria are commonly used to extract copper by oxidising copper sulfide to soluble copper sulfate.

Phytoextraction, on the other hand, uses plants that absorb metal ions from the soil through their roots. These 'hyperaccumulator' plants store the metal ions in their biomass, often in leaves or shoots. Once the plants have matured and accumulated sufficient metal, they are harvested and typically incinerated. The ash produced is then a concentrated source of the metal, from which it can be extracted. This method is particularly effective for removing metals from contaminated soils or extracting metals from low-grade ores where the metal is dispersed.

Both bioleaching and phytoextraction are considered sustainable alternatives because they require less energy and produce fewer harmful pollutants compared to conventional methods. However, they are much slower processes and typically yield smaller quantities of metal. The choice between biological and traditional methods depends on factors like the type of ore, the concentration of the metal, and economic and environmental considerations. For example, bioleaching requires careful control of conditions (pH, oxygen) for optimal bacterial activity, while phytoextraction is limited by plant growth rates and the availability of suitable hyperaccumulator species.

The greenhouse effect is a natural process that warms the Earth's surface. When the Sun's energy reaches the Earth's atmosphere, some of it is reflected back into space, but most of it passes through to the Earth's surface. The Earth absorbs this energy and then re-radiates it as longer-wavelength infrared radiation. Certain gases in the atmosphere, called greenhouse gases (such as carbon dioxide, methane, and water vapour), have a specific molecular structure that allows them to absorb this outgoing infrared radiation. These gas molecules then re-emit the infrared radiation in all directions, including back towards the Earth's surface. This absorption and re-emission of infrared radiation by greenhouse gases traps heat within the atmosphere, preventing it from escaping directly into space. This trapping of heat warms the planet, making it suitable for life. Without the natural greenhouse effect, Earth's average temperature would be much colder, around -18°C, rather than the habitable average of around +15°C. Human activities, particularly the burning of fossil fuels, are increasing the concentration of these greenhouse gases, leading to an enhanced greenhouse effect and global warming.