Global challenges Study Guide

Traditional methods of extracting metals, especially from low-grade ores, often involve energy-intensive processes like smelting, which can produce significant environmental pollutants such as sulfur dioxide and carbon dioxide. Biological methods, such as bioleaching and phytoextraction, offer alternative approaches to recover metals like copper from their ores or contaminated soils. Bioleaching uses bacteria (e.g., Acidithiobacillus ferrooxidans) to oxidise metal sulfides, converting insoluble metal compounds into soluble ones. The metal ions are then leached out and can be recovered, for example, by displacement with a more reactive metal or by electrolysis. Phytoextraction involves growing plants that absorb metal ions from the soil into their biomass. These 'hyperaccumulator' plants are then harvested and incinerated, and the concentrated metal is recovered from the ash. Both methods have significant environmental benefits, as they typically operate at ambient temperatures and pressures, reducing energy consumption and greenhouse gas emissions. They also avoid the release of toxic fumes associated with smelting. However, biological methods are generally much slower than traditional methods, require large areas of land, and the efficiency can be affected by factors such as pH, temperature, and nutrient availability for the microorganisms or plants. The concentration of metal extracted is also often lower per unit of time compared to conventional methods. Evaluating these methods involves weighing their slower extraction rates and logistical challenges against their lower environmental impact and suitability for low-grade ores or contaminated sites where traditional methods are uneconomical or environmentally problematic.

The current scientific understanding of Earth's early atmosphere suggests it was very different from what we experience today. Immediately after its formation, Earth was a molten ball. As it cooled, a primitive atmosphere began to form. This early atmosphere is believed to have been primarily composed of gases released from extensive volcanic activity. Volcanoes erupted frequently, releasing large quantities of key gases. The principal gases released were carbon dioxide, water vapour, and nitrogen. There was also likely some methane and ammonia present, but very little, if any, oxygen. The water vapour released subsequently condensed as the Earth cooled further, forming the first oceans. Over millions of years, the composition of the atmosphere changed significantly. The presence of water allowed for the dissolution of carbon dioxide, forming carbonates which then precipitated to form sedimentary rocks. Later, the evolution of early life forms, particularly photosynthetic bacteria, led to the gradual increase of oxygen, transforming the atmosphere into one capable of supporting complex life as we know it today.