Global challenges Revision Notes

Introduction to Biological Metal Extraction (Higher)

Traditional metal extraction methods like smelting (reduction with carbon or electrolysis) are energy-intensive and can cause environmental damage, including air pollution from sulfur dioxide and carbon dioxide (a greenhouse gas). Biological methods, specifically bioleaching and phytoextraction, offer less environmentally damaging alternatives. These are particularly useful for extracting metals from low-grade ores where traditional methods are not economically viable. While more sustainable, biological methods are typically slower and produce smaller quantities of metal compared to conventional techniques.

Bioleaching Process (Higher)

Bioleaching utilises bacteria to convert insoluble metal compounds within an ore into soluble compounds. These soluble metal compounds can then be washed out and the metal extracted, often by electrolysis or displacement reactions. The process typically involves crushing the ore and mixing it with water and specialised bacteria, such as *Thiobacillus ferrooxidans*. These bacteria oxidise metal sulfides, releasing metal ions into an aqueous solution. For example, copper sulfide ore can be bioleached to produce a solution of copper sulfate.

Phytoextraction Process (Higher)

Phytoextraction uses plants, known as hyperaccumulators, that can absorb metal ions from the soil through their root systems. These accumulated metal ions are then stored in the plant's biomass (e.g., leaves, stems). Once the plants have grown and accumulated a significant amount of metal, they are harvested. The harvested plant material is then often incinerated, and the resulting ash, which is highly concentrated with the metal, is processed to extract the desired metal. This method is suitable for cleaning contaminated land or recovering metals from diffused, low-concentration sources.

Evaluation of Biological Methods (Higher)

Both bioleaching and phytoextraction are more environmentally friendly than traditional methods because they require less energy and produce fewer harmful pollutants. They are also suitable for extracting metals from low-grade ores. However, their main disadvantages are their significantly slower rates of metal extraction and smaller yields compared to smelting or electrolysis. Bioleaching requires careful control of environmental conditions for bacterial activity (e.g., pH, temperature, oxygen levels), and phytoextraction is limited by plant growth rates and the availability of suitable metal-accumulating plant species. The choice of method depends on economic viability, environmental impact goals, and the characteristics of the ore or contaminated site.

Early Earth's Initial Atmosphere

Around 4.5 billion years ago, the Earth's first atmosphere consisted mainly of hydrogen and helium. These light gases were lost into space due to the strong solar winds and Earth's lack of a protective magnetic field in its very early stages. This initial atmosphere did not persist for long, paving the way for the formation of a secondary atmosphere.

Formation of the Secondary Atmosphere

The secondary atmosphere formed largely due to intense volcanic activity on the young Earth. Volcanoes released vast amounts of gases, primarily water vapour (H₂O), carbon dioxide (CO₂), and nitrogen (N₂). Smaller quantities of methane (CH₄) and ammonia (NH₃) were also present. This outgassing provided the raw materials for the atmosphere we have today, although its composition was very different.

Role of Water Vapour and Carbon Dioxide

As Earth cooled, the hot water vapour in the atmosphere condensed, leading to heavy rainfall over millions of years. This rainfall gradually filled depressions on the Earth's surface, forming the early oceans. Carbon dioxide then dissolved from the atmosphere into these newly formed oceans. This oceanic dissolution of CO₂ was a key process in reducing its atmospheric concentration. Furthermore, marine organisms took up dissolved CO₂ to form shells and skeletons, which eventually contributed to the formation of sedimentary rocks like limestone.

Nitrogen Accumulation

Nitrogen was also released by volcanic activity. Unlike carbon dioxide and water vapour, nitrogen is a relatively unreactive gas. This meant it did not readily dissolve in oceans or react significantly with other substances on the Earth's surface. As a result, nitrogen gradually accumulated in the atmosphere over vast periods, eventually becoming the most abundant gas, making up approximately 78% of the atmosphere today.

What is Crude Oil?

Crude oil is a fossil fuel, a finite resource formed from the remains of ancient marine organisms over millions of years under high pressure and temperature. It is a complex mixture of hydrocarbons, which are molecules made up solely of carbon and hydrogen atoms. These hydrocarbons vary significantly in their molecular size, specifically the length of their carbon chains. This variation in chain length is fundamental to separating crude oil, as it directly influences their physical properties, particularly boiling point.

The Fractional Distillation Column

The core of fractional distillation is the fractionating column. This is a tall, cylindrical structure designed to maintain a temperature gradient. It is hottest at the bottom, where the hot crude oil enters, and gradually cooler towards the top. The column contains trays at different levels. As hydrocarbon vapours rise, they cool. When a vapour reaches a height where the temperature matches its boiling point, it condenses into a liquid on a tray and is collected as a specific 'fraction'. Shorter chain hydrocarbons have lower boiling points and rise higher before condensing; longer chain hydrocarbons have higher boiling points and condense lower down.

Process of Separation

1. Crude oil is heated to about 350-400°C in a furnace, causing most of it to vaporise. 2. This hot mixture of liquid and vapour then enters the bottom of the fractionating column. 3. Vapours rise up the column, cooling as they ascend. 4. Hydrocarbons with higher boiling points (longer carbon chains) condense at lower levels and are collected. Examples include bitumen and fuel oil. 5. Hydrocarbons with lower boiling points (shorter carbon chains) continue to rise further up the column. 6. As they cool to their respective boiling points, they condense at higher levels and are collected. Examples include gasoline, naphtha, and refinery gases.

Products of Fractional Distillation

Different fractions collected from the column have distinct uses: Refinery gases (top, very low boiling point) are used for bottled gas and chemical feedstock. Gasoline/petrol (next level down) is used as fuel for cars. Naphtha is used as a chemical feedstock. Kerosene/paraffin is used for jet fuel and heating oil. Diesel/gas oil is used for diesel engines. Fuel oil is used for ships and power stations. Bitumen (bottom, highest boiling point) is used for paving roads and roofing.

Carbon Monoxide (CO)

Carbon monoxide is a toxic gas formed when fuels like petrol, diesel, and coal burn with an insufficient supply of oxygen (incomplete combustion). This often happens in poorly ventilated spaces or when vehicle engines are running inefficiently. The primary source is vehicle exhausts and industrial processes. When inhaled, CO prevents red blood cells from carrying oxygen, leading to suffocation. It is colourless and odourless, making it particularly dangerous.

Sulfur Dioxide (SO2)

Sulfur dioxide is produced during the combustion of fossil fuels (mainly coal and oil) that contain sulfur impurities. Power stations burning fossil fuels are a major source. SO2 dissolves in atmospheric water to form sulfuric acid, which is a key component of acid rain. Acid rain corrodes buildings, damages forests, and acidifies lakes and rivers, harming aquatic life. It can also cause respiratory problems in humans.

Oxides of Nitrogen (NOx)

Oxides of nitrogen, such as nitrogen monoxide and nitrogen dioxide, are formed when nitrogen and oxygen from the air react under the high-temperature conditions found in vehicle engines and some power stations. These gases also contribute to acid rain and can worsen respiratory conditions like asthma. They can also contribute to the formation of photochemical smog, which reduces air quality.

Particulates

Particulates are tiny solid particles and liquid droplets suspended in the air. They are primarily released from the incomplete combustion of fossil fuels, especially diesel, and from burning biomass. Common sources include vehicle exhausts, industrial chimneys, and agricultural burning. Inhaling particulates can lead to serious respiratory problems, including asthma and bronchitis, and can exacerbate heart conditions. They also reduce visibility and cause haze.