Water (H₂O) is a compound that contains oxygen, but this oxygen is chemically bonded to hydrogen atoms, making it unavailable as breathable gas. Extracting oxygen from water involves separating this bonded oxygen or removing oxygen that is already dissolved within the water.
Splitting Water with Electricity
One of the most established methods for producing oxygen from water is electrolysis, which uses electrical energy to decompose water into hydrogen (H₂) and oxygen (O₂). An electrolytic cell typically consists of two electrodes—an anode (positive) and a cathode (negative)—immersed in water. A direct current (DC) power source connects to these electrodes, driving the reactions. Pure water is a poor conductor of electricity due to a lack of ions. Therefore, an electrolyte, such as an acid like sulfuric acid (H₂SO₄) or a base like potassium hydroxide (KOH), is often added to increase the water’s conductivity. This facilitates the flow of ions, crucial for the electrical current. At the negatively charged cathode, water molecules gain electrons in a reduction reaction to form hydrogen gas and hydroxide ions. Simultaneously, at the positively charged anode, water molecules lose electrons in an oxidation reaction, producing oxygen gas and hydrogen ions. The overall reaction is 2H₂O → 2H₂ + O₂. Hydrogen is produced at twice the amount of oxygen, consistent with water’s chemical formula.
Other Ways to Split Water
Beyond electrolysis, scientists explore alternative methods to split water molecules to produce oxygen and hydrogen. One such method is thermolysis, which involves using very high temperatures to split water directly. For example, at temperatures around 2,200°C, a small percentage of water molecules dissociate into hydrogen and oxygen atoms, though much higher temperatures, exceeding 3,000°C, are needed for significant decomposition. Direct thermolysis faces material challenges due to these extreme temperature requirements. To overcome this, thermochemical cycles utilize a series of chemical reactions at lower, yet still high, temperatures to achieve water splitting, producing hydrogen and oxygen in separate steps. Photolysis, or photocatalytic water splitting, uses light energy to dissociate water into hydrogen and oxygen, inspired by natural photosynthesis. Photoelectrochemical cells employ semiconductors that absorb sunlight, initiating water splitting. These systems aim to convert solar energy directly into chemical energy in the form of hydrogen and oxygen. Catalytic methods also play a role, using specific chemical compounds to facilitate the water-splitting reaction, often at lower energy inputs than direct thermal or light-based methods. These catalysts speed up reactions and prevent gas recombination.
Removing Oxygen Already in Water
Water naturally contains dissolved oxygen, which is distinct from the oxygen chemically bonded within the H₂O molecule. This dissolved oxygen is vital for aquatic life but can be detrimental in industrial applications, particularly in closed systems like boiler feedwater and pipelines, where it causes corrosion. Various methods are employed to remove this dissolved oxygen, falling broadly into physical and chemical categories. Physical methods include thermal degassing, such as boiling water, which reduces oxygen solubility as temperature increases. Vacuum deaeration involves reducing pressure over the water, causing dissolved gases, including oxygen, to escape. Gas stripping, commonly using inert gases like nitrogen, bubbles the gas through the water to strip out the dissolved oxygen. This method is often considered effective, though it can be expensive due to the need for large amounts of nitrogen. Chemical methods involve adding oxygen scavengers that react with and remove the dissolved oxygen. Common examples include hydrazine and sodium sulfite, which chemically bind with oxygen to prevent its corrosive effects. Membrane separation technologies also offer a way to filter out dissolved gases using selective membranes.
Where Oxygen Extraction Is Used
Oxygen extraction from water, both by splitting the water molecule and removing dissolved oxygen, finds application across various sectors. In life support systems, such as those aboard spacecraft like the International Space Station and in submarines, electrolysis is a primary method for generating breathable oxygen for the crew. These systems continuously produce oxygen, ensuring a sustainable environment for personnel in isolated settings. Industrially, the oxygen produced from water electrolysis can be used in numerous processes. This includes applications in welding, steelmaking, and the chemical industry, where high-purity oxygen is often required. For instance, oxygen is used to remove impurities from iron in blast furnaces and in the production of various chemicals. While hydrogen is often the primary product of electrolysis, the co-produced oxygen is increasingly being recognized for its value, with potential uses in enhancing combustion processes and optimizing industrial efficiency. Furthermore, removing dissolved oxygen from water is essential in water treatment and power generation. In power plants, deoxygenation of boiler feedwater is necessary to prevent corrosion and pitting of metal surfaces, which can lead to reduced efficiency and costly repairs. Dissolved oxygen removal also plays a role in wastewater treatment, where it can be managed to support specific biological processes or to prevent issues like corrosion in pipelines.