Chemical Pollutants: Water's Silent Killers Robbing Oxygen

Dissolved oxygen (DO) is a measure of how much oxygen is present in water, which is essential for the survival of aquatic organisms. Several factors influence DO levels, including temperature, weather conditions, and organic matter. While DO is crucial, high concentrations can have adverse effects, such as corrosion in boilers and pipes. To address this, various techniques are employed to remove excess DO, including mechanical and chemical methods. The presence of pollutants in water can significantly impact DO levels, and certain chemical pollutants can contribute to oxygen depletion. This raises concerns about the water quality and the health of aquatic ecosystems. Understanding the sources and effects of chemical pollutants that remove oxygen from water is essential for effective water management and ecological preservation.

Characteristics and Values of Chemical Pollutants that Remove Oxygen from Water

Nitrogen stripping

The process of nitrogen stripping involves using large amounts of nitrogen gas to remove oxygen from water. This is done by blowing nitrogen gas through the water, creating a reaction that causes the oxygen to be released into the atmosphere as a gas. This transfer of oxygen from the liquid phase to the gas phase is a common method of removing unwanted gases from a mixture.

While nitrogen stripping is an effective method of oxygen removal, it can be expensive due to the large amounts of nitrogen gas required. This has prompted the exploration of alternative methods, such as "coupling techniques," to reduce nitrogen gas usage and improve the efficiency of the process.

In addition to nitrogen stripping, there are other common techniques for removing oxygen from water, including mechanical and chemical methods. Mechanical methods involve using an oxygen-free gas, typically methane (CH4), to strip oxygen from water through countercurrent contact exchange. Chemical methods, on the other hand, involve adding substances like sulfur dioxide (SO2) or sodium sulfite (Na2SO3) to remove oxygen. These alternative methods are often employed in the oil field industry to protect steel pipes and equipment from corrosion caused by oxygen in the water.

Boiling water at 1atm

The boiling point of water at 1atm (atmospheric pressure) is 100 °C. At this temperature, the vapour pressure of water is 1 atm or 760 mmHg. The boiling point of water decreases with lower pressure, for example, at higher altitudes. This is why water boils more quickly in the mountains than close to sea level, but at a lower temperature.

DO also plays a crucial role in groundwater quality. It supports bacteria that break down pollutants and minimizes harmful substances like iron and manganese. However, in some cases, oxygen can be a harmful substance. For example, in natural waters, DO usually increases the corrosion rate of steel, especially in brines where it causes pitting.

One method to remove DO from water is to use an electrochemical cell. Here, oxygen-rich water flows through a three-dimensional cathode, which removes the DO and releases it in gas form into the air via the anode. This method is more efficient than physical methods and equally efficient as chemical methods due to its low energy consumption and use of non-toxic deoxygenation chemicals. Another effective way to remove DO is through nitrogen stripping.

Boiling water under reduced pressure

The presence of oxygen in water bodies is crucial for supporting bacteria that break down pollutants and minimizing harmful substances. However, oxygen can also be a harmful substance, especially in industries such as power plants, where it can cause corrosion in boilers and pipes. In oil fields, the presence of dissolved oxygen in brine can increase the corrosion rate of steel. This has led to the development of various methods for removing oxygen from water, both mechanically and chemically.

One common technique for removing dissolved oxygen (DO) from water is boiling the water under reduced pressure. This process can be facilitated using a simple manual vacuum pump, which helps lower the air pressure inside the container. Here are the steps to boil water under reduced pressure:

• Boil a pot of water: Start by boiling a pot of water on a stove or any other heat source. You will need enough water to fill a jar or container halfway.
• Prepare the container: While the water is heating up, prepare a glass jar or a suitable container with a wide opening. Make sure the container is clean and dry.
• Pour the boiled water into the container: Once the water has reached its boiling point, carefully pour it into the jar until it is half-filled. It is important to only fill the jar halfway as you will need to leave some air space for creating reduced pressure.
• Cover the container: Place a lid or cover on the jar to seal it. Ensure that the lid has a good seal to prevent air from escaping or entering the jar.
• Use a vacuum pump: Attach a manual vacuum pump to the jar. These pumps are commonly used to keep food fresh and can be purchased online or at specialty stores. Turn on the pump to start removing air from the jar, creating a reduced pressure environment.
• Observe the water: As you pump out the air, you will notice that the boiling point of the water decreases. This is because lowering the pressure also lowers the boiling point. The energetic water molecules will try to escape, but atmospheric air molecules will cause them to collide and return below the liquid surface.
• Continue pumping: Keep using the vacuum pump to draw out more air. Eventually, you will reach a point where the water starts to boil due to the reduced pressure. This demonstrates the relationship between pressure and the boiling point of a liquid.

It is important to note that boiling water under reduced pressure is just one method of removing oxygen from water. Other common techniques include boiling water at atmospheric pressure (1 atm), nitrogen purging, and sonication under reduced pressure. Additionally, in oil fields, sulfur dioxide or sodium sulfite are commonly added to remove oxygen from brine, as they require very little capital investment.

Purging with nitrogen

Nitrogen purging, also known as nitrogen stripping, is a process that uses nitrogen gas (N2) to remove unwanted gases and impurities from a manufacturing system. Nitrogen is an inert gas that is non-combustible and ideal for removing oxygen from water.

The process involves using high-pressure nitrogen pumps to slowly raise the pressure of synthesized nitrogen gas to the necessary levels before purging. The nitrogen gas is then introduced into the system at an inlet point, where it mixes with the contaminants. The mixture is then expelled from an outlet point located as far as possible from the inlet. This method is known as dilution purging and is used to remove unwanted gaseous impurities from complex industrial systems like reactors, columns, and kilns.

Nitrogen purging is commonly used in the oil and gas industry to remove oxygen and eliminate the risk of it mixing with hydrocarbons. It is also used in the food and beverage industry to displace oxygen in packaging, maintaining food quality and removing unwanted gases from beverage piping systems.

In addition to these applications, nitrogen purging is also used in chemical and petrochemical systems for purging organic compounds, metal fabrication to prevent compound contamination, and transformers and volatile electrical environments.

Nitrogen purging is a critical process for maintaining workplace safety and improving productivity in industries such as chemical, pharmaceutical, and oil and gas.

Sonication under reduced pressure

One of the four common techniques used to remove dissolved oxygen (DO) from water is sonication under reduced pressure. This technique is used to accelerate the osmotic dehydration (OD) process and modify the selected properties of cranberries. The cranberries are first placed in an osmotic solution and sonicated for 20 minutes, and then the pressure is lowered for 10 minutes. The frequency of the ultrasound is 21 kHz, and the power is 180 W. The time taken to reduce the pressure is 20 seconds, and the pressure is lowered by 300 mmHg (~40 kPa) by a vacuum pump.

Another application of sonication under reduced pressure is in the activation of flexible printed metal circuits. This process is known as pressure-constrained sonication activation (PCSA). PCSA helps in the interconnection of multilayer circuits and the joining of electronic components to printed circuits without the use of solder. It can be performed at room temperature within 1 second.

While sonication under reduced pressure is an effective method for removing dissolved oxygen from water, it does not lead to the complete removal of oxygen. Other techniques such as nitrogen purging, boiling at 1 atm, and boiling under reduced pressure are also used for oxygen removal. Nitrogen purging, in particular, has been found to be the most effective method among the four common techniques.

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