At present, surfactants are generally used as bactericides and disinfectants in the aquatic industry, and some surfactants are used to improve substrates. However, there are few reports on the impact of surfactants on dissolved oxygen in water bodies.
There are many kinds of surfactants, with more than 20,000 brand names and a production volume of 12 million tons. They have a wide range of uses, involving various fields of industry, agriculture and people’s lives, and are known as “industrial MSG”. Few of them are used in aquaculture, and there are even fewer papers on the application of surfactants in aquaculture.
Surfactant refers to a substance with fixed hydrophilic and lipophilic groups that can be oriented and arranged on the surface of a solution and can significantly reduce the surface tension. The molecular structure of surfactants is amphiphilic: one end is a hydrophilic group and the other end is a hydrophobic group; the hydrophilic group is often a polar group, such as carboxylic acid, sulfonic acid, sulfuric acid, amino or amine The base and its salt can also be hydroxyl, amide group, ether bond, etc.; while the hydrophobic group is often a non-polar hydrocarbon chain, such as a hydrocarbon chain with more than 8 carbon atoms.
Surfactants are divided into ionic surfactants and nonionic surfactants. Surfactants have become a flexible and diverse class of agents due to their series of physical and chemical functions and corresponding practical applications such as wetting or anti-sticking, emulsification or demulsification, foaming or defoaming, solubilization, dispersion, washing, antisepsis, and antistatic. , fine chemical products with a wide range of uses.
At present, surfactants are generally used as bactericides and disinfectants in the aquatic industry. Their bactericidal and disinfectant effects are attributed to their strong interaction with bacterial biofilm proteins, causing them to denature or lose their functions. These disinfectants are in water All have relatively large solubility, such as cationic surfactants and quaternary ammonium salts, which are used for sterilization and disinfection of ponds.
There are also some surfactants used to improve the substrate, but there are currently no relevant reports on the impact of surfactants on dissolved oxygen in water. We have conducted relevant research on this, and the research shows that surfactants have a significant impact on the concentration and duration of dissolved oxygen in aquaculture water bodies. The changes of surfactants on the solubility and time of oxygen in water bodies are as follows:
Surfactants can increase the solubility of oxygen in water bodies. The saturated solubility of oxygen in breeding water bodies generally does not exceed 9 mg/L. After adding a specific type of surfactant to the culture water body, the saturated solubility of oxygen in the water body can exceed 20 mg/L. The time required for the entire process depends on the type of surfactant, and the difference is very large.
We screened more than 200 kinds of surfactants and selected 16 standard cement pools of 500mm×500mm×500mm, filled with surface natural water bodies from large-area lakes (≧1Km2), and used surfactants with an accuracy of 0.01mg/L. The dissolved oxygen meter detects the dissolved oxygen in the water at the bottom of the pool when the water temperature is 25±1°C. Before using surfactant, the average value of dissolved oxygen in the water at the bottom of the pool was 4±0.3mg/L (n=16). Then add different surfactants to each pool, with a concentration of 1mg/L.
The above experimental results show that it only takes 10 minutes for some surfactants to increase oxygen to 6 mg/L, while it only takes 150 minutes for some surfactants to increase oxygen to 6 mg/L. The durations are also different, from only 8 hours to the maximum The longest one can last up to 7 days. At the same time, the effects of different surfactants on the dissolved oxygen in the water body are also significantly different. For example, at 4 hours, the maximum dissolved oxygen in the water at the bottom of the pool was more than 20 mg/L, and the minimum was only 5 mg/L. The above experimental results show that different surfactants can increase the solubility of oxygen in water. The differences lie in the type and dosage of surfactants, the duration and the amount of increased dissolved oxygen.
Based on the experimental results obtained above, the mechanism of surfactant’s effect on the dissolved oxygen level of water body was preliminary discussed:
1. Increase the saturated solubility of oxygen in water
The solubilization effect of surfactant promotes the supersaturation of oxygen molecules in the water solution, which is equivalent to increasing the saturated solubility of oxygen in the water. The saturated solubility of oxygen in aquaculture water generally does not exceed 9 mg/L. Adding surfactant to aquaculture water can increase the saturated solubility of oxygen in aquaculture water. According to the hydrate formation theory and the double-film theory, since the solubility of oxygen in water is very small, water-oxygenates in the water body are generally first formed at the gas-liquid interface, and then slowly diffuse from the gas phase to the liquid phase. In water bodies containing surfactants, due to the solubilization effect of surfactants, a large amount of oxygen is dissolved in the surfactant micelles formed in the aqueous solution. In this way, the formation of water-oxygenates in the surfactant aqueous solution is not only It can occur at the gas-liquid interface, and it can also occur inside the water body, which reduces the formation time of water-oxygenates and increases the solubility of oxygen in the water body.
2. Foaming effect of surfactants
Surface active agents have good foaming effect in water bodies. Since foam is a dispersion system of gas dispersion and liquid, the gas is the dispersed phase and the liquid is the dispersed meson. Water containing surfactant will produce many bubbles under the action of wind, water flow, swimming of aquatic animals, etc. Because surfactant molecules can be adsorbed on the air-liquid interface, they are oriented and arranged to form a strong film to keep the bubbles stable. , at the same time, the surface-active water-based agent has dispersibility and can make bubbles evenly dispersed in the liquid. In the process of generating foam when the water body is disturbed, use�A large number of oxygen-rich air bubbles enter the water body, speeding up the dissolution of oxygen in the water body, thereby bringing the dissolved oxygen in the water body to a higher level.
Especially at night when there is no light, surfactants have a significant effect on increasing the dissolved oxygen in the water body. If there is stirring from the aerator at this time, coupled with the convection of the upper and lower water layers, the water body will not reach the bottom and biological A large amount of oxygen is consumed and there is a sharp lack of oxygen, thereby preventing the pond from flooding of the cultured objects.
3. The effect of surfactants on water density
In aquatic products, hydrophilic (oleophobic) hydrocarbon group surfactants are generally used. When the surfactant molecules enter the aqueous solution, the lipophilic groups of the surfactant have the tendency to escape from the water phase in order to minimize contact with water. trend, but due to the existence of hydrophilic groups in surfactant molecules, they cannot escape from the water phase completely. The result of the equilibrium is that surfactant molecules are enriched on the surface of the solution, that is, the lipophilic groups face the air, while the hydrophilic groups By inserting the water phase, the original interface formed by water and air is gradually replaced by the interface formed by the lipophilic group of the surfactant and air. The surface tension of water is reduced and air enters the water more easily.
Due to the low concentration used and the inability to form micelles, the density of the surface water is lower than that of the bottom water. Under the dispersion of the surfactant, an upper and lower water circulation is formed to bring air to the bottom of the pool.
Especially during the day, due to the photosynthesis of phytoplankton, the oxygen in the water layer above the compensation depth of the culture water body reaches supersaturation, while the oxygen in the water layer below the compensation concentration, especially the bottom water layer, is due to biological respiration and organic matter oxidation. etc., resulting in low oxygen content or even no oxygen.
After the surfactant is splashed into the aquaculture water body during the day, the surfactant begins to be distributed in the upper layer of the aquaculture water body. This increases the density of the upper layer of the aquaculture water body, which will drive the oxygen-rich water in the upper layer to move to the lower water body, making the entire Convection occurs in the upper and lower layers of the aquaculture water body, increasing the oxygen content of the bottom water body and reducing the deterioration of the lower water quality and substrate.
4. The degreasing film effect of surfactants
In high-density aquaculture water bodies, a large amount of feed and other materials are usually put in, which are digested by aquatic animals and excreted a large amount of waste. After microbial decomposition, plankton metabolism and other factors, an oil film is formed on the surface of the water body, blocking the contact between the air and the water body, increasing the size of the water body. of surface tension. The surfactant can gradually curl the oil film into oil droplets, which are detached, rolled, and concentrated to the downwind outlet by the water flow. When the amount of surfactant is large, it can be emulsified and suspended in the water body in the form of particles.
The surface tension of the water body is reduced, making it easier for oxygen in the air to enter the water body, accelerating the dissolution of oxygen in the water body. At the same time, the increased transparency enhances algae photosynthesis, thereby increasing the oxygen content in the water body.
Based on the above, surfactants have a series of physical and chemical properties that can more or less increase the distribution of oxygen in water bodies, which has also been verified in practical applications. At present, there are not many products developed by various companies in terms of surfactants for oxygenating water bodies. Wuhan Tianchen Biotechnology Co., Ltd.’s “Nano Oxygen” is the representative in this regard. As the research on the action mechanism of surfactants becomes more and more in-depth, more and more surfactants will play an important role in aquaculture in the future.
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