There are four kinds of action principles of polyacrylamide. Next, let's learn about four kinds of knowledge related to polyacrylamide compression double electric layer, adsorption neutralization, adsorption bridging and sediment net catcher.
1. Sediment netting mechanism
When metal salts (such as aluminum sulfate or ferric chloride) or metal oxides and hydroxides (such as lime) are used as coagulants, when the dosage is large enough to precipitate metal hydroxides (such as Al (OH) 3, Fe (OH) 3, Mg (OH) 2 or metal carbonates (such as CaCO3) rapidly, the colloidal particles in the water can be trapped by these precipitates when they are formed. When the precipitates are positively charged (Al (OH) 3 and Fe (OH) 3 in the neutral and acid pH range), the precipitation rate can be accelerated due to the presence of anions in the solution, such as silver sulfate ions. In addition, the colloidal particles themselves can be the core of the formation of these metal oxide precipitates, so the optimal dosage of coagulant is inversely proportional to the concentration of the removed substances, that is, the more colloidal particles, the less dosage of metal coagulant.
2. Compressed double electrode layer
The structure of the compressed double electrode layer determines that the concentration of the counter ion is the largest at the surface of the micelle. The greater the distance from the surface of the micelle to the outside, the lower the concentration of the counter ion is, and the final concentration of the counter ion is the same as that in the solution. When the electrolyte is added to the solution, the ion concentration in the solution increases, and the thickness of the diffusion layer decreases.
When two colloidal particles are close to each other, due to the decrease of the thickness of the diffusion layer and the potential, the repulsive force between them decreases, that is to say, the repulsive force between the colloidal particles with high ion concentration in the solution is smaller than that with low ion concentration. The suction between the particles is not affected by the composition of the water phase, but because the diffusion layer is thinner, the distance between them when they collide is reduced, so the suction between them is greater. It can be seen that the repulsion force and attraction force are changed from repulsion force to attraction force (the repulsion potential energy disappears), and the particles can agglomerate rapidly.
This mechanism can better explain the deposition phenomenon in the harbor. As the salt increases, the ion concentration increases, and the stability of fresh water carrying colloidal particles decreases when fresh water enters the seawater, clay and other colloidal particles are easy to deposit in the harbor.
According to this mechanism, when the external electrolyte in the polyacrylamide solution exceeds the critical concentration of agglomeration, there will not be more excess anti ions entering the diffusion layer, and it is impossible for the particles to change the sign and make the particles stable again. Such a mechanism is to explain the effect of electrolyte on the destabilization of colloidal particles by simple electrostatic phenomenon, but it does not consider the effect of other properties (such as adsorption) in the destabilization process, so it can not explain other complex destabilization phenomena, such as excessive dosage of trivalent aluminum salt and iron salt as coagulant, the coagulation effect will decline, or even re stabilize; for example, polymer with the same electrical number as colloidal particles or polymer organics may have good coagulation effect: isoelectric state should have the best coagulation effect, but in production practice, when ξ potential is greater than zero, the coagulation effect is the best.
Actually, adding coagulant to the aqueous solution to destabilize the colloidal particles involves the interaction of colloidal particles and coagulant, colloidal particles and aqueous solution, coagulant and aqueous solution, which is a comprehensive phenomenon
3. Adsorption neutralization
Adsorption neutralization refers to the strong adsorption of different ions, different colloidal particles or chains on the surface of particles. Because this adsorption neutralizes part of its charge and reduces the electrostatic repulsion, it is easy to be close to other particles and absorb each other. At this time, electrostatic attraction is often the main aspect of these actions, but in many cases, other actions exceed electrostatic attraction. For example, Na + and dodecyl ammonium ions (c12h25nh3 +) are used to remove the turbidity caused by the negatively charged silver iodide solution. It is found that the destabilizing ability of organic amine ions with the same valence is much greater than that of Na +, and the excessive addition of Na + will not cause the colloidal particles to be re stable, while the organic amine ions are not, which can cause the colloidal particles to be re stable when the concentration exceeds a certain level, indicating that the colloidal particles have absorbed too many counter ions , so that the original negative charge into a positive charge. When the dosage of aluminum salt and iron salt is high, the phenomenon of re stabilization and charge change will occur. The above phenomena can be explained by the mechanism of adsorption neutralization.
4. Adsorption bridging
Adsorption bridging mechanism mainly refers to the adsorption and bridging of polymer and colloidal particles. It can also be understood that two large colloidal particles of the same size are connected together due to a different size colloidal particle. Polymer flocculants have linear structure, and they have chemical groups that can act on some parts of the surface of the particles. When the high polymer contacts with the particles, the groups can react with the surface of the particles to adsorb each other, while the rest of the polymer molecules extend in the solution and can adsorb with the particles with empty positions on the other surface, so the polymer acts as a bridge connection The function of connection. If there are few colloidal particles, the extension part of the above polymer will not adhere to the second colloidal particle, then sooner or later the extension part will be adsorbed by the original colloidal particle on other parts, the polymer will not play a bridging role, and the colloidal particle is in a stable state. When the dosage of high molecular flocculant is too large, the surface of the particles will be saturated and the phenomenon of re stabilization will occur. If the bridged flocculated colloidal particles are stirred violently for a long time, the bridged polymer may detach from the surface of another colloidal particle, and then roll back to the original colloidal particle surface, resulting in a re stable state.
The adsorption of polymer on the surface of colloidal particles comes from various physical and chemical actions, such as van der Waals gravitation, electrostatic gravitation, hydrogen bond, coordination bond, which depends on the characteristics of the chemical structure of polymer and the surface of colloidal particles. This mechanism can explain the phenomenon that non-ionic or ionic polymer flocculant with the same electric number can get good flocculation effect.