There are four kinds of action principle of polyacrylamide, to understand about the polyacrylamide compression double layer, adsorption electric neutralization, adsorption bridge, sediment trap these four kinds of related knowledge.
1. Mechanism of sediment trapping
When metal salts (such as aluminum sulfate or ferric chloride) or metal oxides and hydroxides (such as lime) are used as coagulants, colloidal particles in water can be netted by these precipitates during their formation 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). When the precipitate is positively charged (Al(OH)3 and Fe(OH)3 in the neutral and acidic pH ranges), the precipitation rate is accelerated by the presence of anions in the solution, such as silver sulfate ions. In addition, colloidal particles in water themselves can be used as the core of the formation of these metal oxygen oxide precipitates, so the optimal dosage of coagulant is inversely proportional to the concentration of removed substances, that is, the more colloidal particles, the less dosage of metal coagulant.
2. Compressed double electric layer
The structure of the micelle double electric layer determines the maximum concentration of counter-ions on the surface of the colloidal particles. The greater the distance from the surface of the colloidal particles, the lower the concentration of counter-ions, and finally equal to the concentration of ions in the solution. When electrolyte is added to the solution, the ionic concentration in the solution increases, 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, the potential decreases, so their mutual repulsion force is reduced, that is, the repulsion force between colloidal particles with high ion concentration in the solution is smaller than that with low ion concentration. The attraction between the colloidal particles is unaffected by the composition of the water phase, but because the diffusion layer is thinner, they collide at a smaller distance and thus attract each other more. It can be seen that the resultant force of repulsion and attraction changes from repulsion to suction (the repulsion potential energy disappears), and the colloidal particles condense rapidly
This mechanism can better explain the deposition phenomenon in the harbor, because when fresh water enters seawater, salt increases, ion concentration increases, and the stability of colloidal particles carried by fresh water decreases, so 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 condensation, there will be no more excess counter-ions into the diffusion layer, and it is impossible to change the sign of colloidal particles to re-stabilize the situation. Such a mechanism is through simple electrostatic phenomena to explain the role of the electrolyte stability of colloidal particles, but it did not consider to take off the stability in the process of the role of other properties, such as adsorption, so you cannot explain some of the other complex unstable phenomenon, such as trivalent aluminium salt and iron salts coagulant dosage is overmuch, condensation effect declines, even to stable; As with the colloidal belt with the same electric number of polymer or polymer organic matter may have good coagulation effect: isoelectric state should have the best coagulation effect, but in production practice when the ξ potential is greater than zero coagulation effect is the best...... And so on.
In fact, it is a comprehensive phenomenon to add coagulant to the aqueous solution to destabilize colloidal particles, which involves the interaction between colloidal particles and coagulant, colloidal particles and aqueous solution, and coagulant and aqueous solution
3. Adsorption electric neutralization
Adsorption electric neutralization effect refers to the surface of particles with different ions, different colloidal particles or chains with different charge parts have strong adsorption, because this adsorption neutralized part of its charge, reduce the electrostatic repulsion, so easy to close to other particles and adsorption. At this time electrostatic attraction is often the principal aspect of these effects, but in quite a few cases other forces override it. For example, when Na+ and dodecylammonium ion (C12H25NH3+) were used to remove the turbidity caused by negatively charged silver iodide solution, it was found that the ability of organic amine ions with the same valence to destabilize was much greater than that of Na+. Excessive addition of Na+ would not result in the re-stabilization of colloidal particles, while organic amine ions did not. It shows that the colloidal particles absorbed too many counter-ions, so that the original negative charge into a positive charge. Stabilization and charge change also occur when aluminum and iron salts are added in high dosage. The above phenomenon is well explained by the mechanism of adsorptive electric neutralization.
4. Adsorption bridging
The mechanism of adsorption bridging mainly refers to the adsorption and bridging between polymer and colloidal particles. It can also be thought of as two large colloids of the same name joined together by the presence of a different colloid in the middle. High polymer flocculant with linear structure, they have with parts play a role of colloidal particle surface functional groups, when high polymer contact with colloidal particles, groups with special reaction and colloidal particles surface adsorption, while the rest of the polymer molecules spread in solution, can have vacancy of colloidal particles with another surface adsorption, The polymer thus acts as a bridge. If the colloidal particles are less, the polymer extension part can not stick to the second colloidal particles, then the extension part will be absorbed in other parts of the original colloidal particles sooner or later, the polymer can not play a bridging role, and the colloidal particles are in a stable state. When the amount of polymer flocculant is too large, it will saturate the colloidal surface and cause the phenomenon of re-stabilization. If the colloidal particles that have been bridged are subjected to intense and prolonged agitation, the bridging polymer may detach from another colloidal surface and reroll back to the original colloidal surface, resulting in a re-stable state.
The adsorption of polymer on the surface of colloidal particles comes from various physicochemical interactions, such as van der Waals attraction, electrostatic attraction, hydrogen bond, coordination bond, etc., and depends on the characteristics of the chemical structure of polymer and colloidal surface. This mechanism can explain the phenomenon that good flocculation effect can be obtained by non-ionic or ionic polymer flocculant with isoelectric number.