polyacrylamide
It is a coagulant commonly used in sewage treatment.
Let me talk about the coagulation mechanism first:
1. Compressed electric double layer: The structure of micellar electric double layer determines that the concentration of counterions is the largest on the surface of colloidal particles, and the greater the distance from the surface of colloidal particles, the lower the concentration of counterions, which is finally equal to the ion concentration in the solution. When electrolyte is added to the solution to increase the ion concentration in the solution, the thickness of the diffusion layer decreases.
When two colloidal particles are close to each other, the zeta potential decreases due to the decrease of the thickness of the diffusion layer, so their mutual repulsion force decreases, that is, the repulsion force between colloidal particles with high ion concentration in solution is smaller than that between colloidal particles with low ion concentration. The attraction between colloidal particles is not affected by the composition of water phase, but because the diffusion layer is thin and the distance between them is reduced, the attraction between them is large. It can be seen that the resultant force of repulsion and gravity changes from repulsion to attraction (repulsion potential energy disappears), and colloidal particles can agglomerate quickly.
This mechanism can better explain the siltation phenomenon in the harbor. When fresh water enters the seawater, the salt content increases, the ion concentration increases, and the stability of colloidal particles carried by fresh water decreases, so colloidal particles such as clay are easy to deposit in the harbor.
According to this mechanism, when the electrolyte added in the solution is much larger than the critical concentration of coagulation, no extra counter ions will enter the diffusion layer, and it is impossible for the colloidal particles to change their signs and stabilize again. The mechanism explains the influence of electrolyte on the instability of colloidal particles by pure electrostatic phenomenon, but does not consider the influence of other properties (such as adsorption) in the process of instability, so it cannot explain other complex instability phenomena, such as excessive dosage of trivalent aluminum salt and ferric salt as coagulant, which reduces or even stabilizes the coagulation effect; For another example, polymer or macromolecular organic matter with the same charge number as colloidal particles may have good coagulation effect: isoelectric point state should have the best coagulation effect, but in production practice, zeta potential is often greater than zero, but coagulation effect is the best.
In fact, the phenomenon of adding coagulant to aqueous solution to make colloidal particles unstable involves the interaction between colloidal particles and coagulant, colloidal particles and aqueous solution, and coagulant and aqueous solution, which is a comprehensive phenomenon.
2. Adsorption neutralization:
Adsorption neutralization means that the particle surface has a strong adsorption effect on different ions, different colloidal particles or parts with different charges of chain ionomer. Because this adsorption neutralizes part of its charge and reduces electrostatic repulsion, it is easy to get close to other particles and adsorb each other. At this time, electrostatic attraction is often the main aspect of these effects, but in many cases, other effects exceed electrostatic attraction. For example, when Na+ and dodecyl ammonium ions (C 12H25NH3+) were used to remove the turbidity caused by negatively charged silver iodide solution, it was found that the destabilization ability of organic amine ions with the same valence was much greater than that of Na+, and excessive addition of Na+ would not cause the destabilization of colloidal particles, while organic amine ions would not. Above a certain dose, the colloidal particles will be unstable, indicating that they have absorbed too much anti-stability effect. When the amount of aluminum salt and iron salt is high, there is also a phenomenon of re-stability, and the charge changes sign. It is appropriate to explain the above phenomenon by adsorption neutralization mechanism.
3. Adsorption bridging effect:
The mechanism of adsorption bridging mainly refers to the adsorption and bridging between polymer and colloidal particles. It can also be understood that two large colloidal particles with the same number are connected by a colloidal particle with a different number. Polymer flocculants have a linear structure, and they have chemical groups that can act on some parts of the surface of colloidal particles. When the polymer comes into contact with the colloidal particles, the groups can react with the surface of the colloidal particles and adsorb each other, while the remaining polymer molecules extend in the solution and can adsorb with another colloidal particle with a vacancy on the surface, so that the polymer can play a bridging role. If there are few colloidal particles, the extended part of the polymer can't attach to the second colloidal particle, and sooner or later, the extended part will be adsorbed to other parts by the original colloidal particle, and the polymer can't play a bridging role, so the colloidal particle is in a stable state. When the dosage of polymer flocculant is too large, the surface of colloidal particles will be saturated and unstable. If the colloidal particles that have been bridged and flocculated are stirred vigorously for a long time, the bridged polymer may detach from the surface of another colloidal particle and roll back to the surface of the original colloidal particle, 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 attraction, electrostatic attraction, hydrogen bond, coordination bond and so on. It depends on the chemical structure characteristics of polymer and colloidal particles. This mechanism can explain the phenomenon that nonionic or ionic polymer flocculants with the same electrical signal can obtain good flocculation effect.
4. The mechanism of sediment net catching.
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) quickly, colloidal particles in water can be captured by these precipitates when they are formed. When the precipitate is positively charged (Al(OH)3 and Fe(OH)3 are in the neutral and acidic pH range), the existence of anions in the solution can accelerate the precipitation speed, such as silver sulfate ions. In addition, colloidal particles in water can be used as the core of the precipitation of these metal oxides, 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.
The four coagulation mechanisms introduced above are often not isolated phenomena in water treatment, but may often coexist, but in some cases, they can be used to explain the coagulation phenomenon of water. However, the mechanism of coagulation is still developing, and further experiments are needed to get a more complete explanation.
Let's talk about the hydrolysis process of the following aluminum salts:
All metal cations exist in the form of various compounds of trivalent aluminum [Al (Ⅲ)] and trivalent iron [Fe (Ⅲ)] in water, no matter which drug morphology is added. Taking aluminum salt as an example, even if Al (Ⅲ) exists in pure ionic state in aqueous solution, it does not exist in the form of Al3+, but in the form of Al(H2O)63+ and aluminum hydrate complex ions.
When the pH value is
The actual reaction is much more complicated than the above reaction. When pH >: 4, hydroxyl ions increase, and the hydroxyl groups of each ion can be bridged (hydroxyl bridging) to produce polynuclear hydroxyl complexes, that is, polymer polycondensation.
The product [Al2(OH)2(H2O)5]4+ can be further bridged into [Al3(OH)4(H2O) 10]5+ through hydroxyl groups. At the same time, the generated polynuclear polymer will continue to hydrolyze.
Therefore, hydrolysis and polycondensation are carried out alternately, and finally neutral aluminum hydroxide with high polymerization degree is obtained. When the number of free radicals exceeds its solubility, aluminum hydroxide precipitates.
According to the above, in the whole reaction, simple components such as Al3+, Al(OH)2+, Al(OH)3, Al(OH)4- and various polymeric ions such as [(Al(OH) 14]4+, [A17 (OH) 650. Will appear at the same time, they will inevitably play a role in the process of coagulation. The polyvalent cation in * * * has a strong ability to neutralize the negative charge of clay colloidal particles and compress their electric double layer, which promotes coagulation.
When the generated inorganic polymer contains negative ions, it is impossible to destabilize clay colloidal particles through charge neutralization, but mainly through adsorption bridging.
This is the water purification mechanism of PAC.
PAM is a polymer coagulant, and its mechanism of action:
(1) is easily adsorbed on the surface of sediment particles through its hydrogen bond because of its polar gene-amide group. (2) Because it has a long molecular chain, the large-scale long chain has a huge adsorption surface area in water, so the flocculation effect is good, and the long chain can act as a bridge between particles to form flocs of large particles and accelerate sedimentation. (3) With the help of polyacrylamide, double ionization compression may occur in the coagulation process of sludge treated by water purification, which reduces the stability of particle aggregation, and the particles combine under the action of molecular gravity, and the simple anions in the dispersed phase can be replaced by polymer anion groups; (4) There is a chemical interaction between the substances in the polymer and natural water, suspended substances in water or previously added hydrolytic coagulant ions, which may be a complex reaction; (5) Because molecular chains are fixed on the surfaces of different particles, polymeric bridges are formed between solid particles. Polyacrylamide is a polymer compound with active chemical properties. Due to the activity of amide groups in molecular side chains, polymers have obtained many valuable properties. Compared with anionic PAM flocculant, nonionic PAM flocculant has the following characteristics because it has no ionic functional group: (1) Flocculation performance is less affected by water PH value and salt fluctuation; Under moderate or alkaline conditions, its flocculation effect (sedimentation rate) is not as good as that of anionic flocculant, but it is better than anionic flocculant under acidic conditions, and its floc strength is stronger than that of anionic polymer flocculant. The molecular weight of anionic PAM flocculant is usually lower than that of anionic or nonionic polymer, and its clarification performance is mainly obtained by charge neutralization. The function of this kind of flocculant is mainly to flocculate negatively charged colloid, which has the functions of turbidity removal and decoloration, and is suitable for water treatment with high organic colloid content.