Mechanism of action of polyacrylamide

Table of Contents

Polyacrylamide’s working principles include particle entrapment, double-layer compression, adsorption charge neutralization, and adsorption bridging.

Particle entrapment

When metal salts (such as aluminum sulfate or ferric chloride) or metal oxides and hydroxides (such as lime) are used as coagulants, the colloidal particles in the water can be trapped by the precipitates formed rapidly, such as metal hydroxides [e.g. Al(OH)₃, Fe(OH)₃, Mg(OH)₂] or metal carbonates (e.g. CaCO₃). When the precipitates carry a positive charge (e.g., Al(OH)₃ and Fe(OH)₃ in the neutral and acidic pH range), the precipitation rate can be accelerated in the presence of anions, such as silver sulfate ions. Moreover, the colloidal particles in the water can act as the core for the formation of these metal oxide precipitates, so the optimal coagulant dosage is inversely proportional to the concentration of the removed substances, i.e., the more colloids, the less metal coagulant dosage required.

Compression of the double layer

The structure of the double layer of a colloid is such that the concentration of counter-ions reaches its maximum at the surface of the colloid. As the distance from the surface of the colloid increases, the concentration of counter-ions decreases, eventually equaling the ion concentration in the solution. When an electrolyte is added to the solution, increasing the ion concentration in the solution, the thickness of the diffusion layer decreases.

When two colloidal particles approach each other, the thickness of the diffusion layer decreases and the potential drops, reducing their mutual repulsion. Thus, the intercolloidal repulsion in a high-ion-concentration solution is smaller than in a low-ion-concentration solution. The attractive forces between colloidal particles are not affected by the composition of the aqueous phase, but as the diffusion layer becomes thinner, the distance at which they collide decreases, increasing their mutual attraction. The net effect of repulsion and attraction changes from being repulsion-dominated to attraction-dominated (the repulsive potential energy disappears), and the colloidal particles quickly coagulate.

This mechanism can explain the sedimentation phenomenon at the estuaries, as the increase in salt content and ion concentration when freshwater enters seawater reduces the stability of colloidal particles carried by freshwater, making clay and other colloidal particles more likely to settle at the estuaries.

According to this mechanism, when the amount of electrolyte added to the polyacrylamide solution exceeds the critical coagulation concentration by a large amount, no more excess counter-ions can enter the diffusion layer, and it is impossible for the colloidal particles to change their charge and re-stabilize. This mechanism, which is a purely electrostatic phenomenon, explains the effect of electrolytes on the destabilization of colloidal particles but does not consider the role of other properties in the destabilization process (such as adsorption), so it cannot explain other complex destabilization phenomena.

Adsorption charge neutralization

Adsorption charge neutralization refers to the strong adsorption effect of colloidal particle surfaces on counter-ions, counter-charged colloidal particles, or chain ions with counter-charged parts. This adsorption neutralizes part of the charge, reducing the electrostatic repulsion, making it easier for particles to approach and adsorb each other. At this time, electrostatic attraction is often the main aspect of these effects, but in most cases, other effects exceed electrostatic attraction. For example, Na⁺ and dodecylammonium ions to remove the turbidity of a negatively charged silver iodide solution, it is found that the destabilization ability of monovalent organic amine ions is much greater than that of Na⁺. Excessive addition of Na⁺ will not cause the colloidal particles to re-stabilize, while the organic amine ions can cause re-stabilization when added beyond a certain amount. This indicates that the colloidal particles have adsorbed too many counter-ions, causing the original negative charge to change to a positive charge. Aluminum salts and iron salts can also cause re-stabilization and charge reversal when added at high concentrations. The above phenomena can be appropriately explained by the mechanism of adsorption charge neutralization.

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Adsorption bridging action

The adsorption bridging action mechanism mainly refers to the adsorption and bridging of high molecular weight substances with colloidal particles. It can also be understood as the connection between two large similarly charged colloidal particles due to the presence of a counter-charged colloidal particle in between. High molecular weight flocculants have a linear structure and contain chemical groups that can interact with certain parts of the colloidal particle surface. When the polymer comes into contact with the colloidal particles, the groups can undergo special reactions with the colloidal particle surface and adsorb each other, while the remaining part of the polymer molecule extends into the solution and can adsorb another colloidal particle with an available surface site. In this way, the polymer acts as a bridging connection.

If there are few colloidal particles, the extended part of the polymer mentioned above may not be able to adhere to a second colloidal particle, and this extended part will eventually be adsorbed onto other parts of the original colloidal particle. In this case, the polymer cannot act as a bridge, and the colloidal particles remain stable. When the amount of high molecular weight flocculant is too large, it can saturate the surface of the colloidal particles, causing a re-stabilization phenomenon. Colloidal particles that have been bridged and flocculated, if subjected to intense and prolonged stirring, may have the bridging polymer detach from the surface of one colloidal particle and coil back to the surface of the original colloidal particle, causing a re-stabilized state.

The adsorption of polymers on the surface of colloidal particles is derived from various physicochemical interactions, such as van der Waals forces, electrostatic forces, hydrogen bonds, and coordination bonds, depending on the chemical structural features of the polymer and the surface of the colloidal particles. This mechanism can explain the phenomenon that non-ionic or similarly charged ionic high molecular weight flocculants can achieve good flocculation effects.

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