The ultimate fate of the coagulant PAC

Polyaluminum chloride coagulation process 1. Coagulation stage: This is the phase in which the chemical solution is injected into the coagulation tank, where it rapidly reacts with the raw water to form fine alum flocs in a very short time; during this stage, the water becomes even more turbid, and intense turbulence in the water flow is required. In beaker experiments, stirring should be carried out at a rapid speed (250–300 revolutions per minute) for 10–30 seconds, generally not exceeding 2 minutes. ⒉Flocculation stage: This is the process during which the floc particles grow and become larger. An appropriate level of turbulence along with sufficient residence time (10–15 minutes) are required; later on, it is possible to observe a large number of floc particles gathering together and sinking slowly, thus forming a layer with a clear surface. In the beaker experiment, stir at 150 revolutions per minute for about 6 minutes, then stir at 60 revolutions per minute for about 4 minutes until a suspension is formed. ⒊Settling stage: This is the process of sedimentation of flocs in a sedimentation tank, during which the water flow must be slow. To improve efficiency, inclined tube (plate) sedimentation tanks are generally used (air flotation is the best method for separating the flocs). A large number of large flocs are blocked by the walls of the inclined tubes (plates) and settle at the bottom of the tank; the water on the upper layer remains clear. The remaining flocs, which are smaller in size and lower in density, slowly descend while continuing to collide with each other and grow larger, such that the residual turbidity remains essentially constant toward the end of this stage. In beaker experiments, it is advisable to stir slowly at 20–30 revolutions per minute for 5 minutes, then let it settle quietly for 10 minutes before measuring the residual turbidity. ⒋Enhancing filtration involves the proper selection of filter media structures and auxiliary filtering agents to improve the removal efficiency of the filter tank; it is an important measure for improving water quality.

This post was last edited by zhaolijun on 2016-3-14 at 15:50. The principle of water purification: compressed double layer. The structure of the colloidal double layer means that the concentration of counterions is highest at the surface of the colloidal particles; as the distance from the particle surface increases, the concentration of counterions decreases, until it eventually equals the ion concentration in the solution. When electrolytes are added to the solution, increasing the ion concentration there, the thickness of the diffusion layer decreases. When two colloidal particles come close to each other, the thickness of the diffusion layer decreases, which leads to a reduction in the ξ potential; as a result, the repulsive force between them diminishes. In other words, the repulsive force between particles in a solution with a high ion concentration is smaller than that in solutions with a low ion concentration. The attraction between colloidal particles is not affected by the composition of the aqueous phase; however, due to diffusion-thinning, the distance between them when they collide decreases, thereby increasing the mutual attraction. It can be seen that the resultant force of repulsion and attraction shifts from being dominated by repulsion to being dominated by attraction (the repulsive potential energy disappears), allowing the colloidal particles to aggregate rapidly. This mechanism can explain the sedimentation phenomena in harbors well; when fresh water mixes with seawater, the salt content increases and the ion concentration rises, which reduces the stability of the colloidal particles carried by the fresh water. As a result, clay and other colloidal particles tend to settle in harbors. According to this mechanism, even when the electrolyte added to the solution exceeds the critical concentration for aggregation by a large margin, no additional excess counterions will enter the diffusion layer, and it is impossible for the colloidal particles to change their charge sign and thereby become stable again. This mechanism uses purely electrostatic phenomena to explain the effect of electrolytes on the destabilization of colloidal particles, but it does not take into account the role of other properties during the destabilization process (such as adsorption); as a result, it is unable to explain some more complex forms of destabilization, such as the situation where excessive amounts of trivalent aluminum salts and iron salts are used as coagulants, leading to a decrease in the coagulation effect or even to the re-stabilization of the particles ； For example, polymers or high-molecular organic substances with the same charge as the Adhesive particles may achieve good coagulation effects; the isoelectric state should yield the best coagulation results, but in practical production, coagulation efficiency is often lowest when the ξ potential is greater than zero. In fact, the destabilization of colloidal particles upon the addition of coagulants in an aqueous solution involves interactions among the colloidal particles and the coagulants, as well as between the colloidal particles and the aqueous solution, and between the coagulants and the aqueous solution; it is a complex phenomenon. Adsorptive electroneutralization: Adsorptive electroneutralization refers to the strong adsorption effect exerted by the particle surface on ions of opposite charge, as well as on colloidal particles or chain-like molecules that carry opposite charges. This adsorption effect neutralizes part of their charge, reducing the electrostatic repulsion, thereby enabling them to approach other particles more easily and adhere to them. At this point, electrostatic attraction is usually the main factor among these forces, but in many cases, other forces become stronger than electrostatic attraction. For example, when using Na+ and dodecylammonium ions (C12H25NH3+) to remove the turbidity caused by a negatively charged silver iodide solution, it was found that monovalent organic amine ions have a much greater ability to destabilize these particles than Na+. An excess of Na+ does not cause the colloidal particles to become stabilized again, but organic amine ions do; when their concentration exceeds a certain level, the particles become stabilized once more, which indicates that the particles have absorbed an excessive amount of counterions, thereby changing their originally negative charge to a positive one. When high amounts of aluminum salts and iron salts are added, re-stabilization occurs as well as a change in charge sign. The above phenomenon can be appropriately explained by the mechanism of adsorptive electroneutralization. Adsorption bridging effect: The mechanism of the adsorption bridging effect refers mainly to the adsorption and bridging of polymer substances with colloidal particles. It can also be understood as two large particles with the same charge being connected to each other by a particle with a different charge. Polymer flocculants have a linear structure; they contain chemical groups that can interact with certain parts of the surface of colloidal particles. When these polymers come into contact with the colloidal particles, these groups undergo specific reactions with the particle surfaces, leading to mutual adsorption. Meanwhile, the remaining parts of the polymer molecules remain in the solution and can adsorb onto other particles that have available sites on their surfaces, thereby enabling the polymers to act as bridges between those particles. If there are few polymer particles, and the extended portion of the aforementioned polymer fails to adhere to a second polymer particle, then this extended portion will eventually be attracted by an existing polymer particle to another location; as a result, the polymer can no longer act as a bridge, while the polymer particles remain in a stable state. When the amount of polymer flocculant added is too high, the surface of the colloidal particles becomes saturated, leading to a re-stabilization phenomenon. For colloidal particles that have been bridged and flocculated, if subjected to intense and prolonged stirring, the bridging polymers may detach from the surface of another particle and roll back onto the surface of the original particle, thereby re-establishing a stable state. The adsorption of polymers on the surface of colloidal particles results from various physicochemical forces, such as van der Waals forces, electrostatic attractions, hydrogen bonds, and coordination bonds, and it depends on the chemical properties of both the polymers and the surface of the colloidal particles. This mechanism can explain the phenomenon that non-ionic or ionically charged polymeric flocculants with the same charge can achieve good flocculation effects. Precipitate trapping mechanism: When metal salts (such as aluminum sulfate or iron chloride) or metal oxides and hydroxides (such as lime) are used as coagulants, and the amount added is sufficient to rapidly precipitate metal hydroxides (such as Al(OH)3, Fe(OH)3, Mg(OH)2) or metal carbonates (such as CaCO3), the colloidal particles in water can be trapped by these precipitates as they are formed. When the precipitate is positively charged (such as Al(OH)3 and Fe(OH)3 at neutral and acidic pH levels), the precipitation rate can be accelerated by the presence of anions in the solution, such as silver sulfate ions. Furthermore, the colloidal particles in water can themselves serve as cores for the formation of these metal oxide precipitates; therefore, the optimal dosage of the coagulant is inversely proportional to the concentration of the substance to be removed – that is, the more colloidal particles there are, the less coagulant needs to be added.

This post was last edited by zhaolijun on 2016-3-14 at 15:51. Treatment of wastewater 1: Wastewater contains colloidal particles (colloidal particles formed by dust, humus, cellulose, etc. in water combined with water), which cannot be removed through natural sedimentation. Certain chemicals (coagulants) must be added to cause the colloidal particles that are difficult to precipitate in water to aggregate and coagulate into larger particles that can then settle. 　　To determine the process parameters of the water flocculation process, such as the type and dosage of flocculants, the pH value of the water, temperature, and the order in which various chemicals are added, simulation experiments are generally conducted. Under conditions of a certain water temperature and appropriate control of stirring intensity and time, experiments were conducted using different flocculants and dosages to adjust the pH value of waters of various colors, in order to observe the flocculation effect. 　　The American Society for Testing and Materials standard ASTM E2035-1980 (revised and confirmed in 1990), \"Flocculation of Water – Method of Test Using a Flocculation Cup\", is an advanced method. In 1997, China adopted the ASTM standard methods on an equivalent basis and issued **standard methods. 　　This method includes three steps: rapid stirring, slow stirring, and static settling. The added animal coagulant is quickly dispersed through rapid stirring and comes into contact with the colloidal particles in water, causing the particles to aggregate and form flocs. Through slow stirring, the microflocs come into further contact with each other and grow into larger particles. After stirring stops, the formed colloidal aggregates settle naturally to the bottom due to gravity. 　　This method is suitable for determining the process parameters of the water flocculation process, including: the type of flocculant, its dosage, the pH value of the water, temperature, as well as the order in which various chemicals are added. 　　By measuring the turbidity and color of water samples in beaker experiments, it is possible to determine the degree of dehydration and aggregation of colloids. 　　2: Process 　　1) The rotation speed of the multi-blade mixer can be adjusted continuously within the range of 20–150 R/min. The impeller blades are made of lightweight, corrosion-resistant material; their dimensions are 60mm*40mm*2mm, and they have a rectangular shape. There should be lighting devices at the base or on the inside of multiple stirrers, through which the formation of flocs can be observed. For multiple mixers and impeller blades, the depth of immersion in water should be 3/4 of that of the beaker. 　　2) Beaker The beakers have the same size and shape, with a capacity of not less than 1500 ml. 　　3. Operating steps: 1) Based on the number of beakers designated for the multi-stirrer, measure 100 ML of water sample for each and place it in the corresponding beakers, then position the beakers properly. Then put the stirrer blades into the water. The axis of the impeller should be offset from the center of the beaker, with at least a 6.4 MM gap between the impeller and the walls of the beaker. Record the temperature at the start of the experiment. 　　2) Put the flocculant into the test tube on the reagent rack. During dosing, dilute the agent in each test tube to 10 ml with water. If the dosage of one of the agents is greater than 10 ml. The other test tubes should also be replenished with water until their volume matches the required amount. When adding the suspension agent, it should be shaken well before addition. 　　3) Turn on the multiple stirrers and stir rapidly at a speed of 120 r/min; add the reagents to each beaker according to the predetermined dosage, then stir for 1 minute. 4) Reduce the speed to 20–40 r/min, at a level that ensures the particles in the beakers remain uniformly suspended. Stir slowly for about 20 minutes. Record the time when the initial flocs are formed. 　　5) After completing the slow stirring, remove the stirrer blades from the water, observe the sedimentation of the flocs, and record the time it takes for most of the flocs to settle. However, under special circumstances, sedimentation is affected by convection; in such cases, the time recorded for sedimentation should be the time when the number of undissolved flocs moving upward and downward is roughly equal. 　　6) After 15 minutes of precipitation, record the thickness of the flocs at the bottom of the beaker. Use a pipette to draw a water sample at the 1/2 level of the clear liquid in the beaker, and determine the burnability, colority, and pH value of the water sample.

Changing the local charge concentration promotes colloidal deposition. So it goes into the water.

What’s explained above is also way too comprehensive! What do others say: Q

It should fall into the sludge along with the precipitates. From the perspective of the adsorption and bridging mechanism, due to its relatively long linear length, PAC can adsorb colloidal particles and form larger flocs~~