Question

Which of the following best describes the primary mechanism by which alum functions as a coagulant?

Accepted Answer

Alum, typically aluminum sulfate (Al₂(SO₄)₃·14H₂O), functions as a coagulant primarily through the formation of aluminum hydroxide [Al(OH)₃] precipitates. Water often contains suspended particles, including clay, silt, organic matter, and microorganisms. These particles are typically negatively charged and remain dispersed throughout the water due to electrostatic repulsion – like magnets with the same poles, they push each other away, preventing them from clumping together. Alum, when added to water, initially dissociates, releasing aluminum ions (Al³⁺) into the solution. These aluminum ions are positively charged. The aluminum ions then undergo a series of hydrolysis reactions. Hydrolysis is the reaction of a chemical species with water. In this case, the aluminum ions react with water molecules, progressively forming various aluminum hydroxide species, such as Al(OH)²⁺, Al(OH)₂⁺, and ultimately, insoluble aluminum hydroxide [Al(OH)₃]. The aluminum hydroxide precipitates are gelatinous and have a large surface area. These precipitates effectively neutralize the negative charges on the suspended particles. By neutralizing the charges, the electrostatic repulsion between the particles is reduced or eliminated. Simultaneously, the large surface area of the aluminum hydroxide allows it to &#x27;sweep up&#x27; and physically entrap the suspended particles. This process is called adsorption – the adhesion of atoms, ions, or molecules from a gas, liquid, or dissolved solid to a surface. The aluminum hydroxide, now laden with trapped particles, forms larger, heavier clumps called &#x27;flocs&#x27;. These flocs are dense enough to settle out of the water due to gravity, or can be easily removed by filtration, thereby clarifying the water. The pH of the water is crucial; alum is most effective within a specific pH range (typically 6.5 to 8.0). Outside this range, the aluminum hydroxide may not form properly or may redissolve, reducing its effectiveness. For example, at very low pH, the aluminum ions may remain dissolved and not precipitate. At very high pH, the aluminum hydroxide may form a very fine precipitate that is difficult to settle or filter.

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Which of the following best describes the primary mechanism by which alum functions as a coagulant?

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