The core function of aluminum sulfate in water treatment is flocculation and precipitation, that is, suspended impurities, colloidal particles, and some pollutants in the water are gathered into "flocs" through chemical action, and then separated through precipitation or filtration, ultimately achieving water purification. This process can be divided into three key stages. The reaction mechanism and operating points of each stage directly affect the treatment effect.
1. Hydrolysis stage: "form transformation" of aluminum ions
When aluminum sulfate is put into water, the Al³⁺ produced by its dissolution will quickly undergo a hydrolysis reaction with water molecules. Since Al³⁺ is a typical "high-valent metal ion", it has a strong attraction to water molecules and will gradually seize the hydroxyl groups (-OH) in water molecules to generate a series of hydroxyl aluminum ions, such as Al (OH)₂⁺, Al (OH)₂⁺, Al₂(OH)₄²⁺, etc.; as the reaction progresses, these hydroxyl aluminum ions will further polymerize to form aluminum hydroxide (Al (OH)₃) colloid. The key to this stage is to control the pH value of the water - when the pH value is between 6.0-7.5 (close to neutral), the hydrolysis reaction of Al³⁺ is the most complete, the production of aluminum hydroxide colloid is the largest, and the stability of the colloid is optimal, providing sufficient "core particles" for the subsequent flocculation process.
2. Adsorption stage: "capture and aggregation" of colloidal particles
Aluminum hydroxide colloid has a huge specific surface area (the surface area of each gram of colloid can reach hundreds of square meters) and strong "positive electricity" - while suspended impurities (such as sediment, clay particles) and colloidal pollutants (such as organic matter, microorganisms) in the water are mostly negatively charged. According to the principle of "opposite charges attract", aluminum hydroxide colloids will actively absorb negatively charged particles in the water and "wrap" them on their own surfaces; at the same time, the colloidal particles will also attract each other through "van der Waals forces", gradually forming smaller "microflocs". In this stage, the stirring intensity is the key influencing factor: moderate stirring (stirring speed is about 100-200 rpm) can fully contact the aluminum hydroxide colloid with the impurity particles to avoid "insufficient adsorption" caused by excessive local concentration. However, excessive stirring will destroy the formed microflocs and reduce the adsorption effect.
3. Sedimentation stage: "Settlement and separation" of floc
As the microflocs continue to gather, their volume and weight gradually increase, eventually forming flocs visible to the naked eye (commonly known as "alum flowers"). At this time, reduce the stirring speed (or stop stirring) and allow the water body to enter the "static sedimentation" stage. The flocs will slowly sink to the bottom of the water body under the action of gravity, forming a "sludge layer"; the upper layer will become a clear water body, and then filter (such as quartz sand filtration, activated carbon filtration) to remove the remaining tiny flocs, and you can obtain standard water quality. At this stage, the "density" and "settling speed" of the floc are the core indicators - high-quality floc should have a tight structure and not easily break. The settling speed can usually reach 0.5-1.0 meters/hour, which can achieve solid-liquid separation in a short time and improve water treatment efficiency.