Why can polyaluminum chloride be used for defluoridation

2025-03-07

In the context of stricter environmental regulations, sewage treatment plants need to achieve standard discharge through technological upgrading and management optimization. As a core water treatment agent, the rational selection and application optimization of polyaluminum chloride (PAC) is key. The following are solutions based on the latest policies and industry practices:

1. Chemical adsorption and precipitation interaction

Aluminum ion release

After PAC solves in water, it quickly dissociates the highly charged aluminum ions (Al³), which are combined with fluoride ion (F) by electrostatic gravity to first form hydrofluoric acid (HF) intermediate, and then further form insoluble aluminum fluoride (AlF ₃) precipitation and precipitation.

response equation:

Al³⁺+3F⁻→AlF₃↓Al³⁺+3F⁻→AlF₃
complexation
Aluminum ions and fluoride ions can also form a variety of stable complexes (such as AlFnAlFn^ {(3-n)}) to reduce the concentration of free fluoride in water.

2.Physical coprecipitation effect

The aluminum hydroxide (Al (OH) 3Al (OH) 3) produced by PAC hydrolysis has a huge specific surface area and positive charge, reinforced by the following methods:

Surface adsorption: the colloids directly adsorb fluorine ions through electrostatic interaction.

Net catching and wrapping: the flocs formed during the flocculation process wrap the fluorine ions in the interior, and finally removed by solid-liquid separation.

3. Key influencing factors in practical applications

Amount: it should be dynamically adjusted according to the concentration of raw water fluorine. Generally, 11-13 kg PAC (Al ₂ O ₃ 30%) should be added per thousand tons of water.

Temperature: when the reactivity decreases below 20℃, the efficiency should be maintained by heating or insulation measures.

Water quality conditions: In the high turbidity water body, PAC can improve the fluoride removal effect by simultaneous removal of suspended matter.

4. Comparison of technical advantages

Compared with traditional aluminum salts (such as aluminum sulfate), PAC has a higher charge density and faster flocculation speed, and the fluorine removal efficiency can be improved by 30% with the same addition amount 17. In typical applications, PAC reduces the fluoride concentration from 14.6 mg/L to 0.4-1.0 mg / L with a removal rate of 33% -75%.