Introduction to the Mechanism of Aluminum Salt Flocculation Treatment for Fluorine-Containing Wastewater

The mechanism of aluminum salt flocculation for removing fluoride ions is relatively complex, mainly consisting of three mechanisms: adsorption, ion exchange, and complexation sedimentation. Below, Jiangsu Haipu Functional Materials, a manufacturer of fluorine-containing wastewater treatment, will introduce these three mechanisms of action in detail as follows:

(1) Adsorption. The process of aluminum salt flocculation and precipitation for fluoride removal is electrostatic adsorption, and the most direct evidence is that AC or PAC fluorine-containing flocs adsorb charged fluoride ions, partially neutralizing positive charges. Under the same pH conditions, their zeta potential is lower than that of their own flocs. Another evidence is that when the concentration of anions such as SO42- and Cl - in water is high, competition can significantly reduce the adsorption capacity of Al (OH) 3 alum flowers formed during the flocculation process for fluoride ions.

(2) Ion exchange. The radius and charge of fluoride ions and hydroxide ions are similar. In the process of aluminum salt flocculation for fluoride removal, polyhydroxyl cations such as Al13O4 (OH) 147+added to water and their hydrolysis form an amorphous Al (OH) 3 (am) precipitate, in which OH - and F - exchange. This exchange process is carried out under equipotential conditions, and the charge carried by the flocs remains unchanged after exchange. The zeta potential of the flocs will not increase or decrease as a result, but the OH - released during this process will raise the pH of the system, indicating that ion exchange is also an important mode of action for aluminum salt fluoride removal.

(3) Complex precipitation. F - can form six types of complexes with Al3+, ranging from AlF2+, AlF2+, AlF3 to AlF63-. The calculation of solution chemical equilibrium shows that in the aluminum salt coagulation and defluorination system with F - concentration of 1 × 10-4~1 × 10-2mol/L, at pH 5-6, it mainly exists in the forms of AlF2+, AlF3, AlF4- and AlF52-. These aluminum fluoride complex ions will form aluminum fluoride complexes (AlFx (OH) (3-x) and Na (x-3) AlFx) or be mixed in the newly formed Al (OH) 3 (am) flocs during the flocculation process, and settle down. The IR and XPS of the flocs The observed aluminum fluoride complex ion AlFx (3-x)+in the spectrum is partly the result of chelation precipitation, while the other part may be the product of ion exchange.

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