Fluoride containing wastewater treatment - flocculation sedimentation methodology

Fluoride containing wastewater treatment - flocculation sedimentation methodology
The predominantly employed flocculant for the flocculation precipitation methodology of fluoride ion wastewater is aluminum salt. Post-addition of aluminum salt to water, the coordination between Al3+and F -, in addition to the intermediary products of aluminum salt hydrolysis and the ultimate produced Al (OH) 3 (am), are harnessed to extract fluoride ions from water via ligand exchange, physical adsorption, and sweeping of alum flowers. Compared with the calcium salt precipitation methodology, the aluminum salt flocculation precipitation methodology boasts the advantages of diminished chemical dosage, enhanced processing capacity, and compliance with national emission standards post one treatment. Aluminum salts such as aluminum sulfate and polyaluminum exhibit commendable coagulation removal effects on fluoride ions.
When utilizing aluminum salts, the optimal pH for coagulation is 6.4-7.2, albeit the dosage is substantial. Depending on the scenario, 150-1000 g per m3 of water needs to be incorporated, which will yield a certain quantity of dissolved aluminum in the water that is detrimental to human health. Following the application of polyaluminum, the dosage can be reduced by approximately half, and the pH range of flocculation precipitation can be broadened to 5-8. The defluorination efficacy of polyaluminum is linked to its intrinsic properties. Polyaluminum with an alkalinity of 75% is optimal for defluorination, and the optimal dosage is when the molar ratio of F to Al in water is approximately 0.7.
The aluminum salt flocculation precipitation methodology also presents notable disadvantages, such as a narrow scope of application, high fluoride content, overuse of coagulants, elevated treatment costs, and a substantial amount of sludge generated; The removal efficiency of fluoride ions is considerably influenced by operational factors such as stirring conditions and settling time, as well as anions such as SO42- and Cl - in the water. The effluent quality is not sufficiently stable, which is linked to the inadequate comprehension of the coagulation mechanism for fluoride removal. Investigating the coagulation mechanism for fluoride removal holds considerable practical significance.
The mechanism of aluminum salt flocculation for extracting fluoride ions is relatively intricate, primarily comprising three mechanisms: adsorption, ion exchange, and complexation sedimentation.
(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-bearing flocs adsorb charged fluoride ions, partially neutralizing positive charges. Under identical 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 diminish 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 comparable. In the course 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 - interchange. This exchange process is executed under equipotential conditions, and the charge borne by the flocs remains unaltered post-exchange. The zeta potential of the flocs will not augment or diminish as a result, but the OH - liberated during this process will elevate the pH of the system, indicating that ion exchange is also a pivotal 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 indicates that in the aluminum salt coagulation and defluorination system with F - concentration of 1 × 10-4~1 × 10-2 mol/L, at pH 5-6, it predominantly 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 interspersed in the newly formed Al (OH) 3 (am) flocs during the flocculation process, and settle down. The IR and... The observed aluminum fluoride complex ion AlFx (3-x)+in the XPS spectrum is partly the consequence of chelation precipitation, while the other part may be the product of ion exchange.