Coagulation Behavior of Hydrolyzed PACl Species: Colloid Destabilization Mechanisms and Flocs Formation Analysis
|關鍵字:||混凝;聚氯化鋁;鋁十三;原子力顯微鏡;碎形維度;Coagulation;PACl;Al13;Atomic Force Microscope;Fractal Dimension|
Polyaluminum chloride (PACl) is the most frequently used to destabilize particles for coagulation in water or wastewater treatment. Effective coagulation by PACl depends on the interaction between hydrolyzed Al species and particles in water. Hydrolyzed Al species, such as polymeric Al or Al(OH)3, affect significantly coagulation mechanisms of colloidal particles, which thereafter influence the formation of flocs. Since hydrolyzed Al species varies with pH as well as concentration of Al, it is very important to realize the Al speciation of various PACl coagulants, and their predominant hydrolyzed Al species at various pH values and dosages for coagulation in practice. Effects of various hydrolyzed Al species on the destabilization of kaolin particles in coagulation were evaluated by jar test as well as Ferron method. Formation of and structure of flocs were also investigated via an in-situ diagnostic technology. In-situ morphology of the flocs formed after coagulation was viewed through a wet SEM assay, and the Al composition of these flocs were further surveyed by XPS. Moreover, in-situ configuration of the Al13 aggregates as well as Al(OH)3 precipitates were also observed by TM-AFM and WSEM, respectively. The formation of sweep flocs by PACl-C coagulation at neutral pH relied on Al(OH)3 precipitates regardless of the dosage applied. By contrast, the PACl-E containing a high percentage of Al13 caused either electrostatic patch or charge neutralization mechanisms with Al13 aggregates at alkaline pH. For high-purity PACl (PACl-Al13) coagulation, electrostatic patch was responsible for particle destabilization at alkaline pH and low dosage. Interparticle bridging becomes the major mechanism at sufficient dosage due to the formation of Al13 aggregates with nearly zero charge. The structure of flocs formed by enmeshment or sweep flocculation and electrostatic patch becomes more compact with dosage, in which the breakage of flocs increases the fractal dimension of flocs. On the contrary, flocs coagulated by PACl-Al13 become looser with dosage. On the other hand, enmeshment or sweep flocculation caused sweep flocs with a rough and ragged contour, while electrostatic patch or charge neutralization induced flocs with a smooth and glossy surface. PACl-Al13 coagulation induced by interparticle bridging brought the flocs of a looser structure with a fluffy contour. At such condition, some larger linear Al13 aggregates composed of a chain of coiled Al13 and several coiled Al13 aggregates with different dimensions can be observed. Intrinsically, the Al(OH)3-rich flocs do not possess well-formed crystalline structure, while the Al13-aggregate flocs possess a Al13-like crystalline structure. There are multitude of amorphous Al(OH)3 precipitates that involve either tetrahedral AlIV(O)4 or octahedral AlVI(O)6 center on the surface of Al(OH)3-rich flocs, while the colloidal Al(OH)3(s) has a sunken surface. It has been proved that the existence of Al13 aggregates on the surface of flocs coagulated by PACl-Al13 at alkaline pH.
|Appears in Collections:||Thesis|
Files in This Item: