聚氯化鋁水解物種之混凝行為：膠體去穩定機制及膠羽形成分析

Abstract

在水及廢水處理過程中，聚氯化鋁(PACl)是最常被使用於混凝程序以去穩定顆粒之混凝劑。聚氯化鋁混凝劑之效用取決於水解物種與水中顆粒間之作用。水解的鋁物種如聚合鋁及氫氧化鋁會嚴重地影響膠體顆粒之混凝機制，然後會影響膠羽的形成。因為水解鋁物種會隨pH及總鋁濃度改變，故瞭解各種聚氯化鋁混凝劑之鋁形態分佈及在各pH值與加藥量下之主要水解鋁物種對於PACl之應用相當重要。 首先，以瓶杯試驗及Ferron法評估混凝過程中各種鋁水解物種對高嶺土顆粒去穩定之影響，並利用即時的診斷技術探究膠羽的形成及表面結構。同時，進行膠羽表面之鋁元素組成分析。此外，藉由輕敲式原子力顯微鏡及濕式掃描式電子顯微鏡觀察Al13聚集體與氫氧化鋁之表面結構。 在中性條件下，無論加藥量多寡，PACl-C混凝膠羽之形成主要依賴氫氧化鋁沉澱物。相對的，在鹼性條件下，具有高Al13含量之PACl-E主要以Al13聚集體行電性補釘及電性中和之混凝機制。在鹼性及低加藥量條件下， 高純度聚氯化鋁(PACl-Al13)混凝主要以電性補釘去穩定顆粒；在足夠加藥量下，由於具有電中性之Al13聚集體形成促使顆粒間架橋變成主要的混凝機制。 在沉澱絆除或掃除機制及電性補釘機制下形成之膠羽，隨著加藥量增加，膠羽結構變成較密實，此時膠羽遭遇破碎會增加膠羽之碎形維度。相反地，PACl-Al13混凝之膠羽結構隨著加藥量增加而變鬆散。另一方面，沉澱絆除或掃除機制下形成之膠羽具有粗糙的外觀，而電性補釘及電性中和機制下形成之膠羽具有光滑的表面。PACl-Al13混凝所形成之架橋作用會造成鬆散的結構且毛茸的外觀。此條件下，存在一些由盤繞的Al13所構成之線條狀Al13聚集體與其他碎形結構之Al13 聚集體。 本質上，富含氫氧化鋁的膠羽不具有良好的晶形結構，而含有Al13聚集體膠羽則具有類似Al13的晶形結構。在富含氫氧化鋁的膠羽表面上，有許多無定形結晶之氫氧化鋁沉澱物具有四面體或八面體結構，而膠體狀之氫氧化鋁具有凹陷的表面。在鹼性條件下，Al13聚集體被證實存在於PACl-Al13之混凝膠羽表面。

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.