Coagulation dynamics of fractal flocs induced by enmeshment and electrostatic patch mechanisms

The size and structure of flocs during floc formation were monitored for various coagulation mechanisms. Two distinctive mechanisms, namely, enmeshment and electrostatic patch, govern the dynamics of kaolin particles coagulation by polyaluminum chloride (PACl). They were investigated by small angle static light scattering (SASLS) and solid-state ²⁷Al NMR. In addition, a novel wet SEM (WSEM) was used in-situ to image the morphology of the aggregate in aqueous solution. Synthetic suspended particles were coagulated by two PACl products, a commercial product (PACl) and one laboratory product (PACl-E). The PACl-E contained more than 60% Al₁₃ while the PACl contained only 7% Al₁₃, with large percentage of colloidal Al. For coagulation by PACl at neutral pH and high dosage where the strong repulsion between particles occurs, the enmeshment ruled by reaction-limited aggregation (RLA) results in larger sweep flocs as well as higher fractal dimensional structure. For coagulation by PACl-E at alkaline pH and low dosage, the flocs were coagulated predominately by electrostatic patch with Al₁₃ aggregates. At such condition, it is likely that diffusion-limited aggregation (DLA) predominately rule PACl-E coagulation. The fractal dimension (D_s) values of PACl and PACl-E flocs formed at enmeshment and electrostatic patch increased with dosage, respectively. When breakage of flocs occurs, the breakage rate of PACl-E flocs is slower than that of sweep flocs. By WSEM imaging, the adsorption of spherical Al precipitates onto the particles was observed to form sweep flocs with a rough and ragged contour, while the PACl-E flocs were formed with a smooth and glossy structure.     

Preparation and characterisation of new-polyaluminum chloride-chitosan composite coagulant

In this study, the formulation of a novel polyaluminum chloride-chitosan composite coagulant that improves the coagulation process for natural organic matter (NOM) removal was investigated. The performance of the composite coagulant was tested using two water sources (synthetic and natural water) to develop a better understanding on the behaviour of the composite coagulant. Fourier Transform-Infra red (FT-IR) spectroscopy, ferron analysis and zeta potential studies were performed to characterise the composite coagulant. FT-IR analysis showed that there is an intermolecular interaction between Al species and chitosan molecules, while ferron analysis indicated that the distributions of Al_a, Al_b, and Al_c in PACl-chitosan are different from those in PACl. At a low Al dosage (2.16 mg L⁻¹), a much higher removal of NOM from synthetic water, as evidenced from UV₂₅₄ and Dissolved Organic Carbon (DOC) measurements, was achieved by the composite coagulants in comparison to that removed by PACl or PACl and chitosan added separately. For natural water from the Myponga Reservoir, both polyaluminum chloride (PACl) and PACl-chitosan composite coagulants demonstrated similar dissolved organic carbon (DOC) percentage removal, whereas PACl-chitosan gave a slight improvement in removing the UV₂₅₄ absorbing components of NOM.     

铝的水溶液化学特征及其聚合物生成机制

聚合氯化铝（PACl）是水处理中应用较广泛的絮凝剂。研究表明,PACl中各种铝聚合物的含量直接影响其絮凝效率。本文综述铝的水解聚合特性以及制备絮凝剂过程中的铝聚合物生成机制,着重介绍了Al13聚合物的结构特性、生成机制以及制备过程中各种因素的影响,为Al13絮凝剂的进一步研究奠定了基础。     

The origin of Al(OH)3-rich and Al13-aggregate flocs composition in PACl coagulation

The composition of hydrolyzed Al species is essential for the understanding of coagulation with Al-based coagulants. Surface characteristics of flocs formed by coagulation with two distinct polyaluminum chloride (PACl) coagulants were identified. One commercial coagulant (PACl-C) with voluminous monomeric Al and colloidal Al(OH)₃ and a custom-made PACl (PACl-Al₁₃) containing high Al₁₃ content were applied to destabilize kaolin particles. The flocs formed by PACl-C and PACl-Al₁₃ at neutral and alkaline pH ranges, respectively, were observed by FE-SEM and HR-TEM. In addition, the Al composition of these flocs was characterized by XPS and HR-XRD, and the imaging of Al(OH)₃ precipitates and Al₁₃ aggregates were conducted by SEM as well as tapping mode AFM in liquid system. The observations of flocs indicate that the morphology of Al(OH)₃-rich flocs are fluffy and porous around the edge of flocs, while the Al₁₃-aggregate flocs have a glossy contour and irregular structure. Both Al(OH)₃-rich and Al₁₃-aggregate flocs do not possess well-formed crystalline structure except for the Al₁₃-like crystal exists in the Al₁₃-aggregate flocs. Among Al(OH)₃ precipitates, colloidal Al(OH)₃ is micro-scale in size, while amorphous Al(OH)₃ is nano-scale. During the formation of Al₁₃ aggregates, some coiled and clustered Al₁₃ aggregates with smoother surface were observed. The XPS study on floc surface showed that tetrahedral (Al^IV) /octahedral (Al^VI) Al ratio on the surfaces of PACl-C and PACl-Al₁₃ flocs is 1:1.6 and 1:9.9, respectively. Of the in situ formed Al₁₃, almost half of Al-hydroxide precipitates on the surface of Al(OH)₃-rich flocs possess the Al^IV center. It also found that the irregularly aggregated Al₁₃ with a similar Al₁₃ crystalline structure subsists on the surface of Al₁₃-aggregate flocs.     

Breakage and re-growth of flocs: effect of additional doses of coagulant species

Several polyaluminum chloride (PACl) coagulants were prepared, with different OH/Al ratios (B values), and characterized by Ferron assay. These were used in studies of floc formation, breakage and re-growth with kaolin suspensions under controlled shear conditions, using a continuous optical monitoring method. Particular attention was paid to the effect of small additional coagulant dosages, added during the floc breakage period, on the re-growth of broken flocs. The results showed that the re-growth ability was greatly dependent on the nature of the PACl species added as second coagulant. The re-growth ability of broken flocs was greatest when the second coagulant was PACl₀ (i.e. AlCl₃, with B = 0) and least with PACl₂₅ (B = 2.5). In the latter case there was no effect on floc re-growth, irrespective of the initial coagulant used. PACls with intermediate B values gave some improvement in floc re-growth, but less than that with PACl₀. Additional dosage of PACl₀ gave re-grown flocs about the same size or even larger than those before breakage. The re-growth of broken flocs is significantly correlated with the species Al_a (monomeric) and Al_b (polymeric), as determined by Ferron assay. The amorphous hydroxide precipitate formed from PACl₀, (mainly Al_a) can greatly improve the adhesion between broken flocs and give complete re-growth. However, for PACl₂₅, mostly composed of Al_b, the nature of the precipitate is different and there is no effect on floc re-growth.     

Floc morphology and cyclic shearing recovery: comparison of alum and polyaluminum chloride coagulants

This study investigated the floc formation and re-aggregation potential for alum, polyaluminum chloride (PACl) and a blend of these coagulants. Bench-scale testing included floc morphology characterization for well-developed floc, post-shear floc, and non-settleable (filter influent) floc. Different applications of coagulants were observed to produce non-settleable floc that was morphologically different. The alum treatment had a decrease in average floc size from coagulated to non-settleable conditions, whereas the PACl and PACl/alum treatments resulted in similar sized floc between these processes. Zeta potential distribution measurements showed that the alum treatment resulted in a negative shift from coagulated to non-settleable conditions whereas the PACl and PACl/alum treatments had no significant shift. A photometric dispersion analyzer (PDA) was employed to compare the differences between coagulant treatments with respect to shear induced aggregate breakup and recovery. The PDA allowed a dynamic monitoring of initial floc aggregation and measured the degree of recovery from cyclic shearing. The degree of recovery from shearing was greatest for the PACl/alum treatment likely as a result of increased collision efficiency due to more effective charge neutralization.     

Comparison of Efficiency between Poly‐aluminium Chloride and Aluminium Sulphate Coagulants during Full‐scale Experiments in a Drinking Water Treatment Plant

Abstract

The efficiency of poly‐aluminium chloride (PACl) and of aluminium sulphate (alum), two commonly applied coagulant agents, was studied comparatively in this work, during full‐scale experiments in a drinking water treatment plant. The removal of suspended solids (turbidity) and the residual aluminium concentrations were carefully monitored and they were used for the evaluation of effectiveness for each coagulant, as well as for the determination of optimal operative conditions. Two alternative treatment processes were examined:

• (a) the conventional coagulation‐flocculation‐sedimentation process, followed by gravity filtration through sand filter beds, and

• (b) the direct filtration process, i.e., coagulation‐flocculation and sand filtration, but without the intermediate sedimentation step.

PACl proved to be a more efficient coagulant than alum, as lower dosages of PACl, about 1.35 mg Al/L in this case, resulted to the production of treated water with low turbidity and residual aluminium content. In addition, the direct filtration process through dual sand‐anthracite filter beds was found to be equally sufficient, as the conventional one, i.e., when applying the sedimentation step; in this case, 0.70 mg Al/L of PACl resulted in low turbidity water (around 0.1 NTU) and residual aluminium content (lower than 150 µg/L). In addition, the operation time of filters was extended to more than 24 hours.     

Mechanism of natural organic matter removal by polyaluminum chloride: effect of coagulant particle size and hydrolysis kinetics

The mechanism of natural organic matter (NOM) removal by AlCl(3) and polyaluminum chloride (PACl) was investigated through bench-scale tests. The fraction distributions of NOM and residual Al after coagulation in solution, colloid and sediment were analyzed as changes of coagulant dosage and pH. The influence of NOM, coagulant dose and pH on coagulation kinetics of AlCl(3) was investigated using photometric dispersion analyzer compared with PACl. Monomeric Al species (Al(a)) shows high ability to satisfy some unsaturated coordinate bonds of NOM to facilitate particle and NOM removal, while most of the flocs formed by Al(a) are small and difficult to settle. Medium polymerized Al species (Al(b)) can destabilize particle and NOM efficiently, while some flocs formed by Al(b) are not large and not easy to precipitate as compared to those formed by colloidal or solid Al species (Al(c)). Thus, Al(c) could adsorb and remove NOM efficiently. The removal of contaminant by species of Al(a), Al(b) and Al(c) follows mechanisms of complexation, neutralization and adsorption, respectively. Unlike preformed Al(b) in PACl, in-situ-formed Al(b) can remove NOM and particle more efficiently via the mechanism of further hydrolysis and transfer into Al(c) during coagulation. While the presence of NOM would reduce Al(b) formed in-situ due to the complexation of NOM and Al(a).     

Coagulation behavior of Al(13) aggregates

The coagulation behavior of Al₁₃ aggregates formed in coagulation of kaolin was investigated by small angle static light scattering (SASLS), solid-state ²⁷Al NMR and tapping mode atomic force microscope (TM-AFM). A kaolin suspension was coagulated by PACl containing high content of Al₁₃ polycation (PACl-Al₁₃). The results indicated that Al₁₃ was predominant in destabilizing kaolin particles for PACl-Al₁₃ coagulation even though at alkaline pH (pH 10). At such high pH, Al₁₃ aggregates were observed when the dosage of PACl-Al₁₃ was increased. In addition, the mechanism of coagulation by PACl-Al₁₃ at alkaline pH was affected by dosage. When the dosage was insufficient, coagulation was caused by electrostatic patch, which led to compact flocs with high fractal dimension (D_f). Interparticle bridging dominated the coagulation when the coagulant dosage approached the plateau of adsorption, which caused the looser flocs with low D_f. The in-situ AFM scanning in liquid system proved that the existence of linear Al₁₃ aggregates composed of a chain of coiled Al₁₃ in coagulation by PACl-Al₁₃ at a high dosage and alkaline pH. Meanwhile, several coiled Al₁₃ aggregates with various dimensions were observed at such condition.