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.     

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.     

Novel ferromagnetic nanoparticle composited PACls and their coagulation characteristics

Effects of magnetic nanoparticles on inorganic coagulants and their coagulation performances were studied in the present work. The Fe₃O₄-SiO₂ core-shell particle (FSCSP) and superfine iron (SI), were compounded with polyaluminium chloride of basicity 2.0 (PACl2.0), providing magnetic PACl2.0s (MPACl2.0s). The physiochemical properties of ferromagnetic nanoparticles were investigated using transmission electron microscopy (TEM), the BET method and a zeta potentiometric analyzer. The Al species distributions of the MPACl2.0s and PACl2.0 were examined by liquid ²⁷Al NMR. Jar tests were employed to evaluate the coagulation performances. Floc properties were assessed by use of the electromotive microscope (EM) and small angle laser light scattering (SALLS). The results showed that modified layers of nanoparticles mitigated agglomeration. FSCSP had a larger specific area and pore volume than SI. The addition of ferromagnetic nanoparticles obviously increased the content of Al_un. MPACl2.0s performed better than PACl2.0 in turbidity removal and DOC removal when dosed less than 0.06 mmol/L as Al. Generally, PACl2.0 + FSCSP (50 mg/L) performed best. Large, loose and weak flocs were produced by MPACl2.0s, which were preferred for the magnetic powder recycling. A plausible structure, Al species-nanoparticles cluster, contributing to the unique properties of MPACl2.0 flocs, was proposed.     

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.     

Polyaluminum chloride with high Al30 content as removal agent for arsenic-contaminated well water

Polyaluminum chloride (PACl) is a well-established coagulant in water treatment with high removal efficiency for arsenic. A high content of Al₃₀ nanoclusters in PACl improves the removal efficiency over broader dosage and pH range. In this study we tested PACl with 75% Al₃₀ nanoclusters (PACl_Al30) for the treatment of arsenic-contaminated well water by laboratory batch experiments and field application in the geothermal area of Chalkidiki, Greece, and in the Pannonian Basin, Romania. The treatment efficiency was studied as a function of dosage and the nanoclusters’ protonation degree. Acid-base titration revealed increasing deprotonation of PACl_Al30 from pH 4.7 to the point of zero charge at pH 6.7. The most efficient removal of As(III) and As(V) coincided with optimal aggregation of the Al nanoclusters at pH 7-8, a common pH range for groundwater. The application of PACl_Al30 with an Al_tot concentration of 1-5 mM in laboratory batch experiments successfully lowered dissolved As(V) concentrations from 20 to 230 μg/L to less than 5 μg/L. Field tests confirmed laboratory results, and showed that the WHO threshold value of 10 μg/L was only slightly exceeded (10.8 μg/L) at initial concentrations as high as 2300 μg/L As(V). However, As(III) removal was less efficient (<40%), therefore oxidation will be crucial before coagulation with PACl_Al30. The presence of silica in the well water improved As(III) removal by typically 10%. This study revealed that the Al₃₀ nanoclusters are most efficient for the removal of As(V) from water resources at near-neutral pH.     

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).     

Effect of shear force and solution pH on flocs breakage and re-growth formed by nano-Al13 polymer

The breakage and re-growth of flocs formed by polyaluminum chloride (PAC) and the Al₁₃O₄ (OH)₂₄⁷⁺ (Al₁₃ for short) polymer were comparatively evaluated for the coagulation of humic acid (HA). A series of jar experiments were conducted to investigate the impacts of shear rate and solution pH on flocs breakage and re-aggregation potential. Results indicated that the responses of flocs to the increasing shear force and solution pH depend on the coagulant used. The ability of flocs to resist breakage decreased with the increasing shear rate. For all levels of shear force investigated in this study, the flocs formed by Al₁₃ polymer were weaker than those of PAC, whereas Al₁₃ polymer displayed a better recoverability than PAC. The similar results were obtained when pH of solution was changed. The flocs generated in acidic conditions were stronger and more recoverable than those generated in alkaline conditions no matter which coagulant was used.     

混凝剂中的铝形态对藻类混凝过程的影响

为了考察混凝剂中的铝形态对藻类混凝过程的影响,使用3种具有不同铝形态分布的混凝剂对含藻水进行了混凝试验。结果表明,硫酸铝由于具有较低含量的Alb,电中和能力较差,故需要较大的投量才能去除藻类,形成絮体;含藻水体系中的有机物主要是腐殖酸及富里酸类物质,微生物代谢产物(SMP)在硫酸铝作混凝剂时得到较好的去除,而腐殖酸及富里酸的去除率较低可能是造成硫酸铝混凝效果较差的原因;Alc(Al(30))在混凝中的作用机理主要是吸附架桥作用,可有效去除水体中的有机物,Al₁₃的主要作用机理是电中和作用,可以有效去除水体中的颗粒物;Al₁₃与Al₃₀由于具有形态的稳定性,其混凝过程受pH值的影响较小。絮体强度因子随着pH值的升高先增大后减小,Al₁₃作混凝剂时絮体恢复因子随pH值的升高先增大后减小,而其他两种混凝剂所形成絮体的恢复因子随pH值的升高而增大。     

Comparison of electrocoagulation and chemical coagulation pretreatment for enhanced virus removal using microfiltration membranes

This research studied virus removal by iron electrocoagulation (EC) followed by microfiltration (MF) in water treatment using the MS2 bacteriophage as a tracer virus. In the absence of EC, MF alone achieved less than a 0.5-log removal of MS2 virus, but, as the iron-coagulant dosage increased, the log virus removal increased dramatically. More than 4-log virus removal, as required by the Surface Water Treatment Rule, was achieved with 6-9 mg/L Fe³⁺. The experimental data indicated that at lower iron dosages and pH (<∼8 mg Fe/L and pH 6.3 and 7.3) negatively charged MS2 viruses first adsorbed onto the positively charged iron hydroxide floc particles before being removed by MF. At higher iron dosages and pH (>∼9 mg Fe/L and pH 8.3), virus removal was attributed predominantly to enmeshment and subsequent removal by MF. Additionally, the experimental data showed no obvious influence of ionic strength in the natural water range of 10⁻⁷-10⁻² M on MS2 virus removal by EC-MF. Finally, EC pretreatment significantly outperformed chemical coagulation pretreatment for virus removal. The proposed mechanism for this improved performance by EC is that locally higher iron and virus concentrations and locally lower pH near the anode improved MS2 enmeshment by iron flocs as well as adsorption of MS2 viruses onto the iron floc particles.     

Breakage and re-growth of flocs formed by charge neutralization using alum and polyDADMAC

The formation, breakage and re-growth of flocs were investigated using alum and polyDADMAC to explore the reversibility of floc breakage. There is a significant reversibility of the breakage process, i.e. the broken flocs can re-grow to the size before breakage, when charge neutralization dominates the coagulation mechanism. However, for higher alum dosage, the break-up process displayed a distinct irreversibility. When coagulated in charge neutralization, the re-growth process of alum was nearly the same as that of polyDADMAC. The average size, coagulation rate and fractal dimension of flocs before and after breakage were nearly the same, including alum and polyDADMAC. While at higher alum dosage, the average size, coagulation rate and fractal dimension of flocs after breakage were much lower than that before breakage. Most important is that the number of small flocs after breakage and re-growth was much less than before breakage when charge neutralization dominated the coagulation mechanism. On the contrary, at higher alum dosage, the small flocs, after breakage and re-growth, increased. The fractal dimension of flocs with alum increased as coagulation time increased until a limiting floc size was reached, while for higher alum dosage, it decreased, whether before or after breakage. The determining parameter for floc re-growth is probably not the fractal dimension, but rather the chemical characteristics of the flocs surface.     

Effects of coagulation processes on aldehydes formation in groundwater treated with common oxidative agents

This work is concerned with the effects of coagulation processes with two different coagulants (polyaluminum chloride (PACl) and Al2(SO4)3) on aldehydes formation during oxidation with common oxidants (ozone, chlorine and chlorine dioxide) in a particular groundwater source in Northern Banat region, Yugoslavia. Aldehydes concentrations in coagulated water were lower than in raw water. In contrast, obtained results showed that specific contents of these disinfection byproducts (microg mg(-1) TOC) showed an increase after coagulation processes in a number of samples. Results indicate that the choice of the coagulant-oxidant combination may be important as well as the type of filtration bed, retention time, and filter washing regime in the removal of aldehydes from water.     

Colour and turbidity removal with reusable magnetic particles—I. Use of magnetic cation exchange resins to introduce aluminium ions

Clarification of hard, turbid surface water by Al³⁺ ions introduced on a sulphonic acid ion exchanger is described. If the resin beads are sufficiently fine, they act as nuclei for floc formation and accelerate the subsequent settling of the flocs. Incorporation of magnetic iron oxide in the resin beads enables them to be conveniently separated from the clarified water. Acidification of the resin-containing sludge then results in partial reloading of the resin with Al³⁺ ions. However, the regenerated resin proved ineffective as a coagulant, a result attributed chiefly to hydrolysis of Al³⁺ ions within the resin. The findings of this work are significant for the theory of coagulation and also suggested the possibility of enhancing clarification by adding fine particles with an absorbing surface.     

Comparing polyaluminum chloride and ferric chloride for antimony removal

Antimony has been one of the contaminants required to be regulated, however, only limited information has been collected to date regarding antimony removal by polyaluminium chloride (PACl) and ferric chloride (FC). Accordingly, the possible use of coagulation by PACl or FC for antimony removal was investigated. Jar tests were used to determine the effects of solution pH, coagulant dosage, and pre-chlorination on the removal of various antimony species. Although high-efficiency antimony removal by aluminum coagulation has been expected because antimony is similar to arsenic in that both antimony and arsenic are a kind of metalloid in group V of the periodic chart, this study indicated: (1) removal density (arsenic or antimony removed per mg coagulant) for antimony by PACl was about one forty-fifth as low as observed for As(V); (2) although the removal of both Sb(III) and Sb(V) by coagulation with FC was much higher than that of PACl, a high coagulant dose of 10.5mg of FeL(-1) at optimal pH of 5.0 was still not sufficient to meet the standard antimony level of 2 microg as SbL(-1) for drinking water when around 6 microg as SbL(-1) were initially present. Consequently, investigation of a more appropriate treatment process is necessary to develop economical Sb reduction; (3) although previous studies concluded that As(V) is more effectively removed than As(III), this study showed that the removal of Sb(III) by coagulation with FC was much more pronounced than that of Sb(V); (4) oxidation of Sb(III) with chlorine decreased the ability of FC to remove antimony. Accordingly, natural water containing Sb(III) under anoxic condition should be coagulated without pre-oxidation.     

Size and structure evolution of kaolin-Al(OH)3 flocs in the electroflocculation process: a study using static light scattering

Electroflocculation (EF) is gaining recognition as an alternative process to conventional coagulation/flocculation. The electrical current applied in EF that generates the active coagulant species creates a unique chemical/physical environment in which competing redox reactions occur, primarily water electrolysis. This causes a transient rise in pH, due to cathodic formation of hydroxyl ions, which, in turn, causes a continuous shift in coagulation/flocculation mechanisms throughout the process. This highly impacts the formation of a sweep floc regime that relies on precipitation of metal hydroxide and its growth into floc. The size and structural evolution of kaolin-Al(OH)₃ flocs was examined using static light scattering techniques, in aim of elucidating kinetic aspects of the process. An EF cell was operated in batch mode and comprised of two concentric electrodes - a stainless steel cathode (inner electrode) and an aluminum anode (outer electrode). The cell was run at constant current between 0.042 A and 0.22 A, and analyses performed at pre-determined time intervals. The results demonstrate that EF is able to generate a range of flocs, exhibiting different growth rates and structural characteristics, depending on the conditions of operation. Growth patterns were sigmoidal and a linear correlation between growth rate and current applied was observed. The dependency of growth rate on current can be related to initial pH and aluminum dosing, with a stronger dependency apparent for initial optimal sweep floc regime. All flocs exhibited a fragile nature and undergo compaction and structural fluctuations during growth. This is the first time size and structural evolution of flocs formed in the EF process is reported.     

Shear-induced flocculation: The evolution of floc structure and the shape of the size distribution at steady state

The flocculation of polystyrene particles in a stirred tank was studied at various shear rates (63–129 s⁻¹) and aluminum sulfate, Al₂(SO₄)₃ 16H₂O, flocculant concentrations. The competition between coagulation and fragmentation during shear-induced flocculation determined the equilibrium or steady state particle (floc) structure and size distribution. The evolution of the floc structure with time was monitored by image analysis of digitized floc images. The average floc structure became less open or irregular as the floc size distribution attained steady state as a result of shear-induced breakage/restructuring. At high alum (flocculant) concentrations, the steady state floc size distribution appeared to be self-preserving with respect to shear rate. In contrast, at lower flocculant concentrations, the steady state floc size distribution narrowed with increasing shear rate as the large tail of the distribution was pushed to smaller particle sizes by shear-induced fragmentation.     

Return sludge employed in enhancement of color removal in the integrally industrial wastewater treatment plant

In this study, the recirculation of chemical sludge and integrated sludge were employed for enhancement of color removal in an integrally industrial wastewater treatment plant. The jar test was conducted for simulating chemical coagulation process with different coagulants, aluminum sulfate and polyaluminum chloride (PACl), after different activated sludge systems (with air or high-purity oxygen). The results showed that with sludge recirculation the process of coagulation for the color removal has up to 35% enhancement in comparison with no sludge recirculation. Meanwhile, the color removal enhancement of coagulant aluminum sulfate was approximately 2 to 3-times the color removal enhancement of coagulant PACl at the optimum ratio of return sludge. In coagulation process with influent color of 1220 true color unit (TCU), the optimum dosage of return sludge was 3010mg/L, when coagulant PACl or aluminum sulfate was employed. In coagulation process with influent color of 536 TCU, the optimum dosages of return sludge were 2340 and 4680mg/L using PACl and aluminum sulfate, respectively.     

Interaction between Cryptosporidium oocysts and water treatment coagulants

The electrokinetic properties of gamma-irradiated Cryptosporidium oocysts in the presence of coagulants (ferric chloride and alum) and coagulant aids (DADMAC based cationic polyelectrolytes) have been studied. The zeta potential of the oocysts was unaffected by the addition of ferric chloride at all pH values (3-10) studied. Addition of alum resulted in reversal of the oocysts charge, which suggests that the initial stage in the coagulation process leading to floc formation proceeds via the adsorption of hydrolysed aluminium species. The cationic polyelectrolyte Magnafloc LT35 was adsorbed onto iron flocs at doses of 0.1 mg/L even against an electrostatic barrier. The cationic polyelectrolyte only adsorbed and caused charge reversal at the oocyst surface at around 0.4 mg/L, suggesting a lower affinity for this surface. These results indicate that the oocysts, unlike inorganic colloidal materials such as metal oxides, appear to possess a lower surface density of active or charged sites. The lower density of sites, combined with the rapid precipitation of iron salts, may be responsible for the lack of specific adsorption of either hydroxylated ferric species or primary iron hydroxide particles on the oocysts. Further, this suggests that a process of sweep flocculation, where oocysts are engulfed in flocs during coagulation and floc formation, is the more likely mechanism involved. By comparison, it is likely that the specific interaction of hydrolysed aluminium species with the oocysts surface would result in a stronger link at the oocyst-floc interface and that the flocculation process may initially proceed via charge neutralisation.     

Effects of humic acid on physical and hydrodynamic properties of kaolin flocs by particle image velocimetry

The physical and hydrodynamic properties of kaolin flocs including floc size, strength, regrowth, fractal structure and settling velocity were investigated by in situ particle image velocimetry technique at different humic acid concentration. Jar-test experimental results showed that the adsorbed humic acid had a significant influence on the coagulation process for alum and ferric chloride. Kaolin flocs formed with the ferric chloride were larger and stronger than those for alum at same humic acid concentration. Floc strength and regrowth were estimated by strength factor and recovery factor at different humic acid concentration. It was found that the increased humic acid concentration had a slight influence on the strength of kaolin flocs and resulted in much worse floc regrowth. In addition, the floc regrowth after breakage depended on the shear history and coagulants under investigation. The changes in fractal structure recorded continuously by in situ particle image velocimetry technique during the growth-breakage-regrowth processes provided a supporting information that the kaolin flocs exhibited a multilevel structure. It was proved that the increased humic acid concentration resulted in decrease in mass fractal dimension of kaolin flocs and consequently worse sedimentation performance through free-settling and microbalance techniques.