A comparative study on the characteristics and coagulation mechanism of PAC-Al₁₃ and PAC-Al₃₀

Abstract

Polyaluminum chlorides with a high Al₁₃ content (PAC-Al₁₃) and a high Al₃₀ content (PAC-Al₃₀) were prepared in a laboratory. The crystalline morphology, SEM images, XRD patterns, aluminum species distribution, and charge neutralization capability of PAC-Al₁₃ and PAC-Al₃₀ with a basic ratio (B = [OH⁻]/[Al³⁺]) of 2.4 were investigated comparatively. The branching morphology and prismatic crystal images of PAC-Al₁₃ and PAC-Al₃₀ were obtained firstly by a simple concentration and crystallization method, and interestingly it was found that these branches appeared to be growing along the diagonal of the tetragonal crystals. The aluminum hydrolytic species distributions of PAC-Al₁₃ and PAC-Al₃₀ were quite different: PAC-Al₁₃ mostly contained the medium polymeric species Al_b while the high polymeric species Al_c was the main ingredient in PAC-Al₃₀. PAC-Al₁₃ and PAC-Al₃₀ exhibited almost the same charge neutralization capability throughout the investigated dosage range, which demonstrated that the coagulation differences of PAC-Al₁₃ and PAC-Al₃₀ investigated in our previous study were probably caused by the stronger bridge-aggregation and sweep-flocculation actions of PAC-Al₃₀ in the coagulation process. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were also used to characterize the powder PAC-Al₁₃ and PAC-Al₃₀ samples. Moreover, the monomolecular aluminum salt of AlCl₃ was investigated in this study as a control.

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1. Introduction

Polymeric aluminum salts are a series of aluminum hydroxyl complexes produced through the hydrolysis, polymerization and precipitation reactions of monomolecular aluminum salts. There are many hydrolyzate forms in hydrolytic aluminum solutions that have been identified by scientists, like Al(OH)₂⁺, Al(OH)²⁺, [A1₆(OH)₁₅]³⁺, [A1₇(OH)₁₇]⁴⁺, [AlO₄Al₁₂(OH)₂₄(H₂O)₁₂]⁷⁺, [Al₁₆(OH)₃₈]¹⁰⁺, [A1₃₀O₈(OH)₅₆(H₂O)₂₄]¹⁸⁺, [A1₉(OH)_n]^(27−n)+ etc.^1–3 Although polyaluminum chloride (PAC) salts have been widely used as coagulants in water treatment recently, researchers have not fully understood the characteristics of all of the hydrolyzed forms of PAC, especially for the medium polymeric species Al_b, and high polymeric species Al_c. It is generally believed that [AlO₄Al₁₂(OH)₂₄(H₂O)₁₂]⁷⁺ (Al₁₃), one of the Al_b polymeric species, is the most effective coagulation species of the polyaluminum coagulants.^4–6 Since the discovery of [A1₃₀O₈(OH)₅₆(H₂O)₂₄]¹⁸⁺ (Al₃₀ being one of the Al_c species) in 2000,^7,8 the coagulation behavior of Al₃₀ has been researched. Chen et al.⁹ found that Al₃₀ is another highly active coagulation/flocculation species which can be used for turbidity removal, and Mertens et al.¹⁰ revealed that Al₃₀ nanoclusters efficiently remove As(V) from water resources. Both Al₁₃ and Al₃₀ possess a strong charge neutralization capability, a high structural stability and a nanoscale molecular size, which would allow them to exhibit a perfect coagulation action.

In addition to their coagulation behavior, the structural models and conversion modes of Al₁₃ and A1₃₀ are also hot topics for scientists. It has been identified by nuclear magnetic resonance (NMR) and X-ray diffraction (XRD) that Al₁₃ is a Keggin polycation, which possesses an AlO₄ tetrahedron as the core and is surrounded by twelve corner- and edge-sharing AlO₆ octahedra; Al₃₀ consists of two δ-Al₁₃ Keggin units linked by four AlO₆ octahedra.¹¹ Allouche et al.¹² demonstrated that aluminum monomers were the species controlling the conversion of Al₁₃ ε-Keggin ions into Al₃₀. Chen et al. ^13,14 investigated the effect of thermal treatment and the total aluminum concentration on the formation and transformation of Keggin Al₁₃ and Al₃₀ species in hydrolytic polymeric aluminum solutions. Chen et al.¹⁵ also studied the hydrolysis/precipitation behavior of Keggin Al₁₃ and Al₃₀ polymers in polyaluminum solutions.

Although the coagulation behaviors, structural characteristics and conversion mode of Al₁₃ and Al₃₀ have been a research hotspot in recent years, the physical and chemical characteristics and coagulation mechanism differences of Al₁₃ and A1₃₀ are not well understood nowadays. In a previous study, we compared the coagulation behaviors of AlCl₃, polyaluminum chlorides with a high Al₁₃ content (PAC-Al₁₃) and polyaluminum chlorides with a high Al₃₀ content (PAC-Al₃₀). The impact of the hydrolysis ratios (B = [OH⁻]/[Al³⁺]) of PAC-Al₃₀ and [SO₄²⁻]/[Al³⁺] molar ratio of polyaluminum chloride sulfate (PACS) on the coagulation effects were also investigated.^16,17 In this study, the crystalline morphologies of PAC-Al₁₃ and PAC-Al₃₀ were investigated comparatively with a simple concentration crystallization method. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were also used to characterize the powder PAC-Al₁₃ and PAC-Al₃₀ samples. The aluminum hydrolytic species distribution and the charge neutralization capability of PAC-Al₁₃ and PAC-Al₃₀ solutions were also investigated, and the coagulation mechanism of them was discussed simultaneously. Moreover, the monomolecular aluminum salt of AlCl₃ was investigated in this study as a control.

2. Materials and methods

2.1 Synthesis of PAC-Al₁₃ and PAC-Al₃₀

All reagents used in the research were of analytical grade. All solutions were prepared using deionized water except for those pointed out specifically. Liquid PAC-Al₁₃ was prepared by slowly neutralizing AlCl₃ solution (1.0 mol L⁻¹) with NaOH solution (0.6 mol L⁻¹) at room temperature under vigorous stirring until the B value reached 2.4. Liquid PAC-Al₃₀ was prepared by heating the PAC-Al₁₃ solution at 95 °C for 12 hours under stirring and refluxing. The AlCl₃ solution was prepared by dissolving a certain amount of AlCl₃·6H₂O in deionized water. The final total aluminum concentration (Al_t) of these PAC-Al₁₃, PAC-Al₃₀ and AlCl₃ solutions was 0.2 mol L⁻¹ and all solutions were aged at room temperature for 5 days before crystallization.

2.2 Characterization of PAC-Al₁₃ and PAC-Al₃₀

Aluminum hydrolytic species distributions in different aluminum solutions were determined with time-developed Al–ferron complex colorimetry using a UV-Vis spectrometer (2550, Shimadzu Co., Japan) at 370 nm.^18,19 Based on the kinetic differences of the reaction between the Al species and 8-hydroxy-7-iodoquinoline-5-sulfonic acid (ferron reagent), the Al species can be divided into three mononuclear and polymer species: oligomer polymeric species Al_a, medium polymeric species Al_b, and high polymeric species Al_c.

The charge neutralization capability of the aluminum solutions was tested with a sol solution which contained 10 mg L⁻¹ humic acid. After rapid mixing of the aluminum solution and humic acid sol solution, samples were immediately taken using a syringe to measure the zeta potential on a Zetasizer Nano ZS (3000HS, Malvern Co., U.K.).

Crystal morphology, SEM and XRD analyses were conducted on the solid samples. As prepared AlCl₃, PAC-Al₁₃ and PAC-Al₃₀ solutions of 10 ml were placed in glass Petri dishes with a diameter of 10 cm, then the dishes were placed in an oven at 60 °C until there was no visible liquid phase in the dishes. The dried aluminum samples were taken out from the oven and cooled to room temperature for photography using a DSC-T99C (Sony) digital camera operated in close-range mode.

Powder samples were obtained for SEM and XRD analysis by finely grinding the crystals in their Petri dishes. The SEM analysis was performed using a JSM-6700F microscope with an operation voltage of 5.0 kV. The XRD analysis was performed using a Bruker D8 diffractometer, using the monochromatized X-ray beam from Cu Kα radiation, and with the operation current, operation voltage and scanning accuracy set to 50 mA, 40 kV and 0.02°, respectively.

3. Results and discussion

3.1 Crystalline morphologies of PAC-Al₁₃ and PAC-Al₃₀

The crystal morphologies of AlCl₃, PAC-Al₁₃ and PAC-Al₃₀ are shown in Fig. 1. It is very interesting to find that, although the AlCl₃ showed no real crystal morphology (Fig. 1a), some cluster branches appeared in PAC-Al₁₃ and these branches appeared to be growing along the diagonal of the tetragonal crystals (Fig. 1b). Comparing the crystal morphology of PAC-Al₃₀ with PAC-Al₁₃, it can be seen that the crystals formed by PAC-Al₃₀ were more complex; the branches were longer and interconnected to form a mesh (Fig. 1c). Wu et al.²⁰ reported that single Al₁₃ was easily combined into linear or branched aggregates, and Fu and Nazar²¹ believed this branched structure of PAC might be caused by the anionic bridging action of the Al₁₃ species. Al₃₀ consists of two δ-Al₁₃ Keggin units linked by four AlO₆ octahedra, which may help it to form more complex aggregation morphologies.

Fig. 1 Crystal morphology of coagulants obtained by the concentration and crystallization method. (a) AlCl₃; (b) PAC-Al₁₃; (c and d) partial view of PAC-Al₃₀.

Although the images of purified Al₁₃ were observed by Wu et al.,²⁰ Gao et al.²² and Feng et al.²³ in previous studies using Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) or Atomic Force Microscopy (AFM), the cluster branches grown from the regular tetragonal crystals and enlarged along the diagonal of the tetragonal crystals have not been reported. The branch structure of PAC-Al₁₃ and PAC-Al₃₀ may enhance the coagulation efficiency by improving the adsorption bonding, bridging and sweeping processes, which corresponds well with our previous reports.^16,17

Furthermore, it can be seen from Fig. 1d that there are some distinct and large prismatic crystals formed in PAC-Al₃₀. Xu et al.²⁴ reported the prismatic crystals of Al₁₃ by SEM, and Shi et al.²⁵ demonstrated that tetrahedrally shaped crystals were the dominant components in the precipitate of Al₁₃ prepared by SO₄²⁻/Ba²⁺ displacement. Both Xu et al.²⁴ and Shi et al.²⁵ believed that the prismatic crystals consisted of single Al₁₃, or a cluster of some single Al₁₃, tetrahedral crystals, and Tang²⁶ pointed out in his monograph on inorganic polymer flocculation theory that the formed large rhombohedral crystals were equivalent to a crystal of Al₃₀. However there has been no sufficient evidence to identify that these big prismatic crystals are the crystals formed by Al₃₀ because, to date, the single crystalline morphology of Al₃₀ has not been accurately observed by scientists.

3.2 SEM images of PAC-Al₁₃ and PAC-Al₃₀

The SEM images of powder AlCl₃, PAC-Al₁₃ and PAC-Al₃₀ samples were investigated to further understand their microscopic morphology. The image of AlCl₃ (Fig. 2a) showed an amorphous morphology, and there were a lot of rough gullies on the surface. The rough gullies of AlCl₃ may be caused by the absorption of water in air, because AlCl₃ is very easy to deliquesce.

Fig. 2 The SEM morphology of coagulants. (a) AlCl₃; (b) PAC-Al₁₃; (c) PAC-Al₃₀.

Comparing the crystal morphology of Fig. 2b with 2c, neither PAC-Al₁₃ nor PAC-Al₃₀ had an obvious crystalline structure in their SEM images, although PAC-Al₁₃ consisted mainly of sheet-like structures, and PAC-Al₃₀ showed irregular or concave shapes. Shi et al.²⁵ also analyzed the SEM images of freeze-dried PAC-Al₁₃ particles and found no regular crystalline morphology like this, which may be caused by the affect of other impurities during the crystallization of unpurified Al₁₃ or Al₃₀ samples.

3.3 XRD spectra of PAC-Al₁₃ and PAC-Al₃₀

Fig. 3 shows the XRD patterns of AlCl₃, PAC-Al₁₃ and PAC-Al₃₀. It can be seen from Fig. 3 that both the PAC-Al₁₃ and PAC-Al₃₀ samples demonstrated significant diffraction peaks at the 2 theta values of 27.293°, 31.684°, 45.420° and 56.453°, which match well with NaCl crystal diffraction peaks. It is well known that NaCl is very sensitive to XRD detection, and even a very small amount of NaCl produces a strong response in the XRD spectrum. Compared with PAC-Al₁₃ and PAC-Al₃₀, there were no crystal diffraction peaks of NaCl in the AlCl₃ sample because there was no introduction of Na⁺ from the preparation process.

Fig. 3 X-ray diffraction pattern of AlCl₃, PAC-Al₁₃ and PAC-Al₃₀.

It is generally believed that the diffraction peaks of Al₁₃ would appear at the 2 theta value of 5–25°.²⁷ By comparison with the spectral library, although there were some hydrated aluminum chloride hydroxides like Al₅Cl₃(OH)₁₂·2H₂O, Al₆Cl₃(OH)₁₅·9H₂O, Al₉Cl₆(OH)₂₁·14H₂O etc. and some alumina hydroxychlorides like Al₁₇O₁₆(OH)₁₆Cl₃, Al₂₄O₁₁(OH)₄₄Cl₆, Al₄₅O₄₅(OH)₄₅Cl etc. in the AlCl₃, PAC-Al₁₃ and PAC-Al₃₀ samples, the diffraction peaks of Al₁₃ were not distinct. Gao et al.²⁸ found that the diffraction peaks of Al₁₃ did not appear in PAC samples with a high Al₁₃ content; only after the SO₄²⁻/Ba²⁺ replacement reaction did the diffraction peaks of Al₁₃ arise. So, the XRD diffraction peaks of Al₁₃ and Al₃₀ may be masked by some impurity peaks.

3.4 Hydrolysis morphology of PAC-Al₁₃ and PAC-Al₃₀

It is generally believed that species that react with the ferron reagent within 1 min are Al_a, while species that react with the ferron reagent between 1 and 120 min are Al_b, and the species that remain unreacted with ferron reagent after 120 min are Al_c. It can be seen from Fig. 4 that the AlCl₃ solution reacted so quickly with the ferron reagent that the reaction reaches equilibrium almost within 1 min, and the absorbance of the reaction product after the reaction was 0.372 cm⁻¹. The reaction of PAC-Al₁₃ and PAC-Al₃₀ with the ferron reagent proceeded more slowly, with both of them ending at about 40 min, but with the absorbance of the reaction products of PAC-Al₁₃ and PAC-Al₃₀ after the reaction reaching 0.301 cm⁻¹ and 0.180 cm⁻¹, respectively. Because the total aluminum concentration (Al_t) of these AlCl₃, PAC-Al₁₃ and PAC-Al₃₀ solutions were all 0.2 mol L⁻¹, the smaller absorbance after the reaction illustrates that the high polymeric species (Al_c) content was higher in the PAC-Al₁₃ and PAC-Al₃₀ solutions.

Fig. 4 The reaction kinetics curves of AlCl₃, PAC-Al₁₃ and PAC-Al₃₀ solutions with the ferron reagent.

According to the reaction kinetics curves of the AlCl₃, PAC-Al₁₃ and PAC-Al₃₀ solutions with the ferron reagent (Fig. 4) and the standard curve of the ferron reagent reaction with standard aluminum solutions (Fig. 5), the aluminum species distribution of AlCl₃, PAC-Al₁₃ and PAC-Al₃₀ was calculated and listed in Table 1.

Fig. 5 The standard curve of the ferron reagent reaction with standard aluminum solutions.

Table 1 The aluminum species distribution of AlCl₃, PAC-Al₁₃ and PAC-Al₃₀

Samples name	Aluminum species distribution%
	Ala	Alb	Alc
AlCl3	93.4	6.6	0.0
PAC-Al13	32.1	62.5	5.4
PAC-Al30	27.3	32.1	40.6

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It can be seen from Table 1 that the AlCl₃ solution contained mostly Al_a and a small amount of Al_b with no Al_c, while the PAC-Al₁₃ solution possessed mostly Al_b, a portion of Al_a and a small amount of Al_c. In comparison to AlCl₃ and PAC-Al₁₃, although Al_c was the main ingredient in PAC-Al₃₀, the aluminum species were more dispersed. The higher polymeric species Al_c may enhance the bridge-aggregation and sweep-flocculation actions of PAC-Al₃₀ in the coagulation process. In addition, there are studies which show that the polydispersity of coagulation systems has a significant effect on the kinetics of coagulation and flocculation.²⁹ If the system is polydisperse and contains large particles, or small colloidal particles in water, it can be coagulated effectively by the fluid shear effect.

3.5 Charge neutralization capability of PAC-Al₁₃ and PAC-Al₃₀

The zeta potential is an important parameter to describe the stability of colloid particles in water. The humic acid water solution is a typical negatively charged sol system, which makes the humic acid particles disperse stably in natural water by electrostatic repulsion. The positively charged aluminum species can neutralize the negative charges on the humic acid particles’ surfaces, making them coagulate and inducing sedimentation. Furthermore, it should be noted that the zeta potential of the sol system is an important factor, but it is not the only one involved in the destabilization, flocculation and precipitation of the humic acid sol. The adsorption bonding, charge neutralization, bridging and sweeping processes caused by different coagulants play a very important role in the coagulation process. When adding the PAC salts to induce coagulation and sedimentation, the kind of dominant hydrolytic species, charge neutralization capability, structural stability of the species, and nanoscale molecular diameter of the PAC salts are all the key factors in the coagulation and sedimentation process.³⁰

The charge neutralization curves of AlCl₃, PAC-Al₁₃ and PAC-Al₃₀ are shown in Fig. 6. It can be seen from Fig. 6 that the zeta potential of the humic acid water system ascended rapidly from negative to positive in the aluminum dosage range of 0.02–0.40 mmol L⁻¹, and then climbed slowly with the further increases in the aluminum dosage. PAC-Al₁₃ and PAC-Al₃₀ exhibited almost the same charge neutralization capability throughout the investigated dosage range, while AlCl₃ showed a slightly weaker charge neutralization capability than PAC-Al₁₃ and PAC-Al₃₀ at the lower dosage but yielded a higher absolute value of the zeta potential with increasing aluminum dosage. The higher absolute value of the zeta potential easily led to the humic acid sol system returning to stability because the particles in the system adopted positive charges and repelled each other.

Fig. 6 The charge neutralization curves of AlCl₃, PAC-Al₁₃, and PAC-Al₃₀.

4. Conclusions

(a) The branching morphology and prismatic crystal pictures of PAC-Al₁₃ and PAC-Al₃₀ were obtained firstly by a simple concentration and crystallization method. The concentration and crystallization method is easy to operate and does not depend on precise and expensive equipment, and could be developed as a simple method for the investigation of the crystalline morphology of polymeric aluminum.

(b) The SEM images of PAC-Al₁₃ and PAC-Al₃₀ did not find well-formed crystalline structures, and the XRD patterns of PAC-Al₁₃ and PAC-Al₃₀ showed significant crystal diffraction peaks of NaCl but weak Al₁₃ or Al₃₀ diffraction peaks, which may result from the fact that the PAC-Al₁₃ and PAC-Al₃₀ samples were not purified and that the test results could have been affected by other impurities.

(c) The results of time-developed Al–ferron complex colorimetry showed that Al_c was the main ingredient and that the aluminum species were more dispersed in PAC-Al₃₀ than in AlCl₃ and PAC-Al₁₃. The higher polymeric species Al_c may enhance the bridge-aggregation and sweep-flocculation actions of PAC-Al₃₀ in the coagulation process.

(d) PAC-Al₁₃ and PAC-Al₃₀ exhibited almost the same charge neutralization capability throughout the investigated dosage range, while AlCl₃ showed a slightly weaker charge neutralization capability than PAC-Al₁₃ and PAC-Al₃₀ at a lower dosage but yielded a higher absolute value of zeta potential when aluminium dosage was increased. The higher absolute value of the zeta potential easily led to the humic acid sol system returning to stability because the particles in the system adopted positive charges and repelled each other.

Acknowledgements

The authors thank the Furong Scholar of Hunan Province, National Natural Science Foundation of China (51178047, 50978088, 51039001) and Colleges and Universities’ Scientific Research Project of Inner Mongolia Autonomous Region (NJZZ16370, NJZZ13017, NJZY16373) for their financial support.

References

1. H. X. Tang and Z. K. Luan, Features and mechanism for coagulation–flocculation processes of polyaluminum chloride, J. Environ. Sci., 1995, 7, 204–211.
2. W. H. Kuan, M. K. Wang, P. M. Huang, C. W. Wu, C. M. Chang and S. L. Wang, Effect of citric acid on aluminum hydrolytic speciation, Water Res., 2005, 39, 3457–3466.
3. J. E. V. Benschoten and K. J. Edzwald, Chemical aspects of coagulation using aluminum salts-I. Hydrolytic reactions of alum and polyaluminum chloride, Water Res., 1990, 24, 1519–1526.
4. C. Z. Hu, H. J. Liu, J. H. Qu, D. S. Wang and J. Rut, Coagulation behavior of aluminum salts in eutrophic water: significance of Al₁₃ species and pH control, Environ. Sci. Technol., 2006, 40, 325–331.
5. Z. Bi, C. H. Feng, D. S. Wang, X. P. Ge and H. X. Tang, Transformation of planar Mögel Al₁₃ coagulant during the dilution and aging process, Colloids Surf., A, 2013, 416, 73–79.
6. R. Y. Jiao, R. Fabris, C. W. K. Chow, M. Drikas, J. Leeuwen and D. S. Wang, Roles of coagulant species and mechanisms on floc characteristics and filterability, Chemosphere, 2016, 150, 211–218.
7. J. Rowsell and L. F. Nazar, Speciation and thermal transformation in alumina sols: structures of the polyhydroxyoxo aluminum cluster [A1₃₀O₈(OH)₅₆(H₂O)₂₆] and its 8-Keggin moiety, J. Am. Chem. Soc., 2000, 122(15), 3777–3778.
8. L. Allouche, C. Gérardin, T. Loiseau, G. Férey and F. Taulelle, Al₃₀: A Giant Aluminum Polycation, Angew. Chem., Int. Ed., 2000, 39(3), 511–514.
9. Z. Y. Chen, B. Fan, X. J. Peng, Z. Zhang, J. Fan and Z. K. Luan, Evaluation of Al₃₀ polynuclear species in polyaluminum solutions as coagulant for water treatment, Chemosphere, 2006, 64, 912–918.
10. J. Mertens, B. Casentini, A. Masion, R. Pöthig, B. Wehrli and G. Furrer, Polyaluminum chloride with high Al₃₀ content as removal agent for arsenic-contaminated well water, Water Res., 2012, 46, 53–62.
11. B. L. Phillips, A. Lee and W. H. Casey, Rates of oxygen exchange between the Al₂O₈Al₂₈(OH)₅₆(H₂O)₂₆¹⁸⁺(aq) (Al₃₀) molecule and aqueous solution, Geochim. Cosmochim. Acta, 2003, 67, 2725–2733.
12. L. Allouche and F. Taulelle, Conversion of Al₁₃ Keggin ε into Al₃₀: a reaction controlled by aluminium monomers, Inorg. Chem. Commun., 2003, 6, 1167–1170.
13. Z. Y. Chen, Z. K. Luan, J. H. Fan, Z. G. Zhang, Y. Z. Li and Z. P. Jia, Effect of thermal treatment on the formation and transformation of Keggin Al₁₃ and Al₃₀ species in hydrolytic aluminium solutions, Colloids Surf., A, 2007, 292, 110–118.
14. Z. Y. Chen, C. J. Liu, Z. K. Luan, Z. G. Zhang, Y. Z. Li and Z. P. Jia, Effect of total aluminium concentration on the formation and transformation of nanosized Al₁₃ and Al₃₀ in hydrolytic polymeric aluminium aqueous solutions, Chin. Sci. Bull., 2005, 50, 2010–2015.
15. Z. Y. Chen, Z. K. Luan, Z. P. Jia and X. S. Li, Study on the hydrolysis/precipitation behavior of Keggin Al₁₃ and Al₃₀ polymers in polyaluminum solutions, J. Environ. Manage., 2009, 80, 2831–2840.
16. P. Y. Zhang, Z. Wu, G. M. Zhang, G. M. Zeng, H. Y. Zhang, J. Li, X. G. Song and J. H. Dong, Coagulation characteristics of polyaluminum chlorides PAC-Al₃₀ on humic acid removal from water, Sep. Purif. Technol., 2008, 63, 642–647.
17. Z. Wu, P. Y. Zhang, G. M. Zeng, M. Zhang and J. H. Jiang, Humic Acid Removal from Water with Polyaluminum Coagulants: Effect of Sulfate on Aluminum Polymerization, J. Environ. Eng., 2012, 3, 293–298.
18. J. L. Lin, C. Huang, J. R. Pan and D. S. Wang, Effect of Al(III) speciation on coagulation of highly turbid water, Chemosphere, 2008, 72(2), 189–196.
19. C. G. Feng, B. Y. Shi, D. S. Wang, G. H. Li and H. X. Tang, Characteristics of simplified ferron colorimetric solution and its application in hydroxy-aluminum speciation, Colloids Surf., A, 2006, 287(1–3), 203–211.
20. X. H. Wu, D. S. Wang, X. P. Ge and H. X. Tang, Coagulation of silica microspheres with hydrolyzed Al(III)-significance of Al₁₃ and Al₁₃ aggregates, Colloids Surf., A, 2008, 330, 72–79.
21. G. Fu, L. F. Nazar and A. D. Bain, Aging processes of alumina sol–gels: characterization of new aluminum polyoxycations by ²⁷Al NMR spectroscopy, Chem. Mater., 1991, 3, 602–610.
22. B. Y. Gao, Q. Y. Yue and B. J. Wang, The chemical species distribution and transformation of polyaluminum silicate chloride coagulant, Chemosphere, 2002, 46, 809–813.
23. C. H. Feng, X. P. Ge, D. S. Wang and H. X. Tang, Effect of aging condition on species transformation in polymeric Al salt coagulants, Colloids Surf., A, 2011, 379, 62–69.
24. Y. Xu, D. S. Wang, H. Liu, Y. Q. Lu and H. X. Tang, Optimization of the separation and purification of Al₁₃, Colloids Surf., A, 2003, 231, 1–9.
25. B. Y. Shi, G. H. Li, D. S. Wang and H. X. Tang, Separation of Al₁₃ from polyaluminum chloride by sulfate precipitation and nitrate metathesis, Sep. Purif. Technol., 2007, 54, 88–95.
26. H. X. Tang, Inorganic polymer flocculants and the flocculation theory, Beijing, Building Industry Press of China, 2006, vol. 5–37, p. 301.
27. C. Z. Hu, G. X. Chen, H. J. Liu, H. Zhao and J. H. Qu, Characterization of flocs generated by preformed and in situ formed Al₁₃ polymer, Chem. Eng. J., 2012, 197, 10–15.
28. B. Y. Gao, Y. B. Chu, Q. Y. Yue, B. J. Wang and S. G. Wang, Characterization and coagulation of a polyaluminum chloride (PAC) coagulant with high Al₁₃ content, J. Environ. Manage., 2005, 76, 143–147.
29. S. Hussain, J. Leeuwen, C. Chow, S. Beecham, M. Kamruzzaman, D. S. Wang, M. Drikas and R. Aryal, Removal of organic contaminants from river and reservoir waters by three different aluminum-based metal salts: coagulation adsorption and kinetics studies, Chem. Eng. J., 2013, 225, 394–405.
30. P. Zhang, H. H. Hahn, E. Hoffmann and G. Zeng, Influence of some additives to aluminium species distribution in aluminium coagulants, Chemosphere, 2004, 57(10), 1489–1494.

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