An alternative method for preparation of polyaluminum chloride coagulant using fresh aluminum hydroxide gels: Characterization and coagulation performance

chemical engineering research and design 1 0 4 ( 2 0 1 5 ) 208–217

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An alternative method for preparation of polyaluminum chloride coagulant using fresh aluminum hydroxide gels: Characterization and coagulation performance Xiaomin Tang a,b,∗ , Huaili Zheng a,b,∗ , Houkai Teng c , Chun Zhao a,b , Yili Wang d , Wanying Xie a,b , Wei Chen a,b , Chun Yang a,b a

Key Laboratory of the Three Gorges Reservoir Region’s Eco-Environment, State Ministry of Education, Chongqing University, Chongqing 400045, PR China b National Centre for International Research of Low-carbon and Green Buildings, Chongqing University, Chongqing 400045, PR China c Tianjin Chemical Research and Design Institute, Tianjin 300131, PR China d College of Environmental Science and Engineering, Research Center for Water Pollution Source Control and Eco-remediation, Beijing Forestry University, Beijing 100083, PR China

a r t i c l e

i n f o

a b s t r a c t

Article history:

The drawbacks, high energy consumption, rigorous preparation condition and high impu-

Received 10 May 2015

rity contents, have been well-recognized in the preparation of widely-used polyaluminum

Received in revised form 4 August

chloride via conventional methods in industrial scale. In order to overcome the drawbacks,

2015

the fresh aluminum hydroxide gels were proposed to prepare the polyaluminum chloride

Accepted 9 August 2015

(PACG ) in this study. The optimal preparation condition of PACG was obtained by optimiz-

Available online 17 August 2015

ing the preparation conditions including temperature, time and acid concentration. The contents of the small/middle polymeric aluminum (Alb ) in an optimized PACG could reach

Keywords:

80%. During preparation, high temperature significantly reduced the preparation time but

Polyaluminum chloride

led to a poor stability. Lower rate constants of reaction between hydroxyl-Al and Ferron,

Aluminum hydroxide gel

kb1 and kb2 , were almost equal at high basicity value. It implied that the middle polymeric

Preparation

aluminum (Alb2 ) was dominant in Alb . Besides, the pseudo-first-order kinetics equation was

Characterization

rearranged. Coagulation by PACG2.0 was found to be the most effective for low-turbidity water

Coagulation

treatment but limited to a narrowest optimum range of dosage compared with other PACG .

Low-turbidity water

The residual aluminum concentration of the water after coagulation by the PACG2.5 with fresh aluminum hydroxide gels was always low even overdosing. Charge neutralization was not the predominant coagulation mechanism in this treatment. © 2015 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

Abbreviations: Ala , mononuclear aluminum; Alb , small/middle polymeric aluminum; Alb1 , small polymeric aluminum; Alb2 , middle polymeric aluminum; Alc , high polymeric or colloidal aluminum; Alm , mononuclear aluminum; Al13 , tridecamer [AlO4 Al12 (OH)24 (H2 O)12 7+ ]; Alun , undetectable aluminum species; PACG , polyaluimnum chloride; NMR, nuclear magnetic resonance; XRD, X-ray powder diffraction; SEM, scanning electron microscopy; NaOH, sodium hydroxide; HCl, hydrochloric acid. ∗ Corresponding author at: Corresponding authors at: Key laboratory of the Three Gorges Reservoir Region’s Eco-Environment, State Ministry of Education, Chongqing University, Chongqing 400045, China. Tel.: +86 023 65120827; fax: +86 023 65120827. E-mail addresses: [email protected] (X. Tang), [email protected] (H. Zheng). http://dx.doi.org/10.1016/j.cherd.2015.08.009 0263-8762/© 2015 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

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1.

Introduction

Coagulation is a basic and essential water/wastewater treatment technique. It aggregates the small suspended and colloidal particles in the water to become bigger ones after coagulant is added, and the aggregated particles are easily removed by gravity settlement or filtration (Alexander et al., 2012; Verma et al., 2012). In recent years, some research efforts have been devoted to aluminum-based coagulants, a kind of efficient and widespread inorganic coagulant (Shirasaki et al., 2014; Bi et al., 2004). Polyaluminum chloride (PAC) as the prehydrolyzed aluminum coagulant attracts increasing attention for wider scope of application, lower alkalinity consumption, less sludge production and lower residual aluminum concentration in the treated water compared with conventional aluminum coagulant (Ghafari et al., 2009; Wu et al., 2007; Yang et al., 2010). The general preparation method of PAC in industrial scale involves two steps. First of all, the raw material containing aluminum (such as, bauxite, gibbsite, granular aluminum etc.) is dissolved by hydrochloric acid (HCl) solution under vigorous condition (high temperature, high pressure and long reaction time). Then, alkaline material (such as, sodium hydroxide (NaOH), sodium carbonate, aluminum acid calcium etc.) is added to adjust the basicity value ([OH− ]/[Al] molar ratio) (Lan et al., 2009; Zhao et al., 2011). In the process, more attention should be paid to the impurity content which sometimes goes beyond the standard. The resistance ability of equipment to corrosion and pressure is strictly required, and much energy is consumed for long preparation time (about 6 h) (Li Fengting et al., 2004). To overcome the defects, granular aluminum is designed to react with strong alkaline solution to generate sodium aluminate before it is dissolved by hydrochloric acid (Zouboulis and Tzoupanos, 2010). Considering the cost of granular aluminum in the application, it will be more meaningful to apply the mineral containing aluminum and aluminum oxide as the raw material. But little information has been found about it. PAC prepared by this method using aluminum acid calcium has the low small/middle polymeric aluminum (Alb ) content even at high basicity value (Li Liu and Li, 2010). Improving Alb content will optimize the characteristics of PAC and enhance its coagulation performance (Shu-xuan et al., 2014). Besides, it is interesting to study the transformations and distributions of aluminum species in PAC prepared by this method, which may be different from that in PAC prepared by the way of mixing aluminum chloride solution with alkaline solution (Xu et al., 2014). It is controversial about the performances of PAC with high and low basicity values. It is generally accepted that the coagulation efficiency of PAC with high basicity value is better than that of one with low basicity value for the large amount of preformed middle/high polymeric aluminum and high charge density (McCurdy et al., 2004; Shirasaki et al., 2014). However, it is reported that PAC with low basicity value perform better in some cases (Yan et al., 2008; Yang et al., 2011). Thus, it makes sense for investigating into the performances of PAC with different basicity values for the treatment of refractory low-turbidity water that is sometimes found in the drinking water plants. In this research, polyaluminum chloride prepared by the fresh aluminum hydroxide gels was named PACG that is different from the conventional PAC in terms of composition and characteristic. And its aluminum species distribution was detected by means of Ferron assay, nuclear

magnetic resonance (NMR), X-ray powder diffraction (XRD) and scanning electron microscopy (SEM). The factors in the preparation, like temperature, time and acid concentration, were mainly considered to investigate their effect on the characteristic of PACG . Aluminum species transformation in the preparation and aging was discussed in detail. The performances of PACG with different basicity values were tested via treating low-turbidity water and the coagulation mechanisms in the application were analyzed.

2.

Materials and methods

2.1.

Coagulants preparation

In this study, all the chemical reagents were analytical grade chemicals. De-ionized water was used to make all the solutions of the reagents. Sodium aluminate solution was prepared by dissolving aluminum hydroxide solid powder (Al(OH)3(s) ) (Chinasun Specialty Products Co., Ltd., China) in NaOH solution (40%) under continuous stirring and heating at 100 ◦ C for about 20 min. After the sodium aluminate solution cooling to room temperature, a certain amount of HCl solution was added into the reaction system. The volumes of acid were varied with the desired basicity values which were chosen as 2.5, 2.0, 1.5, 1.0, 0.5 and 0. Fresh aluminum hydroxide gels formed after adding HCl solution were used to prepare PACG . The main factors, such as preparation temperature, time and acid concentration, were optimized. After fresh aluminum hydroxide gels were dissolved under the proper preparation condition, PACG with different basicity values were prepared. The resultant PACG products were respectively denoted as PACG2.5 , PACG2.0 , PACG1.5 , PACG1.0 , PACG0.5 and PACG0 on the basis of their basicity values. Compared with PACG , the conventional preparation method of PAC in laboratory was represented in the Supplementary information (SI). In the preparation and aging processes of PACG , it is assumed that the following transformations of aluminum species take place based on the experimental phenomena in the study and some reports in relevant literatures (Akitt and Farthing, 1978; Li Liu and Li, 2010; Vermeulen et al., 1975; Zhao et al., 2009). −

Al(OH)3(s) + OH− → Al(OH)4 + H+ → Al(OH)3(gel) + H+ → Al3+ (1)

Ala + OH− → Alb + OH− → Alc

(2)

Ala + Al(OH)3(gel) → Alc

(3)

Alb + Al(OH)3(gel) → Alc

(4)

Ala + Alc → Alb

(5)

Alb + Alb → Alc

(6)

where Ala , Alb and Alc represent the monomeric aluminum, small/middle polymeric aluminum and large/insoluble polymeric aluminum, respectively. Al(OH)3(s) firstly reacts with NaOH to form Al(OH)4 − . Then fresh aluminum hydroxide gels are generated when HCl solution is added. After fresh aluminum hydroxide gels are completely formed, the residual

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acid reacts with them to yield the Al3+ . The reaction rate between them becomes slower and slower resulting from the decrease of acid concentration during the reaction. The additional heating is often required to dissolve the fresh aluminum hydroxide gels. The hydrolysis and polymerization of aluminum are discovered in the solution when the Al3+ is produced. In this study, fresh aluminum hydroxide gels contribute to more Ala transforming to Alb , which is presented in the following resultants. Aluminum species distribution is detected by Ferron assay and 27 Al NMR spectroscopy.

2.2.

Characterization methods

2.2.1.

Ferron assay

Aluminum species distribution is presented by calculating the contents of Ala , Alb and Alc , and it profoundly influences the coagulation efficiency of PAC (Shirasaki et al., 2014; Shu-xuan et al., 2014). The aluminum species in PACG were measured via the application of Al-Ferron timed spectrophotometric method based on the different reaction time of hydroxyl-aluminum with the Ferron reagent (8-hydroxy-7iodoquinoline-5-sulfonic acid) (Sinopharm Chemical Reagent Co., Ltd., China) (Feng et al., 2007). Aluminum species were mainly divided into three categories as follows: Ala which reacted with Ferron within 60 s, Alb which reacted with Ferron within 120 min and Alc which needed much more time to react with Ferron or did not react with Ferron at all. Alc was obtained by subtracting Ala and Alb from the total aluminum (Ng et al., 2012; Shirasaki et al., 2014). The absorbance at 370 nm was measured by a UV–vis spectrophotometer (Beijing Purkinje General Instrument Co., Ltd., China) (Zouboulis and Tzoupanos, 2010).

2.2.2.

27

Al NMR spectroscopy

Al13 as the stable and active aluminum species plays an important role in the coagulation, and it can be directly detected by NMR (Bi et al., 2004; Xu et al., 2014). The 500 MHz 27 Al NMR spectroscopy (Bruker Co., Switzerland) was applied to analyze the aluminum species of PACG . The instrumental settings and experimental conditions were almost the same as previous study (Feng et al., 2007). Sodium aluminate solution (0.2 mol/L) diluted with D2 O was used as inner standard, and its chemical shift was found at 80 ppm. The signals near 0, 3–4 and 62.5 ppm represented mononuclear aluminum (Alm ), dimeric aluminum (Al2 ) and tridecamer [AlO4 Al12 (OH)24 (H2 O)12 7+ ] (Al13 ), respectively (Fig. S1). The concentrations of each species were determined by the ratio of the integrated intensity of their corresponding peaks to that of Al(OD)4 − at 80 ppm. The undetectable aluminum (Alun ) was obtained by subtracting the sum of the detected aluminum species from the total aluminum (AlT ) (Feng et al., 2007; Ye et al., 2009).

2.2.3.

X-ray powder diffraction

Prepared PAC was dried in a vacuum freeze drier. The fine powder samples were analyzed using a D/MAX-1200 X-Ray Diffractometer (Rigaku Corporation, Japan) with a Cu K␣ radiation. Data were collected from 5 to 70◦ 2 with a scanning rate of 1◦ 2 min−1 (Gao et al., 2005).

2.2.4.

Scanning electron microscopy

Prepared PAC was dried in a vacuum freeze drier. The surface morphology of solid PAC was observed using a VEGA II LMU SEM (TES-CAN Company, Czech) (Shi et al., 2007).

2.3.

Alkalimetric/acidometric titration experiments

In the alkalimetric titration experiment, 50 ml of a 0.5 mol/L Al solution prepared by aluminum chloride solution was titrated by 0.5 mol/L NaOH solution with vigorous stirring and with/without heating. The dropping rate of NaOH solution was adjusted to 0.05 mL/min. The pH was recorded throughout the titration. In the acidometric titration experiment, 50 mL of 0.5 mol/L Al solution prepared by sodium aluminate solution was titrated by 6 mol/L HCl solution with vigorous stirring and with/without heating. The pH was recorded throughout the titration.

2.4.

Coagulation performance

The simulated low-turbidity water was provided by the kaolin suspension in the laboratory. Kaolin powder was firstly sieved through a 200-mesh sifter and then mixed it with deionized water to make the stock suspension of kaolin. After the mixed solution was stirred for overnight using magnetic stirrer, lowturbidity water (turbidity = 3.5 ± 0.2 NTU, pH = 7.78 ± 0.1, zeta potential = −13.5 ± 2.5 mV) was prepared via diluting the stock suspension of kaolin (Guan et al., 2014). Coagulation experiments were carried out using a ZR4-6 six-paddle gang stirrer (Shenzhen Zhongrun Water Industry Technology and Development Co., Ltd, China). The jar tests were performed using 500 mL low-turbidity water at room temperature and natural pH. The laboratory prepared PACG and commercial PAC (Chongqing Lanjie Water Cleaning Agent Co., China) were added immediately. The commercial PAC with basicity value of 2.0 was dissolved in a certain volume of water before it was used as comparison. The dosages of coagulants were different due to the various preparation method and basicity value. And the dosage range was generally determined by the pre-experiment. Dosage was given as the concentration of aluminum in milligrams per liter (mg Al/L). The water was mixed at a high speed of 300 rpm for 1 min and then at a low speed of 40 rpm for 10 min. Flocs were formed during the slow mixing, and they were allowed to settle for 30 min. Turbidity and pH of water/treated water were measured using a 2100P turbidity meter (HACH, USA) and a HQ11 pH meter (HACH, USA). The removal rate is calculated using the following equation:

 Removal rate (%) =

1−

Tf Ti

 × 100%

(7)

where Ti and Tf are the initial and the final turbidity in the water, respectively. Zeta-potential was measured by the ZS90 Malvern potential analyzer (Malvern, UK). The residual aluminum species were classified as total aluminum, total dissolved aluminum, dissolved monomeric aluminum, dissolved organically-bound aluminum and dissolved organic monomeric aluminum (Yang et al., 2011). In this study, we chose total dissolved aluminum concentration in the supernatant to calculate the residual aluminum concentration. The supernatant was firstly filtered by 0.45 ␮m membrane filter. It was acidized via adjusting the pH below 1.0 using HNO3 solution and heated at 100 ◦ C for 5 min before it was measured by chrome azurol S colorimetric analysis method according to the national standard of China (GB/T5750.6-2006).

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Fig. 2 – Absorbance spectra (a) and reaction rate (b) of Ferron colorimetric reagent with hydroxyl-Al at different basicity values.

Fig. 1 – XRD pattern of (a) PAC2.0 and (b) PAC1.0 .

3.

Results and discussion

3.1.

Characterization of PACG

The preparation of PACG using fresh aluminum hydroxide gels has the advantages of low energy consumption (considering the short preparation time of about 1 hour) and mild preparation condition compared with conventional preparation method of PAC (Li Fengting et al., 2004). It is found that aluminum species distribution in PACG is similar to PAC prepared by the conventional method except at the basicity value of 2.5 (Table 1 and Table S1). Alb and Alc contents increase with the increase of basicity value. Conversely, Ala content almost decreases to zero when the basicity value is high. The Alb content in PACG2.5 is abnormally less than the Alc content, and insoluble aluminum hydroxide gels are found in the solution. The probable reason is that the acid concentration in the reaction system is too low to completely dissolve the aluminum hydroxide gels in a short time. PACG2.0 possesses the

highest Alb content of 70.04% in the research, and the value is far larger than that in the similar research (Li Liu and Li, 2010). It has been reported that high Alb content resulted in the excellent coagulation performance in the water treatment (Shirasaki et al., 2014; Shu-xuan et al., 2014; Xu and Gao, 2012). The distributions of Al13 and Alm in PACG with different basicity values possess the similar tendency (Table 1 and Fig. S1). And Al13 content is the dominant aluminum species in the PACG2.0 and PACG2.0 . It has been confirmed that the contents of Al13 and Alm in PAC are positively correlated with the contents of Alb and Ala , respectively (Feng et al., 2006). However, the Al13 contents of PACG2.0 and PACG1.5 are almost equal in this study, and Al13 content of PACG2.0 is much smaller than its Alb content for the presence of some undetectable small/middle aluminum species in NMR spectrum. The dimeric aluminum is scarcely found in the NMR spectrum of PACG2.0 and PACG0 (Fig. S1), which represents that dimeric aluminum easily transforms to higher polymeric aluminum and monomeric aluminum at high and low basicity values. Table 1 Aluminum species distributions of PACG as characterized by 27 Al NMR and Ferron methods The XRD patterns of prepared PAC2.0 and PAC1.0 compared with the simulated patterns from Inorganic crystal Database (ICSD) are performed by MDI Jade 6.0 software. The Al13 signals (peaks at about 8.334◦ , 10.124◦ , 23.836◦ ) mainly appearing in the range of 2 from 5 to 25◦ are obviously discovered in PAC2.0 (Fig. 1a). Conversely, the strong diffraction peaks of AlCl3 ·6(H2 O) (peak at about 14.947◦ , 17.190◦ , 22.866◦ , 24.144◦ , 26.064◦ , 27.013◦ , 34.952◦ , 39.045◦ , 41.265◦ , 44.028◦ , 46.734◦ , 51.963◦ ) are observed in the PAC1.0 (Fig. 1b) (Gao et al., 2005). It confirms that PAC2.0 contains Al13 , and monomeric aluminum is dominant in PAC1.0 , which is in accordance with the results from Ferron assay and 27 Al NMR.

Table 1 – Aluminum species distributions of PACG as characterized by 27 Al NMR and Ferron methods. Solution

PACG2.5 PACG2.0 PACG1.5 PACG1.0 PACG0.5 PACG0 a

Undetectable.

27

pH

4.95 4.12 3.77 3.61 3.39 3.08

Al NMR method

Ferron method

Alm (%)

Al13 (%)

Alun (%)

Ala (%)

Alb (%)

Alc (%)

–a 11.33 20.07 55.21 62.69 76.42

– 59.60 54.15 25.46 15.55 –

– 29.07 25.78 19.33 21.77 23.58

5.93 14.78 37.43 61.85 81.11 85.30

37.86 70.04 53.39 36.24 18.67 14.46

56.21 15.19 9.18 1.91 0.22 0.23

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Table 2 – Fitted parameters and the proportion of Alb1 , Alb2 in Alb . Basicity value 2.5 2.0 1.5 1.0 0.5

kb1

kb2

At

A0

A1

A2

Alb1 (%)

Alb2 (%)

R2

0.00050 0.00048 0.01359 0.01702 0.01241

0.00050 0.00048 0.00048 0.00052 0.00045

0.61401 0.94593 1.08095 1.35566 1.29096

0.39647 0.46731 0.59774 0.84815 1.00122

0.00000 0.00000 0.10474 0.21559 0.22209

0.21754 0.47862 0.37847 0.29192 0.06765

0.00 0.00 21.68 42.47 76.66

100.00 100.00 78.32 57.53 23.34

0.99628 0.99948 0.99958 0.99823 0.99758

NaCl is inevitably generated in the preparation, which is sensitive in the XRD even at low content. Its sharp peaks (peak at 31.692◦ , 45.446◦ , 56.477◦ ) are found in the range of 2 > 25◦ . The flaky oval crystals and particles are discovered on the surface of PAC2.0 (Fig. S2), but the tetrahedral-shaped crystal is absent (Shi et al., 2007). The component and relationship with coagulation performance of these crystals and particles should be further studied.

3.2.

Reaction kinetic of hydroxyl-Al with Ferron

As mentioned above, the aluminum species distribution is able to be detected by the Ferron assay. PACG with different basicity values have various reaction kinetics that are used to further analyze the component of PACG . A series of dotted line of absorbance against time are plotted (Fig. 2). The absorbance value increases sharply at the beginning since Al3+ and the monomeric aluminum react with Ferron reagent

instantaneously and the oligomers also rapidly react with it. However, the reaction rate approaches to zero after fast ascending when more time is needed to decompose the middle/large/insoluble polymeric aluminum (Zhou et al., 2006). In the cases of PACG2.0 and PACG2.5 , the phase of fast ascending is not evident and the reaction rates are always low. A lag phase at the beginning of reaction is discovered at the basicity value of 2.5 for fresh aluminum hydroxide gels presumably impede other aluminum species to break down and react with Ferron (Fig. 2b). The curves (Fig. 2a) can be fitted with Eq. (8) using a nonlinear curve with OriginPro 8.0 software: A = At − A1 × exp (−kb1 t) − A2 × exp (−kb2 t)

(8)

A0 = At − A1 − A2

(9)

where A is the absorbance value measured at any time. A0 and At are the absorbance values when time equal to zero and

Fig. 3 – Aluminum species distributions of PACG2.0 under different preparation temperature, (a) 22 ◦ C, (b) 40 ◦ C, (c) 60 ◦ C and (d) 80 ◦ C, and different preparation time.

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213

Fig. 4 – Aluminum species distributions of PACG2.0 under different added acid concentrations, (a) 1 mol/L, (b) 3 mol/L, (c) 6 mol/L and (d) 12 mol/L and different preparation time.

infinite time, respectively. A1 and A2 are the absorbance values of small polymeric aluminum (Alb1 ) and middle polymeric aluminum (Alb2 ), respectively. kb1 and kb2 are the first-order rate constants of Alb1 and Alb2 and they also represent the faster reacting aluminum and the slower reacting aluminum in the phase of Alb -Ferron reaction. t is the reaction time (s) (Feng et al., 2007).The fitted results (Table 2) support the aforementioned conclusion. The value of kb1 is smaller compared with the results in other study (Feng et al., 2007) and it equals to kb2 at the basicity values of 2.0 and 2.5. Thus, ‘A1 ·exp(-kb1 t)’ is considered to merge with ‘A2 ·exp(-kb2 t)’ in the cases. The Eq. (8) is validly inferred to the following one: (10)A = At − A2 × exp(−kb2 t)where A2  is the absorbance value of middle polymeric aluminum. Eq. (10) could be better used to fit the curves of PACG2.5 and PACG2.0 than Eq. (8). It has been confirmed that Alb1 with Al(O)6 structure are easy to break down to react with Ferron (Ye et al., 2009). On the contrary, Alb2 have stronger structure, like ␮4 -O bonded with Al13 , which impedes color reaction (Casey et al., 2000). In this study, the value of kb2 at basicity values of 2.0 is two orders of magnitude smaller than it in previous research (Feng et al., 2007), which confirms that PACG2.0 possesses more middle polymeric aluminum. And a conclusion can be drawn that Alb2 is the dominant aluminum species in PACG2.0 , which explains a better coagulation efficiency of PACG2.0 in the following lowturbidity water treatment. Moreover, Alb2 content gradually decreases when Al13 content is going down (Table 2). It is in

agreement with the view that there is a positive correlation between Alb2 and Al13 (Zhou et al., 2006).

3.3.

Optimization of preparation condition

3.3.1.

Effect of preparation temperature and time

PACG2.0 possessing the highest Alb content is mainly considered in the following research. Preparation temperature and time simultaneously influence the aluminum species distribution of PACG2.0 . Fresh aluminum hydroxide gels formed after adding HCl solution into the sodium aluminate solution are easily dissolved when the mixed solution is heated for a specific period of time, and the reaction rate increases with the increase of temperature (Fig. 3). At the temperature of 80 ◦ C, Alb becomes the dominant aluminum species in the PACG2.0 after heated 0.25 h, and fresh aluminum hydroxide gels are dissolved after 0.5 h (Fig. 3d). The time of dissolving gels, which is always the time of obtaining optimal aluminum species distribution, will be greatly extended to 7 d and 13 h when the temperature is 22 ◦ C (Fig. 3a) and 40 ◦ C (Fig. 3b), respectively. Temperature only speeds up the dissolution rate, but their maximum Alb contents are roughly the same at the separately optimal preparation time (Figs. S4–S6). After fresh aluminum hydroxide gels are dissolved, the hydrolysis, polymerization and decomposition of hydroxyl-aluminum becomes the dominant way of aluminum transformation, which is also influenced by the temperature and time. At the

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Fig. 6 – Alkalimetric titration curve and acidometric titration curve with/without heating. to high aluminum concentration that benefits the storage, transportation and use of PACG2.0 , Alb easily transforms to the other aluminum species at high aluminum concentration (Zhao et al., 2009). And the decrease of Alb content at high acid concentration is more obvious than that at low acid concentration after extending the preparation time. Besides, the concentrated hydrochloric acid is easily volatile especially under heating condition. And a large amount of heat is instantaneously released when it reacts with sodium aluminate. It has been reported that fresh aluminum hydroxide gels will become aged and be hardly dissolved when the temperature is high during their process of formation (Li Liu and Li, 2010). Thus, the moderate acid concentration, 6 mol/L, is recommended.

3.4. Fig. 5 – Aluminum species distributions of PACG2.0 under different preparation temperature, (a) 40 ◦ C and (b) 80 ◦ C, after ageing 30 d. temperature of 80 ◦ C, Alb obviously transforms to Alc after heating 3 h. However, the significant decrease of Alb content is not discovered at other preparation temperature in the experimental period. It indicates that the high temperature, 80 ◦ C, accelerates the aluminum hydrolysis, and the dynamic balance among Ala Alb and Alc is disrupted by extending the preparation time. The desirable aluminum species distribution of PACG2.0 is possibly obtained via controlling the preparation temperature and time. Even though it costs more time for dissolving fresh aluminum hydroxide gels at low temperature, the aluminum hydrolysis is always slow in this case, which improves the stability of PACG2.0 . Besides, the commercial liquid PAC is often used in the water treatment, and its storage time is long enough for the dissolution of fresh aluminum hydroxide gels. Thus, the preparation of PACG at ambient temperature would be more cost-efficient. And it might be a potential preparation method that can be applied in the industrial scale.

3.3.2.

Effect of acid concentration

Effect of added acid concentration on the aluminum species distribution of PACG2.0 is studied under the preparation temperature of 80 ◦ C. It is discovered that Alb content almost reach the highest after heating 0.5 h under four different acid concentrations (Fig. 4). Although high acid concentration leads

Stability of coagulants

Stability is an important factor to describe the characteristic of aluminum-based coagulant (Gao et al., 2002). The mutual transformations among Ala , Alb and Alc are in dynamic balance at a period of time that is determined by the basicity, aging condition and preparation method. Beyond this period, the transformation rate of Alb in PACG2.0 is fastest, and more Alb will transform into Ala and Alc than it is generated. The obvious decrease of Alb often results in the worse coagulation performance (Wang et al., 2004). In this study, the transformation of Alb is slowed down due to the low preparation temperature and moderate preparation time (Fig. 5a and Fig. S7). And the aluminum species distributions of PACG remain largely unchanged after ageing 47 d at preparation temperature of 22 ◦ C and preparation time of 7 d (Fig. S7). However, extending the preparation time will make Alb of PACG2.0 reaching the ‘activated point’, even at 40 ◦ C. Alb content of PACG2.0 prepared at 40 ◦ C for 30 h decreases more than 10% after ageing 30 d (Fig. 5a). Thus, it is considered to make the preparation time little earlier than the optimal time to maintain the Alb content during the aging period. Another way to reach the ‘activated point’ is to raise the temperature. A large amount of Alb , about 25%, will transform to other aluminum species during the aging when the preparation condition of 80 ◦ C for 1 h is found (Fig. 5b and Fig. S10). Conversely, Alb contents of PACG2.5 obviously increase after aging 47 d (Fig. S7–S10). The reason is that the residual aluminum hydroxide gels in PACG2.5 are slowly dissolved and gradually transform to Alb during the aging. Aluminum hydrolysis is also discovered at the basicity

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215

Fig. 7 – Coagulation performance of prepared PACG and commercial PAC with different dosages for low-turbidity water treatment, which was investigated in terms of (a) removal rate, (b) residual turbidity, (c) residual aluminum concentration and (d) zeta potential. value of 0 and 0.5. And the dominant Ala transforming to Alb causes the slight increase of Alb content after 47 d (Fig. S7–S10).

3.5.

Alkalimetric/acidometric titration curves

The possible aluminum species transformations in the preparation of PACG are presented in the earlier discussion. Alkalimetric/acidometric titration curves are plotted to analyze the relationship between basicity value and pH (Fig. 6). At the range of basicity value from 0.5 to 2.0, the change of pH value is not significant with the increase of basicity value since the added OH− is mainly used to assemble the small/middle polymers (Boisvert and Jolicoeur, 1999). In this range, Al3+ will gradually hydrolyze and polymerize to form monomeric aluminum (Ala ), small/middle polymeric aluminum (Alb ) and high polymeric aluminum (Alc ) following the common aluminum hydrolysis process (Bi et al., 2004). But the reaction kinetic of hydroxyl-aluminum with Ferron indicates that PACG2.0 mainly contains Alb2 (Table 2). It is speculated that fresh aluminum hydroxide gels increase the generation of middle polymeric aluminum. When the basicity value is higher than 2.5, the pH increases sharply for the production of aluminum hydroxide precipitate. Besides, at the range of basicity value from 0.5 to 2.0, the pH in acidometric titration curve with heating is higher than that without heating since heating facilitates the free H+ reacting with fresh aluminum hydroxide gels. The pH in acidometric titration

curve with heating is similar with the pH in alkalimetric titration curve without heating, which provides an explanation for the similar aluminum species distribution between PACG and conventional PAC. And the pH in alkalimetric titration curve with heating is lower for accelerating the aluminum hydrolysis. PACG possessing a desired aluminum species distribution during the aging period is used in the water treatment.

3.6.

Coagulation performance

Coagulation performances are investigated via applying PACG in the low-turbidity water treatment. The results reveal that even though most PACG with different basicity values can treat the turbidity below 1 NTU that meet the drinking water standard of China, their removal efficiencies are significantly different (Fig. 7a and b). In the research, the best coagulation efficiency is observed in the application of PACG2.0 for it possesses the maximum number of middle polymeric aluminum that has the high charge density and strong capacity of bridging. At the optimum dosage of 4.40 mg Al/L, the turbidity and residual aluminum concentration in supernatant treated by PACG2.0 are 0.83 NTU and 0.041 mg/L (below the international standard of 0.2 mg/L), respectively (Fig. 7b and c). However, overdosing PACG2.0 makes the surface charge reversal of colloidal particles, which results in the re-stabilization of particles and the performance deterioration in the case. The similar conclusion was found in previous study (Zouboulis and

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Tzoupanos, 2009). It has been claimed that coagulation efficiency was improved with the increase of the basicity value (Shirasaki et al., 2014). But the opposite conclusion is found in the PACG2.5 whose optimal dosage is little more than the PACG2.0 and turbidity in supernatant is higher owing to the presence of fresh aluminum hydroxide gels. Fresh aluminum hydroxide gels possess the low charge and polymerization degree. It weakens the charge neutralization and bridging in the low-turbidity water treatment. Nonetheless, fresh aluminum hydroxide gels might be applied in other special condition, such as heavy metal treatment, due to its potential adsorption capacity. The residual aluminum concentrations are always acceptable even overdosing PAC G2.5 since fresh aluminum hydroxide gels easily transform to colloidal aluminum hydroxide particles that collide and aggregate with other colloidal particles to form the compact flocs (Jiao et al., 2015). Although the higher dosage of PACG with low basicity value and commercial PAC are demanded in this treatment, their optimal dosage ranges are wider compared with PACG2.0 . And a little excess dosage will not significantly result in performance deterioration. These advantages make them more suitable to be applied in the low-turbidity water treatment. The zeta potential of treated water is in correlation with the dosage and the basicity value of PACG (Fig. 7d). The best removal rate is always achieved at the positive zeta potential. It is deduced that charge neutralization should not be the only coagulation mechanism in the low-turbidity water treatment. Other mechanisms, such as adsorption, bridging and entrapment, may contribute to the turbidity removal as well due to the bridging of middle polymeric aluminum and the adsorption of colloidal aluminum hydroxide particles (Huang et al., 2014; Zhao et al., 2013).

4.

Conclusions

An alternative preparation method of PAC was presented in this paper, using fresh aluminum hydroxide gels as the aluminum source. The procedure without the application of extreme conditions in terms of temperature and pressure had the benefit of saving energy. Aluminum species distributions of the self-prepared PACG are discovered similar to that of PAC prepared by the conventional method at the basicity value ranging from 2.0 to 0. And the middle polymers (Alb2 /Al13 ), which play important roles in the coagulation, are the dominant aluminum species in PACG2.0 and PACG1.5 . The presence of Al13 is also confirmed by XRD. Although high temperature decreases the preparation time, the stability of yielding PACG2.0 is worse than that prepared at low temperature. And extending the preparation time will accelerate the aluminum hydrolysis after fresh aluminum hydroxide gels are dissolved. Acid concentration influences the aluminum concentration and aluminum species distribution. The highest Alb content reaches 80% in PACG2.0 at optimal preparation condition. The coagulation performances of PACG are acceptable in the low-turbidity water treatment, especially for PACG2.0 whose treatment efficiency is highest for it possesses the largest number of middle polymers. However, PACG2.0 easily causes the charge reversal and the performance deterioration after overdosing due to its higher charge density. Conversely, PACG0 , PACG0.5 , PACG1.0 and PACG2.5 possess the lower efficiency but they have the wider optimal range of dosage, which is recommended to be applied in the case.

The residual aluminum concentrations are always acceptable even overdosing PAC G2.5 due to the presence of fresh aluminum hydroxide gels. In the analysis of zeta potential, it indicates that charge neutralization is not the main coagulation mechanism, and adsorption, bridging and entrapment might be indispensable in the low-turbidity water treatment.

Acknowledgments We are grateful for the financial support provided by the National Key Technology R&D Program (Grant No. 2012BAJ25B06), the National Natural Science Foundation of China (Grant No. 21177164) and the National 111 Project (Project No. B13041).

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.cherd.2015.08.009.

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