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Effect of fresh aluminum hydroxide gels on algae removal from micro-polluted water by polyaluminum chloride coagulant
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Effect of fresh aluminum hydroxide gels on algae removal from micro-polluted water by polyaluminum chloride coagulant Xiaomin Tang a,b,∗, Huaili Zheng a,b,∗, Yili Wang c, Wei Chen a,b, Jinsong Guo d, Yuhao Zhou a,b, Xiang Li a,b a
Key laboratory of the Three Gorges Reservoir Region’s Eco-Environment, State Ministry of Education, Chongqing University, Chongqing 400045, PR China National Centre for International Research of Low-carbon and Green Buildings, Chongqing University, Chongqing 400045, PR China College of Environmental Science and Engineering, Research Center for Water Pollution Source Control and Eco-remediation, Beijing Forestry University, Beijing 100083, PR China d Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences, Chongqing 400714, PR China b c
a r t i c l e
i n f o
Article history: Received 11 August 2015 Revised 15 January 2016 Accepted 1 March 2016 Available online xxx Keywords: Aluminum hydroxide gel Polyaluminum chloride Preparation Characterization Coagulation Algae
a b s t r a c t Polyaluminum chloride (PAC) coagulant, a most commonly used inorganic coagulant, was prepared using fresh aluminum hydroxide gels under the low energy consumption. The aim of this study was to investigate the influence of fresh aluminum hydroxide gels on the characteristics of PAC and the algae removal from micro-polluted water. The results suggested that PAC with the basicity values (OH/Al) of 2.5 and 2.0 synthesized by this method possessed the high content of middle polymeric aluminum (Alb2 ). And fresh aluminum hydroxide gels contributed to the formation of Alb2 . Adding fresh aluminum hydroxide gels in the aging period temporarily increased the small/middle polymeric aluminum (Alb ) content that rapidly reduced with the time extending since fresh aluminum hydroxide gels accelerated the aluminum hydrolysis. The excessive fresh aluminum hydroxide gels would aggravate the decrease of Alb content. The presence of fresh aluminum hydroxide gels in PAC slightly reduced the coagulation efficiency, but the performance deterioration resulted from overdosing was weakened in the cases. Besides, the residual aluminum concentrations in the treated water were always low as fresh aluminum hydroxide gels induced the generation of colloidal aluminum hydroxide particles. Charge neutralization was not the only coagulation mechanism in this treatment. © 2016 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Polyaluminum chloride (PAC) is the versatile coagulant in the portable water treatment throughout the world [1]. The conventional industrial preparation method of PAC has the defects of high energy consumption, rigorous preparation condition and high impurity content [2]. In order to overcome these weaknesses, granular aluminum is considered to react with alkaline before it is dissolved by hydrochloric acid (HCl), which can be operated
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; Al2 , dimeric aluminum; Al13 , tridecamer [AlO4 Al12 (OH)24 (H2 O)12 7+ ]; Alun , undetectable aluminum species; HCl, hydrochloric acid; NaOH, sodium hydroxide; NMR, nuclear magnetic resonance; NOM, natural organic matter; OH/Al, basicity value; PAC, polyaluminum chloride. ∗ Corresponding authors at: Key laboratory of the Three Gorges Reservoir Region’s Eco-Environment, State Ministry of Education, Chongqing University, Chongqing 40 0 045, China. Tel./fax: +86 23 65120827. E-mail addresses: [email protected] (X. Tang), [email protected], [email protected] (H. Zheng).
under moderate temperature and atmospheric pressure [2]. However, granular aluminum is not cost-effective to be applied in the industrial scale, and other cheap aluminum ore should substitute the pure aluminum in the preparation. Besides, the small/middle polymeric aluminum (Alb ) content of PAC prepared using calcium aluminate by this method is always lower than that of it prepared by conventional method especially at high basicity values [3]. Increasing Alb content is able to improve the coagulation efficiency in most of the studies [4]. And in the preparation, fresh aluminum hydroxide gels as the intermediate product are formed after adding acid solution into the sodium aluminate [3]. But their effect on characteristic of PAC has rarely found in the research. Coagulation as an important water treatment technology has been applied in the algae treatment where PAC plays a vital role [5]. To the best of our knowledge, however, no publication has concentrated on the influence of fresh aluminum hydroxide gels on the treatment of algae, especially for the treatment of algae from micro-polluted water. In this research, PAC was prepared by fresh aluminum hydroxide gels with the heating by sunlight. The effect of fresh aluminum
http://dx.doi.org/10.1016/j.jtice.2016.03.001 1876-1070/© 2016 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Please cite this article as: X. Tang et al., Effect of fresh aluminum hydroxide gels on algae removal from micro-polluted water by polyaluminum chloride coagulant, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.03.001
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hydroxide gels on characteristics of PAC was studied via considering the distributions and transformations of aluminum species in the preparation and aging periods. The micro-polluted water containing algae was prepared to investigate the influence of fresh aluminum hydroxide gels on coagulation performance of PAC in terms of removal rate, residual turbidity, residual aluminum concentration, floc characteristic and zeta potential. 2. Materials and methods 2.1. Coagulants preparation All the chemical reagents were analytical grade chemicals. Deionized water was used to make all the solutions of reagents. The powdered Al(OH)3(s) reacted with NaOH solution (40%) to prepare sodium aluminate solution under rapidly stirring and temperature of 100 °C. And then a certain amount of HCl solution was added into sodium aluminate solution according to the desired basicity value. In the process, the large quantities of fresh aluminum hydroxide gels were formed, and the half-finished PAC solution was prepared. In the pre-experiment, the high temperature and modest heating time could accelerate the dissolution of gels [6]. However, it consumed more energy, and the stability of production was far from satisfactory. In this study, the half-finished PAC solution with fresh aluminum hydroxide gels was placed in the sunlight without further artificial heating in the preparation until gels were dissolved. The weather in the period was presented in the Fig. S1. As the comparison, part of the half-finished PAC solution with fresh aluminum hydroxide gels was placed in the room (about 17 °C) without sunlight and part of it was placed in the water bath (temperature of 22 °C). The prepared PAC solution were respectively denoted as PAC2.5 , PAC2.0 , PAC1.5 , PAC1.0 , PAC0.5 and PAC0 according to the basicity values of 2.5, 2.0, 1.5, 1.0, 0.5 and 0. 2.2. Characterization methods 2.2.1. Ferron assay Aluminum species distributions of PAC were measured by the Al-Ferron timed spectro-photometric method which was based on the different reaction time of various aluminum species with the Ferron reagent (8-hydroxy-7-iodoquinoline-5-sulfonic acid). The monomeric species (Ala ) reacted with Ferron within 60 s, and small/middle polymeric species (Alb ) reacted with Ferron within 120 min. The high/insoluble polymeric aluminum species (Alc ) needed much more time to react with Ferron, or they did not react with Ferron at all. Alc content was obtained by the difference between the content of AlT and that of Ala combined with Alb [7]. 2.2.2.
27 Al
NMR spectroscopy NMR spectroscopy could be also used to analyze aluminum species distribution (Bruker Co., Switzerland). The instrumental settings and experimental conditions were represented in the previous study. Sodium aluminate solution (0.2 mol/L) diluted by deuterium oxide (D2 O) was used as an inner standard, and its chemical shift was 80 ppm. The characteristic peaks of 0, 3-4 and 62.5 ppm represented mononuclear aluminum (Alm ), dimeric aluminum (Al2 ) and AlO4 Al12 (OH)24 (H2O)12 7+ (Al13 ), respectively. The concentrations of aluminum species were determined by the ratio of the integrated intensity of their corresponding peaks to that of peak at 80 ppm. The content of undetectable species (Alun ) was calculated by deducting the sum of the detected aluminum species from the AlT [7]. 27 Al
2.3. Coagulation tests Coagulation tests were performed by a ZR4-6 six-paddle gang stirrer (Shenzhen Zhongrun Water Industry Technology and Devel-
opment Co., Ltd, China) to analyze the coagulation performance of PAC2.5 , PAC2.0 and PAC0 in the treatment of algae from micropolluted water. Microcystis aeruginosa was obtained from the Freshwater Algae Culture Collection at the Institute of Hydrobiology (China). BG11 medium was used as the algae inoculums [8]. The algae were cultured in a constant temperature incubator (Shanghai Boxun Industry & Commerce Co., Ltd, China) under the temperature of 25 ± 2 °C, and illumination of 30 0 0 lx was provided for 12 h every day. The algae with inoculums collected at two cultivation time were used for coagulation tests. The turbidity values of collected solution were 3 ± 0.5 NTU with pH of 7.21 and 10 ± 0.5 NTU with pH of 8.15 respectively. Jar tests were performed at room temperature and natural pH. The self-prepared PAC was added into the water, and the dosage was given as the concentration of aluminum in milligrams per liter (mg Al/L). The commercial PAC with the basicity value of 2.0, as a comparison, was also used in the algae treatment. The water was mixed at a high speed of 300 rpm for 1 min and at a low speed of 40 rpm for 10 min, and then it was allowed to settle for 30 min. Turbidity and pH of water/treated water were measured using a 2100P turbidity meter (HACH, Loveland, USA) and a HQ11 pH meter (HACH, Loveland, USA). Zeta-potential was measured by ZS90 Malvern potential analyzer (Malvern, UK) at the natural pH. The supernatant was acidized using HNO3 solution and was heated at 100 °C for 5 min before the residual aluminum in the supernatant was measured by chrome azurol S colorimetric analysis according to the national standard of China (GB/T5750.62006). The flocs were dried in a vacuum drier, and the morphology of flocs was observed using a VEGA II LMU SEM (TES-CAN Company, Czech). Fractal dimensions of the flocs were calculated by photographic image analysis method using Image-pro Plus 6.0 software [9]. 3. Results and discussion 3.1. Aluminum species distributions in polyaluminum chloride coagulant In this study, PAC is prepared by fresh aluminum hydroxide gels under sunlight. Although the preparation time of this method is long, the temperature of 100 °C is only required in the reaction between powdered Al(OH)3(s) and NaOH for about 20 min and no other artificial heating is needed in the rest of time until PAC is prepared (fresh aluminum hydroxide gels were dissolved). But in the conventional industrial preparation of PAC, the powdered Al(OH)3 or other bauxite firstly reacts with HCl under 100 °C for two hours, and then calcium aluminate is added to adjust the basicity value with the temperature of 80 °C and time of four hours [3]. To compare these preparation conditions, PAC prepared in this study possesses the advantages of short heating time and low energy consumption. Besides, it is often found that high pure PAC solution (without drying) as the product is directly used in the drinking water treatment in the town [3]. And the time interval between preparation and application of PAC solution is long enough for the dissolution of fresh aluminum hydroxide gels. Thus, preparing PAC via fresh aluminum hydroxide gels would be a promising method that could be applied in the industrial scale. Aluminum species distribution is an important factor to characterize aluminum-based coagulants. Alb contributing to the coagulation performance is the vital aluminum species and Al13 is considered as the most effective component among them [4,10]. It is described that aluminum species distributions of self-prepared PAC are similar with PAC prepared by the conventional method at the range of basicity value from 0 to 2.0 (Table 1 and Table S1). The maximum Alb content of self-prepared PAC is obtained at basicity value of 2.0. But Alb content abnormally decreases in the PAC2.5 due to the presence of insoluble fresh aluminum
Please cite this article as: X. Tang et al., Effect of fresh aluminum hydroxide gels on algae removal from micro-polluted water by polyaluminum chloride coagulant, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.03.001
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X. Tang et al. / Journal of the Taiwan Institute of Chemical Engineers 000 (2016) 1–7 Table 1 Aluminum species distributions detected by Ferron assay and Solution
PAC2.5 PAC2.0 PAC1.5 PAC1.0 PAC0.5 PAC0 a
AlT (mol/L)
0.36 0.36 0.32 0.33 0.37 0.37
27
3
Al NMR. 27
Ferron assay 2
Ala (%)
Alb (%)
Alb1 (%)
Alb2 (%)
R
1.65 22.03 43.02 64.24 88.64 86.44
62.72 68.82 46.08 24.32 11.36 13.56
0 0 0 100 100 100
100 100 100 0 0 0
0.99865 0.99999 0.99999 0.99964 0.99999 0.99999
Al NMR spectroscopy
Alc (%)
Alm (%)
Al2 (%)
Al13 (%)
Alun (%)
35.72 9.15 10.91 11.44 0 0
–a 4.56 10.50 33.18 40.90 51.85
– 0 3.27 7.03 7.28 0
– 68.76 48.22 28.37 0 0
– 26.68 38.02 31.41 51.82 48.15
– Undetectable.
Fig. 1. Aluminum species distributions of PAC2.0 before adding fresh aluminum hydroxide gels, after adding fresh aluminum hydroxide gels for 7 days and after adding fresh aluminum hydroxide gels for 14 days. (I) adding 0.25% (m/v) fresh aluminum hydroxide gels; (II) adding 0.5% (m/v) fresh aluminum hydroxide gels; (Ⅲ) adding 1% (m/v) fresh aluminum hydroxide gels.
hydroxide gels. The absorbance values and reaction rates of Ferron colorimetric reagent with hydroxyl aluminum varying with time are described in Fig. S2 and Fig. S3, respectively. Aluminum species in PAC0, PAC0.5 and PAC1.0 rapidly react with Ferron reagent in ten minutes, and then their reaction rates are nearly close to zero. In contrast, the reaction rates do not decrease sharply in the second phase and they keep at a higher level in a long time in the cases of PAC1.5 , PAC2.0 and PAC2.5 . It suggests that the self-prepared PAC with high basicity values possess more middle/high polymeric aluminum which need more time to decompose and react with Ferron reagent [11]. The absorbance spectrums are fitted by an exponential equation, and the contents of small polymer aluminum (Alb1 ) and middle polymer aluminum (Alb2 ) in Alb are obtained via calculating the fitted parameter [12]. The results indicate that Alb2 is the dominant aluminum species in the Alb when basicity value is above 1.5. But Alb2 content is not obviously more than Alb1 at basicity value of 1.5 in the previous study [7]. It is assumed that fresh aluminum hydroxide gels enhance the formation of middle polymeric aluminum. A reason should be that there is the basicity value gradient around the surface of fresh aluminum hydroxide gels, and the basicity value is rather high near the surface. In this condition, monomeric aluminum is easier to transform to middle polymeric aluminum and accumulates around surface of gels. Conversely, Alb2 is hardly found at the basicity value below 1.0 (Table 1). PAC prepared in the sunlight, room and bath water under the optimum preparation time has the similar aluminum species distribution (Fig. S4 and Fig. S5). And their optimum preparation
time are presented in the ascending order of the water bath (6 d), sunlight (7 d) and room (10 d). Furthermore, the contents of Alm , Al2 and Al13 are calculated through 27 Al NMR spectrograms of PAC (Fig. S6). It shows a positive correlation between Al13 and Alb at basicity value ranging from 2.0 to 1.0, which has also been confirmed in other studies [10,12]. The Al13 is almost not detected at basicity values of 0.5 and 0, and the main aluminum species in these basicity values are Alm , which is consistent with the results from Ferron assay. Moreover, dimeric aluminum (Al2 ) is scarcely found in the 27 Al NMR spectrum of PAC2.0 and PAC0 . It represents that Al2 easily transforms to higher polymeric aluminum and monomeric aluminum at high and low basicity values. 3.2. Aluminum species transformation in the aging periods Aluminum hydrolysis and polymerization becomes the main way of aluminum species transformation after fresh aluminum hydroxide gels dissolved. And aluminum species transformation almost follows the traditional processes [6,13]:
Ala + OH− → Alb + OH− → Alc
(1)
Ala + Alc → Alb
(2)
Alb + Alb → Alc
(3)
but in this study, the slow aluminum species transformation is found in the aging period (Fig. S7). The Alb contents of PAC2.5 and PAC2.0 are almost not changed in 120 days. Besides, the aluminum
Please cite this article as: X. Tang et al., Effect of fresh aluminum hydroxide gels on algae removal from micro-polluted water by polyaluminum chloride coagulant, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.03.001
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Fig. 2. Aluminum species distributions of PAC2.0 with the different aging time a) before adding fresh aluminum hydroxide gels, b) after adding 0.25% (m/v) fresh aluminum hydroxide gels for 7 days and c) after adding 0.25% (m/v) fresh aluminum hydroxide gels for 14 days.
species transformation is not obviously in PAC0.5 until 20 days and a considerable amount of Ala transform to Alb after 100 days, which increases the Alb content to about 35%. 3.3. Effect of fresh aluminum hydroxide gels on aluminum species distribution of polyaluminum chloride coagulant Aluminum species distribution significantly influences the coagulation performance of PAC. And the high Alb content always contributes to the high coagulation efficiency [4]. In the research, fresh aluminum hydroxide gels as the aluminum source are available for the preparation of PAC, and they influence the basicity value, pH and aluminum concentration of polyaluminum chloride solution. The molar ratio of fresh aluminum hydroxide gels and acid in the solution influences the aluminum species distribution of PAC. It has been mentioned that fresh aluminum hydroxide gels significantly contribute to the generation of Alb2 at the high basicity values (Table 1). Even so, fresh aluminum hydroxide gels with high concentration seriously decreases the acid concentration in the solution and the reaction rate between gels and acid, which accounts for the longevity of fresh aluminum hydroxide gels in the PAC2.5 . The effect of fresh aluminum hydroxide gels on aluminum species distribution of PAC2.0 at the aging period is also studied
(Figs. 1 and 2). After PAC2.0 was prepared, it is aged for some days. At the beginning of aging, fresh aluminum hydroxide gels are added in PAC2.0 solution. It takes 7 days for the added gels dissolved due to the low acid concentration in the PAC2.0 . And the aluminum species distribution of PAC2.0 is detected after 7 days and 14 days (Fig. 1). It is represented that Alb content increases after adding moderate fresh aluminum hydroxide gels since added gels increase the basicity value of solution. And the Alb content is almost same after 14 days when the amount of added fresh aluminum hydroxide gels of 0.25% is found. However, the excessive fresh aluminum hydroxide gels induce Alb transforming to Alc and reduce the Alb content obviously. Especially, the Alb content of 63.56% is detected after 14 days in the case of adding 1% (m/v) fresh aluminum hydroxide gels. It is confirmed that added fresh aluminum hydroxide gels might accelerate the aluminum hydrolysis in the PAC2.0 solution. The transformation of Alb to Alc /Ala rather than reverse process is found in PAC2.0 solution as the aging time goes on, and Alb content reduces to 23.90% after aging 176 days (Fig. 2a). The 0.25% (m/v) fresh aluminum hydroxide gels are added into the PAC2.0 solution at the different aging time to study their effect on the aluminum species distribution. It also needs 7 days for the dissolution of added gels, and the aluminum species distribution of PAC2.0 was
Please cite this article as: X. Tang et al., Effect of fresh aluminum hydroxide gels on algae removal from micro-polluted water by polyaluminum chloride coagulant, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.03.001
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Fig. 3. Coagulation performance of PAC2.5 , PAC2.0 and PAC0 for the treatment of algae from micro-polluted water in terms of (a) removal rate, (b) residual turbidity, (c) residual aluminum concentration and (d) zeta potential.
analyzed after 7 days and 14 days (Fig. 2b and c). When the added fresh aluminum hydroxide gels are dissolved, the newly generated Ala is found in the solution. And the Ala will continuously transform to Alb and Alc following the traditional aluminum hydrolysis process. In the process, Alb content of PAC2.0 have a temporary increase at the initial phase. However, Alb contents obviously decrease for extending the time to 14 days for the further aluminum hydrolysis [13]. The dash lines present the contents of Ala , Alb and Alc in PAC2.0 without adding gels after 14 days (Fig. 2c). Their Alb contents are higher than those which add fresh aluminum hydroxide gels. It implies that the added fresh aluminum hydroxide gels influence aluminum hydrolysis and the aluminum species distribution of PAC2.0 in the aging period. 3.4. Effect of fresh aluminum hydroxide gels on the treatment of algae from micro-polluted water The self-prepared PAC2.5 , PAC2.0 and PAC0 are applied in the algae removal from micro-polluted water, and their coagulation performances are almost acceptable in the optimal dosage range (Fig. 3). PAC2.5 containing the small amounts of insoluble fresh aluminum hydroxide gels possesses slightly worse removal effi-
ciency than PAC2.0 since fresh aluminum hydroxide gels with the low charge and degree of polymerization weaken the capacity of charge neutralization and bridging per unit volume of PAC. However, fresh aluminum hydroxide gels might own the pore structure and large specific surface area which could conduce to the adsorption [14,15]. Besides, the charge reversal resulting from overdosing in the treatment is not obvious in the case of PAC2.5 due to its low charge density and the presence of fresh aluminum hydroxide gels. And the significant performance deterioration is not discovered in the experimental dosage range whose maximum dosage reaches to 39 mg Al/L. On the contrary, PAC2.0 possessing the higher Alb content contains considerable quantities of positive charges. Thus, performance deterioration is easily happened after excessively dosing PAC2.0 in the water treatment, especially for the micro-polluted water treatment. In the field application, it is complicated to adjust the dosage of PAC according to the raw water quality [16]. Therefore, priority will be given to the PAC that has wide optimal dosage range in the application. Residual aluminum in the treated water has aroused extensive concern in the countries for its potential risk to human health [17]. And it has been reported that dissolved aluminum is almost the major speciation in the residual aluminum, and dissolved monomeric aluminum possess the higher toxicity
Please cite this article as: X. Tang et al., Effect of fresh aluminum hydroxide gels on algae removal from micro-polluted water by polyaluminum chloride coagulant, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.03.001
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Fig. 4. SEM photographs and fractal dimensions of (a) PAC2.5 , (b) PAC2.0 and (c) PAC0 .
[17]. The residual aluminum concentrations in the treated water made by PAC2.5 are always below 0.2 mg/L in the optimal dosage range. The colloidal aluminum hydroxide particles formed by further hydrolysis of middle/high polymer aluminum and fresh aluminum hydroxide gels increase the number of insoluble aluminum species and the flocs size and improve settling performance of flocs, which also reduces the residual aluminum concentration in the treated water [18]. The commercial PAC is also used in the algae treatment. It possesses the similar performance compared with self-prepared PAC (Fig. S8). However, its removal efficiency is little worse than PAC2.0 . Zeta potential is an important indicator of the stability of colloidal dispersions. Colloids with high zeta potential are electrically
stabilized while colloids with low zeta potentials tend to coagulate in the water due to the Brownian motion [19]. Zeta potential slowly increases with the increase of dosage of PAC2.5 for its low charge density (Fig. 3d). And more PAC2.5 is demanded to make the zeta potential of solution approach to zero. It has been claimed that the best coagulation performance is achieved near the zero potential when charge neutralization is the main coagulation mechanism [6,20]. However, the highest removal efficiencies are always obtained below zero potential, which indicates that charge neutralization is not the only coagulation mechanism in this study. And the bridging of Alb /Al13 , the adsorption of aluminum hydroxide gels and the entrapment of flocs also contribute to the algae removal.
Please cite this article as: X. Tang et al., Effect of fresh aluminum hydroxide gels on algae removal from micro-polluted water by polyaluminum chloride coagulant, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.03.001
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The surface morphologies of flocs formed in the test are able to describe the flocs composition and speculate the coagulation process [21]. PAC2.5 containing fresh aluminum hydroxide gels easily forms big colloidal aluminum hydroxide particles in the treatment (Fig. 4), which also improves the capacity of adsorption. And the formed flocs are more compact than those mainly formed by colloidal particle collisions. Besides, the outer surface of flocs formed by PAC2.5 and PAC2.0 is rough and loose. It is suggested that other small particles are swept by the flocs, which explains that some parts of the flocs easily break in the fast re-stirring after settlement in the previous study [22]. The fractal dimension (Df ), an essential character of flocs, is represented by the slope of Log A versus Log P, where A and P are the area and perimeter of flocs respectively. And the slope of scattering plot is measured via fitting a straight line. Normally, the higher fractal dimension is related to the larger floc size and the better settlement [21]. In this study, the flocs formed by PAC2.5 and PAC2.0 have higher fractal dimension, and their better performances have been presented in Fig. 3. 4. Conclusions PAC was able to be prepared by fresh aluminum hydroxide gels under short heating time and low energy consumption, and it would be a promising preparation method applied in industrial scale. PAC2.0 and PAC2.5 prepared in the laboratory maintained the desirable Alb contents in the experimental period, and both of them were high in Alb2 contents. A great deal of Al13 was also found in the PAC2.0 . The presence of fresh aluminum hydroxide gels in preparation and aging period had the effects on characteristics of PAC. Fresh aluminum hydroxide gels were known as the aluminum resource in the preparation, and they facilitated the transformation of monomeric aluminum to middle polymer aluminum. The added fresh aluminum hydroxide gels in the aging period increased Alb content at the initial phase, but Alb content rapidly reduced with the time extending since fresh aluminum hydroxide gels accelerated the aluminum hydrolysis. And adding excessive fresh aluminum hydroxide gels would aggravate the decrease of Alb content. PAC2.5 containing fresh aluminum hydroxide gels performed slightly worse than PAC2.0 in the algae removal from micropolluted water due to the shortage in charge neutralization and bridging. But the charge reversal resulted from overdose effect was mitigated in the application of PAC2.5 for its low charge density. Thus, PAC2.5 was more suitable to be used in the micro-polluted water treatment. Fresh aluminum hydroxide gels enhanced the yield of colloidal aluminum hydroxide particles, which resulted in the formation of compact flocs and the reduction of residual aluminum concentration in the treated water. Charge neutralization was not the only coagulation mechanism via the zeta potential analysis, and adsorption, bridging and entrapment were indispensable in the micro-polluted water treatment. Acknowledgments We are grateful for the financial support provided by the National Natural Science Foundation of China (Project no. 21477010), the National Key Technologies R & D Program of China (Grant no. 2012BAJ25B06) and the National 111 Project (Project no. B13041).
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Effect of fresh aluminum hydroxide gels on algae removal from micro-polluted water by polyaluminum chloride coagulant Xiaomin Tang a,b,∗, Huaili Zheng a,b,∗, Yili Wang c, Wei Chen a,b, Jinsong Guo d, Yuhao Zhou a,b, Xiang Li a,b a
Key laboratory of the Three Gorges Reservoir Region’s Eco-Environment, State Ministry of Education, Chongqing University, Chongqing 400045, PR China National Centre for International Research of Low-carbon and Green Buildings, Chongqing University, Chongqing 400045, PR China College of Environmental Science and Engineering, Research Center for Water Pollution Source Control and Eco-remediation, Beijing Forestry University, Beijing 100083, PR China d Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences, Chongqing 400714, PR China b c
a r t i c l e
i n f o
Article history: Received 11 August 2015 Revised 15 January 2016 Accepted 1 March 2016 Available online xxx Keywords: Aluminum hydroxide gel Polyaluminum chloride Preparation Characterization Coagulation Algae
a b s t r a c t Polyaluminum chloride (PAC) coagulant, a most commonly used inorganic coagulant, was prepared using fresh aluminum hydroxide gels under the low energy consumption. The aim of this study was to investigate the influence of fresh aluminum hydroxide gels on the characteristics of PAC and the algae removal from micro-polluted water. The results suggested that PAC with the basicity values (OH/Al) of 2.5 and 2.0 synthesized by this method possessed the high content of middle polymeric aluminum (Alb2 ). And fresh aluminum hydroxide gels contributed to the formation of Alb2 . Adding fresh aluminum hydroxide gels in the aging period temporarily increased the small/middle polymeric aluminum (Alb ) content that rapidly reduced with the time extending since fresh aluminum hydroxide gels accelerated the aluminum hydrolysis. The excessive fresh aluminum hydroxide gels would aggravate the decrease of Alb content. The presence of fresh aluminum hydroxide gels in PAC slightly reduced the coagulation efficiency, but the performance deterioration resulted from overdosing was weakened in the cases. Besides, the residual aluminum concentrations in the treated water were always low as fresh aluminum hydroxide gels induced the generation of colloidal aluminum hydroxide particles. Charge neutralization was not the only coagulation mechanism in this treatment. © 2016 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Polyaluminum chloride (PAC) is the versatile coagulant in the portable water treatment throughout the world [1]. The conventional industrial preparation method of PAC has the defects of high energy consumption, rigorous preparation condition and high impurity content [2]. In order to overcome these weaknesses, granular aluminum is considered to react with alkaline before it is dissolved by hydrochloric acid (HCl), which can be operated
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; Al2 , dimeric aluminum; Al13 , tridecamer [AlO4 Al12 (OH)24 (H2 O)12 7+ ]; Alun , undetectable aluminum species; HCl, hydrochloric acid; NaOH, sodium hydroxide; NMR, nuclear magnetic resonance; NOM, natural organic matter; OH/Al, basicity value; PAC, polyaluminum chloride. ∗ Corresponding authors at: Key laboratory of the Three Gorges Reservoir Region’s Eco-Environment, State Ministry of Education, Chongqing University, Chongqing 40 0 045, China. Tel./fax: +86 23 65120827. E-mail addresses: [email protected] (X. Tang), [email protected], [email protected] (H. Zheng).
under moderate temperature and atmospheric pressure [2]. However, granular aluminum is not cost-effective to be applied in the industrial scale, and other cheap aluminum ore should substitute the pure aluminum in the preparation. Besides, the small/middle polymeric aluminum (Alb ) content of PAC prepared using calcium aluminate by this method is always lower than that of it prepared by conventional method especially at high basicity values [3]. Increasing Alb content is able to improve the coagulation efficiency in most of the studies [4]. And in the preparation, fresh aluminum hydroxide gels as the intermediate product are formed after adding acid solution into the sodium aluminate [3]. But their effect on characteristic of PAC has rarely found in the research. Coagulation as an important water treatment technology has been applied in the algae treatment where PAC plays a vital role [5]. To the best of our knowledge, however, no publication has concentrated on the influence of fresh aluminum hydroxide gels on the treatment of algae, especially for the treatment of algae from micro-polluted water. In this research, PAC was prepared by fresh aluminum hydroxide gels with the heating by sunlight. The effect of fresh aluminum
http://dx.doi.org/10.1016/j.jtice.2016.03.001 1876-1070/© 2016 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Please cite this article as: X. Tang et al., Effect of fresh aluminum hydroxide gels on algae removal from micro-polluted water by polyaluminum chloride coagulant, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.03.001
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hydroxide gels on characteristics of PAC was studied via considering the distributions and transformations of aluminum species in the preparation and aging periods. The micro-polluted water containing algae was prepared to investigate the influence of fresh aluminum hydroxide gels on coagulation performance of PAC in terms of removal rate, residual turbidity, residual aluminum concentration, floc characteristic and zeta potential. 2. Materials and methods 2.1. Coagulants preparation All the chemical reagents were analytical grade chemicals. Deionized water was used to make all the solutions of reagents. The powdered Al(OH)3(s) reacted with NaOH solution (40%) to prepare sodium aluminate solution under rapidly stirring and temperature of 100 °C. And then a certain amount of HCl solution was added into sodium aluminate solution according to the desired basicity value. In the process, the large quantities of fresh aluminum hydroxide gels were formed, and the half-finished PAC solution was prepared. In the pre-experiment, the high temperature and modest heating time could accelerate the dissolution of gels [6]. However, it consumed more energy, and the stability of production was far from satisfactory. In this study, the half-finished PAC solution with fresh aluminum hydroxide gels was placed in the sunlight without further artificial heating in the preparation until gels were dissolved. The weather in the period was presented in the Fig. S1. As the comparison, part of the half-finished PAC solution with fresh aluminum hydroxide gels was placed in the room (about 17 °C) without sunlight and part of it was placed in the water bath (temperature of 22 °C). The prepared PAC solution were respectively denoted as PAC2.5 , PAC2.0 , PAC1.5 , PAC1.0 , PAC0.5 and PAC0 according to the basicity values of 2.5, 2.0, 1.5, 1.0, 0.5 and 0. 2.2. Characterization methods 2.2.1. Ferron assay Aluminum species distributions of PAC were measured by the Al-Ferron timed spectro-photometric method which was based on the different reaction time of various aluminum species with the Ferron reagent (8-hydroxy-7-iodoquinoline-5-sulfonic acid). The monomeric species (Ala ) reacted with Ferron within 60 s, and small/middle polymeric species (Alb ) reacted with Ferron within 120 min. The high/insoluble polymeric aluminum species (Alc ) needed much more time to react with Ferron, or they did not react with Ferron at all. Alc content was obtained by the difference between the content of AlT and that of Ala combined with Alb [7]. 2.2.2.
27 Al
NMR spectroscopy NMR spectroscopy could be also used to analyze aluminum species distribution (Bruker Co., Switzerland). The instrumental settings and experimental conditions were represented in the previous study. Sodium aluminate solution (0.2 mol/L) diluted by deuterium oxide (D2 O) was used as an inner standard, and its chemical shift was 80 ppm. The characteristic peaks of 0, 3-4 and 62.5 ppm represented mononuclear aluminum (Alm ), dimeric aluminum (Al2 ) and AlO4 Al12 (OH)24 (H2O)12 7+ (Al13 ), respectively. The concentrations of aluminum species were determined by the ratio of the integrated intensity of their corresponding peaks to that of peak at 80 ppm. The content of undetectable species (Alun ) was calculated by deducting the sum of the detected aluminum species from the AlT [7]. 27 Al
2.3. Coagulation tests Coagulation tests were performed by a ZR4-6 six-paddle gang stirrer (Shenzhen Zhongrun Water Industry Technology and Devel-
opment Co., Ltd, China) to analyze the coagulation performance of PAC2.5 , PAC2.0 and PAC0 in the treatment of algae from micropolluted water. Microcystis aeruginosa was obtained from the Freshwater Algae Culture Collection at the Institute of Hydrobiology (China). BG11 medium was used as the algae inoculums [8]. The algae were cultured in a constant temperature incubator (Shanghai Boxun Industry & Commerce Co., Ltd, China) under the temperature of 25 ± 2 °C, and illumination of 30 0 0 lx was provided for 12 h every day. The algae with inoculums collected at two cultivation time were used for coagulation tests. The turbidity values of collected solution were 3 ± 0.5 NTU with pH of 7.21 and 10 ± 0.5 NTU with pH of 8.15 respectively. Jar tests were performed at room temperature and natural pH. The self-prepared PAC was added into the water, and the dosage was given as the concentration of aluminum in milligrams per liter (mg Al/L). The commercial PAC with the basicity value of 2.0, as a comparison, was also used in the algae treatment. The water was mixed at a high speed of 300 rpm for 1 min and at a low speed of 40 rpm for 10 min, and then it was allowed to settle for 30 min. Turbidity and pH of water/treated water were measured using a 2100P turbidity meter (HACH, Loveland, USA) and a HQ11 pH meter (HACH, Loveland, USA). Zeta-potential was measured by ZS90 Malvern potential analyzer (Malvern, UK) at the natural pH. The supernatant was acidized using HNO3 solution and was heated at 100 °C for 5 min before the residual aluminum in the supernatant was measured by chrome azurol S colorimetric analysis according to the national standard of China (GB/T5750.62006). The flocs were dried in a vacuum drier, and the morphology of flocs was observed using a VEGA II LMU SEM (TES-CAN Company, Czech). Fractal dimensions of the flocs were calculated by photographic image analysis method using Image-pro Plus 6.0 software [9]. 3. Results and discussion 3.1. Aluminum species distributions in polyaluminum chloride coagulant In this study, PAC is prepared by fresh aluminum hydroxide gels under sunlight. Although the preparation time of this method is long, the temperature of 100 °C is only required in the reaction between powdered Al(OH)3(s) and NaOH for about 20 min and no other artificial heating is needed in the rest of time until PAC is prepared (fresh aluminum hydroxide gels were dissolved). But in the conventional industrial preparation of PAC, the powdered Al(OH)3 or other bauxite firstly reacts with HCl under 100 °C for two hours, and then calcium aluminate is added to adjust the basicity value with the temperature of 80 °C and time of four hours [3]. To compare these preparation conditions, PAC prepared in this study possesses the advantages of short heating time and low energy consumption. Besides, it is often found that high pure PAC solution (without drying) as the product is directly used in the drinking water treatment in the town [3]. And the time interval between preparation and application of PAC solution is long enough for the dissolution of fresh aluminum hydroxide gels. Thus, preparing PAC via fresh aluminum hydroxide gels would be a promising method that could be applied in the industrial scale. Aluminum species distribution is an important factor to characterize aluminum-based coagulants. Alb contributing to the coagulation performance is the vital aluminum species and Al13 is considered as the most effective component among them [4,10]. It is described that aluminum species distributions of self-prepared PAC are similar with PAC prepared by the conventional method at the range of basicity value from 0 to 2.0 (Table 1 and Table S1). The maximum Alb content of self-prepared PAC is obtained at basicity value of 2.0. But Alb content abnormally decreases in the PAC2.5 due to the presence of insoluble fresh aluminum
Please cite this article as: X. Tang et al., Effect of fresh aluminum hydroxide gels on algae removal from micro-polluted water by polyaluminum chloride coagulant, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.03.001
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X. Tang et al. / Journal of the Taiwan Institute of Chemical Engineers 000 (2016) 1–7 Table 1 Aluminum species distributions detected by Ferron assay and Solution
PAC2.5 PAC2.0 PAC1.5 PAC1.0 PAC0.5 PAC0 a
AlT (mol/L)
0.36 0.36 0.32 0.33 0.37 0.37
27
3
Al NMR. 27
Ferron assay 2
Ala (%)
Alb (%)
Alb1 (%)
Alb2 (%)
R
1.65 22.03 43.02 64.24 88.64 86.44
62.72 68.82 46.08 24.32 11.36 13.56
0 0 0 100 100 100
100 100 100 0 0 0
0.99865 0.99999 0.99999 0.99964 0.99999 0.99999
Al NMR spectroscopy
Alc (%)
Alm (%)
Al2 (%)
Al13 (%)
Alun (%)
35.72 9.15 10.91 11.44 0 0
–a 4.56 10.50 33.18 40.90 51.85
– 0 3.27 7.03 7.28 0
– 68.76 48.22 28.37 0 0
– 26.68 38.02 31.41 51.82 48.15
– Undetectable.
Fig. 1. Aluminum species distributions of PAC2.0 before adding fresh aluminum hydroxide gels, after adding fresh aluminum hydroxide gels for 7 days and after adding fresh aluminum hydroxide gels for 14 days. (I) adding 0.25% (m/v) fresh aluminum hydroxide gels; (II) adding 0.5% (m/v) fresh aluminum hydroxide gels; (Ⅲ) adding 1% (m/v) fresh aluminum hydroxide gels.
hydroxide gels. The absorbance values and reaction rates of Ferron colorimetric reagent with hydroxyl aluminum varying with time are described in Fig. S2 and Fig. S3, respectively. Aluminum species in PAC0, PAC0.5 and PAC1.0 rapidly react with Ferron reagent in ten minutes, and then their reaction rates are nearly close to zero. In contrast, the reaction rates do not decrease sharply in the second phase and they keep at a higher level in a long time in the cases of PAC1.5 , PAC2.0 and PAC2.5 . It suggests that the self-prepared PAC with high basicity values possess more middle/high polymeric aluminum which need more time to decompose and react with Ferron reagent [11]. The absorbance spectrums are fitted by an exponential equation, and the contents of small polymer aluminum (Alb1 ) and middle polymer aluminum (Alb2 ) in Alb are obtained via calculating the fitted parameter [12]. The results indicate that Alb2 is the dominant aluminum species in the Alb when basicity value is above 1.5. But Alb2 content is not obviously more than Alb1 at basicity value of 1.5 in the previous study [7]. It is assumed that fresh aluminum hydroxide gels enhance the formation of middle polymeric aluminum. A reason should be that there is the basicity value gradient around the surface of fresh aluminum hydroxide gels, and the basicity value is rather high near the surface. In this condition, monomeric aluminum is easier to transform to middle polymeric aluminum and accumulates around surface of gels. Conversely, Alb2 is hardly found at the basicity value below 1.0 (Table 1). PAC prepared in the sunlight, room and bath water under the optimum preparation time has the similar aluminum species distribution (Fig. S4 and Fig. S5). And their optimum preparation
time are presented in the ascending order of the water bath (6 d), sunlight (7 d) and room (10 d). Furthermore, the contents of Alm , Al2 and Al13 are calculated through 27 Al NMR spectrograms of PAC (Fig. S6). It shows a positive correlation between Al13 and Alb at basicity value ranging from 2.0 to 1.0, which has also been confirmed in other studies [10,12]. The Al13 is almost not detected at basicity values of 0.5 and 0, and the main aluminum species in these basicity values are Alm , which is consistent with the results from Ferron assay. Moreover, dimeric aluminum (Al2 ) is scarcely found in the 27 Al NMR spectrum of PAC2.0 and PAC0 . It represents that Al2 easily transforms to higher polymeric aluminum and monomeric aluminum at high and low basicity values. 3.2. Aluminum species transformation in the aging periods Aluminum hydrolysis and polymerization becomes the main way of aluminum species transformation after fresh aluminum hydroxide gels dissolved. And aluminum species transformation almost follows the traditional processes [6,13]:
Ala + OH− → Alb + OH− → Alc
(1)
Ala + Alc → Alb
(2)
Alb + Alb → Alc
(3)
but in this study, the slow aluminum species transformation is found in the aging period (Fig. S7). The Alb contents of PAC2.5 and PAC2.0 are almost not changed in 120 days. Besides, the aluminum
Please cite this article as: X. Tang et al., Effect of fresh aluminum hydroxide gels on algae removal from micro-polluted water by polyaluminum chloride coagulant, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.03.001
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Fig. 2. Aluminum species distributions of PAC2.0 with the different aging time a) before adding fresh aluminum hydroxide gels, b) after adding 0.25% (m/v) fresh aluminum hydroxide gels for 7 days and c) after adding 0.25% (m/v) fresh aluminum hydroxide gels for 14 days.
species transformation is not obviously in PAC0.5 until 20 days and a considerable amount of Ala transform to Alb after 100 days, which increases the Alb content to about 35%. 3.3. Effect of fresh aluminum hydroxide gels on aluminum species distribution of polyaluminum chloride coagulant Aluminum species distribution significantly influences the coagulation performance of PAC. And the high Alb content always contributes to the high coagulation efficiency [4]. In the research, fresh aluminum hydroxide gels as the aluminum source are available for the preparation of PAC, and they influence the basicity value, pH and aluminum concentration of polyaluminum chloride solution. The molar ratio of fresh aluminum hydroxide gels and acid in the solution influences the aluminum species distribution of PAC. It has been mentioned that fresh aluminum hydroxide gels significantly contribute to the generation of Alb2 at the high basicity values (Table 1). Even so, fresh aluminum hydroxide gels with high concentration seriously decreases the acid concentration in the solution and the reaction rate between gels and acid, which accounts for the longevity of fresh aluminum hydroxide gels in the PAC2.5 . The effect of fresh aluminum hydroxide gels on aluminum species distribution of PAC2.0 at the aging period is also studied
(Figs. 1 and 2). After PAC2.0 was prepared, it is aged for some days. At the beginning of aging, fresh aluminum hydroxide gels are added in PAC2.0 solution. It takes 7 days for the added gels dissolved due to the low acid concentration in the PAC2.0 . And the aluminum species distribution of PAC2.0 is detected after 7 days and 14 days (Fig. 1). It is represented that Alb content increases after adding moderate fresh aluminum hydroxide gels since added gels increase the basicity value of solution. And the Alb content is almost same after 14 days when the amount of added fresh aluminum hydroxide gels of 0.25% is found. However, the excessive fresh aluminum hydroxide gels induce Alb transforming to Alc and reduce the Alb content obviously. Especially, the Alb content of 63.56% is detected after 14 days in the case of adding 1% (m/v) fresh aluminum hydroxide gels. It is confirmed that added fresh aluminum hydroxide gels might accelerate the aluminum hydrolysis in the PAC2.0 solution. The transformation of Alb to Alc /Ala rather than reverse process is found in PAC2.0 solution as the aging time goes on, and Alb content reduces to 23.90% after aging 176 days (Fig. 2a). The 0.25% (m/v) fresh aluminum hydroxide gels are added into the PAC2.0 solution at the different aging time to study their effect on the aluminum species distribution. It also needs 7 days for the dissolution of added gels, and the aluminum species distribution of PAC2.0 was
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Fig. 3. Coagulation performance of PAC2.5 , PAC2.0 and PAC0 for the treatment of algae from micro-polluted water in terms of (a) removal rate, (b) residual turbidity, (c) residual aluminum concentration and (d) zeta potential.
analyzed after 7 days and 14 days (Fig. 2b and c). When the added fresh aluminum hydroxide gels are dissolved, the newly generated Ala is found in the solution. And the Ala will continuously transform to Alb and Alc following the traditional aluminum hydrolysis process. In the process, Alb content of PAC2.0 have a temporary increase at the initial phase. However, Alb contents obviously decrease for extending the time to 14 days for the further aluminum hydrolysis [13]. The dash lines present the contents of Ala , Alb and Alc in PAC2.0 without adding gels after 14 days (Fig. 2c). Their Alb contents are higher than those which add fresh aluminum hydroxide gels. It implies that the added fresh aluminum hydroxide gels influence aluminum hydrolysis and the aluminum species distribution of PAC2.0 in the aging period. 3.4. Effect of fresh aluminum hydroxide gels on the treatment of algae from micro-polluted water The self-prepared PAC2.5 , PAC2.0 and PAC0 are applied in the algae removal from micro-polluted water, and their coagulation performances are almost acceptable in the optimal dosage range (Fig. 3). PAC2.5 containing the small amounts of insoluble fresh aluminum hydroxide gels possesses slightly worse removal effi-
ciency than PAC2.0 since fresh aluminum hydroxide gels with the low charge and degree of polymerization weaken the capacity of charge neutralization and bridging per unit volume of PAC. However, fresh aluminum hydroxide gels might own the pore structure and large specific surface area which could conduce to the adsorption [14,15]. Besides, the charge reversal resulting from overdosing in the treatment is not obvious in the case of PAC2.5 due to its low charge density and the presence of fresh aluminum hydroxide gels. And the significant performance deterioration is not discovered in the experimental dosage range whose maximum dosage reaches to 39 mg Al/L. On the contrary, PAC2.0 possessing the higher Alb content contains considerable quantities of positive charges. Thus, performance deterioration is easily happened after excessively dosing PAC2.0 in the water treatment, especially for the micro-polluted water treatment. In the field application, it is complicated to adjust the dosage of PAC according to the raw water quality [16]. Therefore, priority will be given to the PAC that has wide optimal dosage range in the application. Residual aluminum in the treated water has aroused extensive concern in the countries for its potential risk to human health [17]. And it has been reported that dissolved aluminum is almost the major speciation in the residual aluminum, and dissolved monomeric aluminum possess the higher toxicity
Please cite this article as: X. Tang et al., Effect of fresh aluminum hydroxide gels on algae removal from micro-polluted water by polyaluminum chloride coagulant, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.03.001
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Fig. 4. SEM photographs and fractal dimensions of (a) PAC2.5 , (b) PAC2.0 and (c) PAC0 .
[17]. The residual aluminum concentrations in the treated water made by PAC2.5 are always below 0.2 mg/L in the optimal dosage range. The colloidal aluminum hydroxide particles formed by further hydrolysis of middle/high polymer aluminum and fresh aluminum hydroxide gels increase the number of insoluble aluminum species and the flocs size and improve settling performance of flocs, which also reduces the residual aluminum concentration in the treated water [18]. The commercial PAC is also used in the algae treatment. It possesses the similar performance compared with self-prepared PAC (Fig. S8). However, its removal efficiency is little worse than PAC2.0 . Zeta potential is an important indicator of the stability of colloidal dispersions. Colloids with high zeta potential are electrically
stabilized while colloids with low zeta potentials tend to coagulate in the water due to the Brownian motion [19]. Zeta potential slowly increases with the increase of dosage of PAC2.5 for its low charge density (Fig. 3d). And more PAC2.5 is demanded to make the zeta potential of solution approach to zero. It has been claimed that the best coagulation performance is achieved near the zero potential when charge neutralization is the main coagulation mechanism [6,20]. However, the highest removal efficiencies are always obtained below zero potential, which indicates that charge neutralization is not the only coagulation mechanism in this study. And the bridging of Alb /Al13 , the adsorption of aluminum hydroxide gels and the entrapment of flocs also contribute to the algae removal.
Please cite this article as: X. Tang et al., Effect of fresh aluminum hydroxide gels on algae removal from micro-polluted water by polyaluminum chloride coagulant, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.03.001
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The surface morphologies of flocs formed in the test are able to describe the flocs composition and speculate the coagulation process [21]. PAC2.5 containing fresh aluminum hydroxide gels easily forms big colloidal aluminum hydroxide particles in the treatment (Fig. 4), which also improves the capacity of adsorption. And the formed flocs are more compact than those mainly formed by colloidal particle collisions. Besides, the outer surface of flocs formed by PAC2.5 and PAC2.0 is rough and loose. It is suggested that other small particles are swept by the flocs, which explains that some parts of the flocs easily break in the fast re-stirring after settlement in the previous study [22]. The fractal dimension (Df ), an essential character of flocs, is represented by the slope of Log A versus Log P, where A and P are the area and perimeter of flocs respectively. And the slope of scattering plot is measured via fitting a straight line. Normally, the higher fractal dimension is related to the larger floc size and the better settlement [21]. In this study, the flocs formed by PAC2.5 and PAC2.0 have higher fractal dimension, and their better performances have been presented in Fig. 3. 4. Conclusions PAC was able to be prepared by fresh aluminum hydroxide gels under short heating time and low energy consumption, and it would be a promising preparation method applied in industrial scale. PAC2.0 and PAC2.5 prepared in the laboratory maintained the desirable Alb contents in the experimental period, and both of them were high in Alb2 contents. A great deal of Al13 was also found in the PAC2.0 . The presence of fresh aluminum hydroxide gels in preparation and aging period had the effects on characteristics of PAC. Fresh aluminum hydroxide gels were known as the aluminum resource in the preparation, and they facilitated the transformation of monomeric aluminum to middle polymer aluminum. The added fresh aluminum hydroxide gels in the aging period increased Alb content at the initial phase, but Alb content rapidly reduced with the time extending since fresh aluminum hydroxide gels accelerated the aluminum hydrolysis. And adding excessive fresh aluminum hydroxide gels would aggravate the decrease of Alb content. PAC2.5 containing fresh aluminum hydroxide gels performed slightly worse than PAC2.0 in the algae removal from micropolluted water due to the shortage in charge neutralization and bridging. But the charge reversal resulted from overdose effect was mitigated in the application of PAC2.5 for its low charge density. Thus, PAC2.5 was more suitable to be used in the micro-polluted water treatment. Fresh aluminum hydroxide gels enhanced the yield of colloidal aluminum hydroxide particles, which resulted in the formation of compact flocs and the reduction of residual aluminum concentration in the treated water. Charge neutralization was not the only coagulation mechanism via the zeta potential analysis, and adsorption, bridging and entrapment were indispensable in the micro-polluted water treatment. Acknowledgments We are grateful for the financial support provided by the National Natural Science Foundation of China (Project no. 21477010), the National Key Technologies R & D Program of China (Grant no. 2012BAJ25B06) and the National 111 Project (Project no. B13041).
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Please cite this article as: X. Tang et al., Effect of fresh aluminum hydroxide gels on algae removal from micro-polluted water by polyaluminum chloride coagulant, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.03.001