ultrafiltration process

Journal of Hazardous Materials 300 (2015) 67–74

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Effect of pH with different purified aluminum species on coagulation performance and membrane fouling in coagulation/ultrafiltration process Lijuan Feng a,b , Shuang Zhao a , Shenglei Sun a , Wenyu Wang a , Baoyu Gao a , Qinyan Yue a,∗ a Shandong Provincial Key Laboratory of Water Pollution Control and Resource Reuse, School of Environmental Science and Engineering, Shandong University, Jinan 250100, China b Key Laboratory of Inorganic Chemistry in Universities of Shandong, Department of Chemistry and Chemical Engineering, Jining University, Qufu, Shandong 273155, China

h i g h l i g h t s • • • •

Coagulation performance of three high content Al species under different pH was first studied. The different floc properties and membrane fouling with Al species were discussed. Membrane fouling was different because of flocs performance. Alb at pH 6 has the best benefit to alleviate the membrane fouling.

a r t i c l e

i n f o

Article history: Received 31 March 2015 Received in revised form 20 May 2015 Accepted 18 June 2015 Available online 23 June 2015 Keywords: pH Membrane fouling Floc properties Purified Al species

a b s t r a c t The influences of solution pH on coagulation/ultrafiltration (C–UF) process were investigated by using three purified Al species of polyaluminium chloride (PACl). A series of online-simulation experiments were developed to assess the coagulation removal efficiencies (turbidity, UV254 ), floc properties and membrane fouling in this paper. The results showed that change of pH had a significant impact on coagulation efficiencies, floc properties, membrane flux as well as the whole process. Under acidic condition, the hydrolysis action of aluminum salts was restrained which is bad for charge neutralization. While under alkaline region, absorption was the dominant mechanism to combine HA–Kaolin. Meanwhile, HA is apt to soluble by deprotonating under alkaline region which is hard to remove. These common effects made the experiment results complex. HA removal efficiency of Ala and Alb were higher than that of Alc , but the turbidity removal by Alc was slightly higher under the same pH condition. Flocs generated by Ala at pH 6 had advantages such as larger size and the most loosely structure which contributed the most to alleviating membrane fouling. Membrane fouling with Alb and Alc in alkaline range was more serious than that in acidic range. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Ultrafiltration (UF) technology has been recognized as an important process for surface water treatment as membrane could entrap particles, colloids and microorganisms [1]. However, serious membrane fouling and low natural organic matters (NOM) removal efficiencies were the main factors impeding the development of UF technology [2]. Thus, various technologies have been studied to change operation conditions or intro-

∗ Corresponding author. Fax: +86 531 88364513. http://dx.doi.org/10.1016/j.jhazmat.2015.06.034 0304-3894/© 2015 Elsevier B.V. All rights reserved.

duce pretreatments to overcome those barriers, includes rinsing, air scouring, chemical cleaning and running modes, adsorption, coagulation, pre-oxidation and pre-filtration, etc. [3]. Recently, coagulation/ultrafiltration (C–UF) combined technology has been used frequently for drinking water treatment [4]. C–UF has the advantage of absorbing NOM and forming large flocs to reduce membrane fouling, which could extend service life of membrane [3]. In addition, the relative low cost of coagulation pretreatment presents a great economic advantage of C–UF. As revealed by previous studies, coagulant types and physical properties of generated flocs significantly affected the performance of C–UF combined technology. Choo stated that the degree of permeable reduction highly depended on coagulants types [5]. Moreover,

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L. Feng et al. / Journal of Hazardous Materials 300 (2015) 67–74

Konieczny et al. stated that aluminum salt coagulant has higher organic removal efficiency than ferric salt in an in-line C–UF process [6]. As aluminum salt coagulant was added into water, there would be a series of hydrolysis reactions and different Al species were formed: monomeric Al species (Ala ) which is composed of molecule, small polymers and instantaneously reacted products such as Al3+ , Al(OH)2+ , Al(OH)2 + , Al(OH)3 and Al(OH)4 − , Al2 (OH)2 4+ dimer, Al3 (OH)4 5+ trimer; medium polymeric species (Alb ); as well as larger polymer and colloidal species (Alc ). According to previous investigations and practical applications, Alb was the most efficient species for removing contaminants due to its high positive electric charge and stability [7]. Thus, many researches focused on the preparation of PACl to enhance the percentage of Alb in industry. The hydrolyzate species were unstable and hard to control in the decoctions of PACl due to a small fluctuation of conditions. For example, the pH and temperature could vary their constitution because of the small fluctuation of conditions. It is also found that pH has significant effects on Al species transformation in raw water. It is well known that the amount of electric charges for hydrolyzed Al was determined by the pH value of solution. So the adjustment of pH is an important and effective way to control hydrolysis product of PACl. Meanwhile, Cho et al. confirmed that the degree of membrane fouling caused by NOM was strongly associated with pH value [8]. However, there are few reports about the influence of pH value on floc characteristics and membrane fouling by three Al species during coagulant/ultrafiltration process. In conclusion, there are large amounts of natural organic matter and suspended solids in surface water, which play crucial roles in membrane fouling [9]. Thus, it is meaningful to study the effect of natural organic matter on membrane fouling. In this study, humic acid (HA) which was the main ingredient in natural organic matter and Kaolin were used to simulate the raw water and the influence of pH on coagulation performance and membrane fouling of three Al species were investigated. 2. Materials and methods 2.1. Preparation of coagulants All reagents used in this experiment were of analytical grade. Ala was prepared by adding 1.7884 g AlCl3 ·6H2 O to 200 mL deionized water directly. Alb : High Alb content of PACl was prepared before purification. Then Alb specie was purified with ethanol–acetone precipitation method [10]. Alc was prepared as follows: the PACl as mentioned above was heated (85 ◦ C) and stirred continuously for 24 h. After 48 h of curing, the PACl was purified by methanol/acetone mixture (1:9). 2.2. Raw water HA is the most important NOM of surface water [11]. Thus, HAKaolin simulated water was used as raw water in this study. The preparation of HA simulated water was as follows: 0.40 g NaOH (Tianjin Damao Co., Tianjin, China) and 1.00 g HA (Aladdin, Shanghai, China) were dissolved in 800 mL deionized water under 1 h magnetic stirring. The solution was diluted to 1000 mL afterwards. Kaolin was added to simulate suspension. Usually, Kaolin stock solution was used instead of Kaolin in this study. The preparation method was as follows: 5.00 g Kaolin (Kermel Co., China) was dissolved into 1000 mL deionized water with 30 min continuous stirring, and then diluted to 2000 mL. The liquor was transferred into a 2000 mL glass measuring cylinder and settled for 30 min. The upper 1200 mL was drawn out for later use. HA stock solution was diluted for 100 times to make HA–kaolin synthetic

water. Meanwhile, kaolin stock solution was used to adjust the initial turbidity to 15.0 ± 0.5 NTU. The properties of synthetic raw water: pH 8.3 ± 0.1, turbidity = 15.0 ± 0.5 NTU, UV254 = 0.29 ± 0.03, zeta = −16 ± 2 mV. 2.3. Jar test Coagulation experiments were conducted in a programmable jar test apparatus (ZR4-6, Zhongrun Water Industry Technology Development Co., Ltd., China). The apparatus consisted of six 1.0 L plexiglass beakers which can be simultaneously operated with different dosage and pH values for comparison in the same condition. A mechanical stirring paddle was used to mix solution and also to change the condition of shear. The size of flat paddle was 50 mm × 40 mm. The stirring conditions were: 200 rpm for 2 min (rapid stir, coagulant was added after the liquor was stirred for 30 s), followed by 40 rpm for 15 min (slow stir). Then after 20 min of sedimentation, the samples were gently withdrawn from 1.0 cm below the surface for subsequent analysis. Turbidity was measured immediately by a turbidimeter (Hach 2100P Co., US) after the sample was transferred. Zeta potential was tested using a Zetasizer (Malvern 3000 Hsa, UK). UV254 absorbance was measured by a UV-754 UV/VIS spectrophotometer (Jinghua Science and Technology instrument Co., Shanghai, China) after the sample was filtered through 0.45 ␮m fiber membrane. 2.4. Floc properties Floc properties were analyzed to measure dynamic floc size by using a laser diffraction instrument (Mastersizer Malvern 2000, Malvern, UK). A 1 L plexiglass beaker was used with transportation system by a peristaltic pump for the inflow/outflow latex tubing which was placed opposite the instrument. Coagulation experiments were conducted in similarly procedure as jar tests mentioned above: 200 rpm for 2 min, followed by a long slow stirring at 40 rpm for 15 min, then a breakage period at 200 rpm for 5 min, followed by a re-growth phase at 40 rpm for 15 min. As the experiment proceeded, flocs size data were automatically recorded by computer every 30 s. In this study, the volumetric equivalent diameter of flocs was used to express floc size. Fifty percentile of the mean size (d50 ) by statistics was adopted. The growth rate, floc strength (Sf ), recovery factors (Rf ) and fractal dimension (Df ) were used to demonstrate the floc characteristics. All of those parameters [12] were determined by Mastersizer as mentioned above. Growth rate =

size time

(1)

Strength factor =

d2 × 100 d1

(2)

Recovery factor=

d3 − d2 d1 − d2

(3)

where d1, d2, d3 are the average floc size of initial steady phase, breakage period and regrowth to stable phase, respectively. A higher strength factor (Sf ) value reveals that flocs would be more resistant to high shear force. It also means the flocs were hard to break. Likewise a higher recovery factor (Rf ) indicates a better recoverability after high shear. The fractal dimension (Df ) expresses the structure of particles aggregates and can be determined by the relationship between I (light intensity) and Q (scatter vector) with the equation as follows [13]: I ∝ Q −Df

(4)

L. Feng et al. / Journal of Hazardous Materials 300 (2015) 67–74

a

80

Turbidity removal efficency (%)

UV 254 remove efficiency(%)

90

80

70

60

50

Ala Alb Alc

40

4

5

6

7

8

9

69

b

60

40

20

Ala Alb Alc

0 4

5

6

pH

7

8

9

pH

10

c

Zeta potencial(mv)

5

0

-5

Ala Alb Alc

-10

-15 4

5

6

7

8

9

pH Fig. 1. Coagulation efficiency of Al species under different pH conditions: (a) UV254 removal efficiency, (b) residual turbidity, (c) zeta potential.

Linearization of the Eq. (5) can be found as a log–log plot of Eq. (4):

constant pressure [14,15]. Four basic fouling types of this model were expressed by Eqs. (6)–(9), respectively:

log I ∝ − Df log Q

Complete blocking model: ln J = ln J0 − kc t

(6)

1 1 = 0.5 + ks t J 0.5 J0

(7)

1 1 + ki t = J J0

(8)

(5)

Df can be calculated as the slope and the values usually range from 1 to 3 in three-dimensional space. A higher Df indicate more dense and compact structures, while a lower Df indicates larger, loosely bound and highly branched flocs.

Standard blocking model :

Intermediate blocking model :

2.5. coagulation/ultrafiltration experiment The ultrafiltration system was composed of a 300 mL-capacity batch cell, a pressure unit which provided constant pressure with nitrogen gas at 150 ± 5 kPa and a data acquisition unit by collecting permeate mass using electronic balance (MSU5201S-000-D0, Sartorius AG Germany). The coagulated water without sedimentation was withdrawn carefully into a cylindrical apparatus which was the key membrane filtration unit. An ultrafiltration flat feet polyethersulfone (PES) membrane (Mosu Co., Ltd., China) with effective diameter 80 mm was installed at the bottom of the apparatus. The specification of polyethersulfone (PES) membrane was molecular weight cut-off of 100 kDa. A magnetic stirring apparatus was employed in the lower cylindrical apparatus to make a uniform suspension. The fouling mechanism may be assessed by Hermia’s semiempirical model under the condition of dead-end filtration and

Cake layer model :

1 1 = 2 + kg t J2 J0

(9)

where J is the permeate volume (L/m2 h), J0 is the certain permeate volume (L) when t = 0, t is the filtration time (min), and kc , ks , ki , kg are constants of the model, respectively. The main fouling mechanism can be confirmed according to the relevant correlation coefficients (R2 ) by calculating experimental data into these formulas. A larger R2 values indicated a better fitting model. The fouling potential of ultrafiltration membrane is explained by modified fouling index (MFI) according to the cake filtration theory [16]. t Rm ␩␣Cb = + V V PA 2PA2

   MFI

(10)

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L. Feng et al. / Journal of Hazardous Materials 300 (2015) 67–74

400

Al a

Al b

200

Floc size d50 (um)

300

Floc size d50(um)

250

pH=4 pH=6 pH=7 pH=9

350

250 200 150 100

150

100

50

pH=4 pH=6 pH=7 pH=9

50 0

0 0

2

4

6

8

10

12

14

16

18

0

2

4

6

Time(min) 250

8

10

12

14

Time(min)

Alc

Floc size d 50(um)

200

150

100

50

pH=4 pH=6 pH=7 pH=9

0 0

2

4

6

8

10

12

14

Time(min) Fig. 2. The trend of floc size under different pH conditions: (a) Ala , (b) Alb , (c) Alc .

The MFI is determined from the gradient of the linear portion of t/V versus V. A lower MFI value indicated less serious membrane fouling. The more detail could be seen in author’s paper [17]. 3. Results and discussion 3.1. Effect of pH on coagulation performance Coagulation jar tests were initially performed under raw water pH conditions to ascertain the optimum dosage of Al species for NOM and turbidity removal. Our previous study showed that all the efficiencies of turbidity and UV254 removal were greater than 80% with lowermost Al dosage of 6 mg/L [17]. Thus, 6 mg/L of Al dosage was chosen for experiments in this study. After the optimum dosage of Al species was determined, the pH of simulated water was adjusted in the range of 4–9 by HCl (0.1 M) and NaOH (0.1 M) solutions. All the coagulation experiments were conducted with the unified program. The trends of UV254 removal efficiency and residual turbidity along with pH were shown in Fig. 1(a) and (b). As pH values varied from 4 to 9, UV254 removal of three Al species increased sharply first, until they reached a maximum at pH 6 and then decreased gradually. Under acidic conditions, the low UV254 removal efficiencies were mainly due to the restriction of Al salt hydrolysis. The dominant coagulation mechanism of Ala is adsorption hydrolization, therefore pH value has an important influence on the Ala coagulation performance, which further leads to the worst coagulation efficiency of Ala . Relatively, Alb and Alc , as the pre-hydrolysis products of Ala , the influences of pH

on hydrolysis action are smaller than Ala [17]. The UV254 removal efficiencies were low under alkaline conditions mainly due to the increasing deprotonation of acidic functional groups on HA macromolecules. When negative charge distributed on the long chain of HA, the macromolecules dissolved in water and became stable according to the resonance theory. It is reported that the electrostatic repulsion would become more obvious with increasing pH, which led to the less dominant hydrophobic interaction [18]. Alc was the minimum with regard to UV254 and turbidity removal in alkaline condition, mainly because the absorption of Al salt was the dominant mechanism [17]. However, Alc was stable colloid which could not absorb soluble HA of negatively charged flocs. Thus, compression of the electric double layer process would not occur. The maximum removal efficiency at pH 6 might be close to the isoelectric point when HA molecules became more hydrophobic and insoluble [19]. As shown in Fig. 1(b), the trends of turbidity removal for three Al species were totally different. For Ala , the efficiency was increased when pH increased. At pH 4, turbidity removal with Ala was zero because Al3+ could not form the primary flocs. For Alb , the efficiency decreased gently with increasing pH and reached bottom value at pH 6. As pH continually increased, the efficiency kept growing dramatically. The turbidity removal efficiency of Alb was the highest under studied pH conditions except pH 6. It was mainly because Alb molecules were quit stable during coagulation process and the influence of pH on the restriction with Al hydrolysis was limited. However, the trend of Alc was on the contrary with Alb . As for UV254 removal efficiency, the turbidity removal efficiency reduced as pH

L. Feng et al. / Journal of Hazardous Materials 300 (2015) 67–74 Table 1 Concentration and percentage of Al species. Coagulants Ala Alb Alc

AlT (mol/L) 0.037 ± 0.001 0.01 ± 0.002 0.009 ± 0.0005

Ala (%) 100 0 4.88

Alb (%)

Alc (%)

0 100 8.75

0 0 86.37

increased, maybe because Alc was hydrolyzed into Alb in acidic range and then enhanced the coagulation effect. Fig. 1(c) displayed the trends of zeta potential with three Al species under various pH. Zeta potential increased initially and then decreased as pH further increased in all cases. The absorption and sweeping ability of Al species were in the following order: Ala > Alb > Alc . As stated by Yu et al. adsorption and charge neutralization are main coagulation mechanisms of Al salts [20]. Fig. 1(c) indicated that the dominant coagulation mechanism was charge neutralization in acidic range while absorption and sweeping mechanisms were dominant in alkaline range. That is the reason why the HA–kaolin removal was higher when pH value was larger than 7. From Fig. 1, it can be generalized that optimal pH for turbidity removal is 9 for Alb and the most effective HA removal occurs at pH 6 with the coagulant of Ala species. The UV254 removal efficiency with Alc (pH 9) and turbidity removal with Ala (pH 4) were the lowest, which indicates that pH has significant influence on these conditions. Based on curvilinear trend of Fig. 1, pH 5 and pH 8 can be omitted to simplify subsequent experiments (Table 1). 3.2. Effect of pH on floc properties The typical pH of natural water from 4 to 9 have been choose. The circumstances of pH 5 and pH 8 have been omitted for the figure is easier to read. Floc growth and sizes of different Al species under pH 4, 6, 7, 9 with time were shown in Fig. 2. Generally, positively charged products by Al salt hydrolyzation were neutralized quickly by negatively charged particles as Al salt coagulants were added into raw water and then primary flocs with small sizes were formed. Subsequently, these primary flocs began to grow rapidly by absorbing HA macromolecules. At the same time, large flocs would be broken by collision in the process of aggregation. When the velocities of aggregate and breakage were equal, a balance plateau of flocs size was achieved [21]. Fig. 2 indicated the largest floc size appeared under pH 6 in all steady-state cases, as well as the fastest growth rate. The flocs sizes formed in acidic range were smaller than those in alkaline range. However, the differences of sizes formed in neutral condition and alkaline range were inconspicuous. Under the same dosage and pH conditions, floc sizes by different Al coagulants were in the following hierarchy: Ala > Alb > Alc , which was due to the differences of Al species distributions. Ala promptly hydrolyzed to Alb and reacted with HA as well as kaolin particles to form large flocs under this pH. As mentioned above, the dominant coagulation flocs were formed by weak hydrolysis of Al(III) by charge neutralization in acidic condition. The dominant form of Al salt was Al3+ , which was hard to react as prime floc. On the other hand, HA unites Kaolin which also reduces the combining capacity. So the floc sizes and growth rate were restrained at the pH of 4 (Table 2). Moreover, the strongest floc appeared (the Sf value was 59.24) when Alb was used at pH 6. Chaignon et al. found that flocs formed by charge neutralization were stronger than those formed by sweeping [22]. At pH 6, the floc sizes of three Al species were in the order of Ala > Alb > Alc . Meanwhile, Boller confirmed that small flocs (<200 ␮m) were stronger against shearing forces [23]. Zhao et al. suggested that the primary flocs formed by sweeping and bridging were larger than those formed by charge neutralization [10]. Tang and Luan reported that

71

Alb aggregates were quite steady to dilution or to changes with pH which only converted slowly [24]. The floc size of Alc grew slowly at pH 4, but at last it was the largest, which was mainly due to hydrolysis of Alc in acidic range and production of prime flocs through adsorption and bridge. The program of coagulation experiment was changed by increasing 5 min of high shear force after floc growth and then back to original program for comparing the variations of different Al salts under different pH conditions. The change of floc sizes were shown in Fig. 3. The data were chosen and calculated according to Eqs. (2)–(5) (Table 2). Fig. 3 showed that the floc size decreased sharply as high shear force was introduced in all cases. The maximum values of Rf and Df appeared simultaneously with Alb at pH 4 and 9. Meanwhile, Cohen and Fong believed flocs formed by sweep have the lowest regrowth ability [25]. Therefore, Df value of Alc was larger than that of Ala , followed by Alb . However, changes in Sf and Rf values of Alc under different pH were not obvious due to the stability of Alc colloid. The data showed that larger Sf value appeared in alkaline condition, which demonstrated that flocs in acidic range were weaker. Jarvis et al. discovered that floc strength decreased as floc size increased [26]. Flocs were larger in neutral and alkaline conditions than those in acidic conditions (Fig. 3), which can be explained by different mechanism of coagulation. As the dominant form of Al salt, Al3+ could aggregate mainly via charge neutralization in acidic condition, while absorption and sweeping were less likely to occur. Moreover, in alkaline range, the formation of Al (OH)4 − could reduce the effect of absorption and sweeping which were the dominant mechanisms of Al salt under unadjusted raw water pH. Wang et al. proposed that flocs formed through sweeping and absorption were more compact than that through charge neutralization [27]. As can be seen from Table 2, the Df values of three coagulants in alkaline range were larger than in acidic range. It indicated that on one hand, the former flocs were looser; and on the other hand, protons of carboxy groups in HA were left at higher pH which made carboxylic exposed to cationic coagulant. In this case, chemical reaction between ions was stronger than electrostatic repulsion. Xu et al. reported that the Alb aggregates were quite steady which had little effect with pH changes, and meanwhile Alb flocs had loose and branched structure which could form bridges within particles [7]. 3.3. Effect of pH on membrane fouling The profiles of normalized membrane permeate flux under different pH conditions with filtration time were shown in Fig. 4. In this part, pH 4, 6, 9 were chosen according to the result of Section 3.2. Fig. 4 demonstrated that flux went down dramatically and then smoothed gradually in all cases. The slightest membrane flux for three Al species occurred at pH 6, and the most severe flux decline appeared at the pH 4 for Ala and pH 9 for Alb and Alc , respectively. In the case of Ala at pH 4, flocs size was quiet small and could not form a quantity of prime participation. Flocs formed at low pH were denser and less porous. For Alb and Alc , the difference of membrane fluxes decline in alkaline range and acidic range was inconspicuous due to the difference of floc properties. Hermia’s model was adopted to interpret the membrane fouling mechanism of coagulated suspension by different Al species under various pH conditions. The experimental data were fitted into Eqs. (6)–(9) and then the corresponding correlation coefficients (R2 ) about mode parameters and time were exported, respectively (presented in Table 3). The R2 values with different models of every line could reveal the well fitted one with declined flux by coagulated suspension and the dominant fouling mechanism. From Table 3, the largest R2 values all appeared at the last column which indicated that the cake layer model played a significant role regardless of

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L. Feng et al. / Journal of Hazardous Materials 300 (2015) 67–74

Table 2 Performance of Al species under different pH conditions. Growth rate

Strength factor

Recovery factor

Fractal dimension

Ala

pH 4 pH 6 pH 7 pH 9

0.49 43.73 31.74 35.52

17.95 27.99 31.51 32.12

15.65 17.70 26.33 35.57

2.90 2.32 2.34 2.35

Alb

pH 4 pH 6 pH 7 pH 9

12.80 29.38 21.33 25.11

28.91 59.24 37.83 36.68

53.23 22.68 30.34 27.04

3.06 2.70 2.37 3.06

Alc

pH 4 pH 6 pH 7 pH 9

10.28 33.51 27.61 22.47

32.79 33.14 34.50 33.26

40.10 31.50 35.20 34.99

2.79 3.01 2.54 2.49

250 350

Ala

200 150 100 50 0 -50

pH=4 pH=6 pH=7 pH=9

200

Floc size d 50 (um)

250

Floc size d 50 (um)

Alb

pH=4 pH=6 pH=7 pH=9

300

150

100

50

0 0

5

10

15

20

25

30

0

35

5

10

Time (min) 250

Alc

20

25

30

pH=4 pH=6 pH=7 pH=9

200

Floc size d50 (um)

15

Time (min)

150

100

50

0 0

5

10

15

20

25

30

35

Time (min) Fig. 3. The trend of floc size after shear force under different pH conditions: (a) Ala , (b) Alb , (c) Alc .

Table 3 Hermia’s model linear equations of data by Al species under vious pH. Complete blocking

Standard blocking

Intermediate blocking

Cake layer

Ala

pH 4 pH 6 pH 9

0.8038 0.8247 0.8315

0.8526 0.8324 0.8539

0.8881 0.8387 0.8727

0.9257 0.8468 0.8991

Alb

pH 4 pH 6 pH 9

0.836 0.9126 0.8267

0.8833 0.9314 0.8776

0.9182 0.9449 0.9166

0.9555 0.9573 0.9618

Alc

pH 4 pH 6 pH 9

0.8247 0.8747 0.8261

0.8603 0.8939 0.8761

0.8885 0.9089 0.9126

0.9254 0.9276 0.9505

L. Feng et al. / Journal of Hazardous Materials 300 (2015) 67–74

Normalized permeate flux, J/J0

1.0

1.1

Ala

Alb

pH=4 MFI=139.77 pH=6 MFI=11.132 pH=9 MFI=31.027

0.9

pH=4 MFI=79.899 pH=6 MFI=45.875 pH=9 MFI=150.13

1.0

Normalized permeate flux, J/J 0

1.1

73

0.8 0.7 0.6 0.5 0.4 0.3

0.9 0.8 0.7 0.6 0.5 0.4 0.3

0.2 0

2000

4000

6000

8000

10000

12000

0.2 -2000

0

2000

4000

6000

Time (s)

Normalized permeate flux, J/J 0

1.1

8000 10000 12000 14000 16000

Time (s)

Alc pH=4 MFI=62.272 pH=6 MFI=16.265 pH=9 MFI=78.899

1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0

2000

4000

6000

8000

10000

12000

Time(s) Fig. 4. The function of filtration of flux under different pH conditions: (a) Ala , (b) Alb , (c) Alc .

pH conditions. This conclusion was consistent with previous study, which suggested that the decline of permeate flux by coagulants was mainly related to porosity of cake layer formation [28]. An intimate relation was reported between floc characteristics and porosity of cake layer [29]. It was believed that specific cake resistance increased with increasing fractal dimension and decreasing floc size. Flocs formed under acidic pH conditions seemed to be less porous and denser [30]. Thus, the membrane fouling in acidic range was more serious in all cases. However, the reason for flux decline in alkaline range was as follows: Al(OH)4 − was the major form hydrolyzed by Alb and Alc which carried negative electricity layer that could not conduct double electrode layer through combination with deprotonated HA. MFI of three Al species with different pH values were also counted using Eq. (10) and shown in Fig. 4. Higher MFI value indicated more serious membrane fouling. MFI of Ala under different pH conditions increased in the following order: pH 6 < pH 9 < pH 4, and the MFI order for Alb and Alc was pH 6 < pH 4 < pH 9. The maximum MFI value appeared at Alb under pH 9, and the minimum at Ala under pH 6 which indicated the slightest membrane fouling. The differences of MFI for three Al species under various pH conditions were remarkable, which arose from the distinct floc properties. Floc formed by Ala at pH 6 were the largest and the looser structure made cake layer polyporous, which could alleviate membrane flux decline. However, coagulated suspension of Alb at pH 9 had flocs of small sizes, weak recovery ability and compact structure, which resulted in more serious membrane fouling. It was noticed that the

flux trend curves of Alb and Alc at pH 4 and pH 9 were similar and nearly overlapped. This resulted from the common effect of characteristics of floc and forms of HA. As for Alb , on one hand, flocs at pH 4 were smaller than that at pH 9 which could made membrane fouling serious. On the other hand, flocs formed by charge neutralization in acidic range could produce lower hydraulic resistance which were looser than by sweep and bridge conditions [31]. Thus, it appeared slightly serious at pH 4 than that at pH 9 due to the two common influences. Flocs of Alc treatment at pH 4 were larger than those at pH 9, which made membrane fouling less serious than at pH 9. However, Alc flocs at pH 9 were looser than those at pH 4, which reduced the difference of membrane flux between them.

4. Conclusions 1. HA removal efficiency of Ala and Alb were higher than that of Alc , while the turbidity removal by Alc was the highest. 2. Change of pH had a significant impact on coagulation efficiency, floc properties, membrane fouling in the whole process. 3. Flocs coagulated by Ala and Alb in acidic ranges were smaller and more compact than those generated in alkaline ranges. However, Alc flocs had larger, stronger and more compact characteristics under acidic conditions. 4. Membrane fouling of coagulated water by Ala at pH 6 was the lowest. Meanwhile, membrane fouling of Alb coagulated water at pH 9 was the most serious.

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