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Coagulation by titanium tetrachloride for fulvic acid removal: Factors influencing coagulation efficiency and floc characteristics
Desalination 335 (2014) 70–77
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Coagulation by titanium tetrachloride for fulvic acid removal: Factors influencing coagulation efficiency and floc characteristics Y.X. Zhao a, B.Y. Gao a,⁎, G.Z. Zhang a, S. Phuntsho b, H.K. Shon b a b
Shandong Key Laboratory of Water Pollution Control and Resource Reuse, School of Environmental Science and Engineering, Shandong University, No.27 Shanda South Road, Jinan 250100, China Centre for Technology in Water and Wastewater, School of Civil and Environmental Engineering, University of Technology, Sydney (UTS), P.O. Box 123, Broadway, NSW 2007, Australia
H I G H L I G H T S
G R A P H I C A L
• TiCl4 achieved higher fulvic acid removal than traditional coagulants. • TiCl4 flocs exhibited higher growth rate and larger size than Fe and Al salts. • Floc strength decayed and recoverability increased with increasing shear rate. • An extension in break-up period deteriorated both floc strength and recoverability. • TiCl4 flocs displayed the weakest recoverability after breakage.
With optimal coagulant doses: (a) growth, breakage and regrowth profile of FA flocs formed by TiCl4, FeCl3 and Al2(SO4)3; (b) PSD of TiCl4–FA flocs; (c) PSD of FeCl3–FA flocs; (d) PSD of Al2(SO4)3–FA flocs.
a r t i c l e
a b s t r a c t
i n f o
Article history: Received 14 September 2013 Received in revised form 3 December 2013 Accepted 18 December 2013 Available online 14 January 2014 Keywords: Coagulation–flocculation Titanium tetrachloride Floc characteristics Shear force Break-up period
A B S T R A C T
Coagulation efficiency of titanium tetrachloride (TiCl4) was investigated for fulvic acid (FA) (as model organic matter) removal compared to traditional coagulants — iron chloride (FeCl3) and aluminum sulfate (Al2(SO4)3). The growth, breakage and regrowth nature of flocs formed by the three coagulants were also comparatively evaluated under different coagulation conditions. Results indicated that TiCl4 achieved higher removal of UV278 (absorbance at 278 nm) and DOC (dissolved organic carbon) than FeCl3 and Al2(SO4)3, and TiCl4 produced the largest flocs with the highest floc growth rate. The responses of floc to different operating parameters depend on the coagulant used, while the floc breakage and re-aggregation were also significantly influenced by shear force and break-up period. Floc strength decayed with the increasing shear force, while the inverse trend was observed for floc recoverability. An extension in break-up period resulted in the deterioration of both floc strength and recoverability regardless of the coagulants used. Flocs formed by TiCl4 displayed the weakest recoverability after breakage by contrast to conventional coagulants. Since TiCl4 is only studied recently as a novel coagulant, the comparative study of coagulation performance and floc characteristics of TiCl4 against the traditional coagulants provided valuable information for its wide application in the future. © 2013 Elsevier B.V. All rights reserved.
⁎ Corresponding author. Tel.: +86 531 88366771; fax: +86 531 88364513. E-mail address: [email protected] (B.Y. Gao). 0011-9164/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.desal.2013.12.016
Y.X. Zhao et al. / Desalination 335 (2014) 70–77
1. Introduction Coagulation–flocculation is one of the most important steps in water treatment plants [1]. Aluminum and iron salts are widely used as coagulants and they are effective in removing a broad range of impurities, including colloidal particles and organic substances [2]. However, the production of large amounts of sludge that need further disposal is the main drawback of using conventional Al and Fe coagulants. The sludge post-treatment is considered as one of the most costly and environmentally problematic challenges of all water treatment works [3,4]. Efforts are therefore made to develop a new coagulant that can reduce either the production of sludge or the influence of sludge disposal on environment. Upton and Buswell [5] first used titanium compounds as coagulant in water treatment. Since then, the coagulation effectiveness of Ti-salts has been proved in terms of particle, nutrients and organic matter removal. Recently, Shon and his co-workers investigated the coagulation efficiency of using titanium tetrachloride (TiCl4) as coagulant, with their results showing that TiCl4 successfully achieved high organic matter removal to the same extent as Al and Fe salts and the resultant flocs had better settleability [6–9]. The most significant advantage was the resultant sludge recovery for the production of a valuable by-product, namely titanium dioxide (TiO2), which is the most widely used metal oxide, whose applications include photocatalysts, cosmetics, paints, electronic paper, and solar cells [10,11]. Shon et al. [7] reported that up to approximately 40 mg-TiO2 nanoparticles/L-wastewater was produced from the wastewater sludge generated by the Ti-salt flocculation. Moreover, titanium does not play any adverse role or toxicity inside the human body and is non-toxic even in large doses [12]. Also, it is used in a gamut of medical applications because of its biocompatible. Therefore, titanium and its compounds are rarely included in any water quality guidelines [13]. Additionally, titanium is one of the most abundant elements on earth and with the development of the titanium industry, the cost of Ti-salts is gradually becoming comparable to that of conventional coagulants. TiCl4 is therefore expected to be a promising alternative coagulant for conventional Al and Fe coagulants. Since TiCl4 is only studied recently as a novel coagulant, the mode of coagulation action and the factors affecting coagulation efficiency and floc characteristics are not completely understood. Floc characteristics after coagulation-flocculation are key points influencing solid/liquid separation process, which would subsequently affect the coagulation efficiency [14]. Small particles generally have lower removal efficiency than the larger ones by coagulationflocculation, since the smaller particles settle down more slowly than the larger ones of similar density [15]. The unit process at water treatment works (WTW) is prevalent with regions of high shear [16]. Therefore, the ability of flocs to withstand rupture and the recoverability after being broken has a significant impact at WTW. The broken flocs were hard to regrow and the reduced floc size would slow down the sedimentation rate and change transport mechanisms [17–19]. There were also reports about the full recovery of the broken flocs formed by charge neutralization, while the sweep flocs showed weak recoverability [18,20,21]. Therefore, floc strength and recoverability are considered as important parameters influencing coagulation efficiency and will provide valuable information in understanding coagulation behavior. Floc properties depend not only on characteristics of raw water source and coagulants, but also various hydraulic parameters, such as shear force and break-up periods, etc. [22,23]. Few researchers have focused on the breakage and regrowth of flocs formed by TiCl4 under different coagulation conditions. Natural organic matter (NOM) is a diverse mixture of organic compounds, including low molecule weight compounds, such as fulvic acid (FA), which is mainly produced by living organisms or formed by secondary synthesis reactions [24]. FA, with the reactive characters of a series of oxygen-containing functional groups, including carboxyl, phenolic-OH, enolic, alcoholic-OH, and carbonyl [25], is easily soluble
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in water and difficult to remove. It causes odor, taste, color and bacterial regrowth in potable water, and have potential to form carcinogenic disinfection-byproducts (DBPs) [22]. Thus, effective removal of NOM present in water has become increasingly important in modern water treatment. The aims of this study are to i) evaluate the coagulation performance of TiCl4 for FA simulated water treatment under different coagulant dose and initial solution pH conditions, ii) investigate the effect of coagulant dose and hydraulic conditions, such as shear force and break-up period, on floc characteristics using a laser diffraction instrument, iii) discuss the relationship between coagulation performance and floc characteristics and investigate the coagulation mechanism. Coagulation performance and floc properties of conventional coagulants (iron chloride (FeCl3) and aluminum sulfate (Al2(SO4)3)) were investigated for comparison. 2. Experimental 2.1. Coagulants and test water TiCl4 solution (20%, density = 1.150 g/ml) was obtained from Photo & Environment Technology Co. Ltd (South Korea), and was used directly without any pretreatment. Stock solutions of Al2(SO4)3 and FeCl3 were prepared at a concentration of 1 g/L by Al and 2 g/L by Fe, respectively. Deionized water was used for all the reagents preparation. The stock solution of FA (1.0 g/L) was prepared by dissolving 1.0 g of FA (biochemical reagent purchased from Yinong Biochemical Technology Co. Ltd., Shanghai, China) directly in deionized water. The simulated water for coagulation experiments was prepared by diluting the FA stock solution by tap water to make 10 mg/L of FA [26] and the raw water turbidity was adjusted to 15.0 ± 0.5 NTU by adding kaolin. The measured UV278 absorbance, dissolved organic carbon (DOC), zeta potential and pH of the suspension was 0.099 ± 0.005 cm−1, 5.638 ± 0.580 mg/L, −19.0 ± 1.0 mV and 8.05 ± 0.05, respectively. 2.2. Jar-test Standard jar tests were conducted using a programmable jar-tester (ZR4-6, Zhongrun Water Industry Technology Development Co. Ltd., China). The water sample (1000 mL) was mixed rapidly for 30 s at 200 rpm before adding coagulant. After coagulant addition, rapid mixing (200 rpm) was performed for 1 min followed by slow mixing at 40 rpm for a duration of 15 min which was then followed by 15 min of quiescent settling. Water samples were collected from 2 cm below water surface for measurements. The water samples were prefiltered using 0.45 μm fiber membrane syringe filter before testing UV278 (absorbance at 278 nm using a UV-754 UV/VIS spectrophotometer) and DOC (measured by a Shimadzu TOC-VCPH analyzer); while turbidity and floc zeta potential were directly measured without filtration using a 2100P turbidimeter (Hach, USA) and Zetasizer 3000HSa (Malvern Instruments, UK), respectively. Coagulation-flocculation experiments under different solution pH values were conducted after the optimal coagulant dosages were determined. The target coagulation pH values were achieved by adding appropriate quantities of HCl and NaOH solutions with the concentration of 0.1 mol/L. 2.3. Floc characterization A laser diffraction instrument (Mastersizer 2000, Malvern, UK) was used to measure dynamic floc size as the coagulation and flocculation process proceeded. The schematic diagram of the on-line monitoring system for dynamic floc size can be found in Zhao et al. [9]. The median equivalent diameter, d50, was selected as the representative floc size, although the same trends were observed for d10 and d90 floc sizes.
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To investigate the effect of coagulant dose on floc properties, the procedures of coagulation tests were as follows: Following the floc growth phase, the aggregated flocs were exposed to a shear force at 200 rpm for 5 min, followed by a slow mixing at 40 rpm for 15 min to allow floc regrowth. For the shear-varying coagulation, the flocs were exposed to the increasing mixing speed to 75, 100, 150, 200 and 300 rpm on the jar-tester for 5 min. Also, a series of jar-tests were conducted to study the effect of break-up period on floc strength and recoverability, with three separate break-up periods at 200 rpm of i) a normal break-up period of 5 min, ii) a short break-up period of 1 min and iii) a long break-up period of 10 min. The break-up period was followed by a restoration of the 40 rpm slow mixing phase for 15 min. The floc growth rate was calculated by the slope of the rapid growth region [27]: Growth rate ¼
Δsize Δtime
ð1Þ
Floc strength factor (Sf) and recovery factor (Rf) are used to compare the floc breakage and recoverability [1,9,28,29]: Sf ¼
d2 100 d1
ð2Þ
Rf ¼
d3 −d2 100 d1 −d2
ð3Þ
a
where, d1 is the average floc size of the plateau before breakage, d2 is the floc size after the floc break-up period, and d3 is the floc size after regrowth to the new plateau.
3. Results and discussion 3.1. Coagulant dose optimization Variation of coagulation performance vs. coagulant dose with TiCl4, FeCl3 and Al2(SO4)3 was investigated in terms of residual turbidity, UV278 removal, DOC removal and floc zeta potential, with the results being shown in Fig. 1. From Fig. 1(a), a sharp decrease in residual turbidity was observed with the increasing coagulant dose for both TiCl4 and FeCl3, followed by a plateau for FeCl3, while a gradual increase was observed for TiCl4 at its dose larger than 46 mg/L. For Al2(SO4)3, the residual turbidity varied inconspicuously from 2.5 NTU to 3.3 NTU within the coagulant dose range investigated. Both UV254 and DOC removal for all three coagulants increased significantly first with the increasing coagulant dose, and then approached a plateau or showed slight increase at high coagulant doses (Fig. 1(b) and (c)). Considering both the coagulation efficiency and cost, the optimum dose for TiCl4, FeCl3 and Al2(SO4)3 was fixed at 46 mg/L, 21 mg/L and 6 mg/L, respectively, where i) TiCl4 gave comparable residual turbidity (around 1.8 NTU) as FeCl3, while it was about 2.9 NTU for Al2(SO4)3, ii) TiCl4 yielded UV278 removal of around 78.6%, approximately 18% higher than that by FeCl3 and
b
7
80
FeCl3 Al2(SO4)3
UV278 removal (%)
Residual turbidity (NTU)
TiCl4
6 5 4 3
60
50 TiCl4 FeCl3
40
2 1
70
0
10
20
30
40
50
30
60
Al2(SO4)3
0
10
Coagulant dose (mg/L)
c
d 60
40
50
60
50
60
10 FeCl3
5
Zeta potential (mV)
FeCl3
DOC removal (%)
30
TiCl4
TiCl4 Al2(SO4)3
50
20
Coagulant dose (mg/L)
40
30
Al2(SO4)3
0
-5
-10
-15
20 0
10
20
30
40
Coagulant dose (mg/L)
50
60
0
10
20
30
40
Coagulant dose (mg/L)
Fig. 1. Variation of coagulation performance vs. coagulant dose with TiCl4, FeCl3 and Al2(SO4)3 measured in terms of : (a) residual turbidity, (b) UV278 removal, (c) DOC removal, and (d) zeta potential. (Dose of TiCl4, FeCl3 and Al2(SO4)3 was calculated as mg-Ti/L, mg-Fe/L and mg-Al/L, respectively; Initial solution pH condition.)
Y.X. Zhao et al. / Desalination 335 (2014) 70–77
Al2(SO4)3, and iii) the DOC removal followed the order of TiCl4 (54.7%) N FeCl3 (39.8%) N Al2(SO4)3 (29.6%). Floc settleability was also investigated to evaluate the merits of TiCl4 coagulation-flocculation performance. The details can be found in S1 of the Supplementary Data (SD). The results indicated that the flocs formed by TiCl4 showed higher sedimentation velocity, and therefor much shorter retention time was required, which offered significant advantage in sedimentation tanks. Variation of floc zeta potential with coagulant dose for the three coagulants is shown in Fig. 1(d). The floc zeta potential increased with the increasing coagulant concentration, reaching isoelectric points at TiCl4 dose of about 46 mg/L, FeCl3 dose of about 33 mg/L and Al2(SO4)3 dose of around 10 mg/L, followed by charge reversal as further increased the coagulant dose for TiCl4 and Al2(SO4)3. Changes in floc zeta potential are often used to evaluate the destabilization ability of coagulants. According to Ye. et al. [30], the coagulant dose, where the charge reversal occurs was defined as iso-electrical-dose (IED). Thus, the IED for TiCl4, FeCl3 and Al2(SO4)3 was 46, 33 and 10 mg/L, respectively, indicating the destabilization capability order of TiCl4 b FeCl3 b Al2(SO4)3. This is quite in contrast to the reports that the higher the charge of a cation, the stronger is its effect on particle zeta potential and the higher the valance, the higher the coagulative power [31]. Changes in floc zeta potential are also generally regarded as effective tool to investigate coagulation mechanism, which is often explained in terms of charge neutralization and sweep flocculation
[32]. Under the optimum coagulant dose conditions (as aforesaid), the floc zeta potential for TiCl4, FeCl3 and Al2(SO4)3 was − 2.1 mV, − 8.4 mV and − 4.5 mV, respectively, indicating that sweep flocculation played an important role for FA removal besides charge neutralization. 3.2. The effect of solution pH on coagulation efficiency Change of residual turbidity, UV278 removal, DOC removal and floc zeta potential as a function of initial solution pH was investigated under optimum coagulant dose conditions for TiCl4, FeCl3 and Al2(SO4)3, with the results being shown in Fig. 2. Overall, the residual turbidity decreased with the increasing pH value, reaching a plateau finally. The TiCl4 coagulant was inferior to FeCl3 and Al2(SO4)3 for turbidity removal in acidic conditions, as reflected by the higher residual turbidity at low pH values. Change of UV278 and DOC removal vs. pH were similar for the three coagulant, showing parabolic shape with the inflection points at pH 8, 6 and 6 for TiCl4, FeCl3 and Al2(SO4)3, respectively. Compared to FeCl3 and Al2(SO4)3, the UV278 and DOC removal of TiCl4 coagulant was the least affected by solution pH, and they increased steadily with increasing pH, and then showed decline trend beyond the inflection point. The pH values of 8, 6 and 6 at the inflection points were therefore selected as the optimum ones for TiCl4, FeCl3 and Al2(SO4)3, respectively, under which condition, TiCl4 and FeCl3 gave comparable UV278 removal of around 75.0%, 10% higher than that by
b
a
80
60
70 50 40
UV278 removal (%)
Residual turbidity (NTU)
73
TiCl4 FeCl3
30
Al2(SO4)3
20 10
60 50 40 30
TiCl4 FeCl3
20
Al2(SO4)3
10 0
0 5
6
7
8
9
-10
10
5
6
7
pH
c
d 50
9
10
10 TiCl4
5
Zeta potential (mV)
DOC removal (%)
8
pH
40
30
FeCl3 Al2(SO4)3
0
-5
-10
TiCl4 FeCl3
20
-15
Al2(SO4)3
5
6
7
8
pH
9
10
5
6
7
8
9
10
pH
Fig. 2. The effect of solution pH on coagulation performance of TiCl4, FeCl3 and Al2(SO4)3 under optimum dose conditions: (a) residual turbidity, (b) UV278 removal, (c) DOC removal, and (d) zeta potential. (The optimum coagulant dose for TiCl4, FeCl3 and Al2(SO4)3 was 46 mg/L, 21 mg/L and 6 mg/L, respectively.)
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Al2(SO4)3, and the DOC removal varied in the order of TiCl4 (54.0%) N FeCl3 (48.3%) N Al2(SO4)3 (36.3%). Variation of floc zeta potential as a function of solution pH is shown in Fig. 2(d), with the results showing that the floc zeta potential decreased from positive side to negative side with increasing pH for both TiCl4 and FeCl3, while for Al2(SO4)3, it increased dramatically from negative side to positive side when pH increased from 5 to 6, followed by gradual decrease to negative side again as further increased pH value. Change of floc zeta potential with pH generally correlates well with the variation of coagulant hydrolyzates under different solution pH conditions. The hydrolysis of Al2(SO4)3 was inhibited at pH 5 and the dominant Al speciation was positive monomer hydrolyzates [33,34], which neutralized the negative charges of particles and organic matters, and further destabilized the colloids. However, the positive charges were not enough for full charge neutralization, yielding the flocs with negative charges. This is also the reason for low UV278 and DOC removal at pH 5 as shown in Fig. 2. When pH increased from 5 to 7, the polymeric hydrolyzates with high positive charges and large surface area formed [33,34]. The adsorption, charge neutralization and co-precipitation between the colloids and the hydrolyzates resulted in high UV278 and DOC removal (Fig. 2) and produced the flocs with positive cha rges. Similarly, for TiCl4 at pH b 8 and FeCl3 at pH b 6, the reaction between the negatively charged pollutants and the positively charged coagulant hydrolyzates generated the flocs with positive charges due to the excess positively charged hydrolyzates adhering to the surface of aggregates. For all the three coagulants, bulk and rapid hydrolysis may occur at high pH values, producing the coagulant species with less positive charges, probably including Ti(OH)4, Fe(OH)3 and Al (OH)3 precipitates. The TiCl4, FeCl3 and Al2(SO4)3 were therefore unable to achieve complete neutralization, producing the flocs with negative charges. This probably led to the decrease in both UV278 and DOC removal at high pH values (Fig. 2) due to mutual repulsion between particles with same charges. The floc zeta potential was + 3.3 mV for TiCl4, +2.9 mV for FeCl3 and +4.6 mV for Al2(SO4)3 under the optimum solution pH conditions, indicating the predominant coagulation mechanism of charge neutralization while adsorption and sweep flocculation also played a significant role during coagulation–flocculation process.
3.3. Floc characterization under different coagulant dose conditions Floc formation, breakage and re-formation with TiCl4, FeCl3 and Al2(SO4)3 under various coagulant dose conditions were monitored by using Mastersizer 2000 and the results are displayed in Fig. S2 of the SD. Different coagulants yielded flocs with different size and growth rate during floc growth phase, and the resultant aggregates varied TiCl4
Floc growth rate Floc size 500
FeCl3
TiCl4
Al2(SO4)3
FeCl3
Table 1 Comparison of floc Sf and Rf by TiCl4, FeCl3 and Al2(SO4)3 under different coagulant dose conditions.
Al2(SO4)3
800 400 600 300 400 200 200
Floc size d50 (µm)
Floc growth rate (µm/min)
over a wide range after floc breakage and reformation. Fig. 3 presents the change of floc size and floc growth rate vs. coagulant dose, with the results showing that both floc size and growth rate followed the order of TiCl4 N FeCl3 N Al2(SO4)3, with an exception that 6 mg/L of FeCl3 yielded extreme smaller flocs than Al2(SO4)3. Variation of floc growth rate and floc size vs. dose by TiCl4 showed similar parabolic trend with the inflection point at 35 and 25 mg/L, respectively, resulting in the optimum floc growth rate of 228.7 μm/min and the optimum floc size of 787.4 μm. For FeCl3, floc growth rate gradually increased with dose, reaching a plateau when the coagulant dose was larger than 21 mg/L, while inconspicuous variation of floc growth rate was observed for Al2(SO4)3, ranging from 18.4 to 26.6 μm/min within the dose range investigated. The floc size ranged from 454 to 517 μm within the dose range of 9 to 21 mg/L, followed by gradual decrease as further increased the coagulant dose for FeCl3, while regarding with Al2(SO4)3, the floc size presented slight decrease from 250 to 180 μm with increasing coagulant dose. Floc strength factor, Sf and floc recovery factor, Rf were calculated using Eqs. (2) and (3) to investigate the floc strength and recoverability. Table 1 displays floc Sf and Rf by TiCl4, FeCl3 and Al2(SO4)3 under different coagulant dose conditions, with the results showed obvious decrease in floc Rf with increasing coagulant dose while the floc Sf was slightly affected by coagulant dose, as reflected by the inconspicuous variation of Sf with coagulant dose within the coagulant dose range investigated. From Table 1, a significant drop in floc Rf at low coagulant doses was observed, irrespective of TiCl4, FeCl3 or Al2(SO4)3, while at high coagulant doses, the decrease in floc Rf was relatively gradual. Previous studies have shown that the flocs formed by sweep flocculation had weaker recoverability after breakage than those by charge neutralization [18,35]. The increase in coagulant dose presumably made the coagulation condition more favorable for sweep flocculation, and therefore resulted in the decrease in floc Rf. However, the flocs could not regrow to anywhere near their previous size for all the three coagulants, for the Rfs of the flocs were all below 100%. Fig. 4(a) presents the growth, breakage and regrowth profiles of flocs formed by TiCl4, FeCl3 and Al2(SO4)3 under the optimum coagulant dose conditions. The particle size distribution (PSD) before floc breakage, after breakage and after regrowth was also analyzed, and is shown in Fig. 4(b), (c) and (d). The growth period of flocs during the TiCl4 coagulation showed a significantly rapid growth during the first 2.5 min, followed by 4 min relatively stable stage of floc size, giving the mean floc size of 591.7 μm. The extended mixing resulted in an apparent decrease in floc size during the subsequent 10 min. It should be noted that the floc size at the end of the slow stir period was selected to calculate the floc strength and recovery factors later. In contrast, the floc size after FeCl3 and Al2(SO4)3 coagulation was more stable than that by
Coagulant TiCl4
FeCl3
100 0 0
Al2(SO4)3
0
10
20
30
40
50
60
Coagulant dose (mg/L) Fig. 3. Change of floc growth rate and floc size (d1) with optimum coagulant dose by TiCl4, FeCl3 and Al2(SO4)3. (Initial solution pH condition.)
Coagulant dose (mg/L) 10 15 20 25 Sf (%) 38.8 40.6 36.2 33.8 Rf (%) 61.1 32.2 14.7 10.4 Coagulant dose (mg/L) 6 9 12 15 Sf (%) – 33.2 30.7 32.6 Rf (%) – 49.1 33.6 28.9 Coagulant dose (mg/L) 2 4 6 8 Sf (%) 38.2 36.9 35.0 32.2 Rf (%) 62.0 27.6 19.3 17.3
30
35
40
43
46
49
30.4
36.1
33.5
36.6
32.6
30.3
8.5
5.6
3.9
3.9
3.8
3.3
18
21
24
27
30
33
36
30.8
33.7
31.1
31.8
31.9
32.8
31.7
24.6
26.8
16.5
14.2
13.5
13.4
13.4
10
12
35.0
33.6
16.5
15.9
Y.X. Zhao et al. / Desalination 335 (2014) 70–77
a
75
700 600
Floc size d50 (µm)
500
TiCl4 FeCl3
400
Al 2(SO4)3 300 200 100 0 -5
0
5
10
15
20
25
30
35
40
T (mins)
b
c
d
10
10
8
8
14
6
Before breakage After breakage After regrowth
4
6 4
2
2
0
0 100
1000
Floc size d50 (µm)
10000
10
Before breakage After breakage After regrowth
Volume (%)
TiCl4
Volume (%)
Volume (%)
12 FeCl3
8
Al 2(SO4)3
6
Before breakage After breakage After regrowth
4 2 0
100
1000
10000
-2
Floc size d50 (µm)
100
1000
10000
Floc size d50 (µm)
Fig. 4. With optimal coagulant doses: (a) growth, breakage and regrowth profile of FA flocs formed by TiCl4, FeCl3 and Al2(SO4)3; (b) PSD of TiCl4-FA flocs; (c) PSD of FeCl3–FA flocs; and (d) PSD of Al2(SO4)3–FA flocs. (The optimum coagulant dose for TiCl4, FeCl3 and Al2(SO4)3 was 46 mg/L, 21 mg/L and 6 mg/L, respectively; Initial solution pH condition.)
TiCl4 and the floc breakage seldom appeared during the floc growth phase. There is a significant advantage of using TiCl4 as coagulant since the resultant flocs were with much larger size regardless of floc growth, breakage and regrowth process. However, the regrowth of flocs formed by TiCl4 was poor with Rf of only 3.8%, which was considerably lower than the value of 26.8% and 19.3% for the flocs formed by FeCl3 and Al2(SO4)3, respectively. As aforesaid, both sweep flocculation and charge neutralization involved during coagulation with the three coagulants under optimum coagulant dose conditions. Sweep flocculation was presumably the predominant coagulation mechanism for TiCl4 coagulation due to the lowest Rf value compared to FeCl3 and Al2(SO4)3, since the adsorption and sweep flocs show limited floc reformation after breakage, while those by charge neutralization have complete recoverability [18,20]. Additionally, the large flocs formed by TiCl4 might be attributed to physical forces such as van der Waals force, rather than intra-molecular bond strength, or there was not so weak recoverability. Analyze of PSD in Fig. 4(b), (c) and (d) showed that, for all the three coagulants, obvious shift of the major peak after floc breakage to under half the original value was observed, followed by the shift of the peak to right side again after floc regrowth, indicating the floc reformation during floc regrowth phase. However, compared to FeCl3 and Al2(SO4)3, fewer change was observed between PSD before and after floc regrowth for TiCl4, indicating the poor floc regrowth after breakage. This is in accordance with the conclusions obtained above that the flocs formed by TiCl4 had weak recoverability after breakage.
3.4. Varying shear rates on floc The effect of high shear force on floc breakage and regrowth properties with TiCl4, FeCl3 and Al2(SO4)3 was investigated under the optimum coagulant dose conditions. The increasing shear rate for 5 min with 75, 100, 150, 200 and 300 rpm were applied after the slow mixing phase, followed by another slow mixing at 40 rpm for 15 min for flocs reformation. The floc growth, breakage and regrowth profiles under various shear rate conditions can be found in Fig. S3 of the SD, and the results showed that both floc breakage and regrowth were intensively affected by shear rate. The responses of floc size to different shear rates were similar for the three coagulants, with the results showing that the floc size decreased with the increasing shear rate, followed by variable floc reformation depending on the intensity of floc breakage. To quantitatively compare the difference of the three coagulants in the response of floc size to increased shear, the floc Sf and Rf under different shear rate conditions were calculated as shown in Table 2. The floc Sf decreased with the increasing shear rate, while a reverse trend was observed for floc Rf. That is, the floc drop degree positively correlated with the applied shear and the high shear force favored floc regrowth. The high shear force was expected to break the flocs in small size with large surface area, producing the broken flocs with more attachment sites. The floc fragments tended to re-aggregate once the high shear rate was removed and the original slow mixing rate was returned. Compared to FeCl3 and Al2(SO4)3, flocs formed by TiCl4 have
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Table 2 Sf and Rf of flocs (d50) formed by different coagulants after 5 min of breakage followed by regrowth for 15 min under different shear rate conditions. Coagulants
G/RPM 75
100
150
200
300
Sf (%) TiCl4 FeCl3 Al2(SO4)3
61.4 72.1 75.3
49.6 57.1 62.1
36.5 37.9 48.2
32.0 32.7 35.0
24.5 25.0 30.4
Rf (%) TiCl4 FeCl3 Al2(SO4)3
0 8.4 14.5
0 9.6 15.1
2.2 20.1 15.3
3.6 24.4 17.2
6.5 25.4 17.3
the weakest ability to resist shear with the lowest Sf value under the same conditions, which corresponded well with the general conception that larger flocs tend to have weak strength than the smaller ones [36]. The larger flocs are more prone to be broken by microscale eddies, while the smaller ones are more likely to be entrained rather than be broken by eddies [1]. The flocs formed by Al2(SO4)3 were with the smallest size but the best ability to stand shear as reflected by the highest Sf value. The polymeric Al hydrolyzates, such as polymerized Al13 aggregates formed under the optimum dose condition maybe another reason that enhanced the floc strength [37]. Generally, the polymerized Al species can complex with FA to form coordinated complexes, which are considered too strong to be broken easily [38,39]. At the slow mixing speed of 75 and 100 rpm, the floc size showed gradual decline during floc breakage process and floc regrowth was barely observed for TiCl4, resulting in the floc Rf value of 0% as shown in Table 2. The floc regrowth became relatively apparent at 150 rpm and above. However, within the shear rate range investigated, flocs formed by TiCl4 presented a significantly weak recoverability as reflected by the lowest floc Rf value compared to those by FeCl3 and Al2(SO4)3. 3.5. Break-up period effect on floc The impacts of various break-up period (1 min, 5 min and 10 min) of high shear (200 rpm) on floc breakage and regrowth properties were comparatively investigated with TiCl4, FeCl3 and Al2(SO4)3 under optimum dose conditions. The floc breakage and regrowth profiles are shown in Fig. S4 of the SD. The floc size decreased to varying degrees due to different break-up periods, followed by various degrees of floc reformation. Table 3 displays the Sf and Rf values under different break-up period conditions, with the results suggesting that both floc strength and recoverability decayed with extension in break-up period regardless of the coagulants used. Compared to TiCl4 and FeCl3, with high shear break-up of 1 and 5 min, Al2(SO4)3 yielded the flocs with the best ability to resist shear as reflected by the highest floc Sf values shown in Table 3, while the floc Sf values by the three coagulants were comparable when extended the break-up period to 10 min. Flocs formed by FeCl3 and Al2(SO4)3 showed comparable floc Rf values with break-up period of 5 and 10 min, while with 1 min of break-up, Al2(SO4)3 has superior advantage over both TiCl4 and FeCl3 given the best floc recoverability. Additionally, the flocs formed by TiCl4 had the weakest recoverability as demonstrated by the lowest Rf values within the shear rate range studied. This is in accordance with the conclusions Table 3 Sf and Rf of flocs (d50) formed by different coagulants after 1 min, 5 min and 10 min of breakage (200 rpm) followed by floc regrowth for 15 min. Coagulants
Sf (%) 1 min
TiCl4 FeCl3 Al2(SO4)3
40.5 41.0 45.5
drawn earlier in Sections 3.3 and 3.4. In the case of 10 min shear period, the regrowth of flocs formed by TiCl4 was poor with floc Rf of only 3.6, which was considerably lower than the value of around 16.0 for the flocs formed by FeCl3 and Al2(SO4)3. The floc aggregation was therefore also greatly influenced by the break-up period besides coagulant characteristics, coagulant dose and shear force as aforementioned.
4. Conclusions Coagulation efficiency of TiCl4, FeCl3, and Al2(SO4)3 was comparatively investigated for simulated fulvic acid water treatment. Floc characteristics were also comparatively studied under different coagulant dose, shear force and break-up period conditions. The main conclusions are as follows: 1. The optimum coagulant dose for TiCl4, FeCl3 and Al2(SO4)3 was 46 mg/L, 21 mg/L and 6 mg/L, respectively, under which condition the TiCl4 coagulant achieved 78.6% of UV278 removal, approximately 18% higher than that by FeCl3 and Al2(SO4)3, and the DOC removal followed the order of TiCl4 (54.7%) N FeCl3 (39.8%) N Al2(SO4)3 (29.6%). 2. The optimum solution pH for TiCl4, FeCl3 and Al2(SO4)3 was 8, 6 and 6, respectively. The TiCl4 had superior advantage over FeCl3 and Al2(SO4)3 with higher UV278 and DOC removal within the pH range investigated. Additionally, the TiCl4 coagulation was the least affected by solution pH regarding to both UV278 and DOC removal. This is particularly significant because of its potential to be applied over a wide pH range. 3. Under the optimum coagulant dose condition, TiCl4 coagulation had the fastest growth rate of aggregated flocs and also the largest floc size compared to the traditional FeCl3 and Al2(SO4)3 coagulants, which suggests that TiCl4 needs much shorter retention time and therefore compact mixing and sedimentation tanks are required. 4. The floc aggregation, breakage and reformation were significantly affected by shear force and breakage period besides coagulant characteristics. The ability of flocs to resist breakage decreased with the increase of shear force, while the inverse trend was observed for floc recoverability. Both floc strength and recoverability decayed with the increasing break-up period regardless of the coagulants used. In all cases, the TiCl4 flocs had the weakest recoverability after breakage, indicating more careful handling of the aggregated flocs during separation and purification process. 5. This study indicated that TiCl4 was an effective coagulant for low molecule weight organic matter removal. Since TiCl4 is only studied recently as a novel coagulant, in the future, the coagulation performance and floc characteristics of TiCl4 will be comparatively studied against the conventional coagulants for other types of organic contaminants, colloidal particles, heavy metals, etc. removal, which will provide valuable information for its wide application in the future.
Acknowledgements This work was supported by grants from the Chinese National Natural Science Foundation (No. 51278283), Australia Research Council Discovery Projects (ARC DP), the Shanghai Tongji Gao Tingyao Environmental Science & Technology Development Foundation (STGEF) and the scholarship from China Scholarship Council.
Rf (%) 5 min 31.6 32.2 35.7
10 min
1 min
29.9 28.0 29.2
14.0 32.3 37.0
5 min 6.6 21.1 19.4
10 min 3.6 16.1 16.9
Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.desal.2013.12.016.
Y.X. Zhao et al. / Desalination 335 (2014) 70–77
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Contents lists available at ScienceDirect
Desalination journal homepage: www.elsevier.com/locate/desal
Coagulation by titanium tetrachloride for fulvic acid removal: Factors influencing coagulation efficiency and floc characteristics Y.X. Zhao a, B.Y. Gao a,⁎, G.Z. Zhang a, S. Phuntsho b, H.K. Shon b a b
Shandong Key Laboratory of Water Pollution Control and Resource Reuse, School of Environmental Science and Engineering, Shandong University, No.27 Shanda South Road, Jinan 250100, China Centre for Technology in Water and Wastewater, School of Civil and Environmental Engineering, University of Technology, Sydney (UTS), P.O. Box 123, Broadway, NSW 2007, Australia
H I G H L I G H T S
G R A P H I C A L
• TiCl4 achieved higher fulvic acid removal than traditional coagulants. • TiCl4 flocs exhibited higher growth rate and larger size than Fe and Al salts. • Floc strength decayed and recoverability increased with increasing shear rate. • An extension in break-up period deteriorated both floc strength and recoverability. • TiCl4 flocs displayed the weakest recoverability after breakage.
With optimal coagulant doses: (a) growth, breakage and regrowth profile of FA flocs formed by TiCl4, FeCl3 and Al2(SO4)3; (b) PSD of TiCl4–FA flocs; (c) PSD of FeCl3–FA flocs; (d) PSD of Al2(SO4)3–FA flocs.
a r t i c l e
a b s t r a c t
i n f o
Article history: Received 14 September 2013 Received in revised form 3 December 2013 Accepted 18 December 2013 Available online 14 January 2014 Keywords: Coagulation–flocculation Titanium tetrachloride Floc characteristics Shear force Break-up period
A B S T R A C T
Coagulation efficiency of titanium tetrachloride (TiCl4) was investigated for fulvic acid (FA) (as model organic matter) removal compared to traditional coagulants — iron chloride (FeCl3) and aluminum sulfate (Al2(SO4)3). The growth, breakage and regrowth nature of flocs formed by the three coagulants were also comparatively evaluated under different coagulation conditions. Results indicated that TiCl4 achieved higher removal of UV278 (absorbance at 278 nm) and DOC (dissolved organic carbon) than FeCl3 and Al2(SO4)3, and TiCl4 produced the largest flocs with the highest floc growth rate. The responses of floc to different operating parameters depend on the coagulant used, while the floc breakage and re-aggregation were also significantly influenced by shear force and break-up period. Floc strength decayed with the increasing shear force, while the inverse trend was observed for floc recoverability. An extension in break-up period resulted in the deterioration of both floc strength and recoverability regardless of the coagulants used. Flocs formed by TiCl4 displayed the weakest recoverability after breakage by contrast to conventional coagulants. Since TiCl4 is only studied recently as a novel coagulant, the comparative study of coagulation performance and floc characteristics of TiCl4 against the traditional coagulants provided valuable information for its wide application in the future. © 2013 Elsevier B.V. All rights reserved.
⁎ Corresponding author. Tel.: +86 531 88366771; fax: +86 531 88364513. E-mail address: [email protected] (B.Y. Gao). 0011-9164/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.desal.2013.12.016
Y.X. Zhao et al. / Desalination 335 (2014) 70–77
1. Introduction Coagulation–flocculation is one of the most important steps in water treatment plants [1]. Aluminum and iron salts are widely used as coagulants and they are effective in removing a broad range of impurities, including colloidal particles and organic substances [2]. However, the production of large amounts of sludge that need further disposal is the main drawback of using conventional Al and Fe coagulants. The sludge post-treatment is considered as one of the most costly and environmentally problematic challenges of all water treatment works [3,4]. Efforts are therefore made to develop a new coagulant that can reduce either the production of sludge or the influence of sludge disposal on environment. Upton and Buswell [5] first used titanium compounds as coagulant in water treatment. Since then, the coagulation effectiveness of Ti-salts has been proved in terms of particle, nutrients and organic matter removal. Recently, Shon and his co-workers investigated the coagulation efficiency of using titanium tetrachloride (TiCl4) as coagulant, with their results showing that TiCl4 successfully achieved high organic matter removal to the same extent as Al and Fe salts and the resultant flocs had better settleability [6–9]. The most significant advantage was the resultant sludge recovery for the production of a valuable by-product, namely titanium dioxide (TiO2), which is the most widely used metal oxide, whose applications include photocatalysts, cosmetics, paints, electronic paper, and solar cells [10,11]. Shon et al. [7] reported that up to approximately 40 mg-TiO2 nanoparticles/L-wastewater was produced from the wastewater sludge generated by the Ti-salt flocculation. Moreover, titanium does not play any adverse role or toxicity inside the human body and is non-toxic even in large doses [12]. Also, it is used in a gamut of medical applications because of its biocompatible. Therefore, titanium and its compounds are rarely included in any water quality guidelines [13]. Additionally, titanium is one of the most abundant elements on earth and with the development of the titanium industry, the cost of Ti-salts is gradually becoming comparable to that of conventional coagulants. TiCl4 is therefore expected to be a promising alternative coagulant for conventional Al and Fe coagulants. Since TiCl4 is only studied recently as a novel coagulant, the mode of coagulation action and the factors affecting coagulation efficiency and floc characteristics are not completely understood. Floc characteristics after coagulation-flocculation are key points influencing solid/liquid separation process, which would subsequently affect the coagulation efficiency [14]. Small particles generally have lower removal efficiency than the larger ones by coagulationflocculation, since the smaller particles settle down more slowly than the larger ones of similar density [15]. The unit process at water treatment works (WTW) is prevalent with regions of high shear [16]. Therefore, the ability of flocs to withstand rupture and the recoverability after being broken has a significant impact at WTW. The broken flocs were hard to regrow and the reduced floc size would slow down the sedimentation rate and change transport mechanisms [17–19]. There were also reports about the full recovery of the broken flocs formed by charge neutralization, while the sweep flocs showed weak recoverability [18,20,21]. Therefore, floc strength and recoverability are considered as important parameters influencing coagulation efficiency and will provide valuable information in understanding coagulation behavior. Floc properties depend not only on characteristics of raw water source and coagulants, but also various hydraulic parameters, such as shear force and break-up periods, etc. [22,23]. Few researchers have focused on the breakage and regrowth of flocs formed by TiCl4 under different coagulation conditions. Natural organic matter (NOM) is a diverse mixture of organic compounds, including low molecule weight compounds, such as fulvic acid (FA), which is mainly produced by living organisms or formed by secondary synthesis reactions [24]. FA, with the reactive characters of a series of oxygen-containing functional groups, including carboxyl, phenolic-OH, enolic, alcoholic-OH, and carbonyl [25], is easily soluble
71
in water and difficult to remove. It causes odor, taste, color and bacterial regrowth in potable water, and have potential to form carcinogenic disinfection-byproducts (DBPs) [22]. Thus, effective removal of NOM present in water has become increasingly important in modern water treatment. The aims of this study are to i) evaluate the coagulation performance of TiCl4 for FA simulated water treatment under different coagulant dose and initial solution pH conditions, ii) investigate the effect of coagulant dose and hydraulic conditions, such as shear force and break-up period, on floc characteristics using a laser diffraction instrument, iii) discuss the relationship between coagulation performance and floc characteristics and investigate the coagulation mechanism. Coagulation performance and floc properties of conventional coagulants (iron chloride (FeCl3) and aluminum sulfate (Al2(SO4)3)) were investigated for comparison. 2. Experimental 2.1. Coagulants and test water TiCl4 solution (20%, density = 1.150 g/ml) was obtained from Photo & Environment Technology Co. Ltd (South Korea), and was used directly without any pretreatment. Stock solutions of Al2(SO4)3 and FeCl3 were prepared at a concentration of 1 g/L by Al and 2 g/L by Fe, respectively. Deionized water was used for all the reagents preparation. The stock solution of FA (1.0 g/L) was prepared by dissolving 1.0 g of FA (biochemical reagent purchased from Yinong Biochemical Technology Co. Ltd., Shanghai, China) directly in deionized water. The simulated water for coagulation experiments was prepared by diluting the FA stock solution by tap water to make 10 mg/L of FA [26] and the raw water turbidity was adjusted to 15.0 ± 0.5 NTU by adding kaolin. The measured UV278 absorbance, dissolved organic carbon (DOC), zeta potential and pH of the suspension was 0.099 ± 0.005 cm−1, 5.638 ± 0.580 mg/L, −19.0 ± 1.0 mV and 8.05 ± 0.05, respectively. 2.2. Jar-test Standard jar tests were conducted using a programmable jar-tester (ZR4-6, Zhongrun Water Industry Technology Development Co. Ltd., China). The water sample (1000 mL) was mixed rapidly for 30 s at 200 rpm before adding coagulant. After coagulant addition, rapid mixing (200 rpm) was performed for 1 min followed by slow mixing at 40 rpm for a duration of 15 min which was then followed by 15 min of quiescent settling. Water samples were collected from 2 cm below water surface for measurements. The water samples were prefiltered using 0.45 μm fiber membrane syringe filter before testing UV278 (absorbance at 278 nm using a UV-754 UV/VIS spectrophotometer) and DOC (measured by a Shimadzu TOC-VCPH analyzer); while turbidity and floc zeta potential were directly measured without filtration using a 2100P turbidimeter (Hach, USA) and Zetasizer 3000HSa (Malvern Instruments, UK), respectively. Coagulation-flocculation experiments under different solution pH values were conducted after the optimal coagulant dosages were determined. The target coagulation pH values were achieved by adding appropriate quantities of HCl and NaOH solutions with the concentration of 0.1 mol/L. 2.3. Floc characterization A laser diffraction instrument (Mastersizer 2000, Malvern, UK) was used to measure dynamic floc size as the coagulation and flocculation process proceeded. The schematic diagram of the on-line monitoring system for dynamic floc size can be found in Zhao et al. [9]. The median equivalent diameter, d50, was selected as the representative floc size, although the same trends were observed for d10 and d90 floc sizes.
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To investigate the effect of coagulant dose on floc properties, the procedures of coagulation tests were as follows: Following the floc growth phase, the aggregated flocs were exposed to a shear force at 200 rpm for 5 min, followed by a slow mixing at 40 rpm for 15 min to allow floc regrowth. For the shear-varying coagulation, the flocs were exposed to the increasing mixing speed to 75, 100, 150, 200 and 300 rpm on the jar-tester for 5 min. Also, a series of jar-tests were conducted to study the effect of break-up period on floc strength and recoverability, with three separate break-up periods at 200 rpm of i) a normal break-up period of 5 min, ii) a short break-up period of 1 min and iii) a long break-up period of 10 min. The break-up period was followed by a restoration of the 40 rpm slow mixing phase for 15 min. The floc growth rate was calculated by the slope of the rapid growth region [27]: Growth rate ¼
Δsize Δtime
ð1Þ
Floc strength factor (Sf) and recovery factor (Rf) are used to compare the floc breakage and recoverability [1,9,28,29]: Sf ¼
d2 100 d1
ð2Þ
Rf ¼
d3 −d2 100 d1 −d2
ð3Þ
a
where, d1 is the average floc size of the plateau before breakage, d2 is the floc size after the floc break-up period, and d3 is the floc size after regrowth to the new plateau.
3. Results and discussion 3.1. Coagulant dose optimization Variation of coagulation performance vs. coagulant dose with TiCl4, FeCl3 and Al2(SO4)3 was investigated in terms of residual turbidity, UV278 removal, DOC removal and floc zeta potential, with the results being shown in Fig. 1. From Fig. 1(a), a sharp decrease in residual turbidity was observed with the increasing coagulant dose for both TiCl4 and FeCl3, followed by a plateau for FeCl3, while a gradual increase was observed for TiCl4 at its dose larger than 46 mg/L. For Al2(SO4)3, the residual turbidity varied inconspicuously from 2.5 NTU to 3.3 NTU within the coagulant dose range investigated. Both UV254 and DOC removal for all three coagulants increased significantly first with the increasing coagulant dose, and then approached a plateau or showed slight increase at high coagulant doses (Fig. 1(b) and (c)). Considering both the coagulation efficiency and cost, the optimum dose for TiCl4, FeCl3 and Al2(SO4)3 was fixed at 46 mg/L, 21 mg/L and 6 mg/L, respectively, where i) TiCl4 gave comparable residual turbidity (around 1.8 NTU) as FeCl3, while it was about 2.9 NTU for Al2(SO4)3, ii) TiCl4 yielded UV278 removal of around 78.6%, approximately 18% higher than that by FeCl3 and
b
7
80
FeCl3 Al2(SO4)3
UV278 removal (%)
Residual turbidity (NTU)
TiCl4
6 5 4 3
60
50 TiCl4 FeCl3
40
2 1
70
0
10
20
30
40
50
30
60
Al2(SO4)3
0
10
Coagulant dose (mg/L)
c
d 60
40
50
60
50
60
10 FeCl3
5
Zeta potential (mV)
FeCl3
DOC removal (%)
30
TiCl4
TiCl4 Al2(SO4)3
50
20
Coagulant dose (mg/L)
40
30
Al2(SO4)3
0
-5
-10
-15
20 0
10
20
30
40
Coagulant dose (mg/L)
50
60
0
10
20
30
40
Coagulant dose (mg/L)
Fig. 1. Variation of coagulation performance vs. coagulant dose with TiCl4, FeCl3 and Al2(SO4)3 measured in terms of : (a) residual turbidity, (b) UV278 removal, (c) DOC removal, and (d) zeta potential. (Dose of TiCl4, FeCl3 and Al2(SO4)3 was calculated as mg-Ti/L, mg-Fe/L and mg-Al/L, respectively; Initial solution pH condition.)
Y.X. Zhao et al. / Desalination 335 (2014) 70–77
Al2(SO4)3, and iii) the DOC removal followed the order of TiCl4 (54.7%) N FeCl3 (39.8%) N Al2(SO4)3 (29.6%). Floc settleability was also investigated to evaluate the merits of TiCl4 coagulation-flocculation performance. The details can be found in S1 of the Supplementary Data (SD). The results indicated that the flocs formed by TiCl4 showed higher sedimentation velocity, and therefor much shorter retention time was required, which offered significant advantage in sedimentation tanks. Variation of floc zeta potential with coagulant dose for the three coagulants is shown in Fig. 1(d). The floc zeta potential increased with the increasing coagulant concentration, reaching isoelectric points at TiCl4 dose of about 46 mg/L, FeCl3 dose of about 33 mg/L and Al2(SO4)3 dose of around 10 mg/L, followed by charge reversal as further increased the coagulant dose for TiCl4 and Al2(SO4)3. Changes in floc zeta potential are often used to evaluate the destabilization ability of coagulants. According to Ye. et al. [30], the coagulant dose, where the charge reversal occurs was defined as iso-electrical-dose (IED). Thus, the IED for TiCl4, FeCl3 and Al2(SO4)3 was 46, 33 and 10 mg/L, respectively, indicating the destabilization capability order of TiCl4 b FeCl3 b Al2(SO4)3. This is quite in contrast to the reports that the higher the charge of a cation, the stronger is its effect on particle zeta potential and the higher the valance, the higher the coagulative power [31]. Changes in floc zeta potential are also generally regarded as effective tool to investigate coagulation mechanism, which is often explained in terms of charge neutralization and sweep flocculation
[32]. Under the optimum coagulant dose conditions (as aforesaid), the floc zeta potential for TiCl4, FeCl3 and Al2(SO4)3 was − 2.1 mV, − 8.4 mV and − 4.5 mV, respectively, indicating that sweep flocculation played an important role for FA removal besides charge neutralization. 3.2. The effect of solution pH on coagulation efficiency Change of residual turbidity, UV278 removal, DOC removal and floc zeta potential as a function of initial solution pH was investigated under optimum coagulant dose conditions for TiCl4, FeCl3 and Al2(SO4)3, with the results being shown in Fig. 2. Overall, the residual turbidity decreased with the increasing pH value, reaching a plateau finally. The TiCl4 coagulant was inferior to FeCl3 and Al2(SO4)3 for turbidity removal in acidic conditions, as reflected by the higher residual turbidity at low pH values. Change of UV278 and DOC removal vs. pH were similar for the three coagulant, showing parabolic shape with the inflection points at pH 8, 6 and 6 for TiCl4, FeCl3 and Al2(SO4)3, respectively. Compared to FeCl3 and Al2(SO4)3, the UV278 and DOC removal of TiCl4 coagulant was the least affected by solution pH, and they increased steadily with increasing pH, and then showed decline trend beyond the inflection point. The pH values of 8, 6 and 6 at the inflection points were therefore selected as the optimum ones for TiCl4, FeCl3 and Al2(SO4)3, respectively, under which condition, TiCl4 and FeCl3 gave comparable UV278 removal of around 75.0%, 10% higher than that by
b
a
80
60
70 50 40
UV278 removal (%)
Residual turbidity (NTU)
73
TiCl4 FeCl3
30
Al2(SO4)3
20 10
60 50 40 30
TiCl4 FeCl3
20
Al2(SO4)3
10 0
0 5
6
7
8
9
-10
10
5
6
7
pH
c
d 50
9
10
10 TiCl4
5
Zeta potential (mV)
DOC removal (%)
8
pH
40
30
FeCl3 Al2(SO4)3
0
-5
-10
TiCl4 FeCl3
20
-15
Al2(SO4)3
5
6
7
8
pH
9
10
5
6
7
8
9
10
pH
Fig. 2. The effect of solution pH on coagulation performance of TiCl4, FeCl3 and Al2(SO4)3 under optimum dose conditions: (a) residual turbidity, (b) UV278 removal, (c) DOC removal, and (d) zeta potential. (The optimum coagulant dose for TiCl4, FeCl3 and Al2(SO4)3 was 46 mg/L, 21 mg/L and 6 mg/L, respectively.)
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Y.X. Zhao et al. / Desalination 335 (2014) 70–77
Al2(SO4)3, and the DOC removal varied in the order of TiCl4 (54.0%) N FeCl3 (48.3%) N Al2(SO4)3 (36.3%). Variation of floc zeta potential as a function of solution pH is shown in Fig. 2(d), with the results showing that the floc zeta potential decreased from positive side to negative side with increasing pH for both TiCl4 and FeCl3, while for Al2(SO4)3, it increased dramatically from negative side to positive side when pH increased from 5 to 6, followed by gradual decrease to negative side again as further increased pH value. Change of floc zeta potential with pH generally correlates well with the variation of coagulant hydrolyzates under different solution pH conditions. The hydrolysis of Al2(SO4)3 was inhibited at pH 5 and the dominant Al speciation was positive monomer hydrolyzates [33,34], which neutralized the negative charges of particles and organic matters, and further destabilized the colloids. However, the positive charges were not enough for full charge neutralization, yielding the flocs with negative charges. This is also the reason for low UV278 and DOC removal at pH 5 as shown in Fig. 2. When pH increased from 5 to 7, the polymeric hydrolyzates with high positive charges and large surface area formed [33,34]. The adsorption, charge neutralization and co-precipitation between the colloids and the hydrolyzates resulted in high UV278 and DOC removal (Fig. 2) and produced the flocs with positive cha rges. Similarly, for TiCl4 at pH b 8 and FeCl3 at pH b 6, the reaction between the negatively charged pollutants and the positively charged coagulant hydrolyzates generated the flocs with positive charges due to the excess positively charged hydrolyzates adhering to the surface of aggregates. For all the three coagulants, bulk and rapid hydrolysis may occur at high pH values, producing the coagulant species with less positive charges, probably including Ti(OH)4, Fe(OH)3 and Al (OH)3 precipitates. The TiCl4, FeCl3 and Al2(SO4)3 were therefore unable to achieve complete neutralization, producing the flocs with negative charges. This probably led to the decrease in both UV278 and DOC removal at high pH values (Fig. 2) due to mutual repulsion between particles with same charges. The floc zeta potential was + 3.3 mV for TiCl4, +2.9 mV for FeCl3 and +4.6 mV for Al2(SO4)3 under the optimum solution pH conditions, indicating the predominant coagulation mechanism of charge neutralization while adsorption and sweep flocculation also played a significant role during coagulation–flocculation process.
3.3. Floc characterization under different coagulant dose conditions Floc formation, breakage and re-formation with TiCl4, FeCl3 and Al2(SO4)3 under various coagulant dose conditions were monitored by using Mastersizer 2000 and the results are displayed in Fig. S2 of the SD. Different coagulants yielded flocs with different size and growth rate during floc growth phase, and the resultant aggregates varied TiCl4
Floc growth rate Floc size 500
FeCl3
TiCl4
Al2(SO4)3
FeCl3
Table 1 Comparison of floc Sf and Rf by TiCl4, FeCl3 and Al2(SO4)3 under different coagulant dose conditions.
Al2(SO4)3
800 400 600 300 400 200 200
Floc size d50 (µm)
Floc growth rate (µm/min)
over a wide range after floc breakage and reformation. Fig. 3 presents the change of floc size and floc growth rate vs. coagulant dose, with the results showing that both floc size and growth rate followed the order of TiCl4 N FeCl3 N Al2(SO4)3, with an exception that 6 mg/L of FeCl3 yielded extreme smaller flocs than Al2(SO4)3. Variation of floc growth rate and floc size vs. dose by TiCl4 showed similar parabolic trend with the inflection point at 35 and 25 mg/L, respectively, resulting in the optimum floc growth rate of 228.7 μm/min and the optimum floc size of 787.4 μm. For FeCl3, floc growth rate gradually increased with dose, reaching a plateau when the coagulant dose was larger than 21 mg/L, while inconspicuous variation of floc growth rate was observed for Al2(SO4)3, ranging from 18.4 to 26.6 μm/min within the dose range investigated. The floc size ranged from 454 to 517 μm within the dose range of 9 to 21 mg/L, followed by gradual decrease as further increased the coagulant dose for FeCl3, while regarding with Al2(SO4)3, the floc size presented slight decrease from 250 to 180 μm with increasing coagulant dose. Floc strength factor, Sf and floc recovery factor, Rf were calculated using Eqs. (2) and (3) to investigate the floc strength and recoverability. Table 1 displays floc Sf and Rf by TiCl4, FeCl3 and Al2(SO4)3 under different coagulant dose conditions, with the results showed obvious decrease in floc Rf with increasing coagulant dose while the floc Sf was slightly affected by coagulant dose, as reflected by the inconspicuous variation of Sf with coagulant dose within the coagulant dose range investigated. From Table 1, a significant drop in floc Rf at low coagulant doses was observed, irrespective of TiCl4, FeCl3 or Al2(SO4)3, while at high coagulant doses, the decrease in floc Rf was relatively gradual. Previous studies have shown that the flocs formed by sweep flocculation had weaker recoverability after breakage than those by charge neutralization [18,35]. The increase in coagulant dose presumably made the coagulation condition more favorable for sweep flocculation, and therefore resulted in the decrease in floc Rf. However, the flocs could not regrow to anywhere near their previous size for all the three coagulants, for the Rfs of the flocs were all below 100%. Fig. 4(a) presents the growth, breakage and regrowth profiles of flocs formed by TiCl4, FeCl3 and Al2(SO4)3 under the optimum coagulant dose conditions. The particle size distribution (PSD) before floc breakage, after breakage and after regrowth was also analyzed, and is shown in Fig. 4(b), (c) and (d). The growth period of flocs during the TiCl4 coagulation showed a significantly rapid growth during the first 2.5 min, followed by 4 min relatively stable stage of floc size, giving the mean floc size of 591.7 μm. The extended mixing resulted in an apparent decrease in floc size during the subsequent 10 min. It should be noted that the floc size at the end of the slow stir period was selected to calculate the floc strength and recovery factors later. In contrast, the floc size after FeCl3 and Al2(SO4)3 coagulation was more stable than that by
Coagulant TiCl4
FeCl3
100 0 0
Al2(SO4)3
0
10
20
30
40
50
60
Coagulant dose (mg/L) Fig. 3. Change of floc growth rate and floc size (d1) with optimum coagulant dose by TiCl4, FeCl3 and Al2(SO4)3. (Initial solution pH condition.)
Coagulant dose (mg/L) 10 15 20 25 Sf (%) 38.8 40.6 36.2 33.8 Rf (%) 61.1 32.2 14.7 10.4 Coagulant dose (mg/L) 6 9 12 15 Sf (%) – 33.2 30.7 32.6 Rf (%) – 49.1 33.6 28.9 Coagulant dose (mg/L) 2 4 6 8 Sf (%) 38.2 36.9 35.0 32.2 Rf (%) 62.0 27.6 19.3 17.3
30
35
40
43
46
49
30.4
36.1
33.5
36.6
32.6
30.3
8.5
5.6
3.9
3.9
3.8
3.3
18
21
24
27
30
33
36
30.8
33.7
31.1
31.8
31.9
32.8
31.7
24.6
26.8
16.5
14.2
13.5
13.4
13.4
10
12
35.0
33.6
16.5
15.9
Y.X. Zhao et al. / Desalination 335 (2014) 70–77
a
75
700 600
Floc size d50 (µm)
500
TiCl4 FeCl3
400
Al 2(SO4)3 300 200 100 0 -5
0
5
10
15
20
25
30
35
40
T (mins)
b
c
d
10
10
8
8
14
6
Before breakage After breakage After regrowth
4
6 4
2
2
0
0 100
1000
Floc size d50 (µm)
10000
10
Before breakage After breakage After regrowth
Volume (%)
TiCl4
Volume (%)
Volume (%)
12 FeCl3
8
Al 2(SO4)3
6
Before breakage After breakage After regrowth
4 2 0
100
1000
10000
-2
Floc size d50 (µm)
100
1000
10000
Floc size d50 (µm)
Fig. 4. With optimal coagulant doses: (a) growth, breakage and regrowth profile of FA flocs formed by TiCl4, FeCl3 and Al2(SO4)3; (b) PSD of TiCl4-FA flocs; (c) PSD of FeCl3–FA flocs; and (d) PSD of Al2(SO4)3–FA flocs. (The optimum coagulant dose for TiCl4, FeCl3 and Al2(SO4)3 was 46 mg/L, 21 mg/L and 6 mg/L, respectively; Initial solution pH condition.)
TiCl4 and the floc breakage seldom appeared during the floc growth phase. There is a significant advantage of using TiCl4 as coagulant since the resultant flocs were with much larger size regardless of floc growth, breakage and regrowth process. However, the regrowth of flocs formed by TiCl4 was poor with Rf of only 3.8%, which was considerably lower than the value of 26.8% and 19.3% for the flocs formed by FeCl3 and Al2(SO4)3, respectively. As aforesaid, both sweep flocculation and charge neutralization involved during coagulation with the three coagulants under optimum coagulant dose conditions. Sweep flocculation was presumably the predominant coagulation mechanism for TiCl4 coagulation due to the lowest Rf value compared to FeCl3 and Al2(SO4)3, since the adsorption and sweep flocs show limited floc reformation after breakage, while those by charge neutralization have complete recoverability [18,20]. Additionally, the large flocs formed by TiCl4 might be attributed to physical forces such as van der Waals force, rather than intra-molecular bond strength, or there was not so weak recoverability. Analyze of PSD in Fig. 4(b), (c) and (d) showed that, for all the three coagulants, obvious shift of the major peak after floc breakage to under half the original value was observed, followed by the shift of the peak to right side again after floc regrowth, indicating the floc reformation during floc regrowth phase. However, compared to FeCl3 and Al2(SO4)3, fewer change was observed between PSD before and after floc regrowth for TiCl4, indicating the poor floc regrowth after breakage. This is in accordance with the conclusions obtained above that the flocs formed by TiCl4 had weak recoverability after breakage.
3.4. Varying shear rates on floc The effect of high shear force on floc breakage and regrowth properties with TiCl4, FeCl3 and Al2(SO4)3 was investigated under the optimum coagulant dose conditions. The increasing shear rate for 5 min with 75, 100, 150, 200 and 300 rpm were applied after the slow mixing phase, followed by another slow mixing at 40 rpm for 15 min for flocs reformation. The floc growth, breakage and regrowth profiles under various shear rate conditions can be found in Fig. S3 of the SD, and the results showed that both floc breakage and regrowth were intensively affected by shear rate. The responses of floc size to different shear rates were similar for the three coagulants, with the results showing that the floc size decreased with the increasing shear rate, followed by variable floc reformation depending on the intensity of floc breakage. To quantitatively compare the difference of the three coagulants in the response of floc size to increased shear, the floc Sf and Rf under different shear rate conditions were calculated as shown in Table 2. The floc Sf decreased with the increasing shear rate, while a reverse trend was observed for floc Rf. That is, the floc drop degree positively correlated with the applied shear and the high shear force favored floc regrowth. The high shear force was expected to break the flocs in small size with large surface area, producing the broken flocs with more attachment sites. The floc fragments tended to re-aggregate once the high shear rate was removed and the original slow mixing rate was returned. Compared to FeCl3 and Al2(SO4)3, flocs formed by TiCl4 have
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Y.X. Zhao et al. / Desalination 335 (2014) 70–77
Table 2 Sf and Rf of flocs (d50) formed by different coagulants after 5 min of breakage followed by regrowth for 15 min under different shear rate conditions. Coagulants
G/RPM 75
100
150
200
300
Sf (%) TiCl4 FeCl3 Al2(SO4)3
61.4 72.1 75.3
49.6 57.1 62.1
36.5 37.9 48.2
32.0 32.7 35.0
24.5 25.0 30.4
Rf (%) TiCl4 FeCl3 Al2(SO4)3
0 8.4 14.5
0 9.6 15.1
2.2 20.1 15.3
3.6 24.4 17.2
6.5 25.4 17.3
the weakest ability to resist shear with the lowest Sf value under the same conditions, which corresponded well with the general conception that larger flocs tend to have weak strength than the smaller ones [36]. The larger flocs are more prone to be broken by microscale eddies, while the smaller ones are more likely to be entrained rather than be broken by eddies [1]. The flocs formed by Al2(SO4)3 were with the smallest size but the best ability to stand shear as reflected by the highest Sf value. The polymeric Al hydrolyzates, such as polymerized Al13 aggregates formed under the optimum dose condition maybe another reason that enhanced the floc strength [37]. Generally, the polymerized Al species can complex with FA to form coordinated complexes, which are considered too strong to be broken easily [38,39]. At the slow mixing speed of 75 and 100 rpm, the floc size showed gradual decline during floc breakage process and floc regrowth was barely observed for TiCl4, resulting in the floc Rf value of 0% as shown in Table 2. The floc regrowth became relatively apparent at 150 rpm and above. However, within the shear rate range investigated, flocs formed by TiCl4 presented a significantly weak recoverability as reflected by the lowest floc Rf value compared to those by FeCl3 and Al2(SO4)3. 3.5. Break-up period effect on floc The impacts of various break-up period (1 min, 5 min and 10 min) of high shear (200 rpm) on floc breakage and regrowth properties were comparatively investigated with TiCl4, FeCl3 and Al2(SO4)3 under optimum dose conditions. The floc breakage and regrowth profiles are shown in Fig. S4 of the SD. The floc size decreased to varying degrees due to different break-up periods, followed by various degrees of floc reformation. Table 3 displays the Sf and Rf values under different break-up period conditions, with the results suggesting that both floc strength and recoverability decayed with extension in break-up period regardless of the coagulants used. Compared to TiCl4 and FeCl3, with high shear break-up of 1 and 5 min, Al2(SO4)3 yielded the flocs with the best ability to resist shear as reflected by the highest floc Sf values shown in Table 3, while the floc Sf values by the three coagulants were comparable when extended the break-up period to 10 min. Flocs formed by FeCl3 and Al2(SO4)3 showed comparable floc Rf values with break-up period of 5 and 10 min, while with 1 min of break-up, Al2(SO4)3 has superior advantage over both TiCl4 and FeCl3 given the best floc recoverability. Additionally, the flocs formed by TiCl4 had the weakest recoverability as demonstrated by the lowest Rf values within the shear rate range studied. This is in accordance with the conclusions Table 3 Sf and Rf of flocs (d50) formed by different coagulants after 1 min, 5 min and 10 min of breakage (200 rpm) followed by floc regrowth for 15 min. Coagulants
Sf (%) 1 min
TiCl4 FeCl3 Al2(SO4)3
40.5 41.0 45.5
drawn earlier in Sections 3.3 and 3.4. In the case of 10 min shear period, the regrowth of flocs formed by TiCl4 was poor with floc Rf of only 3.6, which was considerably lower than the value of around 16.0 for the flocs formed by FeCl3 and Al2(SO4)3. The floc aggregation was therefore also greatly influenced by the break-up period besides coagulant characteristics, coagulant dose and shear force as aforementioned.
4. Conclusions Coagulation efficiency of TiCl4, FeCl3, and Al2(SO4)3 was comparatively investigated for simulated fulvic acid water treatment. Floc characteristics were also comparatively studied under different coagulant dose, shear force and break-up period conditions. The main conclusions are as follows: 1. The optimum coagulant dose for TiCl4, FeCl3 and Al2(SO4)3 was 46 mg/L, 21 mg/L and 6 mg/L, respectively, under which condition the TiCl4 coagulant achieved 78.6% of UV278 removal, approximately 18% higher than that by FeCl3 and Al2(SO4)3, and the DOC removal followed the order of TiCl4 (54.7%) N FeCl3 (39.8%) N Al2(SO4)3 (29.6%). 2. The optimum solution pH for TiCl4, FeCl3 and Al2(SO4)3 was 8, 6 and 6, respectively. The TiCl4 had superior advantage over FeCl3 and Al2(SO4)3 with higher UV278 and DOC removal within the pH range investigated. Additionally, the TiCl4 coagulation was the least affected by solution pH regarding to both UV278 and DOC removal. This is particularly significant because of its potential to be applied over a wide pH range. 3. Under the optimum coagulant dose condition, TiCl4 coagulation had the fastest growth rate of aggregated flocs and also the largest floc size compared to the traditional FeCl3 and Al2(SO4)3 coagulants, which suggests that TiCl4 needs much shorter retention time and therefore compact mixing and sedimentation tanks are required. 4. The floc aggregation, breakage and reformation were significantly affected by shear force and breakage period besides coagulant characteristics. The ability of flocs to resist breakage decreased with the increase of shear force, while the inverse trend was observed for floc recoverability. Both floc strength and recoverability decayed with the increasing break-up period regardless of the coagulants used. In all cases, the TiCl4 flocs had the weakest recoverability after breakage, indicating more careful handling of the aggregated flocs during separation and purification process. 5. This study indicated that TiCl4 was an effective coagulant for low molecule weight organic matter removal. Since TiCl4 is only studied recently as a novel coagulant, in the future, the coagulation performance and floc characteristics of TiCl4 will be comparatively studied against the conventional coagulants for other types of organic contaminants, colloidal particles, heavy metals, etc. removal, which will provide valuable information for its wide application in the future.
Acknowledgements This work was supported by grants from the Chinese National Natural Science Foundation (No. 51278283), Australia Research Council Discovery Projects (ARC DP), the Shanghai Tongji Gao Tingyao Environmental Science & Technology Development Foundation (STGEF) and the scholarship from China Scholarship Council.
Rf (%) 5 min 31.6 32.2 35.7
10 min
1 min
29.9 28.0 29.2
14.0 32.3 37.0
5 min 6.6 21.1 19.4
10 min 3.6 16.1 16.9
Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.desal.2013.12.016.
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