Preparation and Evaluation of Titanium-Based Xerogel as a Promising

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Preparation and Evaluation of Titanium-Based Xerogel as a Promising Coagulant for Water/Wastewater Treatment Xiaomeng Wang, Minghui Li, Xiaojie Song, Zhihao Chen, Bingdang Wu, and Shujuan Zhang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b03321 • Publication Date (Web): 16 Aug 2016 Downloaded from http://pubs.acs.org on August 17, 2016

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Environmental Science & Technology

Preparation and Evaluation of Titanium-Based Xerogel as a Promising Coagulant for Water/Wastewater Treatment

Xiaomeng Wang, Minghui Li, Xiaojie Song, Zhihao Chen, Bingdang Wu, Shujuan Zhang*

State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, 210023, PR China

*Correspondence author. Phone: +86 25 8968 0389, E-mail: [email protected]

Submitted to: Environmental Science & Technology

1

ACS Paragon Plus Environment


Table of Contents (TOC) Art

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ABSTRACT

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The nontoxicity of titanium (Ti) and the potential to produce valuable photocatalysts from the

3

final coagulated sludge constitute the main advantages of Ti-based coagulants over conventional

4

ones. However, the low effluent pH and the too-fast hydrolysis limit the wide application of

5

Ti-salt coagulants. Prehydrolysis, to some extent, is helpful to improve the coagulation

6

performance of Ti-salt coagulants. However, the prehydrolyzed polytitanium chloride (PTC) still

7

suffers from narrow applicable dose/pH range. A novel and efficient Ti-based coagulant, denoted

8

as titanium xerogel coagulant (TXC), was successfully prepared by the sol-gel method with

9

TiCl4 as the precursor and acetylacetone as a modifying agent. Compared with TiCl4, a PTC, and

10

a commercial polyferric sulfate, the resulting TXC possessed a larger floc size, better settling

11

property, and wider applicable coagulant dose/pH range. Moreover, the effluent pH after TXC

12

coagulation was not significantly reduced, avoiding the corrosion problem sometimes caused by

13

the low effluent pH. TXC exhibited good coagulation performance for several real wastewaters,

14

especially for the wastewaters of low turbidity. These results demonstrate that gelation was a

15

more effective strategy than prehydrolysis to overcome the inherent weaknesses of Ti salts as a

16

type of promising coagulants.

17

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18

INTRODUCTION

19

Coagulation is one of the most important and widely used physicochemical processes in water

20

and wastewater treatment.1 Aluminum (Al) and iron (Fe) coagulants have been extensively

21

applied in the past few decades and still dominate the current market share of coagulants.

22

However, the use of Al and Fe in the coagulation process inevitably generates large quantities of

23

unrecoverable and non-reusable sludge. Moreover, Al coagulants have caused concerns because

24

of their adverse effects on human health and the environment.2 Fe coagulants are more efficient

25

at removing dissolved organic carbon than Al coagulants and pose fewer human health risks.

26

However, the effluents of Fe-coagulation are sometimes corrosive and have a high chroma

27

index.3,4

28

Since the first report on the use of titanium (Ti) compounds as coagulants in 1937,5

29

considerable attention has been paid to the development of Ti-based coagulants.6-8 It is reported

30

that Ti coagulants have several merits, including higher removal efficiency for turbidity and

31

natural organic matter,9 larger floc size and faster growth rates,10 and better settling and filtration

32

properties11 than the conventional Al/Fe coagulants. In addition to the better performance in

33

coagulation, the most important advantages of Ti-based coagulants over conventional ones reside

34

in the nontoxicity of titanium12 and the potential to produce valuable TiO2 photocatalysts as

35

byproducts from the final coagulated sludge.13,14

36

Despite the advantages, Ti salt coagulants, mainly titanium tetrachloride (TiCl4, abbreviated

37

as TC) and titanium sulfate (Ti(SO4)2), also have some drawbacks in wastewater treatment,

38

including low effluent pH and too rapid hydrolysis, which hinders the formation of the most

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effective titanic hydrolysates.15 To overcome these shortcomings, prehydrolysis has been

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attempted to improve the coagulation process.16 This approach was inspired by the

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prehydrolyzed Al or Fe coagulants, such as polyaluminum chloride (PAC), polyferric chloride

42

(PFC) and polyferric sulfate (PFS). The hydrolysis of Al and Fe salts in the preparation stage,

43

rather than after the addition to real water, resulted in a better control of the coagulation process.

44

The advantages of prehydrolyzed inorganic Al/Fe coagulants include: reduced need for pH

45

adjustment, better adaptability to low temperature, improved effectiveness at removal of

46

numerous pollutants, and lower cost than organic polymeric coagulants.17,18 To realize these

47

advantages, a series of polytitanium chloride (PTC) with different basicity (OH/Ti molar ratio of

48

0, 0.3, 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0) was synthesized by a slow alkaline titration method.16 The

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PTC with a OH/Ti molar ratio of 1.5 was selected as the optimum one for water treatment.

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Compared with TC, higher or comparable turbidity and organic matter removal were achieved by

51

PTC with improved floc characteristics in terms of size, growth rate, and structure.16 The

52

preparation of PTC is doubtless a successful step-forward in the development of Ti-based

53

coagulants.

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From practical application point of view, long-distance transportation and long-term storage

55

are important factors in the use of coagulants. The PTC developed by Zhao et al.16 were clear

56

and transparent solutions. The ones with low OH/Ti molar ratio could be stable for several weeks

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after preparation, whereas white precipitates appeared gradually with the increase of storage time

58

for the PTCs with OH/Ti molar ratios of 1.5 or higher. Therefore, their experiments were

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conducted with fresh PTC immediately after the preparation to avoid the aging effect.

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Nonetheless, the application of PTC was still hampered by a narrow solution pH and coagulant

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dose range. Therefore, further modification of Ti-based coagulants is still needed.

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Acetylacetone (AcAc) is a commonly used modifier in the sol-gel process for preparation of

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TiO2. The role of AcAc in the sol-gel process is to control the hydrolysis-condensation process

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of the titanic precursors. One of the merits of the sol-gel process is its easy tunability in

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components. The anchored AcAc in a TiO2 xerogel was found to play an important role in a

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photo-controlled reversible sorption process.19 Our preliminary work demonstrates that by

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changing the ratios of the precursors in the sol-gel process, the resulting xerogel could form large

68

flocs under various conditions. Therefore, the main objectives in this work were to (1) fabricate a

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novel Ti-based xerogel coagulant (TXC) with controllable hydrolysis after addition to

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wastewater, (2) evaluate the coagulation performance of TXC compared with TC, PTC, and the

71

widely used commercial PFS as references, and (3) elucidate the mechanisms involved in the

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coagulation process based on material characterization and floc properties.

73 74

EXPERIMENTAL

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Materials. PFS (Fe content: 19%, basicity: 11%) was purchased from the Lvliao Industry Co.,

76

Ltd., China. Humic acid sodium salt (HA) was purchased from Sigma-Aldrich Co., USA. TiCl4,

77

AcAc, ethanol, kaolin powder, NaOH, HCl, and NaF of chemical grade were purchased from the

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Sinopharm Chemical Reagent Co., Ltd., China.

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Simulated Water. An aqueous suspension composed of HA and kaolin powder was

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prepared with tap water (alkalinity: 75 mg/L as CaCO3). The resultant suspension was used as a

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simulated water and had an absorbance at 254 nm (UV254) of 0.333 ± 0.005 cm-1, an initial pH

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of 7.3 ± 0.2, an initial turbidity of 26.0 ± 1.0 NTU, and an initial HA concentration of 20.0 ± 0.2

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mg/L. The initial solution pH was adjusted with 0.1 M HCl or NaOH solutions before the

84

addition of coagulant. Whenever needed, the initial alkalinity was adjusted with NaHCO3. Two

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Cr simulated waters (20 mg/L) were prepared with tap water (pH was adjusted to 5.1) and

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deionized water (pH was adjusted to 10.4) to make the solution consisting of mainly soluble and

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colloidal Cr, respectively. A N/P simulated water was prepared by adding beef extract (100

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mg/L), peptone (100 mg/L), NaNO3 (85 mg/L) and NaH2PO4 (60 mg/L) into tap water.

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Real Wastewater. One tanning wastewater and two textile wastewaters (TW1 and TW2)

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were sampled from a local company in Jiangsu Province, China. A soybean protein wastewater

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was collected from a food company in Shandong Province, China. The pH, initial turbidity, total

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and soluble chromium (Cr) content of the tanning wastewater were 7.3, 2858 NTU, 897.3 and

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15.2 mg/L, respectively. The initial turbidities of the two textile wastewaters were 54 NTU

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(TW1) and 17 NTU (TW2), respectively. Their pH values were both 8.3. The chemical oxygen

95

demand determined with the dichromate method (CODCr), total nitrogen (TN), and total

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phosphorus (TP) of the soybean protein wastewater were 19742, 445.9 and 238.7 mg/L,

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respectively, and the pH was 3.3.

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Preparation of PTC and TXC. TXC was prepared with the sol-gel method. Firstly, 3.1 mL

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of TiCl4 was added dropwise to a mixture of ethanol (20 mL) and AcAc (the molar ratio of

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AcAc/Ti ranged from 1/32 to 3/8) and mixed with a magnetic stirrer at room temperature.

101

Secondly, a mixture of 10 mL of ethanol and a certain volume of ultrapure water (the molar ratio

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of H2O/Ti varied from 1 to 8) were then added dropwise to the TiCl4-ethanol-AcAc solution. The

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mixture was continuously stirred for 90 min to obtain a yellowish and uniform sol. Finally, the

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sol was aged with five aging modes, i.e., air dried at room temperature (A mode), oven dried at

105

50oC in an electric thermostatic dryer (O mode), vacuum dried at 40oC (V mode), and the

106

combinations of V mode with the other two modes (i.e., A/V and O/V), until the sol was turned

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into a gel with a constant weight.

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PTC was obtained using a slow alkaline titration method. Based on the previous work,16

109

considering both the coagulation performance and the stability, a OH/Ti molar ratio of 1.0 was

110

selected for PTC in this work. A volume of 0.58 mL concentrated TiCl4 solution was dropwise

111

added to 225 mL distilled water in an ice-water bath under continuous stirring. Then, 25 mL of

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NaOH solution (0.2 M) was added under intensive agitation to yield a PTC solution with a

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OH/Ti molar ratio of 1.0 (basicity: 25%) and a final Ti concentration of 1 g /L. Both the PTC and

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TC stock solutions were prepared freshly before the coagulation experiment.

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Characterization of TXC. The obtained TXC was systematically characterized, including

116

Fourier transform infrared spectrum (FTIR), X-ray photoelectron spectroscopy (XPS),

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thermogravimetry-differential

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(TG-DTG-DSC), 13C solid-state nuclear magnetic resonance (NMR), zeta potential, transmission

119

electron microscopy (TEM), alkalinity and element analyses. Experimental details for these

120

analyses were available in the Supporting Information (SI Text S1).

thermogravimetry-differential

scanning

calorimetry

121

Coagulation Experiments

122

Jar Test. Coagulation was performed on a program-controlled jar test apparatus (ZR4-6,

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Zhongrun Water Industry Technology Development Co. Ltd., China) with six beakers (1.0 L)

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and six flat paddle impellers (50 mm × 40 mm). The coagulation procedure included an initial

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rapid stirring at 200 rpm (102.5 s-1) for 1 min, a slow stirring at 40 rpm (11.8 s-1) for 15 min, and

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a setting period for 20 min. After these procedures, the supernatants were collected for analysis

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from approximately 2 cm below the water surface using a syringe. When comparison was made

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among TC, PTC, and TXC, the dosage was expressed in mg Ti/L. When PFS was compared, the

129

dosage was expressed as mass of coagulant per liter (mg/L).

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Analytical Methods. For UV254 measurement, the sample solutions were filtered through a

131

0.45 µm fiber membrane and were determined with a UV-Vis spectrophotometer (UV-2700,

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Shimadzu Co., Japan). Residual turbidity (RT) was measured directly with a 2100 P turbidimeter

133

(Hach Co., USA) without filtration. CODCr, TN, and TP were measured using the national

134

standard methods (Text S1). The speciation details for N and P, including the measurements of

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ammonia nitrogen (NH4-N), nitrate nitrogen (NO3-N), organic nitrogen (Org-N), orthophosphate

136

(PO4-P), and organic phosphorus (Org-P), are available in the SI (Text S2). Flame atomic

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absorption spectrometer (AA7000, Shimadzu Co, Japan) was employed to measure the

138

concentration of Cr. Soluble Cr (CrS) and insoluble Cr (CrIns) in the supernatant were

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distinguished with a 0.45-µm cellulose-acetate filter.

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Floc size was analyzed with a laser diffraction instrument (Mastersizer 3000, Malvern Co.,

141

UK) at an emission wavelength of 633 nm. The apparatus was assembled according to a previous

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report.20 Floc formation, breakage, and regrowth tests were conducted following the same two

143

steps as mentioned above. After the slow stirring period, the suspension was exposed to a high

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shear force (200 rpm) for 3 min and another 15 min of slow stirring at 40 rpm for floc regrowth.

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Size measurements were taken every 30 s. The details on the measurement and calculation of the

146

floc properties were available in the SI (Text S2).

147 148

RESULTS AND DISCUSSION

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Optimized Preparation Conditions for TXC. The ratios of the precursors and the aging mode

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are important factors in determining the properties of the resultant xerogels from the sol-gel

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process. With TiCl4 as the reference, xerogels could be successfully prepared with the molar

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ratio of AcAc/TiCl4 and H2O/TiCl4 in the range of 1/32-3/8 and 1-8, respectively. As shown in

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Figure 1a, the TXC samples prepared under such conditions were different in color. Most of

154

them were yellowish, and a few ones were reddish. It has been reported that the properties of

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PTC depends strongly on the basicity.16 Interestingly, there was no correlation between the

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turbidity removal efficiency and the preparation conditions of the xerogels. All of the TXC

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samples possessed similar turbidity removal efficiencies (Figure 1b), suggesting that the

158

preparation of TXC was feasible, with no need for strict control of both the precursor

159

composition and the aging process. The combination of air dry (A mode) with vacuum dry (V

160

mode) significantly shortened the preparation time. Therefore, the procedure for sample 11 was

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selected for further study.

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Coagulation Performance in Simulated Water

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HA-kaolin Simulated Water. The coagulation performances of TC, PTC, and TXC were

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evaluated in the HA-kaolin simulated water under various coagulant dose and solution pH. As

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shown in Figure 2a, the organic matter (indicated by UV254) removal by TXC was slightly lower

166

than those by TC or PTC. TC was effective at turbidity removal within a dose range of 10-20 mg

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Ti/L, whereas the applicable dose range for PTC was 12.5-25 mg Ti/L. The larger dose

168

requirement for PTC than for TC might be a result of prehydrolysis. With further increased dose,

169

both TC and PTC rapidly lost their ability to remove turbidity. In other words, the suspensions

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were re-stabilized at high doses. There was a good correlation between the re-stabilization and

171

the change of zeta potential (Figure 2a), demonstrating that charge neutralization played a key

172

role in the coagulation with TC and PTC. Comparatively, the workable dose range of the TXC

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was much wider. More than 85% of the turbidity could be removed at a TXC dose of 7.5 mg

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Ti/L. The turbidity removal reached 95% at 15 mg Ti/L and did not show any deterioration until

175

the TXC dose was up to 40 mg Ti/L. The stability of TXC in turbidity removal was also reflected

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by its stable zeta potential within the dose range of 7.5-40 mg Ti/L.

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As shown in Figure S1, the increase of alkalinity broadened the applicable dose and pH

178

ranges for TC and PTC. Even so, the turbidity removal abilities of TC and PTC were still not as

179

good as that of TXC. In the alkalinity range of 75-325 mg/L (as CaCO3), TXC performed well in

180

the dose range of 5-40 mg Ti/L, whereas the minimum required dosages for TC and PTC were

181

20 and 15 mg Ti/L, respectively. These results demonstrate that gelation (the process used for the

182

preparation of TXC) is a more efficient way than prehydrolysis (the process used for the

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preparation of PTC) in many respects, including better stock stability, wider applicable pH and

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dose ranges, and better performance in low alkalinity solutions. In addition to the turbidity removal, another significant difference between TXC and

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TC/PTC was the effluent pH. Prehydrolysis of TC to PTC slightly inhibited the pH reduction.

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However, with increased dose of TC/PTC, the effluent pH was quickly decreased to below 4.

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The effluent pH of the TXC samples remained stable at around 7.0 through the entire dose range

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(Figure 2a). As shown in Figure 2b, TXC worked well within the pH range from 5-10, whereas

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both TC and PTC were effective only at pH values above 7. The effluent vs initial solution pH

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profiles (Figure 2b) further demonstrated the better performance of TXC in terms of workable

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pH range and mild pH change.

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Compared with TC and PTC, a much higher dose of PFS was required to achieve the same

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residual turbidity under conditions identical to those used for TXC (Figure S2). TXC had

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obviously wider applicable dose and pH ranges than PFS, although PFS was better at the

196

removal of organic matter. This was consistent with previous reports.21,22 The stronger binding

197

ability of the ferric-oxyhydroxides generated from Fe3+ salts with organics compared with that of

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the titanic hydrolysates might account for the better organic matter removal by PFS. However,

199

the residual metal concentration in the PFS system was much higher than that of the TXC (1.5

200

mg Fe/L vs 0.4 mg Ti/L at a dose of 60 mg/L).

201

N/P Simulated Water. The N/P simulated water contained both organic and inorganic N/P.

202

As shown in Figure S3, among the three Ti-based coagulants, TXC showed the highest TN and

203

TP removal (69.9% and 74.1% for TN and TP, respectively). TC and PTC had a very similar

204

performance (39.1% and 44.1% for TN and TP, respectively).

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Cr Simulated Water. Coagulation experiments were run in two Cr simulated waters (20

206

mg/L), one with an initial pH of 5.1 and one with a pH of 10.4. According to the speciation-pH

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profile of Cr (Figure S4), the main species in the pH 5.1 solution was Cr(OH)2+, whereas that in

208

the pH 10.4 solution was colloidal Cr(OH)3. The experiments in the former solution reflected the

209

ability of these coagulants to remove soluble Cr and the experiments in the latter one showed the

210

coagulation performance for colloidal particles. After the addition of TC, PTC, and PFS, the pH

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of the Cr simulated solution (pH 5.1) was significantly decreased, but with no floc formation in

212

the dose range of 1-25 mg Ti/L or 10-120 mg/L of PFS. This agrees well with the pH effect in

213

Figure 2. When TXC was dosed, the removal of Cr was increased from 23.0% to 31.9% with the

214

dose of TXC from 10 mg Ti/L to 25 mg Ti/L (Figure 3a). It is well known that AcAc could

215

chelate with transition metals through the enolic OH to form chelated complexes.23 The chelated

216

AcAc in TXC was believed to play a role in the removal of the soluble Cr.

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The turbidity of the pH 10.4 Cr solution was 79.8 NTU. Among the four coagulants, TXC

218

had the best Cr removal efficiency. After coagulation, the residual Cr concentrations were 3.6,

219

2.9, 1.1, and 1.9 mg/L for TC, PTC, TXC, and PFS, respectively. Beyond that, the flocs formed

220

in the TXC sample were the biggest with the fastest settling velocity (Figure 3b). It should be

221

noted that the solution pH was closely related to the coagulant dose, which could cause the

222

dissolution of the colloidal Cr. Therefore, the required dose for the different coagulants was not

223

the same. The above results demonstrate that TXC advantages over the other three coagulants for

224

the removal of both soluble and colloidal Cr.

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Coagulation Performance in Real Wastewater

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Tanning Wastewater. Tanning wastewater usually contains a large amount of Cr, which is

227

a hazardous heavy metal. Coagulation and precipitation are the two most widely used methods in

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the treatment of tanning wastewater. As shown in Figure 3c, the Cr removal ability of TXC was

229

much greater than that of the commercially used PFS, especially for the insoluble Cr. Besides,

230

TXC was better than PFS in terms of floc size and settling property. The better performance of

231

TXC in removal of Cr could be attributed to the two advantages over PFS: (1) the faster

232

hydrolysis of the titanate than that of the iron salt (data was given in the next section), which led

233

to an enhanced sweep flocculation, and (2) the chelating ability of the AcAc moiety in the TXC

234

to the soluble Cr.

235

Soybean Protein Wastewater. Soybean protein wastewater is rich in nutrient elements,

236

including both organic and inorganic N and P. The TN removal by TXC was slightly higher than

237

that by PFS (Figures 4a and 4b), whereas PFS showed better performance in the removal of TP

238

(Figures 4c and 4d). It was generally assumed that P was removed by a complicated combination

239

of interactions and adsorption with the metal hydrolysates in the coagulation process.24 Fe3+ has

240

the ability to adsorb a considerable amount of phosphate via a strongly bonded binuclear

241

complex,25 which might account for the better performance of PFS in removal of TP. It was clear

242

that TXC was more efficient in the removal of inorganic N species. The removal percentages of

243

NO3-N and NH4-N in the TXC system were several times to those of the PFS system.

244

Textile Wastewater (TW1). Textile production is a major industry in China and produces

245

huge amounts of wastewater. Coagulation and decolorization are necessary processes in the

246

treatment of textile wastewater. As shown in Figure 5a, both TXC and PFS could form flocs in

247

the textile wastewater. However, once the wastewater was filtrated before coagulation, PFS

248

could no longer form flocs, whereas the flocs formed by TXC remained almost the same floc

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size, growth rate, and settling velocity as those in the raw TW1 water (Figure 5b). Moreover, the

250

flocs formed by PFS, if any, were too small to settle. On the contrary, the flocs in the TXC

251

samples were rapidly deposited on the bottom of the vessels (Figure 5c). As a result, the solution

252

was partially decolorized. The coagulation experiments in another textile wastewater (TW2)

253

showed similar results (Figure S5). These results, together with the experiments in the Cr

254

simulated water (Figure 3a), demonstrate that TXC was applicable to low turbidity water. This is

255

important for practical applications, because low turbidity has been a key restricting factor in

256

coagulation.

257

Floc Properties of TXC

258

Floc Size and Growth Rate. Floc size and growth rate are important criteria in the

259

evaluation of coagulants. Among the four studied coagulants, TXC produced the largest flocs

260

(1470 µm) at the fastest growth rate (945 µm/min) in hydrolysis (Figure 6a and Table S1). Flocs

261

with larger size generally settle more quickly than smaller ones,26 which might explain the better

262

performance of TXC than the other three in both the simulated and real waters.

263

Floc Strength and Recoverability. High shear forces are usually employed in the

264

separation of the aggregated particles from coagulation. Therefore, floc strength and

265

recoverability are important parameters in the optimization of the overall coagulation process.27

266

Floc strength factor (Sf) and recovery factor (Rf) are used to assess the floc strength and

267

recoverability.28,29 Sf refers to the ability to resist rupture by a velocity gradient, whereas Rf

268

indicates floc recovery ability. As listed in Table S1, TXC had a slightly larger Sf than TC and

269

PTC but had the smallest Rf. In other words, the particles formed in the TXC system were not so

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susceptible to shear force, but once broken were less likely to recover. Zhao et al.16 have

271

proposed two possibilities to explain the irreversible floc breakage with limited floc regrowth: (1)

272

floc cohesion is dominated by chemical bonding rather than physical bonding, and (2)

273

coagulation is controlled not only by charge neutralization but also by sweep flocculation. In this

274

study, the large floc size might also be attributable to the small Rf. As shown in Figure 6a,

275

although TXC had the poorest floc recoverability, the flocs in the regrowth stage were still the

276

largest among the four samples. By contrast, PFS had the largest Rf but smallest particle size. In

277

the final separation, it is the particle size rather than the Rf that determines the dewaterability.

278

Therefore, the results here suggest that appropriate stirring speed and time were important in the

279

application of TXC. On the other hand, the results indicate that the use of Rf as an evaluation

280

index for coagulants should be based on specific cases.

281

Flocs are highly porous and fractal-like aggregates.30 Fractal dimension (Df), as an index

282

representing the degree of compactness of the primary particles, is a vital parameter that

283

influences the solid/liquid separation efficiency.31 For all four coagulants, the Df values of the

284

formed flocs were increased after breakage and reduced again after regrowth. The Df of TXC

285

was generally lower than those of TC and PTC but was higher than that of PFS.

286

TEM was employed to observe the hydrolysates formed in the TC and TXC solutions. As

287

shown in Figure 7a, the hydrolysates from TXC in pure water were mixtures of lamellar

288

structures with fine particles, whereas abundant hollow spheres of submicron sizes were formed

289

in the TC solution (Figure 7b). The difference in the three dimensional structures explained the

290

difference of the flocs from TXC and TC in terms of particle size and Df.

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Components of TXC. The chemical composition of TXC was analyzed with FTIR, XPS,

291 292

13

293

spectrum (Figure S6 and Text S4). The high-resolution spectra of O1s and C1s (Figure S7 and

294

Text S4) proved that AcAc was chelated with Ti and was successfully embedded into the

295

framework of the xerogel. The shift of the C=O peaks to lower frequency region in the FTIR

296

spectrum of TXC further demonstrated the chelation of AcAc with Ti4+ (Figure S8 and Text S5).

297

The 13C NMR spectrum of TXC revealed many feature characteristics of AcAc, which was direct

298

evidence for the existence of AcAc in TXC (Figure S9 and Text S6). There were three mass loss

299

steps in the TG-DTG-DSC profiles (Figure S10 and Text S7), corresponding to the evaporation

300

of adsorbed or weakly bounded water and solvent (96°C), the release of residual Cl (206°C), and

301

the decomposition of AcAc (418°C), respectively. Based on the analysis above and the

302

determined mass contents in Table S2, the suggested composition of TXC was

303

Ti4Cl4O16AcAc(OH)2, with a schematic structure as illustrated in Figure S11. The mass content

304

of Ti in TXC was about 20%, comparable to the effective content of PFS (19%).

C NMR, and TG-DTG-DSC. Four elements, Ti, Cl, C, and O, were observed in the XPS survey

305

Coagulation Mechanism of TXC. Coagulation is a very complicated process. The

306

performance of TXC in removal of turbidity; inorganic species, including both heavy metals and

307

nutrient elements; and organic matter suggests several operating factors:

308

(1) The use of AcAc in the sol-gel process for the preparation of TXC made the

309

hydrolysis-condensation reactions more controllable. The hydrolysis of the resulting TXC in

310

waters was sufficiently rapid but milder than that of the precursor (TiCl4) and the prehydrolyzed

311

PTC. The release and volatilization of protons in the form of hydrochloric acid during the

17



Page 18 of 30

312

long-time aging process prevented the dramatic decline of the effluent pH. The quantity of

313

effective hydrolysates and the solution pH were the principal factors that influenced the required

314

coagulant dose. With better-controlled hydrolysis and more stable surface charges, TXC had

315

larger applicable dose and pH ranges than TC and PTC.

316

(2) The chelated AcAc in TXC was not only favorable for controllable hydrolysis but also

317

provided many binding sites for pollutants. This might account for the ability of TXC to remove

318

dissolved Cr, because AcAc could form complexes with many metals, as discussed in Text S5.

319

(3) Charge neutralization and sweep flocculation are two basic mechanisms in coagulation.

320

In the removal of N and P, charge neutralization mechanism dominated in the removal of anionic

321

species (NO3-N and PO4-P), whereas sweep flocculation played a key role in the removal of

322

NH4-N, Org-N, and Org-P. TXC had a higher positive charge density than PFS (0.022 mmol/g vs

323

0.012 mmol/g). As a result, TXC was more effective in the removal of NO3-N. However,

324

because of the stronger binding forces between Fe and P/organic matter, the ability of TXC to

325

remove P and organic matter was relatively weaker than that of the commercial PFS. Except for

326

the weak adhesion force between Ti hydrolysates and organic matter, the small surface area as a

327

consequence of the large floc size might also be attributable to the relatively low efficiency of

328

TXC in removal of organic matter.

329

Overall, the good performances of TXC in coagulation, including larger floc size, better

330

settling property, wider applicable coagulant dose/pH range, and milder effluent pH change than

331

PTC and PFS, demonstrate that gelation might be a more efficient way than prehydrolysis in

332

improving the performance of Ti salt coagulants. Whether TXC could be economically feasible

18


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333

as a coagulant will depend on the cost of the Ti precursors and the recoverable value of the

334

coagulated sludge.

335

336

ASSOCIATED CONTENT

337

Supporting Information

338

Further details of the analytical methods and experimental data are presented free of charge on

339

the Internet at http://pubs.acs.org. These materials include: characterization of TXC (Text S1),

340

analytical methods for N and P (Text S2), determination of floc properties (Text S3),

341

characterization details (Texts S4-S7), floc properties of the studied coagulants (Table S1), mass

342

components of TXC (Table S2), effect of solution alkalinity (Figure S1), turbidity removal from

343

the simulated water (Figure S2), removal for TN and TP in simulated water (Figure S3),

344

speciation-pH profile of Cr (Figure S4), floc formation in the textile wastewater TW2 (Figure

345

S5), XPS survey spectrum of TXC (Figure S6), high-resolution XPS spectra of TXC (Figure S7),

346

FTIR spectra of TXC and AcAc (Figure S8),

347

TG-DTG-DSC curves of TXC (Figure S10), schematic structure of TXC (Figure S11), and

348

related references.

13

C NMR spectrum of TXC (Figure S9),

349 350

AUTHOR INFORMATION

351

Corresponding Author

352

*Phone: +86 25 8968 0389; Tax: +86 25 8968 0569; E-mail: [email protected]

19



353

Notes

354

The authors declare no competing financial interests.

Page 20 of 30

355 356

ACKNOWLEDGMENTS

357

This work was supported by the National Natural Science Foundation of China (21522702).

358 359

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438 439

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Figure Captions

440 441 442

Figure 1. (a) Photos of TXC (S1-12). (b) Turbidity removal efficiency of TXC. Initial turbidity: 26 NTU, pH: 7.2, TXC: 40 mg/L.

443 444

Figure 2. Coagulation performance of TC, PTC, and TXC in the HA-kaolin simulated water as a function of coagulant dose (a) and solution pH (b).

445 446

Figure 3. (a) Cr removal in the Cr simulated water (pH 5.1). (b) Floc formation in the Cr

447

simulated water (pH 10.4). (c) Residual Cr in the tanning wastewater as a function of

448

coagulant dose at pH 7.3.

449

Figure 4. Removal of N and P from the soybean protein wastewater (pH was adjusted to 6.0) as

450

a function of coagulant dose. Insets in b and c: speciation and content of N and P in the

451

wastewater.

452

Figure 5. Floc formation and settling in the raw and filtrated (-F) textile wastewater TW1. (a)

453

and (b): top view pictures during stirring, (c): side view pictures at the slow stirring

454

period and after setting. Initial turbidity: 54 NTU, pH: 8.3, dose: 40 mg/L.

455

Figure 6. Floc size (a) and fractal dimension (b) evolution in the formation, breakage, and

456

regrowth processes of flocs at pH 7 and the optimized dose of coagulants. TC and PTC:

457

15 mg Ti/L, TXC and PFS: 40 and 60 mg/L.

458

Figure 7. TEM images of the hydrolysates of TXC (a) and TC (b). Ti: 10 g/L.

459

25



460

461

Figure 1.

462

26


Page 26 of 30

Page 27 of 30


463

Figure 2.

464 465

27



466

Figure 3.

467 468

469

Figure 4.

470

28


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Page 29 of 30


471

Figure 5.

472 473

474

Figure 6.

475 476

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477

Figure 7.

478

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Preparation and Evaluation of Titanium-Based Xerogel as a Promising

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