Preparation and Characterization of Novel Polytitanium Tetrachloride

Article pubs.acs.org/est

Preparation and Characterization of Novel Polytitanium Tetrachloride Coagulant for Water Purification Y. X. Zhao,† S. Phuntsho,‡ B. Y. Gao,*,† X. Huang,† Q. B. Qi,† Q. Y. Yue,† Y. Wang,† J.-H. Kim,§ and H. K. Shon*,‡ †

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, People’s Republic of 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 § School of Applied Chemical Engineering & The Institute for Catalysis Research, Chonnam National University, Gwangju 500-757, South Korea S Supporting Information *

ABSTRACT: Polymeric metal coagulants are increasingly being used to improve coagulation efficiency, yet the research on the development of titanium and particularly polytitanium salts remains limited. This study is the first attempt in the synthesis, characterization, and application of polytitanium salts as coagulants. Polytitanium tetrachloride (PTC) solutions with different basicity values B (OH/Ti molar ratio) were prepared using a slow alkaline titration method. Jar tests were conducted to assess coagulation performance using both synthetic and real raw water samples, and the floc characteristics were monitored online using a laser diffraction particle size analyzer. Electrospray ionization time-of-flight mass spectrometry (ESI-TOF-MS) was utilized to identify various Ti species, with the results providing strong evidence of the presence of various hydrolyzed Ti species in the titanium aqueous phase. Compared to titanium tetrachloride (TiCl4), higher or comparable turbidity and organic matter removal efficiency could be achieved by PTC with improved floc characteristics in terms of size, growth rate, and structure. Besides, the water pH after PTC coagulation was significantly improved toward neutral pH. This study indicates that PTC is an effective and promising coagulant for water purification. Besides, the PTC flocculated sludge was able to recycle and produce functional TiO2 photocatalyst.

1. INTRODUCTION

require further disposal either at landfills and/or ocean dumping. To overcome the sludge disposal problem, a novel Ti coagulants was proposed and studied recently by Shon and his co-workers5−8 for water purification. The most significant advantage of Ti-based coagulant is that the final coagulated sludge can be recovered to produce a valuable titanium dioxide (TiO2) as byproduct. TiO2 is the most widely used metal oxide with its applications including photocatalysts, cosmetics, paints,

Coagulation or flocculation is one of the most common and important water treatment processes for particle and natural organic matter (NOM) removal.1 Although the use of aluminum (Al) coagulants for water treatment dates back centuries, there have been disputes on the possible adverse effects of Al on human health and environment.2 This has led to the increasing use of iron (Fe) coagulants, which have higher dissolved organic carbon (DOC) removal efficiency than Al coagulants3,4 and less adverse effect to human health. Nonetheless, the coagulation−flocculation process using Al and Fe coagulants produces large quantities of sludge, from which nothing can be recovered or reused and which then © 2013 American Chemical Society

Received: Revised: Accepted: Published: 12966

April 16, 2013 September 29, 2013 October 10, 2013 October 10, 2013 dx.doi.org/10.1021/es402708v | Environ. Sci. Technol. 2013, 47, 12966−12975

Environmental Science & Technology

Article

electronic paper, and solar cells.9,10 Moreover, previous studies reported that the Ti-based coagulants could achieve greater removal efficiency of particle and NOM compared to conventional coagulants.11,12 Additionally, titanium is one of the most abundant elements on the earth and is found in almost all living things, rocks, water bodies, and soils. Titanium is nontoxic even in large doses and does not play any adverse role or toxicity inside the human body.13 Because of its biocompatibility, titanium is used in a gamut of medical applications including surgical implements and implants. Therefore, for these reasons titanium and its compounds are rarely included in any water quality guidelines.14 During the last 20 years, considerable attention has been paid to the development of prehydrolyzed inorganic coagulants based on Al or Fe, such as polyaluminum chloride (PAC), polyferric chloride (PFC), and polyferric sulfate (PFS).15−20 The advantage of the prehydrolyzed coagulants is that the hydrolysis of Al or Fe ions occurs during the preparation stage of the coagulants, not after their addition to the real water, which consequently results in a better control of the coagulation process.21 Furthermore, they are useful for reducing the need for pH adjustment through prehydrolysis, are less sensitive to low temperature, and are highly effective in removing numerous pollutants.4,16 They are also cheaper than organic polymeric coagulants.22,23 However, none of the previous studies reported the development of polytitanium salts as coagulants. When titanium salts were used as coagulants, the charge neutralization (adsorption−destabilization) of optimum coagulation efficiency occurred at low pH values of 3.5−5.0 after coagulation due to the large amount of H+ release during the titanium hydrolysis process.7,11 On the basis of the earlier research on inorganic polymeric coagulants,15,16,23 the low and narrow pH range for titanium coagulation could be solved by developing polytitanium salts that might minimize H+ release through prehydrolyzed titanium coagulants. Also, similar to other inorganic polymer coagulants, polytitanium salts could also perform better than titanium salts in terms of organic matter removal and pH dependence. PAC is a commonly used prehydrolyzed coagulant, which could achieve higher particulates and/or organic matter removal than the traditional AlCl3 and alum coagulants under different coagulation conditions.24,25 It has been well studied and therefore could serve as a prototype for modeling polytitanium coagulant behavior in water purification. PAC can be produced by adding a base to aluminum chloride26 and the relative amount of OH− compared to Al determines the basicity (B, OH/Al molar ratio) of the PAC product. In the same manner, titanium tetrachloride could be modified to polytitanium tetrachloride with different basicities (B, OH/Ti molar ratio). The higher coagulation efficiency of PAC as compared to traditional AlCl3 and alum coagulants is often ascribed to the characteristics of the prehydrolyzed Al species.27,28 Electrospray ionization mass spectrometry (ESI-MS) has been used as a method to determine Al speciation and proved suitable for identifying Al species in AlCl3 and PAC coagulant solutions.29−33 The ESI-MS of metal salts in aqueous solution correlated well with the ions present in solution on the basis of known stability constants.34 However, nobody has done the distribution and transformation pattern of Ti species in titanium and polytitanium coagulant solutions. A thorough understanding of the speciation of hydrolyzed Ti species would

be of great help in developing highly efficient coagulants and understanding their coagulation mechanisms. The main objectives of this study are to (i) synthesize polytitanium tetrachloride (PTC) and identify Ti species using electrospray ionization time-of-flight mass spectrometry (ESITOF-MS), (ii) evaluate the coagulation performance of PTC using both simulated and real wastewater and characterize the floc properties using a laser diffraction instrument, (iii) investigate the mechanisms involved in the coagulation/ flocculation process based on coagulation performance, floc properties, and zeta potential measurement, and (iv) recover the PTC coagulated sludge by incineration to produce functional TiO2 photocatalyst.

2. EXPERIMENTAL SECTION 2.1. Preparation of PTC. Two steps were used for PTC preparation. First, a predetermined volume of concentrated TiCl4 solution (purity ≥99%) was slowly added to cubes of frozen distilled water drop by drop under continuous stirring to obtain 20% TiCl4 solution (density (ρ) = 1.26 g/mL). Then, the predetermined amount of concentrated sodium hydroxide (NaOH) solution (200.0 g/L) was added to the TiCl4 (20%) solution using a slow alkaline titration method under intensive agitation. The chosen basicity (B, OH/Ti molar ratio) values were 0.3, 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0 until irreversible precipitation occurred at the final stage of titration. The resultant samples were thereafter denoted as PTC03, PTC05, PTC10, PTC15, PTC20, PTC25, and PTC30. Clear and transparent PTC solutions with different B values were observed without any visual precipitates and were found to be stable for several weeks after preparation. However, visible white precipitates appeared in the PTC30 coagulant solution after 2 weeks of storage, and after 2 months, some precipitates were identified in PTC20 and PTC25. The PTC15 coagulant solution did not have a clear appearance after three months of storage. The pictures of PTC coagulants with aging period are given in the Supporting Information, Figure S1. The initially formed Ti species were presumably only transitional species that slowly converted to more stable species during aging. The experiments in this study were, however, conducted immediately after coagulant preparation to avoid the aging effect. 2.2. Coagulation Performance of PTC. Coagulation experiments were performed using (i) synthetic water (SW) containing humic acid (HA) as model NOM, and (ii) biologically treated sewage effluent (BTSE) withdrawn from Sydney Olympic wastewater treatment plant, Australia, and (iii) river water (RW) from Parramatta. Standard jar tests were conducted using a programmable jar-tester. Details about the experimental procedures and characteristics of the water samples are described in Supporting Information, section S2. 2.3. Determination of Dynamic Floc Properties. 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 online monitoring system for dynamic floc size can be referred to Zhao et al.11 Following the floc growth phase, the aggregated flocs were exposed to a shear force at 200 rpm for 1 min, followed by a slow mixing at 40 rpm for 15 min to allow floc regrowth. The median equivalent diameter, d50 , was selected as the representative floc size. 12967

dx.doi.org/10.1021/es402708v | Environ. Sci. Technol. 2013, 47, 12966−12975


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Figure 1. Comparative coagulation performance experiments using homemade PTC coagulants with different B values in terms of (a) residual turbidity, UV254, and DOC removal; (b) floc zeta potential and pH after coagulation (coagulant dose 10 mg-Ti/L, see Supporting Information, section S2 for water characteristics).

The floc growth rate was calculated by the slope of the rapid growth region:35 growth rate =

Δsize Δtime

2000.36,38,39 The total scattered light intensity I, the scattering vector Q, and Df followed a power law:40

I ∝ Q−Df (1)

The scattering vector Q is the difference between the incident and scattered wave vectors of the radiation beam in the medium:39

Floc strength factor (Sf) and recovery factor (Rf) are used to compare the floc breakage and recoverability:11,36,37

Sf =

d2 100 d1

d − d2 100 Rf = 3 d1 − d 2

(4)

Q= (2)

4nπ sin(θ /2) λ

(5)

where, n, λ, and θ are the refractive index of the medium, the laser light wavelength in vacuum, and the scattering angle, respectively. DA densely packed aggregate has a higher Df value, while a lower Df value results from a large, high branched and loose bound structure. 2.4. Sludge Recovery. The sludge generated after coagulation of BTSE using PTC was dewatered and then dried at 100 °C for 12 h, followed by calcination at 600 °C for

(3)

where d1 is the average floc size of the plateau before breakage, d2 is the floc size after the floc breakage period, and d3 is the floc size after regrowth to the new plateau. Previous researches have reported the determination of aggregate mass fractal dimension (Df) by using Mastersizer 12968



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12 h. This temperature (600 °C) had been observed to be the most efficient in terms of minimum energy cost and high photocatalytic activity.7 The catalyst produced was, hereafter, referred to as “as-prepared TiO2”. 2.5. Electrospray Ionization Time-of-Flight Mass Spectrometry (ESI-TOF-MS). The prehydrolyzed Ti species of the PTC solutions with B values of 0, 0.3, 0.5, 1.0, 1.5, and 2.0 were analyzed using ESI-TOF-MS. A concentration of 0.01 mol-Ti/L was chosen since more concentrated solutions would contaminate the equipment. Mass spectra were recorded with a high performance liquid chromatography/hybrid quadrupole time-of-flight mass spectrometer (Micromass Q-TOF Micro, Waters, USA). The PTC samples were clear and transparent, and were injected into the spectrometer directly without filtration at a flow rate of 10 μL/min. The detailed instrumental conditions can be referred to Sarpola.32 The sample cone voltage of 70 V was the optimum voltage, because the baseline was then clean enough even for signals with intensities below 5%.33

For PTC with high B values of 2.0, 2.5, and 3.0, the positive charges of the prehydrolyzed Ti species did not satisfy the complete charge neutralization, demonstrating their relatively weak charge neutralization ability. This led to the formation of flocs with negative zeta potential (Figure 2b), and thus sweep

3. RESULTS AND DISCUSSION 3.1. Effect of B Value on Coagulation Performance for SW Treatment. Figure 1 shows the effect of B value on coagulation performance in terms of residual turbidity, UV254 and DOC removal, and zeta potential as well as pH after coagulation. The coagulation performance of TiCl4 (PTC0) was investigated for comparison. 3.1.1. Coagulation Efficiency. The increase of B value first led to a sharp decline in residual turbidity, with the lowest residual turbidity of 0.95 NTU at B = 1.5, and a gradual increase was then observed as the B values were further increased (Figure 1a). The decrease in UV254 and DOC removal was insignificant by up to the B value of 1.5, and beyond the B value of 1.5, the reduction in removal efficiency became more obvious. At the B value of 3.0, the UV254 and DOC removal efficiency was decreased by 15.4% and 24.1%, respectively, compared to that of PTC0. The decrease in UV254 and DOC removal efficiencies was accompanied by an increase in pH after coagulation and a decrease in floc zeta potential (Figure 1b). The increase of pH after coagulation at higher B values suggests that the low pH after coagulation associated with TiCl4 coagulation7,11 can be solved to a certain degree by developing PTC which minimized H+ release through the prehydrolysis of the Ti-species. 3.1.2. Coagulation Mechanisms. Coagulation is generally explained in terms of charge neutralization and sweep flocculation.41 For PTC0, the high HA removal was achieved mainly by charge neutralization between in situ formed soluble hydrolyzed Ti species and negatively charged HA molecules, resulting in a high floc zeta potential of +9.0 mV. Klute42 found that the prehydrolyzed species in PAC are more stable than those formed in situ after AlCl3 and alum addition. Accordingly, the prehydrolyzed Ti species were expected to be stable and reacted with HA directly by charge neutralization after the addition of PTC. With the increasing B value, the floc zeta potential decreased from positive to negative with B = 1.5 as the inflection point (Figure 1b), indicating the change of the predominant coagulation mechanism. The lowest residual turbidity was obtained at B = 1.5 where the floc zeta potential was close to zero, suggesting complete charge neutralization between PTC15 and HA molecules. The relatively high residual turbidity with other B values may be attributed to charge repulsion among the flocs because of similar charges.

Figure 2. Floc characteristics with different B values in terms of (a) online monitoring of floc growth, breakage, and regrowth profiles; (b) size of the flocs (d1, d2, and d3) and floc growth rate.

flocculation may be the dominant mechanism for HA removal. The distinct decrease in HA removal by PTC with high B values (2.0, 2.5, and 3.0) (Figure 1a) demonstrates that the prehydrolyzed and highly polymeric Ti species were less effective in HA removal than the in situ formed Ti species by PTC0. 3.2. Effect of B Value on Floc Properties for SW Treatment. As shown in Figure 2a, the floc size first showed a significant increase after slow mixing at 40 rpm, implying a balance between the rate of aggregation and breakage.43 Then, a significant drop in floc size was observed after the introduction of shear force (200 rpm, 1 min), followed by gradual floc regrowth. 3.2.1. Floc Size and Growth Rate. Figure 2b presents the change in floc size (d1, d2, and d3) and floc growth rate (calculated by eq 1) as a function of B values. The floc growth rate increased sharply with increasing B values, reaching the maximum value of 450.6 μm/min at B = 2.5. This indicates that the prehydrolyzed Ti species have absolute advantage over the 12969



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in situ formed Ti species in terms of floc growth rate, which is consistent with the assumption that the prehydrolyzed species can destabilize particles more quickly.44 The decrease in zeta potential with the increasing B values (Figure 1b) may also favor the floc aggregation due to the decrease in repulsive forces between the flocs, which also resulted in the gradual increase of d1 (Figure 2b). At B > 2.0, the slight decrease in d1 might be attributed to the floc restabilization. The prehydrolyzed Ti species were also superior to the in situ formed Ti species in d2 and d3, especially at the high B values. Figure 2b suggests the continuous increase in d3 with the increasing B values, while d2 was nearly invariable when B < 1.5 but showed a sharp increase when B > 1.5. 3.2.2. Floc Strength, Recoverability, and Fractal Dimension. Figure 3 shows the floc Sf, Rf, and Df values at different B

compact structures. The curve of Df vs B after floc regrowth was almost similar to the curve after floc breakage, demonstrating that the floc compaction degree was not noticeably influenced by floc rearrangement during the floc regrowth process. Coagulation treatment by sweep floc enmeshment is likely to have a reduced recovery rate from the shear-induced breakup than treatment by charge neutralization, and the flocs formed by charge neutralization should give total recovery.46,47 Jarvis et al.36 also observed that the recoverability of flocs gave a certain indication of the floc internal bonding structure. Thus, the irreversible floc breakage with limited floc regrowth was seen as evidence that (i) the flocs formed were held together by chemical bonding rather than physical bonding, and (ii) the coagulation mechanism was not the pure charge neutralization, rather sweep flocculation was involved in the floc reaggregation process. PTC15 yielded high turbidity and HA removal by charge neutralization with the floc zeta potential very close to zero (−0.1 mV). The resultant flocs should therefore have total recovery after shear-induced breakup. Interestingly, PTC15 flocs exhibited the weakest recoverability, as reflected by the lowest Rf value, while the Rf values of PTC20, PTC25, and PTC30 flocs were higher than those of PTC with B values lower than 2.0. Completed floc recovery was observed for PTC25 and PTC30 given the high Rf (99.2% and 112.3%, respectively). This suggests that PTC20, PTC25, and PTC30 included a higher concentration of prehydrolyzed polymer, which may bind with the floc fragments after the floc breakage stage due to bridging, resulting in higher floc Rf values. When PTC0 was used as a coagulant, it may initially have formed TiOCl2 followed by Ti hydrolysis forming insoluble titanium hydrolyzates-organic matter and producing a colloidal sol that settled very slowly, which can be removed by incorporation into Ti(OH)4 flocs.48,49 For the PTC with high B values, the prehydrolyzed Ti species were expected to react with HA directly, resulting in the formation of different primary flocs compared to those formed by in situ formed Ti species. For PTC with B < 2.0, the more compact floc structure than that created by PTC0 indicates that the corresponding primary flocs may have more attachment with one another, or that the repulsion between the flocs is reduced due to the decrease in floc zeta potential (Figure 1b). For PTC with B > 2.0, the dominant prehydrolyzed Ti species were possibly prehydrolyzed polymer, which may tend to favor an extended conformation away from the interface due to their chain structure, resulting in the formation of flocs with a more open structure.50 The continuous increase of floc Sf values (Figure 3a) was not accompanied by the sustainable increase in floc Df values, which was not in good agreement with previous reports indicating a close relationship between floc strength and floc structure.45 On the basis of the research results above, the B value 1.5 was selected as the optimum one for HA water treatment in that (i) PTC15 resulted in the lowest residual turbidity and high HA removal (comparable with that by PTC0), and (ii) the resultant flocs exhibited larger size, higher floc growth rate, and more compact structure than those by PTC0. 3.3. Coagulation Optimization and Floc Characterization for SW Treatment. Coagulation optimization performances for PTC15 and PTC0 were comparatively investigated to ascertain the optimum coagulant dose and initial solution pH for turbidity and organic matter removal. Detailed results and discussion can be found in the Supporting Information, section S3.

Figure 3. Floc properties with different B values measured in terms of (a) Sf and Rf; (b) Df.

values. Within the B range of 0 to 1.5, the Sf values showed little variation, while the PTC with high B values (2.0, 2.5 and 3.0) had a strong advantage over PTC0 because the resultant flocs exhibited higher floc strength given the higher Sf values. The Rf value first decreased, approaching the lowest value of 33.8% at a B of 1.5 (PTC15), and then significantly increased as the B values further increased. Regardless of the floc growth, breakage and regrowth processes, the variation of floc Df vs B values showed a parabolic trend with the inflection point at B = 2.0 (Figure 3b), at which point the resultant flocs showed the largest Df value and therefore the most compact structure. The flocs became more compact upon exposure to high shear because they were broken at weak points and rearranged into more stable structures.45 The PTC flocs exhibited this change, as indicated by the increase of the floc Df values when the flocs were broken into smaller sizes and subsequently formed more 12970



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Table 1. Coagulation Efficiency and the Selected Floc Parameters under Optimum Coagulation Conditions of PTC0 and PTC15 for SW Treatmenta Df

PTC0 PTC15 a

residual turbidity (NTU)

UV254 removal (%)

DOC removal (%)

zeta potential (mV)

floc growth rate (μm/min)

d1 (μm)

d2 (μm)

d3 (μm)

Sf (%)

Rf (%)

before breakage

after breakage

after regrowth

1.8 1.2

88.1 89.2

61.6 61.5

−0.1 −1.4

93.2 276.2

838.6 966.8

227.2 316.5

364.2 461.8

27.1 32.7

22.4 22.3

2.46 2.74

2.55 2.77

2.59 2.80

The optimum coagulant dose was 8 and 10 mg-Ti/L, respectively for PTC0 and PTC15; the optimum solution pH before coagulation was pH 9.

< 1.5, the floc regrowth was barely observed (Supporting Information, Figure S10). 3.4.2. Sludge Recovery. The coagulated sludge after BTSE treatment using PTC underwent calcination to recover and produce “as-prepared TiO2”. The particle structure and photocatalytic property of as-prepared TiO2 were studied using X-ray diffraction pattern and the photodecomposition rate of acetaldehyde gas, respectively (Figure 4), and its performance compared with the commercially available P-25 TiO2. As-prepared TiO2 exhibited only anatase structure,

As shown in Table 1, the optimal coagulant dose for PTC0 and PTC15 was 8 and 10 mg/L, respectively, and the optimal solution pH before coagulation was chosen at pH 9, where the lowest residual turbidity (1.8 and 1.2 NTU for PTC0 and PTC15, respectively) can be achieved based on the high UV254 and DOC removal. The zeta potentials of flocs formed by both PTC0 and PTC15 are close to zero (−0.1 and −1.4 mV, respectively), indicating that the predominant coagulation mechanism is charge neutralization, and the weak particles repulsive force favors floc aggregation. PTC15 produced the flocs with larger d1, d2, and d3 size, higher floc growth rate by an additional 196%, and higher floc strength than those produced by PTC0. Recoverability of the flocs formed by PTC15 was also comparable with that of PTC0. Generally, it is accepted that the larger the floc median diameter is , the smaller is the fractal dimension.15 However, regardless of the floc growth, breakage, and regrowth process, PTC15 yielded flocs with higher Df values than PTC0, although the flocs formed by PTC15 were larger than those formed by PTC0. That is, the flocs formed by prehydrolyzed Ti species exhibited a higher degree of compaction than those formed by the in situ formed Ti species, which is of high significance on solid/liquid separation processes,51,52 such as sedimentation, flotation, and filtration. 3.4. Application of PTC for BTSE and River Water (RW) Treatment. Coagulation performance and floc characteristics of PTC for BTSE and RW treatment were investigated and the detailed information can be found in Supporting Information, sections S4 and S5. 3.4.1. Coagulation Efficiency and Floc Characterization. Similar coagulation behavior was observed when PTC was tested for both BTSE and RW treatment. As presented in Supporting Information, sections S4 and S5, an apparent reduction in residual turbidity and improvement in DOC removal was observed with increasing B value, while UV254 removal efficiency was comparable. For BTSE treatment, PTC05 could improve DOC removal by 10.0% while for RW treatment, the DOC removal was 7.0% higher with PTC15. Additionally, water pH after coagulation increased with increasing B value for both BTSE and RW treatment similar to SW treatment. This study therefore proved the effectiveness of PTC as a coagulant for real water purification. For BTSE treatment, both floc growth rate and size increased with increasing B value, while the floc Sf and Df were improved to different degrees depending on B value. Floc recoverability however varied depending on the B values. For RW treatment, PTC coagulants with B < 1.5 yielded the flocs with comparable floc sizes (d1, d2, and d3) regardless of floc growth, breakage, and regrowth processes. Higher floc size d3 was obtained using PTC15 compared to those formed using PTC with B < 1.5. PTC15 yielded the flocs with Rf of 15.7%, while for PTC with B

Figure 4. Comparison of as-prepared TiO2 and P-25: (a) XRD pattern; (b) photocatalytic results (initial CH3CHO concentration, 2000 mg/L; UV irradiation, black light of three lamps of 10 W each). 12971



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Figure 5. ESI-TOF mass spectra of hydroxyl Ti solutions with different B values (PTC0, PTC03, PTC05, PTC10, PTC15, PTC20) (The PTC samples were equilibrated under normal conditions for one week before the mass spectra was measured).

3.5. Ti Species in PTC Solutions Using ESI-TOF-MS. ESI-TOF-MS was used to identify Ti species in PTC aqueous solutions. According to previous investigation and identification principles of Al species with ESI-MS,29−33 the Ti complexes in solutions are expected to be transformed first into a gas phase in the ESI mass spectrometer, and then be detected as various peaks at different mass to charge ratios (m/z) in mass spectra. Information of Ti species (e.g., molecular formula, structure, polymerization degree, etc.) should be inferred from the

whereas P-25 TiO2 showed both anatase and rutile structures (Figure 4a). The photocatalytic efficiency of as-prepared TiO2 was comparable to P-25 TiO2 (Figure 4b). The results showed inconspicuous adsorption capability of both as-prepared and P25 TiO2 in the absence of light, while the concentration of acetaldehyde showed a sharp decline trend with time under UV light irradiation, followed by removal of a majority of acetaldehyde after 150 min of photocatalytic reaction. 12972



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117, and 224 were marginal, whereas the species at m/z 339 (100%) dominated the spectra. When B was increased to 1.0, the median polymeric Ti complexes at m/z 321 dominated the mass spectra. Additionally, the peak intensity at m/z 259 increased by 30% in comparison to PTC05, and the peaks at m/ z 199 began to appear. At B = 1.5 and 2.0, Ti species were further hydrolyzed to large polymers, reflected by intense peaks at m/z 431 and 491 and reaching about 20% and 10%, respectively (Supporting Information, Table S5). The Ti species at m/z 321 and 339 decreased, while the ions at m/z 257 dominated the mass spectra. The polymers with high polymerization degrees formed at high B values are not stable and hence decomposed into small or medium species. Thus, a rapid decrease of large Ti polymers at m/z 321 and 339 and an apparent increase in small Ti species at m/z 199 occurred. Therefore, it can be proposed that the hydrolysis of the TiCl4 coagulant with the increase of the B values is a process of polymerization toward decomposition. Our study has three implications. First, like the prehydrolyzed Al and Fe coagulants, the TiCl4 coagulant could be prehydrolyzed using a slow alkaline titration method to form a coagulant, named PTC. Second, our study proves the applicability of PTC as effective coagulant for suspended particles and natural organic matter removal. The prehydrolyzed Ti species also could produce the improved floc characteristics compared to the in situ formed Ti species. Third, ESI-TOF-MS results show that it is suitable for identifying Ti species in PTC coagulant solutions. ESI-TOFMS results indicated the presence of a variety of mononuclear and polynuclear complexes ranging from Ti1 to Ti20 cores. Previous studies have reported that Al13 is known to be the most effective and stable polymeric compound of PAC in water and wastewater treatment.19 Likewise, PTC should include the most effective hydrolyzed Ti species. Further investigation of polytitanium salts would provide opportunities to identify the most effective Ti species that could lead to better coagulation performance for water treatment.

distribution characteristics of peaks and the corresponding m/z values in mass spectra. 3.5.1. Interpretation of Mass Spectrometry on Ti Speciation. The Ti complex was mainly composed of titanium, oxo, hydroxo, chloro ligand, and water molecule, and therefore [TixOy(OH)zCln(H2O)m](4x‑2y‑z‑n)+ was suggested as the general formula for all the species in the present study. Figure 5 shows the ESI-TOF-MS spectra of PTC solutions at different B values. The independent Gaussian-shaped clusters observed indicate that the Ti species identified from these peaks consisted of the same polymerization degree. The Ti species identified from peaks splitting every m/z 18 in one Gaussian-shaped cluster are ascribed to the fragmentation of water molecules (or oxo and one hydroxo ligands) from a hydroxyl Ti complex. Most of the complexes are not only composed of hydroxyl and oxo, but also include chloro ligands. The gap between the signals is also 18u when a chloro ligand is replaced by a hydroxo ligand, just as the elimination of a water molecule. In some cases, during the measurement of ESI-TOF-MS, replacement occurs instead of or together with the fragmentation of water. Four kinds of positive charge (i.e., +1, +2, +3, and +4) existed for the identified Ti species using ESI-TOF-MS. Most of the peaks appeared in each mass spectrum with m/z values ranging between 0 and 500. With the increase of m/z value, the polymerization degree of Ti species was expected to become higher. The proposed cationic Ti complexes at different m/z values can be found in Supporting Information, Table S4. In particular, titanium was anticipated to have three valences at the m/z values of 64 and 91; however, no corresponding Ti complexes could be found when titanium had four valences. There are also ambiguities in the assignment of signals to a given formula of TixOy(OH)zCln(H2O)m, because both (OH)22− and O(H2O)2− correspond to identical m/z values and show identical isotopic distributions. For instance, a peak at m/z 339 may be assigned to [Ti4O6(OH)3]+ or [Ti4O7(OH)(H2O)]+. A similar phenomenon was observed for the following cases: (i) Ti2O(OH)2Cl2(H2O)2+ and Ti2O(OH)3Cl(H2O)22+; (ii) Ti4O5Cl4(H2O)22+ and Ti4O6Cl2(H2O)52+. Several Ti species were classified as follows: Ti1−Ti2 species (monomeric and dimeric Ti complexes), Ti3−Ti8 species (small polymeric Ti complexes), Ti9−Ti16 species (median polymeric Ti complexes), and Ti17−Ti20 species (large polymeric Ti complexes). With the increase of the B values, the dominant species transform into high polymers with fewer categories (Figure 5). The abundance of polymeric Ti species can be evaluated by the relative intensity of the peak signals and Supporting Information, Table S5 gives the variation of peak intensity at the selected m/z values. 3.5.2. Distribution of Ti Species Identified from Mass Spectrometry of Polymeric Ti Solutions. As shown in Supporting Information, Table S5, for mass spectra of PTC0, the most intensive peak was assigned to the monomers and dimers (Ti1−Ti2) at mass to charge ratio (m/z) 64 (100%). The Ti species containing more titanium cores (e.g., Ti3, Ti4, Ti7, Ti8, Ti12, and Ti16) at m/z of 117 (63%), 224 (82%), and 339 (85%) were also the dominant species. The Ti species at m/z of 81, 91, and 259 were also observed with the intensity of 53%, 41%, and 37%, respectively. For PTC with high B values, OH− mainly comes from the external addition of base solution (NaOH) and the polymerization process of Ti species can be defined as generated hydrolysis. For PTC03 and PTC05, the Ti speciation distribution changed greatly from that of PTC0. The species at m/z value of 64, 91,

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ASSOCIATED CONTENT

S Supporting Information *

Photos of PTC samples with aging period; characteristics of test water samples and detailed jar-tests processes; detailed results and discussion about the coagulation performance and floc characterization with PTC for both synthetic and real water treatment; analysis of electrospray ionization time-of-flight mass spectrometry (ESI-TOF-MS) of PTC samples with different B values. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(B.Y.G.) E-mail: [email protected]. Tel.:(86) 531 88366771. (H.K.S.) E-mail: [email protected]. Tel.: (61) 2 9514 2629. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a grant from the Chinese National Natural Science Foundation (No. 51278283), Australia Research Council Discovery Projects (ARC DP), the Shanghai 12973



Article

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Tongji Gao Tingyao Environmental Science & Technology Development Foundation (STGEF), and the scholarship from the China Scholarship Council.

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Preparation and Characterization of Novel Polytitanium Tetrachloride

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