Various factors affect the sedimentation behavior of nanotitanium dioxide (n-TiO2) in water. Accordingly, this study aimed to select the dominating factor. An index of sedimentation efficiency related to n-TiO2 concentration was applied to precisely describe the n-TiO2 sedimentation behavior. Ionic strength (IS), natural organic matter (NOM) content, and pH were evaluated in sedimentation experiments. An orthogonal experimental design was used to sequence the affecting ability of these factors. Furthermore, simulative sedimentation experiments were performed. The n-TiO2 sedimentation behavior was only affected by pH and NOM content at low levels of IS. Moreover, divalent cations can efficiently influence the n-TiO2 sedimentation behavior compared with monovalent cations at fixed IS. Seven different environmental water samples were also used to investigate the n-TiO2 sedimentation behavior in aquatic environments. Results confirmed that IS, in which divalent cations may play an important role, was the dominating factor influencing the n-TiO2 sedimentation behavior in aquatic environments.
As nanotechnology rapidly progresses, nanotitanium dioxide (n-TiO2), a promising engineered nanomaterial, has been extensively used in different fields, such as manufacturing industries [
Environmental behavior, including dispersion, transportation, aggregation, and sedimentation, of n-TiO2 has been considerably investigated [
To determine the dominating factor, we included four important stages in our experimental strategy. The individual effect of each single factor on n-TiO2 sedimentation behavior must be first investigated. Then, an orthogonal experimental design, known as systematic method, which has been applied in different fields, such as physical science [
In this study, we used concentration instead of particle size of n-TiO2 as the criterion to evaluate its sedimentation behavior. We also proposed the index of sedimentation efficiency (SE) with the following formula:
The commercial n-TiO2 (99.8% metal basis, 25 nm, anatase, hydrophile, dry powders) was purchased from Aladdin Industrial Corporation (China). The transmission electron microscope (TEM) images (see Figure S1 in Supplementary Material available online at
Six common inorganic salts (NaCl, KCl, Na2SO4, K2SO4, MgCl2, and CaCl2) were utilized in this study. Stock solutions with 1.0 M IS were prepared for each inorganic salt and were filtered through a 0.45
Seven environmental water samples were evaluated in this study: (a) tap water from our laboratory at the Sun Yat-sen University (Guangzhou, China), (b) lake water (LW) from Haizhu Lake (Guangzhou, China), (c) river water (RW) from Pearl River (Guangzhou, China), (d) phreatic water (PW) from Daya Bay (Huizhou, China), (e) sea water (SW) from Nansha Beach (Guangzhou, China), (f) reservoir water (RE) from Mutouchong Reservoir (Zhuhai, China), and (g) waste water (WW) from the Luogang Waste Water Treatment Plant (Guangzhou, China). In a typical procedure, 5 L of each environmental water sample was placed into a 5 L amber reagent bottle to avoid light and stored for 24 h to separate the insoluble substance. Each environmental water sample was filtered through a 0.45
Organic elemental analysis, infrared radiation (IR) spectrum, ultraviolet-visible (UV-Vis) spectrum, and carbon analysis of FA and HA were measured with an elemental analyzer (VARIO EL CUBE, Elementar Analysensysteme GmbH, Germany), Fourier transform infrared spectrometer (NICOLET AVATAR 330, Thermo Fisher, USA), UV-Vis spectrophotometer (TU-1810, Persee, China), and TOC analyzer (Multi N/C 3100 TOC/TN, Analytik Jena AG, Germany), respectively.
Common inorganic ion analysis, carbon analysis, pH, and conductivity of the environmental water samples were measured using an ion chromatograph (ICS-900 and ICS-5000, Dionex Thermo, USA), TOC analyzer (TOC-VCPH, Shimadzu, Japan), pH meter (PHB-3/pH, Shanxin, China), and conductivity detector (DDS-11A, Yulong, China), respectively.
TEM (TECNAI G2 SPIRIT, FEI, USA) images were obtained to observe the original size of n-TiO2 and XRD (D-MAX 2200 VPC, Rigaku, Japan) patterns were utilized to determine the crystal form of n-TiO2. Moreover, an autotitrator (BI-ZTU, Brookhaven, USA) was used to determine the isoelectric point of n-TiO2.
The n-TiO2 suspension sampled in the sedimentation tube was placed into a 15 mL crucible and then vaporized on an 800 W electric stove (ES-0320, Triangle, China). The n-TiO2 can be digested when it is melted with potassium pyrosulfate and becomes titanium sulfate. A stable yellow complex that can be detected by UV-Vis spectrophotometer is generated when acidic hydrogen peroxide is added. According to this property of the complex, we had established a detection procedure of n-TiO2 concentration. In this procedure, 0.5 g (±0.1 g) of K2S2O7 was added into the crucible and melted on the electric stove for 2 min until the suspension was completely evaporated. Subsequently, 3.0 mL (±0.1 mL) of 10% H2SO4 (volume fraction) was added into the crucible and heated on the electric stove for 1 min after the molten mixture cooled down. The solution was then placed into a 10 mL colorimetric tube and immediately added with 1.0 mL (±0.1 mL) of 30% H2O2 (mass fraction). Deionized water was also added to dilute the solution into scale. After 5 min of coloration, the solution was subjected to a UV-Vis spectrophotometer at 400 nm. And a standard curve (Figure S3) was used as the quantitative criterion of n-TiO2 concentration because the linear correlation coefficient is close to 1 (
An orthogonal experiment design [L27(313)] was applied to determine the effect of three factors and their interactions on n-TiO2 sedimentation behavior. In this design, pH, IS of the mixed inorganic salt solution, and TOC concentration of the mixed NOM solution as three factors were evaluated. Three levels were set in each independent factor (Table
Factors and levels of the orthogonal experiment.
Factors | Levels | ||
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1 | 2 | 3 | |
pH | 6.0 | 7.0 | 8.0 |
IS (mM) | 0.5 | 2.5 | 25 |
NOM (mg L−1) | 0.5 | 1.0 | 5.0 |
Sedimentation experiments were conducted in sedimentation tubes (height: 30 cm, radius: 2 cm) for 4 h. Briefly, 0.0250 g (±0.0002 g) of n-TiO2 was weighed with an electronic balance (FA-1104, Hengping, China) and was added in each tube. Pretreated inorganic salt and NOM solutions or environmental water samples were subsequently added. The suspensions were immediately ultrasonicated in an ultrasonic generator (AS3120A, Autoscience, China) for 15 min, and the initial n-TiO2 concentration was 100 mg L−1.
The following eight different sedimentation experiments were conducted: (A) single factor experiment with pH, (B) single factor experiment with IS, (C) single factor experiment with NOM content, (D) orthogonal experiment, (E) double factor experiment with IS of monovalent and divalent cations, (F) double factor experiment with pH and IS, (G) double factor experiment with NOM content and IS, and (H) environmental water sample experiment. Solution conditions in each experiment were shown in Table
The solutions conditions of the sedimentation experiments.
Sedimentation experiment | Solution conditions |
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A | pH from 1.0 to 13.0 |
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B | Ionic strength of KCl from 0.01 mM to 100 mM |
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C | TOC concentration of HA from 0.5 mg L−1 to 10 mg L−1 |
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D | Ionic strength of mix inorganic salt solution (Na2SO4, K2SO4, MgCl2, CaCl2) from 0.5 mM to 25 mM |
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E | Ionic strength of mix inorganic salt solution (NaCl, KCl, MgCl2, CaCl2) from 0.12 mM to 120 mM |
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F | Ionic strength of mix inorganic salt solution (Na2SO4, K2SO4, MgCl2, CaCl2) from 0.12 mM to 120 mM |
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G | Ionic strength of mix inorganic salt solution (Na2SO4, K2SO4, MgCl2, CaCl2) from 0.12 mM to 120 mM |
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H | Pretreated environmental water samples |
In experiment A, the pH of the suspension was adjusted from 1.0 to 13.0 (±0.1) with 1.0 mol L−1 HCl and NaOH. In experiments B, F, and G, the pH was adjusted from 6.0 to 8.0 (±0.1) with 0.1 mol L−1 HCl and NaOH. In experiments C, D, and E, the pH was adjusted to 7.0 (±0.1) with 0.01 mol L−1 HCl and NaOH.
In experiments D and G, the mixed NOM solution was prepared using FA and HA stock solutions with a mass ratio of 1 : 1. In experiments D, F, and G, the mixed inorganic salt solution with an IS ratio of 1 : 1 for monovalent cations and anions and for divalent cations and anions was prepared with Na2SO4, K2SO4, MgCl2, and CaCl2 stock solutions. In experiment E, the mixed inorganic salt solution with a 1 : 1 IS ratio of monovalent and divalent cations was prepared with NaCl, KCl, MgCl2, and CaCl2 stock solutions.
We immediately sampled 1.00 mL (±0.01 mL) of each suspension with a pipette at 1/4 height of each sedimentation tube after 4 h of sedimentation. The concentration of n-TiO2 was detected using a UV-Vis spectrophotometer. All sedimentation tests were performed in triplicate, and data in average were presented.
We determined the relative concentrations of C, N, H, and S in NOM, and C was proven as the major element (Table S1). The relative C concentrations of FA and HA were 41.28% and 61.19%, respectively. Total carbon, TOC, and inorganic carbon (IC) of NOM were further analyzed, and we found that the ratio of TOC/IC ranged from 12 : 1 to 15 : 1 (Table S2). Hence, TOC concentration, instead of the original concentration of NOM, was used in sequential n-TiO2 sedimentation experiments. The IR and UV-Vis spectra presented in Figures S4 and S5 further confirmed the presence of many aromatic organics containing carboxyl and hydroxyl groups in NOM.
The results of analysis on common ions, pH, conductivity, and carbon of environmental water samples were shown in Tables S3, S4, S5, and S6, respectively. The pH of all samples was approximately neutral. Phreatic water, sea water, and waste water samples contained high amount of inorganic ions, and their conductivities were high because of their high salinity. Moreover, waste water samples presented higher TOC concentration than any other sample.
Water composition is very complex in actual aquatic environments and is a crucial factor that governs the stability of nanoparticles and their mobility as colloidal suspension or their aggregation into larger particles and deposition [
Figure
SE of n-TiO2 with KCl (a) and CaCl2 (b) in different IS values, SE of n-TiO2 in different pH values (c), and SE of n-TiO2 with HA in different TOC concentrations (d).
The colloidal system of nanoparticles is generally least stable when nanoparticles are at
The SE values of n-TiO2 with KCl and CaCl2 in different IS values are shown in Figures
At lower than 0.1 mM IS of CaCl2, the SE of n-TiO2 ranged from 20% to 25% after 4 h of sedimentation; this finding was similar to that with KCl. Conversely, 10 times of the IS of KCl must be applied to obtain the same SE level of n-TiO2 at IS higher than 1.0 mM. It can be demonstrated that divalent ions may be more efficient in compressing the electric double-layer on the surface of n-TiO2 than monovalent ions on account of the Schulze-Hardy rule [
The SE of n-TiO2 with HA in different TOC concentrations are shown in Figure
A set of designed orthogonal experiment [L27(313)] with twenty-seven sedimentation tests was conducted. The experimental design consisted of three independent factors and their interactions. Each independent factor included three levels that were set to simulate the water composition of actual aquatic environments. This experiment was performed to investigate the sedimentation behavior of n-TiO2 with multiple factors and determine the most predominant factor. The results of the tests are shown in Table
SE of n-TiO2 based on the orthogonal experiment under different experimental conditions.
Number | Experimental conditions | SE (%) | ||
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pH | IS (mM) | NOM (mg L−1) | ||
01 | 6.0 | 0.5 | 0.5 | 25.56 |
02 | 6.0 | 0.5 | 1.0 | 12.22 |
03 | 6.0 | 0.5 | 5.0 | 5.56 |
04 | 6.0 | 2.5 | 0.5 | 32.22 |
05 | 6.0 | 2.5 | 1.0 | 18.89 |
06 | 6.0 | 2.5 | 5.0 | 15.56 |
07 | 6.0 | 25 | 0.5 | 57.78 |
08 | 6.0 | 25 | 1.0 | 55.56 |
09 | 6.0 | 25 | 5.0 | 51.11 |
10 | 7.0 | 0.5 | 0.5 | 22.22 |
11 | 7.0 | 0.5 | 1.0 | 7.78 |
12 | 7.0 | 0.5 | 5.0 | 3.33 |
13 | 7.0 | 2.5 | 0.5 | 26.67 |
14 | 7.0 | 2.5 | 1.0 | 16.67 |
15 | 7.0 | 2.5 | 5.0 | 11.11 |
16 | 7.0 | 25 | 0.5 | 48.89 |
17 | 7.0 | 25 | 1.0 | 44.44 |
18 | 7.0 | 25 | 5.0 | 37.78 |
19 | 8.0 | 0.5 | 0.5 | 30.00 |
20 | 8.0 | 0.5 | 1.0 | 21.11 |
21 | 8.0 | 0.5 | 5.0 | 12.22 |
22 | 8.0 | 2.5 | 0.5 | 42.22 |
23 | 8.0 | 2.5 | 1.0 | 31.11 |
24 | 8.0 | 2.5 | 5.0 | 24.44 |
25 | 8.0 | 25 | 0.5 | 65.56 |
26 | 8.0 | 25 | 1.0 | 61.11 |
27 | 8.0 | 25 | 5.0 | 58.89 |
To sequence the affecting ability of the three independent factors and their interactions, we performed range analysis. Range analysis is a statistic method that finds out the differences between the maximum and minimum of the average value of target with each factor in different levels. By comparing the range difference of factors, we can directly understand which factor may affect the value of target most. The results (details in Table S7) are shown in Table
Results of range analysis.
Factor |
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pH | 14.20 |
IS | 37.90 |
NOM | 14.57 |
pH × IS | 5.31 |
pH × NOM | 3.58 |
IS × NOM | 6.42 |
Error | 2.84 |
In this study, the value of target is n-TiO2 SE which relates to the sedimentation behavior of n-TiO2. The factor with the highest
Among these three independent factors and their interactions, IS was the most effective factor that influences the sedimentation behavior of n-TiO2. Moreover, the effects of the three independent factors were stronger than those of their interactions. In the single factor sedimentation experiment of n-TiO2, the SE of n-TiO2 simultaneously increased with increasing IS of inorganic salt. By contrast, the SE of n-TiO2 decreased first and then increased with increasing pH and TOC concentration of NOM. Thus, the interactions between two factors may counterbalance their effects and present a weaker effect of n-TiO2 sedimentation behavior than single factor.
The findings of range analysis were confirmed through variance analysis. Variance analysis is a statistic method that analyses the significance of factors influencing the value of target. The results (details in Table S8) are shown in Table
Results of variance analysis.
Factor |
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FR | Significance |
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pH | 101.40 | 2 | 107.42 |
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IS | 787.43 | 2 | 795.38 |
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NOM | 108.41 | 2 | 109.51 |
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pH × IS | 8.73 | 4 | 4.41 |
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pH × NOM | 3.73 | 4 | 1.88 | — |
IS × NOM | 12.37 | 4 | 6.25 |
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Error | 5.94 | 12 |
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By comparing the FR to the
With regard to this orthogonal experiment, the following degrees of freedom were applied: single factor, 2; interactions between two factors, 4; and error, 12. The
Within the range of the orthogonal experiment, the effects of pH, IS, and NOM content on n-TiO2 sedimentation behavior were extremely significant. The effect of the interactions between IS and NOM content was relatively significant and that of the interactions between IS and pH was significant. Only the interaction between pH and NOM content was not significant.
Considering the results of variance analysis, we confirmed that the effect of independent factor on n-TiO2 sedimentation behavior was stronger than that of their interactions. In particular, the effect of IS on n-TiO2 sedimentation behavior was the highest. Moreover, the significance of each independent factor and their interactions was consistent with the order of the factors and the interactions influencing the sedimentation of n-TiO2.
In the orthogonal sedimentation experiment, we discovered that IS, NOM content, and pH were remarkable factors that influence n-TiO2 sedimentation behavior. Hence, we must elucidate the factor that dominatingly affects the sedimentation behavior of n-TiO2.
The SE values of n-TiO2 with different pH values and NOM contents at different IS values are shown in Figure
SE of n-TiO2 with different pH values at different IS values (a) and SE of n-TiO2 with different NOM contents at different ionic IS values (b).
We concluded that pH and NOM content can affect the sedimentation behavior of n-TiO2 at low IS level because the electric double-layer on the surface of n-TiO2 was not compressed. As IS increased, the cations increased and the electric double-layer on the surface of n-TiO2 was completely compressed. Hence, the effects of pH and NOM content on n-TiO2 sedimentation behavior deteriorated. Consequently, IS governed the sedimentation behavior of n-TiO2 and gradually became the dominating factor.
In n-TiO2 sedimentation experiment with single factor, divalent cations can efficiently settle n-TiO2 compared to monovalent cations. The IS of monovalent and divalent cations is a relevant part of the total IS of a solution. In the orthogonal sedimentation experiment, the ratio of monovalent and divalent cations was fixed at 1 : 1. Further studies must focus on the effect of different ratios of monovalent and divalent cations on n-TiO2 sedimentation behavior.
Five different ratios of monovalent and divalent cations were selected in this study, and the results are shown in Figure
SE of n-TiO2 with different ratios of monovalent and divalent cations at different IS values.
To evaluate the sedimentation behavior of n-TiO2 in actual aquatic environments, we obtained seven different representative environmental water samples. Four factors were analyzed to determine the relationship between these factors and the SE of n-TiO2 (Figure
Relationship between SE of n-TiO2 and pH (a), conductivity (b), NOM content (c), and IS (d) of environmental water samples.
On the basis of the expounded theory above and the results of n-TiO2 sedimentation experiments with pH, IS, and NOM content, we considered that the factor exhibiting good correlation with SE of n-TiO2 could be the dominating factor that influences the sedimentation behavior of n-TiO2 in actual aquatic environments.
NOM content and pH of the environmental water samples did not demonstrate good correlations with SE of n-TiO2 (Figures
Conductivity exhibited a positive correlation with IS. The IS of environment water samples presented an excellent linearity with SE of n-TiO2 (Figures
In n-TiO2 sedimentation experiment with single factor and multiple factors, we revealed that divalent cations can efficiently influence the sedimentation behavior of n-TiO2. The relationship between SE of TiO2 and IS of monovalent and divalent cations in environmental water samples was shown in Figures
Relationship between SE of n-TiO2 and IS of monovalent (a) and divalent (b) cations of environmental water samples.
The main challenge in assessing the sedimentation behavior of nanoparticles is the complexity of actual aquatic environments. Many factors can influence the sedimentation behavior of nanoparticles in actual aquatic environments. Individual determination of the effect of every single factor and their interactions on the sedimentation behavior of nanoparticles in actual aquatic environments is unrealistic and impossible. Hence, the dominating factor must be identified because it is crucial in evaluating sedimentation behavior of nanoparticles in actual aquatic environments.
In this study, an orthogonal experimental design was applied to evaluate the sedimentation behavior of n-TiO2. Through range and variance analyses, we statistically found that the IS of water where n-TiO2 settled played the dominating role in influencing the sedimentation behavior of n-TiO2. This finding was confirmed through sedimentation experiments using environmental water samples. Our findings may simplify assessment of n-TiO2 sedimentation behavior in actual aquatic environments and provide an opportunity to artificially govern the n-TiO2 sedimentation process. Thus, we can prevent the uncontrollable dispersion, movement, and transfer of n-TiO2 in actual aquatic environments and reduce the potential risks of n-TiO2 to humans and eco-system in the future. By determining the IS, we can easily compare and precisely estimate the sedimentation behavior of n-TiO2 in actual aquatic environments.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was financially supported by the National Natural Science Foundation of China (no. 21067004, no. 51378494, and no. 51378316), Shenzhen Science and Technology Innovation Committee Program (JCYJ20130331151242230), and One Hundred Talents program, CAS.