September 28, 2007
Preconcentration of Trace Arsenite and Arsenate With Titanium Dioxide Nanoparticles and Subsequent Determination
By Xiao, Yabing Ling, Jie; Qian, Shahua; Lin, Anqing; Et al
ABSTRACT: A novel method of preconcentration of trace arsenite and arsenate by using titanium dioxide nanoparticles as adsorbent was described. The concentrations of preconcentrated arsenite and arsenate were determined by a silver diethyldithiocarbamate spectrophotometric method without desorption. Batch adsorption experiments were carried out as a function of the pH, contact time, amount of titanium dioxide nanoparticles, and solution volume. In the pH range 5 to 6, adsorption rates of arsenite and arsenate were higher than 98%. The calibration coefficient was 0.9991, and the linear range was 0 to 100 [mu]g/L. The developed method was precise, with the relative standard deviation <5% at concentration level of 10 [mu]g/L, with a detection limit (3sigma, n = 6) of 0.44 [mu]g/L. The accuracy of the method for total arsenic was validated by standard reference materials (SRM 3103a) (National Institute of Standards and Technology, Gaithersburg, Maryland). The method was also applied to the analysis of arsenite and arsenate in natural water samples to verify the accuracy. The recovery values remained in a narrow range, from 95 to 103%.Water Environ. Res., 79, 1015 (2007).
KEYWORDS: arsenite, arsenate, titanium dioxide nanoparticles, silver diethyldithiocarbamate spectrophotometric method.
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Arsenic, a widely distributed element in the earth's crust, enters drinking water supplies from the dissolution of minerals and ore or from agricultural and industrial practices. Long-term exposure to arsenic via drinking water causes cancer of the skin, lungs, bladder, and kidneys, and other noncancer effects, including discoloration and thickening of the skin. Increased risks of arsenic- associated lung and bladder cancer and skin lesions have been observed, even at very low arsenic concentrations (less than 50 [mu]g/L). To protect people from the effects of long-term chronic exposure to arsenic, the World Health Organization (Geneva, Switzerland) established an arsenic concentration of 10 [mu]g/L as the guideline for arsenic in drinking water in 1993 (World Health Organization, 1993). In 2001, the U.S. Environmental Protection Agency (Washington, D.C.) also lowered the maximum contaminant level for arsenic permitted in drinking water, from 50 to 10 [mu]g/L (U.S. EPA, 2001).
It is well-known that the toxicity and nutrition of trace elements are highly dependent on their oxidation states and species (inorganic or organic). As for arsenic, it occurs in four oxidation states (+5, +3, 0, and -3), and there are approximately 21 arsenic species commonly detected in environmental and biological systems. In 1974, Penrose compiled the approximate toxicity order of some arsenic compounds, which, in decreasing order, is as follows: arsines > arsenite > arsenoxides > pentavalent arsenicale > arsonium > metallic arsenic. Among all of the arsenic species, only monomethylarsonic acid (MMA), dimethylarsinic acid (DMA), arsenite, and arsenate are commonly present in natural waters. The MMA and DMA, which have much lower contents of arsenic than inorganic arsenic species, are much less toxic and readily eliminated by the body. As a result, it is only necessary to measure the contents of inorganic arsenic species. For two inorganic arsenic species, arsenite [As(III)] is more toxic to biological systems than arsenate [As(V)], because it can bind strongly to thio groups in proteins and result biological body malfunction (Kosnett and Olsen, 1994; Wang and Wai, 2004). In addition, As(V) generally exhibits low mobility in aquifer and sediment systems because of its retention on mineral surfaces controlled primarily by adsorption reactions with metal hydroxide. The As(HI) is more mobile and more toxic (25 to 60 times) than As(V). Arsenite typically exists as arsenous acid (H^sub 3^AsO^sub 3^, pK^sub a^ = 9.22) in the aqueous environment, while arsenates could be AsO^sub 2^(OH)^sub 2^^sup -^ (pK^sub a^ = 2.24), AsO^sub 3^(OH)^sup 2-^ (pK^sub a^ = 6.96), and AsO^sub 4^^sup 3-^ (pK^sub a^ = 11.65). Therefore, the determination of total arsenic concentration is insufficient for clinical and environmental considerations (Gong et al., 2002). To make an accurate evaluation on the arsenic contamination in nature water, it is necessary to determinate the contents of arsenic species individually, especially arsenite and arsenate.
For most water samples with very low arsenic content, direct determination by modem atomic spectroscopic techniques, such as atomic absorption spectrometry (AAS) and inductively coupled plasma- atomic emission spectrometry (ICP-AES), is difficult because of the high matrix interference and limitation associated with the insufficient sensitivity of these techniques. Preconcentration is generally required before analytical determination. It is noticeable that most laboratories in the United States have switched to inductively coupled plasma-mass spectrometry (ICP-MS), by which very low contents of arsenic can be measured without preconcentration. However, ICP-MS can only determine the total arsenic content. To determine the arsenic species individually, ICP-MS must be combined with high-performance liquid chromatography (HPLC). The HPLC-ICP-MS instruments are very costly and are not available in many places in the world, especially in some undeveloped and developing countries and areas.
Currently, many techniques are used for separation and preconcentration of trace elements, including liquid-liquid (Iberhan and Wisniewski, 2003; Mcleod et al., 1981) and solid-phase (Alexandrova and Arpadjan, 1995; Anthemidis and Martavaltzoglou, 2006; Impellitteri, 2004) extractions, co-precipitation (Hiraide et al., 1995; Sun and Yang, 1999; Zhang et al., 2004), and electrochemical deposition (Ciszewski et al., 1989; Dai and Compton, 2006). Among them, solid-phase extraction has found an increasing application because of its higher preconcentration factor, simple procedure, and combination with different analytical techniques, such as AAS and ICP-AES.
Numerous substances have been applied as solid-phase extractants, for example, modified silica (Dogan and Koeklue, 2006; Hassanien and Abou-El-Sherbini, 2006; Tikhomirova et al., 1991; Tong and Yoshifumi, 1990) and alumina (Ghaedi et al., 2006; Hu et al., 2006), active carbon (Bertolino et al., 2006; Gil et al., 2006; Rama and Rama, 1990), and cellulose (Gurnani et al., 2003; Metilda et al., 2003). Titanium dioxide (TiO^sub 2^), which possesses strong affinity toward metal cations (Hlavay et al., 1984; Twidwell et al., 1999), has been widely used as an absorbent for trace elements analysis in the recent decade (Balaji and Matsunaga, 2002; Dutta et al., 2004; Vassileva and Hadjiivanov, 1997). However, research about using nanometer-scale titanium dioxide is seldom mentioned (Gao et al., 2004; Liang et al., 2006; Pena et al., 2005). Owing to their nanoscale size, nanomaterials show a series of novel physical and chemical features from the traditional bulk materials. In nanometer materials, most of the atoms of the nanoparticles are on the surface and highly unsaturated. They can easily bind with other ions or molecules with high selectivity.
In this report, titanium dioxide nanoparticles were applied as an absorbent to preconcentrate arsenite and arsenate in natural water. The influences of pH, contact time, amount of titanium dioxide nanoparticles, solution volume, and interfering ions on the adsorption of arsenite and arsenate were investigated. The standard reference material (SRM) (National Institute of Standards and Technology [NIST], Gaithersburg, Maryland) of arsenic solution was analyzed to determine the accuracy of the method on total arsenic. Three natural water samples were used to determine the accuracy on arsenite and arsenate, respectively.
Measurement. The concentrations of arsenite and arsenate were determined by a UV spectrophotometer (UV-1700, Shimadzu, Kyoto, Japan) using the silver diethyldithiocarbamate spectrophotometric method.
The pH of the solutions was controlled by a pH meter (Mettler Toledo 320-S, Mettler Toledo Instruments, Shanghai, China).
Chemicals and Solutions. Stock solutions of arsenite and arsenate (1 g/L) were prepared by dissolving 0.1733 g NaAsO^sub 2^ and 0.2480 g Na^sub 2^HAsO^sub 4^ in 100 mL deionized distilled water, respectively.
Working solutions of arsenite and arsenate of different concentrations were prepared by diluting their stock solutions with deionized distilled water.
Sample solutions of natural water were prepared by filtering through a 0.45-[mu]m membrane and collected in clean polyethylene containers.
The SRM 3103a of arsenic solution was obtained from NIST. One unit of SRM 3103a consisted of 50 mL of an acidified aqueous solution prepared gravimetrically to contain a known mass fraction of arsenic.
Sodium borohydride (NaBH^sub 4^) solution (0.1 mol/L) was prepared by dissolving 0.378 g NaBH^sub 4^ in 100 mL deionized distilled water.
Potassium iodide solution (0.1 mol/L) was prepared by dissolving 1.660 g potassium iodide in 100 mL deionized distilled water and stored in the dark. Tin dichloride (SnCl^sub 2^) solution (0.1 mol/ L) was prepared by dissolving 2.077 g tin dichloride monohydrate (SnCl^sub 2^ . H2O) in 40 mL hydrochloric acid solution (1 mol/L) and diluted to 100 mL with deionized distilled water.
Silver diethyldithiocarbamate solution was prepared by dissolving 0.250 g silver diethyldithiocarbamate and triethanolamine in 100 mL trichloromethane (CHCl^sub 3^) and then stored in brown bottle in the refrigerator.
Titanium dioxide nanoparticles were provided by the Laboratory of Inorganic Chemistry, Department of Molecular and Chemistry, Wuhan University (Wuhan, China). They were synthesized from titanyl nitrate [TiO(NO^sub 3^)^sub 2^] homogeneous solution by the hydrolysis method; TiO(NO^sub 3^)^sub 2^ solution was heated to boiling at a rate of 2[degrees]C/min, remained boiling for 10 minutes, and then cooled to room temperature (25[degrees]C). Ammonium hydroxide (NH^sub 3^ . H2O) solution was added to neutralize the TiO(NO^sub 3^)^sub 2^. The precipitate was filtered, rinsed, and dried, and then calcined at 850[degrees]C for 1 hour. Then, the rutile titanium dioxide nanoparticles (approximately 100 nm in diameter) were obtained (Zan et al., 1999).
Procedure. Approximately 1000 mL sample solution or working solution was divided into two identical parts. Both parts were mixed with 50 mg titanium dioxide nanoparticles and ultrasonicated for 40 minutes at room temperature (25[degrees]C). The titanium dioxide nanoparticles were separated from the solutions by centrifugation, then transferred into two arsine (AsH^sub 3^) generation flasks, which connected to a tube containing enough silver diethyldithiocarbamate solution (which served as the absorbent of arsine). In one flask, the pH of the solution was adjusted to 5, and the volume of solution was set to 50 mL. Enough prepared NaBH^sub 4^ solution was slowly added to the solution until no more arsine gas was generated. Under this condition (pH = 5), all arsenite would turn into arsine, while arsenate would remain in solution (Howard, 1997). The concentration of arsenite was measured by the silver diethyldithiocarbamate spectrophotometric method. In the other flask, 4 mL potassium iodide solution and 2 mL SnCl^sub 2^ were added in turn. All the arsenate was reduced to arsenite, then enough NaBH^sub 4^ solution was added to convert all arsenite to arsine. The silver diethyldithiocarbamate spectrophotometric method measured the concentration of total arsenic, including arsenite and arsenate. By comparing the difference between the values of two concentrations, the concentration of arsenate could be calculated.
Adsorption Rate (R^sub ad^) Measurement. The adsorption characteristics of titanium dioxide nanoparticles were expressed as the adsorption rate (R^sub ad^), which is generally defined as follows:
M^sub ad^ = amount of metal ion adsorbed on titanium dioxide nanoparticles ([mu]g), and
M^sub i^ = initial amount of metal ion ([mu]g).
Results and Discussion
Effect of pH. Approximately 50 mg of titanium dioxide nanoparticles were added to a series of working solutions (50 mL, 100 [mu]g/L) of arsenite and arsenate for preconcentration. Approximately 0.1 mol/L hydrochloric acid and 0.1 mol/L sodium hydroxide solutions were used to adjust the solutions to desirable pH values.
The absorption rates of arsenite and arsenate on titanium dioxide were investigated at pH range 1 to 10 (Figure 1).
In water solution, the surface of titanium dioxide can be positively or negatively charged, according to eqs 1 and 2. The isoelectric point (IEP) of titanium dioxide nanoparticles is approximately 6.8 (Fernandez-Ibanez et al., 2000).
For arsenate, the absorption rates were higher than 90% when the pH values were lower than 7. That absorption was the result of the electrostatic attraction between arsenate anions and positively charged titanium dioxide nanoparticles. The arsenate anions, including AsO^sub 2^(OH)^sub 2^^sup -^, AsO^sub 3^(OH)^sup 2-^, and AsO^sub 4^^sup 3-^, were produced by deprotonation of nonionic AsO(OH)^sub 3^, according to the following reactions (Goldberg and Johnston, 2001):
When, at higher pH values (pH > IEP), the surface of titanium dioxide nanoparticles had a net negative charge that could no longer absorb arsenate anions, the adsorption rate decreased drastically.
For arsenite, the adsorption onto titanium dioxide nanoparticles remained high in the entire pH range and increased slightly with an increase in pH. Balaji and Matsunaga (2002) suggested that the high adsorption rate was mainly a result of the adsorption of nonionic H^sub 3^AsO^sub 3^, because the pK^sub a1^ value of H^sub 3^AsO^sub 3^ is 9.1 (Martell and Smith, 1979). The slight increase of absorption rates was explained bv Dutta et al. (2004). They thought the release of Protons from H^sub 3^AsO^sub 3^ may remove the hydroxyl ion from the coordinating layer of the titanium dioxide nanoparticle surface. Also, the process could create positively charged sites at the titanium dioxide nanoparticle surface to absorb arsenite anions.
In me PH ran8e 5 to 6- both arsenite and arsenate showed maximum adsorption. For other experiments, the pH of the solutions was set to 5.5.
Effect of Contact Time. A series of working solutions (50 mL, 100 [mu]g/L) of arsenite and arsenate and 50 mg of titanium dioxide nanoparticles were used for this experiment. In the range of contact time from 5 to 40 minutes, the adsorption rates of arsenite and arsenate on titanium dioxide nanoparticles were determined (Figure 2).
From Figure 2, it was found that the adsorption rates of both arsenite and arsenate increased with the increase in contact time, The adsorption rates of arsenite and arsenate both reached a maximum value (approximately 99%) at 30 minutes. Compared with the results of Balaji and Matsunaga (2002) (to absorb arsenite and arsenate from 20 mL of 10 [mu]g/mL solutions with 200 mg titanium dioxide resin, 1 and 4 hours are needed to reach the maximum values of absorption, respectively), our method is proven to be faster for adsorption of both arsenite and arsenate.
It is necessary to note that there is no significant influence of sonication on temperature. With 1 hour of sonication, the temperature of the solution only increased 2.4[degrees]C.
Effect of Amount of Titanium Dioxide Nanoparticles. To investigate the effect of the amount of adsorbent on the adsorption rate, 10, 20, 30, 40, 50, 60, 70, and 80 mg of titanium dioxide nanoparticles were added to a series of arsenite and arsenate (50 mL, 100 [mu]g/L) solutions. Figure 3 showed that the adsorption rates of both arsenite and arsenate reach their maximum when 50 mg of titanium dioxide nanoparticles were added to the solutions. When fewer titanium dioxide nanoparticles were added, the adsorption rate of arsenate was higher than that of arsenite.
Effect of Solution Volume. The solution volume had an effect on the preconcentration efficiency, even though the amount of absorbent added was enough. Experiments were conducted by adding 50 mg of titanium dioxide nanoparticles to a series of arsenite and arsenate working solutions (100 [mu]g/L), with the initial volume ranging from 50 to 500 mL. Table 1 shows that the volume had no significant influence on the recovery values of arsenite and arsenate.
Measurement Adsorption Capacity. The adsorption capacity is defined as the amount (milligrams) of metal ion adsorbed on 1 g of titanium dioxide nanoparticles. Approximately 50 mg of titanium dioxide nanoparticles were added to 50 mL of arsenite and arsenate working solutions, with concentrations ranging from 150 to 750 [mu]g/ L. For arsenite (Figure 4), the adsorption capacity increased with the increase of concentration in the range 150 to 650 [mu]g/L. When the concentration increased to 650 [mu]g/L, the titanium dioxide nanoparticles were saturated, and the adsorption capacity reached the maximum. With a further increase in concentration, the adsorption capacity stabilized, because no more arsenite could be adsorbed. The maximum adsorption capacity was 2.94 mg/g. Arsenate (Figure 5) had a similar curve; the maximum adsorption capacity was 2.89 mg/g.
Interference of Foreign Ions. The possible interferences of 14 kinds of foreign ions (which may have occurred in the samples studied) with the adsorption and measurement of arsenic were surveyed. In this experiment, 100 mL of solutions containing 6.67 pg arsenite or arsenate and one solution with 14 foreign ions were treated under optimum operating conditions. The results listed in Table 2 showed that the presence of the 14 foreign ions had no significant influence on the detection of arsenic under the selected conditions, because the foreign ions absorbed or not absorbed by titanium dioxide nanoparticles cannot influence the formation of arsine.
Detection Limit and Precision. A series of arsenite standard solutions, with concentrations of 20, 40, 60, 80, and 100 [mu]g/L, were prepared by diluting 100 [mu]g/L arsenite working solutions with distilled water. The absorbance of arsenite standard solutions was determined by the silver diethyldithiocarbamate spectrophotometric method.
A calibration curve of arsenite was plotted based on the absorbance and concentration data of the above standard solutions. It had a calibration coefficient of 0.9991, within the linear range 0 to 100 [mu]g/L.
The detection limit was calculated based on 3sigma of the baseline noise at the peak's retention times (n = 6). The detection limit obtained was 0.44 [mu]g/L.
The precision of the determination of arsenite was evaluated under the optimum conditions mentioned above. For this purpose, a series of 500 mL arsenite working solutions (10 [mu]g/L) were measured. The results showed that the relative standard deviation was 4.8%.
Sample Analysis. To study the accuracy of the method, SRM 3103a with 9.941 mg/g +- 0.055 mg/g arsenic (total) was analyzed. By following the procedure described above, the value of 9.872 mg/g +- 0.083 mg/g was attained and matched the SRM very well. However, the analysis of SRM 3103a could only prove the accuracy of the method on total arsenic. To study the accuracy on arsenite and arsenate, respectively, three natural water samples were analyzed, and their recovery values were calculated. Table 3 shows that the recovery values remained in the range 95 to 103%. The method was proved suitable for analyzing arsenite and arsenate, respectively.
Nanometer titanium dioxide was first used as an absorbent for trace arsenic analysis. The preconcentrated arsenite and arsenate were subsequently measured by the silver diethyldithiocarbamate spectrophotometric method.
In the pH range 1 to 7, both arsenite and arsenate showed high adsorption rates (R^sub ad^ > 90%). For arsenite, the adsorption rates increased slowly with an increase in pH, while, for arsenate, the adsorption rates dropped drastically when the pH was higher than 8.6 (IEP of titanium dioxide). In the pH range 5 to 6, both arsenite and arsenate reached the maximum adsorption rate.
The absorption rate of arsenate (50 mL, 100 [mu]g/L) and arsenite (50 mL, 100 [mu]g/L) onto titanium dioxide nanoparticles (50 mg) reached a maximum within 30 minutes, which was much faster than similar work by Balaji and Matsunaga (2002).
Other influences on the amount of titanium dioxide nanoparticles, solution volume, and interfering ions on the adsorption of arsenite and arsenate were investigated. No desorption process of arsenite and arsenate on the titanium dioxide nanoparticles was needed before the photometric determination.
The developed method was precise, with relative standard deviation <5%, at concentration level of 10 [mu]g/L, with a detection limit (3sigma, n = 6) of 0.44 [mu]g/L.
The accuracy of the method on total arsenic was successfully validated by analyzing the standard reference materials (SRM 3103a). The accuracy on arsenite and arsenate was verified by analyzing three natural water samples, with recovery values ranging from 95 to 103%.
Therefore, the method was proven to be suitable for rapid and precise determination of trace arsenite and arsenate in natural water, at a low cost.
Submitted for publication December 8, 2005; revised manuscript submitted November 7, 2006; accepted for publication January 17, 2007.
The deadline to submit Discussions of this paper is December 15, 2007,
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Yabing Xiao1, Jie Ling2, Shahua Qian3*, Anqing Lin1, Wenjie Zheng1, Weiya Xu2, Yuxuan Luo4, Man Zhang1
1 Tianjin Enter-Exit Inspection and Quarantine Bureau, Tianjin, China.
2 Department of Chemistry, Auburn University, Auburn, Alabama.
3 College of Resource and Environmental Science, Wuhan University, Wuhan, China.
4 Hubei Pangu Environmental Protection Engineering & Technology Co. Ltd., Wuhan, China.
* College of Resource and Environmental Science, Wuhan University, Wuhan 430072, China; e-mail: [email protected]
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