Environ. Sci. Technol. 2004, 38, 2248-2251
Selective Separation of La3+ and Lanthanum Organic Complexes with Nanometer-Sized Titanium Dioxide and Their Detection by Using Fluorination-Assisted Electrothermal Vaporization ICP-AES with In-Situ Matrix Removal SHENGQING LI, BIN HU,* ZUCHENG JIANG, PEI LIANG, XUAN LI, AND LINBO XIA Department of Chemistry, Wuhan University, Wuhan 430072, People’s Republic of China
A new method for the determination of free La3+ and La organic complexes in solution using a nanometer-sized titanium dioxide as solid-phase extractant and fluorinationassisted electrothermal vaporization (FETV)-ICP-AES as sensitive detector has been developed. The effect of pH on the adsorption characteristics of La3+ and La complexes of citric acid, 2-hydroxyisobutyric acid (HIBA), and humic acid on nanometer-sized TiO2 was investigated and optimized. On the basis of the difference in volatility between fluoride of analyte (lanthanum) and the fluoride of matrix (titanium), an in-situ removal of the adsorbent matrix (TiO2) from a graphite furnace was realized. Therefore, the free La3+ and adsorbed La complexes on nanometersized titanium dioxide could be determined respectively by FETV-ICP-AES without any other chemical pretreatment. The proposed method was applied for the determination of free ion (La3+) and La complexes in synthetic solutions and soil extracts with satisfactory results.
Introduction Owing to their specific properties, there are widespread applications of rare earth elements (REEs) in variable area, such as functional materials, catalysts, and other products in industry, pharmacy, and agriculture (1, 2). Especially in China, REE-containing fertilizers have been widely practiced for various plants (3). As a result, more and more REEs are getting into the environment, accumulating in organisms, and entering into the food chain (2, 4), which has resulted in an increase in the exposure to REEs and in the dietary intake of REEs. However, it is not sufficient for evaluating the physicochemical behavior, toxicological risk, and bioavailability of REEs with the determination of the components in samples because the physicochemical properties, toxicity, and bioavailability of REEs are strongly dependent on their species. It was recognized that the free rare earth ions would exchange with the Ca2+ easily in vivo to restrain the biological function of Ca2+, while their organic complexes could not do so (2). The soluble extracts of REEs in soil can be assimilated easily by plants. As a result, the various lanthanum species * Corresponding author e-mail:
[email protected]. 2248
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would affect the growth of Narcissus in different ways (5). Thus, speciation analysis of REEs in environmental and biological samples becomes increasingly significant, and some reports have been described recently (5-11). Generally, REEs in soil or sediments could be fractionated by a sequential extraction procedure into three chemically distinct fractions: water soluble, exchangeable, and carbonate bound (B1); iron-manganese oxide bound (B2); and organic and sulfide bound (B3), while only REEs in the B1 fraction were significantly accumulated by wheat (6). It was reported that the accumulation behavior of La, Ce, Pr, and Nd in wheat depends on the concentration of REEs in fertilizer applied, and there was a significant negative correlation between REEs contents in its roots and soil pH value. Another paper (7) also showed that the concentration of REEs in wheat decreased with increasing pH value. Furthermore, the increase of pH value led to transformation from one species to another species. Haraguchi et al. (9) investigated the dissolved states of yttrium and lanthanide elements in lake water with ICP-MS using several sample pretreatment and filtration techniques. It was found that more than 50% of REEs in the natural water samples were present as large organometallic complexes. Nanometer material is a new solid material that has attracted a great deal of attention in recent years due to its special properties (such as small size (1-100 nm), high specific surface area, and high surface energy) (12-14). Consequently, nanometer materials are able to provide selective adsorption ability for trace metal ions and have a high adsorption capacity. Previous study on the adsorption behavior of heavy metal ions demonstrated that nanometersized TiO2 could adsorb heavy metal ions (15) and REEs (16). The fluorination-assisted electrothermal vaporization (FETV)-ICP-AES method, using poly(tetrafluoroethylene) (PTFE) slurry as a fluorinating reagent, has been applied to the determination of refractory elements and carbide-forming elements (17). The aim of this work is to study the adsorption behavior of free ion La3+ and its organometallic complexes on nanometer-sized TiO2, to investigate the optimal separation conditions, and to explore the possibility of in-situ removal of the absorbent matrix (TiO2) in a graphite furnace. On the basis of these results, a new method using nanometersized TiO2 as a selective adsorbent for the separation/ preconcentration of trace analyte, and FETV-ICP-AES for the detection of trace La3+ and its organometallic complexes in environmental samples is proposed.
Experimental Section Instrumentation and Operating Conditions. A 2 kW, 27 ( 3 MHz ICP power source (Beijing Second Broadcast Instrument Factory, Beijing, China) and a conventional plasma torch were used. A WF-4C-type graphite furnace (Beijing Second Optics, Beijing, China) was modified and used as a vaporization device. The radiation from the plasma was focused as 1:1 straight image on the entrance slit of a WDG500-1A monochromator (Beijing Second Optics) with a reciprocal linear dispersion of 1.6 nm/mm. The vaporized analytes were swept into the plasma excitation source through a 0.5 m long Teflon tube (4 mm i.d.) by a stream of carrier gas (Ar). The transient signals were detected with a R456 type photomultiplier tube (Hamamatsu, Japan), a homemade direct current amplifier, and recorded by a U-135 recorder (Shimadzu, Japan). The instrumental operating conditions and wavelength used in the present work are listed in Table 1. 10.1021/es030342f CCC: $27.50
2004 American Chemical Society Published on Web 03/04/2004
TABLE 1. Operating Conditions for Fluorination-Assisted ETV-ICP-AES elements and wavelength (nm) incident power (kW) observation height (mm) carrier gas (Ar) flow rate (L min-1) coolant gas (Ar) flow rate (L min-1) auxiliary gas (Ar) flow rate (L min-1) entrance slit width (µm) exit slit width (µm) drying temp (°C), ramp/hold time (s) ashing temp (°C), ramp/hold time (s) vaporization temp (°C), hold time (s) PTFE concentration (%, w/v) sample volume (µL)
Ti, 334.95 La, 333.75 1.0 12 0.6 16 0.8 20 25 100, 10/20 800, 10/30 2400, 4 6 10
The pH values were measured with a Mettler Toledo 320-S pH meter [Mettler Toledo Instruments (Shanghai) Co. Ltd.] supplied with a combination electrode. An AD-72 centrifuge (Shanghai, China) was used for separation. Standard Solution and Reagents. A stock standard solution (1.000 g L-1) of lanthanum was prepared by dissolving 0.2345 g of La2O3 (Specpure, Shanghai Second Reagent, China) in dilute HNO3 and then diluting to certain volume with doubly distilled water. The La working standard solutions were prepared by diluting the stock solution and adding the appropriate volume of 60% m/v PTFE emulsion (viscosity 7 × 10-3-15 × 10 -3 Pa‚s). Humic acid was purchased from Shanghai Chemical Reagents Company. Trisodium citrate, 2-hydroxyisobutyric acid (HIBA), HNO3, HCl, ammonia, and agar used in this work were all of analytical reagent grade. Doubly distilled water was used throughout. Nanometer-sized TiO2 (anatase with the average diameter of 27 nm and specific surface area of 52 m2/g) was provided by the Laboratory of Inorganic Chemistry, Department of Chemistry, Wuhan University. The synthesis method and characteristics of the nanometer-sized TiO2 are described in the literature (18). Sample Pretreatment. Synthetic Samples. The synthetic solution samples consisted of La3+ and variable amounts of organic complexing reagents. A portion of sample solution containing La3+ was transferred to a 1 mL centrifuge tube, and variable amounts of trisodium citrate were added to form a lanthanum citrate complex, respectively. After adjusting the pH values, an appropriate amount of nanometersized TiO2 was added, and the solution was stirred vigorously for 30 min to facilitate adsorption of the analytes on the adsorbent. After centrifugation (3500 rpm for 20 min), the supernatant was taken out, evaporated to near dryness, and then prepared to 0.5 mL of slurry A containing 6% m/v PTFE by adding the appropriate amount of 60% m/v PTFE emulsion. The adsorbent containing TiO2 and analyte was also prepared to 0.5 mL of slurry B using the mixture of 6% m/v PTFE emulsion and 0.05% m/v agar. Finally, the analyses of slurries A and B were performed by FETV-ICP-AES. Soil Samples. The soil (collected in Xiangfan, Hubei Province, China) was air-dried and fractionated through 200mesh sieves. A total of 1.0 g of the soil sample was weighed into each bottle; 50 mL of 1 mol L-1 (pH 4.8) NH4OAc-HOAc or doubly distilled water was added as extracting solutions. The bottles were kept shaking for 5 h at room temperature. Then the mixture was centrifuged (3500 rpm for 20 min), and 10 mL of soil extract was carefully transferred into another centrifuge tube. After adjusting the pH, 4 mg of nanometersized TiO2 was added for the separation/preconcentration. Both supernatant and adsorbent were prepared to slurries containing 6% m/v PTFE and then determined by FETVICP-AES, respectively.
FIGURE 1. Effect of pH on the adsorption of La and La complexes on nanometer-sized TiO2. The total concentration of La is 0.5 µg mL-1, and the concentration of each ligand is 1 mmol L-1. Determination Procedure. The conventional pneumatic nebulization sample introduction is adopted to select the analytical wavelength. After selecting analytical wavelength, it was disconnected from the plasma torch and replaced by the graphite furnace device. After plasma stabilizing, 10 µL of the prepared sample was pipetted into the graphite furnace with a microsyringe, and the sample inlet hole was then sealed with a graphite cone. Under the selected ETV conditions, the analyte was vaporized and introduced into the plasma by a carrier gas (Ar). The strip chart recorder was used to take down the signal, and the peak height was measured for calibration.
Results and Discussion Effect of pH on Adsorption. The pH value of solution plays an important role in the adsorption of different ions on the oxide surfaces of the nanometer-sized TiO2. To investigate the pH effect on the adsorption of La3+ and La complexes, a series of solutions containing 0.5 µg mL-1 La (i.e., 3.6 µmol L-1) and 1 mmol L-1 trisodium citrate, 2-hydroxyisobutyric acid (HIBA), or humic acid were tested in the pH range of 1.0-8.0, which was adjusted with 1.0 mol L-1 HCl or ammonia. The experimental results are shown in Figure 1. It was found that there is an obvious difference between the adsorption of free La3+ and La complexes on nanometersized TiO2 with pH changing from 4.0 to 6.5. Quantitative adsorption of lanthanum citrate, lanthanum HIBA, and lanthanum humate complexes were achieved in a pH ranging from 4.0 to 8.0; whereas, free La3+ cannot be quantitatively adsorbed until the pH value is up to 6.5. However, if the pH value was lower than 4.0 (pH 3.5 for lanthanum citrate), the adsorption of lanthanum citrate and lanthanum humate complexes declined remarkably due to incomplete complexing between La3+ and the organic ligands (the first acid dissociation constants (pKa) (19) of the citric acid and the 2-hydroxyisobutyric acid are 3.13 and 3.72, respectively). It should be noted that the adsorption mechanism of analytes on nanometer-sized TiO2 is still not clear; further work on it is being undertaken. As a result, the pH value of solution was set to 4.0 for separation in this work. Under the selected conditions, the La complexes tested can be selectively adsorbed by nanometer-sized TiO2. In-Situ Removal of Matrix. Vaporization of refractory elements such as lanthanum and titanium, which tend to form refractory carbides at high temperature in a graphite furnace, is much more difficult. As described in a previous publication (20), a poly(tetrafluoroethylene) (PTFE) emulsion used as the fluorinating reagent in ETV-ICP-AES can not VOL. 38, NO. 7, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Possible Reactions Take Place in Graphite Furnacea ∆rGT (kJ/mol) possible reactions TiO2(s) + 3C(s) f TiC(s) + 2CO(g) TiO2(s) + C2F4(g) f TiF4(g) + 2CO(g) TiC(s) + C2F4(g) f TiF4(g) + 3C(s) 1/
2La2O3(s)
+ 3/4C2F4(g) f LaF3(g) + 3/2CO(g)
800 K
1200 K
1600 K
2000 K
2400 k
2800 K
-587.3
-15.1 -714.7
-147.2 -839.6
-269.1 -954.6 -685.5
-385.1 -1063.0
-419.5
-534.5
-646.9
-757.3
-866.0
Ti -468.1
-309.6
La
a
The molar Gibbs free energy changes of fluorinating reaction were calculated basing on the chemical thermodynamic data quoted from refs 19 and 23.
FIGURE 2. Ashing temperature effect on the intensity of 2 ng of Ti and 2 ng of La with 6% PTFE in slurry. only promote the vaporization of refractory elements and carbide forming elements but also improve the detection limits by 1∼2 orders of magnitude. Research on the fluorinating vaporization processes of lanthanum and titanium in graphite furnace indicated that fluorination reaction between the pyrolysate of PTFE (it degrades in the absence of air to give 95% yield of monomer; 21) and the analyte takes place during the graphite furnace heating cycle in FETV-ICP-AES, and the reaction mechanism could be written as follows:
FIGURE 3. Ashing time effect on intensities of 2 ng of Ti and 2 ng of La. The ashing and vaporization temperatures were 800 and 2400 °C.
PTFE f R-CF2-CF2-CF2• f C2F4(g) + MOx(s) + x
/2C2F4(g) f MF2x (g or s) + xCO(g) (at 415 °C) (ref 22)
where M represents the analyte. However, there is a remarkable difference in the fluorination processes between Ti and La, as indicated in Table 2. The molar Gibbs free energy change of fluorinating reaction calculated being based on the chemical thermodynamic data (19, 23) for Ti is obviously smaller than that for La at the same temperature. Hence, the fluorination reaction between TiO2 and C2F4 may take place more easily than the reaction of La2O3 and C2F4 at some lower temperature. On the other hand, the boiling point of TiF4 (bp 284 °C) is much lower than that of LaF3 (bp 2230 °C). All these data indicate that it is possible to in-situ separate the analyte (La) and matrix (Ti) by selecting an appropriate ashing condition. Figure 2 is the dependence of signal intensity of La and Ti on the ashing temperature. As could be seen, at 800 °C Ti could almost be vaporized completely, while La still remained in the furnace. In other words, by selecting this temperature, the in-situ removal of the TiO2 adsorbent matrix from the graphite furnace is practicable. The effects of ashing time on the intensity of La and Ti are shown in Figure 3; the signal intensity of Ti declines quickly with prolonging the ashing time. On the contrary, the ashing time has no obvious influence on the signal intensity of La at the same temperature (800 °C). Under 2250
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FIGURE 4. Matrix effect of nano-TiO2 on the recovery of 10 ng of La. selected conditions (ashing temperature 800 °C/ashing time 40 s), an in-situ removal of the absorbent matrix (Ti) in a graphite tube was realized. Tolerable Amount of Matrix. As mentioned above, the majority of matrix TiO2 could be removed at the ashing stage; however, the residual matrix may still interfere with the determination of La. This could be attributed to their competitive reactions, which led to incomplete vaporization of analyte (La) or changing the vaporization rate of La. Some experiments were carried out in order to explore the effect of matrix element (Ti) on the intensity of analyte (La). As can be seen from Figure 4. When the concentration of matrix is lower than 4.0 mg mL-1, no obvious change in signal intensity of La was observed. In this work, the concentration of matrix (TiO2) in the prepared slurry was 2.0 mg mL-1. Detection Limit and Precisions. According to the definition of IUPAC, the limit of detection was calculated as the
ilated by plants more easily in an acidic condition, and this method may be used to recognize the species of lanthanum organic complexes in soil extracts.
TABLE 3. Determination Results of La(III) in Synthetic Samples free La (µg/mL)
complexing La (µg/mL)
concn of citrate
measd value
calcd value
measd value
calcd value
10 µmol/L 5.0 µmol/L 2.5 µmol/L
0.06 0.42 0.61
0.01 0.38 0.69
1.02 0.65 0.38
0.99 0.62 0.31
TABLE 4. Determination Results of La(III) in Soil Samples extraction solution
free complexed soluble gross soluble La/ La La La La gross La (µg/g) (µg/g) (µg/g) (µg/g) (%)
water 0.44 NH4Ac/HAc buffer 11.77 (pH 4.8)
1.48 5.46
1.92 17.23
54.22 54.22
3.54 31.8
concentration or absolute mass of analyte yielding a signal equivalent to tree times the standard derivation of the blank value (n ) 11). Then the detection limit (3σ) of the proposed method for La was 6.0 ng mL-1 in concentration and 60 pg in absolute mass. The relative standard deviations (RSDs) for five replicates were 7.4% (lanthanum citrate) and 8.8% (lanthanum humate) (c ) 0.1 µg/mL, n ) 5), respectively. Samples Analysis. Synthetic Samples. The proposed method was applied to synthetic solution samples analysis in which the total concentration of La3+ was 1.0 µg mL-1, the concentrations of trisodium citrate were changed in the range of 2.5-10 µmol L-1, and the pH of the synthetic samples were 4.0. The concentrations of free La3+ and lanthanum citrate complex were determined respectively, and the results are given in Table 3 in which the calculated values (the complexing ratio as 1:1) were also given. Although the measured value of complexing La in Table 3 was a little bit higher than the calculated one in each case (which may attribute to the small amount of La3+ adsorption at pH 4.0), a fairly good agreement between the determined values and the calculated values can still be found. Soil Samples. The soil sample was prepared to soil extracts, and the contents of free La3+ and La-complexing species in the soil extracts were determined by the proposed method. On the other hand, the total amount of La in the soil sample, which was digested with HNO3 and HF, was determined by conventional pneumatic nebulization ICP-AES. The experimental results are both listed in Table 4. As can be seen, the percentage of free La3+ (68%) in the pH 4.8 buffer (NH4OAcHOAc) extracts is much higher than that (23%) in the water extracts. It indicates that lanthanum in soil may be assim-
Acknowledgments Financial support from the National Nature Science Foundation of China and the Nature Science Foundation of Hubei Province, China, are gratefully acknowledged.
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Received for review January 30, 2003. Revised manuscript received July 30, 2003. Accepted January 9, 2004. ES030342F
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