Sensitive determination of trace mercury by UV

Journal of Hazardous Materials 233–234 (2012) 207–212

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Sensitive determination of trace mercury by UV–visible diffuse reflectance spectroscopy after complexation and membrane filtration-enrichment Changhai Yin, Jibran Iqbal, Huilian Hu, Bingxiang Liu, Lei Zhang, Bilin Zhu, Yiping Du ∗ Shanghai Key Laboratory of Functional Materials Chemistry, and Research Center of Analysis and Test, East China University of Science and Technology, Meilong Rd. 130, Shanghai 200237, China

h i g h l i g h t s

A simple, sensitive and selective solid phase reflectometry method is proposed for the determination of trace mercury in real water samples. The merits of the method include high enrichment efficiency, low cost, easy operation and no solvent elution or desorption. Hg–dithizone nanoparticles formed in solution can be enriched on the surface of the membrane. The method will be useful in monitoring the environment in view of mercury pollution.

a r t i c l e

i n f o

Article history: Received 14 March 2012 Received in revised form 4 July 2012 Accepted 5 July 2012 Available online 13 July 2012 Keywords: UV–visible diffuse reflectance spectroscopy (DRUVS) Mercury(II) Dithizone Membrane filtration

a b s t r a c t A simple, sensitive and selective solid phase reflectometry method is proposed for the determination of trace mercury in aqueous samples. The complexation reagent dithizone was firstly injected into the properly buffered solution with vigorous stirring, which started a simultaneous formation of nanoparticles suspension of dithizone and its complexation reaction with the mercury(II) ions to make Hg–dithizone nanoparticles. After a definite time, the mixture was filtered with membrane, and then quantified directly on the surface of the membrane by using integrating sphere accessory of the UV–visible spectrophotometer. The quantitative analysis was carried out at a wavelength of 485 nm since it yielded the largest difference in diffuse reflectance spectra before and after reaction with mercury(II).A good linear correlation in the range of 0.2–4.0 ␮g/L with a squared correlation coefficient (R2 ) of 0.9944 and a detection limit of 0.12 ␮g/L were obtained. The accuracy of the method was evaluated by the analysis of spiked mercury(II) concentrations determined using this method along with those determined by the atomic fluorescence mercury vapourmeter and the results obtained were in good agreement. The proposed method was applied to the determination of mercury in tap water and river water samples with the recovery in an acceptable range (95.7–105.3%). © 2012 Elsevier B.V. All rights reserved.

1. Introduction Mercury is a toxic and hazardous element and its contamination in water is a global problem because it can cause sensorineural hearing loss and impaired speech perception [1]. The allowed level of mercury in drinking water is 1.0 ␮g/L recommended by the World Health Organization (WHO). It is considered as a highly dangerous element because of its accumulative and persistent character in the environment and biota [2]. Therefore, it is quite important to develop reliable methods for its determination in environment. Several analytical techniques have been applied for the determination of mercury, including atomic absorption spectrometry

∗ Corresponding author. Tel.: +86 21 64250551; fax: +86 21 64252947. E-mail address: [email protected] (Y. Du). 0304-3894/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhazmat.2012.07.016

(AAS) [3–5], atomic fluorescence spectrometry (AFS) [6–8], inductively coupled plasma optical emission spectrometry (ICP-OES) [9], inductively coupled plasma-mass spectrometry (ICP-MS) [10,11], electrochemical method [12] and spectrophotometry [13]. Some sophisticated instrumental techniques are extensively used as standard methods in the determination of mercury but they still possess some disadvantages, such as high cost of instruments, matrix interference and time consuming. Spectrophotometry has been most commonly used for quantitative analysis of mercury in solution, due to the relatively inexpensive instrumentation and simple operation procedure, although its biggest drawback is the low sensitivity which only can detect mercury at ppm (mg/L) level normally [14]. However, mercury usually exists in the environment at ppb (␮g/L) level with complicate matrix, so extraction and preconcentration procedures are often indispensable. Solid phase extraction (SPE) is a popular and growing preconcentration technique, which

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has successfully been used for preconcentrating and separating trace mercury from different matrixes [15–20]. Compare to conventional liquid–liquid extraction, solid phase extraction possesses some merits, such as easier manipulation, fewer solvent consumption, higher preconcentration factor and easier couple with analytical instruments such as UV–visible, ICP-MS, ICP-AES and CVAFS. As to the SPE procedure, the final step is the elution of the extracted metal ions from the membrane disk or columns using a solution containing acid or other solvents prior to the determination. However, it is tedious and time-consuming. Thus, it would be very meaningful to develop a practical simple and fast spectrophotometric method for the determination of trace mercury with high sensitivity and accuracy. In recent years, colorimetric solid phase extraction (C-SPE) has been utilized, which is based on the extraction of analytes onto a proper support loaded with a colorimetric reagent and then quantified directly on the solid support surface using reflectance spectroscopy [21–36]. The main advantage of C-SPE is that the elution step of SPE can be completely eliminated. The direct analysis of analytes on the surface of solid support is convenient because of its simplicity and rapidity for routine analyses. However, the reagent is weakly immobilized on a solid support and easily released as it flows through the support. Also the analytes complex with the reagent just on the surface of solid support and the results are sensitive to the filtration rate, filtering of too much water might partially flush the reagent and cause errors [37,38]. To these problems, the aim of this work was to develop a method to detect ppb (␮g/L) level mercury with UV–visible diffuse reflectance spectroscopy. The reagent dithizone employed to detect the mercury was not impregnated on the membrane, but formed colored nanoparticles by complexation with mercury(II) ions in solution. The complexes were then collected on the surface of a membrane by filtering and quantified directly on the surface of the membrane by using integrating sphere accessory of the UV–visible spectrophotometer. Some parameters such as pH, the amount of dithizone, the reaction time, effect of potentially interfering ions and the enrichment efficiency were investigated in detail. The proposed method has been satisfactorily applied to the determination of trace mercury in real water samples. 2. Experimental 2.1. Apparatus A UV–visible spectrophotometer Evolution 220 (Thermo Fisher Scientific Inc., USA) equipped with an integrating sphere accessory was used for UV–visible diffuse reflectance spectra measurements. A model PHS-25 pH meter (Mettler Toledo instrument (Shanghai) Co., Ltd.) was used for pH measurements. Ultra-pure water was obtained from an ultra-pure water purification system (SARTORIUS arium 611DI, 18.2 M, Germany). Atomic fluorescence mercury vapourmeter (German Jena Analytical Instruments AG, Germany) was used to evaluate the enrichment efficiency and validate the accuracy. SHB-III Vadose water vacuum pump (Shanghai Weikai instrument equipment Co., Ltd., China) and Filtration devices (Tianjin Jinteng experiment equipment Co., Ltd., China) were used for membrane filtration-enrichment. 85-2 magnetic stirrer (Shanghai Zhiwei electric appliance Co., Ltd., China) was used for stirring the solution to complex. 2.2. Materials and reagents A stock standard solution of mercury(II) at a concentration of 1.0 g/L was prepared by dissolving 0.1354 g analytical reagent grade HgCl2 (Reagent grade, Sinopharm, Shanghai, China) in 100 mL

water with 5.0 mL 1% (v/v) hydrochloric acid and kept in refrigerator at 4 ◦ C. Working standard solutions were obtained by stepwise dilution of the stock standard solution with ultra-pure water. A complexation reagent solution of dithizone (1.0 ␮g/L) was prepared by dissolving the appropriate amount of this reagent in acetone. The glycine buffer (pH = 2.8) solution was prepared by using glycine and hydrochloric acid. All other reagents were analytical reagent grade, and were dissolved by ultra-pure water to obtain their solutions. WX mixed cellulose membranes (Shanghai Diqing filtration technology Co., Ltd., China) used as solid phase support in the present study were with the pore size of 0.22 ␮m, 50 mm diameter. 2.3. Sample preparation Water samples were collected in prewashed (with detergent, ultra-pure water, dilute HNO3 and ultra-pure water, respectively) polyethylene bottles. Tap water was collected from our laboratory. River water was taken at the depth of 50 cm of Qingchun river of Shanghai, China. All water samples were filtered with 0.22 ␮m micropore membrane acidified with hydrochloric acid before analysis. Organic mercury species were treated into mercury(II) according to Ref. [39]. Briefly, 1000 mL water samples were decomposed with 4.0 mL 50 g/L potassium permanganate solution and 4.0 mL 50 g/L potassium persulfate solution in boiling water bath for 2 h, in order to convert all the mercury species to mercury(II). After that, 100 g/L hydroxylamine hydrochloride solution was added to reduce the residual oxidant. 2.4. Experiment procedure In this method, the dithizone was dissolved in acetone (1.0 g/L) and diluted to working solution (6.0 mg/L). Then, 2.0 mL of this solution was injected into 500 mL of the properly buffered (pH = 2.8) examined solution, under vigorous stirring with magnetic stirrer, which started a simultaneous formation of nanoparticles suspension of dithizone and its complexation reaction with the mercury(II) ions [40]. After 2.0 min, the Hg–dithizone nanoparticles were filtered with WX mixed cellulose membranes with the aid of vacuum pump. The UV–visible diffuse reflectance spectra of the membranes were measured by using integrating sphere accessory of the UV–visible spectrophotometer. 3. Results and discussion 3.1. UV–visible diffuse reflectance spectra of Hg–dithizone complex Dithizone is one of the most popular reagent employed in the spectrophotometric determination of mercury(II) in solution [41–43], and is selected for mercury(II) sensing because of its higher selectivity and sensitivity. Furthermore, dithizone and its complex Hg–dithizone show low solubility in water. This property is particularly advantageous to prepare the dithizone nano-dispersion that may easily be captured by the membrane when filtration [44]. The color of membranes changed from steel blue to reddish brown in the presence of different concentrations of mercury(II) solution at pH = 2.8 when dithizone was added in. As is shown in Fig. 1, the UV–visible diffuse reflectance spectra of Hg–dithizone complex on membranes were recorded in the range of 400–800 nm. Clearly there are two peaks at 485 nm and 602 nm in the diffuse reflectance spectra, which are due to Hg–dithizone complex and dithizone, respectively. The former peak at 485 nm was chosen for analysis. From Fig. 1 we can also find that absorbance at 485 nm that is related to the Hg–dithizone complex clearly increases with mercury(II) concentration, while that at 602 nm related to the dithizone decreases with mercury(II) concentration. This is easy


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Fig. 1. UV–visible diffuse reflectance spectra of membranes with different mercury(II) concentrations (0.2–4.0 ␮g/L).

to understand because with the increase of mercury(II) concentration, the amount of Hg–dithizone complex increases and dithizone decreases. 3.2. Influence of experiment conditions To obtain satisfactory results, some experiment conditions such as pH of the solution, concentration of the reagent, and reaction time were thoroughly investigated and optimized.

low pH should easily cause decomposition of the complex Hg(HL)2 , while high pH may cause hydrolysis of Hg2+ . Therefore, the pH should keep in a suitable range for optimized diffuse reflectance absorbance signal because higher and lower acidic conditions are harmful to make stable Hg–dithizone complex. As is shown in Fig. 2 a relatively favorable pH value range was 2.0–3.0, and the maximum absorbance was obtained at pH = 2.8, therefore pH = 2.8 was selected for further experiments.

3.2.1. Effect of pH The pH is proven to be a critical parameter for the determination of mercury(II). Fig. 2 shows the effect of pH from 1.0 to 5.0, as this pH range is suitable for the formation of Hg–dithizone complex in solution. The influence of pH on the formation of Hg–dithizone complex can be expressed from an complexation reaction equation: Hg2+ + 2H2 L = Hg(HL)2 + 2H+ (where H2 L is dithizone). Clearly

3.2.2. Effect of dithizone concentration The concentration of dithizone solution used for the determination of mercury(II) is probably the main parameter that must be investigated since it primarily determines the diffuse reflectance absorbance. The influence of dithizone concentration on the determination of 2.0 ␮g/L of mercury(II) was investigated in the concentration range of 1.5–9.0 ␮g/L. As is shown in Fig. 3, the diffuse reflectance absorbance signal at 485 nm is increased quickly

Fig. 2. The effect of pH from 1.0 to 5.0 on the diffuse reflectance absorbance, [Hg2+ ]: 2.0 ␮g/L. [dithizone]: 6.0 ␮g/L; reaction time: 2.0 min.

Fig. 3. Effect of dithizone concentration on the diffuse reflectance absorbance [Hg2+ ]: 2.0 ␮g/L. pH: 2.8; reaction time: 2.0 min.

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with the increase of dithizone concentration to 6.0 ␮g/L, and higher dithizone concentration would not increase the absorbance significantly. The increase of absorbance results from the reaction between dithizone and mercury(II) ions and the Hg–dithizone complex coated membrane has maximum absorbance at 485 nm. The amounts of Hg–dithizone increase with dithizone first and then tend to a constant for no enough mercury for formation Hg–dithizone complex. The concentration of dithizone of 6.0 ␮g/L was selected for the further studies because it ensures a sufficient reagent for allowing the total reaction of the mercury(II). The further excess of dithizone will inevitably affect the direct measurement of diffuse reflectance absorbance of the Hg–dithizone complex. 3.2.3. Effect of reaction time The reaction time is the next parameter need to be considered. Generally, the reaction between mercury(II) and dithizone is a time-dependent process and the maximum absorbance signal is attained when the system is at equilibrium. And complete equilibrium needs to attain for accurate and reproducible analysis. The effect of reaction time was studied in our work with the time range of 2.0–20.0 min under room temperature and vigorous stirring. The results showed that the diffuse reflectance absorbance at 485 nm almost kept constant when the reaction time was from 2.0 to 20.0 min. The reaction between mercury(II) and dithizone is so quick under our experiment condition that it has completed within 2.0 min, therefore the reaction time was fixed to 2.0 min for convenience and rapidity. 3.3. Enrichment efficiency The extraction percentage of mercury(II) to the membrane was estimated using atomic fluorescence mercury vapourmeter to detect the amount of mercury(II) in the filtrate after membrane filtration. The flow rate, which affects the enrichment, was controlled by vacuum degree of the vacuum pump. Five different vacuum degrees were set: 0.005 MPa, 0.020 MPa, 0.045 MPa, 0.065 MPa and.095 MPa, the diffuse reflectance spectra of the membranes have no significantly change by varying the vacuum degree from 0.005 to 0.065 MPa, but the absorbance decreased quickly when the vacuum degree reached to 0.095 MPa. Therefore the vacuum was fixed to 0.05 MPa for rapidity and higher absorbance signal. Four mercury(II) solutions with concentrations of 0.5, 2.0, 5.0, 10.0 ␮g/L were used to react with dithizone, the vacuum degree was fixed to 0.05 MPa when the complex was filtrated with membranes. The results of mercury(II) detection using atomic fluorescence mercury vapourmeter showed that no mercury in the filtrate solutions was detected for the four solution samples, that meant the mercury(II) in the filtrate solutions were less than 0.5 ng/L (the limit of detection of the atomic fluorescence mercury vapourmeter), accordingly enrichment efficiency was estimated to be more than 99.9%. The high enrichment efficiency ensures the method developed in the study has high sensitivity and high reliability for ppb (␮g/L) level mercury detection.

Fig. 4. Calibration curve for different Hg (II) concentrations in range of 0.2–4.0 ␮g/L.

as CL = 3SB /m (where CL , SB and m are the limit of detection, standard deviation of six measurements of the blank sample and slope of the calibration curve, respectively), that was 0.12 ␮g/L.

3.5. Effect of potentially interfering ions There are always many ions existed in real water samples, so the effect of potentially interfering ions should be investigated before the application of the proposed method. “The interference from some potential coexisting ions was studied for 2.0 ppb of mercury(II) solution in the presence of different amounts of coexisting ions using the proposed method”. The tolerance ratio was defined as the maximum concentration of a coexisting ion to the concentration of a determined target ion causing an error not greater than 5% in the measurements. The tolerance ratios of coexisting ions to 2.0 ␮g/L Hg2+ were more than 5000 for Na+ , K+ , NH4 + , Ca2+ , Mg2+ , Al3+ , 1000 for Fe3+ , Zn2+ , Mn2+ , Pb2+ , Cd2+ , Co2+ , 5 for Cu2+ under our experiment condition. The copper(II) was found to interfere to the determination of mercury severely at pH = 2.8 because it may form Cu–dithizone complex and yield a strong absorption at 485 nm. Fortunately, to add masking reagents, such as EDTA, would eliminate its interference. The EDTA can mask Cu2+ by formation water-soluble complexes, which cannot be captured by the membrane when filtration. It is noteworthy that EDTA did not interfere with the determination of ppb (␮g/L) level mercury(II). For Cu2+ , the tolerance ratio was enhanced to 1000 after adding 6 mL 1 g/L EDTA. Allowance of the concentration ratios of anions were also examined including Cl− , Br− , NO3 − , PO4 3− , CO3 2− , SO4 2− and CH3 COO− . We set the ratio from 10,000 to 1,000,000, and no interference was observed for Hg2+ determination. The results were listed in Table 1.

3.4. Linear response range of the proposed method Calibration curve was obtained according to the procedure described above. Mercury(II) standard solutions with concentrations of 0.2, 0.5, 0.75, 1.0, 1.5, 2.0, 3.0, and 4.0 ␮g/L were prepared to build calibration curve. Absorbance response signals chosen for calibration were obtained from the diffuse reflectance spectra at 485 nm. As shown in Fig. 4, absorbance and mercury(II) concentration has a good linear correlation in the range of 0.2–4.0 ␮g/L with a squared correlation coefficient (R2 ) of 0.9944. The limit of detection (LOD) was estimated using three times standard deviation defined

Table 1 The tolerance ratios of ions on the determination of 2.0 ␮g/L of Hg (II). Potential interfering ions

Tolerance ratio

Na+ , K+ , NH4 + , Ca2+ , Mg2+ , Al3+ Fe3+ , Zn2+ , Mn2+ , Pb2+ , Cd2+ , Co2+ Cl− , Br− , NO3 − , PO4 3− , CO3 2− , SO4 2− , CH3 COO− Cu2+ a Cu2+

5000 1000 No effect 5 1000

a Tolerance ratio after 6 mL 1 g/L EDTA solution was added before the experiment procedure.


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Table 2 Analytical results of mercury determination in spiked water samples by the proposed method and the atomic fluorescence mercury vapourmeter. Measured (␮g/L)a

Error (␮g/L)b

Samples

Added (␮g/L)

This method

Recovery (%)

Mercury vapourmeter

Recovery (%)

Tap water

0.5 1.5 3.0

0.52 ± 0.04 1.45 ± 0.12 3.14 ± 0.07

104.0 96.7 104.6

0.51 ± 0.03 1.57 ± 0.13 3.15 ± 0.10

102.0 104.7 105.2

0.01 −0.12 −0.01

River water

0.5 1.5 3.0

0.52 ± 0.03 1.50 ± 0.13 2.87 ± 0.11

104.0 105.3 95.7

0.49 ± 0.01 1.46 ± 0.11 3.22 ± 0.13

98.5 99.5 104.0

0.03 0.04 −0.05

a b

Value = mean ± SD (n = 3). Value = mean (this method) − mean (mercury vapourmeter).

3.6. Application to real samples

Acknowledgement

Tap water and river water samples were analyzed by atomic fluorescence mercury vapourmeter first and there was no mercury found. To assess the proposed method, some synthetic water samples were prepared by adding appropriate amounts of standard mercury(II) solution to the tap water and river water to make the concentrations at 0.5, 1.5, and 3.0 ␮g/L. We detected mercury concentrations of the samples with the proposed method and atomic fluorescence mercury vapourmeter to assess accuracy, and each sample was analyzed three times for calculation of standard deviation (SD). As shown in Table 2, the recovery of the added amounts was in the range of 95.7–105.3% and the standard deviation (SD) was from 0.03 ␮g/L to 0.13 ␮g/L, which demonstrated that the proposed method exhibited a good reliability. By comparing the results of various samples obtained from the proposed method with the atomic fluorescence mercury vapourmeter, there were no significant differences in view of SD values and measured concentrations. As can be seen, the errors between the two methods ranged from −0.12 ␮g/L to 0.04 ␮g/L, therefore they are in good agreement with each other. These results demonstrated the applicability of the proposed method to determine ppb (␮g/L) level concentration of mercury in the environmental water samples.

The authors gratefully acknowledge the financial support for this project from the National Natural Science Foundation of China (20975039).

4. Conclusions A sensitive solid phase reflectometry method for mercury determination was developed by UV–visible diffuse reflectance spectroscopy after complexation and membrane filtration-enrichment. Besides high enrichment efficiency, the merits of this method include low cost, low organic reagent consumption and easy operation. No solvent elution or desorption is required because the amount of the colored Hg–dithizone complex is directly measured on the surface of membrane by integrating sphere accessory of spectrophotometer. Mercury(II) ions complex with dithizone in solution and then can be enriched on the membrane surface by filtration-enrichment giving a remarkably high quantification limit. A good linear correlation in the range of 0.2–4.0 ␮g/L with a squared correlation coefficient (R2 ) of 0.9944 and a detection limit of 0.12 ␮g/L were obtained. The recovery was between 95.7% and 105.3% when tap water and river water samples were analyzed, and the standard deviation (SD) was from 0.03 ␮g/L to 0.13 ␮g/L. By comparing the results of various samples obtained from the proposed method with the atomic fluorescence mercury vapourmeter, there were no significant difference. The proposed method had been successfully applied for the determination of trace mercury in real water samples containing a large amount of interfering ions by the addition of EDTA as a masking reagent.

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