Determination of Trace Available Heavy Metals in Soil Using Laser

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Determination of trace available heavy metals in soil using laser-induced breakdown spectroscopy assisted with phase transformation method Rongxing Yi, Xinyan Yang, Ran Zhou, Jiaming Li, Huiwu Yu, Zhongqi Hao, Lianbo Guo, Xiangyou Li, Yong Feng Lu, and Xiaoyan Zeng Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b01756 • Publication Date (Web): 11 May 2018 Downloaded from http://pubs.acs.org on May 17, 2018

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Analytical Chemistry

Determination of trace available heavy metals in soil using laser-induced breakdown spectroscopy assisted with phase transformation method Rongxing Yi, Xinyan Yang, Ran Zhou, Jiaming Li, Huiwu Yu, Zhongqi Hao, Lianbo Guo, Xiangyou Li*, Yongfeng Lu and Xiaoyan Zeng Wuhan National Laboratory for Optoelectronics (WNLO), Huazhong University of Science and Technology (HUST), Wuhan, Hubei 430074, P. R. China *Phone: 86-27-87541423. Fax: +86-27-87541423. E-mail: [email protected]

ABSTRACT: To detect available heavy metals in soil using laser-induced breakdown spectroscopy (LIBS) and improve its poor detection sensitivity, a simple and low cost sample pretreatment method named solid-liquid-solid transformation method was proposed. In this method, available heavy metals were extracted from soil through ultrasonic vibration and centrifuging, and then deposited on a glass slide. Utilize this solid-liquid-solid transformation method, available Cd and Pb elements in soil were detected successfully. The results show that the regression coefficients of calibration curves for soil analyses reach to more than 0.98. The limits of detection could reach to 0.067 and 0.94 ppm for available Cd and Pb elements in soil under optimized conditions, respectively, which are much better than those obtained by conventional LIBS.

Recently, pollutions of heavy metals in soil gradually attract more attentions due to their potential threats to food security. The heavy metals in soil can be absorbed by crops through their roots, and cause health issues for human beings. Therefore, it is of great significance to detect heavy metals in soil. Currently, there are many methods to detect heavy metals in soil. Among them, laser-induced breakdown spectroscopy (LIBS) is a spectrochemical analysis method based on analyzing spectra of plasmas that generated by pulsed lasers. With its advantages of convenient operation, easy setup, and minimal sample preparation1-3, LIBS has been widely applied in many areas, such as steels,4-6 plastics,7 water8,9 and soil,10-12 etc. Some researchers have carried out extensive work on soil detection. For example, Unnikrishnan et al. 13used LIBS to detect copper (Cu) and zinc (Zn) elements in soil. Santos et al. 14 detected Cd in pellet soil using LIBS, and the best calculated limits of detection (LOD) was 1.3 ppm. To improve the LODs of LIBS, some researchers combined LIBS with other techniques. For instance, Liu et al.15 used microwave-assisted LIBS to detect Cu in soil and obtained a 23flod improvement of LOD. Kortenbruck et al. 16 combined LIBS with laser-induced fluorescence (LIF) for soil analysis, and the LOD was 0.3 ppm for Cd. These researchers proved the feasibility of detecting heavy metals in soil using LIBS. However, on one hand, the LODs by conventional LIBS still cannot reach some pollution standards (e.g., 0.2 ppm for Cd in China), even with the microwave and LIF assistance. On the other hand, most of them just detected the total heavy metals, which

were not proper to represent the degree of contamination in soil. In fact, only heavy metals in soil that can be absorbed by crops (available heavy metals) are the threats to the food security, which means that, detecting the available heavy metals in soil is more important than detecting total heavy metals. Many soil researchers have studied how to extract heavy metals from soil samples through hydrochloric acid (HCl), nitric acid (HNO3), Ethylene Diamine Tetraacetic Acid (EDTA), etc. The heavy metals that can be extract by these extraction agents are named as available heavy metals. 17-20 They found that, it is efficient to extract heavy metals from soil by these extraction agents.21 However, nobody adopt this method to detect heavy metals in soil using LIBS. Therefore, it will be very challenging and interesting to combine the available heavy metal extracting method with LIBS. In this work, a pretreatment method named as solidliquid-solid transformation (SLST) was proposed to prepare available heavy metal samples, in which HCl (pH=1) was used as an extractant to extract available heavy metals from soil to solution, and the solution was evaporated on a glass slide, the whole process only costs no more than 20 minutes, which is much shorter than the convenient extraction method. Cd and Pb elements (more than 8% soil in China has been polluted by Cd and Pb elements) were used as two examples to test the proposed method, and the LODs of Pb and Cd elements in soil can reach sub-ppm levels under optimized conditions, which are sufficiently low to meet the pollution standards.

■Experimental setup and method

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Experimental setup. The LIBS experimental setup used in this study is schematically shown in Figure 1. A Qswitched Nd: YAG laser (Quantel Brilliant, maximum energy: 400 mJ/pulse, wavelength: 1064 nm, frequency: 10 Hz) was focused at 2 mm below the target surface using a 100 mm focal length lens to generate plasma. Plasma emission was coupled into an optical fiber by a light collector and then collected by a Czerny-Turner spectrometer (Princeton Instruments, Isoplane SCT320, grating of 3600 lines per mm) equipped with an intensified charge-coupled device (ICCD, Princeton Instruments, Max3, 1024 × 256 pixels). A digital delay generator (SRS, DG535) was adopted to trigger the Nd: YAG laser and the ICCD camera. To avoid over ablation, the sample was mounted on a stepper motor stage, and the laser beam scanned the sample in a straight line. In this work, the laser pulse energy was set to 70 mJ. To minimize the influence of the continuous background, the spectra were collected at an optimized delay time of 1.5 µs and gate width of 2 µs. The time-integrated spectra of laser-induced plasma were obtained in the spectral range of 213-216 nm. Each spectrum was accumulated 200 pulses, and 5 spectra were taken for each sample and averaged.

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sion spectroscopy (ICP-OES), which are listed in Table 1. By adding appropriate amounts of cadmium chloride (CdCl2) and lead nitrate (Pb(NO3)2) solutions in the GSF-5 soil sample, seven new available heavy-metal samples were prepared (added Cd and Pb concentration ranges: 0-5 and, 0-500 ppm, respectively. The original GSF-5 sample was used to prove the detectability of the SLST method, the 7 samples prepared here were used to establish the calibration curve). The concentrations of the Cd and Pb elements added in soil are listed in Table 2. To detect available heavy metals in soil using LIBS, the SLST sample preparation method was used, which contains four steps, as shown in Figure 2. 1. Mix hydrochloric acid solution (HCl, pH=1) and soil powder in a centrifuge tube by ultrasonic shaking in an ultrasonic cleaner. 2. Take the centrifuge tube out of the ultrasonic cleaner, and centrifuge the mixture for 5 min at a speed of 4000 rpm in a centrifuge. 3. After centrifugal process, pipet 5 µl supernatant by a micro-pipette, and drop it on a glass slide. 4. Put the glass slide on a heater for 2 min, and then the SLST sample was obtained. Table 2. Elemental concentrations of prepared samples. Sample No.

Added Cd concentration (ppm)

Added Pb concentration (ppm)

1

0

0

2

0.5

50

3

1

100

4

2

200

5

3

300

6

4

400

7

5

500

Fig. 1. LIBS experimental setup. Table 1. Original concentrations of available heavy metals (HCl) in GSF-5 Sample No.

Cd (ppm)

Pb (ppm)

GSF-5

1.2

75

Sample preparation. A soil sample GSF-5, certified by the State Administration of Quality Supervision, Inspection, and Quarantine of China, was used to prepare samples with different Cd and Pb concentrations. The original concentrations of available Cd and Pb elements (extracted by HCl) were obtained by inductively coupled plasma optical emis-

Fig. 3. Sample area prepared by SLST method and laser treated area.

The diagram of the pretreated sample is shown in Figure 3. The circular areas on the sample contained heavy metals,

ACS Paragon Plus Environment Fig. 2. The procedure of SLST.

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Analytical Chemistry which include the information of available heavy metals in the soil to be detected. However, the areas were not evenly distributed (the concentration around the edge of the circles is higher than that in the center) due to the so-called “coffee ring effects”, 22 which reduces the spectral stability. To address this problem, we scanned the laser beam in a rectangle area (ablation area), which contained the whole “coffee ring”, and then a single spectrum was obtained by accumulating all the pulses in the rectangle area.

■ RESULTS AND DISCUSSION Spectral intensity enhancement. The pellet soil samples generally used in LIBS cannot obtain high detection sensitivity (the LOD was usually 10 ppm level),23-25 but the SLST sample preparing method used in this work can improve the LOD obviously. In the experiment, the original available Cd concentration of the GSF-5 sample is 1.2 ppm. The details to prepare pellet sample were described in our previous work. 25 The SLST sample was obtained using a liquid (HCl) to soil ratio (LSR) of 5:1 and an ultrasonic time of 10 min, respectively. Figure 4 shows the spectra of the SLST and the pellet soil samples. It can be seen that the Cd II 214.44 nm line can hardly be detected in the pellet soil sample but easily be detected in the SLST sample. Therefore, significant spectral enhancement of the Cd element was achieved by the SLST method. Meanwhile, the spectral intensity of Fe II 215.06 nm decreased a lot in the spectrum of SLST sample, which means that the Cd element is easier to be extracted from the soil than the Fe element, this phenomenon was found by other researchers as well, they found that a potential order of metal availability in soils is proposed: Cd > Pb > Zn > Fe,26,27 which may be influenced by the chemical form of these elements.

reduction of ablation threshold, the pre-concentration factor has been studied in our former works.28 Matrix interference suppression is another factor, which was found in this work. As shown in Fig. 4, the intensity of Fe II 215.06 nm in the spectrum of the pellet soil sample was higher than that of the SLST sample, which indicates a higher concentration of Fe element in the soil sample. This phenomenon shows that the concentration of Fe element may influence the spectral intensity of the Cd element. To prove this influence, three SLST samples with different Fe concentrations were prepared (the original extracting solution, a solution with 0.4% FeCl3 addition and a solution with 1.6% FeCl3 addition, respectively). As shown in Figure 5, the spectral intensity of Fe II 215.06 nm increased significantly as the Fe concentration increasing, whilst the spectral intensity of Cd II 214.44 nm decreased correspondingly, which proved that the Fe concentration influences the spectral intensity of Cd II 214.44 nm. The spectral data are listed in Table 3 (the background of all the spectral intensities have been removed). The differences of ionization energies were supposed to be the reason for the difference of the spectral intensities. As shown in Table 4, the ionization energy of Fe atom is lower than that of Cd atom, which means the Fe atom is easier to be ionized than the Cd atom. Therefore, the spectral intensity of Cd element decreased, whilst the concentration of Fe element increased.

Fig. 5. Spectra of the SLST samples with different Fe concentrations in the spectral range of 213-216 nm. Table 3. Spectral intensities at different Fe concentrations (the background of all the spectra have been removed) Sample No.

Fig. 4. Spectra of the SLST and pellet sample in the spectral range of 213-216 nm.

The reason for the intensity enhancement might be attributed to the surface-enhancement effect and matrix interference suppression. Surface-enhancement effect has been studied by many researchers.28-30 Its main process is transforming liquid to solid phase on a metal or a nonmetal substrate, which would lead to enrichment of heavy metals and

Cd II 214.44 nm

Fe II 215.06 nm

73073

7281

43662

171970

28375

247657

Original sample 0.4% FeCl3 added 1.6% FeCl3 added

Table 4. Spectral intensities at different Fe concentrations Element


Atomic No.

1st ionization energy (eV)

2nd ionization energy (eV)


Fe

26

7.93

16.2

Cd

48

9.0

17.0

Influence of the extraction method. In this work, a HCl solution was used to extract heavy metals from soil through ultrasonic vibration. Therefore, both the LSR and the ultrasonic time may influence the spectral intensity. To obtain the best signal for Cd element, the LSR and the ultrasonic time should be optimized. Figure 6 (a) shows the intensities of Cd II 214.44 nm as a function of the LSRs (the ultrasonic time was set to 10 min). It shows that, the spectral intensities of Cd II 214.44 nm kept almost same when the LSRs changed from 2:1 to 5:1, and then decreased after LSR > 5:1. To obtain the highest spectral intensity and the largest amount of sample, the LSR was optimized at 5:1. Figure 6 (b) shows the intensities of Cd II 214.44 nm as a function of the ultrasonic times (the LSR was 5:1). As shown in Fig.6 (b), the spectral intensities of Cd II 214.44 nm reached the maximal value at an ultrasonic time of 10 min, and almost kept the same level from 10 to 120 min, which indicate that most of the available Cd element existed in soil could be extracted in 10 min. To save the sample pretreatment time, the ultrasonic time was optimized at 10 min.

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Calibration Curves and Limits of Detection. To illustrate the quantitative analysis capability of the SLST method, a calibration curve of Cd element was established, as shown in Figure 7. The samples were prepared under the optimized conditions: LSR = 5:1 and ultrasonic time = 10 min. The spectral intensities of Cd II 214.44 nm were used to build the calibration curve in the range of 0-5 ppm, and the error bars indicate the uncertainties of the spectral intensities. From Fig. 7, it was found that, the R2 factor of the calibration curve was 0.9915, the LOD of Cd element calculated by the 3σ criterion29 was 0.067 ppm, which was low enough to satisfy the environmental quality standards for soils in China. As the Cd II 214.44 nm line cannot be detected in the pellet sample at such a low concentration, as shown in Fig. 4. To verify the excellent LOD of the SLST method, it was compared with the LOD of Cd element acquired with conventional pellet method reported in other literatures, as shown in Table 5. It shows that, there were only a few papers reported the quantitative determination of Cd in soil, and all the LODs achieved by pellet method were worse than the LOD of the SLST method in this work, and they cannot meet the environmental quality standards for soils in China (0.2 ppm). These results proved that, the SLST method developed in this work is very effective better than the conventional pellet method.

Fig. 7. Calibration curve of available Cd element. Table 5. The LODs of Cd element obtained by different groups

Fig. 6. Effect of (a) liquid to soil ratios and (b) ultrasonic times on spectral intensities.

LODs (ppm)

matrix

0.3

Pellet soil

[16]

1.3

Pellet soil

[14]

16.5

Pellet soil

[24]

references

Versatility of the SLST method. To verify its multielements detection ability, Pb element was also detected by the SLST method. All the parameters were the same as the Cd element, except for the delay time (from 1.5 to 4 µs) and the gate width (from 2 to 6 µs) were changed. The spectra of SLST and pellet soil samples in the range of 404-408 nm are shown in Figure 8 (a). Compared with


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Analytical Chemistry the pellet soil sample, the spectral intensity for Pb element in the SLST sample was enhanced without exception. Meanwhile, Figure 8 (b) shows the calibration curve of available Pb elements in a range of 0-500 ppm. The spectral intensities of Pb I 405.78 nm were used to build a calibration curve. The results showed that, the R2 factor of the calibration curve was 0.9915, and the calculated LOD of Pb element by the 3σ criterion29 reached 0.94 ppm, whilst the usual LOD of Pb elements in pellet soil samples was at a 10 ppm level using conventional LIBS, as shown in Table 6.24,25 Although, the LOD of Pb element can reach to subppm level with the LIF assistance,31 the costs of the experimental setup will increase a lot at the same time. This result proved that, the SLST method is a simple and sensitive method, which can be used to detect available Pb element in soil in sub-ppm level (much lower than the soil pollution standards, 35 ppm for Pb in China).

Apart from the low LOD for detecting available heavy metals in soil, the SLST method is also more convenient than the pellet method, especially in occasions of large amount of samples. For instance, it takes about 20 min to process 7 soil samples by the SLST method, which is shorter than that of the general pellet method (about 35 min for 7 samples). And more importantly, the SLST method can deal with much more samples (depends on the volume of the ultrasonic cleaner and the centrifuge) at a time, whilst the pellet method can deal with only one sample. As for ICP-OES, it will take hours to dissolve the soil thoroughly, and the use of hydrofluoric acid and perchloric acid make the sample processing method much more complex and dangerous. Therefore, when compared with ICPOES, the SLST method is more effective, convenient and safety.

■ CONCLUSIONS A new method (SLST) to detect available heavy metals in soil using LIBS was developed. By this new SLST method, the spectral intensities of available Cd and Pb elements in soil were enhanced significantly due to the enrichment of these elements and suppression of the matrix interference. Furthermore, the LODs of available Cd and Pb elements acquired by the SLST method were 0.067 and 0.94 ppm, respectively, which were much better than that by the pellet method (at 10 ppm level). The results have shown that the SLST method can be used to detect available heavy metals at sub-ppm level in soil, and it has obvious advantages over the pellet method in improving the LOD for quantitative analysis. It is believed that, the multisample pretreatment ability and low LOD of the SLST method are promising for future investigation of soil pollutions.

AUTHOR INFORMATION Corresponding Author *Phone: 86-27-87541423. Fax: +86-27-87541423. E-mail: [email protected]

Notes The authors declare no competing financial interest.

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT Fig. 8. (a) Spectra of the SLST and pellet soil sample in the spectral range of 404-408 nm, and (b)calibration curve of available Pb element Table 6. The LODs of Pb element obtained by different groups

This research was financially supported by the Major Scientific Instruments and Equipment Development Special Funds of China (No. 2011YQ160017), the National Natural Science Foundation of China (No. 51429501, and 61575073), and the Fundamental Research Funds for the Central Universities of China (HUST: 2015TS075).

LODs (ppm)

matrix

0.6

Pellet soil

[29]

REFERENCES

14.6

Pellet soil

[25]

19

Pellet soil

[24]

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Determination of Trace Available Heavy Metals in Soil Using Laser

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