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1. Introduction
Among heavy metals, mercury (Hg) is one of the most toxic and can be harmful to the brain, kidney, nervous system, and endocrine system in humans and animals [1, 2]. Moreover, the toxicity of Hg is high even at low concentrations because it can accumulate and amplify biologically [3, 4]. Therefore, the standards and guideline values for Hg in drinking water recommended by the World Health Organization and the Environmental Protection Agency are 1 and 2 μg/L, respectively [5, 6]. Therefore, the development of a simple and reliable method for monitoring Hg concentrations in water is of great significance. To determine the trace (less than mg/L) and super trace (less than μg/L) amount of Hg, cold-vapor atomic absorption spectroscopy or cold-vapor atomic fluorescence spectrometry and inductively coupled plasma-mass spectrometry have been used [6, 7]. Although these methods are highly sensitive, their analytical procedures and equipment systems are relatively complex and costly [8, 9]. Currently, numerous studies show that it is possible to determine Hg by using stripping voltammetry, specially anodic stripping voltammetry (ASV). This method possesses several advantages, such as high sensitivity, simple test operation, and low cost of equipment and analysis [8, 9]. A proper working electrode is an important requirement for trace and supertrace ASV methods. Gold electrodes (AuE) [10], gold film electrodes (AuFE) [3, 11–13], and modified gold electrodes are often used to determine Hg because gold has a strong affinity for Hg thus facilitating mercury preconcentration on the electrode surface [9] Recently, several researchers have used copper film electrodes (CuFE) for trace mercury detection in combination with ASV using glassy carbon electrode [12] or boron-doped diamond electrode (BDD) [14, 15]. According to Jovanovski and Hrastnik [15], CuFE exhibits excellent stripping performance comparable or even surpassing that of the gold electrodes for trace mercury determination. The authors also pointed out some attractive properties of CuFE such as simple electrochemical preparation, high sensitivity, well-defined and undistorted stripping signals, and much cheaper than gold film electrodes.
With the aim of developing a low-cost method to determine trace levels of mercury in natural water, based on previously published works in the literature, preliminary tests with stripping voltammetry using glassy carbon and paste carbon electrodes coated with Au or Cu film have been conducted. Attempts using Au or Cu film electrodes in electrolytes containing various types of acid revealed that well-defined stripping peak of Hg appeared constantly on the AuFE but not on the CuFE. In the case of using Au-Cu film formed in situ, the stripping signals for Hg are relatively stable and sensitive. This paper presents the use of gold–copper films on paste carbon working electrode to determine the trace amount of Hg in water.
2. Experimental
2.1. Chemicals and Equipment
Mercury and copper standard solutions (1000 mg/L) were purchased from Merck. HAuCl4·3H2O, 37% HCl, 70% HClO4, ≥65% HNO3, and 30% (
Anodic stripping voltammetry measurements were performed on 797 VA Computrace (Metrohm, Switzerland) with a 50 mL glass electrochemical cell and a system of three electrodes, namely, a carbon paste electrode as working electrodes, an Ag/AgCl (3 M KCl) as the reference electrode, and a platinum rod as the counter electrode.
Water samples were digested using 705 UV Digester (Metrohm, Switzerland).
2.2. Working Electrode Preparation
2.2.1. Substrate Electrode
For the carbon paste electrode preparation (CPE), a hollow Teflon housing having an inner diameter of 3.0 mm with a stainless wire of 1 mm in diameter was tightly filled with a paste prepared from a mixture of graphite powder and paraffin oil (6 : 4,
2.2.2. Working Electrode
Gold/copper film were in situ electrochemical plated on a carbon paste substrate electrode (designated as AuF/CPE, CuF/CPE, and Au-CuF/CPE) from solutions containing Au(III) or/and Cu(II). During the deposition step of the anodic stripping voltammetry, these metal ions were reduced and accumulate onto the electrode surface at a defined potential and time. In this step, working electrode rotates at a constant speed.
2.3. Differential Pulse Anodic Stripping Voltammetry Measurements
Working electrode, reference electrode, and counter electrode were immersed into the electrochemical cell.
The deposition step was carried out at a potential of -1100 mV (
At the end of the deposition step, stirring was stopped for 5–15 s (
2.4. Sample Analysis
Composite river/lake water samples were taken: two one-liter samples were taken at a distance of about 20 meters from the bank at a depth of 50 cm across the river at a location of sampling. These samples were then mixed to make a composite sample. Well-water samples were taken directly from a water pump 15 minutes after the first water was discharged to ensure that it is truly from the depth. The samples were acidified with concentrated HNO3 (2 mL HNO3·L–1 sample) and stored in clean glass bottles placed in a refrigerator at 4°C prior to analysis [16].
In natural water, Hg may exist in various forms, and therefore, the samples were digested before determining total Hg(II). Specifically, 10 mL of the sample was put into a quartz cuvette with a lid, containing 30 μL of 30% (
3. Results and Discussion
3.1. Stripping Signal of Hg(II) at Copper Film Electrodes
Unlike previously published results [12, 15], the stripping signals of Hg at both CuF/GCE (copper film on glassy carbon electrode) [17] and CuF/CPE electrodes in situ prepared were not observed despite changing different analytical conditions, such as acid type and its concentration (HNO3, HCl, and HClO4), the Cu(II) concentration (
As can be seen in Figure 1, when the Hg(II) solution (in HClO4 acid medium) does not contain Cu(II), no peak appears on the differential pulse anodic stripping voltammograms (DP-ASV curve), while if Cu(II) is added to the solution, a stripping peak of the Cu appears at a potential of about 0.2 V. In both cases, no stripping peak of Hg is obtained. During anodic scanning, Cu film on the electrode surface is mostly oxidized and dissolved (at about 0.2 V) before the oxidation of Hg(0) (at about +0.65 V) that releases much of Hg(0) accumulated on the Cu film layer of electrode surface to the solution. This might be the reason why stripping peak of Hg does not appear on the examined electrodes.
[figure omitted; refer to PDF]3.2. Influence of Au(III) Concentration on Performance of Gold Film Electrodes
The concentration of Au(III) (
3.3. DP-ASV Signal of Hg on Au–Cu Film Electrode
3.3.1. Effect of Au(III) and Cu(II) Concentrations
As seen in Figure 2, it is possible to increase the
The effect of
Table 1
The effect of
RSD (%, | |||
0 | 0.20 | - | - |
0.10 | 0 | 1.07 | 9.0 |
0.10 | 0.05 | 1.34 | 1.9 |
0.10 | 0.10 | 1.25 | 2.6 |
0.10 | 0.20 | 1.18 | 3.0 |
0.20 | 0 | 1.84 | 6.1 |
0.20 | 0.05 | 2.56 | 2.0 |
0.20 | 0.10 | 2.18 | 3.1 |
0.20 | 0.20 | 1.79 | 2.8 |
0.30 | 0 | 2.72 | 3.2 |
Experiment results presented in Table 1 show that adding Cu(II) to the Hg(II) analyte solution containing Au(III) causes an increase in the
When using Au-CuF/CPE electrode prepared by using an ex situ technique to record the DP-ASV signal of Hg(II) solution, the obtained stripping peak of Hg was ill-defined, low, and poorly reproducible compared when using an in situ electrode (Figure 3). This might be due to fewer electrochemically active sites formed on the surface of ex situ Au-CuF/CPE than on the in situ one. In addition, the number of active sites might also strongly fluctuate from the time of electrolyzing to the time of electrode transfer to the analyte solution for measurement.
[figures omitted; refer to PDF]
3.3.2. Effect of Acid Types
The stripping voltammetry signals of mercury at Au-CuF/CPE are affected significantly by the supporting electrolytes, especially the type and concentration of acid. The 0.001 M solutions of HCl, HNO3, and HClO4 were used to study their effect on the DP-ASV signals of mercury (Table 2). The
Table 2
Effect of acid types in electrolytic solutions on the
Acid types | 10-3 M HCl | 10-3 M HNO3 | 10-3 M HClO4 |
1.49 | 4.05 | 4.29 | |
RSD (%, | 1.2 | 6.9 | 2.1 |
Conditions:
As shown in Table 3, when the concentration of HClO4 increases from 0.0001 to 0.05 M, the stripping peak current of Hg on the Au-CuF/CPE has the highest value (4.43 μA) at
Table 3
Effect of HClO4 concentration on the
0.0001 | 0.0005 | 0.001 | 0.005 | 0.01 | 0.05 | |
2.52 | 3.55 | 4.10 | 4.43 | 4.43 | 4.30 | |
RSD (%, | 2.4 | 0.7 | 1.3 | 1.3 | 1.6 | 4.4 |
Conditions as in Table 2.
3.3.3. Effect of Deposition Potential
During the deposition step with a negative potential applied to the electrode, Hg(II) is reduced to Hg(0). Then, Hg(0) forms amalgams with Au(0) and Cu(0) on the working electrode surface. When single-metal film electrodes are used to determine the trace analytes with the ASV method, the deposition potential significantly affects the results of the analysis by affecting the microstructure of the metal film on the surface of the working electrode. The
Table 4
Effect of deposition potential on the
-600 | -700 | -800 | -900 | -1000 | -1100 | |
3.20 | 3.26 | 3.96 | 4.06 | 2.88 | 2.83 | |
RSD (%, | 6.0 | 7.6 | 0.7 | 6.3 | 4.9 | 6.3 |
Conditions:
3.3.4. Effect of Deposition Time
As the deposition time increases, the
Table 5
Effect of deposition time on the
160 | 170 | 180 | 190 | 200 | 210 | 220 | 230 | |
3.60 | 3.74 | 3.81 | 3.93 | 4.01 | 4.17 | 4.21 | 4.28 | |
RSD (%, | 6.8 | 3.9 | 0.7 | 1.5 | 0.4 | 0.9 | 1.2 | 0.8 |
Conditions:
3.4. Repeatability, Limit of Detection, and Linear Range
With the appropriate voltammetry conditions presented in Table 5, the 20-time repeated measurements of
The limit of detection (LOD) of the DP-ASV method is determined according to the 3
For the Au-CuF/CPE electrode,
[figures omitted; refer to PDF]
In relatively high concentrations of Hg(II), stripping peaks of mercury on the voltammograms are deformed with a slowing increase of
3.5. Interference Study
In real river or lake water samples, there exist metal ions, such as Fe(III), Cu(II), and Ca(II); anions, such as Cl-, SO42–, and PO43– (with a mg/L concentration); and surfactants. These interferents have much a higher concentration than Hg(II) and may affect the
The reasons for this interference may include the following: the redox potential of the interferent cations is close to that of Hg(II)/Hg0, or these cations may have the stripping potential close to that of Hg, causing the stripping potentials to overlap. The formation of intermetallic compounds may deform the stripping peak of Hg. The anions can form stable complexes or precipitation with Hg, affecting the preconcentration process. The surfactants can strongly adsorb on the surface of the working electrode, preventing the preconcentration and/or the dissolution of Hg. Therefore, it is necessary to study the influence of the interferents (commonly encountered in surface waters) on the determination of Hg.
The influence of the interfering ions/substances is assessed through the relative deviation between
When
3.5.1. Influence of Fe(III), Ca(II), and Cu(II)
The influences of Fe(III), Ca(II), and Cu(II) on the stripping peak current of mercury are presented on Table 6.
Table 6
Influence of Fe(III), Ca(II), and Cu(II) on the
Fe(III) | 0 | 2 | 2.5 | 3 | 3.5 | 4 | |
1.7 | 1.8 | 1.9 | 2.0 | 2.1 | 2.1 | ||
RSD (%, | — | 1.9 | 0 | 0.5 | 1.3 | 3.0 | |
— | 5.9 | 11.8 | 17.6 | 23.5 | 23.5 | ||
Cu(II) | 0 | 0.01 | 0.02 | 0.03 | 0.04 | 0.05 | |
1.2 | 1.1 | 1.1 | 1.0 | 1.0 | 1.0 | ||
RSD (%, | 2.6 | 2.1 | 1.8 | 2.6 | 4.1 | 2.4 | |
0 | 8.3 | 8.3 | 16.7 | 16.7 | 16.7 | ||
Ca(II) | 0 | 40 | 80 | 120 | 160 | 200 | |
1.7 | 1.7 | 1.7 | 1.8 | 2.1 | 2.1 | ||
RSD (%, | 0.7 | 2.2 | 1.5 | 3.7 | 5.8 | 0.7 | |
0 | 0 | 0 | 5.9 | 23.5 | 23.5 |
Conditions:
Fe(III) begins to affect
Ca(II) at 20–120 mg/L does not affect the Hg detection (
The concentration of Cu(II) to 50 μg/L (equal to
3.5.2. Influence of Cl–, SO42–, PO43–, and Triton X-100
In rivers, lakes, and underground waters, the concentration of Cl– and SO42– ions is usually about 10–100 mg/L, and that of PO43– is usually about 0.1–1 mg/L. Such anions can form a complex with Hg(II) and metals present in the solution and, therefore, may affect the Hg determination. As can be seen in Table 7,
Table 7
Influence of Cl–, SO42–, and PO43– on the
Cl– | CCl–, (mg/L) | 0 | 200 | 400 | 600 | 800 | 1000 |
1.2 | 1.1 | 1.2 | 1.3 | 1.2 | 1.2 | ||
RSD (%, | 1.3 | 5.7 | 2.0 | 4.4 | 3.4 | 6.2 | |
0 | 8.3 | 0 | 8.3 | 0 | 0 | ||
SO42– | CSO42–, (mg/L) | 0 | 200 | 400 | 600 | 800 | 1000 |
1.5 | 1.3 | 1.2 | 1.2 | 1.2 | 1.3 | ||
RSD (%, | 3.2 | 1.6 | 1.7 | 5.3 | 2.7 | 2.6 | |
0 | 13.3 | 20 | 20 | 20 | 13.3 | ||
PO43– | CPO43–, mg/L | 0 | 20 | 40 | 60 | 80 | |
1.2 | 1.2 | 1.3 | 1.5 | 1.7 | |||
RSD (%) | 6.1 | 1.0 | 2.3 | 1.6 | 7.5 | ||
0 | 0 | 8.3 | 25 | 41.7 |
Conditions:
Triton X-100 is a nonion surfactant used widely in the production of synthetic domestic as well as industrial detergents. Therefore, the risk of polluting the environment, especially the water environment, is of significant concern [20]. The presence of a surfactant, even at a small concentration, can also significantly affect the quantitative determination with the DP-ASV method. They accumulate on the surface of electrodes and hinder electrolysis [21]. The maximal studied concentration of Triton X-100 is 0.8 mg/L, and the concentration affecting the stripping signals is 0.4 mg/L. This indicates that Triton X-100 substantially affects the determination of Hg with the DP-ASV method with the Au-CuF/CPE.
However, these ions affect the electrolysis in the direction of increasing
3.6. Methods for Hg(II) Determination
So far, numerous studies are reporting the electrolytical determination of Hg(II) with different types of electrodes. The electrodes are mainly those fabricated from gold or modified with gold or nanogold (a precious metal). Moreover, these methods have a long deposit time with high LOD, hence affecting the sensitivity and prolonging the determination. Therefore, our research focuses on the development of a new electrode on the basis of carbon paste and modified with an Au-Cu metal formed in situ. These modified electrodes enable us to determine Hg(II) in water with an appropriate determination time and low LOD. Furthermore, they also meet the EPA standards for drinking water in terms of the level of Hg(II). The comparison is presented in Table 8.
Table 8
Anodic stripping voltammetry method for mercury determination.
No. | Voltammetry | Electrode | Modifier | Deposition time (s) | Linearity (μg/L) | LOD (μg/L) | Real samples | Ref. |
1 | ASV | GC | Submicrometer particulate Cu film | 300 | 10–100 | 0.1 | — | [12] |
2 | ASV | GC | Reduced graphene oxide/Au NPs nanocomposite | 600 | 0.2–30.15 | 0.12 | Tap water | [22] |
4 | Au | — | 120 | 0–50 | 0.05 | Tap water, pond water, wastewater | [23] | |
5 | DP–ASV | ITO | Au NPs | 300 | 0.1–10 | 0.03 | Tap water, lake water, milk, soil sample | [24] |
6 | DP–ASV | Carbon paste | KCdPb3(PO4)3 lacunar apatite | 90 | 0.04–20. | 2.23 | Sea water, fish samples | [25] |
7 | DP–ASV | BDD | — | 30 | 2–10 | 2 | [26] | |
8 | DP–ASV | BDD | — | 60 | 0.005–50 | 0.07 | Power plant samples | [27] |
9 | DP–ASV | Carbon paste | Au NPs-bismuth | 200 | 1–10 | 0.28 | Groundwater, soil samples | [28] |
10 | DP–ASV | GC | Au–Cu | 250 | 1–5 | 0.09 | — | [17] |
11 | DP–ASV | Carbon paste | Au–Cu | 180 | 1–5 | 0.13 | River water, groundwater | This study |
3.7. Practical Applications
3.7.1. Quality Control
To confirm the applicability of the DP-ASV method using Au-CuF/CPE in practice, we first controlled its accuracy via the spike recovery (Rev), the recovery of a known addition, or spike, of analyte to a sample. To determine a spike recovery, a river water (Phuoc Son District, Quang Nam Province, Vietnam, named PS) was split into two portions, and a known amount of a standard solution of Hg(II) was added to one portion. The concentration of Hg(II) was determined for both the spiked,
The recoveries at different standard spiked concentrations were within the expected values of 60-115% at 1-10 μg/L–1 of the analyte (Table 9), issued by international organizations [18, 29].
Table 9
Spike percent recovery for the analysis of Hg(II) in river water sample (PS).
Rev (%) | |||
1.00 | <lod> | 96 | |
2.00 | 95 | ||
3.00 | 97 |
Conditions as in Figure 4.
The data on the Hg concentration in some water sources in Quang Nam Province and Quang Ngai Province, Vietnam, are presented in Table 10.
Table 10
Mercury content in natural water samples analyzed by DP-ASV using Au-CuF/CPE and CV-AAS.
Sample | Type of sample | Location | ||
DP-ASV | CV-AAS | |||
HB | Detention lake water spiked with 1.0 μg Hg/L | Quang Ngai City, Vietnam | ||
HN | Detention lake water spiked with 1.0 μg Hg/L | Quang Ngai City, Vietnam | ||
PS | River water | Phuoc Son District, Quang Nam Province, Vietnam | (NA) | |
SRi | River water | Son Ha District, Quang Ngai Province, Vietnam | (NA) | |
SRe | River water | Son Ha District, Quang Ngai Province, Vietnam | (NA) | |
DM | Hydropower plant reservoir | Phuoc Son District, Quang Nam Province, Vietnam | (NA) | |
GK | Underground water | Tam Ky City, Quang Nam Province, Vietnam | (NA) |
(NA): not available.
Using two-sided
These quality control experiments indicate that the accuracy of the determination result of Hg(II) by using the DP-ASV method with in situ Au-CuF/CPE is reliable. Therefore, this method is suitable for the analysis of Hg(II) in natural water samples.
4. Conclusion
This paper presents the development of gold-copper film electrode formed in situ on the carbon paste substrate. The DP-ASV method developed with these electrodes has a relatively short deposition time and low limit of determination. The method is not significantly affected by the compounds that commonly exist in the water samples. It is possible to develop an analytical process to apply this method in practice.
Acknowledgments
This research is funded by Hue University under Decision 176/QĐ-ĐHH, Code number DHH2021-01-187.
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Abstract
Gold-copper film electrode prepared in situ on carbon paste solid disk substrate (Au–CuF/CPE) was studied as a working electrode. The factors influencing mercury stripping peak currents, such as
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1 Department of Chemistry, Biology, Environment, Pham Van Dong University, Quang Ngai, Vietnam; Department of Chemistry, University of Sciences, Hue University, Hue, Vietnam
2 Department of Chemistry, University of Sciences, Hue University, Hue, Vietnam