Determination of Total Mercury in Solid Samples by Anodic Stripping Voltammetry

The anodic stripping voltammetry (ASV) was investigated to determine total mercury in solid samples using a gold electrode. The mercury was deposited on the gold electrode in a preconcentration step. The oxidation peak of mercury was irreversible. The optimal conditions of the procedure were found to be as follows: 0.05mol L − 1 HCl solution, deposition potential − 0.5 V vs. Ag/ AgCl/KCls, deposition time 40s, and sweep rate 0.04V s − 1 . Under the optimal conditions, the peak current showed a linear dependence on Hg 2+ concentration in the range from 0.01 to 0.1 mgl − 1 . The detection limit and quantiﬁcation limit were 4.28 µ gL − 1 and 12.98 µ gL − 1 , respectively. The mean recovery and relative standard deviation were 91.2% and 2.4% ( n � 9). The procedure was successfully applied for determining total mercury in samples collected from Hanoi light bulb warehouse—The Rang Dong Light Source and Vacuum Flask JSC. The results were compared with cold vapor atomic absorption spectrometry


Introduction
e high-level toxicity of mercury depends on its chemical forms such as the form of methylmercury has more toxicity than the form of elemental mercury [1][2][3]. When mercury enters the food chains, its concentration deposits and increases in the food change, affecting the life of many animals [4,5]. In the past, mercury was used in several applications such as dentistry, industry, and mining. Unlike other heavy metals, Hg appearance is universal due to its unique physicochemical properties (volatility, long persistence in the atmosphere, solubility in water and lipids, and incorporation in organometallic compounds) and the possibility of transportation in atmosphere with deposition at considerable distances from the sources [6].
e Health Canada organizations warned their high level of contaminants in soil with the purpose of protecting human health and environmental quality according to various land use types such as agricultural and commercial industry is 6.6 mg kg −1 [7,8] e mercury concentrations in the nature environment are low due to the breakdown of minerals in rocks and soils caused by the wind and water [9]. However, the human activities (such as the mining and the combustion of fossil fuel and solid waste) release a large amount of mercury concentration in the environment [8,10,11].
About 54% of the global mercury emission from anthropogenic sources was contributed by Asian countries in 2000 [12]. In Vietnam, the pollution of mercury has become a serious problem due to many environmental incidents happened in the recent year. One of the most serious problem accidents happened in Vietnam is the burning of the Rang Dong Light Source and Vacuum Flask Joint Stock Company on August 28, 2019. e fire from the accident released about 15.2-27.2 kilograms of mercury into the environment. erefore, the development of a method for determining total mercury in the environmental sample is very necessary.
e gold electrode has high conductivity and ductility. Moreover, the gold electrode exhibits a high affinity for mercury; hence, it improves selectivity of method. erefore, the gold electrode was used widely to study the electrochemical behaviors and determine mercury in samples [26]. Depending on the analytical methods and types of sample, specific sample preparations are required such as using aqua regia combined with potassium permanganate solution [1], using mixture of strong acids, and using ultrasonication [42]. e solid samples, which were burned at very high temperature from the accident of the Rang Dong Light Source and Vacuum Flask Joint Stock Company, were collected.
In this research, we proposed a new, low cost, high sensitivity and selectivity analytical method for the determination of mercury in the solid samples applying the anodic stripping voltammetry combining with the gold electrode as the working electrode.

Reagents and Instrumentation.
All chemicals were of analytical grade or higher quality.
All measurements for determination of mercury were done on the µAutolab type III (Netherlands), interfaced to the electrode assembly 663 VA Metrohm (Switzerland), and controlled by software 757 VA.
e system consists of a working gold electrode (d � 1 mm; purchased from Metrohm), an AgCl/Ag/KCl reference electrode, a counterelectrode, a stirring glassy carbon rod, and high-purity nitrogen.

Analytical Procedure.
A gold electrode was inserted into the cell, and then, 50.0 mL of the analytical solution containing 0.05 mol L −1 HCl was added. Prior to the voltammograms measurement, the nitrogen flow was passed through the solution for the removal of oxygen. e differential pulse anodic stripping voltammetry was used to determine Hg 2+ . e accumulation step was carried out under potential of −0.5 V (vs. Ag/AgCl/KCl s ) for 40 seconds, while the analytical solution was being stirred. After that, the stirring was stopped for 5 s, and the differential pulse voltammograms were recorded from −0.5 to 0.7 V. All measurements were carried out at room temperature (25 ± 1°C).

Samples Preparation.
e solid samples were collected from the Rang Dong Light Source and Vacuum Flask Joint Stock Company after the burning accidents on August 28, 2019. Stones, gravel, and light were removed the samples. en, we weighed accurately using an analytical balance with uncertainty ±0.0001 g. e amounts of samples (0.1-0.5 g) were transferred to the 50 mL beakers. A 10 mL of 37% (w/v) HCl acid was added to each sample. After that, the beakers were covered with a watch glass and boiled at 100 o C in 60 min. After 60 mins, the sample was hot filtered using a 0.45 µm filter. e obtained solution was transferred into a 50.0 mL volumetric flask and made up to 50.0 mL with ddH 2 O. e analytical procedure of measurement was described in Section 2.2.
e concentrations of Hg 2+ in the solid samples were calculated by the following equation: where C ob is the concentration obtained in solution sample, V is the volume of solution sample, k is the dilution coefficient, and a is the mass of sample.

Method Evaluation.
After optimizing experimental conditions and instrumental parameters, the suitable conditions were selected to determine mercury. e developed method was evaluated through linear range, limit of detection, limit of quantitation, precision, and recovery. e linearity calibration curve was built in standard solutions and spike solid samples from 10 to 100 ppb and 10 to 150 ppb, respectively. e recovery was determined by spiking a known amount of total mercury in the solid sample. en, this sample was prepared using the sample preparation described in Section 2.3. e concentration of mercury was determined by the standard addition method. e recovery was calculated according to the following equation: where C i and C f are the concentrations of mercury found in the sample before addition of standard solution and after addition of standard solution, respectively, and C ad is the To compare results of the anodic stripping voltammetry (AAS) and the cold vapor atomic absorption spectrometry (CV-AAS), we used the paired T-test in 1 tail for checking whether or not the significant error between them [43]. e paired t value was calculated: in that d is the average of the difference between the two results by the following formula: And S d is the standard deviation of the difference and is calculated as follows:

Results and Discussion
3.1. Cyclic Voltammetry. Electrochemical properties of mercury(II) were investigated by cyclic voltammetry method and displayed in Figures 1-3. Figure 1 shows that the voltammograms of mercury(II) were not reversible caused by the i pa to i pc ratio smaller than 1 [44]. e peak current signal appeared in an oxidation zone, which means mercury (II) exhibits oxidative properties. e electrochemical reaction on the surface of working electrode was irreversible, and no reduction peak appeared on the cyclic voltammograms.
To investigate the accumulation behaviors of mercury on the surface of the gold electrode, the cyclic voltammetry was recorded with different deposition time conditions, specifically 0, 30, and 60 seconds at the same deposition potential of −0.5 V of 0.1 mg l −1 Hg 2+ in 0.05 mol. L −1 HCl solution. Figure 2 shows that the peak current of mercury depended on deposition time. When deposition time increased, the peak current increased, which indicated that mercury had deposition on the electrode surface. e effect of scan rate on the oxidation peak current (I pa ) of 0.1 mg l −1 Hg 2+ in 0.05 mol l −1 HCl solution using gold electrode was investigated in the range from 10 to 1000 mV s −1 . Figure 3 indicates that the relationship between the oxidation peak current and sweep rate in cyclic voltammetry was linearly dependent according to the equation log I p � 0.85 log ʋ + 4.94, R 2 � 0.997 (n � 14). eoretically, the slope value of 1.0 suggests the electrochemical reaction occurred on the surface of working electrode, while the value of 0.5 indicated that the current is controlled by semi-infinite diffusion reaction kind in the solution [45]. By fitting the plot of log(i) as a function of log(]), the slope value for mercury oxidation peak was 0.85, which indicated that mercury (II) possessed a strong accumulation character onto the gold electrode surface. erefore, the concentration of mercury was determined by differential pulse anodic stripping voltammetry.

Choice of Supporting Electrolytes.
A high supporting electrolyte concentration is used to minimize the effects of migration to the electrode surface and to reduce the cell resistance which decreases the IR drop. So, the supporting electrolyte is very important factor in voltammetry. To optimize the best medium, some electrolytes such as nitric   Figure 4. e results indicated that presence of hydrochloric acid was essential to obtain a good peak current, so HCl solution was chosen to determine Hg 2+ .

Effect of Hydrochloric Acid Concentration.
e effect of HCl concentration on the signal of Hg 2+ was investigated with the following conditions: the Hg 2+ condition 0.1 mg l −1 , the sweep rate of 0.04 V s −1 , the pulse amplitude 0.05 V, and the pulse time 0.04 s. e results are presented in Figure 5. Figure 5 shows that the HCl concentration affected the current peak of mercury. When HCl concentration increased from 0.01 to 0.05 mol l −1 , the peak current of mercury also increased. However, the peak current did not increase in the concentration range of 0.05-0.15 mol l −1 . When HCl concentration was higher than 0.15 mol l −1 , the mercury signal was decreased, which may be caused by the formation of Hg 2+ complex with the excess chloride ions (Cl − ). erefore, the concentration of 0.05 mol l −1 HCl was chosen for all following experiments for determination of mercury.

Effect of the Anodic Stripping Voltammetry Parameters.
e sensitivity of the anodic stripping voltammetry depends on both the deposition time and deposition potential. e dependence of the stripping peak current on the deposition potential was also investigated in the range from −0.9 V to 0.8 V for 0.05 mg l −1 mercury after preconcentration in 30 s. Figure 6 shows that the peak current was practically depended on the deposition potential. e highest value of peak current could be obtained at a deposition potential of −0.5 V. After that, the value of peak current decreased when deposition potential increased from −0.4 V to +0.3 V. So, the deposition potential of −0.5 V was chosen for the rest of measurements. Figure 7 indicates that the oxidation peak current increased when deposition time increased from 0 s to 80 s. After that, the oxidation peak current became independent with the deposition time, which indicated that the gold electrode surface was completely covered. erefore, 40 seconds deposition potential was chosen for depositing 0.05 mg l −1 Hg 2+ on the surface of working electrode.
In the anodic stripping voltammetry using differential pulse technique, the sweep rate affected the peak current. A well-defined peak current was obtained with 0.04 V s −1 of the sweep rate.

Validation of Method.
e detection limit (LOD) and quantitation limit (LOQ) were calculated from the linearity calibration curves as k × σ/S [46] where k � 3 for LOD and 10 for LOQ, where σ stands for standard deviation and S is the slope of the calibration curve. e detection limit and quantification limit of mercury were found to be as 4.28 µg l −1 and 12.97 µg l −1 , respectively.
Precision was determined by eleven successive measurements of solution containing 50 µg l −1 Hg 2+ in 0.05 mol l −1 HCl solution with deposition time of 40 s at deposition potential −0.5 V. e relative standard deviation (RSD) was 3.07% (n � 11). e results showed that the RSD (%) was smaller than 5%. erefore, the proposed analytical method has been good repeatability.
To evaluate the precision of the method before we applied to determine mercury in solid samples, two levels of concentration 50 µg l −1 and 100 µg l −1 of mercury were added into the solid sample and were analyzed repeatedly nine independent times. e mean recoveries based on nine replicate measurements were found to be 90.51 ± 1.06% and 91.53 ± 0.91%. e relative standard deviation (RSD) was 2.49% and 2.42%, respectively (n � 9). is level of precision was suitable to determine mercury in solid samples at the Rang Dong Light Source and Vacuum Flask Joint Stock Company after burning.

Application.
e proposed procedure was successfully applied to determine mercury in solid samples. e quantitative determination of different concentrations of mercury in a spiked solid sample was carried out by using the proposed anodic stripping voltammetry procedure (Figure 8). e variation in i p versus mercury concentration C was represented by the straight-line equation i p (µA)� (0.1863 ± 0.1302) + (0.0324 ± 0.0015) × C (µg L −1 ) (correlation coefficient 0.999 and n � 5). e limits of detection (LOD) and quantitation (LOQ) of mercury in solid samples were found to be 5.57 µg L −1 and 16.88 µg L −1 , respectively. e accuracy of the method was verified by using the spiked solid samples. e mean recovery was found to be range from 89.8% ± 1.09 to 92.6% ± 1.38 (n � 9 replicate measurements). e proposed method was applied to determine total mercury in solid samples collected at an accidental area. e results indicated that mercury was found in all samples collected from Hanoi light bulb warehouse. e highest concentration of mercury was 182.51 mg kg −1 . According to the standard of Canadian Health Organization, the allowable Hg 2+ content in the soil is 6.6 mg kg −1 . e results showed that the concentration of total mercury in the six solid samples collected at the accidental area was higher than the permitted standard of health organization (Table 1). erefore, we received that each industrial activity has potential risks for the environment.
To validate, the results obtained by the proposed ASV method was compared with those obtained by the CV-AAS   method. e results obtained by anodic stripping voltammetry (ASV) and the cold vapor atomic absorption spectrometry (CV-AAS) are summarized in Table 1.
We find that t calculated (1.56) is less than t table (1.86) listed in Table 1 for 95% confidence and 8 degrees of freedom.
ere is more than a 5% chance that the two sets of results lie "within experimental error" of each other, so we conclude that the results are not significantly different. However, the ASV has highly sensitive, selective, fast, easy to use, low cost, and less expensive equipment. So, the anodic stripping voltammetry can apply to determine total mercury in the solid sample. e proposed method was successfully applied to determine Hg 2+ content in surface water samples collected from To Lich River without sample preparation. e analytical procedure of measurement was the same as in Section 2.2, but deposition time was 300 s. e Hg 2+ content was found in 5 samples collected from To Lich River near Trung Hoa Bridge in May 2020. e Hg 2+ content in samples was determined by 1 µg L −1 Hg 2+ addition standard method. e results obtained for the surface water samples are below the limit values for mercury. e average Hg 2+ content in surface water samples was 0.67 µg L −1 .
e results could confirm that the anodic stripping voltammetry can determine the Hg 2+ content in water with increasing deposition time.

Conclusion
e anodic stripping differential pulse voltammetry was developed for the determination of total mercury in solid samples. e electrochemical behaviors of mercury were an irreversible process. e mercury was deposited on the surface of the gold electrode. With the optimum conditions, the proposed method has high selectivity, sensitivity, precision, accuracy, and well repeatability, with high recoveries from 89.8% to 92.6%. e proposed method successfully applied to the fast determination of mercury in solid samples collected from Hanoi light bulb warehouse-e Rang Dong Light Source and Vacuum Flask JSC.
e results are in good agreement with cold vapor atomic absorption spectrometry.

Data Availability
e data used to support the findings of this study are included within the article.