Determination of arsenic, antimony, bismuth, lead, cadmium and tin in soil by atomic fluorescence spectrometry - Master's thesis - Dissertation

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Analysis of Arsenic, Antimony, Bismuth, Lead, Cadmium, and Tin in Soil Using Atomic Fluorescence Spectrometry

Summary

This study employs hydride generation atomic fluorescence spectrometry (HG-AFS) to determine the levels of arsenic, antimony, bismuth, lead, cadmium, and tin in soil samples. Different pre-treatment methods are applied depending on the element to ensure accurate quantification. The method was validated by analyzing four soil standard reference materials, and the results were found to be both accurate and reliable.

1. Experimental Part

1.1 Reagents

(1) Hydrochloric acid (extra pure, 36%, ρ = 1.19 g/mL); nitric acid (extra pure, 65%, ρ = 1.42 g/mL); perchloric acid (extra pure, 72%, ρ = 1.67 g/mL); hydrofluoric acid (extra pure, 47%, ρ = 1.15 g/mL)

(2) 10% solution of thiourea and ascorbic acid: 10 g of each reagent is dissolved in 100 mL of ultrapure water by heating.

(3) Standard stock solutions of arsenic, antimony, bismuth, lead, cadmium, and tin (1000 µg/mL): provided by the National Standards Research Center.

(4) Potassium hydroxide, potassium borohydride, and iron: analytical grade.

(5) Cd1 and Cd2 special analytical reagents: analytical pure.

1.2 Laboratory Equipment

(1) Atomic fluorescence spectrometer (AFS200T)

(2) High-performance hollow cathode lamps for As, Sb, Bi, Pb, Cd, and Sn

(3) Electric hot plate (EGB5B)

(4) Water bath (HH-4)

(5) Laboratory-grade ultrapure water (RM-200)

1.3 Sample Preparation

(1) Digestion of arsenic, antimony, and bismuth

0.2–0.5 g of soil sample is weighed into a 50 mL stoppered test tube. A small amount of water is added, followed by 10 mL of (1+1) aqua regia. The mixture is shaken and heated in a boiling water bath for 2 hours. After cooling, the solution is filtered into a 50 mL volumetric flask, then 2.5 mL HCl and 5 mL of 10% thiourea + ascorbic acid are added. The volume is adjusted and mixed thoroughly before analysis.

(2) Digestion of lead and tin

0.2–0.5 g of sample is placed in a 100 mL Teflon beaker. After wetting with water, 10 mL of hydrochloric acid is added, and the sample is decomposed under heat in a fume hood. When about 3 mL remains, it is slightly cooled, and 5 mL of nitric acid, 5 mL of hydrofluoric acid, and 3 mL of perchloric acid are added. The beaker is covered and heated at medium temperature for 1 hour. After opening, the solution is further heated to remove silicon. This process may be repeated if necessary. After digestion, the solution is diluted to 100 mL and stored for analysis.

(3) Digestion of cadmium

The procedure used for lead and tin is followed. After digestion, the sample is cooled slightly, and the lid and inner walls are rinsed with water. Then, 1 mL of hydrochloric acid is added to dissolve the residue. The solution is transferred to a 50 mL volumetric flask, 2.5 mL of 0.1% Cd2 special reagent is added, and the volume is adjusted and mixed well for testing.

1.4 Preparation of Standard Curves, Carrier Solutions, and Reducing Agents

Table 1: Configuration of standard solutions for As, Sb, and Bi

Carrier (5% HCl): 25 mL of concentrated HCl is diluted to 500 mL with ultrapure water.

Reducing agent (0.5% KOH + 2% KBHâ‚„): 2.5 g of potassium hydroxide is dissolved in deionized water, and 10 g of potassium borohydride is added. The solution is made up to 500 mL and shaken well. It should be used immediately and not stored overnight.

Table 2: Configuration of lead standard solutions

Carrier (2% HCl): 10 mL of concentrated HCl is diluted to 500 mL with ultrapure water.

Reducing agent (1% KOH + 2% KBH₄ + 1% K₃Fe(CN)₆): 5 g of potassium hydroxide is dissolved in deionized water, followed by 10 g of potassium borohydride and 5 g of iron. The solution is diluted to 500 mL and shaken. It should be prepared fresh and not stored overnight.

Table 3: Configuration of cadmium standard solutions

Carrier (2% HCl): 10 mL of concentrated HCl is diluted to 500 mL with ultrapure water.

Reducing agent (0.5% KOH + 5% Cd1 special reagent): 2.5 g of potassium hydroxide is dissolved in deionized water, followed by 25 g of Cd1 special reagent. The solution is diluted to 500 mL and shaken. It should be used fresh and not stored overnight.

Table 4: Configuration of tin standard solutions

Carrier (2% HCl): 10 mL of concentrated HCl is diluted to 500 mL with ultrapure water.

Reducing agent (0.5% KOH + 2% KBHâ‚„): 2.5 g of potassium hydroxide is dissolved in deionized water, followed by 10 g of potassium borohydride. The solution is diluted to 500 mL and shaken. It should be used immediately and not stored overnight.

2. Results and Discussion

2.1 Linear Equations and Correlation Coefficients

Standard curves were prepared according to the procedures outlined in section 1.4, and the linear equations and correlation coefficients for each element were determined.

Table 5: Linear equations and correlation coefficients for each element

2.2 Measured Values and Recovery Rates

Table 6: Measured values and recovery rates of elements in three soil standards

The results indicate that the recovery of arsenic, antimony, and bismuth using the aqua regia digestion method ranged from 88.3% to 91.8%, 100% to 105%, and 99.7% to 102%, respectively. For lead, cadmium, and tin, which were digested using a combination of nitric acid, hydrochloric acid, hydrofluoric acid, and perchloric acid, the recoveries were 95.2% to 96.5%, 93.5% to 96.7%, and 94.3% to 95.2%, respectively.

2.3 Experimental Considerations

2.3.1 During the digestion of arsenic, antimony, and bismuth with aqua regia, some residual nitric acid remains in the solution. This nitric acid reacts with the thiourea and ascorbic acid, reducing their effectiveness. Therefore, after digestion, low-temperature heating is required to remove the remaining nitric acid.

2.3.2 When measuring lead, the acidity of the digestion solution must be strictly controlled. The solution should be evaporated as much as possible to ensure consistency between the sample digestion solution and the carrier solution.

2.3.3 In the case of cadmium, the addition of Cd2 special analytical reagent enhances sensitivity. The acidity range for the hydride reaction is narrow, so the solution's acidity must be precisely controlled.

3. Conclusion

In conclusion, the contents of arsenic, antimony, bismuth, lead, cadmium, and tin in soil were successfully analyzed using hydride generation atomic fluorescence spectrometry. The results obtained were accurate, reliable, and suitable for environmental monitoring and regulatory compliance purposes.