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

Determination of Arsenic, Antimony, Bismuth, Lead, Cadmium and Tin in Soil by Atomic Fluorescence Spectrometry

Summary

The hydride generation-atomic fluorescence spectrometry method was employed to determine the concentrations of arsenic, antimony, bismuth, lead, cadmium, and tin in soil samples. Different pre-treatment techniques were applied based on the specific element being analyzed, ensuring accurate quantification. This approach was successfully used to analyze four standard soil samples, yielding results that are both precise and reliable.

1,

Experimental Part

1.1

Experimental Reagents

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

(2) 10% thiourea and ascorbic acid solution: Weigh 10 g of each reagent into a beaker, add 100 ml of ultrapure water, and dissolve 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 purity.

1.2

Laboratory Equipment

(1) Atomic fluorescence spectrometer (AFS200T).

(2) High-performance hollow cathode lamps for arsenic, antimony, bismuth, lead, cadmium, and tin.

(3) Electric hot plate (EGB5B).

(4) Water bath (HH-4).

(5) Ultrapure water (RM-200).

1.3

Sample Preparation

(1) Digestion of As, Sb, and Bi

Accurately weigh 0.2–0.5 g of soil sample into a 50 mL stoppered tube. Add a small amount of water to wet the sample, then add 10 mL of (1+1) aqua regia. Shake and heat in a boiling water bath for 2 hours. Evaporate the digestion solution at low temperature, cool, filter, and dilute to 50 mL with 2.5 mL HCl and 5 mL of 10% thiourea + ascorbic acid.

(2) Digestion of Pb and Sn

Accurately weigh 0.2–0.5 g (to 0.0002 g) in a Teflon beaker. Add 10 mL HCl, heat in a fume hood until evaporation. After 3 mL remains, add 5 mL HNO3, 5 mL HF, and 3 mL HClO4. Heat at medium temperature for 1 hour, open, and continue heating to remove silicon. Repeat if necessary. Cool, rinse, and dissolve residue with 2 mL HCl. Dilute to 100 mL and set aside.

(3) Digestion of Cd

Use the same method as Pb and Sn. After digestion, add 1 mL HCl and 2.5 mL of 0.1% Cd2 reagent, dilute to 50 mL, and set aside.

1.4

Standard Curve, Carrier, and Reductant Preparation

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

Add As, Sb, Bi (0.1 µg/mL) / mL	Add concentrated HCl / mL	Add 10% (thiourea + ascorbic acid) / mL	Deionized water final volume / mL	Standard solution concentration / (µg/L)
0.0	5	10	100	0.0
2.0	2.0	4.0	4.0	8.0
8.0	10.0	10.0

Carrier (5% HCl): Dissolve 25 mL concentrated HCl in 500 mL ultrapure water.

Reductant (0.5% KOH + 2% KBHâ‚„): Dissolve 2.5 g KOH in deionized water, add 10 g KBHâ‚„, make up to 500 mL. Do not store overnight.

Table 2: Pb standard solution configuration

Add Pb (1 µg/mL) / mL	Add concentrated HCl / mL	Deionized water final volume / mL	Standard solution concentration / (µg/L)
0.0	2.0	100	0.0
2.0	20.0	4.0	40.0
8.0	80.0	10.0	100.0

Carrier (2% HCl): Mix 10 mL HCl in 500 mL ultrapure water.

Reductant (1% KOH + 2% KBH₄ + 1% K₃Fe(CN)₆): Dissolve 5 g KOH, 10 g KBH₄, and 5 g Fe in 500 mL water. Avoid storing overnight.

Table 3: Cd standard solution configuration

Add Cd (0.1 µg/mL) / mL	Add concentrated HCl / mL	Add 0.1% Cd2 reagent / mL	Deionized water final volume / mL	Standard solution concentration / (µg/L)
0.0	2	5	100	0.0
0.2	0.2	0.4	0.4	0.8
0.8	1.0	1.0

Carrier (2% HCl): 10 mL HCl in 500 mL ultrapure water.

Reductant (0.5% KOH + 5% Cd1 reagent): Dissolve 2.5 g KOH and 25 g Cd1 reagent in 500 mL water. Do not store overnight.

Table 4: Sn standard solution configuration

Add Sn (0.1 µg/mL) / mL	Add concentrated HCl / mL	Deionized water final volume / mL	Standard solution concentration / (µg/L)
0.0	2.0	100	0.0
2.0	2.0	4.0	4.0
8.0	8.0	10.0	10.0

Carrier (2% HCl): 10 mL HCl in 500 mL ultrapure water.

Reductant (0.5% KOH + 2% KBHâ‚„): Dissolve 2.5 g KOH and 10 g KBHâ‚„ in 500 mL water. Avoid storage overnight.

2,

Results and Discussion

2.1

Linear Equations and Correlation Coefficients

Standard curves were prepared according to Section 1.4, and linear equations and correlation coefficients were determined for each element.

Table 5: Linear equations and correlation coefficients

Element	Linear Equation	Linear Correlation Coefficient / r
As	Y = 167.865X + 12.500	0.9999
Sb	Y = 482.280X - 524.504	0.9996
Bi	Y = 223.311X - 35.478	0.9994
Pb	Y = 33.856X + 7.035	0.9996
Cd	Y = 3121.999X + 47.156	0.9992
Sn	Y = 60.062X - 9.991	0.9993

2.2

Test Results and Recovery Rates

Table 6: Measured content and recovery rate of elements in three soil standards

Element	As	Sb	Bi	Pb	Cd	Sn
Measured Value (mg/kg)	GBW07409	GBW07410	GBW07447	GBW07448	GBW07409	GBW07410	GBW07447	GBW07448
1.32	9.64	0.930	0.369	27.9	0.0856	4.00	9.45	0.906	0.256	19.3	0.145	6.92	0.608	0.235	17.8	0.101
Theoretical Value (mg/kg)	1.40	10.5	0.930	0.370	29.2	0.090	4.20	10.7	0.880	0.250	20.0	0.150	7.7	0.580	0.230	18.7	0.108
Recovery Rate (%)	94.3	91.8	100	99.7	95.5	95.1	95.2	88.3	103	102	96.5	96.7	89.9	105	102	95.2	93.5

The results show that the recovery rates for As, Sb, and Bi using the aqua regia digestion method ranged from 88.3% to 91.8%, 100% to 105%, and 99.7% to 102%, respectively. For Pb, Cd, and Sn, using a more complex digestion method involving nitric, hydrochloric, hydrofluoric, and perchloric acids, the recovery rates were between 95.2% and 96.5%, 93.5% and 96.7%, and 94.3% and 95.2%.

2.3

Experimental Considerations

2.3.1

After digesting As, Sb, and Bi with aqua regia, some residual nitric acid may remain in the solution. This can react with added thiourea and ascorbic acid, reducing their effectiveness. Therefore, it is essential to heat the solution at low temperatures to evaporate any remaining nitric acid.

2.3.2

When measuring Pb, the acidity of the digestion solution must be carefully controlled. The solution should be evaporated as much as possible to ensure consistency between the sample and the carrier solution. Using 2% HCl ensures this consistency.

2.3.3

In the case of Cd, adding Cd2 special reagent increases detection sensitivity. The reaction for Cd hydride formation has a narrow pH range, so the acidity must be strictly controlled.

3,

Conclusion

In summary, the hydride generation atomic fluorescence spectrometry method was successfully applied to determine the levels of arsenic, antimony, bismuth, lead, cadmium, and tin in soil samples. The results obtained were both accurate and reliable, demonstrating the effectiveness of the technique in environmental analysis.