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Abstract [Objectives]The contents of copper and lead in the soil of the Yellow River Wetland were determined by flame atomic absorption spectrometry， which provides a theoretical basis for the treatment of soil pollution.[Methods]The soil was digested with a concentrated nitric acidhydrofluoric acidperchloric acid system， and the contents of heavy metals such as copper and lead in the Yellow River Wetland of Shaanxi Province were determined by flame atomic absorption spectrometry.[Results]The correlation coefficients reached 0.999 5 in the range of 0.00-1.00 mg/L， indicating good linearity.[Conclusions]The method is simple in operation， good in reproducibility， high in sensitivity to most elements， and can be widely used.
Key words Flame atomic absorption spectrometry; Wetland soil; Copper; Lead
Wetland is one of the three major ecosystems in the world and is known as "the kidney of the earth". Wetland soil is one of the important environmental factors that constitute the wetland ecosystem. The Yellow River Wetland is the largest wetland delimited by Shaanxi Province at present. In recent years， with the development of science and technology， the three industrial wastes have increased， and there are more and more hormonal pollutants in the Yellow River Wetland， which seriously affects the environment of the wetland and harms various living organisms. For instance， heavy metals such as lead and copper in pollutants are discharged into the wetland， resulting in heavy metals in the wetland soil exceeding standards. Because most of the heavy metals are hardly soluble in water， some microorganisms in the soil cannot decompose them; and on the contrary， organisms can enrich heavy metal elements and possibly convert some heavy metal elements into more toxic metal organic compounds[1]. Therefore， the treatment of soil pollution is very important in todays society. There are many methods for measuring heavy metals in soil， such as inductively coupled plasma atomic emission spectrometry （ICPAES）[2-3]， atomic absorption spectrometry （AAS）[4-5]and laserinducedbreakdown spectrometry （LIBS）[6]. In this study， the contents of copper and lead in the soil of the Yellow River Wetland were determined by flame atomic absorption spectrometry.
Experiment Materials
Experiment principle
Soil is digested with concentrated nitric acidhydrofluoric acidperchloric acid to completely eliminate some insoluble elements such as silicon in the soil and completely dissolve elements to be tested in the sample into the test solution. Then， the test solution obtained by digestion is introduced into an airacetylene flame. In the flame at high temperature， the copper and lead ions selectively absorb corresponding characteristic radiation of the hollow cathode lamp. The measurement is carried out under optimized experimental conditions， for the absorbance of copper and lead. Then， according to the proportional relation between their absorbance and metal ion concentrations， the standard curve method is used to determine the contents of lead and copper in the sample[7]. This method is very simple and rapid. Instruments
ICE3300 atomic absorption spectrophotometer （Thermo Electron）; WFX120A data station; HL1 hollow cathode lamp （copper lamp， lead lamp; manufactured by Hengshui Ningqiang Light Source Co.， Ltd.）; DHG9140A electric thermostat blast oven （Tianjin Taisite Instrument Co.， Ltd.）; universal electric furnace （Beijing Kewei Yongxing Instrument Co.， Ltd.）; electronic analytical balance.
Several volumetric flasks of different volumes; 1， 5 and 10 ml pipettes; several beakers of different specifications; measuring cylinders， suction bulbs， filter paper， funnels， colorimetric tubes.
Reagents
Hydrochloric acid （=1.18 g/ml）， hydrofluoric acid （=1.49 g/ml）; perchloric acid （=1.68 g/ml）; 1% （v/v） nitric acid solution; 1.00 g/L copper metal standard stock solution; 1.00 g/L lead standard stock solution.
Wetland soil samples were collected from Heyang， the Yellow River Wetland.
The parameters of the experimental instrument for the two elements （copper and lead） were set as Table 1.
Experimental Methods
Sample collection
The soil sample was collected by the plumshaped stationing method. During the sampling process， all visible debris on the ground surface was removed. Secondly， the 10-20 cm soil was collected with a plastic container. The collected soil sample was placed in a plastic bag and taken back to the laboratory.
Treatment of soil sample
The collected soil sample （generally not less than 500 g） was poured onto clean A4 paper， dried to a semidry state， and crushed， and visible impurities were removed. The soil was then put in an oven， and dried at 115 °C for 12 h. The dried soil was grinded with a mortar repeatedly until the soil became fine particles， which were put into a bottle （in the preparation process， attention should be paid to the closure of the experiment to prevent contamination of the sample， so as to avoid experimental error）. The soil was reduced by the quadruple method， and 2.000 g of the soil sample was accurately weighed and ovendried at 115 for 12 h until the weight was constant.
Digestion of soil sample
Three parts of identical samples （0.300 0 g each， accurate to 0.000 2 g） were accurately weighed into three 50 ml crucibles， respectively. Each sample in corresponding crucible was wet with water and added with 10 ml of concentrated nitric acid and 5 ml of hydrofluoric acid to soak the soil， and stood for 12 h， to fully digest the soil. On the second day， each sample was heated on an electric heating plate in a fume hood at a low temperature to decompose the sample for the first time. When the solution was evaporated to about 2-3 ml， the sample was slightly cooled and added with 5 ml of concentrated nitric acid and 2 ml of hydrofluoric acid， followed by shaking uniformly and heating on a hot plate at medium temperature. After 1 h， 5-10 ml of concentrated nitric acid and 1-2 ml of hydrofluoric acid can be added according to the digestion condition， and the above digestion process can be repeated as well. When the white smoke was almost completely exhausted again and the liquid in the beaker was colorless and transparent， 1 ml of perchloric acid was added and cooled slightly to room temperature， the beaker was rinsed with water， and 1 ml of a 1% nitric acid solution was added to warmly dissolve the residue. Then， the solution was transferred to a 25 ml colorimetric tube， cooled and diluted with 1% nitric acid to constant volume， followed by shaking well and testing. Because there are various kinds of soils， the types and contents of the contained elements are different. Therefore， in the digestion， it should be observed that the amounts of various acids added can be determined according to the digestion condition， to prevent the experimental results from being affected by the dosage. Results and Analysis
Setting of instrument operating conditions
Condition parameters were set on the workstation as Table 1.
Drawing of standard curve
According to the copper and lead standard solutions prepared in "Reagents"， the absorbance of each solution was measured with 1% dilute nitric acid as a blank， and the AC working curves were drawn. According to the above steps， the absorbance values corresponding to the concentrations of the lead and copper standard solutions were recorded， as shown in Table 2. The working curves of copper and lead elements are shown in Fig. 1 and Fig. 2.
Determination of soil sample
According to the results obtained in Table 2， the absorbance of copper and lead in samples was measured， and substituted to the linear regression equations obtained in Table 2， obtaining corresponding concentrations. Then， copper and lead contents were obtained according to W=C*V/m. The contents are shown in Table 3.
Determination of recovery
In order to verify the accuracy of the analytical method， a recovery test was carried out， and the results are shown in Table 4.
Error analysis
（1） During the experiment， sample weight was strictly controlled. The mass error of the three groups of samples was controlled to be less than 0.001 g （accurate to 0.000 2 g）， so that the data were more accurate， thereby reducing the occurrence of random errors and systematic errors.
（2） Temperature is also one of the factors affecting the experimental data. If the temperature is too low， the sample will not be fully digested， and if the temperature is too high， it will affect the experiment as well， because under a toohigh temperature， the sample will become a paste during digestion， resulting in slightly lower experimental results.
（3） The glass instruments were not soaked with nitric acid， and were stained by other substances， which affected the experimental data.
（4） In the experiment， after the solutions were prepared， they were not measured in time， and flocculation was formed， which affected the experimental data.
Conclusions and Discussion
Soil samples could be pretreated by wet digestion， of which the conditions and operation methods should be grasped， and washed and dried samples should be treated with concentrated nitric acid and perchloric acid. Wet digestion comprises a variety of digestion methods， and depending on the type of the digested sample， the method is also different. In this study， the content of heavy metal copper in the wetland soil was 0.763 mg/kg， and the content of lead was 0.423 mg/kg. It could be seen from the above table that the recoveries of copper and lead were in the range of 97.86%-101.8%， which meets the accuracy requirements of trace element analysis. It indicated that the method of digesting the sample by wet digestion and determining copper and lead by flame atomic absorption spectrometry is accurate and reliable. References
[1]LI XZ. The damage of heavy metals lead and cadmium to soil and crops with reference to the control measures[J]. Nonwood Forest Research， 2000， 18（4）： 12-14. （in Chinese）
[2]YANG KF. ICPOES determination of conventional metals in water[J]. Pollution Control Technology， 2014， 27（6）： 47-49. （in Chinese）
[3]FAN X， ZHOU Y， CHEN D， et al. Analysis of 12 elements in aquatic products by inductively coupled plasma atomic emission spectrometry （ICPAES）[J]. Chinese Journal of Analysis Laboratory， 2013， 32（5）： 91-95. （in Chinese）
[4]WANG XN， WANG ZH， CHEN YN. Determination of lead， cadmium and copper in seawater by solid extractiongraphite furnace atomic absorption spectrometry[J]. Chinese Journal of Analysis Laboratory， 2016， 35（10）： 1157-1160. （in Chinese）
[5]HANG XY， WANG Q， WANG L， et al. Analysis of trace nickel in water samples by magnetic solid extractiongraphite furnace atomic absorption spectrometry[J]. Chinese Journal of Analysis Laboratory， 2016， 34（7）： 807-810. （in Chinese）
[6]ZHAO NJ， GU YH， MENG DS， et al. Research progress of laserinducedbreakdown spectroscopy （LIBS）[J]. Journal of Atmospheric and Environmental Optics： 2016， （5）： 367-382. （in Chinese）
[7]State Environmental Protection Administration， Editorial Committee for Water and Wastewater Monitoring and Analysis Methods. Water and wastewater monitoring and analysis methods （the 4th edition）[M]. Beijing： China Environmental Science Press， 2002： 344-346. （in Chinese）
Key words Flame atomic absorption spectrometry; Wetland soil; Copper; Lead
Wetland is one of the three major ecosystems in the world and is known as "the kidney of the earth". Wetland soil is one of the important environmental factors that constitute the wetland ecosystem. The Yellow River Wetland is the largest wetland delimited by Shaanxi Province at present. In recent years， with the development of science and technology， the three industrial wastes have increased， and there are more and more hormonal pollutants in the Yellow River Wetland， which seriously affects the environment of the wetland and harms various living organisms. For instance， heavy metals such as lead and copper in pollutants are discharged into the wetland， resulting in heavy metals in the wetland soil exceeding standards. Because most of the heavy metals are hardly soluble in water， some microorganisms in the soil cannot decompose them; and on the contrary， organisms can enrich heavy metal elements and possibly convert some heavy metal elements into more toxic metal organic compounds[1]. Therefore， the treatment of soil pollution is very important in todays society. There are many methods for measuring heavy metals in soil， such as inductively coupled plasma atomic emission spectrometry （ICPAES）[2-3]， atomic absorption spectrometry （AAS）[4-5]and laserinducedbreakdown spectrometry （LIBS）[6]. In this study， the contents of copper and lead in the soil of the Yellow River Wetland were determined by flame atomic absorption spectrometry.
Experiment Materials
Experiment principle
Soil is digested with concentrated nitric acidhydrofluoric acidperchloric acid to completely eliminate some insoluble elements such as silicon in the soil and completely dissolve elements to be tested in the sample into the test solution. Then， the test solution obtained by digestion is introduced into an airacetylene flame. In the flame at high temperature， the copper and lead ions selectively absorb corresponding characteristic radiation of the hollow cathode lamp. The measurement is carried out under optimized experimental conditions， for the absorbance of copper and lead. Then， according to the proportional relation between their absorbance and metal ion concentrations， the standard curve method is used to determine the contents of lead and copper in the sample[7]. This method is very simple and rapid. Instruments
ICE3300 atomic absorption spectrophotometer （Thermo Electron）; WFX120A data station; HL1 hollow cathode lamp （copper lamp， lead lamp; manufactured by Hengshui Ningqiang Light Source Co.， Ltd.）; DHG9140A electric thermostat blast oven （Tianjin Taisite Instrument Co.， Ltd.）; universal electric furnace （Beijing Kewei Yongxing Instrument Co.， Ltd.）; electronic analytical balance.
Several volumetric flasks of different volumes; 1， 5 and 10 ml pipettes; several beakers of different specifications; measuring cylinders， suction bulbs， filter paper， funnels， colorimetric tubes.
Reagents
Hydrochloric acid （=1.18 g/ml）， hydrofluoric acid （=1.49 g/ml）; perchloric acid （=1.68 g/ml）; 1% （v/v） nitric acid solution; 1.00 g/L copper metal standard stock solution; 1.00 g/L lead standard stock solution.
Wetland soil samples were collected from Heyang， the Yellow River Wetland.
The parameters of the experimental instrument for the two elements （copper and lead） were set as Table 1.
Experimental Methods
Sample collection
The soil sample was collected by the plumshaped stationing method. During the sampling process， all visible debris on the ground surface was removed. Secondly， the 10-20 cm soil was collected with a plastic container. The collected soil sample was placed in a plastic bag and taken back to the laboratory.
Treatment of soil sample
The collected soil sample （generally not less than 500 g） was poured onto clean A4 paper， dried to a semidry state， and crushed， and visible impurities were removed. The soil was then put in an oven， and dried at 115 °C for 12 h. The dried soil was grinded with a mortar repeatedly until the soil became fine particles， which were put into a bottle （in the preparation process， attention should be paid to the closure of the experiment to prevent contamination of the sample， so as to avoid experimental error）. The soil was reduced by the quadruple method， and 2.000 g of the soil sample was accurately weighed and ovendried at 115 for 12 h until the weight was constant.
Digestion of soil sample
Three parts of identical samples （0.300 0 g each， accurate to 0.000 2 g） were accurately weighed into three 50 ml crucibles， respectively. Each sample in corresponding crucible was wet with water and added with 10 ml of concentrated nitric acid and 5 ml of hydrofluoric acid to soak the soil， and stood for 12 h， to fully digest the soil. On the second day， each sample was heated on an electric heating plate in a fume hood at a low temperature to decompose the sample for the first time. When the solution was evaporated to about 2-3 ml， the sample was slightly cooled and added with 5 ml of concentrated nitric acid and 2 ml of hydrofluoric acid， followed by shaking uniformly and heating on a hot plate at medium temperature. After 1 h， 5-10 ml of concentrated nitric acid and 1-2 ml of hydrofluoric acid can be added according to the digestion condition， and the above digestion process can be repeated as well. When the white smoke was almost completely exhausted again and the liquid in the beaker was colorless and transparent， 1 ml of perchloric acid was added and cooled slightly to room temperature， the beaker was rinsed with water， and 1 ml of a 1% nitric acid solution was added to warmly dissolve the residue. Then， the solution was transferred to a 25 ml colorimetric tube， cooled and diluted with 1% nitric acid to constant volume， followed by shaking well and testing. Because there are various kinds of soils， the types and contents of the contained elements are different. Therefore， in the digestion， it should be observed that the amounts of various acids added can be determined according to the digestion condition， to prevent the experimental results from being affected by the dosage. Results and Analysis
Setting of instrument operating conditions
Condition parameters were set on the workstation as Table 1.
Drawing of standard curve
According to the copper and lead standard solutions prepared in "Reagents"， the absorbance of each solution was measured with 1% dilute nitric acid as a blank， and the AC working curves were drawn. According to the above steps， the absorbance values corresponding to the concentrations of the lead and copper standard solutions were recorded， as shown in Table 2. The working curves of copper and lead elements are shown in Fig. 1 and Fig. 2.
Determination of soil sample
According to the results obtained in Table 2， the absorbance of copper and lead in samples was measured， and substituted to the linear regression equations obtained in Table 2， obtaining corresponding concentrations. Then， copper and lead contents were obtained according to W=C*V/m. The contents are shown in Table 3.
Determination of recovery
In order to verify the accuracy of the analytical method， a recovery test was carried out， and the results are shown in Table 4.
Error analysis
（1） During the experiment， sample weight was strictly controlled. The mass error of the three groups of samples was controlled to be less than 0.001 g （accurate to 0.000 2 g）， so that the data were more accurate， thereby reducing the occurrence of random errors and systematic errors.
（2） Temperature is also one of the factors affecting the experimental data. If the temperature is too low， the sample will not be fully digested， and if the temperature is too high， it will affect the experiment as well， because under a toohigh temperature， the sample will become a paste during digestion， resulting in slightly lower experimental results.
（3） The glass instruments were not soaked with nitric acid， and were stained by other substances， which affected the experimental data.
（4） In the experiment， after the solutions were prepared， they were not measured in time， and flocculation was formed， which affected the experimental data.
Conclusions and Discussion
Soil samples could be pretreated by wet digestion， of which the conditions and operation methods should be grasped， and washed and dried samples should be treated with concentrated nitric acid and perchloric acid. Wet digestion comprises a variety of digestion methods， and depending on the type of the digested sample， the method is also different. In this study， the content of heavy metal copper in the wetland soil was 0.763 mg/kg， and the content of lead was 0.423 mg/kg. It could be seen from the above table that the recoveries of copper and lead were in the range of 97.86%-101.8%， which meets the accuracy requirements of trace element analysis. It indicated that the method of digesting the sample by wet digestion and determining copper and lead by flame atomic absorption spectrometry is accurate and reliable. References
[1]LI XZ. The damage of heavy metals lead and cadmium to soil and crops with reference to the control measures[J]. Nonwood Forest Research， 2000， 18（4）： 12-14. （in Chinese）
[2]YANG KF. ICPOES determination of conventional metals in water[J]. Pollution Control Technology， 2014， 27（6）： 47-49. （in Chinese）
[3]FAN X， ZHOU Y， CHEN D， et al. Analysis of 12 elements in aquatic products by inductively coupled plasma atomic emission spectrometry （ICPAES）[J]. Chinese Journal of Analysis Laboratory， 2013， 32（5）： 91-95. （in Chinese）
[4]WANG XN， WANG ZH， CHEN YN. Determination of lead， cadmium and copper in seawater by solid extractiongraphite furnace atomic absorption spectrometry[J]. Chinese Journal of Analysis Laboratory， 2016， 35（10）： 1157-1160. （in Chinese）
[5]HANG XY， WANG Q， WANG L， et al. Analysis of trace nickel in water samples by magnetic solid extractiongraphite furnace atomic absorption spectrometry[J]. Chinese Journal of Analysis Laboratory， 2016， 34（7）： 807-810. （in Chinese）
[6]ZHAO NJ， GU YH， MENG DS， et al. Research progress of laserinducedbreakdown spectroscopy （LIBS）[J]. Journal of Atmospheric and Environmental Optics： 2016， （5）： 367-382. （in Chinese）
[7]State Environmental Protection Administration， Editorial Committee for Water and Wastewater Monitoring and Analysis Methods. Water and wastewater monitoring and analysis methods （the 4th edition）[M]. Beijing： China Environmental Science Press， 2002： 344-346. （in Chinese）