Analysis Of Heavy Metal Contamination And Chemical Speciation In Farmland Soil In The Suburb Of Xi'an

Based on full field survey and data collection, 207 farmland soil samples were collected from 69 sampling sites. These sampling sites distributed in Eastern suburb, Southern suburb, Western suburb and Northern suburb of Xi'an. At each sampling site, soil samples were collected from three different profiles, named Horizon A (depth from 0 to 10cm), Horizon B (depth from 10 to 20cm) and Horizon C (depth from 20 to 30cm). Some soil physical and chemical properties including soil particle-size, organic matter value, pH value and magnetic susceptibility were determined. Arsenic and mercury in the soil samples were determined by atomic fluorescence spectrometry and copper, chromium, lead, zinc, manganese, nickell in the soil samples were determined by flame atomic absorption spectrometry. Chemical speciation of heavy metal elements copper, chromium, zinc, manganese, nickel was studied using sequential extraction method. Factors influencing heavy metal elements concentration and chemical speciation were discussed. These could provide a reference for agriculture production and prevntion and controlling of heavy metal contamination in farmland soil. The main conclusions were listed below:1. The soil texture type of farmland soil in the suburb of Xi'an is silty sand soil and the average clay content was 11.16%, the average silty sand content is 63.44%, and the average Sand Particles content was 25.41%. The average pH value was 8.6, so soil was the alkaline soil. The average organic matter content was 2.69%, especially higher in the Southern suburb and Western suburb. Low-frequency magnetic susceptibility, high-frequency magnetic susceptibility and frequency magnetic susceptibility were 126.45(10^-8m³/kg),118.49(10^-8m³/kg) and 6.04%, respectively.2. The average content of copper, zinc, chromium, manganese, nickel, lead, arsenic and mercury in the soil samples were 34.02 mg/kg(from10.56 to 115.30 mg/kg), 157.03mg/kg(from 63.61 to 382.80 mg/kg), 67.87 mg/kg(from 26.4 to 156.3 mg/kg), 541.15mg/kg(from 341 to 818.98 mg/kg), 58.67 mg/kg(from 30.55 to 113.80 mg/kg), 67.41mg/kg(from 2.32 to 182.41 mg/kg), 7.75 mg/kg(from 2.08 to 15.94 mg/kg) and 1.42mg/kg(from 0.01 to 17.50 mg/kg). Content of mercury, lead, zinc and copper in farmland soil in the suburb of Xi' an is higher. Content of copper, zinc, manganese and chromium declined with the increasing depth in the soil profile. Other heavy metal elements didn't present this regular pattern: The heavy metal elements content change in the soil profile was influnced by many factors such as human activities, plant uptake, and immersion, impenetrate and loss of water.3. Several pollution evaluate indices of heavy metals in cropland soil samples around Xi' an city were calculated. Single Factor Contaminant Index of arsenic, zinc, manganese, mercury, copper, lead, nickel, and chromium was 0.66, 1.04, 1.0, 11.26, 1.51, 3.51, 2.32 and 2.4, respectively. Synthetical Pollution Index was 8.85 and it indicated that the heavy metal contamination was serious. Geoaccumulation index of arsenic, zinc, manganese, mercury, copper, lead, nickel, and chromium was -1.21, -0.56, -0.59, 1.37, -0.11, 1.23, 0.61 and 0.66, respectively. The potential ecological risk factor of mercury was 450 and the potential ecological was strong, and that of arsenic, chromium, copper, lead, manganese, nickel and zinc was 6.61, 4.81, 7.54, 17.55, 1.99, 11.58 and 1.04, respectively. Potential Ecological Risk Index was 501.25 and it indicated that heavy metal contamination was serious and the ecological risk was high.4. Chemical speciation of different heavy metal elements in Farmland Soil in the Suburb of Xi' an was different. The main speciation of copper was residual form and its average percent was 63.74% (from 51.98 to 78.90%), and the second was Fe-Mn oxide combined form and its average percent was 23.53%(from 7.01 to 42.59%). The average percent of the carbonate combined form, the organic combined form and the exchangeable form was 6.16%(from 0.42 to 23.75%), 4.62%(from 0.36 to 10.55%) and 1.95 %(from 0.49 to 3.75%). The main speciation of zinc was residual form and its average percent was 65.96% (from 47.14 to 80.59%), and the second was Fe-Mn oxide combined form and its average percent was 22.78%(from 12.25 to 34.51%). The average percent of the organic combined form, the exchangeable form and the carbonate combined form was 5.37%(from 2.03 to 20.15%), 2.42 %(from Non-deteced to 11.89%) and 3.48 %(from 0.89 to 13.4%). The main speciation of chromium was residual form and its average percent was 75.35% (from 60.45 to 84.13%), and the second was Fe-Mn oxide combined form and its average percent was 15.06 %(from 8.71 to 21.89%). The average percent of the organic combined form, the carbonate combined form and the exchangeable form was 4.79 %(from 2.23 to 10.89%), 2.95%(from Non-deteced to 6.60%) and 1.85 %(from Non-deteced to 3.37%). The main speciation of manganese was residual form and its average percent was 82.57% (from 78.06 to 88.37%), and the second was Fe-Mn oxide combined form and its average percent was 14.34 %(from 8.96 to 18.44%). The average percent of the carbonate combined forms the organic combined form and the exchangeable form was 2.12 %(from 1.49 to 2.87%), 0.73 %(from Non-deteced to 1.53%) and 0.36 %(from Non-deteced to 1.03%). The main speciation of nickel was residual form and its average percent was 72.05% (from 54.72 to 85.82%), and the second was Fe-Mn oxide combined form and its average percent was 16.36 %(from 8.79 to 26.28%). The average percent of the exchangeable form, the carbonate combined form and the organic combined form was 5.24 %(from 1.97 to 8.11%), 3.84 %(from 1.02 to 7.05%) and 1.88 %(from Non-deteced to 4.66%).5. The statistical data showed that there was a close correlation between copper and zinc, chromium, nickel, manganese, respectively. A close correlation also existed between zinc and chromium, manganese, lead, nickel, respectively. There was also a close correlation between manganese and nickel, mercury, respectively. A close correlation also existed between nickel and arsenic, lead and arsenic, arsenic and mercury, respectively. Low-frequency susceptibility had a close correlation with the content of copper, zinc, chromium, manganese, nickel, and lead, respectively. Soil pH value also had aclose correlation with copper, chromium, nickel, and lead, respectively. Soil clay had a close correlation with the content of copper, zinc, chromium, manganese, nickel, and lead, respectively. Soil organic matter had a close correlation with the content of copper, zinc, manganese, nickel, mercury and lead, respectively.6. The statistical data showed that there was a close correlation between soil physical and chemical properties and different chemical speciation. For copper, there was a close correlation between the exchangeable form and high-frequency susceptibility, soil clay, respectively, and between the residual form and soil clay. For zinc, a close correlation existed between the exchangeable form and soil organic matter, carbonate combined form and soil clay, organic combined form and soil organic matter, residual forms and soil organic matter, respectively. For chromium, a close correlation existed between Fe-Mn oxide combined form and high-frequency susceptibility, organic combined form and susceptibility, respectively. And a negative correlation was found between Fe-Mn oxide combined form and pH value, soil clay, respectively. For manganese, there was a close correlation between carbonate combined form and susceptibility, soil clay, soil organic matter, respectively, and between Fe-Mn oxide combined form and soil organic matter. A close correlation also existed between the residual form and susceptibility, soil clay. For nickel, there was a close correlation between the exchangeable form and susceptibility, arbonate combined form and susceptibility, soil clay, respectively. A close correlation existed between organic combined form and susceptibility, pH value, respectively. A negative correlation was found between Fe-Mn oxide combined form and pH value. There was a close correlation between the residual form and susceptibility, soil clay, soil organic matter, respectively.