Pollution Status And Speciation Of Heavy Metals In Farmland Soil Nearby A Smelter In Baoji

In this paper, farmland soil nearby a smelter in Baoji was selected as researchobject.Total contents of heavy metals, Pb, Cd, Hg, in surface layer soil and wheat in thearea,chemical fractionation of heavy metals by using two sequential extraction methods,namely BCR and Tessier,were determined. Spatial distribution characteristics of total contentsof heavy metals and its chemical fractions were studied by Kriging interpolation method.Available fractions of heavy metals were leached using six single extraction methods, M3,DTPA, EDTA, CaCl2, NH4OAc and H2O, combined with wheat uptake to determine the mostappropriate method of all elements and each element. Relationships between soil physical andchemical properties and total contents of heavy metals in soil and wheat, fractional contentsof heavy metals were studied.The following conclusions were drawn:(1) Results showed that pH value of farmland soil nearby the smelter was around8.52.Average values of the remaining soil properties were organic matter(14.71g/kg),bulk density(1.25g/cm3),CEC(24.04cmol/kg),CaCO3(122.19g/kg),sand(5.84%),silt(85.57%),clay(8.59%).Farmland soil nearby the smelter belonged to calcareous silt loam soil. Apart from sand,coefficient of variation of other physical and chemical properties were small, showingno apparent differences in spatial distribution.(2) Results of total contents of heavy metals in soil showed that average contens of totalPb in soil (55.17mg/kg) exceeded soil background value of Shaanxi Province,while that oftotal Hg in soil (86.0μg/kg) not. Average contens of total Cd in soil reached1.99mg/kg,whichwas much higher than the soil environmental criterion (Ⅱ). Results of spatialinterpolation showed that Pb was richer in southeast, gradually decreasing to northwest. Cdwas higher in northwest and middle but lower in the east, while Hg was evenly distributedamong the whole area with no significant spatial variation.(3) Results of variation analysis and cluster analysis showed that the main sourceof Pb in soil was the fumes releasing from the smelter, while Hg in soil was totally from soilweathering. Accumulation of Cd in soil was due to the smelter, coal-fired power plantand agricultural fertilizer pollutions. The potential ecological risk assessment evaluationresults showed that the potential ecological risk of the whole study area was at light ecological harm level, and Cd was the main pollution contributor, followed by Hg andPb.(4) Average contents of Pb, Cd, Hg in wheat were73.08mg/k g,0.76mg/kg,182.47μg/kg,respectively. Accumulation capability order of heavy metals in wheat was Hg>Pb>Cd.(5) Results of chemical fractionation showed that Pb extracted by BCR sequentialextraction procedure occurred mainly in residual fraction, followed by organic-sulfide,Fe-Mn oxides and exchangeable fractions.Cd extracted by BCR sequential extractionprocedure was occupied by residual and exchangeable fractions, followed by organic-sulfideand Fe-Mn oxides fractions. Pb extracted by Tessier sequential extraction procedure mainlyexisted in residual and Fe-Mn oxides fractions and other fractions were carbonate,organic-sulfide and water soluble fractions. Cd extracted by Tessier sequential extractionprocedure was occupied by residual fraction, while the remaining fractions were water soluble,Fe-Mn oxides, organic-sulfide and carbonate fractions. Exchangeable, carbonate and Fe-Mnoxidation fractions were the main fractions absorbed by wheat.(6) Maps of Kriging interpolation showed that exchangeable fraction of Pb extracted byBCR sequential extraction procedure and water soluble fraction of Pb extracted by Tessiersequential extraction procedure differed widely with total Pb in soil, while other fractions intwo methods were extremely similar with total Pb. Organic-sulfide fraction of Cd extracted byBCR sequential extraction procedure differed widely with total Cd in soil, while theremaining fractions were consistent with total Cd. On the contrast, residual fraction of Cdextracted by Tessier sequential extraction procedure corresponded to total Cd in soil, otherfractions differed with that.(7) Combined with wheat uptake of heavy metals, six extraction methods were compared.The result showed that, M3, EDTA-2Na and DTPA had better extraction effects of availablefractions for both Pb and Cd in soil, in contrast, six methods were not suitable for availablefraction extraction of Hg.(8) Results of correlation between soil physical and chemical properties and totalcontents of heavy metals in soil and wheat, fractional contents extracted by BCR sequentialextraction procedure of heavy metals showed that, all soil physical and chemical propertieshad little effect on total heavy metals in wheat and soil. pH mainly impacted organic-sulfidefraction of Cd. Clay was the main factor affecting available fraction of Cd,while CEC mainlyaffected that of Pb extracted by NH4OAc and that of Hg extracted by NH4OAc and H2O.