Heavy Metal Distribution In Cropland Soils Amended With Sewage Sludge And Heavy Metal Uptake By Soybean

The rapid increase of sewage treatment plants, the disposal and the environmental impact of sewage sludge have been paid more and more attention. Land application of sewage sludge has become an important way for sludge disposal due to its lower cost and higher efficiency. But heavy metals contained in sludge are important confined factor to sludge application on agricultural land on a large scale. The investigation of speciation of metals both in sludge-amended soil and in sludge itself may help to assess the bioavailable metal fractions and the possibility of mobilization of these metals in soil.The sewage sludge was obtained from the second-stage process of Wenchang Sewage Treatment Plant in Harbin.The sewage sludge was applicated in the soils at 0~20 cm deep at different doses. This experiment had been taken for two years continously.The modified three-step sequential extraction procedure proposed by the Community Bureau of Reference (BCR) was used to predict Cd, Cu and Zn fractional distribution in soybean soil amended with sludge at doses of 10 Mg·hm-2, 30 Mg·hm-2 and 60 Mg·hm-2. A high-pressure airtight digestion technique and ICP-MS were used to determine the distribution, translocation and accumulation of Cd, Cu and Zn in the parts of soybean was grown in soils amended with sludge at different doses. The total concentration of Cd in soils amended with sewage sludge at dose of 6 times and 12 times did exceed the safe controlling standards (0.6 mg·kg-1, 7.5>pH>6.5). After the sludge application, the concentration of Cu and Zn in all soil samples was much lower than the control standard levels. The heavy metals in soils applied with sludge were mainly associated with the residual fraction. The relative fractional sequence of the four matels in sludge amended soils predicted here for Pb and Cu was: Residual fraction> Oxidizable fraction> Reducible fraction> Exchangeable and weak acid soluble fraction; for Cd and Zn was Residual fraction> Reducible fraction > Oxidizable fraction > Exchangeable and weak acid soluble fraction. The Exchangeable and weak acid soluble fraction order of Cd, Cu and Zn in weak acid soil was: Zn>Cd>Pb>Cu. In this study, a modified RAC values was proposed to estimate the potential environmental risk associated with heavy metal pollution in cropland phaiozem amended with sewage sludge. According to our modified RAC, for Cd, the soils applied with sludge at dose from 2 times to 6 times the regular amount there was of medium risk and in the case of 12 times the regular amount there was of high risk. For Zn, the classification is similar to that of Cd. The safe period of sludge continuous application should be considered at least 6 years (sludge at dose of 6 times the regular amount) based on experimental results in this study. The capability of metals accumulating by soybean differed with soybean parts and kinds of metals. The relative sequence of uptake by soybean parts predicted here for Pb and Cd was: stems and leaves > roots > seeds; in contrast, for Cu and Zn was: seeds > roots > stems and leaves. The order of enrichment coefficient by soybean for heavy metals was followed with: Cd>Cu>Zn>Pb.