石灰系下水汚泥の長期連用により土壌に蓄積する重金属の存在形態と挙動

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During a 7-year period, plots of an upland field (Grey Lowland Soil, CoSL, initial pH : 6.3) were treated with various rates (0, 0.5, 1.0 and 2.0 kg m^<-2>) of dewatered sewage sludge twice each year (spring and autumn) just before the planting of spinach and cabbage, respectively (a total of 14 applications). The sludge was conditioned using calcium hydroxide as the coagulant aid. Once each year after the cabbage crop, soil samples were analyzed to observe the changes in the chemical forms and contents of heavy metals (Ni, Cu, Zn and Cd). The contents of heavy metals were also measured in the stems and leaves of both crops after each was harvested. 1) Carbonates of Ni, Zn and Cd, and exchangeable Cu were the predominant forms of the heavy metals analyzed in the sludge before application. 2) Sludge application resulted in pronounced increases in the contents of perchloric acid digestible Zn and Cd and slight increases in Ni and Cu, as analyzed in the top soil (0-15 cm), where most of the heavy metals carried by the sludge were found. In plots with the highest rate of sludge, the Zn content in the top soil increased from 76.5 mg kg^<-1> before application to 208 mg kg^<-1> after the 14th application. The Cd content increased from 0.24 to 0.47 mg kg^<-1>. At this time, Zn and Cd contents in the top soil were approximately two times higher than those in the subsoil (15-30 cm). In contrast, total Zn and Cd taken up by the crops during the experimental period decreased from 250 and 1.8 mg m^<-2> in the control plot to 200 and 0.6 mg m^<-2> with the highest rate of sludge, respectively. 3) The Fe-Mn oxides of Ni, Zn and Cu, and carbonates of Cd were the most abundantly increased forms of the heavy metals in the soil due to the sludge applications. In contrast, the contents of exchangeable Zn and Cd decreased with sludge application, thereby concomitantly restricting the mobility and plant availability of the metals. 4) The increase rate of the Fe-Mn oxides fraction of Zn and Ni, and carbonate fraction of Cd started to plateau after the 4th application when the soil pH also plateaued. This phenomenon suggests that an excessive amount of calcium over the cation exchange capacity of a soil might be a factor suppressing the sorption of heavy metals to soil.
社団法人日本土壌肥料学会の論文
2000-04-05

石灰系下水汚泥の長期連用により土壌に蓄積する重金属の存在形態と挙動

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