杭州市城市土壤重金属的潜在可淋洗性研究

POTENTIAL LEACHABILITY OF HEAVY METALS IN URBAN SOILS FROM HANGZHOU CITY

研究了杭州市城市土壤 8个重金属元素 (Cd、Co、Cr、Cu、Ni、Pb、Zn、Mn)的含量、形态和潜在可淋洗性。结果表明 ,该城市土壤中Cd、Cr、Cu、Ni、Pb、Zn和Mn均有明显的积累 ,其中Cd、Co、Cr和Ni主要以稳定的残余态为主 ,而Cu、Pb、Zn和Mn则以可提取态为主 ,因此在强还原、强酸性或有利于有机质分解条件下Cu、Pb、Zn、Mn的释放潜力较高。用荷兰能源研究中心的淋洗方法 (pH4 0 )测试表明 ,该市城市土壤的重金属可淋洗性较低 ,在一般条件下该城市土壤重金属不会有较大的释放 。

The potential risk of surface and ground water contamination by heavy metals leached from contaminated soils have caused considerable concern. Understanding the accumulation and chemical forms of the elements in soils is necessary to predict long term leaching behavior of the heavy metals in the soils. The objective of this study was to evaluate heavy metal leachability of urban soils in Hangzhou City, Zhejiang Province, China. A total of twenty urban soils and twelve suburban agricultural soils were collected from the urban and suburban areas in the Hangzhou city for fractionation and leaching test of heavy metals. An optimized European Community Bureau of Reference three-step sequential extraction procedure was applied to heavy metal fractionation of the soils. Four operationally defined fractions were identified: acid extractable, reducible, oxidizable and residual. The soils were also subjected to determination of potential leachability of heavy metals using a method developed by the Netherlands Energy Research Center. The results showed that the urban soils in the Hangzhou City had been anthropogenically contaminated by heavy metals to a great extent, and heavy metals in the urban soils, including Cd, Cr, Cu, Ni, Pb, Zn, and Mn, were higher than those in the suburban agricultural soils. About 70% of Cd, Co, Cr, and Ni were associated with residual fraction indicating relative low mobility of the metals in the soils. Of the four remaining heavy metals, Cu and Pb were dominantly associated with oxidizable and reducible fractions with low acid extraction fraction. Zinc and Mn had only one-third of their total concentrations in the residual fraction, but one-fourth in the acid extractable, which is susceptible to release. Potential leachability of all the measured eight heavy metals, defined as the maximum metal pool that may become available for leaching at a constant pH 4, was low and decreased in the following order: Cu > Mn > Cd > Ni > Zn > Pb > Co, Cr. Low potential leachability of the heavy metals in the urban soils was due to their low percentage of acid-extractable fraction of the heavy metals, and relatively high organic matter, iron oxides as well as clay contents that could adsorb heavy metals released from dissolution processes. Measured data indicated a low risk for surface and ground water contamination from the urban soils in the Hangzhou City.