The Occurrences of Hg and Cd in Soils around Cities and Rivers and Their Ecological Risk Assessment

BACKGROUNDThe ecological risk of heavy metal anomaly in soil is widespread due to human activities and natural processes. Hg anomaly in urban soil and Cd anomaly along rivers are the typical cases. Recently, the chemical sequential extraction method is widely used for Hg, Cd and other heavy metals to analyze the content of water-soluble fraction, exchangeable fraction, organic bound fraction, and ferric-manganese oxidation in soil. Normally the contents of different heavy metals at the above different chemical extraction forms constitute the basis for ecological effect evaluation. However, no further discussion has been conducted on the natural occurrences of Hg and Cd in soil.

OBJECTIVESTo provide basis for studies on the key factors of ecological risk assessment.

METHODSThe content of Hg was determined by atomic fluorescence spectrometry (AFS) after dissolution by aqua regia. The content of Cd was determined by inductively coupled plasma-mass spectrometry (ICP-MS) after digestion by hydrochloric acid-nitric acid-hydrofluoric acid-perchloric acid complexes. The existence forms of Hg were determined by chemical analysis, pyrolytic Hg method, heavy mineral identification, electronic probe and Raman spectrum. The existing forms of Cd were determined by AB-DTPA extraction, X-ray diffractometer and laser particle sizer analyzer. The ecological effects of Hg and Cd were evaluated based on the content of these elements in rice.

RESULTSThe results show that sulfide was the main natural existence form of Hg in Hg anomaly soil around Changchun, Nanjing, Zhangzhou and Guangzhou. At least a part of Hg was in the form of cinnabar, which resulted in the relatively low percentage of bio-availability content to total content of Hg in soil. The proportion of rice grain with Hg content exceeding the standard of pollution-free food was only 3.4% when the average total content of Hg in soil was up to 500μg/kg. Cd in Cd anomaly area along the Yangtze River and the Pearl River presented as clay adsorption, resulting in about 50% Cd was bio-availability. The ratio of rice grain with Cd content exceeding the standard of pollution-free food was up to 43% when the average content of Cd in soil was 1000μg/kg.

CONCLUSIONSThis reveals that the sensitivity of the ecological effects of heavy metal elements such as Hg and Cd in soil is more directly affected by naturally occurring forms. Hg in the form of cinnabar is 'inert' and is not easily absorbed by crops, so it is not sensitive to ecological effects. Cd in the form of clay mineral adsorption is more active and easily absorbed by crops, resulting in sensitive ecological effects. After the heavy metal elements such as Hg and Cd are absorbed by crops and even the human, their existence forms and their transformation characteristics are the keys to assess whether there is an ecological risk.