BACKGROUND The extractable state of soil heavy metal elements is an important index to measure their bioavailability, but its content changes with soil acid-base properties and other environmental conditions. When determining which method to use for the extractable state of soil heavy metal elements, there are more than 7 kinds of extraction reagents involved in relevant standard methods or technical specifications. These include hydrochloric acid of pH=5.8, 0.1mol/L hydrochloric acid, 0.43±0.02mol/L nitric acid, 0.11mol/L acetic acid, 1mol/L ammonium nitrate solution, 0.005mol/L DTPA leaching agent, 0.01mol/L calcium chloride solution. Different scholars have different research conclusions on different extractants. There is no systematic research report on extractants suitable for different soil types.

OBJECTIVES To select and determine the general extractant suitable for the extractable state of heavy metal elements in soil with different acid-base properties.

METHODS  Using typical farming soil samples as the research objective, 7 kinds of extracting agent were used to extract 8 heavy metals (Cd, Ni, Cu, Zn, Cr, Pb, As and Hg). Inductively coupled plasma-mass spectrometry (ICP-MS) was used to determine Cd, Cr, Cu, Pb, Zn, Ni, whereas atomic fluorescence spectrometry (AFS) was used to determine As and Hg. The extraction rates of 7 kinds of extractants were compared, and the effects of soil acid and alkali properties on the extraction rate of heavy metal elements were studied.

RESULTS Dilute acid solution had a strong ability of leaching heavy metals from soil, and it was not related to the acidity and alkalinity of the soil. Although the extraction ability of 1mol/L ammonium nitrate solution for cadmium indicated that the activity of cadmium in acidic soil was much higher than that in alkaline soil, its extraction rate of lead in alkaline soil was higher than that of cadmium. The extraction rate of DTPA extractant for soil heavy metal elements was significantly higher than that of 0.01mol/L calcium chloride solution, especially for lead, copper and zinc. Moreover, the extraction rate of DTPA extractant for soil heavy metal elements did not change significantly with soil acid-base properties. 0.01mol/L calcium chloride solution had the highest extraction rate of cadmium in soil, which was closely related to the acid-base properties of soil.

CONCLUSIONS 0.01mol/L calcium chloride solution is an ideal general extractant for the extractable state of soil heavy metal elements.