No abstract

Isotope dilution techniques were examined in order to determine the E-value (isotopically exchangeable pool or labile pool) of cadmium (Cd) in uncontaminated soil. In order to avoid spectral interference, coprecipitation separation was applied before taking inductively coupled plasma mass spectrometry (ICP-MS) measurements. An enriched 113 Cd solution was added to a soil suspension (15 g of soil and 150 mL of water) and then a 2 mL aliquot of it was sampled and filtered immediately at different exchange times. By addition of a sodium hydroxide solution to the filtered solution, Cd coprecipitated with major constituents in the filtered solution, such as iron and magnesium. After the supernatant was discarded, the precipitate was dissolved by adding 1.5 mL of 0.2 mol/L nitric acid. After this treatment, the 113 Cd/ 114 Cd ratio in the soil solution could be measured with high precision, within RSD 5%. The correction of mass bias was not required. The proportion of E-values to total Cd concentration in the soil samples ranged between 50 and 60%. These results indicate that the E-value of Cd in uncontaminated soil can be measured by using both coprecipitation separation and isotope dilution ICP-MS.

We determined the natural Cd balance in a cultivated rice paddy field in Japan. The main sources of Cd in the non-polluted paddy field were phosphorus fertilizer and precipitation (annual input of Cd, 2,000 mg ha(-1) and 1,020 mg ha(-1), respectively). These sources account for 95% of the total input of Cd (3,192 mg ha(-1)). The actual increase of Cd in the soil was 0.0016 mg kg(-1), we thus consider Cd in soil increases only slightly as a result of rice culture. This study indicates that it is difficult to reduce Cd loading by irrigation water treatment in a non-polluted paddy field. This further indicates that once a field is polluted by Cd it is difficult to decrease the accumulated Cd by the ordinary cultivation of rice plants.