HEAVY METAL CONTAMINATION OF SOIL AND RICE IN WASTEWATER-IRRIGATED PADDY FIELD IN A SUBURBAN AREA OF HANOI, VIETNAM

Abstract

The present paper deals with the heavy metal contamination of soil and rice grain in the paddy field subjected to the irrigation water polluted with wastes from various industrial plants in Hanoi. Soil and rice grain samples were taken at different distances (0-50 m) from the edge of the paddy field. The retention and potential mobility of heavy metals were assessed based on the contents of total and fractionated heavy metals in the soil and their leachability. The concentration of the fractionated heavy metals was determined by selective sequential extraction method. Heavy metal contents in the rice grain were also assessed. The average concentration of the metals in the soil was in the order: Cu (202 mg kg -1 ) > Zn (192 mg kg -1 ). Cr (185 mg kg -1 ) Pb (159 mg kg -1 ) > Ni (45 mg kg -1 ) > Cd (4 mg kg -1 ). The heavy metal concentrations in the soil exceeded the permissible level of the Vietnamese standard for Cd, Cu, Pb and Zn. The results of selective sequential extraction procedure indicated that dominant fractions were oxide, organic and residual materials for Cd, Ni, Pb and Zn, and organic and residual materials for Cr, and an organic material for Cu. Leaching tests with deionized water and acid solutions indicated that the ratio of leached metal concentration to total metal concentration in the soil decreased in the order: Cd > Ni > Cr > Pb > Cu > Zn. By leaching with deionized water and acid solutions, all heavy metals were released fully from exchangeable fraction, and Cd and Ni were fully from carbonate and oxide fractions. The average concentration of heavy metals in the rice grain was in the order: Zn (14.4 mg kg -1 ) > Cu (6.9 mg kg -1 ) > Cr (3.1 mg kw -1 ) > Pb (2.1 mg kg -1 ) > Ni (1.4 mg kg -1 ) > Cd (0.1 mg kg -1 ). The concentrations of Cu, Ni, and Zn in the rice grain met the WHO standard while the concentrations of Cr and Pb exceeded the permissible level of the standard. The concentrations of Cd met the Vietnamese standard while those o.f Pb exceeded the permissible level of the standard. Transfer coefficient for the metals was in the order: Zn > Ni > Cu > Cd ≈ Cr > Pb.