Investigation of the transport and fate of Pb, Cd, Cr(VI) and As(V) in soil zones derived from moderately contaminated farmland in Northeast, China

Some farmland in Shenyang had been irrigated with industrial wastewater since 1962. Although wastewater irrigation was ceased in 1992, soil had been heavily polluted by heavy metals, especially by Cd. For better understanding processes of soil-heavy metal interactions, in particular, the mobility and retention mechanism of heavy metal in soil, a study on the transport and fate of heavy metals in soil zones from Shenyang suburb was carried out by column leaching tests in laboratory. Breakthrough curves of Pb, Cd, Cr(VI) and As(V) fitted by Thomas model and Yoon-Nelson model. The results of fitted breakthrough curves showed that transport rates of the four heavy metals in the soil zones followed the order: Cr(VI) > As(V) > Cd > Pb, which indicated that Cr(VI) was much more mobile, and Pb was comparatively unmovable. Cr in effluents and As were almost entirely Cr(VI) and As(V), respectively, and no Cr(III) and As(Ill) was ever detected during the leaching tests. The contents of Pb, Cd, Cr and As in leached soils decreased in the order of Pb > Cd > Cr > As, which suggested that adsorption ability of soil to Pb was greatest and to As was least. The methods of selective sequential extraction and solvent extraction were used to determine the fractions of Pb, Cd, Cr, As and the valent states of Cr, As [Cr(VI) or Cr(III), As(V) or As(III)] in original soils and in leached soils. After leaching tests, the relative and absolute concentrations of exchangeable, carbonate, Fe-Mn oxide and organic fraction of each element were all increased, which enhanced the potential mobility and risk of Pb, Cd, Cr and As to soil/groundwater system. The relative concentrations of Cr(III) and As(III) in different depth of the soil zones after leaching tests were increased by about 6.0% and 5.6%, respectively. Cr(III) and As(III) tended to be adsorbed by soils, which reduced the mobility of them into groundwater. (C) 2009 Elsevier B.V. All rights reserved.