Adsorption and desorption of Cu(II) and Pb(II) in paddy soils cultivated for various years in the subtropical China.

The adsorption and desorption of Cu(II) and Pb(II) on upland red soil, and paddy soils which were originated from the upland soil and cultivated for 8, 15, 35 and 85 years, were investigated using the batch method. The study showed that the organic matter content and cation exchange capacity (CEC) of the soils are important factors controlling the adsorption and desorption of Cu(II) and Pb(II). The 15-Year paddy soil had the highest adsorption capacity for Pb(II), followed by the 35-Year paddy soil. Both the 35-Year paddy soil and 15-Year paddy soil adsorbed more Cu(II) than the upland soil and other paddy soils. The 15-Year paddy soils exhibited the highest desorption percentage for both Cu(II) and Pb(II). These results are consistent with the trend for the CEC of the soils tested. The high soil CEC contributes not only to the adsorption of Cu(II) and Pb(II) but also to the electrostatic adsorption of the two heavy metals by the soils. Lower desorption percentages for Cu(II) (36.7% to 42.2%) and Pb(II) (50.4% to 57.9%) ,ere observed for the 85-Year paddy soil. The highest content of organic matter in the soil was responsible for the low desorption percentages for the two metals because the formation of the complexes between the organic matter and the metals could increase the stability of the heavy metals in the soils.