Competitive adsorption of cadmium and lead in paddy soils: a case study of paddy soils in Changsha-Zhuzhou-Xiangtan area of Hunan Province

In this paper, adsorption characteristics of Cd and Pb in three kinds of paddy soils in southern China were studied. The result showed that the sorption characteristics of Cd and Pb in paddy soils were similar, and their adsorption isotherms were all well described by Langmuir equation. Two soil samples with relatively high pH and comparatively low organic matter, CEC, clay content and kaolinite content had greater adsorption capability for Cd and Pb, as estimated by the maximum adsorption parameter (B) of the Langmuir equation. The presence of both metals reduced the tendency for either metal to be absorbed in soils; at the same time, Cd adsorption was affected to a greater extent than that of Pb, but all soils exhibited greater sorption capacity for Pb than for Cd. In three kinds of soils, the Langmuir binding strength parameter (K) was always greater for Pb than for Cd. Meanwhile, K value of binary solution was higher than single solution. This may indicate that the competition for adsorption sites promotes the retention of both metals at some specific adsorption sites, which can cause firm adsorption of metals in specific sites of soils.