ASSESSING TRANSFORMATION OF LEAD AND CADMIUM SPECIATION BY KINETIC APPROACH IN FLUVO-AQUIC SOIL OF TIANJIN AREA, CHINA

摘要

This study was conducted to investigate the transformation of speciation on lead (Pb) and cadmium (Cd) in fluvo-aquic soil of Tianjin area by using a kinetic approach. Heavy metals were added to soil as Pb(NO_3)_2 and Cd(NO_3)_2 at rates of 100 mg-kg1 and 2 mg-kg"1, respectively. Metals in the incubated soil samples were fractionated for 3-58 days by sequential extraction procedure. The heavy metals, spiked in the soil, were time-dependently transferred from the easily extractable fraction (the exchangeable fraction) into less labile fractions (carbonate-, Fe-Mn oxide-, and organic matter-bound fractions). No significant changes were found for the residual fractions of the heavy metals in the soil during the whole incubation period. The distribution of added heavy metals into different solid phase fractions appears to consist of two phases involving the initial rapid retention, followed by a slow continuous retention. Three kinetic equations were used to fit the experimental data, and parabolic equation fitted well. Therefore, the transformation rate of exchangeable fraction in fluvo-aquic soil was estimated by parabolic equation for the above incubation periods. The constant b in parabolic equation was defined as the transformation rate, which presented Pb » Cd. The higher proportion of the exchangeable fraction of spiked Cd in fluvo-aquic soil indicates its higher potential of downward leaching and runoff transport, especially at the early stage of pollution.