Effects of incubation time and moisture on the redistribution of heavy metals in a loessial soil of China

Effects of incubation time (5-150 days) and moisture (75% field capacity, wetting-drying cycle and flooding) on the redistribution of copper (Cu), lead (Pb), cadmium (Cd) and mercury (Hg) in a loessial soil of China were studied The amounts of metals spiked were 100 mg kg (1) for Cu and Pb, 2 mg kg (1) for Cd and Hg, respectively Metals in the incubated soil samples were fractionated termly from 5 to 150 days by the sequential extraction procedure, where the speciation of metals were experimentally defined as exchangeable, carbonate-, Fe-Mn oxide-, organic matter-bound and residual fractions The heavy metals spiked in the soil were time-dependently transferred from the easily extractable fractions (the exchangeable and carbonate-bound fractions) into less labile fractions (Fe-Mn oxide- and the organic matter-bound fractions) No significant changes were found in the residual fractions during the whole incubation The decreases of metals in exchangeable fraction during the incubation could be simulated by a diffusion equation, and the decrease rates were applied to reflect the transformation rates of metal speciation among varying moisture treatments For Cu, Pb and Cd, the transformation rate generally followed the order flooding > wetting-drying cycle > 75% field capacity The soil under flooding treatment had higher metal reactivity for Cu, Pb and Cd, resulting in the more complete movement of these metals toward stable fractions However, such phenomena for Hg were not observed It seemed that moisture effect on Hg speciation was not as significant as on Cu, Pb and Cd