Loss characteristics of Cd in soil aggregates under simulated rainfall conditions.

Soil particles exert a significant influence on the migration of heavy metals in soil and water environments. In this study, a simulated rainfall experiment was conducted to investigate the loss characteristics of cadmium (Cd) from a red soil surface. Two rainfall intensities (50 and 100 mm h) were considered and the distribution of Cd in different fractions (>1, 1-0.25, 0.25-0.05 and <0.05 mm) of aggregates lost from the soil was examined. The result showed that in the artificially polluted soil, the Cd content in different aggregates decreased with particle size, with the lowest Cd content in the <0.05 mm fraction aggregates. In the rainfall experiment, the runoff rate generally increased with the rainfall duration, while the sediment yield rate first increased and then remained stable. The loss rates of various fractions of aggregates in the sediment generally followed the order of (<0.05) mm > 1-0.25 mm > 0.25-0.05 mm > (>1) mm. The proportions of the <0.05 mm-fraction aggregates were the highest in the loss sediment throughout the entire rainfall process. The lost Cd was transported mainly in the sediment-bound form, while only a very small portion of Cd was lost in the water phase of runoff. High rainfall intensity led to greater Cd loss in runoff. In the lost sediment, the concentration of Cd was higher in the macroaggregates. The phenomenon of Cd enrichment in the fine particles was not observed. Although the concentration of Cd in the <0.05 mm fraction was the lowest, the contribution of this fraction to the total Cd content in the sediments was the highest.