Initial soil moisture conditions affect the responses of colloid mobilisation and associated cadmium transport in opposite directions

The effects of initial soil moisture on colloid-associated transport are still poorly understood given the well -recognized significance of colloid-facilitated transport of strongly-sorbing contaminants. In this study, Cd leaching was sequentially conducted in an intact soil column under three initial moisture conditions (near saturation, field capacity and dryness). Soil colloids were always the dominant carriers for Cd. However, upon the lowering of initial soil moisture, increased transport of colloids (96.2-101.0-168.2 mg) was observed, surprisingly, along with decreased transport of colloid-associated Cd (C-Cd) (23.9-10.7-8.2 mu g) and enrich-ment factor (248.4-105.9-48.8 mg/kg) of Cd on colloids, resulting from pH reduction which increased Cd desorption and colloid size increase and/or zeta-potential decrease that showed lower affinity for Cd. Correlation, redundancy analysis and structural equation modelling revealed the dominantly positive role of colloids, EC plus cations (Ca2+ and Mg2+) in the release of C-Cd and dissolved Cd (D-Cd), respectively, under initial moistures of near saturation and field capacity. Under initially dry conditions, soil water potential showed dominantly negative effects on the transport of both C-Cd and D-Cd. These findings highlighted the critical role of initial moisture conditions in modulating colloid-facilitated Cd mobilisation, providing insights into the environmental risk assessment of heavy metals in other leaching scenarios.

Automated summary

This study examined the effect of initial soil moisture on colloid-associated transport, specifically Cd leaching, in a soil column. The results showed that decreased initial soil moisture led to increased transport of colloids, but decreased transport of colloid-associated Cd. This was attributed to pH reduction, increased Cd desorption, and changes in colloid size and zeta-potential. The findings emphasize the importance of initial moisture conditions in modulating colloid-facilitated Cd mobilization, which has implications for assessing environmental risk in leaching scenarios.