Importance of Dynamic Soil Properties in Metal Retention: An Example from Long-Term Cu Partitioning and Redistribution Studies Using Model Systems

The effect of initial conditions and reaction pathways in the long term solid-solution partitioning and solid-phase distribution of Cu among ferrihydrite, leaf compost (LC), and montmorillonite (K-SWy2) were established using compartmentalized batch reactors by varying the sequence of mixing of the sorbents. Copper was allowed to react with a single solid phase for 30 days (1st equilibration) before introducing the other two solid phases and equilibration for 8 additional months (2nd equilibration). The systems were labeled Fe-Ox, Organic, or Smectitic reflecting the single initial solid phase present during the first equilibration. Total dissolved Cu and total Cu in individual solid phases were determined as a function of time during the first and second equilibrations. Results showed that different initial conditions elicited different dynamic responses where the generation of dissolved organic carbon (DOC) and diffusion of colloidal ferrihydrite seemed to influence the long-term partitioning and distribution of Cu. Trends in total dissolved Cu for the systems at the end of the first equilibration were Fe-Ox > Organic > Smectitic, while at the end of the second equilibration the organic system was the least effective in the removal of Cu from solution (Organic > Fe-Ox ≈ Smectitic). Furthermore, our results indicated Cu redistribution toward organic matter and montmorillonite, with small amounts of Cu retained by ferrihydrite. These results are attributed to reaction pathways where the formation of soluble Cu-organic complexes and colloidal Cu-ferrihydrite, and their subsequent reaction with the solids present in the systems, were operative. The experiments reported herein show dynamic properties dictate Cu reaction pathways in multiphase-multicomponent systems and might help to explain unexpected higher mobility of metals after soil remediation.