Accelerating Cu and Cd removal in soil flushing assisted by regulating permeability with electrolytes

Soil flushing is one of the common in-situ remediation technologies, in which the permeability of the soil determines its feasibility. Batch extractions showed that deionized water extracted about 20% Cu and 30% Cd from a soil. Electrolytes of 100 mmol/l NaCl, 500 mmol/l NaCl, and 167 mmol/l CaCl₂ promoted the extractions to about 60% and 90%, respectively, with higher concentration and valence of cations being more effective. Presence of 100 mmol/l EDTA as a chelant further enhanced the extractions to near completion. Extractions appeared to occur concurrently via ion exchange, complexation with Cl^- and predominantly chelation with EDTA. Column leaching in dynamic setups with electrolyte solutions in the presence of EDTA showed similar Cu and Cd removal degrees to the batch extractions. The permeability of soils during leaching decreased by up to 80%, decelerating time-dependent Cu and Cd removal, due to soil swelling by Na⁺. Electrolytes in leaching solutions well defended the permeability of soil against its decrease to as low as 3.5%, maintaining Cu and Cd removal rates. Formulating flushing solution with EDTA and proper electrolytes have advantages of not only enhancing extraction degrees but accelerating heavy metal removal from soil by regulating the permeability, with the potential to be extended to soils with high clay contents and thus low natural permeability.