Choline Chloride–Citric Acid-Based Soil Washing: Metal Removal, Residual Fractionation and Selected Soil Properties

Sustainable remediation requires effective contaminant removal while limiting unnecessary treatment intensity and adverse changes in the treated soil. This study evaluated deep eutectic solvent (DES)-based soil washing by combining process optimization with post-treatment assessment. Three choline chloride-based DES systems containing citric, salicylic or boric acid were screened at different molar ratios and concentrations, followed by kinetic evaluation of the selected formulation and comparison with 0.005 M Na2EDTA. Choline chloride–citric acid (ChCl–CA; 1:1, 50% v/v) showed the highest overall DES performance. After 180 min, removal efficiencies reached 64.5% for Cu, 54.5% for Pb and 68.7% for Zn, corresponding to approximately 94%, 88% and 90%, respectively, of the removal achieved after 2880 min. Extending treatment sixteen-fold beyond 180 min increased removal by only 4.3, 7.8 and 7.6 percentage points for Cu, Pb and Zn, respectively. EDTA achieved higher removal of all three metals and stronger depletion of the dominant F1 and F2 fractions. However, ChCl–CA caused smaller changes in the measured soil properties: pH decreased from 6.6 to 5.8 after ChCl–CA and to 5.4 after EDTA, while organic matter decreased by 8.2% and 20.3%, respectively. Electrical conductivity decreased by 10.8% after ChCl–CA and by 18.9% after EDTA, whereas particle-size composition remained statistically unchanged after both treatments. The results demonstrate the value of combining process optimization, contaminant removal, residual metal fractionation and post-treatment soil condition when evaluating soil washing in the context of sustainable remediation.