Choline Chloride-Urea Deep Eutectic Solvent Enhanced Removal of Lead from Mining Area Soil in the Presence of Oxalic Acid

Choline chloride-urea deep eutectic solvent (ChCl-urea DES) is a potential soil heavy metal eluent due to its advantages of low cost and simple synthesis, easy to biodegrade, and non-toxic. In this paper, the effect of ChCl-urea DES on the removal of lead from soil in the presence of oxalic acid was investigated. The experimental results indicate that ChCl-urea DES has a significantly enhanced effect on lead removal, the lead removal rate obviously increase from 49.8% to 71.1% as 3% of ChCl-urea DES was added to 0.2 mol/L oxalic acid under the condition of shaking time of 4 h and liquid/solid ratio of 10:1, the results suggest that ChCl-urea DES play a synergistic role with oxalic acid in the removal of lead from soil. Speciation analysis of lead was conducted using BCR sequential extraction method before and after washing. The result of speciation analysis indicates that oxalic acid-ChCl-urea DES was more effective in reducing the content of lead in each fraction than oxalic acid alone, particularly for acid-soluble fraction, followed by oxidation fraction and reducible fraction. The increased removal percentage is ranked in descending order: acid extracted fraction (36.2%) > reducible fraction (28.0%) > oxidizable fraction (22.6%) > residual fraction (18.0%). ChCl-urea DES can effectively remove most of mobile and bioavailable lead in contaminated soils in the presence of oxalic acid. The results are helpful to develop ChCl-urea DES based environment-friendly soil heavy metal washing system.