Recyclable water-modified deep eutectic solvents for removal of multiple heavy metals from soil

Removing heavy metals from soil has always been a challenge in terms of safety and effectiveness. Deep eutectic solvents (DESs) are recognized as environmentally friendly reagents with great potential in the removal of heavy metals from soil. In this study, water was introduced as a third component to form new ternary deep eutectic water solvents (DEWSs) to improve their performance. The removal capacity, applicable conditions and mechanisms of sixteen DEWSs for heavy metals were systematically investigated. Experimental results showed that the presence of water significantly enhanced the removal efficiency of three DESs (Choline chloride plus Urea, DEU; Choline chloride plus l-lactic acid, DELA; and Choline chloride plus Ethylene glycol, DEEG) for heavy metals. However, as the molar ratio of water increased, the eutectic systems in the DEWSs weakened and eventually disappeared. Under optimum conditions, DEWLA₇ (DELA : H₂O = 2 : 8) showed the highest removal rate for cadmium, lead, copper and zinc, which were 43.42%, 94.73%, 90.72% and 96.44%, respectively. Hydrogen bonding, adsorption of oxygen functional groups, exchangeable hydrogen substitution, changes in viscosity properties and co-precipitation all contributed to the removal of heavy metals by DEWLA₇. Notably, DEWLA₇ had no significant effect on the content of major minerals and nutrients in the soil. Furthermore, DEWLA₇ proved to be reusable for soil washing, and still retains a high removal rate of 37.32%-83.66% after multi-stage filtration treatment. Therefore, DEWLA₇ was an unexplored and excellent soil washing agent with great potential in economic and social benefits.