Green leaching of cold filter cakes using choline chloride–maleic acid deep eutectic solvent and molecular dynamics simulation

Abstract

Deep eutectic solvents (DESs) represent a novel class of solvents characterized by their favorable biodegradability and compatibility, which have been used for dissolving various metals. In this study, choline chloride–maleic acid (ChCl–Me) DES was synthesized, and the FTIR technique was used to investigate the formation of DES. Then, ChCl–Me DES was applied to dissolve cold filter cakes (CFCs), and FTIR analysis proved the stability of DES during leaching. Response surface methodology (RSM) was used to investigate the effects of time, temperature, DES to CFC ratio, and stirring speed on the leaching efficiency of Zn, Ni, and Cd. Analysis of variance (ANOVA) showed that the leaching time has a significant effect on all three metals' leaching efficiency. Also, the DES to CFC ratio and the interaction between time and the DES to CFC ratio significantly affect Zn leaching efficiency. Moreover, the second order of the DES to CFC ratio significantly affects Cd leaching efficiency. Optimal conditions were determined as follows: a time of 20 hours, a DES to CFC ratio of 70, a temperature of 60 °C, and a stirring speed of 300 rpm. The UV-vis spectra of the leaching solution showed that metals leached as chlorides in DES. Also, CFC characterization by FESEM–EDS before and after leaching via ChCl–Me DES proved almost complete leaching of metals. Therefore, this solvent can be used as a suitable solvent for the cumulative dissolution of Zn, Ni, and Cd metals with a dissolution efficiency of 90%. Molecular dynamics (MD) simulation and density functional theory (DFT) were employed to obtain detailed information about the complexes formed by Zn, Cd, and Ni ions. The surface charge density, obtained from COSMO computations, shows the notable impact of Cl anions and O atoms within ChCl and Me compounds, respectively, on the Zn²⁺, Cd²⁺, and Ni²⁺ complexes. The radial distribution function (RDF) result highlights strong attractive interactions between ions and Cl⁻ in ChCl, extending to bonded atoms. In addition, RDF results show that oxygen atoms in Me have an impact on CFC dissolution in ChCl–Me DES.