Outstanding Performance of the Deep Eutectic Solvent-Based Aqueous Biphasic System Constructed with Sodium Citrate for a Green Gold Separation

Abstract

Aqueous biphasic systems (ABSs) that are based on deep eutectic solvents (DESs) are environmentally benign systems to use for metal ion separation. In this work, a series of DESs was synthesized for the first time with PEG 400 as hydrogen bond donors and tetrabutylphonium bromide (P4Br), tetrabutylammonium bromide (N4Br), or tetrabutylammonium chloride (N4Cl) as hydrogen bond acceptors, and then they were combined with citrate (Na3C6H5O7), which is eco-friendly, to construct an ABS for use in the separation of Au­(I) from an aurocyanide solution. Phase diagrams of DESs + Na3C6H5O7 + H2O systems were constructed using the experimentally measured data. Multiple factors that affect the efficiency of the gold extraction were studied; these factors were the species of salt or DES and their content, the equilibrium pH, the oscillation time, and the initial gold concentration. Gold­(I) is preferentially retained in the DES-rich phase, and the P4Br:PEG 1:2 + Na3C6H5O7 + H2O system has a high extraction efficiency of 100.0% under optimized conditions. FT-IR, NMR, and TEM characterizations and DFT calculations show that the migration of Au­(I) from the salt-rich to the DES-rich phase follows an ion exchange mechanism. Specifically, Au­(CN)2 – replaces Br– in the original P4Br and generates a stable ion pair with the quaternary phosphonium salt cation, P+, and this replacement is driven by electrostatic attractions. A new strong hydrogen bond network simultaneously forms between the anionic Au­(CN)2 – and the −OH group in the PEG 400 component. Finally, the gold of Au­(I)-loaded P4Br:PEG 1:2 can be successfully reduced by sodium borohydride with an efficiency of 100.0%. The strategy to extract gold­(I) from alkaline cyanide solutions using an ABS based on DESs as proposed in this work provides a potential platform for developing green technology for recovering gold.

Green Amino Acid Ionic Liquids-Based Aqueous Biphasic Systems for Efficient Extraction of Gold(I) from Alkaline Solution

“…The weight was again measured by a Mettler Toledo analytical balance with an uncertainty of ±10 –4 g. The process is repeated until enough data are obtained to construct the ATPS phase diagram. The data acquired in the experiment were fitted with Merchuk equation eq w IL = a nobreak0em0.1em⁡ exp ( b w normalK 0.5 − c w normalK 3 ) where w IL and w K represent the mass fractions of Ch-AA IL and K 3 PO 4 (wt %), respectively, and a , b , and c are the fitting parameters.…”

Green Aqueous Two-Phase Systems Constructed by Guanidinium Ionic Liquid for Efficient Recovery of Gold(I) from Thiosulfate Solution

“…Although ILs have been praised as environmentally friendly solvents, the most widely studied ILs for metal separation in the past often consisted of toxic components such as imidazolium, pyridinium, phosphonium cations, or fluorine-containing anions, which are clearly harmful to the environment . In addition, hydrophobic ILs often require dissolution in aliphatic or aromatic diluents to minimize mass-transfer resistance during extraction due to their high viscosity. , Therefore, the traditional mode of “extractant + diluent” is still used in metal ion extraction by ILs, which is in contrast to the original intention of using ILs as eco-friendly extractants . Therefore, it is imperative to develop a system that integrates green ILs with novel extraction manner to achieve the sustainable extraction of gold­(I) in thiosulfate leaching solutions.…”

Formation of Self-Healing Granular Eutectogels through Jammed Carbopol Microgels in Supercooled Deep Eutectic Solvent

“…Deep eutectic solvents (DESs) are eutectic mixtures commonly prepared using two or more substances, which consist of hydrogen bond donors (HBD) and hydrogen bond acceptors (HBA). − Most of the DESs contain one ionic species and are dominated by strong hydrogen bonding interactions, rendering them hydrophilic. − For example, Type III DES is a mixture of quaternary ammonium salt and a hydrogen bond donor. , DESs are nontoxic or have low toxicity, and they are cost-effective, easy to prepare, and biodegradable in nature. , Therefore, DESs can serve as good alternative solvents by replacing toxic and volatile organic solvents to reduce the environmental burden. Additionally, DESs are noteworthy green solvents due to their unique physical properties, including conductivity and surface tension. , Due to these properties, DESs are applicable in various fields such as catalysts, electrochemistry, and many industrial applications. , Specific examples include green gold separation, green self-adhesive tapes, and green and efficient cryopreservation …”