Abstract
There is increasing interest in the development of noble metal separation/recovery processes, especially for applications to “urban mining”. Common separation/recovery processes for noble metals use a solvent (liquid-liquid) extraction technique in hydrometallurgy. However, these processes are time-consuming and not environmentally friendly, because they use organic solvents for sequential metal ion extractions. Electrowinning is an alternative approach for selective metal precipitation that involves controlling the redox potentials of electrodes but requires specialized equipment and generates hydrogen as a byproduct at the cathode surface under dilute conditions. In the present study, we investigated selective gold recovery from a homogenous aqueous solution containing a mixture of dilute HAuCl4 and H2PtCl6 (5.0 × 10−5 M each) and aromatic amino acid-containing peptides (2.0 × 10−4 M each). Gold selectivity was determined by analyzing the compositions of the solids and supernatants obtained from the reaction mixtures. A much higher gold selectivity (gold/platinum (Au/Pt) atomic ratio = 7.5) was obtained using an anthracene-containing peptide compared to peptides containing one or two naphthalene ring(s). Our proposed approach is applicable to the sequential separation of several noble metal ions, such as Au, palladium (Pd), Pt, iridium (Ir) and rhodium (Rh), and simply requires developing aromatics suitable for each noble metal of interest.
Highlights
Processes for noble metal separation/recovery are of increasing interest due to their applicability to sustainable chemistry
We proposed the following mechanism for RU006-directed gold nanoribbon formation: (i) RU006 encapsulates and concentrates AuCl4− in the interior cavities of peptide network architectures through electrostatic interaction with a cationic Lys side chain during peptide self-assembly stabilized by hydrogen bonds, hydrophobic interaction and/or π-π stacking among naphthalene rings; (ii) electrons are transferred from the naphthalene ring to Au(III) and (iii) gold crystals slowly grow to form gold nanoribbons along the template network architecture
We investigated selective gold recovery by RU006, (NaI2)-RU006 or (Ant6)-RU006 from a homogenous aqueous solution comprising a mixture of dilute HAuCl4 and H2PtCl6
Summary
Processes for noble metal separation/recovery are of increasing interest due to their applicability to sustainable chemistry. Metallic elements of interest might be recovered selectively from a homogenous aqueous solution (no organic solvent) by the self-assembly of aromatic amino acid-containing peptides, rapidly enriching metal ions under dilute conditions (Figure 1B). Precipitates from such reaction mixtures must be sufficiently dense to separate by simple centrifugation, meeting the criteria for sustainable, green and noble metal recovery. We investigated the selective gold recovery from a homogenous aqueous solution containing a mixture of dilute gold and platinum ions (each 5.0 × 10−5 M) by mixing the aromatic amino acid-containing peptides RU006 and (Ant6)-RU006. ATR-FTIR spectra were acquired on a Shimadzu IR Prestige-21 FTIR spectrophotometer equipped with a Smiths Detection DuraSample IR II Diamond (Hertfordshire, United Kingdom)
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