Investigation of gold-amino acid interactions using Fourier Transform Infrared spectroscopy and correlation with gold dissolution; Studies at different pH values

Abstract

Recent developments in the field of gold leaching using amino acids have generated an interest in understanding the bonding/interactions involved in gold amino acid complexation. So far, only Density Function Theory (DFT) models have predicted the bonding of gold with amino acids, and there has been limited experimental work to verify the postulated quantum models. This study therefore aims to address the identification of amino-gold interactions using Fourier Transform Infrared spectroscopy (FTIR) and interrogate the predicted DFT interactions of gold and amino acids. The selected amino acids for this investigation were glycine, alanine, cysteine and histidine. Interactions were investigated at different pH levels corresponding to the isoelectric point and deprotonation points of respective amino acids. In addition to the FTIR investigation, gold dissolution experiments were conducted to establish a possible relationship between the interactions and gold dissolution at the explored pH points. FTIR analysis indicated that gold forms complexes with amino acids through N–Au, O–Au, and S–Au bonds. These bonds were evidenced by observed peak changes in the NH3+, COO−, and S-associated IR vibration peaks during the complexation process. In turn, the IR vibration peaks supported the DFT predictions of the active site for the bonding between gold and amino acids. Notably, the analysis revealed significant vibrational shifts in the characterization peaks of ligands at the deprotonation pH across all four amino acids, signifying a high interaction between gold and amino acids with high pH levels. Moreover, the dissolution of pure gold using the four amino acids at 0.5 M concentration of amino acid, 187 mg/L solid w/v, 1 % H2O2 and 300 rpm agitation indicated that gold dissolution was more favourable at the deprotonation pH compared to the isoelectric ph. Overall, the results showed that high dissolution was observed at deprotonation pH that corresponded to high FTIR peak shifts observed on deprotonated amino acid complexation. Lastly, the study hints at a possible correlation between the nature of gold amino-acid complexation and the extent of gold dissolution, which could require further investigation.