Synthesis, sorption-bioreduction and catalytic activity of gold on thioctic acid functionalised silica coated magnetite nanoparticles

Abstract

SGold (Au) is a precious metal that has low toxicity but has vast applications and high market prices. There is an increasing interest in the catalytic application of Au due to its unique properties. However, the global resources of Au are quite limited, thus recovery of Au from waste solution is a challenging and important task. Hence, this research focuses on the adsorption of Au(III) ions on a thiol based adsorbent and its subsequent bio-reduction to Au(0). Thioctic acid functionalised silica coated magnetite nanoparticles (RS-SR-NH-SiO2-Fe3O4) has been synthesised and its ability for the recovery of Au(III) from aqueous solutions was assessed at different parameters. The results of the adsorption thermodynamics and kinetics showed that this magnetic adsorbent has good adsorption capacity for Au(III) and the best interpretation of the experimental data was given by the Langmuir isotherm model. The results showed that the adsorption kinetics followed a pseudo-second-order rate equation with maximum adsorption capacity for Au(III) as 285.71 mg g-1 at 45°C. The thermodynamic parameters ΔG°, ΔH°, and ΔS° were -13.56 kJ mol-1, -24.33 kJ mol-1, and -36.18 J K-1mol-1, respectively. The adsorption was a chemisorption process with activation energy of 11.58 kJ mol-1. In the next step, the adsorbed Au(III) ions were reduced into Au(0) using Phaleria macrocarpa (Scheff.) Boerl fruit aqueous extract. The performance of the biosynthesised Au(0)-RS-SR-NH-SiO2-Fe3O4 catalysts were evaluated by using 4-nitrophenol reduction and styrene epoxidation. Au(0)-RS-SR-NH-SiO2-Fe3O4 catalyst showed a good catalytic performance in the reduction of 4-nitrophenol into 4-aminophenol in the presence of NaBH4 as the hydrogen source. The effect of three different parameters, namely volume of NaBH4, concentration of 4-nitrophenol and amount of catalyst were investigated. Under optimal conditions (0.5 mL NaBH4, 0.05 mM 4-nitrophenol and 2 mg Au-RS-SR-NH-SiO2-Fe3O4), the conversion of 4-nitrophenol were found to be 96% after 60 min. Au(0)-RS-SR-NH-SiO2-Fe3O4 catalyst also showed high reusability as the catalytic activity remained excellent after five successive runs. Meanwhile, the effects of amount of base, reactant to oxidant mole ratio, catalyst amount, solvent volume, temperature and time on the catalytic epoxidation of styrene by Au(0)-RS-SR-NH-SiO2-Fe3O4 catalyst were also investigated. Response surface methodology (RSM) demonstrated the ability to predict the conditions that favour high percentage conversion of styrene. Under the proposed optimised conditions, considering all variables in the model range, namely TBHP molar ratio of 4 and run for 6 h at 80°C, a high percentage conversion of styrene of 61.8% was attained. This catalyst could be easily recovered magnetically and reused for at least four times with satisfactory conversion. Leaching of Au from the RS-SR-NH-SiO2-Fe3O4 surface was extremely small and can be considered negligible. This study showed that RS-SR-NH-SiO2-Fe3O4 acted as dual functional material with excellent properties as adsorbent for the recovery of Au and as support for Au catalyst for the reduction 4-nitrophenol and epoxidation of styrene.