Platinum nanoparticles (Pt(0)NPs) are expected to play a vital role in future technologies as high-performance catalysts. The microbiological route for Pt(0)NPs’ production is considered a greener and simpler alternative to conventional methods. In order to explore the potential utility of extreme acidophiles, Fe(III)-reducing acidophilic bacteria, Acidocella aromatica and Acidiphilium crytpum, were tested for the production of bio-Pt(0)NPs from an acidic solution. Bio-Pt(0)NPs were successfully formed via a simple one-step reaction with the difference in the size and location between the two strains. Intact enzymatic activity was essential to exhibit the site for Pt(0) crystal nucleation, which enables the formation of well-dispersed, fine bio-Pt(0)NPs. Active Ac. aromatica cells produced the finest bio-Pt(0)NPs of mean and median size of 16.1 and 8.5 nm, respectively. The catalytic activity of bio-Pt(0)NPs was assessed using the Cr(VI) reduction reaction, which was shown to be in a negative linear correlation with the mean particle size under the conditions tested. This is the first study reporting the recruitment of acidophilic extremophiles for the production of Pt(0)NPs. Acidophilic extremophiles often inhabit metal-rich acidic liquors in nature and are expected to become the promising tool for metal nanotechnology.

While spent catalysts can cause serious environmental pollution, they can be considered an essential secondary metal source due to their high critical metal grades. The formation of the amino acid-metal complex is often seen in nature, and its potential application in hydrometallurgy can be foreseen. Alanine (Ala) was first screened as the most effective type of amino acid to be used for the selective leaching of spent hydrodesulfurization catalyst (consisting of MoS2 and Co3S4 supported on Al2O3, at 10% Mo and 2.4% Co grades). The sequential 3-step leaching (Step-1: Alkaline Ala leaching at 45°C, Step-2: Hot water leaching at 70°C, Step-3: Second alkaline Ala leaching at 45°C) was conducted where the role of Ala was found to be at least three-fold; 1) maintaining alkalinity by amino acid’s buffering capacity to assist Mo leaching, 2) selectively precipitating Co by forming Co-Ala complex with a distinctive pink color, which can readily re-dissolve in hot water to be separated from spent catalyst particles. 3) Effectively suppressing unwanted dissolution of Al throughout the reaction without needing pH control. Consequently, highly metal-selective, two separate Co-rich (<1% Mo and 79% Co dissolved, Al not detected) and Mo-rich (96% Mo, 19% Co, and 2.1% Al dissolved) leachates were obtained. This study highlighted the potential utility of amino acids as non-toxic, alternative metal lixiviant as well as a metal precipitant for selective leaching of critical metals from spent hydrodesulfurization catalyst.