Production of highly catalytic, archaeal Pd(0) bionanoparticles using Sulfolobus tokodaii

Abstract

The thermo-acidophilic archaeon, Sulfolobus tokodaii was utilized for the production of 20 Pd(0) bionanoparticles from acidic Pd(II) solution. Use of active cells was essential to form well-21 dispersed Pd(0) nanoparticles located on the cell surface. The particle size could be manipulated 22 by modifying the concentration of formate (as electron donor; e-donor) and by addition of 23 enzymatic inhibitor (Cu 2+ ) in the range of 14-63 nm mean size. Since robust Pd(II) reduction 24 progressed in pre-grown S. tokodaii cells even in the presence of up to 500 mM Cl -, it was possible 25 to conversely utilize the effect of Clto produce even finer and denser particles in the range of 8.7-26 15 nm mean size. This effect likely resulted from the increasing stability of anionic Pd(II)-chloride 27 complex at elevated Clconcentrations, eventually allowing involvement of greater number of 28 initial Pd(0) crystal nucleation sites (enzymatic sites). The catalytic activity (evaluated based on 29 Cr(VI) reduction reaction) of Pd(0) bionanoparticles of varying particle size formed under 30 different conditions were compared. The finest Pd(0) bionanoparticles obtained at 50 mM Cl -31 (mean 8.7 nm; median 5.6 nm) exhibited the greatest specific Cr(VI) reduction rate, with 4-times 32 higher catalytic activity compared to commercial Pd/C. The potential applicability of S. tokodaii 33 cells in the recovery of highly catalytic Pd(0) nanoparticles from actual acidic chloride leachate 34 was thus suggested. 35 36 Page 2 of 34 Extremophiles important industrial catalysts. To secure a stable world supply of PGMs and other precious metals, 40 recycling of secondary metal resources (e.g., spent catalysts and e-wastes) is considered 41 increasingly important. For Pd recycling from such secondary resources, strong leaching lixiviants 42 such as aqua regia, HCl, HNO 3 and H 2 SO 4 are used together with an oxidizing agent. To lower 43 environmental impacts, cleaner alternatives, such as using diluted HCl with H 2 O 2 , are also 44 investigated by different groups (e.g., Barakat et al. 2006). Since Pd catalysts today are mostly 45 used as nanoparticles due to their greater specific surface area with higher reactivity (De Corte et 46 al. 2012), developing recycling techniques of the metal in the nanoparticle form would be 47 beneficial. 48 Microbiological production of precious metal nanoparticles is gaining increasing attention 49 as a simple and clean technology which proceeds under ambient conditions without the use of 50 hazardous chemicals (Zhang et al. 2011). Upon utilization of microbial cells, reduction of aqueous 51 Pd(II) ions is triggered by enzymatic activity by an expense of externally added e-donor (or 52 intracellular electron carriers such as NADH accumulated during pre-growth; Okibe et al. 2017). 53 They are then deposited as solid Pd(0) nanoparticles at different cellular locations as a scaffold (on 54 the cell wall, within the periplasmic space and inside cytoplasm), depending on the microbial 55 species and conditions used (De...

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