Biochar is becoming a low-cost substitute of activated carbon for the removal of multiple contaminants. In this study, five biochar samples derived from pine sawdust were produced at different pyrolysis temperatures (300 °C-700 °C) and used adsorbents to remove p-nitrophenol from water. Results indicate that, as the pyrolysis temperature increases, the surface structure of biochar grows in complexity, biochar's aromaticity and number of functional group decrease, and this material's polarity increases. Biochar's physiochemical characteristics and dosage, as well as solution's pH and environmental temperature significantly influence the p-nitrophenol adsorption behavior of biochar. p-nitrophenol adsorption onto biochar proved to be an endothermic and spontaneous process; furthermore, a greater energy exchange was observed to take place when biochar samples prepared at high temperatures were utilized. the adsorption mechanism includes physical adsorption and chemisorption, whereas its rate is mainly affected by intra-particle diffusion. Notably, in biochar samples prepared at low temperature, adsorption is mainly driven by electrostatic interactions, whereas, in their high-temperature counterparts, p-nitrophenol adsorption is driven also by hydrogen bonding and π-π interactions involving functional groups on the biochar surface.

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BACKGROUND Graphene‐supported nanocomposites are promising catalysts for their unique properties. Biosynthesis of such hybrid catalysts possesses several advantages over other methods such as less pollution. The palladium nanoparticle has versatile catalytic activities and its stability and activities can be significantly improved through hybridization with other nanomaterials including graphene. RESULTS In this study, bimetallic palladium‐silver nanoparticles supported on graphene (Pd‐Ag/rGO) was prepared through one‐step biosynthesis using a bacterium Shewanella oneidensis MR‐1. Shewanella oneidensis MR‐1 synthesized Pd‐Ag/rGO through simultaneous reduction of silver nitrate (AgNO3), palladium(II) nitrate (Pd(NO3)2) and graphene oxide (GO). The morphology and composition of the Pd‐Ag/rGO were characterized by transmission electron microscopy, energy dispersive X‐ray, X‐ray diffraction, X‐ray photoelectron spectroscopy, Fourier‐transform infrared spectroscopy and Raman spectroscopy. Furthermore, the hydrogenation activity of synthesized Pd‐Ag/rGO was tested using 4‐nitrophenol as a model chemical at room temperatures and pressure. The Pd‐Ag/rGO showed the best catalytic performance when Pd/Ag ratio was 1:1 and bacterial concentration was 1.0 of OD600 (optical density measured at a wavelength of 600 nm) for the preparation of Pd‐Ag/rGO. Therefore, the kinetic rate constant for 4‐NP reduction was 0.2413 min−1 catalyzed by as‐prepared Pd‐Ag/rGO, which is listed as one of the top level reported catalysts for 4‐NP reduction. CONCLUSION Therefore, this study demonstrated a material and energy‐saving method to synthesize hybrid nanoparticles supported on graphene with high catalytic activity. © 2019 Society of Chemical Industry

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