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Biocompatible materials and biocarriers have attracted great attention in biological wastewater treatment owing to their excellent performance in improving pollutant removal. Graphene-based material, a biocarrier candidate, with excellent adsorbability and conductivity was increasingly applied in anaerobic digestion due to its exceptional potential in the adsorption and electron transfer process. Nevertheless, the green approach for the formation of bio-graphene complexes and their mechanism in dye removal is limited. The aim of this study is to investigate and assess the performance of biological graphene hydrogel (BGH) formed by Shewanella putrefaciens CN32 on the removal of methyl orange (MO) and methylene blue (MB). The results showed that the formation of BGH is determined by the physicochemical characteristics of graphene oxide, including sheet size, oxidation degree, and interlayer distance. BGHs significantly increased the removal efficiency of dyes in comparison to non-graphene samples, with a 24-h removal rate of MO and MB reaching 92.9% and 91%, respectively. The synergetic mechanism of BGH on the enhanced removal rate of organic dye can be ascribed to GO's ability in accelerating extracellular electron transfer and stimulating biodegradation pathways relating to c -type cytochromes, including MtrA and MtrC. These findings provided an understanding of the relationship between graphene-based nanomaterials and Shewanella , which facilitated their future application in environmental biotechnology. [Display omitted] • The reduction of GO by S. putrefaciens CN32 prompt BGH generation. • Sheet area and degree of defects of GO determine the BGH formation. • Bacteria-immobilized rGO hydrogel displayed a synergistic effect in bioremediation. • BGH increased dye removal efficiency by accelerating extracellular electron transfer. [ABSTRACT FROM AUTHOR]

Abstract: The interaction of metalloproteins with oxides has implications not only for bioanalytical systems and biosensors but also in the areas of biomimetic photovoltaic devices, bioremediation, and bacterial metal reduction. Here, we investigate mitochondrial ferricytochrome c (Cyt c) co-sorption with 0.01 and 0.1 M phosphate on hematite (α-Fe2O3) surfaces as a function of pH (2–11). Although Cyt c sorption to hematite in the presence of phosphate is consistent with electrostatic attraction, other forces act upon Cyt c as well. The occurrence of multilayer adsorption, and our AFM observations, suggest that Cyt c aggregates as the pH approaches the Cyt c isoelectric point. In solution, methionine coordination of heme Fe occurs only between pH 3 and 7, but in the presence of phosphate this coordination is retained up to pH 10. Electrochemical evidence for the presence of native Cyt c occurs down to pH 3 and up to pH 10 in the absence of phosphate, and this range is extended to pH 2 and 11 in the presence of phosphate. Cyt c that initially adsorbs to a hematite surface may undergo conformation change and coat the surface with unfolded protein such that subsequently adsorbing protein is more likely to retain the native conformational state. AFM provides evidence for rapid sorption kinetics for Cyt c co-sorbed with 0.01 or 0.1 M phosphate. Cyt c co-sorbed with 0.01 M phosphate appears to unfold on the surface of hematite while Cyt c co-sorbed with 0.1 M phosphate possibly retains native conformation due to aggregation. [Copyright &y& Elsevier]