| Iron oxide minerals are widely found in nature. They can be used as an electron acceptor and be trasformed to secondary minerals during anaerobic respiration of dissimilatory iron reducing bacteria. Graphene is a single layer of carbon atoms tightly packed into a two-dimensional honeycomb sp2 carbon lattice. Due to its unique physical and chemical properties, graphene has aroused people's great interests. In this study, dissimilatory reduction rate and degree of ferrihydrite, morphology and composition of resultant secondary minerals, and simultaneously biological reduction of ferrihydrite and nitrobenzene were investigated in the presence of graphene. Then the biogenic magnetite/reduced graphene oxide (bio-MRGO) nanocomposite was studied for adsorptive removal of organic dyes from aqueous solutions.Reduced graphene oxide (rGO) promoted the reduction of ferrihydrite by Shewanella oneidensis MR-1 at lower concentration (less than 1 mg/L), whereas inhibit the reduction of ferrihydrite at higher concentration (1-100 mg/L). Similar effects on microbial reduction of ferrihydrite were found with other types of graphene materials including nitrogen-doped graphene (N-rGO), Au nanoparticles decorated graphene (Au-rGO) and amino-formylimidazole modified graphene (imidazole-rGO). Suwanee River humic acid (SRHA) and Elliott soil humic acid (ESHA) were selected as representative natural organic matters to study their impacts on microbial reduction of ferrihydrite in the absence and presence of rGO. ESHA inhibited ferrihydrite reduction while SRHA promoted ferrihydrite reduction with the copresence of 0 and 0.5 mg/L rGO. The presence of 50 mg/L rGO together with ESHA or SRHA led to inhibition of ferrihydrite reduction. Riboflavin and flavin mononucleotide (FMN) were used to to study the impacts of cell-excreted organic matter on microbial reduction of ferrihydrite in the absence and presence of rGO. Flavin (Riboflavin and FMN) and 0.5 mg/L rGO could synergistically facilitate ferrihydrite reduction. However, the coexistence of flavin and 50 mg/L rGO inhibited ferrihydrite reduction. Effects of rGO on simultaneous reduction of ferrihydrite and nitrobenzene by MR-1 were investigated. Better nitrobenzene reduction was observed with the presence of higher concentrations of adsorbed Fe(II) on ferrihydrite surface. The products of microbial reduction of ferrihydrite were analyzed by XRD. Magnetite was detected after 5 d when ferrihydrite reduction was promoted. When the reduction was inhibited, the reduct was found to be goethite after 5 d, which was further transformed to magnetite in 30 d.Reduction of graphene oxide and ferrihydrite in sequence by MR-1 resulted in the generation of magnetite/rGO nanocomposite (bio-MRGO). The biogenic bio-MRGO was characterized by transmission electron microscopy, X-ray diffraction, fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and vibrating sample magnetometry. The maximum adsoptive removal rates of methylene blue, malachite green and crystal violet by bio-MRGO were determined to be 99.6%?70.7% and 91.9%, respectively. The adsorption behavior of bio-MRGO for methylene blue could be well described with Langmuir isotherm and pseudo-second-order kinetic model. Thermodynamic analysis revealed the spontaneous and endothermic nature of the adsorption process. Moreover, the bio-MRGO adsorbent could be regenerated through a Fenton-like reaction. Good performance was found with the regenerated bio-MRGO, which could still remove 60% methylene blue after five rounds of operation. |
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