Electron transfer involved in bio-Pd (0) synthesis by Citrobacter freundii at different growth phases.

Gram-negative Citrobacter freundii with high Pd (II) reduction capacity was isolated from electroplating wastewater, and the electron transfer involved in Pd (II) bio-reduction by C. freundii JH was investigated in phosphate buffer saline solution with sodium formate as sole electron donor under anaerobic condition. FTIR spectra indicated that hydroxyl and amine groups on cell wall participated Pd (II) bio-sorption. TEM, XRD, XPS results confirmed that Pd (0) nanoparticles (NPs) could be bio-synthesized intra/extracellularly. Meanwhile, pH turn-over were observed owing to the reduction of cytochrome c (c-Cyt) in bio-reduction process. EPR spectra indicated that free radicals (OH) was generated from high concentration Pd (II), which would cause seriously damage to cell. Despite of the lower tolerance to Pd (II), the cells at logarithmic phase exhibited higher Pd (II) reduction capacity (72.21%) than that at stationary phase (56.21%), which might be related to the relatively stronger proton motive force (PMF) created by the substrate oxidation and the electron transfer, as evidenced by electrochemical experiments (CV, DPV, amperometric I-t curves) and protein denaturalization experiments. Additionally, c-Cyt and riboflavin were confirmed to be important participants in electron transfer. Finally, a putative synthesis mechanism of Pd (0)-NPs was deduced. This study contributed to further understanding the electron transfer in Pd (II) reduction, and provided more information for the bio-synthetic of metal nanoparticles.

Enhanced degradation of triclosan by cobalt manganese spinel-type oxide activated peroxymonosulfate oxidation process via sulfate radicals and singlet oxygen: Mechanisms and intermediates identification.

Spinel is a kind of desirable catalyst to activate peroxymonosulfate (PMS) for chemical oxidation of organic contaminants in wastewater treatment. However, apart from classic sulfate radical based AOPs (SR-AOPs), the generation and oxidative pathways of singlet oxygen (1O2) by Co/Mn spinels have been little explored in PMS catalysis. In this study, spinel-type oxide Co2Mn1O4 was successfully synthesized, and used as highly effective catalyst in PMS activation for heterogeneous degradation of TCS (up to 96.4% within 30 min) at initial pH of 6.8, which was also slightly impacted by coexisting ions. Based on radical scavengers and electron paramagnetic resonance (EPR) experiments, sulfate radicals and singlet oxygen (1O2) were unveiled to be the dominant reactive oxygen species (ROS) in Co2Mn1O4/PMS system. Co2Mn1O4 catalyst exhibited reversible redox properties based on the results of cyclic voltammetry (CV). More importantly, the generation of 1O2 might not only promote the TCS removal rate directly, but also facilitate the metal redox cycle in spinel structure in Co2Mn1O4/PMS system. Finally, degradation pathways of TCS in Co2Mn1O4/PMS system were proposed, which involved the breakage of ether bond and cycloaddition reaction.