| Polyferric sulfate?PFS? is been widely used for treatment of Ni-contaminated wastewater,giving rise to the formation of inorganic sludge laden with Ni and Fe,a hazardous solid waste that requires safe treatment and disposal.Further treamtment of heavy metal-contained sludge is complicated.The commonly used technologies include solidification/stabilization,sanitary landfill,and the resource-based treatment technologies such as pyrometallurgical process,wet process,ferrite process,which require large storage area and high cost,and induce sencondary pollutaion if not well handled.Accordingly,it is important to explore and develop environment-friendly strategies for treatment of heavy metal-contained sludge.Composition analysis reveals that the PFS sludge contains not only high levels of Ni and Fe,but also large amounts of carbon-containing organic matter.It is expectable that pyrolysis of this sludge is conducive to the formation of a heteroatom-doped carbon material,which can find applications in electrocatalysis for oxygen evolution reaction?OER?of electrolyzed water.Taking into account the complicated composition of sludge,how to tune the elemental distribution and the intereaction between atoms in the sludge represents a challenge before the utilization of the obtained carbon as a promising OER electrocatalyst.To address the issue,we adopted a biological acclimation method to obtain the desirable bacteria@mineral precursor by using the dissimilated iron reducing bacteria,Shewanella oneidensis MR-1 to reduce Ni-containing PFS flocs.The heteroatom-doped porous carbon was then synthesized upon the one-step pyrolysis procedure.The effects of the interaction between microorganisms and inorganic minerals,the treatment temperature,and the Fe:Ni weight ratio on the morphology and structure of the resulting materials were investigated.The results show that the strong affinity between microorganisms and minerals is conducive to attaining a carbon material with a larger specific surface area,a higher degree of graphitization,and more dispersed nano-particles?Fe Ni2P?.The increasing temperature results in increases in the graphitization level and the specific surface area;however,when the temperature is too high,the carbon layer structure collapses,the metal particles agglomerate,and the specific surface area decreases.An appropriate Ni content is beneficial to producing the desirable microstructure and chemical composition,but the excessive Ni content has a negative impact on the growth of microorganisms,which affects the process of iron bioreduction and leads to the sintering of the active components of the subsequent material.Comparisons of the electrocatalytic OER performance between the materials synthesized under different conditions show that the mesoporous heteroatom-doped carbon-supported Ni Fe phosphide?Fe Ni2P@HDC?dervived from the biogenic precursor at 800°C exhibits the best activity.Tested in an electrolyte of 1.0 M KOH for OER,the as-synthesized Fe Ni2P@HDC requires a low overpotential of 280 m V to reach a current density of 10 m A cm-2and a small Tafel slope of 56 m V dec-1,which is comparable to that of the majority of the reported Ni and/or Fe phosphide catalysts in the literature and better than the state-of-the art Ru O2 catalyst.Such a good performance seems to be the result of the synergistic effect of the intrinsic activity of Fe Ni2P nanoparticles and the strong interaction between Fe Ni2P and heteroatom-doped graphene-like carbon support.Further study on the electrocatalytic mechanism indicated that during the OER process,a thin layer of MO/MOOH?M=Ni,Fe?is in situ produced on the surfaces of Fe Ni2P particles,forming a core-shell structure of metal phosphide@metal oxide/hydroxide,and the metal oxide/hydroxides on the surface serve as the active site for catalyzing OER.These findings suggest a promising new approach for resource recovery from treating the heavy metal-contained PFS sludge. |
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