| Chromium is a ubiquitous element in nature and mankind.It is necessary to the mankind health.In solution,chromium is found in two stable oxidation states including Cr???and Cr???.The toxicity of Cr???is much more than that of the Cr???.Furthermore,it is a potent carcinogenic and mutagenic element.So the treatment of Cr???got more and more attention in natural water.Zero-valent iron?ZVI?has the characteristics of chemical active and high reducibility.However,ZVI is prone passivation in aqueous solution.In this study,ZVI/Fe3O4/Fe2+were successfully introduced and employed to remove Cr???.Among the iron oxides and hydroxides,goethite??-FeOOH?and akaganeite??-FeOOH?have more stable structure and chemical properties.However,the?-FeOOH and?-FeOOH have disadvantages of easy aggregation.In this paper,immobilization of?-FeOOH and?-FeOOH on carbon microspheres?CMSs?to form?-FeOOH/CMSs and?-FeOOH/CMSs composites was employed to remove Cr???.In this paper,the influencing factors for the removal of Cr???by the ZVI/Fe3O4/Fe2+system,?-FeOOH/CMSs,and?-FeOOH/CMSs were studied.The characterization of simples were examined by scanning electron microscopy?SEM?,transmission electron microscopy?TEM?,Fourier transform infrared spectroscopy?FTIR?,X-ray diffraction?XRD?,the specific surface area analyzer?BET?and X-ray photoelectron spectroscopy?XPS?.?1?ZVI/Fe3O4/Fe2+system had better removal efficiency for Cr???than one-component and two-component systems.The removal efficiencies of Cr???increased with the increasing Fe2+concentration and with the decreasing pH.The removal efficiencies of Cr???approached 100%in the optimum condition.Fe???and Fe???generation–consumption–regeneration cycle in the ZVI/Fe3O4/Fe2+system was found in acidic condition.?2?The removal of Cr???by ZVI/Fe3O4/Fe2+is a complex process including surface adsorption and redox.The removal mechanism of Cr???by ZVI/Fe3O4/Fe2+is as follows:Cr???adsorption by magnetite and ZVI;Cr???direct reduction by ZVI;Cr???reduction by the Fe2+;and reduction of Fe3+to Fe2+by ZVI.In ZVI/Fe3O4/Fe2+system,ZVI was the electron donor.Fe2+overcame the passivation of ZVI.Magnetite served as reactive interface for Cr???reduction and semiconductor for electron transfer.?3??-FeOOH/CMSs had better removal efficiency for Cr???than?-FeOOH/CMSs.The removal efficiencies of Cr???by?-FeOOH/CMSs were all over 90.0%in the pH range from 3.0 to 9.0.The removal efficiency of Cr???by?-FeOOH/CMSs decreased from 85.9%to 44.8%as the solution pH increased from 3.0 to 11.0.The maximum adsorption capacities of Cr???by?-FeOOH/CMSs and?-FeOOH/CMSs composites are 55.37 and 29.37?mg/g,respectively.The coexisting Cl-,Cu2+,and Ni2+had no significant effect on Cr???removal,whereas SO42-and PO43-hindered the removal of Cr???.The adsorption processes of Cr???onto?-FeOOH/CMSs and?-FeOOH/CMSs composites are spontaneous and endothermic.The removal efficiency of Cr???by regenerated?-FeOOH/CMSs still maintained 86.1%in the sixth cycle,while the removal efficiency of Cr???by the regenerated?-FeOOH/CMSs decreased from 85.9%to 52.9%in six cycles.?4?The adsorption mechanism of Cr???by?-FeOOH/CMSs and?-FeOOH/CMSs composites is as follows.Adsorption of Cr???by hydrogen bonding,adsorption of Cr???by electrostatic interaction,reduction of Cr???to Cr???by hydroxyl groups of CMSs surface,reduction of Fe???to Fe???by hydroxyl groups of CMSs surface,and reduction of Cr???to Cr???by Fe???. |
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