| With rapid development of industrial production, huge amount of wastewaters containing lots of hazardous non-biodegradable pollutants were generated. These recalcitrant contaminants are normally chemical stable and difficult to be biodegraded because they can pose toxic effect to microorganisms. Untreated emissions of industrial sewage will definitely cause surface water, groundwater and also soils pollution by those recalcitrant contaminants, which may persistent in environment for long period and bring serious risk to ecosystem and human health. Therefore, it should be urgent to develop some safe and green materials with high efficiency and strong reactivity for treatment of non-biodegradable wastewater.Iron-based materials are abundant in nature environment, and have advantages of easy-available, cost-effective, chemical-stable and environmental-friendly. In recent decades, iron materials, such as iron oxides and zero-valent iron (Fe0), have attracted increasing interest and been wildly used for environmental remediation. For practical application, iron-based materials can be used as Fenton-like catalysts for H2O2 and persulfate activation to produce strong reactive radicals for pollutants degradation. In addition, they could be prepared for adsorption of heavy metal ions and also organic contaminants. Some materials like micro or nano scale F0 have strong reducibility, and can be applied for removal of Cr(Ⅵ), Pb(Ⅱ), Hg(Ⅱ) and reductive degradation of chlorinated organic pollutants as well. Besides, iron materials could be facilely recycled after wastewater treatment by exerting magnetic field due to their good magnetism, thus reducing post-treatment cost. As a result, exploration of reactive, green and easy-available iron-based materials has become an attractive topic for environmental science and engineering research.The present dissertation mainly focused on developing highly reactive iron-based materials via inorganic modification with simple preparation, low energy consumption and good application potential. First of all, the work investigated the preparation of porous sulfur-modified iron oxides for catalyzing H2O2 and persulfate for pollutants degradation. Then, a kind of FeS coated Fe0 was synthesized by sulfide modification for reductive removal of Cr(Ⅵ) from aqueous solution. Finally, we combined the Mn species with iron oxides to explore their synergistic effect on PMS activation for BPA destruction. Each part will investigate the working mechanisms of these novel iron-based materials, and propose the degradation pathways for pollutants degradation.The major works are described as following:(1) This work presents the use of S/Fe composites as heterogeneous PDS activator. The physical-chemical properties of S/Fe composites were characterized by SEM, XRD and XPS. The S/Fe particles appeared as small rod-like or spindle-like clusters, and γ-Fe2O3, α-Fe2O3, iron sulfate hydrate, and FeS2 coexisted in the composites. Compared to the bare iron oxide Fe-300, the S/Fe composites showed greatly improved performance for PDS activation for RhB degradation. Increasing the dosage of S/Fe or PDS resulted in increased oxidation efficiency. In addition, neutral pH was more favorable for the degradation experiments, and different pH conditions could significantly influence the radical species. More sulfate radicals were generated in neutral solutions and more hydroxyl radicals were produced in basic solutions. To test the role of S-modification, thiosulfate was additionally introduced to the Fe-300/PDS system, and a significant promotion of RhB removal was observed. It was proposed that S-modification could accelerate electron transfer between iron species and PDS. Overall, this new material may stimulate interests for iron oxides with sulfide modification for environmental applications.(2) A mesoporous sulfur-modified iron oxide (MS-Fe) was prepared as a heterogeneous H2O2 catalyst for degradation of BPA. The physico-chemical properties of MS-Fe and bare M-Fe were characterized by various characterizations. Both M-Fe and MS-Fe composites appeared as cubic microparticles with abundant pores and cracks as well as large surface area. In contrast to the poor catalytic activity of bare M-Fe, the MS-Fe composites showed greatly improved efficiencies for H2O2 activation for BPA degradation. The high catalytic activity of this new Fenton-like catalyst can be obtained at different initial pH in range of 3.0～9.0. The time evolution of degradation of BPA followed pseudo-first-order kinetics, and the first-order rate constants showed a linear relationship with parameters of initial pH, catalyst dosage and concentration of BPA. However, the H2O2 dosage showed a dual effect on BPA degradation because excessive H2O2 addition lead to scavenging of hydroxyl radicals (OH·). The investigation of working mechanisms of MS-Fe suggested a synergistic effect of homogeneous and heterogeneous degradation reaction, wherein a strong acidic environment, abundant surface-bonded hydroxyl group and electron-mediating effect of sulfur all contributed to fast activation of H2O2. Intermediates of BPA, including 4-isopropenyl phenol, p-benzoquinone, p-methoxystyrene, oxalic acid, were detected via GC-MS analyses. The time-dependent variation of HPLC diagrams showed that these BPA byproducts were accumulated and further decomposed.(3) A magnetic FeS@Fe0 hybrid material was assembled via wrapping Fe0 particles by in situ formed amorphous iron monosulfide. The synthesized FeS@Fe0 has a lower BET surface area but a higher reactivity towards Cr(VI) sequestration than individual nFe0 particles. The reactivity of FeS@Fe0 particles varies with the thickness of FeS cover in terms of different FeS-to-Fe0 molar ratio. The highest Cr(VI) removal efficiency was achieved with the FeS-to-Fe0 molar ratio of 1/9, and further increasing or decreasing the FeS amount played negtive roles. The FeS layer functioned as a condutive channel for electron transmission, which could accelerate the multi-electron transfer from the Fe0 core to Cr(VI) oxyanions and prevent their rapid passivation by iron oxides. Correspondingly, solution pH rise and O2 exposure would significantly inhibited the reduction of Cr(VI). Surface Fe(II), direct electron transfer and reactive sulfide species were validated as the main electron donors for Cr(VI) reduction, which mainly occurred on the solid-liquid interface. Finally, physical characterization of the fresh and Cr-treated FeS@Fe0 particles confirmed that Cr(VI) was converted to Cr(III) and completely immobilized in the solid phase of Fe-Cr hydroxides.(4) A magnetic catalyst was synthesized by immobilizing Mn3O4 onto Fe3O4-GO particles. The Mn-Fe-GO catalyst was used for catalytic PMS oxidation of BPA. The Mn-Fe-GO catalyst exhibited high efficiencies for activating PMS to generate sulfate radicals for BPA degradation. This degradation activity could be enhanced with increasing catalyst dose as well as initial pH of solution. In addition, this magnetic hybrid catalyst exhibited excellent long-term stability with low dissolved metal ions especially at initial neutral or alkaline conditions. The catalytic PMS oxidation of BPA occurred heterogeneously on Mn-Fe-GO surface, and the homogeneous reaction was negligible. On the basis of LC-MS and GC-MS characterization, the decomposition of BPA could proceed via C-C scission between isopropyl and benzene rings, multihydroxylation of the aromatic ring, and also compounds coupling. On basis of XPS characterization, a possible reaction mechanism of catalytic PMS activation involved redox recycle among MnO, Mn2O3 and MnO2, and decomposition of PMS to generate SO4-·, OH· and SO5-·.(5) A porous Fe-Mn catalyst with good magnetism was synthesized by thermal treatment of Fe-Mn oxalate precursor. Compared to individual Mn or Fe oxides, porous Fe-Mn oxides exhibited a high activity in terms of PMS activation for BPA degradation. It was found that 0.1 mmol/L BPAcould be rapidly degraded in 30 min reaction with addition of 2.0 mmol/L PMS and 0.2 g/L Fe-Mn catalyst, and the degradation yield a TOC removal of 65% The reaction efficiency could be enhanced with increased catalyst dose, initial solution pH and solution temperature. The overall activation energy for Fe-Mn catalyst was calculated to be 11.6 kJ/mol accordingt to Arrhenius formula, indicating a good catalytic efficiency of porous Fe-Mn catalyst. The catalyst can be reused in successive runs but the metal leaching and oxides fouling may lead to partial deactivation. The XPS result proved that the redox pairs of Mn(Ⅱ)/Mn(Ⅲ), Mn(Ⅲ)/Mn(Ⅳ) and Fe(Ⅱ)/Fe(Ⅲ) were involved in activation of PMS for BPA degradation. |
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