Modification Of Conjugated Polymer Photocatalysts For Removal Of Chromium (Ⅵ) From Wastewater

The long-term development of human society and sustainability of the earth’s environment have been threatened by the serious environmental pollution.Among the pollution,the heavy metal ion pollution in water was serious and directly affected the human life and health and the restoration of the ecosystem due to the bioaccumulation toxicity and the irreversibly damage of biological organs.Hexavalent chromium（Cr（Ⅵ））widely existed in the trade waste of tanning,printing and dyeing,electroplating,iron and steel,non-ferrous smelting,and petrochemical industries.Cr（Ⅵ）is a common and highly toxic heavy metal ion in water pollution.According to the environmental statistics yearbook of China,about 180 million tons of the trade waste containing chromium were discharged from the industry of tanning.Therefore,an ideal technology for remedying environmental pollution was very urgent which could quickly and effectively remove or decrease the aqueous Cr（Ⅵ）pollution.Remedying the aqueous Cr（Ⅵ）pollution indeed has social significance to environmental protection and human health.Photoreduction technology is an environmental,green,and promising technology for remedying environmental pollution which could photo-reduce the highly toxic and migratory Cr（VI）to nontoxic and sedimentary Cr（III）.Recently,conjugated polymer photocatalysts were widely studied in remedying the aqueous Cr（Ⅵ）pollution because of the facilitated adjustment of the molecular and energy band structure,cheap source,and thermal/optical stability.Nevertheless,there are still many restrictive factors during the photoreduction of aqueous Cr（VI）.Such as the low migration of photon-generated carriers,high recombination of photon-generated electrons and holes,and narrow range of photoexcitation（especially in the visible light area）.To solve the above problems,the key research contents of the doctoral dissertation were carried out from the following five aspects.（1）A ternary metal/non-metal nanomaterial was constructed which could synergize the Z-scheme heterojunction and the plasmonic metal/semiconductor for enhancing visible light photoreduction of aqueous Cr（VI）.The ternary metal/non-metal nanomaterial was composed of g-C₃N₄ nanosheets mutual modified by Ag nanoparticles and Bi₄O₇.By the reasons of the Z-scheme heterojunction of g-C₃N₄ and Bi₄O₇ and the local surface plasmon effect of Ag nanoparticles,not only the visible light photoexcitation and the separation of photon-generated carriers were enhanced,but also the photoreduction activity of aqueous Cr（VI）by the ternary Ag/Bi₄O₇/g-C₃N₄ nanomaterial was strengthened compared with the bulk g-C₃N₄,g-C₃N₄ nanosheets,and Bi₄O₇/g-C₃N₄ nanocomposites.（2）During the thermal treatment,the potassium ion would break the C_2N-NH_x bond of heptazine units of g-C₃N₄,and introduce the cyano group（-C≡N）at the apex of the heptazine units of g-C₃N₄.Further,the Ag nanowires were loaded on the surface of cyano group modulated g-C₃N₄.Owning to the introduced cyano group in the heptazine units of g-C₃N₄,the recombination of photon-generated electrons and holes was restrained,the migration of photon-generated carriers accelerated,and the visible light photoexcitation was enhanced.Simultaneously,the migration and separation of photon-generated carriers were further boosted by the Ag nanowires which possessed a forceful local surface plasmon effect.Hence,the cyano group modulated g-C₃N₄ assisted by silver nanowire exhibited a higher photoreduction activity of aqueous Cr（VI）compared with the bulk g-C₃N₄,and the visible light photocatalytic reaction rate constant of Cr（VI）was about 14 times than that of the bulk g-C₃N₄.（3）Because of the amidoxime group possessed an intense chelation with Cr（VI）and easily obtained by a simple reaction of cyano group and hydroxylammonium chloride,the cyano group modulated g-C₃N₄ was further modified,and an in-site amidoxime functionalized g-C₃N₄ was constructed.Because of the amidoxime group introduced in the heptazine units of g-C₃N₄,the visible light photoexcitation was enhanced apparently,the energy band gap was narrowed,and the recombination of photon-generated electrons and holes was restrained.Benefited from the strong adsorption of Cr（VI）by the amidoxime group and the boosted photocatalytic activity,the in-site amidoxime functionalized g-C₃N₄ displayed higher removal efficiency of Cr（VI）than g-C₃N₄nanosheets.（4）A pyridine-based imprinted conjugated polymer was prepared by a bulk polymerization used4-vinyl pyridine as the selected functional monomer.The content ratio of pyridine aromatic nucleus in the pyridine-based imprinted conjugated polymer was modulated by changing the added quality of4-vinyl pyridine during the bulk polymerization.On account of the protonation of the nitrogen in the pyridine-based imprinted conjugated polymer,the oxyacid anion of Cr（VI）could be electrostatic adsorbed to the pyridine-based imprinted conjugated polymer.That is the pyridine-based imprinted conjugated polymer possessed an admirable adsorption capacity of Cr（VI）.In addition,owing to theπconjugated system of pyridine aromatic nucleus,the pyridine-based imprinted conjugated polymer exhibited satisfactory visible light photoreduction activity of aqueous Cr（VI）and photocatalytic oxidation activities of methylthionine chloride and tetracycline hydrochloride.（5）The morphology nanocrystallization of the pyridine-based imprinted conjugated polymer was implemented through wrapped the pyridine-based imprinted conjugated polymer on a 3D flower-like TiO₂ which was used as the nano substrate.Not only the pyridine-based imprinted conjugated polymer wrapped flower-like TiO₂ displayed strong adsorption and enrichment capacity of Cr（VI）,but also the photoreduction activity of aqueous Cr（VI）was improved significantly.The enhanced removal efficiency of aqueous Cr（VI）was derived from two aspects.One is the utilization efficiency of the photon-generated carriers was increased by the enriched Cr（VI）,the other is the recombination of photon-generated electrons and holes was restrained by the organic-inorganic heterojunction.