Photocatalysts with adsorption property for dye-contaminated water purification

Abstract

Dye-polluted water discharged from textile, dyeing, carpet manufacturing, pulp, and paper industries must be properly treated before it is discharged to the environment because the dyes are hazardous to human beings, also can cause problems to the ecosystem. Adsorption and photocatalytic degradation of dyes are promising methods for the treatment of dye-polluted wastewater. Zinc (Zn) aluminium (Al) layered double hydroxide (ZnAl-LDH) materials, and layered double oxides (LDOs) that have semiconductor properties and anionic exchange properties can be used for the removal of anionic dyes. Correspondingly, graphitic carbon nitride (g-C3N4) is a fascinating metal-free semiconductor with synergistic adsorption property due to its polymeric p-conjugated structure, which is a promising photocatalyst in environmental remediation. Therefore, this thesis focuses on developing novel photocatalysts, which is activated under UV (ultraviolet) and/or visible light irradiation with adsorption property for dyes removal from wastewater. The studies include exploring the optimal experimental conditions for the synthesis of LDOs with the best performance in adsorption and photocatalytic degradation, modifying g-C3N4 with carbon black for tuning its adsorption selectivity and improving the photocatalytic activity, and constructing novel inorganic-organic heterogeneous semiconductor composites (ZnAl-LDH@C3N4) with desired adsorption and photodegradation performance toward both cationic and anionic dyes. The relationship between structures, physicochemical properties of photocatalysts and dye removal performances has also been well studied in the thesis. The first part of the experimental chapters focuses on the zinc aluminium layered double hydroxide based photocatalysts. ZnAl-layered double oxide composites (LDOs) were developed to remove anionic dye Orange II sodium salt (OrgII). Synthetic parameters including the molar ratio of Zn to Al, and calcination temperature were adjusted to study the optimal synthetic condition. The relationship between the structural features and the adsorption properties and photocatalytic activity of LDOs was thoroughly investigated. The second part of the experimental chapter focuses on the modification of g-C3N4. Series of carbon black (CB) modified g-C3N4 samples with efficient adsorption and photocatalytic activity were prepared by heating the mixtures of CB and urea. The adsorption and photocatalytic activities were evaluated by removing both cationic dye methylene blue (MB) and anionic dye OrgII. CB modified g-C3N4 exhibited higher photocatalytic activities than pristine g-C3N4. CB worked as both dopant and reactional site during the polycondensation of urea, which was supposed to increase the crystallisation and condensation degree of g-C3N4. The existing of CB in the g-C3N4 matrix not only enhanced the light absorption for g-C3N4 but also quenched the recombination of charge carriers. Therefore, modified g-C3N4 samples exhibited improved performance for dyesr removal. The third part of the experimental chapters focuses on the composite between inorganic photocatalyst ZnO-LDH and organic semiconductor g-C3N4. A ZnO-layered double hydroxide@graphitic carbon nitride composite (ZnO-LDH@C3N4) was synthesised via co-precipitation method with solvothermal treatment. The ZnO-LDH@C3N4 composite displayed superior performance in both adsorption and photocatalytic degradation of the MB and OrgII, comparing to ZnO-LDH and g-C3N4 individually. For OrgII, ZnO-LDH@C3N4 showed higher adsorption capacity with three synergetic steps including electrostatic and p-p conjugation adsorption followed by ion exchange. For MB, ZnO-LDH@C3N4 exhibited substantial adsorption and high photocatalytic degradation rates under UV and visible light irradiation. The enhanced performance in photocatalytic degradation of MB was induced by the high separation efficiency of photogenerated charges, attributing to the novel inorganic-organic heterogeneous structure of this composite. Sum up, the thesis presents the key findings in this work and raises the perspective for the future research direction.