Abstract

As a semiconducting material with outstanding properties, ZnO has been playing a key role in the chemical industry, light industry, ceramics, electronics, national defence, healthcare and other hightech industries. In recent years, great efforts have been made in studies of ZnO as photocatalyst in the treatment of organic wastewater by virtue of its high reaction rate, excellent applicability and environmentally friendly products. The metal-organic frameworks (MOFs) exhibit large specific surface area and good stability, and calcination of zeolitic imidazolate framework (ZIF)-8 in atmosphere leads to generation of ZnO@MOF. With relatively large specific surface areas, ZnO nanoparticles can generate a large quantity of hydroxyl free radicals under visible light and these radicals can nonselectively oxidise most organics (strong organic-oxidising capability). ZIF-8 was synthesised using the hydrothermal method and calcinated at different temperatures to obtain ZnO nanoparticles of different sizes. Then, the photocatalytic performances of these ZnO nanoparticles were tested by degradation of methylene blue (MB) under visible light. Additionally, the ZnO samples were applied in photocatalytic degradation of dyeing and printing wastewater produced by leather retanning. The chemical oxygen demand (COD) removal rate and the total organic carbon (TOC) mineralisation rate exceeded 70% and 35%, respectively.

Tannery wastewater refers to the industrial wastewater discharged by tannery yards. It contains considerable organics and non-degradable dyes, which increase the oxygen consumption of water. Indeed, organics and non-degradable dyes are severe environmental hazards as their treatments (to meet the emission standards) are extremely challenging. Current treatment methods include physicochemical treatment, biochemical treatment and combined method. However, these methods have their respective limitations in treatment of organic pollutants. Recently, studies of semiconductors as photocatalysts for degradation of various pollutants have attracted great attention both in China and globally.

As a novel nanomaterial, metal-organic frameworks (MOF) are characterised by large specific surface area, adjustable rich porosity, high density of active sites. As a key N-type semiconductor, ZnO has been widely applied in various areas, including environmental protection, optics, electrics and catalysis. With a bandgap of 3.2eV, ZnO can only absorb UV light (with wavelength below 380 nm) in theory, while absorbance of visible light by ZnO is supposed to be extremely low. Hence, catalytic oxidising degradations of wastewater with ZnO as photocatalyst shall be under UV light, which limits their industrial applications. To enhance the photocatalytic performance of ZnO under visible light, researchers have tried several methods, including noble metal deposition, semiconductor composition, non-metallic doping, metal ion doping and carbon loading.

In this study, ZIF-8 (precursor) was synthesised using a hydrothermal method and calcined at different temperatures to obtain ZnO nanoparticles with different sizes. Characterisation by XRD, SEM, TEM and EPR revealed that calcined ZnO samples have large specific surface areas and rich porosity. The photocatalytic performances of different ZnO nanoparticles were evaluated by degradation of methylene blue (MB) under visible light to investigate the effects of morphology, crystal face, surface free radicals, pH and particle size of the ZnO and illumination time on their photocatalytic performances. Additionally, ZnO was employed as a photocatalyst in degradation of dyeing and printing wastewater produced by leather retanning under visible light, and the chemical oxygen demand (COD) removal rates and the total organic carbon (TOC) mineralisation rates were calculated.