Development of novel integrated photocatalytic adsorbents (IPCAs) for organics removal from water & wastewater
Gholamvand, Zahra (2016) Development of novel integrated photocatalytic adsorbents (IPCAs) for organics removal from water & wastewater. PhD thesis, Dublin City University.
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The presence of pharmaceutical compounds in the aquatic environment and their possible effects on living organisms has emerged as a serious environmental concern particularly as conventional wastewater treatments such as adsorption, ozonation, UV and bio-degradation can only partially remove them. Among various treatment technologies, considerable attention has been paid to semiconductor photocatalysis due to complete degradation of organic compounds, minimization of waste disposal and energy consumption and cost reduction. Titanium dioxide (TiO2) is the most widely studied semiconductor photocatalyst for water treatment. However, there are few problems associated with TiO2 photocatalysts such as UV light requirement due to wide band gap, poor photocatalytic efficiency due to high recombination rate of photo-generated electron-hole pares, particle aggregation, small porosity and specific surface area, expensive recovery and reuse which impede its application as an industrial wastewater treatment process. Graphene, the most recent member of carbonaceous nanomaterials has been extensively studied for wide variety of applications due to its unique electronic, mechanical and thermal properties. High surface area exfoliated graphene sheets with tuneable surface chemistry is an attractive two-dimensional platform for anchoring various type of metallic and semiconducting materials in order to enhance their performance for catalysis, sensing, electronic, optoelectronic, thermoelectric, energy conversion and storage application. This research has focused on the synthesis and modification of TiO2 nanomaterials with graphene to overcome TiO2 disadvantages and modify TiO2 photocatalysts with a primary objective of enhancing its photocatalytic performance under UV light irradiation and developing visible-light responsive TiO2-based photocatalysts. For this purpose different TiO2 nanostructures with different dimensionality (nanoparticle, nanotube, nanofiber and beads) were modified with graphene oxide via solution mixing, sol-gel and hydrothermal methods. The result were compared with commercially available Degussa P25 TiO2 particles as a bench mark. Few pharmaceutical model compounds (diclofenac, carbamazepine, famotidine etc.) were used in order to study photocatalytic efficiency of these composites and degradation kinetics of drugs under UV and visible light using HPLC. Promising results were obtained proving the enhancement of UV and visible light photocatalytic activity of graphene incorporated composite comparing to pure TiO2 nanostructures. In synthesis procedure graphene acts like a structure directing agent. For example more uniform TiO2 nanoparticles with smaller crystallite size, higher visible absorption, higher surface area and higher porosity can be synthesised in presence Development of novel integrated photocatalytic adsorbents (IPCAs) for organics removal from water & wastewater Zahra Gholamvand ii of graphene oxide compared to similar materials without graphene. It was found that TiO2 nanotubes with diameters less than 15 nm immobilised on the surface of the graphene showed the highest photocatalytic activity under both UV and visible light as a result of mesoporous structure of nanotubes and facilitated charge transfer between one-dimensional TiO2 and two dimensional graphene sheets. We found that hydrothermal process is an efficient method to produce graphene/TiO2 composites from graphene oxide since crystallisation of TiO2 and reduction of graphene oxide can be done in one pot without need for post annealing or reduction treatments. Analytical techniques such as FTIR, RAMAN and UV-Vis spectroscopy, AFM, SEM and TEM microscopy, XRD, contact angle, four point probe conductivity measurements and BET had revealed that high adsorption and photocatalytic activity is related to graphene oxide planar structure, oxygen containing functional groups, and large surface area. Adsorption properties of graphene oxide were studied using freeze dried graphene oxide sponge under different experimental conditions. The result indicate that graphene oxide is extraordinarily capable of extracting wide range of pharmaceutical compounds rather rapidly (equilibrium reached in less than an hour comparing days for activated carbon) with a higher efficiency than that of activated carbon. According to microscopic images, XRD, EDX, NMR and thermal analysis we suggested that GO initiates crystallisation of the pharmaceutical on its surface and entire solution rather than physical adsorption. It can be due to low surface energy of graphene oxide as a suitable nucleation and growth site for compounds dissolved in water. An important highlight of current work is using Graphene oxide which is produced economically and in large scale with improved dispensability and adsorption properties instead of expensive pristine graphene. Finally nanomaterials especially catalysts prepared by these simple and scalable method are potentially attractive for commercial employment.
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