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Fabrication and characterization of multilayer TiO2 membranes with hierarchically porous structure

Hiyerarşik gözenekli yapılı çok katmanlı TiO2 membranların üretimi ve karakterizasyonu

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  1. Tez No: 496392
  2. Yazar: MERVE BULDU
  3. Danışmanlar: PROF. DR. FATMA BEDİA BERKER
  4. Tez Türü: Yüksek Lisans
  5. Konular: Kimya, Kimya Mühendisliği, Seramik Mühendisliği, Chemistry, Chemical Engineering, Ceramic Engineering
  6. Anahtar Kelimeler: Belirtilmemiş.
  7. Yıl: 2017
  8. Dil: İngilizce
  9. Üniversite: İstanbul Teknik Üniversitesi
  10. Enstitü: Fen Bilimleri Enstitüsü
  11. Ana Bilim Dalı: Kimya Ana Bilim Dalı
  12. Bilim Dalı: Belirtilmemiş.
  13. Sayfa Sayısı: 122

Özet

Su, tüm canlılar için en önemli yaşam kaynağıdır. Dünya üzerinde yaşam var oldukça, temiz ve kullanılabilir suya olan ihtiyaç da artarak var olmaya devam edecektir. Dünya nüfusunun artışı ile birlikte şehirleşme ve sanayileşme son derece hızlı bir şekilde yayılmakta ve gelişmekte, bunun sonucu olarak da ortaya çıkan atık su miktarı ve kirlilik yükü de aynı oranda artış göstermektedir. Tüm bunlara küresel ısınma ve iklim değişikliğinin etkileri de yansıdığında, kullanılabilir su kaynaklarının miktarındaki ciddi azalma küresel su kıtlığı krizine yol açmıştır. Giderek artan temiz ve kullanılabilir su talebini karşılayabilmenin en önemli yolu öncelikle çevreyi ve su kaynaklarını korumaya yönelik çevre politikalarının oluşturulmasıdır. Bunun yanında, gelişen teknoloji ile birlikte, sanayileşme sonucu ortaya çıkan atık suyun kendisi de bir su kaynağı olarak değerlendirilerek, konvensiyonel yöntemlerden çok daha üstün olan son sistem arıtma yöntemleriyle atık suyun kullanılabilir temiz su kaynağına dönüştürülmesi, günümüzde dünya çapında üzerinde en çok çalışılan konulardan biri haline gelmiştir. Bu arıtma yöntemlerinden kullanım kolaylığı bakımından en gelişmişi ve en verimli çalışanı membran sistemlerdir. Membran sistemler; atık sudan temiz su elde edilmesinin yanında, atık su içerisinde bulunan çeşitli moleküllerin geri kazanımına da imkan sağlamaktadır. Membranların kullanım alanları atık su arıtımı ile sınırlı olmayıp; gaz ayırma prosesleri, pervaporasyon, tuzsuzlaştırma prosesleri (acısulardan ve deniz suyundan), enerji üretimi (yakıt hücreleri, güneş pilleri), elektrokimyasal prosesler (diyaliz), sensörler, katalitik prosesler, yiyecek-içecek endüstrisi ve petrokimya endüstrisi gibi birçok endüstriyel proseste ve uygulamada kullanılmaktadırlar. Membran sistemlerinin sınıflandırılmasında iki farklı yaklaşım benimsenmiştir. Birincisi, membranları gözenek boyutlarını baz alarak makrogözenekli (>50 nm), mezogözenekli (2-50 nm) ve mikrogözenekli (

Özet (Çeviri)

Clean water is a fundamental need for humans and other living organisms. It is not only crucial to life on earth, but also is necessary for industrial, agricultural and municipal activities. Due to the rapid population growth and urbanization, the quantity of wastewater produced and the environmental pollution load have drastically increased. Combined with the impacts of global warming and climate change, water scarcity has become a global problem affecting us all. According to the latest researches, global water demand will increase by 55% by 2050, due to the increasing demands arising from daily human activities and industrial processes. Because of the rapidly increasing population and urbanization, insufficient environmental policies, serious environmental pollution and increasing energy consumption, Turkey is ranked as one of the highest water stressed countries, even though it is surrounded by water on three sides. Therefore, development of advanced wastewater treatment technologies is the most necessary action to be taken in order to protect our water sources, to prevent environmental pollution and to recycle and reuse as much water as possible. Membrane filtration is one of the most cost-effective, efficient and environment-friendly technologies, which allows to remove many types of contaminants from water, such as chemicals and biological compounds, based on their size and molecular weight. Besides their considerably long lifetime and high performance, ceramic membranes have tremendeous advantages over polymeric membranes like their bioinert structure, higher chemical, thermal and mechanical resistance, higher stability, higher hydrophilicity, higher separation efficiency, higher permeating flux and higher antifouling property. Thus, they have gained increasing attention in the last few decades. Al2O3 is the most widely used material because of its low cost and chemical and physical stability. However, the stability of the Al2O3 in highly alkaline environments is lower than the TiO2 membranes. Thanks to its outstanding chemical and thermal stability, bioinert and biocompatible structure and photocatalytic activity, TiO2 has wide range of applications in many different fields. In this thesis, multilayer TiO2 membranes with hierarchically porous structure were fabricated and characterized in detail. Several characterization tests such as XRD, DTA/TGA, SEM-EDS, MIP, BET, etc. were performed to investigate the physical, microstructural, chemical and mechanical properties of the prepared materials. The fabricated membranes consist of three main parts: a highly permeable macroporous TiO2 support, a mesoporous TiO2 intermediate layer and a thin, sol-gel derived TiO2 top layer. The macroporous TiO2 support provides mechanical strength to the membrane and acts as a carrier and the mesoporous TiO2 layer is used to gradually decrease the pore size of the support and acts as a bridge between the support and the top layer. The thin, sol-gel derived TiO2 top layer is the selective layer and responsible for the separation. The study includes: 1) fabrication of the disc-type and tubular macroporous TiO2 supports, 2) preparation of the mesoporous TiO2 intermediate layers, 3) synthesis of the TiO2 selective (top) layers, 4) preparation of multilayer TiO2 membranes and 5) characterization studies. Submicron-scale and micron-scale anatase TiO2 powders were used as starting materials for preparing the porous titania ceramic supports. Disc-type TiO2 supports were fabricated by uniaxial pressing in a hydraulic press, whereas the tubular ones were extruded to form single-channel tubular supports using an extrusion machine. Both types of supports were then dried and sintered. The optimum sintering temperatures were 1100 and 1200°C for tubular and disc-type samples, respectively. These temperatures were determined either from thermal analyses and from preliminary experiments. The open porosity and closed porosity for tubular support were calculated as 33.56% and 0%, whereas the bulk density and water uptake were 2.81 g/cm3 and 11.92%, respectively. The average pore diameter values obtained from MIP were between 1.02-2.44 μm for disc-type supports and 0.73 μm for tubular sample. In addition, the porosity given by MIP for the tubular support is about 41.7%, which is higher than the porosity calculated from Archimedes' results. The pure water flux was ~650 Lm−2h−1bar−1 for the selected disc support and was ~1020 Lm−2h−1bar−1 for the tubular one. The intermediate layers were prepared as either supported membranes or unsupported films from homogeneous suspensions of two types of TiO2 powders having different particle sizes, in the presence of a dispersant, an organic binder and a solvent. Once the titania interlayers prepared, they were characterized in terms of their crystalline phase, pore characteristics, microstructural properties and thermal behavior. The pore size distributions of the prepared titania films were measured with Brunauer-Emmet-Teller (BET) nitrogen adsorption and desorption analysis. Considering the BET results together with the visual evaluation of the morphological characteristics of the intermediate layers before and after thermal treatment, it is decided to use TARA-2 in coating experiments. The BET surface area and the mean pore diameter of the TARA-2 were 29.350 m2/g ve 32.38 nm, respectively. Intermediate layers were calcined at 400°C for 1 h by a heating rate of 1°C/min. The sol-gel derived TiO2 selective (top) layers were synthesized via the hydrolysis of titanium (IV) isopropoxide (TIP) in an alcoholic solution in the presence of an acid catalyst. The BET surface area and average pore diameter of the calcined titania gel were 77.180 m2/g and 14.08 nm, respectively. It exhibits Type IV isotherm with H3 hysteresis loop, which is known as a typical isotherm for mesoporous materials. The multilayer ceramic membranes with hierarchically porous structure were produced through coating the support surfaces with intermediate and top layers. The intermediate layer was coated onto the inner surface of the tubular support and onto the disc supports via manually pouring and spray-coating techniques respectively. The desired coating layer thickness was achieved by applying multiple coating steps, if needed. Coated supports were then dried and calcined at 400°C for 1 h by a heating rate of 1°C/min. For the top layer, disc-type samples were coated by manually dipping one side of the disc into the TiO2 sol and tubular ones was coated by pouring the sol through the inner surface of the tube. The coated membranes were dried and then subsequently calcined at 400°C for 1 h by a heating rate of 0.2°C/min. The coating/drying/calcination cycle was repeated until required thickness is achieved. It was concluded that the highly permeable TiO2 supports fabricated in this study are surely suitable for some microfiltration applications as filters themselves, or as supports for membrane layers for ultrafiltration applications. The current results show that the titania top layer exhibits high potential to serve as separation layer for ultrafiltration applications. However, since some of the experiments are still in progress, the complete results are not ready to be reported yet. The further details will be presented in future work.

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