Tekstil atıksularındaki boyar maddelerin fotokatalitik oksidasyon ile giderimi için zno bazlı katalizörlerin geliştirilmesi
Development of zno based catalysts for photocatalytic degradation of dyes in textile wastewater
Yazar: BEYZA DEMİR
Danışman: PROF. DR. MELEK TÜTER ; PROF. DR. ŞEYMA AYDINOĞLU
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Kimya Mühendisliği Ana Bilim Dalı / Kimya Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2019
Konu: Kimya Mühendisliği = Chemical Engineering

Özet:
Su yaşamımız için temel olan kaynaklardan biridir. Fakat artan nüfusa bağlı olarak gelişen sanayileşme sebebiyle su kaynakları kirlenmekte ve tekstil veya diğer boyarmadde endüstrilerinden kaynaklanan su kirliliği çevreyi etkileyen temel sorunlardan biri haline gelmektedir. Kirlilik kalıcı veya biyolojik olarak parçalanamayan organik maddelerden kaynaklanır. Su kirliliğine maruz kalınması tüm canlılara zarar verebilir. Özellikle insanlar üzerinde ciddi hastalıklara, hatta hayat kayıplarına da neden olmaktadır. Dolayısıyla su kirliliği önlem alınması gereken bir konu haline gelmiştir. 
Geleneksel atık su arıtma yöntemlerinin tekstil veya endüstri kaynaklı organik kirlilikleri gidermede etkisiz olduğu bulunmuştur. Fotokataliz ise çeşitli kirleticilerin giderimi için kullanılan ve su kirliliği sorunlarını çözmek için kullanılan gelişmiş bir oksidasyon işlemidir. Fotokatalizör olarak bir yarı iletken kullanılarak sudaki organik kirletici maddelerin bozunması son yıllarda yoğun ilgi görmüştür. 
Bu çalışmada ZnO bazlı fotokatalizörler geliştirerek, tekstil boyaları olan Rodamin B ve metilen mavisi boyalarının fotokatalitik oksidasyon yöntemiyle giderilmesi ve boyar madde giderimini etkileyen parametrelerin incelenmesi amaçlanmıştır.
Sentez çalışmalarında öncelikle belirlenen sol-jel yöntemiyle çinko asetat dihidrat, oksalik asit ve etanol kullanılarak toz formda ZnO elde edilmiştir. Daha sonra yine aynı sol-jel yöntemiyle ZnO sentezi esnasında metal kaynakları eklenerek metal katkılı fotokatalizörler sentezlenmiştir. Metal kaynağı olarak baryum klorür dihidat, kobalt nitrat hekzahidrat, bakır nitrat trihidrat, krom nitrat nanohidrat, demir klorür hekzahidrat, manganez sülfat monohidrat, nikel nitrat hekzahidrat ve kalay klorür dihidrat kullanılarak baryum katkılı Ba/ZnO, kobalt katkılı Co/ZnO, bakır katkılı Cu/ZnO, krom katkılı Cr/ZnO, demir katkılı Fe/ZnO, mangan katkılı Mn/ZnO, nikel katkılı Ni/ZnO ve kalay katkılı Sn/ZnO fotokatalizörleri elde edilmiştir.
Sentezlenen fotokatalizörlerin karakterizasyon testleri XRD, BET, ICP ve UV-Vis DRS analizleri ile gerçekleştirilmiştir. XRD analiz sonuçları değerlendirildiğinde sentezlenen fotokatalizörlerin hekzagonal wurtize yapılı ZnO verileri ile uyumlu olduğu ve metal katkıların ZnO kristal yapısı içerisine entegre olduğu sonucuna varılmıştır. BET analiz sonuçlarına bakıldığında mezogözenekli yapıda fotokatalizörlerin sentezlendiği ve metal katkılarının ZnO'in yüzey alanını, ortalama gözenek çapını ve toplam gözenek hacmini arttırdığı sonucuna varılmıştır. ICP analizi ile katkılanması hedeflenen (%0.5) ve gerçekleşen metal içeriği miktarının birbirine yakın sonuçlar verdiği görülmüştür. UV-Vis DRS analizi ile fotokatalizörlerin bant boşluk değerleri hesaplanmış ve sadece Sn katkısının ZnO bant boşluğunu düşürdüğü diğer metal katkılarının büyük değişiklik yaratmadığı görülmüştür.
Sentezlenen fotokatalizörlerin fotokatalitik performans testleri fotoreaktör içerisinde gerçekleştirilmiştir. Görünür ışık kullanılarak oda sıcaklığında 5 ppm 150 ml boya çözeltisi içerisine 0.15 g fotokatalizör konularak 300 dakika boyunca boya giderimi takip edilmiştir. Rodamin B ile fotokatalitik performans testleri gerçekletirildiğinde ZnO, Ba/ZnO, Co/ZnO, Cu/ZnO, Cr/ZnO, Fe/ZnO, Mn/ZnO, Ni/ZnO ve Sn/ZnO fotokatalizörleri sırasıyla %57, %43, %28, %39, %23, %32, %27 ve %67 Rodamin B giderdiği gözlenmiştir. Ayrıca metilen mavisi boyasına karşı ZnO ve Sn/ZnO sırasıyla %31 ve %38'lik giderim sağlamıştır. Sentezlenen fotokatalizörlerden Sn/ZnO ile her iki boya türünde de en yüksek giderim elde edilmiştir.
Boya giderimine pH etkisini incelemek için ise 3 farklı pH aralığında çalışılmıştır. Sn/ZnO ile pH:10'da Rodamin  B boyasına karşı %72, metilen mavisi boyasına karşı %45 giderim sağlanmıştır. pH:4.5'tan pH:10'a gidildikçe boya giderim oranının arttığı sonucuna varılmıştır.

Abstract:
Water is one of the main sources for our lives. However, due to the development in industralization and the increasing population, water resources are polluted. According to a report published by the World Health Organization in 2017, 844 million people, 84% of whom live in rural areas, still lack a basic drinking water service in 2015. Many of them suffer from severe waterborne diseases due to the use of microbiologically unsafe water. Water pollution from textile or other dyestuff industries is one of the main problems that affects the environment. It is estimated that over 100,000 different dyes and pigments are used industrially in the world and more than 7x105 tons of synthetic dyes are produced annually. These dyes are used in textile, leather, paper and paint industries. The textile industry is at the forefront of industries that consume large amounts of water in processing operations, including pre-treatment, dyeing, printing and finishing. Pollution is caused by persistent or biodegradable organic substances. These comtaminants can have direct effects on entire ecosystems, making life more difficult for humans, plants and animals. It can causes serious diseases and even life losses. Therefore, water pollution has become an issue that needs to be taken precautions.
It has been found that traditional wastewater treatment methods are ineffective for removing the organic pollution from textile or other industries that used dyestuff. Advanced oxidation processes are innovative technologies for wastewater treatment and are extremely useful in removing materials resistant to traditional methods. They provide complete color removal, detoxification and mineralization in the wastewater of textile dye houses. In advanced oxidation processes, aggressive and reactive radicals such as hydroxyl (OH) and superoxide (HO2) are produced to reduce harmful organic compounds to harmless end products such as CO2 and H2O. Among the advanced oxidation processes, the most important one is the heterogeneous photocatalytic oxidation method. This method comprises oxidation of solid organic oxide semiconductors as photocatalysts of organic compounds under UV light having a suitable wavelength.  Photocatalytic materials consist of a valence band containing stable energy electrons and a conduction band with a hidh energy. When the material is not excited by any light sources, the electrons are in the valence band, not in the conduction band. The band gap acts as a barrier between these two bands. When these materials are excited by a light source such as UV, the electrons in the valence band pass into the conduction band to form an electron-space pair. This electron-space pairs migrate to the catalyst surface where they can undergo a redox reaction with other species present on the surface. In most cases, excited electron reacts with the surface-bound water (H2O) molecule to form the hydroxyl (• OH) radical, space reacts with O2 to produce superoxide (O2-) radical anion. These radical anions are known to be primary oxidizing species in photocatalytic oxidation processes. Both types are strong oxidants and react with organic contaminants such as dyes to degradation of dyes. 
Most of the semiconductors used for dye removal from waste water are metal oxides such as TiO2, ZnO, SnO2, WO3, Fe2O3, and metal sulfides such as ZnS, CdS, MoS2. ZnO is the second most commonly used semiconductor after TiO2 and recommended as an alternative to TiO2 due to its low cost, non-toxic structure, good optoelectronic, catalytic and photochemical properties. The main disadvantage of pure ZnO is its wide bandgap. Therefore, it cannot be activated in the visible area of the light. However, considering that only 5-8% of natural daylight is generated by ultraviolet light and 45% of solar photons are in the visible light zone, this restricts the use of ZnO as a catalysts to remove dye from wastewaters. The need for more stable and efficient catalysts that can be active with natural daylight has been the subject of current research. Therefore, it is necessary to make some modifications to obtain active ZnO in the visible region. Various transition metals such as iron (Fe), chromium (Cr), cobalt (Co), nickel (Ni), copper (Cu), vanadium (V) have been used to increase the photocatalytic activity of ZnO in the visible region. These transition metal ions are replaced by Zn2 + ions with tetrahedral O coordination in the ZnO cage. it narrows the band gap by generating an energy level below the conduction band of ZnO particle. And also nonmetals have been used to increase the photocatalytic activity of ZnO in the visible region. When the 2p energy levels of the non-metallic atom and oxygen are similar, their mixtures will raise the valence band level and thus it narrows the band gap of ZnO. Generally, doping on ZnO is made with nonmetals such as carbon (C), nitrogen (N), sulfur (S). Another approach to create more efficient photocatalysts is to make composite semiconductors with using two or more semiconductor metal oxides. In the photocatalytic systems, combining the ZnO with a semiconductor having a narrower band gap expands the absorption wavelenght to the visible light region and reduces the recombination rate of excited electron.
The aim of this study is to develop photocatalysts based on ZnO and to remove the textile dyes by photocatalytic oxidation. And also the parameters that affect the dye removal are investigated.
In the synthesis studies, ZnO was first obtained by sol-gel method using zinc acetate dihydrate, oxalic acid and ethanol. After that, %0.5 metal by weight was doped to ZnO photocatalyst during the synthesis. Barium-doped Ba/ZnO, cobalt-doped Co/ZnO, copper-doped Cu/ZnO, chronium doped-Cr/ZnO, iron-doped Fe/ZnO, manganese-doped Mn/ZnO, nickel doped Ni/ZnO and tin-doped Sn/ZnO photocatalysts were synthesized, since it is known that the metal doping could enhance the photocatalytic activites of ZnO. 
Characterization tests of the synthesized ZnO and metal doped ZnO of photocatalysts were carried out by XRD, BET, ICP and UV-Vis DRS analyzes. When the XRD analysis results are evaluated, it is observed that the synthesized photocatalysts are compatible with the hexagonal wurtized structure of ZnO data. Furthermore, ZnO of the doped metals was included in the crystal structure lattices without causing much effect on the crystal structure. According to the results of BET analysis, it is seen that photocatalysts are synthesized in mesoporous structures. Also metal additives increase the surface area, average pore diameter and total pore volume of ZnO. When the ICP analysis results are evaluated, the target metal content (0.5%) could be achieved in the bulk catalyst samples via using this sol-gel method. UV-Vis DRS measurements were performed to determine the band gap of photocatalysts. For ZnO, this range was found to be 3.27. It was determined that there was not much change in this band gap with the addition of metal. The minimum bandgap is obtained with Sn / ZnOas 3.19. According to the results obtained from the DRS analysis, only  Sn contribution reduced the band gap energy of ZnO. When the results obtained from the analyzes are compared with the studies in the literature, it is concluded that the physical, morphological and optical properties of photocatalysts may vary according to the synthesis method used.
The photocatalytic performance tests of the synthesized photocatalysts were carried out in the photoreactor. By using visible light source, 0.15 g photocatalyst was put into a 5 ppm 150 ml dye solution at room temperature and dye removal was followed for 300 minutes. In the trials against rhodamine B, it can be said that only Sn contribution increases the effectiveness of ZnO. Other metal additives reduced the photocatalytic performance of ZnO in the visible region. 5 ppm Rhodamin B dye degradation of ZnO, Ba/ZnO, Co/ZnO, Cu/ZnO, Cr/ZnO, Fe/ZnO, Mn/ZnO, Ni/ZnO and Sn/ZnO was measured 57%, 43%, 28%, 39%, 23%, 32%, 27% ve 67% in 300 minutes, respectively. The highest photocatalytic performance in Rhodamine B removal was obtained with Sn/ZnO at 67%. The reason why ZnO with Sn doped against Rhodamine B dye shows higher photocatalytic performance compared to pure ZnO and other metal doped ZnOs can be explained by band gap energy. This result explains that Sn/ZnO performs better in visible light than pure ZnO. In the experiments performed to examine the effect of the structure of the dyes on the photocatalytic performance of photocatalysts, photocatalysts for both dyes showed similar tendency but different activity levels. ZnO and Sn/ZnO against the rhodamine B dye had 57% and 67% removal respectively, while ZnO and Sn/ZnO against the methylene blue dye had 31% and 38% removal respectively. Therefore, it can be said that the removal effect of photocatalysts on Rhodamine B dye which is xanthene group dyestuff is higher than methylene blue dye which is azo group dyestuff. It is concluded that the chemical structure of the dyes is effective in the performance of photocatalysts. 
In order to investigate the effect of ambient pH on the photocatalytic performance of catalysts, 3 different pH values were studied. As the ambient pH increases, the  degredation% values for both dyes and both photocatalysts increase. The highest degredation for Rhodamine B was obtained with Sn/ZnO at 72% at pH 10. In methylene blue, the highest removal value is 45% with Sn/ZnO at pH 10. The reason why degradation increases from acid medium to basic medium can be explained by the zeta potentials of photocatalysts. The presence of a large amount of absorbed OH- on the particle surface with increasing pH supports the formation of perhydroxyl radicals, which are considered the primary oxidizing species responsible for the degradation of the dye.