Modifiye edilmiş nanokompozit malzeme sentezi ve sulu ortamdan sezyumun uzaklaştırılmasında kullanımı
Synthesis of modified nanocomposite material and its use on removal of cesium from aqueous media
Yazar: BİLAL ÇETİN
Danışman: PROF. DR. MUSTAFA ÖZCAN ; DR. BEKTAŞ KARAKELLE
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / Kimya Ana Bilim Dalı / Kimya Bilim Dalı
Derece: Doktora, 2022
Konu: Kimya = Chemistry
Dizin: Nanokompozitler = Nanocomposites ; Radyoaktif atıklar = Radioactive wastes ; Sezyum = Cesium ; Sulu ortamlar = Aqueous media

Özet:
Radyoaktif atık, radyoaktif materyallerin sanayi ve tıp sektöründe kullanımının yanı sıra araştırma ve nükleer kuruluşlardan kaynaklanan kaçınılmaz sonuçtur. Bu nedenle, radyoaktif atıkların yönetimi ve bertarafı, neredeyse tüm ülkeleri ilgilendiren bir konudur. Ayrıca, nükleer bilim ve teknolojinin gelişimi, özellikle nükleer enerjinin geniş kullanımı, radyoaktivite kirliliği yoluyla insan çevresini ciddi şekilde tehdit etmektedir. Reaktördeki en önemli fisyon radyonüklitlerinden ikisi olan Cs-137 ve Cs-134, insan sağlığı ve çevre için potansiyel olarak tehlikeli olarak kabul edilir, çünkü sezyumun yüksek çözünürlüğü, uzun yarılanma süresi ve nükleer santrallerde yüksek verimliliğe sahip olması yeraltı sularından biyosfere doğru hareketine neden olmaktadır. Aslında nükleer silah denemeleri ve nükleer santrallerdeki kazalar neticesinde ortaya çıkan Cs-137 ve Cs-134 gibi radyonüklitler atmosfere dağılırlar sonrasında toprakta ve tarımsal ürünlerde kontaminasyona sebep olurlar. Topraktaki radyoaktif sezyumun bitkiler tarafından absorplanması bu bitkilerin hayvanlar tarafında tüketilmesiyle radyoaktif sezyum besin zincirine (et, süt, peynir, bitkisel gıdalar vb.) katılır ve insan bünyesine geçerek bir döngü oluşturur. Bu sebeple radyoaktif sezyumun uzaklaştırılması radyoaktif atık yönetimi açısından büyük önem arz etmektedir. Bu çalışmada kitosan (C56H103N9O39), K4[Fe(CN)6] ve Fe3O4 (Manyetit) ile nanokompozit malzeme sentezlenmiş, FTIR, BET, XRD, SEM ve TGA-DTA ile karakterize edilmiş ve sulu ortamdan sezyumun uzaklaştırılmasında adsorban olarak kullanılmıştır. Bu bağlamda, nanokompozit adsorban üzerine Cs+ adsorpsiyon davranışları pH, sıcaklık, adsorban miktarı, başlangıç sezyum konsantrasyonları, çalkalama hızı, temas süresinin bir fonksiyonu olarak çalışmalar gerçekleştirildi. Bu çalışmalara ilaveten nanokompozit üzerine sezyum adsorpsiyonunda yabancı iyon etkisi, nanakompozitten sezyumun desorpsiyonu, kolon çalışmaları ve radyoaktif sezyum çözeltileri kullanılarak nanokompozit üzerine sezyum adsorpsiyonu çalışmaları yapılmıştır. Adsorpsiyondan elde edilen veriler Langmuir ve Freundlich izoterm modellerine uyarlanmıştır. Nanokompozit adsorban sezyumun adsorpsiyonunda kullanılması neticesinde  maksimum adsorpsiyon kapasitesine Cs+ için 34,36 mg.g-1'da ulaşmıştır. Sezyumun nanokompozit üzerine adsorpsiyonu Langmuir izotermine daha iyi uyduğu sonucuna varılmıştır. Nanokompozit adsorban üzerine sezyum adsorpsiyonunda serbest enerji (ΔG), entropi (ΔS) ve entalpi (ΔH) değişimi gibi termodinamik parametreler incelendi. ΔH0 değeri negatif ve ΔS0 değerinin pozitif olması ekzotermik ve kendiliğinden özellikte adsorpsiyon mekanizmasının gerçekleştiğini, ΔH0 değerinin negatif ve ΔS0 değerinin pozitif olmasının yanısıra ΔG0 değerinin de negatif olması adsorpsiyonun tüm sıcaklıklarda kendiliğinden gerçekleştiğini gösterir. İlave olarak düşük adsorpsiyon entalpisi sebebiyle sezyum, nanokompozit üzerine fiziksel olarak adsorplanmaktadır. Yabancı iyon etkisiyle sezyumun nanokompozit üzerine adsorpsiyonda düşüş olduğu gözlenmiştir. Kolon çalışmalarından elde edilen adsorpsiyon kapasitesinin daha önce elde edilen adsorpsiyon kapasitesinden daha düşük olduğu belirlenmiştir. En düşük desorpsiyon değeri nötr ortamda tespit edilmiştir.

Abstract:
Radioactive waste (RW) is an inevitable residue from the use of radioactive materials (RMs) in industry and the medical sector, as well as from research and nuclear establishments. The management and disposal of such RW is, therefore, an issue relevant to almost all countries. Also, the development of nuclear science and technology, in particular the wide application of nuclear power, seriously threatens the human environment through radioactivity contamination. Two of the most important fission radionuclides from the reactor, 137Cs and 134Cs, are considered potentially dangerous to human health and to the environment, because the relatively high yield in nuclear power plants, long half-lives and high solubility of cesium (Cs) can cause its migration through ground water to the biosphere. In other respects, they can be easily incorporated in terrestrial and aquatic organisms, because of their chemical similarity to potassium (K). The formation of complexes does not have a significant effect on Cs speciation, and the predominant aqueous species in groundwater is the free Cs ion. Different techniques, such as solvent extraction, evaporation and ion exchange, are usually used for the treatment of aqueous waste solutions containing Cs ion. 
Due to their high surface area and shorter diffusion pathways compared to micro sized crystals, nanocrystalline materials (NMs) have recently received a lot of attention in order to expand their industrial uses (MSC). Despite its advantages, there is a challenge with removing used nanomaterials from the aqueous media. The centrifugation procedure, which is both expensive and time demanding, is used to separate the formers. Researchers have recently begun to employ the magnetic separation method (MST) because it is efficient, simple, and quick. Due to their high surface areas and ability to be magnetically collected using an external magnet, magnetic nanoparticles (MNs), which are composed of an MN based core and a functional Shell that can adsorb contaminants such as heavy metals or radioactive nuclides, have been extensively studied for environmental remediation applications. Fe3O4 has low toxicity, small size. Separation of the utilized sorbent is made easier by magnetizing the adsorbent matrix.
Chitosan (Ch) is a biopolymer obtained by the deacetylation of chitin. Moreover, metal nanoparticles can be incorporated with Ch to form a composite. Ch has an excellent adsorption capacity for various metals, such as Cr(VI), Cu(II), Pt(IV), Pd(II), Co(II), Ni(II), and Fe(III). Metal uptake by Ch is primarily attributed to the hydroxyl and amine groups existed in the polymer chain, the functional groups can react with the different metallic species through the mechanism of chelation and/or ion exchange. Ch has two hydroxyl and one amino groups on each glucosamine monomer and the amino groups can be strongly linked to metal ions and amino and hydroxyl groups can be interactive with organic compounds via hydrogen bonding. At the same time, the amino group of Ch is easily protonated in acidic solutions, restricting the application in adsorption process.
Metal hexacyanoferrate (MHCF) analogues are the inorganic complexes known for their versatile applications. Transition metal hexacyanometallates (TMHCM) usually have an open channel framework appropriate for small molecules separation, and their crystal structure is closely related to the coordination adopted by the metal centers. In hexacyanometallates (HCMs), the involved transition metals (TM) are usually found with octahedral coordination within the cubic unit cell (Fm-3m). Some zinc hexacyanoferrates (ZnHCF) have been reported as hexagonal where Zn2+ atom is found tetrahedrally coordinated to four nitrogen (N) atoms from cyano (CN) ligands. Such coordination provides a relatively high thermal stability to these materials and also an interesting porous framework because both metal centers have saturated their coordination sphere with atoms from the bridge group (–CRN–).
Until now, many methods have been used on removal of Cs (I) and Sr (II) ions from aqueous radioactive waste. The most important of these are ions exchange, precipitation, adsorption, solvent extraction, electrochemical and membrane processes. Adsorption process is low cost compared to other methods, thermal stability, resistance to ionizing radiation and compatible with the final form of waste
It is the most preferred for the reasons. 
In this study, nanocomposite material was synthesized with chitosan (C56H103N9O39), K4[Fe (CN)6] and Fe3O4 (Magnetite), characterized by FTIR, BET, XRD, SEM and TGA-DTA and used as an adsorbent for the removal of cesium from aqueous media. In this context, studies were carried out on the adsorption behaviour of Cs+ on the nanocomposite adsorbent as a function of pH, temperature, adsorbent amount, initial cesium concentrations, shaking speed, contact time. In addition to these studies, foreign ion effect in cesium adsorption on nanocomposite, desorption of cesium from nanocomposite, column studies and cesium adsorption studies on nanocomposite using radioactive cesium solutions were carried out. The data obtained from adsorption were adapted to Langmuir and Freundlich isotherm models. As a result of the use of the nanocomposite adsorbent in the adsorption of cesium, the maximum adsorption capacity has reached 34.36 mg.g-1 for Cs+. It was concluded that the adsorption of cesium on the nanocomposite fits the Langmuir isotherm better. Thermodynamic parameters such as free energy (ΔG), entropy (ΔS) and enthalpy (ΔH) change in cesium adsorption on the nanocomposite adsorbent were investigated. Positive ΔH0 and Negative ΔS0 values indicate that the adsorption mechanism is egzothermic and spontaneous, and ΔH0 value is negative and ΔS0 value is positive as well as negative ΔG0 values indicate that adsorption occurs spontaneously at all temperatures. In addition, due to its low adsorption enthalpy, cesium is physically adsorbed on the nanocomposite. It has been observed that there is a decrease in the adsorption of cesium on the nanocomposite due to the effect of foreign ions. A bead-shaped composite was synthesized from the nanocomposite material which is in the form of dust. The beads obtained were used in the adsorption of cesium in column experiment. Column studies with two different flow rates were carried out and the data obtained were applied to the thomas model and the adsorption capacities for both flows were determined. It was determined that the adsorption capacity obtained from the column studies was lower than the adsorption capacity obtained before. Cesium adsorbed nanocomposite adsorbent was used in notr solution and acidic and basic solutions which have different molarities in the determination of desorption amounts. The lowest desorption value was determined in neutral environment. It is extremely important in radioactive waste disposal and storage to ensure that radioactive waste does not move out of the packages in which they are located. From desorption studies we can ensure that if we place radioactive wastes in neutral media we can prevent or decelerate the immigration rate of radioactive cesium from its package.