Trakya dolomiti ile atık sulardan kobalt giderilmesi için radyoaktif izleme tekniğinin kullanılması
The use of radiotracer technique for the removal of cobalt from wastewater with thracian dolomite
Yazar: OĞUZHAN SAÇKIRAN
Danışman: DR. ÖĞR. ÜYESİ AYŞE NUR ESEN
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / Nükleer Araştırmalar Ana Bilim Dalı / Radyasyon Bilim ve Teknoloji Bilim Dalı
Derece: Yüksek Lisans, 2024
Konu: Nükleer Mühendislik = Nuclear Engineering
Dizin: Adsorpsiyon = Adsorption ; Dolomit = Dolomite ; Endüstriyel atık su = Industrial waste water ; Radyoaktif izleyiciler = Radioactive tracers

Özet:
Nüfus artışı, sanayileşme ve kentleşmenin giderek artması, temiz ve içilebilir suya erişim konusunda soruna yol açmaktadır. Evsel, endüstriyel, ticari ve tarımsal faaliyetlerden kaynaklanan su kirliliği hem gelişmiş hem de gelişmekte olan ülkelerde endişe kaynağıdır. Atık suların kontrolsüz deşarjında bulunan kirleticiler ağır metaller, organik ve inorganik partiküller ile toksinler olabilir. Günümüzde nükleer enerji dünyada sürdürülebilir enerji hedeflerine ulaşmada ve enerji güvenliğinin sağlanmasında önemli bir rol oynamaktadır. Nükleer santrallerin işletilmesi, değişen radyoaktivite değerlerine sahip atıklar üretir. Bu atıkların, radyoaktivitelerine ve kullanım amaçlarına bağlı olarak güvenli ve uygun bir şekilde işlenerek bertaraf edilmesi temiz su yönetimini güvence altına almak için gerekmektedir.
Küresel, temiz ve güvenli su tedarikinin sürdürülebilirliği, başlıca çevresel zorluklardan biridir. Su kalitesini iyileştirmek için kirleticilerin ortadan kaldırılması, çevre dostu arıtma teknolojileri gerektirmektedir. Sıvı radyoaktif atıkların arıtılması, çoğunlukla filtrasyon, çöktürme, sorpsiyon, iyon değişimi, buharlaştırma ve membran ayırma gibi uygulamaları içerir. Sorbanların diğer arıtma yöntemleriyle birlikte kullanılması, sıvı radyoaktif atıkların içeriğindeki radyonüklitleri ayırmada etkili bir yöntemdir.
Bu tez çalışmasında sıvı atıklardan kobalt iyonlarının giderilmesi amacıyla Trakya dolomitinin kobalt sorpsiyon performansının incelenmesi amaçlanmıştır. Bu amaçla, radyoaktif izleyici olarak kullanmak üzere kobaltklorür bileşiği İTÜ TRIGA Mark II araştırma reaktöründe ışınlanarak kobalt-60 elde edilmiş ve radyoaktif çözelti hazırlanmıştır. Adsorpsiyon deneyleri, adsorban dozajı, adsorbat konsantrasyonu, çözelti pH'ı, temas süresi ve sıcaklık gibi faktörleri incelemek için kesikli yöntem kullanılarak gerçekleştirilmiştir. Dolomit mineralinin özellikleri XRF, XRD, FTIR, SEM ve partikül boyutu analizi ile belirlenmiştir. Deneysel veriler adsorpsiyon izotermi, kinetiği ve termodinamiği kullanılarak değerlendirilmiştir.
Optimum adsorpsiyon koşulları, katı-sıvı oranı 20 g·L-1, kobalt konsantrasyonu 128 mg·L-1, temas süresi 120 dk. ve pH değeri 6 olarak elde edilmiştir. Maksimum adsorpsiyon kapasitesi 2,32 mg·g-1 bulunmuştur. Kinetik bulgulara göre deneysel veriler sanal ikinci dereceden kinetik modele iyi bir şekilde uymuştur (R2=1). Bu sonuç adsorpsiyon hızını sınırlayıcı adımın kimyasal sorpsiyon olduğunu göstermektedir. Adsorpsiyon süreci Langmuir ve Freundlich yaklaşımlarını bir araya getiren ve geniş bir konsantrasyon aralığında homojen veya heterojen sistemlere uygulanabilen Redlich-Peterson izotermi kullanılarak tanımlanmıştır (R2= 0.958). Langmuir ayırma faktörü değerlerinin 1'den küçük olması adsorpsiyonun elverişli olduğunu göstermiştir. Elde edilen termodinamik parametreler incelendiğinde ΔH0'nin negatif değerleri adsorpsiyonun ekzotermik doğasını, ΔG0'nin negatif değerleri
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adsorpsiyonun 20-60oC'de kendiliğinden gerçekleştiğini ve böylece doğal dolomit-Co(II) adsorpsiyon sürecinin elverişli olduğunu ortaya koymuştur.
Elde edilen sonuçlara göre, Trakya dolomiti radyoaktif ve radyoaktif olmayan kobalt nedeniyle çevre kirliliği önleme çalışmaları için düşük maliyetli bir adsorban olarak önerilebilir. Dolomitin bir adsorban olarak kullanılması suyun kobalt kirliliğini önleyerek çevre koruma çalışmalarına katkı sağlayacaktır. Bu çalışmada Trakya dolomiti ile elde edilen sonuçları geliştirmek için dolomit üzerindeki adsorpsiyon deneylerinin çoklu kirletici içeren çözeltiler ve modifiye edilmiş dolomit kullanılarak devam ettirilmesi önerilebilir. Bunun yanı sıra, gelecekteki çalışmalar arasında gerçek atık su çalışmaları, ölçek büyütme, maliyet analizi ve çevresel etki (örn. yaşam döngüsü analizi) yer almalıdır.

Abstract:
Population growth, industrialization, and urbanization are causing problems with clean and drinkable water access. Water pollution from domestic, industrial, commercial, and agricultural activities is a concern in developed and developing countries. Pollutants in uncontrolled wastewater discharges include heavy metals, organic and inorganic particles, and toxins. Nuclear energy plays an essential role in achieving sustainable energy goals and ensuring energy security in the world. The operation of nuclear power plants produces waste of varying radioactivity. Safe and appropriate treatment and disposal of these wastes, depending on their radioactivity and intended use, is necessary to secure clean water management.
The sustainability of the global supply of clean and safe water is one of the significant environmental challenges. The removal of contaminants to improve water quality requires environmentally friendly treatment technologies. Treatment of liquid radioactive wastes mainly involves filtration, precipitation, sorption, ion exchange, evaporation, and membrane separation. Using sorbents in combination with other treatment methods effectively removes radionuclides from liquid radioactive waste.
Dolomite is a double carbonate compound of calcium and magnesium with the chemical formula CaMg(CO3)2. Dolomite rock is formed from limestone by partially or entirely replacing calcium ions (CaO) in limestone with magnesium ions (MgO). It is, therefore, related to limestones in terms of its composition. Dolomite is a suitable adsorbent due to its wide availability, low cost, and environmental compatibility. Researchers have studied natural dolomite, thermally activated dolomite, and various dolomite composites as a sorbent.
Cobalt is a hard, gray-blue metal that is solid under normal conditions. It is a natural environmental element, often found with other metals such as copper, nickel, manganese, and arsenic. Cobalt has many radioisotopes, the most common radioactive isotope being cobalt-60 (60Co). It emits gamma rays with energies of 1173 keV and 1332 keV and has a half-life of 5.27 years. Cobalt-60 occurs as a by-product of nuclear reactor operations when metal structures such as steel bars are exposed to neutrons. It is used in industrial applications as a radiation source and in medicine for radiation therapy. Therefore, the presence of cobalt-60 in the environment is mainly related to nuclear facilities, and cobalt-60 has been measured in water environments affected by liquid waste discharged from nuclear facilities.
In gamma decay, a nucleus passes from an excited state to a lower-energy state, and the energy difference between the two states is released as a photon. Gamma-ray spectroscopy is an analytical technique to detect, identify, and quantify radionuclides emitting gamma radiation in a sample. It has many advantages, such as the simplicity of sample preparation for radionuclide analysis, the fact that, in most cases, a radiochemical separation process is not required for analysis, and the simultaneous
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analysis of multiple radionuclides. In gamma-ray spectroscopy, the energy of the incident gamma-ray is measured by a detector. The radionuclide is identified by comparing the estimated energy with the known energy of the gamma-ray produced by the radionuclide. A gamma-ray spectrometer is a system that allows the pulses generated in proportion to the energy of the gamma rays reaching the detector crystal to be processed with a preamplifier and amplifier. Then, the signal obtained is digitized and recorded as a spectrum in the memory of the multichannel analyzer.
This study aimed to investigate Thracian dolomite as a low-cost adsorbent for removing stable or radioactive cobalt ions from wastewater using cobalt-60 as a radiotracer. Natural dolomite mineral extracted from deposits in the Thrace region was used without processing. XRF, XRD, FTIR, SEM, and particle size analysis were used to determine the characteristics of dolomite mineral. Cobalt-60 was obtained by irradiating cobalt chloride compound (CoCl2 Merck, 99% purity) for 30 min at 250 kW in the central irradiation tube of the TRIGA Mark II research reactor of Istanbul Technical University. Irradiated and weighed CoCl2 was dissolved in distilled water to prepare a radioactive 500 mL stock solution containing 266 mg·L-1 cobalt. Working solutions of different concentrations were prepared by pipetting and diluting the stock solution to obtain an appropriate concentration for subsequent adsorption experiments.
Adsorption experiments were carried out using the batch method to study parameters such as adsorbent dosage, initial concentration, solution pH, contact time, and temperature. All experiments were done in a water bath at a constant shaking speed of 125 rpm. After the experiments, the suspensions were filtered with Whatman filter papers. 60Co radioactivity was measured before and after the experiments with a gamma-ray spectrometer with a 2"x2" NaI(Tl) scintillation detector at a sample-detector distance of 2.8 cm for counting times of 5-10 minutes. Spectra were analyzed with GammaVision software.
The effect of dolomite dosage on Co(II) adsorption was investigated at the solid-liquid ratio ranging from 2.5 to 80 g·L-1, the effect of the initial concentration of cobalt on the adsorption of Co(II) on dolomite was investigated between 11 and 266 mg·L-1, the effect of contact time on the adsorption of Co(II) on dolomite was studied for contact time ranging from 10 to 300 min, the effect of initial pH on the adsorption of Co(II) on dolomite was investigated for pH values ranging from 2.8 to 8.3, and the effect of temperature on the adsorption of Co(II) on dolomite was investigated at temperatures ranging from 293 to 333 K.
The experimental data were evaluated using adsorption isotherms, kinetics, and thermodynamics. Langmuir, Freundlich, and Redlich-Peterson isotherm models were utilized in this study. The Langmuir isotherm model is semi-empirical, and its basic assumption is that the surface is homogeneous. The adsorption properties of the Langmuir isotherm can be described in terms of a dimensionless constant, the separation factor (RL). Freundlich isotherm model describes multilayer and reversible adsorption on a heterogeneous surface. The Redlich-Peterson isotherm model is an empirical model that combines the Langmuir and Freundlich approaches. Adsorption kinetics controls the adsorption rate and determines the time required for the adsorption process to reach equilibrium. The kinetic data in this study were analyzed with pseudo-first and second-order kinetic equations. The pseudo-first-order model works well in the first phase of the adsorption process (usually the first 20-30 min). The pseudo-second-order kinetic model assumes that the rate-limiting step is chemical sorption and predicts the behavior over the entire adsorption range. To determine the
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spontaneity of the adsorption process, energy and entropy factors should be considered in adsorption processes. ΔH0 and ΔS0 were determined from adsorption experiments at various temperatures using the Van't Hoff equation. ΔH0 and ΔS0 were calculated from the slope and intercept from the lnKD plot as a function of 1/T.
Optimum adsorption conditions were obtained as solid-liquid ratio 20 g·L-1, cobalt concentration 128 mg·L-1, contact time 120 min, and pH 6. The maximum adsorption capacity was found to be 2.32 mg·g-1. This value is consistent with the maximum adsorption capacity of 2.84 mg·g-1 obtained by Ghaemi et al. (2013) for Co(II) ion removal with Iranian dolomite. The optimum pH value for Co(II) was found to be in the range of 6-7. This finding indicated that it is appropriate to work with the natural pH value of the solution, which is 6.3. According to the kinetic results, the experimental data fit the pseudo-second-order kinetic model (R2=1). This result indicates that the rate-limiting step of adsorption is chemical sorption. The adsorption process was described using the Redlich-Peterson isotherm (R2=0.958), which combines the Langmuir and Freundlich approaches and can be applied to homogeneous or heterogeneous systems over a wide concentration range. The calculated Langmuir separation factor values in the 0