Modifiye edilen kitosan ile atık sulardan hekzavalent krom gideriminin incelenmesi
The usage of modified chitosan for the removal of hexavalent chromium from waste water
Yazar: GÖKNUR TÜRSÜN
Danışman: PROF. DR. NURAN DEVECİ AKSOY
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Kimya Mühendisliği Ana Bilim Dalı
Derece: Yüksek Lisans, 2017
Konu: Kimya Mühendisliği = Chemical Engineering

Özet:
Krom VI-B grubunda yer alan, toksik etkisi kanıtlanmış bir geçiş elementidir. +1 ile +6 arasında değişen çeşitli değerlikler alabilmektedir. Krom çevreye verildikten sonra, iki kararlı oksidasyon basamağında Cr (III) ve Cr (VI) bulunur. Üç değerlikli nispeten zararsız iken hekzavalent krom toksik ve kanserojendir. Hekzavalent krom topraktaki ve sudaki sistemde oldukça hareketli ve aynı zamanda deriyle adsorbe edilebilen güçlü bir oksidandır.
Deri, tekstil, maden ve metal endüstrilerinin atık sularında yüksek derişimde Cr (VI) bulunmaktadır. Bu yüksek konsantrasyonlar, alıcı ortama desarj edilmeden önce Su Kirliliği ve Yönetmeği'nde yer alan limit değerlerin altına düşürülmelidir.
Günümüzde ağır metal bağlama kapasitesi yüksek ve kirli sudan istenmeyen ağır metalleri düşük maliyetle ortadan kaldıran çeşitli adsorbanlar üzerine odaklanılmıştır. Kitosan atık su arıtımı uygulamaları için umut verici pek çok avantaj sunmaktadır.
Atık sulardan Cr (VI) gideriminde kullanılan yöntemler, membran prosesleri, kimyasal çöktürme, iyon değişimi ve adsorpsiyon olarak sıralanabilir. Adsorpsiyon, işlem kolaylılığı ve ekonomik yönüyle ön plana çıkmaktadır.
Bu çalışmada öncelikle kitosanın tiyoüre ve glutaraldehit ile modifikasyonu gerçekleştirilmiştir. Modifiye edilen kitosan ve orijinal kitosanın karakterizasyonu FTIR (Fourier Transform Infrared Spektroskopisi), TGA (Termal Gravimetrik Analiz), XRD (X-ray Diffraction) ve DSC (Differential Scanning Calorimetry) analizleri ile yapılmıştır.
Modifiye edilen kitosan ve orijinal kitosanın Cr (VI) tutma kapasiteleri ve giderim yüzdeleri kıyaslandığında, modifikasyonun krom tutma kapasitesini ve giderim yüzdesini oldukça arttırdığı görülmüştür.
Modifiye edilen kitosanın karakterizasyonundan sonra atık sulardan adsorpsiyon yöntemiyle hekzavalent krom giderimi çalışmaları yapılmıştır. Farklı çözelti pH'ı, sıcaklık, başlangıç konsantrasyonu, temas süresi ve adsorban dozunda çalışılarak farklı ortam koşullarının adsorpsiyon prosesi üzerindeki etkileri araştırılmıştır. Belirlenen uygun ortam koşullarında kinetik ve termodinamik sabitler saptanmış, uygun adsorpsiyon izotermleri ile adsorpsiyon tanımlanmıştır.
Yapılan çalışmalar sonucunda modifiye edilmiş kitosan ile atık sulardan hekzavalent krom gideriminde uygun ortam koşulları, çözelti pH'sı 1-3 aralığında, adsorban dozu 1/4 (g/L), temas süresi ise yedi saat olarak belirlenmiştir.
Uygun ortam koşullarında, modifiye kitosan kullanılarak adsorpsiyon yöntemiyle Cr (VI) giderim çalışmalarında giderim yüzdeleri %99 değerlerine ulaşmış ve 442 mg/g yüksek adsorpsiyon kapasitesi gözlenmiştir.

Abstract:
The pollution of water resources due to the disposal of heavy metals has been an increasing worldwide concern for the last few decades. Heavy metals have a harmful effect on human physiology and other biological systems when they are found above the tolerance levelsThe presence of trivalent and hexavalent chromium in the environment is the cause of many well documented toxic effects. The maximum levels permitted in wastewater are 5 mg/l for trivalent and 0.05 mg/l for hexavalent chromium.
Chromium is a transition element which is located to Group VI-B of the periodical table and its toxic effect has been proven. Chromium exists in environment both as trivalent Cr(III) and hexavalent Cr(VI) forms. Hexavalent chromium is toxic and carcinogenic, while trivalent is relatively harmless.
High concentrations of Cr (VI) are present in the wastewater of leather, textile, mining and metal industries. These high concentration must be reduced below the permissible limits before Cr(VI) is discharged into the receiving enviroment.
Activated carbon is the world's most widely used adsorbent for heavy metal removal from waste water. Despite its efficient use, active carbon is expensive. Due to the disadvantage, research interest into the production of alternative adsorbents to replace the costly activated carbon. Because of their low cost, natural materials such as chitosan, zeolites, clay and fly ash are classified as low-cost adsorbents.
In recent years, studies on the use of chitosan as an adsorbent in heavy metal removal have been investigated. Chitosan is a polymer with polysaccharide structure obtained by alkali deacetylation of chitin. Kitin is the second most common biopolymer after cellulose in nature.It is highly reactive as an adsorbent due to the inclusion of amine and hydroxyl groups in the structure.
The degree of deacetylation is one of the most important chemical properties that can affect chitosan performance in many applications. The degree of deacetylation, which determines the content of free amino groups in polysaccharides, can be used to differentiate between chitin and chitosan. FTIR analysis was used to determine the degree of deacetylation of chitosan. The degree of deacetylation of chitosan was found to be % 85,91.
Various techniques have been employed for the treatment of heavy metals, including chemical precipitation, mebran processes, ion-exchange and adsorption. Adsorption
technique is economically suitable and technically practicable to remove hexavalent chromium from waste water as it requires less operative controls.
In this study, firstly chitosan was modified with thiourea and glutaraldehyde. Characterization of the modified chitosan and original chitosan were characterized by FTIR (Fourier Transform Infrared Spectroscopy), TGA (Thermal Gravimetric Analysis), XRD (X-ray Diffraction) and DSC (Differential Scanning Calorimetry.
XRD analysis was also made for the proof of modification. The modification of chitosan with glutaraldehyde and thiourea showed a peak flattening at 11 ° with the resultant original chitosan, indicating a significant decrease in crystallinity as a result of chitosan cross-linking with glutaraldehyde.The crystallinity of the chitosan polymer may play a limiting role in metal retention capacity. The crystallinity parameter of chitosan is an important parameter of accessibility to the inner region for both water and metal ions. Many studies have shown that reducing the crystallinity results in an improvement in metal ion retention properties.
Firstly modified chitosan was synthesized by using thiourea, glutaraldehyde and chitosan at different molar ratios and adsorption capacities compared to the original chitosan were compared. The modified chitosan showed higher Cr (VI) retention performance than the original chitosan. While the original chitosan had a percent removal of 32% at an initial concentration of 50 ppm, the percentage of chitosan removal improved to 99.9%.
The effects of different experimental conditions on the adsorption process were investigated by studying different solution pH, temperature, initial concentration, contact time and adsorbent dose.
As a result of the studies, the optimum environment conditions for hexavalent chromium removal from waste water with modified chitosan were determined as solution pH 1-3, adsorbed dose 1.4 (g / L) and contact time 7 hours.
Kinetic and thermodynamic constants are determined at appropriate ambient conditions and adsorption is defined by appropriate adsorption isotherms. Langmuir isotherms were found to be suitable for characterizing the adsorption of chitosan with the modified Cr (VI) ions. As a result of the thermodynamic analysis, it was found that the adsorption of Cr (VI) ions on the modified chitosan is endothermic. For this reason, increasing the temperature in Cr (VI) ions removal process will have a positive effect on the removal.
Under appropriate experimental conditions, percentages of removal in Cr (VI) removal studies by adsorption using modified chitosan reached 99% and adsorption capacity of 442 mg/g was observed.
Higher capacities can be reached in order to determine the appropriate conditions when working with modified chitosan. It will be appropriate to investigate these conditions in future work.
Synthetic Cr (VI) solution was used in the study. To study the effect of the competing ion, the actual waste water taken from the industry must be taken into account.
Experiments can be carried out in columns filled with modified chitosan in continuous system to determine higher conditions with appropriate conditions in the batch adsorption system.
Experiments should be carried out by changing the particle size and mixing speed of chitosan and the effects of particle size and mixing speed on the adsorption capacity should be investigated.
Applability to desorption processes should be investigated to ensure reuse of adsorbents.