Biyosorbent Kullanılarak Altın Çözeltisinden Altın Geri Kazanımının Uygun Koşullarının Belirlenmesi Ve Kinetiğinin İncelenmesi

Abstract

Altın, parlak sarı renkte çok değerli bir metalik elementtir. Altın günümüzde sadece mücevher olarak kullanılmayıp daha da önemlisi yüksek teknolojilerde önemli derecede kullanılmaktadır. Bu kullanım alanlarından dolayı elektronik endüstrisi için altın ihtiyacı sürekli artmaktadır. Altın varlığının sınırlı olduğu düşünülürse, bu altın talebinin karşılanması amacıyla atık sularla atılan altının geri kazanılması önemli bir teknoloji olarak karşımıza çıkmaktadır. Altının geri kazanılma işleminde yıkama, iyon değişimi ve adsorplama gibi birçok uygulama yöntemi bulunmaktadır. Adsorplama yöntemi kullanıldığında değişik sorbentlerin kullanıldığı bilinmektedir. Bunlardan bazıları tanin, iyon değiştirme reçinesi, aktif karbon ve mantarsı biyokütle olarak sayılabilir. Yapılan son çalışmalar incelendiği zaman adsorpsiyon işleminin biyokütle ile yapılması yani bu işlemde biyosorbent kullanılması gittikçe önem kazanmaya başlamıştır. Biyosorbentlerin hem metal geri kazanımı sonrasında tekrar kullanılabilmesi hem de ucuz ve kolay ulaşılabilir olması nedeniyle adsorpsiyonda kullanılmasını cazip hale getirmektedir. Biyosorbentler, altın gibi değerli maddelerin ve birçok metal iyonlarının geri kazanımını sağladığı gibi atık suların çevreye zarar vermesini engellemesi açısından da önem taşımaktadır. Bu çalışmada 3 farklı biyosorbent olan kitin, kitosan ve suberinle ve inorganik bir yapıya sahip olan vermikülitle yapılan altın adsorplama işlemiyle geri kazanımının yüksek verime ulaşması amacıyla uygun koşullar belirlenmiştir. Bu uygun koşullar, adsorban miktarı, adsorpsiyon süresi, karıştırma hızı, sıcaklık, pH ve başlangıç altın konsantrasyonu gibi fiziksel etkiler şeklinde açıklanmıştır. Adsorpsiyon işlemlerinin ardından çözeltiler içinde kalan altın konsantrasyonları indüktif olarak eşlemiş plazma atomik emisyon spektrometrik (ICP-AES) yöntemle belirlenmiştir. Bu adsorpsiyon işlemlerinin kinetiği belirlenmiş, aktivasyon enerjisi ve Langmuir ve Freundlich modellerindeki uygunluğu incelenmiş ve maksimum altın tutma değeri bulunmuştur. Adsorpsiyon işleminin sonunda adsorbanların tarayıcı elektron mikroskopisi (SEM) ve X-Ray difraksiyonu (XRD) analizleri de yapılmıştır.

Gold is a very precious material which has been used in jewelry and many high technologies in last decade. Gold is used appreciable amounts in plating materials and electronic parts. For example, while a per ton of gold ore contents only 5-30 gr Au, cellular phones consists of 200 g Au per ton of scrap. Therefore, it is obvious that with growing the electronics industry, gold demand has been increased dramatically. Because of this precious material has limited availability, gold recovery is an important technology that is adsorption of gold from wastewater. The traditional methods for gold recovery are ion exchange, leaching and adsorption. Adsorption can be defined as the uptake of ions, atoms or molecules by solid sorbents. Sorbents adsorbs these materials to their surfaces. Many sorbents is used for adsorption of gold including activated carbon, persimmon tannin gel, ion-exchange resins, fungal biomass, inorganic materials etc. Recent studies demonstrate that biosorbents are important materials for gold recovery so interest of these materials increases. Biosorbents consist of inactive, dead or microbial biomass materials that can adsorb metallic ions from aqueous solutions. Biosorbents can be easily regenerated and reuse for metallic recovery. They are also very cheap and reachable materials. Therefore, it is clearly said that biosorbents are one of best option for metallic recovery. In this study, gold was recovered by chitin, chitosan, suberin and vermiculite with different conditions in order to reach high efficient recovery. Best recovery conditions were explained in amounts of sorbent, adsorption time, temperature, shaking rate, pH and initial gold concentration. The remaining gold concentration was measured by inductively coupled plasma atomic emission spectroscopy (ICP). Moreover, this adsorption reaction kinetics was described as reaction rate and activation energy. It was also investigated whether Langmuir and Freundlich adsorption models are fitted to these adsorption processes. Furthermore, X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) analyses were used for observing adsorption process. In results, the best recovery conditions for high efficiency gold recovery by using chitin are 10 mg chitin for amount of sorbent, 40 min for adsorption time, 120 rpm for shaking rate, 30 oC for temperature and nearly 3 and 4 for pH. For kinetic study, reaction rates are determined for every temperature and the activation energy is calculated to be 24.01 kJ/kmol. Gold recovery by chitin is fitted to Langmuir adsorption model and maximum adsorbed gold is 0.99 mg for 10 mg chitin. Secondly, The best recovery conditions for high efficiency gold recovery by using chitosan are 1.5 mg chitosan for amount of sorbent, 60 min for adsorption time, 100 rpm for shaking rate, 30 oC for temperature and nearly 4 for pH. For kinetic study, reaction rates are determined for every temperature and the activation energy is calculated to be 14.04 kJ/kmol. Gold recovery by chitosan is fitted to Langmuir adsorption model and maximum adsorbed gold is 0.93 mg for 1.5 mg chitosan. Moreover, The best recovery conditions for high efficiency gold recovery by using suberin are 5 mg suberin for amount of sorbent, 60 min for adsorption time, 120 rpm for shaking rate, 30 oC for temperature and nearly 1 and 3 for pH. For kinetic study, reaction rates are determined for every temperature and the activation energy is calculated to be 6.48 kJ/kmol. Gold recovery by suberin is fitted to Freundlich adsorption model. Furthmore, the best recovery conditions for high efficiency gold recovery by using vermicilute are 40 mg vermicilute for amount of sorbent, 60 min for adsorption time, 140 rpm for shaking rate, 50 oC for temperature and 1 for pH. For kinetic study, reaction rates are determined for every temperature and the activation energy is calculated to be 79.31 kJ/kmol. Gold recovery by vermiculite is fitted to Langmuir adsorption model and maximum adsorbed gold is 0.92 mg for 40 mg vermiculite. In these results of these experiments, when compared to these four substances, the best substance for gold recovery is chitosan since 1.5 mg chitosan adsorbed nearly all gold ions. The other subtances, which are suberin, chitin and vermicilute, are 5 mg, 10 mg and 40 mg respectively. When adsorption times were investigated, it is clearly said that adsorption is nearly finished at the end of 1 hour. After 1 hour, desorption started to occur. When shaking rates were investigated, gold recovery was increased when shaking rates are increased from 40 rpm to 100, 120 or 140 rpm. When temperature was investigated, 30 oC was best recovery conditions and there was not a strict change with changing temperature for biosorbents that means that adsorption with biosorbents is temperature independent. However, 50 oC was best recovery condition for vermicilute, which means that the adsorption with vermicilute is temperature dependent. Therefore, the operating range for biosorbents is large but it is narrow for vermicilute. When pH values were investigated, larger acidic solution is larger gold recovery for vermicilute because 1 is the best recovery condition for pH. In adsorption models, chitin, chitosan and vermicilute are fitted to Langmuir adsorption model and suberin is fitted to Freundlich adsorption model. When fitting Langmuir, it means that adsorption occurred in limited region of surface and it is one layer adsorption. When fitting Freundlich, it means that the adsorb material is heterogenous. The heterogenous structure of suberin (both contains aromatic and aliphatic structure) verified this information. In Langmuir adsorption model, amount of maximum adsorbed gold is very close for chitin, chitosan and vermicilute. However, 1.5 mg chitosan, 10 mg chitin and 40 mg vermicilute adsorbed this amount gold (nearly 1 mg gold). Therefore, chitosan is more effective than chitin and vermicilute according to Langmuir adsorption model.