Modifiye havuç atıklarıyla bazı ağır metallerin sulu çözeltilerden uzaklaştırılması
Removal of some heavy metals from aqueous solutions by modified carrot peel
Yazar: ZEYNEP KORKUT
Danışman: YRD. DOÇ. DR. FERAH ÇALIŞIR
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Kimya Ana Bilim Dalı
Derece: Yüksek Lisans, 2014
Konu: Kimya = Chemistry

Özet:
Sanayinin gelişmesi ve hızlı nüfus artışı çevre kirliliğini, dolayısıyla su kaynaklarının kirlenmesini de beraberinde getirmiştir. Ağır metallerin önemli bir kirletici grubu oluşturdukları bilinmektedir. Toprakta ve atık sularda bulunan ağır metaller hem insanlar hem de diğer canlılar için oldukça zehirli elementlerdir. Bunların toksik ve kanserojen etkileri olduğu gibi, canlı organizmalarda birikme eğilimi de söz konusudur. Endüstriyel atık sulardaki ağır metallerin yarattıkları çevre tehlikesi ve zehirli etkileri nedeniyle uzaklaştırılması insan sağlığı açısından çok önemlidir. Ağır metallerin gerek endüstriyel atık sulardan ve gerekse ağır metal ile kirlenmiş/kirletilmiş çevresel su kaynaklarından uzaklaştırılmasında çöktürme, iyon değişimi, koagülasyon, kompleksleştirme, ters osmoz, elektrodiyaliz ve çözücü ekstraksiyonu gibi çeşitli yöntemler kullanılmaktadır. Tüm bu yöntemlerin; iyonların tamamının giderilememesi, yüksek kirlilik düzeylerinde etkin olamamaları, yüksek enerji tüketimi gibi dezavantajları vardır. Son yıllarda atık sulardan ağır metal uzaklaştırılmasında mevcut pahalı metodların yerini alan adsorpsiyon yöntemi ise; kullanılan adsorbanın ucuz ve bol miktarlarda bulunabilmesi, hızlı sonuç vermesi, tekrarlanabilirliği ve kolay uygulanabilirliği ile en çok tercih edilen yöntemlerden birisi olmuştur.  Tüketim hızının giderek arttığı dünyada, özellikle tarımsal kökenli atıkların, ağır metal uzaklaştırılmasında adsorban olarak kullanımı yaygınlaşmıştır. Son yıllarda bu konuda yapılan çalışmalar, muz kabuğu, ayçiçeği sapı, pirinç kabuğu, portakal kabuğu gibi tarımsal kökenli atıkların organik ve inorganik bir takım kirleticilerin su ortamından uzaklaştırılmasında önemli rol oynadıklarını göstermiştir.  
Bu çalışmada, bakır, mangan, nikel ve kadmiyum metal iyonlarının sulu çözeltilerden uzaklaştırılması amacıyla havuç atıkları adsorbent olarak seçildi. Havuç atıkları sitrik asit ve okzalik asit ile modifiye edilerek, metal iyonlarını uzaklaştırma kapasiteleri incelendi. Batch ( çalkalama) yöntemi kullanılarak modifiye edilmiş havuç atıklarına metallerin adsorpsiyonu; pH, temas süresi, adsorbent miktarı ve metal iyon konsantrasyonu parametrelerine bağlı olarak incelendi. Süzüntüde kalan metal iyonu konsantrasyonları atomlaştırıcı olarak hava/asetilen alevinin kullanıldığı alevli atomik absorpsiyon spektrofotometresi ile tayin edildi. Optimum değerler pH 5, temas süresi 60 dakika, adsorbent miktarı 0,5 gram olarak bulundu. Modifiye havuç atıkları kullanılarak sulu çözeltilerden Cu(II), Mn(II), Ni(II) ve Cd(II) metallerinin adsorpsiyonu 0,5 gram adsorbent miktarı için sırasıyla %99,5, %84,4, %89,9 ve %98,0 olarak saptanmıştır. Metal iyonlarının Cu+2 > Cd+2 > Ni+2 > Mn+2 sırasında adsorplandığı gözlemlendi.
Modifiye havuç atıklarının yapısını aydınlatmak amacıyla Fourier Dönüşümlü Infrared Spektrometri ile analizleri yapıldı. Ham havuç atıklarının IR spektrumlarına bakıldığında 3336 cm-1' de yayvan O-H gerilme bantları ve 2924 cm-1' de alifatik C-H gerilme bantları gözlenmiştir. Ayrıca 1600-1400 cm-1 civarında alifatik C=C gerilme titreşimlerine ait bantlar gözlemlenmiştir. 1370-1144 cm-1 civarında C-H eğilme titreşimleri ve 1020 cm-1'de C-O titreşim bantları gözlemlenmiştir. Sitrik asit ile modifiye edilmiş havuçta ise, benzer titreşim bantları gözlenmekle birlikte modifikasyonun gerçekleştiğini ortaya koyan karakterize pikler de gözlemlenmiştir. Sitrik asit yapısına ait karboksilli asit gruplarının havuçtaki –OH gruplarıyla esterleşme tepkimesine girerek 1727 cm-1'de –COOR gerilme bantları oluştuğunu görülmüştür. Modifiye havuçtaki –OH gerilme bantları daha yüksek frekansa kaymıştır. Bu da karbonil gruplarıyla hidrojen bağları oluşturan –OH gerilmelerinden kaynaklanıyor olabilir. Hidrojen bağının etkisi modifikasyonla daha da belirgin hale gelmiştir.
Adsorpsiyon denge çalışmalarından elde edilen sonuçlar Langmuir ve Freundlich izoterm modellerine uygulandı. Adsorpsiyon çalışmaları ile Freundlich İzoterm modelinin Langmuir izoterm modeline göre daha iyi uyduğu gözlemlenmiştir. Cu(II), Mn(II), Cd(II) ve Ni(II) için izoterm verilerinden hesaplanan maksimum adsorpsiyon kapasiteleri sırasıyla 9.61 mg/g, 12.74 mg/g, 11.06 mg7g and 8.67 mg/g olarak bulunmuştur.
Gözlenebilme sınırının tayini için 10 paralel metot körüne, geliştirilen yöntem uygulandı. Yöntemin gözlenebilme sınırı (LOD) N=10 ve 3σ olarak Cu(II), Ni(II), Cd(II) ve Mn(II) için sırasıyla 8,2 µg/L, 18,3 µg/L, 2,8 µg/L ve 13,7 µg/L'dir. Tayin sınırı (LOQ) ise 10σ alınarak sırasıyla 27,4 µg/L, 61,1 µg/L, 9,4 µg/L ve 45,7 µg/L olarak bulunmuştur.
Adsorpsiyon özellikleri incelendiğinde modifiye havuç atıklarının, metallerin sulu çözeltilerden uzaklaştırılmasında adsorbent olarak kullanılabileceğini göstermiştir.

Abstract:
Heavy metal pollution is one of the most important environmental problems today because of the growing industrial activities and discharging wastes containing heavy metals from various industries, such as electrolysis, metallurgy, metal surface treating, surface finishing industry, photography, fuel production, electronic, electroplating, atomic energy installation,  mining, milling,  and battery industry  etc.
Heavy metals are not biodegradable and tend to accumulate in living organisms. They are highly toxic even at very low concentrations. Their presences in soils and waters have been responsible for several health problems with living organisms and human beings. Toxic metal ions can cause adverse health effects and physical discomfort. Removal of heavy metal ions from aqueous solutions is very important because of their hazardous effects on environment and many life forms. 
To remove heavy metal ions from aqueous solutions, some conventional methods have been suggested, such as filtration, chemical precipitation, ion exchange, reverse osmosis, complexation, electrochemical treatment, electrolysis, membrane technologies, evaporation etc. Most of the methods have low efficiency and high operation costs for removing heavy metals from effluents at low concentrations. 
The search for new methods leads to adsorption which one of the easiest and cost effective in removing heavy metal ions. Many research works have been done recently to find the potential of using various alternative adsorbents employed by industrial by-product, natural material, or modified polymers in the treatment of heavy metal contaminated wastewater.  
Adsorption is the forces of attraction exist between adsorbate and adsorbent. These forces of attraction can be due to Vander Waal forces of attraction which are weak forces or due to chemical bond which are strong forces of attraction. On the basis of type of forces of attraction existing between adsorbate and adsorbent, adsorption can be classified into two types: Physical adsorption or chemical adsorption. When the force of attraction existing between adsorbate and adsorbent are weak Vander Waal forces of attraction, the process is called physical adsorption or physisorption. In chemical adsorption or chemisorption, the forces of attraction between the adsorbate and the adsorbent are very strong; the molecules of adsorbate form chemical bonds with the molecules of the adsorbent present in the surface. Besides physical and chemical adsorption, biosorption has gained great attention for removal of heavy metals in recent years. Biosorption is a physiochemical process that occurs naturally in certain biomass which allows it to bind contaminants onto its cellular structure.
Biosorption is based on metal binding capacities of various biological materials. It is considered to be a fast physical or chemical process. According to literature, biosorption can be divided into two main proceses: adsorption of the ions on cell surface and bioaccumulation within the cell.
Biosorption has many advantages like low cost, high efficiency of metal removal from dilute solution, minimization of toxic sludge, no additional nutrient requirement, regeneration of biosorbent and the possibility of metal recovery. For the removal of heavy metal ions, biosorption may provide an attractive alternative to conventional methods.
There has been a significant increase in the studies focused on biosorption. In literature, most studies on the biosorption of heavy metal ions directed to optimization of biosorption parameters to obtain the highest adsorption capacity and some of them are related with the biosorption mechanism. 
Many types of biomass have been reported to have high removal capacities for heavy metals. Living or dead biomass, agricultural waste or industrial by-products can be used as low cost biosorbents. If an adsorbent is widely available in nature or is a by-product or waste material from another industry, it can be named as a low cost adsorbent. In different studies, so many kind of biomass have been used as biosorbents such as citrus pectin, potato peel waste, straw, tree bark, peat moss, hazelnut shells, sugarcane bagasse, rice husk, oil palm shell, coconut shell, coconut husk.
A solid material should have some properties such as; good mechanical resistance and thermal stability, large surface area, high particle size, low pH range, low blank values, little/ none contamination and loss of matter.
The objective of this study is to develop inexpensive and effective biosorbent that is easily available in large quantities and feasible economically for removal of heavy metal ions in solution. For this purpose, carrot peel is tested during this study for the biosorption of copper, cadmium, manganese and nickel metal ions in single and multiple metal solutions under controlled experimental conditions.
 A series of carrot peel biosorbents were prepared by different chemical modifications. Sodium hydroxide, citric acid and oxalic acid were used for modification processes. Citric acid modified carrot peel was chosen as an adsorbent for this study. Carrot peel was stirred in 20% aqueous isopropyl alcohol for about 24 hours at room temperature. The sample was filtered, repeatedly washed with deionized water until it had no color in the filtrate, and then dried in an oven at 60 °C for 8 hours. This sample was stirred with 0.1 mol/L NaOH (10% w/v) for 1 hour at room temperature, then filtered, dried at 60 °C for 8 hours and washed with distilled water to neutral pH. After this step, carrot peel was mixed with 0.6 mol/L citric acid and stirred for 2 hours at 80 °C. The acid/peel slurry was filtered then washed with distilled water until the pH was neutral and dried at 60 °C for 8 hours. The effects of chemical modification methods on the adsorption capacity have been investigated. Changes of the surface properties were characterized using the Fourier transform infrared spectroscopy (FTIR). Maximum adsorption capacities were obtained by modification with citric acid. 
A batch adsorption system was applied to study the adsorption of metal ions from aqueous solutions by citric acid modified carrot peel.  For this purpose, the effects of solution pH, contact time, metal ion concentrations, and sorbent dosage were studied in batch experiments. Solution pH has been reported to be the most important variable governing the adsorption of metal ions by the sorbent. pH affects both the solubility of metal ions and the ionization states of functional groups, such as carboxyl and hydroxyl. In order to establish how pH affects metal ion sorption onto modified carrot peel, batch equilibrium studies were conducted at different initial pH values. The effects of pH ranging from 2.0 to 7.0, contact time ranging from 30 min to 180 min and initial metal concentration ranging from 1 mg/L to 30 mg/L on the removal of metal ions were studied. 
Biosorbent dose is simply optimized by varying the amount of adsorbent in contact with a given volume of aqueous solution, and then, the capacity of the adsorbent for the pollutant and percentage removal with changing adsorbent dose is determined. The adsrobent dose that thene corresponds with the highest precentage heavy metal removal is considered to be the optimum dose for the particular adsorbent. Results show that adsorption of metal ions is pH-dependent and the best results are obtained at pH = 5.0. The heavy metal removal efficiencies were 99.5% for copper, 84.4% for manganese, 89.9% for nickel and 98.0% for cadmium.
 Langmuir and Freundlich adsorption models were applied to describe the isotherms and isotherm constants. These models are based on monolayer adsorption of solute and applied for single solute systems. The maximum metal adsorption capacities were calculated from Langmuir and Freundlich adsorption data. Adsorption data showed that the Freundlich adsorpston model is fitted better than Langmuir isoterm model. The maximum adsorption capasities for Cu(II), Mn(II), Cd(II) and Ni(II) were 9.61 mg/g, 12.74 mg/g, 11.06 mg/g and 8.67 mg/g, respectively.
Changes of the surface properties of modified carrot peel were examined by the Fourier Transformed Infrared Spectroscopic analysis. FTIR spectra showed that the principal functional sites taking part in the sorption process included carboxyl and hydroxyl groups. For this purpose, IR spectrums of carrot peel and modified carrot peel  were analyzed. In the IR spectra of carrot peel, some functional peaks are observed such as those wave number 3336 cm-1 for  –OH, 2924 cm-1 for alifatic –CH, 1600-1400 cm-1 for C=C,1370-1144 cm-1 for C-H bending and 1020 cm-1 for C-O vibration. In the IR spectra of modified carrot peel, some peaks shifted a little, such as –OH streching and –COO streching. 
After optimizing the method, it was applied to 10 parallel blank samples to determine limit of detection (LOD) and limit of quantification (LOQ). The limit of detection (3σ, N=10) values for Cu, Ni, Cd and Mn were 8.2 µg/L, 18.3 µg/L, 2.8 µg/L and 13.7 µg/L and the limit of quantification (10σ, N=10) were 2.4 µg/L, 61.1 µg/L, 9.4 µg/L and 45.7 µg/L respectively.
Results showed that the carrot peel could be an effective and environmentally friendly adsorbent to remove the metal ions from aqueous solutions.