Publication: Investigation of heavy metal ion removal by cellulose based adsorbents with ionic functional groups
Abstract
Ağır metal kirliliği, kentsel ve endüstriyel gelişimin artmasıyla tüm dünyada etkisini hissettirmeye başlamıştır. Nehirlerin, göllerin ve diğer su kaynaklarının ağır metal kirliliği, yetersiz su ve atık su arıtımı ve artan endüstriyel faaliyetlerin bir sonucudur. Bu soruna çözüm üretmek amacıyla, son yıllarda, çok sayıda çalışma, su ve atıksuların ağır metal kirliliğinden arındırılması için yeni adsorbanların geliştirilmesine odaklanmıştır. Bu çalışmada, doğada bol ve yaygın olarak bulunan, ucuz ve erişimi kolay bir kaynak olan selüloz kullanılarak sudaki ağır metalleri tutabilecek iyonik fonksiyonel gruplara sahip yeni bir adsorban malzemenin sentezlenmesi amaçlanmıştır. Adsorban malzemenin üretiminde kullanılacak selülozun yüzeyindeki hidroksil gruplarını daha erişilebilir hale getirmek ve bu sayede selülozun daha reaktif olmasını sağlamak amacıyla sentezlemenin ilk adımında selüloz aktivasyon işlemi uygulanmıştır. PUF sentezinde kullanılan izosiyanat yapısının ağır metal giderim verimine etkisini araştırmak amacıyla, selüloz bazlı poliüretan köpük numunelerinin sentezinde, iki farklı diizosiyanat; alifatik diizosiyanat, 1,6-heksametilen diizosiyanat ve aromatik diizosiyanat, 1,4-fenilen diizosiyanat, kullanılmıştır. Bu yaklaşımla toplam altı farklı selüloz bazlı köpük adsorban sentezlenmiştir. Sentezlenen her köpük örneğinin ağır metal giderim kapasitesinin belirlenmesi için Co2+, Cu2+ ve Ni2+ iyonları model ağır metal iyonları olarak seçilmiştir. Co2+, Cu2+ ve Ni2+ sulu çözeltileri kullanılarak , bu tez boyunca AC-HMPUF, AC-PPUF, CAc-PPUF, CAc-HMPUF, CMC-PPUF ve CMCHMPUF olarak isimlendirilecek olan altı farklı selüloz bazlı adsorban ile Kesikli adsorpsiyon deneyleri gerçekleştirilmiştir. Bu deneylerde, CAc-PPUF'nin, diğer absorban örneklerine göre, 25°C'de, 100 mg/ L'lik başlangıç ağır metal konsantrasyonunda, 10 g/ L'lik adsorban dozunda ve 6,5'lik pH'ta, 24 saatlik çalkalama süresi sonunda, Co2+, Cu2+ ve Ni2+ iyonlarını daha yüksek verimle giderdiği gözlemlenmiştir. Belirtilen koşullar altında, bakır, kobalt ve nikel iyonları için CAcPPUF ile giderim verimleri sırasıyla %62.42, %36.39 ve %23.2 olarak tespit edilmiştir. CAc-PPUF, Fourier Dönüşümü Kızılötesi Analizi (FTIR), Taramalı Elektron Mikroskobu (SEM), BET yüzey alanı (BET) ve Zeta Potential analizleri ile karakterize edilmiştir. Önerilen CAc-PPUF yapısı, FTIR analizi ile doğrulanmıştır. SEM görüntüleri, CAc-PPUF’nin değişken mikro gözenek boşluklarına sahip oldukça gözenekli amorf bir yapı olduğunu göstermektedir. CAc-PPUF'un mikrogözenekli yapısı, BET testleri ile incelenmiş ve malzemenin ortalama gözenek yarıçapının 19.96 nm, yüzey alanının ise 23.15 m2 / g olduğu ortaya konmuştur. pH 4-9 aralığında, CAcPPUF'nin zeta potansiyeli negatif bir yük göstermiştir. Geniş bir pH aralığında zeta potansiyel değerlerinin negatif ölçülmesinin, yüzey fonksiyonel gruplarının Ka değerlerinin düşük olmasına bağlı olabileceği düşünülmektedir. CAc-PPUF'nin zeta potansiyel değerlerinin geniş bir pH aralığında negatif olduğunun belirlenmesi, suda pozitif yüke sahip olan ağır metal iyonlarının uzaklaştırılması için uygun yapıda bir adsorban olabileceğini göstermektedir. CAc-PPUF'nin ağır metal adsorpsiyonunu tanımlayan uygun izoterm modelini belirlemek için doğrusal olmayan regresyon yöntemi kullanılmıştır. Uygunluğu incelenmek üzere, literatürde adsorpsiyon ile ağır metal giderimi çalışmalarında en yaygın şekilde karşımıza çıkan Langmuir, Freundlich, Sips ve Dubinin-Astakhov izoterm modelleri seçilmiştir. Deneysel veriler istatistiksel olarak model tahminleri ile karşılaştırılarak her bir model tahmini için R2 , χ2 ve NRMSE değerleri hesaplanmıştır. Seçilen 4 model ile elde edilen istatistiki değerler (R2 , χ2 ve NRMSE) karşılaştırıldığında 4 modelin de deneysel verilere oldukça yakın tahmin sonuçları verdiği görülmektedir. Freundlich, Sips ve Dubinin-Astakhov modelleri, Cu2+ adsorpsiyonu için göreceli olarak biraz daha uygun istatistiki sonuçlar vermektedir. DA modeli, CAc-PPUF üzerine kobalt iyonu adsorpsiyonu için istatistiksel olarak daha iyi tahmin özelliği sergilemektedir. Üç parametreli izoterm modelinin (Sips ve DubininAstakhov), CAc-PPUF üzerindeki Ni2+ izotermi için deneysel verilere biraz daha iyi uyduğu gözlemlenmiştir. Freundlich, Sips ve Dubinin-Astakhov modelleri heterojen yüzeye sahip adsorbanlar için uygun modellerdir. CAc-PPUF’nin yüzeyindeki asetat grupları, oksijen atomları ve nitrojen atomları, ağır metal iyonları için bağlanma yerleri olma özelliği taşımaktadırlar. Bu nedenle, CAc-PPUF'nin yüzeyinin heterojen bir yüzey olduğunu söyleyebiliriz. Dolayısıyla bu sonuçlar, CAc-PPUF'nin heterojen doğası ile iyi bir uyum içindedir. İzoterm deneyleri ayrıca, CAc-PPUF'nin etkinliğinin, çözeltideki ağır metallerin başlangıç konsantrasyonu ile ters orantılı olduğunu göstermiştir. Sudaki ağır metal başlangıç konsantrasyonu arttığında, giderim veriminin azaldığı gözlemlenmiştir. CAc-PPUF'nin ağır metal adsorpsiyon hızının belirlenmesi için kinetik
Heavy metal contamination is spreading throughout the world, especially as urban and industrial development expands. Heavy metal contamination of rivers, lakes, and other water resources has increased as a result of inadequate water and wastewater treatment, and rising industrial activities. To address this problem, numerous studies have concentrated on developing novel adsorbents for the efficient removal of heavy metals from polluted waters. In this study, we concentrated on the synthesis of a special foam material with particular ionic functional groups utilizing cellulose, a naturally occurring resource that is quite abundant. Synthesis procedures started with the activation of cellulose before its use in the foam formation reaction to make the hydroxyl groups at the surface of the biopolymer more accessible, and thus reactive. In order to investigate the effect of isocyanate structure used for the PUF synthesis on heavy metal removal efficiency, two different diisocyanates; an aliphatic diisocyanate, 1,6-hexamethylene diisocyanate, and an aromatic diisocyanate, 1,4-phenylene diisocyanate, were used for the synthesis of cellulose based polyurethane foam samples. As such, six different cellulose based foam adsorbents were synthesized. Heavy metal removal capacity of each synthesized foam sample was tested for the removal of Co2+, Cu2+, and Ni2+ ions as the model heavy metals. Batch adsorption experiments were conducted using six different cellulose based adsorbents, which throughout this thesis are abbreviated as AC-HMPUF, AC-PPUF, CAc-PPUF, CAc-HMPUF, CMC-PPUF and CMC-HMPUF. Batch adsorption experiments at 25 ℃ for 24 hours demonstrated that CAc-PPUF displayed better Co2+ Cu2+, and Ni2+ removal efficiencies at initial heavy metal concentration of 100 mg/ L, adsorbent dose of 10 g/ L, and pH of 6.5. Under the given conditions, the removal efficiencies were 62.42%, 36.39%, and 23.2% for copper, cobalt, and nickel ions, respectively. CAc-PPUF was characterized by Fourier Transform Infrared Analysis (FTIR), Scanning Electron Microscopy (SEM), BET surface area (BET), and Zeta Potential analyses. The proposed structure of CAc-PPUF was confirmed by FTIR analysis. SEM exhibits a highly porous amorphous structure with variable micropore spaces. The mesoporous nature of CAc-PPUF was studied using BET analysis which reveals that the average pore radius of the material is 19.96 nm while the surface area is 23.15 m2 / g. In the pH range of 4–9, zeta potential of CAc–PPUF demonstrated a negative charge. This is attributed to the low Ka values of surface functional groups. Negative zeta potential of CAc-PPUF over a wide pH range makes it a promising adsorbent for the removal of heavy metals since heavy metal ions are positively charged in water. The isotherm model best suitable for predicting the heavy metal adsorption capacity of CAc-PPUF was identified using the non-linear regression technique. For this purpose, the Langmuir, Freundlich, Sips, and Dubinin-Astakhov isotherm models, which are the most widely used in the literature to simulate the adsorption of heavy metals were selected. By using the experimental observations and the model predictions as constraints, the R2 , χ2, and NRMSE values for each model prediction were determined. Statistical analysis of each isotherm model prediction revealed that the predictions are highly comparable. Freundlich, Sips, and Dubinin-Astakhov models have slightly better fits for Cu2+ adsorption. DA model had statistically better predictions for cobalt ion adsorption onto CAc-PPUF. Three parameter isotherm models (Sips and DubininAstakhov) were observed to show slightly better fits to the experimental data for Ni2+ adsorption onto CAc-PPUF. Freundlich, Sips and Dubinin-Astakhov models are described for adsorbents with heterogeneous surfaces. In CAc-PPUF, acetate groups, oxygen atoms, and nitrogen atoms act as binding sites for heavy metal ions. Hence, the surface of CAc-PPUF is heterogeneous. These results, therefore, agree well with the heterogeneous nature of CAc-PPUF. Isotherm experiments further demonstrated that the heavy metal removal of CAc-PPUF was inversely proportional to the initial concentration of heavy metals in solution. It was found that when the initial concentration of heavy metals in the water increased, the removal efficacy decreased. Kinetic experiments were designed to determine the heavy metal adsorption rate of CAcPPUF. In these experiments, Cu was used as the model heavy metal ion and equal volumes of 100 ppm Cu2+ solutions were subjected to the same amounts of CAc-PPUF adsorption at different time intervals. The compatibility of the experimental data with the pseudo-first-order (PFO), pseudo-second-order (PFO) and Elovich models, which are three commonly used kinetic models in the literature, was also evaluated using the nonlinear regression method. It has been shown that the adsorption kinetics of Cu ion on the CAc-PPUF surface is compatible with heavy metals.
Heavy metal contamination is spreading throughout the world, especially as urban and industrial development expands. Heavy metal contamination of rivers, lakes, and other water resources has increased as a result of inadequate water and wastewater treatment, and rising industrial activities. To address this problem, numerous studies have concentrated on developing novel adsorbents for the efficient removal of heavy metals from polluted waters. In this study, we concentrated on the synthesis of a special foam material with particular ionic functional groups utilizing cellulose, a naturally occurring resource that is quite abundant. Synthesis procedures started with the activation of cellulose before its use in the foam formation reaction to make the hydroxyl groups at the surface of the biopolymer more accessible, and thus reactive. In order to investigate the effect of isocyanate structure used for the PUF synthesis on heavy metal removal efficiency, two different diisocyanates; an aliphatic diisocyanate, 1,6-hexamethylene diisocyanate, and an aromatic diisocyanate, 1,4-phenylene diisocyanate, were used for the synthesis of cellulose based polyurethane foam samples. As such, six different cellulose based foam adsorbents were synthesized. Heavy metal removal capacity of each synthesized foam sample was tested for the removal of Co2+, Cu2+, and Ni2+ ions as the model heavy metals. Batch adsorption experiments were conducted using six different cellulose based adsorbents, which throughout this thesis are abbreviated as AC-HMPUF, AC-PPUF, CAc-PPUF, CAc-HMPUF, CMC-PPUF and CMC-HMPUF. Batch adsorption experiments at 25 ℃ for 24 hours demonstrated that CAc-PPUF displayed better Co2+ Cu2+, and Ni2+ removal efficiencies at initial heavy metal concentration of 100 mg/ L, adsorbent dose of 10 g/ L, and pH of 6.5. Under the given conditions, the removal efficiencies were 62.42%, 36.39%, and 23.2% for copper, cobalt, and nickel ions, respectively. CAc-PPUF was characterized by Fourier Transform Infrared Analysis (FTIR), Scanning Electron Microscopy (SEM), BET surface area (BET), and Zeta Potential analyses. The proposed structure of CAc-PPUF was confirmed by FTIR analysis. SEM exhibits a highly porous amorphous structure with variable micropore spaces. The mesoporous nature of CAc-PPUF was studied using BET analysis which reveals that the average pore radius of the material is 19.96 nm while the surface area is 23.15 m2 / g. In the pH range of 4–9, zeta potential of CAc–PPUF demonstrated a negative charge. This is attributed to the low Ka values of surface functional groups. Negative zeta potential of CAc-PPUF over a wide pH range makes it a promising adsorbent for the removal of heavy metals since heavy metal ions are positively charged in water. The isotherm model best suitable for predicting the heavy metal adsorption capacity of CAc-PPUF was identified using the non-linear regression technique. For this purpose, the Langmuir, Freundlich, Sips, and Dubinin-Astakhov isotherm models, which are the most widely used in the literature to simulate the adsorption of heavy metals were selected. By using the experimental observations and the model predictions as constraints, the R2 , χ2, and NRMSE values for each model prediction were determined. Statistical analysis of each isotherm model prediction revealed that the predictions are highly comparable. Freundlich, Sips, and Dubinin-Astakhov models have slightly better fits for Cu2+ adsorption. DA model had statistically better predictions for cobalt ion adsorption onto CAc-PPUF. Three parameter isotherm models (Sips and DubininAstakhov) were observed to show slightly better fits to the experimental data for Ni2+ adsorption onto CAc-PPUF. Freundlich, Sips and Dubinin-Astakhov models are described for adsorbents with heterogeneous surfaces. In CAc-PPUF, acetate groups, oxygen atoms, and nitrogen atoms act as binding sites for heavy metal ions. Hence, the surface of CAc-PPUF is heterogeneous. These results, therefore, agree well with the heterogeneous nature of CAc-PPUF. Isotherm experiments further demonstrated that the heavy metal removal of CAc-PPUF was inversely proportional to the initial concentration of heavy metals in solution. It was found that when the initial concentration of heavy metals in the water increased, the removal efficacy decreased. Kinetic experiments were designed to determine the heavy metal adsorption rate of CAcPPUF. In these experiments, Cu was used as the model heavy metal ion and equal volumes of 100 ppm Cu2+ solutions were subjected to the same amounts of CAc-PPUF adsorption at different time intervals. The compatibility of the experimental data with the pseudo-first-order (PFO), pseudo-second-order (PFO) and Elovich models, which are three commonly used kinetic models in the literature, was also evaluated using the nonlinear regression method. It has been shown that the adsorption kinetics of Cu ion on the CAc-PPUF surface is compatible with heavy metals.