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Comparison Between FTIR and Boehm Titration for Activated Carbon Functional Group Quantification

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Comparison Between FTIR and Boehm Titration for Activated Carbon Functional Group Quantification Comparison Between FTIR and Boehm Titration for Activated Carbon Functional Group Quantification Chad Spreadbury, Regina Rodriguez, and David Mazyck College of Engineering, University of Florida Activated carbon (AC) is a proven effective adsorbent of contaminants in the air and water phases. This effectiveness is due to the large surface area of AC which hosts functional groups. Particularly, oxygen functional groups (e.g. carboxyls, lactones, phenols, carbonyls) have been noted as important in mercury removal. Hence, determining the quantities of these groups is crucial when AC is used for this purpose. Currently, Boehm titration is the most common method for determining the number of these functional groups. However, this test method is susceptible to high user error and require a lengthy procedure and run time. On the other hand, Fourier transform infrared (FTIR) spectroscopy excels at analyzing the functionality of an AC surface. This study investigates whether or not a correlation between Boehm titration and FTIR can be determined using quantitative and qualitative assessment. The results indicate that while using the current methodology for Boehm titrations and FTIR analysis, there is no clear correlation. However, this study does not rule out that a correlation may indeed exist between these test methods that can be elucidated with improved methodology that accounts for the unique physical and chemical characteristics of carbon. INTRODUCTION mercury (Hg0) to create oxidized mercury (Hg2+), which then undergoes chemisorption with the AC surface. ctivated carbon is used globally as an effective Boehm titrations and Fourier transform infrared (FTIR) adsorbent for removing contaminants like mercury spectroscopy are two common testing procedures for A(Hg) from air and water phases. Usage of activated assessing the functionality of AC. The underlying concept carbon is particularly effective in removing oxidized Hg, a in Boehm titrations is that strong acids/bases neutralize all component of flue gas along with its elemental form (Hg0). bases/acids, while conjugate bases of weak acids only accept The adsorption capacity of AC for various contaminants is protons (H+) from acids of higher strength (Fidel, 2013). reliant upon its surface chemistry more so than its surface These bases include sodium hydroxide (NaOH), sodium area or porosity (Barkauskas & Dervinyte, 2004; Biniak et carbonate (Na2CO3), and sodium bicarbonate (NaHCO3). al, 1997; Kim, 2010). Numerous oxygen functional groups Titrating with NaOH will neutralize carboxyl, lactone, and that populate the pores on activated carbon’s internal and phenol groups, while titrating with Na2CO3 will neutralize external surfaces affect mercury removal: carboxyls, only carboxyl and lactone groups (Fidel, 2013; Salame & lactones, phenols, and carbonyls. Figure 1 is a visual Bandosz, 2001). Lastly, titrating with NaHCO3 will just representation of carboxyl, lactone, phenol, and carbonyl neutralize carboxyl groups (Fidel, 2013; Salame & Bandosz, functional groups (Boehm, 2008). 2001). The procedure consists of first titrating a sample down to pH 2 with hydrochloric acid (HCl), then back- titrating to pH 7 with NaOH. Neutralization occurs as a result of the pKa values of these oxygen functional groups and the differing basicity of the Boehm reactants as shown in Table 1. Table 1. Boehm reactants characteristics including pKa value, pH, and functional group determination. (Fidel, 2013) Figure 1. Oxygen functional groups on carbon surfaces. Particular interest is on (a) carboxyl, (c) lactone, (e) phenols, and (f) carbonyl functional groups. (Boehm, 2008 Analyzing the chemical composition and quantity of these groups can determine the effectiveness of contaminant adsorption. Oxygen functional groups oxidize elemental University of Florida | Journal of Undergraduate Research | Volume 18, Issue 3 | Summer 2017 1 CHAD SPREADBURY, REGINA RODRIGUEZ, AND DAVID MAZYCK These qualities allow for determination of functional In order to investigate this hypothesis, powdered activated group quantities by subtraction (Fidel, 2013). Functional carbon (PAC) and graphite samples were tested using group equations are shown in Figure 2. However, a Boehm Boehm titrations and FTIR. Each PAC and graphite was titrations is a long procedure in which and results may be derived from a different carbonaceous source (e.g. wood, varied and may not be replicated. coal, etc.) and was either untreated or treated using 1M nitric acid (HNO3). The results of these two test methods indicate that there may be a correlation between the sets of data. Table 2. Oxygen functional group IR absorptions.1 (Table of Characteristic IR Absorptions) Figure 2. Calculations to determine number of oxygen functional groups on an AC surface. Fx is concentration of functional groups donating protons to Boehm reactant of pKa “x” (µmol/g). Fx1-x2 gives the concentration of functional groups within given pKa range. (Fidel, 2013) On the other hand, FTIR consists of a much quicker and more straightforward procedure. FTIR has been used to identify oxygen functional groups on an activated carbon surface (Barkauskas & Dervinyte, 2004; Biniak et al, 1997; Kim, 2010; Langley et al, 2006; Mawhinney & Yates, 2001; Pandey, 1999). This identification is possible by the principle of functional groups absorbing incoming IR light (Kim, 2010). The mechanisms of FTIR are demonstrated in Figure 3. This absorption of light results in the covalent bonds of functional groups vibrating in various modes, therby creating characteristic, identifiable spectra for each compound (Barkauskas & Dervinyte, 2004; Kim, 2010; Mawhinney & Yates, 2001; Stuart, 2004). Particularly, FTIR has been shown to be effective in assessing carbonyl functional groups on the AC surface (Barkauskas & MATERIALS & METHODS Dervinyte, 2004; Biniak et al, 1997; Pandey, 1999). Solutions for HNO3 treated activated carbon samples along with those used for Boehm titrations were prepared using nanopure deionized (DI) water. Commercially available activated carbons (A-D) and graphite were ground to a particle size of less than 45 microns and then oven-dried at 110°C for approximately 12 hours. Before being used in this experiment, the carbons were cooled to ambient temperature in a desiccator. The carbonaceous origins of PACs A-D and graphite are described in Table 3. Table 3. PAC and graphite origins. Figure 3. Conceptualization of FTIR mechanisms. (Stuart, 2004) Oxygen functional groups and their respective wavenumbers are shown in Table 2. Still, precise numbers of individual functional groups are not obtained, as FTIR is unable to fully penetrate into the pores of activated carbon. The PACs in this study were tested as untreated and treated There may be a correlation between Boehm titration and with HNO3 at a molarity of 1M, and assessed with Boehm FTIR results that allows for quantitative and qualitative titrations and FTIR to compare activated carbon’s physical determination of oxygen function groups using only FTIR and chemical characteristics. analysis. University of Florida | Journal of Undergraduate Research | Volume 18, Issue 3 | Summer 2017 2 COMPARISON BETWEEN FTIR AND BOEHM TITRATION FOR ACTIVATED CARBON FUNCTIONAL GROUP QUANTIFICATION Oxidation Treatment with Nitric Acid (HNO3) DRIFT was performed using a Nicolet Magna 760 FT-IR instrument with a SpectraTech diffuse reflectance unit Untreated PAC and graphite were measured out to 30g shown in Figure 4. The spectrometer was purged of and placed into a beaker. These samples were dried in an atmospheric contaminants at a flow rate of 50 standard cubic oven heater dryer for 12 hours at 110°C and then stored and feet per minute such that pure N2 gas filled the scan area. cooled in a vacuum desiccator. For HNO3, a 250mL Purge was allowed to run for approximately four hours prior volumetric flask was filled with 15.926mL of nitric acid to any background or samples being analyzed and the with concentration of 70% w/w and nanopure water to the detector was cooled using liquid nitrogen and stabilized for meniscus then mixed. After the nitric acid solution was at least one hour before use. prepared, 30g of PAC or graphite was placed in a 500mL round bottom flask. Next, nitric acid solution was added and mixed with the PAC or graphite on a hot stir plate for three hours. Temperature was set at 80°C throughout this procedure. The mixture was then filtered using a vacuum pump setup and 0.45 um HAWP filter paper to separate the PAC or graphite from the nitric acid solution. Alternatively, treated samples were rinsed with 1500mL boiling DI water then filtered until constant pH was obtained. The samples were dried at 110°C for 72 hours. Boehm Titration Methodology Boehm solutions, HCl, NaOH, Na2CO3, NaHCO3, were prepared to a concentration of 0.05 N. After preparation, magnetic stirrers were added to each volumetric flask, and each was sealed with parafilm. Next, these solutions were placed on magnetic plates to allow the stirrers to continuously mix the solutions. The solutions were then purged with nitrogen gas (N2) for at least two hours before and during their usage in experiments. All samples were placed in a 250mL beaker. The carbon samples used in experiments were prepared by using 0.5g PAC or graphite and 0.2g potassium chloride (KCl) along with 25mL of a chosen base: NaOH, Na2CO3, or NaHCO3. A blank was also used to determine the true molarity of the Boehm titration solutions, so that an accurate count of Figure 4. Nicolet Magna 760 FT-IR equipped with SpectraTech diffuse functional groups on activated carbon could be determined. reflectance unit. These blanks consisted of only 0.2g KCl and a chosen base: Samples were prepared with 0.35g potassium chloride NaOH, Na2CO3, or NaHCO3. A magnetic stirrer inserted (KCl) and ~0.0009g PAC or graphite, and mixed using a into each sample; specific stirrers were designated to carbon Wig-L-Bug for approximately ten seconds. Background samples and to blanks. Samples were then sealed with samples, or samples without PAC or graphite, were prepared parafilm, set up on a magnetic stir plate, and purged with N2 using the same methodology.
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