REMOVAL OF ALKALINE EARTH METALS BY USING POLYBENZOXAZINE AND CARBON-BASED NANOPOROUS MATERIALS

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1 REMOVAL OF ALKALINE EARTH METALS BY USING POLYBENZOXAZINE AND CARBON-BASED NANOPOROUS MATERIALS Chanapon Pongteeraporn a, Sujitra Wongkasemjit a,b, Thanyalak Chaisuwan a,b a The Petroleum and Petrochemical College, Chulalongkorn University, Bangkok, Thailand b Center of Excellence on Petrochemical and Materials Technology, Bangkok, Thailand Keywords: Polybenzoxazine, Nanoporous, Metals removal, Activated Carbon ABSTRACT Because of high demand of water consumption, the lack of water is an essential problem that need to be concerned. Using ground water is common in remote area in Thailand. However, Ca (II) and Mg (II), ions found in this type of water can cause some serious problems such as clogging of pipeline or failure of heat transfer in equipments in industrial machines. The objective of this work is to investigate the removal of alkaline earth metals, Ca (II) and Mg (II), from water by using polybenzoxazine and nanoporous carbon materials as anti-fouling adsorbents. The batch technique was carried out under the influences of ph, contact time and amount of adsorbent. The maximum percentages of metal removal were 93.25% for Ca 2+ and 83.96% for Mg 2+ respectively. The optimum condition was established at a basic condition, ph = 10, with 2.0 g. of adsorbent and equilibrated after 1 h. The results showed that the AC-coconut commercial activated carbon with high surface area could be an effective adsorbents to remove alkaline earth metals and the removal of Ca 2+ was higher than that of Mg 2+. thanyalak.c@chula.ac.th INTRODUCTION By definition, water hardness is a measure of the quantity of divalent ions such as calcium and magnesium in water (Meena et al., 2012). The existence of Ca 2+ and Mg 2+ ions leads to problems in cooling and heating systems including clogging pipeline or the other industrial machines. In addition, these divalent ions can react with soap anions decreasing the cleaning efficiency and hence, high consumption of detergents occurred as a result (Saeed & Hamzah, 2013). In order to removal the divalent ions (Ca 2+, Mg 2+ ) various methods have been widely applied for water softening includes, electrochemical processes, Nanofiltration, electrodialysis, ion-exchange, membranes. Adsorption methods are quite a new separation method that play very important role in separation and purification process. Polybenzoxazine (PBZ), a class of high performance phenolic material, provides many interesting characteristic properties such as excellent mechanical properties, high thermal stability, high chemical resistance, near zero-shrinkage, high char yield, low water absorption and controllable pore structure (Ishida, 2011) which can be used as an adsorbent for ion adsorption. The related works were studied by Chirachanchai and Chaisuwan studied PBZ like absorbent for metal, alkali and alkali earth metals, extraction metal removal from waste water, respectively (Chirachanchai, et al., 2000; Chaisuwan, et al., 2010). Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 1

2 In this study, we purpose to produce nanoporous polybenzoxazine-based that can be synthesized from bisphenol-a, formaldehyde and tetraethylenepentamine (TEPA) via quasi-solvent method and study the efficiency of using polybenzoxazine xerogel as an antifouling adsorbent for ion adsorption including an investigation of appropriate microstructure characteristics for this adsorbent. The optimal condition for ions removal was determined as a function of ph, contact time, amount of adsorbent and selectivity of metals. In addition, nanoporous polybenzoxazine, activated carbon and commercial activated carbon were compared in the metal adsorption efficiency. EXPERIMENTAL A. Materials Bisphenol-A (97%) was purchased from Aldrich, Germany, formaldehyde (37%) was obtained from Merck, Germany. Tetraethylenpentamine (TEPA, 85%) was purchased from Fluka, Switzerland, and dimethylformamide (DMF) was obtained from Labscan Asia Co., Ltd., Thailand. Activated charcoal DARCO, 4-12 mesh particle size, granular were purchased from Aldrich, Germany. Coconut shell activated carbon (CSAC) was supported by Carbokarn Co., Ltd., Thailand. Calcium chloride dehydrate was purchased from Carlo Erba, Italy. Magnesium chloride was obtained from Sigma-aldrich, Germany. All chemicals were used without further purification. B. Synthesis of Carbon Xerogel Derived from Bisphenol-A, Formaldehyde and TEPA The molar ratio of bisphenol-a: formaldehyde: TEPA was 1:4:1. Bisphenol-A was dissolved in dimethylformamide and formaldehyde solution was then added into the bisphenol-a solution and stirred continuously for approximately 20 minutes. The temperature was kept under 10 o C using an ice bath. Tetraethylenepentamine was slowly added dropwise into the mixture and stirred continuously for approximately 1 hour while the reaction was cooled with the ice bath until transparent yellow viscous liquid was obtained. The pre-polymer was transferred into vial and sealed with parafilm like a closed system and left for 1 day at ambient condition, then heating at 80 C for 2 days in an oil bath to set the organogel. Benzoxazine gel was soaked in acetone for 3 days in order to get rid of solvent. After that, the sample was dried at 80 C in oven for 1 day. The resulting xerogel, was cured by slowly raised temperature to 100 C for 30 minutes, 220 C for 8 hours and holding at 220 C for 15 minutes. Finally, cooled down to 30 C for 2 hours to obtain the fully-cured polybenzoxazine. Pyrolysis was started at 30 C and raised to 200 C for 1 hours, 600 C for 6 hours, for 800 C for 2 hours and finally hold at 800 C for 2 hours. This reaction is occurred under nitrogen atmosphere. So, the nanoporous carbon derived from polybenzoxazine was obtained. For the activated carbons, carbon samples were activated under the CO 2 atmosphere at 900 C for 3 hours. Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 2

3 C. Characterization The functional groups that related to structure of materials were investigated by using a Nicolet is5 FTIR analyzer in the frequency range of cm -1 with 16 scans at a resolution of 4 cm -1. A differential scanning calorimeter, DSC 822, was used to study the curing behavior of benzoxazine precursor and polybenzoxazine. The sample was heated from 30 to 300 C with heating rate 10 C/min under nitrogen gas with flow rate 20 ml/min. The N 2 adsorption-desorption isotherm measurement was conducted at 196 C on a Quantachrome-Autosorb-1MP. Before measurement, the samples were degassed at 250 C under vacuum atmosphere. Atomic adsorption spectrometer, Varient Spectra200, was used for the quantitative analysis of metal ions in the model metal solution. The calibration method was applied in analysis. D. Adsorption Experiment Metal solutions were prepared from CaCl 2. 2H 2 O and MgCl 2. In all experiments, 50 ml. of a solution containing the metal ions with 100 ppm initial concentration (C 0 ) was mixed with polybenzoxazine-based nanoporous carbon weighing from 100 mg under constant magnetic stirring at 298 K. The initial and final concentrations were determined by atomic adsorption spectrometer (AAS). The percentage of metal removal was calculated by the following equation. RESULTS AND DISCUSSION A. Characterizations of Benzoxazine Precursors and Polybenzoxazines The benzoxazine precursor and polybenzoxazine were characterized by using FT-IR technique in order to examine the functional groups and to confirm the chemical structure. The FT-IR spectra was shown in Figure 1 (a) showing The asymmetric stretching bands of C N C at 1113 cm -1, C O C at 1257 cm -1, and CH2 wagging of oxazine cm -1 were observed. Additionally, the characteristic absorption peaks assigned to the trisubstituted benzene ring and the out-of-plane bending vibrations of C H were observed at 1477 and 935 cm -1, respectively. According to Dunkers, these adsorption bands confirmed that benzoxazine and polybenzoxazine were obtained (Dunkers, et al., 1995). Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 3

4 Benzoxazine precursor Fully-cured polybenzoxazine Benzoxazine precursor Fully-cured polybenzoxazine Absorbance Heat flow (Exo up) Wavenumber (cm -1 ) Temperature ( o C) Figure 1. FTIR spectra of the benzoxazine precursor (a) and polybenzoxazine (b) and DSC thermograms of the benzoxazine precursor (a) and polybenzoxazine after heat treatment at 220 C (fully-cured) (b). B. Thermal Properties of Benzoxazine Precursors A differential scanning calorimeter (Perkin-Elmer, DSC822) was used to study the curing step of benzoxazine precursor as shown in Figure 1. From the DSC thermogram shows the exotherm peak starting at 180 C to 270 C, attributed to the polybenzoxazine ring-opening polymerization as seen from the Figure 1 (a). After the benzoxazine precursor was fully polymerized at 220 C, the fully cured polybenzoxazines were obtained corresponding with the completely disappeared exothermal peaks of polybenzoxazine because oxazine ring opening process was completed as shown in Figure 1(b). This result was similar to that reported by Takeichi and coworkers (Takeichi et al., 2005). Moreover, the thermal stability was investigated by thermal gravimetric analysis (TGA) under nitrogen gas. The fully-cured polybenzoxazine started to decompose at approximately 250 C and the maximum weight loss was observed at between 250 C and 600 C. The residual char yields at 800 C was around 36 %. C. Surface Characteristics of Adsorbents The specific surface area, total pore volume and average pore diameter of polybenzoxazinebased nanoporous, activated carbon and commercial activated carbon were determined by the N2 physisorption using a Quantachrome Autosorb-1 MP surface area analyzer. The surface and pore characteristics of nanoporous carbon are summarized in Table 1. It was found that the total surface area and total pore volume of polybenzoxazine and activated carbon were increased after pyrolysis and activation process from 1.99 m 2 /g to m 2 /g and 0.01 cm 3 /g to 0.33 cm 3 /g, respectively. The average pore size approached to 2 nm which nearby micropores. Two types of commercial activated carbon, commercial and Accoconut, have BET surface area of 493.4, m 2 /g and average pore size 5.45, 1.02 nm., repectively. Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 4

5 Table 1. Textural properties of nanoporous carbon derived from polybenzoxazine Sample name Surface area Total pore volume Average pore size (m 2 /g) (cm 3 /g) (nm) PBZ Activated Carbon Commercial Ac-Coconut D. Ion Adsorption Effect of ph Figure 2. The percentage of calcium (left) and magnesium (right) removals by varying ph solution. The ph of metal ion solutions is considered as the most substantial parameter affecting the adsorption behavior of metal ions. The removal of water hardness ions was strongly dependent on the ph of the solution. From Figure 2. Four types of adsorbent, polybenzoxazine, activated carbon, commercial activated carbon and ac-coconut, were studied by varied ph of solution at 4, 7 and 10. The results revealed that ac-coconut showed the highest calcium and magnesium removals 55.33% and 37.29% at ph 10, respectively. Consequently, ac-coconut was chosen for optimizing the metal removal conditions. The maximum percentages of metal removal were 93.25% for Ca 2+ and 83.96% for Mg 2+ respectively that were achieved at the ph of 10 as shown in Figure 3. This might be due to the increase of hydroxyl ions (OH - ) concentration in the solution that increases the ionic interaction with the adsorbents. In contrast, at lower ph, the surface of the adsorbent is surrounded by hydrogen ions (H + ). The latter adsorption of hardness ions on to the binding sites of adsorbent through repulsion (Rolence et al., 2012). Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 5

6 Effect of Contact Time The ions removal increased with increasing contact time. The percentage of metal ions adsorbed increased rapidly during a few minutes at first and approached equilibrium within 1 hour. A further increase in contact time had a negligible effect on the percent removal. The initial adsorption rate was very fast may be due to the existence of greater number of active sites available for the adsorption of metal ions (Yu et al., 2012). Effect of adsorbent dose The percentage of metal ions removal was increased with increasing adsorbent mass due to the existence of larger surface area as well as greater number of active binding sites. Figure 3. The removal of calcium and magnesium. (ph: 10; the amount of adsorbent: 2.0 g; time: 1 h; the constant stirring speed.) CONCLUSIONS Polybenzoxazine was successfully synthesized via a sol-gel method used as an adsorbent for alkaline-earth metal ions removal. The ac-coconut commercial activated carbon showed the highest percentages of metal removal were 93.25% for Ca 2+ and 83.96% for Mg 2+, respectively. The optimum condition was established at a basic condition, ph = 10, with 2.0 g. of adsorbent and reached to equilibrium state after 1 h. The ac-coconut showed better results than polybenzoxazine or commercial adsorbent due to the higher surface area. Application of metal removal was successfully accomplished. ACKNOWLEDGEMENTS The authors would like to thank The Petroleum and Petrochemical College, Chulalongkorn University, Thailand. Grant for International Research Integration: Chula-Research Scholar, Ratchadaphiseksompote Endowment Fund and the Center of Excellence on Petrochemical and Materials Technology, Chulalongkorn University. Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 6

7 REFERENCES Chaisuwan, T., Komalwanich, T., Luangsukrerk, S., & Wongkasemjit, S. (2010). Removal of heavy metals from model wastewater by using polybenzoxazine aerogel. Desalination, 256(1), Chirachanchai, S., Laobuthee, A., Phongtamrug, S., Siripatanasarakit, W., and Ishida, H. (2000). A novel ion extraction material using host guest properties of oligobenzoxazine local structure and benzoxazine monomer molecular assembly. Journal of Applied Polymer Science, 77(12), Dunkers, J., & Ishida, H. (1995). Vibrational assignments of 3-alkyl-3, 4-dihydro-6- methyl-2h-1, 3-benzoxazines in the fingerprint region. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 51(6), Ishida, H. (2011). Overview and historical background of polybenzoxazine research. Handbook of benzoxazine resins. New York: Elsevier. Table, 5, 9. Meena, K. S., Gunsaria, R. K., MEENA, K., Kumar, N., & Meena, P. L. (2012). The problem of hardness in ground water of Deoli Tehsil (Tonk District) Rajasthan. Journal of Current Chemical and Pharmaceutical Sciences, 2(1). Rolence, C., Machunda, R. L., & Njau, K. N. (2014). Water hardness removal by coconut shell activated carbon. International Journal of Science, Technology and Society, 2(5), Saeed, A. M., & Hamzah, M. J. (2013). New approach for removal of total hardness (Ca2+, Mg2+) from water using commercial polyacrylic acid hydrogel beads, study and application. International Journal of Advanced Biological and Biomedical Research, 1(9), Takeichi, T., Kano, T. and Agag, T. (2005). Synthesis and thermal cure of high molecular weight polybenzoxazine precursors and the properties of the thermosets. Polymer, 46, Yu, Z., Qi, T., Qu, J., Wang, L., & Chu, J. (2009). Removal of Ca (II) and Mg (II) from potassium chromate solution on Amberlite IRC 748 synthetic resin by ion exchange. Journal of hazardous materials, 167(1), Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 7