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Synthesis and Characterization of Molecularly Imprinted Polymers (MIPs) Using Humic Acid
Tikarahayu Putri1*, Muhammad Ali Zulfikar2, Deana Wahyuningrum2, Iwan Syahjoko Saputra1
1Study Programme of Cosmetic Engineering, Institut Teknologi Sumatera, South Lampung 35365, Indonesia
2Faculty of Mathematics and Natural Science, Institut Teknologi Bandung, West Java 40132, Indonesia
*Corresponding email : tikarahayu.putri@km.itera.ac.id Email author 1: tikarahayu.putri@km.itera.ac.id
Email author 2: zulfikar@chem.itb.ac.id Email author 3: deana@chem.itb.ac.id Email author 4: iwan.saputra@km.itera.ac.id Received 28 December 2021; Accepted 10 November 2022
ABSTRACT
MIPs was synthesized through microwave assisted organic synthesis (MAOS) method by using methyl methacrylate (MMA) as functional monomer, ethylene glycol dimethacrylate (EGDMA) as cross-linker, benzoyl peroxide (BPO) as initiator, N,N-dimethyl formamide (DMF) as porogen solvent, and (HA) as template. The MIPs was successfully synthesized according to the characterization using Fourier transform infrared (FTIR) spectroscopy, scanning electron microscope (SEM), electron dispersive spectrometer (EDS), and UV spectrophotometer. FTIR shows the other functional groups peaks in the FTIR spectra of NIPs and MIPs after leaching were appeared at the wavelength of 2955 cm-1 corresponded to stretching vibration of C-H of 1459 cm-1 and 1160.06 cm-1 corresponded to the vibration bands of CH3 and O-CH3. EDS shows the MIPs after leaching have the elemental compositions of C, O, and Si with a mass of 78.34, 21.43, and 0.23%. UV spectrum shows the MIPs and NIPs have absorbance values of 0.36 nm and 0.44 nm.
Keywords: Molecularly imprinted polymers, Humic acid, Characterization
INTRODUCTION
One of the problems that can arise in the environment is the increasing mobility of toxic heavy metals resulted from the complexation and the formation of precursor trihalomethane during drinking water treatment process. This problem occurs because it contains organic chemical compound of natural organic matter (NOM) such as humic acid (HA), fulvic acid (FA), and humin [1-3]. HA is organic compound which has high molecular weight, complex, and containing carboxylic and phenolic functional groups. The presence of humic acid content in peat water causes brownish color and smelly [4]. This compound is difficult to be degraded, therefore the chemical oxidation through COD will not work [5]. The amount of humic acid concentration in natural environment is 0.1 to 200 ppm [3].
Therefore, a suitable method is required to overcome this problem. In general, biological treatment is often done to remove organic material components, but this method has limitations to reduce the levels of complex organic material. Several physicochemical methods are also frequently used such as anodic oxidation [6], coagulation-flocculation [7], and photocatalysis
for removing humic acid [3]. However, there are deficiencies in such methods as they require substantial cost and the formation of secondary pollutants.
Based on the above explanation, absorption method was selected to overcome the problem of humic acid content in water. Absorption treatment was selected because it is simple and highly efficient, the absorption methods also have been extensively applied to remove organic pollutants in water [4]. On the other hand, the cost and the steps of this method is cheaper, easier, and faster. Several absorbents are made for HA removal such as derivatives of chitosan [8], silica [9], eggshell [10], and metal hydroxide [11]. The absorption method is made in solid form. One of the technologies that can be utilized is molecularly imprinted polymers (MIPs) for humic acid absorption.
Methyl methacrylate MIPs has been successfully synthesized and functioned to absorb humic acid in peat water using batch method [4]. However, in the use of batch method, it took a longer reaction time to synthesize the polymer, therefore MAOS (Microwave Assisted Organic Synthesis) method can be used to synthesize the polymer with a shorter time. MIPs was synthesized utilizing MAOS method by using methyl methacrylate (MMA) as functional monomer, ethylene glycol dimethacrylate (EGDMA) as cross-linker, benzoyl peroxide (BPO) as initiator, N,N-dimethyl formamide (DMF) as porogen solvent, and (HA) as template. The selection of monomers to be used in synthesizing MIPs depends on the capability of monomers to interact with the functional group of templates [4]. The MIPs has been synthesized to form a polymer that has many cavities and shaped according to the target analytes. The cavities have been shaped according to the molecule templates because of the molecule has been removed from polymers. The MIPs and NIPs were fabricated by radical polymerization technique. The physical properties of MIPs were characterized utilizing Fourier transform infrared (FTIR), scanning electron microscope (SEM), and electron dispersive spectrometer (EDS).
EXPERIMENT Chemicals
The MIPs material was prepared using microwave assisted organic synthesis (MAOS).
All materials used in this experiment were analytical grades. The solvents were also analytical grade and used without further purification. HA, MMA, EGDMA, BPO, DMF, NaOH and methanol was purchased from Sigma – Aldrich.
Procedure reaction
Synthesis of MIPs using Humic Acid
The amounts of 0.5 mmol of template HA was mixed with 2.5 mmol of MMA, 7.5 mmol of EGDMA, and 0.8 mmol BPO in 15 mL DMF. The solution was stirred for approximately 20 minutes and subsequently synthesized by stirring and flowing it with 99% nitrogen for 10 minutes in the microwave reactor (MAS II SINEO reactor) at 60 °C. The obtained MIPs was dried in a hot air oven at 70 °C for 1 hour. Furthermore, One gram of the material was extracted utilizing Soxhlet apparatus to leach out the molecule templates using 1 M NaOH solution for 24 hours, and then washed it with methanol until the HA was completely removed. The material was dried by a hot air oven. The material obtained was grounded and sieved into particle sized approximately 70 µm.
Instrumentation
MIPs were observed using Fourier Transform Infrared spectroscopy (FTIR Perkin- Elmer) characterization to determine the functional group in MIPs. The morphological of MIPs
were analyzed using Scanning Electron Microscopy-energy dispersive X-ray spectroscopy (SEM-EDS Evo MA10-ZEISS instrument).
RESULT AND DISCUSSION
The non-imprinted polymers (NIPs) as reference were prepared in similar way of the MIPs preparation without the addition of templates. The preparation of MIPs was described in scheme as shown in Figure 1. The functional groups of MIPs can be determined by Fourier Transform Infrared (FTIR) spectroscopy. Figure 2 shows the different characteristic peaks in FTIR spectrum of NIPs and MIPs before leaching. The FTIR spectrum of MIPs before being extracted showed stretching vibration peaks at the wavelength of 1601 cm-1 and 1565 cm-1. It indicates the presence of hydrogen bonding between N-H groups in amide functional groups as one of the characteristics of HA molecule. The stretching vibration peak also appear at 1038 cm-1 as band of C-O within carbohydrates frameworks in HA structure [12].
Figure. 1 Synthesis scheme of molecularly imprinted polymers of humic acid
Figure 2. FTIR spectrum of (a) MIPs, and NIPs before leaching.
According to Figure 3, humic acid molecule has been removed from the polymers during extraction process. There are no significant spectrum differences between the FTIR spectra of NIPs and MIPs after leaching. The stretching vibration of amide and carbohydrate functional groups did not appear in both spectra. The other functional groups peaks in the FTIR spectra of NIPs and MIPs after leaching were appeared at the wavelength of 2955 cm-1 corresponded to stretching vibration of C-H of 1459 cm-1 and 1160.06 cm-1 corresponded to the vibration bands of CH3 and O-CH3. The band of C=O was also appeared at a wavelength of 1729 cm-1 which may come up from EGDMA structure, which is used as cross-linker agent, as well as from MMA as monomer of the polymer [4].
Figure 3. FTIR spectrum of (a) MIPs, and NIPs after leaching
Molecularly imprinted polymers of humic acid was also characterized using SEM analysis. It was shown in Figure 4.
Figure 4. SEM images of (a) MIPs of humic acid and (b) NIPs
The morphology imaging of MIPs and NIPs was analyzed with a current strength 15 Kv and 10000 times magnification. Image of SEM showed the surface morphology of polymer which was fabricated by MAOS method. The images of SEM shows that surface of molecularly imprinted polymers (MIPs) is rougher than surface of non-imprinted polymers (NIPs), which is corresponded to the forming of cavities within the polymer due to the removing of HA as molecule templates [13]. Hence, MIPs should has the ability to absorb HA molecule stronger than NIPs.
The element compositions of material can be analyzed by EDS technique; therefore, EDS could show which atoms the MIPs possess [14]-[17]. The qualitative determination of atoms which present in the sample can be obtained from the energy spectrum versus the relative counts of X-rays [15].
Figure 5. EDS spectrum of MIPs (a) Before and (b) After leaching.
Table 1. EDS element analysis of MIPs before leaching
Element MIPs after leaching (%)
C 70.26
O 25.94
Na 1.94
Al 0.72
Si 0.72
Cl 0.43
Table 2. EDS element analysis of MIPs after leaching
Element MIPs after leaching (%)
C 78.34
O 21.43
Si 0.23
Figure 5a-b shows the EDS spectrum of the MIPs before and after the leaching process.
MIPs before to leaching process had elemental compositions of C, O, Na, Al, Si, and Cl, with a mass of 70.26, 25.94, 1.94, 0.72, 0.72, and 0.43%, respectively, as shown in Table 1. For MIPs after leaching have the elemental compositions of C, O, and Si with a mass of 78.34, 21.43, and 0.23%, respectively as shown in Table 2.
Figure 6. UV Spectrum of (a) Humic Acid, (b) MIPs, and (c) NIPs
UV spectrophotometer was used to determine the maximum wavelength and absorbance values of the MIPs. Figure 6 shows the UV spectra of the Humic Acid (HA), NIPs, and MIPs samples. MIPs and NIPs have a absorbance values of 0.36 nm and 0.44 nm. The presence of HA compounds affects the shift in the absorbance value of the MIPs.
CONCLUSION
The material of MIPs using HA as template was successfully fabricated utilizing MAOS method as has been confirmed by FTIR spectroscopy. The characterization by FTIR spectroscopy could show the different in the characteristic peaks of amide and carbohydrate functional groups that should be absent in MIPs spectrum. SEM images showed that the surface of MIPs was rougher than the surface of NIPs, which indicated the cavities of molecule templates that presence in MIPs. The analysis results of EDS indicated that the main composition elements of MIPs after leaching are carbon and oxygen.
ACKNOWLEDGMENT
This work is supported by Institut Teknologi Bandung for the financial support through Riset KK and ITB 2018 Inovation.
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