Shion Ishizaki, Risa Katayama, Kazuma Hamano
Institute of Science Tokyo High School
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Introduction
Most of the Earth’s water is in the seas, with only a small fraction of freshwater being suitable for consumption. In fact, at the present, 2.2 billion people cannot ensure safety in drinking water. An effective approach to solve this problem is to convert sea water into fresh water. To achieve this, the focus of this study is on Bacterial Cellulose made by Komagataeibacter sucrofermentans which is a type of acetic acid bacteria. These bacteria are commonly encountered in our daily lives, and are utilized in the production of vinegar. Prior research has shown that bacterial cellulose is a semipermeable membrane. Using this fact, we aim to chemically modify bacterial cellulose to impart ion-exchange properties and to convert seawater into freshwater through electrolysis. The success of this study will contribute to solving SDG 6 “Clean water and sanitation”, to achieve universal and equitable access to safe and affordable drinking water for individuals.
Methods
Preparation of cellulose pellicle: A culture solution and a glycerin solution containing the acetic acid bacteria acquired from RIKEN Bioresource Research Center were placed on a petri dish and placed an incubator set at 30 ℃. After one week, the bacterial cellulose gel (pellicle) was subjected to high-pressure steam sterilization, and cleaned by sodium hydroxide solution and pure water (Fig. 1).
Chemical modification for cellulose: Sulfonation of the pellicle was conducted to produce the cation exchange membrane. The pellicle was immersed in sodium periodate solution, then it was cleaned with pure water. Next, it was immersed in sodium hydrogen sulfite solution which changes its structure by adding a new group, a sulfonated group (Fig. 2). Different concentrations and different immersing times were tested to find the appropriate experimental conditions. Diethylaminoethylation of the cellulose pellicle was conducted to produce the anion exchange membrane. The pellicle was immersed in sodium hydroxide solution. In this procedure, a particular part of the cellulose structure is changed to alkoxide. Next, it was immersed in diethylaminoethyl chloride (DEAEC) solution. In this procedure, the diethylaminoethyl (DEAE) group bonds to alkoxide (Fig. 2). The sulfonation experiments were conducted with different concentrations and duration. Two kinds of functional cellulose membranes were obtained. The chemically modified pellicles were dried at room temperature.
Evaluation of the chemical modified cellulose: FTIR Spectrometer was used to evaluate the cellulose membranes and the chemically modified cellulose membrane. When a molecule is irradiated by infrared rays, it absorbs infrared rays of a specific wavelength range depending on the vibrational state of chemical bonds in the molecule. By observing this, specific signals of the studied cellulose membranes can be obtained.
Electrolysis: The performance of the prepared ion exchange membranes was evaluated by electrolysis with the apparatus shown in Fig. 3. The solution of each electrolysis cell was collected and the conductivity and pH were measured. Tap water was electrolyzed at 30 V with the constant voltage generator so that the ammeter reads 0 mA and no bubbles are observed on the electrodes. The electrolysis of saline water, the constant voltage power supply was also adjusted at 4.5 V as the same way.
Results and discussions
Sulfonate cellulose membranes: Fig. 4 shows the result of infrared spectroscopy for the cellulose membrane and the sulfonate cellulose membranes. The red line shows the IR spectrum of cellulose membrane. Other lines show IR spectrum of cellulose membranes soaked in solutions of different concentrations of sodium hydrogen sulfite. The peak at 810 cm-1 is attributed to S-O stretching vibrations of cellulose disulfate1), as shown in Fig. 4. The peak at 1648 cm-1 is caused by the hygroscopicity of the membrane. This indicates that the membrane has become charged by sulfonation. Also, in the region shown here, the spectrum of the sulfonated membrane differs from that of the cellulose membrane. Compared to the IR spectrum of cellulose, the IR spectrum of the sulfonated membrane differs significantly. It can be seen that 2×10-3 mol/L-NaHSO3(aq) or higher showed significant changes of the baselines. Therefore, the treatment concentration of NaHSO3(aq) was selected at 2×10-3 mol/L. Furthermore, through the same method, the processing time was selected to be 2 hours.
Diethylaminoethyl cellulose membranes: The Infrared spectrum of cellulose membrane and that of DEAE cellulose membranes soaked in solutions for different periods were compared. The spectrum of the membrane containing DEAE group showed a peak between 1490 cm-1 to 1570 cm-1. In this area, it is difficult to identify amino group vibrations of DEAE by FTIR because N-C and C-H absorption bands are overlapping. However, the DEAE membrane spectrum is different from cellulose membrane, as shown in Fig. 5. Therefore, membranes are presumed to undergo diethylaminoethylation. In Fig. 5, the IR spectrum of DEAE membrane differs significantly compared to the IR spectrum of cellulose, the red line. It can be seen that DEAEC processing for longer than 1 hour showed significant change of the spectrum baseline. Therefore, the processing time was determined as 1 hour. Furthermore, through the same method, the treatment condition of concentration of DEAEC aqueous solution was determined to be 0.6 mol/L.
Electrolysis: Theoretically, as the electrolysis progresses, the number of ions of the central cell will decrease, and conductivity will also decrease. The conductivity decreases exponentially as shown in Fig. 6. It was confirmed that each ion exchange membrane carried out its function. After each of these electrolysis experiments, the ion exchange membrane and cell were rinsed with purified water prior to the subsequent electrolysis. Since the three electrolysis runs resulted in similar conductivity plots, the ion exchange membrane can be used repeatedly after cleaning with purified water. As in Fig. 6, the electrical conductivity of tap water is about 25 milli siemens per meter (mS/m). By using tap water, conductivity of solution in the center of the cell after electrolysis for 7 hours decreased from 24.7 mS/m to 2.70 mS/m. In addition, conductivity of the anode side cell increased to 77.2 mS/m, and conductivity of the cathode side increased to 128 mS/m. This indicates presence of ion migration which confirmed that each ion exchange membrane carried out its ion removal function. Regarding the change in pH in each cell, it is thought that the pH increased to 11.2 on the cathode side due to conversion of H+ into hydrogen leaving more OH– in the system, and decreased to 3.3 on the anode side due to produce HCl and HClO, part of the generated chlorine dissolved in water. Bubbles on the electrode were almost never observed, but some amount of oxygen seemed to be generated. Finally, electrolysis using saline water, which has an electrical conductivity equivalent to that of brackish water, was investigated. The conductivity of the saline water decreased about 10 % after the electrolysis for 4.5 hours. Therefore, it is thought that these membranes are also able to function as exchange membranes for brackish water. Similar experiments were conducted with saline water, with an electrical conductivity equivalent to seawater, but the apparatus did not serve its purpose, because the performance of the ion exchange membrane could not keep up with the generation of excessive amounts of gases and ions.
Conclusion
Through this research, the chemically modified membranes were confirmed to have ion exchange functions by electrolysis of tap water and brackish water. However, the phase of using seawater has yet to be reached. Future plans are to investigate the electrolysis system using salt water with the same conductivity as seawater and to consider more efficient reaction conditions for chemical modification and modification cellulose with other different function groups.
References
References
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Wang, Y., Chen, L., & Zhang, H. (2020). Regulating surface sulfonation on cellulose nanocrystals and self-assembly behaviors. Chemical Communications, 56, 10958–10961.
