AN INSIGHT OF TIO 2 AND SPENT BLEACHING EARTH (SBE) EFFECT OF PHASE CRYSTALLINE STRUCTURES FOR PVDF HOLLOW FIBER MEMBRANE

: Polyvinylidene fluoride (PVDF) hollow fiber membranes have excellent mechanical properties and chemical stability. However, their hydrophobicity can lead to fouling and hinder permeability. Modifying this hydrophobicity is challenging due to PVDF's complex polymorphous structures, with the β-phase offering superior fouling resistance. One approach to improve PVDF hollow fiber membranes is the addition of inorganic additive materials to reduce hydrophobicity. The objective of this study is to offer new insights into the preparation of PVDF hollow fiber membranes using wet spinning with various additives, such as TiO 2 and SBE. The investigation focused on the functional groups of PVDF in both the α and β phases. The membrane was fabricated using regenerated SBE and TiO 2 , which were incorporated through dope solution preparation and phase inversion through a wet spinning technique. The study used the Fourier Transform Infrared (FTIR) technique to characterize the PVDF hollow fiber membranes. The study found that the PVDF hollow fiber membranes exhibit differences in α and β phase crystalline structures when combined with various additives. The addition of SBE to PVDF hollow fiber membranes results in a dominant β-phase crystalline structure, as indicated by the relative fraction of 0.70 and the largest peak area of 2.27. This has successfully improved the hydrophilic properties of the PVDF-SBE hollow fiber membrane.

PVDF is a semi-crystalline polymer that exhibits complex polymorphous structures.The most common crystalline structures for PVDF membranes synthesized by the NIPS method are the α and β phases (Elma, Pradana, et al., 2022;Zhang et al., 2019).Among these crystalline structures, PVDF membranes with predominantly β phases showed superior fouling resistance compared to PVDF membranes with predominantly α phase (Elma, Rahma, Mustalifah, Wahid, Lamandau, Fatimah, Huda, Alsiren, Nasruddin, et al., 2023).Therefore, a lot of research work has been devoted to the study of the formation of the b-phase of PVDF by different approaches (Su, Sim, Li, Coster, & Chong, 2021).
One approach to improve PVDF hollow fiber membranes is the addition of inorganic particulate materials to reduce the hydrophobicity of the membrane surface, which can significantly improve membrane performance (Salahshoori et al., 2023;Wei et al., 2019).Numerous prior studies have documented the utilization of particles in the modification of polymer-based membranes, incorporating inorganic materials like SiO2, TiO2, Al2O3, zeolite, Ag, GO, ZnO, ZrO2, and others (Muthia, Rhafiq Abdul, Aulia, Alya Dita, & Novrian, 2022;Pratiwi et al., 2023;Razmgar et al., 2019).One of the outcomes of the incorporation of TiO2 in hollow fiber PVDF membranes are enhancing their antifouling capabilities since it enhances the hydrophilic properties of the membrane surface (Elma, Pradana, et al., 2022;Elma, Saraswati, et al., 2023).However, the particle material used is synthetic and a large amount of chemical addition is required, making it less efficient.Therefore, similar materials that are renewable and practical in use are needed (Maulida, Fitriah, Aliah, Rampun, & Elma, 2023;Sari et al., 2023).
Spent bleaching earth (SBE) is an inorganic material with enough Al2O3 and SiO2 to be a renewable substitute for synthetic materials in membrane manufacturing (Eliche-Quesada & Corpas-Iglesias, 2014;Elma, Lestari, Harivram, Mustalifah, & Rahma, 2023;Rahma et al., 2023).SBE originates from the crude palm oil (CPO) refining process and is being recycled to become a potential membrane component material.Sodium bentonite extract (SBE) is commonly used in the refining process of 2.5-3% per ton of CPO (Rahma et al., 2024).In addition, South Kalimantan has many SBE resources from the CPO refinery mills, which connect the supply chain from 20 CPO mills with a 797.89 tons/hour capacity in this region.(Muhamad, Mokhtar, Lau, Ismail, & Naim, 2022;Triwibowo, Elma, Suhartono, & Riduan, 2023).This study enhanced the PVDF hollow fiber membrane by integrating TiO2 and regenerating local SBE.Additionally, it will investigate the crystallinity phase characteristics of the resulting hollow fiber PVDF membrane through its functional group spectra.
This work aims to provide new insights into the preparation of PVDF hollow fiber membranes with various additives such as TiO2 and SBE through wet spinning.The functional groups of PVDF in both the α and β phases were investigated.Deconvolution was performed using Origin software to determine the material's surface area during vibration and stretching (Elma, Septyaningrum, Rahmawati, & Rahma, 2023;Ghani, Elma, Lestari, Alsiren, & Rahma, 2023).

Membrane Preparation and Characterization
The solution was prepared initially by dissolving TiO2 and regenerating SBE in DMAC, following the guidelines in Table 1.The mixture was agitated at 70°C for a duration of 24 hours.Thereafter, 18% of PVDF was incorporated into the dope solution and thoroughly mixed at 70°C until it reached a homogeneous state.Once cooled, the dope solution was poured into the dope tank for the spinning process of the hollow fiber membrane.The spinning setup of the hollow fiber membrane was illustrated in Figure 1, where the dope solution was extruded.Details of the spinning conditions can be found in Table 2.The resultant hollow fiber membrane underwent immersion in tap water for 24 hours, followed by immersion in 50% ethanol for 1 hour and immersion in 98% ethanol for another hour.Subsequently, the hollow fiber membrane was left to dry at room temperature for 24 hours.The Fourier Transform Infrared (FTIR) technique was used to characterize the hollow fiber membranes utilizing the Attenuated Total Reflectance (ATR) method with Bruker Diamond instrument at a wavenumber of 4,000-500 cm -1 .The results are displayed as a graph of FTIR spectra, and the peaks identified as functional groups.The relative fraction of β phase was calculated using FTIR result through Lambert-Beer Law in equation ( 1) (Pramono, Sejati, Wahyuningsih, & Purnawan, 2023).F(β) represents the proportion of the β phase, while Aα corresponds to the absorbances at 769 cm −1 and Aβ corresponds to the absorbances at 840 cm −1 peaks.The value of 1.26 is obtained from the ratio ratio of Kβ to Kα.The Kα has a value of 6.1104 cm 2 .mol - and Kβ has a value of 7.7104 cm 2 .mol - (Chen et al., 2019).Fourier self-deconvolution was carried out on the 850 cm -1 and 650 cm -1 bands, utilizing Gaussian band shapes, after background substruction with Origin software.

Results
Fourier transform infrared spectroscopy (FTIR) is a method for determining the functional groups of substances (Elma, Mawaddah, et al., 2023;Elma, Septyaningrum, et al., 2023;Febriyanti, Elma, Nata, Saraswati, & Simatupang, 2023b).This work also used FTIR analysis to identify the crystal structure of the manufactured PVDF hollow fiber membrane (Mohammadpourfazeli et al., 2023;Muthia, Aulia, Uun, Reza Satria Kelik, & Alya Dita, 2022).Figure .2 depicts the FTIR spectra of a hollow fiber membrane in the wavenumber range 860 -690 cm - 1 , which corresponds to the vibration of C-C, C-H and C-F in various conformations.The 769 cm -1 peak relates to C-C, C-H and C-F as α phase, whereas the 840 cm -1 peak refers to C-F vibration as β phase (Pramono et al., 2023).The α phase, in particular, adopts the non-polar trans-gauche (TGTG) conformation, whereas the β phase is extremely polar, adopting the all-trans (TTTT) conformation (Pradhana et al., 2021).These crystal phases can be obtained through a different processing method like the addition of additives (Su et al., 2021).In the specific case of this work, the addition of an additive substance such as TiO2 and SBE to the PVDF material indicates a different content of the α and β crystalline phases (Chen et al., 2019).

Figure 2. FTIR spectra of hollow fiber membrane PVDF
To quantify the proportion of β phase in each hollow fiber membrane, F(β) was calculated using equation (1).The relative fraction of the β phase in the PVDF hollow fiber membrane is shown in figure 3. The PVDF-SBE hollow fiber membrane displays the highest β phase fraction, reaching 0.70.Due to its intense vibrational band at about 840 cm -1 , PVDF-SBE hollow fiber membrane demonstrates the highest degree of hydrophilicity among the produced PVDF hollow fiber membranes (Dhatarwal & Sengwa, 2020).The presence of the β phase also indicates the polar nature of the PVDF hollow fiber membrane (Pramono et al., 2023).In contrast, the addition of TiO2 to the PVDF hollow fiber membrane resulted in the lowest β phase fraction among other membranes, which was 0.63.A slight increase in the β fraction of 0.64 is observed when an additive mixture of TiO2 and SBE is applied to PVDF compared to the PVDF-SBE membrane.This is  The results of the FTIR graph can be further analyzed using the deconvolution method to better illustrate the graphical changes of the PVDF hollow fiber membranes (Morali, Mandal, Skorobogatiy, & Bodkhe, 2023;Zahratunnisa et al., 2024).Deconvolution of the FTIR spectra was performed by the Gaussian function after baseline correction, after that overlapping the peaks until the deconvoluted spectra matched the experimental data spectra (He, Mohsenzadeh, Zhang, Rault, & Salaün, 2023).The utilization of the Gaussian model has already proven to be an efficient method for distinguishing the overlaps and revealing "hidden" peaks during spectroscopic analysis (Song et al., 2019).Figure 4 illustrates the Gaussian bands of FTIR spectra of a PVDF hollow fiber membrane.The graph indicates that even after deconvolution, the three membranes still exhibit regions of β phase and α phase, plotted as purple and green curves, respectively.There is a difference in the area under the curve for the three membranes, particularly in the peak area of the β phase (purple curve).The PVDF-SBE hollow fiber membrane displays a higher peak area in the β phase compared to the other membranes.From the deconvolution of the FTIR graph of the hollow fiber membrane, the peak area value of each peak can be ascertained.The peak areas of the FTIR graph were measured for the normalized spectra using a local baseline (Aliah et al., 2022;Aulia, Muthia, Aliah, Uun, & Novrian, 2022;Elma, Septyaningrum, et al., 2023).The peak area values of α phase and β phase are indicated in figure.5.The results show that the yield of the β phase of the PVDF-SBE hollow fiber membrane is 2.27, which corresponds to the widest peak area.The peak area of the β phase is also linear with the relative fraction of the β phase.On the other side, the largest peak area of α phase is shown by the PVDF-TiO2-SBE hollow fiber membrane reaching 0.34.However, the relative fraction of the β phase in the PVDF-TiO2-SBE hollow fiber membrane is still relatively larger than in the PVDF-TiO2 hollow fiber membrane.Therefore, the PVDF-TiO2-SBE hollow fiber membrane is still more hydrophilic than the PVDF-TiO2 membrane (Pradhana et al., 2021).

Conclusion
The study compared the effects of adding TiO2, SBE, and both particles to the PVDF hollow fiber membrane.PVDF hollow fiber membranes exhibit differences in α and β phase crystalline structures when combined with various additives.The addition of SBE to PVDF hollow fiber membranes produces a dominant β-phase crystalline structure, as indicated by the relative fractions is 0.70 and the largest peak area reaching 2.27.So the crystalline structure of the PVDF-SBE hollow fiber membrane has successfully improved its hydrophilicity properties.
Characteristics and performance of PVDF membrane prepared by using NaCl coagulation bath: Relationship between membrane polymorphous structure and organic fouling.Journal of Membrane Science, 579, 22-32.

Figure 1 .
Figure 1.Schematic of spinning hollow fiber membrane

F
fact that TiO2 can reduce the formation of the β phase in PVDF.(Erusappan et al., 2021).

Figure 3 .
Figure 3. ꞵ phase fraction of hollow fiber membrane PVDF

Figure 4 .
Figure 4. Deconvolution of FTIR spectra of hollow fiber membrane

Figure 5 .
Figure 5. Represents peak area of α and ꞵ phase in hollow fiber membrane PVDF.

Table 1 .
Dope Solution compositions of hollow fiber membranes

Table 2 .
Spinning conditions of hollow fiber membranes