The Effect of Silica Concentration on The Absorption Properties of Silica-Based Ceramic Membrane

Water pollution is currently an increasingly widespread environmental problem. Methylene blue is a synthetic dye used in the textile industry which pollutes the aquatic environment. The absorption of Methylene Blue textile dye which pollutes the environment uses silica precipitate as a filler in ceramic membranes with a clay matrix. Silica precipitate added to the ceramic membrane varies with a concentration of 0.3 %, 1.01 %, 1.68 %, 2.34 % and 2.99 %. The membrane will be characterized using Fourier Transform Infrared (FTIR) testing to determine the functional groups of the components it contains. Testing the absorption of methylene blue contaminant content from artificial wastewater using the UV-Vis Spectrophotometry test. The FTIR test carried out compared the functional groups of membrane samples before and after sintering. The results obtained from the FTIR test were that the membrane after sintering contained silica and some PEG, while the membrane before sintering contained other components such as PVA, PEG, water molecules and silica. The UV-Vis test carried out on the membrane gave results that the membrane was 0.3 %, 1.01 %, 1.68 %, 2.34 % and 2.99 % respectively has a removal efficiency of 58.0 %, 54.0 %, 59.3 %, 64.0 % and 66.4 %, where the best removal efficiency of methylene blue was on a membrane filled with 2.99 % of silica. Tests on the membrane showed that an increase in silica concentration was in line with an increase in the quality of the membrane’s performance in rejecting methylene blue contaminants.


Introduction
Environmental problems, especially regarding water pollution, are one of the very detrimental impacts of industrial development.Chemical factories are one of the contributors to waste which causes water pollution.Therefore, handling industrial waste by rejecting the contaminants contained before being discharged into waters is something that is currently being considered.One waste-handling technology that is currently being highly developed is membrane technology.Membrane technology is a technology that is widely developed and chosen because membranes have several advantages.These include separation that can be carried out continuously, relatively low energy consumption, easy to combine with other separation technologies, easy to scale up, and operating conditions for separation that can be adjusted and do not require additional materials [1].The absorption of water contaminants using a membrane will be maximized by adding filler to the membrane mixture.The filler added is a porogen (pore-forming) filler.
Silica was chosen as a filler in ceramic membranes because it has good pore-forming properties, strength and stability [2].Silica can be synthesized from fly ash waste from coal combustion [3].Less compatible clay-silica ceramic membranes are added polymer additives such as PVA and PEG to produce more compatible membranes.Porous ceramic synthesis research [4] was reinforced by the addition of fly ash in porous materials by the research [5] becoming the basis for this silica-filled Clay-PVA-PEG ceramic membrane research.

Coal Fly Ash Preparation
The fly ash preparation procedure refers to the ASTM C618 standard.Preparation begins with sieving fly ash using a 180/200 sieve.Fly ash is then mixed with HCl to remove contaminants.Fly ash slurry is filtered and dried as an ingredient for making sodium silicate solution.

Synthesis Precipitate Silica from Coal Fly Ash
Dry fly ash was mixed with 500 mL of 3.5 N NaOH followed by heating at 105 o C at 500 rpm for 150 minutes.This reaction will produce a sodium silicate solution which is then titrated with H2SO4 until the pH is neutral (marked by the formation of a white gel).The white gel was then filtered and dried to evaporate excess water content at a temperature of 100 o C. The dried gel is crushed and ready to be used as a filler for ceramic membranes.

Synthesis of silica-filled PVA-PEG Membrane
PVA with a mass of 6 grams was mixed with 10 mL of HNO3 1M and 190 mL of distilled water while stirring for 2 hours at 180 o C. The silica that had been obtained previously was added to the 2propanol solution to pass the centrifuge process at 600 rpm for 10 minutes.After the centrifuge, the silica was mixed with NH4Cl solution and stirred for 1 hour.Furthermore, 50 mL of PVA, 20 mL of PEG and 13 grams of clay were added to this mixture followed by stirring at 150 o C for 2 hours.Next, the membrane was printed on a petri dish and aerated for 30 hours.The membrane was then sintered in a furnace at 700 o C for 2 hours.

Transmission Electron Microscopy (TEM) Testing from Silica based Fly Ash
TEM is one method that is widely agreed by scientists that ultrathin specimens of high quality are required for the investigation of almost all materials to the particle level [6].Analysis using TEM has advantages such as being able to provide information about the composition of the sample with high resolution.The TEM testing procedure begins by placing the sample on an electric grid under vacuum conditions.Next, the type of output data is selected, including statistics in the form of size and morphology; sample inspection; and high magnification.In this case, the data selected is statistical data in terms of morphological shape and size (particle size distribution) [7].

Component Function Testing with Fourier Transform Infrared (FTIR) of Silica Based Ceramic
Membrane FTIR is a technique used to observe molecular interactions using electromagnetic radiation at a wavelength of 0.75-1000 µm or a wave number of 13,000-10 cm-1.The FTIR testing step begins with placing the sample in an infrared cell.The FTIR tool is run to analyze samples using infrared rays to produce IR spectra data as output [7].One of the sample variations with two parameters before and after the sintering furnace was analyzed using Fourier Transform Infrared (FTIR).FTIR analysis provides information on what materials are vaporized after furnace sintering treatment through the functional groups of materials contained in the membrane before and after furnace sintering.In this FTIR test, infrared rays will hit the sample and provide a signal that will be read by the detector and produce data.

Morphology Testing with Scanning Electron Microscopy of Fly Ash-Based Silica
SEM is an electron microscope designed to investigate the surface of solid objects directly, which has a magnification of 10 -3000000x, depth of field of 4 -0.4 mm and resolution of 1 -10 nm [7].SEM has a working principle, including an electron gun producing an electron beam which is accelerated by the anode, a magnetic lens focuses the electrons towards the sample, the focused electron beam scans the entire sample directed by the scanning coil, and then the electrons hit the sample thereby releasing new electrons which will be received by the detector.and sent to the monitor (CRT) [7].

Membrane Selectivity Testing with UV-Vis Spectrophotometry of Silica-Based Ceramic Membrane
This test procedure refers to ASTM E275, The UV-Vis test begins with the preparation of methylene blue standard solution with concentrations of 5, 10, 15, 20, and 25 ppm and the creation of a calibration curve.After that, each concentration of methylene blue standard solution will be passed through the highest porosity membrane and produce methylene blue filtrate.This filtrate will then be analyzed using a UV-Vis spectrophotometer and generate data.This data can then be calculated using the regression equation on the methylene blue calibration curve to obtain the filtrate concentration after passing through the membrane.

Transmission Electron Microscopy (TEM) testing result of fly ash-based silica
The result of Transmission Electron Microscopy (TEM) testing of silica is shown in Fig. 1 below.

Figure 1. TEM testing result of silica
Using TEM testing the structural morphology of silica precipitates was identified.From Fig 1 above it can be seen that the distribution of silica particles is not uniform (heterogeneous).The silica precipitate appears to consist of small silica particles and the sample is predominantly spherical-like, with most of the small spherical particles still overlapping and forming lumps (agglomeration).It can be seen from the structure of the silica precipitate that it does not spread evenly to form single particles.Particle size analysis can use TEM micrographs processed using the Image J application.Size analysis using the Image J application is the result of TEM micrographs of silica samples at 16 points spread as in the image above.The results of calculating the size of silica particles at 16 points distributed in the sample are as follows:  From Fig. 2, it can be seen that the size distribution of diameter and surface area of silica precipitate from coal fly ash produced respectively has a range of 10.09 -34.52 nm and 109.75 -932.52 nm 2 with 16 points of silica particles distributed.So the average diameter and average area of the silica precipitate which was produced from coal fly ash is 20.52 nm and 358.45 nm 2 .According to research result from Music et al, (2011), the amorphous silica (SiO2) particles have a primary particle size of 15-30 nm and tend to form larger particles (aggregates).These primary particles of silica can be spherical or randomly shaped [8][9].The results obtained in this study have meet the criteria as an amorphous silica particle.The results of the silica particle size data which are in the range > 1 nm and < 100 nm are in the category between molecular and macroscopic, namely nanometers.Nanoparticles are particles with many advantages compared to large particles, including better adhesion ability on many substrates, high corrosion resistance and resistance to cracking [10].

Scanning Electron Microscopy testing result of fly ash-based silica
The scanning electron microscopy test carried out on fly ash-based silica samples serves to determine the structural morphology of the silica surface.SEM tests on silica were applied with 500x magnification.The surface morphology images of fly ash-based silica are shown in Fig. 3.

Figure 3. SEM image of precipitated silica with magnification (A) 500x (B) 1000x (C) 3000x
From the image above (Fig. 3), it can be seen that the silica surface detected in the SEM test is not even.This is because the constituent particles have various sizes with uneven distribution.The silica produced has predominantly spherical particles.Small particles tend to stick together to form large collections which are called agglomerates (called the agglomeration phenomenon).The agglomerates formed to make the silica surface appear as a single particle.The formation of aggregation by nanosized spherical silica from fly ash as shown in the image indicates attractive interactions between particles [11].

Fourier Transform Infra-Red testing result of silica-based ceramic membrane
The FTIR images of silica-based ceramic membranes are shown in Fig. 4. The two figures, Figs.4(A) and 4(B) show the IR spectra of Clay-PVA-PEG 0.3% ceramic membrane before and after the sintering furnace.In the ceramic membrane before the sintering furnace, the presence of asymmetric O-H stretching vibrations from Si-OH which is O-H stretching at the absorption peak of 3391.9 cm-1 with a wide band was identified.This is to the FTIR analysis conducted by Rumiyanti et al, 2021 [6] which states that the presence of a wave peak of 3421.The absorption at the peak of 2875 cm-1, 1326 cm-1, and 1089 cm-1 is the absorption of aliphatic C-H functional groups, C-O stretching functional groups, and C-O-C bonds of polyethene glycol (PEG), respectively [12].
The second image (Fig. 4(B) is IR spectroscopy for the silica-filled Clay-PVA-PEG ceramic membrane after the sintering furnace.In the ceramic membrane after sintering there are only two functional groups of content that can be detected by the FTIR tool, namely at the absorption peak of 715.6 indicating the presence of silica with symmetry Si-O-Si stretching vibrations, by Rumiyanti et al, 2021 [13] which reveals that at 790.2 cm-1 indicates the presence of symmetrical Si-O-Si stretching vibrations.The same thing is also shown at wave number 1013.8 which is an asymmetric stretching vibration of Si-O-Si.In addition, the wave peak at 1013.8 can also be indicated as a C-O bond [14].

UV-Vis Spectrophotometry testing result of ceramic membrane
UV-Vis testing of ceramic membranes aims to determine the ability of the membrane to filtrate methylene blue-contaminated water.UV-Vis testing of ceramic membranes was taken on membrane samples with the highest porosity after being tested for SEM images on origin software.The standard solution used in this study is methylene blue.After being tested using a UV-Vis spectrophotometer, it was found that the maximum wavelength of methylene blue was 648.5 nm.Using the maximum wavelength value of methylene blue, the absorbance data of each sample will be plotted on a curve that connects the concentration with the absorbance of the sample equipped with the regression equation and the value of the R square obtained.The plot of the relationship between concentration data and absorbance of methylene blue solution in a calibration curve is presented as follows.

Figure 5. Methylene blue calibration curve
A 9-gram silica-filled membrane is used, where methylene blue will be passed through for all initial concentration variations of 5, 10, 15, 20, and 25 ppm.The methylene blue solution after passing through the 9-gram silica filler membrane will then be tested using UV-Vis to obtain the amount of absorbance after passing through the membrane.This absorbance value will then be used to find the concentration value through the regression equation on the calibration curve.The plot of concentration and absorbance of methylene blue solution shows a good straight proportional relationship (mutually related) characterized by the regression equation y = 0.0439x + 0.1416 has a very good R2 value of 0.999, as follows:

Adsorpyion properties testing result of silica-based ceramic membrane
The values of methylene blue filtrate concentration after passing through the silica based membrane and the difference with the concentration of standard solution calculated as the effectiveness of removal are presented in Table 2.  Silica as a clay-PVA-PEG membrane filler material will provide pores with an increasing number and smaller size and density as its concentration increases so that the membrane will become more selective.

Conclusion
Precipitated silica as a filler of clay-PVA-PEG ceramic membrane was found has the ability to clarify water through the pores of the membrane.The resulting clay-PVA-PEG ceramic membrane was also proven to have homogeneity between constituent materials due to the bond between functional groups.This was confirmed by the membrane FTIR test in identifying the components contained in the membrane before and after sintering.Increasing the mass of silica filler can also further increase membrane porosity due to increasing the number of pores on the membrane surface.Increasing porosity makes the membrane's performance in absorbing contaminants better.This is proven through porosity tests that produce porosity values of 58.0%, 54.0%, 59.3%, 64.0% and 66.4%.The membrane research carried out produced membranes of good quality, characterized by an average methylene blue removal efficiency percentage of 50%, respectively 58.0, 54.0, 59.3, 64.0 and 66.4%.

Figure 2 .
Figure 2. Diameter and surface area of silica 7 cm-1 will indicate the asymmetric O-H stretching vibration of Si-OH.Another absorption peak such as at 2885 cm-1 represents the C-H group, To Mansur et al, 2008 the wave band with the interval 2840 and 3000 cm-1 refers to C-H stretching.The double bond of two C carbon atoms at 1647.5 refers to the double vinyl bond of two C atoms or C=C stretching.The presence of PEG was identified through several functional groups including the absorption peak of 1341.6 waves referring to C-O stretching and the peak of 998.9 waves referring to the C-O-C group belonging to polyethylene glycol (PEG).

Figure 4 .
Figure 4. FTIR image of the silica-PVA-PEG ceramic membrane (A) before sintering (B) after sintering

Figure 6 .
Figure 6.Absorbance vs concentration of methylene blue

Table 1 .
Particle size calculation result of precipitated silica Diameter (D) and surface area (L) data from 16 points spread across the sample can be visualized with the histogram in Fig 2.