Fabricating Self-Assembled Porous Silica for Microplastic Adsorption in Electrolyzed Drinking Water

Introduction

Microplastics, defined as particles ranging from 1 µm to 5000 µm in size (Frias & Nash, 2019), are considered emerging contaminants. These particles have been found to pollute aquatic environments (Haribowo et al., 2023; Pariatamby et al., 2020; Ta et al., 2020), including the organisms within them (Asadi et al., 2022; Senathirajah et al., 2023), as well as drinking water (Chowdhury et al., 2023; Koelmans et al., 2019; Y. Li et al., 2020; Novotna et al., 2019), all of which are consumed by humans. Groundwater, tap water, bottled water, and refillable packaged water have all been documented to contain microplastic contamination (Mintenig et al., 2019; Oßmann, 2021; Schymanski et al., 2018). Among various drinking water sources, the highest concentrations of microplastics have been found in bottled water (Singh & Bhagwat, 2022). Since drinking water is a basic daily necessity, the presence of microplastics in water consumed by humans poses a potential food safety risk.

Drinking water treatment may contribute to microplastic (MP) pollution as conventional treatment technologies struggle to remove these particles due to their physicochemical properties (Cherian et al., 2023). Membrane filtration, as synthetic organic membranes, is widely applied as the final barrier to contaminants. However, stresses from mechanical abrasion, chemical cleaning, and age-degradation may result in the release of MPs and NPs from such membranes, raising additional health concerns. Additionally, the hydrophobic nature of MPs and NPs enables them to adsorb halogenated by-products, thereby exposing water distribution to secondary contamination (Ding et al., 2021). To counteract such challenges, several alternative membrane materials have been researched. Ceramic and silica-based adsorbents are considered promising options, with silica in particular offering versatility and demonstrated applicability across multiple fields (Steven et al., 2022; Susmanto et al., 2020). However, their wider applications are hindered by costly, environmentally unsuitable synthesis routes, low recyclability, and diminished performance through pore blockage, as smaller pore diameters restrict adsorption capacity (Omorogie & Helmreich, 2024).

Chemobrionic self-assembly has emerged as a potential strategy to overcome these challenges. This process enables the formation of complex nano- to macroscale structures from organic and inorganic precursors, including tubular structures with diameters of approximately 50-100 μm-sufficient for the adsorption of MPs with sizes below 50 μm (Hughes et al., 2021; Reigl et al., 2025). Silica self-assembly can also be achieved through reactions between sodium sulfate (Na2SiO3) and calcium chloride (CaCl2), providing a practical route to functional silica adsorbent materials (Rieder et al., 2022; Thouvenel-Romans et al., 2004). Moreover, post-synthesis treatment offers a strategy to modify silica hydrophobicity and surface characteristics, thus enabling the design of adsorbents with defined functional properties (Wahyuningsih et al., 2021).

Previous studies on MPs removal have mostly focused on carbon-based adsorbents. These materials typically display microporous and heterogeneous structures, which may limit control over pore structure and surface uniformity (Khamkeaw et al., 2020). In contrast, silica-based adsorbents synthesized via self-assembly offer better control over pore size and distribution (Mohammed et al., 2021). Additionally, the adsorption behavior of microplastics (MPs) on these functionalized silica materials remains insufficiently understood. Contrary to dissolved and colloidal pollutants, MPs are considered particulates (Tsochatzis et al., 2024). Therefore, their adsorption behavior is more likely influenced by contact time and the physical interactions occurring between the MPs particles and the adsorbent surface.

This study aims to investigate the adsorption performance of functionalized self-assembled porous silica toward polyethylene (PE) MPs under controlled experimental conditions. The removal efficiency was quantified using flow cytometry based on Nile Red staining, with its structural as well as chemical changes were examined. The novelty of this work lies in its evaluation of contact time–dependent adsorption supported by multiple characterization techniques, providing clearer insight into how the structural properties of functionalized self-assembled porous silica influence microplastic adsorption.

Materials and Methods

Synthesis of Modified Porous Silica Materials

Porous Silica Synthesis

The silica precursor, sodium silicate (Na₂SiO₃) from Sigma-Aldrich®, was dissolved in distilled water at a ratio of 1 g : 4 mL at 80 °C, and then stirred with a magnetic stirrer at 85 rpm until completely dissolved and cooled to room temperature. Calcium chloride (CaCl2) from Sigma-Aldrich® was then ground and sieved to a particle size of 105–250 µm, and added as a templating agent at a ratio of 1:100 (gr/ml). The mixture was aged for 1 hour, resulting in the formation of CaSiO₃ tubes that served as temporary templates. The mixture was then treated with 1 M HCl (Merck®) until the pH reached 7, resulting in the formation of a silica gel, which was subsequently aged for 24 hours (Behede et al., 2022; Setyawan et al., 2021; Zhu et al., 2006). The silica gel was dried at 55 °C for 72 hours (3 days) to remove residual water, then ground, and sieved through a 105-250 µm mesh to obtain uniformly sized silica particles. A total of 1 g of silica was reacted with 20 mL of 5 M HCl at 80 °C for 2 hours to remove CaSiO₃ and generate microscopic pores. The mixture was then filtered and washed with distilled water to remove residual CaSiO₃ from the template, resulting in porous silica with pore sizes up to 50 µm, which were observed using an optical microscope.

Porous Silica Surface Modification

The synthesized porous silica was surface-modified using a 10% (v/v) solution of APTES (3-aminopropyltriethoxysilane, Sigma-Aldrich®). The procedure was carried out by immersing the silica in the APTES solution at a mass-to-volume ratio of 1:40 (w/v), followed by stirring at 150 rpm at room temperature for 8 hours to allow the surface modification reaction to occur (Mahtabani et al., 2020; Silviana et al., 2022). This process aimed to produce positively charged silica surfaces through the formation of bonds between silanol groups (-SiOH) on the silica and amine groups from APTES. After modification, the mixture was aged at room temperature for 24 hours to ensure complete reaction. The silica was then purified by washing with distilled water and dried at 40 °C under atmospheric pressure to obtain activated silica, where the surface modification was confirmed by analyzing functional group changes using FTIR and measuring the pore size using a microscope.

Results

Characterization of Porous Silica Material

The pore size distribution of 100 silica particles obtained from washing NaSiO₃ with 5 M HCl shows that most particles have pore sizes between 60–100 µm, with a median value (50th percentile) of 80.89 µm. The distribution is slightly skewed to the right: the number of particles is high at intermediate sizes (around 60–80 µm) and then decreases gradually at larger pore sizes.

However, there is still a fraction of particles with large pores (micro-silica) in the range of ≥120 to ±180 µm, although the number is relatively small compared to the intermediate size group. This indicates that the silica material washed with NaSiO₃ with 5 M HCl already contains some large-pore particles, but the proportion can still be increased if the intended application requires a predominance of large-pore micro-silica.

Functional Group

To identify the surface functional group of adsorbents, FTIR spectra were recorded. Figure 3 interprets the functional group of porous silica material before and after surface modification using APTES. Both adsorbent and modified adsorbent have three main characteristics of silica stretching vibration as main functional group at 1400 cm^-1 and 700 cm^-1 which correspond to the vibrations of Si-O groups in the siloxane group (Si-O-Si) (Susmanto et al., 2020). Peak at 1600 and 3400 cm^-1 indicates the formation of Si-OH after porous silica synthesis. The peak at 1631 cm^-1 shifted to the 1643 cm^-1 after modification indicated that -OH groups have been used to bind APTES. The broader peak in the range of 3500 - 3200 cm^-1 has formed due to the overlapping peak between -OH and -NH stretching vibration. The amine group, which is considered as the active site, appears in the peak at 3221 cm^-1 after modification treatment. This functional group plays an important role in the MPs adsorption in electrolyzed drinking water. Peak in wave numbers 1458 cm^-1 showed amine firmly bound to the silica (Rahman et al., 2022). The peak at 3400 cm^-1 from adsorbent is broader than silica reference material due to the formation of -OH from porous silica material and the ability of silica to adsorb water. Porous silica material has 700 of its similarity score with silica reference material and indicates that the synthesis of porous silica material did not change its main silica structure.

Morphology and Element Composition

The porous silica material was characterized using Scanning Electron Microscope-Energy Dispersive X-ray (SEM-EDX) (Figure 4) to see the microstructural of material formed and the composition of atoms contained in its material. SEM figures support the microscope data of pore size variation. Silica material shows the irregular morphology with the pore size in the range of 40-micron (porous) and less than 5-micron (sub-porous) in the magnification of 500x and 10,000x. EDX results revealed the presence of Nitrogen (N) atoms after modification (Figure 5). This supports data for the potential of modified porous material to adsorb negatively charged microplastic from electrolyzed water.

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Adsorption Studies

Flow Cytometry Analysis

The potential of modified silica as PE MPs adsorption in electrolyzed water was proved by flow cytometry method. The absorption process was carried out with three different contact times between adsorbent and water (10, 25, 40 minutes and 1, 2, 24 hours) to know the optimal contact time for the adsorption process. Furthermore, mixing has been used to help physical and chemical interaction between active sites on the surface adsorbent with PE. Stirring was applied five minutes for each sample prior to the varying adsorption time. The volume of the sample analyzed by flow cytometer was measured to estimate the number of microplastic particles per µL. The sample volume for flow cytometry analysis was 500 µL. The particle concentration before and after adsorption can be shown in Table 1. Microplastic particles in field samples were counted based upon their signal in terms of side light scatter (SSC) for complexity in a group and fluorescence in a band of PE (green laser) and PerCP (red laser) as shown in Figure 6.

Contact time	Total MPs in electrolyzed water (particles/500 µL)		Concentration of MP particles adsorbed in electrolyzed water
	Before adsorption	After adsorption	particle/500 µL	particles/L
10 (min)	17.17 ± 3.54	17.17 ± 3.54	0 ± 0	0 ± 0
25 (min)	17.17 ± 3.54	17.17 ± 3.54	0 ± 0	0 ± 0
40 (min)	17.17 ± 3.54	17.17 ± 3.54	0 ± 0	0 ± 0
1 (h)	17.17 ± 3.54	16.67 ± 2.83	0.50 ± 0.70	997 ± 1,409
2 (h)	17.17 ± 3.54	14.50 ± 0.23	2.67 ± 3.77	5,330 ± 7,537
24 (h)	17.17 ± 3.54	0 ± 0	17.17 ± 3.54	34,333 ± 7,071

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Adsorbent Characterization

FTIR characterization of modified silica as adsorbent before and after adsorption was shown in Figure 7. The -OH functional group from silica is at wave number of 3414 cm^-1 and it does not change before and after adsorption application (Caroles et al., 2024). Adsorption of PE on modified silica adsorbent resulted in disappearance of the adsorption band at 1458 cm^-1 characteristic to amine groups vibration. This change may indicate possible interaction between amine groups and PE. A similar finding has been reported and shows that the disappearance of specific bands is associated with the adsorption process (Moritz & Łaniecki, 2012). Whereas, wave numbers at 1328 cm^-1 and 1199 cm^-1 appeared after adsorption which corresponds to the CH₃ symmetric deformation and wagging deformation of PE, respectively (Gulmine et al., 2002). The APTES site at 1643 cm¹ after modification has shited to the 1645 cm¹ after adsorption indicated that PE has interacted with this site.

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Figure 8. EDX result of modified porous silica material: (a) before and (b) after MPs adsorption.

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From Figure 8(a), SEM–EDX analysis shows small particles that distribute evenly throughout a matrix surface. This surface morphology shows minimal particle contact between particles and a small amount of particle cluster ormation. However, a distinct phenomenon is observed in the SEM-EDX image shown in Figure 8(b), with ifferent surface characteristics observed because it contains bigger particles and surface area expansions. Meanwhile, DX spectrum shows a significant rise in carbon (C) signal, which increases from 10.45 %w to 11.01 %w, after the porous silica went through microplastics adsorption as shown in Figure 9. The surface carbon content directly elates to larger particle formation because these areas appear together in the sample.

Discussion

Fabrication of Modified-silica Porous Adsorbent

Porous Silica Material Synthesis

The silica precursor, sodium silicate (Na₂SiO₃), was dissolved in distilled water at a ratio of 1 g : 4 mL at a temperature of 80°C. Water acts as a solvent medium that facilitates the breakdown of the solid silicate structure into active aqueous species, where the presence of hydroxyl ions is essential for the dissolution of the solid silicate (Luukkonen et al., 2020). In an alkaline aqueous environment, hydroxyl groups attack and break the solid silica network into dissolved silica species (Alam et al., 2019). This process is an endothermic reaction that requires the absorption of heat energy (Luukkonen et al., 2020) therefore, a temperature of 40°C is used so that the solid can dissolve completely (Rasuli et al., 248 2022).

Calcium chloride (CaCl₂) was added as a templating agent. Its high hygroscopicity allows it to interact strongly with water molecules, where Ca²⁺ ions can associate with silanol groups (Si–OH) in the forming silica network. Its high solubility also enables this compound to function as a sacrificial template to form temporary spatial domains that will later be removed through washing (Zhu et al., 2006). After aging for 1 hour, the solution was acidified with 1 M HCl until pH 7. The addition of HCl neutralizes the sodium silicate, allowing the gelation process and formation of a solid silica network to occur (Setyawan et al., 2021).

The silica gel was then dried, ground, and sieved to obtain particles sized 105–250 µm. Particles within this micron range have been shown to possess excellent specific surface area and pore volume (Zhi et al., 2018), which greatly contribute to the high adsorption capacity for pollutant molecules (Ulfa et al., 2022). Maintaining a lower limit of 50 microns is also crucial to prevent pore plugging. The silica was then reacted with 5 M HCl solution at 80°C for 2 hours. This acid treatment is recommended to dissolve impurity compounds such as calcium oxide (Chindaprasirt & Rattanasak, 2020), allowing calcium to be released from the silica matrix, thereby increasing purity and opening the pores (Dhaneswara et al., 2020). A duration of 2 hours at high temperature provides optimal conditions for dissolving metallic elements without damaging the main material (Chindaprasirt & Rattanasak, 2020).

Surface Modification of Porous Silica

The silica material was modified using a 10% (v/v) APTES solution to introduce amine groups onto the particle surface. These amine groups can be protonated to –NH₃⁺ in aqueous media, resulting in a positively charged silica surface (Mahtabani et al., 2020). The aging process at room temperature for 24 hours, based on Mahtabani et al. (2020), allows greater opportunity for hydrolysis and condensation reactions to occur completely, thereby increasing the number of APTES molecules bound to the surface.

After rinsing with distilled water, the silica was dried at 40°C. A low drying temperature was used because the grafted organic groups are highly sensitive to heat and may begin to decompose at temperatures above 140°C (Tumurbaatar et al., 2022). Additionally, high temperatures after washing can trigger excessive condensation and cross-linking between APTES molecules (Mahtabani et al., 2021), as well as cause shrinkage of the silica network and dehydroxylation, which leads to the formation of a denser matrix with reduced surface area (Grozdanova et al., 2025). Therefore, mild drying is strongly recommended to prevent shrinkage of the silica pores (Grozdanova et al., 2025).

Application of Modified Porous Silica Adsorbent

The adsorption process was carried out using Activated Electrolyzed Water (AEW) under alkaline conditions. Water produced on the cathode side has a basic pH and a high concentration of hydroxide ions. This alkaline environment was selected so that the adsorption process could generate strong interactions between the positively charged silica surface and the target microplastics in water, which tend to carry negative charges (Shih et al., 2022).

The surface of porous silica contains numerous silanol groups after its initial synthesis process. The surface modification with APTES enables different attachment methods between silica and organosilane molecules because of the available hydroxyl sites. The modification process produces surface arrangements based on the extent of APTES condensation and attachment to the silica surface as shown in Figure 10.

The surface modification process results in various bonding patterns because APTES molecules either undergo partial hydrolysis or form monodentate bonds or they completely condense into bidentate or tridentate attachments and occasionally develop surface oligomers as explained in previous studies (Acres et al., 2012; Sypabekova et al., 2022). Although this study does not show the exact surface arrangement, still, aminopropyl group attachments can be proven through FTIR spectral analysis. As shown in Figure 3, FTIR spectral changes show that organosilane species have bonded with silanol groups because new functional bands appear for N–H, C–H, and Si–O–Si bands which show different characteristics than unmodified silica. The observed spectral changes demonstrate successful surface modification and prove that APTES has been successfully attached to porous silica. The adsorption occurred in the specific pH condition (pH = 7.8) controlled by a water electrolysis machine. Solution pH is one of the factors that significantly influences the adsorption of adsorbates onto adsorbents (Omorogie & Helmreich, 2024). In the basic solution, PE as a targeted polymer tends to be negatively charged due to the presence of OH^-, while the adsorbent surface has a positively-charged surface (NH₃⁺). Modified silica with APTES has successfully shifted the isoelectric point to 9.2 which means that under that pH, the adsorbent surface is positively charged due to protonation (Liu et al., 2024). The electrostatic repulsion occurred between the functional group of adsorbents and adsorbates because of the different ion charges between them (Omorogie & Helmreich, 2024). A study from Omorogie and Helmreich (2025) observed that Polystyrene (PS) MPs became positive charge in acidic conditions and slightly negative when pH was increased. The adsorption varies depending on pH values. The electrostatic interaction occurred between PS MPs with functional groups on the adsorbent surface. Electrostatic repulsion will influence the number of adsorbed MPs (Omorogie & Helmreich, 2024). Additionally, physical entrapment may contribute to the removal of PE due to the dominant size range of silica material being the same and larger than PE size range. This is also supported by the presence of PE MPs on the adsorbent surface of SEM analysis (Figure 9). The shifting of wave number around 1600 cm^-1 in FTIR spectra indicates the usage of these APTES sites to interact with PE.

Conclusion

Modified porous silica material demonstrates potential in reducing PE MPs from electrolyzed drinking water. The adsorption performance of porous silica is enhanced after the fabrication of tubular pore formation and amine functionalization and finds that 34,333 microplastic particles/L can be adsorbed within 24 hours. The adsorption process was confirmed through structural and chemical tests which showed that the material developed pores and surface modifications and microplastic binding occurred through nitrogen and carbon-based chemical changes. The removal efficiency reaches 100% with adsorption capacity of 5.35x10^-3 mg/g. However, reproducibility of material needs to be evaluated for practical applicability. Further studies incorporating adsorption kinetics and isotherm modeling are necessary to provide a more comprehensive understanding of the adsorption mechanism. Further studies incorporating the effects of temperature, adsorption kinetics, adsorption capacity variations, isotherm modeling, and combinations of different polymers are necessary to provide a more comprehensive understanding of the adsorption mechanism. In addition, adsorbent surface characterization could be conducted to further explain the interaction between the adsorbent and adsorbate.

Acknowledgement

Acknowledgments are extended to the Ministry of Education, Culture, and Technology of the Republic of Indonesia for funding this research through the BIMA Regular Fundamental Research Grant scheme, contract number 011/LL6/PL/AL.04/2025. The authors also would like to thank Felix Sholeh Kuntoro for their assistance in field sampling and laboratory analysis.

Compliance with ethics guidelines

The authors declare they have no conflict of interest or financial conflicts to disclose.

This article contains no studies with human or animal subjects performed by the authors.

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