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Article

Application of Surface-Modified Natural Magnetite as a Magnetic Carrier for Microplastic Removal from Water

by
Palot Srichonphaisarn
1,†,
Natatsawas Soonthornwiphat
1,†,
Pongsiri Julapong
1,2,
Thanakornkan Limlertchareonwanit
1,
Thidarat Meekoch
1,
Ilhwan Park
3,
Mylah Villacorte-Tabelin
4,5,6,
Onchanok Juntarasakul
1,
Somsak Saisinchai
1,
Carlito Baltazar Tabelin
7,8,9,* and
Theerayut Phengsaart
1,10,*
1
Department of Mining and Petroleum Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok 10330, Thailand
2
Department of Mining and Materials Engineering, Faculty of Engineering, Prince of Songkla University, Songkhla 90110, Thailand
3
Division of Sustainable Resources Engineering, Faculty of Engineering, Hokkaido University, Sapporo 060-8628, Japan
4
Center for Natural Products and Drug Discovery, PRISM, Mindanao State University-Iligan Institute of Technology, Iligan 9200, Philippines
5
Center for Microbial Genomics and Proteomics Innovation, PRISM, Mindanao State University-Iligan Institute of Technology, Iligan 9200, Philippines
6
Department of Biological Sciences, College of Science and Mathematics, Mindanao State University-Iligan Institute of Technology, Iligan 9200, Philippines
7
School of Minerals and Energy Resources Engineering, The University of New South Wales, Sydney, NSW 2052, Australia
8
Department of Materials and Resources Engineering and Technology, College of Engineering, Mindanao State University-Iligan Institute of Technology, Iligan 9200, Philippines
9
Resource Processing and Technology Center, Research Institute for Engineering and Innovative Technology (RIEIT), Mindanao State University-Iligan Institute of Technology, Iligan 9200, Philippines
10
Applied Mineral and Petrology Research Unit (AMP RU), Department of Geology, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Minerals 2025, 15(4), 425; https://doi.org/10.3390/min15040425
Submission received: 5 February 2025 / Revised: 1 April 2025 / Accepted: 15 April 2025 / Published: 18 April 2025
(This article belongs to the Section Mineral Processing and Extractive Metallurgy)
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Versions Notes

Abstract

:
This study investigates the modification and application of natural, micro-scale magnetite (Fe3O4)—an iron oxide mineral and one of the most abundant iron ores in the world—as a magnetic carrier for removing six common types of microplastics (MPs) from water: polypropylene (PP), polyethylene (PE), acrylonitrile butadiene styrene (ABS), polystyrene (PS), polyethylene terephthalate (PET), and polyvinyl chloride (PVC). Hexadecyltrimethoxysilane (HDTMS) was employed as a surfactant to modify the naturally hydrophilic magnetite, transforming it into a hydrophobic material. The characterization of magnetite treated with HDTMS for 0, 6, 12, 24, and 48 h was performed using a scanning electron microscope with energy-dispersive X-ray spectroscopy (SEM-EDS) and Fourier transform infrared spectroscopy (FT-IR). The results showed HDTMS sorption on the surface of natural magnetite, confirming successful surface modification. Carrier magnetic separation was then performed to remove PP, PE, ABS, PS, PET, and PVC using surface-modified, natural magnetite in two size fractions: +38–75 µm (fine-sized) and +75–150 µm (coarse-sized). Improved performance was observed with longer HDTMS treatment of magnetite, while greater than 90% MP removal was achieved using fine-sized, surface-modified, natural magnetite. These results suggest that surface modification enhanced the heterogenous interactions between magnetite and MPs via hydrophobic-hydrophobic interactions, leading to efficient MP removal via carrier magnetic separation.

Graphical Abstract

1. Introduction

Microplastics (MPs) are plastic particles less than 5 mm in size that have become persistent pollutants in freshwater and marine environments in recent years [1,2,3]. Their harmful effects on marine organisms—from planktons and fishes to large mammals—have been documented by many authors. These impacts range from direct physical injuries and exposure to toxic organic compounds like Bisphenol A (BPA) to developmental abnormalities and even death [4,5]. Rochman et al. [4], for example, reported liver toxicity and pathology in Japanese medaka (Oryzias latipes), a widely studied model fish species, when exposed to virgin and marine waste-derived polyethylene (PE). Similarly, Campos et al. [5] found that high concentrations (5 mg/mL) of MPs of polytetrafluoroethylene (PTFE) led to a 35% mortality rate in Philippine mallard duck embryos (Anas platyrhynchos), along with a reduced vascular density and brain and spinal abnormalities, such as encephalomalacia and spinal cord discontinuities.
MPs fall into two categories: primary MPs, which are intentionally produced for commercial and industrial applications, such as microbeads in personal care products, resin pellets, and abrasives, and secondary MPs, which are inadvertently generated by the degradation of larger-sized plastics via physical, chemical, and biological processes in the environment [1]. Because plastic-based materials and products are widely used in everyday life, the release of MPs from both primary and secondary sources into the environment is inevitable [6]. One inherent property of MPs that makes them problematic is their small size, which allows them to be easily transported by wind, running water, and tidal action, contributing to MP pollution of water bodies such as rivers, lakes, and oceans. Rivers, in particular, act as significant pathways, ultimately transporting MPs to the sea [7,8].
Another significant issue is that MPs can serve as carriers of toxic substances, including persistent organic pollutants (POPs), polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), and heavy metals such as lead, mercury, and cadmium. Due to their large surface area and chemical properties, MPs can accumulate and transport these contaminants, posing serious risks to marine ecosystems and human health by contaminating food and water supplies [9,10]. To mitigate these risks, effective methods for removing MPs from water bodies are essential.
Several techniques have been developed for MP removal, often adapted from mineral processing and resource recycling methods. These include gravity separation [11,12,13], electrical separation [14,15], biomaterial adsorption [16], ultrafiltration [17,18], and surface-based separation [19,20,21,22,23]. Gravity separation, a cost-effective and simple method, relies on density differences between MPs and water, but may be ineffective for small MPs with densities close to water, reducing efficiency [24,25]. Electrical separation manipulates electrostatic forces based on MP conductivity and surface charge [26], providing effective separation for charged MPs but requiring specialized equipment and struggling with neutral or weakly charged particles [14]. Biomaterial adsorption, an environmentally friendly alternative, uses bio-based materials to capture MPs, though its efficiency depends on the adsorbent properties and may require regeneration or disposal of spent materials [27,28]. Ultrafiltration, a membrane-based method, is highly effective for capturing even nanosized MPs. However, membrane clogging remains a significant challenge, leading to frequent maintenance and increased operational costs [29]. Among these methods, surface-based separation techniques, including electrical separation and flotation, have gained attention for leveraging MPs’ hydrophobicity and surface charge to enhance removal. Flotation, which relies on bubbles to capture hydrophobic MPs at the air-water interface, is highly effective for hydrophobic MPs and allows large-scale processing. However, it may struggle with hydrophilic MPs or require chemical additives to improve efficiency [30,31]. Each technique has trade-offs in cost, effectiveness, and applicability, highlighting the need for optimized or integrated approaches to improve MP remediation.
A promising emerging method for removing fine materials (<100 µm) is carrier-based separation, which has been used for treating very fine sulfide minerals from low-grade ores that are difficult to process using traditional mineral processing techniques [31]. This technique modifies the surface properties of carriers to facilitate the attachment of target fine particles via hydrophobic-hydrophobic interactions, allowing their recovery through methods like magnetic separation or flotation [31,32]. Carrier-based separation is also applicable to MPs due to their small size and inherent hydrophobicity, which promote attachment to carrier materials with modified surface properties. Magnetic carriers, in particular, offer significant advantages for MP removal because they can be easily separated using magnetic fields, surface-modified for specific MP types, and reused via demagnetization-magnetization methods.
Several studies have explored synthetic magnetite (Fe3O4), ferrites, and modified-natural minerals as magnetic carriers for MP removal [33]. Shi et al. [34], for example, treated natural sepiolite—a clay mineral composed of magnesium silicate—with a ferrous-ferric solution, achieving about 98% removal of polyethylene (PE) MPs (<48 µm) from the solution. Other studies have improved MP attachment and removal by modifying the surface of magnetic materials with surfactants to enhance hydrophobicity. For example, Storozhuk and Iukhymenko [35] functionalized synthetic nano-magnetite with silane and found significantly improved MP removal from wastewater, making this approach a promising option for large-scale water treatment applications. In addition to nano-magnetite, other synthetic magnetic nanoparticles have been developed with advanced surface chemistries tailored to specific MPs. These nanoparticles can selectively target MPs based on polymer composition, offering accuracy and precision unattainable with traditional methods. Surface charge, hydrophobicity, and chemical affinity are critical factors dictating the interaction between carriers and MPs. Advanced coatings, such as polyethyleneimine (PEI) and polydopamine, further enhance adsorption capacity while maintaining the reusability of carrier materials [36,37].
The sustainability of magnetic carriers is another key selection consideration in MP removal. Magnetite-based carriers can be regenerated through simple washing or mild chemical treatments, ensuring long-term reusability [38,39]. Additionally, proper end-of-life management, such as recycling carrier materials or ensuring minimal environmental impacts during disposal, is crucial. MPs could even be repurposed as reductants in iron (Fe) smelting, which could enhance reducing conditions in smelters and improve the structural stability of pyrometallurgical products while minimizing waste generation [40]. Integrating magnetic carrier technology with existing remediation systems could also amplify these benefits, offering a more comprehensive solution for plastic pollution.
Despite the promising potential of surface-modified magnetite for MP removal from water, most published works have focused on synthetic and nano-sized magnetite. The main drawbacks of synthetic, nano-sized magnetite are the complexity of synthesis protocols, challenges in product characterization, difficulties in scaling up, and complications in carrier regeneration [33]. To address these issues, this study proposes the modification of natural, micro-scale magnetite with surfactants as an alternative magnetic carrier for MP removal from water.
In this study, natural, micro-scale magnetite was modified with hexadecyltrimethoxysilane (HDTMS) and applied for the removal of six types of common MP types—polypropylene (PP), polyethylene (PE), acrylonitrile butadiene styrene (ABS), polystyrene (PS), polyethylene terephthalate (PET) and polyvinyl chloride (PVC)—from water. Specifically, this work aims to (i) explore the role of surface properties in MP removal, (ii) assess the effectiveness of HDTMS-modified micro-scale magnetite in MP removal, (iii) evaluate a simple modification method for natural magnetite, and (iv) examine the life cycle, recycling, reuse, and sustainable disposal of HDTMS-modified magnetite. By addressing these objectives, this study seeks to advance our understanding of MP removal from water and contributes to innovative solutions for tackling plastic pollution effectively.

2. Materials and Methods

2.1. Samples

2.1.1. Microplastic Samples

The model plastic samples used in this study represented the plastic types commonly found in waste streams, including wastewater, as MPs [38]. Six types of virgin plastic boards (1000 × 2000 × 2 mm) (Table 1) were cut using a reciprocating saw before crushing in a cutting mill (Orient mill, VH16, Seishin Enterprise Co., Ltd., Tokyo, Japan) to reduce their particle size. The crushed plastics were then ground in a cryogenic mill using liquid nitrogen and sieved to obtain a size fraction of +250–500 µm, which was employed for MP removal experiments.

2.1.2. Magnetic Carrier

The magnetite sample used in this study was obtained from Arizona, USA. The magnetite sample, initially with D50 of ~120 μm, was sieved to isolate the size fraction of +75–150 µm as the coarse fraction. Subsequently, the sample was ground using a disc mill (Kawasaki Heavy Industries, Tokyo, Japan) to obtain a fine-sized fraction of +38–75 µm. The chemical composition of ground samples was analyzed using an X-ray fluorescence spectrometer (XRF; Epsilon 4, Malvern Panalytical Ltd., Malvern, UK) and the analytical results showed that the magnetite sample is mainly composed of 91.9% Fe (as Fe2O3) with trace amounts of Si (4.62% as SiO2), Al (1.13% as Al2O3), and Ti (0.97% as TiO2).

2.2. Surface Modification of Natural Magnetite

The surface modification of natural magnetite was carried out using hexadecyltrimethoxysilane (HDTMS; H3C(CH2)15Si(OCH3)3, ≥85% (GC) Sigma-Aldrich, Darmstadt, Germany). A total of 0.5 mL of HDTMS was dissolved in 50 mL of methanol (99.9% (AR), Fisher Chemical, Leicestershire, UK), and 1 g of magnetite sample was added into an Erlenmeyer flask for the surface modification experiments. These tests were carried out in a closed system by covering the flask with parafilm and shaking it at 200 rpm in a thermostat water bath shaker (OLS26, Grant Instrument, Hertfordshire, UK) for 6, 12, 24, and 48 h at 25 °C (Figure 1). The modified natural magnetite was then collected by filtration using a 2.7 µm pore filter (with a diameter of 47 mm) and dried in a desiccator.
After drying, the modified natural magnetite particles were analyzed using a scanning electron microscope (SEM; Prisma E SEM, Thermo Fisher Scientific, Waltham, MA, USA) coupled to an energy dispersive spectroscopy (EDS; X-MaxN, Oxford Instruments, Bristol, UK) and Fourier transform infrared spectroscopy (FT-IR; Alpha II, Bruker, Billerica, MA, USA) to characterize the surface properties of the magnetic carrier, ensuring its efficacy in MP removal.

2.3. Floatability Experiment

The floatability experiment of natural magnetite with and without modification was carried out using a Hallimond tube. The experiment was conducted under the following conditions: 5 g of magnetite, size fractions of +38–75 µm and +75–150 µm, and 150 mL of water. The conditioning step involved agitating water and methyl isobutyl carbinol (MIBC) as the frother at an impeller speed of 600 rpm for 1 minute using a mechanical stirrer (R 50, Ingenieurbüro CAT, Ballrechten-Dottingen, Germany). After this, 50 mL of the water-MIBC mixture was introduced into the Hallimond tube, followed by 5 g of magnetite, and the volume was increased to 150 mL using the remaining water-MIBC mixture. The aeration rate used in this study was 3 L/minute, with a flotation time of 1 minute. The magnetite in the froth product was collected and dried in the oven at 105 °C for 24 h and then weighed to calculate the floatability.

2.4. Microplastic Removal Experiment

In the MP removal experiments, natural magnetite modified with HDTMS at different treatment times (i.e., without modification, 6, 12, 24, and 48 h modification time) was used. One gram of magnetite (w/and w/o surface modification; in the size fraction of +38–75 µm and +75–150 µm) and one gram of a MP sample (i.e., PP, PE, ABS, PS, PET, and PVC) was introduced into a 50 mL beaker with 10 mL of deionized water and then manually mixed using a glass rod for 2 min to increase the collision probability between magnetite and MPs. MPs were removed together with magnetite using a 0.5 tesla (T) permanent magnet (Miyoshi Mfg. Co., Ltd., Tokyo, Japan) (Figure 2). The MPs left in water were quantified by gravimetry using a digital balance after drying the remaining suspension in an oven at 40 °C for 24 h.

3. Results and Discussion

3.1. Surface Modification of Natural, Micro-Scale Magnetite

3.1.1. Scanning Electron Microscopy with Energy Dispersive X-Ray Spectroscopy

A SEM was used to investigate the changes in surface properties of magnetite with and without HDTMS modification. The surface of magnetite without modification did not exhibit any significant morphological changes (Figure 3a). However, the surface of magnetite modified with HDTMS showed increasing morphological changes with longer modification times. Magnetite particles were subjected to 6 h of modification. Figure 3(b-1) had slight morphological changes and texture alterations; that is, a fine, slime-like, material coating was observed on magnetite particles. These morphological changes became more pronounced with longer modification times, as observed at 12 h (Figure 3(b-2)). Increasing the modification time to 24 h (Figure 3(b-3)), and to 48 h (Figure 3(b-4)) revealed even more pronounced surface changes. Notably, the morphology at 48 h was similar to that at 24 h, with no significant additional changes beyond this point.
To identify the chemical composition of the deposited material on magnetite, EDX analysis was conducted. The elemental changes are shown in Figure 4. The chemical structure of HDTMS is composed of O, C, and Si. Since C and Si are not components of unmodified magnetite, their presence can be used to identify modifications to the magnetite surface. The EDX analysis revealed changes in the percentages of Fe (Figure 4a) and O (Figure 4b), which are also present on the surface of unmodified magnetite. In contrast, the percentages of C (Figure 4c) and Si (Figure 4d) increased with longer modification times compared to unmodified magnetite, which showed no trace of these elements on its surface. The morphological and elemental changes observed by SEM-EDS confirmed the successful modification of magnetite through the sorption of HDTMS on the mineral surface.

3.1.2. Fourier Transform Infrared Spectroscopy Analysis

Changes on the surface of magnetite were further examined by Fourier transform infrared spectroscopy (FT-IR) after surface modifications. The wavelengths of magnetite with different modification times are illustrated in Figure 5. The characteristic infrared absorption bands of unmodified magnetite were found at the same wavenumber, regardless of the size distribution. However, after modification, changes were observed. The peaks at 2905 cm−1 and 2850 cm−1 are assigned to the asymmetrical C–H stretching of alkyl groups [–(CH2)n–], corresponding to the adsorption of HDTMS on the magnetite surface. Furthermore, the 1080 cm−1 IR absorption band, which is associated with Si–O–H and Si–O–Si groups, became more prominent after surface modification with HDTMS [35]. It is also interesting to note that changes in the fine-sized magnetite (Figure 5a) were more significant than those of the coarse-sized magnetite, likely because of the higher surface area of the former than the latter (Figure 5b).
The FT-IR results supported the SEM-EDX observations, confirming the successful modification of the natural, micro-scale magnetite surface. The long-chain hydrocarbon tails (Figure 6a) act as hydrophobic agents, reducing the surface free energy of the magnetite particles. The surface modification protocol relied on the chemical interactions between hydroxyl groups on magnetite and organosilicon compounds in HDTMS. HDTMS can hydrolyze methoxy groups to generate alkyl silanol groups, forming chemical bonds with the surface hydroxyl groups on magnetite. This process involves immersing magnetite in a methanol solution of hydrolyzed HDTMS, leading to the polycondensation of alkyl silanol and forming alkyl siloxane. This creates energetically stable siloxane bonds (–Si–O–Si–) and results in a thin layer through self-assembly. The reaction mechanism between magnetite and HDTMS is illustrated in Figure 6b.

3.1.3. Floatability Experiments

To determine hydrophobicity, floatability tests were carried out for magnetite without and with modifications for 6, 12, 24, and 48 h. The floatability of coarse-sized magnetite (+75–150 µm) (Figure 7) was low for unmodified magnetite due to its naturally hydrophilic properties. The floatability was approximately 7%, but it significantly improved after treatment with HDTMS, reaching around 99% for magnetite treated for 6 h. However, with increased treatment time, floatability showed minimal further improvement. This was likely due to excessive hydrophobicity, which led to agglomeration, causing the magnetite particles to clump together and preventing bubbles from carrying them. Nevertheless, the floatability remained above 97%. From the experimental results, it was confirmed that the modification of magnetite using HDTMS can change the wettability of the magnetite surface from hydrophilic to hydrophobic.
In contrast, fine-sized magnetite (+38–75 µm) was more challenging to remove. Due to its small particle size, magnetite could pass through the porous medium, leading to blockages. Additionally, at high hydrophobicity levels, a layer of magnetite formed on the water’s surface, preventing bubble overflow and the flotation of magnetite. This may be due to the generation of bubbles that are too large to effectively remove very fine-sized magnetite.

3.2. Microplastic Removal Using the Modified Micro-Scale Magnetic Carrier

The removal rate of MPs by modified natural magnetite is illustrated in Figure 8. The removal rates differed when fine-sized and coarse-sized magnetite were employed. For fine-sized magnetite (Figure 8a), the removal rate without modification was only 4.64% for PP, the highest among the six types of MPs tested, followed by PS, ABS, PET, and PVC. This could be attributed to the low density of PP compared with the other types of MPs. After modification, however, the removal rate of MPs increased to over 90%. These results were comparable in effectiveness to the previous study of Grbic et al. [22], which utilized nano zero-valent iron (ZVI) and reached over 90% MP removal from the hydrophobic interaction, which is a noncovalent force where water-repelling (non-polar) substances tend to cluster together in aqueous environments to minimize contact with water. These interactions facilitate the attachment of particles to each other or to bubbles, playing a key role in separation processes such as flotation [41,42,43]. In this case, the hydrophobic particles—modified magnetite and microplastics—attach to each other through hydrophobic interactions and are subsequently removed using magnetic separation.
In contrast, coarse-particle magnetite exhibited a lower removal rate due to its reduced surface area. The highest removal rates were observed for ABS and PS, followed by PET and PVC, with PP and PE having the lowest rates, respectively. As the modification time increased to 12, 24, and 48 h, the order of removal efficiency remained consistent. The removal order appeared to be influenced by the specific gravity (SG) of MPs, which was categorized as high-SG, middle-SG, and low-SG. This could be explained by the higher number of particles in low-SG MPs, which resulted in weak attachment to the magnetite carrier due to limited surface area. In comparison, the middle-SG and high-SG MPs had fewer particles, enhancing removal efficiency by forming more stable “agglomerates” with magnetite.
For the coarse-sized magnetite (Figure 8b), the MP removal rate without modification was only 1.01% for PP, the highest among the six types of MPs tested, followed by PS, ABS, PET, and PVC. After the magnetite modification with HDTMS, the MP removal rate increased significantly. The removal order remained the same as that observed with fine-sized magnetite; however, the removal rate improved considerably as the treatment time increased. The highest MP removal rate was 60%, achieved using the coarse-sized magnetite treated for 24 h. This improvement may be attributed to the surface area and SG of the MPs, which influenced the removal efficiency. The order of removal efficiency was observed as follows: ABS > PS > PET > PVC > PP > PE.

4. Conclusions

This study utilized natural, micro-sized magnetite particles to remove MPs from water, demonstrating the feasibility of modifying natural magnetite with HDTMS. The HDTMS layer deposited on magnetite significantly enhanced the mineral’s hydrophobicity. SEM images showed surface changes in magnetite due to HDTMS treatment, while FT-IR results revealed the appearance of new IR peaks associated with Si–O–Si stretching after HDTMS treatment, both of which confirmed the successful modification of natural magnetite. When fine-sized magnetite was used, the MP removal efficiency was highest, exceeding 90% for all MP types. The SG of MPs had little influence on the removal order, with lighter plastics (lower SG) being removed more effectively than heavier MPs when using unmodified magnetite. However, the size of the magnetite particles played a crucial role in MP removal efficiency. Fine-sized magnetite demonstrated significantly higher removal rates than coarse-sized magnetite, likely due to the larger surface area of the former, which enhanced interactions between the carrier and MPs. These findings suggest that on the micro-scale, smaller magnetite particles are more effective for removing MPs from contaminated water. The importance of surface hydrophobicity in MP removal was also highlighted in this work, linking the effectiveness of modified magnetite to its hydrophobic properties. Modification time also had a significant effect on the removal rate of coarse-sized magnetite, with removal rates increasing up to 24 h of modification time and stabilizing at 48 h. Additionally, the floatability of modified magnetite particles was higher than that of unmodified magnetite, confirming a change in wettability toward increased hydrophobicity. This suggests that modified magnetite can be effectively used for MP removal through carrier flotation. This study emphasized the critical role of surface modification and particle size optimization in improving the efficiency of MP removal from polluted waters, providing a foundation for advancements in environmental remediation technologies. The findings of this work can be applied to the utilization of magnetite as a carrier for MPs in smelters, which can be explored in future studies. Additionally, other materials should be explored as carriers in future research work for MP removal in environmentally important matrices.

Author Contributions

Conceptualization, S.S. and T.P.; methodology, N.S., O.J. and T.P.; software, P.J.; validation, N.S. and T.M.; formal analysis, N.S., P.J. and O.J.; investigation, N.S., T.L. and T.M.; resources, S.S. and T.P.; data curation, P.S., N.S., P.J. and O.J.; writing—original draft preparation, P.S., N.S., I.P., M.V.-T., O.J., C.B.T. and T.P.; writing—review and editing, P.S., I.P., M.V.-T., C.B.T. and T.P.; visualization, P.S and P.J.; supervision, S.S., C.B.T. and T.P.; project administration, P.S., N.S., O.J. and T.P.; funding acquisition, N.S., C.B.T. and T.P. All authors have read and agreed to the published version of the manuscript.

Funding

This research work is supported by the Second Century Fund (C2F) of Chulalongkorn University, Thailand.

Data Availability Statement

Data are contained within the article.

Acknowledgments

The authors gratefully acknowledge the Laboratory of Mineral Processing and Resources Recycling, Division of Sustainable Resources Engineering, Faculty of Engineering, Hokkaido University, Japan for sample preparation. The authors also wish to thank the editor and reviewers for their valuable input to this paper.

Conflicts of Interest

The authors declare no conflicts of interest.

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Minerals 15 00425 g001
Figure 1. A schematic diagram of the magnetite surface modification process employed in this study.
Figure 1. A schematic diagram of the magnetite surface modification process employed in this study.
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Figure 2. A schematic illustration of microplastic removal process employed in this study.
Figure 2. A schematic illustration of microplastic removal process employed in this study.
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Figure 3. SEM photomicrographs of magnetite (a) without modification and with modification at (b-1) 6 h, (b-2) 12 h, (b-3) 24 h, and (b-4) 48 h.
Figure 3. SEM photomicrographs of magnetite (a) without modification and with modification at (b-1) 6 h, (b-2) 12 h, (b-3) 24 h, and (b-4) 48 h.
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Figure 4. Elemental percentage abundance of (a) Fe, (b) O, (c) C, and (d) Si on the magnetite surface measured under different modification conditions.
Figure 4. Elemental percentage abundance of (a) Fe, (b) O, (c) C, and (d) Si on the magnetite surface measured under different modification conditions.
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Figure 5. FT-IR spectra of magnetite without and with modification: (a) fine-sized, and (b) coarse-sized.
Figure 5. FT-IR spectra of magnetite without and with modification: (a) fine-sized, and (b) coarse-sized.
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Figure 6. Schematic illustrations of the (a) chemical structure of HDTMS, and (b) reaction mechanism of HDTMS and magnetite surface.
Figure 6. Schematic illustrations of the (a) chemical structure of HDTMS, and (b) reaction mechanism of HDTMS and magnetite surface.
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Figure 7. The floatability of magnetite without modification and with modification at 6, 12, 24, and 48 h.
Figure 7. The floatability of magnetite without modification and with modification at 6, 12, 24, and 48 h.
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Figure 8. Removal of PP, PE, ABS, PS, PET, and PVC MPs as a function of magnetite modification time: (a) fine-sized magnetite and (b) coarse-sized magnetite.
Figure 8. Removal of PP, PE, ABS, PS, PET, and PVC MPs as a function of magnetite modification time: (a) fine-sized magnetite and (b) coarse-sized magnetite.
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Table 1. The specific gravity (SG) of plastics used in this study.
Table 1. The specific gravity (SG) of plastics used in this study.
Type of PlasticsAbbreviationSGProvider
PolypropylenePP0.92Showa Denko Materials Co., Ltd., Tokyo, Japan
PolyethylenePE0.97Showa Denko Materials Co., Ltd., Tokyo, Japan
Acrylonitrile Butadiene StyreneABS1.03Sumitomo Bakelite Co., Ltd., Shizuoka, Japan
PolystyrenePS1.06Kyoei Sangyo Co., Ltd., Tokyo, Japan
Polyethylene terephthalatePET1.31Sanplatec Corp., Osaka, Japan
Polyvinyl chloridePVC1.38Sanplatec Corp., Osaka, Japan
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MDPI and ACS Style

Srichonphaisarn, P.; Soonthornwiphat, N.; Julapong, P.; Limlertchareonwanit, T.; Meekoch, T.; Park, I.; Villacorte-Tabelin, M.; Juntarasakul, O.; Saisinchai, S.; Tabelin, C.B.; et al. Application of Surface-Modified Natural Magnetite as a Magnetic Carrier for Microplastic Removal from Water. Minerals 2025, 15, 425. https://doi.org/10.3390/min15040425

AMA Style

Srichonphaisarn P, Soonthornwiphat N, Julapong P, Limlertchareonwanit T, Meekoch T, Park I, Villacorte-Tabelin M, Juntarasakul O, Saisinchai S, Tabelin CB, et al. Application of Surface-Modified Natural Magnetite as a Magnetic Carrier for Microplastic Removal from Water. Minerals. 2025; 15(4):425. https://doi.org/10.3390/min15040425

Chicago/Turabian Style

Srichonphaisarn, Palot, Natatsawas Soonthornwiphat, Pongsiri Julapong, Thanakornkan Limlertchareonwanit, Thidarat Meekoch, Ilhwan Park, Mylah Villacorte-Tabelin, Onchanok Juntarasakul, Somsak Saisinchai, Carlito Baltazar Tabelin, and et al. 2025. "Application of Surface-Modified Natural Magnetite as a Magnetic Carrier for Microplastic Removal from Water" Minerals 15, no. 4: 425. https://doi.org/10.3390/min15040425

APA Style

Srichonphaisarn, P., Soonthornwiphat, N., Julapong, P., Limlertchareonwanit, T., Meekoch, T., Park, I., Villacorte-Tabelin, M., Juntarasakul, O., Saisinchai, S., Tabelin, C. B., & Phengsaart, T. (2025). Application of Surface-Modified Natural Magnetite as a Magnetic Carrier for Microplastic Removal from Water. Minerals, 15(4), 425. https://doi.org/10.3390/min15040425

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