Testing an Iron Oxide Nanoparticle-Based Method for Magnetic Separation of Nanoplastics and Microplastics from Water

Abstract

Nanoplastic pollution is increasing worldwide and poses a threat to humans, animals, and ecological systems. High-throughput, reliable methods for the isolation and separation of NMPs from drinking water, wastewater, or environmental bodies of water are of interest. We investigated iron oxide nanoparticles (IONPs) with hydrophobic coatings to magnetize plastic particulate waste for removal. We produced and tested IONPs synthesized using air-free conditions and in atmospheric air, coated with several polydimethylsiloxane (PDMS)-based hydrophobic coatings. Particles were characterized with scanning electron microscopy (SEM), transmission electron microscopy (TEM), superconducting quantum interference device (SQUID) magnetometry, dynamic light scattering (DLS), X-ray diffraction (XRD) and zeta potential. The IONPs synthesized in air contained a higher percentage of the magnetic spinel phase and stronger magnetization. Binding and recovery of NMPs from both salt and freshwater samples was demonstrated. Specifically, we were able to remove 100% of particles in a range of sizes, from 2-5 mm, and nearly 90% of nanoplastic particles with a size range from 100 nm to 1000 nm using a simple 2-inch permanent NdFeB magnet. Magnetization of NMPs using IONPs is a viable method for separation from water samples for quantification, characterization, and purification and remediation of water.

Keywords: PDMS; aminopropylsiloxane; amphiphilic polymer; hydrophobic coatings; hydrophobicity; interparticle interactions; iron oxide nanoparticles; microplastics; nanoplastics; plastic pollution; polydimethylsiloxane nanocomposite; separation science; water remediation.

Figure 1

(A) Functionalization of IONPs with PDMS via hydroxy-terminated PDMS (PDMS-OH, upper structure) or carboxy-terminated (C-PDMS, lower structure), rendering hydrophobic IONPs, (B) Functionalization with PAA:PDMS-co-APMS, PAA coating followed attractive electrostatic interaction of PDMS-co-APMS with PAA coating (C). TEM image of IONPs produced in argon (later coated in C-PDMS) demonstrated cubic morphology (D), TEM image of IONPs produced in air (later coated with PAA:PDMS-co-APMS or PDMS-OH).

Figure 1

(A) Functionalization of IONPs with PDMS via hydroxy-terminated PDMS (PDMS-OH, upper structure) or carboxy-terminated (C-PDMS, lower structure), rendering hydrophobic IONPs, (B) Functionalization with PAA:PDMS-co-APMS, PAA coating followed attractive electrostatic interaction of PDMS-co-APMS with PAA coating (C). TEM image of IONPs produced in argon (later coated in C-PDMS) demonstrated cubic morphology (D), TEM image of IONPs produced in air (later coated with PAA:PDMS-co-APMS or PDMS-OH).

Figure 2

Size distribution histogram for IONPs coated with Siliclad (A). C-PDMS (B), PDMS-OH (C), and PAA:PDMS-co-APMS (D), line represents geometric mean.

Figure 3

XRD spectra for IONPs. (A) XRD spectrum of IONPs produced using air-free techniques, under argon flow shows that the crystal phase is 59.6% wüstite and 40.4% *spinel phase (corresponds to starred indices in image), and (B) XRD spectrum of IONPs produced in ambient air shows that the crystal structure is 72.7% *spinel and 27.8% hematite.

Figure 4

DLS size distribution histograms in water. Line represents cumulative size total. C-PDMS IONPs with SDS (A), PDMS-OH with SDS (B), PAA-coated (C), and PAA:PDMS-co-APMS IONPs (D).

Figure 5

SQUID magnetometry hysteresis results, showing no coercivity and demonstrating the range of m_sat values for three different types of PDMS coated IONP samples; PAA:PDMS-co-APMS (4%) blue, carboxydecyl-PDMS green, and PDMS-OH purple.

Figure 6

Zero-field cooled (ZFC, lower) and field-cooled (FC, upper) curves for the C-PDMS IONP sample (A); PDMS-OH IONP sample (B); and PAA:PDMS-co-APMS IONPs (C) taken at 10 Oe.

Figure 7

FTIR spectra for functionalized IONPs.

Figure 8

UV-vis absorption spectra for all IONP samples (upper image, full scan) and close-up of absorption in the visible range between 500–700 nm (lower image).

Figure 9

Static water contact angles, θ_c, measured for all functionalized IONPs on glass substrates. Images for IONPS coated with (A) Siliclad, (B) hydroxy-PDMS, (C) oleate, (D) PAA:PDMS-co-APMS 4%, (E) PAA:PDMS-co-APMS 2%, and (F) carboxydecyl-PDMS.

Figure 10

Scanning electron microscope (SEM) images showing topography of functionalized IONPs on glass wafers and surface plots (inlayed) for (A) Siliclad, (B) hydroxy-PDMS, PDMS-OH, (C) Oleate, and (D) PAA:PDMS-co-APMS 4%.

Figure 11

Attachment of IONPs to the nurdle samples collected from the environment. The attraction of environmental nurdles to the IONPs coated with C-PDMS. (A), 4% PAA:PDMS-co-APMS (B), 2% PAA:PDMS-co-APMS (C), PDMS-OH (D), PMMA (E), and Siliclad (F), was tested. The nurdles covered with the IONPs were collected by magnetic bars (G).

Figure 12

Binding of NPs by IONPs. After incubation with the IONPs, NPs in water were precipitated to form agglomerates (A). The NPs (arrows) and numerous IONPs (B). The removal of NPs with 100 (C), 500 (D), and 1000 (E) nm was tested using NPs with fluorescent tags.