The most abundant MP polymer-type across all the samples was polyethylene (PE; ~65%). Polypropylene (PP) and co-polymer (CP; mixed polymers) recorded 18% and 4% of the total sub-sampled MP respectively using FT-IR (Figure S7). Although about 13% of the particles give a convincing feature of polymers when visually observed under the stereo microscope, however, no confident material match could be determined hence they were named as unclassified. (Figure 2B).Chemistry of the separation technique. POM is a complex environmental matrix that ensnares materials, including buoyant and non-buoyant macro and microplastics, as it moves across surface waters. The comparable density and size of POM to several microplastic polymers (PE, PP, foamed polystyrene, etc.) limit the effectiveness of the most used method, density separation, to partition microplastics from POM. The use of a binary solvent mixture enables the separation of these similar density materials by altering POM surface chemistry to induce wetting and decrease buoyancy.

POM consists of humic substances having hydrophilic and hydrophobic groups, charged sites, and counter ions.^28–30Depending on parent material, POM has varying portions of polysaccharides and lignin. POM is difficult for water to penetrate (wetting) due to its chemical characteristics, surface cations, and interactions with other organic moieties and minerals in the marine environment.²⁹ For instance, when cations (e.g., H⁺, Na⁺, NH⁴⁺, Fe³⁺, Ca²⁺, Mg²⁺, Hg²⁺ Al³⁺, and organic cations) are bound to the POM’s exchange sites, intramolecular charge repulsion is minimized preventing POM wetting that results in hydrophobicity.^31,32 Furthermore, multivalent cations may also form crosslinked intra and intermolecular bonds with multiple charged sites on the POM. This crosslinking further compacts the POM molecular structure, restricting wetting of the organic matrix, resulting in hydrophobicity that drives buoyancy.³²There are also other mechanisms that influence POM wetting including dispersion or van der Waal’s forces, polarizability, polarity or dipole interactions, permanent dipole-dipole interactions, or hydrogen bonding interactions (electron exchange).³⁰ The overall effect of these mechanisms produces strong bonding on the POM surface which contributes to hydrophobicity.

Therefore, understanding the physicochemical behaviors of the POM composition helps when selecting the optimal method for MP separation. For example, one way to easily separate MPs, particularly buoyant polymers, from POM, is get one material to float while the other sinks. This can be done by displacing insoluble cations present on the POM surface while disrupting intra- and inter-molecular associations (bonding) and surface interactions. This was achieved through a binary solvent medium that penetrated the POM and exposed the bonding within its moiety.

Herrera et al. ¹⁸ used 96% ethanol for density separation of MP from organic matter which yielded improved and simplified method of MP extraction in organic matter. However, the study was limited to particle size of 1-5 mm, with no mention of MP <1 mm that are more toxicologically problematic. Also, water-ethanol-water interactions were not employed in their method as only ethanol (96%) was used. The method described here can be optimized to use 50% ethanol, reducing costs and solvent use. Therefore, relative to Herrera et al. (2018), this method can be optimized to use as little as 50-60% ethanol to reduce costs and solvent use, while also separating smaller MP sizes (<1 mm) from POM.

Second, Herrera et al. reported that their method does not capture polystyrene, and polyurethane foams because they are buoyant in 96% ethanol used. These two polymers were not identified during our FT-IR analysis. However, because of the additional water solvation step in this method, foamed polystyrene and polyurethane, which are buoyant in water, will be captured with this method.

Limitations. Our observation in the current procedure revealed that this technique may not be suitable for microfiber extraction as we could not find any microfiber particle in all our isolated microplastics against our expectation. However, microfibers were also not observed in the extracted organic matter when examined under the stereo microscope. It is also possible that some microfibers are lost during sieving.

Also, improved methods for separating MP from complex environmental matrixes with little or no alteration to the chemistry of the isolated MP are needed, especially for studies that quantify contaminants sorbed to environmentally weathered plastic. This method, because it uses diluted ethanol, may not be sufficient for hydrophilic chemicals sorbed to plastic. However, because of ethanol’s higher polarity, it may not interfere with the stronger bonds of hydrophobic chemicals on microplastic surfaces. This also requires additional investigation in our future studies.

Applications. This simple and cost-effective extraction method efficiently separates microplastics from 30 µm to 1 mm. It can be combined with existing density separation methods to capture higher density MP by using an aqueous salt solution instead of DI water for the water solvation step. Furthermore, it can be adapted and optimized to extract MP from POM in other environmental settings that have not been heavily studied, particularly riparian zone POM as well as soils and sediment that contain organic matter. Binary solvent mixtures could also be used to extract MP from wastewater with its high organic matter content. In addition to versatility, POM separation from MP uses only water and EtOH (which can be easily recovered and reused), aligning with the principles of Green Chemistry,³³ which should be a point of emphasis in microplastic research. Last, this simplistic and low-cost approach with broad applicability can expand research potential for areas where plastic pollution is significant, but resources to study and address the problem are limited.