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.