INTRODUCTION
Microplastics (MPs) are found in various environmental matrices
globally. They possess unique properties which render them mobile with
the ability to sorb contaminants and permeate
membranes.1 MPs have been found in human
stool,2lungs,3,4colons,5 placentae,6 and
blood.7 While little is known to date regarding its
effects on humans, studies are beginning to enumerate MP impacts on
organisms across species and ecosystems.8,9 Though
innovative studies on MP occurrence and abundance are emerging daily,
there are still limitations in the methods of detection, extraction,
identification, quantification, and monitoring of microplastics in the
environment.10 These issues coupled with limited
access to analytical instrumentation may result in under or
overestimation of MP abundance depending on study design, long sample
processing times, hazardous waste generation, and
cost.11,12,13 Additionally, these limitations are
responsible for the lack of harmonized and universally accepted methods
of sampling, extraction/separation, analysis, and reporting. Hence,
improvements to existing methods or new methods are needed to extract MP
from environmental matrices.
Unlike other environmental samples, the separation of MP from
particulate organic matter (POM) is particularly challenging. Its low
density and subsequent buoyancy results in its capture alongside a
myriad of MP polymers with most density separation methods. The POM
captured interferes with recoveries, identification, and quantification
of MPs, particularly MPs ≤1 mm12 which are more toxic
to aquatic organisms.14,15 For instance, MP combined
with organic matter was found to enhance the accumulation and induce
higher toxicity of heavy metals in fish tissue.16Furthermore, these smaller sized items have relatively larger surface
areas to sorb pollutants and are able to penetrate epithelial barriers
due to their high mobility in various mediums.17
Methods to extract and quantify MPs from organic matter depend on
environmental matrices, target particle sizes, and the proposed research
questions. Some methods include manual removal via naked eye or
microscope, enzymes digestion, chemical oxidation, and physical
separation based on plastic density relative to aqueous salt solution
(NaCl, ZnCl2, NaI, etc.).12,13 The
density approach has also been used with only ethanol (96%; 0.8 g
cm3) by Herrera et al. 18 to
separate MP from vegetal-rich samples. Herrera et al. approach is
notable as it deviates from most aqueous salt density separation
methods. It follows a binary mixture separation method of ethanol to
water (96:4), since 100% ethanol may not be obtainable because ethanol
normally forms an azeotrope with water. Binary mixture separation
techniques have been described in literature for over three
decades,19–21 but has not been explicitly optimized
in microplastic research.
Binary solvent mixtures overcome the hydrophobic interaction/effect of
non-polar molecules, such as those on the surface of POM that prevent
wetting. A solvent-water mixture acts as an ”anti-hydrophobic
agent”22 that solvates hydrophobic surfaces and allows
water to penetrate the POM’s internal matrix. The use of a binary
solvent mixture can advance our understanding of MP presence, fate,
behavior, and impact on the environment because it is an easy, fast, and
cheap method of separating microplastics from organic
matter.23 The method discussed below may also preserve
aspects of MP chemistry allowing for the extraction of sorbed
contaminants, particularly hydrophobic chemicals.
A new method of separating, quantifying and verifying smaller MP
(<1 mm) from estuarine POM using a binary solvent
mixture24 of ethanol and water is discussed below.
This method can be used with various environmental matrices and combined
with existing methods, particularly density separation using salt
solutions, to target a variety of polymers. The binary solvent mixture
used accelerates POM wetting, resulting in its loss of buoyancy. When
the binary mixture is removed and only water is reintroduced to the
sample, POM sinks while MP polymers float. The work focuses on
extraction methods and their chemical mechanisms as well as the
quantification and identification of the extracted MP polymers.