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.