Poor extraction efficiencies of polystyrene nano- and microplastics from biosolids and soil

Abstract

Extraction and quantification of nano- and microplastics from sediments and soils is challenging. Although no standard method has been established so far, flotation is commonly used to separate plastic from mineral material. The objective of this study was to test the efficiency of flotation for the extraction of nano- and microplastics from biosolids and soil. We spiked biosolids and soil samples with polystyrene nano- and microbeads (0.05, 1.0, 2.6, 4.8, and 100 μm diameter). Different extraction methods (w/ and w/o H2O2 digestion) were tested, and plastic beads were separated from mineral particles by flotation in a ZnCl2 solution. Plastic particles were quantified by UV-Vis spectrometry and gravimetrically. While large beads (100 μm) could be quantitatively extracted (∼100%) from both biosolids and soils, smaller beads had low extraction efficiencies (ranging from 5 to 80%, with an average of 20%). Except for the 100 μm beads, oxidation with H2O2 negatively impacted the extraction efficiencies. For the soil, extraction with water only, followed by flotation in a ZnCl2 solution, resulted in relatively high extraction efficiencies (>75%) for beads larger than 1 μm, but low efficiencies (<30%) for the 0.05 and 1.0 μm beads. Our results indicate that while flotation generally works to separate plastic nano- and microbeads in a solution, the challenge is to quantitatively extract nano- and microbeads from a biosolids or soil matrix. Samples high in organic matter content require removal of the organic matter, but the common method of H2O2 oxidation leads to poor extraction efficiencies for nano- and microbeads.

Microplastics and Nanoplastics in the Freshwater and Terrestrial Environment: A Review

“…However, synthetic fibres and microbeads are more commonly discussed due to their pervasiveness in daily life. Textiles, such as clothing, result in the release of synthetic fibres through ordinary use and laundering [7,46]. Similarly, microbeads are commonly found in everyday beauty and hygiene products, as well as cleaning products, which are washed down drainage systems [6,35,47].…”

“…Many everyday activities contribute to innumerable quantities of micro-and nanoplastics in the environment [7]. Laundering clothing that contains synthetic materials, such as acrylic, fleece and polyester, contributes to one million tons per year of synthetic fibres entering wastewater streams, 50% of which enters the environment [7,46]. Another consistent producer of microplastics in wastewater streams are exfoliants and toothpaste: per 1.6 g (the average toothpaste application) there can be upwards of 4000 microbeads [51].…”

“…Separating microplastics from natural organic matter (NOM) and other biogenic materials, while preserving the plastic particles, is the largest obstacle when it comes to removing microplastics from soil, WWTP effluents, sludge and other freshwater sources [46]. Treatment methods need to preserve microplastic samples so that critical information, such as the size, shape, colour and surface texture, can be retained to help determine the source, pathway and fate of the microplastics.…”

“…Oxidation using hydrogen peroxide (H 2 O 2 ) has thus become one of the more common methods of separation [86]; however, studies have reported that only small fractions of biogenic material are removed [91] and the NOM (along with the plastic particles) is often bleached rather than removed [89]. Additionally, some polymers such as PE, PP and PS have been found to degrade under these conditions [46,89].…”

“…Hydrodynamic flotation utilizes hydrodynamic flow to assist density separation of the particles. There is also a multitude of different solutions that can be utilized in these methods: distilled water, saturated NaCl, concentrated ZnCl 2 and concentrated NaI [46]. However, flotation…”