Polycyclic aromatic hydrocarbon accumulation in aged and unaged polyurethane microplastics in contaminated soil

Highlights

• A three-step non-digestive separation procedure developed isolating MPs from soil

• Biodegradable polyurethane MPs were more prone to accumulate PAHs from soil

• PAH concentrations in biodegradable polyurethane MPs were 70× higher than those in soil

• The driving factor for sorption was the rubbery or glassy state of MPs

• Aging of MPs did not influence the degree of sorption

Abstract

The interaction of microplastics (MPs) and common environmental organic pollutants has been a frequently discussed topic in recent years. Although the estimated contamination caused by MPs in terrestrial ecosystems is one order of magnitude higher than that in the oceans, experiments have been conducted solely in an aqueous matrix. Therefore, an experiment was carried out with two soils differing in their concentrations of polycyclic aromatic hydrocarbons (PAHs) and polyurethane foams used for scent fences along roads and crop fields. Two types of polyurethane foam (biodegradable and conventional in aged and unaged form) were exposed to soils containing PAHs that originated from historically contaminated localities. The exposure lasted 28 days, and a newly developed three-step procedure to separate MPs from soil was then applied. Biodegradable polyurethane MPs exhibited a strong tendency to accumulate PAHs after 7 days, and their concentrations significantly grew over time. In contrast, the sorption of PAHs on conventional polyurethane MPs was substantially lower (a maximum of 3.6 times higher concentration than that in the soil). Neither type of foam changed their sorption behaviors after the aging procedure. The results indicate that the flexibility of the polyurethane polymeric network could be the main driving factor for the sorption.

Introduction

Plastics production has been rapidly increasing in the last decades and the attention of the scientific community was drawn to the problem of microplastics (MPs; polymer particles <5 mm) pollution. Generally, MPs originate either from larger plastic litter during disintegration or can be found as primary particles used as industrial feedstocks, (e.g., in cosmetic and cleaning products) (Cole et al., 2011).

Research studies have focused mainly on detecting residual plastic in marine environments (Andrady, 2011; Browne et al., 2011; Cole et al., 2011), in other aquatic environments (Mason et al., 2016), and recently also in drinking water (Pivokonsky et al., 2018). Despite the fact, that terrestrial ecosystems are considered as the main sink for MPs, the information about occurrence and effects of MPs on the land is limited (Nizzetto et al., 2016; Horton et al., 2017; Rochman, 2018).

One of the possible reasons for this is that quantification of MPs in solid samples is rather difficult. When separating MPs from the matrix, fluidisation and flotation with high density salt solution is often coupled to oxidative digestion (Nuelle et al., 2014; Li et al., 2019). The digestion step results in oxidative destruction of organic compounds to reduce the organic matter content. Further fluidization and flotation separate particles according to their density. The Fenton's reagent was suggested for the digestion step, because it is very efficient in removing the organic matter and at the same time it is nondestructive towards MPs (Hurley et al., 2018).

To date, the concentration of MPs in solids has been studied using various approaches and it is expressed by concentration units in weight or in the number of particles. For example, up to 55.5 mg/kg of MPs (corresponded to 593 particles/kg) were found in the floodplain soils in Switzerland (Scheurer and Bigalke, 2018). A much higher concentration was recorded in an industrial area in Australia where up to 67,500 mg/kg of MPs were detected in the soil (Fuller and Gautam, 2016). The agricultural fields in Canadian Ontario were contaminated comparably to the Swiss floodplain soils with the maximum concentration of 541 particles/kg and even after sewage sludge application containing up to 14,407 particles/kg (Crossman et al., 2020). In the Belgian sea harbor sediments, up to 390 particles in 1 kg of dry sediment were found (Claessens et al., 2011). More data about MPs in sediments and soils are reviewed in Horton et al. (2017) and Guo et al. (2020).

The environmental consequences, in other words, as the influence of MPs on terrestrial biota and soil were also investigated. For example, Huerta Lwanga et al. (2016) observed increased mortality and reduced growth rates after 60 day exposure of earthworms (Lumbricus terrestris) to MPs <150 μm. Rodriguez-Seijo et al. (2017) detected histological damage and inflammation in the gut of earthworms (Eisenia andrei). Boots et al. (2019) studied an impact of high-density polyethylene and polylactic acid MPs on the microcosmos involving plants, earthworms and soil. All of the three compartments were influenced by the addition of MPs. de Souza Machado et al. (2018) observed alterations in fundamental soil properties such as bulk density and water holding capacity after an exposure to MPs. Nevertheless, MPs concentrations in soils used in the laboratory studies differ among authors because there is a lack of information about MPs concentrations in terrestrial environments due to the reasons discussed previously.

Several studies have focused on the sorption of persistent organic pollutants (POPs) on MPs mainly in the aquatic environments, and have typically examined polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCB), dichlorodiphenyltrichloroethane (DDT), polybrominated diphenyl ethers (PBDEs) and their metabolites (Mato et al., 2001; Velzeboer et al., 2014; Rodrigues et al., 2019; Lin et al., 2019) as well as more polar compounds, such as antibiotics or bisphenols (Li et al., 2018; Wu et al., 2019). Some pilot studies compared the sorption potentials of MPs and sediment (or soil) towards organic pollutants (Mato et al., 2001; Velzeboer et al., 2014). Another study compared the adsorption capacities of high-density PE fragments and different soils for zinc. The MPs exhibited similar sorption properties for the zinc from solutions as well as from soils (Hodson et al., 2017). To date, the sorption of organic pollutants on MPs in the soil environment has not been studied yet. This is probably due to the digestion steps included in the separation protocols that can have negative effects on the organic pollutants. Nevertheless, the general sorption behavior of organic pollutants to plastic particles has been studied and the main factors influencing their behavior have been identified e.g. surface morphological changes, crystallinity changes and density changes during degradation (Guo and Wang, 2019).

It is noteworthy, that the combined effects of POPs and MPs on organisms were studied in aquatic (Besseling et al., 2017) and terrestrial ecosystems (Wang et al., 2019) showing that MPs can serve as vectors of POPs to biota. However, opinions on this potential risk differ among authors (Rodrigues et al., 2019; Wang et al., 2019). Wang et al. (2020) exposed the earthworm Eisenia fetida to POPs preconcentrated on various MPs. The results showed different routes of POPs bioaccumulation in the earthworms. Interestingly, pre-contaminated MPs increased accumulation of POPs in the earthworms, while pristine MPs added to a contaminated soil decreased bioaccumulation. Therefore, the fate of pollutants in POPs-contaminated environments containing MPs should be discovered.

Considering the limited attention paid to MPs in terrestrial ecosystems, we focused on MPs in soil and the potential sorption of PAHs on MPs from contaminated soil. This study simulated a situation after an application of scent fences, which were made of polyurethane foams. The fences are located at the edge of an agricultural landscape to protect crops against deer or beside roads to prevent car accidents with animals. At the end of the lifetime of the scent fences, the polyurethane foams began to break down and the MPs fell onto the soil. According to our hypothesis, these MPs, with their large specific surfaces and nonpolar characteristics, easily adsorbed organic pollutants, namely, PAHs that are typically released by traffic. The aims of our study were (i) effectively separate MPs from these soils using a newly developed three-step separation procedure, (ii) evaluate the sorption of PAHs mixture on MPs over time, (iii) compare the sorption of PAHs on conventional and biodegradable polyurethane foams (both aged and unaged), and (iv) discover the main driving factor of the presumed sorption. To the best of our knowledge, this is the first study that uses soil as a carrying medium and source with a complex mixture of organic pollutants for secondary contamination of MPs.