Investigation into the impact of aged microplastics on oil behavior in shoreline environments

Highlights

• The changes of microplastics under seawater-aging and UV-aging are distinctive.

• Aged microplastics can impact the transport of oil in shoreline environments.

• A higher amount of oil droplet contributes to the aggregation with microplastics.

• C–O and C–H functional groups play an important role in the binding process.

• Effects are determined by the trade-offs between oil-microplastic interactions.

Abstract

Understanding the interactions between oil and other particles in shoreline can help determine the environmental risk and cleanup strategy after oil spill. Nevertheless, far less has been known regarding the impact of aged MPs on oil behavior in the shoreline environment. In this study, the aging course of polyethylene (PE) in shaking seawater and ultraviolet (UV) radiation conditions was investigated. The seawater aging mainly affected the physical properties of MPs, increasing its surface pores and hydrophilicity. UV aging significantly affected both the physical and chemical properties of MPs, which increased its hydrophilicity and crystallinity, decreased its mean particle size and introduced oxygen-containing functional groups onto MPs. The two-dimensional correlation spectroscopy (2D COS) analysis confirmed the evolution of oxygen-containing functional groups from C–O to CO. The effects of aged MPs on oil behavior in water-sand system were further explored. The oil remaining percentages were non-linearly changed with the increasing aging degree of MPs. The particle size of the aqueous phase after washing was inversely related to the oil remaining percentage. Further FTIR analysis revealed that C–O and C–H functional groups played an important role in the process of oil adsorbed on MPs.

Introduction

Marine oil spills are an environmental concern to the public, the oil industry, and regulators. Anthropogenic oil disasters in marine have been regarded as the most damaging category of spills (Nelson and Grubesic, 2018). The estimated global petroleum release to the ocean exceeds 1.3 million metric tonnes per year, this being from natural seeps (46.9%), consumption (37.5%), transportation (12.5%), and petroleum extraction (3.0%) (National Research Council, 2003). Oil spilled into ocean waters may be driven to shorelines by currents and winds, while coastal pipeline and oil tank leakage may also cause shoreline oiling (Feng et al., 2021). This is especially concerning considering that residue oil can remain in shoreline environments for decades (Sauer et al., 1998, Yang et al., 2018). Oil penetrates the ground until it reaches fine-grained sediment, bedrock, the water table, or other penetration-limiting layers. The major oil residues are heavy fraction that is insusceptible to weathering. The degradation of the residues is governed by a number of factors, such as the geomorphic characteristics, wave energy, slope, and beach substrate (Wang et al., 2020). Meanwhile, the interactions between oil and other particles present in the environment underscore the uncertainty of the oil’s fate. For instance, natural clay particles may facilitate the removal of oil by forming oil-particle aggregates, which may, in turn, transfer the oil from the shoreline to the water column or even to the seafloor (Gustitus and Clement, 2017, Owens et al., 2021). Tremendous effort would be demanded to clean the affected areas, being the research focus of shoreline response (Grottoli and Ciavola, 2019). Understanding the oil behaviors in the shoreline environment is important for assessing the environmental risk and identifying the appropriate response strategy (Bi et al., 2020, Vahabisani and An, 2021).

Apart from natural solid particles, microplastics (MPs) are ubiquitous in aquatic environments. Since the beginning of plastic production in the 1940s, global production and application of plastic have soared exponentially to 368 million tonnes per year (PlasticsEurope, 2020). Up to 94% of plastic products presently end up in landfills or are released to the environment, while only 6–26% is recycled. Plastics often enter lakes, rivers, and agricultural land, depositing in sediments or oceans (Alimi et al., 2018). At least 8 million tons of plastic release into our oceans every year, making up 80% of all marine debris from sea surface to sea floor (IUCN, 2018). MPs are an emerging pollutant threatening marine environment (Patrício Silva et al., 2021). Classified by their origin, MPs are either primary or secondary. Primary MPs are small particles designed for commercial usages, such as plastic beads in cosmetics. On May 21, 2021, a cargo ship at Sri Lanka’s Colombo Harbor caught fire and released billions of industrial plastic pellets into shore (Mongabay, 2021). Secondary MPs are particles that result from the breakdown of large plastic weathered through UV radiation, fluctuating temperatures, mechanical abrasion, and biodegradation (Eerkes-Medrano et al., 2015). Polyethylene (PE), a type of linear saturated polyolefin, is the most commonly used plastic today because of its adaptability, toughness, and ease of manufacture (Singh and Sharma, 2008). PE is mainly used in packaging (e.g., bags, films, bottles). Although widespread use of plastic material has brought wide-ranging social and economic benefits, once fulfilling the designed purpose, bulk plastics will be discarded, then being subjected to weathering processes. MPs have been found to be ubiquitous in the ocean (Desforges et al., 2014, Uurasjarvi et al., 2021), coast (Mai et al., 2020, Zhang et al., 2021), sediment (Corcoran et al., 2020, Duan et al., 2021b), seafloor (Dasgupta et al., 2020), vegetation, and animals (Payton et al., 2020).

Roughly half of the world’s population resides in coastal region, which makes the coast a focused zone for MPs. Coastal MPs may originate directly from land sources (e.g., shoreline littering, urban or rural runoff, sewage treatment plants), or may derive from maritime activities (e.g., aquaculture, transportation, resource exploitation), river or lake discharge, or atmospheric dust (Zhang, 2017, Doustmohammadi and Babazadeh, 2020). Previous studies have observed the nature and extent of MPs in coastal areas. For instance, a study conducted at a coastal fishery region in China found that the mean MPs concentration was 5.0 particles/m³, and most of the fibers and fragments were observed to be PE and PP (Zhang et al., 2021). In the subsurface seawater in the northeastern Pacific Ocean, meanwhile, MPs concentrations have been found to range from 8 to 9200 particles/m³, with the highest concentrations found in coastal areas (Desforges et al., 2014). In a recent study conducted in the Persian Gulf, the concentration of MPs in intertidal sediments was found to range between 36 to 228 particles/m² (Abayomi et al., 2017). Additionally, both field studies and modeling have suggested that the majority of the plastics released into the ocean remain within the coastal area (Lebreton et al., 2012). Coastal seas are energetic regions subject to strong hydrodynamic forces, e.g., wind, tides, waves, and thermohaline gradients. In these regions, plastic pollutants undergo beaching, drifting, or settling until they reach a temporary or permanent pool (Zhang, 2017). In spite of the work done in this domain, though, the trajectory and behavior of MPs in coastal environments are not yet well understood.

Some previous studies have investigated MPs as a vector for transporting pollutants. By virtue of their small size and strong hydrophobicity, MPs are well suited to serve as vectors for many organic pollutants. Polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons, and polybrominated diphenyl ethers have been frequently observed in plastic waste gathered from seawater and shoreline sediments (Mai et al., 2018). Zarfl and Matthies (2010) reported that the annual flux of PCBs mediated by plastics to the Arctic Ocean varied from 250 g/year to 130 kg/year. In light of the long residence time and transport distance of plastics, plastic-mediated pollutants may conduce to the marine chemical budget. Moreover, MPs can also remain in organisms for a long period of time, and can be transferred and concentrated in the food web, thereby disrupting the equilibrium of the ecosystem and adversely affecting human health (Batel et al., 2016). However, the interactions between MPs and oiled shorelines have not yet been well documented.

After entering the environment, MPs undergo a series of weathering processes that alter their physical and chemical properties, resulting in alterations in their environmental behavior. Liu et al., 2019a, Liu et al., 2019b indicated that the capacity to adsorb hydrophilic organic chemicals is higher in aged MPs than in pristine MPs. UV-induced abiotic oxidation is one of the main sources of damage on PE, and the initial and rate-determining step for PE degradation in the environment (Singh and Sharma, 2008). Other studies have demonstrated that photoirradiation induces the creation of reactive oxygen species, leading to the appearance of oxygen-containing functional groups on MPs surface (Mao et al., 2020, Shi et al., 2021). Furthermore, the long chains of MPs may be broken, leading to changes in crystallinity. However, there is a scarcity regarding the aging course of PE that has relatively exceptional rigidity and aging resistance. Weathering of MPs under different environmental stresses also raises uncertainties concerning their environmental impacts. In this context, the objectives of the research described in this paper were to explore (1) the aging mechanism of MPs in seawater and UV radiation environments; (2) the effect of MPs on oiled sand environments; and (3) the bonding sites of functional groups between MPs and oiled sand.