Sorption behaviors of phenanthrene on the microplastics identified in a mariculture farm in Xiangshan Bay, southeastern China

Highlights

• Sorption behaviors of plastic fibers identified in a mariculture farm were studied.

• Both plastic fibers showed strong linear isotherms towards phenanthrene.

• Surface functional groups played an important role in microplastic's sorption.

• Small size and rough surface promoted the sorption of phenanthrene.

• Microplastics showed clear selectivity towards hydrophobic organic contaminants.

Abstract

Recently, with the accumulation of evidence that microplastic can be ingested by a variety of marine organisms, microplastic sorption behaviors towards organic contaminants (OCs) have become the subject of more studies due to the concerns about the contaminant vector effect. In this study, the priority microplastics identified in a mariculture farm in Xiangshan Bay, China, including polyethylene (PE) and nylon fibers (i.e., derived from new fishing ropes and nets), were examined for their sorption behaviors. The results indicate that both plastic fibers show linear isotherms towards phenanthrene, a common target hydrophobic organic contaminant (HOC), revealing the characteristics of a partitioning mechanism. The sorption capacity of PE fiber was found to be 1–2 orders of magnitude higher (evaluated by Freundlich parameter log K_F) than that of nylon fiber, suggesting the importance of plastic surface functional groups (i.e., with or without hydrophilic groups). By comparing carbon normalized log K_F with literature data, the organic affinity of PE fiber was found to be 1–2 orders of magnitude lower than that of vectors, such as carbonaceous geosorbents (CG), but was 1–2 orders of magnitude higher than that of marine sediments. Small size and rough surface tended to enhance the sorption of plastic fibers of phenanthrene. In addition, phenol (log K_OW: 1.46), a low-hydrophobicity compound, showed approximately 3 orders of magnitude lower sorption amounts onto both fibers compared to phenanthrene (log K_OW: 4.46), indicating the selectivity of hydrophobicity. The results of this study demonstrate that the high abundance of plastic fibers distributed in mariculture farms could lead to a higher contaminant transfer effect than marine sediments, and their effects on cultured seafood (e.g., crab and fish) need further investigation.

Introduction

Evidence is accumulating that microplastics can be an important vector for transporting organic contaminants (OCs) to organisms (Browne et al., 2013; Chua et al., 2014; Kwon et al., 2017; Sleight et al., 2017; Teuten et al., 2007; Wardrop et al., 2016; Li et al., 2015). The hydrophobic nature of plastic (except for a few plastics such as Teflon) causes microplastic surfaces to potentially sorb various hydrophobic organic contaminants (HOCs) from aqueous solutions (Lee et al., 2014; Wang et al., 2015a; Llorca et al., 2018). Due to the tiny size, microplastics can be ingested by the marine organisms that are at the bottom of the food chain (Cole et al., 2013; Setala et al., 2014), and thus, their contaminant transfer effects could be more significant than those of larger plastic debris.

The reported works concerning the sorption behavior of microplastics mainly include two parts: laboratory and field studies. Like many other environmental vectors of contaminants, e.g., black carbon (Cornelissen et al., 2005), soot (Jonker and Koelmans, 2002), sediments (Cornelissen and Gustafsson, 2004), and carbon nanomaterials (Yang et al., 2006), the development of a theoretical understanding of the sorption behavior of microplastics is the first step for further risk assessment and management. A few laboratory studies reported the sorption behaviors of OCs, particularly HOCs, on microplastics or plastic debris. Teuten et al. (2007) compared the sorption of phenanthrene to standard plastic powders and two natural marine sediments (with an organic carbon content of 0.67 ± 0.15% and 0.18 ± 0.09%) and found that sorption to plastics greatly exceeded sorption to two different natural sediments. Lee et al. evaluated the sorption capacity of plastic debris for HOCs, and their results suggest that the partition coefficients of HOCs between plastic debris and seawater can be as high as their octanol-water partition coefficients (K_OW) (Lee et al., 2014). Polycyclic aromatic hydrocarbons (PAHs) are the most commonly used target contaminant HOCs in laboratory studies, but more HOCs such as PFOS/FOSA (Wang et al., 2015a), DDT (Napper et al., 2015) and PCBs (Velzeboer et al., 2014; Zhan et al., 2016) have been tested recently. In addition to laboratory study, field studies focusing on the sorption of OCs on plastic beads under natural conditions are also conducted (Mato et al., 2001; Rochman et al., 2013). From these reports, it can be found that most studies used standard plastic particles/beads as surrogates for the large variety of microplastics detected in the marine environment. Although these standard plastic samples are easily obtained and handled for sorption tests, there would be uncertainty regarding the differences between those standards and real microplastics. For example, fibrous plastics (e.g., lines, threads and strands) have been widely detected across the marine environment (Hidalgo-Ruz et al., 2012), but relevant reports on their sorption behaviors are absent. In this case, significant inaccuracy would occur when merely rely on the sorption data of standard samples to infer the behavior of microplastics in marine environment.

In this work, priority microplastics including polyethylene (PE) and nylon fibers identified in a mariculture farm in Xiangshan Bay, a semi-enclosed long-narrow bay in southeastern China, were examined for their sorption behaviors towards OCs, i.e., phenanthrene and phenol. In China, a large part of coastal waters has been developed for mariculture (Feng et al., 2004; Liu and Su, 2017) due to large seafood consumption and export. According to the Food and Agriculture Organization (FAO), China accounts for >60% of global fish production from aquaculture (FAO, 2016). The process of mariculture production uses large amounts of plastic materials, such as plastic ropes/nets (i.e., used for enclosure culture) and buoyant plastic foams (i.e., used for shellfish culture). For example, our estimation indicates that an average of 180 kg of plastic netting per square kilometer is used as fencing in the studied mariculture zone. Due to multiple aging factors, such as UV radiation and wind/wave abrasions, large plastics can be decomposed to microplastics or smaller through cracking and fragmentation (Song et al., 2017). Our preliminary sampling results showed that PE and nylon fibers are the predominant microplastics in a mariculture farm, which is mainly developed for enclosure culture of crab and fish in Xiangshan Bay. Xiangshan Bay has a long-history of mariculture (Xiong et al., 2015), and various waste plastics (e.g., tires and foams) are dumped into the seawater for different cultured objectives.

In addition to plastic pollution, high concentrations of OCs such as PAHs (Gu et al., 2016; Zong et al., 2014), polybrominated diphenyl ethers (PBDEs) (Wang et al., 2017) and polychlorinated biphenyls (PCBs) (Wang et al., 2011) are widely detected in the Chinese coastal waters, resulting from various sources including freshwater input, petrochemical production, coal combustion and biomass burning. For the studied mariculture farm, a large coastal power plant (installed capacity: 4 × 600 MW) is located just 5 km away, and some studies have reported high concentrations of OCs in this region (Li et al., 2012; Wang et al., 2015b; Yang et al., 2011a, Yang et al., 2011b). Therefore, the microplastics that occur in the mariculture farm have the potential to be a vector transferring toxic OCs to seafood such as fish and shellfish directly. Currently, sorption data of these practical microplastics are very limited.

The goals of this study are as follows: (1) investigate the theoretical sorption behavior of priority microplastics identified in a mariculture farm through isotherms and kinetics experiments; (2) assess how the factors of microplastic size, surface roughness, material and hydrophobicity of OCs affect sorption performance; and (3) compare the sorption capacity of studied microplastics with the literature data of other environmental vectors. This study is intended to provide basic data for assessing the transfer effect of microplastic contaminants in Chinese mariculture farms.