Plastics and the Ocean. Группа авторов

Figure 2.3 Transport and fate of hydrophobic organic chemicals to/from marine plastic particles, seawater, and biota.

      Source: Adapted from Kwon et al. (2017).

      2.3.3.1 Transport of Plastic Additives to/From Marine Sediment

      Marine sediments are thought of as the final sink compartment for plastic additives and often have the highest concentrations among all compartments. Sources of plastic additives to sediment include leaching from sunken plastic debris into the sediment, partitioning from overlying water or pore water, and settlement of sinking organic material (Figure 2.2). Resuspension of sediment constituents into the water column from bioturbation or currents can transport additives back to the water column (Gallo et al. 2018). Additives with low polarity and high K _oc values likely accumulate in sediment. K _oc values are commonly correlated with hydrophobicity or lipophilic properties, measured by K _ow.

      2.3.3.2 Transport of Plastic Additives to/From Marine Biota

      Marine organisms may be exposed to plastic additives via inhalation, dermal sorption, or ingestion of plastics or the free additives. Significant debate exists in the scientific literature whether the dominant route of exposure is from additives leaching from ingested plastics in the gut or from direct exposure to additives in water or prey. Early studies argued that plastic ingestion is a dominant exposure route since a wide range of marine animals eat plastics, and experimental studies have proven this mechanism in laboratory animals (Browne et al. 2013; GESAMP 2015; Rochman et al. 2013; Tanaka et al. 2013; Teuten et al. 2009). Other studies argue that the contribution of plastics to the bioaccumulation of additives in marine organisms is likely small; rather bioaccumulation is predominantly from ingesting prey tissues that already contain these compounds (Bakir et al. 2014; Clukey et al. 2018; Koelmans et al. 2014, 2016; Rochman et al. 2013; Zarfl and Matthies 2010). In fact, ingesting “clean” plastics reduces HOC concentrations in the body by sorption of the compounds to plastic as it moves through the gut and elimination via feces (Koelmans et al. 2014). Koelmans et al. (2016) provides a critical review of the literature concerning the role of plastic as a carrier/vector of additives and concluded that for the majority of marine habitats, bioaccumulation of HOCs from microplastic is likely overwhelmed by uptake via natural diet. However, in some cases, exposure to additives by the ingestion of plastic may be substantial if the amount of additives in ingested plastic is sufficiently larger than the amount in other diet items (Koelmans et al. 2016).

2.4 Degradation of Plastic Additives in the Marine Environment

      Like their plastic counterparts, plastic additives are also susceptible to oxidative degradation and biodegradation. The final products from degradation of plastic additives and the kinetics of these processes in the ocean are not well understood. Basic understanding of the effects of UV, oxygen, water, pH, and temperature certainly allow scientists to predict potential degradation/transformation structures to some extent. The biological degradation pathways that can occur in marine environments remain, for the most part, a mystery.

With phthalates, biodegradation is likely the most important removal process from water (Net et al. 2015). Many phthalates biodegrade into less toxic metabolites, for instance DEP to MEP and phthalic acid (PA) in soils (Cartwright et al. 2000). Both aerobic or anaerobic microorganisms, including marine‐derived fungi, biodegrade phthalates, and higher order animals, also can rapidly biotransform phthalates (Net et al. 2015, Carstens et al. 2020). Paluselli et al. (2019) found that DnBP, DiDP, BzBP, and DEHP were >85% degraded within 49 days of incubation in aerobic seawater. In marine sediments, half‐lives of monoalkyl phthalate esters were between (18 ± 4 and 35 ± 10) hours (Otton et al. 2008). Phthalates can also degrade in the marine environment via photodegradation and hydrolysis of the ester moiety to PA and the corresponding alcohols via the monoesters. These processes, however, are recognized to be less important than biodegradation (Yan et al. 1995).

2.5 Detection in the Marine Environment

      To write this chapter, we compiled a database of 193 studies that reported concentrations of plastic additives in marine plastic pollution, seawater, marine sediment, and marine organisms from 1978 to 2021.

      2.5.1 Plastic Samples

The diversity of detected chemical additives in plastic samples from the marine environment is staggering (Gauquie et al. 2015), but often, the measured concentrations of pollutants are lower than typical loadings expected in plastic consumer goods (Table 2.1). In fact, all additives were lower than 1% by weight of the debris (Figure 2.4). Possible reasons for the lower than expected levels are numerous.

The first reason is that not all polymers are expected to have high loadings of particular additives. Polymer composition of the marine debris samples is fundamental to interpreting the measured additive concentrations, as only some of the diverse polymers require high additive loadings. For example, flexible PVC products often contain high loadings of phthalates, but PVC is a rare polymer in marine debris unless the study focuses on the seafloor (Brignac et al. 2019). PE and polypropylene (PP) that do not contain phthalates are abundant in ocean surface and beach debris (Brignac et al. 2019; Hermabessiere et al. 2017; Figure 2.4). In another example, HBCDs were found at the highest concentrations within plastic debris in expanded polystyrene (EPS) fishing buoy debris (Figure 2.4). But, the HBCD levels were four orders of magnitude lower in PE and PP fragments, indicating that HBCD was intentionally added to the EPS, but not to the PE or PP products.