Distribution & Type of Marine Debris Polymers on Hawaiian Island Beaches, Sea Surface, and Seafloor.

[NNadir]

The paper I'll discuss in this post is this one: Marine Debris Polymers on Main Hawaiian Island Beaches, Sea Surface, and Seafloor (Jennifer M. Lynch et al. Environ. Sci. Technol. 2019, 53, 21, 12218-12226).

As bird populations fall, accelerated by the wondrous goal of converting all of our continental shelves into industrial parks for wind farms, as well as because of the topic of this post, plastic, a part of the phosphorous cycle will be disrupted, specifically the sea to land portion. Many of the world's mined sources of phosphorous are actually bird droppings on Islands. For a short while, the island nation of Nauru in the Pacific Ocean had the world's highest per capita wealth in the world because it exported bird shit, phosphorous, deposited by sea birds over centuries. (The Nauruan Government "invested" all of this wealth in stocks and bonds which collapsed, and now the nation is one of the world's poorest, the bird shit is depleted, and the Island makes its living by imprisoning refugees deported from Australia.) The importance of birds to the phosphorus cycle is described in the interesting book Why Birds Matter, CAGAN H. SEKERCIOGLU, DANIEL G. WENNY, AND CHRISTOPHER J. WHELAN, Eds., University of Chicago Press, 2016, pp 274-275, 279-282.

I mention this, because I often think about the recovery of important elements and compounds from seawater by raising it to supercritical temperatures. This would serve to recover both phosphorous and carbon dioxide in cases where the seawater is dead from deoxygenation owing to agricultural run-off, as in the Mississippi River Delta, the ecosystem of which has been destroyed by runoff to make "renewable" corn ethanol. The eutrophication process which killed it, involves the explosive growth of micro-organisms which sink to the bottom of the sea as they die after getting killed off by the thickness of the mats they form which restricts sun light, are rotted by oxygen depleting bacteria, killing everything else, fish, crustaceans, and other species.

When I muse on this subject of supercritical water oxidation (SCWO) to recover phosphorous and carbon dioxide, I often reflect that a side product of the process would be to destroy microplastics, which are contaminating the ocean in ever larger amounts, and as another side product would be fresh water, since at supercritical temperatures and pressures, seawater separates into two separate supercritical phases, one containing salts, and one free of salts.

Future generations may need to do these sorts of things, because we have screwed them.

(I may discuss a few interesting papers I came across on polymer reprocessing engineering I just came across that were published in the last few days; not processes I necessarily endorse, but interesting engineering nonetheless, in future posts here.)

One thing I had not considered in my musings is the density of plastics, which is a topic covered in the paper under discussion.

From the paper's introduction:

Plastic marine debris has received increased international attention.(1−4) The Hawaiian Islands, one of the most remote archipelagos with high rates of endemism and endangered species,(5,6) accumulate some of the highest reported amounts of marine debris.(7−10) Hawaii is located south of the Subtropical Convergence Zone (STCZ) and southwest of the Eastern North Pacific Garbage Patch, where the highest concentration of floating plastic pollution on the planet accumulates because of wind-driven convergence.(7,11,12) The Northeasterly trade winds are speculated to be the main driving force pushing floating marine debris from these accumulation areas to Hawaii.(13,14)


Since Hawaii accumulates debris from a variety of sources, understanding the chemical composition of plastic marine debris is necessary.(15) Seven standardized resin codes are assigned to the most commonly produced polymers: (16) polyethylene terephthalate (PET, #1), high-density polyethylene (HDPE, #2), polyvinyl chloride (PVC, #3), low-density polyethylene [LDPE, #4, which includes linear low-density polyethylene (LLDPE)], polypropylene (PP, #5), polystyrene (PS, #6), and other polymers (#7). Some consumer goods are stamped with their resin code, but weathered fragments are often missing these stamps, requiring chemical analyses for identification.

Polymer identification of plastic marine debris is crucial for understanding sources, fate, transport, and effects in the environment. Because different polymers have various chemical structures, their physical, chemical, and biological interactions within the environment will differ. Sorption rates and concentrations of organic and heavy metal pollutants vary among polymers, making certain polymers a greater threat of contaminant exposure to organisms.(17) Chemical reactions during environmental degradation processes can lead to various polymeric degradation products that have not been widely studied.(18−23) The release of additives, fillers, and greenhouse gases(21,24) are highly variable among polymer type and in some cases even toxic.(25,26) Polymer identification tools also provide indicators of the extent of the debris weathering, a sign of aging or possibly a time estimate since littering.(20,27) Each polymer has a different chemical density, which is hypothesized to be a major (but not the only) influence in vertical stratification and fate of plastic debris in the ocean (Table 1).(28,29) For instance, polymers less dense than seawater (e.g., PE and PP) float and are commonly found at the sea surface,(30−34) while denser polymers predominantly sink to the seafloor.(29,35,36) In addition, polymer identification can confirm that debris samples are in fact plastic and other material is not visually mistaken as plastic.(37) These reasons, plus the need to understand which polymers may affect different marine habitats, provided justification for the present study.

The authors collected plastic samples from seawater, from the beaches, and the benthic zones of the Hawaiian islands.

They were collected by divers, by collecting plastics in trawlers, and by picking them up the beaches. The types of plastics were determined simply, by FTIR, using a Perkin Elmer library. (An alternative, and possibly superior approach to polymer identification is differential scanning calorimetry, DSC, but FTIR is pretty good.

Here's a description of the handling of the samples and the samples themselves:

Each plastic piece was categorized by type (fragment, sheet, foam, line, pellet, other, or whole),(28) color, longest measurable dimension, and a weathering intensity rank (1 = mild, 2 = moderate, and 3 = severe). These physical characteristics for each debris piece are provided in Supporting Information Table S2. Photos of cataloged transects from each compartment are shown in Figure S1. “Whole” pieces were recognizable consumer goods that did not fit into the other type categories (e.g., cigarette filters, toothbrushes, and bottle caps); monofilament fishing line, rope, or net materials were classified as “line”; “sheets” were food wrappers, bags, and films; “foams” were expanded cellular plastics (blown with air); “other” was a category primarily for fabrics; “pellets” were preproduction polymers (e.g., nurdles); and “fragments” were unidentifiable pieces that did not fit in the other categories. Multicomponent pieces (e.g., sunglasses) were disassembled; each component was counted, weighed, and analyzed separately. The weathering intensity rank was based on the degree of visual square fracturing and white oxidation on the surface of the plastic (Figure S2). Mild was no square fracturing. Moderate was minimal square fracturing and/or a thin layer of white oxidation on the surface. Severe was deeply embedded square fracturing and/or a thick layer of white oxidation. The weathering rank focused on chemical weathering from photo-oxidative degradation as opposed to mechanical weathering (e.g., abrasions and bite marks). This method was used as an alternative to carbonyl index (CI) analysis because CI analysis has only been applied to polyolefins.(19,43,51)

...um...delicious...

A little more on polymer ID:

Polymers were identified manually from spectra as described previously(15) for 17 polymers in our in-house spectral library. If confirmation was required or the sample could not be manually identified, spectral libraries installed with the PerkinElmer software were used only if the search score was ≥0.90. For LDPE and HDPE differentiation, the presence/absence of a band at 1377 cm–1 was used; (15) however, undifferentiated samples were classified as “Unknown PE” without using a float/sink test. “PE/PP mixture” were samples that produced spectra with both PE and PP transmittance bands as previously described.(15) “Other PE” were samples that produced high-intensity PE transmittance bands along with low-intensity bands associated with other functional groups, such as chlorinated PE. “Other” was a grouping of rare polymers [latex, petroleum wax, acrylic, PP/PET mixture, polycarbonate, and poly(vinylidene fluoride)]. “Unidentifiable” spectra were too noisy to interpret or were suspected copolymers. Samples were categorized as “additive-masked” when spectra produced bands characteristic of additives, mostly phthalic acid esters, which masked the underlying base polymer. These samples were typically elastomers, which consist of large percentages of phthalate plasticizer mixed with a base polymer.(52) All “additive-masked” samples were searched with spectral libraries and will be the subject of a forthcoming manuscript.

Anyway, here is the table, from the paper, detailing the density of various plastics.



Here is a map of the sampling site beaches:



The caption:

Figure 1. Sampling sites in the MHI (n = 17). Percentages after beach names indicate the percent of land development.

(The authors studied the effect of land development on beach plastic accumulation (see the excerpt below).

...Debris Abundance Greater on Windward than Leeward Beaches. Across 11 beaches, a total of 3931 plastic pieces were collected with a total mass of 20552.3 g. Mean (±SD) plastic abundance levels ranged from 0.404 (±0.549) to 68.3 (±41.5) pieces per square meter and from 0.320 (±0.280) to 188 (±234) g/m2. The overall averages were 18.1 (±22.9) piece/m2 and 48.8 (±59.8) g/m2. Kahuku, located on the northern windward coast of Oahu, had the highest plastic marine debris abundance (Figure 2). Kamilo, known to be one of the worst plastic polluted beaches in the MHI, had lesser amounts than Kahuku. This unexpected trend was likely because of the predetermined sampling locations not overlapping with the most polluted portion of Kamilo beach (Figure S3a). All three leeward beaches had concentrations, <1 piece/m2 or 1 g/m2, 1−2 orders of magnitude lesser than windward beaches (Figure 2; ANOVA, p < 0.0001, Supporting Information Appendix 1). These field surveys on smaller-sized debris corroborate aerial surveys that found greater abundance of macro- to mega-sized debris on windward versus leeward beaches in the MHI.13,14

Debris amounts are higher in the MHI than many other places. Ribic et al.10 reported that Oahu has higher debris loads than the US Pacific coast. MHI beaches sampled in the current study were more plastic polluted than South Korean beaches (means = 13.2 items/m2 and 1.5 g/m2 of 0.5−2.5 cm each)53 even though they sampled additional particles in smaller size classes (<1 cm), which inflates their abundances compared to the current study. The current results are also 2 orders of magnitude greater than the North Atlantic Azores (0.62 pieces/m2 of >2 cm) of a similar size range.54 It is challenging to compare the present data with published debris abundances on beaches because of the differences in particle sizes targeted. This emphasizes the need to report multiple measurements (piece counts, size distributions, and mass) to understand the type of debris in a region...

The abundance of debris:



The caption:

Figure 2. Plastic debris abundance (pieces ≥ 1 cm) on three leeward MHI beaches (brown line) is significantly less than that on eight windward beaches (green line) (p < 0.0001). Values are mean ± one SD

.

Where the plastic ends up by form:




The caption:

Figure 3. Types of MHI plastic marine debris sampled across compartments, percentages of pieces. The seafloor (A) and leeward beaches (B) are different from windward beaches and the sea surface (C) (MRPP, p < 0.0001). Values are mean ± one SD.

The degree of weathering (probably somewhat subjective).



The caption:

Figure 4. Weathering rank of MHI plastic marine debris across compartments, percentages of pieces. Debris on the seafloor and leeward beaches (A) are less weathered than windward beaches and the sea surface (B) (MRPP, p < 0.0001). Values are mean ± one SD.

Composition by area of collection:



The caption:

Figure 5. Comparison of MHI marine debris polymer composition across four compartments. Percentages are based on mass (top) and on the number of pieces (bottom). Compartments underlined with different letters are different from each other (MRPP p < 0.0001). Blue shades are floating polymers; brown and red shades are sinking polymers. Polymer abbreviations: low-density polyethylene (LDPE), ethylene vinyl acetate (EVA), high-density polyethylene (HDPE), polypropylene (PP), polystyrene (PS), acrylonitrile butadiene styrene (ABS), cellulose acetate (CA), polyethylene terephthalate (PET), and polyvinyl chloride (PVC). Values are mean ± one SD.

Polymer composition as a function of the type of debris.



The caption:

Figure 6. Polymer composition of different types of MHI plastic marine debris, calculated by pieces. Blue-shaded polymers float in seawater; brown-shaded polymers sink. Polymer abbreviations: low-density polyethylene (LDPE), ethylene vinyl acetate (EVA), high-density polyethylene (HDPE), polypropylene (PP), polystyrene (PS), acrylonitrile butadiene styrene (ABS), cellulose acetate (CA), polyethylene terephthalate (PET), and polyvinyl chloride (PVC).

The relationship between land use and polymer concentration is an interesting discussion:

Land Development Correlations with Marine Debris. Correlations between percent of coastal land developed and marine debris variables helped describe the potential influence of local population on debris found on the 11 beaches (Figure S10). The percent of land developed was weakly, insignificantly, and negatively correlated with the debris quantities by pieces/m2 or g/m2 (Figure S10A, Pearson R2 = 0.277, p = 0.096 and R2 = 0.213 p = 0.153, respectively). Regions with more land development had less debris. This contrasts with Brazil, where debris abundance decreased with distance from urban centers.68 These results further suggest that MHI beach debris, which is in largest abundance on the windward coasts, is primarily originating from nonlocal sources.

These correlations could be confounded by beach cleanups, but we believe that this possible confounder is a minor variable. Cleanup effort is undoubtedly higher on tourist beaches, such as Waikiki, but large-scale cleanup events are scheduled frequently for the less developed beaches. The exact timing of cleanup effort before our sampling was often unknown. Kahuku on windward Oahu has less land development, is located within the James Campbell National Wildlife Refuge, and received the largest debris amounts of all sampling sites.14 Portions of Kahuku are cleaned up approximately weekly to monthly. It was obvious that a recent cleanup had occurred at one of our three Kahuku transects. Still, Kahuku had the highest debris abundance, suggesting that recent cleanup had little impact on our overall findings.

Percent land development and weathering intensity showed a strong negative correlation (Figure S10B, Pearson R2 = 0.600, p = 0.0051). Waikiki, the most developed, had the least weathered debris, suggesting that the small abundance of debris on this beach is from local sources with minimal exposure to environmental conditions. The least developed beaches (Kamilo, Lanai, and Molokai) had the most weathered debris. Weathering intensity for pieces exposed to sunlight could reflect environmental exposure time. The more weathered pieces on the sea surface and windward beaches were in the environment longer, arriving to Hawaii via wind and ocean currents from distant sources, compared to more recently littered debris on leeward beaches.

Types of debris were correlated with land development (Figure S10C). More fragments were found on less developed beaches (Pearson R2 = 0.362, p = 0.050), while more sheets were found on more developed beaches (Pearson R2 = 0.443, p = 0.025). Fragments are formed from mechanical and chemical weathering after extended environmental exposure. As such, the less developed windward beaches received debris dominated by fragments that were presumably washed ashore from older litter of distant sources...

There is quite a bit in the full paper, and, in any case, it certainly is sobering to contemplate this mess we're leaving for future generations.

While supercritical water oxidation may serve to reduce floating polymers it's not clear how to address sunken or buried polymers, and in any case, the industrial infrastructure to do this would need to be massive, and utilize sustainable energy, which does not include the solar and wind industry.

The paper's conclusion:

Globally, this is the largest known study to identify polymers of Hawaiian plastic marine debris with novel comparisons across space and habitat depths. Furthermore, this is the first known study to identify Hawaiian seafloor plastic debris and to identify additives, such as phthalates, as a major component of certain debris pieces. This is also the first study to create an efficient weathering rank. Floating, severely weathered polymers wash ashore from distant sources on windward beaches at a much greater abundance than denser, less weathered polymers found on leeward beaches and seafloor. These results support prior conclusions that the majority of marine debris in Hawaii is coming from distant sources,(69) often composed of maritime gear.(10) Novel information suggests that the leeward beaches receive smaller quantities of litter, but from local activities (e.g., fishing, diving, boating, and picnicking). Stratification of polymers throughout the environment is evident because of the varying polymer densities that result in significantly different transport and fate of marine debris. Debris composed of denser polymers is more likely to sink near their source, while lighter polymers can travel great distances on the sea surface. This stratification leads to exposure of different debris types and chemicals in different habitats and associated biota. Thus, the chemical methodology of polymer identification is critical for understanding sources, fate, transport, and effects of this emerging global contaminant.

Scary, but interesting.

Have a nice day tomorrow.