The extent of plastic pollution
Almost 2500 miles from California and over 4000 miles from Japan, the Hawaiian Islands are the most remote location in the northern hemisphere. And yet, the beaches of Hawaiʻi contain vast amounts of plastic pollution from far away sources. Understanding the quantities and sources of this pollution is a major undertaking at the Center for Marine Debris Research at Hawaiʻi Pacific University on Oahu. The center’s co-director, Dr. Jennifer Lynch, joined a video chat with Rui Chen, one of our application managers for molecular spectroscopy.
“If you take a walk on one of the beaches on the windward side of Oahu, you will be shocked by what you see. If you go to the James Campbell National Wildlife Refuge, your jaw will drop. It’s in a very remote location. No one lives there. But you walk along the beach it looks like a trash dump,” she said. Locations such as these are the site of several long-term studies to understand the nature of marine debris in the environment.
Mezoplastics and microplastics
Plastic pollution in marine environments is classified as mezoplastics (particles larger than 10 mm), macroplastics (larger than 5 mm), microplastics (smaller than 5mm) and nanoplastics. While microplastics have gotten a lot of attention due in part to their discovery in marine digestive systems and even in drinking water, Dr. Lynch reports that macroplastics are a serious problem in ocean waters because they can entangle and kill marine life. As for microplastics, if you scoop a handful of sand from Oahu’s North Shore, you’ll likely find several dozen particles among the grains of sand.
“We don’t think of plastics as being incredibly toxic,” said Dr. Lynch. “However, if plastics are mismanaged, and the plastic wastes are put into the ocean, it actually causes significant harm.”
Dr. Lynch’s team has examined debris contents found in deceased sea turtles for one biomonitoring project. “It’s usually shocking for people to see what we are seeing,” said Dr. Lynch. “It is not uncommon for us to open a turtle’s digestive tract and find about 200 pieces of plastic about this size [a US half-dollar coin].”
While turtles can normally pass these pieces of plastic through their system and excrete the pieces, there is evidence of lethal damage from gut obstruction, perforation, and gut torsion where food cannot pass through the system. A particular problem is with long monofilament fishing lines that can cause intestines to twist and cause a lethal blockage.
FTIR for plastic identification
The Center for Marine Debris Research uses vibrational spectroscopy to help identify the plastics they find on beaches and in animals. Fourier Transform Infrared (FTIR) spectroscopy was developed in parallel with the development of polymers and is recognized as the go-to technique for qualitative and quantitative analysis of plastics and polymer compounds. Complementing FTIR spectroscopy is Raman spectroscopy. While FTIR is adept at providing information about functional groups of compounds, Raman spectroscopy provides information about lower frequency modes, and vibrations that give insight into crystal lattice and molecular backbone structure. Generally speaking, samples that do not lend themselves to FTIR are good Raman samples, and vice versa.
Raman analysis of marine debris
Dr. Lynch states that a Raman microscope and an FTIR microscope, along with pyrolysis gas chromatography, form the trinity on instrumentation to study microplastics. These tools enable researchers to count the number of microplastics, assess particle sizes and perform a chemical identification of the sample.
One of the major challenges in studying marine debris is in comparing the spectra of samples collected in the environment with reference spectra. Spectra libraries based on pristine polymer materials are different from the spectra of samples that have be weathered by salt water, wave actions and UV light.
Because weathered samples undergo chemical changes over time, their spectra tend to be noisier and more difficult to match with commercial spectral libraries. This requires the spectroscopist to visually inspect the spectrum and “apply some commonsense assumptions and knowledge of the chemistry.”
Common polymers in plastic waste
Two of the most common polymers are high-density and low-density polyethylene, which exhibit similar spectral characteristics but follow different paths in uses and in the waste cycle, with HDPE recycled more than LDPE. This makes more difficult tracking patterns in how polymers are used and discarded. For example, as HDPE weathers in the marine environment, a particular band begins to appear that is not seen in pristine HDPE. This band is similar to a band you would find in LDPE.
Dr. Chen asked about collaborative efforts among organizations dealing with the challenges of the spectroscopy of microplastics and marine debris. Dr. Lynch was happy to report that a colleague, Dr. Chelsea Rochman at the University of Toronto, is building a growing spectral library of microplastic particles that have been aged in the environment. The Rochman Lab has also created a library of 148 reference spectra for comparison. Dr. Lynch and others continue to add to the libraries.
Another development in microplastic spectral identification is Open Specy, an online hub for analyzing, sharing, and identifying Raman and FTIR spectra of microplastics developed at the University of California-Riverside.
Polymer accumulation is a recognized global problem, but the effects of plastics on the marine life is still a large question. What are the effects of additive materials such as phthalates as plastics breakdown in the environment? Can microplastics become toxic? How do we isolate nanoparticles in the marine environment, and what problems might they cause? And perhaps the most important question: How do we mitigate the volume of plastic pollution in the environment and prevent its growth? Fortunately, scientists such as Dr. Lynch and the team at the Hawaiʻi Center for Marine Debris Research and other organizations are working to answer these vital questions.
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