This week marks “Research Week” on our blog and we will be highlighting marine debris research projects throughout the week! Research is an important part of addressing marine debris, as we can only effectively address it by understanding the problem the best we can.
By: Rob Hale, Guest Blogger and Professor in the Department of Aquatic Health Science at the Virginia Institute of Marine Science (VIMS)
Plastics are an increasing problem in our ocean and waterways. The plastic products we use, and hence those that find their way into the environment, are made of different polymers. These include products ranging from disposable water bottles, fishing gear, electronics, microbeads from personal care products, to furniture. Chemical additives are inserted into many plastic polymers to modify plastic properties such as color, flexibility, weather resistance, and flame retardancy. These additives may leach out over time, depending on the chemical structure of both the plastic polymer and the additive. Unfortunately, some additives are persistent, bioaccumulative, or toxic. In addition, pollutants already in the water, such as polychlorinated biphenyls (PCBs), can sorb to the surface of plastic debris. After exposure to light or abrasion, plastics fragment into ever smaller particles called microplastics. Microplastics can have different shapes and sizes, which not only affect their leaching and sorption behavior, but can influence what organisms either intentionally or inadvertently consume them. Knowledge of the relative importance of all these factors is critical to our understanding of the effects of discarded plastics in our ocean.
We chose to study these interactions and used a laboratory environment so we could individually control the factors of interest. We investigated different combinations of plastics (polyurethane foam, polyethylene, polystyrene, and polyvinyl chloride or “PVC”), additives, and water pollutants. We also examined the effect of microplastic size and the degree of weathering. We first ground various types of plastics to different size ranges using an ultra-cold grinder to make them brittle (check out this video of the process). Following the grinding of the plastics to microplastic size, we took photos of the microplastics through an electron microscope (see image below) and measured their surface areas. The plastics were then weathered by being exposed to ultraviolet light, as they might experience at the water’s surface or on a beach. Next, we placed a small amount of weathered or unweathered microplastics in a column containing sand (see figure below) and leached them with waters of differing temperature, salinity, and organic carbon content (including imitation animal digestive fluids). The water exiting the column was then collected and analyzed to see what additives were released.
We found that the type of plastic polymer greatly affected its surface area after grinding. Smaller microplastic particles, which have a greater surface area ratio, generally released additives at an increased rate. Polyurethane foam was particularly interesting due to the amount of flame retardants released to the water. Salinity had little effect on additive leaching. In contrast, higher water temperature, such as found in the tropics and the digestive tract of warm-blooded animals, caused more chemicals to be released. Leaching the microplastics with water containing humic acids caused the release of even greater amounts of additives, especially of flame retardants. Humic acids are natural chemicals found in high levels in estuaries, coastal sediments, swamps, landfills, and wastewater treatment plants (where many microplastics are trapped). Leaching the microplastics with synthetic digestive fluids, similar to what occurs in an organism, caused the most additives to be released to the water.
The results of this research allowed us to distinguish what factors increase the release of potentially harmful chemicals from discarded plastics to the environment. Such information is critical to protect living resources, as well as human health. Results will also allow us to design safer plastic products in the future.