What do we know about the time needed for plastics to degrade in the environment?

Very little.

In a new study just published in ACS Sustainable Chemistry & Engineering, my colleagues and I at the University of California, Santa Barbara ventured to find the answer to this very question (Chamas et al, 2020).

But wait a minute. Don't we (or perhaps shouldn't we) already know the answer? The globe is producing as much as 400 million metric tonnes of plastics every year (Geyer et al, 2017), and surely we should know how long it takes for it to degrade, right? Certainly, there are some numbers being circulated. I have seen popular media throwing out numbers like 450 years or 1,000 years. But where are all these numbers come from?

Well, mostly from thin air, we found.

So began our journey to synthesize the evidences found in the peer-reviewed literature. It eventually took us more than a year to complete. Reviewing the literature, we quickly realized that the shapes and the thicknesses of the plastics specimens used for degradation experiments varied widely among studies, making it impossible to draw any meaningful conclusions by simply collating them. We had to develop a method that translates disparate measures into a commensurate metric: the Specific Surface Degradation Rate (SSDR) (Fig. 1).

Figure 1. Representative structures for (a) a section of a flat produce bag with total thickness h and (b) a sphere of radius r. The specific surface degradation rate (SSDR) is defined as the volume of material lost by removal of a layer of thickness Δd in a specified time.

Given that the degradation of plastics takes place on the surface of a material, SSDR can be interpreted as a linear speed at which materials lose their thickness in the course of environmental degradation. We translated the measurements in existing peer-reviewed studies into SSDRs, and here is what we found (Fig. 2).

Figure 2. Specific surface degradation rates for various plastics, in μm/year. Vertical columns represent different environmental conditions (L, landfill/compost/soil; M, marine; B, biological; S, sunlight) and plastics types (represented by their resin identification codes). Plastics type 7, "others", corresponds to various nominally biodegradable plastics. The range and average value for plastics types 1–6 are shown on the right as lines and squares, respectively, as well as for biodegradable "others". Data points representing degradation rates that were unmeasurably slow are shown on the x-axis. Gray columns represent combinations for which no data were found.

First of all, we couldn't locate even a single peer-reviewed study for some plastics type-environment combinations (marked in gray columns in Fig. 2); for example, as you can see from Fig. 2, we didn't have much luck with PVC (type 3). Second, the SSDRs of plastics ranged widely even for the same plastics type-environment combinations. Overall, SSDRs ranged from practically zero to hundreds μm/year. While it is hard to generalize given the paucity of the data points, we observed higher SSDRs for type 7 plastics as compared to the rest. The SSDRs of LDPE (type 4) were higher than HDPE (type 2). Third, polymer pretreatment or the use of a filler (collectively referred to as 'accelerating conditions' in our study) generally, though not always, increased SSDRs.

How does these findings translate to degradation time? When it comes to the time for a plastics material to degrade, the shape of the material in question matters--a lot; the same 2.75g of plastics may take less than 2 years (for thin film with constant area) or over 1,400 years (for bead with shrinking radius) to degrade, depending on its shape (Fig. 3). This is simply because of the differences in the surface area.

Figure 3. Comparison of predicted degradation profiles for HDPE pieces with the same mass, density, and SSDR but different shapes (thin film, fiber, and bead). The dashed lines correspond to extrapolations assuming constant surface area; the solid lines correspond to a model which assumes the radius, and therefore the surface area, decrease over time.

Using SSDRs and the typical shapes of plastics products, we estimated the times needed for common plastics products to degrade in the environment (Table 1). According to Table 1 (you'd better download and blow it up to read it), common single-use plastics items such as LDPE plastic bags and HDPE milk bottles have estimated half-lives of 5 and 250 years, respectively, in landfill/compost/soil conditions. In the marine environment, the estimated half-lives are shorter, at 3.4 and 58 years, respectively. As indicated in the table, however, these numbers come with wide uncertainty margins.

What did we learn from the study? More than anything else, our study highlights the urgent need for additional data on plastics degradation rates based on standard metrics and experimental conditions. After all, we know surprisingly little about how long plastics will last in the environment--and what they will do to our ecosystem and human health.

For details, please refer to our full paper, which is freely available as an open-access article at: https://pubs.acs.org/doi/10.1021/acssuschemeng.9b06635

Acknowledgement

The authors acknowledge funding from the NSF (DGE-1633764 and CBET-1604095). H.M., J.Z., and Y.Q. thank the UC Santa Barbara’s Mellichamp Academic Initiative in Sustainability for partial fellowship support. Y.Q., J.Z., and S.S. are supported by the U.S. Environmental Protection Agency’s Science to Achieve Results (STAR) Program under Grant No. 83557907. Open-access of the paper made possible by the support from NSF.

References

Chamas, A., Moon, H., Zheng, J., Qiu, Y., Tabassum, T., Jang, J.H., Abu-Omar, M.M., Scott, S.L. and Suh, S., 2020. Degradation Rates of Plastics in the Environment. ACS Sustainable Chemistry & Engineering. DOI: 10.1021/acssuschemeng.9b06635

Geyer, R.; Jambeck, J. R.; Law, K. L. Production, Use, and Fate of All Plastics Ever Made. Sci. Adv. 2017, 3 (7), e1700782 DOI: 10.1126/sciadv.1700782