Degradation of bio-based and biodegradable plastics in a salt marsh habitat: Another potential source of microplastics in coastal waters

Abstract

Degradation of bio-based (polylactic acid [PLA] cups, Mater-Bi® [MB] bags) and biodegradable plastics (biodegradable extruded polystyrene [bioPS] plates, biodegradable high density polyethylene [bioHDPE] bags) were compared to conventional plastics (recycled polyethylene terephthalate [rPET] cups, HDPE bags, extruded PS plates) in a salt marsh over a 32-week period. Following 4 weeks, biofilm developed on all plastics, resulting in an increased weight and concomitant decrease in UV transmission for most plastics. All plastics produced microplastic particles beginning at 4 weeks, with single-use bags producing the most microplastics over the 32-week period. At 32 weeks, SEM revealed microcracks and delamination for all plastics except PLA and MB, the latter of which degraded through embrittlement. IR spectral analysis indicated degradation for all plastics except PLA. Results suggest that degradation rates of bio-based and biodegradable plastics vary widely, with MB bags and bioPS plates demonstrating the greatest degradation, while PLA cups demonstrated the least degradation.

Keywords: High density polyethylene; Microplastics; Polyethylene terephthalate; Polylactic acid; Polystyrene.

Graphical abstract

Fig. 1

Photograph of field-deployed apparatus with plastic strips in salt marsh.

Fig. 2

Percentage of collected plastic strips showing evidence of periwinkle grazing over the 32-week exposure in a salt marsh habitat. HDPE = high density polyethylene. PS = extruded polystyrene. rPET = recycled polyethylene terephthalate. bioHDPE = biodegradable HDPE. bioPS = biodegradable extruded PS. PLA = polylactic acid. MB = Mater-Bi®. No strips were available at 32 weeks for MB.

Fig. 3

Photograph of control and post-deployment plastic strips. Post-deployment strips were collected after 32 weeks of exposure in salt marsh except for MB, which was collected following 16 weeks of exposure. HDPE = high density polyethylene. PS = extruded polystyrene. rPET = recycled polyethylene terephthalate. bioHDPE = biodegradable HDPE. bioPS = biodegradable extruded PS. PLA = polylactic acid. MB = Mater-Bi®. * indicates 16-week MB sample.

Fig. 4

Surface area of strips (A), weight of strips with biofilm (B), and number of microplastic particles produced in the laboratory fragmentation test (C) over a 32-week exposure in a salt marsh habitat. For each plastic type, significant differences (p ≤ 0.05) relative to the week 0 strips (control) are denoted by an *. No microplastic particles were produced by the week 0 (control) strips for all plastic types. Mean ± SD. HDPE = high density polyethylene. PS = extruded polystyrene. rPET = recycled polyethylene terephthalate. bioHDPE = biodegradable HDPE. bioPS = biodegradable extruded PS. PLA = polylactic acid. MB = Mater-Bi®. Sample sizes between 0 and 16 weeks were as follows: surface area (n = 4), weight (n = 7), and fragmentation (n = 4). At 32 weeks, samples sizes were reduced due to loss of some strips as follows: surface area and fragmentation (n = 3 for PS), weight (n = 6 for HDPE, bioPS, and bioHDPE). No strips were available to test at 32 weeks for MB.

Fig. 5

Percent transmittance of UV-A (A), UV-B (B) and visible light (C) through plastic strips over a 32-week exposure in a salt marsh habitat. Percent transmittance was significantly reduced for all plastic strips at weeks 4, 8, 16, and 32 relative to week 0 strips (control). Mean ± SD. HDPE = high density polyethylene. PS = extruded polystyrene. rPET = recycled polyethylene terephthalate. bioHDPE = biodegradable HDPE. bioPS = biodegradable extruded PS. PLA = polylactic acid. MB = Mater-Bi®. Sample sizes between 0 and 16 weeks were n = 4. At 32 weeks, samples sizes were reduced to n = 3 for PS due to loss of some strips. No strips were available to test at 32 weeks for MB.

Fig. 6

FT-IR spectra of each plastic sample at 4 weeks (orange), 8 weeks (gray), 16 weeks (yellow), and 32 weeks (green) compared to controls (blue). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 7

SEM images of the surface topography of 0-week (control) and post-deployment plastic strips. Post-deployment strips for SEM were collected after 32 weeks of exposure in salt marsh except for MB, which was collected following 16 weeks of exposure. Surface topography for the 0-week (control) strips was nearly featureless for most plastic types, including HDPE (A), PS (B), bioHDPE (C), rPET (E), PLA (F), and MB (G). The only exception was bioPS (D), which exhibited numerous ovoid depressions. Surface topography of post-deployment plastic strips following removal of biofilm exhibited minor to moderate degraded surfaces, including HDPE (H and O), bioHDPE (J and Q), PS (I and P), bioPS (K and R), and rPET (L and S). Post-deployed PLA (M and T) and MB (N and U) strips both exhibited limited surface erosion. HDPE = high density polyethylene. PS = extruded polystyrene. rPET = recycled polyethylene terephthalate. bioHDPE = biodegradable HPDE. bioPS = biodegradable extruded PS. PLA = polylactic acid. MB = Mater-Bi®.

Fig. 7

SEM images of the surface topography of 0-week (control) and post-deployment plastic strips. Post-deployment strips for SEM were collected after 32 weeks of exposure in salt marsh except for MB, which was collected following 16 weeks of exposure. Surface topography for the 0-week (control) strips was nearly featureless for most plastic types, including HDPE (A), PS (B), bioHDPE (C), rPET (E), PLA (F), and MB (G). The only exception was bioPS (D), which exhibited numerous ovoid depressions. Surface topography of post-deployment plastic strips following removal of biofilm exhibited minor to moderate degraded surfaces, including HDPE (H and O), bioHDPE (J and Q), PS (I and P), bioPS (K and R), and rPET (L and S). Post-deployed PLA (M and T) and MB (N and U) strips both exhibited limited surface erosion. HDPE = high density polyethylene. PS = extruded polystyrene. rPET = recycled polyethylene terephthalate. bioHDPE = biodegradable HPDE. bioPS = biodegradable extruded PS. PLA = polylactic acid. MB = Mater-Bi®.