Bacterial Degradation of Low-Density Polyethylene Preferentially Targets the Amorphous Regions of the Polymer

Abstract

Low-density polyethylene (LDPE) is among the most abundant synthetic plastics in the world, contributing significantly to the plastic waste accumulation problem. A variety of microorganisms, such as Cupriavidus necator H16, Pseudomonas putida LS46, and Pseudomonas chlororaphis PA2361, can form biofilms on the surface of LDPE polymers and cause damage to the exterior structure. However, the damage is not extensive and complete degradation has not been achieved. The changes in polymer structure were analyzed using Time-domain Nuclear Magnetic Resonance (TD-NMR), High-Temperature Size-Exclusion Chromatography (HT-SEC), Differential Scanning Calorimetry (DSC), and Gas Chromatography with a Flame Ionization Detector (GC-FID). Limited degradation of the LDPE powder was seen in the first 30 days of incubation with the bacteria. Degradation can be seen in the LDPE weight loss percentage, LDPE degradation products in the supernatant, and the decrease in the percentage of amorphous regions (from >47% to 40%). The changes in weight-average molar mass (Mw), number-average molar mass (Mn), and the dispersity ratio (Đ) indicate that the low-molar mass fractions of the LDPE were preferentially degraded. The results here confirmed that LDPE degradation is heavily dependent on the presence of amorphous content and that only the amorphous content was degraded via bacterial enzymatic action.

Keywords: LDPE; biodegradation; low-density polyethylene; microbial degradation; polymer structure.

Figure 1

C. necator H16, P. putida LS46, and P. chlororaphis PA2361 growth curves in minimal media with no other carbon source besides LDPE (1% w/v). No growth was observed in the control and therefore it was not included in the graph.

Figure 2

Percent weight loss of LDPE after exposure to C. necator H16 (H16), P. putida LS46 (LS46), and P. chlororaphis PA2361 (PA2361) over 21 days of incubation.

Figure 3

Changes in the structure of LDPE after incubation with three polymer-degrading bacteria. Changes in (A) the weight-average molar mass (M_w), (B) the number-average molar mass (M_n), and (C) the dispersity (M_w/M_n = Đ) of LDPE powder after different incubation times with three bacteria, C. necator H16, P. putida LS46, and P. chlororaphis PA2361, analyzed by HT-SEC. LDPE standard is the LDPE that had no bacteria treatment.

Figure 3

Changes in the structure of LDPE after incubation with three polymer-degrading bacteria. Changes in (A) the weight-average molar mass (M_w), (B) the number-average molar mass (M_n), and (C) the dispersity (M_w/M_n = Đ) of LDPE powder after different incubation times with three bacteria, C. necator H16, P. putida LS46, and P. chlororaphis PA2361, analyzed by HT-SEC. LDPE standard is the LDPE that had no bacteria treatment.

Figure 4

HT-SEC molar mass distributions of (A) C. necator H16-, (B) P. putida LS46-, and (C) P. chlororaphis PA2361-treated LDPE at different time points (Day 2, 15, 21, 30, 60, and 95). The black line represents the standard LDPE, which was not subjected to bacterial treatment.

Figure 4

HT-SEC molar mass distributions of (A) C. necator H16-, (B) P. putida LS46-, and (C) P. chlororaphis PA2361-treated LDPE at different time points (Day 2, 15, 21, 30, 60, and 95). The black line represents the standard LDPE, which was not subjected to bacterial treatment.

Figure 5

T₂ relaxation times for LDPE samples after different periods of incubation with (A) C. necator H16, (B) P. putida LS46, and (C) P. chlororaphis PA2361, measured by TD-NMR.

Figure 5

T₂ relaxation times for LDPE samples after different periods of incubation with (A) C. necator H16, (B) P. putida LS46, and (C) P. chlororaphis PA2361, measured by TD-NMR.