In-Situ Product Removal for the Enzymatic Depolymerization of Poly(ethylene terephthalate) via a Membrane Reactor

Abstract

Poly(ethylene terephthalate) (PET) is a common single-use plastic and a major contributor to plastic waste. PET upcycling through enzymatic depolymerization has drawn significant interests, but lack of robust enzymes in acidic environments remains a challenge. This study investigates in-situ product removal (ISPR) of protons and monomers from enzymatic PET depolymerization via a membrane reactor, focusing on the ICCG variant of leaf branch compost cutinase. More than two-fold improvements in overall PET depolymerization and terephthalic acid yields were achieved employing ISPR for an initial PET loading of 10 mg_PET ml_buffer ^-1. The benefit of ISPR was reduced for a lower initial loading of 1 mg_PET ml_buffer ^-1 due to decreased need for pH stabilization of the enzyme-containing solutions. A back-of-envelop analysis suggests that at a modest dilution ratio, ISPR could help achieve savings on caustic base solutions used for pH control in a bioreactor. Our study provides valuable insights for future ISPR developments for enzymatic PET depolymerization, addressing the pressing need for more sustainable solutions towards plastic recycling and environmental conservation.

Keywords: Enzymatic depolymerization; In-situ product; Membrane reactor; Poly(ethylene terephthalate); Recycling.

Figure 1

(a) Degradation efficiency and yields of (b) TPA, (c) MHET, and (d) BHET from enzymatic depolymerization of Ex‐RPET under different enzyme concentrations. The theoretical maximum TPA yield for 30 mg of Ex‐RPET was approximately 25.9 mg (i. e., 86.4 % of the mass of Ex‐RPET). Error bars are a single standard deviation of the experiments repeated in triplicates. Experimental conditions: initial substrate concentration ([S]) of 10 mg_PET ml_buffer ⁻¹, enzyme concentration ([E]) of 0.5, 1, 2, and 3 μM purified LCC_ICCG (corresponding to 1.45, 2.9, 5.8, and 8.7 mg_enzyme g_PET ⁻¹, respectively), reaction temperature of 55 °C, and 100 mM potassium phosphate buffer at pH of 8.

Figure 2

Comparison of (a) overall PET degradation efficiency, (b) TPA, (c) MHET, and (d) BHET yields from the enzymatic depolymerization of Ex‐RPET for DRs 0 and 4.5. For experiments of DR at 4.5, both ISPR (with the membranes) and non‐ISPR experiments (without the membranes) were conducted. Experimental conditions: initial enzyme to substrate ratio of 2.9 mg_enzyme g_PET ⁻¹, reaction temperature of 55 °C, and 100 mM potassium phosphate buffer at pH≈8. Error bars are a single standard deviation of three repeated experiments.

Figure 3

Comparison of degradation efficiency and solution pH values from the enzymatic depolymerization of Ex‐RPET for (a) (c) initial substrate and enzyme concentrations of 1 mg_PET ml_buffer ⁻¹ and 0.1 μM purified LCC_ICCG, respectively, at DRs of 0, 4.5, and 67 and (b) (d) initial substrate and enzyme concentrations of 10 mg_PET ml_buffer ⁻¹ and 1 μM purified LCC_ICCG, respectively, at DRs of 0, 4.5, 7, and 67. Experimental conditions: initial enzyme to substrate ratio of 2.9 mg_enzyme g_PET ⁻¹, reaction temperature of 55 °C, and 100 mM potassium phosphate buffer at pH≈8. Error bars are a single standard deviation of three repeated runs. For membrane experiments, the reported pH values were those measured inside the membrane cassettes.

Figure 4

Comparison of (a) TPA (theoretical maximum TPA yield $\approx$ 25.9 mg), (b) MHET, and (c) BHET yields from the enzymatic depolymerization of Ex‐RPET for DRs of 0, 4.5, 7, and 67. Experimental conditions: initial enzyme to substrate ratio of 2.9 mg_enzyme g_PET ⁻¹, [S]=10 mg_PET ml_buffer ⁻¹, [E]=1 μM purified LCC_ICCG, reaction temperature of 55 °C, and 100 mM potassium phosphate buffer at pH≈8. Error bars are a single standard deviation of three repeated experiments.

Figure 5

Estimated ratios of NaOH use comparing ISPR to the control (non‐ISPR) environments as functions of dilution ratio and initial PET loading (where pH in the bioreactor is assumed to be controlled at 7.5).