04/29/2026
By Danielle Fretwell

The Francis College of Engineering, Department of Chemical Engineering, invites you to attend a Doctoral Dissertation Proposal defense by Muhammad Umer Abid titled: Integrating Scalable Enzymatic Depolymerization and Hybrid Biomanufacturing for Postconsumer Waste Plastic Valorization

Candidate Name: Muhammad Umer Abid
Degree: Doctoral
Defense Date: Monday, May 4, 2026
Time: 11 a.m. - 1 p.m.
Location: Perry Hall 315

Committee:

  • Advisor: Dongming Xie, Associate Professor, Chemical Engineering, University of Massachusetts Lowell
  • Carl Lawton, Associate Professor, Chemical Engineering, University of Massachusetts Lowell
  • Hsi-Wu Wong, Professor, Chemical Engineering, University of Massachusetts Lowell
  • Margaret J. Sobkowicz Kline, Professor, Plastics Engineering, University of Massachusetts Lowell

Abstract:
Poly(ethylene terephthalate) (PET) and polyethylene (PE) are widely used plastics whose high chemical stability leads to significant environmental persistence. This study presents an integrated strategy for plastic waste valorization by combining enzymatic biorecycling of polyester plastics with hybrid thermochemical–biological upcycling of PE. An engineered ICCG variant of leaf-branch compost cutinase (LCC_ICCG) was codon-optimized and expressed in Escherichia coli BL21(DE3). To reduce downstream processing costs, a pelB secretion signal was fused to produce PelB-LCC_ICCG, enabling extracellular enzyme recovery and improved process efficiency. Small-scale depolymerization experiments demonstrated that temperature, enzyme loading, and substrate crystallinity significantly influence degradation performance. Operating at 65 °C enhanced polymer chain mobility and enzyme activity, while lower crystallinity substrates showed faster depolymerization. In 1-L stirred tank reactors, PelB-LCC_ICCG achieved >80% degradation of 200 g L⁻¹ recycled PET within 3 days, outperforming intracellularly expressed enzyme systems. A kinetic model integrating Langmuir adsorption and modified Michaelis–Menten kinetics was developed to describe enzymatic depolymerization of PET and related polyesters (PTT and PBT). The model indicated that enzyme loading must scale proportionally with substrate concentration to maintain high degradation efficiency. Although LCC_ICCG also depolymerized PTT and PBT, conversion efficiencies were lower than PET, highlighting the need for further enzyme engineering. To address process limitations, a mechanistic model incorporating crystallinity evolution, temperature, agitation, and product inhibition was established. Model-guided optimization identified an optimal enzyme loading of ~1.4 mg g⁻¹ RPET, enabling >95% degradation within 48 h at 65 °C and 350 RPM. Under optimized conditions, complete depolymerization of 100–300 g L⁻¹ RPET was achieved within two days, demonstrating strong potential for industrial-scale PET recycling.

For PE, which lacks hydrolysable bonds, a hybrid strategy using pyrolysis-derived hydrocarbons was explored. Mixed alkanes and alkenes were utilized as carbon sources for microbial production of value-added chemicals. Candida tropicalis strains ATCC 20336 and 20962 achieved long-chain dicarboxylic acid (LCDA) titers up to ~36 g L⁻¹ in fed-batch fermentation. Engineered Yarrowia lipolytica strains were developed for platform chemical production. Initial strains expressing G2PS1 under the TEF promoter produced low triacetic acid lactone (TAL) titers (~3.6 g L⁻¹ from glucose). Incorporation of 17 UAS1B activator elements significantly enhanced transcription, achieving ~38 g L⁻¹ TAL from glucose and ~29 g L⁻¹ from C13 alkane in fed-batch fermentation. Similarly, a PhlD-expressing strain under the YAT1 promoter produced ~5.28 g L⁻¹ phloroglucinol (PG) using glucose, though hydrocarbon-based PG production remained limited.

Overall, this work demonstrates a scalable approach for converting plastic waste into valuable chemicals by integrating enzymatic polyester recycling with thermochemical–biological PE upcycling. These findings provide a strong foundation for advancing sustainable plastic waste conversion technologies and supporting a circular bioeconomy.