Mechanoenzymatic Depolymerization of Polyethylene Terephthalate in Moist Solids: Exploring the Roller Mill

Abstract

Plastic pollution has emerged as a critical global environmental challenge. Effective end-of-life management of plastics remains a pressing issue. Recent advances in enzymatic technology offer promising solutions for the closed-loop recycling of many plastics. For example, numerous enzymes capable of depolymerizing poly(ethylene terephthalate) (PET) into its building blocks have been identified. Notably, enzymatic hydrolysis conducted in moist-solid reaction mixtures is garnering interest as a more sustainable alternative to traditional dilute aqueous conditions. When combined with intermittent mechanical mixing, this approach, termed mechanoenzymology, can enhance enzyme performance, while addressing solubility issues and avoiding the need for substrate pretreatment. Despite these advances, current research in mechanoenzymology predominantly relies on laboratory-scale experiments using shaker mills. This study aims to broaden the scope of mechanoenzymatic transformations by exploring the use of a roller mill instead. Roller mills find widespread use industrially (e.g., in the mining and concrete industries). Utilizing commercial cutinase Novozym 51032 (abbreviated HiC), we investigated how varying milling conditions, moisture levels, and enzyme loadings impact the efficiency and scalability of PET depolymerization to terephthalic acid. The results demonstrate the scalability of enzymatic reactions in moist solids from 300 mg to 3 g and to 30 g scales at a solids loading of 40% w/w and daily milling at 300 rpm for 30 min. This study lays the groundwork for advancing PET recycling technologies on a larger scale.