Enzymatic Hydrolysis of Polyester-Based Soil-Biodegradable Mulch Films: Pathways, Kinetics, and Temperature Dependence

Abstract

Biodegradable mulch films (BDMFs) are designed to biodegrade in soil, yet the enzymatic hydrolysis of the blended polyester components in commercial films, a central step in their soil biodegradation, has not been studied. Here, we systematically investigate the enzymatic hydrolysis of four commercial BDMFs composed primarily of poly(butylene adipate-co-terephthalate) (PBAT) with poly(lactic acid) (PLA) as a minor blend component. Hydrolysis was tested in batch incubations in phosphate buffer (50 mM, pH 7.0) with combinations of one of the four BDMFs and one of the two fungal esterases, Humicola insolens cutinase (HiC) or Rhizopus oryzae lipase (RoL). HiC catalyzed extensive hydrolysis of all BDMFs, resulting in complete conversion of film carbon into soluble products. RoL showed no measurable activity, highlighting strong enzyme specificity in polyester hydrolysis. Proton nuclear magnetic resonance spectroscopy revealed complete HiC-catalyzed transformation of PBAT and PLA into mono-, di-, and trimers, with PBAT hydrolysis governing overall hydrolysis dynamics and preceding PLA hydrolysis. HiC-catalyzed hydrolysis followed Arrhenius-type temperature dependence between 4 and 35 °C with apparent activation energies of 105 and 114 kJ mol–1, implying a strong temperature sensitivity of enzymatic PBAT hydrolysis. Besides providing mechanistic insight into enzymatic BDMF hydrolysis, this work enables prediction of temperature dependence for this central step in soil biodegradation models.