Turbidimetric analysis of the enzymatic hydrolysis of polyethylene terephthalate nanoparticles

The heterogeneous enzymatic hydrolysis of polyethylene terephthalate (PET) by a polyester hydrolase (TfCut2) from Thermobifida fusca KW3 was determined by measuring the change of intensity of transmitted light due to the scattering effect of PET nanoparticles immobilized in an agarose gel. Nanoparticles with a mean diameter between 100 and 160 nm were prepared from PET samples of different crystallinity to provide a large surface area for the adsorption of the enzyme. The turbidity decrease of the PET nanoparticle suspensions was correlated to the surface erosion process resulting from the enzymatic degradation, and enabled a direct estimation of the kinetic parameters of the enzymatic hydrolysis of PET based on a model for heterogeneous biocatalysis. A comparison of the hydrolysis rate constants and the adsorption equilibrium constants of the enzymatic hydrolysis of PET nanoparticles prepared from recycled PET granulate, film and fibres showed that the biodegradability of PET was mainly influenced by the mobility of the polyester chains, which determined the affinity and accessibility of the ester bonds to the enzyme. Differential scanning calorimetric analysis of the partially hydrolyzed PET nanoparticles provided indirect evidence for an endo-type hydrolytic mechanism of TfCut2 in the heterogeneous degradation of aromatic polyesters.