Quantification of Polybutylene Adipate Terephthalate-based Micro- and Nano-plastics from Soil Using Proton Nuclear Magnetic Resonance Spectroscopy

A method to recover and quantify micro- and nano-plastics (MPs and NPs) formed in the soil during biodegradation is needed to accurately assess the degradation and environmental impact of biodegradable plastic products. The presence of MPs and NPs in soil may alter soil properties like aggregation behavior or have toxic effects on soil biota. Existing MP recovery methods are not always suitable for measuring biodegradable polymers like polybutylene adipate terephthalate (PBAT); some common digestion procedures with acids or oxidizers can destroy PBAT-based biodegradable MPs. Identification methods like micro-FTIR and micro-Raman spectroscopy are also limited by the minimum size of particles that can be recovered and analyzed. Therefore, this method was developed to extract and quantify PBAT from soil to assess the mass fraction of MPs and NPs in the soil without chemically transforming PBAT. In the protocol, a chloroform-methanol solution is used to selectively extract PBAT from the soil. The solvent is evaporated from the extract, and then the extract is redissolved in deuterated chloroform. The extract is analyzed by proton nuclear magnetic resonance spectroscopy (¹H-QNMR) under quantitative parameters to quantify the amount of PBAT in each sample. Solvent extraction efficiencies for PBAT range from 76% in a shady loam soil to 45% in an Elkhorn sandy loam soil. PBAT recovery may be reduced for photo-oxidized materials compared to pristine ones and may be reduced in soils with high clay content. Extraction efficiencies do not depend on PBAT concentration within the test range, but lower extraction efficiencies were observed for NPs than for MPs. PBAT quantification results were comparable to the quantification of plastic degradation by measuring cumulative soil respiration in a laboratory incubation study.