Monitoring biodegradation of poly(butylene sebacate) by Gel Permeation Chromatography, (1)H-NMR and (31)P-NMR techniques

Biodegradation of plastics in the pelagic environment of the coastal zone - Proposed test method under controlled laboratory conditions

The increasing quantities of plastic litter accumulated in the oceans, including microplastics, represent a serious environmental threat. Despite the recent legislative actions, the plastic littering problem will not disappear in a short time. It may, however be ameliorated by replacing conventional non-degradable plastics with bio-based materials biodegradable in marine environment (targeting the non-recycled or mismanaged plastic waste). Although priority is set to prevention of plastic litter by means of the circular economy principles, biodegradability is a means of controlling unintentional plastic pollution. In this effort, the development of reliable test methods that would be used along with standard specifications for determining the biodegradability of novel polymeric materials or plastics in marine environments, is a necessary complementary component of the whole strategy to control the marine plastic litter and micro-, nano-plastics threat. The present work focuses on identifying gaps and improving available laboratory test methods for measuring the aerobic biodegradation of plastics in the seawater column within the coastal zone (pelagic environment). The research work followed a methodology that is based on recommendations of ASTM D6691:2017 concerning biodegradation of plastics in the seawater and the similar ISO 23977-1:2020. Three different implementation schemes of the test method were applied using different experimental setups and measuring techniques for monitoring the evolved CO2. The effect of critical parameters affecting nutrient adequacy (concentration in inoculum) and oxygen adequacy (bioreactor size, sample size, frequency of aeration) on the biodegradation of four tested materials was explored, and optimal values are proposed. The results allowed for the refinement of the proposed test method to improve reliability and reproducibility.

Biodegradation of poly(butylene succinate) in soil laboratory incubations assessed by stable carbon isotope labelling

Using biodegradable instead of conventional plastics in agricultural applications promises to help overcome plastic pollution of agricultural soils. However, analytical limitations impede our understanding of plastic biodegradation in soils. Utilizing stable carbon isotope (¹³C-)labelled poly(butylene succinate) (PBS), a synthetic polyester, we herein present an analytical approach to continuously quantify PBS mineralization to ¹³CO₂ during soil incubations and, thereafter, to determine non-mineralized PBS-derived ¹³C remaining in the soil. We demonstrate extensive PBS mineralization (65 % of added ¹³C) and a closed mass balance on PBS−¹³C over 425 days of incubation. Extraction of residual PBS from soils combined with kinetic modeling of the biodegradation data and results from monomer (i.e., butanediol and succinate) mineralization experiments suggest that PBS hydrolytic breakdown controlled the overall PBS biodegradation rate. Beyond PBS biodegradation in soil, the presented methodology is broadly applicable to investigate biodegradation of other biodegradable polymers in various receiving environments.

This study applies stable carbon isotope labelling to study polymer biodegradation in soils. This labelling enables accurate and precise tracking of polymer carbon during biodegradation and, thereby, provides a holistic picture of this process.

Bio-Based Poly(lactic acid)/Poly(butylene sebacate) Blends with Improved Toughness

A series of poly(butylene sebacate) (PBSe) aliphatic polyesters were successfully synthesized by the melt polycondensation of sebacic acid (Se) and 1,4-butanediol (BDO), two monomers manufactured on an industrial scale from biomass. The number average molecular weight (M_n) in the range from 6116 to 10,779 g/mol and the glass transition temperature (T_g) of the PBSe polyesters were tuned by adjusting the feed ratio between the two monomers. Polylactic acid (PLA)/PBSe blends with PBSe concentrations between 2.5 to 20 wt% were obtained by melt compounding. For the first time, PBSe’s effect on the flexibility and toughness of PLA was studied. As shown by the torque and melt flow index (MFI) values, the addition of PBSe endowed PLA with both enhanced melt processability and flexibility. The tensile tests and thermogravimetric analysis showed that PLA/PBSe blends containing 20 wt% PBSe obtained using a BDO molar excess of 50% reached an increase in elongation at break from 2.9 to 108%, with a negligible decrease in Young’s modulus from 2186 MPa to 1843 MPa, and a slight decrease in thermal performances. These results demonstrated the plasticizing efficiency of the synthesized bio-derived polyesters in overcoming PLA’s brittleness. Moreover, the tunable properties of the resulting PBSe can be of great industrial interest in the context of circular bioeconomy.

Sample Preparation Techniques for the Analysis of Microplastics in Soil—A Review

Although most plastic pollution originates on land, current research largely remains focused on aquatic ecosystems. Studies pioneering terrestrial microplastic research have adapted analytical methods from aquatic research without acknowledging the complex nature of soil. Meanwhile, novel methods have been developed and further refined. However, methodical inconsistencies still challenge a comprehensive understanding of microplastic occurrence and fate in and on soil. This review aims to disentangle the variety of state-of-the-art sample preparation techniques for heterogeneous solid matrices to identify and discuss best-practice methods for soil-focused microplastic analyses. We show that soil sampling, homogenization, and aggregate dispersion are often neglected or incompletely documented. Microplastic preconcentration is typically performed by separating inorganic soil constituents with high-density salt solutions. Not yet standardized but currently most used separation setups involve overflowing beakers to retrieve supernatant plastics, although closed-design separation funnels probably reduce the risk of contamination. Fenton reagent may be particularly useful to digest soil organic matter if suspected to interfere with subsequent microplastic quantification. A promising new approach is extraction of target polymers with organic solvents. However, insufficiently characterized soils still impede an informed decision on optimal sample preparation. Further research and method development thus requires thorough validation and quality control with well-characterized matrices to enable robust routine analyses for terrestrial microplastics.

Aerobic biodegradation of bio-based plastics in the seawater/sediment interface (sublittoral) marine environment of the coastal zone - Test method under controlled laboratory conditions

Pollution of the seas due to plastic litter is a rapidly growing environmental problem. Among several actions, legal and technological, undertaken to alleviate this problem, included are the control of single use conventional plastics and the replacement of conventional non-biodegradable plastics with innovative biodegradable-in-the-sea polymers, both aiming at the mitigation of marine litter accumulation. Laboratory tests have been used to characterize plastics regarding their biodegradation in various environments. Biodegradation of plastics depends on the inherent characteristics of the polymer and the particular marine habitat conditions. In the present work, the international standard test method ISO 19679 (2016) for determining the aerobic biodegradation of non-floating plastic materials in a seawater-sediment interface of the coastal marine zone under laboratory simulated conditions is evaluated. Modifications are proposed to improve the reliability of this test method in some aspects. Agitation of the seawater surface in the bioreactor was found to enhance the continuous availability of oxygen at the seawater-sediment interface, thus assuring aerobic biodegradation conditions throughout the test simulating real sublittoral conditions. Additional recommended improvements include the use of larger samples and supplementary nutrients to optimise biodegradation conditions. The laboratory measurements were validated by field experiments. The proposed modifications and refinements optimise the robustness and reliability of the test method for determining aerobic biodegradation of plastic materials in a simulated seawater-sediment interface of the coastal zone.

Quantification of Synthetic Polyesters from Biodegradable Mulch Films in Soils

Soil biodegradable mulch films composed of the polyester polybutylene adipate-co-terephthalate (PBAT) are being increasingly used in agriculture. Analytical methods to quantify PBAT in field soils are needed to assess its soil occurrence and fate. Here, we report an analytical method for PBAT in soils that couples Soxhlet extraction or accelerated solvent extraction (ASE) with quantitative protonnuclear magnetic resonance (q-¹H NMR) spectroscopy detection. The ¹H NMR peak areas of aromatic PBAT protons increased linearly with PBAT concentrations dissolved in deuterated chloroform (CDCl₃), demonstrating accurate quantitation of PBAT by q-¹H NMR. Spike-recovery experiments involving PBAT addition to model sorbents and soils showed increased PBAT extraction efficiencies into chloroform (CHCl₃) with methanol (MeOH) as cosolvent, consistent with MeOH competitively displacing PBAT from H-bond donating sites on mineral surfaces. Systematic variations in solvent composition and temperatures in ASE revealed quantitative PBAT extraction from soil with 90/10 volume % CHCl₃/MeOH at 110-120 °C. Both Soxhlet extraction and ASE resulted in the complete recovery of PBAT added to a total of seven agricultural soils covering a range of physicochemical properties, independent of whether PBAT was added to soils dissolved in CHCl₃, as film, or as particles. Recovery was also complete for PBAT added to soil in the form of a commercial soil biodegradable mulch film with coextractable polylactic acid (PLA). The presented analytical method enables accurate quantification and biodegradation monitoring of PBAT in agricultural field soils.

Disintegration behaviour of bio-based plastics in coastal zone marine environments: A field experiment under natural conditions

The accumulation of plastic wastes in the marine environment represents a steadily increasing global environmental threat. The replacement of conventional plastics with bio-based biodegradable materials may contribute to alleviating the problem in the long run. This work studies the disintegration behaviour of three bio-based plastic materials, namely Polyhydroxybutyrate (PHB), Polybutylene sebacate (PBSe), Polybutylene sebacate-co-terephthalate (PBSeT), in three different coastal zone marine environments under natural conditions. The three studied environments were: 1) the seashore zone which is periodically covered by the seawater due to waves or tide, called eulittoral or intertidal zone; 2) the water column zone of small depth (about 10 m), called pelagic zone; and 3) the interface zone between the water column and the seabed sediment at small depth (about 20 m), called sublittoral or benthic zone. The experiments took place in the Aegean Sea at the SW coast of Salamis Island. The results showed that disintegration, as an indicative measure of biodegradation, occurs in all three tested environments, even though the rate depends on the material, the habitat, and the prevailing during the testing period environmental conditions. The degrees of disintegration of all materials in the three environments exhibited significant differences: Benthic > Intertidal > Pelagic. The observed disintegration can be attributed to biodegradation since the negative reference Low-density Polyethylene (LLDPE) material did not disintegrate.

Biodegradation of Polymeric Mulch Films in Agricultural Soils: Concepts, Knowledge Gaps, and Future Research Directions

The agricultural use of conventional, polyethylene-based mulch films leads to the accumulation of remnant film pieces in agricultural soils with negative impacts for soil productivity and ecology. A viable strategy to overcome this accumulation is to replace conventional with biodegradable mulch films composed of polymers designed to be degraded by soil microorganisms. However, understanding polymer biodegradation in soils remains a significant challenge due to its dependence on polymer properties, soil characteristics, and prevailing environmental conditions. This perspective aims to advance our understanding of the three fundamental steps underlying biodegradation of mulch films in agricultural soils: colonization of the polymer film surfaces by soil microorganisms, depolymerization of the polymer films by extracellular microbial hydrolases, and subsequent microbial assimilation and utilization of the hydrolysis products for energy production and biomass formation. The perspective synthesizes the current conceptual understanding of these steps and highlights existing knowledge gaps. The discussion addresses future research and analytical advancements required to overcome the knowledge gaps and to identify the key polymer properties and soil characteristics governing mulch film biodegradation in agricultural soils.