Evaluation of the mulch films biodegradation in soil: a methodological review

Plastic mulch films are widely used in agriculture, but most are not biodegradable in soil. Biodegradable mulch films are blends of different polymers whose composition-ratios vary notably from one product to another. Their degradation rates vary greatly according to the physio-chemical characteristics of the product and according to the properties of the soil and its microbial activity. The objective of this review is to provide an overview of the methods used to estimate the biodegradation performances of biodegradable plastics in the soil. In line with this objective, 80 papers were selected and systematically analyzed to extract information on the characteristics of the soil used in the experiments, the type of polymer analyzed, and the methods used to estimate biodegradation in soil environment. Our systematic analysis showed that studies were carried out under both laboratory-controlled and open-field conditions, with different approaches involving visual analysis, mass loss measurements, spectroscopy, and CO2 measurements. A linear estimation of biodegradation performance for four of the most common biodegradable polymers (i.e., polybutylene succinate, polybutylene succinate-co-adipate, polylactic acid, and polybutylene adipate-co-terephthalate), either pure or blended, showed a very wide range of results that appear only partially comparable. Many of the analyzed papers did not report soil characteristics at all, despite soil being one of the most important factors in the biodegradation process. Although methodologies for estimating biodegradation are well developed, at least under laboratory-controlled conditions, there is a need for a shared methodology to make results comparable among different experiments. Within such a shared methodology, visual analysis or mass loss measurements, despite not being able to scientifically prove the biodegradation of polymers, should not be discarded a priori as they might be useful indicators especially for open field experiments. When using indirect biodegradation indicators such as visual analysis or mass loss, it is necessary to couple them with CO2 measurements or to use materials whose biodegradability in the soil environment has already been tested.

Development and Characterization of Pectin Films with Salicornia ramosissima: Biodegradation in Soil and Seawater

Pectin films were developed by incorporating a halophyte plant Salicornia ramosissima (dry powder from stem parts) to modify the film’s properties. The films’ physicomechanical properties, Fourier-transform infrared spectroscopy (FTIR), and microstructure, as well as their biodegradation capacity in soil and seawater, were evaluated. The inclusion of S. ramosissima significantly increased the thickness (0.25 ± 0.01 mm; control 0.18 ± 0.01 mm), color parameters a* (4.96 ± 0.30; control 3.29 ± 0.16) and b* (28.62 ± 0.51; control 12.74 ± 0.75), water vapor permeability (1.62 × 10−9 ± 1.09 × 10−10 (g/m·s·Pa); control 1.24 × 10−9 ± 6.58 × 10−11 (g/m·s·Pa)), water solubility (50.50 ± 5.00%; control 11.56 ± 5.56%), and elongation at break (5.89 ± 0.29%; control 3.91 ± 0.62%). On the other hand, L* (48.84 ± 1.60), tensile strength (0.13 ± 0.02 MPa), and Young’s modulus (0.01 ± 0 MPa) presented lower values compared with the control (L* 81.20 ± 1.60; 4.19 ± 0.82 MPa; 0.93 ± 0.12 MPa), while the moisture content varied between 30% and 45%, for the film with S. ramosissima and the control film, respectively. The addition of S. ramosissima led to opaque films with relatively heterogeneous microstructures. The films showed also good biodegradation capacity—after 21 days in soil (around 90%), and after 30 days in seawater (fully fragmented). These results show that pectin films with S. ramosissima may have great potential to be used in the future as an eco-friendly food packaging material.

Study of the Biodegradation of PLA/PBAT Films after Biodegradation Tests in Soil and the Aqueous Medium

The intense consumption of conventional plastics has been generating a series of problems for nature due to the accumulation of municipal solid waste because of its difficult degradation. Therefore, the use of biodegradable polymers becomes a good option to minimize these effects. Poly (lactide acid)/poly(butylene adipate-co-terephthalate) (PLA/PBAT) is a biodegradable blend that can be used mainly in applications that have a short shelf life. So, it is important to know the total biodegradation time of this blend. For this reason, PLA/PBAT films (1.5 x 1.5 x 0.15 cm) were produced by thermal compression molding to be subjected to biodegradation tests in soil and aqueous medium for 180 days. The films were characterized by visual analysis, weight loss measurements, differential scanning calorimetry (DSC), Fourier transforms infrared spectroscopy (FT-IR), contact angle, and scanning electron microscopy (SEM). DSC showed an increase of 0.7% in PLA crystallinity subjected to the aqueous medium, while FT-IR showed a reduction in the bands at 1710 cm-1 and 1100 cm-1, as a result of hydrolysis for both methodologies. The blend's hydrophilic character was increased after both degradation processes, presenting a reduction of 34.5% in the contact angle after biodegradation in soil. From the results, it was possible to conclude that PLA/PBAT films did not degrade completely, as expected, but showed signs that indicated the beginning of the degradation. The degradation was more effective in the aqueous medium.

Reduction in lignin peroxidase activity revealed by effects of lignin content in urea crosslinked starch under aerobic biodegradation in soil

This study characterized the lignin peroxidase (LiP) activity of soil via an enzyme assay to determine the reaction rates and activation energies for 5 wt%, 10 wt%, 15 wt%, and 20 wt% lignin loads in urea crosslinked starch biocomposites. The results revealed that a mixed mode of LiP inhibition occurred after the soil was mixed with these biocomposites with different loads of lignin. Loading of lignin at 5 wt% and 10 wt% lignin resulted in higher values of catalytic activity of LiP: -39.58 and 49.14 µM h-1 g-1 soil, respectively. In comparison, with higher loading of lignin at 15 wt% and 20 wt%, decreases in the catalytic activity of LiP were found and were 28.72 to 37.25 µM h-1 g-1 soil, respectively. The activation energy of LiP increased approximately 1.11- to 1.22-fold when 15 and 20 wt% of lignin was loaded in biocomposites. Research findings established the possibility of unfavorable binding of the LiP to lignin with an increase in the load of lignin, possibly due to the complex structure of intact lignin and presence of inhibitory biodegradation products of lignin accumulates during lignin biodegradation in biocomposites. It was concluded that higher lignin contents (15 wt% and 20 wt%) were effective in reducing the activity of the soil LiP. Hence, higher lignin content possibly protects against losses of lignin, while acting as a filler in the formulation of biocomposites.