Compounds released from unused biodegradable mulch materials after contact with water (2023)

Biodegradable mulches (BMs) can be tilled into soils to mitigate disposal and environmental problems. However, the content of biodegradable microplastics (BMPs) would increase with the addition of biodegradable macroplastics (BMaPs). The fragmented particles have a strong affinity to soil pollutants, having the potential to transfer via the terrestrial food web in an agroecosystem. Based on the spectral analysis and particle size analysis, this study explored the physicochemical characteristics of weathered BMaPs and BMP-derived dissolved organic matter (DOM_BMP). Ultraviolet (UV) irradiation reduced the mechanical strength of BMaPs and induced oxygenated functional groups, thus increasing surface roughness and hydrophilicity. This promoted the adsorption of aromatic compounds and heavy metals from soils to BMPs. After entering the water environment, the pH of the solution with DOM_BMP decreased, whereas the concentration of dissolved organic carbon (DOC) increased. Compared with paper mulch, bioplastic mulch contributed a higher amount of DOM_BMP, such as aromatic structure-containing chemicals and carboxylic acids, to the water environment but released fewer and smaller plastic particles. The findings from this study can help manage environmental risks and determine disposal strategies after the use of mulching.

Microplastics (MPs) and nanoplastics (NPs) from mulch films and other plastic materials employed in vegetable and small fruit production pose a major threat to agricultural ecosystems. For conducting controlled studies on MPs' and NPs' (MNPs') ecotoxicity to soil organisms and plants and fate and transport in soil, surrogate MNPs are required that mimic MNPs that form in agricultural fields. We have developed a procedure to prepare MPs from plastic films or pellets using mechanical milling and sieving, and conversion of the resultant MPs into NPs through wet grinding, both steps of which mimic the degradation and fragmentation of plastics in nature. The major goal of this study was to determine if cryogenic exposure of two biodegradable mulch films effectively mimics the embrittlement caused by environmental weathering in terms of the dimensional, thermal, chemical, and biodegradability properties of the formed MNPs. We found differences in size, surface charge, thermal and chemical properties, and biodegradability in soil between MNPs prepared from cryogenically treated vs. environmentally weathered films, related to the photochemical reactions occurring in the environment that were not mimicked by cryogenic treatment, such as depolymerization and cross-link formation. We also investigated the size reduction process for NPs and found that the size distribution was bimodal, with populations centered at 50 nm and 150–300 nm, and as the size reduction process progressed, the former subpopulation's proportion increased. The biodegradability of MPs in soil was greater than for NPs, a counter-intuitive trend since greater surface area exposure for NPs would increase biodegradability. The result isassociated with differences in surface and chemical properties and to minor components that are readily leached out during the formation of NPs. In summary, the use of weathered plastics as feedstock would likely produce MNPs that are more realistic than cryogenically-treated unweathered films for use in experimental studies.

Bioplastics are promoted as safer alternatives to tackle the long-term persistence of conventional plastics. However, information on the potential release of additives and non-intentionally added substances (NIAS) in the surrounding environment is limited, and biological effects of the leachates have been little studied. Leachates produced from three bioplastics, i.e. compostable bags (CB), bio-polyethylene terephthalate bottles (bioPET) and polylactic acid cups (PLA), and a control polymeric material, i.e. rubber tire (TR), were examined. The chemical nature of bioplastic polyesters PET, PLA and poly (butylene adipate-co-terephthalate) (PBAT) in CB, was confirmed by analytical pyrolysis. Fragments were incubated in artificial sea water for 14 days at 20°C in darkness and leachate contents examined by GC-MS and HPLC-MS/MS. Catalysts and stabilizers represented the majority of chemicals in TR, while NIAS (e.g. 1,6-dioxacyclododecane-7,12-dione) were the main components of CB. Bisphenol A occurred in all leachates at a concentration range 0.3–4.8μg/L. Trace metals at concentrations higher than control water were found in all leachates, albeit more represented in leachates from CB and TR. A dose response to 11 dilutions of leachates (in the range 0.6–100%) was tested for biological effects on early embryo stages of Mytilus galloprovincialis. Embryotoxicity was observed in the whole range of tested concentrations, the magnitude of effect depending on the polymers. The highest concentrations caused reduction of egg fertilization (CB, bioPET, TR) and of larvae motility (CB, PLA, TR). TR leachates also provoked larvae mortality in the range 10–100%. Effects on adult mussel physiology were evaluated after a 7-day in vivo exposure to the different leachates at 0.6% concentration. Nine biomarkers concerning lysosomal functionality, neurotransmission, antioxidant and immune responses were assessed. All lysosomal parameters were affected, and serum lysozyme activity inhibited. Harmonized chemical and biological approaches are recommended to assess bioplastic safety and support production of sustainable bioplastics.

Biodegradable plastic mulch films (BDMs) are employed for sustainable production of vegetables and other specialty crops, as replacements for conventional polyethylene (PE) mulches. Unlike PE mulch, BDMs are disposed through mineralization in soil or compost. Yet, a comprehensive understanding of the long-term impacts of BDM incorporation on the viability of soil ecosystems is needed before BDMs will be widely adopted by farmers. This paper reviews BDM-related standards and suggests additional criteria to consider. The biodegradation process and the impact of microplastics and nanoplastics (MNPs) derived from BDM fragments on soil health are described. Findings show that soil health is minimally impacted by BDM incorporation, but that the behavior of NMPs and BDM minor components in soil is not fully understood.

Biodegradable plastics attract public attention as promising substitute for non-degradable plastics that trigger serious plastic pollution, and they are claimed to be environmentally harmless and biodegradable by microorganisms. However, not all biodegradable plastics are completely degradable under natural conditions. Some of them may be disintegrated into microplastics more rapidly than conventional plastics, emerging as another threat to soil environments. As a part of microplastics, biodegradable microplastics may pose stronger negative effects on several soil species than oil-based microplastics under some conditions. Currently, there is a fiercely increasing trend to replace nondegradable plastic commodities with biodegradable ones. Therefore, to discuss the ecological safety of biodegradable plastics is essential before promoting wide application of them during commercial use. This review provided a brief introduction on biodegradable plastics and summarized their deterioration behaviors in terrestrial environments, together with evidences on releases of additives and biodegradable microplastics. Then, potential adverse effects of biodegradable microplastics in soil ecosystems, including responses on soil properties, microbial communities, and several soil species were discussed, suggesting biodegradable microplastics as a potential threat to ecological safety of soil ecosystems. By this token, biodegradable plastics might not be a panacea to the existing “white pollution” and need further exploring.

Scientia Horticulturae, Volume 263, 2020, Article 109111

More than half of the plastic used in Spain for mulching in agriculture is black polyethylene (PE) mulch. It has been widely adopted due to its low price and ease of installation. The mass use of this material entails a number of disadvantages: the cost of removing fragments from fields after use, the difficulty of managing the waste, and deficient weed control of some species such as purple nutsedge (Cyperus rotundus L.), which is capable of piercing the film. The objective of this work was thus to find alternative materials in horticultural crops in order to reduce these drawbacks. In the present document, we describe the trials carried out in Zaragoza (Spain) from 2012 to 2015 in pepper with various different biodegradable materials in order to find agronomically viable alternatives to PE. Plots were distributed randomly in four blocks and 6–9 different treatments were studied depending on the year, including an unweeded plot. In total, 11 different mulches were tested: 5 biodegradable plastic films, 5 paper mulches and PE. Weed density, mulch degradation and pepper production were evaluated. Biodegradable plastic films and PE were not good options for purple nutsedge control (mean efficacy of 48 %) and only paper mulches controlled this species effectively, as the leaves were unable to pierce the material. The above-soil part of most mulches was intact 15 days after transplant (DAT) and degradation increased in time in all materials excepting PE. In-soil degradation was generally faster for papers than for biodegradable plastics, except in 2015, when the plastic mulching materials suffered early fractures due to a windy spring with exceptionally high solar radiation. Rapid degradation of some paper mulches occurred before the crop covered the soil, which can be a serious problem in windy areas because the material can be lifted by gusts of wind. Earthing up along the edges of the materials 10–15 days after transplanting would solve this problem. Pepper production was similar for all materials in 2012–14 and the differences found in 2015 cannot be attributed to the mulches but to the irregular crop density due to high mortality of pepper seedlings during that year. The biodegradable mulches tested are thus considered to be technically viable alternatives to PE. Paper mulches are recommended for purple nutsedge infestations, while biodegradable films can be used for annual weed infestations.

Journal of Molecular Liquids, Volume 273, 2019, pp. 33-36

The present paper focuses on the study of the incorporation of low levels of natural fillers in mulching films to understand how the fillers can change the mulching degradation capability in soil. This work prepared, biodegraded, and evaluated biodegradable mulching films composed of poly (butylene adipate-co-terephthalate) (PBAT), poly (lactic acid) (PLA), and three natural fillers (carbon black, organic fertilizer, and silica rice ash) prepared by double-screw extrusion. The films were characterized by wettability, sorption of water, biodegradation in simulated soil and ecotoxicity in bean cultivation. The results show that the addition of natural fillers can increase or decrease wettability, and this behavior is depending on the polarity of the fillers. However, all samples show sorption and biodegradation superior to pure blend, as expected. The fertilizer has contributed more for the accumulation of biomass and SiO₂ to the structural growth of the bean. The least satisfactory performance for germination, growth and biomass of the crop was presented by the blends without addition compatibilizers of these natural fillers.

Polymer Degradation and Stability, Volume 175, 2020, Article 109126

Polyethylene (PE) and other non-degradable plastics are used in vast quantities as agriculture mulch to help protect the soil surface, conserve water, and improve soil microclimatic factors. Unfortunately, their continued use poses several environmental problems, so an environmentally friendly solution needs to be found. The use of biodegradable plastics in place of conventional PE is one well studied solution, and here we investigate how different controlled environmental conditions affect the water conservation efficacy, and rate of biodegradation of a novel, biodegradable, sprayable polycaprolactone based polyurethane mulch. The effect of soil moisture content, sunlight, soil type, and polymer pigmentation are investigated using several different characterization techniques and measurements. It was found that the polymer studied is effective at conserving soil moisture, and that it biodegrades at different rates via a bulk erosion mechanism. The rate at which it degrades mostly depends on the soil type to which it is applied, and the moisture content of that soil. This was confirmed using soil CO₂ emissions, polymer mass loss, polymer molecular weight reduction, and scanning electron microscopy. Results are discussed.

Polymer Degradation and Stability, Volume 188, 2021, Article 109578

The examination of the aging behavior of polymers, blends, composites, items, etc. under different environmental conditions is a priority issue for assessing the possible applications and establishing the lifetime. In agriculture, plastic items (mulch films, irrigation tubes, etc.) are widely used and subjected to solar UV exposure that can induce photo-oxidation on macromolecular chains. Therefore, weathering under outdoor conditions and accelerated degradation tests are performed for simulating aging processes during the lifetime of materials and select appropriate stabilizers to be used. In the last decades, oxo- and biodegradable polymers have been investigated and used in agricultural items. The influence of photooxidation or outdoor weathering on their structures, performance and biodegradation behavior in soil has been evaluated. This review aims to provide an overview of the studies focused on the effects of photo-degradation processes on chemical-physical properties and degradation in soil of oxo- and biodegradable polymers or items designed for agricultural purpose, including an outline of the available standards. Additionally, a comparison between the behaviors induced by photo-oxidation of the two different families of degradable polymers used in agricultural plastic items is carried out.

Polymer Testing, Volume 67, 2018, pp. 99-109

Bio-based products represent a key component of the growing global bio-economy. New markets develop for innovative products made from bio-based feedstocks. The functionality of bio-based and conventional plastic films used in the agricultural sector was investigated for a set of representative mulching films. The evaluation was based on testing critical mechanical and other functional properties of the films. Specific mulching film characteristics were addressed, including durability and sensitivity of tensile properties to external field conditions, tear and impact resistance, and penetration strength. Other functional properties tested include water vapour (WV) permeability, CO₂ transmission rates and radiometric properties. Differences were identified in the performance of the various bio-based biodegradable in soil mulching films. Differences were also identified in the performance of bio-based films as compared to the performance of a conventional reference polyethylene film. Higher water vapour permeability was measured for the bio-based films and there were differences in the penetration and impact resistance, tensile strength and degradation behaviour. Adaptations are proposed for some standard test methods to be applicable to testing bio-based plastic films. The overall functionality of the bio-based mulching films was found to be satisfactory. The characteristics of the bio-based biodegradable in soil films offer new design possibilities for environmental friendly, efficient and sustainable agricultural practices.

Science of The Total Environment, Volume 806, Part 1, 2022, Article 150238

Soil-biodegradable plastic mulch films are a promising alternative to polyethylene mulches, but adoption has been slow, in part because of uncertainties about in-field degradation. The international biodegradability standard EN-17033 requires 90% degradation within 2years in an aerobic incubation at constant temperature (20–28°C). However, in-laboratory biodegradability does not guarantee in-field degradation will follow the same timeframe. Field test protocols are needed to assess biodegradable mulches under a range of environmental conditions and collate site-specific information to predict degradation. Our objectives were to (1) monitor in-field degradation of soil-biodegradable plastic mulches following successive applications and incorporations, (2) quantify mulch recovery 2years after the final incorporation, and (3) compare in-field degradation with the laboratory standard in terms of calendar and thermal times based on a zeroth-order kinetics model. A field experiment was established in spring 2015 in Mount Vernon, WA testing five biodegradable mulches laid each spring and incorporated each fall until 2018. Mulch recovery was quantified every 6months until fall 2020, 2 years after the final incorporation. While mulches were incorporated annually, recovery of visible fragments (>2.36mm) was constant or decreasing over time, indicating mulch deterioration kept pace with new additions. In fall 2020, mulch recovery was 4–16% of total mulch mass incorporated. A zeroth-order kinetics model was used to analyze mulch degradation after the final application. Model extrapolations indicate it would take 21 to 58months to reach 10% recovery (90% degradation), exceeding the laboratory standard's 24-month benchmark by up to a factor of 2.4. However, when the analysis is done with thermal time, better agreement between in-field and laboratory degradation rates is observed. While other factors, including soil type, soil moisture, and mulch fragment size are also at play, thermal time, rather than calendar time, will be more applicable for assessing site-specific, in-field mulch degradation.