In-soil degradation of polymer materials waste – A survey of different approaches in relation with environmental impact

Abstract

Vegetal fibers from different sources, including wood fibers and plant-derived fibers, together with polymer plastics per se (natural, synthetic, and their blends), as well as their combinations as composite materials may generate significant amounts of wastes. These will undergo degradation process under exposure to different environmental factors including microorganisms, climatic changes – e.g. droughts, oxygen, temperature, soil dynamics, UV radiation, etc. This survey offers a concise review of degradation under environmental conditions, mainly after in-soil exposure, of waste made of polymer materials and natural fibers. It also describes the most common methods for evaluation of bioconversion and degradation, as well as the structural properties after degradation (e.g. macroscopic investigation; weight loss; spectrometry – UV, FTIR, NMR; X-ray diffraction for crystalline changes; SEM microscopy; and thermal stability).

Full Article

In-Soil Degradation of Polymer Materials Waste –A Survey of Different Approaches in Relation with Environmental Impact

Carmen-Alice Teacă,^a,* Mauruşa-Elena Ignat,^a Marioara Nechifor,^b Fulga Tanasă,^b and Leonard Ignat ^a

Vegetal fibers from different sources, including wood fibers and plant-derived fibers, together with polymer plastics per se (natural, synthetic, and their blends), as well as their combinations as composite materials may generate significant amounts of wastes. These will undergo degradation process under exposure to different environmental factors including microorganisms, climatic changes – e.g. droughts, oxygen, temperature, soil dynamics, UV radiation, etc. This survey offers a concise review of degradation under environmental conditions, mainly after in-soil exposure, of waste made of polymer materials and natural fibers. It also describes the most common methods for evaluation of bioconversion and degradation, as well as the structural properties after degradation (e.g. macroscopic investigation; weight loss; spectrometry – UV, FTIR, NMR; X-ray diffraction for crystalline changes; SEM microscopy; and thermal stability).

DOI: 10.15376/biores.18.1.Teaca

Keywords: Wastes; Wood fibers; Plant fibers; Polymers; Wood-polymer composites; Plant fibers-polymer composites; Degradation; Environmental impact; Evaluation of degradation

Contact information: a: Center of Advanced Research in Bionanoconjugates and Biopolymers, “Petru Poni” Institute of Macromolecular Chemistry, 41A Grigore-Ghica Vodă Alley, 700487 Iaşi, Romania; b: Polyaddition and Photochemistry Department, “Petru Poni” Institute of Macromolecular Chemistry, 41A Grigore-Ghica Vodă Alley, 700487 Iaşi, Romania

* Corresponding author: cateaca@icmpp.ro; cateaca14@yahoo.com

GRAPHICAL ABSTRACT

INTRODUCTION

Polymer materials play a significant role in improving the quality of life. They are omnipresent and fulfilling almost all daily needs of society. Polymer materials comprising natural and/or synthetic components are widely employed in many applications, such as packaging, agriculture, industry, transportation, construction, as well as in different defense strategies. This widespread usage is due to their good resilience conferred by remarkable properties (mechanical strength, resistance to chemical degradation, resistance to mechanical wear, relatively low density, and low cost), and this is all despite their incomplete degradation and ability to persist for a long time under exposure to environmental conditions. It follows that implementation of proper disposal and recycling strategies are required in order to avoid harmful effects on the natural environment (water, soil, air, plants, animals, and even human beings). In this context, it is essential to consider all issues related to ensuring the effectiveness of long-term strategies for the environment, economy, and waste management.

Significant accumulation of solid waste and plastics litter, as a consequence of increased use of different polymer materials, represents a pronounced causative factor of environmental pollution in direct relation with their resistance to biodegradation. In general, global strategies for sustainable waste management are based on methods that involve waste prevention and recycling (Rudnik 2019). All these measures applied to cope with waste are aimed in fact to:

• prevent waste in the first place;
• recycle waste (the oldest known recycling method is composting);
• optimize the final disposal of waste.

Currently, many research studies are oriented towards development of biodegradable polymer materials which can be further cleaved to their constitutive units (oligomers, dimers, and monomers) during the biodegradation process. This occurs by applying an effective and safe disposal strategy using soil or composting environments, and successfully reintroducing them into carbon cycles (Batista et al. 2010; Phetwarotai et al. 2013; Palsikowski et al. 2017; Lv et al. 2018; Salehpour et al. 2018). The resulting degradation products (Lucas et al. 2008), including carbon dioxide, water, biomass, inorganic compounds, are generally non-toxic and non-harmful to the environment. The main stages of biodegradation process are schematically represented in Fig. 1 (Shimao 2001; Lucas et al. 2008; Emadian et al. 2017; Yuan et al. 2020; Kotova et al. 2021).

Fig. 1. Schematic representation of biodegradation stages for polymer materials waste

Forestry industrial activities generate very large amounts of waste, which are currently pelletized, burned, or allowed to decay naturally, often producing environmental problems such as contamination of water and soil (including underground), as well as air pollution. Wood fibers waste represents a significant proportion of the waste stream. Wood processing sectors, namely forestry harvesting/sawmills and the pulp and paper production, produce significant amounts of wood waste, along with other activities, such as construction and demolition, wood packaging production, furniture, pallets and fencing manufacturing, roads and railway construction, housing, etc. (Cai et al. 2013).

Wood and other lignocel­lulosic fibers waste, from both virgin and recycled sources, represent principally a relatively inert, but organic material, which becomes more and more a priority polymer material, considering the rapid evolution of its processing strategies and end markets of this significantly redundant waste material (Coudert et al. 2013; Laleicke 2018; Berger et al. 2020; Li et al. 2020; Xing et al. 2020; Pandey 2022). Some new lignocellulosic sources can be mentioned, and these include paper mill sludge and biorefinery residues (Jaria et al. 2017; Panzella et al. 2019; Kwon et al. 2020; Du et al. 2020; Husanu et al. 2020; Moccia et al. 2022).

Wood waste (sawdust, wood chips, shavings, bark, etc.) present disposal problems for industries that generate them. Tree bark is usually disposed in landfill sites, which are often adjacent to the rivers. After the inherent degradation processes, various toxic products are leaching into the ground waters and rivers, thus raising serious environmental pollution issues. Relatively recent technologies are applied for capitalizing wood waste through compounding with plastics, mostly recycled, in order to produce a large and useful variety of wood-plastic composites (WPCs). These are commonly used for door frames, windowsills, decking and fencing, panels for interior design and compartmenting, sheets and shingles for buildings and roofing applications (Sommerhuber et al. 2015; Sommerhuber et al. 2016; Teuber et al. 2016; Keskisaari and Karki 2018; Akinyemi et al. 2019; Basalp et al. 2020; Boubekeur et al. 2020; Kaho et al. 2020; Mrówka et al. 2020). Wood and lignocellulosic fibers lead to WPC with superior properties by acting more as reinforcement than filler; thus a wise trend is to move toward the use of these fibers in applications requiring additional strength (Cai et al. 2013).

Bulky waste represents a significant and increasing waste stream in every country due to changes in habits and economic status, which results in finding affordable goods for fast replacement. This category of waste comprises, accordingly to the US Environmental Protection Agency (US EPA Terms of the Environment 2018), large items of solid waste such as household appliances, furniture, large auto parts, trees, branches, stumps, and other oversize waste whose large size precludes or complicates their handling by normal solid wastes collection, processing, or disposal methods. Manufacturing of WPCs from bulky waste at industrial scale can reduce the production cost, considering both the lower costs of employed materials and the visible reduction of the environmental effects of plastic waste (Basalp et al. 2020).

Degradation is a very complex and complicated process and depends on many factors, as represented in Fig. 2. Natural decay (biodegradation) represents an essential component of the carbon recycling system in nature. Biodegradable polymer materials of natural origin – wood fibers, plant fibers, as well as polymer plastics, per se or combined in different composite formulations – are permanently exposed to degradation processes of an environmental, chemical, or microbial nature.

The extent of the deterioration depends on the environment in which these materials are usually found in relation to their envisaged applications. Biodegradation of polymer materials waste occurs in relation with their properties (Tokiwa et al. 2009), being strongly related to both chemical and physical ones including: surface properties (area, hydrophilic, and hydrophobic behavior); molecular weight and polydispersity; thermal behavior (glass transition temperature, melting temperature); and crystalline structure and modulus of elasticity.

Fig. 2. Main factors influencing the complex degradation process of polymer materials

Most of the definitions of biodegradation process refer to the presence of microorganisms (bacteria, fungi) and their action on different polymer materials (natural, synthetic, or their combinations, per se or as waste resulting from different activities) when they cause, as preferable last stage, their conversion into carbon dioxide or methane and water, inorganic compounds, and biomass. Nevertheless, there are many situations in which the resulting products from degradation, as small pieces or powders, are not used by microorganisms as carbon and energy sources. This means that materials are degradable, but not biodegradable. It is also possible that materials cannot be subjected to composting as a recycling method, even though they are susceptible to being degraded or, moreover, biodegraded (Stevens 2002; Rudnik 2019).

Therefore, given the complexity of this assembly of correlated and interdependent phenomena which forms the in-soil degradation of polymer materials waste, and considering the variety of polymer waste that ends up intentionally or accidentally in soil, an extensive survey would be of interest. The aim of this paper was to illustrate, with examples from the most recent literature data, the main categories of polymer waste disposed in a controlled or irresponsible manner in soil, namely lignocellulose-sourced polymers, synthetic polymers, and natural fibers-polymer composites. At the same time, the degradation of each waste category is presented and discussed taking into account their specificity, but not comparing them. Simple comparison was not the goal of this work, since this type of environmental degradation occurs and evolves through a wide variety of mechanisms depending not only on the nature of waste and location (soil characteristics), but on season, vicinity, and degree of exposure, etc.

METHODS EMPLOYED FOR EVALUATION OF IN-SOIL DEGRADATION PROCESSES FOR POLYMER WASTES

Advances in the field of characterization techniques and devices have enabled scientists to use single or combined methods in order to evaluate the in-soil degradation of polymer materials. When experiments under controlled conditions (simulated media) are considered, it is very important to provide data collected before and after degradation for comparison reasons. Thus, it is possible to observe changes recorded at preset time intervals and/or monitor the evolution of certain characteristics.

If these characterization methods are used on samples collected from disposal sites, the progress of degradation can be assessed. The credibility of the results is raised when the polymers have been identified with a certain amount of validity, in comparison with technical data available in literature, if any, provided either by manufacturers or by other studies. Even more, these samples can be further submitted to other degradation experiments, under controlled conditions.

Data obtained from different characterization techniques provide information on the following aspects:

• Morphological changes occurring on the surface of the material, such as the presence of cracks, fractures, pores, etc. (microscopy – light microscopy, SEM, TEM, AFM);
• Modification of thermal stability and glass transition temperature (TGA, DTG, DSC);
• Alteration of crystallinity (FTIR, XRD) and mechanical properties (mechanical tests);
• Decrease in the molecular weight and formation of oligomers and other low molecular by-products (NMR, GPC, FTIR, mass spectroscopy).

By combining characterization techniques and discussing results in an integrative manner, reliable assessments can be made and further used to reach pertinent conclusions and provide insightful prognosis. For example, wood decay can be evidenced by using both transmission electron microscopy (TEM) and ultraviolet micro-spectrophotometry measurements, these providing details on the micromorphology of wood that is affected by the brown-rot fungi, and lignin degradation, mainly produced by white-rot fungi (Schmidt et al. 2016).

Scanning electron microscopy (SEM) makes it possible to obtain high-resolution 3D images and offers information on topography, morphology, and composition of various materials. SEM images are utilized in medical and biological sciences, soil and rock analysis, forensic examination, semiconductor, and microchip fields. This is one of the most commonly exploited methods for the imaging characterization of solid objects due to its resolution of about 2.5 nm.

Atomic-force microscopy (AFM) is a non-destructive surface scanning technique that picks up data for imagining surface topography studies and affords the investigation of the functional, electrical, or mechanical properties of materials at the nanoscale. Its lateral resolution is about 30 nm due to the convolution, while the vertical resolution can be down to 0.1 nm.

Thermogravimetric analysis (TGA) measures the thermal stability and composition of materials. This technique affords the evaluation of the composition of products and supplies information about physical phenomena (phase transitions, absorption, adsorption/ desorption) or chemical phenomena (chemisorption, thermal degradation). Differential scanning calorimetry (DSC) is one of the most commonly employed thermal investigation methods, together with thermogravimetric analysis. It is a quick, sensitive, and simple method. DSC measures the enthalpy variation as a function of temperature because of the changes in the physical and chemical properties of materials during their heating. This method permits the identification of glass transition values and the finding of melting and crystallization behavior. DSC is used for various utilizations in various fields of industry.

Contact angle determination is a straightforward method used for the study of the wettability of a solid substrate by a liquid. Its value depends on the nature of solid substrate, liquid and the environment. The hydrophobicity of materials can be evaluated by using this technique.

Tensiometry is the method of determining the tensile strength and the elongation of materials. Tensile strength at break correlates to the toughness of products and it measures the maximum pressure that a sample can endure while being extended before fracture. Elongation is evaluated by utilizing tensile force and establishes the change in length from original. Tensiometry is widely employed in material science, mechanical and structural engineering for quality control and for the finding of materials’ resistance to changeable forces.

Fourier-transform infrared spectroscopy (FTIR) is a quick and easy method employed for the identification of different compounds by the detection of their functional groups that occur in the chemical structure. FTIR provides both qualitative and quantitative analysis, and it is often employed in the study of polymers or in pharmaceutical and forensic investigation.

Nuclear magnetic resonance spectroscopy (NMR) is an analytical method that affords information on the content and purity of a product along with its chemical structure. NMR can be employed to establish molecular conformation in solution and physical properties at the molecular level (solubility and diffusion, conformational and phase changes).

X-ray diffraction (XRD) analysis is one of the most appropriate methods for usual quantitative analysis in comparison with any other technique, such as FTIR or electron microscopy (SEM, TEM). XRD entails the irradiation of the materials with a beam of monochromatic X-rays. The diffracted rays are then examined based on angle of diffraction, and the intensity of the diffracted rays provides information about the crystallinity percent, crystallite shape, size, orientation, and interplanar atomic distance. This technique is a useful tool in pharmaceutical and forensic science, microelectronics, or geological analysis.

X-ray photoelectron spectroscopy (XPS) is a technique used for the investigation of material’s surface chemistry. Thus, XPS provides information about elemental composition, empirical formula, chemical and electronic state, binding energy of functional groups, and the thickness of the superior part of surfaces.

Gel permeation chromatography (GPC) affords the separation of analytes in organic solvents on the basis of their size. This technique enables the finding of dispersity value together with viscosity molecular weight (M_v) and founded on other data, number average molecular weight (M_n), the weight average molecular weight (M_w), and the size average molecular weight (M_z).

Table 1 summarizes examples of the methods employed to investigate polymer materials degradation.

Table 1. Main Methods Applied to Identify Polymer Materials Degradation

BIODEGRADATION OF LIGNOCELLULOSICS

Lignocellulosic wastes (debris) are of real significance for both forest and stream ecosystems as an unceasing source of nutrients, basic structural components, and part of any natural environment. They are basically constituted by three main polymeric components, namely cellulose, hemicellulose and lignin, their recycling being an essential part of the carbon cycle in nature. All three polymers with distinct chemical structure are degraded by a large variety of microorganisms through different enzymatic mechanisms, which act synergistically. Among these, fungi are the well-known degraders of lignocellulose substrates, but some bacteria can be also effectively involved in degradation processes. Considering the insolubility specific characteristics of such substrates, their degradation is facilitated by both fungal and bacterial pathways, which occur usually by means of extracellular mode of action. Such mechanisms rely on hydrolytic reactions that involve both enzymes called hydrolases, responsible for cellulose and hemicellulose degradation, and a specific enzymatic system involved in lignin degradation. Many soil bacteria, especially Actinomycetes sp., are well-known microorganisms that react with lignin to both depolymerize it and produce a high molecular weight metabolite named acid-precipitable polymeric lignin (APPL). Some details related to microorganisms and enzymes involved in degradation of lignocellulose-based sources (Crawford and Pometto 1988; Eriksson et al. 1990b; Uffen 1997; Pérez et al. 2002; Hammel and Cullen 2008; Horn et al. 2012; Brown and Chang 2014; Rytioja et al. 2014; Houfani et al. 2020) are presented in Scheme 1.

In general, lignocellulosic wastes are slowly degraded, at the same time offering good life support for many organisms (fungi, moss, insects, birds, small mammals). Composting can be considered a valuable way to appropriate handling and exploiting lignocellulose waste resulted from different activities (i.e. forestry and agricultural practices, timber industries, agroindustries – meat processing, for example-, yards, sewage). Composting lignocellulose waste can represent an interesting recycling strategy in order to provide useful amendments (fertilizers or organic substrates) for land applications in agriculture and silviculture with beneficial effects on the environment (Hubbe et al. 2010; Hubbe 2014). At the same time, recycling such complex waste by composting can be a real challenge for researchers, given its low decomposition rate (i.e. cellulose and hemicellulose decompose slowly, whilst lignin is resistant to decomposition). In order to optimize the composting of lignocellulosic waste, there are considered different alternatives (Reyes-Torres et al. 2018), including homogenization (waste is pre-treated by applying shredding and extraction stages), addition of co-substrates (such as inoculating microbial agents, amendments, bulking materials), and changes in processing operations (using aeration and two-stages composting processes, temperature control). All these above-mentioned can effectively reduce the duration of composting process, can ease the decomposition of recalcitrant organic compounds such as lignin and cellulose, and can provide high-quality products. Employment of wood waste for turnery and furniture construction applications are also suitable options to recycle them, alongside granulation into wood chips, conversion to charcoal or burning as fuel.

Among biodegradable materials, wood is considered to be a durable material that withstands weathering well without losing much of its structural properties (except for microbial attack). However, a number of environmental (non-biological) parameters contribute significantly to the degradation of wood, including moisture, temperature, light, atmospheric ozone content, and pollution. Another important aspect that may significantly affect the degradation rate of wood is the type of wood, i.e. softwood or hardwood. Hardwood and softwood species differ in several aspects, such as fiber dimensions, chemical composition, mainly in both lignin and cellulose contents, as well as lignin type. The hardwood presents a vessel element and lignin comprising both guaiacyl and syringyl units in the structure. Softwood does not contain vessel elements, while its lignin structure presents mostly just guaiacyl units (Fengel and Wegener 1983).

Scheme 1. Schematic representation of the polymeric components from lignocellulose waste and their degradation under microorganisms’ action through different enzymatic pathways

Forest residues (twigs, bark, sawdust, branches, underground) as well as other waste of vegetal origin (e.g., those resulted from annual plants processing, namely different retting approaches) are likely to be quite variable in chemical composition, texture, and moisture content. Degradation processes in an outdoor environment, including soil, are influenced by many factors such as moisture, temperature, ultraviolet radiation, soil reaction, and microorganisms.

All types of degradation have been observed when lignocellulose-based materials, per se or as components in different composite formulations, were exposed to different environmental conditions. These processes depend to a large extent on materials applications and structural chemistry particularities in relation with conferred properties, as presented in Fig. 3 (John and Thomas 2008; Beg and Pickering 2008; Methacanon et al. 2010; Suardana et al. 2011; Matuana et al. 2011; Dittenber and GangaRao 2012; Azwa et al. 2013).

Fig. 3. Inter-relations between the chemical composition in main polymer components of lignocellulose sources and their behavior under exposure to environmental conditions in relation with properties

The rhizosphere microorganisms can be used as promoters to further accelerate the biodegradation process of different polymer materials in cultivated soil and to stimulate plants’ growth and development, as well (Gerhardt et al. 2009; Janczak et al. 2018; Janczak et al. 2020; Beltrán-Sanahuja et al. 2021; Zhang et al. 2022). These microorganisms act upon complex polymer materials during degradation through releasing exo-enzymes with affinity for such substrates that are depolymerized to low molecular weight intermediate products, namely oligomers, dimers, and monomers. This process, named phytoremediation, is efficient to decontaminate both soils and wood treated using inorganic preservatives (Xing et al. 2020).

In the case of wood, fungi, mainly those belonging to Basidiomycetes species, are the major microorganisms involved in decay processes. These include both brown-rot and white-rot types, and they have significant nutrient recycling implications in forest ecosystems, being also implicated in the weathering of soils (Eriksson et al. 1990a,b). The structure and chemical composition of wood have a significant influence on its degradation by microorganisms and the resulting patterns of decay. Significant decay of wood fibers and plant fiber-based materials is promoted by microorganisms, mainly by the fungal and bacterial communities occurring in nature. In general, wood decay is classified into brown-rot, white-rot, and soft-rot types (Shimada and Takahashi 1991). White-rot fungi are able to fragment the major structural polymers of wood and other lignocellulose sources – cellulose, lignin, and hemicelluloses – and to further metabolize the fragments (Shimada and Higuchi 1991; Kirk and Cullen 1998). Brown-rot fungi selectively decompose holocellulose components (cellulose and hemicelluloses) via extensive depolymerization, leaving lignin partially intact (Goodell et al. 2008; Schilling et al. 2012; Shang et al. 2013) and release more carbon to soils in lignin residues than as atmospheric CO₂. Some Actinomycetes fungi species are significantly implicated in the degradation of lignocellulose materials (Hamed 2013), having the ability to decompose lignin components, and alongside species of Eubacteria, they may exert strong antagonism toward other wood-inhabiting microorganisms (Eriksson et al. 1990b). Their mode of deterioration wood in soil may have a real significance for better knowledge the ageing of wooden cultural heritage objects buried in soil and proper conservation approaches to be implemented. Different microorganisms communities present in soil, mainly fungi and bacteria, can promote degradation through interactions such as commensalism or mutualism, and even competitive or antagonistic (Haq et al. 2014; Johnston et al. 2016; Haq et al. 2022), which influence to a large extent the deadwood environment (Tláskal et al. 2017; Christofides et al. 2019). In the soil of forest ecosystems, the proportion of brown-rotted wood residues is very significant and has an important role in their optimal functioning through high microbial activity and consequently further optimal supplying nutrients and moisture. Moreover, these residues allow survival of ecosystems even in the case of drought periods by maintaining a high moisture content in soil (Eriksson et al. 1990b). So, such decayed wood residues in the soil appear to be essential to sustain a good site quality. Considering wood waste disposal at landfill sites, the soil dynamics evaluation and determination of wood chemical composition, as well, are very important for environmental impact assessment. Such evaluation includes determination of pH value, humus, mineral elements (N, P, K), and salinity as total content of soluble salts. A previous investigation (Teacă et al. 2008) considered three wood waste landfill disposal sites, namely Bicaz (coded as P1), Tasca (coded as P2), and Borca (coded as P3), located in the Neamt County, North-Eastern Romania, in a mountain region, being surrounded by mixed coniferous and deciduous forests. The soils in these forest ecosystems are typical brown earths, with a structure of siliceous sandstones and stones. The forests are mainly composed by coniferous tree species, such as fir (Abies alba L.) and spruce (Picea abies L.), along with some deciduous tree species, namely beech (Fagus sylvatica) and birch (Betula alba). The wood waste disposed of at dumps in this landfill region are usually generated through forestry and sawmilling activities. The authors’ study evidenced that wood wastes influenced to some extent the soil chemistry on disposal area through slightly increasing the acidity (pH value as high as 7.8 to 8, while a typical brown soil has a pH value of 4.8 to 5). A decrease in humus content was noticed, with variation in the mineral elements content (P and K contents), as represented in Figs. 4 a-c.