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Biodegradability of Cellulose Fibers, Films, and Particles: A Review
Martin A. Hubbe ,a,* Jesse S. Daystar
,b Richard A. Venditti
,a Joel J. Pawlak
,a Marielis C. Zambrano
,a Morton Barlaz
,c Mary Ankeny
,b and Steven Pires
b
Cellulose fibers are an abundant material that is well known for its biodegradability. Various forms of cellulose, such as cotton, paper pulp fibers, and microcrystalline cellulose can be regarded as benchmarks for biodegradability, when comparing other materials. However, as revealed by the literature, broad ranges of time and extent of biodegradation have been reported for cellulose. These large ranges can be attributed not only to environmental factors but also to the presence of lignin, the degree and perfection of crystallinity, the size and density of the physical specimens, and chemical modifications to the cellulose, if any. Studies also have shown differences in biodegradability associated with the selection of test methods. Although cellulose is subject to well-known enzyme-promoted mechanisms of biodegradation, the evolution of plant materials has favored development of some resistance to decay, i.e. recalcitrance. Cellulosic materials are clearly less biodegradable than starch. However, they are more biodegradable than various synthetic or bio-based plastics, as well as some cellulose derivatives, which persist in ocean water or soils for very long periods. This review indicates that cellulose biodegradability, while generally rapid and natural, has a rate and extent that depends on a complex and sometimes subtle set of environmental and chemical factors.
DOI: 10.15376/biores.20.1.Hubbe
Keywords: Cellulase; Recalcitrance; Seawater; Soil; Wastewater; Textiles; Environmental impacts
Contact information: a: Department of Forest Biomaterials, North Carolina State University, Campus Box 8005, Raleigh, NC, 27695-8005 USA; b: Cotton Incorporated, 6300 Weston Parkway, Cary, NC 27516, USA; c: Dept. of Civil, Construction, and Environmental Engineering, North Carolina State University, Campus Box 7908, Raleigh, NC, 27695-7908 USA; *Corresponding author: hubbe@ncsu.e
INTRODUCTION
This article reviews published findings related to the relative biodegradability of cellulose fibers, including factors affecting their accessibility to enzymatic attack. Interest in this topic has been spurred by reports of fibers and related materials remaining either after conventional wastewater treatment (López Alvarez et al. 2009; Ghasimi et al. 2016; Libardi et al. 2022), or when cellulosic matter is discharged to other environments as rinsewater after laundering (Ladewig et al. 2015; Zambrano et al. 2019). In addition, there have been concerns about slow biodegradation of cellulosic fibers that reach ocean environments (Zambrano et al. 2020; Nagamine et al. 2022; Royer et al. 2023). Issues to be reviewed include the rates and extent of cellulose fiber biodegradation under a variety of conditions, including fresh water, aerobic and anaerobic wastewater treatment environments, seawater, soil burial, and composting. Literature is examined to shed light on mechanisms of biodegradation, as well as factors that can promote or inhibit those natural processes. Common examples of cellulose fibers include sanitary tissue fibers, cotton and rayon textile fibers, and pulp fibers present in packaging, as well as in printing and writing papers.
Biodegradability of cellulose fibers can be viewed as a continuum, in which specific materials exposed to defined environmental conditions can be compared to reference materials. Such a perspective is illustrated in Fig. 1. In this article the term environmental will be used broadly, including both natural conditions and those in wastewater treatment facilities, unless specified.
Fig. 1. Relative biodegradability of various other polymers compared with that of some different forms of cellulose and wood
As an example of a reference material, every year in temperate climates one can expect masses of leaves to fall from deciduous trees, such that they not only blanket the ground, but many of them pass into streams and eventually into oceans. The aquatic biodegradation of leaf litter has been studied (Sakamaki and Richardson 2008; Raposeiro et al. 2014). It was reported that half a year was sufficient for biological breakdown of leaves placed in tidal flat environments (Sakamaki and Richardson 2008). Raposeiro et al. (2014) reported between about 15% and 95% mass loss of leaves with no added nutrients or bacteria in 28 days in the North Atlantic island of São Miguel in the Azores, depending on the tree species in different fresh-water streams. In addition to such natural reference points, microcrystalline cellulose (MCC) is often used by researchers as a benchmark from which to judge the relative biodegradability of other cellulosic materials in selected environments.
The fact that bioplastics, despite their plant-based origins, are not equally biodegradable was highlighted in recent work by Kwon et al. (2024), who compared biodegradation in aerobic water conditions. Pure poly(lactic acid) (PLA) was not degradable under the studied conditions, whereas poly(β-hydroxybutyrate) (PHB) readily degraded. Moreover, incorporation of just 25% of PLA into a mix of the two polymers, followed by melt-bending and extrusion, yielded fibers that showed only 11% degradation. The differences in biodegradability among bioplastics have been shown to be attributable to differences in crystallinity, hydrophobicity, and chemistry (Kwon et al. 2023b).
Table 1. Highlights from Studies Considering the Biodegradability of Microcrystalline Cellulose (MCC)
Further work showed that mixtures of the more degradable PHB with less degradable polymers such as PLA and polypropylene gave rise to micro- or nanoplastic particles, which tend to build up in the tissues of marine organisms (Kwon et al. 2023a, 2024).
Table 1 provides highlights from studies that have considered the biodegradation of MCC, often in comparison with cellulosic fibers of various types. Other studies have considered various less-biodegradable classes of material, such as bioplastics (Bhagwat et al. 2020; Royer et al. 2023) and synthetic plastic items, including fibers and fabrics (Cooke 1990; Li et al. 2010; Zambrano et al. 2020; Kwon et al. 2021; Royer et al. 2021, 2023).
Another aspect to be considered in this article is the different environments in which biodegradation is important. As shown in Fig. 2, a rough division of categories can be drawn based on relatively dry to water-saturated environments (e.g. soils, composting, and landfilling) vs. aqueous environments (e.g. fresh water, seawater, wastewater treatment). Note that although the figure illustrates the possibility of collecting methane that forms within landfills, such collection is often incomplete or may be absent. Further information about composting (Hubbe et al. 2010; Reyes-Torres et al. 2018; Ruggero et al. 2019; Wu et al. 2022) and wastewater treatment technologies (Hubbe et al. 2016; Srivastava et al. 2022; Wang et al. 2022a) has been published.
Fig. 2. Seven contrasting environments in which biodegradation of cellulose fibers can be expected to have different rates and controlling factors. The figure is arranged such that the gentler environments, with correspondingly slower biodegradation, are towards the top, whereas more biodegradative environments, often with higher temperatures, appear towards the bottom.
Though broader ranges of cellulosic material are considered in this review, the primary focus will be on lignin-free fiber-based products, such as cotton and bleached kraft fibers, the latter of which is the main component of most flushable sanitary paper products. The distinction can be important, since, as will be described in more detail later, lignin can substantially slow down biodegradation (Reyes-Torres et al. 2018; Wu et al. 2022).
Synthetic plastic materials generally were not considered in the literature covered in this article. A general finding is that many such plastics show much lower rates of biodegradation, or even no measurable biodegradation under conditions that lead to substantial degradation of MCC and other cellulosic materials. For instance, Zambrano et al. (2019) reported about 4% biodegradation of polyester after about 245 days in an aqueous aerated system. Under the matched conditions, the biodegradation of cotton was 76% and MCC was 83%. Royer et al. (2021) compared seawater biodegradation based on measurements of fiber diameter. Polyester fibers showed a 5% decrease after 7 months of exposure, whereas lyocell regenerated cellulose fibers showed a 20% decrease after about one month. Li et al. (2010) reported about 13% biodegradation of polyester fabric under conditions giving 23% biodegradation of cotton under large-scale compositing conditions (ASTM D 5988-03). Royer et al. (2023) reported essentially undetectable levels of marine biodegradation of polylactic acid (PLA), polyethylene terephthalate (PET), and polypropylene (PP), under various marine conditions and times that gave substantial biodegradation of cotton, rayon, lyocell, and modal cellulosic fibers (e.g. about 50% to 70% degradation in 7 days and about 80% in 28 days). In a review article, Cooke (1990) draws a distinction between so-called biodegradable synthetic plastics, such as aliphatic polyesters, polyurethanes, some polyamides, polyvinyl alcohol, polyvinyl acetate, polyacrylates, vs. non-biodegradable ones, including polyolefins, polystyrene, and aromatic polyesters. Even if a plastic material is prepared from plant materials, as in the case of PLA, one should not automatically assume that it is biodegradable (Royer et al. 2023). Thus, Bhaghwat et al. (2020) urge testing of each material under the environments of interest, following available standards when possible.
Various plant-based materials, such as starch, chitin, proteins, hemicelluloses, lignin, lipids, and natural rubber, generally fall outside of the primary focus of this article. Attention has been paid to these substances in other reviews, some of which are listed in Table 2. A general rule, which is supported by entries in this table, is that specific enzymes are needed, often in combination, to achieve effective biodegradation of each unique natural polymer.
Table 2. Natural Polymers and Enzymes Associated with their Biological Degradation
Scope of the Problem
The term “cellulose fibers,” which defines the focus of this article, generally will follow the literal meaning of the words, thus excluding cellulose derivatives such as cellulose acetate. Cellulose derivatives will be considered only briefly, to show how they contrast with either natural cellulosic fibers or regenerated cellulose fibers, such as rayon. A reason to focus on biodegradability of cellulose fibers at this time is that large amounts are routinely discharged to natural environments.
As illustrated in Fig. 3, two of the most prominent sources are from the flushing of toilets and the discharge or rinse water from laundering. An average US citizen, in 2018, used 12.7 kg of toilet paper, whereas lesser per capita amounts were used elsewhere (Armstrong 2018). Table 3 summarizes reported information of the total amounts of cellulosic fibers discharged to wastewater treatment systems, with emphasis on toilet paper as a major source. As reported by Gupta et al. (2018), the relative amounts of cellulose in various wastewater and sludge specimens can be determined with high accuracy.
Table 3. Reports of Cellulosic Fibers, Mainly Toilet Paper, Routinely Discharged to Wastewater Treatment Facilities or the Natural Waterways
Likewise, Table 4 highlights studies that have helped to quantify amounts of textile cellulose fibers discharged to wastewater. Two factors that tend to promote detachment of fine particles, including some of the fibers, in the course of laundering are the agitation and the usage of detergents (Zambrano et al. 2019). As described in a recent review article (Hubbe et al. 2022), laundry detergents are designed to promote separation between fibers such as cotton and other attached solids.
Fig. 3. Illustrate of two major sources of cellulosic fibers in municipal wastewaters. Part of figure (washing machine) previously published as an original drawing by the author (Hubbe et al. 2022)
Table 4. Reports of Cellulosic Fibers Discharged to Wastewater Due to the Laundering of Textile Items