Bachelor Thesis Scheikunde
Cigarette butt degradation: the replacement of plastic cigarette filters with
biodegradable alternatives
doorKoen Oostinga
30 november 2020 Studentnummer 11885297Onderzoeksinstituut Verantwoordelijk docent
IBED Prof. dr. G.J. Gruter
Begeleider Naam Onderzoeksgroep dr. A. Praetorius
Department of Ecosystem & BKO gecertificeerd assessor
Table of contents
Abstract 3
Introduction 4
Research Outline 5
Materials and methods 6
Information Sources 6
Kinetic Decay Model 6
Results 8
Alternatives to conventional filters 8
Degradation Pathways 9
Kinetic Decay Model 12
Discussion 15
Conclusion 16
Abstract
Cigarette butts (CBs) are the most collected form of litter globally. They consist of cigarette filters, made of cellulose acetate, an environmentally persistent polymer. Alternatives for this polymer are starch and cellulose, which have a lower persistence. Degradation of cellulose acetate polymers was studied by elaborating on the degradation process itself, and subsequently modelling the biodegradation process, calculating the cigarette butt mass and the chemicals released in water, soil and compost environments. The replacing of conventional cellulose acetate cigarette filters with biodegradable filters may decrease the persistence of the CBs in the environment. This may cause a faster release of chemicals, posing larger risks for the environment.
1. Introduction
The Cigarette Butt problem (Broad Context)
High quantities of cigarettes are consumed annually. On a global scale, the usage amounts up to 4.5 trillion units per year.1,2 After the consumption of cigarettes, filters from smoked cigarette, from here on named cigarette butts (CBs), are frequently left behind and discarded into the environment. CBs are found to be the most numerously littered product worldwide and are collected in great quantities at global beach cleanup programs.1,3,4 In the United States alone, an estimated 49.8 kt CBs was littered in 2011.3 Conventional CBs are quite persistent in the environment, as they are made from cellulose acetate (CA), a synthetic polymer originating from cellulose.1
The health risks of cigarette smoking is universally known. Over 4000 chemicals can be released into the smoker’s lungs and into the environment from cigarettes, of which 50 are carcinogenic to humans.5 The environmental risks of its waste however has received significantly less attention. Potential environmental problems of CB littering can be rephrased as the effect of the disposed cigarettes on the environment. This thesis will focus this effect by analyzing the degradation process of conventional and biodegradable cigarette filters. The main focus of the analysis will be on the degradation rates of CBs and the chemicals released during the degradation process.
CB Littering (Social relevance)
Studies estimate global mean littering percentages of used cigarettes to range from 76 to 84 percent.2,6,7 The inclination of people to litter CBs is a great concern in the solution of the CB problem. The waste of 4.5 trillion CBs per year due to high littering rates amounts up to almost 600 cigarettes wasted per person on the planet per year. While this is a very sizeable amount of waste, compared to the total amount of plastic waste, estimated to be 274 million tons in 2015, the impact on the total plastics flow is relatively small.8 However, CBs have a much higher littering rate versus other plastic wastes because the consumption of cigarettes largely takes place in open public areas, where CB deposition points are often absent. CB littering was shown to not be uniformly distributed over the globe, but have high concentrations mostly in urban areas, possibly influencing the exposure to CB toxicity.
The influence of cigarette butts relative to the entire plastic waste cycle in terms of weight may seem quite small due to the small size and weight (0.17 g per CB) of cigarette butts3 but the global weight of cigarettes that is discarded annually amounts to 1.2 million tons.2
When compared to the total annual plastic ocean pollution instead of total plastic flow, the impact of CBs becomes evident. It is estimated that ocean plastic pollution has a value of 4.8 to 12.7 million tons per year in 2010.9
Conventional Cigarette Filters (Scientific Relevance)
97 percent of all conventional cigarette filters are made from plasticized cellulose acetate.10,11 Cellulose acetate is a polymer derived from cellulose. It is synthesized by a process called acetylation, involving the addition of excess acetyl anhydride, in combination with an acidic catalyst.1,12 The cellulose acetate used in conventional filters has a high density, and in combination with a high degree of acetylation makes the fibers inaccessible for biological microbial degradation.1,12,13 This reducing effect on the biological degradation is increased by the plasticizers that are integrated in the filters.1 Used filters have an even lower microbial activity rate than unused filters due to the presence of tars and other chemicals (that will be discussed in the modelling section) that are released from the tobacco mixture during smoking. These chemicals are toxic to microbial organisms.10
The dissociation speed and the chemical release of conventional filters are perceived to be the main environmental problems that arise from CA CBs apart from the release of smoke from usage. As
mentioned earlier, the chemicals present in tobacco and the filters have a toxic effect on the environment. These chemicals can originate from the tobacco cultivation or product preparation.1 During tobacco cultivation, products such as herbicides and insecticides are used.5 Residues of these products may be found in finalized cigarettes.5 Tobacco plants naturally contain nicotine, which is found to great extent in urban waters.14 Tobacco also contains heavy metals, which originate from the soil and accumulate inside the plant.2,15,16 PAHs were shown to enter the environment through the release of tar and smoke from cigarette consumption.5
Cellulose acetate, the main component in CBs, is a polymer consisting of the glucose subunits, with a certain degree of acetylation. In figure 1, the chemical structure of cellulose acetate is presented. The degree of acetylation (DA) for conventional cigarettes was reported to have a mean value of 2.5.12 The cellulose acetate of cigarette filters thus comprises mostly of a 1:1 mixture of glucose diacetate and glucose triacetate. The high degree of acetylation (DA) is the main reason for the lack of biodegradability of cellulose acetates. Because of this DA, the microbial activity is
reduced, as the enzymes that are responsible for the breakdown of polymers like cellulose acetate cannot reach the vulnerable R-O-R bond between two glucose units.
Biodegradability
While the biodegradability of a product is normally a desired feature, some misunderstandings may be caused by using this term. The degradation process of conventional cigarettes may not only degrade a polymer to carbon dioxide and water, but degradation mechanisms may include the formation of smaller polymers, called microplastics.17 The persistence of the microplastic particles is closely related to the lack of biodegradability for conventional filters, and increases the impact of the persistence problems.
Due to the environmental harm that CBs can cause, biodegradable alternatives to conventional cigarette filters have been developed. These alternatives strive to reduce the impact of CBs on the environment. The biodegradability of CBs however is only a part of the cigarette butt issue, as the great amount of annual released chemicals from CBs poses a similar environmental risk. This thesis aims to compare biodegradable and conventional cigarette filters to assess their separate impacts on the environment. This assessment will be done by discussing the current biodegradable alternatives, analyzing the degradation pathways of CBs, and modelling the release of chemicals and degradation rates from CBs.
Research outline
HypothesisIt is expected that biodegradable cigarette filters are less harmful to the environment than the conventional filters, because of a decreased lifetime in the environment, and are thus expected to be a suitable option for lowering the impact of CBs on the environment. This decreased lifetime is expected not to yield more microplastics due to a different degradation mechanism. The CBs are expected to still be toxic due to the PAHs, heavy metals and nicotine that is present in tobacco. To research whether the transition to biodegradable CBs will have a significant impact on the environmental burden of CBs, the research question: “To what extent are biodegradable cigarette filters a suitable option for lowering the impacts of cigarette butts in the environment?” will be analysed and answered, following three subquestions to form a complete image of the impact of biodegradable CBs on the environment.
Figure 1: Structure of a cellulose acetate polymer with DS = 3.
SQ1: What alternatives are there for conventional cigarette filters?
SQ2: What type of degradation products are released during the degradation of conventional and biodegradable cigarette filters?
SQ3: How fast do CBs degrade in different environments?
For this research, a comparison between conventional and biodegradable cigarette filters is presented. The chemical degradation paths of both conventional and the previously discussed biodegradable filters is presented, including the release of any toxic microplastics or chemicals. In figure 3, an outline of the degradation pathways is presented, highlighting the part of the degradation this paper will focus on. Then, a decay model is set up for an overview of the comparison between biodegradable and conventional filters.
2. Materials and Methods
Information sourcesThe focus of this paper on the gathering of information and data was through literature research. The majority of the information that was utilized during the research originated from academic literature, with only a small amount of non-academic information sources. The non-academic information sources were however evaluated as being necessary for the research, and were utilized only if the information was deemed reliable after in-depth research on the specific topics.
For the degradation model, a quantity of parameters were derived from data presented inside papers. A second source of information for the data used in the degradation model was the supporting information provided by authors of the respective papers.
For the acquiring of academic papers, two sources were used. The first one was Web of Science, an academic literature database, and the second source was Google Scholar. The literature search was performed for a period of 3 months, from the end of April to the start of August 2020.
Within the search for literature, sources before 2005 were excluded from usage, as the information within these sources had a high probability of being outdated. The recent increase in interest towards the CB problem magnifies this effect.
During literature search in these two databases, a number of search terms were used. These search terms are “Cellulose acetate”, “Biodegradable cigarette filter”, “Plastic cigarette filter”, “Cigarette butts”, “Cigarette filters”, “Cigarette butt degradation”, “Cigarette butt chemical release”, “Cigarette biodegradation”, “Cigarette butt microplastics release”, “Cigarette butt toxicity” and “Cigarette butt degradation process”.
Kinetic decay model
To assess how fast CBs degrade in different environments, with differing conditions and ingredients, a kinetic decay model was created. Using parameters for the size, density and degradation times derived from literature, estimations were made to analyze different parameters in the CB degradation problem. By utilizing these literature parameters, chemical leaching ratios and degradation rates were estimated. The model serves the purpose of giving a quantitative overview of the different factors that influence CB degradation. The model results are divided in two sections. The first section being the degradation rates of the total CBs, measured in percentage of mass until a time of 1800 days, as this was the longest used time in literature over which a degradation experiment was conducted.22 This length is desirable for modelling, as the degradation of CBs was reported to be a long process.11,22 The second part of the modelling results analyses the leaching of chemicals from CBs, showing different conditions and their respective influence on the leaching process over time. The degradation of conventional and biodegradable filters was modelled in two environments. These environments are
soil and compost. The soil environment was selected for its similarity to the urban environments. The compost environment was selected to assess the impact of a maximum amount of microbial activity. The modelling of chemical leaching was done only for the water environment, due to a lack of literature on other environments.
For the creation of the CB degradation model, a number of formulas were used. The formulas and parameters used to create the model are explained in this section.
To acquire more detailed model outcomes, some elemental formulas need to be used. The first of these elemental formulas is the surface area of the CB. The surface area is assumed to be a perfect cylinder and modeled to retain its shape throughout the degradation process, and simultaneously keeping the ratio between length and radius identical. The degradation process is modeled as a core of cigarette butt, assuming the microbial activity only takes place at the outside of the CB due to high density fibers and smoking tars. Due to the density of fibers within CBs, combined with the tars and chemicals that persist inside these fibers, no internal biodegradation was assumed to occur.
Formula 1: 𝐴𝐶𝐵(𝑚2) = 2𝜋𝑟 ∗ 𝑙 + 2𝜋𝑟2
Formula 2 is a calculation for the CB mass at a certain time. Mass change over time is a measurement for a degradation process, and thus was calculated in the model for this paper. The surface area was used, in combination with a surface dependent degradation factor (SDDF). The SDDF is the factor, that represents the 50 percent mass % degradation time (DT50) and it is derived from literature.11,22 The derivation was performed by fitting the degradation process lengths with the 50% mass loss values that were given in the literature data. It is assumed that the SDDF is reliable for usage in the model and remains constant throughout the degradation process. The value for initial CB mass was derived from literature, and thus density was not incorporated in formula 2.5
Formula 2: 𝑚𝐶𝐵 (𝑘𝑔) = 𝑚(𝑡) − (2𝜋 ∗ 𝑟 ∗ 𝑙 + 2𝜋𝑟2) ∗ 𝑆𝐷𝐷𝐹 ∗ ∆𝑡
Formula 3 and 4 calculate the radius and length at a certain CB weight and time respectively. Length and radius were acquired from mass and density parameters, as these are literature values. Through the usage of the fact that mass divided by density equals volume, the length and radius of the CB can derived from formula 3 and 4. The shape ratio (SR) is a measurement to indicate and calculate the long and thin cylindrical shape of a CB.
Formula 3: 𝑟𝐶𝐵 (𝑚(𝑡)) = ( 𝑚(𝑡) 𝜌 𝑆𝑅∗𝜋) 1 3 Formula 4: 𝑙𝐶𝐵 (𝑚(𝑡)) = ( 𝑚(𝑡) 𝜌 𝑆𝑅∗𝜋) 1 3 ∗ 𝑆𝑅 = 𝑟 ∗ 𝑆𝑅
To assess the mass loss per timestep, the mass at time (t) is subtracted from the mass at time (t-1), as shown in formula 5.
Formula 5: 𝛥𝑚𝐶𝐵 (𝑘𝑔) = 𝑚(𝑡 − 1) − 𝑚(𝑡)
The mass percentage degraded per timestep was calculated using formula 6. This calculation is necessary for the total CB degradation, as the literature that was found on the total degradation subject used the mass percentage degradation in their data.11,14,22 This way, the scenarios and model calculations performed with the model in this research can be compared with literature models.
Formula 6: 𝛥𝑚𝐶𝐵 (%) = 𝑚(𝑡) 𝑚(0)∗ 100
The chemicals that are leached from a CB are calculated as being a certain percentage of the CB weight. This calculation will be performed on two separate groups of chemicals: PAHs, and heavy metals.
Formula 7: 𝐶ℎ𝑒𝑚𝑖𝑐𝑎𝑙 𝑟𝑒𝑙𝑒𝑎𝑠𝑒 (𝑘𝑔) = 𝛴𝐶ℎ𝑒𝑚𝑖𝑐𝑎𝑙 𝑟𝑒𝑙𝑒𝑎𝑠𝑒 (𝑡 − 1) + 𝛥𝑚 ∗ [𝑐ℎ𝑒𝑚]
The initial values, constants and parameters derived from literature are listed in table 1. The corresponding SDDFs shown in table 2 were derived from the DT50s in literature, by altering it until the SDDF corresponded with the DT50 times.
Constants and initial values used in Kinetic Decay Model
Values
Initial radius [m]7 0.003
Initial length[m]11 0.015
Density [kg][m-3]5 401.08
Shape Ratio (of CB cylinder) [m m-1] 5 Chemicals content [kg kg-1]15 0.0077
Initial CB mass [g] 0.17
Table 1: Constants from literature used in the kinetic decay model
DT50 (time until 50% remaining mass) DT50 time (days) Corresponding surface dependent degradation factor [kg m-2 s-1] DT50 Biodegradable CB10 733 4.9E-09 DT50 Conventional CB10 772 4.7E-09
Table 2: DT50 values from literature and scenario modelling
3. Results
3.1 Alternatives to conventional cigarette filters
A number of alternative compounds have been researched to potentially replace cellulose acetate as the main building block for cigarette filters. To assess whether an alternative to CA as a base for cigarette filters is suitable, besides environmental concerns, mechanical properties of the substance itself have to be noted briefly. The main properties that the filter material must possess are heat resistance during the smoking and filtering capacity for the chemicals in the smoke mixture. By incorporating additives to CA alternatives, these properties can be achieved, however at the potential cost of degradability. The most researched alternatives to conventional cigarettes are discussed in this section.
Starch-based filters
The most prominent of the researched alternatives is starch.18 Starch consists of amylose and amylopectin, with their respective structures shown in figure 2.19 Thermoplastic starch (TPS) has been developed for its properties to replace conventional petroleum-based plastics in general. It proved to be suitable in blended form with cellulose acetate, due to their similar chemical structure.18 The downsides for starch as a replacement to cellulose acetate were stated to be the poor processability. It also lacks mechanical properties and suffers from low
sheet-Figure 2: Structure of amylopectin and amylose
forming capacity.18 These downsides are mostly occurring at high humidities.18 The poor mechanical and barrier properties are a smaller concern for usage in cigarette filters, as these will not be exposed to a harsh environment before usage. These properties may even be useful, as they may cause the filter to degrade faster.
Cellulose-based filters
Cellulose filters are, to a certain extent, already commercially available.10 An example of this is GreenButts. GreenButts is a biodegradable cigarette filter brand, that claims to manufacture hemp-based cigarette filters that degrade in one week (in a compost environment) instead of in multiple years in urban environments. The extent of the degradation was however not shown. Greenbutts CBs are designed to dissolve in a water-rich environment in only minutes.20 However, cigarette filters like these are not being used to the extent that they should be, as a majority of cigarette filters still consist of cellulose acetate. This has two main causes, the first one being the mechanical strength that the cellulose acetate has, compared to more biodegradable alternatives such as cellulose.18 The second reason being that most biodegradable filters are made from hemp, a member of the cannabis family, which has been illegal in the United States until 2018.21 The cellulose filters were shown to degrade faster than the cellulose acetate filters in both a compost pile and a controlled enzyme rich environment.10,22 However, for the environment that corresponds more directly with the surroundings in which littering takes place, being the top of the soil, one source found no significant differences were found for degradation rates between CA and cellulose filters.10
Nanofibrous filters
Nanofibrous filters are a second alternative for the current cigarette filters. While it is in itself not a more biodegradable alternative to conventional cigarettes, it may help other biodegradable alternatives achieve a better filtering capacity, making alternatives more viable for replacement. Nanofibrous filters are created by the electrospinning method.23 Solution electrospinning involves a solution containing the polymer, that is exposed to a strong electric field.24 This field produces shear stress onto the polymers and solution, and if due to high enough voltage this stress becomes too strong, the solution is excreted, creating ultrathin fibers and sheets with very high surface areas.24 This method can thus create high-density polymer sheets. So far, the filtering properties of filters acquired from this method have only been tested with cellulose acetate.23 While this method rests on many different parameters to work properly due to the sensitive nature of the manufacturing process, it does provide possibilities for materials that have a surface area that is too low for filtering, to still gain the desired filtering properties. This is however not the scope of this research and no literature was found regarding this subject, so the usage is still largely hypothetical.
The current state of literature regarding alternatives for conventional cigarette filters has been presented. Next, an assessment needs to be made of the viability of these alternatives for usage and replacement of conventional cigarette filters.
3.2 Degradation pathways
After enlightening on some alternatives that may be used to decrease the potential environmental harm that CBs cause, it is useful to elaborate on the degradation pathways that the different CB types are subject to. This will be done by analyzing the processes that occur during the degradation process, as depicted in figure 3. Determining the rate-determining steps and the release of substances with potential toxicity and subsequently being able to roughly estimate their separate impacts or release quantities is important for both modelling purposes and creating more understanding in a rather undeveloped research field.
In figure 3, an overview of the degradation pathway is presented. In this section, the enzymatic or bacterial degradation, the chemical leaching, the mechanical degradation and the release of microfibers will be discussed.
Figure 3: Degradation pathway overview for CBs. *For unsmoked CBs, the release of toxic tobacco ingredients equals 0. **For biodegradable filters, microfibers may not persist long enough to be significantly harmful to the environment.
3.2.1 Enzymatic Degradation
The enzymatic degradation process of cellulose acetate filters consists of two steps, that are presented in figure 4. First, deacetylation of the separate polymer units occurs. The processes involving the enzyme mechanisms will not be discussed in this paper, as these falls beyond the scope of this research. Through a number of possible esterases, the acetate group is removed from the polymer unit.12,25 This removal occurs by the process of hydrolysis, resulting in alcohol groups remaining attached to the polymer unit and several separate acetate groups depending on the original DS of the polymer.8 The deacetylation of the cellulose acetate polymer has been determined to be the rate-determining step in the degradation process, leaving the cellulase working in the second part of the process to surmount to significantly higher rates.25
The second part within the enzymatic degradation of the cellulose acetate polymer is the cleavage of the polymer backbone.22 Through another process of hydrolysis, involving cellulases, the ester bond between the two cellulose monomers can be broken.12,25 Through further degradation processes, the entire monomer can be broken down to H2O and CO2. Due to the high surface density of the CA polymers in cigarette filters, a large majority of the enzymatic activity involved in the degradation processes takes place on the outside of the cigarette filter, and likely causing mostly monomers to be released from the backbone cleavage process. The degradation into purely monomers from the outer layer is hypothetical and needs to be evaluated and researched further.
Figure 4: Cellulose acetate degradation with complete deacetylation and a cellulose polymer intermediate.
3.2.2 Photodegradation
Photodegradation is a second very important process in cellulose acetate degradation.25 As CB litter is mostly situated on top of the soil, it can be subject to UV radiation in significant quantities. It has been stated that photodegradation of pure cellulose acetate is challenging due to its photostability.25 Photosensitizers and additives may however increase the tendency of cellulose acetate polymers to degrade under exposure from sunlight. It was also shown that higher relative humidity and higher temperatures stimulate the photodegradation process.25
3.2.3 Filter ingredients
Cigarette filter ingredients are cellulose acetate for conventional filters, or cellulose for biodegradable filters such as GreenButts, cellulose for the wrapping of the cigarette filter and some adhesives to increase the mechanical strength and filtering properties of the filter. Cellulose is readily degradable, but the cellulose acetate fibers may have an adverse effect on the environment in the form of microfibers, which is discussed in the section below.
3.2.4 Microfibers
A result of mechanical degradation is the possible release of microfibers. The initial surface area of a CB is relatively small which, in combination with the high density of the CB cellulose acetate fibers, causes the mechanical degradation to be relatively slow. After the mechanical degradation breaks the CB down into smaller pieces, enzymatic and mechanical degradation might both cause the release of microfibers in the environment. The microfibers that may be released from this process form potential threats to the environment.26 Due to the very uncertain nature of this process with regard of the CB problem, the known toxicity of CBs is considered to mainly be caused by the leaching of chemicals, that will be discussed in the next section. Due to a shortage of literature on the microfiber release from CBs it cannot be stated that it is a legitimate cause for CB toxicity.
3.2.5 Chemical leaching
A third important factor within the process described in figure 3 is chemical leaching. Through mechanical degradation, enzymatic degradation and from flushing with water, chemicals that are contained within the filter from usage are released into the environment. These chemicals are, for a significant part, toxic to the environment.14 Three important chemicals or groups of chemicals with potential harmful effects on the environment will be discussed here.
The first chemical that needs to be discussed is nicotine. The presence of nicotine in cigarettes is commonly known and it is thus logical that this chemical is also present in smoked CBs. The health effects of nicotine have been studied extensively on both humans and other organisms.14, 5 The most predominant degradation product of nicotine is cotinine, which is also regarded as a pollutant chemical. The release of nicotine from CBs into water-containing environment has been studied by Roder Green et al., showing in their experiment that after 27.2 minutes, 50% of the total nicotine in a CB was released, amounting to over 3 mg g-1 of CB.14 This quantity of nicotine in a relatively small area transcends the allowed toxic concentration threshold set by the European Union severely.14 Due to the clear surpassing of the toxicity thresholds, it should be taken into account when discussing the
toxicity of CB chemicals. However, the origin of the nicotine released into the environment that is linked to the cigarette problem was found to mainly be the remaining tobacco. Roder-Green et al. (2014) showed results that showed close to 16.2 mg nicotine g-1 tobacco concentration, compared to only 0.22 mg nicotine g-1 filter.14 This thus rules out the filter as a needed replacement for the slowing down in nicotine release to urban environments.
Polycyclic Aromatic Hydrocarbons (PAHs) are a different group of chemicals that also needs to be elaborated on regarding the release of chemicals from CBs. PAHs are persistent in the environment and bioaccumulative. PAHs are toxic to humans due to their lipophilicity when having entered the body.16 The toxicity of PAHs also shows by the affinity of bacteria and cell bodies for aromatic compounds, combined with the previously mentioned properties. Naphthalene, acenaphthylene and acenaphthene are the PAHs that are released by leaching from CBs in the greatest quantity.11 The release of PAHs has not been researched outside a water-rich environment with regard to CBs. The leaching of PAHs occurs at great quantities, with estimations of up to 5 tons of leached PAHs into the water per year globally.16
The third category of chemicals that are leached into the environment that needs to be discussed is heavy metals (HMs). The most prominent HMs, due to their toxicity and quantities present in smoked CBs, include nickel, zinc, arsenic, mercury, aluminum, copper, barium, chromium, iron, titanium, lead and cadmium.11,15,27 The metals in smoked CBs largely originate from the tobacco, due to the tendency of HMs in soil to accumulate in the tobacco plant.15,27 HMs are highly persistent in the environment but can react with other chemicals in the environment. The release of HMs from CBs has, to the extent of this research, only been researched for water-rich environments. In this water-rich environment, the leaching of HMs is highly likely to be at greater rates than leaching on the soil surface. An exception to this are heavy rainfall events, where the leaching on soil environment can approach the rates from the water environment leaching.
3.3 Kinetic decay model
The results of the kinetic decay model are presented in this section. First, the mass percentage degradation of the CB will be shown, followed by the modelling results of the chemical release from different DT50 values. Due to a lack of data on the subject of photochemical CB degradation, this was not incorporated in the modelling. No uncertainties were incorporated in figure 6 and 7, as no literature data for the standard deviations of the results could be found.
3.3.1 CB degradation rates
In figure 5, the modelled mass percentage that is left after a certain time in a soil degradation process is presented, incorporating the DT50 and subsequently the SDDF of the separate literature data. The data for this model is presented in the SI. The calculations present the degradation processes of CBs in the soil environment. In figure 5, the biodegradable CB is degraded faster than the conventional CB. “Conventional CB” is based on a DT50 degradation rate from an experiment that monitored the CB degradation on top of a soil for 1800 days, where a conventional CB was used.28 The data for “Conventional CB” and “Biodegradable CB” are created from DT50 times presented in literature.10,28
Figure 5: Modeled mass percentage degradation of biodegradable and conventional CBs in a soil environment. In figure 6, the mass percentage degradation in a compost environment is shown. The DT50 values were extracted from literature data.10 There is a clear difference in degradation rates between the biodegradable and conventional CBs. The biodegradable CB is fully degraded after 369 days, and the conventional CB is degraded after 989 days.
Figure 6: Modeled mass percentage degradation of biodegradable and conventional CBs in a compost environment.
3.3.2 Chemical leaching from CBs
The second part of the modelling regarded the leaching of chemicals from CBs. The values for figure 7 and 8 were based on literature, deriving the chemical leaching from the total estimated amount of chemicals present in a CB and correlating it to the CB mass. In figure 7, the chemical leaching of PAHs over a period of 34 days is presented. In figure 8, also the HM release in water environment
0 10 20 30 40 50 60 70 80 90 100 Ma ss Perc en ta ge le ft (% ) Time (days)
Soil Environment CB Degradation Percentage
Biodegradable CB Conventional CB 0 10 20 30 40 50 60 70 80 90 100 1 51 101 151 201 251 301 351 401 451 501 551 601 651 701 751 801 851 901 951 Ma ss Perc en ta ge le ft (% ) Time (days)
Compost Environment CB Degradation Percentage
respectively is presented. The figures for PAHs, HMs and nicotine were not combined due to their difference in order of magnitude in terms of leaching mass. The chemical leaching from CBs was only studied in water-rich environment, causing the values from this environment to incorporate less uncertainty than the chemical leaching in the compost and soil environments. The timeslot of 34 days was used for water environments, as this was the literature sample time value for the soaking of the chemicals in the experiment.15 Figure 7 and 8 were acquired by using sampling concentrations from literature.15,16 The leaching processes were assumed to be independent of the degradation process, due to the short timescales found in literature for chemical leaching. The literature values were used to create a trendline, which is depicted in both figures. The error bars originate from the original values, and represent the uncertainty in actual measured concentrations of chemicals. A mean maximum value of 23,4847 µg per CB for the HM leaching was derived from literature.15 The mean maximum value of PAHs is 0,9481 µg per CB.16 No data was found on the chemical release of biodegradable CBs, which could thus not be modeled.
Figure 7: Chemical leaching of PAHs from a CB in a water-rich environment.16
Figure 8: Chemical leaching of heavy metals from a CB in a water-rich environment.15
In the soil environment, flushing with a water-rich environment can occur at a rainfall event. This causes the leachates from CBs to be released at a much higher rate than dry CBs, approaching the leaching rates of the water environment in figure 7 and 8. This will negatively impact the surrounding soils, as it is a possibly less diluted localized leaching event with similar leaching rates.
y = 0,0191ln(x) + 0,0385 R² = 0,4845 0 0,02 0,04 0,06 0,08 0,1 0,12 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 Leach at e PAH s (µ g/C B) Time (days)
PAH Leaching from conventional CB
y = 5,8025ln(x) + 3,5629 R² = 0,8793 0 5 10 15 20 25 30 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 Leach at e H M (µg/CB ) Time (days)
4 Discussion
The first part of the modelling results showed the decline in mass at certain different environments. The second part of the results showed the chemical release in separate environments. The mass decline in soil and water environment is significantly slower than in the compost environment. The chemical release has a very high release rate in the water environment compared to the soil and compost environments.
Due to the uncertainty in the field of CB degradation, an objective and reliable conclusion about the impact of CBs on the environment is very challenging. The data found mostly uses the mass percentage loss of CBs, while leaving out important factors like surface area, mechanical and photodegradation. This mechanical degradation may severely impact the degradation rates, and thus needs to be incorporated in future models when more data for this purpose is available. If mechanical degradation was incorporated in the model, degradation rates would likely increase, chemical release would highly likely accelerate and microfiber release would likely be present.
A second part the degradation part of the model is missing is the inclusion of uncertainties. As no previous modelling has been performed on the degradation of CBs, and the literature consists of experiments rather than models, no uncertainty values could be found. If uncertainties were incorporated in the model, these would be chosen arbitrarily, not aiding the model.
A third part where the model could be improved is with the modelling of the water compartment. As no literature was available on the CB degradation in a water environment, and no experiments were performed in this subject, this important compartment could not be modelled. The incorporation of more compartments and spatial distribution could benefit the model outcomes significantly.
There are severe limitations to the methodology used in this research, as laboratory based research is required to have solid data about the chemical leaching, CB degradation and microfiber release from CBs in different environments. Not only is there very few data available about chemicals in CBs, the CB ingredients and their environmental toxicity, but the data that is available lacks specification. The papers about chemical leaching do not incorporate CB degradation, and the papers about CB degradation do not incorporate the environmental impact of chemical leachates. This is a large knowledge gap and laboratory based research needs to be performed to combine the different factors that were conceptualized in the degradation process description in this thesis. Degradation experiments over longer periods of time need to be performed, with sample analysis on microfiber release, chemical leaching, surface area increase or decrease, and a null experiment with purely mechanical degradation needs to be performed to enlighten the degradation processes, and their respective impact on the total CB degradation. Due to a lack of literature on the mechanical degradation process in CBs, this degradation was not incorporated in the kinetic decay model. The main gaps in the knowledge about biodegradable cigarette filters and their impact on the environment are assumed to be regarding both the degradation process and chemical leaching, as these two subjects require a large number of experiments and further research. A number of subjects for future experiments are suggested in this discussion.
Chemical leaching may become a degradation dependent factor if the timescales of the degradation and the chemical leaching approximate each other. The main knowledge gap regarding biodegradable filters is the mechanical degradation, as no literature was found on this subject with respect to CB degradation. With the absence of literature on mechanical degradation, a lack of standardized degradation data for CBs is also present. This makes it harder to conduct literature research and provide closing conclusions on the subject.
5 Conclusion
To provide an answer as to what extent biodegradable filters can lower the environmental impact of cigarettes, an overview of the CB ingredients, degradation processes and a degradation model was presented. The chemicals that leach from the CBs are likely to be the most impactful factor in terms of CB environmental toxicity.
With the current knowledge and some assumptions made in this paper, faster degradation rates and thus faster chemical leaching will make the chemical release more localized, which is likely to have a more severe negative effect on the environment than the gradual leaching of chemicals over a longer period of time. Apart from chemical leaching, environmental persistence will also pose a threat to organisms and the environment in general. Thus, biodegradable filters are likely to be a better alternative in terms of environmental persistence, but due to the faster chemical leaching, the faster degradation can have some negative aspects as well. If the release of microplastics from CBs is studied, this factor can also be taken into account regarding the CB problem, to comprise a total image of the biodegradable CB viability.
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