A Comprehensive Study of Biodegradation of Cigarette Filters and Bidi Butts

Cigarettes made with five different filter materials including Condensed Tobacco End (CTEC), Infused paper (IP), Cellulose acetate (CA), Combined Material Filter (CMF), DE-Tow^TM Filters (with additives) and Bidi from one of the leading brands were selected for the study.

Barium Hydroxide (Ba(OH)₂) (AR grade), Sodium hydroxide (NaOH) (AR grade), Hydrochloric acid (HCl) (32% aqueous solution, AR grade), Potassium hydrogen phthalate (C₈H₅KO₄) (AR grade), Sulphuric acid (H₂SO₄) (AR grade), Phenolphthalein indicator and pH solutions of within the range of (4.0, 7.0, 10.0) were procured from Merck (Mumbai, Maharashtra, India).

The following glassware and equipments were used in the study: Digital dual range balance (Sartorius-BSA224S-CW, Sartorius-BSA6202S-CW, Sartorius AG, Göttingen, Germany) to weigh test materials, Grinder (Philips Preethi Blue Leaf, Chennai, India) for powdering, deep freezer (Blue Star, Chennai, India), refrigerator (Samsung 230L, Seoul, Korea), orbitek shaker (Scigenic Biotech, LX-D, Kerala, India), micropipettes (Brands, Frankfurt/Main, Germany), UV-Vis spectrophotometer (Shimadzu UV-1700, Kyoto, Japan), muffle furnace (Nabertherm, Type LT 40/12 B410, Lilienthal, Germany) (set to 550 ± 10 °C for volatile solids determination), hot air oven (MMM Venticell Drying Oven, MMM Climacell, Planegg, Germany) (set to 105 ± 10 °C for determination of total dry solids), glassware (burettes, measuring cylinders, beakers, conical flasks from Borosil, Bangalore, India), customized test vessels with test jar, linear smoking machine (Cerulean SM 450, Cerulean, Milton Keynes, UK).

Water bath consisting of a series of 24 test vessels of approximately 3000 mL internal volume with a temperature-controlling system capable of maintaining the temperature of composting vessels at 58 ± 2 °C and equipped with pressurized-air system that provides air to each of the composting vessels at an accurate aeration rate. Each composting vessel was connected to a 5000-mL glass bottles consisting of barium hydroxide (Ba(OH)₂) solution for the trapping of CO₂ (Figure 2).

Figure 2

All the reagents were prepared from analytical grade chemicals and dissolved in Type-2 water with a resistivity of > 1 MΩ•cm, a conductivity of < 1 μS/cm and < 50 ppb of total organic carbons (TOCs).

Well aerated compost with sufficient porosity was obtained from a properly operating, aerobic composting plant, wherein, by controlled conditions organic plant waste is broken down through microbial action. The compost was stabilized by allowing to mature for 2 to 4 months in a well aerated environment to cease the degradation and was afterwards sieved through a screen of 0.9 cm to obtain homogeneous compost, which was used for the preparation of inoculum.

The compost was analysed for all the parameters as per the standard ISO 14855-1 (12) (total dry solids, total volatile solids and pH) and the results were all within the specified limits (Table 1). Additional parameters that characterized the compost quality such as total nitrogen and organic carbon, C/N ratio and microbial population i.e., total plate count (TPC) (22) and yeast and mould count (YMC) (23) were also evaluated.

All the cigarette and bidi samples selected for the study were smoked using a linear smoking machine. Cigarettes were smoked as per regime (puff volume: 35 cc, puff duration: 2 s, puff frequency: 60 s) following the standard ISO 3308 (24) and bidi were smoked as per regime (puff volume: 35 cm³, puff duration: 2 s, puff frequency: 30 s) following the standard ISO 17175 (25). After smoking the respective cigarette filter portion and the whole unburnt portion of bidi (1.5–2.0 cm) were powdered for further study using a grinder.

Customised glass flasks with a volume of 3000 mL with scope for even gas flow in an upward direction were used as test vessels. One set of tests contained 3 parallel vessels for the test material, blank and reference substance (microcrystalline cellulose of 20 μm) respectively. The powdered smoked cigarettes filters and smoked bidi butts were mixed with the inoculum at 1:6 ratio (weight/weight) with a total of 700 g test mixture (100 g of sample and 600 g of inoculum) and introduced into static composting vessels maintaining 50–55% moisture content. The ratio of nitrogen to organic carbon of the test mixtures were in agreement with the specifications, which ranged from 1:10 to 1:40. About 1/4^th of the total volume of test vessels was empty in order to have sufficient headspace for regular manual mixing of test materials to prevent extensive channeling and to provide a uniform attack of the microorganisms like bacteria, fungi etc.

The test vessels were intensively composted under constant flow of air, 58 ± 2 °C temperature and 50% moisture. During this process the organic portion of the cigarette filters and bidi butts undergo aerobic biodegradation by the microorganism present in the inoculum to produce CO₂, H₂O and biomass.

The carbon dioxide produced was trapped in 0.0125 M Ba(OH)₂ solution. The amount of carbon dioxide produced was determined by titrating the remaining barium hydroxide with standardized 0.05 M hydrochloric acid to determine the carbon dioxide production rate (19). The carbon dioxide produced was continuously monitored, measured at regular intervals, in test and blank vessels to determine the cumulative carbon dioxide production by successive addition of carbon dioxide over the period of time. The reaction mechanism is described as follows in the equation CO2+Ba(OH)2→BaCO3+H2OBa(OH)2+2HCl→BaCl2+2H2O \matrix{ {C{O_2} + Ba{{\left( {OH} \right)}_2} \to BaC{O_3} + {H_2}O} \hfill \cr {Ba{{\left( {OH} \right)}_2} + 2HCl \to BaC{l_2} + 2{H_2}O} \hfill \cr }

The mass of CO₂ trapped in the absorption bottle was calculated using the formula given below m(milligrams)=((2CB×VBO)/CA−VA×(VBt/VBz))CA×22 m\left( {milligrams} \right) = \left( {\left( {2{C_B} \times {V_{BO}}} \right)/{C_A} - {V_A} \times \left( {{V_{Bt}}/{V_{Bz}}} \right)} \right){C_A} \times 22 where

m is the mass of CO₂ trapped in the absorption bottle, in mg;

After determining the carbon content (%) of the test material using CHNS analyser, percentage of biodegradation was calculated as percentage of solid carbon of the test material, which was converted to gaseous mineral carbon as CO₂.

The theoretical amount of carbon dioxide (ThCO₂), which can be produced by a total oxidation of added test material and reference material, was calculated by using the formula given below ThCO2=Mt×Ct×4412 ThC{O_2} = {M_t} \times {C_t} \times {{44} \over {12}} where

ThCO₂ is the theoretical amount of carbon dioxide in g

From the accumulated amount of biologically produced carbon dioxide measured in the test vessel (CO₂) _t and in the blank vessel (CO₂) _b, the degree of biodegradation (D_t) was calculated by Dt(%)=((CO2)t−(CO2)b)/ThCO2×100 {D_t}\left( \% \right) = \left( {{{\left( {C{O_2}} \right)}_t} - {{\left( {C{O_2}} \right)}_b}} \right)/ThC{O_2} \times 100 where

D_t is the grade of biodegradation in %

An average degree of biodegradation of parallel vessels was calculated once the deviation in single measurements was less than 20%. In order to obtain a biodegradation curve, the degrees of biodegradation were plotted as a function of experimental days.

The most important part of the study according to the standard method ISO 14855-1 (12) is the validity of the test and as the criteria prescribed for validation (i.e., the degree of biodegradation of the reference material is higher than 70% after 45 days) were fulfilled for the reference (cellulose), as 70% pass level was attained in 38 days. In addition the difference between the percentage of biodegradation of the reference materials in the parallel vessels was within 2.24 – 6.35%, which is less than 20%. Blank compost produced 97.0 mg of CO₂/g of volatile solids after 10 days of incubation, this is well within the range of 50 – 150 mg CO₂/g volatile solids and complies with the standard (Table 2).

Compliance with the validity criteria also indicated the quality of compost and viability of microorganisms, which has contributed to achieve reproducible test results.

The difference in percentage biodegradation between the replicates for all the samples varied from −6.20 to 5.28%, indicating that the results were within the variation limits, i.e., less than 20% and complied with the requirement specified in the standard (Table 3) (26).

Gel Permeation Chromatography (GPC) is the most appropriate technique used to measure the absolute molecular weight of synthetic as well as natural polymers. Herein, GPC technique was considered to measure the molecular weight of the resulting compost from bio-degraded cigarette filter samples of cellulose acetate, CMF and DE-Tow^TM as well as their smoked cigarette filter samples before subjecting to biodegradation, to understand whether these filter samples have completely biodegraded. The resulting compost of Bidi butts, CTEC and infused paper filter samples were not subjected to GPC analysis, as they are made from plant materials.

The resulting compost as well as the smoked cigarette filters were extracted with acetone, dried and extracted with tetrahydrofuran (THF) for GPC analysis. A single solution of the resulting compost of all the samples and corresponding control smoked filter sample were prepared by dissolving 0.5 g of sample in 10.0 mL of THF by sonication. The GPC method was optimized for the sample matrix to produce reliable results (27).

The number average molecular weight (M_n) and weight average molecular weight (M_w) were calculated from molecular weight distributions on the basis of Waters polystyrene calibration standards of known molecular weight. The Waters GPC was run at 35 °C in THF using a column system by Styragel HR3, HR4, HR1 (5 μm, 7.8 × 300 mm), 0.75 mL/min flow and refractive index detection systems.

Data acquisition and data analysis were carried using Waters Empower 3 software.

It is evident from the cumulative degree of biodegradability of different smoked cigarette filters and bidi butts (Figure 3), that the first sample to achieve more than 90% biodegradation was CTEC filter, i.e., 95.3% in 37 days being the fastest, which can be attributed to tobacco stem, used for making these filters.

Figure 3

In the case of bidi butts, 92.1% biodegradation in 54 days was observed. Bidi, with Tendu leaf as outer wrapper was expected to degrade faster, but the result was contrary to the expectation. In order to understand the reason behind the biodegradation pattern, Bidi components were investigated, the findings indicated that the Tendu leaf used in Bidi as a wrapper has lower porosity and combustibility with very hard leaf structure unlike tobacco stem used in CTEC filters. As per literature, Tendu leaf possesses antimicrobial properties (28), which might have inhibited the microbial growth resulting in slower biodegradation rates compared to CTEC filter.

Biodegradation of 93.4% in 55 days similar to bidi butts was recorded in infused paper filters. Faster biodegradation of this filter can be attributed to processed paper being made of cellulose that is used in the production of Infused paper filter. Combined material filter attained 96.2% biodegradation in 86 days. The slower rate of biodegradation of CMF filter (86 days) compared to CTEC filter (37 days), bidi butt (54 days) and infused paper filter (55 days) can be attributed to the presence of cellulose acetate and crimped paper 60:40.

Apart from cellulose acetate, DE-Tow^TM filter contains additives, e.g., an alkaline metal oxide to facilitate faster biodegradation, and has exhibited 91.9% degradation in 97 days. This result points out that incorporation of food grade additives in the cellulose acetate matrix supports the essential microbial activity and thus accelerates the biodegradation process (29). These additives facilitate the hydrolysis of acetate groups, thereby increasing the rate of biodegradation which is reflected in the result of the biodegradation of the DE-Tow^TM filter.

Cellulose acetate filter completed 92.1% biodegradation in 151 days and indicated that cellulose acetate used in cigarette filters is biodegradable in microbially active environments. However, cellulose acetate shows a slower biodegradation rate of 151 days compared to other filter materials, probably caused by the necessity of biofilm formation of at least two different microbes, i.e. a microbe (Bacillus subtilis, Acinetobacter) with acetyl esterase enzyme, which removes the acetyl groups and a microbe (Pseudomonas) with cellulase enzymes which break down the cellulose ring (5). The present study confirmed that the cellulose acetate used in cigarette filters is undoubtedly biodegradable and thus confirming to the standard ISO 14855-1.

When characterizing cigarette filters, we have considered the polydispersity and the molecular weight, since polymers have been characterized by various definitions for molecular weight including the number average molecular weight (M_n), the weight average molecular weight (M_w), and the peak molecular weight (M_p). Polydispersity was calculated based on M_n, and M_w data.

Chromatograms for cellulose acetate filter (CA), combined material filter (CMF), DE-Tow^TM filter control and their resulting compost samples in THF recorded as signal intensity versus time response or elution time (expressed in min) are depicted in Figures 4, 5 and 6.

Figure 4

Figure 5

Figure 6

Molecular weight test results (average in daltons) from GPC analysis indicated that resulting compost of cellulose acetate filters, combined material filters and DE-Tow^TM filters have completely degraded, as no traces of these materials were found in GPC analysis. In order to confirm these results, control samples of each of the respective substances were analysed and the molecular weights are given here:

Cellulose acetate filter: 109357 daltons (Figure 4), combined material filter (CMF): 145918 daltons (Figure 5) and DE-Tow^TM Filter: 87135 daltons (Figure 6).

These result further confirmed that cellulose acetate-based cigarette filters have been completely biodegraded within 180 days under the conditions applied as per ISO 14855-1. Apart from the biodegradation study on cigarette filters and bidi butts, the resulting compost was evaluated for the presence of regulated metal content as per the list in the Indian Solid Waste Management Rules, 2016, and it was observed the concentrations of regulated metals were lower than specified limits and that they were in compliance with the rule.