A Clean Process for Obtaining High-Quality Cellulose Acetate from Cigarette Butts

materials

Article A Clean Process for Obtaining High-Quality Cellulose Acetate from Cigarette Butts

Anna De Fenzo 1, Michele Giordano 2 and Lucia Sansone 2,*

1 Department of Chemical, Materials and Production Engineering (DICMAPI), University of Napoli Federico II, p.zzale Tecchio 80, 80125 Napoli, Italy; [email protected] 2 Institute for Polymers, Composites and Biomaterials, National Research Council of Italy (CNR), 80055 Portici, Italy; [email protected] * Correspondence: [email protected]

Received: 10 September 2020; Accepted: 19 October 2020; Published: 22 October 2020

Abstract: The main purpose of this study is to introduce a modified method for recovering and recycling huge number of cigarette butts (CBs) and convert them into a fashion product. CBs are non-biodegradable waste, generally, discarded improperly into the environment. CBs consist of cellulose acetate, which can be recovered through extraction and purification processes. CBs are the most numerically frequent form of waste in the world. A relevant portion of CBS are discarded improperly into the environment. The principal filter components are plasticized cellulose acetate fibers that have the slowest degradation rate (up to years). In fact, a limiting step is the hydrolysis of cellulose acetate polymer into cellulose and acetic acid, which is extremely slow under ambient conditions and represents a relevant environmental risk. A number of studies have been realized on recycling cigarette butts with encouraging results, and several methods have been studied, including recycling of cigarette butts in asphalt concrete and fired clay bricks, as a carbon source, sound-absorbing material, corrosion inhibitor, biofilm carrier, and many more. In this study, we propose a novel, green, low cost, simple, and efficient extraction method of cellulose acetate fibers (CA) from discarded cigarette butts (DCBs). CBs extraction procedure involves a two-step process consisting of washings in water and some subsequent washings in ethanol. The obtained samples of CA are dried at 60 ◦C for 60 min in the oven. The quality and properties of cellulose acetate extracted and purified are comparable to the pure cellulose acetate fiber used for cigarette butts. The preliminary results obtained on the recovered CA look promising to the use of this recovery material from cigarette butts to obtain a wide consumption fashion product, such as eyeglass frames.

Keywords: cigarette butts; cellulose acetate; extraction process; recovery

1. Introduction Cigarette butts are recognized as toxic residue, thanks to containing chemical contaminants and residues produced during combustion reaction; they are frequently improperly disposed, thrown on the ground or on beaches. This way, contaminants of cigarette butts are going to be carried by rain into surface water, in agriculture land and thereby pollute the environment and ecosystems. Generally, cigarettes have a length of 85 or 100 mm, and a diameter of about 8 mm, they are made of a filter, tobacco, additives, and cigarette wrapper. Cigarette Filters (CFs) usually have a length of 20 to 30 mm long, so a typical cigarette has 55 to 80 mm of tobacco. The role of the filter is to reduce the quantity of smoke, tar, and fine particles inhaled during the combustion and to decrease the harshness of the smoke, and prevent the formation of tobacco flakes in the smoker’s mouth. The filter captures and retains toxic substances; they also prevent tobacco from entering a smoker’s mouth and supply a mouthpiece that may not collapse because the cigarette is smoked. Filters are generally

Materials 2020, 13, 4710; doi:10.3390/ma13214710 www.mdpi.com/journal/materials Materials 2020, 13, 4710 2 of 13 composed of a plug of acetate cellulose filter tow; the cellulose acetate esters are white and packed tightly together to make a filter. CFs are made from cellulose acetate fibers (CA), which are arranged as microscopic-sized white fibers massed together by glycerol triacetate, they are poorly degradable, with a Y-shaped cross-section that is not perpendicular to the flow. These fibers are synthetic plastic which is similar to cotton. To attach the plug to the cigarette wrapper is used a vinyl resin emulsion. Moreover, the filter plug is protected by a tipping paper, that has the role to connect the filter to the column of tobacco, and not to attach to the lips of smokers. The tobacco leaves have different colors, tastes, burning properties, aromas, and nicotine content, depending on the kind of tobacco and its growing location. Tobacco leaves contain several alkaloids, including nicotine. Nicotine is a toxic alkaloid that causes addiction in smokers and it is a strong insecticide. According to the US Department of Health and Human Services (USDHHS), nicotine raises blood pressure, affects the central nervous system, and constricts blood vessels in humans. Nicotine is a colorless liquid that is soluble in water and is readily absorbed through the skin in its pure form. Moreover, a hundred types of additives are mixed with tobacco during the manufacturing process. Tobacco additives include flavorings such as cocoa, rum, licorice, sugar, and fruit juices, and humectants that are used to keep tobacco moist. The tar is constituted by the substances and additives found in tobacco. These substances are composed of organic and inorganic chemicals, including some carcinogens. According to the USDHHS, smokers are exposed to a toxic mix of over 7000 chemicals when they inhale cigarette smoke. Generally, the paper used to wrap the tobacco is composed of flax or linen fiber; moreover, to control or accelerate the burning rate various chemical substances are added to the paper, such as salts, ammonium phosphate, and sodium and potassium citrates. Moreover, calcium carbonate is added to the paper to ensure the creation of attractive ash as the cigarette burns. The wrappers’ seams are glued with an adhesive that is a modified starch or natural gum [1–3]. Although cellulose acetate is a photodegradable polymer, it is not easily biodegradable. It may persist in the environment for years. As already mentioned, this fibrous material is meant to trap tar and other toxic elements during smoking, but when are left on the environment, they can release these substances and this effect implies an increase of risks for the environment. Moreover, in marine ecosystems cigarette butts (CBs) have a big negative impact because the chemicals they contain pose a risk to the organisms of both freshwater and marine environments [4–7]. The recycling of CBs is difficult as there do not seem to be any easy mechanisms or procedures to assure an efficient and economic separation of the butts, or appropriate treatment of the entrapped chemicals. For this aim, different research groups are concentrating on the likelihood of using cigarette butts within various applications that can be classified into different categories: (1) recover the cellulosic fibers in form of nanofiber/nanocrystalline cellulose (NCC), or direct blend into paper products; (2) additives for manufacturing other materials such as bricks and steel; (3) devices for energy storage; (4) electronic components; (5) chemical and medical components; and (6) alternative materials for sound absorption. Zhao et al. demonstrated the use of recovered cigarette butts in the metallurgical industry as corrosion inhibitor for steel in acid solution [8]. In fact, they show that cigarette butts extracted from water have the flexibility to inhibit corrosion and this aptitude increases with increasing CBs concentration; while Mohajerani et al. realized different types of clay bricks with several weight percentages (from 0% to 10% w/w) of CBs by mechanical mixing followed by firing at 1050 ◦C; the fired samples were finally tested for compressive strength, flexural strength, density, and water absorption [9,10]. Ghosh et al. displayed how pyrolysis treatment produces a material with good conductive properties which can be further used in different electronic fields [11]; while Gomez Escobar et al. demonstrated the high sound absorption performance of this material and the potential use of CBs as a component for insulating solutions, which were observed to be competitive even with sale solutions [12]. Ogundare et al. produced NCC from discarded cigarette filters (DCF) [13]. The DCF were treated through an ethanolic extraction, a bleaching in sodium hypochlorite, followed by an alkaline deacetylation, and converted into NCC by sulfuric acid hydrolysis. The study demonstrated that the produced NCC has such a high quality that it could be applied in the field of catalysis or in the biomaterial area. Teixeira et al. Materials 2020, 13, 4710 3 of 13 developed a cellulose pulp production process from cigarette butts employing an alkaline pulping [14]. Moreover, Abu–Danso et al. studied a new method to recover of cellulose from CBs in the form of cellulose nanofibers and cellulose nanocrystals for removing diclofenac pharmaceutical from water [15]. The great versatility of cellulose acetate outlines strong attention to the problem of recycling cigarette butts. Furthermore, due to the problems related to the thermo-oxidative degradation of this product, it is essential to determine operating conditions and energy parameters that can determine a longer life of products made thanks to recycled materials [16–19]. In the present work, cigarette filter extraction by means of water and ethanol is a safe and conservative means to reduce filter waste and the recovered CA can be used for obtaining other products. To evaluate the quality of recovered CA, it is characterized by thermogravimetric analysis in the inert and oxidant ambient and compared to unused CA. Moreover, differential calorimetric analysis has been performed to define the glass transition temperature and the possible crystallinity degree of both the samples and FTIR spectroscopy has been executed to delineate the functional groups after the cleaning process and to determine if any difference were present on the recovered CA respect to the unused specimen. Analyzing onset temperatures and mass loss percentages, respectively, in both the used test ambient were adopted two different modeling methods (Kissinger and the Flynn-WallOzawa (FWO) method) [20]. The Kissinger procedure is proposed to estimate activation energies (Ea) in unused CA compared to recovered CA, in which the relative crystallinity, measured at constant heating rates, can be correlated by the Ozawa model with a temperature-independent exponent. The Ea value obtained for unused CA in different test conditions is similar to Ea values of recovered CA; this means that the recovered CA has as degradation kinetics similar to the one of the unused CA. Finally, the extraction and purification reactions do not affect the CA. We investigated, also, the presence of metals, in particular heavy metals, in recovered samples by means of atomic absorption analysis. Moreover, with the recovered CA we have realized a prototype of eyeglass frames.

2. Experimental

Materials and Methods Used cigarette filters have been collected by Essequadro eyewear Company S.r.l. (Ariano Irpino, Italy). The used cigarette butts were collected from the ashtrays of the local bar and restaurant area of Essequadro Company in Avellino (Ariano Irpino, Italy) and sent to CNR Laboratories in Portici (Portici, Italy), where the cigarette butts, have been sterilized and treated (recovered CA). As comparing material, we have chosen the Rizla cellulose acetate filters (Rizla, Riz La Croix, France). Rizla is a French brand that produces rolling papers and other related paraphernalia, among these also the cellulose acetate filters, in which tobacco, or marijuana, or a mixture, is rolled to make handmade joints and cigarettes. Morphology, thermal, and chemical properties of the Rizla untreated acetate cellulose filter (unused CA) have been compared to the ones of the recovered CA by means of our extracted methods. Our CA extraction method is based on the washing off the discarded cigarette butts (DCBs) in hot water (50 ◦C) for 60 min, after external paper removal, CBs have been washed in cold water three times, so to extend CA fiber. Successively to remove potential organic compounds the butts have been washed in ethanol 99% w/w twice. Finally, the obtained samples of CA were dried at 60 ◦C for 60 min in the oven. Both structural/morphological and thermodynamic/functional properties of unused CA and recovered CA have been studied by means of several experimental techniques to assess the possible material changes. The test analyses have been conducted on five samples of untreated and on five samples of recovered CA. The morphological properties of samples have been analyzed out by means of an optical microscopy analysis, (OLYMPUS BX51 microscope, Olympus, VA, USA) and by means of a scanning electron microscopy (SEM) using a field emission instrument Quanta 200 FEG. Samples were covered with a Materials 2020, 13, 4710 4 of 13 layer of gold/palladium alloy in a high resolution metalized Emitech K575X. X-ray microanalysis was carried out by EDX Inca Oxford 250 instrument. The thermodynamic properties have been evaluated by means of a differential scanning calorimetry (DSC) TA Instruments Discovery DSC (TA Instruments, New Castle, DE, USA using a double scan at 10 C/min from 40 C to 250 C) and by means of thermogravimetric analysis (TA Instruments ◦ − ◦ ◦ Q500 TGA, TA Instruments, New Castle, DE, USA) under dynamic conditions with temperature ramp of 5, 7.5, 10, 15, and 20 ◦C/min from ambient to 800 ◦C in inert and oxidant atmosphere. Sample mass was approximately 10 mg. Collected TGA data have been modelled by Kissinger and Flynn–Ozawa–Wall method. In fact, for dynamic TGA measurements, mass loss is monitored as function of temperature at different heating rates. Kinetics degradation in its general form can be modelled as follows:

dα = f(α(T)) (1) dt The degradation kinetic analysis was done using the Kissinger and Flynn–Wall–Ozawa methods. The Kissinger method is based on the following Equation ! ! β AR E ln ln a (2) 2 T − Ea g(α) − RT × where α = fraction of conversion (defined as mass loss at respective temperature), A = pre-exponential factor, and g(α) = algebraic expression for integral methods [20–22]. From the TGA curves recorded at different heating rates β, temperatures T were determined at the conversions a = 10–90%. The activation energies were calculated from the slope of the straight lines ln(β/T2) versus 1/T. The second method is Flynn–Wall–Ozawa (FWO) method [23,24] is a conversional linear method based on the following equation E ln β c 0.4567 a (3) − − RT 1 where β = heating rate in K min− , c is a constant, T = temperature in K, Ea = activation energy in kJ/mol, and R = universal gas constant. The plot log β versus 1/T, obtained from TGA curves recorded at several heating rates, should be a straight line. The activation energy can be evaluated from its slope. The approximation on which both methods are based is that each degradation step can be considered as a reactive process of the first order and the first derivative function results constant [20,25–28]. The Kissinger and Flynn–Wall–Ozawa methods for calculating the activation energy have the advantage that they do not require knowledge of the reaction mechanism. Moreover, to understand the chemical structure of samples, Fourier-transform infrared spectroscopy (FTIR) have been used (System 2000 FT-IR, Perkin–Elmer, Waltham, MA, USA). In addition, metals in the samples have been analyzed by means of an atomic absorption spectrometer (A PerkinElmer Analyst 700 atomic absorption spectrometer, Norwalk, CT, USA).

3. Results and Discussion

3.1. Morphological Characterization In Figure1a the images of unused and recovered CA samples, acquired by optical microscopy, are reported. It is observed that the volume of the constituent ﬁbers of specimens, is reduced by the cleaning process. Materials 2020, 13, 4710 5 of 13 Materials 2020, 13, x FOR PEER REVIEW 5 of 13

Figure 1. 1. (a()a )Optical Optical microscopy microscopy image image (50×) (50 of) ofcellulose cellulose acetate acetate fibers fibers (CA) (CA) unused unused (left) (left) and andCA × recoveredCA recovered (right); (right); (b) SEM (b) SEMmicrographs micrographs for CA for unused CA unused (left) (left)and CA and recovered CA recovered (right) (right) at 800× at magnification.800 magnification. × In FigureFigure1 b,1b, the the SEM SEM micrographs micrographs for unusedfor unused and recoveredand recovered CA samples CA samples are reported. are reported. It is possible It is possibleto observe to that observe the fibers that constitutingthe fibers constituting the cigarettes the filters, cigarettes after the filters, recovering after the process, recovering appear process, with an appearirregular with surface. an irregular surface. In Table 1 1 we we reported reported the the compositional compositional analysisanalysis ofof recoveredrecovered cigarettecigarette buttsbutts comparedcompared toto unused cigarette filters.filters. TheThe concentration concentration of of titanium titanium is is the the same same for for recovered recovered CA CA and and untreated untreated CA. CA.Generally, Generally, Titanium Titanium gives thegives white the color white to cigarettecolor to filters, cigarette and asfilters, already and mentioned as already nanomaterials mentioned nanomaterialsmade from titanium made dioxide from titanium are used dioxide in cigarette are used filters in to cigarette significantly filters reduce to significantly the number reduce of harmful the numberchemicals of inhaled harmful by chemicals smokers. inhaled The % wby/w smokers.of element The reported % w/w in of Table element1 is a reported mean value in Table calculated 1 is a meanon diff valueerent samplescalculated analyzed. on different Based samples on the dataanalyzed. reported Based in Tableon the1, wedata investigated reported in the Table presence 1, we investigatedor absence of the metals presence and heavyor absence metals of inmetals recovered and heavy CA. We metals examined in recovered the presence CA. We of examined heavy metals the presencein recovered of heavy CA by metals flame atomicin recovered absorption CA by spectrometer. flame atomic The absorption results recorded spectrometer. the absence The ofresults high recordedconcentrations the absence of heavy of metals high concentrations and metals (limit of ofheavy detection metals of theand instrument, metals (limit lower of detection than 0.01 mgof the/L), instrument,only 0.04 ppm lower of titaniumthan 0.01 wasmg/L), detected. only 0.04 Micevska ppm of titanium et al. supposed was detected. that the Micevska toxicity et of al. butt supposed leaches thatis attributed the toxicity to metals of butt and leaches heavy is attributed metals. The to presencemetals and of heavy metals metals. and, in The particular, presence heavy of metals metals and, in incigarettes particular, is imputed heavy metals to the in growth cigarettes and cultivationis imputed ofto tobacco, the growth soil and contamination, cultivation useof tobacco, of pesticide soil contamination,and herbicide, cigaretteuse of pesticide manufacturing and herbicide, process, cigarette and the manufacturing use of brightening process, agents and on the the use paper. of brighteningThe presence agents of Pb andon the Cd paper. is extremely The presence varied inof severalPb and cigaretteCd is extremely brands [varied29–31]. in several cigarette brands [29,30,31]. Table 1. Chemical composition related to the EDX microanalysis.

Table 1. ChemicalSample composition Element related to the % wEDX/w microanalysis. Sample ElementC % 49.3 w/w CA unused OC 50.549.3 CA unused TiO 50.5 0.2 CTi 50.70.2 CA recovered OC 49.150.7 CA recovered TiO 49.1 0.2 Ti 0.2

Materials 2020, 13, x FOR PEER REVIEW 6 of 13

3.2.MaterialsMaterials Thermal 20202020 ,,Degradation 1133,, x 4710 FOR PEER Study REVIEW 66 of of 13 13

3.2. ThermalThe unused Degradation and recovered Study CA were characterized by TGA and DSC analysis. In the Figure 2a DSC3.2. thermograms Thermal Degradation in the first Study heating scan are reported for the unused and recovered CA; the unused The unused and recovered CA were characterized by TGA and DSC analysis. In the Figure 2a and recoveredThe unused samples and recovered present a CA water were desorption characterized peak by in TGA the range and DSC 25–100 analysis. °C and, In thethe Figuremelting2a DSC thermograms in the first heating scan are reported for the unused and recovered CA; the unused temperatureDSC thermograms is near in225 the °C ﬁrst [32]; heating the unused scan areCA reported in the temperature for the unused range and between recovered 100°C CA; and the 210 unused °C and recovered samples present a water desorption peak in the range 25–100 °C and, the melting showsand recovered a broad peak samples due presentto the elimination, a water desorption probably, peak of an in additive the range or 25–100 binder ofC and,commercial the melting CA temperature is near 225 °C [32]; the unused CA in the temperature range between◦ 100°C and 210 °C cigarettetemperature filter. is In near Figure 225 2bC[ DSC32]; thermograms the unused CA in inthe the second temperature heating rangescan are between reported 100 forC the and unused 210 C shows a broad peak due◦ to the elimination, probably, of an additive or binder of commercial◦ CA◦ CAshows and arecovered broad peak CA due samples; to the the elimination, measured probably, glass transition of an additive for unused or binder CA is of193 commercial °C, while for CA cigarette filter. In Figure 2b DSC thermograms in the second heating scan are reported for the unused recoveredcigarette ﬁlter.CA is In192 Figure °C. 2b DSC thermograms in the second heating scan are reported for the unused CA and recovered CA samples; the measured glass transition for unused CA is 193 °C, while for CA and recovered CA samples; the measured glass transition for unused CA is 193 C, while for recovered CA is 192 °C. ◦ recovered CA is 192 ◦C.

Figure 2. (a) differential scanning calorimetry (DSC) thermograms in the first heating scan of unused

CA and recovered CA (b) DSC thermograms in the second heating scan of unused CA and recovered Figure 2. (a) differential scanning calorimetry (DSC) thermograms in the first heating scan of unused CA CA.Figure 2. (a) differential scanning calorimetry (DSC) thermograms in the first heating scan of unused CAand and recovered recovered CA CA (b) DSC(b) DSC thermograms thermograms in the in secondthe second heating heating scan scan of unused of unused CA CA and and recovered recovered CA. InCA. Figure 3a TGA thermograms are reported at 10 °C/min scan rate under the nitrogen inert flow In Figure3a TGA thermograms are reported at 10 C/min scan rate under the nitrogen inert and corresponding derivative and second derivative curves◦ for unused CA sample are reported. It is flow and corresponding derivative and second derivative curves for unused CA sample are reported. possibleIn Figure to observe 3a TGA that thermograms in the temperature are reported range at from 10 °C/min ambient scan to rate 200 under °C one the degradation nitrogen inert step flow is It is possible to observe that in the temperature range from ambient to 200 C one degradation step is presentand corresponding with 10% of derivative mass loss andcorrelated. second Thisderivative mass losscurves is distributed for unused inCA two◦ sample different are reported. contributes: It is present with 10% of mass loss correlated. This mass loss is distributed in two different contributes: Thepossible first liedto observe to the sample that in dehydration the temperature with rangearound from 2% ofambient mass lost to 200and °C the one second degradation one lied stepto the is The first lied to the sample dehydration with around 2% of mass lost and the second one lied to the decompositionpresent with 10% of plasticizer of mass loss compound correlated. present This massin the loss process is distributed of filters realization in two different (8% mass contributes: loss). In decomposition of plasticizer compound present in the process of filters realization (8% mass loss). theThe temperature first lied to therange sample from dehydration250 °C to 450 with °C, the around degradation 2% of mass step lostwith and 80% the of second mass lost one took lied place; to the In the temperature range from 250 C to 450 C, the degradation step with 80% of mass lost took place; thisdecomposition degradation of step plasticizer concerns compound the ◦deacetylation present◦ in process the process of CA of with filters maximum realization rate (8% of massdegradation loss). In this degradation step concerns the deacetylation process of CA with maximum rate of degradation correspondingthe temperature to rangethe temperature from 250 °C of to367 450 °C. °C, The the residue degradation of measurements step with 80% is 9% of ofmass initial lost weight took place; [13]; corresponding to the temperature of 367 C. The residue of measurements is 9% of initial weight [13]; whilethis degradation in Figure 3b step TG concernscurves at the 10 d°C/mineacetylation◦ under processthe inert of flow CA withand correspondingmaximum rate derivativeof degradation and while in Figure3b TG curves at 10 C/min under the inert flow and corresponding derivative and secondcorresponding derivative to thecurves temperature for recovered of ◦367 CA °C. sample The residue are reported. of measurements Compared is to 9% the of previousinitial weight graph, [13]; it second derivative curves for recovered CA sample are reported. Compared to the previous graph, it is iswhile possible in Figure to observe 3b TG that curves the first at 10 degradation °C/min under step thenoticed inert for flow unused and corresponding CA, in this case derivative is absent. and secondpossible derivative to observe curves that the for first recovered degradation CA sample step noticed are reported. for unused Compared CA, in to this the case previous is absent. graph, it is possible to observe that the first degradation step noticed for unused CA, in this case is absent.

Figure 3. (a) TGA(a Thermograms) on unused CA sample (10 ◦C/min, nitrogen(b) ﬂow); (b) TGA

FigureThermograms 3. (a) TGA on recovered Thermograms CA sample on unused (10 ◦C/ min, CA nitrogen sample (10 ﬂow). °C/min, nitrogen flow); (b) TGA Thermograms on recovered(a) CA sample (10 °C/min, nitrogen flow). (b) FigureThe green 3. (a process) TGA Thermograms carried out on on the unused dirty ﬁlters, CA sample erase (10 the °C/min, traces of nitrogen plasticizers flow); in (b the) TGA cellulose acetateThermograms structure. on The recovered principal CA degradation sample (10 °C/min, step occurs nitrogen in theflow). same temperature range (250 450 ◦C) −

Materials 2020, 13, x FOR PEER REVIEW 7 of 13 Materials 2020, 13, x FOR PEER REVIEW 7 of 13 MaterialsThe2020 green, 13, 4710 process carried out on the dirty filters, erase the traces of plasticizers in the cellulose7 of 13 acetateThe structure. green process The principal carried out degradation on the dirty step filters, occurs erase in the the same traces temperature of plasticizers range in the(250 cellulose−450 °C) acetateand the structure. mass lost The in this principal degradative degradation step is aboutstep occurs 85%. The in the residue same oftemperature the measurement range (250 is about−450 10%°C) and the mass lost in this degradative step is about 85%. The residue of the measurement is about 10% andof the the initial mass weight.lost in this The degradative cellulose acetate step is unused about 85%. is burned The residue within of 1.83 the min measurement and the thermostability is about 10% of the initial weight. The cellulose acetate unused is burned within 1.83 min and the thermostability ofof the the initial CA is weight. not affected The cellulose by the processacetate unusedsince the is burnedburning within time values1.83 min are and the the same thermostability for both the of the CA is not affected by the process since the burning time values are the same for both the ofinvestigated the CA is samples.not affected by the process since the burning time values are the same for both the investigated samples. investigatedMoreover, samples. to confirm our data in Figure 4 we reported, the thermogravimetric curves of unused Moreover, to confirm our data in Figure4 we reported, the thermogravimetric curves of unused CA atMoreover, different to heating confirm rates our anddata corresponding in Figure 4 we reported,DTG curves the underthermogravimetric the nitrogen curvesflow. Mass of unused is lost CA at different heating rates and corresponding DTG curves under the nitrogen flow. Mass is lost with CAwith at increasingdifferent heating temperature rates and rate corresponding yielding earlier DTG degradation curves under onsets. the nitrogenIt can be flow. noticed Mass that is lost the increasing temperature rate yielding earlier degradation onsets. It can be noticed that the degradation withdegradation increasing step temperature related to the rate plasticizer yielding decomposition earlier degradation is independent onsets. It from can thebe heatingnoticed scanthat rate.the step related to the plasticizer decomposition is independent from the heating scan rate. DTG curves degradationDTG curves stepdisplay related that tomaximum the plasticizer peak temperatures decomposition are is moderately independent dependent from the upon heating heating scan rates;rate. display that maximum peak temperatures are moderately dependent upon heating rates; while Figure5 DTGwhile curves Figure display 5 shows, that respectively, maximum peak the TG temperatures (a) and DTG are (b) moderately curves related dependent to the recoveredupon heating sample rates; at shows, respectively, the TG (a) and DTG (b) curves related to the recovered sample at different heating whiledifferent Figure heating 5 shows, rates respectively, under the nitrogen the TG inert(a) and flow. DTG It was (b) curves observed related that tothe the absence recovered of degradation sample at rates under the nitrogen inert flow. It was observed that the absence of degradation step of plasticizers differentstep of plasticizers heating rates is underindependent the nitrogen from inertthe heatingflow. It wasrate observedand the deacetylationthat the absence step of isdegradation somewhat is independent from the heating rate and the deacetylation step is somewhat dependent from the stepdependent of plasticizers from the is scan independent rate. Moreover, from the the heating residue rate of all and the the experiments deacetylation carried step out is somewhaton unused scan rate. Moreover, the residue of all the experiments carried out on unused and recovered CA is dependentand recovered from CA the is scan independent rate. Moreover, from the the applied residue scan of rateall the value. experiments carried out on unused andindependent recovered from CA is the independent applied scan from rate the value. applied scan rate value.

Figure 4. Thermogravimetric curves in nitrogen flow (a) unused CA at different heating rate; (b) DTG Figure 4. Thermogravimetric curves in nitrogen ﬂow (a) unused CA at diﬀerent heating rate; (b) DTG Figurecurves 4. for Thermogravimetric unused CA. curves in nitrogen flow (a) unused CA at different heating rate; (b) DTG curvescurves for for unused unused CA. CA.

FigureFigure 5. Thermogravimetric 5. Thermogravimetric curves curves in nitrogen in nitrogen flow ( aflow) TGA (a curves) TGA forcurves recovered for recovered CA at di ffCAerent at heatingFiguredifferent rate; 5. (b heatingThermogravimetric) DTG curves rate; (b for) DTG recovered curves curves CA infor nitrogen atrecovered different flow CA heating (ata) different TGA rate. curves heating for rate. recovered CA at different heating rate; (b) DTG curves for recovered CA at different heating rate. Degradation behavior of the unused CA was also tested in oxidant atmosphere (air flow) and Degradation behavior of the unused CA was also tested in oxidant atmosphere (air flow) and TGA and DTG curves are reported in Figure6. Figure7a,b displays thermogravimetric analysis TGADegradation and DTG curves behavior are reportedof the unused in Figure CA 6.was Figure also 7a,btested displays in oxidant thermogravimetric atmosphere (air analysis flow) and for for recovered CA sample under oxidant conditions. Two different degradation steps in the whole TGArecovered and DTG CA curvessample are under reported oxidant in Figure conditions. 6. Figure Two 7a,b different displays degradation thermogravimetric steps in analysis the whole for investigated temperature range are identified. Slight deviation in the temperature onset with heating recoveredinvestigated CA temperature sample under range oxidant are identified. conditions. Slight Two deviation different in the degradation temperature steps onset in with the heatingwhole rate has been observed corresponding to the deacetylation process (200 C–400 C). The charring stage investigatedrate has been temperature observed correspondingrange are identified. to the Slight deacetylation deviation process in the temperature (200◦ °C–◦400 onset °C). withThe charringheating from 400 C to 550 C results unaffected by heating rate. The mass loss in this degradation step is 18% ratestage has from been◦ 400 observed °C to◦ 550 corresponding °C results unaffected to the bydeacetylation heating rate. process The mass (200 loss °C –in400 this °C). degradation The charring step for all the applied scan rates. The char yield obtained at 700 C, in nitrogen flow, is slightly dependent stageis 18% from for 400all the °C toapplied 550 °C scan results rates. unaffected The char by yield heating obtained rate.◦ Theat 700 mass °C, loss in nitrogen in this degradation flow, is slightly step on heating rate, while in the oxidant atmosphere, the mass loss is total at 600 C and it is not dependent isdependent 18% for all on the heating applied rate, scan while rates. in theThe oxidant char yield atmosphere, obtained theat 700 mass °C, loss in ◦ nitrogenis total at flow, 600 °C is slightlyand it is dependenton the heating on heating rate. Moreover, rate, while from in the the oxidant ambient atmosphere, temperature the to themass 220 loss◦C, is the total identified at 600 °C step and under it is

Materials 2020, 13, x FOR PEER REVIEW 8 of 13 Materials 2020, 13, x FOR PEER REVIEW 8 of 13 Materials 2020, 13, 4710 8 of 13 not dependent on the heating rate. Moreover, from the ambient temperature to the 220 °C, the not dependent on the heating rate. Moreover, from the ambient temperature to the 220 °C, the identified step under nitrogen flow is related to the dehydration process and plasticizer degradation. identifiednitrogen flow step isunder related nitrogen to the flow dehydration is related processto the dehydration and plasticizer process degradation. and plasticizer The degradation. total weight The total weight loss in this temperature range is approximately 10% w/w and result independent Theloss total in this weight temperature loss in this range temperature is approximately range is 10% approximatelyw/w and result 10% independent w/w and result from independent the heating from the heating rate. Moreover, under these conditions, in the temperature range from 250 °C to 600 fromrate. the Moreover, heating under rate. Moreover, these conditions, under these in the conditions, temperature in rangethe temperature from 250 Crange to 600 fromC, 250 two °C di fftoerent 600 °C, two different degradation steps are traceable. The deacetylation process◦ occurs◦ with maximum °C,degradation two different steps degradation are traceable. steps The are deacetylation traceable. The process deacetylation occurs with process maximum occurs peak with temperatures maximum peak temperatures increasing with the heating rate whereas the second stage (charring stage) is peakincreasing temperatures with the heatingincreasing rate with whereas the theheating second rate stage whereas (charring the stage) second is characterized stage (charring by anstage) almost is characterized by an almost constant starting temperature of 400 °C with an end decomposition offsets characterizedconstant starting by an temperature almost constant of 400 startingC with temperature an end decomposition of 400 °C with offsets an between end decomposition 500 C and 550offsetsC, between 500 °C and 550 °C, showing◦ an irregular increasing trend with the scan rate. A◦ mass loss◦ of betweenshowing 500 an irregular °C and 550 increasing °C, showing trend an with irregular the scan increasing rate. A mass trend loss with of 16% the inscan this rate. last A step, mass is noticed loss of 16% in this last step, is noticed independently from heating rate. 16%independently in this last fromstep, heatingis noticed rate. independently from heating rate.

Figure 6. Thermogravimetric curves in air flow (a) unused CA at different heating rate; (b) DTG FigureFigure 6. Thermogravimetric curves curves in in air air ﬂow flow (a) ( unuseda) unused CA CA at di atﬀerent different heating heating rate; ( brate;) DTG (b) curves DTG curves for unused CA. curvesfor unused for unused CA. CA.

Figure 7. Thermogravimetric curves in air flow flow ( a) TGA curves for recovered CA at different different heating Figure 7. Thermogravimetric curves in air flow (a) TGA curves for recovered CA at different heating rate; (b)) DTG curves forfor recoveredrecovered CACA atat didifferentfferent heatingheating rate. rate. rate; (b) DTG curves for recovered CA at different heating rate. Kissinger method has been employed for all TGA scans performed on unused CA and recovered Kissinger method has been employed for all TGA scans performed on unused CA and recovered CA, in order to to evaluate evaluate the the activation activation energy energy of of each each degradation degradation step step in inair air and and nitrogen nitrogen flow. flow. A 2 CA,A comparison in order to between evaluate Kissinger the activation ln(β/Tmax energy) versus of each 1/T degradationmax curves for step the in degradation air and nitrogen stage identified flow. A comparison between Kissinger ln(β/Tmax2) versus 1/Tmax curves for the degradation stage identified comparison between Kissinger ln(β/Tmax2) versus 1/Tmax curves for the degradation stage identified for bothboth the the analyzed analyzed samples samples in in the the inert inert and and oxidant oxidant ambient ambient allowed allowed to collect to collect data discussed data discussed below. for both the analyzed samples in the inert and oxidant ambient allowed to collect data discussed Regardingbelow. Regarding to the inert to the environment inert environment (see Figure (see8 ),Figure the deacetylation 8), the deacetylation process ofprocess the unused of the CAunused sample CA below. Regarding to the inert environment (see Figure 8), the deacetylation process of the unused CA appearssample appears to occur to with occur a greater with a energygreater value energy than value the than recovered the recovered sample (67sample KJ/mol (67 for KJ/mol unused for CAunused and sample appears to occur with a greater energy value than the recovered sample (67 KJ/mol for unused 61CA KJ and/mol 61 for KJ/mol recovered for CA).recovered It is worth CA). notingIt is worth that the noting dehydration that the and dehydration the plasticizer and decompositionthe plasticizer CA and 61 KJ/mol for recovered CA). It is worth noting that the dehydration and the plasticizer takedecomposition place for the take unused place samplefor the unused and the sample energy contributionand the energy of contribution endothermic of process endothermic of dehydration process decomposition take place for the unused sample and the energy contribution of endothermic process couldof dehydration be recovered could with be greaterrecovered effort with to greater complete effort the deacetylationto complete the process. deacetylation process. of dehydration could be recovered with greater effort to complete the deacetylation process.

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FigureFigure 8. 8. ComparisonComparison between between Kissinger Kissinger data data obtained obtained for for the the principal principal degradationdegradat degradationion stage.

AsAs expected, expected, the the activation activation energy energy values, values, computed computed from from the the measurements measurements carried carried out out under under airair flow flow (see Figure 99a,b),a,b), confirmconfirm thatthat thethe recoveryrecovery processprocess providesprovides cellulosecellulose acetateacetate equivalentequivalent perfectly to the never never used used cellulose cellulose acetate acetate from from cigarettesci cigarettesgarettes filter. filter. In In fact, fact, the the activation activation energy energy needed needed forfor deacetylationdeacetylation process,process, for for both both the the investigated investigated sample sample is equalis equal to 40 to KJ 40/mol KJ/mol and theand charring the charring stage stageoccur occur with anwith energetic an energetic contribution contribution of 75 KJof / 75mol KJ/mol indiff erentlyindifferently for unused for unused and recovered and recovered CA. CA.

FigureFFigureigure 9. 9. KissingerKissinger data data obtained obtained for for the the deacetylation deacetylation step step ( (aa))) and and charring charring stage stage ( (bb))) inin in air air flow.flow. ﬂow.

TheThe FOW integral analysis, analysis, based based on on Equation Equation (3) (3) has has been been also also carried out to evaluate the dependencedependence of of apparent activation energy over the extent of reaction.reaction. While the Kissinger method considersconsiders only only one one point point of of thermal thermal degradation degradation curve curve the the FOW FOW model model examines examines different different points, points, eacheach corresponding to to different different conversion values, therefore therefore modeling modeling results results are are considered only only forfor discussion aims. In In Figure Figure 10a,b10a,b the the energy energy profiles profiles are are reported reported for for unused unused (red (red trace) and recoveredrecovered (blue trace) CA, respectively, respectively, inin nitrogen and air atmosphere. In In inert inert flow flow the the degradation degradation reactionreaction is is activated activated in in the the conversion conversion range range 20 20–80%–80% the the value value of of Ea Ea is is 200 200 KJ/mol KJ/mol (see (see Figure Figure 10a).10a). UnderUnder the the oxidant atmosphere, atmosphere, when when a a 10% 10% mass mass is is lost, the energy curve rapidly increases increases as as the decompositiondecomposition gradually takes place for the investigated investigated samples. However, However, in in the the conversion conversion range range 1515–65%,–65%, an energy value of 150 KJ/mol KJ/mol was found found for both the analyzed analyzed samples. For For conversion conversion with with α α > 65%, thethe activationactivation energy energy profiles profiles for for both both samples samples are are characterized characterized by aby positive a positive derivative derivative path pathtoward toward full conversion. full conversion. Moreover, Moreover, two two diff erentdifferent degradation degradation steps steps are are evident, evident, these these are are related related to tottheo the deacetylation deacetylation process process and and char char formation formation (see (see(see Figure Figure 10 10b).10bb).). AsAs expected,expected, also by FOW analysis, analysis, thethe cleaning cleaning process process of of cigarette cigarette filters filters allows to to obtain a recovered sample with behavior similar to to untreateduntreated or “unused” filter. filter.

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Figure 10. Energy profile derived from FOW (a) nitrogen flow; (b) air flow. FigureFigure 10. 10. EnergyEnergy profile profile derived derived from from FOW FOW ( (aa)) nitrogen nitrogen flow; flow; ( (bb)) air air flow. flow. 3.3.3.3. Characterization of Recovered CACA byby FTIRFTIR SpectroscopySpectroscopy 3.3. Characterization of Recovered CA by FTIR Spectroscopy TheThe spectraspectra ofof unusedunused sample sample compared compared to to the the measurement measurement carried carried out out on on recovered recovered sample sample is The spectra of unused sample compared to the measurement carried out on recovered sample reportedis reported in in Figure Figure 11 11.. In In the the region region between between 3700 3700 cmcm−11 andand 3000 3000 cm cm−1and1 and precisely precisely at at3400cm 3400−1 cm, it 1is, is reported in Figure 11. In the region between 3700 cm−1 and 3000 cm−1and− precisely at 3400cm−1, it −is itpossible is possible to observe to observe the the extended extended band band assigned assigned to to OH OH stretching stretching derived derived from from adsorbed OHOH possible to observe the extended band assigned to OH stretching derived from adsorbed OH stretchingstretching derivedderived fromfrom adsorbedadsorbed water.water. SymmetricSymmetric andand asymmetricasymmetric stretchingstretching ofof C-HC-H methylmethyl group stretching derived from adsorbed water. Symmetric and asymmetric stretching of C‐H methyl group areare identifiedidentified correspondingcorresponding toto 29202920 cmcm−11 and 28502850 cmcm−11 bands;bands; these bands are connected to sharpsharp are identified corresponding to 2920 cm−1 and 2850 cm−−1 bands; these bands are connected to sharp −11 peakpeak at at 1432cm 1432cm− due to to –CH–CH22 bending.bending. The The characteristic bands of cellulose acetate have have been been peak at 1432cm−1 due to –CH2 bending. The characteristic bands of cellulose acetate have been highlightedhighlighted andand typicaltypical carbonylcarbonyl stretching band of acetate group is veryvery intenseintense andand cancan bebe easilyeasily highlighted and typical carbonyl stretching band of acetate group is very intense and can be easily identified.identified. TheseThese carbonylcarbonyl band ofof acetylacetyl groupsgroups isis visiblevisible andand appearsappears inin anan isolatedisolated regionregion ofof FTIRFTIR identified. These carbonyl band of acetyl groups is visible and appears in an isolated region of FTIR spectrumspectrum atat 17391739 cmcm−11.. Bending Bending of of the the CH CH group group (belonging to methyl or hydroxylhydroxyl groupsgroups inin thethe spectrum at 1739 cm−−1. Bending of the CH group (belonging to methyl or hydroxyl groups in the plane)plane) atat 13711371 cmcm−11andand a a 1214 1214 cm cm−1 1bandband link linkeded to to the the stretching stretching of of the the CO CO bond of the acetyl groups plane) at 1371 cm−1and a 1214 cm−1− band linked to the stretching of the CO bond of the acetyl groups areare identified. identified. Finally, the asymmetric asymmetric stretching stretching of of the the ester ester group group C–O–C C–O–C at at 1100 1100 cm cm−11 andand the the are identified. Finally, the asymmetric stretching of the ester group C–O–C at 1100 cm−1 and the vibrationalvibrational modesmodes ofof thethe C-OC-O bondbond inin cellulosecellulose molecules,molecules, thatthat generategenerate aa bandband centeredcentered at at 1030 1030 cm cm−11, vibrational modes of the C‐O bond in cellulose molecules, that generate a band centered at 1030 cm−1, areare noticednoticed [[33,34].33,34]. are noticed [33,34].

FigureFigure 11.11. FTIR spectrum on CA unused andand recovered.recovered. Figure 11. FTIR spectrum on CA unused and recovered. By the observation of the FTIR spectra, except for the amount of OH groups on the cellulose By the observation of the FTIR spectra, except for the amount of OH groups on the cellulose structureBy the (unused observation and recovery), of the FTIR it is possiblespectra, to except affirm for and the confirm amount that of the OH extraction groups processon the cellulose produces structure (unused and recovery), it is possible to affirm and confirm that the extraction process structurecellulose acetate(unused compatible and recovery), with the it is unused possible acetate to affirm and reusable and confirm in diff erentthat the applications. extraction process produces cellulose acetate compatible with the unused acetate and reusable in different applications. produces cellulose acetate compatible with the unused acetate and reusable in different applications. 3.4. Reprocessing Waste Cigarette Butts into Usable Material 3.4. Reprocessing Waste Cigarette Butts into Usable Material 3.4. ReprocessingThe best sustainable Waste Cigarette solution Butts for cigarette into Usable butt’s Material environment problems is to recycle cigarette The best sustainable solution for cigarette butt’s environment problems is to recycle cigarette buttsThe and best use sustainable them for the solution manufacturing for cigarette of some butt’s useable environment products. problems We have is demonstrated to recycle cigarette a clean butts and use them for the manufacturing of some useable products. We have demonstrated a clean buttsextraction and use method them offor the the plastic manufacturing cellulose of acetate some from useable the products. waste cigarette We have butts. demonstrated We have been a clean able extraction method of the plastic cellulose acetate from the waste cigarette butts. We have been able extraction method of the plastic cellulose acetate from the waste cigarette butts. We have been able

Materials 2020, 13, 4710 11 of 13 Materials 2020, 13, x FOR PEER REVIEW 11 of 13 to reduce the toxicity level of the used butts and remove the bad sniff sniff from them; we can certainly use them for usable products manufacturing, as for example to produce fashionable products such as eyeframes. Before extruding ourour drieddried recovered recovered CA, CA, we we have have realized realized a casta cast transparent transparent film; film; in fact,in fact, the CAthe CArecovered recovered has has been been dissolved dissolved in acetonein acetone (5% (5%w/ w)/w and) and cast cast in in a a petri petri dish. dish. WeWe obtainedobtained a very transparent filmfilm (see Figure 1212).).

FigureFigure 12. 12. TransparentTransparent film ﬁlm of cellulose acetate recovered from cigarette butts.

The film film in Figure 12 is realizedrealized byby recoveredrecovered CACA dissolveddissolved in acetone.acetone. However, However, the the CA film film exhibits somesome defects,defects, the the film film is is brittle brittle so aso plasticizer a plasticizer needs needs to be to added be added to attain to attain toughness. toughness. The choice The choiceof plasticizer of plasticizer is based onis based compatibility on compatibility with CA, so with we must CA, improveso we must the mechanical improve the properties mechanical and propertiesthermal stability and thermal without stability affecting without the film affecting transparency. the film transparency. The recovered CA granules have been extruded to obtain a brittle, white film film that is is 2 2 mm mm thick. thick. Now we are studying and testing testing the the plasticizers plasticizers and and pigments, pigments, commercial commercial a andnd biodegradable, biodegradable, aimed at giving the desired workability, flexibility,flexibility, toughness,toughness, andand colorscolors toto ourour recoveredrecovered material. material.

4. Conclusions Conclusions Cigarette waste waste pollutes pollutes the the environment, environment, so this so thisthe desired the desired problem problem to be addressed to be addressed by humans. by humans.However, However, such waste such can waste be recycled can be recycled by converting by converting it into raw it into material raw material for the manufacturefor the manufacture of new ofproducts. new products. Cigarette Cigarette butt’s butt’s composition composition includes includes a variety a variety of compounds, of compounds, including including aromatic aromatic and andheterocyclic heterocyclic amines, amines, carbonylated carbonylated compounds, compounds, phenols, phenols, polycyclic polycyclic aromatic hydrocarbons, aromatic hydrocarbons carbon and, carbonnitrogen and oxides, nitrogen and ammonia.oxides, and These ammonia. compounds These showcompounds different show solubility different according solubility to theiraccording polarity. to Withtheir thispolarity. fact, andWith considering this fact, and that considering cellulose acetate that cellulose is the main acetate component is the main and it component is soluble in and acetone it is solubleor ethyl in acetate, acetone we or propose ethyl acetate, a methodology we propose to purify a metho thisdology polymer-based to purify thison several polymer solid-liquid-based on severalextraction solid steps-liquid using extraction solvents steps with using different solvents polarity. with Fourier-transformdifferent polarity. Fourier infrared-transform spectroscopy infrared and spectroscopythermogravimetric and thermogravimetric analysis (TGA) were analysis employed (TGA) to characterize were employed the structural to characterize and thermal the propertiesstructural andof purified thermal CA prope comparedrties of purified to unused CA CA. compared Moreover, to unused the degradation CA. Moreover, thermal the degradation model computed thermal by modelKissinger computed and Ozawa by Kissinger methods and were Ozawa used to methods calculate were the Ea used value to calculate obtained the for recoveredEa value obtained and unused for recoveredCA and to proveand unused the possible CA and variations. to prove We the showed possible that variations. recovered We CA showed has the samethat recove chemical,red thermalCA has theproperties same chemical, of unused thermal CA, the purificationproperties of method unused does CA, not the alter purification the material method properties. does The not recycling alter the materialof CBs and properties. turning this The waste recycling into aof resource CBs and can turning be a solution this waste to cigarette into a resource butt pollution, can be thea solution important to cigaretteresult we butt reached pollution, is that the we important can obtain result a transparent we reached film is from that acetatewe can cellulose obtain a recoveredtransparent by film cigarette from acetatebutts using cellulose a facile recovered and green by extractioncigarette butts method. using a facile and green extraction method.

Author Contributions: Conceptualization, L.S., M.G. and A.D.F.; methodology, L.S. and A.D.F.; validation, Author Contributions: Conceptualization, L.S., M.G. and A.D.F.; methodology, L.S. and A.D.F.; validation, L.S. L.S. and M.G.; formal analysis, A.D.F.; investigation, L.S. and A.D.F.; resources, M.G.; data curation, A.D.F.; writing—originaland M.G.; formal draft analysis, preparation, A.D.F.; L.S. investigation, and A.D.F.; writing—reviewL.S. and A.D.F.; and resources, editing, M.G.; L.S. and data A.D.F.; curation, visualization, A.D.F.; writingM.G.; supervision,—original draft L.S. Allpreparation, authors have L.S. and read A.D.F.; and agreed writing to— thereview published and editing, version L.S. of the and manuscript. A.D.F.; visualization, M.G.;Funding: supervision,No Funding. L.S. All authors have read and agreed to the published version of the manuscript. Funding:Acknowledgments: No Funding.The Authors thank to Essequadro eyewear for providing cigarette butts, funding for the ‘green’ exctraction study and testing support for material development. Acknowledgements: The Authors thank to Essequadro eyewear for providing cigarette butts, funding for the ‘green’ exctraction study and testing support for material development.

Materials 2020, 13, 4710 12 of 13

Conﬂicts of Interest: The authors declare that they have no conﬂict of interest.

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