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Are Cellulose Nanofibers a Solution for a More Circular Economy of Paper Products? Marc Delgado-Aguilar,*,† Quim Tarrés,† M. À ngels Pèlach,† Pere Mutjé,† and Pere Fullana-i-Palmer‡ †

Laboratori d’Enginyeria Paperera i Materials Polímers (LEPAMAP), High Polytechnical School, University of Girona, C/Maria Aurèlia Capmany, 61 − 17071 Girona, Spain ‡ UNESCO Chair in Life Cycle and Climate Change, Escola Superior de Comerç Internacional (UPF), Passeig Pujades, 1, 08003 Barcelona, Spain ABSTRACT: This paper presents the study of the feasibility of incorporating lignocellulosic nanofibers (LCNF) to paper in order to maintain the relevant physical properties and increase the number of cycles that paper can be recycled in the technosphere in a more circular economy. For that purpose, the effect of mechanical refining in recycling processes was compared with that of the novel LCNF addition. In this sense, the behavior of a bleached kraft hardwood pulp when recycled was investigated, as well as the effects of each methodology. Since there are many issues to be considered when trying to replace a technology, the present paper analyses its feasibility from a technical and environmental point of view. Technically, LCNF present greater advantages against mechanical refining, such as higher mechanical properties and longer durability of the fibers. A preliminary life cycle assessment showed that the environmental impacts of both systems are very similar; however, changing the boundary conditions to some feasible future scenarios, led to demonstrate that the CNF technology may improve significantly those impacts.

1. INTRODUCTION 1.1. Paper Recycling. In average, 55% of paper slurries all over the world are made from secondary fibers, i.e. recycled fibers. For some paper grades, such as cardboard or newsprint, this percentage can become close to 100%.1 The end of life of paper may be landfilling, incineration or recycling. Landfilling gives nothing in return to society. Incineration generates energy to be used for other purposes and replaces energy produced by other means; however, burning also entails harmful emissions to the atmosphere and destroys a valuable resource. Recycling is generally seen as a more sustainable option from an environmental point of view, since it allows reusing the fibers to produce new paper and, thus, reduces the consumption of natural resources.2 In this sense, paper industry has clearly opted for recycling. Recycling processes entail, in general terms, a loss in mechanical properties mainly due to the hornification phenomena.3,4 During recycling, fibers become less conformable leading to less and weaker interfiber bonds, which directly affects the tensile strength of paper. For many years, with the purpose of counteracting this situation, mechanical refining has been used for properties recovering. Mechanical refining processes increase the specific surface of fibers, allowing them to bond again with the adjacent fibers and increasing the number of bonds. However, this process has some drawbacks, such as internal fibrillation and many structural damages due to the shearing forces between rotor and stator5 (Figure 16). © 2015 American Chemical Society

Mechanical refining is quite an optimized technique, as properties enhancement depends strongly on how it is performed. In this sense, many efforts have been delivered to reduce the energy consumption and/or to attenuate the structural damage. Some of the proposed improvements require equipment modification, which usually means huge economical investments,7−9 such as the modification of the surface of the refiner disks. 1.2. Cellulose Nanofibers. Cellulose nanofibers (CNF) are stirring a great interest for researchers who deal either with new or well-established scientific and technological domains and many studies have been published in the recent years.10−17 Cellulose nanofibers suspensions are formed by the smallest structural unit of cellulose fibers, with diameters between 3 and 5 nm, although their suspensions may contain aggregates reaching higher dimensions (Figure 2). Due to these tiny diameters, CNF present a huge specific surface, which means that their ability to bond adjacent fibers is greater than the one by cellulose fibers. Their aspect ratio is also high, since they can reach lengths of about 70 μm, which means high intrinsic mechanical properties. A significant number of studies have used CNF for paper reinforceReceived: Revised: Accepted: Published: 12206

June 16, 2015 September 28, 2015 October 1, 2015 October 1, 2015 DOI: 10.1021/acs.est.5b02676 Environ. Sci. Technol. 2015, 49, 12206−12213

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Environmental Science & Technology

Figure 1. Mechanical refining and fiber hornification schematic process.

maintain our standard of living and preserving the natural capital which will sustain future generations. Especially attractive is the “Industrial Symbiosis” metaphor, one of the concepts linked to the discipline long known as “Industrial Ecology”,24 which studies material, energy and information fluxes within industrial systems and their interaction with natural systems. This discipline has been applied to different sectors and geographical locations, including our country, Catalunya (Spain), not only to the paper sector25−27 but also to the leather one.28 Industrial symbiosis aims at using outflows from a plant as inflows to another, either directly or through a new industrial “species” which allows this concrete metabolic pathway. A practical application is the design of industrial parks into which the businesses are welcomed if they use secondary flows or surpluses from others,29 although it can be applied to any system. The final aim is to close cycles and to reduce resource consumption.30 For instance, the Swedish forest industry is organized in clusters of companies compatible among themselves. Wolf found 15 byproducts interchanged in such industrial networks.31 Circular Economy is a term mainly developed by the Ellen MacArthur Foundation 32 and used by the European Commission to define which is the type of economy wanted for the future.33 Circular economy implies two types of flows: those which are biodegradable nutrients, able to go back to nature, and the so-called technical nutrients, designed to be maintained as long as possible within the technosphere thanks to making different types of changes on them so that cycles are possible, for the paper sector as well. For instance,34 second hand books selling is a nondegradative change and cycling. The use of one side used paper to print draft documents increases the amount of ink in the paper but that does not degrade it too much. The extraction of toner from paper or stickers from cardboard boxes does not avoid strongly a future cycle as much as bending or cutting the cardboard. However, other options are more destructive, such as refining cellulose fibers, or more disturbing to future cycles, such as adding chemicals like adhesives. The prioritization of less destructive changes is in line with the waste management hierarchy described by the European Commission since 1997,35 and incorporated in bounding law with the Waste Framework Directive,36 which prioritizes prevention, then reusing, later recycling and finally destruction. If we intend to develop new technologies following circular economy and industrial symbiosis principles, then it is highly important to consider how the material is affected by those technologies. In this paper, we discuss how the compared technologies degrade or not the cellulose fibers. Among the concepts and disciplines toward sustainability, due to the international consensus, its quantitative nature, the huge scientific and application effort already performed, and the availability of software and databases, there is an omnipresent

Figure 2. Size hierarchy: from cellulose fiber to CNF.

ment,10−14,18 showing their strengthening potential as paper additive. These studies mainly use CNF produced from bleached wood fibers, pretreated with TEMPO-mediated oxidation at slightly acidic or basic pH,13 and processed by an intense mechanical treatment in a high pressure homogenizer or microfluidizer. The conditions of TEMPO-mediated oxidation rule the charge density on fiber’s surface, which determines many of the final properties of CNF.19 Pretreatment is supposed to reduce the energy consumption in the homogenization process, avoiding the clogging of the system by fiber entanglement. There are many other pretreatments available in the scientific literature, including enzymatic hydrolysis,20,21 acid hydrolysis,20 carboxymethylation,22 and fully mechanical treatment,23 depending on the desired grade of CNF, as well as the yield of nanofibrillation. Those CNF pretreated with TEMPO-mediated oxidation present a higher yield of nanofibrillation, due to the repulsive forces between carboxylic groups which are beneficial for the following homogenization process. Similarly to TEMPO-mediated oxidation, carboxymethylation reaction is also a methodology that drives to CNF with a high yield of nanofibrillation. The main problem with both pretreatments is the amount of chemicals that are required and, consequently, the costs and the potential risk for the environment. However, CNF may be obtained by mechanical methods alone. These CNF present a lower yield of nanofibrillation but, on the other hand, when they are used as paper reinforcement, they can fairly provide the same tensile strength to the paper that those obtained by chemical methods. 1.3. Paper and Circular Economy. Copying natural processes to deliver less polluting or less resource demanding technologies is a must if we want a more sustainable society, 12207

DOI: 10.1021/acs.est.5b02676 Environ. Sci. Technol. 2015, 49, 12206−12213

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Environmental Science & Technology one: life cycle assessment (LCA).37 LCA analyses38 the material and energy flows in and out of a system, process by process throughout the whole life-cycle, and calculates environmental impacts in different areas of interest (human health, resource depletion and ecosystems quality) thanks to consensual scientific models on impact categories such as climate change, acidification, toxicity, abiotic resource depletion, etc. This methodology addresses the possible rebound effects of using a certain technology. For instance, recycling paper needs a collection system and processes to clean the material or adding CNF needs energy to produce them. Those needed processes, as a whole and per unit of service delivered (functional unit within the LCA argot), might be more polluting than the existing ones. It is even possible that one system delivers the same quantity of material but of a different quality, thus not delivering the function completed but a part of it.39 An LCA deals with these issues. Not always a fully fledged LCA is needed to have an idea of how to proceed in research.40 Sometimes a simplified LCA or the use of the so-called life cycle thinking (LCT) is enough, and balance scientific rigor and practical application has been found essential.41 According to the European Commission, not only LCA but also LCT is a good scientific instrument to define modern environmental policy and to help business decision making.42 For the European Commission,43 LCT is essential to the European Thematic Strategy on the Sustainable Use of Natural Resources. Therefore, we see that life-cycle issues must be considered whenever a technological research is performed in order to optimize resources.

pulp disintegrator for 10 min at 3000 rpm. Afterward, 10 mL were taken and mixed with 25 mL of cationic polymer polydiallyldimethylammonium chloride (polyDADMAC) for 5 min with magnetic stirring. After this time, the mixture was centrifuged in a Sigma Laborzentrifugen model 6 K 15 for 90 min at 4000 rpm. Then, 10 mL of the supernatant was taken to the Mütek equipment. Anionic polymer (Pes-Na) was then added to the sample drop by drop with a pipet until the equipment reached 0 mV. The volume of anionic polymer consumed was used to calculate the cationic demand through the equation below: CD = −

(CpolyD·VpolyD) − (Cpes − Na·Vpas − Na) Wsample

(eq 1)

Where CD is the cationic demand, CpolyD is the cationic polymer concentration, VpolyD is the used volume of cationic polymer, Cpes−Na is the anionic polymer concentration, Vpes−Na is the used volume of anionic polymer, and Wsample is the sample’s dry weight. The carboxyl content (CC) of the obtained LCNF was calculated by conductometric titration. A dried sample (50−100 mg) was suspended in 15 mL of 0.01 M HCl solution; this exchanges Na cations bound to the COOH group by H ions. After 10 min of magnetic stirring, the suspensions were titrated with 0.01 M NaOH, adding 0.1 mL of NaOH to the suspension and then recording the conductivity in mS/cm; this process was repeated until observing a reduction, stabilization and increase in the conductivity. From the conductometric titration curve the presence of strong and weak acid is observed. The CC is given by the following equation:

2. MATERIALS AND METHODS 2.1. Production of CNF and Energy Demand. Stone groundwood pulp (SGW), kindly supplied by Zubialde (Aizarnazábal, Spain), was used to produce the lignocellulosic nanofibers (LCNF) and it was subjected to a pulping process for 1.5 h, at 180 °C and in the presence of a 20 wt % of NaOH with regard to the amount of dry pulp. The liquor ratio was set at 6:1. After the pulping process, the treated SGW pulp was washed with water until constant and neutral pH. Then, the pulp was bleached with sodium hypochlorite at 70 °C for 1 h. This stage was repeated three times, decreasing the Kappa number from 42.1 to 20.3. The pulp was refined at 20 000 revolutions in a PFI mill, rinsed with water and suspended. Next, a 2.5 wt % aqueous suspension was formed and passed 3 and 7 times through a high pressure homogenizer (NS1001L PANDA 2K-GEA) at 300 and 600 bar. This procedure does not use any of the pretreatments described in the introduction, as we have found that the same results could be achieved without adding these extra processes. This fact clearly leads to a less energy demanding and environmentally impacting technology. The production costs were based on the energy consumption of the homogenizer, as some other researchers have considered44 and the rest of the processes were discarded for the calculations, since they are common for any fiber preparation. This energy was determined with an energy consumption measuring equipment: Circutor CVM-C10 and Socomec Diris A20. 2.2. Characterization of LCNF. LCNF were characterized according to Delgado-Aguilar et al. (2015),13 by means of cationic demand, carboxyl content and yield of nanofibrillation. The cationic demand of LCNF was determined using a Mütek PCD 04 particle charge detector. First, 0.04 g of LCNF (dried weight) was diluted in 1 L distilled water and dispersed with a

CC = 162(V2 − V1)c[w − 36(V2 − V1)c]−1

(eq 2)

where V1 and V2 are the equivalent volumes of added NaOH solution (l), c is the NaOH concentration (mol/L) and w the weight of oven-dried sample (g). The results indicate the average mmol of − COOH groups per gram of LCNF. The yield of LCNF was also determined; a LCNF suspension with 0.2% of solid content was centrifuged at 4500 rpm for 20 min in order to isolate the nanofibrillated fraction (contained in the supernatant) from the nonfibrillated and partially fibrillated one retained in the sediment fraction, which was recovered, weighed and oven-dried at 90 C until constant weight. The yield of nanofibrillation was then calculated from the next equation: ⎛ weight of dried sediment ⎞ yield% = ⎜1 − ⎟ × 100 weight of diluted samplex%Sc ⎠ ⎝

where %Sc represents the solid content of the diluted gel sample. Specific surface and diameters were calculated according to Espinosa et al. (2015), which were corroborated by analyzing a dried sample through field emission scanning electron microscopy (FE-SEM) imaging (Hitachi S-3000) at 12 kV. Samples were previously covered with carbon by sputtering. As the CNF distribution or the average diameter within a certain CNF suspension may differ depending on how they are produced, it is important to characterize them properly to be able to draw conclusions. 2.3. Recycling Process. The recycling process was carried out several times. First of all, with the virgin bleached kraft hardwood pulp (BKHP), provided by Ence-Celulosas de Asturias S.A. (Spain), paper sheets were formed following the 12208


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• Paper can be recycled seven times through the proposed CNF process (our findings) • Mechanical refining uses 67.5 kWh of electricity to recycle 1000 kg of pulp • 30 kg of CNF are needed to recycle 1000 kg of pulp • 11.47 kWh are needed to produce 1 kg of CNF • Spanish electricity mix and kraft pulp life cycle data from Ecoinvent 3.1 were used as baseline scenarios. Combining the different assumptions, the systems would be comparable if we use the same functional unit: “one recycling cycle of one ton of paper”. Impacts must be calculated per functional unit for both systems. As for the inventory analysis, we used the software GaBi6 by Thinkstep to make the calculations and the Ecoinvent 3.1 database for the background data. Specifically, the paper pulp data was obtained using the kraft process “RER: sulfate pulp ECF bleached, at plant” and the Spanish mix electricity data from the process “ES: Electricity mix”. Other scenarios were calculated using sulphite pulp “RER: sulphite pulp, bleached, at plant” and a less polluting energy “ES: electricity, hydropower, at power plant”. The foreground data for the CNF production was obtained by laboratory experimentation as explained above. No allocation was needed other than the one implicit in the data sets used for the background system. It is difficult to choose a set of impact categories when LCA is applied to such a novel technology. Of course, no consensus within the sector could have been reached. The ILCD and PEF methodology by the European Commission give clear recommendations on which set of categories use as a start for discussion within the pilot sector PEF studies. After years of discussion, these pilot sectors are (or are not) arriving to a consensus on which categories are the best suited for their sector. Given that we are in no position of generating such a consensus, we have decided to use the initial PEF set included within the GaBi6 software in order to calculate the life cycle impact assessment (impact categories listed in Table 1).

standard ISO 5269-245 and according to Delgado-Aguilar et al. (2015).13 Briefly, 30g of the commercial pulp was suspended in 1.5 L of tap water and pulped in a laboratory pulper (IDM). Then, the amount of pulp slurry was calculated to produce 75 g/m2 of basis weight paper handsheets using a laboratory sheet former. Second, they were repulped in order to prepare paper sheets again and the pulp was treated either by mechanical refinement or by LCNF addition. On the one hand, the mechanical refinement was performed at 1500 revolutions in a PFI mill. On the other hand, LCNF were added at a 3 wt % ratio to the BKHP slurry. This recycling procedure (repulping + treatment) was performed several times. Tensile properties of paper sheets were determined at each cycle, following the standards ISO 1924-1 and ISO 1924-2. The minimum requirement was established at 3700 m of breaking length, since this tensile strength allows the production of usual paper such as writing paper. This procedure was stopped the moment the properties values were not reached anymore. 2.4. Life Cycle Assessment: Goal and Scope Definition and Inventory Analysis. LCA is an ISO standardized methodology 46 ,47 and important guidelines have been published, such as the one by the EC,42 the so-called ILCD Handbook. ISO 1404447 states that “The scope, including system boundary and level of detail, of an LCA depends on the subject and the intended use of the study. The depth and the breadth of LCA can differ considerably depending on the goal of a particular LCA.” The goal of this study is to compare two systems: paper recycling through refining and paper recycling through CNF addition. In relation to the scope of the study, the depth and the breadth of the present LCA is far from being high, so it is clearly not the intention of this study to be taken as a complete LCA and even less as a comparative assertion intended to be disclosed to the public as mentioned in ISO 14044. A life cycle approach is taken to figure out where the main impacts might be. Both systems can be described starting by a certain quantity (1 ton) of paper pulp which is subjected to recycling. The system boundaries are those of the recycling processes. The recycling process can be repeated a different number of times for each system after which virgin pulp is needed to enter the system. On the one hand, because of the different degradative nature of the compared recycling processes, different quantity of virgin pulp is needed. On the other hand, the different nature of the mechanical process of recycling and the mechanical process of CNF production leads to different energy needs (electricity). In summary, both systems have a process of virgin paper production and an electricity consuming process, either recycling through refining or recycling through CNF production. The main hypothesis and assumptions taken within the inventory analysis are the following:

Table 1. Characterization of the Obtained LCNF

LCNF

yield of nanofibrillation(%)

cationic demand (μeqg/g)

SGW

35.18

228.11

carboxyl content (μeq-g/g)

specific surface (m2/g)

diameter in average (nm)

75.8

74.2

33.7

3. RESULTS AND DISCUSSION 3.1. LCNF Characterization. Table 1 shows the yield of nanofibrillation, cationic demand, carboxyl content, specific surface, and the diameter in average of the obtained LCNF. In order to corroborate the obtained diameters and, thus, bear out that the mathematical model described by Espinosa et al. (2015)48 is reliable, dried samples were analyzed with FESEM, as it is reflected in Figure 3. As it is possible to see in Figure 3, there are some LCNF ranging from 21 to 47 nm. That LCNF reflected in the micrograph which seems to have greater diameters were considered as LCNF bundles, as it has been explained above. 3.2. Number of Cycles and Energy Demand. The starting pulp was BKHP either reinforced with LCNF (for the study with LCNF) or mechanically refined. For LCNF addition (3 wt %), the breaking length increase was 103%, while for mechanical refining (1500 rev) was 96%. The properties loss

• 1000 kg of kraft pulp • All processes to obtain the pulp are the same for both systems • The resulting papers have the same quality for both systems • Paper can be recycled three times through refining 12209


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cycles for mechanical refining processes and LCNF addition, respectively. 3.3. Qualitative Sustainability Assessment. The different alternatives of CNF production cited in the introduction have different technological, economical and environmental issues. For instance, the catalytic oxidation with TEMPO results in highly transparent CNF with really tiny diameters, ranging from 3 to 10 nm. The main drawback of this technology is its production cost, which is close to 200 € per kg of CNF. Assuming that a 3 wt % CNF suspension is needed to fit the mechanical requirements stated before, the paper producer should assume a cost related to CNF addition of about 6000 €/tone of paper. In general terms, those CNF that require the use of chemicals during their production become more expensive than those that are obtained by mechanical processes. The production of CNF fully obtained by mechanical treatments costs about 1.50€ per kg of CNF. The main drawback of these CNF is the heterogeneity that they present. Regarding the possible environmental impacts, CNF produced by mechanical treatment have the same composition of the cellulose fibers and no chemicals are added. Therefore, from an industrial symbiosis and circular economy points of view, in addition maintaining the fibers more cycles in the technosphere: the CNF might be produced from degraded cellulose fibers (those not qualified to be recycled anymore); the mechanical treatment has a quite controlled rebound effect (energy demand); the chemical treatment may lead to problems during chemical production or manipulation; the chemical treatment adds chemicals to the fibers, changing the nature of the material and provoking a possible waste management issue. Regarding the effectiveness when applied over paper substrates, both CNF types provide the same properties enhancement, which implies a 100% of breaking length increase over a BKHP substrate when a 3 wt % of CNF is added.49 Taking into account the economical and technical issues, those CNF obtained by fully mechanical treatments were selected for the present study. Regarding the substrate, bleached kraft pulp has been selected for the baseline scenario due to its availability and extended use in paper mills. Therefore, we have finally considered as baseline scenario for quantitative comparison: the Spanish electricity mix; a kraft paper pulp, the most commonly used for paper production; and CNF production without pretreatment and only using an homogenization process. The main hypothesis is that the addition of CNF for paper recycling gives the needed physical properties to the resulting paper while maintaining the fibers more time within the technosphere without a bigger negative trade off due to introducing disturbing chemicals or to having a higher energy demand. Certainly, we have demonstrated that the number of cycles is higher and that no disturbing chemicals are introduced, as CNF have the same nature as cellulose fibers. The calculations below are needed to demonstrate that the higher energy demand does not balance the benefits of the higher number of cycles (the lower need of virgin matter). 3.4. Comparative Life Cycle Impact Assessment. The following life cycle impact results were obtained from the software and database (Table 2). The calculation of impacts uses the impacts of the processes involved (electricity and pulp production) combined in

Figure 3. FE-SEM micrographs of the LCNF.

without treating the pulp at each recycling cycle was also assessed in order to bear out the behavior of the pulp and the effect of the hornification phenomenon. Our experimentation has determined that paper sheets made from BKHP, without adding CNF in bulk nor refining the pulp after each recycling process, experienced a quick properties loss rather proportional to the number of recycling cycles (Figure 4). On the other hand, paper sheets recycled by a refining

Figure 4. Effect of the mechanical refining and CNF addition during recycling cycles.

treatment maintained the tensile strength above the minimum level for three recycling cycles. Finally, recycled paper sheets reinforced with LCNF kept their breaking length higher than the minimum requirement for seven recycling cycles. The differences between the trend that experienced those papers reinforced with LCNF and those that were mechanically refined is understandable. As it has been stated before, mechanical refining causes structural damages to the fibers, reducing their ability to bond adjacent fibers. Comparatively, LCNF have a huge specific surface, involving high hydroxyl groups’ density on LCNF surface capable to bond fibers. In summary, the results reflected in Figure 4 clearly show the better recyclability of those papers reinforced with LCNF in comparison with those mechanically refined. Moreover, it can be stated that refined papers can rather maintain the original properties during three recycling processes but those reinforced with LCNF up to seven. This is equivalent to saying that the fiber must be replaced by virgin fibers each 3 and 7 recycling 12210


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Environmental Science & Technology Table 2. Life Cycle Impacts of Electricity and Pulp for the Baseline Scenario (Ecoinvent 3.1 Database) impact category (ILCD/PEF recommendation)

electricity (per kWh)

pulp (per kg)

Acidification, accumulated exceedance [Mole of H+ eq ] Ecotoxicity for aquatic fresh water, USEtox (recommended) [CTUe] Freshwater eutrophication, EUTREND model, ReCiPe [kg P eq] Human toxicity cancer effects, USEtox (recommended) [CTUh] Human toxicity noncanc. effects, USEtox (recommended) [CTUh] Ionizing radiation, human health effect model, ReCiPe [kg U235 eq] IPCC global warming, excl biogenic carbon [kg CO2-Equiv.] Marine eutrophication, EUTREND model, ReCiPe [kg N-Equiv.] Ozone depletion, WMO model, ReCiPe [kg CFC-11 eq] Particulate matter/Respiratory inorganics, RiskPoll [kg PM2,5-Equiv.] Photochemical ozone formation, LOTOSEUROS model, ReCiPe [kg NMVOC] Resource Depletion, fossil and mineral, reserve Based, CML2002 [kg Sb-Equiv.] Terrestrial eutrophication, accumulated exceedance [Mole of N eq ] Total freshwater consumption, including rainwater, Swiss Ecoscarcity [UBP]

5.49 × 10−03

5.31 × 10−03

1.93 × 1000

4.37 × 1000

2.11 × 10−04

2.26 × 10−04

2.38 × 10−08

5.17 × 10−08

5.79 × 10−08

8.68 × 10−08

2.02 × 1002

1.09 × 1002

5.00 × 10−01

5.21 × 10−01

5.30 × 10−05

3.48 × 10−04

2.97 × 10−08

4.74 × 10−08

3.86 × 10−04

6.31 × 10−04

2.02 × 10−03

3.95 × 10−03

9.02 × 10−07

9.59 × 10−06

7.23 × 10−03

1.38 × 10−02

2.91 × 10−01

1.69 × 10−01

Table 3. Life Cycle Impacts of the Refining and CNF Technologies for the Baseline Scenario

different ways for each system, following the above assumptions and leading to • For the refining system: Impact per kWh * 67.5 + impact per kg pulp * 1000/3 • For the CNF system: Impact per kWh * 11.47 * 30 + impact per kg pulp * 1000/7 Taking these assumptions into account, the impact results are those presented in Table 3 and Figure 5. It is fairly clear that, within the boundary conditions described above, both systems have similar impacts to the environment. On the one hand, CNF recycling technology presents lower impacts for: aquatic ecotoxicity (81.2%), human toxicity on cancer (82.6%), marine eutrophication (56.9%), ozone formation (86.6%), resource depletion (51.6%), and terrestrial eutrophication (87.6%). On the other hand, refining recycling technology presents lower impacts for: acidification (80.8%), freshwater eutrofication (85.5%), ionizing radiation (58.8%, global warming (84.1%), and freshwater consumption (61.1%). Finally, they present almost identical results for: human toxicity noncancer, ozone depletion, and particulate matter. 3.5. Interpretation through Scenario Analysis. Having seen that the two compared technologies deliver similar environmental results for the given conditions, that is, the use of CNF for paper recycling does not give a significant improvement, it is fair to ask if those conditions may change in a way that makes the technology shift advisible. Given that the LCA study is in a preliminary stage, as the technology is far from being mature, and the uncertainty of the different influencing variables, we propose different alternative scenarios to see if the results may vary significantly. No significance, uncertainty or sensitivity analyses are thought to be of much use in addition to the scenario analysis. The geographic situation and the time scale may have an influence on the

impact category (ILCD/PEF recommendation)

refining technology

CNF technology

acidification, accumulated exceedance [Mole of H+ eq ] ecotoxicity for aquatic fresh water, USEtox (recommended) [CTUe] freshwater eutrophication, EUTREND model, ReCiPe [kg P eq] human toxicity cancer effects, USEtox (recommended) [CTUh] human toxicity noncanc. effects, USEtox (recommended) [CTUh] ionizing radiation, human health effect model, ReCiPe [kg U235 eq] IPCC global warming, excl biogenic carbon [kg CO2-Equiv.] marine eutrophication, EUTREND model, ReCiPe [kg N-Equiv.] ozone depletion, WMO model, ReCiPe [kg CFC-11 eq] particulate matter/Respiratory inorganics, RiskPoll [kg PM2,5-Equiv.] photochemical ozone formation, LOTOSEUROS model, ReCiPe [kg NMVOC] resource Depletion, fossil and mineral, reserve Based, CML2002 [kg Sb-Equiv.] terrestrial eutrophication, accumulated exceedance [Mole of N eq ] total freshwater consumption, including rainwater, Swiss Ecoscarcity [UBP]

2.14 × 1000

2.65 × 1000

1.59 × 1003

1.29 × 1003

8.95 × 10−02

1.05 × 10−01

1.88 × 10−05

1.56 × 10−05

3.28 × 10−05

3.23 × 10−05

5.00 × 1004

8.50 × 1004

2.07 × 1002

2.47 × 1002

1.19 × 10−01

6.79 × 10−02

1.78 × 10−05

1.70 × 10−05

2.36 × 10−01

2.23 × 10−01

1.45 × 1000

1.26 × 1000

3.26 × 10−03

1.68 × 10−03

5.09 × 1000

4.46 × 1000

7.59 × 1001

1.24 × 1002

Figure 5. Life cycle impacts of the refining and CNF technologies for the baseline scenario.

results, mainly on the electricity mix and on the maturity of the CNF technology, Scenarios following these assumptions are defined. Scenario 1: Use of bleached sulphite pulp (instead of kraft pulp) for both systems. Scenario 2: 30% of energy improvement for CNF production (Our experiments are new and may be optimized in the future and scaled-up to industrial size. We must keep in mind that the data for the refining are from a very optimized industrial process.) Scenario 3: A combination of scenarios 1 and 2. Scenario 4: Use of Spanish hydropower electricity (instead of the Spanish mix) for both systems. Scenario 5: A combination of scenarios 1, 2, and 4. 12211


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refining and provides more durability to the pulps in the technosphere. In addition, we have found that no pretreatment is needed to produce LCNF by mechanical processes and, therefore, the cost of this production is more than a hundred times lower than by using TEMPO catalytic oxidation. While it is true that the energy demand for each recycling cycle is higher when applying LCNF than when pulp is mechanically refined, a simplified LCA shows that the consumption of natural resources and the impact to the environment or to human health are not higher than those by the mechanical treatment. Moreover, following very plausible future scenarios, such as a decrease in energy demand for such a new technology as CNF production or changing the used electricity mix, recycling using CNF leads to much lower environmental impacts than mechanical refinement. Our final conclusion is that paper recycling by adding CNF is already as sustainable as mechanical refinement in technical, economical and environmental terms and, therefore, it is a promising solution to a more circular economy for the paper sector.

In Figure 6, the results of the CNF system for scenarios 1−4 are presented together with the baseline scenario. The

Figure 6. Impact assessment results of the different scenarios for CNF technology compared to the corresponding one using refining for paper recycling (100% line).

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corresponding Refining system for each scenario would be the line of 100% in the figure. In scenario 1, there is a significant result in favor of the CNF technology, with still only three impact categories with higher impacts (but lower than in the baseline scenario): ionizing radiation, global warming and freshwater consumption. In scenario 2 the numbers are even better, with global warming being similar in both systems. Scenario 3, being the combination of scenarios 1 and 2 is even better in most categories but still with two impact categories with worse results for the CNF technology. Scenario 4 is a big step, showing how depending the result is from the energy profile, with all impact category results for CNF technology being around 40% of those delivered by the refining technology. Scenario 5 gives almost identical results than scenario 4 because taking hydropower electricity makes the biggest difference by far. Finally, it may be possible that, improving the thechnology of CNF production, better nanofibers may be delivered which might increase the number of recycling cycles from our experimental seven to higher numbers. A sensitivity analysis on this parameter has shown that changing from seven to 10 cycles (an increase of 43%) have decreased around 10% of the impact in all categories in respect to the impact of the refining technology. Provided that such an increase in the number of cycles is quite difficult and the result not impressive, this is not a priority for improvement. In sum, the role of mechanical refining and LCNF addition has been investigated. In this sense, it has been demonstrated that the structural damage that mechanical refining causes to fibers results in a lower durability of the fiber in the technosphere when compared to LCNF addition. On the one hand, the initial breaking length increase provided by a 3 wt % of LCNF addition is higher than that provided by 1500 revolutions of mechanical refining. On the other, the residual LCNF in pulp results in a lower decrease of mechanical properties with further recycling cycles, due to the remaining strengthening effect thereof. In this sense, LCNF or CNF (in general terms) are a strong technical alternative to mechanical refining by many reasons: there is no need of huge industrial equipment to apply them in pulps, their strengthening potential is higher than mechanical

AUTHOR INFORMATION

Corresponding Author

*Phone: +34 972 41 8456; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the financial support of the Spanish and Economy and Competitiveness Ministry to the referenced projects: CTQ2012-3686-C02-01 and CTM2011-28506-C0201.

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