Evaluation of an environmental profile comparison for nanocellulose production and supply chain by applying different life cycle assessment methods

Journal Pre-proof Evaluation of an environmental profile comparison for nanocellulose production and supply chain by applying different life cycle assessment methods Janez Turk, Primož Oven, Ida Poljanšek, Anja Lešek, Friderik Knez, Katja Malovrh Rebec PII:

S0959-6526(19)33977-0

DOI:

https://doi.org/10.1016/j.jclepro.2019.119107

Reference:

JCLP 119107

To appear in:

Journal of Cleaner Production

Received Date: 11 April 2019 Revised Date:

15 October 2019

Accepted Date: 30 October 2019

Please cite this article as: Turk J, Oven Primož, Poljanšek I, Lešek A, Knez F, Rebec KM, Evaluation of an environmental profile comparison for nanocellulose production and supply chain by applying different life cycle assessment methods, Journal of Cleaner Production (2019), doi: https://doi.org/10.1016/ j.jclepro.2019.119107. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Evaluation of an environmental profile comparison for nanocellulose production and supply chain by applying different life cycle assessment methods Janez Turk1, Primož Oven2, Ida Poljanšek2, Anja Lešek1, Friderik Knez1, Katja Malovrh Rebec1* 1

Slovenian National Building and Civil Engineering Institute, Dimičeva ulica 12, 1000 Ljubljana, Slovenia 2 University of Ljubljana, Biotechnical Faculty Department of Wood Science and Technology, Jamnikarjeva 101, 1000 Ljubljana, Slovenija *Corresponding author: Fax: + 386 1 2804 484, Phone: + 386 1 2804-514, e-mail: [email protected] E-mail address: [email protected] (J. Turk), [email protected] (P. Oven), [email protected] (I. Poljanšek), [email protected] (A. Lešek), [email protected] (F. Knez), [email protected] (K. Malovrh Rebec).

ABSTRACT The interest in nanocellulose made from woody biomass has been growing rapidly; however, detailed studies on the environmental performance of nanocellulose have only been reported on a few occasions. To fulfill this gap, the environmental performance of nanofibrillated cellulose fabricated from thermogroundwood (removal of extractives, lignin and hemicelluloses, TEMPO oxidation and homogenization processes were included) was evaluated by means of a Life Cycle Assessment. The results show that the purification process contributes more than 95% of the impact. It is associated with a relatively high consumption of electrical energy and ancillary chemicals, i.e., cyclohexane and acetone. The global warming potential of 1 kg of nanofibrillated cellulose is as high as 800 kg CO2 equivalents. Even in the case that in addition to the extractives and the hemiceluloses also lignin is considered as a potentially valuable co-product, and the latter takes over some of the burden, the impact of nanofibrillated cellulose remains relatively high, at around 400 kg CO2 equiv. per kg of nanocellulose. While the primary energy consumption is around 19.000 MJ per kg of nanofibrillated cellulose, or 10.100 MJ in the case that the lignin is considered as a potentially valuable co-product. The study also had a methodological goal, i.e., the impact indicators were calculated using the three most relevant evaluation methods: ILCD/PEF, CML 2001 and ReCiPe 2016. These three methods show similar results for the impact on global warming and acidification. However, in the case of impacts on some other indicators, significant deviations in the obtained impact scores were observed with respect to the results for the three methods. Taking into account the background data of the methods, ReCiPe 2016 was found to be the most up-to-date method and can currently be considered as the preferable Life Cycle Impact Assessment method.

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1 INTRODUCTION Consumers, industry, and governments are increasingly demanding products made from renewable and sustainable resources that are biodegradable, non-petroleum based, carbon neutral, and having low environmental, animal/human-health and safety risks (Khoo et al., 2016; Moon et al., 2011). Cellulose and some of its derivatives are probably the most promising materials to fulfill these criteria. Cellulose is the most abundant polymer on Earth, produced by plants, algae, bacteria and animals Tunicate (Dufresne, 2012). This linear polymer is constructed of anhydrogluycopyranosyl residues connected by β (1 → 4) glycosidic bonds and has the ability to form supramolecular crystalline structures of native cellulose Iα and Iβ (Atalla and VanderHart, 1984; Nishiyama et al., 2002). Cellulose chains are organized in semi-crystalline microfibrils containing alternate crystalline and amorphous regions, whereas the microfibrills aggregate in higher morphological structures, macrobirils or fibrilar aggregates (Donaldson, 2007; Miracle et al., 2001). The isolation of these biological structures leads to new cellulose-based nanomaterials (Herrick et al., 1983; Turbak et al., 1983) having exceptional properties and practically unlimited possibilities for applications. Two types of nanocellulose produced from different biomass raw materials are cellulose nanofibrils and cellulose nanocrystals (Arvidsson et al., 2015). A third type of nanocellulose has a bacterial origin and is referred to as bacterial (nano)cellulose. Nanofibrillated cellulose or microfibrillated cellulose are the terms most frequently used to describe cellulose nanofibrilar objects with diameters of 5–50 nm and lengths of a few micrometers (Moon et al., 2011) (Nechyporchuk et al., 2016). Cellulose nanocrystals are often called nanocrystalline cellulose or cellulose nanowhiskers and have rod-like structures with diameters of 3–35 nm and lengths of 200–500 nm (Dufresne, 2012). The production of nanofibrillated cellulose can be considered as a combination of mechanical, chemical or enzymatic processes (Dufresne, 2012; Hubbe et al., 2008; Rebouillat and Pla, 2013). Most frequently, the production of nanofibrillated cellulose from woody biomass encompasses purification, pre-treatment and the principal treatment. The removal of extractives, hemicelluloses and lignin is achieved by various cooking and bleaching methods, which are similar to those used in the pulping industry (Nechyporchuk et al., 2016). The pre-treatment of purified cellulose pulp is a way to reduce the large energy consumption during nanofibrillated cellulose production and includes enzymatic hydrolysis, carboxylation, carboxymethylation, quaternization, sulfonation and a solvent-assisted pretreatment (Nechyporchuk et al., 2016). The principal treatment is designed to defibrillate the cellulose fibers by intensive mechanical agitation, which includes refining, homogenization, microfluidization, grinding, cryocrusching, high-intensity, ultra-sonication, steam explosion and aqueous counter collision (Kim et al., 2015; Nechyporchuk et al., 2016). The interest in nanocellulose made from woody biomass has been growing rapidly; however, detailed Life Cycle Assessments (LCAs) have only been reported for a few instances where researchers evaluated the environmental impacts associated with the production of nanocellulose. An LCA describes the production phase of products and materials along life-cycle stages and the calculated data can be compared for similar products. Materials designers, consumers and policy makers are increasingly searching for reliable information about the consequences of alternative material substitutes and designs for effective environmental decision making (Ibn-Mohammed et al., 2016). Furthermore, LCA can also be used to identify environmental “hotspots”, which can then guide the process design changes and result in reductions in the environmental footprint of nanocellulose production (Gu et al., 2015). 1

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The environmental performance of nanofibrillated cellulose fabricated from wood pulp was studied by different authors (Arvidsson et al., 2015; Hohenthal et al., 2012; Li et al., 2013). One example was based on experts’ estimations and theoretical data (Hohenthal et al., 2012). They compared three different preparation methods of nanofibrillated cellulose. LCA results were presented in form of five impact categories. However, LCA study was relatively simplified and based on several assumptions, which may influence the certainty of the results. Detailed LCA of nanofibrillated cellulose was performed in two studies (Arvidsson et al., 2015; Li et al., 2013). Li et al. (2013) evaluated four comparable laboratory-scale nanocellulose-fabrication routes; chemical pre-treatment in two of these routes is based on TEMPO oxidation. Arvidsson et al. (2015) also assessed the environmental performance of three production routes for nanofibrillated cellulose (enzymatic production route, carboxymethylation route, homogenization route without pretreatment). Both studies provided comprehensive cradle-to-gate LCA of the nanocellulose fabricated at laboratory scale, with detailed description of processes and inventory data. However, these studies do not provide systematic description of environmental performance of the nanocellulose, as results are presented only for limited (selected) impact categories. Aim of these studies is rather a comparison of different fabrication processes to see which one is the environmentally preferable and identification of environmental hotspots (materials or processes which contribute the most to the total environmental footprint of the product). Interesting laboratory-scale LCAs were conducted by Piccinno et al. (2016; 2018) for the production of cellulose nanofibers and the final product spun yarn. The raw material was vegetable food waste and the release of the fibers was conducted enzymatically. However, LCA results were presented for global warming and human toxicity mid-point impact categories only. In the case of other impact categories, no absolute values were presented, only the relative contribution of different production processes on the certain impact. Furthermore, different authors (De Figueirêdo et al., 2012; Husgafvel et al., 2016; Nascimento et al., 2016) also studied the environmental performance of the production of nanocellulose; however, in this case it was nanocrystalline cellulose. In this LCA study, the environmental performance of nanofibrillated cellulose produced from thermo-groundwood was evaluated with different Life Cycle Impact Assessment (LCIA) methods. The nanofibrillated cellulose fabrication process encompass (1) a purification step starting with Soxhlet extraction, delignification, and the removal of hemicellulose, (2) a pre-treatment of purified cellulose fibers via TEMPO-mediated oxidation and (3) a principal mechanical treatment by homogenization. A laboratory-scale fabrication process was taken into account. This study devoted a lot of effort to collecting in-depth data and researched material-use and energy (resources and wastes) flows associated with the product system, since it is well known that a good LCA relies on a robust Life Cycle Inventory (LCI) and the quantification of the material database and energy being modelled (Zhou et al., 2011). The quality of the results strongly depends on the quality of the underlying LCI data.

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As only limited number of impact category indicators was reported in previous studies dealing with environmental footprint of nanofibrillated cellulose, a holistic report on environmental footprint of the nanocellulose (taking into account cradle-to-gate approach) is still missing. Our research aims to fill this gap. LCA results in this paper are reported for three different, most commonly used impact assessment methods in order to provide a material for comparisons with all future studies. Another

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reason for calculation of results with different methods is to compare the differences and discuss the reasons for this. Three different impact assessment methods are thus critically evaluated.

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2 MATERIALS

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Nanofibrillated cellulose was isolated from thermo-groundwood by a chemo-mechanical method using a high-pressure homogenization technique. Cellulose fibers were extracted from the thermogroundwood in three separate stages: removal of extractives with Soxhlet extraction, delignification and removal of hemicelluloses (Fig. 1). Subsequently, the obtained cellulose fibers were chemically treated with the reagent TEMPO and then mechanically defibrillated into nanofibrils using a highpressure homogenization. Obtained highly viscous TEMPO oxidized nanofibrillated cellulose was characterized by FT-IR, conductometric titration and FE-SEM. FT-IR spectra revealed complete removal of lignin, partial removal of hemicelluoses and the presence of –COOH in a deprotonated state at bands 1604 cm-1. The product had a carboxyl group content of 1.13 mmol g-1. Crystallinity index determined from FT-IR spectra was 48 % and the dimeter of nanofibrils was in the range of 5-20 nm.

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Fig. 1: Processing scheme for the production of 1 kg nanofibrillated cellulose. The scheme also presents the system boundaries of the study.

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Thermo-groundwood was collected at local paper manufacture Vipap Videm Krško d.d. (Krško, Slovenia). The chemical reagents utilized include sodium chlorite, acetic acid, sodium hydroxide, sodium bromide, sodium hypochlorite (10–15% of active chlorine) and (2,2,6,6-tetramethylpiperidin3

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1-yl)oxidanyl (TEMPO), which were purchased from Sigma Aldrich (Germany) and were of reagent grade. Cyclohexane and acetone were of reagent grade and supplied by ECP d.o.o. (Slovenia). All the reagents were used as received and without further purification.

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2.1. Extraction of α-cellulose

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The cellulose fibers were extracted from the thermo-groundwood using the extraction, delignification and removal of hemicelluloses in accordance with ASTM D1104-56 (1978). Cyclohexane (250 mL per 5g of sample) and acetone (250 mL per 5g of sample) were used to remove the lipophilic and hydrophilic extractives with sequential Soxhlet extraction at 120 °C and 110 °C, respectively, for 6 hours. Afterwards, the thermo-groundwood without extractives was suspended in 160 mL of hot distilled water (80 °C) in an Erlenmeyer flask. The Erlenmeyer flask was transferred to a water bath, while the temperature of the system was adjusted to 70 °C. Then 0.5 mL of acetic acid and 1.5 g of sodium chlorite were introduced into the Erlenmeyer flask every hour for a period of 7 hours. The product was washed and filtered with distilled water (450 mL) and acetone (50 mL). It is known as holocellulose. Applying a slightly modified ASTM D1103-60 (1977) method, the produced holocellulose (in 250 mL of water) was converted to α-cellulose with the addition of 9.7 g of sodium hydroxide. The time of the exposure was 7 hours at a temperature of 23±2 ºC. The generated αcellulose was thoroughly filtered and washed with 500 mL of distilled water and 25 ml of ethanoic acid to neutralize the alkaline cellulose. After that, the fibers were continuously washed with distilled water until the pH value reached 7. The resultant cellulose fibers were suspended in water to obtain a 1 % suspension (w/v).

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2.2. Disintegration of cellulose fibers to nanofibrils

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2.2.1. Chemical pre-treatment of α-cellulose –TEMPO oxidation

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The TEMPO oxidation procedure was carried out according to a previously reported routine (Saito et al., 2006). Cellulose pulp (5g) was suspended in 500 mL of distilled water, and 80 mg of TEMPO and 500 mg of sodium bromide were added to the suspension. Sodium hypochlorite was added in a concentration of 5 mmol per gram of cellulose fibers (9.45 mL of 13 % solution). The pH of the suspension was maintained between 10.0 and 10.2 for 60 min using 0.5-M sodium hydroxide. The fibers were filtered out and rinsed several times with warm distilled water to remove any traces of the reagents. They are referred to as TEMPO-oxidized cellulose.

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2.2.2 Mechanical high-pressure homogenization

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The nanofibrillated cellulose was isolated from the TEMPO-oxidized cellulose through a process of high-pressure homogenization. The oxidized fibers were suspended in water at a concentration of 1.5% and passed through a high-pressure homogenizer (Panda PLUS2000, Gea Niro Soavi, Italy) four times, first at 300 bar, and then three times at 1000 bar, to give a TEMPO-oxidized cellulose nanofibril dispersion. The dispersion was diluted between the homogenization steps. Using this process, the fibers were broken down from micro-sized structures to nano-sized structures, forming slurries of nanofibrillated cellulose. The final solids content of the dispersion was 1%.

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3 METHODS

147 148

A Life Cycle Assessment (LCA) is a structured, comprehensive and internationally standardized (ISO 14040, ISO 14044) method. It quantifies all the relevant emissions and resources consumed, as 4

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well as the related environmental and health impacts and resource-depletion issues that are associated with a certain product or process (European Commission, 2010). It modularly calculates the footprints related to material or product production, construction, the use stage, the end-of-life stage and also takes into account the benefits and loads beyond the system boundaries.

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An LCA is also an important tool used to fulfill the requirements laid down in the Construction Products Regulation (Directive 93/68/EEC). This law states that all products traded or sold in Europe must bear a CE mark, which indicates that a product complies with the regulation. The construction products regulation (CE mark) describes seven basic requirements (BWR) for products, sustainable use of natural recourses being one of them (BWR 7). The implementation of BWR 7 by means of an LCA according to EN 15804 is expected at the beginning of 2019. This regulation ensures that reliable information is available to professionals, public authorities, and consumers, so that they can compare the performance of products from different manufacturers in different countries.

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3.1

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The primary aim of this study was to evaluate the environmental performance of nanofibrillated cellulose produced on a laboratory scale. Because this study is part of a larger study, where nanocellulose is supposed to be integrated into different products (for example, the paper industry, the automotive industry, and the textile industry), it will be an important input for the LCAs of products upgraded with nanocellulose.

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Another important goal of our study was to identify production hotspots, i.e., the parts of the fabrication process that contribute the most to the total environmental footprint of the product. It might be helpful when up scaling from the laboratory to large-scale production, while maintaining the smallest-possible environmental footprints.

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The study also had a methodological goal, i.e., to compare the results obtained when using the most relevant impact-assessment methods: ILCD/PEF, CML 2001 and ReCiPe 2016.

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The functional unit was defined as 1 kg of dry nanofibrillated cellulose. The material and energy requirements for the fabrication of 1 kg of the nanocellulose are shown in Table 1. An attributional LCA approach was taken into account, meaning that the environmental burdens were not attributed only to the final product, i.e., nanofibrillated cellulose, but also to all the generated co-products.

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Table 1: Ancillary material (chemicalls) requirements and electricity consumption for production of 1 kg of nanofibrilated cellulose from thermo-groundwood as a raw material.

Definition of the goal and the scope of the study

Purification

Pre-treatment Treatment

Soxhlet extraction

Delignification

Removal of hemicellulos es

TEMPOoxidation

Homogenization

5.5 18,000 85.4 81.7 0 0

194.1 0 0 7.9 4.5 0

341.3 0 0 0 0 0

600 0 0 0 0 1.89

11 0 0 0 0 0

Unit Distilled water Tap water Cyclohexane Acetone Sodium chlorite Sodium hypochlorite

L L kg kg kg kg

5

Acetic acid Sodium hydroxide Reagent TEMPO Sodium bromide Hydrochloric acid

kg kg

0 0

2.3 0

0.86 4.3

0 0.2

0 0

kg

0

0

0

0.016

0

kg

0

0

0

0.1

0

kg

0

0

0

0.0036

0

Electricity

kWh

650

641.3

0.044

0.022

23.66

179

3.2 System boundaries

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System boundaries include the processes beginning with the extraction or production of raw materials for chemical precursors and ending with the fabrication of the final product in the laboratory. Such boundaries are defined as "cradle-to-gate" and include the following processes (see also

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Fig. 1): - Production of raw material (thermo-groundwood). - Synthesis of ancillary materials (chemicals and deionized water), required to support the fabrication process for nanofibrillated cellulose. - Fabrication of nanofibrillated cellulose in the laboratory. All the related energy requirements are accounted for, as well as the emissions associated with the above-mentioned processes. The transport of the wood to the paper mill, where the production of 6

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thermo-groundwood takes place, and the transport of ancillary materials (i.e., chemicals) are also taken into account. LCA studies of nanomaterials are usually production-focused, meaning that they are cradle-to-gate studies, and so do not consider the impacts of nanomaterial release during product use or end-of-life disposal. The reason for excluding downstream processes in this study is similar as in other studies; e.g. lack of inventory data related with further use and end-of-life stage. Significant problem is also uncertainty regarding nanomaterial releases and impacts. Toxicity information of nanomaterials is still not readily available in current LCA databases or impact assessment methods (Eckelman et al., 2012; Gilbertson et al., 2015). To improve this gap, more LCI data would be required. Life cycle impact assessment needs to be extended by impact factors for a wider range of chemical and pharmaceutical compounds (Hischier and Walser, 2012; Kralisch et al., 2015).

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3.3 Life Cycle Inventory Analysis (LCI)

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The Ecoinvent database (version 3.3) was used to evaluate the environmental impacts of the ancillary materials that are required to support the fabrication of nanofibrillated cellulose from thermogroundwood (Table 2). The environmental footprint of the latter raw material was evaluated based on inventory data obtained from a producer. These inventory data include the production and harvesting of stemwood, with the associated forestry activities, its delivery to the paper mill and the production of thermo-groundwood at the paper mill. The latter involves stemwood processing, which requires energy consumption (electricity from the grid mix, steam from the paper mill’s own production, use of chemicals as ancillary materials, use of water and its treatment at the paper mill’s own wastewatertreatment plant). The burdens related to the delivery of chemicals to their users (which in this study are the manufacturers) are already included in the Ecoinvent datasets as market processes. The market processes include inputs from production as well as inputs from transport processes. When the specific supplier is unknown, using the market process is recommended. In the case of the market process, some average deliverable distances for certain geographical regions are assumed, meaning that the burdens related to fuel consumption and combustion represent an approximation. In our case this dataset seemed a reasonable choice, since the production data taken into account are related to laboratory process where specific subcontractors or suppliers are not defined. No inventory data could be found for the synthesis of sodium chlorite (NaClO2), sodium bromide (NaBr) and the TEMPO reagent ((2,2,6,6-tetramethylpiperidin-1-yl) oxidanyl). To cope with this problem, we relied on the inventory of other chemicals, in terms of an environmental burden for a production phase analogous to the originally used chemicals. In such a case, the environmental footprint of a replacement chemical should be similar to that of the originally used chemical (Ravelli et al., 2011). We assumed that the environmental footprint of sodium bromide (NaBr) is similar to the footprint of sodium chloride (NaCl), and therefore the inventory data of the latter were used as proxy data. The same was also assumed by (Li et al., 2013). We also assumed that the synthesis of sodium chlorite (NaClO2) is similar to the synthesis of sodium hypochlorite (NaClO), the inventory data for which are available in the Ecoinvent database. To estimate the environmental footprint of the TEMPO reagent ((2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl), the inventory data for a related chemical, piperidine, were used (Table 2). The electricity consumption was calculated based on the operating time and the equipment’s power specifications. The environmental burdens related to the production of electricity at power plants were evaluated based on the inventory data available in Ecoinvent.

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Table 2: Life cycle inventory for the production of 1 kg of nanofibrilated cellulose from thermogroundwood as a raw material. Inputs

Unit

Amo

Description

Source

unt Thermo-ground kg wood Distilled water L

2.19

Tap water

L

18,000

Cyclohexane

kg

85.4

Acetone

kg

89.6

1151.9

Sodium chlorite kg

4.5

Sodium hypochlorite

kg

1.89

Acetic acid

kg

3.16

Sodium hydroxide

kg

4.5

Reagent TEMPO Sodium bromide Hydrochloric acid

kg

0.016

kg

0.1

kg

0.0036

Electricity

kWh

1315.03

Outputs

Unit

Amo

Thermo-groundwood production

Primary data from Vipap paper mil Market for water, deionised, from tap Ecoinvent 3.3 water (European production) Market group for tap water (European Ecoinvent 3.3 production) Market for cyclohexane, (global Ecoinvent 3.3 production) Market for acetone, liquid (global Ecoinvent 3.3 porduction) Proxy datum: market for sodium Ecoinvent 3.3 hypochlorite, without water, in 15% solution state (global production) Market for sodium hypochlorite, Ecoinvent 3.3 without water, in 15% solution state (global production) Market for acetic acid, without water, in Ecoinvent 3.3 98% solution state (global production) Market for sodium hydroxide, without Ecoinvent 3.3 water, in 50% solution state (global production) Proxy datum: market for piperidine Ecoinvent 3.3 (global production) Proxy datum: market for sodium Ecoinvent 3.3 chloride, powder (global production) market for hydrochloric acid, without Ecoinvent 3.3 water, in 30% solution state (European production) Market for electricity, low voltage Ecoinvent 3.3 (Slovenian grid mix)

Description

unt Extractives

kg

0.03

Co-product

Lignin

kg

0.94

Waste material or eventually co-product

Hemicelluloses kg

0.22

Co-product

Nanofibrillated kg cellulose

1.00

Final product

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3.2

Allocation principles

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Some processes contribute to the provision of more than one function by yielding more than one product (e.g., co-products). In the case of the fabrication of nanofibrillated cellulose, two co-products 8

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are generated, i.e., extractives and hemicelluloses (with further separation to xylans, mannans and glucomannans being possible), lignin is also an output of the process (see

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Fig. 1). Partitioning of the environmental burdens is a common practice in the case that the process involves the production of not only the product of interest, but also co-products during the manufacturing stage. Product systems, such as the one described here, require the issue of allocation to be considered, to determine the proportion of the environmental impacts that will be attributed to the production of each co-product. Allocation by physical relationships (e.g., mass, energy) or the economic value of co-products can be used to partition the inputs and outputs. In the case of physical allocation (by mass), impacts are divided on the basis of the relative mass of each co-product produced by the process under investigation. In the case of economic allocation, the relative values or the price associated with each co-product and the main product should be considered (European Commission, 2010); (Chomkhamsri and Pelletier, 2011); (Lehtinen et al., 2011). Economic allocation is usually encouraged, with the justification being that it is fairer; however, usually it is very difficult to gather the relevant data, especially at the laboratory level of production.

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Extractives and hemicelluloses represent a mixture of various substances, which need to be further processed in order to extract them in the form of valuable materials. Raw-material demand, energy consumption, the requirements of ancillary materials (chemicals), which are associated with environmental burdens, were partitioned between the final product and the two co-products (extractives, hemicellulose). A mass allocation was applied. Due to the relatively high calorific value of lignin, it is usually combusted. Therefore, no allocation was accounted to lignin in the baseline scenario, considering that it is waste. 9

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An economic allocation was not possible, because the extractives and hemicelluloses must be further processed in order to obtain substances with actual economic value. This kind of processing is out of the scope (and the system boundaries) of this study. A total of 2.19 kg of thermomechanical pulp is required to produce 1 kg of nanofibrillated cellulose (a functional unit). During the process of pre-treating the pulp, 0.03 kg of extractives, 0.94 kg of lignin and 0.22 kg of hemicelluloses are co-produced (

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Fig. 1). The environmental burdens associated with Soxhlet extraction (consumption of chemicals and water, energy requirements) were partitioned between the extractives and the processed thermogroundwood that needs to be subjected to further purification processes (delignification, removal of hemiceluloses), before being chemically pre-treated (via TEMPO oxidation) and mechanically disintegrated (via a homogenization process). The amount of thermo-groundwood after being pretreated with Soxhlet extraction is 2.16 kg and the amount of generated extractives is 0.03 kg. Taking into account the mass allocation, 98.5% of the burden is allocated to the processed thermogroundwood and the remainder of the 1.5% burden to the extractives. The burdens associated with the purification processes of delignification and the removal of hemicelluloses are partitioned between the hemicelluloses and the rest of the raw material (alpha cellulose) that is used for the nanocellulose production. In the case of mass allocation, 82% of this burden is partitioned to nanofibrillated cellulose as a final product and 18% to hemicellulose. No burden was attributed to lignin, which is considered as a waste material. Lignin can be burnt and the energy generated can be used. Paper and cellulose mills usually burn lignin to generate power and recycle the energy in the process (Nascimento et al., 2016). 10

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3.4.1 Scenario analysis

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If lignin is also considered as a potentially valuable co-product (Renders et al., 2017), then the allocation of environmental burdens should be conducted in a different way to that referred to above. A total of 44% of the burdens associated with delignification should be attributed to lignin and 56% to the rest of the raw material (holocellulose), which is further purified by the process of removing the hemicelluloses. In the latter processing stage, 18% of the burdens are attributed to hemicelluloses and 82% to the rest of the raw material (alpha cellulose), which is further processed to obtain the endproduct, i.e., the nanocellulose. Using this approach, the environmental footprint for nanofibrillated cellulose is relatively lower. Scenario analysis was conducted also for potential fabrication of the nanofibrillated cellulose at industrial scale, taking into account some reliable expert predictions.

297

3.5 Life cycle impact assessment – the use of different evaluation methods

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A number of methods used for the Life Cycle Impact Assessment (LCIA) convert the anthropogenic emissions and the extractions of natural resources (mineral, fossil) into impact category indicators at the midpoint level (global warming, for example), while others employ impact category indicators at the endpoint level such as damage to the ecosystem quality, human health and resource availability (Goedkoop et al., 2013). In this study, the impact categories at the midpoint level were calculated using three LCIA methods (ILCD/PEF version 1.09, CML 2001 version Jan. 2016 and ReCiPe 2016 version 1.1). One reason for doing this is that the results of various studies are presented with different assessment methods and we are interested in a comparison of the results of these studies. Moreover, the results obtained using three different evaluation methods were compared and in the case of differences, the reason for this was discussed. Unfortunately, there are still many studies where only a particular aspect, most commonly greenhouse gases (for example, Kirchofer et al., 2012) are observed, but all the other parameters are omitted, which should not be promoted in the future.

310

3.2.1

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The International Reference Life Cycle Data System (ILCD) published ‘Recommendations for Life Cycle Impact Assessment in the European context’, which chooses the methodology that has been evaluated as the best within the impact category. This is further developed as part of the Product Environmental Footprint (PEF) projects (European Commission, 2012). The recommended method, which is referred to as ILCD/PEF, is compiled by assessing various characterization models from different LCIA methodologies and choosing the most appropriate. The midpoint impact categories addressed by ILCD/PEF (version 1.09) are shown in the electronic supplementary material Table S1.

318

3.2.2

319 320 321 322 323 324

Among the numerous LCIA methodologies the CML 2001 method aims to provide one of the best practices for midpoint indicators, operationalizing the ISO14040 series of standards. Explicit scientific foundations consistently support all the important choices made and the improvements of CML 2001 (European Commission, 2010). The results of the CML 2001 impact-assessment method can be presented in terms of 12 impact potentials (Table S1 of the supplementary material). The last update of the method, which is used in this study, is from January 2016.

325

3.2.3

ILCD/PEF impact-assessment method

CML 2001 impact-assessment method

ReCiPe 2016 impact-assessment method

11

326 327 328 329 330 331 332 333 334 335 336 337

The ReCiPe method was first developed in 2008 to align two families of methods for LCIA: the midpoint-oriented CML 2001 method and the endpoint-oriented Eco-indicator 99 method. The ReCiPe method thus provides harmonized characterization factors at the midpoint and endpoint levels. The method, which is supported by the Dutch government, is commonly use in Europe (Huijbregts et al., 2016). ReCiPe 2016, which is an updated version (1.1), was used in this study. The methodologies and the data used in the most recent ReCiPe models are up to date with the current scientific knowledge. In this study, we were only interested in the impact categories addressed at the midpoint level (Table S1 of the supplementary material). Three perspectives are included in ReCiPe 2016. The hierarchic perspective, which is based on a scientific consensus with regards to the time frame and the plausibility of impact mechanisms, was applied in this study. The time horizon of the hierarchic perspective is typically 100 years, which is referenced in the ISO standards on LCA (Goedkoop et al., 2013).

338

3.2.4

339 340 341

The primary energy demand (from renewable and non-renewable resources considering the net calorific value in MJ) related to the fabrication of the nanocellulose was evaluated independently, as it is not included in any of the above-mentioned LCIA methods.

342

4 RESULTS AND DISCUSSION

343 344 345 346 347 348 349 350

The contribution of different processes to the environmental footprint of nanofibrillated cellulose is shown in Fig. 2 and in Tables 3 to 5, for three different LCIA methods (ILCD/PEF, CML 2001 and ReCiPe 2016). These results refer to cradle-to-gate system boundaries in the life cycle of nanocellulose. The results are shown for each LCIA methodology separately, as the three applied LCIA methods address several impact categories at the midpoint level, which are not all exactly the same (Table S1 of the supplementary material). Detailed information on environmental footprint of applied raw material, ancillary chemicals and electricity calculated by three LCIA methods are shown in Tables S2 to S4 of the supplementary material.

351

4.1

352 353 354 355 356 357 358 359

Soxhlet extraction was found to be environmentally the most burdening process in the discussed life cycle of nanofibrillated cellulose (see Tables 3 to 5). Its contributions to certain impact-category indicators range between 63 to 80%, taking into account the ILCD/PEF evaluation method (Table 3). The reason for this is that Soxhlet extraction is an energy-intensive process and a relatively large quantity of ancillary chemicals are consumed during this process. Soxhlet extraction is a traditional procedure, which is nowadays often replaced by more advanced laboratory techniques, like speed or accelerated solvent extraction, which are characterized by a lower consumption of solvents and energy.

360 361 362 363 364 365 366

The purification of thermo-groundwood, which encompasses Soxhlet extraction, delignification and the removal of hemicelluloses, together contributes more than 95% of the burden, in all the impact categories. The contribution of the other processes of chemical pre-treatment via TEMPO oxidation and mechanical disintegration via homogenization processes is relatively minor (see Tables 3 to 5). The reason are low energy requirements and slight (in the case of the TEMPO oxidation process) or even no need to use ancillary chemicals (in case of the homogenization process). Several ancillary chemicals are required to support the TEMPO oxidation, but all in relatively low amounts (Table 1).

Primary energy demand

Identification of hotspots

12

367 368 369 370 371 372 373

The distribution of environmental burdens through the life cycle of nanocellulose is directly associated with the primary energy demand (e.g., the consumption of renewable and non-renewable energy resources). Just the Soxhlet extraction is responsible for 79% of the primary energy demands in the life cycle of nanofibrillated cellulose. Around two-thirds of the required electrical energy is consumed during the process of Soxhlet extraction. Moreover, the syntheses of cyclohexane and acetone, required to support the Soxhlet extraction, are also associated with a significant consumption of primary energy (see Fig. 3).

374 375 376 377 378 379 380 381 382 383 384

A detailed analysis shows that the contribution of the raw material (i.e., the thermo-groundwood) to the environmental footprint of the final product (i.e., nanofibrillated cellulose) is minor (see Tables 3 to 5). The electricity requirements for the nanocellulose fabrication process represent the main hotspot in all the impact categories, considering the cradle-to-gate approach in the life cycle of nanofibrillated cellulose. For example, electricity contributes around 50% to the total impact on global warming (Fig. 2), taking into account the environmental footprint of the electrical power produced in Slovenia. The Slovenian electricity grid mix consists of electricity that is mostly produced at nuclear power plants (27%), followed by electricity produced at thermal power plants (25%) and at hydro power plants (18%) (Turk et al., 2017). Electricity produced at thermal power plants (in this case run on lignite) is responsible for a large share of the various emissions, especially the greenhouse-gas emissions that affect global warming.

385 386 387 388 389 390 391 392 393 394 395 396

The secondary hotspots are attributed to the syntheses of cyclohexane and acetone, as ancillary materials. These two chemicals belong to the group of petrochemicals, the characteristics of which are relatively high environmental footprints. This is directly associated with the consumption of primary energy (Fig. 3). Both chemicals are required for the purification of thermo-groundwood. The relative contribution of the other ancillary chemicals to the impact categories is relatively minor (see also Fig. 3, showing the primary energy demands of various chemicals, water production and electricity production). The environmental footprint of the TEMPO reagent ((2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl) is also relatively high. It is comparable to the footprint of the petrochemicals, and in some impact categories it is even higher. However, because a very small amount of TEMPO reagent is required (Table 1), the TEMPO oxidation process has only a minor contribution to the environmental burdens in the life cycle of nanofibrillated cellulose.

13

397 398 399 400

Fig. 2: Contribution of different materials and electricity to various environmental indicators, taking into account the production of 1 kg of nanofibrillated cellulose (comparison of ILCD/PEF, CML 2001 and ReCiPe 2016 methods).

401 402 403

14

404 405 406

Fig. 3: Primary energy demand from renewable and non-renewable resources (net calorific value), taking into account the production of 1 kg of nanofibrillated cellulose.

407 408 409 410

Table 3: Table represents total impact scores attributed to nanofibrillated cellulose and relative contribution (%) of different processes included in nanofibrilated cellulose production to various impact categories (results based on ILCD/PEF evaluation method).

Impact scores 8.84 mole of H+ Acidificatio eq. n Climate change, excl 764.77 biogenic kg CO2 eq. carbon Climate change, incl 767.58 biogenic kg CO2 eq carbon 13025. Ecotoxicity 46 CTUe freshwater Eutrophica 0.76 kg tion P eq. freshwater 0.77 kg Eutrophica N eq. tion

Thermogroundwo od (%)

Soxhlet extraction (%)

Removal of TEMPO Delignificat hemicellul. oxidation ion (%) (%) (%)

Homogeniz ation (%)

0.45

69.42

27.25

0.47

0.23

2.18

0.33

76.53

20.45

0.80

0.40

1.49

-0.25

76.80

20.71

0.80

0.41

1.52

0.26

65.72

30.77

0.44

0.27

2.54

0.50

66.25

30.08

0.44

0.24

2.49

0.39

73.59

22.85

0.84

0.61

1.72

15

marine Eutrophica tion terrestrial Human toxicity, cancer effects Human toxicity, non-cancer effects Ionizing radiation, human health

Land use

Ozone depletion Particulate matter/Re spiratory inorganics Photoche mical ozone formation Resource depletion water Resource depletion, mineral, fossils and renewable s

6.82 mole of N eq.

0.36

75.77

20.95

0.92

0.48

1.51

6.95E05 CTUh

0.42

69.80

26.50

0.66

0.47

2.14

2.44E04 CTUh

0.43

67.03

28.55

1.04

0.64

2.30

190.10 kBq U235 eq.

0.56

63.26

32.73

0.47

0.25

2.73

388.89 kg C def. eq.

4.90

69.21

21.10

2.32

1.03

1.44

5.70E05 kg CFC11 eq.

0.48

63.21

26.87

5.25

2.28

1.92

0.71 kg PM2.5 eq.

0.38

72.55

23.23

1.42

0.62

1.80

3.04 kg NMVOC eq.

0.28

80.85

16.75

0.64

0.31

1.18

33.41 m³ eq.

0.58

67.33

28.62

0.55

0.54

2.37

0.01 kg Sb eq.

0.43

71.48

21.63

2.80

2.20

1.46

411 412 413 414 415

Table 4: Table represents total impact scores attributed to nanofibrillated cellulose and relative contribution (%) of different processes included in nanofibrilated cellulose production to various impact categories (results based on CML 2001 evaluation method). Impact scores

ThermoSoxhlet groundwo extraction

Delignifica Removal tion (%) of 16

TEMPO oxidation

Homogeni zation (%)

od (%)

Abiotic Depletion elements Abiotic Depletion fossil Acidificatio n Potential Eutrophica tion Potential Freshwate r Aquatic Ecotoxicity Pot. Global Warming Potential Global Warming Potential excl biogenic carbon Human Toxicity Potential Marine Aquatic Ecotoxicity Pot. Ozone Layer Depletion Potential Photoche m. Ozone Creation Potential Terrestric Ecotoxicity Potential

(%)

hemicellul. (%) (%)

1.21E03 kg Sb eq.

0.21

68.69

20.71

5.81

3.43

1.15

13682. 36 MJ

0.19

84.90

13.25

0.60

0.25

0.80

7.87 kg SO2 eq.

0.45

69.14

27.52

0.46

0.23

2.20

2.64 kg PO4 eq.

0.48

67.21

29.11

0.51

0.30

2.39

568.05 kg DCB eq.

0.39

65.75

30.60

0.47

0.26

2.53

776.61 kg CO2 eq.

-0.24

76.91

20.61

0.80

0.41

1.50

773.82 kg CO2 eq.

0.33

76.64

20.36

0.80

0.40

1.48

849.07 kg DCB eq.

0.20

84.60

13.31

0.51

0.32

1.05

12821 37.22 kg DCB eq.

0.47

66.54

29.45

0.74

0.40

2.40

5.71E05 kg R11 eq.

0.48

62.96

27.14

5.24

2.27

1.92

0.73 kg ethene eq.

0.23

83.05

14.99

0.45

0.21

1.06

5.33 kg DCB eq.

0.23

70.16

25.71

1.14

0.74

2.02

416 417 418 419

Table 5: Table represents total impact scores attributed to nanofibrillated cellulose and relative contribution (%) of different processes included in nanofibrilated cellulose production to various impact categories (results based on ReCiPe 2016 evaluation method). 17

Impact scores Climate change, default, excl biogenic carbon Climate change, incl biogenic carbon Fine Particulate Matter Formation Fossil depletion Freshwate r Consumpti on Freshwate r ecotoxicity Freshwate r Eutrophica tion Human toxicity, cancer Human toxicity, non-cancer

Ionizing Radiation

Land use Marine ecotoxicity

Thermogroundwo od (%)

Soxhlet extraction (%)

Removal of TEMPO Delignificat hemicellul. oxidation ion (%) (%) (%)

Homogeniz ation (%)

806.92 kg CO2 eq.

0.32

76.71

19.86

1.29

0.39

1.44

810.65 kg CO2 eq.

-0.23

76.99

20.08

1.30

0.41

1.46

2.22 kg PM2.5 eq.

0.42

69.97

25.94

1.26

0.35

2.06

402.80 kg oil eq.

0.25

80.85

16.68

0.84

0.24

1.15

28.20 m3

0.96

82.83

10.18

2.87

2.42

0.74

25.28 kg 1,4 DB eq.

0.18

64.50

32.07

0.43

0.15

2.68

0.76 kg P eq.

0.50

66.05

29.99

0.74

0.24

2.49

49.38 kg 1,4-DB eq.

0.42

69.31

26.60

1.06

0.45

2.16

1051.7 2 kg 1,4DB eq.

0.42

66.84

28.58

1.33

0.51

2.32

180.50 Bq C-60 eq. to air

0.56

63.08

32.76

0.64

0.22

2.74

20.48 Annual crop eq.·y

12.96

59.26

24.01

1.33

0.50

1.94

0.19

64.70

31.81

0.49

0.17

2.65

31.17 kg 1,4-DB

18

eq. Marine Eutrophica tion Metal depletion Photoche mical Ozone Formation, Ecosystem s Photoche mical Ozone Formation, Human Health Stratosphe ric Ozone Depletion Terrestrial Acidificatio n

Terrestrial ecotoxicity

0.05 kg N eq.

0.50

65.09

29.86

1.02

1.06

2.47

0.75 kg Cu eq.

0.25

71.88

20.82

3.70

1.88

1.46

3.89 kg NOx eq.

0.26

75.45

19.47

2.36

1.11

1.35

3.01 kg NOx eq.

0.28

75.43

19.74

2.18

0.98

1.38

2.12E04[kg CFC11 eq.

0.51

63.23

28.36

4.50

1.20

2.20

6.44 kg SO2 eq.

0.45

68.92

27.49

0.72

0.22

2.20

55958. 11 kg 1,4DB eq.

0.01

99.50

0.39

0.05

0.02

0.03

420

4.2

Sensitivity/scenario analysis

421

4.2.1

422 423 424 425 426 427 428 429 430

A relatively large amount of lignin is extracted from the thermo-groundwood during the delignification process (i.e., 0.429 kg from 1 kg of the pulp). Considering lignin as a co-product, which is potentially valuable in a subsequent application, instead of as a waste material, means that the lignin can take over a considerable part of the environmental burdens in the two most environmental and energy-demanding processes, i.e., Soxhlet extraction and delignification. In such a case, the environmental footprint of the nanofibrillated cellulose would decrease by around 40%, taking into account the results obtained with the ILCD/PEF impact assessment method (see Fig. 3). Taking into account the results obtained using the CML 2001 method, the decrease is greater, i.e., between 45 and 50% (Fig. 2).

Lignin as a potentially valuable co-product

19

431 432 433 434 435 436

Fig. 4: Relative comparison of LCA results for two scenarios. The reference is the baseline scenario (lignin considered as a waste material). In the case of the alternative scenario, lignin is considered as a potentially valuable co-product, which takes over part of the burdens related to Soxhlet extraction and the delignification pre-treatment processes. The CML 2001 (a) and ILCD/PEF (b) methods were used to compare the two scenarios.

437

4.2.2

438 439 440 441 442 443 444 445 446

The process of nanofibrillated cellulose fabrication was tested at laboratory scale only. With the transition to a higher technological level (pilot or industrial production), comparable final product could be achieved without the process of Soxhlet extraction, which requires large amount of cyclohexane, acetone and water. This process is also energy demanding (see Table 1). At industrial scale of the nanocellulose production, the extractives can be largely removed during the delignification and removal of hemicelluloses, meaning that the need of cyclohexane as ancillary chemical would be totally omitted. Acetone and energy requirements would be significantly reduced. In such a case, the environmental footprint of the nanofibrillated cellulose would decrease by between 60 and 85% (Fig. 5).¸

Prediction for fabrication of nanofibrillated cellulose at industrial level

447

20

448 449 450

Fig. 5: Relative comparison of LCA results based on laboratory scale and at industrial scale (assumption) fabrication of the nanofibrillated cellulose. The CML 2001 (a) and ILCD/PEF (b) methods were used to compare the results.

451

4.3

452 453 454 455 456 457

In this section the results of selected midpoint impact categories are compared, taking into account the ILCD/PEF, CML 2001 and ReCiPe 2016 methods and the reasons for the differences are interpreted. The global warming potential (GWP), ozone-depletion potential (ODP), photochemical ozone-creation potential (POCP), acidification potential (AP) and human-oxicity potential (HTP) impact categories were included in the comparison. These impact categories are common to all three discussed methodologies.

458 459 460 461 462 463 464 465 466 467 468

The differences in the impact scores obtained using the different impact-assessment methods can be attributed to the differences in the background model, the difference in the substance coverage and also to the errors occurring during the implementation of characterization factors into the modeling software, as found by (Owsianiak et al., 2014). For a given impact category, the background model comprises a category indicator, a characterization model and characterization factors derived from the model (Guinee, 2002). The sources of background models per each selected impact category of the ILCD/PEF, ReCiPe 2016 and CML 2001 impact-assessment methods are shown in Table S5 of the supplementary material. Substance coverage was also checked for each impact category, separately for the ILCD/PEF, CML 2001 and ReCiPe 2016 methods (Table S5 of the supplementary material). Potential errors occurring during the implementation of the characterization factors into the modeling software were also looked for.

469

4.3.1

470 471 472 473 474 475 476

The source of the ILCD/PEF, CML and ReCiPe characterization models for climate change at the midpoint is the IPCC (2013) report (Table S5 of the supplementary material). However, the characterization factors of certain substances affecting GWP are differently weighted in the discussed methods. For example, the GWP of 1 kg of methane emissions into the air is equal to 28 kg of CO2, taking into account the CML 2001 method, 34 kg CO2, taking into account the ReCiPe 2016 method (hierarchist perspective) (Huijbregts et al., 2016) and 36.75 kg CO2, taking into account the ILCD/PEF method (Fazio et al., 2018).

477 478 479 480 481 482 483 484 485

The results of this study show that the three applied LCIA methods show a very similar effect on the global warming potential, without any significant difference in the obtained impact scores (see Fig. 2 and Table S1). The impact of nanofibrillated cellulose on the global warming potential is the lowest in the case of the calculation based on the ILCD/PEF method (770 kg CO2 equiv.) and the highest in the case of the calculation based on the ReCiPe method (814 kg CO2 equiv.) (see Tables 3 to 5). The substance coverage is much higher in the ReCiPe 2016 method compared to the ILCD/PEF and CML 2001 methods, see electronic supplementary material Table S5, which might also represent an important reason for the difference in the impact scores. However, the differences in the GWP impact scores for nanofibrillated cellulose among the three methods are relatively minor, e.g., 6%.

Comparison of the results obtained using different impact-assessment methods

Global Warming Potential (GWP)

21

486 487 488 489

Fig. 6: Contribution of different manufacturing processes to various environmental indicators, taking into account the production of 1 kg of nanofibrillated cellulose (comparison of the ILCD/PEF, CML 2001 and ReCiPe 2016 methods).

490

4.3.2

491 492 493 494 495 496

The CML 2001 and ILCD/PEF impact-assessment methods show very similar impacts on the ozonedepletion potential for the life cycle of nanofibrillated cellulose (the difference is negligible, being less than 0.5%). However, the total impact score obtained using the ReCiPe 2016 method is significantly different, being almost 400% higher (see Tables 3 to 5). Compared to the CML 2001 and ILCD/PEF methods, the ReCiPe 2016 method shows a higher impact on the ozone depletion for all the material (thermo-groundwood, ancillary chemicals) and energy (electricity) requirements (Fig. 2).

497 498 499 500 501 502 503 504 505 506 507 508

For all three discussed methods, the characterization factors for the ozone-depleting substances are primarily implemented by the World Meteorological Organization (WMO). However, these methods use different versions of stratigraphic ozone-depletion models developed by the WMO: CML 2001 refers to the (World Meteorological Organization, 2003) report, ReCiPe 2016 to the (World Meteorological Organization, 2011) report, and ILCD/PEF to the most recent (World Meteorological Organisation, 2014) report. The higher impacts for the ozone-depletion potential obtained with the ReCiPe 2016 method are the result of the following: (i) new semi‐empirical ODPs were included in the ReCiPe 2016 characterization model, with a more detailed specification for the various chlorofluorocarbons (CFCs), and (ii) the ODPs for N2O emissions to the air are included in the characterization model. However, some caution is needed with the latter point. As stated by (Huijbregts et al., 2016), the ODPs for N2O should be considered as preliminary, since the mode of action is different from the other ODPs.

Ozone-Depletion Potential (ODP)

22

509 510 511 512 513 514 515 516 517

Neglecting ODP of N2O substance may represent a significant weakness of CML 2001 and ILCD/PEF impact assessment methods. Several researchers (Ravishankara et al., 2009; Wang et al., 2014) argued N2O as the most important ozone depletion emission. It has been postulated that an ODP factor for N2O emissions might be 0.017 kg CFC-11 equiv. emissions (Ravishankara et al., 2009). These values were calculated for atmospheric conditions in the year 2000, when the stratospheric concentration of reactive chlorine was the highest (Lane and Lant, 2011). In ReCiPe 2016 method, 1 kg of N2O emissions refers to 0.011 kg CFC -11 equivalents, taking into account a decrease of stratospheric concentration of reactive chlorine in last decade. All these ODP factors were generated with steady state modelling for N2O, consisted with the favored approach in LCA.

518 519 520 521 522 523 524

Researchers also justified incorporation of N2O into ozone depletion (ODP) models (Lane and Lant, 2011). However, they emphasized the importance of further investigation on how best to specify a range of ODP values for N2O that can support best sensitivity analysis in LCIA. The net ODP of N2O is influenced by atmospheric chlorine concentrations, which vary through time and reached its historical peak near the year 2000. Climate change has a dampening effect on the ODP of N2O (Wang et al., 2014), meaning that current atmospheric conditions may not be that relevant for time-integrated LCIA modelling based on long-term time frames (100 years or more).

525 526

Photochemical ozone-formation potential (POCP)

527 528 529 530 531

It should be emphasized that the ReCiPe 2016 method addresses separately the POCP impact with regard to the damage to human health and the POCP impact with regards to damage to the ecosystem, while in case of the CML 2001 method, both impacts are aggregated in a single POCP category. The ILCD/PEF method addresses only the POCP impact with regards to damage to human health (see the electronic supplementary material Table S1).

532 533 534 535 536 537 538 539 540 541

The POCP impact with regards to damage to human health and the POCP impact with regards to damage to the ecosystem were aggregated into a joint POCP impact for the ReCiPe 2016 method in order to enable a comparison with the POCP impact calculated using the CML 2001 method. However, these two LCIA methods use different reference units (Table S5 of the supplementary material). In order to enable a direct comparison for the POCP footprint of the nanocellulose calculated with the ReCiPe 2016 and CML 2001 methods, the impact scores were converted into common metrics (kg NMVOC eq.), using the approach of (Dreyer et al., 2003). The results show that the aggregated POCP impact scores of the ReCiPe 2016 method are significantly higher (the only exception is cyclohexane) than the impact scores of the CML 2001 method, taking into account the material and energy requirements in the life cycle of nanofibrilate cellulose (Fig. 2).

542 543 544 545 546 547 548 549 550

In contrast, the ReCipe 2016 and ILCD/PEF 2016 methods give very similar impacts scores, considering just the POCP impact on human health; 3.09 versus 3.04 kg NMVOC eq. (see Tables 3 to 5). Taking into account the relative contribution of the ancillary chemicals and the electricity to the POCP’s impact, the ReCipe 2016 and ILCD/PEF 2016 methods also show significant differences. For example, the synthesis of cyclohexane, as an important hotspot, contributes 34% to the total POCP human-health impact in the case of using the ILCD/PEF method, and 25% in the case of using the ReCiPe 2016 method. The situation is just the reverse with regards to the relative contribution of electricity to the total POCP impact. The ILCD/PEF method shows a relatively lower impact for the

23

551 552

electricity (contributing 41% to the total POCP impact) than the ReCiPe 2016 method (contributing 47%) (see also Fig. 2).

553 554 555 556 557 558 559 560 561 562 563 564

The reason for such differences is that the LCIA methods use different background models (Table S5 of the supplementary material). The ILCD/PEF and CML 2001 methods still use the LOTOS-EUROS characterization model (van Zelm et al., 2008), as also applied in ReCiPe 2008 (an older version of ReCiPe), while the updated ReCiPe 2016 method uses the more recent TM5-FASST characterization model (van Zelm et al., 2016). For example, the human-health POCPs of the NOx and NMVOC emissions are supposed to be equal in the ILCD/PEF method (e.g., 1 kg of NOx equals to 1 kg of NMVOC equivalents), while in the case of the ReCiPe 2016 method, the NOx emissions are supposed to have a relatively lower POCP affecting human health (e.g., 1 kg of NOx equals 0.18 kg of NMVOC equivalents). The main changes introduced into the characterization model of the latest version of ReCiPe (e.g., ReCiPe 2016) are that the European factor was replaced by a world average factor, and additional substances affecting the POCP and respiratory mortality have been included (Huijbregts et al., 2016).

565 566

Human-toxicity potential (HTP)

567 568 569 570

With regards to human toxicity, cancer and non-cancer effects are addressed separately in the ILCD/PEF and ReCiPe 2016 methods (see electronic supplementary material Table S1). Both effects can be aggregated into a single human-toxicity category in order to enable a comparison with the CML 2001 method.

571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586

In the case of the ILCD/PEF and ReCiPe 2016 impact-assessment methods, the relative contribution of the materials (thermo-groundwood and ancillary chemicals) and the electricity (required in the nanocellulose fabrication process) on the human-toxicity impact category is very similar. However, the units of the two methods are different. The ILCD/PEF method uses comparative toxic units (CTUh), which estimate the increase in morbidity in the total human population per unit mass of a chemical emitted, assuming equal weighting between the cancer and non-cancer impacts. The ReCiPe 2016 method uses kg 1,4 dichlorobenzene (1,4-DCB) equivalent units to express the toxicity of the substance affecting human health (both cancer and non-cancer impacts are considered) (Fazio et al., 2018; Huijbregts et al., 2016). The impact scores obtained using the ILCD/PEF method were converted from CTUh into kg 1,4-DCB equivalents in order to enable a comparison with ReCiPe 2016 - see also (Owsianiak et al., 2014). The difference in the total impact scores attributed to the nanocellulose and calculated using the two methods is minor, e.g., less than 2%. However, taking into account the CML 2001 method, the results are different (see Fig. 2 and Table S1). This method uses the same reference units as the ReCiPe 2016 method, but the amount of 1,4-DCB eq. emissions (kg), which affect human health, are estimated to be significantly lower for all the materials and electricity used in the life cycle of the nanocellulose (the only exception is cyclohexane, see Fig. 2).

587 588 589 590 591 592 593

The updated USES-LCA version 3.0, e.g., the Uniform System for the Evaluation of Substances adapted for LCA (van Zelm et al., 2008), is taken into account in ReCiPe 2016 and CML 2001 (Huijbregts et al., 2016; Pre’ Consultants, 2018). Despite the fact that CML 2001 uses the same model as ReCiPe 2016 to describe the chemical fate and exposure, HTPs of various toxic substances are weighted differently in these two methods (European Commission, 2011). Significant differences can be observed, especially for the HTPs of heavy metals that are released to fresh water or soil. Secondly, it was also observed that in case of the ReCiPe 2016 method, some heavy metals (Ni, for example) do 24

594 595 596 597 598 599 600 601

not have a characterization factor assigned in the modelling software, which is an error in the software implementation. Thirdly, the substance coverage in the CML 2001 method is much lower than in ReCiPe 2016 (179 versus 3093). These reasons might explain the significant differences in the final results of the LCA analysis, if conducted using the ReCiPe 2016 and CML 2001 methods. For example, the HTP of the nanofibrillated cellulose amounts to 850 kg of 1,4-dichlorobenzene emissions, taking into account the CML 2001 impact-assessment method and to 1105 kg of 1,4dichlorobenzene emissions, taking into account the ReCiPe 2016 impact-assessment method (see Tables 3 to 5). The difference is around 35%.

602 603 604 605

Despite the fact that ILCP/PEF and ReCiPe 2016 use different characterization models (Table S5 of the supplementary material), both methods show very similar effects on human toxicity (see Fig. 2 and Table S1). It can be argued that the principles of the USEtoxTM and USES-LCA models are very similar (European Commission, 2011).

606 607 608

According to (European Commission, 2011), USEtox is the preferred choice as a default method for the calculation of characterization factors. This is supported by the fact that USEtox offers the largest substance coverage and reflects the latest scientific consensus.

609 610

Acidification potential (AP)

611 612 613 614 615 616 617 618 619 620 621 622 623

The three compared LCIA methods show very similar results, considering the relative contributions of the different chemicals and processes to the acidification potential in the life cycle of nanofibrillated cellulose (see Fig. 2). The results also are similar when comparing the absolute values (i.e., the impact scores) obtained using the three methods. The acidification footprint of 1 kg of nanocellulose calculated using the CML 2001 method amounts to 7.88 kg SO2 equiv. and to 6.45 kg SO2 equiv. calculated using the ReCiPe 2016 method (see Tables 3 to 5). The acidification footprint calculated using the ILCD/PEF method is expressed in different reference units, i.e., 8.85 mole of H+ equiv., which corresponds to 6.76 kg SO2 equivalents. It should be noted that the CML 2001 indicator result is expressed in kg SO2 emitted in Switzerland equivalent, meaning that the emission of 1 kg of SO2 substance into the air is weighted by 1.2 (i.e., corresponding to 1.2 kg SO2 equiv. in Switzerland) (Guinee, 2002). In the case of neglecting the weighting of SO2 emissions, an acidification footprint of 1 kg of nanocellulose calculated using the CML 2001 method would amount to 6.57 kg SO2 equiv, which is very close to the values obtained using the ILCD/PEF and ReCiPe 2016 methods.

624 625 626 627 628

The results of the three methods are similar, despite using different characterization models (Table S5 of the supplementary material). In ILCD/PEF the acidification potential of substances is modeled as the accumulated exceedance above the critical load in sensitive areas, in CML 2001 as the critical load (the actual load is compared to the critical load weighted over ecosystems and region) and in ReCiPe 2016 as an increase of the proton concentration (H+ concentration) in natural soils.

629 630 631 632 633

The characterization factors for acidification do not consider the reaction of acidifying substances with the soil, which is a weakness of the CML 2001 method. The strength of the ILCD/PEF method is that it provides for the deposition of acidifying substances onto different land-cover categories, including forests, surface waters, and semi-natural vegetation. The characterization factors of ILCD/PEF and CML 2001 are site-dependent; however, they only refer to Europe.

634 635 636

ReCiPe 2016 seems to be the most up-to-date acidification impact-assessment method, and has introduced several recent changes. An important change is that the European factor was replaced by a world average factor, based on grid-specific factors. Another important change refers to the indicator 25

637 638 639 640 641 642 643 644

expressing acidity, e.g., the increase of proton concentration (H+ concentration) in natural soils. An acidification substance deposited on the soil leaches into the soil, resulting in a change to the soil solution’s H+ concentration. This change in acidity can affect the plant species living in the soil, causing them to disappear. A geochemical steady-state model PROFILE is used to calculate the spatially explicit changes in the soil’s H+ concentration due to a 10% change in the deposition (Huijbregts et al., 2016; Roy et al., 2012). However, the characterization factors of ReCiPe 2016 are based on the acidification of terrestrial ecosystems only, which could be a potential limitation of the method in comparison with the ILCD/PEF method.

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Important aspect with regard to acidification potential was discussed by some researchers (Andrae, 2019). The current mid-point terrestrial acidification indicators are not precise, as they include a limited view of the soil protons acidification mechanism, namely the number of protons that can be released when an acidifying compound is hydrolyzed or oxidized (Andrae, 2019). To improve the method, this author introduced the acid strength into the model. Taking into account proton release and acid strength of hydrogen chloride emitted to air, this compound contributes significantly to terrestrial acidification potential (Andrae, 2019). Hydrogen chloride yields 1,144 stronger impacts on terrestrial acidification than sulphur dioxide and 68,522 stronger impacts than nitrogen oxides. While in case of ILCD/PEF and ReCiPe 2016 methods, hydrogen chloride is not considered as compound affecting acidification at all. Considering results based on described acidification model, the results can change significantly; however this model still needs to be evaluated before being accepted in acidification models of commonly used LCIA methods.

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4.4 Comparison with other LCA studies

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The environmental footprint of the discussed nanofibrillated cellulose was compared with the footprints of nanocellulose from other available studies. As these studies provide midpoint impact scores only for a very limited number of impact categories, the comparison is focused on the GWPs of nanocelluloses and the consumption of primary energy.

664 665 666 667 668 669 670 671 672 673 674 675 676 677 678 679

In this study the GWP associated with the fabrication of 1 kg of cellulose reaches a significantly high value, i.e., between 770 and 814 kg CO2 equivalents. Even in the case that besides extractives and hemiceluloses also lignin is considered as a potentially valuable co-product, and the latter takes over some of the burdens, the impact of nanofibrillated cellulose remains relatively high (approx. 400 kg CO2 equiv. per kg of the nanocellulose). Taking into account the results of other studies (Arvidsson et al., 2015; Hohenthal et al., 2012; Li et al., 2013), the environmental footprint of nanofibrillated cellulose is relatively low. For example, the GWP of the nanofibrillated cellulose in the comparable study (Li et al., 2013), also fabricated via TEMPO oxidation and homogenization, reaches 190 kg CO2 equivalents. On the other hand a more burdening process (via chloroacetic acid etherification and homogenization) reaches 360 kg CO2 equivalents. But another research group (Arvidsson et al., 2015) that calculated the GWP of nanofibrillated cellulose, presented significantly lower impacts, i.e., 99 kg CO2 equivalents for the fabrication via the carboxymethylation route, 0.79 kg CO2 equivalents for fabrication via the enzymatic route, and only 1.2 kg CO2 equivalents for the fabrication without pre-treatment, taking into account only the homogenization treatment. In both studies the raw material is the kraft pulp, which is already de-lignified. Researchers (Arvidsson et al., 2015; Li et al., 2013) used generic LCI data for de26

680 681 682 683 684 685

lignified kraft pulp, i.e., sulfate pulp from the Ecoinvent database. Our study showed that the production of nanofibrilated cellulose from thermo-groundwood without a pre-treatment was not possible. The procedure used in this study required large quantities of chemicals and energy, mainly due to a comprehensive fabrication process for nanofibrillated cellulose, where the extractives, lignin and hemicellulose were removed, and the hemicelluloses were further separated into individual groups.

686 687 688 689 690 691 692 693 694 695 696 697 698 699 700 701 702 703 704 705 706

Our study showed that the purification (Soxhlet extraction, delignification and removal of hemicelluloses) contributes 95% or even more of the burden to the total environmental footprint of the nanocellulose, considering a cradle-to-gate approach. For example, the purification of thermogroundwood contributes around 755–800 kg CO2 equivalents, while the contribution of the chemical pre-treatment and the principal treatment (mechanical disintegration) is around 15 kg CO2 equivalents. The environmental footprint of the electricity used plays an important role, especially in the process of purification, where the consumption of electricity is the highest. The environmental footprint for electricity varies from country to country, since it depends on the share of electricity derived from renewable and non-renewable resources, and the types of power plants in a specific country. The primary energy consumption in our study was around 19.000 MJ per one kg of nanofibrillated cellulose (or 10.080 MJ if lignin is also considered as a potentially valuable co-product). Compared to another similar research (Li et al., 2013), the primary energy consumption related to the fabrication of nanofibrillated cellulose via the TEMPO oxidation and homogenization is 3470 MJ/kg. Our study showed that purification of the raw material (thermo-groundwood obtained from stemwood) via Soxhlet extraction, delignification and the removal of hemicelluloses is energy intensive, consuming 18.640 MJ of energy per one kg of produced nanocellulose (98% of the energy in the life cycle of the nanocellulose). A total of 15.170 MJ of energy is consumed during the processes of Soxhlet extraction and the removal of hemicelluloses, which were not included in previous studies of manufacturing nanofibrillated cellulose. Without these two purification processes, energy consumption would be very comparable to that presented by Li et al., 2013.

707 708

5. CONCLUSIONS

709 710 711 712

In this study, a full list of inventory data related to fabrication of nanofibrillated cellulose from thermo-groundwood at laboratory scale was provided, making the study totally transparent. Three most commonly used LCIA methods were quantified across a range of environmental impacts, thus providing a material for benchmarking with other studies on nanocellulose.

713 714 715 716 717 718 719 720

The environmental footprint of nanofibrillated cellulose discussed in this paper is relatively high. Reason for this can be attributed to the fact that Life cycle inventory data from laboratory-scale production of cellulose nanofibers were taken into account. This study shows that the purification of thermo-groundwood, which includes the Soxhlet extraction, delignification and removal of hemicelluloses, is responsible for more than 95% of the impacts in the life cycle of nanocellulose. Soxhlet extraction and removal of hemicelluloses are two additional purification steps, which are not included in other LCA studies related to fabrication of nanofibrillated cellulose via TEMPO oxidation. Without these two purification steps, the results would be more comparable to results of other studies.

721 722

The consumption of electricity for the purification of the thermo-groundwood (especially during the process of Soxhlet extraction) and the syntheses of acetone and cyclohexane (both are required as 27

723 724 725 726 727 728 729 730

ancillary materials during the process of Soxhlet extraction) represent significant environmental hotspots. However, soxhlet extraction process would be omitted at industrial scale production, since the extractives can be largely removed during the delignification and removal of hemicelluloses. It means that cyclohexane would not be needed and also energy (e.g. electricity) requirements would lower for about half. So it is expected that at industrial scale, the impact on global warming associated with fabrication of nanocellulose could be reduced for at least 77 %, while impacts in other categories could be reduced between 60 and 85%. The impact on global warming could be further reduced by using an electricity mix derived from a larger share of power plants that run on renewable energy

731 732 733 734 735 736 737 738

Important limitation of this study is that it deals only with fabrication of the nanofibrillated cellulose. It would be interesting to extend the study taking into account use and end-of-life stages also. In such a case, the impacts of nano-particles should be considered. The problem is that the toxicity of nano-particles is not present in commonly available life cycle inventory databases due to limited knowledge of the effects of these materials on the environment and on human health. Uncertainties still exist regarding the fate and transport of nano-particles in the environment and regarding environmental and human exposure. From this point of view, there is a challenge to conduct whole cradle-to-grave LCA of nanomaterials.

739 740 741 742 743 744 745 746 747

The study also deals with the methodological issues of LCAs. Three LCIA methods were used in this study (ILCD/PEF, CML 2001 and ReCiPe 2016) in order to observe the differences in the final results and to discuss the reasons for the differences. The three LCIA methods show similar results for the impact on global warming and acidification. In the case of the ozone-depletion potential, photochemical ozone formation and the human-toxicity potentials of the nanocellulose evaluated by the three methods, a significant deviation in the obtained impact scores was observed for the results of one of the three methods. ReCiPe, which is also included in many impact categories of the ILCD/PED method, is currently preferred method for LCIA. However, also this method needs to be more frequently upgraded with new scientific findings.

748

ACKNOWLEDGMENTS

749 750 751 752 753 754

The research was partially supported by the Ministry of Education, Science and Sport of the Republic of Slovenia within the Programme Group P4-0015. The authors also wish to acknowledge the financial support provided by the Slovenian Research Agency (Research Core Funding No. P2-0273). The authors are also grateful to Mrs. Breda Ogorevc, Mrs. Ana Tea Kos and Mr. Damir Jakovina from the VIPAP VIDEM KRŠKO company for providing data on the environmental footprint of thermogroundwood.

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FIGURE CAPTIONS

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Fig. 1: Processing scheme for the production of 1 kg nanofibrillated cellulose. The scheme also presents the system boundaries of the study.

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Fig. 2: Contribution of different materials and electricity to various environmental indicators, taking into account the production of 1 kg of nanofibrillated cellulose (comparison of ILCD/PEF, CML 2001 and ReCiPe 2016 methods).

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Fig. 3: Primary energy demand from renewable and non-renewable resources (net calorific value), taking into account the production of 1 kg of nanofibrillated cellulose.

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Fig. 4: Relative comparison of LCA results for two scenarios. The reference is the baseline scenario (lignin considered as a waste material). In the case of the alternative scenario, lignin is considered as a potentially valuable co-product, which takes over part of the burdens related to Soxhlet extraction and the delignification pre-treatment processes. The CML 2001 (a) and ILCD/PEF (b) methods were used to compare the two scenarios.

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Fig. 5: Relative comparison of LCA results based on laboratory scale and at industrial scale (assumption) fabrication of the nanofibrillated cellulose. The CML 2001 (a) and ILCD/PEF (b) methods were used to compare the results.

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Fig. 6: Contribution of different manufacturing processes to various environmental indicators, taking into account the production of 1 kg of nanofibrillated cellulose (comparison of the ILCD/PEF, CML 2001 and ReCiPe 2016 methods).

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TABLE CAPTIONS

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Table 1: Ancillary material (chemicals) requirements and electricity consumption for the production of 1 kg of nanofibrillated cellulose from thermo-groundwood as a raw material.

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Table 2: Life cycle inventory for the production of 1 kg of nanofibrilated cellulose from thermogroundwood as a raw material.

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Table 3: Total impact scores attributed to nanofibrillated cellulose and the relative contribution (%) of the different processes included in nanofibrillated cellulose production to various impact categories (results based on the ILCD/PEF evaluation method).

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Table 4: Total impact scores attributed to nanofibrillated cellulose and the relative contribution (%) of the different processes included in nanofibrillated cellulose production to various impact categories (results based on the CML 2001 evaluation method).

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Table 5: Total impact scores attributed to nanofibrillated cellulose and the relative contribution (%) of the different processes included in nanofibrillated cellulose production to the various impact categories (results based on the ReCiPe 2016 evaluation method).

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34

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TABLE CAPTIONS (ELECTRONIC SUPPLEMENTARY MATERIAL)

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Table S1: Mid-point indicators addressed by three different impact assessment methods.

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Table S2: Table represents impact scores of raw material (thermo-groundwood), ancillary material (chemicalls) and of electricity required for production of 1 kg of nanofibrilated cellulose (results based on ILCD/PEF evaluation method).

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Table S3: Table represents impact scores of raw material (thermo-groundwood), ancillary material (chemicalls) and of electricity required for production of 1 kg of nanofibrilated cellulose (results based on CML 2001 evaluation method).

946 947 948

Table S4: Table represents impact scores of raw material (thermo-groundwood), ancillary material (chemicalls) and of electricity required for production of 1 kg of nanofibrilated cellulose (results based on ReCiPe 2016 evaluation method).

949 950

Table S5: Background data of selected impact categories calculated by means of three different LCIA methods: ILCD/PEF, CML 2001 and ReCiPe 2016.

35

Highlights

Environmental footprint of the nanocellulose is considerable (e.g. 800 kg CO2 eq.). The purification contributes 95% to the environmental footprint of the nanocellulose. Three the most relevant LCIA methods results were compared. Current LCIA methods could be improved with regard to AP and ODP models.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: