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Titania-cellulose hybrid monolith for in-flow purification of water under solar illumination Mattia Lucchini, Erlantz Lizundia, Simon Moser, Markus Niederberger, and Gustav Nystrom ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b09735 • Publication Date (Web): 07 Aug 2018 Downloaded from http://pubs.acs.org on August 8, 2018

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ACS Applied Materials & Interfaces

1

Titania-cellulose hybrid monolith for in-flow

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purification of water under solar illumination

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Mattia A. Lucchini1§, Erlantz Lizundia1,2,3§, Simon Moser1, Markus Niederberger1, Gustav

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Nyström4*

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1

8

Vladimir-Prelog-Weg 5, 8093 Zürich, Switzerland.

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2

Laboratory for Multifunctional Materials, Department of Materials, ETH Zürich.

Department of Graphic Design and Engineering Projects, Bilbao Faculty of Engineering.

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University of the Basque Country (UPV/EHU), Bilbao 48103, Spain.

11

3

12

Science Park, 48940 Leioa, Spain.

13

4

14

Switzerland

BCMaterials, Basque Center for Materials, Applications and Nanostructures, UPV/EHU

Laboratory for Applied Wood Materials, Empa, Überlandstrasse 129, 8600 Dübendorf,

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§: These two authors contributed equally to this work

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*: Corresponding author: [email protected]

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KEYWORDS: cellulose nanofibrils, TiO2, nanoparticles, catalysis, water purification. 1 ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT: In this work we report a versatile approach for the development of an in-flow

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purification water system under solar illumination. Cellulose nanofibrils (CNF) were

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impregnated with TiO2 nanoparticles using water as a solvent to obtain hybrid CNF/TiO2

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monoliths with 98% porosity. The opposite surface potential enables an electrostatically

25

induced direct conjugation between TiO2 and CNFs. Scanning electron microscopy (SEM)

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analysis of the surface morphology of the CNF/TiO2 monolith shows a homogeneous dense

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coating of titania nanoparticles onto the interconnected nanofibrils network, providing a

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Brunauer–Emmett–Teller (BET) surface area of about 80 m2·g-1 for the hybrid monolith.

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Furthermore, compression tests reveal a good shape recovery after unloading thanks to the

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highly flexible and mechanically stable three-dimensional structure. Finally, the CNF-based

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hybrids were tested as catalyst for the decomposition of organic pollutants under solar

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illumination. The tests were performed using a continuous flow reactor with a customized

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holder allowing the solution to pass through the monolith. The results reveal

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photocatalytic activity and long-term stability of the hybrid CNF/TiO2 monolith towards

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the decomposition of Methyl Orange and Paracetamol. These features provide a proof of

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concept for the applicability of the hybrid CNF/TiO2 monoliths for in-flow purification of

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water under solar illumination, not only for model dyes, but also for organic pollutants of

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high practical relevance.

good

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2 ACS Paragon Plus Environment

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INTRODUCTION

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Water resources are fundamental for the sustainability and the development of human

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society. On the other side, the increasing world population and the related increase in usage

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of chemicals in daily life is one of the causes of the increased concentration of chemical

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substances in water reservoirs.1–3 One example is the increasing levels of antibiotics in

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groundwater.1,4,5 Antibiotic molecules can reach the water reservoirs due to unwanted

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release in the environment or through human excretion.5 The presence of these active

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compounds in water pools can lead to the development of immunized bacterial colonies and

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to unwanted human exposure. Pharmaceutical molecules have been reported to be

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detrimental also for fishes, with a feminization of the population and a consequence

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negative effect on their preservation.6 Pesticides are another category of potentially harmful

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substances whose concentration in water reserves is increased by human activity.7 Also in

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this case, the extensive use can cause an accumulation of these pollutants due to the drain

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effect of rain water with negative impact on agricultural areas and wild nature.8

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With this in mind, the purification of water from organic pollutants, and in particular

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wastewater from municipal areas, represents a major challenge for the coming years.

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Purification of wastewater from organic molecules has since long time been object of both

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industrial and academic research and many different solutions have been proposed. A first

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option is based on the removal of the molecule of interest by adsorbing them on the surface

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of an active element. An example is the adsorption of antibiotics on activated carbon or

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bentonite.9,10 Despite a high removal efficiency (90-100% at mg·L-1 concentration ranges)

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this method is intrinsically discontinuous since the adsorbent must be reactivated regularly.

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Furthermore, the need of regeneration increases with increasing concentration of the 3 ACS Paragon Plus Environment


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pollutants. A second option is based on reverse osmosis.11 This technique, mostly known

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for desalination of water, can be adopted also for organic molecules and uses a

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semipermeable membrane in order to filter out the organic molecules from the

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groundwater.12 Also in this case, regeneration of the membrane is required regularly.

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A third alternative strategy is based on the decomposition of the organic pollutants while

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they are still dispersed in water rather than on their collection.13,14 In this case, a catalyst is

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used to chemically modify the molecules into a less active species. This idea has been

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pursued with many different catalysts like metals or metal oxides.15–17 Among them,

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particularly appealing is the possibility to use metal oxides with low toxicity, like ZnO or

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TiO2,18–20 together with external radiation to promote photocatalytic decomposition.21 Even

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if the possibility to use metal oxide particles for heterogeneous catalytic decomposition has

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already been demonstrated, there are three main drawbacks associated with this approach

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that remain mostly unsolved. First, most of the studies use nanoparticles (NPs) as active

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species and the decomposition is obtained by dispersing them in the media to treat. This

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approach increases the contact between pollutants and active species but it contaminates the

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media with nanosized objects, whose removal represents an additional difficult step for the

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real application. Second, this approach cannot be used for in-flow applications and

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therefore only small volumes can be treated in a discontinuous way. Third, most of the

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reports use ultraviolet (UV) light as radiation to promote the photocatalytic activity. Even if

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this high-energy radiation increases the activity of the catalyst, the use of UV lamps

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increases the costs of the process, limits the possible application in direct field use and

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decreases the industrial appeal of the strategy.


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In this work, we used the known material combination of cellulose and titania22 as model

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system and developed a general strategy for the preparation of hybrid monoliths with

88

photocatalytic activity towards organic molecule decomposition under solar illumination.

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Where many previous systems either rely on advanced deposition techniques,23,24 specific

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bioconjugation25 or co-gelation26,27 our approach is modular, relying only on electrostatic

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interactions between the material components thereby allowing a straightforward extension

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of the preparation protocol to other organic-inorganic aerogels. The hybrid monolith is

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composed of TiO2 NPs adsorbed on the surface of a cellulose-based monolith. The

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combination of the two materials gives the opportunity to benefit of the NP properties (high

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activity and high surface area) while retaining the stable monolithic shape offered by the

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organic scaffold. The hybrid monolith displays good photocatalytic activity under solar

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illumination and it remains stable under prolonged water flow. This hybrid material is thus

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compatible with continuous flow-through operation, and the demonstrated activity of TiO2

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NPs in presence of solar illumination allows further development for on-field use without

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any external support, not only for model dyes, but also for ubiquitous drugs such as

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Paracetamol.

102 103

MATERIALS AND METHODS

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Starting materials

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Benzyl alcohol (Sigma-Aldrich, > 99.8%), titanium (IV) chloride (Sigma-Aldrich, 99.9%),

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ethanol (EMSURE, absolute), 2,2,6,6-Tetramethylpiperidinyloxy (TEMPO, Sigma-Aldrich,

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98%), sodium bromide (Sigma-Aldrich, ≥ 99%), sodium hypochlorite (Sigma-Aldrich, 10-

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15% available chlorine), sodium hypophosphite (SHP, Sigma-Aldrich, ≥ 99%), 1,2,3,45 ACS Paragon Plus Environment


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butanetetracarboxylic acid (BTCA, Sigma-Aldrich, 99%) and acetaminophen (Sigma-

110

Aldrich, ≥ 99%), also known as Paracetamol, have been used as received without any

111

further purification.

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Synthesis of TiO2

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TiO2 nanoparticles were synthesized according to a method previously reported.28 Briefly,

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3.5 mL (41.15 mmol) of titanium (IV) chloride were added slowly to 10 mL of anhydrous

116

ethanol under stirring. Later, 60 mL of benzyl alcohol were added and the final solution

117

was heated up to 80 °C in an oil bath and was kept under stirring for 24 hours. After the

118

synthesis the particles were washed with diethyl ether, centrifuged off, washed 3 times with

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chloroform (centrifuging after every wash) and finally dispersed in deionized water to yield

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a concentration of 50 mg·mL-1.

121 122

Synthesis of CNF monolith

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Cellulose nanofibrils (CNF) were prepared from never-dried softwood pulp using TEMPO-

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mediated oxidation combined with mechanical treatment (ultrasonication) and

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centrifugation to remove larger fibril aggregates.29 In brief, 1 g of pulp was dispersed in a

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100 mL solution containing TEMPO (0.016 g, 0.1 mmol) and NaBr (0.1 g, 1 mmol) and

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stirred at 500 rpm. 3 mmol of sodium hypochlorite (NaClO) pre-adjusted to pH 10 was

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added to initiate the reaction and the pH was maintained at 10 until no further consumption

129

of –OH was noted. The oxidized pulp was washed with filtration and dialysis against MQ-

130

water (5 bath changes). The total amount of charge was measured using conductometric


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titration on the oxidized pulp revealing a gravimetric charge density of 0.67 mmol·g-1.

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Oxidized pulp was then dispersed at a concentration of 0.6 g·L-1 and cellulose nanofibrils

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were extracted using probe ultrasonication for 2 minutes (Hielscher UP200S, operated at

134

200 W and 20% amplitude setting) followed by centrifugation at 12.000 g for 30 minutes.

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CNF monoliths were thereafter prepared according to a previously developed protocol.30 In

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short, fibrils at a 0.6 g·L-1 concentration were mixed with BTCA and SHP at a 1:1 mass

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ratio (BTCA) and a 2:1 mass ratio (SHP) followed by 15 min of stirring using an Ultra

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Turrax T25 (IKA, Germany), at 10.000 rpm. The dispersion was subsequently frozen in

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aluminum moulds using liquid nitrogen and then freeze-dried. Finally, the freeze-dried

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monoliths were heated to 150 °C for 5 min, to permanently cure the ester cross-links. This

141

step is critical in order to obtain a mechanically robust monolith, which keeps its structure

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upon drying. All monoliths were thoroughly rinsed with deionized water after the cross-

143

linking, to ensure that the residuals from the cross-linking were washed out.

144 145

Preparation of the hybrid CNF/TiO2 monolith

146

The hybrid monolith was prepared by direct wet impregnation. Firstly, the CNF monolith

147

was immersed in circa 10 mL of TiO2 NPs water dispersion (50 mg·mL-1) for 4 hours.

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Later, the monolith was removed from the solution and immersed in 100 mL of deionised

149

water under slow magnetic stirring overnight to remove the excess of particles from the

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monolith surface. Samples were kept wet until needed for UV-Vis, mechanical and

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photocatalytic tests. For structural, morphological and thermal characterization (XRD,

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FTIR, BET, SEM and TGA), samples were air-dried overnight and stored in air closed

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vials. 7 ACS Paragon Plus Environment


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Characterization

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Zeta-potential measurements on CNF and TiO2 were performed on a Malvern Zetasizer

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Nano-ZS. The measurements were performed in water-based dispersions at a particle

158

concentration of 1 mg·mL-1. X-ray powder diffraction (XRD) patterns were recorded using

159

a PANalytical Empyrean powder diffractometer in reflection mode using Cu Kα radiation

160

and operating at 45 kV and 40 mA. Attenuated total reflectance Fourier transform infrared

161

spectroscopy (ATR-FTIR) measurements were performed on a Bruker Alpha FT-IR

162

Spectrometer equipped with diamond ATR optics. Thermogravimetric analysis (TGA) was

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done using a Mettler Toledo TGA/SDTA 851e instrument under air atmosphere at a heating

164

rate of 10 °C·min-1 and an air flow of 50 mL·min-1. UV-Visible (UV-Vis) spectroscopy

165

measurements were performed with a JASCO V-770 instrument. Absorption spectra of

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aqueous dispersions containing synthesized TiO2 and CNF at a 0.1 mg·mL-1 and

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transmittance spectra of water-soaked cellulose monolith and cellulose/TiO2 monolith were

168

collected. Scanning electron microscopy (SEM) analyses were performed on a LEO 1530

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Gemini instrument working at 2 kV. The samples were sputtered with 3 nm thin gold-

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palladium layer prior to SEM imaging. Nitrogen sorption experiments were carried out on a

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Quantochrome Autosorb-iQ-C-XR at 77K, with nitrogen (99.999%) and helium (99.999%)

172

provided by PanGas AG, Switzerland. Before the measurement, the samples were degassed

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in vacuum at 60 °C for 24 h. The surface area was determined via the Brunauer-Emmet-

174

Teller (BET) method. Mechanical response of 5 mm thick wet cellulose and cellulose/TiO2

175

monoliths was studied in compression mode on a universal testing machine (Trapezium


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Shimadzu AGS-X) equipped with a 100 N load cell in displacement control mode at a rate

177

of 0.5 mm·min−1.

178 179

Photocatalytic decomposition of Methyl Orange and Paracetamol

180

The photocatalytic activity of the hybrid monolith was tested through the decomposition of

181

Methyl Orange (MO) and Paracetamol under solar illumination. For the tests, the hybrid

182

monoliths were placed in a custom made PMMA cell (Figure S1) behind a quartz window.

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The cell was connected to a peristaltic pump and to a reservoir with Teflon tubes. The

184

reservoir was filled with 100 mL of a 20 ppm solution (MO or Paracetamol) and the flow of

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the pump was set at 10 mL·min-1. For the MO photocatalytic tests, the solution was kept at

186

25 °C by a thermostatic water bath while the experiments with Paracetamol were performed

187

at 10 °C. Before the decomposition experiments, the solution was allowed to flow through

188

the monolith in the dark for 60 min and the concentration was checked regularly. The

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decomposition was induced by solar illumination provided by a Newport solar simulator

190

equipped with a 300 W Xe lamp operated at 242 W. The intensity was adjusted to 1000

191

W·m-2 (equivalent to 1 sun) tuning the distance between the cell and the lamp. The

192

concentration of the pollutant (MO or Paracetamol) was checked at different time periods

193

through UV-Vis spectroscopy by extraction of 1 mL aliquots that were discarded after

194

analysis. The possibility to relate the absorption maximum of MO, Paracetamol and TiO2

195

NPs in water dispersion to its actual concentration was confirmed by calibration

196

experiments (Figure S2 and S3). Additionally, UV-Vis allows us to check any possible

197

TiO2 nanoparticle release into the surrounding medium thanks to their strong absorbing



Page 10 of 32

198

nature in the UV region, which displays a maximum absorption peak at λ=244 nm (Figure

199

S4a).

200 201

RESULTS AND DISCUSSION

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Monolith preparation and characterization

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Figure 1(a) shows a scheme for the preparation of hybrid CNF/TiO2 monoliths by direct

204

impregnation of preformed CNF monoliths with an aqueous dispersion of TiO2

205

nanoparticles. The TiO2 nanoparticles were synthesized according to the protocol

206

developed by Kotsokechagia et al.28 This synthesis protocol was adopted since it brings to

207

ligand free particles with high crystallinity and small dimension (diameter smaller than 10

208

nm, see Figure S5) that are easily dispersible in water. All these characteristics make this

209

product an ideal candidate for the preparation of a photocatalytic system. Cellulose

210

nanofiber (CNF) monoliths were prepared through freezing CNF dispersion containing

211

BTCA and SHP in liquid nitrogen followed by freeze-drying. Dried monoliths were

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thereafter heat cured at 150 °C to promote the solid-state crosslinking reaction rendering

213

the monoliths stable against rehydration (see Figure 1(b) for the physical appearance of

214

dried and wet cellulose/TiO2 monoliths). After curing, the samples were washed in

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deionized water to remove the residuals from the cross-linking and dried in air. Dry CNF

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monoliths presented an apparent density (ρ0) of 0.02847 ± 0.0013 g·cm-3 (n = 3) as

217

estimated from the equation:

218

𝜌𝜌0 =

𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀

𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴 𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣

(1)


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219


The porosity of the monolith can be calculated as: 𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃 (%) = �1 −

220

𝜌𝜌𝑎𝑎 𝜌𝜌𝑐𝑐

� 𝑥𝑥 100%

(2)

221

where ρc is the bulk density of cellulose taken as 1.6 g·cm-3 and ρa is the monolith apparent

222

density. Accordingly, an average porosity of 98.22% was calculated for the CNF monolith.

223

We expect that such a highly porous structure will provide an accessible surface to interact

224

with the pollutant and allow rapid mass transport and easy water flow through the

225

monolith.

226

The conjugation of titania particles to the surface of cellulose fibrils is possible thanks to

227

the surface chemistry of the two components. TiO2 nanoparticles were found to be highly

228

dispersible in water without additional stabilizers up to a concentration of about 80 mg·mL-

229

1

230

water dispersion of TiO2 particles shows a pH value of about 2.1. The acidic pH can be

231

related to the release of HCl from the particle surfaces.31 According to Zeta-potential

232

measurements , Figure 1(c), in these conditions the particles show a surface potential of +

233

44 mV, suggesting strong electrostatic repulsion between the particles. If the pH is

234

increased adding NaOH solution, the surface potential decreases, reaching 0 mV at pH 6.5

235

(isoelectric point). If the pH is further increased, a negative potential can be induced up to -

236

24 mV at pH 9.5. Thus, the pH strongly influences the surface potential and, consequently,

237

the aggregation behavior of the particles in dispersion. Indeed, at pH lower than 4.5, titania

238

nanoparticles remain stable in aqueous dispersion for weeks, while at higher pH the

239

particles agglomerate severely and precipitate within a few minutes. Accordingly, the

240

conjugation between TiO2 and CNF was performed at a pH of 2.3 to avoid particle

and without observable precipitation within more than a month. In these conditions, the



241

agglomeration. At pH 4.5, the surface potential of the dispersed CNFs is -35 mV, which

242

arises from the presence of carboxylic acid groups on the CNF surfaces. Therefore,direct

243

conjugation between TiO2 and CNF is electrostatically induced by the opposite surface

244

potential in mild acidic aqueous environment (pH < 4.5). The stability of the interaction

245

between particles and fibrils was tested maintaining the impregnated monolith 24 hours in

246

deionized water under stirring. After this interval, the suspension was analyzed via UV-Vis

247

(Figure S4(b)) and a weak signal with a maximum absorption peak at λ=244 nm related to

248

TiO2 NPs was observed. This peak can be related to the concentration of TiO2 NPs in a

249

concentration range from 0.001 mg·mL-1 to 0.5 mg·mL-1 (see calibration curve reported in

250

Figure S3). The absorption peak observed after the experiment was related to a

251

concentration of 0.0042 mg mL-1 of titania in the washing medium. This corresponds to

252

0.42 mg of released TiO2 nanoparticles from the hybrid monolith, which is only 2.4% of

253

the complete loading of nanoparticles within the monolith (as calculated from TGA

254

measurements, see below).

a)

CNF monolith

b)

TiO2 aqueous dispersion

CNF/TiO2 composite monolith

c) Zeta potential (mV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 32

50

Cellulose nanofibers Synthesized TiO2

25

0

-25

-50

2

3

4

5

6

7

8

9

10

pH

255


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256

Figure 1. Scheme depicting the impregnation method to obtain cellulose/TiO2 monolith (a), its physical

257

appearance under dry and wet conditions (b) and Zeta-potential of NFC and TiO2 when dispersed in water at a

258

concentration of 1 mg mL-1 (c).

259 260

Powder X-ray diffraction (XRD) was used to identify the crystalline phase of

261

synthesized TiO2 nanoparticles. Results shown in Figure 2(a) confirm the anatase TiO2

262

crystal phase (01-070-6826) without the presence of any detectable crystalline impurities.28

263

The average crystallite size was estimated from the most intense diffraction peak, (101) at

264

2θ angle of 25.2°, to be about 5 nm according to the Scherrer equation:32

𝜏𝜏 =

265

𝐾𝐾∙𝜆𝜆

(3)

𝛽𝛽𝜏𝜏 ∙𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐

266

where τ is the average size of the crystals, λ is the wavelength of the incident radiation, θ is

267

the diffraction angle, K is the Scherrer constant (set at 0.9) and βτ is the peak width at half

268

height in radians obtained. Four main peaks located at 2θ angles of 14.9, 16.5, 22.7 and

269

34.4° are observed for CNFs, which correspond to the (1̄1̄0), (101), (200) and (004) planes

270

of cellulose I.33,34 The CNF/TiO2 composite monolith displays the diffraction peaks of neat

271

TiO2 nanoparticles and CNFs.

272

Fourier transform infrared (FTIR) spectroscopy measurements were performed to

273

investigate the functional groups present on the surface of the nanoparticles. In Figure 2(b),

274

the spectrum for TiO2 obtained in attenuated total reflectance (ATR) mode displays two

275

main peaks at 3500-3000 and 1623 cm-1 assigned to –OH stretching and –OH bending

276

respectively.28 The CNF monolith presents the characteristic peaks of cellulose at 3650-

277

3200, 2902, 1337, 1160 and 897 cm-1 which are assigned to the O-H stretching vibration,



Page 14 of 32

278

asymmetric stretching vibration of C–H, C–O–H bending, C–O–C bending and to the C–

279

O–C asymmetric stretching at the β-(1→4)-glycosidic linkage, respectively.35 In addition to

280

these vibrations, the CNF monolith also shows a peak around 1720 cm-1 resulting from the

281

-COOH functionalization of the native cellulose as well as the ester carbonyl bands formed

282

as a result of the BTCA crosslinking reaction.30,36 The spectra corresponding to the

283

CNF/TiO2 monolith does not present any band shifting and it is a superposition of the

284

spectra of its individual constituents.

285

Thermogravimetric analysis (TGA) on dried TiO2 samples showed a 5% weight

286

decrease upon heating to 150 °C, indicating a release of the adsorbed water, Figure 2(c). At

287

higher temperature, an additional decrease in weight was observed (additional 10% at 600

288

°C). This weight loss can be attributed to the combustion of organic molecules present on

289

the surface of the titania particles or to the condensation of titanols to titanoxanes.28 The

290

CNF thermodegradation process proceeds in two stages and it ends with the production of

291

an amount of char equivalent to about 3 wt% of the initial sample. Taking into account the

292

TGA traces for CNF and CNF/TiO2 monoliths, the loading of titania nanoparticles in the

293

impregnated monolith can be estimated to be around 17.1% by weight.

294

Figure 2(d) shows the transmission-mode UV-Vis spectra of the as prepared and of the

295

impregnated CNF monoliths. The presence of TiO2 nanoparticles in the impregnated

296

monolith can be observed through the increased absorption of the sample in the region λ

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