Functional nanocellulose filters for water purification Sehaqui H., de Larraya U., Liu P., Pfenninger N., Mathew A., Mautner A., Michen B., Marty E.

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1 Functional nanocellulose filters for water purification Sehaqui H., de Larraya U., Liu P., Pfenninger N., Mathew A., Mautner A., Michen B., Marty E., Schaufelberger L., Tingaut P., and Zimmermann T

2 Introduction Industrialization and human activities have led to an increasing number of pollutant mixtures entering water supply -> we need a universal and sustainable water purification technology. Cellulose nanofibers (CNF) are interesting for that purpose since: They have hydroxyl groups on their surface permitting chemical functionalization to target specific contaminants. X C They have a large surface area that could interact with contaminants. Their structure can be tuned permitting retention of particles via size exclusion and/or adsorption.

3 Outline 1. Examples of chemical functionalization of CNF and analytical tools relevant for water purification. 2. Examples of material structure from CNF. 3. Combining material structure and chemical functionalization in CNF filters with optimum performance.

4 Functionalizations and contaminants Functions: COO - : carboxylate groups N + :trimethyl ammonium groups Contaminants: M + : Heavy metal ions NOM: Natural organic matter (humic acid) NPs: Nanoparticles

5 Carboxylated cellulose and chitin nanofibers via TEMPO-oxidation Scheme of the reaction Characteristics of the carboxylated nanofibers Cellulose nanofibers Chitin nanofibers The reaction is performed in water on wood pulp / crab shell residues, this is followed by mechanical disintegration to yield carboxylated nanofibers Saito, T. et al. (2007) Biomacromolecules, 8, Surface area is 300 m 2 /g Average diameter is 8 nm COOH content: 1.5 mmol/g Surface area is 230 m 2 /g Average diameter is 12 nm COOH content: 0.88 mmol/g Sehaqui, H. et al. (2014) Cellulose, 21,

6 General procedure Heavy metal ions belong to micropollutants of toxicological concern as they do not degrade in natural environments. Nanofibers + adsorbed Cu (II) film XPS, WDX Filtration Nanofibers + Cu (II) Non-adsorbed Cu (II) solution ICP, UV-Vis Spectrophotometry

7 Results Cu adsorbed (mmol/g) UV spectrophotometry ph TOCNF TOChNF ChNF CNF CNF : Unmodified cellulose nanofibers ChNF : Unmodified chitin nanofibers TOCNF : Tempo-oxidized cellulose nanofibers TOChNF : Tempo-oxidized chitin nanofibers Cu adsorbed (mg/g) Wavelength dispersive spectrometery (WDX) Increase DO Increase ph Nanofibers functionalisation with carboxyl groups considerably enhanced their adsorption capacity towards copper (II), particularly around neutral ph where the carboxylate (COO - ) form is present. Carboxylated cellulose diplayed a higher performance than carboxylated chitin due to its higher carboxyl content.

8 humic acid Humic acid removal using cationic CNF Scheme of the reaction TEM of the fibrils The reaction is performed in water according to: PEI, A. et al. (2013) Soft Matter, 9, Width is ~4 nm Humic acid is a major organic constituent of the soil resulting from the biodegradation of dead organic matter. Humic acid transport into natural waters causes Undesirable color and taste. Serves as food for bacterial growth. Binds to heavy metals, viruses and biocides Enhances transport of these substances in water

9 Sehaqui, H. et al. (2015) Soft Matter, 11, Humic acid removal using cationic CNF ζ-potential (mv) Zeta potential ph UV spectrophotometry The twofold reversal of ζ-potential from negative values for CNF-0 to positive values for cationic CNF-1 and then again to negative values for CNF-1/HA complex confirms the success of the cationization reaction as well as the adsorption of HA at the surface of CNF-1. Humic acid adsorbed (mg.g -1 ) CNF-0 CNF-1 CNF-2 CNF-3 a c The highest adsorption capacity ever reported (310 mg of humic acid per gram of CNF) was reached due to the combined high surface area of CNF and high N + content. Humic acid concentration (mg.g -1 )

10 Humic acid Interaction between CNF and Humic acid via QCM f (Hz) Buffer CNF CNF-2 Buffer ph 4.5 ph 6.2 ph 10 Buffer time (min) - A quartz crystal microbalance (QCM) measures, in real time, a mass variation per unit area by measuring the change in frequency of a quartz crystal. - The kinetic of humic acid adsorption process onto CNF is faster at low ph and for highly charged CNF.

11 Filters from CNF Pression Liquid Solid Gas Critical point Temperature 1. Evaporation CNF nanopaper 2. Supercritical drying CNF aerogel 3. Freeze drying CNF foam

12 1. CNF nanopaper Surface SEM Pression Liquid 12 Solid 1 Gas Critical point Temperature Henriksson et al. Biomacromolecules (2008) Sehaqui et al. Biomacromolecules (2010) 12

13 2. CNF aerogel by supercritical CO 2 drying Pression Liquid Solid 2 Gas Critical point Temperature Goal: preserve surface area of NFC Sehaqui et 13 al. Biomacromolecules (2011)

14 CNF aerogel Structure SSA 482 m 2 /g Porosity 56% Pore size 12 nm

15 3. NFC foams by freeze-drying Pression Solid 3 Gas Critical point Temperature Paakko et al. Soft Matter (2008) Sehaqui et al. Soft Matter (2010)

16 Functional nanocellulose filters How does the CNF filter structure affect its performance in terms of flux and retention of contaminants Freeze drying Cationic CNF FD PM Vacuum filtration Exchange to ethanol SE ScCO 2 Hot pressing Supercritical drying FD: freeze drying PM: paper-making SE: solvent exchange ScCO 2 : supercritical CO 2 drying Sehaqui, H. et al. (2016) ACS sustainable chemistry and engineering. 4,

17 Filter performance a Flux (L m -2 h -1 ) PM SE ScCO2 FD Freeze-drying results in filters with large pores and pore volume. This results in a much higher flux of the corresponding filter.

18 Filter performance Humic acid adsorbed (mg g -1 ) b Volume (ml) PM SE ScCO 2 FD b Filter before HA filtration Filter after HA filtration HA before and after filtration Photographs of CNF filter and humic acid solution before and after humic acid filtration. The amount of humic acid adsorbed does not depend on the processing route since all filters swell in water making their adsorption sites available for humic acid adsorption.

19 NPs and virus filtration Nanotechnology will inevitably lead to discharge of NPs into the environment with unknown/hazardous effects. Bacteria and viruses transport into drinking water inflicts major health deseases. NPs and viruses are charged particles that can be retained by CNF filters of opposite charges. Colloidal silica PS nanospheres NPs with negative zeta-potential have their concentration reduced to 0 mg/l upon filtration onto cationic CNF.

20 Filter regeneration It is possible to tune the interaction between CNF and the contaminant and regenerate the filter for multiple utilizations: HA adsorbed (g HA m 2 ) a Cycle 1 Cycle 2 Cycle 3 HA desorbed (g HA g HA -1 ) b Cycle 1 Cycle 2 Cycle time (min) time (min) Heavy metal ions desorption by HCl Humic acid desorption from filter by NaCl

21 Conclusions Water purification via filtration onto functional CNF filters is a promissing and sustainable route. A wide range of pollutants can be adsorbed onto the filters by e.g. electrostatic interactions. Freeze-drying results in filters with higher flux compared to conventional filters by paper-making whitout loss of adsorption capacity. CNF filters could be regenerated for multiple utilizations. Thank you for you attention