Research Article pubs.acs.org/journal/ascecg
Functional Cellulose Nanofiber Filters with Enhanced Flux for the Removal of Humic Acid by Adsorption Houssine Sehaqui,*,† Benjamin Michen,†,‡ Eric Marty,† Luca Schaufelberger,† and Tanja Zimmermann† Applied Wood Materials Laboratory, Empa, Swiss Federal Laboratories for Materials Science and Technology, Ü berlandstrasse 129, CH-8600, Dübendorf, Switzerland ‡ Institute for Building Materials, ETH Zurich, CH-8092 Zurich, Switzerland †
ABSTRACT: Despite great promises of cellulose nanofibers for water treatment, current technologies have lacked the exclusive use of cellulose nanofibers (CNF) in high-flux filters having an affinity for a desired contaminant. To tackle this, we prepared porous and functionalized filters via solvent exchange, supercritical drying, and freeze-drying of cationic CNF and compared them to conventional CNF filters obtained by the paper-making process. Porosity and pore size were evaluated in the dry state qualitatively and quantitatively via scanning electron microscopy and mercury intrusion porosimetry, respectively. The permeance of water and a solution containing a negatively charged model molecule (humic acid) through these filters was measured at various pressures and correlated to the filters’ structure. As compared to the CNF filters made via paper-making, the porosity, pore size, and permeance were increased after processing via solvent exchange, supercritical drying, and freeze-drying routes. Those filters which were prepared via freeze-drying displayed the highest permeance reported so far for CNF filters, which is about an order of magnitude higher than the permeance of CNF filters made via paper-making and having the same grammage. While the permeability was clearly affected by the processing technique, the functional filters showed a comparable adsorption capacity for humic acid. The filtration of a humic acid solution provided an initial removal of nearly 100% without noticeable reduction in flow. Considering the diluted concentration of HA in natural waters, we expect that large volumes of HA solution could be treated with the present CNF filters, with the possibility to regenerate these filters for multiple utilizations. The present concept of utilizing functional cellulose nanofibers in highly permeable filters working on the adsorption principle may be extended to encompass removal of other water contaminants for a better supply of clean water. KEYWORDS: Filter, Cellulose nanofibers, Water purification, Humic acid, High flux, Adsorption
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INTRODUCTION Water across the globe is experiencing widespread contamination from the ever growing human activity, and access to clean water and sanitation is a serious problem facing many people throughout the world and afflicting over one million deaths per year.1 Addressing this problem in different regions of the world will call for chemically, energetically, and operationally straightforward technologies for water treatment having a minimum impact on the environment.2 In this context, water purification by membranes/filters seems to represent an efficient strategy to enhance the quality and supply of water without intensive chemical treatment.2,3 Here, a deep understanding of the filter’s porous structure as well as the aqueous interface between constituents in water and the filter is essential for achieving higher fluxes and improved contaminant retention with higher selectivity, low fouling, and good resistance to a given chemical environment. While surface separation processes based on membranes are in some cases energy demanding due to high operation pressure and maintenance, the separation of contaminants by filters can be operated at lower pressure because size exclusion and adsorption principles act in concert. Moreover, depth filtration has the advantage that it is less susceptible to fouling. © 2016 American Chemical Society
Recently, the beneficial role of cellulose nanofibers (CNF) in environmental remediation was demonstrated with further opportunities for water purification applications.4 Unique features of cellulose nanofibers, including natural abundance, high surface area and aspect ratio, low environmental impact, high strength and stiffness, chemical resistance and functionalizability are promising for the development of next-generation water purification membranes and filters.5 CNF are isolated in water from plant cell walls through mechanical disintegration.6 Based on the chemistry of alcohols, a wide array of functional groups may be attached to the surface of CNF thus widening the range of water contaminants they can immobilize.7 For instance, carboxyl or phosphate groups have been attached to CNF and used for the uptake of heavy metal ions,8−11 radioactive species,12 or positively charged dye.13 Quaternary ammonium groups onto CNF have good affinity for anionic dyes,14 humic acid,15 nitrates, and fluoride16 while displaying antibacterial properties17 that could result in better resistance to biofouling. Also viruses, which carry a net negative surface Received: April 7, 2016 Revised: June 6, 2016 Published: July 18, 2016 4582
DOI: 10.1021/acssuschemeng.6b00698 ACS Sustainable Chem. Eng. 2016, 4, 4582−4590
Research Article
ACS Sustainable Chemistry & Engineering charge,18 show a high affinity to cationic polymer brushes grafted onto cellulose nanofibers.19 Silylated CNF permits the selective oil absorption from an oil/water mixture.20 Functionalization possibilities of cellulose nanofibers are numerous, and these are expected to deal with the increasing number of contaminants entering water supplies. For their use as membranes/filters for water purification, cellulose nanofibers were generally processed into dense films using paper-making process21−23 or were added as thin-film coating on conventional ultrafiltration or microfiltration membrane supports.13,24 Although films prepared by papermaking are mechanically robust,25 they suffer from both low porosity and flux.21−23 On the other hand, thin nanocellulose films (∼100 nm thickness) on top of a mechanical support have higher fluxes and permit rejection of contaminants via size exclusion. However, they contain a relatively low amount of functional nanofibers, which does not allow for adsorption of a large amount of water contaminants.13,24 Hence, the combination of size exclusion and adsorption principle in filters based on cellulose nanofibers with various functionalities offers a highly promising and sustainable purification technology to be exploited. In previous works, different pathways for water removal from CNF suspension were shown to result in a wide range of porous CNF materials.25−32 CNF films (also called nanopapers) prepared via paper-making or casting an aqueous CNF suspension have low porosity, small pores, high mechanical strength, optical transparency, and high barrier properties to gases.21,25,33−37 Exchanging water to a less polar solvent prior to the solvent evaporation can enhance the nanopaper’s porosity by reducing capillary forces acting on the fibrils during drying.25 For example, Henriksson et al. reported a porosity of 19% for a nanopaper prepared from water versus a porosity of 38% for a nanopaper prepared via solvent exchange to ethanol.25 The porosity was further increased up to 60% by Toivonen et al., who used a low polarity solvent, namely octane.38 Nanopaper obtained by supercritical CO2 drying has an even higher porosity of 86% and a large surface area reflecting a preserved structure from the wet to the dry states stemming from the low polarity of CO2 and its gradual transition from the liquid to the gas phase during drying.30 Another processing technique to increase porosity of CNF materials is freeze-drying (also termed lyophilization).27,31 Here, CNF dispersed in water are first frozen forming ice crystals that are subsequently sublimated by lowering their surrounding pressure. During the freezing step, ice crystals are forming and push the fibrils to interstices between the crystals, leaving, after sublimation, large pores surrounded by CNF sheets/cells and a porosity that could reach 99.5%.27,39 Foam, aerogel, and cryogel are common names given to these materials. Ice templation approaches leading to the formation of porous structures have recently gained interest particularly as drying at atmospheric conditions without the need of a vacuum is possible.40,51,52 The goal of the present work is to develop high-flux CNF filters for water purification using the adsorption principle. Filters are prepared from cationic CNF according to papermaking (PM), solvent exchange to ethanol (SE), supercritical drying (ScCO2), or freeze-drying (FD). The positively charged filters are evaluated regarding the removal of an anionic contaminant model, i.e. Aldrich humic acid (HA), by electrostatic interactions. HA is a complex macromolecular product of the chemical and biological degradation of plant and
animal residues, causing in natural water undesirable color and taste. It may cause various environmental and health problems and leads to a massive membrane fouling which limits the use of membrane technologies for drinking water treatment.15,41,42 Hence, functional CNF filters may be used to reduce negatively charged contaminants, such as HA or viruses, in a single step or enhance the performance of membrane separation processes in a pretreatment step by reducing fouling and consequently the frequency of backwash operations. To the best of our knowledge, this is the first study addressing the adsorption of contaminants in functionalized CNF filters of high flux.
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MATERIALS AND METHODS
Pulp residue, a waste product from the pulp and paper industry also called “fiber sludge”,43 with a cellulose and hemicellulose content of 95% and 4.75%, respectively, was kindly provided by Processum AB, Sweden. 2,3-Epoxypropyl trimethylammonium chloride (EPTMAC) and humic acid were purchased from Sigma-Aldrich. Cationic CNF Preparation. Cationic CNF were prepared according to a previously reported method by using 1.25 mL of EPTMAC per gram of the pulp, and conducting the reaction at 80 °C for 8 h.14,16 The ammonium group content of the cationic CNF is 0.37 mmol g−1 as determined by conductometric titration of chloride ions using AgNO3.44 Filter Preparation. Cationic CNF in water with a dry weight of 250 mg and a total volume of 150 mL was vacuum filtered on a glass filter funnel (8 cm in diameter) using a 0.65 μm DVPP filter membrane from Millipore, thus forming a CNF cake on top of the membrane. The cake was hot-pressed giving a paper-making (PM) filter or was immersed in pure ethanol (without drying) at least 3 times to form an alcogel which was subsequently hot-pressed resulting in the solvent exchange (SE) filter. Filters by supercritical CO2 drying (ScCO2) were prepared as reported elsewhere30 by placing the alcogel in a critical point dryer chamber (E3000, Quorum technologies) and injecting liquid carbon dioxide into the chamber under a pressure of ca. 50 bar for the exchange of ethanol to CO2. The chamber was then brought above the CO2 critical point conditions to ca. 100 bar and 36 °C and depressurized for evaporation of supercritical CO2 to form ScCO2 filter. For filter preparation via freeze-drying, we poured 1.125 g (dry weight) of cationic CNF in water at a total volume of 150 mL into a 15 cm × 15 cm mold, which was subsequently subjected to liquid nitrogen. This led to the freezing of the suspension and the breakage of the resulting solid to several pieces of which only large ones were considered for further use. The ice in the frozen suspension was then sublimated under vacuum at ambient temperature with a Lyovac freeze-dryer (SRK system technik GMBH) yielding CNF foam of ∼99.5% porosity, which was subsequently hot-pressed giving FD filters. Hot-pressing was always performed at 105 °C for 20 min under a pressure of 2.2 bar (Carver Inc., USA). Filters prepared via the four different processes have the same grammage (weight per unit area) of 50 g m−2. Characterization of Filters. The density of the filter (ρfilter) was determined by measuring its dry weight and dividing it by its volume. The volume was calculated from the thickness of the filter (determined by a digital calliper) and its area. Porosity (P) was calculated as P = 1 − ρfilter/ρcellulose by taking 1500 kg m−3 as density of cellulose (ρcellulose). The swelling of the filters was taken as the volume of the wet filter divided by the volume of the dry filter. The specific surface area of the filters was determined according to the multipoint Brunauer− Emmett−Teller (BET) method54 by N2 physisorption at relative vapor pressure of 0.05−0.2 on a surface area and pore size analyzer (Coulter SA3100, Beckman coulter Inc.). Prior to each measurement, samples were degassed at 105 °C for 1 h. Similarly, Moisture sorption measurements were performed in a VTI-SA+ dynamic vapor sorption analyzer (DVS, TA-Instruments, New Castle, USA) at a temperature of 25 °C and at a relative humidity range of 0−30% for the estimation of the specific surface area with water as adsorbate according to the 4583
DOI: 10.1021/acssuschemeng.6b00698 ACS Sustainable Chem. Eng. 2016, 4, 4582−4590
Research Article
ACS Sustainable Chemistry & Engineering BET equation.55 Total porosity and average pore size of the dry filters were evaluated by mercury intrusion porosimetry (MIP) at a pressure range of 0.01−200 MPa (Pascal 140/440; Thermo Fisher; Germany). Scanning electron microscopy was carried out using a FEI Nova NanoSEM 230 instrument (FEI, Hillsboro, Oregon, USA) at an accelerating voltage of 5 kV and a working distance of 5 mm. Before SEM observation, tensile fractured cross sections of the filters were placed on a specimen holder, sputter-coated with a platinum layer of about 7.5 nm (BAL-TEC MED 020 Modular High Vacuum Coating Systems, BAL-TEC AG, Liechtenstein) in Ar as a carrier gas at 5 × 10−2 mbar. Filtration Performance. Permeance of water and humic acid through the filters was determined in a dead-end Sterlitech device (HP4750, Kent, USA). Discs of filters with a diameter of 49 mm were cut, soaked in deionized water for at least 3 days to ensure equilibration, and placed on a porous stainless steel plate. Deionized water or HA solution at 55 mg L−1 and pH of 6.5 (similar pH to that of natural waters) were forced through the filters at room temperature using nitrogen at a head pressure of 0.5, 1, or 2 bar, and the permeance for the active filtration area (1460 mm2) was obtained by measuring the volume permeated per unit area and time. After an initial equilibration for 20 min, the filtration was continued for another 20 min during which the quasi-constant permeance was noted. Two to three replicates of each filter were used for the permeance test and the results were averaged. HA solution was prepared as reported elsewhere15 by dissolving HA in water at a concentration of 1 g L−1, followed by prefiltration through a 0.65 μm membrane (DVPP, Millipore) to remove undissolved particles. HA was then diluted to desired concentration by the addition of deionized water. HA concentrations were measured with a UV−vis spectrophotometer (Spectronic Camspec Ltd., UK) at 254 nm by constructing beforehand calibration curves. Humic acid retention was determined by filtering HA solution at pH of 6.5 through CNF filters using the Sterlitech device at a head pressure of 1 bar. Aliquots of the filtrate were taken and their humic acid concentration was measured by UV−vis spectrophotometry. The retention of humic acid is defined as
Figure 1. Processing routes for the preparation of porous filters from cationic cellulose nanofibers.
Table 1. Density, Porosity, Swelling, and Specific Surface Area SSA of CNF filters
paper-making (PM) solvent exchange (SE) supercritical CO2 drying (ScCO2) freeze-drying (FD)
density kg m−3
porosity (%)
SSA (N2) m2 g−1
SSA (H2O) m2 g−1
swelling (%)
950 810
37 46
