In situ modified nanocellulose/alginate hydrogel composite beads for purifying mining effluents

Biobased adsorbents and membranes offer advantages related to resource efficiency, safety, and fast kinetics but have challenges related to their reusability and water flux. Nanocellulose/alginate composite hydrogel beads were successfully prepared with a diameter of about 3–4 mm and porosity as high as 99%. The beads were further modified with in situ TEMPO-mediated oxidation to functionalize the hydroxyl groups of cellulose and facilitate the removal of cationic pollutants from aqueous samples at low pressure, driven by electrostatic interactions. The increased number of carboxyl groups in the bead matrix improved the removal efficiency of the adsorbent without compromising the water throughput rate; being as high as 17 000 L h−1 m−2 bar−1. The absorptivity of the beads was evaluated with UV-vis for the removal of the dye Methylene Blue (91% removal) from spiked water and energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) elemental analyses for the removal of Cd2+ from industrial mining effluents. The modified beads showed a 3-fold increase in ion adsorption and pose as excellent candidates for the manufacturing of three-dimensional (3-D) column filters for large-volume, high flux water treatment under atmospheric pressure.


Introduction
Industrial wastewater treatment before its discharge is crucial to minimize the contamination of water streams with hazardous or potentially hazardous substances, for instance, heavy metal ions and dyes.][3][4][5] Cellulose and nanocellulose (either cellulose nanocrystals CNC or cellulose nanobril (CNF)) have emerged in recent years as versatile biobased choices for water treatment [6][7][8][9][10][11] thanks to their meritorious properties, such as low environmental impact, high natural abundancy, and versatile surface chemistry which allows for functionalization. 6,7Cellulose offers an abundance of hydroxyl groups (-OH) which are susceptible to modication.
An efficient method to introduce carboxyl groups (-COO-) and increase the affinity of cellulose towards cationic species is by selectively oxidizing the primary alcohol of its structure via 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) mediated oxidation 12 Currently, TEMPO oxidation constitutes the most used pretreatment method (before mechanical disintegration of cellulose pulps) for the preparation of highly charged CNF (oen denoted as TEMPO-CNF or TO-CNF) and has been reported as a successful in situ modication method of cellulosic membranes. 13Despite the broad use of cellulose membranes in water purication, there are certain limitations in their capabilities.
The typically dense structure of the bodies of membranes leads to a high-pressure drop when used in dead-end ow and cross-ow.Pressure drop is dened as the pressure difference between the inlet and the outlet of a lter and it can result in the movement or even fracture of the lter.One way to reduce the pressure drop is to increase the permeability of a membrane either by reducing its thickness or increasing its porosity.However, both approaches involve a tradeoff between the mechanical properties and the efficiency of the lter.A less dense membrane matrix entails that a smaller amount of material is used and fewer functional groups are present to facilitate the adsorption.9][20] The advantages of these adsorbents, oen denoted as aerogel or hydrogel beads, are that they offer a higher number of functional groups per area compared to membranes, can be easily removed aer water treatment, and can be packed in cylindrical congurations for the manufacturing of lters that resemble the ion exchange resins and allow high water permeance.
2][23][24] Alginate or alginic acid (commercially available as a sodium salt (SA)) is a linear polysaccharide extracted from brown algae. 21It consists of blocks of (1,4)-linked b-D-mannuronate (M) and a-L-guluronate (G) residues and as a copolymer, it is composed of either consecutive G or M units, for instance, GGGG or MMMM, or alternating M and G units, for instance, GMGMGM.Fig. 1 depicts the GGMMG conguration, as an example of an alginate structural motif.
Due to its structure, alginate forms hydrogels upon interaction with ionic cross-linking agents such as divalent cations.When aqueous alginate solutions are poured dropwise into CaCl 2 solutions they instantly form spherical-shaped hydrogels because of the ionic-crosslinking of the G units of adjacent polymer chains, forming a so-called egg-box structure. 6Notably, it is believed that only the G units bind to Ca 2+ because of their higher degree of coordination. 21he carboxyl and hydroxyl groups of alginate and cellulose make cellulose/alginate composites highly promising for use in water treatment applications for the removal of cationic pollutants.Stand-alone alginate 25,26 as well as cellulose/alginate beads 27,28 have already been reported for heavy metal ion removal.
We aimed to investigate the effect of in situ TEMPO oxidation on nanocellulose/alginate composite beads to control and enhance the performance which has not explored to date.We hypothesize that by oxidizing the available hydroxyl groups of cellulose, the amount of carboxyl groups would increase and thereby improve the removal capacity of the beads.For this purpose, commercial-grade debrillated cellulose and SA were used for the preparation of beads which were then modied in situ by TEMPO-mediated oxidation.The effect of this modication was monitored by comparing the adsorption capacity of the pristine hydrogel beads (denoted as C-SA) with that of TEMPO-oxidized beads (denoted as TO-C-SA) toward Methylene Blue (MB, a cationic model dye) and metal ion (Cd 2+ from mining effluent), respectively.
Composite hydrogel and beads preparation.SA (1 g) was vigorously mixed in 100 mL distilled H 2 O at 50 °C until a transparent solution was obtained.The SA solution was then mixed with a 1% (w/w) cellulose dispersion in a 1 : 1 ratio for 1 h.The mixture was then collected with a syringe and was poured dropwise into a CaCl 2 (0.7 M) solution, through a 2 mm needle (Sterican, Braun) using a syringe pump at a rate of 0.5 mL min −1 (A video of the process is shown in the ESI, † S1).Despite their instantaneous formation, the beads were le in the CaCl 2 solution for 30 more min for stabilization.Finally, the beads were collected, washed with H 2 O, and stored in H 2 O in the fridge.Before every use hereaer, unless otherwise noted, the beads were placed on dry paper for a few seconds to remove the excess absorbed water and have better control over their amount.
In situ TEMPO oxidation of beads TEMPO (0.03 g, 0.2 mM) was dispersed and 0.5 g NaBr (5 mM) was dissolved in 100 mL H 2 O.Then, 4.5 mL NaClO (aq) (10% v/ v, 6 mM) was added and the pH of the reaction mixture was adjusted to 10 with dilute HCl (aq).Then, 10 g of partially dried hydrogel beads were added to the reaction mixture for 5 min.The beads were removed and transferred to an EtOH : H 2 O (1 : 4 ratio) mixture to quench the reaction for 1 min.Finally, the beads were washed with H 2 O and stored in H 2 O in the fridge until further use.

Characterization of surface chemistry of the beads
Fourier transfer infrared spectroscopy (FTIR) spectra of lyophilized beads were recorded in the range of 400-4000 cm −1 , through 64 scans, and a resolution of 4 cm −1 with an ATR-FTIR spectrometer (670-IR, Varian).All obtained spectra were baseline-corrected.

Determination of surface charge density
Beads (1 g) were dispersed in 100 mL Milli-Q H 2 O.The mixture was vigorously mixed with UltraTurrax (IKA) at 15 000 rpm until a homogeneous dispersion was obtained.Aliquots (1 mL) of the dispersions (pristine and in situ oxidized beads, respectively) were diluted with Milli-Q H 2 O to 100 mL and titrated with PDADMAC using a Stabino (ParticleMetrix) system.The charge density of PDADMAC is 0.307 meq mL −1 .The measurements were replicated three times for each sample and the surface charge densities were calculated according to eqn (1): where V 0 is the volume of titrant consumed to neutralize the sample, V sample is the volume of the sample, and x is the concentration of the sample.

Morphology of the beads
Images of the surface of the prepared beads were recorded with a USB optical microscope (AM73915MZT, Dino-Lite Digital Microscope).Moreover, the morphology of the surface and the cross-section of lyophilized beads were observed using a scanning electron microscope (SEM, TEM3000, Hitachi) with an acceleration voltage of 5 kV.

Porosity
The porosity of the beads was calculated by the gravimetric method using eqn (2). 23 where W w and W d are the weights in grams of the wet and dry beads, respectively, and r m and r w are the densities of the bead matrix and water, respectively, in g cm −3 .

Determination of MB removal
Aqueous solutions of MB (5 mg L −1 ) were prepared.Beads (1 g) were immersed in 100 mL dye solutions and aliquots were removed at specic times (aer 1, 5, 10, 30, 60, 120, 240, 300, and 720 min).The aliquots were then analyzed with UV-vis spectroscopy (Agilent Cary 5000 UV/vis/NIR, Agilent) in the spectral range 200-800 nm and the maximum absorbance at 665 nm (corresponding to MB) was recorded.

Determination of Cd 2+ removal
Aqueous solutions of Cd 2+ (400 mg L −1 ) were prepared.Beads (1 g) were immersed in 100 mL Cd 2+ solutions and le for 1 h.The beads were then collected and dried at 40 °C under vacuum.Elemental analysis of the beads was performed with energydispersive X-ray spectroscopy (EDS) using a scanning electron microscope equipped with an X-ray detector (TEM3000, Hitachi), at an acceleration voltage of 15 kV.
Similarly, X-ray photoelectron spectroscopy (XPS) analysis of beads aer interaction with Cd 2+ solution was performed via Thermo Fisher (K-Alpha) with micro-focused Al Ka radiation (energy of 1486.6 eV).A carbon peak at a binding energy of 284.2 eV was used as a reference.For these experiments, samples from effluents of a mining industry were used to study the selectivity of the beads towards Cd 2+ .

Flux measurement
The time needed for 100 mL H 2 O to pass through a chromatographic column with a beaded rim (Lenz, 30 mm diameter) was recorded.The measurement was repeated with the use of commercial lter paper (average pore size 6 mm, Munktell, Grade 3) and 1, 5, and 10 g of the beads.The reported values are averages of 5 replicates.

Results and discussion
Surface chemistry of the beads Functional groups of the prepared beads before and aer the in situ modication were characterized with FT-IR (Fig. 2a).The recorded spectra show absorption peaks at 1606, 1420, and 1029 cm −1 which correspond to stretching vibrations of --COO − (asymmetric), -COO − (symmetric), and C-O bonds, respectively. 24The increase in the intensity of the peaks of TO-C-SA compared to C-SA is attributed to the increase of carboxyl groups aer oxidation.
The increase in the number of carboxyl groups in the composition of the beads aer oxidation is also supported by an increase in surface charge density from 291 ± 51 mmol kg −1 for the C-SA to 477 ± 47 mmol kg −1 for TO-C-SA.An earlier report from our group on in situ TEMPO oxidation of cellulose membranes (sludge-CNF/CNC BE membranes, "BE" herein stands for "bioethanol" because this CNC was produced from the residue of bioethanol production), showed an increase in acidic group content from ∼7 to ∼42 mmol kg −1 , corresponding to a signicant 6-fold charge increase. 13In the current case, the charge density of the beads aer modication is signicantly higher (almost 10 times), however, the increase between modied and unmodied beads showed only a two-fold increase which can be attributed to the abundance of carboxyl groups in the alginate phase used for beads processing.The

Nanoscale Advances
Paper concept of bead formation by nanocellulose and alginate by Ca 2+ crosslinking and subsequent TEMPO oxidation is schematically shown in Fig. 2b.

Morphology of the beads
The morphology of the beads was monitored with a digital optical microscope, as well as SEM imaging for their surface and cross-section (Fig. 3).
From the obtained micrographs, there is no signicant change in the morphology of the beads aer in situ oxidation.The average dry weight of the TO-C-SA beads was about half of the C-SA beads while their average wet weight was almost identical.This is probably attributable to the fact that the modied beads are more hydrophilic and, therefore, absorb more H 2 O. 12 TO-C-SA contains more carboxyl groups than C-SA, explaining the difference in hydrophilicity.Nevertheless, the differences in average dry and wet weights did not signicantly affect the porosity of the beads before and aer the modication (Table 1).

MB removal
The effect of the modication on the adsorptivity of the beads toward cationic dyes was monitored with MB, which is positively charged at neutral pH. 29The obtained data from UV-vis measurements showed that the pristine beads have an adequate removal efficiency of 75% aer 12 h (for initial MB concentration).In situ oxidation of the beads increased the total MB removal efficiency, reaching 91% aer 12 h (Fig. 4).Adsorption of positively charged dyes shows a systematic increase with carboxyl group content indicating that the adsorption process is primarily driven by electrostatic interactions.
A summary of beads used for dye adsorption is tabulated in Table 2. Chitosan (CTS) beads with montmorillonite (MT) were investigated for methyl green (MG) adsorption from aqueous solutions. 30Aer optimizing many parameters, the beads offered efficiency of 99%.Alginate beads containing polyamidoamine/halloysite nanotubes (Alg/Hal_PAMAM beads) was also reported for MG adsorption. 31These composite beads exhibit good adsorption efficiencies.However, it contains inorganic clays e.g.montmorillonite or halloysite that can be a source of contamination for long-term exposure.Our beads are pure organic biopolymers without any inorganic materials with good efficiency.It should be also notted that most of these efficiencies are based on the biopolymer of the beads e.g., chitosan. 31Cellulose-based beads are economically cheaper than chitosan based beads. 32Copper ions were suggested to replace Ca 2+ for the formation of Graphene oxide (GO)/SA/Carrageenan (GO/Alg-Car) beads (Ala : Car : GO = 2 : 2 : 1). 33The use of Ca 2+ is environmentally benign compared to Cu 2+ ions that can be released into the aqueoutic system causing secondary pollution.SA beads of meso-tetrakis(2,4,6-trimethylphenyl)porphyrinto) zinc(II) complex (Zn(TMP)) containing 3% of SA was reported for MB adsorption. 34TO-C-SA beads exhibit comparable adsorption efficiency with a suitable equilibrium time to reach the steady state (Table 2).

Cd 2+ removal
The adsorption capacity of the prepared beads towards Cd 2+ was studied with EDS and XPS elemental analyses.For the former, spiked Cd 2+ aqueous samples were used while for the latter, samples from a water recipient of mining discharge were used in an attempt to monitor the selectivity of the beads towards Cd 2+ .
EDS elemental analysis of the beads aer immersion in aqueous Cd 2+ samples showed an increase in Cd 2+ atomic percentage (at%) from 2.53 to 6.98 (Table 3).This almost 3-fold increase in Cd 2+ atomic percentage can be attributed to the carboxyl groups generated in the in situ oxidation of the beads.In addition, an increase in the atomic percentage of Ca 2+ can be observed.
The at% of adsorbed Cd 2+ on the surface of the beads was estimated from the XPS elemental survey.The data indicate an improvement in ion adsorption; the at% of Cd 2+ increased from 0.84 for the pristine beads to 1.76 for the modied ones (Fig. 5).This again supports the concept of ion adsorption driven by electrostatic interactions.Sehaqui et al. 35 studied the adsorption of divalent metal e.g., Cu 2+ onto oxidized cellulose and showed that the adsorption increases inearly with carboxylate content with maximum adsorption capacities of 55 mg g −1 at neutral pH.It was established that Cu 2+ ions are adsorbed onto the TCNF via electrostatic interactions involving the carboxyl groups on the oxidized cellulose ber surface. 36The proton of the carboxyl group is exchanged with a metal ion during the adsorption process.Although the obtained data from EDS and XPS follow the same trend, there are signicant differences in at% values.XPS is a surface analysis technique using a lower energy X-ray beam compared to EDS.In XPS, the beam penetration depth is about 10 nm while EDS assesses the entire bulk.In addition, the XPS measurements were performed on beads that were used to treat samples of actual effluents from mining.he deconvolution of the high-resolution O 1s spectrum of C-SA beads aer Cd 2+ adsorption reveals peaks at BE of 530.9, 531.9, 532.9, and 533.9 eV, which correspond to C-O, C]O, O-C]O, and O-Cd, respectively (Fig. 7a).From the deconvolution of the O 1s spectrum of TO-C-SA beads aer adsorption, peaks at BE of 530.4,531.4,532.5, and 533.9 eV are revealed (Fig. 7b).The peaks correspond to the same type of O as in C-SA; however, the intensity of the peak that corresponds to C]O increases, presumably due to the contribution of the carboxyl O atoms that are introduced with the TEMPO oxidation. 38,39nally, the deconvolution of the high-resolution spectrum of Cd 3d2 of C-SA beads aer Cd 2+ adsorption reveals two distinct peaks at BE of 405.5 and 412.3 eV, corresponding to Cd 3d 5/2 and Cd 3d 3/2 types of Cd, respectively (Fig. 8a).Cd 2+ is adsorbed on the surface of the beads due to interactions with the O atoms of the carbonyl groups of alginate.However, the deconvolution of the Cd 3d2 spectrum of the TO-C-SA beads reveals two additional lower-intensity peaks at 404.8 and 413.2 eV (Fig. 8b).These two peaks presumably correspond to Cd 2+ ions that are adsorbed on the surface of the beads due to interactions with the carboxyl O atoms that are introduced with the TEMPO oxidation.Binding energies of approximately 7 eV of Cd 3d 5/2 and Cd 3d 3/2 are characteristic of the two states of Cd 2+ . 8

Loose-ll column lters
To evaluate the potential to use the beads in column lters and to compare the effect on water ux, the required time for 100 mL H 2 O to pass through a chromatographic column with a beaded rim under the effect of atmospheric pressure was measured.As a reference, the dewatering time without the use  9).The obtained dewatering times clearly show that a single layer of beads with 1 g of beads did not affect the water ow.An amount of 10 g of beads, which formed a stack of layers of approximately 2.5 cm, showed better water ux than commercial lter paper.Additionally, the average dewatering times for TO-C-SA and C-SA beads are the same which indicates that, in this range, the functionalization does not affect the water ux.Furthermore, the water ux was found to be higher than that reported for cellulose-based layered membrane systems in previous studies from our group which uctuated from 3417 to 14 742 L h −1 m −2 bar −1 . 13,40-43

Conclusions
We devised a facile method of preparing nanocellulose/alginate composite beads with increased adsorption performance derived from the in situ TEMPO oxidation of the hydroxyl groups of cellulose.Oxidation of the beads increased the number of carboxyl groups which can facilitate the removal of cationic impurities from aqueous samples.The enhancement of removal efficiency of the cationic dye MB and Cd 2+ was conrmed with UV-vis measurements (for MB), EDS, and XPS (for Cd 2+ ).Interestingly, the EDS elemental analysis of the beads showed an almost 3-fold improvement in Cd 2+ removal aer the in situ oxidation.Furthermore, the obtained water ux data indicates that the prepared beads can be used for the manufacturing of column lters.This approach offers lters with a higher amount of adsorbent and better water permeability than 2D membrane lters.Overall, the abundance of the component feedstock, as well as the ease and efficiency of the modication route indicate that the nanocomposite beads are promising candidates for larger production and manufacturing of column lters for water purication.Nevertheless, how the mechanical properties of the beads were affected by the oxidation and how the presence of several charged contaminants in the same sample affects the adsorption performance of the beads will be a subject of future investigation.The long-term stability and recyclability of the beads will also be a topic of further investigation.Nanoscale Advances Paper

Fig. 2
Fig. 2 (a) FT-IR spectra and (b) cellulose-SA bead formation via Ca 2+ complexation followed by surface modification by TEMPO oxidation.

Fig. 3
Fig. 3 Top (a) digital micrograph, SEM micrographs of (b) surface, and (c) cross-section of C-SA.Bottom, (d) digital micrograph, SEM micrographs of (e) surface, and (f) cross-section of TO-C-SA.

Fig. 4
Fig. 4 MB removal efficiency for C-SA and TO-C-SA beads.

Fig. 5 Fig. 6 C
Fig. 5 XPS survey of C-SA and TO-C-SA beads after Cd 2+ adsorption.

Fig. 9
Fig. 9 Illustration of loose-fill column filter setup for TO-C-SA beads.1, 5 and 10 g of beads provided 0.3 cm, 1.3 cm, and 2.5 cm of beaded column respectively.

Table 1
Morphological characteristics of C-SA and TO-C-SA beads

Table 2
A comparison among different beads used for dye adsorption

Table 3
The atomic percentage of Cd 2+ and Ca 2+ in C-SA and TO-C-SA beads