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Journal of Water Process Engineering xxx (2014) xxx–xxx

Contents lists available at ScienceDirect

Journal of Water Process Engineering

journa l h om epage: www.elsev ier .com/ locate / jwpe

ead adsorption with sulfonated wheat pulp nanocelluloses

erhi Suopajärvi ∗, Henrikki Liimatainen, Mikko Karjalainen,eikki Upola, Jouko Niinimäki

iber and Particle Engineering Laboratory, P.O. Box 4300, FI-90014 University of Oulu, Finland

r t i c l e i n f o

rticle history:eceived 13 December 2013eceived in revised form 2 June 2014ccepted 26 June 2014vailable online xxx

eywords:

a b s t r a c t

Low-cost sorbents derived from abundant natural resources, industrial by-products, or waste materialsare considered amongst the most viable novel materials for heavy metal removal. In this study, wheatstraw pulp fine cellulosics were used as a biosorbent for the removal of Pb(II) in aqueous solutions afternanofibrillation and sulfonation pretreatments. The effect of the initial lead concentration, sorption time,and solution pH were studied, and the isothermal data were modeled with the Langmuir and Freundlichisotherm models. Pb(II) was adsorbed efficiently (1.2 mmol/g) from the model solution by sulfonated

−1

dsorption isothermsgricultural residuesnionic celluloseiosorbenteavy metalsanocellulose

nanocelluloses which had a width between 5 and 50 nm and a sulfonic acid content of 0.45 mmol g .This adsorption capacity is comparable to those of commercial adsorbents. The Pb(II) adsorption ontonanofibrillated and sulfonated cellulosics followed the Langmuir isotherm model and showed rapid initialkinetics. Thus, nanofibrillated and sulfonated cellulosics are promising green alternatives for the recoveryof metals from aqueous solutions.

© 2014 Elsevier Ltd. All rights reserved.

. Introduction

Environmental toxins, including heavy metals, enter the foodhain through drinking water and crop irrigation, which is anxtensive problem in several countries [1]. Additionally, indus-rial activities produce wastewaters that are contaminated byoxic heavy metals. The mining, metallurgical, electroplating, elec-ronic, nuclear and other water-intensive industries are majorauses of ground water contamination by heavy metals [2]. Com-only, dissolved metal ions are removed from dilute wastewaters

sing conventional physicochemical methods, such as chemicaleduction, an electrochemical treatment, ion exchange, precipi-ation, evaporative recovery, or adsorption onto activated carbon3–5]. However, in many cases, these methods are expensivend inefficient [2–4]. Consequently, low-cost sorbents derivedrom abundant renewable resources, industrial by-products, oraste materials are considered amongst the most viable novel

lternatives for heavy metal removal [1,3,6,7]. Cellulosic wasteaterials, such as sugarcane bagasse [8,9], cotton linters [10], andheat straw [11,12] have been investigated for the adsorption

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(2014), http://dx.doi.org/10.1016/j.jwpe.2014.06.003

f, for example, Cd(II), Cu(II), Mn(II), Mg(II), Sr(II) and Pb(II) ions.heat straw pulp is produced from the agricultural residues of

ood production and, therefore, is a potential sustainable renew-

∗ Corresponding author. Tel.: +358 294 482415; fax: +358 85532405.E-mail address: Terhi.suopajarvi@oulu.fi (T. Suopajärvi).

ttp://dx.doi.org/10.1016/j.jwpe.2014.06.003214-7144/© 2014 Elsevier Ltd. All rights reserved.

able cellulosic source. The fine material fraction (usually <200 �m)of wheat straw pulp in particular is a potential cellulosic raw mate-rial for use as a biosorbent because it decreases the feasibility ofusing wheat pulp for papermaking. However, fine fractions of frac-tionated wheat straw pulps have not yet been investigated as aheavy metal adsorbent.

Nanofibrillation, i.e., the liberation of nanosized constituentsfrom the cell wall matrix of plant fibers, produces a cellulosic mate-rial with a very high surface area of 170–246 m2/g (as measuredfor a nanocellulose and polypyrrole aerogel composite [13]) thatis covered with anionic hydroxyl groups, the latter assumed to bebeneficial for adsorption. Cellulose can be easily chemically modi-fied, which enables the addition of various functional groups to thecellulose backbone [14]. In turn, this addition enhances the uptakeof specific metal ions [15] and improves the adsorption capacity ofcellulosics. One potential, environmentally sustainable method forintroducing reactive aldehyde functionalities into cellulose is aque-ous periodate oxidation [16,17]. Aldehyde groups of 2,3-dialdehydecellulose (DAC) can be further converted easily and selectively withvarious functionalities, such as carboxylic acids [18,19], sulfonates[20–22] or imines [23]. Therefore, periodate-oxidized and selec-tively functionalized nanocellulose from the cellulosics of wheatpulp fines may be a suitable material for metal ion adsorption in

with sulfonated wheat pulp nanocelluloses, J. Water Process Eng.

water. Sulfur containing ligands are known to act as soft acidsthat have high affinity toward Pb(II) ions with high selectivityand reversibility [24,25]. Our recent studies have shown that theperiodate-sulfonation reaction is a potentially green method to

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cyclon

pwa

wpatat

2

2

LTpftucbtlTiFpF

atDpc

tSt

Fig. 1. Obtained wheat straw pulp fine fractions using a hydro

roduce nanocellulose, as it avoids the production of halogenatedastes, because the periodate used can be efficiently regenerated

nd recycled back to the oxidation reaction [26].The objective of the present study was to use the fine fractions of

heat straw as a biosorbent after nanofibrillation and subsequenteriodate-sulfonation pretreatments for the removal of Pb(II) inqueous model solutions. The effects of the initial lead concentra-ion, sorption time, and solution pH were studied. Moreover, thedsorption isotherms of lead were determined in order to studyhe adsorption mechanism.

. Materials and methods

.1. Raw material and chemicals

Dry sheets of bleached wheat straw pulp (Triticum aestivum.) were used as a cellulosic raw material for the experiments.he pulp was fractionated into fines and fiber fractions using aressure screen after disintegration in deionized water. The fineractions (39% w/w from the whole pulp) were fractionated fur-her into two fractions, according to density and size differences,sing a hydrocyclone (Fig. 1). These two fractions, i.e., the hydrocy-lone overflow (Fines 1) and underflow (Fines 2), were thickenedy settling and were used as raw materials in the homogeniza-ion and sulfonation pretreatments. The silicate, hemicellulose andignin contents of the pulp were determined using the ISO 1762,APPI-T-212 and TAPPI-T-222 standards, respectively. The phys-cal parameters of the fine fractions were measured by a MetsoiberLab image analyzer (Finland). The chemical compositions andhysical parameters of the wheat pulp fine fractions are shown inig. 1.

The fine fractions were suspended in deionized water with 0.5% consistency and converted into nanofibrils using awo-chamber high-pressure homogenizer (Invensys APV-2000,enmark) with a pressure of 400–900 bar. The suspensions wereassed through the homogenizer eight times to obtain nanofibrillarellulose gels (NFC 1 and NFC 2).

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All of the chemicals that were used in the adsorbent syn-hesis and characterization were obtained as p.a. grade fromigma–Aldrich, Germany and were used without further purifica-ion. Deionized water was used throughout this work.

e, and the physical and chemical parameters of the fractions.

2.2. Synthesis of the sulfonated nanofibrillar cellulose adsorbents

Anionic sulfonated cellulose derivatives (SNFC 1 and SNFC 2)were synthesized by consecutive periodate oxidation and sodiummetabisulfite sulfonation using NFC 1 and NFC 2 as the startingmaterials. The synthesis was conducted as reported previouslyfor chemical bleached birch pulp [20] with small modifications.In brief, the vicinal hydroxyl groups at positions 2 and 3 of thehomogenized cellulose fines (NFC 1 and NFC 2) were first oxi-dized using sodium metaperiodate to produce aldehyde groups(1.31 mmol/g), and the aldehyde groups were then sulfonated usingsodium metabisulfite (NaS2O5 which reacts with water to formNaHSO3) (Fig. 2).

The suspensions were centrifuged after 72 h of the sulfonationreaction, which was conducted at room temperature in an aqueoussolution, and were washed until the conductivity of the suspen-sions was <20 �S/cm. The anionic charge density of the sampleswas analyzed by conductometric titration using a procedure thatwas described by Katz et al. [27] and by Rattaz et al. [28]. The massyields of the reactions were measured by weighing the obtainedproducts on an analytical balance.

2.3. Characterization of the sulfonated nanofibrillar celluloseadsorbents

2.3.1. Size fractions of the adsorbentsA fractional size analysis of the nanocellulose adsorbents was

conducted with a chromatographic washer using a continuouswater flow in a long tube. The nanocelluloses were washed in a longplastic tube (diameter of 4 mm) with 1000 ml of deionized waterfor 108 s at a flow rate of 7.5 ml/s. In this tube, the particles flowwith different average velocities according to their size such thatthe largest particles tend to stay in the middle of the tube, i.e., in thefaster flow, longer. Thus, these particles emerge first from the end ofthe long tube [29]. A 5 ml sample was injected into the washer witha 0.1% consistency in a constant water flow, and four different sizecategories were obtained by taking samples after separation timesof 60, 77, 86 and 95 s. The largest particles were visualized with a

with sulfonated wheat pulp nanocelluloses, J. Water Process Eng.

CCD camera (resolution of 1.6 �m), and the amounts of material inthe different fractions were measured by filtering the samples on amembrane (retention of 0.2 �m), followed by weighing. Fractionswere also collected for further analysis with FESEM.

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ulfite

2i

tcssfsd

2

wb0oavaawE

l

q

wtt

2

u

q

woetmib

tb

R

of cellulose, which increased the repulsion between particles anddecreased the amount of FR 1. The second fraction (FR 2) consistedof fibril bunches. The widths of these particles, as measured byFESEM, ranged from ∼300 nm to ∼20 nm, while the length was

Table 1The anionic charge densities of the wheat pulp, fine fractions and sulfonated andnanofibrillated fines (SNFC 1 and 2).

Sample Charge density [mmol/g]

Wheat pulp 0.08

Fig. 2. Periodate oxidation and bis

.3.2. Field emission scanning electron microscopy (FESEM)maging

FESEM (Zeiss Ultra Plus, Germany) was used to studyhe morphologies and sizes of the different fractions of thehromatographically-washed nanofibrils. As a pretreatment, theamples were filtered to a polycarbonate membrane with a poreize of 0.2 �m, followed by rapid freezing with liquid nitrogen andreeze-drying in vacuum overnight. The dried samples were thenputter-coated with platinum. A voltage of 10 kV and a workingistance of 5 mm were used when imaging the samples.

.4. Adsorption experiments

Adsorption experiments were performed in batch modeith a chosen lead model waters. In each experiment, a 25 g

atch of lead(II)nitrate (Pb(NO3)2) (the amount of lead was.24–6.38 mmol/l) and 50 mg of nanocellulose were mixed. The pHf the suspensions was adjusted using HCl or NaOH, and 5 ml of

0.1 M acetate buffer were added to keep the pH at a constantalue of 5. The suspensions were agitated for 20 h at room temper-ture, after which they were centrifuged for 20 min at 15,000 × gnd a temperature of 4 ◦C. The lead concentrations of the solutionsere analyzed using an atom absorption spectrometer (AAS, Perkin

lmer 4100, USA).The amount of lead that was adsorbed per unit mass of nanocel-

ulose adsorbent qe (mmol/g) was calculated as follows:

e = (Ci − Ce)m

V, (1)

here Ci and Ce are the initial and equilibrium lead concentra-ions (mmol/L). m and V are the weight of the absorbent (g) andhe volume of the solution (L).

.5. Adsorption isotherm models

In this study, the Langmuir and Freunlich isotherm models weresed. The Langmuir model is given as follows [30]:

e = Q 0bCe

1 + bCe(2)

here qe (mmol/g) and Ce (mmol/L) are the adsorbed metal ionsn the adsorbent and the metal ion concentration in the solution atquilibrium, respectively. b (L/mmol) is a constant that is related tohe affinity of the binding sites. Q0 (mmol/g) is known as the Lang-

uir constant, which represents the monolayer sorption capacity,.e., the maximum amount of metal ions per unit weight of adsor-ent that is needed to form complete monolayer coverage [3,15,24].

The separation factor, RL, which describes the affinity betweenhe adsorbate and the adsorbent, can be calculated using parameter

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(2014), http://dx.doi.org/10.1016/j.jwpe.2014.06.003

as follows [3,15,24]:

L = 11 + bC0

(3)

sulfonation reactions of cellulose.

where C0 is the initial metal ion concentration. For favorableadsorption, RL lies between 0 and 1, while RL > 1 represents unfa-vorable, RL = 1 linear and RL = 0 irreversible adsorption [3].

The Freundlich model is given as follows [3,15]:

qe = KF Cne (4)

where KF (mg1−n/g Ln) and n (dimensionless) represent the Freund-lich constants. KF is indicative of the relative adsorption capacity,whereas n is a measure of the nature and strength of the sorptionprocess and the distribution of active sites [15,24]. The smaller thevalue of n, the greater the heterogeneity; if n = 1, all of the sur-face sites are equivalent, if n < 1, the bond energies increase withincreasing surface density and, if n > 1, the bond energies decreasewith increasing surface density [15,24,31].

3. Results and discussion

3.1. Characteristics of the adsorbents

3.1.1. Amount of anionic groupsTable 1 shows the anionic charge densities of the sulfonated

nanocellulose adsorbents (SNFC 1 and 2), fine fractions (Fines 1and 2) and initial wheat straw pulp. The charge densities (CD) weresimilar for the initial pulp and both fines fractions, but, after thesulfonation reaction, the charge density of the samples increasedmarkedly; the SNFC 2 fraction had a slightly higher CD than SNFC1. Fines 2 contained more lignin and silicates that Fines 1 (Fig. 1). Itis known that lignin and fatty acids contain functional groups, suchas alcohol, ketone and carboxylic groups that may also be oxidizedinto charged groups that have a high affinity toward metals ions[15,32–34].

3.1.2. The size fractions of the adsorbentsFig. 3 shows the mass proportions for the different size frac-

tions of the samples after using the chromatographic washer, andFig. 4 presents the typical particle types of these fractions. Fraction1 (FR 1) mainly contained non-fibrillated material and, there-fore, the largest particles. Moreover, FR 1 also included flocks thatformed from individual micro- and nanofibrils. The fibril floccula-tion decreased after sulfonation due to the increased charge density

with sulfonated wheat pulp nanocelluloses, J. Water Process Eng.

Fines 1 0.08SNFC 1 0.41Fines 2 0.07SNFC 2 0.45

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Fig. 3. Mass proportions of the different fractions (FR 1–4) of the initial wheat pulpfines, nanofibrillated fine fractions (NFC 1 and 2) and sulfonated and nanofibrillatedfi

atatr

rials are reacting at lower pH. An increase in pH also increasedthe adsorption efficiency. To avoid the precipitation of Pb, the

FF

nes (SNFC 1 and 2).

ssumed to be tens of microns. FR 3 (Fig. 4 (bottom right)) con-ained highly fibrillated material with widths from 50 nm to 5 nmnd lengths of several microns. Compared with FR 2 or 3, FR 4 had

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he finest material and more roundish particles with lower aspectatios.

ig. 4. First row: images from the chromatographic washer. Left column: FR1 from Fines

R2 and FR3 from SNFC 2 (scale bar 1 �m).

PRESSess Engineering xxx (2014) xxx–xxx

3.2. Adsorption

3.2.1. Effect of the contact time and pH on adsorptionThe effect of pH on adsorption can vary substantially due to

the different functional groups on the adsorbent surfaces, and thesorption capacity may therefore increase or decrease as a func-tion of pH [35]. At low pH levels, the anionic binding sites onthe modified cellulose materials are protonated, which resultsin the poor adsorption of metal cations. An optimum pH leavesthe binding sites unprotonated, i.e., in the anionic form, and theadsorption capacity is consequently maximized. Generally, opti-mum pHs for heavy metal binding onto modified celluloses arein the range 4.0–6.0 [1,4,35]. Above this optimum range, mostmetals tend to precipitate out of the solution in hydroxide form[4,35]. Pb(II) removal with SNFC 2 is shown in Fig. 5a as a func-tion of pH after 20 h reaction time. The adsorption efficiency wasfound to be poor below pH 4, which was the minimum capacity ofthe adsorption, indicating that the binding sites of the adsorbentswere protonated. However, the adsorption capacity was higherat pH 3 than at pH 4. Same kind of effect was seen with grassadsorbent derived from wheat straw and grass by Chojnacka [12].Probably the silicates and/or lignin of grass and straw raw mate-

with sulfonated wheat pulp nanocelluloses, J. Water Process Eng.

solution pH was only raised to a maximum value of 5 in thisstudy.

2 and SNFC 2 (scale bar 100 �m). Middle: flocks of NFC 1 (scale bar 100 �m). Right:

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T. Suopajärvi et al. / Journal of Water Process Engineering xxx (2014) xxx–xxx 5

Fig. 5. Effect of (a) pH and (b) contact time on lead (II) adsorption by SNFC 2. A lead concentration of 1.45 mmol/l and an adsorbent amount of 50 mg were used.

F angmd

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stfiinhgrcitPiA

TI

ig. 6. Adsorption isotherms for Pb(II) onto the NFC and SNFC adsorbents; (a) the Lata (dashed line).

Fig. 5b shows the percentage removal of Pb(II) as a function ofime for SNFC 2 in the presence and absence of a buffer solution.he adsorption reached a maximum value of approximately 99.5%mmediately after the experiment began, but lead started to beeleased back into the solution after 4 h when a buffer solution wasot used. It was observed that the pH of the solution decreased from

to 3 in the non-buffered solution, which caused the desorptionf lead from the adsorbent surface. However, constant solution pHnd adsorption capacity were obtained for buffered solutions overhe studied contact time (24 h).

Sulfonated nanocelluloses possess a high amount of bindingites due the high surface area of nanofibrils, and sulfonation fur-her increases the number of binding sites and accessibility ofbrils, so that the diffusion of Pb(II) to active adsorption sites

ncreases. The adsorption rate was extremely high at the begin-ing and the degree of ionization of sulfonic groups was probablyigher than that of other groups such as carboxyl- or hydroxylroups [36]. Rapid adsorption indicates that Pb(II) was adsorbed byeadily available adsorption site [36,37]. Soft acid like sulfur formomplexes with ligands having more covalent character rather thanonic [24]. At the beginning of the Pb(II) adsorption on SNFC, the ions

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ie with two ligands to form complexes. At low concentrations, allb(II) ions present in the solution could interact with two bind-ng sites, leading to a higher adsorption performance [37] (Fig. 6).nother possible mechanism is the ion-exchange with an adjacent

able 2sotherm constants and regression coefficient for Langmuir and Freundlich isotherms for

Adsorbent Langmuir

Q0 [mmol/g] b [L/mmol] RL

NFC 1 0.03 140.81 0.002

SNFC 1 0.90 1.65 0.137

NFC 2 0.08 49.14 0.005

SNFC 2 1.21 0.69 0.276

uir model fitted to the data (dashed line) and (b) the Freundlich model fitted to the

hydroxyl group, but as with many other bioadsorbent materialsthere is also a possibility that both mechanisms may participateduring adsorption, since there are several different active groupspresent in the surface of an adsorbent [15,24].

3.2.2. Adsorption isothermsFig. 6 shows the adsorption capacity of the nanofibrillated sam-

ples (NFC 1 and 2) and the nanofibrils after sulfonation (SNFC 1and 2) as a function of the solution equilibrium lead concentration.It can be clearly seen that sulfonation significantly increased theaffinity of Pb toward cellulosics. In addition to the highest charge,the sulfonated nanofibrils also had the highest surface area becausethe sulfonated groups decreased the flocculation of the particles(Figs. 3 and 4), which likely promoted adsorption. SNFC 2, whichalso had the highest charge density, had the best affinity towardPb(II) of the samples.

Langmuir and Freundlich isotherms were used to study theadsorption mechanism of Pb(II) in the samples. The isothermconstants of the models are shown in Table 2. The regression coef-ficient indicates that the Langmuir isotherm model had a betterfit than the Freundlich model. This confirms the monolayer cov-

with sulfonated wheat pulp nanocelluloses, J. Water Process Eng.

erage of Pb(II) onto nanocellulose fibrils. A similar trend has alsobeen noticed with other biomaterial adsorbents [4,15]. The sul-fonated adsorbents had RL values between 0 and 1, indicatingthat the adsorption was favorable for both of these samples, while

the adsorption of Pb(II) onto treated wheat pulp fines adsorbents.

Freundlich

R2 KF [mmol1−n/mol Ln] n R2

0.91 0.014 1.65 0.900.93 0.355 0.58 0.880.64 0.014 1.48 0.740.97 0.351 0.66 0.93

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ARTICLEWPE-41; No. of Pages 7

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he nanofibrillated samples had RL values close to zero, indicatingrreversible adsorption. The values of n were below 1 for the sul-onated samples and above 1 for the nanofibrillated samples. Thus,he sulfonated samples likely had a higher surface heterogeneietyhan the nanofibrillated samples [15,24]. The adsorption capacities,.e., the values of Q0 (Table 2), show that the sulfonated samplesad much higher sorption capacities than the nanofibrillated sam-les. The maximum adsorption capacities of the sulfonated samples0.9 mmol/g for SNFC 1 and 1.2 mmol/g for SNFC 2) are higherhan those of many other adsorbents. The adsorption capacitiesf a Pb(II) ion with different lignocellulosic adsorbents and acti-ated carbon have been reported to be below 0.5 mmol/g [15].igher capacities for Pb(II) adsorption have only been reported

or specific aquatic plants (1.2 mmol/g), bacteria (2.7 mmol/g),lgae (2.6 mmol/g), fungi (1.3 mmol/g) or fly ash (2.5 mmol/g) [15].hemical modifications, such as mercerization and succinylation0.93–2.4 mmol/g [8,38,39]), phosphorylation (0.55–3.08 mmol/g40,41]) or sulfurization (0.97 mmol/g [42]), have also been foundo increase the adsorption capacities of lignocellulosic materi-ls. In this investigation, the obtained adsorption capacities withulfonated samples are comparable to other chemically modifiedignocellulosic adsorbents.

. Conclusions

Sulfonated nanofibrillated wheat straw pulp fines are promis-ng green alternatives for the recovery of metals from aqueousolutions. Pb(II) was efficiently adsorbed from a model solutiony nanofibrillated and sulfonated fine cellulosics, but unmodifiedanocelluloses exhibited poor adsorption capacities. The adsorp-ion capacity of nanofibrillated and sulfonated fines cellulosics was.2 mmol/g at pH 5, which is comparable to the capacities of com-ercial adsorbents. The higher content of lignin increased the

dsorption efficiency of the sulfonated adsorbent. The Langmuirsotherm model fit the data better than the Freundlich model for thedsorption of Pb(II) onto treated wheat straw pulp fine adsorbents.

cknowledgments

The authors would like to acknowledge the funding that wasrovided by the Nano- and Microcellulose Based Materials forater Treatment Applications Project of the European Regional

evelopment Fund, European Union and the Future Biorefineryrogram of Forestcluster Ltd.

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