Cellulose based organogel as an adsorbent for dissolved organic compounds

Industrial Crops and Products 49 (2013) 33–42

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Cellulose based organogel as an adsorbent for dissolved organic compounds Wafa Maatar, Sabrine Alila ∗ , Sami Boufi ∗ Laboratoire Sciences des Matériaux et Environnement (LMSE), University of Sfax, Tunisia

a r t i c l e

i n f o

Article history: Received 14 February 2013 Received in revised form 6 April 2013 Accepted 17 April 2013 Keywords: Organic pollutants Cellulose organogel Adsorption Nanofibhrillated cellulose (NFC)

a b s t r a c t Highly porous cellulose organogels were prepared from nanofibrillated cellulose hydrogels and their adsorption properties towards a wide range of organic pollutants were investigated. Here, we show that by functionalizing the native cellulose nanofibrils of the organogel with the hydrophobic hydrocarbon chains, the adsorption capacity is meaningfully boosted, making possible to use the modified organogel as an adsorbent for organic compounds. The chemical modification was confirmed by infrared spectroscopy (FTIR) and solid NMR spectroscopy. The kinetics and adsorption isotherms of several aromatic compounds, including herbicides were investigated. It was proposed that the adsorption process is the result of the diffusion of the organic solute inside the grafted hydrocarbon chains acting as a reservoir on which the organic compounds would be accumulated. The results showed that the modified cellulose organogels could be easily regenerated and reused without any loss of the adsorption capacity, which constitutes one of the main advantages of this category of the adsorbents derived from a renewable resource. © 2013 Elsevier B.V. All rights reserved.

1. Introduction The rapid industrialization along with modern agricultural methods have resulted in the generation of large amounts of wastewater containing a huge number of pollutant molecules, namely those in the dissolved form, that are harmful both to human and animal lives. It has also contributed to the contamination of surface and ground waters by run-off, drainage, and leaching, from agricultural areas, deposition from aerial applications, discharge of industrial waste, thus resulting in a constant increase in their level. The most common of the dissolved pollutants are heavy metals, dyes, phenols, detergents, pesticides, fungicide and polychlorinated biphenyls (PCBs). Their harmful effects and toxicity are well documented and readily available from online databases (ECOTOX, 2003). Although a number of innovative water treatment methods, such as filtration, ultrafiltration, or oxidation, have been developed for the efficient removal or decomposition of organic solvents. The cost and the additional costs of regeneration, if required, are decisive factors in the choice of the treatment methods to be implemented. Hence, looking for cost-effective, safe and sustainable

∗ Corresponding authors at: University of Sfax, BP 1171, Tunisia. Tel.: +216 52 972 227; fax: +216 74 274 437. E-mail addresses: sabrine [email protected] (S. Alila), sami.boufi@fss.rnu.tn (S. Boufi). 0926-6690/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.indcrop.2013.04.022

technology for the wastewater treatment still a relevant research matter. Amongst the various treatment techniques mentioned above, adsorption is one of the effective methods for lowering the concentration of less soluble organic pollutants from effluent. Together with being efficient and easy to implement at any scale, this method has been widely used to remove dissolved organic pollutants and other hazardous chemicals from water. However, it is worth to mention the shortcoming in using adsorption processes in waste water treatment, among which we can cite the dependence of the sorption capacity on the chemical structure of the material and its characteristics e.g. porosity, specific surface area, swelling and diffusion. Although activated carbon is the most widespread adsorbent, available in a variety of configurations and sizes, it suffers from some drawbacks such as the high cost, expensive regeneration and progressive loss of the adsorption capacity with regeneration (Hamdaoui et al., 2005; Hamdaoui and Naffrechoux, 2007). Therefore, the exploitation of new and cheap adsorbents which can be more easily regenerated has become the focus of intense research (Akhtar et al., 2007a,b; Memon et al., 2007, 2008). In particular, adsorbent derived from ubiquitously available renewable resources is an interesting approach arousing much interest (Crini, 2005; Crisafully et al., 2008; Gupta and Ali, 2002). A number of inexpensive renewable materials, including industrial and agricultural wastes, have already been used as such or with minor treatment to remove different pollutants from industrial effluents (Memon et al., 2007, 2008; Carrott et al., 2009).

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W. Maatar et al. / Industrial Crops and Products 49 (2013) 33–42

However, the adsorption capacity of these naturally occurring products remains very low and fluctuates according to the adsorbent origin. The surface modification is one method to boost the adsorption capacity of the biosorbent and to extend its application to a wide range of organic pollutants (Aloulou et al., 2004, 2006; Alila et al., 2007, 2010, 2011; Alila and Boufi, 2009). In the present work, highly porous organogel from cellulose nanofibrils hydrogel were prepared using solvent exchange approach and their adsorption properties towards a wide range of organic solutes were investigated. It was shown that by functionalizing the native cellulose nanofibrils of the organogel with hydrophobic hydrocarbon chains the adsorption capacity of the material is meaningfully boosted. 2. Experimental 2.1. Materials Commercial bleached eucalyptus pulp (Eucalyptus globulus from Ence-Spain) was used as starting material for the preparation of nanofibrillated cellulose organogel (NFCo). The other chemical products were purchased from Sigma Aldrich and used without purification. 2.2. Preparation of the nanofibrillar cellulose organogel (NFCo) The dispersion of the nanofibrillated cellulose NFC was prepared following a method that was well described in our previous work (Besbes et al., 2011a,b). The procedure involved the following steps: (i) the eucalyptus fibres were first TEMPO-oxidized to a carboxyl content of 500 ␮mol g−1 , (ii) the oxidized fibres suspension (1.5 wt.% consistency) were then fibrillated using high pressure homogenizer (10 passes at 600 bar) giving rise to a highly consistence transparent gel, (iii) the gel was packed in a glass cylindrical tube (h/D: 20/10 mm) and kept at −10 ◦ C during 24 h to freeze the gel, (iv) the frozen gel was removed from the glass mould and kept in acetone to proceed in a solvent exchange and convert the NFC gel into a highly porous organogel structure, and (v) the organogel was then subjected to a surface modification via grafting with linear hydrocarbon chains according to the following. 2.3. Chemical modification of the NFCo The organogel was placed in a three neck flask containing a mixture of toluene/DMF (80/20 vol), equipped with a Dean-Stark system, and kept under reflux until all the residual water retained by the organogel was completely removed by azeotropic distillation. The suspension was then cooled to 60 ◦ C and carbodiimidazole (CDI) was added under dry nitrogen atmosphere and kept under magnetic stirring for 3 h. Subsequently, the reaction mixture was rapidly filtered and washed twice with dry toluene in order to eliminate all the residual CDI. The activated fibres were then brought in a three neck flask containing the hexadecylamine, (2 time the equivalent number of hydroxyl groups of cellulose), and kept under magnetic stirring and dry nitrogen atmosphere at 60 ◦ C for 3 h. Finally, the recovered product was purified by soxhlet extraction with toluene for 24 h, exchanged to ethanol and kept in water for a later use. 2.4. FTIR analysis The FTIR spectra were recorded using a Perkin-Elmer BX II spectrophotometer operating in transmission mode with a resolution of 2 cm−1 in the range 400–4000 cm−1 .

2.5. CP/MAS NMR Cross polarization/magic angle spinning CP/MAS solid state NMR experiments were performed with a Bruker 300 spectrometer operating at a frequency of 75 MHz. The spinning speed was set at 300 Hz. The contact time for CP was 1 ms and then delay time for acquisition was 5 s. Chemical shifts were referred to tetramethylsilane (TMS). 2.6. Surface area measurement The specific surface areas were determined by N2 adsorption measurements at the temperature of liquid nitrogen (ASAP 2020, Micromeritics, US). Prior to measurement, the samples were dried at room temperature. The surface area was determined from the adsorption results using the Brunauer–Emmet–Teller (BET) method. 2.7. Adsorption isotherms All the adsorption experiments were performed in batch conditions. Aqueous solutions of the tested organic solute at different concentrations (ranging from 5.0 × 10−5 to 5.0 × 10−3 M) were first prepared and then the sample of wetted cellulose organogel (corresponding to about 100 mg in dry form) was brought in the flask. The flasks were shaken at 100 rpm in a thermostatic bath for 4 h to reach the adsorption equilibrium. The residual concentration of the organic solute was determined by UV spectroscopy after 15 min of centrifugation at 2000 tr/min at max of the given organic solute. 2.8. Regeneration of the adsorbent The regeneration of the adsorbents has been performed as follows: the cellulose based organogel was removed from the solution and centrifuged at 2000 rpm for 5 min to remove the solution retained in the pores. Then, they were soaked in 40 mL of ethanol for 1 h at room temperature and the desorbed amount of the organic solute was determined by UV spectroscopy. The procedure was repeated two times in order to ensure the complete release of the adsorbed organic solute on the cellulose organogel. Then, the cellulose organogel was removed from ethanol, subjected to centrifugation at 2000 rpm for 5 min and soaked three times in water to remove and exchange ethanol with water. 3. Results and discussion Two approaches were adopted aiming to prepare the cellulose organogel, namely freeze-drying and solvent exchange procedure. Although the former method is the most widely used, the ensuing organogel were more compact and higher in the apparent density than the one issued from solvent-exchange (Aulin et al., 2010). It is likely that shrinkage during the drying process may account for this behaviour. Therefore, the NFCo adsorbent was prepared by the solvent-exchange process in what follows. In order to enhance the adsorption capacity of the NFCo, a surface modification was implemented by grafting hexadecylamine chains after the activation of the cellulose hydroxyl groups of the surface with CDI, according to the approach developed in our previous work (Alila et al., 2009a,b). This modification strategy relies on the activation of the surface cellulose hydroxyl groups with CDI to form a highly reactive imidazole ester followed by a coupling reaction with hexadecylamine generating carabamate function. Furthermore, this modification is carried under mild condition, namely a temperature in the range 50–60 ◦ C and apolar solvent, which prevent any risk of collapse of the organogel’s cell.

W. Maatar et al. / Industrial Crops and Products 49 (2013) 33–42

35

Fig. 1. Aspect of the cellulose organogel at different levels of magnification (a) photograph of cellulose organogel, (b) scanning electron microscopy (SEM) image of the interior part, and (c) Field-emission scanning electron microscopy (FE-SEM) of the cell wall.

Adopting the as-reported procedure, rigid cellulose organogel with a sponge-like aspect foams and a well defined shape were successfully prepared from NFC suspensions without collapse and shrinkage This is better highlighted in Fig. 1 showing the different level of the architecture of the cellulose based adsorbent. The ensuing organogel, exhibited highly porous structures with a cellular texture build up by a thin cell wall formed by the entangled cellulose NFC network. The scale of the cell size was in the range 50–200 ␮m and the FE-SEM image of the cell wall revealed the random in-plane orientation of the nanosized fibrils with widths in the order of 10–40 nm. It is worth noting that the chemical modification of the organogel contributed to preserve the whole integrity of the samples when immersed in water even for a long period. This is attested by the fact that all of the investigations and adsorption runs, carried during several months, were carried out using the same samples after regeneration by ethanol rinsing. The modified cellulose organogel is characterized by a high porosity estimated to 98% and a specific surface determined by N2 adsorption measurements attained about 84 m2 g−1 . The porosity P of the organogel is defined as P = (1 − da /db ) × 100, where da and db (1.6 g cm−3 ) are the densities of the organogel and the density of the bulk cellulose fibril, respectively. 3.1. Characterization of the adsorbent In the first part of our work, the chemical characterstics of the adsorbent is analyzed to evidence the occurrence of the surface modification.

Fig. 2 shows the FTIR spectrum of the adsorbent at the different steps of the modification. The unmodified NFCo (spectrum a) illustrates the characteristic bands of cellulose skeleton at 1456 and 1423 cm−1 assigned to OH in plane bending and CH2 symmetric bending, respectively. The region between 1200 and 1000 cm−1 is typical of fingerprint of the carbohydrate cycle at 1160, 1110 and 1060–1030 cm−1 corresponding to C O C asymmetric stretching, ring asymmetric stretching, and C O stretching, respectively. As for the peak at 1625 cm−1 , it is related to the OH bending of water, tightly absorbed on cellulose. Besides, the spectrum b of the CDI activated intermediate is mainly characterized by the emergence of a new peak at 1780 cm−1 typical of the imidazole ester C O . Moreover, in spectrum c relative to the modified NFC, the disappearance of the peak at 1780 cm−1 along with the emergence of the two peaks at 1710 cm−1 and 1540 cm−1 assignable to C O stretching and NH deformation of the carbamate function, respectively, confirmed the occurrence of the condensation reaction between the imidazole ester and the amino function of the hexadecylamine. CP–MAS solid NMR analysis provided the undeniable affirmation of the high yield grafting of the hexadecylamine chains on the surface of the cellulose fibrils. Indeed, compared to the pristine NFCo, the spectra of the modified NFCo exhibited new peaks in the region between 15 and 30 ppm typical of methylene groups of the grafted hexadecylamine. The respective assignments of the different peaks are reported in the NMR spectra (Fig. 3). It is to be noted that the peaks of the cellulose backbone around 64, 72–74, 88–90 and 103–107 ppm pertaining to the carbons C6, C2–C3–C5, C4 and C1 of the anhydroglucosic unit, respectively, did not undergo

c 2923

2848

1710

1540

Transmittance (%)

b

1780

a

NFCa-modified

1625

NFCa-CDI

1423 1160

NFCa

1110 4000

3500

3000

2500

2000

1500

1000

1030 500

-1

Wave number (cm ) Fig. 2. FTIR spectra of virgin cellulose (a), activated organogel by CDI (b) and modified NFCo with hexadecylamine (c).

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W. Maatar et al. / Industrial Crops and Products 49 (2013) 33–42

Fig. 3. CP-MAS NMR spectra of virgin cellulose organogel and the modified NFCo.

significant change after the modification process, thus proving that the grafting reaction was limited to the surface. Furthermore, from the ratio between the signal integration of the terminal methyl of the attached hexadecylamine and that of the C1 cellulose peaks, the DS was found to be 0.12. Bearing in mind that only the surface hydroxyl groups are concerned with the coupling reaction, this DS value attested for a high extent of surface modification. Actually, If we consider that the surface chains represent about 10–20% of the whole cellulose unit, depending on the square section of the cellulose fibrils, and that only 1.5 hydroxyl groups per anhydro glucosic unit (AGU) of the surface layer are accessible to the reagent, then a DS of 0.12 might be indicative of a high surface modification as discussed in a previous work (Ben Mabrouk et al., 2013). 3.2. Adsorption aptitude of NFCo To investigate the adsorption properties of the modified NFCo, three herbicides and several organic compounds, were selected as pollutant models, and their adsorption behaviour was analyzed. The structure and physical characteristics of the selected organic compounds, as well as their adsorption beahvaiour are reported in Table 1. The first clear evidence of the huge improvement of the adsorption capacities after the modification treatment is given in Table 1. Indeed, compared to the unmodified NFCo, the adsorption capacities are amplified by a factor exceeding 10 for all the solutes. Furthermore, these adsorption capacities are within the range reached for the activated carbon even if these capacities are dependent on the type of activated carbon used (Aksu, 2005; Kyriakopoulos and Doulia, 2006). 3.3. Adsorption kinetic With the aim to investigate the adsorption kinetics and to gain further knowledge on the adsorption mechanism, the effect of the adsorbed amount and initial concentration of the solute were studied using 2-naphthol as a solute model. From Fig. 4, it can be noted that adsorption kinetic is quite high during the first 10 min, and then it progressively decreases until it reaches the adsorption equilibrium within 50 min. A set of runs conducted at different adsorbent amount (result not shown) showed an enhancement of the adsorption efficiency as the dosage

of the adsorbent exceeds 8 g L−1 , which is in line with the higher concentration of the available sites on which the solute could be adsorbed. Moreover, as shown in Table 2, an improvement in the adsorption capacity is noted as the initial solute concentration is going up, which is likely the consequence of the increase in the gradient of concentration on the boundary layer of the adsorbent. However, the initial solute concentration did not affect considerably the adsorption efficiency, since decade amplification in the initial concentration brings about less than 20% improvements in the adsorption efficiency. Given the porous structure of the adsorbent, the diffusion of the adsorbate from the external surface to the pores via boundary layer diffusion and within the pores of the adsorbent could not be ignored. The most widely used models for studying the adsorption mechanism are Boyd’s (Boyd et al., 1947) and Weber’s intraparticle-diffusion models (Furusawa and Smith, 1973; Weber and Morris, 1963). Many studies have shown that the boundary layer diffusion is the rate controlling step in systems characterized by dilute concentrations of adsorbate, poor mixing, and small particle size of adsorbent (Ray, 1996; Mohan and Singh, 2004). Whereas the intraparticle-diffusion controls the rate of adsorption in systems characterized by high concentrations of adsorbate, vigorous mixing, and large particle size of adsorbent. Also, it has been noticed in many systems that film diffusion (external mass transfer) is dominant during the initial adsorbate uptake, and then the adsorption rate becomes controlled by intraparticle-diffusion after the adsorbent’s external surface becomes loaded with the adsorbate (Koumanova et al., 2003; Ray, 1996; Malash and El-Khaiary, 2010). The film-diffusion model of Boyd is a single-resistance assuming that the main resistance to diffusion is in the boundary layer surrounding the adsorbent particle, this model is expressed as follow:

F(t) = 1 −

˛   6   1

2

n=1

n2

exp(−n2 Bt )

(1)

where F(t) is the fractional attainment of equilibrium, at different times, t, and Bt is a function of F(t): F(t) =

q qe

(2)

W. Maatar et al. / Industrial Crops and Products 49 (2013) 33–42

37

Table 1 Physical characteristics of the different solutes and their maximum adsorbed amount on both the unmodified and modified NFCo. Chemical structure

Molar volume (cm3 )

Water solubility (mmol L−1 )

Qmax a unmodified NFC (␮mol g−1 )

Qmax b modified NFC (␮mol g−1 )

Aromatic organic solutes

OH 121.9

5

15

455

101.2

16.9

13

210

121.9

1

10

52

105.6

2.86

10

80

12

165

2-naphthol O N

O

Nitrobenzene H3C

CH3

Xylene Br

Bromobenzene OH

87.8

1041

Phenol Chemical structure

Molar volume (cm3 )

Water solubility (mmol L−1 )

Qmax a unmodified NFC(␮mol g−1 )

Qmax b modified NFCo (␮mol g−1 )

224.4

240

–

70

176.5

81

–

47

169.8

28

–

25

Herbicides H3C

O

O Cl

N

H3C

CH3

Alachlor CH3 N

NH

O

CH3

O

Cl Cl

Linuron Cl N H3C

NH

N N

CH3 NH

CH3

Atrazine a b

Maximum adsorption capacity on the unmodified cellulose organogel. Maximum adsorption capacity on the modified cellulose organogel.

where q and qe are the solute uptake (␮mol g−1 ) at time t and at equilibrium, respectively. By applying the Fourier transform and then integration, Reichenberg (Reichenberg, 1953) obtained the following equation: for F(t) values > 0.85,

Bt = 0.4977 − Ln(1 − F(t))

(3)

 and for F(t) values < 0.85,

Bt =

√ −



 −

2 F(t) 3

 2 (4)

Eqs. (1)–(4) can be used in predicting the mechanistic steps involved in the adsorption process, i.e. whether the rate of

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W. Maatar et al. / Industrial Crops and Products 49 (2013) 33–42

Table 2 Kinetic parameters for the adsorption of 2-naphthol onto NFCo at various initial concentrations. T = 20 ◦ C, adsorbent dose = 8.5 g L−1 . Initial concentration (mol L−1 ) Experimental Qe (␮mol g−1 ) % ads

1 × 10−4 15 88

5 × 10−4 50 85

1 × 10−3 90 80

5 × 10−3 450 70

Film diffusion Slope (first/second segment) Intercept (first/second segment) R2 (first/second segment)

0.032/0.017 0.005/0.449 0.987/0.987

0.032/0.015 0.076/0.416 0.994/0.994

0.012/0.036 0.079/0.413 0.999/0.993

0.036/0.025 0.079/0.230 0.992/0.971

Intraparticle (pore) diffusion Kd (␮mol min1/2 g−1 ) R2 Intercept (first segment)

1.5 0.9714 2

4.4 0.9706 10

9.5 0.9782 11

43 0.9816 117

adsorption takes place via particle diffusion or film-diffusion process. This is done by plotting Bt against time: if the plot is linear and passes through the origin then intraparticle-diffusion controls the rate of mass transfer. If the plot is nonlinear or linear but does not pass through the origin, then it is concluded that film-diffusion control the adsorption rate (Boyd et al., 1947; Weber and Morris, 1963; Reichenberg, 1953). The possibility of intra-particle diffusion was explored by using the intra-particle diffusion model (Furusawa and Smith, 1973; Weber and Morris, 1963). qt = kd t 1/2 + C

(5)

where qt is the adsorbed solute at time t and kd is the intra-particle diffusion rate constant (␮mol min1/2 g−1 ), and c is the intercept. This model assumes that: (i) the external resistance to mass transfer (film diffusion) is not significant or only significant for a very short period at the beginning of diffusion; (ii) the direction of diffusion is radial and the concentration does not change with angular position; (iii) the intraparticle diffusivity is constant and does not change with neither time nor with position. Both Boyd and Weber plots revealed two linear segments attesting of the existence of two different diffusion stages (Table 2 and Fig. 3b and 3c). From results collected in Table 2, one can note that the intercept values of the first segments are significantly different from zero, and this strongly suggests that film diffusion is the rate controlling mechanism during the first 20 min of adsorption. This result was confirmed by the intercept values of the Weber plots which are also significantly different from zero. In addition,

Weber plots (Fig. 3b) show that the slopes of the second segments slow down which indicates a change in the diffusion regimes and a probable involvement of intraparticle diffusion process before the set up of the equilibrium. The calculated values of kd (Weber’s model) increase with the increase of the adsorbent dose and the initial concentration of organic solute. This can be due to the concentration gradient existing between the bulk solution and the surface of the substrate, acting as a driving force for mass transfer. Indeed, the diffusion rate is directly proportional to the concentration gradient; the greater the difference in concentration is, the higher is the diffusion rate. Furthermore, the values of the intercept shown in Table 2 (Weber’s model) give an idea about the film transfer caused by the diffusion of the adsorbate from the bulk solution to the adsorbent surface. The larger the intercept is, the greater is the boundary layer effect (McKay et al., 1985). From these results we can propose a sorption mechanism of the organic solute. The uptake process could be divided into three steps, namely (i) external mass transfer of sorbate molecules to the adsorbent particle–solution interface (bounding liquid film), (ii) intra-particle diffusion of sorbate molecules, and (iii) sorption of sorbate molecules on the active sites of the surface. This last step is very rapid in comparison to the first two steps, and therefore, the overall rate of adsorption is controlled by a diffusional process, in the course of which the film diffusion was the rate limiting step. 3.4. Adsorption equilibrium The adsorption isotherm analysis is of a meaningful in the design of adsorption systems as well as it allows accessing

Table 3 Parameters of the Freundlich, and Temkin isotherms for the adsorption of the tested solutes onto NFCo. Freundlich model

Xylene 2-Naphthol Nitrobenzene Alachlor Linuron Atrazine Phenol Bromobenzene

1/n

KF (␮mol g−1 )

R2

0.8304 0.854 0.8958 1.065 0.4688 0.8459 1.13 1.17

0.388 1.51 0.274 0.176 4.853 0.595 0.032 0.017

0.959 0.961 0.957 0.954 0.815 0.973 0.975 0.950

Temkin model

2-Naphthol Nitrobenzene Xylene Alachlor Phenol Bromobenzene Linuron

H1 (kJ mol−1 )

H2 (kJ mol−1 )

Ktem

R2

−16.82 −15.43 −7 −11 −15.76 −7 −4

−3.36 −2.27 −2.34 −2.14 −3.76 −3.4 −19.4

0.991/0.989 0.991/0.992 0.989/0.99 0.989/0.991 0.991/0.989 1/10.992

0.905/0.98 0.939/0.82 0.906/0.97 0.94/0.938 0.99/0.952 0.93/0.94 0.92/0.93

W. Maatar et al. / Industrial Crops and Products 49 (2013) 33–42

(a) 500

450

A

400

Nitrobenzene Bromobenzene

400

350

2-Naphthol

300

0,0001 mol.L-1 0,0005 mol.L-1

250

Qads (µmol.g-1)

qt (µmol.g-1 )

39

0,001mol.L-1 0,005 mol.L-1

200 150 100

xylene

300

Phenol

200

100

50 0 0

50

100 150 Time (min)

200

0

250

0

0.0005

0.0015

0.002

0.0025

Csol (mol.L-1)

500

(b) 80

B

450

70

400 350

60

0.005mo:L-1

300

0.0001 mol.L-1

250

0.0005mol.L-1

200

0.001mol.L-1

Qads (µmol.g-1)

qt (µmol.g-1 )

0.001

150 100 50

50 40 Atrazine

30 20

Alachlore

10

Linuron

0

0 5

0

10

15

0

20

0.00005

0.0001

0.00015

0.0002

0.00025

0.0003

-1

Csol (mol.L )

t1/2

1.6

Fig. 5. Adsorption isotherms of aromatic organic solutes (a) and herbicides (b) onto NFCo (T = 20 ◦ C).

C

Bt

1.2

0.8 0.0005 mol.L-1 0.0001 mol.L-1

0.4

0.001 mol.L-1 0.005 mol.L-1

0 0

10

20

30

40

50

60

70

Time (min) Fig. 4. Kinetic adsorption of 2-naphthol onto NFCo, the effect of the initial concentration: (a) Adsorbed amount vs time, (b) Boyd and (c) Weber plots.

the thermodynamic parameters governing the interaction process. The adsorption isotherms of five organic solutes and three herbicides, differing in their water solubility and chemical structure, are represented in Fig. 5. All the isotherms show two or more plateaux indicating a particular process. The adsorption capacity of the organic solutes onto modified NFCo is the highest for 2-naphthol followed by nitrobenzene, xylene, phenol and bromobenzene. For the herbicides, the higher capacity is attained with alachlor followed by linuron. No clear correlation between the water solubility or the molecules polarity and the adsorption capacity could be drawn. In fact, numerous parameters such as the hydrodynamic

volume, the shape of the molecule, their hydrophobic character, the interaction potential between the adsorbent and adsorbate are likely to intervene in the adsorption process, making a frank correlation with one parameter difficult to predict. The experimental data could be fitted with the appropriate mathematical models that may be helpful to better understand the adsorption mechanism. In the present work, Langmuir, Freundlich and Temkin were selected as appropriate models; the most appropriate of which will be selected on the basis of the plot of the linearized form and the correlation coefficient (Table 3). The Langmuir model was developed for monolayer adsorption onto a surface containing well-defined finite adsorption sites of uniform energies with no interaction between adsorbed molecules. The isotherm equation is expressed as: qe =

Qmax KL Ce 1 + KL Ce

(6)

where qe is the amount of solute adsorbed at equilibrium per unit weight of adsorbent (␮mol g−1 ). Ce is the equilibrium concentration of solute in the bulk solution (mmol L−1 ). Qmax is the maximum adsorption capacity (␮mol g−1 ) and KL is the constant related to the free energy of adsorption. The linearized form of the equation can be written as follows: Ce 1 Ce = + qe KL Qmax Qmax

(7)

From the data of Ce /qe vs. Ce , KL and Qmax can be determined from the slope and intercept (Table 3). The Freundlich isotherm is an empirical equation appropriate for the adsorption processes where non-uniformity of the surface

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W. Maatar et al. / Industrial Crops and Products 49 (2013) 33–42

of adsorbent is expected with an adsorption energy varying as function of the surface coverage. The Freundlich equation may be written as:

1.2

Bromobenzene 2-Naphthol

1

Nitrobenzene

qe = KF Ce 1/n

(8)

log qe = log KF +

1 log Ce n

0.4 0.2 0

The Temkin isotherm (Temkin, 1941) equation assumes that the heat of adsorption of all the molecules in the layer decreases linearly with coverage due to adsorbent–adsorbate interactions, and that the adsorption is characterized by a uniform distribution of the binding energies, up to some maximum binding energy. Temkin model is given by:

-0.2

qe qmax

=

RT ln K0 Ce Q

(10)

where  is the fractional coverage, R the universal gas constant (kJ mol−1 K−1 ), T the temperature (K), Q = (−H) the variation of the adsorption energy (kJ mol−1 ), and K0 is the Temkin equilibrium constant (L mg−1 ). From the correlation coefficient R2 values it was shown that the Langmuir model failed to fit correctly the experimental data. However, as shown in Table 3, the Freundlich model yields to a correlation coefficient higher than 0.95 at the exception of Linuron. Interestingly, for all the solutes studied, good fitting with the Temkin model was noted with two linear parts attesting for an adsorption process involving two different adsorption energies (Fig. 6). The first one involves interaction energy within 7–17 kJ mol−1 , with the higher value being observed for the polar solute, while the lower value were noted for the less polar solute. The second one involves low adsorption energy in the order of 3 kJ mol−1 . Presumably, these two adsorption energies stand for two adsorption sites within which the interaction is driven by two different interaction process. Based on the magnitude of the adsorption energy, we infer that the adsorption is driven by polar and hydrogen interaction within the first site, while they were

phenol

0.6

(9)

=

Xylene

θ

where qe is the amount of solute adsorbed per unit weight of adsorbent (␮mol g−1 ), Ce the equilibrium concentration (mol L−1 ), KF the constant indicative of the relative adsorption capacity of the adsorbent (␮mol g−1 ) and 1/n is the constant, indicative of the intensity of the adsorption. This equation can be linearized as follows:

0.8

-11

-10

-9

-8

-7

-6

Ln (Csol) Fig. 6. Linearization of Temkin isotherms of organic solutes adsorbed on NFCo.

limited to dispersive interaction on the second one. Given the chemical structure of the modified NFCo surface, we presume that the former sites encompass the polar part of the grafted chains formed by the carbamate function, whereas the later sites involved the apolar hexadecyl chains dangling from the surface. A schematic illustration of the adsorption mechanism of the organic compounds onto modified NFCo is depicted in Fig. 7, where the grafted hydrocarbon chains generated hydrophobic shell on which the organic solutes could be entrapped and held through dispersive interaction with the methylene groups and polar interaction with carabamate function. 3.5. Regeneration of the cellulose based organogel adsorbent To test the regeneration and the reusability of the NFCo adsorbent, the exhausted NFCo adsorbent was submitted to a washing treatment with conventional solvent such as ethanol or acetone. Following the procedure reported in the experimental section, it was observed that one washing cycle using ethanol is enough to completely extract the adsorbed organic solute. After washing with water to remove ethanol, the regenerated organogel could be reused for another adsorption cycle. The absorption capacity of the organogel does not change upon repeated adsorption–desorption

Fig. 7. Schematic illustration of physi-sorption of herbicides onto modified NFC (a) diffusion from the bulk solution, (b) accumulation on the external surface of the organogel, and (c) diffusion inside the grafted hexadecyl chains.

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and reusability without any measurable loss in the adsorption capacity. Based on the result obtained, the present new approach presented here for the uptake of organic pollutants from water exhibits the following advantages and disadvantages: as advantages one can invoke the high available surface area on which the organic solute is likely to be adsorbed; the sustainable character of the adsorbent as it derived from renewable cellulose material, and the possibility to reuse the adsorbent in a multitude of cycles without loss of the adsorption capacity. As disadvantage, one can invoke the necessity to implement the surface chemical modification in order to boost the adsorption capacity. This treatment is likely to increase the cost of the adsorbent. However, given the reusability of the adsorbent, this extra-cost might be supported and recovered during the utilization of the material. Fig. 8. Evolution of the adsorption capacity as a function of the number washing cycle for 2-naphthol, nitrobenzene and phenol.

References for more than 10 cycles, as shown in Fig. 8. Moreover, it is worthy noting that no alteration in the form of the adsorbent was noted following the multiple regeneration treatment. Actually, all the investigations conducted in the present work were performed with several samples of the modified NFCo which were recurrently reused. The efficient adsorption properties of the modified cellulose organogel along with the simple regeneration method motivated us to explore the adsorption behaviour of the CNFa during continuous operation. For this purpose, a laboratory column filled with modified CNFa was designed and fed from the bottom to the top with effluent using a precision peristaltic pump. This work is ongoing and preliminary results showed that adsorption properties are preserved under dynamic condition. 4. Conclusion Sponge-like modified cellulose organogels based on nanofibrillated cellulose were prepared by solvent exchange approach followed by the surface grafting of hexadecylamine under mild condition. The occurrence of the coupling reaction through carbamate linkage was confirmed by FTIR and CPMAS spectroscopy. The chemical modification of the cellulose organogels was found to be essential in converting the porous substrate as an efficient adsorbent towards dissolved organic pollutants such as aromatic compounds and herbicides. The data gathered from adsorption experiments under batch conditions showed that the adsorption capacity ranged from 50 up to 400 ␮mol g−1 , depending on the structure of the organic solutes. The kinetic analysis showed that the adsorption process was quite rapid and reached equilibrium within 40 min. The applicability of diffusional models suggests that the adsorption process of organic solutes onto NFCo is rather a complex process involving both boundary layer and intraparticle diffusion. Based on the adsorption isotherms, it was proposed that the adsorption process involved a multistep mechanism starting with the diffusion of the organic solute from the continuous medium towards the external surface of the adsorbents via a film diffusion process. Then, the diffusion of the molecules inside the hydrocarbon chains of the hexadecyl moieties takes place where Van der Waals interaction would hold the molecules within the hydrophobe reservoir. The cellulose organogel made from nanofibrillated cellulose is obtained from natural cellulose fibres after high pressure homogenization treatment. Although a surface functionalization is needed to boost the adsorption capacity of the material, the present adsorbent arouse much interest given their ease of regeneration

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