Study of Methylene Blue adsorption on keratin nanofibrous membranes

Study of Methylene Blue adsorption on keratin nanofibrous membranes

Journal of Hazardous Materials 268 (2014) 156–165 Contents lists available at ScienceDirect Journal of Hazardous Materials journal homepage: www.els...

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Journal of Hazardous Materials 268 (2014) 156–165

Contents lists available at ScienceDirect

Journal of Hazardous Materials journal homepage:

Study of Methylene Blue adsorption on keratin nanofibrous membranes A. Aluigi a,∗ , F. Rombaldoni b , C. Tonetti b , L. Jannoke c a b c

CNR-ISOF, National Research Council-Institute of Organic Synthesis and Photoreactivity, Via P. Gobetti, 101, 40129 Bologna, Italy CNR-ISMAC, National Research Council-Institute for Macromolecular Studies, C. so G. Pella, 16, 13900 Biella, Italy Politecnico di Torino, Department of Materials Science and Chemical Engineering, Duca degli Abruzzi 24, 10129 Torino, Italy

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• Membranes of keratin nanofibers • • • •

(220 nm diameter) were prepared by electrospinning. The membranes were tested as adsorbents for Methylene Blue dye from water. The adsorption capacity increases with increasing the initial dye concentration and pH. The adsorption capacity decreases with increasing the adsorbent dosage and temperature. Results suggest that keratin nanofibrous membranes could be promising dye adsorbents.

a r t i c l e

i n f o

Article history: Received 18 July 2013 Received in revised form 22 November 2013 Accepted 8 January 2014 Available online 18 January 2014 Keywords: Keratin Nanofibres Methylene Blue Adsorption model

a b s t r a c t In this work, keratin nanofibrous membranes (mean diameter of about 220 nm) were prepared by electrospinning and tested as adsorbents for Methylene Blue through batch adsorption tests. The adsorption capacity of the membranes was evaluated as a function of initial dye concentration, pH, adsorbent dosage, time and temperature. The adsorption capacity increased with increasing the initial dye concentration and pH, while it decreased with increasing the adsorbent dosage and temperature, indicating an exothermic process. The adsorption results indicated that the Langmuir isotherm fitted the experimental data better than the Freundlich and Temkin isotherm models. A mean free energy evaluated through the Dubinin–Radushkevich model of about 16 kJ mol−1 , indicated a chemisorption process which occurred by ion exchange. The kinetic data were found to fit the pseudosecond-order model better than the pseudo-first-order model. The obtained results suggest that keratin nanofibrous membranes could be promising candidates as dye adsorption filters. © 2014 Elsevier B.V. All rights reserved.

1. Introduction The effluents from industries using dyes as food, textiles, papers, cosmetics and other industries, are the main sources of dye pollution [1]. Since many dyes and their break down products are toxic

∗ Corresponding author. Tel.: +39 0516399785; fax: +39 0516399844. E-mail address: [email protected] (A. Aluigi). 0304-3894/$ – see front matter © 2014 Elsevier B.V. All rights reserved.

for human and living organisms, removal of dyestuffs from wastewater has received considerable attention over the past decades. Synthetic dyes are stable to biodegradation, therefore biological aerobic wastewater treatment systems are not successful in removing color from wastewater. Moreover, degradation products of some dyes are toxic. For these reasons several physic-chemical methods as filtration, flocculation, chemical and electrochemical oxidation, ozone treatment and adsorption were developed [2]. Among the aforementioned methods, the adsorption process is

A. Aluigi et al. / Journal of Hazardous Materials 268 (2014) 156–165

one of the most effective and low cost technique widely studied in recent years to remove dyes from wastewater. The adsorption process uses an adsorbent, a material able to bind the toxic substance or molecule to its surface. Adsorption methods are superior to other techniques not only in terms of cost, but also in terms of flexibility, simplicity of design, ease of operation, etc. Moreover, adsorption methods do not produce secondary harmful substances and the surface of an adsorbent can be functionalized and designed in order to increase the adsorption performances toward the toxic substances to be removed [3]. Powders of activated carbons and of various low cost biomaterials have been demonstrated to be good adsorbents for many organic compounds included dyes [4,5]. However, the major drawbacks of using adsorbents in form of fine powder for wastewater treatment are the production of large amount of sludge which may cause a secondary pollution and problems in regeneration [6,7]. Membrane technologies offer a potential solution for wastewater treatment and, for this reason, their industrial applications have considerably expanded in the last 50 years. Although membrane processes are well established technologies for water remediation, many efforts are being made to design new membranes with enhanced adsorption performances toward toxic substances [8,9]. In this context, electrospun nanofibrous membranes, due to their interesting characteristics such as high porosity, high specific surface area, high water permeability, small interfibrous porous size, and interconnected open pore structures, are potentially advanced systems that can offer removal of pollutants from water at lower energy and cost [10]. Electrospinning is the most simple and low cost process which produces ultrafine polymer fibres through the action of an external electric field imposed on a polymer fluid (polymer solution or melt) [11]. At the same pressure drop, filters made of fibres having a mean diameter finer than half a micron (as nanofibres) show a higher capability to collect finer particles compared to conventional filter fibres because the slip flow around the nanofibres increases the diffusion, interception and inertial impactions efficiencies [12]. More specifically, nanofibrous membranes were studied for microfiltration and ultrafiltration. By electrospinning polymers having chemical functional groups able to bind specific toxic adsorbents, it is possible to prepare membranes for water depuration that can offer both adsorption and filtration. Therefore, electrospun nanofibrous membranes of various synthetic and natural polymers as polyvinyl alcohol [13], amidoxime-modified polyacrylonitrile [10], silk fibroin [14], or nanofibrous membranes functionalized through the introduction of inorganic nanoparticles like montmorillonite (MMT) into the polymer matrix [15], were prepared and tested for adsorption of specific materials from aqueous solutions. The great advantage of using nanostructured membranes with high surface area is in their good adsorption performances even at low adsorbent dosages. Research interest in the metal adsorption capacity of new membranes made of protein nanofibres has been intensified in recent years. Among the natural proteins, keratin, being the major component of wools, hairs, horns, nails, and feathers, is an abundant non-food protein characterized by a large number of hydrophilic amino acids with high affinity to ionic species (e.g. heavy-metals or dyes). Moreover, keratin wastes as feathers, horns-nails from butchery, poor quality raw wools from sheep breeding and byproducts from the textile industry account worldwide for more than five millions tons per year. For the aforementioned reasons, keratin is an interesting low cost biomass to be exploited. Electrospun nanofibrous membranes based on keratin have been widely investigated for the heavy-metal ions removal from water both in batch and in dynamic conditions [16–18]. However, to the best of our knowledge, there is no literature focusing on the adsorption capacity of cationic dyes onto the keratin-based nanofibrous membranes.


In this work, nanofibrous membranes made of keratin extracted from wool were prepared by electrospinning process and characterized in their morphology, diameter distribution, thickness, porosity and specific surface area. Afterwards, the prepared membranes were tested as adsorbents for Methylene Blue from aqueous solution through preliminary adsorption tests carried out in batch. In particular, the adsorption capacity and the removal efficiency of keratin nanofibrous membranes were evaluated in function of initial dye concentration, pH, adsorbent dosage, time and temperature. Moreover, equilibrium (Langmuir, Freundlich, Dubinin– Radushkevich and Temkin) and kinetic (pseudo-first order, pseudo-second order) models were used to fit experimental data and the isosteric heat of adsorption was calculated. 2. Experimental 2.1. Materials Keratin protein was extracted from Australian Merino wool (21 ␮m fineness). Methylene Blue (MB) is a basic and cationic dye with CI Classification Number 52015. All chemicals were of analytical grade and were purchased from Sigma–Aldrich. 2.2. Keratin nanofibrous membrane preparation and characterization Keratin was extracted from wool by sulphitolysis and purified as described in a previous work [16]. Afterwards, keratin powder was transformed into nanofibrous membranes through electrospinning process by slightly modifying the electrospinning parameters used in a previous work [17]. Practically, keratin solutions with a final concentration of 15 wt%, were prepared by dissolving the protein in pure formic acid (98%), under shaking, at room temperature, overnight. The prepared keratin solution was placed in a 5 mL syringe with a stainless needle tip having an internal diameter of 0.2 mm. The needle tip (cathode) was connected to a high voltage generator (SL50 Spellman High Voltage Electronics Corporation, USA) and the polymer solution was electrospun toward a grounded stainless steel collector (20 cm × 20 cm) using a “bottom up” configuration, in which the jet-emitting source was positioned below the grounded collector [17]. The electrospinning process was carried out using a voltage of 25 kV, a needle tip-collector distance of 20 cm, a solution feeding rate of 1 ␮l min−1 , controlled with a syringe pump (KDS, KD Scientific nc., USA) and a deposition time of 60 min. During the electrospinning process, the environmental conditions were controlled to be the following: a temperature in the range 20–25 ◦ C and a relative humidity in the range 45–50%. In order to increase the stability in water, the prepared keratin nanofibrous membranes were treated at 180 ◦ C for 2 h. The electrospun nanofibre morphology was observed under a scanning electron microscope (SEM) using a LEO 435 VP (LEO Electron Microscopy Ltd, UK), with and acceleration voltage of 15 kV, a current probe of 100 pA and a working distance of about 20 mm. Before SEM analysis, the membranes were sputter coated with a gold layer using an Emitech (UK) K550 sputter coater setting a current of 20 mA for 240 s. The fibre diameters of the keratin nanofibrous membranes were analyzed by means of a freely distributed software GIMP 2.8 (GNU Image Manipulation Program). In particular the nanofibre mean diameter and the diameter distribution were evaluated from 100 measurements randomly gathered from several SEM photos of several nanofibrous membranes. The thickness of keratin nanofibrous membranes was measured by using


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a FAST-1 compression meter (CSIRO, Australia), under a load of 2 gf cm−2 . The porosity and the specific surface area of the keratin nanofibrous membranes were calculated on a square sample of 4 cm2 as suggested by Ki et al. [14] and described in previous works [16,17]. Practically, the specific surface area (S) of the membranes is expressed by the ratio between the total surface area A (m2 ) and the mass of nanofibrous membranes m (g), as shown in Eq. (1): S=

A m


Assuming nanofilaments as cylinders of indefinite length, A of each membrane can be calculated by using the following equation:

Table 1 Keratin nanofibrous membranes characteristics. Material

Keratin nanofibers

Mean diameter Thickness (␮m) Porosity (%) Specific surface (m2 g−1 )

223 ± 74 nm 50 90 13.59

All assays were carried out in triplicate and the mean values were reported. Data processing was carried out with the ORIGIN 8.1 software (OriginLab Corporation, MA, USA). 3. Results and discussion

4m A= D


where D is the mean fibre diameter and  is the fibre density. Therefore,

3.1. Membrane properties

where Vtot is the total (apparent) volume, obtained by measuring the area and the thickness of the mat, and V is the volume, calculated from measured mass and density of the nanofibrous membrane.

Keratins extracted by sulphitolysis were characterized by proteins having molecular weights of about 60 and 45 kDa (low sulphur content keratins) and proteins having molecular weights ranging from 28 and 11 kDa (high sulphur content keratins) [19]; the isoelectric point of the keratins fell in the range between 4 and 4.5 [20]. Moreover, keratins contained several amino acids with polar side residues able to bind cationic substances: amino acids as aspartic acid, glutamic acid, arginine, cystine and cystein-S-sulphnoate residues composed more than 38% of total amino acid content [21]. The keratin nanofibrous membrane was robust, free-standing and flexible; therefore it could be formed into various membrane modules (Fig. 1a). The membrane was made of randomly oriented nanofibres having a mean diameter of about 220 nm (Fig. 1b). The nanofibres did not show defects as beads or flat morphologies (Fig. 1b); however the diameter distribution curve was rather broad extending from 70 to 430 nm (as shown in Fig. 1c). As regards the keratin nanofibrous membrane properties, the specific surface area was of 13.59 m2 g−1 and the porosity was of about 90% (Table 1).

2.3. Adsorption tests

3.2. Effects of various parameters on MB adsorption

For the adsorption tests, the dye stock solution was prepared by dissolving accurately weighted MB in distilled water, in order to obtain a solution at a concentration of 1000 mg L−1 . The experimental solutions were prepared by properly diluting the stock solution at the desired initial MB concentration. The pH of the starting solutions was adjusted by adding, when necessary, NaOH [1 M] or HCl [1 M]. The adsorption tests were carried out in batch conditions by shaking the keratin nanofibrous membranes immersed in 10 ml of starting MB solutions. The initial and final dye concentrations were determined with a PerkinElmer spectrophotometer at wavelength of 665 nm. The adsorption capacity q (mg g−1 ) and the percentage removal efficiency (R%) were calculated using Eqs. (6) and (7), respectively:

In this section, the effect of different parameters as concentration, time, pH, adsorbent dosage and temperature were discussed.


4 D


The density of the nanofibre mat was calculated by using the mass of the mat in air (m1 ) and in n-hexane (m2 ) through the following equation:  = (m1 h )/(m1 − m2 )


where h is the density of n-hexane at the temperature of measurement.The porosity of the membrane (ε) is obtained by using the equation: ε=


Vtot − V Vtot


(c0 − cf )V

%R =

m c0 − cf c0

× 100



where C0 (mg L−1 ) and Cf (mg L−1 ) are the initial and final MB concentrations, respectively, V (L) is the volume of MB solutions and m (g) is the mass of nanofibrous membranes. The effect of initial MB concentration, pH, adsorbent dosage (ratio between membrane mass and volume of the solution), time and temperature on adsorption was investigated.

3.2.1. Effect of initial MB concentration and contact time The driving force for the dye mass transfer between the aqueous and solid phase is correlated to the initial dye concentration. The effect of the initial MB concentration on the adsorption capacity and removal efficiency, evaluated at pH 6 and 20 ◦ C, using an adsorbent dosage of 1 g L−1 and a contact time of 24 h is shown in Fig. 2a and Fig. 3a, respectively. As can be seen, the adsorption capacity increased with increasing the initial MB concentration until a plateau was reached (Fig. 2a): the maximum adsorption capacity was of about 170 mg g−1 and it was reached at the initial MB concentration of 250 mg L−1 . This behavior can mainly attributed to the concentration gradient increase which results in an enhanced driving force for the mass transfer. As regards the removal efficiency, it decreased from 98% to 50% as the initial MB concentration increased from 50 to 350 mg L−1 (Fig. 3a). At constant adsorbent dosage and higher initial dye concentrations, the available adsorption sites of adsorbent became fewer, therefore a decrease in the removal efficiency occurs. Fig. 4 shows the effect of contact time on the adsorption capacity and removal efficiency evaluated at two different initial MB concentration of 50 and 250 mg L−1 , at pH 6 and 20 ◦ C, using an adsorbent dosage of 1 g L−1 . As shown, there was a rapid uptake in the first 60 min, afterwards a plateau was reached.

A. Aluigi et al. / Journal of Hazardous Materials 268 (2014) 156–165


Fig. 1. Keratin nanofibrous membrane (a) and related SEM micrograph (b) and diameter distribution curve (c).

3.2.2. Effect of adsorbent dosage In order to investigate the effect of adsorbent dosage, MB uptake was evaluated using different ratios between keratin nanofibrous membrane mass (g) and volume (L) of MB solution (adsorbent dosage). The experiments were carried out using a MB solution at initial dye concentration of 250 mg L−1 at pH 6, at a temperature of 20 ◦ C, and a contact time of 24 h. The behavior of adsorption capacity and removal efficiency in function of adsorbent dosage are plotted in Figs. 2b and 3b, respectively. Along with the adsorbent dosage increase from 0.1 g L−1 to 2 g L−1 , the adsorption capacity decreased from 1320 to 127 mg g−1 (Fig. 2b). For constant initial dye concentration, with increasing the adsorbent dosage the interfacial tension between the two phases increases and, as a consequence, the driving force for the mass transfer decreases, reducing in this way the adsorption capacity. On the other hand, the removal efficiency increased from 59% to 97% (Fig. 3b) and this can be attributed to higher availability of more adsorption sites. It is worth noting that the high specific surface area and porosity of nanostructured membranes conferred a great removal efficiency of dye also at relatively low adsorbent dosages. 3.2.3. Effect of pH MB exists in aqueous solutions in form of positively charged ions (cationic dye) and the degree of its adsorption on the membrane

surface is primarily influenced by the membrane surface charge, in turn influenced by the solution pH. Figs. 2c and 3c show the behavior of adsorption capacity and removal efficiency in function of pH, respectively, both evaluated using the starting solution at 250 mg L−1 , an adsorbent dosage of 1 g L−1 , an equilibration time of 24 h and a temperature of 20 ◦ C. As can be seen, the adsorption capacity increased with increase of the pH up to 10. This was due to both higher amount of negatively charged groups of keratin at pH higher than its isoelectric point (4–4.5) and to lesser competition between protons (H+ ) and the MB molecules at higher pH. For the same reasons an increase of removal efficiencies with increasing the pH occurred. 3.2.4. Effect of temperature In order to study the effect of temperature on the adsorption process of MB onto keratin nanfibrous membranes, the experiments were carried out at temperatures of 20, 30, 40 and 50 ◦ C, using a solution at initial MB concentration of 250 mg L−1 , pH 6 and an adsorbent dosage of 1 g L−1 . Figs. 2d and 3d show the influence of temperature on the adsorption capacity and removal efficiency, respectively. In particular, both adsorption capacity and removal efficiency slightly decreased with increasing the temperature; this suggests that the interactions between the cationic dye and active chemical groups of keratin were lower at higher temperatures.


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Fig. 2. Effect of various parameters on the adsorption capacity.

The evaluation of the heat of adsorption can provide useful information related to the nature of adsorption process. The heat of adsorption determined at constant amounts of adsorbed sorbate is known as isosteric heat of adsorption (H) and it can be calculated using the Clausius–Clapeyron equation (8) [22] H − R

 d(ln C )  e


d(1/T )

where R is the molar gas constant (8.314 J mol−1 K−1 ) and Ce (mol L−1 ) is the MB concentration at the equilibrium. The H can be evaluated from the slope of the ln Ce versus 1/T (Fig. 5). The calculated enthalpy change was −12.1 kJ mol−1 indicating that the adsorption process was exothermic in nature. The same behavior was found by Khan et al. for the adsorption of MB on sheep wool [23].

3.3. Adsorption isotherms study The adsorption isotherms study, through the fitting of the isotherm experimental data with different isotherm models, is important for the description of the adsorption process and it is critical in optimizing the use of an adsorbent. Several mathematical models can be used to fit the experimental adsorption values: in this work the Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich models were employed for the interpretation of experimental data (Fig. 6). The validity of the applied models was determined by calculating the average relative error (ARE) defined by the following equation: ARE −

100 p p t=1

|qcal − qexp | qexp


Table 2 Parameter values of the isotherms for the MB adsorption on keratin nanofibre membranes. Model

Isotherm parameters


qmax (mg g−1 ) 167 kf (mg g−1 ) (L mg−1 )1/n 62.54 qmax (mg g−1 ) 345 b (J mol−1 ) 112.3

Freundlich Dubinin-Radushkevich Temkin

KL (L mg−1 ) 0.385 n 4.7 E (kJ mol−1 ) 16.2 kT (L g−1 ) 18.9

R2 0.9927 R2 0.8080 R2 0.8556 R2 0.8433

ARE (%) 8.37 ARE (%) 15.4 ARE (%) 12.7 ARE (%) 13.1

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Fig. 3. Effect of various parameters on the removal efficiency.

where the subscripts exp and cal refer to experimental and calculated data, respectively, and p is the number of experimental data. An average relative error smaller than 13% indicates that the model closely fitted the experimental results [24]. The Langmuir adsorption isotherm (Fig. 6a), which assumes that the adsorption occurs at specific homogeneous sites within the adsorbent and which has been successfully applied to many monolayer adsorption process [25], can be written using the following linearized equation [26] (10) Ce 1 1 = Ce + qe qmax KL qmax


where qe (mg g−1 ) is the adsorption capacity at equilibrium, qmax (mg g−1 ) is the maximum adsorption capacity, Ce (mg L−1 ) is the equilibrium concentration of MB in solution, and KL (L mg−1 ) is the effective dissociation constant. The Freundlich isotherm (Fig. 6b), which describes the adsorption process on energetically heterogeneous surfaces, can be written using the following linearized equation [27] (11) ln qe = ln kf +

1 ln Ce n


where kf ((mg g−1 ) (L mg-1 )1/n ) is a constant indicative of the adsorption capacity of the adsorbent and n is an empirical constant related to the magnitude of the adsorption driving force. In particular, an n value falling between 1 and 10 indicates a favorable adsorption.

The Dubinin–Radushkevich (D–R) model (Fig. 6c) is useful to determine the adsorption type and it can be described using Eq. (12) [28]: qe = qmax exp(−kε2 )


where k (mol2 kJ−2 ) is the constant related to the adsorption energy and ε is the Polanyi potential calculated using Eq. (13):

ε = RT ln 1+

1 Ce


where R is the gas constant (kJ kmol−1 ) and T is the temperature (K). The mean free energy E (kJ mol−1 ) can be calculated by using the constant k through the following formula (14): E = (2k)



Finally, the Temkin model (Fig. 6d) is based on the assumption that some indirect sorbate/adsorbate interactions are responsible of the linear decrease of the adsorption heat of all the molecules in the layer with coverage [29]. The Temkin model is described by the following Eq. (15): qe −

RT ln(kT Ce ) b


where kT (L g−1 ) is the equilibrium binding constant, b (J mol−1 ) is related to heat of adsorption, R is the universal gas constant (8.314 J mol−1 K−1 ) and T (K) is the absolute temperature.


A. Aluigi et al. / Journal of Hazardous Materials 268 (2014) 156–165

Fig. 4. Effect of time on adsorption capacity and removal efficiency.

In Table 2, the parameter values of the applied isotherms, the related correlation coefficient R2 and average relative error (ARE) are reported. As can be seen, the Langmuir model showed the higher R2 value (0.9927) and the lowest ARE, suggesting that this model yielded the best fit compared to other models. By using the Langmuir isotherm it is possible to predict whether an adsorption system is favorable or unfavorable through a dimensionless constant referred to as separation factor RL defined by the following Eq. (16) [30]. RL =

1 1 + kL C0


where KL (L mg−1 ) is the Langmuir constant and C0 (mg L−1 ) is the initial concentration of MB in solution. The value of RL indicates if the adsorption process is irreversible (RL = 0), favorable (0 < RL < 1), linear (RL = 1) or unfavorable (RL > 1). RL values related to MB adsorption on keratin nanofibre membranes were less than 1 and greater than 0 for all initial MB concentration considered (Fig. 7) indicating a favorable adsorption. Moreover, the mean free energy evaluated using the D–R model is of about 16 kJ mol−1 , indicating that the adsorption of MB on keratin nanofibre membranes was a chemisorptions process which occurred by ion exchange reactions [30].

Fig. 5. Plot of ln Ce versus 1/T.

Table 3 Adsorption kinetic parameters for the MB adsorption on keratin nanofibre membranes. Model

qexp (mg g−1 )*

C0 (mg L−1 )

Kinetic parameters qe (mg g−1 )

k1 (min−1 )


Pseudo-first order

46 176

50 250

24.8 111.04

0.0136 0.0124

0.9092 0.8931

Pseudo-second order

46 176

50 250

47.6 178.6

0.00138 0.000349

0.9991 0.9969

qexp (mg g−1 ): experimental value of adsorption capacity.

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Fig. 6. Plots of the fitting of the experimental data with Langmuir (a), Freundlich (b), Dubinin-Radushkevich (c) and Temkin (d) isotherm models.

3.4. Adsorption kinetics The adsorption kinetics, that shows the evolution of the adsorption capacity during time, is important to understand the types of adsorption mechanism of a system. The experimental values of adsorption capacities collected during time using the

solutions at initial MB concentration of 50 mg L−1 and 250 mg L−1 were fitted using the pseudo-first order and pseudo-second order models. The linearized forms of pseudo-first order (17) [31] and pseudosecond order (18) [32] kinetic models are shown below: log(qe − qt ) = log qe − t 1 1 = + t q1 qe kz q2e

Fig. 7. Separation factor for the adsorption of MB on keratin nanofibrous membranes.

k1 t 2.303



where k1 (min−1 ) and k2 (mg g−1 min−1 ) are rate constants of adsorption, qt (mg g−1 ) is the adsorption capacity at time t, qe (mg g−1 ) is the equilibrium adsorption capacity. All the kinetic parameters determined from the intercepts and the slopes of respective plots (Fig. 8) are summarized in Table 3. The adsorption process was best described by the pseudo-second order kinetic model for both initial concentration of dye; in fact this model shows a correlation coefficient (R2 > 0.99) higher than that of pseudo-first order model and the calculated values of the adsorption capacities (qe ) were very close to the experimental ones (qexp ). The applicability of the pseudo-second order model suggests that chemical reactions were responsible for the adsorption of MB on keratin nanofiber membranes.


A. Aluigi et al. / Journal of Hazardous Materials 268 (2014) 156–165

Fig. 8. Plots of the pseudo-first and pseudo-second order kinetics related to the adsorption of MB on keratin nanofibrous membranes.

4. Conclusion In this work, free-standing and flexible keratin-based membranes, made of nanofibres having a mean diameter of about 220 nm, a specific surface area of 13.59 m2 g−1 and a porosity of 90%, were prepared by electrospinning. Preliminary MB adsorption tests, carried out in batch, showed that these membranes have high adsorption capacity toward the cationic dye. In particular, the adsorption capacity increased with increasing the initial dye concentration and pH, while it decreased with increasing the adsorbent dosage. Temperature affected the adsorption process, in fact the adsorption capacity decreased with increasing the temperature and the negative value of the enthalpy change indicated that the adsorption process was exothermic in nature. Among the Langmuir, Freundlich and Temkin isotherm models applied to the adsorption data, the Langmuir was the best model to describe the experimental data and the maximum adsorption capacity obtained at 20 ◦ C, pH 6 and an adsorbent dosage of 1 g L−1 was of about 170 mg g−1 . A mean free energy of 16 kJ mol−1 , evaluated using the D–R model indicated that the adsorption process was chemisorptions process which occurred by ion exchange reactions. Finally, the adsorption kinetics of MB on keratin nanofibre membranes was well described by a pseudo-second order kinetic model. The present preliminary study showed that the very large surface area and the high porosity confer high adsorption performances to the keratin nanofibrous membranes. The adsorption tests in dynamic conditions and desorption tests useful to evaluate if the membranes can be regenerated and reused will be the subject of a further work. Acknowledgement This work was supported through the NanoTWICE project (call Progetto Bandiera - La Fabbrica del Futuro, FdF-SP1-T1.2).

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