A novel structure for removal of pollutants from wastewater

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 216–223

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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa

A novel structure for removal of pollutants from wastewater Nabila S. Ammar a, Hanan Elhaes b, Hanan S. Ibrahim a, Walid El hotaby c, Medhat A. Ibrahim c,⇑ a

Water Pollution Research Department, National Research Centre, 12311 Dokki, Cairo, Egypt Physics Department, Faculty of Women for Arts, Science, and Education, Ain Shams University, 11757 Cairo, Egypt c Spectroscopy Department, National Research Center, 12311 Dokki, Cairo, Egypt b

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

 Quantum mechanical model for

aquatic plant.  A microsphere from natural

resources.  Removal and recovery of Pb(II).

a r t i c l e

i n f o

Article history: Received 16 July 2013 Received in revised form 10 October 2013 Accepted 17 October 2013 Available online 26 October 2013 Keywords: Lead Microspheres Chitosan PM5 Kinetic models Isotherm models

a b s t r a c t Dried water hyacinth was subjected to molecular modifications using quantum mechanical calculations. The model simulates the modified plant as 3 cellulose units, one lignin and some metal oxides namely CaO; FeO and Al(OH)3 are attached through O-Linkage. The model suggests the ability to remove inorganic pollutants from wastewater according to unique hydrogen bonding and high total dipole moment. Based on this model microspheres are synthesized in the laboratory from dried water hyacinth and chitosan following self-assembly method. FTIR spectrum of microspheres exhibits only the characteristic bands for raw materials which give strong evidence that the formed material is a composite. The analysis of SEM micrographes of microspheres showed that the fibers of water hyacinth are imbedded in the crosslinked chitosan matrix. Batch adsorption kinetic models revealed that the sorption of lead ions on microsphere was very fast and the equilibrium was rapidly attained within 30 min. and properly correlated with the second-order kinetic model. Different models of isotherm sorption were used to describe the Pb (II) adsorption onto microspheres. From Langmuir isotherm, the maximum adsorption capacity (qmax) for Pb(II) was 312.5 mg/g, which is about 3 times higher than that of the crude hyacinth. The free energy (E) was 15.798 kJ/mol which shows that the sorption process is endothermic and the mechanism of reaction is an ion-exchange. Even after four cycles of adsorption–desorption, the adsorption capacity was maintained and the decline in efficiency was less than 10%. Ó 2013 Elsevier B.V. All rights reserved.

Introduction Pb(II) is being intensively released into the environment as a result of several industrial activities, such as metal plating, ⇑ Corresponding author. Tel.: +20 12 2727636; fax: +20 23 3370931. E-mail address: [email protected] (M.A. Ibrahim). 1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.10.063

microelectronics, battery manufacture, tanneries, oil refining, and mining. It spreads into the environment through soils and water streams and accumulates along the food chain, resulting in a high risk to human health [1,2]. As it does not degrade biologically, the control of Pb(II) pollution has special importance for both organisms that live in water and those that benefit from water [3]. Hence, the development of efficient techniques for the removal

N.S. Ammar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 216–223

of Pb(II) ions from wastewaters is an important task in terms of protection of public health and environment. A number of technologies such as adsorption, chemical oxidation, precipitation, solvent extraction, ion exchange, membrane processes, and reverse osmosis have been developed for the removal of metal ions from aqueous solutions [4]. Among them, the adsorption technique is found to be the most effective treatment process with selection of a proper adsorbent [4,5]. Application of new trends in water treatment is an environmental challenge. Chitosan is easily prepared from chitin, the most abundant in nature after cellulose [6]. Chemical modification of chitosan produces materials with a variety of physical and mechanical properties [7–9]. For example, chitosan films and fibers can be formed utilizing cross-linkers and adapted techniques for altering from other polysaccharides, such as treatment of amylose with epichlorohydrin [10]. Like hyaluronic acid, chitosan is not antigenic and is a well-tolerated implanted material [11]. Chitosan can easily be prepared in many forms, including, films and membranes. The basic technique for the casting of chitosan films and fibers was early developed [12,13]. Water hyacinth has received attention because of its ability to remove pollutants from aquatic systems [14]. While it reduces sunlight penetration and lowers oxygen content in water, which has a great impact on water ecosystem [15,16]. Water hyacinth could be used in its dry form after modification to remove Cd from polluted water [17]. Furthermore the plant was successfully used to remove both organic and inorganic pollution from water [18]. Based upon the above considerations, the molecular structure of water hyacinth is simulated using semiempirical PM5. Furthermore microspheres were prepared from dried water hyacinth and chitosan, and then examined by FTIR. The ability of this novel structure for removal and for recovery of lead is tested.

Preparation of microspheres About 1 g chitosan is dissolved in 30 mL dilute acetic acid (2% (v/v)), a suspension of water hyacinth dry matter was dropped into the chitosan solution with steering for 1 h and the mixture was treated with ultrasonic path for 30 min. The solution was dropped through a Peristaltic Pump into a gently shaken 100 mL of Sodium tripolyphosphate (TPP) solution at pH 8.6. The chitosan solution was dropped into the TPP solution and the gelled spheres are formed instantaneously. The solidified spheres were filtered; rinsed; air dried and kept in a dry container for further use. Batch sorption experiments In order to explore the effect of contact time, quantity of adsorbent, and the initial concentration of adsorbate, a series of batch experiments were conducted. Batch adsorption was performed by agitating specified amount of adsorbent in 100 mL of metal solution of desired concentration at constant temp. 25 ± 0.2 °C and pH 5.0 ± 0.1 in 125 mL stoppered bottles. The samples were filtered (using Whatman No. 42 filter paper) and analyzed for the concentration of Pb(II) ions remaining in the solution. Calculation of cation uptake by microspheres All the experiments were carried out in triplicate. The percent relative standard deviation of the measurements was calculated and considered acceptable if the value was lower than 5%; otherwise, the data were discarded. The percentage of Pb(II) removal by the microspheres during the series of batch investigations was determined using the following equation:

Removal ð%Þ ¼ ½ðC0  Cf Þ=C0   100 Materials and methods Calculation details All calculations were performed on personal computer using Cigress program system at Spectroscopy Department, National Research Centre [19]. Geometries of the studied structures were optimized at semiempirical PM5 [20,21] method and vibrational spectra were calculated at the same level of theory. The frontier molecular orbitals (highest occupied molecular orbital, HOMO and lowest unoccupied molecular orbital, LUMO) were calculated at the same level of theory.

Reagents and sampling Chitosan low molecular weight was purchased from ABCO Laboratories (Eng. Ltd., Gillingham, England). Pb(NO3)2 is the salt used in the preparation of stock standard solution (1000 mg/L) of analytical grade from Merck (Darmstadt, Germany). The synthetic solutions were then prepared by diluting the stock standard. Sodium tripolyphosphate (TPP) Na5O10P3, molecular weight 367.86 was purchase from Mallinckrodt. Inc. (Paris, France).

Area of plant collections Triplicates of water hyacinth were collected from River Nile at Rod El-Farag, Cairo. Samples were washed with deionized water and then divided into root and shoot. Dried separately at 105 °C for 24 h. The dried plants (root and shoot) were ground in a bladed mixer to less than 0.2 mm.

217

ð1Þ

where C0 and Cf are the initial and equilibrium concentration (mg/L) of Pb ions in solution, respectively. Sorption kinetics of microspheres for Pb(II) uptake The kinetic studies were carried out by shaking 2 g/L of the dried microsphere in 100 mL of Pb(II) ions with an initial concentration of 50 mg/L at different time periods (5, 10, 20, 30, 60, 90, 120 min). The pH of the synthetic solution was 5.0 ± 0.1. Various models were evaluated to describe sorption kinetics. The most commonly used models are pseudo first-order, pseudo-secondorder and Elovich kinetic model. Effect of microsphere dosage The microsphere dosage was varied from 0.25–3.0 g/L using a fixed volume of 100 mL of Pb(II) with an initial concentration 50 mg/L at the equilibration time for lead removal and at constant temp. 25 ± 0.2 °C and pH 5.0 ± 0.1. Sorption isotherms The sorption isotherms were measured by varying the initial Pb (II) ions concentration from 100 to 400 mg/L and the other optimum operating conditions for the highest removal efficiency of microspheres are kept constant. Different sorption models described by Volesky [22] were used for comparison with experimental data. Desorption and reusability studies The recovery and reusability of microspheres is an important parameter related to the potential application of adsorption technology. In this study, microspheres bound to Pb (II) were transferred to a flask containing 100 mL of brine solution (0.5 g NaOH and 5% NaCl) as a desorbing agent to determine desorption

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properties of the microspheres. The mixture was shaken for 30 min at 120 rpm using a rotary shaker. The eluted microspheres were washed repeatedly with deionized water to remove any residual desorbing solution and placed into Pb (II) solution for the next adsorption cycle. The adsorption–desorption cycle was repeated four times using the same microspheres with an initial lead concentration of 50 mg/L each time. The adsorption efficiency of Pb (II) from the microspheres for each cycle and the distribution ratio (Kd) were calculated. The distribution ratio (Kd) was calculated using the equation: K d ¼ ðamount of metal in adsorbent=amount of metal in solutionÞ  V=m

ð2Þ

where V is the volume of the solution (ml) and m is the weight of the adsorbent (g). The percent of metals adsorption and Kd (ml/g) can be correlated by the following equation suggested by Khan et al. (1995):

Adsorption ð%Þ ¼ ð100  K d Þ=ðK d þ V=mÞ

ð3Þ

Instrumentations FTIR Jasco FTIR 430 Fourier Transform Infrared Spectrometer was used for recording the obtained IR spectra. Spectra were recorded in a spectral range of 4000–400 cm1, resolution of 4 cm1 and scan speed is 2 mm/s. Samples were introduced as KBr pellet. Electron microscope Electron microscope Inspect S FEI Company, Philips, Holland was used to record SEM micrograph for water hyacinth as well as water hyacinth/chitosan microsphere. Atomic absorption The concentrations of Pb in all samples were determined according to standard method [23] using Atomic Absorption Spectrometer Varian SpectrAA (220) with graphite furnace accessory and equipped with deuterium arc background corrector. Quality control Precision of the metal measurements was determined by analyzing (triplicate) the metal concentration in samples and for each series of measurements an absorption calibration curve was constructed composed of a blank and five standards. Accuracy of the metal measurements was confirmed using certified standard reference material from National Institute Standards and Technology (NIST).

Results and discussions Molecular modeling study In our previous work, water hyacinth is described as cellulose like material. While examination indicated that water hyacinth is composed of cellulose; lignin and some metal oxides. Of course the plant contains protein but for dried plant at 105 °C over night the protein has suffered from degradation. Accordingly the plant in its dry form is tested as a composite of 3 units of cellulose; one unit of lignin; metal oxides like CaO; FeO that interacted with this composites through the O-Linkage connecting the cellulose and lignin. Al(OH)3 has also interacted through O-Linkage. Semiempirical calculations indicate that this assumption could be a real one. Furthermore, the results indicate that the structure corresponds to the optimized structure at PM5 level of theory. The calculated vibrational spectra indicate no negative frequency which is also an indication for the occurrence of this model molecule. Fig. 1 indicates the optimized structure of this model while Fig. 2 presents the surface morphology of this model. A modification of this model could be achieved also with PM5 as one decides to apply water hyacinth, in dry form for water treatment. The advantage is that dry form is found in small volume with no oxygen consumption. In order to let the dry form looks like the water hyacinth one must add structures resembling protein with its unique hydrogen bonding to act like the real plant. Fig. 3 presents the same model like that in Figs. 1 and 2 with the existence of chitosan. Chitosan possesses NH2 so that it could be added to improve the structure of our proposed model. The model shows optimized structure at PM5 with no negative frequencies in their calculated vibrational spectra. Fig. 4 indicates the surface morphology of the new structure. The calculated vibrational spectra of both models are indicated in Fig. 5. Vibrational spectra are indicated only as a test for the optimization of a given structure. We suppose that the hydrogen bonding from cellulose; lignin and chitosan could perform an attractive electrostatic interaction to deal with heavy metals. Furthermore, the existence of metal oxides and/or hydroxides offers a wide surface for adsorption process which is also a good platform for adsorption of heavy metals. Some physical parameters such as total dipole moment (TDM); ionization potential (IP) and HOMO/LUMO band gap energy (DE) of the studied models are presented in Table 1. Total dipole moment has increased gradually up to 50.535 debye corresponding to the proposed microsphere structure. While the ionization potential has decreased (4.754 eV) with a noticeable decrease in the value of band gap energy (2.849 eV). This indicates that the model which describes the composite of chitosan; cellulose; lignin and metal

Fig. 1. PM5 optimized model (1) the model consists of 3 cellulose units linked through O-Linkage with lignin unit. CaO; MgO and Al(OH)3 are weakly liked through hydrogen bonding with the O-Linkage.

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219

Fig. 2. PM5 calculated surface morphology of the previous model (1).

oxides is suitable for interaction with surrounding metals. This statement is based on its high total dipole moment, lower ionization potential and band gap energy. Preparation of microspheres One of the most important tasks now is to check the model through experimental work. It is well known that the surface of sphere is the largest among all geometrical surfaces, so that we choose to prepare a composite of dried water hyacinth with chitosan in the form of microspheres. This assumption is to make the molecular structure of microsphere which is produced from chitosan and water hyacinth root similar to the molecular structure of the plant root. The same is also indicated for the molecular structure of chitosan and dried shoot as compared with plant shoot. The most important point is to apply the prepared microsphere to a known pollutant such as Pb(II) in order to assess its abilities for removal and/or their recovery. Fig. 6 shows the FTIR spectrum of water hyacinth/chitosan composite. As is shown in the figure the characteristic bands of crosslinked chitosan appear at 1640 cm1

Fig. 4. PM5 calculated surface morphology of the previous model (2).

and 1549 cm1 corresponding to stretching vibration of P@O of the APOH groups and stretching vibration of NH3+ group respectively. Also the spectrum shows the fundamental peaks of water hyacinth dry matter which appears at 1383 cm1 and 1319 cm1 which corresponds to CH3 umbrella mode. The band near 1160 cm1 is representative of the antisymmetric bridge stretching of CAOAC groups in cellulose. From the above mentioned results it could be concluded that there is no new bands appearing in the spectrum of water hyacinth/chitosan composite which give strong evidence that the prepared material is a composite in which the particles of water hyacinth are embedded in the matrix of crosslinked chitosan polymer. Regarding the Fig. 7a, the prepared water hyacinth/chitosan microsphere before immersion in the pollutants looks as plate like structure. As far as the structure is immersed in solution containing the studied pollutants it looks like microsphere as indicated in Fig. 7b. The scanning electron microscope SEM results as

Fig. 3. PM5 optimized modified model (2); the model consists of one chitosan unit; 3 cellulose units and one lignin unit all units are linked together through their O-Linkage. CaO; MgO and Al(OH)3 are weakly linked through hydrogen bonding with the O-Linkage.

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200 180

hyacinth/chitosan microspheres showed that the fiber structure of water hyacinth which is mainly cellulose and lignin is imbedded in the crosslinked chitosan matrix. This data could be a good confirmation for those obtained above by FTIR.

Cel3

Transmittance %

160 140

Cel3-Lig1

Removal of lead from synthetic wastewater

120 100 80

Cel3-Lig1-MO Cs-Cel3-Lig1-MO

60 40 20 0 4000 3500

3000 2500

2000 1500 1000

500

0

Wavenumber Cm-1 Fig. 5. PM5 calculated vibrational spectra for the studied structures. Cellulose (Cel3); cellulose–lignin (Cel3:Lig1); cellulose–lignin–metal oxides (Cel3:Lig1:MO) and finally chitosan–cellulose–lignin–metal oxides (Cs:Cel3–Lig1–MO). No negative frequencies which indicate that the frequencies were calculated for optimized structures.

Table 1 Calculated PM5 semiemperical total dipole moment (TDM) as debye; ionization potential (IP) as eV and HOMO/LUMO band gap energy (DE) as eV.

Cel3 Cel3–Lig1 Cel3–Lig1–MO Cs–Cel3–Lig1–MO

TDM

IP

DE (eV)

3.617 5.340 17.495 50.535

10.627 8.901 6.839 4.754

12.089 3.758 5.737 2.849

HOMO: highest occupied molecular orbital. LUMO: lowest unoccupied molecular orbital. Cel3: 3 cellulose units. Cel3–Lig1: 3 cellulose units linked with lignin unit through O-Linkage. Cel3–Lig1–MO: 3 cellulose units linked with lignin unit and metal oxides through O-Linkage.

Fig. 6. FTIR absorption spectrum of water hyacinth/chitosan microsphere.

The prepared microspheres represent a cheap source of sorbent. For testing its ability to remove lead ions from wastewater, equilibrium time is one of the important parameters for selecting a wastewater treatment system. Where the time consumed for wastewater disposal should be considered. As shown in Table 2, the sorption of Pb(II) ions onto the microspheres, was rapid for the first 5 min and equilibrium was reached within 30 min. Therefore, the period of 30 min. was considered as the optimum time, where the removal percentages of lead ions reached 79.59%. Kinetic characteristic in a microsphere depends not only on the presence of active Pb(II) ions site on it but also depends on the accessibility of the Pb(II) ions site without sterical hindrance which is greatly determined by the matrices of the sorbent. The rate kinetics of Pb(II) ions sorption onto microspheres was analyzed using pseudo first-order, pseudo-second order and Elovich kinetic model. The conformity between experimental data and the model-predicted values was expressed by the correlation coefficients (R2, values close to 1). The relatively higher value is the more applicable model to the kinetics of Pb (II) ions sorption. Pseudo first-order kinetic model The kinetic data were treated with the Lagergren first-order model [24] which is the earliest known one describing the adsorption rate based on the adsorption capacity. The integral form of the pseudo first-order model generally expressed as follows:

logðqe  qt Þ ¼ log qe  k1;ads  t=2:303

ð4Þ

where qe (meq/g) and qt are the amounts of adsorbed Pb ions on the sorbent at the equilibrium and at any time t, respectively; and k1,ads is the Lagergren rate constant of the first-order sorption (min1). The model is based on the assumption that the rate is proportional to the number of free sites. If the pseudo first-order kinetic is applicable, a plot of log (qeqt) versus t should provide a linear relationship from which k1,ads and predicted qe can be determined from the slope and intercept of the plot, respectively (Suppl. 1). The variation in rate should be proportional to the first power of concentration for strict surface adsorption. However, the relationship between initial solute concentration and rate of adsorption will not be linear as pore diffusion limits the adsorption process. It was observed from (Suppl. 1) Table 3, that first-order model failed to provide a realistic estimate of qe of adsorbed Pb ions since the experimental value of qe (0.188 meq/g) was higher than the fitted value (0.046 meq/g) for microsphere. This underestimation of the amount of binding sites is probably due to the fact that qe was determined from the y-intercept (t = 0). The intercept is most strongly affected by the short term metal uptake, which is usually much lower than the equilibrium uptake. It can be concluded that this is a general disadvantage of using the linearized first-order model, where the first order adsorption reaction of Pb(II) ions onto microsphere is not appropriate to describe the entire process even when the correlation coefficient R2 is relatively high. Pseudo-second order model

indicated in Fig. 7c and d presents the SEM images for a-water hyacinth, b-water hyacinth/chitosan microsphere. The result of SEM showed that there was some modification in water hyacinth’s surface structure as a result of introducing chitosan to form plate like structure in dry phase. The analysis of SEM micrograph of water

The kinetics of adsorption process may also be analyzed by pseudo second order rate equation [25]. The pseudo-second-order model is based on the assumption that sorption follows a secondorder mechanism, whereby the rate of sorption is proportional to

N.S. Ammar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 216–223

221

Fig. 7. Prepared water hyacinth/chitosan microsphere (a) before immersion in Pb(II) solution and (b) after immersion in Pb(II) solution. SEM images for (c) water hyacinth and (d) water hyacinth/chitosan microsphere.

values for the second-order kinetic model were 0.999 for the adsorption of Pb ions onto microsphere (Table 3). Based on theoretical considerations, the reaction of a divalent metal ion (M) binding to two free binding sites (B) can be explained by the following expressions [26].

Table 2 Effect of contact time on the removal of Pb(II) by adsorption onto water hyacinth/chitosan microsphere at an initial concentration 50 mg/L, at constant pH 5.0 ± 0.1, temperature 25 ± 0.2 °C and 2 g/L adsorbent weight. Contact time (min)

Percentage of lead removal

001 005 010 020 030 060 090 120

57.14 69.39 71.43 75.50 79.59 79.59 79.59 79.59

M þ 2B $ B2 M

This means, the sorption rate would be proportional to the metal concentration and the square of the number of free sites, which corresponds to the term (qe – qt)2 in the second-order model. The better fit of the second-order model indicated that a 1:2 binding stoichiometry applies, where one divalent metal binds to two monovalent binding sites.

Table 3 Summary of Pseudo-first and -second order rates constant and equilibrium for Pb (II) uptake using water hyacinth/chitosan microsphere. Pseudo-first-order rate parameters K1,ads qe model (meq/g) 0.082 0.046

R 0.928

qe exp. (meq/g) 0.188

Pseudo-second-order rate parameters K2,ads qe model (meq/g) 6.06 0.191

R2 0.999

qe exp. (meq/g) 0.188

2

the square of the number of unoccupied sites. The linearized form of the equation is expressed as

t 1 t ¼ þ qt K 2;ads  q2e qe

r ¼ k½M½B2

Elovich kinetic model Elovich kinetic equation is another rate equation based on the adsorption capacity, which is generally expressed as [27]:

dqt ¼ a expðbqt Þ dt

where a is the initial adsorption rate (mg g1 min1) and b is the desorption constant (g mg1) during any one experiment. It is simplified by assuming abt » t and by applying the boundary conditions qt = 0 at t = 0 and qt = qt at t = t. Eq. (6) becomes form as followed:

qt ¼ ð5Þ

where k2,ads is the rate constant of second-order sorption of microsphere (g/meq min). The linearized second-order plot of t/q against t (Suppl. 2) according to Eq. (5) resulted in straight lines for Pb ions and led to the determination of the second-order rate constants (k2,ads) and qe from the slope and the y-intercept (Table 3). The qe values were very close to the experimentally determined ones which is a first sign of the appropriateness of this model. The R2

ð6Þ

1 1 ln ðabÞ þ ln ðtÞ b b

ð7Þ

If Pb(II) adsorption by microspheres fits the Elovich model, a plot of qt versus ln (t) should yield a linear relationship with a slope of (1/b) and an intercept of (1/b)  ln (ba) (Suppl. 3). Thus, the constants can be obtained from the slope and the intercept of the straight line. Correlation coefficient obtained by Elovich model was higher than that obtained from pseudo first-order model and comparable to that obtained from pseudo second-order model. Elovich model basically supports chemisorptions [28].

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N.S. Ammar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 216–223 Table 4 Effect of different dose of the water hyacinth/chitosan microsphere on Pb(II) removal at an initial concentration of 50 mg/L at constant pH 5.0 ± 0.1 and temperature 25 ± 0.2 °C. Dose of sorbents (g/L)

Percentage of lead removal

0.25 0.5 0.1 0.2 0.3

81.0 83.0 82.0 79.8 39.0

Table 5 Summary of isotherm model parameters for uptake of lead ions by water hyacinth/ chitosan microsphere. Langmuir model qmax (mg/g)

R2

Kf

n

R2

9.88

312.5

0.989

96.69

4.971

0.983

log qe ¼ log K f þ

Effect of adsorbent dose Various amounts of adsorbent microsphere ranging from 0.25– 3.0 g/L were used (Table 4). The removal percentage of Pb(II) ions increased with an increase in adsorbent dosage due to the availability of larger surface area and more adsorption sites. At adsorbent dosage more than 0.25 g the incremental of Pb (II) removal becomes very low, as the surface metal ions concentration and the solution metal ions concentration come to equilibrium with each other [29]. However, the decrease in removal percentage with increase in the adsorbent dose is mainly due to unsaturation of adsorption sites through the adsorption reaction. Where the aggregation of adsorption sites would lead to a decrease in the total surface area of the adsorbent [30]. Equilibrium studies and isotherm modeling Langmuir and Freundlich models Adsorption equilibrium measurements are used to determine the maximum or ultimate capacity. Adsorption equilibrium data are formulated into an isotherm model. The most commonly used models include Freundlich, Langmuir and Dubinin–Kaganer–Radushkevich (DKR) isotherms [31]. The uptake of Pb(II) onto microspheres as a function of its con centrations was studied at temp. 25 ± 0.1 C and at pH 5.0 ± 0.1 by varying the metal concentration from 100 to 400 mg/L while keeping all other parameters constant with respect to optimum dose and time for Lead removal. Blank tests without sorbent were performed to confirm that lead precipitation did not interfere with lead ion sorption. Due to their simplicity, the Langmuir and Freundlich equations are the most widely used models to describe the relationship between equilibrium Pb(II) ions uptake (qe) and final concentrations (Ce) at equilibrium. The Langmuir isotherm relationship is given as

Ferundlish model

K

1 log C e n

ð11Þ

The choice between Langmuir and Freundlich isotherms depends mainly on the nature of equilibrium data [32]. Where, the adsorption phenomena at the solid–liquid interface were commonly described by the adsorption isotherm model, and adsorption isotherms are essential data source for practical design of adsorption systems and understanding of relation with adsorbent and adsorbate [33]. Langmuir and Freundlich isotherm parameters are listed in Table 5. Langmuir isotherm assumes monolayer adsorption, the R2 values for Lead ions was greater than 0.98, which revealed the extremely good applicability of the Langmuir model to these adsorptions. The Freundlich model has generally been considered as an empirical equation based on adsorption on a heterogenous surface and has also been used widely to fit experimental data [34]. Compared with Langmuir isotherm, the R2 values of Freundlich model was also 0.98. As indicated from the Table 5 (Suppl. 4 and 5), the coefficients of determination (R2) of both models were P0.98 indicating that both models were adequately describing the experimental data of Lead sorption. Dubinin–Kaganer–Radushkevich isotherms (DKR isotherms) Langmuir and Freundlich isotherms do not give any idea about sorption mechanism. The DKR isotherm is an analogue of Langmuir type but it is more general because it does not assume a homogeneous surface or constant sorption potential [35a,b,c]. The linearized DKR isotherm equation can be written as shown in

ln qe ¼ ln X m  be2

ð12Þ

where qe is the number of metal ions adsorbed per unit weight of adsorbent (mol/g), Xm is the maximum sorption capacity, b is the activity coefficient related to mean sorption energy, and e is the Polanyi potential, which is equal to:



e ¼ RT ln 1 þ

1 Ce

 ð13Þ

where K (L/g) is the equilibrium adsorption constant which is related to the affinity of the binding sites and qmax (mg/g) is the maximum amount of lead ion per unit mass of sorbent when all binding sites are occupied. The Langmuir parameters can be determined from a linearized form of Eq. (8) (by plotting Ce/qe versus Ce), represented by:

where R is the gas constant (J/mol K) and T is the temperature. The saturation limit Xm may represent the total specific micropore volume of the sorbent. The sorption potential is independent of the temperature but varies according to the nature of sorbent and sorbate [36]. The slope of the plot of ln qe versus e2 gives b (mol2/J2) and the intercept yields the sorption capacity, Xm (mol/g). The sorption space in the vicinity of a solid surface is characterized by a series of equipotential surfaces having the same sorption potential. The sorption energy can also be worked out using Eq. (14):

Ce 1 Ce ¼ þ qe Kqmax qmax

1 E ¼ pffiffiffiffiffiffiffiffiffiffi 2b

qe ¼

Kqmax C e ð1 þ KC e Þ

ð8Þ

ð9Þ

The Freundlich equation is given by:

qe ¼ K f C e1=n

ð10Þ

where kf and n are the Freundlich constants and are related to the adsorption capacity of the sorbent and the adsorption intensity. To simplify the determination of kf and 1/n, Eq. (10) can be linearized in logarithmic form, which allows the determination of the unknown parameters by plotting log qe versus log Ce:

ð14Þ

The DKR parameters are calculated from the slope of the line and listed in Table 6. (Suppl. 6) represented ln qe against e2 for Pb(II) sorption on microspheres. The result shows that, the E value is 15.798 kJ/mol for Pb(II) on microspheres. The positive value of E indicated that the sorption process is endothermic and the mechanism of reaction is an ion-exchange and higher solution temperature will favor the sorption process. One possible interpretation of the endothermicity of the adsorption process was that the metal

N.S. Ammar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 216–223 Table 6 Summary of DKR model parameters for uptake of lead ions by water hyacinth/ chitosan microsphere. Xm (mol/g)

b (mol2/j2)

Sorption energy (E, kJ/mol)

Correlation coefficient R2

2.45  103

0.201

15.798

0.982

ions were highly solvated in water. In order for these ions to be adsorbed, the hydration sphere must be removed, and this process requires energy intake. This energy of dehydration supersedes that required for getting the ions attached to the surface of the adsorbent [37], hence resulting to an overall endothermic nature. The free energy (E) was 15.798 kJ/mol which shows that the sorption process is endothermic and the mechanism of reaction is an ion-exchange [38]. Adsorption/desorption process for lead ions uptake by microspheres Stability is especially important when the same sorbent material is reused in multiple adsorption and desorption cycles [39]. Therefore, the reusability of microspheres was examined based on adsorption/desorption ability. The viability of the process is assured by the high efficiency of lead ions recovery from the exhausted microspheres with brine solution (0.5 g NaOH and 5% NaCl) as a desorbing agent. Furthermore, in order to investigate the reproducibility of microspheres, the adsorption–desorption cycle was repeated four times using the same microspheres at optimum operating conditions. The Pb2+ adsorption percentages (%) and distribution ratio (Kd) were calculated, whose initial concentrations were 50 mg/L, as shown in (Suppl. 7). The obtained results show that, in the first cycle, the removal efficiency for Pb(II) was more than 82%. From the second to four cycles, the microspheres had slightly changed adsorption ability with removal efficiency ranged from 82% to 76%. Conclusion Molecular modeling, FTIR and SEM studies indicate that chitosan is crosslinked into water hyacinth fiber to produce novel plate like structure which looks like spheres as it dropped in solution. The novel structure with compact size; no oxygen consumption are dedicated for the removal of Pb from industrial wastewater in short time. From this study, it can be concluded that, ease and self-assembly method for the inexpensive sorbent made of dried water hyacinth/chitosan show high-sorption performance of Pb(II) and high feasibility of desorption and regeneration of sorption capacity. Moreover, a comparison of different isotherm models revealed that microsphere adsorption data perfectly fitted the Langmuir and Freundlich adsorption isotherms models with regression coefficient R2 > 0.98 for Pb(II). The kinetic studies indicated that the experimental data followed the second-order kinetic reaction. The most interesting result of this microsphere approach is that the mechanism of removal process is an ion-exchange. These appropriate features will contribute to the reuse of microspheres as a novel sorbent material in practical application for removal of heavy metals from industrial wastewater. It is worth to mention that there are so many techniques are utilized in the removal of pollutants some of which are natural while the others depends on chemical methods [40–43]. Acknowledgement This work is supported financially by the National Research Centre, NRC, Grant No. P91001.

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