Biodegradable polymer based ternary blends for removal of trace metals from simulated industrial wastewater

International Journal of Biological Macromolecules 83 (2016) 198–208

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International Journal of Biological Macromolecules journal homepage: www.elsevier.com/locate/ijbiomac

Biodegradable polymer based ternary blends for removal of trace metals from simulated industrial wastewater N Prakash a,∗ , S Arungalai Vendan b a b

Department of Chemistry, Thangavelu Engineering College, Chennai, India Industrial Automation and Instrumentation Division, SELECT, VIT University, Vellore

a r t i c l e

i n f o

Article history: Received 5 August 2015 Received in revised form 29 August 2015 Accepted 27 September 2015 Available online 30 September 2015 Keywords: Chitosan (CS) Montmorillonite clay (MM clay) Nylon 6 (Ny 6) Cross-linking angent Trace metals

a b s t r a c t The ternary blends consisting of Chitosan (CS), Nylon 6 (Ny 6) and Montmorillonite clay (MM clay) were prepared by the solution blending method with glutaraldehyde. The prepared ternary blends were characterization by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), Thermo gravimetric analysis (TGA), Differential scanning calorimetry (DSC) and Scanning electron microscope (SEM). The FTIR results showed that the strong intermolecular hydrogen bondings were established between chitosan, nylon 6 and montmorillonite clay. TGA showed the thermal stability of the blend is enhanced by glutaraldehyde as Crosslink agent. Results of XRD indicated that the relative crystalline of the pure chitosan film was reduced when the polymeric network was reticulated by glutaraldehyde. Finally, the results of scanning electron microscopy (SEM) indicated that the morphology of the blend was rough and heterogenous. Further, it confirms the interaction between the functional groups of the blend components. The extent of removal of the trace metals was found to be almost the same. The removal of these metals at different pH was also done and the maximum removal of the metals was observed at pH 4.5 for both trace metals. Adsorption studies and kinetic analysis have also been made. Moreover, the protonation of amine groups is induced an electrostatic repulsion of cations. When the pH of the solution was more than 5.5, the sorption rate began to decrease. Besides, the quantity of adsorbate on absorbent was fitted as a function in Langmuir and Freundlich isotherm. The sorption kinetics was tested for pseudo first order and pseudo second order reaction. The kinetic experimental data correlated with the second order kinetic model and rate constants of sorption for kinetic models were calculated and accordingly, the correlation coefficients were obtained. © 2015 Published by Elsevier B.V.

1. Introduction Recently, natural polymers have been viewed as a biological and biomedical resource due to their unique properties including non toxicity [1], bio degradability and biocompatibility [2,3]. However, natural homopolymer by itself is inadequate to meet the diversity of demands for biomaterials. Biocompatibility had been considered as ‘the ability of a material to perform with an appropriate host response in a specific application [1], taking into account the interactivity between the biomaterial and the host. Some of the prominent applications for biomaterials are: controlled drug delivery [4,5] discussed water treatment and recycling techniques. [6] Presented a super selectivity potentiometric methodology, using

∗ Corresponding author. Tel.: +91 4426810111. E-mail addresses: [email protected], k [email protected] (N. Prakash), [email protected] (S. Arungalai Vendan). http://dx.doi.org/10.1016/j.ijbiomac.2015.09.050 0141-8130/© 2015 Published by Elsevier B.V.

an ion-selective electrode, for determination of mercury ion (II) in aqueous solution, orthopedic devices [7], sutures, cardiac pacemakers, and vascular grafts. Natural polymers such as konjac glucomannan [8], chitosan [9] and gelatin [10] have remained attractive as they offer features viz., economical, easy availability, potentially degradability and compatible nature. Chitosan was also blended with several polymers such as polyamides, polyurethane foam, poly (acrylic acid), gelatin, silk fibroin and cellulose to enhance mechanical properties [11–13]. [14] synthesized Alumina-coated multi-wall carbon nanotubes are characterized by scanning electron microscopy, X-ray diffraction, and FTIR. They were used as an adsorbent for the removal of lead ions from aqueous solutions in two modes, batch and fixed bed. Chromium and cadmium are highly toxic heavy metals and have to be removed from the water sources. These metals are from various industrial effluents such as tanneries, electroplating and paints. The Chromium toxicity is mainly induced from its hexavalent form, Cr (VI) and easy solubility in water. Its concentration should not exceed 0.05 mg/L in drinking water and it

N. Prakash, S. Arungalai Vendan / International Journal of Biological Macromolecules 83 (2016) 198–208 Table I Ternary polymer blends preparation ratio. S. No

Polymer Blends

1 2 3

Chitosan/Nylon 6/Montmorillonite(1:1:1)-Glutaraldehyde Chitosan/Nylon 6/Montmorillonite(1:2:1)-Glutaraldehyde Chitosan/Nylon 6/Montmorillonite(2:1:1)-Glutaraldehyde

is more toxic with potential carcinogenic effects [15]. Cadmium belongs to the hazardous metal group. It is fairly mobile in soil and primarily present as an organically bound, exchangeable and water-soluble species [16,17]. For Cadmium, the upper limit level in drinking water should be 0.01 mg/L or less. It is shown from the toxicological studies that long term effects of Cd (II) damages kidney, liver and blood. Short-term effects include nausea, vomiting, diarrhoea, and cramps. [18] Reported that the dye degradation rates followed pseudo-first order kinetics with respect to the substrate concentration under the prevailing experimental conditions. Parameters namely the temperature, pH and presence of electron acceptor for different experimental trials were investigated. This was followed by analyzing the effect of pH which emphasized inverse dependency. [19] Synthesized manganese dioxide-coated multiwall carbon nano tube (MnO2 /CNT) based nano composite for experimentation and subsequent optimization. The pH range was varied and the optimum removal was attained when the pH was equal to 6 and 7. They also reported that due to slower the flow rates of the feed solution the higher the removal because of larger contact time. [20] Determined optimal parameters by monitoring different attributaries such as effect of pH, effect of concentration of the dye, amount of adsorbents, contact time, and temperature. Differentiation between particle and film diffusion mechanisms operative in their study was carried out. The removal of Cadmium from the wastewater by various techniques such as chemical precipitation, electro deposition, electro coagulation process, ion exchange and emulsion liquid membrane [21–23] have been employed. These techniques are expensive and ineffective at low concentration of metal ions. Adsorption method is reported to be the most suitable method due to low cost and high efficiency even for low concentration of metal ions [24]. Novel adsorbents prepared from orange peel and Fe2 O3 nano particles have been used [25] to remove Cadmium from aqueous solutions. The removal of Cadmium from electroplating industry effluent is reported and is shown to have desorption and reusability without loss of efficiency. [26] Assessed the applicability of waste materials-bottom ash and deoiled soya-for the removal of the colorant Congo red from wastewaters. [27] Described the use of bottom ash [a power plant waste] and de-oiled soya [an agricultural waste] as effective adsorbents for the removal of a hazardous azo dye [Chrysoidine Y] from its aqueous solutions. [28] Investigated the removal of the dye-tartrazine by photodegradation using titanium dioxide surface as photocatalyst under UV light. [29] Synthesized carbon nanotube, a composite of multi-walled carbon nanotubes and titanium dioxide (MWCNT/TiO2 ) to hybridize the photocatalytic activity of TiO2 with the adsorptivity. A low cost fertilizer industry waste material, carbon slurry, has been chemically treated, activated and used as adsorbent to remove hexavalent Chromium from aqueous solutions [30] and the kinetics of adsorption follows pseudo second order rate equation based on batch experiments. The removal of lead and Chromium from aqueous solutions has been reported using inexpensive Red mud, an Aluminium industry waste and bagasse fly ash. [31] revealed from their experimental study a faster kinetics and efficiency of MNP–OPP in comparison to those of MNP and OPP and further confirmed a complexation and ion exchange mechanism to be operative in metal binding. [32] Attempted to degrade aniline in the synthetic effluent by homogeneous and heterogeneous

199

Fenton oxidation process. The kinetic constants and the thermodynamic parameters for the oxidation of aniline in synthetic wastewater were determined. [33] Treated and activated blast furnace dust to prepare low-cost adsorbents. The blast furnace waste generated in steel plants have been used for the removal of lead and Chromium [34] and concluded that the uptake of lead is greater than that of Chromium. [35] studied four isothermal models, Langmuir, Freundlich, Tempkin, and DubininRadushkevich, and important thermodynamic parameters were calculated. The biodegradable polymer using, cross linking treatment has emerged as another important strategy to improve the performance of the blends. In this study, chitosan/nylon 6/montmorillonite clay blends were prepared by the casting process. In order to produce chitosan film with homogeneous dispersion of fine clay Particles, chitosan, the naturally cationic polymer was used as a compatibilising agent for montmorillonite clay modification, glutaraldehyde was added to the blend as a crosslinking agent in various ratios and evaluates the performance of CTS/NY 6/MM clay blend. The hydrophobic behavior and an increase in spacing between the layers of silicate are important factors which make organophilic montmorillonite compatible with most hydrophobic polymers. Clay and silica have already proved to be an effective way to improve the mechanical, electrical, and thermal properties of polymers. This biopolymer can intercalate into the inorganic clay by means of cationic exchange process in the absence of other organic modifiers. The strong interactions between have been clay and chitosan. The enhanced thermal stability properties of chitosan were reported [36]. Modified Ball clay (MBC) and chitosan composite (MBC–CH) was prepared by [37]. In the present study, solution blending of chitosan with nylon 6 and polyurethane foam with and without cross linking agent and subsequent characterization of the obtained blends using IR, TGA, XRD and SEM is reported. Chitosan alone will not have enough strength and it will not have free standing film which will have enough strength to remove the metals. Hence it is blended with silk which is an artificial fiber. This will have a rein forcing effect to hold the chitosan. To improve the strength it is also blended with starch which will have binding effect. This will facilitate the binding of silk and chitosan.

2. Materials and methods 2.1. Chemicals and materials Chitosan was purchased from India Sea Foods, Cochin, Kerala which is 92% deacetylated. Glacial acetic acids were purchased from Sisco Research Laboratories PVT, LTD, India. Montmorillonite (MM clay) employed in this study was K10 montmorillonite purchased from Sigma-Aldrich. Vermiculite was obtained from Fisher Scientific Pvt Ltd, India. The cross linking agent glutaraldehyde was purchased from SD Fine-Chem Ltd, India.

2.2. Preparation of chitosan/nylon 6/montmorillonite clay blend Ternary blend films were prepared by casting Methods. Chitosan solutions were prepared by dissolving chitosan in 2% glacial acetic acid solution at room temperature with stirring. Ion-exchanged nylon 6 powder was added to water. Montmorillonite clay dissolved in water was also prepared. Subsequently, the dispersion was mechanically stirred at 1000 rpm for one hour. The product was poured into petri dish and allowed to dry. The same procedure was carried out in presence. The prepared ratios were tabulated below in Table I.

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Fig. 1. FTIR spectra of ternary blends (a) 1:1:1+Glu, (b) 1:2:1+Glu, (c) 2:1:1+Glu.

2.3. Characterizations FTIR analysis of the prepared ternary polymer blend was recorded by Fourier transform infra-red spectrophotometer (FT-IR) using the Perkin Elmer 200 FTIR spectrophotometer, in the range of 400–4000 cm−1 at 25 ◦ C with a resolution of 4 cm−1 . The thermal analysis (TGA and DSC) was carried out on a Perkin Elmer thermal analysis instrument. TGA was recorded with 2.0 mg of this sample was heated from 30 ◦ C to 870 ◦ C at a heating rate of 10 ◦ C/min at N2 ATM. In this technique thermal decomposition of polymer blend is measured as a function of temperature. For DSC analysis the pierced lid in the nitrogen atmosphere at a heating rate of 10 K/min was used. The XRD Pattern of the various polymeric blend samples was tested by an X-ray scattering SHIMADUZ XD-DI Diffractometer using Ni filter Cu K␣ radiation source ( = 0.154 nm), set as scan rate = 10◦ /min, using a voltage of 40 kv and a current of 30 mA. The particles are having cubic to cuboid shape with 100–300 nm particle size as given by Nanotrac particle analyzer 2.4. Results and discussion 2.4.1. FTIR-spectra analysis Fig. 1 shows FTIR spectra for (a) 1:1:1+Glu, (b) 1:2:1+Glu, (c) 2:1:1+Glu. The absorption bands of ternary blends occur at 3418,

2921, 2131, 1160, 1111, 1059, 858,717,667, 611, 443 cm−1 . The peak at 3418 can be due to the OH-stretching, which overlaps with NH-stretching in the same region. The peak for chitosan occurs at 3454 cm-1 has been shifted to 3418 cm-1 in the case of ternary system and the O-H stretching frequency has been shifted to lower frequency values and this shift may be due to the complexation that might have occurred due to the reaction of chitosan with Glutaraldehyde. There is not much change in the peak values near the frequency at 2921 cm-1 as this is due to C-H stretching whereas another possible additional peak at 2131 cm-1 occurs and this may arise because of the N-H stretching which might overlap O H stretching in this region. This holds good for increasing content of chitosan or clay. Similar observations are made from Fig. 1. Increase in clay and chitosan content has no significant effect on the O H stretching frequencies. The band occurs at 1160 cm−1 is shifted from 1150 cm−1 and this corresponds to asymmetric stretching of C O C stretching of C O C Bridge. The band occurs in the case of ternary at 1111 cm−1 in Fig. 1 and is shifted from 1096 cm−1 (for pure chitosan). The band occurs at 1059 cm−1 in Fig. 1 is shifted from 1021 cm−1 (for pure chitosan). Again, these bands correspond to C O stretching. These bands are characteristics of chitosan structure. The shifting of the bands to the higher frequency has occurred because of complexation of chitosan and this happens due to blending with nylon 6. This may be attributed to

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201

Fig. 2. XRD pattern of ternary blends of (a) 1:1:1+Glu, (b) 1:2:1+Glu, (c) 2:1:1+Glu.

the effect of hydrogen bonding formed between chitosan and nylon 6. This lower energy shift of the bands occurs when chitosan content is increased and this may be attributed to the formation of inter hydrogen bonds if the macromolecules are from different polymers. This is because of the inter hydrogen bonds formed between two different macromolecules are usually stronger than those formed between the molecules of the same polymer. This may be due to chelating effect caused by the coordination interaction between NH2 groups of the chitosan and nylon 6. 2.5. X-ray diffraction analysis A pure chitosan film shows four main diffraction peaks 13.17◦ , 19.69◦ , 23.40◦ , and 41.67◦ . The diffractograms of pure chitosan and blend with nylon 6 (1:1+Glu), (1:2+Glu) and (2:1+Glu) of chitosan indicate the presence of chitosan in general and miscibility of the polymer with blended polymer in particular. Chitosan and nylon 6 molecules form blends interact and the blend in the XRD pattern shows peak at 2␪ = 19.69◦ (d = 4.505) to 2␪ = 41.67◦ . (1:1+Glu), (1:2+Glu) and (2:1+Glu) the peak of nylon 6, 2␪ = 23.40◦ and the

peak of chitosan shifted to 2␪ = 19.69◦ with increasing nylon 6 contend in the blend (Fig. 2). In the case of glutaraldehyde the peak at 2␪ = 19.69◦ (d = 4.505) decreases and as the chitosan content increases two peaks emerge in the presence of excess nylon 6 and this approaches crystalline nature whereas in the case of 2:1+Glu, crystalline nature is attained again. The change of clay structure before and after chemical modification can be better understood from X-ray studies. From the XRD data it is seen that there are distinct crystalline peaks and this is due to plenty of hydroxyl and amino groups existent in the chitosan structure. These hydroxyl and amino groups form stronger intermolecular and intra molecular hydrogen bonds. Thus, due to certain regularity chitosan molecules can retain its structure even in the blend [38]. However, in the clay blended polymers the characteristic peak at 2␪ = 10.29◦ appears at (1:1:1+Glu) and (1:2:1+Glu) with glutaraldehyde and the peak at 2␪ = 22.36◦ is diminishing. In general, the diffraction pattern changes from sharp peaks to broader one and this may be due to the deformation of the strong hydrogen bond in original chitosan. This might have happened because of the substitution of amino and hydroxyl groups by glutaraldehyde. This

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Fig. 3. TG/DTG of ternary blends of (a) 1:1:1+Glu, (b) 1:2:1+Glu, (c) 2:1:1+Glu.

indicates that chitosan is modified and the structure becomes more amorphous than the crystalline structure of chitosan. 2.6. Thermo gravimetric analysis (TGA) The degradation profile of the blends containing (1:1+Glu) and (2:1+Glu) possess the events that follows the pure chitosan pure clay. When chitosan ratio is increased to (2:1+Glu) from (1:1+Glu) the de-methoxlation/degradation temperature gets much lowered. This means that thermal stability is lost when biodegradable polymer content is increased. On the other hand chitosan with clay particles chitosan films are found to degrade at 286–297 ◦ C. When chitosan content is increased initial decomposition is affected (from 175 ◦ C to 150 C). When the content of chitosan is increased from (1:2+Glu) to (2:1+Glu) the first decomposition temperature of the first peak gets lowered. At the second stage of decomposition the temperature is shifted to higher values where chitosan content has increased from 1:1:1+Glu to 2:1:1+Glu at 450–500 ◦ C to 450–630 ◦ C. The third stage of decomposition for 1:1:1+Glu occurs at 450 to 600 ◦ C whereas at 2:1:1+Glu occurs at

690 to 750 ◦ C in 1:1:1+Gl where there is increase in weight instead loss of weight during decomposition and this may be due to the intake of oxygen while the product gets oxidized. Whereas in the case of (2:1:1+Glu) there is no increase in weights during decomposition and this suggests that chitosan stabilizes and prevents oxidation even at higher temperature 650–750 ◦ C. H values were found to decrease as it moves from 1:1:1+Glu to 1:2:1+Glu and 2:1:1+Glu. Between1:2:1+Glu and 2:1:1+Glu increase of H values was observed. As chitosan content is increased H values were found to have the increasing trend and this is understandable from the fact that chitosan helps in crystallization and this adsorbs more energy, whereas when clay content is increased enthalpy decrease is more and this can be understood from the interaction of clay with chitosan as the surface area ratio of clay to chitosan (Fig. 3). 2.7. Differential Scanning Calorimetry (DSC) In the DSC for pure chitosan and nylon 6 glass transition temperatures are not observed. From the Fig. 4 it is seen that initial endothermic peak is shifted to higher temperature from 150 to

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203

Fig. 4. DSC of ternary blends of (a) 1:1:1+Glu, (b) 1:2:1+Glu, (c) 2:1:1+Glu.

200 ◦ C in the case of additional clay added to the blend (1:2:1+ Glu) whereas in the case of 2:1:1+Glu which is shown in the top of the Fig. 4, it is again reverted to 150 ◦ C with a hump. This shows that the melting point of chitosan changes when clay is added as impurity and depression of melting point occurs. The same is increased to 210 ◦ C when the clay is increased in the blend which is indicated in the middle of the (Fig. 4). Thermal degradation of pure chitosan is around 215 ◦ C. The temperature of endothermic peak is shifted towards lower temperature with increasing chitosan content. Two endothermic peaks are obtained for all films. The first endothermic peak that occurs in the temperature range of 86.8 ◦ C may be attributed to solvent evaporation which in the present case is higher due to the blend with clay and this occurs around 150 ◦ C (26–28). The peaks in the range of 230.52 ◦ C and −270 ◦ C are due to crystallization of the chitosan. This process is not inhibited by clay particles. This is evident from the appearance of the peak and there is no shift in the peak position as the content of clay is increased. From the Fig. 4 it is seen that there are two endothermic peaks and one exothermic peak except in the case of (1:2:1+Glu) the main endothermic peak in three figures occurs more or less the same except at (1:2:1+Glu) where it is lower.

2.8. Scanning Electron Microscope (SEM) analysis It is seen from the surface morphology is found to be nearly smooth whereas the cross sectional morphology was porous and channel type. Porous nature and channel type have resulted from the blend of chitosan with montmorillonite clay and nylon 6 [39]. A configuration is clearly seen from the Fig. 5 this type of structure will help in diffusion of ions faster compared to pure chitosan with nylon 6 and chitosan with montmorillonite clay. In this polymeric of ternary blend, the separation of heavy metals will be maximum. This has happened because of the blend architecture of porous nature with channel type which facilitates the faster diffusion of heavy metal ions. 2.9. Adsorption experiments 2.9.1. Percentage of metal removal as a function of time Percentage removal of copper and cadmium using chitosan, montmorillonite clay and nylon 6 at different time intervals is depicted in Fig. 5.7. It can be seen that the Fig. 6 copper had better recoveries when to compared cadmium. But since the recoveries were parallel it can

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copper cadmium

90 80 70 60 50 40 30

copper cadmium

85

80

75

70

65

60

+2

+2

+2

% of Cu & Cd from industrial wastewater

100

% of Cu & Cd recovery by chitosan,nylon 6 and clay

Fig. 5. SEM images of ternary blends with (a) Surface morphology (1:2:1+Glu), (b) Cross sectional morphology (1:2:1+Glu).

20

55

+2

10 0

50

100

150

200

250

300

350

400

T im e in m in u te s % of C u

+2

& Cd

+2

removed from industrial wastewater as function of time

Fig. 6. percentage of copper and cadmium removed from wastewater as a function of time.

be concluded that Cu+2 and Cd+2 complexes were stable. This may be due to the fact that Palling electro negativity values are closer (Cu+2 = 1.9 and Cd+2 = 1.69). Also the radius of hydrated ion values are almost same, (Rehydrated ion of Cu+2 = 2.07A´˚ and Rehydrated ´˚ The results indicate that the percentage copper ion of Cd+2 = 2.28 A). removal increases from 18.00, 35.00, 40.00 and 90.00 during 30, 60, 90 and 270 min of shaking respectively with ternary blend. Also, it can be seen that the percentage removal of copper remains constant (91.00%), which shows that equilibrium is reached at 300 min. However, in the case of cadmium the percentage removal is increased to 84.00% at 270 min. Also, it can be noted that from 300 min to 360 min the percentage removal of cadmium remained constant (84.00%) indicating that 300 min of contact time is enough for the maximum removal of cadmium from wastewater. 2.10. Percentage of metal removal as a function of pH It is seen from the Fig. 7 the maximum recoveries of copper and cadmium lies in the pH range 3 to 6. This is understandable from the fact that in these ranges the stable species is Cu2+ or Cd2+ also it can be seen from reported by [40,41]. The pH of aqueous solutions and wastewater effluents had a great effect on the recovery as reported by [42–44]. It is found from the graph that the removal of heavy metal ions was pH dependent. Each result presents a characteristic

50 4

5

6

7

8

pH

Fig. 7. percentage of copper and cadmium recovery by chitosan, montmorillonite clay and nylon 6 with glutaraldehyde ternay blend at different pH.

variation of pH with amount adsorbed depending on the adsorbent type, metal ion and/or initial concentration of metal ions. In the present investigation when the adsorption study was conducted in the pH range of 3 to 8, it was observed that the percentage removal of copper increased with an increase in the pH from pH 3 to 5, after which the recovery was reduced, which is due to the fact of hydrolysis of ions. In the case of cadmium containing wastewater the maximum removal of metal occurred at pH 6, after which there was a regular decrease in the percentage removal of the metal. The effect of pH on metal ion adsorption by chitosan and biopolymers can be explained by understanding the potential of zero charge (PZC), which is for chitosan 6.7. At low pH values adsorption is low where surfaces have strong positive charge similar to that of the ions. However, adsorption takes place at low pH values which may be due to limited contribution of chemical adsorption that is caused by the unpaired electrons of nitrogen at acetamido and amino functional groups of chitosan. Also, maximum adsorption occurs near to potential of Zero charge (PZC) in the pH range between 5 to 7 Cu+2 and Cd2+ absorption. It is seen from the above Fig. 7 that cadmium and copper recoveries are maximum in pH range 6 and 5 whereas it decreases after that. The decrease in adsorption is due to the formation of the metal hydroxyl complex for both copper and cadmium.

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205 Copper Cadmium

coppe r cadmium

100

2 .0

+2

1 .5

80

log qe/(qe-q)

+2

% Cu & Cd removal

90

70

60

1 .0

0 .5

50 0 .0

40

0

1.0 gms

2.0 gms

3.0 gms

4.0 gms

5.0 gms

50

100

6.0 gms

Dosage g/10 0ml Fig. 8. the percentages of copper and cadmium removal using chitosan, montmorillonite clay, nylon 6 and glutaraldehyde with different adsorbent dosage.

150

200

250

300

Time in minutes

Fig. 9. Copper and cadmium separation using chitosan, montmorillonite clay and nylon 6 blends.

Copper Cadmium

4.5

2.11. The percentage of copper and cadmium removal using chitosan, montmorillonite clay, nylon 6 and glutaraldehyde with different adsorbent dosage

4.0

3.5

t

3.0

t/q

Various operational parameters are employed to determine adsorption characteristics of chitosan biopolymers for heavy metal ions adsorption from aqueous solutions. These parameters are solid/liquid blends, pH, contact time, metal ion concentration and temperature. The effect of solid/liquid blends for Cu2+ , Cd2+ ions adsorption on chitosan biopolymer was examined. In these experiments operational parameters were kept constant T = 298 K Co = 100 mg/g pH was varied initially for various times. Fig. 8 indicates that percentage of adsorption of copper and cadmium increases steadily with, solid to liquid blends of 2 to 5 g whereas at 5 to 7 g there is saturation. Increasing solid to liquid blends for chitosan biopolymers increases the number of active sites available for adsorption.

2.5

2.0

1.5 0

50

100

150

200

250

300

350

400

Time in minutes

Fig. 10. Copper and cadmium separation using chitosan, montmorillonite clay and nylon 6 blend.

copper cadmium

2.12. Kinetics of adsorption 200

(22)

Where q is the amount of metal adsorbed at any time mg/g qe is the amount of metal adsorbed at equilibrium time mg/g k1 Pseudo first order rate constant (min−1 ) Integrating equation with respect to boundary conditions at and at t = t then the equation becomes In

 q  e qe − q

= k1 T

(23)

−1 Thusthe rate  constant k1 (min ) can be calculated from the plot qe of In qe −q versus time From the Fig. 9 for copper the slope is 0.00747 and k1 value is calculated and the value is found to be 0.01720 min−1 and this is the value of k1 at 298 K. For cadmium from the Fig. 9 the slope is 0.00712 and k1 value is calculated to be 0.01539 min−1 and this is or the value of k1 at 298 K.

1

dq = k1 (qe − q) dt

180

q quanity of metal adsorbed,mg/l

2.12.1. Pseudo first order kinetics for copper and cadmium The pseudo first order kinetic model has been widely used to predict the metal adsorption kinetics. The metal adsorption kinetics following the pseudo first order model.

160 140 120 100

2

R = 0 .9 9 7 4 2 80 60 40 20 4

6

8

10

12

t

14

16

18

20

0 .5

Fig. 11. Plot of q versus square root of time for the amount of copper and cadmium removed.

Table II Freundlich isotherm for copper and cadmium. Metal

Kf

1/n

Copper Cadmium

1.4609 2.0290

0.8137 0.6382

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2.13. Pseudo second order kinetics The adsorption kinetic data can be further analyzed using Ho’s pseudo second order kinetics. 1 t t = + q qe k2 q2e

(24)

2.14. Where q is the amount of metal adsorbed at any time mg/g And qe is the amount of metal adsorbed at equilibrium time mg/g k2 is pseudo Second order rate constant 9 g/mg min Separating the variable and integrating the equation for the boundary conditions at to and gives A plot between versus t gives the value of the constants (g/mg h) and qe (mg/g) can be calculated. The constant is used to calculate the initial adsorption rate h, at t→0, as follows:

From the plot Fig. 10 of t/q versus time for the pseudo second order kinetics the value of k2 is obtained. So initial absorption rate of copper is h = 88.43 mg/g and pseudo second order value of the rate constant k2 is 0.004421 g/mg h. Similarly Fig. 10 depicts the variation of t/q versus time and from this the initial absorption rate of cadmium is of pseudo second order value and the rate constant is 0.003343 g/mg. and the value of h = 59.52 mg/g which is the initial absorption rate of cadmium. The experimental data obtained from varying the contact time was used to study the type of mechanism adopted by the adsorption process. Pseudo-first-order and pseudo-second-order kinetic models were used in this study. The results showed that the adsorption process follows pseudo-second-order kinetics, which indicates that the concentration of both the metal ion and adsorbent were participating in the rate determining step. On comparing the R2 values obtained from pseudo-second-order kinetics model, the values

Copper 2 R = 0.9858 2.0

1.8

1.8

1.6

1.6

1.4

log qe

1.4

log qe

Cadmium 2 R = 0.9982

2.0

1.2

1.2 1.0

1.0

0.8 0.8

0.6 0.6 0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

0.4 1.0

2.4

1.2

1.4

1.6

1.8

2.0

2.2

2.4

log C e

log C e

a

b

Fig. 12. Freundlich isotherm studies for (a) copper and (b) cadmium.

cadmium

copper 0.43

0.27

0.42

0.26

Y =-0.15075+0.0193 X

Y =-0.05052+0.01503 X 0.41

0.25

0.23 0.22

2

0.40

2

R =0.9997

ce/qe

ce/qe

0.24

R =0.9998

0.39 0.38

0.21

0.37

0.20

0.36 0.19

0.35 0.18 16

17

18

19

20

21

26

27

28

ce

A

ce

B

Fig. 13. Langmuir isotherm studies for (a) copper and (b) cadmium.

29

30

N. Prakash, S. Arungalai Vendan / International Journal of Biological Macromolecules 83 (2016) 198–208 Table III Langmuir isotherm for copper and cadmium. Metal ions Cu(II) Cd(II)

Langmuir constants KL (dm3 /g)

Cmax (mg/g)

R2

0.2974 0.1280

88.43 59.52

0.9997 0.9998

0.95208 for Cu2+ and 0.93209 for Cd2+ showing that the linearity is more for copper than cadmium. 3. Intra particle diffusion The intra particle diffusion coefficient for the adsorption of heavy metal ions in chitosan bipolymers is calculated from the slope of the plot between qt versus t0.5 . Fig. 11 depicts the amount of cadmium and copper removal versus square root of time. Fig. 11 shows multi linearity. This is true for both copper and cadmium. This suggests that there are two or more steps involved in the adsorption process [45–47]. The two curves for copper and cadmium show that there is an initial linear portion ended with a smooth curve. The second curve may be considered as another linear portion. Thus the two portions suggest clearly that there are two mechanisms operating and they may be intra particle diffusion and surface adsorption. The initial curved portion of the plot indicates boundary layer effect whereas the second linear portion suggests intra particle diffusion or pore diffusion. The slope of second linear portion of the plot is characterized as the intra particle diffusion parameter K1 (mg/l) the slope is zero in the present case as it is almost parallel to X axis as it is evidenced from Fig. 11. The slope for copper and cadmium for the first region is 18.00 and 35.00. They are almost parallel and the slope of copper is slightly higher compared to cadmium. The intercept of the plot reflects the boundary layer effect. The intercept is larger in both the cases. This suggests that there is greater contribution and this is due to surface adsorption in the rate determining step. 3.1. Adsorption isotherm models The analysis and design of the adsorption process requires the relevant adsorption equilibrium. Adsorption equilibrium provides fundamental physiochemical data for evaluating the applicability of the process as a unit operation. The data obtained are now analyzed using the Freundlich and Langmuir isotherms. 3.2. Freundlich isotherm The logarithmic form of Freundlich model is given by the following equation log qe = log KF +

1 n log ce

(25)

207

Where qe is the amount adsorbed per unit weight of adsorbent at equilibrium (mg/g) And ce is the equilibrium concentration of adsorbate in solution after absorption (mg/g) KF is empirical Freundlich constant or capacity factor (mg/g) and 1n is Freundlich exponent (Table II). The exponent 1/n is less than 1 in these cases. Sorbates are bound with weaker and weaker free energies. Between the two copper and cadmium, 1/n value for copper is higher and is approaching 1 much faster indicates that the copper adsorption is better in chitosan with montmorillonite clay and nylon 6 than cadmium. There is a no limit to the amount adsorbed and there is multilayer adsorption as this is indicated by fitting the Freundlich isotherm. This is provided in Fig. 12. 3.3. Langmuir isotherm Langmuir isotherm in its linear form is given by the equation Ce Ce 1 = + qe Q0 bQ0

(26)

3.4. Where Q0 represents the total number of surface sites per mass of adsorbent The Langmuir isotherm parameters are obtained by fitting the data of adsorption by the elemental concentration in solid and liquid are equilibrium at the end of the experiments (4–7 h). The q0 represents the total number of surface sites per mass of adsorbent. A plot of Cqee versus ce will yield Q0 from the slope and from the intercept KL can be obtained. This is provided in Fig. 13. From the Table III q0 for copper and cadmium are calculated and they are 88.43 and 59.52 and this reflects that the rate of adsorption for copper is faster than cadmium as there are more sites for copper adsorption than for cadmium. KL for copper is 0.01403/0.04042 = 0.2974 and for cadmium = 0.0193/ 0.14074 = 0.1280. This Langmuir constant indicates that the process of adsorption of copper and cadmium takes place in a similar fashion. This also indicates that the equilibrium constant for cadmium and copper are more or less the same for the adsorption reaction and adsorbate affinity of the polymer is the same for both copper and cadmium and depending on the adsorbable sites available for the extraction of copper and cadmium will take place. Table IV discuses with Comparison of the adsorption studies on the Langmuir and Freundlich isotherm parameters for various composites. The results obtained in this study were compared with the reports of the available literatures for techno-economical evaluations. The comparison of various governing and significant factors involved in adsorption study for different polymer composites are comprehensively presented in table. It is clearly observed that R2 is high for chitson/silk/starch that is being used in the present study. From this, it may be concluded that the adsorption effectiveness is greater for the proposed composite.

Table IV Comparison of the adsorption studies on the Langmuir and Freundlich isotherm parameters for various composites. Metal ions Cu (II) Cd (II) Metal ions[48] Cu (II) Ni (II) Metal ions[49] Cr (VI)

Langmuir constants KL (dm3 /g) 0.2974 0.1284 Langmuir constants KL (dm3 /g) 2.482 2.904 Langmuir constants KL (dm3 /g) 1.0630

b (dm3 /mg)

Cmax (mg/g)

R2

– –

88.43 59.52

0.9997 0.9998

b (dm3 /mg) 0.007550 0.00971

Cmax (mg/g) 328.74 299.35

R2 – –

b (dm3 /mg) 0.004017

Cmax (mg/g) 443.71

R2 0.8716

Freundlich constants KF n 1.4009 0.8137 2.0290 0.6382 Freundlich constants KF n 0.8173 1.3103 0.9764 1.4716 Freundlich constants KF n 1.0630 1.2681

R2 0.9858 0.9982 R2 0.9754 0.9829 R2 0.9576

208

N. Prakash, S. Arungalai Vendan / International Journal of Biological Macromolecules 83 (2016) 198–208

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