Polymer Composite — A Potential Biomaterial for the Removal of Reactive Dye

Poly Pyrrle saw dust composite was prepared by reinforcement of natural wood saw dust (obtained from Euphorbia Tirucalli L wood) and Poly Pyrrole matrix phase. The present study investigates the adsorption behaviour of Poly Pyrrole Saw dust Composite towards reactive dye. The batch adsorption studies were carried out by varying solution pH, initial dye concentration, contact time and temperature. The kinetic study showed that adsorption of Reactive Red by PPC was best represented by pseudo-second order kinetics with ion exchange adsorption. The equilibrium data were analyzed by Freundlich and Langmuir isotherm model. The equilibrium isotherm data were fitted well with Langmuir isotherm model. The maximum monolayer adsorption capacities calculated by Langmuir model were 204.08 mg/g for Reactive Red at 303 K. The thermodynamic parameters suggest the spontaneous, endothermic nature of ion exchange adsorption with weak Vader walls force of attraction. Activation energy for the adsorption of Reactive by Poly Pyrrole Composite was 11.6387 kJ/mole, Isosteric Heat of adsorption was 48.5454 kJ/mole also supported the ion exchange adsorption process in which forces of attraction between dye molecules and PPC is weak.


Introduction
In textile industry, the modern dyeing processes and machinery are designed for the usage of synthetic dyes.Dye containing wastewater discharged from such industries is a serious threat to the receiving water bodies around the industrial areas 1 because these toxic organic dyes can affect plant life and thus destroy the entire ecosystem 2 .Reactive dyes are extensively used in many textile industries due to their favourable characteristics, and have been identified as problematic compounds in textile wastewater because they are water soluble and found in the waste water at higher concentrations than other dye classes mainly in their spent, hydrolyzed form [3][4][5][6] .It is estimated that 10-20% of reactive dyes remain in the waste water during production and nearly 50% of reactive dyes may be lost to the effluents during dyeing process, and their removal from effluent is difficult by conventional physical / chemical as well as biological treatment 7 .Therefore, removal of dyes from effluent becomes environmentally important and many treatment methods have been applied such as physical, chemical and biological methods [8][9][10] .However these methods have one or more limitations such as low efficiency, high cost, greater energy consumption and generation of sludge.To overcome these problems, there is an urgent requirement for the development of innovative, economic and effective technology, by which dye from the effluent can be removed.A great interest has been recently directed to polymer bio composites for the removal of dyes from waste water.Polymer composites or Green composites are a viable alternative for the exiting waste water treatment technologies.Polymer composites are materials formed by matrix (polymer) and a reinforcement of natural wood fibre derived from renewable resources.These are environmentally friendly, cheap and bio renewable material.Polymer composites would contribute to solve the water pollution problem around the world and could be the efficient and promising adsorbent for the removal of synthetic dyes from aqueous solution.One efficient way of increasing adsorption capacity of saw dust is the polymerization of monomer on the surface of saw dust.In recent years, conducting electro active polymers such as poly aniline and poly pyrrole have attracted with great attention due to their electrical conductivity and electro activity 11- 13 .Poly Pyrrole, a conducting polymer doped with releasable or exchangeable counter ions coated on saw dust has been utilized for the removal of anionic dyes from aqueous solutions based on ion exchange properties of these polymers.The dye removal technology utilizing bio composite is a viable option because of its economic, eco-friendly, abundantly available, green way and efficient technique.
In this study, Poly Pyrrole-Saw dust composite prepared by polymerizing pyrrole on saw dust surface via chemical route at room temperature.The main objective of this study is to evaluate the potential of polymer composite (PPC) prepared from Euphorbia Tirucalli wood saw dust.The study includes an evaluation of the effects of various operational parameters such as initial dye concentration, contact time, pH and temperature on the dye removal process.The adsorption kinetic models, equilibrium isotherm models and thermodynamic parameters related to adsorption process were also performed and reported.

Adsorbent Preparation of Poly Pyrrole Composites
Euphorbia Tirucalli L wood saw dust used for the preparation of polymer composite.The saw dust was first washed with distilled water in order to remove impurities and finally dried at 333 K for 2 hours.The poly pyrrole was synthesized on saw dust surface, which was previously soaked in monomer Pyrrole solution (0.2 M) for 12 hour at room temperature followed by slow addition of chemical oxidants 0.5 M FeCl 3 at room temperature for 4 hours 14 .Polymerisation was carried out on the surface of saw dust immediately after the addition of oxidant solution.The polymer coated saw dust designated as PAC were filtered, washed with distilled water and dried.

Character Studies
Physico-chemical characteristics of Poly Pyyrole composite were studied as per the standard testing methods 15,16 .In order to characterize the surface structure and morphology of Poly Pyrrole saw dust composite, SEM analysis was carried out using Scanning Electron Microscope as shown in Figure 1.A stock solution of Reactive Red 195 was prepared by dissolving appropriate amount of dye (based on the percentage purity) and suitably diluted as and when required.The concentration of the dye was determined using Elico make UV-Vis spectrophotometer at wavelength 543 nm.All chemicals used were analytical reagent grades and used without further purification.

Batch Adsorption Experiments
Adsorption experiments were conducted at room temperature by agitating 0.10 gm of adsorbent with 100 ml of dye solution of desired concentration in 250 ml stoppered flask using a shaker at a speed of 120 rpm for equilibrium time except for contact time experiments.The effect of solution pH on the equilibrium adsorption of dyes (50 mg/L) was investigated under similar experimental conditions between pH 2 to 12.The effect of temperature on the equilibrium adsorption was studied under similar conditions between temperatures of 303 to 318 K.

Equilibrium Isotherm Studies
Equilibrium studies were conducted by agitating 100 ml of dye solution with 0.1 gm of adsorbent at different initial dye concentration (10-170 mg/lit) upto equilibrium time.After equilibrium, the solution was analysed for remaining dye concentration using Elico make UV-Vis spectrophotometer at appropriate wavelengths.The equilibrium experiments were conducted at different temperatures.

Analysis of Adsorbent Characteristics
The Physico-chemical characteristics of PPC prepared from Euphorbia Tirucalli L wood was summerised in Table 1.

Analysis of Adsorption Parameters Effect of initial dye concentration and contact time
The rate of adsorption is a function of the initial dye concentration and contact time which is an important factor for the effective adsorption.The Figure .3depicts the adsorptions of RR195 by PPC at various initial dye concentrations with contact time.When the initial dye concentration increased from 25 to 100 mg/L, the adsorption capacity of PPC increases from 46.88 to 157.33 mg/g, but the percentage of dye adsorption decreases from 93.75 to 78.67%.The percentage removal of dyes decreases when the initial concentration increases from 25 to 100 mg/L for PPC because the adsorbent has a limited number of active sites which becomes saturated at a certain concentration.Similar results have been observed for the removal of Eosin Y using conducting electro polymers 14 , adsorption of Reactive Orange on loofa activated carbon 17 .The equilibrium adsorption increased with rise in contact time and equilibrium reached at 60 minutes for RR195.Initially the rate of adsorption was rapid, slow and reaches equilibrium, further increase in contact time did not enhance the adsorption.The initial rapid adsorption may be attributed to the presence of large number of available binding sites for adsorption and slower adsorption is due to saturation of binding sites and equilibrium attained.

Effect of pH
The pH of the dye solution has been recognized as one of the most important factors influencing the adsorption process.The effect of initial pH on the dye removal efficiency of PPC was studied at different pH ranging from 2-12 and shown in Fig. 4. The adsorption of RR195 was pH dependent.The maximum percentage removal of RR195 occurs at acidic pH 3-4 and adsorption decreases with increase in pH.In acidic medium, the surface of the adsorbent is positively charged due to higher concentration of H + ions, so the electrostatic attraction between PPC and RR195, is enhanced.But in alkaline conditions, electrostatic repulsion occurs resulting in decreased adsorption.Similar results have been reported for the adsorption of Eosin Y using conducting electro polymers 14 .

Effect of Temperature
The effect of temperature on dye adsorption was studied at 30, 35, 40 and 45º C. The results indicated that the amount of dye adsorbed at equilibrium increases with increasing temperature.This may be a result of increase in the mobility of the dye molecules with increase in temperature 18 .The equilibrium adsorption increased from 88.37% to 95.35% for RR195, indicates that the adsorption is endothermic process.

Adsorption Kinetics
In order to investigate the mechanism of adsorption, characteristic constants of adsorption were determined using Pseudo-first order equation 19 of Largergren based on solid capacity and Pseudo-second order equation 20 based on solid phase adsorption.Pseudo-first order Lagergren kinetic equation can be expressed as follows ) ( The integrated form of equation The dye adsorption described by a modified second order equation is expressed as The values of second order rate constant k 2 and qe were calculated from the intercepts and slopes of the plot of t/q t vs t as shown in Figure .5 and the results are summarized in table 2. The value of k 2 decreases with increase in dye concentration due to decrease in available vacant sites for adsorption.The values of r 2 suggested that pseudo first order equation does not fit well with whole range of adsorption process, as it is applicable for the initial stages of adsorption processes 21 .Based on the values of correlation co-efficient which is above 0.98, the second order kinetic model was more suitable to describe the adsorption process for reactive dye adsorption than pseudo-first order model.But to identify the mechanism of adsorption, the kinetic results were further analysed by ion exchange process.

Ion Exchange Process
In case of polymer saw dust composite the increase in adsorption capacity is due to its ion exchangeable active sites.Poly Pyrrole has positively fixed charged sites which are balanced with the anion.The small size dopant anions can be exchanged with other anionic species in the dye solutions which have stronger interactions with the polymer 14 .Therefore, higher percentage removal of anionic dyes by the polymer saw dust composite is supposed to occur by ion-exchange mechanism due to exchangeable active sites and explained by pseudo-second order kinetics.However, the importance of other processes such as electrostatic interactions between highly polar and positively charged polymer and negatively charged anionic dye molecules cannot be ignored.

Adsorption Isotherm
The equilibrium adsorption isotherm is fundamental in describing the interactive behavior between adsorbate and adsorbent.It is important for predicting the adsorption capacity of adsorbent, which is the main parameters required for design of an adsorption system.The Langmuir 22 and Freundlich 23 models were used to describe the adsorption of RR onto PPC.
The Langmuir equation can be written as Where, C e is the equilibrium concentration (mg/L), q e is the amount of dye adsorbed at equilibrium (mg/g) and Q 0 (mg/g) and b L (L/mg) are Langmuir constants related to adsorption capacity and energy of adsorption respectively.This equation has been successfully applied to many adsorption processes.The Langmuir isotherm is based on assumption of structurally homogeneous adsorbent and monolayer coverage with no interaction between the sorbate molecules.Once a dye molecule occupies a site, no further adsorption can take place at that site 24 .The values of Q 0 and b L calculated from the slopes and intercepts of the linear plots of Ce/qe vs Ce as shown in Figure 6 and the results are summarized in table 3. The values of adsorption efficiency Q 0 and adsorption energy b L increases with increasing the temperature suggested that the maximum adsorption corresponds to a saturated monolayer of dye molecules on all the adsorbents.Further it confirms the endothermic nature of processes involved in the system 25 .The maximum adsorption capacity Q 0 varies from 204.08 to 222.22 mg/g for RR while increasing the temperature from 30°C to 45°C.The essential characteristics of Langmuir isotherm can be expressed by a dimensionless constant called equilibrium parameter R L 26 that is defined by the following equation Where, Co is the highest initial solute concentration.R L value indicated the type of adsorption isotherm to be either unfavourable (R L >1), favourable (R L <1), linear (R L =1) or irreversible (R L =0).Langmuir model is more appropriate to explain the nature of adsorption of RR with correlation coefficient of 0.994 to 0.9979.The Freundlich model is employed to describe the heterogeneous system, which is characterized by heterogeneity factor 1/n.It considers multilayer adsorption with heterogeneous energetic distribution of active sites accompanied by interactions between adsorbed molecules.The Freundlich isotherm is expressed as Where, K f and 1/n are Freundlich constants related to the adsorption capacity and adsorption intensity of the adsorbent respectively.Q e is the amount of dye adsorbed per unit mass of adsorbent (mg/g); ce is the equilibrium concentration of adsorbate (mg/L).The values of K f and 1/n are calculated from intercept and slopes of linear plot of log q e versus log c e (figure not shown).The value of 1/n is below one for RR studied indicating that the adsorption of dyes is favourable.The results of isotherms are summarized in Table .3.
An analysis of the correlation coefficients obtained for these isotherms showed that both isotherm equations describe the adsorption, but Langmuir model was found to be more appropriate to explain the adsorption of RR onto PPC.

Activation Energy
The second order rate constant of the dye adsorption is expressed as a function of temperature by Arrhenius relationship.(7)   where Ea and A refers to Arrhenius activation energy and Arrhenius factor obtained from the slope and intercepts of a graph by plotting lnk 2 vs 1/T as shown in Fig 7 .The activation energy was found to be 11.6387 kJ/mole for adsorption of RR onto PPC.The physisorption usually have energies in the range of 5-40 kJ/mole, while higher activation energies 40-800 kJ/mole suggests chemisorption 27 .The activation energy < 40 KJ/mole for RR dye indicates the physisorption.In adsorption process, energy consideration must be taken into account in order to determine what process will occur spontaneously.Thermodynamic parameters values are the actual indicators for practical application of a process 28 .

RT Ea
The thermodynamic parameters such as change in standard free energy (ΔG 0 ), enthalpy (ΔH 0 ) and entropy (ΔS 0 ) can be determined by using VantHoff equation ΔG 0 = ΔH 0 -TΔS 0 … (9) Where, T is the absolute temperature and K L (L/g) is the standard thermodynamic equilibrium constant.By plotting a graph of ln K L vs 1/T, the values of ΔS 0 and ΔH 0 can be estimated from the slopes and intercepts (figure not shown).The results are summarized in Table 4.The Gibbs free energy change (ΔG 0 ) is an indication of spontaneity of a chemical reaction therefore it is an important criterion for spontaneity.The change in free energy for physisorption is between -20 and 0 kJ/mole, but chemisorption is in a range of -80 to -400 kJ/mole [29][30] .The values of ΔG 0 for all the three dyes were with in the range of -20 and 0 kJ/mole, indicating that the physisorption is the dominating mechanism.The positive values of ΔS 0 confirming physical adsorption nature and increased randomness at the solid-solution interface during adsorption and indicate affinity of the dye onto adsorbents 31 .Physisorption and chemisorptions can be classified to a certain extent by the magnitude of enthalpy change.Bonding strengths of <84 kJ/mole are typically considered as those of physisorption bonds.Chemisorption bond strengths can be 84-420 kJ/mole 32 .

Isosteric Heat of Adsorption
Isosteric Heat of Adsorption ΔH X is defined as the heat of adsorption determined at constant amount of adsorbate adsorbed.It is the basic requirements for the characterization and optimization of an adsorption process and also very important for equipment and process design.The isosteric heat of adsorption at constant coverage is calculated using Clausius -Clapeyron equation 33

K RT
Where, Ce is the equilibrium dye concentration in solution (mg/L), ΔH X is the isosteric heat of adsorption (kJ/mole).The isosteric heat of adsorption is calculated from the slope of the plot lnCe versus 1/T (figure not shown) is given in table 4. The magnitude ΔH X of provides an information about the nature and mechanism of the process.For physical adsorption ΔH X should be below 80 kJ/mole and for chemical adsorption it ranges between 80-400 kJ/mole 34 .The value of ΔH X for the adsorption of RR onto PPC is 48.5454 kJ/mole which is within the range of physisorption and suggested that the adsorption process is physisorption.Therefore, the values of ΔG 0 , ΔH 0, ΔH X and Ea all suggest that adsorption of RR onto polymer coated saw dust was driven by ion exchange adsorption process involving weak Vander Walls force of attraction (as the adsorption energy is very less).

Conclusion
The present investigation showed that Poly Pyrrole -saw dust composites can be effectively and efficiently used for the removal of Reactive dyes from aqueous solution.The adsorption process was dependent on pH, maximum adsorption occurs at pH 3-4 for RR.The pseudosecond order kinetic model fitted well with the dynamical adsorption behavior of anionic dyes suggested ion exchange adsorption.The maximum monolayer adsorption capacities calculated by Langmuir model were 204.08 mg/g for Reactive Red at 303 K.The adsorption capacity of PPC increased with rise in temperature indicating endothermic nature of adsorption.The thermodynamic parameters such as enthalpy change, free energy change indicated the physisorption nature of adsorption process.Arrhenius activation energy calculated was 11.6387 kJ/mole on PPC and lower values of isosteric heat of adsorption also supported the forces of attraction between dye and Poly Pyrrole Composite was weak.
Lower bonding energy suggested that only weak bond is formed between dye and polymer composite.Based on the results obtained in this study, it can be concluded that Poly Pyrrole composite is an effective, economic and alternative biomaterial for the removal of Reactive dyes.

Figure 1 .
Figure 1.SEM image of PPC.Adsorbate Reactive Red195 (M.Wt:1136.31,Mol.Formula: C 31 H 19 ClN 7 O 19 S 6 Na 5 , λmax: 543 nm) used in this study were of commercial quality and used without further purification.The structure of Reactive Red 195 is shown in Figure.2.

Figure 3 .
Figure 3.Effect of Initial Dye concentration and contact time for the adsorption of RR onto PPC.

Figure 4 .
Figure 4. Effect of pH for Adsorption of Reactive Red onto PPC.

Figure 5 .
Figure 5. Pseudo-Second Order plot for Adsorption of RR onto PPC.

Figure 6 .
Figure 6.Langmuir Isotherm for Adsorption of RR onto PPC.

Table 2 .
Kinetic Model values for adsorption of Reactive Red.

Table 3 .
Comparison of the coefficients of Isotherm parameters of Reactive Red.

Table 4 .
Thermodynamic Parameters at different Temperatures.