Investigation on Synthesis and Applications of Novel Polymeric Composites for the Removal of Heavy Metal Ion

Conducting polymers were synthesized by doping polyaniline chloride with Schiff bases such as (E)-2-((4s-chlorobenzylidene)amino)-3-(3a,7a-dihydro-1H-indol-3-yl)propanoic acid (TP4ClB) and (E)-3-(3a,7a-dihydro-1H-indol-3-yl)-2-((2-hydroxy-3,5-diiodobenzylidene)amino) propanoic acid (TPDISA). A chemical oxidation method using potassium persulphate as an oxidising agent was followed for the preparation of polymers. The prepared polymers were characterized by UVVisible, FTIR and H1NMR spectroscopic techniques. The adsorption characteristics of Polymeric composites for the removal of mercuric ion was studied through batch process. Sorption isotherms were constructed using Langmuir, Freundlich, Dubinin Radush Kevich, Temkin and Redlich Peterson models. SEM micrograph, PXRD and EDX spectra were also recorded to confirm the metal ion adsorption on the surface of investigated composites. keywords: Adsorption, Polymeric composite, Mercuric chloride, Tryptophan, Schiff bases.


INTROdUCTION
Environmental contamination due to minerals and heavy metals is the major challenging problem in our country. The concentration of heavy metals due to industrial and agricultural waste materials have been increased to the levels of high risk [1][2][3][4] . Among the various metals like lead, mercury, chromium, arsenic, nickel, uranium, selenium, silver, cadmium, zinc and gold present in industrial waste water, discharge of Hg (II) ions cause severe environmental contamination as it is non-degradable and bio-accumulate in nature [5][6][7][8] . Fertilizers industry, power generation plants, paper manufacturing, rubber processing and oil refineries are found to be the great sources for the emission of mercury into the universe. Health issues such as dialysis and problem in central nervous system occur due to circulation of mercury in the blood and storage in brain, liver, spleen, kidney and bone. Hence it is right time to carryout the research on the removal of mercuric ion from the environment. Though the number of methods such as reverse osmosis, floatation, adsorption, coagulation, flocculation, electrodialysis, chemical precipitation, nanofiltration, ion exchange, ultrafiltration, chemical precipitation and nanofiltration have been practiced for discarding the heavy metals, adsorption is an economical and more suitable one as it is simple to perform, easy to operate at low budget for the removal of toxic contaminants. Hence an attempt has been made to synthesize a novel organic polymer by doping polyaniline chloride with the potential Schiff bases. The synthesized organic polymers have been characterized by UV-Visible, FTIR and H 1 NMR spectroscopy. The adsorption characteristics of PANI-Cl composite for the removal of mercuric ion was studied through batch process [9][10][11] . Langmuir, Freundlich, Dubinin Radushkevich, Temkin and Redlich Peterson isotherms have been verified. Adsorption of the metal on the organic polymer has been confirmed by PXRD, SEM analysis and EDX spectra.

MATERIALS ANd METHOdS
The reagents and solvents were used as such(AR grade). Aniline (BDH) was distilled under reduced pressure before use. All the experiments were carried out using Double distilled water. Hydrochloric acid & sodium hydroxide solutions were used to study sorption behavior of the solutions (BDH) at different pH.

Preparation of polyaniline hydrochloride
Aniline (0.1M) was dissolved in aqueous solution of hydrochloric acid (1M). The mixture was stirred using a magnetic stirrer and 50 mL of potassium persulphate (0.1M) was added drop wise to the aniline-acid mixture for about two hours with continuous stirring at 303K. The contents were stirred further for 30 min to ensure complete polymerization. A dark green coloured PANI-Cl thus formed was filtered using a Whatmann No. 1 filter paper. The excess acid content and oligomers of aniline were removed by repeated washings with distilled water, ethanol, acetone and diethyl ether 11 . The green coloured polymer obtained was dried in an air oven for four hours till constant weight was reached.
The colour changes that appeared during the addition of potassium persulphate is given below.
Straw yellow → yellow → brown → green colour

Preparation of polymeric composite
PANI-Cl (1 g) and Schiff base (TP4ClB, TPDISA) (1 g) were mixed together and suspended in 15 mL of diethyl ether and sonicated in an ultra sonic cleaning bath (NEY, 50 Khz) for 30 minutes. The slurry obtained was centrifuged and the supernatant liquid was removed. The resulting product was washed with ether and centrifuged. The dark green coloured mass obtained was dried in a vacuum desiccator to get a free flowing powder.

Spectral analysis
UV-Visible spectra of the polymeric composites were recorded in DMSO using Lambda 35 Perkin Elmer UV-Visible spectrophotometer (200-1100nm). FTIR spectra were recorded as KBr pellets using a Perkin Elmer FTIR spectrometer in the range of 4000-400 cm -1 . H 1 NMR spectra were carried out in DMSO on a Bruker 400 MHz Nuclear Magnetic Resonance spectrometer. The Powder X-ray diffraction pattern of mercuric chloride after adsorption was performed using Bruker D8 X-ray diffractometer, Germany at a scan rate of 1 step/ second. The sample was irradiated with CuKα radiation and studied in the range 20-80 o (2q) with a step size of 0.001 o . The SEM and EDX spectra of pani matrix were studied using Field Emission Scanning Electron Microscope.

Sorption experiments
Polymeric composites (2 mg) were suspended in a solution of mercuric chloride (10 mL, 0.02M) and sonicated for half an hour at room temperature and centrifuged. Five mL of the supernatant liquid was pipetted out and titrated against KI solution by conductometric method 11,13 . The mercuric chloride concentration in the centrifugate was calculated from the plot of conductance Vs volume of KI. The sorption curves were constructed from the concentration of mercuric chloride after adsorption (C e ). The initial concentrations (C o ) were in the range of 0.01-0.05M.

Effect of temperature
The adsor ption of HgCl 2 solutions on Polymeric composites were studied at 5°C intervals ranging from 303-318 K to understand the temperature effect. Polymeric composites (2 mg) and HgCl 2 (10 mL, 0.02M) were sonicated for 30 min, after which the concentration of mercuric chloride was determined.

Effect of contact time
A known amount (2 mg) of adsorbent was sonicated with 10 mL of 0.02M of HgCl 2 solution at different equilibrium time (10-60 minutes). The optimum time required for maximum adsorption of HgCl 2 was found to be 30 minutes.

Effect of pH
The influence of pH on the adsorption capacity of polymeric composites were investigated in the pH range 1-13. The compounds (10 mg) were suspended in 2.5 mL of conductivity water. HgCl 2 (2.5 mL, 0.05M) and hydrochloric acid (10 -1 to 10 -6 M) were added and sonicated for 30 min to understand adsorption behaviour in low pH. Similarly, NaOH solutions (10 -1 to 10 -6 M) were used for the study at alkaline pH.

UV-Visible spectra
Electronic spectra of the composites exhibit two bands from 290 nm to 357 nm due to π-π* transitions in benzenoid rings and carbonyl present in carboxylic acid group of tryptophan moiety while polyaniline hydrochloride shows absorptions 11 at 310 and 583 nm. This shift in λ max value to lower wavelength confirms the doping of schiff bases on polymer. A small shift observed in both the compounds confirms the adsorption of mercuric ion.

FTIR spectra
The presence of absorption bands in the range 3500-4000 cm -1 may be ascribed to OH stretching frequency of -COOH group of tryptophan molecule in schiff base doped on PANI chloride. Appearance of bands at 1645-1656 cm -1 due to CH=N stretching vibrations of imines confirms the doping of schiff base on polymer. The composites exhibit peaks between 2312-2358 cm -1 due to the presence of iminium sites on PANI matrix. The vibrations around 1487-1514 cm -1 indicate the signature peaks of the PANI back bone arising due to the stretching modes of the imino benzenoid rings. The broad peaks in the region 1000-1100 cm -1 may be attributed to C-N stretching modes of primary amine. The bands around 940-990 cm -1 are generally assigned to bending mode of O-H bond. The bending vibrations of the PANI back bone are observed around 600-690 cm -1 . Both PTP4ClB and PTPDISA exhibit a band at about 3300-3400 cm -1 due to N-H stretching of amine present in schiff base. A peak at 597 cm -1 due to C-Cl and 555 due to C-I further supports the presence of schiff base on the composite. The peaks at 1224 cm -1 , 1261 cm -1 and 1004 cm -1 may be attributed to C-N stretching vibration.

H 1 NMR spectral analysis of polymeric composites
Proton H 1 NMR spectral data (Fig.1a, b) further supports the doping of imines on pani back bone. Signals at 10.1 & 9.98 ppm for both the compounds envisage the presence of carboxylic acid proton in indole derivative of adsorbents. The signals around 8.0 and 8.3 are due to imine protons of PTP4ClB and PTPDISA. Appearance of multiplets in the range 6.6-7.7 for both the composites may be attributed to NH & CH protons of the aromatic rings.

Langmuir Isotherm
This model is used to calculate the amount of Hg 2+ ions adsorbed (q m ) by Polymeric composites. From the plot of 1/q e vs 1/C e , it is observed that the process obeys the Langmuir model. The Langmuir constants q m and K L determined from the slope and intercept of the plot are presented in Table 3. 1/q e = 1/ K L q m C e +1/q m (1) In Langmuir equation (1) q m is the maximum monolayer adsorption capacity of the adsorbent (mg/g).
The essential characteristics of the Langmuir isotherm can be expressed by a dimensionless constant called the separation factor R L .
Where K L is Langmuir constant (mg g -1 ) and C 0 is initial concentration of adsorbate (mg g -1 ).
The R L values indicate that the adsorption process using the adsorbents under investigation is favourable (0<R L <1) 17 . q m value is greater (0.884 mg/g) for PTP4ClB composite with K L (11.06 L/g) and hence the Langmuir model is best fitted for PTP4ClB in comparison with PTPDISA.

Freundlich isotherm
The Freundlich adsorption isotherm gives the sorption equilibrium for both monolayer and multilayer adsorption and it is evidenced by equation (3).
log q e = log k F + 1/n log C e (3) Where n is related to the intensity of adsorption and k F denotes adsorption capacity. From the plot of log q e Vs log C e the Freundlich isotherm constant k F (sorption capacity) and 1/n (intensity of sorption) are calculated [17][18][19][20][21] . The correlation coefficients R 2 are shown in Table 3. The higher value of k F is found for PTPDISA (k F = 1.873) indicating multilayered formation.

dubinin Radush-kevich isotherm
The linear for m of DR isother m is represented by equation (4). A plot of lnqe Vs ε 2 is found to be a linear trace and the constants q D and B calculated from the slope and intercept are presented in Table 3. Dubinin Radush-kevich (DR) isotherm has been usually applied to know whether the adsorption of metal ion is physisorption or chemisorption. The isotherm constant B is relatively

Adsorption studies of mercuric chloride on PANI composites
Experiments on adsorption of mercuric ion by two new organic adsorbents were carried out by batch process. Concentration of mercuric ion in the filtrate after adsorption was measured conductometrically.

Effect of variation of mercuric ion
Langmuir, Freundlich, Temkin, Dubinin Radush-Kevich and Redlich-Peterson models are employed to evaluate the adsorption capacity and the surface properties of adsorbent related to adsorption 14-16 . higher for PTP4ClB (2.198). From this model it is found that physical nature of adsorption is more suitable for both the composites.

Temkin isotherm
This isotherm is used to understand the adsorbent-adsorbate interactions. The linearity of Temkin isotherm is represented by equation (5).
The Temkin isotherm equilibrium binding constant (A T ) and the Temkin heat of adsorption (B T ) are calculated using the slope and intercept of the plot q e Vs lnC e . Temkin heat of adsorption B T is found to be high for PTP4ClB. The higher values of binding constants are observed for PTP4ClB.

Redlich-Peterson isotherm
Redlich-Peterson isotherm can be applied for homogeneous or heterogeneous systems due to its versatility [17][18][19][20][21] . The mathematical relation of this isotherm is given by equation (6). C e /q e = 1/K RP + α RP /K RP C e β Where, K RP α RP and β are constants. A plot of C e /q e Vs C e is linear and the constants K RP and α RP are determined from the slope and intercept (Table 3). K RP value of 4.878 is found for PTPDISA (R 2 =0.993) indicating heterogeneous adsorption. The K RP value of 0.9115 for PTP4ClB suggesting homogeneous adsorption.

Effect of variation of Temperature
In order to ver ify the influence of temperature variation on the process of adsorption, the experiment was conducted at the temperatures between 303-318 K. The distribution coefficients (K d ) are calculated 22 using the equation (7).
Where q e -the amount of metal adsorbed at equilibrium and C e -equilibrium concentration of metal ions in solution.
The standard free energy change (ΔG o ) is calculated from Van't Hoff equation (8).
The slope and intercept values of the plot of lnKd Vs 1/T are used to determine the values of ΔH o and ΔS o .
Thermodynamic parameters 23,24 such as standard free energy of adsorption (ΔG o ), standard enthalpy of adsorption (ΔH o ) and the standard entropy of adsorption (ΔS o ) are very helpful in understanding the adsorbent-adsorbate system. The negative value of ΔG o indicates the feasibility of adsorption process ( Table 4). The exothermic nature of adsorption is understood from ΔH o values. The positive value of ΔS o for both the investigated polymers shows the spontaneity of the adsorption process 19 . It also shows the affinity of adsorbent towards adsorbate species.

Powder XRd study
The XRD patterns show several sharp peaks (Fig. 2a) in the region of 10 o -60 o (2q) with a Where λ = wavelength of CuKα radiation (1.54Å), β = full width at half maximum of peaks 25 . The Particle size varies from 40-160 nm for the composites. The sharp peaks in the region of 20-62 o illustrate the crystalline nature of polymeric composite (Figure 2a,b).

SEM analysis
The difference in morphology is observed in SEM picture of Polymeric composites before and after adsorption of HgCl 2 confirming the adsorption of metal ion (Fig. 3a, d). The particles are of small spherical in nature before adsorption while after adsorption the particles are larger, rod shaped and non-uniform in their size distribution. The embedded molecules of HgCl 2 on Pani backbone are clearly visible.

EdX analysis
Energy dispersive X-ray technique is used to find out the elemental composition and chemical characterisation of samples. The EDX spectra of the chosen composites present sharp peaks at 2-3 KeV strongly supporting the adsorption of mercuric ions on pani matrix (Figure 4a,b).

CONCLUSION
Two Schiff base (TP4ClB and TPDISA) doped polymer composites(PTP4ClB amd PTPDISA) were prepared and characterized by UV-Visible, FT-IR and proton H 1 NMR spectroscopic techniques. The adsorption behaviour of the adsorbents were understood from Langmuir, Freundlich, Dubinin Radushkevich, Temkin and Redlich Peterson isotherm models. It is observed that adsorption of mercuric chloride on both the new polymeric compounds is a favorable process as the adsorbents are best fitted for all the isotherms studied. The thermodynamic parameters calculated in the present work further confirms the feasibility of the adsorption process. SEM micrograph, PXRD and EDX spectra recorded after adsorption are the supporting tools to indicate the adsorption of mercuric ion on the chosen adsorbents.