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Removal of mercury ions from aqueous solutions by composite of polyaniline with polystyrene
Separation and Purification Technology 38 (2004) 225–232
Removal of mercury ions from aqueous solutions by composite of polyaniline with polystyrene R.K. Gupta a,∗ , R.A. Singh a , S.S. Dubey b a
Molecular Electronics Laboratory, Department of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi 221005, India b Nuclear and Radiation Chemistry Laboratory, Department of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi 221005, India Received in revised form 19 November 2003; accepted 19 November 2003
Abstract Radiotracer technique has been used to study the removal of Hg(II) ions from aqueous solutions by composite of polyaniline with polystyrene. This study reveals that the increase of adsorbent concentration (10−8 –10−3 M) and temperature (293–333 K) enhances the removal of Hg(II) ions. Concentration-dependence data agree well with the Freundlich isotherm. Thermochemical data show that this process is endothermic in nature. The radiation stability of the composite was also investigated by exposing it to a 11.1-GBq (Ra–Be) source associated with ␥ dose of 1.72 Gy h−1 . Irradiation has practically no effect on the adsorption capacity of the composite. Desorption experiments show that the process of adsorption of mercury ions is almost irreversible and chemisorptive in nature. © 2003 Elsevier B.V. All rights reserved. Keywords: Composite; Polyaniline; Chemisorption; Desorption; Mercury
1. Introduction High level of toxic heavy metals, present in environment produces significant adverse effect not only to the plants and the animals but also to the human being due to their toxicity and non-biodegradability [1,2]. These toxic heavy metals are introduced in to the environment as waste materials from the industrial effluent and human activities. The prescribed limit of 0.001 ppm for mercury [3,4] is the lowest among all heavy metal ions. Hence it becomes necessary to remove these toxic metal ions from municipal and in∗
Corresponding author. Fax: +91-5422368174. E-mail address: [email protected] (R.K. Gupta).
dustrial effluents to protect plants, animals and human beings from their adverse effect before discharging into natural water bodies or onto land. Although a large number of separation methods are available, the process of adsorption is the most versatile and plays a significant role in removal of heavy metal contamination from water bodies [5–7]. Several polymers, clay minerals, dead biomass and metal oxides have been used as adsorbent [8–11]. Considerable attention was given in recent years for the removal of heavy metal toxic ions such as mercury, nickel, cadmium, zinc, etc. by polymeric beads [12,13]. Polyaniline is used as ion-exchanger, for electrochromic displays, and for fabrication of solid-state devices [14–18]. Composites of organic conductors
1383-5866/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2003.11.009
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R.K. Gupta et al. / Separation and Purification Technology 38 (2004) 225–232
and anionic-type insulating polymers have a great application [19–21] as showed by “charge-controllable membrane” with cation-exchange properties [22] that has been used for water deionization [23]. In this paper, we study the adsorption behavior of Hg(II) ions from aqueous solutions on composite of polyaniline with polystyrene as a function of concentration and temperature. Desorption studies of pre-adsorbed species in Hg(NO3 )2 bulk solution (1.0 × 10−5 M) and effect of irradiation on the composite material is also discussed.
2. Experimental
concentration and pH labeled with 203 Hg was placed in centrifuge tube, equilibrated with 100 mg of composite of polyaniline. The -activity of the aliquots withdrawn before and after addition of composite material at regular intervals of time was measured with the help of an end-window GM counter (ECIL-1660). The amount adsorbed (at , in mol g−1 ) at any time ‘t’ and percentage adsorption (P) were estimated using the following equations: at =
A0 − At cV A0 m
(1)
P=
A0 − At × 100. A0
(2)
Polyaniline was synthesized by the chemical oxidation of aniline (Aldrich, A.R. Grade) using ammonium perdisulphate (Qualigens, A.R. Grade) as an oxidizing agent in hydrochloric acid (Qualigens) medium [24]. The yield of this polymer is very high (∼70%) and does not melt up to 300 ◦ C. The polystyrene (G.S.C., India) was used as received. All other chemicals used were of A.R. Grade. For preparation of composite (50% polyaniline by weight) the required amount of polystyrene was dissolved in excess of benzene (Merck, A.R. Grade) and was introduced in polymer matrix by diffusion [25] followed by grinding [26]. Synthesized polyaniline and composite were characterized by a Philips X-ray diffractometer (PW 1729) X-ray generator and found to be amorphous in nature. IR spectra were recorded in KBr in the wavelength of 4000–400 cm−1 on a Jasco (FT/IR-5300) infrared spectrophotometer.
The percentage desorption was calculated with the relationship
2.1. Sorption measurement
Adsorption of Hg(II) ions on composite of polyaniline as a function of time at various concentrations (10−8 –10−3 M) is shown in Fig. 1. It can be seen from Fig. 1 that the uptake of mercury ions is initially fast with the most mercury being adsorbed within the first few minutes and a complete equilibrium between the two phases was established in 40 min at all the concentrations studied. The results indicate that the amount adsorbed at equilibrium increases from 0.791 × 10−9 to 0.456 × 10−4 mol g−1 with the increase of concentration of the metal ions from 10−8 to 10−3 M. However, a relative change in the uptake occurs, i.e. the percentage adsorption increases from 45.6 to 79.1% for Hg(II) ions with the increase of dilution of the
The radiotracer 203 Hg (t1/2 = 47.0 days, Activity = 159.10 ± 10% MBq in dilute HNO3 ) were obtained from the Board of Radiation and Isotope Technology (BRIT), Mumbai (India) and were used for labeling of sorptive solutions. Sorptive solutions were prepared by dissolving mercury nitrate (Aldrich) in double-distilled water and standardized by an EDTA titration method [27]. Solutions of varying concentration (10−3 –10−8 M) of mercury nitrate were prepared by successive dilution of the stock solution (1.0 × 10−1 M).Time rate variation of adsorption was studied by taking 10.0 ml of adsorptive solution of the desired
Desorption (%) =
A2 1 cV × 100, A 0 − A 1 ae m
(3)
where ‘A0 ’ and ‘At ’ are radioactivity of adsorptive solutions at time zero and ‘t,’ respectively; ‘c’ is the initial concentration of adsorptive solutions (mol l−1 ); ‘V’ is the total volume (l) of adsorptive solutions; ‘m’ is the mass of adsorbent (g); ‘A2 ’ is the radioactivity of desorptive solution; ‘A1 ’ and ‘ae ’ are the radioactivity of adsorptive solution and amount adsorbed at equilibrium, respectively.
3. Results and discussion 3.1. Effect of concentration
R.K. Gupta et al. / Separation and Purification Technology 38 (2004) 225–232
227
1.0
-1
Amount adsorbed ( mol g )
0.8
0.6
0.4 4
x 10 5 x 10 6 x 10 7 x 10 8 x 10 9 x 10
0.2
0.0 0
10
20
30
40
50
60
70
80
90
Time (min) Fig. 1. Time variation of adsorption of Hg(II) ions on composite at various concentrations of mercury nitrate solution. Temperature, 303 K, pH ∼ 4.12.
metal ion solution from 10−3 to 10−8 M. Such an increase in adsorption takes place due to the availability of a smaller number of adsorptive species for an equal number of surface sites. 3.2. Adsorption isotherm Concentration-dependence study of Hg(II) ions over wide range of concentration (10−3 –10−8 M) at 303 K and constant pH = 4.12 has been carried out. The results of these studies have been fitted to the Freundlich isotherm in its logarithmic form as 1 log ae = (4) log ce + log K n where ae is the amount adsorbed at equilibrium, ce is the equilibrium bulk concentration, K and 1/n are Freundlich constants corresponding to the adsorption capacity and intensity of adsorption, respectively. Fig. 2 represents a linear Freundlich plot showing its validity for these systems. These constants were evaluated from the slope (1/n) and intercept (log K) of the straight line and the values are found to be 0.896 and 1.58 × 10−1 mol g−1 . The fractional value
of (1/n) indicates that the surface of adsorbent is of the heterogeneous-type with an exponential distribution of energy sites [28–30]. A relatively large value of K, further confirms the higher affinity of Hg(II) species for the composite polyaniline (Fig. 3). The probable mechanism of adsorption of Hg(II) ions on polyaniline may be as follows – [B–N–Q–N–B–N–Q]n – Hg(II) −−→ – B–N (Hg) –Q–N (Hg) –B–N (Hg) –Q n –
The polyaniline has alternating benzenoid and quinoid rings connected by nitrogen [24]. Nitrogen atoms in polyaniline act as adsorption sites for Hg(II) ions. 3.3. Kinetic studies To establish the practical utility of adsorption, kinetic data have been treated by the models given by Boyed et al. [31], which is valid under the experimental conditions used. This is in accordance with the
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R.K. Gupta et al. / Separation and Purification Technology 38 (2004) 225–232
10
2
- log ae( mol g-1)
8
R =0.9985
6
4
2
0 0
2
4
6
8
10
-1
- log Ce(mol L ) Fig. 2. Freundlich adsorption isotherm for Hg(II) on composite of polyaniline at 303 K.
2.60 1.00
2.55
log KD
1.08
2.45
2.40
1.12
2.35
1.16
2.30 2.9
3.0
3.1
3.2 3
3.3
- log k 1
1.04
2.50
1.20 3.4
-1
1/ T x 10 (K ) Fig. 3. Variation of log k1 and log KD with 1/T for adsorption of Hg(II) on composite. Initial concentration of Hg(II) solution, 1.0 × 10−5 M; pH ∼ 4.12.
R.K. Gupta et al. / Separation and Purification Technology 38 (2004) 225–232
229
3.0
2.5
Bt
2.0
1.5
1.0
-8
1x10 M -3 1x10 M
0.5
0.0 5
10
15
20
25
time (min) Fig. 4. Bt vs. t plots for mercury nitrate solution of 1.0 × 10−8 (䊏 ) and 1.0 × 10−3 M (䊉) concentrations.
observations of Reichenberg [32] as given by the following equations: ∞
F =1−
61 exp(−n2 Bt) π2 n2
(5) 3.4. Effect of temperature
n=1
and πDi B= 2 , r
(6)
where F is fractional attainment of equilibrium at time t, Di the effective diffusion coefficient of adsorbates in the adsorbent phase, r the radius of adsorbent particle assumed to be spherical and n is an integer. The fractional attainment of the equilibrium can be determined with the following equation: F=
Qt , Q∞
at higher concentrations the same were linear up to a certain point and passed through the origin indicating the adsorption to be a particle diffusion in nature [33].
(7)
where Qt and Q∞ are the amounts adsorbed after time t and infinite time (24 h), respectively. For every calculated value of F, corresponding values of Bt are calculated from Eq. (5). The linearity test of Bt versus t plots (Fig. 4) is employed to find out the particle diffusion-control mechanism. At the lower concentration, the Bt versus t plots did not pass through the origin signifying the adsorption to be film diffusion, but
The adsorption of Hg(II) ions on composite of polyaniline with polystyrene was studied as a function of temperature (303–333 K). The initial concentration of Hg(II) being kept at 1.0 × 10−5 M (pH = 4.12). The results (Table 1) reveal that the amount adsorbed increases with increasing the temperature from 303–333 K. However, the time required to attain the equilibrium remains almost unaffected. This may be Table 1 Temperature-variation study on amount of Hg(II) ions adsorbed and desorbed on composite of polyaniline at equilibrium Temperature (K)
Amount of Hg(II) adsorbed
303 313 323 333
0.671 0.715 0.748 0.785
(mol g−1 ) ± ± ± ±
×
106
0.005 0.003 0.007 0.002
Adsorption (%) 67.1 71.5 74.8 78.5
Desorption (%) (at equilibrium) 2.7 3.2 3.1 3.3
Initial concentration of Hg(II), 1.0 × 10−5 M; pH 4.12.
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R.K. Gupta et al. / Separation and Purification Technology 38 (2004) 225–232
either due to acceleration of some originally very slow adsorption steps or due to creation of some new active sites on the surface of adsorbent [34]. Also it is quite possible that a diffusion process take place for adsorption and is partly contributing as the rate of adsorption increases on increasing temperature [35]. The kinetics of Hg(II) adsorption follows first-order rate law obeying the Lagergren equation:
log KD versus 1/T will be equal to −H◦ /2.303R. Enthalpy change was evaluated to be 16.3±0.1 kJ mol−1 which confirms the process to be endothermic and ion exchange in nature [33]. Furthermore, the changes in standard free-energy and entropy have also been calculated (Table 1) by using following equations: G◦ = −RT ln K
(10)
k1 t log(ae − at ) = log ae − , 2.303
G◦ = H ◦ − T S ◦
(11)
(8)
where at and ae are the amount adsorbed per gram at time t and at equilibrium, respectively and k1 is the rate constant of adsorption. The plot of log(ae −at ) versus t for the adsorption of Hg(II) gives a straight line confirming the process to follow a first-order rate law. The activation energy for the uptake of Hg(II) was calculated through the Arrhenius plot (viz. log k1 versus 1/T) using adsorption data at different temperatures. The value of activation energy is found to be 9.47 ± 0.03 kJ mol−1 for Hg(II) on composite materials. It indicates that the forces of attraction operating during adsorption are strong enough so that the process of uptake can even proceed under ordinary conditions. The low value of activation energy for the associated process favored at higher temperature is indicative of non-physical adsorption which has been further verified by a desorption study. The enthalpy change for adsorption was estimated from the classical Clausius–Clapeyron relationship: log KD = −
−H ◦ + constant, 2.303RT
(9)
where KD , H◦ , R and T have their usual meanings. The slope of the straight line obtained by plotting
where symbols have their usual significance. The value of G◦ was found to be negative. It is indicative of the fact that the adsorption of Hg(II) ions on composite of polyaniline is spontaneous in nature (Table 2). The positive value of S◦ infers an increase in the degree of freedom of sorbate ions operative due to more randomness following the adsorption process. 3.5. Effect of irradiation The effect of irradiation on radiation stability of the material towards removal behavior of Hg(II) ions has been studied by carrying the experiments using a 11.1-GBq (Ra–Be) neutron source having a neutron flux of 3.85 × 106 neutrons cm−2 s−1 , associated with a nominal ␥ dose rate of 1.78 Gy h−1 . The amount adsorbed at equilibrium on the irradiated as well as the un-irradiated polyaniline at 303 K in 1.0×10−5 M concentration of adsorptive solution is shown in Table 3. It is evident from the results that the amounts adsorbed at equilibrium were slightly decreased by different length of exposure of radiation. This decrease is attributed to (a) decrease in the surface area of adsorbent and (b) sintering of surface active sites of solid materials. Similar results were reported earlier for the removal of some metal ions [36,37].
Table 2 Kinetic parameter for adsorption of Hg(II) ions on composite as a function of temperature Temperature change (K)
Rate constant (min−1 ) (×102 )
303 313 ± 0.2 323 333
7.60 8.51 9.46 10.5
± ± ± ±
0.03 0.05 0.02 0.01
Initial concentration of Hg(II), 1.0 × 10−5 M; pH 4.12.
Energy of activation (kJ mol−1 )
Enthalpy change (kJ mol−1 )
Entropy (kJ mol−1 )
– 9.47 ± 0.03 – –
– 16.3 ± 0.1 – –
– 21.4 – –
R.K. Gupta et al. / Separation and Purification Technology 38 (2004) 225–232 Table 3 Effect of irradiation on composite of polyaniline by neutrons (Ra–Be) source on adsorption of Hg(II) ions Time of irradiation (h)
towards ionizing radiations and may have potential use in waste management in environment protection from mercury ions.
Amount of Hg(II) adsorbed (mol g−1 ) (×105 )
0 24 48 72
231
0.671 0.664 0.656 0.647
± ± ± ±
0.005 0.002 0.003 0.001
Adsorption (%) 67.1 66.4 65.6 64.7
Initial concentration of Hg (II), 1.0×10−5 M; pH 4.12; temperature, 303 K.
3.6. Desorption study Composite polyaniline with pre-adsorbed Hg(II) ions was washed with double-distilled water to ensure the removal of adhering species and subsequently dried at 383 K. The desorption of pre-adsorbed Hg(II) ions was studied in bulk solution (1.0 × 10−5 M) at different temperatures. It was observed that no appreciable activity of adsorbed species was transferred from adsorbent surface to the desorptive solution. These results show that the process of adsorption of Hg(II) ions by composite polyaniline is almost irreversible and chemisorptive in nature. 3.7. Recyclability The pre-adsorbed composite material was thoroughly washed with double-distilled water to remove the adhering species. Then the composite material was used for recyclability test, about 47% of the adsorbed Hg(II) ions was removed when it was treated with 0.1 M HNO3 solutions within 30 min at 303 K. More than 72% of the adsorbed Hg(II) ions were desorbed within 20 min when diphenylthiocarbazone was used as an elution agent. 4. Conclusions The adsorption of Hg(II) ions on composite of polyaniline increases with increase in concentration and temperature of adsorptive solutions. The uptake process follows first-order rate law and obeys Freundlich isotherm. The removal is endothermic in nature. It takes place by particle diffusion at higher concentration and by film diffusion at lower concentration. The adsorbent shows good radiation stability
Acknowledgements Authors thank Head and Prof. S.P. Mishra, Nuclear and Radiochemistry Laboratory, Department of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi for providing necessary facilities. R.K.G. and S.S.D. thank Banaras Hindu University, Varanasi, India for awarding a research fellowship. References [1] V. Subramaniam, R. Griken, L. Don’t Dach, Environ. Geol. Sci. 9 (1987) 91. [2] B. Volesky, Biosorption of Heavy Metals, CRC Press, Boca Raton Fla ,1992, pp. 7–44. [3] World Health Organization, International Standards of Drinking Water, W.H.O., Geneva, 1971. [4] W. Mertz, The essential trace elements, Science 213 (1981) 1332. [5] J.B. Stankovic, S.K. Milonjic, M.M. Kopenci, T.S. Ceranic, Colloids Surf. 46 (1909) 283. [6] S.S. Krishnan, A. Cancilla, E. Jervis, J. Radioanal. Nucl. Chem. Art. 110 (1987) 379. [7] D.K. Bhattacharya, N.C. Datta, J. Nucl. Sci. Technol. 128 (1991) 1014. [8] S.P. Mishra, V.K. Singh, D. Tiwari, Radiochim. Acta 73 (1996) 49. [9] B. Volesky, Biotechnology 2 (1988) 135. [10] W. Mocka, H. Wihlidal, G. Stehlik, J. Washuttl, E. Bancher, Chemosphere 10 (1979) 787. [11] M.A. Spitsyn, V.N. Andreev, Elektrokhimiya 25 (9) (1989) 1171. [12] A. Denizli, C. Arpa, S. Bektas, O. Genc, Adsorp. Sci. Technol. 20 (2002) 91. [13] N. Akiko, Y. Masao, I. Tomoyuki, O. Taiji, T. Kunihiko, Kagaku Kogaku Ronbunshu 28 (2002) 501. [14] T. Matsunaga, H. Raifuker, T. Nakajima, T. Kawagoe, Poly. Adv. Technol. 1 (1990) 33. [15] G. Gustafsson, Y. Coa, G.M. Treacy, F. Klavettey, N. Colaneri, A.J. Heeger, Nature 357 (1992) 477. [16] J.J. Langer, Mater. Sci. 14 (1988) 41. [17] J. Kanicki, Mol. Cryst. Liq. Cryst. 105 (1984) 203. [18] R.A. Singh, R. Singh, D.N. Srivastava, Synth. Met. 121 (2001) 1439. [19] R. Penner, C. Martin, J. Electrochem. Soc. 133 (1986) 310. [20] T. Shimidzu, A. Ohtani, T. Iyoda, K. Honda, J. Electroanal. Chem. 224 (1987) 123. [21] G. Bidan, B. Ehui, J. Chem. Soc. Chem. Commun. 20 (1989) 1568.
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[22] T. Shimidzu, A. Ohtani, T. Iyoda, K. Honda, J. Chem. Commun. 5 (1987) 327. [23] T. Shimidzu, A. Ohtani, T. Iyoda, K. Honda, J. Chem. Commun. 18 (1986) 1415. [24] D.C. Trivedi, Ind. J. Chem. 33A (1994) 552. [25] T.J. Kang, Y. Miyaki, J.H. Han, T. Motobe, Y.E. Whang, S. Miyata, in: Proceedings of the Third Pacific Polymer Conference, 1994, p. 307. [26] V.P. Malhotra, R. Chandra, R.K. Garg, S. Motsara, Ind. J. Chem. 33 (1994) 595. [27] H. Flascha, EDTA Titrations: An Introduction to Theory and Practice, second ed., Pergamon Press, Oxford, 1964. [28] P. Benes, V. Majer, Traces Chemistry of Aqueous Solution, Elsevier, Amsterdam, 1980.
[29] A. Clark, The Theory of Adsorption and Catalysis, Academic Press, New York, 1970. [30] R. Sips, J. Phys. Chem. 16 (1948) 490. [31] G.E. Boyd, A.W. Adamson, L.S. Meyers, J. Am. Chem. Soc. 69 (1947) 2836. [32] D. Reichenberg, J. Am. Chem. Soc. 75 (1953) 589. [33] F. Helfferich, Ion Exchange, McGraw Hill, New York, 1962. [34] J.W. Hensnley, J. Am. Chem. Soc. 34 (1951) 188. [35] Y.P. Ting, W.K. Teo, Bioresource Technol. 50 (1994) 113. [36] S.P. Mishra, D. Tiwari, M. Mishra, R.S. Dubey, Appl. Radiat. Isotope 50 (1999) 631. [37] V.V. Gromov, V.I. Spitsyn, Atom. Ener. (USSR) 14 (1963) 491.
Removal of mercury ions from aqueous solutions by composite of polyaniline with polystyrene R.K. Gupta a,∗ , R.A. Singh a , S.S. Dubey b a
Molecular Electronics Laboratory, Department of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi 221005, India b Nuclear and Radiation Chemistry Laboratory, Department of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi 221005, India Received in revised form 19 November 2003; accepted 19 November 2003
Abstract Radiotracer technique has been used to study the removal of Hg(II) ions from aqueous solutions by composite of polyaniline with polystyrene. This study reveals that the increase of adsorbent concentration (10−8 –10−3 M) and temperature (293–333 K) enhances the removal of Hg(II) ions. Concentration-dependence data agree well with the Freundlich isotherm. Thermochemical data show that this process is endothermic in nature. The radiation stability of the composite was also investigated by exposing it to a 11.1-GBq (Ra–Be) source associated with ␥ dose of 1.72 Gy h−1 . Irradiation has practically no effect on the adsorption capacity of the composite. Desorption experiments show that the process of adsorption of mercury ions is almost irreversible and chemisorptive in nature. © 2003 Elsevier B.V. All rights reserved. Keywords: Composite; Polyaniline; Chemisorption; Desorption; Mercury
1. Introduction High level of toxic heavy metals, present in environment produces significant adverse effect not only to the plants and the animals but also to the human being due to their toxicity and non-biodegradability [1,2]. These toxic heavy metals are introduced in to the environment as waste materials from the industrial effluent and human activities. The prescribed limit of 0.001 ppm for mercury [3,4] is the lowest among all heavy metal ions. Hence it becomes necessary to remove these toxic metal ions from municipal and in∗
Corresponding author. Fax: +91-5422368174. E-mail address: [email protected] (R.K. Gupta).
dustrial effluents to protect plants, animals and human beings from their adverse effect before discharging into natural water bodies or onto land. Although a large number of separation methods are available, the process of adsorption is the most versatile and plays a significant role in removal of heavy metal contamination from water bodies [5–7]. Several polymers, clay minerals, dead biomass and metal oxides have been used as adsorbent [8–11]. Considerable attention was given in recent years for the removal of heavy metal toxic ions such as mercury, nickel, cadmium, zinc, etc. by polymeric beads [12,13]. Polyaniline is used as ion-exchanger, for electrochromic displays, and for fabrication of solid-state devices [14–18]. Composites of organic conductors
1383-5866/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2003.11.009
226
R.K. Gupta et al. / Separation and Purification Technology 38 (2004) 225–232
and anionic-type insulating polymers have a great application [19–21] as showed by “charge-controllable membrane” with cation-exchange properties [22] that has been used for water deionization [23]. In this paper, we study the adsorption behavior of Hg(II) ions from aqueous solutions on composite of polyaniline with polystyrene as a function of concentration and temperature. Desorption studies of pre-adsorbed species in Hg(NO3 )2 bulk solution (1.0 × 10−5 M) and effect of irradiation on the composite material is also discussed.
2. Experimental
concentration and pH labeled with 203 Hg was placed in centrifuge tube, equilibrated with 100 mg of composite of polyaniline. The -activity of the aliquots withdrawn before and after addition of composite material at regular intervals of time was measured with the help of an end-window GM counter (ECIL-1660). The amount adsorbed (at , in mol g−1 ) at any time ‘t’ and percentage adsorption (P) were estimated using the following equations: at =
A0 − At cV A0 m
(1)
P=
A0 − At × 100. A0
(2)
Polyaniline was synthesized by the chemical oxidation of aniline (Aldrich, A.R. Grade) using ammonium perdisulphate (Qualigens, A.R. Grade) as an oxidizing agent in hydrochloric acid (Qualigens) medium [24]. The yield of this polymer is very high (∼70%) and does not melt up to 300 ◦ C. The polystyrene (G.S.C., India) was used as received. All other chemicals used were of A.R. Grade. For preparation of composite (50% polyaniline by weight) the required amount of polystyrene was dissolved in excess of benzene (Merck, A.R. Grade) and was introduced in polymer matrix by diffusion [25] followed by grinding [26]. Synthesized polyaniline and composite were characterized by a Philips X-ray diffractometer (PW 1729) X-ray generator and found to be amorphous in nature. IR spectra were recorded in KBr in the wavelength of 4000–400 cm−1 on a Jasco (FT/IR-5300) infrared spectrophotometer.
The percentage desorption was calculated with the relationship
2.1. Sorption measurement
Adsorption of Hg(II) ions on composite of polyaniline as a function of time at various concentrations (10−8 –10−3 M) is shown in Fig. 1. It can be seen from Fig. 1 that the uptake of mercury ions is initially fast with the most mercury being adsorbed within the first few minutes and a complete equilibrium between the two phases was established in 40 min at all the concentrations studied. The results indicate that the amount adsorbed at equilibrium increases from 0.791 × 10−9 to 0.456 × 10−4 mol g−1 with the increase of concentration of the metal ions from 10−8 to 10−3 M. However, a relative change in the uptake occurs, i.e. the percentage adsorption increases from 45.6 to 79.1% for Hg(II) ions with the increase of dilution of the
The radiotracer 203 Hg (t1/2 = 47.0 days, Activity = 159.10 ± 10% MBq in dilute HNO3 ) were obtained from the Board of Radiation and Isotope Technology (BRIT), Mumbai (India) and were used for labeling of sorptive solutions. Sorptive solutions were prepared by dissolving mercury nitrate (Aldrich) in double-distilled water and standardized by an EDTA titration method [27]. Solutions of varying concentration (10−3 –10−8 M) of mercury nitrate were prepared by successive dilution of the stock solution (1.0 × 10−1 M).Time rate variation of adsorption was studied by taking 10.0 ml of adsorptive solution of the desired
Desorption (%) =
A2 1 cV × 100, A 0 − A 1 ae m
(3)
where ‘A0 ’ and ‘At ’ are radioactivity of adsorptive solutions at time zero and ‘t,’ respectively; ‘c’ is the initial concentration of adsorptive solutions (mol l−1 ); ‘V’ is the total volume (l) of adsorptive solutions; ‘m’ is the mass of adsorbent (g); ‘A2 ’ is the radioactivity of desorptive solution; ‘A1 ’ and ‘ae ’ are the radioactivity of adsorptive solution and amount adsorbed at equilibrium, respectively.
3. Results and discussion 3.1. Effect of concentration
R.K. Gupta et al. / Separation and Purification Technology 38 (2004) 225–232
227
1.0
-1
Amount adsorbed ( mol g )
0.8
0.6
0.4 4
x 10 5 x 10 6 x 10 7 x 10 8 x 10 9 x 10
0.2
0.0 0
10
20
30
40
50
60
70
80
90
Time (min) Fig. 1. Time variation of adsorption of Hg(II) ions on composite at various concentrations of mercury nitrate solution. Temperature, 303 K, pH ∼ 4.12.
metal ion solution from 10−3 to 10−8 M. Such an increase in adsorption takes place due to the availability of a smaller number of adsorptive species for an equal number of surface sites. 3.2. Adsorption isotherm Concentration-dependence study of Hg(II) ions over wide range of concentration (10−3 –10−8 M) at 303 K and constant pH = 4.12 has been carried out. The results of these studies have been fitted to the Freundlich isotherm in its logarithmic form as 1 log ae = (4) log ce + log K n where ae is the amount adsorbed at equilibrium, ce is the equilibrium bulk concentration, K and 1/n are Freundlich constants corresponding to the adsorption capacity and intensity of adsorption, respectively. Fig. 2 represents a linear Freundlich plot showing its validity for these systems. These constants were evaluated from the slope (1/n) and intercept (log K) of the straight line and the values are found to be 0.896 and 1.58 × 10−1 mol g−1 . The fractional value
of (1/n) indicates that the surface of adsorbent is of the heterogeneous-type with an exponential distribution of energy sites [28–30]. A relatively large value of K, further confirms the higher affinity of Hg(II) species for the composite polyaniline (Fig. 3). The probable mechanism of adsorption of Hg(II) ions on polyaniline may be as follows – [B–N–Q–N–B–N–Q]n – Hg(II) −−→ – B–N (Hg) –Q–N (Hg) –B–N (Hg) –Q n –
The polyaniline has alternating benzenoid and quinoid rings connected by nitrogen [24]. Nitrogen atoms in polyaniline act as adsorption sites for Hg(II) ions. 3.3. Kinetic studies To establish the practical utility of adsorption, kinetic data have been treated by the models given by Boyed et al. [31], which is valid under the experimental conditions used. This is in accordance with the
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10
2
- log ae( mol g-1)
8
R =0.9985
6
4
2
0 0
2
4
6
8
10
-1
- log Ce(mol L ) Fig. 2. Freundlich adsorption isotherm for Hg(II) on composite of polyaniline at 303 K.
2.60 1.00
2.55
log KD
1.08
2.45
2.40
1.12
2.35
1.16
2.30 2.9
3.0
3.1
3.2 3
3.3
- log k 1
1.04
2.50
1.20 3.4
-1
1/ T x 10 (K ) Fig. 3. Variation of log k1 and log KD with 1/T for adsorption of Hg(II) on composite. Initial concentration of Hg(II) solution, 1.0 × 10−5 M; pH ∼ 4.12.
R.K. Gupta et al. / Separation and Purification Technology 38 (2004) 225–232
229
3.0
2.5
Bt
2.0
1.5
1.0
-8
1x10 M -3 1x10 M
0.5
0.0 5
10
15
20
25
time (min) Fig. 4. Bt vs. t plots for mercury nitrate solution of 1.0 × 10−8 (䊏 ) and 1.0 × 10−3 M (䊉) concentrations.
observations of Reichenberg [32] as given by the following equations: ∞
F =1−
61 exp(−n2 Bt) π2 n2
(5) 3.4. Effect of temperature
n=1
and πDi B= 2 , r
(6)
where F is fractional attainment of equilibrium at time t, Di the effective diffusion coefficient of adsorbates in the adsorbent phase, r the radius of adsorbent particle assumed to be spherical and n is an integer. The fractional attainment of the equilibrium can be determined with the following equation: F=
Qt , Q∞
at higher concentrations the same were linear up to a certain point and passed through the origin indicating the adsorption to be a particle diffusion in nature [33].
(7)
where Qt and Q∞ are the amounts adsorbed after time t and infinite time (24 h), respectively. For every calculated value of F, corresponding values of Bt are calculated from Eq. (5). The linearity test of Bt versus t plots (Fig. 4) is employed to find out the particle diffusion-control mechanism. At the lower concentration, the Bt versus t plots did not pass through the origin signifying the adsorption to be film diffusion, but
The adsorption of Hg(II) ions on composite of polyaniline with polystyrene was studied as a function of temperature (303–333 K). The initial concentration of Hg(II) being kept at 1.0 × 10−5 M (pH = 4.12). The results (Table 1) reveal that the amount adsorbed increases with increasing the temperature from 303–333 K. However, the time required to attain the equilibrium remains almost unaffected. This may be Table 1 Temperature-variation study on amount of Hg(II) ions adsorbed and desorbed on composite of polyaniline at equilibrium Temperature (K)
Amount of Hg(II) adsorbed
303 313 323 333
0.671 0.715 0.748 0.785
(mol g−1 ) ± ± ± ±
×
106
0.005 0.003 0.007 0.002
Adsorption (%) 67.1 71.5 74.8 78.5
Desorption (%) (at equilibrium) 2.7 3.2 3.1 3.3
Initial concentration of Hg(II), 1.0 × 10−5 M; pH 4.12.
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R.K. Gupta et al. / Separation and Purification Technology 38 (2004) 225–232
either due to acceleration of some originally very slow adsorption steps or due to creation of some new active sites on the surface of adsorbent [34]. Also it is quite possible that a diffusion process take place for adsorption and is partly contributing as the rate of adsorption increases on increasing temperature [35]. The kinetics of Hg(II) adsorption follows first-order rate law obeying the Lagergren equation:
log KD versus 1/T will be equal to −H◦ /2.303R. Enthalpy change was evaluated to be 16.3±0.1 kJ mol−1 which confirms the process to be endothermic and ion exchange in nature [33]. Furthermore, the changes in standard free-energy and entropy have also been calculated (Table 1) by using following equations: G◦ = −RT ln K
(10)
k1 t log(ae − at ) = log ae − , 2.303
G◦ = H ◦ − T S ◦
(11)
(8)
where at and ae are the amount adsorbed per gram at time t and at equilibrium, respectively and k1 is the rate constant of adsorption. The plot of log(ae −at ) versus t for the adsorption of Hg(II) gives a straight line confirming the process to follow a first-order rate law. The activation energy for the uptake of Hg(II) was calculated through the Arrhenius plot (viz. log k1 versus 1/T) using adsorption data at different temperatures. The value of activation energy is found to be 9.47 ± 0.03 kJ mol−1 for Hg(II) on composite materials. It indicates that the forces of attraction operating during adsorption are strong enough so that the process of uptake can even proceed under ordinary conditions. The low value of activation energy for the associated process favored at higher temperature is indicative of non-physical adsorption which has been further verified by a desorption study. The enthalpy change for adsorption was estimated from the classical Clausius–Clapeyron relationship: log KD = −
−H ◦ + constant, 2.303RT
(9)
where KD , H◦ , R and T have their usual meanings. The slope of the straight line obtained by plotting
where symbols have their usual significance. The value of G◦ was found to be negative. It is indicative of the fact that the adsorption of Hg(II) ions on composite of polyaniline is spontaneous in nature (Table 2). The positive value of S◦ infers an increase in the degree of freedom of sorbate ions operative due to more randomness following the adsorption process. 3.5. Effect of irradiation The effect of irradiation on radiation stability of the material towards removal behavior of Hg(II) ions has been studied by carrying the experiments using a 11.1-GBq (Ra–Be) neutron source having a neutron flux of 3.85 × 106 neutrons cm−2 s−1 , associated with a nominal ␥ dose rate of 1.78 Gy h−1 . The amount adsorbed at equilibrium on the irradiated as well as the un-irradiated polyaniline at 303 K in 1.0×10−5 M concentration of adsorptive solution is shown in Table 3. It is evident from the results that the amounts adsorbed at equilibrium were slightly decreased by different length of exposure of radiation. This decrease is attributed to (a) decrease in the surface area of adsorbent and (b) sintering of surface active sites of solid materials. Similar results were reported earlier for the removal of some metal ions [36,37].
Table 2 Kinetic parameter for adsorption of Hg(II) ions on composite as a function of temperature Temperature change (K)
Rate constant (min−1 ) (×102 )
303 313 ± 0.2 323 333
7.60 8.51 9.46 10.5
± ± ± ±
0.03 0.05 0.02 0.01
Initial concentration of Hg(II), 1.0 × 10−5 M; pH 4.12.
Energy of activation (kJ mol−1 )
Enthalpy change (kJ mol−1 )
Entropy (kJ mol−1 )
– 9.47 ± 0.03 – –
– 16.3 ± 0.1 – –
– 21.4 – –
R.K. Gupta et al. / Separation and Purification Technology 38 (2004) 225–232 Table 3 Effect of irradiation on composite of polyaniline by neutrons (Ra–Be) source on adsorption of Hg(II) ions Time of irradiation (h)
towards ionizing radiations and may have potential use in waste management in environment protection from mercury ions.
Amount of Hg(II) adsorbed (mol g−1 ) (×105 )
0 24 48 72
231
0.671 0.664 0.656 0.647
± ± ± ±
0.005 0.002 0.003 0.001
Adsorption (%) 67.1 66.4 65.6 64.7
Initial concentration of Hg (II), 1.0×10−5 M; pH 4.12; temperature, 303 K.
3.6. Desorption study Composite polyaniline with pre-adsorbed Hg(II) ions was washed with double-distilled water to ensure the removal of adhering species and subsequently dried at 383 K. The desorption of pre-adsorbed Hg(II) ions was studied in bulk solution (1.0 × 10−5 M) at different temperatures. It was observed that no appreciable activity of adsorbed species was transferred from adsorbent surface to the desorptive solution. These results show that the process of adsorption of Hg(II) ions by composite polyaniline is almost irreversible and chemisorptive in nature. 3.7. Recyclability The pre-adsorbed composite material was thoroughly washed with double-distilled water to remove the adhering species. Then the composite material was used for recyclability test, about 47% of the adsorbed Hg(II) ions was removed when it was treated with 0.1 M HNO3 solutions within 30 min at 303 K. More than 72% of the adsorbed Hg(II) ions were desorbed within 20 min when diphenylthiocarbazone was used as an elution agent. 4. Conclusions The adsorption of Hg(II) ions on composite of polyaniline increases with increase in concentration and temperature of adsorptive solutions. The uptake process follows first-order rate law and obeys Freundlich isotherm. The removal is endothermic in nature. It takes place by particle diffusion at higher concentration and by film diffusion at lower concentration. The adsorbent shows good radiation stability
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