Journal of Hazardous Materials 280 (2014) 20–30
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Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Removal of aqueous Hg(II) and Cr(VI) using phytic acid doped polyaniline/cellulose acetate composite membrane Renjie Li a , Lifen Liu a,b,∗ , Fenglin Yang a a MOE, Key Laboratory of Industrial Ecology and Environmental Engineering, School of Environmental Science and Technology, Dalian University of Technology, Dalian, China b School of Food and Environmental Science and Technology, Dalian University of Technology, Panjin, China
h i g h l i g h t s • • • • •
PANI/CA membrane was studied for adsorbing Hg(II) and Cr(VI). The new membrane preparation method is convenient, does not use toxic solvent. The doping agent-Phytic acid improved the adsorptive capacity. PANI-PA/CA membrane possesses higher BET surface area and electrical conductivity. PANI-PA/CA membrane has high adsorptive capacity for Hg(II) and Cr(VI).
a r t i c l e
i n f o
Article history: Received 19 April 2014 Received in revised form 28 June 2014 Accepted 24 July 2014 Available online 2 August 2014 Keywords: Polyaniline Cellulose acetate Hg(II) Cr(VI) Adsorption
a b s t r a c t Conductive composite membrane—phytic acid (PA) doped polyaniline (PANI)/cellulose acetate (CA) (PANI-PA/CA) was prepared in a simple and environmental-friendly method, in which aniline was blended with CA/PA solution and polymerized before the phase conversion. The resultant composite membranes were characterized by SEM, EDX, FTIR-ATR, BET and electrical resistance measurements. When used as adsorbent for Hg(II) and Cr(VI) ions, the prepared composite membrane exhibits excellent adsorption capability. The adsorption of Hg(II) and Cr(VI) follows a pseudo-second-order kinetic model and best fits the Langmuir isotherm model, with the maximum adsorption capacity reaching 280.11 and 94.34 mg g−1 , respectively. The heavy metal loaded composite membrane can be regenerated and reused after treatment with acid or alkali solution, making it a promising and practical adsorbent for Hg(II) and Cr(VI) removal. Tests with river water were also carried out, indicating good performance and application. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Heavy metal pollution has become a world-wide issue affecting the existence and development of human society due to industrializations. As toxic and potential carcinogenic metal, mercury pollution is found from industries [1], agriculture [2] and waste incineration [3,4], and chromium pollution is generated from electroplating, mining and cement/chemical production [5]. Adsorption is often used as one feasible technology due to its low initial cost, simplicity in design, ease of operation and insensitivity to toxic substances. Governments enforce strict emission control
∗ Corresponding author at: MOE, Key Laboratory of Industrial Ecology and Environmental Engineering, School of Food and Environmental Science and Technology, Dalian University of Technology, Panjin, China. Tel.: +86 427 2631799; fax: +86 411 84708083. E-mail addresses: [email protected], [email protected] (L. Liu). http://dx.doi.org/10.1016/j.jhazmat.2014.07.052 0304-3894/© 2014 Elsevier B.V. All rights reserved.
policies for mercury and hexavalent chromium [6]. However, more effective adsorbents and more simple technologies are still in great demand to remove aqueous heavy metal ions. The low-cost, highly stable and capacitive polyaniline (PANI) is often used for heavy metal removal. Its nitrogen-containing functional groups can complex with Hg(II) [7–9], and adsorb anions through electrostatic interaction or hydrogen bonding [8,10]. Although PANI show good performance for removal of heavy metal ions from aqueous solutions, the mechanical strength and solubility of PANI are very poor and a separation step has to be taken after adsorption. To overcome the difficulties in its preparation and applications, PANI was often loaded on carriers such as Fe3 O4 [11,12], carbon cloth [13,14] and polymer fiber mat [15,16]. Membrane adsorbents are easy to separate and the conductive ones may be used as electrodes in electro-reduction or other electrochemical applications. Recently, PANI was incorporated as composites to improve membrane separation characteristics [17–20]. The PANI oligomer
R. Li et al. / Journal of Hazardous Materials 280 (2014) 20–30
was reported to perform well as a pore forming substance [19,21,22]. To obtain PANI composite membrane, NMP or DMF solvents are usually needed to dissolve PANI, that is not only bad for environment, but also the product membranes have poor conductivity. For preparing high performance conductive membrane with PANI, a new environmental-friendly method was proposed in this paper. Using acetic acid solution (HAc:water = 72:28) as solvent, aniline was polymerized in the presence of dissolved CA [23]. Thus, polyaniline (PANI)/cellulose acetate (CA) composite membrane was prepared by casting CA solution containing in-situ polymerized PANI, without any separation and drying processing. Phytic acid (PA) as a doping agent could not only influence the polymerized PANI, but also complex with heavy metal ions [24–26]. So the effect of PA on the adsorptive property of PANI/CA composite membrane was investigated. The characterizations (SEM, EDX, ATR-FTIR, BET and electrical resistance measurement) and adsorptive property of the membrane for Hg(II) and Cr(VI) were studied. The optimal composite proportion and the effect of pH, adsorption time and initial concentration were investigated. Kinetic and thermodynamics, reproducibility and reusability were also studied. 2. Experimental 2.1. Reagents Zinc powder (99.0%) was purchased from Tianjin Chemical Works. Chromic nitrate, copper sulphate anhydrous (CuSO4 , 99.0%), Aniline (99.5%) and phytic acid (50%) were purchased from Tianjin Bodi Chemical Co. Ltd. Cellulose acetate (CA, acetyl content: 54.6%) and lead nitrate (Pb(NO3 )2 , 99.0%) were purchased from Sinopharm Chemical Reagent Co. Ltd. Ammonium persulfate (APS, (NH4 )2 S2 O8 , 98%), sodium hydroxide (NaOH, 96%), Cadmium nitrate tetrahydrate (Cd(NO3 )2 ·4H2 O, 99.0%) and ammonium chloride (NH4 Cl, 99.5%) were purchased from Tianjin Damao Chemical Reagent Co. Ltd. Nitric acid (65–68%) was purchased from Beijing Chemical Works. Acetic acid (99.5%) and ammonia solution (25%) were purchased from Tianjin Fuyu Fine Chemical Co. Ltd. Mercury (II) nitrate (Hg(NO3 )2 ·1/2H2 O, 97%) was obtained from Jiangyan Huanqiu Chemical Works. Potassium dichromate (K2 Cr2 O7 , 99.8%) was purchased from Shenyang Chemical Works. The materials were all used as received. 2.2. Preparation of PANI-PA/CA composite membrane The PANI/CA composite membrane was synthesized via chemical polymerization of aniline in CA solution. CA/acetic acid/water casting solutions were prepared by dissolving CA into acetic acid/water (72/28). Then aniline was blended with the CA solution in ice bath. The mixture was stirred at 0–5 ◦ C for 30 min and APS was added very slowly for well dispersion. The mixture was stirred at this temperature for 8 h. The casting solution was left still for 0.5 h to release bubbles. After that, the casting solution was cast onto a glass plate with a steel knife to obtain a casting film of 400 m thickness. The film was exposed to atmosphere for 10 s, and then immersed in a coagulation bath of pure water over night. The contents of CA and aniline were varied to select the best composition of membrane for Hg(II) removal (Composition of the casting solutions seen in Table 1). To investigate the effect of phytic acid, the PANI-PA/CA composite membrane was prepared by adding a certain amount of phytic acid (3–12 wt.%) in the acetic acid/water solution. 2.3. Characterization of PANI-PA/CA membrane The surface and cross-section morphology of PA/CA and PANIPA/CA membrane (component proportion seen in Table 2) were
21
Table 1 Composition of the casting solutions. Prepared membrane
CA (%)
Aniline (%)
APS (%)
Acetic acid solution (%)
M-0 M-1 M-2 M-3 M-4 M-5 M-6 M-7 M-8
10 10 10 10 10 8 8 8 8
0 1 2 3 4 1 2 3 4
0 1.23 2.45 3.68 4.9 1.23 2.45 3.68 4.9
90 87.77 85.55 83.32 81.1 89.77 87.55 85.32 83.1
Table 2 Component proportions of PA/CA and PANI-PA/CA. Prepared membrane
CA (%)
Aniline (%)
PA (%)
APS (%)
Acetic acid/water (%)
PA/CA PA-PANI/CA
11 8
0 3
6 6
0 3.68
83 79.32
investigated using the scanning electron microscope (SEM, FEI Quanta 200 FEG). PANI-PA/CA membrane before and after adsorption were analyzed with energy dispersive X-ray (EDX, FEI Quanta 200 FEG). ATR-FTIR and BET measurement were recorded using Shimadzu 8400s spectrometer and Quantachrome NOWA 4000 apparatus using N2 adsorption (77.3 K), respectively. The electrical resistance of dry membrane was measured repeatedly by fixing two probes (distance: 2 cm) on that 3 cm × 3 cm membrane. 2.4. Adsorption experiments Batch adsorption experiments were conducted to investigate the Hg(II) and Cr(VI) adsorption by the prepared membrane. Hg(NO3 )2 and K2 Cr2 O7 were used as the source of Hg(II) and Cr(VI), respectively. A piece of 3 cm × 3 cm membrane was added into a 100 mL conical flask containing 50 mL of metal ions solution at 303 K and 150 rpm. The solution pH was adjusted with 0.1 M NaOH, 0.1 M HNO3 (Hg) or 0.1 M H2 SO4 (Cr). The membrane after adsorption was cleaned with deionized water, dried in vacuum oven at 60 ◦ C and weighed. Kinetic studies were carried out by immersion of PANI-PA/CA membrane in Hg(II) or Cr(VI) solutions, with an initial concentration of 223.33 or 67.28 mg L−1 , respectively. During adsorption, the solutions samples were withdrawn at certain time intervals for the measurement of heavy metal ions concentrations. The adsorption capacity qt (mg g−1 ) at time t was obtained using the following equation (1): qt =
C0 − Ct ×V W
(1)
where C0 (mg L−1 ) is the initial concentration of metal ions. Ct (mg L−1 ) is the concentration of metal ions at any time t, V (mL) is the volume of solution and W (g) is the amount of adsorbents. For the isotherm studies, batch adsorptions were conducted for 24 h to reach complete equilibrium with different initial concentrations of heavy metal ions. The solution pH (5.0 for mercury, 2.0 for hexavalent chromium) and temperature (303 K) were kept constant. The adsorption capacity of the absorbents at equilibrium was obtained using the following equation (2): qe =
C0 − Ce ×V W
(2)
22
110.49
Cd(II) Pb(II)
0.58 2.91
Cr(VI)
21.60
Zn(II) Cu(II)
40.72 34.17
2.19
PA/CA
802 904
Cr(VI)containing
3452
14.53
where qe (mg g−1 ) is the equilibrium adsorption capacity of absorbent. Ce (mg L−1 ) is the equilibrium concentration of metal ions.
3446
Hg(II)
3251
Hg(II)containing
2927
Chemical oxygen demand (CODCr , mg/L)
2935
Initial concentration (mg/L)
1736
Metal
Absorbance
Wastewater sample
904 1036 1157 1159 1230 1313 1371 1369 1431 1491 1604 1639 1736
1036
Table 3 Characteristics of synthetic wastewater samples.
1232
R. Li et al. / Journal of Hazardous Materials 280 (2014) 20–30
PA-PANI/CA 600
900
1200
2.5. Regeneration tests
1500
1800
2100
2400
2700
3000
3300
3600
-1
Wavenumber (cm )
For desorption studies, a piece of 3 cm × 3 cm PANI-PA/CA membrane firstly adsorbed heavy metal ions in 50 mL solution (Hg(II) 236.67 mg L−1 or Cr(VI) 45.89 mg L−1 ) for 5 h at 303 K. Then using 50 mL solutions at different pH for desorption, the heavy metal loaded membranes were placed in these medium and stirred for 5 h at room temperature. The above procedure was repeated 4 times to investigate the reusability of the adsorbents.
Fig. 1. ATR-FTIR spectra of PA/CA and PANI-PA/CA.
802, 1313, 1491, 1604 and 3251 cm−1 are observed in the spectra of PANI-PA/CA. They respectively correspond to para-disubstituted benzene ring, aromatic (C–N)+ stretching, benzenoid ring stretching, quinoid ring stretching and N–H stretching [29]. The ATR-FTIR spectrum of PANI-PA/CA indicates that the compositing of PANI, PA and CA is successful.
2.6. Simulated wastewater tests with mixed heavy metal ions
3.1.2. SEM images and EDX analysis The SEM was used to study the morphologies of the prepared membranes. As shown in Fig. 2, the membranes consist of a dense top layer and a porous sub-layer. For PA/CA membrane, the cross-section (Fig. 2b) shows an asymmetrical structure with a dense layer and finger-like macro-voids. The bottom surface (Fig. 2c) shows a fibre-network structure. For PANI-PA/CA membrane (Fig. 2d–f), there exists a remarkable difference in the pore structure. With PANI-PA, the macro-voids become larger due to the large amount of pore-forming agent like APS, PANI oligomer and water. In the image of bottom surface (Fig. 2f), a network structure of fiber and particles is attributed to the PANI. Moreover, the EDX results are shown in Fig. 3 and Table 4, confirming Hg and Cr elements exist in the PANI-PA/CA after adsorption. For Cr adsorbed membrane, despite of the addition of H2 SO4 , which brings elemental sulfur and part of elemental oxygen, the great increase of oxygen content indicate a portion of Cr adsorbed on PANI-PA/CA was in the form of Cr(VI) oxyanions.
To investigate the practical applicability of PANI-PA/CA composite membrane, two kinds of synthetic wastewater containing various heavy metal ions were prepared according to previous literatures [27,28]. Water from a local Linshui River was used in dissolving the heavy metal salts. The characteristics of the samples were listed in Table 3. For treatment, pH of mercury-containing and chromium-containing wastewater samples were adjusted to 5.0 and 2.0, respectively. A certain weight of PANI-PA/CA composite membrane was added to 50 mL simulated wastewater sample and shaken continuously for 8 h at 25 ◦ C. The concentrations of Pb(II), Zn(II), Cu(II) and Cd(II) in solution before and after adsorption were determined using inductively coupled plasma spectrometer (Optima 2000 DV, Perkin Elmer). The amount of Hg(II) and Cr(VI) in the solution before and after adsorption were measured spectrophotometrically, using dithizone and diphenylcarbazide as the corresponding complexing agents (GB 7496-87 and 7467-87, China). 3. Results and discussion
3.1.3. BET analysis The porous structure parameters of PA/CA and PANI-PA/CA were measured and shown in Fig. 4 and Table 5. It can be found that the BET surface area of PANI-PA/CA membrane displays nearly 67.7% increase compared with that of PA/CA membrane. As shown in Table 5, PANI-PA/CA membrane has higher pore volume than PA/CA. However, the average surface pore size of membrane decreases with PANI incorporated. The changes of membrane pore
3.1. The characterization and properties of the prepared membrane material 3.1.1. ATR-FTIR spectra The ATR-FTIR spectra of PA/CA and PANI-PA/CA membrane are shown in Fig. 1. The characteristic bands of CA and phytic acid are very intensive in both spectra. Compared to PA/CA, new peaks at Table 4 Element composition of PANI-PA/CA before and after adsorption. Sample
Before adsorption After adsorption of Hg(II) After adsorption of Cr(VI)
Atomic percent (%) C
N
O
P
Hg
71.66 70.44 56.44
1.80 3.01 4.07
24.49 24.43 35.04
2.04 1.98 1.36
0.15
Cr
S
2.23
0.86
R. Li et al. / Journal of Hazardous Materials 280 (2014) 20–30
23
Fig. 2. SEM images of PA/CA (top surface (a), bottom surface (b) and cross-section (c)) and PANI-PA/CA (top surface (d), bottom surface (e) and cross-section (f)).
Fig. 4. Nitrogen adsorption–desorption isotherm and pore size distributions for PA/CA and PANI-PA/CA membrane.
Table 5 Porous structure parameters of PA/CA and PANI-PA/CA.
Fig. 3. EDX spectra of PANI-PA/CA: (a) before adsorption, (b) after adsorption of Hg(II) and (c) after adsorption of Cr(VI).
Sorbent sample
BET surface area (m2 g−1 )
BJH desorption average pore diameter (nm)
BJH desorption Cumulative volume of pores (cc/g)
PA/CA PANI-PA/CA
10.43 17.49
3.79 3.36
0.061 0.090
structure can be explained by two reasons: (1) a portion of PANI oligomer may be leached out of the casting film during phase separation and acts as a pore forming reagent, which increases the BET surface area and pore volume [22]; (2) with increasing PANI, the exchange between solvent and non-solvent may be hindered due to the increased viscosity, which leads to the decrease of membrane surface pore size [21,30].
24
R. Li et al. / Journal of Hazardous Materials 280 (2014) 20–30
120
a
b Adsorption capacity (mg/g)
Adsorption capacity (mg/g)
60
40
20
0
110
100
90
80
S1
S2
S3
S4
S5
S6
S7
S8
S9
0
3
6
9
12
Phytic acid (wt.%)
Sample
Fig. 5. Effect of PANI content and (b) phytic acid content on adsorption of Hg(II) (C0 (Hg) = 125.45 mg L−1 ; pH = 5.0; adsorption time = 3 h; temperature = 303 K).
Remaining rate (C/C0)
1.0
PANI-PA/CA PANI/CA
0.8
0.6
0.4
improvement is limited. To further investigate the impact of phytic acid, PANI-PA/CA membrane was de-doped by immersed in 8.0 pH buffer solution (0.1 M NH4 Cl/NH4 OH) for 12 h, then dipped with deionized water. PANI-PA/CA and the treated membrane called PANI/CA were added to 50 mL of various heavy metal ions solutions and the adsorption results were summarized in Fig. 6. It shows that doping of phytic acid can significantly improve the adsorption of heavy metal cations on polyaniline composite membrane. The mechanism of interaction between heavy metal ions and PANIPA/CA would be discussed later. In addition, the average electrical resistance of PANI-PA/CA membrane is around 20.11 k, while those of PANI/CA and CA membrane are about 270.76 k and ∞, respectively.
0.2
0.0
Cd(II) Pb(II) Cu(II) Zn(II) Hg(II) Total Cr Cr(VI)
Fig. 6. Adsorption of various heavy metal ions by PANI-PA/CA and PANI/CA (initial concentration: Hg (98.51 mg/L), Cu (68.85 mg/L), Cd (54.7 mg/L), Zn (65.78 mg/L), Pb (91.88 mg/L), Cr (96.6 mg/L); pH of Hg, Cu, Cd, Zn and Pb = 5.0; pH of Cr = 2.0; adsorption time = 8 h; temperature = 303 K).
3.2. Adsorption of Hg(II) and Cr(VI) by the PANI-PA/CA composite membrane 3.2.1. Effect of composition To select the optimal composition, varying the casting solution composition (Table 1), the adsorption capacity of prepared membranes for Hg(II) was tested. As shown in Fig. 5a, membranes with lower CA content have better adsorption property. However, if CA content is further reduced to less than 8 wt.%, the membrane can hardly form via phase inversion. The [aniline]/[APS] radio was kept as 1:1 for higher yield. The Hg(II) removal keeps increasing with the increase of aniline content (up to 3 wt.%) in casting solution, afterward it starts decreasing. 3 wt.% is the optimal aniline content for preparing PANI/CA with the highest adsorption property. Further increase of aniline and APS concentrations could possibly reduce the mobility of casting solution, which affect polymerization and yield of PANI. The trend in Hg(II) removal using PANI-PA/CA prepared with different phytic acid content is shown in Fig. 5b. Phytic acid improved the adsorption capacity of PANI-PA/CA membrane for Hg(II). The optimal phytic acid content is 6 wt.%, above which the
3.2.2. The effect of solution pH on adsorption of Hg(II) and Cr(VI) on PANI-PA/CA. The effect of pH (2.0–8.0) on Hg(II) and Cr(VI) removal was significant (Fig. 7). The optimum pH value is ∼5.0 for Hg(II) and 2.0 for Cr(VI), with the corresponding maximum adsorption capacities reaching 187.00 and 75.03 mg g−1 , respectively. The observed adsorption of Hg(II) and Cr(VI) are consistent with results reported [7,9,10]. For Cr(VI), the effect of solution pH could be explained through electrostatic attraction and redox mechanism. The predominant Cr(VI) species are anion (HCrO4 − , Cr2 O7 2− ) at pH 2.0–6.0, which could be adsorbed on positively charged polyaniline (PZCPANI ∼ 5.8). As the pH increases, the competition between OH− and Cr(VI) species results in the decrease of adsorption capacity. Moreover, due to the high redox potential value of Cr(VI), it is possible that Cr(VI) species are reduced to Cr(III) ones by polyaniline. The reduction reaction of Cr(VI) increases with the decrease of solution pH. For Hg(II), the de-protonation of the functional groups on adsorbent results in the increased uptake of Hg(II) as pH increases from 2.0 to 5.0. At pH value > 6.0, the adsorption of Hg(II) on negatively charged PANI decreases as Hg(OH)2 dominates gradually [31,32].
3.2.3. Adsorption kinetics for mercury (II), Cr(VI) and effect of contact time The adsorption of Hg(II) and Cr(VI) were investigated in the ranges of 3–600 min (Fig. 8). For both Hg(II) and Cr(VI), the adsorptions increase with the time until the equilibrium are reached (600 min). For Hg(II) and Cr(VI), 80% of the adsorption capacities are reached within 180 min and 240 min, and the measured
R. Li et al. / Journal of Hazardous Materials 280 (2014) 20–30
80
Cr(VI) 190 60 180 40 170
a
0.15
Hg(II) Cr(VI)
0.10
t/qt
Hg(II)
Adsorption capacity for Cr(VI) (mg/g)
Adsorption capacity for Hg(II) (mg/g)
200
25
0.05
20
160 2
3
4
5
6
7
8
pH
0.00 0.0
0.1
0.2
Fig. 7. Effect of pH on Hg(II) and Cr(VI) removal by PANI-PA/CA (C0 (Hg) = 250.23 mg L−1 ; C0 (Cr) = 67.29 mg L−1 ; adsorption time = 12 h; temperature = 303 K).
6
Hg(II) Cr(VI)
Hg(II) Cr(VI) 150
4
t/qt
Adsorption capacity (mg/g)
0.5
1/t (min )
b
100
2 50
0
0 0
60
120 180 240 300 360 420 480 540 600 660
0
50
Time (min)
corresponding equilibrium adsorption capacities are 192.92 and 71.98 mg g−1 , respectively. The adsorption kinetic data was analyzed using the pseudofirst-order equation and pseudo-second-order equation which are expressed as follows: 1 k1 1 = (3) + qt qe t qe t 1 t = + qt qe k2 qe 2
100
150
200
250
300
350
400
t (min)
Fig. 8. Effect of adsorption time on adsorption capacity (Hg(II): C0 = 223.33 mg L−1 ; pH = 5.0 and Cr(VI): C0 = 67.28 mg L−1 ; pH = 2.0; temperature = 303 K).
(4)
where k1 (min−1 ) and k2 (g/mg/min) are the equilibrium rate constants of pseudo-first-order equation and pseudo-secondorder equation, respectively. And the initial adsorption rate h (mg/(g min)) could be calculated with k2 and qe value: h=
0.4
-1
200
k2 q2e
0.3
(5)
The fitting plots are shown in Fig. 9a and b. The kinetic parameters acquired from fitting results are summarized in Table 6. According to the correlation coefficients, the pseudo-second-order equation is the better model to describe the adsorption processes of Hg(II) and Cr(VI) on PANI-PA/CA membranes. The qe calculated from the pseudo-second-order equation appeared to be close to the
Fig. 9. Pseudo-first-order kinetic model (a) and pseudo-second-order kinetic model (b) for adsorption of Hg(II) and Cr(VI) onto PANI-PA/CA.
experimental value. The initial adsorption rate of Hg(II) and Cr(VI) on PANI-PA/CA are 2.91 and 1.50 mg/(g min), respectively. 3.2.4. Adsorption equilibrium isotherms For isotherm and capacity studies, adsorption experiments were conducted with different initial concentrations of Hg(II) or Cr(VI). The adsorption time was 24 h for complete equilibrium and the adsorption isotherms for Hg(II) and Cr(II) are shown in Fig. 10a and b. The Langmuir and Freundlich isotherms were used to analysis the isotherm data (Fig. 11a–d.). Langmuir: Ce 1 Ce = + qe qm bqm
(6)
Freundlich: ln qe = ln KF +
ln Ce n
(7)
where qm (mg g−1 ) is the theoretical adsorption capacity, respectively. Ce (mg L−1 ) is the equilibrium concentration of heavy metal ions, b (L mg−1 ) is the adsorption equilibrium constant of
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R. Li et al. / Journal of Hazardous Materials 280 (2014) 20–30
Table 6 Kinetic parameters for the adsorption of Hg(II) and Cr(VI) by PANI-PA/CA composite membrane. Metal
Pseudo-first-order
Hg(II) Cr(VI)
Pseudo-second-order
qe (mg g−1 )
K1 (min−1 )
R2
qe (mg g−1 )
k2 (g mg−1 min−1 )
R2
179.86 52.16
51.20 12.68
0.9910 0.9404
204.92 77.5
6.92 × 10−5 2.49 × 10−4
0.9982 0.9921
270
100
a
b 90
qe (mg/g)
80
210
70
180 200
300
400
50
500
100
150
C0 (mg/L)
C0 (mg/L)
Fig. 10. Adsorption isotherms of Hg (a) and Cr (b) by PANI-PA/CA (temperature = 303 K; contact time = 24 h).
0.9
a
b
5.6
0.6 lnqe
Ce/qe
5.5
5.4
0.3
5.3
0.0 0
60
120
180
3
240
0.8
4
5
6
lnCe
Ce
4.6
c
d 4.5
0.4
lnqe
Ce/qe
qe (mg/g)
240
4.4
4.3 0.0
4.2 0
20
40
Ce
60
80
-4
-2
0
2
4
lnCe
Fig. 11. Isotherm models for the Hg (Langmuir (a) and Freundlich (b)) and Cr (Langmuir (c) and Freundlich (d)) adsorption on PANI-PA/CA.
R. Li et al. / Journal of Hazardous Materials 280 (2014) 20–30
Langmuir model, KF is the binding energy constant. n is the Freundlich exponent related to adsorption intensity. The fitting results are summarized in Table 7. The Langmuir correlation coefficient of Hg(II) adsorption is larger than that of Freundlich, which suggests a monolayer adsorption. In comparison, both isotherms correlated well with Cr(VI) by PANI-PA/CA, indicating adsorption of Cr(VI) is not limited to just a monolayer mechanism. In addition, the qm of PANI-PA/CA determined from the Langmuir isotherm are 280.11 and 94.34 mg g−1 for Hg(II) and Cr(VI), respectively. The qm of PANI-PA/CA are higher than some reported qm values of other adsorbents (Table 8).
a
Adsorption capacity (mg/g)
210
1st 3rd
2nd 4th
140
70
3.2.5. Desorption To investigate the desorption and regeneration potential of adsorbent, solutions with various pH were used as the desorption agent for regenerating metal loaded PANI-PA/CA composite membrane. As shown in Fig. 12a, both acid and alkali are effective desorption medium for Hg(II) loaded PANI-PA/CA. The membrane has the maximum reproducibility in acidic solution, which reserves 85% adsorption capacity after three recycles. Compared to Hg(II), the regeneration potential of Cr(VI) loaded PANI-PA/CA membrane is relatively weak (Fig. 12b), which can be attributed to oxidation of PANI by Cr(VI). Electrochemical regeneration may be more effective, which is under study in the next stage.
0 1
2
3
10
8
pH
b
Adsorption capacity (mg/g)
80
1st 3rd
2nd 4th
60
40
20
0 7
8
9
27
10
pH Fig. 12. Desorption of Hg (a) and Cr (b) from metal-loaded PANI-PA/CA using solutions with different pH (50 mL of desorption medium; desorption time = 5 h; temperature = 303 K).
3.2.6. Simulated wastewater tests with mixed heavy metal ions The prepared PANI-PA/CA membrane was tested to remove Hg(II) and Cr(VI) in simulated wastewater. As shown in Fig. 13a and b, the remaining rate, respectively, drop to 0.241 and 0.001 for Hg(II) and Cr(VI) as the dosage of adsorbent increase to 0.759 g/L. For Hg(II)-containing wastewater, PANI-PA/CA also has high adsorption capacity for Pb(II), consistent with the fact that doping of phytic acid could improve the practical ability of adsorbent. The removal of Zn(II) and Cu(II) in Cr(VI)-containing water sample are not as well as in single treatment due to the low pH. On the basis of this study and previous literatures [33,34], a possible schematic mechanism for metal ions adsorption by PANIPA/CA was proposed (Scheme 1a and b). The nitrogen-containing functional groups of polyaniline could form a much more stable complex with Hg(II) ions than other heavy metal cations. The doping of phytic acid increases adsorption of all studied cations on polyaniline composite, which can be explained by the reason that the phosphate ions can form stable complexes with these metal ions [26]. The adsorption of Cr(VI) could be attributed to the interaction between Cr(VI) oxyanions and the protonated nitrogencontaining functional groups of polyaniline at low pH. A portion of 1.0
1.0
a
b 0.8
Remaining rate C/C0
Remaining rate C/C0
0.8 0.4
0.6
0.2
0.0
Pb(II)
Cd(II)
0.4
0.6
0.2
0.4
0.2
0.0
Cu(II)
Total Cr
Zn(II)
0.2
Hg(II) wastewater 0.2 0.0
0.4
Cr(VI) wastewater 0.4
Adsorbent dose (g/L)
0.6
0.8
0.0 0.0
0.2
0.4
0.6
0.8
Adsorbent dose (g/L)
Fig. 13. The effect of adsorbent dosage on the adsorption of Hg(II) and Cr(VI) from simulated wastewater samples (Inset is the adsorption of other coexistence elements).
28
R. Li et al. / Journal of Hazardous Materials 280 (2014) 20–30
Scheme 1. The proposed adsorption mechanisms of (a) heavy metal cations and (b) Cr(VI) oxyanions on PANI-PA/CA.
R. Li et al. / Journal of Hazardous Materials 280 (2014) 20–30
29
Table 7 Isotherm parameters for the adsorption of Hg(II) and Cr(VI) by PANI-PA/CA composite membrane. Metal
Hg(II) Cr(VI)
Langmuir
Freundlich
qm (mg g−1 )
b (L mg−1 )
R2
KF
n
R2
280.11 94.34
21.93 140.85
0.9982 0.9994
129.10 79.92
7.30 21.88
0.9711 0.9962
Table 8 Comparison of the maximum adsorption capacities of Hg(II) and Cr(VI) on different adsorbents. Adsorbents
qmax (mg/g) Hg(II)
Poly(vinylalcohol)/poly(ethyleneimine) complexing membrane Polyacrylonitrile/polyaniline core/shell nanofiber Macroporous styrene divinylbenzene copolymer Poly(vinylalcohol)/poly(vinylimidazole) complexing membrane p(HEMA/Chitosan) membrane Aminated resin NH2 -functionalized cellulose acetate/silica composite membrane Polyvinyl-chloride inclusion membrane Procion green H-4G immobilized poly(hydroxyethylmethacrylate/chitosan) composite membrane PA-PANI/CA
Cr(VI) adsorbed on polyaniline could be reduced to Cr(III) due to the high oxidizing ability of Cr(VI). In addition, it is well known that synthesis of polyaniline under more acidic condition could increase yield and achieve more excellent performance [35,36]. 4. Conclusions In this work, we presented a novel and green method for the preparation of PANI-PA/CA composite membrane. Polymerization of aniline in CA solution and followed phase-inversion process simplified the preparation process of PANI composite membrane. Compared with PA/CA, PANI-PA/CA composite membrane obtained with this method possesses higher pore volume and surface area. The optimum pH for the adsorption of Hg(II) and Cr(VI) were respectively 5.0 and 2.0 due to the different removal mechanism. The mercury (II) removal was caused by complexation between mercury and the N-containing functional groups. Meanwhile the Cr(VI) removal involved electrostatic attraction and redox mechanism. For both adsorption of Hg(II) and Cr(VI), the process fitted pseudo-second-order kinetic model. The maximum adsorption capacity of Hg(II) and Cr(VI) reached 280.11 and 94.34 mg g−1 . The PANI-PA/CA composite membrane reserved the adsorption property of PANI and possesses higher practical applicability in control and remediation of mercury and chromium pollutions. Acknowledgement The financial support from China National Natural Science Foundation (No. 21177018) is greatly acknowledged. References [1] Y. Wu, S. Wang, D.G. Streets, J. Hao, M. Chan, J. Jiang, Trends in anthropogenic mercury emissions in China from 1995 to 2003, Environ. Sci. Technol. 40 (2006) 5312–5318. [2] R. Nakagawa, Y. Yumita, Change and behavior of residual mercury in paddy soils and rice of Japan, Chemosphere 37 (1998) 1483–1487. [3] O. Lindqvist, Special issue of first international on mercury as a global pollutant, Water Air Soil Pollut., 56 (1991). [4] M.C. Alvim-Ferraz, S.E.R.A. Afonso, Incineration of different types of medical wastes: emission factors for particulate matter and heavy metals, Environ. Sci. Technol. 37 (2003) 3152–3157. [5] K.J.L. Kroschwitz, Encylopedia of Chemical Technology, 1991. [6] U. EPA, National primary drinking water regulations, Total Coliforms (Including Fecal Coliforms and E. Coli), 54 (2009).
Reference Cr(VI)
215 88.11 156.25 118.3 42.1 155.99 19.46 50.85 56.49 280.11
94.34
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Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Removal of aqueous Hg(II) and Cr(VI) using phytic acid doped polyaniline/cellulose acetate composite membrane Renjie Li a , Lifen Liu a,b,∗ , Fenglin Yang a a MOE, Key Laboratory of Industrial Ecology and Environmental Engineering, School of Environmental Science and Technology, Dalian University of Technology, Dalian, China b School of Food and Environmental Science and Technology, Dalian University of Technology, Panjin, China
h i g h l i g h t s • • • • •
PANI/CA membrane was studied for adsorbing Hg(II) and Cr(VI). The new membrane preparation method is convenient, does not use toxic solvent. The doping agent-Phytic acid improved the adsorptive capacity. PANI-PA/CA membrane possesses higher BET surface area and electrical conductivity. PANI-PA/CA membrane has high adsorptive capacity for Hg(II) and Cr(VI).
a r t i c l e
i n f o
Article history: Received 19 April 2014 Received in revised form 28 June 2014 Accepted 24 July 2014 Available online 2 August 2014 Keywords: Polyaniline Cellulose acetate Hg(II) Cr(VI) Adsorption
a b s t r a c t Conductive composite membrane—phytic acid (PA) doped polyaniline (PANI)/cellulose acetate (CA) (PANI-PA/CA) was prepared in a simple and environmental-friendly method, in which aniline was blended with CA/PA solution and polymerized before the phase conversion. The resultant composite membranes were characterized by SEM, EDX, FTIR-ATR, BET and electrical resistance measurements. When used as adsorbent for Hg(II) and Cr(VI) ions, the prepared composite membrane exhibits excellent adsorption capability. The adsorption of Hg(II) and Cr(VI) follows a pseudo-second-order kinetic model and best fits the Langmuir isotherm model, with the maximum adsorption capacity reaching 280.11 and 94.34 mg g−1 , respectively. The heavy metal loaded composite membrane can be regenerated and reused after treatment with acid or alkali solution, making it a promising and practical adsorbent for Hg(II) and Cr(VI) removal. Tests with river water were also carried out, indicating good performance and application. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Heavy metal pollution has become a world-wide issue affecting the existence and development of human society due to industrializations. As toxic and potential carcinogenic metal, mercury pollution is found from industries [1], agriculture [2] and waste incineration [3,4], and chromium pollution is generated from electroplating, mining and cement/chemical production [5]. Adsorption is often used as one feasible technology due to its low initial cost, simplicity in design, ease of operation and insensitivity to toxic substances. Governments enforce strict emission control
∗ Corresponding author at: MOE, Key Laboratory of Industrial Ecology and Environmental Engineering, School of Food and Environmental Science and Technology, Dalian University of Technology, Panjin, China. Tel.: +86 427 2631799; fax: +86 411 84708083. E-mail addresses: [email protected], [email protected] (L. Liu). http://dx.doi.org/10.1016/j.jhazmat.2014.07.052 0304-3894/© 2014 Elsevier B.V. All rights reserved.
policies for mercury and hexavalent chromium [6]. However, more effective adsorbents and more simple technologies are still in great demand to remove aqueous heavy metal ions. The low-cost, highly stable and capacitive polyaniline (PANI) is often used for heavy metal removal. Its nitrogen-containing functional groups can complex with Hg(II) [7–9], and adsorb anions through electrostatic interaction or hydrogen bonding [8,10]. Although PANI show good performance for removal of heavy metal ions from aqueous solutions, the mechanical strength and solubility of PANI are very poor and a separation step has to be taken after adsorption. To overcome the difficulties in its preparation and applications, PANI was often loaded on carriers such as Fe3 O4 [11,12], carbon cloth [13,14] and polymer fiber mat [15,16]. Membrane adsorbents are easy to separate and the conductive ones may be used as electrodes in electro-reduction or other electrochemical applications. Recently, PANI was incorporated as composites to improve membrane separation characteristics [17–20]. The PANI oligomer
R. Li et al. / Journal of Hazardous Materials 280 (2014) 20–30
was reported to perform well as a pore forming substance [19,21,22]. To obtain PANI composite membrane, NMP or DMF solvents are usually needed to dissolve PANI, that is not only bad for environment, but also the product membranes have poor conductivity. For preparing high performance conductive membrane with PANI, a new environmental-friendly method was proposed in this paper. Using acetic acid solution (HAc:water = 72:28) as solvent, aniline was polymerized in the presence of dissolved CA [23]. Thus, polyaniline (PANI)/cellulose acetate (CA) composite membrane was prepared by casting CA solution containing in-situ polymerized PANI, without any separation and drying processing. Phytic acid (PA) as a doping agent could not only influence the polymerized PANI, but also complex with heavy metal ions [24–26]. So the effect of PA on the adsorptive property of PANI/CA composite membrane was investigated. The characterizations (SEM, EDX, ATR-FTIR, BET and electrical resistance measurement) and adsorptive property of the membrane for Hg(II) and Cr(VI) were studied. The optimal composite proportion and the effect of pH, adsorption time and initial concentration were investigated. Kinetic and thermodynamics, reproducibility and reusability were also studied. 2. Experimental 2.1. Reagents Zinc powder (99.0%) was purchased from Tianjin Chemical Works. Chromic nitrate, copper sulphate anhydrous (CuSO4 , 99.0%), Aniline (99.5%) and phytic acid (50%) were purchased from Tianjin Bodi Chemical Co. Ltd. Cellulose acetate (CA, acetyl content: 54.6%) and lead nitrate (Pb(NO3 )2 , 99.0%) were purchased from Sinopharm Chemical Reagent Co. Ltd. Ammonium persulfate (APS, (NH4 )2 S2 O8 , 98%), sodium hydroxide (NaOH, 96%), Cadmium nitrate tetrahydrate (Cd(NO3 )2 ·4H2 O, 99.0%) and ammonium chloride (NH4 Cl, 99.5%) were purchased from Tianjin Damao Chemical Reagent Co. Ltd. Nitric acid (65–68%) was purchased from Beijing Chemical Works. Acetic acid (99.5%) and ammonia solution (25%) were purchased from Tianjin Fuyu Fine Chemical Co. Ltd. Mercury (II) nitrate (Hg(NO3 )2 ·1/2H2 O, 97%) was obtained from Jiangyan Huanqiu Chemical Works. Potassium dichromate (K2 Cr2 O7 , 99.8%) was purchased from Shenyang Chemical Works. The materials were all used as received. 2.2. Preparation of PANI-PA/CA composite membrane The PANI/CA composite membrane was synthesized via chemical polymerization of aniline in CA solution. CA/acetic acid/water casting solutions were prepared by dissolving CA into acetic acid/water (72/28). Then aniline was blended with the CA solution in ice bath. The mixture was stirred at 0–5 ◦ C for 30 min and APS was added very slowly for well dispersion. The mixture was stirred at this temperature for 8 h. The casting solution was left still for 0.5 h to release bubbles. After that, the casting solution was cast onto a glass plate with a steel knife to obtain a casting film of 400 m thickness. The film was exposed to atmosphere for 10 s, and then immersed in a coagulation bath of pure water over night. The contents of CA and aniline were varied to select the best composition of membrane for Hg(II) removal (Composition of the casting solutions seen in Table 1). To investigate the effect of phytic acid, the PANI-PA/CA composite membrane was prepared by adding a certain amount of phytic acid (3–12 wt.%) in the acetic acid/water solution. 2.3. Characterization of PANI-PA/CA membrane The surface and cross-section morphology of PA/CA and PANIPA/CA membrane (component proportion seen in Table 2) were
21
Table 1 Composition of the casting solutions. Prepared membrane
CA (%)
Aniline (%)
APS (%)
Acetic acid solution (%)
M-0 M-1 M-2 M-3 M-4 M-5 M-6 M-7 M-8
10 10 10 10 10 8 8 8 8
0 1 2 3 4 1 2 3 4
0 1.23 2.45 3.68 4.9 1.23 2.45 3.68 4.9
90 87.77 85.55 83.32 81.1 89.77 87.55 85.32 83.1
Table 2 Component proportions of PA/CA and PANI-PA/CA. Prepared membrane
CA (%)
Aniline (%)
PA (%)
APS (%)
Acetic acid/water (%)
PA/CA PA-PANI/CA
11 8
0 3
6 6
0 3.68
83 79.32
investigated using the scanning electron microscope (SEM, FEI Quanta 200 FEG). PANI-PA/CA membrane before and after adsorption were analyzed with energy dispersive X-ray (EDX, FEI Quanta 200 FEG). ATR-FTIR and BET measurement were recorded using Shimadzu 8400s spectrometer and Quantachrome NOWA 4000 apparatus using N2 adsorption (77.3 K), respectively. The electrical resistance of dry membrane was measured repeatedly by fixing two probes (distance: 2 cm) on that 3 cm × 3 cm membrane. 2.4. Adsorption experiments Batch adsorption experiments were conducted to investigate the Hg(II) and Cr(VI) adsorption by the prepared membrane. Hg(NO3 )2 and K2 Cr2 O7 were used as the source of Hg(II) and Cr(VI), respectively. A piece of 3 cm × 3 cm membrane was added into a 100 mL conical flask containing 50 mL of metal ions solution at 303 K and 150 rpm. The solution pH was adjusted with 0.1 M NaOH, 0.1 M HNO3 (Hg) or 0.1 M H2 SO4 (Cr). The membrane after adsorption was cleaned with deionized water, dried in vacuum oven at 60 ◦ C and weighed. Kinetic studies were carried out by immersion of PANI-PA/CA membrane in Hg(II) or Cr(VI) solutions, with an initial concentration of 223.33 or 67.28 mg L−1 , respectively. During adsorption, the solutions samples were withdrawn at certain time intervals for the measurement of heavy metal ions concentrations. The adsorption capacity qt (mg g−1 ) at time t was obtained using the following equation (1): qt =
C0 − Ct ×V W
(1)
where C0 (mg L−1 ) is the initial concentration of metal ions. Ct (mg L−1 ) is the concentration of metal ions at any time t, V (mL) is the volume of solution and W (g) is the amount of adsorbents. For the isotherm studies, batch adsorptions were conducted for 24 h to reach complete equilibrium with different initial concentrations of heavy metal ions. The solution pH (5.0 for mercury, 2.0 for hexavalent chromium) and temperature (303 K) were kept constant. The adsorption capacity of the absorbents at equilibrium was obtained using the following equation (2): qe =
C0 − Ce ×V W
(2)
22
110.49
Cd(II) Pb(II)
0.58 2.91
Cr(VI)
21.60
Zn(II) Cu(II)
40.72 34.17
2.19
PA/CA
802 904
Cr(VI)containing
3452
14.53
where qe (mg g−1 ) is the equilibrium adsorption capacity of absorbent. Ce (mg L−1 ) is the equilibrium concentration of metal ions.
3446
Hg(II)
3251
Hg(II)containing
2927
Chemical oxygen demand (CODCr , mg/L)
2935
Initial concentration (mg/L)
1736
Metal
Absorbance
Wastewater sample
904 1036 1157 1159 1230 1313 1371 1369 1431 1491 1604 1639 1736
1036
Table 3 Characteristics of synthetic wastewater samples.
1232
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PA-PANI/CA 600
900
1200
2.5. Regeneration tests
1500
1800
2100
2400
2700
3000
3300
3600
-1
Wavenumber (cm )
For desorption studies, a piece of 3 cm × 3 cm PANI-PA/CA membrane firstly adsorbed heavy metal ions in 50 mL solution (Hg(II) 236.67 mg L−1 or Cr(VI) 45.89 mg L−1 ) for 5 h at 303 K. Then using 50 mL solutions at different pH for desorption, the heavy metal loaded membranes were placed in these medium and stirred for 5 h at room temperature. The above procedure was repeated 4 times to investigate the reusability of the adsorbents.
Fig. 1. ATR-FTIR spectra of PA/CA and PANI-PA/CA.
802, 1313, 1491, 1604 and 3251 cm−1 are observed in the spectra of PANI-PA/CA. They respectively correspond to para-disubstituted benzene ring, aromatic (C–N)+ stretching, benzenoid ring stretching, quinoid ring stretching and N–H stretching [29]. The ATR-FTIR spectrum of PANI-PA/CA indicates that the compositing of PANI, PA and CA is successful.
2.6. Simulated wastewater tests with mixed heavy metal ions
3.1.2. SEM images and EDX analysis The SEM was used to study the morphologies of the prepared membranes. As shown in Fig. 2, the membranes consist of a dense top layer and a porous sub-layer. For PA/CA membrane, the cross-section (Fig. 2b) shows an asymmetrical structure with a dense layer and finger-like macro-voids. The bottom surface (Fig. 2c) shows a fibre-network structure. For PANI-PA/CA membrane (Fig. 2d–f), there exists a remarkable difference in the pore structure. With PANI-PA, the macro-voids become larger due to the large amount of pore-forming agent like APS, PANI oligomer and water. In the image of bottom surface (Fig. 2f), a network structure of fiber and particles is attributed to the PANI. Moreover, the EDX results are shown in Fig. 3 and Table 4, confirming Hg and Cr elements exist in the PANI-PA/CA after adsorption. For Cr adsorbed membrane, despite of the addition of H2 SO4 , which brings elemental sulfur and part of elemental oxygen, the great increase of oxygen content indicate a portion of Cr adsorbed on PANI-PA/CA was in the form of Cr(VI) oxyanions.
To investigate the practical applicability of PANI-PA/CA composite membrane, two kinds of synthetic wastewater containing various heavy metal ions were prepared according to previous literatures [27,28]. Water from a local Linshui River was used in dissolving the heavy metal salts. The characteristics of the samples were listed in Table 3. For treatment, pH of mercury-containing and chromium-containing wastewater samples were adjusted to 5.0 and 2.0, respectively. A certain weight of PANI-PA/CA composite membrane was added to 50 mL simulated wastewater sample and shaken continuously for 8 h at 25 ◦ C. The concentrations of Pb(II), Zn(II), Cu(II) and Cd(II) in solution before and after adsorption were determined using inductively coupled plasma spectrometer (Optima 2000 DV, Perkin Elmer). The amount of Hg(II) and Cr(VI) in the solution before and after adsorption were measured spectrophotometrically, using dithizone and diphenylcarbazide as the corresponding complexing agents (GB 7496-87 and 7467-87, China). 3. Results and discussion
3.1.3. BET analysis The porous structure parameters of PA/CA and PANI-PA/CA were measured and shown in Fig. 4 and Table 5. It can be found that the BET surface area of PANI-PA/CA membrane displays nearly 67.7% increase compared with that of PA/CA membrane. As shown in Table 5, PANI-PA/CA membrane has higher pore volume than PA/CA. However, the average surface pore size of membrane decreases with PANI incorporated. The changes of membrane pore
3.1. The characterization and properties of the prepared membrane material 3.1.1. ATR-FTIR spectra The ATR-FTIR spectra of PA/CA and PANI-PA/CA membrane are shown in Fig. 1. The characteristic bands of CA and phytic acid are very intensive in both spectra. Compared to PA/CA, new peaks at Table 4 Element composition of PANI-PA/CA before and after adsorption. Sample
Before adsorption After adsorption of Hg(II) After adsorption of Cr(VI)
Atomic percent (%) C
N
O
P
Hg
71.66 70.44 56.44
1.80 3.01 4.07
24.49 24.43 35.04
2.04 1.98 1.36
0.15
Cr
S
2.23
0.86
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23
Fig. 2. SEM images of PA/CA (top surface (a), bottom surface (b) and cross-section (c)) and PANI-PA/CA (top surface (d), bottom surface (e) and cross-section (f)).
Fig. 4. Nitrogen adsorption–desorption isotherm and pore size distributions for PA/CA and PANI-PA/CA membrane.
Table 5 Porous structure parameters of PA/CA and PANI-PA/CA.
Fig. 3. EDX spectra of PANI-PA/CA: (a) before adsorption, (b) after adsorption of Hg(II) and (c) after adsorption of Cr(VI).
Sorbent sample
BET surface area (m2 g−1 )
BJH desorption average pore diameter (nm)
BJH desorption Cumulative volume of pores (cc/g)
PA/CA PANI-PA/CA
10.43 17.49
3.79 3.36
0.061 0.090
structure can be explained by two reasons: (1) a portion of PANI oligomer may be leached out of the casting film during phase separation and acts as a pore forming reagent, which increases the BET surface area and pore volume [22]; (2) with increasing PANI, the exchange between solvent and non-solvent may be hindered due to the increased viscosity, which leads to the decrease of membrane surface pore size [21,30].
24
R. Li et al. / Journal of Hazardous Materials 280 (2014) 20–30
120
a
b Adsorption capacity (mg/g)
Adsorption capacity (mg/g)
60
40
20
0
110
100
90
80
S1
S2
S3
S4
S5
S6
S7
S8
S9
0
3
6
9
12
Phytic acid (wt.%)
Sample
Fig. 5. Effect of PANI content and (b) phytic acid content on adsorption of Hg(II) (C0 (Hg) = 125.45 mg L−1 ; pH = 5.0; adsorption time = 3 h; temperature = 303 K).
Remaining rate (C/C0)
1.0
PANI-PA/CA PANI/CA
0.8
0.6
0.4
improvement is limited. To further investigate the impact of phytic acid, PANI-PA/CA membrane was de-doped by immersed in 8.0 pH buffer solution (0.1 M NH4 Cl/NH4 OH) for 12 h, then dipped with deionized water. PANI-PA/CA and the treated membrane called PANI/CA were added to 50 mL of various heavy metal ions solutions and the adsorption results were summarized in Fig. 6. It shows that doping of phytic acid can significantly improve the adsorption of heavy metal cations on polyaniline composite membrane. The mechanism of interaction between heavy metal ions and PANIPA/CA would be discussed later. In addition, the average electrical resistance of PANI-PA/CA membrane is around 20.11 k, while those of PANI/CA and CA membrane are about 270.76 k and ∞, respectively.
0.2
0.0
Cd(II) Pb(II) Cu(II) Zn(II) Hg(II) Total Cr Cr(VI)
Fig. 6. Adsorption of various heavy metal ions by PANI-PA/CA and PANI/CA (initial concentration: Hg (98.51 mg/L), Cu (68.85 mg/L), Cd (54.7 mg/L), Zn (65.78 mg/L), Pb (91.88 mg/L), Cr (96.6 mg/L); pH of Hg, Cu, Cd, Zn and Pb = 5.0; pH of Cr = 2.0; adsorption time = 8 h; temperature = 303 K).
3.2. Adsorption of Hg(II) and Cr(VI) by the PANI-PA/CA composite membrane 3.2.1. Effect of composition To select the optimal composition, varying the casting solution composition (Table 1), the adsorption capacity of prepared membranes for Hg(II) was tested. As shown in Fig. 5a, membranes with lower CA content have better adsorption property. However, if CA content is further reduced to less than 8 wt.%, the membrane can hardly form via phase inversion. The [aniline]/[APS] radio was kept as 1:1 for higher yield. The Hg(II) removal keeps increasing with the increase of aniline content (up to 3 wt.%) in casting solution, afterward it starts decreasing. 3 wt.% is the optimal aniline content for preparing PANI/CA with the highest adsorption property. Further increase of aniline and APS concentrations could possibly reduce the mobility of casting solution, which affect polymerization and yield of PANI. The trend in Hg(II) removal using PANI-PA/CA prepared with different phytic acid content is shown in Fig. 5b. Phytic acid improved the adsorption capacity of PANI-PA/CA membrane for Hg(II). The optimal phytic acid content is 6 wt.%, above which the
3.2.2. The effect of solution pH on adsorption of Hg(II) and Cr(VI) on PANI-PA/CA. The effect of pH (2.0–8.0) on Hg(II) and Cr(VI) removal was significant (Fig. 7). The optimum pH value is ∼5.0 for Hg(II) and 2.0 for Cr(VI), with the corresponding maximum adsorption capacities reaching 187.00 and 75.03 mg g−1 , respectively. The observed adsorption of Hg(II) and Cr(VI) are consistent with results reported [7,9,10]. For Cr(VI), the effect of solution pH could be explained through electrostatic attraction and redox mechanism. The predominant Cr(VI) species are anion (HCrO4 − , Cr2 O7 2− ) at pH 2.0–6.0, which could be adsorbed on positively charged polyaniline (PZCPANI ∼ 5.8). As the pH increases, the competition between OH− and Cr(VI) species results in the decrease of adsorption capacity. Moreover, due to the high redox potential value of Cr(VI), it is possible that Cr(VI) species are reduced to Cr(III) ones by polyaniline. The reduction reaction of Cr(VI) increases with the decrease of solution pH. For Hg(II), the de-protonation of the functional groups on adsorbent results in the increased uptake of Hg(II) as pH increases from 2.0 to 5.0. At pH value > 6.0, the adsorption of Hg(II) on negatively charged PANI decreases as Hg(OH)2 dominates gradually [31,32].
3.2.3. Adsorption kinetics for mercury (II), Cr(VI) and effect of contact time The adsorption of Hg(II) and Cr(VI) were investigated in the ranges of 3–600 min (Fig. 8). For both Hg(II) and Cr(VI), the adsorptions increase with the time until the equilibrium are reached (600 min). For Hg(II) and Cr(VI), 80% of the adsorption capacities are reached within 180 min and 240 min, and the measured
R. Li et al. / Journal of Hazardous Materials 280 (2014) 20–30
80
Cr(VI) 190 60 180 40 170
a
0.15
Hg(II) Cr(VI)
0.10
t/qt
Hg(II)
Adsorption capacity for Cr(VI) (mg/g)
Adsorption capacity for Hg(II) (mg/g)
200
25
0.05
20
160 2
3
4
5
6
7
8
pH
0.00 0.0
0.1
0.2
Fig. 7. Effect of pH on Hg(II) and Cr(VI) removal by PANI-PA/CA (C0 (Hg) = 250.23 mg L−1 ; C0 (Cr) = 67.29 mg L−1 ; adsorption time = 12 h; temperature = 303 K).
6
Hg(II) Cr(VI)
Hg(II) Cr(VI) 150
4
t/qt
Adsorption capacity (mg/g)
0.5
1/t (min )
b
100
2 50
0
0 0
60
120 180 240 300 360 420 480 540 600 660
0
50
Time (min)
corresponding equilibrium adsorption capacities are 192.92 and 71.98 mg g−1 , respectively. The adsorption kinetic data was analyzed using the pseudofirst-order equation and pseudo-second-order equation which are expressed as follows: 1 k1 1 = (3) + qt qe t qe t 1 t = + qt qe k2 qe 2
100
150
200
250
300
350
400
t (min)
Fig. 8. Effect of adsorption time on adsorption capacity (Hg(II): C0 = 223.33 mg L−1 ; pH = 5.0 and Cr(VI): C0 = 67.28 mg L−1 ; pH = 2.0; temperature = 303 K).
(4)
where k1 (min−1 ) and k2 (g/mg/min) are the equilibrium rate constants of pseudo-first-order equation and pseudo-secondorder equation, respectively. And the initial adsorption rate h (mg/(g min)) could be calculated with k2 and qe value: h=
0.4
-1
200
k2 q2e
0.3
(5)
The fitting plots are shown in Fig. 9a and b. The kinetic parameters acquired from fitting results are summarized in Table 6. According to the correlation coefficients, the pseudo-second-order equation is the better model to describe the adsorption processes of Hg(II) and Cr(VI) on PANI-PA/CA membranes. The qe calculated from the pseudo-second-order equation appeared to be close to the
Fig. 9. Pseudo-first-order kinetic model (a) and pseudo-second-order kinetic model (b) for adsorption of Hg(II) and Cr(VI) onto PANI-PA/CA.
experimental value. The initial adsorption rate of Hg(II) and Cr(VI) on PANI-PA/CA are 2.91 and 1.50 mg/(g min), respectively. 3.2.4. Adsorption equilibrium isotherms For isotherm and capacity studies, adsorption experiments were conducted with different initial concentrations of Hg(II) or Cr(VI). The adsorption time was 24 h for complete equilibrium and the adsorption isotherms for Hg(II) and Cr(II) are shown in Fig. 10a and b. The Langmuir and Freundlich isotherms were used to analysis the isotherm data (Fig. 11a–d.). Langmuir: Ce 1 Ce = + qe qm bqm
(6)
Freundlich: ln qe = ln KF +
ln Ce n
(7)
where qm (mg g−1 ) is the theoretical adsorption capacity, respectively. Ce (mg L−1 ) is the equilibrium concentration of heavy metal ions, b (L mg−1 ) is the adsorption equilibrium constant of
26
R. Li et al. / Journal of Hazardous Materials 280 (2014) 20–30
Table 6 Kinetic parameters for the adsorption of Hg(II) and Cr(VI) by PANI-PA/CA composite membrane. Metal
Pseudo-first-order
Hg(II) Cr(VI)
Pseudo-second-order
qe (mg g−1 )
K1 (min−1 )
R2
qe (mg g−1 )
k2 (g mg−1 min−1 )
R2
179.86 52.16
51.20 12.68
0.9910 0.9404
204.92 77.5
6.92 × 10−5 2.49 × 10−4
0.9982 0.9921
270
100
a
b 90
qe (mg/g)
80
210
70
180 200
300
400
50
500
100
150
C0 (mg/L)
C0 (mg/L)
Fig. 10. Adsorption isotherms of Hg (a) and Cr (b) by PANI-PA/CA (temperature = 303 K; contact time = 24 h).
0.9
a
b
5.6
0.6 lnqe
Ce/qe
5.5
5.4
0.3
5.3
0.0 0
60
120
180
3
240
0.8
4
5
6
lnCe
Ce
4.6
c
d 4.5
0.4
lnqe
Ce/qe
qe (mg/g)
240
4.4
4.3 0.0
4.2 0
20
40
Ce
60
80
-4
-2
0
2
4
lnCe
Fig. 11. Isotherm models for the Hg (Langmuir (a) and Freundlich (b)) and Cr (Langmuir (c) and Freundlich (d)) adsorption on PANI-PA/CA.
R. Li et al. / Journal of Hazardous Materials 280 (2014) 20–30
Langmuir model, KF is the binding energy constant. n is the Freundlich exponent related to adsorption intensity. The fitting results are summarized in Table 7. The Langmuir correlation coefficient of Hg(II) adsorption is larger than that of Freundlich, which suggests a monolayer adsorption. In comparison, both isotherms correlated well with Cr(VI) by PANI-PA/CA, indicating adsorption of Cr(VI) is not limited to just a monolayer mechanism. In addition, the qm of PANI-PA/CA determined from the Langmuir isotherm are 280.11 and 94.34 mg g−1 for Hg(II) and Cr(VI), respectively. The qm of PANI-PA/CA are higher than some reported qm values of other adsorbents (Table 8).
a
Adsorption capacity (mg/g)
210
1st 3rd
2nd 4th
140
70
3.2.5. Desorption To investigate the desorption and regeneration potential of adsorbent, solutions with various pH were used as the desorption agent for regenerating metal loaded PANI-PA/CA composite membrane. As shown in Fig. 12a, both acid and alkali are effective desorption medium for Hg(II) loaded PANI-PA/CA. The membrane has the maximum reproducibility in acidic solution, which reserves 85% adsorption capacity after three recycles. Compared to Hg(II), the regeneration potential of Cr(VI) loaded PANI-PA/CA membrane is relatively weak (Fig. 12b), which can be attributed to oxidation of PANI by Cr(VI). Electrochemical regeneration may be more effective, which is under study in the next stage.
0 1
2
3
10
8
pH
b
Adsorption capacity (mg/g)
80
1st 3rd
2nd 4th
60
40
20
0 7
8
9
27
10
pH Fig. 12. Desorption of Hg (a) and Cr (b) from metal-loaded PANI-PA/CA using solutions with different pH (50 mL of desorption medium; desorption time = 5 h; temperature = 303 K).
3.2.6. Simulated wastewater tests with mixed heavy metal ions The prepared PANI-PA/CA membrane was tested to remove Hg(II) and Cr(VI) in simulated wastewater. As shown in Fig. 13a and b, the remaining rate, respectively, drop to 0.241 and 0.001 for Hg(II) and Cr(VI) as the dosage of adsorbent increase to 0.759 g/L. For Hg(II)-containing wastewater, PANI-PA/CA also has high adsorption capacity for Pb(II), consistent with the fact that doping of phytic acid could improve the practical ability of adsorbent. The removal of Zn(II) and Cu(II) in Cr(VI)-containing water sample are not as well as in single treatment due to the low pH. On the basis of this study and previous literatures [33,34], a possible schematic mechanism for metal ions adsorption by PANIPA/CA was proposed (Scheme 1a and b). The nitrogen-containing functional groups of polyaniline could form a much more stable complex with Hg(II) ions than other heavy metal cations. The doping of phytic acid increases adsorption of all studied cations on polyaniline composite, which can be explained by the reason that the phosphate ions can form stable complexes with these metal ions [26]. The adsorption of Cr(VI) could be attributed to the interaction between Cr(VI) oxyanions and the protonated nitrogencontaining functional groups of polyaniline at low pH. A portion of 1.0
1.0
a
b 0.8
Remaining rate C/C0
Remaining rate C/C0
0.8 0.4
0.6
0.2
0.0
Pb(II)
Cd(II)
0.4
0.6
0.2
0.4
0.2
0.0
Cu(II)
Total Cr
Zn(II)
0.2
Hg(II) wastewater 0.2 0.0
0.4
Cr(VI) wastewater 0.4
Adsorbent dose (g/L)
0.6
0.8
0.0 0.0
0.2
0.4
0.6
0.8
Adsorbent dose (g/L)
Fig. 13. The effect of adsorbent dosage on the adsorption of Hg(II) and Cr(VI) from simulated wastewater samples (Inset is the adsorption of other coexistence elements).
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R. Li et al. / Journal of Hazardous Materials 280 (2014) 20–30
Scheme 1. The proposed adsorption mechanisms of (a) heavy metal cations and (b) Cr(VI) oxyanions on PANI-PA/CA.
R. Li et al. / Journal of Hazardous Materials 280 (2014) 20–30
29
Table 7 Isotherm parameters for the adsorption of Hg(II) and Cr(VI) by PANI-PA/CA composite membrane. Metal
Hg(II) Cr(VI)
Langmuir
Freundlich
qm (mg g−1 )
b (L mg−1 )
R2
KF
n
R2
280.11 94.34
21.93 140.85
0.9982 0.9994
129.10 79.92
7.30 21.88
0.9711 0.9962
Table 8 Comparison of the maximum adsorption capacities of Hg(II) and Cr(VI) on different adsorbents. Adsorbents
qmax (mg/g) Hg(II)
Poly(vinylalcohol)/poly(ethyleneimine) complexing membrane Polyacrylonitrile/polyaniline core/shell nanofiber Macroporous styrene divinylbenzene copolymer Poly(vinylalcohol)/poly(vinylimidazole) complexing membrane p(HEMA/Chitosan) membrane Aminated resin NH2 -functionalized cellulose acetate/silica composite membrane Polyvinyl-chloride inclusion membrane Procion green H-4G immobilized poly(hydroxyethylmethacrylate/chitosan) composite membrane PA-PANI/CA
Cr(VI) adsorbed on polyaniline could be reduced to Cr(III) due to the high oxidizing ability of Cr(VI). In addition, it is well known that synthesis of polyaniline under more acidic condition could increase yield and achieve more excellent performance [35,36]. 4. Conclusions In this work, we presented a novel and green method for the preparation of PANI-PA/CA composite membrane. Polymerization of aniline in CA solution and followed phase-inversion process simplified the preparation process of PANI composite membrane. Compared with PA/CA, PANI-PA/CA composite membrane obtained with this method possesses higher pore volume and surface area. The optimum pH for the adsorption of Hg(II) and Cr(VI) were respectively 5.0 and 2.0 due to the different removal mechanism. The mercury (II) removal was caused by complexation between mercury and the N-containing functional groups. Meanwhile the Cr(VI) removal involved electrostatic attraction and redox mechanism. For both adsorption of Hg(II) and Cr(VI), the process fitted pseudo-second-order kinetic model. The maximum adsorption capacity of Hg(II) and Cr(VI) reached 280.11 and 94.34 mg g−1 . The PANI-PA/CA composite membrane reserved the adsorption property of PANI and possesses higher practical applicability in control and remediation of mercury and chromium pollutions. Acknowledgement The financial support from China National Natural Science Foundation (No. 21177018) is greatly acknowledged. References [1] Y. Wu, S. Wang, D.G. Streets, J. Hao, M. Chan, J. Jiang, Trends in anthropogenic mercury emissions in China from 1995 to 2003, Environ. Sci. Technol. 40 (2006) 5312–5318. [2] R. Nakagawa, Y. Yumita, Change and behavior of residual mercury in paddy soils and rice of Japan, Chemosphere 37 (1998) 1483–1487. [3] O. Lindqvist, Special issue of first international on mercury as a global pollutant, Water Air Soil Pollut., 56 (1991). [4] M.C. Alvim-Ferraz, S.E.R.A. Afonso, Incineration of different types of medical wastes: emission factors for particulate matter and heavy metals, Environ. Sci. Technol. 37 (2003) 3152–3157. [5] K.J.L. Kroschwitz, Encylopedia of Chemical Technology, 1991. [6] U. EPA, National primary drinking water regulations, Total Coliforms (Including Fecal Coliforms and E. Coli), 54 (2009).
Reference Cr(VI)
215 88.11 156.25 118.3 42.1 155.99 19.46 50.85 56.49 280.11
94.34
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