- Email: [email protected]
Fast and selective removal of silver(I) from aqueous media by modified chitosan resins
International Journal of Mineral Processing 120 (2013) 26–34
Contents lists available at SciVerse ScienceDirect
International Journal of Mineral Processing journal homepage: www.elsevier.com/locate/ijminpro
Fast and selective removal of silver(I) from aqueous media by modified chitosan resins Khalid Z. Elwakeel ⁎, Gamal O. El-Sayed, Ramy S. Darweesh Environmental Science Department, Faculty of Science, Port-Said University, Port-Said, Egypt Chemistry Department, Faculty of Science, Benha University, Benha, Egypt Water Quality Audit Department, Al-Menofia Water and Wastewater Company, Menofia, Egypt
a r t i c l e
i n f o
Article history: Received 4 September 2012 Received in revised form 12 February 2013 Accepted 16 February 2013 Available online 21 February 2013 Keywords: Chitosan resins 3-Amino-1,2,4-triazole-5-thiol Ethylenediamine Silver removal
a b s t r a c t Chitosan resin was cross-linked using ethylene glycol diglycidyl ether and modified through the reaction with ethylenediamine and 3-amino-1,2,4-triazole-5-thiol to produce chitosan/amine (R-en) and chitosan/ azole (R-az) resins, respectively. The adsorption behavior of the chelating resins towards Ag(I) from aqueous media was studied. Better adsorption behavior was achieved for Ag(I) by R-en resin. The chelating matrix obtained from ethylenediamine showed uptake capacity of 1.13 mmol/g in 20 min and at 25 °C. These resins were evaluated for their recovery of Ag(I) from aqueous solutions using batch methods and column techniques. Various parameters such as pH, agitation time, Ag(I) concentration and temperature had been studied. The kinetics and thermodynamic behavior of the adsorption reaction were defined. The obtained resins achieved promising results in the selective separation of Ag(I) from other metal ions. Both kinetics and thermodynamic parameters of the adsorption process were obtained. The data indicated that the adsorption process is an exothermic reaction and kinetically proceeds according to pseudo-second order model. Regeneration and durability of the loaded resin towards the successive cycles were clarified. These parameters indicated that the resins can be applied in the removal of Ag(I). © 2013 Elsevier B.V. All rights reserved.
1. Introduction Precious metals are applied in many industrial activities including the electronic, pulp and jewelry industries. These industries generate wastewater containing significant amount of Ag(I) and Au(III). The recovery of these metals from wastewater has been of importance in most industrial branches due to economic and environmental factors. Economically, Ag(I) is considered of special economic interest compared with other metals. Silver nitrate is the most common soluble salt that is used in porcelain, mirroring, photographic, electroplating, and ink formulation industries (Patterson, 1985). Most world silver is recovered from scraps such as photographic films, X-ray films and jewelry (Yazici et al., 2011; Bas et al., 2012). Thus it is necessary to treat the waste aqueous solutions and try to recover them economically. Silver ions cause serious environmental problems because of their widespread usage in many industries and applications (Kiani et al., 2011). Environmental poisoning due to the emission of waste silver from the mineral processing industry in the last few decades has been and continues to be of growing concern, also recent developments
⁎ Corresponding author at: Environmental Science Department, Faculty of Science, Port-Said University, Port-Said, Egypt. Tel.: + 20 1061694332. E-mail addresses: [email protected] (K.Z. Elwakeel), [email protected] (G.O. El-Sayed), [email protected] (R.S. Darweesh). 0301-7516/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.minpro.2013.02.007
in environmental quality standards highlight the need for improved wastewater treatment of dilute metal-bearing effluents. During the past three decades, many studies have been recently focused on the extraction and separation of precious metals due to both increasing industrial need for these metals and their limited sources. The conventional methods for the removal of metal ions from water and waste water include electrolysis (Tao et al., 2012), precipitation (Rivera et al., 2012) ion flotation (Acarkan et al., 2011), ion-exchange (Kononova et al., 2009; Çoruh et al., 2010) and adsorption (Abd El-Ghaffar et al., 2009a, 2009b; Elwakeel, 2010a, 2010b). Among all the above methods, adsorption is recognized as an emerging technique for the depollution of heavy-metal polluted streams. A number of adsorbents have been developed and tested for the removal and recovery of silver(I) such as activated carbon nanospheres (Song et al., 2011), silica gel encapsulated by amino functionalized polystyrene (Xu et al., 2010) and chelating resins (Wang et al., 2012). Chelating resins are easily regenerated from metal ions and they differ from activated carbon and ion exchange resins in their high selectivity in sorption processes (Zang et al., 2009). The selectivity for a specific metal ion depends on what kind of complexing agent is introduced into the polymeric chain. According to the theory of hard and soft acids and bases (HSAB) defined by Pearson, metal ions will have a preference for complexing with ligands that have more or less electronegative donor atoms. Chitosan resins are promising materials for the extraction, separation and recovery of metals in hydrometallurgical field (Liu et al., 2010). Chitosan is a poly-N-glucosamine
K.Z. Elwakeel et al. / International Journal of Mineral Processing 120 (2013) 26–34
27
species obtained by the deacetylation of chitin, the most abundant amino-polysaccharide existing in the environment (Liu et al., 2012). It is highly hydrophilic and is characterized by a flexible polymer chain and by a large number of hydroxyl and amino groups that represent potential adsorption sites (Elwakeel et al., 2012). Moreover, it can be considered a low-cost sorbent because it requires little processing, is abundant in nature and represents a by-product of fishery industry (Iqbal et al., 2011). The ultimate goal of this study is to test the ability of modified chitosan resins bearing N and S as donor atoms for Ag(I) uptake from aqueous media. In this paper the application of the obtained resins for recovery of Ag(I) from aqueous solutions was investigated. The studied resins showed high efficiency and selectivity towards Ag(I) ions. Thermodynamic and kinetic analysis for adsorption process was reported.
Analyzer Vario EL III (Germany). For each analysis, the standard sample (i.e., sulfanilic acid) was analyzed for checking the experimental error within ±1%. All measurements were carried out in triplicate.
2. Experimental
where mw and md are masses (g) of the wet and dried polymers respectively.
2.3.2. Water regain Water regain factor, W%, represents the percentage of water held intrinsically by the polymer. For water regain determination, polymer samples were centrifuged for 30 min at 1000 rpm to remove excess water and then weighed. These samples were then dried at 50–60 °C until complete dryness then weighed again. To calculate this factor, the following equation was applied:
W% ¼
100ðmw −md Þ mw
ð1Þ
2.1. Chemicals Chitosan (Mwt = 10,253) with degree of deacetylation of 82% was obtained from SD Fine-Chem Limited, 3-amino-1,2,4-triazole-5-thiol (az), ethylene glycol diglycidyl ether and ethylenediamine were SigmaAldrich products. All other chemicals were BDH Prolabo products and were used as received. Silver nitrate was used as a source of Ag(I). Potassium tetracyanonickelate(II) was prepared as reported elsewhere (Bassett et al., 1978). 2.2. Preparation of the chelating resins Chitosan resins were prepared and characterized according to the previously reported method (Atia, 2005) as follows: 2.2.1. Preparation of chitosan beads Chitosan (5 g) (31 mmol of monomeric units) was dissolved in 2.0% aqueous acetic acid (250 mL). The chitosan solution was dropped into 0.1 M NaOH solution where a gelatinous precipitate of chitosan was formed, filtered off then washed thoroughly with distilled water; the gelatinous precipitate obtained was suspended in 100 mL methanol. A 3 mL (19.25 mmol) portion of ethylene glycol diglycidyl ether was added then stirred at room temperature for 3 h followed by heating at 70 °C for 4 h. Afterwards the product was isolated by filtration and washed with ethanol followed by distilled water. The cross-linked chitosan particles obtained in the previous step were suspended in 70 mL isopropyl alcohol to which 5 mL epichlorohydrin (62.5 mmol) dissolved in 100 mL acetone/water mixture (1:1 v/v) was added. The contents were stirred for 24 h at 60 °C. The solid obtained was filtered off and washed several times with ethanol followed by water and given the designation R–Cl. Five grams of R–Cl resin was suspended in 100 mL ethanol/water mixture (1:1 v/v) then ethylenediamine (5 mL) was added. The reaction mixture was stirred at 60 °C for 12 h then the product obtained was washed with ethanol followed by water. The chitosan–amine produced was dried in air and given the designation R-en. Five grams of R–Cl was suspended in 100 mL of ethanol/water mixture (1:1 v/v) and 3 g of (3-amino-1,2,4-triazole-5-thiol) was added then the reaction mixture was heated at 70 °C for 12 h while stirring. The chitosan–azole produced was washed with ethanol followed by water then dried in air and given the designation R-az. 2.3. Characterization of the resins 2.3.1. Elemental analysis Nitrogen analysis for R-en and R-az as well as sulfur analysis for R-az were performed by standard microanalysis methods at Microanalytical Center, Cairo University, Giza, Egypt using elemental CHNS
2.3.3. Surface area The surface area of the prepared polymers was measured by methylene blue adsorption as this material is known to be adsorbed as a monolayer only on solid sorbents (Elwakeel and Rekaby, 2011). A standard solution of this material was prepared (0.02 g/L). A calibration curve for methylene blue was drawn (λ = 660 nm) by measuring diluents from standard stock. To calculate the surface area, dried resin (0.1 g) was treated with 25 mL of methylene blue of concentration 0.02 g/L. The treatment lasted until there was no further decrease in absorbance. The amount of methylene blue adsorbed was calculated based on concentration difference between the initial and equilibrium values, which were measured by DR5000 spectrophotomer (HACH), USA. The surface area of the resins was calculated using the following equation (Abd El-Ghaffar et al., 2009a):
As ¼
G NAv ∅ 10 MM W
−20
ð2Þ
where As is the polymer surface area in m 2/g, G the amount of methylene blue adsorbed (g), NAv the Avogadro's number (6.02 × 1023), ∅ the methylene blue molecular cross-section (197.2 Å2), MW the molecular weight of methylene blue (319.85 g/mol) and M is the mass of adsorbent (g). 2.4. Preparation of solutions A stock solution of silver nitrate (2.0 × 10 −2 M) was prepared. A stock solution of EDTA (5 × 10 −3 M) was also prepared and standardized against a solution of MgSO4·7H2O using Eriochrome Black-T (EBT). HNO3 and NaOH were used to change the acidity of the medium. Thiourea (0.5 M) acidified by H2SO4 (0.01 M) was used as an eluent for stripping Ag(I) adsorbed on the resin. 2.5. Uptake measurements 2.5.1. Batch method The effect of contact time on the polymer uptake of Ag(I) was done using a batch method. Dried polymer (0.1 g) was placed in a flask containing 100 mL of metal ions. Initial concentrations of 8×10−3 M were used. The flasks were shaken on a shaker at 200 rpm and at 25± 1 °C. Five milliliters of the solution was taken at different time intervals then centrifuged at 3000 rpm. The residual concentration of Ag(I) was determined complexmetrically with EDTA (8× 10 −3 M) by the replacement titration method using potassium tetracyanonickelate(II) and murexide indicator (Bassett et al., 1978). Each data point was taken as the average of three measurements.
28
K.Z. Elwakeel et al. / International Journal of Mineral Processing 120 (2013) 26–34
The adsorbed amount of Ag(I) per gram of the resin beads qt (mmol/g), at time t was calculated from the mass balance equation as: n X Ctði−1Þ −Cti Vtði−1Þ
qt ¼
i¼1
m
ð3Þ
where Cti (mmol/L) is the measured concentration of the drawn sample number i at time t and Ct0 = (C0), Vti (L) is the volume of the solution in the flask at sample number i and time t, and m is the mass of the air dried resin added into the flask. Uptake of Ag(I) ions on the chelating resins obtained under controlled pH was carried out following the above methods. The desired pH was controlled using HNO3 and NaOH while the equilibrium time was fixed at 6 h and 25 ± 1 °C. Studying the adsorption of Ag(I) in strong basic media was avoided due to the precipitation of silver hydroxide. After equilibration, final concentrations and final pH of Ag(I) for all flasks were measured. Complete adsorption isotherms were obtained by placing 0.1 g portions of dried resin in a series of flasks containing 100 mL of Ag(I) ions at pH 7.6. The flasks were shaken at 200 rpm while keeping the temperature at 25, 35 or 45 °C for 2 h. Later on, the residual concentration was determined where Ag(I) uptake was calculated. Selective separation of Ag(I) from binary mixtures with Cu(II), Pb(II), Cd(II), Zn(II) or Ca(II) was studied at pH 2. Therefore 0.1 g of dry resin was introduced in a series of flasks, each flask containing 100 mL of 5 ×10−3 M Ag(I) solution in a binary mixture with 5× 10−3 M Cu(II), Pb(II), Cd(II), Zn(II) or Ca(II). Five milliliters of the solution was taken after 2 h, and then filtered off, where the residual concentration of metal ion was determined via the titration against 5 ×0−3 M EDTA using murexide as an indicator for Cu(II). EBT was used as an indicator for Ca(II), Mg(II), Cd(II), Zn(II) and Pb(II). 2.6. Elution One gram of the resin was placed in a plastic column (5 cm length and 1.0 cm diameter). A solution of (5 × 10 −3 M) Ag(I) was allowed to flow gradually through the column under the force of gravity at flow rate of 2.0 mL/min. Five milliliters of the underflow solution was removed every 15 min where the residual concentration of the metal ion was determined. The experiment was terminated when the concentration of the influent matches the effluent. Thereafter the column was washed carefully by flowing distilled water through it. The resin loaded by Ag(I) was then subjected for elution using 50 mL of thiourea (0.5) acidified with H2SO4 (0.01 M). After elution the resin was carefully washed with water, dilute NaOH and finally with distilled water to become ready for reuse. This process was repeated for 3 cycles. 3. Results and discussions 3.1. Characterization of the resins The elemental analysis of the resins (Table 1) showed that they contain 13.33% (9.52 mmol/g), and 19.06% (13.61 mmol/g) of nitrogen in R-en and R-az, respectively, and 8.57% (2.73 mmol/g) of sulfur in R-az. These data indicates an increase in N and S contents in R-az resin, this increase is due the presence of az moiety in this resin structure. The data obtained from elemental analysis confirm the success of the modification process according to the reactions shown in Fig. 1. Water regain values are 24% and 21% for R-en and R-az respectively, with insignificant differences when changing conditions. This value reflects the hydrophilic character of the resins. R-en showed higher water regain value than R-az, this may be due to the presence of the heterocyclic triazole moiety in the last resin.
Table 1 Elemental analysis of the studied resins. Resin
R-en R-az
Elemental analysis C%
H%
N% (mmol/g)
S% (mmol/g)
44.05% 37.82%
7.85% 5.44%
13.33% (9.52) 19.06% (13.61)
– 8.57% (2.73)
The surface area of the prepared resins was calculated to be 198.2 and 185.5 m 2/g for R-en and R-az, respectively. 3.2. Uptake studies using batch method Although the amine and sulfur contents in R-az are higher than that of R-en resin, the last resin showed higher uptake than R-az resin, this may be due to the low steric hindrance occurred from the presence of ethylenediamine moiety in R-en resin, which facilities the diffusion of Ag(I) into polymer structure. The fact that the polymers include N and S donor atoms, shows that the resins formed are amenable to uptake Ag(I). The coordination number of Ag(I) is 2. One Ag(I) needs 2 electron pairs to form a complex, thus the ligand occupation (%) of the chelating resin towards Ag(I) could be verified using the relation ligand occupationð%Þ ¼
uptake of AgðIÞ in mmol=g 2 100: concentration of active sites in mmol=g
ð4Þ The ligand occupation (%) in R-az chelating resin is 14.57% at 25 °C and pH 6.7. This low value of ligand occupation (%) may be due to the steric hindrance obtained from the presence of triazole moiety in the polymer structure. While ligand occupation (%) in R-en resin is 24.42% at 25 °C and pH 6.7. The observed increase in ligand occupation (%) for R-en resin may be due to the presence of the less steric hindrance ethylenediamine moiety in the resin structure. The data of the effect of the acidity of the medium on the uptake of Ag(I) is shown in Fig. 2. It is clearly seen that the uptake of Ag(I) increases with increasing pH till reaching a maximum value around pH 5 for R-en and pH 6.7 for R-az. The dominant mechanism of interaction is probably due to the presence of free lone pairs of electrons on nitrogen or sulfur atoms that are suitable for coordination with Ag(I) to give the corresponding resin–metal complex. Chitosan−ðN=SÞ þ AgNO3 ¼ Chitosan−ðN=SÞAgNO3 This behavior indicates the complex formation mechanism between Ag(I) and the donor atoms (N and/or S) on the resin. The slow decrease in the uptake of Ag(I) below initial pH 6.7 may be due to the competition between protons and Ag(I) ions to coordinate with free lone pairs of electrons on nitrogen or sulfur atoms. While the decrease in the uptake of Ag(I) prior to initial pH 7 may be due to the masking of Ag(I) ions in the form of soluble hydroxide anions. At pH > 7, almost all of the Ag(I) ions are precipitated in the form of AgOH (Abd El-Ghaffar et al., 2009b). The recorded appreciable uptake of Ag(I) at lower initial pH (pH 2) values may be useful in the selective separation of Ag(I) from other metal ions. The recorded values of final pH have minor differences with that of initial pH. 3.2.1. Selectivity studies Selective separation of Ag(I) from binary mixtures with Cu(II), Pb(II), Cd(II), Zn(II) or Ca(II) using the studied resins was evaluated at pH 2 and same other adsorption conditions. Their uptake values by R-en in the presence of Ag(I) were in following order 0.09, 0.02, 0.07, 0.06 and 0.01 mmol/g, respectively, while their uptake values by R-az were in following order 0.07, 0.01, 0.06, 0.02 and 0.01 mmol/g,
K.Z. Elwakeel et al. / International Journal of Mineral Processing 120 (2013) 26–34
29
OH O O HO
O
NH2
cross-linked chitosan CH2 CH
CH2Cl
O OH O O HO
O
NH H2C CH
OH
H2C Cl N
NH2CH2CH2NH2
HS
N N
NH2
H
OH
OH
O
O
O
O HO
HO
O
NH
H2C
H2C CH
O
NH CH OH
OH
H2C
H2C
N
N
HN
NH
SH N
H2C CH2
NH2
H
Fig. 1. Proposed structures of the studied chelating resins.
respectively. This data indicates very weak adsorption for the mentioned metal ions relative to that of Ag(I). The separation factors for Ag(I) ions over Cu(II), Pb(II), Cd(II), Zn(II) or Ca(II) ions were calculated from the adsorption data using the following equation (Abd El-Ghaffar et al., 2009b): ðC −C Þ C A2 B2 Separation factor SA=B ¼ A1 ðC B1 −C B2 Þ C A2
ð5Þ
where CA1 and CA2 stand for the concentrations of metal ion A (Ag(I)) before and after adsorption, respectively, and CB1 and CB2 stand for the concentrations of metal ion B (Cu(II), Pb(II), Cd(II), Zn(II) or Ca(II)) before and after adsorption. The values of the separation factor (SA/B) of Ag(I) using R-en resin and R-az are reported in Table 2. These values
indicate the applicability of the studied resins for the selective separation of Ag(I) ions from the other studied metal ions.
3.2.2. Kinetic studies The rate of uptake of Ag(I) on used adsorbents is rapid in the beginning and 80.7% adsorption is completed in 20 min and becomes constant after 80 min, which indicates that equilibrium has been achieved (Fig. 3). These results indicated that a rapid initial uptake rate of Ag(I), followed by a slower removal that gradually approaches an equilibrium condition. This fast rate of adsorption makes this resin promising for practical applications in comparison with other reported adsorbents. The equilibrium time of some previously studied resins extends from 2 h up to few days (Abd El-Ghaffar et al., 2009a; Çelik et al., 2010; Wang et al., 2012).
30
K.Z. Elwakeel et al. / International Journal of Mineral Processing 120 (2013) 26–34
2.5
3.5
Uptake, mmol/g
Uptake, mmol/g
Experimental data
3.0
R-en R-az
2.0 1.5 1.0 0.5
(R-en)
Pseudo-first order model Pseudo-second order model
2.5
Fickian diffusion model Elovich model
2.0 1.5 1.0 0.5
0.0 0
1
2
3
4
5
6
7
8
9
0.0 0
10
20
40
60
80
100
120
140
120
140
Time (min)
Initial pH
3.0
Fig. 2. Effect of pH on the uptake of Ag(I) by resins at 25 °C and initial concentration of 8×10−3.
Experimental data
2.5
(R-az)
Pseudo-first order model
To better understand the adsorption kinetics of Ag(I), pseudo-first order and pseudo-second order models were used to simulate the adsorption process. The models are expressed as follows:(i) Pseudo-first order model (Lagergren, 1898): h
−k1 t
qt ¼ qe 1− exp
i
Uptake, mmol/g
Pseudo-second order model Fickian diffusion model
2.0
Elovich model
1.5 1.0 0.5
:
ð6Þ
0.0
0
20
40
60
k1 t logðqe −qt Þ ¼ log qe − 2:303
ð7Þ
where k1 is the pseudo first order rate constant (min−1) of adsorption and qe and qt (mmol/g) are the amounts of Ag(I) adsorbed at equilibrium and time t, respectively.(ii) Pseudo-second order model (Ho and McKay, 1999):
100
k2 t : 1 þ k2 qe t
Fig. 3. Effect of time on the uptake of Ag(I) from initial concentration of 8 × 10−3 by the studied resins at 25 °C and pH 6.7.
a
Its linearized equation is shown as below: t 1 1 ¼ þ t qt k2 q2 qe e
ð9Þ
where k2 is the pseudo second order rate constant of adsorption (g mmol−1 min−1). The kinetic parameters in both two models are determined from the linear plots of log (qe −qt) vs t for pseudo-first order (Fig. 4a) (t/qt) vs t for pseudo-second order (Fig. 4b). The validity of each model is checked by the fitness of the straight line (R2) as well as the
1.0 0.5
ð8Þ
R-en R-az
log (q e -q t )
qt ¼
80
Time (min)
Its linearized equation is shown as below:
0.0 -0.5 -1.0 -1.5 -2.0
b
0
10
20
30
40
50
60
Time (min) 120
Table 2 Values of the separation factor (SA/B) of Ag(I). Resin
Metal ion
Separation factor (SA/B)
R-en
Cu(II) Pb(II) Cd(II) Zn(II) Ca(II) Cu(II) Pb(II) Cd(II) Zn(II) Ca(II)
132.6 604.6 171.1 200.0 1211.5 162.3 1149.5 189.7 573.6 1149.5
R-az
t/q t (min g/mmol)
100 80 60 40 R-en
20 0
R-az
0
20
40
60
80
100
120
140
160
Time (min) Fig. 4. (a) Pseudo first-orders and (b) Pseudo second-order kinetics of the uptake of Ag(I) by the studied resins at 25 °C and pH 6.7.
K.Z. Elwakeel et al. / International Journal of Mineral Processing 120 (2013) 26–34
31
Table 3 Kinetic parameters the adsorption of Ag(I) on the studied resins.
R-en R-az
Pseudo-first order
Pseudo-second order
Fickian diffusion low
qe, calc (mmol/g)
R2
k2 (g/mmol min)
qe, calc (mmol/g)
R2
Ki (mmol/g min−0.5)
X
R2
α (mmol g−1 min−1)
β (g mmol−1)
R2
0.060 0.091
1.040 1.062
0.978 0.984
0.087 0.134
1.517 1.308
0.997 0.997
0.130 0.130
0.498 0.415
0.957 0.849
0.286 0.268
2.200 2.310
0.986 0.996
experimental and calculated values of qe. Accordingly, and as shown in Table 3, it can be found that the adsorption kinetics data are well described by both pseudo-first order and pseudo-second order rate models, however the pseudo-second order rate model is better with a correlation coefficient R2 above 0.99. The adsorption kinetics study is helpful to understand the mechanism of adsorption reactions. The pseudo-second-order kinetic model is based on the assumption that the rate-limiting step may be chemisorption involving valency forces through sharing or exchange of electrons between sorbent and sorbate (Azizian, 2004; Cui et al., 2012), which is suitable for sorptions at low initial concentration. It was found that the kinetic experimental data obtained could be best fitted into the pseudo-second-order rate model. This observation suggests that Ag(I) adsorption onto resin particles involves chemisorption. This observation confirms that Ag(I) sorption is likely to occur through the formation of the metal complex with ethylenediamine or triazole moiety. Most adsorption reactions take place through multistep mechanism comprising (i) external film diffusion, (ii) intraparticle diffusion and (iii) interaction between adsorbate and active site. Since the first step is excluded by shaking the solution, the rate determining step is one of the other two steps. To know if the intraparticle diffusion is the rate determining step or not the uptake/time data was treated according to Fickian diffusion law (Elwakeel, 2010b).
qt ¼ K i t
Elovich equation
k1 (min−1)
0:5
þX
studied resin is promising for Ag(I) removal relative to the early reported one (Abd El-Ghaffar et al., 2009a). 3.2.3. Adsorption isotherms Study on Ag(I) adsorption isotherm was conducted at pH 6.7± 0.1, the optimal pH for Ag(I) adsorption on the sorbent (Fig. 6). Obviously, increasing the Ag(I) concentration involves an increase in the uptake of Ag(I). Both Langmuir and Freundlich isotherm models were used to describe the relationship between the amount of Ag(I) adsorbed and its equilibrium concentration in aqueous solution. Langmuir model is applicable to homogeneous sorption, which the adsorption of each adsorbate molecule onto the adsorbent has equal adsorption activation energy. Langmuir model can be expressed by the following equation (Langmuir, 1918): qe ¼
Q max KL Ce 1 þ KL Ce
a
1.8 1.5
ð10Þ
where qt is the amounts of Ag(I) adsorbed at time t and Ki is intraparticle diffusion rate (mmol/g min −0.5). The Ki is the slope of straight-line portions of the plot of qt vs t 0.5. The plot of qt against t 0.5 gave straight-line portion (Fig. 5a). The Ki values obtained from the slope of the first straight-line portion are 0.130 (mmol/g min−0.5) for both studied resins. This high value of Ki indicates the fast transfer. The small positive values of X (0.13) for both resins indicate the presence of boundary layer effect on the rate of adsorption. Elovich equation was also applied to the sorption of Ag(I) by the chitosan resins according to the relation (Elwakeel and Rekaby, 2011):
1.2 0.9 0.6 R-en R-az
0.3 0.0 0
b 1.8
where qt is the sorption capacity at time t and α the initial sorption rate (mmol g−1 min−1) and β the desorption constant (g mmol−1). Thus, the constants can be obtained from the slope and intercept of a straight line plot of qt versus log t. The linearization of the equation giving the rate of reaction allows obtaining the initial sorption rate, α (mmol g −1 min −1) from the intercept of a straight line plot of qt versus ln t. The values of α for the adsorption of Ag(I) ions on resins R-en and R-az are 0.286 and 0.268 (mmol g −1 min −1), respectively. These values indicate that the initial sorption rate of R-en is high compared with R-az, which may be attributed to less steric hindrance of ethylenediamine moiety compared with triazole one. The values of β (desorption constant) is found to be 2.20 and 2.31 g/mmol for R-en and R-az, respectively. It was observed that these values are small compared with other studies, which indicates the high affinity of the studied resin towards Ag(I) ions. The data obtained indicate that the
1.5
Uptake (mmol/g)
ð11Þ
qt ¼ 1=β ln ðαβÞ þ 1=β ln t
ð12Þ
where qe the adsorbed value of Ag(I) at equilibrium concentration (mmol/g), Qmax is the maximum adsorption capacity (mmol/g) and KL
Uptake, mmol/g
Resin
1
2
3
4
5
6
7
8
9
10 11 12
t 0.5 (min) 0.5
1.2 0.9 0.6
R-en R-az
0.3 0.0 1
2
3
4
5
6
7
ln t (min) Fig. 5. (a) Intraparticle diffusion and (b) Elovich kinetics of the uptake of Ag(I) from initial concentration of 5 × 10−3 by the studied resins at 25 °C and pH 6.7.
32
K.Z. Elwakeel et al. / International Journal of Mineral Processing 120 (2013) 26–34
18
1.6
(R-en)
15
1.2
C e /q e (g/L)
Uptake, mmol/g
1.4
(R-en)
1.0 0.8
Experimental data at 25°C
0.6
Experimental data at 45°C
Experimental data at 35°C
9 6
Langmuir isotherm at 25°C
0.4
12
25°C 35°C
Langmuir isotherm at 35°C
3
Langmuir isotherm at 45°C
0.2 0.0 0
5
10
15
20
25
0 0
30
45°C
3
6
9
C e (mM) 1.6
21
24
(R-az)
15
1.2
C e /q e (g/L)
Uptake, mmol/g
18
18
(R-az)
1.4
1.0 0.8
Experimental data at 25°C
0.6
Experimental data at 45°C
Experimental data at 35°C
12 9 25°C
6
Langmuir isotherm at 25°C
0.4
35°C
Langmuir isotherm at 35°C
0
5
10
15
20
25
45°C
3
Langmuir isotherm at 45°C
0.2 0.0
12 15 C e (mM)
0
30
0
3
C e (mM)
6
9
12 15 C e (mM)
18
21
24
Fig. 6. Non linear Langmuir isotherms for the adsorption of Ag(I) ions by resins R-en, and R-az at different temperatures and pH 6.7.
Fig. 7. Linear Langmuir isotherms for the adsorption of Ag(I) ions by resins R-en, and R-az at different temperatures and pH 6.7.
is the Langmuir binding constant which is related to the energy of adsorption (L/mmol), Ce is the equilibrium concentration of Ag(I) ions in solution (mmol/L). Its linearized equation is shown as below
surface. The observed decrease of both Qmax and KL with increasing temperature may be related to the decrease of the stability of the complex formed between Ag(I) and resin active sites. The observed deviation from linear plot of Freundlich isotherms at high equilibrium concentration may be due that Freundlich isotherms is applicable at low equilibrium concentrations (Freundlich, 1906). Therefore the last datum of the plots of linear Freundlich isotherms was avoided while fitting to obtain the best regression line. The degree of suitability of the obtained resins towards Ag(I) ions was estimated from the values of the separation factor (RL) using the following relation (Qi and Xu, 2004):
Ce q
¼
e
Ce Q max
þ
1 : KL Q max
ð13Þ
The most important multisided adsorption isotherm for heterogeneous surfaces is the Freundlich isotherm, characterized by the heterogeneity factor 1/ n. The Freundlich model is described by (Freundlich, 1906): qe ¼
1=n KF Ce
ð14Þ
where KF and n are the Freundlich constants related to the adsorption capacity and intensity, respectively. Its linearized expression is shown as below: log qe ¼ log Kf −
1 log Ce: n
ð15Þ
The values of Langmuir constants (Qmax and KL) at different temperatures were calculated from the slope and intercept of the plots in Fig. 7, while the values of Freundlich constants (KF and n) at different temperatures were calculated from the slope and intercept of the plots in Fig. 8 and reported in Table 4. The maximum adsorption capacities (Qmax) obtained at different temperatures are in good agreement with the experimental ones, and the values of R2 reported in Table 4, which is a measure of the goodness-of-fit, confirm the better representation of the experimental data by Langmuir model than Freundlich model (Table 5). This indicates the homogeneity of active sites on the resin
RL ¼
1 1 þ KL Co
ð16Þ
where KL is the Langmuir equilibrium constant and Co is the initial concentration of Ag(I). Values of 0 b RL b 1 indicates the suitability of the process. The values of RL for the investigated resins towards the adsorption of Ag(I) lie between 0.077 and 0.431 for all concentration and temperature ranges. This implies that the adsorption of Ag(I) on both R-en and R-az from aqueous solution is favorable under the conditions used in this study. The values of KL at different temperatures were processed according to the following van't Hoff equation (Donia et al., 2007) to obtain the thermodynamic parameters of the adsorption process ln KL ¼
−ΔHo ΔSo þ RT R
ð17Þ
where ΔHo and ΔSo are enthalpy and entropy changes, respectively, R is the universal gas constant (8.314 J/mol K) and T is the absolute temperature (in Kelvin). The values of ΔHo and ΔSo were calculated from the
K.Z. Elwakeel et al. / International Journal of Mineral Processing 120 (2013) 26–34
0.3
Table 5 Freundlich constants for adsorption of Ag(I) ions on the studied resins.
0.2
(R-en)
Temp. (°C)
R-en N
Kf
R2
n
Kf
R2
25 35 45
3.368 3.061 2.588
0.734 0.621 0.505
0.804 0.900 0.959
2.779 2.783 2.024
0.588 0.531 0.385
0.975 0.910 0.923
Log qe
0.1 0.0 -0.1
R-az
Temperature 25°C
-0.2
35°C
2.0
45°C
-0.3
1.5 0.0
0.5
1.0
1.5
2.0
ln KL (L/mol)
-0.4 -0.5
Log Ce 0.2 (R-az)
0.1
Resin R-en R-az
1.0 0.5 0.0 -0.5 -1.0
0.0
Log qe
33
-1.5 -0.1 -2.0 0.0030
Temperature
-0.2
35°C
-0.3
0.0
0.5
1.0
1.5
0.0032
0.0033
0.0034
0.0035
Fig. 9. Van't Hoff plots for the uptake of Ag(I) on the studied resins.
45°C
-0.4 -0.5
0.0031
1/T (1/Kelvin)
25°C
3.3. Elution and regeneration cycles
2.0
Log Ce
Elution of Ag(I) from the resin was carried out using column method at flow rate of 2.0 mL/min. The saturation of the resin was reached after flowing 240 mL of 5 × 10−3 M Ag(I) in the column (Fig. 10). Sorption/desorption cycle runs were carried out for Ag(I) on both R-en and R-az resins. The elution of Ag(I) ions was performed using 50 mL of 0.5 M thiourea acidified with 0.01 M H2SO4. As shown in Fig. 10, the breakthrough curves for recovery of Ag(I) showed no characteristic changes during successive cycles. The maximum adsorption capacities of the column at different three successive cycles are 1.373, 1.320 and 1.276 for R-en resin, and these values are 1.130, 1.081 and 1.088 for R-az resins. This indicates that both chelating resins have good performance for repeated use up to at least three cycles.
Fig. 8. Linear Freundlich isotherms for the adsorption of Ag(I) ions by resins R-en, and R-az at different temperatures and pH 6.7.
slope and intercept of the plots in Fig. 9, respectively, and reported in Table 6. The negative values of ΔHo indicate the exothermic nature of adsorption process. The enthalpic values obtained are coherent with chemical process, which confirm that the complex formation between Ag(I) ion and resin active sits. The small negative values of ΔSo suggest the slight decrease of randomness during the adsorption of Ag(I). Gibbs free energy of adsorption (ΔGo) was calculated from the following relation and given in Table 6.
3.4. Conclusions o
o
o
ΔG ¼ ΔH −TΔS
ð18Þ
Removal of Ag(I) from aqueous solutions was studied using chitosan resin modified with ethylenediamine and 3-amino-1,2,4-triazole-5-thiol. The recovery process was carried out through batch and column methods. The chelating resins showed good selective adsorption of Ag(I) from binary mixtures of different metal ions. Kinetic studies indicated that the adsorption reaction follows the pseudo second order, intraparticle diffusion and Elovich model. Thermodynamic parameters indicated that the adsorption process is endothermic reaction. The regeneration of
The observed increase in the values of ΔG o with increasing temperature may be attributed to the exothermic nature of the reaction between resin active sites and Ag(I) ions. This may also be reflected in the values of KL, the values of KL which decrease as the temperature increases, indicating lower affinity of the resin towards Ag(I) at higher temperature.
Table 4 Langmuir constants for adsorption of Ag(I) on the studied resins. Temp. (°C)
25 35 45
R-en
R-az 2
Qmax, exp (mmol/g)
Qmax, calc (mmol/g)
KL (L/mmol)
R
1.360 1.210 1.150
1.473 1.311 1.273
0.813 0.838 0.599
0.995 0.995 0.996
Qmax, exp (mmol/g)
Qmax, calc (mmol/g)
KL (L/mmol)
R2
1.250 1.190 1.120
1.396 1.348 1.290
0.601 0.468 0.406
0.992 0.993 0.993
34
K.Z. Elwakeel et al. / International Journal of Mineral Processing 120 (2013) 26–34
Table 6 Enthalpy, entropy and free energy changes for adsorption of Ag(I) ions on the studied resins. ΔHo (kJ/mol)
Resin
−11.844 −15.458
R-en R-az
ΔSo (J/mol.K)
−0.0409 −0.056
ΔGo (kJ/mol) 298 K
308 K
318 K
0.344 1.230
0.753 1.790
1.162 2.350
1.0 0.9 (R-en)
0.8
C/C 0
0.7 0.6 0.5 0.4 0.3
Regeneration cycle
cycle 1 cycle 2 cycle 3
0.2 0.1 0.0
0
50
100
150
200
250
300
Time (min) 1.0 0.9 (R-az)
0.8
C/C 0
0.7 0.6 0.5 0.4 Regeneration cycle
0.3
cycle 1 cycle 2 cycle 3
0.2 0.1 0.0
0
50
100
150
200
250
300
Time (min) Fig. 10. Effect of successive desorption cycles on the breakthrough curves for the recovery of Ag(I) at flow rate of 2.0 mL/min.
both studied resins was performed using 50 mL of 0.5 M thiourea acidified with 0.01 M H2SO4 successfully. References Abd El-Ghaffar, M.A., Abdel Wahab, Z.H., Elwakeel, K.Z., 2009a. Extraction and separation studies of silver(I) and copper(II) from their aqueous solution using a chemically modified melamine resins. Hydrometallurgy 96, 27–34. Abd El-Ghaffar, M.A., Mostafa, M.H., Elwakeel, K.Z., 2009b. Adsorption of silver(I) on synthetic chelating polymer derived from 3-amino-1,2,4-triazole-5-thiol and glutaraldehyde. Chem. Eng. J. 151, 30–38. Acarkan, N., Bulut, G., Gül, A., Kangal, O., Karakaş, F., Kökkiliç, O., Önal, G., 2011. The effect of collector's type on gold and silver flotation in a complex ore. Sep. Sci. Technol. 46, 283–289.
Atia, A.A., 2005. Studies on the interaction of mercury(II) and uranyl(II) with modified chitosan resins. Hydrometallurgy 80, 13–22. Azizian, S., 2004. Kinetic models of sorption: a theoretical analysis. J. Colloid Interface Sci. 276, 47–52. Bas, A.D., Yazici, E.Y., Deveci, H., 2012. Recovery of silver from X-ray film processing effluents by hydrogen peroxide treatment. Hydrometallurgy 121–124, 22–27. Bassett, J., Denney, R.C., Jeffery, G.H., Mendham, J., 1978. Textbook of Quantitative Inorganic Analysis. ELBS, p. 323. Çelik, Z., Gülfen, M., Aydın, A.O., 2010. Synthesis of a novel dithiooxamide–formaldehyde resin and its application to the adsorption and separation of silver ions. J. Hazard. Mater. 174, 556–562. Çoruh, S., Şenel, G., Ergun, O.N., 2010. A comparison of the properties of natural clinoptilolites and their ion-exchange capacities for silver removal. J. Hazard. Mater. 180, 486–492. Cui, H., Li, Q., Gao, S., Shang, J.K., 2012. Strong adsorption of arsenic species by amorphous zirconium oxide nanoparticles. J. Ind. Eng. Chem. 18, 1418–1427. Donia, A.M., Atia, A.A., Elwakeel, K.Z., 2007. Recovery of gold(III) and silver(I) on a chemically modified chitosan with magnetic properties. Hydrometallurgy 87, 197–206. Elwakeel, K.Z., 2010a. Environmental application of chitosan resins for the treatment of water and wastewater: a review. J. Dispers. Sci. Technol. 31, 273–288. Elwakeel, K.Z., 2010b. Removal of Cr(VI) from alkaline aqueous solutions using chemically modified magnetic chitosan resins. Desalination 250, 105–112. Elwakeel, K.Z., Rekaby, M., 2011. Efficient removal of Reactive Black 5 from aqueous media using glycidyl methacrylate resin modified with tetraethelenepentamine. J. Hazard. Mater. 188, 10–18. Elwakeel, K.Z., Abd El-Ghaffar, M.A., El-Kousy, S.M., El-Shorbagy, H.G., 2012. Synthesis of new ammonium chitosan derivatives and their application for dye removal from aqueous media. Chem. Eng. J. 203, 458–468. Freundlich, H.M.F., 1906. Uber Die Adsorption in Losungen, Z. J. Phys. Chem. 57, 385–470. Ho, Y.S., McKay, G., 1999. Pseudo-second order model for sorption processes. Process. Biochem. 34, 451–465. Iqbal, J., Wattoo, F.H., Wattoo, M.H.S., Malik, R., Tirmizi, S.A., Imran, M., Ghangro, A.B., 2011. Adsorption of acid yellow dye on flakes of chitosan prepared from fishery wastes. Arab. J. Chem. 4, 389–395. Kiani, G.R., Sheikhloie, H., Arsalani, N., 2011. Heavy metal ion removal from aqueous solutions by functionalized polyacrylonitrile. Desalination 269, 266–270. Kononova, O.N., Shatnykh, K.A., Prikhod'Ko, K.V., Kashirin, D.M., 2009. Ion-exchange recovery of gold(I) and silver(I) from thiosulfate solutions. Russ. J. Phys. Chem. A 13, 2340–2345. Lagergren, S., 1898. About the theory of so-called adsorption of soluble substances. K. Svenska Vetenskapsakad. Handl. 24, 1–39. Langmuir, I., 1918. The adsorption of gases on plane surfaces of glass, mica and platinum. J. Am. Chem. Soc. 40, 1361–1403. Liu, Y., Cao, X., Hua, R., Wang, Y., Liu, Y., Pang, C., Wang, Y., 2010. Selective adsorption of uranyl ion on ion-imprinted chitosan/PVA cross-linked hydrogel. Hydrometallurgy 104, 150–155. Liu, B., Lv, X., Wang, D., Xu, Y., Zhang, L., Li, Y., 2012. Adsorption behavior of As(III) onto chitosan resin with As(III) as template ions. J. Appl. Polym. Sci. 125, 246–253. Patterson, J.W., 1985. Industrial Wastewater Treatment Technology, 2nd ed. Butterworth, p. 405. Qi, L., Xu, Z., 2004. Lead sorption from aqueous solutions on chitosan nanoparticles. Colloids Surf., A Physicochem. Eng. Asp. 251, 186–193. Rivera, I., Juárez, G., Patiño, F., Reyes, I.A., Roca, A., Reyes, M.I., 2012. Silver precipitation using sodium dithionite in cyanide media. J. Mex. Chem. Soc. 56, 156–162. Song, X., Gunawan, P., Jiang, R., Leong, S.S.J., Wang, K., Xu, R., 2011. Surface activated carbon nanospheres for fast adsorption of silver ions from aqueous solutions. J. Hazard. Mater. 194, 162–168. Tao, H.-C., Gao, Z.-Y., Ding, H., Xu, N., Wu, W.-M., 2012. Recovery of silver from silver(I)-containing solutions in bioelectrochemical reactors. Bioresour. Technol. 111, 92–97. Wang, Y., Ma, X., Li, Y., Li, X., Yang, L., Ji, L., He, Y., 2012. Preparation of a novel chelating resin containing amidoxime–guanidin group and its recovery properties for silver ions in aqueous solution. Chem. Eng. J. 209, 394–400. Xu, Q., Yin, P., Zhao, G., Sun, Y., Qu, R., 2010. Adsorption selectivity and dynamic adsorption behaviors of Cu(II), Ag(I), and Au(III) on silica gel encapsulated by amino functionalized polystyrene. J. Appl. Polym. Sci. 117, 3645–3650. Yazici, E.Y., Deveci, H., Yazici, R., 2011. Recovery of silver from X-ray film processing effluents using trimercapto-s-triazine (TMT). Sep. Sci. Technol. 46, 2231–2238. Zang, Z., Hu, Z., Li, Z., He, Q., Chang, X., 2009. Synthesis, characterization and application of ethylenediamine-modified multiwalled carbon nanotubes for selective solid-phase extraction and preconcentration of metal ions. J. Hazard. Mater. 172, 958–963.
Contents lists available at SciVerse ScienceDirect
International Journal of Mineral Processing journal homepage: www.elsevier.com/locate/ijminpro
Fast and selective removal of silver(I) from aqueous media by modified chitosan resins Khalid Z. Elwakeel ⁎, Gamal O. El-Sayed, Ramy S. Darweesh Environmental Science Department, Faculty of Science, Port-Said University, Port-Said, Egypt Chemistry Department, Faculty of Science, Benha University, Benha, Egypt Water Quality Audit Department, Al-Menofia Water and Wastewater Company, Menofia, Egypt
a r t i c l e
i n f o
Article history: Received 4 September 2012 Received in revised form 12 February 2013 Accepted 16 February 2013 Available online 21 February 2013 Keywords: Chitosan resins 3-Amino-1,2,4-triazole-5-thiol Ethylenediamine Silver removal
a b s t r a c t Chitosan resin was cross-linked using ethylene glycol diglycidyl ether and modified through the reaction with ethylenediamine and 3-amino-1,2,4-triazole-5-thiol to produce chitosan/amine (R-en) and chitosan/ azole (R-az) resins, respectively. The adsorption behavior of the chelating resins towards Ag(I) from aqueous media was studied. Better adsorption behavior was achieved for Ag(I) by R-en resin. The chelating matrix obtained from ethylenediamine showed uptake capacity of 1.13 mmol/g in 20 min and at 25 °C. These resins were evaluated for their recovery of Ag(I) from aqueous solutions using batch methods and column techniques. Various parameters such as pH, agitation time, Ag(I) concentration and temperature had been studied. The kinetics and thermodynamic behavior of the adsorption reaction were defined. The obtained resins achieved promising results in the selective separation of Ag(I) from other metal ions. Both kinetics and thermodynamic parameters of the adsorption process were obtained. The data indicated that the adsorption process is an exothermic reaction and kinetically proceeds according to pseudo-second order model. Regeneration and durability of the loaded resin towards the successive cycles were clarified. These parameters indicated that the resins can be applied in the removal of Ag(I). © 2013 Elsevier B.V. All rights reserved.
1. Introduction Precious metals are applied in many industrial activities including the electronic, pulp and jewelry industries. These industries generate wastewater containing significant amount of Ag(I) and Au(III). The recovery of these metals from wastewater has been of importance in most industrial branches due to economic and environmental factors. Economically, Ag(I) is considered of special economic interest compared with other metals. Silver nitrate is the most common soluble salt that is used in porcelain, mirroring, photographic, electroplating, and ink formulation industries (Patterson, 1985). Most world silver is recovered from scraps such as photographic films, X-ray films and jewelry (Yazici et al., 2011; Bas et al., 2012). Thus it is necessary to treat the waste aqueous solutions and try to recover them economically. Silver ions cause serious environmental problems because of their widespread usage in many industries and applications (Kiani et al., 2011). Environmental poisoning due to the emission of waste silver from the mineral processing industry in the last few decades has been and continues to be of growing concern, also recent developments
⁎ Corresponding author at: Environmental Science Department, Faculty of Science, Port-Said University, Port-Said, Egypt. Tel.: + 20 1061694332. E-mail addresses: [email protected] (K.Z. Elwakeel), [email protected] (G.O. El-Sayed), [email protected] (R.S. Darweesh). 0301-7516/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.minpro.2013.02.007
in environmental quality standards highlight the need for improved wastewater treatment of dilute metal-bearing effluents. During the past three decades, many studies have been recently focused on the extraction and separation of precious metals due to both increasing industrial need for these metals and their limited sources. The conventional methods for the removal of metal ions from water and waste water include electrolysis (Tao et al., 2012), precipitation (Rivera et al., 2012) ion flotation (Acarkan et al., 2011), ion-exchange (Kononova et al., 2009; Çoruh et al., 2010) and adsorption (Abd El-Ghaffar et al., 2009a, 2009b; Elwakeel, 2010a, 2010b). Among all the above methods, adsorption is recognized as an emerging technique for the depollution of heavy-metal polluted streams. A number of adsorbents have been developed and tested for the removal and recovery of silver(I) such as activated carbon nanospheres (Song et al., 2011), silica gel encapsulated by amino functionalized polystyrene (Xu et al., 2010) and chelating resins (Wang et al., 2012). Chelating resins are easily regenerated from metal ions and they differ from activated carbon and ion exchange resins in their high selectivity in sorption processes (Zang et al., 2009). The selectivity for a specific metal ion depends on what kind of complexing agent is introduced into the polymeric chain. According to the theory of hard and soft acids and bases (HSAB) defined by Pearson, metal ions will have a preference for complexing with ligands that have more or less electronegative donor atoms. Chitosan resins are promising materials for the extraction, separation and recovery of metals in hydrometallurgical field (Liu et al., 2010). Chitosan is a poly-N-glucosamine
K.Z. Elwakeel et al. / International Journal of Mineral Processing 120 (2013) 26–34
27
species obtained by the deacetylation of chitin, the most abundant amino-polysaccharide existing in the environment (Liu et al., 2012). It is highly hydrophilic and is characterized by a flexible polymer chain and by a large number of hydroxyl and amino groups that represent potential adsorption sites (Elwakeel et al., 2012). Moreover, it can be considered a low-cost sorbent because it requires little processing, is abundant in nature and represents a by-product of fishery industry (Iqbal et al., 2011). The ultimate goal of this study is to test the ability of modified chitosan resins bearing N and S as donor atoms for Ag(I) uptake from aqueous media. In this paper the application of the obtained resins for recovery of Ag(I) from aqueous solutions was investigated. The studied resins showed high efficiency and selectivity towards Ag(I) ions. Thermodynamic and kinetic analysis for adsorption process was reported.
Analyzer Vario EL III (Germany). For each analysis, the standard sample (i.e., sulfanilic acid) was analyzed for checking the experimental error within ±1%. All measurements were carried out in triplicate.
2. Experimental
where mw and md are masses (g) of the wet and dried polymers respectively.
2.3.2. Water regain Water regain factor, W%, represents the percentage of water held intrinsically by the polymer. For water regain determination, polymer samples were centrifuged for 30 min at 1000 rpm to remove excess water and then weighed. These samples were then dried at 50–60 °C until complete dryness then weighed again. To calculate this factor, the following equation was applied:
W% ¼
100ðmw −md Þ mw
ð1Þ
2.1. Chemicals Chitosan (Mwt = 10,253) with degree of deacetylation of 82% was obtained from SD Fine-Chem Limited, 3-amino-1,2,4-triazole-5-thiol (az), ethylene glycol diglycidyl ether and ethylenediamine were SigmaAldrich products. All other chemicals were BDH Prolabo products and were used as received. Silver nitrate was used as a source of Ag(I). Potassium tetracyanonickelate(II) was prepared as reported elsewhere (Bassett et al., 1978). 2.2. Preparation of the chelating resins Chitosan resins were prepared and characterized according to the previously reported method (Atia, 2005) as follows: 2.2.1. Preparation of chitosan beads Chitosan (5 g) (31 mmol of monomeric units) was dissolved in 2.0% aqueous acetic acid (250 mL). The chitosan solution was dropped into 0.1 M NaOH solution where a gelatinous precipitate of chitosan was formed, filtered off then washed thoroughly with distilled water; the gelatinous precipitate obtained was suspended in 100 mL methanol. A 3 mL (19.25 mmol) portion of ethylene glycol diglycidyl ether was added then stirred at room temperature for 3 h followed by heating at 70 °C for 4 h. Afterwards the product was isolated by filtration and washed with ethanol followed by distilled water. The cross-linked chitosan particles obtained in the previous step were suspended in 70 mL isopropyl alcohol to which 5 mL epichlorohydrin (62.5 mmol) dissolved in 100 mL acetone/water mixture (1:1 v/v) was added. The contents were stirred for 24 h at 60 °C. The solid obtained was filtered off and washed several times with ethanol followed by water and given the designation R–Cl. Five grams of R–Cl resin was suspended in 100 mL ethanol/water mixture (1:1 v/v) then ethylenediamine (5 mL) was added. The reaction mixture was stirred at 60 °C for 12 h then the product obtained was washed with ethanol followed by water. The chitosan–amine produced was dried in air and given the designation R-en. Five grams of R–Cl was suspended in 100 mL of ethanol/water mixture (1:1 v/v) and 3 g of (3-amino-1,2,4-triazole-5-thiol) was added then the reaction mixture was heated at 70 °C for 12 h while stirring. The chitosan–azole produced was washed with ethanol followed by water then dried in air and given the designation R-az. 2.3. Characterization of the resins 2.3.1. Elemental analysis Nitrogen analysis for R-en and R-az as well as sulfur analysis for R-az were performed by standard microanalysis methods at Microanalytical Center, Cairo University, Giza, Egypt using elemental CHNS
2.3.3. Surface area The surface area of the prepared polymers was measured by methylene blue adsorption as this material is known to be adsorbed as a monolayer only on solid sorbents (Elwakeel and Rekaby, 2011). A standard solution of this material was prepared (0.02 g/L). A calibration curve for methylene blue was drawn (λ = 660 nm) by measuring diluents from standard stock. To calculate the surface area, dried resin (0.1 g) was treated with 25 mL of methylene blue of concentration 0.02 g/L. The treatment lasted until there was no further decrease in absorbance. The amount of methylene blue adsorbed was calculated based on concentration difference between the initial and equilibrium values, which were measured by DR5000 spectrophotomer (HACH), USA. The surface area of the resins was calculated using the following equation (Abd El-Ghaffar et al., 2009a):
As ¼
G NAv ∅ 10 MM W
−20
ð2Þ
where As is the polymer surface area in m 2/g, G the amount of methylene blue adsorbed (g), NAv the Avogadro's number (6.02 × 1023), ∅ the methylene blue molecular cross-section (197.2 Å2), MW the molecular weight of methylene blue (319.85 g/mol) and M is the mass of adsorbent (g). 2.4. Preparation of solutions A stock solution of silver nitrate (2.0 × 10 −2 M) was prepared. A stock solution of EDTA (5 × 10 −3 M) was also prepared and standardized against a solution of MgSO4·7H2O using Eriochrome Black-T (EBT). HNO3 and NaOH were used to change the acidity of the medium. Thiourea (0.5 M) acidified by H2SO4 (0.01 M) was used as an eluent for stripping Ag(I) adsorbed on the resin. 2.5. Uptake measurements 2.5.1. Batch method The effect of contact time on the polymer uptake of Ag(I) was done using a batch method. Dried polymer (0.1 g) was placed in a flask containing 100 mL of metal ions. Initial concentrations of 8×10−3 M were used. The flasks were shaken on a shaker at 200 rpm and at 25± 1 °C. Five milliliters of the solution was taken at different time intervals then centrifuged at 3000 rpm. The residual concentration of Ag(I) was determined complexmetrically with EDTA (8× 10 −3 M) by the replacement titration method using potassium tetracyanonickelate(II) and murexide indicator (Bassett et al., 1978). Each data point was taken as the average of three measurements.
28
K.Z. Elwakeel et al. / International Journal of Mineral Processing 120 (2013) 26–34
The adsorbed amount of Ag(I) per gram of the resin beads qt (mmol/g), at time t was calculated from the mass balance equation as: n X Ctði−1Þ −Cti Vtði−1Þ
qt ¼
i¼1
m
ð3Þ
where Cti (mmol/L) is the measured concentration of the drawn sample number i at time t and Ct0 = (C0), Vti (L) is the volume of the solution in the flask at sample number i and time t, and m is the mass of the air dried resin added into the flask. Uptake of Ag(I) ions on the chelating resins obtained under controlled pH was carried out following the above methods. The desired pH was controlled using HNO3 and NaOH while the equilibrium time was fixed at 6 h and 25 ± 1 °C. Studying the adsorption of Ag(I) in strong basic media was avoided due to the precipitation of silver hydroxide. After equilibration, final concentrations and final pH of Ag(I) for all flasks were measured. Complete adsorption isotherms were obtained by placing 0.1 g portions of dried resin in a series of flasks containing 100 mL of Ag(I) ions at pH 7.6. The flasks were shaken at 200 rpm while keeping the temperature at 25, 35 or 45 °C for 2 h. Later on, the residual concentration was determined where Ag(I) uptake was calculated. Selective separation of Ag(I) from binary mixtures with Cu(II), Pb(II), Cd(II), Zn(II) or Ca(II) was studied at pH 2. Therefore 0.1 g of dry resin was introduced in a series of flasks, each flask containing 100 mL of 5 ×10−3 M Ag(I) solution in a binary mixture with 5× 10−3 M Cu(II), Pb(II), Cd(II), Zn(II) or Ca(II). Five milliliters of the solution was taken after 2 h, and then filtered off, where the residual concentration of metal ion was determined via the titration against 5 ×0−3 M EDTA using murexide as an indicator for Cu(II). EBT was used as an indicator for Ca(II), Mg(II), Cd(II), Zn(II) and Pb(II). 2.6. Elution One gram of the resin was placed in a plastic column (5 cm length and 1.0 cm diameter). A solution of (5 × 10 −3 M) Ag(I) was allowed to flow gradually through the column under the force of gravity at flow rate of 2.0 mL/min. Five milliliters of the underflow solution was removed every 15 min where the residual concentration of the metal ion was determined. The experiment was terminated when the concentration of the influent matches the effluent. Thereafter the column was washed carefully by flowing distilled water through it. The resin loaded by Ag(I) was then subjected for elution using 50 mL of thiourea (0.5) acidified with H2SO4 (0.01 M). After elution the resin was carefully washed with water, dilute NaOH and finally with distilled water to become ready for reuse. This process was repeated for 3 cycles. 3. Results and discussions 3.1. Characterization of the resins The elemental analysis of the resins (Table 1) showed that they contain 13.33% (9.52 mmol/g), and 19.06% (13.61 mmol/g) of nitrogen in R-en and R-az, respectively, and 8.57% (2.73 mmol/g) of sulfur in R-az. These data indicates an increase in N and S contents in R-az resin, this increase is due the presence of az moiety in this resin structure. The data obtained from elemental analysis confirm the success of the modification process according to the reactions shown in Fig. 1. Water regain values are 24% and 21% for R-en and R-az respectively, with insignificant differences when changing conditions. This value reflects the hydrophilic character of the resins. R-en showed higher water regain value than R-az, this may be due to the presence of the heterocyclic triazole moiety in the last resin.
Table 1 Elemental analysis of the studied resins. Resin
R-en R-az
Elemental analysis C%
H%
N% (mmol/g)
S% (mmol/g)
44.05% 37.82%
7.85% 5.44%
13.33% (9.52) 19.06% (13.61)
– 8.57% (2.73)
The surface area of the prepared resins was calculated to be 198.2 and 185.5 m 2/g for R-en and R-az, respectively. 3.2. Uptake studies using batch method Although the amine and sulfur contents in R-az are higher than that of R-en resin, the last resin showed higher uptake than R-az resin, this may be due to the low steric hindrance occurred from the presence of ethylenediamine moiety in R-en resin, which facilities the diffusion of Ag(I) into polymer structure. The fact that the polymers include N and S donor atoms, shows that the resins formed are amenable to uptake Ag(I). The coordination number of Ag(I) is 2. One Ag(I) needs 2 electron pairs to form a complex, thus the ligand occupation (%) of the chelating resin towards Ag(I) could be verified using the relation ligand occupationð%Þ ¼
uptake of AgðIÞ in mmol=g 2 100: concentration of active sites in mmol=g
ð4Þ The ligand occupation (%) in R-az chelating resin is 14.57% at 25 °C and pH 6.7. This low value of ligand occupation (%) may be due to the steric hindrance obtained from the presence of triazole moiety in the polymer structure. While ligand occupation (%) in R-en resin is 24.42% at 25 °C and pH 6.7. The observed increase in ligand occupation (%) for R-en resin may be due to the presence of the less steric hindrance ethylenediamine moiety in the resin structure. The data of the effect of the acidity of the medium on the uptake of Ag(I) is shown in Fig. 2. It is clearly seen that the uptake of Ag(I) increases with increasing pH till reaching a maximum value around pH 5 for R-en and pH 6.7 for R-az. The dominant mechanism of interaction is probably due to the presence of free lone pairs of electrons on nitrogen or sulfur atoms that are suitable for coordination with Ag(I) to give the corresponding resin–metal complex. Chitosan−ðN=SÞ þ AgNO3 ¼ Chitosan−ðN=SÞAgNO3 This behavior indicates the complex formation mechanism between Ag(I) and the donor atoms (N and/or S) on the resin. The slow decrease in the uptake of Ag(I) below initial pH 6.7 may be due to the competition between protons and Ag(I) ions to coordinate with free lone pairs of electrons on nitrogen or sulfur atoms. While the decrease in the uptake of Ag(I) prior to initial pH 7 may be due to the masking of Ag(I) ions in the form of soluble hydroxide anions. At pH > 7, almost all of the Ag(I) ions are precipitated in the form of AgOH (Abd El-Ghaffar et al., 2009b). The recorded appreciable uptake of Ag(I) at lower initial pH (pH 2) values may be useful in the selective separation of Ag(I) from other metal ions. The recorded values of final pH have minor differences with that of initial pH. 3.2.1. Selectivity studies Selective separation of Ag(I) from binary mixtures with Cu(II), Pb(II), Cd(II), Zn(II) or Ca(II) using the studied resins was evaluated at pH 2 and same other adsorption conditions. Their uptake values by R-en in the presence of Ag(I) were in following order 0.09, 0.02, 0.07, 0.06 and 0.01 mmol/g, respectively, while their uptake values by R-az were in following order 0.07, 0.01, 0.06, 0.02 and 0.01 mmol/g,
K.Z. Elwakeel et al. / International Journal of Mineral Processing 120 (2013) 26–34
29
OH O O HO
O
NH2
cross-linked chitosan CH2 CH
CH2Cl
O OH O O HO
O
NH H2C CH
OH
H2C Cl N
NH2CH2CH2NH2
HS
N N
NH2
H
OH
OH
O
O
O
O HO
HO
O
NH
H2C
H2C CH
O
NH CH OH
OH
H2C
H2C
N
N
HN
NH
SH N
H2C CH2
NH2
H
Fig. 1. Proposed structures of the studied chelating resins.
respectively. This data indicates very weak adsorption for the mentioned metal ions relative to that of Ag(I). The separation factors for Ag(I) ions over Cu(II), Pb(II), Cd(II), Zn(II) or Ca(II) ions were calculated from the adsorption data using the following equation (Abd El-Ghaffar et al., 2009b): ðC −C Þ C A2 B2 Separation factor SA=B ¼ A1 ðC B1 −C B2 Þ C A2
ð5Þ
where CA1 and CA2 stand for the concentrations of metal ion A (Ag(I)) before and after adsorption, respectively, and CB1 and CB2 stand for the concentrations of metal ion B (Cu(II), Pb(II), Cd(II), Zn(II) or Ca(II)) before and after adsorption. The values of the separation factor (SA/B) of Ag(I) using R-en resin and R-az are reported in Table 2. These values
indicate the applicability of the studied resins for the selective separation of Ag(I) ions from the other studied metal ions.
3.2.2. Kinetic studies The rate of uptake of Ag(I) on used adsorbents is rapid in the beginning and 80.7% adsorption is completed in 20 min and becomes constant after 80 min, which indicates that equilibrium has been achieved (Fig. 3). These results indicated that a rapid initial uptake rate of Ag(I), followed by a slower removal that gradually approaches an equilibrium condition. This fast rate of adsorption makes this resin promising for practical applications in comparison with other reported adsorbents. The equilibrium time of some previously studied resins extends from 2 h up to few days (Abd El-Ghaffar et al., 2009a; Çelik et al., 2010; Wang et al., 2012).
30
K.Z. Elwakeel et al. / International Journal of Mineral Processing 120 (2013) 26–34
2.5
3.5
Uptake, mmol/g
Uptake, mmol/g
Experimental data
3.0
R-en R-az
2.0 1.5 1.0 0.5
(R-en)
Pseudo-first order model Pseudo-second order model
2.5
Fickian diffusion model Elovich model
2.0 1.5 1.0 0.5
0.0 0
1
2
3
4
5
6
7
8
9
0.0 0
10
20
40
60
80
100
120
140
120
140
Time (min)
Initial pH
3.0
Fig. 2. Effect of pH on the uptake of Ag(I) by resins at 25 °C and initial concentration of 8×10−3.
Experimental data
2.5
(R-az)
Pseudo-first order model
To better understand the adsorption kinetics of Ag(I), pseudo-first order and pseudo-second order models were used to simulate the adsorption process. The models are expressed as follows:(i) Pseudo-first order model (Lagergren, 1898): h
−k1 t
qt ¼ qe 1− exp
i
Uptake, mmol/g
Pseudo-second order model Fickian diffusion model
2.0
Elovich model
1.5 1.0 0.5
:
ð6Þ
0.0
0
20
40
60
k1 t logðqe −qt Þ ¼ log qe − 2:303
ð7Þ
where k1 is the pseudo first order rate constant (min−1) of adsorption and qe and qt (mmol/g) are the amounts of Ag(I) adsorbed at equilibrium and time t, respectively.(ii) Pseudo-second order model (Ho and McKay, 1999):
100
k2 t : 1 þ k2 qe t
Fig. 3. Effect of time on the uptake of Ag(I) from initial concentration of 8 × 10−3 by the studied resins at 25 °C and pH 6.7.
a
Its linearized equation is shown as below: t 1 1 ¼ þ t qt k2 q2 qe e
ð9Þ
where k2 is the pseudo second order rate constant of adsorption (g mmol−1 min−1). The kinetic parameters in both two models are determined from the linear plots of log (qe −qt) vs t for pseudo-first order (Fig. 4a) (t/qt) vs t for pseudo-second order (Fig. 4b). The validity of each model is checked by the fitness of the straight line (R2) as well as the
1.0 0.5
ð8Þ
R-en R-az
log (q e -q t )
qt ¼
80
Time (min)
Its linearized equation is shown as below:
0.0 -0.5 -1.0 -1.5 -2.0
b
0
10
20
30
40
50
60
Time (min) 120
Table 2 Values of the separation factor (SA/B) of Ag(I). Resin
Metal ion
Separation factor (SA/B)
R-en
Cu(II) Pb(II) Cd(II) Zn(II) Ca(II) Cu(II) Pb(II) Cd(II) Zn(II) Ca(II)
132.6 604.6 171.1 200.0 1211.5 162.3 1149.5 189.7 573.6 1149.5
R-az
t/q t (min g/mmol)
100 80 60 40 R-en
20 0
R-az
0
20
40
60
80
100
120
140
160
Time (min) Fig. 4. (a) Pseudo first-orders and (b) Pseudo second-order kinetics of the uptake of Ag(I) by the studied resins at 25 °C and pH 6.7.
K.Z. Elwakeel et al. / International Journal of Mineral Processing 120 (2013) 26–34
31
Table 3 Kinetic parameters the adsorption of Ag(I) on the studied resins.
R-en R-az
Pseudo-first order
Pseudo-second order
Fickian diffusion low
qe, calc (mmol/g)
R2
k2 (g/mmol min)
qe, calc (mmol/g)
R2
Ki (mmol/g min−0.5)
X
R2
α (mmol g−1 min−1)
β (g mmol−1)
R2
0.060 0.091
1.040 1.062
0.978 0.984
0.087 0.134
1.517 1.308
0.997 0.997
0.130 0.130
0.498 0.415
0.957 0.849
0.286 0.268
2.200 2.310
0.986 0.996
experimental and calculated values of qe. Accordingly, and as shown in Table 3, it can be found that the adsorption kinetics data are well described by both pseudo-first order and pseudo-second order rate models, however the pseudo-second order rate model is better with a correlation coefficient R2 above 0.99. The adsorption kinetics study is helpful to understand the mechanism of adsorption reactions. The pseudo-second-order kinetic model is based on the assumption that the rate-limiting step may be chemisorption involving valency forces through sharing or exchange of electrons between sorbent and sorbate (Azizian, 2004; Cui et al., 2012), which is suitable for sorptions at low initial concentration. It was found that the kinetic experimental data obtained could be best fitted into the pseudo-second-order rate model. This observation suggests that Ag(I) adsorption onto resin particles involves chemisorption. This observation confirms that Ag(I) sorption is likely to occur through the formation of the metal complex with ethylenediamine or triazole moiety. Most adsorption reactions take place through multistep mechanism comprising (i) external film diffusion, (ii) intraparticle diffusion and (iii) interaction between adsorbate and active site. Since the first step is excluded by shaking the solution, the rate determining step is one of the other two steps. To know if the intraparticle diffusion is the rate determining step or not the uptake/time data was treated according to Fickian diffusion law (Elwakeel, 2010b).
qt ¼ K i t
Elovich equation
k1 (min−1)
0:5
þX
studied resin is promising for Ag(I) removal relative to the early reported one (Abd El-Ghaffar et al., 2009a). 3.2.3. Adsorption isotherms Study on Ag(I) adsorption isotherm was conducted at pH 6.7± 0.1, the optimal pH for Ag(I) adsorption on the sorbent (Fig. 6). Obviously, increasing the Ag(I) concentration involves an increase in the uptake of Ag(I). Both Langmuir and Freundlich isotherm models were used to describe the relationship between the amount of Ag(I) adsorbed and its equilibrium concentration in aqueous solution. Langmuir model is applicable to homogeneous sorption, which the adsorption of each adsorbate molecule onto the adsorbent has equal adsorption activation energy. Langmuir model can be expressed by the following equation (Langmuir, 1918): qe ¼
Q max KL Ce 1 þ KL Ce
a
1.8 1.5
ð10Þ
where qt is the amounts of Ag(I) adsorbed at time t and Ki is intraparticle diffusion rate (mmol/g min −0.5). The Ki is the slope of straight-line portions of the plot of qt vs t 0.5. The plot of qt against t 0.5 gave straight-line portion (Fig. 5a). The Ki values obtained from the slope of the first straight-line portion are 0.130 (mmol/g min−0.5) for both studied resins. This high value of Ki indicates the fast transfer. The small positive values of X (0.13) for both resins indicate the presence of boundary layer effect on the rate of adsorption. Elovich equation was also applied to the sorption of Ag(I) by the chitosan resins according to the relation (Elwakeel and Rekaby, 2011):
1.2 0.9 0.6 R-en R-az
0.3 0.0 0
b 1.8
where qt is the sorption capacity at time t and α the initial sorption rate (mmol g−1 min−1) and β the desorption constant (g mmol−1). Thus, the constants can be obtained from the slope and intercept of a straight line plot of qt versus log t. The linearization of the equation giving the rate of reaction allows obtaining the initial sorption rate, α (mmol g −1 min −1) from the intercept of a straight line plot of qt versus ln t. The values of α for the adsorption of Ag(I) ions on resins R-en and R-az are 0.286 and 0.268 (mmol g −1 min −1), respectively. These values indicate that the initial sorption rate of R-en is high compared with R-az, which may be attributed to less steric hindrance of ethylenediamine moiety compared with triazole one. The values of β (desorption constant) is found to be 2.20 and 2.31 g/mmol for R-en and R-az, respectively. It was observed that these values are small compared with other studies, which indicates the high affinity of the studied resin towards Ag(I) ions. The data obtained indicate that the
1.5
Uptake (mmol/g)
ð11Þ
qt ¼ 1=β ln ðαβÞ þ 1=β ln t
ð12Þ
where qe the adsorbed value of Ag(I) at equilibrium concentration (mmol/g), Qmax is the maximum adsorption capacity (mmol/g) and KL
Uptake, mmol/g
Resin
1
2
3
4
5
6
7
8
9
10 11 12
t 0.5 (min) 0.5
1.2 0.9 0.6
R-en R-az
0.3 0.0 1
2
3
4
5
6
7
ln t (min) Fig. 5. (a) Intraparticle diffusion and (b) Elovich kinetics of the uptake of Ag(I) from initial concentration of 5 × 10−3 by the studied resins at 25 °C and pH 6.7.
32
K.Z. Elwakeel et al. / International Journal of Mineral Processing 120 (2013) 26–34
18
1.6
(R-en)
15
1.2
C e /q e (g/L)
Uptake, mmol/g
1.4
(R-en)
1.0 0.8
Experimental data at 25°C
0.6
Experimental data at 45°C
Experimental data at 35°C
9 6
Langmuir isotherm at 25°C
0.4
12
25°C 35°C
Langmuir isotherm at 35°C
3
Langmuir isotherm at 45°C
0.2 0.0 0
5
10
15
20
25
0 0
30
45°C
3
6
9
C e (mM) 1.6
21
24
(R-az)
15
1.2
C e /q e (g/L)
Uptake, mmol/g
18
18
(R-az)
1.4
1.0 0.8
Experimental data at 25°C
0.6
Experimental data at 45°C
Experimental data at 35°C
12 9 25°C
6
Langmuir isotherm at 25°C
0.4
35°C
Langmuir isotherm at 35°C
0
5
10
15
20
25
45°C
3
Langmuir isotherm at 45°C
0.2 0.0
12 15 C e (mM)
0
30
0
3
C e (mM)
6
9
12 15 C e (mM)
18
21
24
Fig. 6. Non linear Langmuir isotherms for the adsorption of Ag(I) ions by resins R-en, and R-az at different temperatures and pH 6.7.
Fig. 7. Linear Langmuir isotherms for the adsorption of Ag(I) ions by resins R-en, and R-az at different temperatures and pH 6.7.
is the Langmuir binding constant which is related to the energy of adsorption (L/mmol), Ce is the equilibrium concentration of Ag(I) ions in solution (mmol/L). Its linearized equation is shown as below
surface. The observed decrease of both Qmax and KL with increasing temperature may be related to the decrease of the stability of the complex formed between Ag(I) and resin active sites. The observed deviation from linear plot of Freundlich isotherms at high equilibrium concentration may be due that Freundlich isotherms is applicable at low equilibrium concentrations (Freundlich, 1906). Therefore the last datum of the plots of linear Freundlich isotherms was avoided while fitting to obtain the best regression line. The degree of suitability of the obtained resins towards Ag(I) ions was estimated from the values of the separation factor (RL) using the following relation (Qi and Xu, 2004):
Ce q
¼
e
Ce Q max
þ
1 : KL Q max
ð13Þ
The most important multisided adsorption isotherm for heterogeneous surfaces is the Freundlich isotherm, characterized by the heterogeneity factor 1/ n. The Freundlich model is described by (Freundlich, 1906): qe ¼
1=n KF Ce
ð14Þ
where KF and n are the Freundlich constants related to the adsorption capacity and intensity, respectively. Its linearized expression is shown as below: log qe ¼ log Kf −
1 log Ce: n
ð15Þ
The values of Langmuir constants (Qmax and KL) at different temperatures were calculated from the slope and intercept of the plots in Fig. 7, while the values of Freundlich constants (KF and n) at different temperatures were calculated from the slope and intercept of the plots in Fig. 8 and reported in Table 4. The maximum adsorption capacities (Qmax) obtained at different temperatures are in good agreement with the experimental ones, and the values of R2 reported in Table 4, which is a measure of the goodness-of-fit, confirm the better representation of the experimental data by Langmuir model than Freundlich model (Table 5). This indicates the homogeneity of active sites on the resin
RL ¼
1 1 þ KL Co
ð16Þ
where KL is the Langmuir equilibrium constant and Co is the initial concentration of Ag(I). Values of 0 b RL b 1 indicates the suitability of the process. The values of RL for the investigated resins towards the adsorption of Ag(I) lie between 0.077 and 0.431 for all concentration and temperature ranges. This implies that the adsorption of Ag(I) on both R-en and R-az from aqueous solution is favorable under the conditions used in this study. The values of KL at different temperatures were processed according to the following van't Hoff equation (Donia et al., 2007) to obtain the thermodynamic parameters of the adsorption process ln KL ¼
−ΔHo ΔSo þ RT R
ð17Þ
where ΔHo and ΔSo are enthalpy and entropy changes, respectively, R is the universal gas constant (8.314 J/mol K) and T is the absolute temperature (in Kelvin). The values of ΔHo and ΔSo were calculated from the
K.Z. Elwakeel et al. / International Journal of Mineral Processing 120 (2013) 26–34
0.3
Table 5 Freundlich constants for adsorption of Ag(I) ions on the studied resins.
0.2
(R-en)
Temp. (°C)
R-en N
Kf
R2
n
Kf
R2
25 35 45
3.368 3.061 2.588
0.734 0.621 0.505
0.804 0.900 0.959
2.779 2.783 2.024
0.588 0.531 0.385
0.975 0.910 0.923
Log qe
0.1 0.0 -0.1
R-az
Temperature 25°C
-0.2
35°C
2.0
45°C
-0.3
1.5 0.0
0.5
1.0
1.5
2.0
ln KL (L/mol)
-0.4 -0.5
Log Ce 0.2 (R-az)
0.1
Resin R-en R-az
1.0 0.5 0.0 -0.5 -1.0
0.0
Log qe
33
-1.5 -0.1 -2.0 0.0030
Temperature
-0.2
35°C
-0.3
0.0
0.5
1.0
1.5
0.0032
0.0033
0.0034
0.0035
Fig. 9. Van't Hoff plots for the uptake of Ag(I) on the studied resins.
45°C
-0.4 -0.5
0.0031
1/T (1/Kelvin)
25°C
3.3. Elution and regeneration cycles
2.0
Log Ce
Elution of Ag(I) from the resin was carried out using column method at flow rate of 2.0 mL/min. The saturation of the resin was reached after flowing 240 mL of 5 × 10−3 M Ag(I) in the column (Fig. 10). Sorption/desorption cycle runs were carried out for Ag(I) on both R-en and R-az resins. The elution of Ag(I) ions was performed using 50 mL of 0.5 M thiourea acidified with 0.01 M H2SO4. As shown in Fig. 10, the breakthrough curves for recovery of Ag(I) showed no characteristic changes during successive cycles. The maximum adsorption capacities of the column at different three successive cycles are 1.373, 1.320 and 1.276 for R-en resin, and these values are 1.130, 1.081 and 1.088 for R-az resins. This indicates that both chelating resins have good performance for repeated use up to at least three cycles.
Fig. 8. Linear Freundlich isotherms for the adsorption of Ag(I) ions by resins R-en, and R-az at different temperatures and pH 6.7.
slope and intercept of the plots in Fig. 9, respectively, and reported in Table 6. The negative values of ΔHo indicate the exothermic nature of adsorption process. The enthalpic values obtained are coherent with chemical process, which confirm that the complex formation between Ag(I) ion and resin active sits. The small negative values of ΔSo suggest the slight decrease of randomness during the adsorption of Ag(I). Gibbs free energy of adsorption (ΔGo) was calculated from the following relation and given in Table 6.
3.4. Conclusions o
o
o
ΔG ¼ ΔH −TΔS
ð18Þ
Removal of Ag(I) from aqueous solutions was studied using chitosan resin modified with ethylenediamine and 3-amino-1,2,4-triazole-5-thiol. The recovery process was carried out through batch and column methods. The chelating resins showed good selective adsorption of Ag(I) from binary mixtures of different metal ions. Kinetic studies indicated that the adsorption reaction follows the pseudo second order, intraparticle diffusion and Elovich model. Thermodynamic parameters indicated that the adsorption process is endothermic reaction. The regeneration of
The observed increase in the values of ΔG o with increasing temperature may be attributed to the exothermic nature of the reaction between resin active sites and Ag(I) ions. This may also be reflected in the values of KL, the values of KL which decrease as the temperature increases, indicating lower affinity of the resin towards Ag(I) at higher temperature.
Table 4 Langmuir constants for adsorption of Ag(I) on the studied resins. Temp. (°C)
25 35 45
R-en
R-az 2
Qmax, exp (mmol/g)
Qmax, calc (mmol/g)
KL (L/mmol)
R
1.360 1.210 1.150
1.473 1.311 1.273
0.813 0.838 0.599
0.995 0.995 0.996
Qmax, exp (mmol/g)
Qmax, calc (mmol/g)
KL (L/mmol)
R2
1.250 1.190 1.120
1.396 1.348 1.290
0.601 0.468 0.406
0.992 0.993 0.993
34
K.Z. Elwakeel et al. / International Journal of Mineral Processing 120 (2013) 26–34
Table 6 Enthalpy, entropy and free energy changes for adsorption of Ag(I) ions on the studied resins. ΔHo (kJ/mol)
Resin
−11.844 −15.458
R-en R-az
ΔSo (J/mol.K)
−0.0409 −0.056
ΔGo (kJ/mol) 298 K
308 K
318 K
0.344 1.230
0.753 1.790
1.162 2.350
1.0 0.9 (R-en)
0.8
C/C 0
0.7 0.6 0.5 0.4 0.3
Regeneration cycle
cycle 1 cycle 2 cycle 3
0.2 0.1 0.0
0
50
100
150
200
250
300
Time (min) 1.0 0.9 (R-az)
0.8
C/C 0
0.7 0.6 0.5 0.4 Regeneration cycle
0.3
cycle 1 cycle 2 cycle 3
0.2 0.1 0.0
0
50
100
150
200
250
300
Time (min) Fig. 10. Effect of successive desorption cycles on the breakthrough curves for the recovery of Ag(I) at flow rate of 2.0 mL/min.
both studied resins was performed using 50 mL of 0.5 M thiourea acidified with 0.01 M H2SO4 successfully. References Abd El-Ghaffar, M.A., Abdel Wahab, Z.H., Elwakeel, K.Z., 2009a. Extraction and separation studies of silver(I) and copper(II) from their aqueous solution using a chemically modified melamine resins. Hydrometallurgy 96, 27–34. Abd El-Ghaffar, M.A., Mostafa, M.H., Elwakeel, K.Z., 2009b. Adsorption of silver(I) on synthetic chelating polymer derived from 3-amino-1,2,4-triazole-5-thiol and glutaraldehyde. Chem. Eng. J. 151, 30–38. Acarkan, N., Bulut, G., Gül, A., Kangal, O., Karakaş, F., Kökkiliç, O., Önal, G., 2011. The effect of collector's type on gold and silver flotation in a complex ore. Sep. Sci. Technol. 46, 283–289.
Atia, A.A., 2005. Studies on the interaction of mercury(II) and uranyl(II) with modified chitosan resins. Hydrometallurgy 80, 13–22. Azizian, S., 2004. Kinetic models of sorption: a theoretical analysis. J. Colloid Interface Sci. 276, 47–52. Bas, A.D., Yazici, E.Y., Deveci, H., 2012. Recovery of silver from X-ray film processing effluents by hydrogen peroxide treatment. Hydrometallurgy 121–124, 22–27. Bassett, J., Denney, R.C., Jeffery, G.H., Mendham, J., 1978. Textbook of Quantitative Inorganic Analysis. ELBS, p. 323. Çelik, Z., Gülfen, M., Aydın, A.O., 2010. Synthesis of a novel dithiooxamide–formaldehyde resin and its application to the adsorption and separation of silver ions. J. Hazard. Mater. 174, 556–562. Çoruh, S., Şenel, G., Ergun, O.N., 2010. A comparison of the properties of natural clinoptilolites and their ion-exchange capacities for silver removal. J. Hazard. Mater. 180, 486–492. Cui, H., Li, Q., Gao, S., Shang, J.K., 2012. Strong adsorption of arsenic species by amorphous zirconium oxide nanoparticles. J. Ind. Eng. Chem. 18, 1418–1427. Donia, A.M., Atia, A.A., Elwakeel, K.Z., 2007. Recovery of gold(III) and silver(I) on a chemically modified chitosan with magnetic properties. Hydrometallurgy 87, 197–206. Elwakeel, K.Z., 2010a. Environmental application of chitosan resins for the treatment of water and wastewater: a review. J. Dispers. Sci. Technol. 31, 273–288. Elwakeel, K.Z., 2010b. Removal of Cr(VI) from alkaline aqueous solutions using chemically modified magnetic chitosan resins. Desalination 250, 105–112. Elwakeel, K.Z., Rekaby, M., 2011. Efficient removal of Reactive Black 5 from aqueous media using glycidyl methacrylate resin modified with tetraethelenepentamine. J. Hazard. Mater. 188, 10–18. Elwakeel, K.Z., Abd El-Ghaffar, M.A., El-Kousy, S.M., El-Shorbagy, H.G., 2012. Synthesis of new ammonium chitosan derivatives and their application for dye removal from aqueous media. Chem. Eng. J. 203, 458–468. Freundlich, H.M.F., 1906. Uber Die Adsorption in Losungen, Z. J. Phys. Chem. 57, 385–470. Ho, Y.S., McKay, G., 1999. Pseudo-second order model for sorption processes. Process. Biochem. 34, 451–465. Iqbal, J., Wattoo, F.H., Wattoo, M.H.S., Malik, R., Tirmizi, S.A., Imran, M., Ghangro, A.B., 2011. Adsorption of acid yellow dye on flakes of chitosan prepared from fishery wastes. Arab. J. Chem. 4, 389–395. Kiani, G.R., Sheikhloie, H., Arsalani, N., 2011. Heavy metal ion removal from aqueous solutions by functionalized polyacrylonitrile. Desalination 269, 266–270. Kononova, O.N., Shatnykh, K.A., Prikhod'Ko, K.V., Kashirin, D.M., 2009. Ion-exchange recovery of gold(I) and silver(I) from thiosulfate solutions. Russ. J. Phys. Chem. A 13, 2340–2345. Lagergren, S., 1898. About the theory of so-called adsorption of soluble substances. K. Svenska Vetenskapsakad. Handl. 24, 1–39. Langmuir, I., 1918. The adsorption of gases on plane surfaces of glass, mica and platinum. J. Am. Chem. Soc. 40, 1361–1403. Liu, Y., Cao, X., Hua, R., Wang, Y., Liu, Y., Pang, C., Wang, Y., 2010. Selective adsorption of uranyl ion on ion-imprinted chitosan/PVA cross-linked hydrogel. Hydrometallurgy 104, 150–155. Liu, B., Lv, X., Wang, D., Xu, Y., Zhang, L., Li, Y., 2012. Adsorption behavior of As(III) onto chitosan resin with As(III) as template ions. J. Appl. Polym. Sci. 125, 246–253. Patterson, J.W., 1985. Industrial Wastewater Treatment Technology, 2nd ed. Butterworth, p. 405. Qi, L., Xu, Z., 2004. Lead sorption from aqueous solutions on chitosan nanoparticles. Colloids Surf., A Physicochem. Eng. Asp. 251, 186–193. Rivera, I., Juárez, G., Patiño, F., Reyes, I.A., Roca, A., Reyes, M.I., 2012. Silver precipitation using sodium dithionite in cyanide media. J. Mex. Chem. Soc. 56, 156–162. Song, X., Gunawan, P., Jiang, R., Leong, S.S.J., Wang, K., Xu, R., 2011. Surface activated carbon nanospheres for fast adsorption of silver ions from aqueous solutions. J. Hazard. Mater. 194, 162–168. Tao, H.-C., Gao, Z.-Y., Ding, H., Xu, N., Wu, W.-M., 2012. Recovery of silver from silver(I)-containing solutions in bioelectrochemical reactors. Bioresour. Technol. 111, 92–97. Wang, Y., Ma, X., Li, Y., Li, X., Yang, L., Ji, L., He, Y., 2012. Preparation of a novel chelating resin containing amidoxime–guanidin group and its recovery properties for silver ions in aqueous solution. Chem. Eng. J. 209, 394–400. Xu, Q., Yin, P., Zhao, G., Sun, Y., Qu, R., 2010. Adsorption selectivity and dynamic adsorption behaviors of Cu(II), Ag(I), and Au(III) on silica gel encapsulated by amino functionalized polystyrene. J. Appl. Polym. Sci. 117, 3645–3650. Yazici, E.Y., Deveci, H., Yazici, R., 2011. Recovery of silver from X-ray film processing effluents using trimercapto-s-triazine (TMT). Sep. Sci. Technol. 46, 2231–2238. Zang, Z., Hu, Z., Li, Z., He, Q., Chang, X., 2009. Synthesis, characterization and application of ethylenediamine-modified multiwalled carbon nanotubes for selective solid-phase extraction and preconcentration of metal ions. J. Hazard. Mater. 172, 958–963.