chitosan composite adsorbents

Hydrometallurgy 100 (2009) 65–71

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Hydrometallurgy j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / h yd r o m e t

Adsorption of Au(III) from aqueous solution using cotton fiber/chitosan composite adsorbents Rongjun Qu ⁎, Changmei Sun, Minghua Wang, Chunnuan Ji, Qiang Xu, Ying Zhang, Chunhua Wang, Hou Chen, Ping Yin School of Chemistry and Materials Science, Ludong University, Shandong 264025, China

a r t i c l e

i n f o

Article history: Received 30 January 2009 Received in revised form 9 October 2009 Accepted 17 October 2009 Available online 25 October 2009 Keywords: Cotton fiber/chitosan adsorbent Adsorption kinetics Adsorption isotherm Adsorption selectivity Au(III)

a b s t r a c t Two kinds of cotton fiber/chitosan composite adsorbents (SCCH and RCCH) were employed to adsorb Au(III) ions from aqueous solution. The adsorption kinetics and adsorption isotherms of the two fibers for Au(III) were investigated. The experimental results revealed that the adsorption kinetics of SCCH and RCCH fibers for Au(III) was described by the pseudo second-order reaction model. The adsorption isotherm data of Au (III) on the surface of SCCH and RCCH fibers were fitted by linear and non-linear methods of the Freundlich, Langmuir and Redlich-Peterson isotherms. The results confirmed that both linear and non-linear forms of the above-mentioned three models can be used to describe adsorption of Au(III) on the surface of the two fibers and to predict the isotherm parameters. The linear Langmuir and non-linear Langmuir and Redlich-Peterson models are best-fit isotherms for the experimental data. Redlich-Peterson is a special case of Langmuir when the Redlich-Peterson isotherm constant g is set equal to unity. The investigation of adsorption selectivity showed that the SCCH and RCCH fibers displayed strong affinity for gold in the solution and both exhibited 100% selectivity for the gold in the presence of Ni(II), Cd(II), Zn(II), Co(II), and Mn(II). © 2009 Elsevier B.V. All rights reserved.

1. Introduction Gold, as a safe haven for investors, is one of the precious metals and held an allure for thousands of years. Moreover, the further demand for gold has shown an increasing trend due to the increasing uses of gold in industry. There are several new and emerging applications, such as electronics (Goodman, 2002; Ellis, 2004), catalysts (Haruta, 2004), and medical instruments (Navarro, 2009). Nevertheless, the separate and recovery of gold is not actually simple, which is due to the low concentration of gold in environmental, geological and metallurgical materials and insufficient sensitivity. This gives a compelling reason for developing more efficient and environmental-friendly methods for their extraction and recovery from mineral ores and waste materials (e.g., e-wastes, industrial effluents). At present, many methods, such as, co-precipitation (Zhao, 2006), ion exchange (Gomes et al., 2001; Al-Merey et al., 2003; Alguacil et al., 2005) and solvent extraction (Kordosky et al., 1992; Akita et al., 1996) and adsorption (Lam et al., 2007; Chang and Chen, 2006) have been used to separate and enrich gold. Comparatively, the adsorption of a solute on a solid support has the advantage over liquid–liquid extraction in that no mixing and settling requirements have to be fulfilled and organic phase loss through entrainment is

⁎ Corresponding author. Tel.: +86 535 6673982. E-mail address: [email protected] (R. Qu). 0304-386X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2009.10.008

eliminated. And the chelating adsorption seems to be the most suitable method for the recovery of precious metals in the case of low concentration due to low cost and high efficiency. According to the research study carried out by Dubois et al. (1995), some ion exchange resins (especially anionic resins), however, produce hazardous products in the process of pyrolysis and incineration (Dubois et al., 1995). Therefore, how to eliminate the secondary pollution of exhausted resins at the end of their life cycles to the environment is a remarkable problem, and more and more attention should be attached to it. For this reason, several research studies have been focused on the utilization of the natural adsorbents which are extracted from agriculture wastes and seafood by-products (Ngah and Liang, 1999; Guibal et al., 1999; Rao and Khan, 2009) due to their biodegradability and environmental-friendly properties. Among these natural materials, chitosan has proved to be an extremely promising adsorbent. Chitosan is a linear polysaccharide based on a glucosamine unit and is obtained through chitin deacetylation, which is a major component of crustacean shells and one of the most abundant biopolymers in nature. Several researchers have demonstrated that chitosan can be used as a suitable biopolymer adsorbent for the removal of metal ions from wastewater (Penichecovas et al., 1992; Kawamura et al., 1993; Merrifield et al., 2004; Chu, 2002; Ngah et al., 2002; Guibal, 2004; Bassi et al., 2000), because the amino and hydroxyl groups in chitosan can coordinate with metal ions to form stable complexes. Before use, chitosan is generally modified by means of crosslinking or grafting, in order to prevent its dissolution in acidic

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solutions or to improve its metal adsorption properties (to increase adsorption capacity or selectivity). A coating of chitosan on the surface of a solid substance including ceramic alumina (Boddu et al., 2008), Fe3O4 nanoparticles (Chang and Chen, 2005), nanoporous glass beads (Liu et al., 2003, 2002), and cotton fiber (Liu et al., 2001) via physical or chemical methods is a novel strategy to create effective chitosancontaining adsorbents. In particular, a coating of chitosan on the surface via chemical covalent bonding not only solves the problem of dissolution of chitosan in acidic solutions, but also increases the utilization ratio efficiently and improves the mechanical properties of chitosan. Recently, we have prepared chitosan-coated cotton fiber via a Schiff-base bond (SCCH) and a C–N single bond (RCCH) and investigated the adsorption of these fibers for Cu(II), Ni(II), Pb(II), Cd(II), and recovery of Hg(II) in aqueous solution (Zhang et al., 2008; Qu et al., 2009). The reason for choosing cotton fiber as supporting materials of adsorbents is that the major ingredient of cotton fiber is cellulose, which is the most abundant biopolymer in nature and has characteristic properties such as excellent hydrophilicity, biocompatibility and biodegradability. In that study, we found that the above two fibers exhibited good affinity and adsorption selectivity for Au(III) in aqueous solution. The objectives of the present study were to evaluate the adsorption kinetics, adsorption isotherms, and adsorption selectivity of SCCH and RCCH fibers for Au(III) from aqueous solutions. Furthermore, the factors affecting adsorption of Au(III) on SCCH and RCCH fibers such as temperature, solution pH and coexistent ions were investigated systematically. 2. Experimental 2.1. Materials and methods Cotton fiber/chitosan composite adsorbents, SCCH and RCCH fibers with 4.49% and 4.25% of chitosan, respectively, were prepared according to the method described in our previous work (Zhang et al., 2008; Qu et al., 2009). The 1.0 cm fractions of fibers were used in the adsorption experiments. All other reagents were of analytical grade. Doubly distilled deionized water was used in all experiments. Stock solution of Au(III) with a concentration of 1 mg mL− 1 in 2.0 mol L− 1 HCl was used. The working and standard solutions were prepared by diluting this stock solution using suitable ratios. Dilute hydrochloric acid and sodium hydroxide solutions were added to adjust the pH of the solutions. And the pH values were controlled with a Seven Multi pH meter, Mettler Toledo Instruments (Shanghai) Co. Ltd., China. Atomic absorption analysis of the various metal ions was performed with a flame atomic absorption spectrophotometer (AAS, GBC Model 932B, made in Australia).

was filled with 25 mL of Au(III) solution of varying concentrations and the pH was adjusted to 3.0. A known amount of adsorbent (about 100 mg) was added into each flask and agitated continuously at 25 °C for 20 h. The adsorption capacities are calculated also by using Eq. (1), where C is the equilibrium concentrations of Au(III) in solution. 2.4. Adsorption selectivity The selectivity experiments were done by placing 100 mg of SCCH (or RCCH) in a 50-mL flask, and then 2 mL of Au(III), 2 mL of 0.1 mol L− 1 of coexistence metal ion, and 16 mL of buffer solution with pH 3.0 was added in turn. The mixture was shaken for 20 h at 25 °C. The mixture was filtered off and the concentrations of the Au(III) and coexisting metal ion were determined by atomic absorption spectrometry. The above-mentioned adsorption experiments were repeated three times. The adsorption capacities of Au(III) and coexisting metal ion are calculated according to the Eq. (1). The adsorption selective coefficient (α) is defined as follows: α=

The adsorption capacity of AuðIIIÞ on adsorbents ðmmol g−1 Þ The adsorption capacity of coexisting metal on adsorbents ðmmol g−1 Þ

3. Results and discussion 3.1. Effect of pH on adsorption As is well known, the pH of solution is an important parameter for the adsorption of metal ions on adsorbents as it not only affects metal species in solution, but also influences the surface properties of the adsorbents in terms of dissociation of functional groups and surface charges. In order to evaluate the effect of pH on the adsorption of Au (III) on SCCH and RCCH fibers, the pH values of sample solutions were adjusted to a range of 1.0–8.0. The experimental results are shown in Fig. 1. As can be seen from Fig. 1, the solution pH has a significant effect on the adsorption capacity. In the case of solution pH range of 1.0–3.0, the adsorption capacities of Au(III) increase with an increase in pH. The two fibers exhibited low affinities for Au (III) and a small amount of Au (III) were adsorbed onto the SCCH and RCCH fibers below pH 2.0. It is because the competition adsorption between Cl− and AuCl− 4 influenced the Au(III) adsorption on the positively charged sits of SCCH and RCCH fibers in the event that high concentration of Cl− (pH 1.0–2.0) (Chang and Chen, 2006). The optimum pH value appeared at about 3.0 for both SCCH and RCCH fibers are probably due to electrostatic − attraction between R–NH+ 3 and AuCl4 . When the pH was greater than 3.0, the adsorption capacities of both SCCH and RCCH fibers for Au(III) decreased with the increase of pH. From the point of chemical

2.2. Adsorption kinetics The adsorption kinetics of SCCH (or RCCH) fiber for Au(III) was studied by shaking a mixture of 100 mg of adsorbents and 25 mL of Au (III) solution (pH 3.0) in a 50-mL flask at different temperatures. One milliliter of the solution was taken at different time intervals, where the residual concentration of metal ion was determined via AAS. The adsorption capacity of Au(III) is calculated according to the Eq. (1) Q = VðC0 −CÞ = W

ð1Þ

where C0 and C (mmol·mL− 1) are the initial and equilibrium concentration of Au(III), respectively; V(mL) is the volume of solution, and W(g) is the dry weight of adsorbents (SCCH or RCCH fibers). 2.3. Isothermal adsorption The procedure for carrying out isothermal batchwise adsorption tests was as follows: a series of 50-mL flasks were employed. Each flask

Fig. 1. The effect of pH on adsorptions of Au(III) on SCCH and RCCH fibers.

R. Qu et al. / Hydrometallurgy 100 (2009) 65–71

67

structure of chlorogold complexes, evidence showing that the predominant complex of gold is AuCl− 4 at low pH below 3.0. Increasing solution pH causes the hydrolysis reaction of AuCl− 4 to proceed, and thus hydrolyzed chlorogold complexes such as AuCl3(OH)− appear in the aqueous chloride solution (Ogata and Nakano, 2005). The equilibrium constants are as follows: AuðOHÞ4 + Hþ + Cl− ⇄AuClðOHÞ− 3 + H2 O

K1 = 108:51

þ − − AuClðOHÞ− 3 + H + Cl ⇄AuCl2 ðOHÞ2 + H2 O

K2 = 108:06

þ − − AuCl2 ðOHÞ− 2 + H + Cl ⇄AuCl3 ðOHÞ + H2 O

K3 = 107:00

AuCl3 ðOHÞ− + Hþ + Cl− ⇄AuCl− 4 + H2 O

K4 = 106:07 :

Thus, the reversal above pH 3.0 might because the increasing OH− in solution compete with Cl− and form hydroxo-containing gold complex. The hydroxo complex of gold in solution and deprotonation of amine lead to a decrease in electrostatic attractions between the negatively charged Au(III) anions and the positively charged adsorption sites of SCCH and RCCH fibers. For the practicality of environment samples, pH 3.0 was selected in the following work. 3.2. Adsorption kinetics The adsorption kinetics data of Au(III) on SCCH and RCCH fibers is illustrated by Fig. 2. It is observed from Fig. 2 that the temperature has effect on the adsorption of Au(III) on the surfaces of SCCH and RCCH fibers to same extent. The time required to reach adsorption equilibrium shortened with the increasing of temperatures. For example, the adsorption equilibrium time of Au(III) on the surface of SCCH fiber at 5, 15, 25, and 35 °C was 4, 7, 9, and 10 h, respectively. A similar phenomenon can be found from the adsorption of RCCH fiber for Au(III) and the time required for RCCH reaching equilibrium were 5, 7, 9, and 7 h at 5, 15, 25, and 35 °C, respectively. In order to ensure the adsorption completely, the optimum contact times for SCCH and RCCH fibers to Au(III) were fixed 20 h in the following experiments. Adsorption kinetics parameters, which can control the residence time of the adsorbate uptake at the solution–solid interface and provide valuable insights into water treatment process design, are of great importance for the application of adsorbents. Both pseudo firstand second-order models can be used to express the adsorption process of SCCH and RCCH fibers for Au(III). According to the previous investigations (Ramnani and Sabharwal, 2006; Ngah et al., 2004; Rojas et al., 2005), the pseudo first-order model and pseudo secondorder model can be expressed by Eqs. (2) and (3), respectively: dQ = k1 ðQ0 −QÞ dt

ð2Þ

dQ 2 = k2 ðQ0 −QÞ : dt

ð3Þ

Integrating Eqs. (2) and (3) for the boundary conditions t = 0 to t = t and Qt = 0 to Qt = Q0, the equations can be rearranged to obtain the following Eqs. (4) and (5) logðQ0 −QÞ = log Q0 − t 1 1 + t = Q Q0 k2 Q02

k1 t 2:303

ð4Þ

Fig. 2. Adsorption kinetics of SCCH and RCCH fibers for Au(III) at different temperatures.

at any time (mmol g− 1), k1 and k2 are the rate constant of pseudo first-order (h− 1) and second-order (g mmol− 1 h− 1) adsorption. The plots of log(Q0 − Q) versus t and t/Q versus t were employed to test the pseudo first- and second-order models, the results are shown in Figs. 3 and 4, respectively. As can be seen from Figs. 3 and 4, the results obtained for adsorption of Au(III) onto SCCH and RCCH fibers are only fitted well to Eq. (5) instead of Eq. (4). The fitting kinetic parameters are listed in Table 1. The results in Table 1 clearly show that the pseudo secondorder model provides better correlation coefficients than pseudo firstorder model for the adsorption the two fibers, suggesting the pseudo second-order model is more suitable to describe the adsorption kinetics processes of SCCH and RCCH fibers for Au(III). This reveals that the rate limiting step may be chemical sorption involving valency forces through sharing or exchange of electrons between chitosan and Au(III) (Ho and McKay, 1999). Based on the pseudo second-order model, the initial adsorption rate (u, mmol g− 1 h− 1) and half-adsorption time (t1/2, h) of SCCH and RCCH fibers for Au(III) are estimated in Table 2 according to the following Eqs. (6) and (7) (Wu et al., 2005; Amarasinghe and Williams, 2007): 2

u = k2 Q0 ð5Þ

where Q0 is the amount of metal adsorbed at equilibrium per unit weight of adsorbent (mmol g− 1), Q is the amount of metal adsorbed

t1 = 2 =

1 : k2 Q0

ð6Þ ð7Þ

Half-adsorption time, which is defined as the time required for the adsorption to take up half as much Au(III) as its equilibrium value, is

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R. Qu et al. / Hydrometallurgy 100 (2009) 65–71

Fig. 3. Pseudo first-order model of SCCH and RCCH fibers for Au(III) at different temperatures.

an effective measurement of adsorption rate. As shown in Table 2, the initial adsorption rates (u) of SCCH and RCCH fibers increased with the increasing of temperatures in principle. The data of t1/2 listed in Table 2 indicates that the times required reaching equilibriums for SCCH and RCCH fibers were much shorter at higher temperatures than at lower temperatures. These results are in accordance with those obtained from Fig. 2. The thermodynamic parameters for the adsorption process such as free energy of adsorption (ΔG), enthalpy of adsorption (ΔH), and entropy of adsorption (ΔS) are calculated by the following Eqs. (8) and (9) (Fujiwara et al., 2007). ΔG = ΔH−TΔS

log Kd =

ΔS ΔH − 2:303R 2:303RT

Fig. 4. Pseudo second-order model of SCCH and RCCH fibers for Au(III) at different temperatures.

randomness due to the adsorption of Au(III) ions. The negative value of ΔG indicates that the feasibility of the process and the spontaneous nature of adsorption of Au(III) on the surface of SCCH and RCCH fibers. 3.3. Isothermal adsorption Langmuir, Freundlich and Redlich-Peterson isotherms are the most commonly used isotherms for different adsorbent/adsorbate systems to explain solid–liquid adsorption systems and to predict their equilibrium parameters (Ho et al., 2004; Ho, 2003, 2004). The Langmuir isotherm is based on the monolayer adsorption on the

ð8Þ Table 1 The kinetic models of SCCH and RCCH fibers for Au(III) at different temperatures.

ð9Þ

where Kd = CAe/Ce, is the equilibrium constant, CAe is the equilibrium concentration of Au(III) on adsorbent (mmol L− 1), and Ce is the equilibrium concentration of Au(III) in the solution (mmol L− 1); R is the gas constant (8.314 J mol− 1 K− 1), T is the temperature (K), The results are illustrated in Fig. 5 and the parameters are listed in Table 3. The positive value of ΔH for the processes indicates that the adsorption of SCCH and RCCH fibers for Au(III) are both endothermic processes. The positive value of ΔS resulted from the increased

Fibers Temperature Pseudo first-order model (K) k1 Q0 R2 (h− 1) (mmol g− 1) SCCH 308 298 288 278 RCCH 308 298 288 278

0.84 1.02 0.012 0.50 0.94 0.35 0.43 0.59

0.014 0.149 0.173 0.056 0.014 0.026 0.033 0.034

Pseudo second-order model Q0 R2 k2 (h− 1) (mmol g− 1)

0.9376 115.3 0.8209 30.1 0.9543 31.1 0.8246 25.5 0.9323 128.0 0.9380 28.2 0.9346 28.5 0.8831 36.8

0.239 0.241 0.239 0.235 0.242 0.241 0.240 0.235

1.0000 0.9999 1.0000 0.9999 1.0000 0.9999 1.0000 0.9999

R. Qu et al. / Hydrometallurgy 100 (2009) 65–71 Table 2 Adsorption kinetics parameters of SCCH and RCCH fibers for Au(III) at different temperatures. Fibers

Temperature(K)

u (mmol·g− 1·h− 1)

t1/2 (h)

SCCH

278 288 298 308 278 288 298 308

1.41 1.77 1.74 6.56 2.03 1.62 1.64 7.49

0.17 0.13 0.14 0.036 0.12 0.15 0.15 0.032

RCCH

active sites of the adsorbents. The Langmuir isotherm can be expressed as: Q =

Q 0 bC 1 + bC

ð10Þ

69

Table 3 The parameters of kinetic models of SCCH and RCCH for Au(III). Fibers

T(K)

ΔG(KJ mol− 1)

ΔH (KJ mol− 1)

ΔS(J mol− 1)

SCCH

308 298 288 278 308 298 288 278

−19.6 −18.3 −17.1 −15.9 −20.0 −18.8 −17.5 −16.2

17.4

120.1

19.4

127.9

RCCH

between 0 and 1. The linearized form of the Redlich-Peterson model is given by Eq. (15):   AC −1 = g ln C + ln B: ln Q

ð15Þ

where A (l g− 1) and B (l mmol− 1) are the Redlich-Peterson isotherm constants and g is the Redlich-Peterson isotherm exponent, which lies

Redlich-Peterson isotherm equation contains three unknown parameters A, B and g. By varying the isotherm parameters, A, the maximum value of the correlation coefficient for the regression of against can be obtained. Therefore, the minimization procedure is adopted to maximize the correlation coefficient of determination r2 (as a function of A), between the theoretical data for Q predicted from the linearized form of Redlich-Peterson isotherm equation and the experimental data (Ho et al., 2002). In this study, the experimental data in Fig. 6 were fitted using Langmuir, Freundlich and Redlich-Peterson equations by linear and non-linear methods and the corresponding adsorption parameters along with regression coefficients are listed in Tables 4 and 5. From Tables 4 and 5, it can be seen that the regression coefficient r2 of linear and non-linear Langmuir, Freundlich and Redlich-Peterson isothermal models are more than 0.95, suggesting that all the three isothermal models can be used to describe the isotherm adsorptions of SCCH and RCCH resins for Au(III) under the present conditions. In the case of linear methods, Langmuir isotherm is the best-fit model to predict the experimental data as it has the best r2 among the three models. In the non-linear methods, both Langmuir and RedlichPeterson models are the best-fit models to predict the experimental data. Redlich-Peterson is a special case of Langmuir when the RedlichPeterson isotherm constant g was unity. The best-fit experimental equilibrium data in Langmuir isotherm suggests the monolayer coverage and chemisorption of Au(III) onto SCCH and RCCH fibers. The n values are between 2 and 5, indicating the adsorption processes are carried out easily (Kitagawa and Suzuki, 1983).

Fig. 5. The logKd v s 1/T plots for Au(III) adsorption on SCCH and RCCH fibers.

Fig. 6. The adsorption isotherms of SCCH and RCCH fibers for Au(III) at 25 °C.

where Q0 indicates the monolayer adsorption capacity of adsorbent (mmol g− 1) and the Langmuir constant b (l mg− 1) is related to the energy of adsorption. For fitting the experimental data, the Langmuir model was linearized as: C 1 C + = Q bQ0 Q0

ð11Þ

The Freundlich isotherm explains the adsorption on a heterogeneous (multiple layer) surface with uniform energy. The Freundlich model is represented by the equation: Q = KF C

1=n

ð12Þ

where KF (mmol g− 1) is the Freundlich constant related to adsorption capacity of adsorbent and n is the Freundlich exponent related to adsorption intensity (dimensionless). For fitting the experimental data, the Freundlich model is linearized as follows: ln Q =

1 ln C + ln KF n

ð13Þ

The Redlich-Peterson model is represented by the equation: Q =

AC 1 + BC g

ð14Þ

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R. Qu et al. / Hydrometallurgy 100 (2009) 65–71

Table 4 Isotherm parameters obtained by using linear method. Equations

Parameters

SCCH

RCCH

Linear Langmuir

Q0 b r2 kF n r2 A B g r2

0.39 1.36 0.9989 0.19 2.42 0.9779 0.70 2.34 0.84 0.9985

0.45 0.82 0.9956 0.17 2.21 0.9376 0.35 0.84 0.90 0.9834

Linear Freundlich

Linear Redlich-Peterson

Table 6 The adsorption selectivity of SCCH and RCCH fibers for Au(III) from binary metal ion systems. Fibers

System

Metal ions

Adsorption capacity (mmol·g− 1)

Selective coefficient (α)

SCCH

Au(III)–Pb(II)

Au(III) Pb(II) Au(III) Cu(II) Au(III) Ni(II) Au(III) Cd(II) Au(III) Zn(II) Au(III) Co(II) Au(III) Mn(II) Au(III) Pb(II) Au(III) Cu(II) Au(III) Ni(II) Au(III) Cd(II) Au(III) Zn(II) Au(III) Co(II) Au(III) Mn(II)

0.38 0.02 0.37 0.02 0.39 0 0.39 0 0.39 0 0.39 0 0.38 0 0.34 0.02 0.32 0.02 0.36 0 0.35 0 0.34 0 0.34 0 0.35 0

19.0

Au(III)–Cu(II) Au(III)–Ni(II) Au(III)–Cd(II) Au(III)–Zn(II) Au(III)–Co(II)

From Table 5, it can be observed that the maximum monolayer adsorption capacities of Au(III) adsorbed on SCCH and RCCH fibers were found to be 0.39 and 0.45 mmol/g (calculated based on linearized Langmuir equation), respectively, suggesting that RCCH fiber is more easier to adsorb Au(III) than SCCH fiber.

A series of representative binary metal ion systems were chosen to investigate the adsorption selectivity of SCCH and RCCH fibers for Au (III) and the results are shown in Table 6. Obviously, Au(III) was readily adsorbed by the SCCH and RCCH fibers from the systems of Au (III)–Pb(II), Au(III)–Cu(II), Au(III)–Ni(II), Au(III)–Cd(II), Au(III)–Zn (II), Au(III)–Co(II), and Au(III)–Mn(II), respectively, indicating that the two fibers exhibited good adsorption selectivity for Au(III). This high adsorption selectivity was due to high affinity of the Au(III) ion for the amine group in chitosan. According to the hard–soft acid–base (HSAB) theory, Au(III) is classified as a soft ion. Soft ions form very strong bonds with groups containing nitrogen and sulfur atoms (Volesky, 1990). Therefore, chitosan with amino groups shows a very high adsorption capacity for Au(III) ions (Jeon and HÖll, 2003). These findings suggest that SCCH and RCCH fibers can probably be used in the extraction, separation and recovery of Au(III) from a multi-ionic aqueous system. 4. Conclusions The adsorption of Au(III) on SCCH and RCCH fibers were investigated in this study, and the following conclusions were drawn: (1) The adsorption kinetics of the SCCH and RCCH fibers for Au(III) fit the pseudo second-order reaction model. (2) Langmuir, Freundlich and Redlich-Peterson models well described the adsorption isotherms of the SCCH and RCCH fibers for Hg(II) over the temperature range of 278 to 308 K. However, the linear Langmuir and non-linear Langmuir and Redlich-Peterson isotherm models are the best-fit model to predict the experimental data. Redlich-Peterson is a special

Table 5 Isotherm parameters obtained by using non-linear method. Equations

Parameters

SCCH

RCCH

Non-linear Langmuir

Q0 b r2 kF n r2 A B g r2

0.39 1.29 0.9981 0.20 2.88 0.9722 0.51 1.32 1.0 0.9981

0.45 0.84 0.9963 0.19 2.47 0.9565 0.37 0.84 1.0 0.9963

Non-linear Redlich-Peterson

RCCH

Au(III)–Pb(II) Au(III)–Cu(II) Au(III)–Ni(II)

3.4. Adsorption selectivity

Non-linear Freundlich

Au(III)–Mn(II)

Au(III)–Cd(II) Au(III)–Zn(II) Au(III)–Co(II) Au(III)–Mn(II)

18.5 ∞ ∞ ∞ ∞ ∞ 17.0 16.0 ∞ ∞ ∞ ∞ ∞

Test conditions: initial concentration Au(III): 0.00408 mol L− 1; initial concentration metal ions: 0.00408 mol L− 1; pH = 3.0; T = 25 °C.

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