Removal and Recovery of Au(III) from Aqueous Solution Using a Low

Article pubs.acs.org/IECR

Removal and Recovery of Au(III) from Aqueous Solution Using a LowCost Lignin-Based Biosorbent Zhi-Wei He, Li-Hong He, Jun Yang, and Qiu-Feng Lü* College of Materials Science and Engineering, Fuzhou University, 2 Xueyuan Road, Fuzhou 350116, People’s Republic of China ABSTRACT: A lignin-based composite, PANI−EHL, was prepared in a facile manner from enzymatic hydrolysis lignin (EHL) and aniline. The effects of initial pH value, biosorbent concentration, sorption time, and initial adsorbate concentration on the biosorption of Au(III) ions onto PANI−EHL composite were investigated. The results showed that the biosorbent exhibited high biosorption of Au(III) ions. The PANI−EHL composite favored treatment of Au(III) ions with concentrations below 2 mmol L−1 with an adsorptivity of 100%. The maximum biosorption capacity and adsorptivity for Au(III) ions were up to 4193.0 mg g−1 and 70.9%, respectively, at a Au(III) ion concentration of 20 mmol L−1 at 30 °C. Furthermore, the biosorption of Au(III) ions was found to be a redox sorption process, as confirmed by XRD, TEM, and FTIR and UV−vis spectroscopic analyses of the biosorption product. The sorption kinetic and equilibrium data were described well with the pseudo-second-order model and the Freundlich isotherm model, respectively, for the Au(III) ion biosorption process.

1. INTRODUCTION Because of the demand for the use of precious metals, the generation of a large amount of electronic and electrical waste has done harm to the environment.1 In fact, precious metals, such as Au, are present in higher amounts in such discarded wastes rather than in ore form.2 Therefore, the recycling and recovery of Au(III) ions plays a significant role not only in economizing ore resources, but also in reducing pollution. Conventional methods for the recovery of Au(III) ions from aqueous solutions include precipitation, ion exchange, and solvent extraction.3 However, some drawbacks, for example, ineffectiveness and high cost, still limit the wide application of these techniques. Unlike conventional methods, adsorption exhibits unique advantages, such as low operating costs, high efficiency, minimal volume of sludge, and so on.3,4 Recently, many adsorbents, such as activated carbon,5,6 chitosan-based adsorbents,4,7−12 gels,1,13,14 mesoporous silica,3 alga Fucus vesiculosus,15 and Durio zibethinus husk,16 have been used for the sorption and recovery of Au(III) ions from aqueous solution. However, only a few adsorbents have been reported to have a high adsorption capacity for Au(III) ions.1,10,13,14 Enzymatic hydrolysis lignin (EHL), a waste discarded from the ethanol industry, has great potential application in wastewater treatment.17 However, little research has been reported on lignin-based sorbents for the recovery of Au(III) ions from aqueous solutions. This might be because the structure of EHL lacks NH and NH2 functional groups, which play an important part in the redox reactions of heavy metal ions. Therefore, the introduction of nitrogen-containing functional groups is expected to improve the redox and adsorption performance of EHL. Polyaniline (PANI), a conducting polymer, is also an excellent adsorbent for heavy metal ions because of the highly reactive sensitivity of the amine and imine functional groups on its chains.18−22 These nitrogen-containing groups play a role not only in chelation but also in the reduction process.23,24 However, the adsorption © 2013 American Chemical Society

capacity of polyaniline-based adsorbents for Au(III) ions still needs to be improved.25 In this study, a low-cost lignin-based biosorbent, polyanilinemodified EHL, was used for the removal and recovery of Au(III) ions from aqueous solution. Synergistic effects of oxygen-containing functional groups from EHL and nitrogencontaining functional groups from polyaniline chains were expected to result in a high performance of the lignin-based biosorbent with respect to Au(III) ions.

2. EXPERIMENTAL SECTION 2.1. Materials. Enzymatic hydrolysis lignin (EHL) made from cornstalk residues was supplied by Shandong Longlive Biotechnology Co. Ltd. (Shangdong, China) in powder form. Aniline, ammonium persulfate (APS), and chloroauric acid (HAuCl4) were obtained from Sinopharm Chemical Reagent Co. Ltd. and used without further treatment. 2.2. Preparation of Lignin-Based Biosorbent. The lignin-based biosorbent, PANI−EHL composite, was prepared in a facile manner from EHL and aniline, as reported previously.23 A typical polymerization of the PANI−EHL composite was as follows: EHL (0.93 g) was dissolved in an aqueous solution of ammonia (0.1 M, 35 mL) in a 150 mL glass flask placed in a 25 °C water bath. Then, aniline (0.91 mL, 10 mmol) was added to the EHL solution, which was stirred vigorously to form a mixture of aniline and EHL. APS (2.28 g, 10 mmol) was dissolved separately in an aqueous solution of ammonia (0.1 M, 15 mL) to prepare an oxidant solution. The APS solution was poured into the aniline−EHL mixture solution at 25 °C. Then, the polymerization was carried out in a 25 °C water bath for 24 h; the PANI−EHL composite was isolated from the reaction mixture by filtration and washed with Received: Revised: Accepted: Published: 4103

December 11, 2012 February 14, 2013 February 21, 2013 February 21, 2013 dx.doi.org/10.1021/ie303410g | Ind. Eng. Chem. Res. 2013, 52, 4103−4108

Industrial & Engineering Chemistry Research

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3. RESULTS AND DISCUSSION 3.1. Sorption of Au(III) Ions onto PANI, EHL, and PANI−EHL Composite. The values of the sorption capacity (Q) and adsorptivity (q) of Au(III) ions on PANI, EHL, and PANI−EHL composite are reported in Table 1. It was found

an excess amount of aqueous ammonia (0.01 M) solution to remove the residual oxidant and EHL. The product remaining was dried in a 60 °C vacuum oven for one week to obtain PANI−EHL powder. 2.3. Characterization. Morphological measurement of the PANI−EHL composite after sorption was performed on a transmission electron microscope (JEM-2010, Jeol). Wideangle X-ray diffraction scan was obtained using an Ultima III Xray model diffractometer (Rigaku, Tokyo, Japan) with Cu Kα radiation at a scan rate of 10° min−1 in reflection mode over a 2θ range from 5° to 90°. FTIR spectra were recorded on a Nicolet FTIR 5700 spectrophotometer in KBr pellets. UV−vis spectra were measured on a Varian Cary50 Conc spectrometer in the wavelength range of 200−900 nm. 2.4. Biosorption Experiments. Biosorption tests of Au(III) ions on lignin-based biosorbent at varying initial pH values (1.0−4.0), biosorbent concentrations (0.2−1.6 g L−1), sorption times (0−48 h), and initial Au(III) ion concentrations (1−20 mmol L−1) were carried out using a conventional batch method. Typically, 25 mL of test solution containing 5 mmol L−1 Au(III) ions was mixed with 10 mg of dry biosorbent. The sorption solution was stirred for 5 min, and then sorption carried out under static conditions at 30 °C for 24 h to attain equilibrium. After sorption, the biosorbent was filtered immediately, and the Au(III) ion concentration in the filtrate was quantitatively measured by atomic absorption spectrophotometer (Varian, SpectrAA-220, Mulgrave, Victoria, Australia). The biosorption capacity and adsorptivity of Au(III) ions were calculated according to the methods described in an earlier report.13 2.5. Mathematical Models. The sorption kinetic equations were fitted by pseudo-first-order (eq 1) and pseudosecond-order (eq 2) kinetic models7 ln(Q e − Q t ) = ln Q e − k1t 1 1 1 = + 2 Qt Qe k 2Q e t

Table 1. Sorption Capacity (Q) and Adsorptivity (q) of Au(III) Ions onto PANI, EHL, and PANI−EHL Composite

ln Q e = ln KF +

1 ln Ce n

Q (mg g−1)

q (%)

PANI EHL PANI−EHL

1187.6 427.9 1452.2

80.4 29.0 98.3

that the capacity of PANI−EHL composite after adsorption was 1024.3 and 264.6 mg g−1 more than those of PANI and EHL, respectively. Compared with PANI−EHL composite, the adsorptivities of PANI and EHL were low. This was due to the existence of a synergistic effect between PANI and EHL in the PANI−EHL composite,23 which resulted in the improved capacity for Au(III) ions on the PANI−EHL composite. 3.2. Effect of Initial pH Value. The initial pH value of the Au(III) ion solution is a very important parameter for the biosorption of Au(III) ions. The effect of the initial pH value on Au(III) ion biosorption onto PANI−EHL is illustrated in Figure 1a. It was found that, when the initial pH was 2.5, the

(1) Figure 1. (a) Effect of initial pH value on biosorption capacity (Q) and adsorptivity (q) of PANI−EHL composite at a Au(III) ion concentration of 5.0 mmol L−1 and a PANI−EHL composite concentration of 0.4 g L−1 at 30 °C for 24 h and (b) effect of PANI−EHL composite concentration on biosorption of PANI−EHL composite at a Au(III) ion concentration of 5.0 mmol L−1 and an initial pH value of 2.5 at 30 °C for 24 h.

(2)

where k1 is the pseudo-first-order rate constant (h−1) and k2 is the pseudo-second-order kinetic rate constant (g g−1 h−1). Qe and Qt are the amounts of Au(III) ions adsorbed per unit adsorbent (g g−1) at equilibrium and at time t (h), respectively. The Langmuir and Freundlich isotherm models were used to describe and analyze sorption equilibrium, as described by the equations5 Ce C 1 = e + Qe Qm KLQ m

sample

biosorption capacity and adsorptivity of Au(III) ions on PANI−EHL reached maximum values of 1457.0 mg g−1 and 98.2%, respectively. Typically, the initial pH for the recovery of Au(III) ions is between 2.03,6−8,14 and 3.0.9 This is not only because of the application scope for the industrial recovery of gold,6 but also because of efforts to limit the hydrolysis reaction of AuCl4− and the appearance of AuCl3(OH)− in aqueous chloride solution.14 Therefore, subsequent biosorption studies in this work were carried out at a pH of 2.5. 3.3. Effect of Biosorbent Concentration. The effect of biosorbent concentration on biosorption of Au(III) ions onto PANI−EHL was investigated at an initial Au(III) ion concentration of 5 mmol L−1 for PANI−EHL concentrations ranging from 0.2 to 1.6 g L−1 (Figure 1b). It was found that the biosorption capacity decreased with increasing biosorbent concentration, whereas the adsorptivity increased. This was because the surface are per unit mass of PANI−EHL composite exposed to Au(III) ions decreased as the amount of PANI− EHL composite was decreased.16 When the PANI−EHL composite concentration was 0.2 g L−1, the biosorption

(3)

(4)

where Qe is the amount of Au(III) ions adsorbed (mg g−1) at equilibrium and Ce is the equilibrium concentration (mg/L). Qm (g g−1) and KL (mL g−1) are the Langmuir constants related to the saturated sorption capacity and sorption energy, respectively. KF [(mg g−1)(L mg−1)1/n] is the Freundlich constant related to sorption capacity, and n is the sorption equilibrium constant. The values of these constants can be determined from the intercepts and slopes of linear plots of Ce/ Qe versus Ce and lnQe versus lnCe for the Langmuir and Freundlich models, respectively. 4104

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capacity reached as high as 2731.0 mg g−1, and the adsorptivity was 92.4%. However, the biosorption capacity was only 370.0 mg g−1, and the adsorptivity was up to 100%, at a biosorbent concentration of 1.6 g L−1. The results revealed that the adsorptivity was above 90% for all PANI−EHL composite concentrations in the range from 0.2 to 1.6 g L−1. Yet, when the concentration of biosorbent was above 0.4 g L−1, the adsorptivity improved slightly. All in all, a PANI−EHL concentration of 0.4 g L−1 was deemed satisfactory on account of its biosorption capacity of 1437.0 mg g−1 and adsorptivity of 97.2%. Because increasing the PANI−EHL composite concentration was not a practical way to improve the adsorptivity, the biosorbent concentration of 0.4 g L−1 was selected for the following experiments. 3.4. Effect of Sorption Time. The time allowed for sorption between the adsorbate and biosorbent is an important factor for sorption at equilibrium. Figure 2a shows the effect of

Table 2. Sorption Capacities of Different Biomass-Based Adsorbents on Au(III) ions at 303 K adsorbent cross-linked polysaccharide gels glycine-modified cross-linked chitosan lysine-modified cross-linked chitosan cross-linked persimmon tannin gel Durio zibethinus husk microalgal residues lignophenol derivative dimethylamine-modified persimmon chemically modified chitosan cotton cellulose cross-linked paper gel PANI−EHL composite

Qm (mg g−1)

conditions

1491.3 170.0

0.1 M HCl pH 2

1 7

pH 2

11

0.1 M HCl pH 2 0.1 M HCl 0.5 M HCl 0.1 M HCl pH 0.5 0.1 M HCl 0.1 M HCl pH 2.5

13 16 26 27 28 29 30 31 this work

70.34 1516.9 339.6 640.3 1418.4 1109.1 709.2 1223.4 994.9 4193.0

ref

years.1,7,11,13,16,26−31 It can easily be seen that the PANI− EHL composite displayed a high sorption capacity for Au(III) ions. This was due to the redox adsorption of Au(III) ions and the existence of a synergistic effect in the PANI−EHL composite between PANI and EHL. 3.6. Biosorption Mechanism of Au(III) Ions. To investigate the biosorption mechanism of Au(III) ions, two samples from before (PANI−EHL) and after (PANI−EHL/ Au) biosorption were studied by X-ray diffraction (XRD; Figure 3a) and transmission electron microscopy (TEM; Figure 3b).

Figure 2. (a) Effect of sorption time on biosorption of PANI−EHL composite at a Au(III) ion concentration of 5.0 mmol L−1 and an initial pH value of 2.5 at 30 °C and (b) effect of initial Au(III) ion concentration on biosorption of PANI−EHL composite at an initial pH value of 2.5 at 30 °C for 24 h.

sorption time on the biosorption of Au(III) ions onto PANI− EHL composite. It was found that the biosorption capacity and adsorptivity went up rapidly in the beginning as the sorption time increased. When the sorption time was 2 h, the adsorptivity was 42.0%, and the biosorption capacity reached 621.0 mg g−1. The sorption rate decreased gradually during the remaining sorption time, which might be a result of the sorption and desorption processes occurring after saturation of Au(III) ions onto PANI−EHL composite surfaces.16 When the sorption times were 8, 16, and 48 h, the adsorptivity values were 71.6%, 95.7%, and 98.3%, respectively. However, the biosorption capacity and adsorptivity were almost constant at 1452.2 mg g−1 and 98.3%, respectively, after 48 h, signifying that an equilibrium balance of sorption was reached. 3.5. Effect of Initial Au(III) Ion Concentration. The effect of variations in the initial Au(III) ion concentration on the biosorption of PANI−EHL composite was studied (Figure 2b). It was observed that the biosorption capacity increased whereas the adsorptivity decreased with increasing Au(III) ion concentration. When the initial Au(III) ion concentration was 20 mmol L−1, the biosorption capacity and adsorptivity were 4193.0 mg g−1 and 70.9%, respectively. In addition, when the initial Au(III) ion concentration was 2 mmol L−1, the biosorption capacity and adsorptivity were 297.0 mg g−1 and 100%, respectively. Therefore, PANI−EHL composite seems to favor treatment of Au(III) ions with concentrations below 2 mmol L−1, because the biosorption was completed with an adsorptivity of 100%. Table 2 compares the maximum Au(III) ion sorption capacities (303 K) of PANI−EHL composite with those of different biomass-based adsorbents reported in recent

Figure 3. (a) Wide-angle X-ray diffraction curve, (b) TEM image, and (c) electron diffraction pattern of PANI−EHL composite after adsorption (i.e., PANI−EHL/Au) and (d) variation of the pH value of Au(III) ion solution with sorption time.

The XRD pattern for PANI−EHL/Au has four diffraction peaks at 2θ = 38.2°, 44.4°, 64.4°, and 77.6°, corresponding to the (111), (200), (220), and (311) Miller indices of Au, respectively.1,32 This verified the reduction of Au(III) ions into Au(0) crystals, indicating that a reduction reaction occurs during the biosorption process of Au(III) ions. Furthermore, irregular hexagons with edge lengths ranging from 0.52 to 3.29 μm were found in the TEM image of PANI−EHL after biosorption (Figure 3b,c). Because of the strong intensity of the 4105



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Scheme 1. Possible Reactive Sorption Mechanism of Au(III) Ions onto the PANI−EHL Composite

rings of PANI chains.35,37 It was found that the intensities of these three peaks of PANI−EHL/Au were higher than those of PANI−EHL composite, indicating that the number of quinoid rings in PANI increased. The band at 1299−1302 cm−1 is associated with the CN stretching vibration in an alternating unit of quinoid−benzenoid−quinoid.37 Moreover, a peak at 1150−1152 cm−1 is assigned to a vibration mode of the  NH+ structure, and a peak at 1110−1123 cm−1 could be due to aromatic CH in-plane-bending modes.38 The intensities of these two peaks of PANI−EHL/Au were much lower than those of PANI−EHL, which implies that the NH+ structures and benzenoid rings were transformed into  NH structures and quinoid structures, respectively. Therefore, a redox process occurs in the adsorption of Au(III) ions onto PANI−EHL. Moreover, there were few obvious peaks of oxygen-containing groups, such as the phenolic OH and C OC groups. The reason for this is not only the weak intensity of these peaks of EHL39,40 but also the low mass ratio of EHL to aniline in the polymerization of PANI−EHL. UV−vis spectra of PANI−EHL composite before and after Au(III) ion adsorption are shown in Figure 4b. Three absorptions of PANI−EHL composite at around 362, 429, and 859 nm can be assigned to a π−π* transition, a polaron−π* transition, and a π−polaron transition, respectively, of the emeraldine salt.41,42 A band of PANI−EHL/Au at about 588 nm should be due to hexagonal Au plates with a red shift to longer wavelengths compared to the corresponding band of spherical Au particles.43,44 Furthermore, compared with three bands of PANI−EHL composite, the absorptions of PANI−EHL/Au at about 318, 414, and 643 nm showed a large blue shift, which indicates the reduction of π conjugation of PANI−EHL composite after Au(III) ion adsorption.23,45,46 Therefore, it could be speculated that many benzenoid rings were transformed into quinoid structures, in good agreement with the FTIR results. 3.8. Mathematical Models. Pseudo-first-order and pseudo-second-order kinetic models were used to investigate the process of Au(III) ions biosorption on PANI−EHL (Figure 5). The correlation coefficients of the pseudo-first-order and pseudo-second-order equations for the sorption of Au(III) ions

(111) facet of Au, the as-produced Au crystals exhibited preferential growth along the (111) facet.33 The variation of the pH value of Au(III) ion solution with increasing sorption time was also studied (Figure 3d). Clearly, the pH value decreased sharply in the beginning, and then it decreased gradually until reaching the sorption equilibrium. The decrease of the pH value was because of the generation of H+ ions. A possible mechanism (Scheme 1) is as follows: First, the surface physical sorption of Au(III) ions onto PANI−EHL composite occurs through weak van der Waals forces, and the chelation of Au(III) ions onto PANI−EHL by nitrogencontaining functional groups from polyaniline and oxygencontaining functional groups from EHL also occurs. Then, the NH, NH+ groups and benzenoid rings of PANI− EHL composite are oxidized to form N groups and quinoid rings, respectively, resulting in the transfer of electrons. Finally, the reduction of Au(III) into Au(0) occurs as the gold ions gain electrons in an electron-transfer process.13,23 3.7. FTIR and UV−Visible Spectra of PANI−EHL and PANI−EHL/Au. FTIR spectra of PANI−EHL composite and PANI−EHL/Au are presented in Figure 4a. The peak at 3450 cm−1 can be associated with adsorbed water because of the hydrophilicity of the presence of nitrogen.34 The two absorptions at 1636−1640 and 1577−1584 cm−1 are assigned to the stretching of quinoid rings of PANI chains,35,36 and the peak at 1505−1507 cm−1 is due to the stretching of benzenoid

Figure 4. (a) FTIR and (b) UV−vis spectra of PANI−EHL composite and PANI−EHL/Au. 4106



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of metal ions. The maximum biosorption capacity and adsorptivity for Au(III) ions was up to 4193.0 mg g−1 and 70.9%, respectively, at a Au(III) ion concentration of 20 mmol L−1 at 30 °C. Because of the synergistic effect between oxygencontaining and nitrogen-containing functional groups, the PANI−EHL composite can not only remove metal ions but also recover the metal at the same time. Hexagonal Au plates were obtained after the biosorption of Au(III) ions onto PANI−EHL composite. Because lignin is obtained from renewable biomass resources, this low-cost lignin-based biosorbent has great potential to be an economical and effective biosorbent for the removal and recovery of Au(III) ions from wastewater.

Figure 5. (a) Pseudo-first-order and (b) pseudo-second-order kinetic models.

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onto PANI−EHL were found to be 0.9574 and 0.9918 (Table 3), respectively. Comparison of the correlation coefficients of Table 3. Parameters of the Pseudo-First-Order, PseudoSecond-Order, Langmuir Isotherm, and Freundlich Isotherm Models for Au(III) Ion Biosorption onto PANI−EHL Composite parameter

value

Pseudo-First-Order Model Qe (g g−1) 1.141 k1 (h−1) 0.1522 R2 0.9574 Langmuir Isotherm Model Qm (g g−1) 13.76 KL(mL g−1) 1.1714 × 10−4 2 R 0.9286

parameter

AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 22 866 540. Fax: +86 22 866 539. E-mail: [email protected], [email protected]. Notes

The authors declare no competing financial interest.

■

value

ACKNOWLEDGMENTS This project was supported by the Natural Science Fund of Fujian Province (Grant 2012J01201), and the ScienceTechnology Foundation of Education Bureau of Fujian Province (Grant JA12031).

Pseudo-Second-Order Model Qe (g g−1) 1.569 k2 (g g−1 h−1) 0.2228 R2 0.9918 Freundlich Isotherm Model 1/n 0.8894 KF [(mg g−1)(L mg−1)1/n] 2.940 R2 0.9940

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REFERENCES

(1) Pangeni, B.; Paudyal, H.; Abe, M.; Inoue, K.; Kawakita, H.; Ohto, K.; Adhikari, B. B.; Alam, S. Selective Recovery of Gold Using Some Cross-Linked Polysaccharide Gels. Green Chem. 2012, 14, 1917. (2) Cui, J.; Zhang, L. Metallurgical Recovery of Metals from Electronic Waste: A Review. J. Hazard. Mater. 2008, 158, 228. (3) Huang, X.; Wang, Y.; Liao, X.; Shi, B. Adsorptive Recovery of Au3+ from Aqueous Solutions Using Bayberry Tannin-Immobilized Mesoporous Silica. J. Hazard. Mater. 2010, 183, 793. (4) Mack, C.; Wilhelmi, B.; Duncan, J. R.; Burgess, J. E. Biosorption of Precious Metals. Biotechnol. Adv. 2007, 25, 264. (5) Soleimani, M.; Kaghazchi, T. Adsorption of Gold Ions from Industrial Wastewater Using Activated Carbon Derived from Hard Shell of Apricot StonesAn Agricultural Waste. Bioresour. Technol. 2008, 99, 5374. (6) Kononova, O. N.; Kholmogorov, A. G.; Danilenko, N. V.; Kachin, S. V.; Kononov, Y. S.; Dmitrieva, Z. V. Sorption of Gold and Silver on Carbon Adsorbents from Thiocyanate Solutions. Carbon 2005, 43, 17. (7) Ramesh, A.; Hasegawa, H.; Sugimoto, W.; Maki, T.; Ueda, K. Adsorption of Gold(III), Platinum(IV) and Palladium(II) onto Glycine Modified Crosslinked Chitosan Resin. Bioresour. Technol. 2008, 99, 3801. (8) Chang, Y.-C.; Chen, D.-H. Recovery of Gold(III) Ions by a Chitosancoated Magnetic Nano-Adsorbent. Gold Bull. 2006, 39, 98. (9) Qu, R.; Sun, C.; Wang, M.; Ji, C.; Xu, Q.; Zhang, Y.; Wang, C.; Chen, H.; Yin, P. Adsorption of Au(III) from Aqueous Solution Using Cotton Fiber/Chitosan Composite Adsorbents. Hydrometallurgy 2009, 100, 65. (10) Liu, L.; Li, C.; Bao, C.; Jia, Q.; Xiao, P.; Liu, X.; Zhang, Q. Preparation and Characterization of Chitosan/Graphene Oxide Composites for the Adsorption of Au(III) and Pd(II). Talanta 2012, 93, 350. (11) Fujiwara, K.; Ramesh, A.; Maki, T.; Hasegawa, H.; Ueda, K. Adsorption of Platinum(IV), Palladium(II) and Gold(III) from Aqueous Solutions onto L-Lysine Modified Crosslinked Chitosan Resin. J. Hazard. Mater. 2007, 146, 39. (12) Chen, X.; Lam, K. F.; Mak, S. F.; Yeung, K. L. Precious Metal Recovery by Selective Adsorption Using Biosorbents. J. Hazard. Mater. 2011, 186, 902.

the two equations for Au(III) ion biosorption indicates that the pseudo-second-order kinetic model provides a better fit, in line with results reported previously.7,10 Furthermore, the pseudosecond-order model is based on the assumption that the ratelimiting step might be a chemical sorption,47,48 which is in good agreement with the possible mechanism described in Scheme 1. In addition, the Langmuir and Freundlich sorption isotherm models were also used to investigate the process of Au(III) ion biosorption onto PANI−EHL composite (Figure 6). The

Figure 6. (a) Langmuir and (b) Freundlich isotherm models.

correlation coefficients of the two models were found to be 0.9286 and 0.9940, respectively (Table 3). Therefore, the Freundlich sorption isotherm model fits better than the Langmuir sorption isotherm model for the biosorption of Au(III) ions onto PANI−EHL.5

4. CONCLUSIONS The lignin-based biosorbent PANI−EHL composite was found to exhibit excellent redox biosorption characteristics for Au(III) ions. The nitrogen-containing functional groups on PANI− EHL composite chains greatly improved the biosorption ability 4107



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Removal and Recovery of Au(III) from Aqueous Solution Using a Low

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