Chemically Modified Cellulose for Selective Biosorption of Gold from

Apr 27, 2014 - ... California State University, Long Beach, California 90814, United States ... subsequently reduced to the elemental form, yielding m...
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Sustainability in the Metallurgical Industry: Chemically Modified Cellulose for Selective Biosorption of Gold from Mixtures of Base Metals in Chloride Media Manju Gurung,† Birendra Babu Adhikari,†,‡ Xiangpeng Gao,† Shafiq Alam,*,† and Katsutoshi Inoue†,§ †

Faculty of Engineering and Applied Science, Memorial University of Newfoundland, St. John’s, NL A1B 3X5, Canada Department of Chemistry and Biochemistry, California State University, Long Beach, California 90814, United States § Department of Applied Chemistry, Faculty of Science & Engineering, Saga University, 1-Honjo, 840-8502 Saga, Japan ‡

ABSTRACT: In an effort to develop sustainable materials and methods for the recovery and recycling of precious metals, we recently developed two different adsorbents by cross-linking pure cellulose with either epichlorohydrin or concentrated sulfuric acid and then modifying the surface with N-aminoguanidine functional groups. The adsorption behavior of these sorption materials toward Au(III) from a multicomponent mixture of Pt(IV), Pd(II), and some base-metal ions was studied in hydrochloric acid media. Both of the adsorbents exhibited outstanding selectivity toward precious metals, with a selectivity order of Au(III) ≫ Pd(II) > Pt(IV), over base metals in a wide range of acid concentrations. These materials contain a number of positive centers in HCl media that function as sorption active sites for chloroanionic species of Au(III), Pd(II), and Pt(IV). The chloroanionic species of the corresponding precious metals were thus adsorbed on these materials through anion-exchangecoupled electrostatic interaction. The sulfuric acid cross-linked material exhibited improved selectivity and greater adsorption capacity compared to the epichlorohydrin cross-linked adsorbent. As equilibrium was achieved within an hour with quantitative adsorption, the effectiveness of the sulfuric acid cross-linked material with regard to Au(III) was exemplified by the fact that 1 kg of the dry adsorbent material had a capacity to load 9.2 mol, that is, nearly 1.8 kg of Au(III). Moreover, the adsorbed Au(III) was subsequently reduced to the elemental form, yielding metallic gold particles, thereby demonstrating the considerable improvements in efficiency and effectiveness of the novel adsorbents for the recovery of gold in comparison to current commercial resins.

1. INTRODUCTION The development of procedures for the recovery of precious metals from primaryresources (ores) and secondary resources (automobile, chemical, and electronics industries) has become an attractive and imperative issue because of the growing demand for such metals in advanced technological applications. Moreover, because the market values of precious metals in many cases are very high and are constantly rising, the total recovery and reuse of precious metals is undoubtedly justified from economic, environmental, and sustainability perspectives. Recently, because of the gradual depletion of high-grade ore resources and the passage of increasingly stringent environmental legislation, significant proportions of metals are increasingly obtained through secondary resources such as ewastes.1 Although several methods such as ion exchange, liquid−liquid extraction, and membrane filtration are used to recover precious metals from aqueous solutions, these methods are often expensive, lacking in selectivity, slow, and difficult to operate.2 Consequently, it is essential to develop economically viable, more selective, and more efficient alternative methods and materials for the recovery and recycling of target metals. In recent years, biohydrometallurgy has been gaining popularity for the recovery of valuable metals,3 as well as the detoxification of metal-bearing effluents.4 Enriched with a number of functional groups as proper ligating sites, certain types of biomass materials can selectively bind and concentrate metals. In addition, they can be modified with ligands that are very © 2014 American Chemical Society

selective to the target metals, thereby dramatically enhancing their selectivity. Biopolymers not only are biocompatible but also represent an environmentally friendly, sustainable resource and have recently received a great deal of attention within this area of research.5 Among biopolymers, cellulose, the most abundant natural β(1−4)-polysaccharide in the biosphere, has been attracting significant attention as a bioadsorbent because of its biocompatibility, its outstanding physical and chemical behavior, and its abundance in nature. The hydroxyl groups (OH) of cellulose at the C-2, C-3, and C-6 atoms can be partially or even fully reacted with various reagents to afford derivatives with useful properties.6 A ready availability in addition to modest cost and ease of chemical modification make this biopolymer an interesting and attractive class of bioadsorbent. Chemically modified cellulose has shown enormous applicability as heavy-metal adsorbents as thoroughly reviewed by O’Connell et al.7 Moreover, the adsorbents prepared by chemical modification of waste paper have shown promise for preconcentration and separation of precious-metal ions.8−10 Hence, the use of commercial cellulose to fabricate precious-metal-selective adsorption materials is a Received: Revised: Accepted: Published: 8565

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2.2. Instrumentation. A temperature-controlled shaking incubator (Thomas AT24R) was used to agitate the samples for the sorption experiments. The metal concentrations of aqueous samples were measured using a Perkin-Elmer Optima 5300 DV inductively coupled plasma optical emission spectrophotometer. IR spectra of the samples were recorded with a Bruker Tensor 27 FT-IR spectrometer with a Pike ZnSe MIRacle attenuated total reflectance (ATR) accessory with Opus 4.0 software. A Leica DMLP microscope was used to take digital micrograph images of the gold-loaded adsorbent. The X-ray diffraction (XRD) pattern of the gold-loaded sample was recorded with a Rigaku Ultima-IV XRD spectrophotometer in Bragg−Brentano mode. 2.3. Preparation of the Adsorbents. The adsorbents were prepared by cross-linking pure cellulose with either epichlorohydrin or concentrated sulfuric acid and then modifying the surface with N-aminoguanidine (AG) functional groups. 2.3.1. Epichlorohydrin Cross-Linking of Cellulose. The epichlorohydrin cross-linking of cellulose was achieved by employing the procedure described by Šimkovic et al.18 for the cross-linking of starch (see Scheme 1). In a typical run, 30 mL

promising concept for the recovery and recycling of secondary resources and for the development of a sustainable environment. However, very little work has been reported to date utilizing pure cellulose for the adsorptive recovery of precious metals. Commercially available cellulose is crystalline in nature and has a poor adsorption capacity.11 A highly crystalline cellulose sample has a tight structure with cellulose chains closely bound to each other so that the hydroxyl groups are not easily accessible to chemical modification. Certain chemical reactions, in particular, cross-linking and/or grafting reactions, can improve the structural stability and adsorption capability of native cellulose toward precious-metal ions. Recently, it has been reported that the treatment of cellulose with concentrated sulfuric acid disrupts the crystalline structure, yielding an amorphous material.12 Furthermore, the sulfuric acid treated cellulose exhibited a high selectivity and adsorption capacity toward Au(III) but with relatively slow kinetics at ambient temperature.12 Because the adsorption of metal ions takes place mainly on the surface, increasing the sorption active sites on the surface of the polymer matrix is an effective approach to enhancing the sorption kinetics and overall sorption performance of the adsorbent. Covalent linking of amine functions has been an interesting approach for the sorption of precious metals, because the nitrogen atom is a softer base and generally prefers to interact with soft acidic precious-metal ions such as Au(III), Pd(II), and Pt(IV). Guanidine and its derivatives are very interesting ligands for precious metals.13,14 In previous works, tannin-based adsorbents were developed either by cross-linking or by immobilizing different functional groups onto the polymer matrix.15−17 Persimmon tannin extract modified with N-aminoguanidine ligand was found be highly effective for the adsorption of Au(III), Pd(II), and Pt(IV) from hydrochloric acid solution with a high adsorption capacity and fast kinetics.16 In an approach to improve the kinetics of Au(III) adsorption with cellulose-based adsorbents, it is anticipated that such improvements could be achieved by covalently linking Naminoguanidine functional groups onto the polymer backbone. In the present work, two different adsorbents were developed by cross-linking cellulose with epichlorohydrin and concentrated sulfuric acid and then chemically modifying the surface with N-aminoguanidine functional groups. The adsorption behaviors of the prepared adsorbents toward precious metals were studied in terms of selectivity, adsorption isotherm, and kinetics. Herein, we report our findings.

Scheme 1. Epichlorohydrin Cross-Linking of Cellulose

of 17.5% (w/v) NaOH was added to 25 g of cellulose, and the mixture was stirred for 10 min. Then, epichlorohydrin (50 g) and 30% ammonium hydroxide (30 mL) were added sequentially, and the mixture was stirred for another 15 min. The resulting cloudy solution was then gradually heated to 338 K, where it was maintained overnight with constant stirring. Once cooled, the solid was collected by vacuum filtration and washed repeatedly with distilled water until the filtrate displayed neutral pH toward pH indicators. The collected solid was then washed with acetone, followed by an acetone/ water mixture and finally distilled water. After the washed solid was dried, 26 g of cross-linked product was obtained. 2.3.2. Sulfuric Acid Cross-Linking of Cellulose. For the sulfuric acid cross-linking of cellulose (see Scheme 2), a mixture of 25 g of cellulose and 50 mL of concentrated sulfuric acid was first stirred at room temperature for 15 min, and then the temperature was gradually increased to 363 K. During heating, the mixture was converted to a thick paste, preventing additional stirring. Therefore, an additional 20 mL of concentrated sulfuric acid was added, yielding a considerably less viscous mixture that could be easily stirred. The entire mixture was stirred overnight at 363 K, Once this mixture had

2. EXPERIMENTAL SECTION 2.1. Chemicals and Materials. Reagent-grade chloride salts of various metals such as gold, palladium, platinum, iron, cobalt, nickel, copper, zinc, and lead were used to prepare stock solutions of the respective metals. The stock solutions were further diluted to the desired concentrations with 0.1 M HCl to prepare the test solutions. The microcrystalline commercial cellulose powder (Merck, Darmstadt, Germany) employed in the present study was of chromatographic grade. The thionyl chloride (SOCl2), aminoguanidine hydrochloride (CH6N4· HCl), epichlorohydrin (ECH), and sulfuric acid used in this study were purchased from Sigma-Aldrich and were used as received. All other chemicals used in the experimental work were of analytical grade and were used without further purification. 8566

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Scheme 2. Concentrated Sulfuric Acid Cross-Linking of Cellulose

Scheme 3. Chemical Modification of Cross-Linked Cellulose

2.4. Batch Adsorption Tests. Adsorption tests of various metal ions, namely, Au(III), Pd(II), Pt(IV), Fe(III), Co(II), Ni(II), Cu(II), Zn(II), and Pb(II), were carried out batchwise to observe the adsorption behaviors of the prepared adsorbents in varying HCl concentrations ranging from 0.1 to 5.0 M. In a typical set, 15 mL of test solution containing the mixture of metal ions was added to 20 mg of the adsorbent, and the resulting heterogeneous mixture was agitated in a shaking incubator maintained at 303 K and 200 rpm for 24 h. The sample was then filtered to separate the adsorbent, and the concentration of metal ions was determined by inductively coupled plasma optical emission spectrometry (ICP-OES). From the measured initial and equilibrium concentrations of metal ions in solution, the percentage adsorption (A, %) of different metals was evaluated using the relation

been cooled, it was slowly added to a 10% NaHCO3 solution so as to neutralize the acid. The black solid was collected by vacuum filtration and washed repeatedly with water until the filtrate was neutral toward pH indicators. After this solid was dried, 14 g of cross-linked product was obtained. 2.3.3. Modification of Cross-Linked Cellulose with Functional Groups. Surface modification of cross-linked cellulose with N-aminoguanidine (AG) was carried out according to the reaction sequence shown in Scheme 3. In a typical run, 20 mL of thionyl chloride was slowly added to the stirred suspension of 13.0 g of cross-linked cellulose (epichlorohydrin or sulfuric acid cross-linked) in 100 mL of pyridine over a period of 2 h at room temperature, with occasional cooling of the reaction flask in ice water. After the addition of thionyl chloride, the reaction mixture was first allowed to come to room temperature and then slowly heated to a temperature of 343 K, which was maintained overnight with continuous stirring. Then, 30 mL of ice-cold water was slowly added, vacuum filtration was applied, and the solid was collected. The residue was repeatedly washed with water, followed by ethanol and then water again. After drying of the residue, it was determined that the chlorinated product was obtained in quantitative yield. In the next step, a slurry consisting of 6.0 g of chlorinated cellulose, 3.53 g of Na2CO3, and 5.0 g of N-aminoguanidine hydrochloride in 80 mL of dimethylformamide (DMF) was stirred at 353 K for 24 h. Upon cooling, water (50 mL) was added, and the solid was collected by filtration. The residue was washed repeatedly with distilled water until the filtrate was neutral toward pH indicators and was then washed with ethanol. After being dried in an oven, the chemically modified cellulosic materials were obtained in almost quantitative yield. The products of epichlorohydrin cross-linked and sulfuric acid cross-linked cellulose are denoted as ECH-AG-cellulose and Sulf-AGcellulose, respectively.

A (%) =

C i − Ce × 100 Ci

(1)

where Ci and Ce are the initial and equilibrium concentrations (mM), respectively, of metal ions in the aqueous solution. Adsorption isotherm tests were conducted to evaluate the maximum adsorption capacities of the prepared adsorbents for Au(III), Pd(II), and Pt(IV). In these experiments, 20 mg of the adsorbent was shaken with 15 mL of test solution containing varying concentrations of individual metal ions in 0.1 M HCl at 303 K for 96 h to attain the thermodynamic equilibrium of adsorption. The amounts of metal ions adsorbed (q, mol kg−1) were calculated from the relation q=

C i − Ce V W

(2)

where W (g) and V (L) are the weight of adsorbent and the volume of the test solution, respectively. 8567

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molecular-weight water-soluble compounds, makes the material highly amorphous, and enhances the sorption active properties of the material.20 The resulting cross-linked products were then subjected to a surface modification reaction as depicted in Scheme 3. Conversion of alcohol to alkyl chloride with thionyl chloride followed by nucleophilic substitution of the chlorine with Naminoguanidine yielded the desired product in moderate yield. The prepared adsorbents were characterized by FT-IR spectroscopy, as shown in Figure 1. The FT-IR spectrum of

Adsorption kinetics experiments for Au(III) were carried out at four different temperatures between 303 to 318 K. In these experiments, 150 mL of 3 mM Au(III) solution prepared in 0.1 M HCl was added to 200 mg of the prepared adsorbent at each temperature studied. Shaking of the heterogeneous mixture was started immediately after mixing, and a definite volume (5 mL) of the mixture was sampled at different time intervals for the analysis of residual metal concentration in the solution. From the measured concentrations at the beginning and at any time t, the amount of metal adsorbed, qt (mol kg−1), was calculated using the relation qt =

C i − Ct V W

(3)

where Ci and Ct (mM) are the concentrations of Au(III) in the initial solution and remaining in solution at any time t, in the solution, respectively. 2.5. Dynamic Method of Metal Separation. The effective use of the prepared adsorbents for metal separation under a continuous flow of the analyte was investigated by a continuous column method. A glass column with an internal diameter of 8 mm and a length of 24.5 cm was employed for column experiments at ambient temperature. The glass column was tightly packed with a known weight of adsorbent in the middle, and both ends of the bed were supported by glass beads and cotton. Prior to the addition of the test solution, deionized water was continuously passed through the column for a few hours followed by 0.1 M HCl for another few hours for conditioning of the bed. The synthetic feed solution containing mixture of 20 mg L−1 Au(III), Pd(II), and Pt(IV) and nearly 100 mg L−1 Cu(II) prepared in 0.1 M HCl was passed through the column at a constant flow rate of 4 mL h−1 controlled by a peristaltic pump (Cole-Parmer, model 77200-50, Vernon Hills, IL). The effluent solution was collected at hourly intervals using a fraction collector (Spectra/Chrom CF-1 Fraction Collector, Spectrum Chromatography, Houston, TX) and was analyzed periodically to determine the residual metal concentrations in the effluent. Once the bed was saturated with metal ions, the adsorbed metal ions were eluted using acidic thiourea (0.5 M thiourea in 0.1 M HCl) solution as the eluting agent at the same constant flow rate of 4 mL h−1. Also, in this case, before the elution test, the column was washed with deionized water to expel any unbound metal ions. The eluted solution was collected at hourly intervals and analyzed by ICP-OES.

Figure 1. FT-IR spectra of pure cellulose, ECH-AG-cellulose, and SulfAG-cellulose.

pure cellulose consists of a broad band with two wide peaks at 3332 and 3289 cm−1 due to OH stretching. The bands at 1314, 1160, 1105, and 1029 cm−1 are attributed to OH bending and both symmetrical and asymmetrical COC stretching vibrations. The guanidine compounds display four characteristic peaks in the IR spectrum for the following group frequencies: NH stretching at about 3300 cm−1, CN stretching at 1689−1650 cm−1, NH bending at about 1640 cm−1, and CN stretching at about 1300 cm−1.21 As expected from the corresponding group frequencies of the aminoguanidine function, new absorption bands were observed at 1683, 1629, and 1351 cm−1 in the IR spectrum of ECH-AGcellulose. Similarly, significant changes were observed in the 1620−1540, 1340, and 1160−1140 cm−1 regions of the IR spectrum of Sulf-AG-cellulose. This corroborates the introduction of the aminoguanidine group on the cellulose matrix. Once the FT-IR spectra indicated the successful modification of cross-linked cellulose with N-aminoguanidine groups, the degree of functional-group immobilization was evaluated from the nitrogen content of the material. As shown in Table 1, the nitrogen contents of ECH-AG-cellulose and Sulf-AG-cellulose were found to be 4.67% and 11.2%, respectively. This significant increase in nitrogen content as compared to that of pure cellulose suggests the successful immobilization of aminoguanidine functional groups on the surface of the cellulose feed material. From the observed elemental

3. RESULTS AND DISCUSSION 3.1. Preparation and Characterization of the Adsorbents. A review by Doelker summarized a number of methods that have been reported for the cross-linking of cellulose,19 which has been achieved through the formation of acetal and/ or hemiacetal, ester, and ether linkages. Because acetal and ester linkages are more susceptible to acidic conditions than ether linkages, we preferred cross-linking through ether linkages. For this purpose, reagents such as epoxides and compounds containing methylol groups or labile halogens have been used.19 Among epoxides, epichlorohydrin is a widely used cellulose cross-linker that links two cellulose units through the formation of a new ether linkage between two C-6 alcohol groups of glucopyranose units (Scheme 1). The concentrated sulfuric acid-enhanced condensation reaction between two hydroxyl groups to form ether linkages is another method of cross-linking biopolymers (Scheme 2). This method makes the biopolymer rigid through cross-linking, removes the low8568

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Table 1. Elemental Analysis of Commercial Pure Cellulose Powder and Sulf-AG-Cellulose elemental composition (%) material

C

N

N density (mol kg−1)

pure cellulose ECH-AGcellulose Sulf-AGcellulose

42.9

0.00

−

0

40.07

4.67

3.3

0.82

47.6

11. 20

8.0

4.0

functional-group density (mol kg−1)

composition, the nitrogen densities of the prepared materials were calculated as 3.3 and 8.0 mol kg−1 of dry adsorbent for ECH-AG-cellulose and Sulf-AG-cellulose, respectively. Consequently, the functional-group densities were 0.82 and 2.0 mol kg−1 for ECH-AG-cellulose and Sulf-AG-cellulose, respectively. The specific surface properties of the prepared adsorbents were evaluated by Brunauer−Emmett−Teller (BET) surface area analysis and Barrett−Joyner−Halenda (BJH) pore size and volume analysis (Table 2). Pure cellulose, the feed material Table 2. Surface Properties of the Prepared Adsorbents material pure cellulose ECH-AGcellulose Sulf-AGcellulose

BET surface area (m2 g−1)

total pore volume (cm3 g−1)

mean pore diameter (nm)

1.64 5.02 × 10−1

8.39 × 10−3 5.61 × 10−3

20.41 44.68

2.33 × 10−3

3.34 × 10−3

5736.70

employed in this study, is a crystalline mesoporous material. Its porosity was slightly enhanced upon treatment with ECH followed by chemical modification. Concentrated sulfuric acid treatment, on the other hand, significantly increased the porosity of the material, resulting in the formation of a macroporous adsorption material. The commercially available cellulose is microcrystalline in nature.11 The results presented in Table 2 indicate that the commercial cellulose and ECH-AGcellulose were mesoporous materials whereas Sulf-AG-cellulose was macroporous in nature. Concentrated sulfuric acid treatment of cellulose ruptures some of the glycosidic linkages, destroys the rigid three-dimensional network of the polymer, and changes it to a porous material.12 In our case, the porosity of the sulfuric acid-treated material was enhanced nearly 300 times compared to that of the feed material. We observed the formation of porous, beadlike particles upon sulfuric acid treatment of the commercial cellulose. This phenomenon increased the porosity but decreased the specific surface area of the material. Epichlorohydrin cross-linking, however, did not break the crystalline nature of the cellulose matrix. 3.2. Adsorption Behavior of the Adsorbents toward Various Metal Ions. In preliminary experiments, the adsorption behavior of commercial pure cellulose powder toward various metal ions in the mixture was examined at varying concentrations of HCl, and the results are presented in Figure 2a. As is evident from the results in Figure 2a, none of the metallic species were adsorbed on the unmodified cellulose. Then, adsorption experiments were conducted using ECH-AGcellulose under conditions identical to those used for the native cellulose. In this case, a notable improvement in adsorption behavior was observed. The results showing the adsorption

Figure 2. Adsorption behaviors of various metal ions on (a) commercial pure cellulose, (b) ECH-AG-cellulose, and (c) Sulf-AGcellulose for varying hydrochloric acid concentrations. Initial concentration of metal ions = 0.2 mM, weight of dry adsorbent = 20 mg, volume of solution = 15 mL, shaking time = 24 h, shaking speed = 200 rpm, temperature = 303 K.

efficiency, expressed in terms of the percentage adsorption (A, %) of ECH-AG-cellulose toward various metal ions from HCl media of different concentrations, are presented in Figure 2b. As can be seen in this figure, ECH-AG-cellulose exhibited good adsorption behavior toward precious metals with a selectivity order of Pd(IV) ≈ Au(III) > Pt(IV). It is interesting to note that Pd(IV) was adsorbed almost quantitatively from solutions with HCl concentrations of up to 1.0 M. This significant enhancement in adsorption behavior toward precious metals is attributed to the contribution from the functional groups anchored on the surface of the cellulose. Figure 2c shows the results of the adsorption tests for SulfAG-cellulose toward various metal ions at varying HCl 8569

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Figure 3. Adsorption isotherms of Au(III), Pd(II), and Pt(IV) on (a) ECH-AG-cellulose and (b) Sulf-AG-cellulose (experimental plots). [HCl] = 0.1 M, weight of dry adsorbent = 20 mg, volume of solution = 15 mL, shaking time = 96 h, shaking speed = 200 rpm, temperature = 303 K.

nature of the polymer matrix. As already stated, treatment of commercial cellulose with concentrated sulfuric acid destroyed the crystalline nature of the cellulose, forming a highly porous material, but the porosity of the epichlorohydrin cross-linked material was scarcely improved from that of the feed material. Consequently, the significant improvement in adsorption efficiency of the sulfuric acid cross-linked material compared to the epichlorohydrin cross-linked material is attributed to the destruction of the crystalline nature of the cellulose, thereby producing a porous material having enhanced adsorption affinity. 3.3. Adsorption Isotherm. Once it became clear that the present adsorbents could serve as potential candidates for the selective adsorption of precious metals from acidic chloride media, the maximum metal uptake capacities of these adsorbents toward precious metals were evaluated in terms of adsorption isotherm tests. The adsorption isotherm plots of Au(III), Pd(II), and Pt(IV) on ECH-AG-cellulose and Sulf-AGcellulose are shown in panels a and b, respectively, of Figure 3. As revealed by these plots, the amounts of metals adsorbed on both of the adsorbents increased with increasing metal concentration of the test solution, which tended to approach constant values at higher concentrations corresponding to each metal species, thereby exhibiting typical monolayer-type adsorption. The maximum adsorption capacities of each adsorbent toward Au(III), Pd(II), and Pt(IV) ions were evaluated from the constant values in the plateau region of Figure 3 and are summarized in Table 3. The data obtained from adsorption isotherm experiments were treated with the well-known Langmuir isotherm model. The Langmuir equation for adsorption isotherm is expressed as

concentrations. As is evident from the results, this adsorbent exhibited superior performance in comparison to ECH-AGcellulose and a phenomenal improvement in adsorption behavior toward precious metals as compared to the native material. As can be seen in this figure, the adsorption of Au(III) and Pd(II) was quantitative up to 2 M HCl concentrations and decreased very slightly with further increasing acid concentration. The sorption efficiency of the adsorbent toward Pd(II) was also enhanced when cross-linking was conducted using concentrated sulfuric acid, and the adsorbent exhibited moderate adsorption of Pt(IV) for a wide range of HCl concentrations. The metals forming stable anionic chlorocomplexes in HCl media exhibited decreasing adsorption behavior with increasing acid concentration.15,16 ECH-AG-cellulose also displayed similar adsorption behavior, which is likely attributable to the competitive adsorption of anions. In some cases, the adsorbents were practically nonfunctional in HCl concentrations in excess of 3 M.16 The most important point to note with regard to the adsorption behavior of Sulf-AGcellulose is that increasing the concentration of competitive ions had almost no effect on the adsorption of precious metals and that the adsorbent performed highly efficiently for HCl concentrations of up to 5 M. On the other hand, the adsorption of base-metal ions on both of the adsorbents was observed to lie at the baseline for the entire range of HCl concentrations, suggesting that the present adsorbents are outstanding materials for the adsorptive separation of precious metals from mixtures of a considerable number of base metals. The nitrogen atoms of N-aminoguanidine functional groups attached to the surface of the cellulose are converted to positively charged centers in acidic media. Structurally, crosslinked cellulose consists of a number of ether linkages and hydroxyl groups that are also prone to undergo protonation under acidic conditions. Therefore, these adsorbents contain an enormous number of positively charged centers in contact with such solutions. Precious metals, on the other hand, are converted to chloroanionic species within the entire range of HCl concentrations studied. Consequently, the selective adsorption of precious metals on the present adsorbents is believed to be a result of anion exchange followed by electrostatic interaction. Because base metals exist as cationic species in HCl media, no preferential interactions occur, and thus, they are not adsorbed. The superior performance of the sulfuric acid cross-linked material over the epichlorohydrin cross-linked material for adsorption of precious metals is believed to be due to the

Table 3. Maximum Uptake Capacities and Related Parameters of Cellulose-Based Adsorbents toward PreciousMetal Ions Evaluated from Isotherm Experiments qmax (mol kg−1) adsorbate

adsorbent

observed

calculated

b (×10−3 dm3 mol−1)

Au(III)

ECH-AG-cellulose Sulf-AG-cellulose ECH-AG-cellulose Sulf-AG-cellulose ECH-AG-cellulose Sulf-AG-cellulose

1.45 9.21 1.00 1.12 0.53 0.61

1.51 9.25 1.06 1.12 0.54 0.63

17.10 54.05 14.72 20.24 12.72 4.96

Pd(II) Pt(IV)

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Figure 4. Langmuir plots for the adsorption isotherms of Au(III), Pd(IV) and Pt(IV) on (a) ECH-AG-cellulose and (b) Sulf-AG-cellulose.

Ce C 1 = + e qe qmax b qmax

Table 4. Comparison of Adsorption Capacities of the Present Adsorbent with Various Other Cellulose-Based Adsorbents for Precious-Metal Ions Reported in the Literature

(4)

where Ce is the equilibrium concentration of the metal ion remaining in solution after adsorption (mmol dm−3), qe is the amount of the metal ion adsorbed per unit weight of the gel (mol kg−1), qmax is the maximum amount of the metal ion adsorbed on the adsorbent (mol kg−1), and b is the Langmuir constant or equilibrium constant related to the energy of adsorption (dm−3 mmol−1). The parameters qmax and b were obtained from the slope and intercept, respectively, of the equation in the plot of Ce/qe versus Ce. Panels a and b of Figure 4 show Langmuir plots for the adsorption of Au(III), Pt(IV), and Pd(II) on ECH-AGcellulose and Sulf-AG-cellulose, respectively. As is evident from the figure, the experimental data were well fitted to the Langmuir model, and linear plots were obtained with regression coefficients (R2) of >0.99. The various parameters calculated from the Langmuir isotherm model are presented in Table 3. As indicated in the table, the adsorption capacitiues of both adsorbents for Au(III), Pd(II), and Pt(IV) obtained from the Langmuir relation were close to the experimental values obtained from the plateau region of the plots presented in Figure 3. The best fit of the experimental values with those obtained from linear equations and the high values of the regression coefficient obtained using the Langmuir isotherm model suggest that the Langmuir isotherm model accurately fits the experimental data. This implies that the adsorption of Au(III), Pd(II), and Pt(IV) on our adsorbents follows the Langmuir model, which indicates monolayer-type adsorption. The sulfuric acid cross-linked adsorbent not only exhibited a significant improvement in adsorption efficiency over the epichlorohydrin cross-linked adsorbent with regard to precious metals, but also displayed a much higher uptake capacity toward Au(III) in comparison to Pd(II) and Pt(IV). Hence, a comparative evaluation of the maximum adsorption capacity of Sulf-AG-cellulose toward Au(III), Pd(II), and Pt(IV) with some cellulose-based adsorbents reported in the literature was carried out. The data presented in Table 4 demonstrate that other adsorption materials derived from cellulose also exhibit notably high adsorption capacities toward Au(III) over Pd(II) and Pt(IV). Whereas the adsorption capacities of the present adsorbents toward Pd(II) and Pt(IV) were comparable to those of the other adsorbents reported in the literature, Sulf-AGcellulose displayed a markedly higher Au(III) uptake capacity, suggesting that the N-aminoguanidine functional groups significantly contributed to the adsorption of Au(III). The

adsorption capacity (mol kg−1) adsorbenta

Au(III)

Pd(II)

Pt(IV)

ref

PAB paper gel DMA waste paper cross-linked pure cellulose gel IDA-waste paper cross-linked paper gel cotton cellulose gel Sulf-AG-cellulose

5.1 4.6 7.57 3.3 5.53 6.21 9.25

1.5 2.1 − 1.42 − − 1.12

0.5 0.9 − − − − 0.63

8 9 12 22 23 24 this work

a DMA, dimethylamine-modified; IDA, iminodiacetic acid-modified; PAB, p-aminobenzoic acid-modified.

enhancement of the Au(III) loading capacity by nearly 1.7 mol kg−1 in the case of Sulf-AG-cellulose compared to cross-linked pure cellulose gel12 is also attributable to the contribution of functional groups. Therefore, Sulf-AG-cellulose appears to be a highly promising material for the selective preconcentration and recovery of gold contained in wastewater generated from metal refineries or other industries such as electroplating industries. 3.4. Reduction of Adsorbed Au(III) to Metallic Gold. The notable adsorption behavior of cellulose-based adsorbents toward Au(III), as is evident from the data summarized in Table 3, has been attributed to the successive reduction of adsorbed Au(III) to elemental gold.9,12 The Au(III) loading capacity of Sulf-AG-cellulose is higher than that of concentrated sulfuric acid cross-linked pure cellulose,12 paper,23 and cotton.24 Such a high Au(III) loading capacity is not surprising, as gold flakes were observed in the equilibrium solutions of the adsorption isotherm experiments because of the subsequent reduction of the adsorbed Au(III) to elemental gold. The formation of elemental gold by the subsequent reduction of adsorbed Au(III) is evident from the XRD spectrum (Figure 5a) of the dried equilibrium sample of a Au(III)-loaded adsorbent from the adsorption isotherm experiments. The sharp signals at 2θ values of 38.18°, 44.39°, 64.57°, 77.54°, 81.72°, 98.13°, 110.80°, and 115.26° correspond to those of metallic gold. Clear evidence for the formation of elemental gold particles was obtained from the digital microphotograph of the Au(III)-loaded adsorbent shown in Figure 5b. This image was recorded at 200× magnification, and metallic gold flakes 8571

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nitrogen present on the adsorbent are converted to positive centers through protonation in HCl media. Consequently, the interaction of anionic species of precious metals and the positive centers of the adsorbent proceeds through an anionexchange mechanism followed by electrostatic interaction, as shown in Scheme 4. The positive centers of the Naminoguanidine functional group act as the main sorption active sites at which the anionic species AuCl4−, PdCl42−, and PtCl62− replace the chloride ions and maintain electroneutrality. Similar interactions generated at positive centers due to the protonation of hydroxyl groups of cellulose cannot be overlooked. Thus, ionic interaction is considered to be primarily responsible for the adsorption of Au(III), Pd(II), and Pt(IV) on aminoguanidine-modified cellulose. The occurrence of ion exchange and electrostatic interactions is also supported by the fact that the adsorption of metallic species decreases with increasing acid concentration due to the increase in concentration of competitive chloride ions. The electroneutral N-aminoguanidine can form five-membered chelate complexes with Pd and Pt chlorocomplexes in which aminoguanidine is coordinated to the metal by the N-4 amino group and the deprotonated N-1 imino group.28 In the present case also, chelation through nitrogen atoms can take place at lower acid concentrations, leading to the formation of a five-membered chelate complex as depicted in Scheme 5.

Figure 5. (a) XRD spectrum and (b) digital microphotograph of SulfAG-cellulose after Au(III) adsorption.

are distinctly visible. In addition to cellulose-derived adsorbents, the adsorption-coupled reduction of Au(III) to elemental gold has been observed in a number of studies with adsorbents derived from tannin15,16 and lignin.25 This unique behavior of biopolymers toward Au(III) adsorption is believed to be due to the higher reduction potential of Au(III) as compared to Pt(IV) and Pd(II) [oxidation/reduction potential (ORP) values: Au(III) = 1.00 V, Pt(IV) = 0.73 V, and Pd(II) = 0.62 V].26 The exceptional Au(III) selectivity of Sulf-AGcellulose, its very high Au(III) uptake capacity, and the phenomenon of reduction of adsorbed Au(III) species leading to the formation of metallic gold flakes are worthy of praise. 3.5. Proposed Adsorption Mechanism. Considering the mechanism of adsorption, ionic interactions, and/or a chelation mechanism, or a combination of the two are reported for the sorption of soft metals such as Au(III), Pt(IV), and Pd(II) with ligands containing soft donor atoms such as N and S.15,16 As stated above, the major species of Au(III), Pd(II), and Pt(IV) in acidic solutions containing 0.1 M and higher chloride concentrations are the chloroanionic species AuCl4−, PdCl42−, and PtCl62−, respectively.27 On the other hand, the atoms and/ or functional groups with pair of electrons such as oxygen and

Scheme 5. Adsorption of PdCl42− and PtCl62− Species on Electroneutral N-Aminoguanidine Ligands of the Adsorbent through the Formation of Cyclic Chelates

Unique behavior has been observed for the adsorption of Au(III) with concentrated sulfuric acid cross-linked cellulose,12 tannin,20 lignin,29 and so on. The sulfuric acid cross-linked materials from these biopolymers do not show any sorption affinity toward PdCl42− and PtCl62−, but preferentially adsorb

Scheme 4. Adsorption of Anionic Species of Precious Metals on Positive Centers of the Adsorbent Due to Ionic Interactionsa

a

C represents the bulk of cellulose, and MClxn− represents the chloroanionic species of precious metals. 8572

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Au(III).12,20,29 It is believed that this peculiar phenomenon of sulfuric acid cross-linked materials comes from the favorable structural contribution for the uptake of Au(III) species. In the case of biopolymers having (poly)hydroxyl groups, crosslinking with concentrated sulfuric acid takes place through a dehydration reaction, leading to the formation of numerous new ether linkages as proposed in Scheme 2. Careful evaluation of these structures shows that cross-linking generates new structural units in which oxygen atoms are positioned in such a way that a stable five-membered metallocomplex can be formed in which the metal center can coordinate with three oxygen atoms (Scheme 6). The outstanding adsorption behavior of Scheme 6. Formation of a Proposed Five-Membered Metallacomplex of AuCl4− with Cross-Linked Cellulose Figure 6. Adsorption kinetics of Au(III) on Sulf-AG-cellulose. [HCl] = 0.1 M, weight of dry adsorbent = 200 mg, volume of solution = 150 mL, shaking speed = 200 rpm, temperature = 303 K.

One of the motivations for conducting this research was to improve the kinetics of Au(III) sorption with cellulose-based adsorbents. Hence, a comparative study of the time required to reach the equilibrium of Au(III) adsorption on Sulf-AGcellulose was carried out with other cellulose-based adsorption materials as well as other biomass-based materials under identical experimental conditions. The results summarized in Table 5 indicate that the Sulf-AG-cellulose developed in the Au(III) with Sulf-AG-cellulose is therefore believed to be due to the structural contribution of cross-linked materials to the sorption of AuCl4− species, which are then subsequently reduced to metallic gold with adjacent reducing functional groups. Sorption of Au(III) on Sulf-AG-cellulose proceeds by the dual mechanism of adsorption followed by reduction. Kuyucak and Volesky proposed a reaction for the reduction of Au(III) with adsorbents having abundant hydroxyl groups in which the hydroxyl groups are oxidized to carbonyl groups as30

Table 5. Comparison of the Equilibrium Times of Au(III) Sorption on Different Cellulose-Based Adsorbents equilibrium time (h)

adsorbenta PAB paper gel DMA paper gel cross-linked pure cellulose gel cross-linked paper gel cross-linked cotton gel Sulf-AG-cellulose gel

AuCl4 − + 3ROH → Au(0) + 3R(O) + 3H+ + 4Cl− (5)

a

The hydroxyl groups are abundant in polysaccharides, tannin, lignin, and so on, and their participation in the reduction of Au(III) has been confirmed in many studies.9,12,15,16,25 Consequently, in the redox chemistry of Au(III) to Au(0) with cellulose-based adsorbents, the hydroxyl groups are oxidized to carbonyl groups. 3.6. Effects of Shaking Time and Temperature on the Adsorption of Au(III) on Sulf-AG-cellulose. Because SulfAG-cellulose exhibited a remarkable affinity toward Au(III) ions from HCl solutions, kinetic experiments were carried out only for the adsorption of Au(III) ions on Sulf-AG-cellulose. Preliminary experiments were conducted using 1 and 3 mM Au(III) solutions at 303 K, and the results are presented in Figure 6. The amount of metal adsorbed on Sulf-AG-cellulose initially increased rapidly and then gradually decreased, finally approaching a constant value after a certain period of shaking, which is the time required to reach equilibrium. It is understandable from the figure that adsorption equilibrium was attained within 1 h for 1 mM Au(III) whereas it took 6 h to come to equilibrium with quantitative adsorption when the initial concentration of Au(III) was increased to 3 mM.

conditions

ref

24 5 30

0.5 mM/1 M HCl 0.5 mM/1 M HCl 2.0 mM/0.1 M HCl

8 9 12

28 30 0.5

2.0 mM/0.1 M HCl 1.0 mM/0.1 M HCl 1.0 mM/0.1 M HCl

23 24 this work

DMA, dimethylamine-modified; PAB, p-aminobenzoic acid-modified.

present study exhibited remarkably fast kinetics of adsorption. The rapid adsorption kinetics of Au(III) on Sulf-AG-cellulose is attributable to the easier access of chloroaurate anion to the positively charged sites (quaternary nitrogen atoms) of the adsorbent and reflects its tremendous affinity toward Au(III). Temperature is one of the crucial factors affecting reaction rates; hence, the effect of temperature on the kinetics of Au(III) adsorption was also studied at four different temperatures from 303 to 318 K using 3 mM Au(III) solutions. The results showing the effect of shaking time on the amount of Au(III) adsorbed at different temperatures, as presented in Figure 7a, indicate that increasing temperature had a beneficial effect on the sorption kinetics of Au(III) on Sulf-AG-cellulose. Although increasing the temperature from 303 to 308 K increased the initial sorption rates, the time required to reach equilibrium did not appear to change. However, a significant improvement in reaction rate was observed for a 10 K increase in temperature from 303 K, and adsorption equilibrium was achieved within 2 h with quantitative adsorption. 8573

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Table 6. Adsorption Rate Constants and Corresponding Correlation Coefficients for the Pseudo-Second-Order Kinetic Model at Different Temperatures temp (K)

rate constant (k1, h−1)

R2

303 308 313 318

1.29 1.71 2.76 6.11

0.98 0.94 0.98 0.96

in this system becomes favorable at temperatures higher than ambient temperature. 3.7. Energy of Activation for the Adsorption of Au(III). The energy of activation (Ea) not only is important for understanding the dependence of the reaction rate on temperature, but is also a useful parameter for predicting whether the adsorption process is physical or chemical. As the forces involved in physical adsorption are usually weak, the energy requirements in adsorption processes involving physisorption are small, and the activation energies are usually no more than 4.2 kJ mol−1. Chemical adsorption of adsorbate species, on the other hand, involves much stronger forces than physical adsorption. Therefore, the activation energy required for chemisorption is of the same magnitude as the heat of chemical reactions. The activation energy in chemisorption usually varies between 8.4 and 83.7 kJ mol−1.31 The Arrhenius equation relates the activation energy and the rate of reaction according to

k = Ae−Ea / RT

(8)

which can also be written as Figure 7. Effect of shaking time on the amount of Au(III) adsorbed on Sulf-AG-cellulose at different temperatures: (a) experimental plots, (b) pseudo-first-order plot. Initial concentration of Au(III) = 3 mM, concentration of HCl = 0.1 M, shaking speed = 200 rpm.

ln(k) = −

k1 t 2.303

(6)

or ⎛ q ⎞ ln⎜⎜1 − t ⎟⎟ = −k1t qe ⎠ ⎝

(9)

where k is the rate constant, Ea is the activation energy (kJ mol−1), T is the absolute temperature (K), A is the Arrhenius constant, and R is the universal gas constant. An Arrhenius plot was obtained from the relationship of the rate constant and temperature according to eq 9 and is shown in Figure 8. A straight line with a slope of −Ea/R was obtained by plotting ln k versus 1/T, from which the energy of activation was calculated as 84.26 kJ mol−1. The significantly high value of Ea and the variation of the reaction rate with temperature suggest that the adsorption of Au(III) on Sulf-AG-cellulose is a

The adsorption kinetics data obtained at four different temperatures were analyzed in terms of a pseudo-first-order kinetic model according to the equation log(qe − qt ) = log qe −

Ea + ln(A) RT

(7) −1

where qe and qt are the amounts (mol kg ) of metal adsorbed at equilibrium and at any time t, respectively; k1 (h−1) represents the pseudo-first-order rate constant; and t is the shaking time (h). According to eq 7, the rate constant (k1) is determined experimentally by plotting ln[1 − (qt/qe)] versus t as shown in Figure 7b. As can be seen in this figure, all plots corresponding to the four different temperatures lie on proportionally straight lines with R2 ≥ 0.94. The rate constants at each temperature were evaluated from the slopes of these straight lines. The values of the rate constants along with their corresponding correlation coefficients are presented in Table 6. As is evident from the data presented in Table 6, the rate constants for adsorption of Au(III) on Sulf-AG-cellulose increased significantly with increasing temperature, indicating that adsorption

Figure 8. Arrhenius plot for the pseudo-first-order rate constants for Au(III) adsorbed on Sulf-AG-cellulose at different temperatures. 8574

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indicates the volume of solution that can be passed through the packed bed of the adsorbent ensuring quantitative adsorption of the target species. Hence, these results clearly indicate that Sulf-AG-cellulose is highly effective for achieving quantitative adsorption and preconcentration of Au(III) from large volumes of dilute solutions containing elevated concentrations of base metals. The effective adsorption capacities calculated from the breakthrough profiles were 0.47 mol kg−1 for Pd(II) and 0.05 mol kg−1 for and Pt(IV), respectively, which were 42% and 8% of their corresponding equilibrium sorption capacities evaluated from the batch adsorption tests. The low adsorption capacities can be attributed to various phenomena such as competitive adsorption and insufficient contact time to reach equilibrium in continuous mode. Although complete saturation of the column for Au(III) adsorption was not achieved, elution of the loaded metal ions was carried out using 0.5 M thiourea prepared in 0.1 M HCl to recover the adsorbed metals in preconcentrated form. Figure 9b shows the elution profiles of Au(III), Pd(II), and Pt(IV) with acidic thiourea solution. As shown in this figure, loaded precious-metal ions were successfully eluted from the packed bed with preconcentration factors of up to 61.8 for Pd(II), 11.0 for Pt(IV), and 19.0 for Au(III) as compared to the feed solution. The results of the column experiments support the conclusion that Sulf-AG-cellulose adsorbent would be useful in achieving selective and effective separation of trace amounts of valuable metals from industrial effluents and waste streams.

chemisorption process. Chemical adsorption of Au(III) has also been reported with other biomass materials such as lysinemodified cross-linked chitosan.12,20,32 3.8. Dynamic Method of Metal Separation. Based on the promising results obtained in batch adsorption experiments, we extended our study to investigate the possibility of using Sulf-AG-cellulose under dynamic conditions for continuous adsorption and separation of Au(III) from a multicomponent mixture. The experiments were conducted by percolating a mixture consisting of 20 mg L−1 Au(III), Pd(II), and Pt(IV) and 100 mg L−1 Cu(II) through a column packed with 0.15 g of the adsorbent. Figure 9a shows the breakthrough profiles of the

4. CONCLUSIONS New, efficient, and sustainable adsorption materials of biological origin were prepared by cross-linking pure cellulose with epichlorohydrin or sulfuric acid, and then modifying the surface with N-aminoguanidine functional groups. Evaluation of the adsorption behavior of the adsorbents toward precious-metal ions coexisting with several base-metal ions displayed excellent results. The adsorbents exhibited phenomenal selectivity for the adsorption of precious metals from HCl media. Base-metal ions, however, were not adsorbed at all on the adsorbents. Because Au(III), Pd(II), and Pt(IV) form chloroanionic species in hydrochloric acid media, adsorption of these metal ions occurred through ion pairing at the positively charged quaternary nitrogen atoms of the adsorbent. The sulfuric acid cross-linked material exhibited improved selectivity and adsorption capacity over the epichlorohydrin cross-linked material. The adsorption behavior of Sulf-AG-cellulose toward Au(III) is particularly interesting, whereby, after adsorption, Au(III) is reduced by the adsorbent to the elemental form as evidenced by the observed gold aggregates. The sulfuric acid cross-linked material was highly efficient in terms of adsorption kinetics, and equilibrium was achieved within a few hours with quantitative adsorption even in 3 mM Au(II) solutions. The adsorbent also exhibited outstanding performance in continuous-mode experiments and is thus a highly promising material for the selective preconcentration and recovery of gold from large volumes of solutions containing small amounts of gold with substantial quantities of competing base metals. All of these findings led us to conclude that the aminoguanidinemodified cellulose-based adsorbents are potential candidates and sustainable alternatives to commercial polymeric resins for the adsorptive separation and recovery of precious metals from acidic chloride media.

Figure 9. (a) Breakthrough profiles and (b) elution profiles of Au(III), Pd(II), Pt(IV) and Cu(II) using a column packed with the Sulf-AGcellulose gel. Feed concentrations of Au(III), Pd(II), Pt(IV) = 20 mg dm−3, feed concentration of Cu(II) = 100 mg dm−3, flow rate = 4.0 mL h−1, eluent = 0.5 M thiourea in 0.1 M HCl. Ci is the initial concentration of metal ions, and Ce is the metal-ion concentration of the effluent solution. BV stands for bed volume, which is the ratio of the volume of the packed bed of adsorbent to the volume of the effluent solution passed through it.

tested metal ions. The results in this figure demonstrate that the breakthrough of copper took place as soon as it was passed through the column, indicating that the adsorbent had no affinity to adsorb Cu(II); similar results are expected for the other base metals. The breakthroughs of Pd(II) and Pt(IV) started at 510 BV (50 h) and 92 BV (9 h), respectively, whereas complete saturation of the adsorbent bed was achieved at 2551 BV (250 h) and 510 BV (50 h), respectively. It is interesting to note that, in the case of Au(III) adsorption, breakthrough did not occur even after 2756 BV (270 h). Breakthrough volume 8575

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +1-709-864-2331. Fax: +1-709864-4042. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was financially supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) in the form of Discovery Grants. The authors also acknowledge partial support from the Harris Centre at Memorial University and the Multi-Materials Stewardship Board (MMSB), Newfoundland, Canada.

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