- Email: [email protected]
Enhancing the efficiency of AuCl4− ion removal from aqueous solution using activated carbon and carbon nanomaterials
Materials Chemistry and Physics 141 (2013) 454e460
Contents lists available at SciVerse ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Enhancing the efficiency of AuCl 4 ion removal from aqueous solution using activated carbon and carbon nanomaterials ska M. Bystrzejewski*, K. Pyrzyn University of Warsaw, Department of Chemistry, Pasteur 1 Street, 02-093 Warsaw, Poland
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Removal efficiency in non-oxidized sorbent is dictated by crystallinity and porosity. Introduction of surface acidic groups increases removal efficiency two times. Carbon encapsulates have high removal efficiency for broad range concentration of Au(III).
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 August 2012 Received in revised form 10 May 2013 Accepted 20 May 2013
The removal of AuCl 4 ion from acidic aqueous solutions is studied using a series of non-oxidized and surface oxidized carbon materials (activated carbon, carbon nanotubes, carbon-encapsulated iron nanoparticles and carbon black). The studied sorbents differ in crystallinity, porosity and morphology. In the case of non-oxidized carbon materials the maximum removal efficiency (74%) is found for activated carbon, whilst graphitized nanomaterials (i.e. carbon nanotubes and carbon-encapsulated iron nanoparticles) are able to remove 42e45% of gold ion from the solution. The oxidation in nitric acid significantly improves the removal efficiencies. The uptake of Au(III) increases two times (to 91e92%) for oxidized carbon nanotubes and carbon-encapsulated iron nanoparticles. The same oxidation procedure applied to activated carbon and carbon black moderately enhances the uptake efficiency to 88% and 55%, respectively. The observed substantially distinct uptakes are discussed in the frames of textural properties, morphology, surface chemistry characteristics and crystallinity of the studied carbon materials. Moreover, the possibility of a galvanic exchange reaction between AuCl 4 and metallic Fe in the carbon encapsulate core is also evaluated. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Microporous materials Nanostructures Chemisorption Surface properties
1. Introduction The recovery of precious metals, such as gold, platinum and palladium, is an attracting challenge due to the limited sources and growing industrial demand. It has been shown that sorbents based on carbon materials (activated carbon, carbon fibers, carbon nanotubes) have a high potential for extraction and pre-concentration of precious metal complexes, including gold [1e4]. The high redox potential of typical precious metal halide complexes (e.g. AuCl 4,
* Corresponding author. Fax: þ48 22 822 59 96. E-mail address: [email protected] (M. Bystrzejewski). 0254-0584/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2013.05.044
PtCl2 6 ) implicates that their adsorption occurs presumably via a chemisorption process. The adsorption process is accompanied by a spontaneous deposition of small crystallites of the metal which is adsorbed [5,6]. This work is motivated by recent studies showing that crystallites of precious metals (Au, Pt) are spontaneously deposited onto various carbon nanomaterials (carbon nanotubes and carbonencapsulated magnetic nanoparticles) during the contact with a solution containing the metal complex [7,8]. It has been demonstrated that even very short contact times (3 min) result in a complete removal of gold metal from the solution [8]. Here, a morein-depth and comparative studies are presented, in which four various carbon sorbents are investigated, i.e. activated carbon (AC),
ska / Materials Chemistry and Physics 141 (2013) 454e460 M. Bystrzejewski, K. Pyrzyn
multi-wall carbon nanotubes (CNTs), carbon-encapsulated iron nanoparticles (CEINs) and carbon black (CB). These carbon materials have different morphological, textural, surface chemistry characteristics and structural features. The aim of this work is to compare the efficiency of solid phase extraction of gold onto the aforementioned sorbents, and evaluate the parameters, which are the most crucial for the efficient removal of gold. A substantial attention in this paper is also focused on mobile magnetic sorbents and their feasibility in the recovery of gold. Magnetic sorbents dedicated to sorption of typical heavy metals (e.g. Cu, Cd, Co) are widely studied since at least 10 years [9,10]. However, there are only a few reports describing the adsorption of gold and other precious metals onto magnetic sorbents. [11,12]. The ability for rapid separation of the sorbent from the solution is the main advantage of magnetic sorbents. This feature can be used in the carbon-in-pulp process to increase the recovery rate of gold. In the case of mobile sorbents based on magnetic nanoparticles (which may have large available surface area) the time required to reach the sorption equilibrium is shorter in comparison with activated carbon [12]. In this paper carbon-encapsulated iron nanoparticles are studied as a prospective mobile sorbent for gold recovery. These hybrid nanomaterials are composed of a magnetic core (typically 5e100 nm in diameter) and thin carbon coating (1e 5 nm in thickness), which is built of curved graphene layers [13,14]. The carbon shell completely covers the encapsulated magnetic nanoparticle and protects it from oxidation, corrosion and agglomeration. In other words, the specific physical and chemical properties of the nanocrystalline magnetic core are fully preserved. It has been shown that CEINs have a great potential for sorption of heavy metals [15e17] and organic molecules [18,19]. Moreover, the carbon coating in CEINs can be functionalized in a controlled way to develop a magnetic sorbent with enhanced sorption efficiency or selectivity [20].
2. Experimental Activated carbon and multi-wall carbon nanotubes were purchased from SigmaeAldrich and used as received. Carbon black N330 was donated by the Research Institute of Rubber Industry (Piastów, Poland). Carbon-encapsulated iron nanoparticles were synthesized by a carbon arc route and then subjected to the purification procedure to remove the non-encapsulated nanoparticles [14]. The morphology of carbon sorbents was studied by scanning electron microscopy (Zeiss Leo 1530). Raman spectra were acquired with a resolution of 2 cm1 using a 515 nm Ar excitation laser. The nitrogen adsorption studies were conducted on ASAP 20120 analyzer (the samples were outgassed at 200 C under vacuum prior to the measurement). The removal of AuCl 4 ion was studied on raw (i.e. non-oxidized) and surface-oxidized carbon materials. The oxidation procedure included 12 h of soaking of the carbon material in 8 M HNO3 at room temperature, with subsequent washing with water until the neutral pH was obtained. The total number of surface acidic groups was evaluated by the Boehm titration method [21]. The removal of AuCl 4 was investigated at pH 2 at constant temperature (298 1 K). The stock solution of HAuCl4 (1000 mg L1 in respect to Au(III)) was purchased from Spex Cer1 in respect to tiPrep. The diluted solutions of AuCl 4 (1e30 mg L Au(III)) were prepared for the calibration of the atomic absorption spectrometer. For the studies of AuCl 4 removal a 50 mg sample of carbon material was added to 10 mL solution of HAuCl4 (initial concentration 30 mg L1 with respect to Au(III)), with subsequent vigorous shaking for 5 min. Then, the sorbents were filtered off by centrifugation and the concentration of AuCl 4 in the supernatant was evaluated by atomic absorption spectroscopy.
455
3. Results and discussion 3.1. Morphological, structural and textural features of carbon sorbents The studied carbon sorbents have distinct morphological features (Fig. 1, aed). Activated carbon comprises of relatively large grains (hundreds of microns, images not shown), which have a well-developed pore structure with a narrow diameter distribution (70e100 nm in diameter). Carbon nanotubes are several mm in length, and their diameters are in the range between 50 and 100 nm. Carbon-encapsulated iron nanoparticles consist of oval nanoparticles with sizes below 100 nm. CEINs have typical coree shell structure, i.e. they are comprised of metallic cores which are completely covered by a thin carbon coating (Figure S1, Supplementary data). Carbon black consists of uniform bulk nanoparticles with diameters between 20 and 50 nm (Figure S1). The oxidation in nitric acid does not change the morphological features (especially the size distribution and shape) of the studied carbon materials (Fig. 1, eeh). Raman spectroscopy is a widely used technique for studying the electronic structure of carbon materials. The first-order Raman spectrum shows two bands, the so-called G and D bands [22]. In a perfect graphite crystal the G band is located at 1582 cm1 and corresponds to stretching vibrations of CeC bonds in the lattice. The D-band, which appears at lower wavenumber, is associated with structural and topological disorder. The ratio between the integral intensities of the G and D bands is an accepted indicator of the structural quality of carbon materials. This ratio is linearly correlated with the La size (La is the size of a single graphene layer) [22]. The low value of the G/D ratio points to a carbon material, which has a large number of defects and therefore is built of very small graphene layers. The G bandwidth is related to the extent of disorder and also brings useful structural information. Recent studies have shown that this parameter correlates with the Lc crystallite size (the Lc parameter visualizes the stacking order between the adjacent graphene layers) [23]. The G/D ratio values and G bandwidth are listed in Table 1. The G/D ratio is calculated from the integral areas of the bands (the spectra were deconvoluted using the Lorentzian function, see Figures S2eS5). Activated carbon and carbon black have the lowest G/D ratio and highest G bandwidth, which points to their large structural disorder. This is an expected result, because these materials are formed during carbonization of carbon precursors at relatively low temperature (1000e1300 K), which is too low to induce the graphitization processes. Carbon-encapsulated iron nanoparticles have higher G/D ratio and lower G bandwidth in comparison to AC and CB samples. This finding points to higher crystallinity (graphene layers are larger, however they have large numbers of defects which are necessary to introduce the curvature in a carbon coating in CEINs). Carbon nanotubes have the largest structural order among all studied carbon materials. The treatment of carbon materials in nitric acid is frequently used to introduce surface functional groups [24]. The formation of these groups is always accompanied by structural degradation of external graphene layers and decreasing the overall crystallinity of a carbon material. The largest structural changes are observed for carbon nanotubes (Table 2), in which the G/D ratio diminishes 6 times and the G bandwidth increases of 27 cm1 (63%). The crystallinity of carbon-encapsulated iron nanoparticles also becomes smaller, however, this effect is less pronounced in comparison to carbon nanotubes (the G/D ratio decreases of 18% and the G band broadens of 4 cm1 (10%)). In the case of carbon black the G/D ratio virtually does not alter (the change is 3%), whilst the G bandwidth increases only of 2 cm1 (2%). Interestingly, the G/D ratio increases
456
ska / Materials Chemistry and Physics 141 (2013) 454e460 M. Bystrzejewski, K. Pyrzyn
Fig. 1. SEM images of non-oxidized (left column) and surface oxidized (right column) carbon sorbents: activated carbon (a, e), carbon nanotubes (b, f), and carbon-encapsulated iron nanoparticles (c, g) and carbon black (d, h).
of 10% for activated carbon. These intriguing results are discussed in the next section. Importantly, the nitric acid treatment does not change the order of crystallinity, which is as follows for the pristine and oxidized samples: CNTs > CEINs > AC z CB. The studied carbon materials have very distinct textural properties. The values of the specific surface area evaluated from the BET equation and the pore volume are shown in Tables 1 and 2 (nitrogen adsorption and desorption isotherms are shown in Figures S6e S13). Activated carbon has the highest surface area among all studied materials. This is a consequence of high micropore volume
(49% of the total pore volume), which has the largest contribution to the surface area. Carbon nanotubes have the lowest surface area and pore volume. Carbon-encapsulated iron nanoparticles and carbon black have higher surface area in comparison to carbon nanotubes. However, all of these nanomaterials cannot be regarded as highly porous materials. The observed relatively low surface area and low micropore volume can be explained on the basis of morphological similarities. The primary particles of CNTs, CEINs and CB have a smooth surface, which is devoid of any pores (see Figs. 1 and S1). Interestingly, the adsorptionedesorption isotherms
ska / Materials Chemistry and Physics 141 (2013) 454e460 M. Bystrzejewski, K. Pyrzyn Table 1 Structural and textural properties of non-oxidized carbon materials. Material
G/D
G bandwidth (cm1)
SBET (m2 g1)
Total pore volume (cm3 g1)
Micropore volume (cm3 g1)
Activated carbon Carbon nanotubes Carbon encapsulates Carbon black
0.41 7.72 1.14 0.40
56 27 41 86
642 15 77 78
0.516 0.034 0.316 0.438
0.227 0.003 0.010 0.013
of AC and CEINs samples are characterized by a hysteresis, which is absent for other studied samples. This is associated with the presence of mesopores, in which capillary condensation takes place and it limits the nitrogen uptake over a high range of p/p0 [25]. The oxidation in nitric acid changes the textural properties of all studied carbon materials (Table 2). The surface area and the micropore volume in activated carbon decrease of nearly the same values, i.e. 13% and 15%, respectively. This result shows that the sites, at which the micropores are located, primarily undergo degradation. It is obvious, that these sites have larger structural disorder in comparison to the bulk material [26]. Hence, their elimination should improve the crystallinity of AC. In fact, the G/D ratio increases of 10% after oxidation (Table 2). The surface area and total pore volume in carbon nanotubes increase more than 10-fold after oxidation. This is a consequence of amorphization of external graphene layers (the G/D ratio decreases) and opening the cups of nanotubes (the inner core space is then available for nitrogen molecules). Carbon encapsulates and carbon black follow the same trend as carbon nanotubes and also increase their porosity. The results of Boehm titration and values of the point of zero charge are presented in Table 3. The non-oxidized carbon materials have relatively low and similar surface acidity, which ranges between 0.08 and 0.48 mmol g1. Please note, that the evaluated acidity refers to the total content of surface functional groups (carboxylic, phenolic and lactonic). The corresponding point of zero charge values are in agreement with titration results and show that the studied carbon materials have weak acidic properties. Nevertheless, the mean distribution of surface acidic groups differs significantly, because of distinct pore volume. The mean distribution density of functional groups can be expressed as the surface charge density (SCD). This parameter can determined as follows: SCD ¼ (SAC$F/SA)[C m2], where SAC denotes the surface acidity (in mmol g1), SA is the surface area (in m2/g) and F is the Faraday constant (96.5 C mmol1). The surface acidic groups are the most densely packed in carbon nanotubes, and it directly results from their very low porosity and low specific surface area (15 m2 g1). The as-obtained carbon-encapsulated iron nanoparticles (i.e. before the purification procedure) have slightly basic properties. Their point of zero charge is found to be 7.42 and the total number of surface functional groups is 0.05 mmol g1. The observed surface charge in the as-obtained CEINs likely originates partially oxidized non-encapsulated Fe nanoparticles. It should be noted that nonencapsulated Fe particles, which are always present in the asobtained product, are in a continuous contact with the ambient atmosphere and may be oxidized. Table 2 Structural and textural properties of surface oxidized carbon materials. Material
G/D
G bandwidth (cm1)
SBET (m2 g1)
Total pore volume (cm3 g1)
Micropore volume (cm3 g1)
Activated carbon Carbon nanotubes Carbon encapsulates Carbon black
0.45 1.27 0.95 0.39
65 44 45 88
561 162 95 78
0.469 0.376 0.338 0.348
0.193 0.005 0.017 0.015
457
The nitric acid treatment results in 15e30-fold increase of the surface acidity. All surface oxidized samples have acidic properties. It is expressed by the values of the point of zero charge, which is between 3.95 and 4.60 (Table 3). Interestingly, the surface acidity is on a similar level, i.e. 2.50e2.92 mmol g1 (Table 3). This finding shows that the applied oxidation procedure has a nearly identical functionalization potential for various carbon materials and does not depend on the crystalline or the morphological features. Similarly to the non-oxidized samples, the distribution of surface functional groups is different because of diverse porosity. The most dense packing of functional groups is found for carbon black and carbon-encapsulated iron nanoparticles. 3.2. Studies of AuCl 4 removal The Au(III) uptake for the non-oxidized carbon materials varies between 42 and 74% (Fig. 2) and changes in the following order: AC > CB > CNTs z CEINs. To explain this behavior one has to take into account the factors that may accelerate or inhibit the uptake of AuCl 4 ion. In general, the adsorption of ionized species, e.g. heavy metal cations, is usually favored when the surface of a carbon sorbent starts to carry the negative charge (i.e. the surface functional groups undergo deprotonation). In this case the electrostatic attraction occurs and the sorption is enabled. Precious metals can be also adsorbed onto carbon materials, however the mechanism of sorption is more complex in comparison to the mentioned route describing the electrostatic binding of metal cations. As it was stated in Introduction, the sorption of precious metal ions (e.g. Au, Pt and Pd) occurs through spontaneous chemisorption with simultaneous deposition of zero-valent metal clusters onto the surface of a carbon material [5,6]. This effect results from the high redox potential of precious metal ion complexes. The carbon sorbents having higher crystallinity should possess higher resistance toward the oxidation processes and should be less pronounced for the oxidation by AuCl 4 . In fact, CNTs and CEINs are substantially better graphitized than AC and CB (see G/D values in Table 1), and therefore they are more inert in respect to the reaction with AuCl 4. This is supported by the lowest uptakes of Au(III) for CNTs and CEINs. Activated carbon and carbon black are materials of similar crystallinity, however, the uptake efficiency for activated carbon is ca. 1.5 times higher in comparison with carbon black. This observation can be explained by the differences in the porous structure, i.e. activated carbon is a sample with well-developed micropores, whilst carbon black is a non-porous material. The micropores are more pronounced for oxidation by AuCl 4 anions, because they are located at the sites of lower crystallinity. The increased activity of these sites for the oxidation is already demonstrated in experiments with nitric acid, in which the micropore volume decreases after acid treatment (Table 2). The above findings show that the uptake of Au(III) onto nonoxidized carbon sorbents primarily depends on their porous structure and crystallinity, whilst the morphological features seem to have no effect. It is especially seen for carbon black and carbonencapsulated iron nanoparticles. These materials have nearly the same morphological characteristics (Fig. 1), whilst AuCl 4 is preferentially adsorbed onto the material of lower crystallinity, i.e. carbon black. The values of AuCl 4 uptake onto CEINs and CNTs are comparable, however, these samples have different morphology and similar crystallinity. This observation again shows that the morphology of a carbon material does not influence the uptake of Au(III) ion. The uptake of Au(III) is also slightly affected by the surface chemistry characteristics and this observation is found for the non-porous carbon materials only. The removal efficiency increases in the same order as the changes of the surface acidity, i.e. CB > CEINs z CNTs.
ska / Materials Chemistry and Physics 141 (2013) 454e460 M. Bystrzejewski, K. Pyrzyn
458
Table 3 Point of zero charge, surface acidity and surface charge density of non-oxidized and surface oxidized carbon materials. pHzc
Surface charge density (C m2)
Non-oxid.
Surf.-oxid.
Non-oxid.
Surf.-oxid.
Non-oxid.
Surf.-oxid.
6.82 6.45 6.71 5.90
4.44 4.35 3.95 4.60
0.08 0.02 0.18 0.03 0.16 0.02 0.48 0.03
2.75 0.05 2.83 0.07 2.93 0.04 2.50 0.08
0.01 1.15 0.20 0.59
0.47 1.69 2.98 3.09
Next, the removal efficiency of AuCl 4 was studied onto oxidized carbon sorbents. It is well known that the treatment of carbon materials in nitric acid results in formation of surface acidic groups, primarily carboxylic, phenolic and lactonic [24]. One may expect, that the presence of surface groups should favor the adsorption process and increase the removal efficiency. These surface functional groups in oxidized carbon materials theoretically should carry the positive charge (due to the protonation) at highly acidic conditions. The occurrence of positive charge should allow the electrostatic interactions with negatively charged AuCl 4 anions. Unfortunately, this mechanism is highly unlikely, because the surface acidic groups onto carbon materials do not protonate, even at low pH [1]. Nevertheless, the removal efficiency notably increases after oxidation (Fig. 2). The enhancement of Au(III) uptake is the most pronounced in CNTs and CEINs and in this case the removal efficiency rises more than two times, and exceeds the performance of activated carbon and carbon black. The relative removal rate enhancement (RRE) has been calculated and referred to the global surface acidity to verify, whether the observed increase of the removal efficiency has any correlation with the amount of surface acidic groups. The formula for the RRE calculation is as follows: RRE ¼ (Eox Enon-ox/Enon-ox) 100%, where Eox and Enon-ox refer to the uptake of gold onto oxidized and non-oxidized sorbent, respectively. Fig. 3 shows that the RRE parameter linearly changes with the total surface acidity for CEINs, CNTs and CB. This observation clearly evidences that the surface acidic groups readily favor the uptake of Au(III) ion in the case of non-porous carbon sorbents. The AuCl 4 anion has a high redox potential and is able to decompose the surface functional groups. This reaction can be written in a simplified form as follows: jCOOH þ 0 þ AuCl 4 / jAu þ 4Cl þ H þ CO2[, where jCOOH denotes a carboxylic group, which is covalently bound to the surface of a carbon material. This is very likely scenario, since it has been shown that oxidized carbon fibers have decreased surface acidity after adsorption of gold [27]. Activated carbon shows a different behavior and does not follow the linear trend presented in Fig. 3.
92%
80 60
CNTs
100 80 60 40 20
CB
AC
0 2,40
2,55
2,70
2,85
3,00
Surface acidity (mmol/g) Fig. 3. Dependence of relative removal rate enhancement of AuCl 4 and surface acidity.
This finding demonstrates, that surface acidic groups introduced onto activated carbon have a lower activity in the removal process in comparison to carbon nanotubes, carbon-encapsulated iron nanoparticles and carbon black. The acidic groups onto activated carbon are primarily located within the meso- and micropores [28]. This emplacement implies the diffusion resistance for AuCl 4 ions, and therefore the observed relative sorption enhancement is lower (plausibly one may expect the comparable enhancement of the removal rate for a longer contact time). The surface functional groups onto the non-porous sorbents (CNTs, CEINs, CB) are located directly onto the surface of a sorbent, and are easily accessible to the adsorbate. It has been also verified if the distribution of functional groups also influences the removal process of Au(III). No correlation between the surface charge density and RRE has been found (see Figure S14). This finding demonstrates that the amount
100
90
80
70
CB
s IN CE
CN
20
CEINs
120
60
Ts
40
AC
Removal rate (%)
100
Non-oxidized Oxidized
Removal rate enhancement. (%)
Activated carbon Carbon nanotubes Carbon encapsulates Carbon black
Surface acidity (mmol g1)
Removal rate (%)
Material
0
20
40
60
80
100
Initial concentration (mg/L)
0 AuCl 4
Fig. 2. Removal rate of onto non-oxidized and oxidized carbon sorbents (sorbent weight 50 mg, solution volume 10 mL, pH ¼ 2, contact time 5 min).
Fig. 4. Influence of initial concentration of AuCl 4 on removal rate onto oxidized CEINs (sorbent weight 50 mg, solution volume 10 mL, pH ¼ 2, contact time 5 min).
610 -
AuCl4 +
605
600
H -
AuCl4 +
45
AuCl4
H
+
H
-
+
H
30 15 0
CEINs
CEINs-ox
CEINs-defected
Amount of Fe released ( g)
ska / Materials Chemistry and Physics 141 (2013) 454e460 M. Bystrzejewski, K. Pyrzyn
CEINs-ox
Fig. 5. Amount of released Fe during sorption of AuCl 4 onto non-oxidized, oxidized and defected carbon-encapsulated iron nanoparticles. Second bar from the left shows the amount of Fe released in a reference test (without AuCl 4 ).
of surface functional groups (not their distribution) is the main factor that enhances the removal efficiency of AuCl 4. The surface oxidized carbon-encapsulated iron nanoparticles have the best sorption performance in removal of AuCl 4 among all of the studied carbon sorbents. Thus, it is of great importance to check the uptake of Au(III) on CEINs under various experimental conditions. Therefore, additional experiments have been conducted with various starting concentration of AuCl 4 (50 mg of oxidized CEINs, pH ¼ 2, 10 mL solution of HAuCl4, contact time 5 min) (Fig. 4). The largest removal rates (ca. 90%) are observed for the concentration range between 5 mg L1 and 30 mg L1. The removal efficiency gradually decreases above this range. It is an expected result, because the active surface sites are consumed at a higher concentration of AuCl 4. 3.3. Evaluation of galvanic reaction between AuCl 4 and zero-valent Fe in CEINs The results presented above show that the surface acidic groups significantly enhance the sorption efficiency of gold. The largest sorption enhancement is observed for carbon-encapsulated iron nanoparticles. One has to notice that this improvement of sorption properties may be not only caused by the surface effects. According to the thermodynamic predictions, Fe (which is located in the CEIN core) can be oxidized by AuCl 4 ions. However, the access to the iron core is hampered, because the carbon coating is generally impermeable to all constituents of the surrounding solution, including the AuCl 4 ions either. To make the oxidization of the iron core possible, the gold anion must diffuse through the carbon coating. The course of this galvanic exchange reaction can be monitored by
459
evaluating the amount of iron, which is released to the solution. The uptake of Au(III) onto non-oxidized and oxidized CEINs releases 45 mg and 32 mg Fe, respectively (Fig. 5). The reference test (without AuCl 4 ) conducted on oxidized CEINs at acidic conditions (pH ¼ 2) leads to release of 35 mg Fe. The amount of Fe released in the reference test and in the sorption process is on a very similar level. It should be highlighted that the amount of released Fe is only a very small fraction of the total iron (less than 0.2%), which is accumulated in the CEIN sample used in this measurement. Hence, the released amount of Fe cannot originate from the galvanic exchange reaction between AuCl 4 and the iron core. The released Fe must stem from those encapsulates in which the carbon coating is semipermeable. The hydronium ions likely diffuse through such semipermeable barrier and get into the core, and subsequently initialize the etching process of encapsulated zero-valent Fe particles (the schematic pathway is shown in Fig. 6a). This observation points that AuCl 4 does not pass through the carbon coating in surface oxidized CEINs. In the case of non-oxidized CEINs, in which the surface acidity is 18 times lower, the AuCl 4 ion may etch and perforate the carbon coating (the thinnest coating is the most pronounced to perforation). The perforated coating obviously facilitates the diffusion of hydronium ion into the core and accelerates the etching process. In fact, the amount of released Fe in nonoxidized CEINs reaches 42 mg (vs. 32 mg for non-oxidized encapsulates). Next, the etching efficacy of both oxidizing agents (i.e. þ AuCl 4 and H ) in leaching of metallic Fe from the CEIN core has been demonstrated for the as-obtained CEIN product (before its purification). The as-obtained CEINs always contain nonencapsulated Fe particles and defected encapsulates, which have a non-continuous carbon coating. Therefore, in this case, the metallic core is not well protected and is easily accessible to the surrounding environment (see Figure S15 in Supplementary data). The experiment conducted on the as-obtained CEINs results in the largest amount of released Fe (609 mg, Fig. 5). In this experiment the Au (III) uptake is high and reaches 94%. Obviously, the amount of released Fe significantly exceeds its stoichiometric ratio related to the reaction with AuCl 4 . Hence, the released Fe also originates from etching these Fe nanoparticles, which are encapsulated in defected coatings. This result authenticates the pathway depicted in Fig. 6b and confirms that the sorption of Au(III) onto non-oxidized CEINs is likely accompanied by the partial perforation of the coating. This observation also highlights the importance of a high quality carbon coating and its role in the protection of encapsulated particles against the attack of various corrosion agents. The above considerations bring a new view on the recent findings published by Stark group [8]. They studied the removal of Au(III) at acidic conditions (pH ¼ 2.1) onto non-oxidized carbonencapsulated cobalt nanoparticles and observed the release of cobalt ions. They postulated a surface redox mechanism, in which AuCl 4 ion diffuses directly through the carbon coating and initialize the galvanic reaction. Our results argue to the opposing pathway, in which the coating is first perforated during the sorption process, and in the subsequent step the hydronium ion diffuses into the core and initializes the etching process. 4. Conclusions
Fig. 6. Possible routes of Fe release in surface oxidized (a) and non-oxidized (b) carbon-encapsulated iron nanoparticles.
The studies of removal of AuCl 4 from aqueous acidic solutions onto four carbon materials (activated carbon, carbon black, carbon nanotubes and carbon-encapsulated iron nanoparticles) are presented. The investigated carbon materials have different morphology, crystallinity, surface chemical characteristics and textural properties. In the case of non-oxidized carbon sorbents the AuCl 4 ion is preferentially adsorbed onto materials having high porosity and low graphitization degree, whilst their morphological features do not play any role. The
460
ska / Materials Chemistry and Physics 141 (2013) 454e460 M. Bystrzejewski, K. Pyrzyn
maximum uptake of Au(III) for activated carbon is 74%, whilst for other carbon materials this parameter varies between 42 and 45%. The oxidation in nitric acid introduces surface acidic groups and alters the porosity, whilst morphology features are not changed. A substantial increase (up to 120%) of AuCl 4 uptake is observed for all surfaceoxidized sorbents. The distribution of surface acidic groups does not influence the removal efficacy. The greatest enhancement of AuCl 4 uptake is found for carbon-encapsulated magnetic nanoparticles. The mechanistic considerations excluded the possibility of a galvanic reaction between the iron core in carbon encapsulates and AuCl 4 ions. Acknowledgments This work was supported by the Ministry of Science and Education through the Department of Chemistry, Warsaw University under Grant IP2011 006071. The microscopic images were obtained with the use of CePT infrastructure financed by the European Union e the European Regional Development Fund within the Operational Programme “Innovative economy” for 2007e2013. Appendix A. Supplementary data Supplementary data associated with this article can be found in online at http://dx.doi.org/10.1016/j.matchemphys.2013.05.044. References [1] L.R. Radovic, C. Moreno-Castilla, J. Rivera-Utrilla, Carbon materials as adsorbents in aqueous solutions, in: L.R. Radovic (Ed.), Chemistry and Physics of Carbon, vol. 27, New York, Basel, 2001, pp. 227e405.
[2] P. Navarro, C. Vargas, M. Alonso, F.J. Alguacil, Gold Bull. 39 (2006) 93e97. ska, Anal. Chim. Acta 741 (2012) 9e14. [3] K. Pyrzyn [4] Z.R. Yue, W. Jiang, L. Wang, H. Toghiani, S.D. Gardner, C.U. Pitmann, Carbon 37 (1999) 1607e1618. [5] P.A.M. Teirlinck, F.W. Petersen, Miner. Eng. 9 (1996) 923e930. [6] K. Pac1awski, M. Wojnicki, Arch. Metal. Mater. 54 (2009) 853e860. [7] H.C. Choi, M. Shim, S. Bangsaruntip, H. Dai, J. Am. Chem. Soc. 124 (2002) 9058e9059. [8] M. Rossier, F.M. Koehler, E.K. Athanassiou, R.N. Grass, B. Aeschliman, D. Günther, W. Stark, J. Mater. Chem. 19 (2009) 8239e8243. [9] J.H. Jung, J.H. Lee, S. Shinkai, Chem. Soc. Rev. 40 (2011) 4464e4474. [10] M. Bystrzejewski, Przem. Chem. 90 (2011) 399e406. [11] C. Wang, Q. Liu, X. Cheng, Z. Shen, J. Mater. Sci. Technol. 10 (1994) 151e153. [12] Y.C. Chang, D.W. Chen, Gold Bull. 39 (2006) 98e102. [13] R.S. Ruoff, D.C. Lorents, B. Chan, R. Malhotra, S. Subramoney, Science 259 (1993) 346e348. [14] J. Borysiuk, A. Grabias, J. Szczytko, M. Bystrzejewski, A. Twardowski, H. Lange, Carbon 46 (2008) 1693e1701. ska, A. Huczko, H. Lange, Carbon 47 (2009) [15] M. Bystrzejewski, K. Pyrzyn 1201e1204. ska, Coll. Surf. A 377 (2011) 402e408. [16] M. Bystrzejewski, K. Pyrzyn [17] H. Wang, Y.F. Yu, Q.W. Chen, K. Cheng, Dalton Trans. 40 (2011) 559e563. [18] H. Niu, Y. Wang, X. Zhang, Z. Meng, Y. Cai, ACS Appl. Mater. Interfaces 4 (2012) 286e295. [19] R. Fuhrer, I.K. Herrmann, E.K. Athanassiou, R.N. Grass, W.J. Stark, Langmuir 27 (2011) 1924e1929. [20] F.M. Koehler, M. Rossier, M. Waelle, E.K. Athanassiou, L.K. Limbach, R.N. Grass, D. Günther, W.J. Stark, Chem. Commun. 32 (2009) 4862e4864. [21] H.P. Boehm, Carbon 40 (2002) 145e149. [22] L.G. Cancado, K. Takai, T. Enoki, M. Endo, Y.A. Kim, H. Mizusaki, N.L. Speziali, A. Jorio, M.A. Pimenta, Carbon 46 (2008) 272e275. [23] A. Yoshida, Y. Kaburagi, Y. Hishiyama, Carbon 44 (2006) 2333e2335. [24] M.V. Lopez-Ramon, F. Stoeckli, C. Moreno-Castilla, F. Carrasco-Marin, Carbon 37 (1999) 1215e1221. [25] D.D. Do, C. Nguyen, H.D. Do, Coll. Surf. A 187e188 (2001) 51e71. [26] P.J.F. Harris, Z. Liu, K. Suenaga, J. Phys. Condens. Matter 20 (2008) 362201. [27] D.A. Bulushev, I. Yuranov, E.I. Suvorova, P.A. Buffat, L. Kiwi-Minske, J. Catal. 224 (2004) 8e17. [28] S. Liu, R. Wang, J. Porous Mater. 18 (2011) 99e106.
Contents lists available at SciVerse ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Enhancing the efficiency of AuCl 4 ion removal from aqueous solution using activated carbon and carbon nanomaterials ska M. Bystrzejewski*, K. Pyrzyn University of Warsaw, Department of Chemistry, Pasteur 1 Street, 02-093 Warsaw, Poland
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Removal efficiency in non-oxidized sorbent is dictated by crystallinity and porosity. Introduction of surface acidic groups increases removal efficiency two times. Carbon encapsulates have high removal efficiency for broad range concentration of Au(III).
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 August 2012 Received in revised form 10 May 2013 Accepted 20 May 2013
The removal of AuCl 4 ion from acidic aqueous solutions is studied using a series of non-oxidized and surface oxidized carbon materials (activated carbon, carbon nanotubes, carbon-encapsulated iron nanoparticles and carbon black). The studied sorbents differ in crystallinity, porosity and morphology. In the case of non-oxidized carbon materials the maximum removal efficiency (74%) is found for activated carbon, whilst graphitized nanomaterials (i.e. carbon nanotubes and carbon-encapsulated iron nanoparticles) are able to remove 42e45% of gold ion from the solution. The oxidation in nitric acid significantly improves the removal efficiencies. The uptake of Au(III) increases two times (to 91e92%) for oxidized carbon nanotubes and carbon-encapsulated iron nanoparticles. The same oxidation procedure applied to activated carbon and carbon black moderately enhances the uptake efficiency to 88% and 55%, respectively. The observed substantially distinct uptakes are discussed in the frames of textural properties, morphology, surface chemistry characteristics and crystallinity of the studied carbon materials. Moreover, the possibility of a galvanic exchange reaction between AuCl 4 and metallic Fe in the carbon encapsulate core is also evaluated. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Microporous materials Nanostructures Chemisorption Surface properties
1. Introduction The recovery of precious metals, such as gold, platinum and palladium, is an attracting challenge due to the limited sources and growing industrial demand. It has been shown that sorbents based on carbon materials (activated carbon, carbon fibers, carbon nanotubes) have a high potential for extraction and pre-concentration of precious metal complexes, including gold [1e4]. The high redox potential of typical precious metal halide complexes (e.g. AuCl 4,
* Corresponding author. Fax: þ48 22 822 59 96. E-mail address: [email protected] (M. Bystrzejewski). 0254-0584/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2013.05.044
PtCl2 6 ) implicates that their adsorption occurs presumably via a chemisorption process. The adsorption process is accompanied by a spontaneous deposition of small crystallites of the metal which is adsorbed [5,6]. This work is motivated by recent studies showing that crystallites of precious metals (Au, Pt) are spontaneously deposited onto various carbon nanomaterials (carbon nanotubes and carbonencapsulated magnetic nanoparticles) during the contact with a solution containing the metal complex [7,8]. It has been demonstrated that even very short contact times (3 min) result in a complete removal of gold metal from the solution [8]. Here, a morein-depth and comparative studies are presented, in which four various carbon sorbents are investigated, i.e. activated carbon (AC),
ska / Materials Chemistry and Physics 141 (2013) 454e460 M. Bystrzejewski, K. Pyrzyn
multi-wall carbon nanotubes (CNTs), carbon-encapsulated iron nanoparticles (CEINs) and carbon black (CB). These carbon materials have different morphological, textural, surface chemistry characteristics and structural features. The aim of this work is to compare the efficiency of solid phase extraction of gold onto the aforementioned sorbents, and evaluate the parameters, which are the most crucial for the efficient removal of gold. A substantial attention in this paper is also focused on mobile magnetic sorbents and their feasibility in the recovery of gold. Magnetic sorbents dedicated to sorption of typical heavy metals (e.g. Cu, Cd, Co) are widely studied since at least 10 years [9,10]. However, there are only a few reports describing the adsorption of gold and other precious metals onto magnetic sorbents. [11,12]. The ability for rapid separation of the sorbent from the solution is the main advantage of magnetic sorbents. This feature can be used in the carbon-in-pulp process to increase the recovery rate of gold. In the case of mobile sorbents based on magnetic nanoparticles (which may have large available surface area) the time required to reach the sorption equilibrium is shorter in comparison with activated carbon [12]. In this paper carbon-encapsulated iron nanoparticles are studied as a prospective mobile sorbent for gold recovery. These hybrid nanomaterials are composed of a magnetic core (typically 5e100 nm in diameter) and thin carbon coating (1e 5 nm in thickness), which is built of curved graphene layers [13,14]. The carbon shell completely covers the encapsulated magnetic nanoparticle and protects it from oxidation, corrosion and agglomeration. In other words, the specific physical and chemical properties of the nanocrystalline magnetic core are fully preserved. It has been shown that CEINs have a great potential for sorption of heavy metals [15e17] and organic molecules [18,19]. Moreover, the carbon coating in CEINs can be functionalized in a controlled way to develop a magnetic sorbent with enhanced sorption efficiency or selectivity [20].
2. Experimental Activated carbon and multi-wall carbon nanotubes were purchased from SigmaeAldrich and used as received. Carbon black N330 was donated by the Research Institute of Rubber Industry (Piastów, Poland). Carbon-encapsulated iron nanoparticles were synthesized by a carbon arc route and then subjected to the purification procedure to remove the non-encapsulated nanoparticles [14]. The morphology of carbon sorbents was studied by scanning electron microscopy (Zeiss Leo 1530). Raman spectra were acquired with a resolution of 2 cm1 using a 515 nm Ar excitation laser. The nitrogen adsorption studies were conducted on ASAP 20120 analyzer (the samples were outgassed at 200 C under vacuum prior to the measurement). The removal of AuCl 4 ion was studied on raw (i.e. non-oxidized) and surface-oxidized carbon materials. The oxidation procedure included 12 h of soaking of the carbon material in 8 M HNO3 at room temperature, with subsequent washing with water until the neutral pH was obtained. The total number of surface acidic groups was evaluated by the Boehm titration method [21]. The removal of AuCl 4 was investigated at pH 2 at constant temperature (298 1 K). The stock solution of HAuCl4 (1000 mg L1 in respect to Au(III)) was purchased from Spex Cer1 in respect to tiPrep. The diluted solutions of AuCl 4 (1e30 mg L Au(III)) were prepared for the calibration of the atomic absorption spectrometer. For the studies of AuCl 4 removal a 50 mg sample of carbon material was added to 10 mL solution of HAuCl4 (initial concentration 30 mg L1 with respect to Au(III)), with subsequent vigorous shaking for 5 min. Then, the sorbents were filtered off by centrifugation and the concentration of AuCl 4 in the supernatant was evaluated by atomic absorption spectroscopy.
455
3. Results and discussion 3.1. Morphological, structural and textural features of carbon sorbents The studied carbon sorbents have distinct morphological features (Fig. 1, aed). Activated carbon comprises of relatively large grains (hundreds of microns, images not shown), which have a well-developed pore structure with a narrow diameter distribution (70e100 nm in diameter). Carbon nanotubes are several mm in length, and their diameters are in the range between 50 and 100 nm. Carbon-encapsulated iron nanoparticles consist of oval nanoparticles with sizes below 100 nm. CEINs have typical coree shell structure, i.e. they are comprised of metallic cores which are completely covered by a thin carbon coating (Figure S1, Supplementary data). Carbon black consists of uniform bulk nanoparticles with diameters between 20 and 50 nm (Figure S1). The oxidation in nitric acid does not change the morphological features (especially the size distribution and shape) of the studied carbon materials (Fig. 1, eeh). Raman spectroscopy is a widely used technique for studying the electronic structure of carbon materials. The first-order Raman spectrum shows two bands, the so-called G and D bands [22]. In a perfect graphite crystal the G band is located at 1582 cm1 and corresponds to stretching vibrations of CeC bonds in the lattice. The D-band, which appears at lower wavenumber, is associated with structural and topological disorder. The ratio between the integral intensities of the G and D bands is an accepted indicator of the structural quality of carbon materials. This ratio is linearly correlated with the La size (La is the size of a single graphene layer) [22]. The low value of the G/D ratio points to a carbon material, which has a large number of defects and therefore is built of very small graphene layers. The G bandwidth is related to the extent of disorder and also brings useful structural information. Recent studies have shown that this parameter correlates with the Lc crystallite size (the Lc parameter visualizes the stacking order between the adjacent graphene layers) [23]. The G/D ratio values and G bandwidth are listed in Table 1. The G/D ratio is calculated from the integral areas of the bands (the spectra were deconvoluted using the Lorentzian function, see Figures S2eS5). Activated carbon and carbon black have the lowest G/D ratio and highest G bandwidth, which points to their large structural disorder. This is an expected result, because these materials are formed during carbonization of carbon precursors at relatively low temperature (1000e1300 K), which is too low to induce the graphitization processes. Carbon-encapsulated iron nanoparticles have higher G/D ratio and lower G bandwidth in comparison to AC and CB samples. This finding points to higher crystallinity (graphene layers are larger, however they have large numbers of defects which are necessary to introduce the curvature in a carbon coating in CEINs). Carbon nanotubes have the largest structural order among all studied carbon materials. The treatment of carbon materials in nitric acid is frequently used to introduce surface functional groups [24]. The formation of these groups is always accompanied by structural degradation of external graphene layers and decreasing the overall crystallinity of a carbon material. The largest structural changes are observed for carbon nanotubes (Table 2), in which the G/D ratio diminishes 6 times and the G bandwidth increases of 27 cm1 (63%). The crystallinity of carbon-encapsulated iron nanoparticles also becomes smaller, however, this effect is less pronounced in comparison to carbon nanotubes (the G/D ratio decreases of 18% and the G band broadens of 4 cm1 (10%)). In the case of carbon black the G/D ratio virtually does not alter (the change is 3%), whilst the G bandwidth increases only of 2 cm1 (2%). Interestingly, the G/D ratio increases
456
ska / Materials Chemistry and Physics 141 (2013) 454e460 M. Bystrzejewski, K. Pyrzyn
Fig. 1. SEM images of non-oxidized (left column) and surface oxidized (right column) carbon sorbents: activated carbon (a, e), carbon nanotubes (b, f), and carbon-encapsulated iron nanoparticles (c, g) and carbon black (d, h).
of 10% for activated carbon. These intriguing results are discussed in the next section. Importantly, the nitric acid treatment does not change the order of crystallinity, which is as follows for the pristine and oxidized samples: CNTs > CEINs > AC z CB. The studied carbon materials have very distinct textural properties. The values of the specific surface area evaluated from the BET equation and the pore volume are shown in Tables 1 and 2 (nitrogen adsorption and desorption isotherms are shown in Figures S6e S13). Activated carbon has the highest surface area among all studied materials. This is a consequence of high micropore volume
(49% of the total pore volume), which has the largest contribution to the surface area. Carbon nanotubes have the lowest surface area and pore volume. Carbon-encapsulated iron nanoparticles and carbon black have higher surface area in comparison to carbon nanotubes. However, all of these nanomaterials cannot be regarded as highly porous materials. The observed relatively low surface area and low micropore volume can be explained on the basis of morphological similarities. The primary particles of CNTs, CEINs and CB have a smooth surface, which is devoid of any pores (see Figs. 1 and S1). Interestingly, the adsorptionedesorption isotherms
ska / Materials Chemistry and Physics 141 (2013) 454e460 M. Bystrzejewski, K. Pyrzyn Table 1 Structural and textural properties of non-oxidized carbon materials. Material
G/D
G bandwidth (cm1)
SBET (m2 g1)
Total pore volume (cm3 g1)
Micropore volume (cm3 g1)
Activated carbon Carbon nanotubes Carbon encapsulates Carbon black
0.41 7.72 1.14 0.40
56 27 41 86
642 15 77 78
0.516 0.034 0.316 0.438
0.227 0.003 0.010 0.013
of AC and CEINs samples are characterized by a hysteresis, which is absent for other studied samples. This is associated with the presence of mesopores, in which capillary condensation takes place and it limits the nitrogen uptake over a high range of p/p0 [25]. The oxidation in nitric acid changes the textural properties of all studied carbon materials (Table 2). The surface area and the micropore volume in activated carbon decrease of nearly the same values, i.e. 13% and 15%, respectively. This result shows that the sites, at which the micropores are located, primarily undergo degradation. It is obvious, that these sites have larger structural disorder in comparison to the bulk material [26]. Hence, their elimination should improve the crystallinity of AC. In fact, the G/D ratio increases of 10% after oxidation (Table 2). The surface area and total pore volume in carbon nanotubes increase more than 10-fold after oxidation. This is a consequence of amorphization of external graphene layers (the G/D ratio decreases) and opening the cups of nanotubes (the inner core space is then available for nitrogen molecules). Carbon encapsulates and carbon black follow the same trend as carbon nanotubes and also increase their porosity. The results of Boehm titration and values of the point of zero charge are presented in Table 3. The non-oxidized carbon materials have relatively low and similar surface acidity, which ranges between 0.08 and 0.48 mmol g1. Please note, that the evaluated acidity refers to the total content of surface functional groups (carboxylic, phenolic and lactonic). The corresponding point of zero charge values are in agreement with titration results and show that the studied carbon materials have weak acidic properties. Nevertheless, the mean distribution of surface acidic groups differs significantly, because of distinct pore volume. The mean distribution density of functional groups can be expressed as the surface charge density (SCD). This parameter can determined as follows: SCD ¼ (SAC$F/SA)[C m2], where SAC denotes the surface acidity (in mmol g1), SA is the surface area (in m2/g) and F is the Faraday constant (96.5 C mmol1). The surface acidic groups are the most densely packed in carbon nanotubes, and it directly results from their very low porosity and low specific surface area (15 m2 g1). The as-obtained carbon-encapsulated iron nanoparticles (i.e. before the purification procedure) have slightly basic properties. Their point of zero charge is found to be 7.42 and the total number of surface functional groups is 0.05 mmol g1. The observed surface charge in the as-obtained CEINs likely originates partially oxidized non-encapsulated Fe nanoparticles. It should be noted that nonencapsulated Fe particles, which are always present in the asobtained product, are in a continuous contact with the ambient atmosphere and may be oxidized. Table 2 Structural and textural properties of surface oxidized carbon materials. Material
G/D
G bandwidth (cm1)
SBET (m2 g1)
Total pore volume (cm3 g1)
Micropore volume (cm3 g1)
Activated carbon Carbon nanotubes Carbon encapsulates Carbon black
0.45 1.27 0.95 0.39
65 44 45 88
561 162 95 78
0.469 0.376 0.338 0.348
0.193 0.005 0.017 0.015
457
The nitric acid treatment results in 15e30-fold increase of the surface acidity. All surface oxidized samples have acidic properties. It is expressed by the values of the point of zero charge, which is between 3.95 and 4.60 (Table 3). Interestingly, the surface acidity is on a similar level, i.e. 2.50e2.92 mmol g1 (Table 3). This finding shows that the applied oxidation procedure has a nearly identical functionalization potential for various carbon materials and does not depend on the crystalline or the morphological features. Similarly to the non-oxidized samples, the distribution of surface functional groups is different because of diverse porosity. The most dense packing of functional groups is found for carbon black and carbon-encapsulated iron nanoparticles. 3.2. Studies of AuCl 4 removal The Au(III) uptake for the non-oxidized carbon materials varies between 42 and 74% (Fig. 2) and changes in the following order: AC > CB > CNTs z CEINs. To explain this behavior one has to take into account the factors that may accelerate or inhibit the uptake of AuCl 4 ion. In general, the adsorption of ionized species, e.g. heavy metal cations, is usually favored when the surface of a carbon sorbent starts to carry the negative charge (i.e. the surface functional groups undergo deprotonation). In this case the electrostatic attraction occurs and the sorption is enabled. Precious metals can be also adsorbed onto carbon materials, however the mechanism of sorption is more complex in comparison to the mentioned route describing the electrostatic binding of metal cations. As it was stated in Introduction, the sorption of precious metal ions (e.g. Au, Pt and Pd) occurs through spontaneous chemisorption with simultaneous deposition of zero-valent metal clusters onto the surface of a carbon material [5,6]. This effect results from the high redox potential of precious metal ion complexes. The carbon sorbents having higher crystallinity should possess higher resistance toward the oxidation processes and should be less pronounced for the oxidation by AuCl 4 . In fact, CNTs and CEINs are substantially better graphitized than AC and CB (see G/D values in Table 1), and therefore they are more inert in respect to the reaction with AuCl 4. This is supported by the lowest uptakes of Au(III) for CNTs and CEINs. Activated carbon and carbon black are materials of similar crystallinity, however, the uptake efficiency for activated carbon is ca. 1.5 times higher in comparison with carbon black. This observation can be explained by the differences in the porous structure, i.e. activated carbon is a sample with well-developed micropores, whilst carbon black is a non-porous material. The micropores are more pronounced for oxidation by AuCl 4 anions, because they are located at the sites of lower crystallinity. The increased activity of these sites for the oxidation is already demonstrated in experiments with nitric acid, in which the micropore volume decreases after acid treatment (Table 2). The above findings show that the uptake of Au(III) onto nonoxidized carbon sorbents primarily depends on their porous structure and crystallinity, whilst the morphological features seem to have no effect. It is especially seen for carbon black and carbonencapsulated iron nanoparticles. These materials have nearly the same morphological characteristics (Fig. 1), whilst AuCl 4 is preferentially adsorbed onto the material of lower crystallinity, i.e. carbon black. The values of AuCl 4 uptake onto CEINs and CNTs are comparable, however, these samples have different morphology and similar crystallinity. This observation again shows that the morphology of a carbon material does not influence the uptake of Au(III) ion. The uptake of Au(III) is also slightly affected by the surface chemistry characteristics and this observation is found for the non-porous carbon materials only. The removal efficiency increases in the same order as the changes of the surface acidity, i.e. CB > CEINs z CNTs.
ska / Materials Chemistry and Physics 141 (2013) 454e460 M. Bystrzejewski, K. Pyrzyn
458
Table 3 Point of zero charge, surface acidity and surface charge density of non-oxidized and surface oxidized carbon materials. pHzc
Surface charge density (C m2)
Non-oxid.
Surf.-oxid.
Non-oxid.
Surf.-oxid.
Non-oxid.
Surf.-oxid.
6.82 6.45 6.71 5.90
4.44 4.35 3.95 4.60
0.08 0.02 0.18 0.03 0.16 0.02 0.48 0.03
2.75 0.05 2.83 0.07 2.93 0.04 2.50 0.08
0.01 1.15 0.20 0.59
0.47 1.69 2.98 3.09
Next, the removal efficiency of AuCl 4 was studied onto oxidized carbon sorbents. It is well known that the treatment of carbon materials in nitric acid results in formation of surface acidic groups, primarily carboxylic, phenolic and lactonic [24]. One may expect, that the presence of surface groups should favor the adsorption process and increase the removal efficiency. These surface functional groups in oxidized carbon materials theoretically should carry the positive charge (due to the protonation) at highly acidic conditions. The occurrence of positive charge should allow the electrostatic interactions with negatively charged AuCl 4 anions. Unfortunately, this mechanism is highly unlikely, because the surface acidic groups onto carbon materials do not protonate, even at low pH [1]. Nevertheless, the removal efficiency notably increases after oxidation (Fig. 2). The enhancement of Au(III) uptake is the most pronounced in CNTs and CEINs and in this case the removal efficiency rises more than two times, and exceeds the performance of activated carbon and carbon black. The relative removal rate enhancement (RRE) has been calculated and referred to the global surface acidity to verify, whether the observed increase of the removal efficiency has any correlation with the amount of surface acidic groups. The formula for the RRE calculation is as follows: RRE ¼ (Eox Enon-ox/Enon-ox) 100%, where Eox and Enon-ox refer to the uptake of gold onto oxidized and non-oxidized sorbent, respectively. Fig. 3 shows that the RRE parameter linearly changes with the total surface acidity for CEINs, CNTs and CB. This observation clearly evidences that the surface acidic groups readily favor the uptake of Au(III) ion in the case of non-porous carbon sorbents. The AuCl 4 anion has a high redox potential and is able to decompose the surface functional groups. This reaction can be written in a simplified form as follows: jCOOH þ 0 þ AuCl 4 / jAu þ 4Cl þ H þ CO2[, where jCOOH denotes a carboxylic group, which is covalently bound to the surface of a carbon material. This is very likely scenario, since it has been shown that oxidized carbon fibers have decreased surface acidity after adsorption of gold [27]. Activated carbon shows a different behavior and does not follow the linear trend presented in Fig. 3.
92%
80 60
CNTs
100 80 60 40 20
CB
AC
0 2,40
2,55
2,70
2,85
3,00
Surface acidity (mmol/g) Fig. 3. Dependence of relative removal rate enhancement of AuCl 4 and surface acidity.
This finding demonstrates, that surface acidic groups introduced onto activated carbon have a lower activity in the removal process in comparison to carbon nanotubes, carbon-encapsulated iron nanoparticles and carbon black. The acidic groups onto activated carbon are primarily located within the meso- and micropores [28]. This emplacement implies the diffusion resistance for AuCl 4 ions, and therefore the observed relative sorption enhancement is lower (plausibly one may expect the comparable enhancement of the removal rate for a longer contact time). The surface functional groups onto the non-porous sorbents (CNTs, CEINs, CB) are located directly onto the surface of a sorbent, and are easily accessible to the adsorbate. It has been also verified if the distribution of functional groups also influences the removal process of Au(III). No correlation between the surface charge density and RRE has been found (see Figure S14). This finding demonstrates that the amount
100
90
80
70
CB
s IN CE
CN
20
CEINs
120
60
Ts
40
AC
Removal rate (%)
100
Non-oxidized Oxidized
Removal rate enhancement. (%)
Activated carbon Carbon nanotubes Carbon encapsulates Carbon black
Surface acidity (mmol g1)
Removal rate (%)
Material
0
20
40
60
80
100
Initial concentration (mg/L)
0 AuCl 4
Fig. 2. Removal rate of onto non-oxidized and oxidized carbon sorbents (sorbent weight 50 mg, solution volume 10 mL, pH ¼ 2, contact time 5 min).
Fig. 4. Influence of initial concentration of AuCl 4 on removal rate onto oxidized CEINs (sorbent weight 50 mg, solution volume 10 mL, pH ¼ 2, contact time 5 min).
610 -
AuCl4 +
605
600
H -
AuCl4 +
45
AuCl4
H
+
H
-
+
H
30 15 0
CEINs
CEINs-ox
CEINs-defected
Amount of Fe released ( g)
ska / Materials Chemistry and Physics 141 (2013) 454e460 M. Bystrzejewski, K. Pyrzyn
CEINs-ox
Fig. 5. Amount of released Fe during sorption of AuCl 4 onto non-oxidized, oxidized and defected carbon-encapsulated iron nanoparticles. Second bar from the left shows the amount of Fe released in a reference test (without AuCl 4 ).
of surface functional groups (not their distribution) is the main factor that enhances the removal efficiency of AuCl 4. The surface oxidized carbon-encapsulated iron nanoparticles have the best sorption performance in removal of AuCl 4 among all of the studied carbon sorbents. Thus, it is of great importance to check the uptake of Au(III) on CEINs under various experimental conditions. Therefore, additional experiments have been conducted with various starting concentration of AuCl 4 (50 mg of oxidized CEINs, pH ¼ 2, 10 mL solution of HAuCl4, contact time 5 min) (Fig. 4). The largest removal rates (ca. 90%) are observed for the concentration range between 5 mg L1 and 30 mg L1. The removal efficiency gradually decreases above this range. It is an expected result, because the active surface sites are consumed at a higher concentration of AuCl 4. 3.3. Evaluation of galvanic reaction between AuCl 4 and zero-valent Fe in CEINs The results presented above show that the surface acidic groups significantly enhance the sorption efficiency of gold. The largest sorption enhancement is observed for carbon-encapsulated iron nanoparticles. One has to notice that this improvement of sorption properties may be not only caused by the surface effects. According to the thermodynamic predictions, Fe (which is located in the CEIN core) can be oxidized by AuCl 4 ions. However, the access to the iron core is hampered, because the carbon coating is generally impermeable to all constituents of the surrounding solution, including the AuCl 4 ions either. To make the oxidization of the iron core possible, the gold anion must diffuse through the carbon coating. The course of this galvanic exchange reaction can be monitored by
459
evaluating the amount of iron, which is released to the solution. The uptake of Au(III) onto non-oxidized and oxidized CEINs releases 45 mg and 32 mg Fe, respectively (Fig. 5). The reference test (without AuCl 4 ) conducted on oxidized CEINs at acidic conditions (pH ¼ 2) leads to release of 35 mg Fe. The amount of Fe released in the reference test and in the sorption process is on a very similar level. It should be highlighted that the amount of released Fe is only a very small fraction of the total iron (less than 0.2%), which is accumulated in the CEIN sample used in this measurement. Hence, the released amount of Fe cannot originate from the galvanic exchange reaction between AuCl 4 and the iron core. The released Fe must stem from those encapsulates in which the carbon coating is semipermeable. The hydronium ions likely diffuse through such semipermeable barrier and get into the core, and subsequently initialize the etching process of encapsulated zero-valent Fe particles (the schematic pathway is shown in Fig. 6a). This observation points that AuCl 4 does not pass through the carbon coating in surface oxidized CEINs. In the case of non-oxidized CEINs, in which the surface acidity is 18 times lower, the AuCl 4 ion may etch and perforate the carbon coating (the thinnest coating is the most pronounced to perforation). The perforated coating obviously facilitates the diffusion of hydronium ion into the core and accelerates the etching process. In fact, the amount of released Fe in nonoxidized CEINs reaches 42 mg (vs. 32 mg for non-oxidized encapsulates). Next, the etching efficacy of both oxidizing agents (i.e. þ AuCl 4 and H ) in leaching of metallic Fe from the CEIN core has been demonstrated for the as-obtained CEIN product (before its purification). The as-obtained CEINs always contain nonencapsulated Fe particles and defected encapsulates, which have a non-continuous carbon coating. Therefore, in this case, the metallic core is not well protected and is easily accessible to the surrounding environment (see Figure S15 in Supplementary data). The experiment conducted on the as-obtained CEINs results in the largest amount of released Fe (609 mg, Fig. 5). In this experiment the Au (III) uptake is high and reaches 94%. Obviously, the amount of released Fe significantly exceeds its stoichiometric ratio related to the reaction with AuCl 4 . Hence, the released Fe also originates from etching these Fe nanoparticles, which are encapsulated in defected coatings. This result authenticates the pathway depicted in Fig. 6b and confirms that the sorption of Au(III) onto non-oxidized CEINs is likely accompanied by the partial perforation of the coating. This observation also highlights the importance of a high quality carbon coating and its role in the protection of encapsulated particles against the attack of various corrosion agents. The above considerations bring a new view on the recent findings published by Stark group [8]. They studied the removal of Au(III) at acidic conditions (pH ¼ 2.1) onto non-oxidized carbonencapsulated cobalt nanoparticles and observed the release of cobalt ions. They postulated a surface redox mechanism, in which AuCl 4 ion diffuses directly through the carbon coating and initialize the galvanic reaction. Our results argue to the opposing pathway, in which the coating is first perforated during the sorption process, and in the subsequent step the hydronium ion diffuses into the core and initializes the etching process. 4. Conclusions
Fig. 6. Possible routes of Fe release in surface oxidized (a) and non-oxidized (b) carbon-encapsulated iron nanoparticles.
The studies of removal of AuCl 4 from aqueous acidic solutions onto four carbon materials (activated carbon, carbon black, carbon nanotubes and carbon-encapsulated iron nanoparticles) are presented. The investigated carbon materials have different morphology, crystallinity, surface chemical characteristics and textural properties. In the case of non-oxidized carbon sorbents the AuCl 4 ion is preferentially adsorbed onto materials having high porosity and low graphitization degree, whilst their morphological features do not play any role. The
460
ska / Materials Chemistry and Physics 141 (2013) 454e460 M. Bystrzejewski, K. Pyrzyn
maximum uptake of Au(III) for activated carbon is 74%, whilst for other carbon materials this parameter varies between 42 and 45%. The oxidation in nitric acid introduces surface acidic groups and alters the porosity, whilst morphology features are not changed. A substantial increase (up to 120%) of AuCl 4 uptake is observed for all surfaceoxidized sorbents. The distribution of surface acidic groups does not influence the removal efficacy. The greatest enhancement of AuCl 4 uptake is found for carbon-encapsulated magnetic nanoparticles. The mechanistic considerations excluded the possibility of a galvanic reaction between the iron core in carbon encapsulates and AuCl 4 ions. Acknowledgments This work was supported by the Ministry of Science and Education through the Department of Chemistry, Warsaw University under Grant IP2011 006071. The microscopic images were obtained with the use of CePT infrastructure financed by the European Union e the European Regional Development Fund within the Operational Programme “Innovative economy” for 2007e2013. Appendix A. Supplementary data Supplementary data associated with this article can be found in online at http://dx.doi.org/10.1016/j.matchemphys.2013.05.044. References [1] L.R. Radovic, C. Moreno-Castilla, J. Rivera-Utrilla, Carbon materials as adsorbents in aqueous solutions, in: L.R. Radovic (Ed.), Chemistry and Physics of Carbon, vol. 27, New York, Basel, 2001, pp. 227e405.
[2] P. Navarro, C. Vargas, M. Alonso, F.J. Alguacil, Gold Bull. 39 (2006) 93e97. ska, Anal. Chim. Acta 741 (2012) 9e14. [3] K. Pyrzyn [4] Z.R. Yue, W. Jiang, L. Wang, H. Toghiani, S.D. Gardner, C.U. Pitmann, Carbon 37 (1999) 1607e1618. [5] P.A.M. Teirlinck, F.W. Petersen, Miner. Eng. 9 (1996) 923e930. [6] K. Pac1awski, M. Wojnicki, Arch. Metal. Mater. 54 (2009) 853e860. [7] H.C. Choi, M. Shim, S. Bangsaruntip, H. Dai, J. Am. Chem. Soc. 124 (2002) 9058e9059. [8] M. Rossier, F.M. Koehler, E.K. Athanassiou, R.N. Grass, B. Aeschliman, D. Günther, W. Stark, J. Mater. Chem. 19 (2009) 8239e8243. [9] J.H. Jung, J.H. Lee, S. Shinkai, Chem. Soc. Rev. 40 (2011) 4464e4474. [10] M. Bystrzejewski, Przem. Chem. 90 (2011) 399e406. [11] C. Wang, Q. Liu, X. Cheng, Z. Shen, J. Mater. Sci. Technol. 10 (1994) 151e153. [12] Y.C. Chang, D.W. Chen, Gold Bull. 39 (2006) 98e102. [13] R.S. Ruoff, D.C. Lorents, B. Chan, R. Malhotra, S. Subramoney, Science 259 (1993) 346e348. [14] J. Borysiuk, A. Grabias, J. Szczytko, M. Bystrzejewski, A. Twardowski, H. Lange, Carbon 46 (2008) 1693e1701. ska, A. Huczko, H. Lange, Carbon 47 (2009) [15] M. Bystrzejewski, K. Pyrzyn 1201e1204. ska, Coll. Surf. A 377 (2011) 402e408. [16] M. Bystrzejewski, K. Pyrzyn [17] H. Wang, Y.F. Yu, Q.W. Chen, K. Cheng, Dalton Trans. 40 (2011) 559e563. [18] H. Niu, Y. Wang, X. Zhang, Z. Meng, Y. Cai, ACS Appl. Mater. Interfaces 4 (2012) 286e295. [19] R. Fuhrer, I.K. Herrmann, E.K. Athanassiou, R.N. Grass, W.J. Stark, Langmuir 27 (2011) 1924e1929. [20] F.M. Koehler, M. Rossier, M. Waelle, E.K. Athanassiou, L.K. Limbach, R.N. Grass, D. Günther, W.J. Stark, Chem. Commun. 32 (2009) 4862e4864. [21] H.P. Boehm, Carbon 40 (2002) 145e149. [22] L.G. Cancado, K. Takai, T. Enoki, M. Endo, Y.A. Kim, H. Mizusaki, N.L. Speziali, A. Jorio, M.A. Pimenta, Carbon 46 (2008) 272e275. [23] A. Yoshida, Y. Kaburagi, Y. Hishiyama, Carbon 44 (2006) 2333e2335. [24] M.V. Lopez-Ramon, F. Stoeckli, C. Moreno-Castilla, F. Carrasco-Marin, Carbon 37 (1999) 1215e1221. [25] D.D. Do, C. Nguyen, H.D. Do, Coll. Surf. A 187e188 (2001) 51e71. [26] P.J.F. Harris, Z. Liu, K. Suenaga, J. Phys. Condens. Matter 20 (2008) 362201. [27] D.A. Bulushev, I. Yuranov, E.I. Suvorova, P.A. Buffat, L. Kiwi-Minske, J. Catal. 224 (2004) 8e17. [28] S. Liu, R. Wang, J. Porous Mater. 18 (2011) 99e106.