Elsevier Editorial System(tm) for Colloids and Surfaces A: Physicochemical and Engineering Aspects Manuscript Draft Manuscript Number: Title: Enhancing the efficiency of Au adsorption onto activated carbon and carbon nanomaterials Article Type: Research Paper Keywords: Gold adsorption, carbon-encapsulated magnetic nanoparticles, carbon nanotubes, activated carbon, carbon black, precious metals Corresponding Author: Dr Michał Bystrzejewski, Corresponding Author's Institution: Warsaw University First Author: Michał Bystrzejewski Order of Authors: Michał Bystrzejewski; Krystyna Pyrzyńska Abstract: Sorption of gold from acidic aqueous solutions was studied onto a series of non-oxidized and oxidized carbon materials (activated carbon, carbon nanotubes, carbon-encapsulated iron nanoparticles and carbon black). The studied sorbents differed with graphitization degree, porosity and morphology. In the case of non-oxidized carbon materials the maximum sorption efficiency (74%) was found for activated carbon, whilst graphitized nanomaterials (i.e. carbon nanotubes and carbon encapsulates) were able to adsorb 42-45% of the gold ions that were present in the solution. The oxidation in nitric acid significantly changed the sorption efficiencies. The uptake of gold increased two times (to 91-92%) for oxidized carbon nanotubes and carbon-encapsulates. The same oxidation procedure applied to activated carbon and carbon black moderately enhanced the sorption efficiency to 88% and 55%, respectively. The observed substantially distinct gold uptakes were discussed in the frames of textural properties, morphology and graphitization degree. Moreover, the possibility of the galvanic exchange reaction between AuCl4- and metallic Fe in the carbon encapsulates core was evaluated. Suggested Reviewers: Alicja Bachmatiuk Leibniz Institute for Solid State Research, Dresden, Germany A.Bachmatiuk@ifw-dresden.de Dr Bachmatiuk is an expert in synthesis and properties of carbon materials. Janos Szepvolgyi Hungarian Academy of Sciences, Budapest, Hungary szepvol@chemres.hu Prof. Szepvolgyi is an expert in the field of synthesis and applications of carbon nanomaterials. Gervais Soucy University of Sherbrooke, Sherbrooke, Canada Gervais.Soucy@USherbrooke.ca Prof. Soucy is an expert in the field of synthesis and environmental applications of carbon nanomaterials.

Franziska Schaeffel University of Oxford, Dept of Materials, Oxford, UK franziska.schaeffel@materials.ox.ac.uk Dr Schaeffel is an expert in characterization of composite nanomaterials. Seref Gucer Uludag University, Bursa, Turkey sgucer@uludag.edu.tr Prof Gucer is an expert in analitycal chemistry. His research interest mainly involve solid phase extraction of heavy metals.

Prof. Dr. M. Adler

Editor

Colloids&Surfaces A

Dear Editor,

Please find enclosed our contribution “Enhancing the efficiency of Au adsorption onto

activated carbon and carbon nanomaterials” by M. Bystrzejewski and K. Pyrzyńska. Both

authors contributed equally to the manuscript. We believe the paper will be of particular

interest to readers of your journal because we present systematic comparative studies on

sorption of AuCl4- ions onto four carbon materials: activated carbon, carbon black carbon

nanotubes and carbon-encapsulated magnetic nanoparticles. The native sorbents present

distinct sorption efficiencies varying between 42 and 74%. The simple modification of surface

resulted in twofold increase of the sorption efficiency, which appeared for carbon

encapsulates and carbon nanotubes. The obtained original results are discussed in the frames

of morphology, structure and texture. Moreover, a critical evaluation of sorption mechanism

onto carbon encapsulates is presented. We have shown, that AuCl4- ions are not able to

undergo the galvanic exchange reaction with Fe crystallites in carbon encapsulates (contrary

to the previous literature predictions). We would like to highlight that magnetic carbon

encapsulates are considered as practical and prospective mobile sorbents, which combine

properties of magnetic core and modifiable carbon coating. Two our recent paper from

Colloids&Surfaces A are devoted to those nanomaterials (vol. 362, p.102; vol. 377, 402).

Our contribution is an original work and all authors are aware of its content and its

submission. This paper is not under consideration elsewhere, including the Internet. There is

no conflict of interest. Furthermore, we declare that the article (if accepted) will not be

published elsewhere in the same form, in any language, without the written consent of the

publisher.

We thank you for your time and efforts and look forward to hearing from you at your

earliest convenience.

Sincerely yours,

Michał Bystrzejewski, corresponding author

Cover Letter

*Graphical Abstract

Oxidation in nitric acid increases twofold the sorption efficiency of gold onto carbon

nanotubes and carbon-encapsulated iron nanoparticles

The increase of sorption efficiency in carbon encapsulates is not cause by the galvanic

exchange reaction between Fe0 and AuCl4

-

Carbon encapsulates have high sorption efficiency (>90%) for wide range of adsorptive

concentration

*Highlights

Enhancing the efficiency of Au adsorption onto activated carbon

and carbon nanomaterials

M. Bystrzejewski* and K. Pyrzyńska

Warsaw University, Department of Chemistry, Pasteur 1 str., 02-093 Warsaw, Poland

Abstract

Sorption of gold from acidic aqueous solutions was studied onto a series of non-oxidized and

oxidized carbon materials (activated carbon, carbon nanotubes, carbon-encapsulated iron

nanoparticles and carbon black). The studied sorbents differed with graphitization degree,

porosity and morphology. In the case of non-oxidized carbon materials the maximum sorption

efficiency (74%) was found for activated carbon, whilst graphitized nanomaterials (i.e. carbon

nanotubes and carbon encapsulates) were able to adsorb 42-45% of the gold ions that were

present in the solution. The oxidation in nitric acid significantly changed the sorption

efficiencies. The uptake of gold increased two times (to 91-92%) for oxidized carbon

nanotubes and carbon-encapsulates. The same oxidation procedure applied to activated carbon

and carbon black moderately enhanced the sorption efficiency to 88% and 55%, respectively.

The observed substantially distinct gold uptakes were discussed in the frames of textural

properties, morphology and graphitization degree. Moreover, the possibility of the galvanic

exchange reaction between AuCl4- and metallic Fe in the carbon encapsulates core was

evaluated.

Keywords: Gold adsorption, carbon-encapsulated magnetic nanoparticles, carbon nanotubes,

activated carbon, carbon black, precious metals

* Corresponding author. Fax: + 48 22 822 59 96. E-mail address: mibys@chem.uw.edu.pl (M. Bystrzejewski)

*ManuscriptClick here to view linked References

2

1. Introduction

Carbon materials are widely used as sorbents for various applications that include separation

and remediation processes. The surface of carbon materials can be modified in a controlled

way in order to meet the criteria required for specific applications [1]. As for example, typical

non-polar hydrocarbons are preferentially adsorbed onto carbon materials having well

developed surface areas and large amounts of micropores with narrow diameter distribution

[1]. Sorption of ionized species, e.g. heavy metal cations, is usually favored when the surface

of carbon sorbent starts to carry the negative charge, e.g. when the surface functional groups

undergo deprotonation [2]. Precious metals can be also sorbed onto carbon materials, however

their mechanism of sorption is more complex in comparison to the mentioned route involving

electrostatic binding of metal ions. As for example, several works clearly proved that sorption

of the most common precious metals (Au, Pt and Pd) occurs through spontaneous

chemisorption with simultaneous deposition of the zero-valent metal clusters onto the surface

of carbon material [3-5]. These metal cations have sufficiently high red-ox potential and

oxidize the surface of a carbon sorbent. This effect results in generating the new surface

oxygen-containing groups, as it was demonstrated by XPS studies [5].

This work is motivated by recent studies showing that metal crystallites (Au, Pt) can be

spontaneously deposited onto various carbon materials (carbon nanotubes, activated carbon

fibers, and magnetic carbon encapsulates) during contact with the metal salt solution [6-8]. It

was shown that even very short reaction times (5 minutes) resulted in complete removal of

metal ions from the solution [7]. Herein, a more-in-depth and comparative study is presented,

in which four various carbon sorbents are investigated, i.e. activated carbon (AC), multi-wall

carbon nanotubes (CNTs), carbon-encapsulated iron nanoparticles (CEINs) and carbon black

(CB). The studied carbon materials have different morphological, textural and structural

features. The aim of this work is to compare the efficiency of solid phase extraction of gold

onto various sorbents, and evaluate the parameters, which are the most crucial for sorption

3

process of Au ions. A substantial attention in this paper is focused on magnetic carbon

encapsulates, due to their inherent mobility and ability for remote control by external

magnetic fields. Recent work have shown that CEINs have a great potential and applicability

in removal of heavy metals [11-13] and organic molecules [14, 15].

2. Experimental

Activated carbon and multi-wall carbon nanotubes were purchased from Sigma Aldrich

and used as received. Carbon black N-330 was donated by the Research Institute of Rubber

Industry (Piastów, Poland). Carbon-encapsulated iron nanoparticles were synthesized by a

carbon arc route and subjected to the purification procedure to remove the non-encapsulated

nanoparticles [16]. The morphology of carbon sorbents was studied by scanning electron

microscopy (Zeiss Leo 1530). Raman spectra were acquired using a 532 Ar excitation laser

with 2 cm-1

resolution. Nitrogen adsorption studies were conducted on ASAP 20120 analyzer

(the samples were outgassed at 200°C under vacuum prior to the measurements). The sorption

studies were conducted on non-oxidized and surface oxidized carbon sorbents. 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 a titration method. To estimate the sorption efficiency

of gold ions, 50 mg of carbon material was added to 10 ml solution of HAuCl4 (initial

concentration 30 mg/L with respect to Au), with subsequent vigorous shaking for 5 minutes.

When the sorption was completed, the sorbents were filtered off by centrifugation and the

concentration of AuCl4- ion in solution in the supernatant was evaluated by atomic absorption

spectroscopy.

3. Results and discussion

3.1 Morphological, structural and textural features of carbon sorbents

The studied carbon sorbents have distinct morphological features (Figure 1). Activated carbon

comprises of relatively large grains (hundreds of microns, images not shown), which have a

4

well-developed pore structure with a narrow diameter distribution (70-100 nm in diameter).

Carbon nanotubes are several µm in length, and their diameters are in the range between 50

and 100 nm. Carbon encapsulates consist of oval nanoparticles with sizes below 100 nm.

CEINs have typical core-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).

Oxidation in nitric acid has not changed the morphological features (especially the size

distribution and shapes) of the studied carbon sorbents.

As shown in the Introduction the sorption of AuCl4- anions causes oxidation of the

surface of carbon material. The affinity for oxidation should primarily depend on the

graphitization degree, i.e. the particles comprised of larger and well crystallized graphene

layers should have higher resistance to oxidation. In fact, this effect was demonstrated many

times e.g. in thermal oxidation in oxygen of carbon materials. The burning temperature was

downshifted for materials of low graphitization degree [17]. The graphitization degree can be

facilely evaluated from the first order Raman spectrum, as the intensity ratio between two

bands, the so-called G and D bands [18]. The first spectral feature is located at ca. 1580 cm-1

and its intensity increases for highly graphitic carbons. The D band, which appears at lower

wavenumbers, is associated with structural and topological disorder. The G/D values are

listed in Table 1 (Raman spectra are shown in Figures S2-S5). Activated carbon and carbon

black have the lowest G/D ratios, because their particles are comprised of small graphene

layers with very short ordering (a few nanometers). The G/D value for carbon encapsulates is

ca. 3 times higher and is typical for carbon materials having medium graphitic structure (the

graphene layers are larger, however they possess defects that introduce a curvature). The

highest graphitization degree is found for carbon nanotubes. Oxidation in nitric acid changes

the G/D ratios (Table 2). For the case of activated carbon the acid treatment increases slightly

the graphitization degree. Generally, the processing of carbon materials in oxidizing agents

5

destroys the external graphene layers and therefore decreases the overall graphitization

degree. The AC sample exhibit contradictory behavior and it might be deduced that nitric acid

heals structural defects in activated carbon and increases the graphitization. Nevertheless, the

improved graphization results from irreversible etching and elimination of the surface carbon

atoms, which are characterized by the largest surface heterogenicity (i.e. those which form the

micropores) and therefore are the most pronounced to the reaction with nitric acid (this effect

is also accompanied by the loss of surface area, see the discussion below). Carbon black

follow does not change the surface area after oxidation and the variation in the G/D ratio

remains unchanged (within the experimental error). The nitric acid treatment of carbon

encapsulates and carbon nanotubes causes a substantial decrease of graphitization (the biggest

drop is observed in CNTs, in which the G/D ratio diminishes 6 times). The observed changes

point to partial amorphization of external graphene layers. The oxidization by nitric acid also

introduces surface acidic groups [19]. The studied carbon sorbents have comparable values of

surface acidity, which is expressed as the total amount of acidic groups (Table 2). This

finding shows that the applied oxidation procedure results in similar functionalization yield

for all studied carbon materials, i.e. the functionalization yield does not depend on the

morphology and graphitization degree. The initial carbon samples were also subjected to the

titration evaluation of the surface acidity. The values of total surface acidity were between

0.30 and 0.50 mmol/g.

The studied carbon materials have very distinct textural properties. The values of

surface area evaluated from the BET equation and the micropore volume are given in Table 1

(nitrogen adsorption and desorption isotherms are shown in Figures S6-S13). Activated

carbon is typical microporous material with high contribution of the micropores (49% of the

total pore volume). Carbon nanotubes have the lowest surface area and pore volume. This is

an effect of their high graphitization degree. Interestingly, although the distinct crystallinities,

carbon encapsulates and carbon black have comparable values of surface area and total pore

6

volume. This finding may be explained on the basis of similarities in their morphological

features – both materials are composed of uniform spherical-like particles that have smooth

surfaces (see Figure S1). The very low amounts of micropores in CNTs, CEINs and CB stem

from cavities that exist when individual grains form larger agglomerates. Interestingly, the

adsorption-desorption isotherms of AC and CEIN samples are characterized by a hysteresis,

which is absent for other studied materials. The occurrence of the hysteresis results from the

capillary condensation effect and indicates that activated carbon and carbon encapsulates

contains some amounts of mesopores [20]. Oxidation in nitric acid changes the textural

properties of all investigated 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. The worsening of the textural parameters of microporous carbon is a well know

phenomena [21]. The carbon atoms that form the micropores have significantly larger

structural disorder (in comparison to other aromatic graphene layers, e.g. in the core of a AC

grain) and undergo favorable etching during nitric acid oxidation. In fact, the observed

decrease of the amounts of micropores is in good agreement with the increase of the

graphitization degree (the G/D ratio increases of 10% after oxidation). The surface area and

total pore volume in carbon nanotubes increase more than 10-fold after oxidation (this

phenomena results from amorphization of external graphene layers, see also the decrease of

the G/D ratio). Carbon encapsulates and carbon black follow the same trend as carbon

nanotubes and also increase their porosity.

3.2 Gold adsorption studies

The non-oxidized carbon materials have distinct efficiency for Au sorption, which varies

between 42 and 74% (Figure 2) and changes in the following order: AC>CB>CNTs≈CEINs.

To explain this trend one has to take into account the factors that may accelerate or inhibit the

sorption rate between the AuCl4- ion and the surface of the carbon material. The samples of

higher graphitization should be less pronounced for oxidation by AuCl4- (this reaction can be

7

regarded as the analogue of combustion, in which carbons of lower graphitization start to burn

at lower temperatures). In fact, CNTs and CEINs are substantially better graphitized than AC

and CB, and therefore they have higher resistance to oxidation and are more resistant to

AuCl4- ions attack (see G/D values in Table 1). Activated carbon and carbon black are

materials of similar and low crystallinity (Table 1), however, the sorption efficiency for the

AC sample is ca. 1.5 times higher in comparison with the CB material. This observation can

be explained by the differences in porous structure, i.e. activated carbon is a sample with

well-developed micropores, whilst carbon black is practically non-porous material. The

micropores are more pronounced for oxidation by AuCl4- anions than the poorly graphitized

aromatic graphene layers. The affinity for oxidation of the micropores has been shown above,

i.e. the treatment in nitric acid decreases the micropore volume. Carbon nanotubes and carbon

encapsulates have very low content of micropores and these carbon materials are substantially

better graphitized than activated carbon and carbon black (and therefore more resistant to

AuCl4- attack).

The above findings show that the uptake of gold in non-oxidized carbon primarily

depends on their porous structure and graphitization degree, whilst the morphological features

seem to have no effect. It is especially seen for carbon black and magnetic carbon

encapsulates, which form the agglomerates consisted of uniform spherical nanoparticles with

diameters well below 100 nm (Figure 1). These sorbent have nearly the same morphological

characteristics and the gold is preferentially adsorbed onto the material of lower

graphitization, i.e. carbon black. The values of Au uptake onto CEINs and CNTs are

comparable (different morphology, similar graphitization) and this again shows that

morphology does not influence the sorption efficiency.

Next, the sorption efficiency of Au was studied onto oxidized carbon sorbents. It is well

known, that the treatment of solid carbons by nitric acid results in formation of surface acidic

groups, primarily carboxylic, phenolic and lactonic [19]. The sorption of Au is expected to be

8

greater for the case of oxidized sorbents, because the AuCl4- anions should form strong

complexes with positively charged surface acidic groups. However, it has been demonstrated

that typical surface acidic groups do not carry the hydronium ions even at very low pH values

[22]. Nevertheless, the sorption efficiency increases notably after oxidation (Figure 2). The

sorption enhancement is the most pronounced in CNTs and CEINs, in which the Au uptake

rises more than two times and exceeds the performance of activated carbon and carbon black.

The relative sorption enhancement (RSE) has been calculated and referred to the global

surface acidity to verify, whether the observed increase of sorption efficiency has any

correlation with the amount of surface acidic groups. The formula for the RSE is as follows:

%100

oxnon

oxnonox

E

EERSE , where Eox and Enon-ox refer to the uptake of gold onto oxidized

and non-oxidized sorbent, respectively. Figure 3 shows that the RSE parameter linearly

changes with the global surface acidity for CEINs, CNTs and CB. This clearly evidences that

the surface acidic groups readily favor the uptake of gold in non-porous carbon sorbents. The

tetrachlorogold anions have high red-ox potential and are able e.g. to gasify the surface

carboxylic groups:COOH + Au3+

→ Au0 + H

+ + CO2↑, whereCOOH denotes the

carboxylic groups covalently bound to the carbon surface. A substantial decrease of

carboxylic groups after gold deposition was showed for carbon fibers treated in nitric acid

[23] and the same reaction may occur on oxidized carbon encapsulates, carbon black and

carbon nanotubes. Activated carbon shows a different behavior and does not follow the linear

trend shown in Figure 3. This observation demonstrates, that the acidic groups that had been

introduced after oxidation have lower activity in comparison to the acidic groups formed onto

non-porous sorbents. The reason of such difference is likely due to the steric effects. The

acidic groups in activated carbon are located within the meso- and micropores [24], whilst

what causes the diffusion resistance for the AuCl4- ions (please notice that the acidic groups in

non-porous sorbents are placed directly onto a surface of the sorbent particle). Therefore, this

9

finding explains why the observed sorption enhancement for activated carbon is lower in

comparison to the studied non-porous carbon sorbents.

Carbon-encapsulated iron nanoparticles have the best sorption performance in solid

phase extraction of gold. It is of great importance to check the sorption characteristics of

CEINs under distinct experimental conditions. Therefore, additional experiments were

conducted with various starting concentrations of AuCl4- (50 mg of oxidized CEINs, pH=2, 10

ml solution of HAuCl4, contact time 5 minutes). The largest sorption rates (above 90%) are

achieved for moderate concentrations, i.e. those between 20 and 40 mg/L (Figure 4). The

uptake of gold at the lowest concentration is smaller, because the adsorptive has lower

activity and statistically has lower chance to contact the sorbent. The higher concentrations

diminish the sorption efficiency and this is caused to the limited number of active sites

accessible to the adsorptive.

3.3 Mechanism of gold adsorption onto carbon-encapsulated iron nanoparticles

The results presented above show that the surface acidic groups significantly enhance

the sorption efficiency of gold. The largest enhancement is observed for carbon encapsulates.

One has to notice that the sorption improvement may be not only caused by the surface

effects. According to the thermodynamic predictions Fe (which forms the cores in CEINs) can

be oxidized by AuCl4- ions. However, the access to the core is hampered, because the carbon

coating is generally impermeable to all constituents of the surrounding solution, including the

AuCl4- ions either. In other words, to oxidize the metallic Fe core the gold anions should have

diffused through the carbon coating. The possibility to course that reaction can be monitored

by evaluating the amount of released Fe3+

to the solution (the concentration of Fe were

measured by atomic absorption spectroscopy). The sorption of gold onto non-oxidized and

oxidized CEINs expels 45 µg and 32 µg Fe, respectively (Figure 5). The reference test

(without AuCl4-) conducted onto oxidized CEINs at acidic conditions (pH=2) leads to release

35 µg Fe. The amounts of Fe cations released in the reference test and during the sorption

10

onto oxidized CEINs are very similar. This finding points that AuCl4- does not pass through

the carbon coating. Moreover, the released amount of Fe is only a very small fraction of the

iron present in the CEIN core (less than 0.2 %). According to the presented discussion, the

gold anions oxidize the acidic groups (which are more pronounced for oxidation in

comparison to the solid carbon from the coating) and then the AuCl4- is transformed to zero-

valent particles. Thus, the released Fe originates from those encapsulates in which the

coatings are semipermeable, i.e. they can let in the hydronium ions which oxidize the metallic

Fe to the ionic form (the schematic pathways are shown in Figure 6). However, this

explanation does not make clear the response from the non-oxidized carbon encapsulates. In

the case of non-oxidized CEINs, in which the amount of surface acidic groups is at very low

level, the AuCl4- ions likely etch the carbon coating, destroys it and make it perforated. The

perforation obviously facilitates diffusion of hydronium ions into the core and leads to release

larger amounts of Fe (42 µg vs. 32 µg for the oxidized CEINs). The potential of both

oxidizing agents (i.e. AuCl4- and H

+) in leaching the metallic Fe from the CEIN core has been

also demonstrated in the as-obtained CEIN product. The as-obtained CEINs always contain

defected encapsulates, which have non-continuous carbon coating and therefore the metallic

core is not properly protected and is easily accessible to oxidizing ions (see Figure S14 in

Supplementary Data). The experiment conducted on the defected CEINs released the largest

amount of Fe (609 µg, Figure 5) and removed 94% of Au from the solution. This result shows

the importance of high quality coating and its role in protecting the core against the attack of

various corrosion agents. Two next tests have been performed to strengthen the finding that

the AuCl4- does not diffuse through the coating. PtCl6

2- and PdCl6

2- ions were chosen as the

oxidizing agents, because they have lower (+0,72 V) and higher (+1,29 V) red-ox potential in

comparison with the AuCl4- anion (+0,93 V). These metals are adsorbed with high efficiency

exceeding, in each case, 90%. The amount of Fe released after sorption changes in the

following order Pt≈Au(≈HCl)<<Pd and it agrees with the ascending sequence of the red-ox

11

potentials. The comparable Fe amounts expelled in tests with Au and Pt point to the proposed

lack of a direct contact between the CEIN core and the metal complex ion. The case of

sorption of Pd strongly suggests that due to the high oxidation potential, the PdCl62-

ions

oxidize and make the carbon coating perforated. These results bring a new light on the recent

view of the Stark group [7]. They studied sorption of gold at acidic conditions (pH=2.1) onto

carbon-encapsulated cobalt nanoparticles and found that cobalt ions are released after

sorption. They postulated the surface redox mechanism with diffusion of gold anions through

the carbon coating. Our results argue to a different pathway, i.e. the coating can be oxidized

and perforated during the sorption, and in the subsequent steps the hydronium ions may

diffuse to the core and start to leach it. Moreover, in the case of carbon encapsulates having

well developed coating the core material does not influence the sorption of gold and other

precious metals.

4. Conclusions

The investigation on sorption of gold from aqueous acidic solutions onto four carbon

materials (activated carbon, carbon black, carbon nantubes and carbon-encpasulated iron

nanoparticles) was studied. The carbon materials had different morphology, graphitization

degree and textural properties. In the case of non-oxidized sorbents were preferentially

adsorbed onto porous and low graphitized carbon materials. The maximum uptake of Au was

found for activated carbon (74%), whilst studied carbon nanomaterials were able to remove

42-45% of Au. The oxidation in nitric acid introduced surface acidic groups, altered the

porosity, whilst morphology features were not changed. The increased removal of gold was

observed in all oxidized sorbents. The greatest enhancement of sorption efficiency was found

in carbon encapsulates and carbon nanotubes. The oxidized activated carbon had worse

performance, because of the diffusion resistance which appeared after nitric acid treatment.

The possibility of galvanic exchange reaction between AuCl4-

and Fe0 from the core of

12

carbon-encpasulated iron nanoparticles was excluded. The precious metal chlorocomplex ions

did not pass through the encapsulate coating. They oxidized the surface acidic groups or

external graphene layers from the coating. Nevertheless, this process did not alter the stability

of encapsulated Fe crystallites, which were still protected from the external environment.

Acknowledgements. This work was supported by the Ministry of Science and Education

through the Department of Chemistry, Warsaw University under Grants N N204 132137.

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15

Table 1. Structural and textural properties of non-oxidized carbon materials.

Sorbent G/D SBET (m2 g

-1) Total pore volume

(cm3/g)

Micropore volume

(cm3/g)

Activated

carbon

0,41 642 0,52 0,27

Carbon

nanotubes

7,72 15 0,03 0,01

Carbon

encapsulates

1,14 77 0, 32 0,02

Carbon black 0,40 78 0,44 0,02

16

Table 2. Structural and textural properties of surface oxidized carbon materials.

Sorbent G/D SBET

(m2/g)

Total pore volume

(cm3/g)

Micropore volume

(cm3/g)

Surface acidity

(mmol/g)

Activated

carbon

0,45 561 0,47 0,23 2,77

Carbon

nanotubes

1,27 162 0,38 0,04 2,81

Carbon

encapsulates

0,95 95 0,34 0,02 2,90

Carbon black 0,39 78 0,35 0,02 2,45

17

18

Figure 1. SEM images of carbon sorbents: activated carbon (a), carbon nanotubes (b), and

carbon-encapsulated iron nanoparticles (c) and carbon black (d).

1 2 3 40

20

40

60

80

10092%

CB

CEIN

s

CN

TsA

C

Au

so

rpti

on

ra

te (

%)

Non-oxidized

Oxidized

Figure 2. Au sorption rate onto non-oxidized and oxidized carbon sorbents (sorbent weight 50

mg, solution volume 10 ml, pH=2).

19

2,40 2,55 2,70 2,85 3,00

0

20

40

60

80

100

120

Surface acidity (mmol/g)

AC

CEINs

CNTs

CBRel. s

orp

t. e

nh

an

c. (%

)

Figure 3. Dependence of relative sorption enhancement of Au and surface acidity.

20

0 20 40 60 80 100

60

65

70

75

80

85

90

95

Au

so

rpti

on

ra

te (

%)

Initial concentration (mg/L)

Figure 4. Influence of initial concentration of AuCl4- on Au sorption rate onto oxidized CEINs

(sorbent weight 50 mg, solution volume 10 ml, pH=2).

21

1 2 3 4 5 60

50

100

600

605

610

CEINs CEINs-ox

CE

INs

-de

fec

ted

Au

Pd

PtHClAu

Au

Am

oun

to

fFe

rele

ased

(mg)

45 32 35 27 108

609

Figure 5. Amount of Fe released during sorption of various precious metals onto oxidized,

non-oxidized and defected CEINs.

22

Figure 6. Possible routes of Fe release in surface oxidized (a) and non-oxidized carbon-

encapsulated iron nanoparticles.

Supplementary MaterialClick here to download Supplementary Material: Suppl Mater.doc