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The investigation of electroreduction of AuCl4- in the case of gold electrosorption using activated carbon
Materials Today: Proceedings xxx (xxxx) xxx
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Materials Today: Proceedings journal homepage: www.elsevier.com/locate/matpr
The investigation of electroreduction of AuCl4 in the case of gold electrosorption using activated carbon Zh. Supiyeva a,c,⇑, Kh. Avchukir b,c, V. Pavlenko c, M. Yeleuov a,d, A. Taurbekov a, G. Smagulova a,c, Z. Mansurov a,c a
Institute of Combustion Problems, 172, Bogenbai Batyr str., Almaty, Kazakhstan Center of Physical Chemical Methods of Research and Analysis, 96a, Tole bi str., Almaty, Kazakhstan al-Farabi Kazakh National University, 71, al-Farabi ave., Almaty, Kazakhstan d Satpayev University, 22A, Satpayev str., Almaty, Kazakhstan b c
a r t i c l e
i n f o
Article history: Received 4 September 2019 Received in revised form 30 October 2019 Accepted 2 November 2019 Available online xxxx Keywords: Gold Electroreduction Adsorption Activated carbon Rice husk
a b s t r a c t
This paper reports a study of the kinetics of the initial stages of electroreduction of AuCl4 in the case of gold electrosorption using activated carbon. The activated carbon was synthesized from rice husk by the chemical activation. Obtained sorption materials were characterized by Raman Spectroscopy, Transmission electron microscopy, Scanning electron microscopy and SORBTOMETR-M techniques. The kinetics of electrodeposition of gold (III) on platinum from chloride electrolytes by the cyclic voltammetry and chronoamperometry methods have been studied. Chronoamperometric measurements were performed at the potential of +0.2 V vs. Ag/AgCl with varying gold concentration and temperature. Diffusion coefficient of Au3+ ions determined by the cyclic voltammetry method on the basis of the Randles-Ševcˇik equation is in good agreement with the value determined by the chronoamperometry using the Cottrell law. Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the 7th International Conference on Nanomaterials and Advanced Energy Storage Systems.
1. Introduction Activated carbon is a carbon-containing adsorbent having a developed porous structure and large surface area. Activated carbon can be produced from nearly any carbon-containing material by physical or chemical activation methods [1–6]. The authors of this study selected rice husk (RH) as a raw material for preparation of activated carbon, subsequently used as an adsorbent. RH is a renewable, low-cost agricultural waste, which is widely spread in south Kazakhstan. The sorption material obtained from the RH has a wide field of application. Some Kazakhstan scientists investigate RH from different regions of Kazakhstan using it mainly as an adsorbent for waste water treatment and similar purposes [7–12], production of technical silicon [13] or for the supercapacitors [14,15]. According to the literature data [16], it was claimed that the series of adsorbents based on carbonized RH (CRH) have a rather ⇑ Corresponding author at: Institute of Combustion Problems, 172, Bogenbai Batyr str., Almaty, Kazakhstan. E-mail address: [email protected] (Zh. Supiyeva).
low redox potential and the stationary potential is in 0.229 V (Ag/AgCl). The measured stationary (real) potential of [AuCl4 ] in hydrochloric acid medium is equal to 0.760 V (Ag/AgCl). The potential difference between gold – oxidizing agent and sorbent – reducer is 0.531 V. For complete procedure (99.9%) of any redox reaction, a potential difference of 0.3 V is required [17]. From these data, it can be concluded that there is a real possibility for reducing Au3+ to a metallic state. This possibility is confirmed by electron microscopic images [16]. It follows that the sorbents of CRH possess reducing properties, due to the presence of reducing groups such as carboxyl, phenolic, hydroxyl, amine on the surface of these carbon materials [18]. Carbonized adsorbents are not only ion-exchanged [16], but also oxidation-reduction adsorbents. It follows that the process of metallic gold extraction and oxidation of reduction groups of the sorbent is electrochemical, i.e. there are cathode and anode sites. Cathode sites on which their further proceeds reduction of gold (III) are formed at the initial moment of sorption. The chemistry of Au3+ sorption on CRH can be represented as:
Cathodic process : AuCl4 þ 3 e ! Au0 þ 4Cl
ð1Þ
https://doi.org/10.1016/j.matpr.2019.11.013 2214-7853/Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the 7th International Conference on Nanomaterials and Advanced Energy Storage Systems.
Please cite this article as: Z. Supiyeva, K. Avchukir, V. Pavlenko et al., The investigation of electroreduction of AuCl4 in the case of gold electrosorption using activated carbon, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.11.013
2
Zh. Supiyeva et al. / Materials Today: Proceedings xxx (xxxx) xxx
Anodic process: CRH—Red 3 ! CRH—Ox
ð2Þ
where Red and Ox –R-COOH; R-Ph; R-OH; R-NH2 and oxidized forms of carbon groups of the sorbent. Pesic and Storhok [19] investigated the kinetics of gold (III) bromide complex adsorption and observed that gold is accumulated in its original ionic form on the surface of activated carbon. This phenomenon is different from the gold (III) chloride complex behavior, where adsorption process is accompanied by the reduction of the ions [20]. In recent years, many works have been published on gold adsorption using various biosorbents [21]. However, the literature is still insufficient to cover this research area, and more work and studies are needed in this field to develop other locally available and economical adsorbents. In this research, results showing the possibility of activated carbon application in electroreduction of gold from aqueous solution are presented. The studies were performed with synthesized activated carbon based on RH. The influence of amount of activated carbon used, initial concentration of gold (III) chloride complex ions, temperature was investigated.
2. Experimental details 2.1. Synthesis of the porous carbon and their characterization The cleaned and dried RH was collected from local farms of Almaty region, Kazakhstan, and subjected for carbonization at 773 K in an argon atmosphere. Carbonized RH was mixed with potassium hydroxide by use the weight proportion of 1:5 (precursor to KOH) and activated at 1123 K under argon atmosphere. The resulting mixture was subjected to washing by distilled water until the neutral pH. The obtained adsorbents composed of amorphous silica and carbon. Specific surface of carbonized and activated RH was determined by BET method (Brunauer-Emmett-Teller); it was 2818 m2 g1, pore specific volume was 1.59 cm3 g1 and average pore size was within 1.0 2.0 nm. Measurements were carried out using SORBTOMETR-M device.
The characterization of the sorbents has been carried out by means of the methods such as Raman spectroscopy (NTEGRA Spectra Raman, laser wavelength is 473.0 nm, signal with an area diameter of 80 nm), scanning electron microscopy (SEM, Quanta 3D 200i Dual System, FEI) and the transmission electron microscopy (TEM, JEM-2100) with high stability of high voltage and beam current along with an excellent electro-optical system). 2.2. Electrochemical measurements and atomic absorption spectroscopy All electrochemical measurements were carried out in a threeelectrode cell using an Autolab PGSTAT 302 N galvanostat/potentiostat (Metrohm, the Netherlands). A working electrode with surface area of 0.071 cm2 was represented by the platinum rotating disk Autolab RDE 80725. A platinum plate was used as an auxiliary electrode, and the silver-chloride electrode (Ag/AgCl) was used as a reference electrode. 0.1 mol L1 potassium chloride solution containing HAuCl4 with a pH of 1.5 was served as an electrolyte. The gold chloride solutions (HAuCl4) with a gold concentration of 7.65; 23.13; 45.34 and 80.79 mg L1 were prepared by dissolution of metallic gold in aqua regia. In order to study the kinetics of gold (III) chloride complex adsorption the change in concentration of Au3+ was analyzed using the atomic absorption spectroscopy (Perkin Elmer AAnalyst 200, USA). Determination of the content of Au3+ in the solutions was conducted using a lamp with a wavelength of 242.8 nm.
3. Result and discussion In order to identify the surface morphological features of synthesized samples, the SEM has been used. Samples have a complex structure. They exhibit a characteristic macrostructure with diameter of 4–22 mm (Fig. 1a). Next, the TEM image of graphene obtained from activated RH is shown in Fig. 1b. Samples have few-layer graphene with defects and inclusions of an amorphous carbon component, but there are sections of layers without defects with a homogeneous surface structure.
Fig. 1. (a, b) SEM images of carbon from the rice husk; (c, d) TEM images of the graphene layers.
Please cite this article as: Z. Supiyeva, K. Avchukir, V. Pavlenko et al., The investigation of electroreduction of AuCl4 in the case of gold electrosorption using activated carbon, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.11.013
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Fig. 2. Raman spectra of activated carbon.
The typical Raman spectrum of graphene exhibits three peaks: peak D at 1351 cm1, peak G at 1580 cm1, and peak 2D at 2700 cm1. The ratio between the intensities of peak G (IG) and peak 2D (I2D), IG/I2D gives an estimate of the number of layers [22]. Analysis of Raman spectra (Fig. 2) of carbon material obtained from activated RH showed the content of graphene films with three or more layers (IG/I2D = 0.63; 0.50; 0.43; 0.30). The kinetics of electroreduction of gold on a platinum surface from chloride electrolytes by the cyclic voltammetry (CV) with variation of the scan rate in the range of 10–50 mV s1 has been studied. Chronoamperometric (CA) measurements were performed at the potential of +0.2 V vs. Ag/AgCl with varying gold concentration and in the temperature range from 288 K to 308 K. Cyclic voltammograms were obtained in the study process of gold electroreduction on a platinum electrode from chloride solutions (Fig. 3). As can be seen from Fig. 3, the cathode peak corresponds to reaction (1), the anode peak corresponds to the oxidation of gold to Au3+. Complete electroreduction of gold ions proceeds at a potential greater than Ep (c), and electrooxidation at a more positive potential from 800 mV. An increase in the potential scan rate from 10 mV s1 to 50 mV s1 leads to an increase in the cathodic peak current density (ipc) and a shift in the recovery peak
potential (Ep(c)) of gold to the cathode region. The dependence of the values of the current density peaks on the square root of the p potential scan rate ( t) is linear (Fig. 3 Inset) and passes through the origin, which indicates the diffusion nature of the process. The shift in the peak potential of the gold recovery depends on the concentration of gold ions in the solution (Fig. 4). An increase in the gold concentration from 7.65 to 80.79 mg L1 at a potential scan rate of 10 mV s1 causes a shift in the potential of the peak of gold ion reduction on the Pt electrode by 300–400 mV. The use of electrolytes with a high concentration of the potential determining ions leads to an increase in the potential difference between the reduction of gold and electronegative impurities, such as Cu, Ag, etc. p The dependence of ipc on t presented below was calculated on the basis of the Randles-Ševcˇik equation, while the diffusion coefficient of Au3+ ions was calculated at various concentrations (Table 1). For reversible processes, the relationship between the current density of the electroreduction peak and the potential scan rate was described by the Randles-Ševcˇik equation [23].
F3 ip ¼ 0; 4463 RT
!1 ^ n3 A ^ AA ^ D1 A ^ CA ^ t1 A
ð3Þ
Fig. 3. Cyclic voltammograms of electroreduction of gold on a platinum electrode in the solution of 80.79 mg L1 HAuCl4 + 0.1 mol L1 KCl at various scan rates, T = 298 K; Inset: the dependence of the peak current density (ipc) related to the cathode processes on the square root of the potential scan rate.
Please cite this article as: Z. Supiyeva, K. Avchukir, V. Pavlenko et al., The investigation of electroreduction of AuCl4 in the case of gold electrosorption using activated carbon, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.11.013
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Fig. 4. Cyclic voltammograms of gold at 298 K on a Pt electrode in 0.1 mol L1 KCl solution with different HAuCl4 contents at a sweep scan rate of 10 mV s1.
Table 1 The values of the diffusion coefficient of gold ions during electroreduction of gold on the platinum electrode in 0.1 mol L1 KCl solution. Method
C AuCl4 , mg L1 D AuCl4 , 1014 cm2 s1
CV 7.65 0.15
23.13 1.39
45.34 5.38
80.79 17.06
where ip is the cathode current of the peak; n is the number of electrons involved in the electrode reaction; A is the area of the electrode, cm2; F is the Faraday constant, Pendant mole1; D is the diffusion coefficient of Au3+ ions, cm2 s1; C is the volume concentration of Au3+, mol cm3; m is the potential scan rate, V s1; R is the gas constant, J mol1 K1; T is the temperature, K. The diffusion coefficient calculated for aqueous gold solutions is shown in Table 1. It is observed that with an increase in the concentration of gold, the diffusion coefficient increases. The electrochemical reduction of gold ions occurring on the surface of the carbon adsorbent obtained from RH is limited by diffusion, and this proves the presence of the Randles-Ševcˇik dependence. The electrochemical stage, i.e. the electron transfer
process proceeds quickly, and it should be assumed that the charge transfer rate constant is several orders of magnitude higher than the mass transfer constant. Next, chronoamperograms of the electroreduction of gold ions on a Pt electrode at various potentials were obtained (Fig. 5). As can be seen from the Fig. 5, gold electroreduction at the potential of 0.6 V does not occur, it proceeds with a limiting diffusion current at potentials of 0.4; 0.2 V. Two regions are clearly visible from the current transient at a constant potential. The region of decrease in the cathode current is due to the diffusion limitation of the process, which is described by the Cottrell law, which is characteristic of non-stationary linear diffusion processes. The limiting current region is described by the following equation [24].
ik ðt Þ ¼ id ðt Þ ¼ nFC Au3þ
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi DAu3þ =pt
ð4Þ
where DAu3þ – the diffusion coefficient, cm2 s1; t – time, s. In this region, the effective thickness of the diffusion layer is established, which is constant in time. It is shown from the equation that the value of the limiting current of electrodeposition of gold ions at a constant value of the diffusion layer thickness depends only on the concentration of gold in the volume of the
Fig. 5. Chronoamperograms of gold electroreduction in the solution of 80.79 mg L1 HAuCl4 (pH = 1.5) with the addition of sorbent from activated RH at different potentials on a platinum electrode.
Please cite this article as: Z. Supiyeva, K. Avchukir, V. Pavlenko et al., The investigation of electroreduction of AuCl4 in the case of gold electrosorption using activated carbon, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.11.013
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Fig. 6. (a) Chronoamperograms of gold electroreduction in the solution of 28.98 mg L1 HAuCl4 (pH = 1.5) at different temperatures with the addition of carbon adsorbent; (b) Gold adsorption capacity (mg g1) as a function of duration of the contact with activated carbon.
Table 2 Gold recovery at different temperatures. T, K The adsorption percentage, % The adsorption capacity q, mg g1
288 98.92 143
298 95.99 139
308 97.95 142
solution. During electrosorption of gold, there is a decrease in the concentration of gold in the volume of the solution, which can be determined directly by measuring the limiting current. This determines the effectiveness of chronoamperometry in studying the kinetics of the gold sorption process. Kinetic curves (I-t) of Au3+ adsorption were obtained at the constant potential +200 mV corresponding to the limiting current of Au3+ ion reduction on a platinum electrode with varying potential-determining ions concentration and temperature (Fig. 6a). The completeness of the gold electroreduction reaction on a platinum electrode during chronoamperometric measurements was ensured by applying a potential of +200 mV to the electrode and a disk electrode rotation rate of 1000 rpm. 1.5 g of sorbent was introduced into the electrolytic cell. As a result, the concentration of gold ions in the electrolyte decreased; there was a decrease in the limiting current of Au3+ reduction. The curve (It) had a falling form. The residual concentration of Au3+ ions in the solution was determined using a flame atomic absorption spectrometer Perkin Elmer AAnalyst 200 (Fig. 6b). As a result, the adsorbent obtained from RH has a sorption degree of 95–98%, a sorption capacity of 139–143 mg g1 at a temperature in the range of 288–308 K (Table 2). Activated carbon from RH was wrapped with filter paper and then immersed in a thermostatically controlled electrochemical cell with a constant convection. Convection in the cell was provided by a rotating disk electrode. Fig. 6a shows that the limiting diffusion currents of the electroreduction of gold ions decrease with time, which indicates a decrease in the concentration of gold ions into the depth of the solution afterwards by its electrosorption by the adsorbent. This trend persists at temperatures from 288 to 308 K. The exception is the chronoamperogram at temperatures of 308 K with a steep slope. It is also clear that the limiting currents at the initial point are directly proportional to temperature, which once again emphasizes the diffusion nature of the process. It can be seen from Fig. 6a that the slope angles linearly depend weakly on temperature with the exception at 308 K. The linearity of the dependence of current on time at all temperatures is explained by a constant convection rate, in other words, mass transfer of gold
ions. In this case, the supply of the potential of the determining ion to the surface of activated carbon is limited with the convective flow rate (angular velocity of rotation of the disk electrode). At the end of the experiment, it was revealed that the electrosorption of gold ions occurred only on the surface layer of the wrapped sorbent in filter paper. Free immersion of the sorbent would provide a strong difference in the concentration of gold ions in the electrolyte, which can be recorded by the method of chronoamperometry, however, the contact of the sorbent with the indicator electrode creates noise of the recorded signal (limiting diffusion current). Therefore, the use of a chronoamperogram for free immersion of the sorbent is difficult, and it requires special organization of the experiment to study the kinetics of sorption. In addition to chronoamperometric measurements, an atomic absorption analysis of the concentration of gold ions after their adsorption was performed. In this case, the sorbent was immersed in the solution in the form of powder and the solution was stirred with a magnetic stirrer at a constant rate of 200 rpm. The adsorption curves representing the concentration of gold ions plotted versus time are shown in Fig. 6b. From the Fig. 6b it can be seen that the change in the concentration of gold decreases exponentially with time (up to 40 min). Further adsorption of gold ions proceeds with a limiting velocity by a characteristic linear dependence. The form of the curve representing the concentration plotted as a function of time weakly depends on time. This once again confirms the statement made earlier about the diffusion nature of this process, since the diffusion coefficient depends only slightly on temperature than the charge transfer rate constant (or the constant of the electrochemical reaction). From this we can conclude that the reaction (1) is reversible. The SEM and optical images clearly demonstrate that a gold chloride is reduced to metallic gold and deposited on the surface of activated carbon. The smallest gold nanoparticles that could be revealed were 70–200 nm in diameter on the surface of activated RH (Fig. 7a–d). The morphology of activated RH examined using SEM and BET analysis revealed that, the adsorbent represented by carbonized and activated RH is highly porous. In total, the outstanding characteristics of the resulting activated carbon such as an BET specific surface area equal to 2818 m2 g1 and the total pore volume of 1.59 cm3 g1, along with a high adsorption degree of gold ions equal to 95–98% and sorption capacity of 139–143 mg g1, allow to declare this material as an effective adsorbent applicable for extraction of noble metals.
Please cite this article as: Z. Supiyeva, K. Avchukir, V. Pavlenko et al., The investigation of electroreduction of AuCl4 in the case of gold electrosorption using activated carbon, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.11.013
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Fig. 7. SEM images of the surface of the carbon electrode after adsorption of gold at different magnifications of (a) 1 mm, (b) 2 mm; (c) optical surface image and (d) elemental analysis.
4. Conclusion
References
In this study, the features of gold electroreduction on the activated carbon produced from carbonized and activated RH were shown. The kinetics of electroreduction of gold ions investigated by means of cyclic voltammetry and chronoamperometry showed several important trends which haven’t been adequately described before this study by other authors. It is revealed that the electrochemical reduction of gold ions occurring on the surface of the carbon sorbent is limited by diffusion, and this proves the presence of the Randles-Ševcˇik dependence. In this regard, the electrochemical stage, i.e. the electron transfer process proceeds quickly. Meanwhile, the charge transfer rate constant was several orders of magnitude higher than the mass transfer constant. Moreover, the optimal conditions for the process of gold adsorption on a carbon adsorbent were established. Evaluation of the features of reaction mechanism between Au3+ chloride complexes and activated carbon which was performed within the study, allows to control the processes of electroreduction of gold ions more precisely and to enhance the overall efficiency of this method.
[1] M. Streat et al., Ion Exchange and Sorption Process in Hydrometallurgy, John Wiley, Chichester, 1987. [2] R.C. Bansal et al., Active Carbon, Marcel Dekker Inc., New York, 1988. [3] F. Derbyshire et al., Porosity in Carbons, Edward Arnold, London, 1995. [4] M.J.B. Evans et al., Carbon 37 (1999) 269–274. [5] J. Hayashi et al., Carbon 40 (2002) 2381–2386. [6] R. Zanzivet al., in: 6th World Congress of Chemical Engineering, Melbourne, 2001, p. 37. [7] K. Kudaybergenov et al., J. Non-Cryst. Solids 358 (2012) 2964–2969. [8] Z.R. Ismagilov et al., Catal. Today 147 (2009) 58–65. [9] S. Azat et al., Adv. Mater. Res. 535–537 (2012) 1041–1045. [10] A. Merkel et al., Eurasian Chem. Technol. J. 18 (2016) 299–304. [11] K. Ongarbayev Ye et al., Eurasian Chem. Technol. J. 17 (2015) 41–45. [12] S.V. Yefremova et al., Solid Fuel Chem. 50 (2016) 52–57. [13] Z. Issagulov et al., Metalurgija 53 (2014) 685–688. [14] Z.A. Mansurov, World Scientific and Engineering Congress ‘‘Future Energy: innovative scenarios and methods of their implementation”, Astana (2017) 67–75. [15] V.V. Pavlenko et al., Eurasian Chem. Technol. J. 20 (2018) 99–105. [16] Z.A. Mansurov, Carbon Nanostructured Materials Based on Plant Materials, alFarabi Kazakh University, Almaty, 2010, p. 310. [17] O.A. Songina, V.A. Zakharov, Amperometric Titration, Chemistry, Moscow, 1979, p. 303. [18] Z.A. Mansurov et al., Chem. Sustain. Dev. 10 (2002) 339–346. [19] W. Davison et al., Discuss. Faraday Soc. 56 (1973) 171. [20] J.A. Harrison et al., J. EIectroanal. Chem. 53 (1974) 145. [21] I.R. Burrows et al., J. Electroanal. Chem. 58 (1975) 241. [22] Sitansu Sekhar Nanda et al., Trends Anal. Chem. 80 (2016) 125–131. [23] E. Jaeger, A. Zalkind, Measurement Methods in Electrochemistry, World, Moscow, 1977, p. 582. [24] Kh. Avchukir et al., Russ. J. Electrochem. 54 (2018) 1096–1103.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement Ministry of Education and Science of the Republic of Kazakhstan financially supported this work, Project No. AP05134691.
Please cite this article as: Z. Supiyeva, K. Avchukir, V. Pavlenko et al., The investigation of electroreduction of AuCl4 in the case of gold electrosorption using activated carbon, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.11.013
Contents lists available at ScienceDirect
Materials Today: Proceedings journal homepage: www.elsevier.com/locate/matpr
The investigation of electroreduction of AuCl4 in the case of gold electrosorption using activated carbon Zh. Supiyeva a,c,⇑, Kh. Avchukir b,c, V. Pavlenko c, M. Yeleuov a,d, A. Taurbekov a, G. Smagulova a,c, Z. Mansurov a,c a
Institute of Combustion Problems, 172, Bogenbai Batyr str., Almaty, Kazakhstan Center of Physical Chemical Methods of Research and Analysis, 96a, Tole bi str., Almaty, Kazakhstan al-Farabi Kazakh National University, 71, al-Farabi ave., Almaty, Kazakhstan d Satpayev University, 22A, Satpayev str., Almaty, Kazakhstan b c
a r t i c l e
i n f o
Article history: Received 4 September 2019 Received in revised form 30 October 2019 Accepted 2 November 2019 Available online xxxx Keywords: Gold Electroreduction Adsorption Activated carbon Rice husk
a b s t r a c t
This paper reports a study of the kinetics of the initial stages of electroreduction of AuCl4 in the case of gold electrosorption using activated carbon. The activated carbon was synthesized from rice husk by the chemical activation. Obtained sorption materials were characterized by Raman Spectroscopy, Transmission electron microscopy, Scanning electron microscopy and SORBTOMETR-M techniques. The kinetics of electrodeposition of gold (III) on platinum from chloride electrolytes by the cyclic voltammetry and chronoamperometry methods have been studied. Chronoamperometric measurements were performed at the potential of +0.2 V vs. Ag/AgCl with varying gold concentration and temperature. Diffusion coefficient of Au3+ ions determined by the cyclic voltammetry method on the basis of the Randles-Ševcˇik equation is in good agreement with the value determined by the chronoamperometry using the Cottrell law. Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the 7th International Conference on Nanomaterials and Advanced Energy Storage Systems.
1. Introduction Activated carbon is a carbon-containing adsorbent having a developed porous structure and large surface area. Activated carbon can be produced from nearly any carbon-containing material by physical or chemical activation methods [1–6]. The authors of this study selected rice husk (RH) as a raw material for preparation of activated carbon, subsequently used as an adsorbent. RH is a renewable, low-cost agricultural waste, which is widely spread in south Kazakhstan. The sorption material obtained from the RH has a wide field of application. Some Kazakhstan scientists investigate RH from different regions of Kazakhstan using it mainly as an adsorbent for waste water treatment and similar purposes [7–12], production of technical silicon [13] or for the supercapacitors [14,15]. According to the literature data [16], it was claimed that the series of adsorbents based on carbonized RH (CRH) have a rather ⇑ Corresponding author at: Institute of Combustion Problems, 172, Bogenbai Batyr str., Almaty, Kazakhstan. E-mail address: [email protected] (Zh. Supiyeva).
low redox potential and the stationary potential is in 0.229 V (Ag/AgCl). The measured stationary (real) potential of [AuCl4 ] in hydrochloric acid medium is equal to 0.760 V (Ag/AgCl). The potential difference between gold – oxidizing agent and sorbent – reducer is 0.531 V. For complete procedure (99.9%) of any redox reaction, a potential difference of 0.3 V is required [17]. From these data, it can be concluded that there is a real possibility for reducing Au3+ to a metallic state. This possibility is confirmed by electron microscopic images [16]. It follows that the sorbents of CRH possess reducing properties, due to the presence of reducing groups such as carboxyl, phenolic, hydroxyl, amine on the surface of these carbon materials [18]. Carbonized adsorbents are not only ion-exchanged [16], but also oxidation-reduction adsorbents. It follows that the process of metallic gold extraction and oxidation of reduction groups of the sorbent is electrochemical, i.e. there are cathode and anode sites. Cathode sites on which their further proceeds reduction of gold (III) are formed at the initial moment of sorption. The chemistry of Au3+ sorption on CRH can be represented as:
Cathodic process : AuCl4 þ 3 e ! Au0 þ 4Cl
ð1Þ
https://doi.org/10.1016/j.matpr.2019.11.013 2214-7853/Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the 7th International Conference on Nanomaterials and Advanced Energy Storage Systems.
Please cite this article as: Z. Supiyeva, K. Avchukir, V. Pavlenko et al., The investigation of electroreduction of AuCl4 in the case of gold electrosorption using activated carbon, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.11.013
2
Zh. Supiyeva et al. / Materials Today: Proceedings xxx (xxxx) xxx
Anodic process: CRH—Red 3 ! CRH—Ox
ð2Þ
where Red and Ox –R-COOH; R-Ph; R-OH; R-NH2 and oxidized forms of carbon groups of the sorbent. Pesic and Storhok [19] investigated the kinetics of gold (III) bromide complex adsorption and observed that gold is accumulated in its original ionic form on the surface of activated carbon. This phenomenon is different from the gold (III) chloride complex behavior, where adsorption process is accompanied by the reduction of the ions [20]. In recent years, many works have been published on gold adsorption using various biosorbents [21]. However, the literature is still insufficient to cover this research area, and more work and studies are needed in this field to develop other locally available and economical adsorbents. In this research, results showing the possibility of activated carbon application in electroreduction of gold from aqueous solution are presented. The studies were performed with synthesized activated carbon based on RH. The influence of amount of activated carbon used, initial concentration of gold (III) chloride complex ions, temperature was investigated.
2. Experimental details 2.1. Synthesis of the porous carbon and their characterization The cleaned and dried RH was collected from local farms of Almaty region, Kazakhstan, and subjected for carbonization at 773 K in an argon atmosphere. Carbonized RH was mixed with potassium hydroxide by use the weight proportion of 1:5 (precursor to KOH) and activated at 1123 K under argon atmosphere. The resulting mixture was subjected to washing by distilled water until the neutral pH. The obtained adsorbents composed of amorphous silica and carbon. Specific surface of carbonized and activated RH was determined by BET method (Brunauer-Emmett-Teller); it was 2818 m2 g1, pore specific volume was 1.59 cm3 g1 and average pore size was within 1.0 2.0 nm. Measurements were carried out using SORBTOMETR-M device.
The characterization of the sorbents has been carried out by means of the methods such as Raman spectroscopy (NTEGRA Spectra Raman, laser wavelength is 473.0 nm, signal with an area diameter of 80 nm), scanning electron microscopy (SEM, Quanta 3D 200i Dual System, FEI) and the transmission electron microscopy (TEM, JEM-2100) with high stability of high voltage and beam current along with an excellent electro-optical system). 2.2. Electrochemical measurements and atomic absorption spectroscopy All electrochemical measurements were carried out in a threeelectrode cell using an Autolab PGSTAT 302 N galvanostat/potentiostat (Metrohm, the Netherlands). A working electrode with surface area of 0.071 cm2 was represented by the platinum rotating disk Autolab RDE 80725. A platinum plate was used as an auxiliary electrode, and the silver-chloride electrode (Ag/AgCl) was used as a reference electrode. 0.1 mol L1 potassium chloride solution containing HAuCl4 with a pH of 1.5 was served as an electrolyte. The gold chloride solutions (HAuCl4) with a gold concentration of 7.65; 23.13; 45.34 and 80.79 mg L1 were prepared by dissolution of metallic gold in aqua regia. In order to study the kinetics of gold (III) chloride complex adsorption the change in concentration of Au3+ was analyzed using the atomic absorption spectroscopy (Perkin Elmer AAnalyst 200, USA). Determination of the content of Au3+ in the solutions was conducted using a lamp with a wavelength of 242.8 nm.
3. Result and discussion In order to identify the surface morphological features of synthesized samples, the SEM has been used. Samples have a complex structure. They exhibit a characteristic macrostructure with diameter of 4–22 mm (Fig. 1a). Next, the TEM image of graphene obtained from activated RH is shown in Fig. 1b. Samples have few-layer graphene with defects and inclusions of an amorphous carbon component, but there are sections of layers without defects with a homogeneous surface structure.
Fig. 1. (a, b) SEM images of carbon from the rice husk; (c, d) TEM images of the graphene layers.
Please cite this article as: Z. Supiyeva, K. Avchukir, V. Pavlenko et al., The investigation of electroreduction of AuCl4 in the case of gold electrosorption using activated carbon, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.11.013
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Fig. 2. Raman spectra of activated carbon.
The typical Raman spectrum of graphene exhibits three peaks: peak D at 1351 cm1, peak G at 1580 cm1, and peak 2D at 2700 cm1. The ratio between the intensities of peak G (IG) and peak 2D (I2D), IG/I2D gives an estimate of the number of layers [22]. Analysis of Raman spectra (Fig. 2) of carbon material obtained from activated RH showed the content of graphene films with three or more layers (IG/I2D = 0.63; 0.50; 0.43; 0.30). The kinetics of electroreduction of gold on a platinum surface from chloride electrolytes by the cyclic voltammetry (CV) with variation of the scan rate in the range of 10–50 mV s1 has been studied. Chronoamperometric (CA) measurements were performed at the potential of +0.2 V vs. Ag/AgCl with varying gold concentration and in the temperature range from 288 K to 308 K. Cyclic voltammograms were obtained in the study process of gold electroreduction on a platinum electrode from chloride solutions (Fig. 3). As can be seen from Fig. 3, the cathode peak corresponds to reaction (1), the anode peak corresponds to the oxidation of gold to Au3+. Complete electroreduction of gold ions proceeds at a potential greater than Ep (c), and electrooxidation at a more positive potential from 800 mV. An increase in the potential scan rate from 10 mV s1 to 50 mV s1 leads to an increase in the cathodic peak current density (ipc) and a shift in the recovery peak
potential (Ep(c)) of gold to the cathode region. The dependence of the values of the current density peaks on the square root of the p potential scan rate ( t) is linear (Fig. 3 Inset) and passes through the origin, which indicates the diffusion nature of the process. The shift in the peak potential of the gold recovery depends on the concentration of gold ions in the solution (Fig. 4). An increase in the gold concentration from 7.65 to 80.79 mg L1 at a potential scan rate of 10 mV s1 causes a shift in the potential of the peak of gold ion reduction on the Pt electrode by 300–400 mV. The use of electrolytes with a high concentration of the potential determining ions leads to an increase in the potential difference between the reduction of gold and electronegative impurities, such as Cu, Ag, etc. p The dependence of ipc on t presented below was calculated on the basis of the Randles-Ševcˇik equation, while the diffusion coefficient of Au3+ ions was calculated at various concentrations (Table 1). For reversible processes, the relationship between the current density of the electroreduction peak and the potential scan rate was described by the Randles-Ševcˇik equation [23].
F3 ip ¼ 0; 4463 RT
!1 ^ n3 A ^ AA ^ D1 A ^ CA ^ t1 A
ð3Þ
Fig. 3. Cyclic voltammograms of electroreduction of gold on a platinum electrode in the solution of 80.79 mg L1 HAuCl4 + 0.1 mol L1 KCl at various scan rates, T = 298 K; Inset: the dependence of the peak current density (ipc) related to the cathode processes on the square root of the potential scan rate.
Please cite this article as: Z. Supiyeva, K. Avchukir, V. Pavlenko et al., The investigation of electroreduction of AuCl4 in the case of gold electrosorption using activated carbon, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.11.013
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Fig. 4. Cyclic voltammograms of gold at 298 K on a Pt electrode in 0.1 mol L1 KCl solution with different HAuCl4 contents at a sweep scan rate of 10 mV s1.
Table 1 The values of the diffusion coefficient of gold ions during electroreduction of gold on the platinum electrode in 0.1 mol L1 KCl solution. Method
C AuCl4 , mg L1 D AuCl4 , 1014 cm2 s1
CV 7.65 0.15
23.13 1.39
45.34 5.38
80.79 17.06
where ip is the cathode current of the peak; n is the number of electrons involved in the electrode reaction; A is the area of the electrode, cm2; F is the Faraday constant, Pendant mole1; D is the diffusion coefficient of Au3+ ions, cm2 s1; C is the volume concentration of Au3+, mol cm3; m is the potential scan rate, V s1; R is the gas constant, J mol1 K1; T is the temperature, K. The diffusion coefficient calculated for aqueous gold solutions is shown in Table 1. It is observed that with an increase in the concentration of gold, the diffusion coefficient increases. The electrochemical reduction of gold ions occurring on the surface of the carbon adsorbent obtained from RH is limited by diffusion, and this proves the presence of the Randles-Ševcˇik dependence. The electrochemical stage, i.e. the electron transfer
process proceeds quickly, and it should be assumed that the charge transfer rate constant is several orders of magnitude higher than the mass transfer constant. Next, chronoamperograms of the electroreduction of gold ions on a Pt electrode at various potentials were obtained (Fig. 5). As can be seen from the Fig. 5, gold electroreduction at the potential of 0.6 V does not occur, it proceeds with a limiting diffusion current at potentials of 0.4; 0.2 V. Two regions are clearly visible from the current transient at a constant potential. The region of decrease in the cathode current is due to the diffusion limitation of the process, which is described by the Cottrell law, which is characteristic of non-stationary linear diffusion processes. The limiting current region is described by the following equation [24].
ik ðt Þ ¼ id ðt Þ ¼ nFC Au3þ
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi DAu3þ =pt
ð4Þ
where DAu3þ – the diffusion coefficient, cm2 s1; t – time, s. In this region, the effective thickness of the diffusion layer is established, which is constant in time. It is shown from the equation that the value of the limiting current of electrodeposition of gold ions at a constant value of the diffusion layer thickness depends only on the concentration of gold in the volume of the
Fig. 5. Chronoamperograms of gold electroreduction in the solution of 80.79 mg L1 HAuCl4 (pH = 1.5) with the addition of sorbent from activated RH at different potentials on a platinum electrode.
Please cite this article as: Z. Supiyeva, K. Avchukir, V. Pavlenko et al., The investigation of electroreduction of AuCl4 in the case of gold electrosorption using activated carbon, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.11.013
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Fig. 6. (a) Chronoamperograms of gold electroreduction in the solution of 28.98 mg L1 HAuCl4 (pH = 1.5) at different temperatures with the addition of carbon adsorbent; (b) Gold adsorption capacity (mg g1) as a function of duration of the contact with activated carbon.
Table 2 Gold recovery at different temperatures. T, K The adsorption percentage, % The adsorption capacity q, mg g1
288 98.92 143
298 95.99 139
308 97.95 142
solution. During electrosorption of gold, there is a decrease in the concentration of gold in the volume of the solution, which can be determined directly by measuring the limiting current. This determines the effectiveness of chronoamperometry in studying the kinetics of the gold sorption process. Kinetic curves (I-t) of Au3+ adsorption were obtained at the constant potential +200 mV corresponding to the limiting current of Au3+ ion reduction on a platinum electrode with varying potential-determining ions concentration and temperature (Fig. 6a). The completeness of the gold electroreduction reaction on a platinum electrode during chronoamperometric measurements was ensured by applying a potential of +200 mV to the electrode and a disk electrode rotation rate of 1000 rpm. 1.5 g of sorbent was introduced into the electrolytic cell. As a result, the concentration of gold ions in the electrolyte decreased; there was a decrease in the limiting current of Au3+ reduction. The curve (It) had a falling form. The residual concentration of Au3+ ions in the solution was determined using a flame atomic absorption spectrometer Perkin Elmer AAnalyst 200 (Fig. 6b). As a result, the adsorbent obtained from RH has a sorption degree of 95–98%, a sorption capacity of 139–143 mg g1 at a temperature in the range of 288–308 K (Table 2). Activated carbon from RH was wrapped with filter paper and then immersed in a thermostatically controlled electrochemical cell with a constant convection. Convection in the cell was provided by a rotating disk electrode. Fig. 6a shows that the limiting diffusion currents of the electroreduction of gold ions decrease with time, which indicates a decrease in the concentration of gold ions into the depth of the solution afterwards by its electrosorption by the adsorbent. This trend persists at temperatures from 288 to 308 K. The exception is the chronoamperogram at temperatures of 308 K with a steep slope. It is also clear that the limiting currents at the initial point are directly proportional to temperature, which once again emphasizes the diffusion nature of the process. It can be seen from Fig. 6a that the slope angles linearly depend weakly on temperature with the exception at 308 K. The linearity of the dependence of current on time at all temperatures is explained by a constant convection rate, in other words, mass transfer of gold
ions. In this case, the supply of the potential of the determining ion to the surface of activated carbon is limited with the convective flow rate (angular velocity of rotation of the disk electrode). At the end of the experiment, it was revealed that the electrosorption of gold ions occurred only on the surface layer of the wrapped sorbent in filter paper. Free immersion of the sorbent would provide a strong difference in the concentration of gold ions in the electrolyte, which can be recorded by the method of chronoamperometry, however, the contact of the sorbent with the indicator electrode creates noise of the recorded signal (limiting diffusion current). Therefore, the use of a chronoamperogram for free immersion of the sorbent is difficult, and it requires special organization of the experiment to study the kinetics of sorption. In addition to chronoamperometric measurements, an atomic absorption analysis of the concentration of gold ions after their adsorption was performed. In this case, the sorbent was immersed in the solution in the form of powder and the solution was stirred with a magnetic stirrer at a constant rate of 200 rpm. The adsorption curves representing the concentration of gold ions plotted versus time are shown in Fig. 6b. From the Fig. 6b it can be seen that the change in the concentration of gold decreases exponentially with time (up to 40 min). Further adsorption of gold ions proceeds with a limiting velocity by a characteristic linear dependence. The form of the curve representing the concentration plotted as a function of time weakly depends on time. This once again confirms the statement made earlier about the diffusion nature of this process, since the diffusion coefficient depends only slightly on temperature than the charge transfer rate constant (or the constant of the electrochemical reaction). From this we can conclude that the reaction (1) is reversible. The SEM and optical images clearly demonstrate that a gold chloride is reduced to metallic gold and deposited on the surface of activated carbon. The smallest gold nanoparticles that could be revealed were 70–200 nm in diameter on the surface of activated RH (Fig. 7a–d). The morphology of activated RH examined using SEM and BET analysis revealed that, the adsorbent represented by carbonized and activated RH is highly porous. In total, the outstanding characteristics of the resulting activated carbon such as an BET specific surface area equal to 2818 m2 g1 and the total pore volume of 1.59 cm3 g1, along with a high adsorption degree of gold ions equal to 95–98% and sorption capacity of 139–143 mg g1, allow to declare this material as an effective adsorbent applicable for extraction of noble metals.
Please cite this article as: Z. Supiyeva, K. Avchukir, V. Pavlenko et al., The investigation of electroreduction of AuCl4 in the case of gold electrosorption using activated carbon, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.11.013
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Fig. 7. SEM images of the surface of the carbon electrode after adsorption of gold at different magnifications of (a) 1 mm, (b) 2 mm; (c) optical surface image and (d) elemental analysis.
4. Conclusion
References
In this study, the features of gold electroreduction on the activated carbon produced from carbonized and activated RH were shown. The kinetics of electroreduction of gold ions investigated by means of cyclic voltammetry and chronoamperometry showed several important trends which haven’t been adequately described before this study by other authors. It is revealed that the electrochemical reduction of gold ions occurring on the surface of the carbon sorbent is limited by diffusion, and this proves the presence of the Randles-Ševcˇik dependence. In this regard, the electrochemical stage, i.e. the electron transfer process proceeds quickly. Meanwhile, the charge transfer rate constant was several orders of magnitude higher than the mass transfer constant. Moreover, the optimal conditions for the process of gold adsorption on a carbon adsorbent were established. Evaluation of the features of reaction mechanism between Au3+ chloride complexes and activated carbon which was performed within the study, allows to control the processes of electroreduction of gold ions more precisely and to enhance the overall efficiency of this method.
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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement Ministry of Education and Science of the Republic of Kazakhstan financially supported this work, Project No. AP05134691.
Please cite this article as: Z. Supiyeva, K. Avchukir, V. Pavlenko et al., The investigation of electroreduction of AuCl4 in the case of gold electrosorption using activated carbon, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.11.013