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Abstract

Pristine Macadamia nutshell-based activated carbons were chemically oxidized with different concentrations of H₃PO₄ and HNO₃ to increase their surface adsorption properties and further explore if they could be an attractive alternative low-cost adsorbent for gold recovery from cyanide-leached liquors. The modified activated carbons were labeled MACN₂₀, MACN₄₀ and MACN₅₅ to signify the materials prepared from 20%, 40% and 55% (v/v) HNO₃, respectively. Similar nomenclature was followed for H₃PO₄-modified activated carbons. Brunauer-Emmet-Teller, scanning electron microscopy, thermogravimetric analysis, Fourier transform infrared spectroscopy, elemental analysis and X-ray diffraction spectroscopy were used to characterize the prepared activated carbons. The physical properties were attained through determining attrition, ash content, volatile matter and moisture content of all the activated carbons. Various parameters that affect selective adsorption such as the effect of initial concentration, time, agitation speed, interfering species and the dose of the adsorbent were investigated. Optimal parameters for gold ion adsorption were as follows: solution pH, 10; contact time, 6 h; agitation speed, 150 r/min; sorbent amount, 4 g and initial concentration, 5.5 mg/L. The observed selectivity order was not the same for all the adsorbents, but the adsorption of gold was found to be mostly influenced in the presence of nickel and least influenced by copper. MACN₅₅ was found to be the most efficient adsorbent with 74% of gold adsorption from a real-world sample and displayed a similar performance to coconut-based activated carbons.

Keywords

Adsorption activated carbon Macadamia gold ion isotherms kinetics

Introduction

Gold is a precious metal used for decades by humankind in multitudes of applications including jewelry, electronics, medicine, chemical systems and in the banking industry (Das, 2010; Khosravi et al., 2017). The extraction of gold from gold ores was done using the conventional cyanidation process (expression (1)) for decades (Kondos et al., 1995). Although thiosulfate leaching (expressions (2) and (3)) was proposed as the alternative lixiviate to cyanidation (Molleman and Dreisinger, 2002; Navarro et al., 2002; Yu et al., 2015) for the recovery of gold due to its non-toxicity and faster leaching process (Navarro et al., 2007), it has not replaced cyanidation because of a lack of suitable recovery methods from the pregnant solution (Zhang and Dreisinger, 2004) and the consumption of a large number of reagents (Navarro et al., 2007). $4 {Au}_{(s)} + 8 {CN}_{(aq)}^{-} + 2 H_{2} O_{2 (l)} + O_{2 (g)} \leftrightarrow 4 Au {(CN)}_{2 (aq)}^{-} + 4 {OH}_{(aq)}^{-}$ (1) ${Au}_{(s)} + 5 S_{2} O_{3}^{2 -} + Cu {({NH}_{3})}_{4 (aq)}^{2 +} \to Au {(S_{2} O_{3})}_{2 (aq)}^{3 -} + 4 {NH}_{3 (g)} + Cu {(S_{2} O_{3})}_{3 (aq)}^{5 -}$ (2) $4 Cu {(S_{2} O_{3})}_{3 (aq)}^{5 -} + 16 {NH}_{3 (g)} + O_{2 (g)} + 2 H_{2} O_{(l)} \to 12 S_{2} O_{3}^{2 -} + 4 Cu {({NH}_{3})}_{4 (aq)}^{2 +} + 4 {OH}_{(aq)}^{-}$ (3)

Hence, gold mining industries continued to use cyanidation and the focus of researchers shifted at finding more suitable methods for the recovery of gold from cyanide-leached liquors. Merrill–Crowe zinc cementation (Mpinga et al., 2014a), ion exchange (Murakami et al., 2015), chemical precipitation (Soylak and Erdogan, 2006), liquid–liquid extraction (Pan and Gu, 2012) and adsorption by activated carbon (AC) (Khosravi et al., 2017) are some of the methods that have emerged from the literature. Of these methods, adsorption onto ACs is still more popular among researchers (Khosravi et al., 2017; Mpinga et al., 2014b) because it is economical for treating large volumes of very dilute solutions (Mpinga et al., 2014b). ACs are carbonaceous materials characterized by a high degree of surface reactivity, large surface area and a well-developed microporous structure (Poinern et al., 2011). It has been demonstrated that precursors from various agricultural or agro-waste by-products can be utilized in the preparation of ACs. These include biomass (Gueye et al., 2014), coconut shell (Khosravi et al., 2017), date stones (Amor and Ismail, 2013), Macadamia nutshell (Pakade et al., 2017), mango kernels (Rai et al., 2016), oil palm endocarp (Silgado et al., 2014), peanut shell (Al-Othman et al., 2012), tamarind wood (Acharya et al., 2009) and white marudah (Terminalia arjuna) (Mohanty et al., 2005). Of these, coconut shell-based ACs have found more use and acceptance for the recovery of gold in the gold mining industry (Khosravi et al., 2017; Souza et al., 2014; Yu et al., 2015) because of their high mechanical strength and loading capacity (Yalcin and Arol, 2002).

The use of non-coconut shell-based activated carbon should be explored by evaluating ACs prepared from other cost-effective and abundantly available precursors for the recovery of gold.

However, to be effective, the ACs that are produced must be resistant to attrition and transportation within the system because fine AC is undesirable in the mining industry as it will lead to loss of adsorbed Au through the screen (Yalcin and Arol, 2002). Macadamia nutshells have a high cracking pressure, a property which is attractive for producing high-quality ACs that are resistant to attrition. Poinern et al. (2011) compared Macadamia and coconut shell-based ACs for the recovery of gold cyanide ion. The ACs were prepared by CO₂ activation at different temperatures and characterized by scanning electron microscopy (SEM) and Brunauer-Emmet-Teller (BET) surface area. However, this preliminary study was never followed by detailed adsorption studies to evaluate the mechanism of adsorption and rate of uptake. Therefore, this paper investigated the efficacy of using virgin Macadamia nutshell-based ACs prepared by steam activation as well as its derivatives chemically oxidized by different concentrations of nitric and phosphoric acids for the recovery of gold. Secondary treatment of pristine or commercial ACs by oxidants like KMnO₄ (Zhang, 2013; Zhang et al., 2017), HNO₃ (da Costa Lopes et al., 2015), H₂O₂ (Song et al., 2010) and ozone (Jaramilloa et al., 2010) not only increases the surface oxygenated groups such as carboxylic acid and hydroxyl groups of the ACs but also modifies the pore structure as well, thus impacting on the adsorption performance (da Costa Lopes et al., 2015; Jaramilloa et al., 2010; Song et al., 2010; Zhang, 2013; Zhang et al., 2017). It has been further stated that the concentration of the activating agent used for modification influences the final properties of ACs like pore structure, pore volume, surface area and the density of oxygenated functional groups (Yavuz and Zeki, 2014). The resultant surface could either be negatively or positively charged depending on the type of reagent employed during modification. Also, the basicity and acidity properties, as well as the hydrophilicity and hydrophobicity, are imparted by the type of modification used. The importance of surface modification of adsorbents was demonstrated by Yin et al. (2018a, 2018b) during adsorption of phosphate (PO₄^3–) and nitrate (NO₃^–) by biochar materials. It was reported that the presence of Al and Mg–Al modifiers increased the adsorption performance of the biochar significantly when compared to pristine materials.

But sometimes secondary modification can lead to obliteration and total disintegration of the internal structure of the ACs. Therefore, it is paramount to investigate the optimum concentrations of activating agents needed for improved performance. Thus, the focus of this research was to conduct a study in which ACs prepared from waste Macadamia nutshells were investigated for the recovery of gold. The ACs were modified with different concentrations of HNO₃ and H₃PO₄ to improve their adsorption properties. This was done in order to explore if the modified ACs prepared from Macadamia nutshells could be an alternative low-cost adsorbent for the extraction of gold ions in aqueous solutions.

Materials and methods

Chemicals and materials

Potassium aurocyanide, calcium chloride, potassium chloride, boric acid, hydrochloric acid, sodium hydroxide, sodium cyanide, lanthanum chloride, nitric acid and phosphoric acid were purchased from Labchem (Johannesburg, South Africa). Macadamia-activated carbon (MAC) was prepared as previously reported in Pakade et al. (2016).

Characterization

The elemental composition of ACs was obtained using a ThermoFlash 2000 series CHNS/O Organic Elemental Analyzer. Thermogravimetric analysis was performed on a TGA 4000 Thermogravimetric Analyzer (PerkinElmer, Waltham, MA). Infrared absorption spectra were obtained using a PerkinElmer Spectrum 400 FT/IR spectrometer. The CL10 Centrifuge (ThermoScientific, Johannesburg, South Africa) was used for centrifugation. BET instrument (Micromeritics Tristar 3000) supplied by Poretech (Krugersdorp, South Africa) was used for the determination of surface area and pore size. Surface morphological information of ACs was attained using an SEM JOEL Model JSM 6700F (Tokyo, Japan). The extent of graphitization in prepared ACs was determined by X-ray diffractometer (Shimadzu X-RD-7000) (Kyoto, Japan). Scans were run with a step size of 0.02°/s of 2θ, typically in the angle range between 10° and 80°.

Proximate analysis

The moisture content was determined by weighing 50 g ACs and placing them in an oven for 12 h at 110°C. The sample was cooled in a desiccator and weighed to a constant mass. For the determination of ash content, 1 g of the ACs was weighed into a crucible and placed in an electric furnace at 650°C for 1 h. It was then removed from the furnace and placed in a desiccator for cooling and then weighed. Ashing of the AC was done for 3 to 8 h depending on the type of AC and its particle size. Ashing was completed only when a constant mass had been achieved. For the determination of the volatile matter, 1 g of the ACs was weighed into a crucible and placed in an electric furnace set at 900°C for 7 min. The AC was then transferred into a desiccator to cool and then weighed. The moisture of the ACs was pre-determined in order to calculate the volatile matter.

A wet attrition method adapted from Toles et al. (1997) was employed for the determination of hardness of the ACs. Briefly, 1.0 g of ACs was placed in 100 mL of 0.1 M acetate buffer in a 150 mL beaker. The solution was stirred at 500 r/min for 2 h at 25°C. The sample mixtures were then poured on a 0.30 mm screen and washed sequentially with 250 mL of distilled water. The material retained on the screen was transferred onto an aluminum pan and dried in an oven at 110°C for 2 h. The samples were finally cooled in a desiccator.

Preparations of stock and working solutions

Dried potassium aurocyanide K₂Au(CN)₂ (analytical reagent grade) was used to prepare a stock solution (100 mg/L) of gold in distilled water. Each of the other stock solutions consisting of 10.60 g of lanthanum chloride, 18.84 g of sodium cyanide and 27.69 g of calcium chloride was also prepared by dissolving appropriate amounts of salts in 1000 mL flasks and made to the mark using distilled water. The stock solutions were stored at room temperature when not in use. Standard and working solutions were prepared daily from serial dilutions of stock solution. The pH was adjusted using dilute NaOH or HCl solutions. The purpose of lanthanum chloride was to correct the interferences in atomic absorption spectrophotometry, while that of calcium chloride was to simulate the plant condition.

Preparation of 0.1 M borate buffer was carried out as follows: 3.09 g of boric acid (H₃BO₃) was combined with 3.73 g of potassium chloride (KCl) and transferred into a 1000 mL volumetric flask containing distilled water. This buffer solution was used to wet the carbon before use to displace the air entrapped in the carbon.

Preparation of MACs

About 10 g of AC was impregnated with 30 mL of 20% (v/v) HNO₃ solution. The conical flask was enclosed with parafilm. Few holes were punched onto the parafilm for the gases to escape. The mixture was placed in a Protech 721 orbital shaker for three days (200 r/min) to dissolve the solution. After drying, the product was transferred to a Pyrex petri dish which was pre-weighed. The thick liquid that formed was further dried in the oven at 120°C for 24 h. Thereafter, the impregnated AC was weighed and placed in the muffle furnace and heated to 700°C. Following this process, the product was neutralized with the borate buffer solution and washed with distilled water until the pH was neutral. The modified product was labeled MACN₂₀. The same procedure was followed using 40% and 55% (v/v) of HNO₃ and the resulting products were labeled MACN₄₀ and MACN_55. The same procedure was used to prepare MACP₂₀, MACP₄₀ and MACP_60.

Adsorption studies

AC amounts (1, 2, 3, and 4 g) were shaken at 150 r/min for 60 min in different 2.5 L stoppered bottles containing 1 L aliquots of 5.7 mg/L gold cyanide at pH 10 at ambient temperature. The following time intervals were used for contact time studies (60, 120, 180, 240 and 300 min) for initial concentrations (1, 2, 3, 4, 5 and 6 mg/L), while other parameters were kept constant. Following the lapse of the experimental times, a 5 mL aliquot of a sample was then pipetted into a 25 mL volumetric flask and lanthanum chloride (5 mL) was added. The residual concentration of gold in the solution was analyzed using atomic adsorption spectroscopy AA-7000 (Shimadzu, Kyoto, Japan). All experiments were performed in two replicates, and mean results were reported. The adsorption capacity, q_e and percentage adsorption (%A) were calculated from equations (4) and (5), respectively. $q e = \frac{(C_{0} - C_{e}) \times V}{M}$ (4) $% A = \frac{(C_{o} - C_{e})}{C_{o}} \times 100$ (5)where q_e is adsorbate adsorption capacity (mg/g), C_o and C_e are initial and equilibrium concentration (mg/L), respectively, M is the adsorbent mass (g) and V is the volume of solution (L).

The selectivity was tested by employing a multi-element mixture of Ni²⁺, Cu²⁺, Fe²⁺ and Pb²⁺. In a multi-element analysis, all the four cations together with the gold ion were prepared in a single solution and the initial concentration of each analyte was 6 mg/L. The experiment was conducted at optimum conditions. Regeneration of ACs was investigated by bringing into contact 4 g of the adsorbent with a 6 mg/L gold solution for 6 h. After extraction and equilibration, the aqueous solution was filtered and the AC particles were transferred to another sample vial for further adsorption. To strip out the adsorbed gold ions, AC particles were boiled in HCl and then agitated in a 1 L solution of NaOH (0.1 M) and NaCN for 30 min. The solution was filtered and the remaining gold ion in solution was then analyzed. The AC particles were used for the next adsorption for five cycles without any conditioning.

Application to real-world samples

The applicability of the prepared ACs for the adsorption of gold species from real-world samples was carried out using a Barren sample solution from a gold extraction plant. The pH of these samples was measured and subsequently adjusted to pH 10 where necessary. The concentration of other base metals like Ni, Cu, Pb and Fe was measured using an atomic absorption spectrophotometer. For the experiment, 4 g of AC in 1000 mL of the abovementioned solutions was agitated for 6 h, and the concentration of metal ions was measured as stipulated before.

Results and discussion

Characterization of modified ACs

FTIR spectra of various ACs

The FTIR spectra of CAC, MAC, MACN_20, MACN₄₀ and MACN₅₅ are shown in Figure 1. Differences or changes in the spectra were observed around 3500 cm⁻¹, 1400 cm⁻¹ and 1000 cm⁻¹. Specifically, a broad band at 3400 cm⁻¹ assigned to OH^– of alcohols was observed in MAC and CAC. The disappearance of this peak in MACN₄₀ and MACN₅₅ could be attributed to the elimination of alcohols in the presence of heat and acid to form alkenes. In addition, MAC and CAC exhibited signals located at 1374 cm⁻¹ and 1366 cm⁻¹, respectively. This could be attributed to the C = O vibration of carboxylate groups. Another notable change in the spectra is the presence of C–O–C bands at 987, 985 and 982 cm⁻¹ in MACN_20, MACN₄₀ and MACN_55, respectively. Elsewhere, bands attributed to C–O stretch bands characteristic of ester linkages were also observed at 1027 cm⁻¹ (Moyo et al., 2017). The absence of clear OH^– and carboxylate functional groups in nitric acid-treated MAC, coupled with the presence of new ester bands suggests that there was a nucleophilic acyl substitution reaction taking place on the adsorbent surface producing ester linkages. The formation of esters and alkenes will result in the loss of oxygen, perhaps causing the disappearance of the broad band at 3400 cm⁻¹ in treated MAC. Furthermore, the fact that the new C–O–C bands occurred at different wavenumbers implied that they were in different chemical environments. Therefore, one can conclude that the surface chemistries of MACN_20, MACN₄₀ and MACN₅₅ were different from one another and to that of MAC, signifying a successful modification. The FTIR spectra of MACP₂₀, MACP₄₀ and MACP₆₀ displayed similar trends, but peak shifts and intensities were slightly different (results not shown) corroborating the different chemistries of the two activating acids used.

Figure 1.

FTIR spectra of MAC (a), CAC (b), MACN₅₅ (c), MACN₄₀ (d) and MACN₂₀ (e) adsorbents.

Thermogravimetric analyses

Thermogravimetric analyses were conducted to determine the decomposition patterns of ACs. Thermograms and differential curves are depicted in Figure 2(a) to (e). All the samples showed a weight loss at 50–150°C which is due to the evaporation of moisture. The estimated moisture contents were 4%, 10%, 2%, 8% and 4% for CAC, MAC, MACN₂₀, MACN₄₀ and MACN₅₅, respectively. The second decomposition stage observed between 200 and 600°C was ascribed to the gasification of volatile organic matter in charcoal (Kumar and Jena, 2015). The third weight loss was due to decomposition of AC structure and loss of strongly bound compounds at temperatures above 750°C. Similar trends were observed in H₃PO₄-treated carbons (results not shown). The functionalized MACs and the CAC showed a lower percentage loss of moisture compared to the MAC. All the ACs in this study had a residue greater than 75% due to the absence of volatile compounds. The exact temperatures of decompositions are shown in the differential curves.

Figure 2.

Thermograms and derivative weight loss graphs for CAC (a), MAC (b), MACN₂₀ (c), MACN₄₀ (d) and MACN₅₅ (e).

Elemental analysis

The percent of carbon, hydrogen, nitrogen and oxygen content for the studied ACs is shown in Table 1. The treatment of MAC with various concentrations of HNO₃ resulted in an improved %C. The removal of oxygenated functional groups during modification led to an increase in %C of treated MAC. There was a 5% increase in C content of MAC as it increased from 83.27% to 87.68% for MACN₅₅. On the contrary, treatment with H₃PO₄ led to a loss of C. Meanwhile, the oxygen content decreased in HNO₃-treated MACs compared to pristine MAC, while an increase was observed in H₃PO₄-treated materials. Since the %O was calculated by difference, it is possible that there were some other elements present in pristine MAC such as metal ions. However, upon treatment of MAC with HNO₃ solutions, some of these elements were removed, thus leading to the increased estimated %O value. Sharifan (2013) noted that the increase in carbon content following carbonization was an indication of non-carbon elements being released during pyrolysis. The percentage C content of the MACN₅₅ was also found to be closer to that of CAC with just a difference of about 1.5%. There was no definite trend in oxygen content and increase in HNO₃ concentration in the current study, even though Valdes et al. (2002) observed an increase in oxygen content as the concentration of activating agent was increased.

Table 1.

Elemental analysis, BET analysis and proximate analysis of ACs.

	Elemental analysis				BET analysis			Proximate analysis
Type of carbon	C	H	N	O*	BET Surface area (m²/g)	Pore volume (cm³/g)	Pore size (nm)	Moisture(%)	Ash(%)	Volatile%	Attrition%	Fixed carbon*
CAC	89.05	<0.1	0.17	10.78	788	0.476	2.416	0.4	4.3	4.6	0.3	90.7
MAC	83.27	1.53	0.50	14.70	546	0.354	2.590	0.7	4.7	11.7	0.8	82.9
MACN₂₀	86.30	1.05	0.19	12.46	–	–	–	2.1	6.2	12.4	1.1	79.3
MACN₄₀	86.61	1.30	0.52	11.57	–	–	–	2.7	6.2	13.6	1.6	77.5
MACN₅₅	87.68	<0.1	<0.1	12.32	748	0.489	2.613	2.3	8.8	14.1	1.9	74.8
MACP₂₀	79.66	1.0	<0.1	19.34	–	–	–	3.0	6.9	14.7	3.3	72.1
MACP₄₀	81.66	<0.1	0.26	18.08	–	–	–	3.4	10.7	15.3	3.5	67.1
MACP₆₀	78.22	<0.1	<0.1	21.78	824	0.494	2.399	4.4	14.2	16.3	4.8	60.3

*Computed by difference						Quality specification limits		0–15	0–10	0–40	0–5

BET: Brunauer-Emmet-Teller.

BET surface determination of various carbons

The surface area, pore volume and pore size of CAC, MAC, MACP₆₀ and MACN₅₅ are shown in Table 1. CAC had the highest BET surface area of 788 m²/g, while modification of MAC with 55% (v/v) HNO₃ increased the surface area from 545 to 748 m²/g a value that was closer to that of CAC. The BET surface area for MACP₆₀ was the highest at 824 m²/g. Macadamia-based ACs have been reported to have BET surface areas ranging from 600 to 1500 m²/g (Dejang et al., 2015; Pakade et al., 2016; Pezoti et al., 2014). Similarly, the pore volume of pristine MAC increased from 0.354 cm³/g to 0.489 cm³/g, which was very close to 0.476 cm³/g for CAC. The closeness of the surface area and pore volume values for CAC and MACN₅₅ suggests that the two ACs have similar physical characteristics. The increase in the surface area following treatment of ACs, with nitric acid has been observed by other researchers (da Costa Lopes et al., 2015; Vasu, 2008; Yavuz and Zeki, 2014). According to IUPAC definition of pore size characterization, the ACs generated in this study possessed mesopores as all pore size diameters were greater than 2 nm.

X-ray diffraction analysis

The X-ray diffraction patterns for CAC, MAC, MACN₂₀, MACN₄₀ and MACN₅₅ are depicted in Figure 3. The AC samples were found to have both broad peaks and few sharp peaks which revealed a predominantly amorphous structure. Zhao et al. (2009) pointed out a link between porous and amorphous structures of adsorbents, while Tongpoothorn et al. (2011) stated that amorphousness could be ideal for adsorption. Notable peaks were observed at 2θ = 24°, 37°, 44° and 77°. The broad peaks found at around 24° for all the samples suggested that the ACs were non-graphitized, amorphous and could exhibit a high microporous structure (Tongpoothorn et al., 2011; Zhao et al., 2009). The broad peak at 24° was attributed to amorphous carbon (JCPDF-500926) and graphitic carbon (JCPDF-020456) at 26.8° (Dejang et al., 2015). Similar trends were observed with H₃PO₄ modified ACs (results not shown).

Figure 3.

XRD patterns of CAC (a), MAC (b), MACN₅₅ (c), MACN₄₀ (d) and MACN₂₀ (e).

SEM analysis of ACs

The SEM images of CAC, MAC, MACN₅₅ and MACP₆₀ are presented in Figure 4. All the adsorbents possessed a heterogeneous surface with a rough texture and a variety of randomly distributed pores which act as channels to the mesoporous network of ACs. It was also evident from the SEM images that HNO₃ and H₃PO₄ modifications were responsible for altering the physical properties and porosity of the materials, as there are major differences in the surface morphologies of MAC, MACP₆₀ and MACN₅₅. The surface morphology of CAC was similar to that of MACN₅₅. The similarity in pore structure for CAC and MACN₅₅ could be linked to the BET surface area values which were closer to one another. Therefore, it shows that modification of MAC with 55% (v/v) concentrated nitric acid yielded a material with similar physical attributes to CAC. Similar results were observed in a study done by da Costa Lopes et al. (2015) on the modification of pristine MAC with nitric acid. On the other hand, MACP₆₀ pores seemed to be extending from one side to another which showed a disruption of the pore channels.

Figure 4.

SEM images of for CAC (a), MAC (b), MACN₅₅ (c) and MACP₆₀ (d).

Physical characterization of ACs

Ash content, moisture content, volatile matter, attrition and fixed carbon were determined and the values are displayed in Table 1.

Ash content

The ash content for CAC, MAC, MACN_20, MACN₄₀, MACN₅₅, MACP_20, MACP₄₀ and MACP₆₀ was 4.3, 4.7, 6.2, 6.2, 8.8, 6.9, 10.7 and 14.2% (Table 1). Table 1 shows a trend of increasing ash content with increasing concentration of modifying agents (HNO₃ and H₃PO₄). The higher ash content after acid treatment implies that the modification of MAC with nitric and phosphoric acid resulted in a carbon with greater inorganic material content due to the extraction of organic components. That is, high ash content could be attributed to the residual of entrapped dehydrated acid products (Vunain et al., 2017). Even though the nitric acid-treated ACs displayed higher ash content than both CAC and MAC, it was still within the specified limits for mining applications. In contrast, the H₃PO₄-treated MACs (MACP₄₀ and MACP₆₀) exhibited a higher ash content than the specification limit and these can be construed as less quality AC.

Moisture content

There was no noticeable trend in the ACs as a function of nitric acid-activating agent concentration except that untreated MAC (0.7%) had a lower moisture content than treated MAC (2.1 to 2.7%). The moisture content in HNO₃ modified ACs decreased in the order MACN₄₀>MACN₅₅>MACN₂₀. Thermogravimetric analysis (TGA) also displayed the same trend. However, in H₃PO₄-modified ACs, the moisture content increased from 3.0 to 4.4%, as the concentration of activating agent increased. Accurate determination of moisture content in ACs is important because too much moisture may lead to an incorrect solid–liquid ratio dosage for industrial applications. However, the moisture content for all ACs was found to be within the quality specification limits. According to Madhavakrishnan et al. (2008), no correlation existed between moisture content and adsorption power of carbon. An increase in the concentration of the activating agent led to attenuation of fixed carbon content from 83 to 75% for MACN and 72.1 to 60.3% for MACP ACs probably because the treatment of MAC with acids yielded a slightly combustible product. Malik et al. (2006) reported a fixed carbon content of 71.4% for ACs produced following activation of groundnut shell with ZnCl₂.

Volatile matter and attrition

The volatile content of quality ACs can go up to 40%, and it is perceived that the higher the volatile content, the greater will be resistant to attrition of the AC (Chen et al., 2010). The volatile content of H₃PO₄-treated ACs increased from 14.7 to 16.3%, as the H₃PO₄ concentration increased from 20 to 60% (v/v). The HNO₃-treated ACs increased from 12.4 to 14.1% as the concentration of the activating agent increased. All the ACs investigated had attrition resistances that were below 5%, which was an indication of good quality carbons. MAC and CAC volatile contents were 3.3% and 4.6%, respectively. Therefore, an increase in impregnation led to an increase in the volatile matter (Chen et al., 2010).

Adsorption studies

Effect of adsorbent dosage concentration

The results obtained from the optimization of AC sorbent dosage concentration for gold adsorption is shown in Figure 5. The amount of gold adsorbed on AC increased as the adsorbent dosage concentration was increased from 1 to 4 g in all the samples. As the AC dose was increased, free sorption surface and adsorption sites also increased leading to greater adsorption of gold ions on the surface (Liu et al., 2010). Therefore, 4 g was regarded as optimum sorbent dosage concentration and was subsequently used in succeeding experiments. The flattening of the curves from 3 to 4 g suggested that no drastic adsorption increases were expected beyond 4 g. To save on costs and material, it was decided to work with 4 g. The adsorption efficiency decreased in the order: CAC≍MACN₅₅>MACN₄₀>MACP₆₀> MACP₄₀>MACN₂₀>MACP₂₀>MAC. It can be inferred that CAC and MACN₅₅ could exhibit similar chemical attributes for gold ion adsorption.

Figure 5.

Effect of adsorbent dosage concentration adsorption of gold ions (conditions: Solution pH 10; solution Volume, 1000 mL; contact time, 1 h, agitation speed, 150 r/min and initial concentration, 5.622 mg/L).

Effect of initial gold concentration

The influence of initial gold ion concentration on adsorption is illustrated in Figure 6. The percent adsorption decreased as the initial concentration of gold ions increased from 5.5 to 9.5 mg/L. The decrease can be explained on the basis that at lower concentrations, the number of available adsorption sites to gold ion concentration was higher (Rangabhashiyam and Selvaraju, 2015). At a higher concentration, there are fewer adsorption sites available as opposed to the amount of gold species in solution. Thus, the adsorption of gold species onto ACs is dependent on the initial concentration. While the percent adsorption decreased with increase in initial concentration, the adsorption capacity of all adsorbents increased as the concentration of the gold ion in solution increased. This indicated that the initial concentration plays an important role in the adsorption capacity of gold ions on AC. Maximum adsorption was observed at 5.5 mg/L for all the carbons. The adsorption rate ranged from 70% to 90% and the specific adsorption rates were 70%, 78%, 85%, 90% and 91% for MAC, MACN₂₀, MACN₄₀, MACN₅₅ and CAC, respectively. Again, the trend in adsorption efficiency at low concentrations was almost unchanged as the tendency mentioned above, with CAC≍MACN₅₅>MACN₄₀ performing better than the phosphate-modified ACs.

Figure 6.

Effect of initial concentration on adsorption rate (a) and adsorption capacity (b) (conditions: Solution pH 10; contact time, 1 h, agitation speed used 150 r/min and dosage of carbon, 4 g/L).

Effect of agitation

The influence of shaking speed was examined by varying the agitation speed from 50 to 250 r/min, whilst other parameters were kept constant (Figure 7). The percentage adsorption increased steeply at 50 to 150 r/min and then slowed down almost reaching equilibrium between speeds of 200 and 250 r/min. Some of the carbon remained at the bottom of the bottle when speeds below 150 r/min were used implying less contact, and at 200 to 250 r/min, the bottles were shaking vigorously leading to spillages. The optimum shaking speed was found to be 200 r/min. However, in the succeeding experiments, a speed of 150 r/min was used to avoid spillage. The increased adsorption (%) observed at higher agitation speeds was attributed to the improved surface contact (interactions) between the adsorbate and adsorbent because of the higher turbulence and thinning of the liquid boundary layer (Kuśmierek and Świątkowski, 2015). MACN₅₅ and CAC were overlapping and displaying the highest adsorption percentage compared to other carbons. Similar results were obtained in a study done by Soleimani (2007) in which an increase in agitation speed led to an increase in gold adsorption. The order of percentage adsorption decreased according to the following trend: CAC ≍ MACN₅₅> MACN₄₀> MACP₆₀> MACP₄₀> MACN₂₀> MACP₂₀> MAC. Therefore, functionalization of MAC with nitric acid improved its adsorption percentage for gold ions.

Figure 7.

Effect of initial agitation speed on gold adsorption (conditions: Solution pH 10; solution volume, 1000 mL; contact time, 1 h, dosage of carbon, 4 g/L and concentration used was 5.7 mg/L).

Effect of contact time

The results obtained from varying the time during gold adsorption are displayed in Figure 8. The percentage adsorption increased rapidly, as the time of agitation was increased from 1 to 5 h, and then slowed down from 5 to 6 h as it attained equilibrium. The observed rapid increase in the rate of adsorption initially was because the adsorption sites were more accessible for the adsorbent–adsorbate interactions, resulting in a higher adsorption rate. However, the functional groups started to get saturated as the time increased beyond 5 h resulting in lower adsorption rates and consequently equilibrium. MACP was inferior to MACN carbons between 2 and 4 h, while MACN₄₀ and MACN₅₅ performed similarly to CAC at this range. After 5 h, all the carbons seemed to attain a similar adsorption capacity and 6 h was regarded as the optimum time for adsorption.

Figure 8.

Effect of time on gold adsorption (conditions: Solution pH 10; solution volume, 1000 mL; dosage of carbon, 4 g/L, agitation speed used 150 r/min and concentrations used were 5.7 mg/L).

Selectivity (effect of interfering species)

Nickel (Ni: Aw 58.69 g/moL: ionic radius 69 pm), lead (Pb: Aw 65.39 g/moL: ionic radius 74 pm), iron (Fe: Aw 55.85 g/moL: ionic radius 64 pm), and copper (Cu: Aw 63.55 g/moL: ionic radius 72 pm) were used for the investigation of selectivity. A mixture containing 6 mg/L of each of the four metal ions together with gold ions was prepared and used to investigate the effect of co-existing ions. The parameters employed for the experiment were: time 6 h, pH 10, adsorbent mass 4 g and 1000 mL solution volume used at an agitation speed of 150 r/min. Atomic absorption spectroscopy was used to determine the concentration of the unadsorbed competing cations, and the results are illustrated in Table 2.

Table 2.

Effects of competing ions on gold adsorption.

	Adsorption (%)
Adsorbents	Copper	Iron	Nickel	Lead	Gold
CAC	3.9	4.3	22.5	16.5	80.8
MAC	6.6	17.2	21.3	14.5	64.2
MACN₂₀	5.1	11.3	16.2	14.3	69.7
MACN₄₀	5.9	9.4	19.4	16.5	72.5
MACN₅₅	7.2	7.9	17.5	14.9	73.6
MACP₂₀	9.1	8.2	20.7	18.6	60.8
MACP₄₀	5.4	5.8	22.5	19.5	61.0
MACP₆₀	8.7	4.4	24.4	17.8	62.8

The ACs were found to have adsorption percentages lower than 100%, which indicated that the presence of other metals interfered with the sorption of gold ions. About 64% adsorption was achieved for MAC, 70% for MACN₂₀, 72% for MACN₄₀, 73% for MACN₅₅ and 81% for CAC. The removal of gold by phosphate-modified AC was 60.8% for MACP₂₀, 61.0% for MACP₄₀ and 62.8% for MACP₆₀ and this performance was inferior compared to MACN carbons. Copper was found to interfere the least, while nickel was found to interfere the most with all the ACs investigated. Similar results were observed by Sayiner and Acarkan (2014), who investigated the effect of nickel, silver and copper cyanides on adsorption of gold cyanide by ACs from leached samples. Their studies have shown that nickel had the highest adsorption rate and Cu had the lowest.

Reusability of ACs and application to real-world samples

The results presented so far demonstrated that MACN₅₅ exhibited higher adsorption capabilities compared to MACN₂₀ and MACN₄₀ while MACP₆₀ performed better than MACP₂₀ and MACP₄₀. Consequently, the investigation of reusability of the ACs was carried out using MACN₅₅, MACP₆₀, MAC and CAC for comparison. After five cycles, MACN₅₅ demonstrated an adsorption efficiency greater than 80% (Figure 9(a)) which was similar to that of CAC. MAC achieved just above 60% adsorption efficiency after five cycles, and MACP₆₀ was sandwiched just in-between at 70% efficiency. These results verified that MACN₅₅ could be used as an alternative to CAC for gold adsorption in the mining industry waste.

Figure 9.

Re-usability studies for MAC, MACN₅₅, MACP₆₀ and CAC (a) and Application of ACs on real-world sample (b) (conditions: Solution pH 10; contact time, 1 h, dosage of carbon, 4 g/L, agitation speed used 150 r/min and concentrations used were 5.7 mg/L).

A barren sample solution containing 5.88 mg/L of Au, 97.80 mg/L of Ni, 4.12 mg/L of Fe, 5.80 mg/L of Cu and 1.11 mg/L of Pb collected from West Wits Chemical Lab was used to investigate the applicability of ACs on real-world samples. The pH of the barren sample was measured and adjusted to pH 10, and the adsorption was carried out as stipulated previously utilizing optimum conditions with samples taken every hour for analysis. It was observed that about 60% adsorption of gold ion was achieved with MAC, 65% with MACP₆₀, while MACN₅₅ and CAC attained 74% adsorption after 1 h (Figure 9(b)). These results were similar to those observed in the study of interfering species particularly because the real sample (barren solution) contained the same metal species (Ni, Cu, Pb and Fe) which were selected for the study of interfering species. None of the investigated ACs reached 100% adsorption percentage from barren samples, proving that the adsorption rate of gold in the presence of other metals does indeed decrease. Again, MACN₅₅ showed similar adsorption capabilities to CAC.

Adsorption isotherms for HNO₃ ACs

Equilibrium data from the effect of concentration were fitted in the Langmuir and Freundlich isotherms. The Langmuir isotherm assumes that an adsorbent homogeneous surface is covered by a monolayer of adsorbate molecules. The linearized Langmuir equation is given in equation (6) $\frac{Ce}{qe} = \frac{Ce}{qm} + \frac{1}{b . qm}$ (6)where q_e is the amount of adsorbate sorbed on the surface of adsorbent (mg/g), b is the Langmuir adsorption constant (L/mg), Ce is the equilibrium gold cyanide concentration (mg/L), and q_m is the adsorption capacity (mg/g), and. A plot of Ce/qe against Ce gives the values of q_m (slope = 1/q_m) and b (intercept = 1/q_m.b). Freundlich adsorption isotherm, which entails a multilayer adsorption on heterogeneous sites, is described by the following linearized form, equation (7) $log qe = log K_{F} + \frac{1}{n log Ce}$ (7)where K_F is the Freundlich constant and n is the Freundlich exponent. A plot of log C_e versus log q_e gives the slope 1/n and intercept log K_F. Table 3 shows the results of the equilibrium constants for the Freundlich and the Langmuir isotherms. Using the coefficient of determination error terms (R²), it can be observed that all the ACs followed a Langmuir, implying a monolayer adsorption as the R² values were greater than 0.99. Elsewhere, adsorption of benzene and toluene on nitric acid modified ACs fitted in Langmuir isotherm (da Costa Lopes et al., 2015).

Table 3.

Adsorption isotherms and kinetic rate models constants for ACs.

	Adsorption isotherms						Kinetic models
	Langmuir			Freundlich			PFO			PSO
Adsorbents	q_m (mg/g)	b (L/mg)	R²	K_f [(mg/g) /(mg/L)1/n]	1/n	R²	k₁ (1/h)	q_e (mg/g)	R²	k₂ (g/mg/h)	q_e (mg/g)	h_o (mg/g/h)	R²
CAC	12.64	0.33	0.895	5.44	0.25	0.866	0.80	0.48	0.848	3.04	1.47	6.56	0.999
MAC	9.13	1.41	0.987	3.69	0.48	0.884	0.44	0.68	0.957	0.85	1.58	2.12	0.995
MACN₂₀	10.87	0.62	0.977	8.67	0.33	0.938	0.73	0.81	0.954	1.35	1.53	3.17	0.999
MACN₄₀	9.66	1.01	0.984	5.06	0.30	0.899	0.71	0.71	0.955	1.55	1.52	3.57	0.999
MACN₅₅	9.30	1.29	0.987	5.39	0.26	0.856	0.67	0.59	0.926	1.81	1.50	4.08	0.997
MACP₂₀	11.56	0.46	0.948	4.08	0.43	0.902	1.20	1.22	0.932	2.10	1.49	4.72	0.999
MACP₄₀	10.82	0.58	0.966	4.40	0.38	0.896	1.25	1.13	0.874	2.71	1.48	5.95	0.999
MACP₅₅	10.09	0.78	0.978	4.77	0.33	0.883	1.37	1.15	0.803	3.37	1.47	2.15	0.998

Kinetic modeling

The pseudo-first-order and pseudo-second-order kinetic rate models were applied in their linearized forms, equations (8) and (9), respectively, to evaluate the process of adsorption. $\log (q_{e} - q_{t}) = \log q_{e} - \frac{k_{1}}{2.303} t$ (7) $\frac{t}{q_{t}} = \frac{1}{k_{2} q_{e}^{2}} + \frac{1}{q_{e}} t$ (8)where q_t (mg/g) is the amount of Au(CN)₂^– adsorbed at any time t, k₁ (1/h) is the pseudo-first-order (PFO) rate constant of the adsorption process, k₂ (g/mg/h) is the pseudo-second-order (PSO) rate constant, and k₂q_e² is the initial adsorption rate (h_o, mg/g/h). The values of q_e and k₁ were calculated from the linearized form of equation (5). A linear plot of t/qt against t was used to calculate the intercept (k₂) and slope (q_e). The results are depicted in Table 3. It was observed that correlation coefficient values (R²) of the PSO rate models showed better fighting and linearization compared to those of PFO kinetics as they were found to be closer to unity. A PSO fitting further suggested that the type of interaction between Au(CN)₂^– and the adsorbent surface in all ACs was chemisorption (Pakade et al., 2017).

Conclusions

The disappearance of a broad OH^– band at 3400 cm⁻¹ from treated MAC and the appearance of new bands at about 980 cm⁻¹ signaled a successful modification of MAC by the nitric acid-activating agent. Furthermore, the new peaks in treated MACs appeared at slightly different wavenumbers which signified that they were of different chemical environments. The different concentrations of HNO₃ produced materials with different chemistries. The changes were credited to the formation of ester linkages between adjacent hydroxyl and carboxyl groups leading to attenuation of the hydroxyl band at 3400 cm⁻¹. Condensation of alcohols to alkenes was another chemical change attributed to the heating of pristine MAC in the presence of acid. The attenuation of OH^– was supported by elemental analysis that showed a decrease in oxygen content after modification. TGA, SEM and XRD also revealed different structural changes of the surface of MAC as a consequence of acid treatment. MAC treated with concentrated HNO₃ acid (MACN₅₅) displayed superior adsorption properties than the diluted concentrations but its performance matched that of coconut-based ACs. Even when applied to real-world samples, MACN₅₅ and CAC performed the same. All ACs obeyed the Langmuir monolayer adsorption and the PSO kinetic rate model which predicted a chemisorption type of interaction between Au ions and adsorbents. In general, all these results showed that MACN₅₅ could be used as an alternative and/or a complementary supplement to CAC which is currently being used in the mining industry.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research,authorship,and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research,authorship,and/or publication of this article: The financial support from Vaal University of Technology is gratefully appreciated.

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An alternative low-cost adsorbent for gold recovery from cyanide-leached liquors: Adsorption isotherm and kinetic studies

Abstract

Keywords

Introduction

Materials and methods

Chemicals and materials

Characterization

Proximate analysis

Preparations of stock and working solutions

Preparation of MACs

Adsorption studies

Application to real-world samples

Results and discussion

Characterization of modified ACs

FTIR spectra of various ACs

Thermogravimetric analyses

Elemental analysis

BET surface determination of various carbons

X-ray diffraction analysis

SEM analysis of ACs

Physical characterization of ACs

Ash content

Moisture content

Volatile matter and attrition

Adsorption studies

Effect of adsorbent dosage concentration

Effect of initial gold concentration

Effect of agitation

Effect of contact time

Selectivity (effect of interfering species)

Reusability of ACs and application to real-world samples

Adsorption isotherms for HNO3 ACs

Kinetic modeling

Conclusions

Footnotes

Declaration of Conflicting Interests

Funding

References

Adsorption isotherms for HNO₃ ACs