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Investigation of mercury cyanide adsorption from synthetic wastewater aqueous solution on granular activated carbon
Journal of Water Process Engineering 34 (2020) 101154
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Investigation of mercury cyanide adsorption from synthetic wastewater aqueous solution on granular activated carbon
T
Paula Aliprandinia,*, Marcello M. Veigab, Bruce G. Marshallb, Tatiana Scarazzatoc, Denise C.R. Espinosaa a
Chemical Engineering, University of São Paulo, São Paulo, SP, Brazil Norman B. Keevil Institute of Mining Engineering, University of British Columbia, Vancouver, Canada c Programa de Pós-Graduação em Engenharia de Minas, Metalúrgica e de Materiais – PPGE3M, Universidade Federal do Rio Grande do Sul, Porto Alegre, Brazil b
A R T I C LE I N FO
A B S T R A C T
Keywords: Cyano-complexes Gold mine tailings Mercury contamination Remediation systems Wastewater Adsorption
Mercury (Hg) is one of the most harmful metals to human health. Combined with cyanide (CN−), Hg forms a stable complex (Hg(CN)2) that is difficult to remove from solution. Artisanal gold mining effluents containing mercury cyanide complexes are discharged into rivers and streams, which can be potentially bioavailable to aquatic organisms. In this study, activated carbon adsorption was evaluated for its capacity to remove mercury cyanide from solution. A solution with Hg(CN)2 and free cyanide was prepared at pH 12, corresponding to 1 mg/ L Hg2+ and 0.5 mg/L CN. Analysis in batch mode showed that 180 min were required to achieve adsorption equilibrium. The adsorbent dosage was fixed at 0.1 g of activated carbon, which represented an adsorption capacity of 0.14 mg/g, equivalent to adsorption of 81 % Hg(CN)2. The free cyanide concentration did not affect the adsorption of mercury cyanide. Experimental data indicated that the adsorption follows pseudo-second order kinetics and equilibrium results corresponded to the Freundlich adsorption isotherm.
1. Introduction Mercury from chemical waste disposal is known to impact the environment. Mercury occurs in several chemical forms [1]. However, all mercury species are harmful both to human health and the environment [2–6]. Gold mining activities, especially artisanal gold mining, are the main sources of mercury releases and pollution in the environment [7,8]. For recovering gold, a concentration process called amalgamation is used. The ore is contacted with liquid mercury, which forms a solid amalgam with gold. The next step in the process is to heat the amalgam to distill the mercury and recover the gold [7]. Due to the process being grossly, inefficient, typically recovering less than 30 % of the gold [9] a secondary gold extraction method is required [10]. The gold-rich and Hg-contaminated tailings then undergo a leaching process using cyanide [3,10,11], which removes the remaining gold, extracting more than 90 % of the metal [3,8]. However, at the same time, the combination of mercury and cyanide form soluble Hg(CN)2 complex, which have been shown to be potentially bioavailable to aquatic organisms. Mercury cyanide (Hg(CN)2) is a strong and stable cyanide complex, even at pH values lower than 9, making this complex difficult to remove from effluents [9,10]. The discharge of untreated gold processing effluents rich in Hg-CN complexes into rivers downstream from ⁎
gold processing centers is a growing problem around the world [12]. Adsorption methods are the most commonly used water treatment techniques for effluents containing organic and inorganic contaminants [1,13–20]. According to Di Natale et al. [21] and Samad et al. [5], adsorption techniques are the most indicated methods to remove mercury from effluents. Czarna et al. [22] synthesized zeolites from fly ash for adsorption of ionic mercury from wastewater, which reached ion adsorption of 89 % for a solution with 575 mg/L Hg. In comparison, palm oil fuel ash was used by Samad et al. [5] as an adsorbent to remove mercury (II) from industrial wastewater, achieving 91 % ion removal at a mercury concentration of 5 mg/L. Activated carbon is considered the main adsorbent used in the removal of impurities from effluents, including mercury [23–27]. A study by Asasian et al. [28] investigating mercury adsorption using activated carbon prepared from agricultural waste showed that Hg(II) adsorption occurred in relation to physical and chemical mechanisms. Di Natale et al. [21], who investigated correlations between mercury adsorption on activated carbon and different concentrations of dissolved Hg species, found that HgOH and Hg(II) were captured to a higher extent than HgCl, but their adsorption was more sensitive to solution pH. Another study by Zhang et al. [23] showed that 80 % of Hg (II) was removed from aqueous solution using activated carbon
Corresponding author. E-mail address: [email protected] (P. Aliprandini).
https://doi.org/10.1016/j.jwpe.2020.101154 Received 20 August 2019; Received in revised form 4 January 2020; Accepted 15 January 2020 2214-7144/ © 2020 Elsevier Ltd. All rights reserved.
Journal of Water Process Engineering 34 (2020) 101154
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developed from organic sewage sludge. Zabihi et al. [29] evaluated the adsorption of Hg+2 on activated carbons from walnut shell, whereby the maximum adsorption capacity of the mercury ion on the activated carbon was 151.5 mg/g. In other numerous studies, the adsorption of ionic mercury on activated carbon has been investigated with varying results [24,26,30,31]. For the present study, the aim was to identify different parameters associated with the adsorption of mercury cyanide on activated carbon, including the equilibrium time for adsorbing the complex, the mass of activated carbon required to adsorb 1 mg/L of Hg(CN)2, and the influence of free cyanide concentrations. From the experimental results, adsorption isotherms and kinetic models were evaluated to explain the adsorption process.
mercury ion in solution, V is the volume and W is the weight of activated carbon used.
2. Materials and methods
2.3.2. Influence of activated carbon dosage To investigate the effect of mercury cyanide on the capacity of activated carbon, different dosages of adsorbent were used: 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, 0.15, 0.2, 0.3, 0.4 and 0.5 g. All of the other parameters were kept constant: 15 mL solution; 25 °C temperature; 180 min contact time; and shaken at 200 rpm.
2.3.1. Effect of contact time on adsorption To investigate the effect of contact time on the adsorption process, samples were analyzed at time intervals of 10, 20, 30, 40, 60, 90, 100, 120, 150, 180, 240, 360, 540, 960, 1200 and 1440 min. All the other parameters were kept constant: activated carbon dosage of 0.03 g, volume of solution at 15 mL, temperature of 25 °C and shaken at 200 rpm. The data obtained for the contact time was used to investigate the adsorption processes of mercury cyanide on the activated carbon, whereby the pseudo-first order and pseudo-second kinetic models were evaluated.
2.1. Work solution Mercury cyanide solutions were prepared by dissolution of mercuric cyanide Hg(CN)2, (reagent grade) and potassium cyanide (KCN) in an aqueous solution of sodium hydroxide at pH 12. The chemical stability of mercury cyanide complexes in solution was determined by the development of chemical equilibrium diagrams for Hg and CN in NaOH solution at 25 °C, using Hydra-Medusa software and associated database. For this step, a mercury concentration of 5μM (∼1 mg/L) and a total cyanide concentration of 19μM (∼0.5 mg/L) were used together with varying pH from 9 to 14. As artisanal gold mining is characterized by rudimentary techniques [10,32–35], the amount of mercury and cyanide used in gold processing at different mines can vary greatly. According to Tassel et al. [36], the Hg concentration in gold processing effluents ranges from 0.2 to 0.98 mg/L. In addition, the cyanide concentration exceeds the amount required to form the complex with mercury. For this study, a synthetic Hg (CN)2 solution containing 1 mg/L of mercury and 0.5 mg/L of total cyanide (corresponding to 0.25 mg/L free cyanide) was used.
2.3.3. Effect of free cyanide concentrations In order to investigate the influence of the free cyanide concentration on mercury cyanide adsorption, solutions with different concentration ratios of Hg(CN)2 and free CN were used. The Hg(CN)2:CNfree ratios varied from 1:0, 10:1, 4:1, 2:1, 1:1, 1:5, 1:10 and 1:50, while maintaining the mercury concentration constant at 1 mg/L. Other parameters, such as activated carbon dosage (0.1 g) and solution volume (15 mL) were kept constant. For this stage of the study, the cyanide concentration of the samples was also evaluated. The measurement was made using a polarographic method for determining cyanide. 2.3.4. Adsorption isotherms The adsorption data of Hg(CN)2 onto activated carbon was analyzed using three adsorption models: Langmuir, Frundlich and Temkin. The Langmuir equation can be written as Equation (2) [4,23,39,40]
2.2. Sorbent characteristics
Ce/qe = (1/qm KL) + Ce/qm
Experiments were conducted with a commercial granular activated carbon produced from coconut shells. The original material was sieved to a size of 75−250 μm. The textural properties of the adsorbent were examined for surface area measurements by N2 adsorption (BET) and the morphology was performed using scanning electron microscopy (SEM). The surface charge of the material was determined in accordance with the point of zero charge (pHPZC), which was set up using a 0.01 mol/L NaCl solution and adjusted to the required pH (1–12) by adding 0.1 mol/L NaOH and 0.1 mol/L HCl. The resulting solution (20 mL) at each pH was poured into an Erlenmeyer flask containing 0.1 g of activated carbon and shaken at 150 rpm for 24 h. The final pH and pHPZC were then determined [37].
where Ce (mg/L) is the equilibrium concentration of Hg(CN)2 in solution, qe (mg/g) is the volume of complexes adsorbed, qm (mg/g) is the maximum adsorption capacity of complexes and KL (L/mg) is the Langmuir adsorption equilibrium constant. The Freundlich isotherm is expressed as Equation (3) [4,23,40]. log qe = log Kf + (1/n) log Ce
(3)
where Kf (mg/g) and n (g/L) are the Freundlich constants related to adsorption capacity and intensity. The Temkin isotherm can be expressed as Equation (4) [38,40]. qe = (RT/bT) ln KT + (RT/bT) ln Ce qe
2.3. Sorption procedure
(4)
where T (K) is the temperature, R is the universal gas constant equal to 8.314 J/mol.K, bT (J/mol) and KT (L/g) are the constants related to the adsorption heat and to maximum binding energy, respectively. The resulting data were obtained by varying the concentration of mercury cyanide and free cyanide, while the Hg concentrations varied between 0.25 and 2.5 mg/L. The Hg:CNtotal ratio 2:1 was maintained for all of the study solutions.
The experiments were carried out in batch mode. The activated carbon was weighed in 25 mL Erlenmeyer flasks and a 15 mL working solution of mercury cyanide was prepared in different concentrations and directly added to it. Each sample was shaken at 200 rpm and 25 °C. The contact time was determined for each experimental step. The mercury concentration in solution was measured by atomic absorption spectrometry (mercury analyzer RA-915 M with pyrolyzer PYRO915+) and the adsorption capacity (qe) of Hg(CN)2 on the activated carbon was calculated using Equation (1) [4,29,38,39]. qe = ((C0 – Ce)/W) V
(2)
3. Results and discussion To understand the stability of Hg(CN)2 and free cyanide behavior, a speciation diagram for the working solution was constructed using Hydra-Medusa software. According to Drace et al. [3], mercury cyanide can form different complexes, such as: Hg(CN)2, Hg(CN)− 3 and Hg
(1)
where Co and Ce are the initial and equilibrium concentrations of 2
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Fig. 1. Speciation plot for the Hg-CN system as a function of pH. Hg = 1 mg/L and CNtotal = 0.5 mg/L.
Fig. 3. Effect of contact time in relation to the adsorption of Hg(CN)2 on activated carbon.
(CN)24. Fig. 1 shows the speciation diagrams predicted by the Hydra-Medusa software. In the selected pH range (9–14), mercury is mainly present as the Hg(CN)2 complex. For free cyanide at pH above 11, the CN− ion is the predominant species against HCN (hydrogen cyanide). However, the presence of HCN increased with the lowering of pH values, as established in the literature [3,9]. Based on the speciation diagram, all of the mercury present was considered to be tied up as a Hg(CN)2 complex, while the other complexes were considered in low concentration or disregarded.
3.2. Sorption procedure 3.2.1. Effect of contact time on adsorption The influence of contact time on the removal of Hg(CN)2 by activated carbon can be seen in Fig. 3. The removal efficiency increased rapidly in the first minutes, reaching about 0.27 g/mg at 10 min, which was equal to 58 % adsorption. This behavior is a result of the number of available active sites on the activated carbon surface. Equilibrium was reached in approximately 180 min, giving an adsorption capacity of 0.40 mg/g, which was equivalent to 81 % adsorption. Therefore, the adsorption time of mercury cyanide was set to 180 min in each experiment. Based on the contact time results, the adsorption kinetics of mercury cyanide on activated carbon were investigated. The pseudo-first order and pseudo-second order kinetic models were applied to the kinetic study, whereby the model that best fits a straight line (R2 values) describes the probable adsorption mechanism. These kinetic expressions are defined as [39,42,43]
3.1. Sorbent characteristics The SEM micrograph of activated carbon can be seen in Fig. 2, which shows that the morphology of the adsorbent has an irregular shape and the presence of pores in the material is a characteristic typical of activated carbon [41]. The BET surface area of the activated carbon was found to be 506.3m2/g with a total pore volume of 0.24cm3/g. The pHPZC was approximately 7.7, showing that the activated carbon had a slightly basic nature.
ln(qe – qt) = ln qe – k1t
(5)
(1/k2qe2)
(6)
t/qt =
+ t/qe
where qt and qe are mercury cyanide absorbed at time t and at equilibrium, respectively, and k1 and k2 is the first-order and second-order reaction rate equilibrium constant, respectively. Figs. 4 and 5 show the results of the kinetic data modeling. The pseudo-second order model describes the adsorption of Hg(CN)2 better
Fig. 2. Scanning electron microscopy (SEM) of activated carbon at a magnification of 275×.
Fig. 4. Pseudo-first order model for Hg(CN)2 adsorption on activated carbon. 3
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Fig. 5. Pseudo-second order model for Hg(CN)2 adsorption on activated carbon.
Fig. 6. Effect of adsorbent dosage on Hg(CN)2 removal.
Table 1 Kinetic parameters for the adsorption of mercury cyanide onto activated carbon. Model
Parameters
R2
Pseudo-first order kinetic
K1 = 0.0238 min−1 qe, calc. = 0.1348 mg/g qe, exp. = 0.40 mg/g K2 = 0.881 g/mg.min qe, calc. = 0.397 mg/g qe, exp. = 0.40 mg/g
0.745
Pseudo-second order kinetic
0.999
than that of the pseudo first-order. According to the data in Table 2, the pseudo second-order kinetic model had the best correlation coefficient, which was corroborated by the similarity between the calculated qe value and the experimental value. These results suggested that the adsorption data were well represented by this model (Table 1). According to the pseudo-second order kinetic, the adsorption of mercury cyanide occurs by chemisorption and ion-exchange [18,28]. The adsorption capacity for the Hg(CN)2-activated carbon system was low compared to other systems. Synthetic and surface activated materials can achieve theoretical adsorption capacities greater than 18 mg/g for wastewater containing dyes and endocrine-disrupting chemicals or even greater than 42 mg/g for dibenzothiophene, which are both treated with synthesized adsorbents [18,19]. However, even with lower adsorption capacity compared to other adsorption processes, activated carbon was efficient enough to remove the mercury complex without surface pretreatment.
Fig. 7. Effect of the initial concentration of free cyanide on the adsorption of mercury cyanide.
other ratios, adsorption was reduced with increasing CN− ion concentration. Based on this, the selectivity for the complex was higher than for the cyanide ion.
3.2.4. Adsorption isotherms In this study, Langmuir, Freundlich and Temkin models were used to describe the equilibrium experimental data for the adsorption of mercury cyanide on activated carbon. The results are reported in Table 2 and the isotherms shown in Figs. 8–10. The experimental data fit the Freundlich isotherm model (R2 = 0.9738) better than the Langmuir and Temkin models. Although the Langmuir model is the model that best fits for contaminant removal systems using both natural [27] and synthetic adsorbents [18,20],
3.2.2. Influence of activated carbon dosage A significant increase in Hg(CN)2 adsorption was observed when the dosage of activated carbon was increased from 0.01–5.0 g, as this leads to the introduction of more active sites available for adsorption [42,44]. Fig. 6 shows that, even with a small amount of activated carbon (0.01 g), it was possible to remove 47 % of mercury cyanide. Between 0.01 and 0.1 g, adsorption ranged from 47 to 93 %. However, between 0.1 and 0.5 g of activated carbon, adsorption varied between 93–98%. The adsorption capacity for 0.1 g of adsorbent was 0.14 mg/g.
Table 2 Langmuir, Freundlich and Temkin model parameters for the adsorption of Hg (CN)2 on activated carbon.
3.2.3. Effect of free cyanide concentrations The effect of free cyanide concentrations was investigated using solutions with different initial concentration between zero and 50 mg/ L, while maintaining the mercury concentration constant at 1 mg/L. The results can be seen in Fig. 7, which shows that Hg(CN)2 adsorption remained constant and above 92 % for all of the free cyanide concentrations. This means that the amount of free cyanide had little influence on the adsorption of mercury cyanide by activated carbon. For the 10:1 and 4:1 ratios, the free cyanide was 100 % adsorbed. For the
Models
Parameters
Langmuir
KL (L/mg) qm (mg/g) R2 Kf (mg/g) n (g/L) R2 KT (L/g) bT (J/mol) R2
Freundlich
Temkin
4
1.57 1.66 0.6144 1.28 1.31 0.9738 30.48 9475.61 0.8433
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the pollutant [29]. However, if the value of n is equal to 1, this indicates linear adsorption leading to identical adsorption energies for all sites [48]. As the n value in this case was greater than one, there is good indication of a strong physical bond between the activated carbon and Hg(CN)2 [38]. 4. Conclusions Mercury and cyanide form a complex that remains in solution as the Hg(CN)2 species. Even at pH values where the cyanide ion is transformed into hydrogen cyanide, the Hg(CN)2 complex remains stable, which was confirmed by the speciation diagram. Adams [49] had previously predicted that activated carbon had a better affinity for neutral species as Hg(CN)2 than for negatively charged species. The commercial granular activated carbon a presented surface area of 506.3m2/g. In addition, the pHPZC of the material was 7.7. In this study, commercial granular activated carbon was effective for the removal of mercury cyanide at pH 12. Even with the rapid adsorption of the complex on activated carbon, the adsorption equilibrium was achieved in 180 min. At that time, the adsorption capacity for 0.03 g of carbon was 0.40 mg/g, corresponding to 81 % Hg(CN)2 removal. Between 0.1 and 0.5 g of activated carbon, adsorption varied from 93–98%. Using 0.1 g of adsorbent, the adsorption capacity was 0.14 mg/g. The adsorption of Hg(CN)2 remained constant and above 92 % for all of the free cyanide concentrations. The presence of free cyanide did not affect the adsorption of the complex on the activated carbon. The present study showed that the adsorption of mercury cyanide complex on activated carbon followed the Freundlich adsorption isotherm model, with the pseudo-second order model being the best for describing the adsorption kinetics of Hg(CN)2. Based on these theories, it was concluded that adsorption occurred by chemisorption and ion-exchange. In addition, the system was determined to be heterogeneous, with reversible and multilayer adsorption, as well as being a physical process. Adsorption was favorable and the bonds between the activated carbon and the Hg(CN)2 were strong.
Fig. 8. Plot of Langmuir adsorption isotherm for activated carbon adsorption of Hg(CN)2.
Fig. 9. Plot of Freundlich adsorption isotherm for activated carbon adsorption of Hg(CN)2.
Statement of novelty The main objective of this work is to evaluate the possibility of absorption in activated carbon for mercury cyanide. Even though many researchers were worked with activated carbon, very few researchers were reported about mercury cyanide complexes. The adsorption capacity for mercury is known, but the conditions of the artisanal gold mining effluent make the parameters different for the adsorption. The combination of highly toxic compounds, mercury and cyanide are potentially hazardous when exposed to the environment and workers. The evaluation of the removal of these complexes is in accordance with the journal proposal and its search for new approaches. 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.
Fig. 10. Plot of Temkin adsorption isotherm for activated carbon adsorption of Hg(CN)2.
Acknowledgments
isotherms in activated carbon systems have been shown to fit well with the Freundlich model [18,20,45,46]. The Freundlich model assumes a multilayer and reversible adsorption, within a heterogeneous system [28,47]. The n value represents the adsorption intensity. For the experimental data, n was calculated at 1.31 g/L. It is important to point out that n values between one and ten indicates that the adsorption was favorable between the adsorbent and
Financial support to this study was supplied by the São Paulo Research Foundation (FAPESP - grant 2012/51871-9). This study was partly financed by the Coordination for the Improvement of Higher Education Personnel - Brazil (CAPES) Finance Code 001 and Finance Code 88887.362657/219-00. by National Council for Scientific and Technological Development (CNPq) Project number 426816/2018-8. The authors thank Ana Carolina Fadel Dalsin for her help on the project. 5
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Special thanks to Lumex Instruments for their partnership in this research project.
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Investigation of mercury cyanide adsorption from synthetic wastewater aqueous solution on granular activated carbon
T
Paula Aliprandinia,*, Marcello M. Veigab, Bruce G. Marshallb, Tatiana Scarazzatoc, Denise C.R. Espinosaa a
Chemical Engineering, University of São Paulo, São Paulo, SP, Brazil Norman B. Keevil Institute of Mining Engineering, University of British Columbia, Vancouver, Canada c Programa de Pós-Graduação em Engenharia de Minas, Metalúrgica e de Materiais – PPGE3M, Universidade Federal do Rio Grande do Sul, Porto Alegre, Brazil b
A R T I C LE I N FO
A B S T R A C T
Keywords: Cyano-complexes Gold mine tailings Mercury contamination Remediation systems Wastewater Adsorption
Mercury (Hg) is one of the most harmful metals to human health. Combined with cyanide (CN−), Hg forms a stable complex (Hg(CN)2) that is difficult to remove from solution. Artisanal gold mining effluents containing mercury cyanide complexes are discharged into rivers and streams, which can be potentially bioavailable to aquatic organisms. In this study, activated carbon adsorption was evaluated for its capacity to remove mercury cyanide from solution. A solution with Hg(CN)2 and free cyanide was prepared at pH 12, corresponding to 1 mg/ L Hg2+ and 0.5 mg/L CN. Analysis in batch mode showed that 180 min were required to achieve adsorption equilibrium. The adsorbent dosage was fixed at 0.1 g of activated carbon, which represented an adsorption capacity of 0.14 mg/g, equivalent to adsorption of 81 % Hg(CN)2. The free cyanide concentration did not affect the adsorption of mercury cyanide. Experimental data indicated that the adsorption follows pseudo-second order kinetics and equilibrium results corresponded to the Freundlich adsorption isotherm.
1. Introduction Mercury from chemical waste disposal is known to impact the environment. Mercury occurs in several chemical forms [1]. However, all mercury species are harmful both to human health and the environment [2–6]. Gold mining activities, especially artisanal gold mining, are the main sources of mercury releases and pollution in the environment [7,8]. For recovering gold, a concentration process called amalgamation is used. The ore is contacted with liquid mercury, which forms a solid amalgam with gold. The next step in the process is to heat the amalgam to distill the mercury and recover the gold [7]. Due to the process being grossly, inefficient, typically recovering less than 30 % of the gold [9] a secondary gold extraction method is required [10]. The gold-rich and Hg-contaminated tailings then undergo a leaching process using cyanide [3,10,11], which removes the remaining gold, extracting more than 90 % of the metal [3,8]. However, at the same time, the combination of mercury and cyanide form soluble Hg(CN)2 complex, which have been shown to be potentially bioavailable to aquatic organisms. Mercury cyanide (Hg(CN)2) is a strong and stable cyanide complex, even at pH values lower than 9, making this complex difficult to remove from effluents [9,10]. The discharge of untreated gold processing effluents rich in Hg-CN complexes into rivers downstream from ⁎
gold processing centers is a growing problem around the world [12]. Adsorption methods are the most commonly used water treatment techniques for effluents containing organic and inorganic contaminants [1,13–20]. According to Di Natale et al. [21] and Samad et al. [5], adsorption techniques are the most indicated methods to remove mercury from effluents. Czarna et al. [22] synthesized zeolites from fly ash for adsorption of ionic mercury from wastewater, which reached ion adsorption of 89 % for a solution with 575 mg/L Hg. In comparison, palm oil fuel ash was used by Samad et al. [5] as an adsorbent to remove mercury (II) from industrial wastewater, achieving 91 % ion removal at a mercury concentration of 5 mg/L. Activated carbon is considered the main adsorbent used in the removal of impurities from effluents, including mercury [23–27]. A study by Asasian et al. [28] investigating mercury adsorption using activated carbon prepared from agricultural waste showed that Hg(II) adsorption occurred in relation to physical and chemical mechanisms. Di Natale et al. [21], who investigated correlations between mercury adsorption on activated carbon and different concentrations of dissolved Hg species, found that HgOH and Hg(II) were captured to a higher extent than HgCl, but their adsorption was more sensitive to solution pH. Another study by Zhang et al. [23] showed that 80 % of Hg (II) was removed from aqueous solution using activated carbon
Corresponding author. E-mail address: [email protected] (P. Aliprandini).
https://doi.org/10.1016/j.jwpe.2020.101154 Received 20 August 2019; Received in revised form 4 January 2020; Accepted 15 January 2020 2214-7144/ © 2020 Elsevier Ltd. All rights reserved.
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developed from organic sewage sludge. Zabihi et al. [29] evaluated the adsorption of Hg+2 on activated carbons from walnut shell, whereby the maximum adsorption capacity of the mercury ion on the activated carbon was 151.5 mg/g. In other numerous studies, the adsorption of ionic mercury on activated carbon has been investigated with varying results [24,26,30,31]. For the present study, the aim was to identify different parameters associated with the adsorption of mercury cyanide on activated carbon, including the equilibrium time for adsorbing the complex, the mass of activated carbon required to adsorb 1 mg/L of Hg(CN)2, and the influence of free cyanide concentrations. From the experimental results, adsorption isotherms and kinetic models were evaluated to explain the adsorption process.
mercury ion in solution, V is the volume and W is the weight of activated carbon used.
2. Materials and methods
2.3.2. Influence of activated carbon dosage To investigate the effect of mercury cyanide on the capacity of activated carbon, different dosages of adsorbent were used: 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, 0.15, 0.2, 0.3, 0.4 and 0.5 g. All of the other parameters were kept constant: 15 mL solution; 25 °C temperature; 180 min contact time; and shaken at 200 rpm.
2.3.1. Effect of contact time on adsorption To investigate the effect of contact time on the adsorption process, samples were analyzed at time intervals of 10, 20, 30, 40, 60, 90, 100, 120, 150, 180, 240, 360, 540, 960, 1200 and 1440 min. All the other parameters were kept constant: activated carbon dosage of 0.03 g, volume of solution at 15 mL, temperature of 25 °C and shaken at 200 rpm. The data obtained for the contact time was used to investigate the adsorption processes of mercury cyanide on the activated carbon, whereby the pseudo-first order and pseudo-second kinetic models were evaluated.
2.1. Work solution Mercury cyanide solutions were prepared by dissolution of mercuric cyanide Hg(CN)2, (reagent grade) and potassium cyanide (KCN) in an aqueous solution of sodium hydroxide at pH 12. The chemical stability of mercury cyanide complexes in solution was determined by the development of chemical equilibrium diagrams for Hg and CN in NaOH solution at 25 °C, using Hydra-Medusa software and associated database. For this step, a mercury concentration of 5μM (∼1 mg/L) and a total cyanide concentration of 19μM (∼0.5 mg/L) were used together with varying pH from 9 to 14. As artisanal gold mining is characterized by rudimentary techniques [10,32–35], the amount of mercury and cyanide used in gold processing at different mines can vary greatly. According to Tassel et al. [36], the Hg concentration in gold processing effluents ranges from 0.2 to 0.98 mg/L. In addition, the cyanide concentration exceeds the amount required to form the complex with mercury. For this study, a synthetic Hg (CN)2 solution containing 1 mg/L of mercury and 0.5 mg/L of total cyanide (corresponding to 0.25 mg/L free cyanide) was used.
2.3.3. Effect of free cyanide concentrations In order to investigate the influence of the free cyanide concentration on mercury cyanide adsorption, solutions with different concentration ratios of Hg(CN)2 and free CN were used. The Hg(CN)2:CNfree ratios varied from 1:0, 10:1, 4:1, 2:1, 1:1, 1:5, 1:10 and 1:50, while maintaining the mercury concentration constant at 1 mg/L. Other parameters, such as activated carbon dosage (0.1 g) and solution volume (15 mL) were kept constant. For this stage of the study, the cyanide concentration of the samples was also evaluated. The measurement was made using a polarographic method for determining cyanide. 2.3.4. Adsorption isotherms The adsorption data of Hg(CN)2 onto activated carbon was analyzed using three adsorption models: Langmuir, Frundlich and Temkin. The Langmuir equation can be written as Equation (2) [4,23,39,40]
2.2. Sorbent characteristics
Ce/qe = (1/qm KL) + Ce/qm
Experiments were conducted with a commercial granular activated carbon produced from coconut shells. The original material was sieved to a size of 75−250 μm. The textural properties of the adsorbent were examined for surface area measurements by N2 adsorption (BET) and the morphology was performed using scanning electron microscopy (SEM). The surface charge of the material was determined in accordance with the point of zero charge (pHPZC), which was set up using a 0.01 mol/L NaCl solution and adjusted to the required pH (1–12) by adding 0.1 mol/L NaOH and 0.1 mol/L HCl. The resulting solution (20 mL) at each pH was poured into an Erlenmeyer flask containing 0.1 g of activated carbon and shaken at 150 rpm for 24 h. The final pH and pHPZC were then determined [37].
where Ce (mg/L) is the equilibrium concentration of Hg(CN)2 in solution, qe (mg/g) is the volume of complexes adsorbed, qm (mg/g) is the maximum adsorption capacity of complexes and KL (L/mg) is the Langmuir adsorption equilibrium constant. The Freundlich isotherm is expressed as Equation (3) [4,23,40]. log qe = log Kf + (1/n) log Ce
(3)
where Kf (mg/g) and n (g/L) are the Freundlich constants related to adsorption capacity and intensity. The Temkin isotherm can be expressed as Equation (4) [38,40]. qe = (RT/bT) ln KT + (RT/bT) ln Ce qe
2.3. Sorption procedure
(4)
where T (K) is the temperature, R is the universal gas constant equal to 8.314 J/mol.K, bT (J/mol) and KT (L/g) are the constants related to the adsorption heat and to maximum binding energy, respectively. The resulting data were obtained by varying the concentration of mercury cyanide and free cyanide, while the Hg concentrations varied between 0.25 and 2.5 mg/L. The Hg:CNtotal ratio 2:1 was maintained for all of the study solutions.
The experiments were carried out in batch mode. The activated carbon was weighed in 25 mL Erlenmeyer flasks and a 15 mL working solution of mercury cyanide was prepared in different concentrations and directly added to it. Each sample was shaken at 200 rpm and 25 °C. The contact time was determined for each experimental step. The mercury concentration in solution was measured by atomic absorption spectrometry (mercury analyzer RA-915 M with pyrolyzer PYRO915+) and the adsorption capacity (qe) of Hg(CN)2 on the activated carbon was calculated using Equation (1) [4,29,38,39]. qe = ((C0 – Ce)/W) V
(2)
3. Results and discussion To understand the stability of Hg(CN)2 and free cyanide behavior, a speciation diagram for the working solution was constructed using Hydra-Medusa software. According to Drace et al. [3], mercury cyanide can form different complexes, such as: Hg(CN)2, Hg(CN)− 3 and Hg
(1)
where Co and Ce are the initial and equilibrium concentrations of 2
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Fig. 1. Speciation plot for the Hg-CN system as a function of pH. Hg = 1 mg/L and CNtotal = 0.5 mg/L.
Fig. 3. Effect of contact time in relation to the adsorption of Hg(CN)2 on activated carbon.
(CN)24. Fig. 1 shows the speciation diagrams predicted by the Hydra-Medusa software. In the selected pH range (9–14), mercury is mainly present as the Hg(CN)2 complex. For free cyanide at pH above 11, the CN− ion is the predominant species against HCN (hydrogen cyanide). However, the presence of HCN increased with the lowering of pH values, as established in the literature [3,9]. Based on the speciation diagram, all of the mercury present was considered to be tied up as a Hg(CN)2 complex, while the other complexes were considered in low concentration or disregarded.
3.2. Sorption procedure 3.2.1. Effect of contact time on adsorption The influence of contact time on the removal of Hg(CN)2 by activated carbon can be seen in Fig. 3. The removal efficiency increased rapidly in the first minutes, reaching about 0.27 g/mg at 10 min, which was equal to 58 % adsorption. This behavior is a result of the number of available active sites on the activated carbon surface. Equilibrium was reached in approximately 180 min, giving an adsorption capacity of 0.40 mg/g, which was equivalent to 81 % adsorption. Therefore, the adsorption time of mercury cyanide was set to 180 min in each experiment. Based on the contact time results, the adsorption kinetics of mercury cyanide on activated carbon were investigated. The pseudo-first order and pseudo-second order kinetic models were applied to the kinetic study, whereby the model that best fits a straight line (R2 values) describes the probable adsorption mechanism. These kinetic expressions are defined as [39,42,43]
3.1. Sorbent characteristics The SEM micrograph of activated carbon can be seen in Fig. 2, which shows that the morphology of the adsorbent has an irregular shape and the presence of pores in the material is a characteristic typical of activated carbon [41]. The BET surface area of the activated carbon was found to be 506.3m2/g with a total pore volume of 0.24cm3/g. The pHPZC was approximately 7.7, showing that the activated carbon had a slightly basic nature.
ln(qe – qt) = ln qe – k1t
(5)
(1/k2qe2)
(6)
t/qt =
+ t/qe
where qt and qe are mercury cyanide absorbed at time t and at equilibrium, respectively, and k1 and k2 is the first-order and second-order reaction rate equilibrium constant, respectively. Figs. 4 and 5 show the results of the kinetic data modeling. The pseudo-second order model describes the adsorption of Hg(CN)2 better
Fig. 2. Scanning electron microscopy (SEM) of activated carbon at a magnification of 275×.
Fig. 4. Pseudo-first order model for Hg(CN)2 adsorption on activated carbon. 3
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Fig. 5. Pseudo-second order model for Hg(CN)2 adsorption on activated carbon.
Fig. 6. Effect of adsorbent dosage on Hg(CN)2 removal.
Table 1 Kinetic parameters for the adsorption of mercury cyanide onto activated carbon. Model
Parameters
R2
Pseudo-first order kinetic
K1 = 0.0238 min−1 qe, calc. = 0.1348 mg/g qe, exp. = 0.40 mg/g K2 = 0.881 g/mg.min qe, calc. = 0.397 mg/g qe, exp. = 0.40 mg/g
0.745
Pseudo-second order kinetic
0.999
than that of the pseudo first-order. According to the data in Table 2, the pseudo second-order kinetic model had the best correlation coefficient, which was corroborated by the similarity between the calculated qe value and the experimental value. These results suggested that the adsorption data were well represented by this model (Table 1). According to the pseudo-second order kinetic, the adsorption of mercury cyanide occurs by chemisorption and ion-exchange [18,28]. The adsorption capacity for the Hg(CN)2-activated carbon system was low compared to other systems. Synthetic and surface activated materials can achieve theoretical adsorption capacities greater than 18 mg/g for wastewater containing dyes and endocrine-disrupting chemicals or even greater than 42 mg/g for dibenzothiophene, which are both treated with synthesized adsorbents [18,19]. However, even with lower adsorption capacity compared to other adsorption processes, activated carbon was efficient enough to remove the mercury complex without surface pretreatment.
Fig. 7. Effect of the initial concentration of free cyanide on the adsorption of mercury cyanide.
other ratios, adsorption was reduced with increasing CN− ion concentration. Based on this, the selectivity for the complex was higher than for the cyanide ion.
3.2.4. Adsorption isotherms In this study, Langmuir, Freundlich and Temkin models were used to describe the equilibrium experimental data for the adsorption of mercury cyanide on activated carbon. The results are reported in Table 2 and the isotherms shown in Figs. 8–10. The experimental data fit the Freundlich isotherm model (R2 = 0.9738) better than the Langmuir and Temkin models. Although the Langmuir model is the model that best fits for contaminant removal systems using both natural [27] and synthetic adsorbents [18,20],
3.2.2. Influence of activated carbon dosage A significant increase in Hg(CN)2 adsorption was observed when the dosage of activated carbon was increased from 0.01–5.0 g, as this leads to the introduction of more active sites available for adsorption [42,44]. Fig. 6 shows that, even with a small amount of activated carbon (0.01 g), it was possible to remove 47 % of mercury cyanide. Between 0.01 and 0.1 g, adsorption ranged from 47 to 93 %. However, between 0.1 and 0.5 g of activated carbon, adsorption varied between 93–98%. The adsorption capacity for 0.1 g of adsorbent was 0.14 mg/g.
Table 2 Langmuir, Freundlich and Temkin model parameters for the adsorption of Hg (CN)2 on activated carbon.
3.2.3. Effect of free cyanide concentrations The effect of free cyanide concentrations was investigated using solutions with different initial concentration between zero and 50 mg/ L, while maintaining the mercury concentration constant at 1 mg/L. The results can be seen in Fig. 7, which shows that Hg(CN)2 adsorption remained constant and above 92 % for all of the free cyanide concentrations. This means that the amount of free cyanide had little influence on the adsorption of mercury cyanide by activated carbon. For the 10:1 and 4:1 ratios, the free cyanide was 100 % adsorbed. For the
Models
Parameters
Langmuir
KL (L/mg) qm (mg/g) R2 Kf (mg/g) n (g/L) R2 KT (L/g) bT (J/mol) R2
Freundlich
Temkin
4
1.57 1.66 0.6144 1.28 1.31 0.9738 30.48 9475.61 0.8433
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the pollutant [29]. However, if the value of n is equal to 1, this indicates linear adsorption leading to identical adsorption energies for all sites [48]. As the n value in this case was greater than one, there is good indication of a strong physical bond between the activated carbon and Hg(CN)2 [38]. 4. Conclusions Mercury and cyanide form a complex that remains in solution as the Hg(CN)2 species. Even at pH values where the cyanide ion is transformed into hydrogen cyanide, the Hg(CN)2 complex remains stable, which was confirmed by the speciation diagram. Adams [49] had previously predicted that activated carbon had a better affinity for neutral species as Hg(CN)2 than for negatively charged species. The commercial granular activated carbon a presented surface area of 506.3m2/g. In addition, the pHPZC of the material was 7.7. In this study, commercial granular activated carbon was effective for the removal of mercury cyanide at pH 12. Even with the rapid adsorption of the complex on activated carbon, the adsorption equilibrium was achieved in 180 min. At that time, the adsorption capacity for 0.03 g of carbon was 0.40 mg/g, corresponding to 81 % Hg(CN)2 removal. Between 0.1 and 0.5 g of activated carbon, adsorption varied from 93–98%. Using 0.1 g of adsorbent, the adsorption capacity was 0.14 mg/g. The adsorption of Hg(CN)2 remained constant and above 92 % for all of the free cyanide concentrations. The presence of free cyanide did not affect the adsorption of the complex on the activated carbon. The present study showed that the adsorption of mercury cyanide complex on activated carbon followed the Freundlich adsorption isotherm model, with the pseudo-second order model being the best for describing the adsorption kinetics of Hg(CN)2. Based on these theories, it was concluded that adsorption occurred by chemisorption and ion-exchange. In addition, the system was determined to be heterogeneous, with reversible and multilayer adsorption, as well as being a physical process. Adsorption was favorable and the bonds between the activated carbon and the Hg(CN)2 were strong.
Fig. 8. Plot of Langmuir adsorption isotherm for activated carbon adsorption of Hg(CN)2.
Fig. 9. Plot of Freundlich adsorption isotherm for activated carbon adsorption of Hg(CN)2.
Statement of novelty The main objective of this work is to evaluate the possibility of absorption in activated carbon for mercury cyanide. Even though many researchers were worked with activated carbon, very few researchers were reported about mercury cyanide complexes. The adsorption capacity for mercury is known, but the conditions of the artisanal gold mining effluent make the parameters different for the adsorption. The combination of highly toxic compounds, mercury and cyanide are potentially hazardous when exposed to the environment and workers. The evaluation of the removal of these complexes is in accordance with the journal proposal and its search for new approaches. 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.
Fig. 10. Plot of Temkin adsorption isotherm for activated carbon adsorption of Hg(CN)2.
Acknowledgments
isotherms in activated carbon systems have been shown to fit well with the Freundlich model [18,20,45,46]. The Freundlich model assumes a multilayer and reversible adsorption, within a heterogeneous system [28,47]. The n value represents the adsorption intensity. For the experimental data, n was calculated at 1.31 g/L. It is important to point out that n values between one and ten indicates that the adsorption was favorable between the adsorbent and
Financial support to this study was supplied by the São Paulo Research Foundation (FAPESP - grant 2012/51871-9). This study was partly financed by the Coordination for the Improvement of Higher Education Personnel - Brazil (CAPES) Finance Code 001 and Finance Code 88887.362657/219-00. by National Council for Scientific and Technological Development (CNPq) Project number 426816/2018-8. The authors thank Ana Carolina Fadel Dalsin for her help on the project. 5
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Special thanks to Lumex Instruments for their partnership in this research project.
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