Adsorption of Ag⁺ ions using hydroxyapatite powder and recovery silver by electrodeposition

Cite this paper: Vietnam J. Chem., 2021, 59(2), 179-186 Article DOI: 10.1002/vjch.202000148 179 Wiley Online Library © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH Adsorption of Ag+ ions using hydroxyapatite powder and recovery silver by electrodeposition Pham Thi Nam 1 , Dinh Thi Mai Thanh 2,3 , Nguyen Thu Phuong 1 , Nguyen Thi Thu Trang 1 , Cao Thi Hong 1 , Vo Thi Kieu Anh 1 , Tran Dai Lam 1,3 , Nguyen Thi Thom 1* 1 Institute for Tropical Technology, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Cau Giay, Hanoi 10000, Viet Nam 2 University of Science and Technology of Hanoi, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Cau Giay, Hanoi 10000, Viet Nam 3 Graduate University of Science and Technology, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Cau Giay, Hanoi 10000, Viet Nam Submitted August 31, 2020; Accepted November 9, 2020 Abstract Nowadays, waste of electrical and electronic apparatuses generated in huge amount surround the earth and has become a global environmental issue. Electronic waste contains large amounts of metal ions, such as Au, Ag, Cu, Pd, Pb and Cd etc., resulting in a threat to the environment, ecosystems and human health. Therefore, removal of metal ions and recovery of precious metals are extremely necessary. Hydroxyapatite material was reported that they can remove heavy metal ions in water with high efficiency. In this work, Ag + ions in water were adsorbed using hydroxyapatite (HAp) powder and recovery silver by electrodeposition. The adsorption efficiency of silver was about 61 % at 50 o C after 60 minutes of contact time. The Ag + adsorption process using HAp powder followed Langmuir adsorption isotherms with the maximum monolayer adsorption capacity of 18.7 mg/g. 60 % of silver can recovery by electrodeposition after 4 hours at the apply current of 10 mA at 50 °C. Keywords. Ag + ion, Adsorption, hydroxyapatite (HAp), recovery of silver, electrodeposition. 1. INTRODUCTION Among industries, the electronic industry is the world’s largest and fastest growing manufacturing industry. [1,2] Today, electrical and electronic waste are the type of waste that is most interested in the current waste stream because they are the fastest growing waste stream and grow 3 times faster than other types of waste (about 4 percent growth a year). [3] The amount of electrical and electronic waste are created about 40 million tons each year. Electronic waste contains a lot of heavy metals, chemical compounds that easily penetrate soil and water, threatening the environment and human health. [4-7] This seriously affects human health such as cancers, respiratory tract, cardiovascular and neurological. [4-7] Since the early part of 19 th century, physicians have known that silver compounds can cause some areas of the skin and other body tissues. Skin contact with silver compounds has been caused mild allergic reactions, such as rash, swelling, and inflammation. The inhalation with high amount of silver compounds such as silver nitrate or silver oxide may cause breathing problems, lung and throat irritation and stomach pain. [8] Nowadays, a large amount of electronic waste has been discharged into the environment without proper treatment. It carries the risk of polluting heavy metals into the ground and water. Therefore, the treatment of electronic waste is necessary. In addition, electronic waste also contains a big amount of many precious metals such as Au, Ag, Pd, etc. Recovery of precious metals prevents the pollution as well as prodigality. In Vietnam, some materials were synthesized to remove heavy metal ions such as: coffee husk, MnFe2O4/GNPs composite and chitosan/graphene oxide/magnetite nanostructured (CS/Fe3O4/GO) composite. [9-11] The adsorption capacity for Ni(II) of coffee husk is 21.14 mg/g, reported by Do Thuy Tien et al. [9] The CS/Fe3O4/GO can remove 60 % of Fe(III) with adsorption capacity of 6.5 mg/g. [11] Nguyen suggested that MnFe2O4/GNPs composite removed Pb 2+ with high adsorption capacity of 322.6 mg/g. [10] Vietnam Journal of Chemistry Nguyen Thi Thom et al. © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 180 The studies used HAp to adsorbed heavy metal ions in water which were reported for few years ago. [12-15] The adsorbent of HAp showed a good removal ability of heavy metal ions. In our reports, hydroxyapatite (HAp, Ca10(PO4)6(OH)2) powder can remove some ions such as Pb 2+ , Cd 2+ and Cu 2+ with the efficiency of about 86 % corresponding to the adsorption capacity of 281 mg/g. [16] However, the researches for using of HAp to adsorb Ag + ions in water are not reported. The aim of this work is to study the mechanism of Ag + adsorption using hydroxyapatite powder and silver deposition. Herein, HAp powder was used to adsorb Ag + ions in water and recovery of silver by electrodeposition. 2. MATERIALS AND METHODS The chemical precipitation was used to synthesize of hydroxyapatite powder from Ca(NO3)2 (M = 100.09 g/mol, 99.0 % of pure), (NH4)2HPO4 (M = 132.05 g/mol, 99.0 % of pure) and NH4OH (M = 35.05 g/mol, 28 %). These chemicals were purchased from VWR chemicals, Belgium. The obtained hydroxyapatite powder has cylinder shape with size of 18 × 29 nm and the SBET = 75 m 2 /g. [17] Sulfuric acid (M = 98.08 g/mol, 95-97 %) and silver nitrate (M = 169.87 g/mol, 99.0 % of pure) are pure chemical of Merck. The adsorption of Ag + ions was conducted with a 50 mL of AgNO3 solution at various initial concentrations from 10 to 100 mg/L at different contact time of 5, 10, 20, 30, 40, 50, 60, 70 and 80 minutes. The adsorbent amount of HAp was 0.1 g. The concentration of Ag + ions after adsorption process was determined by atomic absorption spectrophotometry (AAS). The capacity (Q) and efficiency (H) of Ag + adsorption process were calculated by the equations (1) and (2):   100 (%)o i o C C H C    (1)   ( / )o i C C V Q mg g m    (2) where, C0 (mg/L) is the initial Ag + concentration; Ce (mg/L) is Ag + concentration at an equilibrium in the solution after adsorption process; V (L) is the solution volume (V = 50 mL) and m (g) is the mass of adsorbent (HAp, m = 0.1 g). The kinetics of Ag + adsorption process using HAp powder were investigated by the effect of contact time (changed from 5 to 80 minutes) following Lagergren’s pseudo-first-order law and McKay and Ho’s pseudo-second-order law. The equations of the two models are showed in (3) and (4) equations, respectively. (3) (4) where: t is the contact time (min); Qt is the adsorption capacity following the time (mg/g); Qe is the adsorption capacity at the equilibrium (mg/g); K1 is adsorption constant following the pseudo-first- order law (min -1 ) and K2 is the equilibrium constant (g/mg.min). From the data of the effect of initial Ag + concentration, we studied the isothermal adsorption model following Langmuir and Freundlich adsorption isotherms ((5) and (6) equations, respectively). (5) lnQe = lnKF + 1/n lnCe (6) where: Qe is the equilibrium adsorption capacity; Qm is maximum single layer adsorption capacity per unit mass of adsorbent; Ce is the equilibrium concentration of Ag + ; KL and KF are Langmuir and Freundlich adsorption constant and n is experimental constant. The effect of pH solution, temperature and adsorbent mass on Ag + adsorption capacity was investigated. 0.1 g HAp was used to remove 50 mL Ag + 50 g/L for 60 min at different pH values from 2 to 8. The treatment temperature was adjusted at 20, 30, 40, 50, 60 and 70 o C using a thermostatic with water bath. The mass of HAp changed from 0.05 to 0.15 g. The Fourier transform infrared spectroscopy (FTIR) is used to identify of functional groups of HAp before and after Ag + adsorption process. The FTIR spectra were recorded by an IS10 (NEXUS) using KBr pellet technique at room temperature over the frequency range from 400 to 4000 cm -1 with a 32 scans and 4 cm -1 resolution. The phase component of HAp before and after adsorption process was analysed by X-ray diffraction (XRD) (Siemens D5000 Diffractometer, CuKα radiation (λ = 1.54056 Å) with a step angle of 0.030°, scanning rate of 0.04285 °/s and 2-theta range of 20-70°). The surface morphology of HAp before and after adsorption of Ag + ions was analyzed by scanning electron microscopy (SEM S4800, Hitachi). The element component of AgHAp was determined using energy dispersive X-ray analysis (Jeol 6490 JED 2300). The recovery of silver was performed in an electrochemical cell containing 0.5 g of Ag-HAp which was dispersed into 5 mL of 0.1 M H2SO4 solution with a cell of three electrodes: the working electrode was a plate of Au (S = 0.0201 cm 2 ), the Vietnam Journal of Chemistry Adsorption of Ag + ions using hydroxyapatite © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 181 counter electrode was Pt plate (S = 0.0201 cm 2 ) and the reference electrode was Ag/AgCl. Silver was deposited on the surface of Au plate by applying current at 2, 4, 6, 8 and 10 mA with the different time from 30 minutes to 4 hours at 50 °C. The remaining Ag + ion concentration in 0.1 M H2SO4 solution was also determined by AAS method. 3. RESULTS AND DISCUSSION 3.1. Standard curve The standard curve of Ag + was constructed with the change concentration of Ag + from 0 to 30 mg/L. The variation of absorbance according to the concentration of Ag + was shown in figure 1. The results showed that the absorbance value increased with increasing of concentration of Ag + . The concentration of Ag + after adsorption process can be extrapolated from the standard curve. 0 5 10 15 20 25 30 0.0 0.5 1.0 1.5 2.0 y = 0. 05 89 C A g+ R 2 = 0 .9 99 2 C Ag+ (mg/L) A ( A b s) Figure 1: The variation of absorbance as a function of the Ag + concentration 3.2. Effect of contact time The change of the efficiency and adsorption capacity of 0.1 g HAp powder in 50 mL of Ag + solution (50 mg/L, pH0 = 5.9) at 20 °C according to the contact time was shown in figure 2. The contact time increased from 5 to 60 minutes, the adsorption efficiency and capacity increased rapidly and reached stability after 60 minutes. It is clear that there is an initial rapid uptake of metal ions, but as time progresses the uptake around the 60 minutes mark no further adsorption takes place. The efficiency reaches about 50 % corresponding to the adsorption capacity of 12 mg/g after 60 minutes. At the contact time of 70 and 80 minutes, the adsorption efficiency and capacity were not significantly changed. From these results, they are possible to conclude that after 60 minutes, Ag + adsorption process reached the adsorption equilibrium. The adsorption kinetics was investigated to determine sufficient residence time on the absorber surface. This is reflected by the change of Ag + ion concentration adsorbed during the batch adsorption studies following the contact time. The experimental data were analyzed using two models: [14,16,18] the pseudo-first-order law and the pseudo-second-order law. 0 20 40 60 80 6 8 10 12 14 Q Time (min) Q ( m g /g ) 30 35 40 45 50 55 H H ( % ) Figure 2: The variation of adsorption efficiency and capacity as a function of the contact time The pseudo-first-order law and the pseudo- second-order law equations were constructed and shown in figures 3 and 4. The correlation coefficient (R 2 ) of two models showed that the pseudo- second- order law described better for the Ag + adsorption process. The parameters of the pseudo-second-order law were calculated and were shown in table 1. 0 10 20 30 40 50 0.00 0.25 0.50 0.75 y = -0.0111x + 0.6841 R 2 = 0.9816 L o g ( Q e- Q t) t (min) Figure 3: The model of the kinetic of Ag + adsorption process using HAp powder according to Lagergren's pseudo-first-order law Table 1: The parameters of Ag + adsorption process using HAp powder The pseudo-second-order law K2 (g/mg.min) Qe (mg/g) R 2 0.0034 12.32 0.9938 Vietnam Journal of Chemistry Nguyen Thi Thom et al. © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 182 0 20 40 60 80 0 1 2 3 4 5 6 7 y = 0. 07 49 x + 0. 54 15 R 2 = 0 .9 93 8 t/ Q t ( m in g m g -1 ) t (min) Figure 4: The model of the kinetic of Ag + adsorption process using HAp powder according to McKay and Ho's pseudo- second-order law 3.3. Effect of initial Ag + concentration The change of the efficiency and adsorption capacity of 0.1 g HAp powder in 50 mL of Ag + solution with different initial concentrations varying from 10 to 100 mg/L at pH0 = 5.9, temperature of 20 °C after 60 minutes of the contact time was shown in figure 5. It is found that adsorption capacity was the result of increasing equilibrium metal ion concentrations in solution. The increased concentrations were able to increase the numbers of Ag + ions at the absorber surface and enhance the probability of adsorption. The data were analyzed based on two isothermal adsorption models of Langmuir and Freundlich (figures 6 and 7). The equilibrium equations are widely used for modelling equilibrium data obtained from adsorption systems. 0 20 40 60 80 100 0 5 10 15 20 25 30 Q Initial Ag + concentration (mg/L) Q ( m g /g ) 20 40 60 80 100 H H ( % ) Figure 5: The variation of adsorption efficiency and capacity as a function of the initial Ag + concentration From the correlation coefficient (R 2 ) of the two equations, it is shown that the Langmuir isothermal described better for the Ag + adsorption process than the Freundlich isothermal. It can be said that Ag + adsorption process on the surface of HAp was monolayer. The value of the maximum adsorption capacity calculated from the Langmuir isothermal model was about 18.7 mg/g. 0.50 0.75 1.00 1.25 1.50 1.75 0.7 0.8 0.9 1.0 1.1 1.2 L o g Q Log C e y = 0. 33 33 x + 0. 56 85 R 2 = 0 .9 66 0 Figure 6: The Ag + adsorption isotherm follows the Freundlich isothermal model using HAp powder 0 10 20 30 40 50 60 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 C e/ Q ( g /L ) C e (mg/L) y = 0. 07 78 7 x R 2 = 0.9 83 42 Figure 7: The Ag + adsorption isotherm follows the Langmuir isothermal model using HAp powder 3.4. Effect of pH solution The effect of pH solution in the range of 2 to 8 on Ag + adsorption ability using HAp powder is presented in figure 8. The pH solution increases leading to the increase of adsorption efficiency. It is clear that at low pH values (pH ~ 2 or 3), the efficiency of Ag + removing is low because of proton-competitive sorption reactions between H + ions and Ag + ions. When the pH solution increases, the competing effect of H + ions decreases leading to the efficiency of removal Ag + increases. In the pH range of 6 to 8, the Ag + removal efficiency does not change. So, pH value of 5.9 (pH0) was the optimum pH value for the Ag + removal process. 1 2 3 4 5 6 7 8 9 4 6 8 10 12 14 16 Q pH solution Q ( m g /g ) 10 20 30 40 50 60 H H ( % ) Figure 8: The variation of Q and H as a function of the initial pH solution Vietnam Journal of Chemistry Adsorption of Ag + ions using hydroxyapatite © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 183 3.5. Effect of temperature treatment In this section, the Ag + treatment temperature was adjusted from 20 to 70 o C using a thermostatic. The results show that the temperature increases from 20 to 50 o C, the adsorption efficiency and capacity increase strongly (figure 9). It is clear that the temperature promotes movement of ions as well as ion exchange reaction. The temperature continues to increase, the adsorption efficiency and capacity nearly do not change. Therefore, the temperature value of 50 o C is chosen to remove Ag + ions. 10 20 30 40 50 60 70 80 11 12 13 14 15 16 Q Temperature ( o C) Q ( m g /g ) 45 50 55 60 65 H H ( % ) Figure 9: The variation of Q and H according to temperature 3.6. Effect of adsorbent mass The effect of HAp mass from 0.05 to 0.15 g on the Ag + adsorption ability is presented in figure 10. The data show that the amount of Ag + removed increases rapidly by increasing of HAp mass from 0.05 to 0.15 g. However, HAp mass increases leading to the adsorption capacity decreases strongly. Therefore, the adsorbent mass of 0.1 g is suitable in this study. 0.05 0.10 0.15 12 14 16 18 20 22 24 26 Q HAp mass (g) Q ( m g /g ) 45 50 55 60 65 70 75 80 85 H H ( % ) Figure 10: The variation of Q and H as a function of HAp mass From the above data, the suitable condition to remove 50 mL Ag + 50 g/L are chosen in this study including: 0.1 g HAp, pH0 = 5.9, temperature of 50 o C for 60 minutes of the contact time. 3.7. Characterization of HAp before and after treatment The characterizations of HAp powder before and after adsorption process were analyzed using FT-IR and XRD. The functional groups in the HAp molecule before and after Ag + adsorption process were determined using FTIR spectra (figure 11). It can be seen clearly that Ag + adsorption process does not change the functional groups in HAp molecule. For both of spectra, the characteristic peaks of OHˉ and PO4 3- groups in HAp were observed. A wide range at 2500 to 3700 cm -1 was characterized for vibration of OHˉ in water. The vibrations at 1040 and 1105 cm -1 are attributed to the P-O stretching of PO4 3- groups. The flexural vibration of the phosphate group was observed at the wave number of 570 to 605 cm -1 . The result is coincident with another report. [18] 4000 3500 3000 2500 2000 1500 1000 500 570-605 1040 1105 AgHAp HAp Wavenumber (cm -1 ) T ( % ) PO 4 3- PO 4 3- OH - Figure 11: FTIR spectra of HAp before and after adsorption process The X-ray diffraction patterns of HAp powder before and after Ag + adsorption process were shown in figure 12. The XRD patterns of HAp and Ag-HAp samples were similar, which presented the characteristic peaks for HAp crystal (JCPDS No. 00- 009-0432). [19] This result is in accordance with previous reports. [20-22] 20 30 40 50 60 70 JCPDS: 00-009-0432 HAp 2 (degree) In te ns it y Ag-HAp Figure 12: XRD patterns of HAp before and after Ag + adsorption process Vietnam Journal of Chemistry Nguyen Thi Thom et al. © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 184 The SEM images of HAp and AgHAp are shown in figure 13. The surface morphology of HAp has cylinder shape. After Ag + adsorption process, there is no significant change in particle’s size and shape. The EDX spectra confirms the present of silver in HAp after adsorption process (figure 14). Figure 13: SEM images of HAp and AgHAp Figure 14: EDX spectrum of AgHAp The cathodic polarization curve of Au electrode in 5 mL of H2SO4