Green chemical synthesis of new chelating fiber and its mechanism for recovery gold from aqueous solution

Journal of Hazardous Materials 378 (2019) 120674

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Green chemical synthesis of new chelating fiber and its mechanism for recovery gold from aqueous solution

T

Weiquan Zhanga, Lina Wub, Xiaoxiang Hana, Lanying Yaoa, Shengze Zhaoa, Jing Suna, ⁎ Yanping Xua, Jionghui Lic, Chunhua Xionga, a

Department of Applied Chemistry, Zhejiang Gongshang University, No.149 Jiaogong Road, Hangzhou, 310012, PR China School of Foreign Languages, Zhejiang Gongshang University, Hangzhou 310012, China c School of Environmental Science and Engineering, Zhejiang Gongshang University, Hangzhou 310012, China b

GRAPHICAL ABSTRACT

ARTICLE INFO

ABSTRACT

Keywords: Polyacrylonitrile fiber Adsorption Chelating fiber Gold Green chemical

A novel environmentally-friendly polyacrylonitrile-2-amino-2-thiazoline chelating fiber (PANF-ATL) with good adsorption performance and thermal stability was synthesized in one step by nucleophilic addition reaction using water as a solvent. The optimum synthesis conditions for the chelating fibers are determined by controlling the synthesis temperature and the molar ratio of the reagents. The sulfur content and functional group capacity of the finally synthesized PANF-ATL were 3.82% and 1.19 mmol/g, respectively. PANF-ATL was characterized by elemental analysis, FTIR, TGA, SEM and XPS. Meanwhile, the adsorption characteristics and mechanism of PANF-ATL were evaluated. The Langmuir model and the pseudo-second-order model well described the adsorption of Au(Ⅲ) by PANF-ATL. The adsorption capacity of PANF-ATL obtained from Langmuir isotherm model towards Au(Ⅲ) was 130.58 mg/g (298 K). In addition, Au(Ⅲ) adsorbed on the fibers was completely eluted using a mixed solution of 4 mol/L HCl and 12% thiourea. It still has good adsorption performance after 5 adsorption-desorption cycles. Overall, PANF-ATL is a cost-effective adsorbent that can effectively adsorb Au(Ⅲ) in aqueous solution.

⁎

Corresponding author. E-mail address: [email protected] (C. Xiong).

https://doi.org/10.1016/j.jhazmat.2019.05.067 Received 18 January 2019; Received in revised form 4 April 2019; Accepted 25 May 2019 Available online 27 May 2019 0304-3894/ © 2019 Elsevier B.V. All rights reserved.

Journal of Hazardous Materials 378 (2019) 120674

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1. Introduction

prepare a thio-functionalized fibrous adsorbent (PANMW-Thio) [30]. Xiang Li et al. reported a thioamide-group chelating nanofiber membrane prepared by a combination of chemical modification and electrospinning process [31]. In this study, with a simple green chemistry method, we prepared a polyacrylonitrile 2-amino-2-thiazoline chelating fiber by using water as a solvent for adsorption of Au(Ⅲ) from aqueous solution. The prepared chelating fiber has a high adsorption capacity and high selectivity to Au (Ⅲ). A detailed study has been carried out by exploring the optimal preparation parameters, characterizing the chelating fiber, and the metal ions and acidity were used as variables to investigate the static adsorption performance of chelating fibers on metal ions. The thermodynamics and adsorption kinetics were studied by using temperature and time as variables.

Gold, the well-known precious metal, has long been used as currency or jewelry [1,2]. With its high corrosion resistance, stability, good electrical conductivity and thermal conductivity, these superior properties enable a wide usage of gold in modern industrial fields such as aerospace technology, information technology and medical technology [3,4]. However, gold resources are limited, which makes recovering gold from electronic waste water or any secondary sources an economic and ecological issue worthy of attention [5]. In the last decades, several methods have been reported the recovery of gold from an aqueous solution, such as precipitation, evaporation, filtration, electrochemical treatment, cyanide leaching, ion exchange resins, reverse osmosis, and the like. However, these gold recovery technologies are beset with difficulties like high cost, low efficiency, and harm to the environment [6,7]. Thus, it is urgent to develop a new efficient precious metal enrichment and separation technology. Adsorption separation technology, renowned for its high selectivity, large concentration ratio, easy operation, etc., has its unique virtues as low concentration of pollutant removal and precious metal ion enrichment. Since the performance of the adsorbent determines the application of the adsorption separation technology, the research and development of the adsorbent has always been the focus of the adsorption separation technology. In recent years, more and more new adsorbents have been reported, including biomaterials, polymer materials, modified materials, etc [8,9]. Currently, people are more interested in functional group-modified adsorbents because of its the good adsorption and desorption properties [10–12]. The functional group in the adsorbent is the main structure for adsorbing metal ions. The functional groups can be classified into amino-type chelating adsorbents containing N and O, chelating adsorbents containing S and N, chelating adsorbents containing P, O and N and a chelating adsorbent containing N, O, S, and P. Adsorbents containing N, S atoms can be used to selectively adsorb precious metals [13]. Organo-functional groups, for example, imidazole [14], thiourea (-N(C]S) NH2 [15], amide (−CONH2) [16,17], alkyl sulphide (-RSR'-) [18] and Schiff bases (RC]N-) [19], adsorbents containing the above functional groups have been successfully applied to the recovery of tracing precious metals [20]. The chelating resin is a chelating material that has a certain adsorption capacity for metal ions [21,22], The heterocyclic ligand is grafted onto the chelating resin, such as imidazole [23], bis(2-benzimidazolyl methyl) amine [24], pyrimidine [25], pyridine [26], 2amino-1,3,4-thiadiazole [27], and 2‑aminothiazole [28], these chelating resins can efficiently adsorb precious metals. But some adsorption reactions, influenced by the adsorption reactions that are affected by the crosslinked network, will occur inside the microspheres and then have a tailing phenomenon during elution, which greatly reduces the reuse efficiency of the chelating resin. The chelating fiber has a small diameter and large specific surface area; its special physical form provides it with a large contact area with the adsorbed material and a small fluid resistance, which not only has a fast adsorption rate and a large capacity, but also makes it easy to desorb for a trace amount of precious metal. Ion adsorption is of great efficiency as well. Polyacrylonitrile fiber (PANF) is a significant raw material for the textile industry and carbon fiber industry because of its high strength, good elasticity, heat resistance and light resistance. The modified PAN integrated fiber’s good hydrophilicity, large specific surface area, and capability of being coordinated with many metal ions make contribution to the its characteristics of selectivity, high speed, large capacity, and renderability, which endow a broad application prospect for the enrichment and recovery of rare metals in industrial wastewater as well as for antibacterial materials [29]. However, there are some problems with the current PAN chelating fiber, complicated synthesis method, organic solvents polluting the environment, and inadequate elution. For example, Shen Deng et al. reported a microwave (MW) assisted method to

2. Material and method 2.1. Materials The Polyacrylonitrile fiber (PANF) was purchased from Hangzhou Jianqing fiber products Ltd. Chelating ligand 2-Amino-2-thiazoline (ATL) (97%) was purchased from Alfa Aesar, AuCl3 (64.4%) from Shanghai Yuanye Biological Technical Ltd., HCl (37%) from Hangzhou Shuanglin Chemical Reagent Factory, Thiourea (≥98.0%) from Sinopharm Chemical Reagent Co., Ltd.; stock solution of metal ions were prepared from Cu(NO3)2·3H2O (≥99%), Cd(NO3)2·4H2O (≥99.0%), ZnSO4·7H2O (≥99.5%), Pb(NO3)2 (≥99.0%), NiSO4·6H2O (≥98.5%) (Sinopharm Chemical Reagent Co., Ltd) in deionized water, respectively. 2.2. Preparation of PANF-ATL First, PANF (0.15 g) and deionized water (30 mL) were placed in a 100 mL three-necked flask and swelled for 12 h. Then, added ATL to the flask while filling nitrogen (5 mL/min) with 150 rpm stirring speed for eliminate air. After 1.5 h, the reaction system was rapidly heated to desired temperature and continued for desired time. When the reaction was over, the reacted fibers were washed with deionized water until no ligand remained, then dried under vacuum condition (50 ℃). The functional group capacity (Fc, mmol/g)) of the chelating fiber can be calculated on the basis of the following equations:

FC =

SC × 1000 32.07 × nS

(1)

where Sc is the sulfur content of the chelating fiber (%) and is measured by an EuroEA3000 elemental analyzer. ns is the number of sulfur atoms of ligand molecules. 2.3. Characterization To verify the synthesis results of the chelating fibers, Fourier transform infrared spectroscopy (FTIR) experiments were performed using an FTIR spectrometer (Nicolet 380, Thermo Fisher Scientific), and the spectra were recorded at a wave number of 400 cm−1 to 4000 cm−1. In order to evaluate the thermal stability of the chelating fibers, thermogravimetric analysis (TGA) was carried out on a TGA instrument (TGA/DSC1, METTLER TOLEDO) under a nitrogen atmosphere at a temperature ranging from 25 ℃ to 1000 ℃ by 20 ℃/min. Xray photoelectron spectroscopy (XPS) (ESCALAB 250Xi, Thermo Fisher Scientific) was used to investigate the adsorption mechanism of PANFATL for Au(Ⅲ). Surface morphology of the fiber was viewed and photographed by scanning electron microscope (SEM, Phenom G2 pro). 2.4. Batch adsorption experiments of metal ions In the batch adsorption experiment, the metal ions and acidity were 2

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used as variables to investigate the static adsorption performance of the chelating fiber on the metal ions, and the adsorption parameters of the chelating fiber to the metal ions were determined to obtain the optimal adsorption conditions. The adsorption kinetics was studied by using temperature and time as variables. The adsorption thermodynamics was studied by using the initial concentration and temperature of metal ions as variables, and the adsorption mechanism was described. Each experiment has been repeated for three times, and the average of results was obtained. The concentration of metal ions was measured using ICPOES. The adsorption capacity (Q, mg/g) was calculated with the following expression:

Q=

C0

Ce W

V

(2)

where C0 is the initial concentration of Au(Ⅲ) (mg/L), Ce is the residual concentration of Au(Ⅲ) in solution(mg/L), V is the solution volume (L), and W is the fiber dry weight (g).

Fig. 1. Influence of the molar ratio of reagents on the sulfur content and functional group capacity of PANF-ATL.

2.5. Desorption and regeneration The PANF-ATL which had reached the adsorption equilibrium was collected, washed with distilled water to remove metal ions which were not adsorbed on the surface, and dried in a vacuum oven at 50 ℃ overnight. Then, the fibers were placed in a mixture containing various concentrations of HCl and thiourea, and were shaken at 25 ℃ for 2 h. Next, the metal concentration in the solution was measured using ICPOES. The eluted PANF-ATL was washed and dried, and then subjected to the next adsorption-desorption experiment. The desorption rate (E) is calculated as follows:

E=

(C0

Cd Vd × 100% Ce ) V

(3)

where Cd is the concentration of Au(Ⅲ) in the eluent solutions, Vd is the volume of the desorption solution, C0, Ce, and V are the same as defined above. 3. Results and discussion

Fig. 2. Influence of reaction temperature on the sulfur content and functional group capacity of PANF-ATL.

3.1. Synthesis of the PANF-ATL The PANF used in this study had a nitrogen content of 24.0% and a functional group content of 17.1 mmol CN/g. 150.0 mg of dried PANF was accurately weighed, placed in 30 ml of deionized water and swelled overnight, then a ligand with a molar ratio (ATL/CN) of 1, 2, 3, 4, 5 was added, protected by nitrogen, and reacted at 90 ℃ for 12 h. The synthesized fibers were subjected to elemental analysis, and the results are shown in Fig. 1. The results show that with the increase of the molar ratio, the sulfur content of PANF-ATL increases rapidly until the molar ratio reaches 4, which is the highest. As the concentration of ligand increases, the contact chance of ligand and fiber increases, and the reaction is promoted. When the molar ratio increases to 5, the sulfur content slightly reduces, which may be due to the excessive concentration of the ligand, and the change in the properties of the fiber is not favorable for the reaction. Therefore, we chose a ligand with a molar ratio of 4 to synthesize PANF-ATL. Accurately weigh 150.0 mg of dried PANF, swell it in 30 ml of deionized water for one night, then add 4 ligands in a molar ratio, protected with nitrogen, and react at 50 ℃, 60 ℃, 70 ℃, 80 ℃, 90 ℃ with stirring for 12 h, respectively. The synthesized fibers were subjected to elemental analysis, and the results are shown in Fig. 2. The results show that with the increase of temperature, the sulfur content of PANF-ATL increases rapidly and maximizes at 90 °C. This is because PANF swells with the increase of temperature, with the melt of paracrystallite, ligands and fiber internal groups. Increased exposure opportunities promote the response. Judging from the evidence above, a reaction temperature of 90 ℃ was chosen to synthesize fibers.

Table 1 The dates of element analysis of PANF, PANF-ATL. Sample

PANF PANF-ATL

Element content (%)

FG(mmol /g)

C

N

H

S

66.3 63.7

24.0 19.5

8.21 9.31

0 3.82

1.191

The results of the elemental analysis of C, N, O, S of the PANF-ATL are given in Table 1. About 3.82% of sulfur content newly appeared in PANF-ATL fiber, indicates that the five-membered rings had been successfully attached onto it. 3.2. Characterization of PANF-ATL fiber Surface chemical structures of PANF, ATL, mixture of PANF & ATL and PANF-ATL were obtained by FTIR spectroscopy, as shown in Fig. 3. The peaks at wavenumbers 2932 and 1451 cm−1 are characteristic peaks of −CH2- group, and 2243 cm−1 are assigned to the characteristic adsorptions of -C≡N group in the PANF framework. The adsorption peak at 2932 cm−1 has changed slightly after modification, which indicates the stability of basic PANF structure throughout the reaction. 3

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should attribute to the mass of Au(Ⅲ) adsorbed on the fiber. Thermogravimetric analysis showe that PANF-ATL has good thermal stability and can reach an operating temperature of 190 °C, which meets the requirements of adsorption applications. XPS is a powerful technique to investigate how metal ions bind to the adsorbents. In order to further identify the functional groups in PANF-ATL fiber and its interaction with Au(Ⅲ), XPS analysis of PANFATL fiber before and after the adsorption of Au(Ⅲ) was conducted. The vacuum of the analysis chamber is lower than 1*10−9Pa, the excitation source is Al ka ray (hv = 1486.6 eV), the working voltage is 15KV, the filament current is 12 mA, the pass energy is 30 eV, and the C1 s standard energy is 284.8 eV. To elucidate the adsorption mechanism of PANF-ATL fibers, XPS analysis was performed for the fibers before and after adsorption of Au (Ⅲ) ions. As shown in Fig. 7(a), after adsorption of Au(Ⅲ), the characteristic peaks of Au 4f5 and Au 4f7 appeared in the spectrum. The appearance of these peaks proves that Au(Ⅲ) ions are adsorbed on the PANF-ATL surface. In addition, A peak at 531.9 eV was observed in the full spectrum, which was the O 1s binding energy of the fiber. The peak of 531.9 eV in the full spectrum is the binding energy of O 1s. It may be due to the hydrolysis of some cyano groups during the synthesis of PANF-ATL, forming an amide bond (−CO−NH−) and Carboxyl group (−COOH), or air pollution on the surface of the fiber. As shown in Fig. 7(b), the extent of peak changes before and after the adsorption of the C 1s spectrum is small, indicating that the C atom may not participate in the adsorption. Fig. 7(c) is the spectrum of N 1s. When the gold is not adsorbed, the N 1s spectrum has two peaks, 399.02 eV belongs to the signal of the -NH group, and 397.43 eV belongs to the Ngroup of the thiazoline in the fiber. After adsorption of Au(Ⅲ), the N 1s band moves to 399.17 eV due to the formation of a bond between N and Au(Ⅲ). [35] As shown in Fig. 7(d), after adsorption of Au(Ⅲ), the peak of 164.57 eV corresponding to S thiophen was lower, and new peaks appeared at 167.57 and 169.08 eV. This is due to the chelating of sulfur atoms with Au(Ⅲ). [36] The proposed adsorption mechanism for Au (Ⅲ) on the PANF-ATL fibers is shown in Fig. 8.

Fig. 3. FTIR spectra of (a) PANF, (b) ATL, (c) mixture of PANF & AT and (d) PANF-ATL.

Nucleophilic addition reaction transfers a cyano group into C]N. Thus, the tensile vibration peak at 2243 cm−1 is greatly reduced as the tensile vibration peak of the five-membered ring eC]N-group at 1645 cm−1 appears [32]. Comparing the spectra of PANF&AT mixture and PANFATL, the PANF&AT mixture has a stretching peak of -NH2- group at 3388 cm−1 and 3419 cm−1, and a broad peak of secondary amine (-NH) appeared at 3422 cm−1 in PANF-ATL spectrum, so there is no free ligand in PANF-ATL [33]. The synthetic route of PANF-ATL was estimated based on the FTIR spectrum, as shown in Fig. 4. In order to observe the fiber morphology of PANF visually, PANFATL, PANF-ATL-Au, an electron micrograph of the fiber surface was taken using SEM, and the results are shown in Fig. 5. Fig. 5(a) shows the surface state of PANF with wrinkles and no cracks. After the combination with ATL, as shown in Fig. 5(b), the surface of PANF cracked, the surface was uneven, and the diameter also increased slightly. The macroscopic representation was fiber shrinkage, and the color changed from white to pale yellow. It is apparent that the properties of the fibers of the graft functional groups have changed. On the fiber adsorbed with Au(Ⅲ), as shown in Fig. 5(c), the surface of PANF-ATL became rougher and some flocs were attached, indicating that Au(Ⅲ) was adsorbed on the surface of the fiber. The thermal stabilities of the PANF, PANF-ATL and PANF-ATL-Au fiber were performed and compared, and the results are shown in Fig. 6. The degradation of PANF-ATL was divided into three stages. In the first decomposition stage, the temperature raised from 25 °C to 100 °C, and the weight loss of PANF-ATL was 1.44%. At the same temperature, the weight loss of PANF-ATL adsorbed with Au (Ⅲ) was 0.85%. The weight loss at this stage can be attributed to the evaporation of residual moisture. In the second stage, the weight loss started at 190 °C and continued to 450 °C. The weight loss of PANF-ATL was 30.8%, and the weight loss of PANF-ATL-Au was 33.5%. The weight loss at this stage can be attributed to the degradation of functional groups. The second stage started at a lower temperature than PANF and had a shorter duration, indicating that the introduction of new functional groups resulted in a slight decrease in the thermal stability of the fiber. In the third stage, starting from 450 °C to over 1000 °C may belong to the degradation of PANF chains. [34] The weight loss of PANF-ATL and the adsorbed fiber at 1000 ℃ differed by 11.7%. This part of the mass

3.3. Adsorption behaviors of PANF-ATL 3.3.1. Effect of acidity Under different acidities, the adsorption capacity of chelating fibers is different, and static adsorption experiments are needed to find the optimum acidity for adsorption. In order to study the optimum acidity of adsorption and avoid the hydrolysis of metal ions, the effect of acidity on the adsorption capacity of chelating fiber within the range of 0.001-0.5 mol/L HCl acidity was investigated. 15.0 mg of PANF-ATL was added to 50 mL of a concentration of 80 mg/L Au(Ⅲ) solution, and after shaking at 25 ℃ for 5 h, the residual metal concentration was determined by ICP-OES. The results obtained were presented in Fig. 9. In the lower acidity (0.001 mol/L HCl), the adsorption capacity of fiber was small, Au(Ⅲ) ion was in the form of a hydrolyzed chlorogold, such as AuCl3 (OH) , and the degree of protonation of functional groups was low, resulting in a decrease in electrostatic attractions between the negatively charged Au(Ⅲ) anions and the positively charged adsorption sites of chelating fiber [37]. The adsorption amount of PANF-ATL on Au (Ⅲ) increased with the increase of acidity, and reached the highest at 0.1 mol/L HCl, as in 0.1 mol/L HCl the Au(Ⅲ) ion was in the form of AuCl 4 . This chloro-complex is stable in 0.1 mol/L HCl and can easily interact with the positive potential point of PANF-ATL by electrostatic Fig. 4. Synthesis routes for PANF-ATL.

4

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Fig. 5. (a)PANF (b)PANF-ATL and (c)PANF-ATL-Au.

forces. Therefore, the adsorption capacity of PANF-ATL fiber for Au(Ⅲ) increased obviously in 0.1 mol/L HCl compared with 0.001 mol/L HCl. However, as the acidity continued to rise, the amount of adsorption began to decrease. This may be due to the fact that the protonated functional group was faced with chloride ion competition while adsorbing Au(Ⅲ) [38]. The higher the acidity, the higher the chloride ion concentration, so the adsorption amount of Au(Ⅲ) is less.

3.3.2. Selective adsorption behaviors In order to verify the selective adsorption capacity of PANF-ATL, selective adsorption experiments under five acidity values were designed. Add 15 mg of PANF-ATL to 50 ml of a mixed solution with a metal ion concentration of 40 mg / L of metal ions, such as Au(Ⅲ), Zn (Ⅱ), Cd(Ⅱ), Cu(Ⅱ), Ni(Ⅱ) and Pb(Ⅱ). The adsorption was carried out for 24 h at 25 ℃. The metal ion concentration was measured using ICP5

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equilibrium (mg/g), Qt is the amount of adsorption at each time point; Q1 and Q2 are the adsorption capacity of the pseudo-first-order model and the pseudo-second-order model (mg/g), respectively; k1 and k2 are the rate constant of the pseudo-first-order model (1/min) and the pseudo-second-order model (g/(mg·min). Experimental data was fitted by OriginPro 2018C (64-bit) SR1 b9.5.1.195 software. From Table 2, it can be seen that the correlation of the pseudo-second-order kinetic model is better than that of the pseudo-first-order kinetic model, and the correlation is extremely high (R22 > 0.99). The results indicate that PANF-ATL follows the quasi-secondary kinetic equation based on common chemical reaction when adsorbing Au(Ⅲ). Therefore, the adsorption process of Au(Ⅲ) by PANF-ATL is mainly controlled by the chemical adsorption mechanism [12]. In addition, the magnitude of the activation energy can reflect the controlled rate of adsorption, it can be calculated by the Arrhenius equation:

ln k = ln A

OES, and the adsorption amount (Q), distribution coefficient (D) and separation coefficient (βRE1/RE2) were caculated. The distribution coefficient(D) represented the affinity of an adsorbent. Then, calculate the distribution coefficient (D), and the separation coefficient (βRE1/RE2) according to the following equation:

RE1 RE 2

C0 Ce V WCe

(4)

DRE1 DRE 2

(5)

=

where C0 is the initial concentration of Au(Ⅲ) (mg/L), Ce is the residual concentration of Au(Ⅲ) in solution (mg/L), V is the solution volume (L), W is the fiber weight (g), and RE1/RE2 is different metal ions. The results are shown in Fig. 10. The results illustrated that under different acidity, PANF-ATL adsorbed more Au(Ⅲ) than the other five metal ions and had obvious selective adsorption capacity. When the acidity was 0.1 mol/L HCl, the maximum separation coefficient was βAu/Pb =14.46, and the minimum separation coefficient was βAu/ Cd = 7.76. PANF-ATL adsorbed more of Au(Ⅲ) even in complex aqueous systems.

Ce 1 C = + e Qe Qm KL Qm

log Qe =

3.3.3. Kinetics of adsorption In order to investigate the equilibrium time of PANF-ATL adsorption of Au(Ⅲ), an adsorption kinetics experiment was conducted t. 15 mg of PANF-ATL was added to 50 mL of Au(Ⅲ) solution at a concentration of 80 mg/L. 1 mL of the supernatant was taken from the mixture at the required time for further analysis, the time range was 5–240 minutes at 288, 298 and 308 K with an acidity of 0.1 mol/L HCl. The change in fiber absorption of Au(Ⅲ) is shown below in Fig. 11. In the first 60 min, Au(Ⅲ) was rapidly adsorbed on the surface of the fiber, and the adsorption amount reached 92.89 mg/g (298 K). Then the adsorption rate gradually slowed down, and the adsorption amount reached 110.93 mg/g (298 K) at 120 min. After 240 min, the amount of adsorption no longer increased, reaching equilibrium adsorption capacity. It can be clearly seen that higher temperature favors PANF-ATL adsorption of Au(Ⅲ). The kinetic models include pseudo-first-order kinetics and pseudosecond-order kinetics, which can be used to describe the adsorption rate of metal ions on chelating fibers and explain the experimental data. Their functional equations are as follows [39,40]:

ln(Qe

Qt ) = ln Q1

t 1 t = + Qt Q2 k2 Q22

k1 t

(8)

Where k is the reaction rate constant, Ea is the activation energy, R is the gas constant (8.314 J/(mol·K)), A is the preexponential factor. Linear fitting of lnk2 versus 1/T, the activation energy of the adsorption process is obtained from the slope (-Ea/R). Through the calculation, the activation energy is 7.94 kJ/mol. According to Y. S. Ho et al. [41], when the adsorption activation energy is less than 25–30 kJ/mol, it is a diffusion control process. It indicates that the speed control process of the adsorption process is dominated by the diffusion control process. Isotherm of adsorption The adsorption isotherm can describe the relationship between the equilibrium concentration of the adsorbate in the solution at the equilibrium of adsorption and the adsorbate on the surface of the adsorbent. It also can be used to reveal the adsorption mechanism and evaluate whether it is a favorable adsorption. Two models of Langmuir [42] and Freundlich [43] are currently used to describe the adsorption behavior of adsorbents, 15 mg of PANF-ATL was immersed into 50 mL Au(Ⅲ) solution with the initial concentration ranging from 50 to 100 mg/L by maintaining reaction temperature at 288, 298 and 308 K. The functional equations of the Langmuir and Freundlich isothermal models are as follows:

Fig. 6. TGA curves of PANF, PANF-ATL and PANF-ATL-Au.

D=

Ea/RT

1 log Ce + log KF n

(9) (10)

Where Ce is the concentration of metal ions in the solution during adsorption equilibrium (mg/L); Qe is the saturated adsorption amount of adsorbent to the metal ion (mg/g); Qm is the theoretical saturated adsorption capacity of the Langmuir model (mg/g); KL is the adsorption constant of the Langmuir model (L/mg); KF is the adsorption constant of the Freundlich model((mg/g)/(mg/L)1/n); 1/n is the Freundlich constant, when the value of 1/n is between 0˜1, it is favorable adsorption, otherwise it is non-favorable adsorption. Experimental data was fitted by OriginPro 2018C (64-bit) SR1 b9.5.1.195 software. It is known from the correlation coefficient R2 in Table 3 that the isotherm adsorption curve of the Langmuir model is significantly better than the Freundlich model, that is, the adsorption behavior of PANF-ATL on Au(Ⅲ) accords with the Langmuir adsorption model. According to the Freundlich model, the values of the parameters 1/n between 0-1, indicating that the adsorption process is favorable adsorption [44]. Adsorption thermodynamics In any adsorption process, temperature is an important factor affecting the amount of adsorption. Generally speaking, the temperature rise is favorable for adsorption. The thermodynamic parameters can be used to describe the effect of temperature on the adsorption capacity during the adsorption process. The Gibbs adsorption free energy (ΔG) change can determine whether the adsorption reaction can proceed spontaneously or not. In this experiment, the thermodynamic

(6) (7)

where Qe is the amounts of Au(Ⅲ) adsorbed on the adsorbent at 6

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Fig. 7. XPS spectra of the PANF-ATL fiber (a) wide scan XPS spectrum, (b) C 1s XPS spectrum, (c) N 1s XPS spectra, (d) O 1s spectra and (e) Au 4f spectra.

parameters of the adsorption system were obtained by the adsorption data of PANF-ATL at three temperatures of 288 K, 298 K and 308 K. Gibbs free energy (ΔG), enthalpy (ΔH) and entropy (ΔS) can be obtained by the following equation [45,46]:

G=

RTLn (K e0 )

G= H ln(K e0 ) =

K e0 =

T S H S + RT R

isotherm model fitted, such as K of the Liu equilibrium constant, or K of the Sips isotherm model or KL, the Langmuir equilibrium constant), that are given initially in L/mg into L/mol. The value of KL of Langmuir, Sips or Liu isotherm expressed in L/mg should first be multiplied by 1000 to convert the units in L/g and then multiplied by the molecular weight of the adsorbate (Au = 196.97 g/mol), to transform K in L/mol, in order to use KL in the thermodynamic calculations. γ is the coefficient of activity (dimensionless), [Adsorbate]° is the standard concentration of the adsorbate (1 mol/L); considering the activity coefficient of the adsorbate and also regarding that the unitary activity of pure adsorbate is 1 mol/L by definition, the equilibrium constant becomes dimensionless. It is calculated that during the adsorption of Au(Ⅲ) by PANF-ATL, ΔG (kJ/mol) (288 K, −26.77; 298 K, −28.78; 308 K, −30.80) is negative and the absolute value increases with the increase of temperature, indicating that the adsorption process is Spontaneous, active, and elevated temperatures are beneficial to the adsorption process. ΔH

(11) (12) (13)

(1000.K g . molecular weight of adsorbate). [Adsorbate]° (14)

Where R is the gas constant (8.314 J/(mol·K)); T is the absolute temperature (K); K°e is the thermodynamic equilibrium constant that is dimensionless. It is calculated by converting the units of Kg (the best 7

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Fig. 11. Adsorption kinetics and capacity Q(mg/g) at different times and different temperatures. Table 2 Kinetics model constants for adsorption of Au(Ⅲ) by PANF-ATL.

Fig. 8. Proposed mechanism for Au(Ⅲ) adsorption on the PANF-ATL fibers.

Qe(mg/g) Pseudo-first-order 288 K 112.65 298 K 124.34 308 K 133.47 Qe(mg/g) Pseudo-second-order 288 K 112.65 298 K 124.34 308 K 133.47

k1 (1/min) ×102

Q1(mg/g)

R2

3.43 4.12 4.53 k2 (g/(mg·min)) ×104

103.55 115.80 127.12 Q2(mg/g)

0.9595 0.9457 0.9486 R2

3.22 3.74 3.99

120.32 131.74 142.79

0.9917 0.9905 0.9922

Table 3 Isotherm parameters for Au(Ⅲ) adsorption by PANF-ATL.

Langmuir 288K 298K 308K Freundlich 288K 298K 308K

Fig. 9. Effect of acidity on the adsorption capability of PANF-ATL for Au(Ⅲ).

Qm (mg/g)

KL (L/mg)

R2

120.17 130.58 138.99 1/n

0.3565 0.5875 0.8289 KF((mg/g) (L/mg)1/n)

0.9990 0.9971 0.9985 R2

0.084 0.065 0.055

81.75 98.20 109.90

0.9512 0.9667 0.9258

(31.17 kJ/mol) is a positive value, which indicates that the adsorption process is an endothermic process, and it needs to rely on external heat, which is also the reason why the adsorption amount increases with increasing temperature [47]. ΔS (201.17 J/(mol·K)) is greater than zero, which is presumed to increase the degree of chaos during the adsorption process [48,49]. 3.3.4. Desorption and regeneration studies In the actual application process, in order to save cost, the adsorbent used should have good desorption performance and reutilization performance. 15 mg of Au(Ⅲ) saturated PANF-ATL were eluted with 15 mL HCl (0.5–4 mol/L) and 15 mL thiourea in various concentrations (3%–12%) under 100 rpm at 298 K for 24 h. The results of desorption experiments showed that the saturated PANF-ATL could be completely desorbed by a mixed solution of 4 mol/L HCl and 12% thiourea. Subsequently, the eluted fibers were dried overnight, and then subjected to an adsorption-desorption experiment, which was cycled 5 times. The results of desorption experiments showed that the saturated

Fig. 10. Adsorption capacities of the PANF-ATL for Au(Ⅲ), Pb(Ⅱ), Cd(Ⅱ), Ni (Ⅱ), Cu(Ⅱ), and Zn(Ⅱ). 8

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Langmuir isotherm model towards Au(Ⅲ) was 130.58 mg/g (298 K, 0.1 mol/L HCl). XPS analysis indicated that Au(Ⅲ)coordinated with the N, S atom of the functional group. In summary, this readily prepared and high adsorption capacity fiber adsorbent may have potential applications in the field of waste electronics recycling and other precious metal enrichment systems. Acknowledgements The work is supported by the Program of Science and Technology of Zhejiang Province, China (No. 2017C31103, No. LGG19B070001), and the Key Research and Development Program of Zhejiang Province (No. 2017C03049). References [1] L. Liu, C. Li, C. Bao, Q. Jia, P. Xiao, X. Liu, Q. Zhang, Preparation and characterization of chitosan/graphene oxide composites for the adsorption of Au(Ⅲ) and Pd (Ⅱ), Talanta 93 (2012) 350–357, https://doi.org/10.1016/j.talanta.2012.02.051. [2] D. Parajuli, C.R. Adhikari, H. Kawakita, K. Kajiyama, K. Ohto, K. 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Fig. 12. The re-adsorption efficiency of PANF-ATL after 5 times regeneration. A number of new adsorbents have also been reported in recent years, and their data on the adsorption capacity and desorption rate of Au(Ⅲ) are listed in Table 4. Some of these adsorbents showed higher adsorption capacity, but the desorption rate did not perform as well as PANF-ATL. It had a higher adsorption capacity and a desorption rate of 100% compared with the adsorbents in the Table 4, and wass an adsorbent with excellent overall performance. Table 4 Au(Ⅲ) Adsorption and Desorption from Single and Binary Solutions. Adsorbents

Au capacity (mg/g)

Desorption Ratio (%)

Reference

EA/AN/DVB copolymer TU*ZIF-8 1-methylimidazole (gel) XAD7HP 1EDA Humic acid TF resin PS-AT resin ODPA-BH PANF-ATL

87.75 4262.8 11.25

100 18.4

[50] [51] [52]

14 19.7 90.91 29.5 733.8 465.16 134.9

100 93.8 92.63 100

[53] [54] [55] [56] [28] [57] This work

PANF-ATL could be completely desorbed by a mixed solution of 4 mol/ L HCl and 12% thiourea. The eluted fibers were dried overnight afterwards, and then subjected to an adsorption-desorption experiment, which was cycled 5 times. The experimental results of the five experiments are shown in Fig. 12. The experimental data showed that after 5 cycles of adsorption-desorption, the adsorption capacity of PANF-ATL decreased by about 10%, and the adsorption capacity remained above 90%. These results indicate that the recyclability of PANF-ATL is sufficient for the removal of recovered Au(Ⅲ) from waste liquid (Table 4). 4. Conclusions In this study, a novel adsorbent, PANF-ATL, was synthesized directly from polyacrylonitrile fibers and 2-amino-2-thiazoline without the use of organic solvents. The optimum conditions for the preparation were determined by analyzing the sulfur content. The results of TGA showed that the chelating fibers had good thermal stability below 280 °C. The adsorbent can be used for separation and enrichment of Au (Ⅲ) in aqueous solution; the adsorption equilibrium can be quickly achieved within 240 min; Au(Ⅲ) can be completely eluted from the adsorbent; the adsorption capacity after 5 cycles of adsorption is still More than 90% of the maximum adsorption capacity. The adsorption process follows a pseudo second-order kinetic model and a Langmuir model. The high adsorption capacity of PANF-ATL obtained from 9

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