Adsorption properties of ion recognition rice straw lignin on PdCl42−: Equilibrium, kinetics and mechanism

Colloids and Surfaces A: Physicochem. Eng. Aspects 514 (2017) 260–268

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Adsorption properties of ion recognition rice straw lignin on PdCl4 2− : Equilibrium, kinetics and mechanism Baoping Zhang a,b,1 , Zhongchen Ma a,b,∗,1 , Fang Yang a,b,1 , Yun Liu a,b,1 , Meichen Guo a,b,1 a

The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan, Hubei, 430081, China Key Laboratory for Ferrous Metallurgy and Resources Utilization of Ministry of Education, Wuhan University of Science and Technology, Wuhan, Hubei, 430081, China b

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• A low cost biosorbent for selective adsorption of PdCl4 2− based on rice straw. • The adsorbent showed prominent adsorption capacity for PdCl4 2− up to 122.49 mg g−1 . • The adsorption mechanism were ion exchange and coordination reaction. • The adsorption obeyed the Langmuir model and pseudo-secondorder kinetics model.

a r t i c l e

i n f o

Article history: Received 13 September 2016 Received in revised form 27 November 2016 Accepted 29 November 2016 Available online 30 November 2016 Keywords: Lignin Ion recognition PdCl4 2− Adsorption mechanism

a b s t r a c t A low cost biosorbent was synthesized based on rice straw, which showed the ability of selective adsorption on PdCl4 2− . Adsorption behaviors, including the effects of the initial concentration of PdCl4 2− , the adsorption time and the concentration of hydrochloric acid on the effect of adsorption, were explored systematically. The results showed that the saturation adsorption capacity of the sorbent on PdCl4 2− was 122.49 mg g−1 and the equilibrium adsorption time was 60 min. In addition, the adsorption obeyed the Langmuir isotherm model and pseudo-second-order kinetics model. Low hydrochloric acid concentration could promote the selectivity of adsorption and the adsorption rate went up to 92.71% when the concentration of hydrochloric acid was 0.5 mol L−1 . Furthermore, the adsorption mechanism was investigated through the analyses of FTIR and XPS and it indicated that Pd(II) coordinated with the N of primary amine and the O of alcoholic hydroxyl except ion exchange with Cl− . © 2016 Published by Elsevier B.V.

1. Introduction

∗ Corresponding author at: School of Materials and Metallurgy, Wuhan University of Science and Technology, Room 6715, NO. 947, Heping Road, Qingshan District, Wuhan, 430081, China. E-mail addresses: [email protected] (B. Zhang), [email protected] (Z. Ma), [email protected] (F. Yang), [email protected] (Y. Liu), [email protected] (M. Guo). 1 School of Materials and Metallurgy, Wuhan University of Science and Technology, Room 6715, NO. 947, Heping Road, Qingshan District, Wuhan, 430081, China. http://dx.doi.org/10.1016/j.colsurfa.2016.11.069 0927-7757/© 2016 Published by Elsevier B.V.

With the excellent physical and chemical properties, palladium is widely used in the fields of life, industry and medicine [1,2]. Currently, the resources of palladium include mineral and secondary resources. Palladium exists in many nonferrous metallic ores as an associated mineral, while the secondary resources are mainly composed by waste electronic products and spent catalysts [3–6]. The traditional processes to extract palladium from palladium resources include chemical precipitation, solvent extraction,

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ion exchange and microbiological extraction [3]. However, the raw materials need to be pretreated to make palladium enriched and then separated from the mixture system through a long complex process by chemical precipitation and solvent extraction, which is of low recovery rate, high cost and serious pollution [7]. Ion exchange has the advantages of high recovery rate and short process, but ion exchange resin is expensive and pernicious when being burned because of the tar and benzene [8,9]. Although it has the advantages of simple operation, low cost and environmental friendly, microbiological extraction is still at the theoretical study stage because of the technical obstacle and the unclear mechanism [10]. Therefore, achieving the extraction metallurgy to be efficient, energy-saving and environmental friendly has been the most urgent problem for the present. In order to achieve this goal, the biological materials were used as biosorbent in the field of extractive metallurgy [11–20]. Lignin is a very good biological material. It distributes widely in plants and contains a large amount of phenolic hydroxyl, alcohol hydroxyl, carbonyl, methyl, carboxyl, aromatic groups, etc. Now lignin has usually been adopted to make functional materials [21–23]. Especially, because of the characteristic of selectivity adsorption on Pd(II), lignin and its derivatives, as the renewable biomass resources and environmental materials, have been used to the recovery of Pd(II) from the mixture system [24–29]. At present, researchers have made great progresses on the extraction of base metals by lignin and its derivatives [30–33]. But the influence factors, especially the adsorption mechanism on precious metals had not been explored systematically, which would have great research significance. So far, a large number of agriculture and forestry byproducts are idled or burned due to the inadequate utilization every year, which should cause the waste of resources and pollution of environment [21]. Based on the concept of green environmental protection and comprehensive utilization of resources, strengthening the research on extraction or synthesis and application of lignin and its derivatives in the field of extractive metallurgy has the great significance for the sustainable development strategy. The ion recognition rice straw lignin (IRRSL) was synthesized in this paper by phenolation, polycondensation, chloration, amination and quaternarization reactions based on rice straw, which is of selectivity on PdCl4 2− in the mixture metallic ion system. The influence factors, including the initial concentration of PdCl4 2− , the adsorption time and the concentration of hydrochloric acid, had been explored systematically. Furthermore, the adsorption mechanism was investigated. Based on the study, the target of high efficiency, energy saving and environmental friendly to extract palladium can be realized.

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2. Materials and methods 2.1. Reagents and materials The rice straw was from Hubei province of China. PdCl2 , NiCl2 ·6H2 O, CuCl2 ·2H2 O, FeCl3 ·6H2 O and ZnCl2 as crystal solid and glycidyltrimethylammonium chloride (GTA) as a liquid were purchased from Sigma-Aldrich (Shanghai, China). H2 SO4 , paraformaldehyde, NaHCO3 , pyridine, thionyl chloride, N,NDimethylformamide (DMF) and Tetraethylenepentamine (TEP) were supplied by Sinopharm Chemical Reagent Co., Ltd (Beijing, China). All of the reagents were analytical reagents. The metal ion solutions were prepared by dissolving the respective chloride salts in hydrochloric acid solution directly. 2.2. Preparation of IRRSL The rice straw lignin-phenol (RSLP) was prepared by sulfuric acid method [32], and then amination RSLP was synthesized based on RSLP by polycondensation, chloration, amination [29]. 3.5 g of RSLP was mixed with 35 mL of 72 wt.% H2 SO4 and stirred at 100 ◦ C for 10 min and then 5.0 g paraformaldehyde was added to the solution interacting for 24 h. The mixed solution was then added dropwise to 1500 mL of 5 wt.% NaHCO3 and stirred for 3 h, subsequently filtered and washed with distilled water of 60 ◦ C. The crosslinked straw lignin-phenol was obtained after being dried in an oven at 90 ◦ C for 48 h. After that, 3.5 g crosslinked straw ligninphenol suspended with 120 mL pyridine, stirring for 5 min and keeping the solution cold in ice bath, then warmed to 70 ◦ C. After that, 18 mL of thionyl chloride was joined in 1 h and interacted for 5 h. The solution was washed with distilled water after being cooled to room temperature until the filtrate was colorless and transparent. The chloride crosslinked RSLP was obtained after being dried in an oven at 50 ◦ C for 5 h. Subsequently, 4.0 g of chloride crosslinked RSLP was mixed with 40 mL of DMF and stirred at 80 ◦ C for 10 min, and then 10 mL of TEP was added to the solution and interacted for 48 h. The Amination RSLP was washed with distilled water followed by methanol washing and then vacuum drying. Finally, 6.0 g of Amination RSLP was suspended in 75 mL DMF/water mixture (1:1, v/v), stirring for 10 min at 60 ◦ C. After that, 18 mL of glycidyltrimethylammonium chloride (GTA) was added and then being kept at 60 ◦ C for 24 h. The product was washed with distilled water and then by methanol. The product obtained was IRRSL. The synthetic route was shown in Scheme 1. 2.3. Analysis and characterization ICP-OES (Themo Elemental IRIS Advantage, America) was used to measure the concentrations of PdCl4 2− before and after adsorp-

Scheme 1. The synthetic route of IRRSL.

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B. Zhang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 514 (2017) 260–268 Table 1 Saturation adsorption capacity of IRRSL and the other adsorbents.

Fig. 1. The initial concentration of PdCl4 2− on adsorption capacity, weight of IRRSL = 10 mg, volume of test solution = 10 mL, concentration of HCl = 0.5 mol × L−1 , shaking time = 100 h, shaking speed = 300 rpm, room temperature.

tion to calculate the adsorption capacity of IRRSL on PdCl4 2− and the change of functional groups of IRRSL before and after adsorption were analyzed through FTIR spectra (from 4000 to 400 cm−1 ) detected by FTIR-spectroscopy (Brvker VERTEX70, Germany). The surface morphology and components of IRRSL was confirmed by Xray Photoelectron Spectroscopy (Perkin Elmer PHI-5400, America). 2.4. Adsorption experiments In batch experiment, a certain volume and concentration of metal ions were mixed with a certain amount of IRRSL in test tubes, and then kept the test tubes in a shaker at a speed of 300 rpm for a certain time. Influencing factors including the initial concentration of PdCl4 2− , adsorption time and concentration of hydrochloric acid were studied. Besides, the selectivity of IRRSL on PdCl4 2− was also studied. All the adsorption tests were performed twice and calculated the average to avoid any experimental errors. Related parameters were calculated according to Eqs. (1) and (2): = qe =

C0 − Ce × 100% C0 C0 − Ce ×V m

(1) (2)

where  is the adsorption ratio, C0 and Ce are the initial and equalized concentrations of PdCl4 2− (mmol L−1 ), respectively, V is the volume of mixed solution (mL), m is the quality of IRRSL (mg), qe is the adsorption capacity (mg g−1 ). 3. Results and discussion 3.1. Effect of initial concentration of PdCl4 2− on adsorption capacity To get the saturation adsorption capacity, the effects of the varying initial concentration of PdCl4 2− (from 0.2 to 7.0 mmol L−1 ) on the adsorption capacity were tested. It could be observed that the adsorption capacity of PdCl4 2− increased along with the increasing of the initial concentration until it achieved the saturation adsorption capacity from Fig. 1. In the range of concentration from 0.2 to 4.0 mmol L−1 , the adsorption capacity obviously increased and then trended to be flat since the initial concentration exceeded 4.0 mmol L−1 . Therefore, the saturation adsorption capacity obtained was 122.49 mg g−1 when the initial concentra-

Adsorbent

Adsorption capacity (mg g−1 )

References

G600 Pd(II)-ion-imprinted porous polymer particles Murexide functionalized halloysite nanotubes Commercial AC 2-Mercaptobenzothiazole PPF resin Aliquat-336 impregnated chitosan IRRSL

23.40 38.90

[34] [24]

42.86

[25]

43.50 50.00 111.11 187.61 122.49

[35] [36] [37] [38] This work

tion reached 4.0 mmol L−1 . When the initial concentration was less than 4.0 mmol L−1 , the consumption of adsorption sites continuously increased with the increasing of the initial concentration of PdCl4 2− , which leaded to the decreasing of the remaining of adsorption sites on PdCl4 2− . During this stage, the adsorption capacity was constantly increasing. Increasing the initial concentration, the total available adsorption sites would be entirely replaced by PdCl4 2− and the saturation adsorption capacity was achieved. The result showed that the IRRSL had the excellent adsorption capacity on PdCl4 2− from hydrochloric acid medium. Table 1 showed the saturation adsorption capacity of IRRSL on Pd(II) and that of the recently reported sorbents [24,25,34–38]. The results indicated that IRRSL had the excellent adsorption capacity. However, the saturation adsorption capacity of IRRSL on Pd(II) was lower than that of Aliquat-336 impregnated chitosan. It could be attributed to the higher content of −NH2 and −OH of Aliquat-336. Nevertheless, the source of lignin distributes widely in plants, IRRSL was cheaper and more environmental friendly. 3.2. Adsorption isotherms The interaction between PdCl4 2− and IRRSL was investigated by adsorption isotherms. Three isotherm models, namely Freundlich, Langmuir and Temkin, were used to explore the relationship between the adsorption capacity and equalized concentration. More information about the three models was as follows [39]. Freundlich model reflects the effect of varying initial concentration on adsorption capacity of absorbent. The model is defined by the equation: log qe = log KF + 1/n log Ce

(3)

where qe is adsorption capacity (mg g−1 ), Ce is the equalized concentration of PdCl4 2− (mmol L−1 ), KF is adsorption equilibrium constant, n is concentration index. When n is more than 1.0, it has been reported that when 1/n is in the range of 0.1–0.5, the adsorption is favorable. The adsorption is difficult when 1/n is moer than 2.0. Langmuir model assumes that the surface of adsorbent was equilibrium and the adsorption belongs to mono-layer adsorption. Besides, the ions absorbed on the surface of absorbent have no interactions. The model can be described by the equation: Ce Ce 1 = + qe qm qm KL

(4)

RL = (1 + KL C0 )

(5)

where qm is the saturation adsorption capacity (mg g−1 ), RL is the model parameter of Langmuir model, KL is adsorption equilibrium constant (mg L−1 ). In addition, when RL is in the range of 0–1, the adsorption is easy. The adsorption is difficult when RL is more than 1.

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263

Fig. 2. (a) Freundlich, (b) Langmuir and (c) Temkin isotherm plots of the adsorption of IRRSL on PdCl4 2− .

However, Temkin model assumes that adsorption attributes to multi-layer and the ions on the surface of absorbent are interactions. The model expressed by the equation: qe = RTb−1 ln KT + RTb−1 ln Ce

(6)

where b and K are constants. The results of linear fitting plots for experimental data of the adsorption of IRRSL on PdCl4 2− were shown in Fig. 2 and the parameters from linear fitting equations were summarized in Table 2, respectively. As shown in Fig. 2(b) and Table 2, the Langmuir model fitted quite well with the high correlation coefficient (R2 = 0.99915). While the correlation coefficients of Freundlich and Temkin (0.88704 and 0.94411, respectively) were lower, which meant that the isotherm models of Freundlich and Temkin fitted bad for the experimental data. In addition, the saturation adsorption capacity calculated by the linear fitting equation of Langmuir was 124.53 mg g−1 and it was very close to the experimental adsorption capacity (122.49 mg g−1 ). According to Eq. (5), the value of RL was in the range of 0.077–0.339, it meant that the adsorption of IRRSL on PdCl4 2− was easy. Therefore, the adsorption process of IRRSL obeyed the formation of mono-layer molecule sorption. Actually, the adsorption isotherm model of IRRSL on PdCl4 2− conformed to that of the other sorbent [34,37,40–42]. 3.3. Effect of adsorption time on adsorption capacity As shown in Fig. 3, the adsorption was very quickly. The adsorption capacity was up to 19.21 mg g−1 and the adsorption rate was as high as 1.28 mg g−1 min−1 during the first 15 min. In the next 45 min, however, there was only incremental quan-

Fig. 3. Effect of adsorption time on adsorption capacity, weight of IRRSL = 10 mg, volume of test solution = 10 mL, initial concentration of PdCl4 2− = 0.2 mM, concentration of HCl = 0.5 mol L−1 , shaking speed = 300 rpm, room temperature.

tity of 1.34 mg g−1 and the adsorption rate suddenly dropped to 0.03 mg g−1 min−1 . Then prolonging the adsorption time, the adsorption capacity was unchanged. In the first 15 min, the adsorption sites on IRRSL were adequate and the concentration of PdCl4 2− was high, so the effective collision was at high frequency. When the adsorption time exceeded 15 min, the effective collision sharply reduced due to the consumption of adsorption sites and PdCl4 2− , which leaded the adsorption rate to falling. 60 min later, almost

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Table 2 Parameters of isotherm models of the adsorption of IRRSL on PdCl4 2− . Freundlich

*

Langmuir

KF

1/n

27.29480

0.26852 −1

q m (experimental) = 122.49 mg g

Temkin

R2

qm1

KL

R2

RTb−1

KT

R2

0.88704

124.53

0.09803

0.99915

16.163

5.09025

0.94411

, qm1 (calculated).

Table 3 Parameters of kinetic models of the adsorption of PdCl4 2− onto IRRSL.

*

Parameters

Pseudo first order

Pseudo second order

W-M

R2 K

0.49068 0.00368

0.99999 0.04808

0.73051 0.33269

Elovich 0.93330 0.01013

0.60434 0.29627

qe (experimental) = 20.80 mg g−1 .

all available PdCl4 2− were adsorbed and the change of adsorption capacity was teeny. In conclusion, the equilibrium of adsorption time and capacity were 60 min and 20.80 mg g−1 , respectively. 3.4. Adsorption kinetics The kinetic mechanism is studied by adsorption kinetics. Four famous kinetic models, namely pseudo-first-order, pseudosecond-order, W-M an Evolich, were used to fit the experimental data of adsorption time and capacity by linear fitting. Particular information about the four kinetic models was as follows [43]. The external diffusion control of adsorption can be proved by Pseudo-first order kinetic. The model is described by the following Eq: log(qe − qt ) = log qe − K1 t/2.303

(7)

where qt is the adsorption capacity of t min (mg g−1 ), K1 is the adsorption constant of pseudo-first order (min−1 ), t is adsorption time (min). Pseudo-second order kinetic model is applied to describe the chemical adsorption process. The model is expressed by the following Eq: t 1 1 = + qt qe K2 q2e

(8)

where K2 is the adsorption constants of pseudo-second order (mg g−1 min−1/2 ). Intra-particle diffusion model demonstrates the internal diffusion process of adsorption. In detail, the adsorption will be controlled by internal diffusion if the fitting line passes through the origin. The adsorption will be controlled by hybrid control if the fitting line does not pass through the origin. The model is defined by the following Eq: qt = K3 t 1/2 + C

(9)

where K3 is the adsorption constant of W-M (mg g−1 min−1/2 ) and C is constant. Elovich diffusion model expounds the adsorption of solid surface of adsorbent, which assumes that the distribution of adsorption energies of adsorbent is unequally. In other words, there are different activated adsorption sites on the adsorbent surface. The model is described by the following Eq: qt = A + K4 ln t

(10)

where K4 and A are rate constant and constant, respectively. Based on Eq. (7)–(10), the adsorption kinetics plots by fitting the experimental data for adsorption of PdCl4 2− onto IRRSL were showed in Fig. 4. All parameters calculated from equations of straight lines were summarized in Table 3. The highest

correlation coefficient (R2 = 0.99999) suggested pseudo-secondorder was the best applicable to fit the experimental data. While the correlation coefficients of pseudo-second-order, W-M and Elovich kinetic models were bad (Table 3), so these models failed to describe the adsorption process. In addition, the equilibrium adsorption capacity was 20.78 mg g−1 according to the equation of pseudo-second-order, which was very close to experimental equilibrium adsorption capacity (20.80 mg g−1 ) according to Table 3. The adsorption kinetics indicated that the adsorption of PdCl4 2− onto IRRSL was a chemisorbed process. In addition, some recently reported literatures about the kinetic models of sorbents on Pd(II) also obeyed the pseudo-second-order [26,34,35,37,42]. 3.5. Effect of hydrochloric acid concentration on selectivity To investigate the selectivity of IRRSL towards the adsorption of PdCl4 2− from the mixture of PdCl4 2− and base metal ions, the adsorption tests were carried out in different concentrations of hydrochloric acid medium. As shown in Fig. 5, the adsorption ratio of PdCl4 2− decreasd with the increasing of hydrochloric acid concentration, while the adsorption ratios of base metal ions increased. When the hydrochloric acid concentration was 0.5 mol L−1 , the adsorption ratio of PdCl4 2− was as high as 92.71%, while the adsorption ratios of Zn2+ , Ni2+ , Fe3+ and Cu2+ were 2.52%, 3.66%, 1.97% and 0.87%, respectively. In this situation, the IRRSL showed excellent selectivity on PdCl4 2− . The adsorption ratio of PdCl4 2− declined to 38.99% when the hydrochloric acid concentration was 5 mol L−1 . While the adsorption ratio of Zn2+ , Ni2+ , Fe3+ and Cu2+ raised to 12.31%, 10.99%, 9.4% and 4.88%, respectively. The ability of selective adsorption of PdCl4 2− onto IRRSL declined evidently. Because of the increasing of hydrochloric acid concentration, the increasing of chloride ion concentration would suppress the dissociation of Cl− of IRRSL due to the common-ion effect, which reduced the amount of adsorption sites for ion exchange of PdCl4 2− . In addition, the base metal ions and chloride ion could form chlorine anion complex under the conditions of high chloride ion concentration [44]. The chlorine anion complex would occupy the adsorption sites and leaded the adsorption capacity of PdCl4 2− to falling. However, the result was encouraging in respect of selective adsorption of PdCl4 2− from the multicomponent mixture solution. 3.6. Adsorption mechanism It is known that the adsorption mechanism of IRRSL contained ion exchange (as shown by Eq. (11)) and the chemical structure of PdCl4 2− absorbed by IRRSL through ion exchange was shown in Fig. 6. 2R − Cl- + PdCl42− → 2R − PdCl42− + 2Cl−

(11)

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265

Fig. 4. (a) Pseudo-first order, (b) pseudo-second order, (c) W-M and (d) Evolich kinetic fit for adsorption of IRRSL on PdCl4 2− . Table 4 Key changes in FTIR peaks of IRRSL and their significance.

Fig. 5. Effect of hydrochloric acid concentration on adsorption ratios of PdCl4 2− , Zn2+ , Ni2+ , Fe3+ and Cu2+ , weight of IRRSL = 10 mg, volume of test solution = 10 mL, initial concentration of metal ion = 0.2 mM, shaking time = 10 h, shaking speed = 300 rpm, room temperature.

However, the adsorption mechanism also contained coordination [45]. The functional groups participated in the adsorption and the coherent elements coordinated with Pd(II) were investigated through analyzing the FTIR pattern and XPS survey spectrum of IRRSL taken before and after adsorption of PdCl4 2− . As shown in

Wavenumber (cm−1 )

Functional group

3426 1647 1160 1107 669

Stretching of N H and O H Inner surface bending of N H of primary amine Bending of C O of alcohol Outer surface bending of N H and O H

Fig. 7 and Table 4, after adsorption, the wide peak of N H and O H stretching vibration about 3426 cm−1 was weaker evidently as well as the peak around 1647 cm−1 belonged to inner surface bending of N H of primary amine [46]. Meanwhile, the disappeared peak about 669 cm−1 was attributed to the outer surface bending of N H and O H. Besides, the weaker peaks about 1160 and 1107 cm−1 were attributed to bending of C O of alcohol [47]. All above analyses adequately indicated that the primary amine and alcoholic hydroxyl groups of IRRSL took part in the adsorption of IRRSL on PdCl4 2− . The XPS spectra of C1s, O1s, N1s, Cl2p and Pd3d (a) of IRRSL before and (b) after the adsorption were shown in Fig. 8. After the PdCl4 2− was loaded on IRRSL, the binding energies of new peaks of Pd3d appeared at 336.30 and 342.34 eV, respectively. While the binding energies of C1s and Cl2p did not show any shift. However, the binding energies of O1s and N1s shifted from 530.57 and 398.00 eV to 530.81 and 400.30 eV, respectively. The shift of the binding energies meant the O of alcohol and N of primary amine took part in adsorption. Furthermore, the binding energies of

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Fig. 6. The chemical structure of PdCl4 2− absorbed onto IRRSL through ion exchange.

Fig. 7. FTIR patterns of IRRSL taken (a) before and (b) after the adsorption on PdCl4 2− .

Scheme 2. Coordination route of PdCl4 2− with (a) the N of primary amine, (b) the intramolecularly O of alcoholic hydroxyl, (c) the intermolecular O alcoholic hydroxyl.

336.30 and 342.34 eV correspond to Pd3d5/2 and Pd3d3/2 of Pb(II)O proved that the new coordination bond formed between Pb(II) and the O of alcohol through coordination reaction [48]. In addition, both FTIR and XPS spectrum of IRRSL suggested that the N of primary amine might also formed new chemical bond with Pb(II). The coordination routes could be described in Scheme 2.

4. Conclusions (1) IRRSL showed the selectivity on PdCl4 2− . Adsorption behaviors showed that the saturation adsorption capacity of IRRSL on PdCl4 2− was 122.49 mg g−1 when the initial concentration of PdCl4 2− was 7.0 mmol L−1 and the equilibrium adsorption

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Fig. 8. XPS spectra of IRRSL taken (a) before and (b) after the adsorption on PdCl4 2− .

time was 60 min when the initial concentration of PdCl4 2− was 0.2 mmol L−1 , respectively. Low hydrochloric acid concentration could promote the selectivity and the adsorption rate went up to 92.71% when the concentration of hydrochloric acid was 0.5 mol L−1 and the initial concentration of PdCl4 2− was 0.2 mmol L−1 . (2) The adsorption isotherm obeyed the Langmuir model and kinetics studies fitted well with pseudo-second-order, which indicated that the adsorption was in the form of mono-layer and chemisorption. (3) The adsorption mechanism of IRRSL on PdCl4 2− was ion exchange and coordination. Furthermore, the ion exchange was produced between Cl− and PdCl4 2− and the Pd(II) coordinated with the N of primary amine and the O of alcoholic hydroxyl. Acknowledgements This work was financially supported by The Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry (the 47th batch) and the open fund of The Key Laboratory for Ferrous Metallurgy and Resources Utilization of Ministry of Education (FRMU201203), Wuhan University of Science and Technology, we sincerely acknowledge their help during the research. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.colsurfa.2016.11. 069. References [1] Y.H. Su, Noble Metals, first ed., Janeza Trdine, Rijeka, 2012. [2] F.A. O’Connor, B.M. Lucey, J.A. Battend, D.G. Baure, The financial economics of gold-a survey, Int. Rev. Financial Anal. 41 (2015) 186–205. [3] S. Syed, Recovery of gold from secondary sources-a review, Hydrometallurgy 115–116 (2012) 30–51. [4] R. Ranjbar, M. Naderi, H. Omidvar, Gh. Amoabediny, Gold recovery from copper anode slime by means of magnetite nanoparticles (MNPs), Hydrometallurgy 143 (2014) 54–59. [5] V. Kumar, J. Lee, J. Jeong, Recycling of printed circuit boards (PCBs) to generate enriched rare metal concentrate, J. Ind. Eng. Chem. 21 (2015) 805–813. [6] A. Akcil, C. Erust, C.S. Gahan, M. Ozgun, M. Sahin, A. Tuncuk, Precious metal recovery from waste printed circuit boards using cyanide and non-cyanide lixiviants-a review, Waste Manage. 45 (2015) 258–271. [7] S.J. Liu, Metallurgy of Platinum Group Metals, second ed., Central South University Press, Changsha, 2013, pp. 321–429.

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