Quasi-complete Separation Re(VII) from Mo(VI) onto Magnetic

Jan 13, 2015 - However, in our previous work, the adsorption capacity of Re(VII) on the DIOA-OCS gel prepared from corn stalk was decreased sharply li...
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Quasi-complete Separation Re(VII) from Mo(VI) onto Magnetic Modified Cross-Linked Chitosan Crab Shells Gel by Using Kinetics Methods Zhenning Lou, Li Wan, Chunfang Guo, Shuqin Zhang, Weijun Shan, and Ying Xiong* College of Chemistry, Key Laboratory of Rare-scattered Elements of Liaoning Province, Liaoning University, Shenyang 110036, P. R. China S Supporting Information *

ABSTRACT: In this work, a novel di-2-ethylhexylamine modified magnetic chitosan (FCS-DIOA) was successfully synthesized and used to recovery Re(VII) in Mo−Re binary mixtures. The percentage content and the chemical structure of adsorbent were characterized by elemental analysis, gas sorption, magnetic properties, X-ray diffraction analysis, infrared spectroscopy, and Boehm titration, respectively. We discussed the effects of steric hindrance and basic strength of the secondary, the tertiary ammonium, or the quaternary ammonium gel on rhenium adsorption selectivity and capacity. More important, Re−Mo binary component separations on the FCS-DIOA gel consistently showed that the kinetic separation method exhibited a greater sorption capacity and selectivity (SelRe/Mo = 1.70−4.40). It was found that rhenium(VII) was adsorbed on the FCS-DIOA via charge-interactions and complexation by an infrared spectroscopy and X-ray photoelectron spectroscopy analysis. Relationships between adsorption mechanism and pH, adsorption isotherms, adsorption kinetics, adsorption thermodynamics, desorption, and regeneration were also discussed.

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INTRODUCTION Rhenium, as a rare metal with high melting point, has been widely used in high temperature superalloy productions and catalysts in oil refineries. However, rhenium has no mineable ore naturally and invariably existing in rocks, pegmatites, and especially in molybdenites. It is especially arduous to recover and separate rhenium from molybdenum in an actual aqueous solution because of their similar chemical properties. Recently, in a world with rising rhenium shortages, its recovery and separation from molybdenite or industrial wastewater has become an urgent problem to be solved. A host of techniques have been adopted to recover Re(VII) such as ion exchange, precipitation, adsorption, and solvent extraction.1−6 Comparing biosorption techniques and other methods, it has high selectivity, large adsorption capacity, and low cost, consequently biosorption was broadly exploited and used. As a biomaterial, chitosan (CS) has a host of advantages, such as being nontoxic, biodegradable, renewable, multifunctional, highly reactive, abundant, and low in cost.7−11 By referring to the related literature,12−15 chitosan is appropriate to adsorption metal ions from aqueous systems because the hydroxyl groups and/or amino groups presented on the chitosan can act with metal ions as chelation sites. In recent years, different with the traditional methods, like filtration, sedimentation, and centrifugation, magnetic separation technology has been attracting attention due to its efficiency, lowcost, and rapidity. Moreover, since magnetic adsorbents can easily be separated using a magnetic field, an army of researchers have focused on the modification of magnetic chitosan for enhancing adsorption behavior on the basis of introducing a chemical functional group and applying in recovery of metal ions from an aqueous system.16−21 © XXXX American Chemical Society

In this study, crab shells were used to prepare the magnetic chitosan gel, which was a waste product of the seafood industry. Modified magnetic chitosan gel (FCS-DIOA) was prepared by introducing di-2-ethylhexylamine onto the magnetic chitosan (FCS). The high content of amine groups made it feasible to achieve the purpose of improving its adsorption capacity and selectivity of rhenium from molybdenum. We investigated the selectivity and adsorption properties for Re(VII) from Mo(VI) aqueous system as well as equilibrium, thermodynamics, and kinetics studies, which proved to be that FCS-DIOA gel was an environmentally friendly, low-cost, and effective adsorbent.

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EXPERIMENTAL SECTION

Materials. Molybdenum and rhenium stock solutions (1 g L−1) were prepared by dissolving hexaammonium heptamolybdate tetrahydrate and ammonium perrhenate in hydrochloric acid, respectively. All reagents were of analytical grade and were used without further purification. Preparation of Modified Magnetic Chitosan Gel. Crab shells (Eriocheir sinesis) were bought from the local market. The process of crude crab chitin processing was described in our previous work,6 including washing, drying, crushing, treating with 1.0 mol L−1 HCl aqueous solution at room temperature, filtering, washing, and then drying to complete the process. The synthesis of modified magnetic chitosan gel includes three steps. Received: August 25, 2014 Revised: January 8, 2015 Accepted: January 13, 2015

A

DOI: 10.1021/ie503362j Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research (i) Step 1: Preparation of Crude Crab Chitosan. By way of N-deacetylation, 1.0 g of crude crab chitin was added in 30 mL of 50% NaOH aqueous solution and stirred for 4 h at 378 K. After filtration, washing to neutrality with deionized water, and drying, the product obtained was designated as crude crab chitosan. The crude crab chitosan is abbreviated hereafter as CS.

remaining liquid was directly measured by using a PE model atomic absorption spectrophotometer. Adsorption kinetics were actualized by mixing 10 mg of the gel in 5 mL adsorbate solution (25 mg·L−1) at varying temperatures and/or at varying hydrochloric acid concentration. The percentage adsorption for metal ions on the adsorbent was calculated according to eq 1, as well as the adsorption selectivity in binary system was determined according to eq 2. %A =

(Ci − Ce) × 100% Ci

Sela / b = log

(ii) Step 2: Preparation of Magnetic Chitosan Gel. Magnetic Fe3O4 nanoparticles were prepared according to the methods described elsewhere. First, FeCl3·6H2O (0.27 g) and FeCl2·4H2O (0.30 g) were dissolved in 10 mL boiled deionized water. Second, the mixture was stirred and heated to 353 K under the protection of nitrogen. Then, the precipitant of NaOH solution (3.0 mL, 2.5 mol L−1) was added dropwisely into the mixture and stirred for about 3 h until stable. Afterward, the Fe3O4 nanoparticles were separated by using magnetic field and washed to neutrality. The chitosan solution in 5% acetic acid medium (20 mL, 1%) was mixed with 0.30 g Fe3O4 nanoparticles, 0.5 mL span-80, and 40 mL paraffin, and then dispersed for 20 min using ultrasonic irradiation. Afterward, 3 mL 50% glutaraldehyde solution was added and stirred for 4 h. The magnetic chitosan was washed with petroleum ether and acetone and finally dried at 333 K. The magnetic chitosan gel is abbreviated hereafter as FCS.

(1)

(qe /Ce)a (qe /Ce)b

(2)

where the index a pertains to one of the metals and index b refers to the remaining metals ions in the solution. According to eq 3, the amount of metal ion adsorbed (q, mg g−1) on the adsorbent was calculated by the measured initial and equilibrium concentration of metal ions: q = (Ci − Ce) × V

(3)

where V (L) stands for volume of solution, Ce stands for the equilibrium concentration measured after adsorption, and Ci is the initial concentration of adsorbate. The values of pH were measured with a S-3C model pH meter. Adsorption isotherm studies22−25 and adsorption kinetics26−29 and thermodynamics studies are shown in the Supporting Information. Desorption and Regeneration Studies. Re(VII)-loaded adsorbent was prepared by mixed with Re(VII) containing solution (100 mg L−1) for 48 h and washed for removing metal ions stuck to on surface of the adsorbent. The Re(VII)-loaded FCS-DIOA gel was put into various concentrations of NH4SCN (1−20%) and shaken at 303 K for achieving equilibrium. Afterward, it was separated from backwash solution by the external magnetic field, washed, and dried at 333 K. In addition, cyclic adsorption experiment was carried out by mixed regenerative adsorbent with a fresh solution five times. Characterization of Modified Magnetic Chitosan Gel. The content of nitrogen of crude crab chitosan and modified magnetic chitosan gels was determined with a Flash EA 1112 elemental analyzer. FTIR spectra of adsorbents were recorded on a Nicolet 5700 FTIR spectrophotometer. Powder XRD patterns at 2θ angles from 20° to 90° were recorded at an interval of 0.02° on a Bruker D8 diffractometer using Cu Kr radiation at room temperature (40 kV, 120 mA). BET specific surface area was measured by a Quantachrome ASiQ-C gas sorption analyzer. XPS spectra were measured by a Thermo ESCALAB 250 X-ray Photoelectron Spectrometer with Al Kα X-ray source and were fitted using XPSPEAK4.1 software. Magnetic properties of the gels were measured on a Lake Shore 7407 vibrating sample magnetometer (VSM) at room temperature. In our previous work,30 the measurement of point of zero charge (PZC) was described in detail. Acidic surface functional groups on adsorbents were determined by the Boehm titration method.

(iii) Step 3: Modification of Magnetic Chitosan Gel with nOctylamine, Di-2-ethylhexylamine, Di-n-octylamine, Tri-isooctylamine. Epichlorohydrin (100 mL) was taken together with 30 mL n-octylamine, di-2-ethylhexylamine, di-n-octylamine, tri-iso-octylamine, respectively, and the solution was stirred for 4 h at 333 K. Then, 1.0 g sodium hydroxide was further added to the above solution and stirred until dissolved. Then, FCS (5.0 g) was added and stirred at 383 K for 8 h. The product was filtered, washed to neutrality, and dried at 333 K. The aminated magnetic chitosan gels are abbreviated hereafter as FCS-NOA, FCS-DIOA, FCS-DNOA, and FCS-TIOA, respectively.

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Batch Adsorption Studies. Adsorption experiments of on the adsorbent were conducted at 303 K for plenty of time by shaking 5 mL of 20 mg L−1 adsorbate solution and 10 mg of the gel at various acid concentrations in 20 mL flask. After attaining equilibrium, the adsorbent and solution was separated by an external magnetic field. The metal ions concentration in the

RESULTS AND DISCUSSION Characterization of Adsorbent. Elemental Analysis and BET Surface Area. An elemental analysis experiment was performed so as to measure the degree of introducing the amine functional group on the adsorbent. Table 1 showed the B

DOI: 10.1021/ie503362j Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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charged above this pH and positively charged below this pH. It can be observed that the PZC value of FCS was acidic (4.60), whereas after modification with di-2-ethylhexylamine, the PZC value has been shifted from 4.60 to 7.61. The change of PZC value of FCS after chemical modification also indicated successfully introducing of amine functional groups on the FCS gel. Fourier Transform Infrared Spectroscopy Analysis. The FTIR spectra of FCS and FCS-DIOA gel were taken in the present work, as shown in Figure 1a. For the FCS, the wide band at 3427 cm−1 was clear, which was corresponded to the overlapping of −NH/−OH stretching. The peak at 2923 cm−1 was assigned to the stretching vibration of −CH and −CH2. The peak of CN at around 1621 cm−1 indicated forming a Schiff base after chitosan reacting with glutaraldehyde. The peak at 1066 cm−1 was corresponding to the C−OH stretching vibration. The appearance of a new peak for the FCS gel at 578 cm−1 (Fe−O bond vibration of Fe3O4) indicated the successful coating on the outside of Fe3O4, and also justified the reaction of step 2 as mentioned above. Compared with that of the FCS and the FCS-DIOA gel, the intensity of the peak at 3427 cm−1 (−NH/−OH) strengthened and shifted to 3413 cm−1. The absorption peak at about 1158 cm−1 was corresponding to C−N stretching vibration. More importantly, the appearance of a new peak at 2360 cm−1 indicated that doped quaternary ammonium salt was formed.31 These observations also revealed that di-2-ethylhexylamine modified magnetic chitosan was successfully synthesized as the reaction of step 3. X-ray Diffraction Analysis. The crystalline structures of Fe3O4, FCS and FCS-DIOA were identified with X-ray diffraction analysis (XRD). As shown in Figure 2a, six characteristic diffraction peaks of Fe3O4 (2θ = 30.1°, 35.5°, 43.1°, 53.4°, 57.2°, and 62.5°),21 marked by their indices ((220), (311), (400), (422), (511), and (440)) were observed for the Fe3O4 prepared in our lab. The same sets of characteristic peaks were also found for the FCS gel and the FCS-DIOA gel, suggesting the stability of the crystalline phase of Fe3O4 nanoparticles in the case of being coated and amine modified. It was also observed that two new peaks appeared at 31.6° and 45.5° for FCS-DIOA indicated that new substance was formed as a result of chemical modification, which agreed well with the elemental analysis and FTIR results. Magnetic Separation Performance. The industrial demand for new magnetic adsorbents keeps increasing, and the

Table 1. Variation in Characteristics of Modified Crab Shell Chitosan Gels

content of nitrogen (N%) surface area (m2 g−1) magnetization (emug g−1) PZC carboxylic functional group (mmol g−1) hydroxyl functional group (mmol g−1) lactonic functional group (mmol g−1) total acidic group (mmol g−1)

CS

FCS

FCSDIOA

FCSNOA

FCSDNOA

FCSTIOA

7.15

1.93

2.15

2.28

1.93

2.23

6.98

0.55

0.77

0.79

0.82

0.69

0

32.80

55.47

51.23

59.82

54.68

7.42 0

4.62 0

7.60 0

7.21 0

7.45 0

7.90 0

8.83

8.75

0.60

0.67

0.61

0.64

0.12

0

0

0

0

0

8.95

8.75

0.60

0.67

0.61

0.64

elemental analysis results of CS, FCS, and FCS-DIOA. It can be clearly observed that the content of nitrogen of FCS (1.93%) decreased sharply after coating crab chitosan (7.15%) on the surface of magnetic particles. The observed value for nitrogen in the prepared di-2-ethylhexylamine functionalized gel (FCSDIOA) was 2.15%, which was just a little higher than that of FCS due to the large molecular weight of FCS. The nitrogen density in the FCS-DIOA gel was calculated as 1.54 mmol g−1. Moreover, the BET specific surface area of CS, FCS, and FCSDIOA fitted by nitrogen sorption isotherms was 6.98, 0.55, and 0.77 m2 g−1, respectively. Boehm Titration and Determination of Point of Zero Charge. Oxygen functionalities of adsorbent, such as lactonic, hydroxyl, and carboxylic groups could be checked by Boehm titrations. Table 1 showed that crude crab chitosan (CS) almost has not the content of lactonic functional (0.12 mmol g−1) and carboxylic functional (0 mmol g−1). Although the hydroxyl group appeared in high levels in FCS, the content all decreased from 8.75 to 0.60 mmol g−1 for the FCS-NOA, FCS-DIOA, FCS-DNOA, and FCS-TIOA gels. The results indicate that the chemical modification by amine group decreases the total acidity, which justifies the chemical modification of the crab shell as the reaction of step 3 as mentioned above. The PZC values of the FCS and FCS-DIOA were 4.62 and 7.60, respectively. The surface of an adsorbent is negatively

Figure 1. FTIR spectra of (a) CS, FCS, and FCS-DIOA gel, and (b) FCS-DIOA gel after Re(VII) adsorption at pH 2.0, 5.0, and pH 10.0. C

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Figure 2. XRD patterns (a) and magnetic hysteresis curves (b) of Fe3O4 (1), FCS gel (2), and FCS-DIOA gel (3).

Figure 3. (a) Adsorption behavior of Fe3O4, CS, FCS, FCS-NOA, FCS-DNOA, FCS-DIOA, and FCS-TIOA for Re(VII) at varying hydrochloric acid concentrations and (b) effect of pH value on final pH on the FCS-DIOA gel: weight of gel = 10 mg, initial concentration of metal ions = 20 mg L−1, solid-to-liquid ratio = 2 g:1 L, shaking time = 48 h, temperature = 303 K.

groups, Fe3O4, CS gel, and FCS gel had no adsorption or limited adsorption for Re(VII), which is indicative of the amine groups as chelation sites for rhenium ions. The focus of this study was on the effect of molecular structure like sterically hindered configurations and different amine groups, for example, secondary, tertiary amine, and quaternary ammonium salt. A comparison of adsorption of Mo(VI) and Re(VII) on the FCS-NOA, FCS-DNOA, FCSDIOA, and FCS-TIOA gels was also shown in Figure 4. It seemed that adsorption patterns for Re(VII) on each gel modified by different amine groups were exhibited similarly, while it had different adsorption patterns for Mo(VI). According to the results, the rhenium(VII) adsorption percentage on the FCS-NOA, FCS-DNOA, and FCS-DIOA gels had no significant change from pH 2.0 to 10.0, whereas the molybdenum adsorption capacity was sharply decreased with increase of the pH (FCS-NOA > FCS-DNOA > FCS-DIOA). This might be due to the fact that the progressively increasing number of alkyl groups on the nitrogen atom of the amine group eventually tend to increase the strong steric hindrance effect on Mo(VI) adsorption over that on Re(VII), although alkyl groups in the amine molecular structure tend to increase the basicity at the same carbon chain. Moreover, unlike the secondary or tertiary ammonium cations, the quaternary ammonium cations are independence on the pH of the solution due to having permanent charge. Thus, as a consequence of the different adsorption mechanism, a sharp decrease was observed in the case of pH 10.0 for both the adsorption capacity of rhenium(VII) and molybdenum on the FCS-TIOA gel.

magnetic property of the adsorbent is critical for application. With a successive coating either by chitosan or by an amine group on the magnetic particles Fe3O4, the reduction in magnetization should be minimal. Consequently, the magnetic performance of the naked Fe3O4, FCS, and FCS-DIOA prepared in this study were investigated. Figure 2b showed the hysteresis loops of Fe3O4, FCS, and FCS-DIOA, and the saturation magnetization of these were 89.04, 32.80, and 55.47 emu g−1, respectively. Although the magnetic saturation of the Fe3O4 nanoparticle after being embedded obviously decreased, the magnetic susceptibility value of the FCS-DIOA gel was sufficient to be used for solid−liquid separation. Quick separation of the FCS-DIOA gel from the solution can be achieved within 2 min, implying that the modified magnetic chitosan gel is suitable for industrial application as magnetic carriers. Comparison of Re(VI) and Mo(VI) Adsorption Behavior of the CS, FCS, FCS-NOA, FCS-DNOA, FCS-DIOA, and FCS-TIOA gels. In our work, we found magnetic Fe3O4 leaked after the adsorption at acid medium (pH ≤ 1), and it is stable at pH > 2 even after six adsorption−elution tests. Thus, in order to keep the stable of magnetic Fe3O4 gel, the adsorption of metal ion solution on the gel was carried out at the range of pH 2.0 to 10.0. The adsorption behavior of Re(VII) on the Fe3O4, CS, FCS, FCS-NOA, FCS-DNOA, FCS-DIOA, and FCS-TIOA gels at varying hydrochloric acid concentrations (pH 2.0−10.0) is shown in Figure 3a. The FCS-NOA, FCS-DNOA, FCSDIOA, and FCS-TIOA gels all exhibit the excellent adsorption of Re(VII) in a wide range of acid concentrations from pH 2.0 to 10.0. Unlike magnetic chitosan gels modified by the amine D

DOI: 10.1021/ie503362j Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 4. Adsorption of Re(VII) and Mo(VI) on the (a) FCS-NOA, (b) FCS-DNOA, (c) FCS-DIOA, and (d) FCS-TIOA gels at varying hydrochloric acid concentrations: weight of gel = 10 mg, initial concentration of metal ions = 20 mg L−1, solid-to-liquid ratio = 2 g:1 L, shaking time = 48 h, temperature = 303 K.

Compared with the adsorption behavior of the FCS-DNOA gel, the FCS-DIOA gel had low adsorption capability of molybdenum for reasons of steric hindrance effects, which results by having an alkyl group side chain at the β-carbon next to the amine group in the structure. From this case, we can conclude that adsorption selectivity and capacity was affected mainly by the steric hindrance of the tertiary amines based adsorbent resulting from its molecular structure. It is interesting to notice that adsorption capacity of Re(VII) on the FCSDNOA and FCS-DIOA gel was similar at varying hydrochloric acid concentrations. However, in our previous work, the adsorption capacity of Re(VII) on the DIOA-OCS gel prepared from corn stalk was decreased sharply like the Mo(VI) adsorption in the FCS-DIOA gel,32 which could be explained because Schiff’s base exists in the FCS-DIOA gel. Adsorption Isotherms of Re(VII) on the FCS-DIOA Gel. Adsorption isotherms are paramount for the depiction of how to interact within metal ions and surface reactive sites on the adsorbent and aggregation degree of metal ions onto adsorbent surface. The adsorption isotherm of Re(VII) on the FCS-DIOA gel was studied at various levels of acidity (pH 10.0, 5.0, and 2.0). The adsorption isotherm data for Re(VII) on the FCSDIOA gel at varying levels of acidity by nonlinear curve fit were tabulated in Table S1. In the case of pH 10.0, 5.0, and 2.0, Langmuir isotherms (determination coefficient R2 = 0.99) all showed a better fit than Temkin, Freundlich, and Dubinin− Radushkevich isotherm equations. This means that the FCSDIOA gel containing the homogeneous surface amine groups binding rhenium(VII) followed the Langmuir equation assumption.33

The maximum adsorption capacity of Re(VII) on the FCSDIOA gel at pH 10.0, 5.0, and 2.0 was evaluated as 60.81, 67.68, and 143.40 mg g−1, respectively. The maximum adsorption capacity of the unmodified magnetic chitosan gel (FCS) obtained at pH 2.0 was just 36.72 mg g−1, which was lower than that of the FCS-DIOA gel. The reason why FCSDIOA has higher maximum adsorption capacity is that the incorporation of di-2-ethylhexylamine onto the FCS produces more amine groups to interact with Re(VII). In order to search for the optimal separation condition for Mo(VI) and Re(VII) onto the FCS-DIOA gel, uptake capacity (q max ) of molybdenum at pH 10.0, 5.0, and 2.0 were also evaluated as 5.05, 38.86, and 77.62 mg g−1, which indicated that FCS-DIOA possessed a high affinity for Re(VII) as well as powerful selectivity toward its adsorption compared with Mo(VI) at pH 10.0. In addition, with a comparison of the uptake capacity of Re(VII) on the FCS-NOA, FCS-DNOA, FCS-DIOA, and FCSTIOA gels, it was shown that the adsorption capacity (qmax = 127.70 mg g−1 or 0.69 mmol g−1) of FCS-DIOA toward Re(VII) was considerably lower than that of FCS-TIOA (197.01 mg g−1 or 1.06 mmol g−1), FCS-NOA (177.90 mg g−1 or 0.96 mmol g−1), and FCS-DNOA (140.34 mg g−1 or 0.75 mmol g−1), resulting from the increasing steric hindrance effect with progressive number of alkyl groups on the nitrogen atom of the amine group. Adsorption Kinetics and Thermodynamics of Re(VII) on the FCS-DIOA Gel. The determination coefficient (R2) and the kinetic parameters from the pseudo-first-order and pseudo-second-order linear equations were calculated, and the E

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when it cannot be separated in the equilibrium state. Thus, kinetic separation Re(VII) from Mo(VI) using the FCS-DIOA gel was studied as follows. Kinetic Separation of Re(VII) from Mo(VI). For the sake of the quasi-complete separation of Re(VII) from Mo−Re binary mixtures by using the FCS-DIOA gel, a kinetic separation was carried out at the different contact time when the acid concentrations of the sample was pH 2.0, 5.0, and 10.0, respectively. As shown in Figure 5, Re(VII) could not be separated from Mo(VI) in the case of pH 2.0, because the adsorption percentage of Mo(VI) was almost the same with that of Re(VII) when it contacts with the gel only for 5 min. However, in the case of pH 6.0, about 61.23% adsorption percentage of Re(VII) was observed when the contact time attained in 10 min, while Mo(VI) adsorption percentage was insignificant. Especially, in the case of pH 10.0, adsorption of Re(VII) attained to 72.46% at the contact time as 1 h, while Mo(VI) adsorption percentage was 0%. Moreover, solid−liquid ratio adsorption tests were conducted by mixing 5 mL of solution with various mass of the gel. The results showed that, in the case of pH 10.0, almost 100% adsorption of Re(VII) at a solid−liquid ratio as 16 g:1 L was in 10 min. In order more clearly to understand the optimal separation condition for rhenium from molybdenum onto the FCS-DIOA gel, adsorption selectivity (SelRe/Mo) of molybdenum and rhenium is shown in Table 3. It is can be concluded that FCS-DIOA gel exhibited a greater sorption capacity and separation character for rhenium from molybdenum by using the kinetics method. To understand the affinity of the FCS-DIOA gel for Re(VII) in the Mo−Re binary mixtures, Mo:Re ratio adsorption tests (100:30, 100:20, 100:10, and 100:3) were carried out. As shown in Figures 6 and S2, when increasing the content of Mo(VI), the sorption capacity for Re(VI) was hardly changed at all, which means an even higher concentration of Mo(VI) ions did not affect the selective adsorption of Re(VI). On the other hand, as the concentration of Mo(VI) in the binary system was several times higher than that of rhenium (100:30− 100:10), adsorption of Mo(VI) was almost negligible for 10− 30 min contact time. However, with continuing increasing the content of Mo(VI), the maximal Mo(VI) adsorption (16%) was

results of adsorption of Re(VII) onto the FCS-DIOA gel were shown in Table S2. The results confirmed that the pseudosecond-order model was suitable to explain the adsorption of low molecular weight compounds on small adsorbent particles.34 The details of adsorption thermodynamics and kinetics of Re(VII) on the FCS-DIOA gel were in the Supporting Information. Desorption and Regeneration. In order to reduce the cost and repeated use of the FCS-DIOA gel, desorption experiments were conducted by mixed the Re(VII)-loaded FCS-DIOA gel with various concentrations of NH4SCN (1− 20%), respectively. As shown in Figure S1, it can be seen clearly that the desorption efficiency was almost the same when NH4SCN concentration was higher than 5%. Hence, 5% NH 4SCN was chosen as a suitable concentration for regeneration of Re(VII) loaded on the gel. To examine the reusability of the adsorbent, six successive adsorption and desorption cycles were carried out in Mo−Re binary mixtures. As shown in Table 2, the results showed that the FCS-DIOA Table 2. Performance of the FCS-DIOA Gel in Consecutive Adsorption−Elution Cycles in Mo−Re Binary Component Mixturesa adsorption % cycle cycle cycle cycle cycle cycle a

1 2 3 4 5 6

Re(VII)

Mo(VI)

100 99.99 98.07 87.54 97.54 93.27

12.01 14.73 19.99 16.25 13.34 17.56

Solid-to-liquid ratio = 16 g:1 L.

gel could be used repeatedly at least six times without significant change for rhenium adsorption while almost 20% Mo(VI) was simultaneously adsorbed in Mo−Re binary solution. It is implied that separation and recovery of Re(VII) from Mo(VI) on the FCS-DIOA gel is not ideal under the equilibrium state. We harbor the idea that kinetic separation is a feasible method for the quantitative separation of metal ions

Figure 5. Effect of contact time on adsorption of Re(VII) and Mo(VI) by FCS-DIOA at (a) pH = 2, (b) 6, and (c) 10: initial concentration of Re(VII) = 45 mg L−1, initial concentration of Mo(VI) = 100 mg L−1, temperature = 303 K, solid-to-liquid ratio = 8 g:1 L. F

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be inferred as an ion exchange reaction, as shown in the following equation (eq 4):

Table 3. Comparison Adsorption Selectivity for Re(VII) on the FCS-DIOA Gel in Mo−Re Binary Component Mixtures SelRe/Mo Mo/Re = 100/45

pH = 10

Mo/Re Mo/Re Mo/Re Mo/Re

5 min 10 min 20 min 60 min 120 min

= = = =

100/30 100/20 100/10 100/3

pH = 10

pH = 6

pH = 2

3.90 4.19 4.12 4.40 1.47

4.11 4.21 1.39 1.06 0.78 SelRe/Mo

0.14 0.22 0.30 0.36 0.37

10 min

20 min

30 min

2.91 1.55 1.70 0.70

3.10 2.04 0.91 0.91

3.12 2.07 0.84 0.91

Moreover, the peaks of CN and C−N stretching vibration were shifted to a lower frequency, which means the involvement of the CN and C−N groups in binding of Re(VII) and formation of a complex due to the change in energy of the functional group. Therefore, the results obtained from the data of Figure 1 revealed that −RCN− and −RC− N− could play a crucial role either in an alkaline or acid system. The complexation reactions mechanism of rhenium(VII) on the FCS-DIOA gel could be expressed by eqs 5 and 6.

observed at the Mo:Re ratio of 100:3 in 30 min. As shown in Table 3, the data of SelRe/Mo also came to the same conclusion, that is, when the value SelRe/Mo > 1.5, Re(VII) could be quasicompletely separated from Mo(VI) by using the FCS-DIOA gel at the different contacting time. Therefore, we can be sure that this biosorbent based on the FCS-DIOA gel is promising to be exploited for applications in the treatment of a certain ratio Re−Mo containing industrial wastewaters by the kinetic method. Adsorption Mechanism of Re(VII) Adsorption on the FCS-DIOA Gel. Adsorption Mechanism Characterized by FTIR Analysis. To explain the adsorption mechanism of Re(VII) on the FCS-DIOA gel, FTIR spectra of the FCS-DIOA gel before and after loading rhenium(VII) from pH 2.0, 5.0, and 10.0 medium, respectively, were recorded. As demonstrated in Figure 1b, comparing the spectra of the FCS-DIOA gel before and after loading rhenium(VII), the intensity of peak at 3413 cm−1, which indicates −RNH, was decreased after Re(VII) adsorption. The peaks at 1600 and 1158 cm−1 assigned to C N and C−N stretching were shifted to lower frequency after Re(VII) adsorption at pH 2.0, 5.0, and 10.0, which indicates the groups containing N atom play an important part in Re(VII) adsorption. It is more important that the disappearance of a strong peak at 2360 cm−1, which corresponds to R3NH+, indicated the ion exchange adsorption process. Thus, the adsorption mechanism of Re(VII) on the FCS-DIOA gel could

Adsorption Mechanism Confirmed by the XPS Analysis. XPS analysis was performed for further evidence the adsorption mechanism inferred by the FTIR spectrum, and the results were presented in Figure 7. From the XPS spectrum of the FCS-DIOA gel after Re(VII) adsorption, the following can be observed: the disappearance of binding energy of XPS Cl 2p3/2 at 201.2 eV and the presence of XPS Re 4f7/2 at 47.1 eV. Also, its binding energies were consistent with those of Re(VII), which might indicate the ion exchange adsorption process between R3NH+ and ReO4− inferred by FTIR analysis. Moreover, the C 1s spectra could be deconvoluted into three individual component groups (C−C, C−N, CN) at 284.53 ± 0.2, 285.72 ± 0.2, and 287.51 ± 0.2 eV, respectively. It could be found that the intensity of C−N and CN increased and had a shift, which was also indicated that C−N and CN were involved in the adsorption of Re(VII) onto the adsorbents. Relationship between Adsorption Mechanism and pH. On the other hand, pH is an important influencing factors in metal

Figure 6. Effect of contact time on adsorption of Re(VII) and Mo(VI) by FCS-DIOA at (a) Mo(VI): Re(VII) = 100:30, (b) 100:20, (c) 100:10, and (d) 100:3: initial concentration of Mo(VI) ions = 100 mg L−1, solution pH = 10, temperature = 303 K, solid-to-liquid ratio = 8 g:1 L. G

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Industrial & Engineering Chemistry Research

has tremendous promise for selective adsorption of rhenium from molybdenum containing wastewater.

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ASSOCIATED CONTENT

S Supporting Information *

Elution curve of Re−Mo binary component separations on the FCS-DIOA gel with NH4SCN; effect of contact time on adsorption of Re(VII) on FCS-DIOA at varying solid−liquid ratio; Langmuir, Temkin, Freundlich, and Dubinin−Radushkevich isotherm constants for adsorption of Re(VII) on the FCSDIOA gel; comparison of the pseudo first- and second-order equations, intraparticle diffusion equation, and Elovich equation, calculated at varying hydrochloric acid concentrations; adsorption isotherm studies; adsorption kinetics and thermodynamics studies; thermodynamics and kinetics of adsorption of Re(VII) on the FCS-DIOA gel. This material is available free of charge via the Internet at http://pubs.acs.org/.

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Figure 7. XPS and typical C 1s spectra of the FCS-DIOA gel before and after Re(VII) adsorption.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

ion adsorption because it could affect the protonation/ deprotonation of functional groups on the surface of the gel and/or metal ions speciation.35 The surface of FCS-DIOA is generally covered with a multitude of amine groups and hydroxyl groups. The N atom of amine groups on the surface of chitosan can donate and share the lone pair electrons with the empty orbit of other cations, featuring thus with weak-base characteristics. The final pH for Re(VII) was also measured and the graph was plotted between initial and final pH in Figure 3b. At pH values 2.0−6.0, the value of final pH was higher than the initial value, which indicates that the active sites are protonated. When the value of initial pH was from 8.0 to 10.0, the final value was invariably lower than the initial value, which indicates deprotonation because of the binding of some compounds to the surface of the FCS-DIOA gel. Agreeing with the results of point of zero charge, as pH was higher than 7.60, uptake of Re(VII) was almost zero because the adsorption surface sites having a negative electric charge increased electrostatic repulsion between ReO4− and the FCS-DIOA gel resulting in the competition adsorption between OH− and ReO4−. This suggestion agrees with the results of 3.3 section, that is the maximum adsorption of rhenium on the FCS-DIOA at pH 10.0 (q = 60.81 mg g−1 or 0.33 mmol g−1) was lower than that at pH 2.0 (q = 143.40 mg g−1 or 0.77 mmol g−1) mainly due to OH− competitively reacting with some of the amine active sites on the FCS-DIOA gel in alkaline medium.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project is supported by National Natural Science Foundation of China (21171080, 21201094), the Scientific Research Found of Liaoning Provincial Education Department (L2014004), and Liaoning BaiQianWan Talents Program.

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REFERENCES

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CONCLUSIONS The effects of steric hindrance and basic strength of magnetic chitosan gels modified by n-octylamine, di-2-ethylhexylamine, di-n-octylamine, and tri-iso-octylamine, respectively, on rhenium adsorption selectivity and capacity were discussed. FCSDIOA gel modified by di-2-ethylhexylamine showed the better adsorption selectivity for Re(VII) than the other adsorbents. Adsorption of Re(VII) on the FCS-DIOA gel was found to fit well with pseudo-second-order kinetic model and Langmuir isotherm. The adsorption mechanism could be expressed by the ion exchange and complexation reactions with C−N and CN groups. Moreover, by using a kinetic separation method, the quasi-complete separation conditions of Re(VII) from Mo−Re binary mixtures on the FCS-DIOA gel were successfully obtained. The result further supports that the FCS-DIOA gel H

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DOI: 10.1021/ie503362j Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX