Mesoporous

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Quaternary phosphonium modified hierarchically macro/ mesoporous silica for fast removal of perrhenate Xingxiao Li, Dong Han, Taotao Guo, Jing Peng, Ling Xu, and Maolin Zhai Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b03306 • Publication Date (Web): 10 Sep 2018 Downloaded from http://pubs.acs.org on September 17, 2018

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Industrial & Engineering Chemistry Research

Quaternary phosphonium modified hierarchically macro/mesoporous silica for fast removal of perrhenate Xingxiao Li, † Dong Han, † Taotao Guo, † Jing Peng,*, † Ling Xu, ‡ and Maolin Zhai*, † †

Beijing National Laboratory for Molecular Sciences, Radiochemistry and Radiation Chemistry

Key Laboratory of Fundamental Science, the Key Laboratory of Polymer Chemistry and Physics of the Ministry of Education, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China. ‡

State Key Laboratory of Molecular Vaccinology and Molecular Diagnostics, School of Public

Health, Xiamen University, Xiamen, Fujian 161102, China

ABSTRACT: A new quaternary phosphonium modified hierarchically macro/mesoporous silica (named HPS-P) was prepared by a sol-gel method accompanied by phase separation by using PEG-10000 as surfactant and tetramethoxysilane as silicon source, followed by reacting with quaternary phosphonium. The resultant HPS-P containing large macroporous skeleton with an average diameter of 0.67 µm and mesopores with an average diameter of 10.3 nm has a specific area of ca. 254.7 m2⋅g-1. The adsorption of HPS-P for ReO4- was investigated, and the adsorption equilibrium could be achieved within 1 min, the adsorption isotherm data for ReO4- could be well fitted with modified Langmuir model. Moreover, HPS-P has excellent adsorption selectivities towards ReO4- with competing anions, including NO3-, SO42- and Cl-. The ∆HΘ and ∆SΘ of the

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Cl-/ReO4- ion-exchange reaction of HPS-P were calculated to be -4.5 kJ.mol-1 and 12.6 J.mol-1.K1

, respectively, indicating that the adsorption of ReO4- onto HPS-P is a spontaneous physical

adsorption process. In dynamic column experiment, HPS-P could be recycled by 2.2 mol⋅L-1 HNO3 with a good performance within 3 min. IR and XPS analysis confirmed that the adsorption mechanism is ion-exchange. INTRODUCTION Tc is the first artificial radioactive element, and 99Tc is one of the most important isotopes with a half-life of 2.13 × 105 years. 99Tc has a high fission yield, and its thermal neutron fission yield from 235U is 6.16%.1-2 During the PUREX processes, 99Tc produced by fission of 235U finally enters high-level waste mainly in the form of 99TcO4-. Although the radiotoxicity of 99Tc is relatively low, in consideration of the long half-life, high environmental mobility, and high volatility during glass solidification, the removal of Tc from aqueous solution is of great importance.3-4 Since all the isotopes of Tc are radioactive, the separation of Tc cannot be carried out in the ordinary lab. Re, whose chemical properties resembles that of Tc, has been widely used as the chemical analogue of Tc.4 Re is one of the rare dispersed elements, and its alloys have been widely used in petrochemical industry, aviation and so on. So it is environmentally and economically important to recover Re from waste solution.5-6 Several methods have been applied for the separation of TcO4- and ReO4-, such as solvent extraction, adsorption and precipitation.4 Although extraction has the advantages of large extraction capacity and high separation efficiency, it suffers from slow kinetics and secondary waste which is difficult to handle and harmful to the environment due to the use of toxic extraction solvent. On the contrary, adsorption


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has the advantages of less pollution to the environment, regeneration ability, low cost, and more convenient operation, which can overcome these shortcomings of extraction.4, 7 Many kinds of adsorbents for TcO4-/ReO4- have been investigated, such as inorganic, organic and organic-inorganic hybrid materials. Inorganic materials include biochar,8 Al2O3,9 LDHs,10-11 etc., which usually have low adsorption capacities and selectivities because of the lack of specific functional groups on the surface. MOFs is another new type of adsorbents which has extremely high adsorption capacities and good selectivity for TcO4- and ReO4-,12-17 but it suffers from high cost and difficulty of synthesis, and may also result in secondary radioactive wastes.18 Organic materials including resins,1, 19 gels18, 20-21 and grafted materials,7, 22 have a lot of advantages of high adsorption capacities and easy synthesis, while they always suffer from slow adsorption kinetics. Introducing the functional groups into the inorganic matrix can greatly enhance the adsorption performance of the adsorbent, without any effect on the advantages of matrix, for example, excellent mechanical properties.21 Among all the inorganic matrix, silica has been widely used as matrix to prepare adsorbents because of its high surface area, good chemical and thermal stability.22 Xiao et al.22 synthesized two new organic-inorganic adsorbents for ReO4- by radiation-induced grafting method, and the adsorption capacities of 1-vinylimidazole and 4vinylpyridine grafted silica for ReO4- were 145.99 and 71.08 mg⋅g-1, respectively. Recently, hierarchically porous silica has attracted considerable attention due to its large specific surface area, low density, and interconnected hierarchical porous structures, which facilitate the mass loading and diffusion.23-24 Wang et al.25 prepared a new hierarchically porous silica coated with a poly(pyrogallol) layer whose adsorption capacities for Hg2+ ion and neutral red were 157.2 and 671.1 mg⋅g-1, respectively. Introducing inorganic nanoparticles into polymer is another way to


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combine the remarkable adsorption properties and excellent physicochemical properties.26 Gao et al.27 reported a new adsorbent for Re(VII) removal, polyaniline/titanium(IV) (PANI/Ti(HPO4)2), with the maximum adsorption capacity of 47.62 mg⋅g-1. Quaternary ammonium, especially pyridinium7 and imidazolium,18, 21 is commonly used for the separation of TcO4- and ReO4-. However, little attention has been devoted to the use of phosphonium for the removal of TcO4- and ReO4-. The tetraphenylphosphonium salt can extract TcO4- and ReO4- from organic solvents,28-29 and the tetraphenylphosphonium ion can precipitate TcO4- in water.30-31 Pepper et al.32 investigated the solvent extraction of ReO4- from chloride solutions by Cyphos 101-IL (hexyl(tetradecyl)phosphonium chloride) in toluene, and it showed better distribution coefficients than that of Aliquat-336. It seems that quaternary phosphonium groups would show desirable adsorption selectivity towards TcO4- and ReO4-. However, to the best of our knowledge, the adsorption performance of HPS modified with quaternary phosphonium salts for ReO4- and TcO4- has been seldom investigated. Herein, we prepared the hierarchically porous silica (named HPS) by a sol-gel method accompanied by phase separation process according to the method of Wang and co-workers,33 by using PEG-10000 as surfactant and tetramethoxysilane as silicon source. Then the surface of HPS was modified with quaternary phosphonium salt to obtain HPS-P. Afterwards, the adsorption behavior of ReO4- was investigated in detail, including kinetics, adsorption isotherms and selectivity, adsorption thermodynamics and column experiments, and finally the mechanism of adsorption was investigated. EXPERIMENTAL SECTION


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Chemicals and Materials. Tetramethoxysilane (TMOS, 98%) was purchased from Meryer Chemical Technology Co., Ltd. γ-(Chloropropyl)triethoxysilane (CPTES, 98%) was purchased from Sinopharm Chemical Reagent Co., Ltd. Polyethylene glycol (PEG, Mw=10,000) was purchased from Sam Chemical Technology (Shanghai) Co., Ltd. Triphenylphosphine (PPh3), diethyl ether, toluene, acetonitrile, acetic acid and ammonium hydroxide solution were purchased from Beijing Tongguang Fine Chemicals Company. NaCl, H2SO4 and HNO3 were purchased from Xilong Scientific Co., Ltd. KReO4 was purchased from Alfa Aesar. Re standard solution was purchased by NCS Testing Technology Co., Ltd. Other chemicals were analytical-grade reagents, all reagents were used as received and without further purification. Deionized water was used in all the experiments. γ-(triethoxysilyl)propyltriphenylphosphonium Chloride (PCPTES) was easily synthesized from CPTES and PPh3 according to Ref34(see SI). Synthesis and characterization of HPS-P Synthesis of HPS HPS was prepared according to the method reported by Wang and co-workers.33 Firstly, 2.4 g PEG was homogeneously dissolved in 20.0 mL 10 mmol⋅L-1 acetic acid. Then 8.0 g TMOS was added into the solution. The semi-transparent sol was obtained after vigorously stir for about 30 min at 0 oC and then transferred into a Teflon rector. The Teflon rector was sealed and kept at 40 oC for 24 h. The resultant white gels was washed with deionized water for several times followed by immersed into 100 mL of 1.0 mol⋅L-1 ammonium hydroxide solution and was kept at 60 oC for 24 h. Afterwards the wet white gel was washed by deionized water and 0.05 mol⋅L-1 HNO3 to neutrality and dried at 40 oC for 24 h. Finally, HPS was obtained by calcination up to


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650 oC at a heating rate of 5 oC⋅min-1 and kept at the same temperature for 5 h in air to remove organic substances. HPS was crushed and sieved to obtain particles with size of 60-100 mesh. Synthesis of HPS-P For the synthesis of HPS-P, 4 g HPS was firstly heated at 110 oC for 1 h to remove the water adsorbed on the surface. Secondly, a homogeneous solution of 12 g PCPTES and 50 mL acetonitrile was introduced into the reactor containing HPS and the mixture reacted at 120 oC in Teflon reactor for 24 h. Finally, ethanol was used to remove the residual reagents, and the product was dried at 60 oC for 24 h to obtain HPS-P. Characterization of HPS and HPS-P IR spectra were got from Spotlight200 (PE, USA). XPS was measured by Axis Ultra (Kratos Analytical Ltd.). Elemental Analysis (EA) was measured on Vario EL (Elementar Analysensysteme GmbH, Germany). SEM images were obtained from Hitachi S-4800 (Hitachi, Japan). Nitrogen sorption measurements was performed on ASAP2020 (MICROMETER). Mercury porosimeter was performed on Autopore IV 9510 (Micromeritics, USA). Thermogravimetric analysis was carried out in Q600 SDT (TA, USA) at a heating rate of 10 o

C⋅min−1 under an air flow rate of 100 mL⋅min-1.

Adsorption of ReO4- onto HPS and HPS-P The typical batch experiments were performed as following: HPS and HPS-P with a certain weight were mixed with KReO4 solution, and the system was shaken at 25 oC, except in the adsorption thermodynamics study. In all experiments the ratio of the weight of HPS and HPS-P


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to the volume of the solution was usually 5 mg⋅mL-1. All adsorption experiments were performed for 1 h, except in the adsorption kinetics study. The mother solution of ReO4- (ca. 1000 ppm) was prepared by dissolving KReO4 in deionized water, and then the mother solution was diluted to the corresponding concentration for experiment. Inductively coupled plasma atomic emission spectroscopy (ICP-AES, Prodigy) was used to measure the concentration of ReO4-, thereby the relative error of Re concentration was estimated as 2%. The uptake of ReO4-, q (mg⋅g−1), was calculated as following:

=

( )×

(1)

Here c0 (ppm) and c (ppm) stand for the concentration of ReO4- in solution before and after adsorption, respectively, while ms (g) and ma (mg) stand for the weight of the solution and adsorbents, respectively. Adsorption kinetics Each of ca. 30 mg HPS-P were mixed with 6 mL solutions with the concentration of ca. 483 ppm at 25 oC, respectively. And the solutions were vibrated vigorously by hand. Then at different intervals, the solution was separated from the adsorbents by filter and then it was diluted for the measurement of Re concentration. Adsorption isotherms and selectivity The adsorption isotherms curve was obtained from relationship between uptake of ions and various c0 .For adsorption selectivity, NO3-, SO42- and Cl- were chosen as anion competitiors,


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which were produced by HNO3, H2SO4 and NaCl, respectively, under the same conditions at c0 ca. 470 ppm, the 0.1 mol⋅L-1 competing anions. Separation factor (SF) was used to evaluate the adsorption selectivity of HPS-P for ReO4-. The separation factor was defined as:35

=

() ( )

()×()

= ()×()

(2)

Where Kd(Re) and Kd(c) represent the distribution coefficient of ReO4- and the corresponding ion, respectively, q(Re) and q(c) (mmol⋅g-1) are the equilibrium capacity of the corresponding ion. And c(c) and c(Re) (mmol⋅kg-1) represent to the equilibrium concentration of the corresponding ion. Adsorption thermodynamics Before adsorption, the c0 was 472 ppm, and the concentration of NaCl in solution was 0.1 mol⋅kg-1. The adsorption temperature changed from 284 K to 328 K. Then the solution was filtered immediately after adsorption and diluted to measure the Re concentration. Column experiments Column adsorption experiment was performed at ambient temperature in a 1 mL syringe containing 81.6 mg HPS-P and both sides of the column were blocked with cotton and a piece of 200 mesh nylon mesh. A peristaltic pump provided the continuous flow of liquid which was connected to the bottom of the column. The feed solution was 24.51 ppm ReO4-, which was pumped through the column at a flow rate of 2.00 mL⋅min-1. The desorbing reagent was 2.2 mol⋅L-1 HNO3. And the solution was then pumped through the column till the concentration of


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the eluents was approximately 0. The leakage concentration of ReO4- was measured online by ICP-AES. After 4 cycles, the resulted HPS-P was measured by IR and XPS to confirm the desorption. RESULTS AND DISCUSSION Preparation and characterization of HPS and HPS-P The synthesis of HPS-P was illustrated in Scheme 1. SEM images of HPS and HPS-P were shown in Figure 1. HPS exhibited a continuous and well defined porous structure, which was consisted of interlaced skeletons with macropores. The average macropore diameter of HPS was 0.88 µm obtained by mercury intrusion method, which was in agreement with the results observed from the SEM images. After modifying with PCPTES, the average macropore diameter slightly decreased to 0.67 µm.

Scheme 1. Schematic illustration of the synthetic procedure of HPS-P


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Figure 1. SEM images of (a) HPS, (b) HPS-P The N2 adsorption-desorption isotherm curves of HPS and HPS-P were shown in Figure 2. (a). Figure 2. (b) showed the mesopore size distribution of HPS and HPS-P. HPS and HPS-P exhibited typical type IV curves with a hysteresis loop. Almost no N2 adsorption in lower P/P0 (0 - 0.7) obviously, indicating few micropores exist in HPS and HPS-P. The hysteresis loop in P/P0 (0.7 - 0.9) is related to the capillary condensation of N2 in the mesopores of the skeletons. The hysteresis loop in P/P0 (0.9-1.0) was probably associated with the macropores.36-38 The similarity of the two curves in Figure 2. (a) indicated that the modification did not destroy the hierarchically porous structure of HPS. The surface area (SBET), pore volume (VP) and average pore size (DP) of HPS were 272.7 m2⋅g-1, 1.029 cm3⋅g-1 and 13.4 nm, respectively. However, after modification all of them decreased slightly, which were 254.7 m2⋅g-1, 0.785 cm3⋅g-1 and 10.3 nm, respectively.


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Figure 2. (a) N2 adsorption-desorption isotherm curves of HPS and HPS-P; (b) mesopore size distribution of HPS and HPS-P IR spectra of HPS and HPS-P were showed in Figure 3. (a). The peaks at 811 cm-1 and 1091 cm-1 belonged to Si-O-Si bonds, respectively. The –OH group at 3747 cm-1 disappeared in HPSP. New peaks at 1440 cm-1 and 665 cm-1 appeared, and it should be corresponding to the C-PPh3+ groups.39-40 The peaks at 744 cm-1 and 690 cm-1 were ascribed to the C-H at phenyl ring, which was mono-substituted. The peak at 721 cm-1 indicated the existence of -CH2-. Figure 3. (b) was the XPS pattern of HPS and HPS-P. The peak of C sharply increased, and new peaks of Cl and P appeared. The above results suggested the successful synthesis of HPS-P.


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Figure 3. (a) IR spectra of HPS and HPS-P; (b) XPS of HPS and HPS-P From the TGA curves (Fig.S1), the weight loss of HPS and HPS-P in the temperature up to 80 oC mainly belonged to the water remained in HPS and HPS-P. And the weight loss of HPS-P in the temperature range of ca. 200 - 550 oC was mainly because of the decomposition of the quaternary phosphonium group modified on the surface. The weight of the quaternary phosphonium group on the surface of HPS-P calculated from the TGA curves was about ca. 12.2%. While the weight of the quaternary phosphonium group on the surface of HPS-P calculated from EA of HPS and HPS-P (Table. S1) was about ca. 0.4061 mmol⋅g-1. Thus, plenty of the functional groups had been chemically grafted on the surface of HPS. The adsorption kinetics of ReO4- onto HPS-P


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The adsorption kinetics of ReO4- onto HPS-P was shown in Figure 4. Notably, the equilibrium could be reached within 1 min. Thus, adsorption kinetics of HPS was much faster than that of the gels21 or resins,20 which needed several hours or even several days to reach equilibrium(Table S2). For the practical usage of TcO4- removal, the rapid kinetics can effectively reduce the contact time between the adsorbents and the radioactive waste solution, thereby reducing the radiation damage to the adsorbent.15 Li et al.41 compared the adsorption kinetics of the hierarchical macro/meso porous hybrid silica and the thiol-functionalized meso porous silica for the adsorption of Pb2+ and Cd2+. They found that the hybrid hierarchical silica reached the adsorption equilibrium within 2 min, while 30 min were needed for mesoporous silica, which indicated that hierarchical hybrid silica had faster adsorption kinetics than the mesoporous silica. Therefore, the extremely fast adsorption of resultant HPS-P was attributed to the special hierarchical structure of HPS-P. The open, macroporous structure could reduce mass transfer resistance to achieve fast adsorption.

Figure 4. Adsorption kinetics of ReO4- onto HPS-P. The c0 was 270 ppm. The adsorption isotherms and selectivity of HPS-P towards ReO4-


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The adsorption isotherms of HPS and HPS-P was shown in Figure 5. The result showed that the adsorption uptake of HPS-P towards ReO4- increased together with c0, while HPS showed almost no adsorption for ReO4-. It indicated the successful modification of quaternary phosphonium on HPS which played an important role for the adsorption. The most commonly used isotherm models in adsorption were Langmuir and Freundlich models. And the equations can be expressed by Eq. 3 and 4, respectively:

= ×

(3)

=

(4)

Where qm is the maximum adsorption capacity, and KL is the Langmuir constant. KF was the Freundlich constant and n was the Freundlich exponent. Although the Langmuir model had been used to describe the adsorption isotherms, it may not be very suitable for ion-exchange processes over the entire exchange isotherm. During the adsorption process, with the target ion (marked as A) adsorbed by the adsorbent, an equal amount of counter-ion (marked as B) would be generated in the solution, which may cause competitive effects to the adsorption. Thus in homovalent ion-exchange processes, the Langmuir model should be modified and can be expressed as the modified Langmuir model.42

= × = " !

(5)

Where Kv is Vanselow selectivity coefficient, and cB is proportional to q.


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The experimental data was fitted by Langmuir, Freundlich and modified Langmuir isotherm models. And the related parameters together with correlation coefficients (R2) were listed in Table 1. According to the R2, it was found that the experiment data were fitted well by the modified Langmuir isotherm equation for HPS-P. From the modified Langmuir isotherm equation, qm was 74.4 mg⋅g-1, which is consistent with the EA data.

Figure 5. Adsorption isotherms of ReO4- onto HPS and HPS-P Table 1. Isotherm parameters of the Langmuir model, Freundlich model and Modified Langmuir model. Langmuir

Freundlich

Modified Langmuir

qm (mg⋅g-1)

KL (L⋅mg-1)

R2

Kf (g-1⋅mg(1-1/n)⋅g-1)

n

R2

KL’

qm (mg⋅g-1)

R2

62.24

0.0499

0.997

12.91

3.4

0.990

0.899

74.4

0.998


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The maximum ReO4- adsorption capacities of various types of adsorbents and that of asprepared HPS-P were compared in Table S2. The adsorption capacity of HPS-P is higher than some of the other adsorbents such as nano Al2O3, nano-SiO2, biochar and so on, owing to the modification with quaternary phosphonium on the surface of HPS. Since only one layer of molecules was modified instead of the grafted polymer, the adsorption capacity of HPS-P is lower than that of grafted materials and gels. But in practical applications of TcO4- removal, the selectivity of the adsorbent is more important than the adsorption capacity. Recycle of adsorbents can make up for the limitation of small adsorption capacity but there are little way to improve the selectivity. The selectivity of anion exchange towards ReO4- was studied by adsorption of ReO4- in presence of competing anions, including NO3-, SO42- and Cl-. The values of the SF of HPS-P towards ReO4- relative to NO3-, SO42- and Cl- were 9.1, 34.6 and 28.9, respectively. We estimated the value of SF of adsorbents reported before by the value of the removal rate, ReO4- capacity and so on. The value of the SF of PC2vimNO3 towards ReO4- relative to NO3- was 2.818, 21. And for SS-MPTSVIMH22, the values of the SF towards ReO4- relative to Cl- and SO42- were about 12 and 2, respectively. Compared to these reported data, HPS-P has comparable adsorption capacity and higher anion exchange selectivity toward ReO4-. Thus, it can be used as a candidate for recovering ReO4- from the aqueous solution. The adsorption thermodynamics of HPS-P towards ReO4In the homovalent ion-exchange processes, the SF could be considered as the equilibrium constant K of the follow equation: MB+A- = MA+B-

(6)


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Where MA is HPS-P containing counter-ion, Cl-, and A- is the target ion, ReO4-. Van’t Hoff equation (Eq. 7) and Gibbs function (Eq. 8) were used to calculate the thermodynamic parameters.

ln = −

∆' ( )

+

∆+ (

∆, - = ∆. - − /∆ -

(7)

(8)

Where R is ideal gas constant, which is 8.314 J⋅mol-1⋅K-1, and T is the temperature (K). The K was measured under different temperatures, and Figure 6 showed the relationship between ln K and 1000/T fitted by Van’t Hoff equation (R2 > 0.999). The thermodynamic parameters ∆HΘ and ∆SΘ can be calculated from the value of the slope and the intercept of the line. According to the calculated results, the value of ∆HΘ and ∆SΘ were -4.5 kJ.mol-1 and 12.6 J.mol-1.K-1, respectively. The value of ∆HΘ was negative indicating that the adsorption process by HPS-P towards ReO4- was an exothermic process, and lower temperature was more favorable for the adsorption process. According to Eq. 8 and the value of ∆HΘ and ∆SΘ, the value of ∆GΘ was negative (-8.08 to -8.63 kJ⋅mol-1) when the temperature was 284.15 to 328.15 K, indicating the adsorption process was spontaneous. Previous studies had found that the value of enthalpy change can be used to distinguish the type of adsorption. The adsorption can be considered as physical adsorption, when the range of enthalpy change is 2.1 to 20.9 kJ⋅mol-1.43 Thus, the adsorption process of HPS-P towards ReO4- can be speculated as a spontaneous physical adsorption process.


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Figure 6. Fitted curves of ln K and 1000/T by using Van’t Hoff equation Column experiments Column experiments were performed to analyse the column adsorption behaviour of HPS-P for ReO4-. In the first three cycles, small amount of feed solution was used so that the column would not be penetrated. Then 2.2 mol⋅L-1 HNO3 was used for the desorption of ReO4-, and the output solution was collected to measure the concentration of ReO4- instead of being sent to ICPAES, and the desorption efficiency of the first three cycles can be calculated. In the fourth cycle, a larger amount of feed solution was used to ensure the column penetrated. Figure 7. (a) and (b) showed the relationship between the relative ReO4- concentration and the bed volume of the feed solution of the four cycles. In the first 3 cycles of the column experiments (Figure 7. (a)), as the volume of the feed solution increased, the relative ReO4- concentration c/c0 remained at ca. 0. Thus the column could be handled at least 14 mL waste liquid at the concentration of 24.50 ppm without flowing out of ReO4-. As shown in Figure7. (b), the 1% breakthrough of ReO4- started at 27 mL. Additionally, it could be seen that the adsorbed ReO4- could be easily desorbed under 2.2 mol⋅L-1 HNO3 in the volume of 5 mL, and the process completed within 3 min. Such a small


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amount of eluent and fast desorption indicated that the column has an enrichment effect on ReO4. As a result, it can remarkably reduce the time and cost practically. Desorption efficiency of first three cycles was shown in Figure 7. (c). Desorption efficiency of the first cycle was 96.4%, and kept well in the next two cycles, which indicated the column had excellent reusability.


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Figure 7. (a) The first three cycles and (b) the forth cycle column experiment of HPS-P. The feed solution, deionized water and 2.2 mol⋅L-1 HNO3 pumped through the column were represented by orange, blue and green, respectively; (c) The desorption efficiency of first three cycles Figure 8 showed the IR and XPS diagrams of HPS-P before and after adsorption as well as desorption. From Figure 8. (a), after adsorption the peak at 910 cm-1 of ReO4- existed. After desorption by 2.2 mol⋅L-1 HNO3, the peak of ReO4- disappeared in IR spectra of HPS-P after desorption. While the other peaks corresponding to HPS-P did not change in IR spectra especially the peak at 1440 cm-1 for the C-PPh3+ groups. For the XPS pattern in Figure 8. (b), the peak of Cl disappeared after adsorption, and the strong peak of Re existed, which indicated that the Cl- was replaced by ReO4-. The peak of Re disappeared after desorption, which was in accord with the results of IR. No peaks of NO3- can be detected both in IR and XPS after desorption, which may be due to the little capacity of NO3- in desorption. Therefore, it can be concluded that ReO4- were adsorbed onto HPS-P through anion exchange mechanism.


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Figure 8. (a) IR spectra and (b) XPS of HPS-P before and after adsorption as well as after desorption CONCLUSIONS In conclusion, a new kind of quaternary phosphonium modified hierarchically macro/mesoporous silica (named HPS-P) for ReO4- removal was synthesized through sol-gel method accompanied by phase separation followed by surface modification. SEM, N2 adsorption performance, IR, XPS and TG analysis confirmed the synthesis of HPS-P. The HPS-P adsorbents has fast adsorption kinetics towards ReO4- due to the unique structure of HPS-P, which has large macroporous skeleton with a diameter of 0.67 µm and mesopores with a diameter of 10.3 nm. The adsorption capacity of HPS-P for ReO4- calculated from modified Langmuir model was 74.4


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mg⋅g-1. HPS-P exhibited good selectivities for adsorption of ReO4- over other competing anions, including NO3-, SO42- and Cl-. The ∆HΘ and ∆SΘ of the Cl-/ReO4- ion-exchange reaction of HPSP were determined to be -4.5 KJ.mol-1 and 12.6 J.mol-1.K-1, respectively, indicating the adsorption of ReO4- onto HPS-P was a spontaneous physical adsorption process. Besides, HPS-P had good performance in dynamic column experiment, and could be recycled by 2.2 mol⋅L-1 HNO3 within 3 min. The dominant adsorption mechanism of HPS-P towards ReO4- was ionexchange testified by IR and XPS. Therefore, resultant HPS-P can be served as a promising candidate adsorbent for fast removal of ReO4- and TcO4- from waste water. ASSOCIATED CONTENT Supporting information Synthesis of the PCPTES, TGA and EA analysis of HPS and HPS-P, Adsorption capacities and the equilibrium time of reported ReO4- adsorbents. AUTHOR INFROMATION Corresponding author *

E-mail Address: [email protected] Tel: +86-010-62753794 (Maolin Zhai)

*

E-mail Address: [email protected] Tel: +86-10-62757193 (Jing Peng)

Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS


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TOC A new quaternary phosphonium modified hierarchically macro/mesoporous silica was synthesized for fast removal of perrhenate.


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Mesoporous

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