Functionalized Magnetic Silica Nanoparticles for Highly Efficient

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Functionalized magnetic silica nanoparticles for highly efficient adsorption of Sm3+ from a dilute aqueous solution Yue Wang, Hari Katepalli, Tonghan Gu, T. Alan Hatton, and Yundong Wang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b04010 • Publication Date (Web): 05 Feb 2018 Downloaded from http://pubs.acs.org on February 10, 2018

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Functionalized magnetic silica nanoparticles for highly efficient adsorption of Sm3+ from a dilute aqueous solution Yue Wanga, Hari Katepallib, Tonghan Gub, T. Alan Hattonb*, Yundong Wanga* a. The State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing, 100084, P. R. China b. Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, 02139, USA

* Corresponding author. [email protected] (T. Alan Hatton), [email protected] (Yundong Wang) Abstract Separation of Sm3+ from a dilute solution via conventional solvent extraction is often plagued by emulsion and third phase formation. These problems can be overcome with functionalized magnetic nanoparticles that can capture the target species and be separated from the raffinae phase rapidly and efficiently on application of a magnetic field. Magentic silica nanoparticles (Fe2O3/SiO2) were synthesized by a modified Stöber method and functionalized with carboxylate (Fe2O3/SiO2/RCOONa) and phosphonate (Fe2O3/SiO2/R1R2PO3Na) groups to achieve high adsorption capacity and fast adsorption kinetics. The adsorbents were characterized by X-ray diffraction analysis, transmission electron microscopy, BET measurements, magnetization property evaluation, Fourier infrared spectroscopy and thermogravimetric analysis. Equilibrium adsorption of Sm3+ on Fe2O3/SiO2/RCOONa particles was attained within 10 min, and within 20 min on Fe2O3/SiO2/R1R2PO3Na nanoparticles. The kinetic data were correlated well with a pseudo-second-order model. Adsorption capacities of Fe2O3/SiO2/RCOONa and Fe2O3/SiO2/R1R2PO3Na were 228 mg/g and 180 mg/g, respectively. The recovery of the adsorbed Sm3+ using 2 mol/L HCl as desorption agent was evaluated. The adsorption mechanism is discussed based on FTIR analysis, carboxylate group/Sm3+ molar ratio, phosphonate group/Sm3+ molar ratio and pH. The adsorbents show significant potential for Sm3+ recovery in industrial applications.

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Introduction Samarium (Sm) is a member of the lanthanide series and is widely used in samarium-cobalt magnets, modification of ceramics and optical glass, and as a neutron absorber1. In China’s rare earth industry, more than 7.2 million tons of wastewater are generated annually, containing low concentrations (0-500 mg/L) of rare earth ions2. The tail liquid and extraction raffinate from a factory in Ganzhou (China) contain Sm3+ at concentrations of 0-100 mg/L. In order to meet the discharge standards as well as to recycle valuable Sm3+, it is essential to recover and enrich Sm3+ from dilute solution before the streams are released into the environment. Many studies have been conducted over the years to recover lanthanides and actinides from dilute solution, e.g., adsorption3-6, ion exchange7, 8, precipitation2, 9, solvent extraction10-14 and membrane filtration11. Among them solvent extraction is the most mature technology for rare earth separation using saponified 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (P507) as extractants. However, a third phase forms easily and phase separation is a time-consuming process. Furthermore, on the basis of the Pearson acid base concept (HSAB), the saponified P507 is readily soluble in aqueous phases due to interactions between the hard acid NH4+ and hard base water15. Particularly, for large phase ratio extraction procedures, the loss of saponified P507 would definitely increase and cause phosphorous content in the released aqueous solution to exceed regulatory standards. This issue is not only of concern with respect to processing costs, but also to the impact such discharge has on the environment. Comparatively, adsorption is a promising method due to the advantages of simple phase separation, minimal environmental impact, facile operation and high efficiency over a wide concentration range. However, a good adsorption

performance

strongly

depends

on

appropriate

adsorbents

and

effective

functionalization. Although various materials such as biomass16-18, synthesized composite beads and polymers5, 19-21 have already been reported as adsorbents, they still have problems such as slow adsorption kinetics and small adsorption capacity. Therefore, alternative materials with both enhanced adsorption rates and improved capacities should be developed. Wang’s22, 23 group reported excellent metal-organic frameworks for selective uranium detection with high sensitivity and uptake from aqueous solutions with high capability. Organic-inorganic composites such as the electrospun poly(acrylic acid)/silica hydrogel nanofibers scaffold were also studied for highly efficient adsorption of lanthanides ions3. The material presented high 2 ACS Paragon Plus Environment

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adsorption capacity owing to the loose and spongy porous network. However, due to the swelling property of polyacrylic acid, the material was not stable in basic environment and cycling use. Recently, magnetic nanoparticles have been studied as novel adsorbents and applied in separation technology by some groups24-30. Our group has made important progress in magnetic nanoparticles applications in organic compounds extraction30 and protein separations27,

29

.

Magnetic nanoparticles are composed of magnetic cores and functionalized shells. Compared to traditional adsorbents, they exhibit good performance owing to the absence of internal diffusional resistances and can be recovered rapidly via an external magnetic field. Furthermore, through chemical modification of shells, the surface functionality can be tailored for specific targets. According to HSAB theory, Sm3+ as a Pearson hard acid prefers to bind with Pearson hard bases such as oxygen, sulfur, nitrogen and phosphorus atoms which could be found in a number of functional groups, including carboxylate6, 20, 31, phosphonate32-34, sulfonate21, 35 and amine24, 36. They have proved to be effective at removing rare earth ions from aqueous solution. Wu et al.19 prepared magnetic alginate microcapsules containing P507 and selectively separated Nd3+ from mixed Nd3+/Co2+ and Nd3+/Zn2+. However, the magnetic core was quite unstable for cyclic operations; the quantity of iron decreased to 91% after only three cycles. The main reason for this reduced performance was the poor coating and weak mechanical strength of alginate. Sun’s group37 developed a magnetic silica hybrid material containing P507 for rare earth adsorption, but the adsorption equilibrium (60 min) and especially the maxium adsorption amount (7.34 mg/g) remained far from satisfactory because of the low content of P507 in the material (0.676 mmol/g). Accordingly, there could be significant value in further investigating applications of functionalized magnetic nano-adsorbents for rare earth recovery owing to the inherent advantages of these materials outlined above. For instance, the direct grafting of functional groups on magnetic nanoparticles is difficult and they are very unstable in desorption acid, and more facile methods for synthesis and stabilization are required. These problems can be overcome by coating a silica layer around magnetic nanoparticles, which not only protects the magnetic core from the outside solution, but also allows for easy attachment of functional groups via the reaction between Si-OH group and silane coupling agents38-40. The adsorbent stability is expected to be improved through control of the silica content and the magnetic core. The adsorption performance can be optimized by choosing proper functional groups and adjusting corresponding concentrations. 3 ACS Paragon Plus Environment

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In this work, γ-Fe2O3 was applied as the magnetic core considering both the saturation magnetization and chemical stability. Magnetic silica nanoparticles were prepared by the Stöber method and then modified by carboxylate and phosphonate groups. The physical and chemical properties were characterized systematically. Sm3+ was chosen as the target ion for adsorption study as it is typical of lanthanides and has important applications. The optimal pH for adsorption was determined. Adsorption equilibria, adsorption kinetics and desorption performance were evaluated. The adsorbent stability in hydrochloric acid was carefully evaluated. The adsorption mechanism is discussed based on FTIR analyses, pH effects, and the carboxylate group/ Sm3+ and phosphonate group/ Sm3+ molar ratios. Experimental Materials

SmCl3•6H2O (≥99.99%) and γ-Fe2O3 nanoparticles (99%) and ammonium hydroxide (28%-30%) were purchased from Alfa Aesar and used without further purification. N-[(3-trimethoxysilyl) propyl] ethylenediamine triacetic acid trisodium salt (45% in water) and sodium 3-(trihydroxysilyl) propyl methylphosphonate (42% in water) were purchased from Gelest. The structures are shown in Fig.1.

(a)

(b)

Fig. 1 The chemical structure of (a) N-[(3-trimethoxysilyl) propyl] ethylenediamine triacetic acid trisodium salt and (b) sodium 3-(trihydroxysilyl) propyl methylphosphonate.


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Adsorbent synthesis

Fe2O3 nanoparticles were coated with a SiO2 layer and then fucntionalized in a two step procedure by a modified Stöber method41, 42. First, TEOS reacted with hydroxyl groups on the surfaces of Fe2O3 nanoparticles to form a silica network coating as shown in Fig. 2(a). Then, carboxylate and phosphonate silanes were reacted with silicon hydroxyl as displayed in Fig. 2(b) and Fig. 2(c), respectively. The detailed process was as follows. 0.5 g γ-Fe2O3 and 50 mL ethanol were mixed and ultrasonicated for 1 h. Ammonium hydroxide (6 mL) and deionized water (5 mL) were then added and stirred for 5 min. TEOS (1 mL) was then added and reacted for 45 min at 25 °C. The precipitate was separated under an applied magnetic field and then washed three times with ethanol. The magnetic silica nanoparticles, denoted as Fe2O3/SiO2, were dried for 1 h at 120 °C. 0.2 g of Fe2O3/SiO2 and 10 mL of ethanol were ultrasonicated together until well dispersed. Then a suitable amount of N-[(3-trimethoxysilyl) propyl] ethylenediamine triacetic acid trisodium salt was added and reacted for 10 h under stirring at 30 °C. Similarly, 0.2 g of Fe2O3/SiO2 and 10 mL of ethanol were mixed well before a suitable amount of sodium 3(trihydroxysilyl) propyl methylphosphonate) was added and the reaction proceeded for 10 h at 30 °C. Next, the solid was separated via an external magnetic field and washed three times with ethanol. Finally, the carboxylate and phosphonate modified magnetic silica nanoparticles were obtained after being dried in an oven for 1 h at 120 °C; they are denoted as Fe2O3/SiO2/RCOONa and Fe2O3/SiO2/R1R2PO3Na, respectively.


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Fig. 2 Synthesis of (a) Fe2O3/SiO2, (b) Fe2O3/SiO2/RCOONa and (c) Fe2O3/SiO2/R1R2PO3Na Physical and chemical characterization

Transmission electron microscopy (TEM) was performed on a JEM-2010 instrument (JEOL). X-ray diffraction was performed on powder samples with a Bruker D8 advance diffractometer. Pore volume, surface area and average pore diameter were obtained by BET measurements (Quadrasorb S1). Magnetization curves for the material were recorded on a vibrating sample magnetometer (VSM, USA model Lakeshore 730T) with an applied magnetic field of up to 1 T. The functional groups on adsorbents were identified qualitatively by Fourier transform infrared spectroscopy (FTIR, Nicolet 8700). Spectra were acquired between 4000 and 400 cm-1 using the KBr disc technique. The thermostability of samples was checked by thermogravimetric analysis (TGA, SDT Q600) at a heating rate of 10 °C/min with nitrogen flushing at 100 mL/min. Adsorption and desorption experiments

Adsorption experiments were conducted by adding 0.01 g of adsorbent into 10 ml Sm3+ solutions at various concentrations in a 20 ml glass vial. Measurements of Sm3+ concentrations by ICP-OES (Perkin Elmer, optima 8000) as a function of time allowed the adsorption kinetics to


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be obtained. The equilibirium adsorption at increasing Sm3+ concentrations yielded the adsorption capacities and isotherms. The pH values of the solutions were adjusted to desired values using NaOH and HCl solutions. To ensure the adsorption equilibrium was attained, the solutions with adsorbents were shaken in a shaker bath for at least 2 h for equilibrium adsorption. After adsorption, the adsorbents were separated from the suspension under an applied magnetic field. The desorption of Sm3+ was facilitated by the addition of a HCl solution to resuspend the recovered magnetic nanoparticles, with higher acid concentrations leading to more concentrated recovered Sm3+ solutions. For high HCl concentrations, however, large amounts of alkali would be needed to adjust the acidity of the concentrated Sm3+ solutions prior to further separation by solvent extraction or production of oxide, carbonate, sulfate and chloride. Thus, an intermediate concentration 2 N HCl solution was used as the desorption agent in this work to guarantee both relatively high desorption efficiency and that the resulting solution would not be too strongly acidic. The amounts of Sm3+ adsorbed per unit of adsorbent at time t (qt, mg/g) and at equilibrium (qe, mg/g) were calculated using the following Eqs. (1) and Eqs. (2), respectively. qt =

(C0 − Ct )V m

(1)

qe =

(C0 − Ce )V m

(2)

where C0 is the initial Sm3+ concentration (mg/L), Ct is Sm3+ concentration at time (mg/L), Ce is Sm3+ concentration at equilibrium (mg/L), V is the volume of solution (L) and m is the weight of adsorbent (g). Results and discussion Preparation of adsorbents

The adsorption capacity of adsorbents is strongly dependent on the type and concentration of functional groups grafted to their surfaces. The concentration of functional groups was optimized by control of the amount of silane coupling agent used in the preparation of the adsorbents. Based on HSAB, hard base RCOO- and R1R2PO3- should form stable complexes with the hard acid rare earth ions. Therefore, carboxylate and phosphonate groups were introduced by adding N-[(3-trimethoxysilyl) propyl] ethylenediamine triacetic acid trisodium salt and sodium 3(trihydroxysilyl) propyl methylphosphonate), respectively. By changing the relative amounts of Fe2O3/SiO2 and silane coupling agent, a series of adsorbents were obtained as shown in Table 1. 7 ACS Paragon Plus Environment

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The adsorption capacities of Fe2O3, Fe2O3/SiO2 and the modified adsorbents were checked to determine the optimal adsorbent. For all adsorption processes, 0.01 g of adsorbents was mixed with 10 ml of a 340 mg/L Sm3+ solution. The adsorption amounts were 9.9 mg/g and 9.7 mg/g for Fe2O3 and Fe2O3/SiO2, respectively. The adsorption capacity increased significantly after modification of the surfaces by carboxylate and phosphonate groups on reaction with silane coupling agents. For Fe2O3/SiO2/RCOONa, the adsorption capacity improved from 137 mg/g to 168 mg/g when the amount of silane coupling agent increased from 0.45 g to 0.85 g. The adsorption capacity was almost unchanged when the amount of silane coupling agent was increased further to 1.13 g. For Fe2O3/SiO2/R1R2PO3Na, the maximum adsorption was obtained for a silane coupling agent mass of 0.79 g. That means the reaction sites were saturated when the amount of silane coupling agents reacting with 0.2 g silica nanoparticles were 0.85 g and 0.79 g, respectively. Therefore, compositions No.5 and No.9 were applied in the following experiments to obtain the optimum adsorption performance. Table 1 The preparation conditions and adsorption amount of adsorbents. No. 1 2 3 4 5 6 7 8 9 10

Fe2O3 Fe2O3/SiO2 Fe2O3/SiO2/RCOONa

Fe2O3/SiO2/R1R2PO3Na

Mass of Fe2O3/SiO2 (g) — — 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2

Mass of silane coupling agent* (g) — — 0.45 0.57 0.85 1.13 0.42 0.53 0.79 1.05

Mass fraction of silane (%) 0 0 69.4 73.9 81.0 85.0 67.7 72.4 79.7 84.0

Mass Adsorbed (mg/g) 9.9 9.7 137 168 201 200 110 146 150 150

Note: The silane coupling agent for No.3, 4, 5, 6 is N-(trimethoxysilylpropyl) ethylene diamine triacetic acid, trisodium salt. The silane coupling agent for No.7, 8, 9, 10 is 3-trihydroxylsilylpropylmethyl phosphonate, sodium salt.

Physical and chemical characterization

Fig.

3

shows TEM

images of Fe2O3, Fe2O3/SiO2, Fe2O3/SiO2/RCOONa and

Fe2O3/SiO2/R1R2PO3Na particles. As displayed in Fig. 3(a), Fe2O3 particles had a broad size distribution. In addition, due to dipole-dipole magnetic attraction43, Fe2O3 particles aggregated significantly during the preparation of Fe2O3/SiO2 particles. Therefore, Fe2O3/SiO2 particles did 8 ACS Paragon Plus Environment

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not have a uniform size, but had a distrbution of sizes over the range of 20 nm to 100 nm as shown in Fig. 3(b). There were distinct contrasts in the morphology of composite particles Fe2O3/SiO2. The dark areas were attributed to the crystalline Fe2O3 core, while the bright regions show the SiO2 coating. It is evident that Fe2O3 particles were coated by a thin layer of SiO2 with negligible fraction of exposed surfaces. The multi-core structure was retained after modification by carboxylate and phosphonate groups as shown in Fig. 3(c) and (d) since the corresponding silane coupling agent only reacted with SiO2 on the surface.

Fig. 3 The TEM micrograph of (a) Fe2O3, (b) Fe2O3/SiO2, (c) Fe2O3/SiO2/RCOONa and (d) Fe2O3/SiO2/R1R2PO3Na. The diffractograms are shown in Fig. 4. The reflection peak positions and relative intensities of the adsorbents agree well with XRD patterns of γ-Fe2O3 in the literature 44. For γFe2O3, diffraction peaks with 2θ at 30.5º, 35.9º, 43.4º, 57.5º and 63.2º were observed, indicative of spinel structure of the maghemite (γ-Fe2O3). The same sets of characterization peaks with same width and relative intensity were also found for Fe2O3/SiO2, Fe2O3/RCOONa and Fe2O3/SiO2/R1R2PO3Na, suggesting the stability of crystallinity of γ-Fe2O3 when coated and functionalized.


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Fe2O3 Fe2O3/SiO2 Fe2O3/SiO2/RCOONa Fe2O3/SiO2/R1R2PO3Na

Intensity (Counts)

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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20

30

40

50

60

70

2-theta (degrees)

Fig. 4 XRD data of Fe2O3, Fe2O3/SiO2, Fe2O3/RCOONa and Fe2O3/SiO2/R1R2PO3Na.

Surface area, pore volume and average pore diameter of adsorbents are important parameters in the adsorption process. As listed in Table 2, the average pore diameter was 1.421.44 nm. The diameter of Sm3+ is 0.0964 nm, much smaller than the pore size. Therefore, Sm3+ could enter into the pores of adsorbents. However, whether the adsorption took place inside pores still depends on the distribution of functional groups. Due to serious aggregation, the surface area decreased from 76.6 m2/g to 1.68 m2/g and 0.28 m2/g after being modified by carboxylate group and phosphonate group, respectively. The pore volume also reduced greatly from 153.2×10-3 mL/g to 5.794×10-3 mL/g and 0.531×10-3 mL/g, respectively. Due to the limited surface area and pore volume, the adsorption presumedly took place mainly on the surface active sites. Table 2 Surface area, pore volume and average pore diameter of adsorbents. Average pore

Surface area

Pore volume

(m2/g)

(×10-3 mL/g)

Fe2O3/SiO2

76.6

153.2

1.43

Fe2O3/SiO2/RCOONa

1.68

5.79

1.44


0.28

0.53

1.42

Material


diameter (nm)

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The

saturation

magnetization

of

Fe2O3,

Fe2O3/SiO2,

Fe2O3/SiO2/RCOONa

and

Fe2O3/SiO2/R1R2PO3Na particles was 69.26, 43.54, 6.63 and 9.08 emu/g, respectively. There was an obvious decrease in saturation magnetization of Fe2O3/SiO2, Fe2O3/SiO2/RCOONa and Fe2O3/SiO2/R1R2PO3Na compared with Fe2O3, most likely due to the decrease in Fe2O3 content per unit weight of the samples. The magnetic intensity is higher than that of reported studies35, 37 (0.51, 4.2 emu/g) and sufficiently strong for rapid separation of the particles from the aqueous phase on application of an external magnet field. The phase separation phenomenon of solvent extraction and adsorption is compared in Fig. 6. For solvent extraction, Sm3+ was extracted by a saponified P507-kerosene solvent. The two phases separated slowly and a thick layer of a third phase formed between the upper oil phase and the lower aqueous phase within two minutes. The third phase gradually became a little thinner with time, but did not disappear even after standing for 6 minutes. For Sm3+ adsorption by magnetic nano-adsorbents, most of the adsorbents were attracted by the magnet in 10 s. In 50 s, the adsorbent particles were collected completely, and much more simply and quickly than the phase separation in the solvent extraction process.


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Fig. 5 Saturation of magnetization of Fe2O3, Fe2O3/SiO2, Fe2O3/SiO2/RCOONa and Fe2O3/SiO2/R1R2PO3Na.

Fig. 6 Phase separation of (a) Sm3+ extraction by P507-kerosene system and (b) Sm3+ adsorption by magnetic nano-adsorbents


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IR analysis was performed to further characterize the Fe2O3/SiO2, Fe2O3/SiO2/RCOONa and Fe2O3/SiO2/R1R2PO3Na particles. As shown in Fig. 7, for Fe2O3/SiO2 the characteristic peaks around 3445 cm-1 and 400-750 cm-1, attributed to γ-Fe2O3, and the peak around 1089 cm-1 due to the Si-O-Si stretching vibration, confirm the coating of SiO2 layer on surface of Fe2O3. After modification with the carboxylate and phosphonate groups, the characteristic peaks of CH2- and -CH3 in the range 2800 to 3000 cm-1 were observed. For the Fe2O3/SiO2/RCOONa spectrum, the bands at 1592 cm-1 and 1408 cm-1 were assigned to C=O and C-O stretching vibration of the carboxylate group, respectively. The characteristic peaks of the phosphonate group appeared around 900-1050 cm-1, 1100-1200 cm-1 and 1230-1260 cm-1. The peak at 1198 cm-1 was ascribed to the P=O stretching vibration. The results revealed that carboxylate and phosphonate groups were successfully grafted onto the surface of Fe2O3/SiO2.

Fig. 7 FTIR spectra of Fe2O3/SiO2, Fe2O3/SiO2/RCOONa and Fe2O3/SiO2/R1R2PO3Na R R PO Na. As presented in Fig. 8, the thermal stability of Fe2O3/SiO2, Fe2O3/SiO2/RCOONa and Fe2O3/SiO2/R1R2PO3Na was investigated. Fe2O3/SiO2 revealed good thermal stability from 200 °C to 900 °C; its weight loss before 100 °C was attributed to water evaporation. For Fe2O3/SiO2/RCOONa, there were three stages of weight loss, over the ranges 0-200 °C, 200500 °C and 500-900 °C, which were attributed to the decomposition of N-[(3-trimethoxysilyl) propyl] ethylenediamine triacetic acid trisodium salt. Finally, the total weight decreased to 51.4%, indicating the mass of carboxylate groups grafted to the particle surfaces. For Fe2O3/SiO2/R1R2PO3Na, there are two obvious weight losses over temperature ranges of 013 ACS Paragon Plus Environment

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300 °C and 300-600 °C. The weight loss was due to the decomposition of C-P, C-O, C-C and CSi bond. The final weight loss was 26.0% of the initial weight. The concentrations of carboxylate and phosphonate groups grafted on the particle surfaces could be determined roughly from these weight losses to be 3.77 mmol/g and 1.60 mmol/g, respectively.

Fig. 8 TGA curves for magnetic silica nanoparticles Adsorption studies The effect of pH on the adsorption

The adsorption was driven primarily by ion exchange between H+ and Sm3+ according to the reactions in Eq.3 and Eq.4. Therefore, pH affected the adsorption process through several phenomena such as adsorbent surface charge (Eq.5-6), metal speciation (Eq.7-9) and competitive adsorption of H+ (Eq.3-4). In this work, the initial pH of the solution was adjusted by addition of HCl and/or NaOH following suspension of the adsorbents in the aqueous solution. nRCOOH + Sm 3 + ⇔ [( RCOO ) n Sm ]3 − n + nH +

(3)

nR1 R 2 PO3 H + Sm 3 + ⇔ [( R1 R 2 PO3 H ) n Sm ]3 − n + nH + (4) RCOO − + H + ⇔ RCOOH

R1 R 2 PO3 − + H + ⇔ R1 R 2 PO 3 H

(5) (6)

S m 3 + + H 2 O ⇔ Sm (OH) 2 + + H +

(7)

S m 3 + + 2 H 2 O ⇔ Sm (OH) 2 + + 2 H +

(8)


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S m 3 + + 3 H 2 O ⇔ Sm (OH) 3 + 3 H +

(9)

To determine the appropriate pH range suitable for Sm3+ adsorption, the precipitation experiments were conducted at pH’s from 2 to 11 for 500 mg/L Sm3+. The equilibrium concentrations of Sm3+ was determined under different acidities. As shown in Fig. 9, the concentration of Sm3+ was constant at pH≤7 and decreased slightly at pH≥7. When the pH value was higher than 8, the Sm3+ concentration decreased drastically due to Sm(OH)3 precipitation. The precipitation pH could also be obtained by the solubility product of Sm(OH)3 at room temperature (6.8×10-22). For 0-500 mg/L Sm3+, Sm(OH)3 begins to form at pH>7.77, which coincides with the experimental results. Additionally, according to the speciation diagram reported21, at pH7.5 and becomes predominant at pH>11.5. All the above results demonstrated that Sm3+ solution should be adjusted at pH≤7 to avoid Sm3+ precipitation.

Fig. 9 The effect of pH on Sm3+ precipitation The effect of pH on Sm3+ adsorption was evaluated over the range 1 to 7 with Fe2O3/SiO2/RCOONa and Fe2O3/SiO2/R1R2PO3Na as adsorbents. As shown in Fig. 10, the adsorption increased with pH and peaked at pH 7.0. That was because at very low pH the concentration of hydrogen ions was sufficiently high to compete with Sm3+ for occupation of active binding sites33 and thereby inhibited the adsorption of Sm3+. At pH values above the point 15 ACS Paragon Plus Environment

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of zero charge (pHpzc), i.e., the pH at which the electrical charge density on a surface is zero, the adsorbent surface was electronegative and was able to attract strongly positively charged metal ions. To avoid precipitation of Sm(OH)3 at higher pHs, however, the initial pH of solution was adjusted to 6.5 for the adsorption experiments described below. It should be noted that the adsorption capacity of Fe2O3/SiO2/R1R2PO3Na was less sensitive to pH than that of Fe2O3/SiO2/RCOONa. This was due to the lower pHzpc of Fe2O3/SiO2/R1R2PO3Na relative to that of Fe2O3/SiO2/RCOONa as displayed in Fig. S1 (Supporting Information), i.e., means Fe2O3/SiO2/R1R2PO3Na was more electronegative than Fe2O3/SiO2/RCOONa at the same pH value.

Fig. 10 Effect of pH on Sm3+ adsorption using different magnetic particles. Cini=565 mg/L, T=298 K, m=0.01 g, V=10 mL; contacting time, 240 min. Adsorption equilibrium

Adsorption equilibrium was analyzed by contacting the adsorbents with different concentrations of Sm3+ in solution for 2 hours. The adsorbed amount increased with initial concentration and then peaked at 500 mg/L, at which concentration the adsorption sites were saturated. At the higher concentrations below this saturation limit, the driving force for Sm3+ towards the adsorption sites on the adsorbent increased and higher adsorption capacity was achieved. It is clear from from Fig. 11 that in the range of 0-133 mg/L, the adsorption by Fe2O3/SiO2/RCOONa

was

less

than

20

mg/g.

In

the

range

of

0-280

mg/L,

Fe2O3/SiO2/R1R2PO3Na presented a larger adsorption capacity than did Fe2O3/SiO2/RCOONa.


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These differences can be attributed to the difference in intrinsic chemisorption equilibrium between Sm3+ and functional groups. It has been reported that the adsorption of metal ions by carboxylate and phosphonate groups is due mainly to ion exchange phenomena and complexion5, 19, 34, 45

. The phosphonate group is more negative due to the lower pHzpc and hence

Fe2O3/SiO2/R1R2PO3Na particles can bind with positively charged rare earth ions more easily than can Fe2O3/SiO2/RCOONa at low Sm3+ concentration range. At Sm3+ concentrations higher than 330 mg/L, the adsorption capacity was close to the saturation vlaues of 228 mg/g (1.516 mmol/g) and 180 mg/g (1.197 mmol/g) for Fe2O3/SiO2/RCOONa and Fe2O3/SiO2/R1R2PO3Na particles, respectively. The main reason for the difference in adsorption capacities was the higher concentration of carboxylate groups on the particle surfaces than of the available phosphonate groups of the relevant particles, as discussed in TG results. A comparison of these sorption capacities to the carboxylate (3.77 mmol/g) and phosphonate (1.60 mmol/g) group availability indicates that the molar ratios carboxylate/Sm3+ and phosphonate/Sm3+ were 2.49 and 1.34, respectively, below the theoretical 3:1 molar ratio expected from the stoichiometry of ion exchange between Sm3+ and H+ on these groups. This probably indicates other functional groups such as OH- may be involved in the binding of Sm3+. Similar phenomena have also been reported elsewhere46.

Fig. 11 Adsorption equilibrium of Fe2O3/SiO2/RCOONa and Fe2O3/SiO2/R1R2PO3Na. T=298 K, m=0.01 g, V=10 mL, pH=6.5; contact time, 240 min.


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Adsorption kinetics

The adsorption kinetics determined experimentally are plotted in Fig. 12. For Fe2O3/SiO2/RCOONa, the adsorption was very fast and reached equilibrium in 10 min. For Fe2O3/SiO2/R1R2PO3Na, a rapid uptake took place at the beginning and then the adsorption increased gradually with contact time, finally reaching equilibrium after 20 min contact time. The fast adsorption rates were attributed to the absence of internal diffusion resistances common in most adsorbent systems since the carboxylate and phosphonate groups were on the surfaces of particles. The final adsorption amounts using the two types of adsorbents were 225 mg/g and 171 mg/g, respectively, consistent with the adsorption equilibrium data shown in Fig.11. The different adsorption rates could be explained by the fact that concentration of carboxylate groups was higher than phosphonate groups as noted above. Three kinetic models, pseudo-first-order, pseudo-second-order, and intraparticle diffusion models as expressed by Eq. (10), Eq. (11) and Eq. (12), respectively, have been used to analyze the adsorption kinetics. k1 t 2.303

(10)

1 1 1 1 t = + t= + t 2 qt k 2 qe qe v0 q e

(11)

lg( qe − qt ) = lg qe −

qt = k p t 1/ 2 + I

(12)

where k1 (min-1), k2 (g/mg.min) and kp (mg/g.min0.5) are the pseudo-first-order rate constant, pseudo-second-order rate constant and intraparticle diffusion rate constant, respectively. qe (mg/g) and qt (mg/g) refer to the particle loadings of Sm3+ at equilibrium and at time t (min), respectively. v0 (mg/g·min) is the initial adsorption velocity. I denotes a constant about the thickness of the boundary layer. In the analysis of experimental data, the linear fitting method was applied. By plotting lg(qe-qt) versus t, t/qt versus t and qt versus t1/2, the kinetic parameters and correlation coefficents were obtained; these are listed in Table 3. The linear correlation coefficients for pseudo-firstorder model and intraparticle diffusion model fits were much less than unity, demonstrating that the adsorption kinetics did not conform to these two models. The pseudo-second-order kinetic equation fit the experimental data best for the two kinds adsorbents with high correlation coefficients (R2) of 0.999. A basic assumption in the pseudo-second-order kinetic model is that 18 ACS Paragon Plus Environment

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Langmuir

the rate-controlling step may be chemical sorption or chemisoption involving valency forces through sharing or exchange of electrons between sorbent and sorbate47. It is assumed that the adsorption capacity is proportional to the availability of adsorption sites on the surface of adsorbent rather than adsorbant concentration in bulk solution48. Therefore, this high adsorption rate could also be retained for more dilute solution. The predicted equilibrium adsorption amounts were 227.27 mg/g and 169.49 mg/g, close to the experimental results. The adsorption involves two steps, the diffusion of Sm3+ in the bulk solution to the reaction interface and the adsorption between Sm3+ and functional groups. Because of the strong shaking of the particle suspensions, the diffusion resistance in the liquid film was neglible, and could be ignored. Therefore, the adsorption on surface was the rate limiting step. Table 3 Comparison of adsorption kinetic constants and linear correlation coefficients for pseudo-first-order, pseudo-second-order and intraparticle diffusion models

Adsorbent

Fe2O3/SiO2/ RCOONa Fe2O3/SiO2/ R1R2PO3Na

qe,exp (mg/g)

Pseudo-first-order model

Pseudo-second-order model

k1 (min-1)

qe,cal (mg/g)

225

0.007

2.3

0.731

0.0121

227.3

0.999

0.431

-90

0.688

171

0.290

1165.2

0.823

0.0003

169.5

0.999

0.169

-22

0.881

R2

k2 (g/mg·min)

qe,cal (mg/g)

Intraparticle diffusion model


R2

kp (mg/g·min0.5)

I

R2

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Fig. 12 Adsorption kinetics of Fe2O3/SiO2/RCOONa and Fe2O3/SiO2/R1R2PO3Na. Cini=550 mg/L, T=298 K, m=0.01 g, V=10 mL, pH=6.5. Desorption and concentration

Desorption and regeneration studies are important to clarify the Sm3+ adsorption onto the adsorbents. The enrichment experiments were performed as follows. 40 mL of 113 mg/L Sm3+ solution was contacted with 0.01 g adsorbents in a shaker bath for 2 hours. The obtained adsorption amounts were 154 mg/g and 125 mg/g, respectively, as shown in Table 4. Then the adsorbents were collected and washed three times with deionized water to remove the residue ions. Subsequently, HCl was mixed with the adsorbent particles for 2 hours to ensure desorption equilibrium. With increasing amounts of HCl, the desorption efficiency improved commensurately, and the enrichment ratio decreased. When 0.5 mL of 2 mol/L HCl was introduced as the desorption agent, the dilute solutions were concentrated up to 22.3 times and 17.0 times, respectively. Overall, it was a slightly easier to remove Sm3+ from Fe2O3/SiO2/RCOONa than from Fe2O3/SiO2/R1R2PO3Na. It is safe to predict that higher concentrations of HCl used as the desorption agent would result in higher desorption efficiency and larger enrichment ratio.


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Table 4 Desorption of Sm3+ from adsorbent by 2 N HCl. Adsorbent

Fe2O3/SiO2/RCOONa


Adsorption Amount (mg/g)

Volume of 2 mol/L HCl (mL)

Desorption efficiency

Enrichment ratio

0.5

80%

22.3

1

83%

11.6

1.5

89%

8.3

0.5

76%

17.0

1

79%

8.9

1.5

84%

6.3

154

125

Adsorbents stability in acid

The stability of the adsorbents in the acidic desorption solution was investigated to assess their long term viability in industrial applications. Fe2O3/SiO2, Fe2O3/SiO2/RCOONa and Fe2O3/SiO2/R1R2PO3Na were immersed in 0.1 M, 1 M and 2 M HCl for 336 h, respectively, a much longer time than that in reported studies4, 19, 37, 49. The solid/liquid ratio was 6 mg/10 mL. The solution iron content was determined by ICP-OES to detemine the weight loss of adsorbents exposed to the acidic solutions. The results are shown in Fig.13. Fe2O3/SiO2/RCOONa and Fe2O3/SiO2/R1R2PO3Na were more stable in acid than Fe2O3/SiO2 because the carboxylate and phosphonate groups grafted onto the surface protected Fe2O3/SiO2 from the acid. After immersion in the different HCl solutions, the weight of all adsorbents barely changed, and magnetic properties were unscathed by this esposure to the acids. They are rather stable in comparison with reported studies4, 19 in which more than 4.6% of the iron was released into solution after only a single adsorption and desorption cycle (0.01 g/10 ml, 0.1 M HCl). Again, this stability can be attributed to the effective SiO2 layer coating the Fe2O3 particles and the high concentration of grafted functional groups.


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(a)

(b)

(c)

Fig. 13 The weight change of adsorbent with time in 0.1 mol/L, 1 mol/L and 2 mol/L HCl. (a) Fe2O3/SiO2, (b) Fe2O3/SiO2/RCOONa, and (c) Fe2O3/SiO2/R1R2PO3Na


Page 22 of 31

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Langmuir

Adsorption mechanism

The mechanisms involved in the binding of metal ions with adsorption sites are numerous, and include electrostatic interactions, surface complexation, ion-exchange and precipitation, all of which can occur individually or in combination46. The adsorption mechanism in this work was studied from two perspectives. On one hand, FTIR spectra of Fe2O3/SiO2/RCOONa and Fe2O3/SiO2/R1R2PO3Na before and after adsorption of Sm3+ were compared. On the other, the acidity of Sm3+ solution before and after adsorption was recorded and compared. As presented in Fig. 14, the FTIR spectra of Fe2O3/SiO2/RCOONa before and after adsorption of Sm3+ show a significant change in the adsorption band at 3437 cm-1, corresponding to O-H, which shifted to a lower wavenumber 3427 cm-1. The peaks at 1074 cm-1 assigned to C=O asymmetry stretching became sharper and shifted to a higher wavenumber 1092 cm-1. Therefore, the carboxylate group changed after desorption, revealing that the carboxylate group participated in the adsorption process. For Fe2O3/SiO2/R1R2PO3Na, the peak at 1198 cm-1 assigned to P=O stretching vibration, became sharper and shifted to a much lower wavenumber 1159 cm-1. The bending vibration of –CH3 bonding with the phosphonate group at 1452 cm-1 shifted to 1416 cm-1,confirming that the phosphonate group was involved in the adsorption process. Sm3+ is a Pearson hard acid with low electronegativity, and therefore tends to react readily with Pearson hard bases with high electronegativity and low polarizability such as oxygen, sulfur, and phosphorous atoms, which exist in carboxylate, phosphonate and sulfonate groups.

Fig. 14 FTIR spectra of (a) Fe2O3/SiO2/RCOONa before and after adsorption and (b) Fe2O3/SiO2/R1R2PO3Na before and after adsorption


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It is commonly reported that the adsorption of heavy metals by carboxylate and phosphonate groups follows a cation exchange mechanism5, 19, 34, 45. In this case, pH changed after adsorption as shown in Table 5, indicating H+ exchange was involved in the adsorption. The acid dissociation, Sm3+ adsorption and Sm3+ hydrolysis jointly contributed to the change in acidity. In addition, as discussed above, the molar ratios carboxylate/Sm3+ and phosphonate group/Sm3+ were 2.49 and 1.34, below the expected 3, indicating that OH- may also bind with Sm3+. Therefore, it can be concluded that the adsorption was mediated by both cation exchange and complexation. For the adsorption experiments with Fe2O3/SiO2/RCOONa, the equilibrium pH of the Sm3+ solution was larger than the initial pH, indicating a decrease in H+ concentration. Due to its relatively large pKa value, the carboxylate group tends to combine with H+ especially under the low pH conditions in which the protons competed with Sm3+ for exchange with Na+. As a result, the H+ concentration decreased and the pH value increased as adsorption proceeded. When the pH increased from its initial value, more Sm3+ instead of H+ was able to bind with the carboxylate; consequently the pH increase was less than it would have been in the absence of Sm3+. Conversely, for Fe2O3/SiO2/R1R2PO3Na, the equilibrium pH was lower than the initial pH; because of the relatively small pKa of the phosphonate group, Sm3+ was able to bind with it more readily than could H+ even under high acidic conditions as shown in Fig. 7. The adsorption of Sm3+ caused the release of H+ to the solution and a decrease in the solution pH. When the initial pH value changed to some degree, the adsorption process was mainly the exchange between Sm3+ and Na+. The acidity was influenced primarily by Sm3+ hydrolysis. The Sm3+ concentration in bulk solution decreased; hydrolysis of Sm3+ weakened; H+ concentration produced during the hydrolysis of Sm3+ became lower; and ultimately pH value did not decrease further. Table 5 The pH change for Sm3+ solution of different initial pH during Sm3+ adsorption (10 mL 565 mg/L, 0.01 g adsorbent) Fe2O3/SiO2/RCOONa


Initial pH

1.06

2.09

3.03

4.01

5.16

6.04

Final pH

1.63

2.47

3.31

4.26

5.39

6.09

Initial pH

1.26

1.92

2.86

3.99

5.03

5.85

Final pH

1.02

1.67

2.81

3.20

4.77

5.85


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Langmuir

For Sm3+ solutions of different initial concentrations, the pH value changed differently as well, as shown in Table 6. The initial pH of the Sm3+ solution was 6.5, and thus, the concentrations of H+ and OH- were 10-6.5 M and 10-7.5 M, respectively. The Na+ concentration was estimated from the concentration of either the carboxylate or the phosphonate group per unit weight of adsorbent (3.77 mmol/g or 1.60 mmol/g, respectively), the mass of adsorbent (0.01 g) and Sm3+ solution volume (10 ml). The Na+ concentration was determined to be 3.77×10-3 M and 1.60×10-3 M for adsorption system with Fe2O3/SiO2/RCOONa and Fe2O3/SiO2/R1R2PO3Na, respectively, significantly higher than H+ concentration 10-6.5 M. Therefore, the ion exchange process was mainly the exchange between Na+ and Sm3+. With an initial Sm3+ concentration in the range 23 mg/L to 112 mg/L, the equilibrium pH was higher than its initial value. In contrast, the equilibrium pH was lower than the initial value for initial Sm3+ concentrations in the range 224 mg/L to 785 mg/L. As the adsorption of Sm3+ proceeded, hydrolysis of Sm3+ in bulk solution decreased and the pH increased accordingly. When the initial Sm3+ concentration was high, the acidity was controlled mainly by Sm3+ hydrolysis50 because only small amounts of Sm3+ could be adsorbed due to the limited number of adsorption sites. With the hydrolysis of Sm3+, the H+ concentration in bulk solution increased gradually and the equilibrium pH decreased relative to its initial value. Table 6 The pH change for Sm3+ solution of different initial concentration during Sm3+ adsorption (10 mL, 0.01 g adsorbent) Initial Sm3+ Adsorbent concentration (mg/L) Initial pH Fe2O3/SiO2/RCOONa Final pH Initial pH Fe2O3/SiO2/R1R2PO3Na Final pH

23

46

67

112

224

337

449

785

6.53 6.69 6.59 7.24

6.48 6.86 6.54 7.52

6.49 7.37 6.41 7.63

6.50 7.20 6.47 6.72

6.57 6.52 6.44 5.97

6.46 6.17 6.53 5.95

6.47 6.13 6.49 5.87

6.48 6.03 6.47 6.07

Comparison with other adsorbents

The rate and capacity for Sm3+ adsorption characteristic of the magnetic nanoparticle adsorbents were compared systematically with those of previously reported materials in Table 7. The adsorption capacity varied from 32 mg/g to 370 mg/g. Reported equilibrium times are in the range 30-150 min, much longer than the 10 min required for Fe2O3/SiO2/RCOONa and 20 min 25 ACS Paragon Plus Environment

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Page 26 of 31

for Fe2O3/SiO2/R1R2PO3Na. Among the reported adsorbents, nano-hydroxyapatite presented the largest adsorption capacity, 370 mg/g, but it exhibited slow adsorption rates (150 min to reach 210 mg/g). Overall, adsorbents Fe2O3/SiO2/RCOONa and Fe2O3/SiO2/R1R2PO3Na showed comparable adsorption capacity to reported adsorbents and the fastest adsorption kinetics ever observed. Table 7 Comparison of different adsorbents for rare earth ion adsorption in terms of the adsorption capacity and adsorption kinetics. Adsorbent

pH

Nano-hydroxyapatite Sargassum biomass PAN@SDS SBA-15-ZMVP Oxidized multiwalled carbon nanotube Fe2O3/SiO2/RCOONa Fe2O3/SiO2/R1R2PO3Na

5.5 5.0 4.8 4

Equilibrium time and corresponding adsorption amount 150 min, 210 mg/g 60 min, 48 mg/g 30 min, 9 mg/g 79 min, 1.8 mg/g

5 6.5 6.5

qmax (mg/g)

References

370 98 98 32

51

64 min, 14 mg/g

89

52

10 min, 225 mg/g 20 min, 171 mg/g

228 180

This work This work

16 21 33

Conclusions In this study, functionalized magnetic silica nanoparticles Fe2O3/SiO2/RCOONa and Fe2O3/SiO2/R1R2PO3Na were prepared and investigated for adsorption of Sm3+ from dilute solutions. The influence of pH on the mass of the rare earth ions adsorbed indicated an optimal adsorption performance occurs at pH=6.5. The adsorbents demonstrated very high adsorption capacity

and

fast

adsorption

equilibrium.

The

Sm3+

adsorption

capacities

with

Fe2O3/SiO2/RCOONa and Fe2O3/SiO2/R1R2PO3Na were 228 mg/g and 180 mg/g, respectively. It took 10 min and 20 min for Fe2O3/SiO2/RCOONa and Fe2O3/SiO2/R1R2PO3Na, respectively, to reach adsorption equilibrium. The adsorption kinetics data fit well the pseudo-second order model. The desorption experiments indicated that Sm3+ could be concentrated at least 22 times and 17 times by Fe2O3/SiO2/RCOONa and Fe2O3/SiO2/R1R2PO3Na, respectively. The adsorbents were strongly acid-resistant and stable in 0.1 M, 1 M and 2 M HCl for 346 h with negligible weight loss. The analysis of FTIR spectra, and effects of carboxylate/Sm3+ and


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Langmuir

phosphonate/Sm3+ molar ratios and pH suggested that the carboxylate, or the phosphonate group and probably OH- were involved in Sm3+ adsorption by ion exchange and complexation. Acknowledgements This work was performed under the support of National Natural Science Foundation of China (NSFC)(31636004), National Key Basic Research Program of China (2012CBA01203), the China Scholarship Council to which the authors wish to express their thanks.


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Wang, F.; Zhao, J.; Pan, F.; Zhou, H.; Yang, X.; Li, W.; Liu, H. Adsorption Properties toward Trivalent Rare Earths by Alginate Beads Doping with Silica. Industrial & Engineering Chemistry Research 2013, 52, (9), 3453-3461. 6. Roosen, J.; Pype, J.; Binnemans, K.; Mullens, S. Shaping of Alginate–Silica Hybrid Materials into Microspheres through Vibrating-Nozzle Technology and Their Use for the Recovery of Neodymium from Aqueous Solutions. Industrial & Engineering Chemistry Research 2015, 54, (51), 12836-12846. 7. W. Zhu, E. W. B. D. Study of the preconcentration and determination of ultratrace rare earth elements in environmental samples with an ion exchange micro-column. Fresenius J Anal Chem 1998,, (360), 74-80. 8. M. F. Ei-Shahat, E. A. S. N. Ion-Exchange Equilibria between Some Rare Earth Metal Ions and Hydrogen Ions in Iron(III) and Tin(IV) Antimonates as Cation Exchangers. Microchemical Journal 1998,, (60), 95-100. 9. R. Vijayalakshimi, S. L. M. H. 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Functionalized magnetic silica nanoparticles for highly efficient adsorption of Sm3+ from a dilute aqueous solution Yue Wanga, Hari Katepallib, Tonghan Gub, T. Alan Hattonb*, Yundong Wanga*


Functionalized Magnetic Silica Nanoparticles for Highly Efficient

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