Research Article pubs.acs.org/journal/ascecg
Cite This: ACS Sustainable Chem. Eng. 2017, 5, 10947-10958
Novel Polyethylenimine−Acrylamide/SiO2 Hybrid Hydrogel Sorbent for Rare-Earth-Element Recycling from Aqueous Sources Qiuming Wang,*,†,‡ Walter C. Wilfong,†,‡ Brian W. Kail,†,§ Yang Yu,†,§ and McMahan L. Gray*,† †
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U.S. Department of Energy, National Energy Technology Laboratory, 626 Cochrans Mill Road, Pittsburgh, Pennsylvania 15236, United States ‡ Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831, United States § AECOM, 626 Cochrans Mill Road, Pittsburgh, Pennsylvania 15236, United States S Supporting Information *
ABSTRACT: Recycling rare-earth elements (REEs) becomes increasingly important because of their supply vulnerability and increasing demands in industry, agriculture, and national security. Hybrid hydrogel sorbents are outstanding, because of their high stability and selectivity. Organic− inorganic hybrid hydrogels were synthesized by thermopolymerization of acrylamide onto PEI polymer chains with N,N′-methylene bis(acrylamide) as a cross-linker. The grafted network was evidenced by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and X-ray photoelectron spectroscopy (XPS). The porous structure was observed by scanning electron microscopy (SEM). The degree of cross-linking, the degree of PEI grafting, and the SiO2 concentration were studied to optimize the adsorption of REEs. The pH value of the medium greatly affected REE adsorption capacity, where the almost-neutral conditions gave the strongest bonding of REEs to active sites. Moreover, kinetic studies showed that the rate-determining step of the adsorption process was chemical sorption, and that REE diffusion within micropores was the control step for, specifically, intraparticle diffusion. The adsorbents showed excellent selectivity and recyclability for REEs through five adsorption−desorption cycles in contact with synthetic acid mine drainage solution. A high separation toward REEs over fouling metals was achieved by using a citrate-based buffer eluent solution. This hybrid hydrogel shows promise for the recycling of REEs from aqueous solutions. KEYWORDS: Rare-earth elements (REEs), Hybrid hydrogel, PEI, Acrylamide, SiO2, Adsorption kinetic, Selectivity, Recycle, Acid mine drainage (AMD)
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INTRODUCTION Rare-earth elements (REEs) are playing an increasing role in modern technologies, such as automobile catalysts, phosphors in cell phones or laptops, and permanent magnets in hybrid engines, because of their unique catalytic, optical, and magnetic properties. In 2010, the global REEs production was ∼133 600 tons, with an estimated yearly demand of 136 100 tons.1 Approximately 95% of the world’s supply comes from a few localities in China.2 However, China has reduced the production and exportation of REEs in recent years for environmental concerns and to protect domestic industries.3 Accordingly, the price of REEs is rapidly increasing. The price of dysprosium metal soared from $250/kg in April 2010 to $2840/kg by July 2011, while the price for neodymium rose from $42/kg to $334/kg over the same period.1 Growing threats to REE supplies and increasing demands in industry and national security make domestic recycling of REEs a critical task. In addition to economic impact, importantly, REEs in waste solution have the potential to threaten human health. Studies found REEs can accumulate in blood, brain, liver, hair, © 2017 American Chemical Society
and bone after entering the human body, increasing concerns about effects of continuous exposure to REEs on human health.4−7 REEs leached in stream waters, alluvial aquifer, and well water are also very significant to evaluations of water quality, and they affect the accumulation of REEs into soils, plants, and animals. As a result, skilled recovery of REEs from industrial effluents to avoid environmental contamination and reduce human health risks is urgently necessary. The substantial quantities of REEs in different water sources amplifies the attractiveness of their recovery from these mediums. Aqueous acid mine drainage runoff (AMD) is regarded as an environmental pollution concern and is attributable to the contamination of nearby stream waters and well waters with appreciable amounts of dissolved REEs and additional fouling metals/non-REEs. Because of the overall low concentrations of dissolved REEs, it is a challenging task to Received: August 17, 2017 Revised: September 12, 2017 Published: September 14, 2017 10947
DOI: 10.1021/acssuschemeng.7b02851 ACS Sustainable Chem. Eng. 2017, 5, 10947−10958
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addition, the adsorptive reusability of the sorbent for REEs recycle were studied through five adsorption−desorption cycles in simulated recovery from a synthetic acid mine drainage solution.
selectively recover them from water sources. REEs in AMD or in rare-earth mining effluent are observed at the ppb (μg/L) level,8−12 and in rivers with large drainage areas or discharges the concentrations are diluted down to the ppt (ng/L) level.13 The REEs coexist with highly dissolved ppm (mg/L) levels of transition metals and post-transition metals, such as copper, iron, aluminum, manganese, etc.8,10 Because of their large ionic radii, REEs usually form complexes with high coordination numbers, which restricts the overall bulk diffusion of ions toward the active sites of various sorbents. This makes this heterogeneous adsorption process slow and complex.14,15 Thus, it is complicated to scavenge REEs from aqueous solutions with binding ligands due to the strong competitive adsorption between ppb-level REEs and ppm-level transition metals. Conventional chemical precipitation, ion exchange, membrane separation, and adsorption are commonly used for the collection of metals from aqueous sources. Among these methods, adsorption is technically simple with low energy requirements and is particularly effective to recover metal ions from low-concentration sources.16,17 The traditional natural clay mineral absorbents, such as montmorillonite and bentonite, are inexpensive but have lower absorption efficiency.18,19 Clays modified with chelating agents greatly enhanced adsorption efficiency, but had poor tolerance toward harsh chemical environments.20 Hydrogels, which are known as highly porous cross-linked polymer networks, were proven to be excellent sorbents for metal ions. Hydrogels are typically water-insoluble, hydrophilic copolymers endowed with bulky and flexible chelating groups oriented within the sorbents 3-D networks.21 Importantly, incorporation of inorganic fillers showed encouraging prospect in facilitating advanced properties of hydrogel networks.22 Hybrid gels were found to overcome the disadvantages of classic hydrogels by combining the advantages of the organic and inorganic components. Clays, ashes, magnetic nanoparticles, graphene, SiO2, and others have been frequently reported as inorganic components in hydrogelbased sorbents for removing organic/inorganic pollutants from solution.22−28 The use of hydrogels in absorbing heavy metals have been widely studied in the literature; however, a deep investigation into REE recovery using hydrogels is lacking.21,23,29−33 Only a small number of REE hydrogels have been investigated in any detail.24,34−39 Borai et al. developed a series of hydrogel sorbents and studied their adsorption capacity, kinetics, and sorption behaviors toward La3+, Ce3+, Nd3+, Eu3+, and Pb2+ single-metal solutions.24,34 Zhu et al. prepared monolithic open-cellular hydrogel adsorbents for La3+ and Ce3+ adsorption.36 One of the hydrogel sorbents reached adsorption equilibrium within 30 min and had an adsorption capacity of 384.62 mg/g for La3+ and 333.33 mg/g for Ce3+. A SA-PGA gel reported by Xu et al. possessed a considerable adsorption capacity for 15 REE elements in both single REE solutions and mixed REE solutions.35 In addition, it had good regeneration capability after 10 adsorption−desorption cycles. Despite this research, unfortunately no report is available on the use of hydrogels to recover rare earths from complex aqueous solutions and industrial waste solutions. In the present work, we have prepared PEI-pAAm-SiO2 organic−inorganic hybrid gels and investigated their adsorption behaviors toward REE metals in different solution environments. The effect of reaction conditions (cross-link degree, grafting degree, and SiO2 concentration) and testing conditions (contact time and pH) on the adsorption efficiency were investigated, and the adsorptive mechanism was proposed. In
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MATERIALS AND METHODS
Materials. Polyethylenimine with a molecular weight (Mw) of 25 000 (PEI25000), acrylamide (AAm), N,N′-methylenebis(acrylamide) (MBAA), ammonium persulfate (APS), N,N,N′,N′-tetramethylethylenediamine (TMEDA), lanthanum chloride (LaCl3), neodymium chloride (NdCl3), europium chloride (EuCl3), dysprosium chloride (DyCl3), ytterbium chloride (YbCl3), HCl, NaOH, and KOH were purchased from Sigma−Aldrich (St. Louis, MO) and used as arrived. A pH 8.6, citrate-based buffer solution was prepared and used in some experiments for simultaneously releasing/desorbing the REEs and regenerating the adsorbent. Further details of the buffer solution cannot be revealed, because of the filing of a patent.40 SiO2 macro particles (80 μm) was obtained from PQ Corporation (Malvern, PA). Preparation of Sorbents. PEI-pAAm-SiO2 hydrogel was prepared via thermopolymerization by grafting AAm monomer onto PEI25000 as a template polymer in the presence of MBAA as a cross-linker at 70 °C. Typically, 30 g of 25 wt % PEI25000 was degassed for 30 min by purging with nitrogen gas in a flask. Then, 0.3 g APS was added and the resulting mixture was gently stirred for an additional 15 min at 70 °C to generate free radicals on PEI. Meanwhile, a well-mixed solution of AAm (30 g), MBAA (6.48 g), SiO2 (6 g), and 120 mL of deionized (DI) water was degassed for 20 min and transferred into the flask. Following this, 1 mL of the reducing agent TMEDA was added. The solution was stirred at 300 rpm under a nitrogen atmosphere. After 2 h of reacting, the nitrogen purge gas was stopped and the flask was sealed to prevent solvent loss. The mixture was stirred for ∼24 h to finish the reaction. The resulting products were precipitated solid grains, which were washed repeatedly with DI water until the solution became clear. The precipitate then was dialyzed for an additional 24 h in a 2 L water bath, with five complete water changes. Finally, the grains were filtered and dried in an oven at 70 °C overnight to produce the final sorbents. The sorbents with different recipes were prepared according to the same procedure. Characterization of Sorbents. The morphologies of the hydrogels were assessed on a field-emission scanning electron microscopy (FESEM) system that was equipped with secondary and backscatter electron detectors (FEI, Model Quanta 600F). SEM images were analyzed by ImageJ (Apache License). The atomic element composition of the hydrogels was analyzed by X-ray photoelectron spectroscopy (XPS) (Physical Electronics, Model PHI 5600 ci). XPS measurements were conducted using Al Kα X-ray source with a monochromatic incident photon energy of 1486.6 eV, combined with charge neutralization. The XPS samples were measured at a 45° takeoff angle and were kept under high vacuum (∼10−9 Torr) during measurement. The pass energy of the hemispherical analyzer was 58.7 eV, and the scan step size was 0.8 eV for survey scans over the binding energy range of 0−1000 eV. The XPS spectra were then calibrated using the binding energy of adventitious carbon at 284.8 eV. The chemical structures of the solid sorbents were determined by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) (Thermo Scientific). The samples were ground to small particles and loaded into the DRIFTS cell. The cell directs IR light into the sample bed, which diffusely scatters unabsorbed light back to the IR detector to provide information about the bulk chemical features of the sorbent. The cell plus sample were then heated at 105 °C for 20 min in flowing N2 to remove preadsorbed water and CO2 from the ambient air, and then were cooled to 55 °C. The single-beam spectra were collected from 400 cm−1 to 4000 cm−1 and averaged from 25 accumulated scans at a resolution of 4 cm−1. Water Swelling Behavior of Sorbents. A quantity of 0.5 g of sorbents was placed into 8 mL of DI water, and the mixture was placed into a 10 mL syringe that was equipped with a 0.2 μm PVDF syringe filter (Whatman, GE Healthcare). Water was squeezed out after contacting the sorbents from 5 min to 24 h. An increase in sorbent 10948
DOI: 10.1021/acssuschemeng.7b02851 ACS Sustainable Chem. Eng. 2017, 5, 10947−10958
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ACS Sustainable Chemistry & Engineering weight was measured, and the sorbent swelling rate was calculated as shown in eq 1:
swelling (%) =
W3 − W1 × 100 W2 − W1
the REE adsorption sites. When using the citrate based buffer as an eluent, an adsorption−desorption cycle was terminated by desorbing the sorbent with 20 mL of 5% citrate-based buffer solution (pH 8.6). When using 1 M HCl solution as an eluent, an adsorption−desorption cycle was terminated by desorbing the sorbent with 20 mL of 1 M HCl, followed by regeneration with 20 mL of 1 M KOH solution and 50 mL of DI water. The flow through eluent was collected for ICP-MS analysis. The REEs adsorption ratios for each flow cycle were calculated as shown in eq 6, desorption ratios for each cycle were determined as shown in eq 7, and recycle ratios for each cycle was calculated as shown in eq 8:
(1)
where W3 is the weight of the swollen sorbent with the syringe and filter, W2 is the weight of dry sorbent with the syringe and filter, and W1 is the weight of the syringe and filter. Inductively Coupled Plasma Mass Spectrometry (ICP-MS) Analysis. Inductively coupled plasma−mass spectrometry (ICP-MS) (PerkinElmer, Model Nexion 300 D) was used to analyze metal concentrations as reported in our previous work.40 The instrument was operated in kinetic energy discrimination (KED) mode, using helium as the collision gas. Standard reference materials (SRM) were analyzed in parallel with each batch of the studied samples for general validation of ICP-MS sample analysis repeatability. The standard reference materials used in this study were prepared by the United States Geological Survey and include natural waters spiked with reagent-grade chemicals (T221, T225, and T227). Data accuracy was determined using recovery of SRM and spiked sample recovery. During ICP-MS data collection, 3 replicates were measured with 10 sweeps per replicate. These values are averaged to give each data point. In addition, at least 1 sample out of every 10 was prepared and analyzed in duplicate to give an estimate of precision. The detection limits under different testing conditions are listed in Table S1 in the Supporting Information. REEs Adsorption Behavior of Sorbents with Batch Method. Stock solutions of 500 ppm LaCl3, NdCl3, EuCl3, DyCl3, and YbCl3 were prepared, and the working solutions were diluted from the stock solution. REE solutions with different pH values were adjusted by 0.1 M NaOH and 0.1 M HCl solutions. Generally, for REE adsorption tests, 0.5 g of a sorbent was placed into 20 mL of 100 ppm REE solution with stirring at 100 rpm. The sorbents were then removed by filtering the solution through a 0.2 μm PVDF syringe filter. The filtered solution was then analyzed by ICP-MS for trace rare-earth elements. The REEs adsorption ratios for batch method were calculated based on eq 2:
Ln 3 + adsorption (%) =
C i − Cf × 100 Ci
Ln 3 + adsorption (%) =
C i − Cads × 100 Ci
(6)
Ln 3 + desorption (%) =
Cdes × 100 C i − Cads
(7)
Ln 3 + recycle (%) =
Cdes × 100 Ci
(8)
where Ci is the initial concentration of REEs, Cads the concentration of flow through REEs solution after adsorption, and Cdes the desorption concentration of REEs.
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RESULTS AND DISCUSSION Characterization of PEI-pAAm-SiO2 Hydrogel Sorbent. PEI-pAAm-SiO2 hybrid gel sorbents were synthesized via free radical-initiated graft polymerization (see Scheme 1). APS and Scheme 1. Preparation of Grafted Hybrid Hydrogel Network for Adsorption of Rare-Earth Metal
(2)
where Ci and Cf are the initial and final concentrations of REEs, respectively. Adsorption Kinetics of REEs in the Batch Method. Kinetic studies were performed by shaking sorbent-REE solutions from 5 min to 6 h and analyzing the filtered solutions taken at different times via ICP-MS. Pseudo-first-order (eq 3), pseudo-second-order (eq 4), and intraparticle diffusion (eq 5) models were used to simulate the adsorption kinetics.35,36,41
log(Q e − Q t) = log Q e − k lt
(3)
t 1 t = + Qt Qe (k 2Q e 2)
(4)
Qt = k idt 0.5 + C id
(5)
TMEDA as a redox initiator pair generated SO4− · free radicals while heating the solution to 70 °C.42 Amine groups on PEI were presumably oxidized by SO4− · free radicals. The free radicals were simultaneously transferred to AAm monomers, followed by a cross-linking reaction of MBAA with PEI.43,44 SiO2 particles were entrapped into the hydrogel matrix as a filler to increase the surface area of the final hydrogel product.25,28 SEM, DRIFTS, and XPS were used to characterize and reveal the morphology and structure of hydrogel sorbents (see Figure 1). A typical sorbent prepared with a starting mixture of 3.4 wt % MBAA, 0.16 wt % APS, and 3.1 wt % of SiO2 was used for characterizations. First noticed was that the dry sorbents were light yellow powders, resulting from PEI oxidation during the graft polymerization (Figure 1a). Surface porosity had great impact on the swelling behavior of sorbents, and would affect metal ion penetration into the networks during REE adsorption tests. The SEM analysis showed that the macrostructure of the sorbent is composed of close-packed fiberlike structures with an average diameter of ∼3.5 μm. Generally, irregular porous structures with dimensions ranging from ∼0.6 μm × 0.6 μm to ∼1.7 μm × 9.6 μm were observed (see Figure 1b). To determine the chemical structure of the formed product, the IR spectra of the pure PEI and the sorbent were collected and are
where Qe and Qt (mg/g) represent the adsorption capacities of sorbents at equilibrium and at time t, respectively; k1, k2, and kid represent adsorption rate constants for pseudo-first-order, pseudosecond-order, and intraparticle diffusion models, respectively. Adsorption−Desorption Behavior of REEs with the Flow Column Method. The reusability of the hydrogel sorbents was studied by using five consecutive adsorption−desorption cycles. 0.5 g of sorbent were settled in a packed-bed REE adsorption column (diameter = 1 cm, length = 5 cm). Twenty milliliters (20 mL) of AMD solution was flowed through the sorbent at a rate of 0.5 mL/min. The resulting solutions were collected in a vial and analyzed for their REE concentrations with ICP-MS. Either a citrate-based buffer solution (5% in DI water, pH 8.6) or HCl solution (1 M, pH 0.36) was used to remove the adsorbed ions from the sorbent materials and regenerate 10949
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Figure 1. (a) Optical image of a typical hydrogel sorbent, (b) typical SEM image of sorbents, (c) DRIFTS spectra of PEI and sorbent, and (d) elemental analysis of sorbent with SiO2 (blue trace) and sorbent without SiO2 (black trace) by XPS.
Figure 2. (a) 100 ppm La3+ adsorption by sorbents prepared with different MBAA concentrations (wt %); (b) water adsorption of sorbents prepared with different MBAA concentrations (wt %); (c) 100 ppm La3+ adsorption of sorbent prepared with different APS concentrations (wt %); and (d) 100 ppm La3+ adsorption of sorbents prepared with different SiO2 concentrations (wt %). [Adsorption conditions: pH 5.5 ± 0.1, contact time = 40 min, contact temperature = 25 °C, and stirring speed = 120 rpm.]
NH2 groups of PEI and AAm. A small peak at 1190 cm−1 was assigned to the Si−O−Si asymmetric stretching vibrations, indicating that SiO2 was trapped inside the polymer networks.48,49 To further identify the existence of SiO2 in the three-dimensional (3-D) networks, elemental analyses of the sorbent with and without SiO2 were compared using XPS (Figure 1d). Major components identified included O, N, and C of the organic polymer networks, and Si of the inorganic SiO2 trapped inside the organic networks. These data are consistent with the DRIFTS spectra. Effects of Various Factors on Hydrogel Adsorption Capacity. The adsorption capacity of sorbents is governed by a
shown in Figure 1c. Prior to the obtaining the IR spectra, all the samples were heated in the IR cell to 105 °C for 20 min in flowing N2 to remove preadsorbed water and carbon dioxide gas from the ambient atmosphere. The characteristic bands of pure PEI at 3600−3200 cm−1 are for N−H stretching vibrations, 2945−2850 cm−1 are the C−H stretching vibrations, 1605 cm−1 is the N−H bending vibrations, 1463 cm−1 is the C−H bending vibration, and 1350−1000 cm−1 are the C−N stretching vibrations.45−47 An absorption peak for secondary amines generated at 3460 cm−1, along with the suppression of the N−H bending band at 1605 cm−1, shows that the primary amines were consumed by the grafting reaction between the − 10950
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Figure 3. Adsorption kinetics of sorbent in contact with different REE metal solutions, using the modeled pseudo-second-order kinetics equation. [Adsorption conditions: Ci = 100 ppm, pH 5.5 ± 0.1, contact temperature = 25 °C, and stirring speed = 120 rpm.]
achieved within 20 min of immersion during a rapid uptake phase, then a slower equilibrium process followed (see Figure S1 in the Supporting Information). Ferfera-Harrar et al. found that an increased MBAA content resulted in strongly crosslinked networks, restricting the flexibility of PAAm chains, which, in turn, reduced the free spaces and restricted water diffusion into the matrix.54 The reduced total water uptake capacity (swelling ratio) with increased cross-linker concentration indicates a decrease in hydrogel pore size and restricted chain relaxation. Sargin et al. studied the effect of cross-linker on ion adsorption performance. They prepared microcapsule sorbents with three different cross-linker concentrations. Sorbent prepared with 0.9 mL of cross-linker exhibited better Cu(II) ion sorption performance than sorbent prepared with 0.3 or 1.5 mL of cross-linker. The degree of cross-linking affected the shape of microcapsules and their performance toward ion adsorption.52 In addition to a relatively rigid pore structure yet flexible overall polymer network, as implied by the data in Figures 2a and Figure 2b, the optimum proximity of active REE adsorption sites is critical in forming amine−REE complexes. The degree of grafting was controlled by varying the APS ratio. APS generates free radicals onto primary amines, providing grafting sites for pAAm growth. Upon tethering with pAAm, PEI randomly becomes attached to/dispersed within the cross-linked pAA network, preventing PEI loss when the hydrogel sorbent is in contact with aqueous solutions. While fixing the other reaction parameters, the APS concentration was varied from 0.05 wt % to 0.26 wt %. Figure 2c shows that the maximum La3+ recovery ratio reached ∼82% when 0.16 wt % APS was used. APS is highly hygroscopic and begins to degrade almost immediately when coming into
combination of factors, such as physical ion penetration, chemical binding of the ion by the active adsorption sites, and elasticity of the polymer chains of the 3-D matrix possessing the chelating groups.33 Three key physical factors affecting the adsorption capacities of our hydrogels were investigated, including cross-linking degree, grafting degree, and SiO2 concentration. A 100 ppm La3+ solution at pH 5.5 ± 0.1 was used to evaluate sorbent uptake capacity prepared with different compositions. The cross-linking degree of the hydrogel presumably controls pore size and chain flexibility of polymerized 3-D networks and can dictate the metal penetration speed and the coordination of REE chelating groups. Cross-linking not only makes polymers mechanically stronger, but also more selective toward some metal ions. An optimized cross-linker concentration is able to advance metal uptake capacity of the hydrogel sorbent by optimizing free space for metal diffusion and metal-sorbent interaction.50−52 The effect of cross-link degree between MBAA (cross-linker) and AAm was investigated by varying the MBAA ratio from 2 wt % to 7.1 wt %, corresponding to a cross-linking degree of 11.7%−43.3%. As shown in Figure 2a, the La3+ recovery ratio of sorbents increased to ∼82% when up to 3.4 wt % of MBAA was used (cross-linking degree of ∼21.7%), then decreased with further increases in cross-linker concentration. Water uptake capacity and pore size were shown to have a direct relationship, where a higher swelling degree of sorbents correlated to a larger pore size and more flexible polymer chains.53 To study the porosity change of sorbents with different cross-linker concentrations, water uptake behavior of sorbents was studied and the results are shown in Figure 2b. The observed water uptake behavior for all materials are very similar. Meaning that 90% of equilibrium swelling was 10951
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Figure 4. Kinetic modeling of REEs adsorption, using the modeled intraparticle diffusion kinetics equation. [Conditions: Ci = 100 ppm, pH 5.5 ± 0.1, contact temperature = 25 °C, and stirring speed = 120 rpm.]
0.1 g of SiO2 as an inorganic filler.24 The hydrogel prepared with 0.03 g of SiO2 showed the highest sorption capacity toward the rare-earth ions, which is consistent with our discovery. Generally, the best sorbent with an optimized La3+ recovery ratio of 82% was achieved by using the recipe with 3.4 wt % MBAA, 0.16 wt % APS, and 3.1 wt % of SiO2. This sorbent will be used in the following adsorption kinetics study, pH effect on REE adsorption, and recycle of REEs from AMD. REE Adsorption Kinetics. Contact time of the feed solution with the sorbent is an important parameter that affects the completion of the REE-sorbent adsorption reaction. Optimizing the contact time is critical for minimizing the amount of sorbent needed to achieve a fixed percentage of REE removal.35,36,58,59 Figure 3 presents the kinetic data of the sorbent in contact with 100 ppm single-element REE solutions at pH 5.5. More than 80% of the REEs were adsorbed within 40 min, indicating fast adsorption kinetics. A selective adsorption behavior toward REEs was observed, where heavy rare-earth elements (HREEs) (e.g., Eu, Dy, Yb) had faster adsorption kinetics and light rare-earth elements (LREEs) (e.g., La, Nd) had slower kinetics. LREEs achieved adsorption equilibrium after ∼120 min, while HREEs achieved adsorption equilibrium after ∼40 min. A similar trend was reported for REE sorption on clay minerals14 and nanoiron sorbents.60 This behavior was attributed to the decrease in ionic radius with the increasing atomic number. LREEs may form hydration complexes with higher coordination numbers, because of their larger size, compared to HREEs, restricting the bulk diffusion of LREE ions toward the chelating sites.14,15,61 These data could help tune the overall selectivity of our sorbents toward specific REEs simply by adjusting the contact times.
contact with water, resulting in a partial loss of activity.55 When the APS ratio is 0.16 wt %, the extra APS should convert more amine sites (can capture REEs) to grafting sites (cannot capture REEs). This would greatly reduce the amount of metal binding functional groups and change PEI chain flexibility in the 3-D networks. These changes would lead to a decrease in REE uptake of the hydrogels. A similar APS effect in graft polymerization was also observed in the preparation of superabsorbent hydrogels.26,56 Studies have shown that the incorporation of inorganic fillers, such as ashes, clays, and inorganic particles, into organic polymeric networks can improve or provide new properties of the original organic materials.23−26 Incorporating fine inorganic fillers into a hydrogel network can compensate for this loss by providing extra binding sites, selectivity, and diffusion channels for metal ions. As shown in Figure 2d, the recovery of La3+ increased remarkably from ∼34% at 0% SiO2 to ∼82% at 3.1 wt % SiO2. The inorganic phase of the hybrid hydrogel provided additional cross-linking points in the 3-D network by introducing physical and chemical reactions between inorganic material and the polymer matrix.26,57 In addition, SiO2 particles also greatly enhanced the surface area and porosity of the sorbent, giving higher REE adsorption capacity.28 However, any further increase of the inorganic content will reduce the ion uptake capacity, because the key factor of our hybrid gel sorbent that affects REE uptake capacity is the amine functional groups. A continuously decreased organic content will eventually reduce REE uptake capacity. Similarly, Borai et al. prepared a series of hydrogels with 0.01− 10952
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ACS Sustainable Chemistry & Engineering The pseudo-first-order model, pseudo-second-order model, and intraparticle diffusion model were fitted to the experimental data to elucidate the adsorption rate and rate-determining step. The pseudo-first-order model did not predict the REE adsorption capacities of the sorbent, as evidenced by poor model fitting to the experimental data (Figure S2 in the Supporting Information). Thus, the pseudo-second-order model, which supposes the chemical surface reaction to be the rate-determining step, was applied. The results in Figure 3 demonstrate that the studied REE elements had highly significant linear relationships between t/Qt and t with high correlation coefficients (R > 99.5%). This suggests that the ratedetermining step of the adsorption process is chemical sorption.59 Our sorbent is a typical amine-based sorbent containing an interpenetrated PEI and pAAm organic network with inorganic SiO2 filler. Amine groups were found to have strong reactivity with Ln3+ ions and are responsible for the uptake of Ln cations by a chelation mechanism in our materials.62 In addition, stable amine−Ln complexes were evidenced by N−Ln infrared vibrational bands.63,64 Collectively, these studies and our experimental data support the hypothesis that the amine-REE chelating process is both pseudo-second order and controls the overall REE adsorption rate. Furthermore, an intraparticle diffusion model was fit to the REE uptake data. This was done to investigate the effect of REE pore diffusion on the overall REE mass-transfer processes occurring in the system (Figure 4). Three portions of linear fitting of the model to the data suggests that REE mass transfer occurred in three steps, which are bulk-exterior surface diffusion (kid,1), mesopore diffusion (kid,2), and micropore diffusion (kid,3).65,66 Rate constants for each step listed in Table 1 follow
Figure 5. Effect of pH on the adsorption capacity of REE metals.
atoms in PEI are protonated.67 At low pH, the 3-D networks of our sorbent likely had a higher cationic degree, because most of the −NH2 and −NH groups were converted to −NH3+ and −NH2+ ammonium ions, respectively. Thus, Ln3+ ions would be repelled from the sorbent’s positively charged surface, resulting in low REE uptake capacity. By increasing solution pH, the electrostatic repulsion between the adsorbent and Ln3+ ions is gradually decreased, because of diminished formation of ammonium ions. As a result, electrostatic attractions become the dominant force between the hydrogel sorbent and REEs at higher pH values. Hence, REE adsorption capacity increased by increasing the solution pH. In addition, water as a weak Lewis base initially coordinated with REEs in the fresh solution. Upon adjusting the fresh solution at pH ∼5.5 to low pH (∼2.5), the smaller coordinated H2O molecules were converted to larger H3O+ species. This increased the overall volume of the coordinated complexes and prevented bulk diffusion of the Ln3+ ions toward the chelating site. Recovery of REEs from Simulated AMD Solution. Recovery/adsorption and release/desorption of REEs is an important concern for sorbent development. It is critical and quite challenging to recycle REEs from actual water sources, because of their extremely low concentration (ppt−ppb), compared to the concentration of fouling metals. To assess the practical application of our optimized hydrogel sorbent, we evaluated its reusability by recovering REEs from simulated AMD solution, followed by REE release and sorbent regeneration.12,40 Table 2 lists the composition of the AMD
Table 1. Fitted Intraparticle Diffusion Model Kinetic Parameters for the Removal of Rare-Earth Elements (REEs) La Nd Eu Dy Yb
kid,1
R2,1
kid,2
R2,2
kid,3
R2,3
0.42 0.52 0.62 0.60 0.72
0.99 0.99 0.94 0.97 0.94
0.15 0.19 0.14 0.15 0.056
1 0.87 0.96 0.97 0.78
0.011 0.023 0.013 0.012 0.0099
0.68 0.99 0.99 0.98 0.78
the order of kid,1 > kid,2 > kid,3, and they indicate that the faster adsorption step was external bulk phase diffusion to the exterior surface of the sorbent. The parameter kid,3 is close to zero, supposing weak diffusion within micropores, which controlled the overall adsorption rate. Adsorption of REEs at Different pH Values. To maximize the removal of REEs with sorbents, it is crucial to optimize solution pH. This is because acidity of the solution affects the ionization of REE ions and the concentration of the counter H+ ions, which would protonate and deactivate the sorbent’s amine chelating groups. It was reported that REE ions will form hydroxide precipitates under alkaline pH conditions;35,36 thus, the adsorption of REEs at different pH values (from pH 2.5 to pH 6.5) was studied on our optimized sorbent (3.4 wt % MBAA; 0.16 wt % APS; 3.1 wt % SiO2) in Figure 5. It is remarkable to see that REE adsorption capacities almost doubled for La, Eu, and Yb, increased by ∼20% for Nd, and increased by ∼30% for Dy by raising the pH from 2.5 to 6.5. Amine groups from PEI and AAm are the main functional groups in the 3-D networks and are highly sensitive to solution pH. Amara et al. found that, in acidic solution, 77% of the N
Table 2. Simulated AMD Solution Composition Determined by ICP-MS Analysisa element Na Mg Al Ca Mn Fe a
ppm 276.4 10.2 13.6 58.1 54.0 265.2
± ± ± ± ± ±
element 8.2 0.4 0.3 1.9 0.8 5.4
La Nd Eu Dy Yb
ppb 15.5 24.8 2.6 8.4 4.5
± ± ± ± ±
0.3 0.3 0.02 0.1 0.1
At pH 2.4 ± 0.1, based on an average of three observations.
solution, where the concentrations of REE ions are at the ppb level. This concentration is ∼104−105 times lower than the concentration of fouling metals. Because adsorption is pHdependent, acidic eluents, such as HCl and HNO3, were reported for REE ion release.36,59,68 Here, we have compared the regeneration effect of 1 M HCl solution and 5% citratebased buffer solution as eluents in five adsorption−desorption 10953
DOI: 10.1021/acssuschemeng.7b02851 ACS Sustainable Chem. Eng. 2017, 5, 10947−10958
Research Article
ACS Sustainable Chemistry & Engineering
Figure 6. Adsorption (%), desorption (%), and recycle (%) of AMD elements in contact with sorbent for five adsorption−desorption cycles with 5% citrate-based buffer as an eluent. [Cn is the cycle number (where n = 1, 2, 3, 4, 5), and AC denotes a citrate-based buffer; for each metal, the bars from left to right represent Cycles 1−5, respectively.]
ideal for the collection of valuable REEs (see Figure S3 in the Supporting Information). Starting from the second cycle, the adsorption ratio of fouling metals gradually increased and their desorption ratio from the second cycle is ∼48%−82% for Na, ∼35% for Mg, ∼92%−98% for Al, ∼69%−77% for Ca, 94%− 98% for Mn, and ∼71%−80% for Fe, respectively. LannicelliZubiani et al. studied the capture−release of REEs with clays as sorbent material and nitrate acid as an eluent.18 At pH 1, protons fully replaced the captured La and Nd ions. Instead of the simple pH effect on sorbent material, an ion-exchange mechanism was proposed prevalent in the reaction. Similarly, in our work, the acid solution (pH 0.36) introduced abundant H+ ions, which displaced metal ions from the sorbent binding sites. On the other hand, they also found that the strong acid caused an interlayer shrinkage of the clay material, leading to an increased competitiveness of fouling metals toward Nd. The restriction of the sorbent matrix is important for the formation of artificial recognition sites in a polymer matrix.69 The gradually increased REE adsorption during cycling probably corresponded to shrinkage of the entrapped SiO2 by acid wash. The shrinking of silica served to imprint new REE adsorption sites within the PEI-pAAm polymer network by detaching from the network and exposing newly available amine sites for ionsorbent chelation.38 Unlike the acid REE release method, the 5% citrate-based buffer (pH 8.6) REE release method showed a high separation
cycles. We defined the adsorption (%) as the ratio between the amount of adsorbed metal ion and the amount of metal ion in the original solution (see eq 6). The desorption (%) was defined as the ratio between the amount of metal ions washed off and the amount of metal ion adsorbed by the sorbent (see eq 7). The recycle (%) was defined as the ratio between the amount of metal ion washed off and the amount of metal ion in the original solution (see eq 8). As shown in Figure 6 for the single-step citrated-based buffer and Figure S3 in the Supporting Information for the two-step acid (HCl)-base (KOH) solution, REE metals were successfully adsorbed and released at pH values as low as 2.4 and REE concentrations as low as 2.6−24.4 ppb in all five cycles, under both elution conditions. Almost 100% REEs via HCl method and ∼80% REEs via the citrate-based buffer method were recycled in the five consecutive adsorption−desorption cycles, demonstrating the high-performance of the hybrid gel sorbent for REE enrichment. In addition, our fresh sorbent showed high REE/fouling metal selectivity (SLn/FM) of 105−450 (Figure S4 in the Supporting Information) during the first three adsorption−desorption cycles. This shows the excellent performance of our sorbent in selectively adsorbing valuable REEs over unwanted fouling metals, despite a fouling metal/ REE ratio of ∼12 000/1. However, the HCl acid method not only washed REEs off, but also washed off most of the adsorbed fouling, which is not 10954
DOI: 10.1021/acssuschemeng.7b02851 ACS Sustainable Chem. Eng. 2017, 5, 10947−10958
Research Article
ACS Sustainable Chemistry & Engineering
volume of 1.36 and 2.06, 10−30-fold of the HREEs were enriched, while LREEs and other base metals were not enriched.73 The same material exhibited a
