SiO2 Hybrid Hydrogel Sorbent for

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Novel Polyethylenimine-Acrylamide/SiO2 Hybrid Hydrogel Sorbent for Rare Earth Elements Recycling from Aqueous Sources Qiuming Wang, Walter Christopher Wilfong, Brian Kail, Yang Yu, and McMahan L. Gray ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02851 • Publication Date (Web): 14 Sep 2017 Downloaded from http://pubs.acs.org on September 17, 2017

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ACS Sustainable Chemistry & Engineering

Novel Polyethylenimine-Acrylamide/SiO2 Hybrid Hydrogel Sorbent for Rare Earth Elements Recycling from Aqueous Sources Qiuming Wang,a,b,* Walter C. Wilfong,a,b Brian Kail, a,c Yang Yu,a,c and McMahan Graya,* a

U.S. Department of Energy, National Energy Technology Laboratory, 626 Cochrans Mill Road,

Pittsburgh, PA 15236, United States b

Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831, United States

c

AECOM, 626 Cochrans Mill Road, Pittsburgh, PA 15236, United States

*

Corresponding authors

*

Qiuming Wang

626 Cochrans Mill Road, Pittsburgh, PA 15236, United States Email: [email protected] Phone: (+1) 412-386-4922 *

McMahan Gray

626 Cochrans Mill Road, Pittsburgh, PA 15236, United States

ACS Paragon Plus Environment

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Email: [email protected] Phone: (+1) 412-386-4826

ABSTRACT: Recycling rare earth elements (REEs) becomes increasingly important due to their supply vulnerability and increasing demands in industry, agriculture, and national security. Hybrid hydrogel sorbents were outstanding due to their high stability and selectivity. Organicinorganic hybrid hydrogels were synthesized by thermo-polymerization of acrylamide onto PEI polymer chain with N,N’-methylene bisacrylamide as a crosslinker. The grafted network was evidenced by DRIFTS and XPS. The porous structure was observed by SEM. Crosslink degree, PEI grafting degree, and SiO2 concentration were studied to optimize the REEs adsorption. The pH value of the medium greatly affected REE adsorption capacity, where the nearly 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 5 adsorptiondesorption 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 due to their unique catalytic, optical, and magnetic properties. In 2010, the global REEs production was around 133,600 tons, with an estimated yearly demand of 136,100 tons. 1 About 95% of the world's supply comes from a few localities in China.

2

However, China reduced

REEs production and exportation in recent years for environmental concerns and to protect domestic industries.

3

Accordingly, the price of REEs is increasing rapidly. The price of

dysprosium metal soared from $250/kg in April 2010 to $2,840/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 and bone after entering 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 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 run-off (AMD) is regarded as an environmental pollution concern and due to their contamination of nearby stream waters and well waters with appreciable amounts of dissolved REEs and additional fouling metals/non-


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REEs. Because of the overall low concentrations of dissolved REEs, it is a challenging task to selectively recover them from water sources. REEs in AMD or in rare earth mining effluent are 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 co-exist with highly

dissolved ppm (mg/L) levels of transition metals and post transition metals, such as copper, iron, aluminum, manganese, etc..

8, 10

Due to 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’s 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 cheap but have lower absorption efficiency. 18, 19 Clays modified with chelating agents greatly enhanced adsorption efficiency, but had poor tolerance towards harsh chemical environments. 20 Hydrogels, known as highly porous cross-linked polymer networks, were proven an excellent sorbent 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


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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 but a deep investigation into REE recovery using hydrogel is lacking. REE hydrogels have been investigated in any details.

21, 23, 29-33

24, 34-39

Only a small number of

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 opencellular hydrogel adsorbents for La3+ and Ce3+ adsorption.

36

One of the hydrogel sorbents

reached adsorption equilibrium in 30 min and had an adsorption capacity of 384.62 mg/g for La3+ and 333.33mg/g for Ce3+, respectively. 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 addition, the adsorptive reusability of the sorbent for REEs recycle were studied through 5 adsorption-desorption cycles in simulated recovery from a synthetic acid mine drainage solution. MATERIALS AND METHODS


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Materials. Polyethylenimine with 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 simultaneously releasing/desorbing the REEs and regenerating the adsorbent. Further details of the buffer solution cannot be revealed due to 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 thermo-polymerization by grafting AAm monomer onto PEI25000 as a template polymer in the presence of MBAA as a crosslinker at 70 oC. Typically, 30 g of 25wt% 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 oC 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 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 product were precipitated solid grains, which were washed repeatedly with DI water until the solution became clear. Then the precipitate was dialyzed for an additional 24 h in a 2 L water bath, with 5 complete water changes. Finally, the grains were filtered and dried in an oven at 70 oC overnight to produce the


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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 FEI Company Quanta 600 field-emission scanning electron microscope equipped with secondary and backscatter electron detectors scanning electron microscope (SEM, FEI 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, 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° take-off angle and were kept in high vacuum of ~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 into 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 sorbents bulk chemical features. The cell plus sample were then heated at 105 oC for 10 min in flowing N2 to remove preadsorbed water and CO2 from the ambient air, and were then cooled to 55 oC. The single-beam spectra were collected from 400 to 4000 cm-1 and averaged from 25 accumulated scans at a resolution of 4 cm-1. Water Swelling Behavior of Sorbents. 0.5 g of sorbents was placed into 8 mL of DI water in a 10 mL syringe equipped with a 0.2 µm PVDF syringe filter (WhatMan, GE Healthcare). Water


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was squeezed out after contacting the sorbents from 5 min to 24 h. An increase in sorbent weight was measured and sorbent swelling rate was calculated as shown in equation (1): Swelling %= (W3-W1) / (W2-W1) * 100

(1)

where W3 is the weight of the swollen sorbent with syringe and filter, W2 is the weight of dry sorbent with syringe and filter, and W1 is the weight of syringe and filter. Inductively Coupled Plasma Mass Spectrometry (ICP-MS) Analysis. Inductively Coupled Plasma Mass Spectrometry (ICP-MS, Perkin Elmer 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 He 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. Additionally, at least one sample out of every ten was prepared and analyzed in duplicate to give an estimate of precision. The detection limits in different testing conditions were listed in table S1. 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 0.1M 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


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analyzed by ICP-MS for trace rare earth elements. The REEs adsorption ratios for batch method were calculated based on equation (2): Ln3+ Adsorption % = (Ci-Cf)/Ci*100%

(2)

where Ci and Cf are the initial and final concentrations of REEs, respectively. REEs Adsorption Kinetics in 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 (equation 3), pseudo-second-order (equation 4), and intraparticle diffusion (equation 5) models were used to simulate adsorption kinetics. 35, 36, 41 lg(Qe-Qt)=lgQe-k1t

(3)

t/Qt=1/(k2Qe2)+t/Qe

(4)

Qt=kidt0.5+Cid

(5)

where Qe and Qt (mg/g) represent adsorption capacities of sorbents at equilibrium and at time t, respectively, k1, k2, and kid represents adsorption rate constants for pseudo-first-order, pseudosecond-order, and intraparticle diffusion models. Adsorption-Desorption Behavior of REEs with Flow Column Method. The reusability of the hydrogel sorbents was studied by 5 consecutive adsorption-desorption cycles. 0.5 g of sorbent were settled in a packed-bed REE adsorption column (diameter: 1cm; length: 5 cm). 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 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).


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When using 1 M HCl solution as 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 equation (6), desorption ratios for each cycle was shown in equation (7), and recycle ratios for each cycle was calculated as shown in equation (8): Ln3+ Adsorption % = (Ci-Cads)/Ci*100%

(6)

Ln3+ Desorption % = Cdes/(Ci-Cads)*100%

(7)

Ln3+ Recycle % = Cdes/Ci*100%

(8)

where Ci is the initial concentration of REEs, Cads is the concentration of flow through REEs solution after adsorption, Cdes is the desorption concentration of REEs, respectively. RESULTS AND DISCUSSION Characterization of PEI-pAAm-SiO2 Hydrogel Sorbent. PEI-pAAm-SiO2 hybrid gel sorbents were synthesized via free radical-initiated graft polymerization (Scheme 1). APS and TMEDA as a redox initiator pair generated SO4-·free radicals while heating the solution to 70 oC. 42

Amine groups on PEI were presumably oxidized by SO4-·free radicals. The free radicals were

simultaneously transferred to AAm monomers, followed by a crosslinking 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


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Scheme 1. Preparation of grafted hybrid hydrogel network for adsorption of rare earth metal. SEM, DRIFTS, and XPS were used to characterize and reveal the morphology and structure of hydrogel sorbents (Fig. 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 (Fig. 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 fiber-like structures with an average diameter of ~3.5 µm. In general, irregular porous structures with dimensions ranging from ~0.6x0.6 µm to ~1.7x9.6 µm were observed (Fig. 1b). To determine the chemical structure of the formed product, the infrared spectra of the pure PEI and the sorbent were collected and shown in Fig. 1c. Prior to the obtaining the IR spectra, all the samples were heated in the IR cell to 105 oC for 10 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 suppression of the N-H bending band at 1605 cm-1, shows that the primary amines were consumed by the grafting reaction between the –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 3-D networks, elemental analyses of the sorbent with and without SiO2 were compared using XPS (Fig. 1d). Major components identified included O, N and C of the organic polymer


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networks, and Si of the inorganic SiO2 trapped inside the organic networks. This data is consistent with the DRIFTS spectra.

Figure 1. (a) Optical image of a typical hydrogel sorbents; (b) typical SEM image of sorbents; (c) DRIFTS spectra of PEI and sorbent, and (d) element analysis of sorbent with SiO2 (Blue) and sorbent without SiO2 (black) by XPS. Effects of Various Factors on Hydrogel Adsorption Capacity. The adsorption capacity of sorbents is governed by a 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 crosslinking degree, grafting degree, and SiO2 concentration. 100 ppm La3+ solution at pH=5.5±0.1 was used to evaluate sorbent uptake capacity prepared with different compositions.


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Figure 2. (a) 100 ppm La3+ adsorption by sorbents prepared with different MBAA wt%; (b) water adsorption of sorbents prepared with different MBAA wt%, (c) 100 ppm La3+ adsorption of sorbent prepared with different APS wt%; and (d) 100 ppm La3+ adsorption of sorbents prepared with different SiO2 wt%. Adsorption conditions: pH: 5.5±0.1; contact time: 40 min; contact temperature: 25 oC; stir speed: 120 rpm. The crosslinking 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. Crosslinking not only makes polymers mechanically stronger, but also more selective toward some metal ions. An optimized crosslinker 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 crosslink degree between MBAA (crosslinker) and AAm was investigated by varying the MBAA ratio from 2 wt% to 7.1 wt%, corresponding to a cross link degree from 11.7~43.3%. As shown in Fig. 2a, the La3+ recovery ratio of sorbents increased to ~82% up to a 3.4 wt% of MBAA used (crosslinking degree of ~21.7%), then decreased with further increased crosslinker concentration. Water uptake capacity


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and pore size was 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 crosslinker concentrations, water uptake behavior of sorbents was studied and the results are shown in Fig. 2b. The observed water uptake behavior for all materials are very similar. Meaning, 90% of equilibrium swelling was achieved within 20 min of immersion during a rapid uptake phase, then a slower equilibrium process followed (Fig. S1). 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 crosslinker concentration indicates a decrease of hydrogel pore size and a restricted chain relaxation. Sargin et al. studied crosslinker effect on ion adsorption performance. They prepared microcapsule sorbents with 3 different crosslinker concentrations. Sorbent prepared with 0.9 mL of crosslinker exhibited better Cu(II) ion sorption performance than sorbent prepared with 0.3 or 1.5 mL crosslinker. Crosslinking degree 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 Fig. 2a and Fig. 2b, optimum proximity of active REE adsorption sites is critical in forming amine-REE complexes. Grafting degree 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 crosslinked 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%. Fig. 2c shows that the maximum La3+ recovery ratio reached ~82% when 0.16 wt% APS was used. APS


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is highly hygroscopic and begins to degrade almost immediately by contacting with water, resulting in partial loss of activity. 55 When the APS ratio is lower than 0.16 wt%, the free radical sites on PEI chains where AAm monomers could be grafted cannot be adequately formed, restricting the effective grafting process. On the other hand for reactions at APS ratios higher than 0.16 wt%, the extra APS should convert more amine sites (can capture REEs) into grafting sites (can’t 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 superabsorbent hydrogel preparation. 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 Fig. 2d, 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 crosslinking 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 in affecting REE uptake capacity is amine functional groups. A continuously decreased organic content will eventually reduce REE uptake capacity. Similarly, Borai et al. prepared a series of hydrogels with 0.01g-0.1 g SiO2 as an inorganic filler.

24

The


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hydrogel prepared with 0.03g SiO2 showed the highest sorption capacity towards the rare earth ions, which is consistent with our discovery. In general, 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 important parameter that affects the completion of REE-sorbent adsorption reaction. Optimizing the contact time is critical to for minimizing the amount of sorbent needed to achieve a fixed percentage of REE removal.

35, 36, 58, 59

Fig.3 presents the kinetic data of the sorbent in contact with 100 ppm

single-element REE solutions at a pH of 5.5. More than 80% of the REEs were adsorbed within 40 mins, indicating fast adsorption kinetics. A selective adsorption behavior towards REEs was observed, where heavy rare earth elements (HREEs, Eu, Dy, Yb) had faster adsorption kinetics and light rare earth elements (LREEs, La, Nd) had slower kinetics. LREEs achieved adsorption equilibrium at about 120 min, while HREEs achieved adsorption equilibrium at about 40 min. A similar trend was reported for REE sorption on clay minerals

14

and nano-iron 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 towards the chelating sites. 14, 15, 61

This data could help tune the overall selectivity of our sorbents towards specific REEs

simply by adjusting the contact times. 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


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pseudo-first-order did not predict the REE adsorption capacities of the sorbent as evidenced by poor model fitting to the experimental data (Fig. S2). Thus the pseudo-second-order model, which supposes the chemical surface reaction to be the rate-determining step, was applied. The results in Fig. 3 demonstrate that the studied REE elements had highly significant linear relationships between t/Qt and t with high R-correlation coefficients (> 99.5%). This suggests that the rate-determining 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. addition, stable amine-Ln complexes were evidenced by N-Ln infrared vibrational bands.

62

In

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 to investigate the effect of REE pore diffusion on the overall REE mass transfer processes occurring in the system (Fig. 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 the order of kid,1>kid,2>kid,3, and indicate that the faster adsorption happened at bulk phase to the exterior surface of sorbent. kid,3 is close to zero, supposing weak diffusion within micropores, which controlled the overall adsorption rate.


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Figure 3. Adsorption kinetics of sorbent in contact with different REE metal solutions with modeled pseudo-second-order kinetics equation. Adsorption conditions: Ci: 100 ppm, pH: 5.5±0.1, contact temperature: 25 oC; stir speed: 120 rpm.


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Figure 4. Kinetic modeling of REEs adsorption with modeled intraparticle diffusion kinetics equation. (Ci: 100 ppm, pH: 5.5±0.1, contact temperature: 25 oC; stir speed: 120 rpm.) Table1. Fitted intraparticle diffusion model kinetic parameters for the removal of REEs. kid,1

R2,1

kid,2

R2,2

kid,3

R2,3

La

0.42

0.99

0.15

1

0.011

0.68

Nd

0.52

0.99

0.19

0.87

0.023

0.99

Eu

0.62

0.94

0.14

0.96

0.013

0.99

Dy

0.60

0.97

0.15

0.97

0.012

0.98

Yb

0.72

0.94

0.056

0.78

0.0099

0.78


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Page 20 of 34

Adsorption of REEs at Different pH Values. To maximize the removal of REEs with sorbents, it’s crucial to optimize solution pH. This is because acidity of the solution affects 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, 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 Fig. 5. It is remarkable to see that REE adsorption capacities almost doubled for La, Eu, and Yb, increased ~20% increased for Nd, and increased ~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 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 were groups were converted into -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 due to 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 into larger H3O+. This increased the overall volume of the coordinated complexes and prevented bulk diffusion of the Ln3+ ions toward the chelating site.


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Figure 5. Effect of pH on the adsorption capacity of REE metals. Recovery

of

REEs

from

Simulated

AMD

Solution.

Recovery/adsorption

and

release/desorption of REEs is an important concern for sorbents development. It’s critical and quite challenging to recycle REEs from actual water sources due to their extremely low concentration (ppt-ppb) comparing with the concentration of fouling metals. For assessing 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 solution, where the concentrations

of REE ions are in ppb level. This concentration is ~ 104-105 times lower than the concentration of fouling metals. Because adsorption is pH dependent, acidic eluents, such as HCl and HNO3, were reported for REE ion release. 36, 59, 68 Here we compared the regeneration effect of 1 M HCl solution and 5% citrate-based buffer solution as eluents in 5 adsorption-desorption cycles. We defined the adsorption% as the ratio between the adsorbed metal ion and the metal ion in the original solution (equation 6). The desorption% was defined as the ratio between the wash off


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Page 22 of 34

metal ion and the metal ion adsorbed by sorbent (equation 7). And the recycle% was defined as the ratio between the wash off metal ion and the metal ion in the original solution (equation 8). Table 2. Simulated AMD solution composition determined by ICP-MS analysis (pH=2.4±0.1, an average of 3 observations). Element ppm

Element ppb

Na

276.4 ± 8.2

La

15.5 ± 0.3

Mg

10.2 ± 0.4

Nd

24.8 ± 0.3

Al

13.6 ± 0.3

Eu

2.6 ± 0.02

Ca

58.1 ± 1.9

Dy

8.4 ± 0.1

Mn

54.0 ± 0.8

Yb

4.5 ± 0.1

Fe

265.2 ± 5.4


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Figure 6. Adsorption %, desorption %, and recycle % of AMD elements in contact with sorbent for 5 adsorption-desorption cycles with 5% citrate-based buffer as eluent. (Cn: cycle number, n=1, 2, 3, 4, 5; AC: a citrate-based buffer; for each metal, the bars from left to right represent Cycles 1 to 5, respectively). As shown in Fig. 6 for the single-step citrated-based buffer and Fig. S3 in the supporting information (SI) for the two-step acid (HCl)-base (KOH) solution, REE metals were successfully adsorbed and released at a pH as low as 2.4 and REE concentration as low as 2.6-24.4 ppb in all 5 cycles under both elution conditions. Almost 100% REEs by HCl method and ~ 80% REEs by the citrate-based buffer method were recycled in the 5 consecutive adsorption-desorption cycles, demonstrating the hybrid gel sorbent’s high-performance for REE enrichment. In addition, our fresh sorbent showed high REE/fouling metal selectivity of between SLn/FM=105 and 450 (Fig. S4) 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 most of the adsorbed fouling off, which is not ideal for the collection of valuable REEs (Fig. S3). 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, ~6977% for Ca, 94-98% for Mn, and ~71-80% for Fe, respectively. Lannicelli-Zubiani et al. studied the capture-release of REEs with clays as sorbent material and nitrate acid as 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


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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 ion-sorbent chelation. 38 Unlike the acid REE release method, the 5% citrate-based buffer (pH=8.6) REE release method showed a high separation ability between REEs and fouling metals. As shown in Fig. 6, the adsorption ratio, desorption ratio, and recycle ratio of each element had no significant difference between each cycle, suggesting that the hybrid 3-D network is stable after citratebased buffer treatment. Extraordinarily, Na, Mg, and Ca were fully separated from the eluent solution. And only ~50% of Al, ~20% of Mn, and ~30% of Fe remained in the solution. The REE recycle ratios were ~80% for La, ~70% for Nd, Eu, and Yb, and ~85% for Dy, separately. The sorbent showed weaker affinity toward acidic trivalent metal ions (Al, Fe, and Ln) but higher affinity toward lower Lewis acidic metal ions (Na, Mg, Ca, and Mn), suggesting that the attractiveness of basic amines toward ions is a tunable factor for the optimization of the regeneration solution. Citrate was reported as a chelating agent that forms soluble multidentate complexes with metals.

70

A sodium citrate solution was reported to remove recovered REEs

from bacteria cells and restore the cells active REE capture sites through the formation of strong REE-citrate complexes.

71

Our citrate based buffer solution presumably works via a similar

mechanism as that of sodium citrate. It was reported that PEI has a pKa of 7.11 to 8.6 with different molecular weights.

72

By adjusting the pH of the eluent above that sorbent’s pKa, one


24

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can effectively neutralize functional groups and disrupt their electrostatic interactions, allowing for REE elution to occur. With the REEs ability to form highly coordinated species with various ligands, the citrate-based buffer (multiple coordinations per buffer molecule) showed higher enrichment capability than HCl (presumably one coordination per molecule). Within the 5 adsorption/desorption cycles, REEs were 6-7 times enriched by citrated-based buffer method and 2-5 times enriched by HCl method (Fig. S5). It is worth noting that the stronger coordination between eluent and REEs, the more challenging in post processing of the eluent solution. Considering the cost effectiveness, there is a balance between enrichment of REEs and post processing of REEs in practical application. The citrated-based buffer in the present study has shown promising ability to selectively wash off metal ions without any optimization. We believe that the coordination between eluent and metal ions is tunable by adjusting different parameters (i.e. pH). The performance of our sorbent in recovery of REEs from practical waste or simulated waste solutions were quite comparable with the sorbents reported in literatures (Table S2). A recent publication from our group reported that porous amine-epoxy monolith sorbents showed a high selectivity (REEs/fouling metals) of ~87 in sorption of a simulated AMD solution. Despite a 98.3% total REEs recovery, only small or 0% of the Na, Ca, and Mg were remain in the sorbenteluent treated solution. 40 Arkhipova ea al. reported non-covalently immobilized β-diketones on low-polar matrices showed 98-100% recovery of ppb levels of REEs from seawater with 2 M HNO3 as eluent. They didn’t provide the seawater composition and no data on the selectivity of REEs over fouling metals of the sorbents is available.

68

In Zhao’s work, ppb levels of REEs

were 100 fold enriched with a 4 h batch adsorption by EDTA-cross-linked β- cyclodextrin from seawater.

59

Ogata’s group studied the practical selective recovery of heavy REEs from apatite


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ores. EDASiDGA, a silica gel based sorbent coupled with 1M H2SO4 as eluent selectively recovered HREE from apatite ores leaching solution (REE/fouling metal~1/400). With a bed volume of 1.36 and 2.06, 10-30 fold of the HREEs were enriched, while LREEs and other base metals were un-enriched.

73

The same material showed

SiO2 Hybrid Hydrogel Sorbent for

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