Selective Adsorption of La3+ Using a Tough ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

Selective Adsorption of La3+ Using a Tough Alginate-ClayPoly(n‑isopropylacrylamide) Hydrogel with Hierarchical Pores and Reversible Re-Deswelling/Swelling Cycles Dongbei Wu,*,† Yawei Gao,† Wenjun Li,† Xiangning Zheng,† YongGui Chen,‡ and Qigang Wang*,† †

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School of Chemical Science and Engineering, Shanghai Key Lab of Chemical Assessment and Sustainability, Advanced Research Institute and ‡Department of Geotechnical Engineering, Key Laboratory of Geotechnical & Underground Engineering of the Ministry of Education, Tongji University, 1239 Siping Road, Shanghai 200092, P. R. China S Supporting Information *

ABSTRACT: Rare earth elements are an important strategic resource, and it is urgent that the rare earth industry continue to explore and develop novel separation methods and technologies. Herein, we fabricated an efficient semi-IPN alginate-clay-poly(n-isopropylacrylamide) (NIPAm) hydrogel by a frozen polymerization method with the help of UV light irradiation, where alginate was employed as the main adsorption functional compound. The as-prepared hydrogel exhibits tough, sponge-like hierarchical macroporous and reversible temperature-responsive characteristics. The maximum adsorption capacity of La3+ is 182 mg/g for the hydrogel composition of 5.0% NIPAm, 4.0% clay, and 3.0% alginate. The Langmuir isotherm fits the data very well, and the adsorption follows the pseudo-second-kinetic equation. The trace of La3+ ions can be effectively separated from the coexisting metal ions. After six repeated adsorption−desorption cycles, no obvious deformation of the shape and or loss of adsorption capacity of the bulk hydrogel is found, but the stress level of the original hydrogel is significantly enhanced. Our results indicate that the green, sustainable, adsorbent hydrogel may serve as a versatile platform for recovery, separation, and purification of rare earth ions and suggest its potential applications in the fields of hydrometallurgy industries and wastewater treatment. KEYWORDS: Alginate, Clay, Poly(n-isopropylacrylamide) (PNIPAm), Adsorption, La (III)

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INTRODUCTION Rare earth elements are an important strategic resource. With increasing demands for rare earth elements in superconductors,1 magnetics,2 rechargeable batteries,3 and catalysts,4 the demand for rare earth elements continues to grow. Traditional separation techniques such as solvent extraction, ion exchange, chemical precipitation, and so on suffer from various disadvantages and are hard-pressed to meet the growing needs of high-tech materials.5−8 Moreover, in the process of rare earth separation and postproduction treatment, rare earth relevant contaminations have been produced and are leading to a serious of pollution problems. Large amounts of rare earth ions are being directly introduced into the environment through various pathways without any treatment, resulting in a waste of rare earth resources and the accumulation of these elements in soil and groundwater.9,10 Therefore, the recovery and recycling of rare earth elements is an urgent issue. Biosorption is a promising technology for the recovery of metal ions from solution. Sustainable bioadsorbents have gained considerable attention due to their low cost, environmental friendliness, and wide origins. Among these biosorbents, alginate is considered to be the most promising candidate. It is © 2016 American Chemical Society

a natural polysaccharide biopolymer composed of chains of 1,4linked β-D-mannuronic (M) blocks and α-L-guluronic (G) blocks. Besides its nontoxicity, biocompatibility, and reusability, alginate has a special advantage for adsorption because it contains abundant free carboxyl and hydroxyl groups along its backbone. In particular, the adjacent G block can quickly chelate divalent or multivalent ions to form a so-called egg-shell structure, which would lead to a high adsorption capacity and selectivity for metal ions. Recently, Caroline et al. extracted alginate products from Brazilian brown seaweed Sargassum f ilipendula for Cr (VI) and Cr (III) biosorptions.11 Alginatecarboxymethyl cellulose (CMC) gel beads were prepared by Ren et al. for efficient removal of Pb (II).12 Simultaneous reduction of Cu (II) and toxicity in semiconductor wastewater using protonated alginate beads was studied by Choi et al.13 By the use of batch and column experiments, Choi et al. investigated the competitive sorption of divalent metal ions such as Ca2+, Cu2+, Ni2+, and Pb2+ on alginate hydrogel beads.14 Received: July 20, 2016 Revised: September 11, 2016 Published: September 15, 2016 6732

DOI: 10.1021/acssuschemeng.6b01691 ACS Sustainable Chem. Eng. 2016, 4, 6732−6743

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. Preparation process and optical images of PNIPAm-clay nanocomposite hydrogel. (a) All of the components of the precursor solution; SA denotes sodium alginate. (b) Formation of the PNIPAm-clay nanocomposite cryogel (−15 °C), a photograph of a self-standing cryogel (−15 °C), and a schematic of the semi-IPN. (c) After adsorption, the two types of polymer networks are intertwined, semi-IPN changes to be an interpenetrating polymer network; demonstrated is a scheme of the strong chelating action between La3+ and G blocks in the alginate chains.

precursor solution and noncovalently interacted with NIPAm chains, resulting in a significant enhancement on the mechanical level of the composites.25 Moreover, clay is widely accepted as an inexpensive and efficient adsorbent and has attracted significant interest in the fields of biology, energy, and environmental, among others, due to its relative abundance, chemical and mechanical stability, high specific surface area, and favorable structural properties.26 Among the various techniques to produce permanent macroporous hydrogels, frozen polymerization exhibits an attracting feature. Through this technique, the polymerization was conducted below the frozen point, when most of the solvent (water, alcohol, dimethyl sulfoxide, and so on) formed crystals. After frozen polymerization of the precursor and thawing of the gel at room temperature, the macroporous hydrogel was generated. It was reported that a PNIPAmsodium methacrylate copolymer hydrogel prepared by this technique displayed a quick temperature response of 2 min temperature oscillations.27 Photoinitiation polymerization is a facile and easily operated method to prepare polymeric hydrogels as soft-wet materials for applications in pharmaceutical, biomedical, and industrial fields. Recent advances in our group demonstrate that semiconductor nanoparticles are one type of promising photoinitiator for the polymerization of tough hydrogels.28−30 Semiconductor nanoparticles can be functionalized not only as photoinitiators but also as crosslinkers and nanofillers to enhance the mechanical strength of hydrogel and enrich its functionalities. Semiconductor nanoparticle-based gels can be used as a recyclable platform for the collective treatment of industrial pollutants due to low its residue and high mechanical strength.31 Inspired by the pioneering works in this field, in this paper we synthesized temperature-sensitive alginate-clay-PNIPAm semi-IPN hydrogels via the semiconductor nanoparticleinitiated frozen polymerization technique. The results demonstrated that this nanocomposite hydrogel has tough mechanical stability, fast responsivity to temperature, and can withstand repeated re-deswelling/swelling behaviors. These properties of the gels recommend them as potential sorbents for rare earth ions. Therefore, the aim of this work is to evaluate semi-IPN alginate-clay-PNIPAm nanocomposite hydrogels as biosorbents in the selective recovery of rare earth ions from the aqueous solution with La3+ being taken as a model. The effects of the

However, most studies focused on the removal of heavy metal ions such as Cu, Cr, Pb, and so on. Not much information on the adsorption of rare earth ions is available. The data on the use of biomass for the adsorption of rare earth elements is limited.15 Furthermore, alginate was mostly used in the form of gel beads, which behaved with weak mechanical strength, slow molecular diffusion, and long adsorption equilibrium time. To improve the mechanical strength of the alginate hydrogel, researchers have developed some innovative methods. Among these approaches, chemical modification of alginate by other natural polysaccharides, such as propylene glycol and xanthan gum, exhibits the most promising application for sustainable animal agriculture reducing phosphorus pollution.16 However, chemical modification is usually complicated, inefficient, and time-consuming. Introducing an insoluble salt into the spinning mixture is another accessible method to improve the mechanical strength of alginate hydrogels. By providing multivalent ions in the form of insoluble carbonate, the strength of the alginate hydrogel can be enhanced because slow release of metal ions would form a homogeneous network structure.17,18 However, the excess acid (HCl or HNO3) would unavoidably cause environmental pollution and chemical waste. In the meantime, -COOH groups in alginate were occupied through a strong supermolecule interaction between -COOH and metal ions, which would lead to a decrease in active sites for target ion uptake. Therefore, developing novel strategies to design and construct powerful alginate-based sorbents with outstanding mechanical levels, a fast adsorption/desorption rate, and repeated regeneration ability are desirable. In this context, a semi- or interpenetration polymer network (IPN) hydrogel composed of alginate, clay, and poly(nisopropylacrylamide) (PNIPAm) was fabricated. This multicomponent hydrogel was characterized by enhanced mechanical strength, supermacroporous network structure, and reversible re-deswelling/swelling behavior. NIPAm was used as a main component based on the fact that PNIPAm can absorb or lose large amounts of water due to its lower critical solution temperature (LCTS) around 33 °C,19 exhibiting featured applications in the fields of artificial organs,20 controlled drug delivery,21 solute extraction and separation,22 scaffolds23 and matrices for tissue engineering.24 Clay nanosheets were employed as another reliable and powerful component because they can be uniformly dispersed into the 6733


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Figure 2. SEM images of a (a) hydrogel prepared at the room temperature (25 °C), (b,c) cryogel prepared at low temperature (−15 °C), and (d) cryogel after adsorption reaches equilibrium (CLa(initial) = 42 mg/L (0.3 mmol/L), 25 °C). interpenetrating polymer network was generated as illustrated in Figure 1c. Characterization of Gels. The morphologies of the nanocomposite gels were examined by a field-emission scanning electron microscope (SEM, Hitachi S-4800, JEOL, Japan). The gel samples prepared at room and low temperatures were freeze-dried using a freeze drier for 1.5 days until all of the solvent was sublimed. FTIR spectra of the cryogels before and after La3+ adsorption were taken in the range of 4000−400 cm−1. The XPS spectra were conducted for chemical analyses of the blank and La-loaded cryogels (PHI 5000C ESCA system, USA). The operating condition is Mg Kα X-ray working at 300 W, 14 kV, and 25 mA. The surface area, pore size, and distribution were conducted by a mercury instrument method (AutoPore IV 9500).

various parameters including pH, contact time, temperature, initial concentration of the La3+ ions, and interference of the adsorption capacity of the hydrogels was investigated in this paper. The adsorption mechanism was explored by FTIR and XPS characterizations. Cycling and reuse of the hydrogel was repeated at least six times. Of particular interest, a change in the microstructure of the composite hydrogels from semi-IPN to IPN was observed after La (III) adsorption. As a sequence, the stress level of the original hydrogels is substantially increased, exhibiting a very helpful and meaningful feature for its practical applications.

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EXPERIMENTAL SECTION

RESULTS AND DISCUSSION Formation and Morphologies of the Gels. Figure 1 illustrates the formation process of the gels. First, the mixture of NIPAm, alginate, clay nanosheets, and ZnO was put in a refrigerator at −15 °C and irritated by a UV-light with which ZnO nanoparticles, a kind of excellent semiconductor material, provided photogenerated holes for the oxidation of OH− to produce OH radicals and polymerize NIPAm chains at the surface of the clay nanosheets. Clay nanosheets were employed as physical cross-linkers to noncovalently connect with NIPAm chains and form the first polymer network (Figure 1a). A similar polymerization mechanism of p(N,N-dimethylacrylamide)-clay nanocomposite hydrogel initiated by ZnO nanoparticles under UV−vis light irradiation was discussed in detail in an earlier work. Sodium alginate plays an important role in fabricating a semi-interpenetrating polymer network (semiIPN) in which the transport of the solutes became easier than in the noninterpenetrating polymer network. The hydrogel components can be initiated by ZnO nanoparticles and effectively polymerized at subzero temperature (Figure 1b). After La (III) adsorption, semi-IPN changed to be an interpenetrating polymer network (IPN) due to the strong chelate action between La3+ and G blocks in the alginate chains, resulting in further strengthening of the original hydrogel

Chemicals and Reagents. Monomer n-isopropylacrylamide (NIPAm) from TCI (Shanghai) Development Co., Ltd. was used as received. Clay nanosheets (Laponite XLG; Mg5.34Li0.66Si8O20(OH)4) were purchased from Rockwood Ltd. The dispersion of ZnO nanoparticles with sizes of approximately 10−30 nm was obtained from Yicheng Ruijing New Mater. Reagent Co. All other chemical reagents, including LaCl3, were analytical grade and used as supplied. Preparation of the Nanocomposite Gels. The process for onepot preparation of alginate-clay-PNIPAm nanocomposite gels via the photopolymerization technique at subzero temperature is shown in Figure 1. As a typical example, 1.8 mL of precursor solution containing 0.1 g of NIPAm, 0.08 g of clay nanosheets, 0.02 g of ZnO nanoparticles, and 3.0 mg of sodium alginate (SA) were first placed into a transparent vial with rigid stirring (Figure 1a). The vial containing precursor was then sealed and moved to a refrigerator (−15 °C). After 2 h of UV light irradiation in the refrigerator, a frozen hydrogel with a semi-interpenetrating polymer network was formed (Figure 1b). The average intensity of the UV light is approximately 22.4 mW cm−2 at 365 nm. Finally, the as-prepared hydrogels were thawed at room temperature (25 °C) and repeatedly washed three times by fresh distilled water every 4 h to replace any unreacted monomers within the gel matrix. For differentiating the hydrogels synthesized at the room temperature, the PNIPAm gels prepared in a frozen state are labeled as cryogels. Following a similar procedure, cryogels without and with various concentrations of sodium alginate were synthesized. After La (III) adsorption, a cryogel with an 6734


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Figure 3. (a) Temperature dependence of swelling ratio of the cryogel. (b) Deswelling kinetics (time dependence) for the cryogel. (c) Swelling kinetics of the vacuum-dried cryogel in distilled water at 25 °C. (d) Swelling/deswelling cycles trigged by changing temperature; the cryogel (original size = 16.0 mm height × 4.0 mm height) was alternatively transferred into water at 25 and 50 °C.

more like a sponge (Figure 2b and c). This structure of the cryogel would support itself as a suitable candidate for adsorption in the form of bulk material where quick solute transportation and gel swelling/deswelling cycles are required. The possible reason for the cryogel having such a hierarchical porous network might be attributed to the pore-forming ability of ice-crystals occurring during the polymerization at a temperature below the H2O melting point. Similar results were reported by Chu, who synthesized thermos-sensitive PNIPAm cryogels in DMSO organic solvent.32,33 After La (III) adsorption, typical IPN structure can be observed in Figure 2d in which the pores of the cryogel become more dense and network-like. It seems that alginate chains and the PNIPAm network are tightly connected by La3+ ions to construct an interconnected structure, resulting in an improvement in the mechanical strength of the original nanocomposite hydrogel, which would be confirmed in the following results. Mechanical Properties of the Cryogel. Mechanical properties of the cryogels were investigated by determining the values of the compressive level at gel strain of 90%. It was found that, with an increase of ZnO concentration from 0.6 to 1.0%, the compressive stress of the cryogel was enhanced from 324.5 ± 16.2 to 487.8 ± 23.4 kPa and the corresponding modulus improved from 11.3 ± 0.85 to 12.7 ± 0.99 kPa. However, when the concentration of the ZnO increased to 1.2%, the value of stress of the cryogel decreased to 393.2 ± 14.3 kPa (Figure S2a). Therefore, in the following experiments, a ZnO concentration of 1.0% was used for initiation of the PNIPAm-clay cryogel. Figure S2b illustrates the effect of clay concentration on the mechanical strength of the cryogel. Similar to the effect of ZnO concentration on the hydrogel strength, with increasing concentrations of the clay nanosheets

(Figure 1c). As expected, the formation of the semi-IPN nanocomposite hydrogel strongly depends on the component concentration and polymerization temperature. An increase in the component concentration may facilitate the formation of the gels. On the basis of the preliminary experiments and the data in Table S1, we found that the gels containing hydrogel and cryogel could not be effectively formed when the concentrations of NIPAm, ZnO, and clay sheets were below 5.0, 0.6, and 3.0%, respectively. From the viewpoint of conversion, high conversion of the gels can usually be achieved at −15 °C. The optimal composition for generating a cryogel is 5.0% NIPAm, 0.15% alginate, 4.0% clay, and 1.0% ZnO. Figure 2 gives the interior morphologies of the gels prepared at room temperature (hydrogel) and −15 °C (cryogel). It can be observed that the network structure of the cryogel is different than that of the hydrogel. The hydrogel has a single interior structure with a pore size of around 10 μm. After drying treatment, the network of the hydrogel easily collapses and forms some semiclosed pores. The morphology of pores looks like a honeycomb (Figure 2a). The cryogel, on the other hand, has an obvious hierarchical structure with 20−100 μm large pores and 1−10 μm interconnected pores. In light of the large pore size of the hydrogel, the surface area, pore volume, porosity, and pore size distribution were determined by the mercury instrument method, and the results were recorded as follows: The surface area of the hydrogel was 6.40 m2/g with a pore volume of 3.59 cm3/g at a diameter of 2.23 μm and a total porosity of 96.7%. The distribution of the pore size was very wide, ranging from 1 to 100 μm. The prevalent pore sizes were around 1, 10, and 100 μm, exhibiting a hierarchical pore structure. The pore size distribution plot is presented in Figure S1. After drying, the interior morphology of the cryogel looks 6735


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Figure 4. (a) Adsorption kinetics fit by the pseudo-second-order model. Experimental conditions: CLa(initial) = 42 mg/L (0.3 mmol/L), initial pH = 5.0, m = 1.0 g, T = 298 K, V = 30 mL. (b) Langmuir isotherm fitting plot. Experimental conditions: initial pH = 5.0, m = 1.0 g, T = 298 K, V = 30 mL, t = 120 min. (c) Effects of temperature on the La (III) adsorption. Experimental conditions: CLa(initial) = 42 mg/L (0.3 mmol/L), initial pH = 5.0, m = 1.0 g, V = 30 mL, t = 120 min. (d) Adsorption and desorption cycles. For the adsorption experiments: CLa(initial) = 42 mg/L (0.3 mmol/L), initial pH = 5.0, m = 1.0 g, T = 298 K, V = 30 mL; for the desorption experiments: elution is 2.0 mol/L HCl.

between the alginate and La (III) ions.34 Homogeneous distribution of La (III) into the hydrogel has the benefit of the formation of a rigid network.35 La-cryogels also exhibit the inherent property of great mechanical strength after swelling for 24 h (Figure S3). The data of the compressive stress for the various gel samples are listed in Table S2. Swelling, Shrinking and Re-Deswelling/Swelling Cycles of the Cryogel. The swelling ratios of the cryogel were studied as a function of temperature, and the results are shown in Figure 3a. It is obvious that the cryogel exhibits a classic temperature stimulant property with the low critical solution temperature around 35 °C. The magnitude of the thermos-induced swelling ratio changes (ΔSR = SR25 °C − SR50 °C) of the cryogel over the temperature range of 25−50 °C is estimated to be 60 g/g, nearly equal to that of conventional PNIPAm hydrogels. The shrinking kinetics for the gels are illustrated in Figure 3b; it can be observed that the shrinkage rate of the cryogel is quite fast. The value of WR was reduced from 1.0 to 0.5 within 4 min and to 0.25 within 20 min, respectively. A reasonable explanation is that the interconnected super-porous network of the cryogel might contribute to the water release via capillary action.36 Figure 3c shows the swelling kinetics of the cryogel at room temperature. The dry cryogel can absorb 30% water within 2 h, ∼55% within 9 h, and nearly 90% within 20 h. These values of water uptake of the cryogel are much higher than those of the hydrogel over the same time intervals. The significant improvement of the water uptake rate is also attributed to the interpenetrating and interconnected pore structure of the cryogel. The intercon-

from 3.0 to 5.0%, the values of compressive stress increased obviously from 295.9 ± 14.8 to 590.4 ± 29.5 kPa. A reasonable explanation is the fact is that clay nanosheets are uniformly merged into the cryogel network and cross-linked with NIPAm chains via supermolecular interactions, resulting in an improvement of the mechanical strength. Because excess clay nanosheets would hinder the polymerization of the cryogel in the presence of alginate, a 4.0% concentration of clay nanosheets was employed for the following experiments. The effect of sodium alginate concentration on the mechanical strength of the cryogel is given in Figure S2c. Over the concentration range of alginate from 0.0 to 0.20%, the cryogel can be formed with comparable mechanical properties but the values of stress sharply decrease from 487.8 ± 23.4 to 278.8 ± 26.8 kPa. In particular, when the concentration of sodium alginate is beyond 0.20%, the precursor of the gels becomes sticky, and the yield of the cryogel is not as high as expected (∼78%). Therefore, an alginate concentration of 0.15% was used for subsequent experiments. It is worth noting that the mechanical strength of the cryogel after La (III) adsorption can increase nearly 540 kPa higher than that of the original cryogel, as shown in Figure S2d, exhibiting an interesting application feature. Moreover, with increasing La (III) concentrations, the compressive level of the hydrogel improved significantly. The maximum stress press can reach 1078 ± 13.6 kPa, which is much higher than that of the original cryogel. A possible reason for the improved mechanical strength of the cryogel after La (III) adsorption might be due to the formation of a second network in the cryogel through supermolecule physical action 6736


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ACS Sustainable Chemistry & Engineering Table 1. Reported Works on La Adsorption by Various Adsorbents

adsorption conditions adsorbent

adsorption capacity (mg/g)

adsorption time (min)

regeneration cycles

385 168 123.5 97.1 100 200 160.2 120 270 333.3 175.4 65.6 55.9

1440 1440 1680 600 360 360 360 480 120 40 120 60 30

3 3 3 3 2 6 6

37

zeolitic imidazolate frameworks ZIF-8 zeolitic imidazolate frameworks ZIF-9038 magnetic alginate gel bead39 magnetic alginate-chitosan gel bead40 Desmodesmus multivariabilis41 fish scales42 Neem sawdust43 bamboo charcocal44 acrylic acid on hydroxypropyl cellulose with attapulgite45 hydroxypropyl cellulose-acrylic acid-attapulgite46 activated carbon from fly ash47 SnO2−TiO2 nanocomposites48 magnetic silica 2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester nanocomposite49 magnetic bentonite grafted by di-(2-thylhexly)phosphoric acid50 magnetic ZnO clay nanocomposite hydrogel51 GMZ bentonite52 magnetic GMZ bentonite53 magnesium silicate functionalized with tetrabutylammonium dihydrogen phosphate54 amberlite XAD-4 impregnated with Aliquat-336 resin55 alginate-clay-PNIPAm hydrogel (this work)

40.5 58.8 26.8 18.4 9.13

3 80 20 3 120

4.73 50.6

40 90

5 5

10 2 6 3

6

C0 (mg/L)

dosage (g/L)

pH

138 138 100 300 250 217 300 400 50 125.0 35

1.0 1.0 0.3 0.3 1.0 0.8 0.2 1.0 1.0 1.0 1.0

5.0 5.0 6.0 6.0 6.0 7.2 5.0 6.0 3.0 5.0 5.5

27.5 42 36 27.5 10

1.0 1.0 1.0 1.0 10.0

4.5 5.0 5.0 6.5

10 1.0

6.5 5.0

1000 42

suddenly decreased after that, which might be due to breaking of the alginate hydrogel network during the shaking process. Adsorption Properties. Prior to the other adsorption experiments, the time-dependent La3+ adsorption capacities of the gels were investigated at 25 °C by fixing the initial concentration of La3+ at 0.3 mmol/L at pH 5.0. For the cryogel, there is a sharp increase of La3+ adsorption in the first 50 min followed by a slow adsorption process, as demonstrated in Figure 4a. The adsorption can reach equilibrium within 90 min with an adsorption capacity around 24.5 mg/g. Therefore, the contacting time was fixed at 120 min in the following experiments to attain adsorption equilibrium. Compared with the cryogel, the hydrogel has a slower adsorption rate and lower adsorption capacity. The complete equilibrium needs at least 150 min with an adsorption capacity of 21.3 mg/g. The fact that the cryogel possesses a higher adsorption capacity than that of the hydrogel might be attributed to the hierarchical and interpenetrating network structure of the cryogel. To further explore the adsorption mechanism, pseudo-first-order and pseudo-second-order kinetic models were used for explaining the experimental data. It was confirmed that the pseudosecond-order model fit the experimental data better than the pseudo-first-order model. In addition, the value of adsorption capacity calculated from the pseudo-second-order model closely matches the experimental values. Therefore, we can deduce that the La3+ adsorption followed pseudo-second-order kinetics over the entire period. The data treatment for the pseudo-second-order and pseudo-first-order equations are provided in Figure S6 and Table S3. The effect of La3+ concentration on the adsorption was investigated for the maximum adsorption capacity, and the results are shown in Figure 4b. For describing the interaction between the adsorbent and adsorbate, Langmuir and Freundlich isotherms were employed for fitting the experimental data. As usual, the Freundlich isotherm is applied for the assumption that all of the binding sites on the adsorbent are

nected porous network of the cryogel is beneficial to the water molecule transportation from the solution to the cryogel inside and subsequent relaxation of hydrated polymer chains. For comparison purposes, the swelling and shrinking kinetics for the hydrogel were investigated under the same conditions as those of the cryogel and demonstrated an unsatisfying result (Figure S4). Figure 3d demonstrates distinct re-deswelling/ swelling cycles by alternating temperature changes. It can be seen that the cryogel has an oscillatory shrinking-swelling feature upon cycling the temperature between 25 and 50 °C. Each complete swelling and deswelling process can be completed within 19 h, and such a cycle can be repeated at least three times. Over three cycles, the whole shrinkingswelling cycles appeared identical to each other. Therefore, the prevailing though is that our cryogel has tough, stable, and rapid response properties, which might be suitable for use in many fields, such as hydrometallurgy and wastewater treatment. In addition, the raw alginate hydrogel was prepared by slowly adding 5.0% CaCl2 solution (1.0 mL) to 3.0% alginate sodium solution (1.5 mL). After removing residual water of the precursor, the calcium alginate hydrogel was obtained with a diameter of 16.0 mm and height of 7 mm (Figure S5a). Tests of the mechanical strength, molecular diffusion, and adsorption time were carried under the same conditions as those of the multicomponent hydrogel. The results demonstrated that the compressive stress and modulus of the alginate hydrogel were approximately 10 ± 0.12 and 4.2 ± 0.08 kPa at a strain of 80%, respectively, which are much lower than those of the prepared multicomponent hydrogel (Figure S5b). The swelling rate of the alginate hydrogel is also quite slow. Almost no swelling was observed after soaking the hydrogel in distilled water for 24 h (as shown in Figure S5c). The effect of time on the La (III) adsorption for the alginate hydrogel was studied, and the results are given in Figure S5d. It was found that the equilibrium time was approximately 80 min; however, the adsorption capacity 6737


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it can be directly used in form of bulk, membrane, and column, and separation of the hydrogel from aqueous solution is convenient and easy to operate; (ii) the hydrogel synthesis method is facile and effective as no excess modification is necessary. Beside monomers, almost no polluting organic reagents were used, and the adsorption materials were green and compatible. Of particular interest, the multicomponent hydrogel overcame shortcomings of the single-component hydrogel and possessed tunable adsorption capacity, mechanical level, and equilibrium time. It is characterized by enhanced mechanical strength after La (III) adsorption, reversible redeswelling/swelling behavior, temperature-stimulatory response properties, and exhibits a unique charm for practical applications. Figure 4c depicts the effect of temperature on the La3+ adsorption. The adsorption capacity of La3+ linearly decreased from 25.0 to 22.6 mg/g in the temperature range from 283 to 303 K, slowly decreased from 22.6 to 22.2 mg/g from 303 to 313 K, and gradually increased from 22.2 to 22.6 mg/g from 313 to 323 K. This trend is consistent with temperature dependence on the swelling ratio of the cryogel. A reasonable explanation is that adsorption is a synergistic and competitive process between the H2O molecule and La3+ ions. At a lower temperature, the cryogel tends to absorb a large amount of water to become a swollen gel, and as a result, more La3+ ions are easily transported into the cryogel network and bound with adsorption functional groups. Upon increasing the temperatures from 283 to 313 K, the swelling ability of the cryogel becomes weaker and weaker, leading to difficulty of La3+ ions moving into the cryogel, and the adsorption capacity of La3+ ions thus decreased concomitantly. By using thermodynamics equations, thermodynamics parameters such as enthalpy change (ΔHΘ), entropy change (ΔSΘ), and free energy (ΔGΘ) can be evaluated in the temperature range of 283−303 K. The negative value of ΔHΘ (−19.49 kJ mol−1) suggests the exothermic nature of adsorption. The negative value of ΔGΘ at 298 K (−3.40 kJ mol−1) means that adsorption is a spontaneous forward process, whereby no energy input from outside of the system is required. The negative value of ΔSΘ (−53.97 J mol−1 K−1) indicates decreasing randomness during the adsorption process. When the temperature was larger than 313 K, the cryogel tends to shrink, and water molecules are extruded from the cryogel. Figure 4d describes adsorption and desorption cycles of the cryogel to the La3+ ions. The cycles were repeated at least six times using a bulk cryogel. The initial La3+ ion concentration was fixed at 0.30 mmol/L and a volume of 30 mL. Fifty milliliters of a 2.0 mol/L HCl solution was employed as a stripping reagent. The elution % is equal to the ratio of the desorbed amount to the adsorbed amount of La3+ ions. As shown in Figure 4d, even at the sixth adsorption/desorption cycle, the cryogel still maintain 93.3% adsorption and 92.4% desorption efficiency, indicating outstanding reusability of the cryogel. For practical applications, running water from the chemical lab at Tongji University was used for the selective removal of La3+ ions. The composition of the running water is give in Table S5. As illustrated in Figure S9a, the uptake percent was 87.8%, which is a little lower than that for single La3+ adsorption in distilled water, which might be a consequence of competitive adsorption among the interference ions. However, 100% desorption of La (III) could be achieved by 1.0 mol/L HCl with a volume of 30 mL for 4 h. The same uptake and desorption percent was repeated for at least six

not equal, and stronger binding sites are preferentially occupied. With an increase in the ratio of site occupation, the binding strength decreases. The Langmuir sorption isotherm assumes monolayer sorption in which the adsorbent surface contains a finite number of active sites. The Langmuir sorption isotherm is considered to be the most commonly used isotherm model and plays a decisive role in calculating the value of adsorption capacity. Figure 4b indicates that the Langmuir isotherm matches the data very well and that the La3+ adsorption occurs at the surface of the binding sites in a monolayer sorption manner. According to the Langmuir isotherm model, the maximum adsorption capacity is determined to be 50.6 mg/g for the hydrogel sample used, whose composition is 5.0% NIPAm, 4.0% clay, and 0.15% alginate. Data treatments on the Langmuir and Freundlich isotherm equations are given in Figure S7. Some of the parameters related to the Langmuir and Freundlich isotherm equations are listed in Table S4. In earlier work, magnetic alginate gel beads and magnetic alginate-chitosan gel beads have been synthesized for the adsorption of La3+; the maximum adsorption capacities were determined to be 123.5 and 114.5 mg/g, respectively, which are much higher than that obtained from the present work. A reasonable explanation is due to the lower amount of alginate in the cryogel (0.15 wt %) than that in the beads (1.5 wt %). For the adsorption capacity of the hydrogel to La (III) to be improved, the concentration of alginate was increased to 1.5 and 3.0% for separate preparations of hydrogel samples. Then, digital images of the hydrogel samples were taken, and compressive strength and adsorption experiments were redetermined. The results demonstrated that the color of the hydrogel sample was yellow, which is closer to that of pure alginate. The hydrogels were self-standing, soft, and flexible (Figure S8a and b). It was determined that the stress strengths of the hydrogels decreased to 37.6 ± 0.14 and 30.3 ± 0.08 kPa at a strain of 80%, respectively, but still maintained selfsupporting shapes (Figure S8c). The maximum adsorption capacities were 145 and 182 mg/g for the samples with alginate concentrations of 1.5% (S1) and 3.0% (S2) (Figure S8d and Table S4). After La (III) adsorption, the compressive stresses of the original hydrogels returned to 194 ± 0.23 kPa for S1 and 208 ± 0.15 kPa for S2, exhibiting satisfactory mechanical levels for their practical application (Figure S8c). Moreover, there are many other materials in the removal of REE from aqueous solution, such as other biosorbents, MOFs, metal oxide nanoparticles, bentonite, active carbon, and polymer resin. Therefore, a comparison of reported works and the La adsorption in this work is given in Table 1. MOFs, active carbon from fly ash, and most of the biosorbents possessed very high adsorption capacities; however, they usually demonstrated long adsorption times. Metal oxide nanoparticles functionalized by various organic ligands had a comparable adsorption capacity and equilibrium time, but they usually had difficulty separating the adsorbent from the adsorbate solution. Although magnetic nanoparticle adsorbents could overcome the shortcomings of the solid/liquid separation profile, they faced some drawbacks, such as low adsorption capacity, complicated modification operation, and excess usage of organic reagents. Bentonite and modified bentonite-based adsorbents had low adsorption capacities but quick equilibrium times. Amberlite XAD-4 impregnated with Aliquat-336 had the lowest adsorption capacity. Therefore, this provides evidence that our multicomponent hydrogel has some impressive features: (i) 6738


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Table 2. Adsorption Capacities of La, Co, Cu, and Ni as Well as Separation Coefficients between La/Co, La/Ni, La/Cu, La/Ce, La/Gd, La/Eu, and La/Yb at Different Aqueous pH Conditions qe (mmol/g) pH

La

Co

Cu

Ni

SLa/Co

SLa/Ni

SLa/Cu

SLa/Yb

SLa/Eu

SLa/Ce

SLa/Gd

2.0 3.0 4.0 5.0 6.0 7.0

0.092 0.134 0.219 0.269 0.247 0.252

0.076 0.079 0.096 0.144 0.150 0.142

0.028 0.047 0.060 0.093 0.094 0.097

0.022 0.027 0.038 0.047 0.053 0.065

1.00 2.45 4.42 2.38 1.52 1.77

0.39 2.46 11.07 8.40 3.87 4.41

3.56 4.44 7.54 3.11 2.49 2.53

2.17 1.33 0.65 20.12 5.12 35.35

2.99 0.91 0.38 14.30 9.54 17.40

6.93 0.50 1.44 162.9 25.37 48.53

5.74 0.63 1.05 144.6 33.56 34.09

Ce and other elements. In addition, the higher coordination number of La (IIII) would lead to a better separation selectivity of La (III) over other heavy metal ions; herein, they are Co, Ni, and Cu. The hydrated ion radii of the above-mentioned metal ions are 106 pm for La (III), 103 pm for Ce (III), 95 pm for Eu (III), 93.8 pm for Gd (III), 85.8 pm for Yb (III), 70 pm for Ni (II), and 72 pm for Cu (II) and Co (II). The competition adsorption of La3+ with interference ions was performed in a two-component system. The mass ratio of K+, Na+, or Li+ compound to La3+ compound is 1000:1. The mass ratio of Ba2+, Zn2+, or Co2+ compound to La3+ compound is 100:1. The mass ratio of Pb2+, Cd2+, Al3+, Fe3+, or REE3+ compound to the La3+ compound is 10:1. As shown in Table 3,

adsorption−desorption cycles, indicating satisfying stability and sustainability of the hydrogel. Furthermore, no evident changes in the morphology or shape of the hydrogel were found. For visualizing the sustainability of the hydrogel, blue Cu2+ ions were employed for the adsorption−desorption cycle, and a similar conclusion could be drawn (as shown in Figure S9b). Therefore, it is acceptable for us to classify the multicomponent hydrogel as a green and sustainable chemistry material. For investigating the adsorption selectivity of La3+ ions to the relative heavy metal, the adsorption experiments of single Co2+, Ni2+, or Cu2+ ions with the same concentration to the La3+ ions at different pH conditions were studied. The data in Table 2 shows that, with increasing pH from 2.0 to 7.0, the adsorption capacity of the cryogel for these ions increases. The adsorption capacity follows the order La3+ > Co2+ > Cu2+ > Ni2+ at the same pH, which might be attributed to the strong affinity of -COOH groups in alginate toward La3+ ions. By calculating the values of separation coefficients, it was found that La3+ ions could be effectively separated from equal mole concentration of a La3+/Co3+ mixture at pH lower than 5.0 and from the La3+/ Ni2+ and La3+/Cu2+ mixtures over the entire pH range. Moreover, it was also suitable for the cryogel to effectively separate Cu2+, Co2+, and Ni2+ from the Cu2+/Co2+, Cu2+/Ni2+, and Co2+/Ni2+ mixtures, especially at higher pH. Moreover, we studied the aqueous pH effect on adsorption for Ce3+, Eu3+, Gd3+, and Yb3+ as the representative typical light, middle, and heavy rare earth ions, and plots on the removal % at various pH levels are shown in Figure S9c. The corresponding values of the separation coefficient are listed in Table 2. According to the separation coefficient, it was deduced that La3+ ions could be effectively separated from the mixtures of La/Yb, La/Eu, La/ Ce, and La/Gd at initial pH ranges from 2.0 to 7.0. For pH 5.0 in particular, the values of SLa/Yb, SLa/Eu, SLa/Ce, and SLa/Gd were calculated to be 20.1, 144.6, 14.3, and 162.9, respectively. Thus, high values of these separation coefficients exhibited a promising application in the field of rare earth element separation. After mixing 0.3 mmol/L La (III) ion solution with 10-fold mole concentration of other rare earth ions at pH 5.0, we found that the removal % of La could exceed 80%, exhibiting a satisfying separation selectivity of the hydrogel for La (III) ions. In summary, the selectivity of La over Ce (or other elements) might be due to a fact that La (III) has a bigger hydrated ionic radius than Ce or other elements. It is wellknown that metal ions generally exist in the form of hydrated ions. A larger hydrated ionic radius of La (III) than Ce or other metal ions means a weaker interaction between La (III) and water molecules. On the other hand, the adsorption was considered to be a competitive process between organic ligands and water molecules. Therefore, as a competitive result, a weaker interaction between La (III) and water molecules would cause a stronger affinity of La (III) to -COOH in alginate over

Table 3. Effect of Interfering Ions on the Adsorption of La (III) ion

compound

concentration ratio

removal of La3+ (%)

removal of interfering ions (%)

La3+ K+ Na+ Li+ Ba2+ Zn2+ Co2+ Pb2+ Cd2+ Ce3+ Eu3+ Gd3+ Yb3+ Al3+ Fe3+

LaCl3 KCl NaCl LiCl BaCl2 Zn(NO3)2 CoCl2 Pb(NO3)2 CdCl2 Ce(NO3)3 EuCl3 GdCl3 YbCl3 AlCl3 FeCl3

1000:1 1000:1 1000:1 100:1 100:1 100:1 10:1 10:1 10:1 10:1 10:1 10:1 10:1 10:1

99.5 99.0 99.2 99.2 93.7 88.2 88.0 83.6 86.3 76.0 82.6 83.1 85.2 99.5 99.5

0.1 0.08 0.2 0.8 0.1 0.9 1.4 2.6 7.0 7.2 8.1 8.3 precipitation precipitation

no obvious decrease in the removal of La3+ was found when 1000-fold K+, Na+, or Li+ was added. A slight decrease in the removal of La3+ was seen in the presence of 100-fold Ba2+, Zn2+, or Co2+ ions. When 10-fold Pb2+, Cd2+, or REE3+ compound was added, the removal of La3+ was evidently decreased, but the removal % of La was still approximately 80% and the removal of other REE ions was less than 9.0%. Whereas for Al3+ and Fe3+ ion solutions, both precipitated at a pH of 5.0. Therefore, it was determined that coexisting Al3+ and Fe3+ ions had no effect on the La (III) adsorption. Furthermore, it was found that Cl− and NO3− had almost no effect on La3+ adsorption, suggesting an excellent selectivity of the cryogel toward La3+ ions. Adsorption Mechanism. Among the various adsorption parameters, aqueous pH plays an important role in the adsorption capacity, selectivity, and mechanism. As shown in Figure 5a, with increasing initial pH values from 1.0 to 5.0, the 6739


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Figure 5. (a) pH effect on the adsorption of PNIPAm-clay nanocomposite gels. CLa(initial) = 42 mg/L (0.3 mmol/L), m = 1.0 g, T = 298 K, V = 30 mL, t = 120 min. (b) FTIR of cryogel before and after La (III) adsorption. (c) XPS spectra of the cryogel before La (III) adsorption. (d) XPS spectra of the cryogel after La (III) adsorption. (e) O 1s XPS spectra of the cryogel before La (III) adsorption. (f) O 1s XPS spectra of the cryogel after La (III) adsorption.

mechanism. As shown in Figure 5b, a clear change in the wavenumber of the hydrogel before and after La adsorption can be observed. A wide peak assigning to the νO−H vibration in the alginate shifted from 3413 to 3433 cm−1, an absorption peak ascribing to the NH2 groups in the PNIPAm shifted from 1546 to 1540 cm−1, and the peak due to the νC−O asymmetrical stretching vibration of -COOH in alginate from 1245 to 1265 cm−1, clearly indicating a strong interaction between alginate, PNIPAm, and La (III) ions. Figure 5c illustrates the typical XPS wide scan spectra and O 1s spectra of the alginate-modified PNIPAm-clay nanocomposite cryogel before and after La (III) ion adsorption. Comparing the spectrum in Figure 5c and d, the peak at the binding energy of approximately 1069.6 eV assigned to Na disappeared, and a new peak at a binding energy of approximately 835.9 eV assigned to La appeared after La (III) adsorption, indicating an ion exchange mechanism during the adsorption process. The O 1s spectrum in Figure 5e and f,

adsorption capacity significantly increased from 4.0 to 16 mg/g for the cryogel without alginate embedded and from 4.0 to 24.5 mg/g for cryogel with alginate incorporated. This fact might be due to the competitive adsorption of La (III) ions with H+ ions. At lower pH, H+ ions were preferentially loaded onto the adsorbent as they had small ionic radii and a higher concentration than that of La3+ ions. Whereas at higher pH, La3+ ions were preferentially adsorbed onto the adsorbent because they had a higher ionic charge and concentration than those of H+ ions. When pH values are beyond 5.0, the adsorption tends to reach a platform, which might be attributed to the saturation of binding sites on the adsorbent. In addition, the cryogel with alginate incorporated had a higher adsorption capacity of La (III) than that of the cryogel without alginate embedded, indicating a strong affinity of carboxyl groups in the alginate toward the La (III) ions. For further investigating the role of alginate in the La (III) adsorption process, FTIR was used for studying the adsorption 6740


Research Article

ACS Sustainable Chemistry & Engineering at a binding energy of approximately 530 eV, was split into two individual component peaks. The peaks at binding energies of 531.35 and 533.44 eV (assigned to -CO and -C−O) moved to 531.12 and 532.46 eV after La (III) adsorption, suggesting the formation of a chemical bond between the carboxyl groups in alginate and La (III) ions. The phenomena further support the fact that the adsorption proceeds via an ion exchange mechanism involving the O atoms in alginates. Stability and Cost of the Prepared Multicomponent Hydrogel. It is well-known that the stability of the adsorbents is a very important parameter for the evaluation of their performance and application potential. Therefore, the stability of the prepared hydrogel in the acid, base, and organic solution were evaluated, and the results are provided in Table S6. The stability tests were performed by soaking the fresh hydrogel in the various solutions containing acid, base, distilled water, ethanol, toluene, cyclohexane, ethyl acetate, and dichloromethane, respectively, for 24 h followed by determining the values of mass, diameter, and stress modulus of the soaked hydrogel. It was revealed that the mass of the hydrogel was obviously reduced in the organic solution, especially in ethyl acetate and dichloromethane solutions, but slightly increased in the acid and base solutions, as well as evidently increased in distilled water. Correspondingly, the diameter of the hydrogel was clearly reduced in the ethyl acetate and dichloromethane solutions and clearly increased in distilled water, and there was no obvious change in the ethanol, toluene, cyclohexane, acid, and base solutions (Figure. S10). The compressive modulus is a third parameter used to judge the stability of the hydrogel. It was seen that the values of the compressive modulus of the hydrogel were obviously improved after soaking the hydrogel in ethyl acetate and dichloromethane, slightly improved in the toluene solution, and sharply decreased in the acid, base, cyclohexane, and ethanol solutions while still maintaining a quite high mechanical level. Therefore, we can deduce that our hydrogel had excellent acid and alkali-resistance and organic reagent-resistance abilities. For evaluating the cost of the multicomponent hydrogel, the price of each component of the hydrogel was investigated in China. Alginate at the industrial level had the cheapest price at $9.0 /kg, ZnO nanoparticles dispersion of $29/kg, analytical-grade n-isopropylacrylamide of $523/kg, and clay nanosheets of $1.5/kg. If the cost of water is neglected, $26/kg is enough to synthesize 1 g of hydrogel adsorbent in comparison with the market price of most of active carbon products of $15/kg. On the other hand, it was confirmed that the adsorption capacity is low for the REE ions, and the separation selectivity for the REE as well as high regeneration cost is unsatisfying; therefore, we think that the cost for the multicomponent hydrogel is acceptable.

The maximum adsorption capacity of La (III) ions is 182 mg/g. After La (III) adsorption, the mechanical strength of the cryogel shows significant improvement due to the interaction between La3+ ions and alginate. It is expected that our hydrogel might be useful in the fields of wastewater treatment, hydrometallurgy, and biotechnology.

CONCLUSIONS In summary, we fabricated a semi-IPN alginate-clay-PNPAm nanocomposite hydrogel by a mild and versatile frozen polymerization technique with semiconductor ZnO nanoparticles as initiators and clay nanosheets as cross-linkers. This nanocomposite hydrogel exhibits a strong mechanical level, reversible re-deswelling/swelling behaviors, and efficient adsorption properties for La (III) ions. The maximum compressive modulus can reach 16.1 kPa. Distinct swelling and deswelling cycles can be trigged at least three times by alternating temperature changes from 25 to 50 °C. Bulk cryogel can be directly used for the selective adsorption of La (III) from the aqueous solution with a short equilibrium time of 90 min.

ABBREVIATIONS SR swelling ratio Ws weight of swollen gel at temperature T Wd weight of dry gel WR water retention Wt weight of the shrunk gel at specific time t WU water uptake St (Wt −Wd)/Wd ratio at time t S50(60) swelling ratio after the first 60 min of retention at 50 °C St/S50(60) swelling ratio after the first 60 min of retention at 50 °C ΔSR swelling ratio change

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b01691. Data treatment on the pseudo-first-order and pseudosecond-order model equations, Langmuir isotherm equation, Freundlich isotherm equation, and thermodynamics parameter equations; some specifications for the calculation and equation of swelling ratio and shrinking rate; formation of the gels; compressive stress for the various gel samples; values of the compressive parameter; compressive performances of the La cryogel after swelling; adsorption kinetics via the pseudo-first-order and the pseudo-second-order model; Langmuir and Freundlich isotherms of cryogels toward La3+; kinetic model parameters of hydrogel toward La3+; Langmuir and Freundlich isotherm constants and correlation coefficients of the cryogels toward La3+; composition of running water at Tongji University; stability test of the multicomponent hydrogel in the acid, base, and organic solutions; pore size distribution of the hydrogel and calcium alginate hydrogel; adsorption/desorption cycle for the running water; sustainable hydrogel illustration for Cu adsorption; effect of aqueous pH on adsorption for the other REE ions; digital images of the hydrogel in the organic solution; and digital images, strength tests, and adsorption behaviors for the hydrogel samples with alginate concentrations of 1.5 and 3.0% (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos. 21274111 and 51473123) and the Recruitment Program of Global Experts

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ACS Sustainable Chemistry & Engineering ΔHΘ ΔSΘ ΔGΘ Kd Cad (mg/L)

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enthalpy change entropy change free energy equilibrium constant concentration of La (III) loaded onto the cryogel at equilibrium R gas constant T (K) temperature in kelvin qe adsorption capacity uptake % uptake percentage D distribution ratio S separation factor Cini (mg/L) concentration at initial status Ce (mg/L) concentration at equilibrium V (L) volume of the ionic solution m (g) mass of the dry bulk gel sample qt La3+ adsorption amount at time t k1 rate constants of the pseudo-first-order model k2 rate constants of the pseudo-second-order model R2 correlation coefficient KF Freundlich constant n Freundlich parameter qmax maximum of adsorption capacity b (L/mg) Langmuir constant R% recovery percentage

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