Recovery of Lithium(I), Strontium(II), and ... - American Chemical Society

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Recovery of Lithium(I), Strontium(II), and Lanthanum(III) Using Ca− Alginate Beads Dongsu Song,† Seong-Jik Park,‡ Hyun Woo Kang,⊥ Seung Bin Park,† and Jong-In Han*,§ †

Department of Chemical and Biomolecular Engineering, and §Department of Civil and Environmental Engineering, Korea Advanced Institute of Science and Technology, 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea ‡ Department of Bioresources and Rural Systems Engineering, Hankyong National University, 327 Chungang-no, Anseong-si, Kyonggi-do 456-749, Republic of Korea ⊥ Korea Atomic Energy Research Institute, 989-111 Daedeok-daero, Yuseong-gu, Daejeon 305-353, Republic of Korea ABSTRACT: In this paper, Ca−alginate beads were explored as a potent adsorbent for trace metals and rare earth ions. The biosorption ability of Ca−alginate beads toward trace metal ions (i.e., Li, Sr, and La) was investigated under different conditions of contact time, initial concentration, pH, and existence of competing ions. Adsorption characteristics were examined by means of a kinetic model and Langmuir and Fruendlich adsorption isotherms. The electrostatic interaction between the trace metal ions and alginate beads and the effect of the metal ion on adsorbents were studied by X-ray photoelectron spectroscopy (XPS). The results showed that Ca−alginate beads, which are well-known for their special ability to sorb various metals, are also able to take up precious trace metal ions from aqueous solutions.

1. INTRODUCTION The value of inorganics, especially trace metals and rare earth metals, has rapidly increased because of the development of electronic industries. Some major global trace metal suppliers enforce a policy on weaponizing these elements and are also less likely to produce more of such material because of tremendous geographical risk potential. Thus, concerns of shortage and unreasonable increase in price have spread all over the world, especially in industrially integrated countries. Lithium is one such metal that has been used in high-value industries, such as batteries, lubricants, and the ceramics industry.1 Lithium reservoirs are mainly distributed around Bolivia, and proven reserves are forecast to last for only a few decades. Both strontium and lanthanum are also regarded as trace metals because of their limited reserves.2,3 Recently, alternative routes, such as extraction from seawater and urban mining, have been discussed intensely. Sea water contains a substantial amount of trace metals. For instance, concentrations of lithium, gallium, lanthanum, and europium in seawater are 0.173 mg, 3 × 10−5 mg, 3 × 10−6 mg, and 1 × 10−8 mg, respectively.4 Retrieval of metal ions from extremely diluted and complex sources, such as seawater, however, is neither easy nor economical. Sorption methods are some of the most practical approaches that can be suited for such a purpose. Ion-exchange and chelating agents by synthetic materials have been widely used as effective and low-cost adsorbents to collect radionuclides, toxic, poisonous, precious, and base metals from aqueous media.4 There are increasingly growing interests in the search for economically feasible, easily available, and environmentally © 2013 American Chemical Society

friendly substances (e.g., biologically derived substances) appropriate for the efficient removal of target species.5 Sorption with these kinds of biologically originated polymers as adsorbents is collectively called biosorption. Biopolymers with the ability to adsorb multivalent metal ions have been studied to extract specific components. Biosorption phenomena occur naturally in certain biomass, such as algae.6 These natural phenomena in algae have mainly been attributed to the cellular wall properties, where both electrostatic attraction and complexation play an important role in adsorbing metal ions. One unique biopolymer found in brown algae is alginic acid. This biopolymer features carboxyl groups capable of forming complexes with metal ions and has recently been reported to have the ability to sorb heavy metals.7−9 Alginate, a salt of alginic acid, is a kind of polysaccharide biopolymer and is composed of β-1,4-linked D-mannuronic acids (M) and L-guluronic acids (G) arranged along a chain in a heterogeneous form.10 As mentioned above, alginates have a carboxylic group, which is the most frequent acidic functional group in the chain, and has affinities to metallic cations. Specifically, aqueous alginate with divalent or multivalent metal ions is insoluble because of the interaction between two adjacent carboxylic groups in the chain and metal ions. Alginate solution can be converted into insoluble gels (alginate beads) with Ca2+ ions as a cross-linking agent, which is known as the “egg-box model”.11,12 The advantages of using alginate beads as Received: March 27, 2013 Accepted: July 29, 2013 Published: August 15, 2013 2455

dx.doi.org/10.1021/je400317v | J. Chem. Eng. Data 2013, 58, 2455−2464

Journal of Chemical & Engineering Data

Article

in 50 mL conical tubes. All experiments, except for pH tests, were performed at neutral pH conditions. A total of 3 mL of aqueous solution was removed before and after each biosorption experiment, and its concentration was measured using ICP−OES (PerkinElmer, 5300DV). Calcium alginate beads were separated by a stainless-steel filter before measuring the concentration. The amount of adsorbed trace metals was determined by the difference in solution concentration before and after the adsorption. The adsorbed quantity of trace metal ions and the adsorption percentage were evaluated according to the following equations:

biosorbents include biodegradability, environmental regeneration, easy manipulation of hydrophilicity, and unlimited supply. In the present study, we investigated the biosorption property of alginate beads for the purpose of adsorbing trace metal ions from aqueous solutions. Equilibrium model fitting was attempted. To unravel the principal sorption mechanisms, functional groups, surface morphology, and characteristics were characterized by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and Brunauer−Emmett−Teller (BET) analysis. Inductively coupled plasma−optical emission spectrometry (ICP−OES) was also used to quantify the biosorption property.

qe = [(Co − Cf )V ]/M

(mg·g −1)

percent adsorption = [(C0 − Ce)/ C0]100

2. EXPERIMENTAL SECTION 2.1. Reagents. To prepare Ca−alginate beads, sodium alginate powder and calcium chloride dehydrate (CaCl2·2H2O) were obtained from Aldrich. Distilled water (Easy Science, μPure water system) was used for the preparation of aqueous solutions. Trace metal solutions, such as lithium, strontium, and lanthanum, were prepared from lithium nitrate (LiNO3, Aldrich), strontium nitrate [Sr(NO3)2, Aldrich], and lanthanum nitrate hydrate [La(NO3)3·H2O, Aldrich], respectively. The concentration of each trace metal solution was in the range of 10 mg·L−1, 25 mg·L−1, 100 mg·L−1, 250 mg·L−1, and 500 mg· L−1, and prepared solutions were further diluted when other concentrations were necessary. All reagents were used without further purification. The pH of each test solution was adjusted with HCl and NaOH solutions to attain required values (i.e., pH 3, 5, 9, and 11). 2.2. Characterization. The concentrations of trace metal ions of lithium, strontium, and lanthanum were measured by ICP−OES (PerkinElmer, 5300DV). Biosorption by the Ca− alginate beads was examined using a shaker (WiseShake, SHO1D) in batch process at 100 rpm. Both a vacuum-assisted pump (Gast Manufacturing, Inc., DOA-P704-AA) and an oven (Jeio Tech, OF-12GW) were used to dry Ca−alginate beads. SEM (FEI Company, Nova 230) was used for the surface image of beads, and FTIR (Bruker Optiks, IFS66 V/S and HYPERION 3000) was used for functional groups of Ca−alginate beads. In addition, BET analysis (Micromeritics, ASAP2010) was conducted to obtain surface morphological characteristics. To confirm the metal adsorption on Ca−alginate beads, X-ray photoelectron spectroscopy (XPS, Thermo VG Scientific, Sigma Probe) was employed to detect the electron density on Ca−alginate beads. 2.3. Preparation of the Ca−Alginate Beads. Ca− alginate beads were produced using 2 % (w/v) sodium alginate solution and 0.1 mol·L−1 calcium chloride solution. The preparation of Ca−alginate beads was carried out by dropwise addition of sodium alginate solution into CaCl2 solution through a syringe pump (KD Scientific, Inc., KD200) with a stainless-steel needle (25 G) for 1 h with mild stirring. Sodium alginate reacted with Ca2+ in the CaCl2 solution, producing insoluble beads via the cross-linking of the alginate chain. The prepared Ca−alginate beads were gently stirred for at least 1 h to stabilize them. After stirring, the beads were filtered with the help of the vacuum-assisted pump and washed with excess distilled water to remove unreacted CaCl2 solution. The beads were collected and dried at room temperature for at least 24 h. 2.4. Biosorption Experiments of Ca−Alginate Beads. All biosorption experiments were conducted using 1 g of alginate beads with 30 mL solutions containing trace metal ions

(1) (2)

where qe is the amount of adsorbed metal ions per unit amount of the biosorbents, Co and Cf are initial and final trace metal ion concentrations, respectively, V is the volume, and M is the amount of biosorbents used. Adsorption kinetic behaviors and equilibrium sorption tests were carried out in accordance with eqs 1 and 2. Competition tests were also conducted with (1) competition of lithium, strontium, and lanthanum and (2) competition with other metal ions of Na, Co, Cu, Fe, and Al. A blank was maintained as a control. All experiments were performed in duplicate, and the limit of error at each duplicate was ± 5 %.

3. RESULTS AND DISCUSSION 3.1. Ca−Alginate Preparation and Characterization. Ca−alginate beads were prepared by cross-linking sodium alginate with a calcium divalent ion (Ca2+). A schematic representation of the Ca−alginate bead structure is shown in Figure 1. Cross-linking agents, Ca2+ ions (gray-colored circles),

Figure 1. Schematic representation on the macromolecular bead formation of alginate based on the “egg-box model”.

bound adjacent alginate chains, and consequently, insoluble beads were developed. This phenomenon was explained by the “egg-box model”,11,12 and free carboxyl groups, which did not participate in bead formation, act as biosorption sites of metal ions.13 Surface morphology of prepared Ca−alginate beads was confirmed by SEM, as shown in Figure 2. The surface was rather rough, with some wrinkles and pores (Figure 2a). Pores with various sizes were also observed at the surface of beads (Figure 2b). These pores play a role in the sorption by 2456



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Figure 3. FTIR spectrum of (A) Ca−alginate beads and (B) sodium alginate powder.

3.2. Kinetic Model Test of Trace Metal Ions on Ca− Alginate Beads. Adsorption kinetic behaviors of the trace metal ions onto the Ca−alginate beads were examined by means of kinetic models. Initial pH values of aqueous solution containing trace metal ions remained unaltered during the entire experiment. Strontium and lanthanum ions were found to be adsorbed on alginate beads more rapidly than lithium ions. Most of all, trace metal ions were adsorbed during the first 2 h, and equilibrium was reached in approximately 8 h. Thus, the equilibrium contact time was determined to be 8 h, and subsequent experiments were thus carried out under this condition. Kinetic model tests were performed in two series, with high (250 ppm) and low (25 ppm) initial concentrations of trace metal ions to investigate the adsorption behavior at high and low concentrations. The results showed that the kinetics were not affected by the initial concentration, implying that the biosorption process involved not only physical interaction between cations and beads but also chemical equilibrium in aqueous medium. More precisely, the adsorbed amount was at least more than 25 ppm under the high concentration. On the other hand, under the low concentration, all cations were not adsorbed on Ca−alginate beads and showed equilibrium after specific elapsed time. Furthermore, the amount of adsorption was increased in the order of La(III), Sr(II), and Li(I). It is noteworthy that there is a relationship between ionic strength and the amount of adsorbed metal ions. These results support that the process of the biosorption by Ca−alginate may be related to electrostatic interaction between cations and the carboxyl group in beads. As shown in Figure 4, concentrations of all tested metal ions adsorbed during the first 2 h. Under high concentration (250 ppm), lithium was adsorbed on adsorbents approximately 10 %, while strontium was about 40 % and lanthanum was more than 80 %. A similar tendency was observed at 25 ppm (under low concentration): 11 % adsorption of lithium, 83 % adsorption of strontium, and 99 % adsorption of lanthanum. These results indicate that there is indeed a distinct relationship between the valence of metal ions and the amount of adsorption. The pseudo-first-order and pseudo-second-order kinetics15−18 can be represented as follows, respectively:

Figure 2. SEM images of Ca−alginate beads before biosorption at (a) 1000× magnification, (b) 10000× magnification, and after trace metal adsorption at 5000× magnification of (c) lithium, (d) strontium, and (e) lanthanum.

increasing the specific surface area of Ca−alginate beads. BET analysis results are summarized in Table 1. Table 1. BET Analysis Data of Ca−Alginate Beads numerical value BET surface area (m2·g−1) total pore volume (cm3·g−1) pore diameter (nm)

4.7882 0.003251 2.71609

Ca−alginate beads with adsorbed trace metal ions displayed different surface morphology from controls without metal ions (panels c, d, and e of Figure 2). The number of pores decreased, which was probably due to the adsorption. Strontium- and lanthanum-adsorbed beads displayed particularly unique surface morphology. Figure 3 shows FTIR spectra of sodium alginate powder and Ca−alginate beads. FTIR spectra indicate four significant peaks, in contrast with sodium alginate powder, which has only two significant peaks. The Ca−alginate beads appeared to have characteristic band peaks at around (1) v(O−H), 3309 cm−1, (2) v(CO), 1631 cm−1, (3) v(C−OH), 1429 cm−1, and (4) v(OC−OH), 1025 cm−1 and 1072 cm−1,14 while sodium alginate peaks appeared at about (1) v(O−H), 3268 cm−1 and (2) v(C−C, long aliphatic chain), 768 cm−1.

qt = qe(1 − e−k1t ) 2457

(3)



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Figure 4. Effect of time on (a and b) lithium, (c and d) strontium, and (e and f) lanthanum adsorption at low and high concentrations, respectively.

t 1 t = + qt qe k 2qe 2

Table 2. Kinetic Equilibrium Parameters of Trace Metal Ion Adsorption on Ca−Alginate Beads in Terms of Low Initial Concentration

(4)

where K1 (min−1) is the pseudo first-order rate constant and K2 (g·mg−1·min−1) is the rate constant of the pseudo-second-order adsorption process. qt and qe (mg·g−1) are the adsorption capacities at time t (min) and equilibrium, respectively. The adsorption behavior of lithium was found to be best described with a pseudo-second-order model, regardless of the initial concentration. In the case of strontium and lanthanum, different kinetic models had to be employed: at low concentration, both ions were suited to both pseudo-first order and pseudo-second order, whereas at high concentration, the kinetics of strontium were well-modeled with pseudo-first order and those of lanthanum were well-modeled with pseudosecond order. The obtained kinetic parameters are summarized in Tables 2 and 3. 3.3. Equilibrium Sorption Test. To study equilibrium isotherm, Langmuir and Freundlich isotherms were fitted to the

(a) Pseudo-First Order qe lithium strontium lanthanum

lithium strontium lanthanum

K1

0.079 1.910 0.557 3.228 0.794 5.998 (b) Pseudo-Second Order

R2 0.781 0.999 1.000

qe

K2

R2

0.087 0.560 0.794

26.013 48.950 571.831

0.825 0.999 1.000

experimental data. The Langmuir isotherm mainly describes chemisorption processes and can explain monolayer adsorption between adsorbents and solids.19 The Langmuir isotherm model can be expressed as follows: 2458



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Table 3. Kinetic Equilibrium Parameters of Trace Metal Ion Adsorption on Ca−Alginate Beads in Terms of High Initial Concentration

Table 4. Isotherm Parameters of Trace Metal Ion Adsorption on Ca−Alginate Beads Langmuir parameters

(a) Pseudo-First Order qe lithium strontium lanthanum

lithium strontium lanthanum

qe =

K1

0.616 407.407 3.258 2.487 6.373 1.166 (b) Pseudo-Second Order

Qm

KL

0.835 0.996 0.996

0.562

0.00433

6.695

0.0120

8.625

0.484

R

2

qe

K2

R2

0.589 3.297 6.746

12.511 3.620 0.326

0.851 0.994 0.998

Q mKLCe X = m 1 + KLCe

Freundlich parameters R2

KF

(a) Lithium 0.814 0.000869 (b) Strontium 0.927 0.324 (c) Lanthanum 0.928 2.105

n

R2

0.846

0.983

0.498

0.974

0.310

0.981

(5)

A linear form of this equation is Ce C 1 = + e qe Q mKL Qm

(6)

where X is the initial concentration of solute minus the final concentration of solute in aqueous media at equilibrium (mg· L−1), m is the concentration of adsorbent (g·L−1), Ce (mg·L−1) is the equilibrium concentration of metal ion in solution, qe is the amount of metal ions adsorbed on adsorbents (mg·g−1), KL is the Langmuir equilibrium constant (L·g−1), and Qm is the maximum sorption capacity (theoretical monolayer saturation capacity). The value of KL relates to the adsorption capacity and thermodynamic characteristics. The Freundlich isotherm can be discriminated by heterogeneous surfaces and the multilayer adsorption process. The Freundlich process can be demonstrated by the following equation: qe = KFCe1/ n

(7)

A linear form of this equation is log qe = log KF +

1 log Ce n

(8)

where qe is the amount of solute adsorbed, Ce is the amount of unadsorbed solutes in solution, KF (L·g−1) is the Freundlich constant, and n is the Freundlich exponent. The 1/n value is mainly related to the intensity and substance of adsorption. In the present study, nonlinear methods were used rather than linear methods. The linear regression was the method of least-squares and supposes that the scattering points around the line follow a Gaussian distribution. Also, parameters derived from linear methods could be easily variable with respect to the different linearized forms of isotherm equations.20,21 The adsorption parameters are summarized in Table 4. The sorption data of trace metal ions have been known to be properly described with either the Langmuir or Fruendlich isotherm.22 Interestingly, however, this was not the case with lithium over the initial concentration range from 0 ppm to 50 ppm, as shown in Figure 5a. This phenomenon might be caused by the limited accessibility of the monovalent charge of lithium ions to the available functional groups of alginate beads. In the case of multivalent ions, such as strontium and lanthanum ions, the isotherms were well-fitted, as shown in panels b and c of Figure 5.

Figure 5. Langmuir and Freundlich sorption isotherms of trace metal ions on Ca−alginate beads: (a) lithium, (b) strontium, and (c) lanthanum.

The Freundlich isotherm exhibited much higher r2 values than the Langmuir model, implying that the sorption by Ca− 2459



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alginate beads was dominated by multilayer adsorption. Furthermore, as mentioned above, the index of adsorption strength 1/n increased with the valences of cations, supporting that the biosorption process on Ca−alginate beads is related to electrostatic interaction. 3.4. Effect of Initial pH. Just like typical sorption reactions, pH is known to substantially influence the biosorption process on Ca−alginate beads by means of changing the status of metal ions and surface functional groups of the adsorbent. To see if this is the case, the effect of pH, ranging from 3.0 to 11.0, was investigated at high (250 ppm) and low (25 ppm) initial concentrations at room temperature. As shown in Figure 6, pH indeed affected the sorption and the highest biosorption capacity was generally observed in neutral pH. However, metals

at 25 ppm did not follow this general trend, suggesting that the adsorption on Ca−alginate beads is rather a kind of physical process, and thus, pH plays a minimal role, particularly at low concentration. The highest adsorption of lithium ions occurred at around pH 7. The adsorbed amount, albeit lower than the other multivalent ions, increased from 4.4 % to 7.4 % with an increase in pH from 3 to 7 and then decreased within 20 % in the basic region, as seen in Figure 6a. Strontium was adsorbed best in the neutral pH region with no change in the amount of adsorbed metal ions in the basic domain. Lanthanum, with the highest adsorption because of the nature of trivalent charges of ions, showed the highest adsorption property in the pH range of 5 to 7 (above 90 %). 3.5. Competition Test. The effects of coexisting ions on the sorption phenomenon were also investigated. This information is of prime importance, especially for the industrial application of Ca−alginate beads for trace metal extraction from seawater. In fact, ions in seawater are known as potent competitors for extracting trace metals of interest.23 All experiments were performed in duplicates, and the mean value was represented. The bars indicate metal ion uptakes as a percentage in Figures 7 and 8. As shown in Figure 7a, the

Figure 7. Study on the effect of competition between trace metal ions on adsorption by Ca−alginate beads: (a) high (250 ppm) initial concentration and (b) low (25 ppm) initial concentration.

adsorbed amounts of high initial concentration were found to be 1.64 % (lithium), 93.82 % (strontium), and 99.30 % (lanthanum). Similarly, in the case of low initial concentration, the uptakes of lithium, strontium, and lanthanum ion were 5.62 %, 82.28 %, and 98.45 %, respectively (Figure 7b). Values from panels a and b of Figure 7 were not much different from those

Figure 6. Influence of pH on biosorption property of trace metal ion on Ca−alginate beads: (a) lithium, (b) strontium, and (c) lanthanum. 2460



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Figure 8. Study on the effect of competition on adsorption of trace metal ions by Ca−alginate beads: competition of other cations with (a) lithium, (b) strontium, (c) lanthanum and (d) Li, Sr, and La.

between trace metal ions and functional groups of Ca−alginate beads. Figure 9 shows the typical XPS spectrum of alginate beads before and after the adsorption of the trace metal ions. The

in individual adsorption experiments (Table 5), meaning that the adsorption on Ca−alginate beads was not affected by the coexistence of other metal cations. Table 5. Competition Test of Lithium, Strontium, and Lanthanum on Ca−Alginate Beads lithium (%) high initial concentration (250 ppm) low initial concentration (25 ppm) differences (ppm)

strontium (%)

lanthanum (%)

1.64

93.82

99.30

5.62

82.28

98.45

−3.98

11.54

0.85

Four parallel experiments were also performed to prove the potential of industrial application of the Ca−alginate beads to recover trace metal ions. Strontium and lanthanum but not lithium exhibited interference in the sorption property by up to 50 %, which was probably due to the competition at active sites on the beads, as shown in panels a, b, c, and d of Figure 8. 3.6. XPS Analysis. Explanation of the phenomenon of trace metal adsorption on Ca−alginate beads was attempted through the analysis of the electron configuration of specific elements on beads by means of XPS.24 XPS can be used to characterize ligand effects in transition-metal complexes. Normally, electrondonating groups lower the binding energy (BE) of the core level electrons, and electron-withdrawing groups acted reversely.25 Qualitative and quantitative analysis on the biosorption process on Ca−alginate beads were performed by XPS. Qualitative adsorption mechanisms between cations and Ca−alginate beads were evaluated by XPS analysis. The electron configurations of carbon and oxygen on biopolymer chains were investigated in terms of electrostatic interaction

Figure 9. XPS spectra of alginate beads (A) before and after (B) Li+, (C) Sr2+, and (D) La3+ adsorption.

presence of lithium ions on alginate beads was not obvious, but strontium and lanthanum ions showed distinguishable peaks compared to the controls with no metal adsorbed, which was in good agreement with the amount of adsorbed metal ions. A strong La 3d peak was observed on alginate beads with trace metal ions adsorbed.26 The typical C 1s XPS spectrum of alginate beads before and after the biosorption is shown in Figure 10. Without metal 2461



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Figure 11 indicates the O 1s XPS spectrum of alginate beads before and after trace metal adsorption. The analogous tendency of peak movement was observed in the O 1s XPS spectrum. The electron cloud density of oxygen atoms was reduced because of the trace metal ion adsorption. Figures 10 and 11 strongly supported that trace metal ions were indeed bound to the alginate chain, especially at carboxyl groups. However, the behavior of XPS spectrum peaks was different from valences of adsorbed metal ions. Before trace metal adsorption, the ratio of oxygen functional groups was 0.47 (O2−):1 (O−CO):0.48 (C−O).28,29 In the case of oddvalence ions (e.g., Li+ and La3+), the ratio of the O−CO group was much higher than those of O2− and C−O groups. On the other hand, in the case of even-valence ions (Sr2+), when adsorbed on the alginate beads, the ratios of O2− and C− O groups were much higher than that of the O−CO group. It is possible that +2 valence ions favor adjacent two carboxyl groups, so that the ratio of the O−CO group is largely decreased. Table 6 indicates the experimental oxygen group ratios for alginate beads. Figure 12 shows the XPS spectrum of Li(I),30,31 Sr(II),32,33 and La(III)34,35 trace metal ions adsorbed on alginate beads. The peak intensity of the XPS spectrum indicates the relative amount of adsorbed metal ions. As shown in Figure 12c, the peak intensity of La 3d was biggest. This phenomenon was well-reflected by the adsorbed amount of La ions on alginate beads, as shown in Figure 4c.

Figure 10. C 1s XPS spectra of the alginate beads (A) before and after (B) Li+, (C) Sr2+, and (D) La3+ adsorption.

adsorption, there was one peak in the C 1s spectrum at a BE of 286.8 eV. On the other hand, after the metal adsorption, the peak was moved to a slightly lower BE as the valence of trace metal ions increased. This means that C atoms in the alginate beads became more reduced, which was caused by the adsorption.27 From the C 1s XPS spectrum, the electron cloud density between carbon and oxygen atoms was moved to carbon atoms. Furthermore, because of the trace metal ion adsorption on alginate beads, electrons in the carbon atoms were donated, which was also supported by Figure 11.

Figure 11. O 1s XPS spectra of the alginate beads (A) before and after (B) Li+, (C) Sr2+, and (D) La3+ adsorption. 2462



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Table 6. Assignments of O 1s Bands Based on Their BE and Relative Ratio (A) before and after (B) Li+, (C) Sr2+, and (D) La3+ Adsorption BE (eV)

relative ratio

type of beads

O2− (scan A)

O−CO (scan B)

C−O (scan C)

O2−/O−CO

C−O/O−CO

A B C D

531.05 531.46 531.19 531.38

533.4 532.96 533.5 532.82

534.71 534.53 535.55 534.79

0.47 0.16 3.86 0.24

0.48 0.02 4.76 0.13

4. CONCLUSION The adsorption of trace metal ions from aqueous solution was performed using Ca−alginate beads and conventional adsorption techniques. Ca−alginate beads were added to trace metal ion solution to adsorb metal ions. The results of this study indicated that Ca−alginate beads can act as sorbents for extraction of trace metal ions from aqueous media. The adsorption amount mainly depends upon valences of metal ions, because the adsorption process is related to electrostatic interaction. Kinetics were fitted well to a pseudo-second-order model, and isotherms of the adsorption process were more fit to the Freundlich equation than the Langmuir equation. Furthermore, coexisting ions were not largely affected by the adsorption properties of trace metal ions. In summary, the results of this study suggest that Ca−alginate beads may be good sources for extracting trace metals and rare earth ions from aqueous environments.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +82-42-350-3629. Fax: +82-42-350-3610. E-mail: [email protected]. Funding

This work was supported by the Advanced Biomass Research and Development Center (ABC) of Korea Grant funded by the Ministry of Education, Science and Technology (20100029728). Notes

The authors declare no competing financial interest.

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REFERENCES

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Figure 12. Trace metal ion XPS spectra of the alginate beads after (a) Li+, (b) Sr2+, and (c) La3+ adsorption.

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Recovery of Lithium(I), Strontium(II), and ... - American Chemical Society

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