Recovery of rare-earth metal neodymium from aqueous solutions by poly-γ-glutamic acid and its sodium salt as biosorbents: Effects of solution pH on neodymium recovery mechanisms

Accepted Manuscript Recoveryof rare-earth metal neodymium from aqueous solutions by poly-γ-glutamic acid and its sodium salt as biosorbents: Effects of solution pH on neodymium recovery mechanisms Misaki Hisada, Yoshinori Kawase PII:

S1002-0721(18)30090-5

DOI:

10.1016/j.jre.2018.01.001

Reference:

JRE 126

To appear in:

Journal of Rare Earths

Received Date: 15 August 2017 Revised Date:

17 December 2017

Accepted Date: 24 January 2018

Please cite this article as: Hisada M, Kawase Y, Recoveryof rare-earth metal neodymium from aqueous solutions by poly-γ-glutamic acid and its sodium salt as biosorbents: Effects of solution pH on neodymium recovery mechanisms, Journal of Rare Earths (2018), doi: 10.1016/j.jre.2018.01.001. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Recoveryof rare-earth metal neodymium from aqueous solutions by poly-γγ-glutamic acid and its sodium salt as biosorbents: Effects of solution pH on neodymium recovery mechanisms Misaki Hisada, Yoshinori Kawase* (Research Center for Biochemical and Environmental Engineering, Department of Applied Chemistry, Toyo

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University, 2100 Kujirai, Kawagoe, Saitama, 350-8585, Japan)

Foundation item: Project supported by Toyo University (g-019-247)

*Corresponding author: Yoshinori Kawase(E-mail: [email protected], [email protected]; Tel.: +81-49-239-1377)

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Abstract: For recovery of metals from low-concentration sources, biosorption is one of promising technologies and poly-γ-glutamic acid (γ-PGA) has been known as apotential biosorbent for recovery of heavy metals from aqueous solutions. Effects of solution pH on recovery of rare-earth metal Nd were systematically examined to clarify

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mechanisms of Nd recovery by γ-PGA and its sodium salt (γ-PGANa). The recovery efficiency of Nd by γ-PGA increases from 2.4% to 19.6 % as pH increases from 2 to 4. Subsequently the Nd recovery efficiencies for γ-PGA and γ-PGANa remain almost constant in the range of pH from 4 to 7. For pH>7 the increase in Nd recovery is significant and 100% recovery of Nd was achieved at pH 9. The pH dependency on Nd recovery by γ-PGANa was similar to that by γ-PGA. Contributions of adsorption and precipitation/coagulation to Nd recovery process were quantified. Whereas the adsorption dominates Nd recovery at lower pH (<~4), the precipitation/coagulation

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controlles Nd recovery process for pH>7. At higher pH, purple gel precipitates are observed. The maximum adsorption capacities for γ-PGA and γ-PGANa are 215 mg-Nd/(g-γ-PGA) at pH 4 and 305 mg-Nd/(g-γ-PGANa) at pH 3, respectively. From the spectra of FT-IR and XPS, the biosorption of Nd onto γ-PGA and γ-PGANavia electrostatic interaction with carboxylate anions at pH 3 is verified. The Nd complexation with amide and

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carboxylate anion groups onγ-PGA and γ-PGANa may also contribute to the Nd recovery. The biosorption isotherms for Nd recovery by γ-PGA and γ-PGANa can be satisfactoryilyfitted by the Langmuir model.

The

thermodynamic studies suggest that the biosorptions of Nd by γ-PGA and γ-PGANa are endothermic. The

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utilization of γ-PGA and γ-PGANa as potential and eco-friendly biosorbents for the highly effective recovery of Nd from aqueous solution is confirmed. Keywords: rare-earth metal Nd recovery; poly-γ-glutamic acid; biosorption; precipitation; coexisting cations Rare-earth metals have been utilized in diverse industrial applications due to their magnetic and conductive properties such as permanent magnets, lamp phosphors, catalysts and rechargeable batteries. wastewaters discharged to aquatic environments sometimes contain rare-earth metals

[1-3]

.

Their presence in the

aquatic environment is of great concern because of their toxicity even at lower concentrations. non-biodegradable and their removal from the environment is very difficult.

Industrial

They are

They are not only highlytoxic but

also significantly precious. Because of their low and localized reserves, recovery of valuable rare-earth metals from industrial wastewaters is very important as a secondary resource. Therefore, the recovery of rare-earth has recently

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ACCEPTED MANUSCRIPT received wide attention from environmental and economical viewpoints.

The recovery of heavy metals from

aqueous solutions has been conventionally carried out by physicochemical processes such as coagulation, membrane separation and solvent extraction [3]. low.

The concentration of rare earth metals in wastewaters is usually

Some of conventional physicochemical processes are costly and inefficient at low metal concentrations and

furthermore generate undesirable secondary residues.

For example, in chemical precipitation processes a large

amount of toxic sludge are generated and extra costs for sludge treatment and disposal are required[4, 5].

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Biosorption has been identified as one of potential technologies for recovery of metals from low-concentration sourcesdue to its high efficiency and simple operation[2,6–10].

Biosorption by poly-γ-glutamic acid (γ-PGA) may offer an alternative to conventional techniques for heavy metal recovery

γ-PGA is an anionic biopolymer made of numerous glutamic acid units (Fig. S1).

[2, 11-13]

.

Itconsists of a large number of carboxyl groupsand amide groups, which provide high cation exchange capacity and

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interaction with metals, respectively [11]. Therefore, γ-PGA has been expected to be a potential and highly efficient biosorbent for the recovery of heavy metals from aqueous solutions. It is a nontoxic and biodegradable extracellular polymeric substance biosynthesized by Bacillus sp. Commercial products of γ-PGA and its salts have been

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manufactured using fermentation technology[11,12].

The aim of this study is to quantify biosorption recovery of rare-earth metal neodymium (Nd) from aqueous solutions by γ-PGA and its sodium salt (γ-PGANa). Nd chosen as a model rare-earth metalin this study is one of the most insufficient rare-earth elements and development of more efficient recovery technologies of Nd has been desired. Nd recovery by adsorption has been investigated using several adsorbents [14-19].

Recovery of rare earth

metals by biosorption has recently gained attention since biosorption is efficient to recover metal ions from Wang et al. [20] studied adsorption of rare earths(III) by calcium alginate-poly glutamic

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low-concentration sources.

acid hybrid gels and obtained the maximum adsorption capacity of Nd(III) of 237.6 mg-Nd/(g-adsorbent). Biosorption of Nd has been examined by Oliveira and Garcia

[21]

, Heilmann et al.

[22]

and Kucuker et al.

[23]

.

In

their studies, however, the bindings between the metal species and functional groups on biosorbents were not

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examined via the characterization of biosorbents. No systematic studies have been reported on the recovery of Ndby γ-PGA and its sodium salt. To quantifythe capability ofγ-PGA and γ-PGANa for Nd recovery, the surfaces of γ-PGA and γ-PGANa before and after Nd biosorption were examined using Fourier transform infrared spectroscopy

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(FT-IR) and X-ray photoelectron spectroscopy (XPS).In this study, the roles of biosorption in the Nd recovery process affected by solution pH were particularly concerned. This is the first paper quantifying the contributions of biosorption and precipitation/coagulationon Nd recovery by γ-PGA and γ-PGANa in the wide range of pH from 2 to 9. The effect of coexisting cations, Cu and Cs, on biosorption of Nd by γ-PGA and γ-PGANa was also studied. 1 Materials and methods 1.1 Chemicals Neodymium (III) chloride hexahydrateas a Nd source was purchased from Wako Pure Chemical Industries, Ltd. (Japan).Reagent-grade cesium chloride and copper(II) sulfate pentahyd rate used to clarify effects of coexisting cations were purchased from KantoChemical Co. (Japan). The reagent-grade γ-PGA (MW: (1.5~2.5)×106) and sodium salt form poly-γ-L-glutamic acid (γ-PGANa) (Mn: 260,000) were purchased from Wako Pure Chemical

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ACCEPTED MANUSCRIPT Industries, Ltd. (Japan) and supplied by Toyobo Co., Ltd. (Japan), respectively. While the poly-γ-glutamic acid was insoluble in water, its sodium salt was soluble.

All reagents were used as received without further purification.

1.2Biosorption of Nd byγγ-PGA and γ-PGANa A 0.5 L Pyrex glass cylindrical reactor was used to conduct experiments of Nd recovery in abatch mode. The Nd loading was altered from 20 to 150 mg/L to obtain biosorption isotherms. Although this initial Nd concentration

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range is larger than the typical concentrations in the aquatic environment, it was chosen for highly accurate determination of Nd concentration as well as the studies in the literature[20,21,24]. The dosages of both γ-PGA and γ-PGANa were fixed at 100 mg/L on the basis of the preliminary experimental results in the range of dosage <400 mg/L (Fig. S2). The reactor was put in a water bath controlled at specified temperatures. The experiments for biosorption isotherm were conducted at three different temperatures, i.e., 283, 298 and 313 K. The liquid samples

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were withdrawn using a syringe and filtrated bya 0.20 µm disposable membrane filter [12]. The Polarized Zeeman Atomic Absorption Spectrophotometry Z-5300 (HITACHI Ltd., Japan) was utilized to measure the concentrations

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of Nd, Cs and Cuin the liquid sample was measured.

1.3 Contributions of biosorption and precipitation/coagulation to Nd recovery process The precipitation/coagulation of Nd might play an important role in the Nd recovery process at higher pH[25]. In fact, as described below, the precipitate/coagulation was observed for pH

4. Therefore, the contributions of

“adsorption” and “precipitation/coagulation” to Nd recovery process were quantitatively distinguished. The definition of their contributions was carried out after the Nd recovery batch experiments. The measurement steps

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for the definition of their contributions to Nd recovery are as follows[26]: (1) separation of the precipitate/ coagulation from the solution using the filtration, (2) dissolution of filtered precipitate/coagulation with 10 wt% nitric acid at pH ~2, (3) measurement of dissolved Nd concentration and estimation of the efficiency of Nd recovery via precipitation/coagulation and (4) estimation of the efficiency of Nd recovery via adsorption by

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subtracting the Nd recovery efficiency via precipitation/coagulation from the total Nd recovery efficiency. 1.4 Characterization of γ-PGA and γ-PGANa

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To clarify the interaction between metal species and main functional groups on the surfaces of γ-PGA and γ-PGANa in liquid samples before and after biosorption, the Fouriertrans form infrared (FT-IR) attenuated total reflectance (ATR) spectroscopy was utilized. The transmission spectra in the frequency range of 500–4000 cm–1 were measured with 4 cm–1 resolution using a Nicolet iS50 FT-IR spectrometer (Thermo SCIENTIFIC, Inc., USA). To identify the compositions of γ-PGA and γ-PGANa in the spectra range of 0–1100 eV, X-ray photoelectron spectroscopy (XPS) measurements were performed on PHI Quantera IITM (ULVAC-PHI, Inc., Japan).

1.5 Biosorption isotherm The Langmuir model is one of well-known models for adsorption isotherm [7, 9].

qe =

qm K L C e 1 + K L Ce

(1)

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The model can be written as:

ACCEPTED MANUSCRIPT Where qe and Ce denote the amount of Nd adsorbed on the biosorbent and the equilibrium concentration in aqueous solution, respectively. Here, qm and KL are the asymptotic maximum biosorption capacity of biosorbent and the equilibrium biosorption constant, respectively. The Freundlich model can be represented as [9]:

qe = K FCe1 n

(2)

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where KF and n are the constant for relative biosorption capacity and the heterogeneity factor, respectively. The constants in the models were determined by fitting the models to the biosorption isotherm data.

The values

of qm and KL inthe Langmuir model were obtained from the slope and the intercept of the plots of Ce/qe vs Ce, respectively. The constants in the Freundlich model, n and KF, were evaluated from the slope and the intercept of

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the plot of ln(qe) vs ln(Ce), respectively. 1.6 Biosorption thermodynamics

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To evaluate the thermodynamic parameters for biosorption of Nd by γ-PGA and γ-PGANa, the following equations were utilized.

∆G0=−RTln(KL) ∆H0=−Rln(KL1/KL2)/(1/T1−1/T2)

(5)

∆S =−(∆G −∆H )/T 0

0

(4)

0

(6)

where ∆G0, ∆H0 and∆S0are standard Gibbs free energy change, standard enthalpy change and standard entropy change, respectively. R and Tare gas constant and temperature, respectively. KL1 and KL2 denote the Langmuir

2 Results and discussion

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constants at T1 and T2, respectively.

2.1Effect of solution pH on Nd recovery by γ-PGA and γ-PGANa

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It has been known that solution pH is one of the most important parameters in biosorption processes. Effects of pH on Nd recovery by γ-PGA and γ-PGANa were examined in the range of pH from 2 to 9. As seen in Fig. 1(a), the Nd recovery efficiencies by γ-PGA and γ-PGANa at pH 2were practically nominal. With increasing pH from 2 to 4,

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the recovery efficiencies by γ-PGA and γ-PGANa increased to 19.6% and 25.6%, respectively. Subsequently the Nd recovery efficiencies for γ-PGA and γ-PGANa remained almost constant in the range of pH from 4 to 7. These dependencies of solution pH on Nd recovery for pH<7 coincide with that by hybrid alginate-silica microspheres [15]. Thereafter the recovery efficiencies significantly increased as solution pH increased and finally reached 100 % at pH 9.

Since γ-PGA consists of a large number of carboxyl groups, chemical reactions through electrostatic interaction might take place between anionic carboxylate ion (−COO−) and cationic ion Nd3+. The pKa of glutamic acid is ~4[12] and the pKa of functional group COOH is in the range of 2–5 [27]. For pH<4, therefore, carboxyl groups inγ-PGA and γ-PGANa were partially ionized and the adsorption of Nd3+ onto γ-PGA and γ-PGANa by electrostatic interaction was rather restricted[25]. At lower pH, moreover, γ-PGA formed intermolecular hydrogen bonds, which is attributed to more compact α-helical conformation and as a result decreased the number of functional groups

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ACCEPTED MANUSCRIPT available for Nd3+adsorption[11,25]. Accordingly, when pH was lower than 4, the Nd recovery efficiency was not high. At higher pH (pH>pKa~4), surface sites on the biosorbent were negatively charged. This was favorable to the biosorption of cations by electrostatic binding.

At higher pH, therefore, the biosorption of Nd3+might be enhanced.

In the pH range from 5 to 6, the Nd adsorption was enhanced by both the deprotonation of carboxyl groups on γ-PGA and the transition from α-helical conformation to random-coil conformation in γ-PGA[24]. Thisα-helix-coilconformation transition increased the number of active site on γ-PGA surface available for Gooding

al.[25] reported

et

that

the

midpoint

of

the

conformation

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Nd3+biosorption.

transition

fromα-helixtorandom-coil is about pH 5.1–6.0. Furthermore, the interaction of Nd with γ-PGA might be 3+

intensified by forming its complexation with amide and carboxylate anion groups[28]. Amide functional groups having weak electron donating nature interact with Nd species [12]. In the pH range from 4 to 7, however, the Nd recovery efficiencies for γ-PGA and γ-PGANa remained almost constant.

In the literature [13, 24, 29] it is reported

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that the hydrolysis of Nd3+ formed NdOH2+ (pH>2.2) and Nd(OH)2+ (pH>5.5) and the precipitation of Nd (Nd(OH)3) occurred for pH>6. Furthermore, the pKa of amine groups is 6.3 [29]. Consequently, the adsorption of Nd

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might be largely prohibited for pH>5.5.

It can be seen in Fig. 1(a) that the Nd recovery efficiency significantly increased for pH>7 and reached 100% recovery at pH 9. As shown in the photograph in Fig. 1(a), in this study the precipitation/coagulation of Nd was observed for pH>4. The purple colored gel precipitate was formed. With increasing pH, the amount of formed precipitate increased and its color deepened. The precipitate might include the complexation of Nd species and γ-PGA or γ-PGANa involving amide and carboxylate anion groups [12]. In this study, therefore, the experiments to quantify the contributions of adsorption and precipitation/coagulation to the Nd recovery were conducted. The

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experimental results in Fig. 1bclearly reveal that the adsorption was dominant in the Nd recovery for pH<4 and the precipitation/coagulation governed the Nd recovery for pH>7. In the range of 44. When pH

3, the contribution of adsorption

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to Nd recovery was 100%. For pH 8, on the other hand, the contribution of precipitation/coagulation was 100 %. Fig. 1(c) depicts the adsorption efficiencies of Nd by γ-PGA and γ-PGANa evaluated by subtracting the amount of Nd in precipitation/coagulation from the total Nd recovery amount. The adsorption efficiency (%) or adsorption

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capacity (mg-Nd/(g-γ-PGA)) significantly increased with increasing pH from 2 to 3, subsequently decreased and finally reached zero at pH 8. The maximum adsorption capacities for γ-PGA and γ-PGANa were 215 mg-Nd/(g-γ-PGA) at pH 4 and 305 mg-Nd/(g-γ-PGANa) at pH 3, respectively. The electronegativity of Nd (1.14) is larger than that of Na (0.93) and smaller than that of H (2.20). This suggests that Nd3+ was more favorable to replace Na+ as compared with H+ in carboxyl groups. Therefore, the Nd adsorption capacity by γ-PGANa was slightly better than that by γ-PGA. Themaximum adsorption capacities for γ-PGA and γ-PGANa in this study are comparable to and higher than that of 238 mg-Nd/(g-adsorbent) obtained by the calcium alginate-ploy glutamic acid hybrid gel

[20]

, respectively. Xiong et al.

[18]

reported the maximum adsorption capacity of 232.56

mg-Nd/(g-adsorbent) for D113-III resin, which is comparable to that for γ-PGA and about 25% lower than that for γ-PGANa in this study. The maximum adsorption capacity of 126.5 mg-Nd/(g-adsorbent) estimated for the lanthanide-ion imprinted polymers by Yusoff et al. [10] is approximately half of those for γ-PGA and γ-PGANa

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ACCEPTED MANUSCRIPT obtained in this study. The maximum adsorption capacity of 189 mg-Nd/(g-adsorbent) was obtained using Chlorella vulgaris by Kucuker et al. [23]. It is around 22% less than the present result for γ-PGA. Heilmann et al. [22] reported the maximum adsorption capacities of 69 and 107 mg-Nd/(g-adsorbent) using Chlamydomonas reinhardtii and Calothrix brevissima, respectively. They are considerably lower as compared with the present results. The maximum adsorption capacity of 101 mg-Nd/(g-adsorbent) found using Sargassum sp. by Oliveira and Garcia [21] is less than the half of the present result for γ-PGA.

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FT-IR analyses for the elucidation of the mechanisms for Nd biosorption by γ-PGA and γ-PGANa were conducted. Since, as seen in Fig. 1(b), the recoveries of Nd by γ-PGA and γ-PGANa for pH 3 were completely controlled by adsorption, the FT-IR ATR spectra of γ-PGA and γ-PGANa before and after biosorption were obtained at pH 3. The FT-IR spectra at pH 3 are depicted in Fig. 2. For reference, the FT-IR ATR spectra of γ-PGA and γ-PGANa at pH 6 and 9 are given in Figs. S3 and S4, respectively. While no distinguished peaks at 1400 cm–1

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representing –COO─ (symmetric) practically appeared, the peaks at 1600 cm–1 assigned for –COO─ (asymmetric) before biosorption for γ-PGA and γ-PGANa sifted to 1640 cm–1 after biosorption. Although the complexation between H+ and –COO─ group was rather firm, some cation exchanges took place between H+ and Nd3+ or Na+ and

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Nd3+.These results were attributed to electrostatic interaction between the negatively charged –COO─ groups ofγ-PGA and γ-PGANa and the positively charged Nd3+ [30]. The broad peak for NH stretch of amides attributed to the overlap of O−H, N−H and C−H stretching vibrations was placed at 3250 cm–1 [20]. The peak of broad band for γ-PGA shifted to lower frequency after biosorption of Nd[31]. On the contrary, as shown in Fig. 2(b) the peak of band attributed to O–H/N–H for γ-PGANa sifted to higher frequency due to the replacement of Na by Nd as well as the results for Cd2+ biosorption by extracellular polymeric substances [32]. It is clear from Figs. 2 S3 and S4 that the

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peaks for –COO─ and O−H/N−H or the sites for adsorption of Nd decreased with increasing pH. This confirms that the adsorption was suppressed at higher pH.

Fig. 3 depicts the XPS spectra before and after biosorption of Nd at pH 3. As seen in the XPS survey spectra (Fig. 3(a)), it is clear that C, O, N, Nd and Na were the main elements on γ-PGA and γ-PGANa surfaces. For γ-PGA, of

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course, the peak at ~1071 eV assigned to Na (Na 1s) was not obtained both before and after biosorption. It is seen in Fig. 3(a) that for γ-PGANa the peak corresponding to Na found before biosorption practically disappeared after biosorption of Nd. This reveals that most of Na ions on the surface of γ-PGANa were replaced by Nd ions.

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The C 1s spectra for γ-PGA in Fig. 3(b) indicate that the two peaks at ~285 and ~288 eV assigned to –C−C− and N−C=O, respectively [29] could be found but no peaks at ~ 292 eV assigned to –COO− [27] practically appeared both before and after biosorption. This reveals that a small number ofcarboxylates in ionized form existed for γ-PGA. For γ-PGANa, on the other hand, the peaks at 284.5 and 293 eV assigned to −C−C− and –COO−, respectively were found. Furthermore, the peak corresponding to N−C=O shifted to higher binding energy after biosorption. The increase in electronegativity of the attached atoms due to the replacement of Na by Nd might be responsible to the shift of the peak [30, 31]. The O 1s spectra for γ-PGA and γ-PGANa before and after biosorption are shown in Fig. 3(c). For γ-PGA, the peak at ~532.5 eV assigned to –COO was foundand shifted from ~532.5 to ~531 eV after biosorption [33]. This shift might be due to the transition from –COOH to –COONd. The electronegativity for Nd is less as compared with that for H. Therefore, the positive charge of oxygen bonded with H+ in –COOH before biosorption decreased after

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ACCEPTED MANUSCRIPT biosorption of Nd. As a result, the energy for emission of an electron from oxygen atom decreased and the peak shifted to lower after adsorption. For γ-PGANa, as seen in Fig. 3(c), the observed shift from ~532 to 532.5 eV after biosorption might be attributed to the change from –COONa to –COONd. Since the electronegativity of Nd is slightly larger than that of Na, the necessary energy to emit an electron from oxygen atom slightly increased. The N 1s core-level spectra for γ-PGA and γ-PGANa are given in Fig. 3(d). The peaks assigned to –NH2 or –NH for γ-PGA and γ-PGANa[34] slightly shifted to lower and higher binding energy, respectively.

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The peaks at ~983 eV presented in Fig. 3(e) are assigned to COONd [35]. The increases in intensity of the Nd 3d5/2 spectra for γ-PGA and γ-PGANa after biosorption clearly confirmed the adsorption and precipitation/coagulation of Nd on the surface of γ-PGA and γ-PGANa. 2.2 Biosorption equilibrium and thermodynamics

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Since, as described above, at pH 3 the Nd recovery process was dominated by adsorption rather than precipitation/coagulation, the isotherm experiments were conducted at pH 3. Fig. 4 represents adsorption isotherm data obtained at 283, 298 and 313 K. When the temperature was increased from 283 to 313 K, the amounts of Nd

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adsorption at equilibrium for both γ-PGA and γ-PGANa increased. This indicates that the adsorptions of Nd onto γ-PGA and γ-PGANa were endothermic. This result coincides with thedata for Nd recovery by magnetic iron oxide Fe3O4[14] and those by silica-based urea-formaldehyde composite material[24]. Dasgupta et al. [16], on the contrary, reported that the adsorption of Nd by functionalized multiwalled carbon nanotubes was exothermic. The determined constants in the biosorption isotherm models (Eqs. (1) and (2)) for Nd biosorption by γ-PGA and γ-PGANa are presented in Table 1 with the model correlation coefficients. The fit of the isotherm data with

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Langmuir isotherm (R2=0.988) was rather better than that with Freundlich model (R2=0968). As expected from the increase in the amount of biosorption with temperature presented in Fig. 4, with increasing temperature from 283 to 313K, the asymptote maximum biosorption capacity, qm, for γ-PGA increased by 27% from 143 to 182 mg-Nd/(g-γ-PGA). For γ-PGANa the maximum biosorption capacity increased by 6% from 223 to 237

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mg-Nd/(g-γ-PGANa). These values are similar to those for Nd adsorption by calcium alginate-poly glutamic acid hybrid gels [20] and larger than that obtained by Sargassum sp

[21]

. The equilibrium adsorption constant, KL, for

γ-PGA increased from 0.0220 to 0.0315 L/(mg-Nd) with increasing temperature from 283 to 313 K. The KL for

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γ-PGANa also increased from 0.0135 to 0.0587 L/(mg-Nd). The values of KL obtained in this study are one order of magnitude less than those of 0.122 and 0.182 L/(mg-Nd) reported for Nd adsorption by calcium alginate-poly glutamic acid hybrid gels [20]. In the Freundlich model, as presented in Table 1, with increasing temperature the relative adsorption capacity constants, KF, significantly increased and the heterogeneity factors, n, also increased for both γ-PGA and γ-PGANa. The values of n and KF obtained at 298 K in this study are somewhat smaller as compared with those at 298 K for Nd adsorption reported in the literature [20, 21]. The calculated thermodynamic parameters (Eqs. (4), (5) and (6)) for Nd biosorption by γ-PGA and γ-PGANa are given in Table 1. The negative Gibbs free energies reveal that the biosorption of Nd on γ-PGA and γ-PGANa spontaneously took place. The positive ∆H0 values confirm the endothermic nature of Nd biosorption. As well as the present biosorption isotherm data, as described above, Tu et al. [14]and Smith et al. [17] observed the endothermic nature of Nd adsorption by magnetic iron oxide Fe3O4 and carbon black derived from recycled tires, respectively.

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ACCEPTED MANUSCRIPT Incidentally, Gao et al.

[13]

found that the biosorption processes of La3+ and Ce3+ by γ-PGA crosslinked with

polyvinyl alcohol were also endothermic. 2.3Effect of coexisting cations, Cs+ and Cu2+, on Nd recovery by γ-PGA and γ-PGANa Effects of coexisting cations of univalent cation Cs+ and bivalent cation Cu2+on trivalent cation Nd3+biosorption by γ-PGA and γ-PGANa at pH 3 were studied. It can be seen in Fig. 5(a) that the Nd biosorption efficiencies by

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γ-PGA and γ-PGANa in the presence of Cs were 12.5% and 13.8 %, respectively, which were somewhat lower than 13.8% and 21.2% in the control, respectively. The biosorption efficiency of Nd3+ declined due to competition with Cs+ for available active sites on the biosorbent surface. In the coexistence of Nd3+ and Cs+, the Cs biosorption capacitiesby γ-PGA and γ-PGANa were 6.3% and 5.9%, respectively, which were smaller than those for Nd because of lower electronegativity of Cs compared with that of Nd. Higher electronegativity of an atom (Cu(1.90),

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Nd(1.14), Na(0.93), Cs(0.79)) enhanced adsorption of its ionic form on the charged surface of biosorbent[33]. Therefore, the presence of Cs ion insignificantly affected the biosorption efficiency of Nd. It is seen in Fig. 5(b) that due to the addition of 144 mg/L of Cu2+ the biosorption efficiencies of Nd by γ-PGA

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and γ-PGANa declined from 13.8% and 21.2% to 13.7% and 9.1%, respectively. Competition for the biosorption sites took place between Nd3+ and Cu2+ and as a result the Nd biosorption efficiencies decreased due to the existence of Cu2+. The decline of Nd biosorption efficiency for γ-PGANa with the presence of Cu was more significant as compared with that forγ-PGA. Since the electronegativity of Cu is rather larger than that of Na and smaller than that of H, Cu could readily replace Na in –COONa rather than H in −COOH. The Cu biosorption capacitiesby γ-PGA and γ-PGANa were 42.9% and 52.3 %, respectively. Due to the larger electronegativity of Cu

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as compared with that of Nd, thebiosorption capacities of Cu forγ-PGA and γ-PGANa were rather larger than thoseofNd.

Fig. 6(a) shows the FT-IR spectra and the XPS survey spectra before and after biosorption of Nd by γ-PGA in the coexistence of Cs or Cu. For reference, those in the absence of cationic ions, Cs and Cu, are also plotted. The

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characteristic broad peak at 3200 cm–1 corresponding to O–H/N–H in the coexisting Nd and Cs was close to that for single Nd biosorption and no peaks shifted. The presence of Cu2+ strongly affected the Nd biosorption and the peak attributed to overlap of O-H and N–H significantly shifted to 3100 cm–1. Furthermore, the peak at 1100 cm–1 [11,20, 36]

appeared when Cu coexisted. This was attributed to the formation of cyan precipitates of

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assigned to C-N

Cu2+ chelated with γ-PGA [10]. The amide nitrogen playing a role as an electron donor was complexed with cationic metals. Amide groups contributed to the complexation and ion-exchange with metals [37]. Yuan et al. [38] confirmed the complexation of amine groups with Cu. The biosorption efficiencies of Cu by γ-PGA, which were considerably higher as compared with those of Nd, indicate that Cu preferably complexed by γ-PGA via amide functional groups in γ-PGA. As seen in the XPS survey spectra for γ-PGA, the peaks at ~740 and ~933 eV for Cs and Cu, respectively, which were not found before and after Nd biosorption in the absence of Cs and Cu, appeared besides the peak at ~980 eV for Nd after Nd biosorption in the coexistence of Cs and Cu. In Fig. 6(b), the FT-IR spectra and the XPS survey spectra before and after biosorption of Nd in the coexistence of Cs or Cu by γ-PGANa are shown. The changes in FT-IR spectra for functional groups and in XPS survey spectra for biosorption of metals due to the coexistence of Cs+ or Cu2+ for γ-PGANa were similar to those for γ-PGA. The

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ACCEPTED MANUSCRIPT appearance of the peak at ~1100 cm–1 assigned C-N and the shift of the peak at 3200 cm–1 assigned O–H/N–H to lower frequency were obtained for γ-PGANa as well as γ-PGA. It is seen in the XPS survey spectra for γ-PGANa that after biosorption of Nd in the coexistence of Cs and Cu the peaks at ~740 and ~933 eV corresponding to Cs and Cu, respectively appeared besides the peak at ~980 eV corresponding to Nd. The peak at ~1070 eV assigned to Na observed before and after Nd biosorption in the coexistence of Cs disappeared after Nd biosorption without

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coexisting cation ions and with the presence of Cu.

3 Conclusions

To clarify mechanisms of Nd recovery by γ-PGA and γ-PGANa effects of solution pH on recovery of rare-earth metal Nd were quantified in the pH range from 2 to 9. The recovery efficiency of Nd by γ-PGA increases from 2.4% to 19.6% with increasing pH from 2 to 4. In the range of pH from 4 to 7, the Nd recovery efficiencies for

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γ-PGA and γ-PGANa remains approximately constant. For pH>7, the Nd recovery significantly increases and 100 % recovery is achieved at pH 9. The pH dependency on Nd recovery by γ-PGANa practically coincides with

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that by γ-PGA. Contributions of adsorption and precipitation/coagulation to Nd recovery process were quantified. Whereas the adsorption dominated Nd recovery at lower pH (<~4), the precipitation/coagulation controlled Nd recovery at higher pH (>7). The maximum adsorption capacities for γ-PGA and γ-PGANa are 215 mg-Nd/(g-γ-PGA) at pH 4 and 305 mg-Nd/(g-γ-PGANa) at pH 3, respectively. The coexistence of univalent cation Cs+ or bivalent Cu2+ somewhat inhibits the biosorption of Nd by γ-PGA and γ-PGANa. This study clarified the utilization of γ-PGA and γ-PGANa as potential biosorbents for the highly effective recovery of Nd from aqueous

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solutions.

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Table 1 Parameters of isotherm models and thermodynamics for Nd biosorption on γ-PGA and γ−PGANa (R2: correlation coefficient) Biosorbent Temperature/

Langmuir model

Freundlich

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Thermodynamic

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model KL

qm

L/

(mg-Nd)/

(mg-Nd))(g-biosorbent)

n

(mg-Nd)n-1 -

parameter R

2

∆G

0

∆H

(kJ/mol) (kJ/mol)

─

∆S 0

0

(kJ/(mol·K))

-n 1/n

L·(g-biosorbent) )

0.0220

143

0.979

8.38

1.84

0.948

-19.0

15.9

0.123

298

0.0309

159

0.993

17.4

2.37

0.985

-20.8

0.994

0.0732

313

0.0315

182

0.995

20.3

2.38

0.995

-21.9

283

0.0135

223

0.985

5.5

1.45

0.985

-17.8

34.3

0.184

298

0.0281

227

0.991

16.5

1.94

0.934

-20.6

38.1

0.197

313

0.0587

237

0.987

52.9

3.41

0.961

-23.5

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283

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γ-PGANa

KF

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γ-PGA

R

2

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ACCEPTED MANUSCRIPT Figure captions Fig. 1Effects of pH on recovery of Nd by γ-PGA and γ-PGANa (temperature: 298 K, biosorbent dosage: 100 mg/L, and initial Nd concentration: 144 mg/L) (a) Nd recovery efficiency (b) Contributions of adsorption and precipitation/coagulation on Nd recovery efficiency

(c) Nd adsorption efficiency and adsorption capacity Fig. 2 FT-IR spectra of biosorbents before and after Nd recovery at pH 3 (a) γ-PGA; (b) γ-PGANa

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(upper: γ-PGA, lower: γ-PGANa)

Fig. 3 XPS spectra of biosorbents before and after Nd recovery at pH 3 (left: γ-PGA and right: γ-PGANa) (a) survey spectra; (b) C 1s spectra; (c) O 1s spectra; (d) N 1s spectra; (e) Nd 3d5/2 spectra

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Fig. 4 Adsorption isotherm at 283, 298 and 313 K (a) γ-PGA; (b) γ-PGANa

Fig. 5Effect of coexisting cations, Cs+ and Cu2+, on biosorption of Nd by γ-PGA and γ-PGANa at pH 3

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(a) Cs+; (b) Cu2+

Fig. 6FT-IRspectra and XPS survey spectra of biosorbents for Nd biosorption in the coexistence of Cs+ and Cu2+at pH 3

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(a) γ-PGA; (b) γ-PGANa

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Fig. 1

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a) γ-PGA; b) γ-PGANa

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Fig. 2

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(a) survey spectra; (b) C 1s spectra; (c) O 1s spectra; (d) N 1s spectra; (e) Nd 3d5/2 spectra Fig. 3

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Fig. 4

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(a) γ-PGA (b) γ-PGANa

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Fig. 5

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(a) Cs+; (b) Cu2+

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Fig. 6

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(a) γ-PGA; (b) γ-PGANa

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ACCEPTED MANUSCRIPT Highlights

• The Nd recovery by poly-γ-glutamic acid (γ-PGA) was systematically studied. • Effects of pH on mechanism for Nd recovery were clarified.

• The precipitation/coagulation controlled Nd recovery process for pH>7.

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• The biosorption dominated Nd recovery at lower pH (<~4).

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•γ-PGA and its sodium salt were confirmed to be potential biosorbents for Nd recovery.

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Supplementary data

γ-PGA

0.7

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80 70 60 50 40

0.5 0.4 0.3

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30

0.6

20

0.2

Nd adsorption rate 0.1

adsorption capacity

10

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0 0

100

200

300

adsorption capacity [mg-Nd/mg-γ-PGANa]

90

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γ-PGANa

Fig. S1 Structures of γ-PGA and γ-PGANa

Nd adsorption rate [%]

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Appendix

0 400

500

γ-PGA concentration [mg/L]

Fig. S2 Effect of γ-PGA dosage on adsorption capacity and adsorption rate for Nd removal.(initial Nd concentration: 100 mg L-1, pH: 3, temperature: 298 K )

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ACCEPTED MANUSCRIPT 101.5

before after

Transmittance (au)

101 100.5

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100

99.5

98.5 500

1000

1500

2000

2500

Wavenumbers

3000

3500

(cm-1)

4000

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a) γ-PGA

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99

101.5

before after

101

Transmittance (au)

100.5

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100

99.5

98.5 1000

1500

2000 2500 3000 Wavenumbers (cm-1)

3500

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500

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99

4000

b) γ-PGANa

Fig. S3 The FT-IR ATR specta of γ-PGA and γ-PGANa at pH 6

22

ACCEPTED MANUSCRIPT 101

before after

Transmittance (au)

100.5 100

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99.5 99

98 500

1000

1500

2000

2500

3000

Wavenumbers

(cm-1)

4000

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a) γ-PGA

3500

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98.5

101.5

before

101

Transmittance (au)

after

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100.5 100

99 98.5 1000

1500

2000 2500 3000 Wavenumbers (cm-1)

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500

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99.5

3500

4000

b) γ-PGANa

Fig. S4 The FT-IR ATR specta of γ-PGA and γ-PGANa at pH 9

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γ-PGANa

─

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─COOH or ─COONa→ ─COO + Nd

3+

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γ-PGA

-

- -

-

-

- -

-

Random ｰ coil

αｰ helix

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Increasing pH

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adsorption

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Graphical abstract

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precipitate/coagulation (purple gel)