Cellulose and Saccharomyces cerevisiae Embark To Recover

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Article Cite This: ACS Omega 2019, 4, 940−952

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Cellulose and Saccharomyces cerevisiae Embark To Recover Europium from Phosphor Powder Balasubramanian Arunraj,† Talasila Sathvika,† Vidya Rajesh,*,‡ and N. Rajesh*,† Department of Chemistry and ‡Department of Biological SciencesBirla Institute of Technology and Science, Pilani, Hyderabad Campus, Shameerpet, Hyderabad 500078, India

ACS Omega 2019.4:940-952. Downloaded from pubs.acs.org by 83.171.253.36 on 01/19/19. For personal use only.

†

ABSTRACT: The mounting demand for rare earth elements (REEs) and the similarities in their chemical and physical properties render their separation and selective recovery quite challenging. Microbe-based adsorbents are quite effective owing to their diverse functional groups on the cell wall and are perceived to be environmentally benign. This work reports the application of yeast (Saccharomyces cerevisiae) embedded in cellulose matrix as an efficient adsorbent for the separation of Eu(III) from aqueous medium. The fungi−biopolymer combination acts as a good host to welcome Eu(III) on its surface through effective coordination with the diverse functional groups. The pH, adsorbent dosage, isotherm studies, and thermodynamic and kinetic parameters were studied, and the characterization was done using FT-IR, XRD, XRF, XPS, SEM-EDX, BET, and confocal microscopy techniques. Ion chromatography was used to monitor the quantitative measurement of Eu(III) during the course of adsorption and desorption process. The regeneration of the biosorbent was achieved using EDTA as the complexing agent for Eu(III). The biosorbent gives a maximum adsorption capacity of 25.91 mg g−1 through Langmuir isotherm model. Further, the biosorbent was employed to recover Eu(III) from a phosphor powder containing other rare earths as well as phosphor powder from a spent fluorescent lamp.

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INTRODUCTION The past few decades have witnessed huge electronic revolution with the immense application of rare earth elements (REEs) in electronics, laser technology, and magnetic materials.1,2 Fluorescent materials are used in television screens, phones, computers, and lamps in the form of REEs oxide powders,3 commonly known as phosphor powder. These coatings accelerate the conversion of incident white light into colorful designs based on the composition of the phosphor powder.4 The composition of lamp phosphor consists of halophosphate phosphor (45 wt %), glass particles, alumina (12 wt %), silica (20−30 wt %), rare-earth phosphors (10−20 wt %), and a residual fraction (5 wt %).5 Fluorescent lamps contain major phosphors, namely, red phosphor powder (Y2O3:Eu3+), green phosphor powder (LaPO4:Ce3+), and blue phosphor powder (BaMgAl10O17:Eu2+).5 The economic reasons and the landfilling of electronic wastes have led to the entry of REE in the environment. Hence, the recycling and recovery of REEs from spent lamp phosphors and other electronic wastes are important due to its abundance in the waste, high commercial value, and the maintenance of elemental balance in the biosphere. Since the physical and chemical properties of REEs are similar, the separation through conventional methods is difficult.6 As reduction of europium from the (III) oxidation state to the (II) state is plausible, its separation from other REEs is possible.7 Economic and environmentally friendly methods to recycle and separate Eu and other REEs are in © 2019 American Chemical Society

great demand. Efficient biosorption techniques using living or dead biomass are promising as they bind to heavy metals effectively.8 Sequestration of heavy metals on biomass is believed to occur through coordination, complexation, chelation, adsorption, microprecipitation, etc. The functional groups found on the cell wall of bacteria and fungi are the key factors in adsorption.9 Murray et al. reported biocatalytic recovery and separation of europium and neodymium using Serratia sp. biofilm.10 Accumulation of Co(II) and Eu(III) from radionuclide contaminated soil using Aspergillus sp. was reported by Song et al.11 The bioengineering approach to extract REEs from solution using lanthanide binding tags, which are made of the S-layer of Caulobacter crescentus and engineered Escherichia coli cells, was studied by Park et al.12,13 The biosorption of lanthanides from mixed lanthanide solution on the surface of Roseobacter sp. AzwK-3b was illustrated by Bonificio et al.14 The potential of Bacillus thuringiensis biomass was realized toward its good adsorption capacity for Eu(III).15 It is well known that biopolymers and microbes function as good adsorbents. Polysaccharides are widely available, and the biopolymers with high structural diversity have been explored as a matrix for metal adsorption.16 Cellulose is one such polysaccharide endowed with hydroxyl groups and hydrogen bonding between the layers. The functional groups available on Received: October 17, 2018 Accepted: December 26, 2018 Published: January 10, 2019 940

DOI: 10.1021/acsomega.8b02845 ACS Omega 2019, 4, 940−952

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Figure 1. FT-IR spectrum of the biosorbent before and after adsorption of Eu(III).

the cell surface of the microbe and the hydroxyl groups in cellulose would play a significant role in metal chelation. Keeping in view these aspects, the prospective synergism between cellulose cross-linked with glutaraldehyde and Saccharomyces cerevisiae biomass was explored for the recovery of Eu(III) from the phosphor powder of a lifeless fluorescent lamp taken from our research laboratory.

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RESULTS AND DISCUSSION The yeast (Saccharomyces cerevisiae) cell wall is made of polysaccharides like chitin, mannan, β-glucan, and β-1,6glucan, which are enriched with diverse functional groups.17 From the FT-IR spectrum (Figure 1), the shifting of vibrational frequencies in the biosorbent before and after adsorption accounts for the plausible interactions between Eu(III) and the biosorbent. A strong and wide band at 3348 cm−1 corresponds to the O−H and N−H stretching18 and another strong band in the 2909 cm−1 region corresponds to the C−H stretching16 in polysaccharides. The frequency shift from 3348 to 3342 cm−1 is attributed to the interaction of Eu(III) with O−H and N−H groups. The peak around 1371 cm −1 represents the C−O and C−H bending mode frequencies in polysaccharides.19,20 Significant changes in the bending vibrational frequency of C−O (1371 to 1365 cm−1) after adsorption are due to the electrostatic interaction between Eu(III) and oxygen lone pair electrons. The carbonyl group stretching frequency observed at 1653 cm−1 before adsorption and the shift to 1642 cm−1 after adsorption indicates that Eu(III) could coordinate with the nonbonding electrons of the carbonyl oxygen. The C−O asymmetric and C−O−C symmetric stretching vibrations were observed at 1163 and 1059 cm−1, respectively.16 After adsorption, there was also a marginal shift in these vibrational frequencies (1163 to 1155 cm−1 and 1059 to 1054 cm−1). Thermal decomposition of the biosorbent was investigated from 30 to 800 °C at a heating rate of 10 °C min−1 (Figure 2). Two stages of thermal decomposition were observed in the biosorbent, namely, initial slow-stage pyrolysis and fast-stage pyrolysis. Hydrophilic nature of the biosorbent was associated with the initial slow-stage pyrolysis. Thus, the weight loss of 7% from 30 to 240 °C is due to the vaporization of volatile matter and moisture in the biosorbent. Further, fast pyrolysis with 55% weight loss from 240 to 330 °C is ascribed to the heterolytic depolymerization16 and splitting of polysaccharide

Figure 2. Thermal stability study of the yeast-immobilized crosslinked cellulose.

chains present in the yeast cell wall.21 The continuous weight loss at 350−600 °C and 600−800 °C corresponds to the decomposition of oligosaccharide and char formation, respectively. The porosity and surface area of the biosorbent were investigated by the adsorption/desorption of nitrogen through BET analysis (Figure 3). The yeast-immobilized cellulose has a pore volume and BET surface area of 0.167 cc g−1 and 121.73 m2 g−1, respectively. The pore diameter was found to be 3.3 nm, which shows that the biosorbent is very close to the microporous nature. The shape of the nitrogen adsorption isotherm suggests the maintenance of the structure and the presence of micropores in the biosorbent.22 The large surface area of the biosorbent favors the interaction between Eu(III) and functional groups on the surface of yeast and cellulose. XPS measurements of the biosorbent were performed to confirm the oxidation state(s) of Eu(III) after it gets adsorbed. In Figure 4a, the survey scan binding energy spectra depict the core levels of C1s, O1s, N1s, and Eu3d. The chemical bonding nature of carbon on the surface of the biosorbent, such as C− C, C−H, C−OH, and C−O−C, was observed from the deconvoluted peaks of C1s (Figure 4b). The nonpolar carbons, which are not bonded to oxygen such as C−C and C−H, are observed at 285.0 eV.23 The oxygen-rich functional groups of epoxy/hydroxyl and carbonyl showed higher binding energies 941


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Figure 3. (a) BET nitrogen adsorption isotherm plot. (b) BET surface area and pore size distribution.

at 286.4 and 287.4 eV, respectively.24 The deconvoluted O1s scan (Figure 4c) gives two peaks with low and high binding energies of 530.1 and 532.0 eV, respectively. The small peak at 530.1 eV corresponds to the Eu−OH bond on the surface of the biosorbent,25,26 which confirms that Eu(III) coordinates with oxygen-containing functional groups on the biosorbent surface. The higher binding energy of 532.0 eV is ascribed to the carboxylic group present in the biosorbent26 (Figure 4d). The binding energy between 1180 and 1110 eV shows the Eu3d photoemission lines in the XPS spectrum. Two intense peaks appeared at 1165.1 and 1135.1 eV, corresponding to the energy levels of Eu 3d3/2 and Eu 3d5/2, respectively, for Eu(III).27 The existence of two additional less intense peaks at binding energies of 1154.3 eV (Eu 3d3/2) and 1125.1 eV (Eu 3d5/2) is due to Eu(II). Hence, it is possible that some amount of Eu(III) was reduced to Eu(II) due to an adsorption-coupled reduction phenomenon.28 The crystalline nature of cellulose was ascertained using Xray diffraction as shown in Figure 5. From the powder XRD diffraction patterns, it is observed that cellulose has two types of crystallinity.29 A strong peak at 2θ = 22.68° and other three

peaks at 2θ = 14.86°, 16.63°, and 34.24° represent the native cellulose I crystal structure.30,31 A less intensity peak at 2θ = 14.86° corresponds to the amorphous structure, while a sharp and high diffraction intensity peak at 2θ = 22.68° accounts for the crystalline structure of cellulose. The crystallinity index and percentage crystallinity are calculated from the following equations29 Ic =

I002 − Iam I002

%Cr =

I002 I002 + Iam

(1)

(2)

where I002 and Iam refer to the maximum intensity of the (002) lattice diffraction and the maximum intensity of the amorphous phase, respectively. The calculated crystallinity index (Ic) and percentage crystallinity for native cellulose (%Cr) and before and after adsorption of Eu(III) on the biosorbent were 0.67, 0.61, and 0.72 and 75.2, 72.4, and 78.7%, respectively. The decrease in the crystallinity from 0.67 to 0.61 is ascribed to the 942


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Figure 4. (a) XPS survey spectrum of the biosorbent after adsorption of Eu(III). (b) High-resolution XPS spectrum of C1s. (c) High-resolution XPS spectrum of O1s. (d) High-resolution XPS spectrum of Eu3d.

the biosorbent was studied using scanning electron microscopy, and the images are given in Figure 6. The presence of yeast on cellulose after immobilization is observed in the SEM images (Figure 6b). After adsorption of Eu(III), there was no significant change in the morphology (Figure 6c). The EDX analysis of the surface confirms that Eu(III) is adsorbed on the yeast-immobilized cellulose matrix (Figure 6d−f). Effect of pH. The initial pH of the aqueous solution plays a crucial role in surface chemistry, and the speciation of europium shows that till pH 6.0, it exists as Eu3+; a further increase in the pH leads to the formation of Eu(OH)2+, Eu(OH)2+, and Eu(OH)3.32 The adsorption of metal ion on the adsorbent is dependent on the complexing ability arising due to the protonation/deprotonation of the reactive functional groups.33 Indeed, such multiple interactions through functional groups in proteins and immobilized metal ions are known in metal affinity chromatography.34 The adsorption pattern of Eu(III) varies depending on the acid−base properties of the hydroxyl, carboxyl, and amine functional groups. Hence, the investigation of adsorption from pH 2.0 to 7.0 was conducted with two different concentrations of Eu(III) (Figure 7). An increase in adsorption capacity at equilibrium is observed as the pH is increased. This phenomenon is due to the replacement of protons on the adsorbent surface by Eu(III) through a proton-exchange mechanism.35,36 The

Figure 5. (a−c) X-ray diffraction patterns of (a) native cellulose and (b) before and (c) after adsorption of Eu(III).

amorphous biomass of Saccharomyces cerevisiae biomass embedded in between cellulose fibers. The marginal increase in crystallinity from 0.61 to 0.72 is observed after adsorption of Eu(III) ions onto the biosorbent surface. The morphology of 943


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Figure 6. SEM-EDX images of (a, d) native cellulose, (b, e) after yeast immobilization, and (c, f) after adsorption of Eu(III).

surface charge of cellulose−yeast is positive, whereas the surface charge is negative when the pH is greater than pHpzc. This indicates that higher adsorption can be ascribed to the electrostatic interaction36−38 between Eu(OH)2+ and the negatively charged biosorbent surface. The interaction between the functional groups present in the biosorbent and Eu(III) is shown in Figure 8. The kinetics, isotherm, and column studies were performed at pH 6.0 in order to avoid the precipitation of Eu(III) as its hydroxide. The effect of biosorbent dosage on the adsorption of Eu(III) at a fixed 50 mg L−1 concentration was studied at pH 6.0 (Figure 9). The adsorption of Eu(III) increased with the amount of biosorbent dosage. Quantitative adsorption at 12 g L−1 biosorbent was attained, beyond which the saturation of active sites results in a flat plateau region. Isotherm, Kinetics, and Thermodynamics. Isotherm studies were examined with the initial concentration (Co) between 20 and 600 mg L−1 at an optimum pH of 6.0. Interactions between the cellulose−yeast biosorbent and Eu(III) were studied using established isotherm models such as Langmuir, Freundlich, and Temkin. In biological studies, heterogeneous protein adsorption is described through Langmuir and Temkin isotherm models.39 The equilibrium concentration (Ce) of the adsorbate and the amount adsorbed at equilibrium (qe) can be explained through isotherm studies.

Figure 7. . Effect of pH for the adsorption of Eu(III) on the biosorbent.

higher initial concentration also favors the adsorption by increasing the interaction between the adsorbent and the adsorbate. Sathvika et al. reported the zero point charge of yeast-immobilized cellulose at pH 4.0.16 At pH < pHpzc, the 944


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Figure 8. Interaction mechanism of Eu(III) with the biosorbent surface.

4) can be calculated from the Langmuir constant b, which gives the favorable nature of adsorption42 RL =

1 1 + bCo

(4)

where Co and b are the initial concentration of Eu(III) and the Langmuir constant, respectively. The separation factor RL value higher than unity indicates unfavorable adsorption, while an RL value of zero indicates the irreversible adsorption process.42,43 The calculated RL value (0.6818) for the adsorption of Eu(III) lies between 0 and 1, which indicates that the adsorption process is favorable. The linearized Freundlich isotherm42 was also utilized to study the adsorption process through the equation as log(qe) = log(KF) + Figure 9. Effect of biosorbent dosage.

(5)

where Ce (mg L−1) is the equilibrium concentration of Eu(III) and qe (mg g−1) is the amount adsorbed at equilibrium. Freundlich constants such as KF (adsorption capacity) and n refers to the adsorption intensity obtained through the plot of log(qe) against log(Ce) (Figure 10c). The Temkin isotherm was studied using a reversible adsorption process, and the linear form of Temkin equation44 is given below

The increase in adsorption capacity at equilibrium with respect to the initial Eu(III) concentration is attributed to the fast and effective interaction between Eu(III) and the surface of the biosorbent.40 The maximum Langmuir adsorption capacity was calculated through the plot of Ce/qe against Ce (Figure 10b), with the linear form of the equation as41 Ce 1 1 = Ce + qe qo qob

1 log(Ce) n

qe = (3)

RT RT ln a Te + ln Ce bTe bTe

(6)

where bTe is the Temkin constant associated to the heat of adsorption (J mol−1), aTe is the Temkin isotherm constant (L mg−1), R is the universal gas constant (J mol−1 K−1), and T is the absolute temperature (K) obtained through the plot of qe against ln Ce. The isotherm parameters, which are obtained from the above three models, are given in Table 1. The higher correlation coefficient value of Langmuir isotherm shows that the adsorption of Eu(III) on the yeast-immobilized cellulose matrix follows monolayer adsorption.

where Ce (mg L−1) and qe (mg g−1) refer to the equilibrium concentration and the amount of Eu(III) adsorbed at equilibrium, respectively. qo(mg g−1) is the maximum adsorption capacity, and b (L mg−1) is the Langmuir constant related to the energy of adsorption. The maximum adsorption capacities of native cellulose, Saccharomyces cerevisiae, and yeast-immobilized cellulose were found to be 18.5, 14.2, and 25.9 mg g−1, respectively. The dimensionless parameter RL (eq 945


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Figure 10. (a) Plot of Ce against qe, (b) Langmuir linear plot, (c) Freundlich isotherm linear plot, and (d) Temkin isotherm linear plot for the adsorption of Eu(III) on the biosorbent.

where qe is the amount of Eu(III) adsorbed at equilibrium condition, qt is the amount of Eu(III) adsorbed at time t, and k1 and k2 are the corresponding rate constants of pseudo-firstorder and pseudo-second-order equations, respectively. The kinetic data plots are represented in Figure 11, and the kinetic parameters are given in Table 2. The best fit of a pseudosecond-order equation having a higher correlation coefficient conveys that the adsorption of Eu(III) onto the cellulose− yeast surface follows pseudo-second-order kinetics. The intraparticle mass-transfer diffusion model was studied through Weber and Morris model46 for the adsorption of Eu(III) on the biosorbent (Figure 11c). The transport of Eu(III) from the solution phase to the biosorbent surface can be controlled by film diffusion, pore diffusion, and particle diffusion.47 The Weber and Morris model is represented as

Table 1. Isotherm Models and Parameters for the Adsorption of Eu(III) model

parameters

values

Langmuir

qo (mg g−1) bL (L mg−1) R2 χ2 KF (mg1−1/n g−1 L1/n) n R2 χ2 bTe (L g−1) aTe (J mol−1) R2 χ2

25.91 0.0234 0.97 0.261 5.783 4.4022 0.89 0.140 0.0855 659.28 0.82 0.288

Freundlich

Temkin

qt = k it 1/2 + C

Adsorption of Eu(III) on the biosorbent also increased with increasing contact time. The kinetics data from the adsorption process were fitted to the Lagergren pseudo-first-order equation (Figure 11a) and pseudo-second-order equation (Figure 11b). The linear forms of pseudo-first-order and pseudo-second-order equations are given below45 log(qe − qt) = log(qe) − t 1 t = + qt qe k 2qe2

k1 t 2.303

(9)

where ki the intraparticle rate constant; calculated parameters are given in Table 2. Adsorption of Eu(III) on the cellulose− yeast biosorbent showed a combination of two diffusion processes. The initial and later linear portions represent the macropore and micropore diffusion processes,48 and the micropore diffusion process attained equilibrium at the point where C = qo. The thermodynamics of the adsorption process was evaluated at four different temperatures (303, 313, 323, and 333 K), and the free energy, enthalpy, and entropy changes were obtained from the plot of ln K against 1/T42,48 (Figure

(7)

(8) 946


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Figure 11. (a) Pseudo-first-order kinetics plot, (b) pseudo-second-order kinetics plot, (c) intraparticle diffusion plot, and (d) van’t Hoff plot for the adsorption of Eu(III) on the biosorbent.

Furthermore

Table 2. Kinetic Parameters for the Adsorption of Eu(III) model pseudo-first-order

pseudo-second-order

intraparticle diffusion

parameters

ΔGr = ΔHCellulose ‐ yeast ‐ Eu + T ΔSCellulose ‐ yeast ‐ Eu

values

−1

k1 (min ) qe1 (mg g−1) R2 k2 (g mg−1 min−1/2) qe2 (mg g−1) R2 C1 (mg g−1) Ki1 (mg g−1 min1/2) R2 C2 (mg g−1) Ki2 (mg g−1 min1/2) R2

0.0257 0.19 0.95 0.0397 5.5725 0.99 5.205 0.0157 0.96 5.316 0.0049 0.98

At equilibrium Cellulose‐yeast‐Eu(solution) F Cellulose‐yeast‐Eu(surface) (12)

ΔGr = ΔGr0RT ln

a[Cellulose ‐ yeast ‐ Eu]surface a[Cellulose ‐ yeast ‐ Eu]solution

(13)

Activity (a) is directly proportional to the concentration (C) of Eu(III) ions in dilute solution.Therefore ΔGr = ΔGr0RT ln

[Cellulose‐yeast‐Eu]surface [Cellulose‐yeast‐Eu]solution

(14)

Hence

11d). The reaction Gibbs free energy change associated with the adsorption process involves the following ΔGr = ΔGCellulose ‐ yeast surface + ΔGCellulose ‐ yeast ‐ Eu

(11)

ΔGr = ΔGr0 + RT ln K

(10)

(15)

At equilibrium

Table 3. Thermodynamic Parameters Associated with the Adsorption of Eu(III) temperature(K)

ΔH°(kJ mol−1)

ΔS°(kJ mol−1 K−1)

ΔG°(kJ mol−1)

average Ea(kJ mol−1)

R2

303 313 323 333

−29.143

−0.0727

−7.175 −6.264 −5.679 −4.954

−26.499

0.995

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(16)

where ΔH°, ΔG°, and ΔS° are the standard enthalpy change, standard free energy, and entropy changes, respectively. The results obtained through the van’t Hoff plot are given in Table 3. The negative free energy change (ΔG°) implies the spontaneity,49 and the negative standard enthalpy change (ΔH°) value shows that the nature of the adsorption process is exothermic.50 The enthalpy change in the range of 80−400 kJ mol−1 accounts for chemical adsorption, and less than 80 kJ mol−1 implies physical adsorption.51 The negative entropy change ΔS° reflects the decrease in randomness. The activation energy of the adsorption process at various temperatures can be calculated using Ea = ΔH°ads + RT. The average activation energy for the exothermic adsorption process was found to be −26.499 kJ mol−1. Desorption Studies. The glass column (1 cm width and 4.5 cm length) was packed with 5.0 g of the biosorbent. Initially, 50 mL of 50 mg L−1 Eu(III) solution was adsorbed into the column. Ten milliliters of 0.1 M EDTA solution was used to regenerate the column.52 A higher formation constant for Eu−EDTA complex fosters the desorption by weakening the interaction between the adsorbate and the adsorbent. A large fraction of the adsorbed Eu(III) was desorbed by EDTA successfully. The regeneration of the biosorbent was 97.5 ± 1.2% for the first cycle. After two cycles of regeneration, the percentage adsorption of Eu(III) decreased from 95.2 ± 1.4% to 84.7 ± 2.1%, whereas the fourth cycle resulted in the adsorption of Eu(III) as 83.2 ± 1.24% (Figure 12).

Figure 13. X-ray fluorescence spectroscopy for native biosorbent and before and after desorption of Eu(III).

EDTA and the low intensity of the peak show that the maximum amount of Eu(III) is desorbed from the biosorbent surface. Europium Recovery in Phosphor Powder. The fluorescent lamp phosphor powder contains Eu(III) and other rare earth elements. In order to recover and separate the precious metals, a phosphor powder containing other rare earths was prepared and studied for the adsorption process. Initially, the efficacy of adsorption was tested with a lower concentration (2.0 mg L−1) of Eu(III) and a higher concentration of other ions (Y: 40.0 mg L−1; Ca: 3.0 mg L−1; Al: 20.0 mg L−1; Ce: 5.0 mg L−1; La: 5.0 mg L−1) in accordance with the composition. Yttrium was specifically chosen at a higher concentration since it is known to affect the recovery of Eu(III) in the red phosphor powder. A 50 mL volume of this simulated phosphor powder solution was taken in the biosorbent column (5.0 g), and the pH of the solution medium was maintained at 6.0. Complete adsorption of Eu(III) was achieved (100%), and desorption was also quantitative with 0.1 M EDTA. A 0.23 g weight of the commercially available europiumdoped yttrium oxide (red phosphor powder) was digested using 10 mL of concentrated hydrochloric acid. After complete digestion, the solution was diluted to 100 mL with deionized water. The resultant red phosphor powder solution was analyzed using atomic absorption spectroscopy and ion chromatography to determine the leached Eu(III) concentration. The concentration of Eu(III) and Y(III) in the leached solution was found to be 97.5 and 1717 mg L−1, respectively. Adsorption studies were carried using a fixed-bed column with a biosorbent weight of 5.0 g and 50 mL of the leached solution at pH 6.0 ± 0.2. The adsorption of Eu(III) present in red phosphor powder reached a maximum of 86 ± 5.0%, and the recovery as Eu−EDTA complex from the biosorbent surface was found to be 83.7 ± 2.0%. Similarly, a phosphor powder sample leached from a used fluorescent lamp was subjected to the adsorption process using the biosorbent. The spent compact fluorescent lamp from the analytical chemistry laboratory was broken into small pieces (100 g) and leached using 500 mL of 1.0 M nitric acid for 24 h.54 The leached solution was filtered to remove undissolved particles and glass pieces. The concentration of Eu(III) in the leached solution was found to be 40 ± 2.0 mg L−1 as

Figure 12. Regeneration cycles of the biosorbent after adsorption of Eu(III).

A nondestructive elemental analysis technique, X-ray fluorescence spectroscopy (Figure 13), was used to analyze the elemental composition present in the biosorbent at various stages. A unique characteristic peak corresponding to X-ray fluorescence lines of the L shell present in Eu(III) was observed by scanning in the range of 1−10 keV. The fluorescence emission due to the transition of electron from the M → L shell and from the N → L shell is represented as Lα and Lβ, respectively. The Lα and Lβ values for Eu(III) peaks were observed at 5.848 and 6.456 keV, respectively, in the biosorbent after adsorption of Eu(III).53 The residual amount of Eu(III) observed in the biosorbent after desorption using 948


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indicated that the proposed cellulose−yeast combination has good potential to adsorb Eu(III) with a Langmuir adsorption capacity of 25.91 mg g−1. The adsorption of Eu(III) at pH 6.0 was found to be spontaneous and exothermic with a negative entropy change. The adsorption process follows pseudosecond-order kinetics with a high correlation coefficient (R2) value. The proof of concept was established through the recovery of Eu(III) in phosphor powder leached from the spent fluorescent lamp. Our studies in this work have indicated that the simple cross-linking of cellulose has the ability to recover up to 60% Eu(III) from a spent fluorescent lamp phosphor powder. However, with surface functionalization of cellulose, it is possible to enhance the recovery of Eu(III) to a higher extent. The biosorbent could be regenerated using 0.1 M EDTA, and this uncomplicated efficient adsorption points to the fact that the cellulose−yeast combination can serve as a sustainable alternative to recover Eu(III) as well as other rare earth elements from diverse matrices.

determined using atomic absorption spectroscopy and ion chromatography. A 50 mL volume of this leached phosphor solution was treated with 5.0 g of the biosorbent, and the pH of the column bed was maintained at 6.0 ± 0.2 using sodium hydroxide. The adsorption of Eu(III) was 55 ± 4.6%, and desorption was achieved completely using 10 mL of 0.1 M EDTA solution. The lower adsorption is attributed to the presence of a higher concentration of other metal ions like Y, La, Ca, Al, and Ce (Table 4) present in the leachate from a Table 4. . Quantification of Phosphor Powder Leached from a Fluorescent Lamp elements

concentration (mg L−1)

Y Eu La Ce Ca Al

388.6 40.2 1.13 27.7 40.9 3.9

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EXPERIMENTAL SECTION Materials. Reagents and chemicals were used as obtained from the respective sources. Cellulose (97%), aluminum nitrate (95%), and hydrochloric acid (35%) were purchased from HiMedia Laboratories Pvt. Ltd, India. A 25% glutaraldehyde solution was purchased from Tokyo Chemicals Industries Ltd. Europium trichloride hexahydrate (99.99%), yttrium nitrate hexahydrate (99.8%), cerium nitrate hexahydrate (99%), ethylenediamine (99.5%), and α-hydroxyisobutyric acid (99%) were obtained from Sigma-Aldrich. Lanthanum chloride hexahydrate (99%), calcium chloride (98%), and EDTA disodium salt (99.5%) were procured from SD Fine Chemicals Ltd., India. Sodium hydroxide (97%) was obtained from Merck Specialities Private Limited, India. Commercial red phosphor powder (Y2O3:Eu-99%) was procured from Sigma-Aldrich. Instrumentation. Quantitative determination of Eu(III) was carried out using an ion chromatograph (Metrohm 883 Basic IC Plus coupled with a conductivity detector; column: Nucleosil 5SA) using α-hydroxyisobutyric acid and ethylenediamine as eluents. Atomic absorption spectroscopy (Shimadzu AA-7000) analysis was also utilized for quantification of Eu(III) using nitrous oxide as the oxidant and acetylene as the fuel gas. A microwave oven (Sharp R-23GT) was employed in the immobilization of yeast on cellulose matrix. An orbital incubator shaker (Bio Technics, India) was used for batch adsorption studies. pH measurements were done using the Metrohm 867 pH module with a pH electrode calibrated with standard pH buffer solutions. The functional groups, which are present in the biosorbent, were characterized by scanning in the range of 400−4000 cm−1 using a Jasco FT-IR 4200 spectrometer. Carl Zeiss Supra 55 combined with Oxford Xmax EDX analyzer were utilized to study the surface morphology and elemental composition of the biosorbent, respectively. The surface area and pore diameter were determined by Brunauer−Emmett−Teller analysis using Quantachrome ASiQwin instrument. Thermal degradation profile of the yeast-immobilized biopolymer was investigated in a nitrogen atmosphere using a NETZSCH STA 2500 differential thermal analyzer in the temperature range of 35− 800 °C. The X-ray diffraction (XRD) pattern of the biosorbent was recorded on a Rigaku Ultima IV X-ray diffractometer using Cu Kα radiation (1.5405 Å). XPS measurements were performed in PHI 5000 Versa Prob II (FEI Inc.) to explore the oxidation states of europium on the biosorbent. XRF

fluorescent lamp. These elements also compete for the active sites in the biosorbent. Although, in this work, a simple crosslinking of the polysaccharide ensures around 60% recovery, it could be possible to enhance the adsorption of Eu(III) from the phosphor powder leached from the fluorescent lamp sample by hosting more accessible functional groups in cellulose through surface functionalization.

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CONCLUSIONS Toward the convergence of biopolymer and a microbe, this joint venture between cellulose and yeast produces a simple and efficient methodology to tap the potential of greener alternatives to recover precious europium. In conclusion, it is evident that yeast immobilized on cellulose matrix enhances the adsorption capacity of cellulose toward Eu(III). Functional groups (NH2, COOH, and OH) available on the yeast cell wall and the polysaccharide coordinate with Eu(III), thereby increasing the adsorption capacity. A comparison of the adsorption capacities against related adsorbents (Table 5) also Table 5. Comparison of Adsorption Capacities against Few Adsorbents

adsorbents

Langmuir adsorption capacity for Eu(III) (mg g−1)

thiourea-functionalized cellulose Mycobacterium smegmatis Sargassum polycystum Ca-loaded MnO2@ polypyrrole nano-hematite Al-substituted goethite palygorskite P-Al2O3 ZSM-5 zeolite Fe3O4@MnOx native cellulose

27.0 19.2 62.3 54.7 13.0 6.8 24.3 112.1 3.3 138.1 18.5

Saccharomyces cerevisiae

14.2

Saccharomyces cerevisiaeimmobilized cross-linked cellulose

25.9

references 55 56 57 58 59 45 60 61 62 63 present work present work present work

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Figure 14. Confocal images of Saccharomyces cerevisiae.

Langmuir adsorption capacity of native cellulose, Saccharomyces cerevisiae, and yeast-immobilized cellulose.

spectrum was recorded using a PANalytical Epsilon 1 spectrometer. The confocal images for the microorganism were obtained using a Leica DMi8 confocal microscope. Preparation of the Biosorbent. The biosorbent was prepared in the ratio of 1:1 (w/w) by mixing 5.0 g of cellulose with 5.0 g of Saccharomyces cerevisiae. The culturing of yeast and the subsequent cross-linking of cellulose with glutaraldehyde were done as previosuly reported by our group.16 The biosorbent was vacuum-dried at 50 °C and used for analytical characterization and adsorption studies. The purity of the grown culture was verified using confocal fluorescence microscopy prior to the preparation of the biosorbent. Diluted rhodamine B solution was mixed with a minimum amount of grown yeast culture and mixed well. The mixture was used to prepare the microscopic slide and analyzed at 553 nm (excitation wavelength) through confocal microscopy. Confocal images of the grown culture are shown in Figure 14. Batch Adsorption Experiments. Europium trichloride hexahydrate (EuCl3·6H2O) was used to prepare the stock standard solution (1000 mg L−1), and further, working standard solutions were prepared accordingly. Preliminary batch adsorption study was carried for the adsorption of Eu(III) by taking 0.2 g of the biosorbent and 25 mL of 50 mg L−1 Eu(III) solution in a 100 ml conical flask. Adsorption equilibrium was achieved in an orbital incubator shaker (120 rpm) for 3 h at 25 ± 2 °C. The concentration of Eu(III) was calculated as qe =

(Co − Ce)V W

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (N.R.) *E-mail: [email protected]. (V.R.) ORCID

N. Rajesh: 0000-0003-1546-9904 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the project grant from the Department of Science and Technology, SERB, India (EMR/ 2016/005231). We are thankful to Sprint testing solutions, Mumbai and the surface characterization laboratory, IIT Kanpur, India for their assistance in SEM, EDX, and XPS characterizations. We acknowledge the Central Analytical Laboratory, BITS Pilani, Hyderabad Campus, India for supporting the other analytical characterizations of the biosorbent.

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REFERENCES

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(17)

where qe, Co, and Ce refer to the adsorption capacity at equilibrium, the initial concentration, and the equilibrium concentration of Eu(III), respectively. V and W refer to the volume of Eu(III) solution (in liters) and the weight of the biosorbent (in grams), respectively. Isotherm studies were performed separately with 0.2 g of native cellulose, Saccharomyces cerevisiae, and yeast-immobilized cellulose by equilibrating 25 mL of Eu(III) solution in the concentration range of 20−600 mg L−1. The pH of the solution was adjusted to 6.0, and the mixture was allowed to reach equilibrium using an orbital incubator shaker for 3 h. The respective concentrations of Eu(III) remaining in the solution phase (Ce) were determined using ion chromatography. The linear graphical correlation between Ce/qe and the equilibrium concentration Ce was used to find the maximum 950


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