Turning Spent Li-ion Battery Anode into a Highly ... - ACS Publications

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Mesocarbon Microbead Carbon-Supported Magnesium Hydroxide Nanoparticles: Turning Spent Li-ion Battery Anode into a Highly Efficient Phosphate Adsorbent for Wastewater Treatment Yan Zhang,†,‡ Xingming Guo,†,‡ Feng Wu,†,‡ Ying Yao,*,†,‡ Yifei Yuan,§,∥ Xuanxuan Bi,§,⊥ Xiangyi Luo,# Reza Shahbazian-Yassar,∥ Cunzhong Zhang,†,‡ and Khalil Amine§ †

School of Materials Science and Engineering, Beijing Key Laboratory of Environmental Science and Engineering, Beijing Institute of Technology, Beijing 100081, China ‡ National Development Center of High Technology Green Materials, Beijing 100081, China § Chemical Sciences and Engineering Division and #Materials Science Division, Argonne National Laboratory, 9700 South Cass Avenue, Lemont, Illinois 60439, United States ∥ Materials Science and Engineering Department, Michigan Technological University, 1400 Townsend Drive, Houghton, Michigan 49931, United States ⊥ Department of Chemistry and Biochemistry, The Ohio State University, 100 West 18th Avenue, Columbus, Ohio 43210, United States S Supporting Information *

ABSTRACT: Phosphorus in water eutrophication has become a serious problem threatening the environment. However, the development of efficient adsorbents for phosphate removal from water is lagging. In this work, we recovered the waste material, graphitized carbon, from spent lithium ion batteries and modified it with nanostructured Mg(OH)2 on the surface to treat excess phosphate. This phosphate adsorbent shows one of the highest phosphate adsorption capacities to date, 588.4 mg/g (1 order of magnitude higher than previously reported carbon-based adsorbents), and exhibits decent stability. A heterogeneous multilayer adsorption mechanism was proposed on the basis of multiple adsorption results. This highly efficient adsorbent from spent Li-ion batteries displays great potential to be utilized in industry, and the mechanism study paved a way for further design of the adsorbent for phosphate adsorption. KEYWORDS: spent Li-ion battery, phosphate, adsorption, magnesium hydroxide, anode ascorbic, and aspartic acid.8 However, recovering and exploring the usage of waste anode materials (mostly carbon-based materials) from spent LIBs is also desirable, which has not been conducted in detail yet. Carbon residues from spent Li-ion batteries have advantages such as large quantity, relatively pure components, and superior carbon matrix structures, which may provide suitable alternatives for preparation of a novel carbon adsorbent that has potential application in wastewater treatment.9,10 Phosphorus, as a limiting nutrient and an essential element for the growth of organisms and plants, is an indicator of surface water quality. Eutrophication of freshwater and deteriorated aquatic ecosystems will occur in the case of excessive discharge of phosphate into water bodies such as lakes, rivers, and seas. Therefore, it is very important to remove

1. INTRODUCTION Rechargeable lithium-ion batteries (LIB) are one of the most successfully commercialized electrochemical power sources, due to high energy density, high power density, and high energy efficiency. The wide application of LIB in portable electronic devices and electric vehicles has been inevitably generating a large amount of spent batteries every year. It is expected that, by 2020, the quantity and weight of discarded LIBs in China will surpass 25 billion units and 500 000 metric tons, respectively.1 Such a huge amount of battery waste byproduct is no doubt a serious environmental nuisance; hence, recovering and recycling this waste resource are in accordance with the global paradigm shift toward sustainable development. Unfortunately, current research focusing on spent LIB recycling processes has been limited to merely the recovery of cathode materials.2−7 The hydrometallurgical approach is most widely used to recycle various metals from spent LIBs by solvent extraction.3 High extraction of Co and Li has been also achieved via leaching with organic acids including citric, malic, © 2016 American Chemical Society

Received: May 7, 2016 Accepted: July 27, 2016 Published: July 27, 2016 21315

DOI: 10.1021/acsami.6b05458 ACS Appl. Mater. Interfaces 2016, 8, 21315−21325

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ACS Applied Materials & Interfaces Scheme 1. Preparation Diagram for Mg-MCMB Composite

mesoporous pores, was used to synthesize nanostructured magnesium hydroxide-modified mesocarbon microbeads (MgMCMB) composite. A series of laboratory experiments were also conducted to determine its phosphate sorption capacities, mechanisms, and characteristics. Here, we are able to demonstrate the feasibility of recovered MCMB usage in preparation of efficient phosphate adsorbent and assess the phosphate removal ability of such a nanocomposite. In addition, the mechanisms governing adsorption of phosphate onto Mg-MCMB were identified through characterizing the physicochemical properties of original and postsorption MgMCMB.

phosphate from aqueous solutions prior to its discharge into surface runoff and natural water bodies to avoid eutrophication.11 Current phosphate removal technologies for municipal sewage and industrial effluents mainly include biological, chemical, and physical treatment methods.12 Among them, simple physical adsorption has attracted considerable attention in recent years, owing to its comparative usefulness and costeffectiveness.13 While most of the traditional adsorbents such as activated carbon, fly ash, slag, and oxide tailings showed very low phosphate adsorption capacity of less than 20.0 mg/g,13−16 recent studies showed engineered carbon embedded with MgO nanoparticles showed improved adsorption capacity for aqueous phosphate. The improved phosphate sorption ability is attributed to the presence of nanosized MgO particles on the carbon surfaces, which helps the precipitation of phosphate from the solution.17−19 In this sense, fabricating nanosized Mgcontaining compounds on the recovered carbon materials from spent LIB anode has great potential to strongly adsorb phosphate as a wastewater treatment adsorbent, considering the features of carbon materials harvested from the spent Li-ion battery anode, especially the presence of large amounts of functional groups on the carbon surface after extended cycles during the cell operation. These functional groups are extremely beneficial to adsorb Mg-containing species. In this study, mesocarbon microbeads (MCMB) carbon recovered from the commercial spent LIB anode was modified by nanosized Mg(OH)2 on the surface, which demonstrated extremely high phosphate adsorption ability as a wastewater treatment adsorbent. To the best of our knowledge, this is the first report on LIB anode material recovery and its potential application in environmental protection, that is, wastewater treatment. Moreover, we were able to achieve the highest phosphate removal capacities (∼600 mg/g) using this recovered nanocomposite, which is 1 order of magnitude higher than that of the previously reported carbon-based adsorbents. As shown in Scheme 1, spent mesocarbon microbeads graphite powder, with large surface area and

2. EXPERIMENTAL SECTION 2.1. Preparation of Nanostructured Mg-MCMB from Spent LIBs. Commercial, cylindrical 18650 spent LIBs were used for our starting materials. A discharging pretreatment step was carried out before dismantling of the battery steel crusts in order to avoid shortcircuiting and self-ignition of battery rolls. After the batteries were dismantled, anodes and cathodes were manually uncurled and separated. The anodes were then immersed into N-methyl-2pyrrolidone (NMP, Beijing Chemical Works) at 80 °C for 4 h to separate support substrates copper foil and anode carbon materials. NMP could be reused multiple times. The anode materials (i.e., graphite) were filtered off, washed thoroughly three times with deionized (DI) water, and dried in an oven at 80 °C to remove NMP and other impurities. The obtained graphitized carbon was denoted MCMB. Then 3.5 g of MCMB was immersed into 1.4 M magnesium nitrate solution [Mg(NO3)2·6H2O, Beijing Chemical Works], stirred at 50 °C for 4 h, and dried in an oven at 80 °C. The dehydrated mixture was heated in a tube furnace (T60/10, SGM, China) at a rate of 10 °C/min up to 600 °C for 1 h under N2 flow to maintain an oxygen-free environment. The resulting sample was then washed with DI water several times, dried in the oven at 80 °C, and ground into fine powder. The obtained sample was stored for further experiments and referred to as Mg-MCMB. 2.2. Characterization of MCMB and Mg-MCMB. Elemental C, H, N, and S analyses were performed on a CHNS elemental analyzer (vario EL cube, Elementar, Germany) via high-temperature combustion of carbon and Mg-MCMB samples. Major metal elements 21316


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ACS Applied Materials & Interfaces Table 1. Elemental Composition of MCMB and Mg-MCMB Nanocomposites mass-based (%) sample

C

H

N

S

K

Na

Ca

MCMB Mg-MCMB

87.18 53.57

0.811 1.874

0.10 0.04

0.07 0.04

0.004

0.002

0.023

Mg 18.13

Fe

Zn

Mn

Co

Ni

Cu

Li

0.939 0.930

0.021 0.010

Pb

Cd

Cr

Figure 1. (A) Raman and (B) FTIR spectra of MCMB and Mg-MCMB composite. of the MCMB and Mg-MCMB were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES, Optima 8300, PerkinElmer) after the AOAC (Association of Official Analytical Chemists) method of nitric acid digestion of samples for multielemental analysis.18,20 Physiochemical features of MCMB and Mg-MCMB were investigated by Raman spectroscopy. Raman spectra were recorded from 100 to 2000 cm−1 on a HR800 confocal Raman spectrometer (Jobin Yvon, French) using a 532 nm argon ion laser. Surface functionalities of MCMB and Mg-MCMB were examined by Fourier transform infrared spectroscopy (Spectrum One FTIR spectrometer, PerkinElmer). The samples were blended with KBr, which acted as the background at a sample/KBr mass ratio of 1:100 and then were pressed into a disk for FTIR analysis. FTIR spectra were obtained at 4 cm−1 resolution in the range from 450 to 4000 cm−1. High-energy synchrotron X-ray diffraction (XRD) was carried out at the 11-ID-C beamline of the Advanced Photon Source (APS), Argonne National Laboratory. The X-ray wavelength was 0.10798 Å. The samples were prepared by tapping the powder into a Kapton capillary, which was then sealed at both ends. XRD patterns were collected in the transmission mode on a PerkinElmer large area detector. The collected two-dimensional (2D) patterns were then integrated into conventional one-dimensional (1D) patterns (intensity vs 2q) for final data analysis by use of Fit2d software. Surface structures, morphology, and elemental composition of MCMB, Mg-MCMB, and postsorption Mg-MCMB samples were analyzed by a scanning electron microscope (SEM; Hitachi S-4800, Japan) equipped with energy-dispersive X-ray spectroscopy (EDS; Oxford instruments, England). Transmission electron microscopy (TEM) was employed to analyze the particle sizes as well as the morphology of the nanoparticles on the surface of MgMCMB and postsorption Mg-MCMB. Prior to TEM imaging, we obtained a dilute suspension from the sample that had been ultrasonically dispersed in ethanol for 5 min. The suspension was then dropped onto a copper grid and dried in air. TEM was carried out inside a JEOL JEM-3010 instrument operating at 300 kV. X-ray photoelectron spectroscopic analysis (XPS) of Mg-MCMB and postsorption Mg-MCMB was carried out with a Phi Quantera-II spectrometer (Ulvac-Phi, Inc., Japan) to determine the surface composition and electronic structures. 2.3. Adsorption Experiments. Batch adsorption experiments of phosphate by Mg-MCMB were performed in 50 mL plastic centrifuge tubes. Stock phosphate solutions were prepared by dissolving

potassium phosphate dibasic anhydrous (K2HPO4, Beijing Chemical Works) in DI water. The adsorption kinetic experiments were conducted with an initial phosphate concentration of 153.2 mg/L (50 mg of P/L), and 0.05 g of Mg-MCMB was added to 25 mL of phosphate solution. The mixtures were shaken in a mechanical shaker at room temperature and withdrawn at different intervals from 20 min to 24 h, then promptly filtered through mixed cellulose membrane filters with 0.22 μm pore size. The phosphate concentration of supernatant was examined by the ascorbic acid method (ESS Method 310.1)21 with a spectrophotometer (UB 2800, Hitachi, Japan). Adsorption isotherm experiments were performed with different initial phosphate concentrations ranging from 61.29 to 4596.77 mg/L (20−1500 mg of P/L). Mg-MCMB (0.05 g) was added to 25 mL of various phosphate solutions. The mixtures were placed on a shaker and agitated for 24 h at room temperature to ensure complete reaction of phosphate solutions with Mg-MCMB. Other experimental steps were the same as the kinetic experiments. Postadsorption Mg-MCMB samples were collected and dried at 80 °C for further characterization. The influence of coexisting anions on phosphate adsorption by MgMCMB was studied by adding anions into phosphate solutions. Chloride, nitrate, carbonate, sulfate, and bicarbonate were investigated by adding 0.01 M NaCl, NaNO3, Na2CO3, Na2SO4, or NaHCO3 to the 153.2 mg/L (50 mg of P/L) phosphate solutions. Mg-MCMB (0.05 g) was added to 25 mL of the obtained solution, and the mixture was shaken for 24 h. The same procedure was then used to determine aqueous and adsorbed phosphate concentrations. Desorption experiments were carried out by desorbing phosphate from Mg-MCMB in 0.5 M NaOH solution. Spent Mg-MCMB was added to NaOH solution and then agitated on a shaker for 24 h. The regenerated Mg-MCMB was then washed several times and used for adsorption again. The regeneration experiment was conducted for eight cycles to evaluate reusability of Mg-MCMB. All experiments were conducted repetitively and the results were averaged for the following analysis. 2.4. Statistics. Coefficient of determination (R2), p-value, and other statistics were analyzed by OriginPro 8.5. Matlab Toolbox was used to fit all kinetics and isotherm data. The variation in phosphate removal ability with coexisting ions was statistically analyzed by oneway analysis of variance (ANOVA) with a significance level of 0.05 (p < 0.05). 21317


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ACS Applied Materials & Interfaces

Figure 2. XRD of prepared MCMB, Mg-MCMB, and postadsorption Mg-MCMB.

Figure 3. SEM images and corresponding EDX spectra of (A−C) Mg-MCMB and (D−F) postsorption Mg-MCMB, and TEM images of (G) MCMB, (H) Mg-MCMB and (I) postsorption Mg-MCMB. The polycrystalline patterns in panel H belong to Mg(OH)2. The illustrated particle in panel I is single-crystalline Mg3(PO4)2(H2O)8 being viewed along the [1 1̅ 0] direction. (Top right inset) Simulated diffraction pattern of [1 1̅ 0] Mg3(PO4)2(H2O)8. Scale bars in all images are 100 nm.

3. RESULTS AND DISCUSSION

elements), as listed in Table 1. MCMB exhibits high carbon content up to 87.18% in the presence of other elements such as Cu, Ca, and Li. The small amount of Cu (0.94%) and Li (0.02%) is most likely from copper foil and graphite anode in the spent anode of LIBs. Compared with the high carbon content in MCMB, Mg-modified MCMB shows Mg content of 18.13 wt % and lower carbon content of 53.57%, indicating successful embedding of Mg into MCMB matrix. Owing to

3.1. Characteristics of MCMB and Mg-MCMB. Mesocarbon microbeads (MCMB) were obtained by separating the anode from the spent LIBs and were further modified by Mg solution to form Mg-MCMB. The composition of obtained MCMB and Mg-MCMB were carefully measured by elemental analyzer (for C, H, N, and S) and ICP-OES (for metal 21318


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The XPS spectrum of Mg-MCMB shows strong signals of Mg on the carbon surface (Figure S2 in Supporting Information), which is also consistent with the XRD analysis. The Mg 2p peak of Mg-MCMB resolves to two main peaks at 49.1 and 51.4 eV, which are attributed to the presence of Mg(OH)2 and overlap with Li 1s spectrum (Figure 4).32,33

addition of Mg, the relative mass contents of C, N, S, Cu, Li and Ca in Mg-MCMB are slightly lower than those of MCMB. Other heavy metal elements such as Fe, Zn, Co, Ni, Pb, Cd, Cr, and Mn were not detected in either MCMB or Mg-MCMB, and thus they will not contribute to phosphate adsorption process. Raman and FTIR spectroscopy were employed to further investigate MCMB and Mg-MCMB. In Raman spectra, two prominent peaks at 1349 and 1578 cm−1 are typical shifts for the D and G bands of carbon, respectively (Figure 1A). D band is associated with amorphous carbon, structural defects, and functionalities (e.g., -OH, -CO, -COOH, and -CC) of carbon, and G band represents the stretching vibration of sp2 carbon atoms, corresponding to first-order scattering of the E2g mode.22−24 High intensity of G band in both MCMB and MgMCMB samples detected as the raw material is mainly graphite from LIB battery anode. Compared to MCMB, the intensity ratio of D band to G band (ID/IG) of Mg-MCMB increases from 0.14 to 0.29, indicating the increase of amorphous phase, structural defects, or functional groups after Mg modification.23,25 FTIR spectra were used to determine vibration frequencies in the functional groups. A number of peaks were observed in FTIR, indicating the complex nature of the samples. These abundant functional groups might benefit phosphate adsorption, since phosphate could be removed by Mg-MCMB through interaction with functional groups (Figure 1B).13The appearance of two peaks at 3445 and 1644 cm−1 could be attributed to stretching and bending vibrations of water molecules, respectively.26 The peaks at 1401 and 1090 cm−1 result from the bending vibration of O−H and the stretching vibration of C−O bond.22,26,27 For Mg-MCMB sample, the sharp and intense peaks at 3697 and 463 cm−1 are indicative of the O−H antisymmetric stretching vibration of magnesium hydroxide and the Mg−O stretching vibration, respectively.28,29 The results suggest the formation of Mg(OH)2 on Mg-MCMB sample after Mg modification. The structure and phase purity of MCMB and Mg-MCMB composite were investigated by X-ray diffraction (XRD), as shown in Figure 2. Compared with the pure crystalline structure of MCMB, it is clear that well-crystallized Mg(OH)2 nanoparticles (JCPDS no. 07-0239) are formed after treatment of recovered MCMB in Mg-containing solution (red curve). The grain size of Mg(OH)2 on Mg-MCMB is roughly estimated to be 18.4 ± 4.0 nm by the Debye−Scherrer equation.30,31 The morphologies and structures of prepared MCMB and Mg(OH)2-modified MCMB (Mg-MCMB) were observed by SEM and TEM. MCMB shows a microsphere structure with rough surface and average particle size of ∼24 μm (Figure S1A) and the EDX spectrum confirms that the major element is carbon with low peaks of oxygen, fluorine, and copper (Figure S1B in Supporting Information), which are consistent with the results of elemental analysis. After magnesium modification, the surfaces of carbon microspheres are full of nanosized flakes with an average thickness of 15.6 nm (Figure 3A,B), which is in accordance with XRD results, and the particle size of MgMCMB carbon microspheres slightly increases to ∼28.5 μm. The EDX spectrum of Mg-MCMB indicates these nanoflakes mainly to be magnesium species because of the higher peaks of oxygen and magnesium compared to MCMB (Figure 3C), which further confirmed the dominance of those Mg(OH)2 nanoparticles on the carbon matrix.

Figure 4. XPS spectra of Mg 2p for Mg-MCMB.

Based upon multiple techniques, we have proven that nanosized Mg(OH)2 has been successfully embedded onto the spent carbon, which will play a key role in phosphate adsorption process, as demonstrated below. 3.2. Adsorption Kinetics and Isotherms. Adsorption of phosphate in this study was conducted by adding Mg(OH)2modified MCMB particles (Mg-MCMB) to stock K2HPO4 solutions. The effects of contact time on adsorption of phosphate by Mg-MCMB are presented in Figure 5. With increasing time, the adsorption of phosphate increases rapidly in the first 3 h and then slows down until adsorption equilibrium at approximately 12 h. This agrees with previous studies that more accessible active sites contribute to the fast kinetics at the beginning of the reaction, but the occupation of active sites leads to a sluggish rate for the following steps.26,34 In our study, the commonly used pseudo-first-order and pseudo-second-order models, Ritchie nth-order model, and Elovich model were applied to simulate the kinetic data. Table 2 summarizes the related kinetic fitting parameters. The coefficient of determination (R2) indicates that all these models were suitable at describing the kinetic data (Figure 5A). The first-order and Ritchie nth-order kinetic model fitted the data slightly better than second-order and Elovich models, with R2 being 0.9964 and 0.9967, respectively. This result shows that both mononuclear and polynuclear adsorption of phosphate would be favored in the kinetics experiment, perhaps explaining why fittings from the nth-order model were slightly better than those of either the first- or second-order model. To further understand the kinetics of the adsorption reaction, intraparticle diffusion plot was employed here. Previous studies showed that intraparticle diffusion is an important process controlling contaminant adsorption on carbon materials.34,35 As shown in Figure 5B, the intraparticle diffusion plot shows two linear portions, suggesting a two-step process. The high linear dependency on the square root of time (R2 = 0.9803) of preequilibrium kinetic plot (the first stage) reveals that intraparticle/pore diffusion is the rate-limiting step at the gradual 21319


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Figure 5. (A) Adsorption kinetic data and modeling and (B) intraparticle diffusion plot for phosphate on Mg-MCMB. Symbols represent experimental data, and lines show model results.

Table 2. Best-Fit Kinetics and Isotherm Model Parameters for Phosphate Adsorption to Mg-MCMB model

equation

parameter 1

parameter 2

parameter 3

R2

Adsorption Kinetics first-order

dqt dt

second-order

dqt

nth-order

dqt

Elovich

dqt

dt dt

dt

= k1(qe − qt )

k1 = 0.4123 h−1

qe = 47.02 mg/g

0.9964

= k 2(qe − qt )2

k2 = 0.009 029 g/mg·h

qe = 53.77 mg/g

0.9902

= kn(qe − qt )n

kn = 0.4275 gn−1/mgn−1·h

qe = 47.42 mg/g

= α exp(− βqt )

β = 10.63 g/mg

α = 18.46 mg/g·h

0.9569

n = 1.107

0.9967

Adsorption Isotherms Langmuir

KQCe qe = 1 + KCe

K = 0.002 003 L/mg

Q = 588.4 mg/g

0.9955

Freundlich

qe = K f Ce n

Kf = 21.85 mg1−n·Ln/g

n = 0.3981

0.9238

Kif = 0.007 659 Ln/mgn

Q = 545.7 mg/g

n = 1.188

0.9982

KT = 0.8961 (L/g)

BT = 9.750 × 10−5 Ln/mgn

n = 1.448

0.9982

n

K if QCe 1 + K if Ce n

Langmuir−Freundlich

qe =

Toth

qe =

dual-mode

qe =

bQCe + KDCe 1 + bCe

KD = 2.628 × 10−6 L/g

Q = 580.4 mg/g

b = 0.002 070 L/mg

0.9954

BET

qe =

QKSCe (1 − KBCe)(1 − KBCe + KSCe)

KS = −5.167 × 10−5 L/mg

Q = 732.7 mg/g

KB = 0.001 448 L/mg

0.9972

K TCe (1 + BT Ce n)1/ n

several isotherm models were used to fit and analyze the isotherm data, including Langmuir, Freundlich, Langmuir− Freundlich, Toth, dual-mode, and BET models (see eqs 6−11 in Supporting Information). Isotherm parameters and regression coefficients from all models are given in Table 2. The results show that, except for the Freundlich model, all the adsorption isotherm models fit well with the experimental data and have high regression coefficients (R2) above 0.99. The maximum monolayer adsorption capacity is 588.4 mg/g (i.e., 192.0 mg of P/g) based on the Langmuir model, which is one of the highest phosphate sorption capacities reported so far. Furthermore, the Langmuir−Freundlich and Toth models, with R2 above 0.998, fit the data slightly better than the other four models. The results suggest that phosphate adsorption on MgMCMB was likely affected by both Langmuir and Freundlich

adsorption stage. The intercept of the plot (Ci) reflects the boundary layer effect, and the larger the intercept is, the greater the contribution of the film diffusion is shown in the ratecontrolling step.36 The first fitted line has a small intercept of only −2.12 mg/g, which may be attributed to the large particle sizes and scarcity of pores for Mg-MCMB, and the negative intercept indicates the boundary layer thickness retarded the intraparticle diffusion step.37 The second linear portion of the plot is ascribed to the final equilibrium stage, where adsorption slows down to reach equilibrium due to lower phosphate concentration in the solution.37−39Adsorption equilibrium isotherms of Mg-MCMB showed increasing sorption of phosphate with increasing concentrations of aqueous phosphate until a maximum adsorption capacity of over 500 mg/L was reached (Figure 6). To be more mechanistically reasonable, 21320


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which range phosphate exists only in the species HPO42− and PO43−. Hence, it can be speculated that Mg(OH)2 on the surface of Mg-MCMB was gradually converted into Mg3(PO4)2·8H2O and MgHPO4 compounds. After phosphate adsorption, the SEM image shows that a large number of nanosized spindle-shaped flakes appear and attach to the carbon surface (Figure 3D,E). These nanoflakes turn out to be Mg−P compounds, based on EDX at the same spot with SEM (Figure 3F). SEM and EDX results show great accordance with the results of XRD analysis and imply that phosphate was successfully adsorbed on the carbon surface and new bond formation dominated the sorption process. The TEM images in Figure 3G−I illustrate the morphological and structural evolution of the carbon materials before and after P adsorption. Figure 3G shows the original morphology of the recovered MCMB, where the porous carbon matrix is clearly shown to mix with some particle-shaped cycling byproducts. In Figure 3H, where the samples were treated by Mg solution, the nanosized needle-shaped Mg(OH)2 is clearly identified with the corresponding selected area electron diffraction (SAED) given in the inset. After P adsorption, Figure 3I captured the existence of a single-crystalline particle, which is analyzed to be Mg3(PO4)2(H2O)8 phase viewed along the [1 1̅ 0] zone axis. Therefore, the TEM results convincingly demonstrate phase evolution of the carbon anode, and they agree well with the XRD results. In XPS, signals of P were detected on postadsorption MgMCMB sample (Figure S3 in Supporting Information). From Figure 7A, the Mg 2p spectrum shows signal at 49.1 eV, corresponding to Mg−OH binding energy.32,33 At 51.1 and 52.4 eV, signals of Mg3(PO4)2 and MgHPO4 are observed.32,42 Additionally, in the P 2p spectrum, two sets of peaks of Mg3(PO4)2 and MgHPO4 are shown, corresponding to the 2p 3 /2 spectrum.32,42,43 The Mg 2p and P 2p spectra not only demonstrate the presence of Mg3(PO4)2 and MgHPO4 but also reveal a large amount of Mg(OH)2 on postsorption MgMCMB (Figure 7), indicating that the adsorbent could remove phosphate successfully and be capable of taking up more phosphate from aqueous solution. In summary, findings from sorption experiments and postsorption characterization indicated that removal of P by Mg-MCMB from spent lithiumion batteries is mainly controlled by precipitation and formation of Mg−P compounds. These results are in line with published mechanism studies of P adsorption onto metal hydroxides.44−48

Figure 6. Adsorption isotherm data and modeling for phosphate on Mg-MCMB. Symbols represent experimental data, and lines show model results.

processes with heterogeneous sorption. The dual-mode model also fit phosphate uptake data well, with R2 of 0.9954. The first term of the dual-mode isotherm represents precipitation of Mg3PO4 and MgHPO4, which is considered linearly related to Ce, whereas the second term corresponds to adsorption.26 The BET model interpreted the isotherm data well with high correlation coefficients (R2 = 0.9972). The affinity between phosphate and Mg-MCMB was strong at the first step, while phosphate in the aqueous phase began to bind to previously adsorbed phosphate, causing coadsorption in the high concentration range.40 Hence, phosphate adsorption should be onto a heterogeneous surface with multilayer adsorption, and the mechanisms of this process may incorporate precipitation and adsorption, which is consistent with the kinetics results.41 More details of the 11 kinetic and isotherm models can be found in the Supporting Information. 3.3. Adsorption Mechanisms of Phosphate on MgMCMB. In order to clarify the mechanism of phosphate adsorption by Mg-MCMB, structure and morphology changes of the adsorbent before and after adsorption were measured by XRD, SEM, EDX, TEM, and XPS. In the XRD spectrum of postsorption Mg-MCMB, strong peaks of newly formed Mg3(PO4)2·8H2O and MgHPO4·1.2H2O crystals are detected (Figure 2). In addition, the final pH of the phosphate solution after adsorption increased to 9.5 from the original pH of 5.2, at

Figure 7. XPS spectra of (A) Mg 2p and (B) P 2p region for postsorption Mg-MCMB. 21321


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ACS Applied Materials & Interfaces Table 3. Summary of Phosphate Adsorption Capacities by Different Adsorbents in the Literaturea adsorbent

max adsorption capacity (mg/g)

adsorbent dosage(g/L)

pH

ref

coir-pith activated carbon tamarind nut shell activated carbon mesoporous MgO microspheres MgAl-LDH/cottonwood biochar Thalia dealbata biochar magnetic iron oxide biomass char magnetiteb iron impregnated coir pith Mg-enriched tomato leaves biochar Fe−Al−Mn trimetal oxidec oak sawdust biochar with lanthanum ALOOH/biochar composites nanobimetal ferrites CuFe2O4 lanthanum hydroxide-doped active carbon fiber amorphous zirconium oxide magnetic orange peel biochar nanostructured Fe−Cu binary oxidesd alunite peanut shell biochar dolomite zirconium-loaded orange waste fly ash iron-loaded skin split waste BOF slag treated by milling Fe−Mg−La composite Fe−Zr binary oxide La(III)-,Ce(III)-,Fe(III)- loaded orange waste digested sugar beet tailing biochar Mg-MCMB

7.74 4.98 75.13 410 4.96 15.41 15.11 27.15 70.92 116.6 55.73 142.7 135.04 41.31 15.3 99.01 3.8 39.8 118 7.57 52.91 174.68 10.88 72.00 60.7 415.2 41.83 42.70 133.09 588.40

4 6 0.4 2 4 0.6 1.0 1.0 2 2 0.1 2 2 0.6 2.5 0.1 6.25 0.2 10 2 2 1.67 2.5 1 10 0.2 1 1.67 2 2

6.0 6.0 ± 0.2 5.0 5.2 7.0 6.0 6.0 7.0 ± 0.3 3.0 5.2 6.8 5.6 6.0 2.64 5.5 6.2

14 16 50 28 55 56 57 58 59 19 60 61 62 63 64 65 66 67 68 69 12 70 71 72 73 11 74 75 13 this study

5.0 ± 0.1 5.0 7.0 ± 0.1 7.0 7.0 9.8 7.0 7.0−7.2 6.0 4.0 7.5 7.0 5.2

a

All experiments were conducted at room temperature. bWith 3 mM NaHCO3 as competing species. cWith 0.1 M NaCl as competing species. dWith 0.01 M NaNO3 as competing species.

Figure 8. (A) Effects of coexisting anions on amount of phosphate adsorption onto Mg-MCMB. (B) Recycle study of spent adsorption Mg-MCMB.

3.4. Comparison with Other Adsorbents for Removal of Phosphate. Increasing numbers of adsorbents have been used in recent years to remove phosphate from aqueous solutions, including activated carbons, slag, fly ash, dolomite, and oxide tailings.11,28,38−40 To illustrate the potential for use of Mg-MCMB for removal of phosphate from aqueous solutions, a comparative evaluation of the adsorption capacities of various adsorbents is provided in Table 3. Most carbon-based and other commercial adsorbents reported in the literature have very limited phosphate adsorption capacity of 100 mg/g. Here, our MgMCMB adsorbent reaches an adsorption capacity as high as 588.4 mg/g, which is one of the highest capacities so far. On the basis of our previous study, MCMB itself shows very limited ability to remove phosphate from aqueous solutions.49 Although modifying carbon matrix with Mg(OH)2/MgO could strongly enhance the adsorbent’s phosphate removal ability, directly applying Mg(OH)2/MgO without carbon support to wastewater is not practically suitable or effective because 21322


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the cost of the adsorbent is fairly low as it is prepared from spent Li-ion batteries, which makes it possible that the adsorbent does not even need regeneration. We are currently conducting more experiments to improve the performance of this system by further modification of the adsorbent.

Mg(OH)2/MgO as adsorbents are either hard to separate from wastewater or their phosphate removal ability is relatively low.48,50 After adsorption processes with Mg-MCMB, the hazardous metal concentrations including Mn, Co, Ni, Cu, Li, Pb, Cd, and Cr in the postsorption solutions were very low or even below the detection limit. The leaching toxicity of Mg-MCMB is within the secure level and will not lead to secondary water pollution (Table S1 in Supporting Information).51 Therefore, Mg-MCMB is a highly efficient and cost-effective adsorbent, which has great potential to be used in practical wastewater treatment application. 3.5. Effects of Coexisting Anions. In addition to studying the adsorption capability of Mg-MCMB, we also investigated the influence of other anions coexisting with Mg-MCMB, as shown in Figure 8A. Chloride, nitrate, carbonate, sulfate, and bicarbonate were added to phosphate solutions in the presence of Mg-MCMB. Among the five anions, bicarbonate exhibits the highest adsorption capacity. The adsorption of phosphate slightly decreases in the presence of chloride and nitrate (10.9% and 12.9% decrease, respectively), indicating weak competition between phosphate and these two anions. The existence of sulfate in the solution reduced the phosphate adsorption by about 22.3%, suggesting that bivalent anions have stronger competitiveness compared to monovalent anions. Carbonate reduced phosphate uptake ability to only 5 mg/g, and the results are statistically significant (p = 0.0003) after analysis by one-way ANOVA with a significance level of 0.05. The decrease in phosphate adsorption could be attributed to blocking and covering the surface adsorption sites by formation of magnesium carbonate precipitation. This evidence further supports the key role of surface precipitation with magnesium in controlling phosphate adsorption on Mg-MCMB. Hence, removal of carbonate anions in wastewater from other sources rather than CO2 from the air prior to addition of Mg-MCMB adsorbent is strongly advised in practical wastewater treatment processes. 3.6. Adsorbent Regeneration and Readsorption of Phosphate. Besides the superior adsorption capability, the stability of Mg-MCMB is another key to evaluate the feasibility of this adsorbent. The desorption and readsorption performance of Mg-MCMB was assessed in eight successive cycles as illustrated in Figure 8B. This adsorbent regeneration study was carried out with initial P concentration of 50 mg of P/L (i.e., adsorbed phosphate amount of about 50 mg/g) because typical P contents in domestic wastewater are under 50 mg of P/L, and conducting experiments at this P level could be more convincing and meaningful for practical wastewater treatment.52−54 The adsorption ability of phosphate on Mg-MCMB decreases gradually with increasing cycle numbers and it retains a decent capacity for phosphate adsorption within eight cycles. In the second cycle, phosphate adsorption capacity was reduced to 45.45 mg/g, which is about 89.3% of the original capacity. The adsorption capacity after seven cycles remains at about 20 mg/g. These results show that P adsorption on Mg-MCMB is not fully reversible, which is probably due to the strong bonding between magnesium and phosphate. Besides, Mg(OH)2 nanoparticles on the surface of the adsorbent could be gradually consumed and the active sites become saturated during the consecutive cycles. The implication from these results is that Mg-MCMB adsorbent has the potential for practical phosphate removal application in wastewater treatment since it shows decent regeneration ability and, especially,

4. CONCLUSION Mg(OH)2-modified MCMB adsorbent was successfully fabricated by salvaging the spent anode materials in LIBs and exhibited an extremely high phosphate adsorption ability in wastewater treatment. A highest adsorption capacity of 588.4 mg/g was obtained in our study. Effects of coexisting anions were also discussed in this work, and we found that the presence of carbonate had a detrimental effect on the adsorption. The adsorption mechanism was proposed by considering multiple kinetic models and Langmuir−Freundlich showed the best fit, indicating heterogeneous multilayer adsorption. The superior adsorption ability of Mg-MCMB, combined with decent stability, shows great potential to be commercially applied in industry.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b05458. Two figures showing SEM images and corresponding EDX spectra of MCMB and XPS spectra for pre- and postsorption Mg-MCMB; one table listing elemental composition of post-sorption solution; additional text and 11 equations describing adsorption kinetic and isotherm models (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(Y.Yao) E-mail [email protected]; tel 86-10-68912657. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Natural Science Foundation of China (NSFC) through Grant 51402018 and the National Key Program for Basic Research of China through Grant 2015CB251100. Use of the Advanced Photon Source was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract DE-AC02-06CH11357.

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