Simultaneous Optimization of Adsorption Capacity and Stability of

Jun 17, 2019 - Finally, the acid-treated ion sieves were filtered, washed with ... adsorbed LiCoxMn2–xO4 samples in 50 mL of 0.5 M HCl solution ...
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Article Cite This: Ind. Eng. Chem. Res. 2019, 58, 12207−12215

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Simultaneous Optimization of Adsorption Capacity and Stability of Hydrothermally Synthesized Spinel Ion Sieve Composite Adsorbents for Selective Removal of Lithium from Aqueous Solutions Majid Bazrgar Bajestani,† Ahmad Moheb,† and Mohammadali Masigol*,‡ †

Department of Chemical Engineering, Isfahan University of Technology, Isfahan 8415683111, Iran Tim Taylor Department of Chemical Engineering, Kansas State University, Manhattan, Kansas 66506, United States

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‡

ABSTRACT: The main aim of this paper was synthesis of spinel type lithium manganese oxide adsorbent with high adsorption capacity and stability for selective removal of lithium ion. In contrast with the previous works reported by other researchers, simultaneous improvements of adsorbent stability and adsorption capacity were investigated by insertion of cobalt into the spinel structure of lithium manganese oxide and optimization of the adsorbent preparation conditions. To this aim, the effects of calcination temperature and molar ratios of Li/Mn and Co/Mn on adsorbent capacity and stability were investigated via hydrothermal method for adsorbent preparation. Experiments were designed by using Design Expert Software and response surface methodology considering three independent variables each at three levels. It was found that the adsorbent synthesized at optimum conditions has high adsorption capacity of 53.52 mg/g and with only 2.52% adsorption capacity loss for two consecutive adsorption cycles. This achievement was also confirmed by XRD analysis. The structural morphology of the optimized adsorbent was characterized by SEM analysis. The result of BET analysis showed that the specific surface area of the optimized adsorbent was 2.564 m2/g. Finally, the results of selectivity adsorption experiment revealed that the optimized adsorbent can be considered as a promising tool for selective separation of lithium ions from sodium ions with molar selectivity of 90.32.

1. INTRODUCTION Lithium is one of the most used and valuable alkali metals with a wide range of applications including lithium batteries, adsorption chillers fluid, lubricants, manufacturing of glass and ceramics, and air conditioning in space crafts and submarines.1−4 Due to the increasing need for lithium compounds in various industries, recovery of lithium ions from existing waste sources has become one of the major concerns of the related industries.5 Inorganic ion sieve type ion exchangers have been of great interest recently due to their high selectivity for separation of particular ions.6 Among these, the relatively inexpensive spinel type lithium manganese oxides with nontoxic properties are known as suitable materials for some applications such as adsorption of lithium ions, manufacturing of lithium batteries’ cathodes, and lithium ion sensor production.7 A spinel unit cell consists of 32 oxygen atoms and 64 and 32 tetrahedral and octahedral sites, respectively.8,9 In the spinel structure of lithium manganese oxides, lithium ions occupy 1/8 of the tetrahedral sites, whereas 1/2 of the octahedral sites are occupied by manganese ions.10−13 Generally speaking, the spinel type adsorbents can be produced by the molding reaction, where a metal ion plays the role of a template to make the adsorbent suitable for selective adsorption of that specific metal ion. For example, when lithium ions are used as template, the resultant adsorbent has a tendency for selective adsorption of lithium ions from an aqueous phase by ion exchanging © 2019 American Chemical Society

mechanism. In order to improve the structural stability of lithium manganese oxide, usually small amounts of transition metals are added to the structure. Over the past few years, many studies have been conducted on the synthesis of lithium ion adsorbents with high adsorption capacity. The most important of these studies in terms of adsorption capacity and selectivity are discussed in the following paragraphs. In 1991, Chang et al. synthesized an adsorbent with the composition of Li1.33Mn1.67O4 which had an adsorption capacity 28.2 mg/g for separation of lithium ions from seawater.13 Chitrakar et al. used antimony to improve structural stability of spinel-like lithium adsorbent and investigated the effect of calcination temperature on the adsorbent properties. At the optimized conditions, adsorbent with chemical formula of Li1.16Sb(V)0.29Mn1.54O4 had adsorption capacity of 38.78 mg/g for Li+ ions from a LiCl enriched seawater solution with initial lithium ion concentration of 5 mg·dm−3.14 In another work in 2001, Chitrakar et al. synthesized a nanoadsorbent with chemical composition of Li1.6Mn1.6O4 by applying refluxing and hydrothermal methods for the recovery of lithium ions from seawater. The adsorbents obtained by the hydrothermal Received: Revised: Accepted: Published: 12207

February 10, 2019 June 2, 2019 June 17, 2019 June 17, 2019 DOI: 10.1021/acs.iecr.9b00804 Ind. Eng. Chem. Res. 2019, 58, 12207−12215

Article

Industrial & Engineering Chemistry Research

XRD analyses were employed for specific surface area determination and structural study of the optimized adsorbent, respectively. Finally, selectivity of the optimized adsorbent for lithium ions adsorption in the presence of sodium ions was investigated.

method had better performance in comparison with those synthesized by the reflux method. Furthermore, the best adsorbents showed the adsorption capacity of 40 mg/g.15 In 2010, Li-wen Ma et al. produced adsorbents with raw materials of lithium carbonate, manganese oxide, and Sb2O3. The Sb/Mn molar ratio of 0.05 and Li/(Mn + Sb) molar ratio of 0.5 showed the maximum lithium ions adsorption capacity (33.33 mg/g) from a solution containing 0.05 M of lithium ions.16 In 2011, Kim et al. produced adsorbents through a hydrothermal process by adding transition metals (M: Cu, Co, Ni, Fe) to lithium manganese oxide, and it was seen that the structural stability of the adsorbents [Li1.6(MnM)1.6O4] was improved. The best results with adsorption capacity of 35 mg/g were obtained by using cobalt as the added transition metal.17 In 2014, Zandevakili et al. studied the effects of synthesis parameters on the properties of lithium manganese oxide adsorbents. The evaluated parameters were the type of manganese and lithium source compounds, lithium to manganese ions molar ratio, calcination temperature, the type of oxidizing agent, and the calcination time. The highest adsorption capacity obtained in their work was 62.47 mg/g when lithium ions were adsorbed from a feed solution containing 0.02 M of lithium ions. The optimized adsorption performance was obtained by using Na2S2O8 as oxidizing agent and lithium nitrate and manganese nitrate as the source of metal ions. At this situation, the lithium to manganese ions molar ratio was 0.6, and calcination was conducted for 6 h at 450 °C.18 Kim et al.19 and Wang et al.20 added iron ions to the structure of spinel type lithium ion adsorbents to facilitate the collection and separation of adsorbents by employing a magnetic field after termination of adsorption process. The synthesized adsorbents by Kim et al. and Wang et al. showed lithium ions adsorption capacity of 6.84 and 53.3 mg/g, respectively. Also, some studies were conducted on synthesis of other inorganic adsorbents for ease of use according to the type of applications. These studies showed adsorption capacities between 14 and 74 mg/g for Li+ ions.21−27 As mentioned before, Zandevakili et al.18 are the only researchers who have investigated and optimized all the factors of lithium manganese oxide synthesis process by hydrothermal method in order to maximize adsorption capacity. However, they did not investigate the stability of the structure. On the other hand, according to previous studies, introducing cobalt ions into the structure of spinel type of lithium manganese oxide adsorbents exhibited the best results to improve the stability of the adsorbents without significant reduction of adsorption capacity.17 None of the previous studies considered simultaneous optimization of stability and adsorption capacity of spinel type lithium ions adsorbents. Therefore, in the current study an investigation was conducted on the synthesis conditions of cobalt introduced lithium manganese oxide adsorbents to optimize the adsorption capacity and structure stability at the same time. To this aim, the raw materials for lithium and manganese ions sources as well as oxidizing agent were chosen according to the results obtained by Zandevakili et al.18 The independent parameters considered for optimization of the objective functions of adsorption capacity and structural stability included the molar ratios of lithium to manganese and cobalt to manganese ions as well as calcination temperature. Optimization of the parameters was carried out by Design Expert software using response surface methodology (RSM). Detailed basis of experimental design and optimization by using RSM can be found in the literature.28−30 BET, SEM, and

2. MATERIAL AND METHODS 2.1. Materials. Cobalt(II) nitrate hexahydrate was purchased from Sigma-Aldrich, and lithium hydroxide was supplied by Samchun Chemical Company. All other chemicals, including sodium persulfate, hydrochloric acid, manganese(II) nitrate tetrahydrate, and lithium nitrate, were supplied by Merck and used as received. Also, deionized water was used for preparation of solutions. 2.2. Synthesis of Adsorbent. In this study, hydrothermal method was used for synthesis of lithium ion sieves. At the first step, manganese oxide particles were prepared by oxidation reaction of manganese nitrate. To this aim, sodium persulfate (as oxidizing reagent) and manganese(II) nitrate tetrahydrate (0.083 mol each) were added to 100 mL of deionized water in a volumetric flask and mixed together. Then, the mixture was transferred into an autoclave and heated at 110 °C for 12 h to complete the oxidation reaction to form the black color manganese oxide particles. Afterward, the black product was separated by filter paper, washed with deionized water, and dried at 120 °C for 12 h in static air. In the next step, LiCoxMn2−xO4 precursor was synthesized by wet impregnation of manganese oxide by lithium and cobalt ions by mixing of manganese oxide black solid with an aqueous solution of lithium nitrate and cobalt(II) nitrate hexahydrate (with the desired Li/Mn and Co/Mn mole ratios). After removal of water by heating the resultant mixture for 12 h at 120 °C in an oven, the dried solid was calcinated at the designated Table 1. Values of Independent Parameters for Adsorbent Synthesis variables

amounts of variables

A: Li/Mn molar ratio B: Co/Mn molar ratio C: calcination temperature (°C)

0.5−1.0−1.5 0.05−0.075−0.1 450−650−850

Table 2. Designed Experiments and Their Results run 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 12208

A: Co/Mn B: Li/Mn 0.05 0.075 0.1 0.075 0.075 0.075 0.075 0.075 0.075 0.05 0.1 0.075 0.075 0.1 0.05 0.05 0.1

0.5 1.5 1.5 1 1 1 1 1.5 1 1.5 1 0.5 0.5 0.5 1 1 1

C: Tc (°C)

first cycle Li+ adsorption (mg/g)

adsorption capacity loss (%)

650 450 650 650 650 650 650 850 650 650 450 450 850 650 850 450 850

44.6 51.22 37.85 42.5 44.1 43.7 43.17 42.25 41.8 50.78 38.11 33.91 34.2 30.22 46.67 55.3 32.6

6.03 3.9 0.45 4.06 3.6 4.48 3.61 1.91 2.96 3.28 1.03 5.83 3.94 2.2 3.8 5.98 0.73

DOI: 10.1021/acs.iecr.9b00804 Ind. Eng. Chem. Res. 2019, 58, 12207−12215

Article

Industrial & Engineering Chemistry Research temperature for 6 h. The following step involved Li+ ions extraction from the synthesized ion sieves which was carried out by stirring them in 0.5 M hydrochloric acid solution for 24 h until the lithium ions were partially extracted. Finally, the acid-treated ion sieves were filtered, washed with deionized water, and dried at 120 °C for 12 h. 2.3. Characterization Tests. 2.3.1. XRD Analysis. The adsorbent crystalline structure was examined using X-ray diffraction (XRD) at room temperature, with Cu Kα radiation (k = 1.5406 Å, 40 kV, 30 mA) and scanning from 10° to 80° at scanning rate of 10°/s by using XRD instrument model (XRD Philips X’pert). XRD results were analyzed and plotted by X’Pert HighScore software version1.0 d. In this analysis, the size of the adsorbent crystalline particles was determined by the Debye−Scherrer equation as follows: kλ t= Bcos β

In this equation C1 and C2 are the lithium ions concentrations (mg/L) for the feed solution and the solution after the adsorption process, respectively. ν denotes feed solution volume, and M stands for the mass of adsorbent (g) added to the solution. Finally, A represents the adsorption capacity (mg/g). In order to determine the stability of adsorbents, the cycles of adsorption−desorption experiments were repeated for each adsorbent. By calculating the adsorption capacity for two consequent cycles, the adsorption capacity loss was determined by eq 4: Adsorption capacity loss (%) =

(1)

(2)

where h, k, and l are Miller index values and d is the interplanar spacing data of the determined 2θ positions of the XRD pattern. 2.3.2. BET and SEM Analyses. Specific surface area of the ion sieve adsorbent was determined by Brunauer−Emmett− Teller (BET) analysis using a Nano Sord Dynamic instrument (SENS IRAN Ltd. Co.). The carrier gas was a mixture of nitrogen and helium (15 vol % N2) with gas flow rate of 10 standard cubic centimeters per minute (sccm), attenuation of 32, and detector temperature of 70 °C. Pore size and pore distribution of the prepared adsorption at the optimized conditions were determined with nitrogen adsorption and desorption isotherms at −196/ 954 °C using a Quantachrome ChemBET 3000 with relative pressure (P/Po) ranging from 0.989 to 0.002. The structural morphology of spinel adsorbent was characterized by scanning electron microscopy (SEM, Philips XL30). 2.4. Characterization of Adsorption Behavior. Lithium ions adsorption experiments were carried out by stirring 0.1 g of the synthesized LiCoxMn2−xO4 samples in 50 mL of 0.5 M LiOH solution (pH = 12) for 24 h at room temperature to reach equilibrium. Also, Li+ ions desorption experiments were conducted by stirring lithium adsorbed LiCoxMn2−xO4 samples in 50 mL of 0.5 M HCl solution for 24 h at room temperature. The concentrations of the lithium ions in the solutions were determined by atomic absorption spectroscopy (Buck Scientific, Model 210 VGP), and the adsorption capacity of samples was determined by using eq 3: A=

C1 − C2 ×v M

(4)

where A1 and A2 represent the adsorption capacity (mg/g) of the first and second adsorption cycles, respectively. The selectivity test of the adsorbent was done by stirring 0.1 g of the optimized adsorbent in 50 mL of an equimolar solution containing lithium and sodium hydroxides (0.5 M each) for 12 h. After the lithium and sodium ions concentrations in the remaining solution were determined by atomic absorption spectroscopy, the adsorption capacity of each ion was determined by using eq 3 to calculate the selectivity of the adsorbent for lithium ions separation in the presence of sodium ions. Design of experiments, optimization, and plotting of figures were carried out by Design Expert 11 (trial version) using response surface methodology. The values of the three independent parameters used by the DOE software are given in Table 1. According to the result of experimental design method, a set of 17 experiments was suggested by the DOE software, which is introduced in Table 2. It has to be noted that five similar experiments were designed by the software at

In this equation, t and B are the crystal size and the line width at half the maximum intensity (fwhm), respectively. Also, λ and β represent the X-ray wavelength and the Bragg angle, respectively, and k is the crystallite shape factor that varies between 0.5 and 1 (in this study it was adjusted at 0.9).31 For calculating lattice parameter, at the first stage, the 2θ positions of the first few peaks in the XRD pattern were determined by comparing the XRD pattern of the sample under study with the expected standard patterns. Then, the lattice parameter (a) of the cubic structure was obtained by using the following standard formula:32 1 h2 + l 2 + k 2 = d2 a2

A1 − A 2 × 100 A1

Table 3. Mathematical Models Suggested for the Objective Functions

(3)

response

equation

first cycle Li+ adsorption adsorption capacity loss

first cycle Li+ adsorption (mg/g) = 41.94 − 7.32A + 4.90B − 2.85C adsorption capacity loss (%) = 3.74 − 1.83A − 1.06B − 0.795C + 0.25AB + 0.47AC − 0.025BC − 0.881A2 + 0.129B2 + 0.024C2

Figure 1. Li/Mn molar ratio effect on adsorption capacity. 12209

DOI: 10.1021/acs.iecr.9b00804 Ind. Eng. Chem. Res. 2019, 58, 12207−12215

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Industrial & Engineering Chemistry Research

Figure 2. Effect of synthesis conditions on adsorption capacity loss.

adsorption capacity for the first cycle was linear. On the other hand, a quadratic model was suggested by the software for adsorption capacity loss (%). Therefore, in the following discussions about the effects of independents parameters on the objective functions, two-dimensional graphs are used for the first objective function with a linear model, and the results for adsorption capacity loss with a quadratic model are presented with contour plots. 3.2. Effects of Li/Mn Molar Ratio. According to Figure 1, by increasing the Li/Mn molar ratio, the adsorption capacity of the adsorbents was enhanced. Enhancement of adsorption capacity can be described by the fact that increasing the molar ratio of the Li/Mn leads to enlarged number of adsorbent sites of the spinel structure that contain lithium ions.33 As seen in Figure 2, at any calcination temperature, increasing the Li/Mn molar ratio improved the adsorption performance by reducing adsorption capacity loss of the adsorbents. However, it has to be noticed that this positive effect on the

central points. In fact, the results of the similar experiments are used by the software as a tool to examine the reproducibility of the experiments and their results and to prove that the experiments are conducted at a controlled situation.

3. RESULTS AND DISCUSSION 3.1. Effects of Independent Parameters on Adsorption Behavior of the Adsorbents. The results of the experiments designed by DOE software are presented in Table 2. These results include adsorption capacity for the first cycle and adsorption capacity loss (%) after one cycle of adsorption and desorption. The obtained results were used by the software for analyzing and modeling the experiments, and the outcome of this process is the models suggested by the software which give a mathematical relation between each objective function (response) and three independent parameters. These models are presented in Table 3. According to the models reported in Table 3, the model obtained for objective function of 12210

DOI: 10.1021/acs.iecr.9b00804 Ind. Eng. Chem. Res. 2019, 58, 12207−12215

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Industrial & Engineering Chemistry Research

aforementioned destructive effect of Mn3+ ions and therefore improves the structural stability which in turn leads to lowered adsorption capacity loss. On the other hand, insertion of cobalt ions to the adsorbent structure increases the Mn valence (3.5+ < Mn) and the stability of the spinel structure. As a result, the adsorption capacity loss was reduced significantly at simultaneous high values of Li/Mn and Co/Mn molar ratios. However, the effect of Co/Mn molar ratio on adsorbent stability is stronger than the effect of Li/Mn molar ratio, and therefore at high Co/Mn molar ratios the effect of Li/Mn ratio on adsorbent stability is weakened. As seen in Figure 2, at a constant value of the Co/Mn molar ratio, adsorption capacity loss decreased with increasing the Li/Mn molar ratio at any calcination temperature. However, this effect is more significant at lower values of temperatures. This may be attributed to consideration of the fact that the adsorbent stability decreased with reducing the calcination temperature, and therefore the effect of Li/Mn molar ratio was more significant. 3.3. Co/Mn Effect. As seen in Figures 2 and 3, increasing the Co/Mn molar ratio in the adsorbent structure reduced adsorption capacity and adsorption capacity loss. Addition of a transition metal, such as cobalt, to a spinel type adsorbent increases the manganese average oxidation state to more than 3.5+, which enhances the stability of spinel structure.17,34,35 The reason is that the Co−O covalent bond is stronger than the Mn−O bond, and therefore formation of the Co−O bonds increases ionicity of Mn−O bonds.38,39 3.4. Influence of Tc. According to Figures 2 and 4, increase in calcination temperature reduced the characteristic parameters of adsorbent capacity and adsorption capacity loss. The latter means that the adsorbent stability was improved by increasing the calcination temperature. By applying higher temperature for calcination process, the crystallinity of the spinel structure and size of the adsorbent particles increased. By enlarging the adsorbent particles, the effective surface area of them is reduced which in turn leads to lowered adsorption capacity. On the other hand, increasing the calcination temperature leads to formation of impurities such as Mn2O3, which is accompanied by oxygen loss in the spinel structure of the adsorbent particles.40−42 The formation of impurities which uses a part of Mn3+ ions of the spinel structure increases the average state valence of the remaining Mn ions in the spinel structure. Increasing the average state valence of Mn ions reduces the bond length of the Mn−O bonds and consequently decreases crystal lattice parameter. This effect improves the stability of the adsorbent particles (decreases adsorption capacity loss). 3.5. Optimization Results. For optimization studies by DOE software, the input factors were set “in range”, and the goals were set to be maximizing “the adsorption capacity of the first cycle” and minimizing “adsorption capacity loss”. The results of optimization by DOE software and confirmation test are shown in Table 4. The results of confirmation test showed a very good agreement with the results predicted by the DOE software for the optimized conditions.

Figure 3. Effect of Co/Mn molar ratio on adsorption.

Figure 4. Effect of calcination temperature on adsorption capacity.

performance of the adsorbent was more significant at lower amounts of the Co/Mn molar ratio. In general, Mn3+ ions are inherently unstable cations, and once they are in contact with aqueous solution (especially in contact with acidic solution) these ions are easily converted to Mn2+ and Mn4+ ions in the spinel structure. As a result, Mn2+ ions are dissolved in the solution which causes relative destruction and therefore reduces adsorption capacity of the spinel type adsorbent.17,34,35 On the other hand, increasing the Li/Mn ratio to more than the stoichiometric value of 0.5 causes the LiMn2O4 compound to be converted to Li1+xMn2−xO4. Raising the number of Li ions in the spinel structure (0 < x) reduces the length of the Mn−O and Li−O bonds and consequently decreases crystal lattice parameter, which is due to increase in the average valence of Mn to more than 3.5+ in the spinel structure.36,37 Increasing the Mn capacity diminishes the

Table 4. Optimized Conditions and Responses Obtained by Optimization and Confirmation Test Design Expert results confirmation test

Co/Mn

Li/Mn

Tc (°C)

first cycle Li+ adsorption (mg/g)

second cycle Li+ adsorption (mg/g)

adsorption capacity loss (%)

0.05 0.05

1.50 1.50

849.9 850.0

51.305 53.520

50.150 52.170

2.252 2.520

12211

DOI: 10.1021/acs.iecr.9b00804 Ind. Eng. Chem. Res. 2019, 58, 12207−12215

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Figure 5. Desirability of optimization condition and responses.

Figure 6. XRD result of adsorbent: (a) after synthesis and (b) after two cycles of the adsorption and desorption.

reveals that the adsorbent has a cubic spinel structure. The intensities and positions of eight diffraction peaks are in good match with LiCo0.5Mn1.5O4 [Inorganic Crystal Structure Database (ICSD) No. 01-070-4214] that has Fd3̅m space group. The crystal size of 409 nm was calculated by using Debye−Scherrer equation, which was in agreement with the previously published results.43,44 The crystal lattice parameter for the optimized sample analyzed by XRD was calculated by using eq 2. The obtained value of 8.166 Å was smaller than the values (8.22−8.24 Å) reported by others for lithium manganese oxide without transition metal.38,39 The reduction of crystal lattice parameter after adding a transition metal to the spinel structure was in agreement with the observations reported by others who explained that addition of cobalt to spinel structure reduced the crystal lattice parameter (8.206, 8.1317 Å). This effect is due to reduction in the Mn3+ concentration in the adsorbent structure and ionic size effect.44,45 The nitrogen-adsorption isotherms curve of the adsorbent presented in Figure 7 showed a typical III classification, which is characteristic of mesoporous materials with nonporous. The pore size distribution of mesoporous adsorbent with the peaks centered at 4 nm is shown in Figure 7. BJH adsorption cumulative volume of pores with size ranging between 1.7 and 300 nm is 0.0146 63 cm3/g, and adsorption average pore

Mayers et al. defined a multiple response method entitled desirability for simultaneous optimization of several responses. Desirability is a mathematical technique to find the conditions at which all responses are optimized at the same time to meet the goals of responses. In this method each response has a desirability function, and the functions of all responses get united into one desirability function.28 Desirability function is used in DOE software to judge the success of the optimization process. It is worth mentioning that a weight function is used to determine the contribution of the desirability function of each response to show the importance of that response. In our work we considered equal degree of importance for both responses and therefore gave a weight function of unit to each of them. The values of individual and combined desirability functions determined by DOE software are shown in Figure 5. An overall desirability value of 0.754 51 reveals that the optimization of process conditions was satisfactory. 3.6. Characterization Results. The XRD patterns of the optimized LiCoxMn2−xO4 adsorbent after synthesis and after two cycles of the adsorption and desorption are shown in Figure 6. This figure reveals that the crystalline structure of the adsorbent was almost unchanged after two cycles of adsorption and desorption processes which is evidence of the structural stability of adsorbent. Also, the XRD pattern shown in Figure 6 12212

DOI: 10.1021/acs.iecr.9b00804 Ind. Eng. Chem. Res. 2019, 58, 12207−12215

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Figure 7. BET N2 adsorption−desorption isotherms for prepared spinel adsorbent at optimized conditions. Inset shows the pore size distribution calculated from adsorption isotherms.

Figure 8. SEM image of the prepared spinel adsorbent in optimized conditions.

diameter (4 V/A by BET) is 10.498 82 nm. Also, results of BET analysis showed that the specific surface area of the adsorbent synthesized at the optimized conditions was 2.564 m2/g, which is a low value compared to the results reported in previous studies on the synthesis of spinel adsorbents (5.1 m2/g at Tc = 800 °C and 23 m2/g at Tc = 400 °C). The lower specific surface area was due to reduced calcination temperature.46,47 SEM image of the spinel adsorbent prepared at optimized conditions is shown in Figure 8. As can be seen, the synthesized adsorbent is composed of cubic particles with spinel

Table 5. Adsorption Selectivity of Optimized Adsorbent Li+ adsorption (mg/g)

Na+ adsorption (mg/g)

50.34

1.846

molar selectivity (

Li+adsorption capacity

Na+adsorption capacity

)

90.32

structure, and the size of adsorbent particles was about 800− 900 nm. 3.7. Selectivity Results. Results of the adsorption selectivity tests of the adsorbent synthesized at optimized conditions are reported in Table 5. The results show high 12213

DOI: 10.1021/acs.iecr.9b00804 Ind. Eng. Chem. Res. 2019, 58, 12207−12215

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Industrial & Engineering Chemistry Research selectivity of the adsorbent for Li+ relative to Na+. This is due to difference between the sizes of these two ions which causes the adsorbent to act like a lithium ion sieve. Conducting research to investigate stability of optimized adsorbent in more cycles (20−50) in a nonsynthetic solution such as seawater with multicomponent ions is suggested as a direction for future studies in this field.

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4. CONCLUSION The goal of the present work was synthesis of a spinel structure lithium manganese oxide adsorbent with proper adsorption capacity and stability. To this aim, cobalt ions were inserted to the adsorbent structure to improve its stability, and then the effects of Co/Mn and Li/Mn molar ratios as well as calcination temperature on the adsorption capacity and stability of the adsorbent were studied. The results showed that increasing Li/Mn molar ratio had positive effects on both characteristics of adsorption capacity and stability. On the other hand, increasing Co/Mn molar ratio and calcination temperature had opposite effects on the adsorption capacity and stability of the adsorbents. By increasing Co/Mn molar ratio and calcination temperature, adsorbent stability was improved and adsorption capacity was reduced. Therefore, to reach compromised conditions for synthesis process of the adsorbent, the optimization was carried out by using DOE software. At the optimized conditions, the adsorbent capacity of 53.52 mg/g was obtained, which was only about 14% smaller than the relatively high value of 62.4 mg/g reported by Zandevakili et al.18 On the other hand, the optimized adsorbent was fairly stable (proved by adsorption capacity loss values and XRD analysis), and its adsorption capacity was about 53% higher than the adsorption capacity (35 mg/g) of the stable adsorbent synthesized by Kim et al.17 The above-mentioned results showed that a stable adsorbent with proper adsorption capacity was achieved in the present work. Finally, the adsorption selectivity test was conducted by using the optimized adsorbent for separation of lithium ions from sodium ions. The results revealed that the optimized adsorbent with the molar selectivity of 90.32 can be considered as a promising tool for selective separation of the lithium ions from sodium ions.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: (785)-477-0092. ORCID

Mohammadali Masigol: 0000-0002-3367-7646 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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REFERENCES

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DOI: 10.1021/acs.iecr.9b00804 Ind. Eng. Chem. Res. 2019, 58, 12207−12215

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Industrial & Engineering Chemistry Research

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DOI: 10.1021/acs.iecr.9b00804 Ind. Eng. Chem. Res. 2019, 58, 12207−12215