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A novel composite adsorbent for the separation and recovery of indium from aqueous solutions
Hydrometallurgy 186 (2019) 73–82
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A novel composite adsorbent for the separation and recovery of indium from aqueous solutions ⁎
T
⁎
Min Lia, , Xiaojing Menga, Kun Huangb, Jian Fenga, , Songshan Jianga a b
Department of Chemical Engineering, Chongqing University of Science and Technology, Chongqing 401331, China Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Graphene oxide Silica gel Adsorption In(III) Chemical grafting
In this study, a novel approach for preparing SiO2@GO-PO3H2 composite using a simple method has been reported, which can be used as an efficient adsorbent for separating In(III) ions from aqueous solutions. The adsorbent was characterized on the basis of Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and Brunauer–Emmett–Teller (BET) technique. The adsorption behavior of In(III) ions on the adsorbent was investigated based upon batch experiments. The adsorption equilibrium was achieved within 50 min and the adsorption kinetics of In(III) ions followed the pseudosecond-order model. The adsorption equilibrium study exhibited good agreement with the Langmuir isotherm model and the maximum adsorption capacity was as high as 149.93 mg·g−1. Furthermore, competitive adsorption demonstrated that the adsorbent maintained good affinity for In(III) ions even in mixed solutions containing K(I), Ca(II), Na(I), Mg(II), Al(III), Zn(II), Fe(II), Pb(II) and Cu(II). The mechanism of the adsorption of In(III) ions SiO2@GO-PO3H2 surface was mainly attributed to the fact that P element in the adsorbent coordinated with In(III) ions to generate IneP bond, which confirmed that the adsorption of In(III) ions on SiO2@GO-PO3H2 belonged to a chemical adsorption process. The SiO2@GO-PO3H2 can be repeatedly used and regenerated ten times without obvious decrease in adsorption capacity, demonstrating that the adsorbent is of high stability and reusability. The results indicated that SiO2@GO-PO3H2 could serve as a promising adsorbent for efficient recovery and elimination of In(III) ions from aquatic environment.
1. Introduction Indium and its compounds have been extensively used to fabricate various devices in the electronic, military and other industries, whereas indium is labeled as a critical strategic metal by the European Commission (Alguacil et al., 2016). In particular, the excellent performance of copper‑indium‑gallium-selenide solar cells has attracted a lot of research attention in recent years. Due to this reason, it can be predicted that the demand for indium will continue to increase in the future (Sasaki et al., 2017). Nevertheless, the content of indium in the crust of Earth is approximately 10−4% and so far not a single mineral, containing indium as the major component, has been identified (Chen and Huang, 2016; Li et al., 2015). Due to the increase of the price caused by the low substitutability and low recycling rates of indium, a raise of its production is needed (Deferm et al., 2017). In addition, indium ions are suspected to be carcinogenic and can cause damage to heart, kidney and liver (Hwang et al., 2013). Therefore, efforts to find methods to recover and recycle indium are of great significance.
⁎
Among several separation methods for metal ions from aqueous solutions, adsorption is widely considered as one of the most promising methods for removing or separating metal ions from aqueous solutions due to its simplicity, low cost and high efficiency (Li et al., 2018a, 2018b). Another challenge in this area is the selective separation of target ions from aqueous solutions, which contain numerous other metal ions. Therefore, it is clear that the development of an adsorbent, which presents excellent selectivity for targeted ions, is an important research direction in the field of adsorption and purification (Sherlala et al., 2018). Unfortunately, adsorbents reported in recent years tend to lack of selectivity for indium ions, although they exhibit large adsorption capacity and fast adsorption rate(Alguacil et al., 2016; Hassanien et al., 2017; Hwang et al., 2013; Jeon et al., 2015; Li et al., 2012; Roosen et al., 2017). In a previous study, two adsorbents were prepared to selectively separate indium ions from real mine wastewater and the results demonstrated that the two adsorbents possessed fast adsorption rate and good affinity for indium ions (Li et al., 2018a, 2018b; Li et al., 2015). However, further development is needed in this field to obtain a
Corresponding authors. E-mail addresses: [email protected] (M. Li), [email protected] (J. Feng).
https://doi.org/10.1016/j.hydromet.2019.04.003 Received 22 November 2018; Received in revised form 17 March 2019; Accepted 1 April 2019 Available online 02 April 2019 0304-386X/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. The scheme for preparation of SiO2@GO–PO3H2.
solutions. The objectives of the present study were to focus on preparing the SiO2@GO-PO3H2 composite and employ it as an adsorbent to selectively separate indium ions from aqueous solutions. The morphologies and structures of the resultant composite were investigated using Fourier transform infra-red spectroscopy (FT-IR), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and Brunauer–Emmett–Teller (BET) technique. The adsorption performance and the mechanism of adsorption were studied in detail based upon batch experiments. The results indicated that SiO2@GO-PO3H2 could be used as a promising adsorbent for separating and recycling indium from aqueous solutions.
cheaper and more readily available adsorbent, which has good performance for selective adsorption of indium ions. Graphene oxide (GO) is a type of graphite with one or several atomic layers, possessing special two-dimensional structures, high surface area (theoretical value of 2620 m2/g) and plenty of oxygencontaining functional groups, such as hydroxyl, epoxy and carboxyl groups on the surface (Zhao et al., 2011). These oxygen groups can be combined with metal ions through electrostatic attractions or complexation. As a result, a large number of GO-based adsorbents are used to eliminate or separate metal ions from aqueous media (El-Maghrabi et al., 2017; Li et al., 2018a, 2018b; Liang et al., 2018; Pan et al., 2016; Sheng et al., 2018). However, the selectivity of GO for targeted ions is limited owing to the fact that oxygen-containing functional groups on the surface of GO can interact with a variety of metal ions, even though adsorbents reported in recent years exhibit large adsorption capacities and good chemical and physical stabilities (Li et al., 2018a, 2018b). On the other hand, the nano-sized structure and excellent dispersion of GO in aqueous solutions make it difficult to be separated even using traditional centrifugation and filtration methods (Liu et al., 2011). The present study attempted to introduce GO onto the surface of silica gel to decorate/functionalize GO through specific functional groups, which can improve its selectivity and separation capability for targeted ions. The manuscript is an extension of our previous work (Li et al., 2015; Li et al., 2018a, 2018b; Li et al., 2014), where the separation of metal ions, including In(III), Cd(II), Zn(II), Cu(II) and Ni(II) from aqueous solutions using modified silica gel, was investigated and the results revealed that the phosphonic acid groups exhibited high affinity for indium ions. It is extended preparing a novel adsorbent by introducing GO onto the surface of silica gel, after which, GO is decorated with the phosphonic acid groups to selectively separate and recycle indium from aqueous
2. Experimental 2.1. Apparatus FT-IR analyses of all samples were performed using a Bruker (TENSOR-27, Germany) spectrometer. The spectra were measured in the absorbance mode within the wavelength range of 4000–400 cm−1. Inductively coupled plasma atomic emission spectrometry (ICP-AES) (Perkin-Elmer-7000DV, USA) was used to determine the concentrations of metal ions in the solutions. A pHS-3C digital pH meter (Shanghai Precision & Scientific Instrument Co., Ltd., China) was used for pH adjustments. The surface morphology of samples was examined using SEM (Hitachi S4800, Japan) at the desired magnification. The N2 adsorption/desorption isotherms were recorded using a NOVA3200e surface area and pore size analyzer (Quantachrome Instruments, USA) at 77 K. Furthermore, XPS was carried out on an ESCALAB 250 (Thermo, USA) unit using monochromatic Al Kα radiation, for which, 74
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(III) ions solutions at various pH values. The pH values of solutions were adjusted via diluted sulphuric acid and ammonia water. The concentrations of In(III) ions in the solutions were measured after stirring for 4 h. The effect of contact time on the removal of indium ions using SiO2@GO-PO3H2 was examined by adding 25.0 mg of adsorbent into 50.00 mL indium ions' solution with the concentration of 140 mg·L−1 at 298 K and pH of 2.5. The samples were extracted from their respective solutions at different intervals until the end of the process. About 10 mg of SiO2@GO-PO3H2 was added to 20.00 mL of indium ions solution of various concentrations (10–150 mg·L−1) at different temperatures. The initial pH value of the solution was adjusted to 2.50. The competitive adsorption of Cu(II), Pb(II), Zn(II), Ca(II), Na (I), K(I), Mg(II), and Fe(II) with respect to In(III) was studied over SiO2@GO-PO3H2. For that, 25.0 mg of adsorbent was mixed with 50.00 mL of aqueous solutions containing 100 mg·L−1 of the aforementioned metal ions at the pH 2.50 for determining the adsorption equilibrium. The distribution coefficient (D) and the separation factor (β) were calculated using Eqs. (2) and (3), respectively (Zhou et al., 2018).
energy normalization was already conducted. 2.2. Materials and reagents Ethanol, indium sulfate, dicyclohexylcarbodiimide (DCC), toluene, 3-aminopropyltriethoxysilane (APTS), vinylphosphonic acid and N, Ndimethyl formamide (DMF) were purchased from Sinopharm Chemical Reagent Co., Ltd.,China). Silica gel (80–120 mesh) was purchased from Qingdao Ocean Chemical Co., Ltd., China. Graphene oxide was supplied by the Institute of Metal Research, Chinese Academy of Sciences, China. All the other reagents used were of analytical grade and purchased from Beijing Chemical Plant, China. These analytical grade reagents were used without any further purification. 2.3. Preparation of SiO2@GO-PO3H2 composite The synthesis of SiO2@GO-PO3H2 composite consists of four steps, which are schematically presented in Fig. 1. (i) SG-APTS. Silica gel was grafted using 3-aminopropyltriethoxysilane and it is described in detail in a previous work (Li et al., 2014). In short, about 8.0 g of silica gel was mixed with 60.0 mL of 6 mol/L hydrochloric acid and refluxed under stirring for 8 h. The activated silica gel was filtered, washed with double-distilled water to neutral and dried under vacuum at 70 °C for 12 h, which resulted in the production of activated silica gel. 5.00 g of activated silica gel was dispersed in toluene (50.0 mL) in a flask followed by gradually adding APTS (5.00 mL) into the solution with continuous stirring. The mixture was refluxed under stirring for 24 h. The product was filtered off, washed with toluene and alcohol and dried under vacuum at 70 °C for 12 h. (ii) SiO2@GO. 0.60 g of GO was dispersed in 400 mL of DMF using sonication for 30 min. Then, about 20.0 g of SG-APTS and 1.50 g of DCC were added into the yellow-brown solutions. The reaction system was stored in a water-bath at 60 °C for 24 h. The product was washed using DMF and ethanol followed by drying under vacuum at 50 °C for 12 h. (iii) SiO2@GO-NH2. First, 5.00 g of SiO2@GO was dispersed in toluene (50.0 mL) in a flask. APTS (15.00 mL) was gradually added into the solution with continuous stirring. The mixture was refluxed under stirring for 24 h. The obtained product was filtered, washed with toluene and alcohol followed by drying under vacuum at 70 °C for 12 h. (iv) SiO2@GO-PO3H2. 5.00 g of SiO2@GO-NH2 was dispersed in ethanol (50.0 mL) in a flask. Then, vinylphosphonic acid (10.00 mL) was gradually added into the solution with continuous stirring. The mixture was refluxed under stirring for 24 h. The final product was filtered, washed with alcohol and dried under vacuum at 50 °C for 12 h.
D=
β = DIn DM
(3)
2.5. Column experiments The fixed-bed adsorption was conducted in glass columns with 6 mm in internal diameter and 100 mm in height. The columns were packed with 1.0 g of SiO2@GO-PO3H2. For In(III) ions adsorption, 100 mg·L−1 of In(III) ions solutions was pumped upwards through the SiO2@GO-PO3H2 packed column, at a steady flow rate 0.50 mL·min−1 by a peristaltic pump. Effluents were collected after every 5.00 mL of the solution passed through the column. The column experiments were performed at room temperature and pH 2.5. For the In(III) ions elution from the fixed-bed, HCl (0.5 mol·L−1), HCl (1.0 mol·L−1), H2SO4 (0.5 mol·L−1) and H2SO4 (1.0 mol·L−1) were chosen as the eluent, and the elution process was carried out at a fixed flow rate of 0.25 mL·min−1. After the column was exhausted, regeneration of SiO2@GO-PO3H2 and desorption of In(III) ions was carried out. Desorption of In(III) ions from the loaded material was performed with 1.0 mol·L−1 of H2SO4 at a constant flow rate of 0.25 mL·min−1.
The stock Cu(II), Pb(II), Zn(II), Fe(II), Al(III) and In(III) solutions were prepared by dissolving CuSO4·5H2O, Pb(NO3)2, ZnSO4·7H2O, FeSO4, AlCl3 and In2(SO4)3 in double-distilled water, respectively. All of these solutions were stored at ambient temperature. For the adsorption experiments, the concentrations of all metallic ions were determined using ICP-AES. The adsorption capacity can be calculated using Eq. (1) (Wu et al., 2018).
3. Results and discussion 3.1. Characterization of SiO2@GO-PO3H2 The synthesized SiO2@GO-PO3H2 composite was confirmed using FT-IR, BET, SEM and XPS analyses. Fig. 2a displays the FT-IR spectra of activated silica gel, SG-APTS, GO, SiO2@GO, SiO2@GO-NH2, free and loaded In(III) SiO2@GO-PO3H2. As shown in Fig. 2a (a,b), all absorption bands in the spectra of activated silica gel and SG-APTS were consistent with those reported in a previous work (Ramasamy et al., 2017). In Fig. 2a(c), the broad and strong absorption band at 3400 cm−1 is attributed to OH, while other bands at 1735 cm−1, 1630 cm−1, 1395 cm−1and 1097 cm−1 are assigned to carbonyl/carboxyl C]O stretching vibration, C]C stretching vibration, OH deformation vibration and CeO stretching vibration, respectively. These results are consistent with those reported in a previous work (El-Maghrabi et al., 2017; Liu et al., 2016; Zhao et al., 2012). Fig. 2a(d) demonstrates that
(ci − cf ) × V (1)
m
(2)
where ci and ce are the initial and equilibrium concentrations of M (mg·L−1) (‘M' represents the metal ions), respectively, V is the volume of the solution (mL), m is the mass of SiO2@GO-PO3H2 (g), and DIn and DM represent the distribution coefficients of indium ions and other metallic ions, respectively.
2.4. Batch adsorption experiments
q=
(Ci − Ce ) V × Ce m
−1
where q represents the adsorption capacity (mg·g ), ci and cf are the initial and final concentrations of metallic ions (mg·L−1), respectively, V is the volume of the solution (L), and m is the mass of SiO2@GOPO3H2 (g). The SiO2@GO-PO3H2 static adsorption studies for indium ions were performed using a batch method in a constant-temperature oscillator and stroke speed maintains 50 r/min. In order to investigate the effect of pH values on adsorption of In(III) ions, about 25 mg adsorbent was added into a capped conical flask containing 50 mL of 140 mg·L−1 In 75
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Fig. 2. (a) The FT-IR spectra of activated silica gel (a), SG-APTS(b), GO(c), SiO2@GO(d), SiO2@GO–NH2(e),free(f) and In(III) ion-loaded SiO2@GO–PO3H2; (b) XPS of O 1 s; (c) Nitrogen adsorption and desorption isotherms of SiO2@GO–PO3H2 before adsorption; (d) Nitrogen adsorption and desorption isotherms of SiO2@GO–PO3H2 after adsorption In(III) ions.
the peaks at 1390 cm−1 and 1100 cm−1 can be attributed to NeH stretching vibration and CeN deformation vibration, respectively (Hosseinzadeh and Ramin, 2018). As shown in Fig. 2a(e), a new adsorption band at 1560 cm−1 can be allocated to NeH bending vibration of the eCONHe group, which was formed by the reaction between the carboxyl group belonging to SiO2@GO and the amide group of APTS (Pytlakowska et al., 2016; Rajabi et al., 2018). Another new peak at 1495 cm−1 is assigned to CeN stretching vibration, while the peak at 700 cm−1 is assigned to NeH waging (Pytlakowska et al., 2016; Rajabi et al., 2018). Several absorption bands confirmed that phosphate groups were grafted onto the surface of SiO2@GO. They were observed in Fig. 2a(f) as peaks at 2325 cm−1 and 1695 cm −1, which were assigned to P(O)-OH stretching vibration (Wu et al., 2013). A band at 1077 cm−1 can be attributed to the symmetrical stretching vibration of C-O-P (Wu et al., 2013; Wu et al., 2018), while the absorption bands at 966 cm−1 and 918 cm−1 are assigned to the stretching vibration of POH (Yin et al., 2011). It is confirmed that SiO2@GO-PO3H2 was prepared successfully through chemical grafting. Nitrogen adsorption isotherm for SiO2@GO-PO3H2 is shown in Fig. 2c. It was observed that SiO2@GO-PO3H2 was Type IV with a clear hysteresis loop according to the IUPAC classification, indicating that the pores in SiO2@GO-PO3H2 composite belonged to a mesoporous structure (Li et al., 2008). In addition, the adsorption and desorption lines are approximately parallel, exhibiting that the pores of the material possess a uniform radius (Li et al., 2008). For SiO2@GO-PO3H2, the BET surface area, pore volume and average pore size were
determined to be 141.531 m2·g−1, 0.246 cm3·g−1 and 7.827 nm, respectively. For In(III)-loaded SiO2@GO-PO3H2, the values of BET surface area, pore volume and average pore size were calculated to be 146.403 m2·g−1, 0.116 cm3·g−1 and 2.743 nm, respectively. The adsorbent surface is rough and porous, which is likely to be due to large quantity of In(III) aggregated at the adsorbent surface after adsorption. However, these pores were formed due to numerous lumpy objects, which overlapped mutually, as observed in the SEM image shown in Fig. 3b. Fig. 3a shows the SEM image of SiO2@GO-PO3H2. Obvious wrinkles belonging to GO are observed on the adsorbent surface, demonstrating that GO was successfully introduced onto the surface of silica gel. Furthermore, some attachments are also observed on the surface of SiO2@GO-PO3H2, which can be the result of numerous phosphate groups grafted due to the chemical grafting method. The chemical composition of elements in SiO2@GO- PO3H2 was investigated using XPS. Peaks were observed in the full spectra of SiO2@GO-PO3H2 (shown in Fig. 4a) at 284.60, 401.28, 532.30, 153.54, 103.58 and 133.06 eV, which can be assigned to C, N, O, Si and P elements, respectively (Amaral et al., 2005; Wu et al., 2013). These results further confirm that phosphate groups were successfully introduced onto the surface of SiO2@GO.
3.2. Effect of pH on adsorption The effect of varying pH on In(III) ions uptake was investigated using the batch experiment. The adsorption experiments were 76
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Fig. 3. (a) XPS survey spectra of free(e) and In(III) ion-loaded SiO2@GO–PO3H2; (b) XPS of P 2p of SiO2@GO–PO3H2; (c) XPS of In 3d; (d) XPS of P 2p of SiO2@GO–PO3H2 after adsorption In(III) ions.
process that functional groups in the adsorbent combine with In(III) ions was retarded. Another reason could be that the existence of a high abundance of H+ competing with In(III) ions at low pH value.
triplicated. Considering the formation of precipitation of In(III) ions at higher pH, pH above 3.0 was not investigated. The pH of the solutions is a significant factor to influence a sorption process of metallic ions, since it has an important impact on solubility of metal ions, species distribution of metal ions in the solution and overall charge of the adsorbent. Fig. 5 shows the effect of pH on the adsorption of In(III) ions on SiO2@GO-PO3H2. As can be seen, the adsorption capacity increased remarkably as the pH of the solution increased from 0.5 to 3.0 and the maximum adsorption capacity was 149.5 mg·g−1 at pH of 3.0. It can be explained that at low pH, the vast majority of the functional groups distributed in the adsorbent were protonated, which indicates the
3.3. Adsorption kinetics The efficiency of SiO2@GO-PO3H2 composite was evaluated by studying the adsorption kinetics. Briefly speaking, 25.0 mg of adsorbent was added to 50.00 mL of aqueous solutions containing indium(III) ions at the pH of 2.50 and the temperature of 298 K. Fig. 6a shows the adsorption capacity of indium(III) ions on SiO2@GO-PO3H2 against the
Fig. 4. (a) SEM image of SiO2@GO–PO3H2 before adsorption; (b) SEM image of SiO2@GO–PO3H2 after adsorption In(III) ions. 77
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sites have been occupied by indium(III) ions, and therefore, further adsorption relies mainly on the diffusion of solute into the intra-particle space. In addition, it was observed that the adsorption capacity increased with the increase in the concentration of indium(III) ions, which can be attributed to a higher driving force for mass transfer at higher concentrations of indium(III) ions. In order to clarify the mechanism underlying the adsorption process, the Lagergren pseudo-first-order and pseudo-second-order kinetic models were employed to evaluate the experimental kinetic data. The pseudo-first-order and pseudo-second-order models are expressed using Eqs. (4) and (5), respectively (Li et al., 2015).
ln(qe − qt ) = ln qe − k1 t
(4)
t 1 1 = + t qt qe k2 qe2
(5) −1
−1
where t is the contact time (min), qt (mg·g ) and qe (mg·g ) are the adsorption capacity of indium (III) ion at any time t and at equilibrium, respectively, and k1 (min−1) and k2 (mg·g−1·min−1) represent the adsorption rate constants of first-order and second-order models, respectively. Fig. 6(b, c) shows the kinetic fitted plots of the pseudo-first-order and pseudo-second-order for the adsorption of indium(III) ions onto SiO2@GO-PO3H2, while the calculated values of different parameters are listed in Table 1. The results presented in Table 1 show that the adsorption process for In(III) ions could be better described using the pseudo-second-order rather than the pseudo-first-order model and that the process is based upon chemisorption (Luo et al., 2017). All the obtained correlation coefficients (R2) from the pseudo-second-order model were close to unity, whereas all the calculated qe values were in
Fig. 5. Influence of pH values on the adsorption capacity of SiO2@GO–PO3H2.
contact time at different concentrations. As seen in Fig. 6a, the adsorbent exhibits a fast adsorption process for the targeted ions. The three adsorption kinetic curves followed an increasing trend with time and the required equilibrium time was around 50 min. The adsorption of overwhelming majority of indium(III) ions was rapidly achieved within the first 35 min, which can be attributed to large number of available adsorption sites on the surface of SiO2@GO-PO3H2. These sites are available for the adsorption of indium(III) ions at an early stage. However, a slow adsorption rate was observed after 35 min, which is likely caused by the fact that most of the active external vacant
Fig. 6. (a) Influence of contact time on the adsorption of In(III) ions onto SiO2@GO–PO3H2; (b) Lagergren pseudo-first-order kinetics of In(III) ions onto SiO2@GO–PO3H2; (c) Lagergren pseudo-second-order kinetics of In(III) ions onto SiO2@GO–PO3H2; (d) Comparison of Langmuir and Freundlich isotherm models for In(III) ions adsorption onto SiO2@GO–PO3H2. 78
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Table 1 Kinetic parameters for the indium (III) ions adsorption onto the SiO2@GO-PO3H2. ci (mg·L−1)
qe,
exp(mg·g
−1
)
Pseudo-first-order rate equation −1
k1 (min 49.35 85.26 99.47
77.86 132.3 149.5
0.03344 0.02292 0.0277
)
qe,
cal
Pseudo-second-order rate equation
(mg·g
−1
)
56.31 111.9 124.3
3.4. Adsorption isotherms and equilibrium The adsorption isotherms of In(III) ions on SiO2@GO-PO3H2 were investigated at three temperatures of 283, 293 and 303 K at the pH of 3.0. Two classical isotherm models, the Langmuir and Freundlich models, were used to analyze the experimental data. The Langmuir model assumes that the adsorption process occurs on a homogeneous surface with a fixed number of active sites and without any interaction among the adsorbed molecules. The non-linear form of the Langmuir isotherm model is expressed using Eq. (6) (Liu et al., 2016).
KL qm ce 1 + KL ce
0.9601 0.9701 0.9820
0.001037 0.00337 0.00326
82.51 137.3 152.0
cal
(mg·g−1)
R22 0.9989 0.9949 0.9988
In order to investigate the selective separation behavior of SiO2@GO-PO3H2, competitive adsorption for In(III) ions was performed in aqueous solutions containing Cu(II), Pb(II), Zn(II), Ca(II), Na(I), K(I), Mg(II) and Fe(II). The results of this experiment are shown Fig. 7, while the calculated relevant parameters are listed in Table 3. These interfering ions were chosen in the present study due to their co-existence with indium in natural sources. As seen from the results presented in Fig. 7 and Table 3, the adsorption capacity of SiO2@GO-PO3H2 for In (III) ions was much larger than that for other interfering ions, indicating that SiO2@GO-PO3H2 possesses a stronger affinity for In(III) ions. According to hard and soft acids and bases theory (HSAB), In(III) ions belonged to hard acid can be more easily captured by the functional groups (ePO3H2) on the adsorbent. Certainly, With the exception of In (III) ions, Ca(II), Na(I), K(I), Al(III) and Mg(II) ions also belong to hard acid in the system. However, the difference in the ionic radius or charge number might make In(III) ions adsorbed.
(6)
1 1 + KL c0
qe,
3.5. Selectivity
where qe is the amount of In(III) ions adsorbed per gram of adsorbent (mg·g−1), and ce is the equilibrium concentration of In(III) ions in the solution. Furthermore, qm and KL are the Langmuir constants related to the maximum adsorption capacity (mg·g−1) and the adsorption energy (L·g−1), respectively. In order to predict the suitability of adsorption process, the essential characteristic of Langmuir equation can be described using the dimensionless separation factor RL, which is defined according to Eq. (7) (Shaheen et al., 2017).
RL =
k2(mg·g−1·min−1)
respectively, indicating that the adsorption of In(III) ions on SiO2@GOPO3H2 follows the mechanism of monolayer adsorption (Luo et al., 2017). This result was expected due to the abundance of functional groups on the surface of adsorbent. Similar results were reported previously in the literature (Chen and Chen, 2015; Cheng et al., 2018). In addition, all qe values obtained from the Langmuir model increased when the temperature increased from 283 K to 303 K, suggesting that the adsorption process was endothermic in nature (Singh et al., 2009). Furthermore, the values of RL lied between 0 and 1 for In(III) ions' adsorption on SiO2@GO-PO3H2 composite, indicating that the adsorption of In(III) ions was favorable (Li et al., 2018a, 2018b).
good agreement with the experimental qe values. Previously, similar observations were reported for the adsorption of Cd(II) on a Cd(II)-ion imprinted polymer (Li et al., 2015) and for U(VI) on functionalized GO (Sun et al., 2015).
qe =
R12
(7) 3.6. Effect of eluting agents on the recovery
−1
where c0 is the initial concentration of In(III) ions (mg·L ). The value of RL serves as an indicator for the nature of adsorption process. The condition of RL > 1 indicates that the isotherm is unfavorable, whereas RL = 1 indicates that the isotherm is linear. Additionally, the condition of 0 < RL < 1 indicates that the isotherm is favorable, while RL = 0 indicates that the isotherm is irreversible. The Freundlich isotherm is suitable for both the monolayer and multilayer adsorptions with heterogeneous binding sites. The nonlinear form of Freundlich isotherm is represented by Eq. (8) (Liu et al., 2016).
For the elution of In(III) ions adsorbed onto the SiO2@GO-PO3H2 from the column, four kinds of eluting agents, H2SO4 (0.5 mol·L−1), H2SO4 (1.0 mol·L−1), HCl (0.5 mol·L−1) and HCl (1.0 mol·L−1), were applied to investigate the recovery efficient of In(III) ion. Among these eluting agents, 1.0 mol·L−1 of H2SO4 was found to be quite effective for desorbing and recovering In(III) ions from the sorbent and the result was shown in Fig. 8a. With a single wash of 1.0 mol·L−1 of H2SO4, 99.2% of In(III) ions was recovered from the fixed–bed column saturated with In(III) ions.
1
qe = Kf ce n
(8)
3.7. Desorption indium(III) ions and reusability of the SiO2@GO-PO3H2
where Kf is roughly an indicator of the adsorption and the factor ‘1/ n‘represents the adsorption intensity. A smaller 1/n value indicates a more heterogeneous surface, whereas a value that is closer to or equal to unity indicates that the adsorbent has relatively more homogenous binding sites. Fig. 6d shows the comparison of Langmuir and Freundlich isotherm models for In(III) ions' adsorption on SiO2@GO-PO3H2 using nonlinear regression. The calculated relevant parameters according to the two models are listed in Table 2. The results presented in Table 2 and Fig. 6d show that the equilibrium data fitted using Langmuir model was better than the Freundlich model. The maximum adsorption capacities were calculated to be 143.1, 146.0 and 149.9 mg·g−1 at 283, 293 and 303 K,
To investigate desorption of In(III) ions from the fixed bed is of crucial importance due to the fact that not only the In(III) ions adsorbed can be separated from the adsorbent but also the adsorbent can be regenerated, making it can recover the ability to capture adsorbate from aqueous solutions. In this study, the adsorption-desorption cycles were repeated ten times by dynamic experiment and 1.0 mol·L−1 of H2SO4 was used as the eluting agent. The results of tens cycles of adsorptiondesorption of In(III) ions onto the SiO2@GO-PO3H2 are shown in Fig. 8b. As can be seen from Fig. 8b, the dynamic adsorption capacity of In(III) ions did not significantly decrease even after ten cycles. The adsorption capacity of In(III) ions for the recycles SiO2@GO-PO3H2 79
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Table 2 Langmuir and Freundlich isotherm constants for In(III) ions adsorption onto SiO2@GO-PO3H2. Temperature(K)
qe,
exp(mg/g)
Langmuir isotherm qm,
283 293 303
138.1 141.7 144.3
cal(mg/g)
143.1 146.1 149.9
Freundlich isotherm 2
KL (L/mg)
R
1/n
Kf (mg/g)
R2
0.4363 0.4642 0.4789
0.9957 0.9964 0.9973
0.2994 0.2936 0.2957
52.49 55.09 58.89
0.9196 0.9052 0.9133
in the adsorbent surface becoming rough and porous. However, these pores were formed through the mutual overlapping of lumpy objects, which is in accordance with the results obtained from SEM. Fig. 3b shows the SEM image of SiO2@GO-PO3H2 after the adsorption of In(III) ions. A large number of attachments were observed onto the surface of adsorbent, whereas these adsorbates were accumulated in a disordered arrangement. These findings confirmed that In(III) ions were captured through functional groups onto the surface of adsorbent. The XPS spectra of free and In(III)-loaded SiO2@GO-PO3H2 are shown in Fig. 4a. No signal belonging to indium was observed in the XPS spectra of SiO2@GO-PO3H2, while two obvious energy bands assigned to In 3d3/2 and In 3d5/2 were observed in the XPS spectra of SiO2@GO-PO3H2-In at 452.95 eV and 445.41 eV, respectively (Bermudez et al., 2006; Chen et al., 2017). These results indicated that In(III) ions were adsorbed onto the surface of adsorbent, which is consistent with the results obtained from the FT-IR analysis. Compared with the spectrum before the adsorption, the band energy for P shifted from 133.06 eV to 132.82 eV after the adsorption, which can be ascribed to the formation of In(III)PO3H2 coordination bonds that resulted in the increase of extra-nuclear electron cloud density surrounding the P atoms (Pan et al., 2016). Additionally, the high-resolution In 3d spectrum is shown in Fig. 4c. The results showed that the bands at 452.95 eV and 445.41 eV can be attributed to the characteristic peaks of In 3d3/2 and In 3d5/2, respectively (Bermudez et al., 2006). As shown in Fig. 4b, two peaks at 132.87 eV and 133.30 eV in the P 2p XPS spectrum of SiO2@GO-PO3H2 before the adsorption can be attributed to PeO and P]O, respectively (Rajabi et al., 2018) Greater changes were observed in the P 2p highresolution spectrum (Fig. 4d) after In(III) adsorption. Three peaks were observed in the spectrum, which were allocated to P]O (133.82 eV), PeO (133.03 eV) and PeIn (132.41 eV), respectively (Wu et al., 2013). In addition, the high-resolution spectra of O 1 s before and after the adsorption are also shown in Fig. 2b. The binding energy of O 1 s before and after the adsorption did not change markedly, suggesting that it did not contribute to the adsorption of In(III) ions. Based on these results, it can be inferred that the In(III) ions in aqueous solutions adsorbed on the surface of SiO2@GO-PO3H2 composite relied mainly on the complexation reaction between In(III) ions and functional groups (-PO3H2) on the surface of SiO2@GO-PO3H2. In addition, only the P element in the functional groups (ePO3H2) participated in coordination with In(III) ions generating the PeIn bond, whereas the oxygen element in the functional groups (-PO3H2) had no contribution in the adsorption of In(III) ions. Therefore, an important research direction is the development of an adsorbent having greater adsorption capacity for In(III) ions and developed by introducing increased number of functional groups containing P element.
Fig. 7. Comparative adsorption of foreign ions onto the SiO2@GO–PO3H2;
only had 2.6% of loss at the tenth cycle. These results indicate that the SiO2@GO-PO3H2 possesses an excellent stability and reusability for the In(III) ions adsorption. 3.8. Adsorption mechanism In order to reveal the adsorption mechanism of In(III) ions on SiO2@GO-PO3H2, free and In(III) ions-loaded SiO2@GO-PO3H2 (referred to as SiO2@GO-PO3H2-In) were characterized on the basis of FTIR, BET, SEM and XPS techniques. The comparison of FT-IR spectra before and after the adsorption indicated that the band at 966 cm−1 before the adsorption was shifted to 953 cm−1 after the adsorption. In addition, the bands assigned to P(O)-OH at 2325 cm−1 and 1695 cm−1 disappeared after the adsorption. However, two new peaks at 799 cm−1 and 621 cm−1 were observed after the adsorption. These results suggested that In(III) ions were captured on the surface of SiO2@GOPO3H2. Fig. 2d shows the nitrogen adsorption isotherm for SiO2@GOPO3H2-In. The shape of SiO2@GO-PO3H2-In isotherm remained unchanged after the adsorption. Therefore, this material could also be assigned to Type IV according to the IUPAC classification. Additionally, a hysteresis loop was observed, which was representative of a mesoporous structure (Guo et al., 2013). The volume adsorbed by SiO2@GOPO3H2-In rapidly increased at the relative pressure (P/P0) of about 0.7, implying capillary condensation of nitrogen within the uniform mesoporous structure (Li et al., 2008). The BET surface area, pore volume and average pore size changed and it could be explained based upon the accumulation of a significant amount of In(III) onto the adsorbent surface after adsorption, resulting Table 3 Selective adsorption of In(III) ions from multi-ionic mixtures by SiO2@GO-PO3H2. Metal
In(III)
Cu(II)
Pb(II)
Zn(II)
Ca(II)
Na(I)
K(I)
Mg(II)
Fe(II)
ci (mg/L) ce (mg/L) DM β(In(III)/M)
99.98 43.24 2624.4 1
100.1 94.74 113.1 23.2
100.0 95.63 91.4 28.7
99.93 89.35 236.8 11.0
99.75 75.36 647.3 4.0
99.89 97.63 46.3 56.7
100.2 98.46 35.3 74.3
99.92 82.53 421.4 6.2
99.94 83.64 389.8 6.7
80
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Fig. 8. (a) Comparison of various eluting agents for elution of In(III) ions; (b) Ten cycles of indium(III) ions adsorption-desorption. Table 4 Comparison of SiO2@GO-PO3H2 with other adsorbents of indium(III) ions. Adsorbent
Equilibrium concentrations /(mg·L−1)
pH
T/K
Adsorption capacity/(mg·g−1)
Equilibrium time /min
References
HBAAS Magadiite Phosphorylated sawdust beads CCB CCR IIP In(III)-IIP SiO2@GO-PO3H2
1150 41 0.87 70 84 128 114 27
2.0 2.5 3.5 4.0 3.0 2.5 2.5 2.5
298 N.R 298 313 298 318 298 303
81.3 80.5 1.121 17.89 109.07 60.62 47.39 149.93
20 10 900 100 360 30 10 50
(Hassanien et al., 2017) (Homhuan et al., 2017) (Jeon et al., 2015) (Calagui et al., 2014) (Adhikari et al., 2012) (Li et al., 2018a, 2018b) (Li et al., 2015) This work
N.R represents no reported.
the functional groups had no contribution to the adsorption of In(III) ions. The results indicated that SiO2@GO–PO3H2 could be considered as a promising candidate for the recovery and removal of In(III) ions from wastewater.
3.9. Comparison with other adsorbents for indium(III) ions In the present study, the adsorption performance of SiO2@GOPO3H2 for In(III) ions in aqueous solution was compared with that of other adsorbents, which have previously been reported in literature (see Table 4). The results show that the proposed SiO2@GO-PO3H2 adsorbent possesses fast adsorption kinetics, larger adsorption capacity and high affinity for In(III) ions. Notably, the adsorption capacity of the proposed adsorbent for In(III) ions may be the highest reported. The results indicated that SiO2@GO-PO3H2 is a reliable and effective adsorbent for the removal or extraction of In(III) ions from aquatic environment.
Acknowledgements This research is supported by the National Natural Science Foundation of China (No. 51708075). Also, the authors acknowledge the Scientific and Technological Research Program of Chongqing Municipal Education Commission (Grant No. KJ1713335, KJQN201801527). References
4. Conclusions
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In the present study, a novel adsorbent (SiO2@GO-PO3H2) was prepared for the extraction and removal of In(III) ions from the aquatic environment. The characterization results confirmed that various functional groups can be introduced on the surface of SiO2@GO through a chemical grafting reaction. The kinetic data fitted well with the pseudo-second-order model, suggesting that chemical adsorption occurred during the adsorption process. The equilibrium data could be better described on the basis of Langmuir adsorption isotherm, indicating that the adsorption was a monolayer adsorption process. Additionally, the maximum adsorption capacity was calculated to be 149.93 mg·g−1 at the pH of 2.5 and the temperature of 303 K, which is the highest capacity for In(III) ions reported so far. The possible adsorption mechanism for In(III) ions could mainly be attributed to the fact that functional groups (ePO3H2) of the adsorbent surface coordinated with In(III) ions through covalent bonds. It appears that only the P element in the functional groups (–PO3H2) participated in the coordination with In(III) ions to form PeIn bonds, whereas oxygen in 81
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A novel composite adsorbent for the separation and recovery of indium from aqueous solutions ⁎
T
⁎
Min Lia, , Xiaojing Menga, Kun Huangb, Jian Fenga, , Songshan Jianga a b
Department of Chemical Engineering, Chongqing University of Science and Technology, Chongqing 401331, China Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Graphene oxide Silica gel Adsorption In(III) Chemical grafting
In this study, a novel approach for preparing SiO2@GO-PO3H2 composite using a simple method has been reported, which can be used as an efficient adsorbent for separating In(III) ions from aqueous solutions. The adsorbent was characterized on the basis of Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and Brunauer–Emmett–Teller (BET) technique. The adsorption behavior of In(III) ions on the adsorbent was investigated based upon batch experiments. The adsorption equilibrium was achieved within 50 min and the adsorption kinetics of In(III) ions followed the pseudosecond-order model. The adsorption equilibrium study exhibited good agreement with the Langmuir isotherm model and the maximum adsorption capacity was as high as 149.93 mg·g−1. Furthermore, competitive adsorption demonstrated that the adsorbent maintained good affinity for In(III) ions even in mixed solutions containing K(I), Ca(II), Na(I), Mg(II), Al(III), Zn(II), Fe(II), Pb(II) and Cu(II). The mechanism of the adsorption of In(III) ions SiO2@GO-PO3H2 surface was mainly attributed to the fact that P element in the adsorbent coordinated with In(III) ions to generate IneP bond, which confirmed that the adsorption of In(III) ions on SiO2@GO-PO3H2 belonged to a chemical adsorption process. The SiO2@GO-PO3H2 can be repeatedly used and regenerated ten times without obvious decrease in adsorption capacity, demonstrating that the adsorbent is of high stability and reusability. The results indicated that SiO2@GO-PO3H2 could serve as a promising adsorbent for efficient recovery and elimination of In(III) ions from aquatic environment.
1. Introduction Indium and its compounds have been extensively used to fabricate various devices in the electronic, military and other industries, whereas indium is labeled as a critical strategic metal by the European Commission (Alguacil et al., 2016). In particular, the excellent performance of copper‑indium‑gallium-selenide solar cells has attracted a lot of research attention in recent years. Due to this reason, it can be predicted that the demand for indium will continue to increase in the future (Sasaki et al., 2017). Nevertheless, the content of indium in the crust of Earth is approximately 10−4% and so far not a single mineral, containing indium as the major component, has been identified (Chen and Huang, 2016; Li et al., 2015). Due to the increase of the price caused by the low substitutability and low recycling rates of indium, a raise of its production is needed (Deferm et al., 2017). In addition, indium ions are suspected to be carcinogenic and can cause damage to heart, kidney and liver (Hwang et al., 2013). Therefore, efforts to find methods to recover and recycle indium are of great significance.
⁎
Among several separation methods for metal ions from aqueous solutions, adsorption is widely considered as one of the most promising methods for removing or separating metal ions from aqueous solutions due to its simplicity, low cost and high efficiency (Li et al., 2018a, 2018b). Another challenge in this area is the selective separation of target ions from aqueous solutions, which contain numerous other metal ions. Therefore, it is clear that the development of an adsorbent, which presents excellent selectivity for targeted ions, is an important research direction in the field of adsorption and purification (Sherlala et al., 2018). Unfortunately, adsorbents reported in recent years tend to lack of selectivity for indium ions, although they exhibit large adsorption capacity and fast adsorption rate(Alguacil et al., 2016; Hassanien et al., 2017; Hwang et al., 2013; Jeon et al., 2015; Li et al., 2012; Roosen et al., 2017). In a previous study, two adsorbents were prepared to selectively separate indium ions from real mine wastewater and the results demonstrated that the two adsorbents possessed fast adsorption rate and good affinity for indium ions (Li et al., 2018a, 2018b; Li et al., 2015). However, further development is needed in this field to obtain a
Corresponding authors. E-mail addresses: [email protected] (M. Li), [email protected] (J. Feng).
https://doi.org/10.1016/j.hydromet.2019.04.003 Received 22 November 2018; Received in revised form 17 March 2019; Accepted 1 April 2019 Available online 02 April 2019 0304-386X/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. The scheme for preparation of SiO2@GO–PO3H2.
solutions. The objectives of the present study were to focus on preparing the SiO2@GO-PO3H2 composite and employ it as an adsorbent to selectively separate indium ions from aqueous solutions. The morphologies and structures of the resultant composite were investigated using Fourier transform infra-red spectroscopy (FT-IR), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and Brunauer–Emmett–Teller (BET) technique. The adsorption performance and the mechanism of adsorption were studied in detail based upon batch experiments. The results indicated that SiO2@GO-PO3H2 could be used as a promising adsorbent for separating and recycling indium from aqueous solutions.
cheaper and more readily available adsorbent, which has good performance for selective adsorption of indium ions. Graphene oxide (GO) is a type of graphite with one or several atomic layers, possessing special two-dimensional structures, high surface area (theoretical value of 2620 m2/g) and plenty of oxygencontaining functional groups, such as hydroxyl, epoxy and carboxyl groups on the surface (Zhao et al., 2011). These oxygen groups can be combined with metal ions through electrostatic attractions or complexation. As a result, a large number of GO-based adsorbents are used to eliminate or separate metal ions from aqueous media (El-Maghrabi et al., 2017; Li et al., 2018a, 2018b; Liang et al., 2018; Pan et al., 2016; Sheng et al., 2018). However, the selectivity of GO for targeted ions is limited owing to the fact that oxygen-containing functional groups on the surface of GO can interact with a variety of metal ions, even though adsorbents reported in recent years exhibit large adsorption capacities and good chemical and physical stabilities (Li et al., 2018a, 2018b). On the other hand, the nano-sized structure and excellent dispersion of GO in aqueous solutions make it difficult to be separated even using traditional centrifugation and filtration methods (Liu et al., 2011). The present study attempted to introduce GO onto the surface of silica gel to decorate/functionalize GO through specific functional groups, which can improve its selectivity and separation capability for targeted ions. The manuscript is an extension of our previous work (Li et al., 2015; Li et al., 2018a, 2018b; Li et al., 2014), where the separation of metal ions, including In(III), Cd(II), Zn(II), Cu(II) and Ni(II) from aqueous solutions using modified silica gel, was investigated and the results revealed that the phosphonic acid groups exhibited high affinity for indium ions. It is extended preparing a novel adsorbent by introducing GO onto the surface of silica gel, after which, GO is decorated with the phosphonic acid groups to selectively separate and recycle indium from aqueous
2. Experimental 2.1. Apparatus FT-IR analyses of all samples were performed using a Bruker (TENSOR-27, Germany) spectrometer. The spectra were measured in the absorbance mode within the wavelength range of 4000–400 cm−1. Inductively coupled plasma atomic emission spectrometry (ICP-AES) (Perkin-Elmer-7000DV, USA) was used to determine the concentrations of metal ions in the solutions. A pHS-3C digital pH meter (Shanghai Precision & Scientific Instrument Co., Ltd., China) was used for pH adjustments. The surface morphology of samples was examined using SEM (Hitachi S4800, Japan) at the desired magnification. The N2 adsorption/desorption isotherms were recorded using a NOVA3200e surface area and pore size analyzer (Quantachrome Instruments, USA) at 77 K. Furthermore, XPS was carried out on an ESCALAB 250 (Thermo, USA) unit using monochromatic Al Kα radiation, for which, 74
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(III) ions solutions at various pH values. The pH values of solutions were adjusted via diluted sulphuric acid and ammonia water. The concentrations of In(III) ions in the solutions were measured after stirring for 4 h. The effect of contact time on the removal of indium ions using SiO2@GO-PO3H2 was examined by adding 25.0 mg of adsorbent into 50.00 mL indium ions' solution with the concentration of 140 mg·L−1 at 298 K and pH of 2.5. The samples were extracted from their respective solutions at different intervals until the end of the process. About 10 mg of SiO2@GO-PO3H2 was added to 20.00 mL of indium ions solution of various concentrations (10–150 mg·L−1) at different temperatures. The initial pH value of the solution was adjusted to 2.50. The competitive adsorption of Cu(II), Pb(II), Zn(II), Ca(II), Na (I), K(I), Mg(II), and Fe(II) with respect to In(III) was studied over SiO2@GO-PO3H2. For that, 25.0 mg of adsorbent was mixed with 50.00 mL of aqueous solutions containing 100 mg·L−1 of the aforementioned metal ions at the pH 2.50 for determining the adsorption equilibrium. The distribution coefficient (D) and the separation factor (β) were calculated using Eqs. (2) and (3), respectively (Zhou et al., 2018).
energy normalization was already conducted. 2.2. Materials and reagents Ethanol, indium sulfate, dicyclohexylcarbodiimide (DCC), toluene, 3-aminopropyltriethoxysilane (APTS), vinylphosphonic acid and N, Ndimethyl formamide (DMF) were purchased from Sinopharm Chemical Reagent Co., Ltd.,China). Silica gel (80–120 mesh) was purchased from Qingdao Ocean Chemical Co., Ltd., China. Graphene oxide was supplied by the Institute of Metal Research, Chinese Academy of Sciences, China. All the other reagents used were of analytical grade and purchased from Beijing Chemical Plant, China. These analytical grade reagents were used without any further purification. 2.3. Preparation of SiO2@GO-PO3H2 composite The synthesis of SiO2@GO-PO3H2 composite consists of four steps, which are schematically presented in Fig. 1. (i) SG-APTS. Silica gel was grafted using 3-aminopropyltriethoxysilane and it is described in detail in a previous work (Li et al., 2014). In short, about 8.0 g of silica gel was mixed with 60.0 mL of 6 mol/L hydrochloric acid and refluxed under stirring for 8 h. The activated silica gel was filtered, washed with double-distilled water to neutral and dried under vacuum at 70 °C for 12 h, which resulted in the production of activated silica gel. 5.00 g of activated silica gel was dispersed in toluene (50.0 mL) in a flask followed by gradually adding APTS (5.00 mL) into the solution with continuous stirring. The mixture was refluxed under stirring for 24 h. The product was filtered off, washed with toluene and alcohol and dried under vacuum at 70 °C for 12 h. (ii) SiO2@GO. 0.60 g of GO was dispersed in 400 mL of DMF using sonication for 30 min. Then, about 20.0 g of SG-APTS and 1.50 g of DCC were added into the yellow-brown solutions. The reaction system was stored in a water-bath at 60 °C for 24 h. The product was washed using DMF and ethanol followed by drying under vacuum at 50 °C for 12 h. (iii) SiO2@GO-NH2. First, 5.00 g of SiO2@GO was dispersed in toluene (50.0 mL) in a flask. APTS (15.00 mL) was gradually added into the solution with continuous stirring. The mixture was refluxed under stirring for 24 h. The obtained product was filtered, washed with toluene and alcohol followed by drying under vacuum at 70 °C for 12 h. (iv) SiO2@GO-PO3H2. 5.00 g of SiO2@GO-NH2 was dispersed in ethanol (50.0 mL) in a flask. Then, vinylphosphonic acid (10.00 mL) was gradually added into the solution with continuous stirring. The mixture was refluxed under stirring for 24 h. The final product was filtered, washed with alcohol and dried under vacuum at 50 °C for 12 h.
D=
β = DIn DM
(3)
2.5. Column experiments The fixed-bed adsorption was conducted in glass columns with 6 mm in internal diameter and 100 mm in height. The columns were packed with 1.0 g of SiO2@GO-PO3H2. For In(III) ions adsorption, 100 mg·L−1 of In(III) ions solutions was pumped upwards through the SiO2@GO-PO3H2 packed column, at a steady flow rate 0.50 mL·min−1 by a peristaltic pump. Effluents were collected after every 5.00 mL of the solution passed through the column. The column experiments were performed at room temperature and pH 2.5. For the In(III) ions elution from the fixed-bed, HCl (0.5 mol·L−1), HCl (1.0 mol·L−1), H2SO4 (0.5 mol·L−1) and H2SO4 (1.0 mol·L−1) were chosen as the eluent, and the elution process was carried out at a fixed flow rate of 0.25 mL·min−1. After the column was exhausted, regeneration of SiO2@GO-PO3H2 and desorption of In(III) ions was carried out. Desorption of In(III) ions from the loaded material was performed with 1.0 mol·L−1 of H2SO4 at a constant flow rate of 0.25 mL·min−1.
The stock Cu(II), Pb(II), Zn(II), Fe(II), Al(III) and In(III) solutions were prepared by dissolving CuSO4·5H2O, Pb(NO3)2, ZnSO4·7H2O, FeSO4, AlCl3 and In2(SO4)3 in double-distilled water, respectively. All of these solutions were stored at ambient temperature. For the adsorption experiments, the concentrations of all metallic ions were determined using ICP-AES. The adsorption capacity can be calculated using Eq. (1) (Wu et al., 2018).
3. Results and discussion 3.1. Characterization of SiO2@GO-PO3H2 The synthesized SiO2@GO-PO3H2 composite was confirmed using FT-IR, BET, SEM and XPS analyses. Fig. 2a displays the FT-IR spectra of activated silica gel, SG-APTS, GO, SiO2@GO, SiO2@GO-NH2, free and loaded In(III) SiO2@GO-PO3H2. As shown in Fig. 2a (a,b), all absorption bands in the spectra of activated silica gel and SG-APTS were consistent with those reported in a previous work (Ramasamy et al., 2017). In Fig. 2a(c), the broad and strong absorption band at 3400 cm−1 is attributed to OH, while other bands at 1735 cm−1, 1630 cm−1, 1395 cm−1and 1097 cm−1 are assigned to carbonyl/carboxyl C]O stretching vibration, C]C stretching vibration, OH deformation vibration and CeO stretching vibration, respectively. These results are consistent with those reported in a previous work (El-Maghrabi et al., 2017; Liu et al., 2016; Zhao et al., 2012). Fig. 2a(d) demonstrates that
(ci − cf ) × V (1)
m
(2)
where ci and ce are the initial and equilibrium concentrations of M (mg·L−1) (‘M' represents the metal ions), respectively, V is the volume of the solution (mL), m is the mass of SiO2@GO-PO3H2 (g), and DIn and DM represent the distribution coefficients of indium ions and other metallic ions, respectively.
2.4. Batch adsorption experiments
q=
(Ci − Ce ) V × Ce m
−1
where q represents the adsorption capacity (mg·g ), ci and cf are the initial and final concentrations of metallic ions (mg·L−1), respectively, V is the volume of the solution (L), and m is the mass of SiO2@GOPO3H2 (g). The SiO2@GO-PO3H2 static adsorption studies for indium ions were performed using a batch method in a constant-temperature oscillator and stroke speed maintains 50 r/min. In order to investigate the effect of pH values on adsorption of In(III) ions, about 25 mg adsorbent was added into a capped conical flask containing 50 mL of 140 mg·L−1 In 75
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Fig. 2. (a) The FT-IR spectra of activated silica gel (a), SG-APTS(b), GO(c), SiO2@GO(d), SiO2@GO–NH2(e),free(f) and In(III) ion-loaded SiO2@GO–PO3H2; (b) XPS of O 1 s; (c) Nitrogen adsorption and desorption isotherms of SiO2@GO–PO3H2 before adsorption; (d) Nitrogen adsorption and desorption isotherms of SiO2@GO–PO3H2 after adsorption In(III) ions.
the peaks at 1390 cm−1 and 1100 cm−1 can be attributed to NeH stretching vibration and CeN deformation vibration, respectively (Hosseinzadeh and Ramin, 2018). As shown in Fig. 2a(e), a new adsorption band at 1560 cm−1 can be allocated to NeH bending vibration of the eCONHe group, which was formed by the reaction between the carboxyl group belonging to SiO2@GO and the amide group of APTS (Pytlakowska et al., 2016; Rajabi et al., 2018). Another new peak at 1495 cm−1 is assigned to CeN stretching vibration, while the peak at 700 cm−1 is assigned to NeH waging (Pytlakowska et al., 2016; Rajabi et al., 2018). Several absorption bands confirmed that phosphate groups were grafted onto the surface of SiO2@GO. They were observed in Fig. 2a(f) as peaks at 2325 cm−1 and 1695 cm −1, which were assigned to P(O)-OH stretching vibration (Wu et al., 2013). A band at 1077 cm−1 can be attributed to the symmetrical stretching vibration of C-O-P (Wu et al., 2013; Wu et al., 2018), while the absorption bands at 966 cm−1 and 918 cm−1 are assigned to the stretching vibration of POH (Yin et al., 2011). It is confirmed that SiO2@GO-PO3H2 was prepared successfully through chemical grafting. Nitrogen adsorption isotherm for SiO2@GO-PO3H2 is shown in Fig. 2c. It was observed that SiO2@GO-PO3H2 was Type IV with a clear hysteresis loop according to the IUPAC classification, indicating that the pores in SiO2@GO-PO3H2 composite belonged to a mesoporous structure (Li et al., 2008). In addition, the adsorption and desorption lines are approximately parallel, exhibiting that the pores of the material possess a uniform radius (Li et al., 2008). For SiO2@GO-PO3H2, the BET surface area, pore volume and average pore size were
determined to be 141.531 m2·g−1, 0.246 cm3·g−1 and 7.827 nm, respectively. For In(III)-loaded SiO2@GO-PO3H2, the values of BET surface area, pore volume and average pore size were calculated to be 146.403 m2·g−1, 0.116 cm3·g−1 and 2.743 nm, respectively. The adsorbent surface is rough and porous, which is likely to be due to large quantity of In(III) aggregated at the adsorbent surface after adsorption. However, these pores were formed due to numerous lumpy objects, which overlapped mutually, as observed in the SEM image shown in Fig. 3b. Fig. 3a shows the SEM image of SiO2@GO-PO3H2. Obvious wrinkles belonging to GO are observed on the adsorbent surface, demonstrating that GO was successfully introduced onto the surface of silica gel. Furthermore, some attachments are also observed on the surface of SiO2@GO-PO3H2, which can be the result of numerous phosphate groups grafted due to the chemical grafting method. The chemical composition of elements in SiO2@GO- PO3H2 was investigated using XPS. Peaks were observed in the full spectra of SiO2@GO-PO3H2 (shown in Fig. 4a) at 284.60, 401.28, 532.30, 153.54, 103.58 and 133.06 eV, which can be assigned to C, N, O, Si and P elements, respectively (Amaral et al., 2005; Wu et al., 2013). These results further confirm that phosphate groups were successfully introduced onto the surface of SiO2@GO.
3.2. Effect of pH on adsorption The effect of varying pH on In(III) ions uptake was investigated using the batch experiment. The adsorption experiments were 76
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Fig. 3. (a) XPS survey spectra of free(e) and In(III) ion-loaded SiO2@GO–PO3H2; (b) XPS of P 2p of SiO2@GO–PO3H2; (c) XPS of In 3d; (d) XPS of P 2p of SiO2@GO–PO3H2 after adsorption In(III) ions.
process that functional groups in the adsorbent combine with In(III) ions was retarded. Another reason could be that the existence of a high abundance of H+ competing with In(III) ions at low pH value.
triplicated. Considering the formation of precipitation of In(III) ions at higher pH, pH above 3.0 was not investigated. The pH of the solutions is a significant factor to influence a sorption process of metallic ions, since it has an important impact on solubility of metal ions, species distribution of metal ions in the solution and overall charge of the adsorbent. Fig. 5 shows the effect of pH on the adsorption of In(III) ions on SiO2@GO-PO3H2. As can be seen, the adsorption capacity increased remarkably as the pH of the solution increased from 0.5 to 3.0 and the maximum adsorption capacity was 149.5 mg·g−1 at pH of 3.0. It can be explained that at low pH, the vast majority of the functional groups distributed in the adsorbent were protonated, which indicates the
3.3. Adsorption kinetics The efficiency of SiO2@GO-PO3H2 composite was evaluated by studying the adsorption kinetics. Briefly speaking, 25.0 mg of adsorbent was added to 50.00 mL of aqueous solutions containing indium(III) ions at the pH of 2.50 and the temperature of 298 K. Fig. 6a shows the adsorption capacity of indium(III) ions on SiO2@GO-PO3H2 against the
Fig. 4. (a) SEM image of SiO2@GO–PO3H2 before adsorption; (b) SEM image of SiO2@GO–PO3H2 after adsorption In(III) ions. 77
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sites have been occupied by indium(III) ions, and therefore, further adsorption relies mainly on the diffusion of solute into the intra-particle space. In addition, it was observed that the adsorption capacity increased with the increase in the concentration of indium(III) ions, which can be attributed to a higher driving force for mass transfer at higher concentrations of indium(III) ions. In order to clarify the mechanism underlying the adsorption process, the Lagergren pseudo-first-order and pseudo-second-order kinetic models were employed to evaluate the experimental kinetic data. The pseudo-first-order and pseudo-second-order models are expressed using Eqs. (4) and (5), respectively (Li et al., 2015).
ln(qe − qt ) = ln qe − k1 t
(4)
t 1 1 = + t qt qe k2 qe2
(5) −1
−1
where t is the contact time (min), qt (mg·g ) and qe (mg·g ) are the adsorption capacity of indium (III) ion at any time t and at equilibrium, respectively, and k1 (min−1) and k2 (mg·g−1·min−1) represent the adsorption rate constants of first-order and second-order models, respectively. Fig. 6(b, c) shows the kinetic fitted plots of the pseudo-first-order and pseudo-second-order for the adsorption of indium(III) ions onto SiO2@GO-PO3H2, while the calculated values of different parameters are listed in Table 1. The results presented in Table 1 show that the adsorption process for In(III) ions could be better described using the pseudo-second-order rather than the pseudo-first-order model and that the process is based upon chemisorption (Luo et al., 2017). All the obtained correlation coefficients (R2) from the pseudo-second-order model were close to unity, whereas all the calculated qe values were in
Fig. 5. Influence of pH values on the adsorption capacity of SiO2@GO–PO3H2.
contact time at different concentrations. As seen in Fig. 6a, the adsorbent exhibits a fast adsorption process for the targeted ions. The three adsorption kinetic curves followed an increasing trend with time and the required equilibrium time was around 50 min. The adsorption of overwhelming majority of indium(III) ions was rapidly achieved within the first 35 min, which can be attributed to large number of available adsorption sites on the surface of SiO2@GO-PO3H2. These sites are available for the adsorption of indium(III) ions at an early stage. However, a slow adsorption rate was observed after 35 min, which is likely caused by the fact that most of the active external vacant
Fig. 6. (a) Influence of contact time on the adsorption of In(III) ions onto SiO2@GO–PO3H2; (b) Lagergren pseudo-first-order kinetics of In(III) ions onto SiO2@GO–PO3H2; (c) Lagergren pseudo-second-order kinetics of In(III) ions onto SiO2@GO–PO3H2; (d) Comparison of Langmuir and Freundlich isotherm models for In(III) ions adsorption onto SiO2@GO–PO3H2. 78
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Table 1 Kinetic parameters for the indium (III) ions adsorption onto the SiO2@GO-PO3H2. ci (mg·L−1)
qe,
exp(mg·g
−1
)
Pseudo-first-order rate equation −1
k1 (min 49.35 85.26 99.47
77.86 132.3 149.5
0.03344 0.02292 0.0277
)
qe,
cal
Pseudo-second-order rate equation
(mg·g
−1
)
56.31 111.9 124.3
3.4. Adsorption isotherms and equilibrium The adsorption isotherms of In(III) ions on SiO2@GO-PO3H2 were investigated at three temperatures of 283, 293 and 303 K at the pH of 3.0. Two classical isotherm models, the Langmuir and Freundlich models, were used to analyze the experimental data. The Langmuir model assumes that the adsorption process occurs on a homogeneous surface with a fixed number of active sites and without any interaction among the adsorbed molecules. The non-linear form of the Langmuir isotherm model is expressed using Eq. (6) (Liu et al., 2016).
KL qm ce 1 + KL ce
0.9601 0.9701 0.9820
0.001037 0.00337 0.00326
82.51 137.3 152.0
cal
(mg·g−1)
R22 0.9989 0.9949 0.9988
In order to investigate the selective separation behavior of SiO2@GO-PO3H2, competitive adsorption for In(III) ions was performed in aqueous solutions containing Cu(II), Pb(II), Zn(II), Ca(II), Na(I), K(I), Mg(II) and Fe(II). The results of this experiment are shown Fig. 7, while the calculated relevant parameters are listed in Table 3. These interfering ions were chosen in the present study due to their co-existence with indium in natural sources. As seen from the results presented in Fig. 7 and Table 3, the adsorption capacity of SiO2@GO-PO3H2 for In (III) ions was much larger than that for other interfering ions, indicating that SiO2@GO-PO3H2 possesses a stronger affinity for In(III) ions. According to hard and soft acids and bases theory (HSAB), In(III) ions belonged to hard acid can be more easily captured by the functional groups (ePO3H2) on the adsorbent. Certainly, With the exception of In (III) ions, Ca(II), Na(I), K(I), Al(III) and Mg(II) ions also belong to hard acid in the system. However, the difference in the ionic radius or charge number might make In(III) ions adsorbed.
(6)
1 1 + KL c0
qe,
3.5. Selectivity
where qe is the amount of In(III) ions adsorbed per gram of adsorbent (mg·g−1), and ce is the equilibrium concentration of In(III) ions in the solution. Furthermore, qm and KL are the Langmuir constants related to the maximum adsorption capacity (mg·g−1) and the adsorption energy (L·g−1), respectively. In order to predict the suitability of adsorption process, the essential characteristic of Langmuir equation can be described using the dimensionless separation factor RL, which is defined according to Eq. (7) (Shaheen et al., 2017).
RL =
k2(mg·g−1·min−1)
respectively, indicating that the adsorption of In(III) ions on SiO2@GOPO3H2 follows the mechanism of monolayer adsorption (Luo et al., 2017). This result was expected due to the abundance of functional groups on the surface of adsorbent. Similar results were reported previously in the literature (Chen and Chen, 2015; Cheng et al., 2018). In addition, all qe values obtained from the Langmuir model increased when the temperature increased from 283 K to 303 K, suggesting that the adsorption process was endothermic in nature (Singh et al., 2009). Furthermore, the values of RL lied between 0 and 1 for In(III) ions' adsorption on SiO2@GO-PO3H2 composite, indicating that the adsorption of In(III) ions was favorable (Li et al., 2018a, 2018b).
good agreement with the experimental qe values. Previously, similar observations were reported for the adsorption of Cd(II) on a Cd(II)-ion imprinted polymer (Li et al., 2015) and for U(VI) on functionalized GO (Sun et al., 2015).
qe =
R12
(7) 3.6. Effect of eluting agents on the recovery
−1
where c0 is the initial concentration of In(III) ions (mg·L ). The value of RL serves as an indicator for the nature of adsorption process. The condition of RL > 1 indicates that the isotherm is unfavorable, whereas RL = 1 indicates that the isotherm is linear. Additionally, the condition of 0 < RL < 1 indicates that the isotherm is favorable, while RL = 0 indicates that the isotherm is irreversible. The Freundlich isotherm is suitable for both the monolayer and multilayer adsorptions with heterogeneous binding sites. The nonlinear form of Freundlich isotherm is represented by Eq. (8) (Liu et al., 2016).
For the elution of In(III) ions adsorbed onto the SiO2@GO-PO3H2 from the column, four kinds of eluting agents, H2SO4 (0.5 mol·L−1), H2SO4 (1.0 mol·L−1), HCl (0.5 mol·L−1) and HCl (1.0 mol·L−1), were applied to investigate the recovery efficient of In(III) ion. Among these eluting agents, 1.0 mol·L−1 of H2SO4 was found to be quite effective for desorbing and recovering In(III) ions from the sorbent and the result was shown in Fig. 8a. With a single wash of 1.0 mol·L−1 of H2SO4, 99.2% of In(III) ions was recovered from the fixed–bed column saturated with In(III) ions.
1
qe = Kf ce n
(8)
3.7. Desorption indium(III) ions and reusability of the SiO2@GO-PO3H2
where Kf is roughly an indicator of the adsorption and the factor ‘1/ n‘represents the adsorption intensity. A smaller 1/n value indicates a more heterogeneous surface, whereas a value that is closer to or equal to unity indicates that the adsorbent has relatively more homogenous binding sites. Fig. 6d shows the comparison of Langmuir and Freundlich isotherm models for In(III) ions' adsorption on SiO2@GO-PO3H2 using nonlinear regression. The calculated relevant parameters according to the two models are listed in Table 2. The results presented in Table 2 and Fig. 6d show that the equilibrium data fitted using Langmuir model was better than the Freundlich model. The maximum adsorption capacities were calculated to be 143.1, 146.0 and 149.9 mg·g−1 at 283, 293 and 303 K,
To investigate desorption of In(III) ions from the fixed bed is of crucial importance due to the fact that not only the In(III) ions adsorbed can be separated from the adsorbent but also the adsorbent can be regenerated, making it can recover the ability to capture adsorbate from aqueous solutions. In this study, the adsorption-desorption cycles were repeated ten times by dynamic experiment and 1.0 mol·L−1 of H2SO4 was used as the eluting agent. The results of tens cycles of adsorptiondesorption of In(III) ions onto the SiO2@GO-PO3H2 are shown in Fig. 8b. As can be seen from Fig. 8b, the dynamic adsorption capacity of In(III) ions did not significantly decrease even after ten cycles. The adsorption capacity of In(III) ions for the recycles SiO2@GO-PO3H2 79
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Table 2 Langmuir and Freundlich isotherm constants for In(III) ions adsorption onto SiO2@GO-PO3H2. Temperature(K)
qe,
exp(mg/g)
Langmuir isotherm qm,
283 293 303
138.1 141.7 144.3
cal(mg/g)
143.1 146.1 149.9
Freundlich isotherm 2
KL (L/mg)
R
1/n
Kf (mg/g)
R2
0.4363 0.4642 0.4789
0.9957 0.9964 0.9973
0.2994 0.2936 0.2957
52.49 55.09 58.89
0.9196 0.9052 0.9133
in the adsorbent surface becoming rough and porous. However, these pores were formed through the mutual overlapping of lumpy objects, which is in accordance with the results obtained from SEM. Fig. 3b shows the SEM image of SiO2@GO-PO3H2 after the adsorption of In(III) ions. A large number of attachments were observed onto the surface of adsorbent, whereas these adsorbates were accumulated in a disordered arrangement. These findings confirmed that In(III) ions were captured through functional groups onto the surface of adsorbent. The XPS spectra of free and In(III)-loaded SiO2@GO-PO3H2 are shown in Fig. 4a. No signal belonging to indium was observed in the XPS spectra of SiO2@GO-PO3H2, while two obvious energy bands assigned to In 3d3/2 and In 3d5/2 were observed in the XPS spectra of SiO2@GO-PO3H2-In at 452.95 eV and 445.41 eV, respectively (Bermudez et al., 2006; Chen et al., 2017). These results indicated that In(III) ions were adsorbed onto the surface of adsorbent, which is consistent with the results obtained from the FT-IR analysis. Compared with the spectrum before the adsorption, the band energy for P shifted from 133.06 eV to 132.82 eV after the adsorption, which can be ascribed to the formation of In(III)PO3H2 coordination bonds that resulted in the increase of extra-nuclear electron cloud density surrounding the P atoms (Pan et al., 2016). Additionally, the high-resolution In 3d spectrum is shown in Fig. 4c. The results showed that the bands at 452.95 eV and 445.41 eV can be attributed to the characteristic peaks of In 3d3/2 and In 3d5/2, respectively (Bermudez et al., 2006). As shown in Fig. 4b, two peaks at 132.87 eV and 133.30 eV in the P 2p XPS spectrum of SiO2@GO-PO3H2 before the adsorption can be attributed to PeO and P]O, respectively (Rajabi et al., 2018) Greater changes were observed in the P 2p highresolution spectrum (Fig. 4d) after In(III) adsorption. Three peaks were observed in the spectrum, which were allocated to P]O (133.82 eV), PeO (133.03 eV) and PeIn (132.41 eV), respectively (Wu et al., 2013). In addition, the high-resolution spectra of O 1 s before and after the adsorption are also shown in Fig. 2b. The binding energy of O 1 s before and after the adsorption did not change markedly, suggesting that it did not contribute to the adsorption of In(III) ions. Based on these results, it can be inferred that the In(III) ions in aqueous solutions adsorbed on the surface of SiO2@GO-PO3H2 composite relied mainly on the complexation reaction between In(III) ions and functional groups (-PO3H2) on the surface of SiO2@GO-PO3H2. In addition, only the P element in the functional groups (ePO3H2) participated in coordination with In(III) ions generating the PeIn bond, whereas the oxygen element in the functional groups (-PO3H2) had no contribution in the adsorption of In(III) ions. Therefore, an important research direction is the development of an adsorbent having greater adsorption capacity for In(III) ions and developed by introducing increased number of functional groups containing P element.
Fig. 7. Comparative adsorption of foreign ions onto the SiO2@GO–PO3H2;
only had 2.6% of loss at the tenth cycle. These results indicate that the SiO2@GO-PO3H2 possesses an excellent stability and reusability for the In(III) ions adsorption. 3.8. Adsorption mechanism In order to reveal the adsorption mechanism of In(III) ions on SiO2@GO-PO3H2, free and In(III) ions-loaded SiO2@GO-PO3H2 (referred to as SiO2@GO-PO3H2-In) were characterized on the basis of FTIR, BET, SEM and XPS techniques. The comparison of FT-IR spectra before and after the adsorption indicated that the band at 966 cm−1 before the adsorption was shifted to 953 cm−1 after the adsorption. In addition, the bands assigned to P(O)-OH at 2325 cm−1 and 1695 cm−1 disappeared after the adsorption. However, two new peaks at 799 cm−1 and 621 cm−1 were observed after the adsorption. These results suggested that In(III) ions were captured on the surface of SiO2@GOPO3H2. Fig. 2d shows the nitrogen adsorption isotherm for SiO2@GOPO3H2-In. The shape of SiO2@GO-PO3H2-In isotherm remained unchanged after the adsorption. Therefore, this material could also be assigned to Type IV according to the IUPAC classification. Additionally, a hysteresis loop was observed, which was representative of a mesoporous structure (Guo et al., 2013). The volume adsorbed by SiO2@GOPO3H2-In rapidly increased at the relative pressure (P/P0) of about 0.7, implying capillary condensation of nitrogen within the uniform mesoporous structure (Li et al., 2008). The BET surface area, pore volume and average pore size changed and it could be explained based upon the accumulation of a significant amount of In(III) onto the adsorbent surface after adsorption, resulting Table 3 Selective adsorption of In(III) ions from multi-ionic mixtures by SiO2@GO-PO3H2. Metal
In(III)
Cu(II)
Pb(II)
Zn(II)
Ca(II)
Na(I)
K(I)
Mg(II)
Fe(II)
ci (mg/L) ce (mg/L) DM β(In(III)/M)
99.98 43.24 2624.4 1
100.1 94.74 113.1 23.2
100.0 95.63 91.4 28.7
99.93 89.35 236.8 11.0
99.75 75.36 647.3 4.0
99.89 97.63 46.3 56.7
100.2 98.46 35.3 74.3
99.92 82.53 421.4 6.2
99.94 83.64 389.8 6.7
80
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Fig. 8. (a) Comparison of various eluting agents for elution of In(III) ions; (b) Ten cycles of indium(III) ions adsorption-desorption. Table 4 Comparison of SiO2@GO-PO3H2 with other adsorbents of indium(III) ions. Adsorbent
Equilibrium concentrations /(mg·L−1)
pH
T/K
Adsorption capacity/(mg·g−1)
Equilibrium time /min
References
HBAAS Magadiite Phosphorylated sawdust beads CCB CCR IIP In(III)-IIP SiO2@GO-PO3H2
1150 41 0.87 70 84 128 114 27
2.0 2.5 3.5 4.0 3.0 2.5 2.5 2.5
298 N.R 298 313 298 318 298 303
81.3 80.5 1.121 17.89 109.07 60.62 47.39 149.93
20 10 900 100 360 30 10 50
(Hassanien et al., 2017) (Homhuan et al., 2017) (Jeon et al., 2015) (Calagui et al., 2014) (Adhikari et al., 2012) (Li et al., 2018a, 2018b) (Li et al., 2015) This work
N.R represents no reported.
the functional groups had no contribution to the adsorption of In(III) ions. The results indicated that SiO2@GO–PO3H2 could be considered as a promising candidate for the recovery and removal of In(III) ions from wastewater.
3.9. Comparison with other adsorbents for indium(III) ions In the present study, the adsorption performance of SiO2@GOPO3H2 for In(III) ions in aqueous solution was compared with that of other adsorbents, which have previously been reported in literature (see Table 4). The results show that the proposed SiO2@GO-PO3H2 adsorbent possesses fast adsorption kinetics, larger adsorption capacity and high affinity for In(III) ions. Notably, the adsorption capacity of the proposed adsorbent for In(III) ions may be the highest reported. The results indicated that SiO2@GO-PO3H2 is a reliable and effective adsorbent for the removal or extraction of In(III) ions from aquatic environment.
Acknowledgements This research is supported by the National Natural Science Foundation of China (No. 51708075). Also, the authors acknowledge the Scientific and Technological Research Program of Chongqing Municipal Education Commission (Grant No. KJ1713335, KJQN201801527). References
4. Conclusions
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In the present study, a novel adsorbent (SiO2@GO-PO3H2) was prepared for the extraction and removal of In(III) ions from the aquatic environment. The characterization results confirmed that various functional groups can be introduced on the surface of SiO2@GO through a chemical grafting reaction. The kinetic data fitted well with the pseudo-second-order model, suggesting that chemical adsorption occurred during the adsorption process. The equilibrium data could be better described on the basis of Langmuir adsorption isotherm, indicating that the adsorption was a monolayer adsorption process. Additionally, the maximum adsorption capacity was calculated to be 149.93 mg·g−1 at the pH of 2.5 and the temperature of 303 K, which is the highest capacity for In(III) ions reported so far. The possible adsorption mechanism for In(III) ions could mainly be attributed to the fact that functional groups (ePO3H2) of the adsorbent surface coordinated with In(III) ions through covalent bonds. It appears that only the P element in the functional groups (–PO3H2) participated in the coordination with In(III) ions to form PeIn bonds, whereas oxygen in 81
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