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Al-LDH) from aqueous solutions
Journal of Solid State Chemistry 233 (2016) 133–142
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Highly efficient and selective adsorption of In3 þ on pristine Zn/Al layered double hydroxide (Zn/Al-LDH) from aqueous solutions Mary Jenisha Barnabas a, Surendran Parambadath a, Aneesh Mathew a, Sung Soo Park a, Ajayan Vinu b, Chang-Sik Ha a,n a
Department of Polymer Science and Engineering, Pusan National University, Geumjeong-gu, Busan 46241, Republic of Korea Australian Institute for Bioengineering and Nanotechnology, The University of Queensland, #75 Corner College and Cooper Road, Brisbane 4072, QLD, Australia
b
art ic l e i nf o
a b s t r a c t
Article history: Received 2 June 2015 Received in revised form 21 September 2015 Accepted 1 October 2015
A pristine Zn/Al-layered double hydroxide (Zn/Al-LDH) showed excellent adsorption ability and selectivity towards In3 þ ions from aqueous solutions. The adsorption behaviour as a function of the contact time, solution pH, ionic strength, and amount of adsorbent under ambient conditions revealed a strong dependency on the pH and ionic strength over In3 þ intake. The structure and properties of Zn/Al-LDH and In3 þ adsorbed Zn/Al-LDH (In–Zn/Al-LDH) were examined carefully by X-ray diffraction, Fourier transform infrared spectroscopy, N2-sorption/desorption, UV–vis spectroscopy, and X-ray photoelectron spectroscopy. The adsorbent had a sufficient number of active sites that were responsible for the In3 þ adsorption and quite stable even after the adsorption process. The selective adsorption of In3 þ on Zn/AlLDH was also observed even from a mixture containing competing ions, such as Mn2 þ , Co2 þ , Ni2 þ , Cd2 þ , Pb2 þ , and Cu2 þ . The adsorption experiments showed that Zn/Al-LDH is a promising material for the preconcentration and selective removal of In3 þ from large volumes of aqueous solutions. & 2015 Elsevier Inc. All rights reserved.
Keywords: Zn/Al-LDH Indium Metal adsorption Separation and pre-concentration
1. Introduction Layered double hydroxides (LDHs), also known as hydrotalcitelike compounds or anionic clays, are bidimensional solids with a positive charge excess in their brucite like layers that have attracted considerable attention in recent years owing to their excellent properties, such as large interlayer surface with a high specific surface area and large pore volume and recycling capability [1]. The structure of the LDHs consists of positively charged mixed metal hydroxide layers separated by charge-balancing anions and water molecules. LDHs have the structural formula, [ M12−+x M3x + (OH)2](Am )x/m nH2O (M2 þ ¼Mg2 þ , Zn2 þ , Ni2 þ etc., M3 þ ¼Al3 þ , Cr3 þ , Ga3 þ etc.). Am denotes interlayer anions, such 2− as NO3 , SO2− 4 and CO3 , and x typically ranges from 0.17 to 0.33. This charge excess is produced by the isomorphic substitution of a divalent cation with a trivalent cation that is compensated for by the introduction of anions (together with water) in the interlayer space. Because the interlayer anions are easily exchangeable, they can be exchanged with other anions. This offers high anion exchange capability, which is the most remarkable characteristic of LDHs. The ease of preparation and widely variable ways, in which n
Corresponding author. Fax: þ 82 51 513 7720. E-mail address: [email protected] (C.-S. Ha).
http://dx.doi.org/10.1016/j.jssc.2015.10.001 0022-4596/& 2015 Elsevier Inc. All rights reserved.
their components can be combined, make LDHs useful for many applications [2–5]. LDHs can adsorb metal cations from aqueous solution despite having a positive charge on the surface layer. The positive charge of the layer attracts hydroxide ions around the surfaces of the LDH crystals in aqueous solution to induce the formation of metal hydroxides. The charge-compensating carbonate ion attached to the surface and the edge can also come in contact with metal cations to form insoluble metal carbonates. These unique features make LDHs excellent adsorbents for heavy metal cations. On the other hand, only hydrotalcite-like compounds have been used as adsorbents for heavy metals and the use of LDHs for the selective removal of heavy metal ions are quite rare. A range of technologies, i.e., electro-analytical techniques, coprecipitation, nanofiltration membranes, solid phase extraction, supercritical CO2 extraction, biosorption, and extraction resin are employed widely for the recovery of soluble In3 þ ions from waste water [6–8]. These methods, however, suffer from serious drawbacks, such as the incomplete recovery of metal ions, especially from dilute metal solutions, high reagent and/or energy requirements, and generation of toxic sludge or other waste products, and high capital cost. With the increasing environmental awareness and legal constraints being imposed on the discharge of effluents, the development of cost-effective alternative technologies is essential and requires more attention. All indium compounds should
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be regarded as highly toxic as they can damage the vital organs like heart, kidney, and liver and may be teratogenic. Indium and its compounds exhibit semiconducting or optoelectronic characteristics, which are used in the production of liquid crystal displays, semiconductors, low pressure sodium lamps, and infrared photodetectors [9–10]. In is also used widely in thin-films to form lubricated layers as well as for producing low melting point alloys, and it is a component in some lead-free solders. Indium recovery is in strong demand and possible through biosorption system using an environmentally friendly microbial system, such as bacterium S. Algae [11]. The bacterial cells could absorb all the indium from an aqueous solution within 10 min even from solutions containing very low In3 þ concentrations. Chitosan-coated bentonite beads were also found to be good adsorbents for In3 þ from aqueous solutions [12]. These reports surveys highlight the use of chemically active functional groups along with LDH that serve as efficient sites to bind metal ions. Although these materials are highly promising in efficiency and are environmentally benign, they require tedious experimental setups, complicated synthesis process, modification and handling. The use of pristine LDH for selective metal recovery still remains a topic of intense research. This paper reports the adsorption of In3 þ from an aqueous solution of In(NO3)3 onto Zn/Al-LDH. Zn/Al-LDH and In–Zn–AlLDH were characterised by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy and X-ray photoelectron spectroscopy (XPS). N2-adsorption/desorption analysis was employed to determine the surface area, pore diameter and pore volume of the materials. For a potential application, the adsorption behaviour of In3 þ on Zn/Al-LDH was investigated under different conditions, such as the contact time, solution pH, ionic strength, and the amount of adsorbent. In3 þ was selected as a representative of bivalent metal ions because of its ubiquitous presence in wastewater. The competitive adsorption from a mixture of Mn2 þ , Co2 þ , Ni2 þ , Cd2 þ , and In3 þ ions and the effect of the presence of Cu2 þ or Pb2 þ or both in the above mixture on the adsorption of In3 þ over Zn/Al-LDH were have also investigated at a solution pH of 5. To the best of the authors' knowledge, there are no reports on the adsorptive removal of In3 þ from the aqueous solutions using Zn/ Al-LDH.
2. Experimental section 2.1. Materials Aluminium(III) nitrate nonahydrate, zinc(II) nitrate hexahydrate, indium(III) nitrate hydrate, cobalt(II) nitrate hexahydrate, copper(II) nitrate trihydrate, nickel(II) nitrate hexahydrate, manganese(II) nitrate tetrahydrate, cadmium(II) nitrate tetrahydrate, lead(II) nitrate, and NaOH were purchased from Sigma-Aldrich and used as received. The appropriate volume of 0.1 M HNO3 or NaOH was used to adjust the pH of the solution. 2.2. Zn/Al-LDH synthesis Zn/Al-LDH was synthesised using a co-precipitation method [13]. A mixed-metal nitrate solution (0.25 M Zn2 þ and 0.08 M Al3 þ ) was adjusted to the solution pH of 9–10 by adding the required amount of 2 M NaOH. The mixture was heated to 80 °C for 18 h and a white precipitate was formed. Subsequently, it was filtered and rinsed with de-ionised water. After drying at 85 °C, the obtained material was ground gently into a fine powder. 2.3. In3 þ adsorption from aqueous solutions The solution containing In3 þ ions was prepared by dissolving a
known amount of In(NO3)3 H2O in distilled water. The concentration of In3 þ ions ranged from 1 to 5 mM and the adsorption experiments were carried out by suspending 20 mg of Zn/Al-LDH in 20 mL of an aqueous solution of In(NO3)3 H2O at pH 5. The effect of the pH on the metal ion uptake was studied by performing equilibrium sorption experiments by varying the solution pH from 3 to 6. The dissolution of Zn2 þ during adsorption was investigated by varying the concentration of In3 þ ranged from 1 to 25 mM at pH 5. Adjustments in the solution pH were done by adding either 0.1 M NaOH or HNO3. The concentration of In3 þ was analysed by inductively coupled plasma-atomic emission spectrometry (ICP-OES). The amounts of adsorbed In3 þ were calculated from the difference between the initial (C0) and equilibrium (Ce) concentrations in the supernatant after centrifugation. The adsorption percentage [adsorption (%) ¼(C0 Ce)/C0 100] was derived from the difference between C0 and Ce [13]. 2.4. In3 þ adsorption from mixtures The competitive metal adsorption was assessed using a solution containing 1 mM of Mn2 þ , Co2 þ , Ni2 þ , Cd2 þ , In3 þ , Pb2 þ , and Cu2 þ nitrate salts in distilled water. The pH of the solution was adjusted to 5 using a 0.1 M NaOH solution. The effect of the presence of Cu2 þ or Pb2 þ or both in a mixture on the adsorption of In3 þ over Zn/Al-LDH was also investigated by using the same solution after making the necessary variations in combinations. The adsorption experiments were carried out by suspending 10 mg of Zn/Al-LDH in 20 mL of an aqueous mixture and shaken for 12 h. The concentration of each metal ion before and after adsorption was analysed by inductively coupled plasma-optical emission spectroscopy (ICP-OES). 2.5. Characterisation of samples XRD was performed on a Bruker AXN using Cu-Kα radiation. The XRD patterns were collected in the low-angle range from 1.2° to 10° 2θ. The N2 adsorption–desorption isotherms were measured using a Nova 4000e surface area and pore size analyser. The samples were degassed at 120 °C for 12 h before the measurements. The Brunauer–Emmet–Teller (BET) method was used to calculate the specific surface area. The pore size distribution curve was obtained from an analysis of the adsorption branch using the Barrett–Joyner–Halenda (BJH) method. The FT-IR spectra were obtained on a JASCO FTIR 4100 using the KBr pelleting method over the frequency range, 4000–400 cm 1. To examine the surface elements of the materials in their existing state, XPS was carried out on a PHI-1600 ESCA System XP spectrometer (Perkin-Elmer, USA) using non-monochromatic Mg Kα radiation operated at 15 kV, under 10 7 Pa pressure, and with photoelectron energy set at 1254 eV.
3. Results and discussion 3.1. SEM and TEM images Fig. 1 presents the representative (a) SEM and (b) TEM images of pristine-Zn/Al-LDH. Both SEM and TEM images provide excellent evidence of the existences of sheet-like morphology for the material with a mean particle size of 130 nm. A small amount of particle aggregation was also observed from the SEM images of pristine-Zn/Al-LDH. The SEM and TEM images provided the existence of good quality material. This is in good agreement with previous reports [14,15].
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Fig. 1. (a) SEM and (b) TEM images of pristine Zn/Al-LDH.
LDH-25, respectively. The decrease in the intensities of (003) and (006) reflections for In–Zn/Al-LDH-m might be due to the screening of adsorbed In3 þ species along with a decrease in the interlayer distance between the adjacent layers while maintaining strong binding with the nitrate ions on the surface. Also a shift to the (003) peak from 2θ 11.52° (Fig. 2a) to the higher 2θ 11.58° (Fig. 2d) was observed. These results confirm not only the adsorption of In3 þ ions, but also the excellent purity of the samples after the adsorption process. The narrow interlayer distance is suitable for the adsorption of metal ions. In this case, however, the intensity of the XRD peaks decreased but the layered crystal structure of the Zn/Al-LDH was unaffected. The decrease in the intensity of the XRD peak might also be due to the decrease in the overall percentage of zinc oxide or aluminium oxide in the material after adding In3 þ to its surface [15–16]. The observation and inference of the progressive In3 þ adsorption from the XRD patterns of In–Zn/Al-LDH-1, In–Zn/Al-LDH-5 and In–Zn/Al-LDH-25 are described in the In3 þ adsorption section. Fig. 2. XRD patterns of the (a) Zn/Al-LDH, (b) In–Zn/Al-LDH-1, (c) In–Zn/Al-LDH-5, and (d) In–Zn/Al-LDH-25. nIndium species.
3.2. Powder X-ray Diffraction (XRD) analysis Fig. 2 presents the XRD patterns of (a) Zn/Al-LDH and (b–d) In– Zn/Al-LDH-m, where ‘m’ is represents the initial molar concentration (m¼ 1, 5 or 25) of In(NO3)3 solution in mM. Fig. 2a revealed a series of sharp and intense peaks at both low and high 2θ values, indicating the characteristic basal reflections of Zn/Al-LDH. The less intense but sharp peaks corresponding to the (003) and (006) faces at 11° and 23° 2θ values indicate the low degree of crystallinity of the LDH samples after adsorption [14]. The narrow and sharp peaks were observed between 30° and 70° 2θ, which provides evidence of the crystalline nature of Zn/Al-LDH. The reflections corresponding to (012), (015), (018), and (110) planes show that the samples exhibit hexagonal lattice with rhombohedral 3R symmetry. The purity and chemical composition of the pristine-Zn/Al-LDH was confirmed by the elemental analysis, which was found to be Zn0.74Al0.23(OH)2 (NO3)0.23 0.45H2O with a Zn2 þ /Al3 þ ratio of 3.2. The XRD pattern after In3 þ adsorption from a 1 mM solution (Fig. 2b) showed that the intensity of all the peaks was significantly lower than that of the Zn/Al-LDH sample before adsorption. Although the intensity of the sample was decreased after In3 þ adsorption, it still retained the layered structure. The 2θ value of d003 reflection before In3 þ adsorption was found to be 11.52°. After In3 þ adsorption the d003 reflection was shifted to 11.57°, 11.58°, and 11.58° for Zn/Al-LDH-1, Zn/Al-LDH-5 and Zn/Al-
3.3. XPS analysis XPS was used to further study the surface chemical composition of the Zn/Al-LDH and In–Zn/Al-LDH-5 assembly (Fig. 3). The binding energy (BE) calibration for both spectra was referenced to the C 1s signal at 284.6 eV. The analysis disclosed the binding
Fig. 3. XP spectra of (a) Zn/Al-LDH and (b) In–Zn/Al-LDH-5.
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states of the major elements present in the material. The representative intensity and area of the peaks in the XP spectra of Zn/Al-LDH and In–Zn/Al-LDH-5 offered insight into the successful formation and abundance of the elements within the material. Pure Zn/Al-LDH shows (Fig. 3a) the characteristic binding energies of 1195.37, 1044.51, 1021.52, 497.29, 138, and 74.3 eV for Zn 2s, Zn 2p1/2, Zn 2p3/2, O1s, Zn 3p, and Al 2p, respectively [17]. After In3 þ adsorption the material showed (Fig. 3b) the characteristic binding energies of In3 þ along with the binding energies already presented in the pure Zn/Al-LDH. These are 1195.37, 1044.51, 1021.52, 497.29,138, 74.3, 452.75, and 444.20 eV for Zn 2s, Zn 2p1/2, Zn 2p3/2, O1s, Zn 3p, Al 2p, In 3d3/2, and In 3d5/2, respectively. The In3 þ adsorption did not alter the position of the binding energy values of the fundamental elements in the material, indicating the stability of the Zn/Al-LDH crystal lattice. The Al 2p BE at 74.3 eV was assigned to the binding of Al–OH. The presence of two binding energies values of 452.75 and 444.20 eV corresponding to In 3d3/2 and In 3d5/2 , respectively in the XP spectrum of In–Zn/Al-LDH-5 (Inset in Fig. 3) provided the evidence for the adsorption of In3 þ over Zn/Al-LDH [18]. 3.4. FT-IR analysis Fig. 4 the FT-IR spectra of Zn/Al-LDH and In–Zn/Al-LDH-m samples. The broad and intense band around 3500 cm 1 was attributed to the stretching vibration of the hydroxyl groups and water molecules in the layered structure, whereas a weak shoulder peak recorded at 3250 cm 1 was assigned to the O–H–O stretching mode of the interlayer water molecules, and hydrogen-bonded to interlayer carbonate anions. The bending mode band of the water molecules is normally observed close to 1600 cm 1, and is recorded as a weak shoulder at 1640 cm 1 for all materials [19]. The band at 430 cm 1 was assigned to the OH–Al–Zn–OH deformation mode, whereas the band at 955 and 1480 cm 1 corresponds to deformation mode of AlOH and ZnOH, respectively. Two bands at approximately 551 and 766 cm 1 were attributed to the translation mode of AlOH. The interlayer charge density and the coulomb repulsion between interlayer anions increases with decreasing Zn/Al ratio, which results in a stronger peak of inter1 . The intensity of layer CO2− 3 and a lower wave number 1357 cm 3þ this peak is decrease the In . A sharp and intense band was noted at 1380 cm 1, which was assigned to the vibration of the NO−3 ions. These bands confirmed the formation of the characteristic LDH
Fig. 4. FTIR spectra of (a) Zn/Al-LDH, (b) In–Zn/Al-LDH-1, (c) In–Zn/Al-LDH-5, and (d) In–Zn/Al-LDH-25.
Fig. 5. N2 adsorption–desorption isotherms of (a) Zn/Al-LDH, (b) In–Zn/Al-LDH-1, (c) In–Zn/Al-LDH-5, and (d) In-Z/Al-LDH-25.
network. The FT-IR spectra of In–Zn/Al-LDH-1, In–Zn/Al-LDH-5 and In–Zn/Al-LDH-25 exhibit similar bands to those of Zn/Al-LDH; however, a slight decrease in the intensity and broadness of the bands between 1250 and 1520 cm 1 was observed. The characteristic vibrations of indium metal ions were also observed in between 1350 and 1650 cm 1, which is overlapped with the other vibration bands existing in the material. The band at 1380 cm 1 was also observed and shifted to a higher wave number comparing to that of Zn/Al-LDH, showing the presence of NO−3 ions in the interlayer space. The absorption bands below 1000 cm 1 are responsible for the M–O vibrations and the band at 400–500 cm 1 was assigned to the O–M–O vibration related to LDHs layers [13– 20]. 3.5. N2-adsorption/desorption study Figs. 5 and 6 show the typical nitrogen adsorption–desorption isotherms and their corresponding pore size distributions (PD) of the adsorbents. Table 1 lists the specific surface area, pore volume and average pore size. The N2 adsorption and desorption isotherms of the synthesised Zn/Al-LDH before and after In adsorption, exhibited a type IV adsorption isotherms according to the
Fig. 6. Pore size distribution of (a) Zn/Al-LDH, (b) In–Zn/Al-LDH-1, (c) In–Zn/AlLDH-5, and (d) In–Zn/Al-LDH-25.
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Table 1 Physico-chemical properties from N2-Sorption analysis of Zn/Al-LDH materials before and after In3 þ adsorption. Material
Specific surface area (m2 g 1)
Zn/Al-LDH 62.5 In–Zn/Al-LDH-1 51.2 In–Zn/Al-LDH-5 38.7 In–Zn/Al-LDH16.6 25
Pore diameter (nm)
Pore volume (cm3 g 1)
19.2 17.8 17.6 13.1
0.22 0.19 0.15 0.11
IUPAC classification with a H1-type hysteresis loop, which is caused by capillary condensation and is the characteristic feature of layered materials. On the other hand, the isotherm of In–Zn/AlLDH-5 and In–Zn/Al-LDH-25 showed a H3-type hysteresis loop, which was attributed to the presence of slit type pores that are coming from the plate-like particles. The multilayer formation and capillary condensation began at a lower P/P0 value for Zn/Al-LDH but they were shifted to a higher P/P0 for In–Zn/Al-LDH-1, In–Zn/ Al-LDH-5 and In–Zn/Al-LDH-25. The adsorption isotherms of the samples showed neither a plateau nor limiting uptake at high P/P0 values. These results confirmed that the multilayer adsorption that takes place between the aggregates of platelets particles [13]. The surface areas were calculated using the Brunauer–Emmett–Teller (BET) method, whereas the pore size distribution was determined by the Barrett– Joyner–Halenda (BJH) method using the desorption branch of the isotherm. The mean pore size of the samples calculated from the BJH method indicates that the data can be fitted very well to the size range of layered materials. The pore size distribution of the Zn/Al-LDH before the adsorption was much wider than that of the sample after In3 þ adsorption. This might be because the sample before adsorption has many small and large mesopores. However, these larges pores disappeared and the pores were highly uniform after In3 þ adsorption. Table 1 lists the surface area, mean pore diameter and the total pore volume of the Zn/Al-LDH, In–Zn/AlLDH-1, In–Zn/Al-LDH-5, and In–Zn/Al-LDH-25. The BET surface area and the pore volume of Zn/Al-LDH decreased slightly after In3 þ adsorption. This might be due to the increased interaction between the layers of Zn/Al-LDH due to the extra interactions caused by the presence of In3 þ and nitrate ions dispersed between the layer spaces [20]. 3.6. Adsorption of In3 þ from aqueous solution Adsorption processes are normally considered intermolecular interactions among solute and solid phases. An interaction with the surface anions or complexation, chelation, precipitation and isomorphic substitutions are the most suggested mechanisms for the adsorption of metal ions over the LDH surface [21–22]. All the mechanisms depend greatly on the pH, temperature, ionic strength, and adsorbent dosage. Therefore, it is important to determine the nature and mechanism of adsorption and the stability of the adsorbent during the adsorption process. A previous study showed that the solution pH has an adverse effect on the removal of metal ions from the brucite structure of LDH materials [23–24]. If metals are released in the solution during the adsorption of cations, it will pollute the water further. Therefore, it is essential to control the release of the metal ions from the LDH materials and maintain it at a level that is not harmful to health. The World Health Organization (WHO) stated a legal limit of Zn2 þ in drinking water of 5 mg L 1 [24]. Therefore, this study examined the use of Zn/Al-LDH for the removal of In3 þ ions from contaminated water under better conditions. Initially, the adsorption of In3 þ ions from 1 to 5 mM solutions
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were examined under a range of pH conditions using pristine Zn/ Al-LDH. No Zn2 þ release was observed in a 1 mM In3 þ solution at pH 5 and 6, but the negligible amounts of 0.72 and 0.21 mg L 1 g 1 were found out at pH 3 and 4, respectively. Interestingly, the material adsorbed the In3 þ completely from the solution over the entire pH range. But the adsorption in a 5 mM In3 þ solution exhibited a Zn2 þ release of 5.2, 3.1, 0.08, and 0.01 mg L 1 g 1 at pH 3, 4, 5, and 6, respectively. In addition, the Zn/Al-LDH material exhibited almost negligible Zn2 þ release at pH 5 and 6, which indicates the stability of the material. Furthermore, the adsorption of In3 þ from 1, 5 and 25 mM solutions was carried out to determine the optimal concentration of the In3 þ solution at pH 5 using Zn/Al-LDH to avoid the release of Zn2 þ ions. The In3 þ adsorption efficiencies were 1, 1.7 and 16 mmol, respectively, from the 1, 5 and 25 mM initial concentrations. To check the stability of the structure of the adsorbent after adsorption, the samples were analysed by XRD and the results are displayed in Fig. 2. Interestingly, the XRD patterns of In–Zn/Al-LDH-1, In–Zn/Al-LDH-5 and In–Zn/Al-LDH-25 showed additional peaks corresponding to In3 þ species at 22°, 39° and 51° 2θ. The intensities of the corresponding reflections of In3 þ species increased with increasing initial concentration and the subsequent adsorbed amount. The amount of In3 þ ions adsorbed per gram of the calcined Zn/Al-LDH showed a linear relationship with the concentration or ionic strength of the In3 þ solution. The drastic increase in In3 þ adsorption when using 25 mM as the initial concentration was only responsible for the enhanced surface precipitation of In(OH)3 and the isomorphic substitution of Zn2 þ by In3 þ . ICP-OES confirmed the extensive removal of Zn2 þ (32 mg L 1 g 1) under these adsorption conditions. Interestingly, the filtrate after adsorption from a 5 mM solution did not exhibited any Zn2 þ release at pH 5. In Zn/Al-LDH, the Zn2 þ ions occupy the octahedral sites exclusively when they are coprecipitated with trivalent Al3 þ . Therefore, the Zn(OH) 2 dissolution has a linear relationship with the ionic strength of the medium [23]. The amount of Zn ion released from the Zn/Al-LDH decreased with increasing initial pH. This revealed the higher stability of the brucite-like sheets at higher pH, which is expected, because these materials are hydroxides. Interestingly, 5 mM is considered the suitable concentration for In3 þ adsorption without releasing Zn2 þ beyond the harmful limit. By considering both conditions, In3 þ adsorption from water was studied with respect to the following: (a) pH of the medium, (b) time, (c) initial indium solution concentration, and (d) adsorbent dosage. The adsorption of metal ions by solid adsorbents from aqueous solutions depends greatly on the pH of the medium. In addition, it is considered to be an important parameter in the physicochemical reaction at the water–solid interface. In this study, 3, 4, 5, and 6 were selected as the solution pH because at higher pH, In3 þ will precipitate as In(OH)3. The results of the adsorption at various pH are shown in Fig. 7a. The In3 þ adsorbed at pH 3, 4, 5, and 6 are 1.5, 1.65, 1.7, and 1.8 mmol g 1 respectively. The level of In3 þ adsorption increases with increasing solution pH. Indium metal dissolves in mineral acids and generates In3 þ ions [24]. The In3 þ ions in an aqueous solution form the hexa-aquo complex, In (H2 O)63 + , which is hydrolysed to In(H2O)5(OH)2 þ and In(H2O)4 (OH)+2 ions before the electrontransfer process. It was reported that when the concentration of indium was less than 0.001 M, InOH2 þ and In (OH)+2 ions predominated among the hydrolysis products. When concentration more than 0.001 M, polynuclear ions predominates [25–26]. In this study we have considered 1, 5 and 25 mM In3 þ solutions, as a reason InOH2 þ and In (OH)+2 are not important. However, at lower pH, the concentration of H þ ions in the solution was very high and the higher mobility of the smaller sized H þ ions retarded the adsorption of In3 þ by the active sites in the Zn/Al-LDH. Owing to the protonation of the active site, the number of neutralised active
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Fig. 7. (a) Effect of pH on the adsorption of indium ions [Conc. ¼5 mM, adsorbate volume ¼ 20 ml, adsorbent dosage¼20 mg], (b) effect of the contact time on the adsorption of indium ions [Conc. ¼ 5 mM, adsorbate volume¼ 20 ml, adsorbent dosage¼ 20 mg, pH ¼5], (c) effect of the initial metal ion concentration on the adsorption efficiency of indium ions [adsorbate volume¼ 20 ml, adsorbent dosage¼ 20 mg, pH ¼5], and (d) effect of the adsorbent dosage on the adsorption efficiency of indium ions [Conc. ¼5 mM, adsorbate volume¼20 ml, pH¼ 5].
sites increased in the strong acidic medium. As a result, a repulsion was exerted between the indium ionic species (binuclear or polynuclear) and the active site; however, at higher pH, there was a decrease in H þ ions in the solution, and both the competition for binding sites and the electrostatic repulsion decreased; hence, metal ion adsorption is favourable. The adsorption of In3 þ as a function of the contact time was conducted at room temperature at pH 5. The curve (Fig. 7b) showed that the amount of In3 þ removal increased gradually with increasing adsorption time, and the adsorption equilibrium was not reached, even after 24 h of adsorption time. Zn/Al-LDH was found to be quite efficient in the removal of In3 þ ions from aqueous solution and the removal depends greatly on the contact time between the solid adsorbent and the metal ion species at its interface. A gradual In3 þ adsorption rate was observed throughout the adsorption process. After 6 h, a decrease in the adsorption rate was observed due to the repulsive interaction between the adsorbed indium species on the interlayer surface of the Zn/Al-LDH
with the predominant ionic species present in the solution. Moreover, the concentration difference generated during the late hours of adsorption between the adsorbent surfaces with the solution further reduced the rate of adsorption. In particular, the kinetics of adsorption was studied at pH 5, which favoured the increased efficiency of adsorption and dominated over the two factors described above. As a result, Zn/Al-LDH exhibited exceptional adsorption capacity towards In3 þ ions [24]. Fig. 7c shows the effect of the initial concentration of indium ions on the percent removal of Zn/Al-LDH at pH 5. The initial concentration of In3 þ was varied in the range of 1–5 mM. An increase in the initial concentration of In3 þ from 1 to 5 mM caused a corresponding decrease in the percent removal from 100% to 31%. The equilibrium adsorption amount of In3 þ ions from 1, 2, 3, 4 and 5 mM metal ion solutions are, however, 144.8, 199.7, 182.5, 169.9 and 172.2 mg g 1, respectively. The maximum amount of In3 þ adsorption of 199.7 mg g 1 was achieved from a metal ion concentration of 2 mM at pH 5. Also an increase in initial metal ion
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Fig. 8. Competitive adsorption of metal ions from a mixture of (a) Mn2 þ , Co2 þ , Ni2 þ , and Cd2 þ (b) Mn2 þ , Co2 þ , Ni2 þ , Cd2 þ , and Pb2 þ , (c) Mn2 þ , Co2 þ , Ni2 þ , Cd2 þ , and Cu2 þ , and (d) Mn2 þ , Co2 þ , Ni2 þ , Cd2 þ , Pb2 þ , and Cu2 þ . [Conc. of individual metal ions ¼1 mM, adsorbate volume¼ 20 ml, adsorbent dosage¼ 10 mg, pH ¼5].
concentration beyond 2 mM was not influencing considerably in the equilibrium adsorption amount of In3 þ . A high initial concentration means a better concentration gradient, which is an important driving force that will help overcome the mass transfer resistance of In3 þ between the liquid and solid phases [27,30]. Fig. 7d shows the effects of the adsorbent dose on the adsorption efficiency of In3 þ onto Zn/Al-LDH at pH 5. 0.01, 0.02, 0.03, and 0.04 g of adsorbent were selected for the uptake of In3 þ from a 5 mM indium nitrate solution. The percentage removal increased from 22% to 60% as the adsorbent dosage was increased from 0.01 to 0.04 g. The equilibrium adsorption amount of In3 þ ions over the adsorbent dosages of 0.01, 0.02, 0.03, and 0.04 g are 120, 212, 269, and 344 mg L 1, respectively. A higher adsorbent dosage indicates more binding sites available for the uptake of In3 þ onto the Zn/AlLDH surface, which results in high removal percentages. The maximum removal of 60% was achieved at dosage of 0.04 g Zn/AlLDH [23–27]. Fig. 8 presents the weight and mole selectivity of (a) Mn2 þ , Co2 þ , Ni2 þ , and Cd2 þ , (b) Mn2 þ , Co2 þ , Ni2 þ , Cd2 þ , and Pb2 þ , (c) Mn2 þ , Co2 þ , Ni2 þ , Cd2 þ , and Cu2 þ , and (d) Mn2 þ , Co2 þ , Ni2 þ , Cd2 þ , Pb2 þ , and Cu2 þ . Fig. 8a shows that Zn/Al-LDH has no affinity towards Mn2 þ , Co2 þ , Ni2 þ , and Cd2 þ . Instead, the material selectively adsorbs Pb2 þ (Fig. 8b) or Cu2 þ (Fig. 8c) from a mixture
containing either of these ions in the first metal ion solution. When Pb2 þ and Cu2 þ were introduced to the first solution (Fig. 8d), Zn/Al-LDH adsorbed both metal ions with a preparation to Cu2 þ more than Pb2 þ ions. From this experiment, it was concluded that Zn/Al-LDH has greater affinity towards Pb2 þ and Cu2 þ and the adsorption mechanism shows electrostatic attraction followed by precipitation. Finally, the selectivity of In3 þ adsorption from a mixture of selected metal ions was examined to ensure the affinity of Zn/AlLDH towards In3 þ ions. Fig. 9 shows the weight and mole selectivity of (a) In3 þ in presence of Mn2 þ , Co2 þ , Ni2 þ , and Cd2 þ , (b) In3 þ in presence of Mn2 þ , Co2 þ , Ni2 þ , Cd2 þ , and Pb2 þ , (c) In3 þ in presence of Mn2 þ , Co2 þ , Ni2 þ , Cd2 þ , and Cu2 þ , and (d) In3 þ in presence of Mn2 þ , Co2 þ , Ni2 þ , Cd2 þ , Pb2 þ , and Cu2 þ . Metal mixture solutions containing nitrate salts of Mn2 þ , Co2 þ , Ni2 þ , Cd2 þ , In3 þ , Pb2 þ and Cu2 þ with 1 mM concentration of each metal ion in water were prepared. 20 ml of the above solution was mixed with 10 mg of the Zn/Al-LDH and shaken for 12 h to achieve the adsorption equilibrium. Fig. 9 shows the adsorption selectivity of the Mn2 þ , Co2 þ , Ni2 þ , Cd2 þ , In3 þ , Pb2 þ , and Cu2 þ over Zn/AlLDH. Fig. 9a presents the competitive adsorption from a mixture of Mn2 þ , Co2 þ , Ni2 þ , Cd2 þ , and In3 þ . This shows that Mn2 þ , Co2 þ , Ni2 þ , and Cd2 þ have no affinity towards the Zn/Al-LDH surface at
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Fig. 9. Competitive adsorption of metal ions from a mixture of (a) Mn2 þ , Co2 þ , Ni2 þ , Cd2 þ , and In3 þ , (b) Mn2 þ , Co2 þ , Ni2 þ , Cd2 þ , In3 þ , and Pb2 þ , (c) Mn2 þ , Co2 þ , Ni2 þ , Cd2 þ , In3 þ , and Cu2 þ , and (d) Mn2 þ , Co2 þ , Ni2 þ , Cd2 þ , In3 þ , Pb2 þ , and Cu2 þ . [Conc. of individual metal ions ¼1 mM, adsorbate volume¼20 ml, adsorbent dosage¼ 10 mg, pH¼ 5].
pH 5. As a result, In3 þ exhibited 100% mole and weight selectivity from the above mixture. When adding Pb2 þ to the above metal mixture (Fig. 9b), the mole and weight selectivity of In3 þ decreased considerably due to the major adsorption of Pb2 þ and minor or negligible adsorption of Co2 þ , Ni2 þ and Cd2 þ ions over the Zn/Al-LDH surface. The same trend was observed for competitive adsorption from a mixture of Mn2 þ , Co2 þ , Ni2 þ , Cd2 þ , In3 þ , and Cu2 þ under the same experimental conditions (Fig. 9c). From these experiments, it was concluded that Pb2 þ and Cu2 þ have great affinity towards the Zn/Al-LDH surface [23–28]. In addition, Co2 þ , Ni2 þ and Cd2 þ ions were adsorbed only in presence of Pb2 þ or Cu2 þ ions in the mixture. Interestingly, Mn2 þ did not exhibit any affinity towards the Zn/Al-LDH surface. Furthermore, the competitive adsorption of Mn2 þ , Co2 þ , Ni2 þ , Cd2 þ , In3 þ , Pb2 þ , and Cu2 þ ions in the same mixture was observed (Fig. 9d). The result highlighted the superior affinity of In3 þ over Pb2 þ and Cu2 þ , and sharing half of the active surface of the Zn/Al-LDH along with other metal ions in the mixture. Moreover, Mn2 þ , Co2 þ , Ni2 þ , and Cd2 þ showed no affinity towards the active adsorbent surface in the combined presence of highly performing In3 þ , Pb2 þ and Cu2 þ ions.
3.7. Equilibrium study The adsorption isotherm models are normally used to describe the interaction between adsorbents and adsorbates, providing most important parameters for designing a desired adsorption system. Therefore, the In3 þ removal capacity of the pristine Zn/AlLDH was determined, where the Langmuir and Freundlich adsorption models were applied to evaluate the effectiveness of the adsorbents [28]. The Langmuir model is based on the assumption that the maximum adsorption occurs when a saturated monolayer of solute molecules is present on the adsorbent surface, the energy of adsorption is constant and there is no migration of adsorbate molecules in the surface plane. Fig. 10a shows the experimental result of the Langmuir isotherm. According to the Langmuir isotherm,
qe =
qm KL Ce 1 + KL Ce
The constants in the Langmuir isotherm can be determined by plotting Ce versus Ce/qe and making use of the above rewritten as:
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141
Table 2 Parameters for Langmuir and Freundlich isotherm models. Langmuir
Freundlich
qm (mg g 1)
KL (L mg 1)
R2
Kf (mg1 n Ln g 1)
n
R2
194
0.0476
0.952
0.034
4
0.909
Table 3 In3 þ adsorption capacities (qmax) results from the literature. Material
In3 þ (mg g 1)
pH
Reference
Ion-electrode S. algae Chitosan-coated bentonite Ion-exchangeable nano-beads Electro-coagulation Pristine Zn/Al-LDH
78.3 107 96 89 57 205
2.4 3.8 6 8 2.3 6
[6] [8] [12] [21] [32] Our work
model to fit the experimental data was examined. In this case, a plot of ln Ce vs. ln qe was used to generate the intercept of Kf and the slope of n. Table 2 lists the values of the parameters for the Langmuir and Freundlich isotherm models. Fig. 10a shows that the isotherm data fits the Langmuir equation well (R2 ¼0.952). The values of qm and KL were 194 mg g 1 and 0.0476 L mg 1, respectively at pH 5 [29]. The Freundlich constants Kf and n were 0.034 mg g 1 and 4, respectively (Fig. 10b). The R2 (0.909) value of Freundlich isotherm was found to be lower than the same of Langmuir isotherm. The magnitudes of Kf and n show the easy separation of indium ions from the aqueous solution and indicate favourable adsorption. The intercept Kf is an indication of the adsorption capacity of the adsorbent; the slope 1/n indicates the effect of concentration on the adsorption capacity and represents the adsorption intensity. As shown above, the n value was found to be high enough for separation. In addition, Zn/Al-LDH gives the adsorption energy for adsorbing In3 þ . The Langmuir isotherm fitted very well with a high correlation coefficient (R2 ¼0.952), showing that the adsorption of In3 þ followed the Langmuir model [30,31]. Fig. 10. Plots of (a) Langmuir and (b) Freundlich isotherm models.
Ce 1 C = + e qe KL qm qm where qm and KL are the Langmuir constants, representing the maximum adsorption capacity for the solid phase loading and the energy constant related to the heat of adsorption, respectively. The Freundlich isotherm model is an empirical relationship describing the adsorption of solutes from a liquid to a solid surface and assumes that different sites with several adsorption energies are involved. The Freundlich adsorption isotherm is the relationship between the amounts of In3 þ adsorbed per unit mass of adsorbent, qe, and the concentration of the In3 þ at equilibrium, Ce, as shown in Fig. 10b.
qe = Kf Ce1/ n
4. Conclusion Pristine Zn/Al-LDH was found to be highly active and selective towards In3 þ ions. The XRD pattern of Zn/Al-LDH exhibited additional crystalline phases after In3 þ adsorption. The adsorption ability increased with increasing In3 þ solution concentration, which was found to be responsible for the reduction in porosity and surface area. The material exhibited a maximum In3 þ intake of 205 mg g 1 at pH 6, which is high adsorption ability compared to the same reported in the literature (Table 3). The adsorption from a mixture of Mn2 þ , Co2 þ , Ni2 þ , Cd2 þ , and In3 þ exhibited 100% adsorption selectivity for In3 þ ions and maintained superior affinity of In3 þ over the Cu2 þ and Pb2 þ from the other mixtures. Overall, pristine Zn/Al-LDH is a promising material for future applications.
The logarithmic form of the equation becomes
ln qe = ln Kf +
1 ln Ce n
where Kf and n are the Freundlich constants, the characteristics of the system. Kf and n are the indicators of the adsorption capacity and adsorption intensity, respectively. The ability of the Freundlich
Acknowledgements The work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Ministry of Science, ICT, and Future Planning, Korea {Pioneer Research Center Program (NRF-2010-
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0019308/2010-0019482); Acceleration Research Program (NRF-2014 R1A2A111 054584); Brain Korea 21 Plus Program (21A2013800002)}.
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Contents lists available at ScienceDirect
Journal of Solid State Chemistry journal homepage: www.elsevier.com/locate/jssc
Highly efficient and selective adsorption of In3 þ on pristine Zn/Al layered double hydroxide (Zn/Al-LDH) from aqueous solutions Mary Jenisha Barnabas a, Surendran Parambadath a, Aneesh Mathew a, Sung Soo Park a, Ajayan Vinu b, Chang-Sik Ha a,n a
Department of Polymer Science and Engineering, Pusan National University, Geumjeong-gu, Busan 46241, Republic of Korea Australian Institute for Bioengineering and Nanotechnology, The University of Queensland, #75 Corner College and Cooper Road, Brisbane 4072, QLD, Australia
b
art ic l e i nf o
a b s t r a c t
Article history: Received 2 June 2015 Received in revised form 21 September 2015 Accepted 1 October 2015
A pristine Zn/Al-layered double hydroxide (Zn/Al-LDH) showed excellent adsorption ability and selectivity towards In3 þ ions from aqueous solutions. The adsorption behaviour as a function of the contact time, solution pH, ionic strength, and amount of adsorbent under ambient conditions revealed a strong dependency on the pH and ionic strength over In3 þ intake. The structure and properties of Zn/Al-LDH and In3 þ adsorbed Zn/Al-LDH (In–Zn/Al-LDH) were examined carefully by X-ray diffraction, Fourier transform infrared spectroscopy, N2-sorption/desorption, UV–vis spectroscopy, and X-ray photoelectron spectroscopy. The adsorbent had a sufficient number of active sites that were responsible for the In3 þ adsorption and quite stable even after the adsorption process. The selective adsorption of In3 þ on Zn/AlLDH was also observed even from a mixture containing competing ions, such as Mn2 þ , Co2 þ , Ni2 þ , Cd2 þ , Pb2 þ , and Cu2 þ . The adsorption experiments showed that Zn/Al-LDH is a promising material for the preconcentration and selective removal of In3 þ from large volumes of aqueous solutions. & 2015 Elsevier Inc. All rights reserved.
Keywords: Zn/Al-LDH Indium Metal adsorption Separation and pre-concentration
1. Introduction Layered double hydroxides (LDHs), also known as hydrotalcitelike compounds or anionic clays, are bidimensional solids with a positive charge excess in their brucite like layers that have attracted considerable attention in recent years owing to their excellent properties, such as large interlayer surface with a high specific surface area and large pore volume and recycling capability [1]. The structure of the LDHs consists of positively charged mixed metal hydroxide layers separated by charge-balancing anions and water molecules. LDHs have the structural formula, [ M12−+x M3x + (OH)2](Am )x/m nH2O (M2 þ ¼Mg2 þ , Zn2 þ , Ni2 þ etc., M3 þ ¼Al3 þ , Cr3 þ , Ga3 þ etc.). Am denotes interlayer anions, such 2− as NO3 , SO2− 4 and CO3 , and x typically ranges from 0.17 to 0.33. This charge excess is produced by the isomorphic substitution of a divalent cation with a trivalent cation that is compensated for by the introduction of anions (together with water) in the interlayer space. Because the interlayer anions are easily exchangeable, they can be exchanged with other anions. This offers high anion exchange capability, which is the most remarkable characteristic of LDHs. The ease of preparation and widely variable ways, in which n
Corresponding author. Fax: þ 82 51 513 7720. E-mail address: [email protected] (C.-S. Ha).
http://dx.doi.org/10.1016/j.jssc.2015.10.001 0022-4596/& 2015 Elsevier Inc. All rights reserved.
their components can be combined, make LDHs useful for many applications [2–5]. LDHs can adsorb metal cations from aqueous solution despite having a positive charge on the surface layer. The positive charge of the layer attracts hydroxide ions around the surfaces of the LDH crystals in aqueous solution to induce the formation of metal hydroxides. The charge-compensating carbonate ion attached to the surface and the edge can also come in contact with metal cations to form insoluble metal carbonates. These unique features make LDHs excellent adsorbents for heavy metal cations. On the other hand, only hydrotalcite-like compounds have been used as adsorbents for heavy metals and the use of LDHs for the selective removal of heavy metal ions are quite rare. A range of technologies, i.e., electro-analytical techniques, coprecipitation, nanofiltration membranes, solid phase extraction, supercritical CO2 extraction, biosorption, and extraction resin are employed widely for the recovery of soluble In3 þ ions from waste water [6–8]. These methods, however, suffer from serious drawbacks, such as the incomplete recovery of metal ions, especially from dilute metal solutions, high reagent and/or energy requirements, and generation of toxic sludge or other waste products, and high capital cost. With the increasing environmental awareness and legal constraints being imposed on the discharge of effluents, the development of cost-effective alternative technologies is essential and requires more attention. All indium compounds should
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be regarded as highly toxic as they can damage the vital organs like heart, kidney, and liver and may be teratogenic. Indium and its compounds exhibit semiconducting or optoelectronic characteristics, which are used in the production of liquid crystal displays, semiconductors, low pressure sodium lamps, and infrared photodetectors [9–10]. In is also used widely in thin-films to form lubricated layers as well as for producing low melting point alloys, and it is a component in some lead-free solders. Indium recovery is in strong demand and possible through biosorption system using an environmentally friendly microbial system, such as bacterium S. Algae [11]. The bacterial cells could absorb all the indium from an aqueous solution within 10 min even from solutions containing very low In3 þ concentrations. Chitosan-coated bentonite beads were also found to be good adsorbents for In3 þ from aqueous solutions [12]. These reports surveys highlight the use of chemically active functional groups along with LDH that serve as efficient sites to bind metal ions. Although these materials are highly promising in efficiency and are environmentally benign, they require tedious experimental setups, complicated synthesis process, modification and handling. The use of pristine LDH for selective metal recovery still remains a topic of intense research. This paper reports the adsorption of In3 þ from an aqueous solution of In(NO3)3 onto Zn/Al-LDH. Zn/Al-LDH and In–Zn–AlLDH were characterised by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy and X-ray photoelectron spectroscopy (XPS). N2-adsorption/desorption analysis was employed to determine the surface area, pore diameter and pore volume of the materials. For a potential application, the adsorption behaviour of In3 þ on Zn/Al-LDH was investigated under different conditions, such as the contact time, solution pH, ionic strength, and the amount of adsorbent. In3 þ was selected as a representative of bivalent metal ions because of its ubiquitous presence in wastewater. The competitive adsorption from a mixture of Mn2 þ , Co2 þ , Ni2 þ , Cd2 þ , and In3 þ ions and the effect of the presence of Cu2 þ or Pb2 þ or both in the above mixture on the adsorption of In3 þ over Zn/Al-LDH were have also investigated at a solution pH of 5. To the best of the authors' knowledge, there are no reports on the adsorptive removal of In3 þ from the aqueous solutions using Zn/ Al-LDH.
2. Experimental section 2.1. Materials Aluminium(III) nitrate nonahydrate, zinc(II) nitrate hexahydrate, indium(III) nitrate hydrate, cobalt(II) nitrate hexahydrate, copper(II) nitrate trihydrate, nickel(II) nitrate hexahydrate, manganese(II) nitrate tetrahydrate, cadmium(II) nitrate tetrahydrate, lead(II) nitrate, and NaOH were purchased from Sigma-Aldrich and used as received. The appropriate volume of 0.1 M HNO3 or NaOH was used to adjust the pH of the solution. 2.2. Zn/Al-LDH synthesis Zn/Al-LDH was synthesised using a co-precipitation method [13]. A mixed-metal nitrate solution (0.25 M Zn2 þ and 0.08 M Al3 þ ) was adjusted to the solution pH of 9–10 by adding the required amount of 2 M NaOH. The mixture was heated to 80 °C for 18 h and a white precipitate was formed. Subsequently, it was filtered and rinsed with de-ionised water. After drying at 85 °C, the obtained material was ground gently into a fine powder. 2.3. In3 þ adsorption from aqueous solutions The solution containing In3 þ ions was prepared by dissolving a
known amount of In(NO3)3 H2O in distilled water. The concentration of In3 þ ions ranged from 1 to 5 mM and the adsorption experiments were carried out by suspending 20 mg of Zn/Al-LDH in 20 mL of an aqueous solution of In(NO3)3 H2O at pH 5. The effect of the pH on the metal ion uptake was studied by performing equilibrium sorption experiments by varying the solution pH from 3 to 6. The dissolution of Zn2 þ during adsorption was investigated by varying the concentration of In3 þ ranged from 1 to 25 mM at pH 5. Adjustments in the solution pH were done by adding either 0.1 M NaOH or HNO3. The concentration of In3 þ was analysed by inductively coupled plasma-atomic emission spectrometry (ICP-OES). The amounts of adsorbed In3 þ were calculated from the difference between the initial (C0) and equilibrium (Ce) concentrations in the supernatant after centrifugation. The adsorption percentage [adsorption (%) ¼(C0 Ce)/C0 100] was derived from the difference between C0 and Ce [13]. 2.4. In3 þ adsorption from mixtures The competitive metal adsorption was assessed using a solution containing 1 mM of Mn2 þ , Co2 þ , Ni2 þ , Cd2 þ , In3 þ , Pb2 þ , and Cu2 þ nitrate salts in distilled water. The pH of the solution was adjusted to 5 using a 0.1 M NaOH solution. The effect of the presence of Cu2 þ or Pb2 þ or both in a mixture on the adsorption of In3 þ over Zn/Al-LDH was also investigated by using the same solution after making the necessary variations in combinations. The adsorption experiments were carried out by suspending 10 mg of Zn/Al-LDH in 20 mL of an aqueous mixture and shaken for 12 h. The concentration of each metal ion before and after adsorption was analysed by inductively coupled plasma-optical emission spectroscopy (ICP-OES). 2.5. Characterisation of samples XRD was performed on a Bruker AXN using Cu-Kα radiation. The XRD patterns were collected in the low-angle range from 1.2° to 10° 2θ. The N2 adsorption–desorption isotherms were measured using a Nova 4000e surface area and pore size analyser. The samples were degassed at 120 °C for 12 h before the measurements. The Brunauer–Emmet–Teller (BET) method was used to calculate the specific surface area. The pore size distribution curve was obtained from an analysis of the adsorption branch using the Barrett–Joyner–Halenda (BJH) method. The FT-IR spectra were obtained on a JASCO FTIR 4100 using the KBr pelleting method over the frequency range, 4000–400 cm 1. To examine the surface elements of the materials in their existing state, XPS was carried out on a PHI-1600 ESCA System XP spectrometer (Perkin-Elmer, USA) using non-monochromatic Mg Kα radiation operated at 15 kV, under 10 7 Pa pressure, and with photoelectron energy set at 1254 eV.
3. Results and discussion 3.1. SEM and TEM images Fig. 1 presents the representative (a) SEM and (b) TEM images of pristine-Zn/Al-LDH. Both SEM and TEM images provide excellent evidence of the existences of sheet-like morphology for the material with a mean particle size of 130 nm. A small amount of particle aggregation was also observed from the SEM images of pristine-Zn/Al-LDH. The SEM and TEM images provided the existence of good quality material. This is in good agreement with previous reports [14,15].
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135
Fig. 1. (a) SEM and (b) TEM images of pristine Zn/Al-LDH.
LDH-25, respectively. The decrease in the intensities of (003) and (006) reflections for In–Zn/Al-LDH-m might be due to the screening of adsorbed In3 þ species along with a decrease in the interlayer distance between the adjacent layers while maintaining strong binding with the nitrate ions on the surface. Also a shift to the (003) peak from 2θ 11.52° (Fig. 2a) to the higher 2θ 11.58° (Fig. 2d) was observed. These results confirm not only the adsorption of In3 þ ions, but also the excellent purity of the samples after the adsorption process. The narrow interlayer distance is suitable for the adsorption of metal ions. In this case, however, the intensity of the XRD peaks decreased but the layered crystal structure of the Zn/Al-LDH was unaffected. The decrease in the intensity of the XRD peak might also be due to the decrease in the overall percentage of zinc oxide or aluminium oxide in the material after adding In3 þ to its surface [15–16]. The observation and inference of the progressive In3 þ adsorption from the XRD patterns of In–Zn/Al-LDH-1, In–Zn/Al-LDH-5 and In–Zn/Al-LDH-25 are described in the In3 þ adsorption section. Fig. 2. XRD patterns of the (a) Zn/Al-LDH, (b) In–Zn/Al-LDH-1, (c) In–Zn/Al-LDH-5, and (d) In–Zn/Al-LDH-25. nIndium species.
3.2. Powder X-ray Diffraction (XRD) analysis Fig. 2 presents the XRD patterns of (a) Zn/Al-LDH and (b–d) In– Zn/Al-LDH-m, where ‘m’ is represents the initial molar concentration (m¼ 1, 5 or 25) of In(NO3)3 solution in mM. Fig. 2a revealed a series of sharp and intense peaks at both low and high 2θ values, indicating the characteristic basal reflections of Zn/Al-LDH. The less intense but sharp peaks corresponding to the (003) and (006) faces at 11° and 23° 2θ values indicate the low degree of crystallinity of the LDH samples after adsorption [14]. The narrow and sharp peaks were observed between 30° and 70° 2θ, which provides evidence of the crystalline nature of Zn/Al-LDH. The reflections corresponding to (012), (015), (018), and (110) planes show that the samples exhibit hexagonal lattice with rhombohedral 3R symmetry. The purity and chemical composition of the pristine-Zn/Al-LDH was confirmed by the elemental analysis, which was found to be Zn0.74Al0.23(OH)2 (NO3)0.23 0.45H2O with a Zn2 þ /Al3 þ ratio of 3.2. The XRD pattern after In3 þ adsorption from a 1 mM solution (Fig. 2b) showed that the intensity of all the peaks was significantly lower than that of the Zn/Al-LDH sample before adsorption. Although the intensity of the sample was decreased after In3 þ adsorption, it still retained the layered structure. The 2θ value of d003 reflection before In3 þ adsorption was found to be 11.52°. After In3 þ adsorption the d003 reflection was shifted to 11.57°, 11.58°, and 11.58° for Zn/Al-LDH-1, Zn/Al-LDH-5 and Zn/Al-
3.3. XPS analysis XPS was used to further study the surface chemical composition of the Zn/Al-LDH and In–Zn/Al-LDH-5 assembly (Fig. 3). The binding energy (BE) calibration for both spectra was referenced to the C 1s signal at 284.6 eV. The analysis disclosed the binding
Fig. 3. XP spectra of (a) Zn/Al-LDH and (b) In–Zn/Al-LDH-5.
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states of the major elements present in the material. The representative intensity and area of the peaks in the XP spectra of Zn/Al-LDH and In–Zn/Al-LDH-5 offered insight into the successful formation and abundance of the elements within the material. Pure Zn/Al-LDH shows (Fig. 3a) the characteristic binding energies of 1195.37, 1044.51, 1021.52, 497.29, 138, and 74.3 eV for Zn 2s, Zn 2p1/2, Zn 2p3/2, O1s, Zn 3p, and Al 2p, respectively [17]. After In3 þ adsorption the material showed (Fig. 3b) the characteristic binding energies of In3 þ along with the binding energies already presented in the pure Zn/Al-LDH. These are 1195.37, 1044.51, 1021.52, 497.29,138, 74.3, 452.75, and 444.20 eV for Zn 2s, Zn 2p1/2, Zn 2p3/2, O1s, Zn 3p, Al 2p, In 3d3/2, and In 3d5/2, respectively. The In3 þ adsorption did not alter the position of the binding energy values of the fundamental elements in the material, indicating the stability of the Zn/Al-LDH crystal lattice. The Al 2p BE at 74.3 eV was assigned to the binding of Al–OH. The presence of two binding energies values of 452.75 and 444.20 eV corresponding to In 3d3/2 and In 3d5/2 , respectively in the XP spectrum of In–Zn/Al-LDH-5 (Inset in Fig. 3) provided the evidence for the adsorption of In3 þ over Zn/Al-LDH [18]. 3.4. FT-IR analysis Fig. 4 the FT-IR spectra of Zn/Al-LDH and In–Zn/Al-LDH-m samples. The broad and intense band around 3500 cm 1 was attributed to the stretching vibration of the hydroxyl groups and water molecules in the layered structure, whereas a weak shoulder peak recorded at 3250 cm 1 was assigned to the O–H–O stretching mode of the interlayer water molecules, and hydrogen-bonded to interlayer carbonate anions. The bending mode band of the water molecules is normally observed close to 1600 cm 1, and is recorded as a weak shoulder at 1640 cm 1 for all materials [19]. The band at 430 cm 1 was assigned to the OH–Al–Zn–OH deformation mode, whereas the band at 955 and 1480 cm 1 corresponds to deformation mode of AlOH and ZnOH, respectively. Two bands at approximately 551 and 766 cm 1 were attributed to the translation mode of AlOH. The interlayer charge density and the coulomb repulsion between interlayer anions increases with decreasing Zn/Al ratio, which results in a stronger peak of inter1 . The intensity of layer CO2− 3 and a lower wave number 1357 cm 3þ this peak is decrease the In . A sharp and intense band was noted at 1380 cm 1, which was assigned to the vibration of the NO−3 ions. These bands confirmed the formation of the characteristic LDH
Fig. 4. FTIR spectra of (a) Zn/Al-LDH, (b) In–Zn/Al-LDH-1, (c) In–Zn/Al-LDH-5, and (d) In–Zn/Al-LDH-25.
Fig. 5. N2 adsorption–desorption isotherms of (a) Zn/Al-LDH, (b) In–Zn/Al-LDH-1, (c) In–Zn/Al-LDH-5, and (d) In-Z/Al-LDH-25.
network. The FT-IR spectra of In–Zn/Al-LDH-1, In–Zn/Al-LDH-5 and In–Zn/Al-LDH-25 exhibit similar bands to those of Zn/Al-LDH; however, a slight decrease in the intensity and broadness of the bands between 1250 and 1520 cm 1 was observed. The characteristic vibrations of indium metal ions were also observed in between 1350 and 1650 cm 1, which is overlapped with the other vibration bands existing in the material. The band at 1380 cm 1 was also observed and shifted to a higher wave number comparing to that of Zn/Al-LDH, showing the presence of NO−3 ions in the interlayer space. The absorption bands below 1000 cm 1 are responsible for the M–O vibrations and the band at 400–500 cm 1 was assigned to the O–M–O vibration related to LDHs layers [13– 20]. 3.5. N2-adsorption/desorption study Figs. 5 and 6 show the typical nitrogen adsorption–desorption isotherms and their corresponding pore size distributions (PD) of the adsorbents. Table 1 lists the specific surface area, pore volume and average pore size. The N2 adsorption and desorption isotherms of the synthesised Zn/Al-LDH before and after In adsorption, exhibited a type IV adsorption isotherms according to the
Fig. 6. Pore size distribution of (a) Zn/Al-LDH, (b) In–Zn/Al-LDH-1, (c) In–Zn/AlLDH-5, and (d) In–Zn/Al-LDH-25.
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Table 1 Physico-chemical properties from N2-Sorption analysis of Zn/Al-LDH materials before and after In3 þ adsorption. Material
Specific surface area (m2 g 1)
Zn/Al-LDH 62.5 In–Zn/Al-LDH-1 51.2 In–Zn/Al-LDH-5 38.7 In–Zn/Al-LDH16.6 25
Pore diameter (nm)
Pore volume (cm3 g 1)
19.2 17.8 17.6 13.1
0.22 0.19 0.15 0.11
IUPAC classification with a H1-type hysteresis loop, which is caused by capillary condensation and is the characteristic feature of layered materials. On the other hand, the isotherm of In–Zn/AlLDH-5 and In–Zn/Al-LDH-25 showed a H3-type hysteresis loop, which was attributed to the presence of slit type pores that are coming from the plate-like particles. The multilayer formation and capillary condensation began at a lower P/P0 value for Zn/Al-LDH but they were shifted to a higher P/P0 for In–Zn/Al-LDH-1, In–Zn/ Al-LDH-5 and In–Zn/Al-LDH-25. The adsorption isotherms of the samples showed neither a plateau nor limiting uptake at high P/P0 values. These results confirmed that the multilayer adsorption that takes place between the aggregates of platelets particles [13]. The surface areas were calculated using the Brunauer–Emmett–Teller (BET) method, whereas the pore size distribution was determined by the Barrett– Joyner–Halenda (BJH) method using the desorption branch of the isotherm. The mean pore size of the samples calculated from the BJH method indicates that the data can be fitted very well to the size range of layered materials. The pore size distribution of the Zn/Al-LDH before the adsorption was much wider than that of the sample after In3 þ adsorption. This might be because the sample before adsorption has many small and large mesopores. However, these larges pores disappeared and the pores were highly uniform after In3 þ adsorption. Table 1 lists the surface area, mean pore diameter and the total pore volume of the Zn/Al-LDH, In–Zn/AlLDH-1, In–Zn/Al-LDH-5, and In–Zn/Al-LDH-25. The BET surface area and the pore volume of Zn/Al-LDH decreased slightly after In3 þ adsorption. This might be due to the increased interaction between the layers of Zn/Al-LDH due to the extra interactions caused by the presence of In3 þ and nitrate ions dispersed between the layer spaces [20]. 3.6. Adsorption of In3 þ from aqueous solution Adsorption processes are normally considered intermolecular interactions among solute and solid phases. An interaction with the surface anions or complexation, chelation, precipitation and isomorphic substitutions are the most suggested mechanisms for the adsorption of metal ions over the LDH surface [21–22]. All the mechanisms depend greatly on the pH, temperature, ionic strength, and adsorbent dosage. Therefore, it is important to determine the nature and mechanism of adsorption and the stability of the adsorbent during the adsorption process. A previous study showed that the solution pH has an adverse effect on the removal of metal ions from the brucite structure of LDH materials [23–24]. If metals are released in the solution during the adsorption of cations, it will pollute the water further. Therefore, it is essential to control the release of the metal ions from the LDH materials and maintain it at a level that is not harmful to health. The World Health Organization (WHO) stated a legal limit of Zn2 þ in drinking water of 5 mg L 1 [24]. Therefore, this study examined the use of Zn/Al-LDH for the removal of In3 þ ions from contaminated water under better conditions. Initially, the adsorption of In3 þ ions from 1 to 5 mM solutions
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were examined under a range of pH conditions using pristine Zn/ Al-LDH. No Zn2 þ release was observed in a 1 mM In3 þ solution at pH 5 and 6, but the negligible amounts of 0.72 and 0.21 mg L 1 g 1 were found out at pH 3 and 4, respectively. Interestingly, the material adsorbed the In3 þ completely from the solution over the entire pH range. But the adsorption in a 5 mM In3 þ solution exhibited a Zn2 þ release of 5.2, 3.1, 0.08, and 0.01 mg L 1 g 1 at pH 3, 4, 5, and 6, respectively. In addition, the Zn/Al-LDH material exhibited almost negligible Zn2 þ release at pH 5 and 6, which indicates the stability of the material. Furthermore, the adsorption of In3 þ from 1, 5 and 25 mM solutions was carried out to determine the optimal concentration of the In3 þ solution at pH 5 using Zn/Al-LDH to avoid the release of Zn2 þ ions. The In3 þ adsorption efficiencies were 1, 1.7 and 16 mmol, respectively, from the 1, 5 and 25 mM initial concentrations. To check the stability of the structure of the adsorbent after adsorption, the samples were analysed by XRD and the results are displayed in Fig. 2. Interestingly, the XRD patterns of In–Zn/Al-LDH-1, In–Zn/Al-LDH-5 and In–Zn/Al-LDH-25 showed additional peaks corresponding to In3 þ species at 22°, 39° and 51° 2θ. The intensities of the corresponding reflections of In3 þ species increased with increasing initial concentration and the subsequent adsorbed amount. The amount of In3 þ ions adsorbed per gram of the calcined Zn/Al-LDH showed a linear relationship with the concentration or ionic strength of the In3 þ solution. The drastic increase in In3 þ adsorption when using 25 mM as the initial concentration was only responsible for the enhanced surface precipitation of In(OH)3 and the isomorphic substitution of Zn2 þ by In3 þ . ICP-OES confirmed the extensive removal of Zn2 þ (32 mg L 1 g 1) under these adsorption conditions. Interestingly, the filtrate after adsorption from a 5 mM solution did not exhibited any Zn2 þ release at pH 5. In Zn/Al-LDH, the Zn2 þ ions occupy the octahedral sites exclusively when they are coprecipitated with trivalent Al3 þ . Therefore, the Zn(OH) 2 dissolution has a linear relationship with the ionic strength of the medium [23]. The amount of Zn ion released from the Zn/Al-LDH decreased with increasing initial pH. This revealed the higher stability of the brucite-like sheets at higher pH, which is expected, because these materials are hydroxides. Interestingly, 5 mM is considered the suitable concentration for In3 þ adsorption without releasing Zn2 þ beyond the harmful limit. By considering both conditions, In3 þ adsorption from water was studied with respect to the following: (a) pH of the medium, (b) time, (c) initial indium solution concentration, and (d) adsorbent dosage. The adsorption of metal ions by solid adsorbents from aqueous solutions depends greatly on the pH of the medium. In addition, it is considered to be an important parameter in the physicochemical reaction at the water–solid interface. In this study, 3, 4, 5, and 6 were selected as the solution pH because at higher pH, In3 þ will precipitate as In(OH)3. The results of the adsorption at various pH are shown in Fig. 7a. The In3 þ adsorbed at pH 3, 4, 5, and 6 are 1.5, 1.65, 1.7, and 1.8 mmol g 1 respectively. The level of In3 þ adsorption increases with increasing solution pH. Indium metal dissolves in mineral acids and generates In3 þ ions [24]. The In3 þ ions in an aqueous solution form the hexa-aquo complex, In (H2 O)63 + , which is hydrolysed to In(H2O)5(OH)2 þ and In(H2O)4 (OH)+2 ions before the electrontransfer process. It was reported that when the concentration of indium was less than 0.001 M, InOH2 þ and In (OH)+2 ions predominated among the hydrolysis products. When concentration more than 0.001 M, polynuclear ions predominates [25–26]. In this study we have considered 1, 5 and 25 mM In3 þ solutions, as a reason InOH2 þ and In (OH)+2 are not important. However, at lower pH, the concentration of H þ ions in the solution was very high and the higher mobility of the smaller sized H þ ions retarded the adsorption of In3 þ by the active sites in the Zn/Al-LDH. Owing to the protonation of the active site, the number of neutralised active
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Fig. 7. (a) Effect of pH on the adsorption of indium ions [Conc. ¼5 mM, adsorbate volume ¼ 20 ml, adsorbent dosage¼20 mg], (b) effect of the contact time on the adsorption of indium ions [Conc. ¼ 5 mM, adsorbate volume¼ 20 ml, adsorbent dosage¼ 20 mg, pH ¼5], (c) effect of the initial metal ion concentration on the adsorption efficiency of indium ions [adsorbate volume¼ 20 ml, adsorbent dosage¼ 20 mg, pH ¼5], and (d) effect of the adsorbent dosage on the adsorption efficiency of indium ions [Conc. ¼5 mM, adsorbate volume¼20 ml, pH¼ 5].
sites increased in the strong acidic medium. As a result, a repulsion was exerted between the indium ionic species (binuclear or polynuclear) and the active site; however, at higher pH, there was a decrease in H þ ions in the solution, and both the competition for binding sites and the electrostatic repulsion decreased; hence, metal ion adsorption is favourable. The adsorption of In3 þ as a function of the contact time was conducted at room temperature at pH 5. The curve (Fig. 7b) showed that the amount of In3 þ removal increased gradually with increasing adsorption time, and the adsorption equilibrium was not reached, even after 24 h of adsorption time. Zn/Al-LDH was found to be quite efficient in the removal of In3 þ ions from aqueous solution and the removal depends greatly on the contact time between the solid adsorbent and the metal ion species at its interface. A gradual In3 þ adsorption rate was observed throughout the adsorption process. After 6 h, a decrease in the adsorption rate was observed due to the repulsive interaction between the adsorbed indium species on the interlayer surface of the Zn/Al-LDH
with the predominant ionic species present in the solution. Moreover, the concentration difference generated during the late hours of adsorption between the adsorbent surfaces with the solution further reduced the rate of adsorption. In particular, the kinetics of adsorption was studied at pH 5, which favoured the increased efficiency of adsorption and dominated over the two factors described above. As a result, Zn/Al-LDH exhibited exceptional adsorption capacity towards In3 þ ions [24]. Fig. 7c shows the effect of the initial concentration of indium ions on the percent removal of Zn/Al-LDH at pH 5. The initial concentration of In3 þ was varied in the range of 1–5 mM. An increase in the initial concentration of In3 þ from 1 to 5 mM caused a corresponding decrease in the percent removal from 100% to 31%. The equilibrium adsorption amount of In3 þ ions from 1, 2, 3, 4 and 5 mM metal ion solutions are, however, 144.8, 199.7, 182.5, 169.9 and 172.2 mg g 1, respectively. The maximum amount of In3 þ adsorption of 199.7 mg g 1 was achieved from a metal ion concentration of 2 mM at pH 5. Also an increase in initial metal ion
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Fig. 8. Competitive adsorption of metal ions from a mixture of (a) Mn2 þ , Co2 þ , Ni2 þ , and Cd2 þ (b) Mn2 þ , Co2 þ , Ni2 þ , Cd2 þ , and Pb2 þ , (c) Mn2 þ , Co2 þ , Ni2 þ , Cd2 þ , and Cu2 þ , and (d) Mn2 þ , Co2 þ , Ni2 þ , Cd2 þ , Pb2 þ , and Cu2 þ . [Conc. of individual metal ions ¼1 mM, adsorbate volume¼ 20 ml, adsorbent dosage¼ 10 mg, pH ¼5].
concentration beyond 2 mM was not influencing considerably in the equilibrium adsorption amount of In3 þ . A high initial concentration means a better concentration gradient, which is an important driving force that will help overcome the mass transfer resistance of In3 þ between the liquid and solid phases [27,30]. Fig. 7d shows the effects of the adsorbent dose on the adsorption efficiency of In3 þ onto Zn/Al-LDH at pH 5. 0.01, 0.02, 0.03, and 0.04 g of adsorbent were selected for the uptake of In3 þ from a 5 mM indium nitrate solution. The percentage removal increased from 22% to 60% as the adsorbent dosage was increased from 0.01 to 0.04 g. The equilibrium adsorption amount of In3 þ ions over the adsorbent dosages of 0.01, 0.02, 0.03, and 0.04 g are 120, 212, 269, and 344 mg L 1, respectively. A higher adsorbent dosage indicates more binding sites available for the uptake of In3 þ onto the Zn/AlLDH surface, which results in high removal percentages. The maximum removal of 60% was achieved at dosage of 0.04 g Zn/AlLDH [23–27]. Fig. 8 presents the weight and mole selectivity of (a) Mn2 þ , Co2 þ , Ni2 þ , and Cd2 þ , (b) Mn2 þ , Co2 þ , Ni2 þ , Cd2 þ , and Pb2 þ , (c) Mn2 þ , Co2 þ , Ni2 þ , Cd2 þ , and Cu2 þ , and (d) Mn2 þ , Co2 þ , Ni2 þ , Cd2 þ , Pb2 þ , and Cu2 þ . Fig. 8a shows that Zn/Al-LDH has no affinity towards Mn2 þ , Co2 þ , Ni2 þ , and Cd2 þ . Instead, the material selectively adsorbs Pb2 þ (Fig. 8b) or Cu2 þ (Fig. 8c) from a mixture
containing either of these ions in the first metal ion solution. When Pb2 þ and Cu2 þ were introduced to the first solution (Fig. 8d), Zn/Al-LDH adsorbed both metal ions with a preparation to Cu2 þ more than Pb2 þ ions. From this experiment, it was concluded that Zn/Al-LDH has greater affinity towards Pb2 þ and Cu2 þ and the adsorption mechanism shows electrostatic attraction followed by precipitation. Finally, the selectivity of In3 þ adsorption from a mixture of selected metal ions was examined to ensure the affinity of Zn/AlLDH towards In3 þ ions. Fig. 9 shows the weight and mole selectivity of (a) In3 þ in presence of Mn2 þ , Co2 þ , Ni2 þ , and Cd2 þ , (b) In3 þ in presence of Mn2 þ , Co2 þ , Ni2 þ , Cd2 þ , and Pb2 þ , (c) In3 þ in presence of Mn2 þ , Co2 þ , Ni2 þ , Cd2 þ , and Cu2 þ , and (d) In3 þ in presence of Mn2 þ , Co2 þ , Ni2 þ , Cd2 þ , Pb2 þ , and Cu2 þ . Metal mixture solutions containing nitrate salts of Mn2 þ , Co2 þ , Ni2 þ , Cd2 þ , In3 þ , Pb2 þ and Cu2 þ with 1 mM concentration of each metal ion in water were prepared. 20 ml of the above solution was mixed with 10 mg of the Zn/Al-LDH and shaken for 12 h to achieve the adsorption equilibrium. Fig. 9 shows the adsorption selectivity of the Mn2 þ , Co2 þ , Ni2 þ , Cd2 þ , In3 þ , Pb2 þ , and Cu2 þ over Zn/AlLDH. Fig. 9a presents the competitive adsorption from a mixture of Mn2 þ , Co2 þ , Ni2 þ , Cd2 þ , and In3 þ . This shows that Mn2 þ , Co2 þ , Ni2 þ , and Cd2 þ have no affinity towards the Zn/Al-LDH surface at
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Fig. 9. Competitive adsorption of metal ions from a mixture of (a) Mn2 þ , Co2 þ , Ni2 þ , Cd2 þ , and In3 þ , (b) Mn2 þ , Co2 þ , Ni2 þ , Cd2 þ , In3 þ , and Pb2 þ , (c) Mn2 þ , Co2 þ , Ni2 þ , Cd2 þ , In3 þ , and Cu2 þ , and (d) Mn2 þ , Co2 þ , Ni2 þ , Cd2 þ , In3 þ , Pb2 þ , and Cu2 þ . [Conc. of individual metal ions ¼1 mM, adsorbate volume¼20 ml, adsorbent dosage¼ 10 mg, pH¼ 5].
pH 5. As a result, In3 þ exhibited 100% mole and weight selectivity from the above mixture. When adding Pb2 þ to the above metal mixture (Fig. 9b), the mole and weight selectivity of In3 þ decreased considerably due to the major adsorption of Pb2 þ and minor or negligible adsorption of Co2 þ , Ni2 þ and Cd2 þ ions over the Zn/Al-LDH surface. The same trend was observed for competitive adsorption from a mixture of Mn2 þ , Co2 þ , Ni2 þ , Cd2 þ , In3 þ , and Cu2 þ under the same experimental conditions (Fig. 9c). From these experiments, it was concluded that Pb2 þ and Cu2 þ have great affinity towards the Zn/Al-LDH surface [23–28]. In addition, Co2 þ , Ni2 þ and Cd2 þ ions were adsorbed only in presence of Pb2 þ or Cu2 þ ions in the mixture. Interestingly, Mn2 þ did not exhibit any affinity towards the Zn/Al-LDH surface. Furthermore, the competitive adsorption of Mn2 þ , Co2 þ , Ni2 þ , Cd2 þ , In3 þ , Pb2 þ , and Cu2 þ ions in the same mixture was observed (Fig. 9d). The result highlighted the superior affinity of In3 þ over Pb2 þ and Cu2 þ , and sharing half of the active surface of the Zn/Al-LDH along with other metal ions in the mixture. Moreover, Mn2 þ , Co2 þ , Ni2 þ , and Cd2 þ showed no affinity towards the active adsorbent surface in the combined presence of highly performing In3 þ , Pb2 þ and Cu2 þ ions.
3.7. Equilibrium study The adsorption isotherm models are normally used to describe the interaction between adsorbents and adsorbates, providing most important parameters for designing a desired adsorption system. Therefore, the In3 þ removal capacity of the pristine Zn/AlLDH was determined, where the Langmuir and Freundlich adsorption models were applied to evaluate the effectiveness of the adsorbents [28]. The Langmuir model is based on the assumption that the maximum adsorption occurs when a saturated monolayer of solute molecules is present on the adsorbent surface, the energy of adsorption is constant and there is no migration of adsorbate molecules in the surface plane. Fig. 10a shows the experimental result of the Langmuir isotherm. According to the Langmuir isotherm,
qe =
qm KL Ce 1 + KL Ce
The constants in the Langmuir isotherm can be determined by plotting Ce versus Ce/qe and making use of the above rewritten as:
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Table 2 Parameters for Langmuir and Freundlich isotherm models. Langmuir
Freundlich
qm (mg g 1)
KL (L mg 1)
R2
Kf (mg1 n Ln g 1)
n
R2
194
0.0476
0.952
0.034
4
0.909
Table 3 In3 þ adsorption capacities (qmax) results from the literature. Material
In3 þ (mg g 1)
pH
Reference
Ion-electrode S. algae Chitosan-coated bentonite Ion-exchangeable nano-beads Electro-coagulation Pristine Zn/Al-LDH
78.3 107 96 89 57 205
2.4 3.8 6 8 2.3 6
[6] [8] [12] [21] [32] Our work
model to fit the experimental data was examined. In this case, a plot of ln Ce vs. ln qe was used to generate the intercept of Kf and the slope of n. Table 2 lists the values of the parameters for the Langmuir and Freundlich isotherm models. Fig. 10a shows that the isotherm data fits the Langmuir equation well (R2 ¼0.952). The values of qm and KL were 194 mg g 1 and 0.0476 L mg 1, respectively at pH 5 [29]. The Freundlich constants Kf and n were 0.034 mg g 1 and 4, respectively (Fig. 10b). The R2 (0.909) value of Freundlich isotherm was found to be lower than the same of Langmuir isotherm. The magnitudes of Kf and n show the easy separation of indium ions from the aqueous solution and indicate favourable adsorption. The intercept Kf is an indication of the adsorption capacity of the adsorbent; the slope 1/n indicates the effect of concentration on the adsorption capacity and represents the adsorption intensity. As shown above, the n value was found to be high enough for separation. In addition, Zn/Al-LDH gives the adsorption energy for adsorbing In3 þ . The Langmuir isotherm fitted very well with a high correlation coefficient (R2 ¼0.952), showing that the adsorption of In3 þ followed the Langmuir model [30,31]. Fig. 10. Plots of (a) Langmuir and (b) Freundlich isotherm models.
Ce 1 C = + e qe KL qm qm where qm and KL are the Langmuir constants, representing the maximum adsorption capacity for the solid phase loading and the energy constant related to the heat of adsorption, respectively. The Freundlich isotherm model is an empirical relationship describing the adsorption of solutes from a liquid to a solid surface and assumes that different sites with several adsorption energies are involved. The Freundlich adsorption isotherm is the relationship between the amounts of In3 þ adsorbed per unit mass of adsorbent, qe, and the concentration of the In3 þ at equilibrium, Ce, as shown in Fig. 10b.
qe = Kf Ce1/ n
4. Conclusion Pristine Zn/Al-LDH was found to be highly active and selective towards In3 þ ions. The XRD pattern of Zn/Al-LDH exhibited additional crystalline phases after In3 þ adsorption. The adsorption ability increased with increasing In3 þ solution concentration, which was found to be responsible for the reduction in porosity and surface area. The material exhibited a maximum In3 þ intake of 205 mg g 1 at pH 6, which is high adsorption ability compared to the same reported in the literature (Table 3). The adsorption from a mixture of Mn2 þ , Co2 þ , Ni2 þ , Cd2 þ , and In3 þ exhibited 100% adsorption selectivity for In3 þ ions and maintained superior affinity of In3 þ over the Cu2 þ and Pb2 þ from the other mixtures. Overall, pristine Zn/Al-LDH is a promising material for future applications.
The logarithmic form of the equation becomes
ln qe = ln Kf +
1 ln Ce n
where Kf and n are the Freundlich constants, the characteristics of the system. Kf and n are the indicators of the adsorption capacity and adsorption intensity, respectively. The ability of the Freundlich
Acknowledgements The work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Ministry of Science, ICT, and Future Planning, Korea {Pioneer Research Center Program (NRF-2010-
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0019308/2010-0019482); Acceleration Research Program (NRF-2014 R1A2A111 054584); Brain Korea 21 Plus Program (21A2013800002)}.
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