Smart Design of Self-Assembled Mesoporous Î±-FeOOH Nanoparticles

Subscriber access provided by UNIVERSITY OF SASKATCHEWAN LIBRARY

Article

Smart design of self-assembled mesoporous #-FeOOH nanoparticle: High-surface-area sorbent for Hg2+ from waste water Astam Kumar Patra, and Dukjoon Kim ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b00937 • Publication Date (Web): 02 Jan 2017 Downloaded from http://pubs.acs.org on January 3, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Sustainable Chemistry & Engineering is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Sustainable Chemistry & Engineering

Smart design of self-assembled mesoporous α-FeOOH nanoparticle: Highsurface-area sorbent for Hg2+ from waste water Astam K. Patra and Dukjoon Kim* School of Chemical Engineering, Sungkyunkwan University, Suwon, Gyeonggi, 16419 Republic of Korea *Corresponding author: Prof. Dukjoon Kim, Tel: +82-31-290-7250 Fax: +82-31-290-7270 E-mail: [email protected] ABSTRACT Self-assemble mesoporous α-FeOOH nanoparticles with high surface area and controlled self-assemble structure have been synthesized through simple and an environmentally friendly method. The formation mechanisms of self assemble mesoporous structures, as well as the effect of pH on structure of the materials is carefully discussed. The self-assemble mesoporous α-FeOOH nanoparticle has been characterized by small-angle X-ray scattering (SAXS) analysis, powder X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), N2 sorption, X-ray photoelectron spectroscopic (XPS) studies. N2 sorption analysis revealed high surface areas (74-152 m2 g-1) and narrow pore size distributions (2.5 nm) for different samples. The XPS analysis revealed that the materials contain large amount of surface Fe-OH group which are the active suite for Hg2+ adsorption. The adsorption process has been discussed using Langmuir and Freundlich models. These self-assemble mesoporous αFeOOH nanoparticles can act as a very efficient and reusable adsorbent for Hg2+ from polluted water. KEYWORDS. Green synthesis, mesoporous materials, goethite materials, Hg2+ adsorption, reusable adsorbent.

1

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

INTRODUCTION Metal hydroxide/oxide nanoparticles have shown great potential application including catalysis,1, 2 adsorption,3 magnetic devices,4 environmental protection,5 gas sensors,6 drug delivery,7 Li ion batteries,8 water splitting9 etc. Specially as a adsorbent in water treatment process, their reactivity has been proved to be highly size-dependent.10 The surface free energy, specific surface area and the percentage of the atoms at surface increase with decreasing particle size of nanomaterials. This effect increases the adsorption performance of the materials. These size-dependent properties would create great impact on nanotechnologies and find new potential solutions to major environmental concerns, and improve the environment and human health. In 2005, Yean et al.11 reported that the 20 nm magnetite nanoparticles show approximately 18 times superior adsorption capacity of As(III) than the 300 nm ones because 20 nm particle have more large surface to form the stable iron-arsenic complexes homogenously than the 300 nm ones. In 2006, Yavuz et al.12 reported that nanocrystalline 12 nm Fe3O4 and commercially available 300 nm Fe3O4 particle show 99.2, 24.9 % removal efficiency of As(III) and 98.4, 29.2 % removal efficiency As(V) respectively. Madden et al.13 reported that the 7 nm hematite particles showed higher adsorption affinity to the Cu2+ than 25 and 88 nm particles. In 2008, Auffan et al.14 reported that the quantity of arsenic adsorption at the surface and normalized per unit of surface area of maghemite remains constant for particles between 300 and 20 nm. But lesser than 20 nm nanoparticle exhibit amended adsorption capacity due to size-dependent structural variation of the surface of particles like the decrement of the occupancy of the tetrahedral site, decrement of the surface free energy and highly reactive adsorption sites. Recently 2-10 nm γ-Al2O3 minute spherical nanoparticles showed high affinity for the adsorption of arsenic from the contaminated aqueous solutions.15 Wang et al.16 reported that the ferrihydrite materials influenced a similar fundamental structure with a crystallite size changing from 1.6 to 4.4 nm. 2


Page 2 of 32

Page 3 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


When crystallite size of ferrihydrite changed, phosphate adsorption capacity decreased from 1690 to 980 µmol g-1 while the adsorption density normalized to comparable specific surface area was similar. So low dimension nanoparticle are very effective in potentials adsorbent. Here we demonstrated a green and environmental friendly synthesis method for selfassemble mesoporous α-FeOOH nanoparticles with 3 nm particle size and high BET surface areas. The α-FeOOH nanoparticles are very cheap, harmless and ecologically friendly. Additionally, it has higher stability among the other iron oxyhydroxides. The nanoparticle show very high affinity to Hg2+ adsorption. Mercury ion have high toxic effect and lead to severe damage to brains, lungs, kidneys, and other organs.17,

18

Here we study the Hg2+

adsorption over 3 nm α-FeOOH nanoparticles. The material shows the high uptake of Hg2+ ions and large distribution coefficient (Kd). The adsorption kinetics was fitted with pseudo second order model and the adsorption isotherm was fitted with Langmuir and Freundlich adsorption isotherm models. Self-assemble mesoporous α-FeOOH material show high adsorption capacity and recyclability to the Hg2+ cation and this can give potential application in water treatment plant to purify waste water to drinkable water. EXPERIMENTAL SECTION Synthesis of self-assembled mesoporous α-FeOOH nanomaterials Self-assembled mesoporous α-FeOOH (hereafter abbreviated as SMF) nanoparticle was synthesized using simple method. In the typical synthesis, 10 mmol sodium salicylate and 10 mmol NaOH were stirred with 20 mL water for 0.5 hr. Then 10 mmol Fe(NO3)3·9H2O was dissolved in 5.0 g deionised water and this solution was gradually integrated to the above solution. 2N NaOH solution was added to adjust the pH to ca. 4. The solution was stirred for 3 hrs. Then the mixture was kept in room temperature for overnight (12 hrs). Additionally, the same syntheses were done in pH=6 and pH=8 respectively. The resultant material was separated by centrifugation and washed several times with water and ethanol. The FTIR

3



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

analysis (Figure S1) attributed that substantial amount of salicylates were abstracted from the materials. This synthesized material was dried at 298 K in vacuum. The materials were designated as SMF-4, SMF-6 and SMF-8. These Self-assembled mesoporous α-FeOOH nanoparticle were used in the adsorption experiment. Hg(II) adsorption experiments Adsorption experiments were completed over self-assemble mesoporous α-FeOOH materials using mercury (II) chloride solutions of kenned strengths 20–200 mg/L. The adsorption experiments were completed containing 100 mL of the Hg2+ solution and 0.1 g of the adsorbent at 298 K and stirred vigorously. After the adsorption reached equilibrium, the adsorbent removed through filtration and the supernatant was accumulated for metal concentration quantifications. The final concentrations of Hg2+ in these solutions were quantified utilizing an inductively coupled plasma/mass spectrometer (Agilent 7500). Kinetics of adsorption The adsorption kinetics for Hg2+ was studied over the self-assemble mesoporous α-FeOOH materials. In the trial, 0.1 g SMF-6 materials was dispersed in 100 mL solution of Hg2+ with concentration 100 mg/L and stirred vigorously. 5 mL aliquot was accumulated and the quantified the concentration of the solution through ICPMS analysis in every 30 min interval. Effect of different metal ion and pH The competitive effects of Na+, K+, Mg2+ and Ca2+ coexisting ions on the removal of Hg2+, 100 mL solution of 100 mg/L concentration for all cations and 0.1 g adsorbent (SMF-6) were stirred for 4 hrs. The adsorbent was removed through filtration when the experiment reached to equilibrium and the supernatant was amassed for metal concentration measurements. The final concentrations of these cations were determined using an inductively coupled plasma/mass spectrometer and inductively coupled plasma optical emission spectrometer. We also compared the adsorption efficiency of the material in the presence of various coexisting 4


Page 4 of 32

Page 5 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


transition metal cations (Fe3+, Ni2+, Cu2+, and Zn2+). 100 mL solution of 100 mg/L concentration for all cations was used for this adsorption experiment. We also study the effect of coexisting heavy metal ion (Cd2+) on adsorption. The effect of different pH on Hg2+ removal by SMF-6 material was carried out with 100 mL of 100 mg/L solution having pH = 4, 6, 7 and 10. Recycling of adsorbent Recycling experiment was carried out with SMF-6 materials as representative to check the stability and efficiency of the α-FeOOH materials. For this experiment, the SMF-6 material was collected after the competition of the experiment. The material was washed with dilute NaNO3+NH4Cl solution to eliminate the adsorbed cations from the materials and dried in vacuum. The stability of the reused materials was examined by SAXS and PXRD analysis and the result is shown in the Figure S2. Materials Characterization The self-assemble structural feature of α-FeOOH materials were investigated by different characterization techniques. SAXS measurement were done utilizing synchrotron radiation with Cu Kα radiation (λavg =0.15418 nm) at 40 kV and 50 mA at the 4C1 line of the SAXS instrument (SAXSess, Anton Paar GmbH, Austria). Bruker D-8 Advance diffractometer [40 kV voltage, 40 mA current and Cu Kα (λ = 0.15406 nm) radiation] was utilized for PXRD patterns of the samples. JEOL JEM-2100F TEM (200 kV) was utilized for HRTEM images. The self-assemble structure was analyzed by FE SEM (JEOL JEM-7600F). Micromeritics Instrument ASAP 2000 surface area analyzer was used to obtain the Nitrogen adsorption desorption isotherms at 77 K. XPS was performed on a Thermo Scientific (Model No ESCALAB 250Xi) X-ray Photoelectron Spectrometer operated at 15 kV and 20 mA with a monochromatic Al Kα X-ray source. All the spectra were rectified by carbon (C 1s) at 284.6 eV as reference. The concentration of mercury solution was analyzed by inductively coupled 5



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

plasma mass spectrometer (ICP MS) (Agilent Model 7500). The concentration of sodium, potassium, calcium, magnesium and cadmium solution was analyzed by inductively coupled plasma optical emission spectrometer (ICO-OES) (Varian). RESULTS AND DISCUSSION

Scheme 1. Synthetic route of self-assemble mesoporous α-FeOOH nanoparticles Syntheses and Materials Characterization Here the self-assemble mesoporous α-FeOOH nanoparticles were synthesized by simple synthesis method via network formation of uniform α-FeOOH nanoparticles. In this syntheses method iron nitrate nonhydrate was hydrolyzed in the presence of sodium hydroxide and sodium salicylate. Our synthesis strategy to prepare self-assemble mesoporous α-FeOOH nanoparticles is shown in scheme 1. The reaction was performed at different pH (4, 6 and 8) using iron nitrate and sodium hydroxide as the source of iron and hydroxide precursor respectively. Here sodium salicylate, sodium hydroxide and deionised water were used as the capping agent, the bridging ligand and solvent respectively. In this study, we varied only the concentration of sodium hydroxide to control the self-assemble of nanoparticles fixing template, concentration of Fe3+, reaction mixture volume, solvent etc. Initially the nucleation of α-FeOOH molecule takes place in the reaction mixture. Furthermore, the bridging agent (hydroxide anion) and the capping agent (salicylate anion) assists and inhibits the formation of Fe-O-Fe-OH respectively during the reaction.19 Currently sodium salicylate is utilized to tune the morphology and control nanostructure of inorganic

6


Page 6 of 32

Page 7 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


materials.20, 21, 22, 23 This is because there is a covalent interaction between salicylate anions and positively charged metal centers in the reaction medium. In this syntheses method, the physical and chemical parameters are selected for increasing the interaction (such as ionic interactions, hydrogen bonding, and dipole-dipole interactions) between them to facile the self-assemble of nanoparticles. One broad peak in the Small-angle X-ray scattering (SAXS) analysis (Figure 1A) of mesoporous α-FeOOH nanoparticles confirmed the self-assembled structure.15, 22 Figure 1A is shown the SAXS patterns of mesoporous α-FeOOH nanoparticles samples SMF-4, SMF-6, and SMF-8. One peak in the spectra is attributed the disorder nature of pore arrangement. The wide angle powder XRD (PXRD) patterns of mesoporous αFeOOH nanoparticles are shown in Figure S3, suggested that the obtain nanoparticle are semicrystalline in nature of goethite (α-FeOOH) phase (JCPDS 01-081-0463).19 Broad peak for these nanoparticles in PXRD result is attributed to the very smaller particle size and lower crystallinity.

Figure 1. (A) Small-angle X-ray scattering (SAXS) pattern of (a) SMF-4, (b) SMF-6, and (c) SMF-8 and N2 adsorption (●) − desorption (○) isotherms of the (B) SMF-4, (C) SMF-6, and (D) SMF-8. In inset the corresponding pore size distribution (PSD) curve for samples (B)-(D) obtained by NLDFT model.

7



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

N2-adsorption-desorption study The mesostructures are investigated by N2 adsorption-desorption measurement at 77 K to evaluate the Brunauer-Emmett-Teller (BET) specific surface area, pore volume, and average pore diameter of the materials. The N2-adsorption-desorption isotherms of these samples are shown in Figure 1. According to IUPAC nomenclature, these isotherms are categorized as typical type II isotherm.24 In the adsorption branch of these isotherms, there is a gradual increase in N2 adsorption at P/P0=0.05–0.3 region. This incrementation is corresponding to multilayer adsorption and attributed to the presence of mesopores in the materials.25,

26

Furthermore, the materials show sharp

nitrogen uptake at high P/P0 values, which is features of interparticle porosity between agglomerated particles.15, 22, 27 The isotherms of materials SMF-4 and SMF-6 (Figure 1B and C) show the reversibility of adsorption and desorption branches. But the desorption branch of isotherm of the materials SMF-8 (Figure 1D) shows H3-type hysteresis loop which is uncommon for type-II isotherm. The H3 hysteresis loop of this material (SMF-8) indicates the existence of disorder nature of pore arrangement, which agrees with our SAXS analysis result (Figure 1A). The BET surface areas for samples SMF-4, SMF-6, and SMF-8 were 86.6, 152.4, and 74.8 m2 g-1 and pore volume of these materials were 0.078, 0.089, and 0.116 cc g-1 respectively. Pore size distributions of these samples were calculated utilizing nonlocal nonlocal density functional theory (NLDFT) model (SiO2 as a reference).28, 29, 30 SMF-4 material has an average pore diameter of ca. 2.59 nm whereas SMF-6 has an average pore diameter of ca. 2.5 nm. However the material SMF-8 shows different types of pore arrangement. So in different synthesis condition (i.e. different pH), the materials show different selfassemble structure and most favorable structure is obtained in pH 6. Here we see that

8


Page 8 of 32

Page 9 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the surface area decreased in the higher pH. At the higher pH, the loss of selfassembly and further growth or crystallization of the particles could be occurred. Microstructural Analysis. TEM images of self-assembled mesoporous α-FeOOH nanoparticles are shown in Figure 2. As seen from the images (Figure 2a, b and d) that mesoporous α-FeOOH materials are made of self-assembled uniform nanoparticles and form the mesoporous structure. Interestingly uniform particle size for all SMF materials are obtained by this simple method. Further, every particle is seen clearly. Figure 2c displays the higher resolution TEM image of self-assemble α-FeOOH nanoparticles (SMF-6) which was obtained at room temperature with pH 6. The average size of these nanoparticles was ca 3 nm.

Figure 2. TEM images of self-assemble α-FeOOH nanoparticles was prepared at different pH (a) SMF-4, (b) SMF-6, (c) HRTEM image of SMF-6 and (d) SMF-8.

9



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 3. FESEM images of self-assemble α-FeOOH nanoparticles was prepared at different pH (a) SMF-4, (b) SMF-6, (c) high resolution image of SMF-6 and (d) SMF8. Further the field emission scanning electron microscopic (FE SEM) analysis was carried out to investigate the self assembles structure of mesoporous α-FeOOH nanoparticles. FESEM images of these samples (Figure 3) shows that tiny particle are aggregated together to form the porous structure and they are spherical in nature. Size of the particles is well agreed with the HR TEM image (Figure 2c). These spherical particles are generated the inter particle porosity. Hg2+ Adsorption Study Hazardous Hg2+ is selected as a model heavy metal ion to study the adsorption properties of the self-assemble mesoporous α-FeOOH nanoparticle adsorbent. Adsorption equilibrium experiments were set up by adding the adsorbents to the Hg2+ aqueous solution at room temperature. The adsorption experiment over SMF-4, SMF-6, SMF-8 materials is show in Figure S4. SMF-4, SMF-6, and SMF-8 materials show 79.5, 86.7 and 80.1% removal efficiency, respectively. Here SMF-6 material show highest adsorption efficiency may be attributed to the high surface area. Further to reveal the adsorption properties of adsorbent, valence states of the elements and surface composition present in the adsorbent, the XPS 10


Page 10 of 32

Page 11 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


analysis was carried out before and after Hg2+ adsorption and the outcomes are presented in Figure 4. In the XPS investigation, the binding energies of the elements are adjusted for specimen charging by referencing the C 1s line to 284.6 eV. The survey XPS profile (Figure 4a) revealed that the SMF-6 and SMF-6-Hg (after Hg2+ adsorption) samples are composed of Fe, C, O and Fe, C, O, Hg respectively. It is quite clear that after adsorption the SMF-6 materials contain the Hg ion. The SMF-6 material exhibits major peaks at 285.1, 532.1, 711.1, and 724.9 eV, which correspond to C 1s, O 1s, Fe 2p3/2, and Fe 2p1/2 respectively in this XPS profile. SMF-6-Hg materials exhibit a new peak around 103.3 eV along with the C 1s, O 1s, Fe 2p3/2, and Fe 2p1/2 peaks. The new peak attributed to Hg 4f in this XPS profile. In the highresolution Fe spectra (Figure 4b), both materials show two peaks at binding energies of 711.1 eV for Fe 2p3/2 and 724.9 eV for Fe 2p1/2 with a satellite peak at 719.3 eV. This is characteristic of Fe3+ in α-FeOOH.31, 32 The high-resolution O 1s spectra (Figure 4c) show four peaks and after deconvolution four peaks are located at 529.9, 531.8, 533.4 and 533.8 eV. The peaks at 529.9 eV attributed to the lattice oxygen atoms binding with Fe (Fe–O).33, 34 The peaks at 531.8 attributed to the hydroxyl groups (Fe–OH) and Fe-O-Hg (surface complexation).31, 35, 36 The surface Fe-OH group is the active site for ion adsorption in the αFeOOH framework.37, 38 The area ratio of Peak II/Peak I are 2.75 and 2.78 for the materials SHM-6 and SMF-6-Hg respectively. The XPS analysis shows that the materials contain large amount of surface Fe-OH group. The peaks at 533.4 can be assigned to O=C-O- group present on the template molecule (Sodium salicylate) in the framework.39 The peaks at 533.8 attributed to the H–O–H bond.33 The high-resolution Hg 4f spectrum (Figure 4d) show two peaks and these two peaks are located at 101.1 for Hg 4f7/2 and 105.1 for Hg 4f5/2 with a spin– orbit splitting of 4 eV for the 4f7/2 and 4f5/2 states which is characteristic of mercury ion.40 The results indicate mercury adsorption on the surface of the self-assemble mesoporous αFeOOH nanoparticle.

11



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 32

Figure 4. XPS analysis of SMF-6 before and after Hg2+ adsorption (a) survey XPS and High resolution XPS of (b) Fe2p (c) O1s (d) Hg 4f.

Table 1: Ion exchange carried out over SMF-6 and SMF-8 materials in mercury chloride solution.

Adsorbent SMF-6

SMF-8

Hg2+ Cation Content in Solution (mg/L) Before

After

20 50 100 150 200 20 50 100 150 200

0.31 4.46 13.3 26.6 53.6 0.54 7.76 19.9 33.2 59.8

Hg2+ Removal Efficiency (%) 98.45 91.1 86.7 82.2 73.2 97.3 84.5 80.1 77.9 70.1

Hg2+ Distribution coefficient (Kd) (mL/g) 6.35 X 104 1.02 X 104 5.71 X 103 4.63 X 103 2.73 X 103 3.60 X 104 5.44 X 103 4.02 X 103 3.51 X 103 2.34 X 103

The mercury contents in different aqueous solutions before and after adsorption studies are shown in Table 1. As seen from Table 1 that the adsorption efficiency for SMF-6 varies from

12


Page 13 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ca. 98–73% and for SMF-8 varies from ca. 97–70 % for different stock solutions. The removal efficiency of Hg2+ was determined by using the following expression: % =

−

Where C0 (mg/L) and Ct (mg/L) indicates initial concentration and the concentration after time t of Hg2+ respectively. The Distribution coefficient (Kd)15 between solid and aqueous phase was determined by using the following expression: =

! " # " !

=

$%&'( ! ) *+,-. #)

The adsorption capability (Qt) of the sample was calculated using following expression: / =

− 0 1

Where C0 (mg/L) and Ct (mg/L) initial concentration and the concentration after time t of Hg2+ respectively, V (L) is the volume of Hg2+ solution. M represents the amount of adsorbent used. Table 1 show that the SMF-6 materials have larger distribution coefficients in low concentration solution for mercury ion uptake for these kinds of adsorption tests. The Kd of SMF-6 materials are analogous with different functionalized mesoporous materials. Table 1 show that the SMF-6 materials have more sizably voluminous distribution coefficients in low concentration solution for the abstraction of mercury ion for all these adsorption experiments. These values are commensurable with different functionalized mesoporous materials.

13



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Hg2+ Adsorption Kinetics The kinetic mechanism of the mercury adsorption process was evaluated in the surface of self-assemble mesoporous α-FeOOH nanoparticle adsorbent, a time dependent adsorption experiment was carried out. The experimental results are shown in Figure 5. The kinetics graph displays that initially SMF-6 materials adsorbed Hg2+ at high rate about 79 % removed during the first 60 min (Initial Hg2+ con. 100 mg/L). After that there was a gradual decrease in the rate of removal and reaching equilibrium. In the 240 min, 86.7 % mercury ions were adsorbed onto the surface of nanoparticle. This is suggesting that SMF-6 shows a fast adsorption dynamics for the elimination of Hg2+ from aqueous medium. The pseudo-second order kinetic model (Figure 5b) was used here to explain the experimental adsorption data and the removal of different contaminants from the water, using the following expression: 2 2 = + 32 34 67 /7

where t, Qt, Qe, and k2 are the reaction time, amount of Hg2+ adsorbed at time t (mg g-1), amount of Hg2+ ions adsorbed per mass unit of the adsorbent at equilibrium (mg g-1), rate constant of the pseudo-second-order adsorption (g mg-1 min-1) respectively.41 Figure 6b show that a good linear relationship is obtained between time (t) and t/Qt plot with a greater correlation coefficient R2 of 0.9986. The corresponding rate constants (k2) for SMF-6 is calculated from the slopes of the plots and the rate constant is 7.825 × 10−4 g mg-1 min-1. Moreover, the calculated Qe (Qe cal 92.16 mg g-1) value also agrees with the experimental data (Qe exp 85.48 mg g-1) using pseudo-second-order kinetics model for self-assemble mesoporous α-FeOOH nanoparticle.

14


Page 14 of 32

Page 15 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. (a) Time dependence of Hg2+ adsorption on SMF-6 and (b) Kinetic equation fitting lines for adsorptions of Hg2+ over SMF-6 using pseudo-second order kinetic model. Conditions: Hg concentration = 100 mg/L. Hg2+ Adsorption Isotherm The adsorption capacity of the self-assemble mesoporous α-FeOOH nanoparticle was determined using the adsorption isotherm of Hg2+ over HMF-6. The results are shown in Figure 6. At an initial mercury concentration of 200 mg/L, the adsorption capacity of SMF-6 is calculated to be 146 mg/g which is attributed to high surface area related to other similar adsorbents. Further the results of the Hg2+ adsorption were analysed by Langmuir and Freundlich adsorption isotherm models. The Langmuir adsorption assumes that the adsorbent has a homogeneous adsorption surface, which means that all the adsorption positions have equivalent adsorbed affinities. The isotherm is plotted (Figure. 6b) by Ce/Qe vs Ce and others parameter are calculated using the following expression: 84 84 = + 34 39 : # /

where Ce, Qe, Qm and KL are the equilibrium concentration of the adsorbate (mg/L), the amount of adsorbate adsorbed per unit mass of adsorbate (mg/g), Langmuir constants related to adsorption capacity and rate of adsorption, respectively.42, 43 The linear plot between Ce/Qe and Ce is fitted, and the correlation coefficient (R2) of the line reaches up to 0.9968, indicating that the adsorption of Hg2+ ions on the surface of SMF-6 nanoparticle fits well

15



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

with this Langmuir's adsorption model. The calculated Langmuir constants Qm and KL value are 200 mg g-1 and 0.063 L mg-1 respectively. The Qm of mesoporous α-FeOOH is higher than that of many other conventional adsorbents (see table S1). Further Langmuir model is more useful to know the feasibility of the isotherm when the model is expressed in terms of RL. Where RL = 1/ (1 + QmKL) and the values of RL indicate the shapes of isotherms. For unfavorable (RL > 1), linear (RL = 1), favorable (0 < RL < 1) and irreversible (RL = 0). The calculated RL value is 0.073 and this is attributed that the adsorption of Hg2+ on the surface of SMF-6 nanoparticle is favourable.44 The Freundlich adsorption model assumes heterogeneous adsorption because it depends on the variety of the adsorption sites was well as the nature of the metal ions adsorbed. The isotherm is plotted (Figure 6c) by LogQe vs LogCe and others parameter are calculated using the following expression: ; @ ;

Smart Design of Self-Assembled Mesoporous Î±-FeOOH Nanoparticles

Recommend Documents