Smart Design of Self-Assembled Mesoporous α-FeOOH Nanoparticles

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Research Article pubs.acs.org/journal/ascecg

Smart Design of Self-Assembled Mesoporous α‑FeOOH Nanoparticles: High-Surface-Area Sorbent for Hg2+ from Wastewater Astam K. Patra and Dukjoon Kim* School of Chemical Engineering, Sungkyunkwan University, Suwon, Gyeonggi 16419 Republic of Korea S Supporting Information *

ABSTRACT: Self-assembled mesoporous α-FeOOH nanoparticles with high surface area and controlled structure have been synthesized through a simple and environmentally friendly method. The formation mechanism of self-assembled mesoporous structures, as well as the effect of pH on the structure of the materials, is carefully discussed. The selfassembled mesoporous α-FeOOH nanoparticles have 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, and 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 a large amount of surface Fe−OH groups which are the active suite for Hg2+ adsorption. The adsorption process has been discussed using Langmuir and Freundlich models. These self-assembled mesoporous α-FeOOH 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

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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 splitting,9 etc. Especially as an adsorbent in water treatment processes, their reactivity has been proven to be highly size-dependent.10 The surface free energy, specific surface area, and percentage of atoms at the surface increase with decreasing particle size of nanomaterials. This effect increases the adsorption performance of the materials. These size-dependent properties indicate the potential for great impact on nanotechnologies and new solutions to major environmental problems. In 2005, Yean et al.11 reported that 20 nm magnetite nanoparticles show approximately 18 times superior adsorption capacity for As(III) than 300 nm ones because the 20 nm particles have more surface area to form the stable iron−arsenic complexes homogeneously than the 300 nm ones. In 2006, Yavuz et al.12 reported that nanocrystalline 12 nm Fe 3 O 4 and commercially available 300 nm Fe3O4 particles 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 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 particles © 2017 American Chemical Society

smaller than 20 nm 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, 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 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. 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 dimensional nanoparticles are very effective in potential adsorbents. Here, we demonstrated a green and environmental friendly synthesis method for self-assemble 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, they have higher stability among the other iron oxyhydroxides. The nanoparticles show very high affinity to Hg2+ adsorption. Mercury ions have a highly toxic effect and lead to severe damage to brains, lungs, kidneys, and other organs.17,18 Here we study the Hg2+ Received: May 2, 2016 Revised: December 23, 2016 Published: January 2, 2017 1272

DOI: 10.1021/acssuschemeng.6b00937 ACS Sustainable Chem. Eng. 2017, 5, 1272−1279

Research Article

ACS Sustainable Chemistry & Engineering adsorption over 3 nm α-FeOOH nanoparticles. The material shows a high uptake of Hg2+ ions and large distribution coefficient (Kd). The adsorption kinetics were fitted with a pseudo-second-order model, and the adsorption isotherm was fitted with Langmuir and Freundlich adsorption isotherm models. Self-assembled mesoporous α-FeOOH material show high adsorption capacity and recyclability to the Hg2+ cation, and this can lend it to potential application in water treatment plants to purify wastewater to drinkable water.

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Materials Characterization. The self-assembled structural features 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) Xray 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 ICPMS (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).

EXPERIMENTAL SECTION

Synthesis of Self-Assembled Mesoporous α-FeOOH Nanomaterials. Self-assembled mesoporous α-FeOOH (hereafter abbreviated as SMF) nanoparticles were synthesized using a simple method. In the typical synthesis, 10 mmol sodium salicylate and 10 mmol NaOH were stirred with 20 mL water for 0.5 h. Then 10 mmol Fe(NO3)3·9H2O was dissolved in 5.0 g deionized water and this solution was gradually integrated to the above solution. A 2N NaOH solution was added to adjust the pH to ca. 4. The solution was stirred for 3 h. Then the mixture was kept at room temperature overnight (12 h). Additionally, the same syntheses were done at pH = 6 and 8, respectively. The resultant materials were separated by centrifugation and washed several times with water and ethanol. The FTIR analysis (Figure S1) showed that a 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 nanoparticles 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 known 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 was 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 (ICPMS Agilent 7500). Kinetics of Adsorption. The adsorption kinetics for Hg2+ was studied over the self-assembled mesoporous α-FeOOH materials. In the trial, 0.1 g SMF-6 materials was dispersed in 100 mL solution of Hg2+ with a concentration 100 mg/L and stirred vigorously. A 5 mL aliquot was accumulated, and the concentration of the solution was quantified through ICPMS analysis in 30 min intervals. Effect of Different Metal Ions and pH. To determin the competitive effects of Na+, K+, Mg2+, and Ca2+ coexisting ions on the removal of Hg2+, a 100 mL solution of 100 mg/L concentration for all cations and 0.1 g adsorbent (SMF-6) were stirred for 4 h. The adsorbent was removed through filtration when the experiment reached equilibrium, and the supernatant was amassed for metal concentration measurements. The final concentrations of these cations were determined using ICPMS and an inductively coupled plasma optical emission spectrometer. We also compared the adsorption efficiency of the material in the presence of various coexisting transition metal cations (Fe3+, Ni2+, Cu2+, and Zn2+). A 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 a 100 mg/ L solution having pH = 4, 6, 7, and 10. Recycling of Adsorbent. A 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 Figure S2.

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RESULTS AND DISCUSSION Syntheses and Materials Characterization. Here the self-assembled mesoporous α-FeOOH nanoparticles were synthesized by a 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-assembled mesoporous αFeOOH nanoparticles is shown in Scheme 1. The reaction was Scheme 1. Synthetic Route of Self-Assembly for Mesoporous α-FeOOH Nanoparticles

performed at different pH values (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 deionized 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-assembly 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) assist and inhibit 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 materials.20−23 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 self1273

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

volumes of these materials were 0.078, 0.089, and 0.116 cm3 g−1 respectively. Pore size distributions of these samples were calculated utilizing a nonlocal density functional theory (NLDFT) model (SiO2 as a reference).28−30 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 conditions (i.e., different pH), the materials show different self-assembled structure and the most favorable structure is obtained in pH 6. Here we see that the surface area decreased in the higher pH. At higher pH values, loss of selfassembly and further growth or crystallization of the particles can occur. 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) mesoporous αFeOOH materials are made of self-assembled uniform nanoparticles. 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. The field emission scanning electron microscopic (FESEM) analysis was carried out further to investigate the self-assembled structure of mesoporous α-FeOOH nanoparticles. FESEM images of these samples (Figure 3) show that tiny particles are aggregated together to form the porous structure and they are spherical in nature. The size of the particles agreed well with the HRTEM image (Figure 2c). These spherical particles generate the interparticle porosity. Hg2+ Adsorption Study. Hazardous Hg2+ is selected as a model heavy metal ion to study the adsorption properties of the self-assembled mesoporous α-FeOOH nanoparticle adsorbent. Adsorption equilibrium experiments were set up by adding the adsorbents to the Hg2+ aqueous solution at room temperature.

assemble of nanoparticles. One broad peak in the small-angle X-ray scattering (SAXS) analysis (Figure 1A) of mesoporous αFeOOH 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 disordered nature of pore arrangement. The wide angle powder XRD (PXRD) patterns of mesoporous α-FeOOH nanoparticles, shown in Figure S3, suggested that the obtained nanoparticle are semicrystalline in nature and of goethite (α-FeOOH) phase (JCPDS 01-081-0463).19 The broad peak result for these nanoparticles in PXRD is attributed to the much smaller particle size and lower crystallinity. 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 the 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 a feature 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 (SMF8) indicates the existence of the disordered nature of the pore arrangement, which agrees with our SAXS analysis result (Figure 1A). The BET surface areas for samples SMF-4, SMF6, and SMF-8 were 86.6, 152.4, and 74.8 m2 g−1, and the pore 1274

DOI: 10.1021/acssuschemeng.6b00937 ACS Sustainable Chem. Eng. 2017, 5, 1272−1279

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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.

Figure 4. XPS analysis of SMF-6 before and after Hg2+ adsorption (a) survey XPS and high resolution XPS of (b) Fe 2p (c) O 1s (d) Hg 4f.

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 high-resolution 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 highresolution 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 α-FeOOH framework.37,38 The area ratio of Peak II/Peak I are 2.75 and 2.78 for the materials SMF-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. The mercury contents in different aqueous solutions before and after adsorption studies are shown in Table 1. As seen from Table 1 the adsorption efficiency for SMF-6 varies from ca. 98− 73% and for SMF-8 varies from ca. 97−70% for different stock solutions. The removal efficiency of Hg2+ was determined using the following expression:

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) SMF-8.

The adsorption experiment over SMF-4, SMF-6, and 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 showing the highest adsorption efficiency may be attributed to the high surface area. Further to reveal the adsorption properties of the adsorbent, valence states of the elements, and surface composition present in the adsorbent, the XPS 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

removal efficiency (%) =

(C0 − Ct ) × 100 C0

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: 1275

DOI: 10.1021/acssuschemeng.6b00937 ACS Sustainable Chem. Eng. 2017, 5, 1272−1279

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The kinetics graph displays that initially SMF-6 materials adsorbed Hg2+ at a high rate of about 79% removed during the first 60 min (initial Hg2+ conc 100 mg/L). After that, there was a gradual decrease in the rate of removal and equilibrium was reached. In 240 min, 86.7% mercury ions were adsorbed onto the surface of nanoparticle. This suggests 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: t t 1 = + Qt Qe k 2Q e 2

Table 1. Ion Exchange Carried out over SMF-6 and SMF-8 Materials in Mercury Chloride Solution Hg2+ cation content in solution (mg/L) adsorbent

before

after

Hg2+ removal efficiency (%)

SMF-6

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

98.45 91.1 86.7 82.2 73.2 97.3 84.5 80.1 77.9 70.1

SMF-8

Kd =

Kd =

Hg2+ distribution coefficient (Kd) (mL/g) 6.35 1.02 5.71 4.63 2.73 3.60 5.44 4.02 3.51 2.34

× × × × × × × × × ×

104 104 103 103 103 104 103 103 103 103

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

number of ions adsorbed per gram of the solid number of ions present per milliliter solution after exchange

Csolid(mol g −1) Cwater(mol mL−1)

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

(C0 − Ct )V M

Where C0 (mg/L) and Ct (mg/L) initial concentration and the concentration after time t of Hg2+ respectively, and V (L) is the volume of Hg2+ solution. M represents the amount of adsorbent used. Table 1 shows 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 commensurate with different functionalized mesoporous materials. Hg2+ Adsorption Kinetics. The kinetic mechanism of the mercury adsorption process was evaluated on the surface of self-assembled mesoporous α-FeOOH nanoparticle adsorbent. For this purpose, a time-dependent adsorption experiment was carried out. The experimental results are shown in Figure 5.

Figure 6. (a) Adsorption isotherms of SMF-6 and adsorption isotherms fitted by (b) the Langmuir and (c) the Freundlich models.

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 (Qecal 92.16 mg g−1) value also agrees with the experimental data (Qeexp 85.48 mg g−1) using a pseudo-second-order kinetics model for self-assembled mesoporous α-FeOOH nanoparticles. Hg2+ Adsorption Isotherm. The adsorption capacity of the self-assemble mesoporous α-FeOOH nanoparticle was determined using the adsorption isotherm of Hg2+ over HMF6. The results are shown in Figure 6. At an initial mercury concentration of 200 mg L−1, 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 analyzed 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:

Figure 5. (a) Time dependence of Hg2+ adsorption on SMF-6 and (b) kinetic equation fitting lines for adsorption of Hg2+ over SMF-6 using a pseudo-second-order kinetic model. Conditions: Hg concentration = 100 mg L−1. 1276

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Figure 7. (a) Effect of pH in the adsorption of Hg2+ on SMF-6 and (b) recycling of SMF-6 materials.

Hg2+ ion adsorption over the self-assembled mesoporous αFeOOH nanoparticles (SMF-6) was tested at different pH values and is shown in Figure 7a. The graph shows a trend of adsorption capacity of the SMF-6 nanoparticle on Hg2+ ions in the pH range of 4−10. The result indicate that the removal efficiency of SMF-6 nanoparticles is influenced by the pH of the solution during the adsorption process. In the pH range 4−6, the removal efficiency of SMF materials increased and then slightly decreased with pH above 6. The decrease of efficiency at higher pH may be attributed to Hg(OH)2 formation and decrease the interaction between them. Furthermore, we also study the influence of common metal ion on Hg2+ removal as wastewater usually contains alkali and alkaline earth metals. Here we study the adsorption of Hg2+ in the presence of Na+, K+, Mg2+, Ca2+, and the results are presented in Figure S5a. We also study the adsorption of Hg2+ in the presence of Fe3+, Ni2+, Cu2+, Zn2+, and the results are presented in Figure S5b. The results suggest that Hg2+ adsorption is not affected by these metal ions except for Ca2+, Ni2+, and Fe3+. However, the removal efficiency is decreased too much in the presence of Cd2+. The effect of competitor heavy-metal ions suggests that the material does not show any selectivity to Hg2+ adsorption which is expected for this type of adsorbent. Regeneration of Adsorbents. For practical applicability of the adsorbents, it is essential to check their usage upon recycling. Self-assembled mesoporous α-FeOOH material was tested for reiterated adsorption experiments. SMF-6 (used as a representative material) was collected after the test and washed several times with NaNO3 + NH4Cl solution to rinse the adsorbed ions from the materials, and dry it well in vacuum for the next run. The recycling efficiency of the materials is shown in Figure 7b. The nanoparticles are easily recoverable, and the materials demonstrate extremely extraordinary reuse proficiency as they can be reused no less than four times without significant loss of adsorption efficiency.

Ce C 1 = e + Qe Qm KLQ m

where Ce, Qe, Qm, and KL are the equilibrium concentration of the adsorbate (mg L−1), the amount of adsorbate adsorbed per unit mass of adsorbate (mg g−1), and 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 nanoparticles fits well with this Langmuir adsorption model. The calculated Langmuir constants Qm and KL values 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 the Langmuir model better demonstrates 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. When the isotherm is unfavorable, RL > 1; when linear, RL = 1; when favorable, 0 < RL < 1; and when irreversible, RL = 0. The calculated RL value is 0.073, and this is attributed to the adsorption of Hg2+ on the surface of SMF-6 nanoparticle being favorable.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 log Qe vs log Ce, and other parameters are calculated using the following expression: log Q e =

⎛1⎞ ⎜ ⎟log C + log K e F ⎝n⎠

Where KF and n are Freundlich constants: KF is the adsorption capacity of the adsorbent and n is an indication of how favorable the adsorption process.45 The linear plot between log Qe vs log Ce is fitted, and the correlation coefficient (R2) of the line reaches up to 0.968. The adsorption is evidently favorable from the value of the Freundlich constant n which is 2.079, while the constant KF has a value of 23.067 mg g−1 (L/mg)1/n, respectively, indicative of the high affinity of the Hg2+ on the surface of self-assembled mesoporous α-FeOOH nanoparticles (SMF-6). Effect of pH and Competitor Metal Ions. The adsorption of hazardous ion from the solution is too much influence with pH of the solution as the ion exists in different form in different pH of the solution. An appropriate pH value can improve/reduce the adsorption efficiency of the adsorbent. Mercury ions exist as Hg2+, Hg(OH)+, Hg(OH)2, and combination of them in solution in different pH.46 Hg2+ exists below pH 3, Hg(OH)2 above pH 6, and both of these two species along with Hg(OH)+ coexist at a pH of 3−6.46 The

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CONCLUSION In conclusion, a self-assembled mesoporous α-FeOOH material was built up by a simple and ecologically friendly method at room temperature using the supramolecular chemistry of sodium salicylate as a structure directing molecule. In this synthesis method, high surface area (152.4 m2 g−1) mesoporous α-FeOOH materials were obtained due to self-assembly of low dimension α-FeOOH nanoparticles (size ∼3 nm). The XPS analysis confirmed that the materials contain a large amount of surface Fe−OH groups which are the active suite for Hg2+ adsorption. Hg2+ adsorption was explored on the surface of selfassembled mesoporous α-FeOOH material in detail. The SMF6 material shows the highest uptake with 98% removal efficiency of 20 mg L−1 Hg2+ ions within 180 min with a 1277

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ACS Sustainable Chemistry & Engineering large distribution coefficient (Kd = 6.35 × 104) between the solid and aqueous phases. The adsorption kinetics fit well with the pseudo-second-order model, and the adsorption isotherm also fit well with the Langmuir and Freundlich adsorption isotherm models. At an initial mercury concentration of 200 mg L−1, the adsorption capacity of SMF-6 is calculated to be 146 mg g−1. The present synthesis has a remarkable advantage in its simplicity for the synthesis of mesoporous α-FeOOH and not only improved surface area but also superior Hg2+ adsorption performance, which makes this material a promising adsorbent for other hazardous heavy metal ions to meet the demands of environmental protection.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b00937. FTIR analysis, SAXS, and wide angle PXRD, removal efficiency of the materials, effect of different metal ions, comparison with conventional adsorbents, and additional information as noted in text. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +82-31-290-7250. Fax: +82-31-290-7270. E-mail: [email protected] (D.K.). ORCID

Astam K. Patra: 0000-0001-6071-8653 Dukjoon Kim: 0000-0001-6187-0737 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS D.K. received a grant from the National Research Foundation of Korea, which is funded by the Korean Government (MEST) (NRF-2009-0093033, NRF-2010-0027955, and NRF2012R1A2A1A05026313).

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